Synthesis Of Bi₂WO₆ Powders And Their Photocatalytic Properties Under Visible Light Irradation

Semiconductor photocatalyst has become an important research topic in the fields of materials,chemistry,environment and energy due to their excellent performances on degradation of organic dyes,hydrogen and oxygen production by photoelectrochemical water splitting and photocatalytic reduction of CO₂ into renewable fuels.TiO₂ is the most widely studied photocatalyst,but the wide band gap?3.0 3.2 eV?makes it only respond to ultraviolet light with wavelengthes less than 413 nm.As the ultraviolet light energy accounts for only 6.07% of the solar energy,the development of narrow band gap semiconductor materials responsed by visible or infrared light?wavelength greater than 420 nm?is the main research direction of photocatalyst in the future.As one of photocatalysts in respond to both visible and near-infrared light,bismuth tungstate?Bi₂WO₆?has great potential in application.Therefore,the development of synthesis methods with simple processes and high photocatalytic activities has an important significance for the application of Bi₂WO₆ photocatalyst.In this thesis,Bi₂WO₆ powders were prepared by different synthetic methods using Bi?NO₃?₃·5H₂O and Na₂WO₄·2H₂O as bismuth and tungsten sources,respectively.The related influencing factors in the synthesis process were studied,and the photocatalytic performance of prepared Bi₂WO₆ powders was evaluated.The main contents of the thesis are summarized as follows:Firstly,Bi₂WO₆ powders were synthesized by a low-temperature solid state method with a simple mixture of Bi?NO₃?₃·5H₂O and Na₂WO₄·2H₂O.Due to the slow diffusion rate of solid phase,Bi₂WO₆ was the main crystallized phase in the obtained powders.Impurities such as Bi2O₃,Na₂W4O13,Bi14W2O₂7 and unknown phases were found in the product powders at different temperatures.The powders synthesized at 350 ? for 2 h exhibited the highest photocatalytic activity under the irradiation of visible light.After 90 min of visible light irradiation,the photodegradation rate of RhB was 96.1%.The as-synthesized powders had good chemical stability and cycling stability.Secondly,Bi₂WO₆ powders were synthesized by direct molten salt method using a mixture of Bi?NO₃?₃·5H₂O and Na₂WO₄·2H₂O powders as precursors and LiNO₃/NaNO₃ as molten salts,respectively.The introduction of molten salts provided a liquid circustance for the reaction to accelerate the diffusion rate of the solid phases,which significantly reduced the impurity phases in the products.Pure Bi₂WO₆ powders were obtained at 350 ?.The amount of molten salt greatly influenced the phase composition of the products.The optimum quality of the molten salts was 2 or 5 times that of the reactants.The synthesized Bi₂WO₆ powders showed a nano-sheet morphology.With increasing reaction temperature,holding time and the amount of molten salts,the nanosheets gradually grew up.The photocatalytic activity of the synthesized powders decreased with increasing temperatures.When the quality of the molten salts was twice times that of the reactants,Bi₂WO₆ powders synthesized at 350 ? for 4 h had the highest visible photocatalytic activity.After 90 min of visible light irradiation,the photodegradation rate of RhB was 98.3%.The product powders also had good chemical stability and cycle stability.Lastly,Bi₂WO₆ powders were synthesized by a molten salt method with co-precipitates as precursors and LiNO₃/NaNO₃ as molten salts,respectively.The precipitates were obtained from a mixture of aqueous solution of Bi?NO₃?₃·5H₂O,Na₂WO₄·2H₂O and ammonia.Pure Bi₂WO₆ powders were synthesized at a low temperature of 250 ?.The introduction of molten salts significantly enhanced the specific surface area of the as-synthesized Bi₂WO₆ powders wit a maximum value of 21.86 m2 g-1.The band gaps of the as-synthesized Bi₂WO₆ powders were 2.64-2.68 eV.The amount of molten salt and the temperature greatly influenced the morphology of the products.When there were no molten salts,the morphology of the Bi₂WO₆ powders was nearly spherical.With the presence of molten salts and a temperature of 250-300 ?,the obtained Bi₂WO₆ particles aggregated toghether with a namometer flake-like morphology.When the temperature reached 350 ?,Bi₂WO₆ particles showed a spherical morphology again.When the quality of molten salts was 10 times that of the reactants,Bi₂WO₆ powders synthesized at 300 ? for 8 h had the best visible photocatalytic activity with a photocatalytic RhB degradation rate of 97.0 % in 40 min.The powders also had good chemical stability and cycle stability.