Water adsorption on O(2x2)/Ru(0001):: STM experiments and first-principles calculations

We present a combined theoretical and experimental study of water adsorption on Ru(0001) precovered with 0.25 ML (monolayer) of oxygen forming a (2x2) structure. Several structures were analyzed by means of density functional theory calculations for which scanning tunneling microscope (STM) simulations were performed and compared with experimental data. Up to 0.25 ML, the molecules bind to the exposed Ru atoms of the 2x2 unit cell via the lone pair orbitals. The molecular plane is almost parallel to the surface with its H atoms pointing toward the chemisorbed O atoms of the 2x2 unit cell forming hydrogen bonds. The existence of these additional hydrogen bonds increases the adsorption energy of the water molecule to approximately 616 meV, which is similar to 220 meV more stable than on the clean Ru(0001) surface with a similar configuration. The binding energy shows only a weak dependence on water coverage, with a shallow minimum for a row structure at 0.125 ML. This is consistent with the STM experiments that show a tendency of the molecules to form linear rows at intermediate coverage. Our calculations also suggest the possible formation of water dimers near 0.25 ML.