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

We present a density-functional study on the properties of two corundum-type solid surfaces, alpha-Al2O3(0001) and alpha-Ga2O3(0001), and water adsorption and oxidation on these substrates. The relative stability and the geometric and electronic properties of various terminations of alpha-M2O3(0001) (M = Al, Ga), namely, O-3-M-M-(named S-1), O-2-M-M-(S-2), O-1-M-M-(S-3), M-M-O-3-(S-4) and M-O-3-M-(S-5), were first investigated. Our calculations predict that the stoichiometric termination, S-5, is the most stable one over a wide range of oxygen chemical potentials. This termination also shows the strongest interlayer relaxation with respect to the bulk structure. The adsorption energy of molecular water at the monolayer, on alpha-Al2O3(0001) (alpha-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 S-3. Finally, the stability of the hydrated structures of S1-5 of these two metal oxides against an applied bias was investigated, followed by the oxidation of water on the stable surfaces.