US20080279752A1

US20080279752A1 - Method for producing a single-wall carbon nanotube

Info

Publication number: US20080279752A1
Application number: US11/808,208
Authority: US
Inventors: Masaki Suzuki; Kouji Ishibashi
Original assignee: RIKEN Institute of Physical and Chemical Research
Current assignee: RIKEN Institute of Physical and Chemical Research
Priority date: 2006-06-09
Filing date: 2007-06-07
Publication date: 2008-11-13
Also published as: JP2007326751A

Abstract

A method for producing a single-wall carbon nanotube, comprising contacting an organic dehydrated alcohol with a catalyst in a closed space in vacuum at a temperature of 600 to 900° C.

Description

FIELD OF THE INVENTION

The present invention relates to a method for producing a single-wall carbon nanotube, which is hereinafter also called “SWCNT” in some cases.

BACKGROUND

In order to produce a carbon nanotube which is hereinafter also called “CNT” in some cases, there have been conventionally known arc-discharge, laser ablation, and chemical vapor deposition.
Here, the arc-discharge is a method in which multiwall carbon nanotubes that are hereinafter also called “MWCNT” in some cases, are produced on an anode by arc-discharging between carbon rods in an atmosphere such as argon at a pressure slightly lower than the atmospheric pressure. In this case, when arc-discharging is performed in the state that a catalyst such as Ni/Y is mixed into the carbon rods, SWCNTs can be formed on an inner side of a container. This arc-discharging method has an advantage that CNTs having a relatively good quality can be produced with fewer defects. To the contrary, however, this method has the problems that (i) amorphous carbon is simultaneously produced, and (ii) it is costly and (iii) unsuitable for the mass production.
Meanwhile, the laser ablation is a method in which the CNTs are produced by irradiating a strong pulse beam such as YAG laser upon carbon into which catalyst such as Ni/Co is mixed in a high-temperature atmosphere of 900 to 1300° C. Although this method has the advantages that the CNTs having a high purity can be obtained and the tube diameters can be controlled by changing the conditions, the yield is low and it is difficult to implement the method in an industrial scale.
Further, according to the chemical vapor deposition (CVD method), the CNTs is produced by bringing a carbon compound as a carbon source into contact with fine particles of a catalytic metal at 500 to 1200° C., and both of the MWCNTs and the SWCNTs can be produced. In addition, when a catalyst is arranged on a substrate, MWCNTs can be obtained, while oriented vertically onto the surface of the substrate.
As the method for producing the SWCNTs by using the CVD method, a method is known from a pamphlet of WO2003/068676, in which a carbon source composed of a compound having oxygen or a mixture of a compound having oxygen and a compound having carbon is contacted with a catalyst at a heating temperature. However, a method has been sought, which can produce SWCNTs and which can realize high growth rate, growth efficiency, vertical synthesis, etc.

