1. Department of Physics
2. Department of Chemistry and Biochemistry
3. The Notre Dame Radiation Laboratory

We report on systematic studies of size dependent alloy formation of silver-coated gold nanoparticles (NPs) in aqueous solution at ambient temperature using x-ray absorption fine structure spectroscopy (XAFS). Various Au core sizes (2.5 20 nm diameter) and Ag shell thickness were synthesized using radiolytic wet techniques. The equilibrium structures (alloy vs. core-shell) of these NPs were determined in the suspensions. We observed remarkable size-dependence in the room temperature inter-diffusion of the two metals. The inter-diffusion is limited to the sub-interface layers of the bimetallic NPs and depends on both the core size and the total particle size. For the very small particles (4.6-nm initial Au-core size) the two metals are nearly randomly distributed within the particle. However, even for these small Au-core NPs, the inter-diffusion occurs primarily in the vicinity of the original interface. Features from the Ag shells do remain. For the larger particles the boundary is maintained to within one monolayer. These results cannot be explained either by enhanced-self diffusion that results from depression of the melting point with size, or by surface melting of the NPs. We propose that defects, such as vacancies, at the bimetallic interface enhance the radial migration (as well as displacement around the interface) of one metal into the other. Molecular dynamics calculations correctly predict the activation energy for diffusion of the metals in the absence of vacancies and show an enormous dependence of the rate of mixing on defect levels. They also suggest that a few percent of the interfacial lattice sites need to be vacant in order to explain the observed mixing.