Preparation and characterization of Fe_2O_3/Bi_2WO_6 composite and photocatalytic degradation mechanism of microcystin-LR

The long-standing popularity of semiconductor photocatalysis, due to its great potential in a variety of applications, has resulted in the creation of numerous semiconductor photocatalysts, and it stimulated the development of various characterization methods. In this study, Fe₂O₃/Bi₂WO₆ composite with a flower-like microsphere and hierarchical structure was synthesized with the facile hydrothermal-impregnation method without any surfactants. X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy, and photoluminescence spectroscopy were used to characterize the structures of the samples. The specific surface area was estimated with the Brunauer-Emmett-Teller (BET) method, and pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) method. The synthesized Fe₂O₃/Bi₂WO₆ composite had an average diameter of approximately 4 nm, with smaller specific surface area and larger pore diameter than those of pristine Bi₂WO₆. The results of XRD and SEM analyses confirmed that the composite was composed of Fe₂O₃ and Bi₂WO₆. The absorption edge of Bi₂WO₆ was at a wavelength of 460 nm. By contrast, the absorption edge of Fe₂O₃/Bi₂WO₆ to visible light was redshifted to 520 nm, with narrower bandgap width and stronger visible light response. It was also found that the main active substances in the degradation of microcystin-LR (MC-LR) were hydroxyl radicals (·OH) and electron holes (h⁺). Consequently, the results further showed that the heterojunction between Fe₂O₃ and Bi₂WO₆ can improve the charge transfer rate and effectively separate the photoinduced electrons and holes. Compared with Bi₂WO₆, Fe₂O₃/Bi₂WO₆ had no significant difference in the adsorption capacity of MC-LR and had more efficient photocatalytic degradation activity of MC-LR. The degradation rates of MC-LR by Fe₂O₃/Bi₂WO₆ and Bi₂WO₆ reached 80% and 56%, respectively. The degradation efficiency of MC-LR was affected by the initial pH value, initial Fe₂O₃/Bi₂WO₆ concentration, and initial MC-LR concentration.