Process for producing monolayer carbon nanotube with uniform diameter

Abstract

Provided is a process for producing a single-walled carbon nanotube on a substrate on which a lot of fine particles comprising at least one kind of a catalyst metal are formed in a reaction apparatus maintained in vacuum, wherein at least one kind of fullerene C2n's (n is an integer of n≧18) is sublimated at a prescribed temperature or higher to produce a fullerene gas in which a partial pressure is controlled; the fullerene gas is transported on the substrate heated at a sublimation temperature of the fullerene or higher; and the fullerene gas is brought into contact with the catalyst metal fine particles described above to produce a single-walled carbon nanotube. The vacuum degree is preferably 0.5 Torr or less, and the sublimation temperature is preferably 700° C. or higher. The substrate has a thin film of a porous substance or an inorganic oxide, and transition metal catalyst fine particles having a particle diameter of 0.5 to 10 nm are formed on the above thin film.

Description

TECHNICAL FIELD

The present invention relates to a production process for a single-walled carbon nanotube (hereinafter referred to as SWNT) by sublimating fullerene, specifically to a production process for SWNT in which a diameter of SWNT is controlled by fullerene used or chemically modified fullerene used.

BACKGROUND ART

A carbon nanotube (hereinafter referred to as CNT) is a carbon cluster comprising a cylindrically wound graphene sheet and having a cross-sectional diameter of 100 nm or less. It is reported in many cases that particularly a single-walled carbon nanotube comprising one layer of a graphene sheet is useful as a nano structural material because of specific electrical and chemical characteristics thereof. In particular, it has become clear from theoretical calculation that SWNT shows various properties extending from semiconductor to metal according to chirality thereof, and therefore if the chirality can be controlled in production or at a separating and refining step, SWNT is expected to have very high industrial usefulness.

A lot of the above trials are reported in terms of science, but examples which are possible to be industrially put into practice have not yet been reported. On the other hand, SWNT has a structure in which a graphene sheet is wound into a cylinder, and therefore if a diameter of SWNT can strictly be controlled, chirality can approximately be controlled without directly controlling the chirality. On the other hand, a variety of chirality which can substantially be assumed is narrowed by narrowing a range of a diameter of SWNT.

Known as a production process for SWNT are an arc discharge process, a laser ablation process, a high frequency plasma process and a thermal decomposition process (a chemical vapor deposition (CVD) process and a catalytic chemical vapor deposition (CCVD) process). In respect to controlling a diameter of SWNT, reported are scientific trials such as controlling a diameter distribution by controlling production conditions including changing temperatures of a catalyst and a furnace and changing a kind and a pressure of an inert gas and obtaining only SWNT falling in the vicinity of a specific diameter by subjecting a mixture of SWNT to heat treatment. However, the controlled SWNT has not yet been surely separated.

A process in which SWNT is produced by reacting a mixture of linear single-walled or multi-walled carbon nanotubes in a carbon material plasma containing —C≡C— or —C═C— is disclosed in Japanese Patent Application Laid-Open No. 203819/2000. It is described that a length of CNT can be controlled by the above process.

It is a subject in Japanese Patent Application Laid-Open No. 058805/2001 to produce CNT in a high yield in a simple manner by a process in which a mixture of the same or different kind of fullerene molecules is mixed with a transition metal element or an alloy thereof to produce CNT at 500° C. or higher under a reduced pressure in an inert gas atmosphere.

In Japanese Patent Application Laid-Open No. 089117/2001, a compound having a five-membered ring of carbon such as fullerene is added to a laser-irradiation target in producing CNT by a laser ablation process, and a catalyst is further mixed with the target, whereby SWNT is produced at a low temperature. However, it is not described to control a diameter of SWNT.

Disclosed in Japanese Patent Application Laid-Open No. 029717/2002 is a production process for a carbon material in which at least one of fullerene and CNT is mixed with amorphous carbon and the mixture is subjected to heat treatment to turn the amorphous carbon into fullerene or CNT. It is described that CNT having a certain length is obtained, but the diameter thereof is not described.

