JP2010091844A - Method of fabricating carbon nanotube alignment film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To construct a uniformly aligned thin film consisting of only carbon nanotube (CNT) with a large area on a substrate. <P>SOLUTION: A carbon nanotube alignment layer is fabricated by using the fact that CNT molecules are aligned along an electric field when an AC voltage is applied between electrodes. The distance between the electrodes is set in the order of centimeters. The frequency of the AC voltage is set within a predetermined low frequency range. The substrate is immersed in the solution between the electrodes, and then is pulled out. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a substrate having carbon nanotubes (hereinafter referred to as “CNT”) oriented on the surface thereof.

  A substrate having a structure oriented in a uniaxial direction and having electrical conductivity in the orientation direction can be used as an electrode substrate for liquid crystal. In general, an electrode for liquid crystal is obtained by applying polyimide on an ITO transparent electrode and aligning it by rubbing. In this case, dust or static electricity generated by rubbing may cause destruction of the element. If the electrode substrate itself has orientation, it is not necessary to perform a rubbing treatment, so that element destruction can be prevented. As a method for that purpose, a method of orienting CNTs so as to connect the electrode spacing by applying an alternating electric field to the electrode spacing of several μm width has been reported (Patent Document 1). Further, it is known that an aqueous alcohol solution is effective for improving the dispersion of CNT (Patent Document 2).

JP2004-71654 JP2005-324999

  However, in this method (Patent Document 1), since aggregates of CNTs are formed so as to bridge the electrodes, not only oriented CNTs but also electrodes used for orientation exist on the substrate. And the orientation of the substrate are lost. In addition, in order to produce a CNT alignment film, it is necessary to produce an alignment electrode on a substrate in advance, which is expensive and difficult to produce in large quantities.

Therefore, as a result of intensive studies, the present inventors have made use of the fact that when an electrode is inserted into a CNT dispersion solution and an AC voltage is applied thereto, the CNTs are aligned in the dispersion along the electric field. The distance between the electrodes is set to the cm order, the frequency of the alternating voltage is set to a specific low frequency range, and the substrate is immersed between the electrodes and pulled up, so that a uniform and oriented thin film consisting only of CNTs is enlarged on the substrate We found that it can be constructed in area.
That is, the present invention
Prepare a solution in which carbon nanotubes with a length of 0.5 to 10 μm are dissolved in a mixed solvent of water and a low molecular alcohol, and place the two electrodes in this solution at a distance of 0.5 to 2.0 cm. 1 to 50 kHz and 1 to 5 kV / cm alternating voltage was applied between the two electrodes, and then the substrate was immersed in the solution and then lifted to dry the solvent and oriented thereon. This is a method for producing a carbon nanotube alignment film in which only carbon nanotubes exist.

The method of the present invention is simple and efficient, and uses a common solvent such as water or ethanol, so that carbon nanotubes oriented in one direction can be integrated not only on a glass substrate but also on a plastic substrate. In addition, the amount of carbon nanotubes can be increased by repeating the transfer operation.
Since the carbon nanotube film formed by the method of the present invention is uniform and has a thickness of several carbon nanotubes, it has high transparency and a visible light transmittance of 80% or more. It can be used as a transparent electrode or a rubbing-free liquid crystal alignment electrode.

The method of the present invention will be described step by step.
(1) First, a solution in which CNTs are dissolved in a mixed solvent of water and a low molecular alcohol is prepared.
The solvent is a mixed solution of water and a low molecular alcohol. The low molecular alcohol refers to an alcohol having 1 to 3 carbon atoms, preferably methanol or ethanol. The proportion of the low molecular alcohol in the mixed solution is preferably 5 to 20% by volume.
A carbon nanotube (CNT) refers to a carbon-based fiber having a hollow structure made of only carbon and having few branches. CNTs may be single-walled or multi-walled. The size of the CNT is usually an average diameter of 1 to 50 nm, preferably 1 to 20 nm, and an average major axis of 0.5 to 10 μm, preferably 1 to 10 μm.
It is preferable to sufficiently disperse CNTs in a solvent. Therefore, hydrophilic groups such as hydroxyl groups and carboxylic acid groups may be added to the CNT surface, and a surfactant such as sodium dodecyl sulfonate is mixed with the solvent to form micelles. May be dispersed.
The concentration of CNT in the solvent is 1 to 10 mg / L.
In order to dissolve CNTs uniformly in a solvent, treatments such as ultrasonic treatment and stirring treatment may be performed.

