Effect of Interfacial Interactions on Charge Transfer in QD-Molecular Catalyst Complexes Public

Abstract

Over the past few decades, the use of fossil fuels has generated greenhouse gases, resulting in increasing deleterious effects on our environment. Recently, research for alternative fuels has increased and has demonstrated positive advances towards processes such as artificial photosynthesis, CO2 reduction, and water splitting. Photocatalysis for the purpose of converting to these greener fuels is one of the main methods by which the products of these processes are achieved. Many types of materials can be used for this goal, particularly semiconductor nanomaterials and molecular catalysts. Fortunately, semiconductor nanocrystals (NCs), quantum dots (QDs) in particular, have several advantages, making them good materials for photocatalysis. They are efficient light harvesters, size tunable, and their surface chemistry can be manipulated towards improving the performance of the nanocrystal. The surface chemistry (ligands, etc) of QDs can control nanocrystal growth, solvent solubility, and electronic passivation, which dictate charge transfer properties in the NC. This work investigates how different molecules bound to the QD surface affect electron transfer processes with the purpose of CO2 reduction with a molecular catalyst. The first project details how electron transfer from a Cd3P2­ QD to a molecular catalyst, fac-Re(4,4′-R2-bpy)(CO)3Cl (bpy=bipyridine; R=COOH) (ReC0A), is changed upon the addition of a hole scavenger, triethylamine (TEA). ReC0A is highly selective and efficient for catalyzing CO2 reduction under only 400 nm excitation. By using a Cd3P2 QD, which can absorb into the near IR, and reducing the ReC0A catalyst, we can extend the range over which CO2 reduction can occur. The second project describes the Fano resonance (FR) coupling phenomenon that occurs between CdSe QD intraband transitions and ReC0A CO modes as a function of catalyst concentration and QD size. The third work demonstrates the effect of distance on FR coupling using ReCxA catalysts with different chain length linkers and core/shell CdSe/ZnS QDs. Finally, the last project is a collaboration with Hong Kong City University that investigates hot electron transfer in Ag-CdSe heterostructures resulting in high quantum yield. This thesis work demonstrates charge transfer dynamics in NC-molecular catalyst complexes upon ultrafast laser excitation for the application of CO2 reduction and photocatalysis.

Table of Contents

Chapter 1. Introduction. 1

1.1 Background and Motivation. 1

1.2 Quantum Dots and Their Surface Composition. 5

1.3 Effect of Surface on Plasmon Induced Hot Electron Transfer in Plasmonic Nanomaterials. 7

1.4 References. 9

Chapter 2. Experimental Methods. 14

2.1 Time Resolved Spectroscopy Techniques. 14

2.1.1 Transient Infrared Absorption Spectroscopy. 14

2.1.2 Visible Femtosecond Transient Absorption. 14

2.1.3 Time Correlated Single Photon Counting. 15

2.2 Quantum Dot Syntheses. 15

2.2.1 Cd3P2 QDs. 15

2.2.2 TOPO Capped CdSe QD Syntheses. 17

2.2.2 Oleate Capped CdSe QD Syntheses. 18

2.2.4 CdSe/ZnS Core Shell Structures. 19

2.3 Sample Preparation. 21

2.3.1 Cd3P2-ReC0A complexes. 21

2.3.2 CdSe-ReC0A complexes. 21

2.3.3 CdSe-ReCxA complexes. 22

2.3.4 CdSe/ZnS-ReS2-TiO2 films. 23

2.4 Other Characterization Methods. 23

2.4.1 Electrochemistry and Cyclic Voltammetry. 23

2.4.2 High Resolution Transmission Electron Microscopy (HR-TEM). 24

2.5 References. 25

Chapter 3. Amine Hole Scavengers Facilitate both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst 26

