Fluorine and nitrogen co-doped graphene quantum dots and preparation method and application thereof

Abstract

The application provides fluorine-nitrogen co-doped graphene quantum dots and a preparation method and application thereof, and belongs to the technical field of preparation of carbon nanometer catalytic materials.The method is as follows: a mixed aqueous solution of citric acid and urea is used to prepare a carrier graphene quantum dot through a one-step hydrothermal method; and then a fluorine-nitrogen co-doped graphene quantum dot is obtained through a step-by-step doping process.Compared with the oxygen reduction catalytic performance of the graphene quantum dot, the fluorine-nitrogen co-doped graphene quantum dot obtained through the hydrothermal-assisted step-by-step doping process has good oxygen reduction catalytic performance.The application has the characteristics of simple operation, low cost, environmental friendliness and large-scale preparation, can realize controllable and sufficient nitrogen atom doping, and the prepared fluorine-nitrogen co-doped graphene quantum dot has good oxidation reduction catalytic performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of preparation of carbon nanometer catalytic materials, and particularly relates to fluorine-nitrogen co-doped graphene quantum dots and a preparation method and application thereof. BACKGROUND

[0002] With the vigorous development of global economy and the continuous growth of population, sustainable energy has become a vital issue to ensure global prosperity. However, the low energy conversion efficiency has become a factor limiting the development of these sustainable energies. It is worth mentioning that fuel cells just solve this problem and stand out among numerous chemical energies. On such a battery, the oxidation reaction occurs at the anode position, and the oxygen reduction reaction (ORR) occurs at the cathode position. Unfortunately, the slow kinetics of the oxygen reduction process limits the development of fuel cells, metal air-batteries, and the most effective ORR catalyst platinum and its alloys have the disadvantages of high cost, cross effect, poor durability, etc. Therefore, it is an urgent need to find a high-activity, stable, low-cost non-platinum-based electrocatalyst for the oxygen reduction reaction.

[0003] Graphene has attracted widespread attention in various fields due to its low production cost, good electron transfer rate and mechanical properties, and high light transmittance. In particular, the electrochemical performance of graphene materials can be greatly improved by introducing transition metals or heteroatoms. Unfortunately, the highly condensed nature of graphene results in relatively weak solubility in solvents. Under this background, graphene quantum dots have become the best alternative material for manufacturing various new devices due to their good electrical conductivity, better dispersibility, rich edge defects, and stable photoluminescence properties.

[0004] Nitrogen doping can effectively change the charge distribution of the carbon carrier and improve the adsorption capacity of the carbon carrier. CN115947340A discloses a metal-nitrogen-doped graphene quantum dot composite material, a preparation method and an application. Nitrogen-doped graphene quantum dots (NGQDs) are prepared in an aqueous solution, different soluble metal salts are added, and a metal-nitrogen-doped graphene quantum dot composite material (M-NGQDs) is prepared by using a photo-polymerization reaction. Patent CN111682222A discloses a preparation method of a Pt-CdS-nitrogen-doped graphene quantum dot composite material and its catalytic use. Soluble cadmium salt, thiourea, citric acid and urea are used as raw materials, and a solvent-thermal method is used to prepare the same. Although the above methods have prepared graphene quantum dots with a certain amount of nitrogen doping, some metal salts need to be added in order to improve the catalytic performance. In addition, the amount of nitrogen doping is uncontrollable and insufficient. SUMMARY

[0005] To address the problems of uncontrollable and insufficient nitrogen doping in existing technologies, the present invention aims to provide a fluorine-nitrogen co-doped graphene quantum dot, its preparation method, and its application. The present invention employs a hydrothermal-assisted stepwise doping method, which is simple, low-cost, scalable, and exhibits good electrocatalytic performance, providing a novel nanomaterial for the research and application of catalytic materials such as fuel cells.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing fluorine-nitrogen co-doped graphene quantum dots, comprising the following steps:

[0008] (1) Add citric acid and urea to a beaker containing deionized water, mix and sonicate for 5 minutes to obtain a brownish-yellow transparent solution;

[0009] (2) The brownish-yellow transparent solution obtained in step (1) was transferred to a reaction vessel lined with polytetrafluoroethylene for high-temperature hydrothermal reaction. After the reaction was completed, the product was taken out and freeze-dried to obtain graphene quantum dots (GQDs).

[0010] (3) Place ammonium fluoride in the lining of a hydrothermal reactor, mix it evenly with graphene quantum dots, and then place it in an electric heating drying oven for high-temperature reaction. After the reactants are cooled to room temperature, they are ground to obtain fluorine-doped graphene quantum dot powder (F-GQDs).

