Ultrafast Charge and Energy Transfer Dynamics in Photoelectrochemical and QD-based Photon Upconversion Systems Open Access

Xu, Zihao (Summer 2019)

Permanent URL: https://etd.library.emory.edu/concern/etds/5h73px061?locale=en


Photocatalysis at the semiconductor-liquid interface is a complex process, which includes the separation, transport, surface recombination, and ultimately interfacial transfer of photoexcited carriers. The rates and efficiencies of these processes are influenced by the electrostatic fields within the semiconductor at the solid/liquid interface. However, direct in situ time-resolved probe of the interfacial field and their influence on the aforementioned elementary steps have been difficult so far, and as a result, mechanistic understandings of key efficiency limiting factors remains poorly developed. Here in the first section, we use in situ transient reflectance spectroscopy to directly both the dynamics and efficiency of charge separation across the p-GaP/TiO2 interface through the Franz-Keldysh effect. We demonstrate that with more negative applied electrochemical potentials, both rate and efficiency of interfacial carrier separation increases. The efficiency of this initial charge separation event (occurring on the < 200 ps time scale) correlates well with the incident photon to current efficiency (IPCE) of these photoelectrodes over a wide range of potentials and excitation power densities, which provides direct evidence that the steady state IPCE is determined by the transiently separated charge carriers on the ultrafast time scale. This study establishes a powerful and general method for in situ time-resolved probe of carrier dynamics in other semiconductor photoelectrode. Photogenerated carrier eventually migrates to the semiconductor/liquid interface for charge transfer to the redox species in electrolyte. Observation of this step and understanding the reaction mechanism could reveal mechanistic insights on the effect of catalysts and other surface modifications. We have also developed a time resolved electrical field induced second harmonic generation (EFISH) technique probe the extent of band bending and carrier dynamics in the semiconductor, which is general method that is applicable to all semiconductors.

Quantum dot sensitized triplet-triplet annihilation photon upconversion is a promising method to utilize sub band gap photons for photoelectrochemical applications that requires high energy photon. It is a constant challenge to develop efficient up conversion system. In the last section we utilized transient absorption spectroscopy to study the key efficiency limiting factors in quantum dot sensitized triplet energy transfer.

Table of Contents

1. Chapter 1. Introduction 1

1.1 Solar water splitting with semiconductor electrode 1

1.2 Charge transfer in solar water splitting system 6

1.2.1 Semiconductor depletion layer at equilibrium in dark 6

1.2.2 Semiconductor potential-current under applied potential 11

1.2.3 Semiconductor potential-current under illumination 13

1.3 Challenges in studying charge carrier dynamics in semiconductor water splitting systems 15

1.4 Photon upconversion sensitized by quantum dots 19

1.4.1 Intersystem Crossing in molecule 20

1.4.2 Quantum dot sensitized triplet energy transfer 22

1.4.3 Competition of charge and energy transfer in quantum dot sensitized triplet energy transfer 24

1.4.4 Surface chemistry of quantum dot affects the molecular triplet lifetime 25

1.5 References 26

2. Chapter 2. Experimental Methods 32

2.1 Transient absorption and reflectance spectroscopy 32

2.2 Time resolved microscopy platform 34

2.3 Time resolved second harmonic generation spectroscopy 36

2.3.1 Femtosecond time resolution second harmonic generation 36

2.3.2 Microsecond time resolution SHG 36

2.4 Photoelectrochemistry setup 37

2.5 Time resolved photoluminescence spectroscopy 38

2.6 Sample preparation 38

3. Chapter 3. Ultrafast Dynamics of Photoinduced Charge Carriers and Electrical Fields at Semiconductor/Liquid Interface 39

3.1 Introduction 39

3.2 Materials and Methods 41

3.2.1 Materials preparation 41

3.2.2 Photoelectrochemical Setup 41

3.2.3 Transient reflectance spectroscopy 42

3.3 Results and Discussion 44

3.3.1 Characterization of 5 nm TiO2 GaP electrode 44

3.3.2 Potential and excitation power dependent IPCE 45

3.3.3 Charge Separation enhancement by p-n junction 48

3.3.4 Potential and excitation power dependent charge separation efficiency 54

3.3.5 Potential and excitation power dependent charge separation kinetics 58

3.4 Conclusion 62

3.5 References 62

4. Chapter 4. Time resolved Second Harmonic Generation for Probing Band Structure at Semiconductor/Liquid Interface 65

4.1 Introduction 65

4.2 Results and Discussion 69

4.2.1 In situ EFISH 69

4.2.2 Photoinduced EFISH spectroscopy 73

4.2.3 Time resolved EFISH probing carrier dynamics 77

4.2.4 Effect of catalyst on water oxidation 78

4.3 Conclusion 80

4.4 Reference 80

5. Chapter 5. Enhanced Intersystem Crossing from Singlet to Triplet by Radical in BODIPY Molecule 84

5.1 Introduction 84

5.2 Results and Discussion 87

5.2.1 Design and Synthesis of BODIPY-TEMPO Dyads 87

5.2.2 Absorption and Emission Spectra 91

5.2.3 Gibbs Free Energy Calculation 94

5.2.4 Singlet Oxygen Quantum Yield (ϕΔ) 96

5.2.5 Transient Absorption Spectroscopy Study 96

5.2.6 DFT Simulation 105

5.3 Conclusion 109

5.4 Reference 110

6. Chapter 6. Triplet Energy Transfer to Oligothiophene Molecule Sensitized by Quantum Dots 113

6.1 Introduction 113

6.2 Results and Discussion 116

6.2.1 Synthesis, Design and Photophysical Property of T6 116

6.2.2 Singlet, Triplet and Intersystem Crossing of T6 122

6.2.3 DFT Calculation of Energetics of T6 127

6.2.4 Synthesis and Transient response of CdSe QD. 131

6.2.5 CdSe Quantum Dot Sensitized Direct Energy Transfer 133

6.2.6 Determine Intrinsic Triplet Energy Transfer Rate 136

6.3 Conclusion 142

6.4 References 143

7. Chapter 7. Photophysics of a Quantum Dot based Photon Upconversion 146

7.1 Introduction 146

7.2 Results and Discussion 147

7.2.1 Synthesis of PbS QD, PbSCdS QD and 5-CT 147

7.2.2 PbS QD Sensitized Photon Upconversion 149

7.2.3 Determine QD, 5-CT redox potential by Cyclic Voltammetry 153

7.2.4 Transient Absorption Spectroscopy of 5-CT, PbS, PbSCdS and PbS-CT, PbSCdS-CT 155

7.2.5 Kinetics of PbS triplet energy transfer to 5-CT 161

7.3 Conclusion 166

7.4 Reference 168

8. Chapter 8. Surface Chemistry as a Key Factor in Photon Upconversion System 170

8.1 Introduction 170

8.2 Results and Discussion 171

8.2.1 Synthesis of PbS QD and Sample Preparation 171

8.2.2 Photon Upconversion Quantum Yield 173

8.2.3 Comparison of PbS-S and PbS-T QD 178

8.2.4 Transient absorption spectroscopy results 183

8.2.5 Double Difference Spectra to separate CS and Triplet State 186

8.2.6 Competition of Charge separation state and triplet state 188

8.3 Conclusion 192

8.4 References 195

9. Conclusion and Outlook 197

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