Method for purifying carbon nano tube by easily removing polymer

Abstract

The invention discloses a method for purifying a carbon nanotube by easily removing a polymer, and belongs to the technical field of semiconductor materials. The method comprises the following steps: adding a crude carbon nanotube raw material and poly (2-(methylacryloyloxy) dodecyl benzoate) into an organic solvent, and mixing; carrying out dispersion treatment on the mixed solution by adopting non-contact ultrasound, and carrying out non-covalent coating on the carbon nanotubes by utilizing a polymer dispersing agent; carrying out centrifugal settling separation on the dispersed mixed solution, removing supernate containing impurities, and collecting precipitates; and washing the precipitate with a good solvent, dissolving and removing the polymer dispersant on the surface of the carbon nanotube to obtain the purified carbon nanotube. According to the method, the coating agent can be thoroughly removed through solvent washing without acid treatment by utilizing the easily soluble characteristic of a specific polymer, so that chemical residues and damage to the structure of the carbon nano tube are avoided, and the carbon nano tube with a complete structure and extremely high purity is obtained.

Description

Technical Field

[0001] This invention relates to the field of semiconductor materials technology, and more specifically to a method for easily removing polymers to purify carbon nanotubes. Background Technology

[0002] Carbon nanotubes (CNTs), as one-dimensional nanomaterials with excellent electrical, thermal, and mechanical properties, have enormous application potential in conductive additives, composite material reinforcements, transparent conductive films, and next-generation semiconductor devices (such as field-effect transistors and integrated circuits). However, during the preparation of carbon nanotubes (such as arc discharge methods and chemical vapor deposition methods), a large number of impurities inevitably remain, mainly including amorphous carbon, graphite particles, and transition metal catalyst particles. The presence of these impurities not only reduces the purity of carbon nanotubes but also severely scatters charge carriers, leading to a significant decrease in their conductivity, mechanical strength, and device on / off ratio, among other key performance characteristics. Therefore, efficiently removing impurities and obtaining high-purity, structurally intact carbon nanotubes is a crucial prerequisite for realizing their high-end applications.

[0003] Currently, existing carbon nanotube purification technologies mainly include physical purification methods and chemical purification methods: Physical purification methods typically employ ultrasonic dispersion combined with centrifugation and microfiltration. This method primarily utilizes the differences in particle size, shape, density, or electrical properties between carbon nanotubes and impurities for separation. However, physical methods have limited efficiency in removing nanoscale impurities, especially for carbon nanoparticles with sizes similar to the diameter of carbon nanotubes or metal catalysts encapsulated in carbon layers, where complete separation through simple physical sedimentation is difficult.

[0004] Chemical purification methods: The most common is liquid-phase oxidation, which uses strong acids (such as nitric acid and sulfuric acid) or strong oxidants to treat the raw materials. The oxidation reaction selectively etches amorphous carbon and dissolves the metal catalyst. While this method can remove most impurities, the harsh reaction conditions often lead to over-oxidation, destroying the originally perfect conjugated structure of carbon nanotubes, introducing numerous surface defects (such as carboxyl groups and hydroxyl groups) and pores, and even severing the carbon nanotubes, thus severely damaging their intrinsic electrical and mechanical properties. Furthermore, acid treatment processes also suffer from low yields, difficulties in wastewater treatment, and environmental pollution.

[0005] In recent years, selective dispersion and purification of carbon nanotubes using polymer non-covalent coating has become a research hotspot. This method utilizes the π-π interaction between conjugated polymers and carbon nanotubes to specifically encapsulate the carbon nanotubes, achieving separation from impurities due to differences in solubility. However, existing polymer purification techniques still suffer from the following significant drawbacks: Polymers are difficult to remove completely: To achieve stable dispersion, the polymers used are typically tightly bound to carbon nanotubes. Existing technologies often employ conjugated polymers with special structures such as imine bonds, which require subsequent degradation through acidic solutions or complex chemical reactions to peel them off the carbon nanotube surface. This chemical depolymerization process is not only cumbersome, but the acidic environment may also damage the carbon nanotubes again, or incomplete reactions may result in polymer residues, affecting the cleanliness of the final product.

