A nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material and its preparation method and application

Abstract

A nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, and its preparation method and application, relate to the field of photocatalytic technology. The aforementioned preparation method comprises the following steps: first, nitrogen-doped graphene quantum dots are prepared by pyrolysis reaction of ammonium citrate in an oil bath, and then nitrogen-doped graphene quantum dots are in situ precipitated onto flaky sodium bismuthate to prepare nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material. The nitrogen-doped graphene quantum dot / sodium bismuthate composite photocatalytic material prepared by the present invention greatly improves the shortcomings of easy recombination of photogenerated charges generated by light excitation of pure sodium bismuthate, poor charge mobility and insufficient utilization of reaction species, so that it produces higher photocatalytic activity under the action of visible light, and can be widely used in the degradation of toxic organic pollutants. At the same time, the synthesis method provided by the present invention has the advantages of simple process, environmental friendliness, low energy consumption and strong controllability, and is convenient for large-scale production and application.

Description

Technical Field

[0001] The present invention relates to the field of photocatalysis technology, and in particular to a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, a preparation method thereof, and applications thereof. Background Art

[0002] Environmental pollution caused by hazardous organic compounds is becoming increasingly serious, and the effective treatment of organic pollutants in the environment has become a major societal concern. Current methods for removing organic pollutants from wastewater include biological, physical, and chemical treatments. Among chemical treatments, photocatalysis is considered a promising technology because it can mineralize most organic pollutants using only light energy. Among reported photocatalysts, TiO2 has been the most widely studied due to its various advantages, including low cost, chemical stability, and nontoxicity. However, due to its wide bandgap (Eg = 3.0–3.2 eV), it cannot efficiently utilize visible light, which hinders efficient absorption of sunlight. While appropriate volume and surface modifications of TiO2 are beneficial for improving its visible light sensitivity, the development of novel visible-light non-TiO2 photocatalysts, such as BiVO4, BiFeO3, and Ag3PO4, has garnered significant attention.

[0003] Bismuth-based oxides are attractive non-TiO2 photocatalysts due to their unique electronic structure and good photoactivity. 3+ The filled Bi6s band and Bi 5+ Bismuth-based oxides exhibit narrow band gaps, strong visible light absorption, and readily accessible holes due to their vacant 6s band. The pentavalent bismuth-based catalyst NaBiO3 has a unique layered structure with a conduction band composed of mixed Na 3s, Bi 6s, and O 2p orbitals. Its band gap is approximately 2.5 eV and can be excited by visible light. However, rapid electron / hole pair recombination, poor charge mobility, and insufficient utilization of reactive species hinder the excellent photoactivity of NaBiO3. Summary of the Invention

[0004] One of the purposes of the present invention is to provide a method for preparing a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material to prepare a more effective visible light catalyst.

[0005] In order to achieve the above object, the present invention adopts the following technical solution: a method for preparing a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, comprising the following steps:

[0006] Nitrogen-doped graphene quantum dots were first prepared by pyrolysis of ammonium citrate in an oil bath, and then nitrogen-doped graphene quantum dots were in situ precipitated onto flaky sodium bismuthate to obtain a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material.

[0007] Furthermore, the specific steps of preparing nitrogen-doped graphene quantum dots through the oil bath pyrolysis reaction of ammonium citrate include: dissolving ammonium citrate in a container filled with water, then placing the container in an oil bath to heat it, taking it out and cooling it to obtain a black product, dissolving the black product with water and putting it into a dialysis bag, then placing the dialysis bag into a container filled with water, regularly changing the water in the container until the water no longer changes color, and then drying the dialyzed product to obtain a black powder, namely nitrogen-doped graphene quantum dots (N-GQDs).

[0008] The heating temperature in the above step can be 200°C, and the heating reaction time can be 30 minutes. The drying temperature in the above step can be 80°C, and the drying time can be 12 to 24 hours. In addition, after dissolving the black product in water, the pH can be adjusted to 7.

