Graphene composite material and preparation method thereof

Abstract

The invention particularly relates to a graphene composite material and a preparation method thereof, and the preparation method comprises the following steps: ultrasonically dispersing graphene oxide in a buffer solution, sequentially adding ascorbic acid and dopamine, ultrasonically dispersing, and then stirring for reaction; filtering and washing the reacted solution, and dispersing the obtained filter cake in deionized water to obtain a dispersion liquid; adding an iron salt solution into the dispersion liquid, performing ultrasonic dispersion, and performing heating and stirring reaction; adding an alkaline reagent into the dispersion liquid to adjust the pH value, and continuously stirring for reaction; and filtering, washing and drying the reaction liquid to obtain the polydopamine reduced graphene oxide / Fe3O4 composite wave-absorbing material. Graphene oxide can be properly reduced through a green, environment-friendly, non-toxic and harmless process to obtain reduced graphene oxide with adaptive conductivity so as to prepare the wave-absorbing material, the preparation method is simple, and the prepared polydopamine / reduced graphene oxide / Fe3O4 composite wave-absorbing material has good wave-absorbing capacity and can be used for preparing the wave-absorbing material. And the method has certain advancement.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials technology, specifically to a graphene composite material and its preparation method. Background Technology

[0002] With the rapid development of information technology and the increasing convenience of information dissemination channels, the risks of electromagnetic interference and information leakage have become increasingly prominent, making electromagnetic pollution a pressing challenge. Graphene's unique porous structure and single-atom-thickness two-dimensional form give it lightweight, large surface area, excellent performance, and broad application prospects. Furthermore, the many suspending bonds in graphene effectively resist the influence of electric fields, and the polarization relaxation of the outer electromagnetic layer significantly reduces the intensity of electromagnetic waves, thus effectively absorbing and attenuating them. These characteristics make graphene a highly promising electromagnetic wave absorbing material.

[0003] However, due to the poor impedance matching, poor dispersion in the matrix, and limited absorption loss mechanisms of graphene, the single energy loss mechanism of pure graphene is insufficient to meet impedance matching requirements, severely limiting its application in the field of electromagnetic wave absorption. Therefore, current research focuses mainly on graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO). GO is usually synthesized through the oxidation reaction of expanded graphite and contains a large number of oxygen-containing functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and epoxy (-O-). Based on the prepared graphene oxide, reducing it by removing oxygen groups yields RGO, which has a structure similar to graphene, exhibits certain conductivity, and its surface oxygen-containing functional groups make it hydrophilic and easily dispersed in solution.

[0004] Patent CN 103342982 A discloses a hollow spherical magnetite / graphene composite microwave absorbing material and its preparation method. This material uses graphene as a matrix, loaded with hollow spherical magnetite, and is prepared through liquid homogeneous mixing and in-situ nucleation growth. In this patent, ferric salts are adsorbed onto the surface of graphene oxide. The ferric salts and graphene oxide are then reduced to the magnetite / graphene composite material by the reducing agents urea and hydrazine hydrate. However, hydrazine hydrate is a highly toxic, volatile, and potentially carcinogenic substance, and is not environmentally friendly. Furthermore, the physical adsorption of magnetite onto the graphene surface poses a risk of detachment, affecting the material's lifespan.

[0005] Patent CN 109348696 A relates to an iron-doped tin dioxide / reduced graphene oxide (Fe-SnO2 / RGO) nanocomposite microwave absorbing material and its preparation method. Using graphene oxide (GO), tin tetrachloride pentahydrate, and ferric nitrate nonahydrate as precursors, a one-step hydrothermal reaction is used to prepare the Fe-SnO2 / RGO binary nanocomposite material. This invention involves a high hydrothermal reaction temperature, long reaction time, and high energy consumption.

[0006] Patent CN110550626A relates to a method for controlled reduction of modified graphene oxide matrix. This invention uses chitosan as matrix, graphene oxide and chitosan form a composite material, and then graphene oxide is prepared by soaking and reducing it in vitamin C matrix and hot pressing. This solves the problem of easy agglomeration of graphene. However, the high hot pressing temperature will cause severe decomposition of chitosan, resulting in a significant decrease in the performance of the composite material.

