Preparation method of carbon quantum dot MXene composite material

Abstract

The invention discloses a preparation method of a carbon quantum dot MXene composite material, and belongs to the field of capacitor materials. The preparation method comprises the following steps: preparing eicosanoic acid modified carbon quantum dots, performing MXene intercalation modification, and preparing the composite material by a spin-coating method. When the prepared composite material is used as a capacitor material, the cycling stability is good, high capacitance can still be kept after multiple times of cycling, and the capacity retention rate after 20000 times of cycling is 93.3%-94.1%.

Description

Technical Field

[0001] The present invention relates to a method for preparing a carbon quantum dot MXene composite material, and belongs to the field of capacitor materials. Background Art

[0002] As a new type of energy storage device, supercapacitors have shown great application potential in portable electronic devices, electric vehicles, smart grids, aerospace and other fields due to their significant advantages such as fast charging and discharging speed, high power density and long cycle life.

[0003] Despite their numerous advantages, supercapacitors currently have relatively low energy density (typically less than 30 Wh kg-1), which significantly limits their further adoption in long-range battery applications. Therefore, developing high-performance supercapacitor electrode materials to increase their energy density has become a research hotspot and a key breakthrough direction in the current energy storage field.

[0004] In recent years, two-dimensional transition metal carbides / nitrides (MXenes) have become a research focus for supercapacitor electrode materials due to their unique physical and chemical properties. MXenes possess high electrical conductivity, abundant surface functional groups, and a tunable layered structure, all of which demonstrate great potential for energy storage.

[0005] However, MXene nanosheets also face some challenges in practical applications. Due to the action of van der Waals forces, MXene nanosheets are prone to stacking, resulting in a significant reduction in their specific surface area and electrochemically active sites, thereby limiting their electrochemical performance. Taking Ti3C2Tx as an example, its interlayer spacing is small, making it difficult for electrolyte ions to fully enter the interlayer, which not only reduces the specific surface area of ​​the material, but also reduces the available electrochemical active sites. In addition, MXene also has the problem of self-oxidation. When in contact with air or moisture, the titanium atoms on the surface easily react with oxygen or water to form non-conductive TiO2, which further reduces the material's electronic conductivity and capacitive active sites, affecting its long-term stability and practical application.

[0006] Carbon quantum dots (CQDs) are a class of zero-dimensional carbon-based nanomaterials less than 10 nanometers in size. They are composed of carbon atoms and their surface functional groups (such as -COOH and -OH). They exhibit fluorescent properties, good water solubility, and biocompatibility. Research has shown that CQDs can passivate the edges of MXene and delay oxidation, primarily through chemical modification of their surface functional groups and a physical barrier effect. This allows for high capacitance retention in humid environments. Furthermore, CQDs can inhibit MXene stacking, increasing its specific surface area and electrochemically active sites. The emergence of CQD-MXene composites represents the intersection of two-dimensional materials science, nanotechnology, and energy science.

[0007] However, in practical applications, although CQDs intercalation inhibits MXene stacking, can increase its specific surface area and electrochemically active sites, and increase its capacitance, its energy density will drop significantly at high power density, especially when it is used as a component of an asymmetric capacitor. In addition, its cycle stability is poor, and the capacitance will drop significantly after a high number of cycles. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the defects of the existing technology. By using eicosanoic acid to modify carbon quantum dots and intercalate MXene, the two are finally prepared into a composite material by spin coating, so as to achieve the purpose of increasing the capacitance when it is used as a capacitor material, reducing its energy density loss at high power density, and improving its cycle stability.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions: A method for preparing a carbon quantum dot MXene composite material comprises preparing eicosanoic acid-modified carbon quantum dots, MXene intercalation modification, and preparing the composite material by spin coating.

