Water adsorption on a p(2x2)-Ni(111)-O surface studied by surface x-ray diffraction and infrared reflection absorption spectroscopy at 25 and 140 K.

The adsorption of water molecules on an oxygen-predosed p(2x2)-Ni(111)-O surface was studied by surface x-ray diffraction and infrared reflection absorption spectroscopy (IRAS) at temperature of 25 and 140 K. Precise structures including adsorbed water, predosed oxygen, and substrate nickel atoms at these two temperatures were determined by x-ray structural analysis. It was found that water molecules adsorb on oxygen additive sites, forming a hydrogen bond at 25 K. A predosed 2x2 oxygen atom appears to accommodate one, two, or three water molecules at positions relating to threefold rotation symmetry. When the surface temperature was raised to 140 K, water molecules appear at an atop site of Ni. The distance between Ni and the oxygen atoms of a monomer water molecule was found to be 0.2241(22) nm. The adsorbed water molecule induces buckling and a lateral shift of the substrate nickel. The IRAS results provided evidence regarding the existence of two distinct adsorption sites. Water molecules in the low-temperature phase exhibit bands from both hydrogen-bonded nuOD and free OD stretchings, while those in the high-temperature phase lie flat with a molecular plane parallel to the surface.