JP7631664B2 - Method for producing carbon nanotubes - Google Patents

Description

The present invention relates to a method for producing carbon nanotubes, and in particular to a method for efficiently producing high-quality carbon nanotubes.

Carbon nanotubes (hereafter sometimes referred to as "CNTs") have excellent properties such as mechanical strength, optical properties, electrical properties, thermal properties, and molecular adsorption capacity, and are expected to be used as functional materials for electronic devices, optical elements, conductive materials, etc.

Here, chemical vapor deposition (hereinafter sometimes referred to as "CVD") is known as one method for producing CNTs. This CVD method is characterized by synthesizing CNTs by contacting a carbon compound, which is the raw material, with a catalyst consisting of metal particles in a high-temperature atmosphere. The CVD method has attracted attention as a production method that allows a high degree of freedom in the CNT production conditions (e.g., the type or arrangement of the catalyst, the type of carbon compound, or reaction conditions, etc.) and is capable of producing both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).

Among the methods for producing CNTs using the CVD method, the method of synthesizing CNTs using a substrate carrying a catalyst layer made of a catalyst for CNT synthesis (hereinafter sometimes referred to as a "substrate for CNT production") has attracted particular attention because it allows the mass production of many CNTs that are aligned perpendicularly to the substrate for CNT production. For this reason, various proposals have been made regarding the method of producing CNTs by the CVD method using a substrate for CNT production, with the aim of reducing production costs, etc.

Specifically, for example, Patent Documents 1 and 2 propose a technology to reduce the manufacturing cost of CNTs by recovering the CNTs formed on the CNT generating substrate and then reusing the used CNT generating substrate for synthesizing CNTs.

JP 2006-27948 A JP 2007-91485 A

However, the conventional CNT manufacturing method described above, which produces CNTs while repeatedly reusing used CNT production substrates, leaves room for improvement in terms of efficiently producing high-quality CNTs.

The present invention aims to provide a method for producing carbon nanotubes that enables efficient production of high-quality carbon nanotubes.

The inventors have conducted extensive research to achieve the above object. The inventors have discovered that when manufacturing CNTs through a first forming step (A) of forming carbon nanotubes on a catalyst layer of a carbon nanotube generating substrate, a peeling step (B) of peeling off the carbon nanotubes formed on the catalyst layer, and a second forming step (C) of forming carbon nanotubes again on the catalyst layer from which the carbon nanotubes were peeled off, if the carbon atom concentration on the surface of the catalyst layer from which the carbon nanotubes were peeled off in the peeling step (B) is less than a predetermined value, high-quality carbon nanotubes can be efficiently manufactured, and have completed the present invention.

That is, the present invention aims to advantageously solve the above problems, and the carbon nanotube manufacturing method of the present invention is a carbon nanotube manufacturing method for manufacturing carbon nanotubes using a carbon nanotube generating substrate comprising a substrate and a catalyst layer provided on the substrate, the carbon nanotube manufacturing method includes a first forming step (A) of forming carbon nanotubes on the catalyst layer, a peeling step (B) of peeling the carbon nanotubes formed on the catalyst layer, and a second forming step (C) of forming carbon nanotubes again on the catalyst layer from which the carbon nanotubes have been peeled, characterized in that the carbon atom concentration on the surface of the catalyst layer from which the carbon nanotubes have been peeled in the peeling step (B) is less than 50 atomic %. If the carbon atom concentration on the surface of the catalyst layer from which the carbon nanotubes have been peeled in the peeling step (B) is less than 50 atomic %, high-quality carbon nanotubes can be efficiently manufactured.
In the present invention, the term "carbon nanotube generation substrate" means either a "pre-used CNT generation substrate on which CNT formation has not been performed even once" or a "used CNT generation substrate after CNT formation has been performed on a pre-used CNT generation substrate (including a used CNT generation substrate after further CNT formation has been performed on a used CNT generation substrate)."
In the present invention, the "carbon atom concentration" can be measured by the method described in the Examples below.

Here, in the carbon nanotube manufacturing method of the present invention, it is preferable that in at least one of the first formation step (A) and the second formation step (C), the carbon nanotubes are formed using a mixed gas containing a carbon nanotube raw material gas and hydrogen. By forming the carbon nanotubes using a mixed gas containing a carbon nanotube raw material gas and hydrogen in the first formation step (A) and/or the second formation step (C), high-quality carbon nanotubes can be manufactured more efficiently.

Furthermore, it is preferable that the carbon nanotube manufacturing method of the present invention further includes a cleaning step (D) of cleaning the catalyst layer from which the carbon nanotubes have been peeled off in the peeling step (B). By cleaning the catalyst layer from which the carbon nanotubes have been peeled off, it is possible to efficiently manufacture carbon nanotubes of higher quality.

The carbon nanotube manufacturing method of the present invention preferably further includes an oxidation treatment step (E) of oxidizing the catalyst layer from which the carbon nanotubes have been peeled off in the peeling step (B). By oxidizing the catalyst layer from which the carbon nanotubes have been peeled off, it is possible to efficiently manufacture higher quality carbon nanotubes.

In addition, the carbon nanotube manufacturing method of the present invention preferably further includes a polishing process in which the catalyst layer from which the carbon nanotubes have been peeled is polished in the peeling process (B). Such a polishing process is preferably carried out after the cleaning process (D). By polishing the catalyst layer from which the carbon nanotubes have been peeled, carbon nanotubes of higher quality can be efficiently manufactured.

The carbon nanotube manufacturing method of the present invention allows for efficient production of high-quality carbon nanotubes.

Hereinafter, an embodiment of the present invention will be described in detail.
In the carbon nanotube manufacturing method of the present invention, carbon nanotubes (CNTs) are synthesized on the catalyst layer of a carbon nanotube generating substrate including a substrate and a catalyst layer provided on the substrate, and the synthesized CNTs are peeled off from the catalyst layer to obtain CNTs. In the CNT manufacturing method of the present invention, used CNT generating substrates (hereinafter sometimes referred to as "used substrates") are reused from the viewpoint of producing CNTs at low cost.

Specifically, the carbon nanotube manufacturing method of the present invention is a CNT manufacturing method that uses a CNT production substrate having a catalyst layer formed on a substrate, and includes a first formation step (A) of forming CNTs on the catalyst layer, a peeling step (B) of peeling off the CNTs formed on the catalyst layer, and a second formation step (C) of forming CNTs again on the catalyst layer from which the CNTs have been peeled off, and optionally further includes a cleaning step (D) of cleaning the catalyst layer from which the CNTs have been peeled off in the peeling step (B) and/or an oxidation treatment step (E) of oxidizing the catalyst layer from which the CNTs have been peeled off. In the CNT manufacturing method of the present invention, CNTs can be efficiently manufactured while reusing used substrates by repeatedly forming and peeling off CNTs.

<Base material for CNT generation>
As described above, the "substrate for CNT generation" in the present invention may be either a "substrate for CNT generation before use" or a "substrate for CNT generation after use."
The pre-used CNT generating substrate is obtained by forming a catalyst layer on a substrate (sometimes called a "virgin substrate") before it is used for CNT synthesis. Note that a base layer such as a carburization prevention layer or a catalyst support layer may be provided between the substrate and the catalyst layer. In other words, the catalyst layer may be formed on the substrate via any layer, i.e., it may be formed on the surface of the substrate, or it may be formed on the surface of any layer such as a base layer.

