Method for producing carbon nanotube

Abstract

Provided is a novel method with which high-purity single-layer carbon nanotubes can be efficiently produced without worrying about a decrease in the strength of a reaction tube. A method for producing a carbon nanotube, which is a method for producing a carbon nanotube using a floating catalyst chemical vapor deposition method (FC-CVD method), and which comprises a step for producing a carbon nanotube by heating a starting material for the carbon nanotube in the presence of an iron-containing catalyst and an alkali metal compound.

Description

Technical Field

[0001] The present invention relates to a method for efficiently producing carbon nanotubes. Background Art

[0002] Carbon nanotubes are nanometer-sized cylindrical materials composed only of carbon. They have attracted much attention due to their electrical conductivity, thermal conductivity, mechanical strength, chemical properties and other properties derived from their structural characteristics. Their practical application in various applications, including the electronics and energy fields, is being studied.

[0003] There are three main methods for synthesizing carbon nanotubes: arc discharge, laser evaporation, and chemical vapor deposition (CVD). Unlike arc discharge and laser evaporation, CVD (Chemical Vapor Deposition) does not use a solid carbon source but a gaseous carbon source, so it can continuously inject a carbon source into the reactor, making it a method suitable for mass synthesis. In addition, it is also an excellent synthesis method in terms of high purity and low production cost of the obtained carbon nanotubes.

[0004] Among them, the floating catalyst CVD method (FC-CVD method) is a method particularly suitable for synthesizing single-walled carbon nanotubes having various excellent properties such as extremely high electrical and thermal conductivity compared to multi-walled carbon nanotubes.

[0005] As more specific methods for producing carbon nanotubes using the floating catalyst CVD method, for example, there are a method for producing single-walled carbon nanotubes in which the side reaction to produce amorphous carbon is suppressed (Patent Document 1), a method for producing high-purity single-walled carbon nanotubes (Patent Document 2), and a method for producing single-walled carbon nanotubes with a high yield (Patent Document 3).

[0006] These methods for producing carbon nanotubes can produce high-purity single-walled carbon nanotubes, but the yield of carbon nanotubes is only a few percent, and single-walled carbon nanotubes cannot be mass-produced.

[0007] For the purpose of efficiently synthesizing in multilayer carbon nanotubes, it is known that there are studies on adding alkali metals as promoters (promoters) to the main transition metal catalysts (non-patent literature 1, non-patent literature 2). Multilayer carbon nanotubes generally have a high yield compared to monolayers. In this study applicable to catalyst substrate-supported CVD, it is also greater than several tens of%. However, compared with the alkali metal non-additive system, the maximum promotion effect can only be 2 times, and its effect is limited.

[0008] It is reported that by changing the reaction tube from alumina (Al2O3) to mullite (Al 4+2x Si 2-2x O 10-x(x: 0 to 0.4)), the reaction was promoted on the surface of the mullite reactor, and the yield of carbon nanotubes was doubled (Non-Patent Document 3). However, although the yield was increased by about 2 times, Si, a component of the reaction tube, was detected in the obtained carbon nanotubes, and it was concluded that the fragments of the reaction tube were mixed as a path. From this point of view, the durability of the mullite reaction tube in the high temperature range (for example, above 1250°C) is worrying. Prior art literature Patent Literature

[0009] Patent Document 1: Japanese Patent No. 5046078 Patent Document 2: Japanese Patent No. 4968643 Patent Document 3: Japanese Patent Application Laid-Open No. 2007-246309 Non-patent literature

[0010] Non-patent document 1: "The effect of alkaline doped catalysts on the CVD synthesis of carbon nanotubes" Phys. Status Solidi B 248, No. 11 2471-2474 (2011) Non-patent document 2: "CVD-synthesis of multiwall carbon nanotubes overpotassium-doped supported catalysts" Applied Catalysis A: General 344,191-197(2008) Non-patent document 3: "Carbon nanotube synthesis and spinning as macroscopicfibers assisted by the ceramic reactor tube" Scientific Reports volume 9, Article number: 9239 (2019) Summary of the invention Technical problem to be solved by the invention

[0011] The present invention has been made in view of the above-mentioned technical problems, and an object of the present invention is to provide a new method capable of efficiently producing high-purity single-walled carbon nanotubes without worrying about reduction in the strength of a reaction tube. Technical solutions for solving technical problems

[0012] The present inventors have discovered that in a method for producing carbon nanotubes using floating catalyst chemical vapor deposition (FC-CVD), if a raw material for carbon nanotubes is heated in the presence of an iron-containing catalyst and an alkali metal compound to generate carbon nanotubes, high-purity single-walled carbon nanotubes can be produced efficiently.

