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.