Adsorption of water molecules on flat and stepped nickel surfaces from first principles

We present an ab initio density functional study of the adsorption of a series of water oligomers (molecule, dimer and trimer) on nickel surfaces with and without step defects. We investigate the preferred adsorption geometries and adsorption energies and analyze the binding mechanisms by means of electronic density difference maps. Special attention is devoted to the incremental adsorption process, i.e., the way additional molecules attaches to an already adsorbed water. In agreement with recent findings, we show that the first water molecule is bound to the surface with an energy of about 0.2-0.4 eV, i.e., with up to twice the strength of a hydrogen bond. In contrast to this, subsequent water molecules increase the total adsorption energy by typically 0.5 eV. However, electron density difference considerations indicate that this additional attraction is not due to the interaction of the new molecule with the surface but mediated by the first water molecule. The interaction of the additional molecule with the surface appears even to be repulsive. We discuss the implications of these findings for the wetting properties of transition metal surfaces.