Atomic Oxygen on Graphite: Chemical Characterization and Thermal Reduction

The chemisorption of O atoms on graphite and the thermal reduction of the oxidized surface were studied by means of high energy resolution photoelectron spectroscopy with synchrotron radiation. The C 1s and O 1s core levels and the valence band spectra were used to identify the different oxidizing surface species and to evaluate the extension of the sp(2) conjugation as a function of oxidation time and annealing temperature. We found that epoxy groups are the dominant species only at the low oxidation stage, and ethers and semiquinones form as oxidation proceeds. The evolution of the ether/epoxy ratio with increasing oxygen coverage provides evidence for the occurrence of C-C bond unzipping. Epoxy groups are the functionalities with the lowest thermal stability and start to desorb around 370 K, strongly affecting the desorption temperature of other functional groups. The ratio between ethers and epoxy groups determines the balance between epoxy-epoxy and epoxy-ether reactions, the latter promoting the removal of C atoms from the C backbone. Adsorbate spectroscopy during thermal annealing definitely proves the catalytic effect of the basal plane oxygen atoms on the desorption reactions.