Oxygen migration on the graphene surface. 1. Origin of epoxide groups

It is now a widely accepted fact that oxidized graphene surfaces are populated, to a greater or lesser extent, with epoxide groups. And yet the origin of these groups has heretofore been mysterious. We report the results of a computational (DFT) analysis of this issue carried out by combining the theoretical and experimental knowledge of three seemingly unrelated fundamental processes: (i) formation of pentagon–heptagon pairs (or Thrower–Stone–Wales defects); (ii) surface diffusion of oxygen atoms on the basal plane; and (iii) graphene unzipping by oxygen insertion. We provide thermodynamic and kinetic evidence for the hypothesis that a key intermediate step in the stabilization of free adjacent zigzag sites – before they reconstruct to form an armchair site or become quinone surface functionalities upon dissociative O2 chemisorption – is the formation of an epoxide group in the basal plane. The presence of epoxide groups on the graphene surface is therefore a result of spillover of edge oxygen (e.g., nondissociatively adsorbed O2 on carbene-type sites), mechanistically reminiscent of the extensively investigated migration of carbon in the conversion of phenyl carbene to bicycloheptatriene.