JP4306316B2 - Carbon nanotube purification method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for obtaining purified carbon nanotubes from carbon nanotubes containing impurities.
[0002]
[Prior art]
Carbon nanotubes (hereinafter referred to as CNT) are considered as potential materials for nanotechnology, and their application possibilities are being studied in a wide range of fields. Applications include the use of CNT single wires such as transistors and microscope probes, and a large number of CNTs in bulk, such as electron emission electrodes, fuel cell electrodes, or conductive composites with dispersed CNTs. There is a method to use. In any method of use, it is preferable to remove impurities such as amorphous carbon present in the CNTs, and some purification treatments are performed. In general, when a CNT is produced, impurities are contained, and therefore a CNT purification step is essential. For example, the metal catalyst used at the time of CNT manufacture is removed by acid treatment, and the carbon substance that can be removed by heat treatment at a relatively low temperature (200 to 500 ° C.) is removed by firing (Patent Document 1).
[0003]
However, impurities that cannot be removed by acid treatment or baking treatment must be removed by another method, and some other purification methods are known. For example, there is a method in which a CNT containing impurities is dispersed by adding a helical polymer in a toluene solvent, and a complex of precipitated CNT and polymer is taken out (see Patent Document 2). However, a polymer having low conductivity is attached to the obtained CNT, and there is a problem that it is difficult to exhibit the original conductivity of CNT. As another method, there is known a purification method in which CNTs containing impurities are dispersed in a solvent using ultrasonic waves and filtered through a membrane having a pore size in the order of micrometers to nanometers (see Patent Document 3). However, with this method, even if CNTs containing impurities are ultrasonically pulverized in ethanol, the CNTs adhering to the impurities cannot be sufficiently separated, and most of them are preliminarily separated by a glass filter having a 10 μm pore diameter. CNT was separated and removed together with impurities, which was not a practical purification method.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-120409
[0005]
[Patent Document 2]
JP 2000-44216 A
[0006]
[Patent Document 3]
Japanese Patent No. 2522469
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for purifying CNT that can separate and disperse CNT from impurities and can obtain high-purity CNT without impairing the conductivity of the CNT.
[0008]
[Means for Solving the Problems]
The present inventors have found that if a linear conjugated polymer is added to a suspension of CNTs, impurities and CNTs can be well dispersed in the solvent, and further, the polymer is larger than the length of CNTs. The inventors have found that impurities can be made finer than the length of CNTs, and have reached the present invention. That is, the present invention is a method for purifying carbon nanotubes, in which carbon nanotubes are selectively separated by filtration from a carbon nanotube suspension composed of carbon nanotubes containing impurities, a linear conjugated polymer, and a solvent.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
CNTs are produced by arc discharge method, chemical vapor deposition method (hereinafter referred to as CVD method), laser ablation method, etc., but CNTs applicable to the purification method of the present invention were obtained by any method. May be. A single-walled carbon nanotube (hereinafter referred to as SWCNT) in which a single carbon film (graphene sheet) is wound in a cylindrical shape and a multi-layer in which two or more graphene sheets are wound in a concentric shape There is a carbon nanotube (hereinafter referred to as MWCNT). When SWCNT and MWCNT are produced by the above method, fullerene, graphite, and amorphous carbon are simultaneously generated as by-products, and catalyst metals such as nickel, iron, cobalt, and yttrium remain. These by-products and residual catalyst metals become impurities.
[0010]
In the linear conjugated polymer used in the present invention, the term “linear” means that the skeleton structure of the polymer does not take a helical structure in a stable state (a state where no external force is applied) and extends straight. means. Further, the conjugated polymer means a polymer in which single bonds and double bonds are alternately linked with respect to carbon-carbon bonds of the polymer skeleton.
[0011]
Examples of such conjugated polymers include polythiophene polymers, polypyrrole polymers, polyaniline polymers, polyacetylene polymers, poly-p-phenylene polymers, poly-p-phenylene vinylene polymers, and the like. Can be mentioned. In order for these conjugated polymers to be linear, the polythiophene polymer and the polypyrrole polymer need to be linked to monomer units at the 2nd and 5th positions of the thiophene ring and the pyrrole ring, respectively. The poly-p-phenylene polymer and the poly-p-phenylene vinylene polymer are linear because the polymer skeleton is connected at the para position of the phenylene group. Among the above polymers, a polythiophene polymer is particularly preferably used in the present invention.
