Hyperthermal Oxidation of Graphite and Diamond

Abstract

Carbon materials have mechanical, electrical, optical, and tribological properties that make them attractive for use in a wide range of applications. Two properties that make them attractive, their hardness and inertness in many chemical environments, also make them difficult to process into useful forms. The use of atomic oxygen and other forms of oxidation has become a popular option for processing of these materials (etching, erosion, chemical functionalization, etc.). This Account provides an overview of the use of theory to describe the mechanisms of oxidation of diamond and graphite using hyperthermal (few electronvolts) oxygen atoms. The theoretical studies involve the use of Born-Oppenheimer molecular dynamics calculations in which on-the-fly electronic structure calculations have been performed using either density functional theory or density-functional-tight-binding semiempirical methods to simulate collisions of atomic oxygen with diamond or graphite. Comparisons with molecular-beam scattering on surfaces provide indirect verification of the results. Graphite surfaces become oxidized when exposed to hyperthermal atomic oxygen, and the calculations have revealed the mechanisms for formation of both CO and CO(2). These species arise when epoxide groups form and diffuse to holes on the surface where carbonyls are already present. CO and CO(2) form when these carbonyl groups dissociate from the surface, resulting in larger holes. We also discuss mechanisms for forming holes in graphite surfaces that were previously hole-free. For diamond, the (111) and (100) surfaces are oxidized by the oxygen atoms, forming mostly oxy radicals and ketones on the respective surfaces. The oxy-covered (111) surface can then react with hyperthermal oxygen to give gaseous CO(2), or it can become graphitized leading to carbon removal as with graphite. The (100) surface is largely unreactive to hyperthermal atomic oxygen, undergoing large amounts of inelastic scattering and supporting reactions that create O(2) or peroxy radicals. We did not observe a mechanism for the removal of carbon for this surface. These results are consistent with experimental studies that show formation of CO and CO(2) in graphite oxidation and preferential etching on (111) CVD diamond surfaces in comparison with (100) surfaces.