Facile Synthesis, Formation Mechanism And Photocatalytic Activity Of The Nano/Micro-materials Of Tungstentrioxide And Tungstate

In recent years, environmental pollution is becoming increasingly serious that becomes a serious problem because of bringing a threat to human being. Photocatalytic oxidation technology as a new pollution control method, with its low energy consumption, mild reaction conditions, simple operation, high efficiency and no secondary pollution, which is concerned by domestic and foreign researchers. However, the traditional semiconductor photocatalyst such as TiO2can be only activated under ultraviolet (UV) irradiation due to its wide band gap (3.2EV). Only about4%of the sunlight can be utilized by pure TiO2. Thus, it is highly essential to design and prepare new visible-light catalysts with enhanced photoacivity has become the research hotspot.In this work, a series of nano/micro-materials of tungsten trioxide and tungstate with uniform size and morphology have been successfully synthesized using low-cost, facile sodium tungstate and calcium chloride as raw material via a solution route. The products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), fourier transform infrared spectrometer (FT-IR), N2adsorption/desorption measurements (BET), differential scanning calorimetry and thermogravimetry (TG/DSC). The effects of the preparation conditions that have important effects on the final products have also been investigated in detail. In addition, we discussed the formation mechanism of products based on conditional experiments. Furthermore, exceptional photocatalytic performance for decomposing rhodamine B (RhB) associated with the very special WO3architecture was demonstrated. The main contents of this thesis are summarized as following:We present a facile template-free precipitation method for the large-scale preparation of hierarchical CaWO4microspheres with diameter about10μm at room temperature. The chemical composition and morphology of the as-obtained products can be tuned by varying factors, such as pH value of the reaction solution, the concentration of citric acid, the concentration of the starting salt in solution, reaction time and the temperature. We found that the CaWO4microspheres were composed of aggregated nanoparticles. Furthermore, this work might represent a novel synthesis strategy for other tungstate nano/micro-materials. Under the same reaction conditions, the monoclinic SrWO4microspheres were also obtained.Core-shell and hollow WO3microspheres have been obtained by calcining the acid-treated oblate CaWO4precursors at500℃for2h. The shell layer of the hollow WO3microspheres was assembled by ultrafine nanoplates with a thickness of about15nm. In the procedure, CaWO4microspheres were turned to hollow WO3microspheres, whereas the original structures of the precursors were perfectly maintained. It was revealed that the final particles were usually formed through a dissolution-crystal process in the hydrothermal process. Furthermore, the obtained hierarchical WO3microspheres showed superior photocatalytic activities for the degradation of RhB under visible light irradiation. The photocatalytic properties of the obtained products are relevant to their size and shape.