Density functional theory study of the clean and hydrated hematite (1(1)over-bar02) surfaces

The structures of the clean and hydrated hematite alpha-Fe2O3 (1 (1) over bar 02) surfaces were investigated using density functional theory, and the free energies of the surfaces in chemical equilibrium with water were calculated as a function of temperature and oxygen partial pressure using ab initio thermodynamics. At 298.15 K, the predicted lowest-energy surface, in equilibrium with 20 Torr H2O has a stoichiometry of (H2O)(2)-X-(HO)(2)-Fe-2-O-2-R, where X denotes a vacancy of an atomic layer of Fe and R represents the bulk stoichiometric repeat (Fe2O3). This surface stoichiometry results in three types of (hydr)oxo functional groups: Fe-OH2,Fe-2-OH, and Fe-3-O. At temperatures above 435 K, maintaining equilibrium with the same water partial pressure, the predicted lowest-energy stoichiometry is (HO)(2)-(HO)(2)-Fe-2-O-2-Fe-2-O-2-R, consistent with water dissociated on the stoichiometric bulk termination. These results suggest that the experimentally observed surface termination is highly sensitive to thermal annealing and water reaction. Furthermore, differences in protonation states or the surface hydroxyl groups are shown to lead to large differences in energetic stability and layer relaxations of the oxide substrate. Prediction of the stable surface termination and its dependence on environmental variables such as water, O-2, and thermal treatment provide a base line for understanding surface reactivity.