A MOF-based hollow rod-shaped electromagnetic wave absorbing material and its preparation method and application

Abstract

The present invention belongs to the field of new material technology, and specifically discloses a MOF-based hollow rod-shaped electromagnetic wave absorbing material, a preparation method thereof, and an application thereof. The electromagnetic wave absorbing material has a hollow rod-shaped structure, including metal atoms and carbon nanotubes, and the carbon nanotubes are coated on the surface of the electromagnetic wave absorbing material. The present invention first disperses rod-shaped ZnO in an alcohol solution, adds a soluble metal salt to obtain a mixed solution; then adds the mixed solution to a 2-methylimidazole solution, mixes, ages, separates, and obtains a ZnO-MOF precursor; and finally performs an annealing treatment. The obtained electromagnetic wave absorbing material has excellent electromagnetic wave absorption performance. When the material filling amount is 20wt%, the minimum reflection loss can be increased from 3.74 to 66.8dB; the effective absorption bandwidth is increased from 0 to 5.44GHz, which is a huge improvement compared to a single rod-shaped ZnO.

Description

Technical Field

[0001] The present invention belongs to the technical field of new materials, and particularly relates to an electromagnetic wave absorbing material, specifically to a MOF-based hollow rod-shaped electromagnetic wave absorbing material, and a preparation method and application thereof. Background Art

[0002] Developing high-performance electromagnetic wave absorbing materials to reduce or eliminate the adverse effects of electromagnetic waves. An ideal electromagnetic wave absorbing material should effectively capture electromagnetic waves and convert electromagnetic energy into other forms. In addition, for practical applications, it should have low mass and thin thickness.

[0003] Generally, the magnetic adsorption properties of a material are related to its composition and microstructure. Metal-organic framework (MOF) materials have attracted much attention due to their diverse structures, high porosity and large specific surface area. However, due to poor impedance matching and high filling rate, MOF materials alone cannot meet the requirements of practical applications. In order to address this limitation, researchers have focused on the structural design of MOF materials and their derivatives to achieve excellent impedance matching and microwave dissipation capabilities. Although many studies have prepared electromagnetic wave absorption materials with excellent performance based on structural design, there are still complex preparation processes and high preparation costs. At the same time, the prepared electromagnetic wave absorption materials also have problems such as poor absorption performance, narrow effective absorption bandwidth and large loading capacity.

[0004] Therefore, it is of great significance to develop an electromagnetic wave absorbing material with simple preparation process, low preparation cost and excellent performance. Summary of the Invention

[0005] The present invention aims to address at least one of the technical problems existing in the aforementioned prior art. To this end, the present invention provides a MOF-based hollow rod-shaped electromagnetic wave absorbing material, its preparation method, and its application. The MOF-based hollow rod-shaped electromagnetic wave absorbing material exhibits excellent impedance matching and microwave dissipation capabilities, a large effective absorption bandwidth, and further has the advantage of a small loading capacity.

[0006] To solve the above technical problems, the first aspect of the present invention provides an electromagnetic wave absorbing material, which has a hollow rod-shaped structure. The electromagnetic wave absorbing material includes metal atoms and carbon nanotubes, and the carbon nanotubes are coated on the outer surface of the electromagnetic wave absorbing material.

[0007] Specifically, the electromagnetic wave absorption material of the present invention has a hollow rod-like structure and forms M@C (M=Co, Ni, Fe, Cu) nanorods, which are composed of carbon nanotubes coated with metal atoms. The high specific surface area of ​​the hollow structure results in a large amount of incident waves entering the material, rather than unfavorable surface reflection. Simultaneously, the incident electromagnetic waves are scattered and reflected multiple times between the hollow interior and the abundant carbon nanotubes on the surface, extending the transmission path and thus improving electromagnetic wave dissipation. Furthermore, the hollow rod-like structures are easily interconnected to form conductive pathways, providing channels for the transmission of charge carriers and leading to the conversion of electromagnetic energy into heat. Furthermore, positive and negative charges accumulate in the numerous heterogeneous interface regions of the hollow rod-shaped electromagnetic wave absorption material and rearrange in high-frequency electromagnetic fields, leading to the conversion of electromagnetic energy into heat. Defects and oxygen-containing groups within the material induce the generation of dipoles, which, when subjected to an external electromagnetic field, align in an orderly manner, resulting in strong dipole polarization. Therefore, the M@C (M=Co, Ni, Fe, Cu) nanorods of the present invention have excellent electromagnetic wave absorption properties.