SUMMARY OF THE INVENTION

The present invention is aimed at solving the above problems, and is to provide a method for producing an SWCNT, which can realize high growth rate, growth efficiency, vertical synthesis, etc.
Having made keen examinations in view of the above problems, the present inventors found out that those problems can be solved by the following measures.
(1) A method for producing a single-wall carbon nanotube, comprising contacting an organic dehydrated alcohol with a catalyst in a closed space in vacuum at a temperature of 600 to 900° C.
(2) The method for producing a single-wall carbon nanotube according to (1), wherein the pressure of the closed space at the time of contacting the organic dehydrated alcohol is at most 10×10⁻²Pa.
(3) The method for producing a single-wall carbon nanotube according to (1) or (2), wherein the pressure at which the organic dehydrated alcohol is contacted with the catalyst is from 1 Pa to 100 kPa.
(4) The method for producing a single-wall carbon nanotube according to any one of (1) to (3), wherein the organic dehydrated alcohol mainly comprises ethanol.
(5) The method for producing a single-wall carbon nanotube according to any one of (1) to (4), wherein the catalyst is at least one kind selected from Fe, Co, Ni, Mo, Pt, Pd, Rh, Ir, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu and oxides thereof.
(6) The method for producing a single-wall carbon nanotube according to any one of (1) to (5), comprising washing the closed space before contacting the organic dehydrated alcohol with the catalyst.
(7) The method for producing a single-wall carbon nanotube according to any one of (1) to (6), wherein the catalyst is a particle and the probability that a single-wall carbon nanotube grows from one particle is at least 50%.
(8) The method for producing a single-wall carbon nanotube according to any one of (1) to (7), wherein the single-wall carbon nanotube has a length of at least 50 nm.
(9) The method for producing a single-wall carbon nanotube according to any one of (1) to (8), wherein the single-wall carbon nanotube is formed on a substrate.
(10) The method for producing a single-wall carbon nanotube according to any one of (1) to (9), wherein the single-wall carbon nanotube can be vertically grown.
(11) The method for producing a single-wall carbon nanotube according to anyone of (1) to (10), wherein the organic dehydrated alcohol is contracted with a catalyst at a temperature of 700 to 800° C.
(12) The method for producing a single-wall carbon nanotube according to any one of (1) to (10), wherein the pressure of the closed space at the time of contacting the organic dehydrated alcohol is at most 10×10⁻⁴Pa.
(13) The method for producing a single-wall carbon nanotube according to any one of (1) to (12), wherein the pressure at which the organic dehydrated alcohol is contacted with the catalyst is from 100 Pa to 40 kPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a CVD apparatus used in Examples.

FIG. 2 is an AFM photograph of SWCNTs obtained in Example 1.

FIG. 3 is an enlarged photograph of a part of FIG. 2.

FIG. 4 is an enlarged photograph of a part of FIG. 3.

FIG. 5 is a diagram showing growth distribution proportions of the SWCNTs obtained in Example 1.

FIG. 6 shows a result of a Raman spectroscopic analysis (around 1200 to 1800 cm⁻¹) of the SWCNT obtained in Example 1.

FIG. 7 shows a result of a Raman spectroscopic analysis (around 100 to 400 cm⁻¹) of the SWCNT obtained in Example 1.

FIG. 8 shows an SEM photograph of SWCNTs obtained in Example 2.

FIG. 9 is an enlarged photograph of a part of FIG. 8.

FIG. 10 shows a sectional photograph of SWCNTs obtained in Comparative Example 1.

FIG. 11 shows an enlarged photograph of a part of FIG. 10.

FIG. 12 shows a sectional photograph of SWCNTs obtained in Comparative Example 2.

