Surface Modifications and Growth of Titanium Dioxide for Photo-Electrochemical Water Splitting (eBook)

Supervisor’s Foreword 6
Abstract 8
Acknowledgements 9
Declaration 10
Contents 11
List of Figures 17
List of Tables 38
1 Introduction 39
1.1 Scope of Research 39
1.2 Structure of Thesis 39
1.3 Publications and Conferences 41
Reference 41
2 Literature Review 42
2.1 Global Energy Drivers for Solar Water Splitting 43
2.2 Photo-Electrochemical Cells (PECs) 44
2.2.1 Overview 44
2.3 Requirements for Economical PEC Water Splitting 46
2.3.1 Solar to Hydrogen Efficiency 46
2.3.1.1 Efficiency Limits 47
2.4 Materials: Metal Oxides for Water Splitting 48
2.4.1 Bandgap and the Solar Spectrum 48
2.4.2 Band Position 49
2.4.3 TiO2 and Fe2O3 for Water Splitting 50
2.4.4 Other Water Splitting Materials 51
2.4.5 Improving Solar to Hydrogen Efficiency 52
2.5 Plasmonic Water Splitting 52
2.6 Plasmonic Nanoparticle Arrays 54
2.7 TiO2-AuNP Systems for Water Splitting 55
2.7.1 Nishijima et al. [32, 33] 56
2.7.2 Liu et al. (2011) 59
2.7.3 Naseri et al. (2010) 60
2.7.4 Chandrasekharan and Kamat (2000) 60
2.7.5 Tatsuma Group 61
2.8 Mechanisms of Plasmonic Enhancement 63
2.8.1 Direct Electron Transfer (DET) 64
2.8.2 Plasmonic Resonant Energy Transfer (PRET) 67
2.9 Other Plasmonic Effects 69
2.9.1 Effect of Enhanced Near-Field on Absorption 69
2.10 Evidence for Direct Electron Transfer 70
2.10.1 Electrons in the Conduction Band of TiO2 71
2.10.2 Shifting Open Circuit Potential Under Illumination 72
2.10.3 Decrease in Absorbance of Plasmon 73
2.10.4 Opposition to DET Mechanism 74
2.11 Oriented Thin Films of TiO2 75
2.12 Conductive TiO2 Thin Films 77
2.13 Summary of Literature Review 78
References 79
3 Principles of Photo-Electrochemical Cells 83
3.1 Nomenclature 84
3.1.1 Constants 84
3.1.2 Symbols 85
3.2 Hydrogen Production by Water Splitting 86
3.2.1 The Water Splitting Reaction 86
3.2.2 Metallic Versus Semiconducting Electrodes 88
3.2.3 Solution Fermi Level 89
3.3 Photoelectrochemical Cells—Overview 90
3.3.1 Quasi-Fermi Level and Photovoltage 92
3.4 Structure of the Space Charge Region 93
3.4.1 Semiconductor Terminology 93
3.4.2 Flat Band Potential 97
3.5 Mathematical Description of the Space Charge Layer 97
3.5.1 Poisson’s Equation 97
3.5.2 Electric Field Distribution in the Semiconductor 99
3.5.3 Potential Distribution at the Semiconductor-Liquid Interface 100
3.5.4 Width of the Space Charge Region 102
3.5.5 Capacitance—Mott-Schottky Plots 105
3.5.6 Helmholtz Layer Potential Difference Due to PH 114
3.5.7 Full Interfacial Model 116
3.6 Photocurrent 119
3.6.1 Depletion Layer Current 120
3.6.2 Diffusion Photocurrent 121
3.6.3 Total Photocurrent 122
3.6.4 Determining the Characteristics of the Electrode 125
3.6.5 Flat Band Potential Estimation from the Photocurrent 125
References 127
4 Experimental Methods 129
4.1 Outline of Experiments 130
4.1.1 Plasmonic Au NPs on TiO2 130
4.1.2 Thin Film Deposition and Characterization 130
4.2 Single Crystal Preparation 130
4.2.1 Nb-Doped Rutile (110) 130