[0008] Preferably, the metal atom includes at least one of cobalt, nickel, iron, and copper; further preferably, the metal atom is cobalt.

[0009] Preferably, the length and diameter of the electromagnetic wave absorbing material are 1-3 μm and 200-600 nm, respectively; further preferably, the length and diameter of the electromagnetic wave absorbing material are 1.5-3 μm and 300-600 nm, respectively.

[0010] Preferably, the aspect ratio of the electromagnetic wave absorbing material is 1:(2-5); further preferably, the aspect ratio of the electromagnetic wave absorbing material is 1:(3-5).

[0011] A second aspect of the present invention provides a method for preparing the above-mentioned electromagnetic wave absorbing material, comprising the following steps:

[0012] (1) Dispersing rod-shaped ZnO in an alcohol solution and adding a soluble metal salt to obtain a mixed solution;

[0013] (2) adding the mixed solution to a 2-methylimidazole solution, mixing, aging, and separating to obtain a ZnO-MOF precursor;

[0014] (3) Annealing the ZnO-MOF precursor to thermally reduce ZnO to obtain the electromagnetic wave absorbing material having a hollow rod-like structure.

[0015] Specifically, the electromagnetic wave absorbing material of the present invention is prepared using a template sacrificial method. First, a dense MOF material is grown on a rod-shaped ZnO template to produce a core-shell ZnO-MOF precursor. The ZnO template in the core layer is then removed by calcination, and the MOF material in the shell layer is simultaneously carbonized to form dense, curved carbon nanotubes, thereby producing hollow M@C (M=Co, Ni, Fe, Cu) nanorods with a large number of carbon nanotubes grown on their surfaces. This simple and cost-effective preparation process allows the production of MOF-based hollow rod-shaped electromagnetic wave absorbing materials with excellent performance through simple room-temperature mixing (e.g., stirring) and annealing.

[0016] Preferably, in step (1), the alcohol solution includes methanol or ethanol; further preferably, the alcohol solution is methanol.

[0017] Preferably, in step (1), the soluble metal salt includes at least one of a soluble cobalt salt, a nickel salt, an iron salt, and a copper salt; further preferably, the soluble metal salt is a soluble cobalt salt.

[0018] Preferably, the soluble cobalt salt is selected from at least one of cobalt nitrate, cobalt chloride, cobalt sulfate, and cobalt nitrate hexahydrate.

[0019] Preferably, in step (1), the dispersion is carried out by ultrasonic dispersion, and the dispersion time is 1-3 hours.

[0020] Preferably, in step (2), the mass ratio of rod-shaped ZnO, soluble metal salt, and 2-methylimidazole in the raw materials for preparing the ZnO-MOF precursor is (0.05-2):(0.2-5):2.25; further preferably, the mass ratio of rod-shaped ZnO, soluble metal salt, and 2-methylimidazole is (0.1-1):(0.5-2.5):2.25. By controlling the mass ratio of rod-shaped ZnO and soluble metal salt, it is beneficial to optimize the microstructure of the material, thereby improving the impedance matching of the material.

[0021] Preferably, in step (2), the solvent of the 2-methylimidazole solution is an alcohol solution, and the alcohol solution includes methanol or ethanol.

[0022] Preferably, in step (2), the mixing is carried out by stirring at a rate of 100-1000 rpm and for a time of 0.5-2 hours; further preferably, the stirring rate is 400-600 rpm and for a time of 0.5-1.5 hours.

[0023] Preferably, in step (2), the aging time is 2-12 hours; further preferably, the aging time is 2-9 hours.

[0024] Preferably, in step (3), the temperature of the annealing treatment is 400-1000°C; further preferably, the temperature of the annealing treatment is 500-900°C.

[0025] Preferably, in step (3), the annealing treatment time is 1-5 hours; further preferably, the annealing treatment time is 1-3 hours.

[0026] Preferably, in step (3), the annealing treatment is performed under the protection of an inert gas, and the inert gas includes nitrogen or argon.

[0027] Preferably, in step (3), the heating rate of the annealing treatment is 1-20°C / min; further preferably, the heating rate of the annealing treatment is 1-10°C / min.

