Method and device for preparing single-wall and few-wall carbon nanotubes

Abstract

The invention belongs to the technical field of preparation of nanotubes, and particularly relates to a method and device for preparing single-wall and few-wall carbon nanotubes, and the method comprises the following steps: preparation and atomization of a catalyst precursor solution: dissolving a metal catalyst precursor in a volatile solvent to form a homogeneous solution, then atomizing the solution into aerosol liquid drops with the diameter of 1-10 microns through an ultrasonic atomizer or an airflow atomizer; according to the invention, a catalyst precursor solution is atomized into micron-sized aerosol liquid drops, so that each liquid drop becomes a tiny and isolated'microreactor '. And after the solvent is evaporated in the preheating zone, highly dispersed catalyst precursor microcrystals are formed.

Description

Technical Field

[0001] This invention relates to the field of nanotube fabrication technology, specifically to a method and apparatus for fabricating single-walled or few-walled carbon nanotubes. Background Technology

[0002] Carbon nanotubes, especially structurally perfect single-walled carbon nanotubes (SWCNTs) and few-walled carbon nanotubes (FWCNTs), exhibit enormous application potential in electronics, composite materials, and energy storage due to their superior electrical, thermal, and mechanical properties. Achieving high-quality, high-yield, and low-cost mass production is key to their practical application. Chemical vapor deposition (CVD) is one of the most promising preparation methods currently available.

[0003] Existing CVD technologies mainly include matrix methods and flotation methods. Matrix methods typically involve preloading the catalyst precursor onto a planar substrate (such as SiO2 / Si) or a bulk support (such as MgO or Al2O3), followed by the introduction of a carbon source gas for growth. While this method can produce high-quality carbon nanotubes, it has significant limitations: First, catalyst particles are prone to sintering and agglomeration during high-temperature pretreatment and growth, leading to size inhomogeneity and resulting in a wide diameter distribution and uncontrollable chirality of the grown carbon nanotubes. Second, the diffusion of the carbon source gas on the fixed catalyst surface is limited, resulting in low carbon nanotube growth density and low yield. Finally, this method is usually batch-based, making continuous operation difficult, and the process of separating and collecting carbon nanotubes from the substrate is cumbersome and prone to introducing defects and contamination.

[0004] To address the problems of matrix-based methods, the flotation method (or gas-phase flow catalysis) was developed. This method mixes a volatile catalyst precursor (such as ferrocene) with a carbon source gas (such as CO or methane) in the gas phase, where it rapidly decomposes, nucleates, and grows carbon nanotubes within a high-temperature reaction zone. The flotation method avoids catalyst immobilization, theoretically enabling continuous production. However, this method also has inherent drawbacks: minute fluctuations in reaction conditions (such as temperature, gas flow, and precursor concentration) significantly affect the nucleation and growth kinetics of catalyst particles, resulting in a mixture of single-walled, few-walled, and multi-walled carbon nanotubes as the product, containing a large amount of amorphous carbon impurities, leading to poor product purity and structural uniformity. Simultaneously, the short lifespan of gas-phase nucleated catalyst particles limits the growth length of the carbon nanotubes.

[0005] Therefore, there is an urgent need in this field for a novel preparation method that can overcome the shortcomings of the above two methods and combine the advantages of both. That is, it can achieve uniform and controllable formation of catalyst particles, avoid sintering and deactivation, and at the same time ensure sufficient contact between carbon source and catalyst, realize the controllable preparation of high-quality, high-yield single-walled and few-walled carbon nanotubes, and facilitate product collection. Summary of the Invention

