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.
