Design and Photo/Electro-catalytic Performance Study of Bismuth Vanadates Based Catalysts

Author:Lv Cha De

Supervisor:chen gang


Degree Year:2019





Employing solar energy-driven photocatalytic reactions to purify water and using solar/electric energy for nitrogen fixation have emerged as green and high-efficiency approach to tackle with the environmental issues.The fabrication of high-performance catalysts plays the key role in the photo/electrocatalytic reaction.As a mixed metal oxide,bismuth vanadates which possess unique crystalline/band structure and good chemical stability,has been investigated widely.This thesis mainly designs bismuth vanadates based materials with hollow structure,heterojunctions and homojunctions by a simple electrospinning method.The effects of its nanostructure,band alignment,and defects on the photocatalytic activity of as-fabricated bismuth are further investigated.Moreover,amorphous bismuth vanadates and polymeric carbon nitride are designed for the application in photocatalytic Cr(VI)reduction and electrocatalytic nitrogen fixationBiVO4 nanotubes with hollow interior and mesopores,which endow the nanotubes with enhanced surface areas,are fabricated through an organic salt induced electrospinning gradient method.In contrast with BiVO4 nanofibers which are fabricated by using inorganic salt,the formation mechanism of nanotubes is determined.When the ratio,composition and amount are changed,the nanotubes still could be obtained,manifesting the universality of as-proposed formation mechanism.The BiVO4 nanotubes are capable of havesting more light due to the multiple specular reflections of incident light in the interior of the nanotubes.When evaluated as photocatalysts for photocatalytic Cr(VI)reduction,BiVO4 nanotubes exhibit much better performance compared with BiVO4 nanofibers,which could be ascribed to the enhanced surface areas and strengthened light havesting ability.By tuning the ratios of solvents and salts during electrospinning process,Bi2O3quantum dots(QDs)decorated BiVO4 nanofibers and Bi4V2O11 nanocrystals embedded BiVO4 nanofibers are fabricated,respectively.The Bi2O3 QDs decoration and Bi4V2O11 nanocrystals embedding achieve the construction of nanosized heterojunctions.The formation mechanisms of Bi2O3 QDs decoration and Bi4V2O11nanocrystals embedding are further investigated.The Bi2O3 QDs decoration and Bi4V2O11 nanocrystals embedding could effectively boost the interfacial charge transfer in as-constructed nanosized heterojunctions,thus realizing rapid separation of photoinduced electrons and holes.Therefore,as-obtained Bi2O3 QDs decorated BiVO4 nanofibers and Bi4V2O11 nanocrystals embedded BiVO4 nanofibers display enhanced visible-light-driven photocatalytic activity for RhB degradation and Cr(VI)reduction than BiVO4 nanofibers.Through controlling the atmosphere and temperature during the calcination treatment of electrospinning fibers,Bi5+self-doped Bi4V2O11 p-n homojunction andα-βBi4V2O11 homojunction(phase junction)are fabricated,respectively.The Bi5+self-doped Bi4V2O11 p-n homojunction could be obtained when calcined with abundant oxygen.Moreover,abundant oxygen could induce salt migration towards the shell region,hence resulting in nanofibers,which is a new mechanism rather than salt gradient effect for the formation of hollow structure.Theα-βBi4V2O11homojunction could be obtained under a suitable calcination temperature.As-fabricated Bi4V2O11 based homojunction photocatalysts exhibit remarkably facilitated photogenerated charge carriers separation and transport in contrast with Bi4V2O11 without homojunction.Under visible light irradiation,Bi5+self-doped Bi4V2O11 p-n homojunction andα-βBi4V2O11 homojunction photocatalysts achieve superior photocatalytic performance for Cr(VI)reduction.Besides,α-βBi4V2O11phase junction photocatalyst also shows promoted photocatalytic nitrogen fixation performance.The salts with different solubility are employed during the electrospinning process to induce partial salt gradient effect,which is further utilized to fabricate Bi4V2O11/CeO2 heterojunction nanotube photocatalyst with amorphous structure.The amorphous Bi4V2O11 possesses multiple defects,leading to effective separation of photoinduced charge carriers in as-constructed heterojunction.In sharp contrast with the counterpart with crystalline structure,the heterojunction photocatalyst with amorphous structure displays much more outstanding photocatalytic properties for Cr(VI)reduction.Furthermore,the multiple defects endow Bi4V2O11/CeO2 nanotube catalyst with adsorption sites for N2.As a result,Bi4V2O11/CeO2 catalyst with amorphous structure also achieves enhanced electrocatalytic nitrogen fixation performance.When modified by amorphization method,polymeric carbon nitride can own nitrogen vacancies,which leads to the enhanced photocatalytic Cr(VI)reduction and electrocatalytic nitrogen reduction reaction activity.