Preparation process of nitrogen-doped carbon film for modifying surface modification of substrate

Abstract

The invention relates to the technical field of film preparation, in particular to a preparation process of a nitrogen-doped carbon film for modifying the surface of a substrate, which comprises the following preparation steps: S1, pretreating the substrate; s2, composite etching treatment; s3, dynamic surface modification; s4, performing magnetron co-sputtering deposition; and S5, gradient annealing treatment. By introducing the electroactive monomer and adjusting the substrate bias voltage in the deposition process and the annealing temperature after deposition, the chemical bond state of N is controlled, the ORR catalytic activity is optimized, it is proved that the ORR catalytic activity of the nitrogen-doped carbon film directly depends on the content of pyridine N in the film, and it is indicated that pyridine N is the effective catalytic active site of ORR.

Description

Technical Field

[0001] This invention relates to the field of thin film preparation technology, specifically to a process for preparing nitrogen-doped carbon thin films by modifying the substrate surface. Background Technology

[0002] The ORR (Oxygen Reduction Reaction) occurring at the cathode of a fuel cell has a significant impact on the actual operating efficiency of the fuel cell. Platinum-based catalysts, which exhibit high catalytic activity and stability in both acidic and alkaline solutions, are typically used to accelerate the ORR. However, due to platinum's high cost, limited production, and susceptibility to methanol poisoning, fuel cells have not yet achieved widespread commercialization. In recent years, nitrogen-doped carbon-based catalysts (such as nitrogen-doped carbon nanotubes and graphene) have attracted considerable interest because nitrogen doping can effectively modulate the electrical properties of carbon-based nanomaterials, thereby achieving higher catalytic performance and potentially replacing platinum-based catalysts. However, in currently reported methods for preparing nitrogen-doped carbon-based catalysts, transition metals (such as Fe) are typically added as catalysts for growing carbon-based nanomaterials, and then dissolved and removed with an acidic solution. This makes it difficult to determine whether the catalytic activity originates from the nitrogen-doped carbon material itself or from some metal residue. Furthermore, during the removal of the metal with an acidic solution, it is speculated that the pyridine N at the catalytically active site can be protonated to an inactive pyridine N-hydrogen, thereby reducing the material's catalytic activity.

[0003] Therefore, developing a completely metal-free method for preparing nitrogen-doped carbon-based catalysts has become an urgent problem to be solved. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a process for preparing nitrogen-doped carbon thin films by modifying the substrate surface.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A process for preparing nitrogen-doped carbon thin films by modifying the substrate surface includes the following preparation steps: S1. Substrate pretreatment: Clean the substrate material to remove surface impurities and obtain the pretreated substrate; S2. Composite etching treatment: The pretreated substrate is etched by coating with modified etching solution for 1-4 minutes, followed by ultrasonic cleaning with ultrapure water for 20-30 minutes to obtain the etched substrate. S3. Dynamic surface modification: The etched substrate is immersed in an electroactive monomer base solution for surface modification for 3-8 minutes, and then dried by gradient heating to obtain the surface-modified substrate material; S4. Magnetron co-sputtering deposition: At a pressure of 5 × 10⁻⁶ in the vacuum chamber. -4 After Pa, ionized gas Ar, reactant gas N2 and CH4 are introduced into it, and auxiliary gas He is introduced. Nitrogen-doped carbon thin film is deposited on the substrate material using magnetron sputtering process to obtain the substrate material on which nitrogen-doped carbon thin film is deposited. S5. Gradient Annealing Treatment: The substrate material of the nitrogen-doped carbon thin film obtained in step S4 is placed in a high-temperature vacuum annealing furnace, where the pressure in the vacuum chamber reaches 5 × 10⁻⁶. -4 After Pa, a protective gas Ar is introduced, and annealing is carried out at a segmented heating rate to obtain a nitrogen-doped carbon film with modified substrate surface. The mass ratio of the surface-modifying agent to the electroactive monomer base solution in the mixture is 3-5:1; The preparation of the surface modification agent includes the following steps: S41. Dissolve 2-4 parts of polyvinylpyrrolidone and 1-3 parts of 3-aminopropyltriethoxysilane in 50-60 parts of deionized water and stir at 400-500 r / min for 8-10 min. S42. Add 5-8 parts of pyridine-3-carboxylic acid to the solution obtained in step S41, and stir in a 60°C water bath for 18-20 minutes until completely dissolved to obtain a preliminary surface modification solution; S43. Add 0.1-0.3 parts of ammonium persulfate to the preliminary surface modification solution, and sonicate at 200W for 12-15 minutes under nitrogen protection to obtain the surface modification agent.