[0007] Patent CN104399090A relates to a polydopamine-modified reduced graphene oxide and its preparation method. The polydopamine-modified reduced graphene oxide includes reduced graphene oxide and polydopamine. The polydopamine is attached to the surface of the reduced graphene oxide by physical adsorption. The modified material is mainly used for its photothermal and photoacoustic imaging properties as a drug for cancer treatment or for use in the field of photoacoustic imaging. Summary of the Invention

[0008] To address the problems existing in the aforementioned graphene oxide materials, this invention proposes a graphene composite material and its preparation method. This invention enables the reduction of graphene oxide through a green, environmentally friendly, non-toxic, and harmless process, yielding reduced graphene oxide with suitable conductivity, thereby producing a microwave absorbing material. The preparation method is simple, highly reproducible, and exhibits excellent microwave absorption performance.

[0009] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for preparing a graphene composite material includes the following steps: (1) Graphene oxide is ultrasonically dispersed in a buffer solution to form a monolayer or few-layer colloid; ascorbic acid and dopamine are added sequentially and ultrasonically dispersed, and then the mixture is stirred to react. Add ascorbic acid (Vc) reducing agent first to remove some of the oxygen-containing groups of GO, restore the sp² region, and improve conductivity; at the same time, create a weakly acidic environment (pH ≈ 4–5) to inhibit the excessively rapid self-polymerization of dopamine.

[0010] Dopamine (DA) is added post-addition, pre-adsorbing onto the GO sheet surface via hydrogen bonding / π-π bonding, providing uniform nucleation sites for subsequent in-situ polymerization; its catechol groups have chelating ability for metal ions, providing Fe... 3+ / Fe 2+This lays the foundation for fixed and uniform deposition.

[0011] During the stirring reaction, Vc continuously reduces GO (the solution color changes from brownish-yellow to black); dopamine is slowly oxidized, causing it to undergo self-polymerization and assembly on the GO surface, forming an ultrathin polydopamine (PDA) coating layer; forming a "rGO@PDA" two-dimensional platform, while PDA provides a large number of -NH- / -OH coordination groups.

[0012] (2) Filter and wash the solution after the reaction in step (1), and disperse the resulting filter cake in deionized water to obtain a dispersion; add iron salt solution to the dispersion and ultrasonically disperse, then heat and stir to react; then add alkaline reagent to the dispersion to adjust the pH, and continue stirring to react; The purpose of filtration and washing is to remove excess Vc, DA monomers and oligomers, and buffer salts; the filter cake is redispersed in deionized water to obtain high-concentration, pure rGO@PDA colloid with a negatively charged surface, which provides a stable substrate for subsequent Fe3O4 in-situ loading.

[0013] Mechanical stirring from room temperature to 60°C, Fe 3+ / Fe 2+ The mixed salt is uniformly adsorbed onto the PDA surface through catechol / amine chelation; temperature control prevents premature nucleation and ensures monolayer dispersion of ions.

[0014] Rapidly raising the pH to 10⁻¹¹ promotes DA monomer polymerization and triggers Fe. 3+ / Fe 2+ Co-precipitation:

[0015] The -NH- / -OH groups of PDA serve as nucleation sites, enabling Fe3O4 nanocrystals (10–20 nm) to be densely and uniformly anchored on the rGO surface, thus inhibiting aggregation. Simultaneously, the reducing properties of PDA can partially absorb Fe... 3+ →Fe 2+ To ensure the stoichiometric ratio of Fe3O4.

[0016] (3) The reaction solution in step (2) is filtered, washed and dried to obtain a composite microwave absorbing material of polydopamine reduced graphene oxide / Fe3O4.

[0017] The byproducts NaCl / NH4Cl and free Fe oxides were removed by filtration, washing, and drying. A ternary composite powder, "rGO@PDA / Fe3O4", was obtained. rGO provides a conductive network and a lightweight framework. PDA acts as an interface "glue" to enhance interfacial polarization, while its N / O functional groups bring dipole losses. Fe3O4 nanoparticles impart strong magnetic losses (natural resonance and exchange resonance) and residual dielectric losses. The three work together to achieve a "dielectric + magnetic" dual loss mechanism, ultimately obtaining thin, light, wide, and strong microwave absorption properties.

[0018] Preferably, in step (1), the graphene oxide is few-layer graphene oxide prepared by the Hummers method; the buffer solution is a Tris-HCl solution with a pH value of 8.0-8.5.