[0012]
The polythiophene polymer has a structure in which a side chain is attached to a polymer having a poly-p-thiophene skeleton. Specific examples include poly-3-methylthiophene, poly-3-butylthiophene, poly-3-hexylthiophene, poly-3-octylthiophene, poly-3-decylthiophene, and poly-3-dodecylthiophene. 3-alkylthiophene (the carbon number of the alkyl group is not particularly limited, but preferably 1-12), poly-3-methoxythiophene, poly-3-ethoxythiophene, poly-3-dodecyloxythiophene, etc. Alkoxythiophene (the number of carbon atoms of the alkoxy group is not particularly limited, but preferably 1-12), poly-3- (butylthio) thiophene, poly-3- (hexylthio) thiophene, poly-3- (dodecylthio) thiophene, etc. -3- (alkylthio) thiophene (the carbon number of the alkyl group is not particularly limited) Preferably 1-12), poly-3-alkoxy-4-alkylthiophenes such as poly-3-methoxy-4-methylthiophene, poly-3-dodecyloxy-4-methylthiophene (the number of carbon atoms of the alkoxy group and the alkyl group) Although there is no restriction | limiting in particular, Preferably 1-12) is mentioned, 1 type (s) or 2 or more types can be used. Among them, at least one selected from poly-3-alkylthiophene, poly-3-alkoxythiophene, and poly-3- (alkylthio) thiophene is preferable, and poly-3-alkylthiophene and poly-3-alkoxythiophene are particularly preferable. As poly-3-alkylthiophene, poly-3-hexylthiophene is particularly preferable. The polymer need not necessarily have a high molecular weight, and may be an oligomer composed of a linear conjugated system.
[0013]
As the solvent used in the present invention, a solvent capable of dissolving a linear conjugated polymer such as toluene, xylene, chloroform, chlorobenzene is preferably used.
[0014]
The above conjugated polymer not only disperses CNTs well in a solution state, but also has a feature that, in particular, SWCNTs disperse while aggregating CNTs aggregated in a bundle. In general, SWCNTs are aggregated in a bundle shape in the manufactured state, and it is preferable that CNTs are released from this bundle state and dispersed when trying to maximize the characteristics of the CNT with a small addition amount. For this reason, when SWCNT is dispersed, Science magazine vol. As seen in 282, p95 (1998), it is used after imparting dispersibility by chemical modification by a method such as adding a functional group to SWCNT. However, when the CNT is chemically modified, the π-conjugated system constituting the CNT is easily broken, and there is a problem that the original characteristics of the CNT are impaired. In the present invention, CNTs can be dispersed without any particular chemical modification.
[0015]
The weight ratio of the CNT containing impurities of the CNT suspension of the present invention, the linear conjugated polymer and the solvent is not particularly limited, but the weight ratio of the linear conjugated polymer to CNT is 0.1 to 10, It is preferable that the density | concentration of CNT is 0.001-1 g / L. By being in this range, CNT can be purified efficiently.
[0016]
In the purification method of the present invention, any of an ultrasonic irradiation method, a kneading method, a reflux method, and a heating and stirring method can be used for the preparation of the suspension, but it is not limited to these methods. As an apparatus for dispersing the aggregate, an ultrasonic irradiation apparatus, a kneading apparatus such as a bead mill, a reflux dissolution apparatus, a heating and stirring apparatus such as a jacketed mixer, a homogenizer with a stirring blade, etc. can be used. An ultrasonic irradiation device is most efficient for dispersing fine particle aggregates having a primary particle diameter of 1 μm or less. Ultrasonic irradiation devices include an ultrasonic cleaner equipped with an ultrasonic transducer under a water tank and an ultrasonic crusher that inserts a probe directly into a sample solution. Can be dispersed. An ultrasonic crusher is more preferable. Since the probe is directly immersed in the sample and irradiated with ultrasonic waves, it can be dispersed strongly in a shorter time. Ultrasonic irradiation conditions vary depending on the liquid volume and the probe diameter, but for a standard probe having a diameter of 13 mm, it is sufficient to irradiate the liquid volume of 50 mL with an output of 250 W for 10 to 30 minutes. When the amount of liquid is larger than this, the ultrasonic irradiation time may be lengthened or the output increased in proportion to the amount of liquid. On the contrary, when the amount of liquid is smaller than this, it is preferable to reduce the output with a probe having a small diameter and irradiate ultrasonic waves.
[0017]
The filtration performed in the purification method of the present invention is a process of collecting necessary CNTs with a filter and separating and purifying unnecessary impurities through the filter. In order to achieve purification by filtration, it is necessary that the particle size of impurities is much smaller than the length of CNT. Usually, CNTs containing impurities are mixed with amorphous carbon having a primary particle size of several tens to 500 nm and CNTs having a length of about 500 nm to 10 μm. Often has a diameter. According to the present invention, the particle diameter of impurities can be reduced by suspending CNTs containing impurities in a solvent together with a linear conjugated polymer, so that purification by filtration becomes possible.
