Cobalt-based nitrogen-doped carbon nano composite material with hollow cubic structure as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of preparation of wave-absorbing materials, and particularly relates to a cobalt-based nitrogen-doped carbon nano composite material with a hollow cubic structure as well as a preparation method and application thereof. Comprising the following steps: taking ZnCo-ZIF with a nano cubic structure as a core, coating ZnCo-ZIF with polydopamine by adopting a precipitation method to form a precursor with a core-shell structure, and carbonizing the precursor at 800-900 DEG C to obtain the cobalt-based nitrogen-doped carbon nano composite material with the hollow cubic structure. According to the invention, the dielectric loss capability can be enhanced by using the conductivity and interface polarization effect of the coated carbon layer, and the Co particles introduce strong magnetic loss; meanwhile, due to interface polarization caused by abundant heterogeneous interfaces existing between the Co particles and the coating carbon layer, between the magnetic particles and the carbon nanotubes and between the carbon matrix and the carbon nanotubes in the composite material, the impedance matching characteristic can be improved, and high reflection loss and wide effective absorption bandwidth can be achieved under the low matching thickness.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing material preparation technology, specifically relating to hollow cubic cobalt-based nitrogen-doped carbon nanocomposite materials, their preparation methods, and applications. Background Technology

[0002] In recent years, the rapid development of 5G communication, the Internet of Things, and high-density, high-frequency integrated electronic devices has brought enormous electromagnetic compatibility challenges and ubiquitous electromagnetic pollution. These problems seriously affect the performance of electronic devices and also have a significant impact on human health. To address these challenges, electromagnetic wave absorbing materials are the most effective and direct approach. Intrinsic materials and structures are two key aspects, and integrating the advantages of both is crucial for the research of highly efficient electromagnetic wave absorbing materials.

[0003] Electromagnetic wave loss mechanisms include dielectric loss, conductive loss, and magnetic loss. Magnetic loss absorbing materials are typically Fe, Co, and Ni-based ferrites and their doped systems. Electrical loss absorbing materials are divided into resistive loss absorbing materials and dielectric loss absorbing materials, including conductive polymers and carbon-based materials such as graphene, carbon nanotubes, and carbon fibers. Both dielectric and magnetic loss absorbing materials have certain drawbacks in the preparation of high-performance absorbing materials due to their singular electromagnetic wave absorption mechanisms, including poor impedance matching and excessively high density. Dielectric absorbing materials suffer from poor impedance matching caused by excessively high dielectric properties, hindering effective electromagnetic wave absorption, while magnetic loss absorbing materials are characterized by high density. Currently, combining materials with different loss types is a common strategy for absorbing material preparation, enabling a synergistic magnetic-electric loss mechanism at the heterogeneous interface, which significantly optimizes the material's internal electromagnetic wave loss capability.

[0004] Improving electromagnetic wave absorption performance requires materials to possess the highest possible magnetic loss while also exhibiting good dielectric frequency response characteristics. Optimization research on the absorption performance of low-frequency absorbing materials primarily focuses on the synergistic control of the material's complex permittivity and complex permeability. By adjusting the composition and microstructure of the absorbing material, its electromagnetic parameters can be adjusted to the desired range. Utilizing the synergistic dielectric properties, loss mechanisms, and diverse compositional and microstructural designs of carbon-based materials to optimize the overall performance of composite materials remains an important direction in the research of high-performance microwave absorbing materials. Currently, researchers are adjusting the microstructure of carbon materials through multi-scale design strategies, including molecular-scale, micro / nano-scale structures, macroscopic structures, and multi-scale integrated assembly design strategies. Metal-organic frameworks (MOFs) are one of the hot topics in the research of magnetoelectric composite microwave absorbing materials. For example, Professor Wang Yanxiang's team at Rudong University prepared MOF-derived layered Cu9S5 / C nanocomposite fibers through electrospinning and subsequent carbonization-sulfurization processes. The minimum reflection loss value reached -69.6 dB at a thickness of 1.83 mm, and the maximum effective absorption bandwidth reached 5.81 GHz. Professor Huang Ying's team at Northwestern Polytechnical University prepared a three-dimensional flower-like porous carbon framework (Mo / N / S-PCF) co-doped with Mo and N, S through pyrolysis modification of ZIF(Zn)-L. The optimal reflection loss of Mo / N / S-PCF at a thickness of 1.7 mm was -53 dB. dB; Xue Weidong's team at the University of Electronic Science and Technology of China synthesized Ni-MOF-rGO and FeNi-MOF-rGO aerogels of metal-organic framework reduced graphene oxide through hydrothermal and freeze-drying processes. FeNi-MOF-rGO aerogel achieved a reflection loss of -48.3 dB and an ultrawide EAB of 8.32 GHz with a thickness of 2.9 mm.

[0005] However, the key to obtaining high-performance absorbing materials lies in enhancing loss and broadening the absorption bandwidth, particularly by controlling the dielectric constant and impedance matching characteristics of the material in the low-frequency range. Existing technologies, including the aforementioned solutions, suffer from poor impedance matching in the low-frequency range, thus failing to achieve effective electromagnetic wave absorption. Summary of the Invention

[0006] To address the aforementioned issues, this invention provides a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material, its preparation method, and its applications. Through a rational MOF framework design combined with high-temperature pyrolysis, a lightweight, broadband hollow cubic absorbing material is constructed. Using ZnCo-ZIF as the core, polydopamine (PDA) is deposited onto ZnCo-ZIF via a precipitation method to form a core-shell structure. Through simple pyrolysis, hollow cubic carbon nanocomposite materials with varying PDA thicknesses are prepared. The conductivity and interfacial polarization effect of the coated carbon layer enhance dielectric loss, while the metallic Co particles introduce strong magnetic loss. Furthermore, the abundant heterogeneous interfaces between the metallic Co particles and the coated carbon layer, between magnetic particles and carbon nanotubes, and between the carbon matrix and carbon nanotubes in the composite material lead to interfacial polarization, which improves impedance matching characteristics. This results in a absorbing material achieving high reflection loss and a wide effective absorption bandwidth at a relatively low matching thickness.

[0007] The present invention solves the above-mentioned technical problems through the following technical solutions.

[0008] One objective of this invention is to provide a method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material, comprising the following steps: using a nanocubic ZnCo-ZIF structure as the core, coating polydopamine onto the nanocubic ZnCo-ZIF structure by precipitation to form a core-shell structure precursor, and carbonizing the precursor at 800 ℃~900 ℃ under a protective gas atmosphere to obtain a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material.

