# Nitrogen loss and oxygen evolution reaction activity of perovskite oxynitrides

## TL;DR

This study uses density functional theory to explain why nitrogen loss in perovskite oxynitrides reduces oxygen evolution reaction activity, revealing surface vacancy healing, electronic structure changes, and increased overpotential.

## Contribution

It provides an atomic-scale explanation for the decline in OER activity due to surface vacancy dynamics and electronic effects in perovskite oxynitrides.

## Key findings

- Surface vacancies are spontaneously healed by adsorbates.
- Nitrogen vacancies cause electron doping and altered stoichiometry.
- Surface substitution increases OER overpotential.

## Abstract

Perovskite oxynitride photocatalysts were reported by experiment to evolve small amounts of N$_2$ due to the self-oxidation of nitrogen ions by photo-generated holes. The N$_2$ evolution rate was observed to decrease with increasing reaction time and was found to be correlated with a decrease in O$_2$ 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 SrTaO$_2$N. 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 Ta$_4$O$_{8+x}$N$_{4-x}$ 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.