Purification method of carbon nanotubes

Abstract

The invention discloses a method for purifying a carbon nano tube, which comprises the following steps: mixing a carbon nano tube containing impurities with a solid halogen source, putting the mixture into a reactor, introducing inert gas, and carrying out heat treatment to convert the solid halogen source into halogen gas, the halogen gas reacts with impurities in the carbon nanotubes to generate metal halide, and the metal halide is taken away from the reactor in an inert gas atmosphere to obtain purified carbon nanotubes. The carbon nanotubes are purified by using the solid halogen source, so that the use of liquid acid is avoided, no wastewater is generated in the whole process or only by-products which are easy to treat are generated, the energy consumption is low, and the structural integrity of the carbon nanotubes can be better maintained.

Description

Technical Field

[0001] This application belongs to the field of nanomaterial purification technology, specifically relating to a purification method for carbon nanotubes. Background Technology

[0002] Carbon nanotubes, as carbon materials with a unique one-dimensional tubular structure, have shown broad application prospects in composite materials, electronic devices, energy storage, and sensing due to their excellent mechanical strength, superior electrical conductivity, and outstanding thermal stability. However, carbon nanotubes obtained through conventional preparation techniques (such as chemical vapor deposition) often contain impurities such as metal nanoparticles (e.g., iron, cobalt, nickel) used for catalytic growth and amorphous carbon. The presence of these impurities not only masks the intrinsic properties of carbon nanotubes but also severely limits their practical application in high-end functional materials and precision devices.

[0003] Currently, the mainstream method for removing metal and amorphous carbon impurities from carbon nanotubes is liquid-phase chemical oxidation. This method typically uses concentrated nitric acid, sulfuric acid, or a mixture of these acids as the oxidation medium, and processes the carbon nanotubes under reflux conditions for an extended period. Its purification mechanism mainly relies on the oxidative etching effect of strong acids to dissolve metal particles and partially remove amorphous carbon. Due to its relatively simple operation and significant effect on removing metal impurities, this method has become the most widely used purification technique in laboratories and industry.

[0004] Although liquid-phase chemical oxidation has certain advantages in purification efficiency, it still has several significant drawbacks in practical applications: First, the method uses highly corrosive acid, which places stringent requirements on the materials of the reaction equipment and poses a high risk of operational safety. Second, the process generates a large amount of acidic waste liquid containing heavy metal ions, and the subsequent neutralization and treatment steps are cumbersome, resulting in significant environmental pollution and poor overall environmental performance. In addition, while removing impurities, the strong oxidizing acid inevitably attacks the carbon nanotubes themselves, causing the tube walls to be etched, the length to be shortened, and the number of structural defects to increase, thereby causing a decline in their conductivity and mechanical properties. Finally, the process involves multiple steps such as heating, stirring, multiple centrifugation washing, and drying, which is lengthy, energy-intensive, and not conducive to cost control in large-scale production. Summary of the Invention

[0005] This application aims to provide a purification method for carbon nanotubes, which solves the problems of severe equipment corrosion, high wastewater treatment costs, high energy consumption, and damage to the intrinsic structure of carbon nanotubes caused by traditional acid washing methods.

[0006] To solve the above-mentioned technical problems, this application is implemented as follows: This application provides a method for purifying carbon nanotubes, the purification method comprising: Impurity-containing carbon nanotubes are mixed with a solid halogen source and placed in a reactor. An inert gas is introduced and the mixture is heat-treated. The heat treatment converts the solid halogen source into halogen gas. The halogen gas reacts with the impurities in the carbon nanotubes to generate metal halides. The metal halides are carried away from the reactor under an inert gas atmosphere to obtain purified carbon nanotubes.

[0007] Optionally, the molar ratio of impurities in the impurity-containing carbon nanotubes to halogens in the solid halogen source is greater than 1:3.

[0008] Optionally, the solid halogen source includes one of the following: a volatile solid halogen source, a solid halogen source consisting of a mixture of inorganic halide and activator, a metal halide solid halogen source, and a metal chlorate solid halogen source.

[0009] Optionally, the volatile solid halogen source includes chlorides and / or fluorides; The chloride is selected from one or more of ammonium chloride, hexachloroethane, and polyvinyl chloride; and / or, The fluoride is selected from one or more of ammonium bifluoride, polytetrafluoroethylene, polyvinylidene fluoride, fluorinated graphite, and perfluoropolyether compounds.

[0010] Optionally, the solid halogen source in which the inorganic halide is mixed with the activator includes: The inorganic halide is selected from one or more of sodium chloride, potassium chloride, magnesium chloride, and gallium chloride; The activator is selected from one or more of manganese dioxide, ferric oxide, and potassium chlorate.

