Preparation Of MOFs Bismuth-rich Bismuth Oxyhalides Heterojunctions And Their Photocatalytic Performances

With the acceleration of industrialization and urbanization,water pollution caused by heavy metals and organic pollutants has become a serious social problem.Photocatalysis technology has attracted extensive attention due to its various merits,such as mild reaction condition,utilizing sunlight as excitation energy,easy to realize continuous treatment and strong mineralization for organic pollutants.The fabrication of efficient and stable photocatalysts is the core issue of the photocatalytic technology development.As photocatalysts,metal-organic frameworks(MOFs)have many advantages,such as(1)excellent adsorption performance;(2)the structure and function can be modified;(3)the large specific surface area and ordered pore structure are conducive to transfer the target pollutant to the active sites,thus enhancing the utilization efficiency of the photo-generated charge carriers;(4)with clear structural characteristics,it is an ideal model for studying the structure-activity relationship of photocatalytic reactions.However,most of MOFs possess wide band gaps,meaning they can only utilize UV light as energy.Moreover,the poor conductivity of MOFs seriously limits its further applications.Bismuth-rich bismuth oxyhalides(BixOyXz,X=Cl,Br,I)are visible-light-responsive inorganic semiconductors with narrow band gaps,which also have the advantages of non-toxicity and pro-environment.However,BixOyXz also possess some disadvantages,such as small specific surface area and rapid photo-generated hole-electron pairs recombination rate.At the same time,because most BixOyXz semiconductors are layered structures,agglomeration is easy to occur during the photocatalytic process,thereby leading to their inactivation.In order to make full use of the advantages of MOFs and BixOyXz semiconductors,it is an effective and feasible strategy to combine them to construct binary heterojunctions.Therefore,this paper takes MOFs and BixOyXz as the research subjects,a series of MOFs/BixOyXz heterojunction photocatalysts were prepared by ball-milling method.Their phase structures,morphologies and photoelectric properties were systematically studied by various characterization techniques.Selecting Cr(VI),bisphenol A(BPA)and ciprofloxacin(CIP)as modeling pollutants,the corresponding photocatalytic activities were investigated under white light irradiation.The enhancement mechanisms of photocatalysis were revealed by exploring the transfer processes of the photo-generated charge carriers and the produced active radicals,which will provide experimental and theoretical basis for the applications of photocatalytic technology in the field of wastewater treatment.The main contents of this paper are as follows:1.In this study,the Bi24O31Br10 nanosheets were successfully synthesized using hydrothermal method.Subsequently,series binary BUC-21/Bi24O31Br10 heterojunctions were fabricated by facile ball-milling method.The results suggested that the introduction of a specific quality of Bi24O31 Br10 on the surface of BUC-21 could enhance the utilization efficiency of white light and suppress the photo-induced carriers recombination to promote the photocatalytic efficiency.The BB-100(the mass ratio of BUC-21/Bi24O31Br10 being 1)exhibited the best performance,in which 99.9%Cr(?)ions were photoreduced into Cr(?)upon the white light irradiation within 120 min.As well,the influences of small organic acids(SOAs),initial pH,light intensities along with the co-existing matters on the Cr(?)removal were investigated.Furthermore,the Box-Behnken experimental design methodology proved that the photoreduction process was significantly influenced by the co-existing inorganic anions and negatively charged dissolved organic matters(DOM).After 4-run recycling for the adsorption-photoreduction of Cr(?)experiments,the BB-100 still exhibited satisfied reduction activity with good stability and reusability.A possible reaction mechanism was proposed and clarified through the electrochemical determination,time-resolved photoluminescence decay spectra,active species trapping experiments,electron spin resonance analyses(ESR)and density functional theory(DFT)calculation.2.Bifunctional Bi12O17Cl2/MIL-100(Fe)heterojunction(BMx)was firstly constructed via facile ball-milling method.The optimal BM200 was highly efficient for Cr(?)sequestration and activation of persulfate(PS)for BPA decomposition under white light illumination,which was much more remarkable than the pristine MIL-100(Fe)and Bi12O17Cl2,respectively.Furthermore,the photocatalytic reduction efficiency can be significantly improved via the addition of some green SOAs.As well,the BPA degradation can be achieved over an extensive initial pH range of 3.0-11.0.When the PS concentration increased to more than 2.0 mmol/L,the BPA degradation efficiency decreased due to the SO4-·self-scavenging effect.It was also found that the co-existence of inorganic anions like H2PO4-,HCO3-,SO42-,Cl-and NO3-could decelerate the BPA degradation.The excellent photocatalytic Cr(?)reduction and persulfate activation performances originated from both MIL-100(Fe)with excellent PS activation ability and Bi12O17Cl2 with a favorable band position,which not only enabled the efficient separation of charges but also accelerated the formation of SO4-·radicals.The BM200 displayed prominent stability and recyclability.More importantly,the credible degradation pathway was proposed based on UPLC-MS analysis and DFT calculation.This research revealed that the Fe-based MOFs/BiXOyXz heterojunctions possessed great potential in wastewater remediation.3.Direct Z-scheme UiO-66-NH2/Bi5O7I heterojunctions were fabricated via ball-milling method.Their activities were investigated by the CIP degradation under white light irradiation.Being compared with pristine UiO-66-NH2 and Bi5O7I,all UiO-66-NH2/Bi5O7I heterojunctions exhibited enhanced photocatalytic performances.The phase structures,morphologies,photoelectric properties and the composition of chemical elements were comprehensively characterized and analyzed by various techniques.Meanwhile,the effects of photocatalyst dosage,initial pH and inorganic anions on CIP adsorption-photocatalysis degradation process over UiO-66-NH2/Bi5O7I heterojunctions were investigated.Active-species-capture experiments and ESR analysis demonstrated that·OH and·O2-worked together for the CIP degradation.The optimal photocatalyst(BU-5)exhibited stable CIP degradation activity in four cycles repeating experiments.Finally,a possible Z-scheme charge migration mechanism was proposed for the UiO-66-NH2/Bi5O7I heterojunctions.Three possible pathways were proposed for the CIP degradation based on the detected intermediates by UPLC-Q-TOF-MS.This work will provide a theoretical basis for the design and preparation of Z-scheme heterojunctions for the removal of the emerging pollutants in the future.