[0009] Furthermore, the method for preparing a core-shell structured precursor includes the following steps: Dopamine hydrochloride, polyether, and mesitylene were dissolved in a first solvent to obtain a mixed solution. The mixed solution was then added to a nanocubic ZnCo-ZIF dispersion and stirred at room temperature to obtain a core-shell structured precursor.

[0010] Furthermore, the ratio of dopamine hydrochloride, polyether, and mesitylene is 0.5 g to 1.5 g: 2 g: 2 mL; the polyether is a mixture of polyether F-127 and polyether P-123 in a mass ratio of 1:2 to 3.

[0011] Furthermore, the mass ratio of nanocubic ZnCo-ZIF to dopamine hydrochloride in the nanocubic ZnCo-ZIF dispersion is 0.5:0.5-1.5.

[0012] Furthermore, the carbonization time is 1.5 h to 3 h, the protective gas is nitrogen, and the heating rate is 3 ℃ / min to 5 ℃ / min.

[0013] Furthermore, the preparation method of the nanocubic ZnCo-ZIF structure includes the following steps: A soluble zinc salt, a soluble cobalt salt, and a surfactant are dissolved in a second solvent to obtain a mixed solution. After mixing the mixed solution and the organic ligand solution, the nanocubic structure ZnCo-ZIF was obtained by stirring at room temperature.

[0014] Furthermore, the molar ratio of the soluble zinc salt, soluble cobalt salt, and surfactant is 6.5 mmol–12.5 mmol: 0.65 mmol–1.5 mmol: 0.05 g. The concentration of the soluble zinc salt in the mixed solution is 0.08 mol / L–0.1 mol / L, the concentration of the soluble cobalt salt is 0.008 mol / L–0.012 mol / L, the soluble zinc salt is zinc nitrate hexahydrate, the soluble cobalt salt is cobalt nitrate hexahydrate, and the surfactant is hexadecyltrimethylammonium bromide.

[0015] Furthermore, the concentration of the organic ligand solution is 0.75 mol / L to 0.8 mol / L, and the volume ratio of the mixed solution to the organic ligand solution is 1:1 to 1.2.

[0016] The second objective of this invention is to provide a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material, which is prepared using the above-described preparation method.

[0017] The third objective of this invention is to provide the application of the above-mentioned hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material in electromagnetic wave absorption.

[0018] Compared with the prior art, the present invention has the following advantages: This invention uses a nanocubic ZnCo-ZIF structure as the core and polydopamine as the shell. A core-shell structure of polydopamine-coated ZnCo-ZIF nanocubic precursor is formed using a precipitation method. Hollow cubic cobalt / nitrogen-doped carbon nanocomposite material is then prepared through a pyrolysis carbonization strategy. During pyrolysis carbonization, the zinc-cobalt-based metal-organic framework (ZnCo-ZIF) retains its regular cubic structure and hollow cubic morphology. The ZnCo-ZIF core undergoes anisotropic thermal contraction during pyrolysis, forming cavities and thus a hollow structure. The polydopamine-coated shell transforms into a carbon layer during pyrolysis. This carbon layer not only protects the cubic structure of ZnCo-ZIF but also forms a hollow cubic structure during pyrolysis, resulting in a stable outer shell structure. A hollow cubic cobalt / nitrogen-doped carbon nanocomposite material was prepared. Due to its hollow structure, electromagnetic waves undergo multiple reflections within the material, increasing the electromagnetic wave reflection path. The heterojunction structure formed by the rectifying contact between the transition metal nanoparticles and the carbon layer enhances the loss performance of the absorbing material through polarization, enabling rapid charge transport and numerous dipole polarizations. The conductivity and interfacial polarization effect of the coated carbon layer can enhance the dielectric loss capability. The metal Co particles introduce strong magnetic loss. At the same time, the interfacial polarization caused by the abundant heterogeneous interfaces between the metal Co particles and the coated carbon layer, between magnetic particles and carbon nanotubes, and between the carbon matrix and carbon nanotubes in the composite material is beneficial to improving impedance matching characteristics. This allows the absorbing material to achieve high reflection loss and a wide effective absorption bandwidth with a relatively low matching thickness. Attached Figure Description

[0019] Figure 1 This is a microstructure diagram of Co@CPC-1 prepared in Example 1 of the present invention. Figure 1 In the image, (a) to (c) are scanning electron microscope images at different sizes, (d) to (f) are transmission electron microscope images at different sizes, and (g) is an EDS mapping image.

[0020] Figure 2 The X-ray diffraction patterns are those of ZC-ZIF@PDA and Co@CPC prepared in Examples 1 to 5 of this invention. Figure 2 In the diagram, (a) is the X-ray diffraction pattern of ZC-ZIF@PDA, and (b) is the X-ray diffraction pattern of Co@CPC.

[0021] Figure 3 The X-ray photoelectron spectra of Co@CPC prepared in Examples 1 to 5 of this invention are shown below. Figure 3 In the diagram, (a) is the full spectrum, (b) is the C 1s spectrum, (c) is the N 1s spectrum, and (d) is the Co 2p spectrum.

[0022] Figure 4The dielectric and permeability diagrams are for the Co@CPC prepared in Examples 1 to 5 of this invention. Figure 5 In the diagram, (a) represents the real part of the dielectric constant, (b) represents the imaginary part of the dielectric constant, (c) represents the dielectric loss tangent, (d) represents the real part of the permeability, (e) represents the imaginary part of the permeability, and (f) represents the magnetic loss tangent.

[0023] Figure 5 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 1 of this invention. Figure 5 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram.

[0024] Figure 6 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 2 of this invention. Figure 6 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram.

[0025] Figure 7 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 3 of this invention. Figure 7 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram.

[0026] Figure 8 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 4 of this invention. Figure 8 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram.

[0027] Figure 9 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 9 of this invention. Figure 9 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram.

[0028] Figure 10 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 1 of this invention.

[0029] Figure 11 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 2 of the present invention.

[0030] Figure 12 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 3 of the present invention.

[0031] Figure 13 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 4 of this invention.

[0032] Figure 14 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 5 of the present invention.

