Modifications and Growth Mechanisms of Ultrathin Aluminum Oxide Films on NiAl in Water

X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) have been combined to study the modifications and growth of alumina thin films on NiAl(001) induced by exposure to water vapor at low pressure (10−6 mbar) and by immersion in ultrapure liquid water at room temperature. The film thickness and stoichiometry, as well as the alloy composition changes, were analyzed by angle-resolved XPS measurements. Ultrathin (∼0.7 nm) hydroxylated native oxide films were compared to thicker (∼4 nm) anhydrous and oxygen-deficient thermal oxide films grown at 900 °C under low air pressure (10−4 mbar). The results show that immersion in liquid water causes the formation of one to two equivalent additional monolayers of aluminum oxide at the interface between the native oxide film and the alloy, suggesting anion transport along the oxide/alloy interface after entry at intergranular oxide sites. The hydroxylated oxide surface remains unchanged. In contrast, immersion of the thicker thermal oxide films in liquid water causes the formation of ∼3 equivalent additional aluminum oxide monolayers at the oxide/water interface, indicating cation transport through the film. AFM suggests a preferential growth at the boundaries between the oxide grains due to faster ion transport. The ∼4 nm thick thermal oxide film is inert when exposed to water at low vapor pressure (10−6 mbar) confirming the pressure gap for the water-induced modifications of transitional alumina films. Exposure to ambient air partially annihilates the oxygen deficiency observed after formation as shown by the variation of the O−Al stoichiometry and core level shift effects.