[Base material]
Here, any substrate can be used as long as it is possible to support a catalyst for CNT synthesis on its surface. Specifically, any substrate that has a proven track record in the manufacture of CNTs can be used as the substrate. It is preferable that the substrate can maintain its shape even at high temperatures of 400° C. or higher.

Examples of materials for the substrate include metals such as iron, nickel, chromium, molybdenum, tungsten, titanium, aluminum, manganese, cobalt, copper, silver, gold, platinum, niobium, tantalum, lead, zinc, gallium, indium, germanium, and antimony, as well as alloys and oxides containing these metals, or nonmetals such as silicon, quartz, glass, mica, graphite, and diamond, as well as ceramics. Among these, metals are preferred because they are low cost and easy to process compared to silicon and ceramics, and Fe-Cr (iron-chromium) alloys, Fe-Ni (iron-nickel) alloys, and Fe-Cr-Ni (iron-chromium-nickel) alloys are particularly suitable.

The shape of the substrate can be flat, thin, block or particulate, and from the perspective of mass production of CNTs, flat and particulate shapes are particularly advantageous because they provide a large surface area relative to the volume.

Here, when a flat substrate is used, there is no particular limit to the thickness of the substrate, and for example, a thin film of about several μm to about several cm can be used. Preferably, the thickness of the substrate is 0.05 mm to 3 mm. If the thickness of the substrate is 3 mm or less, the substrate can be sufficiently heated when synthesizing CNTs, and the occurrence of poor growth of CNTs can be suppressed. In addition, the cost of the substrate can be reduced. On the other hand, if the thickness of the substrate is 0.05 mm or more, deformation of the substrate due to carburization during CNT synthesis can be suppressed, and the substrate itself is less likely to bend, which is advantageous for transporting and reusing the substrate. In this specification, "carburization" refers to the penetration of carbon components into the substrate.

There is no particular restriction on the shape and size of the flat substrate, but a rectangular or square shape can be used. There is no particular restriction on the length of one side of the substrate, but from the perspective of mass production of CNTs, the longer the side, the more desirable it is.

[Carburization prevention layer]
Here, particularly when a flat substrate is used, the substrate may have a carburization prevention layer as an underlayer on at least one of the front and back surfaces, and the substrate preferably has a carburization prevention layer on both the front and back surfaces. The carburization prevention layer is a protective layer for preventing the substrate from being carburized and deformed during synthesis of CNTs.

The carburization prevention layer is preferably made of a metal or ceramic material, and is particularly preferably made of a ceramic material that has a high carburization prevention effect. Examples of metals include copper and aluminum. Examples of ceramic materials include oxides such as aluminum oxide, silicon oxide, zirconium oxide, magnesium oxide, titanium oxide, silica alumina, chromium oxide, boron oxide, calcium oxide, and zinc oxide, and nitrides such as aluminum nitride and silicon nitride. Of these, aluminum oxide and silicon oxide are preferred as materials for the carburization prevention layer because of their high carburization prevention effect.

The thickness of the carburization prevention layer is preferably 0.01 μm or more and 1.0 μm or less. If the thickness of the carburization prevention layer is 0.01 μm or more, the carburization prevention effect can be sufficiently obtained. On the other hand, if the thickness of the carburization prevention layer is 1.0 μm or less, the thermal conductivity of the base material is suppressed from changing, and the base material can be sufficiently heated during CNT synthesis to allow CNTs to grow well. The carburization prevention layer can be formed (coated) by physical methods such as vapor deposition and sputtering, CVD, painting, etc.

[Catalyst supporting layer]
The catalyst support layer is a base layer other than the carburization prevention layer, and serves as a base layer for the catalyst for CNT synthesis. As the material for the catalyst support layer, various materials can be used as long as they serve as a base layer for the catalyst for CNT synthesis, and for example, ceramic materials such as alumina, titania, titanium nitride, and silicon oxide are preferably used. Among them, it is preferable to use a ceramic material as the material for the catalyst support layer. This is because the CNTs grow better when the substrate is reused to synthesize the CNTs.

The thickness of the catalyst support layer is preferably 10 nm or more from the viewpoint of stable CNT growth and improved yield, and is preferably 30 nm or less from the viewpoint of production efficiency.

[Catalyst layer]
The catalyst layer provided on the above-mentioned substrate is a layer containing a catalyst for CNT synthesis such as a metal. The catalyst for CNT synthesis is preferably made to contain fine particles of the catalyst for CNT synthesis by performing a reduction treatment (formation step described later) with hydrogen or the like before CNT synthesis. The catalyst layer can be formed, for example, by applying a catalyst solution for CNT synthesis. Here, as the catalyst for CNT synthesis constituting the catalyst layer, for example, a catalyst that has a proven track record in the manufacture of CNTs can be appropriately used. Specifically, iron, nickel, cobalt, and molybdenum, as well as chlorides and alloys thereof, can be exemplified as catalysts for CNT synthesis. Here, the amount of catalyst for CNT synthesis used to form the catalyst layer can be, for example, an amount that has a proven track record in the manufacture of CNTs.

The catalyst layer may be formed on the substrate by either a wet process or a dry process. Specifically, a sputtering deposition method or a method of applying and baking a liquid in which metal particles are dispersed in an appropriate solvent may be used. The catalyst layer may also be formed into any shape by combining this with patterning using well-known photolithography or nanoimprinting. By adjusting the shape of the catalyst layer formed on the substrate and the growth time of the CNTs, it is possible to obtain CNT aligned aggregates in various shapes, such as thin films, cylinders, and prisms.

When a flat substrate is used, the catalyst layer may be formed on both the front and back sides of the substrate, or on only one side. By forming a catalyst layer on both the front and back sides of the substrate, CNTs can be grown on both the front and back sides of the substrate, improving production efficiency. Of course, it is possible to provide a catalyst layer on only one side of the substrate depending on production costs, production process convenience, etc.

The thickness of the catalyst layer is preferably 0.1 nm or more and 100 nm or less, more preferably 0.5 nm or more and 10 nm or less, and particularly preferably 0.8 nm or more and 3.0 nm or less.

<First formation step (A)>
In the first forming step (A), carbon nanotubes are formed on the catalyst layer. Here, in the first forming step (A), CNTs may be formed on the catalyst layer of a pre-used CNT generating substrate that has not been subjected to CNT formation even once, or CNTs may be formed on the catalyst layer of a used CNT generating substrate that has been subjected to at least one CNT formation. When using a used CNT generating substrate (used substrate) to manufacture CNTs, a new catalyst layer may be formed on the catalyst layer (used catalyst layer) of the used substrate by the catalyst layer forming step described later, or the catalyst layer (used catalyst layer) of the used substrate may be used as the catalyst layer without forming a new catalyst layer.
The method of forming CNTs on the catalyst layer is not particularly limited, but a method of growing CNTs by chemical vapor deposition (CVD method) is preferred in which at least one of the CNT generating substrate and the mixed gas containing the CNT raw material gas and hydrogen is heated in a state where the surrounding environment of the CNT generating substrate is an environment where the mixed gas is present. According to this method, CNTs can be grown on the CNT generating substrate with high efficiency, and a CNT aligned aggregate in which a large number of CNTs grown from the catalyst are aligned in a specific direction can be easily formed.
In addition, as described above, by forming CNTs using a mixed gas containing hydrogen in addition to the CNT raw material gas, the carbon atom concentration on the surface of the catalyst layer after the formed CNTs are peeled off can be reduced.

