The major objectives of the research proposed herein are to study dealloying in nanoscale structures and to develop a thermodynamic analysis describing this behavior. It is anticipated that the results of this work will be broadly applicable to corrosion of nanoscale structures. The lower bound of the nanoelectrode size that we consider corresponds to that where one would sensibly consider the electrode to behave as a metal with chemical and physical properties monotonically approaching that of the bulk planar electrode. Additionally, our interests are in the regime where the crystal structure of a nanoscale alloy electrode is identical to that of the macroscopic bulk solid. In most cases this lower bound corresponds to a particle diameter of order 3 nm. For close-packed metals (atomic volume ~ 15 3) this is equivalent to a spherical particle containing ~ 1000 atoms. We contend that the predictions of thermodynamics as applied to the behavior of nanometer-scale electrodes should be valid down to these length scales. At yet smaller length scales (molecular clusters) many physical and chemical properties are known to display quantum size effects [Volokitin, et al., 1996, Fan and Bard, 1997], oscillatory behaviors [Henglein,1993], entropically driven icosohedral fcc phase transformations, [Howie and Marks, 1984] etc., as a function of the number of molecules defining the cluster size. In order to accomplish our objective we propose to use nanofabrication techniques to produce nanoscale alloy electrodes with effective diameters in the range of 3-20 nm and to compare the dealloying behavior of these structures to that of their bulk counterpart. The major experimental approaches that we will use involve in situ electrochemical scanning tunneling microscopy (ECSTM), transmission electron microscopy (TEM) and voltammetry. Currently, an important application of dealloying in nanoscale electrodes is related to the design of new catalyst structures for the cathode in polymer electrolyte membrane (PEM) fuel cells. Here for example, Pt is alloyed with another metal M and the metal is selectively leached to some extent (not completely dealloyed) from the particle. Many of these catalysts show significantly enhanced oxygen reduction behavior, but there are still a myriad of questions regarding the long term stability of these structures. Undoubtedly, in the future as nanotechnology evolves corrosion of nanoscale structures will be an important issue. Our interest here is in developing the science of corrosion of nanoscale structures. This proposal is not about developing alloys for PEM fuel cell cathodes. While the current interest in alloy catalysts certainly provides some technological motivation for the work proposed, we believe that the new science likely to emerge from this work will be broadly applicable and impact diverse applications in nanotechnology. In the following paragraph we briefly discuss dealloying of Pt-M alloy catalysts. We should note that there is a vast body of work in the electrocatalysis literature aimed at exploiting the behavior of alloy catalyst particles and in no way can we comprehensively review this field. Instead we consider one example primarily to motivate our proposed work. We provide more general motivation in the section related to the intellectual merit of the work proposed. Consider a 3 nm diameter Pt-M alloy catalyst particle at a composition designed to both reduce Pt loading and enhance oxygen reduction in a fuel cell cathode [Stamenkovic et al., 2007a, 2007b]. Typically, the alloying element (e.g. M = Co, Ni, Fe, etc.) is at a composition well below the parting limit (25 50 at% M) so only near surface dealloying seems possible [Pickering 1983; Sieradzki et al., 1989]. Such a particle is composed of about 1000 atoms and 400 of these are on the surface of the particle. In this case, it is obvious that monolayer levels of dealloying will significantly a
|Effective start/end date||7/15/09 → 6/30/14|
- National Science Foundation (NSF): $500,000.00
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