Nitrogen-doped modified graphene and a preparation method thereof

Abstract

The application discloses nitrogen-doped modified graphene and a preparation method thereof, and comprises the following steps: removing water from graphene oxide and glycine under heating and vacuum extraction; mixing and ball-milling the graphene oxide and glycine after water removal to obtain a precursor mixture, and then grinding, crushing and screening the precursor mixture; dispersing the screened precursor powder in water to form a homogeneous suspension, and then performing a hydrothermal reaction; filtering and washing the suspension after the hydrothermal reaction; and performing high-temperature solid nitrogen heat treatment on the solid after the washing to obtain the nitrogen-doped modified graphene. The application realizes the full mixing and uniform dispersion of the graphene oxide and the nitrogen source, and completes the preliminary doping and close combination of nitrogen atoms through the synergistic effect of vacuum drying pretreatment, ball-milling activation, hydrothermal reaction preliminary doping and high-temperature solid nitrogen, thereby significantly improving the nitrogen doping efficiency and nitrogen fixation stability.

Description

Technical Field

[0001] This invention belongs to the field of novel carbon-based functional nanomaterial preparation technology, and particularly relates to a nitrogen-doped modified graphene and its preparation method. Background Technology

[0002] With the rapid development of energy storage devices, electrocatalytic reactions, conductive functional materials, and novel nanocomposite materials, graphene, with its ultra-high specific surface area, excellent conductivity, and unique two-dimensional layered structure, has become a carbon-based functional material with extremely broad application prospects. Functionalization of graphene through nitrogen atom doping can effectively introduce lattice defects, regulate electronic structure, and optimize surface physicochemical properties, significantly improving the practical application performance of graphene in catalysis, energy storage, and electromagnetic functional materials. Therefore, the efficient and controllable preparation of nitrogen-doped graphene has become a research hotspot in the field of nanomaterials.

[0003] Currently, the mainstream methods for preparing nitrogen-doped graphene in the industry mainly include vapor deposition, plasma doping, single hydrothermal doping, and high-temperature pyrolysis. Existing doping processes generally have significant shortcomings. On the one hand, most processes use high-cost nitrogen sources such as urea, ammonia, and nitrogen-containing organic solvents, resulting in high raw material procurement costs. Furthermore, some nitrogen source reactions easily produce toxic and harmful byproducts, which does not conform to the concept of green and low-carbon production. On the other hand, traditional doping methods often use a single reaction system to complete nitrogen doping, achieving only shallow nitrogen attachment. This leads to problems such as uneven nitrogen atom distribution, weak bonding with the graphene matrix, and easy detachment and loss of nitrogen atoms under high-temperature operating conditions. Ultimately, the resulting nitrogen-doped graphene exhibits poor structural stability and low doping efficiency, greatly limiting the material's overall performance.

[0004] Meanwhile, in traditional preparation processes, graphene oxide raw materials are prone to severe agglomeration, the mixing uniformity of nitrogen source and graphene precursor is insufficient, and the initial raw material pretreatment methods are simple and crude, which cannot effectively remove residual moisture inside the raw materials, further causing instability in subsequent doping reactions and low product purity. Moreover, most mature preparation processes have high equipment requirements, complicated operation procedures, and harsh reaction conditions, making it difficult to achieve low-cost, large-scale industrial production, and failing to meet the market's actual demand for high-quality, high-stability nitrogen-doped graphene mass production. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a highly stable nitrogen-doped modified graphene and its preparation method.

[0006] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: A method for preparing nitrogen-doped modified graphene includes the following steps: (1) Dehydration of graphene oxide and glycine was carried out under heating and vacuum conditions; (2) The dehydrated graphene oxide is mixed with glycine and ball-milled to obtain a precursor mixture. Then the precursor mixture is ground, crushed and sieved. (3) Disperse the sieved precursor powder in water to form a homogeneous suspension, and then carry out a hydrothermal reaction at a temperature of 180~200℃. Filter and wash the suspension after the hydrothermal reaction to remove impurities. (4) The solid after washing and removing impurities is subjected to high-temperature nitrogen fixation heat treatment at a temperature of 600~800℃ to obtain the nitrogen-doped modified graphene.