[0011] (4) F-GQDs were placed in an ammonia atmosphere, heated and kept warm, and then cooled to obtain fluorine-nitrogen co-doped graphene quantum dot powder (C-GQD).

[0012] Furthermore, in step (1), the molar ratio of citric acid to urea is 1:(2-5).

[0013] Furthermore, in step (1), the concentration of citric acid in the mixed aqueous solution is 0.2-0.3 mol / L. -1 .

[0014] Furthermore, in step (2), the reaction temperature is 150-170℃ and the reaction time is 3-5h.

[0015] Furthermore, in step (2), the freeze-drying time is 20-30 hours.

[0016] Furthermore, in step (3), the molar ratio of the graphene quantum dot powder to ammonium fluoride is 1:(3-6).

[0017] Furthermore, in step (3), the reaction temperature is 160-200℃; the holding time is 20-40h.

[0018] Further, in step (4), the ammonia atmosphere refers to an atmosphere with an ammonia flow rate of 20-30 mL min -1 ; the temperature rising and holding treatment refers to rising the temperature to 500-800 DEG C at a temperature rising rate of 5 DEG C min -1 , and then holding for 2-4 h.

[0019] The fluorine and nitrogen co-doped graphene quantum dots prepared by the preparation method.

[0020] The fluorine and nitrogen co-doped graphene quantum dots are applied to fuel cells as catalytic materials.

[0021] The present application has the following beneficial effects:

[0022] (1) The present application uses graphene quantum dots as carriers, and prepares fluorine-doped graphene quantum dots with high defect concentration based on a step-by-step doping method, then adjusts the nitrogen doping amount of the product under different heat treatment conditions, and finally obtains fluorine and nitrogen co-doped graphene quantum dots with controllable nitrogen content. Under different calcination conditions, the content of nitrogen atoms changes, especially when the ammonia flow rate is adjusted to 30 mL min -1 , the holding temperature is 600 DEG C, and the holding time is 2 h, the best nitrogen doping amount can be achieved, which can be increased from the initial 6.06% to 8.86%.

[0023] (2) The introduction of fluorine atoms increases the defect concentration of the material, i.e. provides more oxygen vacancies to improve the number of active sites, and provides a large number of potential doping sites for subsequent nitrogen doping, which can effectively realize high-concentration nitrogen doping to obtain fluorine and nitrogen co-doped graphene quantum dots with high catalytic performance.

[0024] (3) The C-GQDs prepared by the present application exhibit excellent catalytic performance. The nitrogen atoms incorporated into the graphene six-membered ring structure are distributed in four different forms at the edge and center of graphene, i.e. pyridine-N, pyrrole-N, graphite-N and oxidized-N, which can effectively improve the oxygen absorption capacity of graphene materials; and after heat treatment, the content of pyridine nitrogen increases relatively obviously, which is beneficial to produce more ORR catalytic active sites.

[0025] (4) The present application uses a hydrothermal method, which has the characteristics of simple operation, low cost, wide applicability and low energy consumption, can realize controllable and sufficient doping of nitrogen atoms, and the prepared nitrogen-doped graphene quantum dots have good redox catalytic performance. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to make the technical solutions of the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description only need to be some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

[0027] Figure 1 Preparation process flow chart of fluorine-nitrogen co-doped graphene quantum dots of the present application.

[0028] Figure 2 SEM image of graphene quantum dots prepared in Comparative Example 1 of the present application.

[0029] Figure 3 TEM image of graphene quantum dots prepared in Comparative Example 1 of the present application.

[0030] Figure 4 Photoluminescence (PL) image of graphene quantum dots prepared in Comparative Example 1 of the present application under different excitation wavelengths (300-750 nm).

[0031] Figure 5 Raman image of quantum dots in Example 1 and Comparative Example 1 of the present application.

[0032] Figure 6 XPS image of quantum dots in Example 1, Comparative Example 1 and Comparative Example 2 of the present application.

[0033] Figure 7 CV image of quantum dots in Example 1, Comparative Example 1 and Comparative Example 2 of the present application.

[0034] Figure 8 LSV image of fluorine-nitrogen co-doped graphene quantum dots prepared in Example 1 (a), graphene quantum dots prepared in Comparative Example 1 (b) and nitrogen-doped graphene quantum dots prepared in Comparative Example 2 (c).

[0035] Figure 9 LSV image of fluorine-nitrogen co-doped graphene quantum dots prepared in Example 1, graphene quantum dots prepared in Comparative Example 1 and nitrogen-doped graphene quantum dots prepared in Comparative Example 2 at 2500 rpm.

