Method for producing carbon nanotubes

Abstract

This method for producing carbon nanotubes comprises: a step for preparing a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst; a step for inputting the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst into the same reaction furnace, heating the same to a first temperature of 600-1,000ºC, holding the temperature at the first temperature for at least one second to thermally decompose each of the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst, and reacting sulfur obtained by the thermal decomposition of the sulfur-containing auxiliary catalyst with the surfaces of iron nanoparticles obtained by the thermal decomposition of the ferrocene to form core-shell-type nanoparticles each including a core composed of iron and a shell covering the core and having iron and sulfur; and a step for heating the core-shell-type nanoparticles and carbon obtained by a pyrolysate of the carbon-containing gas to a second temperature of 1,200-1,400ºC to grow carbon nanotubes from the core-shell-type nanoparticles, thereby obtaining carbon nanotubes.

Description

Carbon nanotube manufacturing method

[0001] The present disclosure relates to a method for producing carbon nanotubes. This application claims priority from Japanese Patent Application No. 2024-054385, filed on March 28, 2024. The entire contents of the Japanese Patent Application are incorporated herein by reference.

[0002] BACKGROUND ART Floating-Catalyst CVD (FC-CVD) is known as a continuous production method for carbon nanotubes (hereinafter also referred to as "CNTs") (for example, Patent Document 1).

[0003] In the FC-CVD method, ferrocene (Fe(C)) is used as a catalyst metal source together with a carrier gas. ５ H ５ ) ２ )), and thiophene (C) as a co-catalyst ４ H ４ S) is introduced into a reactor and heated, which leads to the formation of iron particles through thermal decomposition of ferrocene, followed by the thermal decomposition of thiophene and the formation of Fe—S on the surface of the iron particles, forming active catalyst particles, from which CNTs grow.

[0004] International Publication No. 2020 / 138378

[0005] The method for producing carbon nanotubes according to the present disclosure includes the steps of: preparing a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst; charging the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst into a reactor and heating them to a first temperature of 600°C or higher and 1000°C or lower; and maintaining the first temperature for one second or longer to pyrolyze the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst; and reacting sulfur obtained by pyrolysis of the sulfur-containing auxiliary catalyst with the surfaces of iron nanoparticles obtained by pyrolysis of the ferrocene to form core-shell nanoparticles including an iron core and a shell containing iron and sulfur covering the core; and obtaining carbon nanotubes by heating the core-shell nanoparticles and carbon obtained by pyrolyzing the carbon-containing gas to a second temperature of 1200°C or higher and 1500°C or lower to grow carbon nanotubes from the core-shell nanoparticles.

[0006] Fig. 1 is a diagram for explaining a conventional method for producing carbon nanotubes. Fig. 2 is a diagram for explaining a method for producing carbon nanotubes according to embodiment 1. Fig. 3 is a transmission electron microscope image of the carbon nanotubes obtained from sample 9.

[0007] [Problem to be Solved by the Present Disclosure] From the viewpoint of cost reduction, there is a demand for improving the synthesis efficiency of carbon nanotubes.

[0008] Therefore, an object of the present disclosure is to provide a method for producing carbon nanotubes that can improve the synthesis efficiency of carbon nanotubes when the same raw materials are used.

[0009] Effect of the Present Disclosure According to the present disclosure, it is possible to provide a method for producing carbon nanotubes that can improve the efficiency of carbon nanotube synthesis when the same raw materials are used.

[0010] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. (1) A method for producing carbon nanotubes according to the present disclosure includes the steps of: preparing a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst; charging the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst into a reactor and heating them to a first temperature of 600°C or higher and 1000°C or lower; and maintaining the first temperature for one second or longer to pyrolyze the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst; and reacting sulfur obtained by pyrolysis of the sulfur-containing auxiliary catalyst with the surfaces of iron nanoparticles obtained by pyrolysis of the ferrocene to form core-shell nanoparticles including an iron core and a shell containing iron and sulfur covering the core; and obtaining carbon nanotubes by heating the core-shell nanoparticles and carbon obtained by pyrolyzing the carbon-containing gas to a second temperature of 1200°C or higher and 1500°C or lower to grow carbon nanotubes from the core-shell nanoparticles.

[0011] According to the present disclosure, it is possible to provide a method for producing carbon nanotubes that can improve the synthesis efficiency of carbon nanotubes when the same raw materials are used. In the present disclosure, the same raw materials mean that the types of carbon-containing gas and sulfur-containing auxiliary catalyst are the same, and the amounts of the carbon-containing gas, ferrocene, and sulfur-containing auxiliary catalyst used as raw materials are the same.