[0019] Preferably, in step (1), the mass ratio of graphene oxide, dopamine, and ascorbic acid is 1:1:0.5-2; the concentration of graphene oxide is 1-3 mg / ml; the stirring reaction conditions are: stirring at room temperature (10-40℃) for 12-24 h, and stirring speed is 300-800 rpm; preferably, the mass ratio of graphene oxide, dopamine, and ascorbic acid is 1:1:0.8-1.8; more preferably, the mass ratio of graphene oxide, dopamine, and ascorbic acid is 1:1:1-1.5; even more preferably, the mass ratio of graphene oxide, dopamine, and ascorbic acid is 1:1:1-1.2.

[0020] Preferably, in step (2), the iron salt is ferrous ammonium sulfate and ferric ammonium sulfate, and the molar ratio of ferrous ammonium sulfate to ferric ammonium sulfate is 1:3 - 1:1.5.

[0021] Preferably, in step (2), the concentration of ferrous ammonium sulfate is 20-80 mmol / L and the concentration of ferric ammonium sulfate is 40-160 mmol / L; the mass-volume ratio of graphene oxide in step (1) to the dispersion after adding iron salt in step (2) is 1-3 mg: 1 ml.

[0022] Preferably, in step (2), the concentration of ferrous ammonium sulfate is 25-60 mmol / L and the concentration of ferric ammonium sulfate is 60-120 mmol / L; more preferably, the concentration of ferrous ammonium sulfate is 25-50 mmol / L and the concentration of ferric ammonium sulfate is 60-120 mmol / L. Preferably, in step (2), the ultrasonic dispersion time is 10-30 min, and the conditions for heating and stirring after adding iron salt are a temperature of 50-80℃, a time of 10-30 min, and a stirring speed of 500-600 rpm.

[0023] Preferably, the alkaline reagent added in step (2) is sodium hydroxide or ammonia; the pH is adjusted to 9-11; and the stirring reaction is continued for 20-60 minutes.

[0024] Preferably, the drying in step (3) is vacuum drying, with a temperature of 40-80℃ and a time of 10-16h.

[0025] Preferably, the graphene composite material prepared by any of the above methods.

[0026] Preferably, the graphene composite material is used in microwave absorbing fibers or fabrics.

[0027] The graphene oxide and dopamine used in this invention can both be purchased directly from the market or prepared using existing technologies.

[0028] For example, the principle of few-layer graphene oxide prepared by the Hummers method: The Hummers method is a method to prepare graphene oxide by oxidizing graphite. The principle is that in a mixture of nitric acid and sulfuric acid, graphite is oxidized to graphene oxide by the action of an oxidizing agent (such as potassium permanganate).

[0029] Few-layer graphene oxide (FL-GO) prepared by the Hummers method (including modifications such as ultrasonication or pre-oxidation) has the following typical characteristics: Number of layers: After ultrasonic-assisted or low-speed centrifugation, more than 90% of the slices have ≤5 layers, and the thickness of AFM is 1-2 nm (single layer ~1 nm).

[0030] Interlayer spacing: The XRD 002 peak is located at 2θ≈11-12°, corresponding to d002≈0.75-0.80 nm, which is more than double that of the original graphite (0.335 nm); the greater the ultrasonic power, the wider the interlayer spacing.

[0031] Oxygen-containing functional groups: C / O atomic ratio 1.8-2.2; mainly containing -OH, -COOH, COC- and a small amount of -C=O. Raman ID / IG ≈0.9–1.1, indicating a moderate in-plane defect density.

[0032] Flake size: lateral dimension 0.5-3 µm (depending on raw graphite particle size and oxidation time), can be further obtained by centrifugation to obtain large / small flakes.

[0033] Dispersibility: It can form a stable colloid of 1-3 mg / mL in water with a zeta potential of about -30 mV and no obvious sedimentation after standing for 1 week; it can maintain dispersion in the pH range of 3-10.

[0034] Performance after reduction: After reduction, the C / O ratio can be increased to 6-10, and the conductivity is 10.2 -10 3 S / m; still retains some defects, suitable for conductive composite fillers or electrode materials.

[0035] In short, the few-layer GO prepared by the Hummers method (including improvements) has the characteristics of "few layers, large interlayer spacing, rich oxygen content, good water dispersion, and easy reduction", making it an ideal precursor for the subsequent preparation of conductive thin films, composite materials and functional coatings.

[0036] This invention selects graphene oxide as raw material and prepares a composite material of polydopamine / reduced graphene oxide / Fe3O4 with microwave absorption properties by reducing it with ascorbic acid and dopamine and functionalizing the surface.

