Nitrogen Loss and Oxygen Evolution Reaction Activity of Perovskite Oxynitrides

Perovskite oxynitride photocatalysts were reported by experiment to evolve small amounts of N2, because of the self-oxidation of nitrogen ions by photogenerated holes. The N2 evolution rate was observed to decrease with increasing reaction time and was found to be correlated with a decrease in O2 evolution (OER) activity, the origin of this latter effect however being unknown. Here, we investigate, by means of density functional theory calculation, anion vacancies at the TaON-terminated (001) surface of the perovskite oxynitride SrTaO2N. We find an energetic preference for oxygen and nitrogen vacancies to reside at the surface, where they are spontaneously healed by *O and *OH adsorbates under OER conditions. For nitrogen vacancies, this self-healing leads to an altered stoichiometry Ta4O8+xN4–x that is accompanied by electron doping. Substitution of N by O at the surface also leads to tensile strain, which confines the excess charge to the very surface layer, affecting the binding energy of reaction intermediates and significantly increasing the OER overpotential. This peculiar change in electronic structure thus provides an atomic-scale explanation for the experimentally observed drop in OER activity of perovskite oxynitrides.