Covalent functionalized graphene nanosheet and preparation method thereof

Abstract

The invention belongs to the technical field of graphene modification, and particularly relates to a covalent functionalized graphene nanosheet and a preparation method thereof. According to the technical scheme, carboxymethyl chitosan (CMCS) is used for assisting ball milling to disperse and modify graphene, and covalent functionalization and physical stripping of graphene are synchronously achieved. According to the method, the technical problem that large-scale stripping and functionalization of graphene are difficult due to interlayer strong Van der Waals force and surface chemical inertness is effectively solved, and the defects that a traditional method is complex in process, harsh in condition or unfriendly to environment are overcome. The method has the advantages that the process is green and efficient, the yield of the functionalized graphene is as high as 72.3%, the dispersion concentration of the functionalized graphene in water can reach 14.8 mg / mL, the stability is excellent, and important theoretical and experimental basis is provided for expanding the practical application of the graphene in the fields of thermal management and the like.

Description

Technical Field

[0001] This invention belongs to the field of graphene modification technology, specifically relating to a covalently functionalized graphene nanosheet and its preparation method. Background Technology

[0002] Graphene is a two-dimensional material composed of a single layer of carbon atoms arranged in a hexagonal lattice with sp² hybridization. Its outstanding physical and chemical properties have made it a hot research topic, with a thermal conductivity as high as 5000 W·m·K. ¹ ¹ Graphene possesses excellent mechanical strength, with a modulus reaching 1 TPa, a strength of 130 GPa, and a theoretical specific surface area of ​​2630 m² / g. Its thermal conductivity far exceeds that of traditional metallic materials, and its outstanding mechanical strength and theoretical specific surface area make it a promising candidate for applications in thermal management, electronic devices, composite materials, and biomedicine. However, the practical application of graphene is limited by its tendency to aggregate due to interlayer van der Waals forces and its surface chemical inertness, making efficient dispersion and functionalization difficult.

[0003] First, the strong van der Waals forces between graphene layers cause it to easily aggregate in solvents or matrices, reducing its effective specific surface area and performance, and limiting its uniform dispersion in nanofluids and composite materials. Second, the sp² carbon structure of graphene forms a stable conjugated system with low chemical reactivity, making it difficult to directly introduce functional groups. Traditional covalent functionalization methods often destroy its structure, leading to a decrease in thermal and electrical conductivity, indicating significant shortcomings in existing functionalization methods. Non-covalent functionalization is achieved through π-π stacking or van der Waals forces, such as using pyrene derivatives for adsorption, which can preserve the conjugated structure of graphene. However, the synthesis process is complex, the adsorption force of the functionalized molecules is weak, and the long-term stability is insufficient. Covalent functionalization can introduce functional groups through radical addition, 1,3-dipolar cycloaddition, or Diels-Alder reactions, improving dispersibility and matrix compatibility. However, this requires high temperature, high pressure, or corrosive chemicals, such as concentrated sulfuric acid or peroxides, resulting in complex processes, high costs, and severe environmental pollution. For example, radical addition may introduce non-selective defects, reducing performance; cycloaddition reactions require specific chemical conditions, making industrialization difficult. The graphene oxide (GO) method prepares GO by oxidizing graphite using the Hummers process. GO has hydroxyl and carboxyl groups on its surface, facilitating functionalization. However, the oxidation process introduces numerous defects, resulting in low thermal conductivity and time-consuming preparation. The use of strong oxidants is environmentally unfriendly, and subsequent reduction processes are complex, making it difficult to fully restore the properties of graphene. Traditional methods often rely on organic solvents or high temperatures, increasing costs and posing hazards to the environment and operators. They also result in low yields. Physical methods, such as ultrasonic treatment, offer limited functionalization and struggle to achieve high-concentration dispersions.

[0004] In recent years, green functionalization methods have attracted attention, and bio-based materials have become a hot topic due to their biocompatibility, biodegradability, and environmental friendliness. For example, glucose and sucrose can be used to exfoliate graphite through ball milling, but the grafting rate is low and the water dispersion stability is poor (concentration <5mg / mL).