[0009] Furthermore, the specific steps of preparing a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material by in-situ precipitation of nitrogen-doped graphene quantum dots onto flaky sodium bismuthate include: first preparing the nitrogen-doped graphene quantum dots and flaky sodium bismuthate into dispersions respectively, then adding the nitrogen-doped graphene quantum dot dispersion into the sodium bismuthate dispersion, stirring the reaction and collecting the precipitate, washing and drying it to obtain the nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material.

[0010] The cleaning operation in the above step can be performed by centrifugal washing, and the centrifugal washing speed can be 8000 r / min, and the centrifugal time can be 10-15 minutes. The drying temperature in the above step can be 80° C., and the drying time can be 12 hours.

[0011] The step of preparing the nitrogen-doped graphene quantum dot dispersion includes: adding nitrogen-doped graphene quantum dots to water (such as deionized water) and ultrasonically dispersing them for 30 minutes to obtain the nitrogen-doped graphene quantum dot dispersion.

[0012] The step of preparing the sodium bismuthate dispersion includes: adding flaky sodium bismuthate to ethanol (such as anhydrous ethanol) and water (such as deionized water), ultrasonically treating for 30 minutes, and stirring at room temperature for 30 minutes to fully disperse the sodium bismuthate to obtain the sodium bismuthate dispersion.

[0013] Furthermore, the mass percentage of nitrogen-doped graphene quantum dots in the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material is 0.6-1.4%.

[0014] The nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material can be recorded as X N-GQDs / NaBiO 3 , wherein X is the mass percentage of N-GQDs, and X is 0.6 to 1.4.

[0015] Preferably, the mass percentage of nitrogen-doped graphene quantum dots in the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material is 0.8%, that is, X in X N-GQDs / NaBiO 3 is 0.8.

[0016] Furthermore, when the ammonium citrate is dissolved in a container filled with water, the mass ratio of the ammonium citrate to water is 1:(30-50). For example, the mass ratio of the ammonium citrate to water is 1:40, or 1g of ammonium citrate corresponds to 40ml of water.

[0017] Furthermore, the dialysis bag is an MD44 dialysis bag with a specification of molecular weight 1000.

[0018] Furthermore, the particle size of the prepared nitrogen-doped graphene quantum dots is 2 to 5 nm.

[0019] Furthermore, the nitrogen-doped graphene quantum dot dispersion includes nitrogen-doped graphene quantum dots and water, wherein every 3.8 to 8.8 mg of nitrogen-doped graphene quantum dots corresponds to 10 ml of water.

[0020] Furthermore, the sodium bismuthate dispersion includes flaky sodium bismuthate, ethanol (such as anhydrous ethanol) and water, wherein the mass ratio of flaky sodium bismuthate, ethanol and water is 1: (20-40): (20-40).

[0021] Preferably, every 0.632 g of flaky sodium bismuthate corresponds to 20 ml of ethanol and 20 ml of water.

[0022] Furthermore, every 3.8-8.8 mg of nitrogen-doped graphene quantum dots corresponds to 0.632 g of flaky sodium bismuthate, or the mass ratio of the nitrogen-doped graphene quantum dots to the flaky sodium bismuthate is (3.8-8.8):632. For example, the nitrogen-doped graphene quantum dots are 3.8-8.8 mg, and the flaky sodium bismuthate is 0.632 g.

[0023] The flaky sodium bismuthate may be a commercial flaky sodium bismuthate, which may be of analytical grade.

[0024] Wherein, the water involved in the above preparation method can be deionized water.

[0025] Another object of the present invention is to provide a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, which is prepared by the above-mentioned preparation method.

[0026] In addition, the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material can be applied to degrade phenol solution. Specifically, the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material is dispersed into a solution containing phenol.

