Two-Stage Melting of Au−Pd Nanoparticles

Abstract

Several series of molecular dynamics runs were performed to simulate the melting transition of bimetallic cuboctahedral nanoparticles of gold-palladium at different relative concentrations to study their structural properties before, in, and after the transition. The simulations were made in the canonical ensemble, each series covering a range of temperatures from 300 to 980 K, using the Rafii-Tabar version of the Sutton and Chen interatomic potential for metallic alloys. We found that the melting transition temperature has a strong dependence on the relative concentrations of the atomic species. We also found that, previous to the melting transition, the outer layer of the nanoparticle gets disordered in what can be thought as a premelting stage, where Au atoms near the surface migrate to the surface and remain there after the particle melts as a whole. The melting of the surface below T m is consistent with studies of the interaction of a TEM electron beam with Au and Au-Pd nanoparticles.

Effect of Equivalent Sites on the Dynamics of Bimetallic Nanoparticles

“…Gold atoms diffuse towards the surface before the melting temperature for all particles, in agreement with other works [19,21]. As a result, high concentrations of palladium are not observed in the surface.…”

“…Using a Sutton-Chen potential model [16] to model the interactions in the Au-Pd bimetallic nanoparticles, we performed the Molecular Dynamics simulations in the canonical ensemble (NVT) making use of the Nosé-Hoover thermostat [17,18], and following the same strategy used in a previous set of simulations of Au-Pd nanoparticles [19]. Sutton and Chen modified the Finnis-Sinclair potential to study the interactions between particles, and developed it for ten fcc metal species:…”

Highly size-controlled synthesis of Au/Pd nanoparticles by inert-gas condensation

“…For some particles, their orientation with respect to the electron beam made it difficult to verify clearly the structure, but nevertheless we can conclude that the most common particle structure in our View Article Online depositions was the icosahedral; however, we also observed decahedral particles with less frequency. In a previous work, 12 we have reported that nanoparticles grown by the polyol method are mostly cuboctahedral in shape with an FCC crystalline structure. This is in sharp contrast with our present case, where the particles have Ic and Dh structures.…”

On the Structure of Au/Pd Bimetallic Nanoparticles

“…Five different concentrations were used: pure Au, Au 3 Pd, AuPd, AuPd 3 , and pure Pd. The final liquid configuration from a previously computed heating series 14 was used as the initial configuration for the freezing series. This choice of the initial configuration is arbitrary but not improper, in the sense that any configuration obtained at thermal equilibrium would be equally useful, since at high temperatures the atoms are not bound to equilibrium lattice sites, the shape of the particle is constantly changing, and any typical configuration, including the last configuration in the previous run, can be used without producing computation artifacts; nevertheless, it is known that different initial configurations from a molten state may lead to significant differences both on the freezing temperature and on the final configuration of the run (see, for example, ref 15).…”