  Solar water splitting has attracted much attention as a clean and renewable
route to produce hydrogen fuel. Since the oxygen evolution half-reaction (OER)
requires high overpotentials, much research has focused on finding catalyst
materials that minimize this energy loss. Oxynitrides with a layered perovskite
structure have the potential to combine the superior photocatalytic properties
of layered perovskite oxides with enhanced visible-light absorption caused by
the band gap narrowing due to less electronegative nitrogen ions. In this
paper, we study the OER on the (001) and (100) surfaces of the layered
oxynitride Sr$_2$TaO$_3$N using density functional theory (DFT) calculations to
obtain the OER free energy profiles and to determine the required
overpotentials at various sites on each surface. We find that the reconstructed
grooved (100) surface is most relevant for photocatalysis due to suitable
band-edge positions combined with a low overpotential and good carrier mobility
perpendicular to the surface.