[0037] The mechanism of the reduction reaction in this invention: This invention utilizes ascorbic acid and polydopamine (PDA) to reduce GO, which is a green and mild reduction method. However, PDA has limited reduction capacity, and the conductivity of graphene oxide reduced by PDA is difficult to meet the microwave absorption conditions. Therefore, this patent adds ascorbic acid for reduction. This process is usually carried out at a low temperature and does not require harsh conditions.

[0038] Ascorbic acid, as a "highly efficient reducing agent," plays a crucial role in rapidly and massively reducing graphene oxide (GO), restoring the conjugated sp² structure of graphene, and providing a highly conductive framework for composite materials. This is a prerequisite for achieving high dielectric loss.

[0039] I. Dopamine, as an "interface functionalization aid," plays a dual role in the reaction process. On the one hand, dopamine can assist ascorbic acid in further repairing the defects of GO and improving the reduction degree. On the other hand, and more importantly, dopamine undergoes oxidative self-polymerization in a weakly alkaline environment, forming a polydopamine (PDA) interfacial layer on the surface of partially reduced graphene oxide (rGO). This PDA layer is rich in functional groups such as catechol and amino groups, which not only improves the chemical stability of the composite material, but more importantly, provides abundant active sites for the uniform nucleation and firm anchoring of Fe3O4 nanoparticles.

[0040] In summary, the ascorbic acid-dominated "bulk conductivity restoration" and the dopamine-dominated "surface interface engineering" proceed simultaneously and complement each other in the reaction. This design cleverly resolves the inherent contradictions of a single reducing agent system: if only ascorbic acid is used, although the conductivity is excellent, the inertness of the rGO surface is not conducive to the uniform and stable loading of magnetic particles; if only dopamine is used, its reduction efficiency is relatively low, making it difficult to guarantee the overall high conductivity of the material. This application, through the combination of the two, achieves for the first time in the field of microwave absorbing material preparation the unified construction of "high conductivity" and "highly active interface".

[0041] II. The “Electromagnetic Cooperative Loss Mechanism” Induced by PDA Interface Layer and the Mechanism for Performance Improvement: Another significant advancement of the technical solution in this application compared to the prior art is that the excellent microwave absorption performance of the Fe3O4 / PDA / rGO ternary composite material prepared by the above method originates from the “electromagnetic cooperative loss mechanism” induced by the PDA interface layer.

[0042] Optimized impedance matching: The PDA interface layer, acting as a dielectric layer between conductive rGO and magnetic Fe3O4, can effectively adjust the overall complex permittivity and complex permeability of the composite material. This structure helps alleviate the problem of impedance mismatch between pure carbon materials and air due to excessively high conductivity, resulting in a large amount of electromagnetic wave reflection. This allows more electromagnetic waves to enter the material and be lost, which is key to achieving strong absorption and wide bandwidth.

[0043] Introducing multiple polarization losses: The PDA layer introduces numerous heterogeneous interfaces between rGO and Fe3O4. These interfaces accumulate space charge, resulting in significant interfacial polarization under an applied electromagnetic field, greatly enhancing polarization relaxation losses. Furthermore, the defects inherent in the PDA itself and those it introduces also contribute to dipole polarization losses.

[0044] III. Overall Innovation and Application Prospects of Method-Structure-Performance: This invention provides a novel strategy for preparing microwave absorbing materials with a specific "core-shell" structure that is relatively simple to operate and environmentally friendly. Compared with traditional methods that require documented composite reduction processes or use highly toxic reducing agents (such as hydrazine), this method achieves one-step controllable construction of functional nanocomposites under mild conditions, exhibiting significant process advantages.

[0045] The resulting Fe3O4 / PDA / rGO composite material exhibits a well-defined multilayer structure (rGO as the conductive core, PDA as the functional interface layer, and Fe3O4 as the magnetic shell). This carefully designed structure ensures synergy between dielectric and magnetic losses, as well as optimized impedance matching characteristics. This method provides a novel approach for designing next-generation, lightweight, broadband, strong-absorbing, and highly stable advanced microwave absorbing materials, showing broad application prospects in electromagnetic protection, stealth technology, and other fields.

[0046] The beneficial effects of this invention are: 1. This invention can solve the problems of poor conductivity and poor impedance matching of graphene oxide in one step, and can functionalize graphene oxide to have wave absorption properties. Moreover, the reaction conditions are mild, the operation is simple, and the repeatability is strong.

