JP2002308610A - Method for purifying carbon nanotube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely remove catalyst metals included in carbon nanotubes by a method with less damages. SOLUTION: Carbon nanotubes(CNT) are immersed in a sulfuric acid solution. Thereby, catalyst metals included in the carbon nanotubes are dissolved and separated from the CNT. Or, immersion in an acidic solution and application of an electric field are combined. By applying an electric field, the catalyst metals converted into cations by the acidic solution are attracted to the cathode side and separated from the CNT depositing in the anode side. Then, amorphous carbon is removed by heat treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying carbon nanotubes for removing impurities contained in carbon nanotubes synthesized from a carbon material.

[0002]

2. Description of the Related Art Carbon nanotubes (hereinafter, referred to as CNTs) are cylindrical molecules composed of only carbon molecules discovered in 1991, and are usually formed by an arc discharge method, a laser ablation method, and a chemical vapor deposition method ( It is synthesized by a method such as a CVD method.

The above-mentioned CNTs are applied to semiconductors, semiconductor devices utilizing superconductivity, applications to magnetic recording media utilizing magnetism, and DNA cutting.
Research on the possibility of practical application is ongoing.

[0004]

By the way, when CNT is synthesized by the above-mentioned method, since the substance other than CNT, for example, an amorphous carbon component or a catalyst metal required for synthesis is mixed into CNT, the purity of CNT is remarkably high. It is low.

Therefore, in order to use CNT,
It is necessary to remove the amorphous carbon component and the catalyst metal contained in the CNT.

Here, it is known that the amorphous carbon component can be selectively combusted with CNT by heat treatment and removed to some extent.

On the other hand, it is known that the catalyst metal can be removed by immersing the CNT in an acidic solution such as a hydrochloric acid solution and a nitric acid solution.

However, the catalyst metal is formed CNT
Due to its deep penetration, the effectiveness of its solubility in the above-mentioned acidic solutions remains questionable and leaves room for improvement.

[0009] The present invention has been proposed in view of such a conventional situation, and is a purification method capable of reliably removing the catalytic metal remaining in the CNT and obtaining a high-quality carbon nanotube. The aim is to provide a method.

[0010]

Means for Solving the Problems In order to achieve the above object, a first invention of the present application is characterized in that carbon nanotubes are immersed in an acidic solution containing at least sulfuric acid to remove metal. It is.

Sulfuric acid has a higher ability to remove catalytic metals than other acids. Therefore, if the synthesized carbon nanotubes are immersed in an acidic solution containing sulfuric acid, the catalytic metals that have entered the carbon nanotubes will surely be removed. Removed.

A second invention of the present application is characterized in that a pair of electrodes is immersed in an acidic solution in which carbon nanotubes are immersed, and a metal is removed by generating a potential difference between the pair of electrodes. Things.

When a potential difference is generated between the electrodes as described above, the catalytic metal ionized by the acidic solution is attracted to the cathode side and is efficiently separated from the carbon nanotube.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for purifying carbon nanotubes according to the present invention will be described with reference to the drawings.

The carbon nanotube is synthesized by, for example, an arc discharge method using a carbon electrode.

FIG. 1 shows an example of an arc discharge apparatus 1 for synthesizing carbon nanotubes by an arc discharge method.

The arc discharge device 1 has a reaction chamber 2 called a vacuum chamber. In the reaction chamber 2, a cathode 3 and an anode 4 made of a carbon rod are opposed to each other with a gap G therebetween. I have. The carbon rod used for the cathode 3 is mainly made of graphite or the like, and is mixed with a catalyst metal such as Ni or Co. The rear end of the anode 4 is connected to a linear motion introducing mechanism 5.
, And the respective poles are connected to the current introduction terminals 6a and 6b, respectively.

When synthesizing carbon nanotubes in the arc discharge device 1, first, the inside of the reaction chamber 2 is evacuated and filled with a rare gas such as helium. Next, cathode 3
When a direct current is applied to the anode 4 and the anode 4, an arc discharge occurs between the cathode 3 and the anode 4, and the inner surface of the reaction chamber 2, that is, the side wall surface, the ceiling surface, the bottom surface and the cathode 3, as shown in FIG. Carbon nanotubes (CNT) 10 are deposited. In addition,
If a small container is installed in advance on the side wall surface of the reaction chamber 2,
The CNTs 10 are also deposited therein.

As shown in FIG. 2, the CNT 10 is a molecule having a cylindrical shape in which a plurality of carbons 11 are bonded.

When the CNTs are formed by arc discharge, an alternating current may be applied instead of a direct current.

The CNT 10 can be synthesized by a method other than the arc discharge method described above, such as a laser ablation method and a chemical vapor deposition method (CVD method).

