JP2002338219A - Method for producing carbon nanoring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for efficiently obtaining circular carbon nanotubes from carbon nanotubes. SOLUTION: Single-walled carbon nanotubes are cut to prepare open-end carbon nanotubes each having reactive functional groups capable of forming chemical bonds by cross reactions at both ends in such a way that open-end carbon nanotubes each of whose length is about 2 times the persistence length of each of the single-walled carbon nanotubes are included. The prepared open- end carbon nanotubes are dispersed in a solvent, each of the open-end carbon nanotubes is cyclized by bonding both ends to each other by way of the reactive functional groups and the resulting bonds are decomposed by heat-treating the cyclized carbon nanotubes to produce the objective circular carbon nanotubes. The reactive functional groups are, e.g. carboxy and hydroxy groups and are bonded using a dicyclohexylcarbodiimide as a coupling agent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of carbon nanotubes, and more particularly to a new technique for obtaining a cyclic carbon nanotube (carbon nanoring) from a single-wall carbon nanotube.

[0002]

2. Description of the Related Art Carbon nanotubs
e: may be abbreviated as CNT hereinafter) is a carbon material having a graphite sheet (graphene sheet) rolled into a cylindrical shape, and a single-walled carbon nanotube (single carbon nanotube) having a single graphene sheet. -walle
d carbon nanotube: a tube of multi-walled carbon (hereinafter sometimes abbreviated as SWCNT) and a multi-walled carbon tube in which a plurality of graphene sheets are concentric cylinders
nanotube: hereinafter may be referred to as MWCNT).

[0003] MWCNT and SWCNT are available in 1990.
Since being discovered one after another at the beginning of the age, it has attracted attention as a research tool based on its unique structure, and has been expected to be applied as a variety of nanometer-scale functional materials (nanodevices) in the field of nanotechnology. Active research and development is underway.

[0004] Previous research and development on carbon nanotubes has focused exclusively on long tubular shapes. If a new shape, for example, a carbon nanotube can be made into a ring (ring shape) while essentially retaining the properties of the carbon nanotube, it is expected that it will contribute to the creation of new materials in the field of nanotechnology. . However, there is hardly any method for systematically preparing a specific new shape from carbon nanotubes, and only a small amount of cyclic CNTs is found by accident in the synthesis of carbon nanotubes. Absent.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a new method for efficiently obtaining a cyclic carbon nanotube from a carbon nanotube.

[0006]

Means for Solving the Problems The present inventor has found suitable conditions for cyclization of SWCNT while conducting analysis based on a statistical model used in the field of polymers, and has derived the present invention. is there.

Thus, according to the present invention, there is provided a step of cutting single-walled carbon nanotubes to prepare open-ended carbon nanotubes having reactive functional groups at both ends capable of reacting with each other to form a chemical bond, Including an open-ended carbon nanotube about twice as long as the sustained length of the single-walled carbon nanotube;
Dispersing the open-ended carbon nanotubes in a solvent, bonding both ends of each open-ended carbon nanotube to each other via the reactive functional group to form a ring, and heat-treating the obtained circularized carbon nanotube. Decomposing the bond via the reactive functional group by the method described above.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below.
As shown in the figure, the annular carbon nanotube
) Will be described along with the manufacturing process.(1) Open-end CN having reactive functional groups at both ends
Preparation of T  Open the carbon nanotube with both ends open.
It is called an open-end nanotube. Arc discharge method
Commercially available carbon nanotubes
Is usually closed at both ends and too long,
It cannot be dissolved or dispersed in a solvent as it is
No.

In order to produce a cyclic carbon nanotube (carbon nanoring) according to the present invention, first, a single-walled carbon nanotube is cut into open ends, and a reaction is formed at both ends thereof so as to react with each other to form a chemical bond. Make sure that a functional group is present.

Single-walled carbon nanotube (SWCNT)
A commonly used means of cleaving an open end is sonication in a strong acid (aqua regia) solution, as is well known. By this operation, the defect site of the carbon nanotube is attacked, and a large number of open-end SWCNTs having different lengths at both ends are obtained.

The open end S thus obtained
The WCNT is then chemically treated to have reactive functional groups at both ends. The reactive functional groups present at both ends of the open-end SWCNT are not limited in principle as long as they can react with each other to form a chemical bond. Examples of particularly preferred chemical treatments for imparting functional functional groups are oxidizing solutions (eg, H ₂ SO ₄ / H ₂ O ₂ solutions)
Etching inside. Thereby, oxygen-containing functional groups appear on the cut surfaces at both ends of the open-end SWCNT. The oxygen-containing functional groups are generally a carboxyl group and a hydroxyl group (phenolic hydroxyl group). As described below, such oxygen-containing functional groups react with each other using dicyclohexylcarbodiimide as a coupling agent to easily form a chemical bond, and the open-end SWCN
Cyclize T.