[0006] To solve the above-mentioned technical and safety problems, the technical solution adopted by the present invention is as follows: A method for preparing single-walled or few-walled carbon nanotubes, characterized by comprising the following steps: (1) Preparation and atomization of catalyst precursor solution: The metal catalyst precursor is dissolved in a volatile solvent to form a homogeneous solution, and then the solution is atomized into aerosol droplets with a diameter between 1 and 10 micrometers by an ultrasonic atomizer or an airflow atomizer. (2) Aerosol transport and reaction gas mixing: The aerosol droplets generated in step (1) are carried into the preheating zone by a carrier gas. The carrier gas is an inert gas or a mixture of inert gas and hydrogen. In the preheating zone, the aerosol droplets are mixed with the reaction gas, which serves as a carbon source, to form a mixed reaction gas flow. The temperature of the preheating zone is controlled in the range of 100°C to 400°C, which allows the solvent to fully evaporate but avoids premature decomposition of the catalyst precursor. (3) Floating pyrolysis chemical vapor deposition: The mixed reaction gas flow formed in step (2) is guided to the high-temperature reaction zone, and the temperature of the high-temperature reaction zone is maintained at 700°C to 1100°C; a floating substrate is set in the high-temperature reaction zone, which is composed of high-temperature resistant porous ceramic material or metal mesh, and its average pore size is 0.1 to 10 micrometers; when the mixed reaction gas flow passes through the floating substrate, the volatilized catalyst precursor is reduced or decomposed into catalytically active nano-metal particles at high temperature, and the carbon source gas is catalytically pyrolyzed on the surface of the nano-metal particles to grow single-walled or few-walled carbon nanotubes; (4) Product collection: After the reaction is completed, the floating substrate with attached single-walled and few-walled carbon nanotubes is cooled to room temperature under an inert atmosphere, or the generated single-walled and few-walled carbon nanotubes are collected from the floating substrate by mechanical scraping, airflow purging or solvent washing.

[0007] Preferably, in step (1), the metal catalyst precursor is an organic acid salt, inorganic acid salt, or organometallic compound of iron, cobalt, nickel, molybdenum, or two or more of these metals; the volatile solvent is deionized water, ethanol, methanol, acetone, or a mixture of two or more of these; and the total concentration of metal ions in the catalyst precursor solution ranges from 0.01 mmol / L to 10 mmol / L.

[0008] Preferably, in step (2), the carrier gas is high-purity argon, high-purity nitrogen, or a mixture thereof, or a mixture of hydrogen with a volume fraction of 10% to 50% and the remainder argon or nitrogen; the carbon source reaction gas is one or more of methane, ethylene, acetylene, propylene, ethanol vapor, acetone vapor, or benzene vapor; and the volume fraction of the carbon source gas in the mixed reaction gas stream is 5% to 40%.

[0009] Preferably, in step (3), the floating substrate is alumina, zirconium oxide, quartz fiber, stainless steel mesh, or nickel mesh film; the shape of the floating substrate is planar mesh, corrugated or three-dimensional porous foam structure, and its placement is at an angle of 0 to 90 degrees with the direction of the mixed reaction gas flow, preferably 30 to 60 degrees, so as to increase the gas-solid contact area and disturbance, and promote the uniform growth of carbon nanotubes.

[0010] Preferably, in step (3), the heating program of the high-temperature reaction zone is as follows: the temperature is increased from room temperature to the target reaction temperature at a rate of 5℃ / min to 20℃ / min, and the temperature is kept constant at the target reaction temperature for 10 minutes to 120 minutes; the residence time of the mixed reaction gas flow in the high-temperature reaction zone is 0.5 seconds to 60 seconds.

[0011] Preferably, a trace amount of promoter gas is also introduced into the mixed reaction gas stream in step (2). The promoter gas is one of water vapor, carbon dioxide, oxygen or sulfur-containing compound gas, with a volume fraction of 10 ppm to 1000 ppm. The introduction of the promoter gas is used to adjust the activity and lifetime of the catalyst and to etch amorphous carbon impurities generated during the growth process, thereby improving the graphitization degree and purity of the obtained single-walled and few-walled carbon nanotubes.

[0012] Preferably, the method of introducing the promoter gas water vapor is as follows: a portion of the carrier gas is first passed through a bubbler containing deionized water and with a controllable temperature, so that the carrier gas is saturated with water vapor, and then mixed with the main gas flow. By precisely controlling the temperature of the bubbler between 20°C and 60°C, the partial pressure of water vapor in the mixed gas flow can be precisely controlled.

[0013] Preferably, the growth mode of the single-walled and few-walled carbon nanotubes in step (3) is the "substrate mode", that is, the carbon nanotubes mainly grow and are anchored on the floating substrate; in order to further realize the directional growth of the carbon nanotube array, a directional magnetic field with an intensity of 0.1 T to 2 T or a directional electric field with an intensity of 100 V / cm to 1000 V / cm is applied outside or inside the reactor, so that the catalyst particles with paramagnetism or polarity are oriented along the direction of the field strength during the growth process, thereby guiding the carbon nanotubes to grow in a specific direction.