More specifically, the formation of CNTs on the catalyst layer in the first formation process (A) is not particularly limited, and can be carried out by sequentially carrying out a formation process in which the catalyst for CNT synthesis in the catalyst layer is reduced, a growth process in which CNTs are grown on the catalyst layer, and a cooling process in which the substrate on which the CNTs have been grown is cooled.

[Formation process]
In the formation step, the surrounding environment of the CNT generating substrate for forming CNTs is changed to an environment containing a reducing gas, and then at least one of the CNT generating substrate and the reducing gas is heated to reduce and microparticulate the CNT synthesis catalyst of the catalyst layer. This formation step produces at least one of the effects of reducing the CNT synthesis catalyst, promoting the microparticulation of the CNT synthesis catalyst, and improving the activity of the CNT synthesis catalyst. The temperature of the CNT generating substrate and/or the reducing gas in the formation step is preferably 400°C or higher and 1100°C or lower. In addition, the implementation time of the formation step is preferably 3 minutes or higher and 30 minutes or lower, and more preferably 3 minutes or higher and 8 minutes or lower. If the implementation time of the formation step is within this range, the catalyst particles are prevented from becoming coarse, and the generation of multi-layered CNTs can be suppressed.

The reducing gas may be, for example, hydrogen gas, ammonia gas, water vapor, or a mixture of these. The reducing gas may also be a mixture of these gases and an inert gas such as helium gas, argon gas, or nitrogen gas. The reducing gas is generally used in the formation process, but may also be used in the growth process as appropriate.

[Growth process]
In the growth process, the surrounding environment of the CNT production substrate is changed to one in which a mixed gas containing the CNT raw material gas and hydrogen is present, and at least one of the CNT production substrate and the mixed gas is heated to grow CNTs on the reduced and microparticulated catalyst by chemical vapor deposition.

Here, the raw material gas for CNTs in the mixed gas is a gas containing a substance that is the raw material for CNTs, for example, a gas having a raw carbon source at a temperature for growing CNTs. Among these, hydrocarbon gases such as methane, ethane, ethylene, propane, butane, pentane, hexane, heptane, propylene, and acetylene are preferred. In addition, lower alcohol gases such as methanol and ethanol may also be used. Mixtures of these can also be used.

The mixing ratio of hydrogen in the mixed gas (volume flow rate of hydrogen/volume flow rate of the entire mixed gas) is preferably 1 vol% or more and 30 vol% or less, and particularly preferably 5 vol% or more and 15 vol% or less.
When the mixing ratio of hydrogen in the mixed gas is within the above range, the carbon atom concentration on the surface of the catalyst layer after peeling off the formed CNTs can be reduced, and high-quality carbon nanotubes can be efficiently produced.

The mixed gas may be diluted with an inert gas. The inert gas is a gas that is inert at the temperature at which CNTs grow, does not reduce the activity of the catalyst, and does not react with the growing CNTs. Examples of the inert gas include nitrogen gas; rare gases such as helium gas, argon gas, neon gas, and krypton gas; and mixed gases thereof. In particular, nitrogen gas, helium gas, argon gas, and mixed gases thereof are preferable.

Moreover, it is more preferable that the surrounding environment of the carbon nanotube generating substrate in the growth process further contains a catalyst activation material that has the effect of increasing the activity of the catalyst and extending the active life of the catalyst (catalytic activation effect). The addition of the catalyst activation material can further improve the production efficiency and purity of the CNT. As the catalyst activation material, for example, a material containing oxygen that does not cause significant damage to the CNT at the growth temperature of the CNT is preferable. Specifically, as the catalyst activation material, for example, oxygen-containing compounds with low carbon numbers such as water, oxygen, ozone, acid gas, nitrogen oxide, carbon monoxide, and carbon dioxide; alcohols such as ethanol and methanol; ethers such as tetrahydrofuran; ketones such as acetone; aldehydes; esters; and mixtures thereof are effective. Among these, water, oxygen, carbon dioxide, carbon monoxide, and ethers are preferable, and water and carbon dioxide are particularly suitable.

Note that substances that contain both carbon and oxygen, such as carbon monoxide and alcohols, can function as both a feed gas and a catalyst activator. For example, carbon monoxide acts as a catalyst activator when combined with a more reactive feed gas, such as ethylene, and acts as a feed gas when combined with a catalyst activator that exhibits significant catalytic activity even in small amounts, such as water.

The temperature to which at least one of the CNT generating substrate and the mixed gas is heated may be any temperature at which CNTs can grow, but is preferably 400°C or higher and 1100°C or lower. A temperature of 400°C or higher allows the effect of the catalytic activation material to be fully expressed, while a temperature of 1100°C or lower prevents the catalytic activation material from reacting with the CNTs.

[Cooling process]
In the cooling step, the CNT generating substrate on which the CNTs have been grown is cooled in an inert gas environment, and the CNTs and the CNT generating substrate in a high temperature state after the growth step are prevented from being oxidized in an oxygen-containing environment. Here, the inert gas may be the same as the inert gas that may be used in the growth step. In addition, in the cooling step, the temperature of the CNT generating substrate on which the CNTs have been grown is preferably reduced to 400° C. or less, more preferably 200° C. or less.

[Characteristics of CNT]
In the first formation step (A) or the second formation step (C) described below, CNTs are usually formed as aligned CNT aggregates on the catalyst layer of the CNT production substrate, and it is preferable that the CNTs have, for example, the following properties:
That is, the preferred BET specific surface area of the aligned CNT aggregate is 800 m ² /g or more, and more preferably 1000 m ² /g or more, when the CNTs are mainly unopened. The higher the BET specific surface area, the more the amount of impurities such as metals and carbon can be suppressed to less than several tens of percent (about 40%) of the mass of the CNTs. In addition, CNTs with a large BET specific surface area are effective as catalyst carriers and energy/material storage materials, and are suitable for applications such as supercapacitors and actuators. The upper limit of the BET specific surface area is usually about 2500 m ² /g.

The weight density of the aligned CNT aggregate is preferably 0.002 g/ ^cm3 or more and 0.2 g/ ^cm3 or less. If the weight density is 0.2 g/ ^cm3 or less, the bonds between the CNTs constituting the aligned CNT aggregate are weak, so that it becomes easy to disperse the aligned CNT aggregate homogeneously when it is stirred in a solvent or the like. If the weight density is 0.002 g/ ^cm3 or more, the integrity of the aligned CNT aggregate is improved and it is prevented from falling apart, so that it becomes easy to handle.

