Bismuth Vanadate And Tungsten Oxide Photocatalysts:Synthesis And Properties Investigation

Semiconductor photocatalytic technology has drawn increasing attention in the application of eliminating aqueous pollutants since early 1970 s. However, the defects of wide band gap and low availability in solar energy utilization for traditional photocatalysts limit their catalytic activities. Therefore, it is of great significance to construct novel photocatalysts with narrow band gaps to efficiently utilize the natural sunlight. With the relatively narrow bandgap energies, BiVO₄ and WOx have been commonly used in organics degradation and water splitting under visible-light irradiation. It is well known that the photocatalytc activity of semiconductor photocatalyst is strongly associated with its crystal structure and morphology.Therefore, in this paper, we studied the preparation of BiVO₄ and WOx with different morphologies, and evaluated their photocatalytic activities by degradation of methylene blue in aqueous solution under artificial solar-light irradiation. The main contents were listed as follows:（1） Preparation and efficient visible-light photocatalytic activity of m-BiVO₄ with different morphologiesThe monoclinic scheelite BiVO₄ crystals with different morphologies were prepared via one step hydrothermal process in the presence of Bi（NO₃）3·5H₂O and NH₄VO₃ as reactant, and sodium citrate as the chelating agent by using Na OH and HNO₃ solutions to regulate the p H value of the solution. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscope,Brunauer-Emmet-Teller and UV-Vis diffuse reflectance spectra, and their photocatalytic activities were evaluated by degradation of methylene blue in aqueous solution under artificial solar- light irradiation. The results showed that m-BiVO₄ crystals with peanut-like, oval, twin-quadrangle and twin-four-pointed star morphologies could be synthesized at different p H values. The band gaps values（Eg）of all the BiVO₄ samples were around 2.37².45 e V, which indicated that samples could strongly absorb in the visible-light region. The sample with peanut-like-shape prepared at p H=1 exhibited the highest activity, and the photocatalytic conversioncould reach above 95% after 225 min of irradiation, and its reaction rate constant was0.0132 min-1.（2） Enhanced visible-light photocatalytic activity of BiVO₄ microstructures via annealing processThe peanut-shaped monoclinic scheelite BiVO₄ crystals were synthesized via a facile one step hydrothermal method without using any templates, followed by annealing at different temperatures. The as-prepared samples were characterized by X-ray diffraction（XRD）, scanning electron microscope（SEM）,Brunauer-Emmet-Teller（BET） and UV-Vis diffuse reflectance spectra（DRS）,showing that the annealing procedure could not only change the surface morphology and crystallinity of BiVO₄ but also increase the specific surface area without damaging the peanut shape. For BV-450, it had a higher crystallinity, and the peanut-like morphology of BiVO₄ could be well kept. BiVO₄ samples annealed at different temperatures showed better photocatalytic performance than the samples without annealing, and the sample annealed at 450 °C exhibited the highest activity,and the photocatalytic conversion of methylene blue could reach above 94% after 150 min of irradiation, and its reaction rate constant was 0.018 min-1, which was 2.3 times higher than that of the BV-0 photocatalyst under visible light irradiation.（3） Preparation of coiling-type WO₃·0.33H₂O with superior photocatalytic activityWO₃·0.33H₂O microcrystal with coiling-type has been synthesized via a facile inorganic hydrothermal route in the presence of PTA aqueous solution, which was obtained from the reaction of W with H₂O₂（30%）. The as-prepared sample was characterized by X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, Brunauer-Emmet-Teller and UV-Vis diffuse reflectance spectra, and the photodegradation capability of the obtained sample towards methylene blue was investigated. It has the conclusion that coiling-type WO₃·0.33H₂O was successfully achieved when the volumes of PTA solution and deionized water were 10 ml and 5 ml,respectively. It exhibited high photodegradation performance in the treatment of methylene blue, and the photocatalytic conversion of methylene blue could reach above 96% after 180 min of irradiation.（4） Synthesis of ultrathin W₁₈O₄₉ nanowires clusters and its photocatalytic activityThe monoclinic W₁₈O₄₉ with highly aligned nanowires clusters shape was synthesized via an ethanol-assisted solvothermal method by using peroxopolytungstic acid（PTA） glass powder as precursor. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscope,X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectra and Brunauer-Emmet-Teller. The obtained sample exhibited a high yield, highly aligned and closely packed nanowires which grow aligned along the same direction. The oxidation states of W atom in W₁₈O₄₉ indicated the existence of many oxygen vacancies on the tungsten oxide surfaces, which led to a large absorption tail in the visible and near infrared（NIR） regions（500 800 nm）. The photocatalytic activity of W₁₈O₄₉ crystal was evaluated by degradation of methylene blue in aqueous solution under artificial solar-light. Results demonstrated that the sample exhibited higher activity than WO₃·0.33H₂O reported before, and the photocatalytic conversion of methylene blue could reach above 98% after 75 min of irradiation.