Orientation and Vibrational Dynamics of Rhenium Bipyridyl CO2-Reduction Catalysts on Model Electrode Surfaces Public

Abstract

The average molecular orientation of several rhenium bipyridyl CO2-reduction catalysts adsorbed onto rutile single crystalline TiO2 surfaces has been determined using vibrational sum frequency generation spectroscopy (VSFGS). The vibrational relaxation dynamics of two such catalysts were also investigated on two model electrode surfaces using time-resolved VSFGS techniques.

Polarization-resolved VSFGS was used to determine the average molecular orientation of ReC0A on TiO2 (001), which was found to be within 0 - 22° from the surface normal. These results were supported by Density Functional Theory (DFT) calculations of optimized adsorption geometries. The influence of the substrate on the molecular organization was investigated by monitoring the degree of surface-induced ordering for ReC0A on TiO2 (110). ReC0A exhibited a well-defined anisotropic arrangement following the C2v symmetry of the TiO2 (110) surface, in contrast to an isotropic distribution on the more symmetric (C4) TiO2 (001) surface. An average orientation angle of 28° was determined for the ReC0A/TiO2 (110) system. The influence of lengthening the molecular linkers in a similar system was determined by investigating the adsorption geometries for the series ReCnA (n = 0 - 4) on TiO2 (001) using phase-sensitive VSFGS and DFT calculations. The orientation angles were found to be closely correlated with the average lengths of the two linking anchoring groups, with the molecules tilting more towards the TiO2 surface with increasing linker length.

Time-resolved VSFGS was used to investigate the vibrational relaxation dynamics of the totally symmetric CO stretch of ReC0A on TiO2 (110) and ReC0-Au. Both systems exhibited bi-exponential relaxation from the υ = 1 state consisting of an ultrafast (sub-picosecond) initial relaxation followed by complete recovery of the ground vibrational state within tens of picoseconds. The ultrafast decay is assigned to rapid υ-υ coupling between the three CO stretching modes, and the slower decay is assigned to vibrational population relaxation from the coupled CO modes. Although both systems exhibited similar ultrafast decay rates of the excited state, the excited a'(1) mode persisted for significantly longer in ReC0A on TiO2 compared to ReC0-Au (τ2 = 30.35 and 14.8 ps, respectively). This is attributed to electronic interactions between ReC0A and TiO2 not present in the ReC0-Au system.

Table of Contents

Table of Contents Chapter 1. Introduction 1

1.1. General Introduction

1

1.2. Vibrational Sum Frequency Generation Spectroscopy

4 1.2.1. General Description 4 1.2.2. Historical Background 6

1.3. Catalytic Reduction of CO2 via Transition Metal Complexes

10 1.3.1. General Description 10

1.3.2. Rhenium Bipyridyl CO2 Reduction Catalysts

12

1.4. Summary and Overview

16 1.5. References 17

Chapter 2. Theoretical Description of Sum Frequency Generation

26 2.1. Origins of the Nonlinear Optical Response 27

2.2. Theoretical Description of Vibrational Sum Frequency Generation Spectroscopy

30

2.3. Using Vibrational Sum Frequency Generation Spectroscopy to Determine Molecular Orientation

35

2.4. Theory of Time-Resolved Vibrational Sum Frequency Generation Spectroscopy

39 2.5. References 42 Chapter 3. Experimental Methods 44 3.1. Sample Preparation 44

3.1.1. Molecular Adsorbates - Re bipyridyl complexes

44

3.1.2. Preparation of TiO2 Single Crystal Substrates

45

3.1.3. Sensitization of Rutile TiO2 Single Crystals

46 3.1.4. Preparation of Au Films 47

3.1.5. Chemisorption of a Rhenium Bipyridyl Complex on Gold Substrates

47

3.2. Static Vibrational Sum Frequency Generation Spectroscopy Measurements

48

3.2.1. The Laser Source

50

3.2.2. Generation of Tunable Broadband Infrared Pulses

52

3.2.3. Generation of Narrowband Visible Pulses

54 3.2.4. Sample Stage 57

3.2.4.1. Static Homodyne-detected Vibrational Sum Frequency Generation Spectroscopy Measurements

57

3.2.4.2. Static Phase Sensitive-detected Vibrational Sum Frequency Generation Spectroscopy Measurements

59

3.2.5. Detecting Generated Sum Frequency Pulses

59 3.3. Time-Resolved Measurements 61

3.3.1. IR-Pump SFG-Probe Homodyne-detected Vibrational Sum Frequency Generation Spectroscopy Measurements

61 3.3.2. Ultrafast Mid-Infrared Transient Absorption Measurements 62

3.4. Density Functional Theory Calculations (performed by Victor Batista et al. at Yale University)

64

3.4.1. Density Functional Theory (DFT) Calculations of ReC0A on TiO2 (001)

64

3.4.2. PS-VSFGS Spectra Simulation

64

3.4.3. Density Functional Theory (DFT) Calculations of ReCnA on TiO2 (001)

65

3.5. Vibrational Sum Frequency Generation Spectroscopy Signal Processing Methods

66

3.5.1. Static Homodyne-detected Vibrational Sum Frequency Generation Spectroscopy

66

3.5.2. Static Phase Sensitive-detected Vibrational Sum Frequency Generation Spectroscopy

66 3.6. References 68

Chapter 4. Orientation of a Rhenium Bipyridyl CO2-Reduction Catalyst Adsorbed onto Rutile Single Crystalline TiO2 (001)