[0033] Figure 15 These are comparison diagrams of optimal reflection loss and effective absorption bandwidth for the Co@CPC prepared in Examples 1 to 5 of this invention. Figure 15 (a) is a comparison chart of optimal reflection loss, and (b) is a comparison chart of effective absorption bandwidth. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0036] Metal-organic frameworks (MOFs) are a hot research topic in magnetoelectric composite microwave absorbing materials. MOFs are self-assembled from metal ions and organic ligands, exhibiting characteristics such as a periodic network crystal structure, large specific surface area, and high porosity. Since the synthesis of the first MOF by Yaghi et al. in 1995, MOFs and their derivatives have been widely used in catalysis, energy storage, and other fields. Since 2015, MOF materials have been increasingly applied to electromagnetic wave absorption. The MOF structure plays a crucial role in material synthesis, serving not only as an excellent provider of metal and carbon sources, but also because its unique microstructure allows for the simultaneous occurrence of reduction and pyrolysis processes. This synchronization mechanism ensures that the composite material retains a series of advantageous properties of the MOF precursor after annealing, including high dispersion of metal nanoparticles, orderly microstructure, and uniform chemical composition. These advantages enable MOF-derived composite materials to exhibit superior performance in multiple application areas.

[0037] The physicochemical properties of MOF-derived composite materials can typically be customized by precisely controlling high-temperature annealing conditions and adjusting the ratio of metal ions to organic ligands in the precursor. Compared to other microwave absorbing agents, a major advantage of MOF-derived composite materials is their ability to maintain the integrity of the carbon framework, thereby endowing the material with high conductivity, which greatly improves the efficiency of conductivity loss. Therefore, controlling the structural design and construction methods of the MOF framework morphology holds promise for effectively improving the electromagnetic parameters of the material. However, currently, how to enhance loss and broaden the absorption bandwidth of high-efficiency low-frequency broadband absorbing materials, especially how to control the dielectric constant and impedance matching characteristics of the material in the low-frequency range, remains a key challenge in the rational design of high-efficiency low-frequency absorbing materials.

[0038] Based on this, the present invention constructs a lightweight, broadband hollow cubic structure microwave absorbing material through a combination of reasonable MOF framework design and high-temperature pyrolysis. Specifically:

[0039] A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material includes the following steps: using a nanocubic ZnCo-ZIF structure as the core, polydopamine is coated onto the nanocubic ZnCo-ZIF structure by precipitation to form a core-shell structure precursor, and the precursor is carbonized at 800 ℃~900 ℃ under a protective gas atmosphere to obtain the hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material.

[0040] This invention uses a nanocubic ZnCo-ZIF structure as the core and polydopamine as the shell. A core-shell structure of polydopamine-coated ZnCo-ZIF nanocubic precursor is formed using a precipitation method. Hollow cubic cobalt / nitrogen-doped carbon nanocomposite materials are then prepared through a pyrolysis carbonization strategy. During pyrolysis carbonization, the zinc-cobalt-based metal-organic framework (ZnCo-ZIF) retains its regular cubic structure and hollow cubic morphology. The ZnCo-ZIF core undergoes anisotropic thermal contraction during pyrolysis, forming cavities and thus a hollow structure. The polydopamine-coated shell transforms into a carbon layer during pyrolysis. This carbon layer not only protects the cubic structure of ZnCo-ZIF but also forms a hollow cubic structure during pyrolysis, resulting in a stable outer shell structure, thus enabling the fabrication of... A hollow cubic cobalt / nitrogen-doped carbon nanocomposite material was prepared. Due to its hollow structure, electromagnetic waves undergo multiple reflections within the material, increasing the electromagnetic wave reflection path. The heterojunction structure formed by the rectifying contact between the transition metal nanoparticles and the carbon layer enhances the loss performance of the absorbing material through polarization, enabling rapid charge transfer and numerous dipole polarizations. The conductivity and interfacial polarization effect of the coated carbon layer can enhance the dielectric loss capability. The metal Co particles introduce strong magnetic loss. At the same time, the interfacial polarization caused by the abundant heterogeneous interfaces between the metal Co particles and the coated carbon layer, between magnetic particles and carbon nanotubes, and between the carbon matrix and carbon nanotubes in the composite material is beneficial to improving impedance matching characteristics. This allows the absorbing material to achieve high reflection loss and a wide effective absorption bandwidth with a relatively low matching thickness.

[0041] In a specific embodiment, the method for preparing a core-shell structured precursor includes the following steps: Dopamine hydrochloride, polyether, and mesitylene were dissolved in a first solvent to obtain a mixed solution. The mixed solution was then added to a nanocubic ZnCo-ZIF dispersion, and the mixture was stirred at room temperature using a precipitation method to obtain a core-shell structured precursor.

[0042] In this invention, polyether is used as a solvent and template agent. Zinc-cobalt-based metal-organic frameworks (ZnCo-ZIF) are dissolved in mesitylene. ZnCo-ZIF particles are then coated with polydopamine by precipitation to promote a uniform reaction and form a core-shell structured polydopamine-coated ZnCo-ZIF nanocubic precursor. Polyether, as a template agent, utilizes the template effect of its micelles to effectively guide the formation of mesoporous structures and precisely control the size and morphology of nanomaterials. This allows for the synthesis of nanoparticles with specific structures and properties, resulting in mesoporous materials with uniform pore size distribution and high order.

[0043] In the preparation of hollow cubic cobalt / nitrogen-doped carbon nanocomposites using a pyrolysis carbonization strategy, the organic components (such as polyethers) in the core-shell precursor gradually decompose and volatilize, leading to the formation of cavities within the material. Simultaneously, the external polydopamine undergoes pyrolysis to form a stable outer shell structure. Due to the anisotropic contraction during pyrolysis, a hollow cubic microcube structure is ultimately formed. Furthermore, mesitylene can regulate the carbonization process through interactions with other components, influencing the final material's microstructure and contributing to the formation of the hollow cubic structure.

[0044] In a more specific embodiment, the mixed solution was added to the nanocubic ZnCo-ZIF dispersion, mechanically stirred for 5 min, and then 2 mL of ammonia solution was added. After stirring continuously at room temperature for 2 h, a core-shell structured precursor product was obtained. After washing, centrifugation and drying, the core-shell structured precursor was obtained, that is, the core-shell structured polydopamine-coated ZnCo-ZIF nanocubic precursor was obtained.

[0045] In a specific embodiment, the ratio of dopamine hydrochloride, polyether, and mesitylene is 0.5g to 1.5g: 2g: 2mL; the polyether is a mixture of polyether F-127 and polyether P-123 in a mass ratio of 1:2 to 3.

[0046] In this invention, polyether P-123 is used as a template agent, which can effectively guide the formation of mesoporous structures and prepare mesoporous materials with uniform pore size distribution and high order. Polyether P-127 has unique thermosensitive gel properties. By utilizing the template effect of its micelles, the size and morphology of nanomaterials can be precisely controlled to synthesize nanoparticles with specific structures and properties.

