Morphological sensitivity of silver nanoparticles to the environment

Abstract

Silver nanoparticles have been modeled by density functional theory calculations and ab initio molecular dynamics simulations to explore their stability in air at room temperature. The role of contaminants is considered by adsorbing monoshells of methanethiol molecules on silver nanoparticles of various morphologies (icosahedral, strongly irregular and regular truncated octahedral structures) in the range of 147–201 atoms (∅ = 1.66–1.80 nm), under atmospheric nitrogen pressure. While ab initio molecular dynamics simulations suggest that the icosahedral and all the face-centered cubic FCC clusters are stable at 300 K under vacuum, the ino-decahedral geometry progressively transforms into a complex structure composed of an irregularly icosahedral outershell and a decahedral core. Nanoparticle surface energies, computed at 0 K and 300 K, show a preference for FCC clusters in vacuum, as previously reported experimentally. In the presence of air at 300 K, the icosahedral cluster presents the largest exothermicity in terms of adsorption surface energy of contaminant monoshells. This energetic gain is understood on the basis of a larger surface silver atomic density for the icosahedral structure, which better accommodates dense contaminant monoshells than FCC clusters. The methanethiol adlayers are composed of a complex phase of chemisorbed molecules bound to silver and in interaction with physisorbed contaminants through hydrogen bonds. This theoretical study agrees with measurements of silver nanoparticles exposed to air after synthesis in vacuum and also investigated in solution, and demonstrates that the air environment tunes the relative stability of morphologies in competition. This work paves the way for the understanding of nanoparticle ageing in environmental conditions at the atomic scale.