Understanding Segregation Behavior in AuPt, NiPt, and AgAu Bimetallic Nanoparticles Using Distribution Coefficients
2016
Divi, Srikanth | Chatterjee, Abhijit
Bimetallic nanoparticles (BNPs) often possess peculiar segregation behavior as the particle size, composition, shape, and temperature are varied. However, a thermodynamic model for this phenomenon has been lacking thus far. We show for the first time that the distribution of metal species within a nanoparticle can be adequately captured in terms of distribution coefficients calculated for the facets, facet edges, and bulk regions. Thermodynamic relations for the distribution coefficients are derived. Only m distribution coefficients from the m(m – 1) distribution coefficients are independent, where m denotes the number of regions. The theory is applied to AuPt, NiPt, and AuAg BNPs. Distribution coefficients are calculated at 400 and 600 K using Monte Carlo (MC) simulations of varying BNP sizes and compositions. A wide range of mixing behavior from alloying to partial or full segregation and core–shell to onion-like structures can be observed. A key finding is that the distribution coefficients are independent of the BNP size. The observed size-dependent segregation can be attributed to the relative availability of surface and bulk sites, i.e., the area-to-volume (A/V) ratio. This implies that two bimetallic nanostructures of different sizes and shapes but the same A/V ratio may exhibit nearly identical segregation behavior. Thus, nanothermodynamic segregation in bimetallic alloys may be described concisely using a handful of distribution coefficients.
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