[0007] Furthermore, the conditions for dehydration in step (1) are: vacuum degree controlled at <1000Pa, heating temperature at 45~55℃, and constant temperature holding for 2~4h.

[0008] Furthermore, in step (2), the mass ratio of graphene oxide to glycine is 1:(4.5~5.5).

[0009] Furthermore, in step (2), the ball milling speed is 250~300 r / min and the ball milling time is 2.5~4 h.

[0010] Furthermore, in step (2), a 100-mesh sieve is used for sieving.

[0011] Furthermore, the hydrothermal reaction time in step (3) is 2-3 hours.

[0012] Furthermore, in step (3), water washing to remove impurities is performed using deionized water at a constant temperature of 35~45℃.

[0013] Furthermore, the heat treatment time in step (4) is 1~2 hours.

[0014] The present invention provides a nitrogen-doped modified graphene, which is obtained by the preparation method described above.

[0015] Furthermore, the nitrogen doping content of the nitrogen-doped modified graphene is 3.0~8.5 wt%.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention overcomes the shortcomings of existing technologies, such as single nitrogen doping process, poor nitrogen fixation effect, and poor product stability. Through the synergistic effect of vacuum drying pretreatment, ball milling activation, hydrothermal reaction preliminary doping and high-temperature nitrogen fixation, it achieves both full mixing and uniform dispersion of graphene oxide and nitrogen source, and completes the preliminary doping and tight bonding of nitrogen atoms. This significantly improves nitrogen doping efficiency and nitrogen fixation stability, and solves the technical problems of easy nitrogen atom detachment and uneven doping in existing technologies.

[0017] Meanwhile, glycine is chosen as the nitrogen source, which is lower in cost and more widely available than the high-priced nitrogen sources commonly used in existing technologies, and produces no harmful byproducts, thus better meeting the needs of green production.

[0018] Furthermore, the entire process is simple and controllable, requiring no complex production equipment, and is easy to operate, enabling large-scale mass production. Compared to the complex and costly preparation processes in existing technologies, it is more practical and economical. The nitrogen-doped modified graphene produced has high purity, stable structure, and superior overall performance, better meeting the application needs in fields such as energy storage, catalysis, and conductive materials. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The nitrogen-doped modified graphene suspension obtained in Example 1 is in a dispersed state after standing for 100 hours; Figure 2 This is a 500x magnification image of the graphene oxide raw material from Example 1; Figure 3 This is a 500x magnification image of the nitrogen-doped modified graphene obtained in Example 1. Figure 4 The image shows the XPS spectrum of the nitrogen-doped modified graphene obtained in Example 1. Figure 5 The XPS spectrum of the nitrogen-doped modified graphene obtained in Example 2 is shown. Figure 6 The following are the suspended mass change curves over time for the embodiments: (a) Embodiment 1, (b) Embodiment 2, (c) Embodiment 3. Detailed Implementation

[0021] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0022] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0023] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0024] To address the shortcomings of existing nitrogen-doped graphene preparation technologies, such as high nitrogen source cost, poor doping uniformity, weak nitrogen fixation effect, unstable product structure, and difficulty in mass production, this invention innovatively uses glycine as an environmentally friendly and low-cost nitrogen source. It combines a multi-step synergistic preparation approach involving vacuum drying pretreatment of raw materials, mechanical ball milling activation, hydrothermal preliminary doping, and high-temperature closed nitrogen fixation. By optimizing the entire process parameters, the invention effectively breaks down the graphene agglomeration structure, achieves uniform and deep nitrogen doping, strengthens the bonding strength between nitrogen atoms and the graphene framework, simplifies the preparation process, lowers the production threshold, and yields modified nitrogen-doped graphene with high purity, stable structure, and excellent doping effect.

[0025] In some specific embodiments, the preparation method of nitrogen-doped modified graphene of the present invention includes the following steps: (1) Graphene oxide and glycine were dehydrated under heating and vacuum conditions.

