Green graphene stripping method combining hydrothermal method and liquid phase stripping method

Abstract

The invention relates to a green graphene stripping method combining a hydrothermal method and a liquid phase stripping method. The method comprises the following steps: immersing graphite in a NaHCO3 saturated solution, promoting expansion of the graphite by a hydrothermal method, expanding the graphite for multiple times, subsequently performing suction filtration and dialysis on the graphite to remove surface impurity ions, then assisting liquid phase stripping of graphene by using a nano cellulose crystal, and performing centrifugal treatment on the obtained product to obtain a supernatant which is the graphene-cellulose hybrid aqueous dispersion. According to hydrothermal treatment, gas is decomposed at high temperature and high pressure to increase the interlayer spacing of graphite so as to increase the yield of liquid phase stripping graphene, and the method has the advantages of being safe, free of pollution, low in energy consumption, capable of achieving large-scale preparation and the like. The invention provides a new technical approach for low-cost and large-scale preparation of graphene.

Description

Technical Field

[0001] The invention belongs to the technical field of nano material preparation, and in particular relates to a green graphene exfoliation method combining a hydrothermal method with a liquid phase exfoliation method. Background Art

[0002] Graphene is widely used in composite materials, sensors, energy storage, conductive coatings and other fields due to its excellent mechanical properties, electrical conductivity and thermal conductivity. However, the preparation and dispersion of graphene is still a technical problem. Most of the existing graphene preparation methods rely on chemical reagents such as organic solvents, strong acids or strong bases, which have the problems of high pollution, high energy consumption and difficulty in large-scale preparation. Therefore, the development of a green, safe, pollution-free, low-energy consumption and easy large-scale preparation method for graphene is an urgent issue to be solved.

[0003] In terms of the research and application of using nanocellulose to assist in graphite exfoliation, there are patents (such as Patent No. CN112500584A and Patent No. CN119118074A) disclosed that the former uses concentrated sulfuric acid, hydrogen peroxide, etc. to expand graphite oxide, and then mixes cellulose for ball milling to obtain the product. The steps are cumbersome, and the reagents used are harmful to the environment and dangerous to operate; the latter grafts ionic liquid molecules onto the cellulose surface, promotes the insertion of ionic liquid molecules into the interlayer of the two-dimensional material under a hydrothermal environment to achieve exfoliation. The ionic liquid is expensive and is not conducive to large-scale production. Summary of the invention

[0004] The purpose of the present invention is to provide a green, low-cost method for exfoliating graphene, which is not only environmentally friendly but also has low energy consumption and can be prepared on a large scale.

[0005] The present invention provides a green, low-cost method for exfoliating graphene, which expands graphite by a hydrothermal method, then removes surface impurities by filtering and dialysis, and finally ultrasonically treats with a nanocellulose crystal (CNC) aqueous dispersion to prepare a stable dispersion system of liquid-phase exfoliated graphene cellulose. The hydrothermal treatment is to decompose gas under high temperature and high pressure to increase the spacing between graphite sheets to increase the yield of liquid-phase exfoliated graphene. The method has the advantages of safety and pollution-free, low energy consumption, and scalable preparation. The present invention provides a new technical approach for low-cost, scalable preparation of graphene.

[0006] The technical solution of the present invention to achieve the above object is as follows:

[0007] A method for green exfoliation of graphene combining a hydrothermal method and a liquid phase exfoliation method comprises the following steps:

[0008] (1) Graphite is impregnated with a certain amount of NaHCO 3The saturated solution is placed in a reactor and sealed, and the reactor is placed in an oven to keep warm. When the reactor is cooled to room temperature, the reactor is opened, and the solid obtained by filtering the mixture is the graphite after expansion once;

[0009] (2) Repeat the operation of step (1) until the graphene is in a fluffy state. After the treatment is completed, the filtered graphite is placed in a semipermeable membrane, and both ends of the sealed semipermeable membrane are placed in deionized water for dialysis to remove impurity ions, and finally dried;

[0010] (3) adding the graphite treated in step (2) to the nanocellulose crystal aqueous dispersion, performing ultrasonic exfoliation in an ultrasonic disperser, and then centrifuging the system to obtain an exfoliation product, i.e., a graphene-nanocellulose hybrid, and the resulting solution is a nanocellulose-graphene aqueous dispersion.

[0011] Preferably, the NaHCO in step (1) 3 The usage of saturated solution is 30-80ml / g graphite.

[0012] Preferably, the reaction kettle described in step (1) should be a polytetrafluoroethylene-lined autoclave.