[0028] Preferably, the preparation process of the rod-shaped ZnO is: mixing zinc salt, hexamethylenetetramine and water, heating and reacting; cooling, washing and drying to obtain the rod-shaped ZnO.

[0029] Preferably, the zinc salt is selected from at least one of zinc acetate, zinc nitrate and zinc chloride.

[0030] Preferably, the water is deionized water.

[0031] Preferably, the mass ratio of the zinc salt to hexamethylenetetramine is (0.2-0.5):(0.1-0.6); further preferably, the mass ratio of the zinc salt to hexamethylenetetramine is (0.2-0.4):(0.2-0.3).

[0032] Preferably, the mass volume ratio of the zinc salt and water is (0.2-0.4) g: (20-100) mL; further preferably, the mass volume ratio of the zinc salt and water is (0.2-0.4) g: (20-80) mL.

[0033] Preferably, the heating temperature is 80-100° C., and the holding time is 8-12 hours; further preferably, the heating temperature is 85-90° C., and the holding time is 10-12 hours.

[0034] Preferably, the mixing method is stirring.

[0035] A third aspect of the present invention provides an electromagnetic wave absorbing device, which includes the above-mentioned electromagnetic wave absorbing material.

[0036] Compared with the prior art, the above technical solution of the present invention has at least the following technical effects or advantages:

[0037] (1) The present invention achieves excellent impedance matching and microwave dissipation capabilities of electromagnetic wave absorption materials by constructing hollow and one-dimensional rod-like structures and forming M@C (M=Co, Ni, Fe, Cu) nanorods coated with metal atoms in carbon nanotubes. On the one hand, the electromagnetic wave absorption material utilizes the large inner cavity, pore volume, specific surface area and low density of the hollow structure to improve impedance matching and promote the entry of electromagnetic waves into the interior of the material; on the other hand, it utilizes the one-dimensional rod-like structure to easily form a conductive network and a large number of heterojunction interfaces, thereby improving the dielectric loss capacity. Under the combined action of multiple electromagnetic wave loss mechanisms, it ensures strong absorption of incident electromagnetic waves, thereby achieving excellent electromagnetic wave absorption performance. When the material filling amount is 20wt%, the minimum reflection loss can be increased from -3.74 to -66.8dB; the effective absorption bandwidth is increased from 0 to 5.44GHz, which is a significant improvement compared to single rod-like ZnO.

[0038] (2) The present invention adopts a rod-shaped ZnO template sacrificial method to prepare a MOF-based hollow rod-shaped electromagnetic wave absorbing material. The preparation process is simple and the preparation cost is low. Only a simple room temperature mixing and annealing process is required to prepare a MOF-based hollow rod-shaped electromagnetic wave absorbing material with excellent performance, which greatly simplifies the preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the X-ray diffraction pattern of the rod-shaped ZnO prepared in Example 1;

[0040] Figure 2 X-ray diffraction patterns of the MOF-based hollow rod-shaped electromagnetic wave absorbing materials prepared in Examples 1-3;

[0041] Figure 3 This is the SEM image of the rod-shaped ZnO prepared in Example 1;

[0042] Figure 4 This is an SEM image of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 1-3;

[0043] Figure 5 This is the SEM image of the rod-shaped MoO3 prepared in Comparative Example 1;

[0044] Figure 6 This is the SEM image of the core-shell MoO3 / Co rod-shaped electromagnetic wave absorbing material prepared in Comparative Example 1;

[0045] Figure 7 This is a graph showing the microwave absorption performance of the rod-shaped ZnO prepared in Example 1;

[0046] Figure 8 Graph showing microwave absorption performance of MOF-based hollow rod-shaped electromagnetic wave absorbing materials prepared in Examples 1-3;

[0047] Figure 9 This is a diagram of the microwave absorption performance of the core-shell MoO3 / Co rod-shaped electromagnetic wave absorption material prepared in Comparative Example 1. DETAILED DESCRIPTION

[0048] The present invention is described in detail below with reference to the examples to facilitate understanding of the present invention by those skilled in the art. It is necessary to point out that the examples are only used to further illustrate the present invention and are not to be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above-mentioned invention should still fall within the scope of protection of the present invention. At the same time, the raw materials mentioned below that are not described in detail are all commercially available products; the process steps or preparation methods that are not mentioned in detail are all process steps or preparation methods known to those skilled in the art.

