RGO-coated MnO composite material and preparation method thereof

Abstract

The invention belongs to the field of new materials, and discloses a preparation method of an RGO-coated MnO composite material, which comprises the following steps: step 1, carrying out surface modification on MnO to obtain modified manganous oxide; 2, under the action of an electric field, graphene oxide and modified manganous oxide are subjected to self-assembly to form a self-assembly material with modified manganous oxide wrapped with graphene oxide; and step 3, reducing the self-assembled material to obtain the graphene coated modified manganous oxide composite material RGO (at) MnO. The RGO-coated MnO composite material obtained by the method disclosed by the invention is good in coating uniformity, thin in coating layer and stable in coating, can effectively prevent manganous oxide from being oxidized, and also can maintain the biological characteristics of the manganous oxide. Meanwhile, the invention further discloses the composite material prepared on the basis of the method.

Description

Technical Field

[0001] The present invention relates to the field of new materials, and specifically to an RGO@MnO composite material and a preparation method thereof. Background Art

[0002] Traditional marine antifouling coatings kill marine organisms by releasing toxic materials such as tin, manganese, mercury, and lead. However, later studies found that organic tin materials can accumulate in various fish, shellfish, and marine plants for a long time, leading to genetic mutations and entering the food chain, causing immeasurable ecological problems. Since then, people have paid more attention to the development of environmentally friendly marine antifouling materials. For example, tin-free self-polishing antifouling materials are one of the most effective antifouling materials in current commercial products, and the antifouling period can reach 3-5 years. The excellent antifouling effect of traditional tin-containing coatings comes from the highly toxic tin-containing groups generated by their self-polishing, and they usually need to be used in combination with a large amount of cuprous oxide (40%-50%) antifouling agents and auxiliary antifouling agents. However, studies have shown that copper ions will accumulate in the ocean, especially in the sea mud of coastal harbors, and have brought serious environmental problems. Therefore, finding a substitute for cuprous oxide materials is the most urgent topic in the field of marine antifouling coatings at present.

[0003] Manganese oxide has strong oxygen catalytic activity, strong capacitance, and nano-confinement characteristics, and has high biological safety and little marine pollution, which is an ideal alternative for the biological antifouling of manganese materials. However, manganese oxide has poor stability and is easily oxidized when exposed to oxygen for a long time.

[0004] A patent application with the publication number CN110964354A and the theme of a preparation method of a graphene-based composite coating proposes to mix a dispersion solution of manganese oxide nanoparticles, a coupling agent, and graphene sheets, and obtain a composite coating of graphene-coated manganese oxide nanoparticles after stirring, filtering, and drying, which solves the problems of short service life, poor corrosion resistance, and poor anti-biofouling adhesion in the preparation of composite coatings in the prior art.

[0005] The above method is realized based on the solution assembly method, and the manganese oxide has a nano-particle size.

[0006] Nanoscale manganese oxide is more easily oxidized. There is no particle size difference between nanoscale graphene-coated nanoscale manganese oxide, and there is a deficiency in coating stability. Generally speaking, in the solution self-assembly process, based on the principle of electrostatic adsorption for coating, the particle size of the material to be wrapped should be significantly larger than that of the wrapping material to achieve the stability of wrapping. Otherwise, either the material to be wrapped is not completely coated or the nanoparticles are coated in an aggregated manner.

[0007] The technical problem solved in this case is: how to prepare a graphene-coated manganese oxide material with better performance. Summary of the Invention

[0008] The object of the present invention is to provide a preparation method of an RGO@MnO composite material. The method of the present invention first makes the surface of manganese oxide positive, enabling it to concentrate on the cathode surface based on the principle of weak adsorption of the electrode in an electric field. At the same time, the hydroxyl and carboxyl groups on the surface of graphene oxide are further hydrated to carry negative charges in the electric field, and assembly is achieved under the action of the electric field and electrostatic adsorption. When the surface of manganese oxide is completely coated with graphene oxide, it is no longer affected by the electric field and the assembly is completed. The coating has good uniformity, a thin coating layer, and stable coating, which can effectively prevent manganese oxide from being oxidized and at the same time maintain its biological properties.

[0009] Meanwhile, the present invention also discloses a composite material prepared based on this method.

