PREPARATION OF SINGLE-WALLED CARBON NANOTUBES

RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application

Serial Number 60/569,139 entitled "Preparation of Single-Walled Carbon Nanotubes," filed May 7, 2004, incorporated by reference herein.

STATEMENT REGARDING FEDERAL RIGHTS This invention was made with government support under Contract No.

W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to carbon nanotubes and more particularly to a method for preparing relatively long, single-walled carbon nanotubes.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) are seamless nanometer scale diameter tubes of graphite sheets with fullerene caps. They have shown promise for nanoscale electronic devices, chemical sensors, high strength materials, field emission arrays, tips for scanning probe microscopy, gas storage, and other important applications.

CNTs may be multi-walled or single-walled. Multi-walled CNTs were discovered in the hard deposit formed on the graphite cathode of an arc- evaporation apparatus used to prepare carbon fullerenes C ₆₀ and C ₇ o (S. lijima, "Helical Microtubules of Graphitic Carbon," Nature, vol. 354, pp. 56-58 (1991 )). Single-walled CNTs were reported shortly thereafter (lijima et al., Nature, vol. 363, p. 603, 1993; and Bethune et al., Nature, vol. 363, p. 605, 1993).

In the years following their initial discoveries, many other reports relating to CNTs have appeared (see, for example: Thess, A. et al., Science 273, 483 (1996); Ivanov, V. et al., Chem. Phys. Lett 223, 329 (1994); Li A. et al., Science 274, 1701 (1996); C. Journet et al. Appl. Phys. A, vol. 67, pp. 1-9 (1998); E.G. Rakov, Russ. Chem. Rev., vol. 69, no. 1 , pp. 35-52 (2000); X. Wang et al., Thin

Solid Films, vol. 390, pp. 130-133 (2001 ); and M. Okai, T. Muneyoshi, T. Yaguchi and S. Sasaki, Appl. Phys. Lett, vol 77, pp. 3468 (2000)). Some of these reports demonstrate the production of single-walled CNTs using arc and laser techniques. Generally, single-walled CNTs are preferred over multi-walled CNTs because single-walled CNTs have fewer defects and are stronger and more conductive than multi-walled CNTs of similar diameter.

There has been some success in producing single-walled CNTs from the catalytic cracking of hydrocarbons. Rope-like bundles of single-walled CNTs were generated from the thermal cracking of benzene using an iron catalyst and sulfur additive at temperatures between 1 100-1200 degrees Celsius ( ⁰ C) (see Cheng, H. M. et al., Appl. Phys. Lett. 72, 3282 (1998); and Cheng, H. M. et al., Chem. Phys. Lett. 289, 602 (1998)). These single-walled CNTs were roughly aligned in bundles and woven together, similar to those obtained from methods involving an electric arc or laser vaporization.

Single-walled CNTs have also been produced from the catalytic disproportionation of carbon monoxide (CO) (see, for example, Dai, H. et al., Chem. Phys. Lett. 260, 471 (1996)). Dai et al. reported that the diameter of SWNT generally varies from 1 nm to 5 nm and seems to be controlled by the metal catalyst particle size.

The longest individual CNTs reported thus far are only a few millimeters long (see Z. W. Pan et al., Nature, vol. 394, pp. 631 (1998); and S. Huang, W. Cai, and J. Liu, J. Am. Chem. Soc, vol. 125, pp. 5636-5637 (2003)). Previous reports of long CNTs have generally been limited to yarns (see, for example, K. Jiang, Q. Li, and S. Fan, Nature, vol. 419, pp. 801 (2002) and strands (see, for example, H. W. Zhu et al., Science, vol. 296, pp. 884 (2002) that were spun from much shorter CNTs. Longer CNTs could be made into CNT fibers that would likely be much stronger than any current engineering fiber. Long CNTs could act as scaffolding for neuronal growth (see, for example, H. Hu et al., Nanoletters vol. 4, no. 3 (2004) pp. 507-51 1 )), and may find use in micro- electric motors, neuronal implants, biological and chemical sensors, optical and electronic cables, micro electro-mechanical systems, and other important

applications. Therefore, methods for preparing longer carbon nanotubes are desirable.

