Nitrogen-doped graphene film material as well as preparation method and application thereof

Abstract

The invention discloses a nitrogen-doped graphene film material as well as a preparation method and application thereof, and belongs to the technical field of material preparation. According to the method, nano graphene oxide sheets are uniformly attached to the surface of a metal substrate through electrophoretic deposition, and then efficient reduction and nitrogen doping of graphene oxide are realized through electrochemical reaction in a nitrogen-containing precursor electrolyte. The prepared nitrogen-doped graphene thin film material is controllable in thickness and good in adhesion with substrate metal and has the effect of obviously inhibiting secondary electron emission on the metal surface, and compared with a metal substrate without deposition of a thin film, the secondary electron emission coefficient of the nitrogen-doped graphene thin film material is reduced by 20-50%; the method has a good application prospect in the field of vacuum electronic devices needing to inhibit secondary electron emission.

Description

Technical Field

[0001] The present invention belongs to the technical field of material preparation, and particularly relates to a nitrogen-doped graphene thin film material, a preparation method thereof, and an application thereof. Background Art

[0002] Many vacuum electronic devices select materials with a high secondary electron emission coefficient (δ) and utilize their electron multiplication effect, such as magnetrons, photomultiplier tubes, plasma flat panel displays, etc. In the fields of particle accelerators, space spacecrafts, high-power microwave devices, etc., the secondary electron emission phenomenon often leads to problems such as the formation of electron clouds, surface charging, and a decrease in vacuum degree, which endanger the reliability and stability of the operation of vacuum electronic devices. For example, on the collector in a high-power traveling wave tube, the accumulation of secondary electrons will increase the noise of the amplified signal; in a radio frequency waveguide, secondary electron multiplication will absorb radio frequency power and cause power loss; in a high-energy particle accelerator, the secondary electron multiplication effect forms an electron cloud, resulting in an increase in dynamic pressure, an increase in transverse emittance, and a decrease in beam quality. Therefore, suppressing secondary electron emission is crucial for the stable operation of some vacuum electronic devices.

[0003] To suppress the secondary electron emission on the surface of materials in vacuum electronic devices, selecting materials with a low secondary electron emission coefficient or coating a thin film with a low δ is one of the effective methods. Graphene materials have a low δ and good chemical stability, and are an ideal choice for suppressing secondary electron emission materials. In addition, researchers have found that doping graphene can change the electronic structure of intrinsic graphene, thereby regulating the optoelectronic properties of graphene materials. For example, nitrogen atoms are similar in size to carbon atoms and can replace some carbon atoms in graphene to form doping, resulting in a change in the electronic structure of graphene and thus changing its optoelectronic properties. Bundaleska used microwave plasma excitation CVD to synthesize nitrogen-doped graphene powder and then pressed it onto a metal substrate, but this method has a relatively complex process, expensive equipment, and the prepared thin film is limited by the shape of the substrate material. In addition, although conventional CVD methods for preparing graphene and doped graphene thin films can be directly deposited on the surface of metal substrates, the preparation time is usually long and the production efficiency is low. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: to provide a nitrogen-doped graphene thin film material, a preparation method thereof, and an application thereof to solve the technical problem of serious secondary electron emission on the surface of materials in vacuum electronic devices.

[0005] To achieve the above object, the technical solution adopted by the present invention is: to provide a preparation method of a nitrogen-doped graphene thin film material, comprising the following steps:

[0006] S1. Pretreat the conductive substrate;

[0007] S2. Deposit a graphene oxide (GO) thin film layer on the conductive substrate, then wash and dry it.

[0008] S3. Perform nitrogen doping on the graphene oxide (GO) thin film layer, and finally perform annealing treatment to obtain a nitrogen-doped graphene thin film material on the surface of the conductive substrate.

[0009] Based on the above technical solutions, the present invention can also be improved as follows:

[0010] Further, the conductive substrate is a metal material.

[0011] Further, S1 is specifically: polish the conductive substrate, then immerse it in an acid leaching solution for 10 - 60 s, then wash and blow dry to obtain the pretreated conductive substrate.

