Quantum Confined Semiconductor Nanocrystals for Efficient Charge Separation and Solar-to-fuel Conversion Público
Zhu, Haiming (2014)
Abstract
The advancement of solar-to-fuel conversion requires not only efficient catalysts but also efficient light harvesting and charge transfer centers. Because of tunable electronic, optical and chemical properties, quantum confined semiconductor nanocrystals represent an interesting system for fundamental and practical charge transfer studies. In this dissertation, we investigated single and multiple charge transfer from various quantum confined semiconductor nanocrystals as well as implemented them in solar-to-fuel conversion.
We first studied single exciton charge separation and recombination from CdS, CdSe, CdTe quantum dots (QDs), CdSe/ZnS type I and CdTe/CdSe type II core/shell QDs. We observed the rate of electron transfer (ET) from CdX QDs increases at decreasing size (and increasing driving force), showing a lack of Marcus inverted regime. We proposed an Auger-assisted ET model, in which the ET can be coupled to the hole excitation. In CdSe/ZnS type I QDs, with increasing shell thickness, both the charge separation and recombination rates decrease exponentially, which agrees well with the exponential decreases of the electron and hole densities at QD surface. In CdTe/CdSe type II QDs, the shell localized electron and core localized hole enable ultrafast ET while simultaneously retards the charge recombination process, which leads to the idea of "wavefunction engineering" for QD charge transfer.
We then studied multiexciton annihilation and dissociation from CdSe/CdS type II QDs and CdSe nanorods (NRs). W e show that the multiexciton dissociation efficiency (MED) can be significantly enhanced by controlling the band alignment and shapes of semiconductor NCs. The enhanced MED efficiencies are due to the electron and hole distributions in these nanomaterials, which simultaneously retard Auger recombination and facilitate interfacial ET.
Finally, we demonstrated efficient redox mediator generation and H2 evolution using asymmetric CdSe/CdS seeded NRs. Time resolved spectroscopy study shows that the higher efficiency of CdSe/CdS NRs is due to ultrafast ET, hole filling and slow charge recombination. Using ZnSe/CdS seeded NRs, we show wavelength dependent photocatalytic behavior and we found that under rod excitation, ultrafast ET from CdS rod and fast localization of hole to ZnSe seed suppressing charge recombination loss thus enhancing photocatalytic performance.
Table of Contents
Chapter 1. Introduction...1
1.1. Introduction of Semiconductor Nanocrystals
for Solar Energy Conversion...1
1.2. Interfacial Electron Transfer from Semiconductor
Nanocrytals...5
1.3. Multiexciton Annihilation and Dissociation in Semiconductor
Nanocrytals...7
1.4. Semiconductor Nanoheterostructures for Charge Transfer and
Solar-to-fuel Conversion...9
1.4. Conclusion...15
References...16
Chapter 2. Experimental Methods...25
2.1. Sample Preparation...25
2.1.1. Synthesis of CdS, CdSe, CdTe and ZnSe
Quantum Dots...25
2.1.2. Synthesis of CdSe/ZnS, CdSe/CdS and CdTe/CdSe Core/Shell
Quantum Dots...27
2.1.3. Synthesis of CdSe Nanorods...28
2.1.4. Synthesis of CdS Nanorod and CdSe/CdS and ZnSe/CdS
Dot-in-rod Nanorods...29
2.1.5. Preparation of Nanocrystal-molecule Adsorbate Complexes for
Spectroscopy Measurement...30
2.1.6. Preparation of Water Soluble Nanocrystals...31
2.1.7. Preparation of Pt Nanoparticle Catalyst...32
2.2. Time Resolved Spectroscopy Setup...32
2.2.1. Femitosecond Transient Absorption
Setup...33
2.2.2. Nanosecond Transient Absorption Setup...34
2.2.3. Time Resolved Fluorescence Setup...34
2.3. Steady State MV2+ Photoreduction...35
