Shape-Controlled Synthesis And Photocatalytic Properties Of M-BiVO₄ Crystals

The monoclinic phase BiVO4 crystal can respond to visible light due to the forbidden band width of 2.4 eV,which has attracted wide attention from researchers.In this paper,bismuth vanadate(BiVO4)crystals were prepared by microwave hydrothermal method using bismuth nitrate(Bi(NO3)3·5H2O)and ammonium metavanadate(NH4VO3)as the main raw materials.Using XRD,SEM,spectrophotometer and other test methods,the influence of different experimental parameters on the phase composition,microstructure and photocatalytic performance of the samples was discussed.The experimental results are as follows:(1)Monoclinic scheelite-structured BiVO4 crystals can be synthesized by the microwave hydrothermal method by adding ethanolamine solution to the reaction system.,The morphology of crystal gradually changes from an irregular shape to a rectangular parallelepiped structure with the increase of the amount of ethanolamine.At the same time,the photocatalytic efficiency also increases with the amount of ethanolamine.When 2 mL of lmol/L ethanolamine solution is added in reaction system,the photocatalytic efficiency of the sample is 48%.With the increase of reaction time,the crystallinity of the crystals increased continuously.When the reaction time was 3 h,the degradation efficiency of the sample was about 50.7%.(2)Monoclinic scheelite BiVO4 crystals were synthesized by adding fluorine ions to the reaction system.When the fluoride ion concentration is increased or decreased,the crystal structure changes in two completely different directions.When the fluoride ion concentration is increased,the crystal morphology changes gradually from bulk to sheet as the fluoride ion concentration increases.When the fluoride ion concentration is decreased,the crystal morphology gradually develops into a cuboid rod-like structure as the fluoride ion concentration decreases.When n(Bi3+):n(F-)=1:3 in the reaction system,the photocatalytic degradation rate of the methylene blue solution can reach 80%,and the sample has good photocatalytic recyclability.In this system,when the reaction temperature is increased,the crystallinity of the crystal is improved.When the reaction temperature is 160℃,many smooth crystals appear and the crystals exhibit a uniform polyhedral morphology.At this time,the photocatalytic degradation rate of the solution is 80%t.When the reaction solution pH is 7,the synthesized sample has highert degree of crystallinity.And the sample has good photocatalytic recyclability.(3)Based on the experimental results of the first two experiments,the microwave hydrothermal method was used to synthesize BiVO4 crystals through the combined action of ethanolamine and fluoride ions.Finally,a pure monoclinic BiVO4 powder was successfully synthesized.When the volume of ethanolamine(1M)solution is 2 mL and the BiVO4 sample prepared under the condition of n[Bi3+]:n[F-]=5:1 has a rectangular parallelepiped structure,and the degradation rate of the methylene blue solution under visible light can reach 81.4%(after 200 min light).