[0010] The following are further improvements to the above technical solution: The method for preparing eicosanoic acid-modified carbon quantum dots comprises dispersing nitrogen-doped lignin carbon quantum dots in anhydrous ethanol, performing ultrasonic dispersion, and maintaining the dispersion temperature at 60-70° C. after the ultrasonic dispersion is completed to obtain a carbon quantum dot dispersion for later use; mixing eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane, and then adding the mixture to the maintained carbon quantum dot dispersion temperature at 60-70° C. and stirring for 23-25 ​​hours; filtering, washing, and drying the mixture after the stirring is completed to obtain eicosanoic acid-modified carbon quantum dots; The mass ratio of the nitrogen-doped lignin carbon quantum dots to anhydrous ethanol is 1:9-11; The nitrogen content of the nitrogen-doped lignin carbon quantum dots is 11.5 at %; The mass ratio of the insulated carbon quantum dot dispersion, eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane is 225-275:1.8-2.2:6.5-7.5:225-275.

[0011] The MXene intercalation modification method comprises mixing MXene with deionized water and performing ultrasonic dispersion for 15-25 minutes. After the ultrasonic dispersion is completed, a MXene dispersion is obtained for standby use. Then, sodium stannate, lithium sulfate, and deionized water are mixed and stirred until completely dissolved to obtain a mixed salt solution. Then, the MXene dispersion, the mixed salt solution, and urea are mixed and the temperature is controlled at 115-125° C. to perform a hydrothermal reaction for 20-26 hours. After the reaction is completed, the mixture is filtered, washed, and dried to obtain the intercalation-modified MXene. The mass ratio of the MXene to deionized water is 1:9-11; The size of the MXene is 170 nm; The mass ratio of the sodium stannate, lithium sulfate and deionized water is 0.9-1.1:1.8-2.2:14-16; The mass ratio of the MXene dispersion, the mixed salt solution, and urea is 6.5-7.5:13-17:13-17.

[0012] The spin coating method for preparing a composite material comprises the following steps: mixing eicosanoic acid-modified carbon quantum dots with deionized water to prepare a carbon quantum dot suspension, mixing intercalation-modified MXene with deionized water to prepare a MXene suspension, then mixing the carbon quantum dot suspension and the MXene suspension, adjusting the pH to 8.3-8.7 with aqueous ammonia, then adding a polyethylene glycol octylphenyl ether solution, continuing to stir for 10-14 hours to obtain a composite liquid, controlling the rotation speed to 2500-3500 r / min, performing spin coating, controlling the temperature to 75-85° C. after spin coating, drying in a vacuum drying oven for 16-20 hours, controlling the temperature to 375-425° C. after drying, annealing under an argon atmosphere for 1.5-2.5 hours, and obtaining a carbon quantum dot MXene composite material after annealing. The concentration of eicosanoic acid-modified carbon quantum dots in the carbon quantum dot suspension is 1.75-2.25 mg / mL; The concentration of the intercalated modified MXene in the MXene suspension is 4.5-5.5 mg / mL; The volume ratio of the carbon quantum dot suspension to the MXene suspension is 1:1.5-2.5.

[0013] The amount of the polyethylene glycol octylphenyl ether solution added is 0.075-0.125% of the total volume of the carbon quantum dot suspension and the MXene suspension.