[0026] Graphene oxide possesses hydrophilic oxygen-containing functional groups, enabling uniform dispersion and mixing. It also exhibits numerous structural defects that provide active sites for nitrogen doping, allowing for simultaneous reduction and nitrogen doping under hydrothermal conditions. In contrast, pure graphene has an inert and highly hydrophobic surface, readily agglomerating in water, making it difficult to uniformly composite with raw materials. Furthermore, it lacks active sites for binding nitrogen atoms, hindering effective doping.

[0027] Compared to other amino acids, glycine has a simple molecular structure, no side-chain alkyl groups, a simple pyrolysis reaction pathway, and produces less residual carbon as a byproduct after pyrolysis. In contrast, long-chain amino acids are prone to generating fused-ring carbon that clogs pores during pyrolysis, which is not conducive to nitrogen doping. The thermal decomposition temperature of glycine is suitable for hydrothermal processes of 180~200℃, which can continuously provide an active nitrogen source during the hydrothermal process. Most macromolecular amino acids do not decompose sufficiently at this temperature, resulting in limited doping efficiency. At the same time, glycine has a small molecular weight and low steric hindrance, which allows it to penetrate between graphene oxide sheets during ball milling and hydrothermal processes, achieving uniform nitrogen doping throughout the material. Macromolecular amino acids are limited by steric hindrance and can only be doped on the surface of the material.

[0028] In some embodiments, the vacuum degree is controlled at <1000Pa, the heating temperature is 45~55℃, and the temperature is kept constant for 2~4 hours to completely remove the adsorbed moisture inside the graphene oxide and glycine, so as to avoid the moisture affecting the uniformity of subsequent mixing and reaction effect.

[0029] In some embodiments, this step is carried out in a tubular furnace, and after the heat preservation is completed, the furnace body is kept sealed and allowed to cool down naturally.

[0030] (2) The dehydrated graphene oxide and glycine were mixed at a mass ratio of 1: (4.5~5.5) and ball-milled to obtain a precursor mixture. The precursor mixture was then ground, crushed and sieved.

[0031] Strictly controlling the mass ratio of graphene oxide to glycine can ensure a sufficient supply of nitrogen source without excessive redundant nitrogen source residue, thereby reducing raw material costs while maximizing the nitrogen doping ratio and doping uniformity.

[0032] In some embodiments, the ball milling speed is 250~300 r / min, and continuous uniform ball milling is performed for 2.5~4 h. The mechanical grinding force breaks down the agglomerated structure of graphene oxide sheets, achieving uniform mixing of graphene oxide and glycine at the molecular level, while simultaneously completing the mechanical activation of the material, ultimately forming a uniform and stable precursor mixture. Mechanical ball milling at specific speeds and durations not only breaks down the stacked agglomerates of graphene sheets but also increases the specific surface area of ​​the raw materials, enhances reactivity, and allows glycine molecules to fully adhere to the surface of the graphene sheets, laying the structural foundation for subsequent doping reactions.

[0033] A 100-mesh sieve is preferred for screening to remove agglomerated and large particles of impurities, and to collect the precursor powder with uniform particle size that passes through the sieve, ensuring the consistency of the material particle size and providing high-quality raw material conditions for the hydrothermal doping reaction.

[0034] (3) Disperse the sieved precursor powder in water to form a homogeneous suspension, and then carry out a hydrothermal reaction. Filter and wash the suspension after the hydrothermal reaction to remove impurities.

[0035] In some embodiments, the hydrothermal reaction is carried out at a temperature of 180–200 °C for 2–3 hours. During the reaction, graphene oxide is reduced, and nitrogen is initially doped into the graphene framework, generating nitrogen-doped graphene precursors. During the hydrothermal reaction, oxygen-containing functional groups in graphene oxide are removed and reduced to reduced graphene oxide; glycine is thermally decomposed to generate nitrogen-containing active groups, completing initial nitrogen doping at defects in the reduced graphene oxide and generating nitrogen-doped graphene precursors.

