Understanding the Surface Oxidation of Ag Nanocrystals Under Different Environmental Conditions

In many cases, the presence of a shell of surface oxide detrimentally alters the properties of nanomaterials, rendering them ineffective for the targeted applications. This issue is yet to be addressed because the growth and removal of oxide(s) from the surface of nanomaterials is still poorly understood. In this article, we used vapor-phase-derived Ag nanocrystals with controlled sizes to elucidate the mechanisms of surface oxidation by exposing them to ambient atmosphere, aqueous environments, and ethylene glycol. Our experimental data suggested that the thickness of an oxide shell directly correlated with the extent of plasmon damping, and the optical properties of nanocrystals below 8 nm in size were more drastically affected by the presence of an oxide shell relative to larger counterparts. In addition, we found that aqueous environments with a relatively high pH gave rise to higher rates of oxidation. The time-dependent changes in optical properties were also validated using theoretical simulations based on the discrete dipole approximation. We concluded this article by discussing three different strategies for effectively removing the surface oxide, including decomposition through thermal annealing, wet-chemical reduction through the addition of a reducing agent, and decomposition and/or reduction by electron irradiation.