The initial stage of the interaction of water molecules with graphene supported on Ni(111) was studied by combining electronic, structural and scanning probe microscopy techniques with theoretical calculations. We demonstrate the occurrence of dissociative water adsorption at the basal plane of graphene, which renders the surface evenly covered by H atoms, whereas the OH counterparts are somehow removed from the graphene surface. Density functional theory calculations show that water splitting on Gr/Ni(111) is an endothermic process and becomes exothermic when occurs close to pre-adsorbed H atoms or in correspondence of C vacancies. However, the dissociation energy barrier for uni-molecular dissociation becomes compatible with a reasonable reaction rate at room temperature only when it takes place in correspondence of Ni atoms trapped at the C vacancies during Gr growth. On the other hand, due to the limited number of single Ni atoms catalysts in the Gr layer, it seems reasonable that collective adsorption processes or autocatalytic dissociative reactions are largely responsible for the extensive water dissociation observed experimentally.
