Cr(III)-Doped ZnO Nanoflowers: Synthesis, Mechanism and Enhanced Photocatalytic Activity.

ZnO is one of the most widely used photocatalyst, however, because of the wide band gap, its utilization for visible light is unsatisfactory. Therefore, current efforts are directed toward reducing the band gap of ZnO. Transition metal doping has emerged as a promising method, being capable of not only effectively reducing the band gap of ZnO, but inhibiting the recombination of electron-hole pairs. Cr(III)-doped ZnO nanoflowers with different Cr/Zn molar ratios were synthesized by a mild, facile and eco-friendly solvothermal process. Subsequently, through various characterizations, it was confirmed that the dopant Cr(III) entered the lattices of ZnO, substituting the Zn. Owing to the synergy of flower-like morphology and Cr(III) dopant, the photocatalytic activity of the products was significantly improved. The optimal Cr/Zn molar ratio was 4%, and the photocatalytic activity of 4% Cr(III)-doped ZnO nanoflowers was 1.3 and 3.4 times higher than that of pure ZnO under UV and visible light irradiation, respectively. In addition, the photocatalytic degradation kinetics of methyl orange were also discussed. Furthermore, the mechanism for the improvement in photocatalytic performance of Cr(III)-doped ZnO nanoflowers was also investigated based on a series of characterization results such as the UV-vis diffuse reflectance spectra, photoluminescence, photocurrent and the first-principles calculation.