71 4.1. Introduction 71 4.2. Results and Discussion 74

4.2.1. Static UV-visible and FTIR absorption spectra of ReC0A in solution

74

4.2.2. VSFGS spectra of ReC0A on TiO2 (001)

76

4.2.2.1. Spectral Processing Details

76

4.2.2.2. Processed VSFGS Spectra of ReC0A on TiO2 (001)

78

4.2.2.3. Doubly-resonant VSFGS Study

80

4.2.3. Orientation Analysis of ReC0A on TiO2 (001)

84

4.2.3.1. Theoretical Considerations

84

4.2.3.2. Molecular Orientation Determination

87

4.2.4. Density Functional Theory (DFT) Calculations of ReC0A on TiO2 (001)

88 4.3. Summary 91 4.4. References 91

Chapter 5. Surface-Induced Ordering of a Rhenium Bipyridyl CO2-Reduction Catalyst on Rutile TiO2 Surfaces

97 5.1. Introduction 97 5.2. Results and Discussion 99

5.2.1. VSFGS Spectra of ReC0A on TiO2 (001) and (110)

99

5.2.2. Azimuthal Angular Dependence of the VSFGS Spectra

103

5.2.3. Orientation Analysis Based on Azimuthal Angular Dependence of the VSFGS Spectra

105

5.2.3.1. Theoretical Considerations

105

5.2.3.2. Fitted Azimuthal Dependence for ReC0A on TiO2 (001) and (110)

106 5.3. Summary 108 5.4. References 109

Chapter 6. Orientation of a Series of Rhenium Bipyridyl CO2-Reduction Catalysts on Single Crystalline TiO2 (001) using Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy (PS-VSFGS)

113 6.1. Introduction 113 6.2. Results and Discussion 116 6.2.1. VSFGS Spectra of ReC0A and ReC1A on TiO2 (001) 116

6.2.2. Theoretical Description of Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy (PS-VSFGS)

119

6.2.3. Raw Phase-Sensitive Vibrational Sum Frequency Spectra of ReC0A/TiO2 (001) and Au and Detailed Signal Processing Methods

122

6.2.4. Processed Phase-Sensitive Vibrational Sum Frequency Generation Spectra of ReCnA (n = 0 - 4) on TiO2 (001)

127

6.2.5. Theoretical Analysis of the PS-VSFGS Spectra of ReCnA (n = 0 - 4) on TiO2 (001)

131

6.2.5.1. Determining Molecular Orientation from PS-VSFGS Spectra

131

6.2.5.2. Density Functional Theory (DFT) calculations of ReCnA on TiO2 (001)

138

6.2.5.3. Comparison with Previous Results

140 6.3. Summary 142 6.4. References 143

Chapter 7. Vibrational Relaxation of a Rhenium CO2-Reduction Catalyst on Semiconductor and Metal Surfaces

147 7.1. Introduction 147

7.2. Theory of Time-Resolved Vibrational Sum Frequency Generation Spectroscopy

151 7.3. Results and Discussion 155 7.3.1 ReC0A on TiO2 (110) 155 7.3.1.1. Static VSFGS Spectra of ReC0A on TiO2 (110) 155

7.3.1.2. Time-resolved VSFGS Spectra of ReC0A on TiO2 (110)

157 7.3.2. ReC0-Au 163 7.3.2.1. Static VSFGS Spectra of ReC0-Au 163 7.3.2.2. Time-resolved VSFGS Spectra of ReC0-Au 168

7.3.3. Transient IR-Pump IR-Probe Spectra of ReC0A in DMF

173

7.3.4. Comparison of All Three Systems

176 7.4 Summary 180 7.5. References 181

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School	Laney Graduate School
Department	Chemistry
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Chemistry, Physical
Mot-clé	Surface nonlinear optics Molecular orientation Surface vibrational dynamics Ultrafast laser spectroscopy Vibrational Sum Frequency Generation Spectroscopy VSFGS
Committee Chair / Thesis Advisor	Lian, Tim, Emory University
Committee Members	Heaven, Michael, Emory University Bowman, Joel M, Emory University

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