Concentration effects on segregation behavior of Pt-Rh nanoparticles

Based on the first-principles calculation combined with the cluster expansion technique and the Monte Carlo statistical simulation, concentration dependence of segregation behavior for Pt-Rh cuboctahedral nanoparticles is quantitatively examined. Several ground-state atomic arrangements for the nanoparticles at T = 0 K are predicted where Pt atoms prefer surface sites, particularly, at vertex sites with the lowest coordination number. We find strong Pt segregation at the surfaces of nanoparticles. Concentration dependence of segregation behavior for the nanoparticle is essentially different from that of the bulk Pt-Rh surface. Temperature dependence of segregation in the nanoparticles only appears for Pt composition at around 30%-70% due to the finite number of constituent atoms and to substantial Pt segregation at the surface. Warren-Cowley short-range-order parameters for vertex-edge and vertex-(100) pairs change their signs with respect to the Pt composition. This can be attributed to the significant Pt segregation at the vertex site, which should disrupt the tendency for the preference of unlike-atom pairs found at low Pt compositions. We find that the magnitude relationship of the Pt composition for surface sites is significantly affected by Pt composition in the nanoparticle.