[0011] Optionally, the metal halide solid halogen source is selected from one or more of ferric chloride, aluminum chloride, titanium chloride, copper dichloride, and cobalt trifluoride.

[0012] Optionally, the solid halogen source of the metal chlorate is selected from one or more of sodium chlorate and potassium chlorate.

[0013] Optionally, the heat treatment includes: The heat treatment temperature is 200~1100℃, the heating rate is 1~10℃ / min, and the holding time is 0.5~2h.

[0014] Optionally, the inert gas includes one or more of nitrogen, argon, and an oxygen-containing argon mixture; In the mixed gas, the volume ratio of oxygen is 1% to 10%.

[0015] Optionally, the impurities in the impurity-containing carbon nanotubes include one or more of Fe, Co, Ni, Al, and Mg.

[0016] Compared with the prior art, this application has at least the following advantages: In the embodiments of this application, carbon nanotubes containing impurities are mixed with a solid halogen source and placed in a reactor. An inert gas is introduced and heat treatment is performed. The heat treatment converts the solid halogen source into halogen gas. The halogen gas reacts with the impurities in the carbon nanotubes to generate metal halides. The metal halides are carried away from the reactor under an inert gas atmosphere to obtain purified carbon nanotubes.

[0017] Purifying carbon nanotubes using a solid halogen source is an environmentally friendly process that eliminates the need for strong acids, strong oxidants, and other liquid chemicals, thus preventing the generation of acidic wastewater at the source. It also boasts low energy consumption, eliminating the energy-intensive steps of repeated centrifugation, washing, and prolonged drying required in traditional acid washing methods, simplifying the process. Furthermore, it offers excellent structural protection, as the solid halogen source is converted into halogen gas, and gas-phase halogen etching, under heat treatment, exhibits high selectivity for metallic impurities in carbon nanotubes, particularly for SP. 2 Carbon structures are far less aggressive than strong liquid acids, thus better preserving the length, graphitization degree, and macroscopic aggregation morphology (such as array structures) of carbon nanotubes; they have high purification efficiency, good diffusion of halogen gas molecules, and can reach partially encapsulated metal particles to achieve deep purification, enabling highly selective removal of specific metal impurities.

[0018] In summary, this application avoids the use of liquid acid, produces no wastewater or only easily treatable byproducts throughout the process, has low energy consumption, and can better maintain the structural integrity of carbon nanotubes.

[0019] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0020] Figure 1 A diagram of an apparatus for purifying carbon nanotubes provided in an embodiment of this application is shown; Figure 2 A schematic diagram of the reaction section in the reaction apparatus provided in Embodiment 1 of this application is shown; Figure 3 A schematic diagram of the reaction section in the reaction apparatus provided in Embodiment 2 of this application is shown; Figure 4 A schematic diagram of the reaction section in a reaction apparatus provided in Embodiment 3 of this application is shown; Figure 5 A schematic diagram of the reaction section in another reaction apparatus provided in Embodiment 3 of this application is shown; Figure 6 A schematic diagram of the reaction section in the reaction apparatus provided in Embodiment 5 of this application is shown. Detailed Implementation

[0021] The embodiments of this application will now be described in detail. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0022] Carbon nanotubes, as a carbon material with a unique one-dimensional nanostructure, have become a research hotspot in the field of materials science since their first report in 1991, thanks to their outstanding comprehensive properties. Their axial tensile strength can reach more than 100 times that of steel, while also possessing excellent flexibility. In terms of electrical properties, the conductivity of some carbon nanotubes far exceeds that of copper, and their carrier mobility can reach tens of times that of traditional semiconductor materials. In terms of thermal properties, their thermal conductivity can reach up to 1.5 times that of diamond, while also exhibiting good thermal stability. These outstanding characteristics make them irreplaceable in several key areas such as flexible electronic devices, high-performance composite material reinforcement phases, lithium-ion battery electrode materials, fuel cell catalyst supports, and supercapacitor electrodes, and they are considered one of the core materials driving the development of the next generation of high-tech industries. However, the raw carbon nanotube products obtained by conventional preparation processes such as chemical vapor deposition (CVD), arc discharge, and laser ablation inevitably contain a large number of impurity components. These mainly include transition metal catalyst nanoparticles (such as Fe, Co, Ni and their oxides) used in the preparation process, incompletely graphitized amorphous carbon, carbon nanoparticles, and a small amount of fullerene derivatives. The presence of these impurities seriously hinders the full realization of the intrinsic properties of carbon nanotubes and greatly limits their practical application in high-precision and high-reliability scenarios. Therefore, efficient purification of carbon nanotubes has become a key step in their industrialization process.