[0047] In a specific embodiment, the mass ratio of nanocubic ZnCo-ZIF to dopamine hydrochloride in the nanocubic ZnCo-ZIF dispersion is 0.5:0.5-1.5.

[0048] In a specific embodiment, the carbonization time is 1.5 h to 3 h, the protective gas is nitrogen, and the heating rate is 3 °C / min to 5 °C / min.

[0049] In this invention, the precursor of the core-shell structure is a zinc-cobalt-based metal-organic framework (ZnCo-ZIF). This framework itself has a regular cubic structure. Because the cubic structure of ZnCo-ZIF maintains a certain stability during pyrolysis, the final material retains a hollow cubic morphology. During pyrolysis, the ZnCo-ZIF core undergoes anisotropic thermal contraction, which leads to the formation of cavities within the material, thus creating a hollow structure. Due to the non-uniformity of thermal contraction, the outer surface of the material may exhibit a concave cubic shape. Before pyrolysis, the ZnCo-ZIF particles are coated with polydopamine, which transforms into a carbon layer during pyrolysis. This carbon layer not only protects the cubic structure of ZnCo-ZIF but also forms a hollow cubic structure during pyrolysis.

[0050] In a specific embodiment, the preparation method of the nanocubic structure ZnCo-ZIF includes the following steps: A soluble zinc salt, a soluble cobalt salt, and a surfactant are dissolved in a second solvent to obtain a mixed solution. After mixing the mixed solution and the organic ligand solution, the nanocubic structure ZnCo-ZIF was obtained by stirring at room temperature.

[0051] In a specific embodiment, the molar ratio of the soluble zinc salt, the soluble cobalt salt, and the surfactant is 6.5 mmol–12.5 mmol: 0.65 mmol–1.5 mmol: 0.05 g. The concentration of the soluble zinc salt in the mixed solution is 0.08 mol / L–0.1 mol / L, the concentration of the soluble cobalt salt is 0.008 mol / L–0.012 mol / L, the soluble zinc salt is zinc nitrate hexahydrate, the soluble cobalt salt is cobalt nitrate hexahydrate, and the surfactant is hexadecyltrimethylammonium bromide.

[0052] In a specific embodiment, the concentration of the organic ligand solution is 0.75 mol / L to 0.8 mol / L, and the volume ratio of the mixed solution to the organic ligand solution is 1:1 to 1.2.

[0053] In a specific embodiment, both the first solvent and the second solvent are formed by mixing ethanol and water in a volume ratio of 1:1.

[0054] The following specific examples will provide further explanation.

[0055] Example 1 A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material includes the following steps: S1, preparation of ZnCo-ZIF nanocubes: Solution A was prepared by dissolving 5 mg of cetyltrimethylammonium bromide (CTAB) with 0.9 mmol of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 0.1 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 10 mL of deionized water (DI).

[0056] 55 mmol of 2-methylimidazole was dissolved in 70 mL of DI to obtain solution B. Then, the above solution A was quickly poured into solution B. After stirring at room temperature for 20 min, the product was filtered, washed thoroughly several times with DI and ethanol, centrifuged, and dried at 65 °C overnight to obtain ZnCo-ZIF with a nanocubic structure.

[0057] S2. Preparation of polydopamine-coated ZnCo-ZIF nanocubic precursor: Weigh 500 mg of ZnCo-ZIF prepared by S1 and ultrasonically disperse it in a 20 mL mixed solution of ethanol / DI with a volume ratio of 1:1. The solution is then ultrasonically treated for 30 min to obtain dispersion A.

[0058] Weigh 500 mg of dopamine hydrochloride (PDA), 0.5 g of polyether F-127, 1.5 g of polyether P-123 and 2 mL of mesitylene (TMB), and dissolve them in 30 mL of a mixed solution of ethanol / DI with a volume ratio of 1:1. After ultrasonic treatment, a white suspension emulsion is formed, which is denoted as solution B.

[0059] Solution B was rapidly injected into dispersion A, and mechanically stirred for 5 min. Then, 2 mL of ammonia solution was added. The resulting mixed solution was stirred continuously at room temperature for 2 h to obtain the product. The product was washed several times with DI and ethanol, centrifuged, and dried overnight at 65 °C to obtain polydopamine-coated ZnCo-ZIF nanocubic precursor, named ZC-ZIF@PDA.

[0060] S3. Preparation of hollow cubic cobalt / nitrogen-doped carbon nanocomposites: The ZC-ZIF@PDA obtained in S2 was placed in an alumina crucible and transferred to a tube furnace. Under a flowing nitrogen atmosphere, it was carbonized at 900 °C for 2 h at a heating rate of 5 °C / min. The sample was then cooled to room temperature to obtain a hollow cubic cobalt / nitrogen-doped carbon nanocomposite material, named Co@CPC-1.

[0061] Example 2 A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material includes the following steps: Preparation of S1, ZnCo-ZIF nanocubes: 5 mg of CTAB was dissolved in 10 mL of DI along with 0.9 mmol of Zn(NO3)2·6H2O and 0.1 mmol of Co(NO3)2·6H2O to obtain solution A.

[0062] 55 mmol of 2-methylimidazole was dissolved in 70 mL of DI to obtain solution B. Then, the above solution A was quickly poured into solution B. After stirring at room temperature for 20 min, the product was filtered, washed thoroughly several times with DI and ethanol, centrifuged, and dried at 65 °C overnight to obtain ZnCo-ZIF with a nanocubic structure.

[0063] S2. Preparation of polydopamine-coated ZnCo-ZIF nanocubic precursor: Weigh 500 mg of ZnCo-ZIF prepared by S1 and ultrasonically disperse it in a 20 mL mixed solution of ethanol / DI with a volume ratio of 1:1. The solution is then ultrasonically treated for 30 min to obtain dispersion A.

[0064] Weigh 500 mg of PDA, 0.5 g of polyether F-127, 1.5 g of polyether P-123 and 2 mL of TMB, and dissolve them in 30 mL of a mixed solution of ethanol / DI with a volume ratio of 1:1. After ultrasonic treatment, a white suspension emulsion is formed, which is denoted as solution B.

[0065] Solution B was rapidly injected into dispersion A, and mechanically stirred for 5 min. Then, 2 mL of ammonia solution was added. The resulting mixed solution was stirred continuously at room temperature for 2 h to obtain the product. The product was washed several times with DI and ethanol, centrifuged, and dried overnight at 65 °C to obtain polydopamine-coated ZnCo-ZIF nanocubic precursor, named ZC-ZIF@PDA.