[0006] Secondary contamination: In polymer dispersion processes, existing technologies commonly employ contact ultrasonic probes directly inserted into the solution for high-intensity ultrasonication. Under prolonged high-energy ultrasonic cavitation, the surface of the metal probe is highly susceptible to erosion, resulting in trace amounts of titanium and other metallic particles falling into the solution. These "secondary impurities" introduced by purification equipment are extremely difficult to remove, severely hindering the preparation of semiconductor-grade ultra-high purity carbon nanotubes (purity requirement >99.99%).

[0007] In summary, there is an urgent need in the existing technology for a method that can efficiently remove the original impurities, avoid introducing secondary pollution, and completely remove the coating agent in a simple and gentle way, so as to obtain ultra-high purity carbon nanotubes with intact structure and clean surface. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for easily removing polymers and purifying carbon nanotubes, comprising the following steps: Step S1: The crude carbon nanotube raw material and the polymer dispersant are added to an organic solvent and mixed. The polymer dispersant is poly(2-(methacryloyloxy)benzoic acid dodecyl ester), and its structural formula is shown below:

[0009] Where R is dodecyl, and n is an integer greater than or equal to 2; Step S2: The mixture is dispersed using non-contact ultrasound, and the carbon nanotubes are non-covalently coated using a polymer dispersant to form polymer-coated carbon nanotubes. Step S3: Centrifuge and sedimentate the dispersed mixture to remove the supernatant containing impurities and collect the precipitate containing polymer-coated carbon nanotubes. Step S4: Wash the precipitate with a good solvent to dissolve and remove the polymer dispersant on the surface of the carbon nanotubes, and obtain purified carbon nanotubes.

[0010] Furthermore, the poly(2-(methacryloyloxy)benzoic acid dodecyl ester) is prepared by free radical polymerization reaction. The specific steps are as follows: the monomer 2-(methacryloyloxy)benzoic acid dodecyl ester is dissolved in toluene solvent, and azobisisobutyronitrile (AIBN) with a mass fraction of 1% of the monomer mass is added as an initiator. Under nitrogen protection and sealing conditions, the reaction is carried out at a constant temperature and stirred at 80°C to obtain the crude polymer product.

[0011] Further, the post-processing steps of the crude polymer product include: cooling to room temperature after the reaction is completed, pouring the reaction solution into the undesirable solvent n-hexane to precipitate the product, filtering and washing the product, and then drying it in an empty drying oven at 80°C until the mass is constant to obtain a pure polymer dispersant.

[0012] Furthermore, the preparation method of the monomer 2-(methacryloyloxy)benzoic acid dodecyl ester is as follows: 2-hydroxybenzoic acid dodecyl ester is used as raw material, dissolved in anhydrous dichloromethane, triethylamine is added as an acid-binding agent, 2-methacryloyl chloride is added dropwise under ice bath conditions of 0-5℃, and after the addition is completed, the temperature is raised to room temperature and the reaction is carried out overnight.

[0013] Furthermore, the preparation method of the raw material 2-hydroxybenzoic acid dodecyl ester is as follows: 2-hydroxybenzoic acid and lauryl alcohol are mixed in a molar ratio of 1:1.5, toluene is added as a dehydrating agent, and concentrated sulfuric acid is added as a catalyst, and the mixture is heated to 105-115℃ for reflux dehydration reaction.

[0014] Further, in step S1, the mass ratio of the polymer dispersant to the crude carbon nanotube raw material is 1:1 to 1:5.

[0015] Furthermore, the mass ratio of the polymer dispersant to the crude carbon nanotube raw material is 1:2.