[0036] Figure 10 K-L image of fluorine-nitrogen co-doped graphene quantum dots prepared in Example 1 (a) and graphene quantum dots prepared in Comparative Example 1 (b). DETAILED DESCRIPTION

[0037] With reference to the accompanying drawings, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of the present application.

[0038] Embodiment 1

[0039] The preparation method of the fluorine and nitrogen co-doped graphene quantum dots in the embodiment is shown in a preparation process flowchart as shown in the figure, and the steps are as follows: Figure 1

[0040] (1) 0.21 g of citric acid and 0.18 g of urea were weighed and added to 5 mL of deionized water for mixing and stirring for 5 min to make them uniformly mixed. The stirred solution was transferred to a hydrothermal reaction kettle, and reacted in a blast drying box at 160℃ for 4 h, and the reaction was cooled to room temperature. Then, freeze-drying was performed, and the graphene quantum dot powder (GQDs) was obtained after drying for 24 h;

[0041] (2) In a glove box (the water and oxygen contents are both less than 1 ppm), 228 mg of ammonium fluoride (GQDs:NH4F=1:5) was weighed and placed in the liner of the hydrothermal reaction kettle, and was uniformly mixed with the graphene quantum dots, and was then laid in the reaction kettle and screwed on a stainless steel sleeve. In a blast drying box, 200℃ was kept for 30 h; after natural cooling, the powder was taken out and ground thoroughly to obtain fluorine-doped graphene quantum dots (F-GQDs);

[0042] (3) 50 mg of the F-GQDs solid powder prepared above was laid in a quartz boat, the quartz boat was placed in the constant temperature interval of a tube furnace, the flow rate of ammonia gas was adjusted to 100 mL min -1 , and after the air in the furnace was exhausted after ammonia gas was introduced for 30 min, the flow rate of ammonia gas was adjusted to 30 mL min -1 . After the gas flow was stable, it was heated to 600℃ at a heating rate of 5℃ min -1 , and kept for 2 h, and then the heating was stopped, and it was naturally cooled to obtain fluorine and nitrogen co-doped graphene quantum dots (C-GQDs).

[0043] Embodiment 2

[0044] The preparation method of the fluorine and nitrogen co-doped graphene quantum dots in the embodiment is shown in a preparation process flowchart as shown in the figure, and the steps are as follows:

[0045] ​(1) Weigh 0.19g of citric acid and 0.12g of urea, add them to 5mL of deionized water and stir for 5min to make them evenly mixed; transfer the stirred solution to a hydrothermal reactor and react in a forced-air drying oven at 150℃ for 5h, and wait for the reactants to cool to room temperature; then freeze-dry for 20h to obtain graphene quantum dot powder (GQDs).

[0046] (2) In a glove box (where the water and oxygen content are both less than 1 ppm), weigh 228 mg of ammonium fluoride (GQDs:NH4F=1:3) and place it in the lining of a hydrothermal reactor. After mixing it evenly with the graphene quantum dots, spread it evenly in the reactor and screw on a stainless steel sleeve. In a forced-air drying oven, keep it at 160℃ for 40 h; after natural cooling, take out the powder, grind it thoroughly, and obtain fluorine-doped graphene quantum dots (F-GQDs).

[0047] (3) Weigh 50 mg of the F-GQDs solid powder prepared above, spread it evenly in a quartz boat, place the quartz boat in the constant temperature zone of the tube furnace, and adjust the flow rate of ammonia gas to 100 mL / min. -1 After purging the furnace by introducing ammonia gas for 30 minutes, adjust the ammonia gas flow rate to 20 mL / min. -1 After the airflow stabilizes, reduce the temperature by 5°C for 5 minutes. -1 The temperature is increased to 600℃ at a certain rate, held for 4 hours, and then the heating is stopped and allowed to cool naturally to obtain fluorine-nitrogen co-doped graphene quantum dots (C-GQDs).

[0048] Example 3

[0049] The preparation method of fluorine-nitrogen co-doped graphene quantum dots in this embodiment includes the following steps:

[0050] (1) Weigh 0.28g of citric acid and 0.45g of urea, add them to 5mL of deionized water and stir for 5min to make them evenly mixed; transfer the stirred solution to a hydrothermal reactor and react in a forced-air drying oven at 170℃ for 3h, and wait for the reactants to cool to room temperature; then freeze-dry for 30h to obtain graphene quantum dot powder (GQDs).