[0005] In summary, providing a method for exfoliating and covalently functionalizing graphene using bio-based materials to achieve high grafting rates and water dispersion stability of graphite would be of significant technical importance. Summary of the Invention

[0006] One objective of this invention is to provide a method for preparing covalently functionalized graphene nanosheets. This method utilizes carboxymethyl chitosan (CMCS)-assisted ball milling to disperse and modify graphene, achieving covalent functionalization and exfoliation of the graphene. The aim is to solve the technical challenges of graphene dispersion and functionalization, promoting its potential applications in thermal management materials and other fields. This method is green, simple, efficient, and scalable, suitable for preparing water-based nanofluids and biodegradable polymer composites.

[0007] To achieve the above objectives, the present invention employs the following technical solution: a method for preparing covalently functionalized graphene nanosheets, comprising the following steps: S1. Graphene nanosheets are mixed with carboxymethyl chitosan at a mass ratio of 1:(3-5) to obtain a mixture; S2. Mix the above mixture with zirconium dioxide balls at a mass ratio of 1:(20~30) and load the mixture into a ball milling jar. Dry ball mill the mixture to obtain the ball milling product. S3. The ball-milled product is ultrasonically dispersed in deionized water, then centrifuged at high speed and the centrifuged product is collected. The centrifuged product is dispersed in water and then centrifuged at low speed. The supernatant is taken and vacuum dried to obtain covalently functionalized graphene nanosheets.

[0008] Further improvements to the preparation method of covalently functionalized graphene nanosheets: Preferably, in step S1, the degree of substitution of carboxymethyl chitosan is ≥90%, and the degree of deacetylation is ≥95%.

[0009] Preferably, the graphene nanosheets in step S1 have a particle size of 0.1 μm to 30 μm.

[0010] Preferably, the zirconium spheres in step S2 are composed of a mixture of zirconium spheres with a diameter of 15 mm and a diameter of 8 mm, with a mixing mass ratio of (6~8):(2~4).

[0011] Preferably, in step S2, the rotational speed of the ball mill in the dry ball mill is 300-500 r / min, and the time is 0.5-24 h.

[0012] Preferably, in step S2, the rotation speed of the ball mill in the dry ball mill is 400 r / min, and the time is 8 h.

[0013] Preferably, in step S1, graphene nanosheets and carboxymethyl chitosan are mixed at a mass ratio of 1:4.

[0014] Preferably, in step S3, the high-speed centrifugation speed is 8000~10000 rpm and the centrifugation time is 20~40 min.

[0015] Preferably, in step S3, the low-speed centrifugation speed is 4000~6000 rpm and the time is 20~40 min.

[0016] The second objective of this invention is to provide a method for preparing covalently functionalized graphene nanosheets as described in any one of the above-mentioned methods.

[0017] The advantages of this invention compared to the prior art are as follows: (1) This invention relates to a green method using carboxymethyl chitosan (CMCS)-assisted ball milling, which combines graphene nanosheets with CMCS through mechanical ball milling to achieve covalent functionalization and exfoliation. The hydroxyl and carboxyl groups of CMCS form covalent bonds with the surface of graphene nanosheets, and the graphite layer is mechanically exfoliated to generate thin nanosheets. This method does not require high temperature or organic solvents, and the resulting nanosheets have high dispersibility, excellent thermal conductivity, and matrix compatibility. Polymer grafting of nanomaterials helps to achieve highly stable dispersion in the liquid phase and composite with polymer matrices. Functionalized graphene nanosheets can be used to prepare high-performance water-based nanofluids and biodegradable polymer composites, significantly improving thermal conductivity and mechanical properties.