[0016] Furthermore, in step S2, the non-contact ultrasound is performed by placing the container containing the mixture in an ultrasonic water bath or using a cup-type ultrasonic processor, wherein the ultrasonic probe does not directly contact the mixture.

[0017] Furthermore, in step S3, the centrifugal sedimentation separation utilizes the differences in size and diffusion coefficient between carbon nanotubes and impurities, causing large-sized carbon nanotubes to settle to the bottom, while small-sized amorphous carbon and metal catalyst particles are suspended in the supernatant and removed.

[0018] Further, in step S4, the good solvent is one or more of toluene, n-hexane, or dichloromethane.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) Extremely high purification purity and impurity removal efficiency: This invention achieves effective spatial separation of carbon nanotubes from amorphous carbon and metal catalysts through the specific selective adsorption of polymers. Experimental results show that this method can improve the purity of carbon nanotubes to 99.9995% with extremely low metal impurity content, fully meeting the manufacturing requirements of high-performance semiconductor devices.

[0020] (2) The polymer is easy to remove and leaves no residue: Unlike existing technologies that rely on chemical bond breaking (acid hydrolysis) to remove polymers, the polymer side chain used in this invention contains a large dodecyl chain, which makes it difficult for the polymer molecules to be arranged in a regular manner, resulting in an amorphous state, and it is extremely soluble in commonly used non-polar organic solvents (such as toluene and n-hexane). Therefore, it can be completely washed away with a simple solvent without the need for acid treatment, thus avoiding chemical residue.

[0021] (3) Non-destructive purification while maintaining structural integrity: The entire purification process is based entirely on physical interactions (van der Waals forces and π-π interactions), without involving strong acid oxidation or violent chemical reactions, thus avoiding damage to the graphene lattice structure of the carbon nanotubes. The purified carbon nanotubes have fewer defects in their walls, and their length and electrical conductivity are well maintained. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the steps of the method for purifying carbon nanotubes using easily removable polymers according to the present invention; Figure 2 This is a schematic diagram of the non-contact ultrasonic dispersion of the present invention; Figure 3 The preparation process of poly(2-(methacryloyloxy)benzoic acid dodecyl ester); Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art should understand that the apparatus, systems, and methods described in the embodiments disclosed in this invention are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention.

[0024] Example 1

[0025] The following is combined Figure 1 The embodiments of the present invention are described in detail, mainly including the following steps: Step S1: Take a certain amount of crude carbon nanotube raw material (containing impurities such as amorphous carbon and metal catalysts), and add it together with a polymer dispersant to the organic solvent toluene. In this embodiment, the polymer dispersant is poly(2-(methacryloyloxy)benzoic acid dodecyl ester). In this embodiment, to achieve the best purification effect, the mass ratio of polymer dispersant to crude carbon nanotube raw material is controlled at 1:2. At this ratio, the polymer is sufficient to cover the surface of the carbon nanotubes, while avoiding the subsequent cleaning burden caused by excessive polymer.

[0026] Step S2: Place the container containing the mixture in a cup-type ultrasonic processor for non-contact ultrasonic dispersion. Unlike traditional ultrasonic methods where the probe is directly inserted into the solution, this invention uses a non-contact method, where the probe does not contact the mixture. This effectively avoids secondary contamination caused by titanium alloy particles generated during probe cavitation erosion entering the solution. Figure 2 As shown, during the ultrasonication process, polymer molecules utilize the aromatic ring structure in their side chains to specifically adsorb onto the graphitized surface of carbon nanotubes through π-π stacking interactions. Simultaneously, the long-chain dodecyl groups in their side chains extend into the solvent, providing steric hindrance and solubility, thereby achieving non-covalent coating and stable dispersion of the carbon nanotubes. Impurities (such as amorphous carbon and metal particles) cannot be effectively coated by the polymer due to a lack of conjugated structures or size differences.

