Hydroxylation-induced modifications of the Al2O3/NiAl(001) surface at low water vapour pressure

STM, STS, LEED and XPS data for crystalline theta-Al2O3 and non-crystalline Al2O3 ultra-thin films grown on NiAl(001) at 1025 K and exposed to water vapour at low pressure (1 X 10(-7) -1 X 10(-5) mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O-1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al3+ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of similar to 2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxyhydroxide (AlOxOHy with x + y similar to 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H2O) = 1 X 10(-7) mbar, only the surface of the oxide film is modified, with formation of nodules of similar to 2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (similar to 7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified. (c) 2005 Elsevier B.V. All rights reserved.