One-Step Growth of Core-Shell (PtPd)@Pt and (PtPd)@Pd Nanoparticles in the Gas Phase

Pt-Pd nanoparticles are grown in the gas phase by a magnetron-sputtering source and characterized by electron microscopy techniques for both Pt-rich and Pd-rich compositions of the metallic vapor. It is shown that this growth procedure can produce different types of core- shell nanoparticles, in one step, with sizes in the range of 4-10 nm, according to the composition of the vapor being rich either in Pt or in Pd. In all cases, the nanoparticles present intermixed cores containing both Pt and Pd and shells made of the majority element, i.e., of (PtPd)@Pt structure for the Pt-rich vapor and (PtPd)@Pd structure for the Pd-rich vapor. Global searches of the optimal chemical ordering show that none of these structures correspond to equilibrium configurations. On the contrary, these core-shell structures are strongly out-of-equilibrium, being the result of kinetic trapping phenomena. This is verified by molecular dynamics growth simulations which are able to reproduce both the different types of chemical ordering and the variety of geometric shapes found in the experiments.