[0027] Step S3: The dispersed suspension is subjected to high-speed centrifugation. Separation is achieved by utilizing the significant differences in size and Brownian diffusion coefficient between the polymer-coated carbon nanotubes and impurity particles. Under the centrifugal field, large-sized, high aspect ratio carbon nanotubes (coated with polymer) settle rapidly to the bottom of the centrifuge tube, forming a precipitate; while tiny amorphous carbon particles (<2 nm) and fine metal catalyst particles (<50 nm) remain suspended in the supernatant. The supernatant containing impurities is carefully removed and discarded, and the precipitate at the bottom is collected. If necessary, the dispersion-centrifugation step can be repeated 1-2 times to further improve purity.

[0028] Step S4: Add a good solvent (such as toluene, dichloromethane, or n-hexane) to the collected precipitate. Utilizing the excellent solubility and non-crystallization properties imparted by the large dodecyl side chain of the polymer, the precipitate is shaken and washed. The polymer layer rapidly dissolves in the good solvent and detaches from the carbon nanotube surface. Subsequently, the solvent containing the polymer is separated by simple filtration or centrifugation again, leaving pure carbon nanotubes. This process does not involve acid treatment or metal salt dissociation reactions, avoiding damage to the carbon nanotube structure by chemical reagents.

[0029] Testing revealed that the carbon nanotubes purified using the method described in this embodiment exhibited extremely high semiconductor purity and overall material purity. The semiconductor nanotube content was measured to be as high as 99.9995%, while the metal nanotube content was only 0.0005%, resulting in an overall purification rating of "Excellent".

[0030] The polymer dispersant used in this embodiment is poly(2-(methacryloyloxy)benzoate dodecyl ester), which is the core material of this invention, and its preparation process is as follows: Figure 3 As shown, the process is mainly divided into three stages: intermediate synthesis, monomer synthesis, and polymerization reaction.

[0031] (1) Preparation of intermediate 2-hydroxybenzoic acid dodecyl ester

[0032] First, the reaction apparatus was set up. In a three-necked flask equipped with a spherical condenser and a water separator, the raw materials 2-hydroxybenzoic acid and lauryl alcohol (dodecane-1-ol) were accurately weighed. To ensure a complete reaction, the molar ratio of the two was controlled at 1:1.5. Then, an appropriate amount of toluene was added as a reaction solvent and dehydrating agent, and concentrated sulfuric acid was slowly added dropwise using a syringe as a catalyst, with the amount being approximately 1% to 2% of the total mass of the reactants. The heating device was turned on, and the temperature was slowly raised to 105-115°C, maintaining the reaction solution under reflux. Within this temperature range, toluene and the generated water form an azeotrope, which is removed by the water separator, thus driving the esterification reaction in the forward direction. The reaction continued overnight. After the reaction was completed, the solution was cooled to room temperature and transferred to a separatory funnel. Saturated sodium carbonate solution was added, and the mixture was shaken and washed to neutralize excess acidic substances until the solution was neutral. The upper organic phase was retained and washed repeatedly with deionized water to remove water-soluble impurities. Then, anhydrous magnesium sulfate was added and dried to remove water. Finally, the desiccant is removed by filtration, and the filtrate is subjected to vacuum distillation to collect the fraction with the corresponding boiling point, thus obtaining preliminarily purified dodecyl 2-hydroxybenzoate.

[0033] (2) Synthesis of monomer 2-(methacryloyloxy)benzoic acid dodecyl ester.