[0010] To achieve the above object, the present invention provides the following technical solutions:

[0011] A preparation method of an RGO@MnO composite material, comprising the following steps:

[0012] Step 1: Modify the surface of MnO to obtain modified manganese oxide, making it in a state where the surface is positively charged in an aqueous solution;

[0013] Step 2: Add a composite solution containing graphene oxide, modified manganese oxide, and a conductive agent to an electrolytic cell, and under the action of an electric field, graphene oxide and modified manganese oxide perform self-assembly to form a self-assembled material in which graphene oxide wraps modified manganese oxide;

[0014] Step 3: Reduce the self-assembled material to obtain a composite material RGO@MnO in which graphene wraps modified manganese oxide;

[0015] The D50 particle size D of the graphene oxide GO is 0.1 - 0.9 μm;

[0016] The D50 particle size D of the MnO MnO is 10 - 40 μm;

[0017] D MnO / D GO = 25 - 400.

[0018] In the present invention, the core innovation points are:

[0019] 1. Positive surface modification of manganese oxide;

[0020] 2. Achieving uniform self-assembly based on the electric field and electrostatic adsorption between particles;

[0021] 3. The coating material and the coated material maintain a good particle size difference, making the coating stable and uniform;

[0022] In the synchronous research of this project, we also studied the coating of graphene on copper powder, and the particle size ratio was carried out according to the relationship of Equation 2 below:

[0023]

[0024] k is 20 - 60;

[0025] d GO is less than or equal to 0.8 microns;

[0026] During the experiment, we found that it was impossible to achieve good coating of graphene oxide on manganese oxide using the relationship of Equation 2 above. After analysis, we believe that the main reasons are the conductivity and density of the particles. The conductivity of copper particles is much stronger than that of semiconductor-like manganese oxide, and its enrichment is better. At the same time, the relative density of manganese oxide is only about 5, while the relative density of copper is 9. The huge difference in density also leads to the inapplicability of the coating relationship for materials with different properties.

[0027] The above three elements are the core factors for achieving good coating uniformity, thin coating layer, and stable coating; only by carrying out positive electrochemical modification can manganese oxide particles be accumulated near the cathode based on the weak adsorption principle of the electrode in the electric field (refer to "Research on the Preparation Process and Antibacterial Properties of Graphene Oxide Modified Nickel Coating by Electrodeposition", master's thesis, Nanjing University of Aeronautics and Astronautics, written by Lou Guibin, Section 3.4 Composite Deposition Theory). Based on the appropriate size relationship and electrostatic adsorption, the modified manganese oxide and graphene oxide are assembled. When the assembly is completed, the weak adsorption of the cathode on the particles disappears, and the particles complete the assembly.

[0028] In the above preparation method, the solvent in the composite solution is water, the concentration of graphene oxide in the composite solution is 10 - 20 g / L; the concentration of modified manganese oxide is 50 - 100 g / L; the concentration of the conductive agent is 20 - 50 g / L.

[0029] In the above preparation method, the conductive agent is one or a combination of soluble sodium salts, soluble potassium salts, and soluble magnesium salts.

[0030] In the above preparation method, the surface modification in Step 1 is to load Mn on its surface by the method of organic acid corrosion 2+ , or, load quaternary ammonium salt on its surface by the method of quaternary ammonium salt modification.

[0031] In the above preparation method, the organic acid is citric acid or ascorbic acid; the quaternary ammonium salt is dodecyl trimethyl ammonium chloride or cetyl trimethyl ammonium chloride.

[0032] In the above preparation method, the voltage range of the electric field is 3.5 - 4.5 V, the current range is 15 - 35 A, and the electroplating time is 30 - 60 min.

[0033] In the above preparation method, the reduction conditions in step 3 are: heat preservation is carried out under the condition of 800 - 900 °C in a reducing atmosphere.

