Abstract
The structure of ultrafine bimetallic ${\mathrm{Co}}_{x}{\mathrm{Rh}}_{1\ensuremath{-}x}$ nanoparticles synthesized in mild conditions by codecomposition of organometallic precursors in the presence of a polymer or a ligand has been studied using high-resolution electron microscopy and wide-angle x-ray scattering techniques. While pure rhodium particles exhibit the main structural features of a face centered cubic ($\mathrm{fcc}$), alloying with cobalt induces a progressive loss of periodicities, leading in high-cobalt-content particles to a polytetrahedral structure close to the one already encountered in pure-cobalt particles. When increasing the synthesis temperature, the polytetrahedral structure remains remarkably stable, while particles with higher rhodium content clearly evolve towards perfect $\mathrm{fcc}$. Increasing the size of the particles up to $5--6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ stabilizes the structural phases encountered in the phase diagram of the bulk alloy. Different element-sensitive techniques, x-ray absorption spectroscopy (XANES and EXAFS) and energy-filtering transmission electron microscopy, have also been implemented in order to get chemical information. Evidence is given for a cobalt surface segregation in these bimetallic particles, highly favorable for magnetic-moment enhancement.
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