Composite photocatalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of water treatment catalytic materials, and particularly relates to a composite photocatalyst as well as a preparation method and application thereof. The specific preparation method comprises the following steps: by taking g-C3N4 as a substrate and nitrogen-doped carbon quantum dots as electron transport bridges, sequentially dispersing g-C3N4 and a nitrogen-doped carbon quantum dot solution into a Bi-MOF solution, reacting at 120-200 DEG C, inducing in-situ growth of Bi-MOF on the surface of g-C3N4, and embedding the nitrogen-doped carbon quantum dots into an interface of Bi-MOF and g-C3N4 to obtain the composite photocatalyst. The composite photocatalyst prepared by the invention has high-activity sites, and can efficiently degrade norfloxacin under the conditions of normal temperature, normal pressure and wide pH. The catalyst is stable and not easy to decompose, avoids secondary pollution of a water body, and can be recycled.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment catalytic materials, and particularly relates to a composite photocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] As a typical fluoroquinolone antibiotic, norfloxacin is widely used in the treatment of bacterial infections in humans and animals. However, its low metabolic rate in organisms leads to a large amount of the drug entering the water body through excretion, causing environmental pollution and accelerating the evolution of drug-resistant bacteria, seriously threatening ecological and health safety.

[0003] The existing norfloxacin degradation technologies mainly include ozonation, advanced degradation technology, membrane filtration, biodegradation, and adsorption treatment. However, due to the molecular structure stability and antibacterial properties of antibiotics, the existing degradation technologies generally have problems such as low removal efficiency, high toxicity of by-products, high energy consumption, and high cost.

[0004] Photocatalytic oxidation technology has become a research hotspot due to its high efficiency and green characteristics. Among them, MOF has a three-dimensional polyhedron crystal structure composed of metal centers and organic ligands, which is conducive to the adsorption and diffusion of pollutants to active sites. The full Chinese name of Bi-MOF is bismuth-based metal-organic framework material, which is a composite material composed of bismuth-based metal-organic framework materials. Among various MOF-based photocatalysts, Bi-MOF is similar to MOF materials and exhibits excellent potential for pollutant adsorption and light response due to its high specific surface area, adjustable pore structure, and chemical stability. However, the broadband gap characteristic of Bi-MOF results in its response only to ultraviolet light, which greatly limits its practical application.

[0005] The full English name of graphitic carbon nitride is Graphitic carbon nitride, and its chemical formula is g-C3N4. As a visible light-responsive substrate material, combining Bi-MOF with g-C3N4 can effectively adjust the energy band structure and enhance the light response ability. However, the interfacial charge transfer efficiency between the two is low, and the composite band gap is still large, resulting in insufficient separation efficiency of photo-generated carriers and difficulty in achieving efficient degradation of norfloxacin. Summary of the Invention

[0006] In order to solve the technical problems in the above-mentioned existing technologies that when Bi-MOF and g-C3N4 are combined, the interfacial charge transfer efficiency is low, the composite band gap is still large, resulting in insufficient separation efficiency of photo-generated carriers and difficulty in achieving efficient degradation of norfloxacin, the present invention provides a composite photocatalyst, a preparation method thereof, and an application thereof.

[0007] The present invention uses g-C3N4 as a substrate, and Bi-MOF is in-situ grown on g-C3N4; meanwhile, nitrogen-doped carbon quantum dots are embedded at the interface between the two to obtain a composite photocatalyst. By introducing nitrogen-doped carbon quantum dots as an electron bridge to connect g-C3N4 and Bi-MOF, the present invention accelerates charge transfer, effectively increasing the density of charge carriers; at the same time, the wide-bandgap Bi-MOF and the narrow-bandgap g-C3N4 achieve full-spectrum absorption of visible light through the mediation of nitrogen-doped carbon quantum dots, improving the utilization rate of visible light; it solves the technical problems of low interfacial charge transfer efficiency and still relatively large composite bandgap when Bi-MOF and g-C3N4 are combined, resulting in insufficient separation efficiency of photo-generated carriers.

[0008] The first object of the present invention is to provide a preparation method of a composite photocatalyst, comprising the following steps:

[0009] Using g-C3N4 as a substrate and nitrogen-doped carbon quantum dots as an electron transport bridge, g-C3N4 and the nitrogen-doped carbon quantum dot solution are successively dispersed in the Bi-MOF solution, and reacted at 120°C to 200°C to induce the in-situ growth of Bi-MOF on the surface of g-C3N4. Meanwhile, the nitrogen-doped carbon quantum dots are embedded at the interface between Bi-MOF and g-C3N4 to obtain a composite photocatalyst.