The synthesized CNT 10 contains amorphous carbon and a catalyst metal, and it is necessary to perform so-called purification to remove the amorphous carbon and the catalyst metal to increase the purity of the CNT 10.

In the present invention, after the catalyst metal is first removed, the amorphous carbon is removed and the purification is performed. As will be described later, the amorphous carbon is removed by performing a heat treatment. However, if the heat treatment is performed before removing the catalyst metal, the catalyst metal precipitates on the surface, and the amorphous carbon is removed. Becomes impossible. Therefore, it is necessary to remove the catalyst metal first.

Therefore, first, the CNT 10 is immersed in an acidic solution containing at least sulfuric acid (hereinafter, referred to as a sulfuric acid solution) and stirred to remove catalytic metals such as Co and Ni contained in the CNT 10. By immersing the CNT 10 in a sulfuric acid solution, for example, for one week, the catalytic metal contained therein is removed.

The concentration of the sulfuric acid solution used at this time is 20
%, More preferably 20% or more and 80% or less. If the concentration of the sulfuric acid solution is less than 20%, the catalytic metal contained in the CNT 10 cannot be sufficiently dissolved, and the catalytic metal is hardly removed. When the concentration of the sulfuric acid solution is higher than 80%, the purified CNT 10 may be damaged.

By immersing the CNT 10 in the sulfuric acid solution as described above, the catalytic metal contained in the CNT 10 can be reliably removed. Also, CNT10
Is immersed in a sulfuric acid solution to remove the catalytic metal, thereby reducing damage to the refined CNT 10, for example, deformation of the shape.

The removal of the catalytic metal is performed by immersing the carbon nanotubes in an acidic solution containing sulfuric acid as described above, or by combining immersion in the acidic solution and applying an electric field to generate a potential difference between the electrodes. It is also effective.

In this case, as the acidic solution, not only an acidic solution containing sulfuric acid but also an acidic solution such as hydrochloric acid, nitric acid and acetic acid can be used.

FIG. 3 shows a method of removing catalytic metal by combining immersion in an acidic solution and application of an electric field. As shown in FIG. 3, an acidic solution (for example, sulfuric acid solution) in which CNT 10 is immersed ) Is placed in the container 30, the cathode 31 and the anode 32 are placed in the sulfuric acid solution, and an electric field is applied. By applying an electric field, the catalyst metal eluted in the sulfuric acid solution is deposited on the cathode 31 side, and CNT is deposited on the anode 32 side.
Accumulates.

At this time, it is preferable to add a hydrogen peroxide solution to the acidic solution. By adding the hydrogen peroxide solution, the catalyst metal can be more reliably removed from the CNT 10.

The electric field applied to the acidic solution is preferably 1 V or more, more preferably 4 V or more. When the electric field applied to the acidic solution is less than 1 V, the catalyst metal cannot be reliably removed from the CNT 10.

As a material of the electrode used as the cathode 31 or the anode 32, an acid-resistant metal such as gold or platinum, or conductive carbon is used. In particular, when conductive carbon is used, elution of the electrode into the acidic solution is eliminated.

When an electric field is applied to separate the catalytic metal, a filter 33 is provided between the cathode 31 and the anode 32, as shown in FIG.
It is preferable to separate into one side and the anode 32 side. The filter 33 has pores large enough to allow passage of metal ions of the catalyst metal without passing through the CNT 10. As a result, the catalyst metal and CNT1 in the acidic solution
0 can be efficiently separated, the catalyst metal can be deposited on the cathode 31 side, and the CNTs 10 can be deposited on the anode 32 side.

As shown in FIG. 4, the inside of the container 30 may be separated into the cathode 31 side and the anode 32 side by covering the anode 32 with a basket-shaped filter 33. And
An acidic solution in which CNTs are immersed is put on the anode 32 side, that is, inside the filter 33. As a result, the catalyst metal and the CNT 10 can be more efficiently separated, and the catalyst metal can be deposited on the cathode 31 side, and the CNT 10 can be deposited on the anode 32 side.

As described above, by immersing the CNT 10 in the acidic solution and applying an electric field to the acidic solution, the catalytic metal dissolved in the acidic solution is cationized and attracted to the cathode 31 side. Since the CNT is deposited on the anode 32 side, the catalytic metal can be reliably removed from the CNT 10. Then, the time required to remove the catalyst metal from the CNT 10 can be reduced, for example, by 3 to 4 days.

At this time, C deposited on the anode 32 side
The orientation of NT10 becomes good. In other words, the catalyst metal can be removed and the linearly elongated C
NT10 is obtained.

