Comparative Study of the Passivation of Al(111) by Molecular Oxygen and Water Vapor

The self-limiting oxidation behavior of Al(111) surface by molecular oxygen and water vapor at room temperature is comparatively studied using X-ray photoelectron spectroscopy (XPS). The XPS Al(2p) and O(1s) core-level photoelectron lines are used to monitor the growth of the oxide passivation film, which showed that the limiting thickness increases with increasing gas pressure from 1 x 10(-8) to 1 x 10(-2) Torr. In comparison to oxide film growth via oxidation by molecular oxygen, the analysis of the Al(2p) and O(1s) peaks showed that oxidation by water vapor results in Al(OH)(3)-Al2O3 bilayer film growth with a relatively constant thickness of the upper layer of Al(OH)(3). By fitting the experimentally measured passivation layer thicknesses, which depends on the oxidation time and gas pressure, with the logarithmic growth law from the Cabrera-Mott theory of metal oxidation, we find that oxidation with molecular oxygen results in a stronger Mott potential and thus a thicker limiting thickness of the oxide film than oxidation with water vapor. These results demonstrate that the passivation properties of a metal surface depend not only on the gas pressure but also on the type of oxidizing species used for the passivation.