[0018]
For the filtration performed in the present invention, it is preferable to use a membrane filter having a pore size of 0.5 to 20 μm. As described above, since the primary particle size of impurities such as amorphous carbon is 500 nm (0.5 μm) or less and the length of CNT is often 500 nm (0.5 μm) or more, the pore size of the filter is 0.5 μm or more. It is desirable that A filter having a pore size of less than 0.5 μm cannot be used because a large amount of impurities are collected on the filter and cannot be separated by filtration. On the other hand, a filter having a pore size larger than 20 μm is not suitable for filtration because a large amount of CNT to be collected passes and the yield is extremely reduced. More preferably, the filter has a pore diameter of 5 to 12 μm. By being in this range, the pulverized impurities can pass through and the dispersed CNTs can be efficiently collected. The shape of the membrane filter can be either omnipore or isopore. More preferably, it is an omnipore membrane filter made of tetrafluoroethylene (PTFE), which can be efficiently filtered and the recovered CNT can be efficiently recovered. Thick cellulose filter paper or glass fiber filter paper can also be used. However, when these filter papers are used, CNTs are adsorbed inside the filter paper, and from the viewpoint of yield, it is preferable to use a membrane filter.
[0019]
In the present invention, the conductivity of the purified CNT can be determined as follows. First, purified CNTs are dispersed in a solvent, and CNTs are collected with a membrane filter having a pore size of 0.1 μm. The collected CNTs are in sheet form and transferred onto a glass substrate to obtain a CNT film. The conductivity of the CNT film can be determined by a four-terminal method using four electrodes.
[0020]
【Example】
Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the following examples.
[0021]
Example 1
10 mg of CNT containing impurities such as amorphous carbon (single-wall CNT: manufactured by Carbolex, Inc., purity 70%) and 10 mg of poly-3-hexylthiophene (manufactured by Aldrich, molecular weight: Mw 20000) which is a linear conjugated polymer Then, 20 mL of chloroform was placed in a 50 mL glass sample tube and subjected to ultrasonic irradiation for 30 minutes using an ultrasonic crusher (Tokyo Rika Kikai's VCX-520: output 250 W, direct irradiation). Suspended. 5 mL of this suspension was collected and subjected to suction filtration using a PTFE omnipore membrane filter (Advantech, 25 mm in diameter) having a nominal pore size of 10 μm. As a result, a large number of separated substances were obtained on the filter. The filter-attached filter was placed in a glass sample tube containing 10 mL of chloroform and irradiated with an ultrasonic cleaner for 5 minutes. As a result, the filtrate was well dispersed in chloroform. A small amount of this liquid was collected with a spatula, dropped onto a silicon wafer, air-dried, and observed with an atomic force microscope (AFM). The results are shown in FIG. Most of the filtered product was CNT. From the area ratio, the purity of CNT was 90%. The area ratio is the sum of the total area of CNTs present in the AFM image and the total area of the aggregates, and the total area of CNTs is assigned to the denominator. Calculated and obtained.
[0022]
On the other hand, a small amount of the filtrate that passed through the filter in the first filtration was collected with a spatula and observed with AFM. As shown in FIG. 2, most of them were lumps and CNT was selectively collected. I support it.
[0023]
Comparative Example 1
10 mg of CNT containing impurities used in Example 1 and 20 mL of chloroform were placed in a 50 mL glass sample tube and used for 30 minutes using an ultrasonic cleaner (US-2 manufactured by Inoue Seieido: output 120 W, indirect irradiation). When irradiated with ultrasonic waves, the liquid was suspended inhomogeneously. A small amount of this liquid was collected with a spatula, dropped onto a silicon wafer, air-dried, and observed with an atomic force microscope (AFM). The results are shown in FIG. Most of them are lumps such as amorphous carbon, and the size is mostly 1 μm or more, and it was found that CNTs exist so as to adhere to the lumps. Next, this non-uniform suspension was subjected to suction filtration using a PTFE omnipore membrane filter (Advantech, diameter 25 mm) having a nominal pore size of 10 μm. As a result, all the suspension was collected and the filtrate was chloroform alone. It was.
[0024]
Comparative Example 2
The same operation as in Example 1 was performed except that poly-3-hexylthiophene was not added to the suspension preparation. When the filtered product was redispersed and AFM observation was performed, a large amount of agglomerates such as amorphous carbon was observed in addition to CNT, and it was not purified at all.
[0025]
Example 2
Poly [2-methoxy-5- (2′-ethylhexyloxy) -1,4-phenylenevinylene] (Aldrich), which is a linear conjugated polymer, instead of poly-3-hexylthiophene in the preparation of the suspension The same operation as in Example 1 was performed except that the molecular weight: Mn 86000) was used. When the separated product was redispersed and observed with AFM, it was purified and the purity was 80% based on the area ratio of CNT to impurities.