(2) Next, two electrodes are immersed in this solution.
Any electrode such as metal or ITO (indium tin oxide) may be used as the electrode. The voltage applying device may be a general one. The shape of the electrode is preferably a flat plate shape.
(3) An AC voltage is applied between the two electrodes. The frequency of the AC voltage is 1 kHz to 50 kHz, preferably 10 to 20 kHz. If the frequency of the AC voltage is less than 1 kHz, the agglomeration occurs on the electrode substrate. The voltage is 1 kV / cm or more, preferably 1 to 2 kV / cm. When the voltage is smaller than 1 kV / cm, the orientation is deteriorated.
At this time, the temperature of the solvent is 10 to 30 ° C., preferably room temperature. By applying an alternating voltage, the CNTs in the solvent are aligned along the direction of the electric field.
(4) Thereafter, the substrate is dipped in the solution and then pulled up. There is no restriction | limiting in the material of this board | substrate, Preferably it is glass. The pulling speed is preferably about 1 to 2 cm / min.
(5) Dry the solvent. There is no particular limitation on the drying means. As a result of the removal of the solvent, the CNTs are oriented on the substrate, and a CNT thin film is formed. Its thickness is usually 1-30 nm.
The method of the present invention makes it possible for the first time to form a thin film in which CNTs are oriented on such a large-area substrate.

The following examples illustrate the invention but are not intended to limit the invention.

First, carbon nanotubes (multi-walled CNT) were synthesized by a template method (Japanese Patent Laid-Open No. 2006-282468).
Two 5 cm square aluminum substrates were separated by 5 cm, placed in a 20 wt% concentrated sulfuric acid aqueous solution, and anodized at 10 ° C., 20 V, 571 sec. A porous alumina template having a one-dimensional pore with a diameter of 10 to 20 nm and a length of 10 μm was obtained by anodization. An acetylene gas having a concentration of 20 vol% was allowed to flow into this alumina mold at 600 ° C. for 2 hours to deposit carbon on the mold. Subsequently, the substrate is subjected to oxygen plasma treatment to selectively remove carbon deposited on the surface, soak the substrate in a 3 mol / L sodium hydroxide aqueous solution, dissolve the substrate, filter the aqueous solution, and wash by centrifugation. Repeatedly, water dispersible CNTs were obtained. When the obtained CNT was observed with a transmission electron microscope, the diameter was 10 to 20 nm and the length was about 10 μm.

Next, the CNTs were aligned on a glass substrate. An outline of the apparatus used is shown in FIG.
10 vol% ethanol was added to the aqueous dispersion of CNT (5 mg / L) in volume fraction.
Subsequently, two ITO electrodes (width 1.5 cm, length 6 cm) and a glass substrate (width 1.5 cm, length 3 cm) were inserted into this dispersion. The distance between the two ITO electrodes was 2 cm. Next, an AC electric field of 20 kHz and 2 kV / cm was applied between the electrodes at room temperature for 3 minutes, and then the glass substrate was pulled up at 10 mm / min while applying the electric field.
The glass substrate was observed with an AFM measuring instrument (SII, SPA-400).
FIG. 2 shows AFM images obtained by measuring AFM images of CNT transferred to a glass substrate under the above experimental conditions in the range of (a) 50 μm ² and (b) 25 μm ² . The arrow in FIG. 2 is the pulling direction of the substrate. FIG. 3 shows the cross-sectional height of the substrate of FIG. Z1 and Z2 are the cross-sectional heights of the CNT film obtained from the AFM, and the difference ΔZ indicates the film thickness.
4 shows the orientation distribution of CNTs when the substrate pulling direction is 0 ° in FIG. From this distribution diagram, it is understood that about 60% of CNTs are oriented in the direction of 0 ° to 30 ° when the immersion direction of the substrate is 0 °.
As a result, it was confirmed that the CNTs were oriented in one direction without agglomeration. The film thickness was 10 to 30 nm, which was 1 to 3 times the diameter (10 to 20 nm) of the CNT used. This indicates that this film is a uniform ultrathin film composed of several CNTs.

