Stability of Core–Shell Nanoparticles for Catalysis at Elevated Temperatures: Structural Inversion in the Ni–Au System Observed at Atomic Resolution

We present in situ transmission electron microscopy (TEM) studies of nanoscale Ni–Au core–shell particles on heatable TEM grids. The bimetallic clusters, grown fully inert within superfluid helium nanodroplets to avoid any template or solvent effects, are deposited on amorphous carbon and monitored through a heating cycle from room temperature to 400 °C and subsequent cooling. Diffusion processes, known to impair the catalytic activities of core–shell structures, are studied as a function of the temperature and quantified through fits of a temperature-dependent diffusion constant directly derived from the experiment. After cooling, spatially resolved energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy measurements prove the inversion of the core–shell structure from Ni–Au to Au–Ni. Furthermore, a strong oxidation of the now exposed Ni shell is observed in the latter case. In combination with theoretical studies employing density functional theory, we analyze the influence of oxygen on the observed intermetallic diffusion.