Mechanism of charge separation and recombination in one-dimensional CdS-Pt nanorod heterostructures for solar energy conversion Open Access

Liu, Yawei (Fall 2021)

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Developing efficient solar energy conversion systems is of great significance to meet the explosively increasing demand for clean and renewable energies in the 21st century. Colloidal nanocrystals with strong light absorption, efficient carrier transport and broad tunability can be combined with catalytic metallic domains to form hybrid photocatalytic systems. The rational design of such hybrid semiconductor/metal photocatalysts for solar energy conversion requires comprehensive understanding of charge separation and recombination processes. Among various photocatalytic systems, Pt-tipped CdS nanorod heterostructures have been reported to undergo ultrafast charge separation to form long-lived charge-separated states and to demonstrate high quantum efficiencies for light driven H2 production in the presence of sacrificial electron donors. In this dissertation, we use CdS-Pt nanorods as a model system to investigate how the light driven H2 production performance of such photocatalysts can be systematically tuned by the size of the semiconductor and metal domains. Specifically, we examine how the effects of Pt size and CdS rod length on the H2 generation quantum efficiency can be understood through their effects on various charge separation, charge recombination rates, and catalytic reaction rates.

First, we demonstrated that reducing the Pt size impedes both electron transfer from CdS to Pt and water reduction at Pt, leading to the decrease of hydrogen production quantum efficiency. Furthermore, we showed that increasing the CdS rod length prolongs the charge separated state lifetime and improve the hole removal efficiency by the hole scavengers, which leads to higher H2 generation quantum efficiencies. Those dependences on the morphology of the heterostructures can be explained by a kinetic model that considers the competition between various forward charge separation and reaction processes (electron transfer, hole transfer, hole removal, water reduction) and background charge recombination processes of key intermediates. The efficiency of ssimultaneous multiple electron transfer from the CdS rod to the Pt tip was also examined as a function the excitation fluence. We observed efficient transfer of multiple electrons to the Pt tip and long lifetime of multiple charge separated states, indicating the feasibility of such systems for efficient light driven H2 generation under concentrated sun light illumination.

Finally, we investigated the mechanism of plasmon-induced hot electron transfer from Au tips to CdS nanorods. The quantum efficiency of this process increases from 1% to 18% as the Au tip diameter decreases from 5.5 ± 1.1 to 1.6 ± 0.5 nm due to both enhanced hot electron generation and transfer efficiencies in small Au particles. Our findings suggest that reducing the Au particle size is an effective approach for enhancing the efficiency of plasmon induced hot electron transfer.

Table of Contents

Chapter 1. Introduction 1

1.1 Band alignment of metal-tipped nanorods. 3

1.2 Charge dynamics and H2 production performance 6

1.3 Multielectron interaction in metal-tipped nanorods 7

1.4 Plasmon-induced hot electron transfer 8

1.5 Conclusion 9

References 10

Chapter 2. Experimental Methods 13

2.1 Sample Preparation 13

2.1.2 Synthesis of CdS Quantum Dots 13

2.1.3 Synthesis of CdS Nanorods (NRs) 14

2.1.4 Synthesis of CdS-Au NRs 14

2.1.5 Synthesis of Au NPs 15

2.1.6 Synthesis of CdS-Pt NRs 15

2.1.7 Ligand exchange of NRs 16

2.2 Time Resolved Spectroscopy Set-up 16

2.3 Photo-catalytical H2 production 17

References 18

Chapter 3. Size- dependent light-driven H2 generation quantum efficiency of CdS-Pt nanorod 19

3.1 Introduction 19

3.2 Results 21

3.2.1 CdS NR Characterization 21

3.2.2 Size-dependent H2 Production Performance 22

3.2.3 Charge separation and recombination kinetics 24

3.3. Discussion 28

3.3.1 Size-dependent efficiencies of charge separation and water reduction 28

3.3.2 Size dependent electron transfer from CdS to Pt 33

3.3.3 Key parameters for hybrid semiconductor/metal photocatalysts 35

3.4. Conclusion 36

References 38

Appendix 1 41

Appendix 2 43

Appendix 3 45

Appendix 4 49

Chapter 4. Length-dependence of photo-driven H2 production in 1D CdS-Pt heterostructures 52

4.1. Introduction 52

4.2 Results 53

4.2.1 Sample Preparation 53

4.2.2 H2 production 55

4.2.3 Length dependent electron transfer kinetics 56

4.2.4 Length dependent charge recombination kinetics in toluene 58

4.3. Discussion 60

4.4. Conclusion 64

References 65

Appendix 1 67

Appendix 2 68

Appendix 3 68

Chapter 5. Auger Recombination and Multiexciton dissociation in CdS-Pt Nanorod Heterostructures 73

5.1. Introduction 73

5.2. Results 76

5.2.1 CdS NR Characterization 76

5.2.2 Single exciton dynamics in CdS and CdS-Pt NRs 78

5.2.3 Auger recombination in CdS NRs 81

5.2.4 Multiexciton dissociation in CdS-Pt NRs 86

5.2.5 Charge separated state recombination in CdS-Pt NRs 90

5.3 Conclusion 93

References 94

Appendix 1 99

Appendix 2 100

Appendix 3 103

Appendix 4 104

Appendix 5 106

Appendix 6 109

Appendix 7 116

Appendix 8 120

Chapter 6. Plasmon-Induced Hot Electron Transfer in Metal-semiconductor Heterostructures 122

6.1 Introduction 122

6.2. Results and Discussion 125

6.2.1 Absorption Spectra and TEM images of CdS-Au NRs 125

6.2.2 Analysis of the plasmon width in UV-vis absorption of CdS-Au NRs 126

6.2.3 Transient IR absorption of CdS-Au NRs 129

6.2.4 Quantification of plasmon-induced hot electron transfer quantum yield 132

6.2.5 Understanding the size-dependence of quantum yields 132

6.3 Conclusion 136

References 137

Appendix 1 141

Appendix 2 142

Appendix 3 144

Appendix 4 146

Appendix 5 148

Appendix 6 151

Chapter 7. Summary and Outlook 153

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