First-Principles Characterization and Comparison of Clean, Hydrated, and Defect α-Al2O3 (110) Surfaces

Structural models of the (110) termination of α-Al2O3 are studied using density functional theory (DFT) calculations and thermodynamics modeling to determine the details of the mineral-water interface structure. It has been established for other facets of both alumina and isostructural hematite that surface preparation conditions can influence the stoichiometry and structure observed during in situ experimental characterization studies. To this end, we use theory and modeling to determine the thermodynamically preferred surface structures as a function of the chemical environment, in terms of the oxygen chemical potential, pressure, and temperature. Consistent with studies of other facets of alumina, we find that thermodynamically unfavorable defect structures, upon hydration and hydroxylation, can show greater stability than the hydrated forms of ideal terminations. The model results are compared to experimental characterization of the hydrated (110) surface, with good agreement in terms of layer spacings and calculated surface-free energies. The electronic structure of the exposed surface functional groups is presented and discussed in terms of structure-reactivity concepts used in geochemical surface science.