A three-dimensional near-theoretical limit nitrogen-doped graphene and its preparation method

Abstract

This invention discloses a three-dimensional near-theoretical-limit nitrogen-doped graphene and its preparation method, relating to the field of graphene nanomaterials technology. The preparation process involves first mixing melamine and cyanuric acid, reacting with water, centrifuging, and freeze-drying to obtain a melamine cyanurate precursor. Then, the precursor is mixed with 2-benzimidazole ethanol, ground, and calcined to obtain the target carbon material. At a calcination temperature of 1000℃, the nitrogen doping content in the obtained nitrogen-doped graphene material reaches as high as 10.10 at%, close to the theoretical limit of 12.5 at%. The obtained material possesses a rich nanoporous structure, solving the problems of easy collapse of the pore structure during high-temperature calcination and significant discrepancies between the nitrogen doping content and the theoretical limit in traditional nitrogen-doped graphene preparation processes. Furthermore, the preparation process disclosed in this application is simple, uses widely available raw materials, and has low production costs, contributing to expanding the application prospects of such materials.

Description

Technical Field

[0001] This invention belongs to the field of graphene nanomaterials technology, specifically relating to a three-dimensional near-theoretical limit nitrogen-doped graphene and its preparation method. Background Technology

[0002] Nitrogen-doped graphene has attracted widespread attention in many fields such as electronics, biology, and catalysis due to its excellent properties such as high conductivity, specific surface area, mechanical strength, catalytic activity, and biocompatibility. Improving and optimizing the preparation scheme of such materials to further enhance their performance has become one of the current research hotspots.

[0003] Currently, the main methods for preparing nitrogen-doped graphene include chemical vapor deposition (CVD), arc discharge (ADM), plasma treatment, biomass pyrolysis, hydrothermal methods, and high-temperature calcination. While CVD, ADM, and plasma treatment can be used to prepare large sheets of nitrogen-doped graphene, these methods require expensive equipment, have low material yields, and often only achieve nitrogen doping levels of approximately 1 at%. This significantly limits the cost reduction for industrial applications, and the extremely low nitrogen doping content also restricts the improvement of nitrogen-doped graphene material properties. Although biomass pyrolysis and hydrothermal methods can increase the nitrogen doping content of graphene, they suffer from uneven nitrogen doping. In contrast, high-temperature calcination is a low-cost, simple, and highly uniform method for preparing nitrogen-doped graphene, demonstrating great potential for large-scale application.

[0004] Currently, scholars both domestically and internationally have conducted extensive research on the preparation of nitrogen-doped graphene through high-temperature calcination. The high-temperature calcination method mainly involves mixing and calcining different carbon and nitrogen sources to prepare nitrogen-doped graphene. Commonly used carbon sources include graphene oxide or carbon nitride; nitrogen sources are primarily nitrogen-containing organic compounds such as melamine and urea. When the carbon and nitrogen sources are mixed, under high-temperature conditions, the nitrogen source decomposes to produce nitrogen-containing gas, which is then incorporated into the carbon source to form nitrogen-doped graphene. However, the preparation of the carbon source precursor can increase the cost of nitrogen-doped graphene preparation to some extent, and the structural integrity of the carbon source precursor itself can hinder further nitrogen doping, limiting the potential for increasing the nitrogen doping content. Furthermore, while increasing the temperature during calcination increases the graphite nitrogen ratio for better application in specific fields, excessively high calcination temperatures (e.g., reaching 1000℃) cause a sharp decrease in nitrogen doping content, accompanied by the collapse of the material's pore structure, which further limits performance improvement. Therefore, further reducing the cost of preparing nitrogen-doped graphene by high-temperature calcination through process improvement, while ensuring that the resulting product has high specific surface area, high nitrogen doping content, and high graphitic nitrogen content, remains an urgent problem to be solved.

[0005] Given that nitrogen-rich organic compounds have high carbon and nitrogen content, they can serve as both carbon and nitrogen sources. In-situ nitrogen doping can occur by using only nitrogen-rich organic compounds as initial raw materials and carbonizing them during high-temperature calcination. In-situ doping not only simplifies the preparation process and reduces production costs, but it also facilitates higher nitrogen doping content. However, there are currently few reports on such processes. In 2021, Wang et al. used a zinc cyanamide coordination method to directly convert organic compounds such as melamine into nitrogen-rich graphene. The resulting nitrogen-doped graphene had a very high nitrogen doping content, reaching 15.01 at% at 900℃ and 5 at% at 1000℃. However, their disclosed preparation method involves an acid leaching step after calcination, which generates a large amount of acid waste liquid, burdening the environment and increasing subsequent treatment costs. Although the nitrogen doping content of the graphene material prepared using this method has reached a relatively high level, the nitrogen doping content in the product obtained at 1000℃ is still far from the theoretical limit (the theoretical limit for nitrogen doping content of nitrogen-doped graphene material prepared at 1000℃ is 12.5 at%).

