Morphology-and Crystal Facet-controlled Synthesis And Photocatalytic Properties Of Monoclinic Bismuth Vanadate

Semiconductor photocatalysis is an ideal strategy to solve energy and environmental problems.Monoclinic scheelite bismuth vanadate?m-BiVO₄?,as one of the representative photocatalysts with visible-light response,has attracted extensive interests because of its sufficient solar energy utilization,high photocatalytic activity and stability,nontoxic,and low cost.However,some limitations,such as poor adsorptive capacity,easy recombination of photogenerated electron-hole pairs,still need to be solved to further improve the photocatalytic activity of m-BiVO₄.In this work,we have improved the photocatalytic performance of m-BiVO₄ by controlling morphologies,tuning exposed facets and nonmetal doping.We have exploited X-ray diffractometer,scanning electron microscope,energy dispersive X-ray spectrometry,transmission electron microscope,high-resolution TEM,selected area electron diffraction,X-ray fluorescence spectroscopy,ultraciolet-visible diffuse reflectance,infrared sepectroscopy and nitrogen adsorption-desorption to characterize and analyze the products.Formation mechanism of the typical products have also been discussed.The relationship between the microstructure and photocatalytic activity of the samples are also investigated in this paper.The m-BiVO₄ with even size and regular shape are synthesized by the glycerol/water mixed solvothermal method without using any surfactants and templates.The influence of higher concentrated glycerol aqueous solution and stronger acid condition on structure and properties of the obtained products have also been investigated.The results show that,when the p H value of the precursor is 2,the rodlike and dumbbell-shaped m-BiVO₄ can be synthesized at higher concentration of glycerol aqueous solution?? 25%?.When the concentration of glycerol aqueous solution is fixed at 50%,m-BiVO₄ can be prepared at p H 2⁴ with the morphology turning from rodlike to olive-like and then to sphere.The existence of glycerol in the bismuth nitrate solution can provide different Bi species,including [Bi3+?Gly]?[Bi O+?Gly] and Bi O+.The Bi species and their concentrations can be modified with different concentrations of glycerol aqueous solution and p H values,which induces the variations of crystal phase and morphology of products.The shape transitions of the particles in the glycerol/water system can be assigned to the “rod-dumbbell-sphere” morphogenesis mechanism.The photocatalytic activity of the rodlike m-BiVO₄ sample obtained with 50% glycerol aqueous solution at p H 2 is higher than the dummbell-like,sphere or other BiVO₄ samples.This enhanced photocatalytic performance can be attributed to the large surface area,strong light absorption,and rodlike morphology.The sheet-like m-BiVO₄ with {010} preferred orientation and highly exposed {010} facets are prepared at a lower concentration of glycerol aqueous solution and weaker acid conditions.The influences of solvent,p H value and reaction time on the microstructure and the photocatalytic performance have been systematically studied.And the growth mechanism of sheet-like m-BiVO₄ with highly exposed {010} facets has also been discussed.The results show that under the weak acid condition?p H 4⁶?glycerol has combined functions of solvent and directing-agent.It directs the preferred orientation of the m-BiVO₄ crystals growth along its {010} facets with highly exposed {010} facets,and possesses the sheet-like morphology.The sheet-like m-BiVO₄ with highly exposed {010} facets,which was prepared by using 15% glycerol aqueous solution at p H 6 and heated for 8 h,has the highest extent of preferred orientation along {010} facets.That is,the?040?/)211(intensity ratio is 2.11,which is about 8.44 times than that of the JCPDS card.Moreover,the highest exposure of {010} facet is verified by HRTEM,SAED and XRD analyses.In addition,with prolonging the reaction time,the phases of the products transform from mixed phase to pure monoclinic phase.In the meantime,it is found that the extent of preferred orientation along {010} facets increases firstly and then reach the maximum at 8 h.However,an excess reaction time would cause dissolution of the {010} facets,and thus reduce the exposure degree of the {010} facets.The formation mechanism of the sheet-like m-BiVO₄ with highly exposed {010} facets can be illuminated by “dissolution-recrystallization” process,Ostwald ripening and oriented lapping mechanism.The sheet-like m-BiVO₄ with the highest exposure of {010} facets exhibits the best sunlight-driven photocatalytic performance for MB degradation.The obtained first-order kinetic reaction rate constant?k?is about 2.2 times than that of the above rodlike m-BiVO₄.The enhanced photocatalytic performance of the sheet-like m-BiVO₄ with highly exposed {010} facets can be attributed to the highly exposed {010} facets which increases the separation efficiency of photogenerated electron-hole pairs and improves the surface active sites.P-doped m-BiVO₄ with exposed {010} facets are synthesized by a one-step hydrothermal route.The P doping did not change the crystal phase of BiVO₄ products,but the P5+ is inserted into V5+ sites of the host BiVO₄ lattice which causes lattice distortion.Furthermore,the P doping influences the growth of m-BiVO₄ crystals along its {010} facets,as well as the morphology.The sheet-like m-BiVO₄ doped with 5% P oxoanion exhibites the best photocatalytic activity,which k is 1.2 times than that of the rodlike m-BiVO₄ samples.Such an enhanced photocatalytic activity can be attributed to the P doping and effective exposure of {010} facets,which is infavor of increasing the separation of photogenerated electron-hole pairs.In addition,high crystallinity,sheet-like morphology,and stronger light absorption are also responsible for improving the photocatalytic activity.