Equilibrium and stability of phase-separating Au-Pt nanoparticles

We report oil a theoretical and experimental investigation of Au-Pt nanoparticles (NPs). The Au-Pt miscibility gap is theoretically reevaluated for NPs of various sizes. The model includes a composition-dependent surface energy evaluated by considering surface segregation. Using precise quantitative energy-dispersive X-ray spectroscopy (EDXS) and high-resolution electron microscopy (HREM), the structural evolution of similar to 20 nm Au(core)-Pt(shell) NPs upon annealing at various temperatures (300-800 degrees C) is studied. At low temperatures, only interdiffusion Occurs between the core and the shell, while above similar to 600 degrees C the NPs evolve into Au- and Pt-rich crystals, separated by an interface. At these temperatures, the Au solubility in the Pt-rich phase is found to be 5-10% higher than the bulk phase diagram and agrees qualitatively with the theoretical model. Based oil the EDXS and HREM results, the nature of the interface separating the Au- and Pt-rich phases within a NP is discussed and an estimate of its energy is obtained. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.