[0014] Preferably, after step (4), a purification post-treatment step is also included: the collected crude product is heat-treated in air at 300°C to 500°C for 30 to 120 minutes to remove amorphous carbon; then it is immersed in hydrochloric acid, nitric acid or a mixture thereof with a concentration of 1 mol / L to 6 mol / L and refluxed at 60°C to 100°C for 2 to 12 hours to dissolve and remove metal catalyst particles; finally, after filtration, washing and drying, high-purity single-walled and few-walled carbon nanotubes are obtained.

[0015] The preparation method based on catalyst precursor atomization and floating substrate pyrolysis provided by the invention has the following significant advantages compared with the prior art: This invention atomizes a catalyst precursor solution into micron-sized aerosol droplets, making each droplet a tiny, isolated "microreactor." After solvent evaporation in the preheating zone, highly dispersed catalyst precursor microcrystals are formed. When these microcrystals are reduced in the high-temperature zone, their size is confined to the original droplet size, fundamentally preventing the migration and aggregation of catalyst particles at high temperatures. Compared to the random distribution and easy aggregation of catalysts on the support surface in traditional matrix methods, and the uncontrollability of gas-phase nuclei in flotation methods, this invention can generate uniform, densely distributed, and highly active nanocatalytic particles. This is a decisive factor in achieving narrow diameter distribution of carbon nanotubes, predominantly single-walled and few-walled structures, and few structural defects. It effectively controls the size and dispersion of catalyst nanoparticles, laying the foundation for the growth of high-quality, uniform single-walled and few-walled carbon nanotubes.

[0016] The porous floating substrate introduced in this invention plays a crucial role. On one hand, it provides stable anchoring points for catalyst nanoparticles, avoiding the short active lifetime problem caused by rapid catalyst drift with the airflow in the flotation method. On the other hand, when the mixed reaction gas flow is forced through the pores of the floating substrate, a strong disturbance effect is generated, greatly enhancing the gas-solid phase mass transfer efficiency and ensuring that carbon source molecules can be fully and uniformly transported to each catalyst active site. This design combines the advantages of the matrix method (substrate anchoring) and the flotation method (airflow transport), ensuring both catalyst stability and efficient reactant supply. This allows carbon nanotubes to grow continuously at a faster rate, achieving a carbon nanotube growth density and yield per unit time far exceeding that of the traditional matrix method. It realizes efficient mass transfer and reaction processes between reactants and catalyst, significantly improving the growth efficiency and yield of carbon nanotubes.

[0017] This invention integrates the reaction process with product collection. After the reaction, a "floating substrate-carbon nanotube" composite structure can be directly obtained, which can be used directly as a functional material (such as an electrode or catalyst support). If powder material is required, the carbon nanotubes can be completely peeled off from the substrate using simple mechanical or physical methods (such as scraping or airflow purging), avoiding the cumbersome steps, high costs, and chemical damage to the carbon nanotubes themselves caused by the use of strong acid to corrode the entire substrate in traditional matrix methods. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the steps in the preparation method and apparatus for single-walled and few-walled carbon nanotubes according to the present invention. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to embodiments: according to Figure 1 A method and apparatus for preparing single-walled and few-walled carbon nanotubes are disclosed. The method employs an integrated process based on catalyst precursor atomization and floating substrate pyrolysis chemical vapor deposition to achieve in-situ, uniform catalyst generation and efficient carbon nanotube growth. Specifically, the method includes the following steps: (1) Precise preparation and efficient atomization of catalyst precursor solution: Select one or more organic acid salts or organometallic compounds of transition metals as catalyst precursors, dissolve them in low-boiling-point, highly volatile polar or non-polar solvents, and form a clear, precipitate-free homogeneous solution by magnetic stirring or ultrasonic treatment for at least 30 minutes; then, use a piezoelectric ceramic ultrasonic atomizer or a gas flow atomizer based on the Venturi principle to convert the above solution into fine aerosol droplets with concentrated particle size distribution and an average diameter between 1 and 10 micrometers. The atomization process is carried out at room temperature, and the atomization rate is controlled between 0.1 mL / min and 5 mL / min by adjusting the solution feed rate and the atomizing gas pressure to ensure the stability and controllability of the precursor supply.