[0034] The prepared 2-hydroxybenzoic acid dodecyl ester was placed in a three-necked flask, and anhydrous dichloromethane was added to completely dissolve it. An appropriate amount of triethylamine was added as an acid-binding agent. Under ice bath conditions of 0-5°C, 2-methacryloyl chloride was placed in a constant-pressure dropping funnel and added dropwise to the flask at a rate of 1-2 drops per minute, strictly controlling the dropping rate to prevent side reactions caused by vigorous exothermic reactions. After the addition was complete, the ice bath was removed, and the reaction system was allowed to naturally warm to room temperature and continue reacting overnight with stirring. The reaction endpoint could be monitored by thin-layer chromatography (TLC). In the post-processing stage, the reaction solution was washed with water to remove water-soluble salt impurities. After separation, the organic phase was dried over anhydrous magnesium sulfate. Subsequently, the dichloromethane solvent was removed by rotary evaporation. The crude product was purified by column chromatography to obtain high-purity monomeric 2-(methacryloyloxy)benzoic acid dodecyl ester.

[0035] (3) Preparation of polymer dispersants.

[0036] Accurately weigh the purified monomers and transfer them to a dry reaction vessel. Add 1% (by weight) of azobisisobutyronitrile (AIBN) as a free radical initiator, using toluene as the solvent. To avoid the inhibitory effect of oxygen on free radical polymerization, strictly evacuate the reaction system and replace it with high-purity nitrogen (repeated at least 5 times) to ensure the reaction is carried out under an inert atmosphere and sealed conditions. Start stirring (approximately 900 r / min), raise the reaction temperature to 80℃, and maintain the temperature fluctuation within ±5℃. Molecular weight changes can be monitored via GPC during the reaction. Once the predetermined conversion rate is reached, stop heating and cool to room temperature. Slowly pour the reaction solution into a large amount of the undesirable solvent n-hexane, separating the polymer through precipitation. Collect the precipitate by filtration and wash repeatedly with n-hexane to remove residual monomers and initiator. Finally, place the product in a vacuum drying oven and dry at 80℃ until constant mass is achieved, yielding the final polymer dispersant, poly(2-(methacryloyloxy)benzoic acid dodecyl ester).

[0037] To investigate the effect of different ratios on purification efficiency, a series of comparative experiments were conducted, as shown in Table 1.

[0038] Example 2

[0039] While keeping other process conditions (such as solvent type, ultrasonic power, and centrifugation speed) consistent, the mass ratio of polymer dispersant to crude carbon nanotube raw material in step S1 was adjusted to 1:1, and other steps were the same as in Example 1. At this time, the semiconductor tube content was measured to be as high as 99.2044%, the metal tube content was 0.7956%, and the overall purification rating was "good".

[0040] Example 3

[0041] While keeping other process conditions (such as solvent type, ultrasonic power, and centrifugation speed) consistent, the mass ratio of polymer dispersant to crude carbon nanotube raw material in step S1 was adjusted to 1:5, and other steps were the same as in Example 1. At this time, the semiconductor tube content was measured to be as high as 99.1806%, the metal tube content was 0.8194%, and the overall purification rating was "good".

[0042] The experimental results above show that the purification effect is good when the mass ratio is between 1:1 and 1:5. In particular, in Example 1, the optimal balance point was reached when the mass ratio was 1:2, at which point the semiconductor tube content was measured as high as 99.9995%, while the metal tube content was only 0.0005%, resulting in an overall purification rating of "excellent". Therefore, the preferred mass ratio range of this invention is 1:1 to 1:5, with 1:2 being the most preferred.

[0043] Example 4

[0044] While keeping other process conditions (such as solvent type, ultrasonic power, centrifugation speed) consistent, the mass ratio of polymer dispersant to crude carbon nanotube raw material in step S1 was adjusted to 1:0.5, and the other steps were the same as in Example 1.

[0045] When the mass ratio is 1:0.5 (polymer excess), although the dispersibility is good, the number of washes required to remove the polymer increases, and the excess polymer is prone to forming micelles that encapsulate some impurities and precipitate together, resulting in a slightly lower final purity, with the metal tube content at 1.1483%, which cannot be completely removed.

[0046] Example 5

[0047] While keeping other process conditions (such as solvent type, ultrasonic power, centrifugation speed) consistent, the mass ratio of polymer dispersant to crude carbon nanotube raw material in step S1 was adjusted to 1:10, and other steps were the same as in Example 1.