Another example of a reactive functional group which is present at both ends of the cut open-ended SWCNT and can react with each other to form a chemical bond is acid chloride (acyl chloride) -C
OCI. This functional group replaces the hydroxyl group of the carboxyl group obtained by the acid treatment as described above with S
Appears by chlorine substitution with OCl ₂ , so that the —COCl group at one end of the open-ended CNT can react with a carboxyl group at the other end of the same CNT to form a bond.

[0013](2) Persistence length estimation based on statistical model
Arithmetic  As described in (1) above, it exists in a relatively short area
With peaks and maximum lengths ranging from several hundred nm to several μm
Open-end SWCNTs distributed in the gap length are obtained
It is. The present inventor has studied statistical techniques used in the field of polymers.
From the analysis based on the model, open-end SWCNT
Find an effective method for estimating a suitable length for circularization
did.

The behavior of rigid macromolecules (semiflexible macromolecules) in solution can be modeled by worm-like chains, the statistical properties of which are of persistent length (persistent length).
is well known. Here, the persistence length refers to the tendency of the polymer chain to maintain linearity,
That is, the rigidity is represented by the length, and is defined as a value obtained by extrapolating the average value of the oblique shadow of the vector connecting the ends to the first combined vector to the degree of polymerization infinity. Therefore,
The higher the value of the duration, the harder the chain.

The inventor of the present invention has stated that, like the semi-flexible polymer, the open-end SWCNT obtained as in the above (1) employs a worm-chain model in consideration of the excluded volume effect, and , A preferable condition for forming a cyclic carbon nanotube is the open end S
It has been found that WCNT is about twice as long as the duration as estimated from this model.

According to the worm chain model, the worm chain (worm)
-like chain) is obtained by integrating the distribution function of the distance between both ends of the chain, and is expressed by the following equation (A) (H. Yamakawa, WH Stockma).
yer, J. Chem. Phys. 57 , 2843 (1972)).

[0017]

Embedded image

Here, r represents L as the length of the chain (path length: co
ntour length), which is a reduced length represented by L / 2p when p is a sustained length. Therefore, G will depend on the given parameter p (persistence length). Therefore, in the present invention, the duration is estimated by the following procedure.

Assuming the value of the persistence length p, a ring formation probability distribution G (L; p) with respect to a path length (length of the open-end SWCNT) L is determined. Distribution W (L) for length L of SWCNT before ring formation
Is measured. For the length L of SWCNT, the actual ring-formed SW
G and W obtained by actually measuring the distribution of CNT and obtaining as described above
Is compared with the distribution function G (L; p) · W (L) obtained from the above, and the value of p at the time of the best fit is defined as the persistence length. The duration is generally determined by X-ray scattering measurement, but it is difficult to determine the duration of the open-end SWCNT used in the present invention by such a measurement method. In the present invention, the duration can be easily estimated using the statistical model as described above.

The cyclic CNT (carbon nanotube) according to the present invention peaks when the open-end SWCNT having reactive functional groups at both ends has a length approximately twice as long as the length estimated by the above procedure. (See Examples below). Therefore, in the present invention, in preparing an open-end CNT having a reactive functional group at both ends from the SWCNT by the treatment as described above, the raw material SWCNT is used.
Is about twice as long as, for example, about 1.5-2.
Include 5 times longer open end CNTs.

At this time, it is not always possible to always obtain open-ended CNTs having a constant length distribution by ultrasonic treatment in a strong acid or etching in an oxidizing solution. If the same SWCNT as raw material
As long as is used, the peak positions of the distribution are different, but substantially the same p (duration) value is obtained. In addition, as a statistical model that simulates the behavior of open-end CNTs, a helical worm chain model (helica
l worm-like chain model) (J. Shimada, H. Yamakawa,
Applying Macromolecules 17 , 689 (1984)) yields substantially the same p-value.

The large factor on which the p-value depends is considered to be the diameter of SWCNT used as a raw material. For example, the sustained length of SWCNT having a diameter of 1.2 nm used in Examples described later is 8 nm.
It was estimated to be 00 nm (0.8 μm). The diameter of SWCNT is generally about 1 to 10 nm, and the duration varies depending on the diameter. Thus, according to the present invention, the size (diameter) of the obtained cyclic CNT (carbon nanoring) can be adjusted if the sustained length of the raw material SWCNT is known. In this case, if it is particularly desired to efficiently produce carbon nanorings of a certain size, open-ended CNTs that deviate by more than about 2 times the sustained length can be removed by appropriate means (eg, ultracentrifugation). May be removed in advance.