[0010] It should be noted that the nitrogen-doped carbon quantum dots, as nano-carbon materials with a size less than 10 nm, are composed of sp 2 / sp 3 hybridized carbon, possessing excellent optoelectronic conversion and high conductivity characteristics. The nitrogen-doped carbon quantum dot-based photocatalyst provides an alternative carrier migration path, increasing the number of charge carriers. Its abundant adjustable functional groups contribute to the formation of chemical bonds at the interface of the composite catalyst. By introducing nitrogen-doped carbon quantum dots as an electron bridge between g-C3N4 and Bi-MOF, the present invention accelerates charge transfer and effectively increases the density of charge carriers.

[0011] Preferably, for every 0.001 mol of g-C3N4, 1 mL to 5 mL of the nitrogen-doped carbon quantum dot solution is added; for every 0.001 mol of g-C3N4, 1 mL to 20 mL of the Bi-MOF solution is added; the concentration of the nitrogen-doped carbon quantum dot solution is 10 -5 mol / L; the concentration of the Bi-MOF solution is 20 mmol / L. This is because excessive nitrogen-doped carbon quantum dots are prone to covering the active sites, reducing the number of active sites; while insufficient nitrogen-doped carbon quantum dots result in low charge transfer efficiency.

[0012] In the process of synthesizing the composite photocatalyst of the present invention, when the reaction temperature is low, it is likely to result in poor crystallinity of the MOF structure; when the reaction temperature is high, the g-C3N4 substrate structure is prone to damage. Preferably, the reaction temperature for preparing the composite photocatalyst is 120°C to 200°C. When the reaction time is short, the growth of MOF is incomplete, and when the time is too long, the particles agglomerate. Preferably, the reaction time for preparing the composite photocatalyst is 10 h to 20 h.

[0013] In the preparation process of the present invention, g-C3N4 is first dispersed in the Bi-MOF solution, and then the nitrogen-doped carbon quantum dot solution is added. This is because adding g-C3N4 first will provide a pre-existing support structure for the nitrogen-doped carbon quantum dots; while dispersing the nitrogen-doped carbon quantum dots in the Bi-MOF solution first may form a specific coating or structure on the surface of the catalyst, thereby affecting the subsequent loading and distribution of g-C3N4.

[0014] Preferably, the specific method for preparing the nitrogen-doped carbon quantum dot solution is as follows:

[0015] Using citric acid as the carbon source and urea as the nitrogen source, dissolve citric acid and urea in water, and react at 150°C to 200°C for 3 h to 6 h to obtain the nitrogen-doped carbon quantum dot solution.

[0016] The reaction temperature and time are important factors affecting the nitrogen-doped carbon quantum dots; when the reaction temperature is too low, carbonization is incomplete and the size of the nitrogen-doped carbon quantum dots is uneven; when the reaction temperature is too high, over-carbonization occurs; preferably, the reaction temperature for preparing the nitrogen-doped carbon quantum dots is 150°C to 200°C. When the reaction time is short, the particle size of the nitrogen-doped carbon quantum dots is small and the yield is low; while when the reaction time is long, the particle size increases, but its dispersibility is affected; preferably, the reaction time for preparing the nitrogen-doped carbon quantum dots is 3 h to 6 h. In addition, after the reaction, filtration is carried out to remove unreacted carbon particles and large particle impurities to ensure the monodispersity of the nitrogen-doped carbon quantum dot solution, thereby obtaining a pure nitrogen-doped carbon quantum dot solution.

[0017] Preferably, the dosage ratio of citric acid, urea and water is 2 g to 5 g: 1 g to 5 g: 30 mL to 50 mL. Preferably, the specific method for preparing the Bi-MOF solution is as follows:

[0018] Using isophthalic acid as the organic ligand, dissolve the bismuth source and isophthalic acid in the solvent, and under the action of stirring, the organic ligand and the bismuth ions in the bismuth source are self-assembled through coordination bonds to form a Bi-MOF precursor network to obtain the Bi-MOF solution.

[0019] Preferably, the molar ratio of the bismuth source to isophthalic acid is 1:1.