Further, Zhang and Iijima showed that when a mixture of C₆₀ powder and 5 at % of Co and Ni is used as a laser-irradiation target in a laser oven process, SWNT could be produced, though in a trace amount and covered with amorphous carbon, even at an electric furnace temperature of 400° C., and that when used graphite, SWNT could not be produced without elevating a temperature of an electric furnace (oven) to about 850° C. (Y. Zhang and S. Iijima, Appl. Phys. Lett., 75 (1999), 3087). In this case, it is considered that a fullerene structure is destroyed by irradiating with laser and that what is useful for synthesis of SWNT is fragments which are not completely broken to small pieces by the laser. It is considered that any carbon material can be a raw material for SWNT by vaporizing with laser, and as a result, the process can not regard as a process for synthesizing SWNT from fullerene. In this connection, the oven temperature is set to 400° C. in order to prevent fullerene from being sublimated. An amount of SWNT is too small, and therefore it is difficult to judge the level of the diameter from Raman spectra, but it is considered that the diameter is almost unchanged from that obtained by using a graphite material.

Further, Champbell et al. tried to produce a nanotube by a CCVD process, but what could be produced was a multi-walled nanotube (L. P. Biro, R. Ehlich, R. Tellgmann, A. Gromov, K. Krawez, M. Tschaplyguine, M. M. Pohl, E. Zsoldos, Z. Vertesy, Z. E. Horvath and E. E. B. Champbell: Chem. Phys. Lett., 306 (1999), 155, O. A. Nerushev, R. E. Morjan, D. I. Ostrovskii, M. Sveningsson, M. Jonsson, F. Rohmund and E. E. B. Champbell: Physica B 323 (2002), 51 and O. A. Nerushev, S. Dittmar, R. E. Morjan, F. Rohmund and E. E. B. Champbell: J. Appl. Phys., 93 (2003), 4185).

It is tried to synthesize a nanotube using fullerene and a multilayer thin film of a catalyst metal, and a structure corresponding to a multi-walled nanotube is produced (E. Czerwosz, P. Dluzewski, G. Dmowska, R. Nowakowski, E. Starnawska and H. Wronka: Appl. Surf. Sci., 141 (1999), 350 and E. Czerwosz and P. Dluzewski: Diamond Related Mater., 9 (2000), 901). After that, a shocking paper that a single crystal of SWNT could be produced when using C₆₀ and a multilayer film of Ni was published on Science by the group of Gimzewski et al. of IBM (R. R. Schlittler, J. W. Seo, J. K. Gimzewski, C. Durkan, M. S. M. Saifullah and M. E. Welland: Science, 292 (2001), 1136). Thereafter, however, it has been apparent that a TEM image which is evidence for the above paper is an image of molybdenum oxide (M. F. Chisholm, Y, Wang, A. R. Lupini, G. Eres, A. A. Puretzky, B. Brinson, A. V. Melechko, D. B. Geohegan, H. Cui, M. P. Johnson, S. J. Pennycook, D. H. Lowndes, S. Arepalli, C. Kittrell, S. Sivaram, M. Kim, G. Lavin, J. Kono, R. Hauge and R. E. Smalley: Science, 300 (2003), 1236b), and the group of IBM also has announced that they accept the above fact (M. E. Welland, C. Durkan, M. S. M. Saifullah, J. W. Seo, R. R. Schlittler and J. K. Gimzewski: Science, 300 (2003), 1236c).

It is known that peapod in which fullerenes form a line in the inside of SWNT is turned into DWNT by subjecting to heat treatment at a high temperature (B. W. Smith, M. Monthioux and D. E. Luzzi: Chem. Phys. Lett., 315 (1999), 31), and it is considered that also in the above case, fullerenes in the inside have been deformed into SWNT. Even if a nanotube formed in the inside is SWNT, it is difficult to take it out, and only the same amount of SWNT as that of SWNT which is originally present can be produced at the most. Accordingly, it can not be technology for producing SWNT from fullerene.

An object of the present invention is to produce SWNT having a controlled diameter by a CCVD process.

