CN115734597A - Graphene wave-absorbing composite material and preparation method thereof - Google Patents
Graphene wave-absorbing composite material and preparation method thereof Download PDFInfo
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Abstract
The invention belongs to the technical field of wave-absorbing materials, and particularly discloses a graphene wave-absorbing composite material and a preparation method thereof, wherein the material comprises the following components: graphene sheet layers, and nickel and/or cobalt uniformly stacked on the graphene sheet layers in a tetragonal structure. The preparation method comprises the following steps: adding a precipitator into a dispersion system containing nickel ions and/or cobalt ions and graphene oxide, and uniformly mixing to obtain a reaction mixture; and (3) preparing the reaction mixture into a suspension by a hydrothermal method, filtering and washing the suspension, freeze-drying, and roasting at a low temperature to obtain the composite material. The graphene wave-absorbing composite material prepared by the invention has the advantages that the reflection loss can be as low as-67.1 dB, and the effective bandwidth can be more than 8 GHz.
Description
Technical Field
The invention belongs to the technical field of wave-absorbing materials, and particularly relates to a graphene wave-absorbing composite material and a preparation method thereof.
Background
With the rapid development and miniaturization of electronic technologies such as wireless communication and mobile devices, excessive electromagnetic radiation has been considered as the fourth leading cause of pollution, seriously threatening human health. The attenuation capability of the wave-absorbing material to microwave radiation can actively absorb the microwave radiation and convert the energy of the microwave radiation into heat energy or interfere with the dispersed microwave, and has very important significance. Much research has been devoted to creating highly efficient microwave absorbing materials to reduce electromagnetic pollution.
The graphene has the advantages of high carrier mobility, low density, large specific surface area and the like, and has huge microwave absorption potential. Only graphene is used as an absorber, and due to the fact that the graphene is high in conductivity and single in loss mechanism, interface impedance matching is reduced, and the microwave absorption performance is generally poor. The method for adjusting and balancing the electromagnetic parameters of the wave-absorbing material by assembling the graphene with the magnetic nanoparticles is a feasible method for improving the microwave absorption performance. Different magnetic nanoparticles such as CoNi and Ag modified graphene, hollow nickel nanometer core modified reduced graphene oxide and Fe are assembled or doped on materials such as nanosheets, fibers or core/shell nanometer structures 3 O 4 Modified reduced graphene oxide, and the like, many composite materials have been successfully prepared to improve pure graphene impedance matching. In addition, the wave-absorbing performance of the composite material is improved by charge transfer between graphene interfaces, polarization relaxation of free charge carriers of graphene, electromagnetic connection between graphene and magnetic particles, and a large amount of scattering and reflection generated by folds superposed in the composite material. However, the problems of narrow bandwidth, complex process and the like still exist in the magnetic particle modified graphene wave-absorbing composite material, and the production and application fields of the magnetic particle modified graphene wave-absorbing composite material are limited.
Patent CN110157266A discloses a high heat conduction graphene wave-absorbing composite material and a preparation method, which adopts a graphene film and a wave-absorbing material to prepare the graphene wave-absorbing material in a composite way, wherein the wave-absorbing material is taken as a substrate, and the raw materials of the graphene film layer comprise: graphene, an organic polymer solution and resin. The patent aims to improve the heat-conducting property of the wave-absorbing material by using the graphene film layer.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a graphene wave-absorbing composite material which has ultra-wide-band microwave absorption and is supported by nickel and/or cobalt particles in a square structure accumulation form and a preparation method thereof.
The invention is realized by the following technical scheme.
A graphene wave-absorbing composite material comprises: graphene sheets, and nickel and/or cobalt; the nickel and/or cobalt are uniformly stacked on the graphene sheet layer in a tetragonal structure.
The preparation method of the graphene wave-absorbing composite material comprises the following steps:
(1) Adding a precipitator into a dispersion system containing nickel ions and/or cobalt ions and graphene oxide, and uniformly mixing to obtain a reaction mixture, wherein the precipitator is oxalate ions;
(2) And (2) preparing the reaction mixture obtained in the step (1) into a suspension by a hydrothermal method, filtering and washing the suspension, freeze-drying, and finally roasting at a low temperature to obtain the graphene wave-absorbing composite material.