[0012] (2) In the above (1), the preparing step includes a step of mixing the ferrocene and the sulfur-containing auxiliary catalyst in an organic solvent to obtain a catalyst raw material-containing solution, and in the step of forming the core-shell nanoparticles, the ferrocene and the sulfur-containing auxiliary catalyst are introduced into the reactor as the catalyst raw material-containing solution, and in the catalyst raw material-containing solution, the number of iron atoms N Ｆｅ and the number of sulfur atoms, N Ｓ The ratio of Ｆｅ :N Ｓ The ratio may be 1:2 to 1:8.

[0013] This further improves the efficiency of carbon nanotube synthesis.

[0014] (3) In the above (1) or (2), the first temperature may be 850° C. or higher and 950° C. or lower, and the holding time at the first temperature may be 1 second or higher and 60 seconds or lower.

[0015] This further improves the efficiency of carbon nanotube synthesis.

[0016] (4) In the core-shell nanoparticles, the percentage of the number of sulfur atoms relative to the total number of iron and sulfur atoms may be 6% or more and 45% or less.

[0017] This further improves the efficiency of carbon nanotube synthesis.

[0018] [Details of the Embodiments of the Present Disclosure] Specific examples of the carbon nanotube manufacturing method of the present disclosure will be described below with reference to the drawings. In the drawings of the present disclosure, the same reference numerals represent the same or corresponding parts. Furthermore, dimensional relationships such as length, width, thickness, and depth have been appropriately changed for clarity and simplification of the drawings, and do not necessarily represent actual dimensional relationships.

[0019] In this specification, the notation in the form of "A to B" means A or more and B or less, and when no unit is specified for A and a unit is specified only for B, the unit of A and the unit of B are the same.

[0020] In the present specification, when a compound or the like is represented by a chemical formula, unless the atomic ratio is particularly limited, it is understood to include any conventionally known atomic ratio, and is not necessarily limited to only those within the stoichiometric range.

[0021] In the present disclosure, when one or more numerical values ​​are listed as the lower limit and the upper limit of a numerical range, the combination of any one numerical value listed as the lower limit and any one numerical value listed as the upper limit is also considered to be disclosed.

[0022] First, to deepen understanding of the carbon nanotube manufacturing method of the present disclosure, a conventional method for manufacturing carbon nanotubes by FC-CVD will be described. The conventional method for manufacturing carbon nanotubes uses, for example, a carbon nanotube manufacturing apparatus 10 shown in FIG.

[0023] The carbon nanotube production apparatus 10 includes a quartz tube 12 as a reactor, and a first end 12c of the quartz tube 12. The first end 12c of the quartz tube 12 contains a carbon-containing gas (methane, ethylene, acetone, etc.), a catalyst (ferrocene), and a co-catalyst (thiophene, sulfur (S ８ The apparatus may include a spray 13 for supplying a catalyst raw material-containing liquid containing the catalyst raw material, such as hydroxyl group 12, and hydrogen gas as a carrier gas, and an electric furnace 11 for heating the quartz tube 12.

[0024] The electric furnace 11 heats the quartz tube 12 so that the inside of the quartz tube 12 reaches a CNT synthesis temperature (approximately 1200 to 1500°C). The components contained in the carbon-containing gas and the catalyst raw material-containing liquid supplied to the inside of the quartz tube 12 are heated and thermally decomposed. The thermal decomposition temperature of ferrocene is approximately 400 to 500°C, that of thiophene is approximately 750 to 850°C, and that of sulfur (S ８ The thermal decomposition temperature of carbon-containing gas is about 800 to 1050°C.

[0025] The temperature of the internal region of the first end 12c of the quartz tube 12 (hereinafter also referred to as the "radiant heat receiving region") is approximately 400 to 900°C due to the radiant heat of the electric furnace 11. In the radiant heat receiving region 12e, iron nanoparticles formed by thermal decomposition of ferrocene and thiophene or sulfur (S ８ ) is decomposed to form sulfur (S ２ ) to form core-shell nanoparticles having a core made of iron, which is an iron nanoparticle, and a shell containing iron and sulfur that covers the core. The core-shell nanoparticles are active catalyst particles.

[0026] An internal region of the quartz tube 12 facing the electric furnace downstream of the radiant heat receiving region 12e (hereinafter also referred to as the "electric furnace heating region") is at a CNT synthesis temperature of approximately 1200 to 1500°C. Carbon formed by decomposition of the carbon-containing gas is supplied to the core-shell nanoparticles that have been transported to the electric furnace heating region 12f by the flow of the carrier gas, and carbon nanotubes grow from the core-shell nanoparticles to form carbon nanotubes.

[0027] In conventional carbon nanotube manufacturing methods, the entire quartz tube 12 was heated to a carbon nanotube synthesis temperature of approximately 1200 to 1500°C, so the length along the longitudinal direction of the quartz tube 12 of the radiant heat receiving area 12e, which has a temperature capable of forming core-shell nanoparticles, was very short, and was even shorter than the length along the longitudinal direction of the electric furnace heating area 12f.