DISCLOSURE OF THE INVENTION

The present invention relates to a process for producing SWNT by a CCVD process, in which fullerens are used as a raw material, and they are sublimated and brought into contact with a heated catalyst to synthesize SWNT, wherein SWNT produced is controlled in a diameter by fullerene used or chemically modified fullerene used.

As described above, the present inventors consider that in producing carbon nanotube by a conventional publicly known technology using fullerene for a raw material, it is indispensable to control a partial pressure of a fullerene gas in order to produce a multi-walled carbon nanotube from fullerene, and the present invention has come to be achieved.

First, at least one kind of fullerene C_2n'S (n is an integer of n≧18, for example, C₆₀, C₇₀, C₇₆, C₈₂ and the like) or chemically modified fullerenes is sublimated at a sublimation temperature of the fullerene or higher in a reaction apparatus evacuated to 0.5 Torr or less.

In respect to a vapor pressure of fullerene, conventional experimental data are collected and reported in Table 3 of Pankajavalli, Thermochimica Acta, 316 (1998), 101 to 108, and they are shown in Table 1.

A vapor pressure of, for example, fullerene C₆₀ can be calculated from the following equation by referring to the above and using an average of conventional experiments:
p(Torr)=7.5×10⁸×10^−9500/T(K)

As one example, results obtained by calculating a vapor pressure of fullerene C₆₀ from the equation described above are shown in Table 2.

TABLE 2
Temperature	Temperature	C60 vapor pressure
(° C.)	(K)	(Torr)
400	673	5.743E−06
450	723	5.437E−05
500	773	3.848E−04
550	823	2.147E−03
600	873	9.841E−03
650	923	3.824E−02
700	973	1.293E−01
750	1023	3.878E−01
800	1073	1.050E+00

The above sublimated fullerene gas is sent to a downstream of a reaction apparatus using a vapor pressure thereof as a driving force and brought into contact with a transition metal catalyst carried on a thin film of a porous substance or an oxide of an inorganic substance which is heated to a vaporization temperature of the fullerene. SWNT is produced from fullerene by bringing fullerene into contact with the catalyst. After prescribed time passes since commencing the reaction, the reaction apparatus is cooled to take out SWNT.

If a partial pressure of a fullerene gas (that is, a feeding speed of fullerene) is suitable, fullerene is decomposed on the surface of a catalyst particle and deposited on the surface in the transition state of a carbon atom or molecule, and then a structure having regularity is formed, whereby a single-walled carbon nanotube can be deposited. This is because chirality of a carbon nanotube deposited is considered to be determined according to the direction and the position of a five-membered ring of a fullerene molecule. Accordingly, a single-walled carbon nanotube in which chirality is put in order can be produced in the present process in which a part of a fullerene molecule can be remained on the surface of a catalyst particle in an original form. A carbon nanotube produced by the process of the present invention takes over the regularity of a molecular structure of the fullerene, and therefore a diameter distribution thereof can be narrowed.

However, a production condition of an elevated partial pressure of fullerene gas (accelerating a feeding speed) can not be a condition suited for producing a single-walled carbon nanotube. This is because an amount of carbon staying in a transition state is increased on the surface of a catalyst particle, whereby a structure such as that of a multi-walled carbon nanotube in which plural sheets having regularity are superposed is formed. Further, it is because solid carbon is deposited before carbon atoms are regularly arranged, so that amorphous carbon is produced.

In the process of the present invention, a single-walled carbon nanotube is produced on a substrate having catalyst particles in which particle diameters are uniformized. A size of the catalyst particle is a factor for determining a diameter of a single-walled carbon nanotube deposited from it, and therefore a distribution of a diameter of the single-walled carbon nanotube can further be narrowed by uniformizing a size of the catalyst particle.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a drawing showing a schematic diagram of a production apparatus for SWNT prepared in Example 1.

FIG. 2 is a transmission electron micrograph of SWNT produced in Example 1.

FIG. 3 is a Raman spectroscopic spectral chart of SWNT produced in Example 1.

FIG. 4 is a drawing showing a schematic diagram of a production apparatus for SWNT prepared in Example 2.

