A multi-level pore hollow carbon sphere and a preparation method and application thereof

Abstract

This invention discloses a hierarchical porous hollow carbon sphere, its preparation method, and its application, belonging to the field of carbon material preparation technology. The preparation method includes uniformly mixing anhydrous ethanol, ultrapure water, and ammonia, adding tetraethyl orthosilicate, and stirring to obtain a silica sphere dispersion; adding a carbon precursor and a pore-forming agent to the silica sphere dispersion, stirring and mixing to induce a polymerization reaction, and constructing a coating layer on the surface of the silica spheres; after the reaction, centrifuging, washing, and vacuum drying the mixture; wherein the carbon precursor includes phenolic compounds and aldehyde compounds; the pore-forming agent is a polysaccharide compound; the vacuum-dried product is carbonized and naturally cooled to room temperature to obtain a carbonized product; the obtained carbonized product is etched to remove silica, and then centrifuged, washed, and vacuum dried sequentially to obtain hierarchical porous hollow carbon spheres. This invention solves the problem that existing technologies cannot simultaneously achieve a hollow structure, multi-scale hierarchical pores, and a simple and controllable preparation process for carbon spheres.

Description

Technical Field

[0001] This invention belongs to the field of carbon material preparation technology, specifically relating to a multi-level porous hollow carbon sphere, its preparation method, and its application. Background Technology

[0002] Porous carbon materials, due to their high specific surface area, good electrical conductivity, and chemical stability, have broad application prospects in electromagnetic wave absorption, supercapacitors, lithium-ion batteries, and catalysis. Among them, hollow carbon spheres, with their unique hollow structure, have become a research hotspot in the field of electromagnetic wave absorption because their internal cavities facilitate multiple reflections and scattering of electromagnetic waves.

[0003] Currently, the mainstream methods for preparing hollow carbon spheres include hard template method and soft template method (such as emulsion template method and bubble template method).

[0004] The hard template method typically uses silica spheres, polystyrene spheres, etc., as templates, coating their surfaces with carbon precursors. After carbonization, the template is removed to obtain a hollow structure. This method offers controllable morphology and good repeatability, but the carbon sphere shells obtained from carbon precursors (such as phenolic resin, dopamine, etc.) are often quite dense and lack hierarchical channels. The dense shell makes it difficult to achieve good impedance matching, limiting its performance in the microwave absorption field.

[0005] Soft template methods (such as using surfactants like F127 and P123 as templates and resorcinol-formaldehyde as a carbon source) can prepare ordered mesoporous carbon spheres (as shown in patent application CN120329500A). However, although ordered mesopores can be generated, they lack internal cavities, and the pore size is mainly limited to the mesopore range of 1.5–10 nm, making it difficult to form a multi-scale hierarchical porous structure with micropores, mesopores, and macropores coexisting. For electromagnetic wave absorption applications, solid structures are not conducive to multiple reflections of electromagnetic waves, and single mesopores are also difficult to optimize impedance matching.

[0006] To increase the pore richness of carbon materials, researchers have attempted to add pore-forming agents (such as surfactants, block copolymers, and small-molecule organic compounds) to carbon precursors. These agents decompose during high-temperature pyrolysis, generating micropores or mesopores within the carbon framework. However, the synergistic effect of pore-forming agents and hard templates has not been fully explored in existing technologies, and common pore-forming agents are either costly or require complex post-processing. In addition, some researchers have used hard template methods to prepare multilayer hollow carbon spheres (such as in patent application CN119873798A), obtaining single-layer, double-layer, or triple-layer carbon spheres by alternately encapsulating silica and phenolic resin and modifying with polyacrylic acid. While this approach improves interfacial polarization, it suffers from drawbacks such as complex processes, dense shells, lack of multi-scale hierarchical pores (especially macropores), and the absence of pore-forming agents leading to a single pore type. Summary of the Invention

[0007] To overcome the shortcomings of the prior art, the present invention aims to provide a multi-level porous hollow carbon sphere, its preparation method and application, which solves the problem that the prior art is unable to simultaneously achieve a hollow structure of carbon spheres, multi-scale multi-level pores (micropores-mesopores-macropores), and a simple and controllable preparation process.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A multi-level porous hollow carbon sphere, wherein the carbon sphere is a single-shell hollow spherical structure, and micropores, mesopores and macropores with a diameter of 50-300 nm are simultaneously distributed on its shell.

