Characterization of Stable NiO x /SrTaO x N y Bilayers Boosting the Oxygen Evolution Reaction for Solar Water Splitting

SrTaO x N y (STON) is a well‐known visible light‐responsive semiconductor with ideally located band edges that allow the operability of overall water splitting. Like many oxynitrides, STON shows evidence of detrimental physicochemical changes under oxygen evolution reaction (OER) conditions involving strong caustic electrolytes. We investigate the development of STON instability with detailed electron microscopy and neutron reflectometry (NR) techniques using epitaxial thin films. Different crystallographic orientations are compared with ex situ analysis before and after OER in photoelectrochemical testing. A remarkable difference in stability of the STON surface is observed depending on the crystalline facets, with the [011] lattice planes being the more favorable orientation as compared to [001]. In addition, we show that the electrochemical stability of the photoelectrode surface can be dramatically improved by a homogeneous coating of NiO x , which significantly improves OER kinetics and surface stability in the alkaline environment. NR is realized in this work as a novel route to monitor this photoelectrochemical environment, and it lies in agreement with its microscopy counterpart to monitor the physicochemical changes. This demonstrates potential of NR as an alternative and complementary tool that also has the feasibility for future in situ experimental design.