Preferably, in step (1), the raw material of nickel ions is at least one of nickel nitrate, nickel sulfate, nickel chloride and nickel acetate, the raw material of cobalt ions is at least one of cobalt nitrate, cobalt sulfate, cobalt chloride and cobalt acetate, and the raw material of oxalate ions is at least one of oxalic acid, ammonium oxalate and sodium oxalate.
Preferably, the mass ratio of the graphene oxide to the nickel and/or cobalt is 10-1: 1; the molar ratio of the oxalate ions to the nickel and/or cobalt is 25-1: 1.
preferably, the mass ratio of the graphene oxide to the nickel and/or cobalt is 6-1: 1; the molar ratio of the oxalate ions to the nickel and/or cobalt is 10-1: 1.
preferably, in the step (1), an additive is added to the reaction mixture, the additive is one or a mixture of polyethylene glycol, polyvinyl alcohol, polyethyleneimine, polydiene dimethyl ammonium chloride and methyl cellulose, and the addition amount of the additive is 0-2% (mass percentage content) of graphene oxide.
Preferably, the additive is added in an amount of 0 to 1% of the graphene oxide.
Preferably, in the step (1), the pH of the reaction system is controlled to be 2-9; in the step (2), the hydrothermal condition is 110-200 ℃ and 1-48 h.
Preferably, in the step (1), the pH of the reaction system is controlled to be 6-9; in the step (2), the hydrothermal condition is 120-180 ℃ and 4-24 h.
Preferably, in the step (2), the freeze-drying is performed for 24 to 96 hours under an environment of a temperature of-45 ℃ or lower and a vacuum degree of 0.1Pa or lower.
Preferably, in the step (2), the mixture is roasted at the low temperature of 200-600 ℃ for 1-12 h under the protection atmosphere of vacuum, nitrogen or argon.
Preferably, in the step (2), the roasting is carried out for 1 to 12 hours at the temperature of 300 to 500 ℃.
The graphene wave-absorbing composite material prepared by the invention has the advantages that the reflection loss can be as low as-67.1 dB, and the effective bandwidth can be more than 8 GHz.
The invention has the beneficial technical effects that:
1) In the graphene wave-absorbing composite material provided by the invention, nickel and/or cobalt particles are accumulated on a graphene sheet layer in a tetragonal structure;
2) The nickel/cobalt particles stacked in the square structure support the layered graphene, so that the interface loss and the magnetic loss are increased, the impedance matching characteristic of the obtained composite material is improved, and the wave-absorbing performance of the composite material is improved due to a large amount of scattering and reflection generated by folds stacked in the composite material, so that the composite material has excellent ultra-wideband wave-absorbing performance.
3) The graphene wave-absorbing composite material provided by the invention is simple in preparation process flow, low in cost and easy to obtain required process equipment and raw materials, and has the potential of industrial production and application.
Drawings
FIG. 1 is an SEM electron micrograph of nickel oxalate, nickel/cobalt oxalate and cobalt oxalate;
FIG. 2 is an SEM electron micrograph of nickel/graphene, nickel/cobalt/graphene, and cobalt/graphene composites;
fig. 3 is an XRD of the nickel/cobalt/graphene composite;
fig. 4 shows the 3D reflectivity and the absorption bandwidth (EAB) of the nickel/graphene composite material at different thermal reduction temperatures.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
Example 1
1) 400mg of graphene oxide was dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 1982mg of Ni (NO) 3 ) 2 ·6H 2 O is dissolved in 10ml of deionized water to obtain a nickel nitrate solution. The brown graphene oxide suspension was combined with a nickel nitrate solution and then stirred for 30 minutes. Dissolve 1719mg H in 10ml deionized water 2 C 2 O 4 ·2H 2 And O, obtaining an oxalic acid solution. And slowly adding the oxalic acid solution into the mixture of the nickel nitrate solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 5-7.
2) And (2) putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 7h at 150 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 24h at the temperature of below-45 ℃ and the vacuum degree of below 0.1Pa, and roasting for 2h at the temperature of 400 ℃ to obtain the graphene composite wave-absorbing material with nickel particles stacked in a tetragonal structure.