[0020] (2) Controlled transport of aerosols and uniform mixing of reactants: Purified inert gas is used as a carrier to load the aerosol droplets generated in step (1) into a temperature-controlled tubular preheating chamber. In the preheating zone, the solvent in the aerosol droplets is rapidly evaporated, leaving solid, highly dispersed catalyst precursor microcrystals. At the same time, gaseous or easily vaporized liquid hydrocarbons, which serve as carbon sources, are precisely metered and injected into the preheating zone to fully mix with the gas flow carrying the catalyst precursor, forming a uniformly mixed reaction gas flow. The temperature of the preheating zone is precisely maintained within a narrow range of 150°C to 350°C by a PID control system. The core purpose is to ensure that the solvent evaporates completely without causing premature decomposition or sintering of the precursor.

[0021] (3) Catalytic pyrolysis and carbon nanotube growth based on a floating substrate: The above mixed reaction gas flow is introduced into the core reaction zone of a high-temperature tubular reactor at a specific flow rate. The temperature of the reaction zone is stabilized at the optimal synthesis window of 800℃ to 1000℃ by resistance heating. A porous floating substrate is placed horizontally or inclined in the center of the isothermal zone of the reaction zone. The substrate is made of a material with high chemical inertness and large specific surface area. When the mixed gas flow penetrates the pores of the floating substrate, the gaseous catalyst precursor is instantly reduced to catalytically active metal nanoparticles under high temperature and possible reducing atmosphere, and adsorbed on the substrate surface. The carbon source gas undergoes catalytic cracking at these active sites. The carbon atoms precipitated dissolve in the metal nanoparticles and are finally precipitated from their surface to form single-walled carbon nanotubes with specific chirality or few-walled carbon nanotubes with 2-5 layers.

[0022] (4) Generating and collecting carbon nanotubes at low temperature and with high efficiency: After the reaction continues for a preset time, heating is stopped and inert gas is continuously introduced to allow the system to be programmed to cool down to below 200°C under atmospheric protection. After that, the reactor can be opened and the floating substrate with a dense carbon nanotube film or array attached can be directly removed to obtain the "substrate-carbon nanotube" composite product. Alternatively, the generated carbon nanotubes can be separated from the substrate by mechanical vibration, scraping with a soft brush, purging with reverse airflow, or ultrasonic washing with a suitable solvent (such as ethanol or N-methylpyrrolidone). After filtration and drying, the final product in powder form can be obtained.

[0023] In this specific embodiment, in step (1), the selection of the metal catalyst precursor aims to regulate the size and composition of the final nano-metal particles, thereby affecting the diameter and chiral distribution of carbon nanotubes; preferably, the organic acid salt is ferric acetylacetone, ferric acetate, ferric ammonium citrate, or cobalt acetylacetone, cobalt acetate; the organometallic compound is ferrocene, cobalt dicene; to obtain a bimetallic or multimetallic catalytic system to optimize catalytic performance, a composite system of the above compounds can be used, for example, the molar ratio of ferrocene to cobalt dicene is between 10:1 and 1:1; the volatile solvent needs to have good solubility and rapid evaporation characteristics, preferably analytical grade ethanol, deionized water, or a mixed solvent of ethanol and water (volume ratio 1:1 to 4:1); the molar concentration of total metal ions in the catalyst precursor solution is a key parameter for controlling the density of catalyst particles, and its optimization range is narrow, from 0.05 mmol / L to 2 mmol / L. Within this range, the growth density of carbon nanotubes can be effectively balanced and the aggregation problem caused by excessive catalyst density can be prevented.

