On Solar Hydrogen & Nanotechnology || Front Matter

ON SOLAR HYDROGEN& NANOTECHNOLOGY

On Solar Hydrogen & Nanotechnology Edited by Lionel Vayssieres

© 2009 John Wiley & Sons (Asia) Pte Ltd. ISBN: 978-0-470-82397-2

ON SOLAR HYDROGEN& NANOTECHNOLOGY

Editor

Lionel Vayssieres

National Institute for Materials Science, Japan

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Library of Congress Cataloging-in-Publication Data

On solar hydrogen & nanotechnology / editor, Lionel Vayssieres.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-82397-2 (cloth)

1. Solar energy. 2. Nanotechnology. 3. Hydrogen as fuel. 4. Water oxidation. 5. Photocatalysis.

I. Vayssieres, Lionel, 1968-

TJ810.O52 2010

621.47–dc22

2009040144

ISBN 978-0-470-82397-2 (HB)

Typeset in 10/12pt Times by Thomson Digital, Noida, India.

Printed and bound in Singapore by Markono Print Media Pte Ltd, Singapore.

This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least

two trees are planted for each one used for paper production.

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Contents

List of Contributors xvii

Preface xix

Editor Biography xxiii

PART ONE—FUNDAMENTALS, MODELING, AND EXPERIMENTALINVESTIGATION OF PHOTOCATALYTIC REACTIONS FOR DIRECT

SOLAR HYDROGEN GENERATION

1 Solar Hydrogen Production by Photoelectrochemical

Water Splitting: The Promise and Challenge 3

Eric L. Miller

1.1 Introduction 3

1.2 Hydrogen or Hype? 4

1.3 Solar Pathways to Hydrogen 5

1.3.1 The Solar Resource 5

1.3.2 Converting Sunlight 6

1.3.3 Solar-Thermal Conversion 7

1.3.4 Solar-Potential Conversion 8

1.3.5 Pathways to Hydrogen 9

1.4 Photoelectrochemical Water-Splitting 10

1.4.1 Photoelectrochemistry 10

1.4.2 PEC Water-Splitting Reactions 10

1.4.3 Solar-to-Hydrogen Conversion Efficiency 13

1.4.4 Fundamental Process Steps 14

1.5 The Semiconductor/Electrolyte Interface 14

1.5.1 Rectifying Junctions 14

1.5.2 A Solid-State Analogy: The npþ Junction 15

1.5.3 PEC Junction Formation 17

1.5.4 Illuminated Characteristics 19

1.5.5 Fundamental Process Steps 20

1.6 Photoelectrode Implementations 23

1.6.1 Single-Junction Performance Limits 23

1.6.2 Multijunction Performance Limits 24

1.6.3 A Shining Example 27

1.7 The PEC Challenge 28

1.7.1 What’s Needed, Really? 28

1.7.2 Tradeoffs and Compromises 29

1.7.3 The Race with PV-Electrolysis 29

1.8 Facing the Challenge: Current PEC Materials Research 29

Acknowledgments 32

References 32

2 Modeling and Simulation of Photocatalytic Reactions at TiO2 Surfaces 37Hideyuki Kamisaka and Koichi Yamashita

2.1 Importance of Theoretical Studies on TiO2 Systems 37

2.2 Doped TiO2 Systems: Carbon and Niobium Doping 39

2.2.1 First-Principle Calculations on TiO2 39

2.2.2 C-Doped TiO2 41

2.2.3 Nb-Doped TiO2 45

2.3 Surface Hydroxyl Groups and the Photoinduced Hydrophilicity of TiO2 51

2.3.1 Speculated Active Species on TiO2 – Superoxide Anion (O2�)

and the Hydroxyl Radical (OH.) 51

2.3.2 Theoretical Calculations of TiO2 Surfaces and Adsorbents 51

2.3.3 Surface Hydroxyl Groups and Photoinduced Hydrophilic

Conversion 53

2.4 Dye-Sensitized Solar Cells 58

2.4.1 Conventional Sensitizers: Ruthenium Compounds and Organic Dyes 58

2.4.2 Multiexciton Generation in Quantum Dots: A Novel Sensitizer

for a DSSC 59

2.4.3 Theoretical Estimation of the Decoherence Time between

the Electronic States in PbSe QDs 60

2.5 Future Directions: Ab Initio Simulations and the Local

Excited States on TiO2 63

2.5.1 Improvement of the DFT Functional 64

2.5.2 Molecular Mechanics and Ab Initio Molecular Dynamics 65

2.5.3 Description of Local Excited States 66

2.5.4 Nonadiabatic Behavior of a System and Interfacial

Electron Transfer 67

Acknowledgments 68

References 68

3 Photocatalytic Reactions on Model Single Crystal TiO2 Surfaces 77G.I.N. Waterhouse and H. Idriss

3.1 TiO2 Single-Crystal Surfaces 78