[0018] (2) CMCS is a product of the N-deacetylation of chitin (the second largest natural polymer), a natural polysaccharide. It is water-soluble, biocompatible, and has abundant functional groups (hydroxyl, amino, and carboxyl groups). It is the only basic (containing amino-NH2) polysaccharide among natural polysaccharides and can be regarded as a polyelectrolyte with active hydroxyl and amino groups. It is soluble in dilute acids such as formic acid, acetic acid, and hydrochloric acid. (The -NH2 group will be protonated in an acidic environment to form NH4+). 3+ While containing ions, it is poorly soluble in solvents such as water. The hydroxyl groups in carboxymethyl chitosan can form covalent bonds with the surface of graphene nanosheets, improving dispersibility and compatibility. This invention utilizes biocompatible CMCS to achieve the green synthesis of covalently functionalized graphene nanosheets; exfoliation and functionalization are performed simultaneously, simplifying the process; ball milling technology is easily industrialized and cost-effective; functionalized graphene nanosheets can be used in nanofluids, composite materials, and other fields. The yield was calculated by comparing the concentration of the dispersion before and after centrifugation of the ball-milled sample; the yield was 72.3%, and the water dispersion concentration reached 14.8 mg / mL, which is superior to traditional methods. Attached Figure Description

[0019] Figure 1 A schematic diagram illustrating the process of generating CMCS-GNSs by exfoliating and covalently functionalizing hexagonal graphene using carboxymethyl chitosan (CMCS) ball milling.

[0020] Figure 2 In Figures (a) and (b), the infrared spectra of graphene nanosheets GNSs and carboxymethyl chitosan functionalized graphene (CMCS-GNSs) in Example 1 are shown respectively.

[0021] Figure 3 In the middle (a) and (b), the TGA curves of graphene nanosheets GNSs and carboxymethyl chitosan functionalized graphene CMCS-GNSs obtained in Example 1 are respectively obtained in a nitrogen atmosphere and at a heating rate of 10℃ / min.

[0022] Figure 4 In the middle (a) and (b), the XRD patterns of graphene nanosheets GNSs and carboxymethyl chitosan functionalized graphene CMCS-GNSs in Example 1 are shown respectively.

[0023] Figure 5 In the middle (a) and (b), the graphene nanosheets GNSs and carboxymethyl chitosan functionalized graphene CMCS-GNSs in Example 1 are transmission electron microscope images, respectively.

[0024] Figure 6 The image shows the static effect of unmodified graphene nanosheets (GNSs) and CMCS-GNSs grafted with carboxymethyl chitosan (CMCS) dispersed in water under the same ultrasonic dispersion and preparation conditions. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0026] Example 1

[0027] This embodiment provides a method for preparing covalently functionalized graphene nanosheets, including the following steps: S1. Graphene nanosheets (GNSs, particle size 0.1 μm ~ 30 μm) are mixed with carboxymethyl chitosan (replacement degree ≥90%, deacetylation degree 95%) at a mass ratio of 1:3 to obtain a mixture; the graphene nanosheets are composite material-specific graphene produced by Donghua Engineering Technology Co., Ltd. S2. The above mixture is mixed with zirconia balls at a mass ratio of 1:25 and loaded into a ball mill jar. The ball mill speed is 400 r / min, and the dry ball milling is carried out for 8 hours to obtain the ball milling product. The zirconia balls include two types with diameters of 15 mm and 8 mm, and the mass ratio of the mixture is 7:3. S3. Add the ball-milled product to deionized water, and ultrasonically disperse it using a CNC ultrasonic cleaner. Then, centrifuge it twice at high speed (9000 r / min, 30 min), collect the precipitate, and remove excess carboxymethyl chitosan. Add deionized water again to disperse it evenly, and centrifuge it once at low speed (5000 r / min, 30 min). Collect the supernatant to obtain a graphene suspension grafted with carboxymethyl chitosan. Place it in a vacuum drying oven to dry for 24 h to finally obtain carboxymethyl chitosan functionalized graphene nanosheets (CMCS-GNSs).