[0023] To address the problem of impurity removal from raw carbon nanotubes, researchers have developed various carbon nanotube purification technologies, which can be categorized into three main types based on the state of the processing system: liquid-phase purification, gas-phase purification, and physical purification. Among these, liquid-phase chemical oxidation has become the most widely used purification technology in industrial production and laboratory research due to its relatively simple operation and high efficiency in removing metal impurities. This method typically uses strong oxidizing acids as the core processing agent, including single strong acids (such as concentrated nitric acid or concentrated sulfuric acid) or mixed acids (such as nitric acid-sulfuric acid mixtures or nitric acid-hydrogen peroxide systems). Through processes such as reflux, ultrasonic-assisted reflux, or microwave-assisted reflux, the metal catalyst particles undergo a dissolution reaction with the acid, while the strong acid's oxidizing effect decomposes amorphous carbon impurities. In addition, some modified liquid-phase methods introduce strong oxidants such as potassium permanganate or potassium dichromate to enhance the oxidation effect. Besides liquid-phase chemical oxidation, existing physical purification methods include filtration, centrifugation, electrophoresis, and magnetic separation. Among them, magnetic separation mainly targets impurities in magnetic metal catalysts, using a high-intensity magnetic field to separate magnetic particles from carbon nanotubes. Gas-phase purification methods mostly use air, oxygen, or carbon dioxide as oxidation media, preferentially oxidizing and decomposing amorphous carbon under high-temperature conditions, thereby achieving separation from carbon nanotubes.

[0024] Although existing purification techniques can reduce the impurity content in carbon nanotubes to some extent, all methods have significant technical defects and cannot meet the core requirements of high purity, low damage and environmental friendliness for industrial applications. Among them, the inherent defects of the most widely used liquid-phase chemical oxidation method are particularly prominent: First, the reagents used, such as concentrated nitric acid and concentrated sulfuric acid, are extremely corrosive, which not only imposes corrosion resistance requirements on the reaction equipment and increases equipment investment costs, but also poses safety hazards such as acid leakage and volatilization during operation; Second, this method generates a large amount of acidic wastewater containing heavy metal ions. The subsequent treatment process, such as neutralization and heavy metal recovery of this wastewater, is complex and costly, and easily causes soil and water pollution, resulting in a heavy environmental burden; Third, the oxidation effect of strong oxidizing acids lacks selectivity. While dissolving metal impurities and decomposing amorphous carbon, it also indiscriminately etches the walls of carbon nanotubes, resulting in a large number of defect sites on the carbon nanotube walls and the cutting of the ends, which in turn shortens their length and reduces their specific surface area, severely damaging their excellent intrinsic properties such as electrical conductivity and mechanical strength; Finally, this process involves multiple steps such as heating and reflux, long-term stirring, multiple centrifugation, repeated washing to neutrality, and vacuum drying. The process is lengthy and consumes a lot of energy, which is not conducive to reducing production costs. Other existing methods also have significant shortcomings: among physical purification methods, magnetic separation can only remove magnetic metal impurities, and its removal effect on non-magnetic impurities and amorphous carbon is extremely poor, and it is difficult to separate small catalyst particles; filtration and centrifugation are limited by the differences in particle size and density between impurities and carbon nanotubes, resulting in low purification accuracy and difficulty in obtaining high-purity products; gas phase purification methods usually need to be carried out at 600~900℃, which are harsh reaction conditions, and the oxidizing medium still has a certain etching effect on carbon nanotubes, which can easily lead to structural damage. At the same time, this method has limited effect on removing metal impurities and needs to be combined with other methods to achieve the expected purity.

[0025] In summary, existing carbon nanotube purification methods cannot simultaneously meet the technical requirements of high purification efficiency, low structural damage, environmental friendliness, and low cost, becoming a key bottleneck restricting the industrial application of carbon nanotubes. Therefore, it is urgent to develop a new non-liquid phase, low-damage, environmentally friendly, and energy-efficient carbon nanotube purification method to break through the existing technical bottlenecks and promote the industrial application of carbon nanotube materials.

[0026] This application aims to address the challenges of achieving high purification efficiency, low structural damage, environmental friendliness, and low cost for carbon nanotubes. An embodiment of this application provides a method for purifying carbon nanotubes, comprising: Impurity-containing carbon nanotubes are mixed with a solid halogen source and placed in a reactor. An inert gas is introduced and the mixture is heat-treated. The heat treatment converts the solid halogen source into halogen gas. The halogen gas reacts with the impurities in the carbon nanotubes to generate metal halides. The metal halides are carried away from the reactor under an inert gas atmosphere to obtain purified carbon nanotubes.