FIG. 13 shows an enlarged photograph of a part of FIG. 12.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In the following, the contents of the present invention will be explained in detail.
Note that a range of “--- to ---” is used in the present specification in a sense that figures before and after “to” are included as a lower limit and an upper limit of this range, respectively.
The method for producing the SWCNTs according to the present invention comprises contacting an organic dehydrated alcohol with a catalyst in a closed space in vacuum at a temperature of 600 to 900° C.
That is, the closed space in the present invention is in vacuum when the organic dehydrated alcohol is contacted with the catalyst. When the space is in the vacuum state, remaining gases, materials, etc. can easily removed, so that the SWCNTs having a high quality can be obtained.
Here, the “vacuum” means a state in which the pressure is reduced and no gas flows, and for example, it means the space obtainable by carrying out a vacuum exhaust of a closed space using a vacuum pump.
The degree of the reduced pressure of the vacuum space, that is, the pressure of the closed space when the organic dehydrated alcohol is contacted with the catalyst is preferably at most 1×10⁻²Pa, more preferably at most 1×10⁻⁴Pa, and further preferably at most 1×10⁻⁵Pa.
As such a closed space, a quartz tube can be recited. Further, the vacuum space can be attained by using one or more vacuum pumps, depending upon the degree of the vacuum.
The temperature inside the closed space in the present invention is set to a temperature at which the SWCNTs are formed from the organic dehydrated alcohol, and that temperature is preferably 600 to 900° C., and more preferably 700 to 800° C. Such temperature conditions are ordinarily set by raising the temperature in the state that the metal catalyst is introduced into the closed space.
According to the present invention, the organic dehydrated alcohol is contacted with the metal catalyst. The organic dehydrated alcohol in the present invention includes not only an organic dehydrated alcohol in which water is completely removed, but also an organic dehydrated alcohol containing water in small amount that it is ordinarily regarded as a dehydrated alcohol. For example, those which contain at most 0.005 wt % of water are included in the organic dehydrated alcohol referred to in the present invention.
The kind, etc. of the organic dehydrated alcohol used in the present invention are not particularly specified, and two or more kinds of the mixed alcohols suffice. As the organic dehydrated alcohol used in the present invention, mention may be made of, for example, methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, sec-butanol, tert-butanol, n-pentanol, iso-pentanol, n-amyl alcohol, iso-amyl alcohol, n-hexanol, n-heptanol, n-octanol, n-nonylalcohol, n-decanol, etc. Among them, the organic dehydrated alcohol including at least one kind selected from methanol, ethanol and iso-propanol as a main component is preferable, and one including ethanol as the main component is more preferable. Here, the main component means a component having the maximum content proportion as calculated by weight, and preferably amounting to at least 99.5 wt %.
The pressure at which the organic dehydrated alcohol is contacted with the catalyst is preferably 1 Pa to 100 kPa, and more preferably 100 Pa to 40 kPa, and further preferably 1 kPa to 4 kPa. When the organic dehydrated alcohol satisfying such conditions is contacted with the catalyst, the method made possible a high growth rate and higher growth efficiency, even in a short-time growth.
In the present invention, the organic dehydrated alcohol is contacted with the catalyst. The catalyst used in the present invention is not particularly limited, and one or more kinds of the catalysts may be used together. The metal catalyst is preferable. More specifically, the metal catalyst is at least one kind of Fe, Co, Ni, Mo, Pt, Pd, Rh, Ir, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu and their oxides, and Co and Fe are more preferable. As combinations of two or more kinds of the catalysts, Fe/Co, Fe/Mo, Co/Mo, Fe/Ti, Fe/TiO₂, Fe/Al, Fe/Al₂O₃, Co/Ti, Co/TiO₂, Co/Al, and Co/Al₂O₃are preferable.
The catalyst used in the present invention is preferably deposited on the substrate. In order to deposit the catalyst on the substrate, a resist method, a dip coating method or the like can be employed.
As the resist method, any one of a negative type electron beam resist (for example, a method described in JAPP 43 (2004) 1356) and a positive type electron beam resist (for example, a method described in J. AM. CHEM. SOC. 127 (2005) 11942) is preferable, and the positive type electron beam resist is more preferable. Further, when the positive type electron beam resist is employed, the catalyst is vapor deposited on a portion of the substrate where the resist is removed. At this time, it is preferable angularly to vapor and deposit the catalyst. This measure is preferably employed, because it can provide a catalyst pattern smaller than an actual pattern. The resist method is preferably for a case where a single CNT is grown on the substrate.
In the dip coating method, a catalyst layer is formed by dipping a substrate into a solution containing a catalyst. When the dip coating method is employed, the amount of the catalyst can be regulated by adjusting the concentration of the catalyst in the catalyst-containing solution. The dip coating method is preferable in a case where fine catalyst particles are formed on the entire surface of the substrate.
Here, the substrate can be appropriately selected depending upon the use, etc. of the SWCNTs produced, and is preferably silicon, SiO₂, and Al₂O₃. The surface of the substrate may be oxidized before the catalyst is deposited. This treatment is preferably employed, because it is likely to suppress the formation of a silicide of the catalyst metal.
Further, any carrier may be provided between the substrate and the catalyst.
In the producing method of the present invention, the inside of the closed space is preferably washed before the organic dehydrated alcohol is contacted therewith. By employing this measure, remaining impurities can be removed, so that the SWCNTs can be more effectively grown.
The closed space is preferably cleaned by oxygen cleaning, ozone cleaning, plasma cleaning, vacuum thermal cleaning or the like. The oxygen cleaning, the ozone cleaning, and the plasma cleaning are preferable.
The pressure of the cleaning gas in the case of the cleaning with oxygen, the ozone cleaning, and the plasma cleaning is preferably 1 to 100 kPa, and more preferably 1 to 3 kPa. On the other hand, the pressure of the cleaning gas in the case of the vacuum thermal cleaning is preferably at most 1 Pa, and more preferably at most 6.6×10⁻⁴Pa.
The temperature on cleaning is preferably 500 to 1000° C., and more preferably 800 to 1000° C.
According to the producing method of the present invention, the SWCNTs can be produced in the state that the probability that the single-wall carbon nanotube grows from one particle is high (the above probability being also called “growth rate” in some cases in this specification). The growth rate in the producing method of the present invention can be preferably at least 50%, more preferably at least 60%, further preferably at least 80%, and further more preferably 100%.
According to the producing method of the present invention, a SWCNT can be grown long. Meanwhile, how long a SWCNT grows is called “growth efficiency” in some case in the present specification. According to the producing method of the present invention, the SWCNT having a length of preferably at least 50 nm, more preferably at least 1000 nm, further preferably at least 3000 nm can be obtained.
Furthermore, according to the present invention, the diameters of the SWCNTs can be 0.5 to 1.7 mm, and further can be 0.5 to 1.2 nm.
Moreover, according to the producing method of the present invention, the SWCNTs can be synthesized vertically to the substrate.
In addition, according to the present invention, variations in the configurations of the SWCNTs obtained can be reduced. For example, it is possible that at least 60% (preferably at least 80%) of the SWCNTs can be adjusted to fall in a diameter range of 0.5 to 1.5 nm.
The SWCNTs grown according to the method of the present invention can be preferably used as a post-silicon material for field-effect transistors and the like, probes for scanning type probe microscopes, field-emission type electron sources, etc.
Particularly, since the SWCNTs can be grown vertically to the carrier in the form of the substrate, they can be expected to be applied to the field-emission type electron source.