[0028] Example 2

[0029] This embodiment provides a method for preparing covalently functionalized graphene nanosheets, including the following steps: S1. Graphene nanosheets (GNSs, particle size 0.1 μm ~ 30 μm) are mixed with carboxymethyl chitosan (replacement degree ≥90%, deacetylation degree 95%) at a mass ratio of 1:5 to obtain a mixture; the graphene nanosheets are composite material-specific graphene produced by Donghua Engineering Technology Co., Ltd. S2. The above mixture is mixed with zirconia balls at a mass ratio of 1:25 and loaded into a ball mill jar. The ball mill speed is 300 r / min, and the dry ball milling is carried out for 24 h to obtain the ball milling product. The zirconia balls include two types with diameters of 15 mm and 8 mm, and the mass ratio of the mixture is 7:3. S3. Add the ball-milled product to deionized water, and ultrasonically disperse it using a CNC ultrasonic cleaner. Then, centrifuge it twice at high speed (9000 r / min, 30 min), collect the precipitate, and remove excess carboxymethyl chitosan. Add deionized water again to disperse it evenly, and centrifuge it once at low speed (5000 r / min, 30 min). Collect the supernatant to obtain a graphene suspension grafted with carboxymethyl chitosan. Place it in a vacuum drying oven to dry for 24 h to finally obtain carboxymethyl chitosan functionalized graphene nanosheets (CMCS-GNSs).

[0030] Example 3

[0031] This embodiment provides a method for preparing covalently functionalized graphene nanosheets, including the following steps: S1. Graphene nanosheets (GNSs, particle size 0.1 μm ~ 30 μm) are mixed with carboxymethyl chitosan (replacement degree ≥90%, deacetylation degree 95%) at a mass ratio of 1:4 to obtain a mixture; the graphene nanosheets are composite material-specific graphene produced by Donghua Engineering Technology Co., Ltd. S2. Mix the above mixture with zirconia balls at a mass ratio of 1:25 and load the mixture into a ball mill jar. The ball mill speed is 500 r / min, and the mixture is dry-milled for 0.5 h to obtain the ball milling product. The zirconia balls include two types with diameters of 15 mm and 8 mm, and the mass ratio of the mixture is 7:3. S3. Add the ball-milled product to deionized water, and ultrasonically disperse it using a CNC ultrasonic cleaner. Then, centrifuge it twice at high speed (9000 r / min, 30 min), collect the precipitate, and remove excess carboxymethyl chitosan. Add deionized water again to disperse it evenly, and centrifuge it once at low speed (5000 r / min, 30 min). Collect the supernatant to obtain a graphene suspension grafted with carboxymethyl chitosan. Place it in a vacuum drying oven to dry for 24 h to finally obtain carboxymethyl chitosan functionalized graphene nanosheets (CMCS-GNSs).

[0032] Figure 1 This diagram illustrates the process of ball milling covalently functionalized graphene (generating CMCS-GNS) using carboxymethyl chitosan (CMCS). This invention utilizes a green method of CMCS-assisted ball milling to combine graphene nanosheets with CMCS through mechanical ball milling, achieving covalent functionalization and exfoliation. The hydroxyl and carboxyl groups of CMCS form covalent bonds with the graphene surface, generating covalently functionalized graphene nanosheets.

[0033] Performance testing

[0034] Graphene nanosheets (GNSs) and carboxymethyl chitosan grafted graphene (CMCS-GNSs) were characterized using infrared spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM).