[0006] To further reduce the cost of preparing nitrogen-doped graphene by high-temperature calcination and increase the nitrogen doping content in the material, developing a new process to directly prepare three-dimensional near-theoretical-limit nitrogen-doped graphene from nitrogen-rich organic matter in one step remains a problem that needs to be solved by those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of high cost, complex operation, easy collapse of pore structure during high-temperature calcination, and nitrogen doping content that is far from the theoretical limit in the traditional nitrogen-doped graphene preparation process. The invention provides a new method for preparing nitrogen-doped graphene, which can directly convert low-cost chemical raw material melamine into the target product in one step, and the obtained product has a nitrogen doping content close to the theoretical limit.

[0008] The technical solution of this invention is: a method for preparing three-dimensional near-theoretical-limit nitrogen-doped graphene, the preparation steps of which are as follows:

[0009] 1) Weigh out melamine and cyanuric acid, mix them, add deionized water, and stir the mixture at room temperature to react;

[0010] 2) Centrifuge to remove the supernatant, and freeze-dry the solid product to obtain the melamine cyanurate precursor;

[0011] 3) Melamine cyanurate precursor was mixed with 2-benzimidazole ethanol, ground and calcined to obtain three-dimensional near-theoretical limit nitrogen-doped graphene.

[0012] Further, in step 1), the mass of melamine is 0.625-5g and the mass of cyanuric acid is 0.645-5.16g.

[0013] Furthermore, in step 1), the stirring reaction time is 12-48 hours.

[0014] Furthermore, in step 2), the centrifugation speed is 10000-15000 rpm and the centrifugation time is 1-10 min.

[0015] Further, in step 3), the mass ratio of melamine cyanurate precursor to 2-benzimidazole ethanol is 1 to 9:1.

[0016] Furthermore, in step 3), during the calcination process: the heating rate is 0.5-5℃ / min, the calcination temperature is 800-1000℃, and the calcination time is 1-3h.

[0017] Nitrogen-doped graphene materials can be prepared using the above method. When the melamine cyanurate precursor is mixed with 2-benzimidazole ethanol and calcined at a calcination temperature of 1000℃, the nitrogen doping content in the carbon material is as high as 10.10 at%, which is close to the theoretical limit of 12.5 at%.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. This application demonstrates that after obtaining the melamine cyanurate precursor, it can be thermally converted into nitrogen-doped graphene material in one step by mixing and calcining it with 2-benzimidazole ethanol. When the calcination temperature is 1000℃, the nitrogen doping content (10.10 at%) in the target carbon material is close to the theoretical limit (12.5 at%). Moreover, the material has a large specific surface area, rich nanoporous structure, and exhibits a three-dimensional spatial structure. This solves the problems of easy collapse of pore structure and large difference between nitrogen doping content and theoretical limit in traditional nitrogen-doped graphene preparation processes, which help to expand the application prospects of such materials.

[0020] 2. This application prepares three-dimensional near-theoretical limit nitrogen-doped graphene by a one-step thermal conversion method. The preparation process is simple and easy to operate. Moreover, the raw materials used are low-cost chemical raw materials melamine and cyanuric acid, which has the advantage of low production cost and is conducive to the expansion of the application of this method.

[0021] 3. The three-dimensional near-theoretical limit nitrogen-doped graphene prepared in this application exhibits excellent performance in activating persulfate to degrade tetracycline. It has important theoretical guiding significance and practical value for the design, preparation and potential large-scale industrial production of nitrogen-doped graphene materials, and provides a new approach for the preparation of high-performance nitrogen-doped carbon materials. Attached Figure Description

[0022] Figure 1 A process flow diagram for preparing three-dimensional near-theoretical limit nitrogen-doped graphene;

[0023] Figure 2 The X-ray spectrum of melamine cyanurate prepared in Example 1;

[0024] Figure 3 The infrared spectrum of melamine cyanurate prepared in Example 1;

[0025] Figure 4 The image shown is a transmission electron microscope (TEM) image of the melamine cyanurate prepared in Example 1.

