The Study Of Synthesis And Photocatalytic Activities Of Bismuth Tungstate Nanoparticles

Semiconductor-based photocatalysis has attracted considerable attentionsowing to its potential applications in environmental cleaning and energyregeneration. Bismuth tungstate, a type of novel photocatalyst, is one of thesimplest Aurivillius oxides, possessing the layered structure with theperovskite-like slab of WO2+6and [Bi2O2]. Compared with TiO2photocatalyst,Bi₂WO₆owns the narrower band gap. Both of UV light and visible light can usesolar energy more efficiently and has attracted more and more interest.In this study, the pure orthorhombic Bi₂WO₆nanoparticles，and Cu-doped,Y-doped, N-doped Bi₂WO₆nanoparticles were synthesized by the microwavehydrothermal method, using Na₂WO₄·2H₂O and Bi（NO₃）₃·5H₂O as the rawmaterials. The effects of microwave-hydrothermal process conditions on phasestructure, morphologies and photocatalytic properties of the Bi₂WO₆nanoparticles were discussed and the microwave hydrothermal reactionmechanism of Bi₂WO₆particles was studied. The effects of ion doping on phasestructure, morphologies and photocatalytic properties of the Bi₂WO₆nanoparticles were investigated. The preparation and photocatalytic properties ofBi₂WO₆film photocatalyst were preliminarily studied to solve the problem ofrecycling of the photocatalyst.The pure orthorhombic Bi₂WO₆crystal phase powders were synthesized bythe microwave-hydrothermal method at140-200℃for60min, usingNa₂WO₄·2H₂O and Bi（NO₃）₃·5H₂O as the raw materials. The morphologies ofparticles synthesized at140℃,160℃,180℃and200℃were bulk-like reuniteddisorderly, sphere-like, porous loose woolen-clew-like and the opening clewflower-like respectively, and the BET surface areas of particles were21.45,38.26,25.63and21.45m2/g; The particles presented the photoabsorptionproperties from the UV light region to visible-light. The formation process ofBi₂WO₆was that Bi（NO₃）₃·5H₂O and Na₂WO₄·2H₂O were hydrolyzed into BiONO₃, and H2WO₄, and then dissolved in the hydrothermal media into BiO⁺and WO_-4², BiO⁺and WO_-4²to combine and form the crystal nucleus, and thecrystal nucleus grew into Bi₂WO₆nanoparticles. The optimal photocatalyticproperty was found in rose-like Bi₂WO₆particles obtained when hydrothermaltemperature was180℃, the filling fraction was60%, the hydrothermal time was60min, and the concentration was0.05mol·L-1, with which the degraded rate ofRhB solution reached to92%of RhB under the visible light for60min, while thedegraded rate of RhB solution got to97%under the ultraviolet light for40min.The Cu doped Bi₂WO₆nanoparticles were prepared by the microwavehydrothermal method using Cu（NO3）2·3H₂O as the Cu source. Cu was insubstitution for Bi into the Bi₂WO₆lattice, with the increasing of dopingconcentration, Bi₂WO₆monoclinic crystal phase appeared and disappearedsuccessively. Cu doping made morphologies change greatly. The morphologiesof woolen-clew wound round with nano line （RCu=2.5%）, the layered structure（RCu=5%） and flower-like （RCu=20%） were found. Photocatalytic reactionexperiments show that the visible photocatalytic activity of Bi₂WO₆nanoparticles can be improved by Cu doping when RCuis5%,7.5%and10%respectively. However, the UV photocatalytic activity of Cu doped Bi₂WO₆particles （RCu=2.5%²0%） is weaker than that of the undoped one.The Y doped Bi₂WO₆nanoparticles were prepared by the microwavehydrothermal method using Y（NO3）3·6H₂O as the Y source. Y was in substitutionfor Bi into the Bi₂WO₆lattice and doping with Y did not result in thedevelopment of new crystal orientations, and made morphologies change slightly.Y doped Bi₂WO₆particles and the undoped one were all of flower clustersassembled with nanoflakes. Photocatalytic reaction experiments show that thevisible photocatalytic activity of Bi₂WO₆nanoparticles can be improved bydoping with10%of Y. However, the UV photocatalytic activity of Y dopedBi₂WO₆particles （RY=2.5%²0%） is weaker than that of the undoped one.The N doped Bi₂WO₆nanoparticles were prepared by the microwavehydrothermal method using CO（NH2）2as the N source. N was in substitution forO into the Bi₂WO₆lattice. Doping with N did not result in the development ofnew crystal orientations and made morphologies become irregular. N dopedBi₂WO₆particles were reunited with nanoflakes. Photocatalytic reaction experiments show that both of the visible photocatalytic activities of Bi₂WO₆nanoparticles can be improved by N doping. The visible photocatalytic activitycan be increased by15%and the UV photocatalytic activity can be increased by9%.The homogeneous porous Bi₂WO₆films on FTO/glass had been prepared bysol-gel method, using Na₂WO₄·5H₂O and Bi（NO₃）₃·5H₂O as the raw materials,HOCH2CH₂OH as the solvent, and C6H8O7·H₂O and C5H8O2as the additionagents. With decreasing the annealing time at450℃, the growth of the filmorientation changed. The films had the photoabsorption properties from the UVlight region to visible light region, and could photocatalytically degrade RhB inaqueous solution under visible light irradiation slowly.