[0020] Preferably, the solvent is a mixed solvent of ethylene glycol and N,N-dimethylformamide; the volume ratio of ethylene glycol to N,N-dimethylformamide is 0.5-1:1. Ethylene glycol can inhibit the hydrolysis of bismuth ions and regulate the growth rate of MOF; N,N-dimethylformamide can promote ligand dissolution and coordination reaction.

[0021] Preferably, the conditions for stirring are: the stirring time is 0.5 h-3 h, and the stirring speed is 200 r / min-500 r / min.

[0022] Preferably, the specific method for preparing g-C3N4 is as follows:

[0023] Melamine is calcined at 500 °C-600 °C for 3 h-8 h at a heating rate of 5 °C / min-10 °C / min to obtain g-C3N4.

[0024] The second object of the present invention is to provide a composite photocatalyst prepared by the above preparation method.

[0025] The third object of the present invention is to provide the application of the above composite photocatalyst in the degradation of norfloxacin pollutants.

[0026] Preferably, the specific application method is as follows:

[0027] The composite photocatalyst is mixed with a norfloxacin pollutant solution and subjected to catalytic degradation under light; wherein, 0.1 mg-0.2 mg of the composite photocatalyst is added to each 1 mL of the norfloxacin pollutant solution; the concentration of the norfloxacin pollutant solution is 10 mg / L-30 mg / L.

[0028] Preferably, stirring is carried out during the catalytic degradation process, the stirring time is 600 s-3600 s, and the stirring speed is 500 r / min.

[0029] Preferably, the pH value of the norfloxacin pollutant solution is 2-7.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention utilizes nitrogen-doped carbon quantum dots embedded at the interface of g-C3N4 and Bi-MOF as an electron bridge to connect g-C3N4 and Bi-MOF, constructing a stable composite photocatalyst with significantly improved interfacial electron transport and transfer efficiency. The charge transfer of the composite photocatalyst is accelerated by the nitrogen-doped carbon quantum dots, effectively enhancing the charge carrier density of the composite photocatalyst. Meanwhile, the wide-bandgap Bi-MOF and the narrow-bandgap g-C3N4 achieve full-spectrum absorption of visible light through the mediation of nitrogen-doped carbon quantum dots, improving the utilization rate of visible light; it solves the technical problems of low interfacial charge transport efficiency and still relatively large composite bandgap when Bi-MOF and g-C3N4 are combined, resulting in insufficient separation efficiency of photo-generated carriers.

[0032] 2. The composite photocatalyst prepared by the present invention maintains the framework structure advantages of Bi-MOF. With its porosity and high specific surface area, it provides sufficient adsorption and active sites, while enhancing the redox performance. It realizes efficient catalytic degradation through both radical and non-radical pathways and is applicable to the treatment of various solution pollutants.

[0033] 3. The composite photocatalyst prepared by the present invention can efficiently degrade norfloxacin under normal temperature, normal pressure and a wide range of pH conditions. This catalyst is stable and not easily decomposed, avoiding secondary pollution of water bodies, and can be recycled. The preparation method of the present invention is simple, the raw materials are easily available, and the cost is cost-effective. Description of the Drawings

[0034] Figure 1 Scanning electron microscope images of the composite photocatalyst prepared in Example 1 at different magnifications; among them, (a) is the scanning electron microscope image at a magnification of 10 μm, and (b) is the scanning electron microscope image at a magnification of 5 μm.

[0035] Figure 2 Degradation test diagrams of norfloxacin by different photocatalysts.

[0036] Figure 3 Pseudo-first-order kinetics of the degradation of norfloxacin by different photocatalysts.

[0037] Figure 4 Photocatalytic degradation test diagrams of norfloxacin by the composite photocatalyst prepared in Example 1 at different dosages.

[0038] Figure 5 Pseudo-first-order kinetics of the photocatalytic degradation of norfloxacin by the composite photocatalyst prepared in Example 1 at different dosages.

[0039] Figure 6 Photocatalytic degradation test diagrams of norfloxacin by the composite photocatalyst prepared in Example 1 at different pH values.

[0040] Figure 7Pseudo-first-order kinetics of photocatalytic degradation of norfloxacin by the composite photocatalyst prepared in Example 1 at different pH values. Detailed implementation manners

[0041] In order to enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0042] In the description of the present invention, unless otherwise specified, the reagents used are commercially available, and the methods used are conventional techniques in the art.