[0026] Figure 5 The isothermal adsorption and pore size distribution curves of melamine cyanurate prepared in Example 1 are shown below.

[0027] Figure 6 The image shows the X-ray diffraction (XRD) spectrum of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1.

[0028] Figure 7 The Raman spectrum of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1;

[0029] Figure 8 X-ray photoelectron spectroscopy of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1;

[0030] Figure 9 Isothermal adsorption curve (pore size distribution curve) of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1;

[0031] Figure 10 The image shows a scanning electron microscope (SEM) image of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1.

[0032] Figure 11 This is a transmission electron microscope (TEM) image of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1;

[0033] Figure 12 The image shows a nitrogen elemental scanning image of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1.

[0034] Figure 13 Here is a scanning electron microscope image of the carbon material prepared in Comparative Example 1;

[0035] Figure 14 Isothermal adsorption and pore size distribution curves of the carbon material prepared in Comparative Example 1;

[0036] Figure 15Here is a scanning electron microscope image of the nitrogen-doped carbon material prepared in Comparative Example 2;

[0037] Figure 16 The X-ray photoelectron spectrum of the nitrogen-doped carbon material prepared in Comparative Example 2 is shown below.

[0038] Figure 17 The isothermal adsorption and pore size distribution curves of the nitrogen-doped carbon material prepared in Comparative Example 2 are shown.

[0039] Figure 18 This is a comparison chart showing the efficiency of carbon materials activated by persulfate in removing tetracycline from carbon prepared in Example 1 and Comparative Example 2. Detailed Implementation

[0040] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0041] Example 1: Preparation of three-dimensional near-theoretical limit nitrogen-doped graphene

[0042] See process flow Figure 1 The specific preparation steps are as follows:

[0043] 2.5 g of melamine and 2.58 g of cyanuric acid were weighed and added to a 100 mL beaker. 50 mL of H2O was added to the beaker, and the mixture was stirred at room temperature for 12 h. After the reaction, the mixture was centrifuged at 10,000 rpm for 5 min, and the supernatant was removed to obtain a solid product. The solid product was freeze-dried to obtain approximately 5.0 g of melamine cyanurate. 5.0 g of melamine cyanurate and 1.0 g of 2-benzimidazole ethanol were thoroughly ground in an agate mortar to ensure uniform mixing. The mixture was loaded into a ceramic boat and calcined in a tube furnace under the following conditions: target temperature 1000 °C, heating rate 2 °C / min, and calcination time 2 h. After the reaction, 110 mg of three-dimensional near-theoretical limit nitrogen-doped graphene was obtained.

[0044] Related performance tests

[0045] 1. First, the melamine cyanurate precursor obtained in the intermediate process was characterized by its composition. Figure 2 The X-ray spectrum of the prepared melamine cyanurate is shown in the figure. It can be clearly observed from the figure that there are two characteristic peaks at 10.8° and 28.0°, which belong to the (100) and (002) crystal planes of melamine cyanurate, respectively. This proves that melamine cyanurate was successfully prepared by assembling melamine and cyanuric acid.

[0046] 2. The obtained melamine cyanurate was further characterized by infrared spectroscopy, and the results are shown in the figure. Figure 3 As can be seen from the figure, the vibrational peak of the triazine ring in melamine is from 771 cm⁻¹. -1 The redshift was 815cm -1 At the same location, the C=O vibration peak in cyanuric acid also increased from 1691 cm⁻¹. -1 and 1751cm -1 Redshifted to 1732cm -1 and 1780cm -1 This demonstrates that melamine and cyanuric acid interact to form melamine cyanurate.

[0047] 3. Figure 4 The image shows a transmission electron microscope (TEM) image of melamine cyanurate. The TEM image shows that the obtained melamine cyanurate has a rod-shaped morphology and no obvious porous structure.

[0048] 4. Figure 5 The isothermal adsorption and pore size distribution curves of melamine cyanurate are shown. The specific surface area of ​​the obtained melamine cyanurate is 38.53 m². 2 / g, pore volume is 0.1156m³ 3 / g proves that the material is a bulk material with no porous structure.

[0049] 5. First, the phase structure of the finally prepared nitrogen-doped graphene material was characterized using XRD, and the results are as follows: Figure 6 As shown, observations revealed that the obtained material has a characteristic peak at approximately 26.22° of 2θ, which is close to the standard peak of carbon material at 2θ = 26.5°, proving that the material may be carbon material.

