Understanding Segregation Behavior in AuPt, NiPt, and AgAu Bimetallic Nanoparticles Using Distribution Coefficients

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. (Graph Presented).