MoS2 단일층의 결함과 산소와 수소의 화학적 상호작용에 대한 관한 이론적 연구

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2017. 2. 한승우.In the past few years, monolayer molybdenum disulfide (MoS2) has gained huge attention due to exotic electronic, optical, mechanical and chemical properties and there have been many studies for application in various fields. In particular, applied researches in chemistry have been actively carried out because defect engineering of monolayer MoS2 is relatively easy. For example, monolayer MoS2 shows interesting gas sensing properties such as oxygen and good catalytic efficiency of hydrogen evolution reaction. However, there are limits to improve efficiency of the chemical reactions without fundamental understanding of mechanism because chemical reaction occurs in atomic scale region. For the fundamental understanding, theoretical investigation can act as key role and the calculation technique based on density functional theory is suitable to reveal the atomistic origins of experimental discoveries. In this thesis, I present theoretical investigations of chemical interactions of O2 and H with defects of MoS2 monolayer. First part is theoretical investigation of O2 sensing mechanism on edge of MoS2. I suggest that O2 molecule can be reversibly adsorbed to bridge site of Mo edge with S monomer and the adsorption of O2 makes flatten band of edge state and it is origin of O2 sensing mechanism of MoS2. Second, I investigate hydrogen evolution reaction on sulfur vacancy of MoS2 monolayer by our theoretical model. I model hydrogen evolution reaction on sulfur vacancy of MoS2 monolayer by considering population of all possible states of sulfur vacancy and turnover frequency of all possible paths. From the theoretical model, I directly estimate polarization curve of hydrogen evolution reaction on VS of MoS2 monolayer and the results are in good agreement with experimental reference. In addition, I investigate strain effect on efficiency of hydrogen evolution reaction on the sulfur vacancy by the theoretical model. From the studies, I can suggest that tensile strain has a good influence to efficiency of hydrogen evolution on sulfur vacancy by increase of population of [2H-VS]1- and decrease of total reaction barrier. By the strict theoretical investigations, I can understand fundamental origin of the chemical interactions and suggest design rules to improve efficiency and performance of application using the chemical interactions.1. Introduction 1 1.1. Basic properties of MoS2 1 1.2. Importance of gas sensor 3 1.3. Importance of H2 as energy carrier 3 2. Density functional theory 5 2.1. Hohenberg and Kohn theorem 5 2.2. Kohn-Sham equation 6 2.3. Exchange-correlation energy 7 3. Oxygen Sensing on Edge of MoS2 8 3.1. Introduction of Part 3 8 3.2. Computational setup 9 3.3. Active sites for O2 sensing 9 3.4. Energetics of O2 adsorption on edge of MoS2 14 3.5. Mechanism of O2 sensing on edge of MoS2 16 3.6. Conclusion of Part 3 18 4. Hydrogen Evolution Reaction on Sulfur Vacancy of MoS2 19 4.1. Introduction of Part 4 19 4.2. Theoretical model for HER on VS 23 4.3. Computational methods 25 4.3.1. Calculation setup 25 4.3.2. Solvation effects 26 4.3.3. Transition state and kinetic barrier 27 4.4. Possible states of sulfur vacancy 28 4.5. Definition of Gibbs free energy 33 4.5.1. At SHE condition 33 4.5.2. Finite pH and zero bias 35 4.5.3. Finite pH and bias 35 4.6. Stability and population of nH[VS]q 38 4.7. Consideration of all possible paths of hydrogen evolution reaction 40 4.7.1. General rule of hydrogen evolution reaction 40 4.7.2. Possible HER paths 42 4.8. Intermediate steps and kinetic barriers 62 4.8.1. Classification of intermediate steps 62 4.8.2. Kinetic barriers of reduction 64 4.8.3. Kinetic barriers of Tafel steps 64 4.8.4. Kinetic barriers of protonation steps 65 4.8.5. Data set of kinetic barriers 70 4.9. Total reaction barrier and turnover frequency 73 4.10. Polarization curve 78 4.11. Strain effect on HER on VS 80 4.11.1. Population of possible states 80 4.11.2. Data set of kinetic barriers 84 4.11.3. Total reaction barriers and TOF 89 4.11.4. Polarization curves of HER on VS of strained MoS2 97 4.12. Conclusion of Part 4 99 5. Conclusion 100 Bibliography 101 국문초록 107Docto