Environment Controlled De-wetting Kinetics of Rh-Pd Bilayer/Alloy Thin Films on Silica: A Physical Approach to Synthesize Core-Shell Nanoparticles

The control of morphology and surface composition of nanoalloys is the key factor in order to tune or to extend the range of their optical, magnetic and chemical properties. Therefore it is one of the major tasks in nanoalloy materials science. As the chemical environment has a profound influence on the structure of supported metallic nanoparticles, it can be used as a powerful tool to tune their structure and properties. This study concerns the in-situ investigation of the oxidizing/reducing environment influence on the de-wetting dynamics and kinetics of a Rh-Pd bilayer/alloy thin film model system.
Thin films of ~3nm thickness were grown at RT by molecular beam epitaxy electron beam evaporation on thermally oxidized silicon substrates. These films were subsequently subjected to different combinations of heating and chemical environment (CO and NO) treatments. The film surface composition and the chemical state was determined in-situ by ambient pressure environmental x-ray photoelectron spectroscopy.
Independently on whether the initial state is an alloy or a bilayer with Rh on the top, the film surface shows an enrichment of Pd upon heating in vacuum. Exposure to NO or CO at ~250-300°C results in the surface enrichment with either Rh or Pd, respectively, and subsequent film rupture. The metallic islands, produced by the film de-wetting, also show a similar response to the environment changes. On the other hand, de-wetting caused by heating in NO or CO shows significant differences in the surface chemical composition evolution and, consequently, in the de-wetting onset temperature.
The results are discussed on the basis of the interplay between thermodynamic and kinetic factors, e.g. the energetics of the metallic surfaces in response to the NO/CO adsorbates, the atomic mobility of metallic and oxide surfaces, and the role of mobile Pd/Rh carbonyl or nitrosyl species. The study demonstrates the effect of the chemical environment on the morphology as well as on the composition of supported nanostructures. The nanoalloy/ bilayer approach presented here opens a new route for the fabrication of supported arrays of core-shell nanoparticles.