On Solar Hydrogen & Nanotechnology

List of Contributors xvii
Preface xix
Editor Biography xxiii
PART ONE—FUNDAMENTALS, MODELING, AND EXPERIMENTAL
INVESTIGATION 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 np
þ
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 37
Hideyuki 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 77
G.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
6.1 Introduction 143
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
6.4.1 Introduction 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 163
Lin X. Chen
7.1 Introduction 163
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 O ¨ sterlund
8.1 Introduction 189
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 Quantum
Dot Sensitized TiO2 Nanocrystalline Electrodes 239
Yasuhiro Tachibana
9.1 Introduction 239
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 CLASSICAL
LARGE BANDGAP SEMICONDUCTORS
10 Ordered Titanium Dioxide Nanotubular Arrays as Photoanodes
for Hydrogen Generation 267
M. Misra and K.S. Raja
10.1 Introduction 267
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
11.1 Introduction 291
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 349
Vibha R. Satsangi, Sahab Dass, and Rohit Shrivastav
13.1 Introduction 349
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 401
Alan Kleiman-Shwarsctein, Peng Zhang, Yongsheng Hu,
and Eric W. McFarland
14.1 Introduction 401
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
15.1.1 Introduction 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
16.1 Introduction 507
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
17.1 Introduction 523
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
18.1 Introduction 559
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 YxIn2xO3 573
18.5.2 ScxIn2xO3 580
18.5.3 YxIn2xGe2O7 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 Introduction 589
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
19.3 Conclusions 614
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 Introduction 623
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
20.5 Conclusions 637
Acknowledgments 638
References 638
21 Solar Thermal and Efficient Solar Thermal/Electrochemical
Photo Hydrogen Generation 641
Stuart 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
21.6 Conclusions 660
References 661
Index 665