Furthermore, it is preferable that the CNT aligned aggregate aligned in a specific direction has a high degree of orientation. Here, having a high degree of orientation means satisfying at least one of the following 1. to 3.
1. When X-rays are incident on a CNT from a first direction parallel to the longitudinal direction and a second direction perpendicular to the first direction and the X-ray diffraction intensity is measured (θ-2θ method), there exist θ angles and reflection orientations where the reflection intensity from the second direction is greater than the reflection intensity from the first direction, and there exist θ angles and reflection orientations where the reflection intensity from the first direction is greater than the reflection intensity from the second direction.
2. When X-rays are irradiated from a direction perpendicular to the longitudinal direction of a CNT and the X-ray diffraction intensity is measured (Laue method) from a two-dimensional diffraction pattern image obtained, a diffraction peak pattern indicating the presence of anisotropy appears.
3. The Herman's orientation coefficient is greater than 0 and less than 1, more preferably greater than 0.25 and less than 1, when using X-ray diffraction intensity obtained by the θ-2θ method or the Laue method.

In addition, in the X-ray diffraction of the aligned CNT aggregate, it is also preferable that the diffraction intensity of the (CP) diffraction peak and (002) peak due to the packing between the single-walled CNTs differs from the diffraction peak intensity of the (100) and (110) peaks due to the six-membered carbon ring structure that constitutes the single-walled CNTs in the parallel (first direction) and perpendicular (second direction) incidence directions.

Furthermore, in order for the CNT aligned aggregate to exhibit high orientation and a high BET specific surface area, it is preferable that the height (length) of the CNT aligned aggregate be in the range of 10 μm or more and 10 cm or less. A height of 10 μm or more improves the orientation. Furthermore, a height of 10 cm or less allows for production in a short period of time, suppressing the adhesion of carbon-based impurities and improving the BET specific surface area.

The G/D ratio of the CNT is preferably 1.5 or more, more preferably 2 or more, and even more preferably 2.5 or more. The G/D ratio of the CNT can be 100 or less, and can also be 10 or less. The G/D ratio is an index generally used to evaluate the quality of CNT. In the Raman spectrum of CNT measured by a Raman spectrometer, vibration modes called G band (near 1600 cm ⁻¹ ) and D band (near 1350 cm ⁻¹ ) are observed. The G band is a vibration mode derived from the hexagonal lattice structure of graphite, which is the cylindrical surface of the CNT, and the D band is a vibration mode derived from the amorphous part. Therefore, the higher the peak intensity ratio (G/D ratio) of the G band to the D band, the higher the crystallinity of the CNT can be evaluated.

In this CNT manufacturing method, the CNTs formed on the CNT generating substrate in the first formation process (A) are peeled off in the next peeling process (B), and the substrate is reused.

<Peeling step (B)>
In the peeling step (B), the CNT formed on the catalyst layer is peeled off from the CNT generating substrate and collected. Here, examples of the method for peeling off the formed CNT from the CNT generating substrate include physical, chemical, or mechanical peeling methods. Specifically, for example, a method of peeling off from the catalyst layer using an electric field, a magnetic field, centrifugal force, surface tension, etc., a method of mechanically peeling off directly from the catalyst layer, and a method of peeling off from the catalyst layer using pressure or heat are applicable. It is also possible to use a vacuum pump to suck the CNT and peel it off from the catalyst layer. In addition, examples of a peeling method that is simple and does not damage the CNT include a method of directly pinching the CNT with tweezers and peeling it off from the catalyst layer, and a method of peeling off the CNT from the catalyst layer using a thin blade such as a plastic spatula or cutter blade with a sharp part. Among them, a method of peeling off the CNT from the catalyst layer using a thin blade such as a plastic spatula or cutter blade with a sharp part is preferable as the peeling method.

The peeling step (B) may further include a cleaning step (D) for cleaning the catalyst layer from which the CNTs have been peeled off and/or an oxidation step (E) for oxidizing the catalyst layer from which the CNTs have been peeled off. In the CNT manufacturing method of the present invention, the carbon atom concentration on the surface of the catalyst layer from which the carbon nanotubes have been peeled off in the peeling step (B) must be less than 50 atomic %, but the carbon atom concentration can be determined by measuring the surface of the catalyst layer immediately after the CNTs have been peeled off when the cleaning step (D) and/or the oxidation step (E) are not performed, or by measuring the surface of the catalyst layer immediately after the CNTs have been peeled off and the steps are performed when the cleaning step (D) and/or the oxidation step (E) are performed. Here, "immediately after" means that no other steps are included between the previous step and the measurement of the carbon atom concentration. In the CNT manufacturing method of the present invention, the carbon atom concentration on the surface of the catalyst layer from which the carbon nanotubes have been peeled in the peeling step (B) is less than 50 atomic %, so that high-quality CNTs can be obtained even if carbon nanotubes are formed again on the catalyst layer from which the CNTs have been peeled in the second formation step (C).

<Cleaning step (D)>
In the washing step (D), the catalyst layer (the catalyst layer of the used substrate) from which the CNTs have been peeled off in the peeling step (B) is washed. Note that the washing treatment of the catalyst layer may be repeated multiple times.

Methods for cleaning the catalyst layer include, but are not limited to, washing the catalyst layer of the used substrate with water and wiping it off with a cloth, contacting the catalyst layer of the used substrate with an acid such as nitric acid, hydrochloric acid, or hydrofluoric acid, incinerating impurities on the catalyst layer of the used substrate using an oxygen plasma reactor or a UV ozone cleaner, and contacting the catalyst layer of the used substrate with a liquid containing fine bubbles with an average bubble diameter of 50 μm or less.

In addition, after the cleaning step (D), a polishing step may be carried out as a step separate from the cleaning step (D). As the polishing step, it is preferable to use a method of polishing the catalyst layer by colliding a liquid with the used substrate, and it is more preferable to use a method of physically polishing the catalyst layer of the used substrate by colliding a wet blasting slurry, which is made by dispersing an abrasive in a dispersion medium, against the surface of the catalyst layer of the used substrate. Here, the surface of the used substrate that has been subjected to the wet blasting treatment may be washed with water, wiped off with a cloth, or dried by blowing air.
In addition, the catalyst layer may remain on the used substrate after the above-mentioned polishing process, or may be removed by wet blasting. Even if the catalyst layer is removed, there will be no adverse effect on the subsequent generation of CNTs if the catalyst layer is formed again by the catalyst formation process described below.

Here, examples of the abrasive contained in the wet blasting slurry include alumina, silicon carbide, resin, glass, zirconia, stainless steel, and combinations thereof. Among these, alumina is preferred. The average particle size of the abrasive is preferably 1 μm or more, more preferably 2 μm or more, and is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less.
If the average particle size is 1 μm or more, impurities such as carbon components remaining on the used base material can be efficiently removed, and if it is 30 μm or less, the surface of the used base material after the wet blasting treatment will not be excessively roughened.
In the present invention, the "average particle size" of the abrasive refers to the particle size at 50% of the cumulative height (d _s -50 value) measured in accordance with JIS R6001 (1998, fine powder for precision polishing/electrical resistance test method).

