Preparation And Photocatalytic Performance Of Bismuth Halide Oxide(BiOX) Composite Materials

In recent years,with the acceleration of industrialization and the rapid growth of population,it is inevitable to increase the emission of organic pollutants in the environment.The composition of organic pollutants is complex,mainly including dyes,antibiotics and other refractory substances,and most of them are toxic.If left unchecked,it will inevitably cause serious consequences such as indoor pollutants that cannot be purified in the short term and water pollution,seriously endangering human health and aggravating biodiversity.Photocatalytic technology has the advantages of simple preparation process,low cost,no pollution and complete and efficient mineralization,which makes it an effective way to control environmental pollution.This technology can completely catalyze the oxidation of pollutants into water and carbon dioxide.The key problems that limit the application of photocatalytic technology are:the visible light response of photocatalyst is weak,the separation rate of photogenerated carriers is low,and the redox of photogenerated carriers is not high.At present,many methods have been used to improve the activity and stability of photocatalysts,but these methods have some disadvantages.Therefore,the development of advanced photocatalysts is still a huge challenge.Bismuth oxyhalide（BiOX,X=Cl,Br,I）,as a new type of semiconductor photocatalyst,has a special layered structure and high chemical stability.There is a strong internal electric field between the bismuth oxide layer and the halogen atom layer,which can effectively promote the separation of photogenerated electron-hole pairs and show good photocatalytic performance.However,due to the wide band gap of BiOCl,it can only absorb ultraviolet light,but the ultraviolet light in sunlight only accounts for less than 4%of its energy,while 60%of visible light energy cannot be used,which greatly limits its effective application in solar energy.In addition,although BiOBr and BiOI can absorb visible light,their photocatalytic activity cannot meet the application of photocatalysis in actual environmental pollution control.Therefore,how to prepare low-cost,convenient,efficient and sustainable synthetic composites by modifying BiOX materials is a problem that needs to be solved.Based on the above problems,Ag/BiOBr,BiOCl_xI_1-xand BiOCl/K-C₃N₄semiconductor composite photocatalysts were prepared in this paper.The physical and chemical properties of the catalyst such as microscopic appearance,crystal structure and elemental composition were characterized by various technical means.The catalytic performance was systematically studied and the corresponding catalytic mechanism was proposed.BiOBr nanosheets were prepared by hydrothermal method.On this basis,noble metal Ag was loaded on the surface of BiOBr nanosheets by solvothermal method to prepare a series of Ag/BiOBr composite photocatalysts with different Ag contents.The samples were characterized by XRD,FT-IR,SEM and other test methods.The test results showed that the noble metal composite photocatalyst was successfully prepared.The experimental results show that Ag/BiOBr_（0.05）composite photocatalyst can degrade 96.6%of MO dye within 60 min and 68.5%of gaseous HCHO within 80min under visible light irradiation.After five photocatalytic cycle degradation experiments,the final Ag/BiOBr composite photocatalyst can degrade more than 90%of MO dye within 60 min,and can still degrade 60%of gaseous HCHO within 80 min.The results of free radical capture experiments showed that the main photocatalytic active species were superoxide radicals（·O₂^-）and the photocatalytic mechanism was proposed.A series of BiOI/BiOCl composite photocatalysts with different Cl:I molar ratios were prepared by solvothermal method.This binary photocatalyst is composed of two different’flower-like’structures.The composite photocatalytic materials were characterized by XRD,FT-IR,SEM and other test methods.The test results showed that the semiconductor composite photocatalyst was successfully prepared.The experimental results show that the degradation efficiency of the binary photocatalyst for methyl orange dye（MO）and gaseous formaldehyde（HCHO）is significantly improved compared with the single material.Among them,the BiOCl_0.4I_0.6composite photocatalyst has a degradation efficiency of 91.8%for MO dye within 60 min and a degradation efficiency of 64.3%for gaseous HCHO within 80 min under visible light irradiation.After five photocatalytic cycle degradation experiments,the final BiOCl_0.4I_0.6composite photocatalyst can still degrade 88.4%of MO dye within 60min,and can degrade 60.8%of gaseous HCHO within 80 min,indicating that the photocatalyst has good stability.The photocatalytic mechanism of the semiconductor composites was discussed by free radical trapping experiments.It was proved that the main photocatalytic active species were superoxide radicals（·O₂^-）and holes（h⁺）in the reaction process of degrading methyl orange solution and formaldehyde gas.Nanoflower-like p-n heterojunction photocatalyst BiOCl/K-C₃N₄（abbreviated as BK-x）was prepared by a simple solution coprecipitation method.The physical and chemical properties of the composite photocatalyst were analyzed by XRD,XPS,SEM and other modern techniques.The characterization results showed that the BiOCl/K-C₃N₄p-n heterojunction had enhanced photocatalytic performance in the degradation of methyl orange and gaseous formaldehyde（HCHO）under visible light irradiation.The results of photodegradation experiments showed that BiOCl/K-C₃N₄heterojunction photocatalyst could degrade 99%of MO dye within 30 min and 56%of gaseous HCHO(0.16 mg L^-1)within 35 min under visible light irradiation（λ>400nm）.After five photocatalytic cycle degradation experiments,the degradation efficiency of BiOCl/K-C₃N₄heterojunction photocatalyst to MO solution can still reach more than 90%,and the degradation of HCHO can still reach more than half.The photocatalytic mechanism of the semiconductor composites was discussed by free radical trapping experiments.It was proved that the main photocatalytic active species were superoxide radicals（·O₂^-）and holes（h⁺）in the reaction process of degrading methyl orange solution and formaldehyde gas.The photocatalytic mechanism of the p-n heterojunction was also proposed.