[0024] In this specific embodiment, in step (2), the carrier gas system is crucial for maintaining the inertness of the reaction environment and participating in the reduction process. High-purity argon (purity > 99.999%) is considered the first choice due to its absolute inertness. However, when a certain reducing atmosphere is required, a mixture of hydrogen and high-purity argon with a volume fraction of 20% to 40% can be used. Hydrogen helps to reduce the metal precursor and activate the catalyst surface. The choice of carbon source reaction gas directly affects the growth rate, quality, and graphitization degree of carbon nanotubes. Methane is beneficial for growing single-walled carbon nanotubes with fewer defects due to its high stability and fewer hydrogen free radicals. On the other hand, carbon sources with higher activity, such as acetylene and ethylene, have a faster growth rate, but may increase amorphous carbon byproducts. The concentration (volume fraction) of the carbon source gas is the key to controlling the growth kinetics. Its optimal range is between 10% and 30%. If the concentration is too low, the growth driving force will be insufficient, and if it is too high, it will easily lead to catalyst poisoning and a large amount of amorphous carbon deposition.

[0025] In this specific embodiment, in step (3), the selection of materials and structural design of the floating substrate are the core of achieving efficient and uniform growth; preferred materials include porous ceramic filters of alumina or zirconia with high porosity and thermal stability, with an average pore size preferably of 0.5 to 5 micrometers and a porosity greater than 40%; or stainless steel or nickel metal sintered mesh with certain flexibility and conductivity can be selected; the macroscopic shape of the substrate is circular, square, or rollable strip, and its microstructure can be a simple planar mesh, but more preferably a foam structure or corrugated pleated structure with three-dimensional interconnected channels to greatly increase the effective growth area; the placement angle of the substrate (the angle with the airflow direction) significantly affects the airflow dynamics. Placing it at an angle, such as 45° with the horizontal direction, can generate turbulence, enhance the mass transfer between reactants and catalyst particles, and help to discharge reaction byproducts, thereby obtaining a more uniform carbon nanotube membrane.

[0026] In this specific embodiment, in step (3), the temperature control program of the high-temperature reaction zone is crucial to the nucleation, growth, and structure of the catalyst particles and carbon nanotubes. The heating program is as follows: first, the temperature is raised from room temperature to 500°C at a rate of 10°C / min to thoroughly remove solvent residues and preliminarily decompose the precursor; then, the temperature is held at 500°C for 5-10 minutes; finally, the temperature is raised to the target reaction temperature (e.g., 850°C or 950°C) at a rate of 15°C / min. After reaching the target temperature, the temperature is held for 30 to 90 minutes, which is sufficient to ensure that the carbon nanotubes grow sufficiently without excessively prolonging the production cycle. The apparent residence time of the mixed reaction gas flow in the high-temperature zone is controlled between 2 and 30 seconds by adjusting the total gas flow rate. This time window ensures that the carbon source has enough time to decompose on the catalyst surface and complete the diffusion and precipitation of carbon atoms.

[0027] Specifically, in this embodiment, the strategic introduction of trace amounts of controllable promoter gas into the mixed reaction gas flow in step (2) is a key innovation to improve the crystal quality and purity of carbon nanotubes. The promoter gas is preferably high-purity water vapor or trace amounts of oxygen. Its mechanism of action is selective etching: during the growth process, these trace amounts of oxidizing species can preferentially react (vaporize) with the amorphous carbon or imperfect graphite layer deposited on the surface of the catalyst particles, while the structurally complete graphene tube wall is preserved due to its higher chemical stability. At the same time, they can also slightly etch the catalyst particles to prevent them from being completely wrapped by amorphous carbon and becoming inactive, thereby effectively extending the life of the catalyst. The concentration of the promoter needs to be precisely controlled at an extremely low level, with a volume fraction range of 50 ppm to 500 ppm. Too high a concentration will etch the carbon nanotubes themselves, causing defects, while too low a concentration will not have an effect.

[0028] Specifically, in this embodiment, the stable and precise introduction of the promoter gas (especially water vapor) is a key technical challenge to ensure the reproducibility of experimental results. The preferred introduction method is to introduce a small stream of carrier gas (accounting for 1%-5% of the total carrier gas flow) into a bubbler placed in a precision constant temperature water bath. The bubbler is filled with high-purity deionized water. By precisely controlling the water bath temperature between 25°C and 50°C, the partial pressure of saturated water vapor carried by the split carrier gas can be accurately calculated and stabilized according to the Antoine equation. Subsequently, this water vapor-containing split carrier gas is uniformly mixed with the main carrier gas, aerosol, and carbon source gas in the main gas flow pipeline. By calculating the split ratio and saturated vapor pressure, the water vapor concentration in the final mixed gas flow can be precisely controlled within the expected range (e.g., 200 ± 20 ppm). This indirect introduction method is more stable and controllable than directly injecting liquid water or water vapor into the reactor.