[0013] Furthermore, it has been found that the production method of the present invention can produce high-purity single-walled carbon nanotubes safely and efficiently at lower temperatures even when a reaction tube made of alumina or SiC having excellent strength at high temperatures is used.

[0014] The present invention was completed through further research based on such findings.

[0015] That is, the present invention provides the following aspects of the invention. Item 1 A method for producing carbon nanotubes, which is a method for producing carbon nanotubes based on a floating catalyst chemical vapor deposition method (FC-CVD method). The production method includes the step of heating a raw material of carbon nanotubes in the presence of an iron-containing catalyst and an alkali metal compound to generate carbon nanotubes. Item 2. The method for producing carbon nanotubes according to Item 1, wherein the alkali metal species of the alkali metal compound includes at least one selected from the group consisting of lithium, sodium, potassium, and cesium. Item 3. A method for producing carbon nanotubes according to Item 1 or 2, wherein the alkali metal compound is selected from at least one of phosphates, acetates, chlorides, sulfates, carbonates, hydrates, silicates, and hydroxides of lithium, sodium, potassium, and cesium. Item 4. The method for producing carbon nanotubes according to any one of Items 1 to 3, wherein the amount of the alkali metal compound supplied is 0.01 parts by mass or more and 15 parts by mass or less relative to 100 parts by mass of the raw material of the carbon nanotubes. Item 5. The method for producing carbon nanotubes according to any one of Items 1 to 4, wherein the iron-containing catalyst is ferrocene. Item 6. The method for producing carbon nanotubes according to any one of Items 1 to 5, wherein the step of producing carbon nanotubes is performed in a ceramic reaction tube. Item 7. The method for producing carbon nanotubes according to Item 6, wherein the ceramic reaction tube contains silicon carbide. Effects of the Invention

[0016] According to the method for producing carbon nanotubes of the present invention, single-walled carbon nanotubes with high purity can be produced efficiently. Furthermore, according to the present invention, even if a reaction tube made of alumina or SiC having excellent strength in a high temperature region is used, single-walled carbon nanotubes with high purity can be produced safely and efficiently at a lower temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The diagram schematically shows an apparatus for producing carbon nanotubes using a floating catalyst chemical vapor deposition method (FC-CVD method). DETAILED DESCRIPTION

[0018] Hereinafter, the method for producing carbon nanotubes according to the present invention will be described in detail.

[0019] The method for producing carbon nanotubes according to the present invention is a method for producing carbon nanotubes by using a floating catalyst chemical vapor deposition method (FC-CVD method).

[0020] The so-called floating catalyst chemical vapor deposition method (FC-CVD method) is one of the CVD methods. It is the following method: without using a substrate as a carrier of the catalyst, the catalyst is introduced into a heated reaction tube, and chemically reacts with a carbon source in a floating and flowing state in a gas phase with a carrier gas flowing, so that carbon nanotubes (CNTs) grow in a floating state.

[0021] The method for producing carbon nanotubes (CNTs) of the present invention is characterized by comprising a step of heating a raw material of carbon nanotubes in the presence of an iron-containing catalyst and an alkali metal compound to generate carbon nanotubes.

[0022] In the method for producing carbon nanotubes of the present invention, high-purity single-walled carbon nanotubes can be efficiently produced by heating the raw material of carbon nanotubes in the presence of an iron-containing catalyst and an alkali metal compound (i.e., bringing the raw material of carbon nanotubes into contact with the iron-containing catalyst and the alkali metal compound under a heating environment).

[0023] For example, in a reaction tube used in a floating catalyst chemical vapor deposition method (FC-CVD method), a raw material of carbon nanotubes is introduced (supplied) into the reaction tube, and the raw material is brought into contact with an iron-containing catalyst and an alkali metal compound under a heating environment to generate carbon nanotubes.

[0024] As the raw material (carbon source) of carbon nanotubes, liquid or gaseous carbon compounds can be used. As specific examples, methane, ethane, propane, ethylene, propylene, acetylene, etc. can be appropriately used as gaseous carbon compounds. Alcohols such as methanol and ethanol, aliphatic hydrocarbons such as hexane, cyclohexane, and decalin, and aromatic hydrocarbons such as benzene, toluene, and xylene are preferably used as liquid carbon compounds. Ethylene, benzene, toluene, and decalin are particularly preferably used. Any carbon compound can also be mixed and used in combination.

