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scanned by dEnSiZ
Masao Kaneko
Ichiro Okura
(Eds.)
Photocatalysis
Science and Technology
With 244 Figures and 35 Tables
@J Kodansha
Springer

Professor Masao Kaneko
Faculty of Science,
Ibaraki University
2-1-1 Bunkyo. Mito 310-8512
Japan
E-mail:
kanekom@mito.ipc.ibaraki.ac.jp
Professor Ichiro Okura
Department of Bioengineering,
Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku
Yokohama 226-8501
Japan
E-mail: iokura@bio.titech.ac.jp
List of Contributors
Numbers in parentheses refer to the chapters.
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Amao, Yutaka (18) Faculty of Engineering, Oita University, Dannoharu, Oita
870-1192, Japan ""
Anpo, Masakazu (10) Department of Applied Chemistry, Graduate School of
Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
Arakawa, Hronori (14) Photoreaction Control Reseach Center (PCRC) National
InstItute of Advanced Industrial Science and Technology (AIST), Higashi,
Tsukuba,lbaraki305-8565,Japan
Domen, Kazunari (16) Chemical Resources Laboratory, Tokyo Institute of
Tehnology, Midori-ku, Yokohama 226-8501, Japan; CREST, Japan
SCIence and Technology Corporation
Fujishima, Akira (2) Department of Applied Chemistry, School of Engineering
Uni,-:esity of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan '
Hara, Michikazu (16) Chemica] Resources Laboratory, Tokyo Institute of
Technology, Midori-:ku, Yokohama 226-8501, Japan
Harada, Hisashi (12) Department of Chemistry, Faculty of Physical Sciences and
Engineering, Meisei University & Advanced Materials Research and
Development Center of Meisei University, Hino, Tokyo 191-8506, Japan
Hashimoto, Kazuhito (7) Research Center for Advanced Science and Technology
(RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan
Ibusuki, Taashi (8) .Institute for Environmental Management Technology,
. NatIOn,al InstItute of Advanced Industrial Science and Technology,
Tsukuba, Ibaraki 305-8569, Japan _
Inoue, Yasunobu (15) Department of Chemistry, Nagaoka University of
Technology, Nagaoka, Niigata 940-2188, Japan
Ishibashi, Ken-ichi (7) Kanagawa Academy of Science and Technology Takatsu-
ku, Kawasaki 213-0012, Japan '
Kaneko, Masao (1, 19) Faculty of Science, Ibaraki University, Mite, l,baraki 310-
8512, Japan '
Kera, Y oshia () Department of Applied Chemistry, Faculty of Science and
Engmeenng, Kinki Univrsity, Higashiosaka, Osaka 577-8502, Japan
ISSN 1618-7210
ISBN 4-06-210615-9 Kodansha Ltd., Tokyo
ISBN 3-54 0 -43473-9 Springer-Verlag Berlin Heidelberg New York
Library of Congress Cataloging in Publication Data
Photocatalysis: science and technology I Masao Kaneko, Ichiro Okura (eds.).
p.cm. _ (Biological and medical physics series, ISSN 1618-7210) Includes bibliographical references (p. ).
ISBN 3540434739 (hc: alk. paper) - ISBN 4062106159
1. Photocatalysis. I. Kaneko, Masao, 1942- II. Okura, Ichiro, 1944- III. Series.
QD716.P45 P44 2002 541.3'95-dc21 200207 2 7 8 9

VI List of Contributors
Kitamura, Takayuki (20) Graduate School of Engineering, Suita, Osaka 565-
0871, Japan
Kominami, Hiroshi (3) Department of Applied Chemistry, Faculty of Science
and Engineering, Kinki University, Higashiosaka, Osaka 577-8502, Japan
Matsumura, Michio (17) Research Center for Solar Energy Chemistry, Osaka
University, Toyonaka, Osaka 560-8531, Japan
Minoura, Hideki (6) Department of Chemistry, Faculty of Engineering, Gifu
Unversity, Yanagido, Gifu 501-1193, Japan
Murakami, Shin-ya (3) Department of Applied Chemistry, Faculty of Science
and Engineering, Kinki University, Higashiosaka, Osaka 577-8502, Japan
Nakata, Yoshihiro (4) Department of Chemistry, Graduate School of Engineering
Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
Narasinga Rao, Tata (2) Department of Applied Chemistry, School of
Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
Nosaka, Yoshio (5) Department of Chemistry, Nagaoka University of Technology,
Kamitomioka, Nagaoka 940-2188, Japan
Ohko, Y oshihisa (2) Department of Applied Chemistry, School of Engineering,
University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
Ohno, Teruhisa (11, 17) Research Center for Solar Energy Cemistry, Osaka
University, Toyonaka, Osaka 560-8531, Japan
Ohtani, Bunsho (3, 11) Catalysis Research Center, Hokkaido University, Kita-ku,
Sapporo 060-0811 , Japan
Okura, Ichiro (1, 18) Department of Bioengineering, Tokyo Institute of
Technology, Midori-ku, Yokohama 226-8501, Japn
Sato, Shinri (13) Catalysis Research Center, Hokkaido University, Kita-ku,
Sapporo 060-0811 , Japan
Sayama, Kazuhiro (14) Photoreaction Control Reseach Center (PCRC) National
Institute of Advanced Industrial Science and Technology (AIST), Higashi,
Tsukuba,Ibaraki305-8565,Japan
Tanaka, Keiichi (9) Faculty of Engineering, Oita University, Dannoharu, Oita
8 70-1192, Japan
Watanabe, Toshiya (7) Research Center for Advanced Science and Technology
(RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan
Yanagida, Shozo (20) Graduate School of Engineering, Suita, Osaka 565-0871,
Japan
Yoshida, Tsukasa (6) Department of Chemistry, Faculty of Engineering, Gifu
Unversity, Yanagido, Gifu 501-1193, Japan .
Preface
Environmental pollution is becoming more and more serious. The green
house effect is brought about by the increase in the carbon dioxide concentration
in the atmosphere. This, as well as acid rain, is closely. related to the use of fossil
fuels. In order to solve such environmental a'nd energy resource problems,
photocatalysis has been attracting a great deal of attention. Water photocatalysis
by ultraviolet light using a titanium dioxide photoanode, reported in Nature in
1972 and called the Honda-Fujishima effect, activated research on solar energy
conversion by a photocatalytic method instead of conventional semiconductor
photovoltaic cells. Intense research carried out over the past two decades has
resulted in improved environmental cleaning. For environmental cleaning, various
industrial products based on TiO z can decompose hazardous substances in our
living environment. As for the energy problem, dye-sensitized nanocrystalline
Ti0 2 film achieved efficient solar energy conversion to electricity with a
conversion yield comparable to that of the amorphous silicone photovoltaic solar
cell, and this new type of photochemical cell, called Graetzel' s cell, is now under
development for commercialization.
In 1982 a symposium in Japan on "Catalytic Chemistry Involving Light"
was organized to study photocatalysis and to exchange knowledge on related
topics.This has become a center of activity for Japanese researchers in this field.
The symposium was at first supported by the Solar Energy Research Group of the
RIKEN Institute (The Institute of Physical and Chemical Research) and later by
the Catalysis Society of Japan. In commemoration of the 10th symposium we
published a Japanese review of photocatalysis. The present volume is being
published to commemorate the 20th symposium. The aim of this monograph is to
review all the important and recent research activities on photocatalysis and to
provide the readers with a comprehensive as well as deep understanding of the
topics to further promote related researches on photocatalysis.
The editors are very thankful to the authors who, in spite of their busy life in
research and teaching, willingly accepted the call to contribute and sent their
manuscripts in time. Weare also thankful to Mr. Ippei Ohta of Kodansha
Scientific Ltd. for his efforts and kind advice in publishing this book.
February 2002
Masao Kaneko
/chiro Okura

Contents
List of Contributors ........ - -- -...,....... ...... ....... ............... ................................. v
Preface ........................................ - - -. - -........................................................... Vll
1 Introduction ...... ........ ...... .......................................... .....--... ............... 1
1.1 Background .................................................................. --............ 1
1.2 Aim and Outline of This Volume ............................................. 2
1.3 SumInary .................................................................................... 4
1.4 Future Perspectives ...................... - --........................................... 4
References ..................................................................... h..................... 5
I Fundamental Aspects of Photo catalysts
"
2 Photoelectrochemical Processes of Semiconductors ................. 9
2.1 Semiconductor Electrodes for Solar Energy Conversion 11
2.2 Reduction of CO 2 at Illuminated Semiconductor Electrodes 15
2.3 Photocatalysis ............................................................................ 18
2.3.1 General Remarks ........................................................... 18
2.3.2 Mechanistic Studies ...................................................... 19
2.3.3 Low Intensity Illumination ........................................... 22
2.3.4 Applications .................. ........ .......... .... ........ ........ ........... 24
References ......................... .......................... .... .................... ................. 26
3 Design, Preparation and Characterization of Highly Active Metal
OXIde Photocatalysts ................................................................ ......... 29
3.1 Introduction ......... ................................. .............. ............... _h. ..... 29
3.2 Photocatalytic Activity .............................................................. 29
3.2.1 Effect of Surface Area on Photocatalytic Activity 30
3.2.2 Effect of Electron-hole Recombination on Photocatalytic
Acti vi ty ................................................................... -...... 32
3.2.3 Design of Photo catalysts of High Activity.................. 33
3.3 Preparation of Titanium(IV) Oxide Powders ........................... 33
3.3.1 Sulfate Method .............................................................. 33
3.3.2 Chloride Method (Vapor Method) ................................ 34
3.3.3 Alkoxide Method ........................................................... 34

x
Contents
3.4
3.5
3.3.4 Specific Methods .... - - .-....-.. -.., - d. -.... U ..0...... uu................ 34
3.3.5 Activation of Ti0 2 Photocatalysts .0.. ........ u............ ...... 36
Preparation of Other Photocatalysts ......................................... 38
Characterization of TiO z Photocatalysts of Both High Crystallinity
and Large Surface Area ............................................................. 38
3.5.1 Photocatalytic Activity of HyCOM TiO z in Aqueous
Suspension Systems ...................... --.............................. 38
3.5.2 Correlation Between Physical Properties and Photocatalytic
Activity of HyCOM TiO z .............................................. 39
3.5.3 Novel Hypothesis for Activity of Photocatalyst 43
Preparation and Characterization of Photocatalytic Thin Films
3.6
3.6.1 Preparation of Photocatalytic TiO z Thin Films ...........
3.6.2 Characterization of Photocatalytic Thin Films Prepared
from HyCOM TiO z Powders ........................................
3.7 Summary ....................................................................................
References ............................................................................................
4 Photoelectrochemistry at Semiconductor/Liquid Interfaces 51
4.1 Introduction ............ ................................ ............. ....................... 51
4.2 Basic Properties of Semiconductor/Li,quid Interface ............... 52
4.2.1 Band Bending ................................................................ 52
4.2.2 Barrier Height and Flat Band Potential........................ 54
4.2.3 Electron Transfer and Corrosion Reactions ................. 57
4.3 Photoe1ectrochemistry at Atomically Well-defined Surfaces.. 59
4.3.1 Atomically Flat H-terminated Si Surfaces ................... 59
4.3.2 Selective 'Exposition of (100) Face on n-TiO z (Rutile) by
Photo etching .................................................................. 62
4.4 Photoelectrochemistry at Metal Dot-coated Semiconductors .. 64
4.4.1 Ideal Semiconductor Electrodes ................................... 64
4.4.2 Metal-loaded TiO z Electrodes ....................................... 66
References ............................................................................................ 67
5 Photoelectrochemical Reactions at Semiconductor Microparticle .
5.1 Introduction ................ ........ ......... ................. ................ ..............
5.2 Energy Structure of Semiconductor Microparticle ..................
5.2.1 Depletion Layer .............................................................
5.2.2 Electric Heterogeneity of Surface ................................
5.2.3 Size Quantization Effect ...............................................
5.3 Kinetics at Semiconductor Microparticle ....................................
5.3.1 Recombination Model .-... -- --........ ..................... ...............
5.3.2 2D Ladder Model..........................................................
5.3.3 Effect of Size ,......... ....... ............................. ............... ....
5.4 Observation of Primary Reaction Intermediates' .........................
5.4.1 ESR Analysis for Irradiated TiO z Particles ..................
5.4.2 Direct Observation of Intermediate Radicals ...............
5.4.3 Chemiluminescent Probe for Active Oxygens .............
References ............................................................................................
Contents
xi
6 Ne pproaches in Solution-phase Processing of Semiconductor
ThIn FIlms .......................................................................................... 87
6.1 Intro.duction .... ........... ...................... ...... ........ ............................, 87
6.2 PrvlOs Methods for Solution-phase Deposition of Semiconductor
ThIn FIlms .................................................................................. 89
6.2.1 Chemical Bath Deposition of Metal Sulfide Thin Films
89
6.2.2 Electrodeposition of Metal Sulfide Thin Films ........... 90
6.2.3 Chemical and Electrochemical Deposition of Metal Oxide
Thin Films .. . . .. .. .. .. .. . .. .. . .. . .. . . . . .. .. .. .. .. . . .. .. .. .. .. .. . . . .. .. . .. .. .. . . . 92
6.3 Elctrchemically Induced Chemical Deposition (EICD) of CdS
ThIn FIlms '" ............. ............................ ......... ........ ...... ............... 93
6.3.1 Idea ..:..... ..... .......................... ............. ............. ................ 93
6.3.2 Morphological and Structural Analysis ........................ 94
6.3.3 Growth Kinetics and Mechanism of EICD Process 95
6.3.4 Modification ofEICD Process ...................................... 97
6.4 True lectrodeposition of Metal Sulfide Thin Films by Reduction
of ThlOcyanato Complexes ......... ..... ............................. .............
6.4.1 Idea .......... ...............,................... ....... ...... ........ ...............
6.4.2 Thermodynamic Consideration .....................................
6.4.3 Electrochemical Layer-by-Iayer Growth of CdS
Thin Films ............. .................,...... ........... ......... ............. 98
6.4.4 Electrodeposition of Other Metal Sulfides ................... 100
6.5 Electrochemical Self-assembly of ZnO/Dye Hybrid
Thin Films ................................................ 100
6.5.1 Idea . ................ ..................................
. , .............................................................. 100
6.5.2 Electrochemical Self-assembly of ZnO/Dye Hybrid
Structure ................................
.........................................
6.5.3 Mechanism of Electrochemical Self-assembly............
6.6 Summary ....................................................................................
References
............................................................................................
44
44
45
47
47
............................................................................................
69
69
69
69
71
72
72
73
74
76
77
78
81
83
85
II Application to Environmental Cleaning
7 Self-cleaning Properties of TiOrcoated Substrates '.................
7.1 Introduction ................................................................................
7.2 Photocatalytic Decomposition ............. .....................................
7.2.1 Air Purifying Effect ......................................................
7.2.2 Sterilization Effect
.................................................
7.2.3 Anti- fouling Effect ........................................................
7.2.4 Photo-induced High Amphiphilicity ............................
7.3 Conclusions ........,.........................
References ....................................... ."......
.............................................................................................
8 Cleaning Atmospheric Environment .............................................
8.1 Introduction .................................................
8.2 Photocatalytic Activities of TiO z ...............:::::::::::::::::::::::::::::::
97
97
98
102
104
104
105
109
109
110
110
111
113
114
120
121
123
123
124

Xll
Contents
8.2.1 Oxidation of Air pollutants by Photogenerated Active
Oxygen Species ........................ ..........,........ ..................
8.2.2 Photocatalytic Reactions of Volatile HYdrocrbon\
8.2.3 Photocatalytic Reactions of Halogenated Hy rocar ons
............................................................................................
8.2.4 Nitrogen Oxides (NO x ) ..................................................
8.3 Development of Air Purifying Materials Based on
Photocatalyst ..............................................................................
8.3.1 Immobilization of Powder Photo catalysts ...................
8.3.2 Preparation of Air-purifying Matrials. .:......................
8.3.3 Performance Characteristics of Alr-punfymg
Materials ........................................................................
8.4 Application of Photocatalysis to Cleaning of Atmospheric
Environment ...............................................................................
8.4.1 Passive Purification of Polluted Air .............................
8.4.2 Active Air purification of Closed Space ......................
8.5 Summary ................................................. ............................ .......
References ............................................................................................
9 Water Purification-Degradation of Aqueous pollutant and
Application to Water Treatment ....................................................
9.1 Introduction . .......... ,......................... ........... .... ...:...... ........... .......
9.2 Photocatalytic Characteristics of Titanium DIOxIde ................
9.3 Photocatalytic Degradation of pollutant ...................................
9.3 .1 Volatile Organohalide Compound ................................
9.3.2 Pesticides .......................... ....... ............ ..........................
9.3.3 Other Organic Compounds ...........................................
9.3.4 Environmental Hormones (Endocrine Disruptors)
9.4 Enhancement of Degradation Rate ...........................................
9.4.1 Pt-Ioading .......... ....... ............ .......... ........... .....................
9.4.2 Addition of HzOz ...........................................................
9.4.3 Ozone ..... .................. ...... ............ .................. .... ..............
9.4.4 [ncrease in Adsorption ..................................................
9.5 Solar System for Water Treatment ............................................
9.6 Immobilization of TiO z and InstrumentatIOn ...........................
9.7 Conclusion and Outlook ............................................................
References ............................................................................................
Contents
Xl1l
124
125
11
136
143
147
147
148
149
151
151
153
154
155
III Application to Photo energy Conversion
Photocatalytic Organic Syntheses Using Semiconductor
Particles ..... ................... ................. .......... ............ ....... ....... ................. 185
11.1 Introduction ................................................................................ 185
11.2 Principle of Photocatalysis by Semiconductor Particles .......... 186
11.3 Photocatalytic Reactions by Semiconductor Suspension 187
11.4 Redox Combined Photocatalytic Processes for Nitrogen-containing
Substrates ........ ........ ........ .'.... .................... .......................... ........ 189
11.5 Further Development to Stereoselective Organic Synthesis of
Nitrogen-containing Compounds ....... ........ ............. .................. 191
11.6 [ntroduction of Oxygen Atoms into Organic Compounds 194
11.6.1 Sterospecific Epoxidation of 2-hexene on Photoirradiated
TiO z Powders Using Molecular Oxygen as Oxidant 195
11.6.2 Selective Oxidation of Naphthalene by Molecular Oxygen
and Water Using TiO z Photocatalysts .......................... 196
11.6.3 Photocatalytic Oxygenation: Summary........................ 198
11.7 Concluding Remarks ................ ...... ........ ...... ..... ........................ 199
References . ........................................ .......... ............ ............................. 199
157
157
157
160
160
162
164
165
166
166
167
169
169
171
171
172
172
12 Sonophotocatalysis-Joint System of Sonochemical and
Photocatalytic Reactions .. ................................................ .......... ..... 203
12.1 Introd,uction-What is Sonophotocatalysis? ............................ 203
12.2 Utilization of Sonophotocatalytic Reaction ............................. 204
12.2.1 Sonophotocatalysis of Water ... .................. ........... ........ 204
12.2.2 Sonophotocatalysis of Artificial Seawater ................... 216
12.2.3 Sonophotocatalyses of Organic Compounds ................ 219
12.3 Conclusion and Future Scopes ................................................., 220
References .... ........ ..................... ........... ...... ............... ........ ........... ........ 221
13 Gas-ph':lnse Water Photolysis by NaOH-coated Photocatalysts .... 223
13.1 [ntroduction ...... .......................... ........ ............. ........ ......... .......... 223
13 .2 Water Photolysis by Pt/TiO z .... ........ .............. ................... ........ 224
13.3 Water Photolysis by Metallized Semiconductor Powders 226
13.3.1 Gas-phase Water Photolysis by NaOH-coating ........... 226
13.3.2 Factors Influencing Yield of Water Photolysis ............ 229
13.4 Concluding Remarks ................................................................. 233
References ....... .... ................. ............................................. .......... ......... 234
10 Second-generation TiO z Photo catalysts Able to lnitiate Reactions
Under Visible .Light lrradiation ... ...... ......... .......... .......... ........ ...... \;
10.1 IntroductIon ......:......................................................................... 175
10.2 Experimental ectIo? ................................................................. 176
10.3 Results nd DISCUSSIOn .............................................................. 182
10.4 ConclusIon ........... .................. ............ .......... .............................. 182
References ............................................................................................
14 Water Photolysis by TiO z Particles-Significant Effect of
Na z C0 3 Addition on Water Splitting ............................................ 235
14.1 Introduction .......... ..... .... ................ .............. ....... ..... ......... .......... 235
14.2 Significant Effect of Carbonate Salt Addition on Water Splitting
from PtfTiO z Water Suspension ................................................ 236
14.3 Role of Carbonate Salts on Water Splitting and Reaction
Mechanism ............. ..................... .............. .................... ............. 240

XIV
Contents
14.4 Effective Screening of Active Photocatalysts for Water Splitting
Using Na2C03 Addition Method ...............................................
14.5 Solar Hydrogen Production Using Na2C03 Addition Method ..
14.6 Conclusion ......................... ........... ........... ................ ..................
References ............................................................................................
15 Water Photolysis by Titanates with Tunnel Structures ..............
15.1 Water Photolysis by RuOiBaTi 4 0 9 with Pentagonal Prism
Tunnel Structure ............... ................................................. ........
15.2 Water Photolysis by RuOiN 2 Ti 6 0 13 with Rectangular Tunnel
Structure ...... - ..................... .........................................................
References ............................................................................................
16 Water Photolysis by Layered Compounds ...................................
16.1 Introduction .................... .................... ........ .......... ................. .....
16.2 Layered Oxides of Transition Metals .......................................
16.3 Nb60I7 ..... ......... .......... ............................ ........ ...... ..................
16.3.1 Structure amI Physico-chemical Properties ..................
16.3.2 Photocatalytic Overall Water Splitting ........................
16.3.3 Structure of Ni-Ioaded Nb60I7 and Reaction
Mechanism .....................................................................
16.4 Perovskite-related Layered Oxides ...........................................
16.5 Summary.............. ........ ..... .................... .................. ............. ......
References ........ ....................................................................................
17 Splitting of Water by Combining Two Photocatalytic Reactions
via Quinone Redox Couple Dissolved in Oil Phase: Artificial
Photosynthesis .. ...... - .-.. .-- - .-. ...................... ...... ... ........ ............. '" ........
17.1 Introduction .... .......... ....... ........ ........ .............................. ..... ........
17.2 Strategy for Water Splitting by Mimicking Photosynthesis
17.3 Photocatalytic Hydrogen and Oxygen Evolution in Separate
Systems ......................................................................................
17.3.1 Photooxidation of Water Using Ti0 2 Particles ............
17.3.2 Photoreduction of Water Using Pt-Ioaded Ti0 2
Particles ..........................................................................
17.4 Approaches to Electrochemical and Chemical Combinations of
Two Photocatalytic Reactions ...................................................
17.5 Splitting of Water by a Combination of Two Photocatalytic
Reactions via DDQ/DDHQ ................. ................ .............. ........
17.6 Conclusions .... .......... .... .................................. .......... ..................
References ............................................................................................
18 Sensitization by Metal Complexes Towards Future Artificial
Photosynthesis ...................................................................................
18.1 Introduction ................................................................................
18.2 Photoinduced Hydrogen Evolution in Homogeneous
Four-component Systems .......... ,- -. --.- -..-.... ......... ............ ...........
242
246
248
248
249
250
257
260
261
261
261
263
263
265
19
267
268
276
276
279
279
280
281
282
285
286
289
291
291
293
293
294
Contents
XV
18.2.1 Photoinduced Hydrogen Evolution with Porphyrin Metal
Complexes and Hydrogenase ........................................ 294
18.2.2 Photoinduced Hydrogen Evolution Using Cytochrome
C3 as Electron Carrier .................................................... 296
18.2.3 Photoinduced Hydrogen Evolution Using
Chemically-modified Chlorophyll ................................ 298
18.3 Photoinduced Hydrogen Evolution with Viologen-linked
orphyrin Metal Complexes ........................................................ 299
18.3.1 Photoinduced Hydrogen Evolution with Water-soluble
Viologen-linked Cationic Porphyrin Metal Complexes
and Hydrogenase ......................................................... -. 300
18.3.2 Photoinduced Hydrogen Evolution with Water-soluble
Viologen-linked Anionic Porphyrin and Hydrogenase
........................................................................................
18.4 Other Systems for Hydrogen Evolution Using Natural
Photo sensitizers .........................................................................
18.5 Conclusion ..... ........ ...-...... ...... ....... ...... ............... ....... ......... ........
References
............................................................................................
Catalyses and Sensitization for Water Reaction Towards Future
Artificial Photosynthesis .................................................................
19.1 Introduction .. -.............................................................................
19.2 Design of Artificial Photosynthesis ..........................................
19.2.1 Photosynthesis and Energy Cycle on Earth .................
19.2.2 Artificial Photosynthesis .. -............................................
19.3 Molecular Catalysts for Water Reactions and CO 2 Reduction
19.3.1 Catalysis in Water Oxidation ..-.....................................
19.3.2 Catalysis in Proton Reduction ......................................
19.3.3 Catalysis in Carbon Dioxide Reduction .......................
19.4 Photo excited State Electron Transfer in Heterogeneous Phases
....................................................................................................
19.5 Sensitization of Ti0 2 Powders and Films in Water .................
19.6 Conclusion and Future Prospects .....................................
References
............................................................................................
20 Photoelectric Ti0 2 Solar Cells
.......................................................
20.1 Introduction . ........ ........................................................ ...... .........
20.2 Dye-sensitization of Semiconductors .......................................
20.2.1 History....... ........... ......... .................. ......... .....................
20.2.2 Innovative Dye-sensitized Solar Cells ........................
20.2.3 Fabrication of Dye-sensitized Ti0 2 Solar Cells ...........
20.2.4 Characterization of Innovative Dye-sensitized
Ti O 2 So lar Cells .. -..........................................................
20.3 Electron-transfer Seqsitization on Ti0 2 ....................................
20.3.1 Bonding Structure of Dye on Ti0 2 Influencing 1Jei ......
20.3.2 Dynmics in Electron Transfer from Photo excited Dye 2
to TI0 2 ............................................................................
302
303
306
306
309
309
309
309
311
312
312
316
316
317
320
322
323
325
325
325
325
327
328
329
330
331
331

20.3.3 Electron Transfer Between Oxidized Dye 2 and 1-/13-
Electrolyte ......................................................................
20.4 Electron Transport in Porous TiO z Electrodes .........................
20.4.1 Electron Transport Models for High 1]et .......................
20.4.2 Time-course Analysis ....................................................
20.4.3 Frequency Analysis . .............................................:.........
20.4.4 Effect of TiO z Films on Performance of Dye-sensItIzed
Solar Cells ....... ................................. ..........................
20.5 Sensitization Dyes ........................................... -.........................
20.5.1 Ruthenium Polypyridine Complexes ............................
20.5.2 Other Metal Complexes ..........................................
20.5.3 Organic Dyes ............,............ ........................................
20.5.4 Natural Dyes .. ....... .................... ............... ............ .......
20.6 Recent Research Progress in Dye-sensitized Solar Cells
20.7 Future Work on Dye-sensitized Solar Cells ..uu......... u ............
20.8 Concluding Remarks .................................................................
References ......................, -................................................................-
Index ............................. ...................................................... -.................
332
333
334
335
335
1
Introduction
337
337
337
339
340
342
343
344
345
346
1.1 Background .
Catalysis under light irradiation, called photocatalysis, is attracting a great
deal of attention from view points of fundamental science and applications for
practical use. A. Fujishima and K. Honda in 1972 1 ) achieved ultraviolet light-
induced water cleavage using a titanium dioxide (TiO z ) photo anode in
combination with a platinum counter electrode soaked in an electrolyte aqueous
soltion and reported it just before the oil shock of 1973. This opened up the
Jb'ssibility of solar energy conversion by semiconductors or sensitizers. Recently,
this type of reaction has been commercially applied to environmental cleaning by
utilizing photocatalytic oxidation of organic compounds by TiO z powders or
coating. This monograph is published for use by researchers interested in energy
and environment since the photocatalysis involves important science and
technology for creating new energy resources from sunshine as well as for
cleaning the environment. The earth is facing difficult problems regarding the
globa environment and energy resources. Global warming due to the increase of
carbon dioxide concentration in the atmosphere (greenhouse effect) as well as acid
rain due to NO x and SOx are typical environmental problems caused by the
combustion of fossil fuels. In this regard it is important to create renewable
energy resources instead of using fossil fuels.
During the 1970s and 1980s much work was carried out on photochemical
solar energy conversion by semiconductors or sensitizers to produce fuels by
solar energy.Z-4) The most typical system for such fuel production is water
cleavage by light.
349
2H z O + h v  2Hz + Oz
(1.1)
Since the bandgap of TiO z is 3 eV and can utilize only UV light below 400 nm,
many attempts have been made to sensitize large bandgap semiconductors or to
utilize narrow bandgap semiconductors that can absorb visible light. However, as
far as water photolysi is concerned, utilization of visible light for water cleavage
has been unsuccessful.
Utilization of water suspension of fine powder semiconductors with deposited
noble metal such as platinum has also been intensively investigated in the effort
to achieve water photolysis. There have been some reports using platinized TiO z

L.
I mlroaucnon
1.2 Aim and Outline of This Volume
3
hv
hv
e
and an entire Chapter 3 is devoted to these aspects. Photoelectrochemistry of
semiconductor at semiconductor/liquid interface, or liquid junction, is the first
concept of the semiconductor photocatalysis as described in Chapter 4. Reactions
of semiconductor microparticles are of special importance for practical use as well
as for obtaining further insight into the fundamental concept of semiconductors
(Chapter 5). New methods developed to prepare semiconductors under mild
conditions in solution are also important fundamentals for future basic science and
practical applications (Chapter 6).
The invention of the TiO z photoanode 1 ) solar energy conversion to fuels was
an important objective of photocatalysis, but it has been difficult to utilize visible
light, which is the major component of solar irradiation. It was discovered that
photocatalytic reaction by semiconductor particles can decompose organic
compounds to carbon dioxide,and this photocatalysis has been applied to
decompose dirty, hazardous, bad-smelling or toxic materials produced in daily life
and the global environment. Now environmental cleaning by photocatalysis has
become an important and beneficial industrial activity for practical use,7) as
described in Part II (see also Chapter 2). Since concentrations of such materials
to be removed from our living environment are usually not very high, even room
light instead of strong solar irradiation is sufficient for photocatalytic cleaning
purposes. TiO z coating on substrates for self-cleaning objective is mentioned in
Chapter 7. Cleaning of the atmospheric environment is an urgent global task.
Photocatalytic decomposition of organic halogen compounds or NO x in the
atmosphere is described in Chapter 8. Purification of water is also an intensively
investigated research subject for future application (Chapter 9). In order to achieve
photocatalysis under visible light, sensitization of large bandgap semiconductors
such as TiO z has been an important subject. One of the researches towards this
direction involving metal ion-implantation is described in Chapter 10.
Various efforts to apply photocatalysis to photo energy conversion are
described in Part III. Synthetic chemistry utilizing photocatalysis by
semiconductors has been attracting attention as discussed in Chapters 11 and 12.
The merits of the photocatalysts for synthetic chemistry are: (a) multiple processes
are possible, (b) catalysts can be separated easily and re-used, and (c) the reactions
can proceed under ambient conditions, etc. (Chapter 11). In Chapter 12 photolysis
and sonolysis are combined to obtain specific effects in addition to photochemical
reactions.
As noted above, water photolysis to give simultaneously Hz and Oz has been
an important subject to create fuels from solar energy and water, but photocatalytic
reaction by Pt/TiO z suspensions in water gives usually only Hz and not Oz.
However, stoichiometric HzlOz (= 2/1) production by Pt/TiO z was achieved by a
gas phase water vapor photolysis with NaOH-coated Pt/TiO z as described in
Chapter 13. The presence of a high concentration of carbonate anions in a water
suspension of PtlTiO z gave also stoichiometric Hz and Oz as described in Chapeter
14. Other efforts have been made to photolyze water, including water photolysis
by tunnel structured titanates such as BaTi 4 0 9 (effective wavelength below ca.
360nm) (Chapter 15), photolysis by layered compounds such as K 4 Nb 6 0 17
(effective wavelength below 360nm) (Chapter 16), and a two-step photoexcitation
system using TiO z (Chapter 17). Although these methods give stoichiometric
HzlOz by water photolysis, they are limited to the use of UV light, and water
(a)
(b)
Donor Sensitizer Acceptor
Semiconductor Particle
Fig. 1.1 Photoinduced charge separation by (a) semiconductor particle and (b) a system composed of
sensitizer with electron donor and/or acceptor.
(Pt/TiO z ) powders suspended in water claiming stoichiometric production of Hz
and Oz (HzlOz = 2/1) by UV irradiation, but reproducibility of these reactions by
TiO z powders, especially real production of 0z, has always been a problem. Only
Hz is produced in most cases.
Photosynthesis of green plants carries out photochemical solar energy
conversion by utilizing a sensitizer called chlorophyll which is a Mg complex of
a porphyrin derivative. 5 ) Over the past 25 years a number of reports have appeared
regarding artificial photochemical conversion systems with sensitizers. z - 6 ) The
target of many of these works is again water cleavage by visible light, but it still
remains a difficult task for the future. Such a sensitized conversion system is
also a photocatalytic process where the sensitizer is regarded as a kind of
photocatalyst.
The most important process of photocatalysis is photoinduced charge
separation and successive dark catalyses by the separated positive and negative
charges. In this sense photocatalysis both by semiconductor and sensitizer is
understood by a scheme similar to that shown in Fig. I.l(a) and (b), respectively.
The most difficult problem of photocatalysis is the rapid recombination of
separated positive and negative charges, which must be overcome to achieve
efficient photocatlysis. Because of this problem many works, especially visible
light energy conversion systems, stay on only a modeling of photochemical energy
conversion using expensive sacrificial electron donor or acceptor to prevent
charge recombination by the sacrificing reaction of the agent. Such model
photochemical conversion reactions should in future extend to a real photo energy
conversion system.
1.2 Aim and Outline of This Volume
In this monograph photocatalysis is reviewed from all the important aspects
including fundamentals (Part I), applications to environmental cleaning (Part II)
and photoenergy conversion (Part III). Japan, where the first photocatalysis of
TiO z was achieved, I) is the most active country in this field.
In Part I the fundamental aspects of photocatalysis are described.
Photoelectrochemical processes at semiconductors are the most important basics
for all photocatalytic reactions (Chapter 2). Design, preparation and
characterization of active photocatalysts have been an important research subject,

Kelerences
.J
4
l Introduction
photolysis by visible light has not yet been achieved.
Another approach towards photocatalysis is to use dyes as a sensitizer instead
of a semiconductor as in photosynthesis. It is not the aim of this book to cover all
the aspects of the sensitized photochemical conversion system, but typical
sensitized systems for photocatalytic reactions of water are described in Chapter
18. The concept of a photochemical conversion system using a sensitizer and
water oxidationlreduction catalysts is mentioned in Chapter 19, accompanied by
a discussion on the sensitization of semiconductors.
Sensitization of large bandgap semiconductors has been investigated
intensively to achieve visible light photo energy conversion, but without great
success before 1991. Sensitization of Ti Oz microparticle layers coated with a Ru
complex dye and soaked in an organic medium containing redox electrolytes
achieved light-to-electricity conversion efficiency of nearly 10%, attracting a
great of attention,8,9) and now the so-called Graetzel solar cell will be produced
commercially (Chapter 20). This system is an interesting combination of
semiconductor and sensitizer, and one of the great achievements in photocatalysis.
The editors and the authors of this volume have no doubt that photocatalysis
is one of the most important key science and technologies for the 21 st century. We
hope that the publication of this monograph will become a valuable milestone in
the quest to solve environment and energy resource problems.
energy is kept constant, and the second law that entropy increases in a spontaneous
process in a closed system. The photosynthesis produces both energy and negative
entropy, which is the basis both for the creature and humane activity. If we
continue our social activity by limiting on the concept of a closed planet further
in the 21 st century, we would hit to a catastrophe some day because of these first
and second laws of the thermodynamics. Photocatalysis for creating renewable
energy resource and cleaning environment essentially involves most important
concept for the future of our planet.
References
1. A. Fujishima and K. Honda, Nature, 238, 37 (1972).
2. Photosensitization and Photocatalysis Using Inorganic and Organomettalic Compounds, (K.
Kalyanasudaram and M. Graetzel, eds.), Kluwer Academic Publishers, Dordrecht (1993).
3. Photochemical and Photoelectrochemical Conversion and Storage of Solar Energy, (Z. W. Tian
and Y. Cao, eds.), International Academic Publishers, Beijing (1993).
4. Photochemical Conversion and Storage of Solar Energy, Proceedings of the 12 th International
Conference, Oldenbourg Wissenshaftsverlag,Munich (2000).
5. Photosynthesis, 6 th Edition, (D. O. Hall and K. K Rao, eds.), Cambridge University Press
Cambridge (1999). '
6. Molecl!lar Level Artificial Photosynthetic Materials, (G. J. Meyer, ed.), Progress in Inorganic
ChemIstry, vo1.44, Interscience Publication, New York (1997).
7. TiO z Photocatalysis-Fundamental and Applications-, (A. Fujishima, K. Hashimoto and T.
Watanabe, eds.), BKC Inc., Tokyo (1999).
8. O'Regan and M.Graetzel, NalUre, 353, 737 (1991).
9. KKalyanasundaram and M.Graeel, Coordination Chemistry Review, 77,347 (1998).
1.3 Summary
Fundamental science, technology and applications of photocatalysis are
described by researchers most active in this field. Photocatalysis by TiO z was
invented in 1972 with UV light cleavage of water by TiO z photoanode. Intensive
studies have been carried out since to cleave water by visible light by utilizing
either semiconductor electrodes or powders, but without success. However, these
works over nearly 30 years have resulted in significant accumulation of
fundamental scientific and technological knowledge on photocatalysis leading
to a practical environmental cleaning system and solar cells, both important for the
future of the earth.
Remarkable efforts have been directed to photocatalytic fuel production from
sunshine and water for creating energy resources, and now UV light cleavage of
water for fuel production is possible. However, conversion of solar visible light
to fuels is still under way, and further concentrated researches in this direction are
strongly desired. Conversion of solar energy into electricity has been attained by
a sensitized TiO z thin layer solar cell to reach nearly 10% efficiency, comparable
to that obtained by the amorphous silicone solid cell. This is an "important
achievement in photocatalysis.
1.4 Future Perspectives
Our earth is limited but not closed. Nearly 100% energy source of living
creatures depends on photosynthesis, including the fossil fuels that are thy main
energy resource of human life. This is the important point for our future that our
earth is open to the universe, more correctly to the sun. If the earth is closed no
animals and plants can exist. The first law of thermodynamics designates that the

2
Photoelectrochemical Processes of Semiconductors
The tremendous amount of research that has been carried out in the two
closely related fields of semiconductor photo electrochemistry and photocatalysis
during the past three decades continues to provide fundamental insights and
practical applications. Several excellent reviews have appeared during this same
time period that cover fundamental and general aspects of
photoelectrochemist ry l-7) and photocatalysis. s - I4 )
Beginning with the work on photo electrochemical water electrolysis or
"splitting" in the late 1960s in Japan,15,16) there has been much work carried out
aimed primarily at solar energy conversion as an alternative approach to that of
the solid state junction photovoltaic cell. The latter have enjoyed continuous
development, with the efficiency now reaching over ca. 24% in laboratory cells, 17)
although practical wafer-based cells are in the 12-16% range. IS) One of the driving
forces behind the photo electrochemical approach is the perceived ability to form
rectifying junctions in simpler ways compared to the relatively sophisticated
techniques required in solid state processing. Another is that barrier height can in
principle be varied easily by choosing appropriate match-ups between
semiconductors and redox couples. These types of ideas continue to drive research
in this area, and interesting results continue to be obtained.
The solar energy impinging on the earth's surface is about 3 X 10 24 J per year,
or approximately 10 4 times the worldwide yearly consumption of energy. The
search for the efficient conversioI:l of solar energy into other useful forms is, in
view of the increasing anxiety over the exhaustion of energy resources, one of the
most important challenges for future research and technology development.
In systems designed for the purpose of converting solar energy into electricity
and/or chemicals (for fuel or other purposes), two principal criteria must be met.
The first is the absorption, by some chemical substance, of solar irradiation,
leading to the creation of electrons (i.e., reduced chemical moieties) and holes
(i.e., oxidized chemical moieties). The second is an effective separation of these
electron-hole pairs with little energetic loss, before they lose their input energy
through recombination.
Plants capture the energy from sunlight and thus grow. During this process,
they produce oxygen by oxidizing water and reducing carbon dioxide. In other
words, the oxidation of water and the reduction of CO 2 are achieved with solar
energy. By analogy with natural photosynthesis, we began to investigate the
photoelectrolysis of water using light energy. 16) This approach involves essentially
a photochemical battery making use of a photoexcited semiconductor (Fig.2.1).

10
2 Photoelectrochemical Processes of Semiconductors
2.1 Semiconductor Electrodes for Solar Energy Conversion
11
-1 ./ excess potential for
./ H2 evolution
- - - - - - - - . - - -. i H + 2 - H
Water splitting, open circu' , 2 + e  2
illuminated, pH 2 0 t i <--.
Ep at counter
electrode
junctions. Specific topics covered include following: photo electrochemical water
electrolysis; compound semiconductor electrodes and their stabilization; the use
of p-type semiconductors as photocathodes; photoelectrochemical reduction of
CO 2 at p-type semiconductor electrodes; and photocatalytic decomposition of
organic pollutants and TiOrbased superhydrophilic self-cleaning surfaces.
0.5
1.23 V
2.1 Semiconductor Electrodes for Solar Energy Conversion
hv
1
-<
In a solar photo voltaic cell, photogenerated electron-hole pairs are driven
efficiently in opposite directions by an electric field existing at the boundary
(junction) of n- and p-type semiconductors (or at semiconductor/metal junctions).
The maximum conversion efficiency for solid-state devices that employ a single
junction has reached ca. 24% and, for two junctions, ca. 30%. A potential gradient
can also be created. at the interface of a semiconducting material and a liquid
phase. Hence, if a semicondutor is used as an electrode that is connected to
another (counter) electrode, photoexcitation of the semiconductor can generate
electrical work through an external load and simultaneously drive chemical
(redox) reactions on the surfaces of each electrode. On the other hand, in a system
where semiconductor particles are suspended in a liquid solution, excitation of the
semiconductor can lead to redox processes in the interfacial region around each
particle. These types of systems have been the focus of numerous investigations
over the past twenty years.
Our work started in the late 1960s at the University of Tokyo in research on
photo electrochemical (PEC) solar cells. 15 ,16) One of the first types of electrode
materials we looked at was semiconducting Ti0 2 , partly because it has a
sufficiently positive valence band edge to oxidize water to oxygen. It is also an
extremely stable material in the presence of aqueous electrolyte solutions, much
I1.l0re so than other types of semiconductors that have been tried.
The possibility of solar photoelectrolysis was demonstrated for the first time
with a system in which an n-type Ti0 2 semiconductor electrode, which was
connected through an electrical load to a platinum black counter electrode, was
exposed to near-UV light (Fig. 2.2).19) When the surface of the Ti0 2 electrode was
H20 + 2h+
a:' 1/2 02 + 2 H+
..-+
(1)

..-+
......
P'
:-'
Distance, nm
,--.,
en

'-'
Fig. 2.1 Schematic representation of photoelectrochemical water electrolysis using an illuminated
oxide semiconductor electrode. Open circuit (or small current), pH 2, illuminated conditions
are shown for an oxide with an E CB of -0.65 V (SHE) and an E VB of 2.35 V (SHE). With an
open circuit, a small excess potential (-0.15 V) is available for 8 2 evolution, assuming a
reversible counter electrode.
Such a photoinduced charge separation can proceed effectively provided an
electric field (potential gradient) has been established at the position where the
primary photoexcitation takes place. In general, a potential gradient can be
produced at the interface between two different substances (or different phases).
For example, a very thin (ca. 50 A) lipid membrane separating two aqueous
solutions inside the chloroplasts of green plants is believed to play the essential
role in the process of photosynthesis, which is the cheapest and perhaps the most
successful solar conversion system available. .
Another well-known example is the photocell or solar photovoItaic cell, in
which the photogenerated electron-hole pairs are driven efficiently in opposite
directions by an electric field existing at the boundary (junction) of n- and p-type
semiconductors (or at semiconductor /metal junctions). A potential gradient can
also be created, by a process to be described later in more detail, at the interface
of a semiconducting material and a liquid phase. Hence, if a semiconductor is used
.as an electrode, which is connected to another (counter) electrode, photo excitation
of the semiconductor can generate electrical work through an external load and
simultaneously drive chemical (redox) reactions on the surfaces of each electrode.
On the other hand, in a system where semiconductor particles are suspended in a
liquid solution, excitation of the semiconductor can lead to redox processes in the
interfacial region around each particle. These types of systems have drawn the
attention of a large number of investigators over the past twenty years, primarily
in connection with solar energy conversion. During the last five years, the area of
particulate semiconductors has also seen tremendous growth in terms of
photo catalyzed air and water purification.
The present chapter deals with the principles and recent advances in the
investigation of light energy conversion systems based on semiconductor/liquid
Electrolyte
Fig. 2.2 Schematic diagram of an electrochemical photocell consisting ofTi0 2 and Pt electrodes in an
aqueous electrolyte.

12
2 Photoelectrochemical Processes of Semiconductors
2.1 :SemIconductor blectroaes lor Olar tcnergy onverSlUIl
1,)
1
HzO + 2 h V  - Oz + Hz
2
(2.4)
shows an upward band bending for n-type semiconductor/electrolyte interface.
The thickness of the space charge layer is usually of the order of 1-10 3 nm,
depending on the carrier density and dielectric constant of the semiconductor. If
this electrode receives photons with energies greater than that of the material's
band gap, Eo, electron-hole pairs are generated and separated in the space charge
layer. In the case of an n-type semiconductor, the electric field existing across the
space charge layer drives photogenerated holes toward the interfacial region (i.e.,
solid-liquid) and electrons toward the interior of the electrode and from there to
the electrical connection to the external circuit. The reverse process takes place at
a p-type semiconductor electrode. These fundamental processes have been
discussed in a number of recent reviews. 3 - 7 )
If the conduction band energy E CB is higher (i.e., closer to the vacuum level,
or more negative on the electrochemial scale) than the hydrogen evolution
potential, photogenerated electrons can flow to the counter electrode and reduce
protons, resulting in hydrogen gas evolution without an applied potential,
although, as shown in Fig. 2.1, E CB should be at least as negative as -0.4 V (SHE)
in acid solution or -1.2 V (SHE) in alkaline solution. Among the oxide
semiconductors, TiO z (acid), SrTi0 3 , CaTi0 3 , KTa03, TazOs and ZrOz satisfy
this requirement. On the other hand, the employment of an external bias or of a
difference in pH between the anolyte and catholyte is required in the case of the
other materials in order to achieve hydrogen evolution. For example, in early n-
TiOz-based photo electrochemical cells used in our laboratory,19) the problem of
E CB being slightly lower (less negative) than that necessary to evolve hydrogen
redox was circumvented by using' an anolyte and catholyte with different pH
values, higher in the fonner and lower in the latter. This effectively decreases the
equilibrium cell potential so there is excess overpotential with which the cell
reactions can be driven to higher current densities. Our group and others also
found that SrTi0 3 , which has a sufficiently negative E CB , was able to
photoelectrolyze water without an additional voltage. With its very large band gap,
however, the efficiency of solar energy conversion is very low.
It is desirable for the band gap of the semiconductor to be near that for
optimum utilization of solar energy, which would be ca. 1.35 eV for a solid-state
p-p junction or for a PEC cell under ideal conditions. ZO) Even when photons are
completely absorbed, excess photon energy (E> E BO ) cannot normally be utilized
in a simple, single band-gap device, since vibrational relaxation occurs in the
upper excited states before the charge transfer takes place. Therefore, a fraction
of the photon energy is dissipated as heat. When semiconductor electrodes are
used as either photoanodes or photocathodes for water electrolysis, the bandgap
hould be at least 1.23 e V (i.e., the equilibrium cell potential for water electrolysis
at 25°C and 1 am), particularly considering the existence of polarization losses
due to, e.g., oxygen evolution.
Stable photoanodes such as TiO z have relatively wide gaps (ca. 3.0 eV) so can
utilize only a small portion of the solar spectrum. Semiconductors with smaller
band gaps have good visible light response and, in this sense, are suitable as
photoelectrodes. The energy levels for some of these materials are shown in Fig.
2.4. However, many of these materils decompose easily through photoanodic
dissolution of the electrode surface. For example, in the case of CdS, the material
is oxidized by photogenerated holes so that Cd z + ions dissolve and elemental
irradiated with light consisting of wavelengths shorter than 415 nm, photo current
flowed from the platinum counter electrode to the TiO z electrode through the
external circuit. The direction of the current reveals that the oxidation reaction
(oxygen evolution) occurs at the TiO z electrode and the reduction reaction
(hydrogen evolution) at the Pt electrode. This fact shows that water can be
decomposed, using UV -visible light, into oxygen and hydrogen, without the
application of an external voltage, according to the following scheme:
(excitation ofTiO z by light)
TiO z + 2 hv  2e- + 2 h+
(2.1)
(at the TiO z electrode)
HzO + 2 h+  1 Oz + 2 H+
2
(2.2)
(at the Pt electrode)
2 H+ + 2 e-  Hz
(2.3)
The overall reaction is
When a semiconductor electrode is in contact with an electrolyte solution,
thermodynamic equilibration takes place at the interface. This may result in the
formation of a space charge layer within a thin surface region of the
semiconductor, in which the electronic energy bands are generally bent upwards
or downwards, respectively, in the cases ofn- and p-type semiconductors. Fig. 2.3
-0.8
-0.4
0.4
Epof
counter
electrode
- - - - - - - - - - - - - - - - - - - 0- - - - - - -
Ep of n-type semiconductor
Eo = 1.6 e V
0.8

1.2 K
o
1.6 0 ]

I

150 125 100 75 50
Distance / nm
25
o
Fig. 2.3 Schematic diagram of the energy levels for an n-type semiconductor in a regenerative
photoelectrochemical cell under equilibrium, dark conditions.

14
2 Photoelectrochemical Processes of Semiconductors
2.2 Reduction of CO 2 at Illuminated Semiconductor Electrodes
15
Cd + 2h+  Cd 2 + + S
(2.5)
case of CdS, the competitive ratio for the oxidation of the added redox species
becomes larger as its redox potential becomes more negative (i.e., more easily
oxidizable). The efficiency of the electrode stabilization is controlled by the
relationship between the dissolution potential ED for the semiconductor and EO for
the redox couple. Similar conclusions were reached by Gerischer,25) who also
pointed out the importance of kinetics. Lewis has discussed the fact that some of
these electrochemical reactions are extremely rapid. 26 ) In addition, the importance
oflstrong adsorption of the redox reactant at the semiconductor electrode surface
has been pointed out, particularly for aqueous electrolytes. 21 ,22,25)
Another important advance in achieving stable semiconductor electrodes was
the use ofp-type semiconductor electrodes. 2I ) The basic idea is as follows. Instead
of the active, minority carrier species at the electrode surface being the hole,
which has sufficient oxidizing power, particularly in aqueous electrolytes, to
photocorrode most semiconductor materials, the active species is the electron.
Most semiconductor materials are much less vulnerable to cathodic corrosion, so
that this strategy should lead to improved stability. Using p-InP with a thin oxide
layer, the Bell Labs group was initially able to achieve 9.3% conversion efficiency
and later, with improved surface preparation, 11.5%.21)
More recently, it has been found that high-quality p-type semiconducting
and even conducting CVD diamond films can be prepared using boron as a
dopant. 27 ,28) Diamond has some unique properties that make it highly attractive as
an electrode material, including- excellent chemical inertness, extremely wide
potential window, extreme hardness, excellent transparency and high thermal
conductance. Our work has focused on the use of diamond as both a highly
conductive electrode materiap9) and as a semiconducting photocathode. 28 ) We
have found that, using highly pure, highly crystalline boron-doped films, the
potential window in aqueous acid solution extends from -1.0 V to + 1.5 V vs. SCE.
We have also found that the flat band potential, which approximates E VB , is
around +0.5 V vs. SCE. Since the band gap is approximately 5.5 eV, this puts E CB
at -5.0 V, i.e., above the vacuum level. Therefore, our photo electrochemical
work has involved the use of excimer lasers to exceed this band gap energy. This
work showed that it is possible to photoelectrochemically generate H 2 on diamond
at an underpotential of ca. 1.8 V. Some very interesting experiments can be
carried out in which extremely hot electrons are produced. It was recently shown
by Tata et al. that the diamQnd surface must be oxidized in order to enhance the
photo electrochemical generation of H 2 . 30)
(a) Oxides: base
-5
-4
-3
-2
-1
o
1
2
3
 4
U')
'-'
pO 'n
J\
D-, VLJV>J
-
-
-5
-4
-3
-2
-1
o
I
2
3
4
r'"J r'"J V") r'"J
r'"J 0 V")
0 r'"J C"1 0 0 0 N o N
C"1 0 0 E 0 E= C"1 0 NO
(1)  E= s:: '" .D  s::  I-<
 VJ N 0:1 VJ
I I I I I I Z I I ,N
s:: s:: s:: s:: s:: s:: '= s:: s:: s:: '=
>
-
..-<

......
.......
 (b) Non-oxides: base
.....
o -5
p..
-4
-3
-2
-1
o
I
2
3
4
T 
I I 1 I T T  pO T-fnjg
...L I ,,-.j,
D" , Vl/VH
-5
-4
-3
-2
-I
o
1
2
3
4
u
 ooauOOUU3
I I I I I I I I I I I
p.. s:: s:: p.. s:: s:: p.. s:: p.. S::p..
Fig. 2.4 Relative energy levels of selected (a) oxide and (b) non-oxide semiconductors in alkaline
solution (12-14 pH).
sulfur deposits on the electrode surface. Thus the photo current at a CdS
photo anode shows an abrupt decay with time due to the filtering effect of the
deposited sulfur.
Such unfavorable phenomena must be alleviated in order for small band-gap
semiconductors to be used in PECs. Much effort has been devoted to the problem
of photocorrosion, particularly in the decade following the first report concerning
PECs. Beginning with independent reports in 1976 by groups at Bell Labs,2I)
MIT2 2 ) and the Weizmann Institute 23 ) that the aqueous S2-/S 2 2- couple could be
used to stabilize CdS and CdSe, it has become well-established that stabilization
of these electrodes can be achieved through the addition of suitable redox species
to the electrolyte.
Early work in our laboratory involved the examination of competitive
photoanodic oxidation at a CdS electrode in electrolytes containing various
reducing agents using the rotating ring-disk electrode (RRDE) technique. 24 ) In the
2.2 Reduction of CO 2 at Illuminated Semiconductor
Electrodes
The increasing levels of carbon dioxide in the atmosphere has now become
a global environmental issue because of the greenhouse effect. There have been
various approaches not only for recycling of this greenhouse gas but also for an
efficient production of fuel alternatives. 3I ,32) The use of solar energy to
photo electrochemically reduce CO 2 is an appealing one and has been studied
"almost as long as the photoelectrochemical water-splitting reaction.
Halmann reported the first work on the reduction of CO 2 at an illuminated p-
GaP photocathode in 1978. 33 ) Our group reported the first work on the use of

16
2 Photoelectrochemical Processes of Semiconductors
2.2 Reduction of CO 2 at lllummated elmconductor hlectrodes
11
semiconductor powder dispersions in 1979. 34 ) This area has been covered recently
in reviews by Halmann 3 !,32) and by Lewis and Shreve. 35 ) Electrode materials that
have been examined include p-GaP, p-GaAs, p-CdTe, p-InP and p-Si.
Semiconductors used in the form of powders have included Ti0 2 , ZnO, CdS,
GaP, SiC, W0 3 , ZnS and CdSe. Various types of metal coatings have been
examined, including Au, Zn, Pb, Cu, In, Pd and noble metal alloys. Both aqueous
and nonaqueous electrolytes have been examined.
During CO z reduction at a p-type semiconductor electrode, the
photogenerated minority carriers (electrons) are swept to the electrode surface at
close to E CB ' Thus the E CB of the particular semiconductor should be negative
enough to reach the thermodynamic redox potential of the predominant CO z
reduction reaction plus any kinetic overpotential. In a dry nonaqueous electrolyte,
in the absence of any species that could react with CO z '- radical anion or catalyze
its disproportionation, E CB should be more negative than -1.67 V (SHE). Fig. 2.5
shows a band diagram with the band edge positions ofp-InP, as a typical example,
in relation to the redox potentials for CO z reduction. It should be noted here also
that, even though the potentials required to reduce CO z in nonaqueous electrolytes
are more negative than those in aqueous electrolyte, an advantage of the former
is that many of the semiconductor electrode materials are more stable. This
advantage, which was already mentioned in relation to n-type photo anodes, is also
true in the case of p-type photocathodes.
A high-pressure COz-methanol system has been found to offer a number of
advantages for the photoelectrochemical reduction of CO Z .36) At high pressures the
mass transport of CO z increases significantly which in turn enhances the current
densitiy. The reduction reaction was examined in a 40-atm COz-methanol medium
using the p-type semiconductor electrodes p-InP, p-GaAs, and p-Si (Fig. 2.6).
With p-InP photocathodes, current densities up to 200 mA cm- z were achieved,
with current efficiencies of over 90% for CO production, while hydrogen gas
evolution was suppressed to low levels. At high current densities and CO z
pressures, the CO z reduction current was found to be limited principally by light
0
50
100
150
t:\I
I
E 0
u
«
E
- 50
>-
:!:::
CJ")
c
tD 100
""C
-
r:
CD
.....
'-
::J 150
u
0
50
100
150
SHE scale
-;2
-1.7 C02 + e-  C02-
-1.5
CB
W////// /h
-r- I
Eg = 1.3 V -0.5
O

VB 0.5
Potential / V vs Ag QRE
-2.0 -1.0 0
,
,
,
,
c. ,
,
C02! co, C03 2 -
A
I .
B
b.
c
,
,
c., ,
,
,
a.
-2.0
-1.0
o
Fig. 2.6 Current-potential curves for (A) p-InP, (B) p-GaAs, and (C) p-Si electrodes in 0.3 M TBAP
in methanol (40 atm CO 2 ) (b) in the dark and (a, c) under illumination. Curves a and c
correspond to the behavior corrected and uncorrected for ohmic losses, respectively. Curve
d was obtained for a metallic Cu electrode. (QRE stands for quasi-reference electrode).
intensity. Of the various factors that were found to influence the product
distribution,-including the concentrations of added water and strong acid, the
CO z pressure was the most critical. It was proposed that the adsorbed (COz)z.-
radical anion complex reaches high coverages at high CO z pressures and is
responsible for both the high current efficiencies observed for CO production
and the low values observed for Hz evolution. Furthennore, this adsorbed complex
is responsible for stabilizing all three semiconductor electrode materials at high
CO z pressures, even at current densities as high as 100 mA cm- z .
/' -0.2 C02 + 2H+ + 2e- HCOOH
-0.1 C02 + 2H++ 2e-CO + H20
1.0
Fig. 2.5 Energy-level diagram for CO 2 photoreduction at p-InP semiconductor.

18
2 Photoelectrochemical Processes of Semiconductors
2.3 Photocatalysis
19
2.3 Photocatalysis
electrons is -0.52 V, which is in principle negative enough to evolve hydrogen
from water, but the electrons can become trapped and lose some of their reducing
power, as shown. However, even after trapping, a significant number are still
able to .reduce dioxygen to superoxide Oz -, or to hydrogen peroxide RzOz.
ependmg upon the exact conditions, the holes, .OH radicals, Oz -, HzOz and Oz
Itself can all play important roles in the photocatalytic reaction mechanisms (see
below). /
In order to avoid the use ofTiO z powder, which entails later separation from
the water, various researchers began to work on ways of immobilizing TiO
partiles, for example, in thin film fonn. One of the first repons on the preparatio
of TIOz films was that of Matthews. 56 ) This idea has also been worked on by
Anderson,57) by Heller 1Z ) and by our group.58-64) Our group has been developing
ways to put photocatalytic TiO z coatings on various types of support materials
e.g., ceramic tiles. .The various types of TiOz-based materials will be discussed
again in connection with applications.
2.3.1 General Remarks
As has been pointed out by Heller, aU of the extensive knowledge that was
gained during the development of semiconductor photoelectrochemistry during the
1970s and 1980s has greatly assisted the development of photocatalysis. z1 ) In
particular, it turned out that TiO z is excellent for photocatalytically breaking
down organic compounds. For example, if one puts catalytically active TiO z
powder into a shallow pool of polluted water and allows it to be illuminated with
sunlight, the water will gradually become purified. Ever since 1977, when Frank
and Bard first examined the possibilities of using TiO z to decompose cyanide in
water,37,38) there has been increasing interest in environmental applications. These
authors quite correctly pointed out the implications of their result for the field of
environmental purification. Their prediction has indeed been borne out, as
evidenced by the extensive global efforts in this area. 8 - 14 ,39-55)
One of the most important aspects of environmental photocatalysis is the
availability of a material such as titanium dioxide, which is close to being an ideal
photocatalyst in several respects. For example, it is relatively inexpensive, highly
stable chemically, and the photogenerated holes are highly oxidizing. In addition,
photogenerated electrons are reducing enough to produce superoxide from
dioxygen.
The energy band diagram for TiO z in pH 7 solution is shown in Fig. 2.7. As
shown, the redox potential for photogenerated holes is +2.53 V vs. the standard
hydrogen electrode (SHE). After reaction with water, these holes can produce
hydroxyl radicals (.OR), whose redox potential is only slightly decreased. Both
are more positive than that for ozone. The redox potential for conduction band
2.3.2 Mechanistic Studies
Although it has been assumed that oxidative and reductive photocatalytic
reactions take place simultaneously on TiO z panicles, it is very difficult to confinn
this assumption experimentally, because the products of both reactions mix
immediately. However, it is possible to set up a simple system that can be used
to model individual particles and is compatible with microscopic detection of
reaction products. 65 -68) Such a system, shown in Fig. 2.8, involves a TiO z film that
has metallic regions, e.g., Pd or ITO, and a scanning microelectrode. The
microelectrode can be positioned as close as -20 Jim to the surface. lts potential
can be set at values at which either Oz or HzOz, for example, can be monitored as
a function of time.
Some examples of the results obtained with this system are summarized
below. In one of the early studies, it was found that HzOz is produced to a much
vs. SHE
8
-0.52 e
-0.45 -------------
8
-1
Potentiostat
Ti 3 + -OH
H2/H20 (-0.413)
02/02.- (-0.28)
02/H202 (+0.28)
Fe( CN)6 4 -/ 3 -( +0.36)
+ 1 02/ H 2 0 (+0.83)
H202/H20 (+1.35)
o
pt
(CE)
SCE
(RE)
+0.28 -------------- ----------------
+2.53
03/H20 (+2.07)
.OH/H20 (+2.27)
Carbon Microelectrode
(WE)
Pd film
h EB
h EB
+3
(pH = 7)
Ti02
(
, Inverted Microscope
Fig. 2.7 Schematic diagram showing the potentials for various redox processes occurring on the Ti0 2
surface at pH 7.
Fig. 2.8 Experimntal setup for te separate monitoring of oxidation and reduction reactions occurring
on the T10 2 -ITO film usmg a carbon microelectrode.

20
2 Photoelectrochemical Processes of Semiconductors
Fig. 2.9 Concentration changes of dissolved oxygen (directly proportional to cathodic current, -1 V
vs. SCE) or H 2 0 2 (proportional to anodic current, + 1 V vs. SCE), measured separately above
Ti0 2 and Pd, caused by the photocatalytic reaction in the absence and presence of ethanol in
aqueous solutions. Hatched areas represent the time duration of the photoirradiation. (A)
Concentration changes of dissolved oxygen due to photocatalytic reactions in aqueous solution
without EtOH (light intensity, 9.5 m W/cm 2 ). (B) Concentration changes of dissolved oxygen
due to photocatalytic reactions in 5 vol % ethanol aqueous solution (light intensity, 2.5
mW/cm 2 ). (C) Concentration changes of H 2 0 2 due to photocatalytic reactions in 5 vol %
ethanol aqueous solution (light intensity, 2.5 mW/cm 2 ).
(A) UV irradiation
-
+-'
C
 -400
....
:J
()
-300
(8) -300
«
c..
::-200
c
Q)
....
....
:J
()
-100
o
o
1 234
Time / min
(C)
« + 1 00
c..
-
+-'
C +50
Q)
....
....
:J
()
o
( n1\bbti6 D
. . . . . . . . . . . . . .
. . . . . . . . . . . . .
-500
«
c..
:: -400
c
Q)
....
....
:J
()
-300
4
-300
«
c..
--200
+-'
c
Q)
....
....
:J
() -100
o
5
« + 1 00
c..
-
+-'
C +50
Q)
....
....
:J
() 0
4
2.3 Photocatalysis
21
UV irradiation
greater extent at the ITO portion of the illuminated TiOz-ITO composite surface,
due to Oz reduction. 65 ) In a subsequent study on the separate detection of dissolved
oxygen in aqueous solution at reducing regions (in this case, Pd) and oxidizing
regions (TiO z ), using a carbon microelectrode, we found an increase in Oz
concentration near the Ti0 2 surface and a decrease in the Oz concentration near
the Pd surface under illumination (Fig.,2.9).68) Oxygen is produced via water
oxidation on TiO z :
2H z O + 4h+  Oz + 4H+
(2.6)
3
Oxygen is consumed at the metallic electrode (ITO or Pd) via reduction,
principally to peroxide.
Oz + 2H+ +2e-  HzOz
(2.7)
although reduction to superoxide
I
I
I I
-j---t-
I I
I I
I I
O 2 + e-  Oz-
(2.8)
or to water may also contribute:
Oz + 4e- + 4H+  2H z O
(2.9)
5
On an actual TiO z microparticle, depending upon whether there is a deposited
catalyst, there can also be varying contributions of these latter three reactions. As
discussed later, some photocatalytic reaction pathways can involve -Oz -, and
therefore it appears to be a desirable product; as such, metallic catalysts may in
fact be counterproductive in this respect, if they promote the two- or four- electron
reduction reactions.
In the presence of ethanol, however, dissolved oxygen was consumed at both
TiO z and Pd sites, the consumption of oxygen being larger at the TiO z surface
(Fig. 2.10).68) A possible scheme that is consistent with Oz consumption at TiO z
IS:
HzO + h+  -OH + H+
(2.10)
-OH + CH 3 CH z OH  CH 3 C-HOH + HzO
(2.11 )
c ............ )
. . . .. . . . . . . . .
. . . . . . . . .. ...
  AP.9V pq  : 
. . . . . . . . . . . . .
CH 3 C-HOH + Oz  CH 3 CH(OH)00-
(2.12)
producing an organoperoxyl radical, which can either participate in a chain-type
process
CH 3 CH(OH)00- + CH 3 CH z OH  CH 3 CH(OH)00H + CH 3 -HOH
(2.13)
CH 3 CH(OH)00H  CH 3 CHO + HzOz
(2.14)
or react further with Oz to produce a tetroxide intennediate, as discussed later. The

22
2 Photoelectrochemical Processes of Semiconductors
2.3 Photocatalysis
23
o
2.0
4.0
6.0
8.0
10.0
There is a large volume of work devoted to the second topic, i.e., conventional
kinetic measurements. This has been reviewed in 1995 by Hoffmann et at. II) Of
particular interest is work that has been carried out with very low intensity UV
light, similar to the intensity that exists in ambient indoor lighting, i.e., nW cm- z
to J1W cm- Z . 74 ,75) One of the major conclusions of this work has been that the
quantum yield (QY) for the photocatlytic decomposition of simple organic
compounds appears to be determined uniquely by the ratio of the amount of light
absorbed and the number of molecules adsorbed, which has been termed the
normalized light intensity Inorm. 74 ) There are a number of interesting and useful
consequences of this result. One of these is that, in order to maximize the light
utilization at low intensity, one should maximize the amount of adsorbed
molecules.
(: For a simple system such as 2-propanol, which decomposes to acetone, with
no appreciable chain reactions, the QY reaches a plateau at very low light
intensity.74) The precise value depends sensitively upon the characteristics of the
particular film, probably depending upon the presence of trace impurities and
defects, which could act as recombination sites. For a system in which there are
radical chain reactions, such as acetaldehyde, QY can continue to increase, even
at very low Inorrn values, and can in principle exceed unity.75)
A subsequent paper also includes results from higher light intensity
illumination, conditions under which mass transport can begin to playa role. 7o ) A
master plot of the log ofthe light intensity vs. the log of the reactant concentration
Ti02
Pd
ethanol present
ethanol, phenol
present
Amount of consumed oxygen /10- 9 mol cm- 2
Fig. 2.10 Amounts of O 2 consumed as a result of photocatalytic reactions in ethanol-containing
aqueous solution, with and without 0.1 M phenol present, under UV irradiation for 1 min.
The amount of O 2 consumed was estimated using a finite difference method from the
oxygen concentration vs. time curves (light intensity, 2.5 mW/cm 2 ; microelectrode potential,
- I V vs. SCE).
importance of radical intermediates was found to be great, as evidenced by the
strong decrease in Oz consumption in the presence of phenol, a radical scavenger.
The size resolution of this technique at present is only on the order of tens of
micrometers, but future work could extend this down to the nanometer scale, at
which it might be possible to examine a TiO z film without any deposited metal
regions and to image nanoscale regions with different types of reactivity. Such
measurements could lead to important insights.
Based on these types of results together with those obtained with other
techniques (see, e.g., the work of Heller and coworkers 1Z ) the detailed mechanisms
of photocatalytic reactions are being worked out. It is becoming apparent that, for
several types of organic reactants, superoxide plays an important role. However,
for one class of compounds, i.e., aldehydes, superoxide plays almost no role, the
mechanism involving molecular oxygen, however, in a complex series of chain
reactions.
10 6
10 18
10 5 .
2.3.3 Low Intensity Illumination
1 0 1 .;targelfor
! ;,indoorair purification
1
-
-
10 17 Sl>
cr
(J)
1 0 16 
0-
ro
10 15 a.
"0
::r
10 14 8
:::s
(J)
10 13 A
c
Sl>
10 12 3-
p)
10 11 3
11 0 4 , i:!$s-trflSROl1t
to) [ ontued regi
. 10 3
5: ---
 1 02 target for outdoor
 air purification,
tIJ
c:
Q)
.....
t:
i
,
u rye A
.....
..c:
C)
,
"
On the fundamental side, the research on photocatalysis has focused on
several topics, including a) the primary processes involved in the production and
trapping of photogenerated electrons and holes, using pulsed femtosecond or
picosecond laser techniques, b) measurements on the kinetics of the
photodecomposition processes on longer time scales, and c) measurements on
the kinetics on small size scales. For the first topic, the reader is referred to
several recent publications. 69 - 73 ) This work is of great practical importance,
because it helps to point out the critical factors involved in the photocatalytic
materials themselves.
> 1 0- 1 '
::>
.
I
I\:)
10- 2
"
,. !
10- 3 10- 2 10- 1 10 1 10 2 10 3
reactant concentration /ppmv
<
1010 rfl
I
Fig. 2.11 Plots of light intensity vs. initial reactant concentration showing pure mass transport-limited
and pure light intensity-limited conditions for photodegradation of gas-phase organics.

24
2 Photoelectrochemical Processes of Semiconductors
2.3 Photocatalysis
25
(Fig. 2.11) can be used to display the regioci,. in which the reaction rate is purely
light intensity-controlled (lower right) and in \v.hich the rate is mass transport-
controlled (upper left). The boundary of the l region depends upon the
effectiveness of the mass transport, e.g., natural or foconvection.
2.3.4 Applications
$1 . .. ;-....
".
(I V .ti.
'\ "
'Q;.....i
'.'
There are a number of examples of applications of environmental
photocatalysis that are already at or near the stage of implementation or
commercialization. Some selected applications of photocatalytic technology are
listed in Table 2.1.
The range of possible applications is massive, and some of these have been
reviewed in a recent book.39) Here we touch briefly on an application related to
tunnel lamps. Workers at the Toshiba Lighting and Technology Corp. in Japan
have been working to develop the technology for selfcleaning cover glasses for
such lamps.40) These lamps are continually exposed to vehicle exhaust fumes and
become progressively less transparent over a period of weeks or months,
depending on the traffic. If the glass is coated with Ti0 2 , it stays transparent
much longer and does not require such frequent cleaning (see Fig. 2.12). The
.. ,
,
Fig. 2.12 Verification of the photocatalytic decomposition effect for exhaust compounds from a
diesel engine in the laboratory (UV illumination intensity in the range of 280-380 nm was
ca. 10 JiW cm- 2 for 8 h) (taken from Ref. 40).
Table 2.1 Selected applications of photocatalysis
Application
Exterior tiles, kitchen and bathroom
components, interior furnishings, plastic
surfaces, aluminium siding, building
stone and curtains, paper window blinds
Translucent paper for indoor lamp
covers, coatings on fluorescent lamps and
highway tunnel lamp cover glass
Tunnel wall, soundproofed wall, traffic
signs and reflectors
Tent material, cloth for hospital garments
and uniforms and spray coating for cars
Room air cleaner, photocatalyst-equipped
air conditioners and interior air cleaner
for factories
Concrete for highways, roadways and
footpaths, tunnel walls, soundproofed
walls and building walls
River water, ground water, lakes and
water storage tanks
Fish feeding tanks, drainage water and
industrial wastewater
cleaning process is time-consuming, expensive and dangerous, necessitating
diversion of tunnel traffic so the cleaning crews can work.
One of the problems in the implementation of this type of TiO z coating on
glass is that, unless special measures are taken, the coating as deposited on
ordinary soda-lime glass is inactive. This is because the Na+ cations from the
glass diffuse into the TiO z film when it is fired, with an adverse effect on the
subsequent photocatalytic activity. When TiO z films are prepared on high-purity
silica glass, this problem does not arise, so an obvious solution is to deposit a thin
intennediate layer of silica prior to depositing the titania film. The silica layer
effectively blocks the diffusion ofNa+ cations from the glass into the titania film.
Heller and coworkers have solved this problem by acid treating the glass to
remove the Na+ cations prior to the deposition of the titania film. IZ }
The tunnel lamp application is a good example of practical use of
photocatalytic cleaning technology. The flux of organic contaminants to the
surface is more or less balanced with the rate at which the photocatalyst can
break them down. In addition, the light source is already built into the system,
which is a fortunate situation. This is of course also true of indoor room lighting
fixtures, which are also beginning to be marketed in Japan.
U is one of the unique aspects of TiO z that there are actually two distinct
photo-induced phenomena: the first is the well-known original photocatalytic
phenomenon, which lea'ds to the breakdown of organics, and the second, more
recently discovered one involves high wettability. This latter phenomenon we
have tenned "superhydrophilicity." Even though they are intrinsically different
processes, they can, and in fact must, take place simultaneously on the same
TiO z surface. Depending upon the composition and the processing, the surface can
have more photocatalytic character and less superhydrophilic character, or vice
versa.
Property
Self-cleaning
Category
Materials for residential and office
buildings
Indoor and outdoor lamps and related
systems
Materials for roads
Others
Air cleaning
Indoor air cleaners
Outdoor air purifiers
Water purification Drinking water
Others
Antitumor activity Cancer therapy
Self-sterilizing
Hospital
Others
Endoscopic-like instruments
Tiles to cover the floor and walls of
operating rooms, silicone rubber for
medical catheters and hospital garments
and uniforms
Public rest rooms, bathrooms and rat
breeding rooms

.GU
.G rnmoelecrrocnemlcal t'rocesses or ::sem1conauctors
References
27
A uv
Q=
A B
Irradiation time
Bockris, eds.), Plenum Press, New York, 30, 187 (1996).
3. W. Jaegennann, in: Modern Aspects of Electrochemistry,(R. E. White, B. E. Conway and J. O.
M. Bockris, eds.), Plenum Press, New York, 30, 1 (1996).
4. A. J. Nozik and R. Memming, J. Phys. Chem. 100:13061 (1996).
5. A. Fujishima and D. A. Tryk, in: Functionality of Molecular Systems, (K. Honda, ed.), Springer-
Verlag, Tokyo, 2, 196 (1999).
6. N. Sato, Electrochemistry at Metal and Semiconductor Electrodes, Elsevier, Amsterdam, (1998).
7. L. M. Peter, in: Applications of Kinetic Modeling, (R. G. Compton and G. Hancock, eds.),
Elsevier, Amsterdam, 37,223 (1999). '
8. Photocatalysis-Fundamentals and Applications, (N. Serpone and E. Pelizzetti, eds.), John Wiley
& Sons, New York, (1989).
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10. Photocatalytic Purification and Treatment of Water and Air, (D. F. Ollis and H. Al-Ekabi, eds.),
Elsevier, Amsterdam (1993).
11. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 95, 69 (1995).
12. A. Heller, Ace. Chem. Res" 28, 503 (1995).
13. A. Fujishima and T. N. Rao, Pure and Appl. Chem., 70,2177 (1998).
14. A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol.C: Photochem. Rev., 1, 1
(2000). _.
15. A. Fujishima, K. Honda and S. Kikuchi, Kogyo Kagaku Zasshi, 72,108 (1969).
16, A. Fujishima and K. Honda, Nature, 238, 37 (1972).
17. 1. Zhao, A. Wang and M. A. Green, Appl. Phys. Left., 73, 1991 (1998).
18. A. Shah, P. Torres, T. Tscharner, N. Wyrsch and H. Keppner, Science, 285, 692 (1999).
19. A. Fujishima, K. Kobayakawa and K. Honda, J. Electrochem. Soc., 122 1487 (1975).
20. S. M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, New York (1981).
21. A. Heller, Ace. Chem. Res., 14,154 (1981).
22. M. S. Wrighton, Ace. Chem. Res., 12, 303 (1979).
23. G. Hodes, J. Manassen and D. Cahen, Nature, 261, 403 (1985).
24. T. Inoue, T. Watanabe, A. Fujishima, K. Honda and K. Kohayakawa, J. Electrochem. Soc., 124,
719 (1979).
25. H. Gerischer, Solar Energy Conversion; Solid-state Physics Aspects., Springer, Berlin, Chapter
4 (1979).
26. N. Lewis, Annual Review of Physical Chemistry, 42, 543 (1991).
27. G. M. Swain, !ldv. Mat., 6, 388 (1994).
28. L. Boonma, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem. Soc., 144,
Ll42 (1997).
29. T. N. Rao and A. Fujishima, Diamond Relat. Mater., 9, 384 (2000).
30. T. N. Rao, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem. Soc., 146,680 (1999).
31. M. M. Halmann, Chemical Fixation of Carbon Dioxide - Methods for Recycling CO 2 into Useful
Products, CRC Press, Boca Raton (1993).
32. M. M. Halmann and M. Steinberg, Greenhouse Gas Carbon Dioxide Mitigation: Science and
Technology, CRC Press LLC, Boca Raton, Florida, U.S.A. (1999).
33. M. M. Halmann, Nature, 275, 115 (1978).
34. T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 277, 637 (1979).
35. N. S. Lewis and G. A. Shreve, in: Electrochemical and Electrocatalytic Reactions of Carbon
Dioxide, (B. P. Sullivan, K. Krist and H. E. Guard, eds.), Elsevier, Amsterdam, p. 263 (1993).
36. K. Hirota, D. A. Tryk, T. Yamamoto, K. Hashimoto, M. Okawa and A. Fujishima, J Phys.
Chem. B, 102, 9834 (1998).
37. S. N. Frank, A. J. Bard, JAm. Chem. Soc., 99, 303 (1977).
38. S. N. Frank, A. J. Bard, J Phys. Chem., 81, 1484 (1977).
39. A. Fujishima, K. Hashimoto, T. Watanabe, TiD 2 Photocatalysis: Fundamentals and Applications,
BKC, Inc., Tokyo (1999).
40. H. Honda, A. Ishizaki, R. Soma, K. Hashimoto, A. Fujishima, J Illum. Eng. Soc. (Winter), 42
(1998).
41. A. Fujishima, T. N. Rao, Proc. Indian Acad. Sci. (Chem. Sci.), 109,471 (1997).
42. J. Peral, X. Domenech, D. F. Ollis, J Chem. Tech. Biotech., 70, 117 (1997).
43. A. Mills, S. LeHunte, J. Photochem. Photobiol. A-Chem., 108, 1 (1997).
44. K. Vinodgopal, P. V. Kamat, Chemtech., 26, 18 (1996).
45. U. Stafford, K. A. Gray, P. V. Kamat, Heterogeneous Chem. Rev., 3, 77 (1996).
46. M. Anpo, H. Yamashita, S. G. Zhang, Current Opinion in Solid State and Materials Science, 1,
630 (1996).
47. A. L. Linsebigler, G. Q. Lu, J. T. Yates, Chem. Rev., 95, 735 (1995).
48. K. Rajeswar, J. Appl. Electrochem., 25, 1067 (1995).
49. G. Stewart, M. A. Fox, Research on Chemical Intermediates, 21, 933 (1995).
oxygen vacancies H H+
A \/
...P, A H 2 0 /0:..... /0.....
Ti TI Ti- Ti Ti TI
(OH",H')
{ e" + Ti4+  Ti 3 ,
4h+ of- 20l"  02i
uvljda
fl
q
Photoinduced oxygen vacancies are
replaced by dissociated water molecules,
resulting in hydrophilic surface
 water droplets
uniform water film
II' rLLr (no fogging)
° °
TI"" .....T( .....TI
H H
I I
/0..... ,...0.....
TI TI Ti
Q)
C:
t:
III
t5

t:
o
()
A Hydrophobic
B Hydrophilic
-
.. -
- -
- -
dark
TiO. substrate
Fig. 2.13 Mechanism of photo-induced hydrophilicity.
The second phenomenon, superhydrophilicity, has only recently been
studied. 77 - 80 } This effect was actually discovered by accident in work that was
being carried out at the laboratories of TOTO, Inc., in 1995. It waS found that if
a TiO z film is prepared with a certain percentage of SiOz, it acquires
superhydrophilic properties after UV illumination. In this case, electrons and
holes are still produced, but they react in a different way. The electrons tend to
reduce the Ti(IV) cations to the Ti(III) state, and the holes oxidize the oz- anions.
In the process, oxygen atoms are ejected, creating oxygen vacancies (Fig. 2.13).
Water molecules can then occupy these oxygen vacancies, producing adsorbed OH
groups, which tend to make the surface hydrophilic. The longer the surface is
illuminated with UV light, the smaller the contact angle for water becomes;, after
about 30 minutes or so under a moderate intensity UV light source, the contact
angle approaches zero, meaning that water has a tendency to spread perfectly
across the surface.
Various companies have been trying to develop self-cleaning surfaces,
especially windows, for a long time. One approach has indeed been to try to
make the surface highly hydrophilic, so that a stream of water is enough to
displace stain-causing organic compounds. Such approaches have often involved
the use of surfactants. The objective, in effect, is to make the glass permanently
surface-active. However, the problem is that such coatings lack durability,
hardness and weather resistance.
In contrast, TiO z coatings can maintain their hydrophilic properties
indefinitely, as long as they are illuminated. Making use of the idea of cleaning
by a stream of water, coated windows can be cleaned by rainfall. Other related
applications for hydrophilic glass include windows that are easily cleaned by
water alone, and anti-fogging or anti-beading windows and mirrors. Beading of
rainwater on automobile side-view mirrors can be a serious safety problem, and
now this problem has virtually been solved.
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A: Chern., 98, 79 (1996).
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415, 183 (1996).
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( 1997).
69. N. Serpone, D. Lawless, R.Khairutdinov and E. Pelizzetti, J. Phys. Chern., 99, 16655 (1995).
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(1997).
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T. Watanabe, Nature, 388, 431 (1997).
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T. Watanabe, Adv. Mater., 10, 135 (1998).
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3
Design, Preparation and Characterization of Highly
Active Metal Oxide Photo catalysts
3.1 Introduction
Semiconduc'tor photocatalysis, chemical reactions occurring in
photoirradiated semiconducting materials, have been explored extensively, and a
number of the photocatalytic reactions have been applied to practical processes,
such as detoxification or mineralization of waste and/or hazardous materials,I,2} as
reviewed in other chapters of this volume. In both fundamental and application
studies, it is necessary to choose the most adequate semiconductor photocatalyst
fom a large number of candidates. Since the semiconductor photo catalysts are
always solid materials, numerous variations, e.g., particle size and distribution,
s,urface area, crystal and surface structure, etc., can be obtained even if the
chemical composition is the same. In fact, one of the most significant and
,
promjsing photocatalysts, titanium(IV) oxide (Ti0 2 ), is available from many
manufacturers or prepared in laboratories in different forms, characteristics, and
photocatalytic activities. A major goal of investigation on design and preparation
(or selection from commercial products) of semiconductor photo catalysts is to
make highly active ones that utilize light energy with high efficiency. However,
no solid strategies to realize this goal have yet been established; we have very little
empirical information, e.g., anatase Ti0 2 tends to show higher activity compared
ith rutile. Consequently, theoretical considerations on the photocatalytic activity
t9 clTify its correlation with characteristics, i.e., physical property and structure
of photocatalysts, are highly anticipated. In this chapter, the design of metal
oxide photocatalysts, especially Ti0 2 , will be discussed based on results of kinetic
investigations 3 - 7 ) and a novel method of preparation of highly active Ti0 2
photocatalysts.
3.2 Photocatalytic Activity
The principle of the semiconductor photocatalytic reaction is simple.
Irradiation by light of energy greater than the band gap, separating vacant
conduction band (CB) and filled valence band (VB), excites an electron in VB into
CB to result in the formation of an excited electron (e-)-positive hole (h+) pair.
These e- and h+ reduce and oxidize respectively chemical species on the surface
of photocatalyst, unless they recombine to give no net chemical reaction but heat.
The o,rigina1 structure (or chemical composition) of semiconducting materials
remains unchanged if equal numbers of e- and h+ are consumed for chemical

30
3 Highly Active Metal Oxide Photo catalysts
3.2 Photocatalytic Activity
31
reaction systems. However, the linear relation can be rationalized by taking into
account the rection between e- and/or h+ with the surface-adsorbed substrate(s).
When the specIfic surface area is increased, i.e., particle size is decreased without
changing the surface properties, how is the photocatalytic reaction modified? If
one assumes that enough (saturated) substrate is adsorbed on the surface, the rate
should be enhanced. Of course, it is impossible to adapt a general kinetic
consideration of ordinary thermal catalytic eactions, where the constant surface
density of active sites makes the rate in proportion with the specific surface area,
to te photocatalytic reactions. In the photocatalytic reaction systems, only a
portIOn of the photocatalyst particles or an outer part of bulk materials can absorb
incidnt photons, and the remainder of the photocatalyst does not take part in the
reactIon. However, the total number of absorbed photons should be constant if a
sufficient amount of photocatalyst absorbs all the incident photons and, therefore,
the total number of e--h+ pairs is expected to be independent of specific surface
area (Fig. 3.1).
reaction and/or recombination, whence the term photocatalyst. It is evident that
the apparent rate of the photocatalytic reaction depends on the photocatalyst,
because efficiency of utilization of incident or absorbed light energy (in the latter
case, the efficiency is called quantum yield (efficiency)) is generally smaller than
unity; the quantum efficiency may vary from zero to unity by changing the
photocatalyst. Consequently, one can attribute the difference in the rate of
photocatalytic reaction to activity of photocatalyst, i.e., photocatalytic activity. In
this sense, photocatalytic activity is defined as the absolute or relative rate of
photocatalytic reaction.
Based on the above considerations, the photocatalytic activity of
semiconductor materials must be controlled by three basic parameters: (1) light
absorption property, e.g., light absorption spectrum and coefficient, (2) rate of
reduction and oxidation of reaction substrate by respectively e- and h+, and (3) rate
(or probability) of e--h+ recombination. The first parameter(s) is mostly governed
by the bulk structure of the semiconductor solid and if we choose one of the
crystal forms ofthe photocatalyst, it is more difficult to modify. Only the diffusion
and reflection properties are controllable by changing, e.g., particle size or surface
texture, but in the case where a powder suspension is used, almost all the incident
photons are absorbed by the particles and may therefore provide negligible
influence on the rate. The second parameter, i.e., the rate of e- and h+ transfer
across the semiconductor-solution (gas) interface, has received much attention of
researchers in this field of chemistry. The potential of e- and h+ can be estimated
from the edge potential of CB and VB, respectively, and is constant unless the
crystal structure of the photocatalyst is changed. Redox potential of the reaction
substrates, adsorbed on the surface, may be influenced slightly by the surface
chemical structure, which interacts with the substrates, but the amount of adsorbed
substrate depends more directly on the nature of the surface, e.g., specific surface
area. Thus, the second parameter is closely related with the surface area and will
be discussed in the following section. The third parameter, the rate of e--h+
recombination, has been neglected, partly because its estimation is rather difficult
compared with the other two parameters. However, it has been proved that the
recombination rate predominates the photocatalytic activity in some
photocatalysts7,8) or under selected reaction conditions,8) as discussed later. This
is understood by the fact that an ordinary semiconductor photocatalytic reaction
gives a quantum efficiency much smaller than unity; if the efficiency is 30%,9)
70% of e--h+ pair disappears by recombination without giving any chemical
reaction. Thus, to design the semiconductor photocatalyst, we can not neglect the
rate of recombination. This will be discussed in Section 3.2.2.
small
small
surface
area
ratio of
substrate
molecule!
electron-
hole
3.2.1 Effect of Surface Area on Photocatalytic Activity
In several photocatalytic reactions, a linear relation between the rate of
photocatalytic reaction and amount of substrate(s) adsorbed on the surface of
photocatalyst has been reported. 3 . 1 0- 12 ) When the Langmuirian adsorption isotherm
was expected, this behavior was sometimes called Langmuir-Hinshelwood (L-H)
mechanism even if only a kind of adsorbed substrate was assumed. Strictly
speaking, however, this is wrong, because L-H mechanism involves the surface
reaction of two kinds of adsorbed species, which is not realized in photocatalytic
large
large
single crystal
larger particles
nanD particles
light
....
....
....
..
Fig. 3.1 Schematic representation of effect of surface area on photocatalytic activity. If constant
density and complete absorption of incident photons are assumed, the number of e- and h+ is
independent of particle size, i.e., surface area. The amount of the substrates adsorbed on the
photocatalyst increases with the increase in the surface area, which, therefore, enhances the
reaction of e- and h+ with the substrates.

J£
J .1Jbl.uy rl.t...U v t.,.. .lV.1'-'LCU VAJUt... .1 llULUt...aLaJY,:)L,,:)
.)..) rreparatton ot 1 Itamum(l V) Oxide Powders
33
Large surface area with constant surface density of substrate leads to faster
rate of e- and h+ reaction with substrates, because of the larger number of
substrates surrounding the e--h+ pairs. In this sense, the larger the specific surface
area, the higher the photocatalytic activity. In some of the literature reporting
photocatalytic activity, the activity is shown by the rate per unit surface area. This
expression of photocatalytic activity makes for some confusion, since the
photocatalyst of relatively small surface area may give larger activity while the
apparent rate by constant light flux is not so fast. As described above, if almost
all the photons are absorbed by the photocatalyst in these reactions, the expression
can be rationalized, and the activity should be an indication of the recombination
rate, as shown in the following section.
3.2.3 Design of Photo catalysts of High Activity
As described above, the rate of e--h+ recombination must be an alternative
decisive factor in photocatalytic activity, since the photogenerated e--h+ pairs
recombine to give no chemical reactions unless they react with the surface-
adsorbed substrates. Unlike the rate of reaction of e- and h+, it is very difficult to
evaluate the recombination rate directly. Consequently, few discussions on
recombination rate relating to the photocatalytic activity have been made.
One of the most probable structural features related to e--h+ recombination is
crystallinity. It is assumed that the recombination occurs at crystal defects. 13 ) In
fact, amorphous TiO z showed negligible photocatalytic activity, presumably due
to the defects in the particles,?) but we have few methods to evaluate the number
of defects in photocatalyst powders. Surface of crystals is, in a sense, a defective
site, where continuity of crystal structure is terminated, and thereby, the larger the
surface area, the faster the recombination. Since the surface area also has a
positive influence, i.e., in a reversed way of e--h+ recombination, on the reaction
rate of e- and h+ with substrates, estimation of overall photocatalytic activity
should be made carefully; when the surface reaction predominates the
recombination, the photocatalyst of larger surface area is better, and vice
versa. 14,15)
Recently we reported that ultrafast laser spectroscopy enabled us to measure
the rate of recombination occurring in a time region of a few tens of picoseconds
in several kinds of TiO z photocatalysts. 16 ) The analyses of very fast decay of
trapped electrons with positive holes showed that the rate obeys second-order
rate expression; this is consistent with the above-mentioned recombination of e--
h+. 17 ) The second-order rate constant obtained in these experiments depended
strongly on the nature of the TiO z powder. For example, the rate constant for
amorphous TiO z with negligible photocatalytic activity was relatively large, while
that of a highly active HyCOM TiO z with high crystallinity, as discussed later, was
small. As far as we know, this second-order rate constant may be the sole reported
measure of the recombination rate, although it is not an absolute value. 16 )
Investigation on the correlation of the recombination rate constant and number of
defective sites in various photo catalysts is now in progress. 18)
Many researchers have claimed that the structural characteristics, e.g., crystal
structure (form), particle size, surface area, etc., of photo catalysts determine their
photocatalytic activities. Judging from the theoretical considerations described
above, however, the relation between the physical properties and the
photocatalytic activities is not so simple, and at least two parameters related to the
surface reactions and recombination of e--h+ must be optimized to obtain highly
efficient semiconductor photocatalysts. As a working hypothesis for the
preparation of highly active semiconductor photocatalysts, we have proposed
that larger surface area and high crystallinity are minimum requisites of
photocatalysts. 5 ,8) This is based on the above-mentioned considerations; the larger
surface area corresponds to higher rate of surface reaction of e- and h+, and the
high crystallinity, i.e., little crystal defects to slower rate of e--h+ recombination.
Of course, other factors no doubt also have appreciable influence on the
photocatalytic activity, but we believe that these two parameters are indispensable
basic requirements.
In the ordinary processes of metal oxide preparation, metal hydroxide or
hydrated metal oxide is precipitated in the first step then calcined to dehydrate into
metal oxide. The as-prepared precipitate is, generally, of large surface and low
crystallinity, and the calcination reduces the surface area and improves the
crystallinity. Thus, preparation of metal oxide powders having both large surface
area and high crystallinity requires precise control of calcination conditions,
because the calcination has negative and positive effects on the surface area and
the crystallinity, respectively. In the following sections, several reported processes
for metal oxide preparation are reviewed from the standpoint of the physical
properties of products required for highly active photocatalysts.
3.2.2 Effect of Electron-hole Recombination on Photocatalytic Activity
3.3 Preparation of Titanium(IV) Oxide Powders
In industrial processes, TiO z powder is mainly produced by sulfate or chloride
vapor) method. In the laboratory, titanium alkoxide is frequently used as the
starting material.
3.3.1 Sulfate Method
Titanium oxysulfate (TiOS0 4 ) solution, produced by dissolving titanium
minerals such as ilmenite (FeTi0 3 ) in sulfuric acid, is neutralized with base
yielding hydrated titanium oxide (TiOz'nHzO), which is calcined to form anatase
?r rutile TiO z . This process, including a calcination step, is called the sulfate
method. Sulfate ion often remains in the intermediate product, i.e., hydrated TiO z ,
and it is generally difficult to remove sulfate ion completely from the intermediate
by washing and/or calcination at high temperatures. The hydrated TiO z possesses
a1large surface area but also has a large number of crystal defects which act as
iecombination center of electron-positive hole (e--h+) resulting in negligible
photocatalytic activity. TiO z derived from hydrated TiO z by calcination is usually
inactive because of contaminants, such as iron coming from starting materials and
sulfate ion remaining in the final product which also induce recombination. In

34
3 Highly Active Metal Oxide Photo catalysts
3.3 Preparation ofTitanium(IV) Oxide Powders
35
another method, TiOS0 4 is hydrolyzed by thermal treatment of the solution in an
autoclave at high temperatures (hydrothermal treatment), affording hydrated
Ti0 2 . 19 ,20)
3.3.2 Chloride Method (Vapor Method)
Nut

[]] [] [] []
f f  f
Thermocouple
Water
Silica Wool
Thermal decomposition (or combustion) of titanium tetrachloride vapor,
which is formed by a reaction of titanium minerals and chlorine gas, at 973-1273
K yielded Ti0 2 (TiCI 4 (g) + O 2 = Ti0 2 (s) + 2Ch). This process including the
thermal decomposition is called the chloride method or vapor method.
Crystallization of intermediate Ti0 2 species into anatase and/or rutile also occurs
because this process is carried out at high temperatures. Degussa P-25 Ti0 2 is
produced by this method, consisting of the mixture of anatase and rutile
crystallites,21) and is widely used by researchers all over the world because it
exhibits higher photocatalytic activity for many kinds of reaction systems. P-25
possesses sufficient surface area (ca. 50 m2g-l) and has fewer defects because of
higher production temperature. Generally speaking, this method is not used in the
laboratory because it is difficult to control reaction conditions, e.g., temperature,
and recover the product. Hydrolysis of TiCl 4 as the intermediate product in this
process under atmospheric pressure yields hydrated Ti0 2 , Ti0 2 fine particles, or
colloidal Ti0 2 depending on the preparation conditions. 22 ,23)
IP:;=
Starting
material
and
organic
solvent
Heater
Fig. 3.2 Schematic illustration of a reaction instrument for HyCOM Ti0 2 synthesis.
3.3.3 Alkoxide Method
3.3.4 Specific Methods
,'  ;:. : 0 tf  1 J: t" ' Z;;
, J .-#'t:-'  . 11  _e.. \  w! .-. \-
4.. i .._,!.:-';:'fo.   7 l- 4of-
j , r' I. f. .\;4¥,..
f: 1 ,In !'il!li,/"! ..
; 1 ' '' 1 1 i II/fll l lil l ";
'! 1 I II jl II . " :
,,1"' ";'. 1'\,."1'<
p.".".... " , :, ',' .. ;
  fi. ..  .. ....;Ji.-'rj,,\o,..;.....,
'": ',-i:;- !/'''' ;;! 5nm ' 
"'-'.._...w.  -1t!I .--> ..,j,::...- <:: __A$::>CI'-'1!-
hi.f'l!jt-":j t? ;';:: ,;4
  -  "..,
" ....'\{'b -I _¥, 1
J"'..b....tI.<;\.. I . , -J : _.,.I.
.,., 'i'!. ,'. 'I;S. r. '.;, '"
; "t:":-:'.'J. \. 't. to lr.., ., .?' """i-
JK-; \tt''''' .....<""\. < Y'J ,.. I ,\(4: ,\ 0 . '- ,  , .;" , ;j
, '"';'i , . '1 .
" "  \ _:A. .o.:.-: ; , J
,';, ,"'.(', $", ' j,
,: Y.<i"  "';,:," ,;.1, .'f'}:
,.,; .' r ;" ',:I'Wt"v;J:'"
. . '. ':: .:.;y(...\ ).-
,\ly, ",, :I'"
;, ,.;tI'}''
.. \ '
./ .f'<: .0' <
....'1t;, ....
'),
Many kinds of metal oxide are prepared by hydrolysis of meta] alkoxide and
a subsequent gelation process. Numerous syntheses of Ti0 2 by the alkoxide
method have been reported. 19,20,24,25) This method can produce Ti0 2 of high purity
because alkoxide is easily purified by distillation, as in the case of TiCI 4 .
Furthermore, in contrast to Ti0 2 's prepared from titanium salts, the alkoxide-
derived product is free from counter anions so the effect of the anions on
photocatalytic activity can be neglected. In this method, alkoxide is dissolved in
an appropriate solvent, e.g., alcohol, and the solution is then mixed with another
alcohol solution containing water, hydrolyzing the alkoxide to give Ti0 2 sol
and/or precipitate. When the Ti0 2 sol thus formed is coated onto a substrate then
gelated, Ti0 2 thin films are produced. The sol solution is gelated, the rate of
which depends on conditions such as concentration of alkoxide or ratio of alkoxide
and water, forming hydrated Ti0 2 .
Amorphous Ti0 2 of large surface area of 300 m 2 g- 1 is obtained by thermal
decomposition of titanium isopropoxide vapor at 563 K.26) Amorphous Ti0 2 is
used as a UV -absorbent because it possesses negligible photocatalytic activity due
to a large number of defects leading to e- -h+ recombination and therefore exhibits
negligible photocatalytic activity.7) Calcination induces crystallization of
amorphous Ti0 2 into the anatase or rutile form and the thus crystallized Ti0 2
sample shows appreciable photocatalytic activity.?)
Several methods for direct synthesis of crystalline Ti0 2 are reported. Molten
. ..fT;
. /1:
4.': :
50nm
- "
Fig. 3.3 TEM photograph of HyCOM Ti0 2 synthesized from titanium butoxide in toluene at 300°C.

