A method for purifying carbon nanotubes

Abstract

The application relates to the technical field of carbon nanotubes, and particularly discloses a carbon nanotube purification method. The carbon nanotube purification method comprises the following steps: (1) weighing carbon nanotube crude products and various raw materials; (2) adding the carbon nanotube crude products and a surfactant into deionized water to perform ultrasonic dispersion, so as to obtain a dispersion liquid; (3) performing gradient centrifugation on the dispersion liquid, and taking upper suspension; (4) mixing hydrochloric acid and nitric acid with the suspension to treat the carbon nanotubes; (5) treating the carbon nanotubes with dilute acid, mildly oxidizing the carbon nanotubes through a low-concentration hydrogen peroxide solution, and then adsorbing magnetic impurities in a magnetic field; (6) performing cross-flow filtration through a polycarbonate microfiltration membrane, and collecting the treated carbon nanotubes; and (7) performing inert gas annealing on the treated carbon nanotubes, and performing freeze drying, so as to obtain purified carbon nanotubes. The purification method can efficiently remove metal catalysts, amorphous carbon and graphite impurities, and meanwhile, the tube wall integrity of the carbon nanotubes is maintained.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube technology, and more specifically, to a method for purifying carbon nanotubes. Background Technology

[0002] Carbon nanotubes (CNTs) have broad application prospects in composite materials, nanoelectronic devices, and energy storage due to their excellent mechanical, electrical, and thermal properties. However, the crude synthesis products often contain impurities such as metal catalyst particles (e.g., Fe, Co), amorphous carbon, and graphite fragments, which severely restrict the intrinsic properties of carbon nanotubes. Therefore, developing efficient and low-damage purification techniques is a key prerequisite for realizing their high-end applications.

[0003] Current mainstream purification methods suffer from a core contradiction: purity and structural integrity cannot be simultaneously achieved. While concentrated acid reflux treatment can effectively dissolve metal catalysts, it erodes the carbon nanotube walls, increasing structural defects; it breaks the aspect ratio of carbon nanotubes, impairing their enhancement efficiency; and residual acid ions reduce conductivity. Although high-temperature air oxidation can burn amorphous carbon, it simultaneously oxidizes the end caps and defect sites of carbon nanotubes, causing irreversible mass loss. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a method for purifying carbon nanotubes.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for purifying carbon nanotubes includes the following steps: (1) Weigh 20-30 parts of crude carbon nanotubes, 4-6 parts of surfactant, 40-60 parts of hydrochloric acid, 40-60 parts of nitric acid, 80-120 parts of hydrogen peroxide solution, and 160-200 parts of deionized water by weight. (2) Add the surfactant to deionized water and stir until uniform, then add the crude carbon nanotubes, stir and disperse, and then place it in an ultrasonic disperser for ultrasonic treatment to obtain a dispersion. (3) Place the dispersion in a centrifuge, centrifuge at low speed for 4-6 min, then centrifuge at high speed for 30-40 min, and take the upper suspension. (4) Add hydrochloric acid and nitric acid to the suspension, stir at 100-200 rpm for 1-2 hours under water bath conditions of 40-60℃, filter, wash with deionized water until neutral, and obtain dilute acid treated carbon nanotubes. (5) Add the carbon nanotubes treated with dilute acid to hydrogen peroxide solution, stir at 80-100 rpm for 2-4 hours at room temperature, then place them in a magnetic field to adsorb magnetic impurities, collect them, and obtain a non-magnetic carbon nanotube suspension. (6) The non-magnetic carbon nanotube suspension was cross-flow filtered through a polycarbonate microfiltration membrane, and the filtered solid was collected to obtain the treated carbon nanotubes. (7) The treated carbon nanotubes are placed in an atmosphere furnace and kept at 200-300℃ for 40-60 min in an argon atmosphere. Then, the temperature is raised to 300-400℃ for annealing for 1-2 h. After cooling to room temperature, the carbon nanotubes are pre-cooled with liquid nitrogen and then freeze-dried for 4-8 h. The purified carbon nanotubes are then sealed and packaged.

[0006] Further, the crude carbon nanotubes in step (1) include at least one of crude multi-walled carbon nanotubes, crude oligo-walled carbon nanotubes, and crude single-walled carbon nanotubes.

