The Nature of Carbon Vacancies Initiating Graphite Oxidation

Abstract

The density functional theory (DFT) calculations at the B3LYP/3–21G(d,p) level of C₄₂H₁₆ cluster model were used to mimic the formation of the mono-vacancy defects on the single graphite sheet (graphene) as a result of carbon atom removal due to reaction with molecular oxygen. The formation of the edge mono-vacancy defect (five-membered rings of carbon atoms terminated by CH₂ group) appeared to be energetically favourable with energy effect of −263 kJ/mol. In contrast, in the case of basal-plane mono-vacancy defect formation (five- and seven-membered rings of carbon atoms), the respective energy is positive and equal to +362 kJ/mol and the considered process is energetically unfavourable. Similarly, in the temperature interval of 300–1200 K, the reaction of edge mono-vacancy defect formation is energetically favourable since the Gibbs free energy is negative (from −267 to −281 kJ/mol), while the reaction of basal-plane mono-vacancy defect formation is energetically unfavourable as the Gibbs free energy remains positive (from +337 to +314 kJ/mol). The obtained results agree well with the available experimental data which demonstrate the anisotropy of the natural graphite surface to oxidation and confirm that defects at graphite basal plane are most likely associated with the original defect sites and cannot be formed as the result of removal of carbon atoms due to oxidation with molecular oxygen.