Using the glass substrate onto which the aligned CNTs were transferred in the operation of Example 1, CNTs were adsorbed on the substrate by the same operation as in Example 1. FIG. 5 is an AFM image of CNT transferred to the glass substrate under the above experimental conditions, and the arrows indicate the direction of pulling up the substrate. FIG. 6 shows a cross-sectional image of this substrate. Z1 and Z2 are the cross-sectional heights of the CNT film obtained from the AFM, and the difference ΔZ indicates the film thickness. As a result, it was confirmed that the CNT alignment film formed by the first operation and the CNT alignment film formed by the second operation did not change in film thickness and increased in density. At this time, the film density was about 60%.
As described above, the conductivity of the aligned CNTs (FIG. 6) that had been adsorbed twice and transferred onto the glass substrate was measured. A gold electrode was vapor-deposited in a direction parallel to the alignment direction and a direction perpendicular to the alignment direction on the glass substrate on which the aligned CNTs were adsorbed (electrode width: 1 mm, electrode interval: 20 μm). FIG. 7 shows a schematic diagram thereof. FIG. 8 shows the current-voltage characteristics. As a result, the difference in conductivity between the alignment direction and the direction perpendicular thereto was 5 times.

  Since it is known that the absorbance of CNT used in this example is 0.006 / 1 nm at a wavelength of 400 nm (Journal of Materials Chemistry "Fabrication of densely packed multi-walled carbon nanotube ultrathin films using a liquid-liquid interface" , 2007, vol 17 pp3806-3811), the transmittance at 400 nm of the substrate on which CNTs were adsorbed twice was required to be about 80%. Since the absorbance at 400 nm to 700 nm is the strongest at 400 nm, this indicates that the alignment film has a transmittance of 80% or more in visible light.

  The present invention produces a CNT thin film oriented on an insulator substrate. Since this film has a thickness of several CNTs, application as a transparent electrode and a liquid crystal alignment electrode can be expected. Further, by using a polymer for the substrate to be transferred, it can be used as a flexible electrode, and development to a photoelectric conversion film such as an EL element or a solar cell can be expected.

It is a figure which shows the schematic diagram of an electric field alignment film manufacturing apparatus. It is a figure which shows the AFM image of multi-walled CNT transferred to the board | substrate. The arrow indicates the substrate pulling direction. It is a figure which shows the cross-sectional image of the board | substrate with which CNT was transferred. It is a figure which shows the orientation distribution of multi-walled CNT calculated | required from the AFM image of FIG. It is a figure which shows the AFM image of the CNT film | membrane produced by repeating transcription | transfer to a board | substrate twice. The arrow indicates the substrate pulling direction. It is a figure which shows the cross-sectional image of the board | substrate with which CNT was transferred. It is a figure which shows the conductivity measuring method of a CNT alignment film. (A) is an orientation direction, (b) shows the conductivity measuring method in a direction perpendicular to the orientation. It is a figure which shows the conductivity anisotropy of a CNT alignment film.

Claims (2)

Prepare a solution of carbon nanotubes with a length of 0.5 to 10 μm in a mixed solvent of water and low molecular alcohol, and place two electrodes in this solution so that the distance between them is 0.5 to 2.0 cm. 1 to 50 kHz and 1 to 5 kV / cm alternating voltage was applied between the two electrodes, and then the substrate was immersed in the solution and then lifted to dry the solvent and oriented thereon. A method for producing a carbon nanotube alignment film in which only carbon nanotubes exist. A method for producing a carbon nanotube alignment film in which each step of claim 1 is repeated a plurality of times, whereby only high-density aligned carbon nanotubes are present thereon.