Theoretical Studies of the Reactions between Hyperthermal O(³P) and Graphite: Holes and the Second Layer

Theoretical direct dynamic calculations on the basis of density functional theory are used to investigate hyperthermal collisions between O(³P) and highly oriented pyrolytic graphite (HOPG). The simulations suggest the HOPG surface becomes functionalized with epoxide groups as in previous works. Also, incoming O atoms can react at the surface to form O₂ by way of an Eley–Rideal mechanism when the surface exposed to colliding O atoms is already functionalized with epoxides. A second layer of pristine graphene, included in the model, absorbs collision energy and significantly reduces the rates of reactions that liberate carbon from the surface, compared to single-layer models. Semiquinone functional groups are thought to be common at graphitic sheet edges and holes and to be a major source of CO and CO₂. We deploy a model to explore the behavior of semiquinones when exposed to hyperthermal oxygen atoms. These groups are found to lead to CO and to play a role in the formation of epoxide groups on the underlayer. Both of these findings are important in explaining the behavior of CO and O atoms observed in a recent set of experiments. Once produced, the CO remains associated with the sheet, interacting with the functional groups surrounding the hole for the remainder of the simulation. An epoxide group on the underlayer forms below the surface layer and would require the surmounting of two energy barriers to desorb, which is consistent with experimental predictions. CO₂ formation was also observed, and it resulted from an Eley–Rideal reaction as did the CO. Lactones form during some of the trajectories, and this functional group is thought to be important in the erosion of graphite caused by exposure to oxygen atoms. The exposure of lactones to hyperthermal O atoms is also explored, and it is found to lead predominantly to CO₂.