Bandgap-adjustment and enhanced surface photovoltage in Y-substituted LaTaIVO2N†
Perovskite-type oxynitrides AB(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 LaTaIVO2N resulting in phase-pure La0.9Y0.1TaIVO2N, La0.75Y0.25TaIVO2N, and La0.7Y0.3TaIVO2N. By using microcrystalline YTaO4 together with an unconventional ammonolysis protocol we synthesized the highest reported weight fraction (82(2) wt%) of perovskite-type YTa(O,N)3. Ta4+ in La1−xYxTaIVO2N 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 (EG,x=0.3 ≈ EG,x=0 < EG,x=0.1 < EG,x=0.25). Surface photovoltage spectroscopy (SPS) of particulate La1−xYxTaIVO2N (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 ∼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 reduced B-site metal cations such as Ta4+ on the photovoltage and charge carrier recombination rate.