[0049] Example 1

[0050] A method for preparing a MOF-based hollow rod-shaped electromagnetic wave absorbing material comprises the following steps:

[0051] (1) Preparation of rod-shaped ZnO: 0.3 g of anhydrous zinc acetate and 0.3 g of hexamethylenetetramine were added to 60 mL of deionized water and stirred for 2 hours to obtain a mixture. The mixture was transferred to a 100 mL PTFE (polytetrafluoroethylene) liner and maintained at 90°C for 12 hours. The mixture was then cooled to room temperature (25°C) and washed with deionized water. The product was dried at 60°C to obtain rod-shaped ZnO.

[0052] (2) Preparation of MOF-based hollow rod-shaped electromagnetic wave absorbing material: 0.3 g of the rod-shaped ZnO prepared in step (1) was added to 40 mL of methanol, and 2 mmol of cobalt nitrate hexahydrate was added and ultrasonically dispersed for 2 hours to obtain a mixed solution A; 2.2528 g of 2-methylimidazole was added to 40 mL of methanol to obtain a solution B; solution B was poured into the mixed solution A, stirred at a rate of 500 rpm, stirred for 1 hour, and aged for 3 hours; the product was collected by centrifugation with methanol and dried at 60°C for 24 hours to obtain a ZnO-MOF precursor; finally, under nitrogen atmosphere protection, annealed at 800°C for 2 hours at a heating rate of 2°C / min to thermally reduce ZnO, and the MOF-based hollow rod-shaped electromagnetic wave absorbing material of this embodiment was obtained, which was recorded as HCCN-1.

[0053] Example 2

[0054] Referring to the preparation method of Example 1 of the present invention, by changing the addition amount of rod-shaped ZnO in step (2) to 0.4 g, the MOF-based hollow rod-shaped electromagnetic wave absorbing material of this example is obtained, which is recorded as HCCN-2.

[0055] Example 3

[0056] Referring to the preparation method of Example 1 of the present invention, by changing the addition amount of rod-shaped ZnO in step (2) to 0.5 g, the MOF-based hollow rod-shaped electromagnetic wave absorbing material of this example is obtained, which is recorded as HCCN-3.

[0057] Comparative Example 1

[0058] A method for preparing a MoO3 / Co core-shell rod-shaped electromagnetic wave absorbing material comprises the following steps:

[0059] (1) Preparation of rod-shaped MoO3: 1.1586 g of ammonium molybdate tetrahydrate was dissolved in 60 mL of deionized water, stirred for 10 minutes, 5 mL of nitric acid was added, and stirring was continued for 20 minutes. The solution was transferred to a polytetrafluoroethylene-lined stainless steel autoclave (50 mL) and maintained at 180°C for 12 hours; then cooled to room temperature (25°C), washed with deionized water, and dried at 60°C to obtain rod-shaped MoO3.

[0060] (2) Preparation method of MoO3 / Co core-shell rod-shaped electromagnetic wave absorption material: 0.4 g of the rod-shaped MoO3 prepared in step (1) was added to 40 mL of methanol, and 2 mmol of cobalt nitrate hexahydrate was added and ultrasonically dispersed for 2 hours to obtain a mixed solution A. 2.2528 g of 2-methylimidazole was added to 40 mL of methanol to obtain a solution B; the solution B was poured into the mixed solution A, stirred at a rate of 500 rpm, stirred for 1 hour, and aged for 3 hours; the product was collected by centrifugation with methanol and dried at 60°C for 24 hours to obtain a MoO3-MOF precursor; finally, the product was annealed at 800°C for 2 hours at a heating rate of 2°C / min under a N2 atmosphere to obtain the MoO3 / Co core-shell rod-shaped electromagnetic wave absorption material of this comparative example.

[0061] Performance Testing

[0062] 1. Component Analysis

[0063] Figure 1 is the X-ray diffraction pattern of the rod-shaped ZnO prepared in Example 1, Figure 1 In: The horizontal axis 2θ (degree) represents the diffraction angle (degrees), and the vertical axis Intensity represents the intensity of the diffraction peak. Figure 1 It can be seen that rod-shaped ZnO has diffraction peaks at 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8°, 66.3°, 67.9° and 69.0°, which are consistent with the peak shape of the ZnO standard card (ZnO standard PDF).