[0009] A method for preparing the multi-level porous hollow carbon spheres includes the following steps: Step 1: Mix 50-80 mL of anhydrous ethanol, 5-15 mL of ultrapure water and 2-8 mL of ammonia water evenly, add 3-6 mL of tetraethyl orthosilicate, stir to react, and obtain a silica sphere dispersion. Step 2: Add a carbon precursor and a pore-forming agent to the silica ball dispersion, stir and mix to allow polymerization to occur, forming a coating layer on the surface of the silica balls; after the reaction is complete, centrifuge, wash and vacuum dry the mixture; wherein, the carbon precursor includes phenolic compounds and aldehyde compounds; the molar ratio of the silica ball dispersion to the phenolic compounds is (13~27):(2~8), wherein the silica ball dispersion is based on tetraethyl orthosilicate, and the molar ratio of the phenolic compounds to the aldehyde compounds is 1:(0.5~4); the mass ratio of the phenolic compounds to the pore-forming agent is 1:(0.05~0.5); Step 3: Carbonize the product after vacuum drying in Step 2 and allow it to cool naturally to room temperature to obtain the carbonized product. Step 4: Etch the carbonized product obtained in Step 3 to remove silicon dioxide, and then centrifuge, wash and vacuum dry in sequence to obtain multi-level porous hollow carbon spheres.

[0010] Furthermore, in step 1, after adding tetraethyl orthosilicate, the mixture is stirred for 10–60 min.

[0011] Furthermore, in step 2, the polymerization reaction time is 24–72 hours, the reaction temperature is room temperature, and it is a single polymerization coating.

[0012] Furthermore, in step 2, the phenolic compound is at least one of resorcinol, phenol, and m-aminophenol; and the aldehyde compound is at least one of formaldehyde, acetaldehyde, and furfural.

[0013] Furthermore, in step 2, the pore-forming agent is any one of β-cyclodextrin, starch, or cellulose.

[0014] Furthermore, in step 3, the carbonization process is carried out under an inert atmosphere, with a heating rate of 1–5 °C / min, a carbonization temperature of 600–900 °C, and a holding time of 3–8 h.

[0015] Furthermore, in step 4, an aqueous sodium hydroxide solution is used for etching at a concentration of 0.5–5 mol / L, an etching temperature of 70–100°C, and an etching time of 2–6 h.

[0016] Furthermore, the centrifugal washing conditions in steps 2 and 4 are 4000-12000 r / min and 3-6 min, with washing three times each using deionized water and anhydrous ethanol; the drying conditions are vacuum drying at 60℃ for 12 h.

[0017] An application of the aforementioned multi-level porous hollow carbon sphere in electromagnetic wave absorption, battery electrodes, energetic materials, photosensitive material carriers, sensor chips, or lubricant materials.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention is based on the Stöber method, using silica spheres as hard templates, phenolic and aldehyde compounds as carbon sources, and polysaccharide compounds as pore-forming agents. After polymerization, carbonization, and etching, hollow carbon spheres with micropores, mesopores, and macropores across a multi-scale pore structure are obtained. This method is simple, low-cost, and highly controllable. The resulting carbon spheres have a hollow structure and abundant multi-level pores, making them suitable as electromagnetic wave absorbing materials and extending their applications to batteries, energetic materials, photosensitive material carriers, sensor chips, and lubricant materials.

[0019] (1) This invention introduces an organic pore-forming agent into the hard template method to prepare single-layer hollow carbon spheres for the first time. The pore-forming agent decomposes during the carbonization process and introduces more abundant micropores and mesopores into the carbon shell. At the same time, its self-assembly behavior forms macropores, which, combined with the cavity structure formed by the hard template, obtain multi-level porous hollow carbon spheres with micropores, mesopores and macropores coexisting, which significantly improves the specific surface area and pore connectivity of the material.

[0020] (2) Compared with the multilayer carbon spheres in the prior art (such as the patent application document with publication number CN119873798A), the present invention does not require multiple alternating coatings of silica and phenolic resin, nor does it require surface modification of polyacrylic acid, making the process simpler and the cost lower. The carbon spheres obtained by the present invention have a single-layer hollow structure with multi-level channels across scales in the shell, avoiding the problems of increased thickness and process complexity caused by multilayer structures. In terms of electromagnetic wave absorption performance, the effective absorption bandwidth of the present invention can reach 8.21 GHz, which is better than the 6.0 GHz of the multilayer carbon spheres in the prior art, demonstrating the advantages of the single-layer multi-level porous structure in broadband absorption.

