Equilibrium and stability of phase-separating Au–Pt nanoparticles

We report on 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 ∼20 nm Au(core)–Pt(shell) NPs upon annealing at various temperatures (300–800 °C) is studied. At low temperatures, only interdiffusion occurs between the core and the shell, while above ∼600 °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 on 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.