4.2.2 Reduced Undoped Rutile (110) 131
4.2.3 Fabrication of Electrodes 131
4.3 Nanoparticle Fabrication 132
4.3.1 Annealing Metal Thin Films 132
4.3.2 Micellar Nanolithography 133
4.3.3 Nano-sphere Lithography 134
4.4 Thin Film Deposition 135
4.4.1 Target Processing 136
4.4.2 Pulsed Laser Deposition 136
4.4.2.1 Film Thickness Calibration 138
4.4.3 Sputtering 139
4.5 Materials Characterization 140
4.5.1 X-Ray Diffraction 141
4.5.1.1 Data Correction 142
4.5.1.2 Peak Fitting 143
4.5.1.3 XRD Texture Analysis (Pole Figures) 143
4.5.2 Raman Spectroscopy 144
4.5.3 Scanning Electron Microscopy (SEM) 145
4.5.4 Atomic Force Microscopy (AFM) 145
4.5.5 Profilometry 146
4.5.6 UV-Vis Spectroscopy 147
4.5.6.1 Bandgap Estimation—Tauc Plots 148
4.6 Electrochemical Techniques 149
4.6.1 Electrochemical Measurement Setup 149
4.6.2 Working Electrode Preparation 152
4.6.3 Reference Electrodes and Electrolyte 153
4.6.4 Voltammetry 154
4.6.5 Chopped Light Voltammetry 157
4.6.6 Chronoamperometry 159
4.6.7 Impedance Spectroscopy 160
4.6.7.1 EIS Theory 160
4.6.7.2 EIS Practical Aspects 162
References 165
5 Results: Plasmonic Photocurrent with AuNP-TiO2 167
5.1 Experimental Aims 168
5.2 Summary of Experimental Results and Conclusions 169
5.3 Future Work 171
5.4 Sample Production—General 172
5.5 Nanoparticle Array Architecture 172
5.5.1 Annealed Gold Thin Films on Indium Tin Oxide (ITO) 173
5.5.2 AuNPs on Rutile (110)—Annealing Gold Thin Films 178
5.5.3 AuNPs on Rutile (110)—Micellar Nanolithography (MNL) 179
5.5.4 Nano-sphere Lithography (NSL) 180
5.5.5 Sample Identification—Batch Numbers 181
5.6 Optical Measurements 182
5.6.1 Transmittance and Reflectance 183
5.6.2 Reflectance 186
5.6.3 Absorption 187
5.7 Photo-Electrochemical Measurements 187
5.7.1 Measuring the Photocurrent in Chronoamperometry 188
5.7.2 AuNPs Versus AuMNLs—Nb-Doped TiO2—Chronoamperometry 192
5.7.3 Effect of Electrode Potential on Plasmonic Photocurrent on Nb-TiO2 AuNPs 199
5.7.4 AuNPs—Reduced Versus Nb-Doped TiO2—Chronoamperometry 202
5.7.5 Chopped Light Voltammetry of Nb-TiO2 AuNPs 203
5.7.6 Effect of Reduction of Rutile (110) on Plasmonic Photocurrent 207
5.8 Discussion of Results 208
References 213
6 Electrochemistry of TiO2—Rutile (110) 214
6.1 Experimental Aims 215
6.2 Summary of Experimental Results and Conclusions 215
6.3 Further Work 217
6.4 Comparing Nb-Doped to Reduced Rutile (110) 218
6.5 Phase Diagrams 219
6.6 Nb-Doped Rutile (110)—Batch 1—Voltammetry 219
6.6.1 ‘Dark’ Voltammetry 219
6.6.2 Photocurrent 223
6.6.3 Photocurrent—Reproducibility 225
6.7 Nb-Doped Rutile (110)—Batch 2—Voltammetry 226
6.7.1 Initial ‘Dark’ Voltammetry 226
6.7.2 Further ‘Dark’ Voltammetry 227
6.8 Voltammetry of Reduced (Undoped) Rutile (110) 228
6.8.1 ‘Dark’ Voltammetry 228
6.8.2 Redox Process on Reduced Rutile 230
6.9 Structure of the Space Charge Layer of Rutile (110) 231
6.9.1 Mott-Schottky Analysis—Nb-Doped TiO2 (Batch 1) 231
6.9.2 Mott-Schottky Analysis—Nb-Doped TiO2 (Batch 2) 234
6.9.3 Mott-Schottky Analysis—Reduced TiO2 236