36
3 Highly Active Metal Oxide Photo catalysts
3.3 Preparation of Titaniurn(IV) Oxide Powders
37
salt reaction of a mixture of titanium oxysulfate and alkaline nitrate at 603-703
K led to the formation of microcrystalline anatase Ti0 2 (TiOS0 4 + 2N0 3 -= Ti0 2
+ SOi- + 2N0 2 + 1/20 2 ),21) The product showed surface area of 137-305 m 2 g- I ,
but it may have been contaminated with a small amount of sulfate ion. Laser-
induced decomposition of alkoxide gave Ti0 2 micro-crystals. 28 ) Rutile Ti0 2 is
synthesized by hydrothermal oxidation of titanium metal at >723 K in an
autoclave. 29 )
Synthesis of crystalline Ti0 2 in organic media is also examined. Thermal
reaction of titanium oxyacetylacetonate in glycols (glycothermal reaction) at
523-573 K yields microcrystalline anatase. 30 ) High-temperature hydrolysis of
alkoxide with water dissolved in organic solvent at high temperatures (423-473
K) and concomitant crystallization (HyCOM: Hydrothermal Crystallization in
Organic Media) gives anatase nano-crystal oflarge surface area (> 100 m 2 g- 1 ).31,32)
Figs. 3.2 and 3.3 show the reaction apparatus and a TEM photograph of the
HyCOM Ti0 2 , respectively.
Thermal decomposition of alkoxide in inert organic solvents such as toluene
(TD: Thermal Decomposition)33) and high-temperature hydrolysis of alkoxide
with water homogeneously liberated from solvent alcohols (THyCA: Transfer
Hydrolytic Crystallization in Alcohols)34) also give nanocrystalline anatase, These
Ti0 2 samples prepared in organic media (HyCOM, TD and THyCA) satisfy the
properties necessary for photocatalysts of higher photocatalytic activity as
described in the previous section. The activity of these Ti0 2 's prepared in organic
media will be discussed later.
Anatase crystallite is observed after calcination of hydrated Ti0 2 at temperatures
higher than 673K. Though surface area is decreased, the activity of thus-obtained
Ti0 2 sample increases dramatically.38) The recombination probability of e--h+
decreases due to the improved crystallinity while the amount of substrate(s)
adsorbed on Ti0 2 particles decreases due to the decrease in surface area. On
calcination, the former positive effect is larger than the latter negative effect,
which reasonably explains the increased activity. Calcination at higher
temperatures induced growth of anatase crystallite. However, in this temperature
nmge, a slight increase in calcination temperature induces dramatic increase in
crystallite and decrease in surface area. Therefore, precise control by calcination
O,fl these factors of Ti0 2 prepared from hydrated Ti0 2 's is generally difficult.
Severe calcination decreases surface area,19,26) which results in large decrease in
.
activity.38) The anatase crystallite in each sample transforms to rutile crystallite on
calcination at around 973 K.20) Contaminants such as sulfate ion in the sample
uppresses crystal growth, decrease in surface area and phase transformation. 39 )
H9wever, as mentioned previously, at the same time, these contaminants often
decrease photocatalytic activity.
"
\ Nanocrystalline Ti0 2 's synthesized in organic media show higher thermal
stability,30-34) i.e., the decrease in surface area on calcination is relatively smaller
compared with Ti0 2 's prepared by other methods. Since these nanocrystals have
negligible amorphous and/or hydrated parts, appreciable dehydration and
crystallization occurring in ordinary hydrated Ti0 2 's do not take place in these
nanocrystals. Therefore, the number of crystal defects ofTi0 2 nanocrystal can be
reduced by calcination without large decrease in surface area.
3.3.5 Activation of TiO z Photo catalysts
c. Hydrothermal Treatment
A. Metal Loading
Potential of the top (edge) of the valence band of Ti0 2 is strongly positive (ca.
3 V vs. SHE), which means Ti0 2 possesses high oxidation ability whereas the
conduction band edge (bottom) sits close to H+/H 2 (0 V at pHO) redox potential,
suggesting relatively lower reduction ability of Ti0 2 . For example, in the system
of photocatalytic dehydrogenation of alcohols accompanying hydrogen (H 2 )
evolution (RCHOHR' = RCOR' + H 2 ), H 2 is liberated negligibly from bare Ti0 2 ,
while oxidation of alcohols is expected to take place. Only a small amount
(usually <1 wt%, depending on surface area of powder) of platinum (Pt)
deposition on Ti0 2 particles dramatically increases the rate of H 2 evolution,35)
indicating that platinum acts as co-catalyst, i.e., site for H+ reduction by electron.
In the case of Ti0 2 particles with diameter of less than 10 nm, the Ti0 2 shows
activity if one Pt fine particle is deposited on each Ti0 2 particle and excess
loading ofPt decreases the activity.) Palladium, rhodium, gold, silver and copper
show the same effect as Pt. 36,37)
Crystallization of hydrated Ti0 2 into anatase or rutile phase occurs by
hydrothermal treatment,40) i.e., thermal treatment in water at temperatures higher
than its boiling point, in which an autoclave is indispensable to keep the
spontaneous pressure (>0.1 MPa) of water. Since solubility in water of inorganic
compounds, including Ti0 2 , increases with temperature, crystallinity of product
is increased and crystal growth by a dissolution-recrystallization mechanism is
accelerated in this treatment. Hydrothermal treatment has been applied in the
preparation of large crystals such as quartz (a-Si0 2 ), but recently it was found that
microcrystalline anatase is obtained by this treatment of hydrated Ti0 2 at
relatively low temperatures. 41 ,42) It is, however, generally difficult to prepare Ti0 2
of both high crystallinity and large surface area by this method because anatase
crystal grows dramatically by the treatment at around 473 K.43)
D. Another Treatment
Hydrated Ti0 2 , the intermediate product in the alkoxide method, exhibits
negligible activity. Usually, the number of crystal defects, which act as,
recombination center of e--h+, is reduced by thermal treatment (annealing). .
Recently, extension of the absorption edge ofTi0 2 from ultraviolet to visible
region (visiblization) by ion implanting into Ti0 2 has been reported, whereby
ions such as vanadium and chromium are implanted into a Ti0 2 pellet by a high
voltage accelerator. 44 ) When these ions are added to Ti0 2 by ordinary methods,
e.g., incipient" wetness, equilibrium adsorption, or ion exchange, absorption of
visible light is actually observed, but photocatalytic reaction does not take place
B. Thermal Treatment (Annealing)

38
3 Highly Active Metal Oxide Photocatalysts
J..J vJl.aJ.a"""""J.JL..a".lVA..1 V.I.  J.'-'1 .I. .I.J.""..vvu...u....J u"u
under irradiation by visible light. Moreover, the activity under UV irradiation was
reduced. On the other hand, the Ti0 2 prepared by this ion-implantation method
exhibits the activity under visible light irradiation. Clarification on the state of
electron or the reaction mechanism is expected.
also used for 2-propanol and acetic acid systems and, as expected, show higher
activity in both systems 33 ,34) as well as HyCOM Ti0 2 . Therefore, our hypothesis
that Ti0 2 powders satisfying both high crystallinity and large surface area should
show higher activity is proved to be effective for design of highly active
photocatal yst.
3.4 Preparation of Other Photocatalysts
3.5.2 Correlation Between Physical Properties and Photocatalytic
Activity of HyCOM TiO z
Potassium niobate with a perovskite-type structure (Nb6017) with a small
amount of nickel oxide (NiO) photocatalytically decomposes water into hydrogen
and oxygen (2H 2 0 = 2H 2 + O 2 ) with relatively high efficiency. The photocatalyst
is prepared by the calcination of a mixture of niobium oxide and potassium
carbonate at 1200-1300°C. 4 5) Titanates active for stoichiometric decomposition of
water are also obtained by the same solid process. 46 ) Recently, photocatalysts of
high activity have been synthesized by new methods. Layered double oxide
(K2La2Ti03012) prepared by a polymerized complex method possesses surface
area larger than the catalyst obtained by a solid process and therefore exhibits
higher activity for splitting of water. 47 ) Orthorhombic strontium tantalate (SrTa206)
prepared by a flux method shows photocatalytic activity for decomposition of pure
water. 48 )
A. Ag Metal Deposition-oxygen Evolution System 52 )
3.5.] Photocatalytic Activity of HyCOM TiO z in Aqueous
Suspension Systems
This reaction has been well characterized so far3.5.38.53-56) and the amount of
silver ion (Ag+) adsorbed on Ti0 2 surfaces ([Ag+]ads) can be measured precisely
under conditions similar to those ofthe photocatalytic reaction. Fig. 3.4 shows the
correlation between the rate of photocatalytic reaction (RAg) and [Ag+]ads'
The linear relations clearly indicate that the rate is proportional to the amount
of substrate adsorbed on the particles and thereby the adsorptivity is one of the
decisive factors in the reaction rate. Another important finding is that the slope for
HyCOM(973) (1.3) was approximately three times larger than that for P-25 (0.45).
The importance of the balance of adsorptivity and recombination probability in
photocatalytic reactions has, as far as we know, been pointed earlier by
Fleischauer and co-workers 57 ) and recently by Martin and co-workers. 14 ) In a
previous study,I6) we investigated the recombination kinetics after photoexcitation
of several Ti0 2 .powders by femtosecond pump-probe diffuse reflectance
measurement and found that the second-order rate constant for e--h+
recombination for HyCOM(973) was 2-2.5 times smaller than that for P-25,
which is almost consistent with the ratio of the slope in this study. Of course, other
possibilities controlling the reaction rate are also considered, e.g., difference in the
3.5 Characterization of Ti0 2 Photocatalysts of Both High
Crystallinity and Large Surface Area
HyCOM Ti0 2 powders were platinized (0.5 wt%) by an incipient wetness
method and used for photocatalytic dehydrogenation of 2-propanol and deaino-
N-cyclization of (s)-lysine 49 ) in aqueous solution under deaerated conditions. The
HyCOM Ti0 2 exhibited 2-3 times higher activity for the former 9 ) and twice or
more for the latter 50 ) than P-25. In the photo deposition of silver metal
accompanying oxygen evolution under de aerated conditions, bare HyCOM Ti0 2
also showed activity twice higher than that ofP-25. 9 ) Taking account of the fact
that P-25 has been recognized to be a highly active photocatalyst that many
researchers use P-25 as the reference catalyst, it can be concluded that HyCOM
Ti0 2 is an ultra-highly active photocatalyst.
Photocatalytic activity of HyCOM Ti0 2 for complete decomposition
(mineralization) of organic compounds under aerated conditions was examined.
Acetic acid was chosen as the model compound and the amount of evolved carbon
dioxide was analyzed.
150
or- 100
,
..s:::
0
E
:::t.
-
OJ
r1 50
CH 3 COOH + 20 2 = 2C0 2 + 2H 2 0
o
o
50
100
150
200
In this system, the rate of CO 2 formation by HyCOM Ti0 2 is approximately three
times larger than those of P-25 and ST-OI (Ishihara),51) clearly indicating that
HyCOM Ti0 2 exhibits excellent photocatalytic activity under aerated conditions
as well as under deaerated conditions. Moreover, TD and THyCA Ti0 2 's were
[Ag+]ads I J.lmol g-1
Fig. 3.4 Correlation between the initial rate of photocatalytic Ag metal deposition (RAJ and Ag+
adsorption ([Ag+]ads) of HyCOM Ti0 2 calcined at 973 K (open circles) and P-25 (closed
circles).

40
3 Highly Active Metal Oxide Photo catalysts
3.5 Characterization of Ti0 2 Photocatalysts
41
150
2.5 Ag+ ion nm- 2 ) and property of adsorption sites on Ti0 2 is independent on the
crystallite size and/or surface area, i.e., the adsorption characteristic and reactivity
of Ag+ toward e- remains constant upon calcination.
- .. If the properties of crystal bulk, e.g., rate of e--h+ recombination, are not
influenced by the calcination, decrease in photocatalytic reaction rate by
calcin(!.tion is attributable to the above-mentioned behavior of [Ag+]ads' However,
on the contrary, the apparent rate of photocatalytic Ag deposition was enhanced
by the higher-temperature calcination, as Fig. 3.6 shows. Similar results for this
reaction were observed for commercial Ti0 2 's54) and Ti0 2 samples prepared by
hydrolysis of titanium isopropoxide,38,55) although the rate was decreased on
severer calcination at 973-1173 K. The effect of calcination on the recombination
rate for HyCOM Ti0 2 was also examined by the femtosecond pump-probe diffuse
reflectance measurement and the rate drastically decreased on calcination. 16) This
indicates that one possible reason for the increase in the photocatalytic reaction
rate is the reduced probability of e--h+ recombination by the annealing of
crystallites. The calcination improves the crystallinity and reduces crystal defects,
which should predominantly act as a recombination center for e--h+, leading to
lower probability of recombination and enhanced photocatalytic activity. Since,
as reported previously,9) the ,quantum efficiency of the photocatalytic reaction
by HyCOM was estimated to be ca. 30% and, thereby, a large part of e--h+
undergoes mutual recombination, such an effect of calcination to diminish the
recombination seems to be reasonable.
A similar effect of high-temperature calcination of rutile TiO z powder has
been reported by Oosawa and Gditzel. 54) They attributed the calcination-induced
enhancement of photocatalytic activity for Ag deposition along with O 2 evolution
to decrease in surface-hydroxyl density, although the role of surface hydroxyls
was not clarified. Sato and Kadowaki pointed out that there are many factors
influencing the overall reaction rate,55,56) e.g., reduction of powders enhances the
Ag deposition rate due to increase in conductivity in the particle. 56) Although
there remains a possibility that another factor, e.g., surface hydroxyl density,
oxygen vacancies, conductivity, light scattering characteristics or steady-state
density of e- -h+ in each particle, affects the overall reaction rate, our hypothesis
average density of e--h+ in the photoirradiated particles which might be affected
by many factors such as impurity content and surface structure of Ti0 2 particles
even though the two Ti0 2 powders have very similar particle size, surface area,
and crystal structure. However, the low e--h+ recombination probability for
HyCOM(973) is most probably attributable to the apparently large rate in spite of
relatively small [Ag+] ads'
Figure 3.5 shows the effect of calcination on the physical properties of
HyCOM TiOz's. The crystallite size of anatase and the BET surface area of as-
prepared sample were 11 nm and 140 m 2 g- 1 , respectively. Upon elevating the
calcination temperature, the crystallite size was increased and the surface area was
decreased, reflecting crystal growth and sintering of the anatase crystallites upon
calcination. It should be noted that even after calcination at 973 K the sample
remained in the anatase phase and had a large surface area of 34 mZg-l. The factor
of adsorptivity, [Ag+]ads, was also reduced by the calcination (Fig. 3.6) and almost
proportional to the BET surface area (Fig. 3.7). This shows that the density (ca.
60
40
E
c
-
0r-
o 20
l:J
o
o
100
50
.-
t
OJ
C\J
E
-
I-
W
co
(f)
500
1000
o
1500
Fig. 3.5 Effect of calcination on crystallite size (squares) and BET surface area (circles) of HyCOM
Ti0 2 .
600
or-
I
OJ
o
E
::t
-
II)
l:J

-+'
OJ

400
200
600
250
or-
I
200 OJ
0 400
.- E
I
..r:: ::t
150 -
0 II)
E l:J
:::t IU
100 - +""' 200
OJ OJ
£- 
50
0
0 0 50 100 150
500 1000 1500 rrf -1
Tc/K SBET I 9
Tc IK
o
o
Fig. 3.6 Effect of calcination on [Ag+]ads (open circles) and RAg (closed circles).
Fig. 3.7 Plots of [Ag+]ads of HyCOM Ti0 2 calcined at various temperatures in air as a function of their
BET surface area.

'T"'-
J lUlUY fi\,.U ye; lYle;Li:1l VAlue; r l1ULU\.Oi:1Li:11YM::>
43
':>.J L-naraCTenzauon 01 1IU2 rnOtocatalYSts
Table 3.1 Physical properties, silver ion adsorptions, and photocatalytic activities of Ti0 2 powders
Ti0 2 Tea!")'/ K SBET d 101 b ) (Ag+]ads Age) 02 e ) Ag/40 2
Im2g-1 /nm Ipmol g-l Ipmol h- 1 Ipmol h- 1
HyCOM 140 II 496 10 <1
HyCOM 573 133 10 485 15 <1
HyCOM 823 78 18 368 25 5 1.36
HyCOM 973 34 26 98 108 28 0.96
HyCOM 1073 8 47 33 149 39 0.95
HyCOM 1173 3 55 18 185 46 1.00
HyCOM 1273 3 13 187 48 0.96
P-25 50 25 192 67 16 1.00
ST-Ol 300 900 15 4 0.85
0), HyCOM Ti0 2 powders were prepared at 573 K in toluene and calcined at Teal,
b) Crystallite size calculated from the 101 diffraction peak of anatase.
e) Rates for photocatalytic Ag metal deposition and O 2 evolution from an aqueous Ag 2 S0 4 solution.
(Inital concentration of Ag+ was 50 mmol dm- 3 .)
appears to be consistent with the results of the other photocatalytic reaction
systems.
B. Mineralization of Acetic Acid and Dehydrogenation of 2-Propanol Systems
The effect of calcination temperature on photocatalytic activities of HyCOM
Ti0 2 for mineralization of acetic acid under aerated conditions 51 ) and
dehydrogenation of 2-propanol under de aerated conditions 9 ) have also been
examined and are shown in Figs. 3.8 and 3.9, respectively.
Although, unfortunately, the relatively higher detection limits did not allow
us to determine the amount of adsorbed substrates under conditions similar to the
photocatalytic reaction, it can be assumed that the amount of adsorbed substrates
is proportional to the specific surface area, as seen in Fig. 3.7. In the former
system, the as-prepared HyCOM showed the highest activity and the activity
decreased monotonically with elevating calcination temperature, i.e., decreasing
surface area; the adsorptivity factor predominates in this case. Compatible with
these results, a commercial Ti0 2 , Ishihara ST -01, having large BET surface area
but showing lower activity for Ag deposition (Table 3.1), exhibited higher
40
30
0
E
:::t
-- 20
C\I
0
0
>-
10
0
0 500 1000 1500
Tc/K
photocatalytic activity.
In the 2-propanol dehydrogenation system, the activity increased with
calcination up to 973 K and decreased at higher temperatures. Since, in this case,
transformation of crystallites from anatase into rutile might affect the ability of
P 0 2 particles,38) the interpretation of the overall rate is somewhat complicated.
fIowever, in the region where crystallites were anatase, the predominant effect of
the calcination ratper than the adsorptivity was clearly seen. While this tendency
is similar to the present Ag-deposition system, it should be noted that the as-
prepared HyCOM sh,owed moderate, but not negligible, activity. Thus, the
calcination influenced the photocatalytic activity in different ways depending on
the type of reaction.
3.5.3 Novel Hypothesis for Activity of Photocatalyst 52 )
Photocatalytic behavior appears to be related to the difference in number of
electrons (or holes) required to complete the photocatalytic reactions: the larger
the number becomes, the higher the recombination probability. For mineralization
of acetic acid, one can expect the reaction to proceed spontaneously after one-
electron (hole) oxidation of the substrate acetic acid, since a proposed intermediate
radical undergoes addition of O 2 followed by thermal degradation. 58) The
dehydrogenation of 2-propanol is formally a two-electron process. Unlike these
reactions, completing the Ag deposition together with O 2 evolution requires four
electrons. In other words, four holes must migrate in the particle, escaping from
recombination, before arriving at the site for O 2 evolution. 1t seems that, under
these circumstances, the recombination becomes a more significant factor
determining the photocatalytic activity: Ti0 2 powders as prepared or calcined at
lower temperatures adsorb large amounts of substrates but, presumably due to
their crystal defects, migration of active species may be markedly prohibited.
On-the contrary, HyCOM calcined at higher temperatures or P-25, which is
prepared via high-temperature gas-phase process, exhibits superior photocatalytic
activity, especially when the reaction requires a multiple number of electrons
(holes).
Consistent with the'above arguments, we observed improvement in
Fig. 3.8 Effect of calcination on yield of carbon dioxide from acetic acid solution
250
200
0 150
E
:::t.
--
C\I 100
I
>-
50
0
0 500 1000 1500
Tc/K
Fig. 3.9 Effect of calcination on yield of hydrogen from 2-propanol solution.

't't
.J Dlgmy i"\cnve lVleml UXlGe rnotocataJysts
45
250
200
,.-
I
..c: 150
0
E
:::t 100
--
Q)
+-'
(1j
"- 50
0
0
3.6 Preparation and Characterization of Photocatalytic Thin Films
Ti0 2 gel. 67) Fabrication of films from TiCl 4 as the starting material has also been
reported. 68 ) It is well known that the photocatalytic activity of these coatings
strongly 'depends on the preparation and post-deposition treatments, since they
have decisive effects on the chemical and physical properties of Ti0 2 included in
the coatings. Therefore, it is necessary to choose adequate processing conditions
to yield highly active photocatalytic coatings.
B. Another Route
500
1500
c;hemical and physical vapor deposition technique has been widely applied
for the preparation of such photocatalytic thin films. Since these vapor methods
need an instrumental setup which enables control of temperature and pressure,
their initial and running costs are generally high and the size of substrate is
limited. Spray method, in which titanium alkoxide and water is sprayed on a
substrate heated at a desired temperature, affords Ti0 2 thin films. 69 ) However, like
the sol-gel route, the physical properties and photocatalytic activity of Ti0 2
strongly depend Oil many factors such as temperature of substrate, flow rate of
carrier gas, and partial pressure of starting material in the system.
As discussed in the previous section, the property of Ti0 2 suitable for each
reaction system depends on the type of reaction. 9,33,34,51) Thus, design and control
of the Ti0 2 properties appropriate to the desired photocatalytic reaction system is
required. A promising alternative strategy for producing highly active
photocatalytic coatings is the attachment of stable Ti0 2 particles of high
photocatalytic activity onto a substrate without reduction of activity. If the size of
the particles is small enough, transparent photocatalytic films are, in principle,
available. HyCOM Ti0 2 , which has been proved to exhibit ultra-high
photocatalytic activity in several reaction systems, is one of the most suitable
candidates for the starting material of Ti0 2 thin films. HyCOM- Ti0 2 powders
were dispersed in aqueous solution of nitric acid to yield a Ti0 2 sol stable for more
than 90 days, and transparent Ti0 2 thin films were successfully produced by dip-
coating from the Ti0 2 soI.70)
1000
Calcination temperature I K
Fig. 3.10 Effect of calcination on the rate of photocatalytic reaction (Ag: open circles, acetone:
closed circles, and O 2 : squares) in an aqueous Ag 2 S0 4 solution (25 rnrnol drn- 3 ) in the
presence of 2-propanol (0.5 rnrnol).
photocatalytic activity of HyCOM calcined at a middle temperature range between
823 and l073 K by adding 2-propanol (0.5 mmol) to the starting Ag 2 S0 4 solution
(Fig. 3.10), e.g., the rates for Ag metal deposition together with acetone formation
(2Ag+ + CH 3 CHOHCH 3 = 2Ag + CH j COCH 3 + 2H+)4) by HyCOM(823) and
HyCOM(1073) were increased from 25 and 149 to 60 arid 245 Jlmol h- I ,
respectively.
Essentially the same behavior was reported earlier for Ti0 2 powders prepared
from alkoxide. 38 ) This may be accounted for by the change in the ,number of
electrons from 4 (0 2 ) to 2 (acetone).
3.6 Preparation and Characterization of Photocatalytic Thin
Films
A key technology for practical application is the preparation of immobilized-
Ti0 2 coatings, e.g., fabrication of transparent Ti0 2 thin film on a glass substrate.
In this section, .preparation and characterization of Ti0 2 thin films are briefly
described.
3.6.2 Characterization of Photocatalytic Thin Films Prepared from
HyCOM TiO z Powders 70 )
Figure 3.11 shows absorption spectra of HyCOM Ti0 2 films with different
thickness as well as the glass substrate. Clearly the Ti0 2 coating absorbed light of
the ultraviolet region and the absorption increased with the thickness (Table 3..2),
while negligible absorption was seen in the visible region. Thus, the method
enables us to immobilize the HyCOM Ti0 2 particles without losing transprency
in th visible region.
Figure 3.12 shows the time course of the absorbance of a dye, malachite
green (MG) solution, in the presence and absence of HyCOM-film-A. Practically
no decrease in absorbance by immersing HyCOM-film-A or by keeping in the
dark for 10 min indicates that adsorption of MG onto the film and thermal
catalytic decomposition of MG can be neglected. In the absence of Ti0 2 film, the
UV irradiation reduced the absorbance negligibly, showing little direct photolysis
of MG. On the other hand, the absorbance, i.e., the MG concentration, was
3.6.1 Preparation of Photocatalytic Ti0 2 Thin Films
A. Sol-gel Route
Sol-gel processes including dip or spin-coating as a final step of preparation
have been used to prepare various kinds of metal oxide thin films, mainly because
of their relatively low cost and flexible applicability to wide ranges of size and
shape of substrates. Through this technique, Ti0 2 films of high photocatalytic
activity have been produced. 59 - 61 ) Preparation of Ti0 2 films from crystalline
colloidal Ti0 2 solution has -also been reported 62 -64) and recently Ti0 2 thin films of
high photocatalytic activity are being obtained from the Ti0 2 sol prepard by
hydrothermal treatment of peroxotitanic acid solution 65 ,66) or alkoxide-derived

46
3 Highly Active Metal Oxide Photo catalysts
References
47
HyCOM-film-A
HyCOM-film-B
STS-film-N)
STS-film-Ba)
50
110
40
100
20
38
25
52
52
73
41
52
HyCOM and STS, the rate increased with the film thickness, but not linearly; the
rate of each thicker film (film-B) was less than that expected from the rate of each
thinner firm (film-A). This could not be attributed to nonlinear photoabsorption
property, since the photoabsorption of the film, estimated by subtraction of
substrate part from the transmission spectrum shown in Fig. 3.11, was almost
J .
proportional to the thickness for each Ti0 2 (Table 3.2). Therefore, one of the
reasons for nonlinearity is that only the outer part of the Ti0 2 was exposed to the
MG solution, i.e., penetration depth of the solution into the film is limited. Along
I. .
with the thickness of Ti0 2 film, the total number of absorbed photons, which
produce electron-hole pairs, should increase, but the number of adsorbed MG
v
molecules can not be increased proportionally to result in nonlinearity of the
1,( .
photocatalytic reaction rate. Based on these considerations, we can compare the
photocatalytic activity of HyCOM and STS Ti0 2 films depending. on the film
thickness, i.e., photo absorption. For each case, thinner (film-A) and thicker (film-
B), the HyCOMfilms showed the higher rate but smaller photoabsorption while
their thickness was even larger than the STS films. This clearly shows the higher
efficiency of utilization of electron-hole pairs in HyCOM Ti0 2 compared with
STS-Ti0 2 ; the ratio of efficiency should be larger than that of apparent rate ofMG
decomposition (1.3 and 1.4 for films-A and films-B, respectively). In the present
stage, we can not determine which of the two significant factors of photocatalytic
activity, larger amount of adsorbed substrate and smaller probability of electron-
hole recombination, predominates in these results. It was clearly demonstrated that
uperior photocatalytic activity of source HyCOM- Ti0 2 particles was preserved
after immobilization on glass substrate.
100 glass substrate
<f- 80 HyCOM-film-A
--
Q)
u 60
c
ru 
E "HYCOM-film-B
E 40
I/)

L-
I- 20
0 300 400 500 600 700
Wavelength I nm
Fig. 3.1] Transmission spectra of Ti0 2 thin films of different thickness (50 nm for HyCOM-film-A
and 110 nm for HyCOM-film-B).
Table 3.2 Rate of photocatalytic decomposition of malachite green by Ti0 2 thin films immersed in its
aqueous solution (2.5 x 10- 4 mol dm- 3 )
film
thickness I nm
% absorption at 320 nm
rate I % h- 1
a) Films prepared from Ishihara STS-Ol Ti0 2 sol.
3.7 Summary
0.6
A A A .6. .6.
.
0.5 -
Q) .
u
c .

.c 0.4 -
L-
a .
I/) 
.c
<x: .
0.3 I-
Black light on
0.2 J
0 10 20 30 40 50 60 70
Time Imin
I Kinetic and theoretical considerations of photocatalytic activity of
semiconducting materials lead to the working hypothesis that large surface area
and higher crystallinity are minimum requisites for photocatalysts of high
efficiency. This is consistent with previous reports on photocatalytic activities.
Several methods for the preparation of photocatalyst in the form of powder and
film, including novel procedures, HyCOM, TD and THyCA, for a highly active
T,i0 2 , have been described and reviewed.
References
Fig. 3.12 Time course of absorbance of MG solution in the presence of slide glass (triangles) or
HyCOM-film-A (circles).l
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4
Photoelectrochemistry at Semiconductor/Liquid
Interfaces
4.1 Introduction
An electron is excited from the highest occupied molecular orbital (HOMO)
to the lowest unoccupied molecular orbital (LUMO) when a molecule in solution
absorbs light. The excited electron in the LUMO may transfer to a neighboring
molecule (oxidant) in solution, leading to the reduction of the oxidant, whereas the
electronic hole (electron vacancy) in the HOMO may transfer to another
neighboring molecule (reductant) in solution, resulting in the oxidation of the
reductant. Quite similar photoinduced reduction-oxidation processes can occur at
the semiconductor/solution (semiconductor/liquid) interface when a
semiconductor in solution absorbs light. Fig. 4.1 schematically illustrates the
0 ) ::
- ..
R ..
hv
e'
. \,
. '8
(o x'-
-R'
electrolyte
'semiconi:!uctor
electrolyte
. C.B.'
.electron.
. .. . . .
hv
»

c::::

Ox
electrolyte
semiconductor
electrolyte
Fig. 4.1 Photoinduced electron transfer processes in a semiconductor/solution system. C.B.:
conduction band, V.B.: valence band.

52
4 Photoelectrochemistry at Semiconductor/Liquid Interfaces
4.2 Basic Properties of Semiconductor/Liquid Interface
53
photoinduced processes at the semiconductor/solution interface. The excited
electron in the conduction band (C.B.) and the (electronic) hole in the valence
band (V.B.) both migrate to the semiconductor surface and then transfer to an
oxidant (Ox) and a reductant (R') in solution, respectively, finally leading to the
reduction of Ox and the oxidation of R ' .
Figure 4.1 indicates that the photoinduced electron-transfer (redox) processes at
the semiconductor/solution interface can be understood in the same way as those
in molecular systems. However, there are some important differences between the
two systems. The first is that, in the semiconductor system, the band bending (the
internal potential gradient or internal electric field), explained later, is present
within the semiconductor near the interface (though it is not shown in Fig. 4.1 for
simplicity). The band bending causes effective electron-hole separation in the
semiconductor, leading to very inefficient electron-hole recombination and thus
very high quantum efficiencies of the photoinduced redox reactions. This is a
strong advantage of the semiconductor system. It is not unusual for the quantum
efficiency to reach nearly 100% for single-crystal semiconductor systems.
The second important difference is that the interface potential is present at the
(outer) Helmholtz layer of the semiconductor/solution interface. The interface
potential is produced by surface dipoles of surface bonds as well as surface
charges due to ionic adsorption equilibria between the semiconductor surface
and the solution. If the interface potential can be regulated by a change in the
chemical structure of the semiconductor surface, then the semiconductor band
energies can be shifted to match the energy levels of the solution species (oxidant
or reductant). This is another advantage of the semiconductor system because
this enables improvement of the electron transfer rate at the
semiconductor/solution interface and the energy conversion efficiency.
Semiconductor systems have other advantages in that the visible and near uv
light can be absorbed effeciently and the electrons and holes in the semiconductor
in general have much higher .mobilities than ions in solution. In the present
chapter, the basic properties of the semiconductor/solution interface are described
followed by discussion of some recent topics of photo electrochemistry at this
interface.
interface,2) where U(R/Ox) is the equilibrium redox potential (or equilibrium
electrode potential) of the (R/Ox) couple and q is the elementary charge.
Figure 4.2 schematically illustrates (a) the charge distribution, (b) the charge-
density distribution, (c) the potential distribution, and (d) the band bending at the
semiconductor/electrolyte interface, under the assumption that no surface charge
or surface dipole is present. The semiconductor is taken to be n-type, but a similar
diagram can be drawn for the p-type. The band bending in the semiconductor,
shown in (d), comes simply from the potential distribution (c), which, in turn,
comes from the charge distribution in (a). The charge distribution (or the electrical
double layer at the interface) comes from the difference in the Fermi level between
the semiconductor and the redox electrolyte before contact. At equilibrium after
contact, the Fermi level of the semiconductor, E F = -qU (U the electrode
potential), lines up with the Fermi level (or the redox level), -qU(R/Ox), of the
redox electrolyte. The line-up is assured by the formation of the electrical double
layer and the potential difference at the interface.
At the electrical double layer in Fig. 4.2(a), the charges on the electrolyte side
are mostly localized at the (outer) Helmholtz layer if the electrolyte concentration
(a)
e e
e
ions at the e  ionized donor
Helmholtz layer = e
e e
I "."
electrolyte: :
I I
I
I
I I
: Depletion :
 Region -..:
n-type semiconductor
(b)
£
C/)
(I) c:
0)(1)
ro"'O
..c:
<..>
o
o
w
x
(c)
4.2 Basic Properties of Semiconductor/Liquid Interface
4.2.1 Band Bending
ro
:.;::::;
c:
(I)
..-
o
c..
iy---i
Ox + e- = R
(4.1)
>.
0)
.....
(I)
c:
(I)
-q<I>B
;-----
:0
I
I
I
I
I
I
I
.w
 x
The band bending at the semiconductor/liquid (electrolyte solution) interface
can be understood by considering the potential distribution at this interface. In a
case where the electrolyte solution contains a redox couple (R/Ox), which causes
an electrochemical redox reaction,
(d)
/1
the potential distribution at the semiconductor/electrolyte interface can be
understood in the same way as the well-known Schottky barrier at the
semiconductor/metal interface. I) The Fermi level of the metal in the laner interface
corresponds to the Fermi level of the redox electrolyte, -qU(R/Ox), in the former
Ev
Fig. 4.2 Schematic illustrations of (a) the charge distribution, (b) the charge-density distribution, ( c)
the potential distribution, and (d) the band bending at the semiconductor/redox electrolyte
interface, assuming that no surface charge nor surface dipole is present.

54
4 Photo electrochemistry at SemiconductorlLiquid Interfaces
4.2 Basic Properties of Semiconductor/Liquid Interface
55
is enough high (2 0.5 M). On the other hand, the charges on the semiconductor
side are distributed deep into the interior of the semiconductor, forming a wide
space charge layer. The n-type semiconductor has an electron donor in the crystal,
doped as an impurity, such as phosphorus in n-Si. The space charges in the n-type
semiconductor are composed of ionized electron-donor ions. The reason why a
space charge layer of finite thickness is formed is because the density of the
electron donor, No, is low, in a range from 10 14 to 10 18 cm- 3 , and also because the
ionized electron-donor ions are spatially fixed and immobile. The principle of
electrical neutrality at the electrical double layer leads to an extended distribution
of charges deep toward the interior of the semiconductor.
The potential distribution in the space charge layer can be obtained by solving
the Poisson equation for a given charge distribution. For a
semiconductor/electrolyte interface such as that shown in Fig.4.2, the
potential,</>(x), at a distance, x, from the semiconductor surface is given as follows:
conduction band in the interior of the semiconductor.
The Ufb can be determined experimentally by measurements of the differential
capacitance of the semiconductor/electrolyte contact. The space charge, Qs, per
unit area is given by
Q, = qNDW = [2 q e o e,ND (U - Ufu - k; )T'
(4.6)
The differential capacitance of the space charge layer, Cd; per unit area is thus
given as follows:
c - () Qs _ [ 2
d - ()U - qEoEsN o
( ) ] -1/2
U - Ufb _ k;
(4.7)
This equation can be rewritten as:
t/J(x) = qN o ( wx - £ ) + t/J(O)
EoEs 2
under the condition that No is constant throughout the semiconductor, where Eo is
the permittivity of vacuum, Es the dielectric constant of semiconductor, t/J(O) the
potential at the surface (x = 0), and W the width of the space charge layer. The </>(0)
and Ware given by
(0 s x S W)
(4.2)
1 2 ( kT )
C/ = qEoEsN o U - Ufb - q
(4.8)
The Ufb can then be determined by obtaining lICl vs. U plots, which are called
(a)
t/J(O) = ( qNoW ) 8
EoEs
( 4.4)
ions at the .--0 - 0
....-" 0
Helmholtz layer 0 = 0
surface -
charge 0 = 0
electrolyte :
I
I
: I
I I
I
: Depletion :
I
:+ Region --:
I
o
(4.3)
W = [( 2q:' ) ( U - U fu - k; ) T'
0""---:>
o "--- ionized donor
o
n-type semiconductor
where 8is the thickness of the (outer) Helmholtz layer, Uthe electrode potential,
and Ufb the flat band potential (the potential at which the semiconductor bands are
flat). It should be noted in Eqs. (4.2)-(4.4) that a change in U results only in a
change in the band bending (potential distribution) in the semiconductor, with </>(0)
(or U fb ), namely the surface band positions, being kept nearly constant because 8
is very small (ca. 0.3 nm). This is an important property of the semiconductor/
solution junction.
4.2.2 Barrier Height and Flat Band Potential
(b)
ro
+==
c
Q)
+-'
0
C.
(c)
>.
0)
L-
Q)
C
Q) ------
-qU(R/Ox)
w
x
Figure 4.2( d) shows that an energy barrier forms at the semiconductor/redox
electrolyte interface, similar to the Schottky barrier at a metal/semiconductor
interface. The most important quantity is the barrier height (q) or the flat band
potential U fb , which essentially determines the surface band positions of the
semiconductor with respect to the energy levels of solution species. The qW B is
given for an n-type semiconductor by
Ev
qW B = q{U(R / Ox) - U fb } + L1
(4.5)
Fig. 4.3 Schematic illustrations of (a) the charge distribution, (b) the potential distribution, and (c) the
band bending at the semiconductor/redox electrolyte interface in case where negative surface
charges are present.
where L1 is a small energy difference between the E F and the bottom of the

56
4 Photoelectrochemistry at Semiconductor/Liquid Interfaces
4.2 Basic Properties of Semiconductor/Liquid Interface
57
Mott-Schottky plots.
We assumed in Fig. 4.2 that no surface charge or surface dipole is present in
the semiconductor. In general, however, both surface charges and surface dipoles
are present in the semiconductor owing to adsorption equilibria for various ions
between the electrolyte and the semiconductor surface as well as formation of
polar bonds at the semiconductor surface. Such surface charges and surface
dipoles change the potential difference in the (outer) Helmholtz layer and thus
cause shifts in the surface band positions, as shown schematically in Fig. 4.3. The
shifts can be expressed as changes in CP(O) or Ufb in the above equations, with the
fonns of the equations themselves kept unaltered. The surface band positions
(U fb or ljJ(O)) are kept nearly constant for a change in U in this case, too, because
Ufb or ljJ(O) is determined solely by an interfacial ionic adsorption equilibrium or
surface dipoles.
Figure 4.4 shows the surface band positions of some typical semiconductors
in aqueous electrolytes (pH 7), calculated from the experimentally determined U fb ,
compared with the redox levels of some important redox reactions. It is known
that the Ufb for most semiconductors, such as n- and p-GaAs, n- and p-GaP, n- and
p-InP, n-ZnO, n-Ti0 2 , and n-Sn02, in aqueous electrolytes is solely determined by
the solution pH and shifts in proportion to pH with a slope of -0.059 V/pH.3,4) This
is explained by the adsorption equilibrium for H+ or OH- between the
semiconductor surface and the solution, for example,
shifts toward the negative in the presence of chalcogenide ions such as S2- and Se 2 -
in solution and shifts toward the positive in the presence of cadmium (Cd 2 +) ions
in solution, both due to the adsorption of the ions on the semiconductor surface.
4,6) The slope of the shift for adsorption of divalent ions is (l/2)xO.059 V per
decade of change in concentration. Furthermore, it is known that the Ufb for some
semiconductor electrodes shifts by a change in the surface termination bond/,8),
as well as by electrode illumination,9,1O) and the presence of a redox couple in the
electrolyte. 10) The Ufb in nonaqueous electrolytes is also reported for some typical
semiconductor electrodes. 1 1,12)
4.2.3 Electron Transfer and Corrosion Reactions
The electron transfer reactions at the semiconductor/electrolyte interface
occur either via the conduction band or the valence band. The total current is
therefore given by the sum of four partial currents, denoted as i e a , ice, i v a , and ivC,
where the subscripts, c and v, represent electron transfer via the conduction and
valence bands, respectively, and the superscripts, a and c, indicate anodic and
cathodic processes, respectively. Let us assume hereafter that the electron transfer
occurs only via the conduction band. In a simple case where the concentration of
the electrolyte is sufficiently high and only the overvoltages at the Helmholtz layer
(T}H) and in the space charge layer (T}se) are important, the ie a and ice can be given
as follows 4 )
Ss - OH + H\q = Ss - OH 2 +
(4.9)
. e - . O ( Cox,s J ( ns J { q(l - ae)T}H }
Ie - -Ie -0 -0 exp -
Cox s ns kT
(4.10)
where Ss - OH refers to te OH group present at the semiconductor surface.
The Ufb for n- and p-SP) and that for metal chalcogenides such as n-CdS, n-
CdSe and CdTe 4 ,6) do not obey the above law, remaining nearly constant in a
range of pH lower than about 6 for Si and about 10 for n-CdS. This is most
probably because the semiconductor surface has no OR group in this pH range.
It shoud be mentioned that the Ufb for the metal chalcogenide semiconductors
. a . O ( Cr,s J ( qaeT}H J
Ie = lc -0 exp
C r , s kT
(4.11 )
!!:.L = ex p ( -qT}sc J
ns ° kT
(4.12)
-2 conduction band (pH 7) redox potential
-1 T T 1 I'l CO 2 +2W+2e--+HCOOH
>-
........ 2.26 1. 4  i H++e--+1/2 H 2
UJ , N 2 +6H++6e--+2NH 3
::L:
z 0 1.1
u) 
>
- O 2 +4 W+4e-.... 2H 2 0
ro 1 Si
+' CdSe 3.0
c
<1> SiC 1
+' CdS
0
0. 2
valence band
3 Ti0 2
T}H= t/>H- </>R0, T}se= C/Jse- C/Jseo
(4.13)
. ° - C OO k e ( t1Gee J - C O N. k a ( t1Ga J
Ie - q ox,s ns e exp - U - q r,s e e exp - U
(4.14)
11G e e == t1G e a == qU(R / Ox) - qUfb + 11 (barrier height)
(4.15)
Fig. 4.4 Surface band positions of some semiconductors in an aqueous electrolyte (pH 7), calculated
from experimentally determined Ufb.
where ie ° is the exchange current, Cox,s and Cr,s are the surface concentrations of Ox
and R, respectively, ns the surface concentration of electrons in the conduction
band, a e the transfer coefficient, l/>H and C/Jse the potential drops at the Helmholtz
layer and the space charge layer, respectively. The superscript, 0, means that the
quantities are the values at equilibrium.
In cases where the ie a and ice are much smaller than the diffusion-limited

currents for Ox and R and the TJH is negligible( TJH == 0), it follows that Cox s =
Cox,so = Cox, C r . s = C,so = C n and 1]se = 1] (total overvoltage). In this case, equatins
(4.10) and (4.11) can be simplified as follows:
 = i/ +' = O{I - ex -k1) )} (4.16)
GaAs + 6H z O + 6h+  Ga(OH)3 + As(OH)3 + 6H+
(4.22)
This equation represents the well-known rectifying property ofthe semiconductor
electrode. In the above assumption, TJH == 0 and thus TJ = 1]se = lpse - lpse 0 = U - uo
= U - U(Ox/R). Therefore, the current ie is nearly zero (or nearly equal to ie O ) in
U> Ufb and starts to rise steeply at a potential of U == U fb , as can be seen from
equations (4.10)-(4.15).
The photo current (i p ) under illumination at a wavelength (A) is expressed by
the following, 1) in the case where the diffusion length of the hole (L p ) is much
larger than the width of the space charge layer (W) and the interfacial electron-
transfer rate of the hole is sufficiently high,
i = q }; ( 1 - R ){ l _ exp( -a).w) }
p). 0,). ). 1 L
+ a). p
It is known that such a corrosion reaction is suppressed by using a redox couple
having the redox potential negative of the corrosion potential,4} or more strictly,
negative of the redox potential of a long-living surface intermediate of the
corrosion reaction. 4 . 13 ) This requirement for electrode stability leads to a significant
decrease in the barrier height qiP B or [U(R/Ox) - U fb ].
The corrosion reactions cause another serious problem. They produce a
number of surface reaction intermediates such as dangling bonds and atomic
vacancies, having electronic levels within the band gap. Namely, corrosion
reactions produce a number of mid-gap surface states, which act as surface
recombination centers for photogenerated carriers. The V oe is decreased largely by
the production of such surface states. 3 ,4)
4.3 Photo electrochemistry at Atomically Well-defined
Surfaces
(4.17)
4.3.1 Atomically Flat H-terminated Si Surfaces
where Jo,'A is the flux of incident photons, R'A the reflectivity of the semiconductor
surface, and a'A the absorption coefficient. The total current (i) is expressed as
follows:
Si + 2H z O + 4h+  SiO z + 4H+
(4.21)
The photoelectrochemistry at atomically well-defined semiconductor surfaces
is one of the current topics related to the nanostructuring of the semiconductor
surfaces. Most studies have been made on silicon (Si) surfaces, and it is now
well established that hydrogen fluoride (HF)-etched Si surfaces are terminated
mainly with Si-hydrogen bonds (SiHn. n = 1, 2, or 3)14-17) and that, for Si (111),
successive etching with 40% ammonium fluoride (NH4F) produces atomically
flat Si(lll) surfaces, terminated mainly with monohydride (== Si_H).Is-ZZ) Alkali
etching under negatively applied biases also produces similar atomically flat Si
(111) surfaces. z3 }
It is generally accepted that H-terminated Si surfaces are fairly stable under
atmospheric conditions and that monohydride (== Si-H)-terminated surfaces are
more stable than di- or tri-hydride (=SiH z or -SiH 3 ) terminated ones due to smaller
steric hindrance. A number of studies have been made on chemistry of H-
terminated Si surfaces. For oxidation, it is reported that H-terminated Si (100)
[hereafter abbreviated as H-Si (100)] surfaces were oxidized in back bond by
long-time (several-hundred hours) exposure to humid air. z4 } The back-bond
oxidation of H-Si (100) and (111) was also reported for immersion in aqueous
HzOz solutions Z5 } and even for immersion in pUre water (containing dissolved air)
for several hundred minutes. z6 } Recently, surface reactions ofH-Si (111), (100),
and (110) were studied comparatively in various solutions, and it is reported that
monohydride (== Si-H) changed to == Si-OH in water containing HzOz and ozone. Z7 }
The alkylation of Si surfaces is an interesting subject. It is reported that the
immersion of H-Si (111) in organic solutions of 1-alkenes, diacyl peroxide, etc.
produced alkyl-monolayers on Si. zS } The formation of organic monolayers on
H-Si (111) via electrochemical reduction of diazonium compounds in aqueous
solutions is also reported. z9 } .
The formation of Si-X (X=halogen) bonds, especially in the form of ordered
arrangements on a nanometer scale, is of much interest from the point of view of
nanostructuring because the Si-X will have a more ionic character and hence a
i  ;,. +  " ;,. + O{I_ ex 1) )}
The open-circuit photovoltage (V oe ), i.e., the absolute value of 1] at i = 0, is given
by
(4.18)
e = kT I n ( i: + I ) <-::: kT I n ( i: )
q Ie q Ie
(4.19)
In general, the following equation is used, I} taking into account the electron-hole
recombination at the surface and in the space charge layer,
Voe = nkT In ( i:). + 1 ) <-::: nkT In ( ip). ) (4.20)
q ,0 q 'o
e 
where n is called the ideality factor. The n ranges between 1 and 2, and no surface
or space charge recombination occurs when n = I (see Eq. 4.19).
In the above argument, we only considered the electron-transfer reactions
between the semiconductor surface and redox species in solution. Actually, the
electrons and holes at the semiconductor surface often cause another type of
redox reaction leading to semiconductor corrosion. For n-type semiconductors
such as n-Si and n-GaAs, for example, the following anodic photo corrosion
reactions occur. 3,4}

60
4 Photoelectrochemistry at Semiconductor/Liquid Il).terfaces
4.3 Photoelectrochemistry at Atomically Well-defined Surfaces
61
larger chemical reactivity than Si-H and can be used as starting bonds for
nanoscale surface modification. It is reported that the exposure of H-Si (111) to
Clz or Brz gases in vacuum gave the formation of Si-Cl (or Si-Br) bonds in
nearly a full coverage. 3D) We found that the surface Si-X bonds can be formed by
the simple immersion of H-Si (111) and (100) in concentrated hydrogen halide
(HX) solutions containing a small amount of an oxidant such as dissolved air
(oxygen) and halogen molecules (XZ).3I-33)
Figure 4.5 shows FTIR spectra in the region of Si-H stretching vibrations for
H-Si (111) (a) before and (b,c) after immersion in 8.6 M HBr.32) The formation
of an ideal H-Si (111) surface is clearly seen by the appearance of a sharp peak
at 2083.7 cm- I , assigned to terrace Si-H stretching vibration. Very weak peaks at
2071.1, 2101.1 and 2134.5 cm- I can be assigned to monohydride, dihydride, and
vertical dihydride at steps, respectively. The spectrum (a) is hardly changed by
immersion in oxygen-free 8.6 M HBr, as shown in Fig. 4.5(b). On the other hand,
it is largely changed by immersion in air (oxygen)-dissolved 8.6 M HBr, as shown
in Fig. 4.5(c). In the latter case, the main 2083.7 cm- I band almost disappears,
accompanied by an appearance of a new broad band on the higher energy side.
The spectral change can be attributed to the reaction of a considerable part of
terrace Si-H to Si-Br bonds, the broad bands on the higher energy side being
assigned to remaining terrace Si-H having the vibrational energies modified by
neighboring Si-Br bonds.
The formation of Si-X bonds was confirmed by measurements ofXPS spectra
and flat band potentials. For (100) surface, back-bond oxidation occurs
simultaneously with the Si-X formation under the same conditions, though it is
negligible for Si (111). The mechanism of the surface reactions can be explained
by hole injection by an oxidant, followed by the nucleophilic attack of halide ions
or water molecules. 32 ,33) For dihydride (=SiH z ) bonds on H-Si (100), for example,
the reactions at the initial stage can be expressed as follows:
Si (100) + OX aq ---7 h+ + OX- aq
hole injection
(4.23)
=SiH z + 2h+ + X- aq ---7 =SiHX + H+ aq
nucleophilic attack (4.24)
Si-SiH z + h+ + HZO aq ---7 Si-OH -SiH z + H\q
"
(4.25)
Si-OH -SiH z + h+ ---7 Si-O-SiH z + H\q
(4.26)
I 5xl 
where h+ is an injected hole and -SiH z refers to a surface Si atom with a dangling
bond. Successive reactions, leading to the formation of =SiXz, (SiO)zSiH z ,
(SiO)3SiH, etc., will occur similarly.
It is interesting to note here that the immersion of H-Si (111) in 7.1 M HI
produced a number of aligned nanorods of iodine (or poly-iodine) on the
surface. 34 ) An example of atomic force images is shown in Fig. 4.6. The
2000
2100
2200
'1' . ,'i\o ,_ '  ' i 1 .  ' 'If" i ., .. t
... .." '& .;' \ ,"'r "I . Z ....J . t .-,.. '\  '.1  t 1f ¥.
t"" t¥ d.....' '''of''';};",., . "",l ,  ,111 '- 4<-. .. oj...'. H«"....f.
 .' "'... ¥ '« ".,.. . .., « .....'"" >
 Jt  .....: t: "t l". th' -.. ... '""":.............. '" .,
.'A  . ..:.:......s;  : 'Ii .... $ ' ... -.....:". ....1... -.... "l.... '" II "', j;
f.., "01 1fI:.-- ...;   {t. .,....." 15' ..........:.... ......" d 1. '¥o 1Jf. ,
t .. .. ,";-.",/ ¥(:' ...,.:::, ':.'\',.\.: :"....." .... /' ,."'...::...¥..IO. .."'..' ""\. ".
- ft. r ,,,,': :<.; '1 r-:.' So", ;....... . ",'  :t ...." ...........J,  I'
i .. . (', .  ,.. c.  .. "'- '4 t . .f .. < 4. 4" . Vi't,I "'", .t
..:.:.tf #.- -:-. j ";'-"'"''' " :1 ""... f" -'", ;,..\ ...!.... 0 75
;, .. . .......  ., , '"' J. . , ..1,. _ . "_. . <, .. \ ., .
1,  -. ";;',,, ..... . >.,  .. ",",I'tIo,,>r! ,..,*"
",,"II. , S:. ,), 1.-6'...I..f.. '';' ",.t ".. . .i "",:t'.'" ._ 
_ ....."1'';' : .t .;". 'f'". r:.. /''''...' .....'.... "_'#
 - '.lit.  ......;;;a. .. .., .. 1. ".c f...! 
. ' .. .'_ ............ ;.j<:t'.....: >'.;# "," ''': 1.",;.. k #...11 
:...;, ''If ... ! _  !',  :,: .. "" "" ,  B> .... " J""'.... ""'''. :,t t'if ""
_ ..... 4,... """"'- &    .....,.
1"¥'16..... -,..., '."'"'!"t It :'.Jf<r,., . '>HI"",.a ..  ... '-.It .
..  .  -".... ,".... ..:. ...'U"'....:,......... 1 '\'.-'
t"-.. ..............,  .... '.' *. ,.'
;."... (). ".' t' ".,................  "- ,-. I",;  ' .'-';: '<:",:;' 0.50
. "'............. J' - ,- ",' .,. ,. ."\... -"" >
,::# ,':.:   At .." n ,:i:. .; ..;' ..:  * ,...
I  "1 1"., tI'''''¥'., '4""! 1ft ..... ...._.,,;-  ""11:""  -.
'" .. J . t J .  ' ... . "\ ... .... - .,: \....,. ... '*'.... ... ',.  '.'
{"  :'" it... A . ,. , ..;. ,!f'  ,. f.. .. " ...: ,:...' "'.
¥>  ",w ... "..... MIJ ,"".( .' .., ¥... .I" '" -P"",,*,"""" .
.t.} Jt:.. ....'1"'..:: I,. 1 1 '" vA"""",. ...."""./.....--: ... ',""\"1- or::"
.. ( ., 1  it- k .h.  .,:.....  , :: '':;'' .;:  " ';'." 'Y' ..
 -...". r/:"''*'b;  ...... "."' .. c.' .' <...;' ,.. :;:Ht"" 0.25
. 1',....... "{.':' "t ......,,'": '"" $- . 't.-...}- ,....... >11-...... . '/." ..... ' i
-..... ''':-'..f....:, ...,:..",-...",:: ..'"  ./ '/f.'<' .. (# t....*-, ..\...
""'''d.'' .- 'I:Iv<o:I>....."<Ii'....:'......' 1 ...... -...p.,>w'\o... '\.,".'
   At I .",," Mt:t""'-<:,. C-9 lI'" ,.. k""".... ..".,.M<Io. ., It-
"'. :: ...11>-. . .. "j ",,: '*--- (', :' .':">. f'. )t........ ...
'\..\« '.- , "".. -- f'  ".. '" .. >$.
..,IM.;>'\" +-,;. '" . f". ,",10';> _t'....::."t, '. . . .
... J J" I .,1. ...." '-11';.'" ....,.. -." ,-; -." "':"
 ':I'.... . #' ,f, #.. ')'.... fI """....  ... .---- .__ '"
'.4.. ....... \. . ,. .-"... -  .. -"'*' it...) . -
 ..",. '", -- ..+ .' , :....' '*'... .,.._ .;r"''r'" <W'fI( Irr .t " .....,.,""': '"
, .0
O. 75 1. 00
1.00
(a)
o
u

.D
I-<
o
(j"J
.D

(b)
(c)
x3
expanded
Wavenumber / em-I
Fig. 4.5 FTIR spectra in the region of Si-H stretching vibrations for H-Si (111). (a) As prepared H-
Si (111 ), (b) after immersion in deoxygenated 8.6 M HBr (i.e., 8.6 M HBr including 0.5 M
(NH 4 hS03) for 10 min, and (c) after immersion in air-dissolved 8.6 M HBr for 10 min.
2083.7 cm- I : Si-H at terrace, 2071.1 cm- I : Si-H at step, and 2134 cm- I : SiH z at step.
o
0.25
0,50
Fig. 4.6 AFM image of nanorods of iodine on H-Si (I II), formed by immersion in 7.1 M HI.

62
4 Photoelectrochemistry at Semiconductor/Liquid Interfaces
4.3 Photoelectrochemistry at Atomically Well-defined Surfaces
63
HBr aq.
light
surface. It is thus not so easy to obtain well-defined Ti0 2 surfaces because Ti0 2
is chemically stable and difficult to etch. Ti0 2 can only be etched (and
photo etched) in concentrated H 2 S0 4 .38) Recently we found that nanosized
rectangular holes or grooves, extending long in the <001> direction, were
produced when n-Ti0 2 (rutile) electrodes were illuminated in 0.05 M H 2 S0 4
under anodic bias. 39 ,40) Fig. 4.8 shows a few examples of SEM's of the photoetched
n-Ti0 2 (rutile) surfaces. 40 ) Similar results were reported for polycrystalline Ti0 2
electrodes. 41 ) Interestingly, only the (100) face was selectively exposed at the
walls of the photo produced rectangular holes and grooves, irrespective of crystal
faces of electrode surfaces. 40)
For rutile Ti0 2 , the (110) face is known to be the thermodynamically most
stable, i.e., to have the lowest surface Gibbs energy. In fact, this face is exposed
after ion-sputtering and annealing in vacuum. No (100) face is exposed at the
walls of rectangular holes produced by chemical etching with hot H 2 S0 4 ,
indicating that the selective exposition of the (100) face is characteristic of
photoetching in H 2 S0 4 , The water photooxidation and photo etching occur at the
illuminated n- Ti0 2 surface under anodic bias competitively with each other. Thus,
a possible explanation may be given 40 ) by assuming that the selective exposition
of the (100) face is due to a kinetics-controlled mechanism, namely, the water
photooxidation at the (100) face is much faster than the photo etching at the same
face, though not so much faster at other surfaces such as (110) and (001). This
result is of much interest, showing the possibility of crystal-face dependence of
the water photooxidation and/or photo etching rates, as well as the possibility of
the formation of a well-defined Ti0 2 (100) surface.
cation exchange membrane
S . Pt
n- I
H T
Fig. 4.7 Photodecomposition of HI into Hz and Iz (or 1]-) by use of one Si chip, coated with Pt
nanodots on both sides.
mechanism of formation of such nanorods has not yet been clarified. Nano-rods
of copper 35 ) and nickep6) are also formed when metals are electrodeposited on
H-Si (Ill).
The formation of Si-X bonds causes shifts in Ufb. This can be used for
regulation of the surface band positions of Si, i.e., for improvement of the solar
energy conversion efficiency of Si photoelectrochemical solar cells. Fig. 4.7
shows an interesting example which photodecomposes hydrogen iodide (HI) into
hydrogen (H 2 ) and iodine (12 or 1 3 -) only with an n-Si chip having Pt dots at the
surface. 31 ) It should be noted that the Si surface on the side of the (HI + 1 2 )
solution is partially covered with Si-I termination bonds, whereas the Si surface
on the side of the HBr solution is partially covered with Si-Br bonds, due to the
above-mentioned surface halogenation reactions, The difference in the bond
dipole between Si-I and Si-Br leads to a difference in the inteface potential and
hence to a difference in the surface band positions. A potential gradient is thus
induced within the Si chip between the two solutions, which assures effective
separation of photo generated electrons and holes in the Si chip. A high solar-to-
chemical conversion efficiency of 3.4% was obtained under simulated solar
AM1.5 (100 mW/cm 2 ) illumination.
--w-
\. .'
, '. ..
,:) r
,.' t
''fill ..
. ,... ..
· J .
. ...
. "c" i:,
.i
. U J.:;...... "
" · f :ri.:<100>
4.':'. i
!"f':.'
4.3.2 Selective Exposition of (100) Face on n-Ti0 2 (Rutile) by
Photoetching
-.. »",.."'- --... -- -.- -' " t. .
Titanium dioxide (Ti0 2 ) has been attracting much attention for its important
role in water photo-oxidation and photocatalyst, as well as a base material for dye-
sensitized solar cells. A number of studies have been conducted on the
mechanisms of interfacial photo-anodic reactions but the reported mechanisms still
remain sketchy, and the detailed molecular mechanism has not yet been clarified.
The main reason for confusion may arise from the possibility that the reaction
mechanism depends on detailed chemical structures of the electrode surface. This
implies that studies with well-defined surfaces are of key importance.
It is noted that commercial Ti0 2 (100), (110), and (001) surfaces, obtained by
crystal cutting and appropriate polishing, have no truly atomically well-defined
I:w_y. "'hA::'-..' :'.  ....
.i>--."
,. """.. .
. .".'" ,-: omtJ;ti

<001>
Fig. 4.8 Scanning electron micrographs of nanosized rectangular holes extending long in the <00 I >
direction, formed at n-TiO z surfaces by photoetching in 0.05 M H z S0 4 . Upper: (001) TiOz
surface and lower: (110) surface.

64
4 Photoelectrochemistry at Semiconductor/Liquid Interfaces
4.4 Photoelectrochemistry at Metal Dot-coated Semiconductors
65
4.4 Photoelectrochemistry at Metal Dot-coated
Semiconductors
energy conversion efficiency of 14.9 % was obtained under solar AMI 100
,
mW/cm z illumination. 44) The high efficiencies reported for Ru- or Os-loaded n-
GaAs 46) may be achieved by the same principle of metal-dot coating.
The generation of such high Voc's comes from the following facts 4Z --45): (1)
Almost no lowering in the energy barrier is caused by metal-dot coating and thus
the maximum barrier height (nearly equal to Eg) can be reached by regulating the
redox level in solution. (2) The surface carrier recombination is suppressed to a
minimum because the major part of the Si surface is covered and passivated by
thin Si-oxide.
Almost no lowering in the energy barrier by metal dots is attained if they are
very small compared with the width of the space charge layer of the
semiconductor, i.e., about 5 nm in diameter. This is because surface band
modulation by metal dots decays sharply toward the interior of the semiconductor
(Fig. 4.1 0) and is limited only to a narrow surface region of nearly the same size
as the metal dots. Accordingly, the "effective" barrier height, qW B ', for metal
dot-coated electrodes is nearly equal to the barrier height, qe, for a naked
semiconductor electrode (Fig. 4.10).
It should be emphasized that the metal dot-coated semiconductor electrodes
can meet all the above-mentioned requirements simultaneously and have the
properties of the ideal semiconductor electrode. The key point is that, for metal
dot-coated electrodes, the reaction-proceeding part is limited to the narrow regions
of metal dots and the remaining major semiconductor surface is kept free from
surface states. On the contrary, for normal semiconductor electrodes with
homogeneous surfaces, interfacial reactions occur over the entire surface
,
producing reaction intermediates (surface recombination centers) all over the
surface.
The metal dot-coated semiconductor electrodes can be used for efficient
4.4.1 Ideal Semiconductor Electrodes
The semiconductor electrodes for high-efficiency solar energy conversion
must meet various requirements, such as formation of high energy barriers, low
surface carrier recombination, and high interfacial electron transfer rates (or high
catalytic activity of the semiconductor surface). The electrodes should also meet
requirements of high chemical stability, harmlessness to the environment, and low
cost for practical application. Highly active semiconductor phoiocatalysts should
also meet various similar requirements. A serious difficulty arises because the
above requirements are not necessarily compatible with each otherY) For example,
a catalyst (a metal or a metal-complex compound) deposited on the semiconductor
surface to improve the catalytic activity in general leads to a large decrease in the
photovoltage because the deposited catalyst acts as a surface carrier recombination
center and enhances surface carrier recombination.
Such a dilemma can be overcome by using semiconductor electrodes coated
with sparsely scattered, extremely small (nanometer-sized) metal dots,4Z--45) such
as shown schematically in Fig. 4.9 with n-Si used as a semiconductor. The naked
Si surface is covered with naturally grown thin SiO z layer and passivated. The
photo current flows through the metal dots. The photo current can be stable in
aqueous redox electrolyte because the Si surface is covered and protected by
coating with metal dots and SiO z .
The effectiveness of metal-dot coating was clearly shown by the generation
of high V oc' s exceeding those of conventional p-n Si solid-state solar cells having
nearly the same simple structure. 43 --45) Photoelectrochemical (PEC) solar cells,
composed ofn-Si (0.2-0.7 .Qcm) with Pt dots of about 5-10 nm in diameter, a Pt-
plate counter electrode, and aqueous 4.8 M HBr + 0.03 M Brz, generated Voc's of
0.62 to 0.66 V. Quantitative analyses using Langmuir-Blodgett layers of Pt
particles for regulation of the particle size and density on n-Si have shown that
ideal minority carrier-controlled solar cells are obtained for this type of cel1. 45 ) An
ex
redox electrolyte
on transparent conductor
. 
«' \'1 '
.o'(\r_\
cf'.
.("' \
'./
,
,
metal particle
(-5 nm wide and -20 nm apart)
i - E(t'i)
.
.
.
bd.7" EF(M)
 ..E. s (£)
. 'J
n-Si
Si oxide
0.-
U'

CQ

<f,
'0......
E. un
surface of semIconductor
Fig. 4.9 Schematic cross-sectional view of a semiconductor (n-Si) electrode coated with sparsely
scattered nanosized metal particles
Fig. 4.10 Two-dimensional energy band diagram for a metal dot-coated n-type semiconductor
electrode.

66
4 Photoelectrochemistry at Semiconductor/Liquid Interfaces
References
67
Metal-loaded Ti0 2 particles are often used as photocatalysts. In most cases,
the loaded metal acts as a catalyst for a reductive reaction such as hydrogen
evolution. Direct measurements of the potential of the loaded metal, however,
revealed that the metal can act not only as a catalyst for a reductive reaction but
also as a catalyst for an oxidative reaction such as oxygen evolution under the
appropriate conditions. 50 ,5I)
Figure 4.11 explains how the loaded metal can act as catalyst for both the
reductive and oxidative reactions. The essential point is that the barrier height at
the metal/semiconductor interface is changeable by the principle of Fig. 4.10 or
other mechanisms. Thus, when the band bending is weak, photoexcited electrons
mostly enter the metal and the metal acts as a catalyst for a reductive reaction (Fig.
4.11(A)). On the other hand, when the band bending is strong, the holes in the
valence band mostly enter the metal and the metal acts as a catalyst for an
oxidative reaction (Fig. 4.11(B)). The prevalence of one over the other depends
on the magnitude of the band bending, i.e., on the relative rates of reaction of the
electrons and holes at the metal-free semiconductor surface.
hydrogen photoevolution 47 ,48) and carbon-dioxide photoreduction. 49 ) A high
efficiency is reported for a Rh-H loaded p-InP electrode,48) similar to that ofp-GaP
and p-Si electrodes.47) Efficient photoreduction of CO 2 to methane and ethylene
with a high energy conversion efficiency is also achieved by the use of copper
particle-coated p-Si immersed in COTsaturated 0.1 M KHC0 3 (pH 6.8).49)
4.4.2 Metal-loaded TiO z Electrodes
References
n- Ti02
metal
particle solution
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30. J. Terry, R. Wigren, C. Mo, R. Cao, G. Mount, P. Pianetta, M. Linford and C. Chidsey, Nuclear
Instruments and Methods in Physics Research E, 133, 94 (1997).
31. M. Fujitani, R. Hinogami, J. G. Jia, M. Ishida, K. Morrisawa, S. Yae and Y. Nakato, Chem. Left.,
1041 (1997).
(A)
......o.o............o............ .
............................................... ..
.. ...................................
.' C B....................................
: ::::: :::: :':: : : : : : : 0::::::::: : : : :::
. ........:.:-:-:-:-:.:.: ..:.:.:......... 
11 E/I(M)
EFdark(M)
h'U
(8)
.................................................. ..
................................................ ..
::- c: '8: ;:::::::::::::::::::::::::::::::::::
......  ........ - ........
........................... ..............
........................... ............
.......................... .. ............
.......................... .. .........
........... ...... ..........
........... .........
.......... ......
E F (n-Ti0 2 )
Fig. 4.11
Energy band diagrams for metal-loaded TiO z particles (photocatalyst). (A) The metal acts
as a catalyst for a reductive reaction such as hydrogen evolution under weak band bending.
(B) The metal acts as a catalyst for an oxidative reaction such as oxygen evolution under
strong band bending.

68 4 Photoelectrochemistry at Semiconductor/Liquid Interfaces
32. X. W. Zhou, M. Ishida, A. Imanishi and Y. Nakato, Electrochim. Acta, 45,4655 (2000).
33. X. W. Zhou, M. Ishida, A. Imanishi and Y. Nakato, J. Phys. Chem. B, L05, 156 (2001).
34. A. Imanishi, M. Ishida, X. W. Zhou and Y. Nakato, Jpn. J Appl. Phys. 39,4355 (2000).
35. K. Hara and I. Ohdomari, Jpn. J Appl. Phys., 37, L1333 (1998).
36. A. Imanishi, K. Morisawa and Y. Nakato, Abstracts of 197th ECS Meeting, Vol. 200-1, No. 615,
Toronto, Spring 2000; Electrochem. Solid-State Leu. submitted (2001).
37. Y. Nakato, J. G. Jia, M. Ishida, K. Morisawa, M. Fujitani, R. Hinogami, S. Yae, Electrochem.
Solid-Stale Left., 1, 71 (1998).
38. L. A. Harris and R. H. Wilson, J Electrochem. Soc., 123, 1010 (1976).
39. Y. Nakato, H. Akanuma, 1. -I, Shimizu, Y. Magari, J. Electroanal. Chem., 396,35 (1995).
40. A. Tsujiko, T. Kisumi, Y. Magari, K. Murakoshi and Y. Nakato, J Phys. Chem. B, 104,4873
(2000).
41. T. Sugiura, T. Yoshida and H. Minoura, Electrochem. Solid-State Leu., 1, 175 (1998).
42. M. Ishida, K. Morisawa, R. Hinogami, J. G. Jia, S. Yae, Y. Nakato, Z. Phys. Chern., 212, 99
(1999).
43. Y. Nakato, K. Veda, H. Yano and H. Tsubomura, J. Phys. Chem., 92,2316 (1988).
44. Y. Nakato and H. Tsubomura, Eleccrochim. Acca, 37,897 (1992).
45. J. G. Jia, M. Fujitani, S. Yae, Y. Nakato, Electrochim. Acta, 42, 431 (1997).
46. B. A. Parkinson, A. Heller, B. Miller, J. Electrochem. Soc., 126, 954 (1979); B. J. Tufts, I. L.
Abrahams, P G. Santangelo, G. N. Ryba, L. G. Casagrande, N. S. Lewis, Nature, 326, 861
(1987).
47. Y. Nakato, S. Tonomura, H. Tsubomura, BeL Bunsenges. Phys. Chem., 80, 1289 (1976); Y.
Nakato, H. Yano, S. Nishiura, T. Veda, H. Tsubomura, J. Electroanal. Chem., 228,97 (1987).
48. E. Aharon-Shalom and A. Heller, J. Electrochem. Soc., 129, 2865 (1982).
49. R. Hinogami, Y. Nakamura, S. Yae and Y. Nakato, J. Phys. Chem. B, 102, 974 (1998).
50. Y. Nakato and H. Tsubomura, Israel J. Chem., 22, 180 (1982).
51. Y. Nakato, M. Shioji and H. Tsubomura, Chern. Phys. Leu., 90, 453 (1982).
5
Photoelectrochemical Reactions at Semiconductor
Microparticle
5.1 Introduction
Photoelectrochemical reactions at the particulate semiconductor are originally
described as a photoelectrochemical cell where a working electrode of
semiconductor and a counter electrode of metal combine directly with each other
to minimize the size. I) However, particulate photocatalysts are not the same as
those of electrochemical cells. The difference in dimensions causes the change in
the energy structure. The kinetics in and around the particle also. differ from
those observed at the electrode plate. In this chapter, the features characteristic of
particulate photocatalysts are described first, followed by the basic analyses of the
photocatalytic reactions at Ti0 2 powders, based on the organic radicals produced
at the initial stage and the active oxygens formed at the photo-illuminated Ti0 2
detected with ESR spectrometry and chemiluminescence method, respectively.
5.2 Energy Structure of Semiconductor Microparticle
5.2.1 Depletion Layer
Hetero junctions, forming a Schottky barrier like a metal-semiconductor
junction, normally change the energy levels of conduction and valence bands.
When the Fermi level of the semiconductor equilibrates with the energy level of
the redox couple in the solution, the electric energy level at the' surface is pinned
and a depletion layer is formed. This is postulated since the rectified current can
be observed at semiconductor plate electrodes. The bending of the band in the
semiconductor at the surface can be described as a solution of the one-dimensional
Poisson-Boltzmann equation.
For a semiconductor microparticle, by employing a Poisson-Boltzmann
equation of spherical symmetry with radius R, a potential drop is obtained as a
functi?n ofr (distance from the center), as shown in Fig. 5.1(A). 2)
l/J = kT 2 (r - R + Lsc)2 ( 1 + 2 R- Lsc )
6e r .
(5.1)
Where, L D and Lsc are Debye length and the space charge layer width,
respectively. k, e, and T are the Boltzmann constant, elementary charge, and
temperature, respectively. L D and Lsc are given by

70
5 Photoelectrochemical Reactions at Semiconductor Microparticle
5.2 energy ::)tructure 01 ::)em1ConGUCtor lYl1croparuc1e
II
(A)
(B)
(C)
5.2.2 Electric Heterogeneity of Surface
Metals, such as platinum, are usually introduced to improve the electron-
hole separation efficiency. In order to analyze the energy structure of the metal-
loaded particulate semiconductor, we solved the two-dimensional Poisson-
Boltzmann equation. 3 ) When the metal is deposited to the semiconductor by, for
example, evaporation, a Schottky barrier is usually formed. 4 ) For the Schottky type
contact, the barrier height increases with an increase of the work function of the
metaI,4 which should decrease the photocatalytic activity. However, higher activity
was actually observed for the metal with a higher work function. 5 ) This results
from the fact that ohmic contact with deposited metal particles is established in
photocatalysts when the deposited semiconductor is treated by heat 6 ) or metal is
deposited by the photocatalytic reaction.7) Therefore, in the numerical computation
we assumed ohmic contact at the energy level junction of the metal and
semiconductor.
Figure 5.2 shows the contour map of energy level in a semiconductor on
which metal particles are deposited. 3 ) The magnitude of the potential is normalized
to the difference between energies of the conduction band bottom edge and the
metal work function. The dimension was not sensitive to the shape of the potential
contour map. At the back of the deposited metal, there is a large steep potential
drop, because the potential at the surface changes sharply from the metal to the
semiconductor. The mechanism of the charge separation was experimentally
elucidated from the logarithmic relationship between the photocatalytic activity
E (red/ox)
1$0 =

t $0
conduct i on
band
$.c
E (red/ox)
EF
EF
...
...
«S
....

::::I
en
...
....
«S
.....



C>
....
::::I
"CI
c:
C>
....
«S
....

valencB band
va I ence band

r
---7


large particle
sma II part i c Ie
metal-deposited particle
Fig. 5.1 Schematic energy band bending for (A) large particle, (B) small particle, and (C) metal-
deposited particle. R , radius of the particle; Lse, space charge layer; E(red/ox), redox level in
solution; E F , Fermi level in semiconductor; CPo, potential drop in semiconductor. Contour map
for (C) is shown in Fig. 5.2
( ) 1/2
1.0 = EoEkT
e 2 N o
(5.2)
Lsc = r 2EoECPsc ) 112
eNo
where BoE and No are dielectric constant of the semiconductor and donor density
for n-type semiconductor, respectively, and cpsc'represents the potential drop
within the space charge layer. Lo is calculated to be 12 nm for a semiconductor
characterized with E = 100 and No = 10 24 m- 3 . Lsc for the semiconductor is
estimated to be 105 nm, when CPsc is 1 Volt.
When the particle radius is much larger than the space harge layer width (R
» Lsd, the potential drop at the center of the particle CPo becomes equivalent to
CPsc, as shown in Fig. 5.1(A). For the small particle with R < 3 112 Lsc, the whole
particle is under the depletion layer (Fig, 5.1(B)) and the maximum potential
drop at the center is represented by CPo = (kTR 2 /6e L02).2) This is the case for most
photo catalysts since the normal particle size is of the order of 10 nm. Then, for the
particle with the radius equal to the Debye length, R = Lo , a potential drop at the
center CPo is very small; it is calculated to be 0.004 V. Thus, in contrast to the flat
semiconductor electrode, the separation of photo generated electron-hole pairs is
not commonly responsible for the depletion layer. When small metal particles are
deposited on the surface, the band bending becomes large (Fig. 5.1(C)) as
described in the following section.
(5.3)
t\\
1
"C
I
--
-+-
......"
, 1
, e
" 4I!"EJ)
I \
.  ,4/!',
:::.... - I
c:::;  I
. c:::; :::
"C;:,'
./
I I
o 0.2 0.4
140 bu\k
I I I
0.6 0.8 '.0
z/d
'.2
Fig. 5.2 Contour map of energy bending for metal deposited n-type semiconductor obtained by
numerical solving of 2D Poisson-Boltzmann equation. 3 ) Parameters: d = 20 nm, N D = 10- 24
m- 3 , and £ = 80. Plausible pathways for electron 8 and hole EB generated by photo-
absorption are shown by arrows.

/2
:> Photoelectrochemical Reactions at Semiconductor Microparticle
5.3 Kinetics at Semiconductor Microparticle
73
of metal-loaded TiO z and the work function of the metaP)
If the reaction of interest is exothermic, the reaction can proceed without
loading rare metals, which are usually quite expensive. In this case, surface
heterogeneity of potentials may be caused by a surface defect or nonstoichiometry.
Adsorption of reactant molecules possessing a positive electron affinity can act as
a source of the potential drop near the surface of the semiconductor.
ligh
A-
5.2.3 Size Quantization Effect
When the size of the semiconductor particle is smaller than that of the exciton
in the semiconductor, the energy structure changes according to what is called the
size quantization effect. The lowest excitation energy Eex of a small semiconductor
is expressed by Eq. (5.4),8)
Fig. 5.3 Processes of photocatalysis at semiconductor particles. The numbers correspond to those in
the text. Path increased by decreasing the particle size are shown by "increase."
Eex = Eg + V e + V h - Ee
(5.4)
subsequent redox reactions at the surface (step CID) are the key processes in the
photocatalysis. Step @ and/or @ sometimes occur too fast to be included in the
reaction steps.
where Eg is the band gap energy of the bulk crystal, V e and V h are energy shifts for
the bottom of the conduction band and the top of the valence band, respectively.
Ee represents the coulombic energy caused by an electron-hole pair formed in a
particle, which can be calculated by E e =2.59/(eR) with R of the particle radius in
units of nm. 8 ) With the approximation method called effective mass
approximation, the energy shifts, V e and Vh, are calculated employing the "infinite"
depth spherically symmetric quantum well model. For example, V e = 0.376/(m/
RZ ), where me. represents the effective mass of the conduction band electron in
the bulk semiconductor. In addition, the authors proposed an empirical method to
calculate the energy shift based on the "finite" depth quantum well model. 8) This
energy model is effective especially for particles whose size is as small as the
Bohr radius of the exciton, which is calculated by 0.0527e(m e O + mh*)/m/ mho
with effective hole mass of mh 0.
Since this calculation of the quantum size effect is demonstrated for direct
band gap, this equation must be carefully applied to the indirect band gap
semiconductor such as TiO z . 9 ) The effective mass ofthe conduction band electron
reported for rutileTiO z ranges from 3 to 30 with a usual value of 20. For anatase
TiO z , on the other hand, the effective electron mass is reported to be 1. 10 ) The
smaller effective mass of anatase can be attributed to the higher mobility of the
electron, which is 40 times larger than that of rutile. 10)
5.3.1 Recombination Model
In order to understand the reaction mechanism of photocatalysis, a simple
kinetic model can be presented as follows.
TiO z + hv  e- + h+ (5.5)
e-+A ke reduction products (5.6)
A- ---7
k
h++D D+ ---7 oxidation products (5.7)
e- + h+ k r (5.8)
 TiO z
k'
A - + D+ A+D (5.9)
5.3 Kinetics at Semiconductor Microparticle
where e- and h+ are conduction band electrons and valence band holes,
respectively. A and D represent molecules which are reduced and oxidized,
respectively. The intermediate species, A-and D+, precede the reaction toward
reduction and oxidation products. The photoinduced electron and hole recombine
with each other and diminish in the solid (Eq. (5.8)). Indirect recombination or a
net recombination may occur between the primary reductant and oxidant, as
indicated by Eq. (5.9).
According to the reaction scheme of Eqs. (5.5)-(5.8), the reaction rates for e-
and h+ can be expressed respectively as follows:
Photocatalytic reactions at the semiconductor surface can be described by the
following six steps as shown in Fig. 5.3. CD Absorption of a unit of light associated
with the formation of a conduction band electron and a valence band hole in the
semiconductor. @ Transfer of an electron and a hole to the surface. CID
Recombination of electron-hole pairs during the reaction processes. @
Stabilization of an electron and a hole at the surface to form a trapped electron and
a trapped hole, respectively. CID Reduction and oxidation of molecules at the
surface. @ Exchange of a product at the surface with a reactant at a medium.
Among these reaction steps, the absorption of light in the bulk (step CD) and
d [e-] = _ ke [e-] - k r [e-][h+] + g
dt
(5.10)

74
5 Photoelectrochemical Reactions at Semiconductor Microparticle
5.3 Kinetics at Semiconductor Microparticle
75
d [h+] = _ k h [h+] - k r [e-][h+] + g
dt
(5.11)
where, [e-] and [h+] are densities (cm- 3 ) of conduction band electrons and valence
band holes in the semiconductor particle, respectively. g represents the generation
rate of photoinduced e- h+ pairs (cm- 3 s- I ), which can be calculated with the flux
of the incident light, absorption coefficient of the syst,em, and the density of the
semiconductor material. ke and k h are the rates of electron and hole transfers (S-I),
respectively. Both of the decay rates must be affected by the molecules, A and D,
adsorbed on the particle surface, respectively. The authors showed how the
depletion of the surface reactant can be included. ll ) Indirect recombination Eq.
(5.9) should also be taken into account to discuss the formation rate when the
oxidation or reduction products, D+ or A -, are monitored.
The differential equations can be solved by employing a steady state
approximation for [e-] and [h+]:
OJ
13
-e
m
Q.
m
.!:
en
s::
e
......
u
OJ
Q)

o
.....
OJ
..c
E
:J
Z
x . . '. . . - .
xn+\xn+\f--xn+\
1 m /, 1 m /1 m +\
X_lX x+\
1 /1/1
x=:x-x:
XX:
1/1
xgx
a
. . . . . ., X m
g - ke [e-] =k r [e-][h+]
(5.12)
Number of holes in a particle
Fig. 5.4 Two-dimensional (2D) ladder model for the photocatalytic reactions at small semiconductor
particles. X" m represents the distribution of particles containing n electrons and m holes at
some instant.
k h [h+] = ke [e-]
(5.13)
photoinduced electron and hole in the particle, respectively.13) The fraction of
the number of the particles is represented by X n m , which is characterized with the
values of two integers, nand m. Transitions concerning X n m involve the following
three kinds of processes as shown by the arrows in Fig. 5.4; ( )" ) photo-generation
for electron-hole pair to increase both n and ill by 1, ( Lf ) charge recombination
to decrease both nand m by 1, and ( .t) electron and ( ) hole transfers from the
particle to surface molecules to decrease nand m by 1, respectively.
In order to simplify the model, the rate constants ke and k h are assumed to be
independent of the number of charges, nand m, in the particles. Then, the
recombination rate can be expressed by nmk/V, where V is the volume of the
particle, and the time differential of the fractionX n m is expressed by Eq. (5.15).
which lead to,the following relationship,
1 - We k r
-=-g
We 2 kekh
(5.] 4)
where We , defined by We = kef e-]/g, is the quantum yield of the electron transfer.
The dependence of quantum yield on the incident light intensity is supported by
the experimental evidence of the electron transfer to methylviologen induced by
laser pulse. 12)
When the quantum yield is so low that the recombination reaction becomes
relatively dominant, Eq. (5.14) is simplified to We = (k e k h /k r g)I/2 , or [e-] =
(krg/krk e ) 112. This relationship indicates that the steady amount of photoinduced
electron is proportional to the square root of the light intensity. A similar
relationship can be also obtained for the case of the photoinduced holes, and an
equation similar to Eq. (5.14) is used to calculate the hole transfer yield,<P h .
dX" k
 = ( n + l ) k X"+I + ( m + l ) k X" + ( n + l )( m + l ) X"+1
dt e m h m+I V m+1
+ gVX'-'m_' - {nk, + mk h + nm i + gv}x':.. (5.15)
5.3.2 2D Ladder Model
Provided that all processes of electron transfer involve the reduction reaction,
the quantum yiel for primary reduction We can be calculated from the amount of
the reduction product formed in the electron transfer processes divided by the
amount of absorbed photons during the reaction. Thus, by summing up all
fractions, the quantum yield is given by Eq. (5.16),
The above relationship between rp and the rate constants is derived based on
the conventional formulation of the rate equations. The unit to measure the amount
of electrons and holes in the particle is density, the same as in bulk
emiconductors. When the particle size is extremely small or the photon density
IS very low, only a few pairs of electron and hole are photogenerated and
recombine with each other in the particle. This means that "photon density" does
not take continuous values as suitably used in the conventional rate equations, but
takes some series of values whose unit is the inverse of the particle volume.
Taking into account this deviation, we proposed a new model in which particles
are assigned by two integers, nand m, which represent the numbers of
L L nkeX"m
(5.16)
A> _ n m
'¥e-
gV
where the denominator g V represents the number of photons formed in a single
particle in unit time. Eqs. (5.15) and (5.16) can be processed by a computer for
various values of kinetic parameters and the volume of the particle. 13)

76
5 Photoelectrochemical Reactions at Semiconductor Micropartic1e
5.4 Observation of Primary Reaction Intermediates
77
In order to test this model, we measured the quantum yield of the electron
transfer to methylviologen as a function of the particle radius of CdS
nanoparticle. 13 ) The dependence of the electron transfer yield on the particle size
well proved the applicability of the 2D ladder model to this system. For a low
excitation limit of gV < 1, the quantum yield is independent of the light intensity,
as expressed by
C. Diffusion of Carrier to Surface
tP e = ke
ke + k r / V
while at gV> 10 the calculated result is approximated with Eq. (5.14), where the
density is assumed to take a continuous value. Thus, the 2D ladder model is
proved applicable to describe the kinetics at micropaticles of various sizes and at
various excitation intensities.
( 5. ] 7)
The distribution of photogenerated electrons and holes in a semiconductor
solid is determined by the intensity of the light at the corresponding location. For
example, in rutile TiO z crystalline, the absorption coefficient for the light at the
wavelength of360 nm is 9 x 10 4 cm-], which means that the intensity of the light
decreases to lie after travel of 110 nm in the solid. Since the size of the
semiconductor photocatalysts is mostly of the order of 10 nm, electrons and holes
are photogenerated even at the center of the photocatalyst particle. This means that
it takes time for the photogenerated electrons and holes to reach the surface and
recombination occurs to some extent during the travel. On the other hand, for
smaller particles of a few nanometers, the photogenerated electrons and holes can
reach the surface immediately after the excitation, since the effective size of
excitons (photogenerated electron hole pairs) in the semiconductor is close to
the size of the particles. Thus, a small particle showing the size quantization
ffect, which originates from the confinement of excitons on the surface boundary,
IS expected to undergo fast surface reaction. The surface density of the conduction
band electrons for CdS of 2 to 3 nm in radius was calculated using the finite
depth spherical well model. The calculated values are within the range of 10- 5 to
10- 6 nm- z and proportional to the electron transfer rate obtained experimentally. 8)
5.3.3 Effect of Size
There are several factors caused by the change of the particle size that affect
the activity of particulate photocatalysis: (1) surface area, (2) band energy shift,
(3) accessibility to the surface, and (4) space for charge separation.
A. Increase of Surface Area
Since the photocatalytic reaction occurs at the surface of a semiconductor the
increase of the surface area as a result of the decrease in the particle sie is
expected to cause increase in the activity. This may be true in many cases, but in
contrast to conventional catalyses there are many factors that influence the activity
on photocatalyses. If the photocatalytic activity is affected by the amount of
reactant at the surface, the surface area must be a factor of the activity. Bowever,
since the photocatalytic reaction proceeds in the vicinity of the area of light
absorption, the surface where the light does not reach would not contribute to the
activity. Thus, it is not easy to find an appropriate relationship between the surface
area and the photocatalytic activity in the literature.
D. Space for Electron-hole Separation
B. Shift in Energy Level
When the size of the particle decreases, the electron-hole separation is
expected to be inefficient. As has been postulated in the 2D ladder reaction model
stated above, the recombination rate is reciprocally proportional to the volume of
the particle. That is, the recombination rate is proportional to the probability at
which an electron encounters a hole. Since the surface electron transfer reaction
competes with the recombination reaction, the transfer efficiency decreases with
the decrease of the size of the particle, if the rate of the surface electron transfer
does not change significantly.
Effect of separation space on yield was found in some experiments. A higher
quantum yield was observed for InzS3 with larger particle size when the electron
transfer to methylviologen in aqueous solution was examined by pulse-laser
irradiation. 13) A decrease in the yield of the products with decreasing size has also
been reported for the photocatalytic polymerization of methyl methacrylate with
ZnO of the size ranging from 4 to 40 nm; ]5) this was attributed to the increase in
the recombination caused by the reduction of the dimensions.
As described above, when the dimensions of the semiconductor reach the
order of nanometers, the energy levels shift according to the quantum size effect.
This shift is one of the most important size effects in photocatalytic reactions. The
shift of the conduction band may accelerate the reduction while that of the valence
band may increase the oxidation reaction. For example, an increase in the activity
by a factor of 50 has been reported for the photocatalytic isomerization of styrene
with CdS, where the energy band is located at a critical position for the reaction. ]4)
One drawback to utilizing the size quantization effect is that the shift in the
energy level causes a shift in the absorption wavelength of light. For instance, the
shift to a shorter wavelength of the absorption spectrum is not favorable for TiO z .
Although the effective absorption of visible light is important for the practical use
of photocatalysis, the intensity of the absorption spectrum of TiO z in the visible
region is extremely small compared with that in the ultraviolet region.
5.4 Observation of Primary Reaction Intermediates
It would be helpful to detect and identify the reaction intermediates at each
step in the photocatalytic reaction to understand the reaction mechanism and
modify it for more convenient use. In the following sections, our research to gain
a better understanding of the detailed photocatalytic reaction procedure on TiO z
powder is reviewed.

78
5 Photoelectrochemical Reactions at Semiconductor Micropartic1e
5.4.1 ESR Analysis for Irradiated Ti0 2 Particles
The photocatalytic properties of TiO z strongly depend on the preparation
conditions. It has been reported that the activity of photo catalysts is affected by
the conditions of heat treatment and by the crystal structure, which changes from
anatase to rutile by the heat treatment. 16) To obtain more useful TiO z powders, it
is necessary to clarify the factors which cause the differences in photocatalytic
activity.
In the process of photocatalysis, the electrons and holes produced on
photoirradiated TiO z powders are trapped at the particle surface to form unpaired-
electron species (step @) in Fig.5.3). Photocatalytic reactions are actually the
reactions of these radicals with reactant molecules at the TiO z surface. Electron
spin resonance (ESR) spectroscopy has been used for the detection of the
photoproduced radicals on TiO z at low temperatures such as 77 K. It has been
reported that photoproduced electrons are trapped at various different sites:
titanium atoms on the surface or inside the particles, or oxygen molecules
adsorbed on the surface. On the other hand, photoproduced holes are trapped at
lattice oxygen atoms near the particle surface or at surface hydroxyl groups. We
analyzed these radical species for several TiO z photocatalysts that are
commercially available, and found that the differences in the photoproduced
radicals resulted from different heat-treatment conditions and the reactivity with
several molecules. 17)
A careful heat treatment under aerated condition has been performed for
Hombikat UVlOO TiO z powder (Sachdeben Chemie, GmbH). The crystal structure
of the powder remains anatase below 800°C. For ESR measurements, TiO z powder
sealed under vacuum in a sample tube was irradiated with a 500 W mercury lamp
through a band-pass glass filter. In order to examine the reactivity of the samples,
1 atm of air, 45 Torr of 2-propanol vapor, or 20 Torr of water vapor was
introduced into the sample tubes.
a
b
c
2.0983
5.4 Observation of Primary Reaction Intermediates
79
c
B : j

2.004
2.0663
2.0343
2.0023
g value
1.9703
1.9383
1.9063
Fig. 5.5 ESR spectra J;Jleasured at 77 K for irradiated UVIOO Ti0 2 powder non-heated(a), heated at
350°C (b), and at 700°C (c) for 5 hoursY>
(From Y. Nakaoka and Y. Nosaka,J. Photochem. Photobiol. A., 110, 301 (1997»
A. Trapped Electrons
no response 9n contact with oxygen. Therefore, this signal must not arie from. the
surface Tj3+ but from the inner Ti 3 + of the particle. In the photocatalysIs on TIOz,
most reduction reactions do not proceed unless metals are deposited on TiO z
surface as an electron trap.I9,ZO) Moreover, it is probable that the Ti 3 + formed
inside particles acts as a recombination center and reduces the activity of the
surface reactions for heat-treated TiO z powders. .
When the sample came in contact with oxygen, a signal (C) appeared at
around g = 2.06. We assigned this signal to Oz-., since one of the components of
the g values of Oz -e radicals produced on ZnO and zeolite has been reporte to be
g = 2.05.1 and 2.057, respectively.21,ZZ) This signal was also observed III the
samples heated at 200°C and 350°C.
It was reported that photoproduced electrons are trapped on TiO z to form Ti 3 +.
The g values, which show the signal position in ESR spectra, are below 2.00 for
the paramagnetic species of Ti 3 +. Although it is difficult to predict the exact
location of Tj3+ radicals from g values, the signal could be assigned from the
reactivity of these radicals on the addition of various molecules to the sample.
For the non-heated sample, ESR signal (B in Fig. 5.5) characterized with a set
of g-values (gl=1.957, gz=1.990, and g3=1.990) appeared. This signal originates
from the electrons trapped at the particle surface because the intensity increased
in the presence of electron donor molecules while it decreased in the presence of
electron acceptor molecules. Since this signal disappeared for the sample heated
at 200°C, this trapped electron may be stabilized by the hydroxyl groups of the
TiO z surface. This observation is consistent with the reported values of hydrated
Ti 3 +, which shows g of gl=1.960, gz= 1.990, and g3=1.990. 18 )
When the sample was heated at 700°C a different signal (E) appeared. The
new signal with a set of g-values (g1=1.961, gz=1.992, and g3=1.992) is very
similar to that of the hydrated TP+ described above. However, this signal showed
B. Trapped Holes
The photoproduced holes in TiO z are trapped to form oxygen radicals o-e
. d . 17) F
whose g-values are in the range of 2.00 to 2.03 as reVIewe III our report. or
untreated UV100 TiO z sample, photoirradiation produces radicals (A) identified

80
5 Photoelectrochemical Reactions at Semiconductor Microparticle
5.4 Observation of Primary Reaction Intermediates
81
A
!
B
!
c
!
D
!
E
!
5.4.2 Direct Observation of Intermediate Radicals
_ /0
OH OH OH 0
I I I "
0" . / 0" . /0" T ., / 0" T . / 0::::: ::l- 0,- ....-: 0 '- T "" 0 '-
Ti / TI T1 I I Tl TI /" i Ti
I "0 / "0/ "0 / "0/ I "0 / "0 / "0./ "0./
o I I I I I - I I I I I I I
"Ti / 0 .+Ti / 0 "Ti / 0 "Ti / 0 "Ti / 0" . / 0" . / 0 " . / 0
0/ I "0/ "0/ "0 / "0/ "0/ Tl" 0 / T1" 0 / TI"
10! 6 I  I  I  I I I I _I?
T ' / " T . / '- T . / " Ti / " Ti / " T . / 0 " T ' / 0 " T " . / 0 '-- '
I / 1 I . I 1 I TI
I "0 1"0/ "0/ 1"0/ "0/ 1"0/ "0/ "0/
I . I I I I I I I,!. I
0" 0" I 0 I 0 0' 0 I I I
Ti / Ti / "Ti / "Ti / " Ti / " Ti / 0 "Ti / 0 ': Ti / 0
Fig. 5.6 Idealized chemical structure for the radicals observed on anatase Ti0 2 by means of low
temperature ESR spectroscopy.17) A, trapped hole with OH group (Ti 4 +O-' Ti 4 +OH-) ; B,
surface trapped electron(Ti 3 +); C. adsorbed superoxide radical (0 2 -.); 0, hole trapped at
surface with less OH groups (Ti 4 +0 2 -Ti 4 +O-'); E, inner trapped electron (Ti3+).
(From Y. Nakaoka and Y. Nosaka, J. Photochem. Photobiol. A., 110, 305 (1997»
The initial process in the photocatalytic reactions, which occurs immediately
after the electron transfer reaction, has not been clearly understood so far. z5 ) Most
of the important reactions involve the initial process of the oxidation of organic
molecules. Since the photocatalytic reaction proceeds usually in aqueous
suspension of TiO z or with adsorbing molecules on TiO z , water is oxidized first
by photogenerated positive holes to form hydroxyl (-OH) radicals. In the
succeeding process, -OH radicals react with organic compounds to form oxidized
species or decomposed products. This process is called indirect oxidation. Another
possible process is the direct oxidation of organic solute compounds at the surface
by photo-produced holes. Although analysis of the reaction products is necessary
to clarify the oxidation pathways, distinct differentiation will not be attainable
directly from the product identification since the products may be identical in two
pathways. Z5) Hence, one way of investigating which pathway is facilitated in the
initial process of the photocatalytic reactions would be to monitor the intermediate
radicals formed during the reaction. Since the -OH radical cannot be monitored
directly by ESR spectroscopy at room temperature, indirect monitoring methods
such as spin trapping Z6) or spin probing 27 ) techniques have been employed.
However, since these reagents are usually reactive they are not suitable to
investigate whether the photocatalytic oxidation is direct or indirect, although in
situ observation for reacting intermediates provides essential information for
understanding the reaction mechanisms.
Bard and co-workers first reported, in their pioneering work in photocatalyses,
the decomposition of aqueous acetic acid with platinized TiO z powders. z8 )
by a set of g-values, gl=2.004, gz=2.014, and g3=2.018. This signal is attributed
to the holes trapped on or near the particle surface because the intensity decreased
in the presence of electron donor molecules and increased in the presence of
electron acceptor molecules. Moreover, the intensity of this signal decreased for
the sample with heat treatment but increased on the addition of water vapor.
These observations indicate that this radical is related to the surface hydroxyl
groups. Howe and Gditzel reported that the photoproduced holes are trapped at the
lattice oxygen atoms located in the subsurface layer of the hydrated anatase to
form the radicals whose structure is Ti 4 +0- e Ti 4 +OH- with the set of g-values of
gI=2.002, gz=2.0 12, and g3=2.0 16. 18 ) Thus the signal observed for untreated
UVIOO TiO z , whose surface is considered to be covered with hydroxyl group, can
be assigned to the radical Ti 4 +O-.Ti 4 +OH-.
For the heated sample the signal described above disappeared and a new
signal (D) at g of 2.004,2.018, and 2.030 appeared. This signal is not related to
the surface hydroxyl group because it was observed only for the heated samples.
Since the g value changed on the addition of water vapor, this radical is considered
to be sensitive to the environmental changes around the surface. Therefore, this
signal would correspond to the radicals formed on the surface of the particle.
Micic et al. reported the radical formation of the trapped holes on the surface of
TiO z colloid and assigned it to the Ti 4 +O z 1i 4 +0- e radical. z3 ) The set of g-values of
the signal of this radical is reported to be gl=2.007, gz=2.018, and g3=2.02[7.
Since the signal observed for the heat-treated TiO z agrees well with this radical,
we assigned this signal to the Ti 4 +O z -Ti4+0- e radical.
In the presence of 2-propanol vapor, the Ti 4 +0- e Ti 4 +OH- radical yielded a
methyl radical while no reaction occurred for the Ti 4 +O z - Ti 4 +0- e radical. This
result shows that the surface hydroxyl group plays an important role in the
oxidation of 2-propanol on the TiO z surface. Thus, the reactivity of these two
radicals is different. The idealized chemical structures of these radicals are
illustrated in Fig. 5.6. 17 ) For several TiO z photo catalysts which are available
commercially, the quantity of the two the radical species shifts continuously
depending on the amount of surface OH groupS.Z4)
. "")!. 
% : '-$   ,y 
,>y
+H
. ., "I.
.;-
x 7'1<
 "'..;-- . )or
. y '"
?
Y'
{x- >S-
o:; .
...
Y- >; +H
....

. . . Ti02
.
+H
... ,..
y.
o.
-OR
-OH
.<
} 1'+
,":/  )
I -CH3
CH3 + C02
>S-
'$

-OR
OH
li9rPli] \ 
r --CH2COOH
CH3COOH -CH 3 + C02
and
Fig. 5.7 Schematic illustration of initial decomposition of acetic acid at illuminated Ti0 2 particles
under deaerated condition.