[0035] Figure 2 In Figures (a) and (b), the infrared spectra of graphene nanosheets (GNSs) and carboxymethyl chitosan-functionalized graphene (CMCS-GNSs) from Example 1 are shown, respectively. The infrared spectra show that the original GNSs has a wavelength of 1628.8 cm⁻¹. -1 A distinct absorption peak appears on both sides, which coincides with the C=C bond stretching vibration peak of GNSs, at 3439.9 cm⁻¹. -1 The peaks appearing on the left and right sides are likely due to the stretching vibration of OH groups caused by moisture absorption. The CMCS peak at 3416.0 cm⁻¹ is... -1There is a strong absorption peak nearby, caused by the overlapping stretching vibrations of the amino (-NH2) and hydroxyl (-OH) groups. This peak is relatively broad due to the hydrogen bonding between the amino and hydroxyl groups. The peak is at 1626.4 cm⁻¹. -1 and 1425.4cm -1 Yes - COO - The presence of both antisymmetric and symmetric stretching vibration peaks indicates the introduction of a carboxyl group into the molecule, consistent with CMCS. The CMCS-GNSs sample peaked at 3427.9 cm⁻¹. -1 A broad and strong -OH stretching vibration peak and a -NH stretching vibration peak are observed, but compared to pure CMCS, the intensity of these peaks is weaker and slightly shifted to the left. This may be due to changes in the -OH environment of CMCS and its consumption, such as caused by hydrogen bonding. (1589.7 cm⁻¹) -1 (or the stretching vibration peak of the C=C bond) and 1404.5 cm⁻¹ -1 Peak is introduced by CMCS -COO - and 1313.8cm -1 1113.8cm -1 and 1050.5cm -1 The shifts in peaks are caused by the CO stretching vibration in CMCS and the sugar ring stretching vibration of COC, with a redshift in the C=C stretching vibration peak. These shifts indicate an interaction between CMCS and GNSs, providing preliminary evidence of the presence of CMCS in the modified sample.

[0036] Figure 3 In Figures (a) and (b), the TGA curves of the original graphene nanosheets (GNSs) and carboxymethyl chitosan-functionalized graphene (CMCS-GNSs) from Example 1 were obtained under a nitrogen atmosphere at a heating rate of 10 °C / min. The thermogravimetric analysis (TGA) data shows that the original graphene exhibited only a small mass change across the entire temperature range, likely due to the decomposition of impurities. For the CMCS-GNSs sample, the TGA experiment recorded a weight loss of approximately 26.5 wt%, indicating that carboxymethyl chitosan was successfully grafted onto the surface of the graphene nanosheets.

[0037] Figure 4 The XRD patterns of graphene nanosheets (GNSs) and carboxymethyl chitosan functionalized graphene (CMCS-GNSs) in Example 1 are shown in (a) and (b), respectively. The XRD patterns show that carboxymethyl chitosan (CMCS) exhibits a broadened diffraction peak at 20.2°, indicating its amorphous polysaccharide structure. GNSs shows a sharp (002) crystal plane diffraction peak at 2θ = 26.2°, indicating its highly crystalline layered structure. For CMCS-GNSs, the (002) peak of graphene shifts to the left to 2θ = 24.9°, with a decrease in peak intensity and an increase in peak width, indicating that CMCS molecular chains are intercalated into graphene, expanding the interlayer spacing.

[0038] Figure 5 Images (a) and (b) are transmission electron microscopy (TEM) images of graphene nanosheets (GNSs) and carboxymethyl chitosan-functionalized graphene (CMCS-GNSs) from Example 1, respectively. Graphene was dispersed in alcohol, sonicated for a period of time, and then dropped onto a copper grid. Testing was performed after the solvent had completely evaporated. TEM characterization showed that the purity of the CMCS-GNSs sample was significantly improved, with a significantly reduced content of nanoparticle impurities compared to the precursor or insufficiently modified graphene samples. Importantly, the CMCS-GNSs sample exhibited low contrast in the TEM images, with its average brightness closely matching the contrast of the substrate supporting the thin-layer sample. This low-contrast characteristic is a key indicator of the thin and uniform structure, suggesting that the CMCS-GNSs sheets have a low layer count. Simultaneously, the clear TEM image outlines clearly show that the CMCS-modified graphene retains its well-continuous two-dimensional sheet morphology. This indicates that carboxymethyl chitosan modification not only effectively improves the dispersibility and functionality of graphene but also does not damage its two-dimensional planar structure. Instead, it promotes the acquisition of high-quality few-layer structures and reduces other impurities that may be introduced.

