Charge Separation and Transport Dynamics in One-Dimensional Colloidal Nanostructures for Solar Energy Conversion Open Access

Wu, Kaifeng (2015)

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Colloidal one-dimensional (1D) semiconductor nanocrystals offer the opportunity to maintain quantum confinement in two dimensions and to tune their light absorption and charge separation capabilities in the remaining dimension. For this reason, they can be implemented into various solar energy conversion applications, such as photocatalysis, which utilizes long distance charge separations, and luminescent solar concentrators, where strong light absorptions are needed. In this dissertation, we investigated carrier separation and transport dynamics in 1D colloidal nanostructures which are crucial for those applications. We first demonstrated efficient and long-lived photoinduced electron transfer from CdS and CdSe/CdS Nanorods (NRs) to Pt tips, enabled by ultrafast hole trapping in the former and hole localization to CdSe seed in the later. These studies reveal that hole immobilization is the key factor for efficient charge separation in 1D NRs. In addition, we showed that when using CdS-Pt and CdSe/CdS-Pt NRs for light-driven H2 evolution, hole removal was the efficiency limiting step, providing guidance for rational improvement of these NRs for solar-to-fuel conversion. We then studied plasmon induced hot electron transfer from metal to semiconductor in Au tipped NRs. Electron transfer in CdS-Au NRs was found to follow a low-efficiency conventional mechanism where the excited plasmon in Au decayed into a hot electron-hole pair and hot electron transfer into CdS competed with ultrafast electron relaxation within Au. In contrast, in CdSe-Au NRs the plasmon band was strongly damped and highly efficient charge separation was observed. For this, we proposed a new plasmon induced charge transfer transition mechanism. These findings suggest an exciting possibility of using plasmons as the light harvesting component for solar energy conversion. Finally, we studied exciton transport dynamics in CdSe/CdS NRs as a model system for luminescent solar concentrators (LSCs). We revealed a competition between band offset driven exciton transport and trap states induced exciton localization processes, which resulted in a universal length dependence of rod-to-seed exciton localization efficiency. We also showed that exciton trapping can be overcome in 2D CdSe/CdTe hetero-nanosheets where unity exciton localization efficiency was realized, showing great potentials for efficient LSCs.

Table of Contents

Chapter 1. Introduction. 1

1.1. Low-Dimensional Nanostructures beyond QDs for Solar Energy Conversion. 1

1.2. Electronic Structure of NRs. 5

1.3. Charge Separation and Recombination Dynamics in Photocatalytic Pt-tipped Nanorods 7

1.4. Plasmon Induced Charge Separation in Au-tipped NRs. 9

1.5. Exciton Transport Dynamics in Hetero NRs for Luminescent Solar Concentration. 12

1.6. Conclusion. 15

References. 18

Chapter 2. Experimental Methods. 24

2.1. Sample Preparation. 24

2.1.1. Synthesis of CdSe and CdS Quantum Dots. 24

2.1.2. Synthesis of CdSe, CdS and CdSe/CdS Nanorods. 25

2.1.3. Synthesis of Metal Tipped Nanorods. 27

2.1.4. Synthesis of CdSe, Pt tipped CdSe, and core/crown CdSe/CdTe Nanosheets. 30

2.1.5. Preparation of Water Soluble Nanorods. 31

2.1.6. Representative Sample Images. 32

2.2. Time Resolved Spectroscopy Setup. 33

2.2.1. Femtosecond Transient Absorption Setup. 34

2.2.2. Nanosecond Transient Absorption Setup. 35

2.2.3. Time Resolved Fluorescence Setup. 36

2.3. Light Driven H2 Evolution. 37

References. 38

Chapter 3. Ultrafast Charge Separation and Long-lived Charge Separated State in Photocatalytic CdS-Pt Nanorod Heterostructures. 39

3.1. Introduction. 39

3.2. Results and Discussion. 42

3.2.1. Sample synthesis and characterizations. 42

3.2.2. Carrier dynamics in free CdS NRs and NR-molecular acceptor complexes. 43

3.2.3. Charge separation and recombination dynamics in CdS-Pt NRs. 49

3.3. Conclusion. 51

References. 52

Appendix 1. 55

Appendix 2. 59

Chapter 4. Ultrafast Exciton Quenching by Energy and Electron Transfer in Colloidal CdSe Nanosheet-Pt Heterostructures. 62

