Preparation, characterization and photocatalytic performance of Mo-doped ZnO photocatalysts

A series of Mo-doped ZnO photocatalysts with different Mo-dopant concentrations have been prepared by a grinding-calcination method. The structure of these photocatalysts was characterized by a variety of methods, including N₂ physical adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, photoluminescence (PL) emission spectroscopy, and UV-vis diffuse reflectance spectroscopy (DRS). It was found that Mo⁶⁺ could enter into the crystal lattice of ZnO due to the radius of Mo⁶⁺ (0.065 nm) being smaller than that of Zn²⁺ (0.083 nm). XRD results indicated that Mo⁶⁺ suppressed the growth of ZnO crystals. The FT-IR spectroscopy results showed that the ZnO with 2 wt.% Mo-doping has a higher level of surface hydroxyl groups than pure ZnO. PL spectroscopy indicated that ZnO with 2 wt.% Mo-doping also exhibited the largest reduction in the intensity of the emission peak at 390 nm caused by the recombination of photogenerated hole-electron pairs. The activities of the Mo-doped ZnO photocatalysts were investigated in the photocatalytic degradation of acid orange II under UV light (?? = 365 nm) irradiation. It was found that ZnO with 2 wt.% Mo-doping showed much higher photocatalytic activity and stability than pure ZnO. The high photocatalytic performance of the Mo-doped ZnO can be attributed to a great improvement in the surface properties of ZnO, higher crystallinity and lower recombination rate of photogenerated hole-electron (e^?/h⁺) pairs. Moreover, the undoped Mo species may exist in the form of MoO₃ and form MoO₃/ZnO heterojunctions which further favors the separation of e^?/h⁺ pairs.