[0006] Preferably, the substrate material is one of monocrystalline silicon or polycrystalline titanium.

[0007] Preferably, in step S1, when the substrate material is monocrystalline silicon, the cleaning method is as follows: ultrasonically clean it with acetone, ethanol and ultrapure water for 20-30 minutes in sequence to remove the stains on its surface. When polycrystalline titanium is selected as the substrate material, the cleaning method is as follows: boil it in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 for 30-40 minutes to remove the oxides on its surface, and then ultrasonically clean it with ultrapure water for 20-30 minutes.

[0008] Preferably, the preparation of the electroactive monomer base solution includes the following steps: S31. Dissolve 10-13 parts of acrylic acid and 8-11 parts of acrylamide in deionized water, stir at 400-500 r / min until completely dissolved and control the pH to 6.5-7.0 to obtain a transparent solution; S32. Dissolve 0.1-0.5 parts of graphene quantum dots in deionized water and sonicate at 200-250W power for 20-30 minutes until no visible agglomeration is observed to obtain a dispersion. S33. Under a nitrogen atmosphere, add 5-8 parts of vinyl isocyanate and the dispersion prepared in step S32 to a transparent solution, and stir at 700-800 r / min for 20-30 min to finally obtain the electroactive monomer base solution.

[0009] Preferably, the flow rates of the ionized gas Ar, the reactant gases N2 and CH4, and the auxiliary gas He introduced in step S4 are 30-35 sccm, 20-25 sccm, 5-10 sccm, and 5-8 sccm, respectively.

[0010] Preferably, the magnetron sputtering process parameters in step S4 are: RF power of 160-170W; substrate bias of -12.3V to 200V; sputtering time of 40-50min; and sputtering pressure of 0.5-1Pa.

[0011] Preferably, the sputtering target used in the magnetron sputtering process in step S4 is a high-purity carbon target with a diameter of 6 cm and a thickness of 1 mm and a purity of 99.95%.

[0012] Preferably, in step S5, the initial stage heating rate of the segmented heating rate for annealing is 5℃ / min to 600℃, the second stage heating rate is 10℃ / min to 800-1000℃, and after holding at that temperature for 10-15 minutes, the furnace is slowly cooled to room temperature.

[0013] Preferably, the preparation of the modified etching solution includes the following steps: S21. Add deionized water to a container, slowly add nitric acid, and stir until well mixed to obtain a nitric acid solution; S22. Gradually add hydrofluoric acid while stirring continuously, controlling the temperature not to exceed 40°C, to obtain an acid mixture; S23. Add the modified solution to the acid mixture obtained in step S22, and stir continuously for 15-20 minutes to finally obtain the modified etching solution; The mass ratio of nitric acid, hydrofluoric acid, and the modified solution is 3:1:0.5-1.

[0014] The preparation of the modified liquid includes the following steps: S231. Add 1-3 parts of citric acid to an appropriate amount of deionized water and stir until completely dissolved to obtain a citric acid solution; S232. Add 0.5-0.8 parts of urea to a citric acid solution and continue stirring until completely dissolved to obtain a mixture; S233. Add 7-10 parts of polyethylene glycol and 0.3-0.6 parts of sodium dodecyl sulfate to the mixture, and stir for 10-20 minutes in a water bath at 40°C to obtain the modified solution.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention controls the chemical bond state of N through the synergistic effect of surface modification agents and electroactive monomer base solutions, thereby optimizing the ORR catalytic activity. It proves that the catalytic activity of nitrogen-doped carbon films for ORR directly depends on the content of pyridine N in the film, indicating that pyridine N is an effective catalytic active site for ORR.