[0051] (2) In a glove box (where the water and oxygen content are both less than 1 ppm), weigh 228 mg of ammonium fluoride (GQDs:NH4F = 1:6) and place it in the lining of a hydrothermal reactor. After mixing it evenly with the graphene quantum dots, spread it evenly in the reactor and screw on a stainless steel sleeve. In a forced-air drying oven, keep it at 190℃ for 35 h; after natural cooling, take out the powder, grind it thoroughly, and obtain fluorine-doped graphene quantum dots (F-GQDs).

[0052] (3) Take 50 mg of the F-GQDs solid powder prepared above, lay it in a quartz boat, and place the quartz boat in the constant temperature interval of a tube furnace. Adjust the flow rate of ammonia gas to 100 mL / min -1 . After the air in the furnace is exhausted by flowing ammonia gas for 30 min, adjust the flow rate of ammonia gas to 30 mL / min -1 . After the gas flow is stable, heat to 600°C at a heating rate of 5°C / min -1 , and keep the temperature for 2 h. Then stop heating and naturally cool down. Fluorine and nitrogen co-doped graphene quantum dots (C-GQDs) are obtained.

[0053] Example 4

[0054] The preparation method of the fluorine and nitrogen co-doped graphene quantum dots in this example is as follows:

[0055] (1) Take 0.28 g of citric acid and 0.45 g of urea, add them to 5 mL of deionized water, and mix and stir for 5 min to make them uniformly mixed. Transfer the stirred solution to a hydrothermal reaction kettle, and react in a blast drying oven at 170°C for 3 h. After the reaction is cooled to room temperature, perform freeze-drying. After 30 h of drying, graphene quantum dot powder (GQDs) is obtained.

[0056] (2) In a glove box (the water and oxygen contents are both less than 1 ppm), take 228 mg of ammonium fluoride (GQDs:NH4F = 1:6) and place it in the liner of a hydrothermal reaction kettle. After mixing the ammonium fluoride with the graphene quantum dots uniformly, lay them in the reaction kettle and screw on a stainless steel cover. In a blast drying oven, keep the temperature at 190°C for 35 h. After natural cooling, take out the powder, grind it thoroughly, and obtain fluorine-doped graphene quantum dots (F-GQDs).

[0057] (3) Take 50 mg of the F-GQDs solid powder prepared above, lay it in a quartz boat, and place the quartz boat in the constant temperature interval of a tube furnace. Adjust the flow rate of ammonia gas to 100 mL / min -1 . After the air in the furnace is exhausted by flowing ammonia gas for 30 min, adjust the flow rate of ammonia gas to 30 mL / min -1 . After the gas flow is stable, heat to 700°C at a heating rate of 5°C / min -1 , and keep the temperature for 3 h. Then stop heating and naturally cool down. Fluorine and nitrogen co-doped graphene quantum dots (C-GQDs) are obtained.

[0058] Example 5

[0059] The preparation method of the fluorine and nitrogen co-doped graphene quantum dots in this example is as follows:

[0060] (1) Take 0.19 g of citric acid and 0.30 g of urea, add them to 5 mL of deionized water, mix and stir for 5 min to make them uniformly mixed; transfer the stirred solution to a hydrothermal reaction kettle, react in a blast drying oven at 165°C for 4 h, cool the reaction product to room temperature; then freeze-dry, and after 26 h of drying, obtain graphene quantum dot powder (GQDs);

[0061] (2) In a glove box (both water and oxygen contents are less than 1 ppm), take 228 mg of ammonium fluoride (GQDs:NH4F = 1:4) and place it in the lining of the hydrothermal reaction kettle, mix it uniformly with the graphene quantum dots, then lay them in the reaction kettle and screw on the stainless steel cover. In a blast drying oven, heat at 200°C for 20 h; after natural cooling, take out the powder and grind it thoroughly to obtain fluorine-doped graphene quantum dots (F-GQDs);

[0062] (3) Take 50 mg of the F-GQDs solid powder prepared above, lay it in a quartz boat, place the quartz boat in the constant temperature interval of the tube furnace, adjust the flow rate of ammonia gas to 100 mL / min -1 , and after the air in the furnace is exhausted, adjust the flow rate of ammonia gas to 30 mL / min -1 . After the gas flow is stable, heat to 600°C at a rate of 5°C / min -1 , and keep the temperature for 3 h, then stop heating and naturally cool to obtain fluorine and nitrogen co-doped graphene quantum dots (C-GQDs).

