MASTER S DEGREE THESIS. Electrochemical Stability of Pt-Au alloy Nanoparticles and the Effect of Alloying Element (Au) on the Stability of Pt

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1 Electrochemical Stability of P-Au Nanoparticles and the Name: Amsalu Efrem Yemene Supervisor: Prof.Chen,Bing Hui Effect of alloying Element (Au) on the stability of Pt Xiamen University, China University Code: Student ID No.: MASTER S DEGREE THESIS Electrochemical Stability of Pt-Au alloy Nanoparticles and the Effect of Alloying Element (Au) on the Stability of Pt By AMSALU EFREM YEMENE (ETHIOPIA) Supervisor: Professor Chen, Bing Hui Major: Chemical Engineering Date of Graduation: September, 2011 A THESIS SUBMITTED IN FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOCHEMICAL ENGINEERING COLLEGE OF CHEMISTRY AND CHEMICAL ENGINEERING XIAMEN UNIVERSITY

2 CERTIFICATION I, Professor Chen, Bing Hui, hereby certify that I have read this manuscript and recommend for acceptance by the Xiamen University a dissertation entitled Electrochemical Stability of Pt-Au alloy Nanoparticles and the Effect of Alloying Element (Au) on the Stability of Pt in fulfillment of degree of Master of Engineering at Xiamen University, People s Republic of China. Signed Supervisor: Professor Chen, Bing Hui Date Department of Chemical and Biochemical Engineering College of Chemistry and Chemical Engineering Xiamen University Xiamen, Fujian Province P.R. China

3 ORIGINAL STATEMENT The research described in this thesis Masters of Engineering was conducted under the supervision of Professor Chen, Bing Hui at the Department of Chemical and Biochemical Engineering, Xiamen University. I hereby declare that the work submitted is my own and that appropriate credit has been given where reference has been made to the work of others. I also confirm that it has not been previously or concurrently submitted for any other degree, diploma or any other qualifications at Xiamen University, P.R China or other institutions. COPYRIGHT DECLARATION All rights reserved. No part of this dissertation may be reproduced, stored in any retrieval system, or transmitted in any form by any means: electronic, mechanical, photographing, recording or otherwise without prior written permission of the author or Xiamen University. AMSALU EFREM YEMENE NAME OF STUDENT Date:

4 Acknowledgments ACKNOWLEDGMENTS First and foremost I praise God for his strength upon all who assisted me. I praise Him for giving us life, health and courage to press on despite of challenges and shortcomings we experienced along the journey. I express my heartfelt gratitude to my supervisor Prof. Chen, Bing Hui, for his constant encouragement, invaluable advice and guidance throughout the course of my research work. I attribute the level of my Masters degree to his encouragement and effort and without him this thesis, too, would not have been completed or written. Also, I am indebted to Mr. Chen, Guoqin for assisting me during this period with research materials, and discussion on points that I lacked understanding. Moreover, I am also grateful to the Chinese Scholarship Council, who has given me the grant to study and the chance to explore my potentials and continue my passion in science and engineering; as well as to my Chemical Engineering faculty and classmates in Xiamen University. Also, I am thankful to the Xiamen University International Students community and Chinese friends who have been supportive. I am also thankful to all my country mates in Xiamen University: Abiy, Abirdu, Rediet, Temesgen, Meb, and Joe for their help and cares that makes me feel like I am in home, Ethiopia. I am indebted to my parents, sisters and brothers, who have been there to let me feel how home, could be so near yet so far and for their prays. A special thank goes to my friends (Solomon legesse, Mesfin W/Senbet and Desalegn Alemu)! i