[0027] The present invention prepares nitrogen-doped graphene quantum dots by an oil bath pyrolysis reaction of ammonium citrate, and then prepares nitrogen-doped graphene quantum dots / sodium bismuthate composite photocatalytic materials by in-situ precipitation of the nitrogen-doped graphene quantum dots onto flaky sodium bismuthate. Among them, the present invention uses a single raw material, ammonium citrate, as a carbon source and a nitrogen source, and prepares nitrogen-doped graphene quantum dots by an oil bath pyrolysis reaction, and the preparation process and raw materials are simple. Moreover, the nitrogen-doped graphene quantum dots / sodium bismuthate composite photocatalytic material prepared by the present invention greatly improves the shortcomings of easy recombination of photogenerated charges generated by light excitation of pure sodium bismuthate, poor charge mobility, and insufficient utilization of reaction species, so that it produces higher photocatalytic activity under the action of visible light, and can be widely used in the degradation of toxic organic pollutants. At the same time, the synthesis method provided by the present invention has the advantages of simple process, environmental friendliness, low energy consumption, and strong controllability, and is convenient for large-scale production and application.

[0028] Moreover, the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material prepared by the present invention has nitrogen-doped graphene quantum dots deposited on the surface of flaky sodium bismuthate, thereby constructing an effective surface separation center, extending the carrier lifetime, increasing the reaction sites and accelerating the interfacial redox reaction, so that it has higher visible light catalytic performance, has an excellent degradation effect on phenol, and has broad application prospects in the treatment of harmful organic compounds. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the SEM image 1 of nitrogen-doped graphene quantum dots (N-GQDs) in Example 1;

[0030] Figure 2 This is the second SEM photo of nitrogen-doped graphene quantum dots (N-GQDs) in Example 1;

[0031] Figure 3 XRD patterns of raw material sodium bismuthate and XN-GQDs / NaBiO3 prepared in each example;

[0032] Figure 4 This is the SEM image of the raw material sodium bismuthate;

[0033] Figure 5 This is the SEM image of 0.8 N-GQDs / NaBiO3 prepared in Example 2;

[0034] Figure 6 Solid UV-visible diffuse reflectance spectra of the raw material sodium bismuthate and 0.8 N-GQDs / NaBiO3 prepared in Example 2;

[0035] Figure 7 The transient photocurrent response diagram of the raw material sodium bismuthate and 0.8 N-GQDs / NaBiO3 prepared in Example 2;

[0036] Figure 8 Electrochemical impedance spectroscopy (EIS) diagram of raw material sodium bismuthate and 0.8 N-GQDs / NaBiO3 prepared in Example 2;

[0037] Figure 9 Graph showing the degradation of phenol by the XN-GQDs / NaBiO3 composite photocatalytic material prepared in each example compared with pure sodium bismuthate under visible light irradiation. DETAILED DESCRIPTION

[0038] To facilitate understanding by those skilled in the art, the present invention is further described below with reference to the examples, which are not intended to limit the present invention. It should be noted in advance that the following examples were performed in the laboratory, and those skilled in the art should understand that the amounts of the components given in the examples merely represent the ratios between the components, and are not intended to be specific limitations.

[0039] The challenges of pure NaBiO₃ (rapid electron / hole pair recombination, poor charge mobility, and insufficient utilization of reactive species) can be addressed by constructing effective surface separation centers to extend carrier lifetime, increase reaction sites, and accelerate interfacial redox reactions. In this regard, ultrasmall nitrogen-doped graphene quantum dots (N-GQDs) are promising candidates for integration with the NaBiO₃ matrix. Compared to larger materials with numerous surface defects that serve as electron-hole recombination centers at the interface, N-GQDs, as zero-dimensional (0D) carbon materials, can reduce the number of interfacial defects. Due to their low cost, limited toxicity, high chemical stability, excellent conductivity, and unique edge and quantum confinement effects, N-GQDs have been extensively studied in electrochemical biosensors, photovoltaic devices, and energy conversion. In particular, their upconversion photoluminescence properties make N-GQDs excellent photosensitizers for harvesting visible and near-infrared light. By leveraging their electron transfer / storage capabilities, N-GQDs can also be used in the development of hybrid photocatalysts to accelerate interfacial charge carrier migration, extend charge lifetime, and reduce light-blocking effects.

[0040] Based on this, the present invention constructs a more effective visible light catalyst by attaching N-GQDs on NaBiO3 nanosheets to form a nanoscale heterojunction.