[0028] In the FC-CVD method, a gas containing a carbon-containing gas and a catalyst raw material-containing liquid (hereinafter also referred to as "raw material-containing gas") passes from the first end 12c to the second end 12d without stagnation. Therefore, in conventional carbon nanotube manufacturing methods, the time it takes for the raw material-containing gas to pass through the radiant heat-receiving region 12e is very short (much shorter than 1 second), and the time for forming core-shell nanoparticles is insufficient. As a result, the catalyst and auxiliary catalyst remain as they are, or coarse iron particles are formed, resulting in a low proportion of core-shell nanoparticles, which are highly active catalyst particles, and low carbon nanotube synthesis efficiency.

[0029] In order to improve the synthesis efficiency of carbon nanotubes, the present inventors have focused particularly on the process of forming core-shell nanoparticles and created a new production method. The production method of carbon nanotubes according to a first embodiment will be described below.

[0030] [Embodiment 1: Method for Producing Carbon Nanotubes] A method for producing carbon nanotubes according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 1") includes the steps of: preparing a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst; charging the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst into a single reactor, heating them to a first temperature of 600°C or higher and 1000°C or lower, and maintaining them at the first temperature for one second or longer to thermally decompose the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst; and reacting sulfur obtained by thermal decomposition of the sulfur-containing auxiliary catalyst with the surfaces of iron nanoparticles obtained by thermal decomposition of the ferrocene to form core-shell nanoparticles including a core made of iron and a shell having iron and sulfur covering the core; and a step of growing carbon nanotubes from the core-shell nanoparticles by heating the core-shell nanoparticles and carbon obtained by pyrolysis of the carbon-containing gas to a second temperature of 1200°C or higher and 1500°C or lower, thereby obtaining carbon nanotubes.

[0031] The carbon nanotube manufacturing method of the first embodiment can use, for example, a carbon nanotube manufacturing apparatus 10 shown in Fig. 2. The carbon nanotube manufacturing apparatus 10 can include a quartz tube 12 serving as a reactor, a spray 13 disposed on one side of a first end 12c of the quartz tube 12 and supplying a catalyst raw material-containing liquid containing a carbon-containing gas, ferrocene and a sulfur-containing auxiliary catalyst, and hydrogen gas serving as a carrier gas into the interior of the quartz tube 12, and a first electric furnace 11a and a second electric furnace 11b for heating the quartz tube 12.

[0032] Before starting the production of carbon nanotubes, the quartz tube 12 is heated in advance in a first electric furnace 11a and a second electric furnace 11b while flowing hydrogen gas. The first region 12a of the quartz tube 12 facing the first electric furnace 11a is heated in the first electric furnace 11a to a temperature (first temperature) at which core-shell nanoparticles can be formed, which is 600°C or higher and 1000°C or lower. The second region 12b of the quartz tube 12 facing the second electric furnace 11b is heated in the second electric furnace 11b to a temperature (second temperature) at which carbon nanotubes can be synthesized, which is 1200°C or higher and 1500°C or lower.

[0033] <Preparing Step> In the preparing step, a carbon-containing gas, ferrocene, and a sulfur-containing co-catalyst are prepared.

[0034] The carbon-containing gas is a carbon source for the carbon nanotubes, and may be a gas containing at least one selected from the group consisting of methane, ethylene, and acetone.

[0035] Ferrocene and sulfur-containing co-catalysts are raw materials for core-shell nanoparticles. The sulfur-containing co-catalysts include thiophene and sulfur (S ８ ) can be used.

[0036] The preparing step may include a step of mixing ferrocene and the sulfur-containing co-catalyst with an organic solvent to obtain a catalyst raw material-containing solution. The organic solvent may be toluene, ethanol, or the like.

[0037] The volume ratio of ferrocene to be mixed with the organic solvent and the sulfur-containing auxiliary catalyst is determined by the ratio of the number of iron atoms N in the catalyst raw material-containing solution. Ｆｅ and the number of sulfur atoms, N Ｓ The ratio of Ｆｅ :N Ｓ The ratio may be adjusted to be 1:2 to 1:8. This further improves the synthesis efficiency of carbon nanotubes. Ｆｅ :N Ｓ is N Ｆｅ :N Ｓ = 1:4 to 1:8, or N Ｆｅ :N Ｓ= 1:4 to 1:6 is also acceptable.

[0038] <Step of forming core-shell nanoparticles> In the step of forming core-shell nanoparticles, a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst are charged into the same reactor, heated to a first temperature of 600°C or higher and 1000°C or lower, and maintained at the first temperature for one second or longer, thereby thermally decomposing the carbon-containing gas, ferrocene, and sulfur-containing auxiliary catalyst, and reacting sulfur obtained by thermal decomposition of the sulfur-containing auxiliary catalyst with the surface of iron nanoparticles obtained by thermal decomposition of ferrocene, thereby forming core-shell nanoparticles including a core made of iron and a shell containing iron and sulfur covering the core.

