Direct Vibrational Probe of Energetics at Semiconductor/Electrolyte Interface Restricted; Files Only
Suo, Sa (Spring 2025)
Abstract
In photoelectrochemical systems, the energetics at semiconductor/electrolyte interface determine the driving force available for surface catalytic reactions, such as water splitting and CO2 reduction reaction (CO2RR). Various surface modification methods and innovative architecture design of the photoelectrodes have been proposed for boosting the device performance. However, for heterogeneous, multi-junction photoelectrodes, typically containing metal nanoparticle cocatalysts, directly probing the energetics at the catalytic sites under operational conditions remains challenging.
We reported the first operando spectroscopic approach to directly measure the quasi-Fermi level of metal catalyst sites in heterogeneous photocathodes via surface-enhanced Raman spectroscopy (SERS). We studied a CO2RR photocathode, which consists of the nanoporous p-type Si modified with Ag nanoparticles, as a prototype and demonstrated a selective probe of ~ 0.59 V photovoltage generated at the Si/SiOx/Ag junctions. This vibrational Stark probing approach offers a new way for the thermodynamic evaluation of multi-junction photoelectrodes with varied architectural designs.
We further applied this method to investigate the energetics in a redox polymer mediated CO2RR photocathode system. In this system, p-type silicon is modified with surface-bound viologen polymer, into which Au nanoparticles are incorporated. Through operando surface-enhanced Raman spectroscopy, we determined the Fermi level of both the viologen polymer and the Au catalyst sites under varying Si Fermi levels. Combined with the photoelectrochemical performance analysis, the determination of the Fermi levels of all three components in the photocathode demonstrated a comprehensive understanding of the electron transfer mechanism. We also employed the Fermi level probing methodology to investigate the Fermi level alignment in a semiconducting polymer/metal catalyst cathode, N2200 functionalized with Au nanoparticles, during the electrical doping of N2200.
Additionally, surface-enhanced resonance Raman spectroscopy was utilized to determine the electric field distribution at p-type and n+-type Si/electrolyte interfaces. A Ru complex chemically attached to the Si surface, was used to probe the interfacial electric field changes at Si/electrolyte junctions under varying potential regimes. The formation of accumulation region and (light-induced) inversion region was revealed by the potential-dependent Stark shift of the probe molecule. These observations directly correlated with the theoretically predicted and experimentally observed semiconductor space-charge capacitance (Csc) behavior as a function of potential.
Table of Contents
Chapter 1 Introduction 1
1.1 Photoelectrochemical Systems for Photoelectrochemical Solar Energy Conversion 1
1.2 Semiconductor/Electrolyte Interface 4
1.2.1 Brief Introduction of Semiconductor Carrier Statistics 5
1.2.2 Energetics at Semiconductor/Electrolyte Junction 7
1.2.3 Accumulation, Depletion and Inversion Region in Semiconductors 11
1.2.4 Photovoltage Generation 13
1.2.5 Junction Behavior of Hybrid Semiconductor/Electrolyte Interface 15
1.3 Spectroscopic Techniques for Investigating the Semiconductor/Electrolyte Interface 18
1.4 References 22
Chapter 2 Experimental Methods 30
2.1 Introduction of Surface-enhanced Raman Spectroscopy 30
2.2 Instrumentation of Surface-enhanced Raman Spectroscopy 32
2.3 Synthesis of Shell-isolated Nanoparticles (SHINs) 34
2.4 Photoelectrochemistry Setup 35
2.5 Material Preparation 35
2.5.1 Ag Electrode Preparation 35
2.5.2 b-Si-Ag Photoelectrode Preparation 36
2.5.3 SAM Layer Preparation for Ag and b-Si-Ag (Photo)electrode 36