[0050] 6. To further confirm that the obtained material is a carbon material, Raman spectroscopy was used to characterize it, and the results are as follows: Figure 7 As shown in the figure, the characteristic peaks belonging to the D and G bands of carbon materials can be clearly observed, which further confirms that the prepared material is a carbon material.

[0051] 7. X-ray photoelectron spectroscopy of the final material obtained ( Figure 8 The results show that the nitrogen doping content of this material is as high as 10.10 at%, which is very close to the theoretical limit of 12.5 at% (prepared at 1000℃).

[0052] 8. The isothermal adsorption and pore size distribution curves of the nitrogen-doped graphene obtained in Example 1 are shown below. Figure 9 As shown in the figure, the specific surface area of ​​this material is 322.83 m². 2 / g, pore volume is 1.8694m³ 2 / g indicates that the material has a large specific surface area and pore structure, and there is no phenomenon of pore collapse.

[0053] 9. Figure 10 The image shows a scanning electron microscope image of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1. As can be seen from the image, the obtained carbon material exhibits a curled silk-like structure, which indicates the successful preparation of the graphene sheet structure.

[0054] 10. TEM characterization Figure 11 The results confirmed that the obtained carbon material was graphene, and that it possessed a three-dimensional porous structure. High-resolution transmission electron microscopy (HRTEM) images revealed that the interplanar spacing of this material was 0.43 nm, which is likely due to the high nitrogen doping content, resulting in an interplanar spacing greater than that of standard graphite (0.35 nm).

[0055] 11. Scanning transmission electron microscopy (STEM) images of the obtained materials and their corresponding elemental scans. Figure 12 It can be observed that nitrogen is uniformly distributed on the surface of the material, indicating successful surface doping with nitrogen.

[0056] Comparative Example 1: A carbon material was prepared by mechanically mixing melamine and cyanuric acid, then mixing with 2-benzimidazole ethanol and calcining.

[0057] 2.5g of melamine and 2.58g of cyanuric acid were weighed and ground thoroughly in an agate mortar until they were evenly mixed. Then, 1.0g of 2-benzimidazole ethanol was added to the mixture and ground again until it was evenly mixed. The mixed powder was loaded into a ceramic boat and placed in a tube furnace for calcination. The calcination conditions were set as follows: target temperature 1000℃, heating rate 2℃ / min, and calcination time 2 hours. After the reaction was completed, the target carbon material was obtained.

[0058] Related performance tests

[0059] 1. Scanning electron microscope images of the carbon material obtained in Comparative Example 1 are shown below. Figure 13 As shown, melamine and cyanuric acid were mechanically mixed rather than assembled and then calcined with 2-benzimidazole ethanol. The resulting material exhibited a blocky structure, which was significantly different from the silk-like graphene structure prepared in Example 1.

[0060] 2. The isothermal adsorption and pore size distribution curves of the carbon material prepared in Comparative Example 1 are shown below. Figure 14 As shown, the obtained material has a very small specific surface area and pore volume, only 25.83 m², respectively. 2 / g and 0.1407m 3 / g proves that the material is a bulk material with a non-porous structure.

[0061] Comparative Example 2: Melamine was directly mixed with 2-benzimidazole ethanol and calcined to generate nitrogen-doped carbon materials.

[0062] Weigh 5g of melamine and 1.0g of 2-benzimidazole ethanol and grind them thoroughly in an agate mortar until they are evenly mixed. Load the mixture into a ceramic boat and calcine it in a tube furnace. The calcination conditions are set as follows: target temperature 1000℃, heating rate 2℃ / min, and calcination time 2 hours. After the reaction is completed, nitrogen-doped carbon material can be obtained.

[0063] Related performance tests

[0064] Figure 15 The image shows a scanning electron microscope (SEM) image of the nitrogen-doped carbon material obtained in Comparative Example 2. As can be seen from the image, the carbon material obtained by directly mixing melamine with 2-benzimidazole ethanol and calcining exhibits a small blocky structure, which is completely different from the morphology of the silk-like graphene prepared in Example 1.

[0065] Figure 16 The X-ray photoelectron spectroscopy of the nitrogen-doped carbon material prepared in this comparative example shows that the nitrogen doping content of the carbon material obtained by directly mixing and calcining melamine with 2-benzimidazole ethanol is 7.7 at%, which is also less than the nitrogen content of the nitrogen-doped graphene material prepared using melamine cyanurate precursor.

