Photophysical and Photocatalytic Properties of a New Series of Visible-Light-Driven Photocatalysts M 3 V 2 O 8 (M = Mg, Ni, Zn)

A new series of visible-light-driven photocatalysts M 3 V 2 O 8 (M = Mg, Ni, Zn) was synthesized by the conventional solid-state reaction method. The crystal structure and photophysical properties were characterized by powder X-ray diffraction and UV−vis diffuse reflectance spectroscopy, respectively. Photocatalytic properties were evaluated by O 2 evolution from an aqueous AgNO 3 solution under visible light irradiation. The results showed that the three compounds of M 3 V 2 O 8 (M = Mg, Ni, Zn) were all crystallized in an orthorhombic system with the space group Abam. However, the substitution of M 2+ cations with different electronic states imposed significant effects on their photophysical and photocatalytic properties. Zn 3 V 2 O 8 showed a higher activity than Mg 3 V 2 O 8, while Ni 3 V 2 O 8 showed almost no activity. Theoretically calculated energy band structures and density of states (DOS) by the plane-wave-density function theory (DFT) revealed that the different photophysical and photocatalytic properties of M 3 V 2 O 8 (M = Mg, Ni, Zn) could be ascribed to their different electronic structures. The Zn 3d and O 2p orbitals were hybridized to construct the valence band of Zn 3 V 2 O 8, favoring the mobility of photoexcited holes in the valence band and thus promoting the O 2 evolution activity. In contrast, no such hybridization effect occurs in Mg 3 V 2 O 8 as the Mg 2p orbitals are not involved in the valence band. The split Ni 3d orbitals inserted between the O 2p and the V 3d orbitals in Ni 3 V 2 O 8 are not suitable for O 2 evolution.