JP2591497B2 - Purification method of carbon single tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying a carbon tube having a diameter on the order of nanometers, and more particularly to a method for purifying a carbon single tube.

[0002]

2. Description of the Related Art Carbon nanotubes having a diameter of about nanometers, so-called carbon nanotubes, are well known as being composed of multiple atomic layers. Embedded in a carbon electrode, and performing arc discharge, it has been discovered that a carbon single tube composed of one layer of atoms can be formed in the gas phase. One example is Nature, vol. 363, p. 603. , 1993. This phenomenon is considered that the evaporated iron atoms re-aggregate in the gas phase to form fine particles, which act as a catalyst to grow a carbon single tube. According to observation using an electron microscope, usually, the formed carbon single tube is bonded to the catalyst fine particles at its end, and hardly exists as a single carbon single tube. At the time of arc discharge, the catalyst fine particles aggregated in the low-temperature portion around the high-temperature plasma also aggregate a large amount of carbon atoms evaporated from the electrode, so that a graphite film is formed on the surface of the catalyst fine particles. Since the graphite film is chemically stable and serves as a protective film against acid, it is difficult to chemically remove the catalyst fine particles bonded to the carbon single tube, and it is difficult to isolate the carbon single tube.

[0003] On the other hand, regarding the purification of carbon tubes composed of multiple atomic layers, since no catalyst is required, fine metal particles having a size larger than the diameter of the tubes are not mixed, and only small spherical carbon particles have impurities. Therefore, the obtained product is heated in an oxygen atmosphere to burn out minute impurities, thereby purifying multiple carbon tubes. This is based on the fact that although the atomic layer outside the carbon tube is eroded by the heat treatment, small-sized impurities are first eliminated. However, in the case of a carbon single tube composed of one atomic layer, since the diameter is smaller than the catalyst fine particles that occupy the majority of impurities, this method cannot be used as it is for purification of the carbon single tube.

As described above, it is theoretically expected that a carbon single tube becomes a semiconductor or a metal due to a difference in the diameter and atomic arrangement of the tube, unlike graphite having a plate-like structure as a semimetal. Although the possibility of a new material has been sought in the field of the electronics industry, there has been a problem that purification is difficult.

At this stage, a method for efficiently producing a carbon single tube without using a catalyst has not been invented. However, following the conventional method for producing a carbon single tube using a catalyst, the carbon single tube is largely destroyed. It is necessary to provide a method for removing fine particles of a catalyst without performing the method. The present invention has been made in view of such a conventional situation.

[0006]

SUMMARY OF THE INVENTION The carbon single tube purification method of the present invention aims at physically removing chemically stable catalyst fine particles to achieve the above object. In a method of purifying a carbon single tube produced by arc discharge using metal fine particles as a catalyst, a heat treatment is performed on the carbon single tube in the presence of gold fine particles in an oxygen atmosphere at a temperature condition that is not oxidized by the carbon single tube alone. The method is characterized in that a portion of the carbon single tube in contact with the gold fine particles is selectively oxidized and cut to separate it from the metal fine particles, and then the carbon single tube is isolated by removing the metal fine particles and the gold fine particles.

According to this method, if the gold fine particles are sprayed on graphite and heated in an oxygen atmosphere or air, even at a low temperature of about 400 ° C., which cannot be oxidized by graphite alone, a portion in contact with the gold fine particles is selectively oxidized. Japanese Journal of Applied Physics,
Vol. 32, p. 2809, based on the example reported in 1993. In the case of flat graphite as reported, holes can be formed in the form of fine gold particles only, whereas the diameter is smaller than the size of the fine gold particles. If it is applied to a carbon single tube having, the tube can be cut. After cutting the carbon single tube, ferromagnetic metal fine particles of iron or cobalt as a catalyst can be physically separated from the carbon single tube by applying a magnetic field.

In the present invention, it is desirable that the fine gold particles for tube cutting required for the carbon single tube purification method be produced simultaneously with the production of the carbon single tube by arc discharge.