EXAMPLES

In the following, the present invention will be more concretely explained with examples. Materials, use amounts, proportions, processing contents, processing procedures, etc. can be appropriately changed, so long as such changes do not deviate from the purpose of the present invention. Therefore, the scope of the present invention is not limited to specific examples given below.

Example 1

Formation of Catalyst Patterns

Polymethylmethacrylate (PMMA) as an electron beam positive resist was coated on that surface of a silicon substrate of which had been thermally oxidized (hereinafter called also “thermally oxidized silicon substrate” in some cases) by spin coating, thereby obtaining a thin film of about 50 nm. Next, rectangular patterns having sizes of 20 to 60 nm were formed by electron beam lithography. Thereafter, Co was deposited, in the average thickness of 0.1 nm, on the resultant by vacuum deposition, and Co patterns were formed by a lift-off technique with acetone.

Outline of an Apparatus

A chemical vapor deposition apparatus (CVD apparatus) constituted by a quartz tube, an electric furnace, a rotary pump and a turbo molecular pump as shown in FIG. 1 was used.

Cleaning of the Quartz Tube

Heat treatment was carried out, while an oxygen gas was being flown to clean the interior of the quartz tube and a quartz boat. The quartz tube was vacuum evacuated down to 2 Pa (coarse sucking) by the rotary pump, and then while the oxygen gas was being flown at a flow rate of 0.5 l/min., the tube was heated up to 800° C., and thermally treated at 800° C. for 10 minutes. At that time, the pressure was adjusted by switching a valve to a needle valve so that the inner pressure of the quartz tube might be about 2.7 kPa. After the cleaning, the oxygen gas was stopped, the quartz tube was cooled, while the tube was being evacuated to vacuum by the turbo molecular pump. The oxygen gas used was at a G1 grade (99.99995%). Cleaning can be also performed by the oxygen plasma cleaning, the ozone cleaning or the like.

