(472e) Structure, Interconversion And Reactivity Of Point-Like Surface Defects On Amorphous Silica

Atomic-level control and accurate determination of the structure and properties of oxide surfaces has long been an issue of great importance due to their many applications in electronics, heterogeneous catalysis, and electrochemistry. However, the difficulty of direct characterization arising from sample charging has impeded progress towards understanding the fundamental behavior and properties of oxide surfaces. Point-like defects may significantly alter the surface properties of amorphous silica, but their formation, structure, and dynamics are still unclear. Earlier experimental investigations have demonstrated that upon electron irradiation a silica surface undergoes changes in not only photosensitvity but also surface reactivity, mainly due to irradiation-induced defects. Oxygen deficient centers (ODCs) are most common point defects in oxide materials While a great deal of effort has been devoted to understanding the nature of ODCs in bulk silica, little is known about the formation, structure, and dynamics of surface defects despite their importance in determining complex silica surface chemistry. In this talk, we present the structure and stability of oxygen vacancy related defects on amorphous silica based on combined classical Monte Carlo and first principles quantum mechanical calculations. Based on model amorphous silica surfaces constructed using combined Monte Carlo and density functional calculations, we determined the structure of point-like surface defects, such as Si-Si dimer, divalent Si, and silanone, and the pathways and barrier their interconversion using extensive ab initio molecular dynamics and static transition-state search calculations. These results suggest that surface oxygen vacancies in the dimer configuration may undergo thermally-activated transformation with moderate activation energies into other stable point-like defects including divalent Si defects, silanone groups, and/or subsurface Si-Si dimers. This in turn implies that under thermal equilibrium the surface concentration of oxygen vacancies in the dimer configuration would be minimal, while providing a theoretical support for the existence of divalent Si defects and silanone groups on the amorphous surface. We will also discuss the reactivity of these surface defects towards various gas species. The detailed understanding greatly assists in understanding the defect-associated complex surface chemistry of amorphous silica.