Diffusion Monte Carlo study of O2 adsorption on single layer graphene

Diffusion Monte Carlo (DMC) calculations were observed for an valid description of the nature of the O2 adsorption on single layer graphene. We investigated the stable orientation of O2 at a specific adsorption site as well as its equilibrium adsorption energy. At equilibrium adsorption distances, an O2 molecule was found to prefer a horizontal orientation, where the O-O bond is parallel to the graphene surface, to the vertical orientation. However, the vertical orientation is favored at O2-graphene distances shorter than the equilibrium distance, which could be understood by the steric repulsion between O and C atoms. In opposition to previous DFT calculations, our DMC calculations reflect that the midpoint of a C-C bond (a bridge site) is energetically preferred for the O2 adsorption to a center of a hexagonal ring (a hollow site). The lowest DMC adsorption energy was found at an intermediate point between a hollow site and a bridge site, where the O2 adsorption energy was estimated to be -0.142(4) eV, which is in very good agreement with the recently reported experimental value. Finally, we have found that O2 is very diffusive on the surface of graphene with the diffusion barrier along a bridge-hollow-bridge path being as small as ~11 meV.