[0016] 2. The nitrogen-doped carbon film with the highest pyridine N content not only exhibits the best ORR catalytic activity, but also demonstrates better stability and methanol resistance in alkaline solutions than commercially available Pt / C catalysts. This metal-free, pyridine N-rich amorphous nitrogen-doped carbon film catalyst holds promise as a replacement for platinum-based catalysts as cathode catalysts in direct methanol fuel cells and alkaline fuel cells, showing significant application value.

[0017] 3. In this invention, the carboxylic acid groups in the modified etching solution can form hydrogen bonds or chemical bonds with subsequent modified electroactive monomers (such as acrylic acid and vinyl isocyanate). The two work together to enhance the adsorption capacity of the monomers on the substrate surface. At the same time, the carboxylic acid groups form a hydrogen bond network with the amino groups (NH2⁻) produced by the decomposition of urea, which enhances the stability of the etching solution. Citric acid complexes metal ions, preventing impurities from interfering with the subsequent magnetron sputtering process. Attached Figure Description

[0018] Figure 1 This is a process flow diagram for preparing nitrogen-doped carbon thin films with modified substrate surfaces according to the present invention; Figure 2 This is a flow chart of the preparation process of the modified etching solution of the present invention; Figure 3 This is a flow chart of the preparation process of the modified liquid of the present invention; Figure 4 This is a process flow diagram for preparing the electroactive monomer base solution of the present invention; Figure 5 This is a flow chart illustrating the preparation process of the surface modification agent of the present invention; Figure 6 This is a bar chart showing the pyridine N and pyrrole N content on the surface of nitrogen-doped carbon thin films deposited under different substrate biases in Example 1 of the present invention; Figure 7 The LSV diagrams of the ORR of nitrogen-doped carbon films deposited under different substrate biases in Example 1 of the present invention, and the nitrogen-doped carbon films obtained in Comparative Example 1 and Comparative Example 2 are shown. Figure 8The KL diagrams of nitrogen-doped carbon films deposited under different substrate biases in Example 1 of the present invention, and nitrogen-doped carbon films obtained in Comparative Example 1 and Comparative Example 2, are shown at -0.60V. Figure 9 The graph shows the performance test results of anion exchange membrane fuel cells using nitrogen-doped carbon films as cathode catalysts, obtained in Example 1 and Comparative Example 2 of this invention. Figure 10 The chronoamperometry diagrams of the nitrogen-doped carbon thin films obtained in Example 1 and Comparative Example 2 of this invention are shown (wherein, the electrolyte used is a 0.1 MkOH solution saturated with O2). Figure 11 The chronoamperometry (COP) of nitrogen-doped carbon films obtained in Example 1 and Comparative Example 2 of this invention is shown in the figure (wherein, the electrolyte used is 0.1 M KOH, saturated with N2 for 0-1000s, saturated with O2 for 1000-2000s, and saturated with O2 and injected with 3 M CH3OH for 2000-3000s). Detailed Implementation