82
5 Photoelectrochemical Reactions at Semiconductor Microparticle
5.4 Observation of Primary Reaction Intermediates
83
Although they suggested the formation of methyl radicals, they did not actually
detect the methyl radicals in aqueous acetic acid. On the other hand, Kaise et al.
have observed carboxymethyl radicals -CH 2 COOH and methyl radicals -CH 3 by
using a flow cell by ESR spectroscopy.29) We examined the dependence of radical
concentrations on the flow rate and found that the amounts of these two
intermediate radicals changed, reflecting the different reaction paths. 30, 31)
Reaction scheme of photocatalytic decomposition of acetic acid under
deaerated condition is analyzed based on the flow-rate dependence of the radical
formation 30 ) combined with the reported mechanism. In this scheme, -CH 3 radicals
are formed via two reaction pathways. One is indirect oxidation via -OH radicals
which are formed by the oxidation of water with photoinduced holes (h+) at the
valence band. Another is direct oxidation of adsorbed molecules by the hole. On
the other hand, -CH 2 COOH radicals are not formed by direct oxidation but only
by indirect oxidation via -OIl radicals. The reaction scheme is proposed as
illustrated in Fig. 5.7.
Since carboxymethyl radicals -CH 2 COOH are formed only by indirect
oxidation, the radical ratio, [-CH 2 COOH]/[ -CH 3 ], can be used to estimate the
ratio of the indirect oxidation via -OH radicals. The ratios of the indirect oxidation
were measured for five different commercially available Ti0 2 photocatalysts 31 ) and
for HyCOM TiOlO J prepared by Kominami. Platinization was essential to detect
the reaction intermediates for these photocatalyst powders. The amounts of two
radicals produced for several photo catalysts are shown in Fig. 5.8. The ratios of
two radicals were found to be characteristic of the different Ti0 2 catalysts. The
heat treatment decreases the amount of -CH 2 COOH radicals, suggesting decrease
in the formation of -OH radicals. 3I )
When Ti0 2 sols, STS-01 and STS-02 supplied by Ishihara Sangyo Co. Ltd.,
were used as photocatalysts, a sufficient amount of radicals was formed without
platinum metal deposition because of the high dispersibility of the particles.
These two samples differ only in the dispersing agents, HN0 3 and HCI for STS-
01 and STS-02, respectively. STS-02 Ti0 2 sol shows a significantly small amount
of -CH 2 COOH, because -OH radical reacts very rapidly with CI- to form the less
reactive radical HCIO- .
The difference in the relative amounts of primary radicals observed for the
different Ti0 2 powders is correlated with their particle properties.31) The BET
surface area does not systematically affect the amount of radicals. The surface
area, which is measured by adsorbing N 2 molecules, correlates naturally with the
primary particle size, which does not affect the amount of radicals. Then the
surface area causes no effect on the total amount of the produced radicals. On the
other hand, the secondary particle size correlates strongly with the total amount
of radicals. One should be reminded that photocatalytic reactions are initiated by
the absorption of photons in the semiconductor grains. Penetration depth of light
into Ti0 2 semiconductor is much shorter than the secondary particle size of
0.4-2.6 J.1m, and only part of the surface of the aggregated powder is illuminated.
Therefore, the entire surface of the photocatalytic particles is not involved in the
photocatalytic reaction. When the secondary particle size becomes smaller, the
more illuminated part of the grain surface is exposed to the bulk solution. This is
the reason for the observed correlation between the activity and the secondary
particle size. This is consistent with the higher activity observed for the highly
dispersed Ti0 2 sols.
5.4.3 Chemiluminescent Probe for Active Oxygens
For photocatalytic reactions under ambient conditions, active oxygen species
such as hydroxyl raicals (-OH), superoxide ions (-0 2 -), and hydrogen peroxide
(H 2 0 2 ) have been noticed as key species to promote the reaction. 25 ) These oxygen
species are formed in the following reactions:
;::J
'"
en
-;;
u
:.c;
'"
<-.
.....
o
en

j
 ': .:': :d.';
o 5 10 15 20
Time / s
250
(A)
200
. In the presence of HzDz
ji
 150

.... 100
....... ';'
o I n the absence of HzD!
-CH3  h -'-, .OH
tD
C'.J
I
P-
.....,
P-
-
=
;::
I
>-
::J
::E::

tD '7'
C'.J
I c.-, 
P- C>
3 u
= C'.J
I <=>
t/.J I
E- </.)
</.) E-
</.)
.CH2COOH  .OH
250
(B)
200
150
100
50
0
400
o slow' decay component
c fast decay component
450
500
wavelength I nm
550
600
Fig. 5.8 Relative amounts of .CH3 and .CH 2 COOH radicals formed in the photocatalytic
decomposition of CH 3 COOH in de aerated 0.1 mol dm- 3 solution by using various Ti0 2
photocatalysts.
Fig. 5.9 (A) Chemiluminescence intensity measured at >420 nm after the end of excitation of Ti0 2 in
aqueous solution containing 0.1 mM luminol and 10 mM NaOH. Addition of 0.2 mM H 2 0 2
causes a shift of the slow-decay part. (B) Chemiluminescence spectra measured for the two-
decay part in the decay profile, showing that both parts originate from luminol.

84
5 Photoelectrochemical Reactions at Semiconductor Microparticle
References
85
e- + O 2  O 2 - 0
h+ + OH- -OH
-OH + -OH  H 2 0 2
(5.18)
L-O + O 2 - 0 + H+  L0 2 H-
(5.22)
(5.19)
L + H0 2 -
 L0 2 H-
(5.23)
(5.20)
where L -0 and L are the one-electron oxidized and two-electron oxidized forms of
luminol. A scheme for the chemiluminescence is shown in Fig. 5.10. Thus, the
amounts of O 2 - 0 and H 2 0 2 can be determined by separating the decay component
of chemiluminescence. Actually we performed two kinds of experiments. One is
the addition ofluminol after the end of the excitation, where L-o is mainly formed
by autoxidation. Therefore, the amount of O 2 - 0 can be estimated. In the other
experiment luminol is present during the excitation. In this case, luminol is
oxidized photocatalytically by -OH radical or h+, and used to estimate the amount
of H 2 0 2 . The steady amount of O 2 - 0 produced during the irradiation of 13 m W on
3.5 cm 3 alkaline suspension of Ti0 2 was estimated to be of the order of 10- 13 M.
Detection of H 2 0 2 as small as 10- 9 M was demonstrated for the photocatalytic
water oxidation. 32 )
The amount of O 2 - 0 produced is independent of the surface properties of
Ti0 2 , but for the larger secondary particle size increase in the amount 0[0 2 - 0 was
observed. 24 ) On the other hand, the amount of H 2 0 2 increases for the smaller
primary particle size with large numbers of surface hydroxyl groups. This
observation suggests that H 2 0 2 is formed from -OH radicals and that the formation
of -OH is suppressed by the oxidation of luminol. The amount of -OH radicals
produced was estimated from the effect of CI- ions on the O 2 - 0 formation 33 ) and
found to be high for the Ti0 2 possessing a large number of surface hydroxy]
groups. This difference is explained to originate from the different structures of
the trapped hole, based on the ESR measurements described above.
O 2 - 0 + e- + H20 H0 2 - + OH-
(5.21 )
where H0 2 - is an ionized form of hydrogen peroxide H 2 0 2 .
The first application of the chemiluminescent (CL) probe to the photocatalytic
reactions was implemented by the authors. 32, 33) When a CL probe, luminol, is
present in Ti0 2 suspension at pH 11, a very weak light emission was observed
after photoirradiation at 387 nm. The intensity of the emission decays as shown
in Fig. 5.9(A), where the two components, that is, fast and slow decays, can be
distinguished. This figure shows that only the slow decay component of the
luminescence increases in the presence of H 2 0 2 . The emission spectra for both
components were measured and shown in Fig. 5.9(B), both of which are
attributable to the chemiluminescence of luminol.
With comprehensive experiments,32) it was found that the intensity of the
fast decay component is proportional to the amount of O 2 - 0 , while the slow decay
component is determined by [L] [H0 2 -]. According to this, the chemiluminescent
reactions of the fast and slow decays are expressed by Eqs. (5.22) and (5.23),
respectively.
NH2 0- NH2 0- 
«:! 'OH or h+ «:1
L.-
 
(02/ 0H - )
OH 0- s 0
Fast
_ecaY decay IT]
 [E] O 2 H2 0 2
References
Mechanism for
Luminol
Chemiluminescence
:*
 O.
I ight emission
(425rvn)
1. A. J. Bard, J. Phocochem" 10, 50 (1979).
2. W. J. Albery and P. N. Bartlett, J. Electrochem. Soc., 131, 315 (1984).
3. Y. Nosaka, Y. Ishizuka and H. Miyama, Ber. Bunsenges. Phys. Chem., 90, 1199 (1986).
4. S. M. Sze, in: Physics of Semiconductor Devices, Chapter 5. 2nd ed, John Wiley and Sons, New
York (1981) . ,
5. Y. Nosaka, K. Norimatsu and H. Miyama, Chem. Phys. Lett., 106, 128 (1984).
6. G. A. Hope and A. J. Bard, J. Phys. Chem., 97, 1979 (1983).
7. Y. Nakato and H. Tsubomura, J. Photochem., 29, 257 (1985).
8. Y. Nosaka, J. Phys. Chem., 95, 5054 (1991).
9. N. Serpone, D. Lawless and R. Khairutdinov, J. Phys. Chem., 99,16646 (1995).
10. M. Voinov and J. Augustynski, in: Heterogeneous Photocatalysis, 1, (M. Schiavello, ed.) John
Wiley and Sons, New York (1997).
11. Y. Nosaka and Y. Nakaoka, Langmuir, 11, 1170 (1995).
12. Y. Nosaka and M. A. Fox, J. Phys. Chem., 92, 1893 (1988).
13. Y. Nosaka, N. Ohta and H. Miyama, J. Phys. Chem., 94, 3752 (1990).
14. P. de Mayo, K. Muthuramu and W. K. Wong, Catal. Lett., 10, 71 (1991).
l5. A. J. Hoffman, H, Yee, G. Mills and M. R. Hoffmann, J. Phys. Chem., 96, 5540 (1992).
16. B. Ohtani and S. Nishimoto, J. Phys. Chem., 97, 920 (1993).
17. Y. Nakaoka and Y. Nosaka, J. Photochem. Photobiol., A, Chem., no, 299 (1997).
18. R. F. Howe and M. Gditzel, J. Phys. Chem., 91, 3906 (1987).
19. A. L. Linsebigler, G. Lu and J. T. Yates, Jr., Chem. Rev., 95, 735 (1995).
20. H. Kominami, T. Matsuura, K. Iwai, B. Ohtani, S. Nishimoto and Y. Kera, Chem. Lett., 693
(1995).
21. J. H. Lunsford and J. P. Jayne, J. Chem. Phys., 44,1487 (1966).
22. P. H. Kasai, J. Chem. Phys., 43,3322 (1965).
.
I Rapid 0 Il02 H -1
Fig. 5.10 Mechanism ofluminol chemiluminescence observed in Ti0 2 photocatalytic reactions.

86 5 Photoelectrochemical Reactions at Semiconductor Microparticle
23. 0.1. Micic, Y. Zhang, K. R. Cromack, A. D. Trifunac and M. C. Thurnauer, 1. Phys. Chem., 97,
7277 (1993).
24. T. Hirakawa, Y. Nakaoka, J. Nishino and Y. Nosaka, 1. Phys. Chem., B, 103,4399 (1999).
25. a) N. Serpone, E. Pelizzetti and H. Hidaka, in: Photocatalytic Purification and Treatment of
Water and Air, (D. F. Ollis and H. Al-Ekabi, eds.), Elsevier, Amsterdam p. 225 (1993).
b) D. Bahnemann, J. Cunningham, M. A Fox, E. Pelizzetti, P. Pichat and M. Serpone, in Aquatic
Surface Photochemistry; (G. Helz, R. Zepp and D. Crosby, eds.) CRC Press: Boca Raton, FL, 261
(1993).
c) M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 95, 69 (1995).
d) H. Gerischer, Electrochim. Acta, 38, 3 (1993).
26. M. A. Grela, M. E. J. Coronel and A. J. Colussi,1. Phys. Chem., 100, 16940 (1996).
27. P.F. Schwarz, N.J. Turro, S.H. Bossmann, AM. Braun, A. A Abdel Wahab and H. Duerr. 1.
Phys. Chem. B, 101, 7127 (1997).
28. B. Kraeutler, G. D. Jaeger and A J. Bard, J. Am. Chem. Soc., 100,4903 (1978).
29. M. Kaise, H. Nagai, K. Tokuhashi, S. Kondo, S. Nimura and O. Kikuchi, Langmuir, 10, 1345
(1994).
30. Y. Nosaka, K. Koenuma, K. Ushida and A Kira, Langmuir, 12, 736 (1996).
31. Y. Nosaka, M. Kishimoto and J. Nishino, 1. Phys. Chem. B, 102, 10279 (1998).
32. Y. Nosaka, Y. Yamashita and H. Fukuyama, J. Phys. Chem. B, 101,5822 (1997).
33. Y. Nosaka and H. Fukuyama, Chem. Leu., 383 (1997).
6
New Approaches in Solution-phase Processing of
Semiconductor Thin Films
6.1 Introduction
Thin films of inorganic and organic semiconductors are playing an essential
role in various optoelectronic devices. Because functions exhibited from materials
are the sum of phenomena occurring at the atomic or molecular level, precise
control of the structure during the film processing can result in dramatic
improvements of the device performance. In othe.r words, the performance of
thin film materials is strongly dependent on the method of preparation. Other
important requirements for thin film processing are the cost of production and
environmental aspects such as reducing toxicity of materials, suppressing emission
of pollutants, and facilitating recycling or waste treatment. Methods of thin film
processing that meet all these conditions must be developed to obtain full benefit
from the new technologies.
Thin film processing can be divided into gas phase techniques and solution
phase techniques. The gas phase techniques are methods that condense ions or
clusters into a solid film in high vacuum such as vacuum evaporation, molecular
beam epitaxy, sputtering or laser ablation and a method employing the chemical
formation of a solid film on a substrate exposed to a stream of precursor
molecules, called chemical vapor deposition (CVD). The advantage of these
methods is high controllability of film growth. Films with highly ordered
structures and controlled composition can be grown by adjusting parameters such
as vapor pressure and substrate temperature. The obvious disadvantage of the
gas phase techniques is the need for extreme conditions such as high vacuum
and/or temperature. Emission of gaseous and often toxic wastes is another
problem. The high cost and environmental stress associated with the gas. phase
techniques are serious drawbacks in the face of increasing demand for thin film
materials. The solution phase techniques, on the other hand, enjoy advantages of
reduced production cost and environmental stress. Since the precursor molecules
or ions are dissolved in solutions, less energy is required 'than the gas phase
techniques. I) Examples of solution phase techniques are those employing solid
film formation by thermal decomposition and crystallization from precursors
dissolved in solution or dispersed in slurries such as spray pyrolysis, screen-
printing or sol-gel processing, that based on solid formation by chemical
decomposition of molecular precursors in solutions called chemical bath
deposition (CBD) by analogy to CVD, and electrodeposition (ED) to form solid
films by electrochemical redox reactions of dissolved precursors at substrates.

88
6 Solution-phase Processing of Semiconductor
6.2 Solution-phase Deposition of Semiconductor
89
Among them, CBD and ED processes in aqueous solutions have the highest
advantages for suppressing energy for film production, because they do not
involve any heat treatment at high temperatures. Despite their economical and
ecological advantages, it is generally thought that such solution processes result
in poor structural and compositional quality of the products, since film growth is
less controllable in solution than in gas phase, suffering from unwanted side
reactions. However, recent progress of scanning probe micrographs (SPMs) has
revealed that electrochemical processes under certain conditions can result in
well-regulated epitaxial growth of thin films both for metals z . 3 ) and compound
semiconductors. 4 . 5 ) The CBD process in some cases has also been shown to be
largely dominated by surface reactions to allow epitaxial film growth.6-8) When the
strong chemical interaction of ions and molecules in solution is properly
controlled, very selective solid film formation is realized. Such an approach
should also be beneficial for thin film processing of organic molecules. Despite
their unique properties, low toxicity and potential for large-scale production, thin
films of organic molecules has been obtained mostly by gas phase techniques. 9 . 10 )
Low-temperature solution phase methods are, in principle, suitable for organic
materials, as they often decompose at high temperatures. Another important and
interesting series of materials is inorganic/organic hybrids. Self-assembly of
unique hybrid structures and new properties can be found from virtually unlimited
combinations of inorganic and organic materials. 11-15) V se of solution is essential
for such a self-assembly process, because free and strong chemical interaction
under mild conditions is required for the constituent ions and molecules to build
up inorganic/organic hybrid structures chosen by themselves.
In this chapter, we will review studies on CBD and ED of compound
semiconductor thin films with main emphasis on cadmium sulfide (CdS), one of
the most important materials in optoelectronic and photoelectrochemical
applications because of its high photochemical activity and favorable band
energies to utilize visible light. Their mechanism, previous achievements in
structure control and their limitations are discussed (section 6.2). Our efforts in
improving structure control in the CBD and ED of CdS thin films are then
described. Electrochemical control of surface conditions introduced to the CBD
process realized an electrochemically induced chemical deposition (EICD) of
CdS thin films to allow a long-range layer-by-layer (homoepitaxial) growth of
CdS crystals (Section 6.3).16.17) Electrochemical reductive decomposition of
chemically stable aqueous thiocyanato complexes of metal cations lead to true
electrochemical growth of metal sulfide thin films (Section 6.4).18) The idea of
utilizing surface reactions for the control of film structure in the ED process was
further developed as an electrochemical self-assembly of zinc oxide (ZnO)/dye
hybrid thin films by carrying out cathodic ED of ZnO from an aqueous bath
containing water-soluble organic dye molecules (Section 6.5). 1 9-Z 1) Solution phase
thin film processing therefore not only replaces previous methods for synthesis of
semiconductor thin films but also enables us to obtain new materials in an
environmentally benign manner.
6.2 Previous Methods for Solution-phase Deposition of
Semiconductor Thin Films
6.2.1 Chemical Bath Deposition of Metal Sulfide Thin Films
Studies on deposition of metal sulfide (typically, PbS and CdS) thin films
driven by their chemical formation in solutions started around 1970. Early studies
are summarized in a review by Chopra et ai. ZZ) When sulfur-containing chemicals
such as thiourea «NHz)zCS, TV), allylthiourea (CH 2 :CH 2 NHCSNH z , A TV) or
thioaceamide (CH 3 CSNH 2 , T AA) are added to aqueous solutions of metal salts,
homogeneous precipitation of corresponding metal sulfides takes place. It has
been revealed by experience that transparent, specular and adherent thin films can
be deposited on a substrate dipped in such solutions when their pH is maintained
basic, while powdery, poorly adherent deposits can only be obtained in acidic
solutions. z3 ) The following studies on CBD have therefore been carried out almost
without unexception in an alkaline medium. Complexing agents such as ammonia,
triethanolamine and ethylenediaminetetraacetate are used to stabilize metal cations
in alkaline solutions. Almost all kinds of binary metal sulfide and some ternary
compounds have been obtained by modification of the reaction system by now. In
the 1990s, the main focus of studies has been placed on obtaining a better
understanding of the deposition mechanism from the relationship between the
deposition conditions and the growth kinetics z4 - Z6 ) or their influence to the film
structure. 27 ) The proposed mechanism for CBD of CdS thin films is illustrated in
Fig. 6.1. The soluble Cd z + species such as [Cd(NH 3 )4Y+ releases Cd z + ion to form
adsorbed Cd(OH)z reaction sites.
[Cd(NH 3 )4Y+ + 20H- + site  Cd(OH)zads + 4NH 3
(6.1)
This is converted into a Cdz+-TV surface complex upon reacting with TV.
Cd(OH)zads + SC(NHz)z  Cd(SC(NHz)z)(OH)zads
(6.2)
The Cdz+-TV intermediate decomposes at the surface to promote layer-by-Iayer
growth of CdS crystals.
Cd(SC(NHz)z)(OH)zads  CdS + CNzHz + 2H z O + site
(6.3)
...'" layer-by-ayer growth of ...
 . ::-:;',: .  
U<r Cd (OH)2ads 'a. i '\ ,....,OH- \J, ,....., . .
,.  'v;,,-.., \ ,,, NH CN + 2H 0 . > ,
:: ! 'l:J <t C{.h. 0 "'tCdS. 2 . 2 to >:X:.\
">" , -:-[Cd(OHh S QNH 2 hlads , \i'i , i . '(f) incorporatIOn 0 ., .." ".."
" c . f  .q ts:.' ,' ,,-, CdS It . I 4'....,. ....lth,"-\.:
{, r!:rmatlon 0, '\ INH2  '>'! :() p IC es ,.\-}ri k"
\" su, ,ace complex S=C randomlY aggregated" \,.-, ",---, .' ."'-).., .
 ' NH hydroxylated CdS particles ""'", \\. random -;tctured
.- thiourea /'"
2 same surface condition porous outer layer
Fig, 6.1 Schematic illustration of CdS film growth by alkaline CBD using thiourea. Layer-by-Iayer
growth of CdS crystallites is followed by adhesion of CdS particles formed in the bath to form
random structuredouter layer.

90
6 Solution-phase Processing of Semiconductor
6.2 Solution-phase Deposition of Semiconductor
91
The existence of the surface Cd hydroxide has been experimentally confirmed. zS )
In order to obtain thin films of good quality, it is important to allow formation of
Cd hydroxide at the surface, while prohibiting precipitation of bulky Cd(OH)2 by
adjusting pH, strength and type of complexing agents. Z9 ) There have been a few
exceptions reporting CBD of metal sulfide thin films such as CdS, CUzS and CuS
from acidic bath containing thiosulfate (SZ03Z-, TS).30,31) However they are
probably based on a totally different reaction mechanism ofTS ions. The reactions
of TS ions with metal cations will be discussed later in Section 6.2.2.
Since CdS film growth in alkaline CBD is controlled by surface reactions,
highly crystallized and structurally ordered thin films can be deposited when
proper conditions are chosen. Indeed, the CBD method has produced
heteroepitaxial growth of CdS thin films by using an InP single crystal substrate
having a small lattice mismatch to CdS. 6 . 7 ) However, such ideal film growth can
only be achieved at the early stage of deposition. As the reaction proceeds,
randomly agglomerated CdS particles formed in the bath begin to adhere to the
film to form the random structured porous outer layer, as shown in Fig. 6.1. Z4 ,3Z)
Because the surface condition of the film and that of the particles in the bath are
the same in the alkaline CBD process, incorporation of the solid particles formed
in solution into the film cannot be avoided. The epitaxial growth mentioned above
can actually only be achieved up to a film thickness of a few tns of nanometers. 6)
In order to avoid the influence of solid formation in the bulk phase to the film
growth, an alternative method based on sequential immersion of substrate to
separate baths, called successive ionic layer adsorption and reaction (SILAR)
method, has been developed. 33 ) For example, a substrate is immersed in a solution
of Cd salt, rinsed by water to leave an adsorbed monolayer of Cd z + ions, then
dipped in a solution of NazS to form one layer of CdS and rinsed with water
again. Repetition of this cycle several hundred times is expected to yield a CdS
film grown in an ideallayer-by-Iayer fashion. Although several reports did appear
for deposition of structurally ordered thin films of CdS,33,34) ZnS 33 - 35 ) and PbS,36)
the film growth is especially sensitive to the rinsing process and does not always
achieve one layer per deposition cycle. The SILAR process is also very time-
consuming and produces large amounts of dilute waste water, thus losing the
economical and ecological advantages of solution phase processing.
Cd Z + + 2e-  Cd (EO = - 0.646 V vs. SCE)
(6.5)
In this system, the formation of CdS is strictly limited to the surface of substrates
so that no influence to the film growth can be considered to result from the
solution phase reaction. Deposition of nanosized epitaxial dots on Au( 111) has
been successfully achieved by using this strategy.40.41) However, long-range
epitaxial crystal growth seems to be difficult, probably due to the very low
solubility of CdS in DMSO.
TS has been frequently used in aqueous systems. 4Z ,43) TS is known to decompose
and release elemental sulfur in acidic solutions. 44 )
SZ03Z- + 2H+  Sbulk + SOz + HzO
(6.6)
In an early report it was assumed that this bulky S colloid was co-reduced with
Cd z + to generate CdS, similar to that in the org'anic medium described above. 4Z )
Cd Z + + Sbulk + 2e-  CdS + Sbulk
(6.7)
However, such an electrode process seems unlikely, because S colloids are large
solid particles exceeding several hundred nanometers in diameter. 44 ) It should be
noted that TS forms stable coordination to various metal cations, for example, as
indicated by the high stability constant for the first coordination of TS to Cd z +_
Cd Z + + SZ03Z-  ([Cd(Sz03)] (lOgKI = 3.92)
(6.8)
Since a large excess of TS is often added to the solution under typical
experimental conditions,4Z) Cd z + exist mostly as thiosulfato complexes. Reductive
decomposition of this soluble species should therefore be more suitable to describe
the electrode process responsible to the CdS film growth_
;
[Cd(Sz03)] + 2e-  CdS + SO/- (EO = - 0.131 V vs. SCE) (6.9)
6.2.2 Electrodeposition of Metal Sulfide Thin Films
,
This reaction prevails with the deposition of metallic Cd (Eq. (6.5», as expected
from its more positive equilibrium potential. However, this thiosulfato complex
is chemically unstable, providing another route for the formation of CdS.
The most simple and straightforward approach for ED of metal sulfide thin
films is the co-reduction of elemental sulfur and metal cations in an organic
medium such as dimethylsulfoxide (DMSO)Y-39) Because elemental sulfur is
dissolved as polymeric species such as SS,39) the overall reaction for ED of CdS
thin film can be written as
[Cd(Sz03)] + HzO  CdS + SO/- + 2H+ (8G Z9SK = - 42.2 kJ mol-I) (6.10)
Cd Z + + Sm + 2e-  CdS + SCm-1)
(6.4)
Precipitation of CdS particles in acidic aqueous solution containing Cd salt and
TS has also been confirmed in our own experiments. Such chemical formation of
metal sulfide might be responsible for the fihn formation in the CBD of CdS 30 ) and
CU Z S 31 ) in acidic bath containing TS, mentioned earlier. These multiple roles of TS
seriously complicate the film growth as summarized in Fig. 6.2. The cathodic
electrochemical deposition in aqueous TS solutions is, in reality, not a true ED
process. Formation of solid particles of S and CdS does contribute to the film
growth, aside from the electrochemical formation of CdS by Eq. (6.9), causing
structural and compositional irregularity to the product.
Since solution phase reactions cannot be neglected even in those systems
for which the equilibrium potential cannot be defined because the sulfur species
is not identified, but is thermodynamically favored to the deposition of metallic
Cd, owing to largely negative Gibbs energy of formation for CdS (f1G.fz9SK =
-145.2 kJ mol-I).

92
6 Solution-phase Processing of Semiconductor
6.3 EICD of CdS Thin Films
93
Cd2  " S2032--.S + S032-
0-+.......
o J Cd(SP3) S £Oid
X HP
Cd(S203) hydrolysis 0
electroreduction 0 &
H SO (]o --.. 
. 2 4 randomly aggregated
Cd - thiosulfate complex CdS particles
(log K 1 = 3.92)
o
SO 2- 
 3
CdS  0 0
O 0
o @ 0
indusion Of S colloid and CdS
particles formed in the bath
generated OH-. Electrochemical base generation can be achieved by reduction of
water, dissolved oxygen or reactive ion such as nitrate according to the following
reactions.
2H z O + 2e-  Hz + 20H- (EO = -1.07 V vs. SCE)
(6.13)
0<6
random structure
S rich composition
Oz + 2H z O + 2e-  HzOz + 20H- (EO = -0.39 V vs. SCE)
(6.14)
Fig, 6.2 Schematic illustration of CdS film growth what is believed to be ED using thiosulfate. The
fate of thiosulfate is highly complicated, being subject to various side reactions in solution to
form solid particles of S and CdS.
Oz + 2H z O + 4e-  40H- (EO = +0.16 V vs. SCE)
(6.15)
N0 3 - + HzO + 2e-  NO z - + 20H- (EO = -0.24 V vs. SCE)
( 6.16)
which are believed to be ED processes, use of separate baths as in the SILAR
process has been attempted for the electrochemical method. In the case of CdS,
a monolayer of Cd can first be deposited on a substrate such as Au(111) at the
more positive potential than that for the deposition of bulk Cd (underpotential
deposition = UPD) in a solution of Cd salt. The substrate is rinsed with water then
the solution is switched to that ofNazS, in which anodic UPD of the S layer is to
be carried out. After rinsing with water again, the whole cycle is repeated as
SILAR. This method is called electrochemical atomic layer epitaxy (ECALE).45)
Epitaxial growth of CdS has indeed been realized up to several atomic layers. 4 . 46 )
However, no long-range epitaxial growth has been reported and the problems of
deposition time and waste production that trouble SILAR remain with the ECALE
method.
N0 3 - + 6H z O + 8e-  NH 3 + 90H- (EO = -0.37 V vs. SCE) (6.17)
6.2.3 Chemical and Electrochemical Deposition of Metal Oxide Thin
Films
The idea to fix metal cations onto the substrate by electrochemical local pH
increase was originally developed for deposition of transition metal hydroxides
such as Ni(OH)z, actively studied as materials for positive electrode in
rechargeable batteries. 55-57) The deposition in this case ceases after the reaction
expressed by Eq. (6.11). Electrolysis by the same strategy, however, ends up
with crystalline materials for CU Z O,56.58.59) CeOZ,60) Pb0 61 ) and ZnO. 6Z ,63) Because
direct crystallization takes place only where the electron flows, deposition of
thin films with highly ordered stru.ctures is expected. Heteroepitaxial
electrochemical growth of Cu z 0 64 ) and Zn0 65 ) has recently been demonstrated
using Au(IOO) and n-GaN single crystal substrates, respectively. Crystallographic
orientation controlled electrodeposition of ZnO has been achieved by employing
more commonly used and economical single crystal substrates such as Si or
graphite. 66 ) When basal planes of graphite are used, electrochemical Van der
Waals epitaxy of ZnO thin films has been achieved owing to' the parallel
arrangement of the (002) planes with the basal planes of graphite. ED of other
metal oxides such as ZrOz ,67) TiO z ,68) BaTi0 3 69) and PZT (PbZrl_xTix03)1°) has also
been achieved, although they are deposited as amorphous hydrous films, which are
subsequently fired for crystallization.
i'
Solution phase deposition of metal oxides is less established than deposition
of metal sulfides. The chemical process for the formation of metal oxides in
water can be considered to be hydrolysis of metal cations and dehydration of
intermediate hydroxides, for divalent cations described as
[MLnF+ + 20H-  M(OH)z + nL
(6.11)
6.3 Electrochemically Induced Chemical Deposition (EICD) of
CdS Thin Films
M(OH)z  MO + HzO
(6.12)
where L stands for ligand ions or molecules stabilizing metal cations in water.
Thin film deposition based on such a mechanism has been demonstrated forI
Cu z O,47) ZnO,48-50) TiOz,51.5Z) SiOl 3 ) and ZrOz.54) Thermal activation,47-49.51)
chemical formation of OH- 50) or chemical consumption of U Z - 54 ) has been utilized
to trigger the reactions of eqs. 11 and 12. Direct crystallization of oxide materials
under mild conditions is a great advantage of these chemical processes compared
with the more widely used sol-gel process, which converts hydroxides to oxides
by heat treatment. However, the oxide formation takes place often indiscriminately
both at the substrate and in solution phase, causing the same difficulties in the
control of film structure as discussed for CBD of metal sulfides.
Electrodeposition of metal oxide thin films in aqueous solutions is made
possible by promoting hydrolysis of metal cations with electrochemically
6.3.1 Idea
As described in the previous sections, formation of S9lid particles, mainly
CdS, causes disordering of the film structure in the hitherto studied techniques for
CBD and ED of CdS thin films. Although it is difficult to avoid spontaneous
chemical formation of CdS in solution because of its high chemical stability,
deposition of ordered thin films should still be possible if a method is developed
to disable such solid formation in solution to contribute to the film growth. In
CVD processes, the surface condition of the substrate is actively controlled by
applying heat and/or light to activate the desired reaction at the substrate and
suppress it in the gas phase. CBD lacks such active control of the surface condition
at the substrate, allowing the inclusion of CdS particles formed in solution to the

94
6 Solution-phase Processing of Semiconductor
C.,; tiLlJ OJ LQ;:' 1 mn rums
":JJ
film. If the condition at the vicinity of the substrate can somehow be differentiated
from that of the bulk phase, film growth may be achieved solely by surface
reactions which are different from those taking place in the solution bulk. As
noted earlier, chemical deposition of CdS thin films does not proceed in an acidic
chemical bath, because Cd(OH)z is needed for the s.urtce reactions for the layer-
by-layer growth of CdS films. When local pH at the surface of the substrate is
raised relative to the bulk, where pH remains below that for the Cd(OH)z
formation, the targeted reaction for CdS film growth should take place only at the
substrate. Local pH increase in water can be easily achieved by electrochemical
reduction as seen in the electrodeposition of oxide thin films described above.
According to such strategies, we have studied electrochemically induced chemical
deposition (EICD) of CdS thin films in which layer-by-Iayer growth of CdS by
surface reactions is triggered by electroreduction of protons in an acidic chemical
bath. 16,17)
Fig, 6.3 SEM photographs of CdS deposits obtained on ITO glass substrates after electrolyses at
-O.4(a), -O.5(b) and -0.6 V (vs, SCE) for 3 h in a 0,05 M CdCh + 0.1 M T AA aqueous mixed
solution (initial pH = 2.4) maintained at 70°C.
(From K. Yama1guchi et al., J Phys. Chern. B, 102,9679 (1998))
6.3.2 Morphological and Structural Analysis
(a)
(b)
.(' ...
.
Cd 2 + + CH 3 CSNH z + 2H z O  CdS + CH 3 COOH + NH 3 + 2H+ (6.18)
100nm
. .-' ( )
\, .. \\ C
'. .
.... '10),
"'''", L
'.. . . '. . j..
. ajj.il\I'''''':.'
". !.; 'f
''.:' ..;,\,' F .'rfi,
...... ' j. \.... .'"'
'.9t (... \ .(' '
: .,: tI'\.''\\' ':' '..
','i.' .;.:'4': ,"" .. Q ;'ld3 58
.  '-: .'j, t...: ....\ 0,' ..,....  .:. .run
I '\;.... .  ..nd.\\';/" .:
... . ;"\.:\.J.:s: I 
" _.t"\
.
(013 )
10: · .
Acidic (pH 2-4.5, adjusted by acid) aqueous mixtures containing a Cd salt
such as CdClz and thioacetamide (T AA), which decomposes more easily than
TV in acidic solutions, are used as the bath. When the mixture is heated to around
70°C in a water bath, yellow turbidity develops in solution because of precipitation
of CdS according to the reaction,
"....:. /1./
(b) . >:",,,"."'\,
O ...... .,', ...':f':
..'. . '';:'f! r.,\ b'r..'-I/#,).,.
I'" ;:'. .....: '".' ,";' " ';' 11'
.  .;". ""..:..,,;. f
....: A .. ....
''>".:'>
-
(113)
(031)
No CdS film can be deposited on a substrate such as an indium tin oxide
(ITO) coated glass by simply placing it in this solution. Cathodic polarization of
the substrate at an appropriately negative potential brings about proton reduction
to raise the local pH matched to the applied potential.
Fig. 6.4 CdS deposits obtained on a Mo mesh after electrolyses at-0.65 V (vs. SCE) for 3 h in a 0.05
M CdCl z + 0.1 M T AA aqueous mixed solution (initial pH = 2.0) maintained at 70°C; TEM
photographs observing the hexagonal particle from its side (a), high magnification image of
the same particle (b), and complementary selected area electron diffraction pattem (c).
(From K. Yamaguchi et aI., J Phys, Chern, B, 102.9682 (1998»
2H+ + 2e-  Hz (EO = -0.244 V vs. SCE)
(6.13)'
indicating that each particle comprising the film is a single crystal of a-CdSY)
The same deposition has been carried out using a Mo mesh as a substrate fo:( TEM
observation (Fig. 6.4). Fig. 6.4(a) corresponds to a side view of the hexagonal
particle. In the higher magnification image (b), a lattice fringe with 3.58 A
spacing, thus corresponding to the distance of(IOO) planes of a-CdS is seen over
the whole particle. The complementary selected area electron diffraction (c)
shows spot pattern which is assigned as indicated in the figure. These TEM
observations therefore confirm that the hexagonal particle is an individual CdS
crystal ,whose top and side correspond to (002) and (100) planes of a-CdS,
respectIvely. Extending the deposition time simply enlarged the hexagonal
particles, achieving a film thickness larger than 500 nm and particle size larger
than 250 nm under certain conditions. 17 ) Such a long-range homo epitaxial growth
of CdS could not be achieved by alkaline CBD. 24 )
In Fig. 6.3 are shown the SEM photographs of the deposits obtained after 3h of
potentiostatic electrolyses at various potentials in the CdCl z + T AA solution with
nitial pH of2.4. Because the equilibrium potential for proton reduction at this pH
IS around -0.4 V, no appreciable current flows during the electrolysis at -0.4 V.
Round particles with diameter of around 0.8 }.lm formed as an assembly of several
smaller particles are found on the substrate (Fig_ 6.3(a». They are of the sam
quality as the precipitates formed in the bath and are only weakly attached to the
substrate; they can be completely removed by rinsing with water. When the
applied potential is shifted negatively to -0.5 and -0.6 V, cathodic current during
the electrolysis gradually increases and the ITO substrate becomes covered with
particles smaller than those of the precipitates (Fig. 6.3(b),(c». Highly transparent
and strongly adherent yellow thin films were deposited. It should be noted that
each particle seen in Fig. 6.I(c) has a hexagonal shape. X-ray diffraction (XRD)
pattern of the film has shown that the film is made of hexagonal a-CdS and the
crystal size estimated from the full width at half maximum of the XRD peaks
closely matched with the size of the particles seen in the SEM photographs,
6.3.3 Growth Kinetics and Mechanism of EICD Process
In the EICD process, film growth ceases when the electrolysis is stopped,
even when the CdS precipitation (Eq. (6.18» in solution continuesY) The role of

96
6 Solution-phase Processing of Semiconductor
6.4 True Electrodeposition of Metal Sulfide Thin Films
97
the electrolysis relative to the film growth must be discussed. The rate of CdS
precipitation in solution phase has been determined at various reaction
temperatures between 50 0 and 80°CY) Hydrolysis ofTAA in acidic solution is a
second-order reaction with respect to the concentrations of TAA and proton. 71 )
Therefore, the co-production of a proton gives autocatalysis to the precipitation
reaction, which is expressed by Eq. (6.18). The second-order reaction rate
constants at various temperatures have been determined accordingly and an
activation energy of 75.3 kJ/mol has been estimated for the precipitation
reaction. 17 ) The rate of film growth is apparently linear to the deposition time as
well as to the passed charge at all temperaturesY) The relationship between the
rate of film growth and reaction temperature has revealed an activation energy of
56.7 kJ/mol for the film growth, remarkably smaller than that of the precipitation
reaction. 17 ) This is a clear indication that the film growth in the EICD process is
not under the control of CdS formation in the solution bulk. The apparent linearity
of the film thickness to the passed charge suggests limitation of the film growth
by that of the electrode reactions. However, the slope of the thickness vs. charge
plots, thus apparent faradic efficiency of the film growth, varied with change in
reaction temperature. Such a relationship is not expected for a typical
electrodeposition process. The activation energy of the electrode reaction
determined from the current densities during the film growth was only 27.7
kJ/mol, much smaller than that of the film growth and almost equal to that of
proton reduction (21 .0 kJ/mol), which was determined from separate experiments.
It is certain that the electrode reaction during the film growth is that of proton
reduction. The deposition mechanism of the EICD process is illustrated in Fig. 6.5.
The electrochemical reduction of proton (Eq. (6.13» raises the local pH at the
surface of the substrate to allow the formation of a Cd-hydroxide reaction site as
in the case of the alkaline CBD.
Cd(OH)Zads + CH 3 CSNH z  Cd(SC(CH 3 )(NH z »(OH)zads
(6.20)
Cd(SC(CH 3 )(NH z »(OH)zads --t CdS + CH 3 COOH + NH 3 + site (6.21)
Reactions 13 and 19-21 can be expressed by an overall equation
Cd Z + + CH 3 CSNH z + 2H z O + 2e-  CdS + CH 3 COOH + NH 3 + Hz(6.22)
Thus the role of the electrochemical proton reduction can be regarded as reducing
the activation barrier of the chemical formation of CdS expressed by Eq. (6.18)
by compensating increase of proton concentration, which is co-produced along
with CdS formation. Activation of surface chemical reactions for selective film
growth is achieved typically by applying heat to the substrate in the CVD process.
We believe that th_e CBD process becomes one step closer to the CVD process by
introducing electrochemical control in the present EICD method.
6.3.4 Modification of EICD Process
Cd Z + + 20H- + site  Cd(OH)zads
(6.19)
Surface reactions play decisive roles in film growth in the EICD process.
Modification of film growth is therefore possible by adding chemical species
which directly affect the surface reactions. When CdCl z is simply substituted by
CdS0 4 , thin films of cubic f3-CdS have been obtained, preserving all the
characteristics of the EICD process such as high crystallinity.7 Z ) This effect of the
counter anion indicates that the actual surface reactions for the film growth is not
as simple as those discussed in Section 6.3.3. Addition of mer cap to ethanol (ME),
which is known to be strongly adsorbed on the surface of CdS, hindered crystal
growth, reslting in the formation of nano-particulate thin films. 72 ) Concomitant
enlargement of bandgap energy (from 2.43 to 2.69 eV) due to size'quantization
effect has been observed upon addition of ME up to 6 mM.7 Z ) EICD of nano-
particulate CdS thin films has also been realized in an acetonitrile bath containing
Cd(CI0 4 )z and T AA. 73) One-step deposition of CdS/ZnS bilayer thin film has
been made possible by EICD in an aqueous mixture of Cd z + and Zn z +, because
chemical deposition of ZnS proceeds by accumulation of small ZnS particles
onto the substrate, namely, in a cluster-by-cluster manner, contrary to the case of
CdS. 74 ) The surface processes of chemical deposition can therefore be adjusted by
means of electrochemical control as well as by co-existing chemical species, so
that thin films of desired structures are selectively synthesized.
T AA molecules coordinate to the surface hydroxides to form intermediates for
layer-by-Iayer growth of CdS.
Electroreduction of proton CdS formation in solution phase CdS film growth
Ea= 21.0kJ/moi Eu= 75.3kJ/mol Eu= 56,7kJ/moi
laYer-bY-laYe J
Surface catalyzed
decomposition ofCd 2 +-TAA growth of
1 _ CdS crystals
C "2 Hz
H' H' scavenge ot co-
'/  0 0 0 h uced proto o
Cd 2 '- TM NH ....." CdS CH COOH + NH '
, I 2 3 4
S=C\CH  0 C 
3 formation of randomly --,.. not incorporated
aggregated CdS particles
6.4 True Electrodeposition of Metal Sulfide Thin Films by
Reduction of Thiocyanato Complexes
6.4.1 Idea
CdS growth not
disturbed by solution
phase reaction
As discussed in Section 6.2.2, previously studied methods for
electrodeposition of metal sulfide thin films involve either chemical reactions in
solution to form solid particles (TS in water)4Z.43) or do not allow lo'ng-range
homoepitaxial crystal growth (S in DMSO),37-41) so are not suitable for deposition
of ordered thin films of useful thickness. If a system where layer-by-Iayer
Fig. 6.5 Schematic illustration of CdS film growth by electrochemically induced chemical deposition
(EICD) method.l7)

98
6 Solution-phase Processing of Semiconductor
6.4 True Electrodeposition of Metal Sulfide Thin Films
99
6.4.2 Thermodynamic Consideration
linearly to the passed charge confirming its electrochemical growth. IS) The SEM
photographs of the electrodeposited CdS thin films are shown in Fig. 6.6. Deposits
on the ITO substrate (a) have a rice-corn shape with a diameter of 100-150 nm
and length of 200-250 nm. The XRD pattern of this film (Fig. 6.7(a» indicates
that the deposits are of hexagonal a-CdS. When poly-Ni is used as the substrate,
these deposits are aligned with their long side vertical to the substrate (Fig.
6.6(b». The corresponding XRD pattern (Fig. 6.7(b» indicates strong diffraction
from the (002) planes, indicating a strong preference in the c-axis vertical to the
substrate orientation. TEM observation of the deposits has confirmed that the
deposits are of single crystal a-CdS and their long axis corresponds to the c-
axis. IS) Because the mismatch between (-J3 x -J3)R30° sites of Ni(111) and
atomic ordering of the six-fold symmetrical (002) planes of a-CdS is as small as
4.17%, epitaxial seeds of a-CdS might be deposited initially so that the CdS
films with the observed strong crystallographic orientation are obtained. The film
growth in the present system can be summarized as illustrated in Fig. 6.8. The
electrochemical growth of metal sulfides is achieved (true ED), as in the case of
electrodeposition of ZnO (section 6.2.3) can be found, the problems of the
previous ED methods can be overcome. The key is to find a chemical species
which is chemically stable in water but is electrochemically reactive for sulfide
formation. Although many sulfur-containing species are easily decomposed in the
presence of metal cations, as seen with TV, TAA and TS, thiacyanates (SCN-, TC)
form stable metal complexes which are chemically stable, Metal-catalyzed
electrochemical ligand reduction to yield metal sulfides has been extensively
studied for metal complexes with TC and selenocyanates (SeCN-).7 5 - 77 ) Although
such reactions have been reported as being less efficient for Cd 2 + than for
transition metals such as C0 2 + and NF+,77) we have found that the reaction does
occur to achieve true electrochemicallayer-by-Iayer growth of CdS thin films. IS)
In the presence of excess TC, soft acidic Cd z + forms tetra-thiocyanato
complexes in which four TC coordinate to Cd z + with their soft basic S atoms as
expected from the high stability constant of complex formation.
Cd 2 + + 4SCN-  [Cd(SCN)4F- (lOgK4 = 3.6)
(6.23)
( ) ,." ....", ...... ,-,.' ...., (b) ' :''1,,:.''':''. '".- ',,' .. <.. ....
a ,', J   ,....... -;!. '.: (It.;. '..;' «":" o..,: ,... .J''Ir.  "?..s. i." ,
. . ....4.. '-. f..;,...... \ ....' ... " \ . ..itlP.f b" ,. 1: ':
. .'f"  ..r;f\v'.'  17 . - "01 \". .).'"tt..  .. . J.
. . "i,:'.' : "'"',\ .....  ... . r:: ' ,; c .... , J  'it;;>,.;" " .'
 . ..... " .-,J .. .,' '''., >. . .,' ". 'j '1'
>/:. .: ;"..1' :.1;- .,. ... J./i!".... :.I)   .' ," " ':;.. '.:(" *'" .J..,.. 4
. If. .,}:. W! r.'  - ..: "'.. .c.. , '''..-: .'t
'. \. . 'd:  ." '!fd,;"....;o I: .,:..!......  ":\D" ..:'1t..i"\ ";.<.3 ,,'
.. .  .. ....c ,. .'V ..', T, '.' "(to"t:....;,..  . . .  '11.. /to: "., '. ' .,.
fI .'" I ., ..'.c::''': .  .' ''I:;t' '-"1' J'.'" . .....If.'1"'., "'" .
"c' t .::. ....... iki."t.,;) .: ......"*...\..:\0....,.. . ''..','r' '...Ii.....
. , , ;:  .,!,\:.. r... \  ,. .. ..,. .;If", 'Tit&4\'t .  ,... t .' 1 . tf!... 
.....111. '. , ..' .,' "iI ',... ,. 1;t't.", ,.:ti'"f'  '. ,A.:, '? " ff\ ...
..... " :'. "£J .f;; .:-; ,'" '...... 5.. ..".... ".. '\'. -'. . ", ' 'i! .,
. '.: h.... . a . ,,:,',:.:. ...WII.',!..;.:,;;.,..  .>'..,..:'.'a.:...T.f...,.'"
' tJ . . .. '1:,)1' . ' . ", .',. \;I '.".!f V . ,',' --..-- ,
. -. I  ; '. ......... {I" -"'. f.It t;,'" . '. 4" . '. .'J> .:Cii"1'" I
.",.. ,'/' ..: 4ft .,..,.:c..... .,:, . ....,.iJt io" .', ".J'.' . ," . . .
. .,. ." ", ." /,.. ,:..,..:i;..:' . ,I:' ..:'
. '.. ,'. ,.;,1'  ,. 500 nm .::.,..",. ... '... :''''' 500 nm
. . oJ .. .J. ,",...;;1 _ ....,:"'. #, ..tJ
The chemical stability of this metal complex against CdS formation should be very
high, because chemical formation of CdS according to
Cd 2 + + 3SCN-  CdS + CN- + (SCN)z (L\G Z9SK = +140.4 kJ/mol) (6.24)
is not a spontaneous reaction as indicated by its Largely positive L\G value. If we
consider the formation of sulfide ion by direct electro reduction of TC, its
equilibrium potential is largely negative.
Fig, 6.6 SEM photographs of CdS thin films electrodeposited on an ITO glass substrate (a) or a
polycrystalline Ni (b) at -0.7 V (vs. SCE) in a 0.08 M CdCh + l.O'M TC aqueous mixed
solution maintained at 70°C.
(From T, Yoshida et aI., J. ElectroanaI. Chern., 473, 214 (1999»
SCN- + 2e-  Sz- + CN- (EO = -1.096 V vs. SCE)
(6.25)
c::;-
o .: CdS
2-
(b) . "': Ni ---
-
-
0: ITO -
.......
....
---
M
0
.......
.
i:' .
'..-<
</)
1=1
(I) ---
..... -
.E 0
-
--- ....... 0
0 .
0 ---
- c::;- M
....... 5' 0
0 -
. - - .......
....... -
....... .
I , I , I I
20 30 40 50
28 (CuKa) / degree
Deposition of metallic Cd (Eq. (6.5» and hydrogen evolution reaction (Eq. (6.13»
thus prevail in this reaction. However, reduction of Cd z + thiocyanato complex
should occur at a much more positive potential,
[Cd(SCN)4Y- + 2e-  CdS + CN- + 3SCN- (Eo = -0.32 V vs. SCE) (6.26)
so that electrodeposition of CdS while avoiding Cd deposition is expected from
a thermodynamic point.
6.4.3 Electrochemical Layer-by-Iayer Growth of CdS Thin Films
When acidic (pH 3 to 4) mixed solution containing 0.08 M CdClz and 1 M
NH 4 SCN (or KSCN) is heated, the solution remains clear because no chemical
solid formation takes place. When ITO glass or polycrystalline Ni substrates are
cathodized at -0.7 V (vs. SCE) in this solution, although very slowly compared
to CBD or EICD, transparent yellow thin films grow. Film thickness increased
Fig, 6.7 XRD patterns of electrodeposited CdS thin films corresponding to those in Fig, 6,6,
(From T, Yoshida et aI., J. Electroanal. Chern" 473, 214 (1999)

100
6 Solution-phase Processing of Semiconductor
6.5 Electrochemical Self-assembly of ZnO/Dye Hybrid Thin Films
101
Cd 2 + SCN- --Jo-[Cd(SCN)4F--)(..no CdS formation
0", ... Cd thiocyanato complex in sol, phase
Cd 2 +-SCN- (log K4 = 2,3) 0
electroreduction of Cd-SCN- complex
([Cd(SCN)4J2' + 2e' -.... CdS + CN- + 3SCN-, E' = -0.32 V vs, SCE)
(Cd 2 + + 2e-.... Cd. E' = -0,643 V vs, SCE)
(SCN- + 2e' -.... S2, + CN- , E' = -1,096 V vs, SCE)
CN-
t
-CdS 0
CdS growth not
isturbed by solution
phase reaction
dye molecules such as Ru-polypyridine complex.7 8 . 79 ) Ultra fast and efficient
photo-induced charge separation is achieved as a consequence of the synergetic
effect at the TiOzidye boundary.80) The sensitizer molecules must be chemically
attached onto the surface of inorganic semiconductor for efficient sensitization. An
ordered monolayer of dye molecules is self-assembled in the TiOziRu complex
system. 81 ) Preparation of dye-modified semiconductor films has been conducted
by stepwise processing in previous studies, that is, formation of porous
semiconductor film by sol-gel based processing, followed by dye adsorption from
solution. 78 ,79) The use of solution for dye adsorption is very important for giving
freedom for the dye molecules to choose their structure on the surface. In contrast,
the processing of semiconductor films starting from colloids inherently results in
a random structure. We have seen in the previous sections that chemical and
electrochemical deposition of semiconductor thin films from ionic or molecular
precursors can yield thin films with ordered structures. Such processes should be
beneficial to enhance the performance of the hybrid materials.
Although direct deposition of crystalline TiO z has not been achieved,
electrodeposition of ZnO yields highly ordered crystalline thin films as seen in
Section 6.2.3. ZnO is one of the most promising materials for dye-sensitized
solar cells. 8Z ) When dyes are adsorbed on electrodeposited ZnO thin films by
dipping them in dye solutions, the films are only slightly colored due to the
limited surface area of highly crystallized ZnO films. However, when dyes are
added to the deposition bath, dye-loaded colored ZnO thin film could be
electrodeposited in a one-step synthesis. 19 ) Through studies with the chemical
and electrochemical deposition of metal sulfide thin films described in the above
sections, we learned how reactions at the deposit/solution interface affect crystal
growth. Adsorption of dyes onto the growing surface of ZnO indeed affected the
crystal growth, creating thin films with unique crystallographic structures. ZO ) At the
same time, chemical interactions between dye and ZnO and those among
neighboring dye molecules themselves lead to self-assembly of ordered dye
layer. zO ) Owing to such free interactions in solution, the entire ZnO/dye hybrid
material is self-assembled as the film growth is driven electrochemically (Fig.
6.9). This new and simple method to obtain inorganic/organic hybrid thin films is
called "electrochemical self-assembly".zl) The simplicity and heat treatment-free
processing are obvious technological advantages of this method, making it
Fig, 6.8 Schematic illustration of CdS film growth by true electrodeposition in thiocyanate system. 18)
layer-by-layer growth of CdS is achieved solely by electrochemical reductive
decomposition of Cd z + - TC complex, making this process the first example of
true electrodeposition of CdS in an aqueous system. It is of great significance that
the present method not only enables formation of structurally ordered thin films
but also provides a benefit for prolonged use of the deposition bath since thin films
with equivalent qualities could be repeatedly produced from the same bath.
6.4.4 Electrodeposition of Other Metal Sulfides
The reductive decomposition of thiocyanato complexes should be applicable
to the electrodeposition of other metal sulfides. We have tried this with Pd z +,
C02+, NiZ+, Zn z + and In 3 +. 18 ) While thin films of PdS, CoS and NiS could be
successfully electrodeposited, other metal sulfides such as ZnS and InzS3 could not
be obtained. This is an interesting series of results when we think of the softness
(hardness) of these metals as acid. TC coordinates with its soft basic S atom to soft.
acidic Cd 2 + and Pd z +, while hard acidic In 3 + only permits coordination with hard
basic N atom to form an isothiocyanato-complex. Other metals are at the
borderline accepting coordination of both Sand N. Because reduction of TC is
catalyzed by a central metal,15.76) such ligand reduction may result in the formation
of metal sulfides only for thiocyanato-complexes. The difference in bahavior
among Coz+, Nj2+ and Zn z + could be reasoned as the consequence of efficient
catalysis of the electron transfer reaction by the transition metals. Such trends fit
nicely with the previous findings by electrochemical analyses. 77 ) It is therefore
understood that the chemical structure of the active species is decisive to the
film formation. Thus, designing such molecular precursors which are chemically
stable but can be electrochemically decomposed to metal sulfides should broaden
the possibilities of electrochemical thin film synthesis.
6.5 Electrochemical Self-assembly of ZoO/Dye Hybrid Thin
Films
I  '

, 
: 
.. 
"f 
f 


6.5.1 Idea
There has been a growing interest in inorganic/organic hybrid materials in
recent years with expectations for new or improved properties, which are not
exhibited or attainable by using the parent materials separately. The usefulness of
such materials has been best manifested by recent successful studies of dye-
sensitized solar cells, which typically employ thin film electrodes of porous oxide
semiconductors such as TiOz, and whose surface is modified with sensitizing
Fig, 6.9 Eiectrochemical self-assembly of ZnO/dye of hybrid thin films,

102
6 Solution-phase Processing of Semiconductor
6,5 Electrochemical Self-assembly of ZnO/Dye Hybrid Thin Films
103
6.5.2 Electrochemical Self-assembly of ZoO/Dye Hybrid Structure
(a)"'  ""; .U (b),' \\'_-r . l\'\'(
'.,/ ,-' r""-J f I \ \ .), "", ' I. '.
J ,', '''' p '' 1'--.. \ ,,''1 ?--
, ., .d' i ,I "" , ...."'....,
I ,......... fi. £ :J' . /  S''''
,. )  .': · f!f,")\\; t
 \. .,A / Q$ " ' H ' ".' I'};I , '" "",  / 'f1r.:-,  ::..,
.  \(, \- r ." I Ii t/I,,, 11/.,1. .,-
...; '> . - ( , ' :': ::->' -- i.:, \//r- V! Z I\\  II-
,'. .."'" ...-... ; "'-;:. . _-'Os . ,'1,_
.,. .j:' ,- ,/(-f'/'/'/"\'\"" 7' 'I'
. ,....) , '- , ,;"....:.: J. //, '/ J " ...,.____.   -
'I . \'. / . ,..,<.- . - " '/ _r/" ::,. .', 1
.'  ' I _. ..,. ...".:;-:."\....'-.",,, J-lm
especially useful for non-heat-resistant organic materials. It is also expected that
such materials having structures chosen by the constituent ions and molecules
themselves will exhibit the highest synergetic effect from the inorganic/organic
boundaries.
A. One-step Electrodeposition
Electrodeposition of ZnO thin films can be triggered by reduction of nitrate
typically in an aqueous solution of zinc nitrate. 62 ) The overall reaction for
deposition of ZnO can be written as follows.
Fig. 6.10 SEM photographs ofZnO (a) and ZnO/TSPcSi (b) thin films e1ectrodeposited on basal-plane
of oriented pyrolytic graphite (OPG) electrode.
(From T. Yoshida and H. Minoura, Adv, Mater" 12, 1220 (2000)
Zn Z + + N0 3 - + 2e-  ZnO + NO z - (EO = +0.246 V vs. SCE) (6.27)
5Zn z + + N0 3 - + 2H z O + 8e-  5ZnO + NH/ (EO = +0.213 V vs. SCE) (6.28)
When a small amount of TSPcSi is added, the film morphology completely
changes as seen in Fig. 6.IO(b). Flat disk-like deposits are assembled in parallel
and stand on their edge on the substrate. XRD and TEM analyses revealed that
each disk is a single crystal of ZnO and its edge and plane correspond to the
(100) and (002) planes of ZnO, respectively.zO) The addition of TSPcSi therefore
changed the crystallographic orientation of the film by 90 degrees, from (002)
parallel with the substrate to (100) parallel with the substrate.
When water-soluble dyes are added to the bath, colored thin films of ZnO
incorporating the dye molecules are obtained by one-step electrodeposition.
Water-soluble dyes such as metal complexes of 2,9,16,23-
tetrasulfophthalocyanines (TSPcMs; M = Zn(II) (TSPcZn), Al(III)[OH] (TSPcAl)
or Si(lV)[OH)z (TSPcSi»,19,ZO) organic dyes such as eosinY (EY)83) and
tetrabromophenol blue (TB)84) were successfully loaded into ZnO by the same
strategy. Characteristics of hybrid thin films such as thickness, morphology,
crystallographic structure, and amunt of incorporated dyes, are strongly
dependent on factors such as deposition potential, kind of dyes added and their
concentration in solution, although the products are in all cases mixtures of
crystalline ZnO and dyes. One of the interesting features of the electrochemically
self-assembled ZnO/dye thin films is the dye loading in very high amounts. For
example, 4.8 x 10- 8 mol of EY per cm z of projected film area can be loaded at a
thickness of only 1.7 f.1m. 83 ) Considering that the loading of the Ru complex at 1.3
x 10- 7 mol/cm z is achieved with a film thickness higher than 10 f.1m by stepwise
processing,18) EY molecules in the electrodeposited ZnO/EY film are concentrated
more than twofold.
C. Formation of Ordered Dye Assemblies
Because dye adsorption takes place during film growth in the electrochemical
self-assembly, it has a significant impact on the crystal growth and' changes the
film morphology. In some cases, it leads to the formation of porous structured film
made of nano-sized ZnO as the crystal growth is hindered.19.zo.83) In other cases,
the dye adsorption influenced crystallographic orientation of the film. ZO.ZI) Fig. 6.10
shows SEM photographs o(ZnO thin film electrodeposited in a'O.1 M Zn(N03)Z
solution and ZnO/TSPcSi thin film deposited under the same condition but with
TSPcSi added at 50 f.1M to the bath. When deposition is carried out on Van der
Waals planes of graphite, ZnO self-orients with its c-axis perpendicular to the
substrate owing to the highest stability of the closest packed (002) planes of ZnO
and consequently exposes its hexagonal planes in the SEM image (Fig. 6.1O(a».66)
While dye adsorption strongly affects the crystal growth to create the unique
structure of ZnO, the loaded dye molecules also form ordered assemblies as found
by UV-vis abso.rption spectroscopy. TSPcZn molecules form II-stacking dimers
in water due to hydrophobic intermolecular attraction. Intermolecular electronic
interaction in such parallel arrangement of the chromophores is observed by a
characteristic blue-shift of the absorption spectrum. Because of such strong
intermolecular affinity, TSPcZn forms multilayers of II-stacking aggregates on
ZnO in the electrodeposited ZnO/TSPcZn films. 19 ,ZO) The aggregates of TSPcZn
were easily extracted from the film by rinsing the film with a solution of a cationic
detergent such as cetyltrimethylammonium chloride (CT AC), only leaving
chemically anchored TSPcZn monomer on the surface of ZnO. ZO ) Formation of
ordered dye assemblies in parallel arrangement (H-aggregation) has also been
observed for ZnO/Ey83) and ZnO/TB84) films prepared by electrochemical self-
assembly. Contrary to these dye molecules, TSPcAl and TSPcSi are present as
monomers in the deposited films because of the hindrance to aggregation imposed
by the axially coordinating OH-.20) It is interesting to note that EY molecules in
the deposited ZnO/EY film are present only on the surface of ZnO, because they
can be completely extracted from the film by 0.1 M KOH without changing the
film thickness and morphology.zO) As all the dyes are accessible by redox
electrolyte when the film is dipped in electrolyte solution, it performs as an
efficient sensitized photoelectrode, making it a promising candidate for application
to dye-sensitized solar cells. 83 )
B. Impact of Dye Adsorption to Crystal Growth

104
6 Solution-phase Processing of Semiconductor
References
105
6.5.3 Mechanism of Electrochemical Self-assembly
small contribution to the ongoing rapid progress being made in the solution phase
processing of thin film materials. The more such processes are understood and
improved, the more the possibilities anticipated. The target of synthesis is not
limited to inorganic materials, but can extend to organic materials and hybrids.
The quality of the products can be as good as those processed by gas phase
techniques. Solid formation from ions and/or molecules in all cases is the process
of self-assembly. We cannot force the molecules and ions to build up ordered
structures, but can guide them to do so. Precipitation of solids in solution is one
of the most basic issues in chemistry, but many aspects are still not understood.
More thinking is needed, but no high energy and big instruments are required for
solution phase processing of thin film materials,
The process of electrochemical self-assembly for ZnO/TSPcM thin films is
illustrated in Fig. 6.11 taking into consideration all the experimental findings. 20 )
The anisotropy for stability of dye adsorption on different crystal faces of ZnO
seems to be the key factor to create the unique crystallographic structure of the
film. As the adsorption of TSPcM takes place most effectively on the (002) planes
of ZnO, crystal growth along the c-axis is strongly hindered. Consequently, ZnO
crystals grow along the (100) direction to create the disk-like deposits. Because
film growth is achieved by crystal growth in this system,65,66) the observed (100)
parallel with substrate orientation of the film is established. Chemical interaction
among the dye molecules as well as that between dye and ZnO determine the
structure of the adsorbed dye layer. It has also been found that electrochemical
reduction of dye molecules is involved during the film growth in the case of
EY83) and TB.84) The increased nucleophilicity of the reduced dye molecules
should enhance their attachment to Zn2+ during the film growth.
Although further studies are needed for a complete understanding of the
electrode processes for the formation of the ZnO/dye hybrid structure, it is built
up as consequence of free interaction of the constituent molecules and ions as seen
in our studies. The use of a solution is essential for such processes. The present
technique has widened the horizons for obtaining inorganic/organic hybrid
materials. Further studies are expected to achieve the synthesis of various new
materials with new and useful properties.
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.  ..
.. prefereritiai
growth of(IOO)
O transport of
N0 3 - i .C
Zn 2 +
Fig. 6.11 Schematic illustration of electrochemical self-assembly for ZnO/TSPcM thin films
(From T. Yoshida and H. Minoura, Adv. Mater., 12, 1220 (2000))

106 6 Solution-phase Processing of Semiconductor
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and M, Gratzel, J. Am, Chern, Soc" 115, 6382 (1993).
80, T, Hannappel, B, Burfeindt, W. Storck and F, Willig, J. Phys. Chern, B, 101,6799 (1997).
81. V, Shklover, Yu,E. Ovchinnikov, L.S. Braginsky, S,M. Zakeeruddin and M. Gratzel, Chern.
Mater" 10,2533 (1998).
82. H. Rensmo, K. Keis, H. Lindstrom, S. Sodergren, A Solbrand, A Hagfeldt, S,-E, Lindquist, L.N,
Wang and M. Muhammed, J. Phys. Chern, B, 101, 2598 (1997),
83. T. Yoshida, K. Terada, T. Oekermann, D, Schlettwein, T, Sugiura and H. Minoura, Adv. Mater"
12, 1214 (2000).
84. T. Yoshida, J. Yoshimura, M, Matsui, T, Sugiura and H. Minoura, Trans. MRS-J, 24, 497 (1999),

7
Self-cleaning Properties of Ti0 2 -coated Substrates
7.1 Introduction
Photocatalytic technology is becoming more and more attractive to industry
today because global environmenta] pollution has come to be recognized as a
serious problem that needs to be addressed immediately. Among the various
semiconductor materials, TiO z has attracted wide interest for its potential use in
industry. As a typical semiconductor, excitation of TiO z with photon energy
higher than its band gap gives rise to excited state electrons and holes at the
conduction band and valence band, respectively, initiating various redox reactions
at the semiconductor surface or interface. The strong 'oxidizing power of the
photogenerated holes, the chemical inertness and nontoxicity of TiO z make it a
superior photocatalyst. After the report on water splitting using TiO z
semiconductor photoelectrode in the early 1970s, I) many studies on
photo electrochemistry have been conducted using TiO z for solar energy
conversion. Z -4) In addition, various applications of TiO z photocatalysis such as
organic synthesis,5-7) CO z reduction,S) cancer treatment,9) and others have been
reported, In particular, the applications to environmental cleanup have become a
topic of current interest,IO-12) e.g., degradation of halogenated compounds in air, 13)
degradation of various surfactants in water,14) sterilization of water,15) and
decomposition of oil spills on water surface,16) are being studied vigorously.
These applications involve use of the strong oxidative power of TiO z when it
absorbs light of wavelength shorter than 380 nm and are based on the premise that
intense ultraviolet light (ca. 10m W cm- Z ) such as that found in indirect sunlight
or provided by mercury lamp can be used as the excitation light source.
One of the basic tenets of photochemistry is, however, that energy provided
by photochemistry has been quantized. This means that, regardless of the intensity
level of te light, the energy of each photon is the same. Converted to thermal
energy, the energy of photons of light whose wavelengths are shorter than 400 nm
is equivalent to greater than 30,000°C in thermal energy. Based on this idea, we
have attempted to develop a new type of photocatalysis. 17) Even though the light
may be weak, chemical reactions can still proceed on the surface of the TiO z at
30,000°C in thermal energy. Our strategy is to use UV light existing in a living
environment. Even though the light may. be weak, chemical reactions can still
proceed on the surface of TiO z at 30,000°c. Ifwe can collect and use this UV light
to drive photochemical reactions, most organic compounds could be "burned" at
room temperature when the reactant's concentration is low enough. At a

temperature as high as this, organic matter can be readily oxidized to carbon
dioxide and water. Moreover, we have recently found a novel property of the TiO z
surface, i.e. UV irradiation of TiO z surface can produce a highly amphiphilic
surface. ]8-Z6) This phenomenon is observed even under weak light irradiation,
Such amphiphilic surfaces have antifogging and self-cleaning properties, etc.,
and possess the possibility of leading to numerous applications. To achieve these
applications, we use various types of materials coated with a transparent but
highly photoactive TiO z colorless thin film.
We will describe in this review the various superior properties of TiO z
surface, i.e., even under extremely weak UV light found in ordinary room light,
photo active TiO z film can decompose fairly large amounts of various organic
compounds accumulating on the surface; in addition, highly amphiphilic surface
can be produced. This photocatalytic decomposition activity and photoinduced
amphiphilicity of TiO z can produce a new type of highly effective self-cleaning
surface.
10 1
-
E
c..
c..
-.-
s::
0
.-
- 10°
«S

-
s::
CJ)
0
s::
0
U
-1
10
0
(a)
-. .
1000 2000
7.2 Photocatalytic Decomposition
Time (5)
7.2.1 Air Purifying Effect
Fig, 7.1 Concentration of gaseous methylmercaptan in the presence of the TiO T coated substrate (a)
in the dark, (b) under fluorescent white light bulb (UV intensity: II m W cm- 2 ) and (c) under
a fluorescent black light bulb (UV intensity: 295 m W cm- 2 ).
Considering the characteristics of TiO z photocatalysis, it is concluded that the
most appropriate applications for the photocatalytic approach would be aimed
against low concentrations of substances that pose serious risks to health or
comfort. A malodorous pollutant might be a good example, because the actual
quantity ofthe chemical may be very smalJ.27) The Japanese government regulates
the concentrations of such major malodorous substances such as ammonia (NH 3 ),
hydrogen sulfide (HzS), methyl mercaptan (CH 3 SH) and acetaldehyde (CH 3 CHO)
in the environment. CH 3 SH, for example, is regulated to concentrations lower than
0.002-0.01 ppm. Higher concentrations are not permitted. The human sense of
smell, however, senses it at a concentration as low as 0.00012 ppm.
Here, we show the examination of the photocatalytic decomposition of
CH 3 SH. As the photocatalyst, we used a sintered TiO z film (anatase, thickness of
ca. 1 J.1m) coated on a ceramic tile (l0 cm x 10 cm)_ TiOz-free (normal) tile was
used in the control experiments. Either a fluorescent white light bulb (10 W) or
a fluorescent black light bulb (lOW) was used as the exciting light source. For the
degradation of gaseous CH 3 SH, a glass-made reactor was used. The concentration
was determined by gas chromatograph. Figur 7.1 shows the changes with time of
the gaseous CH 3 SH concentration in the presence of one piece of the TiOz-coated
substrate. No concentration change was observed in the dark. On the other hand,
CH 3 SH quickly degraded oxidatively under irradiation with relatively strong UV
light (295 m W cm- z ) by either photogenerated holes Z8 ) or excited oxygen atom
(-Oad*) formed from adsorbed oxygen reacting with photogenerated electron and
hole successively.z9,30)
What is surprising is the catalytic activity under an ordinary fluorescent
white light bulb. The UV intensity was only 11 J.1W cm- z , which was ca. 1/30 that
of the fluorescent black light, but the first-order degradation rate constant with the
former amounts to ca. 1/3 that ofthe latter. The quantum efficiency at 11 J.1W cm- z
light intensity, calculated under the assumption that 6 holes are used to degrade
one molecule, is ca. 40%, which is about 10 times larger than that at 295 mW cm- z .
The increase in reaction efficiency at lower light intensity can be explained by
estimating the photon nd reactant fluxes arriving to the photocatalyst surface. The
incident photon flux for a 11 J.1W cm- z light intensity at 350 nm is ca. 2 x ,1 0 13 cm z
Is. For a gas phase molecule with a concentration of 3 ppm in air, the number
diffusing to the surface is estimated to he on the order of 101'3 cm z s. This value
is calculated using it diffusion coefficient of 10- 5 m Z S-1 under the assumption that
the molecules are arriving by the concentration diffusion process, i.e., molecules
are distributed uniformly in air but the adsorbed molecules are oxidized
(disappear) quickly. Those values clearly indicate that the mass-transport process
to the photocatalyst surface is the rate-determining process even under such weak
light excitation conditions. This is because the reactant concentration is very low
(less than 3 ppm).
The important point is that the human sense of smell is so keen that even a
very small number of molecules (about 3 to 4 orders of magnitude lower
concentration than in the present experiment with CH 3 SH) causes trouble in
ordinary living environments. In other words, the amount ofUV light contained
in an ordinary fluorescent light bulb is enough to decompose a CH 3 SH odor. This
should hold true for almost all foul-smelling gases. CH 3 CHO, NH 3 and HzS were
also decomposed even under such a very weak UV illumination. These results
suggest that these TiOz-coated materials are effective in eliminating malodorous
gases without special excitation light systems.
7.2.2 Sterilization Effect
Recently, infectious diseases caused by Escherichia coli, HIV and other
bacteria and viruses have become serious threats. Not long ago, the sharp decline

--.

0
-
8
c:
0 IS.
:;::;
(.) 6
ro
"-
u..
C) 4
c:
.-
>
> 2
"-
:J (c)
CJ)
0 - "\
'--. ..J
0 1 2 3
Time (h)
112
7 Self-cleaning Properties ofTiOrcoated Substrates
in tuberculosis cases and eradication of smallpox, as declared by the Wodd Health
Organization, gave medical students the impression that the subject of infectious
diseases was outdated. However, infectious diseases are back, often stronger than
ever, and new diseases seem to appear almost daily. With the exponential increase
in the movement of people across countries, oceans and continents, helped by
increased affluence and advances in transportation technology, the threat of
infectious diseases to the world is real again.
TiO z photocatalysis is considered to be effective for sterilization purposes as
well. We report the sterilization effect of a TiO z photocatalyst using three different
bacteria, i.e., Escherichia coli (E. coli., K-12 IFO 12713), Methicillin-resistant
Staphylococcus aureus (MRSA, llD 1677) and Pseudomonas aaruguuinosa (IFO
13736). Pre cultured cells of those bacteria were suspended in distilled water.
This suspension was dropped on the substrates and the substrates were covered
with a transparent Pyrex glass plate. After illumination through the glass plate at
room temperature, bacteria were collected by wiping the substrate surface with a
gauze patch. Then the solution of collected bacteria was spread on nutrient agar
medium and the medium cultured at 36°C for 24 h.
Since most organic compounds should be decomposed on photo excited TiO z ,
it is expected that bacteria will also be destroyed on it regardless of species. 9 ,15,3I,3Z)
We tested above three different types of bacteria. Typical results are shown in Fig.
7.2, in which the white spots are colonies of those bacteria. The numbers of
colonies correspond to those of viable cells on the substrates after illumination
with UV intensity of 14 J.1W cm- z for 30 min. Although there existed many
colonies for the TiOz-free substrate, not one is observed for the TiOrcoated one,
indicating strong antibacterial effect under even such a weak UV light.
More detailed data are shown in Fig. 7.3 in which a surviving fraction of E.
coli. on TiOrcoated substrate is plotted as a function of time under different UV
light intensity. In the dark the surviving fraction is almost 100% in this time
period. However, ca. 80% of the bacteria were destroyed in 3 h even under UV
10
7 2 Photocatalytic Decomposition
113
Fig. 7.3 Sterilization of Escherichia coli on TiOrcoated substrate under room light illumination, The
UV intensities were set at (a) 0.8 mW cm 2 , (b) 2,7 mW cm- 2 and (c) 13 mW cm- 2 . The ratio
of numbers of colony for TiOrcoated substrate to that for the TiOrfree substrate is
represented as the surviving fraction,
light of 0.8 mW cm z , which is contained in 200-lx white light illumination from
a fluorescent light bulb or in the l300-lx illumination from an incandescent lamp.
These levels are typical of room lighting. The number of bacteria destroyed in 3
h is ca. 2 x 10 z cm z , whereas the total number of UV photons irradiated amounts
to 2 x 10 16 cm z . By increasing the light intensity, the sterilization time became
shorter. On the average, only ca. 10 14 photons were enough to destroy one
bacterium. The sterilization mechanism is not clear at the present stage, but
various active oxygen species formed on TiO z may be responsible for the
sterilizati on.
Recently, in-hospital infection caused by MRSA (Mechicillin-resistant
taphylococcus aureus )left on ward floors and \yalls after conventional cleaning
and sterilization procedures has become a serious problem. MRSA has become
resistant to most commonly used antibiotics, infecting patients, especially the
aged, and can cause death in the absence of effective antibiotics. The number of
bacteria and viruses causing such problems is, however, very small compared to
that of UV photons contained in typical room light. Judging from the results we
have obtaind, TiO z photocatalyst would also be effective against such problems.
Room light is strong enough to destroy bacteria existing on TiOrcoated substrates
in indoor living spaces.
Escherichia coli
Normal Tile
Staphylococcus
aureus (MRSA)
Pseudomonas
aeruginosa
0 ..",.
,':...... .
7.2.3 Anti-fouling Effect
Fig. 7,2 Colonies of three kinds of bacteria cultured from cells collected from surfaces of the TiO r
coated substrate (right) and Ti0 2 -free substrate (left) illuminated with l4-mW cm- 2 UV light
for 30 min, Each colony corresponds to one viable cell on the substrate.
Next, we report the anti-fouling effect.3-37) The anti-fouling effect has been
confirmed only qualitatively. The time course change of surface gloss of the
substrates was measured by a commercial gloss meter. A self-cleaning function
can also be expected with TiOrcoated substrates when small amounts of organic

114
7 Self-cleaning Properties ofTi0 2 -coated Substrates
115
7.2 Photocatalytic Decomposition
-
en
...
.-
1.0. l
n
-----------
n
(c)
glass
resm
hydrophobic resin
c
::s
.
.c
J...
C1:S
---
Fig, 7.5 Shapes of water droplets on various materials,
the balance between the cohesive forces in the liquid and the adhesive forces
between the solid and the liquid. On glass or other inorganic materials, water
has a contact angle ranging from 20 0 to 30°. With plastics, the contact angle is
typically from 70° to 90°. With water-repellent plastics, such as silicone resins and
fluororesins, the angle could be higher than 90°. Very few substances are known
to show angles of lower than 10°, with the exception of some water-absorbing
substances and surfaces that have been activated with soap or similar agents.
These surface, however, do not retain long-lasting effects.
However, we recently discovered photogeneration of a highly amphiphilic
(both hydrophilic and oleophilic) TiO z surface. ls - Z6 ) The unique character of this
surface is ascribed to the microstructured composition of hydrophilic and
oleophilic phases, produced by ultraviolet irradiation. A thin TiO z polycrystalline
film from anatase solon a glass substrate show a water-contact angle of ca. 70°
before UV irradiation. After irradiation, water droplets spread out on the film,
resulting in a contact angle of ca. 0 0. The contact angle of oily liquids (such as
glycerol trioreate and hexadecane) was also measured, Distinct contact angles
were found for the TiO z film under normal conditions, but all the liquids spread
across the surface upon UV irradiation, with contact angles of ca. 0°. Irradiation
induced the suface to be highly hydrophilic and highly oleophilic. This wettability
change was also observed for both anatase and rutile TiO z surfaces of polycrystals
or single crystals, independent of their photocatalytic activity, suggesting that a
phenonemenon different from the photocatalytic decomposition reaction occurs.
Even though the TiO z samples were stored in the dark for a few days, the high
amphiphilicity of the TiO z surface was maintained. A longer storage period
resulted in a gradual increase in the water-contact angle, indicating a surface
wettability trend towards hydrophobicity/However, high amphiphilicity was
regenerated repeatedly by UV light irradiation.
All of the liquids examined spread completely on a UV -illuminated TiO z
surface, with a contact angle of ca. 0°. This leads to the dramatic conclusion that
UV illumination creates a surface that is both highly hydrophilic and highly
oleophilic. For a certain liquid, the major contribution to the contact angle comes
from the interfacial character of the solid material, which is related to its surface
structure. Therefore, it is assumed that structural change of the TiO z surface via
UV illumination plays an important role in its unique wettability.
Friction force microscopy (FFM) images provide information at a
microscopic level to explain this unique surface wettability. A rutile TiO z (110)
single crystal was used since its wettability behavior induced by UV illumination
is analogous to anatase poly crystalline films. Additionally, a flat surface is
required for FFM measurement. Before UV illumination (Fig. 7.6 (a», no
difference in contrast was observed for either the FFMimage or the topographic
image, indicating microscopically homogeneous wettability on the surface. After
UV illumination (Fig. 7.6(b», however, the formation of hydrophilic (bright)
en
en
Q)
c:
.-
en
en
o
-
"
0.8
.._..
.J
0.6
15
20
o
5
10
Time (day)
Fig. 7.4 Surface gloss under room light illumination: (a) TiOrfree substrate with a UV intensity of
0.12 mW cm- 2 ; (b) and (c) Ti0 2 - coated substrate with UV intensities of 0.12 mW cm- 2 and
3.5 mW cm- 2 , respectively, The initial value of each case was normalized to one.
contaminants accumulate gradually on it. Fig. 7.4 shows one of the examples, i.e.,
the change in surface gloss of the substrates left in a urinal subjected to daily use.
A TiOz-free substrate lost its gloss gradually over a period of several days. On the
other hand, the decrease in gloss of the TiOz-coated substrate was much smaller
under ordinary lighting conditions (UV light intensity of 0.12 mW cm- Z ). When
the UV intensity was increased to 3.5 mW cm- z , the gloss hardly changed for more
than 20 days. Similar results were obtained for the accumulation of cigarette
smoke residue on TiOz-coated substrates. Although it is difficult to analyze this
effect quantitatively, such a self-cleaning effect is anticipated only under
conditions in which the number of molecules of staining substances reaching the
surface is much lower than the number of photons.
However, as far as we have checked, the TiOz-coated substrate shows
unexpectedly effective self-cleaning effect in both ordinary indoor living and
outdoor spaces. Particularly in outdoor spaces, inorganic contaminants such as
sand may be mainly responsible for stains, so photocatalytic oxidative power
may not be considered useful for self-cleaning process. However, we found that
TiOz-coated materials show very effective anti-fouling effect even against
inorganic substanses. This is probably because inorganic ones adhere on the
substrate surface by oil spots, which serve as a binder. Therefore decomposition
of the oil also decreases the amount of inorganic materials on the surface.
7.2.4 Photo-induced High Amphiphilicity
In our daily environment, the surface of a material will repel water to some
degree. The degree of water repellency of a substance can be expressed in terms
of the contact angle of a water drop with the surface (Fig. 7.5). According to
Young's equation, the contact angle of a liquid drop on a solid surface results from

7,2 Photocatalytic Decomposition
117
116
7 Self-cleaning Properties of TiOz-coated Substrates
and oleophilic (dark) areas was clearly seen on the surface. There exist hydrophilic
domains with a regular rectangular shape ranging from 30 to 80 nm in size. A
higher resolution topographic image was also obtained, demonstrating that the
hydrophilic domains are higher in position than the oleophilic areas. The image
was also obtained by rotating the sample stage by 45° with respect to Fig. 7 .6(b),
indicating that the above structure holds irrespective ofthe scanning direction. All
the images show that the rectangular features align particularly along the [00]]
direction of the (110) single crystal surface.
On the TiO z single crystal (110) surface, oxygen bridging sites align along the
same direction. It is well known that the atomic coordinations at the TiO z surface
differ from those in the bulk since the atom arrangements are truncated on the
surface. This gives rise to five-coordinated Ti atoms and two-coordinated 0
atoms, which are more energetically reactive than the six-coordinated Ti and
three-coordinated 0 atoms in the bulk. By UV illumination the holes are
transferred to the surface, creating oxygen vacancies'most likely at the two-
coordinated bridging sites, which are suitable for dissociative water adsorption.
These defects probably influence the affinity to chemisorbed water of the
surrounding five-coordinated Ti sites thus resulting in the formation of hydrophilic
domains, leaving the remaining oleophilic region (Fig. 7.7). Photoproduced
electrons are also transferred to the surface Ti 4 +, forming TP+ followed by the
electron transfer to adsorbed oxygen molecules.
These domain structures were gradually reversed during storage in the dark.
This leads to the conclusion that the highly amphiphilic character of the TiO z
surfaces result from the nanoscale separation between the hydrophilic and the
oleophilic phases. A conventional hydrophilic (or oleophilic) surface is one that
contains uniform hydrophilic (or oleophilic) terminal groups that furnish high
adhesion tension to water (or oil). Therefore a hydrophilic (oleophilic) surface
displays a lower (or higher) water contact angle and a higher (or lower) oil c01)tact
angle. We present a unique TiO z surface, created by UV illumination, that is
composed of a nanoscale distribution of hydrophilic and oleophilic regions. When
a liquid droplet (either water or oil) is several orders greater than the hydrophilic
: ;".'4;f,:';; ..,::".:. .::\ . "',:,' .:  ?:
' h...t:'..J r ' J '.,., tt.1 _1" . 'I , } ,
't'" '. ". \:., ...' i. "t" In"",," ', . ;t.'!
ii-)'j£ij{t . " YliJ('" .' it,.,.  
!'.'.r'1."'J'1"".'!''J '", .'tt . . ..   '.,}k .
,'::'.i.:f?;':i:.:(::t}F;S)f.b;f, ..('t: N
1.,f' ;. "',' (:."'f{?) ,',>, «,.'t.7'<?,.{;.:J -, ,) :... 1
o 234
Before UV
(a)
H
O-
: 6 2 -
...... :-4/ "'--. ,:4+/
/ Tl "'--.02/Tl ......
/ (A)
/ dark
HH
\ /
o

HH HH
0/ H- H- ({
. 0 0 .
! / \ f /
...... T . 4 + T -4+
/ 1 "'--. 0 2 / 1 .....
(B)
H H H H
\ / \ /
? (!.1) ?
...... 4/--:"'--. :4+/
/ Tl "'--. 02/ Tl ......
H 2 0
...
hv

.
dark
Fig. 7.7 Mechanism of photoinduced hydrophilicity.
I,!,: ..:,...' 'f'."'" . : ..<. :..:" * t: '. . .+ .IS)
I "1 .....",......"(  .;- :t'..-- .. . "it . -t  ,'". ,
i 2 3 4 
or, oleophilic domain, it instantaneously spreads on the surface, following the
microstructured flow channels that are formed by the oleophilic walls for water
or the hydrophilic walls for oil, hence resembling a two-dimensional capillary
pen.omenon. Moreover, the hydrophilic and oleophilic regions are alternately
distnbuted so the surface has equal wettability for both water and oil.
. Furier transform infrared (FTIR) spectroscopy provides another way to
Illvesllgate the conversion of the surface wettability. As shown in Fig. 7.8, a
freshly prepared anatase film, which has relatively high surface defects due to the
annealing rocess, shows IR bands that are positioned at 3,695 cm- I , assigned to
the stretchmg of a hydroxyl group that was chemisorbed on a surface defect site
3,300 cm- I , assigned to hydroxyls for both dissiated water and molecularl;
adsorbed water; and 1,623 cm- I , pertaining to H-O-H bending for molecular
water. This observation denotes the coexistence of dissociated and molecular
water on TiO z surface. Storage in the dark for one week resulted in the decrease
of all the bands, suggesting that hydroxyl desorbed at the defect sites and also the
molecular waer is desorbed. The significant increase in the water contact angle
after storage III the dark is consistent with this result. The surface conversion
may be ascribed to the replacement of the chemisorbed hydroxyl groups with
the oxygens in the air.
. Net, let us consider the applicability ofthis unique wettability phenomenon.
With TiOz photocatalyst alone, this highly ampiphilic surface disappears soon
after exposure of the surface to light. When the photocatalyst has been combined
I,:ih a w.ate-sorbig. substance that can hold water within its structure (such as
'silicon dIOxide or sIlica gel), the effect of the surface continues even in the dark.
. ogging of the surfaces of mirrors and glass occurs when steam that has cooled
on these surfaces form many water droplets. On a highly amphiphilic surface, no
If) 0
After UV
(b)
Fig. 7.6 (a) FFM image (5 x 5 fJ.m 2 ) of the Ti0 2 (110) surface before UV irradiation. (b) FFM image
(5 x 511m 2 ) of the same surface after UV irradiation,

118 7 Self-cleaning Properties of Ti0 2 -coated Substrates
0.12
Q) 0.10
(,)
t:::
ro
.e
o
(/)
.D
« 0.08
0.06
4000
3000
-1
VVavenurnber(crn )
bl)
o
'bh
bI)
t8+->
.u
.- Q)

Q)
1800 1500
Fig. 7,8 FTIR reflection absorption spectra of a Ti0 2 anatase film. All the measurements were
performed for the same area on the same film in the following sequence: freshly prepared film
(solid line), after storage in the dark for 7 days (broken line), after UV illumination for 5 h
(dotted line). The inset in the left panel illustrates the enlargement of the 3,900 cm- I to 3,600
cm- 1 region.
least easily
stained
water drops are formed. Instead, a uniform film of water is formed on the surface.
This uniform water film prevents fogging. Fig. 7.9 shows the relationshp between
the contact angle of a surface with water and the tendency t?ward fogging. -
Until now; water droplets have ben rendered easier to remove by imparting
water repellency to the surface of automobile window glass or windshields.
However, with this method, the water must be blown off the glass by wind or
shaken off by vibration. Otherwise, the droplets remain and cloud the surface.
With the superhydrophilic approach, no wind or vibration is needed to combat
fogging. We exposed a highly amphiphilic mirror and a normal mirror to steam
to compare the results. The normal mirror quickly fogged, but the highly
amphiphilic photocatalyst-coated mirror retained its clarity. A mirror once covered
with a highly amphiphilic coating retains its effect semipermanently (several
years at least). We expect that various glass products, for example, mirrors aud
eyeglasses, can be imparted with anti fogging functions using this new technology,
with simple processing and at low cost. In fact, in the near future, many Japanese-
made cars will be equipped with antifogging, antibeading superhydrophilic side-
. .
view mIrrors.
Stain-proofing, self-cleaning effects can also be enhanced by highly
amphiphilic photocatalyst action. For example, a plastic surface smeared with oil
cannot be cleaned unless one uses detergent. A highly amphiphilic TiO z surface
modified with SiOz, however, has a higher affinity for water than for oil. i)..n oil
smear on a plastic utensil is released from the plastic surface when the utensil is
simply soaked in water. Based on this ?haracteristic, a kitchen exhaust fan, which
is likely to be covered with oil, could be easily cleaned by water if the fan blades
were coated with a highly amphiphilic photocatalyst. Outdoor applications of
this technique also are possible. Most exterior walls of buildings become soiled
most easily
stained
7,2 Photocatalytic Decomposition
119
.l:1
bl)
..a
t
superhydrophilic surface


o
.......
small ....-- contact angle -.. large
FIg. 7.9 Water contact angle and anti-fogging effect,
superhydroplilic
./ surface
nomal glass or tile
/
c
.-
0:;::
11.).0
11.) cd
1-0 c::
bI)'-
11.),S
"'Cj<n
fluorocarbon
surface
/
./
reSIn or
painted
surface
I small I - water contact angle - I large
Fig. 7.10 Con t t 1
ac ang e and degree of stainability.
frm .automotive exhaust fumes, which contain oil
bUlldmg materials are coated with a hi hI ' y. omponents. If the original
te walls will wash away with rainfall g k y aphiphIh potocatalyst, the dirt on
times. ' eepmg the bUIldIng exterior clean at all
The susceptibility of a building exterior . . .
to its contact angle with wat Th atenal to sOllIng is closely related
b . ld ' . er. e matenal used th .
Ul Ing IS actuall y more II ' k t b :.. on e outside walls of a
I . . e 0 e sOlled if it .
P astlc is more likel y to be . 1 d h iS more water-repellent. Thus
. SOl e t an sheet I . 1 '
matenal like a fluorocarbon I ' . h g as.s or tl es. A water-repellent
p astlc is t e most lIkely to be soiled. A highly

120
7 Self-cleaning Properties of TiOrcoated Substrates
References
121
amphiphilic material that shows a water contact angle of zero degrees is far less
likely to be smeared than any other material (Fig. 7.10). Smear-resistant effect was
also demonstrated by exterior concrete walls coated with TiO z . When the sample
panels coated with TiO z and regular concrete panels were placed outdoors for six
months, soiling was very conspicuous on regular concrete panels, while the
photocatalytic exterior material was not soiled at all. This photocatalytic building
material is projected to have a life of ten years or longer.
The highly amphiphilic photocatalyst seems to have many extra benefits.
For example, the fact that water droplets are less likely to form means that it
dries quickly. The formation of dew on indoor glass panes, which often occurs in
winter, may be prevented by using glass treated this way. In vinyl greenhouse
farming, coating the inside surface of vinyl sheets with a superhydrophilic
photocatalyst can prevent the formation of dew, which, dropping on the farm
products, can cause rotting. Highly amphiphilic technology can also prevent
underwater bubbles from forming on a surface, suggesting many possible
applications based on this characteristic. The applications of superhydrophilicity
technology will not be limited to antifogging and self-cleaning. The scope ofthis
technology will no doubt extend to as yet unknown fields.
Photocatalytic technology inherently requires some quantity of light, and
highly amphiphilic technology is no exception. When it is to be used indoors, the
quantity of light provided by indoor lighting may not be sufficient. Our research
and development efforts recently solved this problem. Improving the sensitivity
to the weak light irradiation, we combined W0 3 with TiO z . In the future, we will
focus on the areas of material properties and lighting techniques so maximum
possible use can be made of available light to achieve the desired effect.
ceramic tile by sintering the TiO z sol at high temperature. However, we are very
close to developing a coating paint containing photoactive TiO z powders which
is stable under UV irradiation and can be fixed on any type of materials such as
plastics, paper, etc. at near room temperature. Therefore we will be able to add
photofunctionalized functions of TiO z described here to essentially any type of
material.
There already exist various types of materials possessing anti-fouling
functions. However, the self-cleaning function presented here is based on a
completely new and different concept. The already available anti-fouling materials
are treated so as to resist staining, but the materials discussed here can decompose
or remove staining substances automatically even after becoming stained. In
order to maintain a clean environment, we must often use chemicals such as
deodorants, germicides and detergents. In contrast, TiO z materials make it possible
to maintain clean conditions without using any chemicals. We believe that TiO z
photocatalysis (photocatalytic decomposition and photoinduced high wettability)
using very low-intensity light will open new avenues in environmental science.
References
Photocatalytic technology has been applied to the decomposition of noxious
gases, odor-causing gases, dirt, among others. In other words, that particular
technology involves materials that have been carried to the surface from the
surrounding environment. On the other hand, the highly amphiphilic effect is
based on the concept of altering the properties of the surface itself by
photocatalytic action. While both technologies are applied to the prevention of
soiling, their basic mechanisms are quite different from each other. The basic idea
is that the same TiO z material can have both types of properties, photocatalytic and
highly amphiphilic, in varying proportions, depending on the composition and the
manufacturing process. For the application of TiO z photocatalysts in various
fields, the desired proportions of these properties should be "engineered in."
We have described how TiO z coated materials exhibit deodorizing, sterilizing
and self-cleaning effects under weak UV light. So far, TiO z is not a promising
semiconductor for collecting solar energy, because it absorbs only UV light. We
propose here a new field of TiO z photochemistry in which TiO z is used as a
photo active coating material. In this case the disadvantage of TiO z in terms of
solar energy conversion turns out to be an advantage, i.e., it is colorless so that we
can use it as a final coating on colored materials. In addition, only a small number
of photons are used so it is not harmful to humans but effective for organic
compounds existing at low levels. We used photoactive TiO z films prepared on
I. A. Fujishima and K. Honda, Nature, 238, 37 (1972),
2. A. Heller, Acc, Chem. Res,,14, 154 (1981),
3. H. Gerischer, Photovoltaic and Photoelectrochemical Solar Energy Conversion, (F. Cardon, Y.
P. Gomes, W, Dekeyser, eds.), Plenum Press: New York and London, 1981; p L99,
4, B, O'Regan and M. GrazeI, Nature, 353,737 (1991),
5, M. Fujihira, Y. Satoh and T. Osa, Nature, 293, 206 (1981),
6. T. Sakata and K. Hashimoto, Nouv, J. Chim., 9, 699 (1985),
7, T, Sakata, Photocatalysis, (N. Serpone and E. Pelizzetti, eds.), John Wiley and Sons: New York,
1989; p. 311.
8, K. R.Thampi, J. Kiwi and M. Graze!, Nature, 327, 506 (1987).
9. R, Cai, Y. Kubota, T. Shuin, H, Sakai, K. Hashimoto and A. Fujishima Cancer Research 52
2346 (1992). " ,
10. D. F. Ollis, Photochemical Conversion and Storage of Solar Energy, (E. Pe!izzetti and M.
Schiavello, eds.); Kluwer Academic Publishers: Dordrecht, Boston and London, 1991; p. 593,
II. Purification and Treatment of Water and Air, (D. F. Ollis and H. AI-Ekabi, eds,), Elsevier,
Amsterdam, 1993.
12. A. Mills, R .H, Davies and D, Worsley, Chem. Soc, Rev" 417 (1993).
13. S. Yamazaki-Nishida, K. J, Nagano, L. A. Phillips, S. Cervera-March and M. A, Anderson, J.
Photochem. Photobiol. A; Chem" 70, 95 (1993):
14. H. Hidaka, J. Zhao, E, Pelizzeti, N. Serpone, J. Phys, Chem., 96, 2226 (1992),
15. C. Wei, W.-Y. Lin, Z. Zainai, N. E. Williams, K. Zhu, A. p, Kruzic, R. L. Smith and K,
Rajeshwar, Environ. Sci, Techno/., 28, 934 (1994).
16. A, Heller, in: Photocatalytic Purification and Treatment of Water and Air, (D. F, Ollis and H.
AI-Ekabi, eds.), p.139, Elsevier, Amsterdam (1993).
17, A. Fujishima, K, Hashimoto, T. Watanabe, Ti0 2 Photocatalysis - Fundamentals and Applications,
Bkc, Inc,: Japan (1999),
18, R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi
and T. Watanabe, Nature" 88, 431 (1997),
19, R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A, Kitamura, M. Shimohigoshi
and Watanabe, T. Advanced Materials, 10, 135 (1997).
20. A. Nakajima, S, Koizumi, T. Watanabe and K. Hashimoto, Langmuir, 16, 7048 (2000).
21. R. D. Sun, A. Nakajima, T. Watanabe, A. Fujishima and K. Hashimoto, J, Phys. Chem., 105, 1984
(2001),
22. N, Sakai, A. Fujishima, T, Watanabe and K. Hashimoto, J. Phys. Chem., 105, 3023 (2001).
23. N. Sakai, R, Wang, A. Fujishima, T, Watanabe and K. Hashimoto, Langmuir, 14, 5918 (1998).
24. R. Wang, N. Sakai, A, Fujishima, T. Watanabe and K. Hashimoto,J. Phys. Chem. E, 103,2188
(1999).
25. T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima and K, Hashimoto,
Thin Solid Films, 351, 260 (1999).
7.3 Conclusions

122 7 Self-cleaning Properties of TiOTcoated Substrates
26. M. Miyauchi, A. Nakajima, A. Fujishima, K. Hashimoto and T. Watanabe, Chem. Mater., 12,3
(2000),
27, T, Noguchi, P. Sawunyama, K. Hashimoto and A. Fujishima, Environ. Sci. Techno!., 32, 3831
(1998),
28. S, Sitkiewitz and A. Heller, NewJ. Chem., 20, 233 (1996).
29. J. Fan and J. T,Yates, Jr., J. Am, Chem. Soc" 118,4686 (1996),
30. A. Sclafani and J. M. Herrmann, J. Phys. Chem., 100, 13655 (1996).
31. Y. Kikuchi, K. Sunada, T. Iyoda, K. Hashimoto and A. Fujishima, J. Photochem, Photobio. A:
Chem" 106, 51 (1997).
32. K. Sunada, Y. Kikuchi, K. Hashimoto and A. Fujishima, Environ. Sci. Techno!., 32, 726 (1999),
33, p, Sawunyama, L. Jiang, A. Hashimoto and K. Hashimoto,J. Phys, Chem, B, 101, 11000 (1997),
34. P. Sawunyama, A. Fujishima and K. Hashimoto, Chem. Commun" 2229 (1998).
35. P. Sawunyama, A, Fujishima and K. Hashimoto, Langmuir, 15, 3551 (1999).
36, T, Minabe, D. A. Tryk, p, Sawunyama, y, Kikuchi, K. Hashimoto and A, Fujishima, J.
Photochem, Photobio. A: Chem., 137,53 (2000),
37, T. Minabe, p, Sawunyama, Y. Kikuchi, A, Fujishima and K, Hashimoto, Electrochemistry, 67,
1132 (1999).
8
Cleaning Atmospheric Environment
8.1 Introduction
In Jglpan, serious air pollution due to SOx and other pollutants noted since the
1950s, has been significantly diminished by the efforts of both governmental and
industrial sectors. However, even now the concentrations of nitrogen oxides
(NO x ) and suspended particulate matter (SPM) in air remain high, particularly in
big cities, and these often exceed the atmospheric environmental standards. For
example, the atmospheric environmental standard for photochemical ozone or
surface ozone has been met at only 2% of the air monitoring stations. More
recently, much attention is being paid to hazardous air pollutants that are assumed
to affect human health if they are taken for long periods even at very low
concentrations. Atmospheric environmental standards have been set for benzene,
trichloroethylene (TCE) and tetrachloroethylene (PCE). Higher concentrations
of benzene than its standard (3J1g/m 3 as annual average value) have been observed
at many monitoring places. Furthermore, a special law for dioxins
(polychlorodibenzo-para-dioxins, polychlorodibenzofurans and coplanar
polychlorobiphenyls) was passed in late 1999.
Various technologies to deai with these atmospheric environmental issues
have been developed and applied. The amounts of NO x and SPM emitted from
stationary sources have decreased, but" those from motor vehicles, especially
diesel-powered vehicles, have not been suppressed due to technological
NOx
SI'M (Uhlck smokc
Vohltilc Org,mics
Ozonc
lIazm'dous Air'
J'ollulants
Dioxins
Jndocl'inc d iuuptol"5
Soils
Rivcn
L'lkcs
J'ormaldchydc
Tolucnc, I nsccticidcs
II iO-;lcl'oso!5
Indoor
Spaccs
Fig. 8.1 Air pollutants and need for environmental purification technology.