5 Abstract ABSTRACT Understanding and controlling the corrosion behaviour of nano-scale solids are vital in its applications ranging from sensing to catalysis. Herein, Gibbs-Thomson thermodynamic analysis is presented for the case of Pt-Au alloy nanoparticles; and then employed to demonstrate the construction of its particle-size-dependent potential-ph diagram, and potential (E) vs. (2/r) graph. It also discussed the data set required for the construction of such diagrams in general and describes how some parameters are accessible via experiment while others can only be reliably determined from first-principles-based electronic structure calculations. Based on this GT equation, it could be possible to predict the dissolution potential for Pt of Pt-Au alloy NP. In the case of Pt in Pt-Au alloy NPs this analysis predicts that particles of diameter less than 4 nm dissolves via the direct electrochemical dissolution pathway, Pt Pt e -, while larger particles form an oxide and then dissolve at high potential through chemical dissolution of the oxide (indirect dissolution pathway). Therefore stability decreases with its particle size. In addition to this, the effect of alloying element (Au) on the stability of Pt has been analyzed by comparing E vs. (2/r) graphs of Pt in Pt-Au alloy NPs with that of Pt-alone NPs. This comparison shows that the dissolution potential which was predicted by their corresponding GT equations for Pt in Pt-Au alloy NPs is greater than that of pure Pt and this difference is higher when the particle size is getting smaller and smaller. Subsequently, Pt nanoparticles could be stabilized against dissolution by alloying it with gold; this is because of the higher platinum oxidation potential resulted from gold. This increased stability has an important contribution to many technological applications, for example, in fuel cell Pt has used as an electrocatalyst so its instability results in the degradation of the fuel cell. Beside its economical and activity advantages, using Pt-Au alloy NPs as an electrocatalyst decreases the problem of fuel cell degradation which comes through dissolution of Pt. ii

6 Abstract Key Words: Gibbs-Thomson equation (GT), dissolution potential (E), electrochemical stability, Pt-Au alloy nanoparticles, fuel cell. iii

7 摘要 摘要 抗腐蚀能力是实现纳米材料广泛应用的关键 本文主要围绕 Pt-Au 合金纳米粒子的抗腐蚀性能, 通过 Gibbs-Thomson 热力学分析, 建立由纳米粒子的粒径调控的 potential-ph 和 potential (E) vs. (2/r) 的相图 另外讨论了相图中数据的设置准则及如何通过实验设计获得 通过建立的 Gibbs-Thomson 方程提供了一种预测 Pt-Au 合金纳米粒子中 Pt 的溶解电位的可能性 对于 Pt-Au 合金纳米粒子, 粒径在 4nm 以下的颗粒中 Pt 的溶解是以直接电化学溶解途径进行的,Pt Pt e - ; 而较大的纳米粒子以氧化物形式在更高电位下化学溶解出 ( 间接溶解 ); 因此, 粒径越小其稳定性越低 通过比较 Pt-Au 双金属与纯 Pt 纳米粒子的 E vs.(2/r) 图, 探讨了 Au 对 Pt 的稳定化机制 结果表明, 运用 GT 方程预测的 Pt-Au alloy 合金纳米粒子中 Pt 的溶解电位比纯 Pt 的高, 且纳米粒子的粒径越小差异越大 Au 的存在起到了抑制 Pt 的溶解腐蚀, 增强了 Pt 的稳定性 因此,Pt-Au alloy 合金纳米粒子的经济性, 高活性及高稳定性将有望应用于质子交换膜燃料电池的电极材料 关键词 :Gibbs-Thomson 方程 (GT); 溶解电位 (E); 电化学稳定性 ; 金铂合金纳米粒子 ; 燃料电池 iv

8 Abbreviations ABBREVIATIONS CN - Coordination Number DFT - Density Functional Theory ECSTM - Electrochemical Scanning Tunneling Microscope ESA - Electrochemical Surface Area fcc - unit cell - Face centered Cubic Unit cell GT - Gibbs-Thomson MEA - Membrane Electrode Assembly NPs - Nanoparticles ORR - Oxygen Reduction Reaction PSE - Particle Size Effect v

9 Table of Contents TABLE OF CONTENTS Acknowledgements...i Abstract...ii Abbreviations...v Table of Contents...vi List of Figures....ix List of Table. x Chapter 1 Introduction Literature Review Motivation of the Study Objectives of the Study Derivation of Gibbs-Thomson Thermodynamic Equation Constructing the particle-size-dependent potential ph diagram For Pt of Pt-Au alloy nanoparticles The effect of alloying element (Au) on the stability of Pt Scope and Contribution of the Study... 8 Chapter 2 Fundamentals of Calculation Thermodynamic Analysis for the Stability of Substances at their Bulk State Thermodynamic Analysis for the Stability of Substances at Nano-scale Gibbs Thomson Equation Chapter 3 Methodology and Data Source General Thermodynamic analysis Derivation of Equation (3.1) Model of the Study Model equations for the electrochemical equilibrium of P Derivation of Gibbs-Thomson equation for each reaction equations Stated on model Equations for Pt of Pt-Au alloy nanoparticle...24 vi