The dispersion medium for the slurry for wet blasting is not particularly limited as long as it can disperse the abrasive, but for example, water can be used.
The concentration of the abrasive in the wet blasting slurry is preferably 5% by volume or more, more preferably 7.5% by volume or more, even more preferably 10% by volume or more, and preferably 25% by volume or less, more preferably 22.5% by volume or less, and even more preferably 20% by volume or less. If the concentration of the abrasive in the slurry is 5% by volume or more, impurities such as carbon components remaining on the used substrate can be efficiently removed, and if it is 25% by volume or less, a decrease in processing efficiency due to collision between the abrasives can be suppressed.

The wet blasting slurry can then be accelerated, for example with compressed air using a blast gun, before being struck against the catalyst layer of the used substrate.
Here, the pressure of the compressed air is not particularly limited, but is preferably 0.03 MPa or more, more preferably 0.05 MPa or more, and is preferably 0.3 MPa or less, and more preferably 0.2 MPa or less. If the pressure of the compressed air is within the above-mentioned range, the slurry containing the abrasive can be uniformly atomized, and the surface of the used substrate can be uniformly treated.

The distance from the projection nozzle of the projection gun to the surface of the used substrate is preferably 5 mm or more, more preferably 10 mm or more, even more preferably 15 mm or more, and preferably 50 mm or less, more preferably 45 mm or less, and even more preferably 40 mm or less. If the distance from the projection nozzle of the projection gun to the surface of the used substrate is within the above-mentioned range, the surface of the used substrate can be treated efficiently and uniformly.

The projection angle φ (angle with respect to the used substrate surface, 0°≦φ≦90°) of the wet blasting slurry is preferably 80° or more, more preferably 85° or more, and even more preferably 90° (i.e., perpendicular to the used substrate surface). If the projection angle φ is 80° or more, the used substrate surface can be treated efficiently and uniformly.

The type of projection gun used in the wet blasting process is not particularly limited, and examples include nozzle and wide slit projection guns.
In addition, the specific mode of the wet blasting treatment using the projection gun is not particularly limited, but for example, a preferred mode is one in which the used base material is transported in a horizontal direction and the wet blasting slurry is sprayed onto the transported used base material from a projection gun fixed above the used base material. The transport speed of the used base material is not particularly limited, but is preferably about 10 to 100 mm/sec.

<Oxidation treatment step (E)>
In the oxidation treatment step (E), the catalyst layer (the catalyst layer of the used substrate) from which the CNTs have been peeled off in the peeling step (B) is subjected to an oxidation treatment. Note that the oxidation treatment of the catalyst layer may be repeated multiple times.
The method for oxidizing the catalyst layer is not particularly limited, and examples thereof include a method in which an oxygen source (oxidizing gas) is introduced while heating the catalyst layer of the used substrate, and carbon present on the surface of the catalyst layer of the used substrate is converted into carbon dioxide.

<Second Forming Step (C)>
In the second formation step (C), CNTs are formed on the catalyst layer from which the CNTs have been peeled off. That is, in the second formation step (C), CNTs are formed on the catalyst layer of the used CNT production substrate on which at least one CNT synthesis has been performed. The method of forming CNTs on the catalyst layer is the same as that of the first formation step (A) described above.
Here, before forming CNTs on the catalyst layer from which the CNTs have been peeled off, a catalyst support layer formation step of newly forming a catalyst support layer on the catalyst layer from which the CNTs have been peeled off may be included, or a catalyst layer formation step of newly forming a catalyst layer on the catalyst layer from which the CNTs have been peeled off or on the newly formed catalyst support layer may be included.

[Catalyst supporting layer forming step]
The catalyst support layer may be formed on the catalyst layer from which the CNTs have been peeled off by either a wet process or a dry process (such as a sputtering deposition method). From the viewpoints of the simplicity of the film formation apparatus, the high throughput, and the low cost of raw materials, it is preferable to use a wet process.

Hereinafter, as an example, a case where a catalyst support layer is formed by a wet process will be described.
The wet process for forming the catalyst support layer comprises a step of applying a coating solution A, which is prepared by dissolving a metal organic compound and/or a metal salt containing the element to be the catalyst support layer in an organic solvent, onto a substrate, and a subsequent heating step. A stabilizer may be added to the coating solution A to suppress excessive condensation polymerization reaction of the metal organic compound and the metal salt.

Here, for example, when an alumina membrane is used as the catalyst support layer, aluminum alkoxides such as aluminum trimethoxide, aluminum triethoxide, aluminum tri-n-propoxide, aluminum tri-i-propoxide, aluminum tri-n-butoxide, aluminum tri-sec-butoxide, and aluminum tri-tert-butoxide can be used as the metal organic compound and/or metal salt for forming the alumina membrane. Other examples of the metal organic compound for forming the alumina membrane include complexes such as tris(acetylacetonato)aluminum(III). Examples of metal salts include aluminum sulfate, aluminum chloride, aluminum nitrate, aluminum bromide, aluminum iodide, aluminum lactate, basic aluminum chloride, and basic aluminum nitrate. Among these, it is preferable to use aluminum alkoxides. These can be used alone or as a mixture of two or more.

As the stabilizer, it is preferable to use at least one selected from the group consisting of β-diketones and alkanolamines. These compounds may be used alone or in combination of two or more. As β-diketones, acetylacetone, methyl acetoacetate, ethyl acetoacetate, benzoylacetone, dibenzoylmethane, benzoyltrifluoroacetone, furoylacetone, trifluoroacetylacetone, etc. may be used, but it is particularly preferable to use acetylacetone and ethyl acetoacetate. As alkanolamines, it is possible to use monoethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, N,N-dimethylaminoethanol, diisopropanolamine, triisopropanolamine, etc., but it is preferable to use secondary or tertiary alkanolamine.

As the organic solvent, various organic solvents such as alcohol, glycol, ketone, ether, esters, and hydrocarbons can be used, but it is preferable to use alcohol or glycol because of their good solubility for metal organic compounds and metal salts. These organic solvents may be used alone or in combination of two or more. As the alcohol, methanol, ethanol, isopropyl alcohol, etc. are preferable in terms of handling and storage stability.

The amount of the metal organic compound and/or metal salt in Coating Solution A is not particularly limited, but is preferably 0.1 g or more, more preferably 0.5 g or more, and preferably 30 g or less, more preferably 5 g or less, per 100 mL of organic solvent.
The amount of stabilizer in Coating Solution A is not particularly limited, but is preferably 0.01 g or more, more preferably 0.1 g or more, and preferably 20 g or less, more preferably 3 g or less, per 100 mL of organic solvent.

Coating solution A may be applied by any method, including spraying, brushing, spin coating, dip coating, etc., but dip coating is preferred from the standpoint of productivity and film thickness control.

Heating after application of Coating Liquid A can be performed at a temperature range of 50°C to 400°C for a period of 5 minutes to 3 hours depending on the type of catalyst-supported layer. Heating initiates hydrolysis and condensation polymerization reactions of the applied metal organic compound and/or metal salt, and a cured coating (catalyst-supported layer) containing metal hydroxide and/or metal oxide is formed on the surface of the catalyst layer from which the CNTs have been peeled off.

