10 research outputs found
์ฐํํ์ ๋ฐ์ ์ค๊ณ๋ฅผ ํตํ ์ ์ด๊ธ์ ์ฐํ๋ฌผ์ ์ฐํ์ํ ์ ์ด์ ๊ด์ ๊ธฐํํ์ ์๋์ง ๋ณํ ์์ฉ
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ผ๋ฌธ (๋ฐ์ฌ)-- ์์ธ๋ํ๊ต ๋ํ์ : ๊ณต๊ณผ๋ํ ์ฌ๋ฃ๊ณตํ๋ถ, 2018. 8. ์ฃผ์์ฐฝ.Transition metals have various oxidation states because they have electrons in the d-orbital, which has small energy differences. Therefore, transition metals have the redox property that electrons can easily be lost and obtained. These metals can also bond with various anions, forming oxides, nitrides and sulfides with various compositions showing various material properties. Especially, transition metal oxides are widely used for photoelectrochemical catalyst because they show various band structures according to the oxidation states. The oxidation states in transition metal oxides is crucial for the selectivity and the activity of catalyst because they are related to the potential of photogenerated electrons, which is determined by the position of energy band. Among the photoelectrochemical energy conversion reactions, the CO2 reduction reaction is greatly affected by the electron potential of photogenerated electrons for its selectivity and efficiency. Because, in performing the reduction of CO2 in an aqueous condition, which is the requirement of mimicking natural photosynthesis process, the reduction of CO2 has to compete with the hydrogen evolution reaction. Making the CO2 reduction more dominant was difficult because the rate-determining step of CO2 reduction has a higher redox potential than that of hydrogen evolution reaction, which requires the precise control of active materials.
The objective of this thesis is precisely controlling the oxidation state of transition metal oxide to selectively perform the photoelectrochemical CO2 reduction and designing the nanostructure to optimize the activity and stability of active materials.
Firstly, 1-D nanostructured mono-phase Cu2O nanofiber photocathode for selective photoelectrochemical CO2 reduction was fabricated using electrospinning method and thermodynamically programmed calcination. The phase of Cu2O nanofiber should be precisely controlled to mono-phase without the impurity phase as CuO to selectively perform the photoelectrochemical CO2 reduction over hydrogen evolution reaction. Until now, the atmosphere during the calcination process was not precisely controlled either oxidative atmosphere by atmospheric pressure, or reductive atmosphere by Ar or H2 gas resulting mixing with CuO. The phase of copper oxide nanofibers was controlled by the nanoscale gas-solid reaction considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of Cu2O in which mono-phase Cu2O can be fabricated in a reasonable reaction time scale is discovered. Also, a hierarchical structure of Cu2O thin film underlayer and TiO2 passivation layer is developed to optimize the photoactivity and stability of Cu2O nanofiber electrode. By controlling the oxidation state of copper oxide nanofiber electrode and designing the appropriate nanostructure, a faradic efficiency of 93% for CO2 conversion to alcohol was achieved in an aqueous media.
The second focus is to control oxidation state in nanostructured hematite (ฮฑ-Fe2O3) and to develop hierarchically structured photoanode for effective oxygen evolution reaction (OER), which is the counter reaction of photoelectrochemical CO2 reduction. One of the most important challenges of hematite photoanode for water oxidation is the improvement of the electrical conductivity. To date, the conventional approach to overcome the drawback has been to identify heterogeneous dopants. We systematically controlled oxygen vacancy as a new intrinsic dopant source and investigated the interplay with external dopant, such as tin (Sn). Based on this understanding, we demonstrate that controlled generation of oxygen vacancies can activate the photoactivity of hematite significantly. Also we developed hierarchical structure that consists of undoped Fe2O3 underlayer, Ti-doped Fe2O3 nanorod, ฮฒ-FeOOH overlayer to optimize the photoactivity of hematite. Undoped hematite underlayer negatively shifted the onset potential by 40 mV (0.84 V vs. RHE) and ฮฒ-FeOOH overlayer enhanced the photocurrent density to 1.83 mA cm-2 at 1.4 V vs. RHE.
