Photophysical and photocatalytic properties of Zn ₃ M ₂ O ₈ (M = Nb, Ta)

The Zn ₃M ₂O ₈ (M = Nb, Ta) compounds with layered crystal structures were prepared by a conventional solid-state reaction method, and their electronic-band structures, optical properties, and photocatalytic activities were investigated. The prepared compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis diffuse reflectance, and Mott-Schottky measurement. The UV-Vis diffuse reflectance spectra revealed Zn ₃Nb ₂O ₈ and Zn ₃Ta ₂O ₈ exhibited band gaps of 4.1 and 4.5 eV, respectively. From the electronic-band-structure calculations using DFT method, it was found that the valence band in both compounds was constructed by the hybridization of Zn 3d and O 2p orbitals, whereas the conduction band was consisted of Nb 4d (Zn ₃Nb ₂O ₈) and Ta 5d (Zn ₃Ta ₂O ₈) orbitals. The photocatalytic activity of both compounds was examined by the measurement of the H ₂ production rate in an aqueous methanol solution under UV light irradiation. The Zn ₃Ta ₂O ₈ with a larger band gap exhibited higher activity than Zn ₃Nb ₂O ₈, which was attributed to its higher reduction potential resulting from the difference of its electronic-band configuration.