Nitrogen-doped layered oxide Sr5Ta4O15−xNx for water reduction and oxidation under visible light irradiation

Abstract

Development of a photocatalyst with wide visible light absorption is of vital importance in solar-chemical energy conversion. In this work, we introduce a new nitrogen-doped layered oxide, Sr₅Ta₄O_15−xN_x, which exhibits a significantly extended absorption edge compared with the undoped oxide Sr₅Ta₄O₁₅. The extension of the visible light absorption has been ascribed to the substitution of nitrogen for oxygen atoms as well as the formation of Ta–N bonds, which was confirmed by X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Judged by the first principle calculation, the N 2p states mixed with pre-existing O 2p states shift the valence band maximum upward and result in wide visible light absorption. Band structure analysis combined with UV-Vis diffuse reflectance spectrum (DRS) and Mott–Schottky (M–S) measurement shows that the conduction and valence bands of Sr₅Ta₄O_15−xN_x are sufficient for water reduction and oxidation, respectively. The photocatalytic water splitting performances of Sr₅Ta₄O_15−xN_x are strongly related to the deposited cocatalyst. With an optimized cocatalyst, the Sr₅Ta₄O_15−xN_x shows both H₂ and O₂ evolution activities under visible light irradiation using CH₃OH and AgNO₃ as scavengers respectively. Following the optimized cocatalyst deposition of the Sr₅Ta₄O_15−xN_x, the cocatalyst-modified nitrogen-doped tantalum-based layered oxides Sr₂Ta₂O_7−xN_x and Ba₅Ta₄O_15−xN_x also exhibit activities for both the water splitting half reactions. This work demonstrates that the nitrogen-doped tantalum-based layered oxides may be a new type of potential photocatalyst with wide visible light absorption for solar water splitting.