Visible light-enhanced photocatalytic dye degradation and hydrogen evolution performance of BiVO4 thin films prepared at various annealing temperatures

Bismuth vanadate (BiVO4) is an impressive semiconducting photocatalyst in visible light irradiation. It has numerous advantages, such as less toxicity, low production cost, photocorrosive resistance, photostability, a small band gap and high responsiveness under visible light. In this study, we used sol-gel dip coating techniques to prepare high-quality BiVO4 thin films as photocatalysts at various annealing temperatures and investigated the effects of annealing temperatures (200, 300, 400 and 500 degrees C) on structural, optical and photocatalytic dye degradation and hydrogen (H-2) evolution. X-ray diffractometry and Raman spectra analyses were used to evaluate the monoclinic crystallisation structure of BiVO4 thin films.The particle sizes of the prepared samples increased with the annealing temperature. Field-emission scanning electron microscopy and Brunauer-Emmett-Teller surface analyses showed that the morphology of the BiVO4 thin films exhibits a nanoflake-like structure. The surface areas of BiVO4 thin films increased with annealing temperature. X-ray photoelectron microscopy was used to interpret the electronic structure and chemical composition of the prepared BiVO4 thin films, and optical absorption studies revealed that the red shift that exhibits the band gap energy was reduced depending on the annealing temperature. This shift enabled better photoactivation and H-2 evolution in the visible region. The photocatalytic activities of the BiVO4 thin films were also investigated for methylene blue degradation under visible irradiation. The BiVO4@500 degrees C thin film had better first-order kinetics and stability than the other samples. Moreover, experiments with H-2 evolution confirmed that BiVO4@500 degrees C had remarkably higher H-2 evolution activity (27.3 mu mol h(-1)) than BiVO4@200 degrees C (3.1 mu mol h(-1)). The improved dye degradation and H-2 evolution were due to the enhanced surface area and improved electron transfer efficiency in visible light absorption. In addition, a proposed mechanistic model for the dye degradation and H-2 evolution of the BiVO4 photocatalyst is presented.