[0047] 2. This invention utilizes ascorbic acid and dopamine to reduce graphene oxide while simultaneously functionalizing the graphene oxide surface. Dopamine (DA) is added post-processed, providing uniform nucleation sites on the GO sheet surface via hydrogen bonding / π-π pre-adsorption, thus facilitating subsequent in-situ polymerization. The reducing and adhesive properties of PDA are utilized to functionalize the material surface during GO reduction, such as enhancing hydrophilicity and biocompatibility. Its catechol groups possess chelating ability for metal ions, specifically Fe. 3+ / Fe 2+ This lays the foundation for the fixation and uniform deposition of iron oxide particles. It is beneficial for the adhesion of iron oxide particles and their application in the absorption of electromagnetic waves. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a scanning electron microscope image of graphene oxide from Example 1.

[0050] Figure 2 This is a scanning electron microscope (SEM) image of polydopamine / reduced graphene oxide / Fe3O4 from Example 1.

[0051] Figure 3 The graph shows the relationship between reflection loss and thickness / frequency of polydopamine / reduced graphene oxide / Fe3O4 in Example 1.

[0052] Figure 4 This is a 3D graph showing the relationship between reflection loss and thickness / frequency of polydopamine / reduced graphene oxide / Fe3O4 in Example 1.

[0053] Figure 5 Example 8: 3D graph showing the relationship between reflection loss of polydopamine / reduced graphene oxide / Fe3O4 and thickness and frequency (GO:DA:Vc=1:1:0.5).

[0054] Figure 6 The graph shows the relationship between reflection loss and thickness / frequency of polydopamine / reduced graphene oxide / Fe3O4 in Example 8. (GO:DA:Vc=1:1:0.5).

[0055] Figure 7 Example 9: 3D graph showing the relationship between reflection loss of polydopamine / reduced graphene oxide / Fe3O4 and thickness and frequency (GO:DA:Vc=1:1:2).

[0056] Figure 8The graph shows the relationship between reflection loss and thickness / frequency of polydopamine / reduced graphene oxide / Fe3O4 in Example 9 (GO:DA:Vc=1:1:2).

[0057] Figure 9 The 3D plot of the reflection loss of polydopamine / graphene oxide / Fe3O4 as a function of thickness and frequency is shown in Comparative Example 1 (GO:DA:Vc=1:1:0).

[0058] Figure 10 The 3D plot shows the relationship between reflection loss and thickness and frequency of polydopamine / graphene oxide / Fe3O4 in Comparative Example 2 (GO:DA:Vc=1:0:3).

[0059] Figure 11 The 3D plot shows the relationship between reflection loss and thickness and frequency of polydopamine / graphene oxide / Fe3O4 in Comparative Example 3 (GO:DA:Vc=1:3:0). Detailed Implementation

[0060] 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. Example

[0061] This embodiment provides a method for preparing graphene composite materials, including the following steps: (1) Take 100 mg of graphene oxide (few-layer graphene oxide prepared by the Hummers method) in a beaker, add 50 ml of Tris-HCl solution, then add 100 mg of ascorbic acid and sonicate for 20 min, then add 100 mg of DA and sonicate for 10 min. Finally, add a rotor and stir magnetically at 600 rpm at room temperature for 12 h. The scanning electron microscope image of the graphene oxide used in this invention is shown below. Figure 1 As shown, by Figure 1 The SEM image shows that the surface of the graphene oxide material is smooth, the edges of the material are wrinkled, and the boundaries are irregular sheets.

[0062] (2) After washing and filtering the above dispersion with deionized water, place it in a 250 ml three-necked round-bottom flask and add 30 ml of deionized water to obtain dispersion a. Weigh 0.5236 g of ferrous ammonium sulfate and 1.287 g of ferric ammonium sulfate, dissolve them in 20 ml of deionized water and add them to dispersion a. Sonicate for 20 min.

[0063] (3) The dispersion in step (2) is mechanically stirred at 50°C and 500 rpm. After 20 min, ammonia is added dropwise until the pH of the dispersion is about 10-11. The reaction is stopped after stirring for another 30 min.

[0064] (4) Pour out the solution after the reaction, use a strong magnet for magnetic separation, take the lower black precipitate and wash it in deionized water and filter it. Then put it in a vacuum oven at 60°C and dry it for 12 h to obtain the target product, namely the composite microwave absorbing material of polydopamine / reduced graphene oxide / Fe3O4.

[0065] TEM image of the product in Example 1 is shown below. Figure 2 ;according to Figure 2 The SEM images show that after co-precipitation, the iron oxide particles are anchored on the graphene surface, resulting in a rough and granular surface.