[0026]
Comparative Example 3
The same as in Example 1 except that poly-m-phenylene vinylene (PmPV) (method disclosed in JP 2000-44216 A) was used instead of poly-3-hexylthiophene in the preparation of the suspension. The operation was performed. When the separated product was re-dispersed and AFM observation was performed, a lot of agglomerates such as amorphous carbon were observed in addition to CNT, and the purity was not so high.
[0027]
Comparative Example 4
An attempt was made to purify CNT according to a known method (Japanese Patent No. 2522469). When 10 mg of CNT containing impurities and 20 mL of ethanol were placed in a 50 mL glass sample tube and subjected to ultrasonic irradiation for 30 minutes using an ultrasonic crusher (output 250 W, direct irradiation), the liquid was suspended unevenly. . When this liquid was filtered through a glass filter having a pore diameter of 10 μm, all the suspension was filtered out, and even if the filtrate was observed with AFM, it was free of both CNT and impurities and was not purified.
[0028]
Example 3
Except that poly-3-hexylthiophene was changed to poly-3-octylthiophene (manufactured by Aldrich, molecular weight: Mw 67000), the same operation as in Example 1 was carried out, and the redispersed liquid of the filtrate was separated and subjected to AFM observation. As a result, it was purified neatly, and the purity was 90% from the area ratio of CNT to impurities.
[0029]
Example 4
Except that poly-3-hexylthiophene was changed to poly-3-dodecylthiophene (manufactured by Aldrich, molecular weight: Mw 32000), the same operation as in Example 1 was carried out, and the re-dispersed liquid separated was separated and subjected to AFM observation. As a result, it was purified neatly, and the purity was 90% from the area ratio of CNT to impurities.
[0030]
Example 5
The same procedure as in Example 1 was performed except that the CNT was changed to a CNT (single-walled CNT, purity of 10% or less) manufactured by Honjo Chemical Co., Ltd. The purity was 60% from the area ratio of CNT to impurities.
[0031]
Example 6
The CNTs having a high purity obtained by the purification method of Example 1 were irradiated with ultrasonic waves for 1 hour in an ultrasonic cleaner in chloroform, and a PTFE omnipore membrane filter (Advantech, 25 mm diameter) having a nominal 5 μm pore size was used. When suction filtration was performed, a large number of filtered products were obtained again. The filter-attached filter was placed in a glass sample tube containing 10 mL of chloroform, and sonicated for 5 minutes with an ultrasonic washer. As a result, the filtrate was dispersed again in chloroform. A trace amount of this liquid was collected with a spatula and observed with AFM. As a result, the purity was 99% or more.
[0032]
Example 7
The CNT obtained by the purification method of Example 1 was collected on a membrane filter having a pore diameter of 0.1 μm, transferred onto a glass substrate, and the conductivity of the obtained CNT film was measured by a four-terminal method. , 60 S / cm.
[0033]
Comparative Example 5
The CNT obtained by the purification method of Comparative Example 3 was collected on a membrane filter having a pore diameter of 0.1 μm, transferred onto a glass substrate, and the conductivity of the obtained CNT film was measured by a four-terminal method. 2 × 10 ^-3 S / cm.
[0034]
【The invention's effect】
By using the purification method of the present invention, highly pure CNT can be taken out.
[Brief description of the drawings]
1 is an atomic micrograph of CNTs sorted in Example 1. FIG.
FIG. 2 is an atomic microscope photograph of the aggregate contained in the filtrate after CNT filtration in Example 1.
3 is an atomic micrograph of the suspension in Comparative Example 1. FIG.

Claims (6)

A method for purifying carbon nanotubes, comprising suspending carbon nanotubes containing impurities in a solvent together with a linear conjugated polymer, and separating the carbon nanotubes from the resulting suspension by filtration. The carbon nanotube purification method according to claim 1, wherein the suspension is prepared by ultrasonic irradiation. The method for purifying carbon nanotubes according to claim 1 or 2, wherein the separation is performed using a membrane filter having a pore diameter of 0.5 to 20 µm. The method for purifying carbon nanotubes according to any one of claims 1 to 3, wherein the linear conjugated polymer is a polythiophene polymer. The method for purifying carbon nanotubes according to claim 4, wherein the polythiophene polymer is at least one selected from poly-3-alkylthiophene, poly-3-alkoxythiophene and poly-3- (alkylthio) thiophene. The carbon nanotube purification method according to any one of claims 1 to 5, wherein the carbon nanotube is a single-walled carbon nanotube.

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