This study provides useful information for understanding the methodology to precisely control the oxidation state of transition metal oxide and rational design of nanostructured electrodes, and their effects on activity and selectivity of photoelectrochemical reactions.Table of Contents
Abstractโฆโฆโฆโฆโฆ.โฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆ.. i
Table of contentsโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆ. iv
List of Tablesโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆ... viii
List of Figuresโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆโฆ. ix
Chapter 1. Introduction
1.1. Materials for photoelectrochemical CO2 reduction .................. 1
1.2. Phase and structural issues of photoelectrode materials ............. 6
1.2.1. Cu2O photocathodes for CO2 reduction .............................. 6
1.2.2. Fe2O3 photoanodes for water oxidation .............................. 12
1.3. Objective of the thesis ................................................................. 16
1.4. Organization of the thesis ............................................................ 20
Chapter 2. Theoretical Background
2.1. Semiconductor phoelectrochemistry .................................... 21
2.1.1. Energy band and Fermi level ........................................... 21
2.1.2. Semiconductor/electrolyte interface .................................. 26
2.1.3. Potential distributions at the interface ........................... 30
2.1.4. Photoelectrochemical process ........................................ 38
2.1.5. Recombination of carriers ............................................. 44
2.2. Photoelectrochemical CO2 reduction ........................... 50
2.2.1. Theory of photoelectrochemical CO2 reduction ............... 50
2.2.2. Summary of CO2 reduction catalyst and performances ..... 53
2.3. Photoelectrochemical water oxidation ..................................... 56
2.3.1. Theory of photoelectrochemical water oxidation ............ 56
2.3.2. Trends in water oxidation activity of catalyst ................. 60
Chapter 3. Experimental Procedures
3.1. Sample preparation ...................................................................... 63
3.1.1. Fabrication of Cu2O photocathode ................................... 63
3.1.2. Fabrication of Fe2O3 photoanode ................................... 65
3.2. Photoelectrochemical analysis .................................................. 68
3.3. Microstructural and chemical analysis ................................... 70
Chapter 4. Design and Fabrication of Cu2O Nanofiber Photocathode
4.1. Introduction .................................................................................. 71
4.2. Fabrication of mono-phase Cu2O nanofiber .............................. 75
4.2.1. Optimization of the electrospinning process .................... 75
4.2.2. Design of the reaction path ............................................. 81
4.2.3. Design of the reaction parameter ................................... 85
4.3. Fabrication of the hierarchical structure ................................ 94
4.3.1. Optimization of Cu2O thin film underlayer ...................... 95
4.3.3. Optimization of TiO2 passivation layer ........................... 98
4.4. Summary ..................................................................................... 107
Chapter 5. Photoelectrochemical performance of Cu2O photocathode
5.1. Introduction .............................................................................. 108
5.2. Photoelectrochemical performances ........................................... 109
5.2.1. Effects of Cu2O thin film underlayer ................................ 109
5.2.2. Effects of TiO2 passivation layer ..................................... 112
5.2.3. Effects of the phase of copper oxide ................................ 119
5.3. Summary ................................................................................... 128
Chapter 6. Generation of Oxygen Vacancies in Fe2O3 Photoanode
6.1. Introduction ............................................................................... 129
6.2. Characterization of spray-coated Fe2O3 films ............................ 132
6.2.1. Microstructure of Fe2O3 films ........................................... 132
6.2.2. Phase of Fe2O3 films during annealing ............................... 135