[0043] Example 1

[0044] This example provides a preparation method of a composite photocatalyst, including the following steps:

[0045] Step 1, prepare a nitrogen-doped carbon quantum dot solution:

[0046] Dissolve 3 g of anhydrous citric acid and 1 g of urea in 30 mL of deionized water to obtain a mixture.

[0047] Transfer the mixture to a polytetrafluoroethylene-lined autoclave and react at 180 °C for 6 h. After cooling, filter the reacted solution through a 0.22 μM water filter membrane to obtain a pure nitrogen-doped carbon quantum dot solution, denoted as N-CQDs.

[0048] Step 2, prepare Bi-MOF:

[0049] Dissolve 3 mmol of Bi(NO3)3·5H2O in 30 mL of ethylene glycol to obtain a Bi(NO3)3·5H2O solution.

[0050] Dissolve 3 mmol of isophthalic acid in 40 mL of N,N-dimethylformamide to obtain an isophthalic acid solution.

[0051] Under magnetic stirring, slowly drop the isophthalic acid solution into the Bi(NO3)3·5H2O solution and stir at a rotation speed of 400 r / min for 2 h to obtain a Bi-MOF solution.

[0052] Step 3, prepare the composite photocatalyst:

[0053] Load 12 g of melamine into a crucible with a lid, place it in a muffle furnace, heat it to 550 °C at a heating rate of 5 °C / min, and calcine for 7 h to obtain g-C3N4.

[0054] Disperse g-C3N4 and N-CQDs in 10 mL of the Bi-MOF solution in sequence, and stir until completely mixed under magnetic stirring to obtain a precursor solution.

[0055] The precursor solution was transferred to a polytetrafluoroethylene-lined autoclave and reacted at 180 °C for 12 h. It was naturally cooled to room temperature, washed several times with distilled water and ethanol, and dried in an oven at 60 °C to obtain a composite photocatalyst denoted as Bi-MOF / N-CQDs / g-C3N4.

[0056] Example 2

[0057] This example provides a method for preparing a composite photocatalyst, which includes the following steps:

[0058] Step 1: Prepare a nitrogen-doped carbon quantum dot solution:

[0059] Dissolve 5 g of anhydrous citric acid and 5 g of urea in 50 mL of deionized water to obtain a mixture.

[0060] Transfer the mixture to a polytetrafluoroethylene-lined autoclave and react at 200 °C for 3 h. After cooling, filter the reacted solution through a 0.22 μM water filter membrane to obtain a pure nitrogen-doped carbon quantum dot solution denoted as N-CQDs.

[0061] Step 2: Prepare Bi-MOF:

[0062] Dissolve 3 mmol of Bi(NO3)3·5H2O in 30 mL of ethylene glycol to obtain a Bi(NO3)3·5H2O solution.

[0063] Dissolve 3 mmol of isophthalic acid in 40 mL of N,N-dimethylformamide to obtain an isophthalic acid solution.

[0064] Under magnetic stirring, slowly add the isophthalic acid solution to the Bi(NO3)3·5H2O solution and stir at a speed of 400 r / min for 2 h to obtain a Bi-MOF solution.

[0065] Step 3: Prepare the composite photocatalyst:

[0066] Load 12 g of melamine into a covered crucible, place it in a muffle furnace, heat it at a heating rate of 10 °C / min to 500 °C, and calcine for 8 h to obtain g-C3N4.

[0067] Disperse g-C3N4 and N-CQDs in 1 mL of the Bi-MOF solution in sequence and stir magnetically until completely mixed to obtain a precursor solution.

[0068] Transfer the precursor solution to a polytetrafluoroethylene-lined autoclave and react at 120 °C for 20 h. It was naturally cooled to room temperature, washed several times with distilled water and ethanol, and dried in an oven at 60 °C to obtain a composite photocatalyst denoted as Bi-MOF / N-CQDs / g-C3N4.

[0069] Example 3

[0070] This example provides a preparation method of a composite photocatalyst, including the following steps:

[0071] Step 1, prepare a nitrogen-doped carbon quantum dot solution:

[0072] Dissolve 2 g of anhydrous citric acid and 1 g of urea in 30 mL of deionized water to obtain a mixture.