6.9.4 Discussion of Mott-Schottky Analysis 238
6.9.5 Resistance Measurements 239
6.10 Photocurrent Measurements 241
6.10.1 Optical Characteristics—Band Gap and Absorption 241
6.10.2 Modelling the Photocurrent 242
6.11 Impedance Spectroscopy—Equivalent Circuits 246
6.11.1 NbV-Doped Rutile (110)?Sample 2, Batch 2 246
6.11.2 Reduced Rutile (110)—Sample 1, Batch 1 252
References 255
7 TiO2 Thin Films on Fused Silica 256
7.1 Summary of Results and Conclusions 257
7.2 Further Work 258
7.3 Target Production and Pulsed Laser Deposition Methods 259
7.4 General Deposition Conditions 259
7.5 Deposition on Borosilicate Glass 259
7.6 Deposition on Fused Silica 260
7.6.1 Effect of Deposition Pressure on Crystallization 261
7.6.2 Post-annealing of Films Deposited at Room Temperature 263
7.6.3 The Effect of Substrate Heating During Deposition on Film Structure 265
7.6.4 Discussion of Substrate Heating Versus Post-annealing 269
7.6.5 Effect of Growth Conditions on Film Surface Morphology 271
7.6.6 Effect of Film Thickness 276
7.7 Optical Properties of TiO2 Films 279
7.7.1 Optical Characterization—Film Thickness and Refractive Index 279
7.7.2 Optical Characterization—Absorption 281
7.8ƒDiscussion—Stabilisation of Anatase with Nb 288
7.9ƒDiscussion—Conductivity 289
References 293
8 Epitaxial TiO2 Thin Films on Single Crystal Substrates 295
8.1 Summary of Results 295
8.2 Further Work 296
8.3 Substrate Overview 297
8.4 TiO2 Films on STO (100) 298
8.4.1 Effect of Substrate Temperature 298
8.4.2 Effect of Post-annealing 302
8.4.3 Effect of Other Deposition Parameters 303
8.4.4 X-ray Texture Analysis—Epitaxy of Films on STO (100) 306
8.4.5 Optical Measurements—Films on STO 309
8.5 TiO2 on MgO (100) 309
8.5.1 Effect of Temperature—90 mm Substrate-Target Distance 310
8.5.2 Effect of Temperature—50 mm Substrate-Target Distance 313
8.5.3 Effect of Other Deposition Parameters 316
8.5.4 X-ray Texture Analysis—Epitaxy of Films on MgO (100) 319
8.5.5 Optical Measurements—Films on MgO 327
8.5.6 Raman Spectroscopy—Films on MgO 329
References 330
9 Conclusions 332
Appendix A: Defect Chemistry of TiO2 335
A.1 Undoped TiO2—Frenkel Disorder 336
A.2 Undoped TiO2—Schottky Disorder 337
A.3 Undoped TiO2—Non-stoichiometric Disorder 338
A.4 Nb-Doped TiO2—Extrinsic Ionic Disorder 339
A.5 Nb-Doped TiO2—Schottky Disorder as Ionic Defect 339
A.6 Non-stoichiometry 340
Appendix B: Phase Diagrams of TiO2 and H2O 344
B.1 Ti–H2O Phase Diagram 344
B.2 O–H Phase Diagram 349
B.3 Species Expected in Voltammetry from Phase Diagrams 350
Appendix C: Optical Measurements—Films on Fused Silica 352
C.1 TiO2—Effect of Deposition Pressure 352
C.2 TiO2—Effect of Deposition Temperature 354
C.3 1 % Nb-Doped TiO2—Thickness Calibration 355
C.4 1 % Nb-Doped TiO2—Temperature Variation 357
Appendix D: Optical Measurements—Films on MgO (100) 359
D.1 Effect of Deposition Temperature 359
D.2 Effect of Deposition Parameters Other than Temperature 360
Appendix E: Additional XRD Measurements 363
References 365