124
8 Cleaning Atmospheric Environment
8.2 Photocatalytic Activities of TiO z
125
difficulties, the continuous increase in the number of vehicles and other reasons.
Since areas along major roads of big cities are heavily contaminated with NO x and
SPM, the public sector must conduct more countermeasures to reduce NO x and
SPM.
Given the situation, new technologies which can be applied to purification of
polluted air as well as to remediation of contaminated soils, underground water
and others, as shown in Fig 8.1, must be developed. Air pollutants to be treated
are present at concentrations lower than several ppm and at room temperature.
Thus technologies different from the conventional technologies treating air
pollutants from emission sources such as automobiles and boilers must have the
following characteristics. They must I) be able to treat pollutants at room
temperature, and 2) be able to treat huge amounts of air.
We have been investigating heterogeneous chemical reactions among gaseous
pollutants and particulate matters consisting of various metal oxides in the
atmosphere. As one of the results it was found that the adsorption and oxidation
of SOz and NO z on coal-burning fly ashes and soil particles were remarkably
enhanced by photoirradiation. l ) Of the metal oxides in the particles, titanium
dioxide (TiO z ) showed the highest photocatalytic activity for oxidation of SOz and
NO z to sulfuric acid (H z S0 4 ) and nitric acid (HN0 3 ), which were captured on
TiO z . This result suggests that TiO z can be used as a photocatalyst for removing
ambient SOz and NO x .
Table 8.1 Rate constant (k) for the reaction of OH radical with air pollutants at 298 K (Unit: 10- 13 cm 3
mol- l S-l)
Pollutant k Pollutant k Pollutant k
CO 1.3 CH 3 0H 7.9 t-2-C 4 H s 700
NO z 670 CzHsOH 1.6 CH 3 CCl 3 0.1
NH 3 1.6 CH 3 COOH 8,0 CHCl = CClz 21
SOz 20 CH 4 0.06 CCl z = CCl z 1.7
CH 3 SH 330 C Z H6 2.5 C 6 H 6 10
HzS 48 C 3 H S 11 Toluene 61
HCRO 92 Cz 90 m-Xylene 240
CH 3 CHO 200 C 3 H 6 300 C 6 H s CI 6
8.2 Photocatalytic Activities of Ti0 2
The active oxygen species can oxidize hazardous air pollutants (organics) to
carbon dioxide (CO z ) and hydrogen halides (HX) when organics are halogenated
compounds, and can oxidize SOz and NO x (inorganic air pollutants) to H z S0 4
and HN0 3 . Of the oxygen species, OH is a well-known radical, which plays
major roles in atmospheric chemistries in the gas-phase.
The rate constants for reactions of OH with pollutants are summarized in
Table 8.1. Rate constants can be used to estimate how easily a pollutant can be
treated by photocatalysis. NO z shows a higher rate constant with OH than any
other inorganic compound in Table 8.1, while CH 3 SH, CH 3 CHO, propylene, m-
xylene appear to be oxidized faster than other organic compounds.
8.2.1 Oxidation of Air Pollutants by Photogenerated Active Oxygen
Species
8.2.2 Photocatalytic Reactions of Volatile Hydrocarbons
TiO z is considered to be a promising material for treatment of air pollutants
through its highly strong oxidative ability. As shown in Fig. 8.2, on the TiO z
surface illuminated with light shorter than 400 nm, active oxygen species of
hydroxyl radical (OH), 0-, 0 3 -, super oxide (Oz-), and others are produced from
reactions of positive holes (h+) and electrons (e-) with Oz in air, HzO or OH-
group, lattice oxygen of TiO z .
Volatile hydrocarbons contained in gasoline, solvents and other substances are
likely to be emitted into the atmosphere. Since photochemical reactions between
volatile hydrocarbons and NO x cause the formation of photochemical smog
(surface ozone), stricter emission control is strongly desired.
A. Propylene Z )
UV light. (Sunl )
NO, VOCs
I\rl.ifidallight)
/\
/ "
"'z.\ ( I e' '\: :\ VOCs
)" . ()2' / N02
 '\.  11 ----.. + C02. IIX
Ti02 h+ 
i
\
\
We have studied photochemical reactions between propylene and NO x in air
with/without powders of metal oxides dispersed on the bottom of a flat Pyrex glass
'cell, connected to a gas circulation system. The cell was irradiated with a high
pressure Hg lamp, but light shorter than 300 nm was cut off by the Pyrex glass.
Although thorough analysis of reaction products could not be performed due to the
small volume of the reactor, the time profile of acetaldehyde (CH 3 CHO) in Fig.
8.3 and the final yield of CO and,CO z in Table 8.2 were obtained. For the gas-
phase reaction where atomic oxygen and OH played maj or roles in the
photochemical reaction between propylene and NO x , the major product of
CH 3 CHO continuously increased with time, and smaller amounts of CO and CO z
were produced. Very similar results were found for some oxides of metal (Co, Ni,
Cr), i.e., the metal oxides had little effect on the gas-phase reaction. For SnOz and
Fez03, a slight decrease in CH 3 CHO concentration was observed, indicating that
these metal oxides could decompose CH 3 CHO to some extent. The decomposition
ofCH 3 CHO took place more clearly for W0 3 and ZrOz [Curve (c)]. The reaction
1120
Fig. 8,2 Principle of photocatalytic oxidation of various air pollutants under UV light illumination,

]26
8 Cleaning Atmospheric Environment
0,"" r l1VLV\;<1L<11YU'-' .M.'-'U v iUto:, VI IlV2
ILl
(al
was confirmed from the photochemical reaction of CO instead of C 3 H 6 that the
activity of metal oxides for CO oxidation was SnOz, ZrOz, W0 3 « TiO z < ZnO
< CeOz and that the other metal oxides were not active. It is presumed that CO
formed primarily in the gas-phase accumulates at a constant conversion rate from
C 3 H 6 , and is further oxidized to CO z on ZnO and CeOz. For CO oxidation by TiO z ,
more details are given below in connection with benzene oxidation.
Formation of CO z was remarkably affected by the presence of metal oxides.
The amount of CO z formed and the ratio of CO z to CO are given in Table 8.2. The
formation of CO 2 was greatly enhanced over the oxides of Zr, Sn, W, Zn, Ti and
Ce, while it remained unchanged at the blank level for the oxides of Cr, CO, Ni
and Fe. The difference between the two groups of metal oxides is more clearly
demonstrated from the concentration ratio of CO z to CO. The value of the ratio
becomes greater than' 1.0 from SnOz and rapidly increases to CeOz, showing an
increasing activity for oxidation. It should be noted that the active metal oxides
are n-type semiconductor oxides. Fez03 is also an n-type oxide, but the band gap
energy is the lowest of the n-type oxides investigated here.
80
.-
.-
,/ "'-
60 /:.........."'- (bl
E /:/
a. //
a. Ie)
C1J 'l .--
"tJ
>- A __. --
.c. 40
C1J
 ./ ............
:3
C1J 1/ //
u
.q:
20 11/
A .
"'-"'-"'-'" (dl
...- 4_..._
T__ (el
'Y--"'--"'_'Y_
20
40
60
80
o
Reaction time (minI
Fig, 8.3 Profiles of acetaldehyde formation for photochemical reaction of C3H6-NOz-dry air in the
presence of various metal oxides at ambient temperature.(a) blank, CoO, NiO, Cr203, (b)
Fe203, Sn02, (C)Zr02' W0 3 , (d)ZnO, (e)Ti0 2 , Ce02' Initial concentrations of C 3 H 6 and N0 2
are 200 and 100 ppm,respectively.
(From K. Takeuchi, T. Ibusuki, Atmos. Environ., 20, 1155 (1986»
B. Toluene 3 )
Metal oxide CO/C 3 H 6 dec (%) CO 2 (ppm) CO 2 /CO
Blank 9,2 ] 8,9 0,57
Cr203 ILl 19.4 0.4:i
CoO 9.0 18,8 0.56
NiO 7.4 18.2 0,63
Fe203 9.1 18.6 0.57
Sn02 8.0 32.0 1.20
Zr02 10.2 45.1 1.2]
W0 3 8.4 72.0 2,63
ZnO 5.5 122 6.44
Ti0 2 7.5 326 10,19
Ce02 5.3 322 14.76
FAI 10.3 28.9 0.74
FA2 11.4 39,1 0.96
FA3 9.1 27.3 0.82
(From K. Takeuchi, T, Ibusuki, Atmos, Environ" 20, 1155 (1986»
A flow-type photochemical reaction system (Fig. 8.4) was developed for
studying toluene photochemical reactions in the presence of TiO z with/without Oz,
HzO or NO x , because toluene is the most abundant volatile organic compound in
air, and TiO z has such a remarkable photocatalytic activity as mentioned above
and is present in airborne and soil particles at an order of about 1%.
The result obtained for a reaction systm of toluene (80 ppm) - NO z (80
ppm) - Oz (0-20 %) over TiO z (16 g) in the dry condition and a reaction time of
'10.3 min is shown in Fig. 8.5. The result for the reaction system of toluene (80
ppm) - air - HzO (relative humidity: 0-60 %) over TiO z (16 g) and a reaction time
of 10.3 min is shown in Fig. 8.6. As can be seen from Fig. 8.5, even in the
absence ofOz, a reaction took place to produce CO z (5.4 ppm) and benzaldehyde
(BA, 1.3 ppm). This may indicate the oxidation of toluene by 0 (formed according
Table 8.2 Ratio of CO formed to decreased propylene, amounts of CO 2 formed, and ratio of CO 2 to
CO formed at a reaction time of 60 min
I
I c 11=
-
-
sampling
-
k
j
over ZnO gave much smaller amounts of CH 3 CHO with a maximum in the profile,
showing the catalytic decomposition of CH 3 CHO on ZnO. This tendency is
stronger for TiO z and CeOz [Curve (e)]. Even at the initial reaction stage, the yield
of CH 3 CHO decreased strikingly and only a trace amount was detected after 60
min reaction time.
The amount of CO formed after a reaction time of 60 min ranged from 19 to
43 ppm (blank reaction: 33 ppm). As shown in Table 8.2, however, the ratio of CO
formed to decreased propylene lies between 9::1:2% except for ZnO and CeOz. It
Fig. 8.4 Schematic diagram of a flow-type photochemical system. (a) Quartz glass cylinders on which
Ti0 2 was coated, (b) outer cylinder (35mm o.d, x 1200mm), (c) 60W black light, (d) reactant
gas mixture (nitrogen), (e) nitrogen, (f) oxygen, (g) air, (h) thermal mass flow controller, (i)
humidifier, (j) gas-bubblers, (k) pump,
(From S. Kutsuna et aI., Atmos. Environ., 27A, 599 (1993»)

128
8 Cleaning Atmospheric Environment
</I

g 2
"'0
o
...
a.
15
3
E
a.
a.
O,L, rnULUCi:lLi:UYllC .t\.CllYHlt::s UJ J IV2
1.L
c
.Q

a
...

c
Q)
u
c
o
U
o
Ni trotoluenes
10 20
30
concentration was similar to that for BA.
Figure 8.6 shows the effect of water vapor on the formation of CO z and BA
for the toluene oxidation. The concentration of CO 2 increased linearly with the
increase in relative humidity. The yield at 60% relative humidity was greater by
one order than that in the dry condition. It has been pointed out that the amount
of O 2 adsorbed under photo illumination is proportional to that of hydroxyl groups
(OH-) on the TiO z surface. 4 ) The phenomenon has been explained by the following
reactions:
E
a.
20 ,Sl
(\J
0
u
....
0
c
,2
OJ
10 a
...
-I-'
C
Q)
U
C
0
U
Ti0 2 + hv  exiton (h-e)  h+ + e-
OH- ads + h+  C?H ads
02ads + e-  02-ads
O 2 concentration (%)
Fig, 8.5 Effect of O 2 initial concentration (0, 7,5, 15 and 20%) on formation of the products in the
heterogeneous photooxidation of toluene for Ti0 2 -toluene (80 ppm)-N0 2 (80 ppm) system at
a reaction time of 10.3 min.
(From T, Ibusuki, K, Takeuchi, Almas, Environ" 20, 1711 (1986»)
Fig. 8,6 Effect of relative humidity (0, 30, 50 and 60%) on formation of CO 2 ad benzaldehyde in e
heterogeneous photooxidation of tolene for Ti0 2 -toluene (80 ppm)-alr system at a reactIOn
time of 10.3 min.
(From T. Ibusuki, K. Takeuchi, Almas. Environ" 20, 1711 (1986))
E
a.
a.
2
Q)
"'0
>-
L:

a
N
c
Q)
s:J
....
0
c
,Q

a
...
-I-'
C
Q)
U
C
0
U
0
20 40 60
Relative humidity (%)
200
E
a.
a.
(\J
0
u
.....
100 0
c
.2

a
...
-I-'
C
Q)
U
C
0
U
Under photo illumination TiO z produces excitons and positive holes (h+), which
react with OR-ads to generate OH ads radicals. Once the holes are trapped, the
photoelectron (e-) is free to participate in the adsorption of oxygen. (n other
words, OR-ads can promote the adsorption of oxygen to the Ti0 2 surface. The
dependence of CO 2 formation on the relative humidity, shown in Fig. 8.6, suggests
hat the increase in the watr vapor concentration in the gas-phase results in
increase in the amount of OH- ads hence the amount of 02-ads greatly increases. The
QH radicals as well as O 2 - and other oxygen species generated on the surface of
riOz have the potential to oxidize toluene and BA, as has been mentioned before.
It seems reasonable that the yield of BA sharply declined with the increase in
ielative humidity. It should be mentioned that the increase in relative humidity,
i.e., the increase in OH- may compete and/or hinder the adsorption of toluene on
the surface of Ti0 2 , hich may result in retardation of toluene oxidation, since the
photocatalytic oxidation is considered to take place on the Ti0 2 surface. The
effect of water vapor on the photocatalytic oxidation of hydrocarbons,
eDhancement or retardation, may thus depend on their initial concentration, their
adsorptivity and other factors.
'"
C. Benzene 5 )
[
1. Important role of water
Benzene is an important feedstock and component of gasoline. It is widely
used as anti-knock additive and solvent. However, benzene must be removed
from the flue gases that are emitted from petrochemical plants, petroleum tanks,
oke ovens, distillation towers and the installations in which benzene is used as
a'solvent, due to its carcinogenicity.
I Photocatalytic decomposition of benzene over Ti0 2 in gas-phase at room
.f
emperature was studied with a flow-type photochemical reactor similar to that
show in Fig. 8.4, at room temperature. The main objective of the study described
here was to evaluate the dependence of the product distribution on reaction
nditions and to elucidate the role of Oz and H 2 0 in the photoreaction.
Figure 8.7 shows the time course of benzene photooxidation reaction over
Ti0 2 under a humidified condition (relative humidity: 65%). No reaction
to N0 2 photolysis: NO z + h v  NO + 0) and/or 0- (produced by e-:- 0). In the
presence of O 2 , Ti0 2 under photo illumination produces some active oxygen
species as has previously been mentioned, which may oxidize toluene. As shown
in Fig. 8.6, 16 ppm of CO z and 1.1 ppm of BA were produced in the absence of
NO z . It should be noted that the ratio ofBAlC0 2 (1.1/16 = 0.06) is much smaller
than that (1.3/5.4 = 0.26) in Fig. 8.5, This may be interpreted in terms of the
difference in activity between the active oxygen species derived from N0 2 and O 2 .
In Fig. 8.5, the formation of nitrotoluenes is observed, even though the
concentration was smaller than that of CO 2 or BA. Nitrotoluenes may be produced
from the addition of N0 2 to toluene on the surface of Ti0 2 . It is therefore
reasonable that the dependence of nitro toluenes formation on the N0 2 initial

130 8 Cleaning Atmospheric Environment
400
8.2 Photocatalytic Activities of Ti0 2 131
100 100
E
0.. 
0.. 80 80
-- --
0 c.:
U 0 
-0 U'J
..... 60 --
c: (!) .... 60 >-,
'"
> '-'
,.., c.: .
0 0 >
.
u CJ '-'
...... 0 40 40 CJ
0
0 c:: -
c: 0 0
.2 N (()
 c.:
(!) 20 - CO
E  20
0 ' I ..
u...
0 0
0.0 1.0 2.0
H 2 0 concentration / %
benzene feed off t r dark, air purge
100 00-0-00'0'00-0-00-0.00- -- ---------,.- 0-00-0'0
 J
........... ...
 .. .. ..
80 .
500
480
c:
,2
'"
t 60
>
c:
o
tJ
1) 40
c:
V
N
C
1) 20
co
CO 2
/"" benzene feed
I,,'
- 300
(Stage III)
"" - 200
-
henzene feed
100
00000
0
0 60 120
(Stage J)
(Stage II)

co
00000000000000
o
o
60
120
180 240 300 0
60
Time / !nin
Fig. 8.7 Time course of photooxidation reaction of benzene with Ti0 2 under humidified conditions.
Benzene (80 ppm), O 2 (20%), water vapor (2,2.%), total flow-rate (l00 ml min- 1 ), and catalyst
0.24 g.
(From H. Einaga et aI" Phys. Chern, Chern. Phys" 1,4903 (1999))
Fig. 8.8 Change in benzene conversion and selectivity to CO 2 and CO with concentration of water
(From H. Einaga et aI" Phys. Chern. Chern. Phys., 1,4903 (1999))
proceeded without catalyst or in the dark. On irradiation of the catalyst, benzene
conversion reached approximately] 00%, while CO z and CO are formed with an
induction period (Stage I). No other products were detected in the effluent gas
stream. The amount ofCO z and CO formed became constant after ca. 120 min. At
this time, 97% of carbon mass balance was obtained and the seletivities to CO z
and CO were 93% and 7%, respectively. The color of the catalyst changed from
white to light brown. After 6 h photoreaction, irradiation was stopped and the
reactor was purged with humidified air for 30 min When irradiation was started
once again without benzene feed (Stage II), CO 2 and CO were formed and the
color of the catalyst surface changed back to white. The total amount of CO 2 and
CO formed in Stage II is estimated to be 40Jimol g-l, and the agreement for the
overall carbon balance is achieved with respect to the total amount of benzene
decomposed and that of CO z and CO produced in Stages I and II. The
photoreaction was carried out once again with the feed of benzene after the gas-
solid adsorption equilibrium in a reactor was achieved (Stage III). The reaction
profile seems very similar to that in Stage 1.
That the carbon mass balance was not perfect in the initial period of Stage 1
may be interpreted as due to the formation of polymeric compounds on the catalyst
surface. The formation of CO 2 and CO in Stage II is considered as a result of
decomposition of these products. The consistency for the total carbon balance
through Stages I and II suggests that the products on the catalyst surface were
completely oxidized to CO z and CO.
The dependence of the benzene conversion and the selectivity to CO 2 and CO
on the concentration of H 2 0 is shown in Fig. 8.8. The values were measured at the
stationary state. The carbon mass balance is in the range of97-103%. The benzene
conversion decreased with decreasing H 2 0 concentration. The selectivities to
CO 2 and CO were 93% and 7%, respectively, and the values were almost
independent of the H 2 0 concentration in the range of 0.8% to 2.2%. In dry air, the
benzene conversion was 5% and the selectivities for CO 2 and CO 90% and 10 %,
::1

--
..--..
8

u::
3800
3600
3400
3200
Wavenumber / cnr I
Fig. 8.9 Diffuse reflectance IR spectra of the Ti0 2 samples,
(a) fresh sample irradiated in humidified air for 5 h; (b) the deactivated sample after benzene
photooxidation in dry air for 5 h; (c) the sample after the deactivated Ti0 2 was irradiated in
humidified air without benzene feed.
(From H. Einaga et aI" Phys. Chern, Chern. Phys" 1,4903 (1999))
respectively. In this stage, the catalyst was significantly browned. The amount of
carbon deposited on the catalyst surface was estimated to be 310 Jimol g-I. This
value corresponds to 0.4 times the surface monolayer ofthe catalyst. It is therefore
deduced that the accumulation of the carbon deposits contributes to the catalyst
deactivation to some extent.
Figure 8.9 shows the diffuse reflectance IR spectra of Ti0 2 samples after
the photoreaction. The band due to the hydroxyl groups is observed at 3,665 cm- I
for the fresh Ti0 2 (a), consistent with that for anatase. 6 ) The band at 3,630 cm- I
is assignable to the adsorbed water,7) which may originate from the ambient air

132
8 Cleaning Atmospheric Environment
8.2 Photocatalytic Activities of Ti0 2
133
during the preparation of the sample. The band at 3,665 cm- I is completely
diminished for the TiO z sample, which was used for the photoreaction in dry air
for 5 h (b), indicating that the hydroxyl groups were consumed in the absence of
H 2 0. After the Ti0 2 was irradiated in humidified air without benzene feed, the
band was restored ( c), that is, the hydroxyl groups were reproduced. It is thus
inferred that the dehydroxylation-rehydroxylation cycles take place during the
photoreaction in the presence of H 2 0. The band at 3,520 cm- I can be assigned to
another kind of hydroxyl group, since its behavior is similar to that at 3,665 cm- I ,
As described above, the presence of H 2 0 not only retards the formation ofthe
carbon deposits on the catalyst surface, but also enhances their oxidation to CO z
and CO. H 2 0 regenerates the Ti0 2 surface hydroxyl groups which are consumed
in the photoreaction. Based on these results, the following reaction mechanism is
proposed focusing on the role of the hydroxyl groups.
OH radical formed from OH- and h+ rapidly adducts benzene to form a
cyclohexadienyl radical, which is subsequently oxidized to a peroxy radical in the
presence of O 2 . The peroxy radical transforms to the various intermediates
including phenol, hydro quinone and to CO and CO 2 . In the presence of H 2 0, the
surface hydroxyl groups consumed in the photoreaction are regenerated, leading
to a successive catalytic cycle. Although the mechanism for the decomposition of
the polymeric compounds is not well understood, it can be deduced that the OH
radicals produced from the hydroxyl groups playa very significant role in the
decomposition reaction.
In this study, we suggest that CO was not the intermediate of CO 2 in the
photo oxidation of benzene. It was also shown that the selectivities to CO z and CO
were 93% and 7%, respectively, and almost independent of the concentration of
Oz, and H 2 0 in the range of 0.8% to 2.2%. The invariability of the values may
imply that the formation of CO 2 and CO may proceed in different pathways in the
photoreaction and the contribution of each pathway may not be much changed by
varying these conditions.
When the gas stream was changed from humidified air to dry air, the amount
of carbon deposits, very probably attributable to the polymeric products, increased.
In this stage, as the surface hydroxyl groups were consumed, the probability for
the direct reaction of hole with benzene (formation of benzene cation radical) may
be increased. The benzene cation radical formed on the solid surface may react
with benzene, which is one of the main steps in the polymerization. 8 )
It was found that the benzene conversion decreased with increase in the
benzene concentration, while the selectivities to CO z and CO remained constant
at 93% and 7%, respectively. The browning of the catalyst became remarkable as
the benzene concentration increased. Table 8.3 shows the amounts of carbon
deposits on the catalyst surface at the steady state. The amount increases with
increase in benzene concentration. The addition reaction of benzene to the radical
intermediates formed during the photoreaction was promoted with the increasing
amount of benzene, facilitating polymerization as a result.
The result obtained here is considered very valuable for optimizing the gas-
solid photocatalytic system for the purification of the airstream polluted with
benzene: the performance can be improved by the addition of water vapor to the
airstream.
80
132
179
257
79
144
163
220
2. Remarkable effects of pt supported on TiO z on formation and stabilization
of active oxygen species
Ti0 2 can oxidize benzene very efficiently, but about 7% of the benzene was
converted to CO, which is one of air pollutants regulated by air quality law in
Japan. As mentioned before in Section 8.2.2A, TiO z is not so active for oxidation
of CO to CO 2 , compared with ZnO and Ce02. It has been reported that the
efficiency of the photoreaction over Ti0 2 increased with the doping of transition
metals, which has been interpreted in terms of either the suppression of hole-
electron recombination or the electron trapping by the metal. However, it was also
shown that metal dopants generated sites, which increase electron-hole
recombination center. These metallTi0 2 catalysts have been applied to the
photo oxidation of organic compounds, but both positive and negative effects
have been observed. The difference in the reactivity of the photo catalysts may
depend on the kind of active oxygen species and that of reactants. It is thus
necessary to have further and reliable information on active oxygen species
photoformed on the metal/Ti0 2 .
Ti0 2 powders, P-25 were used as the precursors and the reference sample.
Pt/Ti0 2 was prepared by photo deposition from an ethanol-water Ti0 2 suspended
solution containing H 2 PtCI 6 /6H 2 0 in a Pyrex vessel with vigorous stirring under
N 2 atmosphere. The Pt/Ti0 2 powders were washed with purified water and dried
at 110°C for 30 min. The amount of Pt on Ti0 2 was estimated to be about 1.0
wt%.
Photo oxidation of CO was carried out with a flow reactor at room temperature
(303 K), Prior to each experiment, the samples were calcined at 473 K for 2 h in
vacuo. The samples were hydoxylated by being exposed.to water vapor (20 Torr).
UV irradiation was carried out with a UV lamp equipped with a UV-30 cut-off
filter « 300 nm).
The addition of Pt to Ti0 2 enhanced its reactivity for the CO oxidation. Fig.
8.10 shows a time profile for the oxidation. The reaction did not proceed at all
before the UV irradiation (Stage I). After the Pt/Ti0 2 sample was irradiated in
humidified air (reltive humidity: 65%; flow rate: 100 ml min- 1 ) without CO
feed for 30 min at room temperature, the lamp was turned off and the gas flow was
switched to the dry air containing 400 ppm of CO (Stage II). In this stage, CO was
completely consumed in the initial period (5 min), while 340 ppm of CO 2 was
formed. Although the amount of CO consumed and that of CO 2 formed decreased
with time, they were recovered by repeating the same pretreatment for 10 min
(Stage III). The total amount of CO 2 formed in Stage II was estimated to be 23
mmol, which corresponds to twice the amount of Pt on the catalyst. No reaction
Table 8.3 Effect of the benzene concentration on the amount of the carbon deposits
accumulated on the catalyst surface
[Benzene] / ppm
Amount of carbon deposits / llmol g-I
Reaction conditions: water vapor (1.7%), space time 5.3 x 10 5 g min mol-I.
(From H, Einaga et ai" Phys. Chern, Chern, Phys" 1,4903 (1999»

134
8 Cleaning Atmospheric Environment
lI.L t'notoCatalYUc A.Cuvmes or IlV2
1..D
occurred when Ti0 2 was used instead of Pt/Ti0 2 .
These findings indicate that the reactive species responsible for the CO
oxidation are formed on the UV irradiated Pt/Ti0 2 catalyst. In order to identify the
reactive species, ESR measurement was carried out for the Pt/Ti0 2 sample. Fig.
8.1l(a) shows the electron paramagnetic species formed on the TiO z samples as
a reference. First, the samples were irradiated with UV light in the presence of
water vapor (20 Torr) and O 2 (30 Torr) for 10 min at room temperature. After the
irradiation was stopped, the sample was evacuated to ca. 0.05 Torr in the dark,
followed by the ESR measurements at 77 K. On the Ti0 2 surface, the
paramagnetic species assigned to O 2 - were observed with gl = 2.025, gz = 2.009,
g3 = 2.002, together with the reduced TP+ species.
On the other hand, when the Pt/Ti0 2 sample was irradiated with UV light, the
other paramagnetic species appeared with g// = 2.007, gl = 2.000, overlapped with
the signal of O 2 - (Fig.8.ll(b». This species could be assigned to 0 3 -, which
possesses a weak covalent bonding between the p electrons of the oxygen
molecule and the free electron in 0-. It has been reported that this type of 0 3 - was
reactive for the CO oxidation.
(a)
°2
g. g g)
2,0 mT
I------i
III
Ti 3 +
(b)
° 3 " gll gJ.
n
CO + 0 3 -  CO 2 + O 2 -
The 0 3 - species were not detected before the UV irradiation, showing that it
was photochemically formed on the sample. Although 0 3 - was observed on Ti0 2 ,
Ti0 2 /Si0 2 on UV irradiation at low temperature (77-137 K), it was unstable and
readily decomposed at room temperature. 9 ) This is consistent with the fact that 0 3 -
has not been detected on the Ti0 2 sample, as shown in Fig. 8.l1(a). However, 0 3 -
remained on the Pt/Ti0 2 sample in the dark at room temperature. It can thus be
concluded that the addition of Pt enhances the stability of 0 3 - on the Ti0 2 .
It has been suggested that the hole center 0- generated from the lattice oxygen
LJJ
g, gz gJ
Oz"
Fig. 8.11 Electron paramagnetic species formed on (a) Ti0 2 and (b) Pt/Ti0 2 samples, which were UV
irradiated in the presence of water (20 Torr) and O 2 (30 Torr) at room temperature, followed
by evacuation. The spectra were obtained at 77 K.
hv in humidified air !tv in humidified air
for 30 min without CO for 10 min without CO
, ,
400 400
I
I
S 0 I 8
 6
0... I 0...
0... I I 300 0...
300 dark I dark I dark
-- I --
0 I I N
I 1 0
U dry air I dry air 1 dry air
I I U
...... \ I
0 \ ......
200 I I 200 0
c: Stage I) \ (Stage II) I (Stage III)
0 \ 1 c:
\ I 0
0... \ \ '......
, \ 
8 5, \
\ E
;:j 100 \ 100
CIJ , .. , 0
c: , 'Q 
0 I ---a-
U --9---9- '
0 0
0 20 0 20 40 60 0 20 40 60
Time / min
(denoted by O 2 -). on UV irradiation subsequently reated with O 2 to form 0 3 -,
O 2 - + h+  0-, 0- + O 2  0 3 -
'...
Fig, 8.10 Time course for the CO oxidation over PUTi0 2 catalyst in air at room temperature. (D)CO
consumption, (0) CO 2 formation,
When the Pt/Ti O 2 sample was irradiated with UV light in the absence of O 2 ,
the 0 3 - signal was not observed. This indicates that O 2 is essential for the 0 3 -
formation and supports the scheme described by the above equation. In this
study, however, no clear ESR signal assigned to 0- was obtained: Pt/TiO z sample
evacuated to 10- 3 Torr after the UV irradiation did not give the signal of the hole
center 0- adjacent to a cation vacancy or the surface (V- or V s center). -The other
ype of 0- (free hole 0-) could not be detected in the ESR spectrum either, very
probably due to the rather short relaxation time of the species, caused by the
degeneration of the 0- p orbitals.
I The formation of 0- on the UV -irradiated Pt/Ti0 2 sample was presumed
from a result obtained for CO oxidation: when 13 Torr of CO was introduced into
the sample after the UV irradiation, a strong signal assigned to CO 2 - was observed
with gl = 2.003, g// = 1.999. The appearance of this signal suggests the presence
of 0- on the Pt/Ti0 2 surface, since it has been reported that the CO 2 - is generated

8.2 Photocatalytic Activities of Ti0 2 137
40
Ol-Cr
8 ,
30
0.
Co CZHCIJ
--
c consumed
0
.- 20 o CO z
....

....
- d
c
<1.1
c..I
C
0
U 10
10 20 30
Time I s
Fig. 8.12 Time course of C 2 HCI 3 consumed and reaction products for the C 2 HCl r Ti0 2 -air system:
initial concentration of C 2 HCl 3 was 30 ppm. Light intensity was 0.046 min- I .
(From S. Kutsuna et aI., Atmos, Environ" 27A, 599 (1993)
136 8 Cleaning Atmospheric Environment
by the reaction of 0- ions with CO. 10) When O 2 was introduced into the Pt/Ti0 2
sample on which CO 2 - was formed, followed by the evacuation, the CO 2 - signal
was greatly diminished and the Oz- signal appeared. This indicates that CO 2 -
reacts with Oz to form Oz-
CO + 0-  CO 2 -
CO 2 - + Oz  CO 2 + O 2 -
and that 0- is also the reactive species for the CO oxidation. It should be noted
that O 2 - has been reported to be inactive toward CO in the gas-phaseY)
In summary, it can be concluded that the photochemically formed 0- and 0 3 -,
the active oxygen species for the CO oxidation are stabilized on the Pt/TiO z , and
that Pt/Ti0 2 can oxidize CO (benzene) to CO 2 -
8.2.3 Photocatalytic Reactions of Halogenated Hydrocarbons
o
o
There are many kinds of halogenated hydrocarbons. Some of them like TCE
and PCE are widely used as solvents in metal parts, semiconductor washing, dry
cleaning, etc. As mentioned before, however, they are considered to be hazardous
air pollutants and suppression of their emission to the atmosphere is strongly
desired. Others include chlorofluorocarbons (CFCs), 1,1, i-trichloroethane (MC),
hydro chlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). CFCs,
HCFCs and MC have chlorine atom,s and are stable in the troposphere, because of
their slow reaction rates with OH radicals. They can diffuse into the stratosphere,
where chlorine atoms generated from photolysis decompose much ozone through
radical chain reactions. The production of CFCs and MC has been stopped.
Recycling and destruction of these substances are strongly recommended in order
to decrease their emission into the atmosphere. Similar countermeasures are
suggested for HFCs, since they have high global warming potential.
30
(A) ,Ol-cr
/
,
E .6
0.
0. 20
c:::
0
-
C1
I-
-
c:::
-:.>
:J 10 -
c:::
0
u
-5
..-..
><
-
'-' -10
c
--
A. Trichloroethylene (TCE) and Tetrachloroethylene (PCE)12)
.....
A flow-type photochemical reaction system similar to that shown in Fig. 8.4
was used for TCE or PCE - TiO z - air with/without water vapor. Fig. 8.12 shows
the time course of the consumption of TCE (C 2 HCb) and reaction products in the
heterogeneous photochemical reaction between Ti0 2 and TCE under dry
conditions. Within a reaction time of26 s, most TCE was transformed to CO 2 , CO,
COCh, CHCl 3 and trapped chloride ion (t-Cl-). The t-Cl- was determined by
subtracting the Cl- (from hydrolysis of eOCh) from the total CI- trapped in the
water, indicating that it originated mainly from HCI and Cl 2 formed in the
reaction.
Figure 8.13(A) shows the consumption of TCE and reaction products at
different initial concentrations of TCE. For CO, COCl z and CHCI 3 , each
conversion ratio against TCE decay was almost constant in the TCE concentration
range of 10-30 ppm. For CO 2 , the ratio decreased (0.91-0.73) with increase in
TCE initial concentration, probably because the formation rate and/or the amount
of active oxygen species were not high enough to convert TCE of higher
,.0 co
.0'
ro-' " X COCIl
LI .-
-::.:>E- - - CHCI J
o
o 10 20 30 40
C} HCl J concentration I ppm
-15
-20
40
(8)
.
,
,
,
,
,
,
,
,
,
,
,
.
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
.
I
I
-15 -10 -5
CoX Iln(1-X)
o
Fig. 8.13 (A) Dependence ofC 2 HCl 3 consumed and concentration of products on initial concentration
. of C 2 HCI 3 : reaction time was 19 s, light intensity was 0,046 min- I , (B) Plot of CoX/ln( I-X)
vs t/In(I-X): The straight line showed the relationship -l/kK + CoXoIkln(1-X) - mt/In(1-
X), where Co is initial concentration of C 2 HCh, X is the conversion ratio at time == t which
is.calculated as {Co - C t (the concentration at time == t)}/C o m is the mass ofTi0 2 (1.3 g),
k IS the rate constant of the surface reaction and K is the equilibrium binding constant.
(From S. Kutsuna et al., Atmos, Environ., 27A, 599 (1993))

138
8 Cleaning Atmospheric Environment
8.2 Photocatalytic Activities ofTi0 2
139
or Clads + [HCIC-CCI0 2 ]ads.
The formation of Cl through the decomposition of TCE and PCE has been pointed
out also for the gas-phase reaction of TCE and PCE with OH radicals. 15 ) The Cl
fomied reacts with TCE and PCE at much higher rates than OH, as previously
mentioned.
rhotocatalytic oxidation of TCE and PCE over Ti0 2 was investigated under
humid conditions. As shown in Fig. 8.14, TCE decay and the formation of
products decreased considerably with increase in the relative humidity. This
retardation seem to be related to consumption of Cl or Clads by water and to
competitive adsorption between water vapor and TCE to the Ti0 2 surface. Fig.
8.15 shows the effect of the light intensity on the reactions of TCE and PCE
under dry conditions and at 50% relative humidity. Under dry conditions, the
decreasing rates of TCE and PCE increased with the light intensity, although the
slopes became smaller at high light intensities. The TCE and PCE displayed
almost the same decreasing rates at each light intensity. At 50% relative humidity
the reaction rates increased linearly with the light intensity. The decreasing rate
of TCE was higher than that of PCE.
These differences in the light intensity dependence of the decreasing rates
between dry conditions and 50% relative humidity indicate that active species
other than O 2 - and CI are involved in the photochemical reaction in the presence
of water vapor. Since the OH radical can be produced by the reactions of O 2 -
and/or h+ with H 2 0 and/or OH- groups on the surface, OH radicals may become
the main contributor to TCE or PCE degradation under humid conditions. This
assumption seems reasonable considering the relative magnitude in the reaction
rates of OH with TCE and PCE in the gas-phase mentioned above.
From the viewpoint of application of photocatalysis to treating TCE, PCE or
other organic compounds, the results obtained here are very informative. TCE and
PCE can be a source of Cl under dry conditions. Since Cl is in general more
reactive for hydrogen abstraction from organic compounds than OH, it is possible
concentration, CO and COCh to the final oxidation product of CO 2 .
As shown in Fig. 8.13(B), plots of t/ln (I-X) against CoX/In (I-X) gave a
straight line. This relation was derived from a simple Langmuir form equation,
rate = kKC/(I+KC t ). This may suggest that TCE adsorbed on Ti0 2 reacts with
active oxygen species, which are produced on the TiO z surface under
photo illumination.
The results obtained for both TCE and PCE are summarized in Table 8.4. The
decay rate of TCE is almost same as that of PCE, while the reaction rate of PCE
with OH in the gas-phase is smaller by one order of magnitude than that of TCE
(See Table 8.1). In dimethylformamide or acetonitrile solutions, the rate constant
of O 2 - with PCE is approximately 1.7 times that with TCE. B ) A gas-phase reaction
rate of Cl (chlorine atom) with TCE is about twice that with PCE.14) The small
difference between the reaction rates of TCE and PCE may suggest that O 2 -
and/or Cl are the major species reacting with the compounds under dry conditions.
In addition to the final oxidation products of CO 2 , Ch and t-CI-, the oxidation
intermediates of CO, COCI 2 , CHCl 3 and CCl 4 were found. CO and CHCl 3
production were observed only for TCE, and CCl 4 only for PCE.
As Ti0 2 is illuminated with light shorter than 400 nm, e- reacts with Oz to
produce O 2 - and h+ attached to TCE to form C 2 HCh+ads' When these intermediates
react with each other as follows,
C 2 HCl 3 \ds + O 2 -ads  [HCIC-CCh 0 2]ads
[HCIC-CCh02]ads will be produced. This radical may decompose in the following
manner;
[HCIC-CCI 2 0 2 ]ads  COHCl ads + COChads
[HCIC-CCh02]ads  COHChads + COChads + Clads
30
j
e' "
c.. CztICIJ' ,
c.. ..
)
- 20 - consumed
c ,
. 
....
 ,
... \
....
C
<1.1
c..I 10
c: \
0
u CO 0
---- -.
COCl} .1J
- - - - -X--'.,
0 CHCI)  -::......
0 10 20 30 40 50 60
Relative humidity / %
Clads will attack the C-C double bond of TCE to form [HCI 2 C-CCI 2 ]ads. Through
addition of O 2 , this radical may be changed to [HCh-CClzOz]ads, which would be
transformed in a similar way to [HCIC-CCh02]ads with formation of Clads:
or [HCIC-CClz0 2 ]ads + Clads'
Table 8.4 The decrease in C 2 HCl 3 or C 2 CL 4 and the concentration of products (ppm) for heterogeneous
photochemical reactions on Ti0 2 under dry conditions
Substance Decrease COCh CHCI 3 CCl 4 Ch CO CO 2 t' -CL - Mass balance Mass balance
(C) (CI)
C 2 HCl 3 19,2 3.3 0,5 0.0 6.8 4.8 14.2 10,7 0.59 0.56
C 2 Cl 4 19.2 5.4 0,0 0.7 5.7 0.0 16,2 9.6 0,58 0.45
Initial concentration: 30 ppm; reaction time: 19 s; light intensity: 0,046 min- 1 .
t'-CI- was obtained by subtracting CI- (from C1 2 ) from t-Cl-.
Mass balance was obtained by the ratio of the total C or Cl of reaction products to that ofthe substance
decreased.(Inital concentration of Ag+, was 50 mmol dm- 3 .)
(From S, Kutsuna et ai" Atmos. Environ" 27A, 599 (1993)
Fig, 8,14 Dependence of C 2 HCl 3 consumed and concentration of products on relative humidity: initial
concentration of C 2 HC1 3 was 30 ppm, reaction time was 19 s, light intensity was 0.046 min- I .
(From S. Katsuna et ai., Atmos. Environ., 27A, 599 (1993))

140 8 Cleaning Atmospheric Environment
30 30
(A) (8 )
E v E
0. 20 C 1 CI 4 0.
0. - 0.. 20 -
- consumed
c c f
0 0
..... ....
 
... L, /
..... ....
c C
ClJ 
U 10 u 10 / C) Cl 4
C C
0 0
U U ;I consumed
0 0
0 0.05 0.1 0.15 0 o. I 0.2 0.3
Light intensity (k ) / ruin . 1 Light intensity (k) / min'.)
d d
.2 Photocatalytic ActIvItIes o1'Ti0 2 141
However, it is rather difficult to decompose them in liquid-phase as rapidly as in
gas-phase, because the diffusion of pollutant onto the photocatalyst is essential for
the decomposition reaction, and the diffusion rate of molecules in water is much
slower than in the gas-phase. Recently, the idea of TCE or PCE in water treated
by gas-phase photocatalytic oxidation is the object of much attention. As shown
in Fig. 8.16, polluted water is introduced into a stripping column where TCE or
\ PCE is vaporized or separated from water. 16) The air stream containing TCE or
PCE is led to a photocatalytic reactor using sunlight. A scrubber is used for
removing HCl, Cl z and/or COCh, which will be produced. One private company
in the United States is developing a pilot plant at a flow rate of 600 m 3 min- 1
reaction gas containing less than 1,000 ppm TCE.
B. 1,1,1- Trichloroethane (MC)17)
MC was a popular solvent for cleaning of semiconductor and degreasing of
metal parts, but due to its ozone depletion potential, it has been phsed out and its
use is strictly controlled. We have studied the photocatalytic decomposiion of this
as a typical chlorinated saturated hydrocarbon.
The same flow-type photochemical reactor as shown in Fig. 8.4 was used
here, although the reaction time was longer and the reaction temperature was
from 298-353 K, As shown in Fig. 8.17, MC was transformed to dichloroethene
(CH 2 CCl z ) on Ti0 2 in the dark and under dry conditions. The MC consumption
increased with the reaction temperature. The ratio of MC consumed to CH 2 CClz
produced was close to unity, indicating that the elimination of HCl from MC
predominantly occurred in this temperature range.
Under photo illumination, M,C was converted to CO 2 , C1 2 , and HC!. Fig.
8.18(A) shows the dependence of MC consumed and reaction products on the light
Fig. 8,15 Effect ofIight intensity on C 2 HCI 3 or C 2 Cl 4 consumed under (A) dry condition (B) at 50%
r.h, Initial concentration of C 2 HCI 3 or C 2 Cl 4 was 30 ppm, reaction time was 19 s,
(From S. Katsuna et al., Atmos. Environ" 27 A, 599 (1993»
Treated air Photoreactors
c
:: n: E
:: ::: ;:)
t. ,,,
:: ::: 0
': ::: U
rn
c
0...
0...
';::
......
(f)
100
Air
Polluted
groundwater Treated water
80
E
0..
0..
- 60
c
0
.- CH 2 CCl 2
.....
co::1
 ,
.....
c:: 40 : CHJCCl J
Q) .
u decay
c::
0
U
20
Fig. 8,16 Photocatalytic purification of chlorinated hydrocarbons in groundwater: TCE is dispersed
into the airstream, which is treated with photocatalyst.
to enhance the photocatalytic oxidation rates of organic compounds by adding
TCE to a reaction gas. Since humidity retards the decomposition rate of TCE or
PCE, it is better to remove water vapor from a reaction gas before the
photocatalytic reaction zone. Increasing the light intensity is an alternative way to
obtain a reaction rate for the practical decomposition of TCE or PCE.
There have been many studies on photocatalytic degradation of TCE or PCE
in water because of many episodes of groundwater pollution by these substances.
o
20
40 60 80
Temperature / °C
100
Fig. 8,17 Dependence of CH 3 CCI 3 decay and CH 2 CCl 2 production on the reaction temperature in the
dark: reaction time was 78 s. Initial concentration of CH 3 CCh was 90 ppm.
(From S, Katsuna et aI., Atmos, Environ., 28, 1627 (1994»

142 8 Cleaning Atmospheric Environment
30 80
(A) cO z (ll) a: CH)CO) decay
b: CHICO]
c:HO
E 60 d: a]
0..
0.. 20 E
- 0..
0..
t:: --
0 t
.;::; 40
ro .D
t! E
c:: :!
CI) c:
(j 10 0
t::
0 20
U
.L t'nOWCatalYUC A.cuvmes 01 11V2 14j
0.00 0.05 0.10
Ughl inlensily <k.t) I min"
to Ch, that is, the photo oxidation of chloride, is enhanced by the increase in the
light intensity.
The reaction was greatly inhibited by the presence of water. When the relative
humidity was changed from 0 to 5%, the MC decay at 333 K decreased from 13.8
PPI? to zero in the dark and from 15.2 to 1.0 ppm under photo illumination. No MC
decay was detected either in the dark or under photo illumination at relative
humidities higher than 10%. It was found that 30 ppm of CH 2 CCh completely
disappeared in 78 s un,der photo illumination, even at 50% relative humidity at 313
K. From these results, it is concluded that water may hinder the first step, that is,
the elimination of HCI from, MC on TiO z .
This study indicates two important functions of TiO z in the heterogeneous
chemical reaction of MC: one is thermal catalytic dehydrochlorination and the
other is photocatalytic decomposition. The combination of these two processes
can mineralize MC and keep the catalytic activity of TiO z constant. For treatment
of this kind of halogenated hydrocarbons, both thermal and photocatalytic
activities of TiO z are necessary, that is, the temperature of the Ti0 2 reaction
sysyem sould e elevted to around 373 K, which can be achieved by using
sunhght wIth a kmd of lIght concentration system.
a
a 0.05 0.1
Light intensity (k d ) I min- I
o
Fig. 8.18 (A) Dependence of CH 3 CC1 3 decay and concentration of products on light intensity; (B)
change in the distribution of Cl number among products by light intensity; CH 3 CCb
concentration was 30 ppm. Reaction time was 78 s. Reaction temperature was 333 K. t-CI-
was calculated by subtracting lhe amount of CI- (from C1 2 ) from the total amount of CI-
trapped in water, and it thus mainly came from HC!.
(From S, Katsuna et aI., Atmos. Environ., 28, 1627 (1994»)
8.2.4 Nitrogen Oxides (NO x )
intensity. In the presence of photoillumination, no CHzCCh was detected. When
CH 2 CCh of 30 ppm in air was photo illuminated with kd = 0.05 min- I (k d is the
photo dissociation rate of N0 2 within N 2 ), all of it disappeared in a reaction time
of 78 s. These results indicate that MC is decomposed to CO z , Ch and HCl on
Ti0 2 under photo illumination through the following two reaction steps,
s mentioned in the introduction, the concentration of NO x in Japan has
remamed almost constant for the last 10 years and often exceeds the air quality
standard set for nitrogen dioxide (N0 2 ), particularly along heavy traffic roads in
densely populated areas. To counter this situation, the development of a new
technology for removing NO x at sub-ppm level from air of traffic roads, tunnels
or other environmental emission sources is essential.
CH 3 CCl 3 + L1(heat)  CH 2 CCh + HCl
A. Photocatalytic Activity for Oxidation of NO x I 9)
CH 2 CCh + h v  CO 2 + Clz + HCl
The flow-type photochemical reaction system schematically shown in Fig.
8.19 was designed and fabricated. About 200-250 mg of catalyst powders was
coated onto an inner Pyre£( glass cylinder surface and calcinated overnight at 480
K under air. After cooling to room temperature, the cylinder was placed inside an
outer glass cylinder. PhotoiUumination was provided with a cylindrical bank of 12
,b,lack lights (wavelength: 300-400 nm). A reaction gas of 1-2 ppm NO x in air was
pepared fro a NO/N0 2 -N 2 mixture as well as pure O 2 in cylinder and purified
,air. The reactIon gas passed through the reactor at a flow-rate of 500 ml min- 1 for
5-10 hours at different relative humidities (dry - 72%). After each experimental
run, the inner glass cylinder was removed from the reaction system and immersed
in 185 ml of purified water. The catalyst powders were washed away from the
glass cylinder with an ultrasonic cleaner. The content of HN0 3 in the water was
determined as N0 3 - by ion chromatography. NO x (NO and NO z ) in the gas-phase
was cntinuously monitored with a chemiluminescent NO-NO x analyzer.
Figure 8.20 shows a typical experimental result obtained for a mixture of
TiOz-AC (activated carbon) particles, which was exposed to ca. 2 ppm of NO in
air for a period of 5 h at relative humidity of 50%. The lines of NO (blank) and
N0 2 (blank) were recorded in a separate experimental run (blank test) in which
As a first step MC is transformed into CH 2 CClz by the elimination of HCI. As a
second step, the CH 2 CCh formed is photochemically decomposed to CO 2 , Ch and
HCl on Ti0 2 .
From the temperature dependence of MC consumption shown in Fig. 8.17, an
apparent activation energy for the reaction is estimated to be ca. 70 kJ mol-I. This
kind of dehydrochlorination reaction from chloroethanes has previously been
shown to occur on solid acids and bases. IS) Since the Ti0 2 surface has Lewis acid
sites, the dehydrochlorination reaction may proceed on these acid sites. It is
suggested that some of the HCl produced remains on the Ti0 2 surface and may
inhibit this reaction, but under photo illumination HCI would be removed by a
photocatalytic action of Ti0 2 as discussed below.
In Fig. 8.18(B), the data of Fig. 8.18(A) are plotted as a function of the
number and the kind of Cl. The difference in Cl distribution between the dark and
the photo illuminated (k d = 0.05 min- 1 ) conditions suggests that CH 2 CCh is rapidly
decomposed to HCl and Ch under photo illumination, as TCE or PCE could be
decomposed by the Ti0 2 photocatalyst. The difference in the distribution of Cl
between kd = 0.05 min- I and kd = 0.10 min- I implies that the transformation ofHCl

144
8 Cleaning Atmospheric Environment
8,2 Photocatalytic Activities of Ti0 2
145
x
o
z
Humidifier
t. 
.,{'
.....
'.,{'
t ":-. t
.r
.,.-
.,{'
.,."
.,{'
.,.a
.,{'
.....
.,{'
....'
.,{'
. ....
.. .
Table 8.5 Photocatalytic activity of several kinds of materials for NO x removal. ([NO], 3.8 ppm;
relative humidity, 50%)
NOx
analyzer
Photocatalyst NO removed N0 2 removed HN0 3 recovered pH
(l0- mol g-lh-I) (10-6 mol g-'h-') (10- 6 mol g-Ih-l)
Ti0 2 -1 (46 m 2 g-l) 12.8 -5.3 7.7 4.6
TiOrAC 8.7 2,9 6.4 4.7
Ti0 2 -Fe-AC 11.0 2.0 7,5 4,7
Ti0 2 -Fe-Mg-AC 12,5 2.8 13,3 6.9
Ti0 2 -Fe-Ca-AC 13,0 3.2 13.6 5.8
Ti0 2 -2 (320 m 2 g-l) 13.4 0.8 14,2 4,3
Ti0 2 -2-Fe-Mg-AC 13.2 3,6 16,1 6,7
Reactor
Mixing ratio of Ti0 2 to Ac is 3.
Fe(Fe203): 2wt% vs. Ti0 2 , Mg(MgO) or Ca(CaO): 1 wt% vs. Ti0 2 .
pH: the value of water rinsing the photocatalyst after experiment,
Flowmeter
Recorder
,
Ti0 2 -AC, Ti02-Fe203-AC) at a relative humidity of 50%. The total amount of
NO and NO z removed and that of HN0 3 recovered were divided by the
experimental period (5 h) and were normalized by the weight of the catalyst used.
As can be seen from Table 8.5, TiOz- 1 (Degussa P-25) was able to recover a large
amount of HN0 3 , but as indicated by the negative sign, about 40% of NO was
released as N0 2 to the gas-phase. Since the release of N0 2 into air is unfavorable
to the environment, a way to diminish the formation/release of NO z must be
found. It was discovered that AC could adsorb both NO and NO z , whereas the
amount of HN0 3 formed was not large. Thsee findings imply that a mixture of
Ti0 2 and AC may have the catalytic activity necessary for removing NO x .
The catalytic activity of a mixture of Ti0 2 and AC was examined at different
mixing ratios of Ti0 2 to AC and the mixing ratio of TiO z to AC suitable for
removing NO was determined to be 2:3. As can be seen in Table 8.5, Ti0 2 -AC
removed both NO and N0 2 with a slightly smaller value ofHN0 3 recovered than
that for Ti0 2 -1.
Fig. 8.19 Schematic diagram of a flow-type reactor for heterogeneous photocatalytic reaction of
NO x .
(From T. Ibusuki, K. Takeuchi, J. Mol. Catal., 88, 93 (1994))
2,0
E NO (blank)
0.
0.
C
0
co
'-
-
c
C1)
(,,) 1,0 -
c
0
(,,)
N
0
Z
0
Z
N0 2 (Ti0 2 - AC)
0.0 100 200 300
0
time I min
B. Improvement of Catalytic Activity of TiOz-AC Mixture
Fig. 8.20 Time course of the concentration of NO and N0 2 in the presence of catalyst (a mixture of
Ti0 2 and activated carbon) and without catalyst (blank) under photoillumination.
(From T. Ibusuki, K. Takeuchi, J. Mol. Catal., 88, 93 (1994»)
Addition of metal oxides (Fe203, C0 3 0 4 and NiO) to the Ti0 2 -AC mixture
was attempted to improve the catalytic activity, since' it was demonstrated that
those metal oxides have relatively high adsorptive activity for NO.20) It was found
that each metal oxide similarly enhanced the catalytic activity for NO removaL
Most experiments, however, have been conducted with respect to Fez03, because
Co and Ni oxides are not preferred from the environmental point of view. The
result obtained for TiOz-AC-Fe203 was added to Table 8.5. Ti02-AC-Fe203 was
made by simply mixing commercial Fe203 powders (average diameter of 10 J-Lm)
with TiO z -l and AC. The amount of NO removed during 5 h photo illumination
by the Ti02-AC-Fe203 was about 30% increased, compared to that by
Ti0 2 -1-AC. Since the optimum value of Fe203 added was confirmed to be 1-3 wt
% of TiO z for Ti0 2 -AC, and Fe203 itself did not show a high catalytic activity,
FeZ03 may act as a promoter, probably by attracting more NO to the surface ofthe
catalyst. Indeed, Kaneko has pointed out Z1 ) that dispersion of Fe203 on AC resulted
in a marked increase in NO adsorption.
The amount ofHN0 3 recovered by TiOz-AC-Fe203 catalyst was examined at
N0 2 (blank) was produced by oxidation of NO in the pipe lines and the reactor.
In the presence of the catalyst under photo illumination, the concentration of NO
was greatly reduced, although it gradually increased with reaction time, while a
little N0 2 appeared in the gas-phase. The area surrounded by the line of NO
(blank) and that of NO (Ti0 2 -AC) corresponded to the amount of NO removed by
the TiOz-AC catalyst.
A reaction gas of3.8 ppm NO in air was passed over photo catalysts (Ti0 2 -1,

146
8 Cleaning Atmospheric Environment
8.3 Air Purifying Materials Based on Photocatalyst
147
various relative humidities. The HN0 3 yield obtained under humid conditions
(relative humidity: 30% to 72%) seemed to be constant, although they were about
20% less than that obtained under dry conditions. This low sensitivity to relative
humidity is considered to be convenient for the catalytic oxidation of NO x in
ambient air, because relative humidity of ambient air frequently changes.
The catalytic activity seemed to be gradually decreased with the reaction
time (as shown for Ti0 2 -AC in Fig. 8.20, probably due to accumulation ofHN0 3
on the surface. However, by washing the used catalyst with purified water and
drying it, its activity was completely recovered. This suggests that most of the
HN0 3 on the catalyst surface is easily washed away. It is thus concluded that the
Ti02-AC-Fez03 powders can be used repeatedly as catalysts.
Since the photocatalytic activity appeared to depend on the adsorptivity of NO
and/or N0 2 , another Ti0 2 powder with a larger surface area than TiO r 1 was
examined (Ti0 2 -2 in Table 8.5). The photocatalytic activity was much better
than Ti0 2 -1, that is, the amount of HN0 3 recovered was about twice that by
Ti O z-1 and N0 2 was not released. As can be seen in Table 8.5, the pH of the
water washing TiO r 1 and Ti0 2 -2 after the experiment was almost the same,
while the amounts of HN0 3 recovered were different. This can be interpreted in
terms of the acidity/alkalinity of each TiO z ; TiOz- 1 is acidic and Ti0 2 -2 is
alkaline due to its small content of sodium. It is known that alkaline solutions can
absorb NO z well, so the addition of some alkaline metal oxides (MgO, CaO) to
TiOz-Fe20rAC mixtures was conducted. As shown in Table 8.5, MgO and CaO
remarkably increased the activity of photocatalysts for NO x removal, especially the
removal of N0 2 and the recovery of HN0 3 .
light
Fig. 8,21 A reaction scheme presumed for the catalytic oxidation of NO x (NO and N0 2 ) to HN0 3
(N0 3 -) by the Ti02-AC-Fe203 mixture under photoillumination.
(From T, Ibusuki, K. Takeuchi, J. Mol. Catal., 88, 93 (1994))
The mixture of Ti02-AC-Fez03 powders showed excellent catalytic activity
for the removal ofNO x ' This can be attributed to the cooperative effects ofTi02
AC and Fe203, i.e., the strong and rapid photocatalytic oxidation of NO/N0 2 by
TiO z and the adsorption of NO and N0 2 by AC and Fe203' Pichat et al. 22 ) have
pointed out that NO is photo adsorbed on Ti0 2 , each NO molecule capturing one
semiconductor free electron (NO- formation), which is similar to the reaction of
Oz with electron (0 2 - formation). Since their results have been obtained in the
absence of O 2 , it was not ascertained whether NO can have sufficient opportunities
to interact with free electrons, even under our experimental conditions where the
O 2 concentration of20% was much higher than 1-2 ppm ofNO x ' To explain the
present result that N0 2 and HN0 3 are formed from NO, NO must react with
oxygen species produced by photo activated Ti0 2 . Hori et al. 23 ) revealed from
their experiments on photocatalytic oxidation of N0 2 - to N0 3 - in aqueous
suspension of Ti0 2 that the production ofN0 3 - stopped when O 2 was not supplied
to the solution. They suggested the contribution of H0 2 or H0 2 - radical to the
N0 2 - oxidation. On the surface of photo illuminated TiO z , production of super
oxide (0 2 -) takes place. It has been reported that some kinds of reactive oxygen
species such as atomic oxygen (0), 0-, OH radical and H0 2 radical are produced
through reactions among Oz -, positive hole (h+), OZ-, H+/H 2 0 and OH- on the
TiO z surface. Based on the gas-phase chemistry concerning NO x ,24), reactions of
(NO + H0 2 ) and (NO + 0) can be expected to rapidly proceed to form (N02 +
OH) and N0 2 , respectively, and NO z can also be assumed to react rapidly with OH
and 0 to produce HN0 3 /N0 3 as well.
The reaction scheme illustrated in Fig. 8.21 is therefore presumed, where in
practice it can be imagined that much larger particles of AC and Fe203 than Ti0 2
are surrounded with fine particles of Ti0 2 . NO diffuses to the surface of Ti0 2 and
reacts rapidly with the reactive oxygen species of 0, H0 2 or 0- to produce NO z .
N0 2 further reacts with OH and 0 to form HN0 3 . AC may capture N0 2 formed
from NO by Ti0 2 and give Ti0 2 10ng enough time to oxidize NO z with OH or 0
to HN0 3 . Fe203 deposited on Ti0 2 or adjacent to Ti0 2 and AC may help attract
more NO, which will be oxidized by the active oxygen species. The
aforementioned result of the higher activity under dry conditions than under
humid conditions may reflect the change in the relative contribution of each
reactive species (0, OH, H0 2 , and 0-) to NO/N0 2 oxidation reaction with relative
humidity (the amount of H 2 0 or OH- interacting with TiOiAC/ Fe203).
c. Reaction Scheme of NO x Removal by Photocatalysts
8.3 Development of Air Purifying Materials Based on
Photocatalyst
.3.1 [mmobilization of Powder Photocatalysts 25 )
For practical applications, the photocatalyst should be immobilized to provide
flat surfaces that receive sunlight or artificial light. Thin films of Ti0 2 can be
directly prepared on various substrates by sputtering, electrolytic oxidation of
titanium plates, and dip, spin, and spray coating of titanium compounds. However,
the photocatalytic activity for NO x removal of these films is much lower than that
of fine powder Ti0 2 catalysts. Recently, some Ti0 2 coating agents have become
commercially available, but none of these products are efficient for NO x removal,
mainly due to lack of large surface area.
. We tried to use binding materials for immobilization of Ti0 2 particles. The
requirements for the binder are that it
1) supports the catalyst particles firmly, yet allows the particles to function,

148
8 Cleaning Atmospheric Environment
8.3 Air Purifying Materials Based on Photocatalyst
149
2) is chemically and physically stable against the strong photocatalytic action of
TiO z ,
3) has some structures to provide large specific surface area, and
4) is harmless for environmental use.
Specific surface area is crucial for air purifying materials, because unlike
usual catalytic reactions the reaction products (nitric acid or nitrate) remain on the
surface to cover active sites. This has been demonstrated by the fact that
photocatalytic performance is very much improved by using finer TiO z particles,
increasing TiO z content and making porous structures.
Obtaining good binding materials that satisfy the above requirements was
very difficult. Although we tested many materials for use as a binder, most organic
binders decomposed by the strong photocatalytic action of TiO z . To date we have
found three materials that give good results: fluorocarbon polymer (PTFE)-based
sheets, hardened cement paste and alkoxysilane-based paint.
adsorption of N0 2 , and in the case of cement coating, the alkaline constituents
(mostly calcium) attract NO z that is acidic by nature. Therefore, hardened cement
paste is not simply a catalyst, but is eroded by HN0 3 . However, the erosion rate
is very small under normal conditions, as will be discussed below.
8.3.3 Performance Characteristics of Air-purifying Materials 26 ,27}
8.3.2 Preparation of Air-purifying Materials 26 )
A flow-type reactor similar to that shown in Fig. 8.19 was modified to make
it possible to install the purifying materials. Typical conditions for laboratory
testing were: air flow rate, 1.5 I min- 1 , NO feed concentration, 1.0 ppm; relative
humidity, 80% (25°C); testing period, 24 h; UV intensity, 0.5 mW cm- z .
The effect of initial NO concentration on the NO x removal is shown in Fig.
8.22. Although the painted films and cement plates' were less active than the
PTFE sheets, these materials removed NO efficiently from environmental standard
level to seriously polluted level (0.05-10 ppm). They were also able to remove
NO z and SOz with similar efficiency. The lower efficiency at higher
concentrations is explained by the rapid accumulation of reaction products (nitrate
or sulfate) on the surface. Therefore, the photocatalytic method may not be
suitable for treatment at emission sources.
Figure 8.23 shows the effect of UV light intensity on NO x removal. The
remova] percentage did not depend very much on the UV intensity under our
experimental conditions. It is thus concluded that 0.1 mW cm- z is enough for
removing ppm levels of NO x ' This level of UV intensity is observed even on
cloudy days in winter from the sun. Consequently, the air purifying materials
could work in the outdoor environment during the daytime.
After the exposure experiment, the materials were dipped in purified water for
1--4 h. Nitrate and a small amount of nitrite were found in the leachate. The pH
The TiO z used here was supplied from Ishihara Techno Corp. (type ST-Ol,
average crystallite size, 7 nm; specific surface area, 320 m Z g-I). The activated
carbon (type 3GS; 1350 m Z g-I; pore size, 2.0 nm) from Kuraray Chemicals Co.
was ground into particles «0.15 mm diameter).
Fluorocarbon products and Portland cement were purchased from DuPont-
Mitsui Fluorochemicals Co. and Chichibu-Onoda Cement Corp., respectively.
The sheet type photocatalyst was prepared by the following procedures. Powders
of TiO z , PTFE and AC were mechanically mixed with some solvents. The mixture
was pressed into a sheet with a roller. The typical content of TiO z and AC in the
sheet was 30% and 10%, respectively. A 100 mm square portion of the sheet
(0.5 mm thick) weighed ca. 5 g.
To make cement-coated plates, one part TiO z powder was mixed with three
parts white Portland cement then mixed with the appropriate amount of water. The
mixture was set on a slate plate. The typical TiO z coating density was 165 g m- z .
The TiO z paint was prepared by the so-called sol-gel method. Alkoxysilanes
[(RJ)n-Si-(ORz)4-n] were mixed with water for hydrolysis and condensation under
certain conditions. Then TiO z powder was added to the mixture and mixed well
to give paint. The paint-coated films were obtained by spraying the paint to
stainless steel plates. Thickness of the films was typically 10-30 Jlm.
All the materials have a porous structure to provide higher specific surface
area ofTiO z . Fluorocarbon polymers are chemically stable and have little affinity
with any materials. Electron micrographs showed that the polymer kept its
aggregated particle structure after the roll process. Hence, they can hold TiO z
particles without covering the active surfaces. Characteristic porous structures
were also observed for the hardened cement paste and alkoxysilane sol-gel films.
Without activated carbon, all the materials have a white, mat surface. When
activated carbon is mixed, the color becomes grayish. The PTFE sheets are a
rubber-like, flexible material. The cement- and. paint-based materials can be
applied to many products, especially to existing buildings and structures.
It was found that mixing adsorbents (activated carbon) is not required for the
cement and paint coatings. Their porous structure itself may be suitable for
100
NO
80
..-,
,0
0'
-
co 60
>
0
E
ID
....
.. 40
0
z
20
o
[]"" - - - -c _ _ Flourocarbon
-0 _ polymer sheet
Paint film 1:J.....
........ '0
"'6'_."
\.
\
\
o
0.01
0.1 1
NO (ppmv)
10
Fig. 8.22 Dependence of24 h average NO x removal on NO feed concentration, Air-purifying material,
200 Cm 2 ; flow-rate, 1.5 I min- 1 ; UV intensity, 0.5 mW cm- 2 .

150
8 Cleaning Atmospheric Environment
8.4 Application of Photocatalysis to Cleaning of Atmospheric Environment
151
NO
PTFE sheets have the highest efficiency. Increasing TiO z content is very effective
for improving the performance. The efficiency of the painted films depends on
film thickness and the amount of TiO z .
100
0.1 mW/cm2
80 
8.4 Application of Photocatalysis to Cleaning of
Atmospheric Environment
-

0
---
ro 60
>
0
E
ID
....
'" 40
0
z
20
The purifying materials can be used for treating NO x and other air pollutants
by irradiation with artificial light (active purification). If the photocatalyst is le£1
outdoors as a form of flat panels, it is activated by sunlight to remove air
pollutants during the day (passive purification). The material is expected to be
regenerated by rainwater, since the annual average rainfall in Japan is about 1500
mm. Consequently, an air purification system that works with natural energies
only may be realized by photocatalysis.
Materials Catalyst content Thickness R 24h e) t R =O,5 d) QR=O,5 e QR=O,2 e
(%) (J1m) (%) (h) (m mollm 2 ) (m mollm 2 )
PTFE sheet 40 500 90 48 7 20
PTFE sheet b ) 70 500 90 72 >70 >200
Cement plate 25 200 80 36 5 8
Paintedfilm 55 15 80 36 5 8
8.4.1 Passive Purification of Polluted Air 26 )

We conducted a field test for examining the performance characteristics of the
air purifying materials for NO x removal from ambient air. As the test site, a
roadside of a 6-lane, east and west bound highway in Tokyo was selected. The
average traffic was 113,000 vehicles a day.
Two types of plastic panels [effective size, 0.6 x 1.0 m; made from poly(vinyl
chloride) resin] were used for testing' the sheets in polluted areas. One is the open
type (the sheets are exposed to air) and the other a windowed type. The open type
can simulate realistic conditions. The windowed type has a Pyrex glass window
that allows the UV light to reach the sheets. The roadside air was continuously
pumped to the windowed panels at 15-60 I min- 1 . This type was, used for
estimating the amount of air coming into contacted with the materials., The air
purifying materials were attached to the panels using adhesive tape.
The test panels were set facing south at the north side of the highway so as
o 0.2 0.4 0.6
UV intensity (mW/cm 2 )
Fig. 8.23 Dependence of 24 h average NO x removal on UV intensity, Material, cement plates (200
cm 2 ); [NO], 1.0 ppm, flow-rate, 1.5 I min- I .
Table 8.6 Typical performance of the air-purifying materials for NO x removalS)
a)Experimental conditions: sample, 200 cm 2 ; [NO], 1.0 ppm, air flow rate, 1.5 lImin.
b)Sampie, 150 cm 2 , NO 3.0 lImin.
e) Average removal percentage for NO x for 24 h,
d)Half-life of NO x removale)Amount of NO x removed until the removal decreases to 50% or 30%.
3
-
.1
E
U
S
E 2
-

,
>
:J
E 1
0..
0..
-
..
0
Z
0
0
of the leachate was 4-7 (8-9 for cement plates). Nitrogen recovery (ratio of the
nitrate and nitrite found to the NO x removed) was 40-60%. Since it was 90-95%
for the experiment using powder TiO z (see Table 8.4), the rest of the nitrogen
compounds (mostly nitrate) were considered to be trapped in the binder matrix.
Formation of nitrous oxide (NzO) was not observed during the experiment. The
materials did not show any significant changes in efficiency after 10-50 times
exposure-leaching cycles.
For the cement plates, ca. 50 ,umoll- I calcium ion was found in the leachate
even before NO x exposure. The concentration of calcium ion in the leachate
doubled after the exposure. This means that the cement plates neutralize the
removed acids and are eroded by air pollutants. However, simple calculation
showed that the erosion rate was 0.05-0.2 mm y-I at very polluted conditions.
Table 8.6 compares the performance of the air purifying materials. Again the
"
.... ,1- . I,.
, ','" UV-A
.'
,
\
,
,
Roadside air
,
,
,
,
I
,
, , Panel E
'..-.--.-. :
......... Panel 0 - - ..... . \' :
, ......-..-...-- .......-- ....._:.:--'..... ,_.
4
8 12 16
Time (February 17, 1994)
20
24
Fig. 8,24 Variation of NO x concentration at the test site (air pumping rate: panel D, 15,5; panel E,
24,21 min-I).

152
8 Cleaning Atmospheric Environment
8.4 Application of Photocatalysis to Cleaning of Atmospheric Environment
153
to receive sunlight. Concentrations of air pollutants in both roadside air and that
from the windowed panels were monitored. A weather station set near the test
panels automatically recorded the temperature, humidity, pressure, wind direction
and speed, insolation, and rainfall. Intensity of ultraviolet light was monitored with
a UV -A photometer (315-400 nm). During the test period, the air-purifying
materials were replaced (not regularly) and rinsed with purified water in the
laboratory to recover nitrate converted from NO x '
Figure 8.24 shows a typical diurnal variation of NO x concentration (solid
line) observed for the roadside air on a fine winter day. The high NO x
concentration was caused by busy traffic (especially large, diesel engine trucks).
The dashed lines indicate the NO x concentration in the air treated with the sheets
from the windowed panels. Fig. 8.24 clearly shows that the PTFE sheets removed
NO x from the polluted air between 7 am and 5 pm. The UV-A intensity shown by
the bell-shaped curve centered at noon was over 0.1 mW cm- z . The average
removal percentage for NO x with the windowed panels during the field test was
31-69%.
SOz and nonmethane hydrocarbons (NMHC) were removed, 67-78% and
17-20%, respectively, by the windowed panels. For NMHC, it is assumed that the
fractions of lower aldehydes and olefins are removed, since TiO z degrades such
fractions much more rapidly than saturated and aromatic hydrocarbons.
The removal percentages obtained with the windowed panels depend on the
rate of air pumping. Therefore, NO x removal rate by the PTFE sheets was
calculated from the amounts of nitrate recovered from the sheets on both
windowed and open panels. The rate estimated was ca. 3 mmol (NOx) m- z d- I . As
the capacity of the sheet for NO x removal is 20 mmol m- 2 (sheet) (Table 8.6), the
sheets can be used continuously for one week without requiring a regeneration
process.
The field test shows that the photocatalytic method can achieve passive air
purification. Although the open-air, passive application of photocatalysis is not a
perfect measure for improvement of air quality, we believe it will play an
important role in energy- and labor-saving mitigation of air pollution in urban
areas.
Some local governments, including the Tokyo and Osaka metropolitan
governments, of cities that suffer from severe NO x pollution, have started
evaluating the performance of photocatalytic air-purifying materials on larger
scales. We are improving the efficiency of the materials further and preparing
diverse application products, as shown in Table 8.7. We also continue to conduct
a computer modeling study to simulate real streets in order to predict air
purification effects by the photocatalytic materials.
8.4.2 Active Air Purification of Closed Space 27 )
n -The NO x (mainly NO) concentration in traffic tunnels is usually 1-3 ppm,
depending on the traffic conditions, and some ventilation systems including
electric precipitators are installed in these tunnels to maintain visibility. The
ventilated air is thus the emission source of NO x and other organic pollutants
(hydrocarbons, aldehydes, etc.). It is rather difficult to use sunlight for irradiation
of photocatalysts, Active air purification systems using UV lamps are being
developed for treating the air of tunnel ventilators, underground parking places,
etc,
A prototype treatment system of NO x consists of four 40- W fluorescent lamps
which are radially equipped with four sheets ofPTFE photocatalyst (total area of
18 mZ) and shower nozzles over the lamps/reactors as well as a reservoir, which
are used for washing the photocatalyst sheets periodically. A reaction gas
prepared from diesel engine exhaust, containing about 2 ppm of NO x is introduced
into the reactors at a flow rate of 1.0 m S-I for 10 hours. As shown in Fig. 8.25,
although the NO x removal ratio seemed to decline slowly after 6 h, the average
NO x removal ratio obtained was 77.7%. About 80% of NO x was recovered as
nitrate and nitrite ions by the catalysts using the shower system.
The volume of the ventilation gas by washing for a 6-lane tunnel 1 km in
length is approximately 1.5 million m 3 h- I , which is almost equivalent to that
from middle-size power station plants, while the concentration of NO x and other
pollutants is much lower. To treat the ventilation air, about 4,000 units of the
apparatus in Fig. 8.25 may be necessary. This is considered to be a very large
system, but it is estimated that this system is smaller in scale and more cost-
effective than those systems using concentrations of NO x and NH 3 , the selective
catalytic reduction method. Furthermore, the scale can be smaller if more active
air-purifying materials are developed.
This system can be applied to purification of air at underground parking
places, offices, homes and other areas, For example, Daikin Industry Co.
developed an air purifier using thin honeycomb type TiO z photocatalysts irradiated
by small UV lamps. The purifier consists of the photocatalyst and an electric
precipitator for removing both air pollutants of formaldehyde, odorous species,
etc., and particulate matter.
NO x
100
5
E
 4--
::l.
--
........
,--------------------- 60 
Inlet. S
-40 Q.)
I--<

--------------- 80
--
Field
Application
c:..;
c:
8 2
x
o
z
Table 8.7 Possible application of passive air-purifying materials
o
1 _-, ----- Oull('1 ___n_n______________n_____ 20 x
- _------------- 0
--------------------------- 0 Z
2 4 6 8 10
Timel h
o
Building materials
Road construction materials
walls, roofing, tiles, bricks, concrete blocks, sheet glass*
sound barriers, signs, paving, guardrails, curbstones, crosswalk paint,
surfaces of other structures
paint, cement, solar panels*
Fig. 8,25 NO x removal by a treatment system using PTFE Sheets, Flow rate, 0.96 m S-I; Temp.,
14.3°C; Lh" 29%; [NO x ], 2.23 ppm; Average NO x removal, 77.7%.
Other
* As transparent thin films

154
8 Cleaning Atmospheric Environment
References
155
8.5 Summary
References
Our research on photocatalytic degradation of air pollutants has been
reviewed in this chapter. Organic compounds like aromatic hydrocarbons and
olefins can be decomposed to CO z , but some intermediates such as aldehydes are
produced. For benzene, CO is produced to some extent, but Pt-Ioaded TiO z can
oxidize CO to CO z efficiently by 0/0 3 -, which is produced and stabilized by Pt.
Chlorinated hydrocarbons are degraded mainly to CO z and HCI/Ctz, but some
hazardous by-products such as COCtz are produced. Cl atoms released from the
reactants enhance the rate of decomposition. For a saturated chlorinated
hydrocarbon, CCI 3 CH 3 , thermal dehydrochlorination takes place on TiO z surface.
TiO z shows a very high activity for oxidation of NO x to HN0 3 . Modification of
TiO z with activated carbon, Fez03 and/or MgO or CaO significantly improves the
fixation of ambient NO x on the photocatalysts.
Several air-purifying materials such as fluorocarbon polymers, cements or
paints which immobilize powders of TiO z and additives, have been developed.
They are now being applied to purification of ambient NO x in polluted open
and/or semi-closed areas. The photocatalyst business is now beginning, and many
and various private companies are proposing and developing materials, devices
and systems for cleaning the polluted air. Field tests conducted by local
governments and other organizations indicate that the photo catalysts can remove
NO x to some extent, but more active photocatalysts must be developed, In this
connection, we recently found a new TiO z which can absorb visible lights and
oxidize NO x more efficiently than the conventional TiO z , as shown in Fig. 8.26.
TiO z (B) was prepared by irradiating TiO z (A) with plasma under hydrogen
atmosphere, and had Ti01.8 composition. Although the reason why TiO z (B) can
absorb visible light and show catalytic activity requires elucidation, this new
photocatalyst will be important in increasing the application for cleaning of the
atmospheric environment.
1. K. Takeuchi and T. Ibusuki, Heterogeneous Photochemical Reactions and Processes in the
Troposphere, in: Encyclopedia of Environmental Control Technology, ed. by P,N. Cheremisonoff,
GulfPubl.Houston, 279 (1989),
2, K. Takeuchi and T. Ibusuki, , Atmos, Environ., 20, 1155 (1986).
3. T. Ibusuki and K. Takeuchi, Atmos. Environ., 20, 1711 (1986),
4. A. H. Boonstra and A. H. A. Mutsaers, J, Phys. Chem., 79, 1694 (1975).
5, H, Einaga, S, Futamura and T, Ibusuki, Phys, Chem. Chem. Phys" 1,4903 (1999),
6. M. Primet, p, Pichat and M,-V. Mathieu, J. Phys. Chem., 75, 1216 (1971),
7. M. Primet, p, Pichat and M,-V, Mathieu, C. R. Acad, Sci. Ser, B, 267, 799 (1968).
8. J. March, in: Advanced Organic Chemistry, p194, Wiley-Interscience, (1995).
9. A. R. Gonza1ez-Elipe, G, Munuera and J, Soria, J. Chem. Sac" Faraday Trans, 1, 75, 748
(1979); P. Meriaudeau and J. C. Vedrine, J. Chem. Soc., Faraday Trans, 2,72,472 (1976).
10, M, Naccache, Chem, Phys. Lett" 11,323 (1971); A. Kazusaka and J. H. Lunsford, 1. Catal" 45,
25 (1976).
11. J. L. Moruzzi, J, W, Ekin, Jr. andA. V. Phe1ps,J. Chem. Phys" 48, 3070 (1968),
l2, S, Kutsuna, Y. Ebihara, K. Nakamura and T. Ibusuki, Atmos. Environ., 27A, 599 (1993),
13, T. S, Calderwood, R. C. Neuman, Jr. and D. T, Sawyer, J, Amer, Chem. Sac" lOS, 3337 (1983),
14. R. Atkinson and S. M. Aschmann, Int. J. Chem. Kinet" 19, 1097 (1987),
15. N. ltoh, S. Kutsuna and T. Ibusuki, Chemosphere, 28, 2029 (1994),
16. C, S, Turchi, E, J. Wolfrum and R. A. Miller, Abstracrs for the First International Conference on
Advanced Oxidation Technologies, p.125, London, Ontario, Canada (1994).
17. S. Kutsuna, M. Kasuda and T. Ibusuki,Atmos. Environ., 28,1627 (1994),
18. I.Mochida, J,Take, Y.Saito and Y.Yoneda, 1. Org, Chem" 32, 3894 (1967),
19, T. Ibusuki and K. Takeuchi, 1. Malec, Catal., 88, 93 (1994).
20. G, Busca and V. Lorenzellu, J. Catal., 72, 303 (1981),
21. K. Kaneko, Langmuir, 3,357 (1987).
22. P. Pichat, J-M, Hermann, H. Courbon, J,D isdier and M-N. Monzzanega, Can. 1. Chem. Eng., 60,
27 (1982); H, Courbon and p, Pichat,J, Chem. Sac.. Faraday Trans, 1,80,3175 (1984).
23. Y.H ori, A. Nakatsu and S, Suzuki, Chem. Lett" 1429 (1985).
24. R. Atkinson, D.l. Baulch, R. A. Cox, R. F. Hompson, Jr., J. A. Kerr and J. Troe, J. Phys, Chem,
Ref Data, 18, 887 (1989).
25. K. Takeuchi, N. Negishi, S. Kutsuna and T. 1busuki, Proceedings on. Material Solutions for
Environmental Problems, (H. Mostaghaci, ed.), Canadian institute of Mininf, Metallugry and
Petroleum (1997), ISBN 0-919086-72-1,
26. K. Takeuchi, T, Ibusuki, S. Nishikata and T. Nishimura, Proceedings of the 2nd International
Symposium on Environmental Application of Advanced Oxidation Technologies, Report CR-
107581, Electric Research Institute, pp.9-70 (1997).
27. S. Nishikata, T. Nishimura, K. Takeuchi and T. Ibusuki, Proceedings of the 2nd International
Symposium on Environmental Application of Advanced Oxidation Technologies, Report CR-
107581, Electric Research Institute, pp.4-145 (1997).
40
30
0"
..........
b--_-q,
Ti02 (13)
,
'b.

---
--
':U
6 20
E
(j)
L..
o
Z 10
Ti02 (1\)
o
o
300
400 500
\Vav('lpnt hI nm
600
Fig. 8.26 Comparison of Ti0 2 (A) (untreated) with Ti0 2 (B) (plasma treated) for NO removal.

9
Water Purification-Degradation of Aqueous
Pollutant and Application to Water Treatment
9.1 Introduction
Basic studies on photocatalytic water purification started over twenty years
ago. Since then it has been attracting the interest of many scientists and engineers.
In the 1990s, its application to practical use has drawn great attention from
industry.
Nevertheless, water purification has lagged behind air purification in the
process of commercialization. Major problems in practical application to water
treatment, as compared to air purification, can be summarized as follows. CD The
degradation efficiency is lower in water. @ Pollutant concentration in polluted
water is generally higher than in air. G) Pollutant has less contact with the
photocatalyst, due to its slower diffusion in water. @ Recovery of powder
photocatalyst from water causes engineering difficulties in automatic operation.
Despite these difficulties, research and development are still very active. This
chapter introduces some of the progress made in the field.
9.2 Photocatalytic Characteristics of Titanium Dioxide
Many researchers agree that titanium dioxide is the best photocatalyst for
environmental application at present. Advantages of titanium dioxide over other
semiconducters are: CD high activity, @ large stability to light illumination, G) low
price, @ nontoxicity.
The mechanism for the generation of photocatalytic activity is commonly
interpreted as follows. Illumination of TiO z by light with elJ,ergy larger than the
bandgap energy elevates electron in the valence band to the conduction band, and
a positive hole is formed in the valence band after elevated electron (Eq. (9.1 ».
The positive hole oxidizes either pollutant directly (Eq. (9.3» or water to produce
OH radical (Eq. (9.4» whereas the electron in the conduction band reduces oxygen
adsorbed to TiO z (Eq. (9.2». In the photocatalytic degradation of the pollutant, the
reduction process of oxygen (Eq. (9.2» is slower than oxidation of pollutant
(Eqs. (9.3) and (9.4». However, the two processes should proceed simultaneously,
because otherwise electrons accumulate in the conduction band and the
recombination between electron and positive hole increases. Therefore, the
efficient consumption of electrons is essential to promote photocatalytic oxidation.

158 9 Water Purification
hv
TiO z  e- + p+ (9.1)
e- + Oz  Oz- (9.2)
p+ + Pol  --7 CO 2 (9.3)
p+ + HzO  .OH + H+ (9.4)
.OH + Pol --7 --7 CO z (9.5)
where e- is conduction band electron, p+ positive hole and Pol pollutant.
Of two crystal forms of industrial TiO z products, anatase has larger
photocatalytic activity than rutile. Pure rutile shows the lower activity, while
pure anatase shows the higher activity. In the mixed crystal the activity increases
with percentage content of anatase up to 15%, and above this percentage either
nearly levels off or gradually increases, depending on the pollutant (Figs. 9.1
and 9.2).1) Another factor determining the photocatalytic activity of TiO z is the
crystallite size of anatase. A proportional relationship between the degradation rate
of trichloroethylene and crystallite size is demonstrated in Fig. 9.3. Z ) The effect of
crystal structure on the degradation rate is related also to the nature of the
pollutant. In the preparation of TiO z from Ti(OH)4 (or TiO z · n HzO), crystallite
and particle sizes grow with calcination temperature, and above 600°C crystal
form is transformed from anatase to rutile. Fig. 9.4 shows a relation between
degradation rate of different pollutants and calcination temprature of TiO z . It
was demonstrated that pollutants can be classified into two groups: for group 1 the
degradation rate increases with calcination temperature up to 500°C, and for
9.2 Photocatalytic Characteristics of Titanium Dioxide 159
1
o
o 50 100
Anatase content (%)
B
0
7 0
,-..., 6
c
0 00
E 5
 0
0
E 4
r--
'0
 3
2
Fig. 9.2 Initial degradation rate of C 2 HCh vs. anatase content of commercial Ti0 2 .
(From K. Tanaka, M. F. V. Capule, T, Hisanaga, Chern, Phys, Lett., 187, 74 (1991»
6 0
5 o 0
"......., 0
c
'6 0
E 4 0
'"
0
E 3
....
I
0
...- 2
.........
\..9
1
a
a 50 100
Ana tase cant e nt(%)
-5
10
,--...,
c
 10- 7
o
E
'--'
}.!'
10- 8
10
Fig. 9.1 Initial degradation rate of CH 2 CICOOH vs. anatase content of commercial Ti0 2 ,
(From K. Tanaka, M. F, V, Capule, T. Hisanaga, Chern. Phys. Left., 187,74 (1991»
100
o
I (A)
1000
Fig. 9.3 Initial degradation rate of C 2 HCl 3 vs. crystal size of anatase.
(From A. P. Rivera, T, Hisanaga, K. Tanaka, Appl. Cata/, B: Eviron., 3, 37 (1993))

160
(a)
,.....,
c
E
"'-- 0 5
o '
E
l"-
I
o
>2
9 Water Purification
9.3 Photocatalytic Oegradation of Pollutant
161
o
Organochlorine compound
Table 9.1 Photocatalytic degradation rate of volatile organochlorine compounds
(b)
,.....,
c
E 0 5
"".
Oichloromethane CH 2 Cl 2
Chloroform CHCl 3
Tetrachlorocarbon CCI 4
1,I-Dichloroethane 1,I-C 2 H 4 CI 2
1,2- Dichloroethane 1,2-C 2 H 4 CI 2
I, I , I-Trichloroethane 1,1,] -C 2 H 3 C1 3
I, I ,2- Trichloroethane 1,1,2-C 2 H 3 CI 3
I, I, I ,2- Trichloroethane 1, 1,1 ,2-C 2 H 2 CI 4
I, I ,2,2- Trichloroethane 1,] ,2,2-C 2 H 2 CI 4
1,2- Dichloroethylene 1,2-C 2 H 2 Ch
Trichloroethylene C 2 HCI 3
Tetrachloroethylene C 2 Cl 4
o
E
<.D
'0

o
o
500 1000
Calci nation temp(O C)
o
500 1000
Calcination temp(OC)
Ti0 2 - a)
(min)
80
65
480
97
53
125
68
69
55
51
63
48
Pt/Pi0 2 - b)
(min)
47(1.7>-2
54(1.2)
480(1.0)
47(2,1)
3] (1.7)
150(0,8)
53(1.3)
60(1.2)
46(1.2)
17(3.0)
38(1.7)
56(0,9)
Ire)
(ppm . min- I )
k l - d )
(xlO- 3 . S-I)
1.6
4.4
0.18
1.9
0.6
1.1
1.5
2,0
830
6,8
2.3
2,0
a) Time for 50% degradation, organochlorine compound 5 x 10-4 moll-I, Ti0 2 2.8g I-I.
bj Ratio of rate on Ti0 2 per that op Pt I Ti0 2 .
ej Rate constant from Langmuir-Hinshelwood.
d) First-order rate constant.
(From K. Tanaka, Mizu-Kankyou Gakkaishi, 20, 69 (1997»
Fig. 9.4 (a) Initial degradation rate of organic compounds vs. calcination temperature of Ti0 2 . (0):
C 2 HCI 3 (7.6 x lO- S mol/l), (.6.): C 6 H s OH (lO-4 mo l/l), (X): C 2 Cl 4 (6,0 x lO- s mol/I), (e): C 6 H 6
(1 0-4mol/l) ,
(b) Initial degradation rate of organic compounds vs. calcination temperature of Ti0 2 , (0):
CH 2 BrCOOH (10-3 mo lll), (.6.): C 6 H s OH (10-3 mo l/l), (X): C 6 H s COOH (lO-mol/I), (D):
C 2 HCh (5 x 10-4mol/l).
(From A. P. Rivera, T. Hisanaga, K. Tanaka, Appl. Catal. B: Eviron" 3, 37 (1993»
Table 9.2 Comparison of pseudo first-order rate constants for disappearance and initial TOC
elimination of different pesticides
Chemical structure Disappearance TOC DIT
(D) elimination (T)
o H P
Triadimefon (10-4 moll-I) a "-("--"'-CCCH", 0.48 0.033 14,7
CI  I ,
N J
CH'V-"(CH'h
Pirimicarb (10-4 moll-I) CH)  0.48 0,026 33.1
(CH 3 ) 2 NCOO
Asulam (10-4 moll-I) (3 0.68 0.071 2.4
S01NHCOOCH]
Diazinon (1.7 x 10-4 moll-I) CH, VCH( CH,> , 0.17 0,013 12,6
O-P( C2HO h
II
S
OCONUCH 3
MPMC (10- 4 moll-I) QCH' 0,29 0.043 6,7
CU J
group 2 the rate is nearly constant below 500°C. The former is hydrophobic and
the latter hydrophilic.
9.3 Photocatalytic Degradation of Pollutant
9.3.1 Volatile Organohalide Compound
Several organochlorine compounds are suspected to be carcinogenic. Among
them trichloroethylene and tetrachloroethylene, which have often been reported
to contaminate underground water, are degraded relatively fast by photocatalysis,
forming only trace intermediates. Because of these advantanges, photocatalysis
has been considered to be practical for the degradation of trichloroethylene and
tetrachloroethylene. From studies on several organochlorine compounds (Table
9.1),3,4) the following general conclusions were drawn regarding their
photocatalytic degradation. CD Alkene is more degradable than alkane
(trichloroethylene> 1,1,1-trichloroethane). @ Compounds with two or three
chlorines attached to the same carbon are less degradable than those with chlorines
attached to different neighboring carbons (1,2-dichloroethane > 1,1-
trichloroethane). @ The effect of Pt-Ioading decreases with the number of
chlorines. @ The degradability of different halogen compounds increases in the
order CI > Br > F.5) Upon degradation of these compounds a nearly stoichiometric
amount of Cl- is produced simultaneously with their disappearance. Small
quantities of toxic intermediates are formed from the degradation of these
compounds. For example, dichloroacetaldehyde 6 ) and trichloroacetic acid 7 ) were
identified from trichloroethylene and tetrachloroethylene, respectively.
As noted earlier, efficiency of photocatalytic degradation is larger in air than

162
9 Water Purification
9,3 Photocatalytic Degradation of Pollutant
163
water. This difference is large especially in the degradation of organohalide
compound, because halide radicals formed in air accelerate the degradation
reaction, whereas the radical is quenched immediately in water. For example,
the quantum yield of the degradation of trichloroethylene is 0.1 in gaseous phase,S)
while it is 0.017 6 ) in aqueous medium. Therefore it is practical to degrade volatile
organohalide compounds after transferring them from water to air. However, gas
phase degradation produces high concentrations of toxic intermediates and
mineralization products such as Clz, COCl z , and lower amounts of CHCICOOH,
CH 3 COOH and HCOOH.
Several intermediates have been identified in the process of pesticide
degradation, unlike degradation of volatile organochlorine compounds. Stability
o H 
Ct '" c........-c....C( CH))
I «'......
a  \\J

9.3.2 Pesticides
H
0+
t
NH/+ . NO:!'. CO 2
OH
QO"
a
(photolysis)
of the intermediates can be estimated by the difference between disappearance and
TOC removal rates. Table 9.2 compares the ratio of the two rates for several
aromatic and nitrogen-containing heterocyclic pesticides. 9 ) It was shown that the
latter forms more stable intermediates, and hence its mineralization takes longer
time. This was also demonstrated in the degradation of the fungicide triademefon,
which is made up of aromatic and heterocyclic rings (Fig. 9.5). Its aromatic
moiety degrades quickly, while triadole ring degradation takes longer (Fig. 9.6).
Mineralization products of triademefon NH/ and N0 3 - were formed in addition
to COz. Most nitrogen-containing compounds produce both NH 4 + and N0 3 - as
mineralization products. The formation of NH 4 + has been assumed to indicate
that the reduction process is involved in the degradation of these compounds.
The ratio ofNH 4 + to N0 3 - is determined by the chemical structure surrounding the
nitrogen atom in the molecule. Matthews et al. 10) drew the general conclusion from
\
r
1.0
- '\
0
E
..
b
"-
c
0 0.5
:;::;
ro
L..
......
C
QJ
U
C
0
U
,

0 ,
0
o H 
 '" /c.,C( CH 3 ) 3
CI0 \\,
N J
Triadimefon
OH
Q
a
6
+ Aliphatic comounds
t
t
CO 2
1,2,4-triazole

3
4
2
Illumination time / hour
OH
OOH
b
OH
o oro
a
Fig, 9.6 Formation and degradation of 1,2,4-triazole from photocatalytic degradation oftriadimefon,
(
.6.: Triadinefon, 0: 1,2,4-Triazole.
t
t
COOH
CHCOOH
OCONHCH3
OCH3 --
CH 3
OH OH OH
OCH3 - OCH3 -- OOH
C OH OH
I \
I CHO
COOH
CH)COOH t
\ HCOOH
,/
CO z
--.  HCHO  HCOOH CO2
CH3COOH  NH4+
.. .. N03
CH2(OH)COOH
Fig. 9.5 Possible degradation path way of the fungicide triadimefon.
(From K. S, N. Reddy, T. Hisanaga, K. Tanaka, Tox, Eviron, Chern" 68,409 (1999»
Fig. 9.7 Possible degradation pathway of the insecticide MPMC.