[0063] Example 6

[0064] The preparation method of a fluorine and nitrogen co-doped graphene quantum dot in this example is as follows:

[0065] (1) Take 0.28 g of citric acid and 0.18 g of urea, add them to 5 mL of deionized water, mix and stir for 5 min to make them uniformly mixed; transfer the stirred solution to a hydrothermal reaction kettle, react in a blast drying oven at 150°C for 5 h, cool the reaction product to room temperature; then freeze-dry, and after 24 h of drying, obtain graphene quantum dot powder (GQDs);

[0066] (2) In a glove box (both water and oxygen contents are less than 1 ppm), take 228 mg of ammonium fluoride (GQDs:NH4F = 1:5) and place it in the lining of the hydrothermal reaction kettle, mix it uniformly with the graphene quantum dots, then lay them in the reaction kettle and screw on the stainless steel cover. In a blast drying oven, heat at 180°C for 20 h; after natural cooling, take out the powder and grind it thoroughly to obtain fluorine-doped graphene quantum dots (F-GQDs);

[0067] (3) Weigh 50 mg of the F-GQDs solid powder prepared above, spread it evenly in a quartz boat, place the quartz boat in the constant temperature zone of the tube furnace, and adjust the flow rate of ammonia gas to 100 mL / min. -1 After purging the furnace by introducing ammonia gas for 30 minutes, adjust the ammonia gas flow rate to 30 mL / min. -1 After the airflow stabilizes, reduce the temperature by 5°C for 5 minutes. -1 The temperature is increased to 500℃ at a certain rate, held for 3 hours, and then the heating is stopped and allowed to cool naturally to obtain fluorine-nitrogen co-doped graphene quantum dots (C-GQDs).

[0068] Example 7

[0069] The preparation method of fluorine-nitrogen co-doped graphene quantum dots in this embodiment includes the following steps:

[0070] (1) Weigh 0.28g of citric acid and 0.18g of urea, add them to 5mL of deionized water and stir for 5min to make them evenly mixed; transfer the stirred solution to a hydrothermal reactor and react in a forced-air drying oven at 150℃ for 5h, and wait for the reactants to cool to room temperature; then freeze-dry for 24h to obtain graphene quantum dot powder (GQDs).

[0071] (2) In a glove box (where the water and oxygen content are both less than 1 ppm), weigh 228 mg of ammonium fluoride (GQDs:NH4F = 1:5) and place it in the lining of a hydrothermal reactor. After mixing it evenly with the graphene quantum dots, spread it evenly in the reactor and screw on a stainless steel sleeve. In a forced-air drying oven, keep it at 180℃ for 20 h; after natural cooling, take out the powder, grind it thoroughly, and obtain fluorine-doped graphene quantum dots (F-GQDs).

[0072] (3) Weigh 50 mg of the F-GQDs solid powder prepared above, spread it evenly in a quartz boat, place the quartz boat in the constant temperature zone of the tube furnace, and adjust the flow rate of ammonia gas to 100 mL / min. -1 After purging the furnace by introducing ammonia gas for 30 minutes, adjust the ammonia gas flow rate to 30 mL / min. -1 After the airflow stabilizes, reduce the temperature by 5°C for 5 minutes. -1 The temperature is increased to 600℃ at a certain rate, held for 3 hours, and then the heating is stopped and the mixture is allowed to cool naturally to obtain fluorine-nitrogen co-doped graphene quantum dots (C-GQDs).

[0073] Example 8

[0074] The preparation method of fluorine-nitrogen co-doped graphene quantum dots in this embodiment includes the following steps:

[0075] (1) Take 0.28 g of citric acid and 0.18 g of urea, add to 5 mL of deionized water, mix and stir for 5 min to make them evenly mixed; transfer the stirred solution to a hydrothermal reaction kettle, react in a blast drying oven at 150°C for 5 h, cool the reaction product to room temperature; then freeze-dry, dry for 24 h to obtain graphene quantum dot powder (GQDs);

[0076] (2) In a glove box (both water and oxygen contents are less than 1 ppm), take 228 mg of ammonium fluoride (GQDs:NH4F=1:5) and put it in the lining of the hydrothermal reaction kettle, mix it evenly with the graphene quantum dots, then lay it in the reaction kettle and screw on the stainless steel cover. In a blast drying oven, heat at 180°C for 20 h; after natural cooling, take out the powder and grind it thoroughly to obtain fluorine-doped graphene quantum dots (F-GQDs);

[0077] (3) Take 50 mg of the F-GQDs solid powder prepared above, lay it in a quartz boat, put the quartz boat into the constant temperature interval of the tube furnace, adjust the flow rate of ammonia gas to 100 mL / min -1 , and then adjust the flow rate of ammonia gas to 30 mL / min -1 after purging the air in the furnace. After the gas flow is stable, heat to 800°C at a rate of 5°C / min -1 , and then stop heating, and naturally cool to obtain fluorine and nitrogen co-doped graphene quantum dots (C-GQDs).

