Probing the Kinetics of the Metal/Electrolyte Interface Using Nanoporous Gold

技术：脱合金是一种腐蚀过程，其中合金的元素成分选择性地从最初的大块样品中溶解出来。在溶解过程中，剩余的贵重合金成分沿着不断生长的合金/电解质界面扩散，形成一个开放的纳米孔金属，其孔径可从~ 2nm向上调节到许多微米。表现出这种行为的典型材料是纳米孔金（NPG），它是由银/金合金合金化银制成的。虽然现在已经确定了在脱合金过程中导致孔隙演化的一些微观过程，但许多关于扩散和溶解动力学的基本问题，以及它们之间的相互关系，都是不清楚的。PI将采用纳米多孔金作为重点材料，研究纳米尺度下原子扩散和溶解的热力学和动力学，进行耦合实验和理论研究。最近，纳米多孔金孔隙演化的新模型涉及原子沿合金/电解质界面的扩散，以及这种扩散如何与溶解竞争，从而形成复杂的微观结构。这种相互作用在大多数涉及金属沉积或溶解的电化学过程中无处不在，本课程将对所有这些领域做出贡献。以下基本问题将是主要的兴趣：(1)纳米孔金的微观结构是什么？PI将通过先进的透射电子显微镜断层扫描阐明纳米孔金复杂的三维孔隙度。(2)纳米多孔金属粗化过程中界面扩散速度有多快？在电化学电位控制下，特别是在零电荷电位附近，通过对纳米孔金进行粗化处理，进行仔细的表面扩散测量，来探测界面扩散速率。(3)金属/电解质界面快速界面扩散的微观根源是什么？快速界面扩散起源的线索将通过粗化的耦合温度/电位测量来评估。(4)合金中纳米孔隙演化的耦合扩散/溶解模型的普遍性如何？为了测试孔隙度演化模型的普遍性，PI将检查模型非水电解质中的合金化。非技术：了解纳米多孔金属的原子尺度动力学将影响应用的发展，为催化、传感和其他学科提供超高表面积、形态控制的金属。同样的理解也将影响腐蚀科学，无论是开发现有材料的防腐蚀策略，还是开发新的耐腐蚀材料。这些背景为本科研究和高中研究项目提供了很好的重点。本科生将参与研究工作，在各种新的应用中使用纳米孔金，包括支持陶瓷纳米颗粒催化剂和染料敏化太阳能电池中的高表面积电极。作为实验研究的补充，PI将继续开发一种名为MESOSIM的动力学蒙特卡罗模拟和结构可视化工具。虽然PI主要将该程序科学地用于研究脱合金，但该程序的应用范围要广泛得多，可用于研究薄膜生长，纳米颗粒形态稳定性等，以及晶体结构可视化。PI将继续开发MESOSIM作为材料科学教育的工具（该程序的模拟输出已经被纳入流行的材料科学入门教材中），特别是通过开发包含该程序的课程模块，该程序将得到广泛的传播。

TECHNICAL: Dealloying is the corrosion process in which an elemental component of an alloy is selectively dissolved from an initially bulk sample. During dissolution, the remaining more noble alloy component diffuses along the ever-growing alloy/electrolyte interface to form an open nanoporous metal with pore size tunable upwards from ~2 nm to many microns. A prototypical material exhibiting this behavior is nanoporous gold (NPG) made by dealloying silver from Ag/Au alloys. While some microscopic processes that lead to porosity evolution during dealloying have now been identified, many fundamental questions about the kinetics of diffusion and dissolution, and their interrelationship, are ill-understood at best. PI will pursue a coupled experimental and theoretical program using nanoporous gold as a focus material to study thermodynamics and kinetics of atom diffusion and dissolution at the nanoscale. Recent new models for porosity evolution in nanoporous gold involve diffusion of atoms along the alloy/electrolyte interface, and how this diffusion competes with dissolution in a complex dance leading to the formation of a complex microstructure. Such interactions are ubiquitous in most electrochemical processes involving deposition or dissolution of metals, and this program will contribute to all of these area. The following fundamental questions will be of primary interest: (1) What is the microstructure of nanoporous gold? PI will clarify the complex three-dimensional porosity of nanoporous gold via advanced transmission electron microscopy tomography. (2) How fast is interfacial diffusion during coarsening of nanoporous metals? Interface diffusion rates will be probed by making careful surface diffusion measurements via coarsening on nanoporous gold under electrochemical potential control, particularly around the potential of zero charge. (3) What is the microscopic origin of fast interface diffusion at the metal/electrolyte interface? Clues to the origin of fast interface diffusion will be assessed by coupled temperature/potential measurements of coarsening. (4) How universal is the coupled diffusion/dissolution model for nanoporosity evolution in dealloying? To test generality of our models for porosity evolution, PI will examine dealloying in model non-aqueous electrolytes. NON-TECHNICAL: Understanding atom-scale kinetics in nanoporous metals will impact applications development in providing ultra-high surface area, morphologically controlled metals for catalysis, sensing and other disciplines. This same understanding will also impact corrosion science, both in developing corrosion prevention strategies for existing materials, as well as new corrosion-resistant materials. These contexts provide good focus for undergraduate research and high school research projects. Undergraduates will participate in the research effort by using nanoporous gold in a variety of new applications including supports for ceramic nanoparticle catalysts and high surface area electrodes in dye-sensitized solar cells. Complementary to experimental studies, PI will continue to develop a kinetic Monte Carlo simulation and structure visualization tool called MESOSIM. While PI has primarily used this program scientifically to study dealloying, the program is much more general in its application, and can be used to study thin film growth, nanoparticle morphological stability, etc., as well as crystal structure visualization. PI will continue the development of MESOSIM as a tool for education in materials science (simulation output from the program has already been incorporated into a popular introductory materials science text), specifically by developing course modules incorporating the program that will find wide distribution.