Binding of atomic oxygen on graphene from small epoxy clusters to a fully oxidized surface

We used ab initio calculations based on density functional theory (DFT) to investigate the binding of atomic oxygen on graphene, considering adsorption structures with sizes varying from sub-nanometric clusters to infinite overlayers. From an extensive set of studied configurations of oxygen dimers, trimers and tetramers we demonstrated a strong tendency of adsorbates to form close-packed structures. In the high O coverage regime, corresponding to a C/O ratio of 2:1, we identified two very favorable structures with a distribution of O adatoms which enables both the hybridization of all C-2p_z states from the graphene π electron network with the O-2p states, as well as the minimization of the electrostatic repulsion between negatively charged O atoms attached to the same side of the graphene sheet. Using DFT results obtained for small clusters we constructed a simple model able to describe the energetics of the O islands with sizes beyond those that can be directly treated by first principles methods.