The nickel/graphene composite wave-absorbing material prepared in the embodiment and 85% paraffin are prepared into an annular mixture sample with an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-67.1 dB, and the effective bandwidth EAB is 8.66GHz.
Example 2
1) 400mg of graphene oxide was dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 495mg of Ni (NO) 3 ) 2 ·6H 2 O and 495mgCo (NO) 3 ) 2 ·6H 2 O was dissolved in 10ml of deionized water to give a nickel nitrate/cobalt solution. The brown graphene oxide suspension was combined with a nickel/cobalt nitrate solution and then stirred for 30 minutes. Dissolve 10296mg H in 10ml deionized water 2 C 2 O 4 ·2H 2 O, obtainingOxalic acid solution. And slowly adding the oxalic acid solution into the mixture of the nickel nitrate/cobalt solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 4-6.
2) And putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 1h at 200 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 24h, and roasting for 1h at 500 ℃ to obtain the graphene composite material with nickel/cobalt particles stacked in a tetragonal structure.
The square stacking nickel/cobalt/graphene composite material prepared in the example and 85% paraffin are prepared into a circular mixture sample with an outer diameter of 7mm and an inner diameter of 3.04 mm. The coaxial method is adopted for testing, the electromagnetic parameters of the sample in the range of 2-18GHz are measured through a vector network analyzer, and then CST software is used for calculating the reflection loss condition of the composite material, wherein the minimum reflection loss is-27.1 dB, and the effective bandwidth EAB is 8.75GHz.
Example 3
1) 400mg of graphene oxide was dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 658mgCo (NO) 3 ) 2 ·6H 2 O was dissolved in 10ml of deionized water to obtain a cobalt nitrate solution. The brown graphene oxide suspension was combined with a cobalt nitrate solution and then stirred for 30 minutes. 1120mg H dissolved in 10ml deionized water 2 C 2 O 4 ·2H 2 And O, obtaining an oxalic acid solution. And slowly adding the oxalic acid solution into the mixture of the cobalt nitrate solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 5-7.
2) And putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 12h at 120 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 24h, and roasting for 1h at 400 ℃ to obtain the cobalt particle tetragonal stacking graphene composite material.
The cobalt/graphene composite material prepared in this example and 85% paraffin were used to prepare a sample of a ring mixture having an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-45.3 dB, and the effective bandwidth EAB is 8.32GHz.
Example 4
400mg of graphene oxide and 0.4mg of polyethylene glycol were dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 212mgNi (NO) 3 ) 2 ·6H 2 O is dissolved in 10ml of deionized water to obtain a nickel nitrate solution. The brown graphene oxide suspension was combined with a nickel nitrate solution and then stirred for 30 minutes. Dissolve 1719mg of H in 10ml of deionized water 2 C 2 O 4 ·2H 2 And O, obtaining an oxalic acid solution. And slowly adding the oxalic acid solution into the mixture of the nickel nitrate solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 5-7.
2) And putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 4h at 180 ℃ to obtain a graphene suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 24h, and roasting for 12h at 200 ℃ to obtain the nickel particle tetragonal stacking graphene composite material.
The nickel/graphene composite material prepared in the example and 85% paraffin are prepared into a circular mixture sample with an outer diameter of 7mm and an inner diameter of 3.04 mm. The coaxial method is adopted for testing, the electromagnetic parameters of the sample in the range of 2-18GHz are measured through a vector network analyzer, and then CST software is used for calculating the reflection loss condition of the composite material, wherein the minimum reflection loss is-22.3B, and the effective bandwidth EAB is 8.51GHz.
Example 5
400mg of graphene oxide and 0.2mg of polyvinyl alcohol were dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 272mg of Ni (NO) 3 ) 2 ·6H 2 O and 232mg (CH) 3 COO) 2 Co·4H 2 O was dissolved in 10ml of deionized water to obtain a nickel/cobalt ion solution. Combining the brown graphene oxide suspension with a nickel/cobalt ion solution, and thenStirred for 30 minutes. Dissolve 2240mg H in 10ml deionized water 2 C 2 O 4 ·2H 2 And O, obtaining an oxalic acid solution. And slowly adding the oxalic acid solution into the mixture of the nickel/cobalt ion solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 5-7.