[0023](3) Cyclization reaction of open-end CNT  Prepared as described in (1) and (2) above, and
Open-end SWCNTs with reactive functional groups are
After appropriate washing and purification as necessary, disperse in solvent
Both ends of each open-end SWCNT are reactive
Cyclization reaction is performed by bonding to each other through functional groups
To do. For example, as described above, carboxy at both ends
Oxygen-containing functional group consisting of a hydroxyl group or (phenolic) hydroxyl group
Using open-ended SWCNTs with
Dispersed in chilled formamide (DMF)
Silcarbodiimide (DCC) as coupling agent
To react the oxygen-containing functional groups at both ends,
Join the ends. Figure 1 shows that such a reaction
A carbon nanotube (carbon nanoring) is obtained
FIG.

As the solvent used as the dispersion medium, D
In addition to MF, water, ethanol and the like are exemplified. However,
It is preferable to use a solvent having a high dispersibility for open-end SWCNTs having a reactive functional group at both ends, and if the dispersion is poor due to the combination with the open-end SWCNTs, the formation rate of cyclic carbon nanotubes is low. For example, an open-end SW terminated with an acid chloride as described above as a reactive functional group
It has been found that when cyclization reaction is carried out by dispersing CNTs in DMF, the dispersion becomes poor and the yield of cyclic carbon nanotubes is low.

In a solution in which the open-end SWCNT having a reactive functional group is dispersed, in addition to the cyclization reaction by the self-bonding of both ends of the open-end SWCNT, the open-end SWCNT and another open-end SWCNT are separated. Reaction and open-end SWCNT
A reaction in which comrades gather together by van der Waals forces to form a bundle can occur. In order to avoid such undesirable reactions as much as possible and to allow the ring formation reaction to proceed preferentially, it is desirable to keep the concentration of open-end SWCNT in the solution extremely low. For example, in the case of the embodiment described later, the concentration of the open-end SWCNT is a low concentration of 0.05 mg / ml or less.

[0026](4) Heat treatment of cyclized SWCNT  The cyclic SWCNT obtained as in (3) above is
Very stable, both chemically and physically,
So that it is subjected to heat treatment as the final step.
You. This heat treatment step involves an open air during the cyclization reaction.
Solvent used to disperse the SWCNTs (eg, D
MF) in which cyclized SWCNTs are dispersed in acid-free
In a raw atmosphere (for example, nitrogen gas atmosphere),
The bond (organic bond) derived from a reactive functional group is decomposed
In general, at a temperature of 300 ° C. or more (for example, 500 ° C.)
This is done by heating.

When the cyclic SWCNT (carbon nano ring) after such heat treatment is observed with a scanning tunneling microscope (STM), a uniform contrast is recognized over the entire circumference of the ring, and the bond at the end of the open-end SWCNT is decomposed. Indicates that a carbon-carbon bond was formed. Thus, the cyclic SWCN obtained by the present invention
T can maintain the properties of the raw material SWCNT, for example, conductivity, over the circumference.

According to the present invention, SWC used as a raw material
Although it depends on the diameter (thickness) of NT, it is generally several hundred nm.
Can obtain a circular SWCNT having a size (diameter) of several thousand nm. Further, as described above, by adjusting the length of the open-end SWCNT in consideration of the sustained length of the raw material SWCNT, a fixed size of the SWCNT can be obtained. An annular SWCNT can be obtained.

[0029]

EXAMPLES In order to more specifically show the features of the present invention,
Examples will be described below, but the present invention is limited by these examples.
It is not something to be done.Example 1: Production of cyclic SWCNT  Single-walled carbon nanotubes with a diameter of 1.2 nm (Tubes @ Ri
ce) H_TwoSO_Four(98%) / HNO_Three(60%) mixture
Sonicate for 24 hours in mixed solution (3: 1) and open
And cut into an end SWCNT. This open end S
WCNT was filtered through a Teflon filter
Later, the residue is H_TwoSO_Four(98%) / H_TwoO_Two(30%) mixture
Dipping in a mixed solution (4: 1) at 70 ° C for 0.5 hour
End of open-end SWCNT by ching
Carboxyl group and hydroxyl group (phenolic hydroxyl group)
Was made to appear.