[0012] Further, the acid leaching solution is a nitric acid or sulfuric acid solution with a concentration of 2 - 5 wt%.

[0013] Further, S2 is specifically: place the pretreated conductive substrate as an electrode sheet connected to the positive pole of the DC power supply in the graphene oxide (GO) dispersion liquid, deposit at 3 - 6 V for 5 - 60 s, then wash, and finally dry at 80 °C for 2 - 8 h to obtain an intermediate with a graphene oxide (GO) thin film layer deposited on the surface.

[0014] Further, the graphene oxide (GO) dispersion liquid is prepared by dissolving graphene oxide (GO) powder and a polymer binder in water at a mass ratio of 0.05 - 0.1:0.5 - 1, stirring and dissolving at 80 - 90 °C for 10 - 30 min, and finally ultrasonically oscillating for 20 - 40 min; the polymer binder is polyvinyl alcohol or polyacrylic acid.

[0015] The GO sheets in the dispersion liquid are negatively charged due to the surface carboxyl and hydroxyl groups, migrate to the positive pole under the action of the DC electric field and are uniformly deposited on the surface of the metal substrate; the polymer binder forms a multi-level interfacial bond with the GO sheets through hydrogen bonds and van der Waals forces, fills the interlayer gaps, and forms a dense and well-adhering thin film on the surface of the metal substrate.

[0016] Further, S3 is specifically: place the intermediate obtained in step S2 as an electrode sheet connected to the negative pole of the DC power supply into the composite electrolyte solution, react at -1.5 - -0.6 V vs. SCE for 10 - 30 min, and finally place it in an inert gas and anneal at 400 - 500 °C for 30 min. The residual electrolyte and moisture in the surface thin film are decomposed and volatilized and removed during the annealing process to obtain the nitrogen-doped graphene thin film material.

[0017] Furthermore, the composite electrolyte is obtained by dissolving a nitrogen-containing precursor and an electrolyte salt in water at a mass ratio of 1-5:0.6-1; the nitrogen-containing precursor is urea, ammonium nitrate or ammonium chloride; the electrolyte salt is potassium nitrate, sodium nitrate or potassium sulfate.

[0018] The nitrogen-containing precursor in the composite electrolyte decomposes under the action of an electric field, and the generated active nitrogen source binds to the GO defect sites to achieve nitrogen atom doping; at the same time, the electrochemical reduction reaction removes the oxygen-containing groups in GO and partially restores the sp 2 conjugated structure.

[0019] Furthermore, the inert gas is argon or nitrogen, and the gas flow rate is 200-400 sccm.

[0020] The present invention also discloses a nitrogen-doped graphene thin film material prepared by the above preparation method.

[0021] The present invention also discloses the application of the nitrogen-doped graphene thin film material in the preparation of vacuum electronic devices.

[0022] The beneficial effects of the present invention are as follows:

[0023] 1. By using electrophoresis deposition combined with electrochemical technology, the nitrogen-doped graphene thin film material is efficiently prepared in two steps, which can simply and efficiently realize the reduction of the GO thin film on the metal substrate, as well as the control of the nitrogen doping amount and the graphene structure. The deposited thin film is uniformly distributed and the thickness is controllable, and it adheres well to the metal substrate.

[0024] 2. The δ of the nitrogen-doped graphene thin film material prepared by the method of the present invention is as low as below 1.0. Compared with the metal substrate without the deposited thin film, δ is reduced by 20-50%, and it has good application prospects in the field of vacuum electronic devices that require suppressing secondary electron emission.

