Molecular dynamics simulation study of the melting of Pd-Pt nanoclusters

Abstract

Bimetallic nanoclusters are of interest because of their utility in catalysis and sensors. The thermal characteristics of bimetallic Pt-Pd nanoclusters of different sizes and compositions were investigated through molecular dynamics simulations using quantum Sutton-Chen (QSC) many-body potentials. Monte Carlo simulations employing the bond order simulation model were used to generate minimum energy configurations, which were utilized as the starting point for molecular dynamics simulations. The calculated initial configurations of the Pt-Pd system consisted of surface segregated Pd atoms and a Pt-rich core. Melting characteristics were studied by following the changes in potential energy and heat capacity as functions of temperature. Structural changes accompanying the thermal evolution were studied by the bond order parameter method. The Pt-Pd clusters exhibited a two-stage melting: surface melting of the external Pd atoms followed by homogeneous melting of the Pt core. These transitions were found to depend on the composition and size of the nanocluster. Melting temperatures of the nanoclusters were found to be much lower than those of bulk Pt and Pd. Bulk melting temperatures of Pd and Pt simulated using periodic boundary conditions compare well with experimental values, thus providing justification for the use of QSC potentials in these simulations. Deformation parameters were calculated to characterize the structural evolution resulting from diffusion of Pd and Pt atoms. The results indicate that in Pd-Pt clusters, Pd atoms prefer to remain at the surface even after melting. In addition, Pt also tends to diffuse to the surface after melting due to reduction of its surface energy with temperature. This mixing pattern is different from those reported in some of the earlier studies on melting of bimetallics.