[0034] In the present invention, the assembly uniformity can be further improved through the following electrolytic cell, and the structure of the electrolytic cell is as follows:

[0035] At least one flow channel structure in the electrolytic cell is a Tesla valve structure or a flow channel similar to the Tesla valve structure; both ends of the electrolytic cell are a liquid inlet and a liquid outlet; the liquid inlet and the liquid outlet are used to realize the liquid circulation in the electrolytic cell through a pump; the liquid inlet is used to input the composite solution to the inlet of the flow channel; the composite solution discharged from the outlet of the flow channel is discharged through the liquid outlet; the flow channel is composed of a main flow channel and a secondary flow channel, most of the liquid in the composite solution flows through the main flow channel, and a small part of the liquid in the composite solution flows through the secondary flow channel; the cathode and the anode in the fluid are arranged in the secondary flow channel. The coating uniformity can be further improved through the Tesla valve structure or a flow channel similar to the Tesla valve structure. Most of the liquid flows through the main flow channel, and a small part of the liquid flows through the secondary flow channel. The flow velocity of the liquid in the secondary flow channel is lower than that of the main flow channel, and the cathode and the anode are in the secondary flow channel, which is more conducive to the uniform coating and assembly of graphene oxide and modified manganese oxide; the flow channel is composed of several first guide plates, second guide plates, and third guide plates; the second guide plate and the third guide plate form the secondary flow channel; the first guide plate is located on one side of the second guide plate and the third guide plate; the first guide plate and the second guide plate, and the first guide plate and the third guide plate form the main flow channel; an aeration module is arranged below the cathode and the anode; the aeration module is used to provide an inert gas to the composite solution flowing through the cathode and the anode. The aeration module can improve the suspension performance of manganese oxide, increase the contact probability between modified manganese oxide and graphene oxide, improve the coating uniformity, and avoid the problem of reduced coating uniformity caused by the deposition due to the too high density of modified manganese oxide.

[0036] Meanwhile, the present invention also discloses an RGO@MnO composite material prepared by the method described above.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] The method of the present invention first makes the surface of manganese oxide positive, enabling it to concentrate on the cathode surface based on the principle of weak electrode adsorption in an electric field. At the same time, the hydroxyl and carboxyl groups on the surface of graphene oxide are further hydrated to carry negative charges in the electric field, and assembly is achieved under the action of the electric field and electrostatic adsorption. When the surface of manganese oxide is completely coated by graphene oxide, it is no longer affected by the electric field and the assembly is completed. The coating has good uniformity, a thin coating layer, and stable coating, which can effectively prevent manganese oxide from being oxidized and at the same time maintain its biological properties. Description of the Drawings

[0039] Figure 1 It is the XPS test result diagram of the composite material of Example 1 of the present invention;

[0040] Figure 2 It is the electron microscope diagram of the composite material of Example 1 of the present invention;

[0041] Figure 3 It is the schematic structural diagram of the electrolytic cell of the present invention. Detailed Embodiments

[0042] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0043] The structure of the electrolytic cell used in the present invention is as shown in the following equipment embodiment.

[0044] Equipment Embodiment

[0045] An electrolytic cell is provided with two flow channels 1 whose flow channel structures are Tesla valve structures or similar to Tesla valve structures; the two ends of the electrolytic cell are a liquid inlet 2 and a liquid outlet 3; the liquid inlet 2 and the liquid outlet 3 realize the liquid circulation in the electrolytic cell through a pump 4; the liquid in the electrolytic cell is in a flowing state at all times; the liquid inlet 2 is used to input the composite solution into the inlet of the flow channel 1; the composite solution discharged from the outlet of the flow channel 1 is discharged through the liquid outlet 3.

[0046] The flow channel 1 is composed of a main flow channel 5 and a secondary flow channel 6. Most of the liquid in the composite solution flows through the main flow channel 5, and a small part of the liquid in the composite solution flows through the secondary flow channel 6; the cathode 7 and the anode 8 in the fluid are arranged in the secondary flow channel 6.

[0047] To further improve the coating uniformity, we adopted a flow channel 1 with a Tesla valve structure or similar to the Tesla valve structure. Most of the liquid flows through the main flow channel 5, and a small part of the liquid flows through the secondary flow channel 6. The flow rate of the liquid in the secondary flow channel 6 is lower than that in the main flow channel 5. Moreover, the cathode 7 and the anode 8 are located in the secondary flow channel 6, which is more conducive to the uniform coating and assembly of graphene oxide and modified manganese oxide.

