Surface Composition and Crystallinity of Coalescing Silver-Gold Nanoparticles

Bimetallic nanoparticles exhibit catalytic, optical, electronic, and magnetic synergy between their constituent metals. Typically, that synergy is traced to the domain structure and surface characteristics of such particles. Here these characteristics of coalescing Ag-Au nanoparticles of various initial sizes and morphologies (segregated or alloys) are investigated by atomistic molecular dynamics (MD) at different temperatures. Silver atoms exhibit increased mobility over Au and occupy gradually the surface of the coalesced (or sintered) bimetallic particle, consistent with scanning electron microscopy and selective O₂ chemisorption experiments for heterogeneous catalysis of ethylene oxidation. The characteristic sintering time of equally sized Ag-Au nanoparticles is similar to that of pure Au but shorter than that of Ag nanoparticles. When the latter coalesce with substantially bigger Au ones, a patchy Ag layer is formed at the Au particle surface. However, when Ag nanoparticles are bigger, then Au is rather embedded into Ag, consistent with microscopy data. Most notably, X-ray diffraction (XRD) patterns of Ag-Au nanoparticles are obtained by MD, distinguishing segregated from alloyed ones. The latter exhibit a weaker XRD reflection of the (200) crystalline plane and, most distinctly, form smaller crystal size (highly polycrystalline) than coalescing pure and segregated Ag and Au nanoparticles, quantitatively explaining the structure of flame-made Ag-Au nanoparticles for biomaterial applications.