Influence of Composition and Chemical Arrangement on the Kinetic Stability of 147-Atom Au-Ag Bimetallic Nanoclusters

High calcination temperatures are often required in nanoparticle synthesis and an understanding of how heating affects the structure and chemical arrangement of bimetallic nanoclusters is essential to design efficient fabrication processes. We have investigated the kinetic stability of 147-atom Au-Ag nanoalloys with varying composition and chemical ordering using ensemble molecular dynamics simulations to replicate these high temperature conditions. Ag-rich mixed alloys undergo a diffusion-less martensitic structural transition from cuboctahedral to icosahedral; the melting temperature (T-m) of the subsequent icosahedra is dependent on the Au:Ag stoichiometry. Core@shell chemical arrangements do not behave in a similar manner: T. strongly depends on the shell component; additionally Au-55@Ag-92 exhibits increased stability as a result of its icosahedral Au core. We also report a novel dependence of nanocluster phase transitions on the chemical arrangement, as shown by low temperature nonmartensitic atomic diffusion of Ag atoms to the surface for Ag-55@Au-92. This finding establishes why Ag@Au chemical arrangements, in particular, are difficult to maintain experimentally. Overall, the kinetic phenomena observed help to explain why particular morphologies and chemical arrangements are more abundant in experimental synthesis and how postprocessing affects structure and chemical arrangement.