Size effect on the stability of Cu-Ag nanoalloys

Classical molecular dynamics (MD) simulations are used to study the phase stability of Cu-Ag nanoalloys based on the analysis of their thermodynamic mixing properties for both random and core-shell clusters as functions of nanoparticle size, temperature and composition. At 298 K, results for nanoalloys of increasing size at fixed composition suggest that alloying Cu and Ag is thermodynamically feasible only for a nanocluster size range, excluding very small (<1.8 nm) and large clusters (greater than or similar to 4 nm). In the size range of favourable alloy formation, Cu-Ag core-shell structures are more stable than random configurations, and the same conclusion holds for most of the composition range at fixed cluster size and 298 K. Varying temperature at fixed nanocluster size and fixed composition, core-shell structures are preferred up to the melting temperature of the nanoparticle. Also, we test an analytical model to predict the thermodynamic properties of mixing of nanoalloys using bulk enthalpies of mixing of the pure components and those of the corresponding bulk alloy. The enthalpies and Gibbs free energies of mixing obtained from the analytical model qualitatively agree with those obtained from MD simulations, especially when the nanoparticle size increases above 2.8 nm.