[0064] Figure 2 The X-ray diffraction patterns of the MOF-based hollow rod-shaped electromagnetic wave absorbing materials prepared in Example 1-3 are as follows: Figure 2It can be seen that the MOF-based hollow rod-shaped electromagnetic wave absorption materials HCCN-1, HCCN-2 and HCCN-3 prepared in Examples 1-3 all have diffraction peaks at 44.2°, 51.5° and 75.8°, which correspond to the absorption peaks of the metal Co standard card (metal Co standard PDF), respectively, indicating that the main component of the MOF-based hollow rod-shaped electromagnetic wave absorption materials prepared in Examples 1-3 is metal Co.

[0065] 2. Microstructure

[0066] Figure 3 The SEM image of ZnO prepared in Example 1 is shown in FIG. Figure 3 It can be seen that ZnO has a rod-like structure with a diameter of about 100 nm and a large aspect ratio (about 10-18:1).

[0067] Figure 4 The SEM images of the MOF-based hollow rod-shaped electromagnetic wave absorbing materials prepared in Examples 1-3 are shown in FIG. Figure 4 a. Figure 4 b and Figure 4 c are SEM images of MOF-based hollow rod-shaped electromagnetic wave absorbing materials prepared in Examples 1-3; Figure 4 d is a SEM image of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 2 at a high magnification; Figure 4 e is a SEM image of the cross section of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 2. Figure 4 It can be seen that the electromagnetic wave absorbing material prepared by the present invention has a hollow structure, and a large number of carbon nanotubes grow on the surface of the material, which greatly increases the specific surface area of ​​the material; at the same time, the material also has a rod-like structure with an aspect ratio of about 1: (3-5). However, the addition amount of different rod-like ZnO has a significant effect on the apparent morphology of the material: Figure 4 a It can be seen that HCCN-1 (the addition amount of rod-shaped ZnO is 0.3 g) has some irregular particles that are not in rod shape, HCCN-2 (the addition amount of rod-shaped ZnO is 0.4 g) presents a relatively uniform rod-shaped structure, and HCCN-3 (the addition amount of rod-shaped ZnO is 0.5 g) shows some irregular particles that are not in rod shape.

[0068] Figure 5 The SEM image of MoO3 prepared in Comparative Example 1 is shown in FIG. Figure 5 It can be seen that MoO3 has a rod-like structure with a diameter of about 200nm and a large aspect ratio (about 10-18:1).

[0069] Figure 6 This is the SEM image of the electromagnetic wave absorbing material prepared in Comparative Example 1. Figure 6As can be seen in the figure, the material still maintains a rod-like structure and Co nanoparticles are attached to the rough surface.

[0070] 3. Electromagnetic properties

[0071] The electromagnetic performance test method is as follows: 20wt% of the sample to be tested is uniformly mixed with 80wt% paraffin wax, and the mixture is compacted into a ring (Φout = 7mm, Φin = 3.04mm). The electromagnetic parameters in the range of 2-18GHz are collected using a vector network analyzer (VNA, N5234A). The calculation is based on the following formula:

[0072]

[0073]

[0074] where Z in and Z0 is the input impedance of the absorber and free space, μ r and ε r are the complex magnetic permeability and complex permittivity, d is the thickness of the sample, f is the frequency of the microwave, and c is the speed of light.

[0075] Figure 7 This is a graph showing the microwave absorption performance of the rod-shaped ZnO prepared in Example 1. Figure 7 In the figure, the horizontal axis Thickness represents the thickness, the horizontal axis Frequency represents the frequency, and the vertical axis Reflection loss represents the reflection loss. min Indicates the minimum reflection loss, EAB max Represents the maximum effective absorption bandwidth, and d represents the thickness. Figure 7 It can be seen that when the filling amount of rod-shaped ZnO in paraffin is 20wt%, the minimum reflection loss RL min =-3.74dB, corresponding to a thickness of 4mm; the maximum effective absorption bandwidth is 0.