[0027] In practice, raw carbon nanotubes containing metal catalyst impurities are placed in a reactor along with a solid halogen source. Under an inert gas atmosphere, the temperature is increased programmatically for heat treatment. During this process, highly reactive halogen gases (such as Cl2, F2, HCl, and fluorine-containing free radicals) are generated through thermal volatilization, decomposition, or in-situ gas-solid phase reaction. These gases undergo selective chemical reactions with the metal catalyst particles (such as Fe, Co, and Ni) within the carbon nanotubes, generating volatile metal halides which are carried away by the gas flow, thus achieving deep removal of metal impurities. After the reaction is complete, heating is stopped, and the mixture is cooled to room temperature under continuous ventilation. The purified carbon nanotubes are then collected.

[0028] It should be noted that, Figure 1 A diagram of the apparatus for purifying carbon nanotubes provided in an embodiment of this application is shown; as follows: Figure 1 As shown, the carbon nanotube purification apparatus includes the following parts: a heating decomposition device, an absorption and separation device, a reaction device, and a tail gas treatment device. Taking argon as the inert gas, argon gas is introduced throughout the entire carbon nanotube purification apparatus. The heating decomposition device is used to heat and decompose the solid halogen source into halogen gas and other decomposition products at the heat treatment temperature. The absorption and separation device is used to absorb other decomposition products and separate the halogen gas, which is then introduced into the reaction device. The reaction device contains carbon nanotubes containing impurities. The halogen gas reacts with the impurities in the reaction device to generate metal halides. Finally, the tail gas treatment device carries the generated metal halides away from the reaction device, and purified carbon nanotubes are obtained in the reaction device.

[0029] In this embodiment, carbon nanotubes are purified using a solid halogen source. The purification process is green and environmentally friendly, eliminating the need for strong acids, strong oxidants, or other liquid chemicals, thus preventing the generation of acidic wastewater at the source. It is also energy-efficient, eliminating the energy-consuming steps of repeated centrifugation, washing, and prolonged drying in traditional acid washing methods, simplifying the process. Furthermore, it offers good structural protection, as the solid halogen source is converted into halogen gas. Gas-phase halogen etching, under heat treatment, exhibits high selectivity for metallic impurities in carbon nanotubes, particularly for SP. 2 Carbon structures are far less aggressive than strong liquid acids, thus better preserving the length, graphitization degree, and macroscopic aggregation morphology (such as array structures) of carbon nanotubes; they have high purification efficiency, good diffusion of halogen gas molecules, and can reach partially encapsulated metal particles to achieve deep purification, enabling highly selective removal of specific metal impurities.

[0030] In summary, this application avoids the use of liquid acid, produces no wastewater or only easily treatable byproducts throughout the process, has low energy consumption, and can better maintain the structural integrity of carbon nanotubes.

[0031] Optionally, in one embodiment, the molar ratio of impurities in the impurity-containing carbon nanotubes to halogens in the solid halogen source is greater than 1:3.

[0032] It should be noted that the molar ratio of impurities in the impurity-containing carbon nanotubes to halogens in the solid halogen source is greater than 1:3. For example, the molar ratio of impurities to halogens can be one or more of 1:3, 1:4, 1:5, 1:8, and 1:10, or any combination thereof. This maintains the principle that halogens are in excess of impurities during the reaction process to ensure complete removal of impurities.

[0033] In this embodiment, the molar ratio of impurities in the impurity-containing carbon nanotubes to halogens in the solid halogen source is greater than 1:3. Essentially, this provides a stoichiometric excess of halogens to the system, ensuring that all metal impurity particles (including tiny metal particles encapsulated within amorphous carbon) can fully contact and react with the halogens, avoiding impurity residues due to insufficient halogens, thereby improving the purification accuracy of the carbon nanotubes.

[0034] Optionally, in one embodiment, the solid halogen source includes one of the following: a volatile solid halogen source, a solid halogen source consisting of a mixture of inorganic halide and activator, a metal halide solid halogen source, and a metal chlorate solid halogen source.

[0035] It should be noted that, under heat treatment temperature, the volatile solid halogen source itself can directly volatilize or decompose to produce active halogen species (halogen gas).

[0036] It should be noted that the solid halogen source, which is a mixture of inorganic halogen salts and activators, utilizes inexpensive and stable inorganic salts to generate active chlorine species in situ under the action of activators.

[0037] It should be noted that solid halogen sources of metal halides can sublimate or volatilize at heat treatment temperatures, and then decompose to generate halogen gases. These gases react with metal impurities through gas-phase halogen transport reactions to generate volatile metal halides, which are then removed.

[0038] It should be noted that the solid halogen source of metal chlorate is a metal chlorate source that can be decomposed in a controlled manner. This type of metal chlorate needs to be used in an inert atmosphere with high dilution, uniform mixing and strict temperature control. It is purified by directly generating highly active chlorine gas through its controlled thermal decomposition.