[0025] 3. Compared with traditional methods such as CVD, microwave plasma excitation CVD and other high-temperature gas-phase reactions, the electrochemical equipment required by the present invention has lower cost, simpler operation, and is basically not limited by the shape of the substrate metal material. Description of the Drawings

[0026] Figure 1 is the process flow chart of the method of the present invention;

[0027] Figure 2 is the field emission scanning electron micrograph of the nitrogen-doped graphene thin film material prepared in Example 1;

[0028] Figure 3 is the field emission scanning electron microtopography of the nitrogen-doped graphene thin film material prepared in Example 2;

[0029] Figure 4Field emission scanning electron microscopic morphology of the nitrogen-doped graphene thin film material prepared in Example 3;

[0030] Figure 5 Secondary electron emission coefficient (δ) - primary electron energy (E p ) curve test results of the nitrogen-doped graphene thin film material deposited on the surface of oxygen-free copper substrates with different pre-treated oxygen-free copper sheets and different deposition times;

[0031] Figure 6 δ-E of high-purity aluminum sheet and the nitrogen-doped graphene thin film material deposited p curve test results;

[0032] Figure 7 δ-E of stainless steel sheet and the nitrogen-doped graphene thin film material deposited p curve test results. Specific embodiments

[0033] The following describes the specific embodiments of the present invention to facilitate those skilled in the art of the present technology to understand the present invention. For those conditions not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained through commercial purchase. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those ordinary skilled in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

[0034] Example 1

[0035] A preparation method of a nitrogen-doped graphene thin film material, the flow chart is as Figure 1 shown, including the following steps:

[0036] S1. Grind one side of the oxygen-free copper sheet with specifications of 10.0 mm × 10.0 mm × 0.4 mm (purity 99.99%) smooth with 1000# and 2000# sandpapers respectively, then soak it in a nitric acid solution with a concentration of 3 wt% for 10 s, then ultrasonically clean it with deionized water, ethanol and acetone for 2 min in sequence, dry it with nitrogen, finally attach a copper wire to the back of the oxygen-free copper sheet, evenly coat the back of the oxygen-free copper sheet with a peelable protective glue, and leave it to dry for 4 h for standby;

[0037] S2. Electrophoretic deposition of graphene oxide (GO): Disperse 25 mg of GO powder and 0.3 g of polyvinyl alcohol in 50 mL of deionized water, stir for 2 h, dissolve by stirring at 90 °C for 10 min, and finally ultrasonically oscillate for 20 min to obtain a GO dispersion with a concentration of 0.5 mg / mL. Connect the pretreated oxygen-free copper sheet and the wire to the positive electrode of the DC power supply, connect the graphite electrode or stainless steel sheet to the negative electrode of the DC power supply, and place them parallel in the GO dispersion with a distance of 10 mm between the positive and negative electrodes; set the voltage for electrophoretic deposition to 3 V, start the power supply reaction time to 25 s, after the reaction is over, take out the oxygen-free copper sheet, wash it with deionized water, and finally dry it at 80 °C for 2 h to obtain an intermediate with a GO thin film layer deposited on the surface;

[0038] S3. Electrochemical reduction and nitrogen doping of GO: Dissolve 1.5 g of urea and 0.5 g of potassium nitrate in 50 mL of deionized water and stir for 10 min to obtain a composite electrolyte; connect the intermediate as the working electrode to the negative electrode of the electrochemical workstation, connect the metal sheet or graphite electrode as the counter electrode to the positive electrode of the electrochemical workstation, place the saturated calomel electrode as the reference electrode in the composite electrolyte, set the constant potential to -0.6 V vs. SCE, and the reaction time to 10 min; finally, anneal it in an argon atmosphere with a gas flow rate of 200 sccm at 400 °C for 30 min to obtain a nitrogen-doped graphene thin film material (N-rGO@Cu). The field emission scanning electron micrograph of the nitrogen-doped graphene thin film material (N-rGO@Cu) is as Figure 2 shown, and the edges of the graphene sheets with a dispersed distribution can be clearly seen.