Growing

A thermally oxidized silicon substrate on which the catalyst metal of Co was formed in a discrete fashion was placed on the quartz boat, the boat was placed in the heating furnace, the interior of the quartz tube was vacuum evacuated down to 6.6×10⁻⁴Pa by using the rotary pump and the turbo molecular pump, and the temperature was raised from room temperature up to 800° C. in about 15 minutes. After the temperature reached the growing temperature, waiting was done for 10 minutes so that the temperature might be stabilized. At that time, the inner pressure of the quartz tube was 6.0×10⁻⁵Pa (the pressure of the closed space at the time of contracting the organic dehydrated alcohol). Subsequently, dehydrated ethanol for organic synthesis (water content: at most 50 ppm, manufactured by Wako Pure Chemical Industries, Ltd.) (pressure at the time of introduction: 6.6×10⁻⁵Pa) was introduced into the quartz tube, and SWCNs were grown in the closed quartz tube at the inner pressure of 1.5 kPa (the pressure at which the organic dehydrated alcohol contacts with the catalyst) for 1 minutes. Thereafter, while the interior of the quartz tube was being evacuated to vacuum using the rotary pump and the turbo molecular pump, the furnace was cooled, and then the substrate was taken outside, thereby obtaining SWCNTs. The pressure inside the quartz tube immediately before taking the sample outside was 9.3×10⁻⁵Pa.
A photograph of the SWCNTs obtained was taken by means of an atomic force microscope (AFM). Results are shown in FIGS. 2 to 4. FIG. 3 is an enlarged photograph of a part of FIG. 2, and FIG. 4 is an enlarged photograph of a part of FIG. 3.
Observation results confirmed that the yield of the SWCNTs was 60 to 100%, and they grew at a very high efficiency. Further, it was confirmed that the maximum length was at least 3 μm and the diameters were in a relatively narrow range of around 0.5 to 1.2 nm (FIG. 5). Further, FIG. 6 and FIG. 7 show results in the Raman spectroscopic analysis (excitation wavelength: 488 nm) of a single SWCNT obtained. It was also confirmed that the ratio (G/D) between the height of a peak near 1590 cm⁻¹and that of a peak near 1350 cm⁻¹was 9.2 in FIG. 6 and that since a peak was observed near 164 cm⁻¹in FIG. 7, a SWCNT having a high quality was synthesized.

Example 2

Formation of a Catalyst Pattern

Catalyst patterns were formed according to a method described in Chem. Phys. Lett. 403 (2005) 320. Specifically, two kinds of solutions were prepared, which had cobalt acetate tetrahydrate and molybdenum acetate dissolved in an ethanol solutions each in an amount of 0.01 wt %, respectively (cobalt acetate solution and molybdenum acetate solution). A thermally oxidized silicon substrate was dipped in the molybdenum acetate solution, then thermally treated at 400° C. in air for 5 minutes, successively dipped in the cobalt acetate solution, and thereafter thermally treated at 400° C. in air for 5 minutes. Thereby, catalyst patterns in which a catalyst of Co/Mo is deposited on a surface of the substrate at a high density were obtained.
Next, after the quartz tube was cleaned in the same manner as in Example 1, SWCNTs were grown in the same way as in Example 1.
A sectional photograph of the obtained SWCNTs was taken by a scanning electron microscope (SEM). Results are shown in FIGS. 8 and 9.
It is also clear from FIG. 9 that the SWCNTs having the maximum length of 4 μm grew, while being oriented vertically to the substrate.

Comparative Example 1

SWCNTs were grown in the same manner as in Example 2, except that the organic synthesis ethanol was replaced by a special-grade ethanol (manufactured by Kanto Chemical Co., Ltd.) having 0.4% of water at the maximum. The sectional photograph of the obtained SWCNTs taken by the SEM revealed that they did not grow vertically to the substrate, but hugging the substrate as shown in FIGS. 10 and 11, while no vertical growth was observed unlike the present invention.

Example 3

SWCNTs were grown in the same manner as in Example 2, except that the interior of the quartz tube was not cleaned. Although the SWCNTs obtained were inferior to those in Example 2, the better SWCNTs were obtained in Example 3 as compared with Comparative Examples 1 and 2.