[0041] Based on the above inventive concept, the present invention provides a method for preparing a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, which specifically comprises the following steps:

[0042] Step 1: Dissolve amine citrate in a beaker filled with deionized water, place the beaker in an oil bath, heat to 200°C, react for 30 minutes, take out, cool and dissolve the black product in deionized water, adjust its pH to 7, then transfer it to an MD44 dialysis bag and place it in a beaker filled with deionized water. Change the water every once in a while until the water no longer changes color. After dialysis, pour it into a watch glass and place it in a drying oven at 80°C to dry it to obtain black powder, which is nitrogen-doped graphene quantum dots (N-GQDs).

[0043] Step 2: First, N-GQDs were added to deionized water and ultrasonically dispersed for 30 minutes to obtain an N-GQDs dispersion; then, commercial flake sodium bismuthate was added to anhydrous ethanol and deionized water for ultrasonic treatment for 30 minutes, and stirred at room temperature for 30 minutes to fully disperse it to obtain a sodium bismuthate dispersion; finally, N-GQDs dispersion was added to the sodium bismuthate dispersion, stirred for reaction for 2 hours, and then the precipitate was collected in a centrifuge tube, centrifuged and washed three times with deionized water, and after washing, it was placed in a vacuum drying oven to dry, thereby obtaining a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, recorded as XN-GQDs / NaBiO3, where X is the mass percentage of N-GQDs, and X is 0.6~1.4.

[0044] In step 1, the mass ratio of ammonium citrate to deionized water is 1:(30-50); the specification of the MD44 dialysis bag is a molecular weight of 1000, and the drying time is 12-24 h.

[0045] Wherein, the particle size of the N-GQDs particles in step 1 is 2 to 5 nm.

[0046] In step 2, the mass ratio of sodium bismuthate, anhydrous ethanol and deionized water is 1: (20-40): (20-40); the centrifugal washing speed is 8000 r / min, the centrifugation time is 10-15 min, the vacuum drying temperature is 80° C., and the drying time is 12 h.

[0047] Wherein, the commercial flaky sodium bismuthate in step 2 is analytically pure, and X is 0.8. Example 1

[0048] (1) Preparation of nitrogen-doped graphene quantum dots (N-GQDs): Dissolve 1 g of amine citrate in a beaker containing 40 ml of deionized water, place the beaker in an oil bath, heat it to 200 °C, react for 30 min, take it out, cool it down, dissolve the black product in deionized water, adjust its pH to 7, then transfer it to an MD44 dialysis bag (molecular weight of 1000) and place it in a beaker containing deionized water. Change the water every once in a while until the water no longer changes color. After dialysis, pour it into a watch glass and place it in a drying oven at 80 °C for 24 hours. The obtained black powder is N-GQDs (particle size of 2 to 5 nm). Figure 1 and Figure 2 .

[0049] (2) Preparation of N-GQDs dispersion and sodium bismuthate dispersion: 3.8 mg of N-GQDs prepared by oil bath pyrolysis were weighed, and 10 ml of deionized water was added for ultrasonic dispersion for 30 min to obtain N-GQDs dispersion; 0.632 g of commercial flake sodium bismuthate was added to 20 ml of anhydrous ethanol and 20 ml of deionized water for ultrasonic treatment for 30 min, and stirred at room temperature for 30 min to fully disperse it to obtain sodium bismuthate dispersion;

[0050] (3) Preparation of nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material (X N-GQDs / NaBiO3): N-GQDs dispersion was added to sodium bismuthate dispersion, stirred for 2 h, centrifuged at 8000 r / min for 15 min, and washed three times with deionized water. After washing, it was placed in a vacuum drying oven at 80°C for 12 h to obtain nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material, recorded as 0.6 N-GQDs / NaBiO3 (the mass percentage of N-GQDs is 0.6%). Example 2

[0051] Except that the mass of N-GQDs was replaced with 5.0 mg, the rest was the same as in Example 1 to obtain a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, recorded as 0.8 N-GQDs / NaBiO3 (the mass percentage of N-GQDs was 0.8%). Example 3