[0039] In the CNT production apparatus of FIG. 2, a carbon-containing gas and a catalyst raw material-containing liquid can be sprayed from a spray 13 together with a carrier gas (for example, hydrogen or argon) and introduced into a quartz tube 12, which is a reactor.

[0040] The first region 12a of the quartz tube 12 is heated to a first temperature of 600 to 1000°C. Therefore, the carbon-containing gas and the catalyst raw material-containing liquid that reach the first region 12a are heated to the first temperature. The time during which the carbon-containing gas and the catalyst raw material-containing liquid pass through the first region 12a corresponds to the retention time at the first temperature.

[0041] In the carbon nanotube production apparatus 10, the length of the first region 12a along the longitudinal direction of the quartz tube 12 is sufficiently longer than the length of the radiant heat receiving region 12e along the longitudinal direction of the quartz tube 12, which corresponds to the first region 12a in a conventional carbon nanotube composite production apparatus. Therefore, in the carbon nanotube production apparatus 10, the time it takes for the gas containing the carbon-containing gas and the catalyst raw material-containing liquid to pass through the first region 12a can be extended to 1 second or more, ensuring sufficient time for the formation of core-shell nanoparticles. Furthermore, the catalyst and auxiliary catalyst are prevented from remaining as they are, and coarse iron particles are prevented from being formed.

[0042] The first temperature may be 600°C or higher and 1000°C or lower, and the holding time at the first temperature may be 1 second or higher and 60 seconds or lower. Alternatively, the first temperature may be 850°C or higher and 950°C or lower, and the holding time at the first temperature may be 1 second or higher and 60 seconds or lower. The first temperature may be 600°C or higher and 1000°C or lower, or 850°C or higher and 950°C or lower, and the holding time at the first temperature may be 2 seconds or higher and 60 seconds or lower, 3 seconds or higher and 60 seconds or lower, 5 seconds or higher and 60 seconds or lower, 10 seconds or higher and 60 seconds or lower, 20 seconds or higher and 60 seconds or lower, 30 seconds or higher and 60 seconds or lower, 40 seconds or higher and 60 seconds or lower, or 50 seconds or higher and 60 seconds or lower.

[0043] The flow rate of the entire gas containing the carrier gas, the carbon-containing gas, and the catalyst raw material-containing liquid in the first region 12a may be, for example, 0.4 to 10 cm / sec.

[0044] The length of the first region 12a along the longitudinal direction of the quartz tube 12 may be, for example, 100 to 1000 mm. The length of the second region 12b along the longitudinal direction of the quartz tube 12 may be, for example, 300 to 1000 mm.

[0045] The ratio of the length of the first region 12a to the length of the second region 12b along the longitudinal direction of the quartz tube 12 may be, for example, first region 12a:second region 12b=1:1 to 1:10.

[0046] The diameter of the internal space of the quartz tube 12 may be, for example, 10 to 100 mm.

[0047] <Step of Growing Carbon Nanotubes> In the step of growing carbon nanotubes, the core-shell nanoparticles and carbon obtained by pyrolysis of a carbon-containing gas are heated to a second temperature of 1200°C or higher and 1500°C or lower, thereby growing carbon nanotubes from the core-shell nanoparticles and obtaining carbon nanotubes.

[0048] 2, core-shell nanoparticles formed in the first region 12a of the quartz tube 12 and carbon formed by decomposition of the carbon-containing gas are transported to the second region 12b by the flow of the carrier gas. The second region 12b is heated to a second temperature of 1200 to 1500°C. Therefore, carbon is supplied to the core-shell nanoparticles in the second region 12b, and carbon nanotubes grow from the core-shell nanoparticles.

[0049] The second temperature may be 1200°C or higher and 1500°C or lower, and the holding time at the second temperature may be 1 second or higher and 300 seconds or lower. Alternatively, the second temperature may be 1300°C or higher and 1400°C or lower, and the holding time at the second temperature may be 1 second or higher and 300 seconds or lower. The second temperature may be 1200°C or higher and 1500°C or lower, or 1300°C or higher and 1400°C or lower, and the holding time at the second temperature may be 10 seconds or higher and 300 seconds or lower, 30 seconds or higher and 300 seconds or lower, 60 seconds or higher and 300 seconds or lower, 90 seconds or higher and 300 seconds or lower, 120 seconds or higher and 300 seconds or lower, 150 seconds or higher and 300 seconds or lower, 180 seconds or higher and 300 seconds or lower, or 2100 seconds or higher and 300 seconds or lower.

[0050] The total gas flow rate in the second region 12b may be, for example, 0.4 to 10 cm / sec.

[0051] The carbon nanotube grown from the core-shell nanoparticle may then be separated from the core-shell nanoparticle and exist as a single carbon nanotube, or may exist as a carbon nanotube composite in which the core-shell nanoparticle is attached to the carbon nanotube.

