Melting of graphene supported Pd-Pt core-shell nanoparticles: A molecular dynamics study

In this study, molecular dynamics simulation is utilized to investigate thermal behavior of both free and supported pure Pd, Pt and Pd-Pt core-shell nanoparticles during continuous heating. Heating curves calculated between 298 K and 2000 K are used to determine solid-liquid transitions for free and supported nanoparticles. The melting temperature of Pd-Pt core-shell nanoparticles is found to be between the one for pure Pd and Pt nanoparticles. The results show that melting temperature of supported nanoparticles decreases about 120 K due to the metal-carbon interactions. Structural changes during melting of nanoparticles are also detected through the common neighbor analysis, the mean-square displacement method and the atomic segregation. The common neighbor analysis indicates three different melting modes in both free and supported nanoparticles. The analysis demonstrates that the graphene support has no significant effect on melting modes due to the low distortion in the nano-particle structure. In addition, the mean-square displacement of nanoparticles increases in the presence of the graphene support by forming a cap shaped nano-particle and nucleation of Shockley dislocations at the metal-carbon interface. In order to show the effect of metal-carbon interactions on mean-square displacement of nanoparticles, the contact angle is computed displaying the interactions. The statistic radii and atomic distribution reveal the segregation of Pd atoms to the shell and Pt atoms to the core because of dissimilar surface energies. Furthermore, the atomic distribution of Pd and Pt atoms in supported core-shell nanoparticles are more homogenous than free ones.