[0039] Example 2

[0040] A preparation method of a nitrogen-doped graphene thin film material, the flow chart is as Figure 1 shown, including the following steps:

[0041] S1. Polish one side of an oxygen-free copper sheet with specifications of 10.0 mm × 10.0 mm × 0.4 mm (purity 99.99%) with 1000# and 2000# sandpapers respectively, then immerse it in a 3 wt% nitric acid solution for 10 s, then ultrasonically clean it with deionized water, ethanol, and acetone for 2 min in sequence, dry it with nitrogen, and finally attach a copper wire to the back of the oxygen-free copper sheet, evenly coat the back of the oxygen-free copper sheet with a peelable protective glue, and leave it to dry for 4 h for standby;

[0042] S2. Electrophoretic deposition of graphene oxide (GO): Disperse 25 mg of GO powder and 0.4 g of polyvinyl alcohol in 50 mL of deionized water and stir for 2 h. Stir and dissolve at 90 °C for 20 min, and finally ultrasonically oscillate for 20 min to obtain a GO dispersion with a concentration of 0.5 mg / mL. Connect the pretreated oxygen-free copper sheet and the wire to the positive electrode of the DC power supply, and connect the graphite electrode or stainless steel sheet to the negative electrode of the DC power supply. Place them parallel in the GO dispersion with a distance of 10 mm between the positive and negative electrodes. Set the voltage of electrophoretic deposition to 3 V, start the power supply reaction for 25 s. After the reaction, take out the oxygen-free copper sheet, wash it with deionized water, and finally dry it at 80 °C for 4 h to obtain an intermediate with a GO thin film layer deposited on the surface;

[0043] S3. Electrochemical reduction and nitrogen doping of GO: Dissolve 1.5 g of urea and 0.5 g of potassium nitrate in 50 mL of deionized water and stir for 10 min to obtain a composite electrolyte. Connect the intermediate as the working electrode to the negative electrode of the electrochemical workstation, and connect the metal sheet or graphite electrode as the counter electrode to the positive electrode of the electrochemical workstation. Place the saturated calomel electrode as the reference electrode into the composite electrolyte, set the constant potential to -0.8 V vs. SCE, and the reaction time to 20 min. Finally, anneal it in an argon atmosphere with a gas flow rate of 200 sccm at 400 °C for 30 min to obtain a nitrogen-doped graphene thin film material (N-rGO@Cu). The field emission scanning electron micrograph of the nitrogen-doped graphene thin film material (N-rGO@Cu) is as Figure 3 shown, and the edges of the graphene sheets distributed diffusely can be clearly seen.

[0044] Example 3

[0045] A preparation method of a nitrogen-doped graphene thin film material, the flow chart is as Figure 1 shown, including the following steps:

[0046] S1. Polish one side of an oxygen-free copper sheet with specifications of 10.0 mm × 10.0 mm × 0.4 mm (purity 99.99%) with 1000# and 2000# sandpapers respectively, then immerse it in a 3 wt% nitric acid solution for 10 s, and then ultrasonically clean it with deionized water, ethanol, and acetone for 2 min respectively, dry it with nitrogen, and finally attach a copper wire to the back of the oxygen-free copper sheet, evenly coat the back of the oxygen-free copper sheet with a peelable protective glue, and leave it to dry for 4 h for standby;

[0047] S2. Electrophoretic deposition of graphene oxide (GO): Disperse 50 mg of GO powder and 0.5 g of polyvinyl alcohol in 50 mL of deionized water and stir for 2 h. Stir and dissolve at 90 °C for 30 min, and finally ultrasonically oscillate for 40 min to obtain a GO dispersion with a concentration of 1.0 mg / mL. Connect the pretreated oxygen-free copper sheet and wire to the positive electrode of the DC power supply, and connect the graphite electrode or stainless steel sheet to the negative electrode of the DC power supply. Place them parallel in the GO dispersion with a distance of 10 mm between the positive and negative electrodes. Set the voltage for electrophoretic deposition to 3 V and the reaction time of starting the power supply to 25 s. After the reaction, take out the oxygen-free copper sheet, wash it with deionized water, and finally dry it at 80 °C for 6 h to obtain an intermediate with a GO thin film layer deposited on the surface;