[0073] Transfer the mixture to a high-pressure reaction kettle lined with polytetrafluoroethylene and react at 150 °C for 6 h. After cooling, filter the reacted solution through a 0.22 μM water filter membrane to obtain a pure nitrogen-doped carbon quantum dot solution, denoted as N-CQDs.

[0074] Step 2, prepare Bi-MOF:

[0075] Dissolve 3 mmol of Bi(NO3)3·5H2O in 30 mL of ethylene glycol to obtain a Bi(NO3)3·5H2O solution.

[0076] Dissolve 3 mmol of isophthalic acid in 40 mL of N,N-dimethylformamide to obtain an isophthalic acid solution.

[0077] Under magnetic stirring, slowly drop the isophthalic acid solution into the Bi(NO3)3·5H2O solution and stir at a rotation speed of 400 r / min for 2 h to obtain a Bi-MOF solution.

[0078] Step 3, prepare the composite photocatalyst:

[0079] Load 12 g of melamine into a crucible with a lid, place it in a muffle furnace, heat it to 600 °C at a heating rate of 7 °C / min, and calcine for 3 h to obtain g-C3N4.

[0080] Disperse g-C3N4 and N-CQDs in 20 mL of the Bi-MOF solution in sequence and stir until completely mixed under magnetic stirring to obtain a precursor solution.

[0081] Transfer the precursor solution to a high-pressure reaction kettle lined with polytetrafluoroethylene, react at 200 °C for 10 h, naturally cool to room temperature, wash it several times with distilled water and ethanol, and dry it in an oven at 60 °C to obtain a composite photocatalyst, denoted as Bi-MOF / N-CQDs / g-C3N4.

[0082] Comparative Example 1

[0083] This comparative example provides a preparation method of a Bi-MOF photocatalyst.

[0084] Dissolve 3 mmol of Bi(NO3)3·5H2O in 30 mL of ethylene glycol to obtain a Bi(NO3)3·5H2O solution.

[0085] Dissolve 3 mmol of isophthalic acid in 40 mL of N,N-dimethylformamide to obtain an isophthalic acid solution.

[0086] Under magnetic stirring, slowly add the isophthalic acid solution dropwise to the Bi(NO3)3·5H2O solution and stir for 2 h to obtain a Bi-MOF solution.

[0087] Transfer the Bi-MOF solution to a polytetrafluoroethylene-lined autoclave, heat it at 180 °C for 12 h, naturally cool it to room temperature, wash it 3 times with distilled water and ethanol, and then dry it in an oven at 60 °C to obtain a Bi-MOF photocatalyst.

[0088] Comparative Example 2

[0089] This comparative example provides a method for preparing g-C3N4.

[0090] Load 12 g of melamine into a crucible with a lid, place it in a muffle furnace, calcine it at a heating rate of 5 °C / min for 7 h to 550 °C, cool it and grind it into powder to obtain g-C3N4.

[0091] Comparative Example 3

[0092] This comparative example provides a method for preparing nitrogen-doped carbon quantum dots.

[0093] Dissolve 3.0 g of anhydrous citric acid and 1.0 g of urea in 30 mL of deionized water to obtain a mixture.

[0094] Transfer the mixture to a polytetrafluoroethylene-lined autoclave, heat it at 180 °C for 5 h; after cooling, filter the reacted black-green solution through a 0.22 μM water filter membrane to obtain a pure nitrogen-doped carbon quantum dot solution, denoted as N-CQDs.

[0095] Comparative Example 4

[0096] This comparative example provides a method for preparing Bi-MOF / g-C3N4.

[0097] Dissolve 3 mmol of Bi(NO3)3·5H2O in 30 mL of ethylene glycol to obtain a Bi(NO3)3·5H2O solution.

[0098] Dissolve 3 mmol of isophthalic acid in 40 mL of N,N-dimethylformamide to obtain an isophthalic acid solution.

[0099] Under magnetic stirring, the isophthalic acid solution was slowly added dropwise to the Bi(NO3)3·5H2O solution, and stirred for 2 h to obtain the Bi-MOF solution.

[0100] 0.184 g of g-C3N4 was added to the Bi-MOF solution, and then transferred to a high-pressure reaction kettle with a polytetrafluoroethylene lining, and heated at 180 °C for 12 h, naturally cooled to room temperature, and washed 3 times with distilled water and ethanol; then dried in an oven at 60 °C to obtain Bi-MOF / g-C3N4.