164
9 Water Purification
9,3 Photocatalytic Degradation of Pollutant
165
their studies on many nitrogen-containing compounds that amino groups form
predominantly NH/ and nitro groups form N0 3 -. However, it should be noted that
there are many compounds which do not follow this general rule.
A possible degradation pathway of aromatic pestides is proposed for MPMC
(3,4-xylyl N-methycarbamate) in Fig. 9.7. 11 ) Functional groups and / or H on an
aromatic ring are replaced by an OR group. Organophosphorus and suI fer-
containing pesticides form P0 4 3- (H Z P0 4 -, HPO/-) and SO/-, respectively. Since
these atoms are in the side chain of most pesticides, they may not affect TOC
removal rate significantly, unlike nitrogen-containing pesticides. P0 4 3- and SO/-
attach tightly to TiO z particle and often reduce the activity.
1. Oxidation
e
......
02 + e" ...oz.
hll
# Dye + p+ Dye ox
,
p+
2. Electron injection
hll
,. Dye ---.. Dye ox + eO
9.3.3 Other Organic Compounds
-...... O 2 + eO ---..0z.
Photocatalytic degradation of many organic compounds has been reported,
including noxious compounds such as PCB, dioxin lz , 13) and DDT. 4 ) Among them
the chlorophenolic compounds have been studied extensively,J4-16) because these
compounds are toxic, water-soluble and used for synthesis of other chemicals. the
degradation process is similar to that of other phenolic compounds.
Benzenesulfonate compound is another water-soluble aromatic compound. It was
shown that the degradation rate of these compounds with different functional
groups follows Hammett's law, indicating dependence on the electron-negativity
of benzene ring (Fig. 9.8),17)
Many dyes are decolorized and ultimately mineralized by photocatalysis. In
the degradation of azo dye,IS-2I) the degradation rate decreases in the order
monoazo> diazo> triazo. IS ,19) Three processes including oxidation and reduction
are considered to occur simultaneously in the photocatalytic degradation of dye.
These are illustrated in Fig. 9.9. Process 1 is the common photocatalytic
degradation process of organic compounds. Process 2 is spectral sensitization as
observed in a wet-type solar cell. In process 3 one moiety of dye molecule serves
as the electron acceptor, suppressing recombination between electron and positive
hole.
3. Oxidation and reduction
e
............ Dye + e- Dyered
 Dye + p + ........Dye ox
hll
p+
Fig. 9.9 Photocatalytic degradation mechanism of dye.
2,0
Some natural polymers such' as lignin and fumic acid are of environmental
concern. They are tightly adsorbed to TiO z and thus suppress the degradation of
other pollutants. zZ ) Their basic structure is phenolic compounds linked with each
other by a hydrocarbon chain. Carboxylated or/and hydroxylated phenolic
compounds were identified in the photocatalytic degradation.
9.3.4 Environmenta] Hormones (Endocrine Disruptors)
'7
L
..c
0 0
E OH
::i
'-

0) NO z
0
Many chemicals are suspected to be hormone disruptors. They cause
dysfunction in humans and animals at low concentrations. Photocatalysis has the
advantage of complete mineralization of low concentration pollutants. We
confirmed that several chemicals suspected as being hormone disruptors can be
quickly removed if they are present in amounts of less than 10-5mol 1-1 (Fig.
9.10).z3) For large volume of water, sunlight exposure may be applicable. Several
chemicals which are suspected as being hormone disruptors such as DDT,4)
malathion,Z4) parathion,Z4,Z5) pentachlorophenoF6) and phthalic ester Z7 ) have been
studied for photocatalytic degradation.
o
p (meta or para)
Fig. 9.8 Hamett's law in photocatalytic degradation of benzenesulfonate compound. 0: para, .6.:
meta.
1.0
-0.5
0.5

166
9 Water Purification
o
U
--
U
9.4 Enhancement of Degradation Rate
167
1.5
--0-- 5 x 10-5 M
,.,....0........ 10-4 M
IBet
I '
 8tH!
. ".
 \ EB.
b , . !it . - m. .
05 ,. ,
, ib "
0'. 
\ Q ""
\ " "',
, , ,
\ b. "t..
i' Q ....
, ".
'\ .." .....,....................
,Q "
0. ..,. ..,..
...... o. .............
0,..." "''0... ."6---_______
0,75
....0.... 2,5 x 10-4 M
,,-..
C
,-
E 1.0 x
"" /x
0 X
E /
..... X
0

---
\.! 05
0
..--0.----
5xlO-4M
o
x
- . -EE- . -
10.3 M
-m_
1B- . - - .. - - . - - . _ . .. EB
o
0,25
o
X 0
OX x
x"
o
o
100
200
300
400
a
Time / min
Fig. 9.10 Photocatalytic degradation of p-nitrotoluene. 500ml solution, 6W black light.
a 50 100
Ana tase cont ent (%)
9.4 Enhancement of Degradation Rate
Fig. 9.12 Effect ofPt-loading on the degradation of phenol over commercial Ti0 2 sample. 0: Ti0 2 ,
X: Ti0 2 / Pt.
(From K. Tanaka, M. F. V. Capule, T. Hisanaga, Chern. Phys, Lett" 187, 74 (1991»
The effect of platinum has been studied mainly in relation to hydrogen
generation from water because it is essential for hydrogen evolution. Similarly, Pt
enhances the degradation of the polluting substance (Fig. 9.11).z8) The function of
Pt is assumed to promote the transfer of the conduction band electron to Oz (Eq.
(9.2». Other metals such as rhodium, palladium and silver are also effective.
Copper was studied to replace these expensive metals. However, copper is less
effective, and dissolves in water by illumination. The loading method is also
9.4.1 Pt-loading
2
C';'
o
...........
2:
-.:t
'0
..-
'---"
D-.
W
o

o
...........
..-
.........
D-.
>
o
o
important. Pt can be loaded simply by mixing TiO z with Pt black in mortar, but
its efficiency is low. The photocatalytic method reported by Bard et al. is often
employed. The Pt effect appears to be greater with rutile than anatase (Fig. 9.12).1)
9.4.2 Addition of H 2 0 2
1.0
Photocatalytic oxidation is accelerated by scavenging conduction band
electrons thus preventing the recombination between electron and positive hole.
Several oxidants such as HzOz, K Z S Z 0 7 and KI0 4 13) are used for this purpose.
Among them HzOz may be the best from a practical point of view, because no
reaction product is left after the treatment. Several observations have been made
(a) (b)
15 5.0 15
........ "- . :; "-"... -. .........:: =t -.::::.
a ..:;::, 0 a
E 10 0 E 10 E
0.5 b E ''0 "0
Co b .::. 2.5 --
.::' ::; M 
U I U 5 ,-
I 5 u
U U I
N
U
..
,
a a 0
60 120 0 60 120
Illumination lime{min,) Illuininalion lime{min.)
a
o
123
I1lumi nat ion
4 5
time (h )
6
Fig, 9.13 Effect of H 2 0 2 on the degradation of C 2 HCl 3 ,
(a) Rutile (Katayama).
(b) Anatase (Fujititan TP-2), 0, e: Without H 2 0 2 . 6., .A.; With H 2 0 2 . 0, 6.: Degradation.
e, .A.: Cl-.
(From K. Tanaka, A. P. Rivera, M, F. V. Capule, T. Hisanaga, Yousui to Haisui, 38, 280
(1995»
Fig, 9,11 Effect of pt-Ioading Ti0 2 on the degradation of the pesticide DDVP. 0: Ti0 2 (rutile),
e: Ti0 2 / Pt.

168
9 Water Purification
15
;$ 10
0 0
.-<
t5 b.
5
o
9.4 Enhancement of Degradation Rate
169
on the effect of HzOz: there is an optimal concentration of H z O z ,Z9) the effect of
H 2 0 Z is larger on rutile than anatase (Fig. 9.13), and optimal concentration is
higher with rutile than anatase (Fig. 9.14).
The relation between HzOz effect and calcination temperature is illustrated in
Fig. 9.15. Z ) Optimal HzOz concentration and its effect increase with calcination
temperature. Separate experiments show that increase in calcination temperature
brings about the crystal growth of anatase and over 600°C crystal is transformed
to rutile. Thus it can be concluded that the HzOz effect increases with the growth
of crystallite size of anatase, and it was confirmed that the effect is larger on rutile
than anatase. Optimum pH of HzOz effect was around 6.0 for chloral hydrate. 30 )
9.4.3 Ozone
r I
o J 1 cf 1 0 6 16 5 10"
1 0 3 1 cf
Either photocatalysis or ozonation alone achieved rapid disappearance of
aromatic pesticide. (n contrast, mineralization (TOC removal) was slow for both.
However, photocatalytic mineralization was enhanced considerably by ozone
pretreatment (Fig. 9.16).31) This effect may be explained by ozonolytic cleavage
of the aromatic ring and subsequent formation of aliphatic compounds which are
more degradable by photocatalysis. Simultaneous use of photocatalyst and
ozonation (illuminated by 254 urn light) showed synergetic effect on TOC removal
(Fig. 9. 17).3Z) In this process scavenging of electrons by ozone is considered to
play the most important role.
Concentration ofH202 / M
Fig. 9.14 Difference in H 2 0 2 effect on the degradation of C 2 HCl 3 over Ti0 2 between anatase and
rutile. 0: Rutile (Katayama), .6..: Anatase(TP-2).
5
4
c
E 3
 0
0
E
t-- 2 .A
I
a
-
 i
0
<>
0 r . . , , . ,
o /1 0 7 10 6 10 5 1(Y'1O- 3 1C/16 1
H 2 0 2 (mol/l)
9.4.4 Increase in Adsorption
Photocatalysis efficiency can be improved by collecting pollutant in the
50
40
"'-
E 30
u
 20
10
o
o 5 10
Illumination or ozonation
t ime(h)
Fig. 9.15 Effect ofH 2 0 2 concentration on the degradation ofC 2 HC1 3 over Ti0 2 prepared by calcining
at different temperatures, Calcination temperatures, (e): 120°C, (X): 300°C, (D): 500
600°C, (0): 700°C, (+): 800°C.
(From A. P. Rivera, T. Hisanaga, K. Tanaka, App/. Cata/. B: Eviron., 3, 37 (1993))
Fig. 9.16 Effect of ozone pretreatment on photocatalytic degradation of the pesticide DEP, 0: 0 3 , .6..:
Ti0 2 , e: 0 3 + Ti0 2 .
(From K. Tanaka, K. Abe, C. Y. Sheng, T, Hisanaga, Eviron. Sci. Eng., 26, 2535 (1992))

170
9 Water Purification
9.5 Solar System for Water Treatment
171
1.0
solution to the surface of TiO z . Adsorbents 33 - 35 ) such as activated carbon, silica and
zeolite were used to promote the adsorption. Y oneyama et ai. 33 ,34) reported that
TiO z supported on those adsorbents exhibited high photocatalytic activity for the
degradation of pollutants. They also observed that photocatalysis efficiency is
proportional to the adsorption coefficient, and strong adsorption suppresses the
degradation of some gaseous chemicals suggesting that pollutant moves from
adsorbent to TiO z . However, there is a conflicting report that a simple mixture of
TiO z and adsorbent is more efficient than TiO z supported on adsorbent. 36 )
Adsorption of an ionic pollutant was increased by modifying the surface
electric charge of TiO z particle. P0 4 3- and nafion (anion exchange polymer)
attached to TiO z promoted the adsorption of cationic pollutant and in turn
accelerated its degradation (Fig. 9.18).37)
(3
E
,.,
b
'- 0.5
(3
c
Q)
..r::
c...
o
o
(a)
80
9.5 Solar System for Water Treatment
0>
E
" 60
u
o
t-
40
o 10 20 30 40
(b) Illumination time / min
One of the major advantages of photocatalysis is the use of sunlight. Although
anatase absorbs only a small amount of sunlight (3--4%), sunlight is regarded as
a practical light source in some regions of the world. Many solar experiments have
been performed indicating that the solar system is a practical process for the
degradation of many pollutants when their concentration is low. In Fig. 9.19
sunlight was compared with a 500W super-high pressure mercury lamp in the
degradation of the herbicide asulam. A relatively high efficiency of sunlight
degradation was demonstrated. Large-scale experiments using prototype solar
sysytems are in progress in Europe and the United States.
Fig. 9,17 Degradation of phenol (a) and TOe removal (b) by a combination of photocatalyst and
ozone, 0: UV / Ti0 2 /0 3 , .6..: UV / 0 3 , e: UV / Ti0 2 , ....: UV, X :0 3 ,
(From K. Tanaka, K. Abe, T. Hisanaga, J. Photochern. Photobiol. A: Chern" 101, 85 (1996))
9.6 Immobilization of Ti0 2 and Instrumentation
0.75

'1"
S
 0.5
;::I
0"

<1!
p..
0.25
1\
.
\" ,
,q\
\ .,
. i ",
.. \ ".\,
\\\:::>,
. , ,',
\ . \ "'0. \)...
. "' m \ ""0>"8-
... l!t.-----tJ. . ...........
o
o 50 100
-D- plain
nafion
......,.<>,....... 1.2 mg I g
....0.... 2.2
In photocatalytic water treatment the difficulty in recovering TiO z powder
----6---- 4,5
.. .. -83- -- 9
0.75 -D- 50CJW Hg
::.E .......0....,.. solar
'1"
S \
E 0.5 \
<1! \
"5 L
'"
«
0.25
'o
.,.,.-.... 45
150 200
o
o
10
20 30
40.
50
Time / min
Time /min
Fig. 9,18 Effect of nation-coating on Ti0 2 in the degradation of the herbicide paraquat.
Fig. 9,19 Solar experiment on the degradation of the herbicide asulam, compared with a mercury lamp
experiment.

172
9 Water Purification
References
173
from treated water is a major obstacle for the instrumentation. To solve this
problem, several immobilization methods have been studied. Two commonly
employed immobilization methods are the coating of TiO z powder with binder,
and the sol-gel method for the preparation of thin film. In the former silicon
binder is most commonly used because of its stability to photocatalysis. The
physical strength of this coated layer increases with the amount of binder, whereas
its photocatalytic efficiency decreases with it. Although TiO z film prepared by the
sol-gel method has larger physical strength, it generally shows lower efficiency
than powder immobilized by a minimum amount of binder. Immobilized
photocatalysts have smaller surface area than powder, hence less photocatalytic
efficiency. To overcome this problem, TiO z is often coated on supporting
substrates of varying shapes, e.g. particles, fibers and honeycombs. Both powder
and immobilized TiO z are now used on a commercial basis.
31. K, Tanaka, K. Abe, C. Y. Sheng, T. Hisanaga, Environ. Sci, Technol" 26, 2534 (1992).
32. K. Tanaka, K. Abe, T. Hisanaga, J. Photochern. Photobiol. A: Chern., 101,85 (1996).
33. N, Takeda, T. Torimoto, S. Sampath, S. Kuwabata, H. Yoneyama, J.Phys.Chern., 99, 9986
(1995),
34. T. Torimoto, S. Hoh, S. Kuwabata, H. Yoneyama, Environ. Sci. Technol., 30, 1275 (1996).
35. Y. Xu, C. H. Langford, J. Phys. Chern. B, 101, 3115 (1997).
36. J. Matos, J. Laine, J, M. Herrmann, Appl. Catal. B: Environ" 18,281 (1998).
37. S. Vohra, K, Tanaka, Abstract I of 78 th Chern.Soc.Jpn" P.543 (2000).
9.7 Conclusion and Outlook
Intensive and extensive studies on photocatalysts over the past 20 years have
demonstrated their applicability to water treatment and their limitations. A single
application of a photocatalyst is not possible for all types of waste water. It can
be used as secondary and tertiary treatment. An other possible,way is to combine
it with hydrogen peroxide, ozone or others materials. Development of other
applications besides waste water treatment is also under way.
References
1. K. Tanaka, M. F. V. Capule, T. Hisanaga, Chern. Phys, Leu" 187,73 (1991).
2, A. P. Rivera, T. Hisanaga, K. Tanaka, Appl. Catal. B: Environ" 3, 37 (1993).
3, T, Hisanaga, K. Harada, K. Tanaka, J. Photochern. Photobiol, A: Chern., 54, 113 (1990),
4. F. Sabin, T. Turk, A. Volger, J. Photochern. Photobiol. A: Chern., 63, 99 (1992).
5. L. A. Dibble, G. B. Raup, Environ, Sci, Techno/., 26, 492 (1992).
6. A. L. Pruden, D. F. Ollis, J. Catal., 82, 404 (1983),
7, y, Mao, C. Schoneich, K-P, Asmus, J. Phys. Chern" 95, 10080 (1991).
8. D. F. Ollis, Environ. Sci, Technol., 19, 480 (1985).
9. K. S. N, Reddy, T, Hisanaga, K. Tanaka, Toxicol. Environ, Chern" 68, 403 (1999).
10. G. K.- C. Low, S. R. McEvoy, R. W. Matthews, Environ, Sci, Technol., 25,460 (1991).
II. K. Tanaka, S, M. Robledo, T. Hisanaga, R. Ali, Z. Ramli, W. A. Bakar, J. Mol. Catal. A: Chern.,
144,425 (1999).
12. E. Pelizzetti, M. Borgarello, E, Borgarello, N, Serpone, Chernosphere, 17, 499 (1988).
13, E, Pelizzetti, V. Carlin, C. Minero, M, Gratzel, NewJ, Chern., 15, 351 (1991).
14, R. W, Matthews, J. Catal., 111,264 (1988).
15. J-c. D'Oliveira, C. Minero, E. Pelizzetti, P. Pichat, J. Photochern, Photobiol. A:Chern" 72, 261
(1993),
16, C. AI-Sayred, J-c. D'Oliveira, P. Pichat, J. Photochern. Photobiol. A: Chern" 58, 991 (1991).
17. B. Sangchakr, T. Hisanaga, K. Tanaka, J. Photochern. Photobiol. A: Chern., 85, 187 (1995).
18. W, Z. Tang, Z. Zhang, H. An, M. Q. Quitana, D. F, Torres, Environ, Technol., 18, I (1997).
19. K. Tanaka, K. Padermpole, T. Hisanaga, Wat. Res" 33, 327 (1999).
20. M, Muneer, R. Philip, S. Das, Res. Chern, Interrned., 23, 233 (1997).
21. P. Peeves, R. Ohlhausen, D. Sloan, K. Pamplin, T. Scogins, Solar Energy, 48, 413 (1992).
22. K. Tanaka, R, C. R. Calanag, T. Hisanaga, J. Mol. Catal. A: Chern" 138,287 (1999).
23, M. S. Vohra, T. Hisanaga, K. Tanaka, Abscracts of 77th Chern. Soc. Jpn., P.309 (1999).
24. C. K. Gratzel, M. Jirousek, M. Gratzel, J. Mol. Catal., 60, 375 (1990),
25. R. Doong, W. Chang, J. Photochern, Photobiol, A: Chern" 107,239 (1997).
26, G. Mill, M, R. Hoffman, Environ, Sci, Technol., 27, 1681 (1993),
27. M, Halmann, J. Photochern. Photobiol. A: Chern., 66,215 (1992).
28, K. Harada, T, Hisanaga, K. Tanaka, Wat, Res" 24, 1415 (1990).
29. K. Tanaka, T. Hisanaga, K. Harada, New J. Chern" 13, 5 (1989).
30. K. Tanaka, K. Harada, T, Hisanaga, J. Photochern, Photobiol. A: Chern" 54, 113 (1990).

10
Second-generation Ti0 2 Photocatalysts Able to
Initiate Reactions Under Visible Light Irradiation
10.1 Introduction
t
Environmental pollution and destruction on a global scale have drawn
attention to the vital need for totally new, safe and clean chemical technologies
and processes, the most important challenge facing chemical scientists for the 21 st
century. Strong contenders as environmentally harmonious catalysts are titanium
oxide photo catalysts which can operate at room temperature in a clean and safe
manner while applications of such photocatalytic systems are urgently desired for
the purification of polluted water, the decomposition of offensive atmospheric
odors as well as toxins, the fixation of CO z and the decomposition of NOx and
chlorofluorocarbons on a global scale. I -4)
However, unlike photosynthesis in green plants, the titanium oxide
photocatalyst in itself does not allow the use of visible light and can make use of
only 3-4% of solar beams that reach the earth. Therefore, to address such
enormous tasks, photocatalytic systems which are able to operate effectively and
efficiently not only under UV but also under the most environmentally ideal
energy source, sunlight, must be established, To this end, it is vital to design and
develop unique titanium oxide photo catalysts which can absorb and operate with
high efficiency under solar and/or visible light irradiation. 3 ,4)
This chapter deals with the design and development of such unique second-
generation titanium oxide photo catalysts which absorb UV -visible light and
operate effectively under visible and/or solar beam irradiation by applying an
advanced metal ion-implantation method.
10.2 Experimental Section
The main characteristics of the various titanium oxide catalysts used in this
chapter have been summarized in Table 10.1. Titanium oxide thin film
photocatalysts were prepared using an ionized cluster beaql (ICB) method. 5 ,6)
Using ICB, the titanium metal target was heated to 2,200 K in a crucible and Ti
vapor was introduced into the high vacuum chamber to produce Ti clusters. These
clusters then reacted 'with Oz in the chamber and stoichiometric titanium oxide
clusters were formed. The ionized titanium oxide clusters formed by electron
beam irradiation were accelerated by a hIgh electric field and bombarded onto the
glass substrate to form titanium oxide thin films.
The metal ion-implantation of the catalysts was carried out using an ion-

176 10 Second-generation Ti0 2 Photocatalysts Able to Initiate Reactions
Table 10.1 Characteristics of the titanium oxides used in the present study
Catalyst Anatase, BET surface Particle size, Purity as Bandgap
% area, m 2 /g nm Ti0 2 , % energy, e V
F-2 72.3 27.1 23.4 99,97 3.25
F-4 87.5 54.2 15.0 99.97 3,251
F-6 81.0 102 9.30 99,99 3,262
P-25 70.9 50.2 18.6 99.54 3.250
S-l 86.1 30,6 30.2 99.90 3.252
implanter consisting of a metal ion source, mass analyzer, high voltage ion
accelerator (50-200 keY), and a high vacuum pump.7) The metal ions were
expected to be injected into the deep bulk of the catalyst when high acceleration
energy was applied to the metal ions. In fact, as expected, SIMS analyses using
a Shimadzu/Kratos SIMS I 030 clearly showed that the metal ions implanted into
the titanium oxide catalyst exist in a highly dispersed state and are injected into
the deep bulk of the catalyst, exhibiting a distribution maximum at around
1,000-3,000 A from the surface and zero distribution at the surface. 8 - 10 ) Although
such distribution depends on the acceleration energy and the kind of catalyst,
one of the most significant advantages in using the metal ion-implantation method
is to modify the bulk electronic properties of a catalyst. .
The metal ion-implanted titanium oxide catalysts were calcined in Oz at
around 725-823 K for 5 h. Prior to various spectroscopic measurements such as
UV - VIS diffuse reflectance, SIMS, XRD, EXAFS, ESR and ESCA as well as
invesigations on the photocatalytic reactions, both the metal ion-implanted and
unimplanted original pure titanium oxide photo catalysts were heated in Oz at 750 -
K and then degassed in cells at 725 K for 2 h, heated in Oz at the same temperature
for 2 h, and finally outgassed at 473 K to 10-6 Torr. IO - 13 )
Light irradiation of the photocatalyss in the presence of reactant molecules
such as NO x and a mixture of CH 3 C=CH and HzO was carried out using a high-
pressure Hg lamp (Toshiba SHL-100UV) through water and color filters, i.e., A
> 450 nm for visible light irradiation and A < 380 nm for UV irradiation,
respectively, at 275-295 K. The reaction products were analyzed by GC and GC-
MASS. The UV -VIS diffuse reflectance spectra were measured using a Shimadzu
UV -2200A spectrophotometer at 295 K. The ESR spectra were recorded at 77 K
with a Bruker ESP300E and a JEOL RE-2X spectrometer (X-band). The binding
energies and the element composition of the catalysts were measured using a
Shimadzu ECSA-3200 electron spectrometer. The XAFS (XANES and FT-
EXAFS) spectra were measured at the BL-7C facility of the Photon Factory at the
National Laboratory for High-Energy Physics, Tsukuba.
lO.3 Results and Discussion 177
::J
cci
<D
U
C
aj
.c
o
(/)
.c
«
:2:

350 400c 450 500 550 600 650
Wavelength / nm
Fig. 10.1 UV -Vis absorption spectra (diffuse reflectance) of the unimplanted pure Ti0 2 (a) and the Cr
ion-implanted Ti0 2 «b)-(d)), and the solar spectrum which reaches the earth. (amounts of
Cr ion-implanted in 10- 7 moVg, b: 2,2, c: 6.6, d: 13)
::J
cci
-
<D
U
C
aj
.c
a
(/)
.c
«
:2:

250
350
450
550
Wavelength / nm
10.3 Results and Discussion
Fig. 10,2 Shifts in the absorption spectra of various types of Ti0 2 photocatalysts (shown in Table 1)
implanted with the same amounts of Cr ions. a: unimplanted original pure p-25, b: V/F-6,
c: V/F-4, d: V/P-25, e: V/F-2, f: F/S-1. (amount of Cr ions implanted: 6.6 X 10- 7 moVg)
The metal ion-implantation method was applied to modify the electronic
properties of titanium oxide photo catalysts by bombarding them with high energy
metal ions, and it was discovered that metal ion-implantation with various
transition metal ions such as V, Cr, Mn, Fe and Ni accelerated by high voltage
enables a large shift in the absorption band of the titanium oxide catalysts toward
visible light regions, with differing levels of effectiveness. However, Ar, Mg, or
Ti ion-implanted titanium oxides exhibited no shift, showing that such a shift is
not caused by the high energy implantation process itself, but to some interaction
of the transition metal ions with the titanium oxide catalyst. As can be seen in Fig,
10.1 «b )-( d)), the absorption band of the Cr ion-implanted titanium oxide shifts
smoothly to visible light regions, the extent of the red shift depending on the
amount and type of metal ions implanted, with the absorption maximum and

minimum values always remaining constant. The order of the effectiveness in the
red shift was found to be V > Cr > Mn > Fe > Ni ions. Such a shift allows the
metal ion-implanted titanium oxide to use solar beams more effectively and
efficiently, at up to 20-30%. 4,10)
Furthermore, as shown in Fig. 10.2, such red shifts in the absorption band of
the metal ion-implanted titanium oxide photocatalysts can be observed for any
kind of titanium oxide except amorphous types, the extent of the shift changing
from sample to sample. It was also found that such shifts in the absorption band
can be observed only after calcination of the metal ion-implanted titanium oxide
samples in Oz at around 723-823 K. Therefore, calcination in Oz in combination
with metal ion-implantation was found to be instrumental in the shift of the
absorption spectrum toward visible light regions. These results clearly show that
shifts in the absorption band of the titanium oxides by metal ion-implantation is
a general phenomenon and not a special feature of a certain kind of titanium
oxide catalyst.
Figure 10.3 shows th absoprtion bands of the titanium oxide photocatalysts
impregnated or chemically doped with Cr ions in large amounts as compared
with those for Cr ion-implanted samples. The Cr ion-doped catalysts show no shift
in the absorption band, however, a new absorption shoulder appears at around 420
nm due to the formation of the impurity energy level within the bandgap, its
intensity increasing with the amount of Cr ions chemically doped. Such results
indicate that the method of doping causes the electronic properties f the titanium
oxides to be modified in completely different ways, thus confirming that only
metal ion-implanted titanium oxide catalysts show shifts in the absorption band
toward visible light regions.
With unimplanted or chemically doped titanium oxide photocatalysts, the
photocatalytic reaction does not proceed under visible Light irradiation (.I.. > 450
nm). However, we have found that visible light irradiation of metal ion-implanted
titanium oxide photo catalysts can initiate various significant photocatalytic
1.5
 off
0 1
! N2
-
......
c:J
=
'0
0
..
Q.
 0.5
0
"C

....

0 2 4 6 8 10 12
Reaction time / h
Fig. 10.4 Reaction time profiles ofthe photocatalytic decomposition of NO into N 2 and O 2 as well as
N 2 0 on the Cr ion-implanted Ti0 2 photocatalyst under visible light (A> 450 nm) irradiation
at 295 K. Unimplanted original pure Ti0 2 photocatalyst did not show any photocatalytic
reactivity under the same reaction conditions. Nor did Ti0 2 photo catalysts chemically
doped with Cr ions exhibit any photocatalytic reactivity.
::J

300 350
450 550
Wavelength / nm '
reactions. As shown in Fig. lOA, visible light irradiation (.I.. > 450 nm) of the Cr
ion-implanted titanium oxide in the presence of NO at 275 K leads to the
decomposition of NO into N z , Oz, and NzO with a good linearity against the
irradiation time. Under the same conditions of visible light irradiation, the
unimplanted original pure titanium oxide photocatalyst did not exhibit any
photocatalytic reactivity. The action spectrum for the reaction on the metal ion-
implanted titanium oxide was in good agreement with the absorption spectrum of
the photocatalyst shown in Fig. 10.1, indicating that only metal ion-implanted
.titanium oxide photo catalysts were effective for the photocatalytic decomposition
reaction of NO. Thus, metal ion-implanted titanium oxide photo catalysts were
found to enable the absorption of visible light up to a wavelength of 400-600 nm
and were also able to operate effectively as photocatalysts, hence their name,
"second-generation titanium oxide photocatalysts".4,9,I3)
It is important to emphasi?:e that the photocatalytic reactivity of the metal ion-
implanted titanium oxides under UV light (.I.. < 380 nm) retained the same
photocatalytic efficiency as the unimplanted original pure titanium oxides under
the same UV light irradiation conditions. When metal ions were chemically doped
into the titanium oxide photocatalyst, the photocatalytic efficiency decreased
dramatically under UV irradiation due to the effective recombination of the photo-
formed electrons and holes through the impurity energy levels formed by the
doped metal ions within the bandgap of the photocatalyst (in the case of Fig.
10.3).14) These results clearly suggest that metal ions physically implanted do
not work as electron and hole recombination centers but only work to modify the
-
Q.)
()
c:
ro
..0
a
(/)
..0
«


Fig, 10.3 Absorption spectra ofTi0 2 chemically doped with Cr ions. (Cr ions doped in 10- 6 mol/g, a:
undoped original pure Ti0 2 (P-25), b ' ; 1.6, c'; 20, d'; 100, e': 200),

180
10 Second-generation Ti0 2 Photocatalysts Able to Initiate Reactions
10.3 Results and Discussion
181
efficiency of these photocatalysts to increase under visible light irradiation,
passing through a maximum at around 6xlO l6 V/cm 2 of the catalyst, then
decreasing with a further increase in the number of metal ions implanted. Only on
samples implanted with an increased number of metal ions could the presence of
ions at the near surfaces be observed by ESCA measurements. Thus, these results
clearly suggest that there are optimal conditions in the depth and number of metal
ions implanted to achieve a high photocatalytic reactivity under visible light
irradiation.
The ESR spectra of the V ion-implanted titanium oxide catalysts were
measured before and after calcination of the samples in Oz at around 723-823 K,
respectively. Distinct and characteristic reticular V4+ ions were detected only
after calcination at around 723-823 K. It was found that when a shift in the
absorption band toward visible light regions was observed, the reticular V4+ ions
could be detected by ESR. No such reticular V ions or shift in the absorption band
have ever been observed with titanium oxides chemically doped with V ions. 13 ,15)
Figure 10.6 shows the XANES and FT-EXAFS spectra of the titanium oxide
catalysts physically implanted with Cr ions (b and B) and also chemically doped
with Cr ions (a and A), respectively. Analyses of these XANES and FT-EXAFS
spectra show that in the titanium oxide catalysts chemically doped with Cr ions
by an impregnation or sol-gel method, the ions are present as aggregated Cr-
oxides having octahedral coordination similar to CrZ03 and tetrahedral
coordination similar to Cr03, respectively. On the other hand, in the catalysts
physically implanted with Cr ions, the ions are present in a highly dispersed and
isolated state in octahedral coordination, clearly suggesting that the Cr ions are
incorporated in the lattice positions of the catalyst in place of the Ti ions.
Our results clearly show that modification of the electronic state of titanium
oxide by metal ion-implantation is closely associated with the strong and long
distance interaction which arises between the titanium oxide and the metal ions
electronic property of the catalyst. 9,10,12,13)
We have conducted various field work experiments to test the photocatalytic
reactivity of the newly developed titanium oxide photo catalysts under solar beam
irradiation. As can be seen in Fig. 10.5, under outdoor solar light irradiation at
ordinary temperatures, the Cr and V ion-implanted titanium oxide photo catalysts
showed several times higher photocatalytic reactivity for the photocatalytic
decomposition of NO. It was also found that under solar light irradiation at
ordinary temperatures, the V ion-implanted titanium oxide photo catalysts showed
several times higher photocatalytic reactivity for the photocatalytic hydrogenation
of CH 3 C:=CH with HzO than the unimplanted original pure titanium oxide
photocatalysts. These results, together with the results shown in Fig. 10.1, clearly
show that by using second-generation titanium oxide photocatalysts developed by
applying the metal ion-implantation method, we are able to utilize visible and solar
light energy more efficiently.
The relationship between the depth profiles of the metal ions of the metal ion-
implanted titanium oxide photocatalysts having the same number of metal ions,
such as V or Cr ions, and their photocatalytic efficiency under visible light
irradiation were investigated. It was found that when the metal ions were
implanted in the same amounts into the deep bulk of the catalyst by applying high
voltage acceleration energy, the photocatalyst exhibited a high photocatalytic
efficiency under visible light irradiation. On the other hand, when a low voltage
was applied, the photocatalyst exhibited a low efficiency urrder the same
conditions of visible light irradiation. 13)
It was also found that increasing the number (or amounts) of metal ion-
implanted into the deep bulk of the titanium oxides caused the photocatalytic.
=
 8 1
Q,j
I-i
J::
 6

=
: :a
Q:i ::I
..Q4
o I-i
Z 
 -
o
:5
Q,j 2
.
Q,j
.
......

Q:i
 0
Ti02
Crrno 2
Photocatalyst
Jttf
,. '" .:,:.
:J :J
ct! ro
-- --
Q)
0
c:
ct!
..Q

0
(f)
..Q
«
VITi O 2
6010 6050
Energy / eV
Fig, 10,5 Effect of the Cr and V ion-implantation on the photocatalytic reactivity of Ti0 2 under solar
beam irradiation for the photocatalytic decompositon of NO at 295 K. (solar beams: 38.5
mW/cm 2 , amount of photocatalyst: 6,0 g)
024
Distance / A
6
Fig. 10.6 XANES (left) and FT -EXAFS spectra (right) of Cr ion chemically doped Ti0 2 «a) and (A))
and Cr ion implanted Ti0 2 catalysts «b), (B)), respectively.

182 10 Second-generation Ti0 2 Photocatalysts Able to Initiate Reactions
implanted and not by the formation of impurity energy levels within the bandgap
of the titanium oxides which is often observed in the chemical doping of metal
ions.
10.4 Conclusion
The advanced metal ion-implantation method has been successfully applied
to modify the electronic properties of the titanium oxide photo catalysts, enabling
the absorption of visible light. even longer than 550 nm and initiating the
photocatalytic reactions effectively not only under UV but also visible light.
irradiation. The results obtained in the photocatalytic reactions and various
spectroscopic measurements of the photocatalysts indicate that the implanted
metal ions are highly dispersed within the deep bulk of the catalysts and work to
modify the electronic nature of the photocatalysts without any changes in the
chemical properties of the surfaces. These modifications were found to be closely
associated with an improvement in the reactivity and sensitivity of the
photocatalyst, thus enabling the titanium oxides to absorb and operate effectively
not only under UV but also under visible light irradiation. As a result, under
outdoor solar light irradiation at ordinary temperatures, metal ion-implanted
titanium oxide photocatalysts showed several times higher photocatalytic
efficiency than the unimplanted original pure titanium oxide photocatalyst. Thus,
the advanced metal ion-implantation method has opened the way to many
innovative possibilities, and the design and development of such unique titanium
oxide photocatalysts can also be considered an important breakthrough in the
utilization of solar light energy which will advance research in sustainable green -
chemistry for a better environment.
Acknowledgments
The present work was partly supported by the NEDO Grant and the 1998
Mitsubishi Foundation. The author would like to express his thanks to both.
References
1. Photocatalysis, (N. Serpone and E. Pelizzetti eds.), John Wiley & Sons, New York (1989).
2. M. Anpo, Catal. Surveys Jpn., 2, 167 (1997) .
3. M. Anpo, Proc. 1st Int. Conf Protect. the Environ., Rome, 75 (1998),
4. M, Anpo, in: Green Chemistry, (P. Tundo and P. Anastas eds,), 1, Oxford University Press
(2000),
5. K. Takami, N. Sagawa, H, Uehara, and M. Anpo, Shokubai, 41, 295 (1999).
6. H, Yamashita, M. Honda, M, Harada, Y. Ichihashi,and M, Anpo, J. Phys, Chem, B, 102, 10707
(1998).
7, M. Anpo and M, Takeuchi, in Handbook of Ion Engineering, (2001),
8. M, Anpo, H. Yamashita and Y. Ichihashi., Optronics, 186, 161 (1997).
9. M. Anpo, Y. Ichihashi, M. Takeuchi and H. Yamashita, Res, Chem. Intermed" 24, 143 (1998).
10. M. Anpo, Y. Ichihashi, M, Takeuchi and H. Yamashita, Stud. Surf Sci. Catal., (H. Hattori and
K. Otsuka, eds.), 121,305, Kodansha-Elsevier (1999),
11. M, Anpo, M. Takeuchi, S. Kishiguchi and H, Yamashita, Surf Sci.' Jpn., 20, 60 (1999),
12, M. Anpo, et ai" US patent No. 6,077,492, June 20, (2000). ,
13. M. Anpo, in: Stud. Surf Sci. Catal., 130 A, 12th Intern, Congr. Catal., (A. Corma, F, V, Melo,
S. Mendioroz, J, L. and G, Fierro eds.), Elsevier Science V. B., part A, 157 (2000).
14, H, Paul Maruska and A. K. Ghosh, Solar Energy Mater" 1,237 (1979).
15. B, Morin, A. Davidson, M. Che, Y. Ichihashi and M, Anpo, Unpublished data,

11
Photocatalytic Organic Syntheses Using
Semiconductor Particles
11.1 Introduction
The discovery of photoassisted water cleavage at semiconductor electrodes,
known as the Honda-Fujishima effect,I,2) stimulated and developed research on
photocatalytic reactions occurring at the surface of semiconductor particles, as
well as semiconductor electrode systems, even though photoinduced reactions
over solid surface was common knowledge in the nineteenth century, e.g.,
chalking of paint films containing titanium(IV) oxide as a pigment. From the
beginning of the recent research history,3) application of this semiconductor
photocatalytic reaction to organic syntheses, as well as to chemical conversion and
storage of light energy,4,5) was recognized as one of the most important and
attractive targets in this field of chemistry. During the 1980s and early 1990s,
information both on the mechanism of photocatalytic reaction of organic
compounds and on the activities of various semiconducting particles, i.e.,
photocatalysts, accumulated. 6 ) Based on these results, many examples of organic
syntheses through photocatalytic reaction have been discovered over the past
several years. This indicates that the research on photocatalytic reactions is
making significant progress toward practical synthetic processes.
Up to now, researchers in this field have clarified the following advantages
and characteristics of semiconductor photocatalytic reactions: (1) multiple
processes, e.g., reduction and oxidation, proceed successively in a one pot
reaction, (2) catalysts can be separated and re-used easily, (3) the reactions
proceed at ambient temperature under atmospheric pressure, and (4) unlike
ordinary organic synthetic procedures, water can be used as a solvent, enabling the
use of water-soluble organic substrates, such as those from biological sources,
without derivatization. These advantages should be emphasized in comparison
with conventional thermal reactions and utilized for development of novel organic
syntheses, since they satisfy requisites for the sustainable chemical process for the
next century, the Green Chemisty Process'?) This review article describes the
principles and characteristics of the photocatalytic reaction and shows the
possibility of application to practical organic syntheses.

186
11 Photocatalytic Organic Syntheses Using Semiconductor Particles
11.3 Photocatalytic Reactions by Semiconductor Suspension
187
11.2 Principle of Photocatalysis by Semiconductor Particles 8 . 9 )
hv
The electronic energy structure of titanium(JV) oxide (TiO z ) or cadmium(II)
sulfide (CdS), a semiconductor, is indicated by a valence band (VB) filled with
electrons, a vacant conduction band (CB), and a band-gap separating these bands
(Fig. 11.1). Light. of energy greater than the band-gap excites the electron in VB
into CB, leaving a vacant state of electron in VB, i.e., a positive hole. This
electron excitation in Ti0 2 or CdS requires light of wavelength shorter than
approximately 400 nm or 500 nm, respectively. This process of photoexcitation
resembles the excitation of organic molecules: a HOMO level electron is excited
into the LUMO level to leave a vacancy in LUMO. The active species for redox
reactions, an excited electron (e-) and a positive hole (h+), transfer to, i.e., reduce
and oxidize, respectively, molecules, i.e., oxidants and reductants, adsorbed on the
surface of semiconductors. When these redox processes consume the same number
of excited electrons and positive holes, no chemical change occurs in the
photoexcited semiconductor particles; electron flows from the reductant being
oxidized to the oxidant being reduced through the semiconductor particles. In this
sense, the photoinduced reaction that occurs over photo irradiated semiconductor
particles should be called a "photocatalytic" reaction. Although many authors
have defined the term "photocatalytic reaction" differently, in this review the
definition is the reaction induced by the photoabsorption of semiconductor
particles which undergo no net chemical change, without regard to whether change
in the free energy (L1G) before and after the reaction is positive or negative and
whether the quantum efficiency exceeds unity or not.
Thus, the electron flows from the reductant to the oxidant through
photoexcited semiconductor particles. Therefore, the particle suspended in a
solution of reductant and oxidant can be regarded as a very small electrochemical
cell in which the irradiated light energy is used as free energy change and/or
activation energy of the redox reaction of the reductant and the oxidant (Fig,
11.2).
As described in the subsequent section, various types of redox reaction
proceed by the photocatalytic reaction as commonly seen in conventional
electrolysis. Hence, the semiconductor photocatalytic reactions resemble
electrolysis in principle, but several advantages have been pointed out, e.g.,
photocatalytic reactions do not require electrolytes, which are indispensable in
ordinary electrolysis.
................
a photoirradiated
semiconductor (Ti02)
particle loaded wit
platinum deposits
III
Fig, 11.2 Schematic representation of a photoirradiated semiconductor particle loaded with small
amounts of platinum deposits,
11.3 Photocatlytic Reactions by Semiconductor Suspension
conduction
band
excited
electron
( oxidant )
Eon flow
( reductant )
Photocatalytic reactions of organic compounds reported so far were
categorized into two groups: I) organic synthesis or reactions to be operated to
make chemical bonds, including reductive fixation of carbon dioxide, and 2)
decomposition and/or mineralization oftoxic materials and pollutants into carbon
dioxide. 10, II) Examples of the former reported before the early 1990s, except the
carbon dioxide fixation, have been summarized in reviews. 6 ,IZ) All the
photocatalytic synthetic reactions reported so far proceed at ambient temperature
and atmospheric pressure. This is one of the most significant features of
photocatalytic reaction, leading to laboratory safety and easy handling ?f thermally
unstable materials. Since photocatalysts are frequently used metal oXIdes such as
TiO z and metal sulfides such as CdS and zinc(II) sulfide (ZnS), they can be
obtained commercially or by conventional inorganic syntheses in the laboratory
with or without further purification or activation. In some cases, colloidal catalysts
are used as prepared in a reaction vessel. Results of photocatalytic reaction of
suspensions, especially the reaction rate, depend strongly not only on the
photo catalysts but also on the conditions of photo irradiation. The influence of the
nature ofTiO z photocatalysts is discussed in another chapter of this volume.
Since oxidation and reduction by, respectively, a positive hole and an excited
electron are essential steps of the semiconductor photocatalytic reaction, we can
find substrates to be oxidized and reduced in the reported reaction systems. First,
the authors will interpret the cases in which photocatalytic reaction produces the
desired product by the oxidation of a starting material. This category includes
band gap
valence
band
o
Fig. 11,1 Electronic structure of the semiconductor and schematic representation of photocatalysis.

188
1 I Photocatalytic Organic Syntheses Using Semiconductor Particles
11.4 Redox Combined Photocatalytic Processes
189
dehydrogenation and/or dimerization of alcohols, 13-15) and oxygenation of aromatic
rings. 16) The presence of the oxygen molecule (0 2 ), if the suspension is opened to
air or under an air or O 2 flow, is very convenient because it acts not only as an
acceptor of the excited electron to enhance the oxidation by the positive hole, but
also as a reagent for organic radicals, produced by a positive hole, to yield a
peroxy radical chain carrier for an auto oxidation-like reaction. In the latter case,
one photon can produce multiple organic hydroperoxy radicals; the quantum
yield may exceed unity. A silver cation (Ag+) is also used as a sacrificial and
stoichiometric electron acceptor and deposited as rather inert silver metal particles
onto the photocatalysts. 17 - 19 ) In the absence of these electron acceptors, i.e.,
oxidant, water as a solvent or as an impurity of organic solvents accepts the
excited electron to yield hydrogen molecule (H 2 ). Loading of small particles of
noble metals, e.g., platinum (Pt), or their oxide onto the photocatalysts enhances
the H 2 production, presumably because the photoexcited electron tends to move
to the noble metal deposits and they decrease the activation energy of H 2
production. 20 ,21) Co-presence of O 2 and water complicates, in most cases, the
mechanism of photocatalytic reactions and gives a variety of products. Therefore,
neat substrates, e.g., toluene, or solutions in organic solvents are often chosen for
the photocatalytic oxidation in the presence of O 2 - Acetonitrile has been used
frequently because of its sufficient stability toward oxidations. In any case ,in both
the absence and presence of O 2 , positive holes (or hydroxyl radicals produced by
them I4 )) oxidize the starting materials. The total stoichiometry is, therefore,
generally described as dehydrogenation or oxygenation. Photocatalytic
epoxidation of olefins and dihydroxylation of naphthalene are discussed later in
this chapter.
On the other hand, the production of desired compounds through reduction of
starting material requires the electron donors to be oxidized (reductant). Alcohols
are often used not only as a solvent but as the donor to produce useful compounds,
e.g., anilines from nitrobenzenes,22) alcohols from aldehydes,23) and secondary
amines from the corresponding Schiff bases. 24) From the organic synthetic point
of view, however, the separation of undesired products, aldehydes or ktones,
from the alcohols is necessary unless subsequent reaction processes consume
them 25 ,26) or they are easily removed by distillation or other procedures. A recent
report has shown that water acts as the electron donor and is converted into O 2 in
the photocatalytic regio-selective reduction of terpenes mixed with aqueous
suspension of Ti0 2 . 27 ,28) It is notable that isolation of the desired product from the
reaction mixture is simple in this type of photocatalytic reduction.
As is often observed as electrode-dependence in electrolysis,there are some
examples in which the product(s) depends on the photocatalyst. In the
photocatalytic reaction oflactic acid (CH 3 CH(OH)COOH), Ti0 2 tends to give the
decarboxylated oxidized product, acetaldehyde (CH 3 CHO), while CdS gives the
dehydrogenated product, pyruvic acid (CH 3 COCOOH),29-31) The most plausible
reason is the difference in adsorption of lactic acid (sites on adsorbate and/or
structure of adsorbent) or in surface active species between Ti0 2 and CdS. The
difference in adsorption property has been clarified by infrared spectroscopy.32,33)
This is an indication that the product selectivity of photocatalytic reaction over
semiconductor particles is controllable by surface treatment. This is a unique
feature of the heterogeneous reaction, not achieved in homogeneous photoreaction.
11.4 Redox Combined Photocatalytic Processes for Nitrogen-
containing Substrates
Careful inspection of the reported photocatalytic reactions may demonstrate
that reaction products can not be classified, in many cases, into the two above
categories, oxidation and reduction of starting materials. For example,
photoirradiation onto an aqueous suspension of platinum-loaded Ti0 2 converts
primary alkylamines into secondary amines and ammonia, oth of whih ae ,not
redox products. 34 ) In.a similar manner, cyclic secondary ammes, e.g., .pIpendm,
are produced from a,m-diamines. 34 ) Along this line, trials of synthesI of cyhc
imino acids such as proline or pipecolinic acid (PCA) from a-ammo aCIds,
ornithine or lysine (Lys), have been successfu1. 35 ) Since optically pure L-isome of
a-amino acids are available in low cost, their conversion into optically actIve
products is one of the most important and practical chemical rutes fr the
synthesis of chiral compounds. It should be noted that L- and racemIC PCA s are
obtained from L-Lys by Ti0 2 and CdS photocatalyst, respectively. This will be
discussed later in relation to the reaction mechanism.
The stoichiometry of above-mentioned photocatalytic reactions of amines is
generally expressed as,
2RNH 2 = RNHR + NH 3
(intermolecular deamino condensation)
or
H 2 NR'NH 2 = R '-NH + NH 3
--------
(intramolecular deamino condensation)
Again, this is not a simple reduction or oxidation, but intr- and intraolecular
deamino condensation. This reaction is proved to proceed VIa three reactIOn steps,
including both oxidation and reduction. As an example (Fig. 11.3), Ls .undergoes
deaminocyclization through (1) oxidation of an amino group by posItIve hole o
yield an imine (step 0), (2) hydrolysis of the imine into aldeye and .ammoma
and intramolecular condensation of the aldehyde with a remammg ammo group
(step C), and (3) reduction by excited electron, presumably on the platinum
surface of the resulting Schiff base intermediate (step R). To complete these
photoctalytic reactions successfully, it is necessary to eplae ,0 2 in the rection
mixture by an inert gas such as nitrogen or argon before IrradIatIOn; conta1l1ated
O 2 may accept excited electrons to prevent the final step of the SchIff bae
reduction. 36 ) Since the same number (two) of positive holes and electrons IS
consumed for the production of one molecule of PCA, this reacti?n proceeds
catalytically and produces no by-product other than ammonia. ,
It is expected and has been clarified experimentally that there are two racton
pathways depending on which amino group is oxidized first and that th OXIdatIon
of a side-chain E-amino group and an a-amino group leads to a (S)-L-Isomer and
racemate, respectively. The apparent catalyst dependence, L-isomer from Ti0 2 and

190
11 Photocatalytic Organic Syntheses Using Semiconductor Particles
1 1.5 Stereoselective Organic Synthesis of Nitrogen-containing Compounds
191
o
L-Iysine (Lys) COOH
H2NNH2
2h+ -2H+
COOH
imine] _ . l
H2NNH
H 2 0! -NH 3
COOH
H2NO
! -H 2 0
reduction (or ,oxidation) of once oxidized (or reduced) substrate, leading to no
chemical reaction, although such reversed, or short-circuited, reactions have been
utilized in the racemization of amino acids by suspended CdS photocatalysts. 20 ,42)
In spite of this undesirable possibility, the redox-combined photocatalytic reaction,
e,g., deaminocyclization of Lys, proceeds efficiently, This is attributed to the
dark chemical step (C in Fig. 11.3) avoiding the back electron transfer to the
once oxidized (reduced) substrate. 21 ) Thus, chemical reaction of the intermediate
species enhances the total redox-combined reaction. For this purpose, general
chemical reactions such as hydrolysis, condensation, addition, or rearrangement,
can be used as well as photoreaction of intermediates. Semiconductor powders,
especially metal oxides, have also been used as catalysts for such dark chemical
reactions. It is expected, therefore, that the surface of photo catalysts enhances the
transformation of intermediate species to induce highly efficient redox-combined
photocatalytic processes.
oxidation by
positive hole(s)
R
COOH
HNNH :
H 2 0! -NH 3
COOH
HOC
NH 2
! -H 2 0
(j cyclic Schiff (j
..A base _,A
N CO OH N COOH
2e-, 2H+! (pt) 2e', 2H+! (pt)
(j pipecolinic (j
.,A acid (PCA) ..A
N COOH N COOH
H H
11.5 Further Development to Stereos elective Organic Synthesis
of Nitrogen-containing Compounds
C
hydrolysis and
condensation into
a cyclic Schiff base
intermediate
reduction by
photoexcited
electron(s)
L-isomer
racemate
Selective synthesis of a target stereoisomer is one of the most important and
significant processes, Hence, achievement of stereoselective synthesis by
photocatalytic reaction is also strongly desired. There are three possibilities for the
stereoselective syntheses by semiconductor photocatalytic reaction: in the step of
(1) oxidation by a positive hole, (2) included chemical reaction(s), e.g.,
condensation into a Schiff base, and/or (3) reduction by an excited electron, An
example of (1) is the photocatalytic L- PCA synthesis from Lys; the optical purity
depends on the position of the amino group which the positive hole attacks. To the
Fig, 11.3 Reaction mechanism of photocatalytic synthesis ofpipecolinic acid (PCA) from L-Iysine.
racemate from CdS suspensions, is accounted for by the preferential oxidation of,
respectively, E- and a-amino group ofLys. Although the reason for the different
positions for oxidation is unclear at present, it is most likely that the conformation
of Lys adsorbed on the surface of the photocatalyst produces a decisive effect.
Infrared spectroscopic analyses ofLys adsorbed on the surface of photo catalysts
have supported this mechanism. 37 )
Similar redox-combined processes have been reported. For example, it has
been clarified by control experiments using a photoirradiated semiconductor
electrode that the photocatalytic production of indazoles from substituted
azobenzenes is based on the condensation of two intermediates formed through
oxidation and reduction. 38 -40) In the case of oxidation of substituted olefins a
similar redox combined mechanism is assumed; cation and anion radicals are
fOD11ed by the reaction of olefin with positive hole and of O 2 with excited electron,
respectively, and they react to produce a 4-membered ring intermediate, a
dioxethane, to undergo bond cleavages into the desired products. 41 ) In the
photocatalytic reactions, a positive hole and excited electron must react at the
neighboring surface sites of a small semiconductor particle, enabling the
combination of reduction and oxidation without the addition of an electrolyte,
which is an indispensable component in electrolysis. However, in the particulate
system the recombination of positive hole and electron is also likely, as well as
100 5
;:!!.
0 1.14
-
"0 4
Q) 80
':;'
()
0
a.. 60 3 "
(.)
"C .........
c::
ct! (J)
c::
c:: 2 
0 40
"w .....
.....
Q)
>
c::
0 20 1
()
a..
«
0 0 0
a b c d e
CdS photocatalyst
Fig, 11.4 Trans- and cis-PDC yields as a function of molar amounts of Cdo deposited during the
photocatalytic reaction for 24 h. a: Furuuchi CdS as received, d: a was annealed at 1023 K
under a stream of Ar (40 cm 3 min- 1 )-air (1.7 cm 3 min- 1 ), f: Mitsuwa CdS was annealed at
823 K under the Ar-air stream, and g: a was annealed in air at 1023 K and left for 15
months in air. CdS powder (50 mg) was suspended in an aqueous solution of DAP (100
,umol) and irradiated at > 300 nm under Ar atmosphere at 298 K.

192
] I Photocatalytic OrganIc ::'yntheses usmg ::.emlConaucwr rarucies
L i.:J u;n;oselecnve vrganlc YIlLIlt:SIS Ul l-.nrugt:n-CUIlLi:1lIllHg vU1l1PUUHU:;
l::1J
Q ( COOH )
HOOc"'" N/.: cOOH = N
HOOc
CS8
2e-12W
!
H+
1
base catalyst used (CdS or TiOJ, produced cis-PDC almost selectively. On the
other hand, bare CdS powders tend to produce trans isomers preferentially. A
heat-treated CdS powder gave the highest trans/cis molar ratio of larger than 4,
but some other commercial CdS catalysts were less active and yielded a mixture
of PDC isomers. These differences in stereo selectivity may be caused by the
reduction step, as shown in Fig. 11.5,
In the case of the platinum (or its oxide)-loaded photocatalyst, the
photo excited electron may move to the platinum deposit and produce a surface-
adsorbed hydrogen atom. The addition of two hydrogen atoms to the double bond
of the cyclic Schiff base, a proposed intermediate, from same direction (syn-
addition) may preferably produce cis-PDC. Control experiments in which
hydrogenation of pyridine-2,6-dicarboxylic acid over platinum black gives cis-
isomer selectively support this mechanism. On the other hand, the Schiff base
intermediate may be reduced over bare CdS surfaces not by the hydrogen atom
transfer, but by successive electron transfers. It has been clarified that CdS powder
heat-treated under partly aerated conditions, i.e., slightly oxidized to form sulfur
vacancies, undergoes photocatalytic reduction to deposit small amounts of metallic
cadmium (CdO), which acts as a reaction site for trans-PDC production. 44 ) Similar
Cd o deposition has been reported 46 ,47) and related to the photocatalytic activity.48-50)
As shown in Fig. 11.6, the larger the amount of deposited Cdo, the higher the yield
of trans- PDC, evidence that the deposited Cdo enhanced the production of the
trans isomer. When optically active DAP, synthesized according to a previous
report 51 ) was used optically active trans-PDCs ((S,S) and (R,R) from (S,S) and
(R,R)-DAP, respectively) were obtained with >90% selectivity.45)
Several reports on photocatalytic conversion of nitrogen-containing
compounds have been published,22,52,53) in which stereo control of reaction
products were achieved. Thus, we can expect progress in photocatalytic organic
cOOH cOOH
H2NNH2
2h+ ! -2H+
H 2 0 !-NH 3
cOOH cOOH
H2NO
-H 2 0 !
Cd
cOOH

Hooct r
 '
H H
I I
Pt
-, cOOH

HOOc t N
: e'
!
!
Hooc.,,0cOOH
H
",0",
HOOc" N "cOOH
H
trans-PDc
cis-PDc
r 40
g
30
;:!!.
0
-
"0
Q)
':>- 20
0
0
a..
10
Fig. 11.5 Proposed intermediacy of a cyclic Schiff Base (CSB) producing respectively trans- and cis-
PDC through two-step electron transfer and syn-addition of hydrogen atoms at platinum
surface.
authors' knowledge, there are no reports showing stereos election through steps (2)
or (3), except for the trial stereoselective synthesis through step (3) introduced
here. 43 -45)
As expected easily from analogy with Lys, photocatalytic reaction in
deaerated aqueous suspensions of semiconductor particles produces piperidine-
2,6-dicarboxylic acid (PDC) from 2,6-diaminopimelic acid (DAP). The product
PDC was optically inactive when an optically inactive 1: 1:2 mixture of L, D,
and meso isomers of DAP was used , However, two diastereomers, trans and
cis-PDC's were obtained and their molar ratio strongly depended on the kind of
photocatalyst used (Fig. 11.4).
Since almost all natural PDC analogs, 2,6-disubstituted piperidines, are in the
trans (LL (SS)) conformation, the selective synthesis of trans-PDC is most
valuable. The loading of a small amount of platinum or its oxide, regardless of the
o
o
1
2
3
CdO / Jlmol
Fig. 11.6 Trans- and cis-PDC yields as a function of molar amounts of Cdo deposited during the
photocatalytic reaction for 24 h, a: Furuuchi CdS as received, d: a was annealed at 1023 K
under a stream of Ar (40 cm 3 min- 1 )-air (1.7 cm 3 min- I ), f: Mitsuwa CdS was annealed at
823 K under the Ar-air stream, and g: a was annealed in air at 1023 K and left for 15
months in air.

194
11 Photocatalytic Organic Syntheses Using Semiconductor Particles
11.0 IntrOaucuon 01 vxygen .tHorns lULU VIgi:111I\; vUUl[JUUUU,,"
17-'
11.6 Introduction of Oxygen Atoms into Organic Compounds
selective oxidation of naphthalene, water and molecular oxygen are needed as the
reactants. In contrast to earlier reports on the hydroxylation of aromatic
compound, the most striking feature of this reaction is that only 2-
formylcinnamaldehyde and 1 ,3-dihydroxynaphthalene are obtained in the reaction
products, without any mono-hydroxylated naphthalene being detected. Moreover,
the photocatalytic reaction was found to be strongly affected by the incident light
intensity, oxygen concentration, as well as the water content in the mixed solution.
The experimental details and reaction mechanisms of these reactions will be
discussed in the following sections.
synthesis of useful nitrogen-containing compounds, as well as the oxygen.
containing compounds discussed in the following section.
Many studies have been carried out on the epoxidation of olefin
compounds 5 4-61) because epoxides are important starting materials for the synthesis
of polymers, [n laboratories, epoxides are synthesized by partial oxidation of
olefins using iodosylbenzene, hypochlorite, or organic hydroperoxides as the
oxidants. For these reactions, porphyrin compounds are often used as the catalysts.
62-65) Peracetic acid is also known to be an efficient oxidant for the epoxidation of
olefins 66)
Molecular oxygen, which is the most attractive oxidant from the economical
and environmental viewpoint, has also been reported to be an oxidant for the
synthesis of epoxides using some porphyrin compounds as the catalysts,67,68) The
catalysts are, however, deactivated if the reactions are continued for a long period,
For example, Ru-porphyrin loses its catalytic activity as the result of coordination
of carbon monoxide to the nuclear ruthenium (IV) atom. In the case of Nb-
porphyrin, which has the catalytic effect under photoirradiation, it is gradually
photodecomposed. Another disadvantage of these reactions is that isolation of
products is problematic from the reaction mixture in a homogeneous phase. In
contrast to the above molecular catalysts, semiconductor photocatalysts have
advantages in their stability and separability from the reaction mixture. Because
of these advantages, they have been studied with regard to organic
syntheses 35 ,42-44,6<J-71) and treatment of waste. 72-74)
For epoxidation of olefins, Kanno et al. 75) reported that 1, l-diphenylpropylene
was oxidized to 1, 1-diphenylepoxypropane by photoirradiation of Ti0 2 or CdS
particles suspended in oxygenated solutions. However, in their results the main
product was not epoxide but ketone. Fox et af.1 6 ,77) also reported that arylated
olefins were oxidized on photoirradiated semiconductor particles in air-saturated
acetonitrile solutions to yield small amounts of epoxides, carbonyl compounds
being the main product in most cases. 76 ,77) Although the yields in the reports are
low, the generation of epoxides on photocatalysts using molecular oxygen as the
oxidant is noteworthy.
Hydroxylation reaction has been observed for several semiconductors in
aqueous suspensions. Matthews found that benzoic acid can be hydroxylated in
Ti0 2 aqueous suspension. 78 ) Fujihira et at. reported the Ti0 2 -photocatalyzed
hydroxylation of benzene and toluene in water. 79 ) In these systems, the OH radical,
which is obtained as the result of the reaction between photogenerated holes and
water or oxygen and photogenerated electrons, is an important reactant for the
hydroxylation of aromatic compounds.
In this section, new methods for the stereospecific epoxidation of 2-hexene
and the selective oxidation of naphthalene using Ti0 2 as photocatalyst under UV
illumination are introduced. Both these reactions under similar conditions have
been examined by several authors, but the conditions for the selective epoxidation
and selective oxidation was optimized only recently. For the epoxidation of
olefins, only molecular oxygen is needed for the reactant. It is emphasized that the
epoxidation reaction proceeded stereospecifically in high yield. In the case of
11.6.] Stereospecific Epoxidation of 2-hexene on Photoirradiated TiO z
Powders Using Molecular Oxygen as Oxidant
As the starting materials of this work, 2-hexene consisting of trans and cis
isomers in a ratio of 1.7 : 1, as well as pure trans- or cis-2-hexene was used.
Photocatalytic reactions were carried out in Pyrex glass tubes containing Ti0 2
powder and 2-hexene. The oxygen gas was passed through the suspension during
the reaction. A 500 W high-pressure Hg lamp was used as the light source.
Passing the light beam through a cut-off glass filter < 340 nm eliminated deep UV
light. The quantum yield of the reaction was determined at a wavelength of 365
nm. The details of the experimental conditions have been described
previously.8o,81)
Using any Ti0 2 powder as the photocatalyst, 2,3-epoxyhexane was always
obtained as the main product from 2-hexene, as shown in Fig. 11.7. 82 )
 
4000
3500
. Trans isomer
I
o Gis isomer
"6 3000
E
::1.
--
 2500
-0
.x
8. 2000
w
1500
1000
Ratio
Surface area
(m 2 /g)
P-25
(A+R)
4.6
I I I I I .
ST.01 ST.11 TIO.5 PT.101 CR.EL TIO-3 PT.201
(A) (A) (R) (R) (R) (R) (R)
4.6 4.3 4.7 7,1 4.3 3,7 4.7
192.5 65,0 2.1 24,5 5.4 42.5 7.1
500
o
39.3
Fig, 11,7 Comparison of the photocatalytic activities of different Ti0 2 powders for producing trans-
2,3-epoxyhexane and cis-2,3-epoxyhexane from 2-hexene (translcis = 1.7), The reactions
were performed by photoirradiation For 19 h in pure 2-hexene suspended with Ti0 2 powder
under a stream of oxygen, The crystal structures and surface areas are listed underneath the
graph. The main crystalline structure of each Ti0 2 powder is denoted by A (anatase) and R
(rutile). The ratio trans/cis epoxyhexane produced photocatalytically.

196
11 Photocatalytic Organic Syntheses Using Semiconductor Particles
11.6 Introduction of Oxygen Atoms into Organic Compounds
197
cis-2-hexene
trans-2,3-epoxyhaxene cis-2,3-epoxyhaxene
high-pressure mercury lamp was employed as the light source. Deep UV light was
eliminated by passing the light beam through a UV-34 filter (:S; 340 nm) to prevent
direct photoexcitation of naphthalene, Reaction products were analyzed by high-
performance liquid chromatography (HPLC).
Among the Ti0 2 powders, P25, which contains a 30% rutile phase and has a
43 m 2 /g surface area, showed the highest activity, as shown in Fig. 11.8, In this
system, only three kinds of oxidized compounds (2-formylcinnamaldehyde and
1,3- and 1,4-dihydroxynaphthalene) among 10 possible isomers were obtained. 81 )
Furthermore, no mono-hydroxylated compounds, i.e., 1- and 2-naphthol, were
detected in the reaction products, Quantum efficiencies of the reactions were
determined at around 365 nm. After photoirradiation for 1 h, the quantum
efficiencies were determined to be 14.6% and 5.8% for the production of 2-
formylcinnamaldehyde and 1,3-dihydroxynaphthalene, respectively. The
efficiency was determined on the assumption that 4 holes are necessary to form
one oxidized naphthalene molecule.
The presence of water in the solution is essential to the production of oxidized
naphthalene compounds, As shown in Fig. 11.9, the reaction rate increased with
increase of water content and reached the maximum at a water content of about
6%.
Oxygen is also essential, because no products were obtained without oxygen.
These results indicate that both molecular oxygen and water are indispensable to
the selective oxidation of naphthalene. In other words, the intermediate species
formed from O 2 and H 2 0 as the result of the reactions with photogenerated
electrons and holes are considered essential in the reaction.
When the hydroxylation reaction of naphthalene was carried out with Fe 3 + and
H 2 0 2 (the Fenton reaction), which produce the -OH radical, the main products
Ti0 2 powder (P-25, Degussa) showed the highest activity for the epoxidation
of2-hexene. Using this catalyst, the chemical yield of the epoxide reached 83%.
The average quantum efficiency of the epoxidation reaction reached 9.5% under
the photoirradiation for about 19 h.
Interestingly, the ratio of trans- to cis-2,3-epoxyhexane produced
photocatalytically using Ti0 2 (Ishihara, PT -101) reached 7.1, which is about 4
times the trans/cis ratio of 2-hexene used as the starting material. The trans
isomer was always the main product for 8 kinds ofTi0 2 powders investigated, as
shown in Fig. 11.7. This trans selectivity is considered to be related to the surface
properties of the Ti0 2 powders.
In order to clarify the stereochemistry of the epoxidation of 2-hexene, the
reaction was carried out on photo irradiated Ti0 2 powder (Ishihara, PT-I01) using
trans-2-hexene or cis-2-hexene as the starting material. From trans-2-hexene,
trans-2,3-epoxyhexane was obtained as the main product with the ratio between
trans and cis-2,3-epoxyhexane being 98.4 to 1.6 (Scheme 11.1).
trans-2-hexene trans-2,3-epoxyhaxene cis-2,3-epoxyhaxene
 O 2  "\fV
..
Ti0 2
0
98.4 1.6

O 2
Ti0 2
..

"\fV
o
12.0
88.0
o
E
::1.
--
(/)
U
::J
"l:J
o
is..
'0
1:
::J
o
E
«
. 2-formylcinnamaldehyde
o 1,3-dihydroxynaphlhalene
r2I 1,4-dihydroxynaphlhalene
Scheme 11.1
In the case of cis-2-hexene, the ratio of trans to cis-2,3-epoxyhexane was 12.0
to 88.0. These results indicate that the epoxidation of 2-hexene proceeds
stereospecifically on the photo irradiated Ti0 2 powder, i.e., the C2-C3 bond
conformation of the 2-hexene cation radical, which is the reaction intermediate,
was retained during epoxidation because the reactant was adsorbed strongly on the
surface of Ti0 2 powder leading to inhibition of the rotation of C2-C3 bond. The
high yield of trans-2,3-epoxyhexane is attributable to the higher reactivity of
trans-2-hexene than cis-2-hexane and to the retention of the trans and cis
configurations of the substrates during the epoxidation.
11.6.2 Selective Oxidation of Naphthalene by Molecular Oxygen and
Water Using TiO z Photo catalysts
TIO-4 TIO-5 TIO.2 CR-EL TIO-3 ST.11 ST-01 PT. 1 01
(P.25)
PhotocatalysIs
Crystallization R+A R A R R A A R
Surface area 39,3 2.1 18.0 5.4 42.3 65.1 192.5 24.5
m 2 tg
The photocatalytic reactions were carried out in Pyrex glass tubes containing
a mixture of acetonitrile and water and Ti0 2 powders at room temperature.
Oxygen gas was passed through the solution at a rate of 3.0 ml/min. A 500 W
Fig. 11.8 Photocatalytic oxidation of naphthalene on various kinds of TiO z in solution, The reaction
was carried out for 1 h in the mixed solution (acetonitrile-water) containing TiO z and
naphthalene. The solution was bubbled with O 2 . The crystal structures and surface areas are
listed underneath the graph. The main crystalline structure of each TiO z powder is denoted
by A (anatase) and R (rutile).

198
11 Photocatalytic Organic Syntheses Using Semiconductor Particles
l\.tae;;U::1H;e;;:;
1:1:1
80
. 2-formylcinnamaldehyde
. . 1,3-dihydroxynaphthalene
.A. 1,4-dihydroxynaphthalene
(5 60 . .
E .
::l . .
-- .
(/)
- 40
()
:J .
l:J .
0 . .
...
0... . .
.
20
.
.
0
0 5 10 15 20 25
Water content / vol%
11. 7 Concluding Remarks
A large number of recent reports on the photocatalytic reaction of organic
compounds have satisfied the basic requirements for organic synthesis, e.g.,
isolation and identification of products, and they have been published in
specialized journals of organic chemistry, such as The Journal of Organic
Chemistry, These trends indicate that organic chemists have begun to recognize
the semiconductor photocatalytic reaction as a novel synthetic tool. Thirteen
years ago, one of the authors (B. 0.) expressed in a review article 84 ) his dream that
an example of photocatalytic reaction would be introduced in the journal, Organic
Syntheses. The recent trends of research in this field and the unique characteristics
of the photocatalytic reaction as shown here strongly suggest that this dream will
come true in the near future. Further progress in the design of reaction and catalyst
from both organic and catalyst chemical approaches is the key to success in this
area.
References
Fig. 11.9 Effect of water content in a mixed solution (acetonitrile-water) on the amount of products
obtained after photoirradiation for 1 h. Reaction conditions are the same as those shown in
Fig, 11.8.
These reactions using Ti0 2 photo catalysts proceed with high chemical yield
and high quantum efficiency under photoirradiation using molecular oxygen as the
electron acceptor. The failure of early studies in the syntheses of these oxygen-
containing compounds is, presumably, due to lack of detailed optimization and
careful analysis of reaction products. It is strongly suggested that the
photocatalytic reaction system has the potential to be applied to practical synthesis
of oxygenated organic compounds.
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mechanism in which introduction of two hydroxy groups proceeded
simultaneously in the above-mentioned photocatalytic system. The production
of di-hydroxylated compounds (as well as the production of 2-
formylcinnamaldehyde) is, therefore, concluded to be unique to the photocatalytic
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We have recently found that the reaction is remarkably accelerated in the
presence of H 2 0 2 . 83) The rate was enhanced 6 - 40 times by the addition of H 2 0 2 ,
and the quantum yield reached as high as 76% for rutile Ti0 2 powders. On the
other hand, no such enhancement was observed for anatase Ti0 2 powders.
All the results suggest that a unique mechanism is involved in the
photocatalytic reaction of naphthalene on Ti0 2 particles. The details of the
mechanism are not clear at the present stage. However, the interaction between the
intermediate species generated from oxygen and water, which are assumed to be
bound to the surface, and the naphthalene molecules adsorbed on the Ti0 2 surface
is considered to be the key to the reaction.
11.6.3 Photocatalytic Oxygenation: Summary

200
11 Photocatalytic Organic Syntheses Using Semiconductor Particles
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12
Sonophotocatalysis-Joint System of Sonochemical
and Photocatalytic Reactions
12.1 Introduction-What is Sonophotocatalysis?
Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the
simultaneous irradiation of ultrasound and light with photocatalyst. This method
includes irradiation with alternating ultrasound and light. Ultrasound effects on
heterogeneous photocatalytic reaction systems have been demonstrated by
Mason,l) Sawada et al.,2) Kado et at.,3) and Suzuki et al,4) In these papers, not only
acceleration of photocatalytic reactions but increase in product selectivity by
ultrasonic irradiation has also been reported. It was postulated that ultrasound
effects, such as surface cleaning, particle size reduction and increased mass
transfer, were the result of the mechanical effects of ultrasound. 1,5) Lindley
reviewed these and other effects. 5 )
The effects of ultrasonic irradiation on photochemical reactions have been
also reported. 6 - 9 ) In those papers, effects of cavitation were demonstrated.
Cavitation means the process in which micro bubbles, which are formed within a
liquid during the rarefaction cycle of the acoustic wave, undergo violent collapse
during the compression cycle of the wave. 5 ) The dissociation of water to radicals
is an example of these effects. Since activated chemical species such as free
radicals have high reactivity, chemical reactions proceed. In other words, this
phenomenon is a chemical effect of ultrasonic waves.
The decomposition of liquid water and the following reactions are the results
of a typical chemical effect. In this case, however, overall water splitting does not
occur because oxygen is not obtained but hydrogen and hydrogen peroxide are. On
the other hand, it is impossible to decompose water by photochemical reaction
under illumination with a xenon lamp. Although it is possible to decompose
water by photocatalytic reaction using a desirable photocatalyst and photo-
irradiation, it is difficult to decompose in practice because of rapid backward
reaction, the formation and accumulation of intermediates onto the surface of
photocatalyst,IO) and other reasons.
Recently, liquid water was decomposed to hydrogen and oxygen
stoichiometrically and continuously by irradiations of ultrasound and light with
particulate photocatalyst. H) This reaction system is thought to be a joint one for
sonolysis and photocatalysis. Furthermore, this system also is a hybrid of
mechanical effects and chemical effects, In this chapter, the effect of ultrasound
on photocatalytic reaction is considered. The joint system of sonochemical and
photocatalytic reactions, in particular,is explained.

204
12 Sonophotocatalysis
hv
f
12.2 Utilization of Sonophotocatalytic Reaction
205
>
B. Overall Water Splitting)))
Liquid water was decomposed to H 2 and O 2 stoichiometrically and
continuously by sonophotocatalytic reaction, as shown Fig. 12.2. Effectiveness of
simultaneous irradiation of ultrasound and light is also confirmed as shown in
Table 12.1. This table shows the gaseous reaction products from liquid water in
Fig, 12,2 Time dependencies of so no photocatalytic reaction products from pure water. As powdered
photocatalyst, TiOrA (200mg, Soekawa, Commercial Reagent, rutile-rich type and specific
surface area 1.9 m 2 /g) was used without further treatment. Liquid water (I50 cm 3 , Wako,
Distilled water for HPLC) was used as reactant and was purged with argon. A Pyrex glass
bulb (250-300 cm 3 ) was used as a reactor and was placed in a temperature-controlled bath
(EYELA NTT-1200 and ECS-O) all time, After the glass bulb was sealed, the irradiation was
carried out under argon atmosphere at 35°C. Photo and ultrasonic irradiations were
performed from one side with a 500 W xenon lamp (Ushio, UXL500D-0) and from the
bottom with an ultrasonic generator (Kaijo, TA-4021-461 1,200 kHz 200 W), respectively.
Phorocatal ysis
A
Be...
Ultrasound

250
>
o
E
::t 200
.........
+-'
o
:]
"tJ
e 150
c.
en
:]
o
CI)
 1 00 -
bD
'+-
o
+-'
C
:] 50 -
o
E
<
Sonolysis
ABC ...
hv
Ultrasound L
 ;I
Do
lCJlL
>
or
Sonophotocatal ysis
ABC .n
C ...
A
B
Reactants
> Products
Fig, 12.1 Outline of this chapter: Sonophotocatalysis is better system for higher yield and selectivity.
It is also po.ssible to proceed the chemical reaction, which is hardly accomplished by
photocatalysis or sonolysis, independently.
12.2 Utilization of Sonophotocatalytic Reaction*l
12.2.1 Sonophotocatalysis of Water
A. General Considerations
Photocatalytic cleavage of water (H 2 0) is very attractive from the viewpoint
of solar energy conversion. In general, however, it is difficult to decompose pure
water in liquid phase to hydrogen (H 2 ) and oxygen (0 2 ) stoichiometrically and
continuously by photocatalysis under the illumination of a xenon lamp and normal
conditions such as room temperature, ambient pressure, neutralized pH, etc. In a
few successful cases, strict conditions or special photocatalysts were required,12-16)
On the other hand, it is known that H 2 is formed but formation of O 2
corresponding to evolved H 2 is not observed by sonolysis. Consequently, neither
photocatalysis nor sonolysis can accomplish overall water splitting in liquid
phase. The author has attempted to construct a combinational reaction system of
light and ultrasound in order to obtain H 2 and O 2 from water continuously.
I-H 2 !
-C>- 02
o
o
4
Reaction time / h
6
8
2
Table 12.1 Effect of simultaneous irradiation on gaseous reaction products from water under Ar
atmosphere
Photo- Light Ultrasound Product Icm 3 .3h- 1 Ratio of products
No. catalyst H 2 O 2 H 2 I O 2
0 X X X
1 Ti0 2 X X
2 Ti0 2 0 X Trace
3 Ti0 2 0 0 2.65 1.06 2.5
4 X 0 0 2.81 0,19 14.8
5 X X 0 3.26 0,17 19.2
6 Ti0 2 X 0 2,83 0,11 25.7
7 X 0 X
Phtocatalyst: 200 mg; Ultrasound source: 200 kHz 200 W; Light source: 500 W-Xe; Irradiation time:
3 hours; 0: with irradiation and X: without irradiation or without catalyst.
.1 Photo and Ultrasonic irradiations were performed with a 200W ultrasonic generator (200 kHz) and
a 500W Xe lamp.

206
12 Sonophotocatalysls
J£.L Ulll1Lc:1l1Ull Vi UHU}'1.1Vl.V\.la"u..lJu\,.l .I."-,",U",",UVJ..l
different conditions. Sonophotocatalysis of water is No.3. Other cases did not
accomplish overall water splitting. For example, no product was obtained without
irradiation for No.O and NO.1 in Table 12.1. In the cases of photochemical and
photocatalytic reactions (No.7 and No.2 in Table 12.1), the cleavage of water
seldom occurred. In the cases of sonolyses regardless of presence of photocatalyst
and photo-irradiation (Nos. 4, 5 and 7 in Table 12.1), H 2 and a corresponding
amount of hydrogen peroxide (H 2 0 2 ) were produced in gas phase and in liquid
phase, respectively. Fig. 12.3 shows sonication products from water. In these
cases, only a small amount of oxygen could also be detected. However, the ratio
of H 2 to O 2 differs from a desirable value estimated to be 2.0.
reported that the primary process of sonolysis for water was the formation of the
hydrogen radical (-H) and hydroxyl radical (-OH), as shown in Eq. (12.3).
H 2 0  -H + -OH
(12.3)
-H + -H  H 2
(12.4)
-OH + -OH  H 2 0 2
(12.5)
500
1-.-H2 I
-o-H202
It is thought that both active species react with each other and produce H 2 and
H 2 0 2 . It is apparent that H 2 is produced by dimerization of - H radicals and that
H 2 0 2 is produced by dimerization of -OH radicals. The recombination of -H and
-OH radicals may also occur. HoweveI;, there is no distinction between H 2 0
formed by the radical reaction and the one from solvent and/or the reactant. As a
result, water reacted to H 2 and H 2 0 2 by sonolysis. In order to accomplish overall
water splitting, H 2 0 2 must be decomposed to O 2 and H 2 0, as shown in Eq. (12.6).
600
100
2H 2 0 2  O 2 + 2H 2 0
(12.6)
'0
E 400
'-
+'
U
::::J
"'C
o
 300
....
o
+'
C
::::J
o 200
E
<
1
H 2 0  H 2 + - O 2
2
(12.1 )
As suggested in Table 12.1, H 2 0 2 seldom reacted this way by sonication. It
is known that H 2 0 2 is unstable under ordinary conditions. And this material might
be decomposed to O 2 under irradiation of ultraviolet (UV) light. Although the
white light from a xenon lamp includes UV, H 2 0 2 could seldom react (No.4 in
Table 12.1).
1t is also known that a metal oxide such as manganese oxide (Mn02) is
effective for decomposition of H 2 0 2 . An attempt to obtain O 2 was carried out
using metal oxide catalysts without photoirradiation.
In the case of Ti0 2 catalyst, only a small amount of O 2 was obtained by
sonication (No.6 in Table 12.1). On the contrary, in the case ofthe Mn02 catalyst,
the amount of O 2 evolved exceeded several times the corresponding amount of H 2 .
As manganese ion was detected in the solution, it was assumed that Mn02
dissolved into water by sonolysis, i.e. Mn ionized in solution to produce
manganese ion and O 2 was evolved. Thus, at the present time water cannot be
sonocatalytically decomposed into H 2 and O 2 in the dark.
Photocatalytic reaction is another candidate for decomposition ofH 2 0 2 . Fig.
12.4 indicates reduction H 2 0 2 with O 2 evolution by photocatalytic reaction, In
other words, H 2 0 2 was decomposed photocatalyticaly to O 2 continuously and
stoichiometricall y.
Thus, water decomposition is assumed to proceed by a two-step process as
follows:
o
o
5
1 0 15 20
Sonication time / h
25
Fig, 12:3 Time dependence of sonication products from pure water. Ultrasound: 200kHz, 200W;
Atmosphere: Ar; Temperature: 5°C.
Therefore, there are three categories of reactions:
(I) Sonophotocatalytic reaction, which means simultaneous light and ultrasound
irradiations with titanium oxide (Ti0 2 ) photocatalyst
(II) Sonochemical reactions with or without Ti0 2 photocatalyst
1 1
H 2 0  2' H 2 + 2' H 2 0 2
(Ill) Other reactions
No products
(12.2)
I st step: Sonochemical process,
2H 2 0  H 2 + H 2 0 2
(12.7)
2 nd step: Heterogeneous photocatalytic process,
It is known that sonochemical reaction of water consists of several elementary
processes although stable products are H 2 and H 2 0 2 . For example, Mead et aU7)
1
H 2 0 2  2' O 2 + H 2 0
(12.8)

208 12 Sonophotocatalysis
140
E 120
::t
'- 100
III
'C 80
C1)
.....
III
E 60
I+-
0
..... 40 -
C
:J
0
E 20
«
0
0
12,2 Utilization of Sonophotocatalytic Reaction 209
20
40
60
80
100
reactivity can be easily understood from Table 12.1. Each reaction rate was
estimated from the H 2 production rate and compared. The production rate of H 2
for the sonochemical system without photocatalyst (No.5 in Table 12.1) was
slightly more than that with photocatalyst (No.6 in Table 12.1). Thus it is thought
that the suspension of photocatalyst powder affects the reactivity of the
sonochemical reaction.
Figure 12.5 shows the time dependencies of the products for
sonophotocatalysis of water using a very fine powdered photocatalyst (tentatively
named Ti0 2 -B, specific surface area is 48,7 m 2 /g, further details on properties are
listed in Table 12.2), As a result of the first run in Fig. 12.5, there is an obvious
induction period for O 2 production. As oxygen was evolved on a suitable rate after
the induction period, reforming and/or refining on the surface of the photocatalyst
might occur by irradiation of ultrasound, and light, respectively during the
induction period. It was also assumed that the surface of photocatalyst was
covered with the intermediates. The author thought the adsorption of intermediates
on the surface of photocatalyst was the major reason for the induction period.
The adsorption was thought to be strong because ultrasonic vibration was
vigorous. In order to confirm the improvement of the product ratio, irradiations
were repeated twice as shown in Fig, 12.5. After the first run, the subsequent run
was started after the gaseous products were expelled from the reactor. In the case
of second run, product ratio was desirable value from the beginning of the
reaction. As the result, the improvement was held on. In other words, the
improvement of the product ratio was not transient.
Therefore, the intermediates are supposed to adsorb strongly on the surface
of photocatalyst such as chemisorptions. Through this observation, it would be
also confirmed that the photocatalyst was possible to re-use.
In the case of large particle of Ti0 2 (commercial reagent, tentatively named
I H 2 02 1
-0- 02
Reaction time / min
Fig, 12.4 Decrease of HzOz with production ofO z by photocatalytic reaction. Photocatalyst: TiOz-A,
200 mg; Light: 500 W-Xe; Temperature: 25°C; Atmosphere: Ar.
Results from examinations of each process (Fig. 12.3 for the first step and Fig.
12.4 for the second step) were satisfied with Eqs. (12.7) and (12.8). And the
products from water by sonophotocatalysis were also supported two-step
mechanism (Fig. 12.2). However, it is not clear that sonophotocatalysis of water
proceeds via this two-step process because there are a lot of elementary reactions
in this process. For example, O 2 would be produced not from H 2 0 2 but other
intermediates. Therefore, the detail mechanism of sonophotocatalysis must be
examined continuously from this time forth.
Overall water splitting, anyway, was accomplished by a combined effect of
sonolysis and photocatalysis.
In conclusion, the sonophotocatalytic system was clearly effective for the
decomposition of water into H 2 and O 2 .
180 1st run
(; 160 _ 140
E 0
:::s. 140  120 2nd run
:: 120 '- 100
" ..
-5 100 "
" 80
e ""
c- 80 e
.... c- 60
0 60 ....
... 0
c: .. 40
" 40 c:
0 "
E 20 0 20
< E
0 < 0
0 2 4 6 8 10 0 2 4 6
Reaction time / h Reaction time / h
C. Effect of Suspended Fine Particles on So no chemical Reaction l8 )
The sonophotocatalytic system is effective for overall water splitting as
shown in Fig, 12.2 and Table 12.1. This system requires, properly, a photocatalyst
such as particulate Ti0 2 . As ultrasonic waves pass through the solution, the
properties ofthe solution influence a sonochemical reaction. In particular, negative
effects are considered in the presence of powdered photocatalysts. The effects of
fine particles in the solution on the sonochemical reaction have been noted so far.
For example, Yasuda et aI, 19) reported the effects of insoluble particles, such as
silicon oxide (Si0 2 ) or aluminum oxide (AI 2 0 3 ), in the reactant solution on the
sonochemical reaction and demonstrated that the reaction rate constant depended
on particle properties, particle size and number of particles. It is assumed that a
powdered photocatalyst suspended in the solution obstructs the transmission of
ultrasonic waves. In this section, the influence of the photocatalyst powder
suspended in solution on the sonochemical reaction is examined.
The influence of the suspended photocatalyst powder in water on the
Fig. 12.5 Improvement of product ratio by repeated reaction, Photocatalyst: TiOz-B (Nippon Aerosil,
P-25, anatase-rich type and specific surface area 48.7m z /g, 200mg; Light: 500 W-Xe;
Ultrasound: 200kHz, 200W; Atmosphere: Ar,
Table 12.2 Comparison of properties of TiO z photocatalysts
TiO z Manufacture Crystal structure Specific surface area mZ/g
A Soekawa Rutile rich 1.9
B Nippon Aerosil Anatase rich 48.7
C Katayama Anatase 8,1

210
12 Sonophotocatalysis
12,2 Utilization of Sonophotocatalytic Reaction
211
10
50
100
200
3,6
6,3
8.1
24,2
They must be kept distinct one from another. Some differences of character have
been pointed out and Fujishima et al. listed the properties of both crystals. 20 ) The
Catalysis Society of Japan has also published the data for Reference Catalyst
Ti0 2 . *3 In this section, the relationship between crystal structure and product ratio
is examined.
The problem noted above must be pointed out once again. Fig. 12.6 shows the
time dependencies of products from water by sonophotocatalysis using Ti0 2 of
large particle size (tentatively named TiOrC, specific surface area is 8.1 m 2 /g,
further details listed in Table 12.2), The production rate of O 2 was very low
although surface area was small. It was stated above that the larger particle of the
photocatalyst, the smaller difference in product ratio. However, Fig. 12.6
contradicts that explanation.
Figure 12.7 indicates the dependence of product ratio on the specific surface
area. Some samples show great difference regardless of surface area. As illustrated
in Fig. 12.7, anatase type crystal gives bad product ratio. From these results, the
difference between anatase and rutile is clear. It is assumed that anatase combines
with H 2 0 2 to the complex. In other words, H 2 0 2 disappears although it is not
clear whether H 2 0 2 is adsorbed or not. One way or another, rutile rich Ti0 2 is
better for the sonophotocatalytic reaction of water.
Table 12.3 Gradual improvement in product ratios by reducing the photocatalyst
Amount of Photocatalyst I mg
HzlOz
Photocatalyst: TiOz(B specific surface area 48.7 mZlg); Ultrasound:
200 kHz, 290 W; Light: 500 W Xe; Temperature: 15°C;
Atmosphere: Ar; Irradiation time: 2 hours.
TiOrA, specific surface area is 1.9 m 2 /g, further details of properties are listed in
Table 12.2), by contrast, the product ratio was a suitable value even at the
beginning of the reaction as shown in Fig. 12.2. In general, the finer powder has
the higher catalytic activity because oflarge surface area. For the same reason, on
the other hand, higher amounts of intermediates such as H 2 0 2 are adsorbed on the
photocatalyst surface, and this adsorption may cause the induction period.
In order to confirm the dependence of the product ratio on surface area, other
experiment was performed. The amount of photocatalyst was changed and the
sonophotocatalytic reaction was performed. The product ratio in the early stage
of the reaction decreased to the desirable value when the amount of photocatalyst
was reduced, i.e., the surface area was reduced, as shown in Table 12.3. Although
improvement of the product ratio was confirmed, the ratio of H 2 to O 2 indicated
a high value, 3.6, even in the case of a small amount (lOmg) of photocatalyst. In
order to explain the phenomenon, another factor must be considered, i.e., the
difference in produCt ratio and the induction period discussed in the following
section.
Concluding this section, when very fine particles were dispersed in the
reactant solution, i.e., the number of particles and the surface area increased, the
reactivity of the sonophotocatalytic reaction decreased and the product ratio
became lower. In general, for photocatalytic reactions, the finer the photocatalyst,
the better for the reaction. However, for sonophotocatalytic reactions it was found
that the finer the particles such as Ti0 2 -B in the reactant solution, the worse the
product ratio. Since it is impractical to obtain and use a photocatalyst of very large
particle size to increase the activity limitlessly, a suitable particle size must be
selected to obtain high performance in the sonophotocatalytic reaction.
Heterogeneous photocatalytic reaction products and their production rates
depend on the kind of photocatalysts. As noted above, each photocatalyst powder
has different properties. In the case of the sonophotocatalytic reaction, products
or their yields also depend on the kind of photocatalysts. The effect of surface area
on product ratio was disussed in section (12.2.1C). The influence of surface
area on product ratio was noted, but factors other than surface area must be
introduced to explain the difference in product ratios. It is known that there are
several crystal structures of Ti0 2 . The major structures are anatase and rutile. *2
160
 140
 I+H
....... 120 (
....
() -0- 02 .
::3 . I
"'C 100
0
...
c.
CI) 80
::3
0
CI.)
CI) 60
m
bO

0
.... 40
c:
:::s
0
E 20
<.(
0
0 1 2 3 4 5 6
Reaction time / h
D. Dependence of Product Ratio on Type of Ti0 2
Fig. 12.6 Dependencies of sonophotocatalytic reaction products from pure water. Photocatalyst:
TiOz-C(Katayama, Commercial Reagent, anatase type and surface area 8,lm Z lg), 200 mg;
Light: 500 W-Xe; Ultrasound: 200kHz, 200W; Temperature: 35°C; Atmosphere: Ar.
.2 Brookite is reported as another structure
.3 Five kinds of TiO z are listed as reference samples.

212
12 Sonophotocatalysis
12.2 Utilization of Sonophotocatalytic Reaction
213
16
l:::,. l:::,.
14
12 -
l:::,. I Anatase I l:::,.
10
C\J
0 l:::,.
........ 8 -
C\J
:t:
6 -
4 - I Rutile I .
. .
2 -.
0
0 20 40 60 80
Specific surface area (m 2 /g)
reactions. Supporting effects have often been reported for the Ti0 2
photocatalyst. 21 ,22) Sakata et al. reported that platinum (Pt) and ruthenium oxide
(Ru02) were effective for H 2 and O 2 production, respectively.23) In this section, the
effect of a supporting material on the reactivity of sonophotocatalytic
decomposition of water is examined.
In order to diminish the induction period for O 2 evolution and to increase
product yields, metal or metal oxide supported photo catalysts were utilized. Fig.
12.8 shows gaseous reaction products from water by sonophotocatalysis using
three kinds of photocatalysts. Ti0 2 has an induction period for O 2 evolution, as
described above. Although improvement in the ratio of products was expected, 'it
was not confirmed, Because there are many candidates for improvement of
reactivity, trials must be continued.
F. Overall Water Splitting Using Visible Light 24 )-An Attempt at Solar
Energy Conversion
140
120 IDH21
0 02
E
::::s. 100
"-
.....
0
:;,
"C
0 80
l-
e.
en
:;,
0
II) 60
en
1'0
bII
'+-
0
.....
c: 40
;:]
0
E
<
20
0
Ti0 2 Pt/Ti0 2 RU02/Ti02
As a photocatalyst, Ti0 2 has been widely utilized for its strong oxidation
power, stability in solution, reasonable cost, and low poisonous characteristics. As
shown in previous sections, this photocatalyst is effective also for the
sonophotocatalytic reaction system. Because of a large bandgap, however, a
photocatalytic reaction using visible light cannot be put into practical use. If a
visible light-driven photocatalyst is utilized, overall water splitting should be
accomplished under both irradiation by visible light and ultrasound
It is known that cadmium sulfide (CdS) and ion oxide (Fe203) are visible
light-sensitive materials, in particular, one-step photocatalysis of water is possible
for CdS. 25 ) Recently, Kudo et al. reported that bismuth vanadate (BiV0 4 ) showed
photocatalytic activity for O 2 evolution from an aqueous silver nitrate solution
under visible light. 26 ,27) BiV0 4 is a very attractive photocatalyst although it is
impossible to decompose water by a one-step process. Thus this material is a
candidate for an O 2 evolution photocatalyst to construct a two-photon process. 26 )
In other words, this material is expected to be an O 2 evolution photocatalyst to
Fig, 12,7 Relationship between specific surface area and product ratio. All samples are commercial
TiO z (almost all are reagent grade). .6.: anatase or anatase-rich and e: rutile or rutile-rich.
E. Effect of Supporting Material on Reactivity
It is known that supporting materials such as noble metals and their oxides
contribute to improve activity or selectivity in catalytic and photocatalytic
140
E 120
::t
--..... 100
1'0
'C 80
Q)
+-'
1'0
E 60
4-
e
+-' 40
c
:J
e
E 20
<
0
- IDH 2 0 2 1
r:-:- -:-:- r:-:-
12'.]02
-
.,
7:
:
-
I.
Control
Ti02 -A SiVO 4
CdS
Fe203
Fig. 12,8 Effect of supporting materials on sonophotocatalysis of water. Photocatalyst: TiOz-B, 200
mg; LIght: 500 W-Xe; Ultrasound: 200kHz, 200W; Atmosphere: Ar; Reaction time: 3
hours.
Fig. 12,9 Photocatalytic reaction of HzOz using a variety of photocatalysts, Photocatalyst: 200 mg;
Light: 500 W-Xe; Reaction time: 1 hour; Temperature: 25°C; Atmosphere: Ar.

214
12,2 Utilization of Sonophotocatalytic Reaction
215
12 Sonophotocatalysis
illustrated in Eq. (12.8) and Fig. 12.4. Fig. 12.9 shows O 2 production rates from
H 2 0 2 by photocatalysis. In the cases of white light illumination by a xenon lamp,
neither CdS (commercial reagent) nor Fe203 (commercial reagent) was effective,
but BiV0 4 photocatalyst was. BiV0 4 used was prepared using soft solution
processes. 26 ) It was also reported that the bandgap of this material was 2.4 eV and
absorption edge was 51Onm. When shorter wavelength cutting filter of Y -43(>
430nm, Toshiba) was inserted in the light path, BiV0 4 photocatalyst retained the
reactivity, as shown in Fig. 12.10. On the contrary, in the case of Ti0 2
photocatalyst used for the reference, the O 2 production rate decreased markedly.
Thus H 2 0 2 was decomposed to O 2 and H 2 0 using BiV0 4 photocatalyst under
visible light irradiation.
In the next experiment, the simultaneous irradiation of visible light and
ultrasound was carried out 'using BiV0 4 photocatalyst in order to decompose
water. Fig. 12.11 hows the time dependencies of sonophotocatalytic reaction
products from water with the filter of Y-43. H 2 and O 2 evolved continuously,
During the reaction, dissolution of BiV0 4 powder in solution by ultrasonic
irradiation was not observed. Thus, BiV0 4 was driven as the photocatalyst. As
there are many visible light-sensitive materials, the discovery of other visible
light-driven photo catalysts is expected in the near future.
construct a two-step process for water decomposition using photon and ultrasonic
energy.
Before examing water splitting by sonophotocatalysis, photocatalysis ofH 2 0 2
was done because the photocatalytic process in the sonophotocatalytic water
splitting system was assumed to be due to the decomposition of H 2 0 2 , as
140
- .- H 2 0 2 (Ti0 2 )
- .- 02(Ti02)
120 . . -lr-H2(h(BiV04)
- - - - - -o-02(BiV04)
15 - - - -----A-
E 100 --.--.-A
::s.
--.....
III 80
'1: -
C1)
.....
III
E
'+- 60
0
.....
C
:::I 40
0 -
E
«
20
0
0 20 40 60 80 100
Reaction time / min
G. Isolation of Products Using Alternating Irradiation Method 28 )
The sonophotocatalytic reaction system is a novel idea for overall liquid
water splitting. The H 2 obtained, however, was mixed with other products such as
O 2 in the reactor. When H 2 is utilized in practice, e.g., in fuel or chemical
synthesis, etc., it is desirable to isolate H 2 from the product. Namely, isolation of
H 2 is required. Although separation of H 2 from other products may be possible
using a membrane, the method is likely to skip this process, and it is desirable to
avoid the use of additional equipment as much as possible.
In this section, the possibility of isolating products during sonophotocatalysis
is discussed. In order to isolate H 2 and O 2 , an attempt was made to carry out
sonolysis and photocatalysis separately one after the other.
As H 2 was obtained by the first step as shown in Eq. (12.7), the use of only
ultrasonic irradiation is sufficient to obtain H 2 . Along with H 2 evolution, however,
H 2 0 2 accumulates in the solution. Hydrogen peroxide must be decomposed in
order to protect against the changing properties of the reactant. Although
simultaneous irradiation is effective for sonophotocatalytic overall water splitting,
this method is not sufficient for isolation of products. Thus, sonophotocatalysis
with simultaneous irradiation must be modified,
If'a two-step mechanism, as shown in Eqs. (12.7) and (12.8), occurs, each
product will be produced independently by alternating irradiation of ultrasound
and light. An alternating irradiation was performed for overall water splitting
under argon atmosphere. Ultrasonic wave must be irradiated first, followed by
photo irradiation. This order of irradiation was decided from the scheme of
sonophotocatalysis of water, as shown in Eqs. (12.7) and (12.8). As shown in Fig.
12.12, H 2 and O 2 were evolved by sonolysis and photocatalysis, respectively.
Thus each gaseous product was collected independently.
In the case of electrolysis of water, each gaseous product is evolved in a
Fig. 12,10 Decrease of H 2 0 2 with production of O 2 by photocatalytic reaction using visible light.
Photocatalyst: TiOz-A or BiV0 4 , 200 mg; Light: 500 W-Xe equipped with shorter
wavelength cut filter (>430 nm); Temperature: 25°C; Atmosphere: Ar,
60
 50 g
::s. O2
--..... 40
.....
(J
:::I

e 30
Q.
'+-
0 20
.....
C
:J
0
E 10
«
0
0 2 4 6
Reaction time / h
Fig. 12,11 Time dependencies of sonophotocatalytic reaction products from pure water using visible
light. Pre-irradiation was carried out for 3 hours.; Photocatalyst: BiV0 4 , 200 mg; Light:
500 W-Xe equipped with shorter wavelength cut filter (>430 nm); Ultrasound: 200kHz,
200W; Temperature: 25°C; Atmosphere: Ar.