[0078] Example 9

[0079] The preparation method of a fluorine and nitrogen co-doped graphene quantum dot in this embodiment is as follows:

[0080] (1) Take 0.20 g of citric acid and 0.28 g of urea, add to 5 mL of deionized water, mix and stir for 5 min to make them evenly mixed; transfer the stirred solution to a hydrothermal reaction kettle, react in a blast drying oven at 160°C for 4 h, cool the reaction product to room temperature; then freeze-dry, dry for 28 h to obtain graphene quantum dot powder (GQDs);

[0081] (2) In a glove box (both water and oxygen contents are less than 1 ppm), take 228 mg of ammonium fluoride (GQDs:NH4F=1:4) and put it in the lining of the hydrothermal reaction kettle, mix it evenly with the graphene quantum dots, then lay it in the reaction kettle and screw on the stainless steel cover. In a blast drying oven, heat at 200°C for 25 h; after natural cooling, take out the powder and grind it thoroughly to obtain fluorine-doped graphene quantum dots (F-GQDs);

[0082] (3) 50 mg of the F-GQDs solid powder prepared above was placed in a quartz boat, which was placed in the constant temperature interval of a tube furnace, and the flow rate of ammonia was adjusted to 100 mL / min -1 After the air in the furnace was purged for 30 min, the flow rate of ammonia was adjusted to 30 mL / min -1 . After the gas flow was stabilized, heating was performed at a rate of 5 ℃ / min -1 to 600 ℃, and the temperature was maintained for 2 h. Then, heating was stopped, and the sample was naturally cooled to obtain fluorine and nitrogen co-doped graphene quantum dots (C-GQDs).

[0083] Example 10

[0084] A method for preparing fluorine and nitrogen co-doped graphene quantum dots according to the present embodiment includes the following steps:

[0085] (1) 0.25 g of citric acid and 0.39 g of urea were weighed and added to 5 mL of deionized water, and stirred for 5 min to mix uniformly. The stirred solution was transferred to a hydrothermal reaction kettle, and reacted in a blast drying oven at 150 ℃ for 4 h. After the reaction was cooled to room temperature, freeze-drying was performed for 30 h to obtain graphene quantum dot powder (GQDs);

[0086] (2) In a glove box (the water and oxygen contents were both less than 1 ppm), 228 mg of ammonium fluoride (GQDs:NH4F = 1:6) was weighed and placed in the liner of a hydrothermal reaction kettle, mixed uniformly with the graphene quantum dots, and then placed in the reaction kettle and screwed on a stainless steel cover. In a blast drying oven, the temperature was maintained at 180 ℃ for 30 h. After natural cooling, the powder was taken out and ground thoroughly to obtain fluorine-doped graphene quantum dots (F-GQDs);

[0087] (3) 50 mg of the F-GQDs solid powder prepared above was placed in a quartz boat, which was placed in the constant temperature interval of a tube furnace, and the flow rate of ammonia was adjusted to 100 mL / min -1 After the air in the furnace was purged for 30 min, the flow rate of ammonia was adjusted to 30 mL / min -1 . After the gas flow was stabilized, heating was performed at a rate of 5 ℃ / min -1 to 600 ℃, and the temperature was maintained for 4 h. Then, heating was stopped, and the sample was naturally cooled to obtain fluorine and nitrogen co-doped graphene quantum dots (C-GQDs).

[0088] Comparative Example 1

[0089] A method for preparing graphene quantum dot powder (GQD) according to the present comparative example includes the following steps:

[0090] Take 0.21 g of citric acid and 0.18 g of urea, add to 5 mL of deionized water, mix and stir for 5 min to make them evenly mixed; transfer the stirred solution to a hydrothermal reactor, react in a blast drying oven at 160℃ for 4h, cool the reaction to room temperature; then freeze-drying, drying time is 24h to obtain graphene quantum dot powder (GQD).

[0091] The SEM image of graphene quantum dots prepared in Comparative Example 1 is shown in Figure 2 From the figure, it can be seen that the graphene quantum dots at room temperature are in the form of particles, uniformly distributed, and a small amount of obvious agglomeration.

[0092] The TEM image of graphene quantum dots prepared in Comparative Example 1 is shown in Figure 3 From the figure, it can be seen that the prepared graphene quantum dots are a clear and uniformly dispersed spherical shape, with a particle size of 6-8 nm.

