Monitoring water and oxygen splitting at graphene edges and folds: Insights into the lubricity of graphitic materials

The functionality of graphene as lubricant material is affected by extrinsic factors, such as the film thickness and the environmental conditions. Graphite lubricating capability depends as well on air humidity. To accurately describe the tribochemistry mechanisms underlying these behaviours we adopt a Quantum Mechanics/Molecular Mechanics approach. We show that reactive edges are able to cause a huge friction increase, which is quantified for graphene flakes between sliding diamond surfaces. Moreover, folds spontaneously formed in single layer graphene under tribological conditions are shown to be highly reactive due to carbon re-hybridization. This observation offers a new hint for interpreting the dependence of graphene friction on the number of layers. Both water and oxygen molecules are found to be effective in quenching the reactivity of defects by dissociative chemisorption. However, peculiar mechanisms of water molecules makes humidity more effective than oxygen for enabling the lubricity of graphitic media. They include collective processes as Grotthus-like proton diffusion enhanced by confinement, and the strong change in hydrophilic character of the passivated media. This comprehensive study sheds a new light on debated issues of graphene and graphite tribology, and highlights the potentiality of these materials for metal-free catalysis, e.g., for H production by water splitting.