Au-Ag Bimetallic Nanoparticles: Surface Segregation and Atomic-Scale Structure

Monte Carlo simulations were performed to study systematically the surface segregation behaviors and atomic-scale structural features of Au-Ag nanoparticles for a range of alloy compositions, particle sizes, and temperatures. Segregation of Ag to the surface was observed in all the particles considered. The surface segregation was promoted by increasing the particle sizes or Ag compositions and decreasing nanopartides' temperatures. It was found that the most stable mixing patterns are the onionlike structure with Ag-rich shell for small particles, and the alloyed-core/layered-shell structure for large particles. Accordingly, the calculated alloying extents based on Monte Carlo simulations are consistent with experimental EXAFS analysis, which indicates more obvious alloying features in nanoparticles with larger sizes or at higher temperatures, and more obvious segregated features in nanoparticles under the opposite conditions. The size distribution of Au ensembles on different coordinated sites was analyzed quantitatively, which presented varied composition- and temperature-dependent effects. The possible effects of size and shape distribution of surface ensembles on tuning the catalytic activity and selectivity of bimetallic nanopartides were also discussed.