[0048] S3. Electrochemical reduction and nitrogen doping of GO: Dissolve 1.5 g of urea and 0.5 g of potassium nitrate in 50 mL of deionized water and stir for 10 min to obtain a composite electrolyte. Connect the intermediate as the working electrode to the negative electrode of the electrochemical workstation, connect the metal sheet or graphite electrode as the counter electrode to the positive electrode of the electrochemical workstation, and place the saturated calomel electrode as the reference electrode in the composite electrolyte. Set the constant potential to -1.3 V vs. SCE and the reaction time to 30 min. Finally, anneal it in an argon atmosphere with a gas flow rate of 200 sccm at 400 °C for 30 min to obtain a nitrogen-doped graphene thin film material (N-rGO@Cu). The field emission scanning electron micrograph of the nitrogen-doped graphene thin film material (N-rGO@Cu) is as Figure 4 shown, and the edges of the graphene sheets with diffuse distribution can be clearly seen.

[0049] Example 4

[0050] A preparation method of a nitrogen-doped graphene thin film material, the flow chart is as Figure 1 shown, including the following steps:

[0051] S1. Polish one side of a high-purity aluminum sheet with a specification of 10.0 mm × 10.0 mm × 0.4 mm (purity 99.99%) with 1000# and 2000# sandpapers respectively, then immerse it in a sulfuric acid solution with a concentration of 4 wt% for 20 s, and then ultrasonically clean it with deionized water, ethanol and acetone for 2 min respectively, dry it with nitrogen, and finally stick a copper wire on the back of the aluminum sheet, evenly coat the back of the aluminum sheet with a peelable protective glue, and leave it to dry for 4 h for standby;

[0052] S2. Electrophoretic deposition of graphene oxide (GO): 25 mg of GO powder and 0.2 g of polyacrylic acid were dispersed in 50 mL of deionized water and stirred for 3 h, dissolved by stirring at 80 °C for 30 min, and finally ultrasonically oscillated for 40 min to obtain a GO dispersion with a concentration of 1 mg / mL. The pretreated aluminum sheet and wire were connected to the positive pole of the DC power supply, and the graphite or stainless-steel sheet was connected to the negative pole of the DC power supply. They were placed parallel in the GO dispersion with a distance of 10 mm between the positive and negative poles. The voltage for electrophoretic deposition was set at 6 V, and the reaction time after starting the power supply was 5 s. After the reaction, the aluminum sheet was taken out, washed with deionized water, and finally dried at 80 °C for 8 h to obtain an intermediate with a GO thin film layer deposited on the surface of the aluminum sheet;

[0053] S3. Electrochemical reduction and nitrogen doping of GO: 1 g of ammonium nitrate and 0.5 g of potassium sulfate were dissolved in 50 mL of deionized water and stirred for 10 min to obtain a composite electrolyte. The intermediate was used as the working electrode and connected to the negative pole of the electrochemical workstation, and the metal or graphite electrode was used as the counter electrode and connected to the positive pole of the electrochemical workstation. The Ag / AgCl electrode was used as the reference electrode and placed in the composite electrolyte. The constant potential was set at -1.5 V vs. SCE, and the reaction time was 10 min. Finally, it was annealed in an argon atmosphere with a gas flow rate of 400 sccm at 450 °C for 30 min to obtain a nitrogen-doped graphene thin film material (N-rGO / Al).

[0054] Example 5

[0055] A preparation method of a nitrogen-doped graphene thin film material, the flow chart is as Figure 1 shown, including the following steps:

[0056] S1. A stainless-steel sheet with specifications of 10.0 mm × 10.0 mm × 0.4 mm was polished smoothly on one side with 1000# and 2000# sandpapers respectively, then immersed in a 5 wt% nitric acid solution for 60 s, and then ultrasonically cleaned with deionized water, ethanol, and acetone for 2 min in sequence, dried with nitrogen, and finally a copper wire was attached to the back of the stainless-steel sheet, and a peelable protective glue was evenly applied to the back of the stainless-steel sheet and left to dry for 4 h for standby;

