The Structure of Silica Surfaces Exposed to Atomic Oxygen

In this work we use molecular dynamics (MD) simulations with the ReaxFF SiO GSI potential to model the structure of quartz and amorphous silica surfaces exposed to atomic oxygen. The ReaxFF SiO GSI potential is a reactive force field that was specifically parametrized to describe gas surface interactions in silica–oxygen systems (Kulkarni, A. D.; et al. J. Phys. Chem. C 2012, 117, 258–269). We show that the ReaxFF SiO GSI potential accurately describes the experimentally measured bulk structure of quartz and amorphous silica, as well as experimentally and computationally characterized surface features for these materials. A flux boundary condition is implemented in molecular dynamics simulations to model the exposure of silica surfaces to atomic oxygen. We find that the types of defects occurring on silica surfaces under vacuum and exposed to atomic oxygen at high temperatures are in agreement with previous MD simulations and experimental measurements of silica surfaces. The ReaxFF SiO GSI potential predicts a peroxyl defect that has not been observed in previous MD simulations of silica surfaces, but has been experimentally identified. Density functional theory (DFT) calculations are used to validate the extent to which the ReaxFF SiO GSI potential predicts the structure and binding energy of the oxygen molecule on this defect. Through MD simulation and comparison with experiment, we identify the chemical surface defects that exist on real silica surfaces exposed to atomic oxygen at high temperatures and discuss their role in the catalytic recombination of atomic oxygen.