[0019] The present invention will now be clearly and completely described in conjunction with embodiments thereof. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0020] Please see Figure 1-11 The present invention provides a technical solution: Example 1 A process for preparing nitrogen-doped carbon thin films by modifying the substrate surface: S1. Substrate pretreatment: Before using the polycrystalline titanium substrate material, it is cleaned by boiling it in a hydrochloric acid solution of 10ml hydrogen chloride and 30ml water for 30 minutes to remove the oxides on its surface. Then, it is ultrasonically cleaned with ultrapure water for 20 minutes to obtain the pretreated substrate. Before performing the composite etching process, a modified etching solution is prepared, which includes the following steps: S21. Add deionized water to a container, slowly add 30g of nitric acid, stir and mix well to obtain a nitric acid solution; S22. Gradually add 10g of hydrofluoric acid while stirring continuously, controlling the temperature not to exceed 40℃, to obtain an acid mixture; S23. Add 5g of modified solution to the acid mixture obtained in step S22, and stir continuously for 15min to finally obtain the modified etching solution; The preparation of the modified liquid includes the following steps: S231. Add 10g of citric acid to an appropriate amount of deionized water and stir until completely dissolved to obtain a citric acid solution; S232. Add 5g of urea to a citric acid solution and continue stirring until completely dissolved to obtain a mixture; S233. Add 70g of polyethylene glycol and 3g of sodium dodecyl sulfate to the mixture and stir for 10min in a water bath at 40℃ to obtain the modified solution; S2. Composite etching treatment: Take 30g of the modified etching solution prepared above and etch the pretreated substrate by coating. The etching time is 1min. Then, perform ultrasonic cleaning with ultrapure water for 20min to obtain the etched substrate. Before performing dynamic surface modification, the preparation of the electroactive monomer base solution and the surface modification agent includes the following steps: S31. Dissolve 100g of acrylic acid and 80g of acrylamide in deionized water, stir at 400r / min until completely dissolved and control the pH to 6.5 to obtain a transparent solution; S32. Dissolve 1-5g of graphene quantum dots in deionized water and sonicate at 200W power for 20min until no visible agglomeration is observed to obtain a dispersion. S33. Under a nitrogen atmosphere, 50g of vinyl isocyanate and the dispersion prepared in step S32 are added to a transparent solution and stirred at 700r / min for 20min to finally obtain an electroactive monomer base solution. The preparation of surface modification agents includes the following steps: S41. Dissolve 8g of polyvinylpyrrolidone and 4g of 3-aminopropyltriethoxysilane in 200ml of deionized water and stir at 400r / min for 8min. S42. Add 20g of pyridine-3-carboxylic acid to the solution obtained in step S41, and stir in a 60℃ water bath for 18min until completely dissolved to obtain a preliminary surface modification solution; S43. Add 0.4g of ammonium persulfate to the preliminary surface modification solution, and sonicate at 200W for 12min under nitrogen protection to obtain the surface modification modifier. S3. Dynamic surface modification: The etched substrate is immersed in a mixture of 40g of electroactive monomer base solution and 120g of surface modification agent prepared above for surface modification. The immersion time is 3min, and then the substrate is dried by gradient heating to obtain the surface-modified substrate material. S4. Magnetron co-sputtering deposition: At a pressure of 5 × 10⁻⁶ in the vacuum chamber. -4After Pa, ionized gas Ar, reactant gases N2 and CH4 are introduced into the substrate, along with auxiliary gas He (flow ratio Ar:N2:CH4:He = 30:20:5:5 sccm). A nitrogen-doped carbon thin film is deposited on the substrate material using magnetron sputtering. The resulting substrate material with the deposited nitrogen-doped carbon thin film has an RF power of 160 W, a substrate bias of -12.3 V, a sputtering time of 40 min, and a sputtering pressure of 0.5 Pa. S5. Gradient Annealing Treatment: The substrate material of the nitrogen-doped carbon thin film obtained in step S4 is placed in a high-temperature vacuum annealing furnace, where the pressure in the vacuum chamber reaches 5 × 10⁻⁶. -4 After Pa, a protective gas Ar is introduced, and annealing is carried out at a segmented heating rate. The initial heating rate is 5℃ / min to 600℃, and the second heating rate is 10℃ / min to 800℃. After holding at this temperature for 10 min, the furnace is slowly cooled to room temperature to obtain a nitrogen-doped carbon film with modified substrate surface.

[0021] Example 2 The only difference between Example 2 and Example 1 is that in Example 2, the substrate bias voltage is -50V, while the other steps are exactly the same as in Example 1 and Example 2.

[0022] Example 3 The only difference between Example 3 and Example 1 is that in Example 3, the substrate bias voltage is -150V, while the other steps are exactly the same as in Example 1 and Example 3.

[0023] Example 4 The only difference between Example 4 and Example 1 is that in Example 4, the substrate bias voltage is -200V, while the other steps are exactly the same as in Example 1 and Example 4.