6.3. Effects of oxygen vacancy generation on photoactivity ............. 138
6.3.1. Generation of oxygen vacancy ...................................... 138
6.3.2. Interplay with the external dopant .................................... 140
6.3.3. Effects on photoeletrochemical performance ................... 144
6.4. Summary ...................................................................................... 156
Chapter 7. Development of Hierarchically Structured Fe2O3 Photoanode
7.1. Introduction ................................................................................. 157
7.2. Fabrication of hierarchically nanostructured Fe2O3 .................. 161
7.3. Effects of hierarchical structure on photocurrent density ........ 168
7.4. Effects of ฮฒ-FeOOH on charge transfer kinetics ....................... 175
7.4.1. Investigation of the transient photocurrent ....................... 175
7.4.2. Analysis by electrochemical impedance spectroscopy ...... 178
7.5. Summary ................................................................................... 181
Chapter 8. Conclusion
8.1. Summary of Results .................................................................... 182
8.2. Future work and suggested research ........................................ 185
References ............................................................................................... 186
Abstract (In Korean) ............................................................................ 206
Curriculum Vitae ................................................................................. 209Docto
์ ํผ์จ ์ฐ๊ตฌ์์์์ ํฌ๊ด์ ์ ์-์์ฑ์ ์ฐ๋์ ์ด์ฉํ ์ด์ค ์คํ ๋น๋์นญ์ฑ ์ฐ๊ตฌ
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ผ๋ฌธ(๋ฐ์ฌ)--์์ธ๋ํ๊ต ๋ํ์ :์์ฐ๊ณผํ๋ํ ๋ฌผ๋ฆฌยท์ฒ๋ฌธํ๋ถ,2014. 8. ์ต์ ํธ.The spin structure of the proton has been investigated in the high Bjorken x and low momentum transfer Q^2 region. We used Jefferson Lab's polarized electron beam, a polarized target, and a spectrometer to get both the parallel and perpendicular spin asymmetries A_para and A_perp. These asymmetries produced the physics asymmetries A_1 and A_2 and spin structure functions g_1 and g_2. We found Q^2 dependences of the asymmetries at resonance region and higher-twist effects. Our result increases the available data on the proton spin structure, especially at resonance region with low Q^2. Moreover, A_2 and g_2 data show clear Q^2 evolution, comparing with RSS and SANE-BETA. Negative resonance in A_2 data needs to be examined by theory. It can be an indication of very negative transverse-longitudinal interference contribution at W ~ 1.3 GeV. Higher twist effect appears at the low Q^2 of 1.9 GeV^2, although it is less significant than lower Q^2 data of RSS. Twist-3 matrix element d_2 was calculated using our asymmetry fits evaluated at Q^2 = 1.9 GeV^2. d_2 = -0.0087 +- 0.0014 was obtained by integrating 0.47 <= x <= 0.87.Abstract i
List of Figures xii
List of Tables xiv
1 Introduction 1
1.1 Structure Functions and Asymmetries 3
1.2 Spin Structure 7
1.3 World Data 11
1.4 Motivation of the Experiment 17
2 Experimental Setup 23
2.0.1 Overview of the Setup 23
2.1 Electron Beam 25
2.1.1 Electron Source 25
2.1.2 Accelerator 27
2.2 Hall C Beamline 29
2.2.1 Beam Position Monitor 29
2.2.2 Beam Current Monitor 30
2.2.3 Beam Energy Measurement 31
2.2.4 Moeller Polarimeter 31
2.2.5 Raster 34
2.2.6 Chicane Magnet and Helium Bag 37
2.3 Target System 40
2.3.1 Target System 40
2.3.2 Dynamic Nuclear Polarization 42
2.3.3 Target Polarization Measurement 45
2.4 Big Electron Telescope Array 48
2.5 High Momentum Spectrometer 48
2.5.1 Magnets 51
2.5.2 Slit System 52
2.5.3 Drift Chambers 53
2.5.4 Hodoscopes 55
2.5.5 Cherenkov 55
2.5.6 Calorimeter 58
2.6 Trigger and Data Acquisition 60
3 Data Analysis 63
3.1 Calibration and Reconstruction 64
3.2 Packing Fraction 67
3.3 Dilution Factor 73
3.4 Dead Time 77
3.5 Nitrogen Polarization 80
3.6 Asymmetry Calculation 81
3.7 Radiative Correction 82
3.8 Fitting and Error 85
3.9 Measured Asymmetries to Others 87
3.10 Systematic Uncertainty 88
4 Results and Discussions 101
4.1 Asymmetries 101
4.2 Spin Structure Functions 109
4.3 d2 Matrix Element 114
4.4 Summary 116
Appendices 119
A Asymmetry Extraction 121
B Fitting Functions 125
C Data Tables 127
Bibliography 138
์์ฝ(๊ตญ๋ฌธ์ด๋ก)
๊ฐ์ฌ์ ๊ธ(Acknowledgement)Docto