[0014] Compared with the prior art, the present invention achieves the following beneficial effects: In the method for preparing eicosanoic acid-modified carbon quantum dots of the present invention, eicosanoic acid and carbon quantum dots are condensed under the coupling action of 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, resulting in a modification effect on the surface of the carbon quantum dots to obtain eicosanoic acid-modified carbon quantum dots. In the MXene intercalation modification method, tin and lithium are intercalated between MXene layers through the intercalation action of urea in a hydrothermal environment. The eicosanoic acid-modified carbon quantum dots and the intercalation-modified MXene are then spin-coated to prepare a composite material. Experiments have shown that the electrochemical properties of the material can be effectively improved. The composite material prepared by the present invention has a high rate performance when used as a capacitor material. It can maintain a high capacitance at different scan rates and current densities. At a scan rate of 10mV / s, the capacitance is 335-342F / g; at a scan rate of 50mV / s, the capacitance is 332-335F / g; at a scan rate of 100mV / s, the capacitance is 309-315F / g; at a scan rate of 200mV / s, the capacitance is 275-281F / g; at a scan rate of 500mV / s, the capacitance is 243-245F / g; at a scan rate of 1000mV / s, the capacitance is 219-224F / g; at a scan rate of 2000mV / s, the capacitance is 219-224F / g. At a scan rate of mV / s, the capacitance is 208-210F / g; at a current density of 1A / g, the capacitance is 348-353F / g; at a current density of 5A / g, the capacitance is 334-339F / g; at a current density of 10A / g, the capacitance is 307-310F / g; at a current density of 50A / g, the capacitance is 281-288F / g; at a current density of 100A / g, the capacitance is 275-279F / g; at a current density of 500A / g, the capacitance is 254-261F / g; at a current density of 1000A / g, the capacitance is 204-213F / g; When the composite material prepared by the present invention is used as a capacitor material, the cycle stability is good, and the capacity retention rate after 2000 cycles is 99.8%-99.9%, the capacity retention rate after 5000 cycles is 98.6%-98.7%, the capacity retention rate after 10000 cycles is 95.8%-96.2%, the capacity retention rate after 15000 cycles is 95.2%-95.5%, and the capacity retention rate after 20000 cycles is 93.3%-94.1%; When the composite material prepared by the present invention is used as a capacitor material, it can maintain a high energy density at different power densities. When the power density is 1kW / kg, its energy density is 30.9-31.2Wh / kg; when the power density is 5kW / kg, its energy density is 30.1-30.8Wh / kg; when the power density is 10kW / kg, its energy density is 28.5-28.8Wh / kg; when the power density is 50kW / kg, its energy density is 25.3-26.1Wh / kg. DETAILED DESCRIPTION Example 1

[0015] (1) Preparation of eicosanoic acid-modified carbon quantum dots The nitrogen-doped lignin carbon quantum dots were dispersed in anhydrous ethanol and ultrasonically dispersed for 20 min. After the ultrasonication, the mixture was kept warm at 65° C. to obtain a carbon quantum dot dispersion for later use. Eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane were mixed evenly and added to the insulated carbon quantum dot dispersion. The temperature was maintained at 65° C. and stirred for 24 h. After the stirring was completed, the mixture was filtered, washed, and dried to obtain eicosanoic acid-modified carbon quantum dots. The mass ratio of the nitrogen-doped lignin carbon quantum dots to anhydrous ethanol is 1:10; The nitrogen content of the nitrogen-doped lignin carbon quantum dots is 11.5 at %; The mass ratio of the insulated carbon quantum dot dispersion, eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane is 250:2:7:250.

[0016] (2) MXene intercalation modification MXene was mixed with deionized water and ultrasonically dispersed for 20 minutes. After the ultrasonication, a MXene dispersion was obtained for use. Sodium stannate, lithium sulfate, and deionized water were mixed and stirred until completely dissolved to obtain a mixed salt solution. The MXene dispersion, mixed salt solution, and urea were then mixed and the temperature was controlled at 120°C for a hydrothermal reaction for 24 hours. After the reaction was completed, the mixture was filtered, washed, and dried to obtain the intercalated modified MXene. The mass ratio of the MXene to deionized water is 1:10; The size of the MXene is 170 nm; The mass ratio of the sodium stannate, lithium sulfate and deionized water is 1:2:15; The mass ratio of the MXene dispersion, the mixed salt solution, and urea is 7:15:15.

[0017] (3) Preparation of composite materials by spin coating The eicosanoic acid-modified carbon quantum dots were mixed with deionized water to prepare a carbon quantum dot suspension, and the intercalation-modified MXene was mixed with deionized water to prepare a MXene suspension. The carbon quantum dot suspension and the MXene suspension were then mixed, and the pH was adjusted to 8.5 with ammonia water. Then, a polyethylene glycol octylphenyl ether solution was added and stirred for 12 hours to obtain a composite liquid. The speed was controlled at 3000 r / min for spin coating. After the spin coating was completed, the temperature was controlled at 80°C and dried in a vacuum drying oven for 18 hours. After the drying was completed, the temperature was controlled at 400°C and annealed under an argon atmosphere for 2 hours. After annealing, a carbon quantum dot MXene composite material was obtained. The concentration of eicosanoic acid-modified carbon quantum dots in the carbon quantum dot suspension is 2 mg / mL; The concentration of intercalated modified MXene in the MXene suspension is 5 mg / mL; The volume ratio of the carbon quantum dot suspension to the MXene suspension is 1:2.