[0007] Furthermore, the surfactant in step (1) includes at least one of polyethylene glycol octylphenyl ether, polyoxyethylene lauryl ether, and polysorbate-80.

[0008] Furthermore, in step (1), the hydrochloric acid has a mass fraction of 6% to 10%, the nitric acid has a mass fraction of 20% to 30%, and the hydrogen peroxide solution has a mass fraction of 4% to 6%.

[0009] Furthermore, the ultrasonic processing power in step (2) is 200~300W, and the processing time is 10~20min.

[0010] Furthermore, in step (3), the low-speed centrifugation speed is 1000~1200 rpm, and the high-speed centrifugation speed is 14000~16000 rpm.

[0011] Furthermore, the pore size of the polycarbonate microfiltration membrane in step (6) is 0.1~0.3μm.

[0012] Furthermore, the tangential flow velocity of the cross-flow filtration in step (6) is 0.4~0.6m / s.

[0013] Furthermore, the freeze-drying temperature in step (6) is -50~-40℃.

[0014] In summary, this application includes at least the following beneficial effects: This invention employs a purification method that first disperses carbon nanotubes (CNTs) with a surfactant to break down the CNT bundles and expose the impurity interface. Gentle sonication is then used to initially separate the CNTs from the impurities, avoiding excessive sonication that could lead to structural breakage. Gradient centrifugation is then used to remove impurities in layers based on the density difference between the impurities and CNTs. Low-speed centrifugation removes large particles of metal catalyst and graphite fragments, reducing the burden on subsequent acid treatment and minimizing acid usage. High-speed centrifugation collects the CNTs from the upper suspension, precipitating any remaining amorphous carbon. Then, low-concentration hydrochloric acid and nitric acid are used at low temperatures to dissolve small particles of metal catalyst, preventing over-oxidation of the CNTs in subsequent metal-catalyzed oxidation steps. A low-concentration hydrogen peroxide solution is then used to selectively oxidize the amorphous carbon while protecting the CNT structure. The acid-treated CNTs undergo carboxylation, improving the localized oxidation efficiency of hydrogen peroxide. Low-temperature treatment inhibits the erosion of the CNT walls by hydrogen peroxide. Magnetic separation is then used to remove magnetic particles such as Fe / Ni, reducing the filtration burden. A polycarbonate microfiltration membrane is used for cross-flow filtration to trap carbon nanotubes, with a continuous tangential flow washing the membrane surface to allow smaller amorphous carbon particles to pass through, preventing lateral blockage of the polycarbonate membrane by carbon nanotubes. Finally, annealing removes surface-adsorbed chemical residues and repairs surface defects caused by acid / oxidation. Freeze-drying is used instead of traditional high-temperature drying to prevent carbon nanotubes from agglomerating due to capillary forces and maintain their dispersibility. This purification method can efficiently remove metal catalysts, amorphous carbon, and graphite impurities while maintaining the integrity of the carbon nanotube walls. Attached Figure Description

[0015] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0016] Figure 1 This is a transmission electron microscope image of the purified multi-walled carbon nanotubes from Example 1 of the present invention; Figure 2 This is a transmission electron microscope image of the purified multi-walled carbon nanotubes of the present invention. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0018] It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0019] This invention provides a method for purifying carbon nanotubes, comprising the following steps: (1) Weigh 20-30 parts of crude carbon nanotubes, 4-6 parts of surfactant, 40-60 parts of 6%-10% hydrochloric acid, 40-60 parts of 20%-30% nitric acid, 80-120 parts of 4%-6% hydrogen peroxide solution, and 160-200 parts of deionized water by weight. (2) Add the surfactant to deionized water and stir evenly, then add the crude carbon nanotubes, stir and disperse, and then place it in an ultrasonic disperser and ultrasonically treat it with a power of 200~300W for 10~20min to obtain a dispersion. (3) Place the dispersion in a centrifuge and centrifuge at a low speed of 1000~1200 rpm for 4~6 min, then centrifuge at a high speed of 14000~16000 rpm for 30~40 min, and take the upper suspension. (4) Add hydrochloric acid and nitric acid to the suspension, stir at 100-200 rpm for 1-2 hours under water bath conditions of 40-60℃, filter, wash with deionized water until neutral, and obtain dilute acid treated carbon nanotubes. (5) Add the carbon nanotubes treated with dilute acid to hydrogen peroxide solution, stir at 80-100 rpm for 2-4 hours at room temperature, then place them in a magnetic field to adsorb magnetic impurities, collect them, and obtain a non-magnetic carbon nanotube suspension. (6) The non-magnetic carbon nanotube suspension was cross-flow filtered through a polycarbonate microfiltration membrane with a pore size of 0.1-0.3 μm at a tangential flow rate of 0.4-0.6 m / s, and the filtered solid was collected to obtain the treated carbon nanotubes. (7) The treated carbon nanotubes are placed in an atmosphere furnace and kept at 200~300℃ for 40~60 min in an argon atmosphere. Then, they are heated to 300~400℃ and annealed for 1~2 h. After cooling to room temperature, they are pre-cooled with liquid nitrogen and then freeze-dried at -50~-40℃ for 4~8 h. The purified carbon nanotubes are then sealed and packaged.