[0101] Comparative Example 5

[0102] This comparative example provides a preparation method of Bi-MOF / nitrogen-doped carbon quantum dots.

[0103] 3 g of anhydrous citric acid and 1 g of urea were dissolved in 30 mL of deionized water to obtain a mixture.

[0104] The mixture was transferred to a high-pressure reaction kettle with a polytetrafluoroethylene lining and reacted at 180 °C for 6 h. After cooling, the reacted solution was filtered through a 0.22 μM water filter membrane to obtain a pure nitrogen-doped carbon quantum dot solution, denoted as N-CQDs.

[0105] 3 mmol of Bi(NO3)3·5H2O was dissolved in 30 mL of ethylene glycol to obtain a Bi(NO3)3·5H2O solution.

[0106] 3 mmol of isophthalic acid was dissolved in 40 mL of N,N-dimethylformamide to obtain an isophthalic acid solution.

[0107] Under magnetic stirring, the isophthalic acid solution was slowly added dropwise to the Bi(NO3)3·5H2O solution, and stirred for 2 h to obtain the Bi-MOF solution.

[0108] 1 mL of N-CQDs was added to the Bi-MOF solution, and then transferred to a high-pressure reaction kettle with a polytetrafluoroethylene lining, and heated at 180 °C for 12 h, naturally cooled to room temperature, and washed 3 times with distilled water and ethanol; then dried in an oven at 60 °C to obtain Bi-MOF / nitrogen-doped carbon quantum dots, denoted as Bi-MOF / N-CQDs.

[0109] Comparative Example 6

[0110] This comparative example provides a preparation method of g-C3N4 / nitrogen-doped carbon quantum dots.

[0111] 12 g of melamine was loaded into a covered crucible and placed in a muffle furnace, calcined at a heating rate of 5 °C / min for 7 h to 550 °C, cooled and ground into powder to obtain g-C3N4.

[0112] Dissolve 3.0 g of anhydrous citric acid and 1.0 g of urea in 30 mL of deionized water to obtain a mixed solution.

[0113] After adding 0.184 g of g-C3N4 powder to the mixed solution, transfer it to a high-pressure reaction kettle lined with polytetrafluoroethylene, heat it at 180 °C for 5 h, naturally cool it to room temperature, and wash it three times with distilled water and ethanol; then dry it in an oven at 60 °C to obtain g-C3N4 / nitrogen-doped carbon quantum dots, denoted as g-C3N4 / N-CQDs.

[0114] In Examples 1 to 3 of the present invention, composite photocatalysts were prepared to solve the problems of low interfacial charge transfer efficiency and still relatively large composite bandgap when Bi-MOF and g-C3N4 were combined, resulting in insufficient separation efficiency of photo-generated carriers. Moreover, the surface morphologies and photocatalytic performance data of Examples 2 to 3 were similar to those of Example 1. Taking the catalysts prepared in Example 1 and Comparative Examples 1 to 6 as examples for research, the specific research methods and results are as follows:

[0115] Experimental tests:

[0116] 1. SEM characterization.

[0117] As Figure 1 shown, a large number of Bi-MOF nanospheres are loaded on the g-C3N4 nanosheets, and the overall structure is in the shape of a nanoflower, with the petal thickness being about 20 nm.

[0118] 2. Photocatalytic test.

[0119] In the present invention, a solution containing norfloxacin is prepared with ultrapure water, and the initial concentration is 10 mg / L without changing the initial pH. Measure seven portions of 50 mL of norfloxacin solution, and add the photocatalysts prepared in Example 1 and Comparative Examples 1 to 6 respectively, with the addition amount being 10 mg for each. At the same time, irradiate the norfloxacin solution with a xenon lamp light source, control the xenon lamp irradiation current to be 18 A, stir and react for 600 s to 3600 s, and the stirring speed is 500 r / min. The degradation effect is as Figure 2 .