[0021] (3) The carbon spheres obtained by this invention have a unique hollow structure and adjustable-size large pores. When used as electromagnetic wave absorbing materials, the hollow structure is conducive to multiple reflections and scattering of electromagnetic waves, and the multi-level channels can enhance interface polarization and dipole polarization, optimize impedance matching, and achieve broadband strong absorption. At the same time, this structure also has broad application prospects in the fields of battery electrodes, energetic materials, and sensors.

[0022] (4) The process of this invention is simple, the raw materials are cheap and readily available, and there is no need for complicated hydrothermal treatment or template agent pre-burning steps, making it easy to scale up production. Attached Figure Description

[0023] Figure 1 These are scanning electron microscope (SEM) images of the hierarchical porous hollow carbon spheres prepared in Examples 1-4 of this invention. Figure 1 (a) and (b) Scanning electron microscope (SEM) images of the hierarchical porous hollow carbon spheres prepared in Example 1; Figure 1 (c) and (d) are SEM images of Example 2; Figure 1 (e) and (f) are SEM images of Example 3; Figure 1 (g) and (h) are SEM images of Example 4.

[0024] Figure 2 (a) is a nitrogen adsorption-desorption isotherm diagram of the multi-level porous hollow carbon spheres prepared in Example 1 of the present invention; Figure 2 (b) is a pore size distribution diagram of the hierarchical porous hollow carbon spheres prepared in Example 1 of the present invention; Figure 2 (c) is a nitrogen adsorption-desorption isotherm diagram of the multi-level porous hollow carbon spheres prepared in Example 2 of the present invention; Figure 2 (d) is a pore size distribution diagram of the multi-level porous hollow carbon spheres prepared in Example 2 of the present invention.

[0025] Figure 3 (a) is a graph showing the variation of reflection loss as a function of frequency for the multi-level porous hollow carbon spheres prepared in Example 1 of the present invention; Figure 3 (b) is the curve of reflection loss of the multi-level porous hollow carbon sphere prepared in Example 2 of the present invention as a function of frequency.

[0026] Figure 4 This is a 3D graph showing the reflection loss of the multi-level porous hollow carbon sphere prepared in Example 1 of the present invention as a function of frequency.

[0027] Figure 5 This is a 2D graph showing the reflection loss of the multi-level porous hollow carbon sphere prepared in Example 1 of the present invention as a function of frequency. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto. All technologies implemented based on the above content of the present invention are within the scope of the present invention.

[0029] In the following examples, tetraethyl orthosilicate was reagent grade, ≥98%; ammonia was analytical grade, 25-28%; formaldehyde solution was analytical grade, 37-40%; and resorcinol was analytical grade, ≥99.5%.

[0030] Example 1 This embodiment provides a method for preparing hierarchical porous hollow carbon spheres with a particle size of 500 nanometers, including the following steps: (1) Preparation of silica spheres: Add 70 mL of anhydrous ethanol, 10 mL of ultrapure water and 4 mL of ammonia water to a 250 mL beaker, turn on magnetic stirring, mix evenly, slowly add 3.5 mL of tetraethyl orthosilicate, continue stirring for 20 min, the solution gradually changes from clear to milky white, and silica sphere dispersion is obtained.

[0031] (2) Polymerization of carbon precursor: Resorcinol, formaldehyde solution and β-cyclodextrin were added to the silica ball dispersion obtained in step (1). The molar ratio of silica ball dispersion (calculated as tetraethyl orthosilicate) to resorcinol was 16:4, the molar ratio of resorcinol to formaldehyde solution was 1:2, and the mass ratio of resorcinol to β-cyclodextrin was 1:0.1. The reaction was carried out with magnetic stirring at room temperature for 48 hours. The solution color gradually turned yellowish-brown, and a complex coated with carbon precursor was obtained.

[0032] (3) Centrifugation, washing and drying: The solid phase obtained in step (3) was washed three times each with deionized water and anhydrous ethanol. After each washing, the solid phase was centrifuged at 8000 r / min for 3 minutes. The washed product was placed in an oven and dried under vacuum at 60℃ for 12 h to obtain a solid powder.

[0033] (4) Carbonization treatment: The solid powder obtained in step (4) is evenly spread in a corundum crucible and placed in a tube furnace for carbonization under an argon atmosphere: the temperature is increased to 800°C at a heating rate of 2°C / min, held for 5 hours, and then naturally cooled to room temperature to obtain the carbonized product.

[0034] (5) Etching to remove the template: Add the carbonized product obtained in step (5) to a 2 mol / L sodium hydroxide aqueous solution and etch it in an 80°C water bath for 4 hours.