Comparative Example 2

SWCNTs were grown in the same manner as in Example 2, except that instead of the vacuum evacuation of down to 6.6×10⁻⁴Pa, the temperature was raised, while argon gas was filled inside the quartz tube at 0.3 SLM and a pressure of 40 kPa under controlling and that before growing, the interior of the quartz tube was vacuum evacuated to 2 Pa through its one end by the rotary pump. The sectional photograph of the obtained SWCNTs taken by the SEM revealed that they did grow not vertically to the substrate, but hugging the substrate as shown in FIGS. 12 and 13. Thus, it was recognized that no vertical growth was observed unlike the present invention.

Example 4

In Example 2, the interior of the quartz tube was evacuated to vacuum, and the temperature was raised to 800° C., while the pressure was kept at around 2 Pa, and then SWCNTs were grown at 2 Pa. A sectional photograph of the obtained SWCNTs taken by the SEM confirmed that although the SWCNTs obtained were inferior to those in Example 2, the better SWCNTs were obtained in Example 4 as compared with Comparative Examples 1 and 2.
The SWCNTs having excellent growth efficiency and growth rate could come to be produced by employing the SWCNT producing method of the present invention. Further, the SWCNTs could come to be grown vertically to the substrate by employing the producing method of the present invention.
The present disclosure relates to the subject matter contained in Japanese Patent Application No. 160690/2006 filed on Jun. 9, 2006, which is expressly incorporated herein by reference in its entirety.
All the publications referred to in the present specification are also expressly incorporated herein by reference in their entirety.
The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The description was selected to best explain the principles of the invention and their practical application to enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention not be limited by the specification, but be defined claims set forth below.

Claims

1. A method for producing a single-wall carbon nanotube, comprising contacting an organic dehydrated alcohol with a catalyst in a closed space in vacuum at a temperature of 600 to 900° C.

2. The method for producing a single-wall carbon nanotube according to claim 1, wherein the pressure of the closed space at the time of contacting the organic dehydrated alcohol is at most 10×10⁻²Pa.

3. The method for producing a single-wall carbon nanotube according to claim 1, wherein the pressure at which the organic dehydrated alcohol is contacted with the catalyst is from 1 Pa to 100 kPa.

4. The method for producing a single-wall carbon nanotube according to claim 1, wherein the organic dehydrated alcohol mainly comprises ethanol.

5. The method for producing a single-wall carbon nanotube according to claim 1, wherein the catalyst is at least one kind selected from Fe, Co, Ni, Mo, Pt, Pd, Rh, Ir, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu and oxides thereof.

6. The method for producing a single-wall carbon nanotube according to claim 1, comprising washing the closed space before contacting the organic dehydrated alcohol with the catalyst.

7. The method for producing a single-wall carbon nanotube according to claim 1, wherein the catalyst is a particle and the probability that a single-wall carbon nanotube grows from one particle is at least 50%.

8. The method for producing a single-wall carbon nanotube according to claim 1, wherein the single-wall carbon nanotube has a length of at least 50 nm.

9. The method for producing a single-wall carbon nanotube according to claim 1, wherein the single-wall carbon nanotube is formed on a substrate.

10. The method for producing a single-wall carbon nanotube according to claim 1, wherein the single-wall carbon nanotube can be vertically grown.

11. The method for producing a single-wall carbon nanotube according to claim 1, wherein the organic dehydrated alcohol is contracted with a catalyst at a temperature of 700 to 800° C.

12. The method for producing a single-wall carbon nanotube according to claim 1, wherein the pressure of the closed space at the time of contacting the organic dehydrated alcohol is at most 10×10⁻⁴Pa.

13. The method for producing a single-wall carbon nanotube according to claim 1, wherein the pressure at which the organic dehydrated alcohol is contacted with the catalyst is from 100 Pa to 40 kPa.

14. The method for producing a single-wall carbon nanotube according to claim 1, wherein the catalyst is a particle and the probability that a single-wall carbon nanotube grows from one particle is at least 60%.

15. The method for producing a single-wall carbon nanotube according to claim 1, wherein the single-wall carbon nanotube has a length of at least 1000 nm.