[0013] Preferably, the oven insulation temperature in step (1) is 85°C-95°C.

[0014] Preferably, the insulation time in step (1) is 6-12 hours.

[0015] Preferably, the semipermeable membrane in step (2) is an RC membrane with a molecular weight cut-off of no more than 14,000.

[0016] Preferably, the operation of repeatedly expanding the graphite in step (2) means that the graphite after filtration is continuously immersed in the same NaHCO 3 The saturated solution was then hydrothermally treated under the same conditions.

[0017] Preferably, the dialysis in step (2) requires water changes at least 3 times, with an interval of no less than 8 hours between each changes.

[0018] Preferably, the concentration of the nanocellulose crystal aqueous dispersion in step (3) is 2-16 mg / mL.

[0019] Preferably, in step (3), the mass ratio of graphite to nanocellulose crystals is 1:0.25-1:2.

[0020] Preferably, the ultrasonic treatment time in step (3) is 3-6 hours.

[0021] Preferably, in the centrifugation operation described in step (3), the centrifugation rate is 4000-8000 r / min and the centrifugation time is 30-60 min.

[0022] The present invention adopts hydrothermal method to expand graphite. Compared with the commonly used hummers method, on the one hand, this method adopts mild NaHCO 3 As an expansion agent, it is not only harmless to the environment, but also cheap and readily available; on the other hand, the graphene obtained by exfoliation is not oxidized and does not need further reduction, which reduces operating costs and the use of chemical reagents. In addition, nanocellulose crystals are further used to assist in liquid phase exfoliation of graphene, and the combination of hydrothermal expansion and liquid phase exfoliation allows the graphene to be effectively exfoliated. The present invention uses harmless and mild reagents instead of commonly used corrosive and hazardous reagents, and combines the hydrothermal method with the liquid phase exfoliation method to obtain a green graphene exfoliation method, which is not only of great significance for energy conservation and emission reduction and the realization of "carbon peak and carbon neutrality", but also more conducive to the large-scale preparation of graphene.

[0023] The present invention provides a method for green exfoliation of graphene by combining a hydrothermal method with a liquid phase exfoliation method, which has the following beneficial effects:

[0024] (1) The present invention uses mild and pollution-free sodium bicarbonate instead of commonly used concentrated sulfuric acid as a swelling agent, expands graphite by a hydrothermal method, and then uses nanocellulose crystals to assist in exfoliating graphene, combining the hydrothermal method with the liquid phase exfoliation method, using only green, pollution-free, cheap, readily available and safe reagents, reducing the cost of exfoliating graphene, and making it more likely to achieve large-scale preparation of graphene.

[0025] (2) The sodium bicarbonate expansion graphite operation used in the present invention can be repeated, and as the number of repetitions increases, the yield of subsequent nanocellulose-assisted liquid phase exfoliation of graphene continues to increase. Three expansions can increase the subsequent graphene exfoliation yield to 62.2%. The use of nanocellulose crystals to assist in exfoliation of graphene has the following effects: first, the hydrophobic interaction between the nanocellulose crystals and graphite can cause a separation trend between graphite sheets, thereby increasing the yield of ultrasonic exfoliation of graphene; second, the good water dispersibility of the nanocellulose crystals stabilizes the exfoliated graphene, preventing the exfoliated graphene from restacking.

[0026] (3) The nanocellulose graphene aqueous dispersion prepared by the present invention not only has a high solution concentration, but also has a stable dispersion after high-speed and long-term centrifugation. It can be used to prepare graphene slurry or as an additive modifier for aqueous matrix. For example, the nanocellulose graphene aqueous dispersion can be used as a functional filler for PAM-based hydrogels, which can improve the mechanical properties of the hydrogels and give the hydrogels conductive properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 X-ray diffraction patterns of graphite expanded three times by hydrothermal method and untreated graphite.

[0028] Figure 2 Atomic force microscopy image and thickness distribution diagram of graphene nanocellulose hybrid prepared by hydrothermal expansion three times.

[0029] Figure 3 Field emission scanning electron microscopy image of graphene nanocellulose hybrid prepared by hydrothermal expansion three times.

[0030] Figure 4 Digital photo of nanocellulose aqueous dispersion prepared by hydrothermal expansion three times. DETAILED DESCRIPTION

[0031] In order to make the purpose, features and advantages of the present invention clearer and easier to understand, the technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0032] Embodiment 1:

[0033] The graphene was exfoliated and weighed using the following steps:

[0034] 25 mg of 8 mg / ml CNC aqueous dispersion was prepared, to which 0.2 g of untreated graphite was added, and the mixture was placed in a flask and stirred and ultrasonicated in an ultrasonic disperser for 4 h. The obtained product was centrifuged at 8000 r for 60 min, and the upper layer of liquid was freeze-dried to obtain 0-LEG / C-1.

