Environment Controlled Dewetting of Rh–Pd Bilayers: A Route for Core–Shell Nanostructure Synthesis

Chemical environment plays a significant role on the size, shape, or surface composition of nanostructures. Here, the chemical environment effects are studied in the context of core–shell nanoparticle synthesis. The environment driven dynamics and kinetics of Rh/Pd bilayers is investigated by in situ ambient pressure X-ray photoelectron spectroscopy. Thin Rh (∼1.5 nm)/Pd (∼ 1.5 nm) bilayers were grown on thermally oxidized Si substrates. The films were heated in CO or NO environments or heated in vacuum with a subsequent NO/CO cycling. This study demonstrates that not the initial stacking sequence but the chemical environment plays a crucial role in controlling the surface composition. Heating in CO results in a surface enrichment of Pd at ∼200 °C and is followed by film dewetting at ∼300 °C. Heating in NO results in progressive oxidation of Rh starting at ∼150 °C, which stabilizes the film continuity up to >∼375 °C. The film rupture correlates with the thermal destabilization of the surface oxide. Heating in vacuum results in a significant increase in surface Pd concentration, and the following NO/CO cycling induces periodic surface composition changes. The quasi-equilibrium states are ∼50% and ∼20% of Rh/(Rh + Pd) for NO and CO environments, respectively. Possible surface composition change and dewetting mechanisms are discussed on the basis of the interplay of thermodynamic (surface/oxide energy and surface wetting) and kinetic (surface oxidation and thermally induced and chemically enhanced diffusion) factors. The results open alternative ways to synthesize supported (core–shell) nanostructures with controlled morphology and surface composition