Preparation and Photocatalytic Property of BiOX(X=Cl,Br) Based Photocatalytalysts

Author:Li Zuo

Supervisor:liang yu jun


Degree Year:2019





Semiconductor photocatalytic technology has been widely applied in the fields of solar energy conversion,catalytic synthesis and environmental purification in recent years.As a kind of photocatalyst with high efficiency,study on bismuth oxyhalide(BiOX((X=Cl,Br,I)))has caught great attention of researchers.Benefitting from the strong internal electrostatic field(IEF)between the layer structures interleaved with[Bi2O2]slabs and double halogen atoms slabs(perpendicular to the slab layers),separation efficiency of the photogenerated electron and hole can be largely improved.As a consequence,BiOX exhibits high photocatalytic activity,which is of great potential in many photochemistry related fields.Nevertheless,the relatively positive CB bands of BiOX lead to the low reductive ability of the photogenerated electrons,which can not fulfill the demand of solar energy conversion.Besides,the BiOX photocatalysts in nanoscale tend to aggregation in aqueous solution while the less exposed active sites results in the decreased photocatalytic activity.Moreover,the visible-light-driven(VLD)photocatalytic performance of BiOX is relatively weak,which hinders the practical application of BiOX.Therefore,in this study,considerable efforts,such as facet control,heterojunctionconstruction,hierarchicalstructuresconstructionand doping/heterojunction synergism,have been paid on tuning band structure,improving visible light responsibility and separation efficiency of electron-hole pairs,in order to enhance the VLD photocatalytic performance of BiOX based photocatalysts.The main contents of this paper are as follows:(1)Uniform foursquare BiOCl nanosheets with highly exposed{001}facets were synthesized via hydrothermal route.The experimental parameters such as reaction temperature,time and the amount of additives(e.g.,sodium hydroxide,CPAM and sodium citrate)played vital roles in the formation of BiOCl nanosheets.The band gap of the typical-synthesized BiOCl nanosheets was estimated to be 3.33 eV from the UV-Vis diffuse reflectance spectrum(DRS),which could response to UV light.The highly exposed{001}facets led to the increased oxygen vacancy concentration within the BiOCl nanosheets,which could trap electrons and restrict the recombination of photogenerated electron-hole pairs,thus enhancing the photocatalytic activity of as-prepared BiOCl sample,which was 34.5 times higher than commercial P25 in degrading Methyl Orange(MO).Furthermore,a series of BiOCl/Fe2O3(FB)heterojunctions were synthesized through an in-situ oxidation method by using elemental Bi nanospheres as Bi source while FeCl3 as Fe source and oxidizing agent.The optical response range of the composite was extended to the visible region due to the existence of Fe2O3(Eg=2.22 eV).Besides,the formation of heterojunction between BiOCl and Fe2O3 improved the separation efficiency of photo-generated charge carriers,contributing to the enhancement of photocatalytic activity of the product.The degradation efficiency of Rodanmine B(RhB)and MO as well as the reduction efficiency of Cr(VI)by the FB sample were 34.37,2.22 and 40.71 times higher than pure BiOCl,while 50.97,17.92 and 5.48 times higher than pure Fe2O3,respectively.This work demonstrated a new approach for the fabrication of Bi-based heterostructured photocatalytic systems from in-situ oxidation of elemental Bi by metal cations.(2)BiOX(Cl,Br)/g-C3N4(BC-X)heterojunctions were constructed through an in-situ hydrolysis method by taking metallic Bi nanospheres as Bi source.The TEM and HRTEM images showed the heterogeneous nanostructures at the interface between BiOX and g-C3N4.DRS results revealed the visible light responsibility of both BiOCl/g-C3N4(BC-Cl)and BiOCl/g-C3N4(BC-Br),in which the adsorption edges were located at 440 nm and 500 nm,respectively.As to the BiOBr/g-C3N4 composite,the Z-scheme mechanism proved by active species trapping experiments led to the persistence of photoinduced electrons in the CB of g-C3N4 while holes in the VB of BiOBr,thus ensuring the high redox ability of the charge carriers.Benefiting from the Z-scheme mechanism,the BC-Br sample exhibited superior photocatalytic activity,which was 2.6 times higher than BC-Cl(traditional type II)in degrading RhB under visible light irradiation.As an effective strategy to obtain samples with highly dispersed heterointerface,this work could provide a facile route for the surface modification of g-C3N4.(3)A series of BiOBr/Bi2S3 heterojunctions(BS)were synthesized through acid vapor ion exchange method.Basing on the ion exchange between S2-in thioglycolic acid as well as O2-and Br-in BiOBr,BiOBr were partially transformed into Bi2S3,leading to the formation of heterojunction.Benefiting form the generation of Bi2S3,the composites exhibited enhanced optical adsorption capability in the visible region,thus endowing the higher visible light responsibility of the BS samples.Besides,the coexistence of two phases(BiOBr and Bi2S3)within a single nanosheet were in contact with each other closer,while the highly dispersed heterointerface contributed to the separation and transfer of photoinduced charge carriers and resulted in the enhanced photocatalytic activity of the products.Moreover,this method did little damage to the precursor(BiOBr)while the flower-like hierarchical structure is remained.Profited from the reflection of light between the nanosheets,the light usage of the heterojunctions were improved significantly.The degradation efficiency of RhB and tetracycline hydrochloride(TC)as well as the reduction efficiency of Cr(VI)by the BS sample were 4.29,9.26 and 3.69 times higher than pure BiOBr,while 6.08,10.42 and5.70 times higher than pure Bi2S3,respectively.(4)Bi2-xMnxO3/Bi1-y-yMny OBrheterojunctionwassynthesizedthroughthe calcination of Bi/Mn oxalate precursor followed by the in-situ hydrolysis process for the first time.The decomposition of Bi/Mn oxalate under 400 oC resulted in the porous Bi2-xMnxO3.The in situ growth of Bi1-y-y MnyOBr nanosheets on the surface of porous Bi2-xMnxO3 resulted in the hierarchical nanostructure of the final product.The generation of doping level(Mn2+/Mn3+)narrowed the band gaps of the components(Bi2O3 and BiOBr)and increased the concentration of oxygen vacancies(Vo),which could serve as electron trapping agent.Therefore,both visible light responsibility and separation efficiency of charge carriers were enhanced significantly.Moreover,the construction of heterojunction between Bi2-xMnxO3 and Bi1-yMnyOBr prolongs the lifetime of charge carriers,which is attributed to the promoted charge transfer efficiency.The synergistic effect of the above factors made the BM samples exhibit excellent visible-light-driven photocatalytic activity.Notably,the photosensitization of rhodamine B(RhB)improved the photocatalytic activity of h-BM in reducing Cr(VI),which was1.90 times higher than the catalytic system without RhB.Remarkably,the RhB molecules were also decomposed at the same time,which provided a feasible way for the simultaneous treatment of wastewater containing Cr(VI)and multiple organic pollutants.This research proposed a new pathway for the facile construction of novel bidirectional ion-doped heterojunction from metal oxalates.