3.2 Photoreactions Over Semiconductor Surfaces 80

vi Contents

3.3 Ethanol Reactions Over TiO2(110) Surface 81

3.4 Photocatalysis and Structure Sensitivity 83

3.5 Hydrogen Production from Ethanol Over Au/TiO2 Catalysts 84

3.6 Conclusions 87

References 87

4 Fundamental Reactions on Rutile TiO2(110) Model

Photocatalysts Studied by High-Resolution Scanning

Tunneling Microscopy 91

Stefan Wendt, Ronnie T. Vang, and Flemming Besenbacher

4.1 Introduction 91

4.2 Geometric Structure and Defects of the Rutile

TiO2 (110) Surface 93

4.3 Reactions of Water with Oxygen Vacancies 96

4.4 Splitting of Paired H Adatoms and Other Reactions Observed on Partly

Water Covered TiO2(110) 98

4.5 O2 Dissociation and the Role of Ti Interstitials 101

4.6 Intermediate Steps of the Reaction Between O2 and H Adatoms

and the Role of Coadsorbed Water 106

4.7 Bonding of Gold Nanoparticles on TiO2(110)

in Different Oxidation States 112

4.8 Summary and Outlook 115

References 117

PART TWO—ELECTRONIC STRUCTURE, ENERGETICS,

AND TRANSPORT DYNAMICS OF PHOTOCATALYST

NANOSTRUCTURES

5 Electronic Structure Study of Nanostructured Transition

Metal Oxides Using Soft X-Ray Spectroscopy 125

Jinghua Guo, Per-Anders Glans, Yi-Sheng Liu,

and Chinglin Chang

5.1 Introduction 125

5.2 Soft X-Ray Spectroscopy 126

5.2.1 Soft X-Ray Absorption and Emission Spectroscopy 126

5.2.2 Resonantly Excited Soft X-Ray Emission

Spectroscopy 127

5.3 Experiment Set-Up 127

5.3.1 Beamline 128

5.3.2 Spectrometer and Endstation 129

5.3.3 Sample Arrangements 131

5.4 Results and Discussion 132

Acknowledgments 139

References 139

Contents vii

6 X-ray and Electron Spectroscopy Studies of Oxide Semiconductors

for Photoelectrochemical Hydrogen Production 143

Clemens Heske, Lothar Weinhardt, and Marcus B€ar


6.2 Soft X-Ray and Electron Spectroscopies 145

6.3 Electronic Surface-Level Positions of WO3 Thin Films 147

6.3.1 Introduction 147

6.3.2 Sample Handling and the Influence of X-Rays, UV-Light

and Low-Energy Electrons on the Properties of the WO3 Surface 147

6.3.3 Surface Band Edge Positions in Vacuum – Determination

with UPS/IPES 149

6.3.4 Estimated Surface Band-Edge Positions in Electrolyte 151

6.3.5 Conclusions 153

6.4 Soft X-Ray Spectroscopy of ZnO:Zn3N2 Thin Films 154


6.4.2 The O K XES Spectrum of ZnO:N Thin Films – Determination

of the Valence Band Maximum 154

6.4.3 The Impact of Air Exposure on the Chemical Structure

of ZnO:N Thin Films 155

6.4.4 Conclusions 157

6.5 In Situ Soft X-Ray Spectroscopy: A Brief Outlook 158

6.6 Summary 158

Acknowledgments 159

References 159

7 Applications of X-Ray Transient Absorption Spectroscopy

in Photocatalysis for Hydrogen Generation 163Lin X. Chen


7.2 X-Ray Transient Absorption Spectroscopy (XTA) 165

7.3 Tracking Electronic and Nuclear Configurations in Photoexcited

Metalloporphyrins 171

7.4 Tracking Metal-Center Oxidation States in the MLCT State

of Metal Complexes 176

7.5 Tracking Transient Metal Oxidation States During Hydrogen Generation 178

7.6 Prospects and Challenges in Future Studies 180

Acknowledgments 181

References 181

8 Fourier-Transform Infrared and Raman Spectroscopy of Pure

and Doped TiO2 Photocatalysts 189

Lars Osterlund


8.2 Vibrational Spectroscopy on TiO2 Photocatalysts: Experimental

Considerations 191

viii Contents

8.3 Raman Spectroscopy of Pure and Doped TiO2 Nanoparticles 195

8.4 Gas–Solid Photocatalytic Reactions Probed by FTIR Spectroscopy 199

8.5 Model Gas–Solid Reactions on Pure and Doped TiO2

Nanoparticles Studied by FTIR Spectroscopy 205

8.5.1 Reactions with Formic Acid 205

8.5.2 Reactions with Acetone 221

8.6 Summary and Concluding Remarks 229

Acknowledgments 230

References 230

9 Interfacial Electron Transfer Reactions in CdS QuantumDot Sensitized TiO2 Nanocrystalline Electrodes 239

Yasuhiro Tachibana


9.2 Nanomaterials 240

9.2.1 Semiconductor Quantum Dots 240