[0036] Using glycine as a reducing agent and nitrogen source, an integrated reduction and in-situ nitrogen doping reaction of graphene oxide (GO) was achieved in a hydrothermal system. The surface of graphene oxide is rich in hydrophilic oxygen-containing functional groups such as hydroxyl and epoxy groups, possessing good water dispersibility and numerous structural defects, providing ample active sites for the reduction reaction and nitrogen doping. This solves the problems of inertness, strong hydrophobicity, easy aggregation, and difficulty in doping modification of pure graphene. During the reduction and doping process, glycine, with its unique advantages of simple molecular structure, lack of side-chain alkyl groups, and low steric hindrance, can uniformly penetrate into the interlayer gaps of graphene oxide, achieving uniform composite formation throughout the entire process. Compared to long-chain amino acids, glycine has a simpler pyrolysis pathway and less residual carbon, avoiding the formation of fused ring carbon that blocks the material's pores and maximizing the preservation of the material's porous structure. Furthermore, its thermal decomposition temperature is highly compatible with the 180–200℃ hydrothermal process, allowing for continuous decomposition during the hydrothermal reaction and the stable release of reducing active groups and active nitrogen species. Specifically, the reducing amino groups of glycine can effectively break the oxygen-containing functional groups on the surface of graphene oxide, removing surface oxygen-containing groups and realizing the transformation of graphene oxide into reduced graphene oxide (rGO). At the same time, the active nitrogen atoms generated by decomposition can precisely anchor the defective active sites of the graphene carbon skeleton, completing in-situ nitrogen doping modification. In addition, through pretreatment at 45-55℃ in a vacuum, the adsorbed water in the raw materials is completely removed, eliminating the interference of moisture on the interfacial recombination and reaction process, effectively improving the reduction efficiency and doping uniformity of glycine on graphene oxide, and finally obtaining a nitrogen-doped reduced graphene oxide composite system with uniform structure and controllable defects, providing a good microstructure foundation for the material's excellent microwave absorption performance.

[0037] During the high-temperature hydrothermal reaction at 180–200℃, glycine undergoes a controlled deamination and decarboxylation pyrolysis reaction, ultimately converting into reactive ammonia nitrogen (NH3 / NH4). + The process involves small-molecule organic acids and trace amounts of carbon-based intermediates, without the formation of large-molecule fused-ring carbon byproducts. Among these, the ammonia-state active nitrogen generated during decomposition is the core nitrogen source for achieving nitrogen doping in graphene. It can precisely bond to defect sites in the carbon framework to form effective doping configurations such as pyridine nitrogen and graphitic nitrogen. The small-molecule pyrolysis products have good solubility and do not cause pore blockage, thus maximizing the preservation of the porous structure of the material.

[0038] In some embodiments, the filtered solid product is washed with deionized water at a constant temperature of 35~45℃ to remove impurities until the filtrate is neutral, thereby thoroughly washing away unreacted free glycine and various water-soluble impurities in the system and improving product purity.

[0039] (4) The solid after washing and removing impurities is subjected to high-temperature nitrogen fixation heat treatment.

[0040] In some embodiments, the heat treatment is carried out under a nitrogen atmosphere at a temperature of 600-800℃ for 1-2 hours. The high-temperature thermal effect strengthens the chemical bonding between nitrogen atoms and the graphene carbon matrix, transforming unstable nitrogen-containing structures in the precursor, stabilizing the doped structure, and improving the bonding stability between nitrogen and the carbon framework. Simultaneously, it completely removes residual trace amounts of moisture and volatile impurities from the sample, completing deep nitrogen fixation modification and yielding nitrogen-doped modified graphene with stable structure, uniform doping, and excellent purity.

[0041] This invention employs segmented doping: first, shallow preliminary doping of nitrogen and reduction of graphene oxide are completed through a medium-temperature hydrothermal reaction, and then deep nitrogen fixation and bonding are achieved through high-temperature heat treatment. The two-step doping works synergistically to completely solve the defects of easy nitrogen atom detachment and weak bonding force in traditional processes.

