Efficient removal of methylene blue over composite-phase BiVO4 fabricated by hydrothermal control synthesis

Series of composite-phase BiVO4 (scheelite-tetragonal and scheelite-monoclinic) were synthesized by hydrothermal method. The mass content of monoclinic phase (ηmono = Imono/(Imono + Itetra) × 100%) in composite-phase BiVO4 was successfully controlled by adjusting the pH value of precursor solution, which was achieved by manipulating the dosage of 2 M NaOH during the synthetic reaction. Results indicated that the pH value of the precursor solution had great effect on the structure of BiVO4. It took on pure tetragonal form with solid and spherical morphology when pH ≤ 3.8, but presented pure monoclinic structure with quadrate flake appearance when pH ≥ 8.5. Along with the increase of pH (3.8–8.5) and the ηmono, the tetragonal particles and monoclinic particulates began to link together, and the two phases interweave together to form a petal-like structure when ηmono reached over 60% (pH = 4.7). The photocatalytic activities of obtained products were investigated by photodegradation of methylene blue under visible light. It was found that composite-phase BiVO4 with ηmono > 60% presented better photocatalytic activity than that of pure phase BiVO4, either monoclinic or tetragonal. About 95% of the 10 mg L−1 methylene blue was degraded within 120 min catalyzed by the best catalyst BiVO4 with ηmono = 92.73%. Possible mechanism was put forward.