216 12 Sonophotocatalysis
120
0
E 100 -
::::t
......... 80
+'
0
:J
"'0
0 60 -
L-
a.
4-
0 40
+'
C
:J
0
E 20
«
0
0
2
1 2,2 Utilization of Sonophotocatalytic Reaction 217


As described above, each reaction consisting of a two-step process is
influenced by the addition ofNaCl. Thus, it is assumed that the presence ofNaCl
in solution affects sonophotocatalytic reaction products orland their yields. In
this section, the influence of NaCI dissolved in water on the sonophotocatalytic
reaction is considered after reconfirmation of the influence ofNaCI on the reaction
of each process.
B. Effects of NaCI on Photocatalytic and Sonochemical Reactions
First, the photocatalytic decomposition of H 2 0 2 was examined in the presence
and absence of NaCl. The influence of .Cl radicals from the surface of Ti0 2
photocatalyst by irradiation of UV could not be observed. The reducing rate of
H 2 0 2 in NaCI solution was similar to that in pure water even at high
concentrations such as 10% NaCI solution. 28 ) It was clear that the amount of O 2
produced corresponded to that of H 2 0 2 reduced. It is assumed that the
photocatalytic reaction of H 2 0 2 resists the influence of the addition of NaC1.
Next the influence of the addition ofNaCI on a sonochemical reaction was
,
considered. Fig. 12.13 shows the effect of NaCl concentration ,in water on the
yield of product. The amount of product depended on the concentration ofNaCl,
which was different for each product. The gradient for H 2 0 2 was larger than that
for H 2 . Consequently, the product ratio depended on the concentration ofNaCI in
the solution.
3
4
5
6
Reaction time / h
H 2 0 2 + .Cl  H0 2 + RCI
Fig, 12,12 Isolation ofH 2 and O 2 by sonophotocatalytic reaction using alternating madiation method,
Photocatalyst: Ti0 2 -B, 200 mg; Light: 500 W-Xe; Ultrasound: 200kHz, 200W;
Temperature: 35°C; Atmosphere: Ar.
different place (electrode) at the same time. However, in sonophotocatalysis using
alternating irradiation technique, each product was evolved in the same place
(reactor) at a different time.
12.2.2 Sonophotocatalysis of Artificial Seawater 18 ,28)
100
A. General Considerations
As seawater is the most available water on earth, the possibility of application
of sonophotocatalytic reaction system to seawater cleavage is also examined. it is
known that sodium chloride (NaCl) is the principal mineral constituent of seawater
and chloride ion (Cl-) concentration is nearly 2%.29) When the desired amount of
NaCI is added into the system, effects ofNaCI 011 the reaction of each process are
assumed to occur.
Hirakawa et aI. reported the effect of the addition ofNaCl in photocatalytic
reactions using :ri0 2 powders. 30 ) They explained that adsorbed chloride ions and
holes on Ti0 2 reacted to chloride radicals(.Cl), as shown in Eq. (12.9), and these
radicals attacked species such as H 2 0 2 , as shown in Eq. (12.10). Thus, R 2 0 2 did
not decompose to O 2 but was scavenged by .Cl in this reaction system.
It has also been reported that the sonochemical reaction is prevented by the
presence of NaCI (or chloride ions) in the solution. 18 ,31,32) In these studies, it was
demonstrated that the yields of products such as H 2 0 2 decreased in the presence
of Cl-. The greater the concentration of NaCI in the solution, the lower the yield
of product obtained, and the product ratio depended on the concentration ofNaC1.
Cl- + p+  .Cl
o
E
::::t
.........
.....
o
:J
"'C
e 10
a.
4-
o
.....
C
:J
o
E
«
,. H, !
-irH202
(12.9)
(12.10)
1
0.01
1-
NaCL / %
10
100
0.1
Fig, 12.13 Relationship between sonochemica1 products and NaCl concentration, Ultrasound:
200kHz, 200W; Temperature: 25°C; Atmosphere: Ar,

218
12 Sonophotocatalysis
12.2 Utilization of Sonophotocatalytic Reaction
219
The changing character of the reactant is the first reason for the decrease. As
different gradients were observed for each product, other reasons had to be
considered. The chemical effects were thought to be the second reason, The
scavenging of intermediates is the typical chemical effect for decreasing. When
Cl- ions are added to water during sonication, chemical reactions occur as shown
below.
12.2.3 Sonophotocatalyses of Organic Compounds
-OH + Cl-  OH- + -Cl
(12.11)
Photocatalytic and sonochemical reactions have been studied for a long time,
but there are very few reports on sonophotocatalytic reactions. In these articles,
the major targets of researchers who study sonophotocatalytic reactions were not
inorganic materials but organic compounds. For example, Mason,l) Sawada et
al.,2) Kado et al. 3), Suzuki et ai. 4 ) and Stock et ai. 33 ) reported on pentacholrophnol,
organic chloride, 2-propanol, surfactant and dye, respectively. , .
In this section, the sonophotocatalytic reaction of an orgamc compound IS
demonstrated. As a reactant, acetic acid is introduced. The reaction of acetic acid'
is a typical and a very famous photocatalytic reaction, called the Photo-Kolbe
reaction. 34 )
H 2 0 2 + Cl-  H 2 0 + CIO-
(12.12)
H 2 0 2 + CIO-  H 2 0 + O 2 + Cl-
(12.13)
The reason for the scavenging -OH radicals, as indicated by Eq. (12.11), is the
probability that collisions for -OH radicals are reduced and dimerization impeded.
Although H 2 0 2 was assumed to react with Cl- or with CIO-, [Eqs. (12.12) and
(12.13), respectively], decrease in H 2 0 2 was not observed in NaCI-H 2 0 2 solution
without irradiation. On the other hand, decrease in H 2 0 2 was observed when
NaCl was added to H 2 0 2 solution with irradiation of ultrasound. Thus, it is
assumed that H 2 0 2 was attacked by active chlorides such as -Cl radicals,
Consequently, sonochemical reaction of water was sensitive to the addition
of NaCl.
35
30
o Photocat.
0
E 25 !'J Sonophotocat.
::s.
--.....
+'
g 20
"'0
0
\..
a.
..... 15
0
+'
C

0 10
E
-<
5
0
CO 2 CH 4
c. Aim at Decomposition of Seawater
It is true that a sonochemical reaction is prevented by the addition of NaC1.
As shown in Fig. 12.10, however, the ratio of H 2 to H 2 0 2 and the yield of H 2
changed gradually at low concentrations. Thus, only a small difference in
sonochemical reaction products was observed between water and a 4% NaCI
solution like seawater.
Table 12.4 lists the sonophotocatalytic reaction products from pure water
and those from 4% NaCl solution. 18 ) The results from 4% NaCl solution were
obtained under the most desirable conditions. The sonophotocatalytic reaction was
impeded by the addition of NaCI to water. However, only a slight influence was
observed at low concentration such as seawater.
In conclusion, the decomposition of seawater into H 2 and O 2 is expected to be
accomplished using the sonophotocatalytic reaction. Furthermore, H 2 and O 2 were
obtained separately by the alternating irradiation technique, as described in section
12.2.10.
Fig. 12.14 Effect of sonication on yields of photocatalytic reaction products from 10% acetic acid
solution, Photocatalyst: 5% Pt/(TiOz-B), 200 mg; Light: 500 W-Xe; Ultrasound: 200kHz,
200W; Temperature: 20°C; Atmosphere: Ar; Reaction time: 1 hour.
Table 12.4 Sonophotocatalytic reaction products from water and 4% NaCl solution under Ar
atmosphere at 35°C
Table 12.5 Effect of sonication on photocatalytic reaction products from acetic acid at 20°C under Ar
atmosphere
Product / pmol'2h- 1 Product ratio
Salt Hz HzOz Oz Hz/Oz
no 71 2 27 2.6
NaCl(4%) 59 8 22 2.6
System
Hz
co
Product / cm 3 h- 1
CO z CH 4
C Z H6
C Z H4
Photocatalyst, TiOz-A, 200 mg; Ultrasound, 200 kHz, 200 W; Light, 500 W Xe.
Photocat. trace 0.27 0,29 0,03
Sonochem. 4.7 4.9 0.29 0,53 0,03 0,18
Sonophoto, 5.0 5.5 0.34 0,65 0.03 0,20
Photocatalyst: 5% Pt/(TiOz-B), 300 mg; Ultrasound: 200 kHz 200 W; Light: 500 W-Xe,

220
12 Sonophotocatalysis
References
221
CH 3 COOH  CH 4 + CO 2
(12.14)
is greateful to Nippon Aeresil Co. Ltd. for suppling Ti0 2 (P-25), This research
program was supported by Meisei University and by the Advanced Materials R &
D Center of Meisei University.
Photo-Kolbe reaction:
Figure 12.14 shows the effect of ultrasound on the amount of the main
products of the Photo-Kolbe reaction. The reaction appears to have been
accelerated by ultrasonic irradiation. The product ratio of the sonophotocatalytic
reaction, however, was not satisfactory. A reasonable value of methane (CH 4 ) to
carbon dioxide (C0 2 ) must be 1.0 for Photo-Kolbe reaction, as shown in Eq.
(12.14),
References
1 1
CH 3 COOH  "2 C 2 H 6 + CO 2 +"2 H 2
(12.15)
1. T,J,Mason, "Current Trends and Future Prospects", in: Current Trends in Sonochemistry,
(G,J.Price ed.), p.171, The Royal Society of Chemistry, Cambridge (1992),
2. KSawada, T .Moriizumi, KHirano, Proceedings of the 6,h Annual Meeting of the Japan Society
of So no chemistry, 49 (1997)[in Japanese]; KHirano, H.Takigawa, M,Karori and KSawada, '95
Asian Conference on Electrochemistry, IP-40 (1995).
3, Y,Kado, M,Atobe, T.Nonaka, Denki Kagaku (Electrochemistry), 66, 760 (1998) [in Japanese].
4, Y,Suzuki, Warsito, H,Arakawa, A,Maezawa and S,Uchida, International J,Photoenrgy, 1, 1
(1999),
5, J,Lindley, Sonochemistry (T.J,Mason ed,), Chapter 8, p.102, The Royal Society of Chemistry
(1991),
6. A,Gaplovsky, J.Donovalova, S.Toma, R,Kubinec, Ultrason. Sonochem" 4, 109 (1997).
7. H.Sohmiya, T.Kimura, M.Fujita, T,Ando, Proceedings of the 5,h Annual Meeting of the Japan
Society of So no chemistry , 26 (1996)[in Japanese]
8, T,Kimura, M,Fujita, T.Ando, Ultrason. Sonochem" 6, 93 (1999),
9, A.Gaplovsky, J.Donovalova, S,Toma and R,Kubinec, JPhotochem.Photobiol. A: Chemistry,
115, 13 (1998).
10. A.Mills and G.Porter, JChem,Soc.,Faraday Trans. I, 78, 3659 (1982).
11. H.Harada, Ultrason. Sonochem" 8, 55 (2001),
12, A.Kudo, A.Tanaka, K.Domen, K,Maruya, KAika, T.Onishi, JCata/., 111, 67 (1988).
13, KSayama H,Arakawa, JChem,Soc. Chem.Commun., 150 (1992).
14. KSayama, H.Arakawa, JChem,Soc, Faraday Trans, 93, 1647 (1997).
,15, YInoue, T.Niiyama, Y.Asai and KSato, JChem.Soc, Chem.Commun" 579 (1992).
16. S.Sao, Kikan Kagaku Sosetsu, Japan Chemical Society, p.106 (1994)[in Japanese].
17, E,L.Mead, R,G,Sutherland, R,E.Verrall, Can. J. Chem" 54, 1114 (1976),
18, H.Harada, Jpn.J,Appl.Phys" 39, 2974 (2000),
19, K Yasuda, R. Tanigawa, M. Oga, K. Uehara, M. Tachi, Y. Bando and M, Nakamura: Proc, 8th
Ann. Meet, of Jpn. Soc. of Sonochem. Kyoto, 38, (1999)[in Japanese].
20. A.Fujishima, K.Hashimoto and T.Watanabe, in: Ti0 2 Phtocatalysis -Fundamentals and
Applications-, p,123, BKC Inc. (1999).
21. T,Sakata and T.Kawai, J.Chem,Soc. Chem.Commun" 694 (1980).
22. H,Harada, Kikan Kagaku Sosetsu, p,69, Japan Chemical Society (1994) [in Japanese].
23. T,Sakata, K.Hashimoto and T.Kawai, JPhys.Chem" 88,5214 (1988).
24. H.Harada, C. Hosoki and A,Kudo, J,Phtochem,Photobiol" A: Chemistry [to be published].
25. A,Fujishima, KHashimoto and T,Watanabe, in: Ti0 2 Photocatalysis -Fundamentals and
Applications-, p.128, BKC Inc, (1999).
26. A.Kudo, KOmori and H.Kato, J.Am,Chem,Soc" 121, 11459 (1999).
27. KKudo, KUeda, H.Kato and I.Mikami, CataI.Lett" 53, 229 (1998).
28, H.Harada, Int.J.Hydrgen Energy, [in press],
29, Encyclopedia Chimica (Kagaku Daijiten), Vol. 2, p. 250, Kyoritsu, Tokyo, (1963) [in Japanese].
30. Hirakawa T, Nakaoka Y, Nishino J, Nosaka Y. Primary, JPhys,Chem. B, 103,4399 (1999).
31. C,A.Wakeford, R.Blackbum, P.D.Lickiss, Ultrason. Sonochem., 6, 141 (1999),
32. Chohonpa Gijyucsu Binran (J,Saneyoshi, ed,), p.210, Nikkan Kogyo Shimbun-sha, (1971) [in
Japanese]. -
33. N, L. Stock, J. Peller, K. Vinodgopal and P, Kamat, Envion, Sci, Techno/.,34, 1747 (2000),
34. B.Kraeutler, C.D.Jaeger and A.J.Bard, J.Am.Chem,Soc., 100, 4903 (1978).
35. H.Harada, Ultrason, Sonochem" 5, 73 (1998),
36. J-L.Luche, P,Cintasad and N.Kards, Proceedings of the 9th Annual Meeting of the Japan Society
of Sonochemistry, 1 (2000).
As a subreaction, the formation of ethane (C 2 H 6 ) is considered. All of gaseous
products are listed in Table 12.5. The formation rate of C 2 H 6 did not increase when
simultaneous irradiation was performed. Table 12.5 suggests that an ultrasonic
irradiation does not positively affect the formation rates of products except carbon
monoxide (CO), However, one interesting feature of this system was found, CO 2
was reduced with increasing CO, The amount of CO 2 in the sonophotocatalytic
system was less than that found when CO 2 was added to sonochemical and
photocatalytic systems. It is postulated that reducing CO 2 is the result of the
sonochemical reduction of CO 2 .35)
12.3 Conclusion and Future Scopes
The combined effect of sonolysis and photocatalysis on chemical reactions
was illustrated. Liquid water decomposition, in particular, was demonstrated.
Overall water splitting was hardly accomplished by photocatalysis or sonolysis,
independently. Using by sonophotocatalytical technique, however, water was
decomposed to hydrogen and oxygen stoichiometrically and continuously. And
each product was isolated using alternating irradiation method. Sodium chloride
solution like seawater was also decomposed using similar technique. Furthermore,
the visible light-sensitive photocatalyst was applied to this overall water splitting
system.
Sonication is a tool for improvement of chemical processes such as
photocatalytic reaction. The improvements of reaction rates, yields and selectivity,
the generation of reactive intermediate species and so on were reviewed. 36 ) Some
examples have been also shown in this chapter. The development of a new
reaction pass by the combined effect of photocatalysis and sonolysis is expected
in the near future. The contribution to Green Chemistry is one of typical examples.
On other contribution of sonophotocatalytic reaction, it may be thought to
construct the system like Photosynthesis, since oxygen evolution and carbon
dioxide fixation proceed simultaneously by sonophotocatalysis.
Acknowledgments
The author wishes to thank Prof. Akihiko Kudo at the Science University of
Tokyo for fruitful discussions about visible light-sensitive photocatalysts. Author

13
Gas-phase Water Photolysis by N aOD-coated
Photocatalysts
13.1 Introduction
The photodecomposition of water is a key process of artificial photosynthesis,
the target of great ambition among scientists in photochemistry. Since Honda
and Fujishima I) discovered photoinduced water splitting in the
photoelectrochemical (PEC) cell with a Ti0 2 photoanode and a Pt counter
electrode, many efforts have been made to investigate water photodecomposition
using semiconductor photoelectrodes from the viewpoint of light-to-chemical
conversion. 2 -4) A metallized, powdered semiconductor is thought to function as a
micro-PEC cell and has been used as a typical photocatalyst. S ) Because platinized
Ti0 2 (Pt/Ti0 2 ) works like a TiOrPt PEC cell,S) one may suspect that it can
photodecompose water into H 2 and O 2 , When Pt/Ti0 2 is suspended in pure water
followed by band-gap irradiation, however, no O 2 is evolved while a small amount
of H 2 is sometimes produced. This result was often interpreted as the inability of
Ti0 2 to photodecompose water without external bias or the formation of peroxide
species such as H202 On the other hand, Schrauzer and Guth 6 ) reported that water
chemisorbed on Ti0 2 powder is photolyzed into H 2 and O 2 in a molar ratio of2: 1
when conducted under Ar atmosphere. After their report, some workers tried to
reproduce their results but could not observe the formation ofH 2 and O 2 for Ti0 2 .
Among such efforts, Sato and White 7 ) found that Pt/Ti0 2 can photodecompose
water into H 2 and O 2 if the catalyst is in "a wet state", but no products are
observed if the catalyst is in "a dry state" even in the presence of gas-phase water
at the saturation pressure. The reason for this is disscussed below. Shortly before
their finding, Wagner and Somorjai 8 ) reported that platinized SrTi0 3 single crystal
can photodecompose gas-phase water into f!2 and O 2 when it is coated with a
deliquescent basic material such as NaOH. Their NaOH-coating method was
succeeded by Sato and White 9 ) for gas-phase water photolysis by NaOH-coated
Pt/Ti0 2 . The efficiency of water photo splitting by a NaOH-coating method was
found to be much higher than that by "a wet state" method. The NaOH-coating of
metallized metal-oxide semiconductors was thus established as an excellent
method to enhance photocatalytic water photolysis. In this chapter, the studies on
water photolysis over metallized semiconquctors are reviewed from the viewpoint
of factors influencing the formal yield.

224
13 Gas-phase Water Photolysis by NaOH-coated Photocatalysts
13,2 Water Photolysis by Pt/TiO z
225
13.2 Water Photolysis by Pt/Ti0 2

Vacuum
t
a case where, Pt/Ti0 2 suspended in pure water is irradiated. Sato and White,
however, found that both H 2 and O 2 are produced in a ratio of 2: 1 as shown in Fig.
13.2, when Pt/Ti0 2 in the cell is moistened with a small amount of water (this is
referred to as wet state) and illuminated.?) The formation rates of H 2 and O 2
decline with illumination time. The amounts of H 2 and O 2 decrease quickly after
the lamp is turned off, indicating their fast recombination on Pt sites. The product
yield is very sensitive to the amount of water added to the catalyst, and there is
an optimum amount of water. 7) These results lead us to suppose that the reverse
reaction, 2H 2 + O 2  2H 2 0, occurs so rapidly on Pt in a dry or suspension state
that the products are not observed even if water photolysis takes place on the
catalyst. A thin film of water on the catalyst would prevent the reaction ofH 2 with
O 2 to occur rapidly but enables the products to escape from the catalyst surface to
the gas phase. Tabata et al. 10) reported recently that the yield of water photolysis
by Pt/Ti0 2 powder suspended in pure water depends on the direction of
irradiation. Irradiation from the top of the reaction cell gives 10 3 times higher yield
than from the bottom,lO) In this case the suspension must be stirred rigorously by
a magnetic stirrer to float the catalyst up to the suspension surface. The situation
of the catalyst near the surface is basically the same as that in the wet state, i.e.,
the water layer on the catalyst in the suspension surface is very thin.
To examine the occurrence of gas-phase water photolysis in the dry state, CO
was added to the system. H 2 and CO 2 are formed in a ratio of2: 1, indicating that
,the reaction, H 2 0 + CO  H 2 + CO 2 , takes place. II) Since this reaction does not
occur on Ti0 2 alone, H 2 should be formed at Pt,sites and CO 2 at Ti0 2 sites by the
reaction of CO with the oxidation products of water such as oxygen molecules or
OH radicals. Interestingly, a small amount of O 2 is also formed when CO pressure
is low, as shown in Fig. 13.3,u) After the complete consumption of CO, O 2 as well
as H 2 decreases, This result implies that gas-phase water photolysis can occur on
Pt/Ti0 2 in the dry state if the reverse reaction is inhibited by, for example, CO
adsorption on Pt. It was also demonstrated that the reaction of H4 with gas-phase
Gas-phase heterogeneous reactions have been carried out in a closed
circulating system evacuable to high vacuum (Fig. 13.1). For heterogeneous
photocatalytic reactions, a quartz reaction cell with flat bottom (R. C. in Fig.
13.1) has been used. When PtlTi0 2 powder is spread over the bottom of the cell
followed by evacuation (this is referred to as dry state) and illuminated in the
presence of gas-phase water, neither H 2 nor O 2 is observed. The same is true for
Circulation
Pump
Mass
spectrometer
H2 0
Mirr 
/f---- ----<--- 1 500W Hg Lamp l
 Filter(UV-D33S+H20)
R.C.
Trap
Fig, 13,1 Schematic diagram of the circulating reaction apparatus for heterogeneous photocatalytic
reactions. R, Coo Reaction cell, F: Filter (UV-D33S + HzO),
0,3
Light off Light on
 
0.1
t
o

-
(\)
S
{/)
{/)
(\)

Cl..
5
 0.2
!3
l/J
l/J


o
50
100
150
200
250
o
10
20
30
40
Time/min
Time/min
Fig. 13,2 Time dependence of Hz and Oz evolution from illuminated Pt/TiO z moistened with a small
amount of water.
Fig. 13.3 Formation of Oz during the photocatalytic reaction ofHzO(g) with CO over PtlTiO z . Water
vapor pressure is ca. 24 Torr,

226
13 Gas-phase Water Photolysis by NaOH-coated Photocatalysts
13.3 Water Photolysis by Metallized Semiconductor Powders
227
water on illuminated Pt/Ti0 2 leads to the simultaneous formation of both the
reduction product, C 2 H 6 , and the oxidation product, CO 2 . 12 )
150
13.3 Water Photolysis by Metallized Semiconductor Powders
....
I
...r::
13.3.1 Gas-phase Water Photolysis by NaOH-coating
.-4
o
[ 100
--....
v

I-<
P
o
.......
+->
U 50
;:j
""0
o
I-<
0...
N

The above results indicate that a requirement for water photolysis by Pt/Ti0 2
is to prevent the reverse reaction on Pt sites. Wagner and Somorjai 8 ) successfully
carried out gas-phase water photolysis by Pt/SrTiOrcrystal coated with
deliquescent basic materials. Their method is reasonable to suppress the reverse
reaction, because a deliquescent material coated on a substrate absorbs a large
amount of water to form a thin film of its aqueous solution. The film inhibits the
reaction products to readsorb directly on the catalyst, while the products on the
catalyst can escape to the gas phase by diffusion. It is very important that H 2 and
O 2 can desorb from the catalyst surface to the gas phase without making bubbles,
because if they desorb as bubbles then they would inevitably mix with each other
in the growing process of bubbles and recombine on Pt sites. In addition, an
aqueous basic solution would work as an electrolyte which enhances ion transfer
in photoelectrochemical reactions.
When Pt/Ti0 2 powder is coated with NaOH and illuminated in the presence
of gas-phase water, H 2 and O 2 are produced in a stoichiometric ratio of 2: 1 even
in the dry state. 9 ,I3) Rh and Pd loaded Ti0 2 powders also show photocatalytic
activity for gas-phase water photolysis when coated with NaOH.14) The product
formation rates decline with time due to the reverse reaction. The yield of gas-
phase water photolysis depends on the pressure of gas-phase water, as shown in
Fig. 13.4,14) Since the yield is also dependent upon the amount of NaOH coated,
o
10
20
30
NaOH coated/wt 0/0
Fig. 13,5 Dependence of the maximum yield of gas-phase water photolysis on the amount of NaOH
coated on Rh/TiO z , HzlOz ratio is stoichiometric in all runs,
100
I

.-4
0
8
:::l 60
--....
v

I-<
I::
0 40
.......
+->
U
r6
0
I-<
0...
::£'
0
0
10 wt% NaOH
P Sal
the maximum yield is determined by changing water vapor pressure and NaOH
loading, as shown in Fig. 13.5. 14 ) Thus, there is an optimum set of water-vapor
pressure and NaOH loading for gas-phase water photolysis by NaOH-coated,
metallized Ti0 2 powders. This would arise from an optimum thickness of aqueous
NaOH-solution film on the catalyst, since the film thickness determines the
diffusion rates of the products through the film. After the optimization ofNaOH-
loading and water-vapor pressure, the highest quantum yield of water photolysis
was ca. 29% (assuming that two photons are required for one H 2 molecule) when
the Rh/Ti0 2 was used. 14 )
To estimate the optimum thickness of the solution, the yield was measured as
a function of the amount of water on the catalyst. In this experiment, Pt/Ti0 2
powder was immersed in the measured amount of NaOH solution and the
photoyield determined. After the yield was measured, the solution volume was
reduced by pumping for an appropriate time through an outside cold trap to
measure the amount of water removed from the NaOH solution. As shown in
Fig. 13.6, the yield sharply increased when the solution was reduced to a certain
amount and then decreased upon further removal of water. IS ,16) This result
indicates that the yield is mainly influenced by the thickness of the solution on the
catalyst and a concentrated NaOH solution appears to enhance the reaction.
Wagner and Somorjai 8 ) also reported that the yield of gas-phase water photolysis
by NaOH-coated SrTi0 3 increases with increased NaOH loading. The thickness
ofNaOH solution at the optimum condition in Fig. 13.6 is estimated to be less
than 0.1 mm. The rate constant for the reaction of H 2 with O 2 in the dark was
measured as a function of the amount ofNaOH solution. As shown in Fig. 13.7,
the rate constant decreases linearly with increase in the amount of solution and
drops to almost zero at 0.2 ml of the solution. 16 )
It is worth noting that the photocatalytic activity of Pt/Ti0 2 for water
photolysis is significantly enhanced when the catalyst is irradiated for a prolonged
t.3 wt% NaOH
10
20
Water-vapor pressure/Torr
Fig. 13.4 Dependence of the yield of gas-phase water photolysis on water vapor pressure over 1.3 wt%
and 10 wt% NaOH-coated Rh/TiO z , Psat is the saturated vapor pressure at ambient
temperature. HzlOz ratio is stoichiometric in all runs,

228
13.3 Water Photolysis by Metallized Semiconductor Powders
229
13 Gas-phase Water Photolysis by NaOH-coated Photocatalysts
l..d
'0 40
S
::L
........
 30
I-<
t:1
o
'-@ 20
........
o
>-
<1J
£ ]0
50
time in a concentrated NaOH solution. 13 - 16 ) The reason for this is not clear but may
be related to the fact that. the activity of metallized semiconductors for water
photolysis is greatly enhanced by the addition of Na2C03 or NaHC0 3 .17) Since a
small amount of CO 2 is produced during water photolysis on Pt/Ti0 2 from
hydrocarbon impurities in Pt/Ti0 2 ,7) Na carbonates could be formed by irradiation
in NaOH solution.
13.3.2 Factors Influencing Yield of Water Photolysis
Besides the solution thickness on the catalyst and the concentration of
deliquescent material, various other factors which may influence the yield of
water photolysis over metallized semiconductor powders have been examined.
A. Electrolyte Materials
o
o
(19)
0.2
KOH is a deliquescent basic electrolyte like NaOH, and its coating on Pt/Ti0 2
makes possible gas-phase water photolysis as described earlier for SrTi0 3 crystal. 8)
In experiments similar to that shown in Fig. 13.6, the yield is maximized at ca.
0,12 ml ofKOH solution. 16 ) The maximum yield is, however, less than one half
the maximum yield in NaOH solution, 16) In addition, the photocatalytic activity of
Pt/Ti0 2 declines with illumination time in KOH solution by unknown
mechanisms.
LiOH is a nondeliquescent basic electrolyte so that a very thin film of LiOH
solution is not so likely to be formed on the catalyst to achieve gas-phase water
photolysis, but it may work as an electrolyte in wet-state water photolysis. When
Pt/Ti0 2 is immersed in LiOH solution, no O 2 is evolved and a small amount ofH 2 ,
is produced irrespective of the amount of LiOH solution. 16 ) Similar deactivation'
has been observed for PtfTi0 2 in H 2 S0 4 solution (0.1 N).16) The mechanisms of
these deactivations of Pt/Ti0 2 are not clear.
0.1
0.3
Amount of solution/ml
Fig, 13.6 Dependence of the yield of water photolysis over Pt/Ti0 2 in NaOH solution on the solution
amount, The reaction was started in 0.5 ml solution involving 1. 1 mmol ofNaOH. After the
yield measurement at a given amount of solution, the solution amount was reduced by the
method described in the text and the yield measured. After a series of measurements for the
solution involving 1.1 mmol NaOH, NaOH was added to increase its amount to 2.0 mmol.
Values in parentheses denote water vapor pressure (Torr) in the reaction system.
0.4
Since Ti0 2 is quite inactive for H 2 evolution, it needs a co-catalyst of H 2
production from protons for water photolysis. Although Pt is frequently used as
such a co-catalyst, Rh and Pd are also active for the electrochemical reduction of
protons. For gas-phase water photolysis, these metals were loaded on Ti0 2 powder
by a photodeposition method. The activity order of these metals was determined
to be Rh > Pt > Pd when the catalyst was coated with NaOH. 14 ) When SrTi0 3 is
used as a photocatalyst, Rh-Ioaded cata.1yst also shows higher activity than the Pt-
loaded catalyst. 32 ) The photocatalytic activity of the Rh-Ioaded catalysts are,
however, not so stable as to the Pt-Ioaded catalysts. 14) Under the same conditions
as for the water photolysis, the rate constants for the reaction of H 2 with O 2 in the
dark were measured for each Ti0 2 catalyst, and the activity order was l4 ) Pd > Pt
Z Rh. Thse results indicate that the catalytic activity of a metal supported on Ti0 2
for the reverse reaction is not necessarily related to that for H 2 evolution. The
activity order shown here is, however, not so definite, since the activity of a
supported metal catalyst depends upon the preparation method. For Pt/Ti0 2 , the
sample prepared by an impregnation method, which is the most popular method
-
+->
(/)
t:1
o
U'
<1J
(Tj

B. Co-catalyst
0.2
o
o
0.1
Amount of solution/ml
0.2
Fig, 13.7 Dependence of the rate constant for the reaction ofH 2 with O 2 over PtlTi0 2 in the dark on
the amount ofthe solution involving 1. 1 mmol NaOH '

lJU
13 Gas-phase Water Photolysis by NaOH-coated Photocatalysts
13,3 Water Photolysis by Metallized Semiconductor Powders
231
in metal catalyst preparation, showed lower photocatalytic activity for water
photolysis than the other catalysts prepared by photodeposition and chemical
reduction methods. 13)
NiO x is known to work as a good co-catalyst for water photo cleavage by
semiconductor photocatalysts in an aqueous suspension. 18 ) The catalytic activity
of NiO x is very low for the reaction of H 2 with O 2 , while it appears to produce
efficiently H 2 from protons. NiOxlTi0 2 was used for water photolysis in NaOH
solution under the same conditions as shown in Fig, 13.6. H 2 and O 2 are produced
in a stoichiometric ratio even when the amount of NaOH solution is relatively
large. IS) The yield increases with decreasing amount of NaOH solution and
maximized at ca. 0.12 ml quite similarly to the case of Pt/Ti0 2 . IS) Since the
reverse reaction is very slow on NiOxlTi0 2 , this result indicates that some factors
other than the reverse reaction suppress the yield of the water photolysis in
suspensIOn.
In addition to a HTProduction co-catalyst, the loading of an 02-production
catalyst may enhance water photolysis. Thus, Ru02 is sometimes loaded on Ti0 2
photo catalysts together with Pt, since Ru02/Ti02 is a well known excellent
electrode for Ch and O 2 production. 19) Graetzel and his coworkers 20 ) reported that
the addition of Ru02 to Pt/Ti0 2 leads to enhancement in water photolysis.
Yamaguti and Sato,l3) on the other hand, reported no effect of the addition of Ru02
to Pt/Ti0 2 on the yield of water photolysis. Because the upper valence band edge
of Ti0 2 is more than 1.7 eV positive than the O 2 evolution potential from water,
an oxidation catalyst would be unnecessary for O 2 evolution in photocatalytic
water photolysis, In fact, Ti0 2 is active enough for O 2 photo evolution from an
aqueous solution of sacrificial electron donors such as silver ion. 21 ) Ru02 functions
not only as an oxidation catalyst but also as a reduction catalyst,22) the addition of
Ru02 to Pt/Ti0 2 can enhance H 2 evolution in water photolysis ifthe activity of the
Pt is not sufficient to utilize photoinduced electrons.
for water photolysis, and compared its activity with that of platinized anatase
Ti0 2 powder. As shown in Table 13.1, Pt/Ti0 2 (rutile) is less active than
Pt/Ti0 2 (anatase) irrespective of their sources. The low activity of rutile for water
photolysis is not due to its relatively large particle size, because MCB anatase,
which is the most active in this experiment, has a particle size similar to those of
rutile samples judging from their surface areas. The difference in the activity for
water photolysis between anatase and rutile Ti0 2 can be attributed to the
difference in the position of flat band potential. The flat band potential of anatase
Ti0 2 is thought to be ca. 0.2 eV more negative than NHE, while that of rutile Ti0 2
is ca. 0.1 eV more negative. 23 ,24) Therefore, anatase Ti0 2 has a great advantage for
the photoelectrochemical evolution of H 2 from water, since its flat band potentials
is ca. 0.1 e V more negative than that of rutile Ti0 2 . The photocatalytic activity of
Ti0 2 depends sensitively upon impurities. The sample prepared from Ti sulfate
(Kanto anatase, for xample), especially, shows poor activity.l3)
It is noteworthy that the most active sample, MCB anatase, shows a diffuse
reflection spectrum which is closer to the spectrum of rutile than to that of
anatase. B) This result may indicate the inclusion of rutile Ti0 2 in the MCB sample,
although XRD of the MCB sample shows the anatase spectrum. 13) Ti0 2 particles
consisting of both rutile and anatase are sometimes claimed to show high
photocatalytic activity, but the reason for this is not understood yet.
D. Particle Size of Ti0 2
Table 13,1 Photocatalytic activities of Pt/TiOz(anatase) and Pt/TiOz(rutile) for water photolysis
Crystal form Source Nominal purity BET surface area Hz production
(%) (mZlg) rate «,umole/h)
MCB 99 8 284
Anatase Merck 99 12 198
Kanto 99.5 12 56
Katayama 99,98 6 53
Rutile Kishida 99,5 3 20
Furuuchi 99,99 2 15
It appears that very little if any water photolysis occurs on small particles of
Ti0 2 . For example, Pt-Ioaded Ti0 2 particles prepared by the hydrolysis of
Ti(C 3 H 7 0)4 (anatase, surface area> 100 m 2 /g) shows no photocatalytic activity for
water photolysis, while they exhibit much higher photocatalytic activities for H 2
photo evolution from an aqueous solution of alcohol than the commercial Ti0 2
powders used for water photolysis described in this article. IS) Degussa P 25 Ti0 2
is often used as a standard photocatalyst, and its average particle size is reported
to be ca. 30 nm. Ti0 2 (P 25) loaded with Pt by a photochemical deposition method
shows high activity for the sacrificial photo evolution of H 2 , whereas it exhibits no
activity for water photolysis. However, Ni/Ti0 2 (P 25) is found to be active for
water photo splitting in a NaOH solution when oxidized in O 2 at 200°C. IS ) This
indicates that the lack of activity of Pt/Ti0 2 (P 25) is due not to the particle size
of Ti0 2 but to some surface reverse reaction on Pt/Ti0 2 (P 25). Sato lS ) attempted
to modify the surface of Pt/Ti0 2 (P 25) by oxidation with O 2 , and found that
Pt/Ti0 2 (P 25) becomes active for water photolysis in a NaOH solution, only
when it is treated at 350°C in O 2 stream at 1 atm. The maximum activity of O 2 -
treated Pt/Ti0 2 (P 25) is more than two times higher than that of NiOxlTi0 2 (P
25).IS) The spillover of hydrogen atoms from Pt surfaces to Ti0 2 surfaces may take
place significantly on Pt-Ioaded small Ti0 2 particles during water photolysis so
that the reaction of hydrogen atoms with the primary oxidation products of water
occurs spontaneously, and the oxidation of Pt may suppress the spillover. It is
reported that water accelerates the spillover of hydrogen. 2S,26) It has been observed
that hydrogen spillover occurs intensively during the adsorption ofH 2 on Pt/Ti0 2
with highly dispersed Pt,27) The dispersion ofPt on Ti0 2 powder may be enhanced
for fine Ti0 2 particles, If this is the case, then water photolysis on Pt-Ioaded
C. Crystal Form of Ti0 2
Ti0 2 has three crystal forms, anatase, brookite, and rutile. Rutile is a stable
form, and the others are a low-temperature form. The band gap energy of rutile is
ca. 3.0 eV, while those of the other two are ca. 3.2 eV. Since an external bias was
required for the photoelectrolysis of water in a TiOlrutile)-Pt PEC cell, I) one may
assume that rutile powder shows no activity for the photocatalytic decomposition
of water. Yamaguti and Sato 13 ) found that platinized rutile Ti0 2 powder is active
HzIOz ratio is 2 except for Furuuchi TiO z , for which the ratio is ca. 3,

232
13 Gas-phase Water PhotolysIs by NaUH-coated l'hotocatalysts
1."q Loncluumg KemarKS
L-'-'
E. SrTi0 3
around room temperature, there is an optimum water-vapor pressure for the
photolysis yield, but this could not be found at 47°C.16,33) This is because the
water-vapor pressure could not be raised enough due to difficulty in elevating
temperature of the whole reaction system.
For the gas-phase water photolysis on NaOH-coated catalysts, the yield drop
upon increasing water-vapor pressure arises from an increase in the thickness of
the solution layer on the catalyst due to the absorption of water by NaOH. It is
therefore reasonable that the optimum water-vapor pressure shifts to higher
pressures upon elevating temperature, since the solution layer thickness at a given
water-vapor pressure decreases with increasing temperature, The results of Fig.
13.8 show not only the shift in the optimum temperature, which was not definitely
determined at 47°C, but also increase in the yield at 47°C. The apparent activation
energy estimated from Fig. 13.8 is ca. 4 kcal/mol. Since this value is close to the
activation energy of a diffusion rate, the temperature dependent yield would be
attributable to the temperature dependence of the rate of the products diffusing
from the catalyst surface to the gas phase through the solution layer.
Ti0 2 with high surface area would hardly take place. The dependence of the
water-photolysis yield on electrolyte, which was described before, may also be
ascribed to the difference in the extent of hydrogen spillover.
The charge separation of photoinduced electron-hole pairs would be
indispensable for water photolysis by semiconductor photocatalysts, since the
reaction is thermodynamically uphill. The charge separation occurs due to the
band bending in the space charge layer at the surface of the semiconductor; this
layer is thought to lie in a range of more than 100 nm toward the bulk. 28) The fact
that water photolysis using as small a Ti0 2 particle as P 25 is possible may
indicate that the charge separation does occur in the incomplete space charge
layer.
A SrTi0 3 single crystal is active for water photolysis when loaded with Pt. 8,29)
Among conventional, commercial semiconductor powders except for Ti0 2 , SrTi0 3
is the only semiconductor which shows activity for water photolysis. 18 ,30,31)
Yamaguti and Sato 32 ) investigated the photocatalytic activities of Pt- and Rh-
loaded SrTi0 3 for water photolysis in pure water and NaOH and H 2 S0 4 solutions.
The yield increases significantly with decrease in the amount of these solutions.
The promoting effect of NaOH coating on gas-phase water photolysis is not so
remarkable as in the case of metallized Ti0 2 photocatalysts. The maximum
quantum yield was ca. 1.2% for Rh/SrTi0 3 .
G. Ambient Pressure
60
.... 50
',..q
..........
0 40

-
(l)
1Tj 30
I-<
t:1
0
...... 20
"S
..........
0
;>
<!) 10
f'1

0
0 10 20 30
Water vapor pressure/T orf
Photocatalytic decomposition of water on semiconductors is usually
conducted under reduced pressure. There are arguments that O 2 production is
much less than stoichiometric when water photolysis is carried out under
atmospheric pressure. 20 ,34,35) Such nonstoichiometric O 2 evolution is often ascribed
to the photo adsorption of O 2 onto semiconductor partic1es 2o ,34,35) or the formation
of peroxides. 36) On the other hand, the electrochemical potential of H 2 evolution
shifts to the positive direction with increasing ambient pressure according to the,
Nernst equation. Therefore, pressure effect may be negatively significant for'
water photolysis by Ti0 2 'photocatalysts, since the flat band potentials of Ti0 2 are
close to the potential ofNHE.23,24)
Yamaguti and Sato 37 ) examined the influence of ambient pressure on the
stoichiometry and yield of water photolysis over Pt/Ti0 2 (anatase and rutile) in
pure water and NaOH solution using He as diluent gas. Although the H 2 /0 2 ratio
is not exactly stoichiometric, the ratio remains unchanged irrespective of the
crystal form of Ti0 2 when He pressure is changed from zero to 1 atm. 37 ) They also
found that very slow adsorption of O 2 occurs on the catalysts in the dark. 37 )
Therefore, the Hi02 ratio is stoichiometric for highly active photo catalysts or
under a condition in which the solution layer on the catalysts is so thin that O 2 is
easily desorbed from the catalyst surface, but O 2 may not evolve when its
formation is slower than its adsorption on the catalyst. The yield for the rutile
catalyst slightly decreases with increasing He pressure; while the yield for the
anatase catalyst is unchanged. This result may be attributed to the fact that the flat
band potential of rutile is more positive than that of anatase.
F. Temperature
The yield of gas-phase water photolysis over NaOH-coated Pt/Ti0 2 increases
with elevating temperature as shown in Fig. 13.8. 16 ,33) At temperatures below or
13.4 Concluding Remarks
Fig, 13.8 Temperature dependence of the yield of gas-phase water photolysis over NaOH-coated
Pt/Ti0 2 , The amount ofNaOH coated is 15 wt%. Hi02 ratio is stoichiometric in all runs,
After the finding of photocatalytic water decomposition by Pt/Ti0 2 , many
new semiconductor photo catalysts which are able to photolyze water in an
aqueous suspension have been reported. 38 ) These photocatalysts are characterized

234 13 Gas-phase Water Photolysis by NaOH-coated Photocatalysts
by the presence of a co-catalyst that is active for H 2 evolution but inactive for the
reaction of H 2 with O 2 . Even for such photocatalysts, the solution layer on the
catalysts is thought to suppress the products escaping from the catalyst surface to
the gas phase as demonstrated for NiOx!Ti0 2 , IS) on which the reverse reaction
hardly takes place. Therefore, the methods described in this chapter will be useful
for efficient water photolysis by photocatalysts loaded with a co-catalyst inactive
for the reverse reaction.
14
Water Photolysis by Ti0 2 Particles-Significant
Effect of Na2C03 Addition on Water Splitting
References
1. A. Fujishima and K. Honda, Bull. Chem, Soc, Jpn., 44, 1 ]48 (1971); Nature (London), 237, 37
(1972).
2, K. Rajeshwar, P, Singh and J, DuBow, Electrochem, Acta" 23, 1117 (1978).
3. M, S. Wrighton, Ace, Chem, Res., 12,301 (1979),
4. A. Heller, Ace. Chem. Res., 14, 154 (1981).
5. A. J, Bard, J. Photochem" 10,59 (1979); Science, 207, 139 (1980).
6. G. N, Schrauzer and T, D, Guth, j, Am. Chem. Soc., 99, 7189 (1977),
7. S. Sato and J. M, White, Chem, Phys, Lett" 72,83 (1980).
8. F. T. Wagner and G, A. Somorjai, J. Am, Chem. Soc" 102, 5494 (1980).
9, S. Sato and J, M. White, J. Catal., 69, 128 (1981),
10, S. Tabata, N. Nishida, Y. Masaki and K. Tabata, Catal. Lett" 34, 245 (1995).
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13, K. Yamaguti and S. Sato, Nippon Kagaku Kaishi, 258 (1984) [in Japanese].
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15. S. Sato, Nippon Kagaku Kaishi, 1182 (1988) [in Japanese].
16. S. Sato, New J. Chem., 12, 859 (1988),
17. K. Sayama and H. Arakawa, J. Chem. Soc, Chem, Commun., 150 (1992); Chem. Lett., 253
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18, K. Domen, S, Naito, S, Soma, M. Ohnishi and K. Tamaru, J. Chem. Soc,. Chem. Commun., 543
(1980); K. Domen, S, Naito, T, Ohnishi and K. Tamaru, Chem. Phys. Lett., 92, 433 (1982); K,
Domen, S, Naito, T. Ohnishi, K. Tamaru and M, Soma, J. Phys, Chem" 86, 3657 (1982); K.
Domen, A. Kudo and T. Ohnishi, J. Catal. 102,92 (1986).
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20. D, Duonghong, E, BorgareHo and M. Graetzel, J. Am. Chem, Soc" 103,4685 (1981); E.
Borgarello, J. Kiwi, E. Pelizzetti, M. Visca and M. Graetzel, ibid" 103, 6324 (1981).
21. S, Sato and T. Kadowaki, Denki Kagaku, 57, 1151 (1989) [injapanese].
22, P. Keller, A. Moradpour and E. Amouyal, J. Chem, Soc, Faraday Trans, 1,78,3331 (1982).
23, B. Kraeutler and A. J. Bard, J. Am, Chem, Soc., 100, 5985 (1978).
24. H. Minoura, M. Nasu and Y. Takahashi, Ber, Bunsenges, Phys, Chem" 89, 1064 (1985).
25, P. A. Sermon and G, C. Bond, Catal, Rev., 8, 211 (1973),
26, Spillover of Adsorbed Species,(G. M. Pajonk, S. J. Teichner and J. E, Germain, eds,), Elsevier
(1983),
27. S. Sato, J. Catal" 92, 11 (1985).
28. R, Memming, Electrochem, Acta, 25,77 (1980),
29. M. S, Wrighton, p, T, Wolczanski and A. B, Ellis, J. Solid State Chem" 22, 17 (1977).
30. H. Yoneyama, M. Koizumi and H, Tamura, Bull. Chem. Soc. Jpn" 52, 3449 (1979).
31. J. M, Lehn, J. P. Sauvage and R. Ziessel, Nouv, J. Chim" 4, 623 (1980); J, M. Lehn, J, P,
Sauvage, R, Ziesse1 and L. Hilaire, Israel J. Chem" 22, 168 (1982),
32. K. Yamaguti and S, Sato, Nouv. J. Chim., 10, 217 (1986),
33. S. Sato, Denki Kagaku, 54, 977 (1986) [in Japanese],
34. E, Borgarello, J, Kiwi, M, Graetzel, E. Pelizzetti and M. Visca, J. Am, Chem. Soc" 104, 2996
(1982),
35. A. Mills and G, Porter, J. Chem, Soc, Faraday Trans. 1,78,3659 (1982).
36, J, Kiwi and M, Graetze1, J. Phys. Chem" 88, 1302 (1984); J, Kiwi and C, Morrison, J. Phys,
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38. A. Kudo, Hyoumen (Surface), 36, 625 (1998) [in Japanese].
14.1 Introduction
If we could easily and directly produce a mass of hydrogen, which is one of
the most promising clean energy sources, from inexhaustible and inexpensive
resources such as solar light energy and water, energy problems as well as the
global warming issue caused by CO 2 emission which now confront humankind
would be solved once and for all. In other words, sustainable development of the
human race for the future would be established. Stoichiometric and direct water
splitting reaction using a simple oxide semiconductor photocatalyst system under
solar light irradiation is one possible way to realize such a dream.
Since Honda and Fujishima found the water decomposition phenomenon
using a photo electrochemical cell composed of Ti0 2 anode and Pt cathode under
anode illumination in 1969,1) water splitting into hydrogen (H 2 ) and oxygen (0 2 )
using oxide semiconductor photocatalysts has been widely investigated as a
method for the direct conversion of solar energy to chemical energy and for
energy storage similar to photosynthesis. However, the stoichiometric and
photocatalytic decomposition of water to H 2 and O 2 (H 2 :0 2 =2: 1) has proved to be
a very difficult reaction over Pt/Ti0 2 photocatalyst, and it scarcely proceeds
under ambient conditions because the change in the chemical potential of this
reaction is highly positive (b.G=+56.7 kcal/mol) and several undesirable side
reactions take place on the small catalyst particles. When Pt/Ti0 2 catalyst
suspended in pure water was irradiated with UV light, only a small amount of H 2
evolved initially and the rate of H 2 evolution decreased gradually with irradiation
time. 2 ,3) Further, no O 2 evolution was observed. This was a serious problem for
researches on water splitting using oxide semiconductor photocatalysts.
Recently, we discovered by chance that sodium carbonate (Na2C03) addition
to a Pt/Ti0 2 suspension of water is significantly effective in promoting
stoichiometric photo- decomposition ofliquid water. 4 ) This procedure has turned
out to also be applicable to many other photocatalyst systems such as M/Ti0 2 ,
M/Ta20s, M/Zr02, M/SrTi0 3 , etc. (M = metal or metal oxide).s-8) Moreover, our
research group at Tsukuba was the first in the world to demonstrate effective
solar hydrogen and oxygen production using a combination of 3wt%NiOx/Ti0 2
photocatalyst and Na2C03 addition. 9 ) Here, we introduce this unique photocatalytic
water splitting system using a Na 2 C0 3 addition method.

236
14 Water Photolysis by Ti0 2 Particles
14 2 Effect of Carbonate Salt Addition on Water Splitting
237
14.2 Significant Effect of Carbonate Salt Addition on Water
Splitting from Pt/Ti0 2 Water Suspension
Table 14,1 Rate of photocatalytic H 2 and O 2 evolution from a Pt/TiO z catalyst suspended in salt
solution
Additive
Additive amount
(ml)
Evolution rate* (Jimollh)
Hz Oz pH
0 7,9
3 1 10.9
316 158 11.0
568 287 11.0
39 13 11.0
5 1 11.5
20 10 11.8
4 2 8.4
13 3 10,8
5 0 13.3
5 0 6.8
4 0 5,5
5 0 12.0
2 0 8,6
3 0 11.0
The photocatalytic water splitting reaction was performed using the glass
closed gas-circulating system with an inner irradiation quartz reactor shown in Fig.
14.1. The light source was a 400 W high-pressure Hg lamp covered with a quartz
water jacket. The reaction mixture was prepared by introduction ofTi0 2 (P-25 or
self-made materials, 0.3 g), H 2 PtC1 6 (Wako Chemical, Pt: O.3wt% to Ti0 2 ) and a
prescribed amount of additive such as a carbonate salt into distilled water (350 ml)
in the quartz reactor. Then the reaction mixture was mixed well using a magnetic
stirrer and deaerated thoroughly. After the introduction of Ar into this system (35
torr), the first run was started by irradiation. The evolution of H 2 and O 2 was
detected by an on-line gas-chromatograph (TCD, MS-5A, Ar-carrier) and a
pressure sensor. 10)
Table 14.1 shows both H 2 and O 2 evolutions from several kinds of aqueous
suspension of Pt/Ti0 2 . An aqueous suspension includes a prescribed amount of
inorganic salt as an additive. In the case of no additives, a small amount of H 2
evolved; however, O 2 evolution was not observed, Upon the addition of a sodium
salt, i.e., NaOH, NaCl, Na2S04, Na3P04, NaHP0 4 , respectively, the H 2 evolution
rate slightly increased in comparison with the experiment conducted without an
additive, however, it decreased to zero with reaction time, There was no O 2
evolution in the gas phase over long periods. On the other hand, upon addition of
none
Na z C0 3 0.10
0.38
0,76
1.14
K Z C0 3 0.38
1.45
NaHC0 3 0.10
Li z C0 3 0.06
NaOH 0,76
NaCl 0,76
NazS04 0.38
Na 3 P0 4 0.25
NazHP04 0,38
Na3P04(0.003mol) + NaHP0 4 (O, 15mol)
Pt(0.3wt%)-TiO z : 0.3g, water: 350ml, an inner irradiation quartz reactor, high pressure Hg lamp
(400W).
*Rate at steady state,
magnetic
stl rrer
high pressure
Hg lamp
(400W)
Na2C03, the H 2 evolution rate increased dramatically. Furthermore, at the same
time, O 2 evolution occurred in an exactly stoichiometric ratio of H 2 0
decomposition, i.e. H 2 :0 2 =2: 1, Production of CO, CH 4 , HCOOH, CH 3 0H and
carbon was not observed. It is clear that stoichimetric water splitting was
established in the case ofPtITi0 2 photocatalyst system by the addition ofNa2C03'
Figure 14.2 shows '-the reaction time course of H 2 and O 2 evolution from
0.3wt%Pt/Ti0 2 catalyst suspended in Na2C03 aqueous solution. In the first run, the
evolved H 2 /0 2 ratio became nearly 2 several hours after the initiation of
irradiation. The evolution rate decreased gradually with irradiation time, but the
photocatalyst itself was not deactivated since the evolution rate recovered by
pumping away the evolved gases in the gas phase. It is concluded that the increase
of pressure due to evolved gas in the reactor might suppress the further evolution
of H 2 and O 2 . It was also interesting that the initial activity in each run increased
with longer irradiation times and became constant after 70 h (after the 9 th run), i.e.,
an induction period was observed and the intrinsic activity of the catalyst
improved with irradiation time. The rate of gas evolution in Table 14,1 showed the
steady-state activity, which was almost constant after several runs, The total
amount of catalytically evolved gases for 50 runs was 24.3 mmol (544 ml STP)
ofH 2 and 12.1 mmol (27] ml STP) of O 2 in the presence of 0.3 g (3.8 mmol) of
catalyst. This means that the water splitting over Pt/Ti0 2 is indeed a catalytic
reaction. As shown in Table 14.1, both H 2 and O 2 evolved from all the
concentrated carbonate solutions. Stoichiometric evolution of H 2 and O 2 was also
observed for concentrated NaHC0 3 and K 2 C0 3 solutions. The rates ofH 2 and O 2
evolution depended on the concentration of the carbonate salts. The maximum
evolution rate was obtained upon addition of 0.76 mol of Na2C03 to 350 ml of
water at pH 11.0. We performed an experiment at pH 11.0 by adjusting the pH
vacuum line
pressure sensor
to G.C.
circu I ation
pump
Dead Volume
=270ml
reactor
Fig, 14,1 A glass closed gas-circulating system with an inner irradiation quartz reactor.

238
14 Water Phomlysis by Ti0 2 Particles
14.2 Effect of Carbonate Salt Addition on Water Splitting
239

L;
f:.
--
'"
II]
bO
-0
OJ
50 :>
(5
:>
OJ
....
o
25 e
;:I
<I)
<I)
e
0..
o -;;;
.e
II]
0..
stoichiometric splitting and the acceleration of the gas evolution rate over Pt/Ti0 2 .
The dependence of the rate of gas evolution upon the amount of loaded Pt was
measured. There was no activity over Ti0 2 photocatalyst without Pt loading, as
shown in Fig. 14.3. The rate of gas evolution increased with increase in the Pt
loaded over Ti0 2 up to O.3wt%. Then:fore, it is suggested that Pt loading was
essential for the photocatalytic decomposition of water. The loaded Pt might help
charge separation, and act as the H 2 evolution site. However, the activity decreased
drastically with a further increase in Pt loading, and became zero at Pt loading
more than 3.0wt%. The back reaction takes place very rapidly even in the dark
condition. The back reaction is considered to be a serious problem in water
splitting. We compared the rate of the back reaction on several used
photocatalysts, as shown in Fig. 14.4. These photo catalysts were used in several
solutions for a given period of irradiation. Then they were filtered off and washed
well to remove salt and dried. The rate of back reaction was monitored as a
decrease in the pressure of H 2 and O 2 in the closed gas-solid system with ice
trap. It was found that the rate of back reaction decreased with an increase in
photoreaction period for Na2C03 solutions. This suggests that the improvement in
the photocatalytic activity with irradiation time shown in Fig. 14.2 is associated
with the decrease in the back-reaction rate over the catalyst. Furthermore, it was
found that the rate of back reaction over Pt/Ti0 2 was affected by the nature of the
reaction solutions and decreased in the order, pure water> NaOH(aq) >
with several non-carbonate salts, such as Na3P04 and Na2HP04. However, water
splitting did not take place. Therefore, it is concluded that neither the pH of the
solution nor the Na+ cation directly contributes to the water splitting, and that
concentrated carbonate ions (CO/- or HC0 3 -) are essential for both the
1 st run 3rd run
(5 1.0 n
E
E evac
 
CD
"2 0.5 II'
> H2
(5
>
OJ
....
0
C 02
:>
0 0 10 30
E
" irradiation time' h
6th nm 7th 8th 9th run
60
80
irradiation time' h
_ _ _ _ _ _ ./(f) and (g)
...- ...""'
 --
"--'_-::----___ (e)
. \ '--(J-:'------ (d)
, (b) --____ ----__ -------
\ -------- ---'-
--- - - -=-

-
-------
( a) --------_
------
Fig, 14.2 Time course of H 2 and O 2 evolution from the Pt(O,3wt%)-Ti0 2 catalyst suspended in Na2C03
solution. Catalyst: 0.3 g, Na2C03: 80 g, water: 350 ml, an inner-irradiation quartz cell,
high-pressure Hg lamp (400 W). The evolved gas in the gas phase was pumped away at the
moment indicated by the arrow( J- ).
(From K. Sayama and H. Arakawa, J. Chern, Soc., Farady Trans" 93, 1649 (1990»
200
I
..c
o
E
::::i.
---
c:
o
 tOO
>

'"
d
OJ)
......
o
B
d
.....
a 
a
0/\
.
: Z O O \\
/ V2 .
o o
60,

1=
£
U)
>-. 40
U)
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Q)
...
::>
U)
U)
11.)
...
0..
20
o
20
30
10
time's
O.t 1 10
amount of loadtd platinum (wt. %)
Fig. 14.4 Dependence of the rate of the back reaction (2H 2 0 + O 2  2H 2 0) in the gas phase upon the
reaction period and type of reaction solution over Pt(O,3 wt%)-Ti0 2 in the dark.lnitial gas
in the closed system was a mixture ofH 2 (50 Torr) and O 2 (25 Torr). Catalyst: 0,2 g, (a) Pure
I
water for 41 h, (b) NaOH solution for 41 h, (c) Na2C03 for 14 h, (d) Na2C03 for 41 h, (e)
Na 2 C0 3 for 171 h, (f) RuOll wt%)-Ti0 2 and (g) NiOxCl wt%)-Ti0 2 catalysts used in the
Na2C03 solution for 20 h,
(From K. Sayama and H. Arakawa, J. Chern, Seo" Faraday Trans" 93, 1650 (1997»
Fig. 14.3 Dependence of the H 2 and O 2 evolution upon the amount of loaded Pt. Ti0 2 : 0.3 g, Na2C03:
80 g, water: 350 ml.
(From K. Sayama and H. Arakawa, j, Chern. Seo., Faraday Trans., 93, 1650 (1997»

(a) before etching
(b) after etching
(c) r"ference
(a)
°ig)
t T
H.
f /'-/ /
 I ,/V\Jl'J
I N,dJ.., _ J
, JI LJ
PtO
H
o 0-
OH- [
-----
H 2 0 0-0
h+
] h+ !ia f q,
H '
or,O  Ona) _ Oia)
o etc. !
H 2 + OW
H 2 0
./
e
Pt black
(b)
OH'
Oig)
t t
[ H q, <><>-cf> "" ] Yf
c'" I
'9' 0 etc. Cia)
H 2
, O 2
H 2 +Otr __ H 2 0
H 2 0 {\
80 7S 70
binding energy/e V
80 7S 70
binding energy/e V
80 75 70
binding energy/e V
HC0 3 -
/
'i? 0,- o,-;;.o [ q . -0, p ]
C H c,"O Y __ H, c_ o  --
q" '9' 0 (( or 0 etc,
h+
hv(<300nm)orh" e'
Fig, 14.5 XPS spectra ofPt 4f 5 /2 and 4f 712 over the Pt(3 wt%)-TiO z catalysts used in several solutions.
(a) Before etching, (b) after etching by argon ion for 10 min, (c) reference samples,
(From K. Sayama and H. Arakawa, J. Chern, Seo,. Faraday Trans" 93, 1650 (1997»)
/£1.
Fig, 14,6 Speculated reaction mechanism over Pt-TiO z (a) in water and (b) in carbonate salt aqueous
solution.
(From K. Sayama and H. Arakawa, J. Chern. Seo,. Faraday Trans., 93, 1653 (1997»)
Na2C03(aq).
Figure 14.5 shows the XPS spectra of Pt 4Fs12 and 4f 712 over 3wt%Pt/Ti0 2
photo catalysts after photocatalytic reaction for several solutions. These
photocatalysts were used for 41 hours in pure water, NaOH and Na2C03 solutions,
and then they were filtered off and washed well to remove salt and dried. The
amount of the Pt loaded in all photo catalysts was 2.9 :!:: 0.] wt% as confirmed by
XRF. It was found that the spectra of all photocatalysts were similar to the spectra
of zero-valent Pt black, and that the spectrum of photocatalyst in the pure water
was more intense than that in NaOH or Na2C03. In addition, the intensities of the
spectra in both NaOH and Na 2 C0 3 were increased by Ar ion etching, Therefore,
the Pt particles might be covered gradually during the photoreaction in the NaOH
and Na2C03 solutions. There were no Na salts on the XPSsamples. We also
demonstrated that Ti0 2 suspended in NaOH or Na2C03 aqueous solution could be
dissolved gradually by the addition of concentrated H 2 0 2 solution. Therefore, it
is speculated that titanium compounds such as titanium hydroxides might cover
the Pt particles during photoreaction in NaOH or Na2C03' From the results in Fig,
14.4, the rate of H 2 0 formation from H 2 and O 2 over the Pt/Ti0 2 in Na2C03
solution was much less than that in NaOH solution. Therefore, it is concluded that
the effective suppression of the back reaction over Pt particles of catalyst might
be one of the important functions of the carbonate salts.
14.3 Role of Carbonate Salts on Water Splitting and
Reaction Mechanism
proposed reaction mechanistic scheme over Pt/Ti0 2 in water. 10) Many undesirable
side reactions of H 2 and O 2 take place over Pt/Ti0 2 as follows: (I) the back
reaction over Pt to form H 2 0 from H 2 and O 2 , (II) photo adsorption of O 2 and
surface peroxide formation over Ti0 2 , and (III) recombination of electrons and
holes produced under irradiation. The back. reaction over Pt is a very serious
problem in photocatalytic water splitting, because it readily proceeds
thermodynamically (G < 0). The Pt/Ti0 2 catalyst itself shows an intrinsic ability
to split water. Therefore, both H 2 and O 2 are formed on the catalyst surface under
irradiation. The O 2 is photoadsorbed and 02(a) and 02.-(a) are formed, while H 2
can be released from the catalyst surface since H 2 is not affected by
photoadsorption. Some of the adsorbed O 2 might be altered to a peroxide
species. I I) The 02(a), 02+-(a) and peroxide species thus accumulate gradually on
the catalyst. Finally, after extended irradiation, H 2 evolution would stop because
a pair reaction of 02( a) reduction by electron and O 2 .-( a) oxidation by h+ becomes
a major process, as noted by Mill. 3 )
We propose the mechanism shown in Fig. 14.6(b) for water splitting in the
presence of carbonate ions. One of the effects of carbonate salts is suppression of
the back reaction over Pt by modification of Pt surface with dissolved Ti 4 +
complexes. We must also consider the effect of the carbonate on the Ti0 2 itself,
because significant effects of carbonate salts were also observed over NiOxlTi0 2
and Ru02/Ti02 catalysts on which the back reaction did not occur. 3 ) IR study
revealed that the surface of Ti0 2 ofPt/Ti0 2 catalyst was covered with many types
of carbonate species. Based on these facts, we speculate that carbonate species on
Ti0 2 play an important role in the photocatalytic decomposition of water. Eriskin
and Lind have studied the behavior of HC0 3 - and C0 3 +- radicals in NaHC0 3
aqueous solution by pulse radiolysis. 12 ) They demonstrated that most of the OR-
radicals were consumed by HC0 3 +- or CO/- and that the self-recombination of
The Pt/Ti0 2 photocatalyst can theoretically split water in aqueous solution,
because Ti O 2 has the appropriate conduction band and valence band for H 2 and O 2
evolution, respectively. However, there are several reasons why the water splitting
can not occur over Pt/Ti0 2 on carbonate-free solutions. Fig. 14.6(a) shows the

242
14 Water Photolysis by Ti0 2 Particles
14.4 Screening of Active Photocatalysts for Water Splitting
243
14.4 Effective Screening of Active Photocatalysts for Water
Splitting Using Na2C03 Addition Method
to Pt/Ti0 2 catalyst, stoichiometric evolution of H 2 and O 2 (H 2 :0 2 = 2: 1) was
observed over RuOr or NiOx-loaded Ti0 2 catalysts in a Na2C03 aqueous solution,
Kudo and his coworkers reported that NiO/Ti0 2 catalyst was active for water
splitting in aqueous NaOH solution. 14 ) However, no H 2 and O 2 evolution was
observed over any of these Ti0 2 catalysts in'pure water. In the case ofNiO)Ta20s
and RuOiTa20s, the evolution of H 2 and O 2 was observed to be stoichiometric
from pure water as well as Na2C03 aqueous solution. It was found, for the first
time, that Ta20s could be used as a photocatalyst for water splitting. The activity
of NiO)Ta20s appeared to decrease on addition of Na2C03' However, the gas
evolution rate over NiO)Ta20s in pure water decreased gradually and became
lower than that over NiO)Ta20s in Na 2 C0 3 aqueous solution after more than 10
h. On the whole, Na2C03 addition has a beneficial influence on the photocatalytic
decomposition of water over Ta20s catalysts. 7 ). It has been reported that a neat
Zr02 catalyst without loading can decompose water in pure water. 6 . 8 ) The gas
evolution activity of Zr02 decreased significantly by the loading ofPt and Ru02
onto Zr02. In the case of Pt/Zr02 catalyst, the photocatalytic activity was
C0 3 '- radicals to form peroxycarbonate had a negative apparent activation energy.
Peroxocarbonates can also be synthesized by mixing H 2 0 2 and carbonates salts. B )
These results suggest the possibility of peroxycarbonate formation in the
photocatalytic reaction of Pt/Ti0 2 suspended in the carbonate solution. Thus,
most of the holes initially react with carbonate anions on the surface, and
carbonate radicals and bicarbonate radicals are formed. By the coupling of two
radicals, peroxycarbonates are formed and these peroxycarbonates are then easily
decomposed into O 2 and CO 2 by holes under irradiation. CO 2 evolution
accompanying O 2 evolution might aid the desorption of O 2 from the surface; thus
this O 2 desorption process is irreversible, in contrast to the situation in the absence
of carbonate. 10)
A Na2C03 addition method has been found to be very effective for water
splitting using not only Pt/Ti0 2 photocatalyst but also various other kinds of
photocatalysts, Some metals and meta] oxides were applied as support on Ti0 2
photocatalysts as an electron trap center and these catalysts tested as water
splitting photocatalysts. Fig. 14.7 shows the reaction time course of H 2 and O 2
evolution using I wt% RuOiTi0 2 photocatalyst. Hydrogen ecvolution rate was
quite low and O 2 evolution was not observed before addition of Na2C03 into
aqueous suspension of 1 wt%RuOiTi0 2 photocatalysts, However, it is clear that
the H 2 and O 2 evolution rate increased significantly right after the addition of
Na2C03 and the gas evolution ratio of H 2 to O 2 was 2: 1. Furthermore, gas
evolution remained steady for more than 60 hours, Table 14.2 shows th-e rate of
H 2 and O 2 evolution over various kinds of metal-loaded simple semiconductor
catalysts suspended in both Na2C03 aqueous solution and pure water. In addition
()
50
Irradiation time ( h )
100
Table 14,2 Rate of photocatalytic H 2 and O 2 evolution over various kinds of metal-loaded simple
semiconductor catalysts suspended in both Na2C03 aqueous solution and pure water
Semoicon-ductor Loaded material Rate of gas evolutionlJ-lmol-h- 1
in Na2C03 sol. in pure water
H 2 O 2 H 2 O 2
Ti0 2 Pt(O,3wt%) 78 38 2 0
Ru02( I wt%) 34 17 tr 0
NiOil wt%) 64 32 1 0
Ta20S Pt(O.1 wt%) 1 0 1 0
Ru02(l wt%) 68 34 32 17
NiOilwt%) 153 79 190 99
Zr02 Pt(O.lwt%) 53 23 tr 0
RuOD wt%) 12 6 11 5
NiOil wt%) 43 22 129 70
Nb 2 0 s Pt(O.1 wt%) 5 0 1 0
RuODwt%) tr 0 tr 0
NiO x (1 wt%) . tr 0 tr 0
ZnO Pt(O.]wt%?t> tr 0
Ru02(1 wt%) 0
W0 3 Pt(O, 1 wt%) tr 0
Ru02(l wt%) tr 0
Sn02 Pt(O.lwt%) tr 0
Ru02(1 wt%) ] 0
F e203 Pt(O,lwt%) tr 0
In203 Pt(O.1 wt%) tr 0
Bi 2 0 3 Pt(O,lwt%) tr 0
Hf0 2 Pt(O. 1 wt%) tr 0
RU02( 1 wt%) 2 0
Ce02 Pt(O.lwt%) tr 0
La203 Pt(O.lwt%) tr 0
Y2 0 3 Pt(O,lwt%) tr 0
CdS Pt(O.lwt%) tr 0
SiC Pt(O.lwt%) tr 0
Catalyst: Ig (Ti0 2 : O.3g), Water: 350m!.
An inner irradiation type quartz cell, high pressure Hg lamp (400W).
:: . O()()
,,--....
.......
o
S
:::i
,()OO
Hydrogen
'-"
C/J
ro
OJ)
"0
g; 1. 000
.......
o
>-

N azCO] addition
Pure water
o
Fig. 14,7 Effect of Na2C03 addition on water splitting using Ru02rri02 photocatalyst.

244
14 Water Photolysis by TiO z Particles
14.4 Screenmg of Active Photocatalysts tor Water pl1ttmg
1.4)
Table 14.3 Rate of photocatalytic Hz and Oz evolution over mixed oxide semiconductor catalysts
suspended in both Na z C0 3 aqueous solution and pure water
results suggest that the Na2C03 addition method is very useful in the search for a
photocatalyst for water splitting.
From these experimental results, it is possible to classify these
photocatalatysts into five groups as follows.
Group A: Photo catalysts that can split water only with Na2C03 addition
Pt/Ti0 2 , Ru02/Ti02, NiOiTi0 2 , Rh/Ti0 2 , Cu/Ti0 2 , Pt/Zr02,
Pt/K 4 Nb 6 0 17 , Pt/K 2 Ti 6 0 13 , PrINa2 Ti 6 0 13
Group B: Photo catalysts whose activities were enhanced by N a2C03 addition
Ru02/Ta20s, Rh/SrTi0 3 , NiOx/SrTi0 3 , Ru02INa2Ti6013,
RuOiK 2 Ti 6 0 13 , Ru02/BaTi406
Group C: Photo catalysts whose activities did not change by Na2C03 addition
Ru02/Zr02, Pt/SrTi0 3
Group D: Photocatalysts whose activities decreased by Na2C03 addition
NiOiTa20s, NiOiZr02, NiOiNb6017, Ru02/Nb6017
Group E: Photocatalysts whose activities disappeared by Na2C03 addition
Photo catalysts in Group A show significant and typical effect of Na2C03
addition on water splitting. Ti0 2 photo catalysts and photo catalysts promoted by
Pt belonged to Group A. From the result that all Pt-promoted photocatalysts
belonged to Group A, it is concluded that one of the effects of Na2C03 addition
is the suppression of the back reaction of H 2 and O 2 to H 2 0 formation over Pt
surface. Therefore, it promotes, the simultaneous evolution of H 2 and O 2 . In
addition, it is speculated that the stability of surface peroxide materials such as
H 2 0 2 over Ti0 2 is stronger than that over other oxide semiconductor surface and
therefore the effect of Na2C03 addition is significant in the case of Ti0 2
photocatalysts because Na2C03 suppresses the formation of peroxide materials.
In the case of Groups B, C and D, Na2C03 addition allowed carbonate anion
(COl-) adsorb on the semiconductor surface. At the same time, it changes various
factors such as pH, ionic strength and electric conductivity of solution and others,
influencing photocatalytic activity. Changes in these fuctors make the effect of
N a2C03 addition on water splitting very complicated. In order to improve the
photocatalytic activity of an individual photocatalyst, it is necessary to look for
recovered by the addition of Na2C03 to pure water. The rate of gas evolution
over NiOiZr02 decreased to approximately one-third the original value on
addition of Na2C03 to pure water. Moreover, in the case of Ru02/Ti02, Na2C03
addition did not influence the activity, Thus, the effect of Na2C03 addition to pure
water is a complicated phenomenon and depends on the combination of loading
materials and semiconductors. Furthermore, the photocatalytic decomposition of
water over various kinds of semiconductor catalysts, such as Nb 2 0 s , ZnO, W0 3 ,
Sn02, Fe203, In203, Bi 2 0 3 , CdS and SiC, suspended in Na2C03 aqueous solution,
was attempted. However, evolution of O 2 was not observed over any of these
catalysts, although a very small amount of H 2 evolved during the initial stage of
the reaction.
Table 14.3 shows the photocatalytic decomposition of water over mixed
oxide semiconductors loaded with some metals and metal oxides. Photocatalysts
such as Rh/SrTi0 3 ,IS) Ru02INa2Ti6013,16) RuOiBaTi 4 0 9 ,17) and NiOiK4Nb601718)
have been reported to be effective for water decomposition in pure water. The
activity of these catalysts was checked and the decomposition of water into H 2 and
O 2 was confirmed in our experiments, as shown in Table 14.3. The activities of
these catalysts, except Nb6017' increased on addition of Na2C03' Furthermore,
even Pt/Na2Ti6013 and Pt/K 2 Ti 6 0 13 showed activity for the photocatalytic
decomposition of water in Na2C03 aqueous solution. It has been reported that Pt-
intercalated Nb6017 without aqua regia treatment does not show photocatalytic
activity in pure water. 19 ) We found that the addition of Na2C03 to pure water
leads to the highly active photocatalytic decomposition ofwater. 20 ) For SrTi0 3 , the
behavior of the photocatalytic decomposition of water was similar to that of
Ta20s. Thus, both metal oxide-loaded catalysts (NiOiSrTi0 3 and NiOi Ta 2 0 s)
showed activity even in pure water without Na2C03 addition, and non-metal
loaded catalysts (SrTi0 3 and Ta20s) were unable to decompose water. These
No, Semiconductor Loaded metal Rate of gas evolutionl,umol . h- I
(or metal oxide) in Na z C0 3 sol. in pure water
Hz Oz Hz Oz
1 SrTi0 3 none tr 0 tr 0
2 Pt(l wt%) 10 4 9 2
3 Rh(O, 1 wt%) 48 14 20 4
4 NiOxOwt%) 41 20 9 4
5 Nb6017 Pt(O.3wt%)a J 451 217 tr 0
6 NiOil wt%) 60 28 403 197
7 RuOz(l wt%) 41 20 21 I 100
8 Na z Ti 6 0 13 Pt(O.1 wt%) 15 5 tr 0
9 RuOz(l wt%) 55 25 5 2
10 Kz Ti 6 0 13 PT(O.lwt%) 63 17 tr 0
11 RuOz(lwt%) 49 24 11 1
12 BaTi 4 0 9 Pt(O,lwt%) 2 tr tr 0
13 RuOz(l wt%) 36 18 30 13
Catalyst: Ig, Water: 350ml, an inner irradiation type quartz cell, high pressure Hg lamp (400W),
Na z C0 3 : 80g,
[Pt(NH 3 )4]Ch, was used as Pt precursor.
Pt leaded catalysts were not applied to aqua regia treatment.
Table 14.4 Combination of semiconductor and supported metal or oxide for photocatalysts which can
split the water to Hz and Oz in suspension
Semiconductor supported metal or oxide
Pt NiO x RuOz
TiO z (Q) 0 (Q)
SrTi0 3 0 0 0
A 4 Nb 6 0 17 (A = K,Rb) 0 0 (Q)
Na z Ti 6 0 13 (Q) 0
BaTi 4 0 9 X 0
TazOs X (Q) (Q)
ZrOz (Q) (Q) (Q)
KSr z Nb 3 O lO X (Q) (Q)
Kz Ti 6 0 13 (Q) (Q)
A4 Ta x Nb6-x O l7 (Q)
Others
(Q) (Cu)
o (Rh)
(Q) (Not supported, Cu)
o Reported photocatalyst so far.
(Q) Newly developed photocatalyst by carbonate addiction.
X Difficult to split the water.
- Not measured.

I I
Solar light (AM1.5, lOOmW/cm 2 )
o
Table 14,5 Optimum conditions for water splitting over TiO z and ZrOz photocatalysts suspended in
a Na z C0 3 solution
ZrOz photocatalyst system
TiO z photocatalyst system
ZrOz(Nakarai Chemical Co,): l.Og
NaHC0 3 : 2.0g in 350 ml of water
TiOz(Hydrolysis of Ti(OC 3 H 7 )4): O.3g
Pi loading by in situ photoreduction: O.3wt%
Na z C0 3 : 80g
Hz evolution rate: 1123 J1 mol/h (25.2 ml/h)
Oz evolution rate: 614 J1 mol/h (13,8 ml/h)
Quantum efficiency: 2.0%
Hz evolution rate: 1638 J1 mol/h (36.7 mllh)
Oz evolution rate: 875 J1 mol/h (19.0 ml/h)
Quantum efficiency: 71 %
400W High-pressure Hg lamp was used. 350 ml of water was used,
the optimum reaction conditions related to N a2C03 addition.
Table 14.4 summarizes the combination of oxide semiconductor and support
metal or metal oxide for the photocatalyst, which can split the water to H 2 and O 2
stoichiometrically in the suspension system. We now have more than twenty
photocatalysts which can decompose water under UV irradiation. Using this
Na2C03 addition method, more than fourteen photo catalysts for water splitting
were newly discovered.
Optimization of water splitting system was conducted for Zr02 and Pt/Ti0 2
photocatalysts. Table 14.5 shows typical performances. In the case of the non-
promoted Zr02 photocatalyst system, 37 ml/h of H 2 and 19m1/h of O 2 evolved
simultaneously. This Zr02 catalyst is commercially available. Quantum efficiency
for water splitting over Zr02 catalyst was 71 %. In the case of the 0.3 wt% Pt/Ti0 2
photocatalyst system, 25 ml/h of H 2 and 14 ml/h of O 2 evolved and the quantum
efficiency of this system was 2%. This Ti0 2 catalyst was prepared by hydrolysis
of Ti(OC 3 H 7 k
Fig. 14.8 The photoreactor for water splitting under solar light irradiation.
The photo reactor has a flat quartz window II cm in diameter. 50 mg of powder
photocatalyst and 40 ml of Na z C0 3 aqueous solution are introduced in the pohotoreactor.
After solar light irradiation, the evolved gas was analyzed by GC after connecting the
photoreactor to the glass closed gas-circulating system shown in Fig.14,l.
Table 14.6 Water splitting using TiO z photo catalysts under real solar light irradiation
Catalyst Solution Amount of evolved gas Weather
(mol/I) ( ml/m Z ) condition
Hza) OZa)
Pt(O.1 wt%)/TiO z pure water 21 0 fine
Na z C0 3 (2.2) 45 trace fine
NaOH(4.4) 42 0 fine
NiO x (3wt%)/TiO z pure water 38 0 fine
NaOH(4.4) 153 0 fine
Na z C0 3 (2.2) 420 184 fine
Na Z C0 3 (2.2) 170 68 cloudy
Na Z C0 3 (2,2) 0 0 dark
RuOz(3wt%)/TiOz Na Z C0 3 (2.2) 161 74 fine
Catalyst: 0.05g, reaction solution: 40ml, cell window area: 95cm z ,
a)total amount of evolved gas for 6.5 hours (9:30-16:00)
14.5 Solar Hydrogen Production Using Na2C03 Addition
Method
The stoichiometric and steady-state water splitting using a photocatalyst
system under the real solar Light irradiation was has so far not been achieved.
Therefore, we tried a solar hydrogen production using Ti0 2 photocatalyst
suspended in an aqeous solution ofNa2C03 one summer in Tsukuba, Japan. Ti0 2
was prepared by hydrolysis of Ti(OC 3 H 7 )4 and the precipitate was then calcined
in the air stream at 500°C, Three kinds of photocatalysts were prepared,
0.lwt%Pt/Ti0 2 , 3wt%NiOxlTi0 2 and 3wt%Ru02ITi02. 50 mg of photocatalyst, 40
ml of pure water and a fixed amount of N a2C03 were introduced into a glass
photoreactor having a flat quartz window with an area of 95 cm 2 , and it was then
evacuated up to 35 torr. The photoreactor shown in Fig. 14.8 was left on the
shaking machine under real solar light irradiation (AM-1.5) for 6.5 hrs. Table 14.6
shows the results of three catalysts. 0.lwt%PtlTi0 2 photocatalyst was not effective
for stoichiometric water splitting. Only a small amount of H 2 was produced.
However, 3wt%NiOxlTi0 2 and 3wt%RuOiTi0 2 photocatalysts worked well.
3wt%NiOiTi0 2 photocatalyst showed the best results. It should be noted that H 2
evolution rate over NiOxlTi0 2 and Ru02/Ti02 was superior to that over Pt/Ti0 2
under solar light irradiation. This is because the back reaction to form H 2 0 from
H 2 and O 2 over Pt/Ti0 2 was not fully suppressed under the solar light irradiation
condition. The light intensity of solar irradiation was weaker than that of 400W
high-pressure Hg lamp. Production of H 2 increased with increasing solar
irradiation. On one fine day, 420 ml/6.5hrs/m 2 of H 2 and 184 ml/6.5hrs/m 2 of O 2
were produced from water using a system composed of 3wt%NiO)Ti0 2
photocatalyst and 2.2 mol/l of Na2C03' This was the first time in the world that
water was decomposed stoichiometrically and steadily using an oxide
semiconductor photocatalyst under real solar irradiation. The quantum efficiency
of this solar hydrogen production reached at least 1.7%. If we carry out this
experiment using 50 kg ofNiOxlTi0 2 photocatalyst in a large square-shape pond
100m x 100m, about 4200L of H 2 and 1800L of O 2 can be produced from water
)
on a sunny day.

248 14 Water Photolysis by Ti0 2 Particles
15
14.6 Conclusion
Water Photolysis by Titanates with Tunnel Structures
A newly developed Na2C03 addition method for water splitting using simple
oxide semiconductor photocatalyst systems was introduced. This method has
proved to be very useful for water splitting using various kinds of photocatalysts.
The main role ofNa2C03 for water splitting is the significant acceleration of O 2
desorption from oxide semiconductor surface via peroxycarbonate intermediates
fOD11ed by the reaction of surface carbonate species and positive holes in the
valence band area of the oxide semiconductor catalyst by photo excited charge
separation under irradiation. Water splitting under natural solar light irradiation
was conducted stoichiometrically and efficiently for the first time ever using this
Na2C03 addition method with a 3wt%NiO)Ti0 2 photocatalyst.
Solar light irradiation to the earth for 45 minutes is sufficient to provide the
annual energy consumption of humans. However, we humans do not yet utilize
fully this huge solar light energy source. A water splitting photocatalyst system,
which can potentially produce a mass of hydrogen, clean energy for the next
generation, from virtually inexhaustible solar light and water, is an artificial
photosynthesis process. We should meet the challenge to develop a method of
solar hydrogen production using the photocatalytic direct water splitting process.
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8, K.Sayama and H.Arakawa, J, Photochem. Photobiol. A:Chern., 94,67 (1996),
9. H,Arakawa and K.Sayama, Res. Chern. 1nterrned" 26, 145 (2000).
10. K.Sayama and H.Arakawa, J. Chern, Soc., Faraday Trans" 93, 1647 (1997).
11. G,Munuera, AR.Gonzalez-Elipe, AFernandez, P.Malet and J,P,Espinos, J, Chern.Soc" Faraday
Trans, 1, 85, 1279 (1989),
12. T,E,Eriksen and J. Lind, Radiat, Phys. Chern" 26, 197 (1981),
13, D,P.Jones and W,P,Griffith, J. Chern. Soc., Dalton Trans., 76, 2526 (1980),
14. AKudo, K.Domen, K.Maruya and T.Onishi, Chern, Phys, Left., 133, 517 (1989).
15, J.Lehn, J.Sauvage, R,Ziessel and L.Hilaire, 1sr, J. Chern., 22, 168 (1982),
16, Y.lnoue, T.Kubokawa and K.Sato, J, Phys. Chern. 95,4059 (1992).
17. Y.lnoue, T.Niiyama,Y,Asai and K.Sato, J, Chern. Soc" Chern. Cornrnun., 1992,579.
18. AKudo, ATanaka, K.Domen, K.Maruya, K.Aika and T.Onishi, J, Catal., 111, 67 (1988).
19. K.Sayama, ATanaka, K.Domen, K,Maruya and T.Onishi, J. Phys. Chern" 95, 1345 (1991),
20. K,Sayama, K,Yase, H.Arakawa, K,Asakura, ATanaka, K.Domen, T.Onishi, J, Photochem.
Photobiol, A: Chern, , 114, 125 (1998).
In the development of metal oxide photo catalysts with high and stable
photocatalytic activity for water decomposition, the establishment of a correlation
between photocatalytically active sites and metal oxide structures is desirable. In
particular, it is important to see how the local structures of metal oxides are
associated with the essential steps such as photoexcitation, the transfer of excited
charges to the surface, and reduction/oxidation of adsorbed reactants. This chapter
deals with photolysis of water by titanates with tunnel structures. ,The roles of
tunnel-related local structures in the photocatalysis and ofRu02 promoters loaded
on the titanates are presented.
Figure 15.1 shows the representative tunnel structures of two titanates.
Barium tetratitanate, BaTi 4 0 9 , has a chemical twin type tunnel structure: Ti0 6
octahedra are not oriented parallel to each other, giving a pentagonal prism
space. I ,2) Alkaline metal hexa-titanate, M 2 Ti 6 0 13 (M= N a. Ka, Rb), is one of the
Wadsley-Andersson type titanates in which Ti0 6 octahedra share an edge at one
level in linear groups of three, giving a tunnel structure characterized by a
rectangular space. 3 ,4) These titanates are prepared by solid state reaction of the
corresponding alkaline or alkaline earth metal carbonates and Ti0 2 at temperatures
higher than 1273 K,5-8) In addition, BaTi 4 0 9 can also be prepared by a sol-gel
References
QBa
f8J 110 6
Q M==Na,K,Rb
Fig. 15,1 Schematic representations of a pentagonal prism tunnel structure of (a) BaTi 4 0 9 and a
rectangular tunnel structure of (b) M 2 Ti 6 0 13 ( M = Na, K, Rp).

250
15 Water Photolysis by Titanates with Tunnel Structures
15,1 Water Photolysis by Ru02/BaTi409
251
15.1 Water Photolysis by RuOz/BaTi409 with Pentagonal
Prism Tunnel Structure
significantly with increasing temperatures above 973 K. The ratio of Hi02 was
4 for BaTi 4 0 9 calcined at 973 K, decreased to 2.9 at 1073 K, 2.7 at 1173 K and
2.6 at 1273 K. These results show a good correlation between the photocatalytic
activity and the crystallinity of BaTi 4 0 9 .
For the Ba- Ti-O system, there is a series of barium titanates with the
chemical fomula of Ba2(n_I)Ti4n+] o IOn' 1,12) In order to see the roles of a pentagonal
prism tunnel structure, it is of interest to investigate photocatalysis by non-tunnel
structure barium titanates belonging to the same series. A phase diagram of the
Ba- Ti-O system shows three stable titanates such as Ba2 Ti 9 0 20 , Ba4 Ti l3 0 30 and,
Ba6Ti17040 in addition to BaTi 4 0 9 . These three titanates have no characteristic
tunnel structures. Ba2Ti9020 has a hollandite-like structure,13) whereas Ba4 Ti l3 0 30
and Ba6Ti17040 have close-packed arrays of oxygen and barium atoms in which
some of the octahedral voids are filled by titanium atoms. 1,14) Fig. 15.3 shows the
photocatalytic activity of four RuOTloading barium titanates. Quite large
photocatalytic actiity was observed only for Ru02/BaTi409, whereas other three
barium titanates showed very poor photocatalytic performance: only a small
amount of hydrogen was evolved. IS)
There are two factors controlling photocatalytic activity. One is the role of
Ru02 ,and the other is the ability of barium titanates for photo excitation, that is,
the ability of formation of photoexcited charges (electrons and holes) and their
separation. For the former, the dispersed states such as particle size and
distribution are important, since photoexcited charge transfer occurs through
Ru02' Thus, it is necessary to examine whether or not the states of Ru02 are
similar among the four barium titanates. High resolution TEM observations
showed that Ru02 particles with spherical shapes were uniformly dispersed on the
four barium titanates: the average particle size was 2.3 nm for Ba4Ti13030, 2.6 for
BaTi 4 0 9 , 4.4 for Ba2 Ti 9 0 2 0 and 4.7 for B% TiI7040.IS) Thus, there were no intrinsic
differences among the particle size and dispersion of Ru02, excluding the
possibility that Ru02 had significant effects on activity differences.
For the latter, the formation of photo excited charges and their behavior were
examined by electron paramagnetic resonance (EPR). Fig. 15.4 shows EPR signals
obtained in a He atmosphere at 77K under UV irradiation. IS) BaTi 4 0 9 showed a
method. Acetic acid anhydride solutions of barium acetate and titanium
isopropoxide in a 1: 1 molar ratio were mixed, and viscous solutions were obtained
by water addition. 9 ) The solution was dried at 473 K for 5 h and calcined in air at
various temperatures from 873 to 1273 K for 20 h. The titanates thus prepared
showed very poor performance by themselves, but became very active for overall
splitting of pure water when combined with promoters. Different kinds of metals
and metal oxides were investigated as promoters,8,1O) and Ru02 was found to be
useful for H 2 and O 2 production.
RuOTloading titanate photocatalysts were prepared by impregnation with
RuCl 3 aqueous solution or RU3 (CO)12 in THF and subsequent oxidation at around
773 K.II)
Figure 15.2 shows a representative example of H 2 and O 2 production from
water by UV irradiation of3 wt% RuOTloading BaTi 4 0 9 . 7 ) From the initial stage,
both products were evolved in gas phase with constant rates, and the ratio of H 2
to O 2 ( Hi02= 1.92 ) reached nearly the stoichiometric value. Three repetitions of
the reaction after evacuation of the gas phase products showed good
reproducibility, and no deactivation was observed, indicating that the titanate
was a quite stable photocatalyst. The turnover number of H 2 production at 30 h
irradiation was larger than 300.
To examine the relationship between crystallinity and photocatalytic activity,
BaTi 4 0 9 prepared by the sol-gel method was calcined at high temperatures. 9 ) The
X-ray diffraction pattern showed amorphous for a sample calcined at 873 K.
Crystallization started by calcination at 973K, proceeded with increasing
calcination temperatures, and crystallinity reached 83% at ] 273 K. The
photocatlaytic activity strongly depended on the calcination temperature.
Photocatalytic activity was negligible for BaT409 calcined at 873 K and increased
1 st 2nd 3rd
200
0
E
::I..
--
t5
:::J
"C
0
a. 100
-
0
-
c:
:::J
0
E
<:
10 20 30
t I h
150 -
-
I
.r::
-: 100
tJ'I
r4
0
S
:i.
........ 50

a
(a)
(b)
(c)
(d)
Fig. 15.2 Production ofH 2 and O 2 from pure water by 3 wt% RuOrloading BaTi 4 0 9 .
(From y, Inoue, J. Chern, Soc. Chern, Cornrnun" 580 (1992»
Fig. 15.3 Comparison of photocatalytic activity on Ru02/barium titanates of (a)Ba6 Ti 17 0 4o ,
(b)Ba4TiI303o, (c) BaTi 4 0 9 and (d) Ba2Ti902o"

252
15 Water Photolysis by Titanates with Tunnel Structures
15,1 Water PhotolysIs by RuOiHaf1 4 0 9
253
strong EPR signal withg= 2.018 andg= 2.004,16) whereas the rest of the barium
titanates provided no characteristic signals. These EPR results clearly
demonstrated that the radical production with UV irradiation was quite different
between BaTi 4 0 9 and the other titanates. The g values of the signal for BaTi 4 0 9
were close to those of a surface 0- species reported for N 2 0 adsorption on ZnOI7)
and Mo/Si0 2 ,18) One interesting finding was that exactly the same signal for
BaTi 4 0 9 obtained in a He atmosphere appeared in the other gases such as H 2 , O 2
and Ar. 19 )
Figure 15.5 shows changes in the intensity of a signal at g=2.018 for BaTi 4 0 9
with cycles of light-on and -off under different gas phases. 19 ) UV irradiation of
BaTi 4 0 9 in vacuum caused a steep increase in the intensity, followed by a rapid
decrease with a half-life time of 30 s. With light-off, the signal nearly disappeared.
It is of particular interest that in the presence of gases, irrespective of the kind of
gas, the signal remained stable not only with light-on but also with light-off.
The signal (g= 2.018 and g= 2.004) obtained in 30 Torr 0 f O 2 changed to a
new signal with g = 2.018, g=2.0 1 0 and g=2.003 when the temperature was raised
in an O 2 atmosphere to room temperature and then cooled to 77 K. IS) The g values
of the newly produced species were quite analogous to those of adsorbed 0 3 -, On
the other hand, the signals were not observed when the same procedures of
temperature increases were performed in gas atmospheres other than O 2 ,
In order to see how the EPR active species is associated with photocatalysis,
a BaTi 4 0 9 sample was subjected to reduction in a H 2 atmosphere at high
temperatures from 1073 to 1173 K and then to oxidation at 773 K. Changes in the
photocatalytic activity and the intensity of EPR signal were determined. As shown
in Fig. 15.6, the intensity of the signal decreased with increasing reduction
r", "--
,J., ,:-::-: _ _-=-:  _ _, _ _ _.   ,--=-:=t
....,-.-1'v.,/'v..r-.j'J'\".4"Y\r.___--..J"W'<'\'''''JJ....\
I
I
Ib) i
>. _---1_ _, _ . , " - - - . - , - - - - - . , - - - - , - - - - - , - - - - - - - - - - - . - - - - - - -
+' tlj\.f'------.fr--""'I..... .J....J
.r-4  - \,--...,
 i . I
OJ
-g Ie) '. _ _, _ _ _ _ _ _ _ _ _ , , _ _ _ _ _ . , _ _ _ , , _ . _ , _ , . . , . , _, _ _ _ _ _ _. _ _ _
'-4
r;; _..
g, i
. I d) I  , __ _ _ _ . _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ , _ _ _ _ _ _ __
:I
.,
Il: p, ." r - no__. . .,...., ,'.-.."1..,.,. ,_.---....,.'._.-.......-
.,-", , -,--/, -.....
bJ r" ;"".. --.......,........--- -
Ie) I
.¥----------------------------------------------
Mn 2 +
light
off on off on off on
0 2 4 6 8 10 12
t / min
Mn 2 +
I
Fig. 15.5 Changes in intensity ofEPR signal (g = 2.018) with cycles oflight-on and light-off in (a)
vacuum, and 30 Torr of (b) O 2 , (c) H 2 , (d) He, and (e) Ar.
(From y, Inoue, Chern. Phys. Left., 267, 74 (1997»
(a)
(b)
12
20
10 '2
'c
:::J
g=2.018
..
J::
(c) 0
E
::t
__ 10
::-
Ig=2.004
(d)
0
3220 3260 3300 (8) (b) (e) (d)
RIG
.a
B ....


'w
6 a5
......
c:
(\J
4 c:
0>
'(i)
a:
2 0...
W
o
(e)
Fig, 15.4 EPR signal of (a) Ba6Ti1704o, (b) Ba4Ti1303o, (c) BaTi 4 0 9 and (d) Ba2Ti902o under UV
irradiation at 77 Kin 30 Torr of O 2 ,
(From Y. Inoue, J. Chern, Soc" Faraday Trans" 94, 93 (1998»
Fig. 15.6 Photocatalytic activity and EPR signal intensity of BaTi 4 0 9 , (a)BaTi 4 0 9 without heat
treatment, and BaTi 4 0 9 oxidized at 1173 K after reduction with H 2 at (b) 1073, (c) 1123, (d)
1148, and (e) 1173 K.
(From Y. Inoue, J. Chern. Soc" Faraday Trans., 93, 2435 (1997»

54
15 Water Photolysis by Titanates with Tunnel Structures
15.1 Water PhotolYSIS by KuU2/tsa114U9
1.::'::>
9.1",2.01B I
temperatures and nearly disappeared by reduction at 1173 K.20) A similar change
in photocatalytic activity with reduction temperatures occurred with
Ru02/BaTi409' Based on these results, a correlation between the photocatalytic
activity for H 2 and O 2 production and the EPR signal intensity was determined.
Fig. 15.7 shows that the activity increased in proportion to the signal intensity,
indicating that the EPR-active surface species directly contributes to the
photocatalytic active sites. When an isotope oxygen, 17 0 2 , was used instead of
normal O 2 , for oxidation at 773 K following H 2 reduction at 1073K, EPR spectra
showed new signals with g=2.038 and g= 2.026, as shown in Fig, 15.8. 21 ) This
signal is due to hyper fine splitting of 17 0- with A =19.1 G. The appearance of the
isotope signal indicates that the radical results from lattice oxygen. Furthermore,
it should be noted that the formation of the 0- radical occurred at the surface
region, since the signal was stabilized in the presence of gases. 19 ) From these
findings, the unique signal of BaTi 4 0 9 with g=2.018 and g=2.004 was assigned to
surface lattice 0-. The radical undoubtedly appeared as a result of the photoexcited
electron and hole formation, indicating that BaTi 4 0 9 has a higher ability to
produce photoexcited charges, compared to the other three barium titanates. The
high photocatalytic performance of Ru02/BaTi409 is considered to be associated
with the high efficiency of BaTi 4 0 9 for 0- radical formation.
Figure 15.9 shows Raman spectra of the barium titanates. In the range of300
to 1000 cm- 1 , BaTi 4 0 9 has three major bands at 430-450, 590-650 and 860
cm- I . IS ,16) The characteristic feature was that a strong single peak appeared at
25
20
"'I 15
J::
0
E
::t.
-- 10

5
2 4 6 8 10 12
EPR signal intensity (arb. units)
Fig, 15,7 Photocatalytic activity ofH 2 (.) and O 2 (D) production vs. EPR signal intensity,
(From Y. Inoue, J. Chern. Soc., Faraday Trans" 93, 2435 (1997»
A.1" 19.1 
.q
'"
c:
v
.5
c:
'"
E
'"
0::
J \ 
)Vv \j
(a) fv'J flA fA '
(\ /,J'0 ". 'v \J
(b) ./ ,---",- 
g=1. 97
(8)
I 9 u =2 .004
(b)
(d)
(\,.\..
/ ---,'-../
f\
('vVVM-v
/"-\ ,/
.'0_'
I
3150 3200
3250 3300 3350
magnetic field I G
1000
800
600
400
wavenumber f cm- 1
Fig. 15.8 EPR signal of BaTi 4 0 9 treated in (a) 16 0 2 and (b) 17 0 2 .
(From y, Inoue, Chern. Phys. Lett., 319, 457 (2000»
Fig. 15.9 Raman spectra of (a) Ba6Ti1704Q, (b)Ba4Ti1303Q, (c) BaTi 4 0 9 and (d) Ba2Ti9020.
(From Y. Inoue, J. Chern, Soc" Faraday Trans" 94, Q (1997»

256
15 Water Photolysis by Titanates with Tunnel Structures
15,2 Water Photolysis by RuOiN 2 Ti 6 0 13
257
Ccntcr of gravity
of 0 ions
around as high as 860 cm- I . On the other hand, the other three barium titanates
showed complicated peaks consisting of many small peaks. In the region of high
wave number, no strong single peak was present, although considerably broad and
small peaks were observed. Dehnicke showed that the Raman spectrum ofTiOCl 2
compound had a strong peak at 836 cm- 1 , which was assigned to the stretching
vibration of a double bond-like short Ti-O bond. 22 ) Analogous to this, the same
assignment can be given to the characteristic peak of BaTi 4 0 9 . The presence of the
short Ti-O bond indicates that the Ti0 6 octahedron is heavily distorted.
The crystallographic data showed that BaTi 4 0 9 has two kinds of strongly
distorted Ti0 6 octahedra. One Ti0 6 octahedron has displacement of a Ti ion by
0.030 nm from the center of gravity of the surrounding oxygen ions and the other
by 0.021 nm. 2 ) The displacement generates large dipole moments of 5.7 and 4.1
Debye, respectively, which correspond to the presence of internal fields in Ti0 6
octahedra, as shown in Fig. 15.10. It is highly plausible that this field acts so as
to promote the separation of photo excited electrons and holes in photoexcitation.
Gas phase effects showed that the EPR signals were retained in the presence of gas
phases, suggesting that the surface lattice 0- radical was stabilized by th
surrounding gaseous molecules. The stabilization might be effective for 0- radical
ejected from the surface, which is produced as a result of Ti-O bond scission upon
UV irradiation. A model of the photocatalysis is shown in Fig. 15.11.
'L
a
a.2nm
Fig. 15,10
Local structure and dipole moment of a pentagonal prism tunnel structure of BaTi 4 0 9 .
Filled circle Ti 4 +, small circle 0 2 -, large circle Ba 2T . The arrows show dipole moment. The
numericals express the Ti-O bond length in nm.
(From y, Inoue, Chern, Phys. Left., 267, 75 (1997»
15.2 Water Photolysis by RuOz/NzTi60I3 with Rectangular
Tunnel Structure
tH 2
t0 2 + H
Figure 15.12 shows the photocatalytic activity of RuOz /M 2 Ti 6 0 13 (M = Na,
K, Rb, Cs) where M 2 Ti 6 0 13 was impregnated with either RuCh aqueous solution
or RU3(CO)12 in THF. Ru02 thus prepared was referred to as RuOlCL) and
Ru02(CB), respectively. The photocatalytic activity increased in the order
Na>K>Rb»Cs.ll) No activity was observed for M=Cs. The ratio ofH 2 /0 2 was 2.0
for M=Na, 1.82 for M=K, and 1.72 for M=Rb. It is noteworthy that the order of
photocatalytic activity with kinds of the alkaline atoms was the same for both
Ru02(CL) and Ru02(CB) and that RuOlCB)/M 2 Ti 6 0 n gave 1.6-2 fold larger
activity than did Ru02(CL)/M2 Ti 6 0 n . High resolution TEM showed that
Ru02(CB) was more uniformly dispersed and had smaller particle sizes (average
particle size Pav=3.2 nm) than did RuOlCL)(Pav=6,7 nm). These results indicate
that the higher activity of Ru02/M2Ti6013 prepared using RU3(CO)12 is ascribable
to increases in the functions of Ru02 particles and in the concentration of active
sites associated with Ru02'
In the EPR spectra ofM 2 Ti 6 0 13 under UV irradiation at 77 K in the presence
of O 2 , Na2Ti6013 showed a strong signal with g=2.020, g=2.018 and g= 2.004.
K 2 Ti 6 0 13 gave a signal with exactly the same g values but 30% lower intensity.23)
For Rb 2 Ti 6 0 13 , intensity decreased further, whereas for Cs 2 Ti 6 0 13 , no signal was
observed. The order of EPR signal intensity increased in the order
Na>K>Rb»Cs, which was the same as that observed for photocatalytic activity,
A good correlation existed between the photocatalytic activity and the EPR signal
intensity, II) as shown in Fig. 15.13.
The life times of EPR signal with cycles of light-off and -on were measured
in vacuum and in 30 Torr of He and N 2 .1I) In vacuum p. small signal (g=2.020,
JhV
e-

,. . 
t
tH 2 H+

OHt02 +H
h+ 7'
e \0-
.
t
. ,..