[0048] More specifically, the flow channel 1 is composed of a number of first guide plates 9, second guide plates 10, and third guide plates 11; the second guide plates 10 and the third guide plates 11 constitute the secondary flow channel 6; the first guide plates 9 are located on one side of the second guide plates 10 and the third guide plates 11; the first guide plates 9 and the second guide plates 10, and the first guide plates 9 and the third guide plates 11 constitute the main flow channel 5;

[0049] The first guide plates 9 are arranged in two columns in a fishbone form, and there is also fluid flow between the two columns of the first guide plates 9. Therefore, comprehensively Figure 3 It can be seen that the fluid flow form of the present invention is generally divided into the flow through the flow channel 1 based on the Tesla valve structure and the fluid flow between the two columns of the first guide plates 9; among them, in these two flow forms, there is a part of the liquid flowing from the flow channel 1 of the Tesla valve structure into the space between the two columns of the first guide plates 9, and flowing from the space between the two columns of the first guide plates 9 into the flow channel 1 of the Tesla valve structure; however, the overall flow mode is still mainly based on the flow channel 1 of the Tesla valve structure;

[0050] An aeration module 12 is provided below the cathode 7 and the anode 8; the aeration module 12 is used to provide an inert gas to the composite solution flowing through the cathode 7 and the anode 8.

[0051] The aeration module 12 is a ceramic aeration plate, which is connected to an inert gas supply device; a buffer layer such as a transparent epoxy glue layer can be provided at its bottom to prevent the ceramic aeration plate from cracking.

[0052] The aeration module 12 can improve the suspension performance of the modified manganese oxide, increase the chance of contact between the modified manganese oxide and graphene oxide, improve the coating uniformity, and avoid the problem of reduced coating uniformity caused by the deposition due to the excessive density of the modified manganese oxide.

[0053] More specifically, in the following embodiments, the specific parameters of the electrolytic cell of the present invention used (in actual applications, relevant parameters can be adjusted according to factors such as production scale, and the actual protection scope is not limited to the following specific structural parameters) are:

[0054] Electrolytic cell: length: 56 cm; width: 45 cm; height: 18 cm;

[0055] First guide plate 9: 9 cm

[0056] Second deflector 10: 5 cm

[0057] Third deflector 11: 16 cm

[0058] The distance between the second deflector 10 and the third deflector 11 is: 5 cm;

[0059] Both the cathode 7 and the anode 8 are titanium alloy plates with a ruthenium-iridium alloy coating;

[0060] The distance between the anode 8 and the cathode 7 is: 3 - 4 cm

[0061] The volume of the composite solution contained in the electrolytic cell is: 30000 cm 3 ;

[0062] The circulation volume of the pump 4 during operation is: 8 - 12 L / h.

[0063] Example 1

[0064] Preparation of RGO@MnO

[0065] S1: Modification of manganese oxide: Add manganese oxide (particle size D50 is 30 microns) to an aqueous citric acid solution with a concentration of 0.5 mol / L, ultrasonically disperse for 30 s, filter, collect the solid and dry it to obtain modified manganese oxide;

[0066] S2: Preparation of GO dispersion: Weigh a certain amount of GO (D50 sheet diameter 0.4 μm), and then add it to deionized water, and perform ultrasonic treatment for 4 h to form a uniform and stable GO dispersion;

[0067] S3: Preparation of electroplating reaction solution: Add the above-mentioned GO dispersion, modified manganese oxide, and sodium sulfate to ionized water respectively, stir evenly and perform ultrasonic dispersion to obtain a uniformly mixed solution A, where the GO concentration is 10 g / L, the modified manganese oxide concentration is 100 g / L, and the sodium sulfate concentration is 20 g / L;

[0068] S4: Preparation of GO@MnO composite material: Connect the electroplating electrodes of the electrolytic cell to the positive and negative electrodes of an external power supply respectively, change the voltage, current and electroplating time of the electrodes (voltage range 4V, current range 20A, electroplating time 40 min) to synthesize the GO@MnO composite material; during the electroplating process, the temperature of the electrolytic cell is controlled at 35 °C;

[0069] S5: Impurity removal: After electroplating, let the solution obtained after electroplating stand and precipitate to remove the supernatant, and perform repeated precipitation washing with deionized water to mainly wash away the Na + 、SO4 2-, and then vacuum drying treatment is carried out to obtain the final product GO@MnO composite material. The vacuum drying temperature is 100 °C, and the drying time is controlled at 20 h.