[0066] S3. Preparation of hollow cubic cobalt / nitrogen-doped carbon nanocomposites: The ZC-ZIF@PDA obtained in S2 was placed in an alumina crucible and transferred to a tube furnace. It was carbonized at 900 °C for 2 h under a flowing nitrogen atmosphere at a heating rate of 5 °C / min. The sample was then cooled to room temperature to obtain a hollow cubic cobalt / nitrogen-doped carbon nanocomposite material, named Co@CPC-2.

[0067] Example 3 A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material includes the following steps: Preparation of S1, ZnCo-ZIF nanocubes: 5 mg of CTAB was dissolved in 10 mL of DI along with 0.9 mmol of Zn(NO3)2·6H2O and 0.1 mmol of Co(NO3)2·6H2O to obtain solution A.

[0068] 55 mmol of 2-methylimidazole was dissolved in 70 mL of DI to obtain solution B. Then, the above solution A was quickly poured into solution B. After stirring at room temperature for 20 min, the product was filtered, washed thoroughly several times with DI and ethanol, centrifuged, and dried at 65 °C overnight to obtain ZnCo-ZIF with a nanocubic structure.

[0069] S2. Preparation of polydopamine-coated ZnCo-ZIF nanocubic precursor: Weigh 500 mg of ZnCo-ZIF prepared by S1 and ultrasonically disperse it in a 20 mL mixed solution of ethanol / DI with a volume ratio of 1:1. The solution is then ultrasonically treated for 30 min to obtain dispersion A.

[0070] Weigh 500 mg of PDA, 0.5 g of polyether F-127, 1.5 g of polyether P-123 and 2 mL of TMB, and dissolve them in 30 mL of a mixed solution of ethanol / DI with a volume ratio of 1:1. After ultrasonic treatment, a white suspension emulsion is formed, which is denoted as solution B.

[0071] Solution B was rapidly injected into dispersion A, and mechanically stirred for 5 min. Then, 2 mL of ammonia solution was added. The resulting mixed solution was stirred continuously at room temperature for 2 h to obtain the product. The product was washed several times with DI and ethanol, centrifuged, and dried overnight at 65 °C to obtain polydopamine-coated ZnCo-ZIF nanocubic precursor, named ZC-ZIF@PDA.

[0072] S3. Preparation of hollow cubic cobalt / nitrogen-doped carbon nanocomposites: The ZC-ZIF@PDA obtained in S2 was placed in an alumina crucible and transferred to a tube furnace. Under a flowing nitrogen atmosphere, it was carbonized at 900 °C for 2 h at a heating rate of 5 °C / min. The sample was then cooled to room temperature to obtain a hollow cubic cobalt / nitrogen-doped carbon nanocomposite material, named Co@CPC-3.

[0073] Example 4 A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material includes the following steps: Preparation of S1, ZnCo-ZIF nanocubes: 5 mg of CTAB was dissolved in 10 mL of DI along with 0.9 mmol of Zn(NO3)2·6H2O and 0.1 mmol of Co(NO3)2·6H2O to obtain solution A.

[0074] 55 mmol of 2-methylimidazole was dissolved in 70 mL of DI to obtain solution B. Then, the above solution A was quickly poured into solution B. After stirring at room temperature for 20 min, the product was filtered, washed thoroughly several times with DI and ethanol, centrifuged, and dried at 65 °C overnight to obtain ZnCo-ZIF with a nanocubic structure.

[0075] S2. Preparation of polydopamine-coated ZnCo-ZIF nanocubic precursor: Weigh 500 mg of ZnCo-ZIF prepared by S1 and ultrasonically disperse it in a 20 mL mixed solution of ethanol / DI with a volume ratio of 1:1. The solution is then ultrasonically treated for 30 min to obtain dispersion A.

[0076] Weigh 500 mg of PDA, 0.5 g of polyether F-127, 1.5 g of polyether P-123 and 2 mL of TMB, and dissolve them in 30 mL of a mixed solution of ethanol / DI with a volume ratio of 1:1. After ultrasonic treatment, a white suspension emulsion is formed, which is denoted as solution B.

[0077] Solution B was rapidly injected into dispersion A, and mechanically stirred for 5 min. Then, 2 mL of ammonia solution was added. The resulting mixed solution was stirred continuously at room temperature for 2 h to obtain the product. The product was washed several times with DI and ethanol, centrifuged, and dried overnight at 65 °C to obtain polydopamine-coated ZnCo-ZIF nanocubic precursor, named ZC-ZIF@PDA.

[0078] S3. Preparation of hollow cubic cobalt / nitrogen-doped carbon nanocomposites: The ZC-ZIF@PDA obtained in S2 was placed in an alumina crucible and transferred to a tube furnace. It was carbonized at 900 °C for 2 h under a flowing nitrogen atmosphere at a heating rate of 5 °C / min. The sample was then cooled to room temperature to obtain a hollow cubic cobalt / nitrogen-doped carbon nanocomposite material, named Co@CPC-4.

[0079] Example 5 A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material includes the following steps: Preparation of S1, ZnCo-ZIF nanocubes: 5 mg of CTAB was dissolved in 10 mL of DI along with 0.9 mmol of Zn(NO3)2·6H2O and 0.1 mmol of Co(NO3)2·6H2O to obtain solution A.

[0080] 55 mmol of 2-methylimidazole was dissolved in 70 mL of DI to obtain solution B. Then, the above solution A was quickly poured into solution B. After stirring at room temperature for 20 min, the product was filtered, washed thoroughly several times with DI and ethanol, centrifuged, and dried at 65 °C overnight to obtain ZnCo-ZIF with a nanocubic structure.

[0081] S2. Preparation of polydopamine-coated ZnCo-ZIF nanocubic precursor: Weigh 500 mg of ZnCo-ZIF prepared by S1 and ultrasonically disperse it in a 20 mL mixed solution of ethanol / DI with a volume ratio of 1:1. The solution is then ultrasonically treated for 30 min to obtain dispersion A.

[0082] Weigh 500 mg of PDA, 0.5 g of polyether F-127, 1.5 g of polyether P-123 and 2 mL of TMB, and dissolve them in 30 mL of a mixed solution of ethanol / DI with a volume ratio of 1:1. After ultrasonic treatment, a white suspension emulsion is formed, which is denoted as solution B.

