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

Abstract

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. Heatin...