[0042] The method of this invention uses glycine as a green and inexpensive nitrogen source throughout the entire process. No toxic or harmful byproducts are generated in the reaction. The water washing and purification process is simple and mild. The overall preparation process has low equipment requirements and simple procedures, making it suitable for industrial-scale continuous production.

[0043] The nitrogen-doped graphene sheets prepared by the method of this invention exhibit excellent dispersion, uniform nitrogen doping distribution (nitrogen content 3.0~8.5wt%), stable crystal structure, and significantly optimized conductivity, electrochemical activity, and surface physicochemical properties. They can be widely used in various fields such as energy storage electrodes, electrocatalytic reactions, conductive composite materials, and electromagnetic functional materials.

[0044] This invention uses graphene oxide as the carbon matrix and glycine as a low-cost, environmentally friendly nitrogen source. First, the two raw materials undergo low-temperature vacuum dehydration pretreatment. Then, they are mixed according to a fixed mass ratio. Mechanical ball milling with specific parameters is used to activate and uniformly mix the materials. After grinding and sieving, a precursor powder with uniform particle size is obtained. Subsequently, the precursor is dispersed into a suspension, and graphene oxide reduction and initial nitrogen doping are achieved through a medium-temperature hydrothermal reaction. Impurities are removed by constant-temperature deionized water washing and filtration. Finally, high-temperature heat treatment is used to complete deep nitrogen fixation modification, yielding the finished nitrogen-doped graphene. This invention employs a three-step synergistic process of ball milling activation, hydrothermal doping, and high-temperature nitrogen fixation, effectively breaking down graphene agglomeration, achieving uniform nitrogen doping, and strengthening the nitrogen-carbon bonding force. This solves the problems of poor nitrogen fixation, easy nitrogen atom detachment, and uneven doping in traditional doping processes. The raw materials are readily available, the process is simple and green, and it is easy to scale up production. The resulting product has a stable structure, high purity, and excellent comprehensive performance, showing broad application prospects in energy storage, electrocatalysis, and conductive functional materials.

[0045] Example 1 Step 1: Vacuum pretreatment of raw materials S1.1 Material Packaging: Place the graphene oxide raw material and the glycine nitrogen source separately inside a clean alumina covered crucible, and then place the two crucibles containing the raw materials into the tubular furnace cavity in sequence.

[0046] S1.2 Low-temperature vacuum dehydration: After the sealed tubular furnace, adjust the vacuum level inside the furnace to 500Pa, set the heating temperature to 50℃, and keep it at a constant temperature for 3 hours.

[0047] S1.3 Natural Cooling: After the heat preservation is completed, turn off the heating system of the tubular furnace, keep the furnace body sealed and cool down naturally. After the crucible and the raw materials inside have cooled to room temperature, take them out, seal them and store them for later use.

[0048] Step 2: Precise proportioning and mixing with mechanical ball milling activation S2.1 Precise weighing and mixing of ingredients: Take 1g of pretreated graphene oxide and 5g of glycine, and mix them precisely according to the mass ratio of 1:5 to obtain the mixed raw materials.

[0049] S2.2 Closed ball mill activation: Put the mixed raw materials into the ball mill jar, add hard grinding balls at a ball-to-material mass ratio of 10:1, and lock the jar lid to complete the sealing and fixation; start the ball mill, set the operating frequency to 23.21Hz, corresponding to a spindle speed of 298 r / min, and continuously ball mill at a uniform speed for 3 hours.

[0050] S2.3 Static Discharge: After the ball milling process is completed, shut down the equipment, allow it to cool at room temperature, and then remove the mixture from inside the ball mill tank.

[0051] Step 3: Crushing, Grinding, Sieving, and Homogenization S3.1 Fine Grinding: The precursor mixture formed into blocks after ball milling is placed in a clean mortar and manually ground and crushed until it is completely transformed into a fine powder.

[0052] S3.2 Standard sieving and grading: Use a 100-mesh standard test sieve to sieve the ground powder, collect the precursor powder with uniform particle size that passes through the sieve, and store it in a dry and sealed container for later use.