FIG. 5 is a Raman spectroscopic spectral chart of SWNT produced in Example 2.

FIG. 6 is a drawing showing a schematic diagram of a production apparatus for SWNT prepared in Example 3.

FIG. 7 is a drawing showing a heating curve of fullerene and a change in a vapor pressure thereof in Example 3.

FIG. 8 is a transmission electron micrograph of SWNT produced in Example 3.

FIG. 9 is a transmission electron micrograph of SWNT produced in Example 3.

FIG. 10 is a Raman spectroscopic spectral chart of SWNT produced in Example 3.

FIG. 11 is a Raman spectroscopic spectral chart of SWNT produced in Example 4.

FIG. 12 is Raman spectroscopic spectral charts of SWNT produced in Examples 3 and 4 and SWNT produced in Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a schematic drawing showing a production apparatus for carrying out the present invention.

In the process of the present invention, at least one kind of fullerene C_2n's (n is an integer of n≧18) or chemically modified fullerenes is sublimated at a sublimation temperature of the fullerene or higher in a reaction apparatus controlled to a vacuum of 0.5 Torr or less, preferably 0.05 Torr or less. A vaporizing part for fullerenes is in an effusion cell or a quartz tube having a small diameter. Large flow resistance is present between it and an outside reaction tube, and the internal pressure thereof becomes approximately a vapor pressure of the fullerene at a set temperature.

This sublimated fullerene gas is guided by means of a straightening tube and brought into contact with a catalyst in a downstream. In FIG. 1, the stream of the fullerene gas is controlled by a method in which fullerene to be vaporized is put in a closed side of a quartz tube sealed on one side and in which an open end is turned to a vacuum device side to allow the heated and vaporized fullerene to flow with a fullerene vapor pressure used as driving force.

A pressure of the fullerene gas is controlled by a heating temperature thereof, and it is important to control the temperature. If a back pressure of the reaction apparatus is 0.05 Torr, a vapor pressure of fullerene has to be at the lowest 0.05 Torr which is equivalent to the back pressure, which requires it to be heated at 660° C. On the other hand, if the back pressure is 0.5 Torr, it has to be heated at 760° C.

It is reported that when a very pure solid of C₆₀ is heated in pure Ar for 10 minutes, thermal decomposition is commenced at 959° C. or higher and that the thermal decomposition is almost completed at 977° C. or higher (M. R. Stetzer et al., Thermal Stability of C₆₀, Phys. Rev. B. Vol 55 (1997). pp. 127 to 131). On the other hand, it is considered that when a small amount of a solvent, other fullerenes such as C₇₀ and oxygen are present, the thermal decomposition proceeds at a considerably lower temperature than that in the case described above, and it is reported that the thermal decomposition already proceeds, as shown in the following document, at 718° C. even if a raw material having a relatively high purity is used (Y. Piacente et al., J. Phys. Chem., Vol 99 (1995). pp. 14052 to 14057).

Accordingly, it is considered that the decomposition of C₆₀ proceeds at 700° C. or higher, and a temperature of heating fullerene should have an upper limit, so that it is preferred to reduce the back pressure and to lower the sublimation temperature.

Fullerene moved from a vaporizing part collides with a transition metal catalyst to become an initial nucleus of a single-walled carbon nanotube while maintaining a part of the molecular structure thereof, whereby the single-walled carbon nanotube grows from the metal catalyst. It is considered that once the initial nucleus is formed, SWNT is then relatively quickly grown. Accordingly, a speed of elevating the temperature since commencing vaporization of fullerene becomes important. The catalyst is heated to a high temperature required for producing an initial nucleus of SWNT from fullerene. The temperature is preferably 750 to 900° C.

A porous substance or an oxide of an inorganic substance is coated or made into a film on a substrate which can stand the operating temperature, and particles of at least one kind of a metal are carried thereon. The sublimated fullerene described above is allowed to pass on the substrate.

