On Solar Hydrogen and Nanotechnology

More energy from the sun strikes Earth in an hour than is consumed by humans in an entire year. Efficiently harnessing solar power for sustainable generation of hydrogen requires low-cost, purpose-built, functional materials combined with inexpensive large-scale manufacturing methods. These issues are comprehensively addressed in On Solar Hydrogen & Nanotechnology – an authoritative, interdisciplinary source of fundamental and applied knowledge in all areas related to solar hydrogen. Written by leading experts, the book emphasizes state-of-the-art materials and characterization techniques as well as the impact of nanotechnology on this cutting edge field. Addresses the current status and prospects of solar hydrogen, including major achievements, performance benchmarks, technological limitations, and crucial remaining challenges Covers the latest advances in fundamental understanding and development in photocatalytic reactions, semiconductor nanostructures and heterostructures, quantum confinement effects, device fabrication, modeling, simulation, and characterization techniques as they pertain to solar generation of hydrogen Assesses and establishes the present and future role of solar hydrogen in the hydrogen economy Contains numerous graphics to illustrate concepts, techniques, and research results On Solar Hydrogen & Nanotechnology is an essential reference for materials scientists, physical and inorganic chemists, electrochemists, physicists, and engineers carrying out research on solar energy, photocatalysis, or semiconducting nanomaterials, both in academia and industry. It is also an invaluable resource for graduate students and postdoctoral researchers as well as business professionals and consultants with an interest in renewable energy. 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