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Bandgap-Adjustment and Enhanced Surface Photovoltage in Y-Substituted LaTaIVO2N

Abstract

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 unsusal red/orange color of the Ta4+-containing perovskites. In combination with crystal structure analysis the DFT calculations identified the orthorhombic strain as main descriptor for the unexpected trend of the optical bandgap (EG,x=0.3EG,x=0EG,x=0.1EG,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 material. 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.

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Article information


Submitted
22 Feb 2020
Accepted
19 May 2020
First published
19 May 2020

J. Mater. Chem. A, 2020, Accepted Manuscript
Article type
Paper

Bandgap-Adjustment and Enhanced Surface Photovoltage in Y-Substituted LaTaIVO2N

C. Bubeck, M. Widenmeyer, A. T. De Denko, G. Richter, M. Coduri, S. C. Eduardo, E. J. Goering, H. Zhang, S. Yoon, F. Osterloh and A. Weidenkaff, J. Mater. Chem. A, 2020, Accepted Manuscript , DOI: 10.1039/D0TA02136A

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