[0039] In this embodiment of the application, various solid halogen sources are used to solve the problems of severe equipment corrosion, high wastewater treatment costs, high energy consumption, and damage to the intrinsic structure of carbon nanotubes that exist in traditional acid washing methods.

[0040] Optionally, in one embodiment, the volatile solid halogen source includes chlorides and / or fluorides; The chloride is selected from one or more of ammonium chloride, hexachloroethane, and polyvinyl chloride (PVC); and / or, The fluoride is selected from one or more of ammonium bifluoride, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorinated graphite, and perfluoropolyether compounds.

[0041] It should be noted that volatile solid halogen sources undergo thermal decomposition or pyrolysis reactions at 200~800℃, releasing highly active halogen or hydrogen halide species, which then react with metal impurities and amorphous carbon impurities in carbon nanotubes.

[0042] It should be noted that perfluoropolyether compounds include perfluoropolyether waxes (PFPE) and perfluoropolyether oligomers. The backbone of perfluoropolyether compounds consists of alternating carbon-oxygen bonds (-CO-), with all carbon atoms replaced by fluorine atoms (-F, no hydrogen atoms). The end groups are usually fluorine-containing groups (such as -CF3, -OCF3, -COF, etc.), while some oligomers may retain a small number of reactive groups (such as -COOH, -OH) at the end groups. Its general formula can be represented as: Rf-O-(CF2CF2O) m -(CF2O) n -Rf' (where Rf and Rf' are perfluoroalkyl groups, and m and n are the degrees of polymerization). Perfluoropolyether oligomers have relatively low molecular weights (typically 500~5000 Da) and degrees of polymerization m+n=5~30.

[0043] It should be noted that when the solid halogen source is a volatile solid halogen source, the inert gas introduced can be argon or nitrogen to remove air.

[0044] In this embodiment, the volatile solid halogen source can controllably release halogens (Cl2, F2) or hydrogen halides (HCl, HF) under certain temperature conditions to achieve selective removal of impurities while maximizing the protection of the intrinsic structure of carbon nanotubes.

[0045] Optionally, in one embodiment, the solid halogen source in which the inorganic halide is mixed with the activator comprises: The inorganic halide is selected from one or more of sodium chloride, potassium chloride, magnesium chloride, and gallium chloride; The activator is selected from one or more of manganese dioxide, ferric oxide, and potassium chlorate.

[0046] It should be noted that when the solid halogen source is a mixture of inorganic halide and activator, an inert gas can be introduced, or a mixture of oxygen and argon or an inert gas carrying trace amounts of water vapor can be directly introduced. This application preferentially selects a mixture of oxygen and argon.

[0047] In this embodiment, the solid halogen source, which is a mixture of inorganic halide and activator, controls the decomposition path of inorganic halide through activator, and releases active chlorine species in a controllable manner at <800℃, thereby achieving efficient removal of impurities. At the same time, it avoids damage to the carbon nanotube structure, improves the purification accuracy of carbon nanotubes, and is low-cost, easy to industrialize, and suitable for large-scale production needs.

[0048] Optionally, in one embodiment, the metal halide solid halogen source is selected from one or more of ferric chloride, aluminum chloride, titanium chloride, copper dichloride, and cobalt trifluoride.

[0049] It should be noted that when the solid halogen source is a metal halide solid halogen source, the inert gas introduced can be argon or nitrogen to remove air.

[0050] It should be noted that metal halide solid halogen sources have unique thermal decomposition characteristics, and can release highly active halogen species in the heat treatment temperature range of 300~600℃.

[0051] It should be noted that, as Figure 4 , 5 As shown, a metal halide solid halogen source is placed in the upstream low-temperature section (temperature range of 300~600℃), and a carbon nanotube sample containing impurities is placed in the downstream high-temperature section (temperature range of 800~1000℃). The upstream low-temperature section causes the metal halide solid halogen source to sublimate upon heating and flow with the carrier gas to the downstream high-temperature section. The metal halide solid halogen source rapidly decomposes in the low-temperature section (temperature range of 300~600℃), generating highly active chlorine gas in situ. The chlorine gas selectively oxidizes amorphous carbon, carbon nanoparticles, and metal catalyst impurities present in the carbon nanotubes in the high-temperature section (temperature range of 800~1000℃), generating gaseous chlorides or carbon oxides, which are then discharged from the system with the argon gas flow, thus obtaining purified carbon nanotubes.

[0052] In this embodiment, the metal halide solid halogen source has the dual functions of halogen supply and catalytic oxidation at a certain temperature, achieving efficient targeted removal of impurities. At the same time, it reduces damage to the carbon nanotube structure by utilizing the regulatory effect of its own metal ions, without secondary pollution, and simplifies the process to adapt to industrial production.