[0020] The crude carbon nanotubes include at least one of crude multi-walled carbon nanotubes, crude oligo-walled carbon nanotubes, and crude single-walled carbon nanotubes; the surfactant includes at least one of polyethylene glycol octylphenyl ether, polyoxyethylene lauryl ether, and polysorbate-80.

[0021] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0022] Example 1 The purification method for carbon nanotubes in this embodiment includes the following steps: (1) Weigh 20 parts of crude multi-walled carbon nanotubes, 4 parts of polyethylene glycol octylphenyl ether, 40 parts of 6% hydrochloric acid, 40 parts of 20% nitric acid, 80 parts of 4% hydrogen peroxide solution, and 160 parts of deionized water by weight. (2) Add polyethylene glycol octylphenyl ether to deionized water and stir evenly. Then add crude multi-walled carbon nanotubes, stir and disperse, and place in an ultrasonic disperser and ultrasonically treat with a power of 200W for 10 minutes to obtain a dispersion. (3) Place the dispersion in a centrifuge, first centrifuge at a low speed of 1000 rpm for 4 min, then centrifuge at a high speed of 14000 rpm for 30 min, and take the upper suspension. (4) Add hydrochloric acid and nitric acid to the suspension, stir at 100 rpm for 1 h in a water bath at 40°C, filter, wash with deionized water until neutral, and obtain dilute acid treated carbon nanotubes. (5) Add the carbon nanotubes treated with dilute acid to hydrogen peroxide solution, stir at 80~100 rpm for 2 hours at room temperature, then place them in a magnetic field to adsorb magnetic impurities, collect them, and obtain a non-magnetic carbon nanotube suspension. (6) The non-magnetic carbon nanotube suspension was filtered through a polycarbonate microfiltration membrane with a pore size of 0.1 μm at a tangential flow rate of 0.4 m / s, and the filtered solid was collected to obtain the treated carbon nanotubes. (7) The treated carbon nanotubes were placed in an atmosphere furnace and kept at 200°C for 40 min in an argon atmosphere. Then, they were heated to 300°C and annealed for 1 h. After cooling to room temperature, they were pre-cooled with liquid nitrogen and then freeze-dried at -50°C for 4 h. The purified carbon nanotubes were then sealed and packaged.

[0023] The purified carbon nanotubes of this embodiment were imaged using transmission electron microscopy. The transmission electron microscopy image is shown below. Figure 1 As shown.

