Charge Transfer Dynamics in Homogeneous and Heterogeneous Artificial Photosynthetic Systems Open Access

Huang, Zhuangqun (2011)

Permanent URL: https://etd.library.emory.edu/concern/etds/41687j348?locale=en
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Abstract

The direct, efficient, and sustained sunlight-driven water splitting process as a method to store energy in the simplest but highest energy-dense chemical bond, H2, remains one of the most desirable targets to achieve energy and environmental sustainability. The three general goals of this thesis are: (1) utilization of polyoxometalate (POM) WOCs for solar-driven water oxidation; (2) preparation of triads and all inorganic dyads while tuning their charge separation dynamics and O2 evolution properties, and (3) evaluation of the interfacial electron transfer dynamics and O2 evolution properties of these dyads and triads.

This thesis starts from the evaluation of the all-inorganic polyoxometalate WOCs in a homogeneous system. These WOCs are oxidatively, hydrolytically and thermally stable, can accommodate multiple metals with varying potentials, and therefore are promising for efficient solar fuel production. The spectroscopic studies of the charge separation dynamics in homogeneous systems show that an integrated acceptor-photosensitizer-WOC composite with intact, directly-connected structure is a promising approach to accelerate the water oxidation process and hence improve the selectivity and quantum efficiency of the WOC. Before discussing heterogeneous systems, it is vital to understand the photophysics of the polyoxometalate molecules, where the nature of the metal-to-polyoxometalate charge transfer in a POM chromophore is discussed in detail. This investigation facilitates our understanding of the interfacial change transfer dynamics in composite electrodes, and more importantly the development of all inorganic dyadic photoanodes with POM choromophores as photosensitizers. After that, the efforts to prepare and characterize stable nano-assembles of triads and dyads are presented, where the triadic and dyadic electrodes are extensively investigated by transient spectroscopy. Throughout the whole work, various spectroscopic tools are employed to understand the relevant fundamental photophysics, photochemistry, and photoelectrochemistry in order to improve the performances of the existing systems and to guide the designs of new constructs.

Table of Contents

Table of Contents

Part I: Introduction

Chapter 1 Fundamentals and Mechanistics in Natural and Artificial Photosynthetic Systems...1

1.1 Overview...2
1.2 Photosystem II: fundamentals and mechanisms...4
1.3 General challenges in artificial photosynthesis...10
1.4 Polyoxometalate water oxidation catalysts...15
1.5 Goal of this thesis and outline...17
1.6 Reference...19

Part II: Experimental Methods

Chapter 2 Experimental Methods...27

2.1 Materials...28
2.2 Synthesis...28
2.3 Sensitization of TiO2...31
2.4 Sensitization of γ-Fe2O3 nanoparticles...31
2.5 Coating of surface catalysts...32
2.6 General instrumentations...33
2.7 Time-resolved fluorescence measurement...33
2.8 Transient absorption measurement...34
2.9 General procedures...37
2.10 Reference...39

Part III: Homogeneous Light-driven Water Oxidation

Chapter 3 Homogeneous Light-driven Water Oxidation...41

3.1 Introduction...42
3.2 Results and discussion...46

3.2.1 System optimization...46
3.2.2 Oxygen evolution...47
3.2.3 Mechanism...49
3.2.4 Ru4POM and Co4POM...54

3.3 Conclusion...56
3.4 Reference...57

Chapter 4 Charge transfer Dynamics in Homogeneous Systems...61

4.1 Introduction...62
4.2 Spectroscopic methods...64
4.3 Photoinduced electron transfer from Ru dyes to S2O82-...66

4.3.1 Basic theory for fluorescence quenching...66
4.3.2 Stern-Volmer plots...68

4.4 Electron transfer dynamics in photocatalytic systems...71
4.5 Conclusion...75
4.6 Reference...75

Chapter 5 Designs for Improved Water-Oxidation...79

5.1 Introduction...80
5.2 Alternative photosensitizer: [Ru(mptpy)2]4+...81

5.2.1 Photophysics and photochemistry of [Ru(mptpy)2]4+...83
5.2.2 Time-resolved fluorescence decay measurements...85
5.2.3 [Ru(mptpy)2]4+ in solution-based water oxidation system...91

5.3 Microheterogeneous system...92
5.4 Conclusion...94
5.5 Reference...94

Part IV: Photophysics of Polyoxometalates

Chapter 6 Photophysics of Polyoxometalate-based Chromphores...97

6.1 Introduction...98
6.2 Results and discussion...100

6.2.1 Ru4POM...100
6.2.2 {Mo72V30} keplerate...103
6.2.3 [CoII2(H2O)W11O39]8-...104

6.3 Conclusion...107
6.4 Reference...107

Chapter 7 Characterization of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore...112

7.1 Introduction...113
7.2 Results and discussion...114

7.2.1 Static spectroscopy...115
7.2.2 Computation...117
7.2.3 Femtosecond pump-probe spectroscopy...118

7.3 Conclusion...124
7.4 Reference...125

Part V: Heterogeneous Systems: Composite Triadic Photoanodes

Chapter 8 Interfacial Charge Transfer in a TiO2/Ru470/Ru4POM Triad...128

8.1 Introduction...129
8.2 Results and discussion...131
8.3 Conclusion...138
8.4 Reference...139

Chapter 9 Interfacial Charge Transfer Dynamics in Well-defined TiO2/Sensitizer/Ru4POM Triads...143

9.1 Introduction...144
9.2 Results and discussion...146

9.2.1 TiO2/RuP2/Ru4POM triad...146
9.2.2 TiO2/RuC2/Ru4POM triad...153

9.3 Photoelectrochemical performance...155
9.4 Conclusion...159
9.5 Reference...159

Part VI: Heterogeneous systems: composite dyadic photoanodes

Chapter 10 Heterogeneous Systems: Composite Dyadic Photoanodes...162

10.1 Introduction...163
10.2 Results and discussion...166
10.3 Conclusion...170
10.4 Reference...171

Chapter 11 Photophysics of Hematite Photoanode...175

11.1 Introduction...176
11.2 Results and discussion...178

11.2.1 Electronic spectra of hematite...178
11.2.2 Femtosecond visible measurements...181
11.2.3 Nanosecond visible measurements...188
11.2.4 Transient spectral assignment: Stark effect...190
11.2.5 Transient spectral assignment: dye-sensitized approaches...194
11.2.6 Transient spectral assignment: PEC approaches...195

11.3 Conclusion...195
11.4 Reference...196

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