9.2.2 Metal Oxide Nanocrystalline Semiconductor Films 241

9.2.3 QD Sensitized Metal Oxide Semiconductor Films 242

9.3 Transient Absorption Spectroscopy 245

9.3.1 Principle 245

9.3.2 Calculation of Absorption Difference 245

9.3.3 System Arrangement 246

9.4 Controlling Interfacial Electron Transfer Reactions

by Nanomaterial Design 247

9.4.1 QD/Metal-Oxide Interface 248

9.4.2 QD/Electrolyte Interface 250

9.4.3 Conducting Glass/Electrolyte Interface 252

9.5 Application of QD-Sensitized Metal-Oxide Semiconductors to Solar

Hydrogen Production 258

9.6 Conclusion 260

Acknowledgments 260

References 260

PART THREE—DEVELOPMENT OF ADVANCED NANOSTRUCTURES

FOR EFFICIENT SOLAR HYDROGEN PRODUCTION FROM CLASSICALLARGE BANDGAP SEMICONDUCTORS

10 Ordered Titanium Dioxide Nanotubular Arrays as Photoanodes

for Hydrogen Generation 267

M. Misra and K.S. Raja


10.2 Crystal Structure of TiO2 268

10.2.1 Electronic and Defect Structure of TiO2 269

10.2.2 Preparation of TiO2 Nanotubes 272

10.2.3 Energetics of Photodecomposition of Water on TiO2 279

References 288

Contents ix

11 Electrodeposition of Nanostructured ZnO Films and Their

Photoelectrochemical Properties 291

Torsten Oekermann


11.2 Fundamentals of Electrochemical Deposition 292

11.3 Electrodeposition of Metal Oxides and Other Compounds 294

11.4 Electrodeposition of Zinc Oxide 295

11.4.1 Electrodeposition of Pure ZnO 295

11.4.2 Electrodeposition of Doped ZnO 297

11.4.3 P-n-Junctions Based on Electrodeposited ZnO 298

11.5 Electrodeposition of One- and Two-Dimensional ZnO Nanostructures 298

11.5.1 ZnO Nanorods 298

11.5.2 ZnO Nanotubes 301

11.5.3 Two-Dimensional ZnO Nanostructures 302

11.6 Use of Additives in ZnO Electrodeposition 303

11.6.1 Dye Molecules as Structure-Directing Additives 303

11.6.2 ZnO Electrodeposition with Surfactants 307

11.6.3 Other Additives 311

11.7 Photoelectrochemical and Photovoltaic Properties 312

11.7.1 Dye-Sensitized Solar Cells (DSSCs) 312

11.7.2 Photoelectrochemical Investigation of the Electron Transport

in Porous ZnO Films 316

11.7.3 Performance of Nanoporous Electrodeposited

ZnO Films in DSSCs 320

11.7.4 Use of ZnO Nanorods in Photovoltaics 321

11.8 Photocatalytic Properties 322

11.9 Outlook 323

References 323

12 Nanostructured Thin-Film WO3 Photoanodes for Solar Water

and Sea-Water Splitting 333

Bruce D. Alexander and Jan Augustynski

12.1 Historical Context 333

12.2 Macrocrystalline WO3 Films 334

12.3 Limitations of Macroscopic WO3 336

12.4 Nanostructured Films 336

12.5 Tailoring WO3 Films Through a Modified Chimie Douce Synthetic Route 339

12.6 Surface Reactions at Nanocrystalline WO3 Electrodes 342

12.7 Conclusions and Outlook 345

References 346

13 Nanostructured a-Fe2O3 in PEC Generation of Hydrogen 349Vibha R. Satsangi, Sahab Dass, and Rohit Shrivastav


13.2 a-Fe2O3 350

x Contents

13.2.1 Structural and Electrical/Electronic Properties 350

13.2.2 a-Fe2O3 in PEC Splitting of Water 351

13.3 Nanostructured a-Fe2O3 Photoelectrodes 352

13.3.1 Preparation Techniques and Photoelectrochemical Response 353

13.3.2 Flatband Potential and Donor Density 365

13.4 Strategies to Enhance Photoresponse 368

13.4.1 Doping 368

13.4.2 Choice of Electrolytes 373

13.4.3 Dye Sensitizers 374

13.4.4 Porosity 375

13.4.5 Forward/Backward Illumination 375

13.4.6 Loading of Metal/Metal Oxide 377

13.4.7 Layered Structures 377

13.4.8 Deposition of Zn Islands 380

13.4.9 Swift Heavy Ion (SHI) Irradiation 382

13.4.10 p/n Assemblies 385

13.5 Efficiency and Hydrogen Production 386

13.6 Concluding Remarks 388

Acknowledgments 393

References 393

PART FOUR—NEW DESIGN AND APPROACHES TO BANDGAP

PROFILING AND VISIBLE-LIGHT-ACTIVE NANOSTRUCTURES

14 Photoelectrocatalyst Discovery Using High-Throughput Methods

and Combinatorial Chemistry 401Alan Kleiman-Shwarsctein, Peng Zhang, Yongsheng Hu,