[0024] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that, in Comparative Example 1, the addition of surface modification agent and modification liquid is omitted, and the composite etching process is carried out using only nitric acid and hydrofluoric acid. The remaining steps are exactly the same in Comparative Example 1 and Example 1.

[0025] Comparative Example 2 Commercially available Pt / C catalyst was used as Comparative Example 2. The Pt / C catalyst was purchased from Hunan Minsizhuang Technology Co., Ltd.

[0026] Performance testing: The ORR electrocatalytic activity and stability of the thin-film catalyst were characterized using a PARSTAT 2273 electrochemical workstation in a three-electrode electrochemical electrolyzer. The prepared nitrogen-doped carbon thin film with modified substrate surface was used as the cathode catalyst in anion exchange membrane fuel cell. A single cell was fabricated, and the discharge performance of the cell was tested to characterize the catalyst's catalytic activity in the actual working environment of the fuel cell.

[0027] Experiments showed that when the absolute value of the substrate bias voltage in Example 3 increased to 100V, the pyridine N content reached its maximum, at which point the pyridine N content in the film was approximately three times that of the pyrrole N content. Generally, pyridine N and pyrrole N are the most common N types doped in carbon matrices. Because pyridine N has a lone pair of electrons, it can increase electron-donating ability, facilitating the adsorption of O2 molecules and the breaking of OO bonds. Therefore, it is considered to promote a four-electron transfer ORR and exhibits high ORR catalytic activity.

[0028] Appendix Figure 8 The KL diagrams for nitrogen-doped carbon films deposited under different substrate biases in Example 1 of this invention, as well as nitrogen-doped carbon films obtained in Comparative Examples 1 and 2, are shown at -0.60V. According to the KL formula, ω can be plotted at a fixed potential (-0.60V). -1 / 2 and J -1 The relationship curve, i.e., the KL diagram (attached) Figure 8 The electron transfer number and kinetic current density of the catalyst in the ORR process can be calculated from the intercept and slope of the curve in the KL plot. The results are shown in Table 1 below: Table 1. Total N, pyridine N, and pyrrole N content on the surface of nitrogen-doped carbon films deposited under different substrate bias voltages, and the electrocatalytic performance parameters of nitrogen-doped carbon films, pure carbon films, and Pt / C catalysts on ORR.

[0029] As the absolute value of the substrate bias voltage increased from the floating voltage to 100V, the total N content gradually increased from 6.0 at.% to 15.3 at.%. Further increases in the absolute value of the substrate bias voltage led to a decrease in the total N content. This change is because increasing the absolute value of the substrate bias voltage enhances the energy of the bombarding ions during deposition, promoting the doping of nitrogen atoms into the carbon matrix and thus increasing the total N content in the carbon film. However, when the absolute value of the substrate bias voltage continues to increase, excessively high bombarding ion energy can easily induce preferential etching of N atoms deposited on the film surface, i.e., backsputtering, resulting in a decrease in the total N content in the film. Meanwhile, compared to Example 1, Comparative Example 1 without the added modified liquid showed a gap in electrocatalytic performance, indicating that the carboxylic acid groups in the modified etching solution prepared with the modified liquid can form hydrogen bonds or chemical bonds with subsequent modified electroactive monomers (such as acrylic acid and vinyl isocyanate). These two components work together to enhance the adsorption capacity of the monomers on the substrate surface and improve electrocatalytic performance.

[0030] Appendix Figure 6 This is a bar chart showing the pyridine N and pyrrole N content on the surface of nitrogen-doped carbon films deposited under different substrate bias voltages in Example 1 of the present invention. As can be seen from the figure, the pyridine nitrogen content reaches its maximum when the absolute value of the substrate bias voltage increases to 100V. Generally, pyridine nitrogen and pyrrole nitrogen are the most common types of nitrogen doped in carbon matrices. Since pyridine nitrogen has a lone pair of electrons, it can increase electron-donating ability, which is beneficial for adsorbing O2 molecules and breaking OO bonds. Therefore, it is considered to promote a 4-electron transfer ORR and has high ORR catalytic activity.