[0120] As Figure 2As shown, in the photocatalytic system with a single catalyst containing only Bi-MOF, g-C3N4, or N-CQDs, the removal rate of norfloxacin is low and fails to achieve the expected effect of complete removal, indicating that the contribution of catalyst adsorption is far from sufficient. When in the photocatalytic system with two catalysts containing Bi-MOF / g-C3N4, g-C3N4 / N-CQDs, or Bi-MOF / N-CQDs, the degradation efficiency of norfloxacin is significantly enhanced; in the catalytic system of the Bi-MOF / N-CQDs / g-C3N4 composite photocatalyst, its degradation efficiency is nearly 100%. By comparing different catalysts, it can be concluded that under light irradiation, Bi-MOF / N-CQDs / g-C3N4 can better degrade norfloxacin, generating a variety of highly reactive species, increasing the concentration of free radicals and non-free radicals in the solution, acting together on norfloxacin, promoting the removal of norfloxacin, and accelerating the reaction effect.

[0121] As Figure 3 shown, compared with the single Bi-MOF, g-C3N4, and N-CQDs, or compared with the photocatalytic degradation systems containing two catalysts of Bi-MOF / g-C3N4, g-C3N4 / N-CQDs, or Bi-MOF / N-CQDs, the rate constant k value is the largest in the photocatalytic degradation system of Bi-MOF / N-CQDs / g-C3N4, indicating the fastest degradation rate of norfloxacin.

[0122] In this invention, a solution containing norfloxacin was prepared with ultrapure water, with an initial concentration of 10 mg / L. Five portions of 50 mL norfloxacin solution were taken without changing the initial pH. 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg of Bi-MOF / N-CQDs / g-C3N4 prepared in Example 1 were added respectively; at the same time, the norfloxacin solution was irradiated with a xenon lamp source, and the xenon lamp irradiation current was controlled at 18 A, stirred and reacted for 3600 s, and the stirring speed was 500 r / min. The degradation effect is shown in Figure 4 .

[0123] As Figure 4As shown, adding different amounts of Bi-MOF / N-CQDs / g-C3N4 has different effects on the degradation of norfloxacin. When the mass of Bi-MOF / N-CQDs / g-C3N4 is 5 mg, the degradation efficiency of norfloxacin is 82%. The reason may be that the low addition amount of Bi-MOF / N-CQDs / g-C3N4 leads to a slow reaction rate. Moreover, when the catalyst addition amount is insufficient, the number of active sites is limited, and the number of reactant molecules that can be catalyzed also decreases accordingly, resulting in a reduced reaction rate and an unsatisfactory catalytic effect. With the increase in the dosage of the composite photocatalyst, when the dosage of the composite photocatalyst is 10 mg, its degradation efficiency is the best, and the degradation rate is 98%. Continuing to increase the composite photocatalyst, its degradation efficiency decreases instead. This may be because too much catalyst may hinder the effective contact and mixing of reactants, thereby reducing the reaction efficiency. Preferably, 0.1 mg to 0.2 mg of Bi-MOF / N-CQDs / g-C3N4 is added to each 1 mL of norfloxacin solution; more preferably, 0.2 mg of Bi-MOF / N-CQDs / g-C3N4 is added to each 1 mL of norfloxacin solution.

[0124] As Figure 5 shown, when the dosage of Bi-MOF / N-CQDs / g-C3N4 is 10 mg, the value of its rate constant k is the largest, indicating that the concentration of norfloxacin pollutants decreases faster per unit time, the catalyst activity is higher, and the degradation efficiency of norfloxacin is better.

[0125] In the present invention, a solution containing norfloxacin pollutants is prepared with ultrapure water, and the initial concentration is 10 mg / L. Six portions of 50 mL of the pollutant solution are taken, and 10 mg of Bi-MOF / N-CQDs / g-C3N4 is added to each portion, and the pH is adjusted to 2, 3, 5, 7, 9, and 10. At the same time, the norfloxacin solution is irradiated with a xenon lamp light source, and the xenon lamp irradiation current is controlled to be 18 A, and the reaction is stirred for 3600 s at a stirring speed of 500 r / min. The degradation effect is shown in Figure 6 .

[0126] As Figure 6As shown, under the condition of the same xenon lamp intensity, except for pH 9 and 10, the degradation rate of norfloxacin by Bi-MOF / N-CQDs / g-C3N4 can reach over 80% under other pH conditions, indicating that the initial pH of the reaction solution has little effect on its degradation, and this degradation system has a wide pH range. When pH is 9 and 10, the degradation fails, probably because under strong alkaline conditions, the generated free radicals are the main active species for degrading norfloxacin. However, under strong alkaline conditions, these free radicals may be more prone to self-quenching or reacting with other substances, resulting in a decrease in their concentration and activity. This reduces the collision frequency and reaction rate between the free radicals and norfloxacin molecules, thus reducing the degradation efficiency. Secondly, when pH = 2 and 3, the degradation rate of norfloxacin is 83%, and norfloxacin can be removed well. Therefore, under most pH conditions, the photocatalytic degradation of norfloxacin in water by Bi-MOF / N-CQDs / g-C3N4 can be achieved.