2.5.4 Si Electrode Preparation for Si/[(PQ2+/+)n]surf /Metal Electrode 37
2.5.5 Si/[(PQ2+/+)n]surf /Metal Electrode Preparation: Electrode Functionalization and Metal Incorporation 37
2.5.6 Monomer I Synthesis 38
2.5.7 FTO/[(PQ2+/+)n]surf /Metal Electrode Preparation 38
2.5.8 Roughened Gold Electrode Preparation 39
2.5.9 SAM Layer Preparation for Au and Si/[(PQ2+/+)n]surf /Metal Electrode 39
2.5.10 N2200 Fabrication 39
2.5.11 N2200/Au Electrode Preparation 40
2.6 References 41
Chapter 3: Direct Vibrational Stark Shift Probe of Quasi-Fermi Level Alignment in Metal Nanoparticle Catalyst Based Metal-Insulator-Semiconductor Junction Photoelectrodes 43
3.1 Introduction 43
3.2 Results 45
3.2.1 Bare Ag Electrode 48
3.2.2 b-p-Si-Ag Photocathode 51
3.3 Discussion 54
3.4 Conclusion 55
Appendix 3.1 SERS Spectra and Fitting 56
Appendix 3.1.1 Ag-4-MBN Electrode 56
Appendix 3.1.2 b-Si-Ag-4-MBN Photoelectrode 58
Appendix 3.2 Desorption of 4-MBN from Ag and b-Si-Ag Electrodes 60
3.5 References 62
Chapter 4: Electron Transfer Energetics in Photoelectrochemical CO2 Reduction at Viologen Redox Polymer-Modified p-Si Electrodes 66
4.1 Introduction 66
4.2 Results 67
4.2.1 (Photo-)Electrochemical Performance 68
4.2.2 HAADF-STEM Imaging 71
4.2.3 (Photo-)electrocatalytic CO2 Reduction 73
4.2.4 Fermi Level Determination of Viologen Polymer 74
4.2.5 Fermi Level Determination of Au NPs 80
4.3 Discussion 84
4.4 Conclusions 86
Appendix 4.1 Electrochemistry and Product Analysis 87
Appendix 4.2 Electrochemistry for Operando Surface-enhanced Raman spectroscopy 87
Appendix 4.3 UV-vis Absorption Characterization of Viologen Polymer Film 88
Appendix 4.4 X-Ray Photoelectron Spectroscopy (XPS) 89
Appendix 4.5 High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) 92
Appendix 4.6 Cyclic and Linear Sweep Voltammetry 93
Appendix 4.7 Mott-Schottky Analysis 94
Appendix 4.8 Bulk Electrolysis 96
Appendix 4.9 Operando Surface-enhanced Raman Spectroscopy (SERS) Measurement 96
Appendix 4.10 Electrochemical Impedance Spectroscopy (EIS) 97
Appendix 4.11 CO2 Reduction Product Analysis 98
Appendix 4.12 Surface-enhanced Raman spectroscopy (SERS) Results 98
Appendix 4.13 p-Si/[(PQ2+/+)n]surf /Au Photocathode 99
Appendix 4.14 Determination of Viologen Polymer Fermi Level 100
Appendix 4.15 Nitrile frequency vs Au Fermi level Calibration Relation 102
Appendix 4.16 n+-Si/[(PQ2+/+)n]surf /Au Electrode (Spectra Fitting) 105
Appendix 4.17 n+-Si/[(PQ2+/+)n]surf Electrode 105
Appendix 4.18 Time Resolved SERS Measurements 107
4.5 References 109
Chapter 5: Deciphering the Fermi Level Alignment and Electron Transfer Mechanism at Polymer Cathode/Metal Cocatalyst/Electrolyte Interphase 113
5.1 Introduction 113
5.2 Results 115
5.2.1 UV-vis Spectroelectrochemistry of N2200 115
5.2.2 Raman Spectroelectrochemistry of N2200 119
5.2.3 Determination of Au Fermi Level in N2200/Au Cathode 124
5.3 Conclusion 126
Appendix 5.1 Complementary Data for UV-vis Spectroelectrochemistry 127
Appendix 5.2 ATR-IR Spectroelectrochemistry 128
Appendix 5.3 Spectrum Fitting 131
Appendix 5.4 Calibration Relationship between the C≡N frequency and the Au Fermi level 132
5.4 References 134
Chapter 6: Semiconductor Accumulation, Depletion and Light-induced Inversion Region Revealed by Operando Surface Enhanced Raman Spectroscopy 137
6.1 Introduction 137
6.2 Results 139
6.2.1 SERS Probe of the Junction Behavior at p- and n+-type Si/Electrolyte 141
6.2.2 Fluence- and Frequency-dependent Impedance Measurements 143
6.3 Discussion 145
6.4 Conclusion 151
Appendix A6.1 UV-vis Spectrum of Ru005 152
Appendix A6.2 Raman Spectrum of n+-type Si-Ru005 152
Appendix A6.3 Voigt Fitting of C≡N Vibrational Band 153
Appendix A6.4 Mott-Schottky Measurement of p-type Si 153
6.5 References 155
Chapter 7 Conclusion and Outlook 158
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