[0009]

It has been reported that when graphite is heated in an oxygen atmosphere, graphite in contact with the fine gold particles is selectively oxidized even at a temperature at which graphite is not oxidized. If this method is applied to a carbon single tube and is oxidized in the presence of gold fine particles, only the portion in contact with the gold fine particles is oxidized, and the tube can be cut. When a carbon single tube is produced by an arc discharge method, a mixture of a carbon single tube and gold fine particles is obtained by vapor deposition of gold in a gas. This is heated under an oxygen atmosphere under a temperature condition that is not oxidized by the carbon single tube alone. As a result, the portion in contact with the fine gold particles is selectively oxidized, and the carbon single tube is cut. Normally, the catalyst for producing carbon single tubes is iron, cobalt,
Since a ferromagnetic material such as nickel is used, for example, by applying a magnetic field while the product is dispersed in a solvent,
The cut carbon single tube and the catalyst fine particles can be separated. Since the ferromagnetic metal fine particles are coated with graphite at the time of forming the carbon single tube, they are stable to acid and are not easily separated chemically, and the above-mentioned physical removal by applying a magnetic field is effective. The removal of the fine gold particles may be performed using an aqua regia or an organic solvent containing a halide having a high dissolution rate in gold.
By the above method, it becomes possible to purify a carbon single tube.

[0010]

An embodiment of the present invention will be described below with reference to FIG. A helium partial pressure of 180 Torr, a methane partial pressure of 10 Torr, and a hydrogen partial pressure of 10 Torr are introduced into the stainless steel chamber 8 from the gas introduction pipe 9 and a DC voltage is applied to the carbon electrodes 1 and 2 facing each other to cause arc discharge 4. Form. The high-temperature plasma generated by the arc discharge evaporates carbon vapor from both electrodes and iron previously embedded in the cathode, and once atomized carbon and iron aggregate in the low-temperature area around the plasma, producing soot that can be observed with the naked eye .
A single carbon tube is formed in the soot, and contains fine catalyst particles. Under a reduced pressure of about 200 Torr in total pressure, heat generated by the plasma causes a temperature gradient in the chamber to generate convection, so that the soot rises together with the airflow 6 and then deposits on the collection substrate 7.

On the other hand, if a tungsten wire 3 is stretched concentrically with respect to the carbon electrode, and a thin gold wire is wound on the tungsten wire in advance, gold can be evaporated by passing an electric current through the tungsten wire and heating the tungsten wire. . The fine particles of gold aggregated in the gas phase reach the collecting substrate 7 together with the gas flow 5, but by simultaneously performing arc discharge and energization to the tungsten wire, a sample in which the carbon single tube and the fine gold particles are uniformly mixed is obtained. Can be obtained.

After completion of the arc discharge, the atmosphere in the chamber is replaced with oxygen of about 30 Torr, and a collecting substrate made of a heat-resistant thin plate of tungsten or molybdenum is removed for 300 to 300 Torr.
By heating to 400 ° C., the portion of the carbon single tube that is in contact with the fine gold particles can be cut. In addition, since the carbon single tube has higher reactivity than graphite, the carbon single tube is oxidized and disappears by the heat treatment exceeding 400 ° C. even without the fine gold particles.

The sample after the heat treatment can be subjected to ultrasonic dispersion in an organic solvent such as ethanol and then applied with a magnetic field to remove iron fine particles as a catalyst. The fine gold particles used for cutting the carbon single tube were dissolved in aqua regia, and reported by the Japan Institute of Metals, Vol. 32, No. 619.
Page, 1993, by using an organic solvent containing a halide having an extremely low concentration and high solubility of gold, for example, a solution of chlorine and trimethylamine hydrochloride in acetonitrile, whereby gold fine particles can be more effectively used. Can be removed, and a carbon single tube can be isolated.

[0014]

As described above, the method for purifying a carbon single tube according to the present invention makes it possible to purify a carbon single tube, which has been difficult in the prior art. This is a practical method in which a single tube can be formed and pretreated for purification in the same chamber by generating gold simultaneously with the generation.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an example of an apparatus used in an embodiment of the method of the present invention.

[Explanation of symbols]

 1, 2 Carbon electrode 3 Tungsten wire with gold wire wrapped around spiral 4 Arc discharge 5, 6 Airflow 7 Collection substrate 8 Chamber 9 Gas introduction pipe 10 Exhaust pipe

Claims (2)

(57) [Claims] 1. A method for purifying a carbon single tube produced by arc discharge using metal fine particles as a catalyst, wherein the carbon single tube is oxidized in an oxygen atmosphere in the presence of gold fine particles and is not oxidized by the carbon single tube alone. Heat treatment, and selectively oxidize and cut the portion of the carbon single tube in contact with the gold fine particles to separate it from the metal fine particles, and then remove the metal fine particles and the gold fine particles to isolate the carbon single tube. Characteristic carbon single tube purification method. 2. The method for purifying a carbon single tube according to claim 1, wherein the fine gold particles are produced simultaneously with the production of the carbon single tube by arc discharge.