Suitability of Different Sr₂TaO₃N Surface Orientations for Photocatalytic Water Oxidation

Abstract

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.

Water Oxidation Catalysis on the Nitrogen-Deficient SrNbO2N(001) Surface

“…Nevertheless one could expect the different electron density at different surface sites to affect the catalytic activity, which we will investigate in the following. to lean towards surface N atoms, forming very stable O-N dimer structures, the dimers carrying a negative charge 42 . While such negatively charged dimers would be more easily formed with increasing N substitution, the substitution also reduces the number of O-N dimers than can form.…”

Surface Chemistry of Perovskite Oxynitride Photocatalysts: A Computational Perspective

“…These terminations may undergo a combination cies. [43] Dimer formation thus reduces the degree of oxidation the surface undergoes under OER conditions and therefore strongly stabilizes the surface adsorbate structure. In our calculations, we however observed not a complete conversion of all O adsorbates to O-N dimers, but rather about 75% [43] (compare Figs 5a-c).…”

“…[43] Dimer formation thus reduces the degree of oxidation the surface undergoes under OER conditions and therefore strongly stabilizes the surface adsorbate structure. In our calculations, we however observed not a complete conversion of all O adsorbates to O-N dimers, but rather about 75% [43] (compare Figs 5a-c). This finding was rationalized by the fact that occupied states associated with the O-N dimer lie higher in energy than those associated with the upright O adsorbate and that there is hence a balance between the energy gained by O-N formation and the accommodation of electrons is higher-energy states.…”

“…O-N dimers, even if spontaneously formed, may not necessarily have to participate in the OER as the OER could, for example, occur predominantly on the O adsorbates that remain upright. Apart from this possibility, we discovered for Ruddlesden-Popper Sr 2 TaO 3 N that Ta atoms that carry an O-N dimer and are hence formally fully coordinated, may become overcoordinated and catalyze the OER next to the adsorbed O-N dimer with a fairly small overpotential of only 0.51 V [43] (Fig. 5d).…”

“…4a), they expose, along the (001) direction, very similar surface structures as SrTaO 2 N. The layered structure, however, affects the binding energy of OER intermediate species and places Sr 2 TaO 3 N on the weak-binding side of the OER activity volcano, while SrTaO 2 N is located on the strong-binding side. [43] This suggests that layering, such as in the Ruddlesden-Popper structure, can be a way to tune the reactivity of OER catalysts that bind reaction intermediates too strongly. The (001) orientation in the Ruddlesden-Popper structure, however, suffers from very low carrier mobility (very large effective masses) perpendicular to the surface.…”