Next, the CNT 10 from which the catalyst metal has been removed is selectively burned in an annular electric furnace 40 as shown in FIG. 5 in order to remove amorphous carbon. In particular,
The CNT 10 placed in the container 42 is placed inside the tube 41,
While blowing air in the direction of arrow A, at least 250 ° C and 60
By heating at less than 0 ° C. for 30 minutes, CNT 10
To remove amorphous carbon. Amorphous carbon has an unstable structure having an incomplete six-membered ring, and is unstable to heat as compared with CNT10. Therefore, it can be removed by heating at 250 ° C. or more and less than 600 ° C. C
Since NT10 has a stable structure, it is more stable to heat than amorphous carbon, and does not decompose by heating at 250 ° C. or more and less than 600 ° C.

As described above, by immersing the CNT 10 in the acidic solution and applying an electric field to the acidic solution, the catalytic metal contained in the CNT 10 can be reliably removed, and the CNT 10 can be purified. CNT
10 can be reduced.
Further, the catalyst metal is deposited on the cathode 31 side, and CNT is deposited on the anode 3 side.
Since the catalyst metal is deposited on the second side, the catalyst metal can be reliably removed from the CNT 10. Then, the time required to remove the catalyst metal from the CNT 10 can be reduced.

At this time, C deposited on the anode 32 side
The orientation of NT10 becomes good. That is, the CNT 10 having a linearly elongated shape is purified.

[0040]

EXAMPLES Hereinafter, specific examples of the present invention will be described based on experimental results.

<Experiment 1> In this experiment, the CNTs were immersed in sulfuric acid, hydrochloric acid, and nitric acid to examine the dissolution of the catalyst metal and the damage of the obtained CNTs.

Example 1 First, CNTs were manufactured by an arc discharge method. next,
The CNT was immersed in a 67% sulfuric acid solution to remove the catalytic metal. Finally, the CNTs were heated by an annular electric furnace to remove amorphous carbon, and the CNTs were purified.

Example 2 CNT was purified in the same manner as in Example 1 except that a 50% sulfuric acid solution was used instead of the 67% sulfuric acid solution.

Example 3 CNT was purified in the same manner as in Example 1, except that a 33% sulfuric acid solution was used instead of the 67% sulfuric acid solution.

Example 4 CNT was purified in the same manner as in Example 1 except that a 20% sulfuric acid solution was used instead of the 67% sulfuric acid solution.

Comparative Example 1 CNT was purified in the same manner as in Example 1 except that a 3N nitric acid solution was used instead of the 67% sulfuric acid solution.

Comparative Example 2 CNT was purified in the same manner as in Example 1 except that an 18% hydrochloric acid solution was used instead of the 67% sulfuric acid solution.

In Examples 1 to 4 and Comparative Examples 1 and 2, the dissolution of metal and the damage of each CNT obtained at this time were evaluated. Table 1 shows the results. In Table 1, ○ indicates that the catalyst metal in the CNT was sufficiently removed, and △ indicates that the catalyst metal was weakly dissolved but the catalyst metal was removed, indicating that the catalyst metal was not sufficiently removed. What was and the resulting CN
When damage was seen in T, it was evaluated as x.

[0049]

[Table 1]

From the results shown in Table 1, in Example 1 using a 67% sulfuric acid solution and Example 2 using a 50% sulfuric acid solution, it is possible to reliably remove the catalytic metal in the CNT. It turned out to be. Further, it was found that no damage was observed in the CNT obtained at this time.

Example 3 using a 33% sulfuric acid solution
In Example 4 using a 20% sulfuric acid solution, it was possible to remove the catalytic metal in the CNTs, although not as much as in Examples 1 and 2, and at the same time, the obtained CNTs were damaged. Turned out to be impossible.

Comparative Example 1 using a 3N nitric acid solution
Then, damage was observed on the obtained CNT.

Comparative Example 2 using an 18% hydrochloric acid solution
Did not dissolve the catalyst metal and could not remove the catalyst metal.

It should be noted that in the first to fourth embodiments,
It took one week to remove the catalytic metal from the CNT.

From these results, it was found that by immersing the CNT in the sulfuric acid solution, the catalytic metal contained in the CNT can be reliably removed.

<Experiment 2> In this experiment, the CNT was immersed in each of sulfuric acid, hydrochloric acid and nitric acid, and an electric field was applied to examine the dissolution of the catalyst metal and the damage of the obtained CNT.

Example 5 CNTs from which amorphous carbon had been removed were immersed in a 67% sulfuric acid solution, and then the CNTs were purified in the same manner as in Example 1 except that an electric field was applied.

Example 6 CNT was purified in the same manner as in Example 5 except that a 50% sulfuric acid solution was used instead of the 67% sulfuric acid solution.