[Catalyst layer formation process]
The catalyst layer may be formed by either a wet process or a dry process (such as a sputtering deposition method) in the same manner as the catalyst support layer. From the viewpoints of the simplicity of the film forming apparatus, the high throughput, and the low cost of raw materials, it is preferable to use a wet process.

Hereinafter, as an example, a case where a catalyst layer is formed by a wet process will be described.
The wet process for forming the catalyst layer consists of a step of applying a coating solution B, which is made by dissolving a metal organic compound and/or a metal salt containing an element that will be a catalyst for CNT synthesis in an organic solvent, onto a substrate, and a subsequent heating step. A stabilizer may be added to the coating solution B to suppress excessive condensation polymerization reactions of the metal organic compound and the metal salt.

Here, for example, when iron is used as a catalyst for CNT synthesis, the metal organic compound and/or metal salt for forming the iron thin film that becomes the catalyst layer can be iron pentacarbonyl, ferrocene, iron acetylacetonate (II), iron acetylacetonate (III), iron trifluoroacetylacetonate (II), iron trifluoroacetylacetonate (III), etc. Examples of the metal salt include inorganic iron acids such as iron sulfate, iron nitrate, iron phosphate, iron chloride, and iron bromide, and organic iron acids such as iron acetate, iron oxalate, iron citrate, and iron lactate. Of these, it is preferable to use organic iron acids. These can be used alone or as a mixture of two or more types.

The stabilizer and organic solvent for coating solution B may be the same as those for coating solution A. The content of these may also be the same as those for coating solution A.

Furthermore, the coating method for coating liquid B can be the same as that for coating liquid A. In addition, heating after coating coating liquid B can be performed in the same manner as for coating liquid A.

This catalyst layer formation process results in the formation of a used substrate with a new catalyst layer formed on its surface.

The total thickness of the newly formed catalyst support layer and the catalyst layer is preferably 5.0 nm or more, and more preferably 10 nm or more.
In addition, the total thickness of the catalyst support layer and the catalyst layer formed on the substrate (when a used substrate is reused to form a new catalyst support layer and catalyst layer on the used catalyst layer, the total thickness of the used catalyst support layer and catalyst layer and the total thickness of the newly formed catalyst support layer and catalyst layer) is preferably more than 10 nm, more preferably 15 nm or more. In addition, the total thickness of the catalyst support layer and the catalyst layer can be 3 μm or less.

<Carbon atom concentration>
The carbon atom concentration on the catalyst layer surface before forming the CNTs in the first formation step (A) is preferably less than 50 atomic %, more preferably 40 atomic % or less, and particularly preferably 25 atomic % or less. When the carbon atom concentration is in the preferred range, the quality and yield of the obtained CNTs can be further improved.

After the first formation step (A), the carbon atom concentration on the surface of the catalyst layer from which the CNTs have been peeled off in the peeling step (B) must be less than 50 atomic %, preferably 48 atomic % or less, and more preferably 45 atomic % or less. If the carbon atom concentration is in the preferred range, the quality and yield of the obtained CNTs can be further improved.

When the "carbon atom concentration on the surface of the catalyst layer of a pre-used CNT production substrate that has not yet been used for CNT formation even once" is "A0" (e.g., "A0" in Example 1 described below) and the "carbon atom concentration on the surface of the catalyst layer of a used CNT production substrate after the nth CNT formed has been peeled off" is "An" (e.g., "A1 to A3" in Example 1 described below), it is preferable that the carbon atom concentration ratio (An/A0) is less than 2. If the carbon atom concentration ratio (An/A0) is within the above range, the quality and yield of the obtained carbon nanotubes can be maintained at a good level.

The "carbon atom concentration A0 on the surface of the catalyst layer of a pre-use CNT generation substrate that has not been subjected to CNT formation even once" (carbon atom concentration A0 on the surface of the catalyst layer before CNT formation) is preferably less than 50 atomic %, more preferably 40 atomic % or less, and particularly preferably 25 atomic % or less. If the "carbon atom concentration A0 on the surface of the catalyst layer of a pre-use CNT generation substrate that has not been subjected to CNT formation even once" is within the above range, the quality and yield of the obtained CNTs can be further improved.

Continuous Production of CNTs
In the case of manufacturing CNTs while reusing the used substrate, the formation and peeling of CNTs are repeated. In this manner, by reusing the used substrate for the formation of CNTs, the manufacturing cost of CNTs can be further reduced.

Here, when CNT formation and peeling are repeatedly performed, the carbon atom concentration on the surface of the part where the catalyst layer is newly formed when the Xth CNT is formed (where X is an integer of 2 or more) is generally approximately equal to the carbon atom concentration on the surface of the catalyst layer after the X-1th CNT formation and peeling is performed, taking into consideration that a cleaning process (D) etc. is performed in the peeling process (B). For example, the carbon atom concentration on the surface of the part where the catalyst layer is newly formed when the second CNT is formed is generally approximately equal to the carbon atom concentration on the surface of the catalyst layer after the first CNT formation and peeling is performed.

The number of times that CNT formation and peeling are repeated, i.e., the number of times that the CNT generating substrate is reused, is preferably 50 times or more from the viewpoint of reducing costs, and since more is preferable, there is no particular limit, but it is usually 100 times or less. If the number of times that the CNT generating substrate is reused is 100 times or less, deformation of the CNT generating substrate can be suppressed, and the transportability (handling ability) of the CNT generating substrate can be improved.

<CNT Yield>
The yield of CNTs is preferably 1.0 mg/cm ² or more, more preferably 1.2 mg/cm ² or more, and particularly preferably 1.5 mg/cm ² or more.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.
In the examples and comparative examples, the BET specific surface area and carbon atom concentration of the CNTs were evaluated using the following methods.

<BET specific surface area>
The measurement was performed using a BET specific surface area measuring device (HM model-1210, manufactured by Mountec Co., Ltd.).
<Carbon atom concentration>
The carbon atom concentration was measured using X-ray photoelectron spectroscopy (XPS) (Ulvac-PHI, PHI-5000 VersaP), which provides detailed information on elements present within a few nanometers of the surface.
The measurement was performed using a 300 nm fluororesin (300 nm fluororesin) and a 300 nm fluororesin (300 nm fluororesin) . The X-rays were monochromated Al-Kα, with a beam diameter of 100 μm and a beam output of 25 W-15 kV. Three pieces of 1 cm x 1 cm were cut out from the substrate (50 cm x 50 cm: thickness 0.6 mm) used in the CNT synthesis to prepare measurement samples, and wide and narrow spectra were obtained at two different points for each sample. The target limits were C, O, N, Al, and Fe present near the surface, and the obtained spectra were calibrated for charge correction, background processing, and peak smoothing processing were performed. The relative carbon atom concentration of the target element was calculated using a semi-quantitative method based on integral value comparison taking into account the relative sensitivity coefficient, and the average value of six measurements was taken as the carbon atom concentration (atomic%) on the substrate surface.

