Critical Role of the Semiconductor–Electrolyte Interface in Photocatalytic Performance for Water-Splitting Reactions Using Ta3N5Particles

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 Ta3N4.83O0.17 and N deficient Ta3N4.83 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.