[0052] Except that the mass of N-GQDs was replaced with 6.3 mg, the rest was the same as in Example 1, and a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material was obtained, which was recorded as 1.0 N-GQDs / NaBiO3 (the mass percentage of N-GQDs was 1.0%). Example 4

[0053] Except that the mass of N-GQDs was replaced with 7.6 mg, the rest was the same as in Example 1, and a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material was obtained, which was recorded as 1.2 N-GQDs / NaBiO3 (the mass percentage of N-GQDs was 1.2%). Example 5

[0054] Except for replacing the mass of N-GQDs with 8.8 mg, the rest was the same as in Example 1 to obtain a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material, recorded as 1.4 N-GQDs / NaBiO3 (the mass percentage of N-GQDs was 1.4%).

[0055] Characterization of catalysts:

[0056] Powder X-ray diffraction (XRD) was performed on a PANalytical Xpert Pro X-ray diffractometer using Cu K radiation at 40 mA and 40 kV. The morphology of the samples was investigated using scanning electron microscopy (SEM, FEI, Netherlands). Diffuse reflectance spectroscopy (DRS) was recorded on a Shimadzu UV-2700 spectrophotometer using BaSO₄ as a reference. Transient photocurrent responses and electrochemical impedance spectroscopy were measured using an electrochemical workstation (CHI-750E).

[0057] Catalyst degradation performance test:

[0058] 25 mg of photocatalyst was dispersed in 100 mL of phenol (20 mg·L -1 ) solution. Before light irradiation, the mixture was magnetically stirred for 30 minutes in the dark to achieve adsorption-desorption equilibrium. The mixture was then illuminated using a 300W xenon lamp with a 420nm cutoff filter. At regular intervals, 3mL of the mixture was centrifuged and filtered. Finally, the filtrate concentration was analyzed using a Shimadzu UV-2700 UV-Vis spectrophotometer.

[0059] Specific instructions:

[0060] Figure 3 The XRD patterns of raw material NaBiO3 and products XN-GQDs / NaBiO3 prepared in Examples 1 to 5 are shown. It can be seen that they all have strong diffraction peaks, indicating that they all have good crystallinity. Further analysis shows that the raw material NaBiO3 is a rhombohedral crystal system, and most of the peaks correspond to NaBiO3 . 2H2O (JCPDS. No.30-1160), the XRD pattern of product X N-GQDs / NaBiO3 is almost consistent with that of raw material NaBiO3, without obvious characteristic peaks of N-GQDs, which is attributed to the small amount of N-GQDs added and their significant scattering.

[0061] Figure 4 and Figure 5 SEM images of raw material NaBiO3 and product 0.8 N-GQDs / NaBiO3 prepared in Example 2. Figure 4 It was found that the raw material NaBiO3 is mainly in the shape of flake flower clusters, while the morphology of 0.8 N-GQDs / NaBiO3 ( Figure 5 ) is similar to the raw material NaBiO3.

[0062] Figure 6 The following are solid-state UV-visible diffuse reflectance spectra of the raw material sodium bismuthate and the 0.8 N-GQDs / NaBiO3 prepared in Example 2. It is found that the absorption band edge of NaBiO3 is approximately 500 nm, while the absorption band edge of 0.8 N-GQDs / NaBiO3 obtained by attaching N-GQDs to the flaky NaBiO3 is approximately 510 nm, showing a slight red shift. This is attributed to the upconversion photoluminescence properties of the N-GQDs.

[0063] Figure 7 and Figure 8 The transient photocurrent response diagram of the raw material sodium bismuthate and 0.8 N-GQDs / NaBiO3 prepared in Example 2 ( Figure 7 ) and electrochemical impedance spectroscopy (EIS) diagram ( Figure 8 ). Photocurrent response and electrochemical impedance spectroscopy (EIS) can analyze the carrier separation and recombination of photocatalysts. Figure 7 As shown in Figure 2, the photocurrent response intensity of the 0.8 N-GQDs / NaBiO3 composite photocatalyst is more than twice that of NaBiO3, which is attributed to the fact that photogenerated electrons can be quickly transferred through N-GQDs, thereby improving the carrier separation efficiency. Figure 8 As shown in the figure, the 0.8 N-GQDs / NaBiO3 composite photocatalyst has a smaller arc radius than NaBiO3, which also proves that its carrier separation is more efficient. These results show that the carrier separation efficiency of NaBiO3 can be significantly improved by compounding with NGQDs, thereby improving its photocatalytic activity.

