Controllable Synthesis And Photocatalytic Activities Research Of Bismuth Vanadate

Recently, organic contaminants degradation using TiO₂as a catalyst is oneof the focuses in the field of photocatalysis. However, it is only active under UVlight because of its large band gap energy （3.2eV）, which limited its furtherapplication to a great extent in the visible light region. Bismuth vanadate（BiVO₄）, as one of the non-titania based visible-light-driven semiconductorphotocatalysts, has attracted much attention due to its narrow band gap,2.4eV（monoclinic phase） and responding to visible light. Therefore, the research anddevelopment of BiVO₄photocatalytic material will improve the utilization ofsolar energy. And it also has potential practical value in environmentalpurification and new energy development. The photocatalytic properties ofBiVO₄are strongly related to its crystal phase, morphology, size and crystallinity,therefore, the key to improve the photocatalytic activity is to seek an appropriatepreparation method achieving the controllable synthesis of nanocryatals. BiVO₄exists in three phases, monoclinic sheelite, tetragonal zircon and tetragonalsheelite. Among of them, the photocatalytic activity of monoclinic phase is muchhigher than that of the other two.In this paper, BiVO₄photocatalyst with different morphologies wereprepared via a microwave hydrothermal method, and the effect of themorphology on the photocatalytic activity was investigated. The experimentalresults are as follows:（1） BiVO₄microspheres were prepared with Bi（NO₃）₃and NH₄VO₃as startingmaterials at pH of5through the microwave hydrothermal method. The effect ofdifferent calcination temperatures on the crystalline phase and morphology wasinvestigated, and the relationship between crystalline phase of BiVO₄and itsphotocatalytic performance was further studied. The photocatalytic activity ofthe BiVO₄microspheres was determined by degradation of methylene blue （MB）molecules under visible light irradiation. The results indicate that the crystalline phase of as-prepared BiVO₄microspheres is tetragonal phase, and the diameterof microspheres is1-4μm. It is found that the phase transition has undergoneafter being calcined at500C, and monoclinic scheelite BiVO₄microspheresobtained by being calcined after600C has the excellent visible-lightphotocatalysis. Besides, BiVO₄with different crystalline forms can affect thedegradation of methylene blue molecules.（2） Tetragonal BiVO₄microspheres with different surface structures weresynthesized by the microwave hydrothermal method using Bi（NO₃） and NH4VO3as starting materials and with NaOH to adjust pH. The photocatalyticperformance for methylene blue degradation was also investigated for BiVO₄samples calcined at600C under the visible light irradiation. The results indicatethat microspheric BiVO₄with particle sizes in the range of1-4μm can beobtained. The pH value had no effect on the crystal form, however, the effect ofpH value on the surface structure of the microsphere was obvious. The highestdegradation rate was achieved at pH11, reaching to94.7%in210min undervisible light irradiation. In addition, the formation mechanism of themicrospheres was investigated in brief.（3） BiVO₄photocatalyst with controllable morphology was prepared through amicrowave hydrothermal method by adding different additives. BiVO₄hollowmicrospheres have been synthesized in the presence ofethylenediaminetetraacetic acid （EDTA） by a microwave hydrothermal method.The as-prepared hollow microspheres were composed of hundreds of nanorods.Increasing the amount of EDTA produced a new nanocage structure. Theevolution process of BiVO₄with different microstructures indicated that theamount of EDTA added played a crucial role in determining the shape of thesamples. Additionally, a possible formation mechanism of the hollowmicrospheres and nanocages is proposed briefly by a series of experiments of thehydrothermal time and temperature.（4） Monoclinic BiVO₄decahedrons were successfully synthesized withTween-80as the template via a facile microwave hydrothermal method. Theeffect of different hydrothermal temperature on the crystalline phase andmorphology was studied. The photocatalytic performance of the BiVO₄samplesprepared at different hydrothermal temperature was determined by degradation of MB with the assistance of H₂O₂. Additionally, much enhanced photocatalyticperformance for methylene blue degradation was also realized with theassistance of appropriate amount of hydrogen peroxide under the visible lightirradiation. The hydrothermal temperature has an effect on the photocatalyticproperty of the BiVO₄samples.