[0083] Solution B was rapidly injected into dispersion A, and mechanically stirred for 5 min. Then, 2 mL of ammonia solution was added. The resulting mixed solution was stirred continuously at room temperature for 2 h to obtain the product. The product was washed several times with DI and ethanol, centrifuged, and dried overnight at 65 °C to obtain polydopamine-coated ZnCo-ZIF nanocubic precursor, named ZC-ZIF@PDA.

[0084] S3. Preparation of hollow cubic cobalt / nitrogen-doped carbon nanocomposites: The ZC-ZIF@PDA obtained in S2 was placed in an alumina crucible and transferred to a tube furnace. It was carbonized at 900 °C for 2 h under a flowing nitrogen atmosphere at a heating rate of 5 °C / min. The sample was then cooled to room temperature to obtain a hollow cubic cobalt / nitrogen-doped carbon nanocomposite material, named Co@CPC-5.

[0085] The hollow cubic cobalt-based nitrogen-doped carbon nanocomposites prepared in Examples 1 to 5 were subjected to structural and performance tests, and the results are shown below.

[0086] Figure 1 This is a microstructure diagram of Co@CPC-1 prepared in Example 1 of the present invention. Figure 1In the image, (a) to (c) are scanning electron microscope (SEM) images at different sizes, (d) to (f) are transmission electron microscope (TEM) images at different sizes, and (g) is an EDS mapping image. Figure 1 As shown, after pyrolysis, a cubic carbon framework with a uniformly distributed mesoporous structure was obtained. As indicated by scanning electron microscopy (SEM images), the prepared Co@CPC-1 nanoparticles exhibit a uniform cubic shape with an average particle size of approximately 280 nm to 320 nm. Due to the thin carbon coating on the surface, the Co@CPC-1 nanoparticles exhibit a concave cubic shape, which is caused by the anisotropic thermal contraction of the ZnCo-ZIF core during pyrolysis. Transmission electron microscopy (TEM) reveals that the surface of the cubic nanoparticles is surrounded by a polydopamine-derived carbon layer, forming a unique cavity structure. Simultaneously, a small number of Co nanoparticles are embedded within the hollow cubic nanoparticles. EDS mapping further confirms the uniform distribution of five main elements: C, N, O, Zn, and Co. It can be observed that the elemental signals of Co and Zn gradually decrease as the carbon layer thickness increases.

[0087] Figure 2 The X-ray diffraction patterns are those of ZC-ZIF@PDA and Co@CPC prepared in Examples 1 to 5 of this invention. Figure 2 In the diagram, (a) is the X-ray diffraction pattern of ZC-ZIF@PDA, and (b) is the X-ray diffraction pattern of Co@CPC. Figure 2 As shown, the crystal structure and chemical composition of ZC-ZIF@PDA prepared in Examples 1 to 5 and the pyrolyzed Co@CPC were characterized by XRD tests, respectively. Figure 2 As shown in (a), the diffraction characteristic peak positions of the ZC-ZIF@PDA precursor are basically consistent with those of the ZnCo-ZIF reported in the literature. It can be clearly observed that the diffraction peak intensity of ZnCo-ZIF@PDA gradually weakens with the gradual increase of the surface polydopamine coating. This may be due to the increasing coating thickness, background signal interference from the coating layer, and the absorption and scattering effects on X-rays, leading to a gradual weakening of the XRD diffraction peak signal. Figure 2 In (b), it can be observed that the XRD pattern of the pyrolyzed Co@CPC is located at 2 θ A distinct broad diffraction peak appears at 26.15°, corresponding to the (002) crystal plane of carbon (JCPDS#41-1487). Simultaneously, a peak at 2... θ Two distinct diffraction peaks appeared at 44.26° and 51.58°, corresponding to the (111) and (200) crystal planes of metallic Co (JCPDS #97-067-1069), respectively. The strongest diffraction peak appeared at 2... θ=44.26° corresponds to the (111) crystal plane of Co, which is consistent with the HRTEM measurement results. Similarly, as the coating amount increases, the intensity of the corresponding diffraction peak of metallic Co gradually decreases. In summary, this indicates that the Co@CPC series samples mainly contain the crystal structure of metallic Co and graphitic carbon.

[0088] Figure 3 The X-ray photoelectron spectra of Co@CPC prepared in Examples 1 to 5 of this invention are shown below. Figure 3 In the image, (a) is the full spectrum, (b) is the C 1s spectrum, (c) is the N 1s spectrum, and (d) is the Co 2p spectrum. Figure 3 As shown, X-ray photoelectron spectroscopy was used to characterize the chemical composition, elemental valence states, and electronic structure of the composite material surface. Figure 3 As shown in (a), the X-ray photoelectron spectra of the Co@CPC samples prepared in Examples 1 to 5 are shown. Five characteristic peaks were detected in the total X-ray photoelectron spectrum: C 1s, N 1s, O 1s, Zn 2p, and Co 2p, indicating that the Co@CPC samples contain all five elements: C, N, O, Zn, and Co. Figure 3 As shown in (b), in the fine spectrum of C 1s, the positions at approximately 284.8 eV, 285.3 eV, 286.3 eV, and 288.7 eV correspond to CC / C=C, C−O / CN, C=O, and O−C=O bonds, respectively. Abundant carbon-oxygen groups generally facilitate the introduction of polar molecules and asymmetric charge distributions, thereby enhancing dipole polarization and potentially improving the microwave absorption performance of Co@CPC. Figure 3 As shown in (c), the N 1s fine spectrum reveals distinct characteristic peaks corresponding to graphite N, pyrrole N, Co-Nx, and pyridine N, respectively. Figure 3 As shown in (d), in the Co 2p spectrum, metallic Co... 0 The characteristic peak appears at 777.6 eV, Co 3+ The characteristic peak appears at 780.2 eV, Co 2+ The characteristic peak appears at 783.8 eV, and the satellite peak appears at 795.6 eV. This indicates that the Co nanoparticles in Co@CPC mainly exist as Co. 0 Co 2+ and Co 3+ It exists in the form of.