[0070] S6: Preparation of RGO@MnO: GO@MnO is calcined under a reducing gas. Specifically, it is a H2-N2 gas mixture. The GO@MnO powder is heated from room temperature to 300 °C at a rate of 5 °C / min and maintained at this temperature for 30 min; then it is quickly heated to 800 °C (10 °C / min), and then thermally insulated at this temperature for 2 h; finally, the powder is naturally cooled to room temperature to obtain the RGO@MnO composite material.

[0071] Example 2

[0072] Generally the same as Example 1, the differences are as follows:

[0073] The D50 particle size of manganese oxide is 40 μm, and the D50 sheet diameter of graphene oxide is 0.2 μm.

[0074] Example 3

[0075] Generally the same as Example 1, the differences are as follows:

[0076] The D50 particle size of manganese oxide is 10 μm, and the D50 sheet diameter of graphene oxide is 0.2 μm.

[0077] Example 4

[0078] Generally the same as Example 1, the difference is that cationic emulsifier is used to modify manganese oxide. The specific method is as follows:

[0079] 1 g of manganese oxide is added to 1 L of an aqueous solution of dodecyltrimethylammonium chloride with a concentration of 10 g / L, stirred and dispersed with ultrasonic waves for 30 min, the solution is filtered, the solid is retained and dried to obtain modified manganese oxide.

[0080] Example 5

[0081] Generally the same as Example 1, the differences are as follows:

[0082] The concentration of graphene oxide is 20 g / L; the concentration of modified manganese oxide is 100 g / L;

[0083] The voltage range is 3.5 V, the current range is 15 A, and the electroplating time is 30 min.

[0084] Example 6

[0085] Generally the same as Example 1, the differences are as follows:

[0086] The concentration of graphene oxide is 10 g / L; the concentration of modified manganese oxide is 50 g / L;

[0087] The voltage range is 4.5 V, the current range is 35 A, and the electroplating time is 60 min.

[0088] Example 7

[0089] Generally the same as Example 1, the difference is that:

[0090] The cathode and anode are arranged in the main flow channel.

[0091] Comparative Example 1

[0092] Generally the same as Example 1, the difference is that the electrodes in the electrolytic cell are not powered on, and the liquid circulation time is controlled within 40 min.

[0093] Comparative Example 2

[0094] Generally the same as Example 1, the difference is that:

[0095] Manganese oxide is not positively electrified and modified, and directly forms a composite solution with graphene oxide and a conductive agent.

[0096] Comparative Example 3

[0097] Generally the same as Example 1, the difference is that:

[0098] Manganese oxide is not positively electrified and modified, and directly forms a composite solution with graphene oxide and a conductive agent, and the electrodes in the electrolytic cell are not powered on, and the liquid circulation time is controlled within 40 min.

[0099] Comparative Example 4

[0100] Generally the same as Example 1, the difference is that:

[0101] The D50 particle size of graphene oxide is 0.5 μm; the D50 particle size of manganese oxide is 100 μm.

[0102] Comparative Example 5

[0103] Generally the same as Example 1, the difference is that:

[0104] The D50 particle size of graphene oxide is 0.5 μm; the D50 particle size of manganese oxide is 10 μm.

[0105] Comparative Example 6

[0106] Generally the same as Example 1, the difference is that:

[0107] The D50 particle size of graphene oxide is 1.2 μm; the D50 particle size of manganese oxide is 40 μm.

[0108] Performance detection:

[0109] Antioxidant performance test (TG experiment)

[0110] Take 30 - 40 mg of the RGO@MnO composites obtained in the examples and comparative examples respectively for antioxidant experiments (thermogravimetric analysis tests). The test temperature range is from room temperature to 800 °C, and the heating rate is 10 °C / min.

