K₂SO₄-Assisted Hexagonal/Monoclinic WO₃ Phase Junction for Efficient Photocatalytic Degradation of RhB

Fabrication of a phase junction in the photocatalyst is one of the efficient strategies for enhancement of the photocatalytic activity. However, research on the relation between phase composition and photocatalytic property of WO3 is limited because of the barely controllable phase transition process from monoclinic to hexagonal phase. A facile sol–gel synthesis of a composition tunable hexagonal/monoclinic-WO3 (h/m-WO3) phase junction with K2SO4 as stabilizing agent is developed. X-ray powder diffraction, scanning electron microscopy, UV–Raman, high-resolution transmission electron microscopy, and UV–vis diffusion reflectance spectroscopy are employed to investigate the structures, morphologies, crystalline phases, phase composition, and optical properties of the as-prepared samples. Contents of the hexagonal phase in the WO3 samples can be precisely adjusted in a wide range from 0 to 71.1 wt % by regulating the K2SO4 amount, the calcination temperature, and the calcination time. Degradation of rhodamine B of samples indicates that the reaction rate depends significantly on the contents of the hexagonal/monoclinic phase in the WO3 samples. A 7.4 times enhancement in the reaction rate is observed for the h/m-WO3 sample with 71.1 wt % h-WO3 than the pure m-WO3. The increased photocatalytic activity is attributed to the formation of a phase junction between h-WO3 and m-WO3, which exhibits high efficiency of the separation and transfer of photoexcited electron–hole, as evident from electrochemical impedance spectra results. This work provides a new insight into the fabrication of a phase composition designable h/m-WO3 phase junction with high photocatalytic performance, which benefits the application of WO3 in environmental protection issues.