Microstructure Adjustment,formation Mechanism Of Bismuth Vanadate Materials And Their Color Property And Photocatalytic Activity

Semiconductor materials present optical,electric,magnetic,or catalytic properties which have been extensively applied in many fields such as energy,environment,medicine and functional materials.The aforementioned macroscopical properties of materials are closely associated with their microstructures,therefore,getting deep understanding of the structure-activity relationship between microstructure and macroscopical property of materials provides theoretical basis for improving the performance of semiconductor materials.Moreover,the microstructure of materials often depends on the synthetic methods as well as the technological parameters.Hence,clarifying the microstructure adjusting mechanism of semiconductor materials will provide theoretical basis for developing novel technology for preparing semiconductor materials with excellent performance.Bismuth vanadate（BiVO₄）is a kind of semiconductor materials with great application potential,which not only possesses properties such as conductivity,ferroelasticity,color and visible-light photocatalytic activity,but also owns some outstanding advantages,such as environment-friendly property,low-cost,high stability and excellent performance.At present,the application researches of BiVO₄ as pigment and photocatalytic materials have attracted great attention.For example,BiVO₄ has been successfully developed as environmental-friendly yellow pigment（C.I.Pigment Yellow 184）,which substitutes for the lead chromate yellow pigment.The BiVO₄ pigment can be used in the fields which require high performance,such as food,toy,printing ink,plastic,rubber,finishing coat,etc.In addition,monoclinic BiVO₄（mBiVO₄）has become one of the widely used materials in the field of semiconductor photocatalysis due to its narrow band gap,visible-light driven photocatalytic activity and high stability.Nevertheless,both as pigment and photocatalytic materials,improving the nature of BiVO₄ materials certainly will promote its further application.This thesis is aimed to improve the color property and photocatalytic activity of BiVO₄materials.In the thesis,a solvothermal route in glycerol/water mixed solvent was employed to optimize the microstructure of BiVO₄ materials.The structural properties of BiVO₄ materials such as facets,morphology and nanostructure could be adjusted by controlling the experiment parameters such as solvent composition ratio,pH value of precursor solution and structure-directing agent.On the one hand,the influence rule of microstructure of the materials on the color property has been investigated,while the structure-activity relationship between the structural properties and the color property has also been revealed.On the other hand,this thesis emphatically studies the effects of materials on the photocatalytic activity for the degradation of typical simulating organic pollutant so as to select photocatalytic materials with excellent performance,essentially studies the formation mechanism of typical materials,and reveals the microstructure adjusting mechanism of the materials.The thesis also focuses on the influecne rules of microstructure of the materials on the photocatalytic activity,and uncovers the structure-activity resationship between the structural property and photocatalytic activity.This study provides the scientific foundation for the design and development of corresponding materials and their applications in the fields of pigment and photocatalytic materials.The details are summarized briefly as follows:The color and structural properties of the environmental-friendly mBiVO₄ materials have been adjusted by a solvothermal route in glycerol/water mixed solvent.The structural properties such as color property,optical property,morphology,chemical composition,particle size,phase and exposed facets of present BiVO₄ samples have been systematically investigated by means of digital photograph,CIELab color space,UV-Vis diffuse reflectance spectra（UV-Vis DRS）,infrared spectroscopy（FTIR）,scanning electron microscope（SEM）,energy spectrum（EDS）,nitrogen adsorption,X-ray powder diffraction（XRD）,transmission electron microscope（TEM）and high-resolution TEM（HRTEM）.Experimental results indicate that,compared with conventional mBiVO₄materials,implementing the pH value control of precursor solution in the present method can not only adjust the color change of mBiVO₄ materials from yellow to yellow-green,but also control the structural properties,such as exposed facets and proportions,morphology and nanostructure.The mBiVO₄ yellow-green pigments can be obtained in the pH range of 3-5.The corresponding relationship among pH values of the precursor solution,color property and structural properties of the materials has also been