String-like collective atomic motion in the interfacial dynamics of nanoparticles

Abstract

The exploding field of nanotechnology is largely driven by the significant physical and chemical property changes of nanoparticles (NPs) and other nanostructures in comparison to bulk materials and by an increasing capacity for measuring these property changes and synthetically tuning these property changes by controlling particle and feature size, along with surface chemistry. Given the large size-dependent shifts of NP melting temperatures T_m, we can expect at least some of these NP property changes to be associated with an alteration in the NP atomic mobility as the particle size and surface chemistry are varied. Since recent electron microscopy studies on metal NPs of interest in heterogeneous catalysis (e.g., fuel cells, carbon nanotube growth) have often indicated a high NP interfacial mobility and have suggested the relevance of this phenomenon to catalysis, we performed molecular dynamics (MD) simulations for a range of NP sizes in the catalytically relevant temperature range with a focus on quantifying the NP interfacial dynamics. Our illustrative computations were performed for Ni NPs because these particles have been considered in fundamental studies of both fuel cell catalysis and carbon nanotubes growth. Instead of a simple fluid layer on the NP surface, we find a prevalence of string-like collective atomic motions where the geometrical nature of these collective excitations is found to be quantitatively like the collective atomic motions found in glass-forming liquids and in the grain boundary dynamics of polycrystalline materials. We illustrate our new perspective on NP interfacial dynamics by showing metal atom additives (Ag and Pt) alter the length of the string excitations (decreasing and increasing the average length, respectively), as in previous studies of molecular and NP additives in glass-forming polymer liquids.