Spectroscopic Investigation of Semiconductor Charge Carrier Dynamics and Semiconductor/electrolyte Catalytic Interface for Solar Energy Conversion Open Access
Zhao, Fengyi (Fall 2023)
Abstract
Utilizing semiconductor photoelectrode is a promising but challenging method to achieve direct energy conversion from solar energy to chemical energy stored in liquid fuels. A lot of fundamental questions need to be understood on semiconductor/electrolyte junction to achieve efficient energy conversion efficiency, for instance, how to understand the surface minority charge accumulation, the function of the surface co-catalyst and the catalytic interfacial electric field. In this thesis, we intend to investigate the semiconductor/electrolyte catalytic junction through time-resolved and in situ spectroscopic methods. In chapter 3 and 4, we introduced an in-situ bias-dependent Second Harmonic Generation (SHG) technique to investigate potential drop at the semiconductor electrode and solution double layer side without illumination and the minority charge accumulation at the surface under illumination condition for oxygen evolution reaction (OER). It is found that the TiO2 crystal angle and light polarization will have significant effect on its bias-dependent SHG behavior. Under photoexcitation, screening of built-in potential is observed, which is correlated to the surface hole accumulation. Studying accumulated holes provides crucial understanding of water oxidation rate-determining species. In chapter 8, in-situ Raman spectroscopy is developed to study the properties of a novel semiconductor-catalyst/electrolyte junction SMA/CNT/CoPc-NH2 during CO2 photoreduction condition.
In chapter 5, the synthesis of TiO2-Co9POM hybrid photoanode material showed a three-fold OER photocurrent enhancement compared to unmodified nanoporous TiO2, comprehensive transient absorption and photoelectrochemical characterization highlight the function of Co9POM catalyst. Chapter 6 uses transient reflectance spectroscopy to study the photocathode material GaP/TiO2, showcasing the importance of solution electrolyte concentration and potentiostat response in regulating interfacial recombination. Lastly, chapters 7 focused on the charge separation process of a CdS based CO2 reduction catalyst, clearly revealing the initial rapid charge separation process and later charge accumulation on the catalyst through transient absorption spectroscopy, providing mechanistic insight of this novel hybrid photocatalyst.
Table of Contents
1 Introduction. 1
1.1. Description of semiconductor/electrolyte junction under equilibrium.. 1
1.2. Potential-current behavior of semiconductor/electrolyte junction under dark and illumination conditions. 5
1.3. Progress of understanding semiconductor/electrolyte junctions. 9
1.4. Challenges in studying the semiconductor/electrolyte junctions for oxygen evolution reaction, hydrogen evolution reaction and CO2 reduction reactions. 14
1.5. Reference. 16
2 Experimental Methods. 21
2.1. Transient absorption and reflectance spectroscopy. 21
2.1.1. Transient absorption Spectroscopy. 21
2.1.2. Transient reflectance spectroscopy. 23
2.2. Second Harmonic Generation spectroscopy. 24
2.3. In situ Raman spectroscopy. 26
3 In situ Investigation of Electric Field Distribution on TiO2(100)-electrolyte Junction by Azimuthal-Angle-Resolved Second Harmonic Generation. 28
3.1. Introduction. 28
3.2. Materials and Methods. 30
3.2.1 Materials. 30
3.2.2 Experimental Method. 31
3.2.3 Bias-dependent SHG equation derivation. 32
3.3. Results and discussion. 35
3.4. Conclusion. 48
Appendix Chapter 3. 49
3.5. Reference. 51
4 Direct Operando Observation of Surface Charge Build-up on TiO2 Photoanode Under Water Oxidation Conditions by EFISH.. 57
4.1. Introduction. 57
4.2. Materials and Methods. 60
4.3. Results and Discussions. 61
4.3.1 Operando EFISH of TiO2 under illumination. 61
4.3.2 Impedance spectroscopy investigation of light-induced Fermi-level pinning 67
4.3.3 Isotope effect on EFISH and Photocurrent 70
4.3.4 Effect of electrolyte on 72
4.3.5 Nature of surface charge and rate-determining step. 74
4.3.6 Impact of to Incident Photon to Current Efficiency. 78
4.4. Conclusion. 80
Appendix Chapter 4. 81
Appendix A4.1. EFISH and acquired under negative and positive scan directions. 81
Appendix A4.2. Equivalent circuit of semiconductor/electrolyte junction and Gauss fitting of surface state capacitance. 81
Appendix A4.3. Effect of local pH change on 83
Appendix A4.4. Effect of electrolyte on and photocurrent and KIE values. 85
Appendix A4.5. Effect of other solution adsorbate on 86
Appendix A4.6. Separation between flatband (Vfb) and photocurrent onset (Vonset) potential and calculation of IPCE. 87
4.5. References. 89
5 The Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO2 Photoanode for Water Oxidation in Acid. 98
5.1. Introduction. 98
5.2. Material and Methods. 101
5.2.1 Sample synthesis. 101
5.2.2 Generation Characterization Methods. 102
5.2.3 Photoelectrochemical Characterizations. 103
5.3. Results and Discussion. 104
5.3.1 Characterization of TiO2 and TiO2-Co9POM photoelectrodes. 104
5.3.2 Photoelectrochemical Oxygen Evolution Reaction Performance of TiO2-Co9POM 107
5.3.3 Transient Absorption Spectroscopy Probing Charge Transfer Dynamics in TiO2-Co9POM... 112
5.4. Conclusions. 115
Appendix Chapter 5. 116
Supplementary General Characterization. 116
Supplementary PEC Characterization and AC Impedance Fitting Parameters. 118
Supplementary Transient Absorption Spectroscopy. 120
5.5. References. 122
6 Semiconductor Photocatalysis Quantum Efficiency Limited by Electrolyte Concentration. 128
6.1. Introduction. 128
6.2. Material and Method. 129
6.2.1. Material preparation. 129
6.2.2. Photoelectrochemistry. 130
6.2.3. Transient Reflectance Spectroscopy. 130
6.3. Results and Discussions. 130
6.4. Conclusion. 142
Appendix Chapter 6. 143
6.5. References. 147
7 Synergizing Electron and Heat Flows in Photocatalyst for Direct Conversion of Captured CO2 150
7.1. Introduction. 150
7.2. Materials and Methods. 152
7.2.1. Materials. 152
7.2.2. General Characterization Methods. 153
7.2.3. Measuring work function and valence band maximum (VBM) with UPS. 154
7.2.4. TA measurement conditions. 155
7.2.5. Photochemical CO2 reduction measurements. 156
7.2.6. In situ Raman spectroscopy and in situ UV-visible spectroscopy. 156
7.2.7. Modeling of CNT local heating. 157
7.3. Results and Discussion. 158
7.4. Conclusion. 173
Appendix Chapter 7. 173
Bleach decay kinetics fitting. 173
Supplementary Figures and Tables. 177
7.5. Reference. 192
8 Enhanced Methanol Production from Photoelectrochemical CO2 Reduction via Interface and Microenvironment Tailoring. 197
8.1. Introduction. 197
8.2. Material and Methods. 199
8.2.1. Materials. 199
8.2.2. Preparation of catalyst ink. 199
8.2.3. Fabrication of SMA-CFx 200
8.2.4. Fabrication of photocathode. 200
8.2.5. PEC measurement 201
8.2.6. Characterization. 201
8.2.7. Numerical Modeling. 202
8.3. Results and Discussion. 203
8.4. Conclusion. 214
Appendix Chapter 8. 215
Supplementary Results. 215
8.5. References. 226
9 Conclusions and Outlook 229
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