[0025] In addition, as the iron-containing catalyst, metallocene compounds such as ferrocene, acetylacetonate metal salts such as ferric chloride and ferric acetylacetonate, metal carbonyls such as carbonyl iron, more preferably metallocene compounds such as ferrocene, and particularly preferably ferrocene. The iron-containing catalyst used may be only one kind or two or more kinds.

[0026] In the present invention, the lower limit of the supply amount of the iron-containing catalyst is preferably 2 parts by mass or more, more preferably 4 parts by mass or more, and further preferably 6 parts by mass or more, relative to 100 parts by mass of the carbon source. In addition, as an upper limit, it is preferably 14 parts by mass or less, more preferably 12 parts by mass or less, and further preferably 10 parts by mass or less. As the preferred range of the supply amount of the iron-containing catalyst, relative to 100 parts by mass of the carbon source, 2 to 14 parts by mass, 2 to 12 parts by mass, 2 to 10 parts by mass, 4 to 14 parts by mass, 4 to 12 parts by mass, 4 to 10 parts by mass, 6 to 14 parts by mass, 6 to 12 parts by mass, 6 to 10 parts by mass.

[0027] In addition, other catalysts may be used together with the iron-containing catalyst. Other catalysts are preferably transition metal compounds and transition metal particles. As transition metals, cobalt, nickel, palladium, platinum, rhodium are preferred, and cobalt and nickel are more preferred. As transition metal compounds in other catalysts, metallocene compounds such as cobaltocene and nickelocene are preferred; chlorides such as cobalt chloride; acetylacetonate metal salts; carbonyl metals, and metallocene compounds such as cobaltocene and nickelocene are more preferred. The other catalysts used may be only one or more.

[0028] The particle diameters of the iron-containing catalyst and the transition metal fine particles are preferably 0.1 nm to 50 nm, more preferably 0.3 nm to 15 nm, respectively. The particle diameter of the fine particles can be measured using a transmission electron microscope.

[0029] The method for supplying the iron-containing catalyst is not particularly limited as long as it is a method that can supply the iron-containing catalyst to the system (e.g., a reaction tube), and examples thereof include a method of supplying the iron-containing catalyst in a state where the iron-containing catalyst is dissolved in a solvent and a method of introducing the iron-containing catalyst into the reaction tube in a state where the iron-containing catalyst is gasified. In the present invention, a method of supplying the iron-containing catalyst in a state where the iron-containing catalyst is dissolved in a solvent is preferred. When other catalysts are used in combination, the same method can be used.

[0030] In the method of supplying the iron-containing catalyst in a state of being dissolved in a solvent, the solvent is not particularly limited, but is preferably a liquid carbon compound serving as a carbon source.

[0031] In the production method of the present invention, in order to promote the formation reaction of carbon nanotubes, it is preferred to further add a sulfur compound. Examples of sulfur compounds include organic sulfur compounds and inorganic sulfur compounds. Examples of organic sulfur compounds include mercaptans, thiophenes, thionaphthenes, and benzothiophenes. Examples of inorganic sulfur compounds include elemental sulfur, carbon disulfide, and hydrogen sulfide. The sulfur compound used may be only one kind or two or more kinds.

[0032] The method of adding the sulfur compound is not particularly limited, and an example of the method is to dissolve the sulfur compound in a liquid carbon compound used as a carbon source and add the sulfur compound.

[0033] In the present invention, relative to 100 parts by mass of the carbon source, the lower limit of the amount of the sulfur compound supplied is preferably 0.1 parts by mass or more, more preferably 0.25 parts by mass or more, and further preferably 0.5 parts by mass or more. In addition, as the upper limit, it is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and further preferably 2 parts by mass or less. If it is below the lower limit, the promotion effect cannot be obtained. In addition, if it is above the upper limit, it is possible to cause a decrease in quality due to a large amount of residue in the carbon nanotubes, and a decrease in yield due to an increase in the load in the refining process. Relative to 100 parts by mass of the carbon source, the preferred range of the amount of the sulfur compound supplied can be 0.1 parts by mass to 5 parts by mass, 0.1 parts by mass to 3 parts by mass, 0.1 parts by mass to 2 parts by mass, 0.25 parts by mass to 5 parts by mass, 0.25 parts by mass to 3 parts by mass, 0.25 parts by mass to 2 parts by mass, 0.5 parts by mass to 5 parts by mass, 0.5 parts by mass to 3 parts by mass, 0.5 parts by mass to 2 parts by mass.

