Segregation of Pt(28)Rh(27) bimetallic nanoparticles: a first-principles study.

Based on a first-principles calculation combined with the cluster expansion technique and Monte Carlo statistical simulation, the segregation behavior of a Pt(28)Rh(27) cuboctahedral nanoparticle is examined. From the effective cluster interaction of the nanoparticle, we see a similar weak ordering tendency inside the nanoparticle to that for Pt-Rh bulk alloy. Below the bulk melting temperature of around 1700 K, we find strong Pt segregation to the surface of the nanoparticle. This is due mainly to larger Pt on-site segregation energy for surface sites than that for subsurface and core sites. In order to examine the segregation behavior of the Pt(28)Rh(27) nanoparticle, we find that the ordering contribution is essential, which reverses the preferable segregation between edge and (100) sites. A ground-state atomic arrangement of the Pt(28)Rh(27) nanoparticle at T = 0 K is predicted, where all the Pt atoms are located at surface sites, particularly at the vertex site of the lowest coordination number.