Interfacial Electron Transfer Dynamics between Organic Molecular Adsorbates and Semiconductor Nanoparticles 公开
Stockwell, David (2010)
Published
Abstract
Interfacial electron transfer dynamics (ET) from photoexcited organic molecular adsorbates to semiconductor (SC) nanocrystalline thin films have been measured using ultrafast transient mid-infrared and visible spectroscopy. The various factors influencing variation in ET rates, such as the electronic nature of semiconductors, adsorbate driving force and formation of exciplex intermediates were investigated.
Dynamics of ET from C343 to TiO2, SnO2 and ZnO nanocrystalline films were compared in order to study the role of exciplex-mediated ET at molecule/semiconductor interfaces. The deactivation of C343 excited state and formation of oxidized C343 were measured by transient visible spectroscopy to follow the rate of charge separation across the interface, forming the exciplex. Dissociation of the exciplex to form free electrons in the semiconductors was probed by electron absorption in the mid-infrared. Charge separation rates were instrument response limited (< 150 fs) and ~1 ps for C343 on TiO2 and SnO2, respectively. The faster ET rate to TiO2 than to SnO2 is attributed to the higher density of conduction band states in the former. On ZnO, initial charge separation across the interface was shown to be instrument response limited (< 150 fs), but exciplex dissociation over ~12 ps was shown to determine the overall ET rate.
ET dynamics from oligoene dyes to TiO2, SnO2 and ZnO nanocrystalline films, and from fluorescein 548 to In2O3, SnO2 and ZnO were compared in order to study both the semiconductor dependence on ET rate and the role of exciplex formation influencing electron diffusion into bulk SC. We show that the role of exciplexes is negligible in TiO2, In2O3 and SnO2 systems. On ZnO, the exciplex formation is suggested to be dependent on adsorbate coupling to surface states, where oligoene exciplex formation was shown to be instantaneous. Exciplex dissociation mediating electron injection into bulk ZnO is ~5 picoseconds, determining the overall ET rate. Results for fluorescein/ZnO indicate negligible exciplex formation, suggesting that ET is mediated by excited state deactivation, injecting electrons directly into the ZnO conduction band. For the dyes examined, the semiconductor dependence of ET rate follows the previously observed trend of TiO2 >> SnO2 ~ In2O3 > ZnO.