[0052] In the carbon nanotube composite, the core-shell nanoparticles are not particularly limited in terms of the position and form of attachment as long as they are attached to the carbon nanotube. The core-shell nanoparticles may be attached to the ends of the carbon nanotube, or to regions other than the ends of the carbon nanotube. At least a portion of the core-shell nanoparticles may be embedded inside the carbon nanotube. The number of core-shell nanoparticles attached to one carbon nanotube composite may be one or more. The attachment of core-shell nanoparticles to the carbon nanotube composite can be confirmed by observing the carbon nanotube composite with a transmission electron microscope at 100,000 to 1,500,000 magnifications.

[0053] The carbon nanotube (hereinafter also referred to as "CNT") may be a CNT with a known structure, such as a single-walled CNT in which only one carbon layer (graphene) is cylindrical, a double-walled CNT or multi-walled CNT in which multiple carbon layers are stacked in a cylindrical shape, or a cup-stacked CNT in which graphene is stacked in the shape of a paper cup with a bottom.

[0054] The shape of the CNT is not particularly limited, and it may be either closed or open at the end. One or both ends of the CNT may have a cone made of conical graphene.

[0055] The length of the CNTs can be appropriately selected depending on the application. The length of the CNTs may be 1 μm or more and 600 mm or less, 10 μm or more and 600 mm or less, or 100 μm or more and 600 mm or less. The length of the CNTs is measured by observation with a scanning electron microscope.

[0056] The diameter of the CNTs may be 0.6 nm or more and 20 nm or less, 0.7 nm or more and 10 nm or less, or 0.8 nm or more and 5 nm or less.

[0057] In this disclosure, the diameter of a carbon nanotube refers to the average outer diameter of a single CNT. The average outer diameter of a CNT is obtained by directly observing the cross section of a CNT at any two points using a transmission electron microscope, measuring the outer diameter, which is the distance between the two most distant points on the circumference of the CNT, and calculating the average of the obtained outer diameters. If the CNT includes a cone portion at one or both ends, the diameter is measured at a location excluding the cone portion.

[0058] The core-shell nanoparticles include a core made of iron and a shell containing iron and sulfur that covers the core.

[0059] The core may contain inevitable impurities, such as iron carbide, in addition to iron, as long as the effects of the present disclosure are not impaired.

[0060] The shell may contain inevitable impurities, such as iron carbide, in addition to iron and sulfur, as long as the effects of the present disclosure are not impaired.

[0061] In the core-shell nanoparticles, the entire surface of the core may be covered with the shell, but a portion of the surface of the core may not be covered with the shell as long as the effects of the present disclosure are not impaired.

[0062] The core-shell nanoparticles may be composed of a core made of iron and a shell containing iron and sulfur that covers the core. The core-shell nanoparticles may contain carbon and oxygen in addition to the core and shell, as long as the effects of the present disclosure are not impaired.

[0063] The fact that the core-shell nanoparticles contain a core made of iron and a shell containing iron and sulfur that covers the core is confirmed by an energy dispersive X-ray spectroscopy (EDX) device or an electron energy loss spectroscopy (EELS) device attached to a transmission electron microscope (TEM).

[0064] The percentage of sulfur atoms relative to the total number of iron and sulfur atoms in the core-shell nanoparticles (hereinafter also referred to as the "sulfur content of the core-shell nanoparticles") may be 6% or more and 45% or less, 10% or more and 40% or less, or 20% or more and 35% or less. When the percentage is 6% or more, the sulfur content of the carbon nanotube composite containing the core-shell nanoparticles increases. When the carbon nanotube composite is applied to the positive electrode of a lithium-sulfur battery, it contributes to improving the theoretical energy density of the lithium-sulfur battery. When the percentage is less than 6%, sufficient energy density cannot be obtained due to a shortage of sulfur relative to lithium. When the percentage is 45% or less, the conductivity of iron can be maintained, and when the carbon nanotube composite is applied to the positive electrode of a lithium-sulfur battery, it can promote the electrochemical reaction between sulfur and lithium during the charge and discharge process of the lithium-sulfur battery.

[0065] In the present disclosure, the percentage of the number of sulfur atoms relative to the total number of iron and sulfur atoms in core-shell nanoparticles is measured as follows: The core-shell nanoparticles attached to the carbon nanotubes are measured for characteristic X-rays derived from iron (Fe-Kα rays) and characteristic X-rays derived from sulfur (S-Kα rays) using an energy dispersive X-ray spectrometer attached to a scanning electron microscope (SEM), and the atomic ratio of sulfur to iron is calculated from the signal intensities and a correction factor.

[0066] When multiple core-shell nanoparticles are attached to one carbon nanotube, the above measurement is performed for each of all core-shell nanoparticles. In the present disclosure, the sulfur content of the core-shell nanoparticles being 6% or more and 45% or less means that the sulfur content of each of the multiple core-shell nanoparticles attached to one carbon nanotube is within the range of 6% or more and 45% or less. The same applies to other numerical ranges.

