Insights of the Multimode\nOrientation-Dependent Initial\nOxidation of Aluminum by First-Principles Theory Calculations

A fundamental understanding of the oxidation mechanisms\nof aluminum\n(Al) alloys is of great importance for its applications in corrosion,\ncatalysis, sensors, etc. In this work, we systematically investigated\nthe first-stage oxidation behaviors of three low-index Al facets with\nO coverage up to two monolayers (ML) by using density-functional theory\n(DFT). The large negative adsorption energies indicated favorable\noxidation on all three facets. However, distinctive structural and\nelectronic changes induced by the adsorption of oxygen have led to\ndifferent oxidation modes. More specifically, the oxidation process\nproceeded by “intercalating” into the subsurface region\nalong the (111) plane out of the (110) facet with spontaneous O ingress\ninto (110) far below one ML, as revealed by the electron density distribution,\nwhereas the oxide ad-layer grew in a “layer-by-layer”\nmode on Al(111) and (001) facets. Moreover, various Al–O complexes\nwith different atomic coordination numbers (CN), configurations, and\nsizes may be indicators of the tendency of an Al surface to be oxidized.\nBesides, the oxide phases formed on (111)/(001) and (110) assembled\nthe Al–O bond distribution within α-Al<sub>2</sub>O<sub>3</sub> and γ-Al<sub>2</sub>O<sub>3</sub>, respectively.