and Eric W. McFarland


14.2 The Use of High-Throughput and Combinatorial Methods for the

Discovery and Optimization of Photoelectrocatalyst Material Systems 402

14.2.1 The Use of High-Throughput and Combinatorial Methods

in Materials Science 402

14.2.2 HTE Applications to PEC Discovery 405

14.2.3 Absorbers 408

14.2.4 Bulk Carrier Transport 411

14.2.5 Electrocatalysts 412

14.2.6 Morphology and Material System 412

14.2.7 Library Format, Data Management and Analysis 414

14.3 Practical Methods of High-Throughput Synthesis of Photoelectrocatalysts 415

14.3.1 Vapor Deposition 416

14.3.2 Liquid Phase Synthesis 417

14.3.3 Electrochemical Synthesis 419

14.3.4 Spray Pyrolysis 422

Contents xi

14.4 Photocatalyst Screening and Characterization 423

14.4.1 High-Throughput Screening 424

14.4.2 Secondary Screening and Quantitative Characterization 432

14.5 Specific Examples of High-Throughput Methodology

Applied to Photoelectrocatalysts 437

14.5.1 Solar Absorbers 437

14.5.2 Improving Charge-Transfer Efficiency 443

14.5.3 Improved PEC Electrocatalysts 448

14.5.4 Design and Assembly of a Complete Nanostructured

Photocatalytic Unit 451

14.6 Summary and Outlook 453

References 454

15 Multidimensional Nanostructures for Solar Water Splitting:

Synthesis, Properties, and Applications 459

Abraham Wolcott and Jin Z. Zhang

15.1 Motivation for Developing Metal-Oxide Nanostructures 459


15.1.2 PEC Water Splitting for Hydrogen Production 460

15.1.3 Metal-Oxide PEC Cells 460

15.1.4 Dye and QD Sensitization 462

15.1.5 Deposition Techniques for Metal Oxides 462

15.2 Colloidal Methods for 0D Metal-Oxide Nanoparticle Synthesis 463

15.2.1 Colloidal Nanoparticles 463

15.2.2 TiO2 Sol-Gel Synthesis 464

15.2.3 TiO2 Hydrothermal Synthesis 465

15.2.4 TiO2 Solvothermal and Sonochemical Synthesis 466

15.2.5 TiO2 Template-Driven Synthesis 468

15.2.6 Sol-Gel WO3 Colloidal Synthesis 470

15.2.7 WO3 Hydrothermal Synthesis 470

15.2.8 WO3 Solvothermal and Sonochemical Synthesis 470

15.2.9 WO3 Template Driven Synthesis 471

15.2.10 ZnO Sol-Gel Nanoparticle Synthesis 473

15.2.11 ZnO Hydrothermal Synthesis 474

15.2.12 ZnO Solvothermal and Sonochemical Synthesis 475

15.2.13 ZnO Template-Driven Synthesis 479

15.3 1D Metal-Oxide Nanostructures 481

15.3.1 Colloidal Synthesis and Fabrication 481

15.3.2 Synthesis and Fabrication of 1D TiO2 Nanostructures 481

15.3.3 Colloidal Synthesis and Fabrication of 1D WO3 Nanostructures 486

15.3.4 Colloidal Synthesis and Fabrication of 1D ZnO Nanostructures 487

15.4 2D Metal-Oxide Nanostructures 488

15.4.1 Colloidal Synthesis of 2D TiO2 Nanostructures 488

15.4.2 Colloidal Synthesis of 2D WO3 Nanostructures 490

15.4.3 Colloidal Synthesis of 2D ZnO Nanostructures 491

xii Contents

15.5 Conclusion 492

Acknowledgments 493

References 493

16 Nanoparticle-Assembled Catalysts for Photochemical

Water Splitting 507

Frank E. Osterloh


16.2 Two-Component Catalysts 509

16.2.1 Synthetic and Structural Aspects 509

16.2.2 Photocatalytic Hydrogen Evolution 511

16.2.3 Peroxide Formation 513

16.2.4 Water Electrolysis 515

16.3 CdSe Nanoribbons as a Quantum-Confined

Water-Splitting Catalyst 516

16.4 Conclusion and Outlook 518

Acknowledgment 519

References 519

17 Quantum-Confined Visible-Light-Active Metal-Oxide

Nanostructures for Direct Solar-to-Hydrogen

Generation 523

Lionel Vayssieres


17.2 Design of Advanced Semiconductor Nanostructures

by Cost-Effective Technique 524

17.2.1 Concepts and Experimental Set-Up of Aqueous

Chemical Growth 524

17.2.2 Achievements in Aqueous Design of Highly Oriented

Metal-Oxide Arrays 528

