Atomistic Simulations of Al(100) and Al(111) Surface Oxidation: Chemical and Topological Aspects of the Oxide Structure

The chemical and topological aspects of short- and medium-range atomic ordering on oxidized Al(100) and Al(111) surfaces have been studied by employing reactive force field-based molecular dynamics (ReaxFF-MD) simulations as a function of O2 gas density at 300 K. We found two oxide film growth regimes, compatible with experimental and recent modeling data. Trend of changes in oxide film thickness with increasing oxygen gas density agrees with available literature data, while slightly thicker oxide film forms on the Al(100) substrate. Chemical descriptors of short- and medium-range correlation manifest difference in atom environment between two ultrathin oxide films as [3,4]Al and [2,3]O-coordinated species dominate. In turn, a highly liquid-like structure of ultrathin oxide film develops on the Al(100) surface compared to an amorphous nature of the Al(111) oxide film with slightly lower thickness. Three-dimensional analysis of oxide structures reveals a medium-range atomic order formed by the arrangement of dominating corner-sharing configurations over edge-sharing ones with some deviation from the ideal polyhedral units. Three-fold ring is in majority over 2-, 4-, and 5-fold ones, in conjunction with a 2-fold ring forming the most frequent ring linkage. The high-n ring structure can be treated as a measure of a certain degree of “free volume”, incorporated in the oxide film during its growth on the Al(100) or Al(111) surfaces and can initiate nanostructure formation in anodic oxide film. Such diversity in ring abundance also explains the lower mass density of the oxide films compared to crystalline alumina compounds