[0035] (6) Post-processing: After etching, centrifuge at 8000 r / min for 3 minutes, wash with water and ethanol 3 times each, collect the precipitate, and vacuum dry at 60℃ for 12h to obtain multi-level porous hollow carbon spheres.

[0036] Example 2 This embodiment provides a method for preparing hierarchical porous hollow carbon spheres with a particle size of 500 nanometers, including the following steps: (1) Preparation of silica spheres: Add 70 mL of anhydrous ethanol, 10 mL of ultrapure water and 4 mL of ammonia water to a 250 mL beaker, turn on magnetic stirring, mix evenly, slowly add 3.5 mL of tetraethyl orthosilicate, continue stirring for 20 min, the solution gradually changes from clear to milky white, and silica sphere dispersion is obtained.

[0037] (2) Polymerization of carbon precursor: Resorcinol, formaldehyde solution and β-cyclodextrin were added to the silica ball dispersion obtained in step (1). The molar ratio of silica ball dispersion (calculated as tetraethyl orthosilicate) to resorcinol was 16:5, the molar ratio of resorcinol to formaldehyde solution was 1:2, and the mass ratio of resorcinol to β-cyclodextrin was 1:0.5. The reaction was carried out by magnetic stirring at room temperature for 48 hours. The solution color gradually turned yellowish-brown, and a complex coated with carbon precursor was obtained.

[0038] (4) Centrifugation, washing and drying: The solid phase obtained in step (3) was washed three times each with deionized water and anhydrous ethanol. After each washing, the solid phase was centrifuged at 6000 r / min for 4 minutes. The washed product was placed in an oven and dried under vacuum at 60℃ for 12 h to obtain a solid powder.

[0039] (5) Carbonization treatment: The solid powder obtained in step (4) is evenly spread in a corundum crucible (the thickness does not exceed 2 mm), placed in a tube furnace, and carbonized under a nitrogen atmosphere: the temperature is raised to 800℃ at a heating rate of 2℃ / min, held for 5 hours, and then naturally cooled to room temperature to obtain the carbonized product.

[0040] (6) Etching to remove the template: Add the carbonized product obtained in step (5) to a 3 mol / L sodium hydroxide aqueous solution and etch it in an 80°C water bath for 3 hours.

[0041] (7) Post-processing: After etching, centrifuge at 6000 r / min for 4 minutes, wash with water and ethanol 3 times each, collect the precipitate, and vacuum dry at 60℃ for 12 h to obtain multi-level porous hollow carbon spheres.

[0042] Example 3 This embodiment provides a method for preparing hierarchical porous hollow carbon spheres with a particle size of 200 nanometers, including the following steps: (1) Preparation of silica spheres: Add 70 mL of anhydrous ethanol, 10 mL of ultrapure water and 2 mL of ammonia water to a 250 mL beaker, turn on magnetic stirring, mix evenly, slowly add 3.5 mL of tetraethyl orthosilicate, continue stirring for 20 min, the solution gradually changes from clear to milky white, and silica sphere dispersion is obtained.

[0043] (2) Polymerization of carbon precursor: Resorcinol, formaldehyde solution and β-cyclodextrin were added to the silica ball dispersion obtained in step (1). The molar ratio of silica ball dispersion (calculated as tetraethyl orthosilicate) to resorcinol was 16:3, the molar ratio of resorcinol to formaldehyde solution was 1:2, and the mass ratio of resorcinol to β-cyclodextrin was 1:0.05. The reaction was carried out with magnetic stirring at room temperature for 24 hours. The solution color gradually turned yellowish-brown, and a complex coated with carbon precursor was obtained.

[0044] (4) Centrifugation, washing and drying: The solid phase obtained in step (3) was washed three times each with deionized water and anhydrous ethanol. After each washing, the solid phase was centrifuged at 12000 r / min for 3 minutes. The washed product was placed in an oven and dried under vacuum at 60℃ for 12 h to obtain a solid powder.

[0045] (5) Carbonization treatment: The solid powder obtained in step (4) is evenly spread in a corundum crucible (thickness not exceeding 2 mm), placed in a tube furnace, and carbonized under a nitrogen atmosphere: the temperature is increased to 700℃ at a heating rate of 2℃ / min and held for 4 hours. The carbonized product is obtained by natural cooling to room temperature.

[0046] (6) Etching to remove the template: Add the carbonized product obtained in step (5) to a 1 mol / L sodium hydroxide aqueous solution and etch it in an 80°C water bath for 3 hours.