Example 1
<First synthesis of CNT>
[Preparation of virgin CNT generation substrate]
1.9 g of aluminum tri-sec-butoxide, an aluminum compound, was dissolved in 100 mL of 2-propanol, an organic solvent, and 0.9 g of triisopropanolamine, a stabilizer, was further added and dissolved to prepare coating solution A.
Furthermore, 174 mg of iron acetate, an iron compound, was dissolved in 100 mL of 2-propanol, an organic solvent, and 190 mg of triisopropanolamine, a stabilizer, was added and dissolved to prepare Coating Liquid B.
An Fe-Cr alloy SUS430 substrate (manufactured by JFE Steel Corporation, 50 cm x 50 cm, thickness 0.3 mm, Cr 18%) was prepared as a substrate.
The above-mentioned coating liquid A was applied to the surface of the prepared substrate by dip coating in an environment of room temperature 25° C. and relative humidity 50%. Specifically, the substrate was immersed in the coating liquid A, held for 20 seconds, and then pulled up at a pulling speed of 10 mm/sec. Thereafter, the substrate was air-dried for 5 minutes, heated in an air environment at 300° C. for 30 minutes, and then cooled to room temperature to form an alumina thin film with a thickness of 14 nm on the substrate as a catalyst support layer, which is a base layer.
Next, in an environment of room temperature 25° C. and relative humidity 50%, the above-mentioned coating liquid B was applied by dip coating onto the alumina thin film provided on the substrate. Specifically, the substrate having the alumina thin film on the substrate was immersed in the coating liquid B, and then held for 20 seconds, and the substrate having the alumina thin film was pulled up at a pulling speed of 3 mm/sec. Thereafter, the substrate was air-dried for 5 minutes (drying temperature 45° C.) to form a 1 nm-thick iron thin film as a catalyst layer on the substrate, thereby obtaining an unused CNT generating base material.
Then, the carbon atom concentration (A0) of the surface of the catalyst layer formed on the surface of the unused CNT generating substrate (substrate before CNT formation) was measured. The carbon atom concentration (A0) was 23.44 (atomic %).
[Formation of CNTs]
Next, the prepared unused CNT generating substrate was placed in the reaction chamber of a CVD device maintained at a furnace temperature of 750°C and a furnace pressure of ^1.02x105 Pa, and He: 100 sccm and _H2 : 900 sccm were introduced into the reaction chamber for 6 minutes. This reduced the CNT synthesis catalyst (iron) and promoted its microparticulation, resulting in a state suitable for CNT growth (a state in which a large number of nanometer-sized catalyst microparticles were formed) (formation process). The density of the catalyst microparticles at this time was adjusted to ^1x1012 to ^1x1014 particles/ ^cm2 . Thereafter, He: 800 sccm, _C2H4 : 100 sccm, _H2 : 100 sccm, and _H2O : amounts such that the _H2O concentration was 200 ppm were supplied for ¹⁰ minutes into the reaction chamber with the furnace temperature held at 750° C. and the furnace pressure held at 1.02 _× 105 Pa. As a result, single-walled CNTs grew from the catalyst fine particles on the surface of the CNT production substrate (CNT growth process).
After the CNT growth process was completed, only He: 1000 sccm was supplied into the reaction chamber, and the remaining mixed gas (raw material gas + hydrogen gas) and catalyst activation material were removed. As a result, an aligned CNT aggregate was formed on the surface of the catalyst layer formed on the surface of the CNT production substrate (CNT formation process).
[Recovery of CNTs]
Thereafter, the CNT aligned aggregate grown on the catalyst layer was peeled off from the surface of the CNT generating substrate. Specifically, the CNT aligned aggregate was peeled off using a plastic spatula with a sharp part. During peeling, the sharp part of the spatula was placed on the boundary between the CNT aligned aggregate and the substrate, and the sharp part was moved along the surface of the CNT generating substrate so as to scrape off the CNT aligned aggregate from the CNT generating substrate. In this way, the CNT aligned aggregate was peeled off from the CNT generating substrate, and a used substrate was obtained (peeling process (B)).
The yield and specific surface area of the obtained CNTs were measured, and as shown in Table 1, the yield was 1.48 (mg/cm ² ) and the specific surface area was 1502 (m ² /g).
[Cleaning of used substrates]
The obtained used substrate was washed with water (washing step (D)), and the carbon atom concentration (A1) on the surface of the used substrate after washing with water was measured. As shown in Table 1, the carbon atom concentration (A1) was 41.44 (atomic %), which was lower than 50 (atomic %).
Thereafter, the obtained used base material was subjected to a polishing treatment by wet blasting using a wet blasting device (PFE300, manufactured by Maco Corporation) equipped with a wide slit type projection gun (polishing treatment step).
The wet blasting slurry used in the wet blasting treatment was prepared by dispersing an alumina abrasive (abrasive grains) having an average particle size of 7 μm (corresponding to particle size #2000 specified in JIS R6001 (1998, Fine Powder for Precision Polishing/Electrical Resistance Test Method)) in water as a dispersion medium. The abrasive concentration was 15% by volume. The wet blasting treatment was performed once under the following conditions on a used substrate transported horizontally under a blast gun at 50 mm/sec.
Compressed air pressure of the projection gun: 0.09 MPa
Distance from the projection nozzle of the projection gun to the surface of the used substrate: 30 mm
Projection angle φ: 90°

<Second synthesis of CNT>
[Formation of catalyst layer]
On the catalyst layer (catalyst layer formed in the first CNT synthesis) formed on the surface of the washed used substrate, a catalyst support layer (alumina thin film) with a thickness of 14 nm was formed (catalyst support layer forming step), and a catalyst layer (iron thin film) with a thickness of 1 nm was formed (catalyst layer forming step) (number of times the used substrate was reused: 1 time). As a result, the total thickness of the catalyst support layer and the catalyst layer was 30 nm.
[Formation of CNTs]
In the same manner as in the first synthesis of CNTs, CNTs were formed on a catalyst layer (thin iron film) formed on the surface of the cleaned used substrate (CNT formation step).
[Recovery of CNTs]
In the same manner as in the first synthesis of CNTs, the aligned CNT aggregate was peeled off from the surface of the cleaned used substrate on which the CNTs had been formed, to obtain a used substrate (peeling step (B)).
The yield and specific surface area of the obtained CNTs were then measured. As shown in Table 1, the yield was 1.73 (mg/cm ² ) and the specific surface area was 1351 (m ² /g).
[Cleaning of used substrates]
The used substrate was washed in the same manner as in the first synthesis of CNTs (washing step (D)).
Then, the carbon atom concentration (A2) of the surface of the catalyst layer formed on the surface of the used substrate after the cleaning step (D) (after washing with water and before the wet blasting treatment) was measured. As shown in Table 1, the carbon atom concentration (A2) was 35.89 (atomic %).
<Third synthesis of CNT>
[Formation of catalyst layer]
On the catalyst layer (catalyst layer formed in the second synthesis of CNT) formed on the surface of the washed used substrate, a catalyst support layer (alumina thin film) with a thickness of 14 nm was formed (catalyst support layer forming step) and a catalyst layer (iron thin film) with a thickness of 1 nm was formed (catalyst layer forming step) (used substrate reused twice). As a result, the total thickness of the catalyst support layer and catalyst layer was 45 nm.
[Formation of CNTs]
In the same manner as in the first synthesis of CNTs, CNTs were formed on a catalyst layer (thin iron film) formed on the surface of the cleaned used substrate (CNT formation step).
[Recovery of CNTs]
In the same manner as in the first synthesis of CNTs, the aligned CNT aggregate was peeled off from the surface of the cleaned used substrate on which the CNTs had been formed, to obtain a used substrate (peeling step (B)).
The yield and specific surface area of the obtained CNTs were then measured. As shown in Table 1, the yield was 1.76 (mg/cm ² ) and the specific surface area was 1364 (m ² /g).
[Cleaning of used substrates]
The used substrate was washed in the same manner as in the first synthesis of CNTs (washing step (D)).
Then, the carbon atom concentration (A3) of the surface of the catalyst layer formed on the surface of the used substrate after the cleaning step (D) (after water washing and before wet blasting) was measured. As shown in Table 1, the carbon atom concentration (A3) was 43.32 (atomic %).

