Water Oxidation on Oxygen-Deficient Barium Titanate: A First-Principles Study

We present a study of the effects of oxygen vacancies (Q(vac)) on the oxygen evolution reaction (OER) on the TiO2-terminated (001) surface of cubic BaTiO3 (cBTO-TiO2) using spin-polarized DFT+U calculations and the standard (cation-based) four proton-coupled-electron-transfer methodology. We find that the excess electrons associated with O-vac's are involved in charge transfer (CT) to the intermediate adsorbate species HO*, O*, and HOO* and/or new surface oxygen hole states that we identified. The CT is responsible for an increase in these species' binding energies to the oxygen deficient surface (cBTO-TiO2-x) to an extent consistent with their electro-negativity. The much stronger stabilization of HO* and O* compared to HOO* results in an increased overpotential eta(OER) on the reduced oxide. This result is at odds with experiment that shows a significantly increased efficiency for oxygen-deficient BTO, suggesting that a different mechanism and/or surface must be involved under the experimental conditions. We also identify heretofore unreported HO* and O* intermediate adsorbate structures whereby these species oxidize the surface and a surface oxygen hole is formed adjacent to the adsorption site. We assign the facile surface oxidation to the 2-fold coordination of the surface oxygen atoms in Ti-O-Ti surface moieties and a resulting low work function.