[0047] (7) Post-processing: After etching, centrifuge at 12000r / min for 3 minutes, wash with water and ethanol 3 times each, collect the precipitate, and vacuum dry at 60℃ for 12h to obtain multi-level porous hollow carbon spheres.

[0048] Example 4 This embodiment provides a method for preparing hierarchical porous hollow carbon spheres with a particle size of 800 nanometers, including the following steps: (1) Preparation of silica spheres: Add 60 mL of anhydrous ethanol, 10 mL of ultrapure water and 8 mL of ammonia water to a 250 mL beaker, turn on magnetic stirring, mix evenly, slowly add 5.2 mL of tetraethyl orthosilicate, continue stirring for 30 min, the solution gradually changes from clear to milky white, and silica sphere dispersion is obtained.

[0049] (2) Polymerization of carbon precursor: Resorcinol, formaldehyde solution and β-cyclodextrin were added to the silica ball dispersion obtained in step (1). The molar ratio of silica ball dispersion (calculated as tetraethyl orthosilicate) to resorcinol was 23:3, the molar ratio of resorcinol to formaldehyde solution was 1:3, and the mass ratio of resorcinol to β-cyclodextrin was 1:0.33. The reaction was carried out by magnetic stirring at room temperature for 48 hours. The solution color gradually turned yellowish-brown, and a complex coated with carbon precursor was obtained.

[0050] (4) Centrifugation, washing and drying: The solid phase obtained in step (3) was washed three times each with deionized water and anhydrous ethanol. After each washing, the solid phase was centrifuged at 4000 r / min for 6 minutes. The washed product was placed in an oven and dried under vacuum at 60℃ for 12 h to obtain a solid powder.

[0051] (5) Carbonization treatment: The solid powder obtained in step (4) is evenly spread in a corundum crucible (the thickness does not exceed 2 mm), placed in a tube furnace, and carbonized under an argon atmosphere: the temperature is increased to 700℃ at a heating rate of 2℃ / min and held for 6 hours. The carbonized product is obtained by natural cooling to room temperature.

[0052] (6) Etching to remove the template: Add the carbonized product obtained in step (5) to a 4 mol / L sodium hydroxide aqueous solution and etch it in a 90℃ water bath for 2 hours.

[0053] (7) Post-processing: After etching, centrifuge at 4000 r / min for 6 minutes, wash with water and ethanol 3 times each, collect the precipitate, and vacuum dry at 60℃ for 12 h to obtain multi-level porous hollow carbon spheres.

[0054] Performance testing (I) Morphology and pore structure characterization The morphology and pore structure of the prepared hierarchical porous hollow carbon spheres were characterized.

[0055] Scanning electron microscopy (SEM) showed that the carbon spheres obtained in Example 1 were regular spheres with a diameter of approximately 500 nm. Some of the broken carbon spheres exhibited a clear hollow structure (e.g., Figure 1 As shown in (a) and (b), its macropore diameter is approximately 100 nanometers. The carbon spheres obtained in Example 2 have a diameter of approximately 500 nm (as shown in Example 2). Figure 1 As shown in (c) and (d), the macropore diameter is 100-300 nm. The hollow carbon spheres obtained in Example 3 have a diameter of approximately 200 nm, and the partially damaged carbon spheres show obvious hollow structures (e.g., Figure 1 As shown in (e) and (f), its macropore diameter is approximately 50 nanometers. The hollow carbon spheres obtained in Example 4 have a diameter of approximately 800 nm (as shown in Example 4). Figure 1 As shown in (g) and (h), its macropore diameter is approximately 200 nanometers.

[0056] In this invention, the size of the macropores in the shell can be effectively controlled by adjusting the amount of pore-forming agent. When the mass ratio of the pore-forming agent to the phenolic compound varies within the range of (0.05~0.5):1, the macropore diameter can be adjusted between 50~300 nm. This is because the size of the aggregates formed by the self-assembly of the pore-forming agent in the polymerization system increases with its concentration, leaving macropores of corresponding size after carbonization. Therefore, by changing the amount of pore-forming agent, hierarchical hollow carbon spheres with different macropore sizes can be obtained to meet the needs of different application scenarios.