[0053] Step 4: Hydrothermal reaction to achieve initial nitrogen doping S4.1 Preparation of a homogeneous suspension: Take the sieved precursor powder and place it in a clean beaker. Add an appropriate amount of deionized water and stir at a constant speed until the powder is completely dispersed in the aqueous solution to form a stable homogeneous suspension.

[0054] S4.2 Sealed hydrothermal reaction: Transfer the prepared suspension smoothly into a polytetrafluoroethylene-lined hydrothermal reactor, check the reactor's sealing structure and tighten the sealing cap to prevent air and liquid leakage during the reaction process; place the entire hydrothermal reactor into the hydrothermal synthesis equipment, set the reaction temperature to 180℃, and maintain the temperature for 2 hours for hydrothermal decomposition reaction.

[0055] S4.3 Cooling and Material Removal: After the hydrothermal reaction is completed, cut off the heating power supply and allow the reactor to cool naturally to room temperature before removing the mixture from the reaction system.

[0056] Step 5: Washing, filtration, purification and impurity removal S5.1 Negative pressure filtration separation: The suspension (containing nitrogen-doped graphene primary product) after hydrothermal reaction is transferred into the Buchner funnel, and a PES polyethersulfone microporous membrane with a pore size of 30 nm is used as the filter medium. The negative pressure filtration device is turned on to achieve rapid separation of solid products and liquid waste liquid.

[0057] S5.2 Constant Temperature Water Washing for Impurity Removal: During the filtration process, the solid product is repeatedly rinsed with 40℃ constant temperature deionized water. The total water washing volume is controlled at about 1L. The pH value of the filtrate is detected in real time using pH test paper until the pH value of the filtrate is stably maintained at about 7.

[0058] S5.3 Collect crude product: After filtration, collect the solid material on the surface of the filter membrane, which is the purified nitrogen-doped graphene crude product.

[0059] Step Six: High-Temperature Heat Treatment to Enhance Nitrogen Fixation Molding S6.1 Spreading the material evenly: Spread the washed and purified solid sample evenly inside the alumina crucible, ensuring that the material is spread evenly and without any accumulation or clumping.

[0060] S6.2 High-temperature nitrogen fixation heat treatment: Place the crucible containing the sample into a high-temperature heat treatment furnace, and set the heat treatment temperature to 600℃ under a nitrogen atmosphere, and keep it at that temperature for 2 hours.

[0061] S6.3 Cooling and Sealing: After the heat preservation process is completed, the high-temperature furnace is turned off. The sample is taken out after it is cooled to room temperature naturally with the furnace. Finally, a nitrogen-doped modified graphene product with stable structure, uniform doping and excellent purity is obtained. It can be sealed and dried for storage.

[0062] The elemental contents (wt%, as determined by an organic elemental analyzer) of the obtained nitrogen-doped modified graphene product are: N 3.035%, C 91.605%, H 4.361%. Figure 4 The image shows the XPS spectrum of the nitrogen-doped modified graphene obtained in Example 1, where the N peak can be observed.

[0063] Figure 2 This is a 500x magnified microstructure image of the graphene oxide raw material from Example 1. Figure 3 This is a 500x magnification image of the nitrogen-doped modified graphene obtained in Example 1; it can be seen that the graphene oxide changed from a sheet-like structure to a fragmented structure.

[0064] This embodiment uses moderately nitrogen-doped graphene, with a mild process suitable for ordinary operating conditions.

[0065] Example 2 Based on Example 1, this example adjusts the material dosage and reaction parameters. 2 g of graphene oxide and 10 g of glycine are weighed and mixed for ball milling, while the remaining ball milling and sieving processes remain unchanged. The hydrothermal reaction temperature is increased to 200°C and the hydrothermal holding time is shortened to 1 hour. The high-temperature nitrogen fixation stage is set at a heat treatment temperature of 700°C and held at a constant temperature for 1.5 h. The remaining washing, filtration, and cooling processes are consistent with those in Example 1, and a high-nitrogen-doped modified graphene material is prepared.

