Scanning tunneling microscopy studies of carbon-oxygen reactions on highly oriented pyrolytic graphite

The oxidation of highly oriented pyrolytic graphite (HOPG) in air at elevated temperatures was studied by examination of the oxidized HOPG by scanning tunneling microscopy (STM). Etch pits of uniform size and monolayer depth were readily formed on preexisting defects or generated vacancies in the HOPG basal plane by heating freshly cleaved HOPG samples in air at 650 OC. The density of the pits in different samples of H O E varied from 0.1 to 13 pm-2. When the oxidized HOPG samples were reheated and reexamined by STM, the original etch pits had grown and new, smaller etch pits were found. Therefore, pit growth initiation occurred during each cooling-reheating cycle, and consecutive heating cycles could control the pit density. At higher temperatures, above 700 'C, carbon abstraction occurred on the basal plane of graphite, which generated additional vacancies and initiated new etch pits continuously throughout the period of heating at this temperature. The rates of carbon abstraction in both cases depended on the vacancy-density of the HOPG. On HOPG samples treated with different materials, both those expected to be chemically active (e.g., FeCI3, CdS, H2PtCI6) or inert (e.g., NaCI, AI2O3), numerous etch channels were found on the surfaces after oxidation, representing the paths of randomly moving particles. Because similar channels are formed with all of these materials, these channels are probably mainly produced by mechanical interactions by the particles on the graphite surface.