[0024] Example 2 This embodiment of a method for purifying carbon nanotubes includes the following steps: (1) Weigh 25 parts of crude oligowalled carbon nanotubes, 5 parts of polyoxyethylene lauryl ether, 50 parts of 8% hydrochloric acid, 50 parts of 25% nitric acid, 100 parts of 5% hydrogen peroxide solution, and 180 parts of deionized water by weight. (2) Add polyoxyethylene lauryl ether to deionized water and stir evenly. Then add crude oligowalled carbon nanotubes, stir and disperse, and place in an ultrasonic disperser and ultrasonically treat for 15 minutes at a power of 250W to obtain a dispersion. (3) Place the dispersion in a centrifuge, first centrifuge at a low speed of 1100 rpm for 5 min, then centrifuge at a high speed of 15000 rpm for 35 min, and take the upper suspension. (4) Add hydrochloric acid and nitric acid to the suspension, stir at 150 rpm for 1.5 h in a water bath at 50 °C, filter, wash with deionized water until neutral, and obtain dilute acid treated carbon nanotubes. (5) Add the carbon nanotubes treated with dilute acid to hydrogen peroxide solution, stir at 90 rpm for 3 hours at room temperature, then place them in a magnetic field to adsorb magnetic impurities, collect them, and obtain a non-magnetic carbon nanotube suspension. (6) The non-magnetic carbon nanotube suspension was filtered through a polycarbonate microfiltration membrane with a pore size of 0.2 μm at a tangential flow rate of 0.5 m / s, and the filtered solid was collected to obtain the treated carbon nanotubes. (7) The treated carbon nanotubes were placed in an atmosphere furnace and kept at 250°C for 50 min in an argon atmosphere. Then, the temperature was raised to 350°C and annealed for 1.5 h. After cooling to room temperature, the carbon nanotubes were pre-cooled with liquid nitrogen and then freeze-dried at -45°C for 6 h. The purified carbon nanotubes were then sealed and packaged.

[0025] Example 3 This embodiment of a method for purifying carbon nanotubes includes the following steps: (1) Weigh out 30 parts of crude single-walled carbon nanotubes, 6 parts of polysorbate-80, 60 parts of 10% hydrochloric acid, 60 parts of 30% nitric acid, 120 parts of 6% hydrogen peroxide solution, and 200 parts of deionized water by weight. (2) Add polysorbate-80 to deionized water and stir evenly. Then add crude single-walled carbon nanotubes, stir and disperse, and place in an ultrasonic disperser and ultrasonically treat with a power of 300W for 20 minutes to obtain a dispersion. (3) Place the dispersion in a centrifuge, first centrifuge at a low speed of 1200 rpm for 6 min, then centrifuge at a high speed of 16000 rpm for 40 min, and take the upper suspension. (4) Add hydrochloric acid and nitric acid to the suspension, stir at 200 rpm for 2 hours under a water bath at 60°C, filter, wash with deionized water until neutral, and obtain dilute acid treated carbon nanotubes. (5) Add the carbon nanotubes treated with dilute acid to hydrogen peroxide solution, stir at 100 rpm for 4 hours at room temperature, then place them in a magnetic field to adsorb magnetic impurities, collect them, and obtain a non-magnetic carbon nanotube suspension. (6) The non-magnetic carbon nanotube suspension was filtered through a polycarbonate microfiltration membrane with a pore size of 0.3 μm at a tangential flow rate of 0.6 m / s, and the filtered solid was collected to obtain the treated carbon nanotubes. (7) The treated carbon nanotubes were placed in an atmosphere furnace and kept at 300°C for 60 min in an argon atmosphere. Then, they were heated to 400°C and annealed for 2 h. After cooling to room temperature, they were pre-cooled with liquid nitrogen and then freeze-dried at -40°C for 8 h. The purified carbon nanotubes were then sealed and packaged.

[0026] Comparative Example The purification method for the carbon nanotubes in this comparative example was conventional high-concentration acid washing (6M HNO3, 80℃, 6h). Transmission electron microscopy was used to image the purified carbon nanotubes in this comparative example; the transmission electron microscopy image is shown below. Figure 2 As shown.

[0027] Experimental Example The length retention rate, metal residue and conductivity of purified carbon nanotube samples from Examples 1-3 and the comparative examples were measured. The specific results are shown in Table 1.

