Density functional theory study of clean, hydrated, and defective alumina (1102) surfaces

We report an ab initio thermodynamic analysis of the alpha-Al(2)O(3) (1102) surface aimed at understanding the experimentally observed terminations over a range of surface preparation conditions as well as a stoichiometric model for the (2x1) surface reconstruction observed after high-temperature annealing. As temperature is increased under both ultrahigh vacuum and ambient hydrated conditions, the predicted minimum-energy structural model goes through the same series of changes: from the hydroxylated missing-Al surface model (or half-layer model in which the topmost Al site of the stoichiometric surface has zero occupancy), to the hydroxylated stoichiometric model, to another hydroxylated missing-Al surface model with tetrahedral coordinated surface Al, and finally to the clean (1x1) stoichiometric model. These results are in agreement with observations of both missing-Al and bulklike stoichiometries under wet conditions and in agreement with similar trends reported for isostructural hematite. However, we observe that the models with excess oxygen have a relatively higher surface-free energy and distinct surface relaxations in the case of alumina as compared to hematite. At very high temperatures where oxygen defects are generated, we find that a stoichiometric, charge-neutral (2x1) structure becomes the most thermodynamically stable. This is consistent with the observation of a (2x1) electron diffraction pattern when the surface is annealed at 2000 K while a (1x1) pattern persists at lower annealing temperatures. A general rule that emerges from our modeling results is that while the full phase space of hydrated and defective surfaces is expansive, model stoichiometries that can be made charge neutral through either hydration or defects offer the greatest thermodynamic stability. However, the unique trends in structure and relative energies of alumina surface stoichiometries as compared to hematite can be understood based on the difference in electronic structure of the substrate.