[0067] The particle size of the core-shell nanoparticles may be 2 nm or more and 50 nm or less, 4 nm or more and 25 nm or less, or 6 nm or more and 10 nm or less.

[0068] In the present disclosure, the particle size of core-shell nanoparticles is measured as follows. First, a sample containing core-shell nanoparticles is added to ethanol and dispersed by ultrasonic irradiation. This dispersion is dropped onto a dedicated grid and dried. Then, the sample is observed at a magnification of 100,000 to 1,500,000 using a transmission electron microscope (TEM). Next, the outer diameter, which is the distance between the two most distant points on the periphery of each core-shell nanoparticle, is measured in this TEM image. This outer diameter corresponds to the particle size of each core-shell nanoparticle.

[0069] When a carbon nanotube composite contains multiple core-shell nanoparticles, the above measurement is performed for each of all of the core-shell nanoparticles. In the present disclosure, the particle size of the core-shell nanoparticles being 2 nm or more and 50 nm or less means that the particle size of each of the multiple core-shell nanoparticles attached to a single carbon nanotube is within the range of 2 nm or more and 50 nm or less. The same applies to other numerical ranges.

[0070] The particle size of the core may be 1 nm or more and 48 nm or less, 2 nm or more and 23 nm or less, or 3 nm or more and 8 nm or less.

[0071] The thickness of the shell may be 0.5 nm or more and 12 nm or less, 1 nm or more and 6 nm or less, or 1.5 nm or more and 3 nm or less.

[0072] In the present disclosure, the method for measuring the particle size of the core and the average thickness of the shell is as follows. First, core-shell nanoparticles are observed at a magnification of 100,000 to 1,500,000 times using a transmission electron microscope (TEM). Then, in the same field of view, an electron energy loss spectroscopy (EELS) device attached to the TEM is used to measure two-dimensional mapping images of signals derived from Fe-Fe bonds (core region) and Fe-S bonds (shell region). The outer diameter, which is the distance between the two most distant points on the periphery of the region corresponding to the core portion, is measured. This outer diameter corresponds to the particle size of the core. The shell thickness is calculated based on the difference between the particle size of the core-shell nanoparticles and the particle size of the core.

[0073] The various measurements of the core-shell nanoparticles described above are performed on the core-shell nanoparticles attached to the carbon nanotubes, and the measurement results obtained thereby approximately indicate the composition and morphology of the core-shell nanoparticles formed in the process of forming the core-shell nanoparticles.

[0074] The obtained carbon nanotubes may not have coarse particles with a particle size of more than 50 nm attached thereto. The coarse particles may be formed by the catalyst ferrocene (Fe(C ５ H ５ ) ２ ) decomposes into iron particles during the production of carbon nanotubes, and these coarse iron particles become coarse. These coarse particles do not contribute to the synthesis of CNTs and affect the electrical conductivity characteristics of CNTs. If these coarse particles are not attached to carbon nanotubes, CNTs can maintain their inherent electrical conductivity.

[0075] The absence of coarse particles attached to the carbon nanotubes can be confirmed by observing the carbon nanotubes with a TEM. The composition of the coarse particles is measured using an energy dispersive X-ray diffractometer (TEM-EDX) attached to the TEM.

[0076] The carbon content of the obtained carbon nanotube composite in which core-shell nanoparticles are attached to carbon nanotubes may be 15% by mass or more and 99% by mass or less, 30% by mass or more and 95% by mass or less, or 60% by mass or more and 90% by mass or less.

[0077] The carbon content of the carbon nanotube composite is measured by thermogravimetric analysis. The specific measurement method is as follows: Approximately 1 to 5 mg of a sample of the carbon nanotube composite is weighed and immersed in dry air (N ２ :O ２While flowing a mixture of 200 mL / min of ethanol (distilled water, ethanol = 8:2), the sample is heated from 30°C to 1000°C at a temperature increase rate of 3°C / min, and the change in weight of the sample is measured. The carbon components in the sample are burned in the range of 300 to 900°C and released as gases such as carbon dioxide, resulting in a decrease in weight. The sample is further heated to 1000°C, and the weight of the sample remaining after the carbon components have been completely burned corresponds to the content of catalyst particles contained therein before combustion. Therefore, the carbon content can be calculated by measuring the content of catalyst particles.

[0078] [Supplementary Note 1] A process of preparing a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst; a process of charging the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst into a single reactor, heating them to a first temperature of 600°C or higher and 1000°C or lower, and maintaining them at the first temperature for one second or longer to thermally decompose the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst, respectively, and reacting sulfur obtained by thermal decomposition of the sulfur-containing auxiliary catalyst with the surfaces of iron nanoparticles obtained by thermal decomposition of the ferrocene to form core-shell nanoparticles including a core made of iron and a shell having iron and sulfur covering the core; a step of growing carbon nanotubes from the core-shell nanoparticles by heating the core-shell nanoparticles and carbon obtained by thermal decomposition of the carbon-containing gas to a second temperature of 1200°C or higher and 1500°C or lower, thereby obtaining a carbon nanotube composite comprising the carbon nanotubes and the core-shell nanoparticles attached to the carbon nanotubes.

