Oxynitrides with the perovskite structure are promising candidates for photocatalysis under visible light due to their appropriate optical and electronic properties. Recently, layered perovskites have attracted attention for their improved performance with respect to bulk perovskites in photocatalytic water splitting. In this paper, we investigate the structural and electronic properties of the layered Ruddlesden-Popper oxynitride SrTaON and its (001) and (100) surfaces using density functional theory (DFT) calculations. We find that the energetically favoured configuration of the bulk has an in-plane cis anion order and exhibits rotations of the TaON octahedra. Furthermore, we show that the TaON-terminated (001) surface suppresses exciton recombination due to higher-energy surface states, giving a potential explanation for the good photocatalytic performance.

Gathering information on the atomic nature of reactive sites and trap states is key to fine tuning catalysis and suppressing deleterious surface voltage losses in photoelectrochemical technologies. Here, spectroelectrochemical and computational methods were combined to investigate a model photocathode from the promising chalcopyrite family: CuIn0.3Ga0.7S2. We found that voltage losses are linked to traps induced by surface Ga and In vacancies, whereas operando Raman spectroscopy revealed that catalysis occurred at Ga, In, and S sites. This study allows establishing a bridge between the chalcopyrite's performance and its surface's chemistry, where avoiding formation of Ga and In vacancies is crucial for achieving high activity.

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.

Oxide materials with a perovskite-derived layered structure often show better photocatalytic performance compared to nonlayered materials of similar composition. Here we investigate the catalytic activity for the oxygen evolution reaction (OER) on surfaces of the layered Carpy−Galy oxides Sr 2 Ta 2 O 7 and Sr 2 Nb 2 O 7 . We find that the (010) surface parallel to the layering shear plane is not relevant for photocatalysis. The (100) surface perpendicular to this plane, on the other hand, can catalyze the OER, at least for Sr 2 Ta 2 O 7 , whereas Sr 2 Nb 2 O 7 is inactive for the OER without a cocatalyst, both in agreement with experiment. The OER activity of Sr 2 Ta 2 O 7 ( 100) is, however, only retained while the surface does not assume a reconstructed stepped structure via Sr dissolution which we predict to be thermodynamically favored. Engineering Sr 2 Ta 2 O 7 into long-lasting photoelectrodes thus has to focus on exposing (100) facets as well as preventing Sr dissolution, for example, via a high Sr concentration in the electrolyte.

Perovskite oxynitrides are an established class of photocatalyst materials for water splitting. Previous computational studies have primarily focused on their bulk properties and have drawn relevant conclusions on their light absorption and charge transport properties. The actual catalytic conversions, however, occur on their surfaces and a detailed knowledge of the atomic-scale structure and processes on oxynitride surfaces is indispensable to further improve these materials. In this contribution, we summarize recent progress made in the understanding of perovskite oxynitride surfaces, highlight key processes that set these materials apart from their pure oxide counterparts and discuss challenges and possible future directions for research on oxynitrides.