[0053] Optionally, in one embodiment, the solid halogen source of metal chlorate is selected from one or more of sodium chlorate and potassium chlorate.

[0054] It should be noted that when the solid halogen source is a metal chlorate solid halogen source, the inert gas introduced can be argon or nitrogen to remove air.

[0055] It should be noted that sodium chlorate (decomposition temperature about 290℃) and potassium chlorate (decomposition temperature about 356℃) can undergo thermal decomposition reactions in the purification process within the temperature range of 300~500℃, releasing highly reactive chlorine species such as halogen gases.

[0056] In this embodiment, the metal chlorate solid halogen source simultaneously provides highly active chlorine species and an oxidizing atmosphere under certain temperature conditions, achieving efficient synergistic removal of metal impurities and amorphous carbon impurities, while also having the advantages of strong reaction controllability and no secondary pollution.

[0057] Optionally, in one embodiment, the heat treatment includes: The heat treatment temperature is 200~1100℃, the heating rate is 1~10℃ / min, and the holding time is 0.5~2h.

[0058] It should be noted that the heat treatment temperature is 200~1100℃. For example, the heat treatment temperature is one or any two of the following: 200℃, 300℃, 400℃, 450℃, 500℃, 600℃, 700℃, 800℃, 900℃, 950℃, 1000℃, and 1100℃.

[0059] It should be noted that the above heat treatment is carried out by programmed temperature rise, with a heating rate of 1~10℃ / min. For example, the heating rate can be one or any two of the following: 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, and 10℃ / min.

[0060] It should be noted that the heat preservation time is 0.5 to 2 hours. For example, the heat preservation time is one or any two of 0.5 hours, 1 hour, 1.5 hours, and 2 hours.

[0061] 200~500℃: Suitable for low decomposition temperature halogen sources such as metal chlorates (sodium chlorate, potassium chlorate) and ammonium chloride, releasing active halogen species, preferentially removing easily reactive free metal impurities (Fe, Co, Ni particles) and surface amorphous carbon, suitable for the purification of heat-sensitive single-walled carbon nanotubes.

[0062] 500~800℃: Compatible with fluorides such as PTFE, PVDF, and fluorinated graphite, releasing active fluorine species that can remove metal particles and metal oxide impurities encased inside amorphous carbon, while further etching the remaining amorphous carbon.

[0063] 800~1100℃: Suitable for high-stability halogen sources (such as perfluoropolyether wax), and can repair minor defects in the carbon nanotube wall through annealing at a temperature of 800~1100℃, improve its graphitization degree, and ensure intrinsic properties such as electrical conductivity and mechanical strength.

[0064] If the temperature is below 200℃, the solid halogen source cannot be effectively decomposed, and the impurity removal reaction is difficult to start; if the temperature is above 1100℃, it will cause the graphitization structure of the carbon nanotube wall to be destroyed and the tube body to break, while the energy consumption will increase significantly.

[0065] In this embodiment, the temperature range (200~1100℃) is adapted to the decomposition characteristics of different halogen sources, accommodating the removal of multiple types of impurities. The heating rate (1~10℃ / min) avoids local reaction runaway and reduces damage to the carbon nanotube structure. The holding time (0.5~2h) ensures complete impurity reaction, balancing purification efficiency and cost. In summary, heat treatment precisely controls the decomposition behavior of solid halogen sources and the reaction process of impurities, achieving the dual goals of efficient impurity removal and low-damage structure preservation.

[0066] Optionally, in one embodiment, the inert gas includes one or more of nitrogen, argon, and an oxygen-containing argon mixture; In the mixed gas, the volume ratio of oxygen is 1% to 10%.

[0067] It should be noted that the volume percentage of oxygen is 1% to 10%. For example, the volume percentage of oxygen is one or any two of the following: 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, and 10%.

[0068] In this embodiment, nitrogen / argon can isolate air and ensure the controllability of the reaction; the synergistic effect of the argon mixture containing 1%~10% oxygen enhances the removal of amorphous carbon and improves purification efficiency. In other words, the inert gas creates a controllable reaction atmosphere, works synergistically with the halogen source to achieve efficient impurity removal, and simultaneously protects the structural integrity of the carbon nanotubes, balancing process safety and economy.

[0069] Optionally, in one embodiment, the impurities in the impurity-containing carbon nanotubes include one or more of Fe, Co, Ni, Al, and Mg.

[0070] It should be noted that the aforementioned metallic impurities have conductive, magnetic, or semiconductor doping properties. If they remain in carbon nanotubes, they will seriously affect their application in semiconductor devices, flexible electronics, high-frequency communications, and other fields.

