Bandgap-adjustment and enhanced surface photovoltage in Y-substituted LaTaIVO2N

Perovskite-type oxynitridesAB(O,N)(3)are photocatalysts for overall water splitting under visible light illumination. In the past, structurally labile perovskite-type oxynitrides (e.g.YTaON2) were predicted to be highly suitable. In this work, we tackle the challenging YTa(O,N)(3)synthesis by Y-substitution in (LaTaO2N)-O-IV resulting in phase-pure (La0.9Y0.1TaO2N)-O-IV, (La0.75Y0.25TaO2N)-O-IV, and (La0.7Y0.3TaO2N)-O-IV. By using microcrystalline YTaO(4)together with an unconventional ammonolysis protocol we synthesized the highest reported weight fraction (82(2) wt%) of perovskite-type YTa(O,N)(3). Ta(4+)in (La1-xYxTaO2N)-O-IV was verified by X-ray photoelectron spectroscopy (XPS) and X-ray near edge absorption structure (XANES) analysis. Density functional theory (DFT) calculations revealed a transparent conductor-like behavior explaining the unusual red/orange color of the Ta4+-containing perovskites. In combination with crystal structure analysis the DFT calculations identified orthorhombic strain as the main descriptor for the unexpected trend of the optical bandgap (E-G,E-x=0.3 approximate to E-G,E-x=0<E-G,E-x=0.1<E-G,E-x=0.25). Surface photovoltage spectroscopy (SPS) of particulate (La1-xYxTaO2N)-O-IV (x= 0, 0.1, 0.25, 0.3) films revealed negative photovoltages at photon energies exceeding 1.75 eV, confirming that these materials are n-type semiconductors with effective bandgaps of similar to 1.75 eV irrespective of the Y content. The photovoltage values increased with the Y content, suggesting an improved carrier generation and separation in the materials. However, increasing the Y content also slowed down the timescales for photovoltage generation/decay indicating trap states in the materials. Based on our results, we suggest a significantly weaker as classically assumed impact of reducedB-site metal cations such as Ta(4+)on the photovoltage and charge carrier recombination rate.