The Synthesis And Photocatalytic Evaluation Of CQDs/g-C₃N₄/BiOX Composite Photocatalyst

With the development of social economy and people’s living standards,environmental pollution and energy crisis have received much attention.Photocatalysis utilizes solar energy as a driving force,which has the advantages of high efficiency,greenness,and simple operation.It is extremely promising in terms of environment and energy.Traditional photocatalysts such as TiO₂ and ZnO have the disadvantages of wide bandgap,narrow absorption range and high recombination of photo-generated carriers,which limits their practical application.Therefore,developing a new type of catalyst that has visible light response,low recombination of photo-generated electron-hole and high stability is imminent.In this paper,the visible light-responsive g-C₃N₄,BiOX with strong oxidation ability of photo-generated hole,CQDs has up-conversion characteristics,and the three materials were used to synthesize a series of CQDs/g-C₃N₄/BiOX composites.These composites could broaden the absorption range of visible light,reduce the recombination rate of carriers,and enhance the photocatalytic activity.The structure,composition,morphology and photocatalytic performance of the composite materials are discussed through a series of characterization methods.Besides,the mechanism of photodegradation of pollutants was proposed as well.The main research contents are as follows:（1）g-C₃N₄ was prepared by calcination,CQDs was prepared by hydrothermal method,CQDs/g-C₃N₄/BiOBr composites were prepared at room temperatureby stirring.The composition,structure and morphology of the composites were analyzed by XRD,FT-IR,DRS,XPS,TEM and SEM,respectively.It was found that the CQDs,g-C₃N₄ and BiOBr contacted closely and formed a heterojunction structure.In the experiment of degrading rhodamine B under LED light,it was found the 3%CQDs/g-C₃N₄/BiOBr composite had the best photocatalytic degradation performance,the degradation rate of RhB was 44 times of g-C₃N₄ and 6.5 times of BiOBr.At the same time,the 3%CQDs/g-C₃N₄/BiOBr composite also exhibited superior degradation performance on bisphenol A and tetracycline.By using PL,PT and EIS analysis,electrons and holes of CQDs/g-C₃N₄/BiOBr coud be separated effectively.Finally,the photocatalytic degradation mechanism of CQDs/g-C₃N₄/BiOBr composites was proposed based on the capture experiment and the band positions.（2）Porous g-C₃N₄（pg-C₃N₄）was prepared by the template method and CQDs/pg-C₃N₄/BiOI composites were prepared at room temperature by stirring.The composition,structure and morphology of the composites were analyzed by XRD,FT-IR,DRS,XPS,TEM and SEM,respectively.In the degradation experiment,using MO as target pollutant and LED lamp as light source to explore the material degradation performance,it was found that the 5%CQDs/pg-C₃N₄/BiOI composite had the best photocatalytic performance and the degradation rate of MO was 8 and 3.3times that of pg-C₃N₄ and BiOI.Moreover,the 5%CQDs/pg-C₃N₄/BiOI composite also showed good degradation effects on antibiotic substances ciprofloxacin and tetracycline.In the analysis of PT and EIS,it was found that electrons and holes of CQDs/pg-C₃N₄/BiOI composites had the highest separation efficiency.Finally,the photocatalytic degradation mechanism of CQDs/pg-C₃N₄/BiOI composites was described based on the capture experiment and the band positions.（3）The CQDs/pg-C₃N₄/BiOCl composites were synthesized at room temperature by stirring.Through XRD,FT-IR,XPS,TEM,SEM and other characterization methods,it was demonstrated that the CQDs/pg-C₃N₄/BiOCl composites were prepared successfully,and the heterojunction structure was formed between CQDs,pg-C₃N₄and BiOCl.In order to simulate the photocatalytic performance in the natural environment,this experiment used sunlight as light source.The 1%CQDs/pg-C₃N₄/BiOCl composite had the best degradation performance,and the rhodamine B was degraded completely at 20 min.At the same time,the 1%CQDs/pg-C₃N₄/BiOCl composite also showed good degradation of bisphenol A and tetracycline.The PT and EIS results showed that electrons and holes of CQDs/pg-C₃N₄/BiOCl composites had the highest separation efficiency.Based on the capture experiment and the band positions,the photocatalytic degradation mechanism of CQDs/pg-C₃N₄/BiOCl composites was described.