[0071] In the embodiments of this application, the impurities removed include one or more of Fe, Co, Ni, Al, and Mg. The main purpose is to target and remove residual catalytic metal impurities, which is suitable for high-precision electronics / semiconductor fields, eliminates the catalytic poisoning effect of metal impurities, is suitable for energy storage / conversion fields, reduces the biotoxicity of metal impurities, and is suitable for biomedical fields.

[0072] To make the inventive objectives, technical solutions, and beneficial effects of this application clearer, the application is further described below with reference to embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of this application.

[0073] The present application will be described in detail below through embodiments.

[0074] Example 1 direct decomposition method of ammonium chloride like Figure 1-2 As shown, 1g of crude carbon nanotubes prepared by a floating catalytic method with a metal catalyst content of 5wt% was used as impure carbon nanotubes (CNTs), and 3g of solid ammonium chloride (NH4Cl) powder was used as a solid halogen source. The two were lightly mixed evenly in a mortar. The mixture was placed in a quartz boat and placed in the center of a quartz tube furnace. After sealing, argon gas was introduced at a flow rate of 200 sccm for 30 minutes to completely remove air. Subsequently, under an argon atmosphere, the temperature was increased to 950°C at a rate of 5°C / min and held at this temperature for 2 hours. After the holding period, heating was stopped, and argon gas was continued until the furnace cooled to room temperature. The sample was removed and vacuum dried at 60°C for 1 hour to obtain purified carbon nanotubes.

[0075] The purified carbon nanotubes obtained in Example 1 were analyzed by X-ray fluorescence spectroscopy (XRF), and the iron content was reduced to below 0.5 wt%. Raman spectroscopy showed that the intensity ratio of the D peak to the G peak (ID / IG) was lower than that of the sample purified by acid washing, indicating fewer structural defects in the carbon nanotubes.

[0076] Example 2 pyrolysis of polytetrafluoroethylene like Figure 1 , 3 As shown, 1g of arrayed carbon nanotubes were used as impurity-containing carbon nanotubes (CNTs), with residual iron catalyst on the substrate. The arrayed carbon nanotubes were layered with 0.5g of polytetrafluoroethylene (PTFE) powder (solid halogen source) (PTFE at the bottom, carbon nanotube array at the top). Under argon protection, the temperature was increased to 950℃ at a rate of 5°C / min and held for 1 hour. The fluorine-containing active species generated by the pyrolysis of PTFE reacted with iron to form volatile iron fluoride, which was carried away by the gas flow, yielding purified carbon nanotubes.

[0077] The purified carbon nanotube array obtained in Example 2 maintained its morphology well, and the conductivity test showed that its conductivity was better than that of the acid-washed sample of the same purity.

[0078] Example 3 Ferric chloride halogen transport method like Figure 1 , 4 As shown in Figure 5, in a tubular furnace quartz tube, anhydrous ferric chloride solid (solid halogen source) is placed in the upstream low-temperature section, and carbon nanotube (CNT) samples containing impurities are placed in the downstream high-temperature section. After the system is thoroughly purged with high-purity argon, it is heated to 800~1000℃ at a rate of 5℃ / min under argon gas protection, causing ferric chloride to sublimate and flow with the carrier gas to the high-temperature section. Ferric chloride decomposes rapidly at 800~1000℃, generating highly active chlorine gas in situ. The chlorine gas selectively oxidizes amorphous carbon, carbon nanoparticles, and metal catalyst impurities present in the carbon nanotubes at high temperature, generating gaseous chlorides or carbon oxides, which are discharged from the system with the argon gas flow, thus obtaining purified carbon nanotubes.

[0079] Example 3 uses a solid precursor to generate the reaction gas in situ, avoiding the danger of directly using high-pressure chlorine gas. Furthermore, the chlorine concentration can be precisely controlled by the sublimation temperature and the carrier gas flow rate, which features safe operation and excellent purification effect.

[0080] Example 4 NaCl-MnO2-O2 in-situ activation method 1.00 g of impure carbon nanotubes, 2.00 g of NaCl, and 1.00 g of MnO2 powder (solid halogen source) were ball-milled and mixed in a tube furnace. The mixture was then placed in a tube furnace, and a 5% O2 / Ar mixed gas (200 mL / min) was introduced. The temperature was increased to 450°C at a rate of 3°C / min, followed by Ar introduction and holding at that temperature for 3 hours. After cooling, the soluble salts were removed by washing with 60°C hot water to obtain purified carbon nanotubes.

[0081] Example 4 shows that the total amount of metal impurities decreased by >90% after purification, and the raw material cost was extremely low.