Fig. 15,11 A model of photocatalytic active sites of Ru02-1oading BaTi 4 0 9 and M 2 Ti 6 0 13 (M =
Na,K,Rb) titanates for water decomposition.
(From Y. Inoue, J. Chern, Soc" Faraday Trans" 93, 2436 (1997»

258
15 Water Photolysis by Titanates with Tunnel Structures
15.2 Water Photolysis by RuOiN 2 Ti 6 0 13
259
g=2.018 and g= 2.004 ) appeared but rapidly attenuated to a negligible level
under the conditions of light-on. In He and N 2 , large signals were observed and
remained stable under light-on. The signal was maintained to' a considerable
extent, even under light-off. This behavior was quite analogous to those observed
for BaTi 4 0 9 , indicating that the radical species produced on M 2 Ti 6 0 13 (M = Na, K,
Rb) by UV irradiation has the same character and is assigned to the surface lattice
0- radical.
Na
K
Rb
Cs
Raman spectra showed that M 2 Ti 6 0 13 (M = Na, K, Rb) has a strong single
peak in the region 846-873 cm- I , in addition to complicated peaks below 600
m- I . II) This characteristic peak corresponds to the single peak observed in
BaTi 4 0 9 . Thus, it is clear that the Ti0 6 octahedra of M 2 Ti 6 0 n has short Ti-O
bonds of double bond-like character, which was indicative of heavy distortion of
the Ti0 6 octahedra. Wads ley-Andersson demonstrated that M 2 Ti 6 0 13 had three
kinds of Ti0 6 octahedra in which the positions ofTi ions deviated from the center
of gravity of the surrounding six oxygen,3) thus forming dipole moment inside
each Ti0 6 octahedra. The dipole moment was 5.3, 5.8 and 6.7 D for Na2Ti6013,3)
4.2, 6.9 and 6.0 D for K 2 Ti 6 0 13 ,4) and 5.0, 6.9 and 4.5 D for Rb 2 Ti 6 0 13 . 3 ) The
presence of an extremely large dipole moment was similar to the situation of
BaTi 4 0 9 . Thus, it is evident that the internal fields due to the dipole moment in the
distorted Ti0 6 octahedra have a promoting effect on 0- radical formation by UV
irradiation. These findings demonstrated that both M 2 Ti 6 0 13 and BaTi 4 0 9 had
common factors for the origin of photoctalytic activity generation, although the
shape of the tunnel structures was different.
The additional role of tunnel structures is a geometric effect, which allows the
accommodation of small Ru02 particles on their space, i.e., the characteristic
structures consisting of "concave" sites have the advantage of forming small
RU02 particles. 8 ) The space unit is large enough to accommodate. Ru02 particles
ofless than 1 nm,II,IS) As demonstrated in the HRTEM images, such small Ru02
particles were present when prepared using a RU3(CO)12 complex, and it is
plausible that the small RU02 particles were embedded in the space. This provides
the nest model shown in Fig. 15.14. This confoD11ation is likely to enhance the
efficiency of photoexcited electron transfer from the titanate surface to the RU02
particles.
Based on the above results, the following picture might be drawn as a
photocatalysis model in tunnel structure photocatalysts. As shown in Fig. 15.10,
the internal fields of the distorted Ti0 6 octahedra having large dipole moment
facilitate the separation of photoexcited charges, that is, electron transfer from O 2 -
l
lattice ions to Ti ions. This leads to the formation of a stable surface lattice 0-,
which acts as a hole site, whereas the photoexcited electrons move to RU02
particles, through which the electron is transferred to adsorbed H+.
20
Hz
[J RU02 (CL)
D Ru0 2 (CB)
::: 15
-
o
e
::L
-... 10


.-
>
.-

< 5
O 2
o
Fig, 15.12 Changes in photocatalytic activity with M in RuOlCL)/ M 2 Ti 6 0 13 and Ru02(CB)/BaTi40g.
(From Y. Inoue, Phys, Chern, Chern, Phys., 1, 182 (1999»
100
20
o RU02 (CL)
. RuOz (CB)
80

.-
j>
 60
Co)

Q)
j>
;3 40

-
Q)

Rb 2 Ti 6 0 13
.
Cs Ti 6 0 13
o 5 10 15 20 25
Concentration of radical I 10 16 g-1
Fig. 15,13 A correlation between the 0- radical concentration and the relative photocatalytic activity
of Ru02-loading M 2 Ti 6 0 13 (M = Na,K,Rb).
(From Y. Inoue, Phys, Chern. Chern. Phys., 1, 182 (1999»
Fig, 15.14 A "nest" model for Ru02 accommodation by a pentagonal prism tunnel space,
(From Y. Inoue, J. Chern, Soc" Faraday Trans" 90, 801 (1994»

260 15 Water Photolysis by Titanates with Tunnel Structures
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21. M, Kohno, S. Ogura, K. Sato and y, Inoue, Chern, Phys. Lett., 319, 451 (2000),
22. K. Dehnicke, Anorg. Allg, Chern" 309, 266 (1961),
23, S. Ogura, M, Kohno, K. Sato and Y. Inoue, App/. Surf Sci., 121/122, 521 (1997),
Water Photolysis by Layered Compounds
16.1 Introduction
Considerable research effort has gone into the design and study of
photo systems for the cyclic cleavage of water into H 2 and O 2 , Any such system
that could work under visible light irradiation will have genuine applications in
solar energy conversion and storage. Two basic approaches to the subject have
emerged. One is to mimic natural photosynthesis by using organometallic
sensitizers. Another strategy is to use inorganic compounds which are stable
under photochemical conditions. Among inorganic photo catalysts, layered
compounds provide unique characteristics for electron transfer processes due to
their low dimensionality. Layered materials with ion exchange and/or intercalation
capabilities especially, show behavior that is not seen in so-called bulk type
photocatalyst such as Ti0 2 . There are several advantages to using layered
compounds rather than bulk type catalysts for water photolysis. Layered
compounds are rich in variety, and they are easily modified by exchange of
cations in the interlayers. The most important advantage is that some layered
compounds have considerably higher photocatalytic activities than bulk catalysts
because the electrons and holes generated in each thin inorganic sheet readily
reach interlayer spaces as reaction sites in the interlayer spaces. [n the present
chapter, we will focus on photocatalytic cleavage of water using layered
compounds.
16.2 Layered Oxides of Transition Metals
Transition metal elements such as Ti, Nb and Ta, which form dO cations, are
known to form several oxides of layered structure exhibiting interlamellar activity.
Figs. 16.1 and 16.2 show the idealized structures of some of these oxides. All of
these oxides consist of titanate and/or niobate.macropolyanion sheets and alkali
metal cations in between to compensate the negative charges of the sheets. The
alkali metal cations can be replaced by various kinds of cations including protons.
These oxides may be regarded as semiconductors/insulators due to their large
band gap. The electronic state of the oxide sheets is excited when the photons with
energy larger than the band gap (typically in the ultraviolet region) are absorbed.
This process may be regarded as the excitation of electrons from the valence
band, mainly consisting of 0 2p orbitals, to the conduction band, mainly
consisting of Ti or Nb 3d orbitals. Based on the band models of the electronic

262
16 Water Photolysis by Layered Compounds
16.3 Nb6017
263

. . . . . . III»
[
0000000

Table 16.1 Unit Cell Data and Electronic Band Energy Data for Nb6017 and Related Oxides
oo --.00
1011"':-  0 :,11(':- 
";; 1k!llul/""jii1"III_III'
....'... :::!,.. ...."... /..
""......'f. ""......'f
. . . .
. . .
Composition Unit cell dimensions
No,ofH 2 0
of hydration
Estimated band gap Estimated
/ Absorption edge Conduction band
edge 01 vs, NHE)
Ax Ti2."Mx04
K 2 Ti 4 0 q
Nb6017
Orthorhombic, a = 0.758,
b = 3,23, c = 0.640 nm,
a = 0.783, b = 3,32,
c = 0.646 nm,5)
Orthorhombic, a = 0.779, .
b = 3.319, C = 0,650 nm
Orthorhombic,
b = 3.45 nm. 8 )
Orthorhombic,
b = 3,32 - 3.45 nm 8 )
0,3 and 4,5 4 ); 3 5 )
3.52 ey6)
3,3 ey7)
360 nm 8 )
-0.76 6 )
. . .
. . . ..
. . . ..lglIgligl
..IQlIgligl 0 _0_ 
IQlIgliol 0 0 0 1011':'1101.
o 0- 0 101l01l01f".:o'.:o'
1i511i511i5lr.:o....
rrr


o \?J'£fj4:\

Rb 4 Nb 6 0 17
3 5 ,9)
360 nm 8 )
Rb4T017
== 3 8 )
295 nm 8 )
4.4 eYIO)
Na2Ti)07
K] Ti s NbO l4
CsTi 2 Nb0 7
A4 Ta"Nb6-xOI7
(A = K,Rb;
x = 2,3,4,6)
K2H2Nb60'7
295 - 410 nm 8 )
3.45 eV 6 )
-0.265 6 )



101101 101101 10110/
r"'Ii5lr",161-
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states, the excitation process is depicted as the formation of electrons (e-) in the
conduction band and positive holes (h+) in the valence band. If electrons and
holes are localized, they may be regarded as Ti 3 + or Nb4+ and 0- species. The
detailed dynamic behavior of these excited states has not been fully revealed,
Judging from the results of photocatalytic reactions, the behavior of photo excited
electrons and holes seems to be more or less similar to that of bulk type titanates
and niobates. In principle, however, these photo excited electrons and holes should
be regarded as being confined in two-dimensional sheets of the oxides,
Among these oxides, Nb6017 has been studied in some detail although the
structure is rather unique. Studies on this oxide will be introduced first, followed
by those on other layered oxides. Tables 16.1 and 16.2 list the various systems
studied for photocatalysis.
KTiNbO s KNb]Og
Fig, 16,1 Idealized structures of some layered titanates and titanoniobates possessing edge- and
corner-connected octahedra. I)
!I
. .
. .
Ii
. . . .
* !I
. .
. .
. . .
. .
 * . .
. .
. . . .
 . . !I
* Ii . .
. .
 . .
(i) (ii) (iii) (iv)
(llA1F 4) RbLaNb 2 0 7 CsCa2Nb]OIO RbBi]Ti 4 OI]
16.3 KtNb 6 0 17
16.3.1 Structure and Physico-chemical Properties
!I
. .
. .
.
. .
. . 000
* . . ft
000
. . I
  000
 o 0 0
. . . . . .
. . . . . . x 000
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. .
. . . . . . * Ii . .
. . . . . . . .
 
(v) (vi) (vii) (viii) (ix)
K 2 NiF 4 NaLaTi0 4 LizLao.833Nbl.S Ti o ,5 0 7 KzLa2Ti]OIO Li2Laz.2SNbl.Z5 Tiz, 7501)
Single crystals of Nb6017 can be obtained by slow cooling of the melt near
1200°C. 4 ,34,35) Powder samples can be prepared by conventional ceramic methods
at about 1 100°C. X-ray powder patterns have been reported in the literature. 4 )
The BET surface area of the ceramic preparation 8 ) is typically of the order of 1
m 2 g- l . Hydrothermal synthesis can be used to prepare the single-phase oxide at
temperatures as low as 280°C with four times the surface area, as reported in the
literature. 36 ) Catalytic properties of such preparations have not been examined.
This oxide has a layered structure consisting of [Nb 6 0 17 ]4- macropolyanion
sheets interleaved by K+ ions, as shown in Fig. 16.3. The K+ ions can be replaced
by various mono- and multivalent metal cations,22,37,38) protons 6 ,12,38,39) and organic
cations. 40 ) An important aspect of the structure is the presence of two, types of
interlayer regions,5,9) referred to as interlayer J and interlayer II, with differing
interlayer reactivity.37) This makes the [Nb 6 0 17 ]4- sheets nonsymmetric with
respect to mirror reflection about the sheet planes. Besides the anhydrous
compound of the above formula, it also forms two hydrates, one with about three
Fig, 16,2 Idealized structures of some perovskite-related layered oxides and their fluoride relatives. 2)

264
16 Water Photolysis by Layered Compounds
16,3 Nb6017
265
System
Type of catalytic reaction
Table 16,2 Nb6017 and Related Systems Investigated for Photocatalytic Activity
Ref.
Promoter
Nb6011
H 2 and O 2 from H 2 0
H 2 [rom aq, methanol
Ni H 2 and O 2 from H 2 0
Ni Redn, of NO]- to N0 2 -
Ni & a third component, H 2 and O 2 from H 2 0
metal oxide or hydoxide
Pt
Pt
MO x , M = Cr, Mu, Co,
Ni, Cu, Pt, Ru, Rh
Rb 4 Nb 6 0 17 Ni
Rb4Ta6017 Ni
A4Nb6-xTaxOI7' Ni
A=K,Rb, x = 2,3,4,6
H 2 and O 2 from H 2 0
H 2 from aq. methanol,
O 2 from aq, AgNO]
H 2 and O 2 from H 2 0
H 2 and O 2 from H 2 0
H 2 and O 2 from H 2 0
H 2 and O 2 from H 2 0
7, 11
11-13
7,11, 11-17
18
17
II
18
13
7
(I
o
(I
o
I
(I
o
5
8,20
8
13,20
21,22
23,24
18
25
13
II
-xHxNb6011
MY-Nb6017
H 2 from aq, methanol
H 2 from C)-C 4 alcohols
H 2 from aqueous Na2S0]
Redn, of NO]- to N0 2 -
Pt H 2 and O 2 from H 2 0
Pt H 2 from aq. methanol,
O 2 from aq, AgNO]
Pt H 2 from C)-C 4 alcohols
Pt, with RuL] as sensitizer H 2 from aq.KI
Redn, of My 2 + to MV+ in H 2 0
and C1-C] alcohols
Redn, of My2+ to MV+
H 2 from aq, methanol
-xMyNb6017' M =-
Cr, Fe, Co, Ni Cu
(I
o
.
o
.
o
(I
o
I
22
6,26
27
28-31
L2
Fig, 16.3 Schematic structure ofNb6017'])
for water splitting.
Nb6017 crystals exhibit optical birefringence 34 ) and absorb in the UV region
with an optical absorption edge between 320-330 nm. A band gap of 3.3-3.5 eV
has been estimated from diffuse reflectance measurements on powder samples 4 )
and on samples suspended in aqueous media. 6 )
H 2 and O 2 from H 2 0 32
H 2 and O 2 from H 2 0 25
H 2 from aq. Na2S and aq. Na2S0] 24
H 2 from aq. Na2S and aq. Na2S0] 24
H 2 from aq, Na2S0] 23
Redn, of MV2+ to MV+ in H 2 0 and 27
C1-C] alcohols
H 2 from K 2 SO] 33
H 2 from aq, Na2S and aq, Na2S0] 24
_xHxNb601'/Ti02
Pt
Pt
xHxNb601/Fe20] -
Pt
xHxNb601/CdS
Ni
_xHxNb60 17
-Cdo.8Zno,2S
16.3.2 Photocatalytic Overall Water Splitting
In distilled water, Nb6017 without modification evolves H 2 and O 2 under
band gap irradiation. This indicates that the photoexcited electrons and holes
have enough potential to reduce and oxidize water. The rates of H 2 and O 2
evolution are, however, very low. Various transition metal oxides were examined
as promoters, and NiO was found to be the best promoter. 14) Typically, Ni-loading
has been carried out as follows. The powdered Nb6017 is impregnated with
dilute aqueous Ni(N0 3 )2 solution, evaporated to dryness with stirring, then
calcined in air to decompose the nitrate to oxide. It is then subjected to evacuation
and to activate the catalyst, it is reduced in H 2 (at a pressure of about 40 kPa) at
500°C for 2 h then oxidized in O 2 (at a pressure of about 16 kPa) at 200°C for I
h. It is known that K 4 Nb 6 0 17 , on treating with NiCh solutions, exchanges the K+
water molecules of hydration, which is stable in the humidity range 25%-85%,
and the other with about 4.5 molecules of water of hydration, which is stable
above 85% humidity.4) These hydrates are structurally related to the anhydrous
oxide: the H 2 0 molecules are accommodated in interlayer I together with the
already existing K+ ions, leaving the [Nb 6 0 17 ]4- sheets and interlayer II intact. 5 ,9)
The possible existence of higher hydrates being formed under aqueous
environments has also been suggested. 4 ) Such spontaneous intercalation of water
molecules is an important feature in making this oxide a photocatalyst, especially

266
16 Water Photolysis by Layered Compounds
16,3 Nb6017
267
ions in interlayer I with Ni 2 + ions. 37 ) It was proved that a 0.1 wt% loading of Ni
with the above treatment procedure is the optimum condition for overall water
splitting.?) For such a sample, assuming 100% exchange, less than 2 mol% ofK+
ions in interlayer I are expected to be substituted. Fig. 16.4 shows a typical time
course of H 2 and O 2 evolution in distilled water for the optimally prepared catalyst.
H 2 and O 2 are produced in a stoichiometric ratio and the total amounts of evolved
H 2 and O 2 exceed that of catalyst. The quantum efficiency at 330 nm was
estimated to be about 5%17) for the optimally loaded catalyst. It should be noted
that for Nb6017' irrespective of modification, the quantum efficiency reaches
20% at 300 nm under low light intensities. 41 ) At higher light intensities, the rates
of H 2 and O 2 evolution did not increase with light intensity; the dependence of the
evolution rates on the light intensity was of the order of 0.52. This nonlinearity
under a higher light intensity regime was attributed to the dominance of the
recombination reaction of photogenerated electrons and holes over the redox
reactions with water.
The pH of a suspension of Ni-loaded Nb6017 in water is about 11 and the
suspension acts as a kind of buffer for the addition of small amounts of acid or
base. When a small quantity (about 0.05 wt%) of KOH was added, the activity
increased by about 30%17) but the pH remained unchanged. On changing the pH
to lower or higher values by adding considerable amounts of acid or base, the
catalytic activity decreased. 7 ) For example, the rate of water decomposition
decreased to about half the value on changing the pH from 11 to 9 or to 13.
Complete suppression of oxygen evolution was observed on addition of excess
alkali to the reaction mixture, although the activity is regained on neutralizing the
excess alkali with acid.
The substitution of K+ with Rb+ gave Rb 4 Nb 6 0 17 , which was found to
decompose water at twice the rate as the K+-analogue. 8 )
16.3.3 Structure of Ni-loaded K 4 Nb 6 0 n and Reaction Mechanism
4
40
Ni-loaded catalysts were characterized using various techniques such as XPS,
EXAFS, ESR and TEM.
XPS measurements demonstrated that loaded Ni is predominantly located
between the layeres of the catalyst and little remains on the external surface. IS) For
sensitivity reasons, a sample with 1 wt% Ni-loading was used. Comparison of the
Ni2p312 peak intensity in the catalyst with that in a reference sample (which was
also 1% Ni-loaded KNb0 3 with almost the same BET surface area as that of
Nb6017) has shown that the surface concentration ofNi in the former is about
100 times less than that of the reference sample. 7 ) EXAFS spectra for I wt% Ni-
loaded samples both before and after the reduction procedure, as well as for Ni and
NiO as standards, indicated that after reduction by H 2 at 500°C for 2 h the loaded
Ni was completely reduced to the metallic state. IS) Even after reoxidation by O 2
at 200°C for I h, most of the Ni remained metallic. (By XPS, the Ni, which
remained on the external surface, was found to be in the oxidized form.) The
formation of metallic nickel on a 0.1 wt% Ni-loaded catalyst was also confirmed
by ESR measurements. 7 ) The appearance of an intense resonance line after the
reduction and reoxidation indicates the formation of ferromagnetic metallic nickel
in the sample.
TEM images of 0.1 % Ni-loaded samples showed the presence of ultrafine
particles of ca. 0.5-0.7 nm in diameter, whereas for 1 % loaded samples, particles
larger than 3 nm were also observed. IS) The rate of overall water splitting on the
0.1 wt% Ni-loaded catalyst was twice that on the ] wt% Ni-loaded one. Therefore,
the Ni-particles responsible for the enhancement of the activity are ultrafine
particles probably located in interlayer I. As mentioned above, XPS results
indicated that these particles are present in the bulk of the catalyst. :The larger
particles observed in the 1 wt% Ni-loaded sample are regarded as poisonous
because they exist in the bulk of the catalyst possibly leading to local destruction
of the layered structure. X-ray powder diffraction did not show variations in d-
spacing perpendicular to the [Nb 6 0 17 ]4- sheets since the amount of Ni was very
small.
Based on these characterizations, a model structure of 0.1 wt% Ni-loaded
Nb6017 was proposed as shown in Fig. 16.5. During the loading of the catalyst
with nickel, most of the nickel enters the interlayex: region I as Ni 2 + by replacing
K+ ions, leaving a very small fraction on the external surface. During reduction at
700°C, the Nj2+ cations are reduced to metallic nickel in the form of ultrafine
particles of about 0.5 nm size.
Based on this model, a possibl reaction mechanism for overall water splitting
on Ni-loaded K 4 Nb 6 0 17 has been proposed as follows. On band gap irradiation,
electrons and holes are generated in the host lattice of [Nb 6 0 17 ]4-. The electrons
are transferred to the nickel particles on which the hydrogen formation takes
place. The holes migrate to the other side of the niobate sheet where water is
oxidized to O 2 . Such separation across the sheet may be facilitated by an
electrostatic gradient existing across the niobate sheets due to unequal K+
3 ro
30 
"0 ---
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aJ
'"
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(.) aJ
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'0
8 2 :>
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0 0
 
0 '"
E '"
aJ
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1 10
o
o
10
20 30
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40
o
50
Fig. 16.4 Time course of H 2 and O 2 evolution over 0.1 wt% Ni loaded Nb6017' 14)

268
16 Water Photolysis by Layered Compounds
hv
@
flJ!if/ftt//2
II
Fig, 16,5 Proposed mechanism of catalytic water decomposition by Ni-Ioaded Nb6017.15)
environments on the two sides of the sheets. However, since the quantum
efficiency of overall water splitting is at most 20% as described above, such an
electron - hole separation is probably not very efficient. Most of the electrons and
holes recombine within the host lattice. Some holes probably react at interlayer I
with water molecules or Ni metal particles, causing the reverse reactions or
degradation of the activity. The degradation of the catalyst with continued use, as
seen by the decreasing rate of production of gases with time (see Fig, 16.4), has
been attributed to the decreasing amount of metallic nickel in the sample. It
should be noted that this reaction mechanism was inferred from the structure of
the active catalyst, and has not been confirmed by any spectroscopic methods.
16.4 Perovskite-related Layered Oxides
Several perovskite-related layered oxides have been used for photocatalytic
studies, especially for hydrogen and oxygen evolution reactions. These oxides
showing interlamellar activity belong to two main classes: i) the Dion-Jacobson
series (DJ series) of the general fOD11ula AMn-IBn03n+), (e.g. KCa2Nb301042) and ii)
the Ruddlesden-Popper series (RP series) of general fOD11ula A2Mn-JBn03n+l. (e.g.
Na2Gd2Ti301043) and K2La2Ti30 10 44 )) , Their structures are characterized by n-
octahedra thick perovskite-like sheets stacked with interlayer A ions placed
between the sheets. Fig. 16.2 shows idealized structures of some of these oxides.
An intermediate series, where the amount of interlayer ions varies between those
of the above two series, is also known. 45 ) Structurally, three types of stacking
sequences are observed in which the neighboring sheets are i) eclipsed (e.g.
RbCa2M301O), ii) "staggered" (e.g. NaCa2M301O), or iii) partially staggered (e.g.
KCa2M301O, M=Nb 46 ) or Ta. 47 »)
An important aspect of these structures is the possibility of varying the
thickness of sheets in multiples of octahedron thickness, Thus, in the DJ series,
oxides with n=2,46-55) n=3,!3,42,46,53-62) n=4,46,56,63,64) n=5,56, 63,64) n=6,56) and n=7 56 )
are known. In the RP series, compounds with n=1 are not known. However, the
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270
16 Water Photolysis by Layered Compounds
16.4 Perovskite-related Layered Oxides
271
A. Dion-Jacobson Series, AMn-lNbn03n+l
CH 3 0H molecules, which is facilitated by the spontaneous hydration of these
proton exchanged oxides. Platinum loading enhances the activity for all
compounds. Considerable activity has been observed towards O 2 evolution from
aqueous AgN0 3 , though the activity is much less than that of Ti0 2 . This is
attributed to the intercalated Ag+ cations that work probably as recombination
centers for electrons and holes.
Photocatalytic activity towards oxidation of higher alcohols has also been
investigated. 13 ,99) For this purpose, KCa2Nb301O was used as the representative
system. Table 16.5 presents the results together with those of Ti0 2 . For each of
the alcohols, the activity increases in the order, KCa2Nb301O < HCa2Nb301O <
HCa2Nb301O-Pt. Also, the lower the chain length, the higher is the activity,
especially for proton-exchanged fOD11s.
By pillaring the layered oxides, it is possible to increase the interlayer space
to make such space available for guest molecules. Ebina et at. 99 ) have investigated
the catalytic activity of silica-pillared Ca2Nb301O. Effective pillaring was achieved
by intercalation ofTEOS into HCa2Nb301O' The BET surface area of the pillared
material was 200 m 2 g- 1 . Since the interlayer space is now accessible for the
alcohol with longer alkyl chains as well, the activity with these alcohols improves
to a greater extent. Table 16.5 includes the results using various alcohols. The
effect of increased interlayer volume is clearly seen in an increase in perfoD11ance
with all alcohols and more so with I-butanol. The pillars are stable up to 400°C.
At higher temperatures, the pillars collapse in height, as indicated by the shifting
of the (001) line to higher angles in the powder X-ray diffraction patterns.
NaLaTi0 4 -type oxides,65-68) where Na+-occupied interlayer planes alternate with
La 3 + -occupied interlayer planes, or the corresponding H+ analogues may be
considered as an ordered n=1 system, Higher members in this series, namely,
those with n=2,69) n=3,43,44,65,70-75) and n=4 69 ) are known, The possibility of varying
the chemical composition by choosing appropriate elements in three positions,
namely, i) cubooctahedral, ii) octahedral and iii) interlayer sites, is another
attractive feature of these oxides. Accommodation of vacancies in cubooctahedral
sites,64,69,75) interlayer cation sites,45,75) and in oxygen sites 76 ,77) is also known. In
addition, their rich interlayer chemistry, e.g., spontaneous hydration, ion-
exchangeability and intercalation of'alkyl amines,48,56,61) makes them very
attractive systems for catalysis studies. Other related studies include exchange of
interlayer monovalent ions with divalent ions,18-81) pillaring of HLaNb 2 0 7 with
A0 2 (A=Ti, Zr or Si),82) reductive intercalation of small ions,57,62,83-90)
luminescence measurements,65,91-95) and dielectric property measurements. 96 )
The photocatalytic cleavage of water in the Dion-Jacobson series is presented
first followed by studies on Ruddlesden-Popper series and finally on the
intermediate series. Table 16.3 lists the oxides employed for photocatalysis
studies.
The Dion-Jacobson series of oxides of the general formula'AMnINbn03n+1 (A
= K, Rb and Cs, n = 2,3 and 4, as well as protonated analogues) has been
investigated by Domen et at. 13 ) for hydrogen evolution from aqueous alcohol
(with and without Pt-loading) and for oxygen evolution from aq. AgN0 3 , under
UV irradiation. Band gaps were estimated from diffuse reflectance spectra. The
photocatalytic perfoD11ance of these oxides is presented in Table 16.4. In these
samples, Pt was loaded by photo deposition method, from H 2 PtCl 6 in aqueous
methanol solution. Since PtC1 6 2 - is an anion, it is not intercalated in the host
oxide. Therefore, loaded-Pt particles were deposited on the external surface of the
layered perovskite powder and observed by TEM.99)
It is seen in Table 16.4 that the activities of protonated compounds are better
those of their non-protonated counterparts. The higher activity towards hydrogen
evolution has been attributed to the easy accessibility of the interlayer space for
Table 16,5 Activity of Ca2Nb301O -System and Ti0 2 for H 2 Evolution from Various Alcohols
Methanol
Rate ofH 2 evolution (I1 mol h-lg-l) Ref,
Ethanol I-Propanol I-Butanol
7 3 2 10
73 27 19 10
380 40 30 10,99
5500 1I00 1060 99
5200 3500 2800 10,99
Catalyst
KCa2Nb301O 7
HCa 2 Nb 3 0 1O a ) 920
HCa2Nb301O-Pta) 4700
Ca2Nb30w-Si02 8100
Pt-Ti0 2 4000
a)Degree ofH+ exchange: > 95%.
Table 16.4 Photocatalytic Activities of Layered Perovskite Compounds and of TiOP)
1.0
Catalyst Rate of gas evolution (umol h- 1 g-l)
H 2 from aqueous methanol O 2 from
Original compound H+ -exchanged compound original
Alone Pt-loaded Alone Pt-loaded compound
KLaNb 2 0 7 28 54 760 3,800 46
RbLaNb 2 0 7 60 90 740 2,600 2
CsLaNb 2 0 7 12 28 300 2,200 3
KCa2Nb301O 14 100 5,900 19,000 8
RbCa2Nb301O 3 26 3,100 17,000 16
CsC a 2 Nb 3 0 1O 2 10 970 8,300 10
KSr2Nb301O 10 110 8,900 43,000 30
KCa 2 NaNb 4 O lO 5 280 790 18,000 39
Ti0 2 7,400 660
<!)
()
s::1
ro
-e 0.5
o

a KCa2Nb301O
b RbPb2Nb301O
o
200
400
600
800
Wavelength / nm
Fig. 16,6 Diffuse reflectance spectra of (a) KCa2Nb301O and (b) RbPb 2 Nb 3 0 IO ,98)

272
16 Water Photolysis by Layered Compounds
16.4 Perovskite-related Layered Oxides
273
Visible light response was observed when the cubooctahedral Ca 2 + ions in
[Ca2Nb301O]- sheets were replaced by Pb 2 + ions as in RbPb2Nb301O.98) In Fig.
16.6, diffuse reflectance spectra of KCa2Nb301O and RbPb 2 Nb 3 0 IO , are shown.
The reason for the absorption edge of RbPb 2 Nb 3 0 lO being at lower energies has
not yet been fully revealed, but may be due to the interaction between 6s 2 electrons
of Pb 2 + and 2 p 6 electrons of O 2 -, Therefore, the top of the valence band shifts to
higher electron energy resulting in a decrease ofthe band gap. Fig. 16.7 shows the
time course of hydrogen evolution by this compound under visible light (> 420
nm) irradiation. Here also, as in the previous cases, HPb 2 Nb 3 0 lO shows better
activity compared to that of the Rb- form,
In this case, two types of Pt-Ioading procedures were employed: i) use of
H 2 PtCl 6 as the precursor as described above to produce Pt particles on the external
surface of the catalyst, and ii) use of Pt(NH3)4Ch to form Pt particles in the
interlayer region. In the latter method, intercalation of Pt(NH3)/+ was achieved by
treating HPb 2 Nb 3 0 lO with an aqueous solution of Pt(NH3)4Ch for one week. This
enables the formation of Pt particles in the interlayer region during
photodecomposition of Pt(NH3)/+ by UV light. The locations of the Pt particles
PtCI-
pt (NH 3 ) +
I PtCI-
pt (NH 3 ) +


(;\
CH 3 0H CO 2
H 2
150
(a)
(b)
"0
E 100
::::t
Fig. 16.8 Schematic representation of mechanisms of H 2 evolution over Pt / HPb 2 Nb 3 0 IO : Pt exists
in the interlayer region in (a) and only on the external surface in (b).9B)
........
'"0
(!)
;>-
'0
(!)
;>-
(!)
C'1

 50
@
o

Table 16,6 Photocatalytic Activities of A2La2 Ti 3 0 IO , ALa2 Ti 2 NbO IO and AuLa2 Ti 2 .sNb o . S O IO oxides lO1 )
o
b
a
Catalyst Rate of gas evolution Optimun condition
(,umol h-lg-l)
pH of the
H 2 O 2 Ni-Ioading Alkali hydroxide solution
(wt% ofNi) concentration
(mol dm- 3 )
K2La2 Ti 3 O lO 444 221 3 0.1 12.8
Rb2La2 Ti 3 O lO 869 430 4 0.1 12,8
CS2La2 Ti 3 O lO 700 340 3 0 10.5
RbLa2 Ti 2 NbO IO 49 30 0.3 0.1 12.8
CsLa2 Ti 2 NbO IO 115 50 0,3 0 8,5
Rbl.5La2 Ti 2 . s Nb o .s0 1O 725 358 5 0,1 12.6
CS Ls La 2 Ti 2 .sNb o .s0 1O 540 265 4 0 104
o
10
Time / h
20
Fig. 16.7 Time course of visible light induced H 2 evolution from aqueous methanol solution over
modified RbPb 2 Nb 3 0 lO catalysts: (a) RbPb 2 Nb 3 0 IO , (b) Pt (0.1 wt%) from H 2 PtCl 6 /
RbPb 2 Nb 3 0 IO , (c) HPb 2 Nb 3 0 IO , (d) Pt (0.1 wt%) from H 2 PtCl 6 / HPb 2 Nb 3 0 lO and (e) Pt (0.1
wt%) from [Pt(NH 3 )4]CI 2 / HPb 2 Nb 3 0 IO ,9B}

274
16 Water Photolysis by Layered Compounds
were confiD11ed by XPS and TEM. The catalytic activity was found to be higher
(about five-fold) for the internally loaded samples, The difference of the catalytic
activity for the two cases is explained as follows (Fig. 16.8). When Pt particles are
on the external surface, Fig. 16.8(a), some of the photogenerated electrons,
especially those produced in the interior region of the sample, must move from the
bulk towards the Pt site. During this move, some of them may recombine with
holes, making the photocatalytic process less effective. On the other hand, when
the Pt is located in the interlayer region, Fig. 16.8(b), the travel length is
considerably shorter, leading to effective photoreaction.
Oxides, such as ALa2Ti2NbOlO (A=Rb or Cs), which have different sheet
composition compared to KCa2Nb301O, have been investigated for their catalytic
activity for overall water decomposition after Ni-loading,97) The results are
included in Table 16.6 in the following section.
B. Ruddlesden-Popper Series, A2Mn-lBn03n+l
K2La2Ti301O is a typical oxide in the Ruddlesden-Popper series exhibiting
interlayer reactivity.44) It forms a hydrate with about 1-2 H 2 0 molecules per
formula unit under ambient conditions. 44 ,74) Fig. 16.9 shows schematic structures
of anhydrous and hydrated oxides. Variation of the number of waters of hydration
with humidity and exposure time has been reported for the analogous compound
K2Nd2Ti301O.70) A band gap of 3.5 eV has been estimated for K2La2Ti301O from
reflectance measurements. 102)
16.4 Perovskite-related Layered Oxides
275
2
'0 Hz

---
"'0
Q)
;>
'0
;>
Q)
U)
ro
bl)
t.r-<
0
+-'
S
0

1 2 3 4 5 6
Time / h
Fig, 16,10 Time course of gas evolution over K2La2Ti)01O with optimum loading (3.0 wt% Ni) and
optimum KOH concentration (0,] M).IOO)
02
hv
H2 0
H2 

;':',i,:,,,;cht;:e::' ""'.;';':';;<;f;(.:f'{";:;,,, ::-1 SO - 100 A
H2 0 02 H 2 0
:: .: :2:  t <' :i:':-i,,:, L e"' .' ".- . ':'A!' -'i _ !;.;:. .1. .Vi  .;u.
. . . .
 H2
H2 0
> 'cih+ "':';;;<.: ,.  ". ':.
Interlayer
, ',.. -. 2 """; ,
La2Ti301O ;. layec;,:
Ii
. .
.
I
c c
1
Ii
. .
1----:--1
a/b 'a/b l
K2La2Ti)01O K2La2 ThO 10' H 2 O K2La2 Ti)OIO' 2H 2 O
H2 0
ca. 1 J1.m
Fig, 16.11 Proposed reaction mechanism for photocatalytic water decomposition on Ni-]oaded
K 2 La 2 Ti)OIO' 97)
Takata et al. and Ikeda et al. have investigated K2La2 Ti 3 0 lO for
photocatalysis. 97, 100-102) On loading with nickel, the oxide exhibits photocatalytic
activity for overall water splitting under band gap irradiation. The Ni promoter is
loaded on the catalyst in a manner similar to that described above. The optimum
, amount ofloaded nickel was 3 wt%, which is considerably different from that for
Nb6017 (0.1 wt%). The activity depends on the KOH concentration. O.IM of
KOH solution was the optimum concentration for this system. Fig. 16.10 shows
a typical time course of water decomposition on Ni-loaded K2La2Ti301O' Although
Fig. 16,9 Schematic structures ofK2La2Ti)01O' K2La2Ti)01O.H20 and K2La2Ti)01O.2H20,44. 7)

276
16 Water Photolysis by Layered Compounds
References
277
other promoters such as Pt and RU02 were also examined, only Ni was found to
be effective.
A scanning electron microscopic investigation of the morphology of the Ni-
loaded catalysts revealed that most of the Ni was present as small particles on the
external surface of the particles. Based on this observation, the reaction
mechanism depicted in Fig. 16.11 has been proposed. 97 ) According to this
mechanism, electrons and holes are generated in the La2 Ti3 0 10 2 - sheets on band
gap irradiation, The electrons, having higher mobility compared to holes, migrate
on to the external surface reaching the Ni particles where H 2 0 is reduced to H 2 .
The holes travel a much shorter distance and react with water molecules in the
interlayer region to generate oxygen. K2La2Ti301O prepared by the polymerized
complex (PC) method has double the photocatalytic activity of the original. 101,102)
In this case, decrease of the impurity phase contributes to the high activity.
Analogous compound Rb2La2 Ti 3 0 lO and Cs2La2 Ti 3 0 lO were also studied for
overall water splitting. 97 ) The optimum Ni-Ioading for these was slightly higher,
namely 4 wt%. The catalytic activity was about twice that of K2La2 Ti 3 0 IO , as
shown in Table 16.6.
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16.5 Summary
To summarize the foregoing studies, layered compounds exhibit high
photocatalytic activities for the cleavage of water owing to the unique layer
structures composed of thin inorganic sheets and interlayer spaces. However, no
layered compound that works for overall water splitting under visible light
irradiation has been devised to date. One of the biggest stumbling blocks
preventing the construction of such an overall photocatalyst is the scarcity of
layered compounds responding to visible light. As mentioned above, RbPb 2 Nb 3 0 lO
irradiated with visible light decomposes water into H 2 and O 2 in the presence of
a sacrificial donor and acceptor, respectively, suggesting that modification of
inorganic sheets is a key to this problem. Designs and syntheses of layered
compounds absorbing visible light are clearly needed and will be the subject of
future work.
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17
Splitting of Water by Combining Two Photocatalytic
Reactions via Quinone Redox Couple Dissolved in Oil
Phase: Artificial Photosynthesis
17.1 Introduction
Hydrogen is the energy-rich material obtained as a result of splitting water,
the most abundant and clean chemical resource. The counterpart product of water
splitting is oxygen, which can be released into air, simplifying the material
recycling system. Hydrogen has been the focus of much attention as an energy
source, and studies on the utilization, storage, and transportation of hydrogen
have been developed.
Production of hydrogen by splitting water utilizing solar energy has long
been a dream of researchers, and extensive studies have been carried out using
semiconductor photocatalysts. The reaction is not easy despite its simple chemical
reaction equation, which is expressed by
H 2 0 -7 H 2 +  O 2 /)'G = -237 kJ mol-I (-2.46 eV).
2
The essential reason for this difficulty is that this reaction is largely
endothermic. Since the above reaction is a 2-electron process, the average energy
of I-electron steps is -119 kJ mol-I (-1.23 eV). For practical electrolysis of water
an overpotential of about 0.5 V is necessary, i.e., an overall voltage of about 1,7
V is necessary. This large voltage requires 4 Si solar ells connected in a series
to electrolyze water. The difficulty of splitting of water is also due to the
complexity of the reaction. In many cases, even if the reaction is
thermodynamically possible, the reaction rate is very'slow, because there are
many back processes. The back reactions are not only between hydrogen and
oxygen, but also between the intermediates. Therefore, a special scheme is
necessary to drive the reaction efficiently. Thus far much progress has been made
to improve the reaction efficiency from different approaches. This includes the
utilization of new semiconductor photocatalysts,I,2) loading of co-catalysts on
semiconductor particles,3-5) and adsorption of ionic species on the surface,6) among
others. However, the efficient splitting of water using solar energy has still not
been achieved.
High efficiency of the reaction under visible light is the key to the efficient
utilization of solar energy, To utilize visible light, the band gap of the
semiconductor must be smaller than 3 eV. However, it is very difficult to drive a
largely endothenTIic reaction using semiconductors with small band gaps. This is
(17.1)

280
17 Splitting of Water by Combining Two Photocatalytic Reactions
17.3 Photocatalytic Hydrogen and Oxygen Evolution
281
the problem of splitting water molecules using solar energy. One approach to
solve this problem is to divide the reaction into two parts, i.e" to use 4 pairs of
electrons and holes (or 4 photons) to split one water molecule. In such a scheme,
the energy required in each step can be lessened, and it opens the way to utilizing
semiconductors with small band gaps. On the other hand, the disadvantage of this
scheme is that twice the number of photons is necessary to drive the same reaction.
That is, by widening the available spectral region, the number of required photons
is doubled. However, the splitting of water by combined photochemical reactions
is a reasonable approach, because this is the scheme followed in the
photosynthesis of green plants, In photosynthesis, the reaction with stored energy
(per electron - hole pair) as high as the splitting of water is driven by combining
two photosystems. 7 ,8)
Sayama et al. 9) and Lincous et al. 10) have proposed systems in which water is
split by a combination of two photocatalytic and photochemical reactions. The
present authors have also studied the water splitting system based on combined
photocatalytic systems. IH5 ) Our goal is the construction of a water splitting
system by mimicking the photosynthesis of green plants.
Light
Ught
/
Oxygen Evolution System
Hydrogen Evolution System
Fig, 17.2 Schematics of artificial photosynthesis consisting oftwo photocatalytic reactions for oxygen
and hydrogen production,
17.2 Strategy for Water Splitting by Mimicking Photosynthesis
semiconductor paticles have often been compared to the photosynthesis of green
plants. 16-21) However, in most of these reactions sacrificial reagents which undergo
irreversible changes are utilized. These irreversible reactions cannot be utilized in
the splitting of water as the energy conversion system, and a new scheme must be
introduced into the photocatalytic reaction system.
Figure 17.2 illustrates our model for splitting water by solar energy. It is
important that all the redox reactions involved in the system be reversible. The
quinone compound in the organic solvent combines the two photocatalytic
reactions, and its function can be compared to the electron relaying molecules in
thylakoid membranes of chloroplasts. Electron transfer reactions via quinone
compounds in artificial systems have been studied as a model of
photosynthesis 22 ,23) and in an electrochemical system for acid concentration. 24 )
With a view to achieving a system mimicking the photosynthesis of green
plants, we have studied the processes required to construct the whole system.
These processes are discussed below,
Light reactions of the photosynthesis of green plants are good models for the
conversion of solar energy into chemical energy.7,8) The energy diagram of
photosynthesis is shown in Fig. 17,1. In photosynthesis, the energy for each
photosystem is lowered by dividing the energy conversion reaction into two parts.
This enables the utilization of visible light. In addition, the separation of the
solution into two parts by a membrane is considered to be effective to prevent the
back reactions of the intermediates. These mechanisms provide clues to the
construction of a water splitting system consisting of oxygen evolution and
hydrogen evolution.
There are many reports of photocatalytic reactions on semiconductor particles
by which oxygen or hydrogen is produced. These photocatalytic reactions on
17.3 Photocatalytic Hydrogen and Oxygen Evolution in
Separate Systems
hv
It is known that water can be oxidized into oxygen on photo irradiated Ti0 2
using some electron acceptors which can be recycled. A typical example of such
an electron acceptor is Fe(llI) ions, although the reported efficiency of oxygen
evolution is 10w. 25 ,26) Recently, however, we found that the efficiency of this
reaction can be raised using various kinds of rutile Ti0 2 powders,27) As for the
hydrogen evolution, there are many studies on reduction of water using Pt-loaded
TiO/8) and Pt-loaded CdS. 29 ) In most of these cases, hydrogen evolves with the
oxidation of sacrificial electron donors such as alcohols and sulfite ions. In
contrast to these systems, little is known about systems in which hydrogen evolves
using electron donors that can be recycled.
In this section, oxygen and hydrogen production is described. For these
studies we used Ti0 2 particles as the model photocatalyst, although Ti0 2 is not
suitable for the utilization of visible light. Our purpose, at this stage, is to
demonstrate the feasibility of the two-step water splitting system.
P700'
"A o
.......A
.:... e
Fx"
, Fs
FA Fd
-- ___NR ( NA DPH
0"_'0-.__0'" "
NADp?
-1.2
.1.0
.0.8 P680'
-0,6 "-
Phea
W -0.4 '"
I OA
Z -0.2 \
en
> 0 as
> +0.2
--
C> +0.4
W
+0.6 H 2 O
e
+0.8 Mn
+1.0, O 2 'z
+1.2
photosystem I
Cytb-' .0
photosystem II
Fig. 17,1 Energy level diagram of light reactions of photosynthesis of green plants

282
17 Splitting of Water by Combining Two Photocatalytic Reactions
17.3 Photocatalytic Hydrogen and Oxygen Evolution
283
17.3.1 Photooxidation of Water Using Ti0 2 Particles
800
<5 600

---
(j) 400
U
::J
-0
2
0..
200
20
40
Time / min
60
80
Photooxidation of water on rutile Ti0 2 electrodes was first reported by
Fujishima and Honda. 30 ) This achievement triggered intensive studies on Ti0 2
photoelectrodes, revealing that band bending is necessary for efficient oxidation
of waterY) Usually, the photo current for oxidation of water appears in the
potential region more positive than the flat band potential (Ufb), and the difference
between Ufb and the electrode potential gives the magnitude of the band bending.
The electron-hole recombination process near the flat band potential was
confirmed for Ru-doped SrTi0 3 electrodes by a comparison of photocurrent and
photoluminescence. 32 )
We found that that large-size Ti0 2 particles show activity for the oxidation of
water. 27) This finding is consistent with the requirement of band bending for the
oxidation of water, because large particles are advantageous for band bending. On
the other hand, the oxidation of water and splitting of water using small Ti0 2
particles has been reported. 33 - 35 ) However, in the present authors' experience,
some of the results are not reproducible. Although we do not deny the previous
reports, caution must be exercised regarding oxygen evolution from small Ti0 2
particles dispersed in solution.
We obtained many kinds ofTi0 2 particles from the Catalysis Society of Japan,
commercial sources, and laboratories. Among the Ti0 2 particles, we found that
Ti0 2 particles (JRC- TIO-5) supplied by the Catalysis Society of Japan showed the
highest efficiency for oxidation of water when used with chemical oxidants such
as Fe (III) and Ag(I) ions. Some other Ti0 2 particles show efficiency similar to that
of the JRC-TIO-5 powder. These Ti0 2 powders contained the rutile phase as the
main component and particles of the size of the order of 1 pm. The JRC- TIO-5
powder contains the rutile phase at 94% and a surface area of about 2.6 m 2 g-I, A
scanning electron micrograph (SEM) of this Ti0 2 powder is shown in Fig, 17.3.
Fig. 17.4 Photocatalytic oxygen (0) and Fe(lI) (e) production by photocatalytic reaction, Ti0 2
powder (JCR- TIO-5, 13 mg) suspended in an aqueous solution (5.0 ml) of 5,6 mM Fe(lII)
chloride was irradiated using a 500 W high pressure Hg lamp. Light of wavelength shorter
than 340 nm was removed using a cut-off glass filter. Before photoirradiation started, the
solution was bubbled with argon,
(From M. Matsumura et aI., J. Chern. Soc" Faraday Trans., 94, 3707 (1998»
.
,,
/ \
( ,
\/
..-'
"'.\:.
";'
0)
(3
E 40
::t..
--.
(j)
-0

0 30
0.
N
0
i=
c 20
0
-0
(!)
.Q
0
CJ) 10
-0
ctI
c:
:J
0
E 0
«
0
o
.
1: 1
.
1 :2

1 :1
4:1
--''"'"''
5
10
15
20
Concentration of iron ions in solution / mmol dm- 3
Fig, 17.5 Adsorption isotherms of Fe(lII) and Fe(lI) ions on Ti0 2 powder (JCR- TIO-5) at 25°C; 0
for Fe (III) ions, D for Fe (II) ions. When both Fe(lI) and Fe(lII) ions were added to the
solution, their adsorptivity affected each other; for Fe (III) ions, . for Fe(lI) ions. The
molar ratio of Fe(IlI) ions to Fe(lI) ions is described in the figure,
(From M. Matsumura et aI., j, Phys. Chern, B, 101, 6417, 10605 (1997»
500 nm
Fig. 17,3 A SEM picture ofTi0 2 powder (JRC-TIO-5) used for photocatalytic reactions.

on Ti0 2 surface. 27 ) As shown in Fig, 17.5, Fe(III) ions are predominantly adsorbed
on Ti0 2 , when Fe (Ill) and Fe(II) ions are present in solution. It is also noteworthy
that by the addition of Fe(III) ions to the solution, the photocurrent at a rutile Ti0 2
electrode rises steeply near the onset potential of the photo current, as shown in
Fig. 17.6. This indicates that the adsorbed Fe(III) ions have some catalytic effect
on the oxidation of water by holes on Ti0 2 . Such unique properties of Ti0 2
particles are very useful to drive photocatalytic reactions to convert and store
light energy. The Fe(III) adsorbed Ti0 2 surface is also found to have unique
properties for the oxidation of alcohols,37) a topic beyond the scope of this review
on water splitting.
284
17 Splitting of Water by Combining Two Photocatalytic Reactions
(a)
I 0,2 mA cm- 2
17,3 Photocatalytic Hydrogen and Oxygen Evolution
285
17.3.2 Photoreduction of Water Using Pt-loaded Ti0 2 Particles
For the photoreduction of water, Pt-loading ofthe photocatlyst is essential to
lower the overpotential for hydrogen evolution. Using Pt-loaded Ti0 2
(JRC-TIO-5) particles as the photocatalyts, we found that hydrogen evolves in the
presence of halides, i.e., chloride, bromide, and iodide, in solution, 11-13) To split
water by a combination with the water oxidation system, which uses a
Fe(II)/Fe(lII) redox couple, iodide ions cannot be used, because iodine cannot
oxidize Fe(II) ions. On the other hand, chlorine is too reactive. Hence, in our study
we used bromide ions as the electron donor for hydrogen evolution.
Hydrogen was produced by photo irradiation of the photocatalyts in an
aqueous solution of potassium bromide (2.0 M), as shown in Fig. 17.7. In the
experiments, Ti0 2 particles loaded with Pt (0,5 wt%) by the photodeposition
method 38 ,39) was used as the photocatalyst. In this case, the amount of evolved
hydrogen leveled off when it reached 8.0 ,umol under our experimental conditions.
The amount of hydrogen was much less than the amount of bromide ions added
70
0
60
50
a
 40
(f)
t5 30
:J
LJ
e 20
CL
10
20 40 60 80 100
Time/min
(b)
I 0.2 mA cm- 2
-0.4
o
0.4
0,8
1.2
v vs, Ag/AgCI
Fig. 17.6 [-V characteristics of a rutile Ti0 2 electrode in an aqueous solution containing 0,01 mol
dm- 3 sulfuric acid and 0.5 mol dm- 3 potassium nitrate (a), and those obtained after addition
of Fe(III) nitrate (2.0 mmol dm- 3 ) to the solution (b). The potential was swept at a rate of
20 mV S-l,
(From M, Matsumura et aI., J. Phys. Chern, E, 101, 6417 (1997»
We have reported that water is efficiently oxidized on photoirradiated Ti0 2
particles (JRC- TIO-5) in an aqueous solution of Fe(lII) ions at a pH of about
2.4. 27 ) A typical result of oxygen evolution in these systems is shown in Fig.
17.4. The amount of oxygen evolved was quantitatively in good agreement with
the amount of Fe (II) ions generated in the solution. The quantum efficiency of the
reaction reached as high as 25%.33).
It is noteworthy that the reaction rate did not decrease until most F e(IlI) ions
were converted to Fe(II) ions. This result indicates that the oxidation of Fe(lI) ions
on the surface of Ti0 2 is a slow process and that water is oxidized despite
possessing a more positive oxidation potential than Fe(II) ions. The high
efficiency of this reaction has been attributed to specific adsorption of F e(III) ions
Fig. 17.7 Photocatalytic production of hydrogen and Br3 - by photocatalytic reaction. Besides hydrogen
(0) and Br3- (D), PtBr62- (e) was sometimes produced, Pt-loaded Ti0 2 powder «JCR-
TlO-5, 16 mg) was photoirradiated in an aqueous solution (7.0 ml) of 2 M potassium
bromide. Other experimental conditions are the same as those of Fig. 17.4.
(From M. Matsumura et al., J. Chern. Soc., Faraday Trans., 94, 3707 (1998»

286
17 Splitting of Water by Combining Two Photocatalytic Reactions
17.4 Electrochemical and Chemical CombmatlOns of Two Photocatalytic Keactlons
[IS {
-0.2
0,0
0.2
w
I 0.4
z
en
>
> 0.6
---
0
W Fe 3 + / Fe 2 +
0.8
1.0
1.2
O 2 / H 2 O
1.4
potential of Br/Br2 is more positive than that of Fe(I1)/Fe(I1I), the above results
indicate that the splitting of water is theoretically possible. Their relationship of
the redox potentials is shown in Fig. 17.8, the diagram is similar to that of the Z-
scheme of photosynthesis of green plants. If the redox couples are effectively
connected, the concentrations of bromine and Fe(II) are kept low. This will be
very advantageous in preventing the back reactions on the photocatalysts.
Note that very little splitting of water is achieved by photoirradiating Ti0 2 and
Pt-loaded Ti0 2 particles in a solution containing both Fe(III) and bromide ions. In
such a mixed system, hydrogen is not generated because Fe (III) ions and bromine
are more likely to be reduced than protons. Similarly, very little water is oxidized
in the presence of bromide ions in solution. In order to prevent such undesired
reactions, the oxygen and hydrogen evolution reactions must be separated. It is
also necessary to combine the systems to recycle the bromine/bromide and
Fe(lII)/Fe(II) redox couples. Otherwise, the oxygen and hydrogen evolution stops
when Fe (III) and bromide ions are consumed.
One approach to combining the two systems is the electrochemical process
using electrodes and ion exchange membranes, as shown in Fig. 17.9. 14 ) Electrons
and protons are transported from Compartment II (for oxygen evolution) to
Compartment I (for hydrogen evolution) through an electric circuit and ion-
exchange membranes, respectively. By this combination, the consumed material
is only water in Compartment II, and hydrogen and oxygen evolve in
Compartments I and II, respectively. Although there are some technical problems,
the evolution of hydrogen and oxygen from water was successfully demonstrated
by experiments,14) Despite the successful combination of two photocatalytic
to the solution (14 mmol). The termination of the net reaction was attributed to the
fact that bromine fOnTIed in the solution was reduced on the Pt-loaded Ti0 2 more
easily than protons. This is the difficulty in driving the endothermic reaction of
photocatalysts. It is important, however, that hydrogen does evolve while the
concentration of bromine is low. The problem of the termination of the reaction
by bromine can be circumvented in the combined photocatalytic reaction system,
as discussed in the following section.
Another feature of this reaction is that Pt sometimes dissolved into the
solution as PtBr62- ions, as seen in Fig, 17.7. The dissolution became significant
for the powders on which a large amount of platinum was loaded, Dabestani et
aI.40) reported a similar dissolution of platinum in their system, where molecular
oxygen was present as the electron acceptor. By using a photocatalyst on which
only a small amount of Pt was deposited under very low irradiation intensity, we
obtained hydrogen in a stoichiometric amount of bromine produced in the
solution. IS)
17.4 Approaches to Electrochemical and Chemical
Combinations of Two Photocatalytic Reactions
The photooxidation and photoreduction of water in separate reaction systems
are demonstrated in Sections 7.3.1 and 7.3.2, respectively. Since the redox
H. / H 2
e-
e-!
Ie-
Pt
Pt
B( Br2 0 Fe 3 +
Br 2 / B( H 2 hv
hv
2H+ H 2 O
"
Compartment I Nafion membranes Compartment II
Fig, 17,8 Energy levels for a water splitting system with a combination of two photocatalytic reactions
for oxygen and hydrogen production.
Fig. 17.9 Electrochemical combination of two photocatalytic reactions for water splitting.

288
17 Splitting of Water by Combining Two PhotocatalytIc Reactions
11.:) ;:,pllumg OJ water DY Lomomanon or 1 wo rnoWCatalYliC KeaCliuns
LO
reactions in such a system, the structural advantage of photocatalytic reactions,
which occur on particles dispersed in solutions, is lost by using the electrodes.
Another approach to a combined system is the connection of the two systems
through a quinone redox couple dissolved in an oil phase, as shown in Fig. 17.2.
This system is analogous to the combination of photo systems I and II in the
photosynthesis of green plants. Fig. 17.10 illustrates the structure of our model
system, in which the oil-phase corresponds to the lipid bilayer membrane of
chloroplast. Such a system is structurally identical to a liposome and has the
possibility of development for use in a batch reactor.
The key to the construction of the system is the choice of the quinone redox
couple in the oil phase and the oil itself. The quinone compound must be reduced
by Fe(II) ions, and the reduced form must be oxidized by bromine. These
requirements indicate that the redox potential must be in the range between 0.77
and 1.07 V vs. NHE. After investigating of many redox compounds, we found that
2,3-dichloro-5 ,6-dicyano-l ,4-benzoquinone (DDQ) dissolved in n-butyronitrile
may be a good candidate for the system. DDQ has a largely positive redox
potential because of its strong electron withdrawing substituents.
Figure 17.11 shows the spectral changes of DDQ in an n-butyronitrile solution
as the result of the redox reactions with Fe(II) and bromine in aqueous solutions.
The spectra are for just the DDQ solution (A), and after successive contacts first
with an aqueous solution of Fe(II) chloride (B) and then with an aqueous solution
of bromine (C). When the DDQ solution was brought into contact with an Fe(II)
solution, an absorption band appeared at 352 nm, completely agreeing with that
of the reduced form (DDHQ) ofDDQ. This absorption band decreased by bringing
the solution into contact with a bromine solution, as shown in Fig. 17.11. The
reproducible spectral changes indicate the applicability of DDQ as the mediator
between the two photocatalytic reactions producing Fe(I1) ions and bromine,
respectively.
3
I
I
.
. .
: r 1\
2 : I
: I
: I
000
Q)
u
c
co
..0
.....
o
(f)
..0
«
B
N
OOHO
o
250

300
350
400 450
500
Wavelength / nm
Fig. 17.11 Spectral change of an n-butyronitrile solution of DDQ as the result of successive redox
reactions with Fe(I!) ions and bromine in aqueous solutions. A, DDQ (1.1 x 10- 3 M) in n-
butyronitrile(3.0 ml); B, the solution was kept in contact with an aqueous solution (5.0 ml)
ofFe(II) chloride (2.8 x 10- 2 M) for 25 min; C, the solution was kept in contact with an
aqueous solution (8.0 ml) of bromine (1.3 x 10- 3 M) and potassium bromide (0.5 M) for
30 min after the treatment with Fe (I!) ions. The spectra were obtained using a quartz cell
with l-cm path length.
(From M. Matsumura et aI., Zeitschriftfiir Physikalische Chemie, 213, 171 (1999»
Selection of the organic solvent for DDQ and DDHQ is also important. It
must be a good solvent for DDQ and DDHQ, but it must not mix with water. Its
chemical stability is also essential. To meet these requirements, we chose n-
butyronitrile as the solvent.
r
17.5 Splitting of Water by Combination of Two Photocatalytic
Reactions via DDQ/DDHQ
ooHO 000 OOO ooHO
Oil phase.
2H T
') Br2 2Br-
H2
,
Fe(lI) Fe(llI)
"202+2W
H 2 0
hv
To construct photocatalytic reaction systems with oil/water interfaces, an
aqueous solution containing bromide and Pt-Ioaded TiO z particles was placed
under the n-butyronitrile solution containing DDHQ. By photo irradiation of the
aqueous phase, we obtained hydrogen, as shown in Fig. 17.12. With the evolution
of hydrogen, the absorption band ofDDHQ in n-butyronitrile disappeared, owing
to the oxidation ofDDHQ by bromine generated in the aqueous phase. This result
indicates that the system shown in Fig. 17.10 is half achieved. The difference in
the rate ofhydrogeri evolution between the results shown in Figs. 17.7 and 17.12
is mostly due to the different irradiation light intensities used in the experiments.
The ext target is the construction of oxygen evolution in the double phase
system. For this purpose we constructed a double phase system, in which the
aqueous phase contains Fe (III) ions and TiO z particles, and the oil phase was an
n-bmyronitrile solution of DDQ. As shown in Fig. 17 .13, oxygen evolved by
photoirradiation of the aqueous phase. Together with oxygen evolution, DDQ in
hv
Water phase
Compartment I
Compartment II
Fig. 17.10
Combination of two photOcatalytic reactions via a quinone compound. Compartment I (for
hydrogen evolutIOn) and compartment I! (for oxygen production) are connected through
a redox couple of DDQ and DDHQ dissolved in an oil phase.
(From M. Matsumura et aI., Zeitschriftfiir Physikalische Chemie, 213, 170 (1999»

290
17 Splitting of Water by Combining Two Photocatalytic Reactions
References
291
n-butyronitrile was converted into DDHQ. In this case, however, the amount of
oxygen evolved was slightly less than that expected from the amount of Fe(III)
consumed, suggesting that n-butyronitrile hinders the oxidation of water on TiO z
particles. Presumably, some n-butyronitrile molecules dissolved in the aqueous
phase are oxidized on the photo irradiated TiO z particles, although nitriles have
been considered to be stable on photoirradiated TiO z particles. 41 ,4Z)
When we did experiments in a reaction cell with the structure shown in Fig.
17.10, we were able to observe the production of both hydrogen and oxygen.
This experimental result indicates the feasibility of the combined system for the
splitting of water. However, the production rates of oxygen and hydrogen
gradually decreased, probably because of the problem observed in the oxygen
evolution system. The side reaction is a serious problem from the viewpoint of
application. The choice of the organic solvent is the essential point for completely
realizing the target system.
14
12
0
E 10
-'
.......
-
c 2.0
0 8
......
::J
0 1.5
> OJ.
6 u
Q) c
c 2 1.0
Q) 0
CJ) en
4 .J:J
0 « 0.5
"-
-0
>.
I 2 0 350 400 450
300
Wavelength I nm
0 1400
0 200 400 .600 800 1000 1200
Time / min
17.6 Conclusions
0.8
3
Water splitting systems by combining two photocatalytic reactions have been
demonstrated by electrochemical and chemical methods. The system consisting of
two photocatalytic reactions in aqueous phases and a quinone redox couple in an
oil phase is similar to photosynthesis of green plants. To realize more efficient
water splitting without deceleration of the reaction rates by this method, the redox
couple and solvent must be further pursued. Despite the problem of degradation
of the system, we can conclude that a water splitting system consisting of two
photocaalytic reactions is feasible. The merits of such a system are as follows.
1) The energy required for splitting water is lessened, opening the way to the use
of small band gap semiconductors.
2) The concentrations of products in solution can be kept low because of the
redox reactions between the products. This is advantageous for suppressing
back reactions, which lower the reaction efficiency.
3) Hydrogen and oxygen can be obtained in separate chambers.
In order to split water by solar light with high efficienciy, we must utilize
visible light. For oxidation of water, W0 3 43,44) and Ru-doped TiO z 45 ) can evolve
oxygen under visible light irradiation using Fe (III) ions as the electron acceptors.
For hydrogen evolution by visible light, the dye-sensitized TiO z 46 ) appears to be
a good candidate.
Fig. 17.12
Evolution of hydrogen from a double phase system. The aqueous solution (8.0 ml)
contained 2 M potassium bromide and Ti0 2 powder (120 mg). The n-butyronitrile solution
(1.7 ml) contained DDHQ (1.0 x 10- 3 M). During the photocatalytic reaction, only the
aqueous phase was photoirradiated. The inset shows the spectral change of the n-
butyronitrile solution by photoirradiation for 1500 min, indicating that DDHQ was
converted into DDQ.
(From M. Matsumura et aI., Zeitschriftfur Physikalische Chemie, 213, 172 (1999»
o
E
-'
.......
-
0.6
c
o
'';=
::J
o
>
Q)
C
Q)
CJ)
>.
X
o
0.4
0.2
(!)
u
 2
.e
o
en
 1
References
o
300
350 400
Wavelength I nm
450
0.0
o
100
200
300
400
500
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Fig. 17.13
Time / min
Evolution of oxygen from a double phase system. The aqueous solution (20 ml) contained
8.0 x 10- 3 M Fe(I1I) chloride and Ti0 2 powder (30 mg). The n-butyronitrile solution (1.0
ml) contained DDQ (1.9 x 10- 3 M). During the photocatalytic reaction, only the aqueous
phase was photo irradiated. The inset shows the spectral change of the n-butyronitrile
solution by photoirradiation for 480 min, indicating that DDQ was converted into DDHQ.
(From M. Matsumura et aI., Zeilschrift fur Physikalische Chemie, 213, 173 (1999»

292 17 Splitting of Water by Combining Two Photocatalytic Reactions
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17. A J. Bard, J. Electroanal. Chem., 168, 5 (1984).
18. T. Takata, Y. Furumi, K. Shinohara, A Tanaka, M. Hara, J.N. Kondo and K. Domen, Chem.
Mater., 9, 1063 (1997).
19. A. J. Bard and M.A Fox, Ace. Chem. Res., 28, 141 (1995).
20. AM. Linsebigler, G. Lu and J. T. Yates Jr., Chem. Rev., 95, 735 (1995).
21. S. Tabata, H. Nishida, Y. Masaki and K. Tabata, Cat. Lett., 34,245 (1995).
22. J. J. Grimaldi, S. Boilean, J. -M. Lehn, Nature, 265, 229 (1977).
23. D. Gust, T.A Moore and AL. Moor, Z. Phys. Chem., 213, 149 (1999).
24. M. Matsumura, M. Nohara and T. Ohno, J. Chem. Soc., Perkin Trans. 2, 11, 1949 (1995).
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26. T. Sakata, K. Hashimoto and T. Kawai, J. Phys. Chem., 88, 5214 (1984).
27. T. Ohno, D. Haga, K. Fujihara, K. Kaizaki and Matsumura, M. J. Phys. Chem. B, 101, 6415
(1997); errata, 101, 10605 (1997).
28. T. Kawai and T. Sakata, J. Chem. Soc., Chem. Commun., 1980,695.
29. M. Matsumura, S. Furukawa, Y. Saho and H. Tsubomura, J. Phys. Chem., 89, 1327 (1985).
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31. T. Ohnishi, Y. Nakato and H. Tsubomura, Ber. Bunsenges. Phys. Chem., 79,523 (1975).
32. T. Ohno, S. Izumi, K. Fujihara and M. Matsumura, J. Photochem. Photobiol. A. Chem., 129, 143
(1999).
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34. T. Kawai and T. Sakata, Nature, 286,474 (1980)
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(2000)
37. T. Ohno, S. Izumi, K. Fujihara and M. Matsumura, J. Phys. Chem., accepted for publication
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Phys. Chem., 90, 2729 (1986).
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42. M. A. Fox, Ace. Chem. Res., 16,314 (1983).
43. J. R. Darwent and A. Mills, J. Chem. Soc., Faraday Trans. 2,78,359 (1982).
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46. A Fagfeldt and M. Gratzel, Chem. Rev., 95,49 (1995).
18
Sensitization by Metal Complexes Towards Future
Artificial Photosynthesis
18.1 Introduction
The mechanisms of initial photosynthetic processes have recently been
elucidated by X-ray analysis of complexes in the photosynthetic reaction center
of photosynthetic bacteria. A prosthetic group of the photosynthetic reaction
center in Rhodopseudomonas viridis has been elucidated by X-ray crystalline
analysis. 1) This chromophore located near the reaction center absorbs light energy
and transfers it to a bacteriochlorophyll dimer. The electron from the excited
bacteriochlorophyll is subsequently transferred to bacteriopheofitin and then to
low-energy quinone. A charge-separated species with a long lifetime can be
achieved by suppression of back electron transfer. The lack of back electron
transfer results in a long distance between quinone anion radicals and
bacteriochlorophyll dimer cation radicals. The charge-separated species are used
as reducing and oxidizing reagents in the dark reac.tions. The electron transfer
processes in the photosynthetic reaction center occur via the photoexcited singlet
state of chlorophyll within picoseconds and with a quantum yield of ca. 100%.
Photoexcited electron transfer occurs by the fixation of each chromophore in the
reaction center. To elucidate the mechanism of photosynthetic processes in the
reaction center, with the aim of establishing effective artificial photosynthesis
conductors, many electron acceptor-linked porphyrin metal complexes have been
synthesized and their photochemi,cal properties characterized. 2 - 31 )
I Many attempts to establish artificial systems that mimic photosynthesis have
been carried out for solar energy conversion systems. Solar energy is the most
attractive future energy resource, since it is virtually limitless. The amount of total
energy reaching the earth is ca. 3.0 x lO z1 kJ per year. However, as solar energy
is strongly influenced by weather and seasons, it is necessary to convert solar
energy to a storage form of energy. There are two fundamental ways to convert
solar energy into other forms of energy: thermal and quantum conversion. In
thermal conversion, photons with longer wavelengths play important roles in the
conversion process but do not have the ability to excite a molecule to higher
energy levels. During quantum conversion, photons of shorter wavelength are
responsible for exciting molecules that initiate the photochemical reactions.
For the storage of solar energy by photochemical reactions, the following
conditions are required:. 1) the reaction must be endothennic (i.e., /1G > 0), 2) the
photoproducts must be stable, suitable for transportation and convenient to use,
and 3) the photoreaction must have high efficiency and high quantum yield. The

294
18 Sensitization by Metal Complexes Towards Artificial Photosynthesis
18.2 Photoinduced Hydrogen Evolution in Homogeneous Four-component Systems
295
photochemical cleavage of water into hydrogen and oxygen is suitable for storing
solar energy, because this reaction satisfies the above requirements and hydrogen
is expected to be a future fuel resource. Some representative photoinduced
hydrogen evolution systems are described in this chapter.
TEOAOX )( znTPPS.Jl4*znTPP X MV2  + 1ft' H!
H2ase
TEOA ZnTPPS+ MV + H"
18.2 Photoinduced Hydrogen Evolution in Homogeneous Four-
component Systems
Scheme 18.2 Photoinduced hydrogen evolition using water-soluble zine porphyrin and hydrogenase.
TEOA: triethanolamine; ZnTPPS: zinc tetraphynylporphyrin tetrasulfonate; MV2+:
methylviologen; H 2 ase: hydrogen.
Photoinduced hydrogen evolution systems consisting of an electron donor
(D), a photosensitizer (P), an electron carrier (C) and a catalyst (a four-component
system as shown in Scheme 18.1, have been widely studied. Porphyrin metal
complexes are suitable photo sensitizers for photoinduced hydrogen evolution.
Hydrogenase is widely used as a hydrogen evolution catalyst. In this section,
photoinduced hydrogen evolution via a four-component system containing
porphyrin metal complexes a nd hyd rogenase are introduced.
DOX )( P hv . * X C  1/2 H2
Catal st
o P+ C' H +
using laser flash photolysis. The lifetime of the photo excited triplet state of
ZnTPPS and reduced methylviologen is important for successive reactions and its
decay curve follows first-order kinetics. The lifetime of the photoexcited triplet
state of ZnTPPS is relatively long and estimated to be 0.60 ms. In contrast, the
decay curve of the photoexcited triplet state of ZnTPPS in the presence of My2+
follows second-order kinetics. It has been clarified that the photoexcited triplet
state of ZnTPPS is quenched by MVZ+. The quenching rate constant kq is estimated
to be 1.7 x 10 9 mol- 1 dm 3 S-l, whose value shows the process to be diffusion
controlled, as defined by the Stem- Y olmer plot. Quenching efficiency (1}q) of the
photo excited triplet state of ZnTPPS by My2+ is expressed as follows:
18.2.1 Photoinduced Hydrogen Evolution with Porphyrin Metal
Complexes and Hydrogenase
k [MV2+]
1}q = (ko : k q [My2+])
where ko is the inverse of the photoexcited triplet state lifetime of ZnTPPS in the
absence ofMy2+. The 1]q value is estimated to be 0.99. This result indicates that
the photoexcited triplet state of ZnTPPS is quenched by My2+ effectively. The
decay curve of reduced My2+ observed at 605 run (the absorption maximum of the
reduced ¥y2+) follows second-order kinetics. The rate constant (k b ) of back
electron transfer between reduced methylviologen and ZnTPPS cation radicals is
Scheme 18.1 Photoinduced hydrogen evolution using a four-component system. D: Electron donor,
P: Photosensitizer, C: Electron carrier.
Krasnovski et ai. observed photoinduced hydrogen evolution with
hematoporphyrin, the first report of photoinduced hydrogen evolution using
porphyrin as a photosensitizer. 32) Since this report, extensive studies of
photoinduced hydrogen evolution using porphyrin metal complexes (Table 18.1)
have been carried out. Photoinduced hydrogen evolution systems containing
triethanolamine (TEOA) as the electron donor, zinc tetraphenylporphyrin
tetrasulfonate (ZnTPPS) as the photosensitizer, methylviologen (My2+) as the
electron carrier, and hydrogenase as the catalyst (Scheme 18.2) have been
utilized. 39 . 40 ) Reduced My2+ concentration increases positively with irradiation
time (Fig. 18.1). The elementary processes of MY2+ photoreduction can be studied
0.8
-
M
'E 0.6
"C
o
E
'1 0.4
T""
Table 18.1 Typical photoinduced hydrogen evolution systems using porphyrin metal complexes
,.....,
+
>
:a: 0.2
......
Electron donor Photosensitizer Electron carrier Catalyst Ref.
NADH Hematoporphyrin My2+ H 2 ase 32
EDTA Uroporphyrin My2+ Pt colloid 33
RSH ZnTPP My2+ Pt colloid 34
RSH ZnTPP My2+ H 2 ase 34
EDTA ZnTMPyP MY2+ Pt colloid 35,36
RSH ZnTPPS My2+ H 2 ase, Pt colloid 37,38
o
o
20 40
Time I min
60
Fig. 18.1 Time dependence of reduced My2+ formation. A sample solution containing TEOA (0.25
mol dm- 3 ), ZnTPPS (0.13 ,umol dm- 3 ) and My2+ (0.22 mmol dm- 3 ) in 25 mmol dm- 3 Tris-
HCI buffer (pH = 7.4) is irradiated at 30°C.
NADH: Nicotineamide adenine dinucleotide reduced form. EDT A: Ethylenediamine tetraacetic acid.
RSH: Mercaptoethanol. ZnTPP: Zinc tetraphenylporphyrin. ZnTMPyP: Zinc tetrakis( 4-
methylpyridyl)porphyrin. ZnTPPS: Zinc tetraphenylporphyrin tetrasulfornate.

296
18 Sensitization by Metal Complexes Towards Artificial Photosynthesis
18.2 Photoinduced Hydrogen Evolution in Homogeneous Four-component Systems
21.)7
0.8
0.3
0
E
..... 0.6
I
0 0.2
't'""
- m
"0 ..-
Q) "<I'
1= 0.4 Q
0 0
>
w
c: 0.1
Q)
tn 0.2
0
'-
"0
>-
J:
0
0 0 20 40 60
0 1 2 3 Time I min
Time I h
Fig. 18.2 Time dependence of hydrogen evolution. A sample solution containing TEOA (0.25 mol
dm- 3 ), ZnTPPS (0.13 ,umol dm- 3 ), My2+ (0.22 mmol dm- 3 ) and hydrogenase (50 ,ul) in 25
mmol dm- 3 Tris-HCl buffer (pH:= 7.4) is irradiated at 30°C.
Fig. 18.3 Time dependence of reduced cytochrome C3 formation monitored at 419 nm. A sample
solution containing TEOA (0.25 mol dm- 3 ), ZnTPPS (0.13 ,umol dm- 3 ), cytochrome C3 (1.2
,umol dm- 3 ) and My2+ (0.22 mmol dm- 3 ) in 25 mmol dm- 3 Tris-HCl buffer (pH:= 7.4) is
irradiated at 30°C.
found to be 8.9 x 10 9 mol- 1 dm 3 S-l. When a sample solution containing ZnTPPS,
My2+, TEOA, and hydrogenase is irradiated, hydrogen evolution can be observed
(Fig. 18.2). In this experiment, the rate of hydrogen evolution increased with
irradiation time. The quantum yield of this system was ca. 0.03%. To improve the
hydrogen evolution yield, the following conditions are favorable: I) a higher
efficiency of charge separation, and 2) the suppression of back electron transfer.
Although systems with the above characteristics have been investigated, an
adequate system has not yet been established. Effective charge separation and
suppression of back electron transfer, and adequate photoinduced hydrogen
evolution have been accomplished using the system containing a surfactant such
as TEOA, ZnTPPS, MY2+ and hydrogenase in the presence of
cethyltrimethylammonium bromide (CTAB).39,40)
-50
o
E
co
.
40
o
't'""
-
30
>
o
>
QJ 20
c:
QJ
en
e 10
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>.
:r:
18.2.2 Photoinduced Hydrogen Evolution Using Cytochrome C3 as
Electron Carrier
o
o
30
60 90 120 150 180
Time I min
Fig. 18.4 Time dependence of hydrogen evolution. A sample solution containing TEOA (0.25 mol
dm- 3 ), ZnTPPS (0.13 ,umol dm- 3 ), My2+ (0.22 mmol dm- 3 ), cytochrome C3 (1.2 ,umol dm- 3 )
and hydrogenase (50 ,ul) in 25 mmol dm- 3 Tris-HCI buffer (pH:= 7.4) is irradiated at 30°C.
as a catalyst has been describedY} When an aqueous solution containing only
ZnTPPS, cytochrome C3 and TEOA was irradiated, no photoreduction of
cytochrome C3 was observed. In contrast, if My2+ was added to the system,
cytochrome C3 was easily photoreduced (Fig. 18.3). Irradiation resulted in a
decrease in the typical cytochrome C3 absorption band at 410 nm and an increase
in the bands at 419, 528 and 552 nm, indicating that cytochrome C3 was indeed
reduced. The chemical species with 419 nm absorption is a reduced form of
cytochrome C3. Concentration of the reduced species increased rapidly at the
As cytochrome C3 is a natural electron carrier of hydrogenase, it is expected
to be an efficient electron carrier for photoinduced hydrogen evolution (Scheme
3). Photoreduction of cytochrome C3 by the irradiation of a system containing
photosensitizer, cytochrome C3 and TEOA as an electron donor, and hydrogenase
TEOAo)( ZnTPPS hv .' znTPP X CYI c, VMV t
TEOA ZnTPPS+ Cyt c3-A Mv 2+ 1/2 H2
Scheme 18.3 Photoinduced hydrogen evolution using water-soluble zinc prophyrin, cytochrome C3
and hydrogenase. TEOA: triethanolamine; ZnTPPS: zinc tetraphynylporphyrin
tetrasulfonate; Cyt C3; My2+: methylviologen; H 2 ase: hydrogenase.

298
18 Sensitization by Metal Complexes Towards Artificial Photosynthesis
18.3 Photoinduced Hydrogen Evolution with Porphyrin Metal Complexes
299
beginning of the reaction and reached a constant value after 2 h of irradiation,
when almost all of the cytochrome C3 existed in the reduced form. The elementary
processes of cytochrome C3 photoreduction can be studied using laser flash
photolysis. The photo excited triplet state of ZnTPPS in the presence of
cytochrome C3 (1.2 .umol dm- 3 ) produces a decay curve that follows second-order
kinetics and has an estimated lifetime of 292 .us. It has been clarified that the
photoexcited triplet state of ZnTPPS is quenched by cytochrome C3' The
quenching rate constant kq is estimated to be 1.2 x 10 9 mol-I dm 3 S-I as defined by
the Stern- Y olmer plot. When hydrogenase was added to a system containing
ZnTPPS, cytochrome C3, My 2 + and TEOA and the mixture irradiated, hydrogen
evolution was observed (Fig. 18.4).
18.2.3 Photoinduced Hydrogen Evolution Using Chemically-modified
Chlorophyll
5
0
E 4

0
't'""
- 3
'0
C1>

0
> 2
w
c:
C1>
OJ
0 1
L.
'0
>.
J:
0
0 1 2 3
Time I h
Photoinduced hydrogen evolution with another system hydrogenase and
chlorophyll, has been reported. 42 ) In this reaction, NADH is the electron donor,
chlorophyll (ChI) the photosensitizer, methylviologen the electron carrier and
hydrogenase the catalyst. As the chlorophyll is decomposed by irradiation of the
system, steady hydrogen evolution is not achieved. To solve this problem,
chlorophyll (ChI) has been modified with polyethylene glycol (PEG). A
photoinduced hydrogen evolution system with ChI coupled with PEG (PEG-ChI)
(Scheme 18.4) is described. 43 ) Fig. 18.5 shows the accumulation of reduced MV2+
resulting from the irradiation of a sample solution containing 2-mercapoethanol,
PEG-ChI, and My2+. Reduced MV 2 + concentration increased with irradiation
time. When a catalyst (hydrogenase) was included in the sample solution
containing 2-mercapoethanol, PEG-ChI and My2+, and the sample irradiated,
Fig. 18.6 Time dependence of hydrogen evolution. A sample solution containing 2-mercaptoethanol
(33 mmol dm- 3 ), PEG-Chi (4.6 ,umol dm- 3 ), My2+ (0.20 mmol dm- 3 ) and hydrogenase (40
,ul) in 25 mmol dm- 3 Tris-HCl buffer (pH = 7.4) is irradiated at 30°C.
hydrogen evolution was observed as shown in Fig. 18.6.
1/2 RS-SROX )( PEG-Cht-!l4PEG-*Ch X MV 2  + 1/2 H2
H2ase
.J RSH PEG-Chi + MV'+ W
4
Scheme 18.4 Photoinduced hydrogen evolution using polythylene glycol modified chlorophyll and
hydrogenase. RSH: 2-mercaphoethanol; PEG-Chi: polyethylene glycol modified
chlorophyll; My2+: methylviologen; H 2 ase: hydrogenase.
1

18.3 Photoinduced Hydrogen Evolution with Viologen-linked
Porphyrin Metal Complexes
M
IE 3
"C
o
E
Irl 2
o
't'""
-
,.....,
+
20
40 60 80
Time I min
100
Viologen derivatives can be the substrates for hydrogenase. Photoinduced
hydrogen evolution in a four-component system comprising an electron donor,
porphyrin metal complex, an electron carrier and hydrogenase, occurs via the
photoexcited triplet state instead of the photo excited singlet state. To improve this
system some viologen-linked zinc porphyrins (S-C) have been synthesized. In
viologen-linked porphyrins, the photoexcited singlet and triplet states are easily
quenched by the bonded viologen, relative to viologen-free porphyrin. As
viologen-linked porphyrin metal complexes can act as both a photosensitizer and
an electron carrier, these compounds have been applied in photoinduced hydrogen
evolution (Scheme 18.5). In this section, studies of the photochemical and
photophysical properties of water-soluble viologen-linked porphyrin metal
complexes, and a photoinduced hydrogen evolution system containing water-
soluble viologen-linked porphyrin metal complexes and hydrogenase are
described.
o
o
Fig. 18.5 Time dependence of reduced My2+ formation. A sample solution containing 2-
mercaptoethanol (33 mmol dm- 3 ), PEG-Chi (4.6 ,umol dm- 3 ) and My2+ (0.20 mmol dm- 3 )
in 25 mmol dm- 3 Tris-HCI buffer (PH = 7.4) is irradiated at 30°C.

300
18 Sensitization by Metal Complexes Towards Artificial Photosynthesis
18.3 Photoinduced Hydrogen Evolution with Porphyrin Metal Complexes
301
DOX )( P-C  *P.C  +
Catal st
o p +.C 1/2 H2
ZnP(C l1 V)4 can be measured by laser flash photolysis. The lifetimes of
ZnP(C l1 Y)4(1.5-4.2 )1s) are shorter than that of ZnP(C l1 )4(l,300-1,400 )1s),
indicating that the photoexcited triplet state is also quenched by the bonded
viologen. These results indicate that intramolecular electron transfer between
.photoexcited porphyrin site and viologen occurs via the photoexcited singlet and
triplet states. Photoinduced hydrogen evolution with nicotineamide adenine
dinucleotide phosphate reduced form (NADPH), ZnP(C l1 Y)4 and hydrogenase
was carried out under steady state irradiation. For all ZnP(C l1 V)4, the rate of
hydrogen evolution increased with irradiation time. Photoinduced hydrogen
evolution is apparently more effective using ZnP(C 4 V)4 or ZnP(C 5 Y)4, relative to
an individual component system (Fig. 18.8). The mechanism of photoinduced
hydrogen evolution with ZnP(C l1 Y)4 and hydrogenase is hypothesized from the
results. As the porphyrin used in this experiment is cationic, the photoexcited
triplet state of porphyrin is reductively quenched by the electron donor in the
initial stage of the photreduction. Due to the Soret band when ZnP(C l1 Y)4 was
irradiated in the presence ofNADPH, the porphyrin absorption spectrum intensity
at 438 nm decreased, indicating that the reduced porphyrin may be formed by
reductive quenching of the porphyrin photo excited triplet state by NADPH. On the
contrary, bonded viologen was not reduced by NADPH. Absorbance of the
photoexcited triplet state ofZnP(C l1 V)4 depends strongly on the methylene chain
length and is correlated with the photoinduced hydrogen evolution levels. By
measuring the absorbance, it was shown that the photoexcited triplet state of
ZnP(C l1 Y)4 corresponds to the initial hydrogen evolution rate. This indicates that
hydrogen evolution proceeds via the photoexcited triplet state of ZnP(C l1 V)4. The
proposed mechanism is shown in Scheme 18.6. First, the photoexcited singlet state
of ZnP(C l1 V)4 forms following irradiation. Second, the photo excited triplet state
of ZnP(CV)4 forms by an intercrossing reaction. Third, the porphyrin moiety is
Scheme 18.5 Photoinduced hydrogen evolution using viologen-liked porphyrin. D: Electron donor,
P-C: Viologen-liked porphylin.
18.3.1 Photoinduced Hydrogen Evolution with Water-soluble Viologen-
linked Cationic Porphyrin Metal Complexes and Hydrogenase
The photoexcited triplet states of zinc cationic porphyrins such as zinc
tetrakis(N-methylpyridyl) porpnyrin (ZnTMPyP) are rather long (> 1.0 ms). Thus
the photoexcited triplet state is easily quenched by the bonded viologen in
viologen-linked zinc cationic porphyrins. Water-soluble viologen-linked zinc
cationic porphyrins are attractive for use in improving the photoinduced hydrogen
evolution using a four-component system. Water-soluble viologen-linked zinc
cationic porphyrins (ZnP(C l1 V)4; Fig. 18.7) have been synthesized and their
photochemical properties investigated. 44 ,45) While the shape of ZnP(C l1 Y)4
fluorescence spectra and those of viologen-free zinc-porphyrins (ZnP(C l1 )4) are
identical, the fluorescence intensity ofZnP(C l1 V)4 is lower than that ofZnP(C l1 )4'
The fluorescence of porphyrin is quenched by the bonded viologen due to
photoexcited intramolecular electron transfer from the photo excited singlet state
of porphyrin to the bonded viologen. The fluorescence decay of ZnP(C l1 Y)4
consists of two components with short and long lifetimes. The long-lifetime
component makes a large contribution. The lifetimes ofZnP(C l1 V)4 (0.24-0.90 ns)
are shorter than those of ZnP(C l1 )4 (1.7-1.9 ns), indicating that the photoexcited
singlet state of zinc porphyrin is quenched by the bonded viologen. Quenching of
the photoexcited singlet state by the bonded viologen can be estimated by
measuring fluorescence lifetime. The lifetime of the photoexcited triplet state of
R
1+
N
I
20
1J
QJ
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o
>
QJ
c:
QJ
 5
L..
"C
>-
:I:
+
R-
o
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<D
I 15
o
""'"
-
++
R= -(CH 2},,-N N -CH 3
o
o
1 2
Time I h
3
Viologen-linked cationic porphyrins
Zn P(Cn V)4: n=3...6
Fig. 18.8 Time dependence of hydrogen evolution. A sample solution containing (4.0 m!) ZnP(C n V)4
(2.5 ,umol dm- 3 ), NADPH (2.0 mmol dm- 3 ) and hydrogenase (0.35 unit) is irradiated at 30 v e.
+: ZnTMPyP (2.5 ,umol dm- 3 ), NADPH (2.0 mmol dm- 3 ), MV 2 + (10 ,umol dm- 3 ) and
hydrogenase (0.35 unit) is irradiated at 30°C. (0) n = 3; (e) n = 4; (.) n = 5; (D) n= 6.
Fig. 18.7 Chemical structures of water-soluble viologen-linked zinc cationic porphyrins (ZnP(C n V)4)'

302
18 Sensitization by Metal Complexes Towards Artificial Photosynthesis
18.4 Other Systems for Hydrogen Evolution
303
hv
1
1*ZnP-V C
y
"ZnP-V -H ZnP-.V
NADPH NADP
H+
ZnP -V... (n
4 1I2 H2
 Zn P-v
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4
Q)

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Q)
c:
2
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1J
>-
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. .
1 2
Time I h
3
reduced via quenching of the photoexcited triplet state by NADPH. Finally,
electron transfer from the reduced porphyrin moiety to viologen occurs and
hydrogen evolves by electron transfer from the reduced viologen to hydrogenase.
Supporting this model, fluorescence lifetime decay measurements revealed that
electron transfer from the photoexcited singlet state of porphyrin to the bonded
viologen is very rapid, suggesting that no photoinduced hydrogen evolution occurs
via the photoexcited singlet state of porphyrin. Furthermore, the correlation of the
photoexcited triplet state of ZnP(C n V)4 to the initial hydrogen evolution rate is
strong evidence that hydrogen evolution proceeds via the photo excited triplet
state of ZnP(C n Yk
0 6
E
co
I
o
o
18.3.2 Photoinduced Hydrogen Evolution with Water-soluble Viologen-
linked Anionic Porphyrin and Hydrogenase
Fig. 18.10 Time dependence ofhydtogen evolution under steady state irradiation at 30°C. The sample
solution consisting of NADPH (2.0 mmol dm- 3 ), TPPSC 6 V(2.5 JImol dm- 3 ), and
hydrogenase (0.35 unit) in 4.0 ml of25 mmol dm- 3 Tris- HCI (pH = 7.4) containing 1%
Triton X-100 (.). +:NADPH (2.0 mmol dm- 3 ), TPPS (2.5 JImol dm- 3 ), MV 2 + (2.5 ,umol
dm- 3 ), and hydrogenase (0.35 unit) in 4.0 ml of 25 mmol dm- 3 Tris- HCl (pH = 7.4)
containing 1% Triton X-100.
Scheme 18.6 Proposed mechanism of photoinduced hydrogen evolution using viologen-liked
porphyrin. ZnP-V: viologen-liked porphyrin; H 2 ase: hydrogenase.
H03S
+ ++
-(CH2)r1NN-CH3
investigated. 46 ,47) The shape of their fluorescence spectra are the same as that of
TPPS, but the fluorescence intensity of TPPSCnV is lower than that of TPPS.
Recall that the fluorescence of porphyrin can be quenched by bonded viologen.
Furthermore, the finding that the fluorescence lifetimes ofTPPSCnY (9.10-10.4
ns) are shorter than that ofTPPS (12.7 ns) indicates that the photo excited singlet
state of porphyrin is quenched by the bonded viologen. In contrast, as the lifetimes
of TPPSCnV (300-429 f.1s) and TPPS (476 f.1s) as measured by laser flash
photolysis are quite similar, it appears that the photoexcited triplet state is not
quenched. These results indicate that for water-soluble viologen-linked anionic
porphyrin intramolecular electron transfer between the photoexcited porphyrin site
to viologen occurs via the photoexcited singlet state. Photoinduced hydrogen
evolution with nicotinamide adenine (NADPH), TPPSCnY and hydrogenase was
carried out under steady state irradiation. As shown in Fig. 18.10, TPPSC 6 Y
effectively enables photoinduced hydrogen evolution, whereas an individual
component system dose not. In the TPPSC 6 V-containing system, the rate of
hydrogen evolution increased with irradiation time.
The reductive quenching reaction and degradation of zinc porphyrin occurred
by using water-soluble viologen-linked zinc cationic porphyrins. On the other
hand, the oxidative quenching reaction and no degradation of porphyrin occurred
by using anionic porphyrins. Thus, water-soluble viologen-linked anionic
porphyrins are attractive to improve the photoinduced hydrogen evolution system.
Water-soluble viologen-linked anionic porphyrins with sulfo-groups (TPPSCnV;
Fig. 18.9) have been synthesized and their photochemical properties
18.4 Other Systems for Hydrogen Evolution Using Natural
Photosensitizers
S03H
Viologen-linked anionic porphyrins
TPPSCnV: n=3-6
Green plants split water under irradiation by sunlight. The reaction pathway
of the green plant photo system is shown in Fig. 18.11. With the aid of light
energy the plants first extract electrons by oxidizing water. The electrons are
then transported into the Calvin cycle through ferredoxin. The arrows in the
Fig. 18.9 Chemical structures of water-soluble viologen-linked anionic porphyrins (TPPSC n V).

304
18 SensItizatIOn by Metal Complexes lowarus ArtI1ICIa1 l'hotosynthesis
IO." VlIlt:r oC)YSlt:lll:; IUl UYUIugt:ll .r::,VUIULlUll
JUJ
02
H'O-.-4 PS II e
hv
H+
.............. Hydro gen ase '.."H'
: eO
10
o
roE 8
I
e
 To Calvin cycle
0
T""
- 6
"'C
QJ
>
0
> 4
QJ
c:
QJ
en
0
L.. 2
"'C
>-
:r:
e
. Ferredoxin
. PS I
hv
Fig. 18.11 Diagram of electron flows in photosystem in green plant.
scheme show the direction of the electron flow. Following the addition of
hydrogenase, electrons are extracted from ferredoxin and shunted from the Calvin
cycle in the direction shown by the dotted arrow. Hydrogen subsequently evolves
through the reduction of protons. According to the method, photoinduced
hydrogen evolution in chloroplast-ferredoxin-hydrogenase systems has been tried.
Hydrogen evolution has been observed using a chloroplast-ferredoxin-
hydrogenase system. 48 ) In this system, however, the hydrogen evolution rate
decreases with irradiation time, making it difficult to obtain hydrogen on a
continuous basis. The cause of the decrease is simultaneous oxygen evolution,
which strongly inhibits hydrogenase activity. Therefore, to obtain hydrogen over
a long time period, oxygen removal is necessary. One solution is to consume
oxygen by adding glucose and glucose oxidase. Similarly, to avod enzyme
deactivation by oxygen, Greenbaum et al. used a flow system compressed carrier
gas (either helium or nitrogen) to continually purge the reaction cuvettes and
electrolysis cells of oxygen. 49 ) This system enabled measurement of the
simultaneous hydrogen and oxygen photoproduction in marine green algae.
Chlamydomonas species (clone f-9), for example, has a steady state rate of
hydrogen and oxygen production during irradiation with a stoichiometric ratio
close to 2: 1. Another method to present oxygen-induced deactivation of
hydrogenase is to develop a hydrogenase which is unaffected by oxygen. Along
this line, Kamen et al. found that when fixed on glass beads, hydrogenase from
Clostridium pasteurianum is more stable against oxygen. 50)
Hydrogen and oxygen evolution in a system containing grana, FNR and
hydrognease (Scheme 18.7) has been investigated. 51) Sample solutions containing
grana, NADPH, NAD and hydrogenase can indeed support hydrogen evolution
upon irradiation (Fig. 18.12). The rate of hydrogen evolution increases with
irradiation time for 2 h. The amount of hydrogen evolved during 2 h is estimated
to be 1.0 x 10- 7 mol. Furthermore, in this system, oxygen evolution also occurs.
The amount of oxygen evolved increases with irradiation time to a maximum
quantity (8.0 x 10- 8 mol) in 30 min, and then decreases to absence (Fig. 18.13).
o
o
0.5
1.0 1.5
Time I h
2.0
Fig. 18.12 Time dependence of hydrogen evolution by the sample solution (6.0 ml) containing grana
(1.0 ml), NADPH (0.39 mmol dm- 3 ), NAD (0.39 mmol dm- 3 ) and hydrogenase (56 units)
10 9
0
E 8 0
ro E
I 
0 6
T"" 0
- 6 T""
"'C -
QJ "'C
> QJ
0 
> 4 0
QJ >
c: QJ
QJ 3 c:
en QJ
0 C1
L.. 2 >.
"'C ><
>- 0
:r:
0 0
0 0.5 1.0 1.5 2.0 2.5
Time I h
hv
1/2 02  NADPH  ADH  1/2 H2
rana FNR H2aSe
H20 NADP NAD H+
Fig. 18.13 Time dependence of hydrogen (e) and oxygen (.) evolution by the sample solution (6.0
ml) containing grana (1.0 ml), NADPH (0.39 mmol dm- 3 ), NAD (0.39 mmol dm- 3 ) and
hydrogenase (56 units).
Scheme 18.7 Photoinduced hydrogen and oxygen evolution with the system containing grana, FNR
and hydrogenase.
This result suggests that oxygen is utilized by molecules such as NADPH-
dehydrogenase, or consumed in processes such as photorespiration. It is
noteworthy that the maximum value of hydrogen evolution is 10 times larger
than that of oxygen evolution.

