Lubrication of Stone-Wales transformations in graphene by hydrogen and hydroxyl functional groups

First-principles calculations were performed to address the role of functional groups (hydrogen atoms and hydroxyl molecules) in lubricating the fundamental transformation by which a Stone-Wales defect is formed in graphene. Energy barriers in the presence of a single H atom, as well as in the case of two, four, and six H atoms chemisorbed in graphene in several distinct site configurations are found to be smaller than in pristine graphene. Our study examines in detail the electronic mechanism behind the stabilization, by the functional groups, of the transition state of the defect-forming reaction relative to the reactants (functionalized graphene and Stone-Wales defect), due to partial strain relaxation and electronic saturation of the transition-state dangling bonds. We frame these findings in terms of the reactivity, to the functional groups, of the reactants and transition states. Our calculations point to a very favorable kinetic pathway with a strongly reduced activation barrier, in which two H atoms bind to next-nearest-neighbor C atoms, and saturate the two transition-state dangling bonds, resulting in a strong barrier reduction of 5.6 eV (from 9.3 eV without functional groups to 3.7 eV). In the case of two chemisorbed OH molecules, we find a further reduction of the Stone-Wales transformation barrier for one configuration considered, when compared to the similar one with two H atoms, providing additional confirmation of the reactivity-based mechanism.