Molecular Dynamics Study of Diffusion in Palladium Hollow Nanospheres and Nanotubes

1. Introduction The synthesis of nano–scale materials is a rapidly developing field of materials science. Recently, Sun et al. [1] have demonstrated the preparation of a number of hollow nanostructures of Au, Pd and Pt by a replacement reaction with Ag starting with solid Ag templates. Such structures have very considerable promise in a wide range of technological applications such as catalysis, precise drug delivery and many others. However, it has been noted [2,3] that hollow nano-objects should be unstable in principle and, with time, they will tend to shrink. According to [2,3] the mechanism of shrinking can be considered as resulting from the vacancy flux from the inner surface to the external surface. The driving force for this flux is the difference between the vacancy concentrations on the inner and external surfaces. We have described the shrinkage via the vacancy mechanism of a pure element hollow nanosphere and nanotube (see our abstract titled 'Analytical and kinetic Monte–Carlo study shrinkage by vacancy diffusion of hollow nanospheres and nanotubes'). Using Gibbs-Thomson boundary conditions an exact solution was obtained of the kinetic equation. The collapse time as a function of the geometrical sizes of hollow nanospheres and nanotubes was determined. Kinetic Monte-Carlo (KMC) simulation of the shrinkage of these nano-objects was performed: it confirmed the predictions of the analytical model. However, it has been shown on the basis of this simulation that under real conditions reconstruction of the external surface can occur. This reconstruction could not be taken into account either in the theoretical analysis or KMC simulation. In the present paper, we study the diffusion in a pure Pd hollow nanosphere and nanotube by performing a molecular dynamics (MD) simulation using the embedded-atom method (EAM).