10 Table of Contents Gibbs-Thomson equation for Reaction (A ) Gibbs-Thomson equation for Reaction (C ) Gibbs-Thomson equation for Reaction (B ) Definition of Terms and Data Collection to evaluate Equation (3.6) and (3.9) Definition of Terms Surface free energy ( ) and Surface stress ( f ) Molar volume (Ω) Partial molar volume ( ) Average molar volume, < Ω > Standard Electrode Potential Data Collection to evaluate Equation (3.6) and (3.9) Surface stress value for Pt of Pt-Au alloy NPs, f Pt/Au, And also For PtO- Pt of Pt-Au, Interfacial free energy value for Pt of Pt-Au alloy NPs,, and also For PtO-Pt of Pt-Au, Partial Molar volume for Pt of Pt-Au, Pt and for PtO on Pt of Pt-Au Average Molar volume for Pt-Au alloy, Ω Pt/Au and for PtO-Pt, < Ω PtO/Pt > Calculation 35 Chapter 4 Results and Discussion Potential, (E) vs. (2/r) Graph Particle-size-dependent potential-ph diagram Effect of alloying Au on the electrochemical stability of Pt Technological importance of understanding the stability of Pt-Au alloy nanoparticles...50 Chapter 5 Conclusion and Suggestions Conclusion Recommendations vii

11 Table of Contents References Appendices...58 viii

12 List of Tables LIST OF FIGURES Figure 3.1 A solid (S) composed a single soluble component of, n, moles of Component-1, in equilibrium with a multi-component ( ) Liquid (l). The Gibbs dividing surface, is chosen such that there is no surface excess of component 1, n Figure 3.2 To show Pt-Au alloy nanoparticles and then due to condition change it Forms a thin oxide layer cover, PtO Figure 4.1 E vs. 2/r for Pt in Pt-Au alloy Figure 4.2 Particle-size-dependent potential-ph diagram for Pt-Au alloy...45 Figure 4.3 E vs. (2/r) diagram for comparison of Pt in Pt/Au alloy with Pt- alone (Pure)...50 ix

13 List of Tables LIST OF TABLES Table 3.1 Summary of the data set that is required to evaluate eqs. (3.6) and (3.9) Table 4.1 E Pt/Pt 2+ and E PtO/Pt for Pt of Pt-Au alloy Table 4.2 E Pt/Pt 2+ for Pt of Pt-Au alloy Table 4.3 E Pt/PtO for Pt in Pt-Au alloy...44 Table 4.4 E 2+ Pt/Pt and E PtO/Pt for Pt-alone...48 Table 4.5 E value for the same particle size of between Pt-Au alloy NP and That of Pt-alone...49 x

14 Chapter 1 Introduction Chapter 1 1. Introduction Metal particles, wires, and thin films at nano-scale can have structural and electronic properties that differ from those of its bulk macroscopic counterparts. My focus on the electrochemical stability for Pt of Pt-Au alloy nanoparticles is in the regime where the crystal structure of the nano-scale solid is identical to that of the bulk macroscopic solid. In most cases a lower bound for this corresponds to a particle diameter of order 2 nm. For close-packed metals (atomic volume ~14 Å 3 ) this is equivalent to a spherical particle containing ~300 atoms, with 200 atoms at the surface of the particle. At yet smaller length scales ( molecular clusters ) many physical and chemical properties are known to display quantum size effects, oscillatory behaviors, entropically driven icosohedral facecentered-cubic(fcc) phase transformations, etc., as a function of the number of molecules defining the cluster size [1]. For a given bulk substance when its size decreases to a nano-scale, its physical and chemical properties show changes, that is different from its bulk, this phenomenon is called quantum size effect. In fact, the standard electrode potential of a substance at nano-scale shows quantum size effect because at nano-scale which is smaller than that of its bulk. Therefore, predicting the standard electrode potential for a nano-scale substance is quite important to understand its electrochemical stability. This prediction can be done by means of Thermodynamic simulation based on Gibbs-Thomson equation; which is a DFT based thermodynamic equation that makes a simple relation between electrode potential and particle size. This equation contain parameters like surface stress, surface free energy ( ) and atomic volume, the value for some of these parameters could be obtained by first-principle based calculation and some by experiment. As this study has focused on analyzing the electrochemical stability of Pt in Pt- Au alloy nanoparticles, calculating its equilibrium potential by using Gibbs-Thomson thermodynamic equation is a primary step. In order to do this values of parameters in this equation should be determined, some of which are obtained from literature and others by doing calculation, after this solve each equations by substituting the values 1