[0066] The target product powder from Example 1 and paraffin wax were pressed into coaxial samples with an outer diameter of 7.00 mm, an inner diameter of 3.04 mm, and a thickness of approximately 2 mm in a special mold at a mass ratio of 5:5. The electromagnetic parameters were tested using an AV3629D vector network analyzer, and the absorption performance was calculated. The test frequency was 2-18 GHz. The reflection loss curve of the sample from Example 1 as a function of frequency is shown below. Figure 3 As shown in the figure, the relationship between reflection loss and thickness and frequency in 3D is as follows: Figure 4 As shown, when the matching thickness is 4 mm, the minimum reflection loss (Rmin) is -55.2 dB at 7.0 GHz. When the matching thickness is 2.5 mm, the effective absorption bandwidth (EAB) of the sample reaches 4.7 GHz, exhibiting wideband absorption characteristics. Example

[0067] This embodiment provides a method for preparing graphene composite material. The difference from Example 1 is that sodium hydroxide is used instead of ammonia to adjust the pH of the solution, while the rest of the formulation steps are the same. Example

[0068] This embodiment provides a method for preparing graphene composite materials, including the following steps: (1) Take 100 mg of graphene oxide in a beaker, add 50 ml of Tris-HCl solution with pH 8.5, then add 100 mg of ascorbic acid and sonicate for 20 min, then add 100 mg of DA and sonicate for 10 min. Finally, add a rotor and stir magnetically at 500 rpm at room temperature for 12 h.

[0069] (2) After washing and filtering the above dispersion with deionized water, place it in a 250 ml three-necked round-bottom flask and add 30 ml of deionized water to obtain dispersion a. Weigh 0.5236 g of ferrous ammonium sulfate and 1.287 g of ferric ammonium sulfate, dissolve them in 20 ml of deionized water and add them to dispersion a. Sonicate for 20 min.

[0070] (3) The dispersion in step (2) is mechanically stirred at 50°C and 600 rpm. After 20 min, sodium hydroxide is added dropwise until the pH of the dispersion is about 10-11. The reaction is ended after stirring for another 30 min.

[0071] (4) Pour out the solution after the reaction, use a strong magnet for magnetic separation, take the lower black precipitate, wash and filter it in deionized water, and then put it in a vacuum oven at 60°C for 12 h to obtain the target product. Example

[0072] This embodiment provides a method for preparing graphene composite materials, including the following steps: (1) Take 100 mg of graphene oxide in a beaker, add 50 ml of Tris-HCl solution with pH 8.0, then add 200 mg of ascorbic acid and sonicate for 20 min, then add 100 mg of DA and sonicate for 10 min. Finally, add a rotor and stir magnetically at 600 rpm at room temperature for 12 h.

[0073] (2) After washing and filtering the above dispersion with deionized water, place it in a 250 ml three-necked round-bottom flask and add 30 ml of deionized water to obtain dispersion a. Weigh 0.5236 g of ferrous ammonium sulfate and 1.287 g of ferric ammonium sulfate, dissolve them in 20 ml of deionized water and add them to dispersion a. Sonicate for 20 min.

[0074] (3) The dispersion in step (2) is mechanically stirred at 50°C and 500 rpm. After 20 min, sodium hydroxide is added dropwise until the pH of the dispersion is about 10-11. The reaction is ended after stirring for another 30 min.

[0075] (4) Pour out the solution after the reaction, use a strong magnet for magnetic separation, take the lower black precipitate, wash and filter it in deionized water, and then put it in a vacuum oven at 60°C for 12 h to obtain the target product. Example

[0076] This embodiment provides a method for preparing graphene composite materials, including the following steps: (1) Take 100 mg of graphene oxide in a beaker, add 50 ml of Tris-HCl solution with pH 8.5, then add 100 mg of ascorbic acid and sonicate for 20 min, then add 100 mg of DA and sonicate for 10 min. Finally, add a rotor and stir magnetically at 600 rpm at room temperature for 12 h.

[0077] (2) After filtration and washing with deionized water, the above dispersion was placed in a 250 ml three-necked round-bottom flask and 30 ml of deionized water was added to obtain dispersion a. 0.5236 g of ferrous ammonium sulfate and 1.287 g of ferric ammonium sulfate were weighed, dissolved in 20 ml of deionized water and added to dispersion a. The mixture was sonicated for 20 min.