2.4. Light Driven H2 Evolution...36
References...38
Chapter 3. Electron Transfer from CdX Quantum Dots of Different Sizes: the Study of Driving Force...40
3.1. Introduction...40
3.2. Results and Discussion...43
3.2.1. Steady State and Transient Absorption
Spectra of CdX QD and QD-acceptor complexes...43
3.2.2. Size/Driving Force Dependent Electron Transfer Rates of CdX
QDs...48
3.2.3. Computational Simulation of Auger-assisted Electron Transfer
from CdX QDs...61
3.3. Conclusion...63
References...64
Appendix 1...70
Appendix 2...72
Appendix 3...74
Appendix 4...75
Appendix 5...77
Appendix 6...79
Chapter 4. Charge Separation and Recombination from CdSe/ZnS Type I Core/Shell Quantum Dots of Different Shell Thicknesses: the Study of Electronic Coupling...84
4.1. Introduction...84
4.2. Results and Discussion...87
4.2.1. Characterization of QDs and QD-AQ
Complexes...87
4.2.2. Exciton Dynamics in Free QDs...89
4.2.3. Charge Transfer Dynamics in QD-AQ Complexes...92
4.2.4. Shell Thickness Dependent Charge Separation and
Recombination Kinetics...96
4.3. Conclusion...105
References...106
Appendix 1...111
Appendix 2...112
Chapter 5. Charge Separation and Recombination from CdTe/CdSe Type II Core/Shell Quantum Dots: the Idea of Wavefunction Engineering...114
5.1. Introduction...114
5.2. Results and Discussion...119
5.2.1. Characterization of CdTe/CdSe Type II
QDs...119
5.2.2 Exciton Dynamics in Free CdTe/CdSe Type II QD...123
5.2.3. Charge Transfer Dynamics in QD-AQ Complex...129
5.2.4. Comparison between CdSe, CdTe, CdTe/CdSe (type II) and
CdSe/ZnS (type I) QDs...135
5.3. Conclusion...140
References...141
Appendix 1...148
Chapter 6. Multiexciton Annihilation and Dissociation from CdSe/CdS Quasi-type II Quantum Dots: the Effect of Band Alignment...152
6.1. Introduction...152
6.2. Results and Discussion...156
6.2.1. Characterization of CdTe/CdSe Type II
QDs...156
6.2.2. Single Exciton Charge Separation and Recombination Kinetics
in CdSe/CdS QDs...162
6.2.3. Multi-Exciton Dyanmics in CdSe/CdS QDs...164
6.2.4. Multi-Exciton Charge Separation and Recombination...175
6.3. Conclusion...178
References...180
Appendix 1...184
Appendix 2...186
Appendix 3...188
Appendix 4...191
Chapter 7. Multiexciton Annihilation and Dissociation from One-dimensional CdSe Nanorods: the Effect of Nanocrystal Shape...194
7.1. Introduction...194
7.2. Results and Discussion...197
7.2.1. Spectral Signature and Assignment of 1D
Excitons...197
7.2.2. Multi-exciton Dynamics in CdSe QRs...203
7.2.3. Single and Multiple Exciton Dissociation from CdSe
QRs...211
7.3. Conclusion...218
References...219
Appendix 1...223
Appendix 2...227
Appendix 3...228
Appendix 4...229
Chapter 8. Redox Mediator Photoreduction and H2 Evolution Using CdSe/CdS Dot-inrod Nanorod...230
8.1. Introduction...230
8.2. Results and Discussion...234
8.2.1. MV2+ Photoreduction...234
8.2.2. H2 Evolution Coupled with Pt as Catalyst...239
8.2.3. Mechanism for Efficient MV2+ Photoreduction...242
8.3. Conclusion...252
References...253
Appendix 1...257
Appendix 2...257
Appendix 3...258
Appendix 4...259
Chapter 9. Redox Mediator Photoreduction Using ZnSe/CdS Dot-in-rod Nanorod: Wavelength Dependent Quantum Yield...261
9.1. Introduction...261
9.2. Results and Discussion...263
9.2.1. Static Absorption and Emission
Spectra...263
9.2.2. Carrier Localization Dynamics in ZnSe/CdS
Nanorod...267
9.2.2. Stead State MV·+ Radial Generation...271
9.2.3. Charge Separation and Recombination under 555 nm
Excitation...274
9.2.4. Charge Separation and Recombination under 400 nm
Excitation...279
9.3. Conclusion...287
References...288
Appendix 1...294
Chapter 10. Summary and Outlook...295
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