The transition metal is preferably any simple substance of Fe, Co, Mo, Ni, Rh, Pd and Pt or a mixture thereof, and it is more preferably Fe, Co or Mo. The smaller the diameter of the metal particles, the better, and it is preferably 0.1 μm or less, more preferably 10 nm or less and further more preferably 3 nm or less.

The porous substance shall not be restricted in a material as long as it can carry the metal fine particles described above and does not cause a change by a reaction temperature in the apparatus, and it is preferably the porous body of a metal oxide or other inorganic substance. Among them, the porous bodies of zeolite, magnesia, alumina, silica and mesoporous silica are more preferred, and Y type zeolite is particularly preferred. A thin film of an inorganic oxide can also preferably be used, and a silicon oxide film is particularly preferred.

The substrate on which the porous substance is carried or the substrate on which an oxide film of an inorganic substance is formed (hereinafter referred to as the substrate) is put parallel to a flowing direction of a fullerene gas. Or, a plate worked to a shape after the inner wall of the reaction tube is preferred.

The above substrate is cooled after prescribed time passes since starting the production reaction. In respect to the cooling method, heating of the reaction tube is stopped, and air of room temperature is blown thereto from the outside by a fan to quickly cool the reaction tube. After reaching the room temperature, the substrate is taken out, and thus SWNT is obtained on the substrate.

According to the above production process, SWNT having a uniformized diameter can be obtained.

EXAMPLES

The present invention shall be explained below in further details with reference to examples, but the present invention shall not be restricted to the examples described below.

Example 1

As shown in FIG. 1, a quartz tube having an inner diameter of 4.5 mm and a length of 200 mm in which one side was sealed and which was filled in the sealed side with 500 mg of fullerene C₆₀ was disposed in a quartz tube (reaction tube) having an inner diameter of 26 mm which was put in a heating furnace so that a fullerene-filled part was positioned in the middle of the first heating furnace. A quartz plate which was homogeneously coated thereon with Y type zeolite particles (particle diameter: 0.3 to 1 μm) carried thereon with Fe/Co catalyst fine particles (particle diameter: 1 to 2 nm) was put parallel to a flowing direction in the second heating furnace. The reaction tube was evacuated to 0.5 Torr or less by means of a rotary pump. The first heating furnace has a length of 20 cm, and the second heating furnace has a length of 30 cm. The first heating furnace was moved by 20 cm in an opposite direction of the second heating furnace along the quartz tube, and the first heating furnace and the second heating furnace were heated to 850° C. and 900° C. respectively while allowing argon to flow at 350 Torr and 200 sccm in the state that fullerene was not heated. After heated, argon was stopped flowing, and the reaction tube was evacuated again to 0.5 Torr or less. Then, the first heating furnace was returned to the prescribed position to start heating fullerene. After continuing the operation for 10 minutes on the conditions described above, heating was stopped, and air of room temperature was blown thereto by a fan to cool the reaction tube. After cooled, the quartz plate coated thereon with zeolite was taken out to obtain SWNT.

The sample prepared was subjected to supersonic wave treatment in toluene to dissolve and remove fullerene, and then it was observed under a transmission electron microscope (TEM) and analyzed by Raman spectra.

A TEM photograph is shown in FIG. 2, and a Raman spectroscopic spectra thereof are shown in FIG. 3.

It can be found from FIG. 2 that SWNT containing no by-products and having a uniformized diameter is produced.

Observed in FIG. 3 are peaks (1590 cm⁻¹) originating in graphite and peaks in the vicinity of 150 to 300 cm⁻¹ which are characteristic of SWNT. Further, shown in the figure is a diameter estimated from the relationship of a diameter of SWNT and a Raman shift (Jorio et al., Phys. Rev. Lett., Vol. 86 (2001), pp. 1118):
d(nm)=248/ν(cm⁻¹)
and it can be found that the diameter is almost 1 nm.

Example 2

A schematic drawing of the apparatus used is shown in FIG. 4.

Operation was carried out in the same manner as in Example 1. The back pressure was controlled to 0.05 Torr, and used was a quartz plate coated thereon with zeolite which was semi-cylindrical after the inner wall of a reaction tube. A quartz tube in which fullerene was sealed had the same diameter as that in Example 1 but had a length reduced to 100 mm. Also, the first heating furnace was controlled to a temperature of 680° C., and the second heating furnace was controlled to a temperature of 825° C.