[0029] In this specific embodiment, the growth mode of the single-walled and few-walled carbon nanotubes in step (3) is the "substrate mode"; in order to further control the macroscopic orientation of the carbon nanotubes and realize the transition from random network to oriented array, an external physical field can be applied during the reaction; the preferred solution is to apply a steady strong electric field with a direction perpendicular or parallel to the airflow direction, and the field strength range is 300 V / cm to 800 V / cm. The guiding mechanism is as follows: In a high-temperature reaction environment, the newly formed carbon nanotubes have an instantaneous dipole moment or can be polarized due to their sp2 hybrid structure. Under the action of a strong electric field, they are subjected to torque and dielectric force, causing their long axis to align along the electric field lines, thereby forming a carbon nanotube film or bundle structure with a certain orientation on the floating substrate. This oriented structure has important application value in the preparation of anisotropic functional materials (such as high thermal conductivity films and oriented reinforced composite materials).

[0030] In this specific embodiment, after step (4), in order to further remove residual catalyst metal particles and amorphous carbon impurities from the product and obtain ultra-high purity carbon nanotube materials, a step-by-step purification process must be performed; this purification process includes the following steps: a. Oxidation to remove carbon: The crude product is placed in a muffle furnace or tube furnace and heated to 400°C to 450°C in flowing air. It is then calcined at a constant temperature for 60 minutes. This mild oxidation condition can selectively oxidize amorphous carbon into CO2 gas while causing minimal damage to highly crystalline carbon nanotubes. b. Acid washing to remove metals: Transfer the oxidized sample to a round-bottom flask, pour in a 3 mol / L hydrochloric acid solution, and reflux and stir at 80°C for 6 hours. Hydrochloric acid can effectively dissolve exposed iron, cobalt and other metal particles to form soluble chlorides. c. Washing and drying: After the reaction is complete, the filter cake is washed with a large amount of deionized water through a microporous membrane until the filtrate is neutral. Finally, the filter cake is placed in a vacuum drying oven and dried at 60°C for 12 hours to obtain ultra-high purity single-walled and few-walled carbon nanotube powder with less than 2 wt% metal residue and extremely low amorphous carbon content.

[0031] The present invention has been described in detail above. However, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, any modifications or improvements that do not depart from the spirit of the present invention are within the scope of protection of the present invention.

Claims

1. A method for preparing single-walled or few-walled carbon nanotubes, characterized in that, Includes the following steps: (1) Preparation and atomization of catalyst precursor solution: The metal catalyst precursor is dissolved in a volatile solvent to form a homogeneous solution, and then the solution is atomized into aerosol droplets with a diameter between 1 and 10 micrometers by an ultrasonic atomizer or an airflow atomizer. (2) Aerosol transport and reaction gas mixing: The aerosol droplets generated in step (1) are carried into the preheating zone by a carrier gas. The carrier gas is an inert gas or a mixture of inert gas and hydrogen. In the preheating zone, the aerosol droplets are mixed with the reaction gas, which serves as a carbon source, to form a mixed reaction gas flow. The temperature of the preheating zone is controlled in the range of 100°C to 400°C, which allows the solvent to fully evaporate but avoids premature decomposition of the catalyst precursor. (3) Floating pyrolysis chemical vapor deposition: The mixed reaction gas flow formed in step (2) is guided to the high-temperature reaction zone, and the temperature of the high-temperature reaction zone is maintained at 700°C to 1100°C; a floating substrate is set in the high-temperature reaction zone, which is composed of high-temperature resistant porous ceramic material or metal mesh, and its average pore size is 0.1 to 10 micrometers; when the mixed reaction gas flow passes through the floating substrate, the volatilized catalyst precursor is reduced or decomposed into catalytically active nano-metal particles at high temperature, and the carbon source gas is catalytically pyrolyzed on the surface of the nano-metal particles to grow single-walled or few-walled carbon nanotubes; (4) Product collection: After the reaction is completed, the floating substrate with attached single-walled and few-walled carbon nanotubes is cooled to room temperature under an inert atmosphere, or the generated single-walled and few-walled carbon nanotubes are collected from the floating substrate by mechanical scraping, airflow purging or solvent washing.

