Size and composition effects in polymer-protected ultrafine bimetallicPtxRu1−x(0<x<1)particles

Abstract

Ultrafine ${\mathrm{Pt}}_x{\mathrm{Ru}}_{1-x}$ particles (with $0<~x<~1),$ synthesized by decomposition of organometallic precursors in the presence of a stabilizing polymer, are studied using both high-resolution transmission electron microscopy and wide-angle x-ray scattering techniques. In this paper, we focus on the influence of the size and the composition on the atomic organization and structural features of Pt, Ru, and Pt-Ru particles. It is shown that the pure Pt and Ru particles crystallize in the same structure as in the bulk, i.e., face centered cubic (fcc) for platinum and hexagonal close packed (hcp) for ruthenium. A modification of the cell parameters due to size reduction is observed. In particular, evidence is given for the change of the $c/a$ ratio in Ru particles. The alloy particles are also crystalline, whatever the composition. The role of the composition in the structural transition from the fcc structure to the hcp one in nanoscaled particles is demonstrated. The mechanism of structural transition is fully described.