[0089] Figure 4 The dielectric and permeability diagrams are for the Co@CPC prepared in Examples 1 to 5 of this invention. Figure 4In the diagram, (a) represents the real part of the dielectric constant, (b) represents the imaginary part of the dielectric constant, (c) represents the dielectric loss tangent, (d) represents the real part of the permeability, (e) represents the imaginary part of the permeability, and (f) represents the magnetic loss tangent. The electromagnetic parameters of the Co@CPC-1 / 2 / 3 / 4 / 5 series composite microwave absorbing materials obtained through testing are used to further analyze the influence of structural control and the carbon coating layer on the microwave absorption performance. For example... Figure 4 As shown in (a), the real dielectric part of Example 1 Co@CPC-1 is the highest in the low-frequency range, and gradually decreases from 14.41 to about 7 as the frequency increases, indicating that the dielectric polarization effect of Co@CPC-1 is strong in the low-frequency range. The real dielectric parts of Examples 2 Co@CPC-2, 4 Co@CPC-4, and 5 Co@CPC-5 are basically consistent in the low-frequency range. In the mid-to-high frequency range (12 GHz to 16 GHz), the real dielectric part of Example 2 Co@CPC-2 reaches the highest, indicating that its dielectric polarization performance is strong in the 12 GHz to 16 GHz frequency range. The real dielectric part of Example 3 Co@CPC-3 is generally smaller than that of the other examples (decreased from 6.89 to 3.48), indicating that its dielectric polarization ability is relatively weak compared to other samples. As the frequency increases, the real dielectric part of all Co@CPC samples in Examples 1 to 5 shows a decreasing trend. This is because dipole polarization and interfacial polarization are difficult to follow the changes in the applied electric field during the transition from low to high frequency. A higher real dielectric part may originate from enhanced interfacial polarization between the carbon coating layer and Co nanoparticles in the material. The imaginary dielectric part reflects the dielectric loss capability of the material and is directly related to its electromagnetic wave absorption capability. Figure 4 In (b), the imaginary part of the dielectric of Co@CPC-1 is significantly higher than that of other samples in the low-frequency region, indicating that this material has higher dielectric loss in the low-frequency range. In the high-frequency range (>10 GHz), the imaginary parts of the dielectric of Examples 2 (Co@CPC-2), 3 (Co@CPC-3), 4 (Co@CPC-4), and 5 (Co@CPC-5) all show varying degrees of increase, with Example 5 (Co@CPC-5) reaching its maximum value between 12 GHz and 18 GHz, indicating enhanced dielectric loss capability at high frequencies. The dielectric loss factor allows for a direct comparison and analysis of the dielectric loss capability of each sample. Figure 4In (c), Co@CPC-1 and Co@CPC-2 of Example 2 exhibit strong losses in the low-frequency range, while the dielectric loss factor of Co@CPC-5 of Example 5 shows a significant peak (around 0.92) in the high-frequency range, indicating that it has high dielectric loss performance in the high-frequency range. This is because the higher carbon coating content improves the dielectric properties of the material. Co@CPC-3 of Example 3 shows a similar significant peak (around 0.82) to Co@CPC-5 of Example 5 in the 13 GHz to 18 GHz frequency band, indicating that it also has high dielectric loss performance in the high-frequency range, and its loss capability is significantly higher than that of Co@CPC-2, Co@CPC-3, and Co@CPC-4 of Example 4.

[0090] The real part of the permeability in electromagnetic parameters reflects a material's ability to store magnetic energy, and is related to the material's magnetic domains and magnetization. Figure 4 As shown in (d), the real part of the permeability of Example 3 Co@CPC-3 is higher than that of other samples in most frequency bands. The higher real part of the permeability indicates that the magnetic Co nanoparticles in the material are uniformly distributed and have good permeability, indicating that it has a stronger magnetization ability. It is not difficult to find that the real part of the permeability of Example 5 Co@CPC-5, which has the strongest dielectric loss capability, gradually decreases to the lowest value (around 0.78) in the range of 10 GHz to 15 GHz, indicating that it has the weakest magnetization performance in the mid-to-high frequency range. The real part of the permeability of each sample decreases in the high frequency range. This is because the magnetic domain movement and magnetization hysteresis of magnetic Co particles are difficult to follow the changes in high frequency magnetic fields. The imaginary part of the permeability reflects the magnetic loss capability of the material, which is related to mechanisms such as hysteresis loss, eddy current loss, and natural resonance. Figure 4 As shown in (e), Example 3 Co@CPC-3 exhibits relatively high magnetic loss in the low-frequency range, indicating its strong magnetic loss capability in this frequency band. In contrast, Example 5 Co@CPC-5 shows more significant magnetic loss in the high-frequency range, possibly related to the natural resonance and exchange resonance of its internal magnetic particles. Figure 4 In (f), the magnetic loss tangent of Example 5 Co@CPC-5 is shown.

[0091] Figure 5 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 1 of this invention. Figure 5 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram. Figure 6 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 2 of this invention. Figure 6 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram. Figure 7The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 3 of this invention. Figure 7 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram. Figure 8 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 4 of this invention. Figure 8 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram. Figure 9 The images show the three-dimensional and corresponding two-dimensional reflection loss diagrams of the Co@CPC prepared in Example 9 of this invention. Figure 9 In the diagram, (a) is a three-dimensional reflection loss diagram, and (b) is the corresponding two-dimensional reflection loss diagram. Figure 10 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 1 of this invention. Figure 11 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 2 of the present invention. Figure 12 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 3 of the present invention. Figure 13 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 4 of this invention. Figure 14 This is a graph showing the relationship between the reflection loss curve and the matching thickness of the Co@CPC prepared in Example 5 of the present invention.