[0127] It can be seen from Figure 7 that when the pH of the reaction solution is 5, the rate constant k value is significantly better than that under other pH conditions, that is, without changing the original acidity and alkalinity of norfloxacin, the composite catalyst has the fastest degradation rate and the best effect on it.

[0128] The Bi-MOF / N-CQDs / g-C3N4 composite catalytic material prepared in the present invention not only has high active sites, but also can efficiently degrade norfloxacin in water at room temperature, normal pressure and a wide pH range. The Bi-MOF / N-CQDs / g-C3N4 composite catalyst of the present invention participates in and completes the catalytic reaction, does not dissolve in the water body, and does not cause secondary pollution to the water body. The catalytic material has a stable structure, and experiments have been conducted on various water bodies, which also have no impact on the degradation.

[0129] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A preparation method of a composite photocatalyst, characterized in that, It includes the following steps: Using g-C3N4 as the base and nitrogen-doped carbon quantum dots as the electron transport bridge, dispersing the g-C3N4 and nitrogen-doped carbon quantum dot solutions in the Bi-MOF solution in sequence, reacting at 120 °C to 200 °C, inducing the in-situ growth of Bi-MOF on the surface of g-C3N4, and simultaneously embedding the nitrogen-doped carbon quantum dots into the interface between Bi-MOF and g-C3N4 to obtain a composite photocatalyst.

2. The preparation method of the composite photocatalyst according to claim 1, wherein Add 1 mL to 5 mL of the nitrogen-doped carbon quantum dot solution for every 0.001 mol of g-C3N4; Add 1 mL to 20 mL of the Bi-MOF solution for every 0.001 mol of g-C3N4; The concentration of the nitrogen-doped carbon quantum dot solution is 10 -5 mol / L; The concentration of the Bi-MOF solution is 20 mmol / L.

3. The preparation method of the composite photocatalyst according to claim 1, characterized in that The reaction time is 10 h to 20 h.

4. The preparation method of the composite photocatalyst according to claim 1, characterized in that, The specific method for preparing the nitrogen-doped carbon quantum dot solution is as follows: Using citric acid as the carbon source and urea as the nitrogen source, dissolving citric acid and urea in water, and reacting at 150 °C to 200 °C for 3 h to 6 h to obtain the nitrogen-doped carbon quantum dot solution.

5. The preparation method of the composite photocatalyst according to claim 4, wherein, The dosage ratio of the citric acid, urea and water is 2 g to 5 g: 1 g to 5 g: 30 mL to 50 mL.

6. The preparation method of the composite photocatalyst according to claim 1, wherein, The specific method for preparing the Bi-MOF solution is as follows: Using isophthalic acid as the organic ligand, dissolving the bismuth source and isophthalic acid in the solvent, and under the stirring action, enabling the organic ligand and the bismuth ions in the bismuth source to self-assemble through coordination bonds to form a Bi-MOF precursor network, thereby obtaining the Bi-MOF solution.

7. The preparation method of the composite photocatalyst according to claim 6, wherein, The molar ratio of the bismuth source to isophthalic acid is 1:1; The solvent is a mixed solvent of ethylene glycol and N,N-dimethylformamide; the volume ratio of ethylene glycol to N,N-dimethylformamide is 0.5 to 1:

1.

8. A composite photocatalyst, characterized in that, The composite photocatalyst is prepared by using the preparation method of the composite photocatalyst according to any one of claims 1 to 7.

9. The application of the composite photocatalyst according to claim 8 in degrading norfloxacin pollutants.

10. Use of the composite photocatalyst according to claim 9 in the degradation of norfloxacin pollutants, characterized in that, The specific application method is as follows: Mix the composite photocatalyst with the norfloxacin pollutant solution and carry out catalytic degradation under light; Among them, add 0.1 mg to 0.2 mg of the composite photocatalyst to every 1 mL of the norfloxacin pollutant solution; the concentration of the norfloxacin pollutant solution is 10 mg / L to 30 mg / L.

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