[0064] Figure 9The following are the degradation curves of phenol degradation by XN-GQDs / NaBiO3 composite photocatalysts prepared in different embodiments compared with pure sodium bismuthate under visible light irradiation. It can be seen that under visible light irradiation, the degradation rate of phenol by the raw material NaBiO3 reached 91.7% in 50 minutes. The degradation rate of phenol by the XN-GQDs / NaBiO3 composite photocatalysts prepared in different embodiments was faster than that of the raw material NaBiO3. As the N-GQDs content increased from 0.6% to 1.4%, the degradation rate of phenol first increased and then decreased. Among them, the 0.8% N-GQDs / NaBiO3 composite photocatalyst had the fastest degradation rate for phenol, with a degradation rate of 92.6% after 30 minutes of visible light irradiation.

[0065] The embodiments described above are only some embodiments of the present invention and do not limit the scope of implementation of the present invention. Any changes and improvements made in the principles, process conditions (such as the amount of catalyst, solution concentration, etc.) and applications (such as degradation of dyes, antibiotics and other organic toxic wastewater) of the present invention should fall within the scope of protection of the present invention.

Claims

1. Application of a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material in the degradation of phenol solution, characterized in that: The nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material is prepared by the following preparation method: Nitrogen-doped graphene quantum dots were first prepared by pyrolysis of ammonium citrate in an oil bath, and then in situ precipitation of the nitrogen-doped graphene quantum dots onto flaky sodium bismuthate was performed to prepare a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material that retained its original appearance. The specific steps of preparing the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material by in-situ precipitation of nitrogen-doped graphene quantum dots onto flaky sodium bismuthate include: First, nitrogen-doped graphene quantum dots and flake sodium bismuthate are prepared into dispersions respectively, and then the nitrogen-doped graphene quantum dot dispersion is added to the sodium bismuthate dispersion. The mixture is stirred and reacted at room temperature for 2 hours, and then the precipitate is collected, washed, and dried to obtain a nitrogen-doped graphene quantum dot / sodium bismuthate photocatalytic composite material. Wherein, the mass percentage of nitrogen-doped graphene quantum dots in the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material is 0.6-1.4%.

2. The use of the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material in the degradation of phenol solution according to claim 1, characterized in that: The specific steps of preparing nitrogen-doped graphene quantum dots by ammonium citrate oil bath pyrolysis reaction include: Dissolve ammonium citrate in a container filled with water, then place the container in an oil bath to heat it, take it out and cool it to obtain a black product, dissolve the black product in water and put it into a dialysis bag, then place the dialysis bag into a container filled with water, regularly change the water in the container until the water no longer changes color, and then dry the dialyzed product to obtain a black powder, which is nitrogen-doped graphene quantum dots.

3. The use of the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material according to claim 2 in the degradation of phenol solution, characterized in that: When the ammonium citrate is dissolved in a container filled with water, the mass ratio of the ammonium citrate to water is 1:(30-50).

4. The use of the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material according to claim 1 in the degradation of phenol solution, characterized in that: The sodium bismuthate dispersion comprises flaky sodium bismuthate, ethanol and water, and the mass ratio of the flaky sodium bismuthate, ethanol and water is 1:(20-40):(20-40).

5. The use of the nitrogen-doped graphene quantum dots / sodium bismuthate photocatalytic composite material according to claim 1 in the degradation of phenol solution, characterized in that: Every 3.8 to 8.8 mg of nitrogen-doped graphene quantum dots corresponds to 0.632 g of flaky sodium bismuthate.

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