17.3 Quantum Confinement Effects for Photovoltaics

and Solar Hydrogen Generation 529

17.3.1 Multiple Exciton Generation 530

17.3.2 Quantum-Well Structures 531

17.3.3 Intermediate Band Materials 531

17.4 Novel Cost-Effective Visible-Light-Active (Hetero)Nanostructures

for Solar Hydrogen Generation 533

17.4.1 Iron-Oxide Quantum-Rod Arrays 533

17.4.2 Doped Iron-Oxide Quantum-Rod Arrays 541

17.4.3 Quantum-Dot–Quantum-Rod Iron-Oxide

Heteronanostructure Arrays 545

17.4.4 Iron Oxide Oriented Porous Nanostructures 546

17.5 Conclusion and Perspectives 548

References 548

Contents xiii

18 Effects of Metal-Ion Doping, Removal and Exchange on Photocatalytic

Activity of Metal Oxides and Nitrides for Overall Water Splitting 559

Yasunobu Inoue


18.2 Experimental Procedures 561

18.3 Effects of Metal Ion Doping 561

18.3.1 Sr2þ Ion-Doped CeO2 561

18.3.2 Metal-Ion Doped GaN 564

18.4 Effects of Metal-Ion Removal 569

18.5 Effects of Metal-Ion Exchange on Photocatalysis 573

18.5.1 YxIn2�xO3 573

18.5.2 ScxIn2�xO3 580

18.5.3 YxIn2�xGe2O7 582

18.6 Effects of Zn Addition to Indate and Stannate 583

18.6.1 Li1.6Zn1.6Sn2.8O8 584

18.6.2 Ba3Zn5In2O11 584

18.7 Conclusions 585

Acknowledgments 586

References 586

19 Supramolecular Complexes as Photoinitiated Electron Collectors:

Applications in Solar Hydrogen Production 589

Shamindri M. Arachchige and Karen J. Brewer


19.1.1 Solar Water Splitting 589

19.1.2 Supramolecular Complexes and Photochemical

Molecular Devices 590

19.1.3 Polyazine Light Absorbers 591

19.1.4 Polyazine Bridging Ligands to Construct Photochemical

Molecular Devices 594

19.1.5 Multi-Component System for Visible Light Reduction of Water 595

19.1.6 Photoinitiated Charge Separation 596

19.2 Supramolecular Complexes for Photoinitiated

Electron Collection 598

19.2.1 Photoinitiated Electron Collection on a Bridging Ligand 598

19.2.2 Ruthenium Polyazine Light Absorbers Coupled Through

an Aromatic Bridging Ligand 600

19.2.3 Photoinitiated Electron Collection on a Platinum Metal 602

19.2.4 Two-Electron Mixed-Valence Complexes for Multielectron

Photochemistry 604

19.2.5 Rhodium-Centered Electron Collectors 605

19.2.6 Mixed-Metal Systems for Solar Hydrogen Production 613


List of Abbreviations 616

Acknowledgments 616

References 617

xiv Contents

PART FIVE—NEW DEVICES FOR SOLAR THERMAL

HYDROGEN GENERATION

20 Novel Monolithic Reactors for Solar Thermochemical Water Splitting 623

Athanasios G. Konstandopoulos and Souzana Lorentzou


20.1.1 Energy Production and Nanotechnology 623

20.1.2 Application of Solar Technologies 624

20.2 Solar Hydrogen Production 624

20.2.1 Solar Hydrogen Production: Thermochemical Processes 625

20.2.2 Solar Chemical Reactors 626

20.3 HYDROSOL Reactor 627

20.3.1 The Idea 627

20.3.2 Redox Materials 627

20.3.3 Water Splitting: Laboratory Tests 629

20.3.4 HYDROSOL Reactors 630

20.3.5 Solar Testing 631

20.3.6 Simulation 633

20.3.7 Future Developments 636

20.4 HYDROSOL Process 636


Acknowledgments 638

References 638

21 Solar Thermal and Efficient Solar Thermal/Electrochemical

Photo Hydrogen Generation 641Stuart Licht

21.1 Comparison of Solar Hydrogen Processes 641

21.2 STEP (Solar Thermal Electrochemical Photo) Generation of H2 646

21.3 STEP Theory 648

21.4 STEP Experiment: Efficient Solar Water Splitting 653

21.5 NonHybrid Solar Thermal Processes 657

21.5.1 Direct Solar Thermal Hydrogen Generation 657

21.5.2 Indirect (Multistep) Solar Thermal H2 Generation 659


References 661

Index 665

Contents xv

List of Contributors

Bruce D. Alexander, University of Greenwich, UK.