discussed.It can be found that the color property of present mBiVO₄ yellow-green pigment mainly depends on the preferentially exposed{010}facets and the regular sheet-like morphology simultaneously.Hierarchically structured BiO₄ materials oriented along{010}facets with different morphologies have been synthesized by a solvothermal process in glycerol/water mixed solvent.The effects of the volume fraction of glycerol aqueous solution on the phase structure,growth degree of{010}facets,morphology and photocatalytic activity have been investigated.It is found that the volume fraction of glycerol aqueous solution has strong influence on the kind and concentration of the reaction species,which not only influences the crystal growth of BiVO₄ oriented along{010}facets,but also induces different self-assembly modes of BiVO₄ nanocrystals serving as building blocks,and further leads to diverse morphologies,such as sphere,ellipsoid,olive-like,pinnately compound leaf-shape,leaf-shape,sheet-like,strip-like and four-leaved primrose-like.A possible formation mechanism is proposed on the basis of the characterization results of XRD and SEM,that is,the forming process of the products includes precipitation reaction,dissolution-recrystallization,Ostwald ripening and self-assembly process.In this chapter,we focus on the photocatalytic activity of the materials for the photodegradation of methylene blue solution under simulated sunlight irradiation.Among the series of BiVO₄ materials with different microstructures,the four-leaved primrose-like mBiVO₄ material exhibits the highest first-order kinetic reaction rate constant.The enhanced photocatalytic activity can be attributed to the improved separation of photogenerated charge caused by the synergistic effect between the special four-leaved primrose-like morphology and preferential growth of{010}facets.Nanostructured shuriken-like m BiVO₄ materials with preferentially exposed{010}facets have been synthesized by an ethylenediamine tetraacetic acid disodium（EDTA）-assisted solvothermal process in glycerol/water mixed solvent.The results from XRD,SEM and TEM characterization indicate that controlling the addition amount of EDTA can adjust the structural properties such as exposed degree of{010}facets,morphology and nanostructure.When the addition amount of EDTA is 0.4 mmol and 0.5 mmol respectively,the corresponding product is nanostructured shuriken-like m BiVO₄ crystals with preferentially exposed{010}facets and two dimension sheet-like mBiVO₄ single crystals with preferentially exposed{010}facets,respectively.The coordination reactions between the certain forms of Bi and V source ions and glycerol molecules,as well as EDTA have been confirmed through a series of verified experiments.Based on the above experimental results and previous researches,the formation mechanisms of the products are proposed,that is,mBiVO₄ single crystals conform to the traditional growth mechanism of crystals,while nanostructured mBiVO₄ crystals follow non-classical growth mechanism of crystals.The latter growth mechanism mainly includes the following procedures:（1）The formation of building blocks:amorphous BiVO₄nanoparticles produced from the precipitation reaction between the reactive ions crystallize to form mBiVO₄ nanocrystals.This process includes not only the phase transition（kinetically controlled tetragonal zirconium silicate BiVO₄ nanocrystals gradually transform into mBiVO₄ nanocrystals）,but also anisotropic growth of crystals caused by the absorption of glycerol molecule and EDTA on certain facets of BiVO₄nanocrystals,whereafter the building blocks result from the growth and aging of the mBiVO₄ nanocrystals.（2）Nanostructured m BiVO₄ crystals can be formed by directional lapping of building blocks.The BiVO₄ nanocrystals serving as building blocks aggregate with each other so as to decrease the surface energy of BiVO₄ nanocrystal itself.In the process of aggregation,the interaction forces between the glycerol molecules and EDTA adsorbed on the surface of building blocks respectively（such as hydrogen-bonding as well as the coulombic force）dominate the lapping way of building blocks,and finally,determine the morphology,exposed facets and proportions of the products.The photocatalytic activity tests indicate that,among the series of mBiVO₄ materials with different microstructures,the nanostructured shuriken-like mBiVO₄ materials with preferentially exposed{010}facets exhibite the highest first-order kinetic reaction rate constant,which was 1.13 times than the four-leaved primrose-like mBiVO₄ materials.The results of molecular fluorescence spectra indicate that the enhanced photocatalytic activity can be attributed to the synergistic effect of the highest exposure degree of{010}facets and the sepcial shuriken-like morphology,which promote the separation of photogenerated charge.