[0018] The amount of the polyethylene glycol octylphenyl ether solution added is 0.1% of the total volume of the carbon quantum dot suspension and the MXene suspension. Example 2

[0019] (1) Preparation of eicosanoic acid-modified carbon quantum dots The nitrogen-doped lignin carbon quantum dots were dispersed in anhydrous ethanol and ultrasonically dispersed for 15 minutes. After the ultrasonication, the mixture was kept warm at 60°C to obtain a carbon quantum dot dispersion for later use. Eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane were mixed evenly and added to the insulated carbon quantum dot dispersion. The temperature was maintained at 60°C and stirred for 25 hours. After the stirring was completed, the mixture was filtered, washed, and dried to obtain eicosanoic acid-modified carbon quantum dots. The mass ratio of the nitrogen-doped lignin carbon quantum dots to anhydrous ethanol is 1:9; The nitrogen content of the nitrogen-doped lignin carbon quantum dots is 11.5 at %; The mass ratio of the insulated carbon quantum dot dispersion, eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane is 225:1.8:6.5:225.

[0020] (2) MXene intercalation modification MXene was mixed with deionized water and ultrasonically dispersed for 15 minutes. After the ultrasonication, a MXene dispersion was obtained for use. Sodium stannate, lithium sulfate, and deionized water were mixed and stirred until completely dissolved to obtain a mixed salt solution. The MXene dispersion, mixed salt solution, and urea were then mixed and the temperature was controlled at 125°C for a hydrothermal reaction for 20 hours. After the reaction was completed, the mixture was filtered, washed, and dried to obtain the intercalated modified MXene. The mass ratio of the MXene to deionized water is 1:9; The size of the MXene is 170 nm; The mass ratio of the sodium stannate, lithium sulfate and deionized water is 0.9:1.8:14; The mass ratio of the MXene dispersion, the mixed salt solution, and urea is 6.5:13:13.

[0021] (3) Preparation of composite materials by spin coating The eicosanoic acid-modified carbon quantum dots were mixed with deionized water to prepare a carbon quantum dot suspension, and the intercalation-modified MXene was mixed with deionized water to prepare a MXene suspension. The carbon quantum dot suspension and the MXene suspension were then mixed, and the pH was adjusted to 8.3 with ammonia water. Then, a polyethylene glycol octylphenyl ether solution was added and stirred for 14 hours to obtain a composite liquid. The rotation speed was controlled at 2500 r / min and spin coating was performed. After spin coating, the temperature was controlled at 75°C and dried in a vacuum drying oven for 20 hours. After drying, the temperature was controlled at 375°C and annealed under an argon atmosphere for 1.5 hours. After annealing, a carbon quantum dot MXene composite material was obtained. The concentration of eicosanoic acid-modified carbon quantum dots in the carbon quantum dot suspension is 1.75 mg / mL; The concentration of intercalated modified MXene in the MXene suspension is 4.5 mg / mL; The volume ratio of the carbon quantum dot suspension to the MXene suspension is 1:1.5.

[0022] The amount of the polyethylene glycol octylphenyl ether solution added is 0.075% of the total volume of the carbon quantum dot suspension and the MXene suspension. Example 3

[0023] (1) Preparation of eicosanoic acid-modified carbon quantum dots The nitrogen-doped lignin carbon quantum dots were dispersed in anhydrous ethanol and ultrasonically dispersed for 25 min. After the ultrasonication, the mixture was kept warm at 70°C to obtain a carbon quantum dot dispersion for later use. Eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane were mixed evenly and added to the insulated carbon quantum dot dispersion. The temperature was maintained at 70°C and stirred for 23 h. After the stirring was completed, the mixture was filtered, washed, and dried to obtain eicosanoic acid-modified carbon quantum dots. The mass ratio of the nitrogen-doped lignin carbon quantum dots to anhydrous ethanol is 1:11; The nitrogen content of the nitrogen-doped lignin carbon quantum dots is 11.5 at %; The mass ratio of the insulated carbon quantum dot dispersion, eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane is 275:2.2:7.5:275.

