Mechanistic study of defect-induced oxidation of graphite

Oxidation of a graphite initiated from surface defects is investigated by using scanning tunneling microscopy (STM) and density functional (DF) calculation The defects are coated on a graphite surface with controlled depth and density by impact of Ar+ ions at low energies (50-500 eV). Oxidation of the defects with Oz at temperatures of 450-650 degrees C produces shallow pits on the surface via layer-by-layer etching of the carbon layer. The etch pattern and speed vary depending on the depth of a pit. Round shaped pits are produced in most cases, but less frequently anisotropic patterns are also observed for monolayer etching. A multilayer pit grows a few times faster than a monolayer pit via simultaneous etching of the top and inner carbon layers. Kinetic parameters are extracted for the oxidation of depth-differentiated pits. The activation energies are quite comparable for the oxidation of monolayer and multilayer pits, but the pre-exponential factor is larger for multilayer oxidation. which gives rise to a faster etching speed. DF calculations are performed to identify the key intermediates involved in the oxidation reactions. The calculation shows that an Oz molecule dissociates exothermally with no energy barrier and then forms oxide species at the top or bridge sites of a carbon vacancy. A metastable O-2* precursor state, in which an O-2 molecule adsorbs at the bridge site without dissociation, is also found. These surface species can be desorbed under the etching conditions either thermally or via further chemical reactions. Reaction pathways involving these intermediates are proposed to explain the different etching behaviors observed for mono- and multilayer pits.