Skimming the surface : A surface structural approach to understanding silver ion release from silver nanoparticles

Abstract

In the last decades, there has been an exponential increase in the number of publication on the topic of silver nanoparticles. However, little is yet understood about the fundamental processes that underlie the dissolution of silver nanoparticles, which one of the most significant aspects of silver nanoparticle behavior for both application and environmental health. This thesis constitutes a considerable advance towards closing that knowledge gap. A subvalent surface oxide structure is proposed consisting of a single layer of neutral ≡Ag3OH units. When these surface become oxidized, ionic silver is released and a second surface layer is formed, with oxidation over multiple layers. An equilibrium is thought to exist between the two surface strucures, which stabilizes the silver nanoparticle from further oxidation. The subvalent ≡Ag3OH structure is consistent with crystallographic data and computation modelling results. A surface complexation model is designed based on the proposed structure, which is tested and refined through a set of dissolution experiments in which the pH dependency of silver ion release is monitored for particles of different size. Both equilibrium concentrations and the kinetics of silver ion release are consistent with the model. Kinetic data additionally reveal a rate limiting step which is thought to involve the adsorption of molecular oxygen to the particle surface. Equilibrium data suggest the existence two oxidation processes: controlled, reversible oxidation at mildly oxidative conditions, and irreversible oxidation at more oxidative, i.e. acidic, conditions in which complete surface layers are stripped by lateral growth of critical cavities. Equilibrium data also reveal a size dependency which is us used to calculate a surface free energy value for silver nanoparticles covered by the subvalent surface layer. The importance of surface energy for silver nanoparticles, and indeed nanoparticles in general, is such that it warrants an in depth study of the topic, which is performed in the latter two research chapters of this thesis. A long standing controversy on the behaviour of surface energy at the extreme end of the nanoscale is tackled by applying the thermodynamic approach by Tolman to computational data. Using these data, the thermodynamically consistent surface, the surface of tension, is localized for metallic nanoparticles, and found to coincide with the physical surface. When scaled to the latter, surface energies are shown to vary minimally with size, as is also found when using the Tolman lengths that correspond to the physical surface. Lastly, the influence of edge and corner atoms on the stability and the surface energy of metallic nanoparticles is investigated. Using coordination numbers and surface densities of various atoms, a model is developed to which accurately describe the excess energy of nanoparticles with respect to bulk metal. The model can be directly applied to predict the energy of edge and corner atoms. Additionally, the model is used to isolate the energy of twinning from total particle energies as determined by computational modelling. The model is applied to predict the surface energy of the different crystallographic surfaces of various FCC metals, the relative stability of a variety of nanoparticle morphologies and the surface energy of metallic nanoparticles at room temperature.