[0079] The present embodiment will be described in more detail with reference to examples, although the present embodiment is not limited to these examples.

[0080] <Preparation of Apparatus> A carbon nanotube production apparatus having the structure shown in Fig. 2 was prepared. The inner diameter of the internal space of the quartz tube 12 was 46 mm, and the length was 1000 mm. For samples 1 to 26, the length of the first electric furnace 11a was 500 mm, and the length of the second electric furnace 11b was 500 mm. For samples 27 to 31, the length of the first electric furnace 11a was 70 mm, and the length of the second electric furnace 11b was 500 mm.

[0081] <Preparation of Carbon Nanotubes> Each sample contained ferrocene and either thiophene or sulfur (S) as a sulfur-containing co-catalyst. ８ ) was dissolved in toluene to prepare a catalyst raw material containing solution. The content of ferrocene in the catalyst raw material containing solution was 4 mass %. ８ By adjusting the content of the iron atoms in the catalyst raw material-containing liquid, Ｆｅ and the number of sulfur atoms, N Ｓ Ratio to N Ｆｅ :N Ｓ But, N Ｆｅ :N Ｓ A catalyst raw material containing liquid having a ratio of 1:1, 1:2, 1:4, 1:6 or 1:8 was prepared.

[0082] For each sample, the ratio N Ｆｅ :N Ｓ Using a catalyst raw material containing liquid having the above composition, the temperature T2 of the second electric furnace 11b was set to 1400°C, and the temperature T1 of the first electric furnace 11a was set to the temperature shown in the "T1" column of Table 1, and carbon nanotubes were synthesized.

[0083] Specifically, the quartz tube was heated in an electric furnace while hydrogen gas with a hydrogen gas concentration of 100% by volume was supplied from the spray 13 into the internal space of the quartz tube at a flow rate of 7 L / min (flow velocity of 7 cm / sec). The temperature of the first region 12a of the quartz tube 12 facing the first electric furnace 11a was the same as the temperature T1 of the first electric furnace 11a. Therefore, temperature T1 corresponds to the first temperature. The temperature of the second region 12b of the quartz tube 12 facing the second electric furnace 11b was the same as the temperature T2 of the second electric furnace 11b. Therefore, temperature T2 corresponds to the second temperature. Note that when T1 is 1400°C, the first temperature and the second temperature are the same, which corresponds to the conventional manufacturing method.

[0084] Next, while hydrogen gas was flowing, ethylene gas was supplied from the spray 13 into the internal space of the quartz tube at a flow rate of 50 mL / min (flow rate 0.05 cm / sec) and the catalyst raw material-containing liquid was supplied at a flow rate of 0.1 mL / min (flow rate 0.02 cm / sec) for 5 minutes. The flow rate of the entire gas including hydrogen gas, ethylene gas, and catalyst raw material-containing liquid was 7.07 cm / sec. In Samples 1 to 26, the time required for ethylene gas, ferrocene, and thiophene to pass through the first region (retention time at first temperature T1) was 7 seconds. In Samples 27 to 31, the time required for ethylene gas, ferrocene, and thiophene to pass through the first region (retention time at first temperature T1) was 1 second.

[0085] Thereafter, the sample containing the carbon nanotubes formed in the quartz tube was collected.

[0086]

[0087] <Observation with a Transmission Electron Microscope> The collected samples of each specimen were observed with a transmission electron microscope at 200,000 magnification. In all specimens, carbon nanotubes alone or carbon nanotube composites, which are carbon nanotubes with core-shell nanoparticles attached, were confirmed.

[0088] In each sample, a carbon nanotube composite assembled wire in which a plurality of carbon nanotube composites were assembled was also produced.

[0089] A transmission electron microscope image of the sample recovered from Sample 9 is shown in FIG.

[0090] <Evaluation of carbon nanotube synthesis rate> The collected sample was placed in a thermogravimetric analyzer, and its weight W1 (mg) before heating was measured. The weight W1 (mg) was divided by the time (min) during which ethylene gas and the catalyst raw material-containing liquid were supplied to calculate the synthesis sample weight per unit time (mg / min).

[0091] While dry air was flowing into the thermogravimetric analyzer, the temperature was raised to 1000°C at a rate of 3°C ± 1°C / min. The weight W2 (mg) of the sample at 1000°C was measured. The ratio W1 / W2 of weight W2 (mg) to weight W1 corresponds to the proportion of residual catalyst in the sample. The carbon nanotube synthesis rate was calculated using the following formula: Carbon nanotube synthesis rate (mg / min) = synthesis sample weight per unit time (mg / min) × (1 - catalyst residual proportion) In this evaluation, the difference between weight W1 (mg) and weight W2 (mg) is considered to be the weight of carbon nanotubes.

