Critical Role of the Semiconductor–Electrolyte Interface in Photocatalytic Performance for Water-Splitting Reactions Using Ta₃N₅ Particles

Distinct photocatalytic performance was observed when Ta₃N₅ was synthesized from commercially available Ta₂O₅ or from Ta₂O₅ prepared from TaCl₅ via the sol–gel route. With respect to photocatalytic O₂ evolution with Ag⁺ as a sacrificial reagent, the Ta₃N₅ produced from commercial Ta₂O₅ exhibited higher activity than the Ta₃N₅ produced via the sol–gel route. When the Ta₃N₅ photocatalysts were decorated with Pt nanoparticles in a similar manner, the Ta₃N₅ from the sol–gel route exhibited higher photocatalytic hydrogen evolution activity from a 10% aqueous methanol solution than Ta₃N₅ prepared from commercial Ta₂O₅ where no hydrogen can be detected. Detailed surface and bulk characterizations were conducted to obtain fundamental insight into the resulting photocatalytic activities. The characterization techniques, including XRD, elemental analysis, Raman spectroscopy, UV–vis spectroscopy, and surface-area measurements, revealed only negligible differences between these two photocatalysts. Our thorough characterization of the surface properties demonstrated that the very thin outermost layer of Ta₃N₅, with a thickness of a few nanometers, consists of either the reduced state of tantalum (TaN) or an amorphous phase. The extent of this surface layer was likely dependent on the nature of precursor oxide surfaces. DFT calculations based on partially oxidized Ta₃N_4.83O_0.17 and N deficient Ta₃N_4.83 consisting of reduced Ta species well described the optoelectrochemical properties obtained from the experiments. Electrochemical and Mott–Schottky analyses demonstrated that the surface layer drastically affects the energetic picture at the semiconductor–electrolyte interface, which can consequently affect the photocatalytic performance. Chemical etching of the surface of Ta₃N₅ particles to remove this surface layer unites the photocatalytic properties with the photocatalytic performance of these two materials. Mott–Schottky plots of these chemically etched Ta₃N₅ materials exhibited similar characteristics. This result suggests that the surface layer (1–2 nm) determines the electrochemical interface, which explains the different photocatalytic performances of these two materials