Interaction of water with ordered theta-Al(2)O(3) ultrathin films grown on NiAl(100)

The structure of an ordered, ultrathin theta-Al(2)O(3) film grown on a NiAl(100) single-crystal surface was studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED), and its interaction with water was investigated with temperature programmed desorption (TPD) and XPS. Our results indicate that H(2)O adsorption on the theta-Al(2)O(3)/NiAl(100) surface is predominantly molecular rather than dissociative. For theta(H)()2(O) < 1 ML (ML = monolayer), H(2)O molecules were found to populate Al(3+) cation sites to form isolated H(2)O species aligned in a row along the cation sites on the oxide surface with a repulsive interaction between them. For theta(H)()2(O) > 1 ML, three-dimensional ice multilayers were observed to form, which then desorb during TPD with approximate zero-order kinetics as expected. A small extent of H(2)O dissociation was observed to occur on the theta-Al(2)O(3)/NiAl(100) surface, which was attributed to the presence of a low concentration of oxygen atom vacancies. Titration of these defect sites with adsorbed H(2)O molecules revealed an estimated defect density of 0.05 ML for the theta-Al(2)O(3)/NiAl(100) system consistent with the ordered nature of the synthesized oxide film.