[0035] Embodiment 2:

[0036] The graphene was exfoliated and weighed using the following steps:

[0037] 0.4 g of untreated graphite was impregnated in 20 g of NaHCO 3 The saturated solution was placed in a high-pressure reactor and kept at 85°C for 6 hours. When the reactor cooled to room temperature, the reactor was opened and the resulting mixture was filtered and then transferred to a semipermeable membrane for dialysis to remove residual salts. During this period, the water was changed at least 3 times, each time at an interval of 8 hours, and then dried to obtain graphite that had expanded once. 25 mg of 8 mg / ml CNC aqueous dispersion was prepared, and 0.2 g of graphite that had expanded once was added thereto. The mixture was placed in a flask and stirred in an ultrasonic disperser while ultrasonicating for 4 hours. The obtained product was centrifuged at 8000r for 60 minutes to separate the cellulose, and the obtained upper liquid was freeze-dried to obtain graphene.

[0038] Embodiment 3:

[0039] The graphene was exfoliated and weighed using the following steps:

[0040] 0.4 g of untreated graphite was impregnated in 20 g of NaHCO 3 The saturated solution was placed in a high-pressure reactor and kept at 85°C for 6 hours. When the reactor cooled to room temperature, the reactor was opened and the obtained mixture was filtered. The obtained graphite was further immersed in NaHCO 3 In the saturated solution, the hydrothermal expansion treatment was carried out for a total of three times. After the last filtration, it was transferred to a semipermeable membrane for dialysis to remove residual salts. The water was changed at least 3 times, each time with an interval of 8 hours. After that, the graphite expanded 3 times was dried. 25 mg of 8 mg / ml CNC aqueous dispersion was prepared, and 0.2 g of graphite expanded 3 times was added thereto. The mixture was placed in a flask and stirred in an ultrasonic disperser for 4 hours. The obtained product was centrifuged at 8000r for 60 minutes to separate the cellulose, and the upper layer of liquid was freeze-dried to obtain graphene.

[0041] Embodiment 4:

[0042] The graphene was exfoliated and weighed using the following steps:

[0043] 0.4 g of untreated graphite was impregnated in 20 g of NaHCO 3 The saturated solution was placed in a high-pressure reactor and kept at 85°C for 6 hours. When the reactor cooled to room temperature, the reactor was opened and the obtained mixture was filtered. The obtained graphite was further immersed in NaHCO 3 In the saturated solution, the hydrothermal expansion treatment was performed three times in total. After the last filtration, it was transferred to a semipermeable membrane for dialysis to remove residual salts. The water was changed at least three times, each time with an interval of 8 hours. After that, the graphite expanded three times was obtained by drying. 0.2g of the graphite expanded three times was added to 25g of deionized water, and the mixture was placed in a flask and ultrasonicated in an ultrasonic disperser for 4 hours while stirring. The obtained product was centrifuged at 8000r for 60min to separate the cellulose, and the obtained upper liquid was freeze-dried to obtain graphene.

[0044] Comparative Example 1

[0045] The graphene was exfoliated and weighed using the following steps:

[0046] 0.4 g of untreated graphite was impregnated in 20 g of NaHCO 3 The saturated solution was placed in a high-pressure reactor and kept at 85°C for 6 hours. When the reactor cooled to room temperature, the reactor was opened and the obtained mixture was filtered. The obtained graphite was further immersed in NaHCO 3In the saturated solution, the hydrothermal expansion treatment was carried out for a total of three times. After the last filtration, it was transferred to a semipermeable membrane for dialysis to remove residual salts. The water was changed at least 3 times, each time with an interval of 8 hours. Then the graphite expanded 3 times was dried. 25 mg of 2 mg / ml CNC aqueous dispersion was prepared, and 0.2 g of graphite expanded 3 times was added thereto. The mixture was placed in a flask and stirred in an ultrasonic disperser for 4 hours. The obtained product was centrifuged at 8000r for 60 minutes to separate the cellulose, and the upper liquid was freeze-dried to obtain graphene.

