Photocatalytic Degradation of Methylene Blue by Innovative BiVO4/TiO2 Composite Films under Visible Light Irradiation

Bismuth vanadate and titanium dioxide (BiVO4/TiO2) composites, used as visible-light-driven photocatalysts were successfully synthesized with different mole ratios by coupling of a co-precipitation method with a sol-gel method. The phase transitions of the as-prepared BiVO4/TiO2 composites were carried out by X-ray diffraction (XRD). The results clearly indicated that the as-synthesized BiVO4/TiO2 composites presented in two crystalline phases, which were monoclinic BiVO4 and tetragonal TiO2 (anatase) structures. Raman spectroscopy provided further structural evidence as to the composition of the BiVO4/TiO2 composites. In order to investigate the effect of TiO2 on the BiVO4 photocatalyst, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were used to consider the BiVO4/TiO2 composite as comparison with the pure TiO2 and BiVO4. It was found that the BiVO4/TiO2 composite was formed by coupling TiO2 nanoparticles on surface of BiVO4 particles, rather than a doped material. Light absorption of the BiVO4/TiO2 composites across the UV-visible spectrum and band gap energy were also investigated by diffuse reflectance photospectroscopy (UV-vis DR). Furthermore, the BiVO4/TiO2 composites were fabricated into films by doctor blading (on glass substrates) in order to solve the catalysts separation issue for studying their photocatalytic activities evaluated via degradation of methylene blue (MB) dye in aqueous solution under visible light irradiation. Photocatalytic results showed that composite photocatalysts having a higher photocatalytic activity (with pseudo-first order kinetics) than the pure BiVO4 and TiO2 do individually. The BiVO4/TiO2 composite with mole ratio of 1:1 exhibited the highest of the apparent MB degradation rate constant of 1.568 h. It is proposed that electronic interactions between BiVO4 and TiO2 leads to an improved charge separation of the coupled BiVO4/TiO2 system, the electrons can transfer from the conduction band of TiO2 to that of BiVO4, which is responsible for the enhancement in the rate of photocatalytic degradation. Additionally, the MB degradation efficiency over the BiVO4/TiO2 composite film was quite stable in five reuse cycles, indicating the long-term stability of the film. BiVO4/TiO2 composite films are intended to be used in photocatalytic degradation of organic pollutants in wastewater, especially those from the textile and the photographic industries. Keywords—photocatalytic degradation, innovative BiVO4/TiO2 composite films, methylene blue, visible light irradiation