.JVV
J 0 .:>t::I1SIUzanon DY Metal Complexes Towards Artificial Photosynthesis
18.5 Conclusion
In this chapter, we described some artificial photosynthesis systems that
employ porphyrin metal complexes to photoinductively evolve hydrogen.
Although further development is required, the photoinduced hydrogen evolution
systems described in this chapter will likely be useful in the conversion of solar
energy to chemical energy.
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19
Catalyses and Sensitization for Water Reaction
Towards Future Artificial Photosynthesis
19.1 Introduction
Photocatalysi s 1-5) is attracting a great deal of attention as a tool for creating
new energy resources from solar energy to solve global environmental problems
such as the greenhouse effect by carbon dioxide, acid rain, and other pollutants.
Photocatalysis is catalysis induced by light irradiation. Semiconductors such as
TiO z are well-known typical materials for achieving photocatalysis. In a wider
sense, sensitizers are another candidate for photocatalysis, which should be used
in combination with other catalyst(s).
Intensive investigations have been carried out in order to construct an
artificial photosynthetic system to realize solar energy conversion such as water
cleavage by visible light utilizing a photocatalyst to produce fuels by sunshine. 2 - 5 )
However, a total system of artificial photosynthesis has not been achieved yet. The
first report of water cleavage by UV light with Ti0 2 photo anode and platinum
counter electrode 6 ) has evoked much attention to sensitize such large bandgap
semiconductors by coating with a dye sensitizer. The successful sensitization of
the TiO z microparticles layer by coating with a dye sensitizer has led to what is
called a sensitized photoregenerative solar cell with 10% light-to-electricity
conversion efficiency,7) which is attracting much attention for practical use as a
high cost-performance device.
In spite of these remarkable developments in photo energy conversion
research, it is still difficult to establish an artificial photosynthetic system to
create an energy source from solar energy by utilizing photocatalysis. In the
present chapter, the efforts of the present author's group towards realizing an
artificial photosynthetic system are mainly reviewed with a short introduction of
works by others. Our strategy is to develop each unit such as reactions at a
photoexcitation center, dark catalyses and charge transport and later to combine
these units to achieve a complete system. in the future. Important results obtained
to date by our group, including trials of sensitizing TiO z particles and photoanode
in a water phase, are reviewed.
19.2 Design of Artificial Photosynthesis
19.2.1 Photosynthesis and Energy Cycle on Earth
It is important to understand the earth's energy cycle in order to design future

310
19 Catalyses and Sensitization for Water Reaction
19.2 Design of Artificial Photosynthesis
311
CO 2 + 2H z O + 8 photons -) (C6H\z06)1I6 + Oz + H 2 0
(19.1)
electrons originating from water and driven by solar photon energy as illustrated
in Fig. 19.1,8) which is supported by photosynthesis.
Earth and biology are in a non-equilibrium state, but maintains a steady state,
for which it is an important condition that the system is not closed but open to the
outside. Only under open conditions can such non-equilibrium but steady state be
maintained. If earth were closed, all biological life would end, but the Earth is
fortunately open to the universe. Based on this as well as on the first and the
second laws of thermodynamics, it was a natural consequence that we humans
confronted the problem of environment and energy resource only as .far as we
conduct energy cycles in the closed earth. It is important for our future energy
resources that the earth be open to the ut;liverse, i.e., to the sun.
energy resources compatible with the global environment. Almost all biological
energy sources including the main energy source that supports our social life are
provided directly and indirectly by photosynthesis. Photosynthesis can be
understood as a photochemical reaction of carbon dioxide with water using solar
visible light energy to produce carbohydrate as the main product (Eq. (19.1».
This reaction can be simplified by the following three major processes (Eqs.
(19.2) to (19.4», where water is an electron donor which provides electrons to the
whole system obtained at the Mn protein complex (Eq. (19.2», the electrons
from water are excited by solar photons at the chlorophyl reaction center (two
steps) to form high energy electron (e-*)(Eq. (19.3», and the e-* reduces CO z to
carbohydrate (Eq. (19.4) via reduction ofNADP to NADPH.
19.2.2 Artificial Photosynthesis
4e-* + 4H+ + CO z -) (C 6 H 12 0 6 )1/6 + HzO
(19.4)
Based on photosynthesis and the energy cycle on the earth, an artificial
photosynthetic system was proposed (Fig. 19.2) to create fuels from' solar energy
and water by utilizing photocatalytic reactions. 9 ) This scheme takes minimum
requirements to achieve photochemical energy conversion.
Only one photoexcitation center (P) is designed for the first scheme. Dark
catalyses of water oxidation (C z ) and proton (or CO z ) reduction (C 1 ) are desiged,
which are coupled electrically with the photoexcitation center via mediators (M z
and M 1 ). In order to decompose water by this system, the energy gap between the
ground and excited states ofP should be less than 3 eV (wavelength larger than
400 nm), the redox potential of C 2 should be more positive than 0.81 V (vs. SHE,
at pH7), and that of C 1 more negative than -0.41 V (vs. SHE, pH7). Other than
these energy level requirements, it is important to suppress back electron transfer
by (l) kinetic design of the electron transfer reactions and (2) use of a
heterogeneous phase to achieve uni-directional electron transfer.
As for the photoexcitation center, dye sensitizers, small bandgap
semiconductors or dye-sensitized large bandgap semiconductors are candidates.
Our approach is to establish each unit then combine the units by utilizing
molecular aggregates. Our progress towards this goal is described in Sections
19.3 (catalysts), 19.4 (photoexcited state electron transfer in a heterogeneous
phase), and 19.5 (sensitization of TiO z in water).
2H z O -) 4e- + 4H"'" +0 2
(19.2)
4e- + 8 photons -) 4e-*
(19.3)
The photosynthetic product is assumed to store high energy electrons. We ingest
the product as food and the high energy electrons in the food are returned back to
Oz (produced by the photosynthesis) by respiration, thereby liberating free energy
for biological activities to reproduce water and CO 2 , Combustion of fossil fuels
(oxidation by O 2 ) liberates energy and reproduces water and CO 2 in the same
way. Thus the energy cycle on Earth can be represented by the circulation of
4e-
4 e-
4 e-
Respiration
Combustion
(-)
O 2
.....
oj
....
..,
c
Q)
..,
o
P.
"CJ
Q)
CI:
(+)
e
Oxygen-Evolving Center
Mn-Protein complex
2H20 - 0'2 + 4H+ + 4e-
Water Oxidation
h}J
RZ
Reactant
C 2
Catalyst
M Z P
Mediator Photo-
reaction
center
(R 2 )red' + (RI)ox
M l
Mediator
C I
Catalyst
RI
Reactant
Net reaction:
hi)
(R 2 )OX + (RI)red
Fig. 19.1 Energy cycle on the earth represented by electron flow driven by solar energy where
electrons are provided by water. This energy cycle is supported by photosynthesis
represented by Eq.(l9.1).
Fig. 19.2 Proposed artifici.al photosynthetic system. 9 )

312 19 Catalyses and SensitIzatIOn tor Water ReactIOn
[(bpy )zMn(,u-0)2Mn(bpy )z]3+
[(bpy )z(HzO ) Ru(,u-O )Ru(HzO )(bpy )z]4+
1
2
J..J.oJ .lY.l.Vl\';UJ'U. va...u.....1 ... .LV... 1'1' C.U,\.IJ. .I.'\.'-'UV""'V.I..I.U' u........ --- L ..."'....."'"................"'......
Table 19.1 Calculated values of k, rd and reo for various ammine ruthenium complexes as a water
oxidation catalyst incorporated into Nafion membrane
Catalyst Catalysis by k(1O- 2 s- 1 ) rd (nm) reo (nm) Molecular
one molecule size (nm)
..uL
Ru-red 4e- 4.4 1.23 (H) }0.75
lR
[(NII 3 )sFluOFlu(NII 3 )s]s+ 4e- 1.1 1.02 (]3) 1 ° 75
1A
[[(bpyMH 2 0)Ru hO]4+ 4e- 0.21 1.07 (3) 1 0.826
[Ru(NH 3 )6P+ 2e- 0.09 0.78 1.20 e t 0.75
[Ru(NH 3 )sClf+ 2e- 0.27 0.85 1.26 e t 0.75
cis-[Flu(NH 3 )4 CI 2r 4e- 2.1 0.81 e t 0.78
cis-[Ru(NH 3 MH 2 O)2r 4e- 2.7 0.81 @ 1 0.78
trans-[Ru(NH 3 )4 CI 2]+ 2e- 9.0 0.93 1.66 e 1 0.78
19.3 Molecular Catalysts for Water Reactions and CO 2
Reduction
19.3.1 Catalysis in Water Oxidation
Although catalytic water oxidation (dark reaction) is the first and important
reaction of the electron flow in the photosynthesis represented by Fig. 19. I
whereby water is used as the source of electrons provided to the whole system, its
catalyst and reaction mechanism are not yet established. 10 - 13 ) In the photosynthesis
Mn-protein complex works as a catalyst for the difficult four-electron oxidation
of two molecules of water to liberate one Oz molecule (Eq. (19.2)). It is inferred
that at least four Mn ions are involved in the active center, but its structure is not
yet completely elucidated.
Molecular catalyst for water oxidation has been attracting a great deal of
attention not only as a model for the photosynthetic catalyst but also as a
component in an artificial photosynthesis. Many structural models have been
synthesized and investigated as the photosynthetic Mn complex. Tetrakis(2,2'-
bipyridine)( di ,u -oxo )di-Mn complex 111) and tetrakis(2,2' -bipyridine)( ,u -oxo )di-
Ru complex (2, Meyer's complex)14) are typical examples, but many of them
failed to show high activity for water oxidation.
The present authors have been studying water oxidation catalysis by both
chemical (Scheme 19.1, using Ce(IY) oxidant) and electrochemical (Scheme
19.2, using polymer-coated electrode) methods, and established that trinuclear,
dinuclear and mononuclear ammine ligand-based Ru complexes show high
activity as catalysts for water oxidation.
4celV X ComplexO+ O 2 + 4H+
4Ce'" comPleX(n+4)X 2H 2 0
Scheme 19.1 Chemical water oxidation catalysis.
presence of a strong oxidant such as Ce(IY) ion. 15 - 18 )
[(1I3)5Rll-()-Rll(lf3)4-()-Rll(1I3)5]6+
3 (Ru-red)
4e-
Complex{n+4)+
2H 2 0
It has been established that other ammine ligand-based Ru complexes including
mononuclear and dinuclear ones are all active catalysts for water oxidation (vide
infra) in both homogeneous aqueous solution and in heterogeneous phase such as
a polymer membrane and clay. 18)
Important findings on these catalysts are as follows (see also Table 19.1).
1) Trinuclear (Ru-red) and dinuclear complexes are capable of 4-electron
oxidation of water by one molecule. Mononuclear complexes are capable of
either 4-electron or 2-electron oxidation of water, two catalyst molecules being
required for the latter.
2) Bimolecular decomposition of the catalyst takes place at its high oxidation state,
-l:(CF 2 - CF)-(CF 2 -CF2)
[ . ? ] I n
CF 2
t FCFa m
o
I
CF 2 CF 2 SO a -H+
Complex n +
O 2 + 4H+
Electrode
Water oxidation
catalyst layer
Bulk electrolyte
aqueous solution
Scheme 19.2 Electrochemical water oxidation catalysis.
As for the catalyst we have found that the trinuclear Ru-ammine complex
called Ru-red (3) is an active catalyst, and it evolved dioxygen bubbles in the
4 Nafion

314
19 Catalyses and Sensitization for Water Reaction
19.3 Molecular Catalysts for Water Reactions and CO 2 Reduction
315
forming N z by oxidation of the ammine ligands in competition with water
oxidation. Such bimolecular decomposition is prohibited remarkably by
isolating the catalysts from each other in a matrix such as anionic polymer
matrix (e.g., Nafion, 4). The catalytic activity itself remains similar in both
heterogeneous phase as in homogenous aqueous solution.
3) Assuming a random distribution of the catalyst in a matrix, the bimolecular
decomposition distance was estimated to be almost contact distance between
molecules.
4) When two catalyst molecules are needed for the 4-electron water oxidation, the
catalytic activity shows an optimum catalyst concentration in the matrix since
bimolecular decomposition still takes place, nd cooperative distance between
catalyst can be determined also by assuming a random distribution.
5) For the electrocatalytic water oxidation using polyanion(Nafion)-coated
electrode dispersing catalysts in the polymer membrane, charge transport from
the electrode to the catalyst taking place by charge hopping is important. This
charge transport by hopping is facilitated by a high concentration of the catalyst
in the matrix, which is a contradictory requirement for the bimolecular
decomposition, so that optimum and delicate concentration conditions exist for
the electrocatalytic system. The charge hopping distance (ro) and bimolecular
decomposition distance (r d) can also be determined by assuming a random
distribution.
6) Amino acid residue models such as a tyrosine residue model (p-cresol) lengthen
remarkably the charge hopping distance, a phenomenon which can solve the
problem in the electrocatalysis mentioned in the above item 5) and enhance
remarkably the catalytic activity.
Details on some of the above items are given below.
When Ru-red was used as a catalyst in the presence of a large excess of
Ce(IY) oxidant (Scheme 19.1), the rate of Oz evolution was first order with
respect to the catalyst concentration, showing that Ru-red is capable of 4-electron
oxidation of water. By the decomposition of the Ru-red, N z was formed; its
formation rate was second order with respect to the catalyst concentration,
showing that the decomposition is bimolecular. The decomposition distance in a
polymer (Nafion) matrix was estimated by assuming the random distribution of the
catalyst molecule in the matrix. The probability density Per) of the distance
between the nearest neighbor molecules (r nm) is represented by Eq. (19.5)
according to the Poisson statistics
k is the intrinsic rate constant of Oz formation.
- L - 47r (r 1 - s3)N A a c x lO- z4 J
k app - k exp
3
By applying Eq. (19.6) to the data of k app versus c, rd was determined as shown in
Table 19.1. 18 ,19) The results of this table are summarized as follows:
a) Trinuclear and dinuclear Ru complexes are capable of 4-electron oxidation of
water, but for mononuclear complexes only the cis-tetraammine Ru complexes
are capable of 4-electron oxidation, and other mononuclear ones are 2-electron
catalyst by one molecule. For the 2-electron catalysts, cooperative distance (rco)
for the bimolecular water oxidation was also estimated by a procedure similar
to that used for determining rd'
b) Ammine complex shows much higher activity than bipyridine complex.
c) The bimolecular decomposition distance is nearly the contact distance between
the molecules.
d) Although it is not shown in the table, the activity of the molecular complexes
is much higher than that of conventional noble metal oxide catalysts such as
PtO z or RuOz by one to two orders of magnitude based on one molecule and
one repeating unit.
Regarding item 6) above on electrocatalysis, the coexistence of a tyrosine
residue model, p-cresol (p-Cre), enhanced remarkably the catalytic activity of
Ru-red confined in a Nafion membrane coated on an electrode (Fig. 19.3).20) This
was attributed to the nearly twofold lengthening of the charge hopping distance
by p-cresol (from 1.28 nm to 2.25nm).
(19.6)
40 .
b
//.,......._ , ><;--_._-
./'" '0,
.t" ': · " ".
,.' "'"
/ 
!
i .
/ /f
i !
: I
!I t. C
; l 0
i! ,Q...-l!.h... CL
f! ..c.::?-'->".1)---.l1 hir .\.......
If .,'" 0 0 \.Q).....
if "i) a
!.
(1)
(II)
[ -47r(r 3 - s3)N A a c x IO- Z4 ]
Per) = 4nr Z N Aa c x IO- Z4 exp 3
....
I
..c::
....... 30

Q)

::I
I:: 20

Q)

I::
 .10

(19.5)
where N A is Avogadro's number (mol-I), c is the concentration of the catalyst
molecule in the matrix (mol dm- 3 = M), a is the localization factor of the molecule
estimated taking into consideration the hydrophilic region fraction in the matrix,
and s (nm) is the contact distance of the catalyst molecules. When rd (nm) is the
decomposition distance (center-to-center) of the catalyst molecules within which
they undergo decomposition, the fraction of decomposing catalyst (R dec ) is
calculated by integrating the P(r) from s to rd. The apparent rate of Oz formation
(k app ) should be proportional to l-R dec , so that k app is expressed by Eq. (19.6) when
o
0.00 0.05 0.10 0.15 0.20
Ru-red concentration I M
Fig. 19.3
Relationship between turnover number (TN) of Ru-red confined in a coaed Nafion
membrane for electro catalytic O 2 evolution and the catalyst concentration in the absence of
amino acid model compound (a), in the presence of 5.0 x 1O- 2 Mp-Cre (b), and with toluene
(c). The solid line and dashed line are calculated curves. The dash-dotted curve (1) is a
simulated curve in the presence ofp-Cre when ro and rd are the same as those In the absence
of p-Cre, and curve (II) is a simulated curve in the presence of p-Cre when ro and k02 are
the same as those in the absence of p-Cre.
(From M. Yagi, K. Kinoshita and M. Kaneko, J. Phys. Chem. B, 101,3958 (1997»

316
19 Catalyses and Sensitization for Water Reaction
19.4 Photo excited State Electron Transfer in Heterogeneous Phases
317
19.3.2 Catalysis in Proton Reduction
..
+e'
Proton reduction is an important catalysis in water photolysis. Pt and Pt0 2
have been the best known catalysts for process. However, these colloidal or
powder catalysts are not well suited for the construction of a conversion system
based on molecules, and, moreover, incorporation of these strongly colored
materials into photochemical conversion systems should be avoided because of
their possible filter effect. From this point of view it is desirable to use a molecular
catalyst if a highly active one is available.
It was reported that cobalt-tetraphenylporphyrin complex (CoTPP) coated
on an electrode catalyzes electrocatalytic proton reduction/I) but the activity was
not very high. We have found that metal porphyrins and metal phtahlocyanines
when incorporated into a polymer membrane coated on an electrode show high
activity in electro catalytic proton reduction to produce H Z .z Z ,Z3) Some data are
summarized in Table 19.2. It was shown that this catalyst is more active than a
conventional platinum base electrode.
The concentration of the catalyst in the matrix is especially important for this
catalysis. When the catalyst concentration exceeded ca. 0.05 mol dm- 3 in a matrix,
the activity decreased drastically_ Although this result can not yet be interpreted
definitively, it suggests that since the catalysis should be a bimolecular process,
the low concentration of the complex catalyst is important so the two molecules
of a H+-coordinated intermediate can find the best location for a cooperative Hz
formation.
co
I path Ii
('\
H 2 0 H'
19.3.3 Catalysis in Carbon Dioxide Reduction
Fig. 19.4 The mechanism for electrochemical CO 2 reduction catalyzed by CoPc confined in a poly(4-
vinylpyridine) membrane. Possible paths (I and II) are shown. An equilibrium with respect
[0 protons between the polymer matrix and the bulk phase exists.
(From T. Abe, T. Yoshida, S. Tokita, F. Taguchi, H. Imaya and M. Kaneko, J. Electroanal.
Chern., 412, 130 (1996»
Although water photolysis is the simplest photochemical conversion system,
carbon dioxide reduction is still an attractive research subject as a synthetic model
for CO z reduction in photosynthesis. There are numerous reports on chemical
and photochemical CO 2 reduction,Z4) but it is not the aim of this chapter to review
these works.
It was found that when metal phthalocyanine (MPc) is incorporated into a
poly(vinylpyridine) membrane, it works as a very active catalyst for
electrocatalytic CO z reduction to produce carbon monoxide. z5 ) By investigating
visible spectral change of CoPc in a poly(vinylpyridine) membrane coated on a
graphite electrode by an in situ potential step chronospectrometry (PSCS) during
CO z reduction., it has been shown that, after CO z coordination to the two-electron
reduced CoPc, one more electron is injected from the electrode to form CO,
recovering one-electron reduced CoPc (Path II of Fig. 19.4 z5 », different from the
believed reaction scheme (Path I).
The mechanism was different depending on the electron-donating or electron-
accepting substituents on the CoPc ring. Details are found in the literature. 26)
19.4 Photoexcited State Electron Transfer in Heterogeneous
Phases
a bare Pt
PtlN f
Pt/FeTppa)
PtlNf[FeTpp]a)
Pt/MnTppa)
PtlNf[MnTpp]a)
2.15
2.83
2.55
2.68
1.30
4.12
14.4
34.6
5.31
208
17.9
68.1
Electron transfer of sensitizers in the photoexcited state is important for
constructing photochemical conversion systems since energy conversion such as
that shown in the scheme of Fig. 19.2 can be realized only by heterogeneous
phase(s). Heterogeneous electron transfer is also important in the development of
a dye-sensitized solar cell that is attracting the attention of many researcqers
now. This is discussed in the next section.
Photoinduced electron relay in a polymer solid phase was found to take place
for a ethylenediaminetetraacetic acid (EDT A)-tris(2,2' -bipyridine) ruthenium
(I1)(Ru(bpy)/+) - methylviologen (MV2+) system 27 ) utilizing cellulose paper.
Electron transfer from EDT A to the oxidized Ru(bpY)33+ formed after electron
transfer from its excited state to MYz+ accumulated blue MY+ in the cellulose
paper.
In a solid phase, since the reaction components can not move or move only
Table 19.2 Results of potentiostatic electrolysis (1 h) using a modified Pt electrode in a pH 1.0
aqueous solution
System
H 2 produced/pI
PotentialfV "s. Ag/ AgCI
-0.25 -0.30
a) The coated amount of MTPP was 1.0 x 10- 8 mol cm- 2

318
19 Catalyses and Sensitization for Water Reaction
19.4 Photoexcited State Electron Transfer in Heterogeneous Phases
319
slightly during a photo excited state, the electron transfer shows a specific aspect
entirely different from that in a homogenous solution where the reaction takes
place commonly by a dynamic mechanism. In a solid phase electron transfer
takes place usually by a static mechanism in which the reaction components can
not move. Depending on the mobility of the components the mechanism can be
partly dynamic. The mechanism can be determined by investigating emission
intensity and emission decay based on quenching of the emission by electron
transfer. When the mechanism is dynamic (Fig. 19.5(a», the reaction is expressed
by the Stern-Volmer equation (Eq. (19.7»,28)
Quenching model
Table 19.3 Possible quenching mechanisms and kinetic parameters
Ref.c)
Rate constant
first-order second-order
Appearance Eq.b)
of Stern-Volmer
Plots a )
I) I-site models
1-1) Dynamic mechanism
model 1 d)
k ql = 0
1-2) Quenching involving static mechanism
1-2-1) I-step equilibrium models
model 2 k ql
model 3 k q1 e )
model 4 k ql
model 51) k ql e)
.b. = To = 1+ ksv[Q]
I l'
(19.7)
where 10 is the emission intensity in the absence of quencher (Q, here electron
acceptor or donor), I is that in the presence of Q (concentration [Q] mol dm- 3 ), l'
is the corresponding lifetime of the excited state determined by its emission
decay, and ksv is the Stern-Volmer constant.
For a static mechanism the TjT plots do not show a slope (the lifetime is
independent on the Q concentration) (see Fig. 19.5(b», and very typically the 1j1
plots follow the Perrin mechanism (Eq.19.8»),29)
1-2-2) multi-step equilibrium models
model 6 !cql
model 7 k q1 e )
model 8 k ql
model 9 g ) k q1 e )
model 10 ik q . h )
model 11 ik q . h )
2) 2-site models
1
 = exp( K 1 [Q))
I
(19.8)
model 12
model 13
(site 1)
k q21
k q21
where Kl = VN A (V is the quenching sphere volume).
In this mechanism the probe (sensitizer) containing Q in the quenching sphere
around the probe is quenched entirely, but the probe containing no Q is not
quenched at all (see Fig. 19.5 (b). When the process involves both dynamic and
static mechanisms, the 10/1 and 1'0/1' plots are depicted as shown in Fig. 19.5 (c).
These are only typical cases, and actually the electron transfer mechanism in
a solid state is often more complicated. 30 ) Possible electron transfer mechanisms
are summarized in Table 19.3 31 ) based on the following points:
a) The number of different nature sites; 1 or 2 sites.
b) Dynamic or static mechanism as well as their combination.
k q2
k q2
k q2
k q2 =0
k q2 =0
k q2
k q2
k q2 =0
k q2 =0
k q2
k q2 =0
(site 2)
k q22
k q22 = 0
See the rer l )
S
C 11
U 12
D 13
S 14
C 21
U 22
C 23
U 24
C 27
U 28
D
D
29
30
a) S, straight line; U, upward deviating curve; D: downward deviating curve; C: complicated curve,
sometimes showing downward deviation.
b) The corresponding equation shown in the text of the Ref. 31.
CJ The corresponding quenching system to which the mecanism is applicable.
d) Conventional dynamic quenching model.
e) The probe containing a quencher in its quenching sphere does not emit, i.e., k ql » Iho.
I) Conventional static quenching model.
g) The emission intensity follows the so-called Perrin equation.
h) i is the numb;r of quencher molecules in the quenching sphere of the probe.
c) The incorporation equilibrium of Q into the quenching sphere around a probe;
one-step equilibrium or multi-step one.
d) The rate of static quenching; very fast so that emission is negligible, or the
emission should be considered since th emission is comptitive with the
quenching.
e) For multi-step equilibrium between Q and probe; the static quenching rate is
proportional to the number of Q in the quenching sphere, or independent of the
number of Q.
Please refer to the literature 3l ) for the details and corresponding equatiof.ls.
For a complete static quenching following the Perrin mechanism, Eq. (19.9)
is derived, where ro (nm) is the electron transfer distance between donor and
acceptor molecules (molecular center to center).
; = ex p [ 4"'(r - 8' N A 10- 24 [Q] ]
(a)
(b)
I /1, 'r /7:
o 0
(e)
I /1
o
-
1: /T
o
7: 0 /7:
I /1
o
I
or
'1:'/'1:
o
Queneher concentration
Fig. 19.5 Typical plots (relative emission intensity 10/1 and relative emission lifetime '[017:) for the
quenching of photo excited state against quencher concentration. (a) Dynamic mechanism,
(b) Static mechanism, and ( c) Combined mechanism.
(19.9)

320
19 Catalyses and Sensitization for Water Reaction
19.5 Sensitization of Ti0 2 Powders and Films in Water
321
o
conversion efficiency 7.1 % under AM 2 irradiation conditions (l00 m W cm- Z ). 7)
This evoked great attention to produce high cost-performance solar cells with
efficiency of about 10%, comparable to the efficiency of amorphous silicone
photovoltaic cells. A typical dye used is Ru(dcbpy)z(SCN)2' The primary reaction
of this cell is electron injection from the photoexcited state of the adsorbed dye
into the conduction band of Ti0 2 , which takes place very rapidly in a femto
second order.
This so-called photoregenerative solar cell can work efficiently only by using
organic medium, and the efficiency is very low in water medium. We have tried
to investigate dye sensitization of Ti0 2 powder suspension and powder films in.
water medium by various metal complex dyes in order to utilize it for future
artificial photosynthesis. Some results are described in this section.
First a fundamental photochemical catalysis by TiO z powder suspension in
water should be mentioned as a trial to cleave water by UV light. Platinized TiO z
powders (Pt/TiO z ) 'suspended in water have been expected to photolyze water to
produce Hz and O 2 by UV light, and some reports appeared as described in other
chapters of this book. However, established true water photolysis by UV light
takes place only under special conditions, e.g., with high concentration carbonates
anions in water, or with the addition of NaOH in a gas phase.
We have reported that platinized TiO z (TiOz/Pt) suspended in water produced
only H 2 under irradiation 35 ) without giving any detectable oxidized product. After
complete deactivation of the catalyst in a long-term photoreaction, the recovered
TiOz/Pt did not photoproduce Hz even with the addition of alcohol (electron
donor), showing that the TiO z underwent some structural change other than that
caused by the evolution of H 2 due to the presence of some reductive impurities.
As for the change in the catalyst, oxidation of the Ti 4 + to Ti 5 + was suggested from
XPS data giving an average structure of (Ti z 0 5 )(TiO z )z.
Sensitization of TiOz/Pt powders in water by dissolved sensitizer was carried
out under visible light irradiation in the presence of sacrificial electron donor
EDTA to produce Hz. It was found that tris(bipyrimidine)Ru(II) (5) is an efficient
sensitizer for visible light Hz formation.
a
2.5
2.0
0.05
0.10
(MyZ')
0.15
0.20
Fig. 19.6 Plots of In [relative emission yield] vs. My2+ concentration for t.he electron transfer
quenching of photoexcited Ru(bpy)/+ by My2+ in a polysiloxane film 1ll the presence of 0.2
MIND (D) and 0.05 MIND (0) and without IND (0).
(From K. Nagai, J. Tsukamoto, N. Takamiya and M. Kaneko, J. Phys. Chern., 99, 6650
(1995»
The electron transfer from the photoexcited Ru(bpy)l+ to MVz+ confined in
a polysiloxane film showed a complete static quenching following the Perrin
model,3Z) and the electron transfer distance ro was 1.4 nm, which is comparable to
a conventional electron transfer distance in biological systems. The presence of
a tryptophan residue model, 3-methyindole (IND) enhanced much the quenching
efficiency (Fig. 19.6) by lengthening the electron transfer distance, and the
electron transfer distance was estimated to be 2.7 nm, almost twice that without
the mediator. 3Z)
The same electron transfer was investigated in a polyethyleneoxide film
known to be a polymer electrolyte capable of transporting ions. The electron
transfer mechanism was analyzed to take place by both static and dynamic
mechanisms 33) the electron transfer distance was estimated to be 1.7 nm, and the
, .
dynamic rate constant was 4.6 x 10 6 M- 1 s- 1 , which is two orders of magllltude
lower tha that in an aqueous solution.
19.5 Sensitization of Ti0 2 Powders and Films in Water
N:J
C N rJ
h'N. :
....: '/ X N
.Ru.,
C1£)"U
2+
After the first report on UV light water photolysis with n- TiO z photoanode,6)
sensitization of this large bandgap (Eg = 3 eV) semiconductor to utilize visible
light has been an important research subject and dy-senitiation of Tiz
photoanode was tried in the 1980s. AdsorptIOn of tns( 4,4 -dicarboxy-2,2 -
bipyridine)ruthenium(I1) (Ru(dcbpy)l+) onto a TiO z photoaode generated a
photo current by a monochromatic 460 nm visible light (intensIy 0.22  W cm- Z )
with short circuit photo current (Jsc) 36 J.1Acm- z and converSIOn efficIency of
44%.34) In relevant dye-sensitized systems, the photo current has been on the order
of tens of J.1 Acm- 2 .
Dye sensitization of a nanometer-sized Ti0 2 powder film soaked in an organic
medium containing iodine/iodide redox electrolytes successfully generated open
circuit photovoltage (Voc) 0.68 V, Jsc 11.2 mAcm- z , Fill factor (FF) 0.68, and
5 Ru(bpym)/+
A high concentration of dissolved Ru(bpy)l+ was required for the
sensitization, suggesting adsorption of the sensitizer onto the Ti0 2 . tetrakis( 4-
carboxyphenyl)porphine and tetrakis( 4-sulfonic phenyl)porphine sensitizers
dissolved in water were also effective for Hz evolution. The presence of a water

322
19 Catalyses and Sensitization for Water Reaction
.KeH::n:llt:::.
J£J
-1
 0
.-
ro
.......
........ +1
!::
<J)
........
0
p.. +2
+3
e- Ru(bpy)l+'
e-1
Sensitization of TiO z powders and films in water was also described (Section
19.5). Sensitization of layered nanosized TiO z particles was used successfully
for an efficient photoregenerative solar cell, but only under soaking in an organic
medium. Some sensitizing effect of TiO z powders and films in water was found,
which should further be investigated for future application to photoenergy
conversion systems.
The first commercial application of photocatalysts has started to clean our
environment by TiO z powders and films. In order to utilize photocatalysts for solar
energy conversion, sensitization of large bandgap semiconductors is important.
The most difficult task for an artificial photosynthetic system is to establish
visible light-induced charge separation with minimum back charge recombination.
Utilization of a heterogeneous phase such as a semiconductor or polymer will be
a promising approach to achieve this objective.
j
Potentiostat
i e-
e- 2H
7;;TAJEDTAOX
Ru(bpy)l+/3+
Fig. 19.7 Sensitization ofnanosized Ti0 2 films in water dissolving Ru(bpy)l+ and EDTA, a sacrificial
reducing agent, as a trial for future visible light cleavage of water.
References
19.6 Conclusion and Future Prospects
1. n0 2 Photocatalysis - Fundamentals and Application, (A.Fujishima, ed.), BKC Publishers, Tokyo
(1999).
2. Photosensitization and Photocatalysis Using inorganic and Organometallic Compounds,
(KKalyanasundaram and M.Griitzel, eds.), Kluwer Academic Publishers, Dordrecht (1993).
3. Photochemical and Photoelectrochemical Conversion and Storage of Solar Energy, (Z.W.Tian
and Y.Cao, eds.), International Academic Publishers, Beijing (1993).
4. Photochemical Conversion and Storage of Solar Energy, Proceedings of the 12 th International
Conference, Oldenbourg Wissenschaftsverlag, Munich (2000).
5. Molecular Level Artificial Photosynthetic Materials, (G.J.Meyer ed.), Progress in Inorganic
Chemistry, vo1.44, Interscience Publication, New York (1997).
6. A.Fujishima and A.Honda, Nature, 238, 37 (1972).
7. B.O'Regan and M.Gratzel, Nature, 353, 737 (1991).
8. M.Kaneko and D. Woehrle, Adv.Polym.Sci., 84, 141,Springer-Verlag, Berlin (1987).
9. M.Kaneko, 11 th Symposium on Unsolved Problems of Polymer Chemistry, p.21 The society of
Polymer Science, Japan, Tokyo (1976).
10. D.O.Hall and K.K.Rao, Photosynthesis, sixth edition, Cambridge University Press, Cambridge
(1999).
11. W. Ruetinger and G. C. Dismukes, Chem. Rev., 97, 1 (1997).
12. R. Manchanda and G. W. Brudvig, Coord. Chem. Rev. 144, 1 (1995).
13. V. L. Pecoraro, M. J. Baldwin and A.Gelasco, Chem. Rev., 94, 807 (1994).
14. J. A. Gilbert, D. S. Eggleston, W.R.Murphy,Jr., D.A.Geselowitz, S. W. Gersten, D. J. Hodgson,
and TJ.Meyer, J. Am. Chem. Soc., 107, 3855 (1985).
15. R. Ramaraj, A. Kira and M. Kaneko, Angew. Chem. Int. Ed., 25, 1009 (1986).
16. R. Ramaraj, A. Kira and M. Kaneko, J. Chem.Soc., Faraday Tr.i, 83, 1539 (1987).
17. R. Ramaraj and M. Kankeo, Adv. Polym.Sci., 123,215, (1995).
18. M. Yagi and M. Kaneko, Chem. Rev., 101,21 (2001).
19. M. Yagi, S. Tokita, K Nagoshi, I Ogino and M.Kaneko, J. Chem. Soc., Faraday Tr., 92, 2457
(1996).
20. M. Yagi, K .Kinoshita and M. Kaneko, J. Phys. Chem., B, 101,3957 (1997).
21. R. M. Kellett and T. G.Spiro, inorg. Chem., 24, 2373, 2378 (1958).
22. T. Abe, H. Imaya, S. Tokita, D. Woehrle and M. Kaneko, J. Porphyrins & Phthalocyanines, 1,
215 (1997).
23. F. Taguchi, T. Abe and M. Kaneko, J. Mol. Cat. A: Chem.,140, 41 (1999).
24. H. Tanaka, B. C. Tzeng, H. Nagao, S. M. Peng and K. Tanaka, inorg. Chem., 32, 1508 (1993).
25. T. Abe, T. Yoshida, S. Tokita, F. Taguchi, H. Imaya and M. Kaneko, J. Electroanal. Chem.,412,
l25 (1996).
26. T. Abe, H. Imaya, T. Yoshida, S. Tokita, D. Schlettwein, D. Woehrle and M. Kaneko, J.
Pophyrins & Phthalocyanines, 1, 315 (1997).
27. M. Kaneko, J. Motoyoshi and A. Yamada, Nalure, 285, 468 (1980).
28. O. Stern and M. Volmer, Phys.Z., 20, 183 (1919).
29. J. Perrin, Compo Rend. Acad. Sci. Paris, 184, 1097 (1927); 178, 1978 (1924).
30. M. Kaneko, Progress in Polymer Science, submitted (2001).
31. K. Nagai, N. Takamiya and M. Kfineko, Macromol. Chem. Phys., 197,2883 (1996).
32. K Nagai, J. Tsukamoto, N. Takamiya and M. Kaneko, J. Phys. Chem.,99, 6648 (1995).
oxidation catalyst such as Ru ammine complexes or RU02 attached on the Pt/Ti0 2
in the place of EDT A was not effective for Hz formation, showing that catalytic
electron donation from water is still the most difficult and unsolved process for
a complete water photolysis by visible light.
Sensitization of Ti0 2 nanosized particle films soaked in water was tried by
dissolving a sensitizer and a sacrificial electron donor (EDT A) in the water phase
(Fig. 19.7). Photocurrent was strongly dependent on the concentration of
Ru(bpy)l+' reaching saturation at higher concentrations beyond 2 mM. By
analysis of the photocurrent-vs.-concentration curve, a Langmuir-type adsorption
of the dye was suggested.
Sensitization of TiO z powders and films for water photolysis is still an
attractive and as yet unsolved problem in the construction of an artificial
photosynthetic system for creating energy sources from solar energy and water.
Catalysis and sensitization for water reaction towards a future artificial
photosynthetic system for creating a new energy source from solar energy and
water have been described here. A proposed scheme for an artificial
photosynthetic system which combines photoexcitation reaction, dark catalyses of
water oxidation and proton reduction, and dark charge transport has been
explained. (Section 19.2). In order to achieve such photo energy conversion, dark
catalyses are of importance, so catalysts for water oxidation (Oz evolution) and
proton reduction (H 2 evolution) were disussed in Section 19.3. Considerable
advances have been made in this area. However, more and more active catalysts
are desirable in order to compete with rapid back electron transfer at the
photoexcitation center. Electron transfer at the photoexcited state of sensitizers has
been described especially for heterogeneous systems (Section 19.4), where static
mechanism is predominant. Electron hopping distance was estimated by assuming
a random distribution of the redox molecules in a solid matrix. Mediation of
electron transfer by a mediator molecule such as amino acid residue model was
successfully carried out to nearly double the electron hopping distance.

324 19 Catalyses and Sensitization for Water Reaction
33. T. Abe, T. Ohshima, K. Nagai, S. Ishikawa and M. Kaneko, Reactive & Functional Polymers, 37,
133 (1998).
34. J. Desilvestro, M. Gratzel, L. Kavan and J. Moser, J. Am. Chem. Soc., 107,2988 (1985).
35. T. Abe, E. Suzuki, K. Nagoshi, K. Miyashita and M. Kaneko, J. Phys. Chern. B, 103, 1119
(1999).
20
Photoelectric Ti0 2 Solar Cells
20.1 Introduction
Dye-sensitized TiO z solar cells (DSC) are currently under intense
investigation for their respectable photo-conversion efficiency, low cost materials
and production and life cycle assessment (LCA) in mass production. Some recent
reviews l --4) and books 5 - 7 ) deal with the dye-sensitized solar cell systems and
sensitization of wide-bandgap n-type semiconductors like TiO z . The contributed
papers presented at the Twelfth International Conference on Photoenergy
Conversion and Storage of Solar Energy in Berlin, August 9-14, 1998 have been
published as proceedings in Zeitschriftfur Physikalische Chemie in 1999. 8 ) TiO z
nanocrystallites the size of which range from 10 to 30 nm give a mesoporous
structure of DSC, playing a decisive role in photon-harvesting, photo-induced
electron transfer and electron transport at DSC. In this chapter, the history of
dye-sensitization of metal oxide semiconductors is introduced, focusing on the
properties and role of TiO z as an important component of the cells, and recent
progress in sensitizing dye molecules.
20.2 Dye-sensitization of Semiconductors
20.2.1 History
The dye-sensitization phenomena in wide-bandgap n-type semiconductors
are electron-transfer processes from photo-excited dye molecules to the
conduction band of semiconductors, followed by electron injection from electron
donors in the vicinity of the dye to their oxidized state. 9 ,IO) Dye-sensitization is
well known in the technique of silver photography.ll-13) Dye-sensitization has
also recently been recognized as an important process in photon harvesting from
visible'light, and intense scientific studies of the photon-to-electron conversion
have been undertaken with a view to the photochemical and photo electrical
conversion of solar light using wide-bandgap metal oxides since the mid
] 960s. 14,15) At the early stage of the research, kinetic investigatiolls using ZnO,
SnOz, TiO z and SrTi0 3 as semiconductors, and chlorophylls, cyanine dyes,
xanthene dyes, azo dyes and metal complexes such as ruthenium trisbipyridine
complexes, such as Ru Il (bpY)3 (bpy = 2,2'-bipyridine) as sensitizing dye molecules
were studied and empirical data accumulated. 16 ,17) Ruthenium(II) trisbipyridine
complexes have strong MLCT absorption in visible-light region with good

326
20 Pl1otoelectnc l1U 2 :::>olar CeHs
(a) Surface anchoring with amide bond
I
In-OH
/1
(C 2 H 5 0bSi(CH 2 bNH 2
HOCQ-RhB
Dicyclohexy lcarbodiimide
(b) Surface anchoring with carboxylic graoup
I
In-OH
/1
HOCO-RhB
Dicyclohexylcarbodiimide
LVak .LI Y\,J-"""lJi:)J".lLUI.J.V,"". v.... '""....u....."''V.............'''''"'''''''V........
..
 I «C 2 H 5
In-O-Si--(CH 2 h- NH 3
/ I 6C 2 H 5
 I «C 2 H 5 H
In-O-S!-(CH2b-NyRhB
/ I OC 2 H 5 0
efficiency (Fig. 20.1). Tsubomura et al. Z3) also pointed out the importance of dye
adsorption on semiconductor surface, and Hauffe et al. Z4) reported the advantage
of chelation of dye molecules for effective sensitization. In 1979, Goodenough et
al. Z5) also studied the dye-sensitization of ruthenium complexes having carboxyl
groups in the bipyridylligand, cis-RuII(bpY)z(dcbpy)Clz (Dye 1: dcbpy = 2,2'-
bipyridine-4,4'-dicarboxylic acid), on TiO z and SnOz, discussing the effects on
electronic coupling between LUMO localized on the carboxylic group of the dye
[igands and the nature of the conduction band of the semiconductors (Fig. 20.2).
..
20.2.2 Innovative Dye-sensitized Solar Cells
..
I
J'n-On RhB
/1 °
Tsubomura et al. reported pioneering work on DSC systems using ZnO films
as a wide-bandgap semiconductor electrode, Rose Bengal as a sensitizing dye, and
aqueous hydroquihone solution as a redox electrolyte. z6 ,Z7) However, the light
harvesting efficiency of the system was very low, because they used monolayered
dye molecules on micro-sized ZnO. The importance of increasing surface area of
the semiconductor electrode was pointed out. Z8 ) Gratze] et af.29-31) reported
remarkable sensitization of mesoporous Ti0 2 films using Ru II (dcbpY)3 and cis-
Ru ll (dcbpY)z(H 2 0)z as sensitizing dye molecules. Interestingly, they obtained
74% of incident photon-to-current conversion efficiency (IPCE) at maximum
absorption wavelength of Ru ll ( dcbpy h
A milestone in DSC history is the report of Gratzel and his group in [990 3Z ,33)
(Fig. 20.3). They achieved 10% photoconversion efficiency under 1 sun (AM1.5)
conditions using highly porous TiO z films and a reactive ruthenium dye (cis-
RuII(dcbpy)zCNCS)z. See Dye 2 (N3) below). The most important feature ofDSC
is extremely high light harvesting ability due to more than 1,000 times higher
Fig. 20.1 Chemical adsorption of Rhodamin B on Sn02.
o
-0
-0
o
Q
OCJ
I   / c
Ti-O 0
I\) 0
Dye 1
CJ
OQ
+- @ / C
: 0 0
o
,/
- -
- -
- f5 hV
e-
/-
e
,
,
,
,
,
,
,
,
,
,
,
,
, -
- ,
-,'
" -
Dye-adsorped TiO z nanocrystallite
(diameter -20 nm, thicknesswlO m)
I TiO z 1 I Dye 21 I [-1I3 - I
..
hv
HOOC
---
-
Electron flow
Fig. 20.2 Schematic view of molecular orbital interaction between the caboxylic group of Dye I and
surface Ti or Sn sites of metal oxide semiconductors.
stability, and are being extensively studied as sensitizers. 18 ,19) It is worth noting
that Sutin et al. 20) examined the sensitization of single-crystal rutile Ti0 2 using
ruthenium(II) complexes, such as cis-RuII(bpY)3Clz, Ru(5Cl-phen)3Clz (Cl-phen =
5-chloro-l,10-phenanthroline) and cis-RuII(Mez-phen)3Clz (Mez-phen = 4,7-
dimethyl-I, 1 O-phenanthroline). They fourid that the quantum efficiency of electron
injection of cis-RuIJ(Merphen)3Clz depends on pH of the solution, approaching to
unity at low pH conditions.
Interesting research progress of dye-sensitization was achieved in 1977.
Fujihira et al. 21 ,22) reported the importance of chemical bonding between
sensitizing dye molecules and surface of sensitized semiconductors, and
Rhodamine B that is chemically bonded to SnOz shows 20 times higher sensitizing
hv
Organic
electrolyte _
J-1I3- solution
(Hole transport layer)
COOH
hv
11' HOOC
OTE Pt- or Carbon-
(F doped SnOz) coated OTE Dye 2 COOH
lWEI I CEI
Fig. 20.3 Geometric and energetic structures of dye-sensitized solar cells.

328
2U Photoelectnc 1'10 2 Solar Cells
jL.':J
L.U.L. Vye-sensltlzatlon or ;:,emlconauctors
practical surface area than the projected area, defining as roughness factor, RF
> 1,000. The surface of the porous TiO z film that was formed by sintering avo 20-
nm sized nanocrystalline TiO z particles was effectively covered by Dye 2.
J se x e X jJ
1}=
Is
(20.1)
where Is is the power of incident light. Generally, the efficiency is measured
under solar simulator irradiation of air mass 1.5 (AM1.5) light with intensity of
100mW'cm- z . Incident photon-to-current conversion efficiency (IPCE) under
monochromic light irradiation shows wavelength dependence of the performance
of DSC. Maximum IPCE at the absorption peak of the system (dye) exceeds
80%, indicating that the quantum yield (charge collection efficiency) is almost
unity.
20.2.3 Fabrication of Dye-sensitized TiO z Solar Cells
Highly porous Ti0 2 thin films of 10 /lm thickness are produced by spreading
an aqueous dispersion of the 20-nm sized TiO z nanocrystallites on an optically
transparent conductive glass electrode (OTE) and then sintered under air for 30
min at around 450°C. The resulting porous film has mesoporous morphology and
high effective surface area with RF > 1,000. This porous TiO z electrode is dyed
by soaking in an alcoholic solution of Dye 2, which has a wide absorbing range
of solar light and ligands with carboxylic groups for effective anchoring on the
Ti0 2 surface. The resulting dye-adsorbed Ti0 2 electrode is combined with
platinum- or carbon-coated OTE as a counter electrode, and an organic 1-/1 3 -
solution is introduced between them as a redox and hole conductive electrolyte.
Such a dye-adsorbed Ti0 2 structure is very similar to that of the thylakoid
membrane filled with chlorophyll pigment in the natural photosynthetic system,
contributing to efficient light harvesting.
Figure 20.4 shows a respectable photo current-voltage (I-V) profile of DSC
obtained using the most appropriate materials and additives in the electrolyte. The
photo current density in short circuit, J se = 18-22 mA 'cm- 2 , the open circuit
photovoltage, V oe > 650 mY, and the cell fill factor, which is the ratio of obtained
maximum cell output power to J se x Voe,jJ> 0.7, are obtained. The cell provides
nearly 10% photoenergy conversion efficiency of the solar light energy.
The overall photo conversion efficiency is also given by Eq. (20.1)
20
18
16
--.
N 14
I
E
CJ
..;: 12
e
'-'

....
.tiJ 10
c
gJ
"C
- 8
c
gJ
""
""
::I
U 6
4
2
20.2.4 Characterization of Innovative Dye-sensitized TiO z Solar Cells
The DSC consists of five important components, Le., an OTE, a porous TiO z
electrode, a sensitization dye, an 1-/1 3 - redox electrolyte solution and a counter
electrode. They form the four interfaces that will play important roles in the
vectorial electron transfer of DSC. In the innovative DSCs, high photoelectrical
conversion efficiency should eventually be achieved by optimum vectorial
electron transfer between the four interfaces and charge transport in the five
components. The conversion efficiency 17 is expressed by the product of charge
(electron) collection efficiency between conductive glass and porous TiO z film 17ee,
charge (electron) transport efficiency in Ti0 2 layer 17eh electron injection
efficiency from the photo-adsorbed dye to Ti0 2 17ei, solar light harvesting and
charge (electron and hole) formation efficiency of the dye 17th, reduction (hole
injection) efficiency of the oxidized dye by iodide ion 17hb efficiency of the charge
(hole )-transport in the electrolyte 17hh charge (hole) collection efficiency by
reduction of triiodide to iodide on counter electrode 17he (Fig. 20.5, Eq. (20.2).
6
IIlIilllllr
,.--6 :
". :
'- ..... _.. 11111.1111111
9
Dye
rv : : .
: n.I. "" .Inn 1
.. Ilmr n (ox CUP1 .I!mn...l
"lIIlIIilllll e ..
l ' t::0 @  ..
V : Pt or Carbon
.
.
i
i
i
I
I
I
Measured at NREL I
Date: March 25, 1998 I
Sample Area: O,2542ds cm z I
[rradiance: 100 mW'cm- z (AM 1.5) I
Jsc = 17.899 mA'cm- 2 i
Voc= 756 mV I
11= 68% i
Efficiency = 9.203% !
!
.
.
.
 IIlIIillllll.(
o :'
.
.
Mesopq ro 1U!
TiQ2
,
"IIIIQIIIIII _
.
.
'II'
IIlIIillllll
'1IIIilllllll
.
.
.
.
.
.
.
.
.
.
.
,
.
.
.
.
SnOz/F 1
.
.
(1]c X 1]et X 1]e) X 1]lh X ( 1]i'. X 1Jht X 1]c)
'high surface area
.fast electron transport
'high absorption : .fast diffusion of
coefficiency  electrolyte
.wide absorption band : .fast hole transport
-long-term stability
o
o 100 200 300 400 500 600
Voltage (m V)
700
800
'barrier ree
(good contact)
.fast electron injection .fast hole injection
.slow back electron transfer
'barrier free
(catalytic)
Fig. 2004 Example of a respectable photocurrent-voltage profile of dye-sensitized solar cells.
Fig. 20.5 Main factors to be improved in dye-sensitized solar cells.

T] = {( T]ec X T]et x T]ei) x T]lh X (T]hi X T]ht X T]hc)}
(20.2)
charge-transfer. The structure and electronic state of the adsorbed dye molecules
are argued on the basis of the IR and Raman vibrational spectroscopy. The
association constant of the adsorbed molecule to the semiconductor surface is
determined by the absorption and emission spectra as probe. Electrochemical
measurements prove the energetic structures of dyes and semiconductors, and
kinetic analyses with transient spectroscopy reveal the electron transfer dynamics.
The optimization of seven factors will contribute to the improvement of DSC. As
far as TiO z is concerned, characterization of T]et and T]ei is crucial. With regard to
dye molecules, factors affecting T]ei, T]lh and T]hi should be clarified and optimized.
To optimize or solidify the redox electrolyte solution, materials should be
developed without lowering T]hi, T]ht and T]hc'
20.3.1 Bonding Structure of Dye on TiO z Influencing T]ei
20.3 Electron-transfer Sensitization on TiO z
p
o ,0
. -- .\ 
Ti:: e:c Dye 2
. " .f
o '0

\>
Ti----O
. .\ 
O. e.:p
Ti----O
p
o
\ ""..9Q 
Ti::::,., CJ......C
, \j ""'O 0
o
The author and his co-workers 34 ) based on their work on TiOz-modification
with acid chlorides or acid anhydrides 35 ) assumed that the bonding structure of Dye
2 on Ti0 2 should contribute to effective T]ei' They concluded that the adsorbed Dye
2 molecules react with Ti0 2 , giving monodentate ester-like linkages between
carboxyl groups of the dcbpy ligand and Ti IV on Ti0 2 surface. Spectroscopically,
they observed shifts to higher wavenumber of c=o stretching vibration and
decrease in the strength of the o-c-o anti symmetric telescopic motion of ligand
carboxyl group (Fig. 20.6). Finnie et at. 36) recently discussed the adsorption
structure by a comparison of a series of tetraalkylammonium salts of Dye 2. The
shift of C=O in the IR spectrum is due to differences in the extent of hydrogen
bonding, and suggests a bidentate chelate or bridging coordination to the Ti0 2
surface via two carboxylate groups per dye molecule. The crystal structure of Dye
2 was solved recently, and interaction between Dye 2 and the (101) surface of
anatase TiO z was modeled by using molecular dynamics calculations, where two
of four carboxylate groups were attached to the surface. 37 ) In both structures, the
;r* orbital that originally exists for the carboxyl group is delocalized through
interaction with the conduction band of Ti0 2 derived from the 3d orbit (t2) of the
surface Ti IV ion, contributing to high T]ei in DSCs.
The semiconductor nanocrystallites work as electron acceptors from the
photo excited dye molecules, and the electron transfer as sensitization is influenced
by electrostatic and chemical interactions between semiconductor surface and
adsorbed dye molecules, e.g., correlation between oxidation potential of excited
state of the adsorbed dye and potential of the conduction band level of the
semiconductor, energetic and geometric overlapping integral between LUMO of
dye molecule and the density of state distribution of the conduction band of
semiconductor, and geometrical and molecular orbital change of the dye on the
r! 
T '. /C
.1----0 
o

ester bonding
3d (tZ)III1I11I11I11I1t*
20.3.2 Dynamics in Electron Transfer from Photoexcited Dye 2 to TiO z
bidentate chelating
bridging
COOH
The dynamics of the interfacial electron-transfer between Dye 2 and Ti0 2
were examined precisely by laser-induced ultrafast transient absorption
spectroscopy. Durrant et al. 38) employed subpicosecond transient absorption
spectroscopy to study the rate of electron injection following optical excitation of
Dye 2 adsorbed onto the surface of nanocrystalline Ti0 2 films. Detailed analysis
indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec
(instrument response limited) and 50% in 1.2 :f: 0.2 psec.
Willig et al. 39 ) measured the transient absorption of dye adsorbed TiO z film
under ultrahigh-vacuum signal of the injected hot electrons on Ti0 2 with a rise
time of < 25 fsec. The electron transfer reaction reported here did not involve
redistribution of vibrational excitation energy and was thus completely different
from the well-known Marcus-Levich-Jortner-Gerischer type of electron transfer
at the weak electronic interaction.
Ellington et al. 40) used femtosecond pump-probe spectroscopy to probe
directly the arrival of electrons injected into the TiO z film with near- and mid-IR
that probe the absorption at 1.52 /lm and in the range of 4.1-7.0 /lm. Their
measurements indicate an instrument limited -50 fsec upper limit on the electron
injection time. These observations suggest that electron injection from Dye 2 to
Ti
I
Ti-O
\ ",,0
-Ti:::....e\
! .."""0
Ti-O
\ ",,0
....\\ e '
-Tr:.. - ,>
! """'0
Ti-O
\
Ti
Ti0 2 Surface
COOH
Dye 2
bidentate chelating
Fig. 20.6 Three types of chemical bonding structure Dye 2 on Ti0 2 surface.

332
20 Photoelectric TiO z Solar Cells
2004 Electron Transport in Porous Ti0 2 Electrodes
333
Ti0 2 is completed in a few IOs fsec, indicating that the excited electron has
moved to the conduction band ofTi0 2 before relaxation in LUMO of Dye 2.
Back electron-transfer of injected electron from conduction band to the
oxidized Dye 2 was also examined by transient absorption spectroscopy. Durrant
et al. 41 ,4Z) reported that the charge recombination kinetics is multicomponent and
strongly dependent on irradiation intensity, electrolyte composition (ca. 10 6 -fold
change in rate), and the applied bias to the Ti0 2 film. Excitation of more than one
dye on one particle of TiO z results in a rapid acceleration in the charge
recombination kinetics. Under no bias and low irradiance conditions, the charge
recombination reaction occurs on ,Llsec-msec time scales. When the applied
potential is more negative than a threshold potential, a rapid acceleration of the
charge recombination kinetics is again observed, for example from ca. 1 msec at
+0.1 V vs. Ag/AgCl to ca. 3 psec at -0.8 V (ca. 10 8 -fold increase in rate). They
conclude that the charge recombination kinetics in such dye-sensitized films is
strongly dependent on the electron occupation both in trap and conduction band
states of the TiO z film.
Ti
! 50:t 25 fsec
Ti-
/ \ ..",q 
-Ti::"'e\ 
.'1 I
'II
o 
Ti-O
\ Jlsee-msee
,,0 I
.' \
-Ti::" 8>
"1 I
1110
COOH
COOH
50 :f: 25 fsec
In a report on the electron transfer between dye/hole transport electrolyte, the
electron transfer rate from redox electrolytic solution (0.3 M KI and 0.03 M 1 2
ethylene carbonatelpropylene carbonate (1: 1) solution) to oxidized state of Dye 2
was determined to be 110 nsec based on the lifetime of the Dye 2 cation. 41 ) It is
much faster (by an order of 10 3 fold) than the back electron-transfer from Ti0 2 to
oxidized Dye 2.
As for the electron transfer between bare Ti0 2 and redox couple, direct
reverse electron transfer occurs from either conduction band or surface trap levels
to the electrolyte. Salafsky et ai. 43 ) observed time-resolved microwave
conductivity of dyed Ti0 2 film and revealed that the decay consists of two
components on the order of 100 msec and> 1 sec. To avoid charge recombination
of injected electron with 1 3 - on Ti0 2 surface, Frank et ai. 44 ) treated the dye-coated
Ti0 2 electrodes with pyridine derivatives (4-tert-butylpyridine, 2-vinylpyridine,
or poly(2-vinylpyridine)). Both V oc (from 0.57 to 0.73 V) and the cell conversion
efficiency (from 5.8 to 7.5%) at AM 1.5 are improved significantly compared with
the untreated electrode,' The pyridine compounds may lower the back-electron-
transfer rate constant by 1-2 orders of magnitude, but there is no significant effect
on the recombination mechanism or the kinetics of electron injection from excited
dye molecules to Ti0 2 . Kinetic studies of the dye-covered electrodes show that the
rate of recombination is second order in 1 3 - concentration, which is attributed to
the dismutation reaction 2 1 2 - ---7 1 3 - + 1- with Iz as the electron acceptor in the back
reaction, Mass-transport theory is applied to understand the dependence of J sc on
the radiant power at low 1 3 - concentration and to calculate the diffusion coefficient
of 1 3 - ions (7.6 X 10- 6 cm 2 .s- 1 ) in the porous Ti0 2 structure.
The dynamics of electron-transfer at the interfaces ofTiOziDye 2/electrolyte
are summarized in Fig. 20.7. Two forward electron-transfer steps are much faster
than the corresponding reverse electron-transfer (charge recombination) on the
order of 10 3 to 10 6 , The results well explain the high T]ei value due to the efficient
and vectorial electron-transfer in Dye 2-sensitized DSC.
C.B.
n* (dcbpy)
20.3.3 Electron Transfer Between Oxidized Dye 2 and 1-11 3 - Electrolyte
Surface States
100 msec - I see
hv
1- /1 3 -
Ru(II/lII)
V.B.
mesoporous TiO z
Dye 2
electrolyte
Fig. 20.7 Time-scale of interfacial electron transfer at Ti0 2 /Dye 2 and Dye 2/electrolyte interfaces.
20.4 Electron Transport in Porous TiO z Electrodes
The efficiency of DSC depends largely on the behavior of the electrons that
are injected by the sensitizer dye under photoirradiation. Since porous disordered
Ti0 2 electrodes form a large number of boundaries between particles, there was
a consensus among most scientists in the field that such porous semiconductor
structure makes poor photoelectrochemical properties compared to single crystals
of the same semiconductor. However, it has now become possible for the Ti0 2
electrode to collect, keep and so efficiently transport electrons through such
porous network, as seen in DSC using Dye 2, and anatase nanocrystalline Ti0 2 .
When compared with macroscopic polycrystalline or single-crystalline
semiconductors, the nanocrystalline Ti0 2 films exhibit anomalous electron

334
20 Photoelectric Ti0 2 Solar Cells
2004 Electron Transport in Porous Ti0 2 Electrodes
335
transport properties: slow, nonexponential current and charge recombination
transients, and intensity-dependent response times. Such properties affect largely
the electron transport efficiency, 17eb in this system.
( 1 - e- aw )
J=qr
1 + aL
(20.3)
20.4.1 Electron Transport Models for High 17et
where r is the incident light intensity, a is the reciprocal absorption length, ill is
the width of depletion layer, and L, which is expressed by using the diffusion
coefficient and mean charge carrier lifetime as L = (D-r)1/2, is the minority carrier
diffusion length. The equation was based on a semiconductor-metal Schottky
barrier,49) and was later applied to liquid-junction by Butler. 50 ) Lindquist et aJ.5l)
derived models for electron transport in mesoporous Ti0 2 film by using Eq. (20.3)
based on the assumptions that the electron is transported by diffusion and that the
diffusion length is constant through the film, giving steady-state solutions of the
generationlcollection equation for the density of mobile excess electrons under
illumination.
The electron transport mechanism in mesoporous Ti0 2 film is modeled
mainly by using diffusion theory, except in the report by Augustinski et al.,45) who
proposed the explanation that the initial film charging by dye-sensitization, in
terms of the self-doping, causes an insulator-metal (Mott) transition in a donor
band of Ti0 2 , accompanied by a sharp rise in conductivity of the nanopartic1es.
Willig et al. 46) presented the idea that a p-n junction type equilibrium electric
field exists at the OTE/Ti0 2 interface, but not in the Ti0 2 mesoporous film
because the photogenerated electrons are screened effectively due to the high
ionic strength in the electrolyte (Fig. 20.8). Since the electrons travel in Ti0 2
film with slow diffusion but have extremely long lifetime and the screening ions
cannot function as recombination centers, 17et and 17ec are still high. Nelson 47 )
applied a model of dispersive transport based on the continuous-time random
walk to nanocrystalline TiO z electrodes. Electrons perform a random walk on a
lattice of trap states, each electron moving after a waiting time that is determined
by the activation energy of the trap currently occupied. Monte Carlo simulations
of the quantitative kinetics of photo current and charge recombination transients,
and the intensity dependence of photocurrent transients reproduce many of the
features that have been observed in dye-sensitized Ti0 2 electrodes under applied
bias. 41 )
Gerischer4 8 ) developed a model where photogenerated electron-hole pairs in
semiconductors are separated by a band bending. Gartner4 9 ) derived an expression
for the photo current density J as
an _ D a2 n n r -aw
-- ---+a.Le
at ax 2 'r
(20.4)
The time-course and frequency analyses of electron transport in mesoporous
Ti0 2 films evidenced the expression of the Eqs. (20.3) and (20.4) in the following
papers, where extremely high 1Jet and 17ec values of DSC systems are explained.
20.4.2 Time-course Analysis
exponential energy
distribution of
trap site
Kinetic studies of the dynamic processes in the time domain are measured in
time-resolved electrochemical experiments Season et al. 52 ) reported that the
transient photo current response consists of a fast (msec) and a slow (sec)
component that exhibits a power law dependence on light intensity with an
exponent of -0.6 to -0.8. They assumed that the density of photogenerated
electron decreases logarithmically in" depth from the illuminated surface, and that
the diffusion constant of the' electron depends on its density. Hagfeldt and
,
Lindquist 53 ) measured the transient photo current. in anatase TiO z electrodes in
contact with an electrolyte followed by laser excitation. The time profile of
photocurrent depends on film thickness and electrolyte conductivity. The diffusion
coefficient for electon in Ti0 2 in ethanolic solution of 700 mM LiCI0 4 is 1.5 X
10- 5 cm 2 's- l . This is in the range of ion diffusion in solution and several orders of
magnitude smaller than that of electron in the bulk TiO z (ca. 10- 1 cm 2 .s- I ).
Salafsky et al. 43 ) showed time-resolved microwave conductivity
measurements of the decay of electrons in Ti0 2 nanocrystals, following dye-
sensitization. The time scale for this decay process is on the order of 100 msec to
sec and explains the high electron collection efficiency 1Jec in DSC, since the
long time scale allows for slow transport through the TiO z network.
random walk
F-doped
Sn02
. --. 
. ....,.:....i,,
'. ,)';':lt:;::;l____.
:.';: '. < f'.' c ; ./" ,
 , _ .  . e .
;:t': ;'y.' 3-;<-__
'i;;;;OP.'-r2 .";\ .........
"- j.y.;.,:; ....; ;.):.\\/
LUMO
diffusion
thermal excitation
- e- -.....
J - - "::t:7. c.B.
\-! \-! \-! Trap sites
20.4.3 Frequency Analysis
waiting time
In contrast to kinetic studies, frequency resolved experiments analyze the
response of electrochemical systems to periodic or sinusoidal perturbations of
voltage or current. 54 ) However, electrochemical impedance spectroscopy (EIS)
is the only universally accepted electrochemical frequency resolved method
because of the conceptual difficulty involved. Electrochemical perturbation and
p-n junction
Fig. 20.8 Schematic models of electron transport in mesoporous Ti0 2 electrodes.

336
20 Photoelectric Ti0 2 Solar Cells
20.5 Sensitization Dyes
337
response are normally analyzed based on linear equivalent circuit elements, i.e.,
resistors and capacitors. EIS can be used in semiconductor electrodes both in the
dark and steady illumination, where it is termed photo electrochemical impedance
spectroscopy (PElS). Intensity-modulated photo current spectroscopy (IMPS) in
which the magnitude and the phase shift of photo current are measured under
modulated intensity of illumination incident, and intensity-modulated
photovoltage spectroscopy (IMVS) in which modulation of photovoltage in
response is measured under modulated illumination have been employed in DSC.
Season et al. 52) reported that the IMPS response of dye-sensitized Ti0 2
mesoporous film is described by a diffusion model and the diffusion coefficient
in the film depends on light intensity, where the electron transfer process is
explained by thermal excitation from trap sites of the particles.
Peter et ai. 55 ) fitted the experimental IMPS data measured for DSC to the
theoretical model and derived values of the lifetime ('r = 2 x 10- 2 s) and diffusion
coefficient (D = 5 x 10- 5 cmZ's-l) of photoinjected electrons on mesoporous Ti0 2
electrode. The D in porous film is nearly four orders of magnitude lower than that
in the bulk phase such as sputtered thin films or single-crystal anatase, but agrees
with the value obtained by kinetic analysis. 53) They observed the transmission at
near-IR (904 nm), where electrons on conduction band ofTi0 2 have absorbance,
under IMPS irradiation of dye-sensitized TiO z film at 514 nm. 56 ) The net electron
injection yield is unity under short circuit and 0.3 under open circuit, indicating
that the reaction of strongly accumulated electrons on Ti0 2 with the oxidized
dye competes with dye regeneration by J-. The frequency-dependent impedance
under steady illumination indicates that the majority of the detected electrons
are trapped and consumed predominantly via back reaction with 1 3 - in both open-
circuit and short-circuit conditions.
The lifetime 'r and diffusion coefficient D of photoinjected electrons in DSC
measured over five orders of magnitude of illumination intensity using IMVS and
IMPS. 56) 'ris proportional to the r- J12 , indicating that the back reaction of electrons
with 1 3 - may be second order in electron density. On the other hand, D varied with
rO. 68 , attributed to an exponential trap density distribution of the form Nt(E) DC
exp[-f3(E - Ec)/(kBT)] with f3:::: 0.6. Since 'r and D vary with intensity in opposite
senses, the calculated electron diffusion length L = (D''r)1I2 does not change
linearly with the irradiance.
Frank et al. employed IMVS and IMPS methods to investigate the charge-
recombination kinetics and band edge movement, 57) charge collection efficiency
71 58) and electrical P otential distribution59) of DSC. Surface modification of dye-
'lee,
adsorbed TiO z electrodes with 4-tert-butylpyridine or ammonia leads to a
significant negative band edge shift concomitant with a more high V oc ' The
second-order nature of the charge recombination that occurs predominantly via
trapped electrons in surface states with respect to 1 3 - concentration is confirmed.
17ec is estimated from the respective time constants for charge recombination at
open circuit 'roc and the combined process of charge collection and charge
recombination at short circuit 'rsc, indicating that 'roc depends largely on r, and 17ec
relates to 'rocl 'rsc..
20.4.4 Effect of TiO z Films on Performance of Dye-sensitized Solar
Cells
The author and co-workers 60 . 61 ) reported that Ti0 2 mesoporous films prepared
by sintering at 350-550°C of a single-phase anatase nanocrystallite have
transparent, narrow pore size distribution and good electron transport
characteristic, and that DSC made of the Dye 2-adsorbed full anatase Ti0 2 films
as photoanodes achieved better photoenergy conversion efficiency compared to
those prepared using Degussa P25 films. Frank et ai. 6Z ) reported structural and
photoelectrochemical characterization of full rutile Ti0 2 electrode prepared from
hydrolysis ofTiCl 4 . J sc and V oc under AM 1.5 illumination of Dye 2 adsorbed 4.5
/lm thick films increase over the temperature range of IOO-500°C, from 1.1
mA'cm- 2 and 602 mV to 8.7 mA'cm- 2 and 670 mY. The increase with annealing
temperature correlates with increased concentration of adsorbed dye and improved
light scattering properties of the film associated with the particle size of the rutile
TiO z .
20.5 Sensitization Dyes
Dye 2 gives the best photoconversion efficiency of the examined sensitization
dyes for DSC at the present stage. Detailed studies on Dye 2 (Fig. 20.7)
sensitization revealed that the effective interaction with TiO z surface through the
carboxyl groups plays an important role in giving high 17ei' At the same time,
Dye 2 must give favorable 17lh and 17hi' The high 17ei and 17hi mean that the dye
should have a panchromatic property with high absorption coefficient and
appropriate redox potential relevant to the redox couple of the electrolyte. In
addition, the light stability of the dyes is also an important factor. High 17eb 17th and
TJhi will eventually lead to high stability. This section will deal DCS studies from
the viewpoint of sensitization dyes.
20.5.1 Ruthenium Polypyridine Complexes
Gditzel's group energetically examined various ruthenium(II) polypyridine
complexes that have similar structure of Dye 2 as sensitization dyes. 63 ,64) A new
panchromatic dye (Dye 3) named Black Dye 65 ) with 4,4',4"-tricarboxy-2,2':6',2"-
terpyridine absorbs light of the near-IR wavelength range up to 900 nm, providing
higher TJ1h' The National Renewable Energy Laboratory (NREL), USA, states
COONBu4
COOH
HOOC
I
 /, HOOC -::7 /-
\\ N I N
I />NCS  N I /.>NCS
, R "
_ N-RU I I , NCS JXQ N" U I 'NCS
N NCS I
- HOOC 0 ",N I

COONBu4
Dye 3 Dye 4
HOOC
COOH
"N"
10
HOOC
COOH
DyeS

to the iOnIC double layer at the electrodelelectrolyte interface. This fact has
important implications for the optimization of the combination of dye and
electrolyte for both T]ei and T]hi in DSC.
Some metal complexes with polypyridine ligands similar to Dye 2 have also
been investigated as sensitzing dyes. Ferrere and Gregg 73 ) synthesized cis-
FeII(dcbpy)z(CN)z complex (Dye 8), and Bigozzi et af.74;5) compared properties
of cis-OsII(dcbpY)2(CN)z (Dye 9) with comparable cis-RuII(dcbpy)z(CN)z (Dye
10). The efficiencies are much lower than that of Dye 2, but they proposed the
that DSC with Dye 3 shows an electrical conversion efficiency in slight excess of
10% with J sc > 20 mA'cm- 2 , V oc "" 0.72 V andff = 0.7. Dye 3 in the DSC system,
however, seems to have less stability than Dye 2. A tetrabutylammonium salt of
Dye 2 37 ) (Dye 4) shows comparable efficiencies with Dye 2. Recently, the trans-
form of Dye 2 66 ) (Dye 5) was isolated and characterized. Cell efficiency of the
trans-form dye was not documented in the paper, but the red-shifted absorption
bands should improve efficiency.
Arakawa et al. 67 ,68) synthesized phenanthroline dyes and applied successfully
it to DSC. 1,1O-Phenanthroline ligand is more rigid and symmetrical than 2,2'-
bipyridine, but their metal complexes show similar properties. The cell with
tetrabutyl ammonium salt of cis-Ru II (dcphen)2(NCS)z (Dye 6: dcphen = 4,7-
dicarboxy-l, 1 O-phenanthroline) shows 6.1 % conversion efficiency under LOO
mW'cm- 2 (AM 1.5) irradiation. Bonh6te et a1. 69 ) examined electron injection to the
porous thin film of Ti0 2 , Alz0 3 and ZrOz by using a complex (Dye 7) with the
phosphoric acid residue as an anchoring group.
20.5.2 Other Metal Complexes
eaOH
Haoe
Hoae
Hoae
Hooe
eaaNBu4
eOOH
Haae
Dye 8
Haae
eaaNBu4
Dye 6
a
II
a-p
I
a
Dye 11
Dye 7
The oxidation potential of the dye adsorbed onto nanocrystalline Ti0 2
becomes pH-dependent, decreasing 53 m V per unit pH as followed by the flatband
potential of the Ti0 2 . Therefore, the driving forces for both forward and back
electron transfer reactions are practically independent of pH.70) Zaban and
Gregg 71 ,72) examined chemical oxidation, spectro-electrochemical reduction, and
potential-dependent photoluminescence of eight different dyes to investigate their
changes in the redox potential relative to the potentials of semiconductor and
electrolyte solution. The adsorption-induced potential changes depend on both the
electrolyte composition and the position of the specifically adsorbed dye relative
Hooe  t?  COOH
  N '" /,
I -.;: "
'I N, /N-
N Fe" N
-N/ "  ;,
 "-N I h-
I
 COOH
Haoc
Dye 13
eOOH
eaOH
Hooe
Hoae
eOOH
eOOH
Dye 9
Dye 10
HOOC  ;{? eOOH
  N ,,/'
I -.;: "
'I N, /N-
c1 N N gl  N
- N
 "-N I h-
I  I
 COOH
HOOe
Dye 12
(), ,N  b
2: \ 
 N NU  IN;'N
 "- I
N X -:?' I
I 
HO 00
X  (SlOH
N
Dye 14

340
20 Photoelectric Ti0 2 Solar Cells
possibility of applying the complexes with MLCT absorption to DSC. More
recently, Lewis et al. 76,77) examined the electrochemical properties of a series of
osmium(II) polypyridine complexes and compared them with their ruthenium(II)
analogues to show the noteworthy conversion efficiency of Dye 9.
Since the metal complexes of porphyrin 19 ) and phthalocyanine 78 ) have the
excellent thermal and light stability, they are utilized as the pigment in various
fields. Zn lI 5,1O,15,20-tetrakis(carboxyphenyl)-porphyrin (ZnTCPP: Dye ll),79)
Mn IJ and Fe 1ll 4,4',4",4"'-tetracrboxy-phthalocyanine complexes (Dye 12, 13),72)and
Ru ll bis(3 ,4-dicarboxypyridine )-( 1,4,8,11,15, l8,22,25-octamethylphthalocyanato)
(JM3306: Dye 14),80) were examined as the sensitization dyes in DSC. The
photo current is observed on irradiation of both absorption of the Soret band and
Q-bands of each dye.
Br
HO
HOOG
HOOG
Fluorescin
Dye 15
HOOG
Uranine
GOOH
NaO
HOOG
GOOH
Dye 16
HOOG 0
  N
N i )
o GOOH
Dye 17
20.5.3 Organic Dyes
Gregg et al. 81 ) examined photosensitization of perylene pigments (Dye 15-17)
on a porous Sn02 thin film instead of TiO z film as DSC, in view of energy
matching with conduction band of semiconductors and LUMO of the sensitizers.
When perylene-3,4-dicarboxylic acid-9,10-(5-phenanthroline) carboximide (Dye
15) was used, J sc of 3.26 mA'cm- z , V oc of 0.45 V, and a photoelectric conversion
efficiency of 0.89% were observed under AM 1.5 irradiation. IPCE achieves
close to 40% at 460 nm.
Arakawa et a/ 8Z ) employed various 9-phenyl xanthene derivatives as the dye
for DSC, and found that Eosin Y (EY: Dye 18) shows relatively high conversion
efficiency of 1.3% under 100 mW'cm- z irradiance. They clarified the efficiency
of 9-phenyl xanthene derivatives as follows: EY > Dibromofluorescein >
Fluorescein = Fluorescin > Rhodamin B > Dichlorofluorescein > Uranine >
HO
Tetrachlorofluorescein
o HO
20.5 Sensitization Dyes
341
o 
Br Br
Dibromofluorescein
OH EtN
I
Et
Rhodamin B
o
HO
HO
o 
OH OH
Pyrogallol Red
GI
GI
GI
o
Fluorescein
GI-
N+Et HO
I
Et
02 N
Br
Eosin B
o 
GI GI
Dichlorofluorescein
o
Flurorescinamine 2
GI
Br
o
Me
o H-N
I
Et
NTH
I
Et
Rhodamin 60
Pyrogallol Red> Flurorescinamine 2 > Erythrosine B = Rose Bengal = Phloxine
B > Tetrachlorofluorescein > Eosin B > Rhodamin 60. The EY-sensitized DSC
shows stability for 1,700 hours irradiation. Th ey 83) also found that Mercurochrome
(Dye 19) sensitizes ZnO, giving very high conversion efficiency of 2.5% under
AMl.5 (99 m W'cm-Z) irradiation. Merocyanin dyes 84 ) containing a carboxyl group

342
20 Photoelectric TiO z Solar Cells
20.6 Recent Research Progress in Dye-sensitized Solar Cells
343
OH
Dye 19
Br
C 1s H 37
I
N
ssyS
, N'CH2COOH
o
Dye 23
a
a
HO
OH
Dye 20
OH
OH
HO
OH
Dye 21
Dye 22
Dye 24
type, and the color changes from red to violet, where the HOMO localizes on the
chromen ring fragment, but the LUMO exists on the cyanin part and on Ti Iv .
Thus the intermolecular CT character absorption of Dye 24 on TiO z plays an
important role for a respectable T]ei'
and a long alkyl chain, in particular 3-carboxymethyl-5-[2-(3-octadecyl-2-
benzothiazolinyldene )ethylidene ]-2-thioxo-4-thiazolidinone (Dye 20), showed
remarkably high conversion efficiency (4.2%, AM1.5, 100 mW.cm- 2 ).
Tennakone et ai. 85 ) used triphenylmethane type (metallochromic) organic
dye (Dye 21, 22), both of which show a large bathochromic shift on complexing
with metal ions. The molecular orbital calculation of these dyes in chelating
condition with the Ti IV ion revealed that the LUMOs of these dyes are localized
on the Ti IV ion, but the HOMOs are delocalized in the whole dyes. Such MO
distribution similar to the LMCT transition in transition metal complexes should
contribute to the vectorial electron transfer (high lJei) from the excited dye to
Ti0 2 .
20.6 Recent Research Progress in Dye-sensitized Solar Cells
20.5.4 Natural Dyes
The dynamics of electron transfer at the interfaces of TiOz/Dye 2 and Dye
2/electrolyte have been revealed as shown in Fig. 20.7, and the energy relationship
between Ti0 2 and Dye 2 molecule has been elucidated by Swedish scientists at the
Thirteenth International Conference on Photochemical Conversion and Storage of
Solar Energy (IPS-2000).88) These facts mean high T]ei and T]hi in Dye 2 sensitized
Ti0 2 solar cell systems, substantiating the unbelievable stability of the dye
molecules under visible light soaking. In fact, long-term stability of more than two
years under outdoor conditions and 10,000 hours under simulated sunlight (400
W'm-Z) has been confirmed.
Recently, the project Joule 3 in the European Community based on
accelerated aging tests. on large numbers of DSCs, revealed that thermal stress
appears to be one of the most critical factors determining long-term stability and
is strongly related to the liquid composition of electrolytes. 89) They obtained the
following results for DSC's based on pure liquid electrolytes. 1) A minor decrease
in performance of initially 5% solar efficient cells has been found after 2,000
hours at 60°C storage in the dark. 2) After 3,400 hours under combined thermal
stress and continuous I-sun equivalent light soaking at 40°C, good stability with
The cyanidin dyes obtained from flag (iris), or tea leaves, were found to
work as sensitizers of DSC. Tennakone et al. 86) utilized successfully the natural
dye for DSC for the first time. The catechol residue in Santalin pigment (cyanidin
dye: Dye 23) extracted from red sandalwood (Pterocarpus santalinus) was
anchored on a porous TiO z thin film to form a salt with Ti IV on Ti0 2 surface.
Smestad et ai. 87 ) also reported DSC using natural flavonoid anthocyanin dye (Dye
24) extracted from California blackberry (Rubus ursinus), which shows efficiency
of 0.56% (100 mW'cm-Z). When Dye 24 forms a salt with Tpv on Ti0 2 surface,
the cyanin part is converted from the fulavynium type structure to the quinonoidal

344
20 Photoelectric Ti0 2 Solar Cells
20.S Concludmg Remarks
345
15% decrease in maximum power could be demonstrated.
The remaining problem for the liquid-based DSC is efficiency drop at higher
temperatures (over 60°C). The following line of reasoning is conceivable: dye
desorption, electrolyte decomposition due to the electrolysis at the surface of
Ti0 2 , poisoning of electrode surface, difficulty in the hermetic sealing of the
module which suppresses evaporation and leak of electrolytes, detachment of
TiOTlayer from working electrode, migration of Pt-particles into the electrolyte.
In addition to the thermal instability, low temperature causes lowering of the
solubility of the electrolyte salt species, leading to precipitation of the salts.
Solidification of the liquid electrolyte phase is a suitable solution to improve
long-term reliability for all-season, large area power applications.
Several attempts have been made to solidify the hole transport layer using p-
type semiconductors,86,90) room temperature molten salts,91) ionic conducting
polymers,9z.93) conducting organic polymers,94,95) amorphous amine,96) galated
electrolyte,97) or solid state dye-p-n-type solar cells. 98 ) However, the efficiencies
of the solid cells are unsatisfactory compared to those using the liquid phase
electrolyte. The main reason is the difficulty in homogeneous penetration of
solidified electrolytes into the mesoscopic space constructed by the dye-adsorbed
Ti0 2 nanocrystallites. Gelation of electrolyte after introduction of the liquid
electrolyte in the mesoporous space was reponed for the first time to give a
respectable efficiency at I-sun light intensity that was comparable to that of the
liquid electrolyte cells. 6o ,99) The penetrated quasi-solid state electrolyte should
play an important role in smoothing electron injection into the oxidized dye
molecules and minimizing recombination loss under high light intensity. The
higher stability of the quasi-solid state DSC was also confirmed as due to
suppression of evaporation or leak of the nitrile base electrolyte. Toshiba
groupI00.101) recently reported another efficient solid-state DSC employing a novel
polymer gel electrolyte. They used liquid electrolyte as a gel precursor consisting
of organic molten salts with high boiling point, iodide species and monomer form
of gelators, fabricated by injecting the liquid precursor into mesoporous space,
followed by heat polymerization. Interestingly, they confirmed that the resulting
gel electrolytes have three-dimensional networks at 120°C, showing electric
conductivity consistent with that of liquid electrolytes. They observed 7.3%
photoconversion efficiency in their solid system.
The research race aiming at solidification of DSC by replacing the liquid
electrolytes with solid state materials such as conductive polymers and novel
hole transport materials is still on. Tennakone disclosed the use of CuBr as an
exotic hole transport material for solidification of DSC. IOZ) Such success would
open up the possibility of a low-cost printing process to fabricate solid-state
DSC.
redox electrolytes. 6) Solid state cells, hole conductors, sensitized heterojunction.
7) Tandem devices which would be a combination cell of a dye-sensitized n-
type metal oxide and a dye-sensitized p-type metal oxide through iodideliodine
redox couple as recently reported. 104)
The authors have emphasizeu the importance of charge transport in
nanocrystalline films. To our knowledge, TiO z colloids obtainable from Solaronix
S. A. seem to give favorable Ti0 2 films that show high photoconversion
efficiency. Electron transport in the mesoporous TiO z network remains the least
well studied and understood. A theoretical understanding will lead to successful
design of highly effective porous-structured films as the electron transport layer.
Low-temperature Ti0 2 film fabrication based on an understanding of the
electron transport will also open up the possibility for further reduction in
production costs. Study of the environmental aspects of dye-sensitized solar cells
revealed that DSC is a suitable alternative for the electric generator because of its
earth-friendliness. 105 ) Based on a Life Cycle Assessment in ISO 14040 standard,
carbon dioxide emission of the cell is estimated to be 19-47 g COzlkWh, which
is 1/10 that of the gas power plant (450 g CO 2 /kWh). Toxicity of Dye 3 is
negligible because of its negative Ames test. 106 )
DSC is attractive as a low-cost photovoltaic generator. However, there exist
a number of practical difficulties in commercilizing these cells. Optically
transparent conducting glass in DSC is the most expensive component at present.
The development of inexpensive materials such as conducting plastic materials
will lead to very low-cost and lightweight DSC.107) Ruthenium complexes used for
DSC should also be replaced by new sensitized dyes having no noble metal ions.
Carbon-based counter electrodes should be used instead of platinum-coated
electrode for large-scale application of DSC. 108.109)
20.8 Concluding Remarks
20.7 Future Work on Dye-sensitized Solar Cells
To our knowledge, ECN Solar Energy in the Netherlands and a commercial
company in Europe lO8 ) are ready to produce DSC price tags for supermarket
shelves which are linked by radio to the retailer's computer system. DSC has
been confirmed to have long-term reliability for indoor use and is very sensitive
to artificial light, and the tags should be less expensive than those powered by
conventional batteries. The Institute of Applied Photovoltaics in Germany
(lNAP)llO) and Shell Corporation have targeted the outdoor solar power
marketY1,112) In addition, at IPS-2000, Dr. G. Tulloch disclosed that Sustainable
Technology in Australia (STA)113,114) is focusing on DSC wall panels for outdoor
use from the viewpoint of the following features. 1) Less affected by high angles
of incident light. 2) Better relative performance at temperatures above 30°C. 3)
Better relative performance when partially shaded or in diffuse light and hazy
conditions. Manufacturing facilities of 500 kW DSC are being established at
Queanbeyan, New South Wales, Australia, with the assistance of the Australian
Greenhouse Office. The production of .smart windows combined with DSC is
also worth noting.
In view of the recent research progress and novel applications, the authors
believe that DSC will be regarded as the next-generation solar cells that can be
popularized like electric appliances, such as TVs and video tape recorders. The
In Dr. M. Gditzel's plenary lecture at IPS-2000,103) he presented the following
research topics to improve DSC. 1) Mastering the interfaces, electron transfer
dynamics, control of dark current. 2) Charge transport in nanocrystalline films. 3)
Panchromatic sensitizers, dye cocktail, quantum dot charge injection. 4) Light
management, mixed metal oxide films, core-shell metal oxide films. 5) New

346
20 Photoelectric Ti0 2 Solar Cells
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Index
A
absorption band of the titanium oxide
178
absorption coefficient
accumulation of HN0 3
acetaldehyde 125
acetic acid 219
activated carbon (Ae)
active air purification 153
active oxygen 83
- specy 124
adsorbent 171
aggregated powder 83
Ag metal deposition 39
air mass 1.5 (AM 1.5) 329
air purifying effect 110
air purifying material 147
alkaline metal hex a-titanate (M 2 Ti 6 0 13 )
249
alkaline metal oxide
alkoxide method 34
alkoxysilane-based paint
Ah03 338
alternating irradiation method
Ames test 345
ammine ligand-based Ru complex
312
amino acid residue model
amphiphilicity 110, 114
anatase 158,209,210,230
- Ti0 2 198
annealing 36
anthocyanin 342
antifogging 118
anti-fouling affect
aromatic compound
artificial photosynthesis
311
artificial photosynthetic system
309,322
58
146
143
146
148
215
314
113
195
223, 309,
"
B
back electron transfer 295, 296
back reaction 239,241
bacteriochlorophyll 293
band bending 13,52
band gap energy 230
barium tetratitanate 249
barium titanate 251
barrier height 54
BaTi 4 0 9 249,251
Ba2 Ti 9 0 20 251
Ba4 Ti 13 0 30 251
Ba6Ti1704u 251
Ba2{n-I)Ti4n+IOlOn 251
Ba-Ti-O system 251
benzaldehyde 127 ,
benzene 129, 194
- cation radical 132
best promoter 265
bimolecular decomposition distance
314
2,2'-bipyridine 325, 338
2,2' -bipyridine-4,4'-dicarboxylic acid
327
bismuth vanadate 213
BiV0 4 213
C
cadmium sulfide 213
Calvin cycle 303
carbon dioxide 310, 316
carbonate ion 238
carbonate salt 236
carbonyl compound 194
catalysis in water oxidation 312
catalyst preparation 230
catalytic water oxidation 312
catechol 342
cavitation 203
CCl 4 138

350
Index
cell fill factor
Ce02 126
chain reaction 22
charge hopping 314
- distance 314
charge separation
charge transport
CHC1 3 136
chemical bath deposition (CBD)
89
chemical energy
chemiluminescence
chloride method 34
chloride radical (eCl)
chlorine atom (Cl)
chlorophyll 298
chloroplast - ferredoxin- hydrogenase
304
cis-2,3-epoxyhexane 196
c-2-hexene 195, 196
clean energy 248
cleavage of water 206
closed circulating system
CO 130
CO 2 310
CO 2 - 135
cobalt-tetraphenylporphyrin complex
316
co-catalyst
COCh 136
combinational reaction system
combined effect of sonolysis and
photocatalysis 220
concave site 259
contact angle 26
continuous-time random walk
conversion efficiency 15,65
conversion of solar energy 4
CO oxidation 133
CO 2 reduction 16, 316
corrosion 58
- potential 59
Cr ion-implanted titanium oxide
177
crystal form of Ti0 2
crystallinity 250
crystallite size 40, 158
cyanidin 342
cyclic cleavage of water
cytochrome C3 296, 297
328
71,232,296
314
306
84
216
138
224
229,230,234
204
334
230
261
D
dangling bond 61
87,
deactivation 229
Debye length 69
decompose pure water in liquid phase
204
decomposition of NO 179
decomposition of seawater
decomposition of water
Degussa P-25 231
dehydrochlorination reaction
dehydrogenation 42
depletion layer 70
design of photocatalyst 33
4,7 -dicarboxy-l, I O-phenanthroline
338
2,3-dichloro-5,6-dicyano-l,4-
benzoquinone (DDQ)
dichloroethene (CH 2 CC1 2 )
differential capacitance
diffusion length 58
diffusion of carrier 77
diffusion rate 227,233
di-hydroxylated compound
1,3-dihydroxynaphthalene
1,4-dihydroxynaphthalene
dinuclear complex 313
dinuclear Ru complex 315
Dion-Jacobson series 268, 270
dipole moment 257,259
direct oxidation 82
direct water splitting
dissolved oxygen 21
distorted Ti0 6 octahedron
distortion of Ti0 6 octahedron
domain structure L L 6
dye-sensitization 325
dye-sensitized solar cell 103
dye-sensitized Ti0 2 solar cell
dynamic mechanism 318
electrochemically induced chemical
deposition (EICD) 88, 94, 97
electrochemical self-assembly 88,
100, 101, 104
electrochemical Van der Waals epitaxy
93
electrodeposition (ED) 87
electron donor 54
electron-hole pair 13
electron-hole separation 77
eleen-on paramagnetic resonance (EPR) 251
electron transfer 75, 317
- distance 320
emission decay 318
emission intensity 318
energy cycle on earth 309
environmental cleaning 3
environmental purification 18
Eosin B 341
Eosin Y 340
epoxidation 194, 196
epoxide 194, 196
EPR spectra 257
Erythrosine B 34 1
ESCA measurement 181
ESR 78
- measurement L 34
ethylenediaminetetraacetic acid (EDT A)
317,322
EXAFS 267
exchange current 57
218
208
142
288
141
55
198
197
197
235
257
259
F
Fe(IIl) ion 284
Fe203 L45
Fenton reaction 197
Fermi level 52
ferredoxin 303
first and second Laws of the
thermodynamics 5
first law of thermodynamics 4
flat band potential 15, 54,231,233,
282,338
fluorescein 340
fluorescin 340
fluorocarbon polymer (PTFE) 148
2- formylcinnamaldehyde 197
friction force microscopy 115
FT-EXAFS spectrrum 181
325
E
effect of calcination 40
effect of Pt supported on Ti0 2 133
effect of size 76
effect of UV light intensity 149
effect of water vapor 129
electrical double layer 53
electro catalysis 314
electrocatalytic proton reduction
316
electrochemical atomic layer epitaxy
(ECALE) 92
electrochemical impedance spectroscopy
(EIS) 335
G
gas-phase water photolysis 223,
225,226,227,233
Index
351
Graetzel solar cell 4
grana 304
green chemistry 182, 220
green plant 303
- photo system 303
g value 252
H
H 2 204,208,321
- evolution 321
H 2 0 2 198
halogenated hydrocarbon 136
Hammett's law 164
hardened cement paste L 48
heat treatment 78
Helmholtz layer 54
2-hexene 196
high-performance liquid chromatography
197
high-pressure Hg lamp 195
high-pressure mercury lamp 197
HN0 3 143
hole injection 61
hormone disruptor .165
HyCOM Ti0 2 36,38; 39, 45 Ii I
hydration 264 . . ;, :;.-
hydrogen 204,279,285
hydrogenase 294,296, 298, 299,
302,303,304 I'
hydrogen peroxide 84,215
hydrogen photo evolution 66
hydrogen radical (eH) 207
hydrophilic 115, 116
hydrothermal treatment 37
hydroxy group 198
hydroxyl group 80, 117
hydroxylation 195
- reaction 194
hydroxyl radical (eO H) 18,81,
207
hyper fine splitting of 17 0- 255
ideality factor 58
immobilization 172
- of powder photocatalyst 147
impurity energy level 178
incident photon-to-current conversion
efficiency (IPCE) 327,329
indirect oxidation 82
indirect recombination 74
intensity-modulated photocurrent
spectroscopy (IMPS) 336

352
lndex
intensity-modulated photo voltage
spectroscopy (IMYS) 336
intercalation of water 264
interlayer 263
intermediate 162
internal field 257
ion-implanted titanium oxide
iron oxide 213
IR spectra ofTiO z
Ishihara PT -10 I
isolate Hz 215
isolate Oz 215
isotope oxygen ( I7 O z )
176
131
196
255
J
joint system of sonochemical and
photocatalytic reactions 203
Joule 3 343
K
KCa z Nb 3 0 lO
K Z C0 3 237
KzLazThOlo
Nb6017
kinetics
KSr z Nb 3 0 lO
271
274
263
72
245
L
Langmuir-Hinshelwood (L-H)
mechanism 30
layered compounds 261
layered oxides 261
life cycle assessment 324, 325
lifetime of the excited state 318
light-to-chemical conversion 223
liquid-junction 335
M
malachite green 45
Marcus-Levich-Jortner-Gerischer type of
electron transfer 331
mass transport 16
maximum cell output power 328
mediator 311
mercurochrome 341
merocyanin 341
mesoporous film 334
mesoporous structure 325
metal ion-implantation method 175,
176,180,182
metal ion-implanted titanium oxide
178
- catalysts 178
- photo catalysts
metal-loaded TiO z
metal loading 36
metallochromic 342
metal phthalocyanine
methyl radical 82
methylviologen 294, 317
microelectrode 19
microparticle 69
micro-PEC cell 223
microwave conductivity
mineralization 38,42
molecular oxygen ] 94, 195
mono-hydroxylated naphthalene
198
mononuclear complex
Mott-Schottky plot
Mott transition 334
MRSA 112, 113
MYz+ 317,320
- radical 194
.OH radical 197
olefin compound 194 i I
oleophilic 115, 116
open-circuit photovoltage 58, 328.
optically transparent conductive-glass
electrode (OTE) 328
optimization ofNaOH-loading -,' 227
0- radical 259 I
organic compound 219
outdoor solar light irradiation 180,
182
overall water splitting 206, 208,
213,215,275,276
overlapping integral 330
overvoltage 57
oxidant 167
oxidative quenching reaction 302
oxide semiconductor photocatalyst
235
oxidized naphthalene 197
- compound 197
oxygen 204,284
- evolution 39
- radical 79
- vacancy 26
ozone 169
179, 182
72
316
335
315
56
N
NaCI 216
Na Z C0 3 237
- addition method 235,242,246,
248
Nafion 314
NaHC0 3 237
(nanometer-sized) metal dots 64
nanorod 61, 62
nanostructure 59
NaOH-coating method 223
naphthalene 197
I-naphthol 197
2-naphthol 197
nest model 259
NiO 265
NiO)Nb6017 244
NiO)TazOs 243
NiO)TiO z 230,231,241
3wt%NiO)TiO z 246,247
NiOiZrOz 244
nitrogen dioxide 143
nitrogen oxide (NOx) 143,230
nitro toluene 128
number of electrons 43
p
Oz 204,208,310
0 3 - 134, 252
Oz adsorption
Oz evolution
OH 125
P-25 38,337
particle size 208,210,231,257
passive purification of polluted air
151
pentagonal prism tunnel structure
249
perovskite-related layered oxides
268
peroxycarbonate formation 242
Perrin mechanism 318, 319
perylene 340
pesticide 162
1,10-phenanthroline
9-phenyl xanthene
Phloxine B 341
photoadsorption of Oz
photo anode 14
photocatalytic activity ofTiO z
photocatalytic cleavage of water
270
photocatalytic decomposition of NO
180
photocatalytic decomposition of water .
239
326, 338
340
241
o
124
233
314
Index
353
photocatalytic overall water splitting
265
photocatalytic reaction I 2, 176, 311
photochemical catalysis, 321
photochemical energy conversion 2,
311
photochemical reaction 310
photochemical solar energy conversion
1
photocorrosion 14, 58
photocurrent 58,320
- density 328
- -voltage profile 328
photodecomposition 223
photo electrochemical impedance
spectroscopy (PElS) 336
photoelectrochemical (PEe) solar cell
62,64
photo electrochemistry 3,9
photoelectrolysis 11
photoenergy conversion efficiency
328
photo etching 63
photo excitation center 311
photo excited intramolecular electron
transfer 300
photo excited singlet state 299, 300,
302,303
photoexcited triplet state 295, 298,
299,300,301,303
photogenerated electron 194
photogenerated hole 194
photoinduced charge separation 2
photoinduced hydrogen evolution
294,296,298,299,302,303,304,
306
Photo-Kolbe reaction 219
photolysis 3
photon flux III
photooxidation of water 282
photoreduction of CO z 66
photoreduction of water 285
photoregenerative solar cell 321
photosensitizer 294, 296, 299
photosynthesis 2, 5, 10, 220, 280,
293,310
- system 306
photosynthetic reaction center 293
photo system 280
photovoltaic cell 10
phthalocyanine 340
physical property 39,40
pillared Ca z Nb 3 0 lO 271

pillaring 271
p-n junction 334
Poisson-Boltzmann equation 69
polymeric compound 130
polymerized complex (PC) method
276
porous structure 148
porphyrin 340
- metal complex 294,299,306
potassium bromide 285
preparation of titanium (IV) oxide
33
product distribution L 7
product ratio 210,212
promoter 250
propylene 125
proton 311
- reduction 316
Pt 316
P-25 Ti0 2 34
PUTi0 2 223,236,240,246
Pt-loaded Ti0 2 286, 289
Pt-loading 166
Pt/Zr02 243
Pyrogallol Red 341
Q
Q-bands 340
quantum efficiency
266,268
quantum size effect 76
quantum yield 23, 74, 198,227,
232,296
quenching efficiency 295
quenching of the emission
quenching rate constant
quenching sphere 318
quinone 281,288
196,246,247,
318
295
R
radical scavenger 22
Raman spectrum 255
random walk 334
RbPb 2 Nb 3 0 lO 272
reaction intermediate 77
reaction rate 73
reaction scheme 73
- of NO x removal 146
recombination 23,30,32,41, 73,
241
- kinetics 39
- reaction 266
rectangular tunnel structure 249
rectifying property
redox potential
reduction 311
reductive quenching reaction
renewable energy resource 1
reverse reaction 225, 225, 226, 229,
58
53,287
180
solar beam irradiation
solar cell 321
solar energy 279, 293, 306, 311
- conversion 1,213,309
- conversion efficiency 62
solar hydrogen 235
- production 246, 247
sonication 206
sonochemical reaction 206
sonochemical reduction of CO 2
sonolysis 204
sonophotocatalysis 203, 208
- of artificial seawater 216
- of organic compounds 219
- of water 204
sonophotocatalytic reaction
Soret band 340
space charge layer
specific surface area
splitting of water
SrTi0 3 232
ST-Ol 38,42
stain-proofing 118
static mechanism 318
stereospecific 194
stereospecifically 194
stereospecific epoxidation
sterilization III
Stern-Volmer constant 318
Stern-Volmer equation 318
successive ionic layer adsorption and
reaction (SILAR) 90, 92
sulfate method 33
sunlight 171
superhydrophilicity
superoxide 19
supporting material 212
surface area 30,40, 76, 210
surface band position 56
surface charge 56
surface dipole 56
surface lattice 0- 255
- radical 257,258
surface peroxide formation
surface state 59, 65
surface wettability 115
302
230
Rhodamine 6G 341
Rhodamine B 326,340
RhlSrTi0 3 244
room temperature molten salts 344
Rose Bengal 327,341
roughness factor 328
Ru(bpY)3 2 + 317, 320, 321, 322
Ruddlesden-Popper 270
- series 268,274
Ru-doped Ti0 2 291
RU02 230, 250
RuOzlBaTi 4 0 9 244
RuOzlNa2Ti6013 244
RU02 particle 251
RU02 promoter 249
RuOzlTa20S 243
RuOzlTi0 2 241,242
3wt%RuOzlTi0 2 246
Ru-red 312,314
rutile 158,209,210,230
- Ti0 2 198
220
206
12,54,70,232
209,211,212
279
195
s
sacrificial electron donor 321, 322
Schottky barrier 52, 335
seawater 216
second-generation titanium oxide
photocatalyst 175, L 79
selective oxidation 196, 197
self-cleaning 24, 113
semiconductor 3, 194, 311
- electrode 11
- particle 3
sensitization 4
- ofTi0 2 320
sensitizer 2, 4, 311
sensitizing dye 325
simple oxide semiconductor
photocatalyst 248
simultaneous irradiation of ultrasound
and light 205
size quantization effect 72
smart window 345
smear-resistant effect 120
Sn02 326,327,340
sodium chloride 216
25
241
T
TEM 267
temperature dependence
tetrachloroethylene (PCE)
thin film 45
TiCl 4 337
232,233
136, 160
Ti0 2 1,3,12,29,205,208,281,
321
- activation
- electrode
- film 45
- microparticle
- thin film 44
titanium dioxide I, 29
titanium oxide photocatalyst
toluene 127, 194
total carbon balance
traffic tunnel 153
trans-2,3-epoxyhexane 196
trans-2-hexene 195, 196
trans isomer 196
trans selectivity 196
transition 334
- metal ion 177
transparent 24
trapped electron 78
trapped hole 79, 80
4,4' ,4"-tricarboxy- 2,2': 6' ,2"-terpyridine
337
I, I, I-trichloroethane
trichloroethylene (TCE)
trinuclear 313
- Ru complex 315
triphenylmethane 342
tris(2,2'-bipyridine) ruthenium
tryptophan residue model 320
tunnel structure 249
tyrosine residue model
36
282
4
175
130
136, 141
136, 160
317
315
u
ultrafast laser spectroscopy 32
ultrasound 203
underpotential deposition 92
UralUline 340
UV light water photolysis 320
V
viologen-linked porphyrin metal
complex 299
V ion-implanted titanium oxide catalyst
181
V ion-implanted titanium oxide
photocatalyst 180
visible light 213
- irradiation 176, 178, 182
visiblization 37
Voc's 64
volatile hydrocarbon 125

w
XYZ
water 195,311
- cleavage 1
- oxidation 85,311,312
- photolysis 1,3,223,316
- photooxidation 63
- splitting 248
wettability 25
W0 3 291
XANES spectrum 181
xenon lamp 203
XPS 240, 267
XRF 240
yield of water photolysis 229
ZnO 126,341
Zr02 243,246,338
Z-scheme 287