[0066] The elemental contents (wt%, as determined by an organic elemental analyzer) of the obtained nitrogen-doped modified graphene product are: N 5.9%, C 89.376%, H 4.731%. Figure 5 The XPS spectrum of the nitrogen-doped modified graphene obtained in Example 2 shows the N peak.

[0067] This embodiment uses high-nitrogen-doped modified graphene, which has high nitrogen doping efficiency and is suitable for scenarios requiring high nitrogen doping levels.

[0068] Example 3 Based on Example 1, this example focuses on optimizing the high-temperature nitrogen fixation effect. 2 g of graphene oxide and 10 g of glycine were weighed and mixed for ball milling. The ball milling activation, hydrothermal doping at 190°C, and water washing purification processes remained unchanged. The high-temperature heat treatment temperature was increased to 800°C and kept at a constant temperature for 1 h. The nitrogen-carbon bonding strength was enhanced by the higher temperature, and a highly stable nitrogen-doped graphene with extremely strong structural stability and excellent temperature resistance was prepared, which is suitable for functional material applications under high-temperature conditions.

[0069] The elemental contents (wt%, as determined by an organic elemental analyzer) of the obtained nitrogen-doped modified graphene product are as follows: C 89.20 wt%, N 7.20 wt%, H 2.60 wt%.

[0070] This embodiment is a highly stable nitrogen-doped graphene with a stable structure and excellent temperature resistance, making it suitable for high-temperature operating conditions.

[0071] Comparative Example 1 Based on Example 1, glycine was replaced with urea, while all other process parameters remained unchanged.

[0072] Comparative Example 2 Based on Example 1, glycine was replaced with peptone, while all other process parameters remained unchanged.

[0073] Comparative Example 3 Based on Example 3, the high-temperature nitrogen fixation process at 800℃ for 1 hour is omitted, while the remaining process parameters remain completely unchanged.

[0074] Comparative Example 4 Based on Example 1, the nitrogen fixation process of 600℃ high temperature holding for 2 hours was omitted, and the sample was directly dried at room temperature after washing with water to obtain a hydrothermal nitrogen-doped graphene sample.

[0075] Suspension stability performance test To characterize the suspension stability of graphene, a suspension stability test was conducted: 15g of nitrogen-doped modified graphene from the examples and comparative examples was accurately weighed and placed in a 500mL beaker, and 400mL of water was added to prepare a graphene suspension. After ultrasonic treatment to achieve sufficient dispersion, the suspension was allowed to stand at room temperature. After standing for the set time, 2mL of the upper suspension was measured, vacuum dried to remove the solvent, and the mass of the dried solid was weighed using a microbalance. The content of suspended graphene in the system was quantitatively calculated based on this, and a suspension mass-time curve was plotted.

[0076] Figure 6 These are the curves showing the change in suspended mass over time for each embodiment.

[0077] Figure 1 The doped modified graphene of Example 1 was in a dispersed state after being suspended and left to stand for 100 hours, and the dispersion was good.

[0078] The nitrogen-doped graphene prepared in Comparative Example 1 exhibited poor suspension performance (after 10 hours of suspension testing, the suspended mass accounted for only 72.8%). This may be because urea molecules do not contain carboxyl coordination structures, making it difficult for them to bond with oxygen-containing groups on the surface of graphene oxide. In the mixed suspension system, the binding force between the nitrogen source and the graphene sheets is weak, and the dispersion and adsorption uniformity are poor. Nitrogen elements are only attached to the surface of the sheets, making it difficult to achieve uniform doping inside the sheets. During the high-temperature nitrogen fixation stage, the urea decomposition rate is too fast, releasing a large amount of ammonia gas instantaneously. The violent airflow impact causes severe stacking of graphene sheets, collapse and damage of the original pore structure, and a significant decrease in the specific surface area of ​​the material. The suspension stability and micropore structure deteriorate simultaneously.

[0079] The nitrogen-doped graphene prepared in Comparative Example 2 showed poor suspension performance (after 10 hours of suspension testing, the suspended mass accounted for only 81.4%). This may be because the peptone composition is complex, and large molecular peptides are difficult to intercalate uniformly between the graphene oxide sheets, which easily leads to local nitrogen agglomeration, high suspension viscosity, poor dispersion uniformity, and significant deterioration of overall suspension performance.