Example 2
<First synthesis of CNT>
Except for using only the raw material gas (no hydrogen) instead of using a mixed gas containing hydrogen in the formation of CNT, the formation of the catalyst support layer, catalyst layer and CNT, recovery of CNT, cleaning of the used substrate, the yield and specific surface area of CNT, and the carbon atom concentration (A0) of the catalyst layer surface formed on the surface of an unused CNT generating substrate (substrate before CNT formation) were measured in the same manner as in the first CNT synthesis of Example 1. The carbon atom concentration (A0) was 25.61 (atomic %). As shown in Table 2, the CNT yield was 1.96 (mg/ ^cm2 ) and the specific surface area was 1310 ( ^m2 /g).
[Oxidation treatment of cleaned substrate]
Next, the cleaned substrate was placed in a thermostatic chamber for 1 hour under the conditions of oxidizing gas: _N2 containing 1% _O2 , and chamber temperature: 400°C, to carry out oxidation treatment (oxidation treatment step (E)), and the carbon atom concentration (A1) on the surface of the oxidized substrate was measured. As shown in Table 2, the carbon atom concentration (A1) was 40.87 (atomic %).
<Second synthesis of CNT>
Except for using only the raw material gas (no hydrogen) instead of the mixed gas containing hydrogen in the formation of CNT, the formation of the catalyst support layer, catalyst layer and CNT, recovery of CNT, cleaning of the used substrate, and measurement of the CNT yield and specific surface area were carried out in the same manner as in the second CNT synthesis of Example 1. As shown in Table 2, the CNT yield was 1.91 (mg/ ^cm2 ) and the specific surface area was 1285 ( ^m2 /g).
[Oxidation treatment of cleaned substrate]
As in the first synthesis of CNT, the washed substrate was oxidized and the carbon atom concentration (A2) on the surface of the oxidized substrate was measured. As shown in Table 2, the carbon atom concentration (A2) was 46.32 (atomic %).
<Third synthesis of CNT>
Except for using only the raw material gas (no hydrogen) instead of the mixed gas containing hydrogen in the formation of CNT, the formation of the catalyst support layer, catalyst layer and CNT, recovery of CNT, cleaning of the used substrate, and measurement of the CNT yield and specific surface area were carried out in the same manner as in the third CNT synthesis in Example 1. As shown in Table 2, the CNT yield was 1.88 (mg/ ^cm2 ) and the specific surface area was 1244 ( ^m2 /g).
[Oxidation treatment of cleaned substrate]
As in the first synthesis of CNT, the washed substrate was oxidized and the carbon atom concentration (A3) on the surface of the oxidized substrate was measured. As shown in Table 2, the carbon atom concentration (A3) was 46.79 (atomic %).

(Comparative Example 1)
<First synthesis of CNT>
Except for using only the raw material gas (no hydrogen) instead of using a mixed gas containing hydrogen in the formation of CNT, the formation of the catalyst support layer, catalyst layer and CNT, recovery of CNT, cleaning of the used substrate, measurement of the yield and specific surface area of CNT, and measurement of the carbon atom concentration (A0) of the catalyst layer surface formed on the surface of an unused CNT generating substrate (substrate before CNT formation) and the carbon atom concentration (A1) of the surface of the used substrate after the cleaning step (D) (after water washing and before wet blasting treatment) were performed in the same manner as in the first synthesis of CNT in Example 1. The carbon atom concentration (A0) was 23.44 (atomic %). As shown in Table 3, the CNT yield was 1.65 (mg/cm ² ) and the specific surface area was 1411 (m ² /g).
<Second synthesis of CNT>
Except for using only the raw material gas (no hydrogen) instead of the mixed gas containing hydrogen in the formation of CNT, the formation of the catalyst support layer, catalyst layer and CNT, recovery of CNT, cleaning of the used substrate, and measurement of the CNT yield and specific surface area were carried out in the same manner as in the second CNT synthesis of Example 1. As shown in Table 3, the CNT yield was 0.53 (mg/ ^cm2 ) and the specific surface area was 635 ( ^m2 /g).

As shown in Tables 1 and 2, in Examples 1 and 2, in the CNT formation in the second and third CNT syntheses, the carbon atom concentrations (A1 and A2) on the surface of the used catalyst layer on the surface of the used substrate after the peeling process (B) were both less than 50 atomic %, and CNTs with a specific surface area of 800 ^m2 /g or more were obtained at a yield of 1.0 g/ ^cm2 or more.

On the other hand, as shown in Table 3, in Comparative Example 1, in the CNT formation in the second CNT synthesis, the carbon atom concentration (A1) of the surface of the used catalyst layer on the surface of the used substrate after the peeling process (B) was 50 atomic % or more, the specific surface area of the obtained CNT was less than 800 ^m2 /g, and the CNT yield was less than 1.0 g/ ^cm2 .

The results in Tables 1, 2, and 3 show that high-quality CNTs can be produced efficiently if the carbon atom concentration on the surface of the used catalyst layer from which the CNTs have been peeled off after CNT formation is less than 50 atomic %.

The CNT manufacturing method of the present invention allows for efficient production of high-quality CNTs.

Claims (2)

A method for producing carbon nanotubes, comprising the steps of: producing carbon nanotubes using a substrate for producing carbon nanotubes, the substrate comprising a substrate and a catalyst layer provided on the substrate;
A first forming step (A) of forming carbon nanotubes on the catalyst layer;
a peeling step (B) of peeling off the carbon nanotubes formed on the catalyst layer and cleaning the catalyst layer from which the carbon nanotubes have been peeled off;
a polishing step of polishing the catalyst layer after the cleaning;
a second forming step (C) of forming carbon nanotubes again on the catalyst layer from which the carbon nanotubes have been peeled off after the polishing step;
the carbon atom concentration on the surface of the catalyst layer cleaned in the peeling step (B) is less than 50 atomic %;
The second formation step (C) includes a catalyst support layer formation step of newly forming a catalyst support layer on the surface of the catalyst layer before forming carbon nanotubes, and a catalyst layer formation step of newly forming a catalyst layer on the newly formed catalyst support layer, thereby forming carbon nanotubes on the newly formed catalyst layer. 2. The method for producing carbon nanotubes according to claim 1 , wherein in at least one of the first forming step (A) and the second forming step (C), the carbon nanotubes are formed using a mixed gas containing a carbon nanotube raw material gas and hydrogen.