[0034] As the alkali metal compound in the production method of the present invention, preferably lithium, sodium, potassium, cesium phosphate, acetate, chloride, sulfate, carbonate, hydrate, silicate, hydroxide of tetraborate, more preferably lithium, sodium, potassium, cesium chloride, sulfate, carbonate, hydrate, silicate, hydroxide of tetraborate, further preferably sodium, potassium, cesium chloride, sulfate, carbonate, hydrate, silicate, hydroxide of tetraborate, particularly preferably potassium, cesium chloride, carbonate, hydroxide. The alkali metal compound used may be only one kind or two or more kinds.

[0035] There is no limitation on the method of supplying the alkali metal compound, and examples thereof include a method of directly supplying a powder into a reaction tube, a method of preparing an aqueous solution of the alkali metal compound and spraying it from a nozzle, a method of vaporizing the alkali metal compound and supplying it into a reaction tube, etc. From the perspective of ease of handling, a method of preparing an aqueous solution of the alkali metal compound and spraying it from a nozzle, and a method of vaporizing the alkali metal compound and supplying it into a reaction tube are more preferred.

[0036] In the present invention, relative to 100 parts by mass of carbon source, the lower limit of the supply amount of the alkali metal compound is preferably 0.01 parts by mass or more, more preferably 0.03 parts by mass or more, further preferably 0.05 parts by mass or more, and particularly preferably 0.07 parts by mass or more. In addition, as the upper limit, it is preferably 15 parts by mass or less, more preferably 12 parts by mass or less, further preferably 10 parts by mass or less, and particularly preferably 8 parts by mass or less. If it is below the lower limit, the promotion effect is not obtained. In addition, if it is above the upper limit, it is possible to cause a decrease in quality due to a large amount of residual alkali metal compounds in the carbon nanotubes, and a decrease in yield due to an increase in the load in the refining process. Preferred ranges of the amount of the alkali metal compound supplied include 0.01 to 15 parts by mass, 0.01 to 12 parts by mass, 0.01 to 10 parts by mass, 0.01 to 8 parts by mass, 0.03 to 15 parts by mass, 0.03 to 12 parts by mass, 0.03 to 10 parts by mass, 0.03 to 8 parts by mass, 0.05 to 15 parts by mass, 0.05 to 12 parts by mass, 0.05 to 10 parts by mass, 0.05 to 8 parts by mass, 0.07 to 15 parts by mass, 0.07 to 12 parts by mass, 0.07 to 10 parts by mass, and 0.07 to 8 parts by mass, relative to 100 parts by mass of the carbon source.

[0037] As the carrier gas, an inert gas such as hydrogen, argon, helium, or nitrogen can be used. These gases can be used alone or in combination.

[0038] In the present invention, the step of generating carbon nanotubes can be carried out in a reactor. As a reactor, there is no particular limitation as long as it can efficiently produce carbon nanotubes. It is preferred to use any one of a horizontal reactor and a vertical reactor for the reaction, and it is more preferred to use a vertical reactor for the reaction. In addition, as the shape of the reactor, for example, a reactor having a tubular shape can be preferably used.

[0039] The material of the reactor in the present invention is not particularly limited as long as it has excellent strength in a high temperature region. Examples thereof include ceramics such as alumina, silica, silicon carbide, silicon nitride, aluminum nitride, mullite, and ferrite; glasses such as soda glass, lead glass, borosilicate glass, and quartz glass; and metals such as stainless steel and carbon steel. In the present invention, ceramics are preferably used. As a specific material, alumina, silicon carbide, and mullite are preferred, alumina and silicon carbide are more preferred, and silicon carbide is further preferred.

[0040] The reaction temperature in the present invention is not particularly limited as long as it is a temperature at which the iron-containing catalyst and the carbon source can react efficiently, and is preferably 1200°C to 1800°C, more preferably 1200°C to 1500°C, and more preferably 1200°C to 1300°C. By being in this reaction temperature range, the G / D ratio representing the defect rate of carbon nanotubes and the existence ratio of single-walled carbon nanotubes are excellent. On the other hand, if the temperature is too high, the existence ratio of single-walled carbon nanotubes decreases, and if the temperature is low, the yield of carbon nanotubes is significantly reduced.

[0041] The carbon purity of the carbon nanotubes produced by the production method of the present invention is preferably 80% or more, more preferably 84% or more, and particularly preferably 88% or more.