[0092] This invention systematically explores the influence of hollow nanocage carbon coatings on the electromagnetic wave absorption performance of composite materials. The minimum reflection loss and effective absorption bandwidth of the Co@CPC series materials prepared in Examples 1 to 5 were analyzed, and the results are as follows: Figures 5-9 and Figures 10-14As shown. By adjusting the amount of carbon coating on the surface of the hollow nanocage, the influence of interfacial impedance performance on microwave absorption performance was studied, and a detailed analysis was conducted based on the theoretical mechanism. Example 1, Co@CPC-1, achieved a minimum reflection loss of -19.31 dB (7.60 GHz) with a matching thickness of 3.24 mm, and an effective absorption bandwidth of 6.16 GHz (11.84 GHz to 18 GHz) at a thickness of 1.95 mm, covering the entire Ku band. When the carbon layer is thin, electromagnetic waves can easily penetrate the carbon layer and enter the interior of the nanocage, but the low attenuation capability prevents the incident electromagnetic waves from forming an effective conversion inside. With the increase of the carbon coating layer, Example 2, Co@CPC-2, showed a significantly improved minimum reflection loss to -51.23 dB (16.00 GHz, 1.71 mm), and an effective absorption bandwidth of 5.76 GHz (12.24 GHz to 18 GHz) at a thickness of 1.87 mm. Example 3: Co@CPC-3 achieved a minimum reflection loss of -52.63 dB (13.60 GHz) with a matching thickness of 2.53 mm. Its effective absorption bandwidth reached a maximum of 8.56 GHz (9.44 GHz to 18 GHz) at a thickness of 2.88 mm, covering 64% of the X-band and the entire Ku-band. This is because the interface between the Co particles and the carbon layer forms a heterojunction, generating interfacial polarization, which helps to enhance the complex permittivity. Furthermore, the appropriately thick carbon layer balances conductivity and interfacial polarization effects, improving interfacial impedance and achieving impedance matching in the mid-to-low frequency range. Example 4: Co@CPC-4 achieved a minimum reflection loss of -52.68 dB (8.32 GHz) with a thickness of 3.07 mm and an effective absorption bandwidth of 6.32 GHz (11.68 GHz to 18 GHz) at a thickness of 2.02 mm, also exhibiting excellent broadband absorption characteristics. In contrast, Example 5 Co@CPC-5 exhibited a minimum reflection loss of -46.12 dB (7.76 GHz) with a matching thickness of 3.22 mm, and an effective absorption bandwidth of 3.60 GHz (8.32 GHz to 11.92 GHz) with a matching thickness of 2.53 mm, covering part of the X-band. The performance degradation may be due to the excessive thickness of the carbon layer leading to a significant increase in the complex permittivity, which in turn induces the skin effect. The increased interfacial impedance limits the deep propagation of electromagnetic waves within the material, weakens the absorption capacity in the low-frequency range, reduces the impedance matching degree, and causes electromagnetic waves to be reflected at the material surface, resulting in a decrease in absorption performance. It exhibits a peak value in the high-frequency range (>12 GHz), indicating that it has a higher magnetic loss capability at high frequencies. The magnetic losses of Examples 1 Co@CPC-1 and 3 Co@CPC-3 are mainly concentrated in the mid-to-low frequency range (4 GHz to 10 GHz).In summary, Example 3, Co@CPC-3, exhibits higher real and imaginary permeability values, demonstrating better magnetic properties, especially achieving stronger magnetic loss at low frequencies. Co@CPC-5 exhibits the highest dielectric loss tangent and magnetic loss tangent at high frequencies, indicating that it is more suitable for electromagnetic wave absorption at high frequencies.

[0093] Figure 15 These are comparison diagrams of optimal reflection loss and effective absorption bandwidth for the Co@CPC prepared in Examples 1 to 5 of this invention. Figure 15 In the diagram, (a) shows the comparison of optimal reflection loss, and (b) shows the comparison of effective absorption bandwidth. For example... Figure 15 As shown, the reflection loss and absorption bandwidth of the Co@CPC series materials from Examples 1 to 5 were compared. Example 3, Co@CPC-3, exhibited the best absorption performance, with a minimum reflection loss of -52.63 dB and an effective absorption bandwidth of 8.56 GHz. This indicates that the material achieved optimal impedance matching and electromagnetic wave absorption performance through the synergistic effect of magnetic and dielectric losses. In contrast, Examples 1, Co@CPC-1, and 5, Co@CPC-5, performed poorly. Co@CPC-1 had relatively weak attenuation capabilities, resulting in insufficient absorption. Example 5, Co@CPC-5, had the worst absorption performance due to its excessively thick carbon coating and high complex dielectric constant, leading to poor impedance matching. Overall, Example 3, Co@CPC-3, achieved the best balance between strong loss capability and ultra-wideband absorption, making it the best absorber in this series. The precise design of the hollow nanostructure not only adjusted the impedance matching between the material and free space, but also maintained a balance between magnetic and dielectric losses while achieving wideband absorption. These characteristics verify that the effective absorption bandwidth of absorbing materials can be effectively improved by precisely designing special microstructures and optimizing interface composition.

[0094] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

[0095] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material, characterized in that, Includes the following steps: Using a nanocubic ZnCo-ZIF structure as the core, polydopamine was coated onto the nanocubic ZnCo-ZIF structure by precipitation to form a core-shell structure precursor. The precursor was then carbonized at 800 ℃~900 ℃ under a protective gas atmosphere to obtain a hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material.

2. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 1, characterized in that, The method for preparing a core-shell structured precursor includes the following steps: Dopamine hydrochloride, polyether, and mesitylene were dissolved in a first solvent to obtain a mixed solution. The mixed solution was then added to a nanocubic ZnCo-ZIF dispersion and stirred at room temperature to obtain a core-shell structured precursor.

3. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite materials according to claim 2, characterized in that, The ratio of dopamine hydrochloride, polyether, and mesitylene is 0.5 g to 1.5 g: 2 g: 2 mL; the polyether is a mixture of polyether F-127 and polyether P-123 in a mass ratio of 1:2 to 3.

4. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 2, characterized in that, The mass ratio of nanocubic ZnCo-ZIF to dopamine hydrochloride in the nanocubic ZnCo-ZIF dispersion is 0.5:0.5-1.

5.

5. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 1, characterized in that, The carbonization time is 1.5 h to 3 h, the protective gas is nitrogen, and the heating rate is 3 ℃ / min to 5 ℃ / min.

6. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 1, characterized in that, The preparation method of nanocubic ZnCo-ZIF includes the following steps: A soluble zinc salt, a soluble cobalt salt, and a surfactant are dissolved in a second solvent to obtain a mixed solution. After mixing the mixed solution and the organic ligand solution, the nanocubic structure ZnCo-ZIF was obtained by stirring at room temperature.

7. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 6, characterized in that, The molar ratio of soluble zinc salt, soluble cobalt salt, and surfactant is 6.5 mmol–12.5 mmol: 0.65 mmol–1.5 mmol: 0.05 g. The concentration of soluble zinc salt in the mixed solution is 0.08 mol / L–0.1 mol / L, the concentration of soluble cobalt salt is 0.008 mol / L–0.012 mol / L, the soluble zinc salt is zinc nitrate hexahydrate, the soluble cobalt salt is cobalt nitrate hexahydrate, and the surfactant is hexadecyltrimethylammonium bromide.

8. The method for preparing hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 6, characterized in that, The concentration of the organic ligand solution is 0.75 mol / L to 0.8 mol / L, and the volume ratio of the mixed solution to the organic ligand solution is 1:1 to 1.

2.

9. A hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material, characterized in that, It is prepared using the preparation method according to any one of claims 1 to 8.

10. The application of the hollow cubic cobalt-based nitrogen-doped carbon nanocomposite material according to claim 9 in electromagnetic wave absorption.

Images