2. The preparation method according to claim 1, characterized in that, In step (1), the metal catalyst precursor is an organic acid salt, inorganic acid salt, or organometallic compound of iron, cobalt, nickel, molybdenum, or two or more of these metals; the volatile solvent is deionized water, ethanol, methanol, acetone, or a mixture of two or more of these; and the total concentration of metal ions in the catalyst precursor solution ranges from 0.01 mmol / L to 10 mmol / L.

3. The preparation method according to claim 1, characterized in that, In step (2), the carrier gas is high-purity argon, high-purity nitrogen, or a mixture thereof, or a mixture of hydrogen with a volume fraction of 10% to 50% and the balance of argon or nitrogen; the carbon source reaction gas is one or more of methane, ethylene, acetylene, propylene, ethanol vapor, acetone vapor, or benzene vapor; and the volume fraction of the carbon source gas in the mixed reaction gas stream is 5% to 40%.

4. The preparation method according to claim 1, characterized in that, In step (3), the floating substrate is alumina, zirconium oxide, quartz fiber, stainless steel mesh, or nickel mesh film; the shape of the floating substrate is planar mesh, corrugated or three-dimensional porous foam structure, and its placement is at an angle of 0 to 90 degrees with the direction of the mixed reaction gas flow, preferably 30 to 60 degrees, in order to increase the gas-solid contact area and disturbance, and promote the uniform growth of carbon nanotubes.

5. The preparation method according to claim 1, characterized in that, In step (3), the heating program of the high-temperature reaction zone is as follows: the temperature is increased from room temperature to the target reaction temperature at a rate of 5℃ / min to 20℃ / min, and the temperature is kept constant at the target reaction temperature for 10 minutes to 120 minutes; the residence time of the mixed reaction gas flow in the high-temperature reaction zone is 0.5 seconds to 60 seconds.

6. The preparation method according to any one of claims 1 to 5, characterized in that, In the mixed reaction gas flow in step (2), a trace amount of promoter gas is also introduced. The promoter gas is one of water vapor, carbon dioxide, oxygen or sulfur-containing compound gas, with a volume fraction of 10 ppm to 1000 ppm. The introduction of the promoter gas is used to adjust the activity and lifetime of the catalyst and to etch amorphous carbon impurities generated during the growth process, thereby improving the graphitization degree and purity of the obtained single-walled and few-walled carbon nanotubes.

7. The preparation method according to claim 6, characterized in that, The method for introducing the promoter gas water vapor is as follows: a portion of the carrier gas is first passed through a bubbler containing deionized water and with a controllable temperature, so that the carrier gas is saturated with water vapor, and then mixed with the main gas flow. By precisely controlling the temperature of the bubbler between 20°C and 60°C, the partial pressure of water vapor in the mixed gas flow can be precisely controlled.

8. The preparation method according to claim 1, characterized in that, The growth mode of single-walled and few-walled carbon nanotubes described in step (3) is the "substrate mode", that is, the carbon nanotubes mainly grow and are anchored on the floating substrate. In order to further realize the directional growth of carbon nanotube arrays, a directional magnetic field with an intensity of 0.1 T to 2 T or a directional electric field with an intensity of 100 V / cm to 1000 V / cm is applied outside or inside the reactor, so that the catalyst particles with paramagnetism or polarity are oriented along the direction of the field strength during the growth process, thereby guiding the carbon nanotubes to grow in a specific direction.

9. The preparation method according to claim 1, characterized in that, Following step (4), a post-purification treatment step is also included: the collected crude product is heat-treated in air at 300°C to 500°C for 30 to 120 minutes to remove amorphous carbon; It is then immersed in hydrochloric acid, nitric acid or a mixture thereof with a concentration of 1 mol / L to 6 mol / L, and refluxed at 60°C to 100°C for 2 to 12 hours to dissolve and remove the metal catalyst particles. Finally, after filtration, washing and drying, high-purity single-walled and few-walled carbon nanotubes are obtained.

10. An apparatus for preparing single-walled or few-walled carbon nanotubes, characterized in that, The method comprising any one of claims 1-9.

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