Computational study of α-M2O3 (M = Al, Ga): surface properties, water adsorption and oxidation

Abstract

We present a density-functional study on the properties of two corundum-type solid surfaces, α-Al₂O₃(0001) and α-Ga₂O₃(0001), and water adsorption and oxidation on these substrates. The relative stability and the geometric and electronic properties of various terminations of α-M₂O₃(0001) (M = Al, Ga), namely, O₃–M–M– (named S₁), O₂–M–M– (S₂), O₁–M–M– (S₃), M–M–O₃– (S₄) and M–O₃–M– (S₅), were first investigated. Our calculations predict that the stoichiometric termination, S₅, is the most stable one over a wide range of oxygen chemical potentials. This termination also shows the strongest inter-layer relaxation with respect to the bulk structure. The adsorption energy of molecular water at the monolayer, on α-Al₂O₃(0001) (α-Ga₂O₃(0001)) ranges from −0.46 (−0.06) to −1.13 (−0.92) eV, depending on the surface composition. Furthermore, the dissociation of water on all these surface terminations was found to be exothermic, it was also found that water can spontaneously dissociate on S₃. Finally, the stability of the hydrated structures of S_1–5 of these two metal oxides against an applied bias was investigated, followed by the oxidation of water on the stable surfaces.