[0039] Take 0.5 g each of unmodified GNSs and CMCS-GNSs grafted with carboxymethyl chitosan (CMCS) from Example 1, then disperse them in 10 ml of water, sonicate at 600 W for 30 min, and then let stand for observation. Figure 6 As shown, under the same ultrasonic dispersion and preparation conditions, both unmodified GNSs and CMCS-GNSs grafted with carboxymethyl chitosan (CMCS) in Example 1 were black turbid suspensions at 0 h. With prolonged standing time, the GNSs dispersion rapidly showed a clear supernatant and sedimentation interface, and basically settled by 24 h. In contrast, CMCS-GNSs remained a uniform dark suspension after standing for 48 h, without significant stratification or blocky deposition. This comparison indicates that CMCS grafting significantly improves the colloidal stability of graphene in the aqueous phase: the hydrophilic carboxyl / amino groups improve interfacial wettability and impart surface charge, thereby weakening van der Waals attraction and π–π stacking between layers. Furthermore, after standing for 48 h, the supernatant solution was taken, the solvent was evaporated, and the solid residue was weighed to characterize the stable dispersion concentration: CMCS-GNSs in Example 1 was 14.8 mg / mL, while those in Examples 2 and 3 were 18.1 mg / mL and 15.4 mg / mL, respectively. The above results collectively demonstrate that surface functionalization is effective and can impart excellent water dispersion stability to materials, providing a more stable and processable dispersion basis for their application in hydrophilic systems and polymer composites.

[0040] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments. It should be noted that many modifications and improvements can be made by those skilled in the art, and all modifications or improvements not exceeding the scope of the claims should be considered within the protection scope of the present invention.

Claims

1. A method for the preparation of covalently functionalized graphene nanosheets, characterized in that, The method comprises the following steps: S1, mixing graphene nanosheets and carboxymethyl chitosan according to a mass ratio of 1:(3-5) to obtain a mixture; S2, mixing the mixture and zirconium dioxide balls according to a mass ratio of 1:(20-30) and loading them into a ball mill tank to obtain a ball milling product by dry ball milling; S3, ultrasonic dispersing the ball milling product in deionized water, then high-speed centrifuging and collecting a centrifuged product, dispersing the centrifuged product in water, low-speed centrifuging again, taking supernatant, and vacuum drying to obtain covalently functionalized graphene nanosheets.

2. The method for preparing covalently functionalized graphene nanosheets according to claim 1, characterized in that, The degree of substitution of the carboxymethyl chitosan in step S1 is greater than or equal to 90%, and the degree of deacetylation is greater than or equal to 95%.

3. The method for preparing covalently functionalized graphene nanosheets according to claim 1, characterized in that, The particle size of the graphene nanosheets in step S1 is 0.1 μm-30 μm.

4. The method for preparing covalently functionalized graphene nanosheets according to claim 1, characterized in that, The zirconium dioxide balls in step S2 are mixed zirconium dioxide balls with a diameter of 15 mm and a diameter of 8 mm, and the mass ratio of the mixed zirconium dioxide balls is (6-8):(2-4).

5. The method for preparing covalently functionalized graphene nanosheets according to claim 1, characterized in that, The rotation speed of the ball mill in the dry ball milling in step S2 is 300-500 r / min, and the time is 0.5-24 h.

6. The method for preparing covalently functionalized graphene nanosheets according to claim 5, characterized in that, The rotation speed of the ball mill in the dry ball milling in step S2 is 400 r / min, and the time is 8 h.

7. The method for preparing covalently functionalized graphene nanosheets according to claim 1, characterized in that, The graphene nanosheets and the carboxymethyl chitosan are mixed according to a mass ratio of 1:4 in step S1.

8. The method for preparing covalently functionalized graphene nanosheets according to claim 1, characterized in that, The rotation speed of the high-speed centrifuging in step S3 is 8000-10000 rpm, and the time is 20-40 min.

9. The method for preparing covalently functionalized graphene nanosheets according to claim 1 or 8, characterized in that, The rotation speed of the low-speed centrifuging in step S3 is 4000-6000 rpm, and the time is 20-40 min.

10. Covalently functionalized graphene nanosheets prepared by the method of any one of claims 1-9.

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