Shamindri M. Arachchige, Virginia Polytechnic Institute and State University, USA.

Jan Augustynski, Warsaw University, Poland. Email: [email protected]

Marcus B€ar, Helmholtz-Zentrum Berlin f€ur Materialien und Energie, Berlin, Germany.

Email: [email protected]

Flemming Besenbacher, Aarhus University, Denmark. Email: [email protected]

Karen J. Brewer, Virginia Polytechnic Institute and State University, USA.


Lin X. Chen, Argonne National Laboratory, USA; Northwestern University, USA.


Chinglin Chang, Tamkang University, Taiwan.

Sahab Dass, Dayalbagh Educational Institute, India.

Per-Anders Glans, Lawrence Berkeley National Laboratory, USA.

Jinghua Guo, Lawrence Berkeley National Laboratory, USA. Email: [email protected]

Clemens Heske, University of Nevada, Las Vegas, USA.


Yongsheng Hu, University of California, Santa Barbara, USA.

Hicham Idriss, University of Aberdeen, UK; Robert Gordon University, UK.


Yasunobu Inoue, Nagaoka University of Technology, Japan.


Hideyuki Kamisaka, The University of Tokyo, Japan.

Alan Kleiman-Shwarsctein, University of California, Santa Barbara, USA.

Athanasios G. Konstandopoulos, CERTH/CPERI, Greece; Aristotle University, Greece.


Stuart Licht, The George Washington University, USA. Email: [email protected]

Yi-Sheng Liu, Tamkang University, Taiwan.

Souzana Lorentzou, CERTH/CPERI, Greece; Aristotle University, Greece.

Eric W. McFarland, University of California, Santa Barbara, USA.


Eric L. Miller, Hawaii Natural Energy Institute, USA. Email: [email protected]

Manoranjan Misra, University of Nevada, Reno, USA. Email: [email protected]

Torsten Oekermann, Leibniz Universit€at Hannover, Germany.


Frank Osterloh, University of California, Davis, USA. Email: [email protected]

Lars Osterlund, Totalf€orsvarets forskningsinstitut (FOI), Sweden; Uppsala University,Sweden. Email: [email protected]

Krishnan S. Raja, University of Nevada, Reno, USA.

Vibha Rani Satsangi, Dayalbagh Educational Institute, India.


Rohit Shrivastav, Dayalbagh Educational Institute, India.

Yasuhiro Tachibana, Osaka University, Japan; RMIT Australia.


Ronnie T. Vang, Aarhus University, Denmark.

Lionel Vayssieres, National Institute for Materials Science, Japan.


Geoff Waterhouse, University of Auckland, New Zealand.

Lothar Weinhardt, Universit€at W€urzburg, Germany.

Stefan Wendt, Aarhus University, Denmark.

Abraham Wolcott, University of California, Santa Cruz, USA.

Koichi Yamashita, The University of Tokyo, Japan.


Jin Z. Zhang, University of California, Santa Cruz, USA. Email: [email protected]

Peng Zhang, University of California, Santa Barbara, USA.

xviii List of Contributors

Preface

Finding new ways to power the future by making cleaner and safer energy without creating

additional CO2 in the atmosphere is one, if not simply the most important, problem facing

humanity today. Indeed, meeting the current and future demand for energy, which according to

various sources should double by 2050, while managing the environmental consequences of

energy production and consumption, is of crucial importance nowadays, with the industrial and

economic rise of highly populated countries such as India and China. Reducing urban air

pollution and the build-up of greenhouse gases that threaten severe climate change due to

global warming, and lowering dependence on foreign oil is of utmost importance. Indeed, the

transition from fossil fuels to hydrogen is of revolutionary importance, not only for its societal

impact, but also for the new discovery of materials for renewable energy. Energy is not just a

part of the world economy, it is the economy.