[0047] The products obtained from Experimental Examples 1-4 and Comparative Example 1 were weighed to obtain the following table:

[0048] Table 1 Graphene exfoliation yield

[0049]

[0050] From Table 1, we can see that:

[0051] a) In Examples 1-3, as the number of hydrothermal expansions increases, the graphene exfoliation yield is significantly improved. Under the same liquid phase exfoliation conditions, the exfoliation yield of graphite expanded by sodium bicarbonate once can be increased from 14.35% to 38.75%, a 2.7-fold increase in yield; the exfoliation yield of graphite expanded by sodium bicarbonate three times can be increased from 14.35% to 62.20%, a 4.3-fold increase in yield. The increase in yield is due to the fact that in the hydrothermal expansion step of graphite, bicarbonate ions can be inserted between graphite sheets due to the violent Brownian motion under high pressure, and sodium bicarbonate can be rapidly decomposed into CO under high temperature. 2 Gas, the expansion of gas can increase the distance between graphite layers and reduce the interlayer van der Waals force, such as Figure 1 As shown, the peak values ​​of each crystal plane of the expanded graphite are reduced, indicating that the crystal regularity of graphite becomes worse after hydrothermal expansion, which is conducive to further liquid phase exfoliation of graphene.

[0052] b) In Example 3 and Comparative Example 1, as the amount of nanocellulose crystals added increases, the graphene exfoliation yield is improved. When the amount of graphite added is the same and is set to 1, the yield obtained by the same weight of nanocellulose crystals assisted liquid phase exfoliation is 62.20%, but the yield of 0.25 times the weight of nanocellulose crystals assisted liquid phase exfoliation is 41.25%. The gain effect of nanocellulose crystals on graphene exfoliation is due to two reasons: first, the hydrophobic interaction between nanocellulose crystals and graphite can produce a separation trend between graphite sheets, thereby improving the yield of ultrasonic exfoliation of graphene; second, the good water dispersibility of nanocellulose crystals stabilizes the exfoliated graphene and prevents the exfoliated graphene from restacking. The thickness of the graphene in the final product obtained in Example 3 is about 1.2nm, and the number of graphene layers is about 4. Figure 2, Figure 3 shown.

[0053] c) The nanocellulose graphene aqueous dispersion obtained in Example 3 not only has a high solution concentration, but also has a stable dispersion after high-speed and long-term centrifugation. Figure 4 shown.

[0054] d) By comparing Example 3 with Example 4, it can be found that in the absence of nanocellulose crystals to assist in graphene exfoliation, the graphene exfoliation yield is only 5.15%, indicating that the combination of the hydrothermal method and the nanocellulose-assisted liquid phase exfoliation method greatly improves the graphene exfoliation efficiency.

[0055] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for green exfoliation of graphene combining a hydrothermal method and a liquid phase exfoliation method, characterized in that: The following steps are involved: (1) immersing graphite in a saturated solution of NaHCO3 to promote the expansion of graphite by hydrothermal method; (2) filtering, removing surface impurities by dialysis, and drying; (3) adding it to a nanocellulose crystal aqueous dispersion for ultrasonic exfoliation to obtain a liquid-phase exfoliated graphene-cellulose hybrid; the resulting solution is a nanocellulose graphene aqueous dispersion.

2. The method according to claim 1, characterized in that The hydrothermal method in step (1) promotes the expansion of graphite, and the specific steps are as follows: The graphite is immersed in a saturated NaHCO3 solution, the system is placed in a reactor and sealed, the reactor is placed in an oven and kept warm, and when the reactor is cooled to room temperature, the reactor is opened, the mixture is filtered and dried to obtain a solid, which is the graphite after expansion once.

3. The method according to claim 2, characterized in that The amount of NaHCO3 saturated solution used per gram of graphite is 30-80 ml.

4. The method according to claim 2, characterized in that: The insulation temperature is 85-95° C., and the insulation time is 6-12 hours.

5. The method according to claim 2, characterized in that: The reactor should be a polytetrafluoroethylene lined autoclave.

6. The method according to claim 1, characterized in that Repeat step (1) to promote the hydrothermal process of expanding graphite until the graphene is in a fluffy state. After the process is completed, the dried graphite is placed in a semipermeable membrane, and both ends of the sealed semipermeable membrane are placed in deionized water for dialysis to remove impurity ions.

7. The method according to claim 6, characterized in that The semipermeable membrane is an RC membrane with a molecular weight cut-off of no more than 14,000.

8. The method according to claim 1, characterized in that The concentration of the nanocellulose crystal aqueous dispersion described in step (3) is 2-16 mg / mL.

9. The method according to claim 1, characterized in that: In step (3), the mass ratio of graphite to nanocellulose crystals is 1:0.25-1:

2.

10. The method according to any one of claims 1 to 9, characterized in that: The ultrasonic treatment time in step (3) is 3-6 hours.

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