[0042] The intensity ratio G / D of the carbon nanotubes produced by the production method of the present invention is preferably 40 or more, preferably 60 or more, and particularly preferably 70 or more. G / D is measured by a Raman spectrometer, and in the Raman spectrum measured by the resonance Raman scattering method (excitation wavelength 532 nm), the G band (1590 cm -1 Nearby) and D belt (1300cm -1 The higher the G / D ratio, the less defects there are in the structure of the carbon nanotube.

[0043] The diameter of the carbon nanotube produced by the production method of the present invention is preferably 3.0 nm or less, and more preferably 2.5 nm or less.

[0044] The carbon nanotubes of the present invention may be single-walled carbon nanotubes or multi-walled carbon nanotubes, but the production method of the present invention can more appropriately produce high-purity single-walled carbon nanotubes.

[0045] The abundance ratio of the single-walled carbon nanotubes (SWCNTs) in the carbon nanotubes obtained by the production method of the present invention is preferably 75 mass % or more, more preferably 80 mass % or more, further preferably 85 mass % or more, particularly preferably 90 mass % or more. Example

[0046] Hereinafter, the present invention will be further described in detail by way of examples, but the present invention is not limited to the examples.

[0047] use Figure 1 The device shown in the figure produces carbon nanotubes. Figure 1 In the figure, 1 is a raw material spray nozzle, 2 is a reaction tube, 3 is a heater, 4 is a carbon nanotube recovery device, and 5 is an alkali metal compound supply nozzle.

[0048] Carbon nanotubes were synthesized using the following reagents and the like. Toluene: Kanto Chemical Co., Ltd. Ferrocene: Fujifilm Wako Pure Chemical Industries, Ltd. Thiophene: Tokyo Chemical Industry Co., Ltd. Hydrogen: Iwatani Gas Co., Ltd. Sodium chloride: Fujifilm Wako Pure Chemical Industries, Ltd. Sodium tetraborate decahydrate: Fujifilm Wako Pure Chemical Industries, Ltd. Sodium silicate: Fujifilm Wako Pure Chemical Industries, Ltd. Potassium tetraborate tetrahydrate: Fujifilm Wako Pure Chemical Industries, Ltd. Potassium chloride: Fujifilm Wako Pure Chemical Industries, Ltd. Potassium hydroxide: Fujifilm Wako Pure Chemical Industries, Ltd. Potassium carbonate: Fujifilm Wako Pure Chemical Industries, Ltd. Cesium hydroxide: Nacalai Tesque Co., Ltd.

[0049] <Example 1> use Figure 1 The carbon nanotube manufacturing device shown in the figure is used to synthesize carbon nanotubes. As the reaction tube 2, a tube with an inner diameter of 50 mm, an outer diameter of 60 mm, and a length of 1400 mm made of pressure-sintered silicon carbide (SiC) is used. Argon gas is circulated in the tube, and the reaction tube is heated to 1250°C by a heater 3 (effective heating length 900 mm) in the argon gas flow. Then, instead of the supply of argon gas, hydrogen gas is supplied as a carrier gas. A mixed solution of 100 parts by mass of toluene as a carbon source, 8.08 parts by mass of ferrocene as an iron-containing catalyst, and 0.91 parts by mass of thiophene as a sulfur compound is supplied through a nozzle 1, and an aqueous sodium chloride solution (0.10 parts by mass in terms of the supply amount of sodium chloride) as an alkali metal compound is supplied through a nozzle 5. The reaction time is set to 3 hours. As a result of the reaction, a deposit of black carbon nanotubes is generated in a carbon nanotube recovery device 4 arranged at the lower part of the reaction tube. The carrier gas was changed from hydrogen to argon, and the temperature was lowered to room temperature. The deposit was recovered by a recovery device and the following evaluation was performed. The results are shown in Table 1.

[0050] <Yield> The mass of the recovered carbon nanotubes was divided by the mass of the supplied carbon source. The calculation formula is as follows. Yield (%) = (mass of recovered carbon nanotubes / mass of carbon source) × 100

[0051] <G / D ratio> The intensity ratio G / D of the G band (1590 cm-1) and the D band (1590 cm-1) was measured using a laser Raman microscope (RAMANtouch VIS-NIR-DIS, Nanophoton Co., Ltd.) at a laser wavelength of 532 nm. -1Nearby) and D belt (1300cm -1 The peak intensity ratio of the vicinity is calculated.