15 Chapter 1 Introduction of parameters to their corresponding equation and finally, a numerical Gibbs- Thomson thermodynamic equation will be obtained. Based on this equation, graphs like E vs. (2/r) and particle-size-dependent potential-ph diagram can be drawn to demonstrate the stability of Pt in terms of potential and ph range, and to infer the dissolution mechanism for Pt in Pt-Au alloy nanoparticles. In addition to this, it could be possible to analyze the effect of alloying element (e.g. Au in this work) on the stability of Pt by comparing numerical Gibbs-Thomson thermodynamic equation of Pt-Au alloy nanoparticles with that of Pt-alone. 1.1 literatures Review Nanotechnology (sometimes shortened to "nanotech") is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with structures sized between 1 to 20 nm (mostly particles size between 1 to 100nm are regarded as nano-scale though) in at least one dimension, and involve developing materials or devices possessing at least one dimension within that size. Quantum mechanical effects are very important at this scale, which is in the quantum realm. Nanotechnology is believed to be the basis of many technological innovations of the 21 st century. Research and development in this field is growing rapidly throughout the world. The major outputs of this activities are the development of new materials in the nanometer scale, including nanoparticles. [2] These are usually defined as particulate materials with at least one dimension of less than 100 nm, where one nanometer is 10-9 m. By comparison, a human hair is approximately 70,000 nm in diameter, a red blood cell is approximately 5,000 nm wide and simple organic molecules have sizes ranging from 0.5 to 5 nm. Today, nanoparticles have already became an indispensable material for industries because of their unique size dependent properties such as electrical, magnetic, mechanical, optical and chemical properties, which largely differ from those of their bulk. For example, gold at bulk state is inert, whereas at nano-scale it shows extremely special property of catalytic activity. The electrochemistry of nanoscale metal particles should differ from the electrochemistry of bulk metal electrodes. The best reason for believing this is that the physical properties of metal nanoparticles 2

16 Chapter 1 Introduction (smaller than 15 nm or so) differ from those of bulk metal, and these properties are depends on the particle size. In the case of silver, for example, the lattice constant of the fcc-unit cell is compressed by as much as 9% for nanoparticles, and the melting point is depressed by as much as 700 C. Electronic properties including the photoelectron yield and the energy of the Plasmon resonance absorption also exhibit particle size dependence in this size regime. Collectively, this observation leads to the expectation that the electrochemical properties of silver nanoparticles, such as the standard electrode potential, E, should also be size dependent. The thermodynamic analysis based on Gibbs Thomson (or Kelvin) equation predicts that metal nanoparticles will exhibit an E shifted negatively from the E of the bulk metal [3]. As a result of this, nanoparticles tends to dissolve well below the dissolution potential of their bulk counterpart this implies that, they are not stable at bulk standard electrode potential. Nonetheless, so many advantages were obtained by using nano-scale materials in different industries but still there are so many problems those hiders using it for large scale production and commercialization of different technologies. One of these problems is the stability of substances at nano-scale. So understanding and controlling the electrochemical stability or corrosion behaviors of nano-scale solids are quite important in a variety of applications such as nano-scale electronics, sensing, and catalysis, especially understanding the dissolution mechanism of Pt or Pt-M nanoparticle catalysts in fuel cell. There have only been a few reports in the literature aimed at examining the electrochemical stability of small isolated metallic clusters or particles. For instance, in studies which has been done by Klob et al. [4] and Kowk et al. [3] has examined the stability of nanometer-size Cu particles on Au (111) surfaces, and Ag particles on highly oriented pyrolytic graphite surface respectively. Each of these studies has reported an enhanced stability of the nano-scale particle when they are compared to the dissolution behavior of their corresponding bulk electrode, and these results were qualitatively rationalized in terms of the possible nonmetallic behavior of these particles resulting from quantum size effects. In the case of Cu particle study, other research groups like Del Popolo et al. [5] and Maupai et al. [6] has suggested that mechanical alloying effects may have been responsible for the observed enhanced stability but for the case of 8-43 nm diameter Ag particle arrays stability which has been examined by Ivanova et al. [7] showed a decrease in the dissolution potential with 3