If one considers the largest and geographically balanced free resource available on earth, that

is seawater, and that more sunlight energy is striking our blue planet in one hour than all of our

annual energy consumption, the direct solar-to-hydrogen conversion by photo-oxidation of

water is a very straightforward and attractive solution for the production of hydrogen, as it is

clean, sustainable and renewable and so without the use of a sacrificial agent. It offers an

alternative solution to fossil-fuel-based energy sources and explains the tremendous interest in

renewable, sustainable energy sources and materials for energy conversion.

Indeed,materials – natural or manmade, that is, in raw or engineered forms – are the key to

technological development and industrial revolution and thus, to stable, stronger and more

secure societies. The ages of ancient civilization are remembered by theirmaterials. Prehistoric

eras are named after thematerials that defined their technological status, for example, the Stone

Age, the Bronze Age, the Iron Age. Truly, the present age will be remembered as the

Nanomaterials Age, as they should provide our society with the necessary tools to supply

renewable and sustainable energy sources and energy-source carriers, while protecting the

environment and providing better health, if scientists manage to unravel their full potential

while understanding and managing their eventual toxicity.

Newmaterials bring new knowledge and newfields of science, which in turn evolve into new

technologies. In many respects, materials can be considered as the parents of almost all

technologies. Most technological breakthroughs have been achieved by the development of

newmaterials (and technologiesbasedon them).Newtechnologiescreatenewindustries,which

in turn provide jobs, as well as better and more secure living conditions and societies. It is well

established by economists (and not just scientists) that long-term economic growth depends

on innovation and technological progress (which account for about 90% of the economic

growth).

However, to address these new challenges, current materials and conventional technologies

are simply not good enough. The necessity of materials development which is not limited to

materials that can achieve their intrinsic theoretical limits, but makes it possible to raise those

limits by changing the fundamental underlying physics and chemistry is crucial. The demand

of novel multifunctional materials is a major challenge for scientists to address to solve crucial

contemporary issues related to energy, environment and health. Truly, the transition of energy

resources from their fossil-fuel-based beginnings to clean and renewable technologies relies on

the widespread implementation of solar-related energy systems, however the high cost of

energy production and the relatively low efficiency of currently used material combinations

pose an intrinsic limitation. Indeed, (r)evolutionary development is required to achieve the

necessary increases in efficiency and decrease in cost of materials for energy conversion. The

need for low-cost functional materials purposely built from optimized building blocks with

controlled size, morphology, orientation and aspect ratio, fabricated by cost-effective large-

scale manufacturing methods, will play a decisive role in the successful large-scale imple-

mentation of solar-related energy sources. However, fabricating andmanufacturing large areas

of such functionalmaterials still represents a tremendous challenge.Novel smarter and cheaper

fabrication techniques and, just as important, better fundamental knowledge and comprehen-

sive understanding of the structure–property relationships using materials chemistry and

nanoscale phenomena such as quantum confinement, for instance, to create multifunctional

structures and devices, is the key to success.

The materials requirements for water splitting and thus the direct solar-to-hydrogen

generation are drastic. First, the materials must be stable in water. Second, they must be

stable (upon illumination) against photocorrosion and their bandgap must be small enough to

absorb visible light, but large enough not to “dissolve” once illuminated. Finally, their band

edges must be positioned below and above the redox potential of hydrogen and oxygen,

respectively. Bandgap energy and band-edge positions, as well as the overall band structure of

semiconductors are of crucial importance in photoelectrochemical and photocatalytic applica-

tions. The energy position of the band edges can be controlled by the electronegativity of the

dopants and solution pH, as well as by new concepts such as quantum confinement effects and

the fabrication of novel nanostructures. Fulfilling the abovementioned requirements while

keeping the cost of the materials low is a tremendously difficult challenge, which explains why

direct solar-to-hydrogen generation is still in its infancy, compared to photovoltaics.

This book written by leading experts in major fields of physical sciences from USA, Europe

and Asia covers the fundamentals of photocatalysis at oxide interfaces for direct solar-to-

hydrogen conversion, the latest developments in materials discovery and their in-depth

characterization by high-resolution electron scanning probe microscopy and synchrotron-

radiation-based X-ray spectroscopy, as well as the latest development of devices for solar

thermal generation of hydrogen. It consists of five distinctive parts and 21 chapters and

addresses in detail: the fundamentals, modeling and experimental investigation of photo-

catalytic reactions for direct solar hydrogen generation (Part I); the electronic structure,

energetics and transport dynamics of photocatalyst nanostructures (Part II); the development of

advanced nanostructures for efficient solar hydrogen production from classical large bandgap

semiconductors (Part III); the new design and approaches to bandgap profiling and visible-

light-active nanostructures (Part IV) and the development of new devices for solar thermal

xx Preface

hydrogen generation (Part V). While covering the entire spectrum of studies involved in solar

hydrogen, the main focus was intentionally given to novel materials development leading to

stable and cost-effective visible-light-active semiconductors and efficient (sea)water splitting,

the holy grail of photocatalysis.