2) And (2) putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 48 hours at 110 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 minutes each time, freeze-drying for 24 hours, and roasting for 1 hour at 500 ℃ to obtain the graphene composite material with nickel/cobalt particles stacked in a tetragonal structure.
The nickel/cobalt/graphene composite material prepared in this example and 85% paraffin were used to prepare a sample of a ring-shaped mixture having an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-19.1 dB, and the effective bandwidth EAB is 8.92GHz.
Example 6
400mg of graphene oxide and 0.3mg of polydienedimethylammonium chloride were dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 396mg of Ni (CH) 3 COO) 2 ·4H 2 O and 846mg (CH) 3 COO) 2 Co·4H 2 O was dissolved in 10ml of deionized water to give a nickel acetate/cobalt solution. The brown graphene oxide suspension was combined with the nickel acetate/cobalt solution and then stirred for 30 minutes. Dissolve 1693mg (NH) in 10ml of deionized water 4 ) 2 C 2 O 4 To obtain the ammonium oxalate solution. And slowly adding the ammonium oxalate solution into the mixture of the nickel acetate/cobalt solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 7-9.
2) And (2) putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 6h at 160 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 50h, and roasting for 6h at 350 ℃ to obtain the graphene composite material with nickel/cobalt particles stacked in a tetragonal structure.
The nickel/cobalt/graphene composite material prepared in this example and 85% paraffin were used to prepare a sample of a ring-shaped mixture having an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-22.3 dB, and the effective bandwidth EAB is 8.11GHz.
Example 7
400mg of graphene oxide and 0.8mg of polydienedimethylammonium chloride were dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 290mg of Ni (CH) 3 COO) 2 ·4H 2 O and 495mgCo (NO) 3 ) 2 ·6H 2 O is dissolved in 10ml of deionized water to obtain a nickel/cobalt ion solution. The brown graphene oxide suspension was combined with the nickel/cobalt ion solution and then stirred for 30 minutes. 560mg (NH) was dissolved in 10ml of deionized water 4 ) 2 C 2 O 4 To obtain the ammonium oxalate solution. Slowly adding the ammonium oxalate solution into the mixture of the nickel/cobalt ion solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 7-9.
2) And putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 5h at 150 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 24h, and roasting for 10h at 400 ℃ to obtain the graphene composite material with nickel/cobalt particles stacked in a tetragonal structure.
The nickel/cobalt/graphene composite material prepared in this example and 85% paraffin were used to prepare a sample of a ring-shaped mixture having an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-33.2 dB, and the effective bandwidth EAB is 8.00GHz.
Example 8
400mg of graphene oxide and 2mg of polyethylene glycol were dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 290mg of nickel sulfate heptahydrate were dissolved in 10ml of deionized water to obtain a nickel sulfate solution. The brown graphene oxide suspension was combined with a nickel sulfate solution and then stirred for 30 minutes. 320mg of sodium oxalate was dissolved in 10ml of deionized water to obtain a sodium oxalate solution. Slowly adding the sodium oxalate solution into the mixture of the nickel sulfate solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 6-8.
2) And putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 28h at 170 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 24h, and roasting for 6h at 250 ℃ to obtain the graphene composite material with nickel particles stacked in a tetragonal structure.
The nickel/graphene composite material prepared in the example and 85% paraffin are prepared into a circular mixture sample with an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-40.6 dB, and the effective bandwidth EAB is 8.43GHz.
Example 9
400mg of graphene oxide and 3.6mg of methylcellulose were dissolved in 50ml of deionized water and dispersed with ultrasonic waves for 60 minutes to form a brown graphene oxide solution. 495mg of cobalt sulfate heptahydrate was dissolved in 10ml of deionized water to obtain a cobalt sulfate solution. The brown graphene oxide suspension was combined with a cobalt sulfate solution and then stirred for 30 minutes. 560mg of sodium oxalate was dissolved in 10ml of deionized water to obtain a sodium oxalate solution. Slowly adding the sodium oxalate solution into the mixture of the cobalt sulfate solution and the graphene suspension, stirring for 30 minutes to obtain a reaction mixture, and controlling the pH value of the reaction system to be 6-8.