[0024] (2) MXene intercalation modification MXene was mixed with deionized water and ultrasonically dispersed for 25 minutes. After the ultrasonication, a MXene dispersion was obtained for use. Sodium stannate, lithium sulfate, and deionized water were mixed and stirred until completely dissolved to obtain a mixed salt solution. The MXene dispersion, mixed salt solution, and urea were then mixed and the temperature was controlled at 115°C for a hydrothermal reaction for 26 hours. After the reaction was completed, the mixture was filtered, washed, and dried to obtain the intercalated modified MXene. The mass ratio of the MXene to deionized water is 1:11; The size of the MXene is 170 nm; The mass ratio of the sodium stannate, lithium sulfate and deionized water is 1.1:2.2:16; The mass ratio of the MXene dispersion, the mixed salt solution, and urea is 7.5:17:17.

[0025] (3) Preparation of composite materials by spin coating The eicosanoic acid-modified carbon quantum dots were mixed with deionized water to prepare a carbon quantum dot suspension, and the intercalation-modified MXene was mixed with deionized water to prepare a MXene suspension. The carbon quantum dot suspension and the MXene suspension were then mixed, and the pH was adjusted to 8.7 with ammonia water. Then, a polyethylene glycol octylphenyl ether solution was added and stirred for 10 hours to obtain a composite liquid. The speed was controlled at 3500 r / min for spin coating. After the spin coating was completed, the temperature was controlled at 85°C and dried in a vacuum drying oven for 16 hours. After the drying was completed, the temperature was controlled at 425°C and annealed under an argon atmosphere for 2.5 hours. After annealing, a carbon quantum dot MXene composite material was obtained. The concentration of eicosanoic acid-modified carbon quantum dots in the carbon quantum dot suspension is 2.25 mg / mL; The concentration of intercalated modified MXene in the MXene suspension is 5.5 mg / mL; The volume ratio of the carbon quantum dot suspension to the MXene suspension is 1:2.5.

[0026] The amount of the polyethylene glycol octylphenyl ether solution added is 0.125% of the total volume of the carbon quantum dot suspension and the MXene suspension.

[0027] Comparative Example 1 The difference from Example 1 is that the step of preparing eicosanoic acid-modified carbon quantum dots is omitted, and untreated nitrogen-doped lignin carbon quantum dots are directly used as raw materials. While keeping the dosage unchanged, the other steps remain unchanged, and the composite material is prepared by spin coating. The nitrogen content of the nitrogen-doped lignin carbon quantum dots is 11.5 at %.

[0028] Comparative Example 2 Different from Example 1, the MXene intercalation modification step was omitted, and untreated MXene was used as the raw material. While keeping the amount unchanged, the other steps remained unchanged, and the composite material was prepared by spin coating. The size of the MXene is 170 nm.

[0029] Test Example 1: Rate Performance Test The carbon quantum dot MXene composite materials of Examples 1-3 and Comparative Examples 1-2 were subjected to rate performance tests, and the performance of the MXene composite materials was characterized using a three-electrode apparatus, wherein the three-electrode test system selected a mercury / mercurous sulfate electrode as a reference electrode, an activated carbon film as a counter electrode, and 3M sulfuric acid as an electrolyte. The capacitance of the MXene composite materials at different scan rates was tested, and the results are shown in Table 1. The capacitance of the MXene composite materials at different current densities, in F / g, is shown in Table 2.

