Platinum nanoparticles on carbonaceous materials: the effect of support geometry on nanoparticle mobility, morphology, and melting.

Molecular dynamics simulations have been used to investigate the morphology and mobility of platinum nanoparticles of various sizes supported by carbon materials. The embedded-atom method was used to model Pt-Pt interactions, and the Lennard-Jones potential was used to model the Pt-C interactions. The C atoms in the supports were held fixed during the simulations. The supports considered were a single graphite sheet and three bundles of carbon nanotubes. Three sizes of Pt nanoparticles were considered: 130 atoms, 249 atoms, and 498 atoms (Pt(130), Pt(249), and Pt(498) respectively). It was found that for all three sizes, diffusion coefficients were approximately one order of magnitude higher for graphite-supported nanoparticles than for carbon nanotube-supported nanoparticles. In addition, increasing the size of the nanoparticle decreased its diffusion coefficient, with Pt(130) having the highest and Pt(498) the lowest diffusion coefficients. More interestingly, we found that for the Pt nanoparticles of all three sizes the diffusion coefficient increases as temperature increases, reaches a maximum at the melting temperature of the nanoparticle, and then decreases. The melting temperature was found to be strongly dependent on the particle size, but only slightly dependent on the features of the supports. While the size of the nanoparticle was seen to affect the particles' mobility, it did not significantly affect their structure. The nanoparticles supported by graphite have density profiles that indicate a highly ordered, fcc-like structure, while the particles supported by carbon nanotubes have a more disordered structure. An order parameter confirms that the nanoparticles' structure depends on the support morphology.