[0057] Figure 2 (a) is the nitrogen adsorption-desorption isotherm of the hierarchical porous hollow carbon spheres obtained in Example 1. This isotherm exhibits a mixed I / IV characteristic, revealing the multi-scale hierarchical porous structure of the carbon spheres. In the low-pressure region (P / P0 < 0.05), the adsorption capacity rises sharply to approximately 48 cm³ / g, indicating the presence of numerous micropores. In the medium-pressure region (P / P0 = 0.15~0.85), a stable narrow hysteresis loop forms between the adsorption and desorption capacities, indicating abundant mesopores and relatively open pore channels. In the high-pressure region (P / P0 > 0.85), the adsorption capacity continues to rise, from 68.87 cm³ / g to 104.21 cm³ / g, an increase of over 35 cm³ / g, indicating the clear presence of macropores. The three levels of pores—micropores, mesopores, and macropores—together constitute a complete multi-scale hierarchical porous structure.

[0058] The obtained carbon spheres have a BET specific surface area of ​​205.2 m² / g and a total pore volume (single-point adsorption, P / P0 = 0.9945) of 0.161 cm³ / g. Detailed analysis of the pore structure is as follows: Micropores: The t-Plot micropore area was 144.6 m² / g and the micropore volume was 0.0586 cm³ / g. HK pore size distribution analysis showed that the median micropore size was about 0.82 nm, indicating that there are abundant micropores in the shell.

[0059] Mesopores: The cumulative pore volume of BJH desorption (1.7–300 nm) is 0.0960 cm³ / g, and the average pore size of BJH desorption is 7.01 nm, confirming the existence of mesopores.

[0060] Macropores: There is a significant cumulative increase in pore volume in the pore size range greater than 50 nm (for example, BJH pore size distribution analysis shows that the incremental pore volume is 0.0059 cm³ / g in the range of 78.0–99.8 nm; DFT pore size distribution analysis shows that the cumulative pore volume increases from 0.0825 cm³ / g to 0.0904 cm³ / g in the range of 100–230 nm).

[0061] like Figure 2 As shown in (b), the dV / dD differential pore size distribution curve of Example 1 shows pore volume response in the pore size range of 0 to 250 nanometers, which verifies the existence of a multi-scale hierarchical pore structure.

[0062] Nitrogen adsorption-desorption tests were performed on the hierarchical porous hollow carbon spheres obtained in Example 2, such as... Figure 2As shown in (c), the nitrogen adsorption-desorption isotherm of the obtained carbon spheres exhibits a typical type IV curve. In the low-pressure region (P / P0<0.1), the adsorption capacity rises sharply to about 115 cm³ / g, indicating the presence of abundant micropores in the sample; in the medium-pressure region (P / P0=0.4~0.9), a significant hysteresis loop appears, indicating the presence of mesopores; in the high-pressure region (P / P0>0.9), the adsorption capacity rises again, indicating the presence of macropores or stacked pores.

[0063] Its BET specific surface area is 441.4 m² / g, and its total pore volume (single-point adsorption, P / P0 = 0.9944) is 0.380 cm³ / g. Detailed analysis of the pore structure is as follows: Micropores: The t-Plot micropore area was 333.1 m² / g and the micropore volume was 0.1375 cm³ / g. HK pore size distribution analysis showed that the median micropore size was about 0.80 nm, indicating that there are abundant micropores in the shell.

[0064] Mesopores: The cumulative pore volume of BJH desorption (1.7–300 nm) is 0.2615 cm³ / g, and the average pore size of BJH desorption is 7.07 nm, confirming the existence of mesopores.

[0065] Macropores: A significant cumulative increase in pore volume exists in the pore size range greater than 50 nm. (DFT pore size distribution analysis shows that the cumulative pore volume increases from approximately 0.1708 cm³ / g to 0.1818 cm³ / g in the range of 100–216 nm, and pores with a diameter greater than 216.6 nm contribute 46.63 m² / g of surface area.) This further confirms the existence of macropores.

[0066] like Figure 2 As shown in (d), the dV / dD differential pore size distribution curve of the hierarchical hollow carbon spheres obtained in Example 2 has a pore volume response in the range of 0 to 250 nanometer pore size, which proves that the obtained carbon spheres have a multi-scale hierarchical pore structure.

[0067] The above results fully demonstrate that the carbon spheres obtained by this invention have a multi-scale hierarchical porous structure with micropores, mesopores, and macropores coexisting.

[0068] (II) Electromagnetic wave absorption performance test The obtained multi-porous hollow carbon spheres were uniformly mixed with paraffin wax at a mass ratio of 20:80 and pressed into coaxial ring-shaped test samples (outer diameter 7.0 mm, inner diameter 3.0 mm, thickness approximately 2.0 mm). The electromagnetic parameters of the samples were tested in the frequency range of 2–18 GHz using a vector network analyzer, and the reflection loss was calculated based on transmission line theory.