[0078] (3) The dispersion in step (2) is mechanically stirred at 50°C and 500 rpm. After 20 min, sodium hydroxide is added dropwise until the pH of the dispersion is about 10-11. The reaction is ended after stirring for another 30 min.

[0079] (4) Pour out the solution after the reaction, use a strong magnet for magnetic separation, take the lower black precipitate, wash and filter it in deionized water, and then put it in a vacuum oven at 60°C for 12 h to obtain the target product. Example

[0080] This embodiment provides a method for preparing graphene composite materials, including the following steps: (1) Take 50 mg of graphene oxide in a beaker, add 50 ml of Tris-HCl solution with pH 8.0, then add 50 mg of ascorbic acid and sonicate for 20 min, then add 50 mg of DA and sonicate for 10 min. Finally, add a rotor and stir magnetically at 300 rpm at room temperature for 24 h.

[0081] (2) After filtration and washing with deionized water, the above dispersion was placed in a 250 ml three-necked round-bottom flask and 30 ml of deionized water was added to obtain dispersion a. 0.4545 g (1.6 mmol) of ferrous ammonium sulfate and 1.287 g (4.8 mM) of ferric ammonium sulfate were weighed, dissolved in 20 ml of deionized water and added to dispersion a. The mixture was sonicated for 20 min.

[0082] (3) The dispersion in step (2) was mechanically stirred at 80°C and 600 rpm. After 10 min, sodium hydroxide was added dropwise until the pH of the dispersion was 9. The reaction was stopped after stirring for 60 min.

[0083] (4) Pour out the solution after the reaction, use a strong magnet for magnetic separation, take the lower black precipitate, wash and filter it in deionized water, and then put it in a vacuum oven at 40°C for 16 h to obtain the target product. Example

[0084] This embodiment provides a method for preparing graphene composite materials, including the following steps: (1) Take 150 mg of few-layer graphene oxide prepared by the Hummers method into a beaker, add 50 ml of Tris-HCl solution with a pH of 8.0, then add 100 mg of ascorbic acid and sonicate for 20 min, then add 150 mg of DA and sonicate for 10 min. Finally, add a rotor and stir magnetically at 800 rpm at room temperature for 12 h.

[0085] (2) After filtration and washing with deionized water, the above dispersion was placed in a 250 ml three-necked round-bottom flask and 30 ml of deionized water was added to obtain dispersion a. 0.2841 g (20 mmol / L) ferrous ammonium sulfate and 0.5321 g (40 mmol / L) ferric ammonium sulfate were weighed, dissolved in 20 ml of deionized water and added to dispersion a. The mixture was sonicated for 20 min.

[0086] (3) The dispersion in step (2) was mechanically stirred at 80°C and 600 rpm. After 10 min, sodium hydroxide was added dropwise until the pH of the dispersion was 11. The reaction was stopped after stirring for 20 min.

[0087] (4) Pour out the solution after the reaction, use a strong magnet for magnetic separation, take the lower black precipitate, wash and filter it in deionized water, and then put it into a vacuum oven at 80°C for 10 h to obtain the target product. Example

[0088] This embodiment provides a method for preparing graphene composite material. The difference from Example 1 is that in this comparative example, the mass ratio of GO, DA, and Vc is 1:1:0.5, and the concentration of GO is 2 mg / ml. The remaining steps are the same as in Example 1.

[0089] like Figure 5 , 6 As shown, the sample exhibits a minimum reflection loss of -18.7 dB at 5 mm thickness and a maximum effective absorption bandwidth (EAB) of 2.80 GHz at a thickness of 3 mm. The performance at this configuration is acceptable, but not optimal. Example

[0090] This embodiment provides a method for preparing graphene composite material. The difference from Example 1 is that in this comparative example, the mass ratio of GO, DA, and Vc is 1:1:2, and the concentration of GO is 2 mg / ml. The remaining steps are the same as in Example 1.

[0091] like Figure 7 , 8 As shown, the sample exhibits a minimum reflection loss of -12.5 dB at a thickness of 5 mm, and its maximum effective absorption bandwidth narrows significantly to 1.57 GHz at a thickness of 4 mm. This indicates that excessive Vc may have adverse effects on the material structure or the interactions between components.

[0092] This comparative example provides a method for preparing a graphene composite material. The difference between this comparative example and Example 1 is that ascorbic acid is not added, and only dopamine (DA) is used to reduce GO. The ratio is GO:DA:Vc = 1:1:0. The remaining steps are the same as in Example 1.

