The Role of MnOx and Ni:MnOx Co-Catalysts on the Photoelectrochemical Properties of Ta-O-N Photoanodes

Abstract

The ever-increasing energy demand and environmental concerns associated with fossil-based fuels have driven the urgent need for sustainable methods to produce chemical fuels. One approach is to perform solar water splitting to produce hydrogen and oxygen. For this purpose, metal oxide-based semiconductor are promising photoelectrode materials due to their relatively high abundance and stability in aqueous solutions. However, metal oxides typically have only slow surface reaction kinetics for water oxidation [1]. To overcome this, water oxidation co-catalysts, such as CoPi, FeOOH, NiFeOx, and MnOx, have been deposited on the surface of numerous metal oxide semiconductors resulting in an improved performance [2-6]. However, the processes at the semiconductor/co-catalyst interface are not yet well understood. Here, we investigate the role of MnOx and Ni:MnOx co-catalysts (thickness < 10 nm) deposited by atomic layer deposition (ALD) on Ta-O-N thin films as a model system. By systematically controlling the partial pressure of NH3, H2 and H2O during post-annealing of Ta thin films, different phases (e.g. Ta2O5, TaOxNy, Ta3N5) with varying valence band maximum positions were successfully prepared. The valence band position of the co-catalyst was also varied by modifying the Ni content. Photoelectrochemical studies with and without hole scavenger reveal that MnOx and Ni:MnOx enhances the photocurrent of Ta-O-N films by a factor of ~ 4, but they do not affect the charge injection efficiency. Instead, the photocurrent increase is likely a result of higher charge separation efficiency. We attribute this to the formation of a band bending at the Ta-O-N/MnOx and Ta-O-N/Ni:MnOx interface, as suggested by the open circuit potential (OCP) and in-line X-ray photoelectron spectroscopy (XPS) data. This is further supported by analyzing Ta-O-N films with varying thickness of the co-catalysts; thicker co-catalysts result in increasing band bending, which correlates with the photocurrent trend as well as the effective carrier diffusion length measured by time-resolved microwave conductivity (TRMC). Finally, the influence of the band offsets between the different photoanode in the Ta-O-N systems and the co-catalysts is discussed.