3.1 Introduction. 26

3.2 Results and Discussion. 27

3.2.1 Sample Preparation and Characterization. 27

3.2.2 Addition of Hole Scavenger TEA to QD... 33

3.2.3 Electron Transfer in Cd3P2/ReC0A Complexes. 39

3.2.4 Effect of TEA on ET in Cd3P2/ReC0A.. 43

3.3 Conclusions. 47

3.4 Supporting Information. 48

3.4.1 Cd3P2 Size Determination. 48

3.4.2 Attempting to Reverse Exciton Band Blue Shifting. 48

3.4.3 UV-Vis of ReC0A on CdS and CdSe. 50

3.4.4 TA Spectra of ReC0A + TEOA and QD + Methyl Viologen. 50

3.4.5 Transient IR Spectra of ReC0A on Cd3P2 52

3.4.6 Loss of Bleach Amplitude. 53

3.4.7 TA Spectra of Cd3P2 and Different Concentrations of ReC0A on Cd3P2 with and without TEA.. 54

3.4.8 TA Spectra of Cd3P2 + ReC0A with Different Concentrations of TEA.. 56

3.4.9 Time Correlated Single Photon Counting. 57

3.4.10 ¹H NMR and DOSY of Cd3P2 and TEA.. 57

3.4.11 Cyclic Voltammetry of ReC0A and TEA.. 60

3.4.12 Kinetics of QDs and Electron Acceptors with and without TEA.. 62

3.4.13 Ab initio Method Details. 63

3.4.14 Quantum Dot Area Calculation. 65

3.5 References. 66

Chapter 4. Fano Resonance Coupling of Quantum Dot-ReC0A Complexes Reveals Loading and Size Dependence. 70

4.1 Introduction. 70

4.2 Results and Discussion. 72

4.2.1 Characterization of Quantum Dots. 72

4.2.2 Electron Transfer Rates After Visible Excitation. 73

4.2.3 Fano Resonance as a Function of Catalyst Concentration. 75

4.2.4 Fano Resonance as a Function of QD Size. 79

4.3 Conclusions. 81

4.4 Supporting Information. 82

4.4.1 Characterization of ReC0A on CdSe QDs. 82

4.4.2 TA Spectra of ReC0A on QDs. 83

4.4.3 Subtraction of Solvent from TRIR Spectra. 86

4.4.4 Fitted TRIR Spectra of CdSe 545 with and without ReC0A.. 88

4.5 References. 93

Chapter 5. Effect of Distance on Fano Resonance Coupling in CdSe-ReCxA and CdSe/ZnS-ReS2 Complexes. 96

5.1 Introduction. 96

5.2 Results and Discussion. 97

5.2.1 Characterization of Quantum Dot and Rhenium Complexes. 97

5.2.2 Characterization of QD-Rhenium Complexes Using Visible Transient Spectroscopy. 101

5.2.3 Fano Resonance as a Function of Distance: Catalyst Modifications. 103

5.2.4 Effect of Shell Thickness on Fano Resonance. 106

5.3 Conclusions. 109

5.4 Supporting Information. 109

5.4.1 Characterization of Free Rhenium Complexes and Bound to QDs. 109

5.4.2 TA Spectra of Complexes on QDs. 114

5.4.3 Subtraction of solvent from TRIR Spectra: QD-ReCxA.. 117

5.4.4. Fitted TRIR Spectra of CdSe and CdSe/ZnS on TiO2 films. 118

5.5 References. 120

Chapter 6. Epitaxial Growth of Highly Symmetrical Branched Noble Metal-Semiconductor Heterostructures with Efficient Plasmon-Induced Hot-Electron Transfer. 123

6.1 Introduction. 123

6.2 Experimental Methods. 124

6.2.1    Quantum Yield Calculation. 124

6.2.2    Multiexponential Fitting for PHET and Charge Recombination. 126

6.3 Results and Discussion: Ultrafast Plasmon Induced Hot Electron Transfer 126

6.4 Conclusions. 131

6.5 References. 132

Chapter 7. Summary. 134

About this Dissertation

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School	Laney Graduate School
Department	Chemistry
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Chemistry, Physical
Mot-clé	Charge transfer Excitons Quantum dots Ultrafast spectroscopy
Committee Chair / Thesis Advisor	Tianquan Lian, Emory University
Committee Members	Craig Hill, Emory University Brian Dyer, Emory University

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