After the acid treatment, ultracentrifugation (3500 ×
g) to remove the bundle formed from SWCNT, and the remaining SWCNT in the supernatant was dispersed in anhydrous DMF (concentration: 0.05 mg / ml) to obtain a lightly colored solution. To this solution was added excess DCC (1 mg
/ Ml) and the resulting mixture was stirred overnight at room temperature. In order to confirm the formation of cyclic SWCNT, the solid substance was collected, washed with deionized water through a Teflon filter (0.8 μm), and then observed on mica by AFM (atomic force microscope). This AFM image is shown in FIG. As shown in FIG. 2, the formation of a substantially constant-sized ring (annular SWCNT) having a diameter of about 540 nm is observed (see also Example 2 described later). The cyclic SWCNT thus formed was extremely stable chemically and physically, and no ring-opening reaction occurred even when treated with dimethylaminopyridine and n-butylamine or NaOH.

The cyclic SWCNT washed and filtered through the Teflon filter as described above was cast on graphite, and then heated to 500 ° C. for 3 hours in a nitrogen atmosphere. When the cyclic SWCNT after the heat treatment was observed on graphite by STM (in air, bias 1.0 V), a uniform contrast was recognized over the entire circumference of the ring.

[0032]Example 2: Persistence length based on statistical model
Estimate  Statistical model for the case shown in Example 1 above
The duration based on
The utility was examined. The histogram shown in FIG.
Of the length (path length: circumference) of the SWCNTs formed in the ring
(In Example 1, the ring shape by adding DCC)
After the synthesis reaction, the sump before filtration with a Teflon filter
3), and the histogram shown in FIG.
Fig. 1 shows the distribution of the length of SWCNT before formation (see Example 1).
Then, H_TwoSO_Four/ H_TwoO_TwoBefore treatment and before DCC addition
Sample). These histograms
Also cast each sample on mica and AFM
Obtained by counting while observing in
You.

As described in the above (2), the ring formation probability distribution G (L; p) is obtained by assuming various values of the persistence length, and the measured values before the ring formation (the histogram of FIG. )) And a distribution function W (L) · G obtained from the length distribution W (L) of SWCNT obtained above and G (L; p).
When (L; p) was compared with the actually measured value of SWCNT actually formed with a ring (the histogram of A in FIG. 3), the value of p that was the best fit was p = 0.82 μm. The curve shown in FIG. 3A is W (L) · G when p = 0.82 μm.
(L; p), and the curve shown in FIG.
G (L; p) at 0.82 μm.

The circumference of the actually formed annular SWCNT having an average diameter of about 540 nm (0.54 μm × 3.14 = 1.7)
μm) is the value of p (0.82 μm) estimated from the model.
m) about twice. As shown in the histogram of FIG. 3A, the SWC having a length in the range of 1.3 μm to 2.1 μm peaked at about twice the value of the sustained length.
Nanorings are formed from NT, and substantially no nanoring formation occurs outside of this range.

Therefore, by considering the value of the sustained length of the raw material SWCNT, by preparing an open-end SWCNT having a length about twice that value (having reactive functional groups capable of self-bonding at both ends), Cyclic carbon nanotubes (carbon nanorings) of almost constant size
It will be appreciated that

[0036]

As described in detail above, according to the present invention, cyclic carbon nanotubes can be reliably and efficiently obtained from single-wall carbon nanotubes. The cyclic carbon nanotube obtained by the present invention has a unique shape (cyclic) having various properties of the carbon nanotube.
It is expected that this will contribute to the development of new functional materials such as electronic materials, magnetic materials, and optical materials as nanodevice materials.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a state in which an open-ended carbon nanotube having a reactive functional group at both ends is cyclized according to the present invention.

FIG. 2 is an atomic force micrograph illustrating a cyclic carbon nanotube formed in accordance with the present invention.

FIG. 3 shows the length distribution of open-ended SWCNTs when cyclic carbon nanotubes are formed according to the present invention, in comparison with values obtained from a statistical model.

Claims (2)

[Claims] 1. A step of preparing single-walled carbon nanotubes having an open-ended carbon nanotube having reactive functional groups at both ends capable of reacting with each other to form a chemical bond by cutting the single-walled carbon nanotubes. Including about twice as long open-ended carbon nanotubes; dispersing the open-ended carbon nanotubes in a solvent so that both ends of each open-ended carbon nanotube are connected via the reactive functional group. A method of producing a cyclic carbon nanotube, comprising: a step of binding to each other to form a ring; and a step of heat-treating the obtained cyclic carbon nanotube to decompose the bond via the reactive functional group. 2. The reactive functional group at both ends of the open-ended carbon nanotube is an oxygen-containing functional group. The oven-end carbon nanotube is dispersed in dimethylformamide as a solvent, and dicyclohexylcarbodiimide is used as a coupling agent in the oxygen-containing functional group. The method for producing a cyclic carbon nanotube according to claim 1, wherein the both ends are bonded to each other by reacting the contained functional groups.