2) And (2) putting the reaction mixture into a 100ml Teflon-lined stainless steel autoclave, reacting for 15h at 170 ℃ to obtain a suspension, filtering and washing the suspension, centrifuging for 3 times by using deionized water, centrifuging for 3 min each time, freeze-drying for 80h, and roasting for 4h at 220 ℃ to obtain the graphene composite material with the cobalt particles stacked in a tetragonal structure.
The cobalt/graphene composite material prepared in this example and 85% paraffin were used to prepare a sample of a ring mixture having an outer diameter of 7mm and an inner diameter of 3.04 mm. And (3) testing by adopting a coaxial method, measuring the electromagnetic parameters of the sample in the range of 2-18GHz by using a vector network analyzer, and calculating the reflection loss condition of the composite material by using CST software. The lowest reflection loss is-22.4 dB, and the effective bandwidth EAB is 8.12GHz.
TABLE 1 summary of the material ratios of the examples
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention. It should be noted that other equivalent modifications can be made by those skilled in the art in light of the teachings of the present invention, and all such modifications can be made as are within the scope of the present invention.
Claims (12)
1. A graphene wave-absorbing composite material is characterized by comprising: graphene sheets, and nickel and/or cobalt; the nickel and/or cobalt are uniformly stacked on the graphene sheet layer in a tetragonal structure.
2. A preparation method of the graphene wave-absorbing composite material according to claim 1, wherein the method comprises the following steps:
(1) Adding a precipitator into a dispersion system containing nickel ions and/or cobalt ions and graphene oxide, and uniformly mixing to obtain a reaction mixture, wherein the precipitator is oxalate ions;
(2) And (2) preparing the reaction mixture obtained in the step (1) into a suspension by a hydrothermal method, filtering and washing the suspension, freeze-drying, and finally roasting at a low temperature to obtain the graphene wave-absorbing composite material.
3. The preparation method according to claim 2, wherein in the step (1), the raw material of nickel ions is at least one of nickel nitrate, nickel sulfate, nickel chloride and nickel acetate, the raw material of cobalt ions is at least one of cobalt nitrate, cobalt sulfate, cobalt chloride and cobalt acetate, and the raw material of oxalate ions is at least one of oxalic acid, ammonium oxalate and sodium oxalate.
4. The preparation method according to claim 2, wherein the mass ratio of the graphene oxide to the nickel and/or cobalt is 10-1: 1; the molar ratio of the oxalate ions to the nickel and/or cobalt is 25-1: 1.
5. the preparation method according to claim 4, wherein the mass ratio of the graphene oxide to the nickel and/or cobalt is 6-1: 1; the molar ratio of the oxalate ions to the nickel and/or cobalt is 10-1: 1.
6. the preparation method according to claim 2, characterized in that in the step (1), an additive is added into the reaction mixture, wherein the additive is one or more of polyethylene glycol, polyvinyl alcohol, polyethylene imine, polydiene dimethyl ammonium chloride and methyl cellulose, and the addition amount of the additive is 0-2% of that of the graphene oxide.
7. The method according to claim 6, wherein the additive is added in an amount of 0 to 1% based on the amount of graphene oxide.
8. The production method according to claim 2, wherein in the step (1), the pH of the reaction system is controlled to 2 to 9; in the step (2), the hydrothermal condition is 110-200 ℃ and 1-48 h.
9. The method according to claim 8, wherein in the step (1), the pH of the reaction system is controlled to 6 to 9; in the step (2), the hydrothermal condition is 120-180 ℃ and 4-24 h.
10. The process according to claim 2, wherein the freeze-drying in the step (2) is carried out under conditions of a temperature of-45 ℃ or lower and a degree of vacuum of 0.1Pa or lower for 24 to 96 hours.
11. The preparation method according to claim 2, wherein in the step (2), the mixture is baked at a low temperature of 200-600 ℃ for 1-12 h under a vacuum, nitrogen or argon protective atmosphere.
12. The method of claim 11, wherein in the step (2), the calcination is carried out at a temperature of 300 to 500 ℃ for 1 to 12 hours.
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