[0028] Table 1: Performance Test Results of Purified Carbon Nanotube Samples from Examples and Comparative Examples

[0029] As shown in Table 1, compared with Example 1, the length retention rate, metal residue, and conductivity of the comparative example are all reduced. This is because Example 1 uses the purification method of the present invention, which combines gradient centrifugation pre-separation with low-temperature dilute acid treatment. Low-speed centrifugation removes large metal catalyst particles and graphite fragments, reducing the burden of subsequent acid treatment and reducing the amount of acid used. High-speed centrifugation collects carbon nanotubes in the upper suspension, precipitating residual amorphous carbon. Low-temperature dilute acid dissolves small metal catalyst particles, avoiding over-oxidation of carbon nanotubes in subsequent metal catalytic oxidation steps. Then, a low-concentration hydrogen peroxide solution is used to selectively oxidize amorphous carbon while protecting the carbon nanotube structure. This method achieves 398 S / cm, and annealing removes surface-adsorbed chemical residues and repairs surface defects caused by acid / oxidation, effectively restoring the conductivity of the sp² network. Figure 1 and Figure 2 The transmission electron microscopy images show that the carbon nanotubes purified in Example 1 maintain good integrity, while the carbon nanotubes purified in the comparative example show breakage and some amorphous carbon and other impurities are not completely removed.

[0030] Therefore, the purification method for carbon nanotubes provided by this invention can efficiently remove metal catalysts, amorphous carbon and graphite impurities from carbon nanotubes while maintaining the integrity of the carbon nanotube wall, and has good performance and broad application prospects.

[0031] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for purifying carbon nanotubes, characterized in that, Includes the following steps: (1) Weigh 20-30 parts of crude carbon nanotubes, 4-6 parts of surfactant, 40-60 parts of hydrochloric acid, 40-60 parts of nitric acid, 80-120 parts of hydrogen peroxide solution, and 160-200 parts of deionized water by weight. (2) Add the surfactant to deionized water and stir until uniform, then add the crude carbon nanotubes, stir and disperse, and then place it in an ultrasonic disperser for ultrasonic treatment to obtain a dispersion. (3) Place the dispersion in a centrifuge, centrifuge at 1000-1200 rpm for 4-6 min, then centrifuge at 14000-16000 rpm for 30-40 min, and take the upper suspension. (4) Add hydrochloric acid and nitric acid to the suspension, stir at 100-200 rpm for 1-2 hours under water bath conditions of 40-60℃, filter, wash with deionized water until neutral, and obtain dilute acid treated carbon nanotubes. (5) Add the carbon nanotubes treated with dilute acid to hydrogen peroxide solution, stir at 80-100 rpm for 2-4 hours at room temperature, then place them in a magnetic field to adsorb magnetic impurities, collect them, and obtain a non-magnetic carbon nanotube suspension. (6) The non-magnetic carbon nanotube suspension was cross-flow filtered through a polycarbonate microfiltration membrane, and the filtered solid was collected to obtain the treated carbon nanotubes. (7) The treated carbon nanotubes are placed in an atmosphere furnace and kept at 200-300℃ for 40-60 min in an argon atmosphere. Then, the temperature is raised to 300-400℃ for annealing for 1-2 h. After cooling to room temperature, the carbon nanotubes are pre-cooled with liquid nitrogen and then freeze-dried for 4-8 h. The purified carbon nanotubes are then sealed and packaged.

2. The purification method for carbon nanotubes according to claim 1, characterized in that, The crude carbon nanotubes in step (1) include at least one of crude multi-walled carbon nanotubes and crude single-walled carbon nanotubes.

3. The purification method for carbon nanotubes according to claim 1, characterized in that, The surfactant in step (1) includes at least one of polyethylene glycol octylphenyl ether, polyoxyethylene lauryl ether, and polysorbate-80.

4. The purification method for carbon nanotubes according to claim 1, characterized in that, In step (1), the hydrochloric acid has a mass fraction of 6% to 10%, the nitric acid has a mass fraction of 20% to 30%, and the hydrogen peroxide solution has a mass fraction of 4% to 6%.

5. The purification method for carbon nanotubes according to claim 1, characterized in that, The ultrasonic processing power in step (2) is 200~300W, and the processing time is 10~20min.

6. The purification method for carbon nanotubes according to claim 1, characterized in that, The pore size of the polycarbonate microfiltration membrane in step (6) is 0.1~0.3μm.

7. The purification method for carbon nanotubes according to claim 1, characterized in that, The tangential flow velocity of the cross-flow filtration in step (6) is 0.4~0.6m / s.

8. The purification method for carbon nanotubes according to claim 1, characterized in that, The freeze-drying temperature in step (7) is -50~-40℃.

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