[0093] The photoluminescence (PL) image of graphene quantum dots prepared in Comparative Example 1 under different excitation wavelengths (300-750 nm) is shown in Figure 4 From the figure, it can be seen that under 330 nm excitation, the highest emission peak is shown at 455 nm.

[0094] Comparative Example 2

[0095] The preparation method of a nitrogen-doped graphene quantum dot (N-GQD) in this comparative example is as follows:

[0096] (1) Take 0.21 g of citric acid and 0.18 g of urea, add to 5 mL of deionized water, mix and stir for 5 min to make them evenly mixed; transfer the stirred solution to a hydrothermal reactor, react in a blast drying oven at 160℃ for 4h, cool the reaction to room temperature; then freeze-drying, drying time is 24h to obtain graphene quantum dot powder (GQD);

[0097] (2) Take 50 mg of the above prepared GQDs solid powder, lay it in a quartz boat, put the quartz boat into the constant temperature interval of the tube furnace, adjust the flow rate of ammonia gas to 100 mL min -1 , pass ammonia gas for 30 min to exhaust the air in the furnace, then adjust the ammonia gas flow rate to 30 mL min -1 . After the gas flow is stable, heat to 600℃ at a heating rate of 5℃ min -1 , keep for 2h, then stop heating, and naturally cool down to obtain nitrogen-doped graphene quantum dots (N-GQD).

[0098] Effect Example

[0099] (1) Raman spectrum analysis

[0100] Raman tests were performed with the samples of Example 1 and Comparative Example 1 to characterize the degree of defects of each sample. The D peak originates from the breathing vibration of sp 3 atoms in carbon rings, used to characterize the degree of disorder of the graphene material; while the G peak usually originates from the stretching vibration of sp 2 atom pairs, reflecting the degree of crystallinity of the graphene material. Figure 5 For the Raman spectra of the quantum dots in Example 1 and Comparative Example 1, the presence of the crystalline G band (1590 cm -1 ) and disordered D band (1357 cm -1 ) was shown, with the I D / I G ratio changing from 0.88 to 1.44, indicating that the structure is rich in defects.

[0101] (2) XPS spectrum analysis

[0102] Figure 6 For the XPS spectra of the quantum dots in Example 1, Comparative Example 1 and Comparative Example 2, XPS spectrum analysis was performed on the prepared C-GQDs. From the determination results, the C1s characteristic peak at 284 eV, the O1s characteristic peak at 532 eV, and the N1s characteristic peak at 399 eV were all measured, indicating that the basic structure of C-GQDs can be obtained under the reaction conditions. The nitrogen atoms incorporated into the graphene six-membered ring structure are distributed in four different forms at the edge and center of graphene, i.e. pyridine-N, pyrrole-N, graphite-N and oxidized-N, which can effectively improve the oxygen absorption capacity of the graphene material. The different types and contents of nitrogen in GQD, N-GQD and C-GQD are shown in Table 1, it can be seen that compared with the relative content of the untreated sample, the content of graphite nitrogen and oxidized nitrogen in C-GQDs after annealing is reduced, while the content of pyridine nitrogen is relatively significantly increased, which is conducive to generating more ORR catalytically active sites, which means that the ORR performance of C-GQDs is superior to that of GQDs and N-GQDs.

[0103] Table 1 Different types and contents of nitrogen in GQD, N-GQD and C-GQD

[0104]

[0105] (3) Oxygen reduction performance test

[0106] All electrochemical measurements were performed on a CHI660E electrochemical workstation equipped with a rotating disk electrode (RRDE-3A). Platinum wire and Hg / HgO electrode were used as the counter and reference electrode, respectively. To prepare the working electrode, 5 mg of prepared electrocatalyst was ultrasonically dispersed in 0.25 mL of ethanol, 0.045 mL of deionized water and 0.005 mL of naphthol solution. Then, 5 μL of the mixed solution was added to a glassy carbon disc electrode and naturally dried [loading: 0.07065 mg cm -2 (C-GQD catalyst)]. Cyclic voltammograms (CV) were measured in oxygen-saturated 0.1 M KOH solution in the potential range of -1.0 to 0.2 V at a scan rate of 50 mV s -1 The ORR activity of the catalyst was tested by linear sweep voltammetry (LSV) at a rotation speed of 400-2500 rpm and a scan rate of 50 mV s -1 Electrochemical measurements were performed on an electrochemical workstation using a three-electrode system. The Hg / HgO (1 M KOH filling solution) reference electrode was calibrated based on the standard hydrogen electrode (SHE).