Raman spectroscopic spectra of SWNT produced are shown in FIG. 5.

Example 3

A schematic drawing of the apparatus used is shown in FIG. 6.

Operation was carried out in the same manner as in Example 2, and a quartz tube in which fullerene was sealed was equipped with a thermocouple to measure a heating condition of SWNT.

Shown in FIG. 7 are a temperature change of the quartz tube in which fullerene was sealed and a change in a vapor pressure of fullerene since starting the experiment.

The TEM photographs of SWNT produced are shown in FIGS. 8 and 9, and Raman spectroscopic spectra thereof are shown in FIG. 10.

Example 4

Operation was carried out in the same manner as in Example 3, and fullerene C₇₀ was substituted for fullerene C₆₀ as the raw material.

Raman spectroscopic spectra of SWNT produced is shown in FIG. 11. It can be found that the similar SWNT to that in the case of C₆₀ is formed.

Comparative Example 1

Raman spectroscopic spectra of SWNT produced from alcohol by a CCVD process are shown in FIG. 12.

Comparisons between a diameter distribution in SWNT produced from alcohol and those in SWNTs produced in Example 3 (C₆₀) and Example 4 (C₇₀) are shown by Raman spectroscopic spectra. The number of peaks is large in SWNT produced from alcohol, and diameter distributions of SWNTs produced from fullerenes are apparently narrowed.

INDUSTRIAL APPLICABILITY

SWNT obtained by the present invention can widely be used for an FED display, a fuel cell, an electron microscope, a ultra high strength material and an electroconductive composite material.

Claims

1. A process for producing a single-walled carbon nanotube on a substrate on which a lot of fine particles comprising at least one kind of a catalyst metal are formed in a reaction apparatus maintained in vacuum, wherein at least one kind of fullerene C2n's (n is an integer of n≧18) is sublimated at a prescribed temperature or higher to produce a fullerene gas in which a partial pressure is controlled; the fullerene gas is transported on the above substrate heated at a sublimation temperature of the fullerene or higher; and the fullerene gas is brought into contact with the catalyst metal fine particles described above to produce a single-walled carbon nanotube.

2. The process for producing a single-walled carbon nanotube as described in claim 1, wherein a pressure in the reaction apparatus is 0.5 Torr or less.

3. The process for producing a single-walled carbon nanotube as described in claim 2, wherein the pressure in the reaction apparatus is 0.05 Torr or less.

4. The process for producing a single-walled carbon nanotube as described in claim 1, wherein the sublimation temperature is 700° C. or lower.

5. The process for producing a single-walled carbon nanotube as described in claim 1, wherein the fullerene involves chemically modified fullerene.

6. The process for producing a single-walled carbon nanotube as described in claim 1, wherein the catalyst metal is a transition metal belonging to the fifth A group, the sixth A group and the eighth group in the periodic table of elements.

7. The process for producing a single-walled carbon nanotube as described in claim 6, wherein the catalyst metal is any simple substance of Fe, Co, Mo, Ni, Rh, Pd and Pt or a mixture of two kinds thereof or more.

8. The process for producing a single-walled carbon nanotube as described in claim 1, wherein the substrate has a thin film comprising at least one of a porous substance and an inorganic oxide, and the fine particles described above are formed on the thin film.

9. The process for producing a single-walled carbon nanotube as described in claim 8, wherein the porous substance is an inorganic porous substance.

10. The process for producing a single-walled carbon nanotube as described in claim 9, wherein the inorganic porous substance is zeolite.

11. The process for producing a single-walled carbon nanotube as described in claim 10, wherein the zeolite is a Y type.

12. The process for producing a single-walled carbon nanotube as described in claim 8, wherein the thin film of the inorganic oxide is a silicon oxide film.

13. The process for producing a single-walled carbon nanotube as described in claim 1, wherein the fine particles have a particle diameter of 0.5 to 10 nm.