[0057] S2. Electrophoretic deposition of graphene oxide GO: Disperse 50 mg of GO powder and 0.5 g of polyvinyl alcohol in 50 mL of deionized water and stir for 3 h, stir and dissolve at 80 °C for 20 min, and finally ultrasonically oscillate for 30 min to obtain a GO dispersion with a concentration of 1 mg / mL. Connect the pretreated stainless steel sheet and wire to the positive pole of the DC power supply, and connect the graphite electrode or stainless steel sheet to the negative pole of the DC power supply. Place them parallel in the GO dispersion with a distance of 10 mm between the positive and negative poles; set the voltage of electrophoretic deposition to 3 V, start the power supply reaction time to 60 s, take out the stainless steel sheet after the reaction and wash it with deionized water, and finally dry it at 80 °C for 6 h to obtain an intermediate with a GO thin film layer deposited on the surface of the stainless steel sheet;

[0058] S3. Electrochemical reduction and nitrogen doping of GO: Dissolve 1.5 g of ammonium chloride and 0.5 g of sodium nitrate in 50 mL of deionized water and stir for 10 min to obtain a composite electrolyte; connect the intermediate as the working electrode to the negative pole of the electrochemical workstation, connect the metal sheet or graphite electrode as the counter electrode to the positive pole of the electrochemical workstation, place the saturated calomel electrode as the reference electrode in the composite electrolyte, set the constant potential to -1.0 V vs. SCE, and the reaction time to 20 min; finally, anneal it in a nitrogen atmosphere with a gas flow rate of 200 sccm at 500 °C for 30 min to obtain a nitrogen-doped graphene thin film material (N-rGO@Steel).

[0059] Experimental example

[0060] 1. Refer to T / CIE 270-2024 "Test Method for Secondary Electron Emission Coefficient of Metal Materials" to test the secondary electron emission coefficients of the oxygen-free copper sheet pretreated in step S1 and dried with nitrogen and the oxygen-free copper sheet without surface pretreatment in a high-vacuum environment, and its δ-E p curve comparison is as Figure 5 shown. It can be seen from the figure that the δ m of the oxygen-free copper sheet without surface pretreatment is 1.59, while the maximum secondary emission coefficient δ m of the oxygen-free copper sheet after surface pretreatment is 1.26, indicating that there is a thin film oxide layer on the surface of the untreated copper sheet, which increases the δ of copper, and the pretreatment has an inhibitory effect on the secondary electron emission of the material.

[0061] By changing the electrochemical reduction and nitrogen doping time, the effect on the nitrogen content of the thin film was explored. As Figure 5As shown, the electrochemically reduced nitrogen doping time in step S3 of Example 1 was adjusted to 10 min and 30 min respectively, and the nitrogen content in the nitrogen-doped graphene thin film material prepared changed from 5.08% (20 min) to 4.56% (10 min) and 6.75% (30 min). When the nitrogen doping time was extended from 10 min to 30 min, the nitrogen content in the thin film increased from 4.56% to 6.75%, with an increase rate of 48.0%. This indicates that extending the electrochemically reduced time can effectively promote the decomposition of nitrogen-containing precursors and the continuous embedding of active nitrogen species, thereby regulating the nitrogen doping ratio. Compared with the surface-pretreated oxygen-free copper sheet, depositing the nitrogen-doped graphene thin film reduced the δ m of the copper sheet from 1.26 to 1.04 - 1.10, with an average reduction of about 20%.

[0062] 2. Refer to T / CIE 270-2024 "Test Method for Secondary Electron Emission Coefficient of Metal Materials", and test the δ of the pretreated high-purity aluminum sheet and stainless steel sheet obtained in step S1 of Examples 2 and 3 in a high-vacuum environment, and compare them with the finally prepared nitrogen-doped graphene thin film materials (N-rGO@Al and N-rGO@Steel) respectively. Their δ-E p curves are as Figure 6 shown. Compared with the high-purity aluminum sheet only subjected to surface pretreatment, the δ m of the surface-deposited nitrogen-doped graphene thin film material (NrGO / Al) decreased from 2.04 to 1.10, with a decrease rate of 46.1%.

