Using solar energy to split water and produce hydrogen has the potential to become a key technology to sustainably meet our rising energy demands [1]. Techno-economic studies revealed that economically viable solar water splitting should produce hydrogen at a cost of less than 2 USD per kg. To achieve this economic target, particle-based photocatalytic systems need to reach a solar-to-hydrogen (STH) efficiency of 10 % and a lifetime of at least 5 years [2].

Among the material classes currently being investigated for application in solar water splitting, oxynitrides are considered promising candidates [3]. They are typically synthesized by nitridation of oxidic materials introducing the N 2p orbitals that form the valance band edge, resulting in smaller band gaps and therefore in the absorption of a larger part of the visible light in the solar spectrum. A promising example is LaTiO₂N, which has a band gap of about 2.1 eV, and is composed of earth abundant elements [4]. It has already achieved competitive oxygen evolution rates and even overall water splitting [5, 6]. The deposition of cocatalysts such as CoO_x to enhance the performance is thereby essential [7].

Although improving the lifetime of oxynitride photocatalysts is as important as their efficiency, significantly less research efforts have been dedicated to stability investigations [8]. Fully understanding the degradation mechanisms that deactivate the involved photo- and cocatalyst materials is therefore essential. In general, water splitting systems deactivate over time either via charge related degradation induced by photogenerated electrons and holes, via chemical degradation by the electrolyte environment or via mechanical degradation, such as the detachment of cocatalyst particles. Investigating these processes is important to develop photocatalytic systems that are competitive with other technologies.

In this work we investigate the degradation mechanisms of LaTiO₂N particles, both with and without deposited CoO_x cocatalysts. The materials are therefore subjected to well-defined conditions, involving exposure to an electrolyte and light irradiation. The particles are characterized before and after the procedure, to reveal changes caused by degradation. Structural and morphological investigations are carried out by X-ray diffraction (XRD), as well as scanning and (scanning) transmission electron microscopy (SEM/(S)TEM) coupled to electron energy loss spectroscopy (EELS) to investigate the chemical composition. Optical properties are assessed by UV-vis spectroscopy and the photocatalytic activity is investigated by measuring the oxygen evolution rate via gas chromatography.