This research project examines alloy corrosion processes in nanoscale structures. The program involves a combined experimental and theoretical approach focused on developing broadly applicable thermodynamic models of dealloying corrosion. This problem is extraordinarily rich in that there are two intrinsic length scales that must be considered. One length scale is set by the alloy composition and the other is set by the physical dimensions of the sample. The richness of the problem derives from the interaction of these length scales, which leads to entirely new corrosion phenomena. Corrosion of such structures is important at monolayer levels and occurs at compositions which are not vulnerable to attack in corresponding larger macroscopic scale samples. Thus conventional approaches for characterizing dealloying corrosion are not applicable at the nanoscale. In order to perform this work we use a new technique for developing reproducible alloy nanoelectrodes in the size range of 3 ? 10 nm and employ a variety of characterization techniques including underpotential deposition for assaying surface compositions, electrochemical scanning tunneling microscopy and high resolution analytical transmission electron microscopy. The combination of experimental data generated on well characterized nanostructures together with our thermodynamic modeling will provide a general fundamental framework for understanding these important nanoscale corrosion processes. The science evolving from this research will impact diverse applications in nanotechnology.
NON-TECHNICAL SUMMARY:
This research examines corrosion of nanoscale structures such as those used as catalysts in fuel cells, nanoparticle based bio-assays, bio-sensing and environmental monitoring for security and surveillance. In the case of fuel cells for transportable power the stability of metal alloy catalyst particles to corrosion is the key problem and to date has limited this technology to widespread application. This program is evolving new experimental data and modeling that will provide engineering guidelines for the design of nanoscale structures that are stable to corrosion. In order to accomplish these goals students are trained to develop expertise in order to integrate knowledge in the disciplines of electrochemistry and materials science. Today there is a dearth of graduate students trained in the U.S. in this manner. Part of this problem stems from the perception students have that corrosion science is an old developed field and is separated from the current hot areas which include fields such as renewable energy and ?bio-nano?. This research program opens a new arena in corrosion science which should help to get more high quality students interested in corrosion. In support of this, we are developing new courses in materials electrochemistry which specifically focuses on the corrosion and electrochemical behavior of nanoelectrodes. This will be taught at the senior undergraduate/beginning graduate level and is aimed specifically at exciting student interest in this field. Both graduate and undergraduate students are involved in this research program. In addition to active involvement with the NSF funded Minority Graduate Education @ Mountain State Alliance (MGE@MSA) program, the PI is getting individuals from underrepresented groups involved in this research program.
This research program examined corrosion of nanoscale metallic structures such as those used as catalysts in fuel cells, nanoparticle based bio-assays, bio-sensing and environmental monitoring for security and surveillance. In the case of fuel cells for vehicles new catalyst technology uses low content platinum alloys that improves performance while minimizing the amount of this expensive metal required for operation of the fuel cell. However, these alloy catalyst particles are more susceptible to corrosion than pure platinum and this is a key problem that to date has limited the application of this technology. The process responsible for this degradation is called dealloying because corrosion selectively dissolves the non-platinum component of the alloy. This program has focused on the development of high quality experimental data and models that can be used by engineers to design alloy catalyst nanoparticles stable to corrosion. The conditions under which elemental metals corrode are generally described in terms of so-called electrochemical potential-pH diagrams, however these have not been available for nanoscale structures. In this research, we used electrochemical scanning tunneling microscopy to examine the corrosion stability of platinum nanoparticles ranging in size from 2 – 10 nm. The results of this study made it possible to validate thermodynamic models that we used to develop particle size dependent potential-pH diagrams that can be utilized by engineers to ascertain the conditions under which any pure metal nanoparticle is stable to corrosion. These particular results have significant societal implications for the effect of nanoparticles on potable water supplies. In the case of metal alloy nanoparticle corrosion our research focused on defining the electrochemical conditions for which a particular nanoparticle would be stable to dealloying processes. The model system that we used in this portion of our study was silver-gold alloys. We found that for any particular set of conditions such as electrolyte pH, stability was determined by particle size and composition. At a composition rich in silver, particles larger than about 20 nm underwent dealloying processes that would result in the formation of porous nanoparticle structures. Particles of the same composition but smaller than 10 nm in diameter, underwent only a surface dealloying process that spontaneously leaves a thin stable gold shell surrounding the nanoparticle. This morphology is similar to that required for relatively inexpensive (compared to a pure platinum catalyst) platinum alloy catalysts in fuel cell applications since one needs only enough Pt in the nanoparticle to form the shell. Thus the required platinum content is less than 25 percent of that required in conventional pure platinum catalysts. Expertise in materials electrochemistry and corrosion science requires the ability to integrate knowledge in the disciplines of electrochemistry and materials science and today there is a dearth of students trained in the U.S. in this manner. We believe that part of this problem stems from the perception students have that corrosion science is an old developed field and is separated from the current hot areas which include fields such as renewable energy and "bio-nano". This research program has opened a new arena in corrosion science that should help to get more high quality students interested in corrosion. In support of this, we developed new classes in materials electrochemistry, which specifically focuses on the corrosion and electrochemical behavior of nano-scale systems such as those studied in this research program. These classes have been taught at the senior undergraduate/beginning graduate level and are aimed specifically at exciting student interest in this field.