[0107] Figure 7 CV plots of quantum dots prepared for Example 1, Comparative Example 1 and Comparative Example 2. The peak potential (Ep) of C-GQDs (0.7 V) is superior to that of raw GQDs (0.60 V) and N-GQDs (0.61 V). Higher oxygen reduction potential means that the catalyst has weaker affinity for oxygen, which makes it easier for adsorbed oxygen to be reduced, and is conducive to obtaining higher catalytic activity. Therefore, the C-GQDs catalyst exhibits superior catalytic performance. In addition, the CV curve integral area of C-GQDs is significantly increased compared with raw GQDs and N-GQDs, indicating a more complete electron transfer process between the carbonyl functional group and the oxygen atom, corresponding to the superior oxygen reduction catalytic performance of N-GQDs.

[0108] Figure 8 LSV plots of fluorine and nitrogen co-doped graphene quantum dots prepared for Example 1 (a), graphene quantum dots prepared for Comparative Example 1 (b) and nitrogen-doped graphene quantum dots prepared for Comparative Example 2 (c). At a scan rate of 5 mV s -1 , the diffusion current density (jD) of the C-GQD catalyst also significantly increases with increasing rotation speed, and is much larger than the diffusion current densities (jD) of the control catalysts GQD and N-GQD (b and Figure 8 Figure 8 Figure 8 ​​c) because the higher the rotation speed, the faster the mass transfer efficiency, resulting in more oxygen being adsorbed onto the working electrode of the catalyst for reduction. This can be attributed to the modification of the GQD carrier by nitrogen, which makes the nanomaterial have a larger defect concentration and a more uniform distribution profile. In addition, the LSV curve of the catalyst at 2500 rpm is shown in Figure 9 Figure 5, and the onset potentials of GQD, N-GQD and C-GQD are 0.69 V, 0.72 V and 0.89 V, respectively.

[0109] Figure 10 K-L plot of the fluorine and nitrogen co-doped graphene quantum dots prepared in Example 1 (a) and the graphene quantum dots prepared in Comparative Example 1 (b). As can be seen from the figure, the K-L plot of the N-GQD electrode was obtained on the LSV curve at a voltage of -0.3 V, showing a clear linear relationship. According to the slope of the K-L plot and the K-L formula (formula 1 and 2), the average number of electron transfers of the GQD and N-GQD catalysts at a voltage of -0.3 V was calculated to be 2.53 and 3.4, respectively. The results show that nitrogen doping promotes the change of the catalytic reaction from a two-electron transfer pathway to a four-electron transfer pathway.

[0110] 1 / j = 1 / j k + 1 / Bu 0.5 Formula (1)

[0111] B = 0.2nF[(D(O2))] 2 / 3 v -1 / 6 C O2 Formula (2)

[0112] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing fluorine-nitrogen co-doped graphene quantum dots, characterized in that, Includes the following steps: (1) After mixing citric acid, urea and deionized water evenly, the mixture is subjected to a high-temperature hydrothermal reaction. The solution is then cooled to room temperature, freeze-dried, and ground to obtain graphene quantum dot powder. (2) The graphene quantum dot powder obtained in step (1) is mixed with ammonium fluoride at a molar ratio of 1:(3-6), and then reacted at 160-200℃ for 20-40h. The reactants are cooled to room temperature and then ground to obtain fluorine-doped graphene quantum dot powder. (3) The fluorine-doped graphene quantum dot powder obtained in step (2) is subjected to an ammonia flow rate of 20-30 mL / min. -1 Under the atmosphere, after 5℃ min -1 The temperature is increased to 500-800℃, held for 2-4 hours, and then cooled to obtain fluorine-nitrogen co-doped graphene quantum dot powder.

2. The method for preparing fluorine-nitrogen co-doped graphene quantum dots according to claim 1, characterized in that, In step (1), the molar ratio of citric acid to urea is 1:(2-5).

3. The method for preparing fluorine-nitrogen co-doped graphene quantum dots according to claim 1, characterized in that, In step (1), the concentration of citric acid in the mixed aqueous solution is 0.2-0.3 mol / L. -1 .

4. The method for preparing fluorine-nitrogen co-doped graphene quantum dots according to claim 1, characterized in that, In step (1), the reaction temperature is 150-170℃ and the reaction time is 3-5h.

5. The method for preparing fluorine-nitrogen co-doped graphene quantum dots according to claim 1, characterized in that, In step (1), the freeze-drying time is 20-30 hours.

6. Fluorine-nitrogen co-doped graphene quantum dots prepared by the preparation method according to any one of claims 1-5.

7. The fluorine-nitrogen co-doped graphene quantum dots of claim 6 are used as a catalytic material in fuel cells.

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