[0080] The performance of the suspension obtained from nitrogen-doped graphene in Comparative Example 4 was significantly reduced. The suspension mass decreased to 60% of its initial value after 10 hours, and after 100 hours, precipitation occurred, reducing the suspension mass to 20%. Compared to the nitrogen-doped graphene cured at high temperature in Example 1, the performance was extremely poor. This is because this sample only achieved initial adsorption and weak bonding doping of nitrogen through a hydrothermal reaction, without undergoing high-temperature nitrogen fixation and reconstruction. The nitrogen atoms mostly existed in unstable adsorbed states and weakly bonded amino groups on the graphene sheet surface, failing to form a stable CN-C bond structure.

[0081] High temperature stability test The nitrogen-doped graphene samples prepared in Example 3 were subjected to isothermal heat treatment at 700℃ for 5 h and 10 h, respectively. The test results showed that the nitrogen retention rate was higher than 98% and 95% under the two heat treatment durations, respectively. (Nitrogen retention rate = nitrogen content after treatment / nitrogen content before treatment * 100%) This indicates that the high-temperature nitrogen fixation process in Example 3 can form stable CN bonds between nitrogen atoms and the carbon framework. Nitrogen atoms are not easily lost under long-term high-temperature conditions, and the material has excellent high-temperature structural stability, making it suitable for long-term use in high-temperature (below 700℃) conditions.

[0082] The product obtained in Comparative Example 3 was directly heat-treated at 700℃ for 5 h and 10 h, respectively. The nitrogen retention rates were measured to be only 82% and 74%, indicating a significant loss of nitrogen atoms. Without high-temperature nitrogen fixation, nitrogen atoms are mostly attached to the graphene surface in an adsorbed state and weakly bound amino form, with low CN bond energy. Prolonged high-temperature stress on these weak bonding sites causes nitrogen atoms to continuously desorb and escape. Simultaneously, the graphene sheets undergo thermal sintering and stacking, significantly reducing the stability of the doped structure and making long-term stable retention of nitrogen at high temperatures impossible.

[0083] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention should fall within the protection scope of the present invention.

Claims

1. A method for preparing nitrogen-doped modified graphene, characterized in that, Includes the following steps: (1) Dehydration of graphene oxide and glycine was carried out under heating and vacuum conditions; (2) The dehydrated graphene oxide is mixed with glycine and ball-milled to obtain a precursor mixture. Then the precursor mixture is ground, crushed and sieved. (3) Disperse the sieved precursor powder in water to form a homogeneous suspension, and then carry out a hydrothermal reaction at a temperature of 180~200℃. Filter and wash the suspension after the hydrothermal reaction to remove impurities. (4) The solid after washing and removing impurities is subjected to high-temperature nitrogen fixation heat treatment at a temperature of 600~800℃ to obtain the nitrogen-doped modified graphene.

2. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, The conditions for dehydration in step (1) are: vacuum degree controlled at <1000Pa, heating temperature at 45~55℃, and constant temperature holding for 2~4h.

3. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, In step (2), the mass ratio of graphene oxide to glycine is 1:(4.5~5.5).

4. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, In step (2), the ball milling speed is 250~300 r / min and the ball milling time is 2.5~4 h.

5. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, In step (2), a 100-mesh sieve is used for sieving.

6. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, The hydrothermal reaction time in step (3) is 2-3 hours.

7. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, In step (3), deionized water at a constant temperature of 35~45℃ is used for water washing to remove impurities.

8. The method for preparing nitrogen-doped modified graphene according to claim 1, characterized in that, The heat treatment time in step (4) is 1~2 hours.

9. A nitrogen-doped modified graphene, characterized in that, It is obtained by the preparation method described in any one of claims 1 to 8.

10. The nitrogen-doped modified graphene according to claim 9, characterized in that, The nitrogen-doped modified graphene has a nitrogen content of 3.0~8.5 wt%.

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