Hopefully, political leaders, policy makers and programmanagers of governmental funding

agencies, as well as industrial and private investors will soon realize that it is actually not

enough to provide funding for existing devices by just multiplying the current technology, but

rather investment in R&D of new technologies/devices. Moreover, the world electricity

infrastructure needs to be modernized to involve cost-effective and more efficient distribution,

for instance by investingmassively in smart-grid technology and significantly reducing the cost

of renewable technologies. In addition, given that the world is in urgent need of reinventing its

energy sources, changing and improving technology policies (new laws could also actually

help address global warming), and using tax on carbon-related energy to fund innovative spin-

offs and small businesses to induce a shift to noncarbon energy sources, as well as contributing

to the necessary market momentum for helping them to survive the world recession by

increasing productivity and creating newmarkets. Finally, a better perception and understand-

ing by the publicwould certainly help to support and embrace thevery urgent shift to renewable

energy sources and I sincerely hope this book will contribute to it.

Lionel Vayssieres,

Berkeley, USA, July 12, 2009

Preface xxi

Editor Biography

Born in 1968, he obtained his BSc and MSc in physical chemistry in 1989 and 1990,

respectively, and a PhD in inorganic chemistry in November 1995 from the Universit�e Pierreet Marie Curie, Paris, France for his research work on the interfacial and thermodynamic

growth control of metal-oxide nanoparticles in aqueous solutions. Thereafter, he joined

Uppsala University, Sweden as a postdoctoral researcher for the Swedish Materials Consor-

tium on Clusters and Ultrafine Particles to extend his concepts and develop purpose-built

metal-oxide nanomaterials for photoelectrochemical applications, as well as to characterize

their electronic structure by X-ray spectroscopies at synchrotron radiation facilities. He has

been invited as a visiting scientist to: the University of Texas at Austin; the UNESCO Center

for Macromolecules & Materials, Stellenbosch University, and iThemba Laboratory for

Accelerator-Based Sciences, CapeTown, SouthAfrica; theGlennT. SeaborgCenter, Chemical

Sciences Division, at Lawrence Berkeley National Laboratory; Texas Materials Institute; the

Ecole Polytechnique Federale de Lausanne, Switzerland; the University of Queensland,

Australia and Nanyang Technological University, Singapore, developing novel metal-oxide

nanorod-based structures and devices.

He has (co-)authored 60 refereed publications in major international journals which have

already generated over 3000 citations since the year 2000; Essential Science Indicators (as

of November 1, 2009) shows 114 citations per paper for Materials Science and 71 for All

Fields; five ISI highly cited papers (four as first author) for the last 10 years, a single-author

2003 paper ranked no. 1 in the Top 10 hot papers in chemistry (Jul-Aug 05), no. 2 (Sep-Dec 05)

and no. 3 (May-June 05) in the Top 3 hot papers inmaterials science and themost cited paper in

materials science for the country of Sweden for the last 10 years as identified by Essential

Science Indicators. He has been interviewed by In-Cites and by ScienceWatch in 2006 for

this single-authored 2003 paper which has now been cited over 800 times. Two other first-and-

corresponding author 2001 papers have already been cited 400 times, five other papers of his

(three as the first author) have been cited over 100 times during the last 10 years. He has

presented over 125 seminars at universities, governmental and industrial research institutes and

90 talks at international conferences including 70 invited lectures (20 plenaries) in 26 countries

and acted as an organizer, chairman, executive program committee member and advisory

member for major international conferences (ACerS, ICTP, IEEE, IUPAC, MRS, SPIE and

stand-alones) in the fields of Nanoscience and Nanotechnology and projects worldwide. He is,

for instance, the founder (2006) of the SPIE Optics and Photonics symposium entitled Solar

Hydrogen and Nanotechnology, which has been held every year since its creation.

He is currently an independent scientist at the World Premier International Center for

Materials NanoArchitectonics, National Institute for Materials Science, in Tsukuba, Japan; a

guest scientist at Lawrence Berkeley National Laboratory, USA and an R&D consultant. He is

also the founding editor-in-chief of the International Journal of Nanotechnology (Impact

Factor 2008: 1.184) and a referee for 63 SCI scientific journals, as well as for major funding

agencies in USA, Europe, Asia and Africa.

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