[0082] Example 5 Controlled decomposition method of potassium chlorate like Figure 1 , 6As shown, a decomposition zone, a deoxygenation zone, and a purification zone are sequentially set in a continuous gas flow system. Solid potassium chlorate (solid halogen source) is placed in the decomposition zone and heated to 400-450℃ at a heating rate of 3°C / min under an inert argon carrier gas to decompose it, mainly generating oxygen. The gas flow then enters the deoxygenation zone, which is filled with metallic copper and maintained at 400-600℃. Oxygen reacts with copper to form copper oxide and is completely removed. The deoxygenated gas flow finally enters the purification zone at 800-1000℃, where residual chlorine precursors are converted into highly reactive chlorine gas, which reacts with the carbon nanotube sample placed in this zone. The pure chlorine gas selectively etches amorphous carbon, carbon nanoparticles, and metal catalyst impurities, generating volatile substances that are discharged with the carrier gas. The carbon nanotubes are effectively protected because there is no oxygen present, thus obtaining purified carbon nanotubes.

[0083] Example 5, through an integrated continuous process, achieved for the first time the simultaneous in-situ removal of oxygen from the potassium chlorate decomposition gas source, ensuring that the purification process relies solely on highly selective chlorine etching, thereby significantly avoiding oxidative damage to the carbon nanotube structure while improving the efficiency of impurity removal.

[0084] Comparative Example 1 Traditional mixed acid reflux method Take 1.00 g of the original carbon nanotubes from the same batch, place them in a round-bottom flask, add 60 mL of a mixed acid with a ratio of V(concentrated H₂SO₄):V(concentrated HNO₃) of 3:1, and reflux in an oil bath at 80 °C for 4 hours. After cooling, repeatedly centrifuge and wash with water until neutral, then vacuum dry at 60 °C for 12 hours. The final iron content is approximately 0.4 wt%, yielding purified carbon nanotubes.

[0085] The purified carbon nanotubes obtained in the comparative example had a Raman ID / IG ratio as high as 0.45. Scanning electron microscopy showed that the length of the carbon nanotubes was significantly shortened, and about 1 liter of acidic iron-containing wastewater was generated.

[0086] In summary, the carbon nanotube purification method of this application avoids the use of liquid acid, produces no wastewater or only easily treatable byproducts throughout the process, has low energy consumption, and can better maintain the structural integrity of carbon nanotubes.

[0087] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0088] Although embodiments of this application have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for purifying carbon nanotubes, characterized in that, The purification method includes: Impurity-containing carbon nanotubes are mixed with a solid halogen source and placed in a reactor. An inert gas is introduced and the mixture is heat-treated. The heat treatment converts the solid halogen source into halogen gas. The halogen gas reacts with the impurities in the carbon nanotubes to generate metal halides. The metal halides are carried away from the reactor under an inert gas atmosphere to obtain purified carbon nanotubes.

2. The purification method according to claim 1, characterized in that, The molar ratio of impurities in the impurity-containing carbon nanotubes to halogens in the solid halogen source is greater than 1:

3.

3. The purification method according to claim 1, characterized in that, The solid halogen source includes one of the following: a volatile solid halogen source, a solid halogen source consisting of a mixture of inorganic halide and activator, a metal halide solid halogen source, and a metal chlorate solid halogen source.

4. The purification method according to claim 3, characterized in that, The volatile solid halogen source includes chlorides and / or fluorides; The chloride is selected from one or more of ammonium chloride, hexachloroethane, and polyvinyl chloride; and / or, The fluoride is selected from one or more of ammonium bifluoride, polytetrafluoroethylene, polyvinylidene fluoride, fluorinated graphite, and perfluoropolyether compounds.

5. The purification method according to claim 3, characterized in that, The solid halogen source of the mixture of inorganic halide and activator includes: The inorganic halide is selected from one or more of sodium chloride, potassium chloride, magnesium chloride, and gallium chloride; The activator is selected from one or more of manganese dioxide, ferric oxide, and potassium chlorate.

6. The purification method according to claim 3, characterized in that, The metal halide solid halogen source is selected from one or more of ferric chloride, aluminum chloride, titanium chloride, copper dichloride, and cobalt trifluoride.

7. The purification method according to claim 3, characterized in that, The solid halogen source of metal chlorate is selected from one or more of sodium chlorate and potassium chlorate.

8. The purification method according to any one of claims 1-7, characterized in that, The heat treatment includes: The heat treatment temperature is 200~1100℃, the heating rate is 1~10℃ / min, and the holding time is 0.5~2h.

9. The purification method according to any one of claims 1-7, characterized in that, The inert gas includes one or more of nitrogen, argon, and an oxygen-containing argon mixture. In the mixed gas, the volume ratio of oxygen is 1% to 10%.

10. The purification method according to any one of claims 8 or 9, characterized in that, The impurities in the impurity-containing carbon nanotubes include one or more of Fe, Co, Ni, Al, and Mg.

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