[0069] Test results show that the multi-level porous hollow carbon spheres exhibit excellent electromagnetic wave absorption performance. The sample in Example 1, with a thickness of 2.2 mm, achieved a minimum reflection loss of -24.41 dB (i.e., over 99.99% of electromagnetic waves were absorbed), and an effective absorption bandwidth (reflection loss below -10 dB) of 8.21 GHz (9.79–18.0 GHz). Figure 3 As shown in (a), Figure 4 , Figure 5 The 3D and 2D reflection loss diagrams are provided. In Example 2, the multi-level porous hollow carbon spheres, with a thickness of 2.0 mm, achieved a minimum reflection loss of -22.31 dB and an effective absorption bandwidth (reflection loss below -10 dB) of 7.72 GHz (10.28–18.0 GHz). Figure 3 As shown in (b). Compared with existing multilayer hollow carbon spheres (such as those reported in patent application CN119873798A, with RLmin=-43.87dB and EAB=6.0GHz), the effective absorption bandwidth of this invention is significantly wider (8.21GHz vs 6.0GHz), demonstrating the advantages of the multi-level porous hollow structure of this invention in broadband absorption. The excellent absorption performance is attributed to the hollow structure enhancing the multiple reflections and scattering of electromagnetic waves, while the multi-level channels in the shell introduce abundant interfacial polarization and dipole polarization, optimizing the impedance matching characteristics of the material.

[0070] Example 5 This embodiment provides a method for preparing hierarchical porous hollow carbon spheres with a particle size of 400 nanometers, including the following steps: (1) Preparation of silica spheres: Add 50 mL of anhydrous ethanol, 15 mL of ultrapure water and 6 mL of ammonia water to a 250 mL beaker, turn on magnetic stirring, mix evenly, slowly add 3 mL of tetraethyl orthosilicate, continue stirring for 60 min, the solution gradually changes from clear to milky white, and silica sphere dispersion is obtained.

[0071] (2) Polymerization of carbon precursor: Phenol, acetaldehyde and furfural mixture and starch are added to the silica ball dispersion obtained in step (1). The molar ratio of silica ball dispersion (calculated as tetraethyl orthosilicate) to phenol is 13:2, the molar ratio of phenol to the mixture of acetaldehyde and furfural is 1:0.5, and the mass ratio of phenol to starch is 1:0.5. The reaction is carried out at room temperature with magnetic stirring for 60 h. The solution color gradually turns yellowish-brown, and a complex coated with carbon precursor is obtained.

[0072] (3) Centrifugation, washing and drying: The solid phase obtained in step (3) was washed three times each with deionized water and anhydrous ethanol. After each washing, the solid phase was centrifuged at 8000 r / min for 3 minutes. The washed product was placed in an oven and dried under vacuum at 60℃ for 12 h to obtain a solid powder.

[0073] (4) Carbonization treatment: The solid powder obtained in step (4) is evenly spread in a corundum crucible and placed in a tube furnace for carbonization under an argon atmosphere: the temperature is increased to 600℃ at a heating rate of 1℃ / min, held for 8h, and then naturally cooled to room temperature to obtain the carbonized product.

[0074] (5) Etching to remove the template: Add the carbonized product obtained in step (5) to a 0.5 mol / L sodium hydroxide aqueous solution and etch it in a 70°C water bath for 5 hours.

[0075] (6) Post-processing: After etching, centrifuge at 8000 r / min for 3 minutes, wash with water and ethanol 3 times each, collect the precipitate, and vacuum dry at 60℃ for 12h to obtain multi-level porous hollow carbon spheres.

[0076] Example 6 This embodiment provides a method for preparing hierarchical porous hollow carbon spheres with a particle size of 600 nanometers, including the following steps: (1) Preparation of silica spheres: Add 80 mL of anhydrous ethanol, 5 mL of ultrapure water and 6 mL of ammonia water to a 250 mL beaker, turn on magnetic stirring, mix evenly, slowly add 6 mL of tetraethyl orthosilicate, continue stirring for 10 min, the solution gradually changes from clear to milky white, and silica sphere dispersion is obtained.