[0052] <Carbon purity> Using a differential thermal thermogravimetric analyzer (Hitachi High-Tech Science STA 7200RV), about 7 mg of the sample was heated from room temperature to 900°C at a temperature increase rate of 10°C / min at an air flow rate of 200 cc / min, and the weight loss ratio in the temperature range from room temperature to 900°C was evaluated.

[0053] <Example 2> The same operation as in Example 1 was carried out except that the alkali metal compound was sodium tetraborate decahydrate and the supply amount was 0.51 parts by mass.

[0054] <Example 3> The same operation as in Example 1 was carried out except that the alkali metal compound was sodium silicate and the supply amount was 5.07 parts by mass.

[0055] <Example 4> The same operation as in Example 1 was carried out except that the alkali metal compound was potassium tetraborate tetrahydrate and the supply amount was 1.48 parts by mass.

[0056] <Example 5> The same operation as in Example 1 was carried out except that potassium chloride was used as the alkali metal compound and the supply amount was 0.46 parts by mass.

[0057] <Example 6> The same operation as in Example 1 was carried out except that potassium hydroxide was used as the alkali metal compound and the supply amount was 0.07 parts by mass.

[0058] <Example 7> The same operation as in Example 1 was carried out except that potassium carbonate was used as the alkali metal compound and the supply amount was 0.17 parts by mass.

[0059] <Example 8> The same operation as in Example 1 was carried out except that cesium hydroxide was used as the alkali metal compound and the supply amount was 0.44 parts by mass.

[0060] <Comparative Example 1> The same operation as in Example 1 was carried out except that the heating temperature was 1300° C. and the alkali metal compound was not supplied.

[0061] <Comparative Example 2> The same operation as in Comparative Example 1 was performed except that the heating temperature was set to 1300°C and the reaction tube 2 was made of mullite (Al2O3+SiO2 mass = 98%, Al2O3 / SiO2 = 1.8, bulk density = 2.7).

[0062] <Comparative Example 3> The same operation as in Comparative Example 1 was performed except that the heating temperature was set to 1200°C and the reaction tube 2 was made of mullite (Al2O3+SiO2 mass = 98%, Al2O3 / SiO2 = 1.8, bulk density = 2.7).

[0063] [Table 1]

[0064] As shown in Table 1, in Examples 1 to 8, in the method for producing carbon nanotubes using the floating catalyst chemical vapor deposition method (FC-CVD method), the raw material of carbon nanotubes is heated in the presence of an iron-containing catalyst and an alkali metal compound to generate carbon nanotubes, and single-walled carbon nanotubes with high purity can be produced efficiently. In addition, in Examples 1 to 8, compared with Comparative Examples 2 and 3 using a reactor made of mullite, high-yield and high-quality carbon nanotubes were obtained at low temperatures, so by supplying an alkali metal compound, the reaction temperature can be lowered, there is no need to worry about strength, and production can be more safe. Description of Reference Numerals

[0065] 1···Raw material spray nozzle 2. Reaction tube 3. Heater 4···Carbon Nanotube Recycler 5···Nozzle for supplying alkali metal compound.

Claims

1. A method for producing carbon nanotubes, characterized in that: The method is to prepare carbon nanotubes by using a floating catalyst chemical vapor deposition method, namely, FC-CVD method. The production method includes the step of heating a raw material of carbon nanotubes in the presence of an iron-containing catalyst and an alkali metal compound to generate carbon nanotubes.

2. The method for producing carbon nanotubes according to claim 1, wherein: The alkali metal species of the alkali metal compound includes at least one selected from the group consisting of lithium, sodium, potassium, and cesium.

3. The method for producing carbon nanotubes according to claim 1 or 2, wherein: The alkali metal compound is at least one selected from the group consisting of phosphates, acetates, chlorides, sulfates, carbonates, hydrates, silicates, and hydroxides of lithium, sodium, potassium, and cesium.

4. The method for producing carbon nanotubes according to claim 1 or 2, wherein: The supply amount of the alkali metal compound is 0.01 parts by mass or more and 15 parts by mass or less relative to 100 parts by mass of the raw material of the carbon nanotubes.

5. The method for producing carbon nanotubes according to claim 1 or 2, wherein: The iron-containing catalyst is ferrocene.

6. The method for producing carbon nanotubes according to claim 1 or 2, wherein: The step of producing carbon nanotubes is performed in a ceramic reaction tube.

7. The method for producing carbon nanotubes according to claim 6, wherein: The ceramic reaction tube contains silicon carbide.

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