[0030] Table 1

[0031] Table 2

[0032] In Example 1-3, carbon quantum dots were modified with eicosanoic acid, and MXene was intercalated with sodium stannate and lithium sulfate under the action of urea. Finally, a composite material was prepared by spin coating. The material has excellent rate performance and can maintain a high capacitance at different scan rates and current densities. Comparative Example 1 omits the step of preparing eicosanoic acid-modified carbon quantum dots and directly uses untreated nitrogen-doped lignin carbon quantum dots as raw materials. The lack of eicosanoic acid modification will lead to a decrease in the rate performance of the material. Regardless of whether it is at different scan rates or different current densities, the initial capacitance is low, but the capacitance decays slowly. Although there is still a large loss at high scan rates and high current densities, the overall loss is slightly smaller. Comparative Example 2 omits the MXene intercalation modification step and uses untreated MXene as raw material. The lack of intercalation of tin and lithium elements will lead to a decrease in the rate performance of the material. The initial capacitance loss is small, but with the increase of scan rate and current density, the capacitance loss becomes more serious.

[0033] Test Example 2 Cyclic Stability Test Capacitors were prepared using the carbon quantum dot MXene composite materials of Examples 1-3 and Comparative Examples 1-2, wherein nitrogen-doped graphene aerogel was used as the positive electrode material and assembled with the MXene composite material into an asymmetric capacitor. Constant current charge and discharge tests were performed to test their cyclic stability. The test current was 1 A and the voltage window was 0-1.8 V. The capacity retention rate after different cycle times was tested. The results are shown in Table 3.

[0034] Table 3

[0035] In Example 1-3, carbon quantum dots were modified with eicosanoic acid, and MXene was intercalated with sodium stannate and lithium sulfate under the action of urea. Finally, a composite material was prepared by spin coating. The material had excellent cycle stability and could maintain a high capacity retention rate after multiple cycles. Comparative Example 1 omitted the step of preparing eicosanoic acid-modified carbon quantum dots and directly used untreated nitrogen-doped lignin carbon quantum dots as raw materials. The lack of eicosanoic acid modification would lead to a decrease in the cyclic stability of the material. After multiple cycles, the capacity retention rate dropped significantly. In Comparative Example 2, the MXene intercalation modification step is omitted and untreated MXene is used as the raw material. The lack of the intercalation effect of tin and lithium elements will lead to a decrease in the cycle stability of the material. After multiple cycles, the capacity retention rate will drop significantly.

[0036] Test Example 3: Energy density test at different power densities Capacitors were prepared using the carbon quantum dot MXene composite materials of Examples 1-3 and Comparative Examples 1-2, wherein nitrogen-doped graphene aerogel was used as the positive electrode material and assembled with the MXene composite material into an asymmetric capacitor. The energy density was tested at different power densities, and the results were expressed in Wh / kg. Table 4 shows the results.

[0037] Table 4

[0038] In Example 1-3, carbon quantum dots were modified with eicosanoic acid, and MXene was intercalated with sodium stannate and lithium sulfate under the action of urea. Finally, a composite material was prepared by spin coating. The material can maintain a high energy density at different power densities. In Comparative Example 1, the step of preparing eicosanoic acid-modified carbon quantum dots was omitted, and untreated nitrogen-doped lignin carbon quantum dots were directly used as raw materials. Without the modification of eicosanoic acid, the initial energy density of the material decreased significantly, and as the power density increased, the energy density decreased more seriously. In Comparative Example 2, the MXene intercalation modification step is omitted and untreated MXene is used as the raw material. The intercalation effect of tin and lithium elements is lacking. Although the initial energy density of the material decreases, the degree of decrease is not large. However, with the increase of power density, the energy density still decreases to a large extent.