There remains a need for carbon nanotubes that are longer than the carbon nanotubes that are currently available. Therefore, an object of the present invention is a method for preparing carbon nanotubes.

Another object of the present invention is a method for preparing carbon nanotubes that are longer than the carbon nanotubes that are currently available. Yet another object of the present invention is a method for preparing individual carbon nanotubes that do not have to be separated apart from, for example, yarn.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method for preparing carbon nanotubes. The method involves heating a transition metal species on a substrate in the presence of gaseous alcohol. The method may involve flowing the gaseous alcohol past the metal species. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiment(s) of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: FIGURE 1 shows a schematic representation of an apparatus used to prepare carbon nanotubes according to the invention.

FIGURE 2 shows scanning electron micrograph (SEM) images of a segment of an individual carbon nanotube prepared according to the invention, where each image shows a small part of the nanotube.

FIGURE 3 shows an optical photograph of the 48 mm long substrate on which a 4 cm long carbon nanotube was synthesized. The gray line was formed by superimposing on the substrate 230 SEM images of a carbon nanotube having a length of about 4 centimeters that was prepared according to the invention.

FIGURE 4 shows an Atomic Force Microscopy (AFM) image of a segment of a 4 cm long CNT prepared according to the invention; and FIGURE 5 shows a typical G-peak Raman spectrum of a long, semiconducting CNT prepared according to the invention.

DETAILED DESCRIPTION

The invention relates to the preparation of relatively long, single-walled carbon nanotubes. Individual, single-walled carbon nanotubes having a length up to about 4 centimeters in length were prepared. The invention was demonstrated by preparing carbon nanotubes having a length of about 4 centimeters long using an iron catalyst.

The practice of the invention can be further understood with the accompanying figures, which include representations of an apparatus used to prepare carbon nanotubes and SEM images of a carbon nanotube. Similar or identical structure is identified using identical callouts. FIGURE 1 shows a schematic representation of an apparatus used to demonstrate the invention. Apparatus 10 includes quartz tube 12 having inlet end 14 and outlet end 16. Inside tube 12 is placed substrate 18. A thin film of a solution 20 of soluble catalyst is placed on the end of substrate 18 that is near inlet end 14. Solution 20 was an ethanol solution that contained ferric chloride (FeCI ₃ ) catalyst (0.10 molar). The substrate was a silicon (100) substrate that measured about 0.8 cm wide by about 4.8 cm long. Inlet end 14 of quartz tube 12 was connected via connector 22 to inlet gas manifold 24 a manifold capable of sending gas through into tube 12. Outlet end 16 of the tube was connected via connector 26 to an outlet assembly that included optional vacuum pump 28. After putting

substrate 18 in tube 12, with solution 20 thereon, inlet end 14 was connected to inlet gas manifold gas manifold 24. Tube 12 was then placed into tube furnace 30. The furnace was powered on, heating quartz tube 12 and the substrate 18 with solution 20 inside, causing evaporation of solvent from solution 20. A flowing gas mixture (30 cc/min) of argon and hydrogen (about 94 percent argon, about 6 percent hydrogen) was then sent through end 32 of manifold 24, into inlet end 14, and into tube 12 while furnace 30 heated substrate inside to a temperature of about 900 degrees Celsius. After about 30 minutes, ethanol vapor was added to the hydrogen/argon gas mixture by sending hydrogen/argon gas through end 34 of inlet gas manifold 24; the hydrogen/argon gas bubbled through ethanol at a flow rate of about 20 cc/min. After about one hour more, the power to furnace 30 was turned off to allow quartz tube 12 to cool down. The substrate was removed, and the carbon nanotubes that formed on the substrate were examined. Carbon nanotubes that formed on the substrate were examined using scanning electron microscopy. FIGURE 2 shows several scanning electron micrograph (SEM) images of a segment of one of the carbon nanotubes. This carbon nanotube had a total length of about 4 centimeters, and required a total of 230 SEM images to see the entire tube from one end of the nanotube to the other end. Except for small amounts of overlapping, this nanotube was largely isolated from the other nanotubes that were present on the substrate. The images that are linked together to form FIGURE 2 collectively show a 1- millimeter long segment of the carbon nanotube. The legend included in FIGURE 2 indicates a length of 50 micrometers (μm). Two arrows mark the ends of this segment. The diameter of the CNT could not be determined from the SEM images because the bright CNT image in an SEM micrograph is much wider than the real diameter of the CNT (see, for example, Y. Homma, S. Suzuki, Y. Kobayashi, and M. Nagase, Appl. Phys. Lett., vol. 84, pp. 1750-1752 (2004)). Importantly, the ends of the 4 cm long carbon nanotube were located at the edges of the substrate. From this observation, it is believed that an even an even longer CNT would have been the result if a longer substrate had been used.