[0066] Figure 17 The isothermal adsorption and pore size distribution curves of the nitrogen-doped carbon material prepared in Comparative Example 2 are shown. The obtained material has a very small specific surface area and pore volume, only 60.21 m², respectively. 2 / g and 0.3210m 3 / g proves that the material is a bulk material with a non-porous structure.

[0067] Comparing the test data of Example 1, Comparative Example 1, and Comparative Example 2, it can be seen that preparing melamine cyanurate precursor from melamine and cyanuric acid and then calcining it with 2-benzimidazole ethanol to obtain nitrogen-doped carbon material has a significant effect on improving the material's performance. It not only increases the specific surface area of ​​the target material but also increases the nitrogen doping content, making it closer to the three-dimensional near-theoretical limit, which will help broaden the application prospects of such carbon materials.

[0068] Application examples

[0069] The carbon materials prepared in Comparative Example 2 and Example 1 were used to conduct experiments on the removal of organic pollutants from water.

[0070] 1) Weigh 40 mg of tetracycline into a beaker, dissolve it in deionized water, transfer it to a 2 L volumetric flask, and finally dilute to volume to obtain a 20 mg / L tetracycline solution.

[0071] 2) Weigh 10 mg of the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1 and 10 mg of the nitrogen-doped carbon material prepared in Comparative Example 2 into two different 200 mL beakers, and then add the tetracycline solution prepared in step 1) to each beaker. Stir for 5 min to reach adsorption equilibrium.

[0072] 3) Weigh 24 mg of potassium persulfate and dissolve it in tetracycline solution to initiate the degradation reaction. Keep stirring continuously and start timing from the moment of addition. Every once in a while, use a syringe to draw 3 mL of the reaction solution and filter it with a 0.22 μm filter to remove the catalyst. Measure the concentration of the remaining tetracycline in the filtrate by ultraviolet spectroscopy.

[0073] In addition, the results of degradation experiments with only persulfate added were used as a control.

[0074] Degradation results as follows Figure 18 As shown in the figure, only about 10% of the tetracycline was removed from the system after 30 minutes when persulfate was added alone. When both the nitrogen-doped carbon material prepared in Comparative Example 2 and persulfate were added simultaneously, about 25% of the tetracycline was removed after 30 minutes. However, when both the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1 and persulfate were added to the system, the amount of tetracycline removed after 30 minutes reached as high as 83%. This demonstrates that the three-dimensional near-theoretical limit nitrogen-doped graphene prepared in Example 1 can efficiently activate persulfate to degrade organic matter.

[0075] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing three-dimensional near-theoretical-limit nitrogen-doped graphene, characterized in that, The preparation steps are as follows: 1) Weigh out melamine and cyanuric acid, mix them, add deionized water, and stir the mixture at room temperature to react; 2) Centrifuge to remove the supernatant, and freeze-dry the solid product to obtain the melamine cyanurate precursor; 3) Melamine cyanurate precursor and 2-benzimidazole ethanol were mixed at a mass ratio of 1 to 9:1, ground and calcined to obtain three-dimensional near-theoretical limit nitrogen-doped graphene. During calcination: the heating rate is 0.5-5℃ / min, the calcination temperature is 800-1000℃, and the calcination time is 1-3 h.

2. The method for preparing three-dimensional near-theoretical-limit nitrogen-doped graphene as described in claim 1, characterized in that, In step 1), the mass of melamine is 0.625-5 g, and the mass of cyanuric acid is 0.645-5.16 g.

3. The method for preparing three-dimensional near-theoretical-limit nitrogen-doped graphene as described in claim 1, characterized in that, In step 1), the stirring reaction time is 12-48 h.

4. The method for preparing three-dimensional near-theoretical-limit nitrogen-doped graphene as described in claim 1, characterized in that, In step 2), the centrifugation speed is 10000-15000 rpm and the centrifugation time is 1-10 min.

5. A nitrogen-doped graphene, characterized in that, It is prepared by the method for preparing three-dimensional near-theoretical limit nitrogen-doped graphene according to any one of claims 1-4.

6. The nitrogen-doped graphene as described in claim 5, characterized in that, The carbon material prepared by calcining a mixture of melamine cyanurate precursor and 2-benzimidazole ethanol at 1000°C has a nitrogen content as high as 10.10 at.

7. The application of nitrogen-doped graphene as described in claim 6 in the activated persulfate degradation of tetracycline, characterized in that, The nitrogen-doped graphene enables tetracycline to be degraded by 83% within 30 minutes.

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