[0092] The carbon nanotube synthesis rate for each sample is shown in Table 1.

[0093] Same ratio N Ｆｅ :N Ｓ When comparing samples (for example, between Sample 1 and Sample 2) using catalyst raw material-containing liquids having T1 of 600 to 1000°C (Examples), it was confirmed that the carbon nanotube synthesis rate was higher and the carbon nanotube synthesis efficiency was improved compared to the sample (Comparative Example) having T1 of 1400°C.

[0094] <Measurement of Core-Shell Nanoparticles> For each sample, the percentage of the number of sulfur atoms relative to the total number of iron and sulfur atoms in the core-shell nanoparticles attached to the carbon nanotube was measured. The specific measurement method is as described in embodiment 1. The results are shown in the "S / (Fe+S)" column of "Core-Shell Nanoparticles" in Table 2. For example, the description "10-20%" for sample 1 indicates that the sulfur content of each of the multiple core-shell nanoparticles attached to one carbon nanotube is within the range of 10-20% (i.e., the minimum value is 10% and the maximum value is 25%).

[0095] The particle size of the core-shell nanoparticles was measured for each sample. The specific measurement method is as described in embodiment 1. The results are shown in the "Total particle size" column of "Core-shell nanoparticles" in Table 2. For example, the description "3 to 40 nm" for sample 1 indicates that the particle size of each of the multiple core-shell nanoparticles attached to one carbon nanotube is within the range of 3 to 40 nm (i.e., the minimum value is 3 nm and the maximum value is 40 nm).

[0096] <Measurement of Carbon Nanotubes> The length and diameter of the carbon nanotubes were measured for each sample. The specific measurement method is as described in embodiment 1. The results are shown in the "Length" and "Diameter" columns for "CNT" in Table 2. For example, the description of the length of sample 1 as "1 to 100 μm" indicates that multiple carbon nanotubes were produced in each sample, and the length of each of the carbon nanotubes was within the range of 1 to 100 μm (i.e., the minimum value was 1 μm and the maximum value was 100 μm). The same applies to the diameter.

[0097]

[0098] Although the embodiments and examples of the present disclosure have been described above, it is intended from the beginning that the configurations of the above-described embodiments and examples may be appropriately combined or modified in various ways. The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims, not by the above-described embodiments and examples, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims.

[0099] 1 Carbon nanotube, 2 Core-shell nanoparticle, 3 Carbon nanotube composite assembly wire, 10 Carbon nanotube manufacturing apparatus, 11 Electric furnace, 11a First electric furnace, 11b Second electric furnace, 12 Quartz tube, 12a First region, 12b Second region, 12c First end, 12d Second end, 12e Radiant heat receiving region, 12f Electric furnace heating region, 13 Spray.

Claims

1. A method for producing carbon nanotubes, comprising the steps of: preparing a carbon-containing gas, ferrocene, and a sulfur-containing auxiliary catalyst; charging the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst into a reactor and heating them to a first temperature of 600°C or higher and 1000°C or lower, and maintaining them at the first temperature for one second or longer to thermally decompose the carbon-containing gas, the ferrocene, and the sulfur-containing auxiliary catalyst; and reacting sulfur obtained by thermal decomposition of the sulfur-containing auxiliary catalyst with the surfaces of iron nanoparticles obtained by thermal decomposition of the ferrocene to form core-shell nanoparticles having an iron core and a shell containing iron and sulfur covering the core; and obtaining carbon nanotubes by heating the core-shell nanoparticles and carbon obtained by thermal decomposition of the carbon-containing gas to a second temperature of 1200°C or higher and 1500°C or lower, thereby growing carbon nanotubes from the core-shell nanoparticles.

2. The preparing step includes a step of mixing the ferrocene and the sulfur-containing auxiliary catalyst with an organic solvent to obtain a catalyst raw material-containing solution, and in the step of forming the core-shell nanoparticles, the ferrocene and the sulfur-containing auxiliary catalyst are introduced into the reactor as the catalyst raw material-containing solution, and in the catalyst raw material-containing solution, the number of iron atoms N Ｆｅ and the number of sulfur atoms, N Ｓ The ratio of Ｆｅ :N Ｓ 2. The method for producing carbon nanotubes according to claim 1, wherein the ratio of the molten metal to the unmolded metal is 1:2 to 1:

8.

3. The method for producing carbon nanotubes according to claim 1 or 2, wherein the first temperature is 850°C or higher and 950°C or lower, and the holding time at the first temperature is 1 second or higher and 60 seconds or lower.

4. A method for producing carbon nanotubes described in any one of claims 1 to 3, wherein in the core-shell nanoparticles, the percentage of the number of sulfur atoms relative to the total number of iron and sulfur atoms is 6% or more and 45% or less.

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