[0031] Appendix Figure 7 The LSV diagrams for ORR of nitrogen-doped carbon films deposited under different substrate biases in Example 1 of this invention, as well as those obtained in Comparative Examples 1 and 2, are shown. When the absolute value of the substrate bias increases from the floating voltage to 200V, the turn-on potential and limiting current density of the nitrogen-doped carbon film for ORR first increase and then decrease. When the substrate bias is -100V, the nitrogen-doped carbon film exhibits the highest catalytic activity, with a turn-on potential identical to that of the Pt / C catalyst (-0.02V) and a limiting current density of 5.86 mA cm⁻¹. -2 .

[0032] Appendix Figure 9 The figures show the performance test results of anion exchange membrane fuel cells using nitrogen-doped carbon films obtained in Examples 1 and 2 of this invention as cathode catalysts. The performance of the anion exchange membrane fuel cells was tested using the nitrogen-doped carbon films obtained in Examples 1 and 2 as cathode catalysts to evaluate the catalyst's catalytic activity for ORR under actual fuel cell operating conditions. Figure 9As shown, when nitrogen-doped carbon thin films and Pt / C are used as catalysts, the nitrogen-doped carbon thin film catalyst exhibits the same catalytic activity as commercial Pt / C catalysts under actual fuel cell operating conditions.

[0033] Appendix Figure 10 The figures show the chronoamperometry (CMT) curves of the nitrogen-doped carbon thin films obtained in Examples 1 and 2 of this invention for ORR. The test conditions were: electrode potential -0.26 V, electrode rotation speed 1600 rpm; electrolyte was O2-saturated 0.1 M KOH solution. The CMT curves in O2-saturated 0.1 M KOH solution at a fixed electrode potential (-0.26 V) show that during the 18000 s CMT test, the nitrogen-doped carbon thin film catalyst exhibited superior stability compared to commercial Pt / C catalysts.

[0034] Appendix Figure 11 The figures show the chronoamperometry (COP) of the nitrogen-doped carbon films obtained in Examples 1 and 2 of this invention. The test conditions were: electrode potential -0.26V; electrode rotation speed 1600rpm; electrolyte 0.1M KOH, saturated with N2 for 0-1000s, saturated with O2 for 1000-2000s, and saturated with O2 followed by injection of 3M CH3OH for 2000-3000s. The figures show that the nitrogen-doped carbon film exhibits better methanol resistance compared to the Pt / C catalyst.

[0035] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for preparing nitrogen-doped carbon thin films by modifying the substrate surface, characterized in that, The preparation steps include the following: S1. Substrate pretreatment: Clean the substrate material to remove surface impurities and obtain the pretreated substrate; S2. Composite etching treatment: The pretreated substrate is etched by coating with modified etching solution for 1-4 minutes, followed by ultrasonic cleaning with ultrapure water for 20-30 minutes to obtain the etched substrate. S3. Dynamic surface modification: The etched substrate is immersed in a mixture containing a surface modification agent and an electroactive monomer base solution for surface modification. The immersion time is 3-8 minutes. Then, it is dried by gradient heating to obtain the surface-modified substrate material. S4. Magnetron co-sputtering deposition: At a pressure of 5 × 10⁻⁶ in the vacuum chamber. -4 After Pa, ionized gas Ar, reactant gas N2 and CH4 are introduced into it, and auxiliary gas He is introduced. Nitrogen-doped carbon thin film is deposited on the substrate material using magnetron sputtering process to obtain the substrate material on which nitrogen-doped carbon thin film is deposited. S5. Gradient Annealing Treatment: The substrate material of the nitrogen-doped carbon thin film obtained in step S4 is placed in a high-temperature vacuum annealing furnace, where the pressure in the vacuum chamber reaches 5 × 10⁻⁶. -4 After Pa, a protective gas Ar is introduced, and annealing is carried out at a segmented heating rate to obtain a nitrogen-doped carbon film with modified substrate surface. The mass ratio of the surface-modifying agent to the electroactive monomer base solution in the mixture is 3-5:

1. The preparation of the surface modification agent includes the following steps: S41. Dissolve 2-4 parts of polyvinylpyrrolidone and 1-3 parts of 3-aminopropyltriethoxysilane in 50-60 parts of deionized water and stir at 400-500 r / min for 8-10 min. S42. Add 5-8 parts of pyridine-3-carboxylic acid to the solution obtained in step S41, and stir in a 60°C water bath for 18-20 minutes until completely dissolved to obtain a preliminary surface modification solution; S43. Add 0.1-0.3 parts of ammonium persulfate to the preliminary surface modification solution, and sonicate at 200W for 12-15 minutes under nitrogen protection to obtain the surface modification agent.

2. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, The substrate material is either monocrystalline silicon or polycrystalline titanium.

3. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, In step S1, when the substrate material is monocrystalline silicon, the cleaning method is as follows: ultrasonically clean it with acetone, ethanol and ultrapure water for 20-30 minutes in sequence to remove the stains on its surface. When the substrate material is polycrystalline titanium, the cleaning method is as follows: boil it in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 for 30-40 minutes to remove the oxides on its surface, and then ultrasonically clean it with ultrapure water for 20-30 minutes.

4. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, The preparation of the electroactive monomer base solution includes the following steps: S31. Dissolve 10-13 parts of acrylic acid and 8-11 parts of acrylamide in deionized water, stir at 400-500 r / min until completely dissolved and control the pH to 6.5-7.0 to obtain a transparent solution; S32. Dissolve 0.1-0.5 parts of graphene quantum dots in deionized water and sonicate at 200-250W power for 20-30 minutes until no visible agglomeration is observed to obtain a dispersion. S33. Under a nitrogen atmosphere, add 5-8 parts of vinyl isocyanate and the dispersion prepared in step S32 to the transparent solution obtained in S31, and stir at 700-800 r / min for 20-30 min to finally obtain the electroactive monomer base solution.

5. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, The flow rates of the ionized gas Ar, the reactant gases N2 and CH4, and the auxiliary gas He introduced in step S4 are 30-35 sccm, 20-25 sccm, 5-10 sccm, and 5-8 sccm, respectively.

6. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, In step S4, the magnetron sputtering process parameters are: RF power of 160-170W; substrate bias of -12.3V to 200V; sputtering time of 40-50min; and sputtering pressure of 0.5-1Pa.

7. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, In step S4, the sputtering target used in the magnetron sputtering process is a high-purity carbon target with a diameter of 6 cm and a thickness of 1 mm and a purity of 99.95%.

8. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, In step S5, the initial stage of annealing is heated at a rate of 5°C / min to 600°C, and the second stage is heated at a rate of 10°C / min to 800-1000°C. After holding at this temperature for 10-15 minutes, the furnace is slowly cooled to room temperature.

9. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 1, characterized in that, The preparation of the modified etching solution includes the following steps: S21. Add deionized water to a container, slowly add nitric acid, and stir until well mixed to obtain a nitric acid solution; S22. Gradually add hydrofluoric acid while stirring continuously, controlling the temperature not to exceed 40℃, to obtain an acid mixture; S23. Add the modified solution to the acid mixture obtained in step S22, and stir continuously for 15-20 minutes to finally obtain the modified etching solution; The mass ratio of nitric acid, hydrofluoric acid, and the modified liquid is 3:1:0.5-1.

10. The process for preparing nitrogen-doped carbon thin films by modifying the substrate surface according to claim 9, characterized in that, The preparation of the modified liquid includes the following steps: S231. Add 1-3 parts of citric acid to an appropriate amount of deionized water and stir until completely dissolved to obtain a citric acid solution; S232. Add 0.5-0.8 parts of urea to a citric acid solution and continue stirring until completely dissolved to obtain a mixture; S233. Add 7-10 parts of polyethylene glycol and 0.3-0.6 parts of sodium dodecyl sulfate to the mixture, and stir for 10-20 minutes in a water bath at 40°C to obtain the modified solution.

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