17 Chapter 1 Introduction particle size, but the quantitative behavior of these particle assemblies could not be rationalized. In contrary to results shown on the above literatures, the work of Tang et al. [1] [8] which are based up on the Gibbs Thomson thermodynamics analysis and ECSTM experiment result shows that the stability decreases as the particle size. The analysis for the stability of nanoparticles could be done in so many ways like by experiment; using electro chemical scanning tunneling microscope (ECSTM), and theoretically; by DFT based thermodynamics and also kinetic studies. Recently Tang et al. [8] has done an in-situ STM measurement and density functional theory (DFT) based thermodynamic calculations to show the quantum size effect, that is, as a particle size decreases the stability also decreases, for example his in-situ STM analysis showing that a potential-time sequence of 5 Pt particles of different size dissolving in 0.1 M H 2 SO 4. Initially a set of 5 particles are subjected to a potential of 0.600V vs. (NHE) (Normal Hydrogen Electrode) and then the Voltage is pulsed to V at this potential the dissolution of particles (r = 0.58 nm) and (r = 0.62 nm) has been observed. Particles of (r = 0.83 nm) and (r = 0.81 nm) were stable to a potential of V and then dissolved at V. Particle of (r = 1.43 nm) was stable at V after 600 s at this potential. This particle starts to dissolve at a potential of V after 300s, so this experiment result demonstrated that the dissolution potential for the given particle size for example the dissolution potential for (r = 0.58 nm) and (r = 0.62 nm) was in the range of 0.600V to 0.650V. In the above DFT study, the author had developed a Pourbaix diagrams for extended Pt surfaces which was obtained by first-principle calculation, the result which had been obtained from this diagram was in agreement with the above experimental results i.e. this diagram had shown that the direct dissolution of bulk Pt to Pt 2+ occurred only at low ph and high electrode potential [8] but at high ph value Pthydroxide and PtO will be formed, at this condition if Pt 2+ exist in the solution it only comes through the chemical dissolution of the oxide. Finally, results from this experiment and DFT study shown that, when the particles size getting smaller, it dissolved well below the dissolution potential of the bulk so this implied that their stability also decreased but in the case of large sized particles their stability was not affected like the small sized particles because they were passivated by forming a thin oxide layer so it prevents Pt from further dissolution. If Pt 2+ was formed in the solution it only came through the chemical dissolution of PtO. 4

18 Chapter 1 Introduction Another work which has been done by Henglein et al. [9] also shows that the prediction of large negative shifts in the redox potential for small Ag n clusters (n = 1 to 15) as the number of atoms in the cluster decreases and also Plieth et al. [10] similarly predicted a negative shift in the redox potential proportional to (1/radius) for small nanoparticles relative to the bulk metal based on the difference in surface free energy between bulk metal and the same number of atoms dispersed into smaller nanoparticles. Experimentally, [11] Brus et al. and co-workers [11] also showed that large Ag nanoparticles grew at the expense of small NPs on a conductive surface (Ostwald ripening) due to the predicted negative shift in oxidation potential for smaller sizes. Such experimental and theoretical results have shown that the agreement between experimental and theoretical calculation for the prediction of the standard electrode potential by using Gibbs -Thomson thermodynamics equation. 1.2 Motivation of the Study In today s world, the demand for clean and sustainable energy sources has become a strong driving force in continuing economic development, and thus as well in the improvement of human living conditions. In recent year s fuel cells as clean energy converting devices, has drawn a great deal of attention due to its high efficiency, high energy density, and low or zero emissions. The electrochemically active Pt sites in the membrane electrode assembly (MEA) are the only effective ones for fuel cell electrode reactions. So the more the sites of electrochemically active Pt (e.g., electrochemical surface area, ESA), the higher the performance of fuel cell electrodes. Different processes can be involved in the apparent loss of platinum surface area, including (i) platinum dissolution and redeposition (Ostwald ripening process), (ii) coalescence of platinum nanoparticles occurring as a result of particle migration on the carbon support, (iii) platinum nanoparticle agglomeration triggered by corrosion of the carbon support, (iv) platinum dissolution and migration of the dissolved Pt species to the interior of the polymer electrolyte membrane where they can react (be reduced) with H 2 gas diffusing from the anode to the cathode, and (v) platinum dissolution and migration of the dissolved Pt species to water collected from the reactant gas exiting the cell. Also, it should be stressed that the relative importance of these processes might vary with 5