[0111] The test results are shown in Table 1;

[0112] Table 1 Test result table

[0113]

[0114]

[0115] Result analysis:

[0116] 1. It can be seen from Examples 1 to 3 that by regulating the particle size of manganese oxide and the sheet diameter of graphene oxide, products with relatively excellent antioxidant performance can be prepared;

[0117] 2. It can be seen from Example 4 that there are various methods to achieve the surface positive charge modification of manganese oxide. Among them, modification by cationic emulsifier is preferred; modification by cationic emulsifier will increase the surface charge distribution amount and improve the affinity for graphene; at the same time, during the subsequent calcination process, the emulsifier will carbonize, which may improve the combined particle size of graphene and manganese oxide, make their combination closer, and maintain good antioxidant properties;

[0118] 3. It can be seen from Examples 5 and 6 that concentration has an impact on performance. The higher the concentration of graphene oxide, the better its coating performance; if the concentration of manganese oxide is reduced without changing the concentration of graphene oxide, the concentration of manganese oxide at the electrode plate position will be reduced, slightly reducing the antioxidant performance of the product.

[0119] 4. It can be seen from Example 7 that the assembly performance of the product set in the secondary flow channel is better, and a smaller flow rate is more conducive to the electrostatic assembly of the two kinds of particles.

[0120] 5. It can be seen from Comparative Examples 1 - 3 that even when not powered on, the particles can be assembled based on the electrostatic field; positive charge modification is a necessary means to achieve performance improvement; if there is an electric field and no positive charge modification is carried out, it can also have a certain coating effect. The reason is that in the solution, manganese oxide itself will adsorb metal ions to make its surface slightly positively charged, so the result of improved antioxidant performance will be shown.

[0121] 6. As can be seen from Comparative Examples 4 to 6, even within the appropriate range of particle size ratios, as the particle size of manganese oxide increases, its antioxidant performance deteriorates. The possible reason is that the increase in particle size affects its suspension performance in the liquid and reduces its assembly ability with graphene oxide; if the particle size of manganese oxide is too small or the particle size of graphene oxide is too large, it will also affect the coating performance and reduce the antioxidant ability.

[0122] 7. Refer to Figure 1 and Figure 2 , Figure 1 is the XPS test result diagram of the composite material of Example 1 of the present invention. As can be seen from Figure 1 , manganese oxide and graphene can be detected; as can be seen from Figure 2 , a uniform coating layer is formed in the composite material of the present invention.

[0123] In summary, the improvement of the antioxidant ability of the composite of the present invention is a comprehensive technical set, which is closely related to the electric field, the positive electrification transformation of manganese oxide, the selection and optimization of particle size, etc.

Claims

1. A preparation method of RGO@MnO composite material, characterized in that, It includes the following steps: Step 1: Surface-modify MnO to obtain modified manganese oxide, so that it shows a positively charged surface state in an aqueous solution; Step 2: Add a composite solution containing graphene oxide, modified manganese oxide, and a conductive agent into an electrolytic cell. Under the action of an electric field, graphene oxide and modified manganese oxide self-assemble to form a self-assembled material with graphene oxide wrapping modified manganese oxide; Step 3: Reduce the self-assembled material to obtain a composite material RGO@MnO with graphene wrapping modified manganese oxide; The D50 particle size D of the graphene oxide GO is 0.1 - 0.9 μm; The D50 particle size D of the described MnO MnO is 10 to 40 μm; D MnO / D GO = 25 to 400.

2. The preparation method according to claim 1, characterized in that, The solvent in the composite solution is water, the concentration of graphene oxide is 10 - 20 g / L; the concentration of modified manganese oxide is 50 - 100 g / L; the concentration of the conductive agent is 20 - 50 g / L.

3. The preparation method according to claim 2, characterized in that, The conductive agent is one or a combination of soluble sodium salts, soluble potassium salts, and soluble magnesium salts.

4. The preparation method according to claim 1, characterized in that, The surface modification described in step 1 is to load Mn on its surface by the method of organic acid corrosion 2+ , or load quaternary ammonium salt on its surface by quaternary ammonium salt modification.

5. The preparation method according to claim 4, characterized in that, The organic acid is citric acid or ascorbic acid; the quaternary ammonium salt is dodecyl trimethyl ammonium chloride or cetyl trimethyl ammonium chloride.

6. The preparation method according to claim 1, wherein The voltage range of the electric field is 3.5 - 4.5 V, the current range is 15 - 35 A, and the electroplating time is 30 - 60 min.

7. The preparation method according to claim 1, characterized in that, The reduction conditions in Step 3 are: calcine at 800 - 1000 °C in a reducing atmosphere, and then anneal.

8. A RGO@MnO composite material, characterized in that, Prepared by using the method according to any one of claims 1 to 7.

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