Adsorption of water on pristine graphene: a van der Waals density functional study with the vdW-C09 approach

In this work, we employed density functional theory (DFT) with the vdW-C09 functional to investigate the adsorption of a single water molecule on pristine graphene, accurately accounting for long-range dispersion forces. Three high-symmetry adsorption sites-the center of the hexagonal ring, the C-C bond, and the top site-were explored in combination with three molecular orientations: Down, H-bond, and Up configurations. The calculated adsorption energies range from -93 to -145 meV, indicating that the interaction is dominated by weak van der Waals forces characteristic of physisorption. The most stable configuration corresponds to the Down orientation above the center of the hexagonal ring, with an adsorption energy of -145 meV and an equilibrium distance of 3.27 Å defined as the vertical separation between the oxygen atom of the water molecule and the graphene plane. These results are in close agreement with previous theoretical studies and confirm the non-reactive and hydrophobic nature of pristine graphene.