Structural study of bimetallicCoxRh1−xnanoparticles: Size and composition effects

Abstract

The structure of ultrafine bimetallic Co x Rh 1−x nanoparticles synthesized in mild conditions by codecomposition of organometallic precursors in the presence of a polymer or a ligand has been studied using highresolution electron microscopy and wide-angle x-ray scattering techniques. While pure rhodium particles exhibit the main structural features of a face centered cubic (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 fcc. Increasing the size of the particles up to 5 -6 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.

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

“…EXAFS measurements on Rh-rich Co−Rh nanoparticles indicate a segregated stucture, with Co occupying the surface sites, though the analysis was more ambiguous for other compositions . Recent MD and MC simulations (using a Gupta-like many-body potential) obtained a good agreement with the experiments concerning the composition-dependent nearest-neighbor distance.…”

“…Casenove, Respaud, and co-workers synthesized PVP-stabilized Co−Rh nanoalloys via the co-decomposition of organometallic precursors. , It was found that alloying with Co led to successive loss of the fcc ordering of the Rh particles. For particles under 2 nm, HRTEM, EXAFS, and wide-angle X-ray scattering measurements show that Co-rich clusters have noncrystalline polytetrahedral packing while Rh-rich particles have defective fcc structures (though they become more regular at increased temperatures) …”

Magnetic and Structural Properties of Isolated and Assembled Clusters

“…Recent experimental results on the NN bond lengths and magnetic moments in Co-Rh clusters [110,213] and the comparison with available theoretical calculations suggest that these binary clusters are likely to have a Rh core with Co rich outer layers. In fact, the measurements of NN bond lengths yield values that are very similar to that of bulk Rh [213].…”

“…Recent experimental results on the NN bond lengths and magnetic moments in Co-Rh clusters [110,213] and the comparison with available theoretical calculations suggest that these binary clusters are likely to have a Rh core with Co rich outer layers. In fact, the measurements of NN bond lengths yield values that are very similar to that of bulk Rh [213]. Moreover, the measured magnetic moments per Co atom in the size range N þ M ¼ 200-400 is hMi expt T =N ' 2:38m B for x Co ' 0:5 [110], which are much larger than the theoretical values obtained for a Co rich core but which are consistent with the calculations for a Rh-rich core eventually with some degree of mixing at the interface.…”

Development of Bi-Metallic Fe—Bi Nanocomposites: Synthesis and Characterization

“…[9][10] Interestingly, in all these systems, the shell was enriched in the magnetic element while the non-magnetic one remained at the core. As the particles were synthesized in soft conditions, the resulting distribution can hardly be attributed to thermodynamics.…”