Example 7 CNT was purified in the same manner as in Example 5 except that a 33% sulfuric acid solution was used instead of the 67% sulfuric acid solution.

Example 8 CNT was purified in the same manner as in Example 5, except that a 20% sulfuric acid solution was used instead of the 67% sulfuric acid solution.

Example 9 CNT was purified in the same manner as in Example 5 except that a 3N nitric acid solution was used instead of the 67% sulfuric acid solution.

Example 10 CNT was purified in the same manner as in Example 5, except that an 18% hydrochloric acid solution was used instead of the 67% sulfuric acid solution.

In Examples 5 to 10, the dissolution of each metal was examined. As a result, it was possible to remove the catalytic metal without fail. The time required to remove the catalyst metal was 3 to 4 times as compared with when no electric field was applied.
Days shortened. In addition, the purified CNT had good orientation.

[0064]

As is clear from the above description, in the first invention of the present application, since the carbon nanotube is immersed in an acidic solution containing at least sulfuric acid, the catalyst metal contained in the carbon nanotube is reduced. It is possible to reliably remove them. Further, damage to the carbon nanotube can be reduced.

Further, according to the second aspect of the present invention, by immersing the carbon nanotubes in the acidic solution and applying an electric field to the acidic solution, the catalytic metal is deposited on the cathode side, and the carbon nanotubes are Since the catalyst metal is deposited on the anode side, the catalytic metal can be reliably removed from the carbon nanotube. Then, the time required to remove the catalyst metal from the carbon nanotube can be reduced. At this time, the orientation of the carbon nanotubes deposited on the anode side is good. Therefore, the catalyst metal is efficiently removed, and the carbon nanotubes in a linearly oriented state are purified.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of an arc discharge device used for synthesizing carbon nanotubes.

FIG. 2 is a schematic view of a carbon nanotube.

FIG. 3 is a schematic diagram showing a state in which a filter is provided between a pair of electrodes when an electric field is applied.

FIG. 4 is a schematic diagram showing a state in which the periphery of an anode is covered with a filter.

FIG. 5 is a schematic view showing an example of an annular electric furnace used for removing amorphous carbon.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Arc discharge device, 2 reaction chambers, 3 cathodes, 4 anodes, 5 linear motion introduction mechanism, 6a, 6b current introduction terminals, 10 carbon nanotubes, 30 containers, 31 cathodes, 32 anodes, 33 filters, 40 annular electric furnace,
41 tubes, 42 containers,

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Seiji Shiraishi 6-7-35 Kita Shinagawa, Shinagawa-ku, Tokyo F-term in Sony Corporation (reference) 4G046 CC10

Claims (16)

[Claims] 1. A method for purifying carbon nanotubes, comprising immersing carbon nanotubes in an acidic solution containing at least sulfuric acid to remove metals. 2. The method for purifying carbon nanotubes according to claim 1, wherein the concentration of sulfuric acid in the acidic solution is 20% or more. 3. The method for purifying carbon nanotubes according to claim 2, wherein the concentration of sulfuric acid in the acidic solution is 80% or less. 4. The method according to claim 1, wherein after removing the metal, the amorphous carbon is removed by heating.
A method for purifying a carbon nanotube according to the above. 5. The method according to claim 4, wherein the heating is performed in a temperature range of 250 ° C. or more and less than 600 ° C. 6. A method for purifying carbon nanotubes, comprising immersing a pair of electrodes in an acidic solution in which the carbon nanotubes are immersed, and generating a potential difference between the pair of electrodes to remove the metal. 7. The method according to claim 6, wherein the acidic solution contains at least one selected from sulfuric acid, hydrochloric acid, nitric acid, and acetic acid. 8. The method for purifying carbon nanotubes according to claim 6, wherein said acidic solution contains aqueous hydrogen peroxide. 9. The method according to claim 6, wherein the potential difference is 1 V or more. 10. The method according to claim 9, wherein the potential difference is 4 V or more. 11. The method according to claim 6, wherein the electrode is made of carbon. 12. The method according to claim 6, wherein the electrode is made of gold. 13. The method for purifying carbon nanotubes according to claim 6, wherein a filter that does not transmit carbon nanotubes is provided between the pair of electrodes, and the carbon nanotubes are deposited on the anode side. 14. The method for purifying carbon nanotubes according to claim 13, wherein the filter is a basket-shaped filter that covers the periphery of the anode. 15. The method according to claim 6, wherein the amorphous carbon is removed by heating after removing the metal. 16. The method according to claim 15, wherein the heating is performed in a temperature range of 250 ° C. or more and less than 600 ° C.