Claims

1. A method for preparing a carbon quantum dot MXene composite material, characterized in that: The preparation method includes preparing eicosanoic acid-modified carbon quantum dots, MXene intercalation modification, and preparing the composite material by spin coating; The method for preparing eicosanoic acid-modified carbon quantum dots comprises dispersing nitrogen-doped lignin carbon quantum dots in anhydrous ethanol and ultrasonically dispersing the carbon quantum dots; keeping the mixture warm after the ultrasonication to obtain a carbon quantum dot dispersion for later use; uniformly mixing eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane; and adding the mixture to the warmed carbon quantum dot dispersion for stirring. After the stirring is completed, the mixture is filtered, washed, and dried to obtain eicosanoic acid-modified carbon quantum dots. The MXene intercalation modification method comprises the following steps: mixing MXene with deionized water and ultrasonically dispersing the mixture; obtaining a MXene dispersion for later use; then mixing sodium stannate, lithium sulfate, and deionized water and stirring until completely dissolved to obtain a mixed salt solution; then mixing the MXene dispersion, the mixed salt solution, and urea to perform a hydrothermal reaction; and after the reaction is completed, filtering, washing, and drying to obtain the intercalation-modified MXene. The spin coating method for preparing the composite material comprises the following steps: mixing eicosanoic acid-modified carbon quantum dots with deionized water to prepare a carbon quantum dot suspension, mixing intercalation-modified MXene with deionized water to prepare a MXene suspension, then mixing the carbon quantum dot suspension and the MXene suspension, adjusting the pH with ammonia water, adding a polyethylene glycol octylphenyl ether solution, and continuing to stir to obtain a composite liquid, then spin coating the composite liquid, drying the composite liquid after the spin coating is completed, and annealing after drying to obtain the carbon quantum dot MXene composite material.

2. The method for preparing a carbon quantum dot MXene composite material according to claim 1, wherein: In the method for preparing eicosanoic acid-modified carbon quantum dots, the ultrasonic dispersion time is 15-25 minutes; The insulation temperature needs to be controlled at 60-70°C; The stirring method is to maintain the temperature at 60-70° C. and stir for 23-25 ​​hours.

3. The method for preparing a carbon quantum dot MXene composite material according to claim 1, wherein: In the method for preparing eicosanoic acid-modified carbon quantum dots, the mass ratio of nitrogen-doped lignin carbon quantum dots to anhydrous ethanol is 1:9-11; The nitrogen content of the nitrogen-doped lignin carbon quantum dots is 11.5 at %; The mass ratio of the insulated carbon quantum dot dispersion, eicosanoic acid, 1-cyclohexyl-2-morpholinoethylcarbodiimide p-toluenesulfonate, and dichloromethane is 225-275:1.8-2.2:6.5-7.5:225-275.

4. The method for preparing a carbon quantum dot MXene composite material according to claim 1, wherein: In the MXene intercalation modification method, during ultrasonic dispersion, the required ultrasonic time is 15-25 minutes; The hydrothermal reaction method is to control the temperature to 115-125° C. and perform the hydrothermal reaction for 20-26 hours.

5. The method for preparing a carbon quantum dot MXene composite material according to claim 1, wherein: In the MXene intercalation modification method, the mass ratio of MXene to deionized water is 1:9-11; The size of the MXene is 170 nm; The mass ratio of the sodium stannate, lithium sulfate and deionized water is 0.9-1.1:1.8-2.2:14-16; The mass ratio of the MXene dispersion, the mixed salt solution, and urea is 6.5-7.5:13-17:13-17.

6. The method for preparing a carbon quantum dot MXene composite material according to claim 1, characterized in that: In the method for preparing the composite material by spin coating, when ammonia water is used to adjust the pH, the pH needs to be adjusted to 8.3-8.7; When the polyethylene glycol octylphenyl ether solution is added and stirring is continued, the required stirring time is 10-14 hours; The spin coating needs to control the rotation speed to 2500-3500r / min; The drying method is to control the temperature to 75-85°C and dry in a vacuum drying oven for 16-20 hours; The annealing method is to control the temperature to 375-425° C., perform annealing in an argon atmosphere, and control the annealing time to 1.5-2.5 hours.

7. The method for preparing a carbon quantum dot MXene composite material according to claim 1, characterized in that: In the method for preparing the composite material by spin coating, the concentration of eicosanoic acid-modified carbon quantum dots in the carbon quantum dot suspension is 1.75-2.25 mg / mL; The concentration of the intercalated modified MXene in the MXene suspension is 4.5-5.5 mg / mL; The volume ratio of the carbon quantum dot suspension to the MXene suspension is 1:1.5-2.5; The amount of the polyethylene glycol octylphenyl ether solution added is 0.075-0.125% of the total volume of the carbon quantum dot suspension and the MXene suspension.

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