High photocatalytic activity over starfish-like La-doped ZnO/SiO_2 photocatalyst for malachite green degradation under visible light

In this work, the unique starlike La-doped ZnO–SiO₂ photocatalysts were constructed by an evaporation and calcination method and characterized in detail. UV–vis reflectance and DFT calculation confirm that the doping with La allows to obtain a decrease of band gap of ZnO/SiO₂, which enhances visible light absorbance and oxidizing ability. The photoluminescence intensity reduces greatly, indicating more effective separation of the photogenerated carriers of La-doped ZnO-SiO₂. Photocatalytic activities of La-doped ZnO-SiO₂ with different doping ratios under simulated visible light irradiation were evaluated with malachite green (MG) as a model pollutant. Under optimized conditions including solution pH of 8, 15 mg/L of MG solution and 15 mg of catalyst dosage, 0.2% La-ZnO-SiO₂ exhibits the best catalytic activity in photodegradations of MG in water. The removal and mineralization efficiency of MG can reach 96.1% and 70.9% in 140 min, respectively. The as-prepared catalysts present superior stability and recyclability after four times reuse. Moreover, selective quenching experiments indicate that hydroxyl radical (·OH), hole (h⁺) and superoxide radical (·O₂^-) are the main reactive species responsible for MG degradation. Possible mechanism for photocatalytic elimination of MG over La-doped ZnO/SiO₂ photocatalyst is finally proposed.