[0048] When the mass ratio is 1:10 (insufficient polymer), the amount of polymer is insufficient to completely coat the carbon nanotubes, causing some carbon nanotubes to agglomerate and settle along with impurities or disperse unevenly. The purification yield and purity both decrease significantly, and the metal tube content rises to 1.9968%.

[0049] Table 1. Effect of different ratios on purification efficiency

[0050] It should be noted that the above detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for purifying carbon nanotubes with an easily removable polymer, characterized by, The method comprises the following steps: Step S1: mixing crude carbon nanotube raw material and a polymer dispersant in an organic solvent, wherein the polymer dispersant is poly(2-(methacryloyloxy)benzoic acid dodecyl ester) with the following structure: wherein R is dodecyl, and n is an integer greater than or equal to 2; Step S2: dispersing the mixed solution by non-contact ultrasonic treatment, and non-covalently coating the carbon nanotubes with the polymer dispersant to form polymer-coated carbon nanotubes; Step S3: centrifugal sedimentation separation of the dispersed mixed solution to remove the supernatant containing impurities, and collecting the precipitate containing polymer-coated carbon nanotubes; Step S4: washing the precipitate with a good solvent to dissolve and remove the polymer dispersant on the surface of the carbon nanotubes, and obtaining purified carbon nanotubes.

2. The method of claim 1, wherein, The poly(2-(methacryloyloxy)benzoic acid dodecyl ester) is prepared by a free radical polymerization reaction, and the specific steps are as follows: dissolving monomer 2-(methacryloyloxy)benzoic acid dodecyl ester in toluene solvent, adding 1% of the mass of the monomer azobisisobutyronitrile (AIBN) as an initiator, and stirring at 80°C under nitrogen protection and sealed conditions to prepare a polymer crude product.

3. The method of claim 2, wherein, The post-treatment steps of the polymer crude product include: after the reaction is completed, cooling to room temperature, pouring the reaction solution into a poor solvent n-hexane to precipitate and separate out, filtering and washing, and drying in a 80°C vacuum drying oven until the mass is constant to obtain pure polymer dispersant.

4. The method of claim 2, wherein, The preparation method of the monomer 2-(methacryloyloxy)benzoic acid dodecyl ester is as follows: using 2-hydroxybenzoic acid dodecyl ester as a raw material, dissolving it in anhydrous dichloromethane, adding triethylamine as an acid binding agent, and adding 2-methacryloyl chloride dropwise under ice bath conditions at 0-5°C, and then warming to room temperature overnight.

5. The method of claim 4, wherein, The preparation method of the raw material 2-hydroxybenzoic acid dodecyl ester is as follows: mixing 2-hydroxybenzoic acid and lauryl alcohol according to a molar ratio of 1:1.5, adding toluene as a water carrying agent, and adding concentrated sulfuric acid as a catalyst, heating to 105-115°C for reflux water removal reaction.

6. The method of claim 1, wherein, In step S1, the mass ratio of the polymer dispersant to the crude carbon nanotube raw material is 1:1 to 1:

5.

7. The method of claim 1, wherein, The mass ratio of the polymer dispersant to the crude carbon nanotube raw material is 1:

2.

8. The method of claim 1, wherein, In step S2, the non-contact ultrasonic treatment is performed by placing a container containing the mixed solution in an ultrasonic water bath or using a cup-type ultrasonic processor, and the ultrasonic probe does not directly contact the mixed solution.

9. The method of claim 1, wherein, In step S3, the centrifugal sedimentation separation is to make large-size carbon nanotubes settle at the bottom, while small-size amorphous carbon and metal catalyst particles are suspended in the supernatant and removed.

10. The method of claim 1, wherein, In step S4, the good solvent is one or more of toluene, n-hexane or dichloromethane.

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