19 Chapter 1 Introduction the size of the Pt nanoparticles [12]. The degradation mechanism is still being investigated and the approaches for improving the catalyst durability are also under extensive exploration. Still now a great progress has been achieved in this field in the few past years. Generally, after a fuel cell operates for a certain time an irreversible loss of performance has been encountered. Although the mechanism leading to the performance loss of the full cell has not been completely understood but knowing the electrochemical stability for Pt or Pt-M electrocatalyst i.e., the dissolution condition and its mechanism has an important contribution to understand and design a solution to improve the performance of fuel cell. So this thought makes me motivated to do this research work. 1.3 Objectives of the Study For many applications, increased surface to volume ratio achieved by particle size reduction leads to lower materials cost and higher efficiency, but there are questions as to whether the intrinsic stability of materials also decreases with particle size. An important example of this relates to the stability of Pt catalysts in, for example, fuel cells. This study has focused on analyzing the electrochemical stability of catalysts for fuel cell application. [13] In fuel cell normally Pt or Pt-M alloy nanoparticles of size ranging from 2-3 nm were used as an electrocatalyst on both anode and cathode electrodes. Here, for Pt-Au alloy nanoparticles, Gibbs-Thomson thermodynamic analysis has been used for the prediction of its standard electrode potential, to construct its particles-size dependent potential-ph diagram which shows the stability for Pt of Pt-Au alloy nanoparticle of a given size as a function of potential, and ph, and also to show the effect of particle size on the dissolution mechanism change for Pt in Pt-Au alloy nanoparticles. Finally, this GT analysis also could be used to show the effect of alloying element (Au) on the stability of Pt and then discuss on some technological importance of this analysis. The specific objectives of this study include: 6

20 Chapter 1 Introduction Derivation of Gibbs-Thomson Thermodynamic Equation When an equilibrium developed for a finite-size solid immersed in a fluid containing a single soluble component of the solid, there will be a difference in the chemical potential of the component in the solid and fluid phases, which has been given [14] by,, then by using this equation it could be possible to derive Gibbs-Thomson equation to predict the dissolution potential and then to analyze electrochemical stability for a given metal element into consideration as the function of ph and potential. Based on GT thermodynamics analysis, this study has focused on analyzing the electrochemical stability for Pt of Pt-Au alloy nanoparticles. In order to attain this objective GT equation for each electrochemical and chemical equilibria for Pt of Pt- Au alloy nanoparticles should be derived. The electrochemical and chemical equilibria for Pt in Pt-Au alloy NP involve reactions like a direct dissolution of Pt, oxide formation of Pt and chemical dissolution of PtO. After this collect the data sets which are required to evaluate GT equations of each reaction equations. Finally, it could be possible to predict the dissolution potential for each particle size of the focus, develop particle-size-dependent potential-ph diagram, E vs. (2/r) graph and also to analyze the effect of alloying element (Au) on the stability of Pt by using these GT equations Constructing the particle-size-dependent potential ph diagram for Pt In Pt/Au alloy nanoparticles Potential ph diagram is also called pourbaix diagram shows the thermodynamic stability of different species in aqueous solution. In this diagram stability areas were represented as a function of ph and electrochemical potential scales, usually the upper and the lower stability limits of water are also shown in the diagram with dotted lines. Traditionally these diagrams have been taken from different handbook however; in most handbooks these diagrams are available only for a limited number of temperature, concentration and element combinations. Still now most potential ph diagram are only available for bulk metals and its compounds, but it is not the case for a finite size metal particles (nanoparticles) and its 7

21 Degree papers are in the Xiamen University Electronic Theses and Dissertations Database. Full texts are available in the following ways: 1. If your library is a CALIS member libraries, please log on and submit requests online, or consult the interlibrary loan department in your library. 2. For users of non-calis member libraries, please mail to etd@xmu.edu.cn for delivery details.

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