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

BiVO₄crystal as a new photocatalytic material has attracted researchers’attention because of it’s band gap of mono Clinic scheelite structure is 2.4 eV,whichcan respond to visible light.This thesis mainly explores the synthesis and morphology control of nano-scale BiVO₄crystals.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.The phase structure,morphology and photocatalytic properties of as-prepared BiVO₄parti Cles were characterized by X-ray diffraction（XRD）,field scanning electron microscope（FESEM）,selected area electron diffraction（SAED）,energy dispersive X-Ray spectroscopy（EDS）and the degradation of methylene blue,respectively.The relationships of synthesis technology and synthesis parameters with the phase structure,morphology and photocatalytic activity were dicussed.The experimental results are as follows:1.Flaky mono Clinic scheelite BiVO₄crystals can be synthesized by using a microwave hydrothermal method with Bi（NO₃）₃·5H₂O and NaVO₃as Bi and V sources,and chloride ion as a capping agent.BiClO impurities are generated when n（Cl^-）:n(Bi³⁺)>1:1 is added.The flake crystals with smooth surface and relatively uniform particles were obtained.When n（Cl^-）:n(Bi³⁺)=1:1,the reaction temperature was 130℃,and the photocatalytic activity was good.Crystals with irregular size and morphology gradually increased,and the degree of crystal agglomeration increased with the increase of reaction temperature.The crystal has a high degree of crystallinity and is regular flakes when pH=0.5.the crystals transform into rods With the increase of pH.When pH=9,the synthesized crystals become irregular blocks.2.Crystals with smaller particles and more regular morphology can be obtained by changing different synthesis processes.Change the morphology of bismuth vanadate by changing the mixing speed of precursor liquid,It has little effect on the crystal morphology When the mixing speed is less than 3 ml/min,when the mixing speed is greater than 3 ml/min,the synthesized crystals are seriously agglomerated and the size distribution is uneven with the increase of the mixing speed.Change the crystal morphology by changing the heating rate of the crystal,it can synthesize BiVO₄crystals with small agglomeration degree and crystal size（1-2μm）in a chloride system when the temperature is rapidly raised to 130℃,the photocatalytic degradation rate of methylene blue can reach80%.Large crystals（5-6μm）begin to appear and gradually increase as the temperature continues to increase.Crystals with smaller particles are obtained in the fluoride ion system,and the temperature rises rapidly,but the agglomeration phenomenon is more serious.Change the morphology of bismuth vanadate by adding complexing agents,It is possible to synthesize flaky bismuth vanadate crystals with a size of about 1μm when n(Bi³⁺):n（Cl^-）:citric acid=1:1:4,the reaction temperature is 130℃.When the reaction temperature continued to increase,the crystal began to transform into a square rod-like structure.The synthesized crystals became crystals with no fixed morphology and different sizes when the reaction temperature was 160°C.3.Synthesis of bismuth vanadate crystals in a simpler water bath with stirring.Mono Clinic BiVO₄crystals with truncated octahedral morphology can be synthesized in a system containing fluoride ions when n(Bi³⁺):n（F^-）=1:1,pH=0.5,and water bath heating.The mono Clinic phase of BiVO₄crystals can be obtained when the synthesis temperature is65℃and the holding time is 90min in the chloride ion-added system.BiVO₄crystals with flaky crystals and relatively uniform distribution can be obtained when the synthesis temperature is 75℃and the holding time is 2 h.Crystals with certain photocatalytic activity Synthesized by this method.It exhibits high photocatalytic activity when the synthesis temperature is 75℃in the chloride ion system.