Table of Contents
Table of Contents
Chapter 1. Introduction 1
1.1 General Introduction 1
1.2 ET Dynamics From Dyes to Nanoporous Semiconductor Films 4
1.2.1 Measurement of Interfacial Electron Transfer 4
1.2.2 Dependence of ET Injection on Accepting Semiconductor 6
1.3 Investigation of Molecular Orientation 7
1.4 Summary 9
References 10
Chapter 2. Theoretical Considerations 22
2.1 Theory of Interfacial Electron Transfer 22
2.1.1 Electron transfer from donor to acceptor 22
2.1.2 Interfacial electron transfer between adsorbate and
semiconductor 28
2.2 Theory of Sum Frequency Vibrational Spectroscopy 32
References 36
Chapter 3. Experimental Methods 39
3.1 Preparation of Semiconductor Colloid and Nanoporous Films
39
3.1.1 Preparation of TiO2 nanoporous thin films 39
3.1.2 Preparation of SnO2 nanoporous thin films 40
3.1.3 Preparation of ZnO nanoporous thin films 40
3.1.4 Preparation of In2O3 nanoporous thin films 41
3.1.5 Preparation of ZrO2 nanoporous thin films 41
3.2. Preparation and Characterization of Au and TiO2
Nanocrystalline Films
for Sum Frequency Vibrational Spectroscopy 41
3.2.1 Preparation of Au films 42
3.2.2. Preparation of TiO2 nanocrystalline films on CaF2 prisms and
Au-
coated sapphire windows 43
3.3 Dyes Used in This Work 44
3.3.1 Coumarin 343 44
3.3.2 Oligoene Dyes 45
3.3.3 Rhodamine B and Fluorescein 548 45
3.3.4 Re polypyridyl complexes 46
3.4 Sensitization of Samples 47
3.4.1 Sensitization of Semiconductor Nanoporous Thin Films 47
3.4.2 Sensitization of Au and TiO2 Monolayer Films 47
3.5 Ultrafast Transient Absorption Measurements 48
3.5.1 Ultrafast Mid-Infrared Transient Absorption Measurements
48
3.5.2 Ultrafast Visible Transient Absorption Measurements 49
3.6 Time-Resolved Fluorescence Spectroscopy Measurements 50
3.7 Sum Frequency Vibrational Spectroscopy Measurements 50
References 52
Chapter 4. Interfacial electron transfer from Coumarin-343 to
TiO2, SnO2 and ZnO nanocrystalline thin films 54
4.1 Introduction 54
4.2 Results 57
4.2.1 C343/EtOH 57
4.2.2 C343/TiO2 60
4.2.2.1 Transient IR measurement of dye coverage dependence
60
4.2.2.2 Transient IR measurement and excitation power dependence.
63
4.2.2.3 Transient visible spectra of C343/TiO2 64
4.2.2.4 Comparison of IR absorption and visible spectral signatures
66
4.2.3 C343/SnO2 67
4.2.3.1 Transient IR measurement of dye coverage dependence
68
4.2.3.2 Transient IR measurement and excitation power dependence.
69
4.2.3.3 Transient visible spectra of C343/SnO2 71
4.2.3.4 Comparison of IR absorption and visible spectral signatures
73
4.2.4 C343/ZnO 74
4.2.4.1 Transient IR measurement of dye coverage dependence
74
4.2.4.2 Transient IR measurement and excitation power dependence.
76
4.2.4.3 Transient visible spectra of C343/ZnO 78
4.2.4.4 Comparison of IR absorption and visible spectral signatures
80
4.2.5 Comparison of mid-IR electron absorption kinetics from C343
to TiO2, SnO2 and ZnO films 81
4.3 Discussion 84
4.4 Summary 89
References 90
Chapter 5. Interfacial electron transfer from Oligoene Dyes
to TiO2, SnO2 and ZnO nanocrystalline thin films 96
5.1 Introduction 96
5.2 Results 98
5.2.1 EtOH solution 98
5.2.2 Oligoene/SnO2 102
5.2.2.1 Transient IR measurement of electron transfer kinetics.
103
5.2.2.2 Transient visible spectra of O1/SnO2. 106
5.2.2.3 Comparison of IR absorption and visible spectral
signatures. 109
5.2.3 Oligoene/TiO2 110
5.2.3.1 Transient IR measurement of electron transfer kinetics.
111
5.2.3.2 Transient visible spectra of O1/TiO2. 113
5.2.3.3 Comparison of IR absorption and visible spectral
signatures. 114
5.2.4 Oligoene/ZnO 115
5.2.4.1 Transient IR measurement of electron transfer kinetics.
116
5.2.4.2 Transient visible spectra of O1/ZnO. 117
5.2.4.3 Comparison of IR absorption and visible spectral
signatures. 119
5.2.4 Emission lifetime/relative injection yield measurements.
120
5.3 Discussion 121
5.4 Summary 127
References 128
Chapter 6. Interfacial electron transfer from Fluorescein 548 to
SnO2, In2O3 and ZnO nanocrystalline thin films 133
6.1 Introduction 133
6.2 Results 136
6.2.1 Fl/ZrO2 136
6.2.2 Fl/In2O3 138
6.2.2.1 Transient IR measurement of dye coverage dependence.
139
6.2.2.2 Transient IR measurement and excitation power dependence.
141
6.2.2.3 Transient visible spectra of Fl/In2O3. 143
6.2.2.4 Comparison of IR absorption and visible spectral
signatures. 145
6.2.3 Fl/SnO2 147
6.2.3.1 Transient IR measurement of dye coverage dependence.
148
6.2.3.2 Transient IR measurement and excitation power dependence.
149
6.2.3.3 Transient visible spectra of Fl/SnO2. 151
6.2.3.4 Comparison of IR absorption and visible spectral
signatures. 153
6.2.4 Fl/ZnO 154
6.2.4.1 Transient IR measurement of dye coverage dependence.
154
6.2.4.2 Transient IR measurement and excitation power dependence.
156
6.2.4.3 Transient visible spectra of Fl/ZnO. 158
6.2.4.4 Comparison of IR absorption and visible spectral
signatures. 160
6.2.5 Comparison of Electron Injection from Fl to In2O3, SnO2 and
ZnO Films 161
6.3 Discussion 163
6.4 Summary 168
References 169
Chapter 7. Sum frequency vibrational spectroscopy study of
molecular sensitizers to Au/TiO2 interfaces 175
7.1 Introduction 175
7.2 Results and Discussion 177
7.2.1 Au-enhanced Thiol SFVS Spectra 177
7.2.2. Au-enhanced ReC0-thiol SFVS Spectra 182
7.2.3. Total Internal Reflection ReC1P SFVS Spectra 191
7.3 Summary 196
References 197
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