Fabrication of a monoclinic/hexagonal junction in WO3 and its enhanced photocatalytic degradation of rhodamine B

A series of WO 3 samples with different crystalline phases were prepared by the thermal decomposition method from ammonium tungstate hydrate. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and N 2 adsorption-desorption were used to characterize the crystalline phase, morphology, particle size, chemical composition, and surface area of the WO 3 samples. The formation of hexagonal (h-WO 3 ) and monoclinic (m-WO 3 ) crystal structures of WO 3 at different temperatures or different times was confirmed by XRD. m-WO 3 is formed at 600 °C, while m-WO 3 starts to transform into h-WO 3 at 800 °C. However, h-WO 3 , which forms at 800 °C, may transform into m-WO 3 by increasing the calcination temperature to 1000 °C. SEM results indicate that m-WO 3 particles exhibit a bulky shape with heavy aggregates, while h-WO 3 particles exhibit a rod-like shape. Moreover, m-WO 3 crystals are sporadically patched on the surface of the h-WO 3 rod-like particles, resulting in the exposure of both m-WO 3 and h-WO 3 on the surface. It is observed that the monoclinic phase (m-WO 3 )/hexagonal phase (h-WO 3 ) junction was fabricated by tuning the calcination temperature and calcination time. The relative ratios between m-WO 3 and h-WO 3 in the phase junction can readily be tailored by control of the calcination time. The photocatalytic activities of WO 3 with different crystalline phases were evaluated by the photocatalytic degradation of rhodamine B as a model pollutant. A higher photocatalytic activity was observed in the WO 3 sample with the m-WO 3 /h-WO 3 junction as compared with the sample with only m-WO 3 . The improvement of photocatalytic activity can be attributed to the reduction of the electron-hole recombination rate owing to the formation of the phase junction, whose presence has been confirmed by HRTEM and photoluminescence spectra. A monoclinic phase (m-WO 3 )/hexagonal phase (h-WO 3 ) composite has been fabricated by tuning the calcination temperature and time in the thermal decomposition method. The m-WO 3 /h-WO 3 phase junction demonstrates enhanced photocatalytic activity for RhB degradation.