[0076] Figure 8 This is a graph showing the microwave absorption performance of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 1-3 when the filling amount in paraffin is 20 wt%, wherein: Figure 8 a and Figure 8 b is a graph showing the microwave absorption performance of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 1; Figure 8 c and Figure 8 d is a graph showing the microwave absorption performance of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 2; Figure 8 e and Figure 8 f is a graph showing the microwave absorption performance of the MOF-based hollow rod-shaped electromagnetic wave absorbing material prepared in Example 3. Figure 8 c and Figure 8d It can be seen that when the filling amount of HCCN-2 in paraffin is 20wt%, the minimum reflection loss RL min =-66.8dB, corresponding to a thickness of 2.1mm; the maximum effective absorption bandwidth is 5.44GHz (12.56-18GHz), corresponding to a thickness of 1.9mm, and its electromagnetic performance is greatly improved compared with the rod-shaped ZnO prepared in Example 1.

[0077] Figure 9 The microwave absorption performance diagram of the core-shell MoO3 / Co rod-shaped electromagnetic wave absorbing material prepared in Comparative Example 1 is shown in FIG. Figure 9 It can be seen that when the filling amount of MoO3 / Co in paraffin is 20wt%, the minimum reflection loss RL min =-44.9dB, corresponding to a thickness of 2.5mm; the maximum effective absorption bandwidth is 4GHz (14-18GHz) corresponding to a thickness of 1.4mm, and its electromagnetic performance is far inferior to the MOF-based hollow rod-shaped electromagnetic wave absorption material prepared in Example 2 of the present invention.

[0078] For those skilled in the art to which the present invention belongs, a number of simple deductions or substitutions can be made without departing from the concept of the present invention, without having to resort to creative work. Therefore, based on the disclosure of the present invention, simple improvements made by those skilled in the art to the present invention should be within the scope of protection of the present invention. The above embodiments are preferred embodiments of the present invention, and all processes similar to the present invention and equivalent changes made should fall within the scope of protection of the present invention.

Claims

1. A method for preparing an electromagnetic wave absorbing material, characterized in that: The following steps are involved: (1) Disperse rod-shaped ZnO in an alcohol solution and add a soluble metal salt to obtain a mixed solution; (2) adding the mixed solution to a 2-methylimidazole solution, mixing, aging, and separating to obtain a ZnO-MOF precursor; (3) Annealing the ZnO-MOF precursor to thermally reduce ZnO, thereby obtaining the electromagnetic wave absorbing material having a hollow rod-like structure.

2. The method for preparing the electromagnetic wave absorbing material according to claim 1, wherein: In step (1), the alcohol solution includes methanol or ethanol; and / or the soluble metal salt includes at least one of a soluble cobalt salt, a nickel salt, an iron salt, and a copper salt.

3. The method for preparing the electromagnetic wave absorbing material according to claim 1, wherein: In step (2), in the raw materials for preparing the ZnO-MOF precursor, the mass ratio of rod-shaped ZnO, soluble metal salt and 2-methylimidazole is (0.05-2): (0.2-5): 2.

25.

4. The method for preparing the electromagnetic wave absorbing material according to claim 1, wherein: In step (3), the temperature of the annealing treatment is 400-1000° C.; and / or the time of the annealing treatment is 1-5 hours.

5. The method for preparing the electromagnetic wave absorbing material according to claim 1, wherein: The preparation process of the rod-shaped ZnO is as follows: zinc salt, hexamethylenetetramine and water are mixed, heated and reacted; after cooling, the mixture is washed and dried to obtain the rod-shaped ZnO.

6. The method for preparing the electromagnetic wave absorbing material according to claim 5, characterized in that: The mass ratio of the zinc salt to hexamethylenetetramine is (0.2-0.5): (0.1-0.6); and / or the mass volume ratio of the zinc salt to water is (0.2-0.4) g: (20-100) mL.

7. An electromagnetic wave absorbing material, characterized in that: The electromagnetic wave absorbing material is prepared by the preparation method of any one of claims 1 to 6, wherein the electromagnetic wave absorbing material has a hollow rod-like structure, comprises metal atoms and carbon nanotubes, and the carbon nanotubes are coated on the surface of the electromagnetic wave absorbing material.

8. The electromagnetic wave absorbing material according to claim 7, characterized in that The metal atoms include at least one of cobalt, nickel, iron and copper.

9. The electromagnetic wave absorbing material according to claim 7, characterized in that The length and diameter of the electromagnetic wave absorbing material are 1-3 μm and 200-600 nm, respectively; and / or the aspect ratio of the electromagnetic wave absorbing material is 1:(2-5).

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