4.1. Introduction. 62

4.2. Results and Discussion. 64

4.2.1. Sample synthesis and characterizations. 64

4.2.2. Carrier dynamics in free CdSe NSs and NS-molecular acceptor complexes. 66

4.2.3. Exciton quenching mechanism in Pt tipped CdSe NSs. 72

4.3. Conclusion. 78

References. 79

Appendix 1. 83

Appendix 2. 87

Appendix 3. 88

Chapter 5. Hole Removal Rate Limits Photo-driven H2 Generation Efficiency in CdS-Pt and CdSe/CdS-Pt Semiconductor Nanorod-metal tip Heterostructures. 93

5.1. Introduction. 93

5.2. Results and Discussion. 98

5.2.1 Absorption and emission properties of CdSe/CdS-Pt 98

5.2.2. Effect of electron donors on H2 generation efficiency. 100

5.2.3. Charge separation and recombination in CdSe/CdS-Pt and CdS-Pt 103

5.2.4. Comparing charge separation and recombination in CdS-Pt and CdSe/CdS NRs 110

5.2.5. Hole filling of CdSe/CdS and CdS NRs by electron donor 113

5.3. Conclusion. 120

References. 121

Appendix 1. 125

Appendix 2. 126

Appendix 3. 129

Appendix 4. 130

Appendix 5. 133

Chapter 6. Plasmon-Induced Hot Electron Transfer from the Au Tip to CdS Rod in CdS-Au Nanoheterostructures 139

6.1. Introduction. 139

6.2. Results and Discussion. 142

6.2.1. Static absorption and emission spectra of CdS-Au. 142

6.2.2. Electron transfer from excited CdS NRs to Au tips. 146

6.2.3. Plasmon induced hot electron transfer from Au to CdS. 155

6.3. Conclusion. 165

References. 166

Appendix 1. 171

Appendix 2. 177

Appendix 3. 183

Appendix 4. 185

Appendix 5. 187

Chapter 7. Plasmon Induced Interfacial Charge Transfer Transition for Efficient Hot Electron Transfer from Metal Nanostructures. 189

7.1. Introduction. 189

7.2. Results and Discussion. 192

7.2.1. Absorption spectra of CdSe-Au. 192

7.2.2. Electron transfer from excited Au to CdSe NRs. 196

7.2.3. Excitation Wavelength Independent Charge Separation Yields. 199

7.2.4. Polarization Dependent Charge Separation Yields. 202

7.2.5. Reduction of methyl viologen by plasmon-generated electrons. 204

7.3. Conclusion. 208

References. 208

Appendix 1. 211

Appendix 2. 214

Appendix 3. 217

Appendix 4. 219

Appendix 5. 224

Chapter 8. The Competition between Band Alignment and Hole trapping in CdSe/CdS Quasi-type II Dot-in-rod Nanorods. 227

8.1. Introduction. 227

8.2 Results and Discussions. 230

8.2.1 Static Absorption and Emission Spectra. 230

8.2.2 Nature of Lowest Energy Band Edge Exciton (X3) State. 235

8.2.3 Excitation Wavelength Dependent Exciton Relaxation Dynamics. 240

8.2.4. Assignment of Three Long-Lived Exciton States. 244

8.2.5. Charge Separation from Three Types of Excitons. 252

8.3. Conclusion. 256

References. 259

Appendix 1. 265

Appendix 2. 267

Appendix 3. 269

Chapter 9. Universal Length-Dependence of Rod-to-Seed Exciton Localization Efficiency in CdSe/CdS Dot-in-rod Nanorods. 271

9.1. Introduction. 271

9.2 Results and Discussions. 273

9.2.1. Preparation and Morphological Characterization of CdSe/CdS NRs. 273

9.2.2. Electronic structure of type I and quasi-type II CdSe/CdS NRs. 279

9.2.3. Length-dependent exciton localization efficiency. 283

9.2.4. Exciton Trapping on Nanorods. 285

9.2.5. Mechanism of universal length-dependent exciton localization efficiency. 287

9.3. Conclusion. 291

References. 292

Appendix 1. 296

Appendix 2. 298

Chapter 10. Unity Efficiency Formation of Charge-transfer Exciton State in Atomically-thin CdSe/CdTe Type-II Heteronanosheets. 300

10.1. Introduction. 300

10.2 Results and Discussions. 303

10.2.1. Sample Preparations and Characterizations. 303

10.2.2. Efficiency of Charge Transfer Exciton formation. 306

10.2.3. Ultrafast Charge Separation and Recombination Dynamics. 308

10.3. Conclusion. 317

References. 317

Appendix 1 321

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