[0063] The δ-E p curves of the polished stainless steel sheet and the deposited nitrogen-doped graphene thin film material are compared as Figure 7 shown. Compared with the stainless steel sheet only subjected to surface pretreatment, the δ m of the deposited nitrogen-doped graphene thin film material (N-rGO@Steel) decreased from 1.57 to 1.14, a decrease of 27.4%.

[0064] The above results all show that the nitrogen-doped graphene thin film material prepared by the method of the present invention has an obvious inhibitory effect on the secondary electron emission of the metal matrix surface. Similar to other graphene and amorphous carbon thin film materials, nitrogen doping treatment has an obvious inhibitory effect on the secondary electron emission coefficient of oxygen-free copper sheets. Without complex equipment and high-temperature conditions, the method is simple and efficient, providing an effective solution for the controllable preparation of high-stability and low-secondary electron emission thin films in vacuum electronic devices.

Claims

1. A method for preparing a nitrogen-doped graphene thin film material, characterized in that: The following steps are involved: S1. Pretreatment of the conductive substrate; S2, depositing a graphene oxide film layer on a conductive substrate, and then washing and drying; S3, nitrogen doping the graphene oxide film layer, and finally annealing to obtain a nitrogen-doped graphene film material.

2. The method for preparing the nitrogen-doped graphene thin film material according to claim 1, characterized in that: The S1 specifically comprises: polishing the conductive substrate, then soaking it in an acid immersion solution for 10 to 60 seconds, and then washing and drying it to obtain a pretreated conductive substrate.

3. The method for preparing the nitrogen-doped graphene thin film material according to claim 2, characterized in that: The acid leaching solution is a nitric acid or sulfuric acid solution with a concentration of 2-5wt%.

4. The method for preparing the nitrogen-doped graphene thin film material according to claim 1, characterized in that: The S2 is specifically as follows: placing the pretreated conductive substrate as an electrode sheet connected to the positive electrode of a DC power supply in a graphene oxide dispersion, depositing it at 3 to 6 V for 5 to 60 seconds, then washing it, and finally drying it at 80° C. for 2 to 8 hours to obtain an intermediate with a graphene oxide thin film layer grown thereon.

5. The method for preparing the nitrogen-doped graphene thin film material according to claim 4, characterized in that: The graphene oxide dispersion is prepared by dissolving graphene oxide powder and a polymer binder in water at a mass ratio of 0.05-0.1:0.5-1, stirring and dissolving at 80-90° C. for 10-30 minutes, and finally ultrasonically oscillating for 20-40 minutes. The polymer binder is polyvinyl alcohol or polyacrylic acid.

6. The method for preparing the nitrogen-doped graphene thin film material according to claim 1, characterized in that: The S3 is specifically as follows: placing the intermediate obtained in step S2 as an electrode sheet connected to the negative electrode of a DC power supply into a composite electrolyte, reacting at -1.5 to -0.6 V vs. SCE for 10 to 30 minutes, and finally placing it in an inert gas and annealing it at 400 to 500° C. for 30 minutes to obtain a nitrogen-doped graphene film material.

7. The method for preparing the nitrogen-doped graphene thin film material according to claim 6, characterized in that: The composite electrolyte is prepared by dissolving a nitrogen-containing precursor and an electrolyte salt in water at a mass ratio of 1-5:0.6-1; the nitrogen-containing precursor is urea, ammonium nitrate or ammonium chloride; and the electrolyte salt is potassium nitrate, sodium nitrate or potassium sulfate.

8. The method for preparing the nitrogen-doped graphene thin film material according to claim 6, characterized in that: The inert gas is argon or nitrogen, and the gas flow rate is 200-400 sccm.

9. A nitrogen-doped graphene film material, characterized in that: The method is prepared by the method according to any one of claims 1 to 8.

10. Use of the nitrogen-doped graphene film material according to claim 9 in preparing vacuum electronic devices.

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