| Bismuth tungstate, the simplest member of the Aurivillius family of layered perovskites with general formula Bi2An-1BnO3n+3 (A=Ca, Sr, Ba, Pb, Bi, Na, K and B=Ti, Nb, Ta, Mo, W, Fe) and energy gap of 2.75eV, has a band structure composed of the valence band (VB, the hybrid orbitals of O2pand Bi6s) and the conduction band (CB, the W5d orbital). It has excellent intrinsic physical and chemical properties, such as ferroelectricity, piezoelectricity, pyroelectricity, catalytic behavior and nonlinear dielectric susceptibility and has been widely used in the relevant field. Bi2WO6 material is a promising vis-responsive photocatalyst because of its absorption in the visible light area.In this paper, we synthesized Bi2WO6 and Sr doped Bi2WO6 powders through hydrothermal and solvothermal processes and studied the influence of synthesis conditions and doping concentrations on the morphologies and properties. Various test methods and analytic tools were applied to characterize the microstructure and phase composition.Hydrothermal synthesis of Bi2WO6:The effects of process parameters such as pH values of the precursor, temperature and holding time on the morphologies and properties were investigated. Experimental results indicate that 180℃and 24h is the proper conditions for preparing Bi2WO6 powders. Influences of the pH values on the microstructures and light absorption properties were discussed through XRD,SEM and UV-vis techniques. Results show that all samples have absorption in the visible light area, the product obtained at the pH values of 2 has the best photocatalytic activities under UV-light irradiation and no obvious loss of photocatalytic property is observed after four cyclic tests. After 150min visible light irradiation, the degradation rate of dye reached 38%.Solvothermal synthesis of Bi2WO6:The effects of solvents on the microstructures and phase composition were studied. The products were almost crystal nucleus, crystal growth didn't happen at 180℃and 24h when taking alcohol as solvent. Pure orthorhombic Bi2WO6 were obtained in EG solvent, and the morphology was nanoplates composed of nanoparticles. The effects of temperature and holding time on the photocatalytic activities were also investigated when in EG solvent. After increasing the temperature to 240℃, the phase composition changed, while the time didn't affect the structure significantly. The sample obtained at 180℃for 36h show superior photocatalytic properties over that for 24h under visible light irradiation, both of which reveal higher properties than that prepared by hydrothermal process.Hydrothermal synthesis of Sr doped Bi2WO6:The influences of doping concentration on the morphology and properties were discussed. The result suggests that Sr modification does not change the phase structrue of the Bi2WO6 photocatalyst but restrains the growth of grains. Morphology observation revealed that as synthesized Bi2WO6 were self-assembled three-dimensional (3D) nest-like structures, which were constructed from nanoplates. When doping concentration reached 2.0mol%, the nest-like structures began to collapse. XPS analysis indicated that ion had entered the crystal lattice. All strontium modified Bi2WO6 show increased photocatalytic activity over that of pure Bi2WO6 in the photodegradation of RhB under UV light illumination, among which the highest is obtained when the concentration is 0.5mol%, with reaction rate constant k of 0.03975min-1. Photocatalytic activity under visible light illumination (λ>420nm) was also performed. about 57% of RhB can be photodegraded after 120min irradiation. Samples subsequently thermal treated at 500℃show higher photocatalytic activities. |
PDF Full Text Request