[0077] (2) Polymerization of carbon precursor: Phenol and m-aminophenol mixture, acetaldehyde and cellulose are added to the silica ball dispersion obtained in step (1). The molar ratio of silica ball dispersion (calculated as tetraethyl orthosilicate) to phenol and m-aminophenol mixture is 27:8, the molar ratio of phenol and m-aminophenol mixture to acetaldehyde is 1:4, and the mass ratio of phenol and m-aminophenol mixture to cellulose is 1:0.2. The reaction is carried out with magnetic stirring at room temperature for 72 hours. The solution color gradually turns yellowish-brown, and a complex coated with carbon precursor is obtained.

[0078] (3) Centrifugation, washing and drying: The solid phase obtained in step (3) was washed three times each with deionized water and anhydrous ethanol. After each washing, the solid phase was centrifuged at 6000 r / min for 4 minutes. The washed product was placed in an oven and dried under vacuum at 60℃ for 12 h to obtain a solid powder.

[0079] (4) Carbonization treatment: The solid powder obtained in step (4) is evenly spread in a corundum crucible and placed in a tube furnace for carbonization under an argon atmosphere: the temperature is increased to 900°C at a heating rate of 5°C / min, held for 3 hours, and then naturally cooled to room temperature to obtain the carbonized product.

[0080] (5) Etching to remove the template: Add the carbonized product obtained in step (5) to a 5 mol / L sodium hydroxide aqueous solution and etch it in a 100℃ water bath for 6 hours.

[0081] (6) Post-processing: After etching, centrifuge at 6000 r / min for 4 minutes, wash with water and ethanol 3 times each, collect the precipitate, and vacuum dry at 60℃ for 12h to obtain multi-level porous hollow carbon spheres.

[0082] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A multi-level porous hollow carbon sphere, characterized in that, The carbon sphere is a single-shell hollow spherical structure, with micropores, mesopores and macropores with a diameter of 50-300 nm distributed on its shell.

2. A method for preparing multi-level porous hollow carbon spheres as described in claim 1, characterized in that, Includes the following steps: Step 1: Mix 50-80 mL of anhydrous ethanol, 5-15 mL of ultrapure water and 2-8 mL of ammonia water evenly, add 3-6 mL of tetraethyl orthosilicate, stir to react, and obtain a silica sphere dispersion. Step 2: Add a carbon precursor and a pore-forming agent to the silica ball dispersion, stir and mix to allow polymerization to occur, forming a coating layer on the surface of the silica balls; after the reaction is complete, centrifuge, wash and vacuum dry the mixture; wherein, the carbon precursor includes phenolic compounds and aldehyde compounds; the molar ratio of the silica ball dispersion to the phenolic compounds is (13~27):(2~8), wherein the silica ball dispersion is based on tetraethyl orthosilicate, and the molar ratio of the phenolic compounds to the aldehyde compounds is 1:(0.5~4); the mass ratio of the phenolic compounds to the pore-forming agent is 1:(0.05~0.5); Step 3: Carbonize the product after vacuum drying in Step 2 and allow it to cool naturally to room temperature to obtain the carbonized product. Step 4: Etch the carbonized product obtained in Step 3 to remove silicon dioxide, and then centrifuge, wash and vacuum dry in sequence to obtain multi-level porous hollow carbon spheres.

3. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, After adding tetraethyl orthosilicate in step 1, the mixture is stirred for 10–60 min.

4. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, In step 2, the polymerization reaction time is 24–72 h, and the reaction temperature is room temperature, which is a single polymerization coating.

5. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, In step 2, the phenolic compound is at least one of resorcinol, phenol, and m-aminophenol; the aldehyde compound is at least one of formaldehyde, acetaldehyde, and furfural.

6. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, In step 2, the pore-forming agent is any one of β-cyclodextrin, starch, or cellulose.

7. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, In step 3, the carbonization process is carried out under an inert atmosphere, with a heating rate of 1–5 °C / min, a carbonization temperature of 600–900 °C, and a holding time of 3–8 h.

8. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, In step 4, sodium hydroxide aqueous solution is used for etching at a concentration of 0.5–5 mol / L, an etching temperature of 70–100℃, and an etching time of 2–6 h.

9. The method for preparing a multi-level porous hollow carbon sphere according to claim 2, characterized in that, The centrifugation washing conditions for steps 2 and 4 are 4000-12000 r / min and 3-6 min, respectively, with washing with deionized water and anhydrous ethanol 3 times each; the drying conditions are vacuum drying at 60℃ for 12 h.

10. The application of a multi-level porous hollow carbon sphere as described in claim 1 in electromagnetic wave absorption, battery electrodes, energetic materials, photosensitive material carriers, sensor chips, or lubricant materials.

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