[0093] like Figure 9 As shown, when GO is reduced using only dopamine (DA) (GO:DA:Vc = 1:1:0), the minimum reflection loss of the resulting sample (PDA / rGO / Fe3O4) is only -7.22dB, indicating that the electromagnetic wave absorption efficiency of this sample material is low and there is no practical effective absorption bandwidth.

[0094] This comparative example provides a method for preparing a graphene composite material. The difference from Example 1 is that this comparative example does not add dopamine and uses a single reducing agent, ascorbic acid V. C Restore GO, Vc:Go = 3:1, and the remaining steps are the same as in Example 1.

[0095] When ascorbic acid is used alone to reduce GO, although GO can be partially reduced, its absorption performance is only slightly improved. For example... Figure 10 As shown, at 4.5 mm, the minimum reflection loss is only -8.8 dB (RL≤-10 dB is considered effective absorption), indicating virtually no effective absorption bandwidth. This suggests that rGO prepared by reducing ascorbic acid alone cannot meet the electromagnetic wave absorption performance requirements.

[0096] This comparative example provides a method for preparing a graphene composite material. The difference between this comparative example and Example 1 is that ascorbic acid is not added, and only dopamine (DA) is used to reduce GO. The ratio is GO:DA:Vc = 1:3:0. The remaining steps are the same as in Example 1.

[0097] like Figure 11As shown, when GO is reduced using only dopamine (DA) (GO:DA:Vc = 1:3:0), the minimum reflection loss of the resulting sample (PDA / rGO / Fe3O4) is only -8.0 dB. This data clearly indicates that the material obtained by reduction using only dopamine has low electromagnetic wave absorption efficiency and does not meet the practical requirements for effective absorption bandwidth. This demonstrates the limitations of using a single dopamine reducing agent in constructing highly efficient microwave absorbing materials.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a graphene composite material, characterized in that, Includes the following steps: (1) Graphene oxide was ultrasonically dispersed in a buffer solution, ascorbic acid and dopamine were added sequentially and ultrasonically dispersed, and then the mixture was stirred to react. (2) Filter and wash the solution after the reaction in step (1), and disperse the resulting filter cake in deionized water to obtain a dispersion; add iron salt solution to the dispersion and ultrasonically disperse, then heat and stir to react; then add alkaline reagent to the dispersion to adjust the pH, and continue stirring to react; (3) The reaction solution in step (2) is filtered, washed and dried to obtain the graphene composite material, namely the polydopamine reduced graphene oxide / Fe3O4 composite microwave absorbing material.

2. The method for preparing the graphene composite material according to claim 1, characterized in that: In step (1), the graphene oxide is few-layer graphene oxide prepared by the Hummers method; the buffer solution is Tris-HCl solution with a pH value of 8.0-8.

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3. The method for preparing the graphene composite material according to claim 1 or 2, characterized in that: In step (1), the mass ratio of graphene oxide, dopamine, and ascorbic acid is 1:1:0.5-2; the concentration of graphene oxide is 1-3 mg / ml; the stirring conditions are: stirring at room temperature for 12-24 h, and stirring speed is 300-800 rpm.

4. The method for preparing the graphene composite material according to claim 1, characterized in that: In step (2), the iron salts are ferrous ammonium sulfate and ferric ammonium sulfate, and the molar ratio of ferrous ammonium sulfate to ferric ammonium sulfate is 1:3 - 1:1.

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5. The method for preparing the graphene composite material according to claim 1, characterized in that: In step (2), the concentration of ferrous ammonium sulfate is 20-80 mmol / L, the concentration of ferric ammonium sulfate is 40-160 mmol / L, and the mass-volume ratio of graphene oxide in step (1) to the dispersion after adding iron salt in step (2) is 1-3 mg: 1 ml.

6. The method for preparing the graphene composite material according to claim 1, characterized in that: In step (2), the ultrasonic dispersion time is 10-30 min, and the heating and stirring reaction conditions after adding iron salt are 50-80℃, 10-30 min, and 500-600 rpm.

7. The method for preparing the graphene composite material according to claim 1, characterized in that: The alkaline reagent added in step (2) is sodium hydroxide or ammonia; the pH is adjusted to 9-11; and the reaction is continued for 20-60 minutes.

8. The method for preparing the graphene composite material according to claim 1, characterized in that: The drying temperature in step (3) is 40-80℃ and the time is 10-16h.

9. A graphene composite material prepared by the method according to any one of claims 1-8.

10. The application of the graphene composite material of claim 9 in microwave absorbing fibers or fabrics.

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