FIGURE 3 shows an optical photograph of the 48 mm long substrate on which the 4 cm long carbon nanotube was synthesized, side by side with a ruler. The nanotube is shown sketched onto the substrate. The grey line on the substrate indicates the position of the carbon nanotube as determined by superimposing on the substrate 230 SEM images taken of a 4 cm long carbon nanotube prepared according to the invention.

The diameter of the 4 cm long CNT was measured by atomic force microscopy (AFM). FIGURE 4 shows an Atomic Force Microscopy (AFM) image of a segment of the 4 cm long CNT. The image width is 2.5 μm. The inset is a height profile across the CNT, with a diameter determined as 1.4 nm from the peak height. The measured diameter of 1.4 nanometers indicates that the CNT is a single walled CNT.

Raman Spectra were obtained for about 20 different CNTs. These spectra indicate that the CNTs produced according to the invention include both metallic CNTs and semiconducting CNTs, and from the observed Raman frequencies in the radial breathing mode, which indicate a range of CNT diameters from about 1.30 nm to about 2.25 nm, they all appear to be single walled (see M. S. Dresselhaus et al., Carbon, vol. 40, pp. 2043-2061 (2002). FIGURE 5 shows a typical G-peak Raman spectrum of a long, semiconducting CNT produced according to the method of the invention. The G-peak

Lorentzian line shape and 13 cm ^"1 peak separation indicates that this CNT is a 1.9 nm (+ 0.2 nm) diameter semiconducting nanotube (see Y. Homma, S. Suzuki, Y. Kobayashi, and M. Nagase, Appl. Phys. Lett., vol. 84, pp. 1750-1752 (2004). Any change in the chirality of the CNT would have been indicated by a change in the Raman spectrum of the CNT along the length of the CNT. Only minor changes in the Raman spectra of the CNT over lengths up to about 2 mm were observed, indicating the chiral stability of the CNT along the its length. The density of CNTs was affected by the concentration of catalyst used in the solution, the calcination temperature, the thickness of the oxide layer on the silicon substrate, and other parameters. The initial heating rate and gas

flow rate were found to have only minor effects on the CNT length and CNT density. A sample with low CNT density was chosen in order to trace individual CNTs using scanning electron microscopy (SEM) and measure the length of individual CNTs. CNTs longer than 1 inch were consistently obtained using a variety of experimental parameters.

Initially, an ethanol solution of the transition metal containing species (FeCI ₃ in the demonstration example) was put onto one end of the substrate prior to calcinations. It should be understood that while ethanol was used to demonstrate the invention, any solvent (acetone, water, to name a few) capable of dissolving the transition metal containing species could also be used. The purpose of the solvent is to provide finely divided metal catalyst on the support after evaporation of the solvent.

While ferric chloride was used to demonstrate the invention, other transition metal catalysts, such as other iron containing species, and cobalt- containing species, and nickel containing species could also be used. Carbon vapor deposition using transition metal catalysts such as iron, cobalt, and nickel containing species, have been used to produce multi-walled CNTs as a main product instead of single-walled CNTs (see, for example, U. S. Patent Number 4,663,230). This invention relates to using these types of catalysts in the presence of alcohol vapor to produce long, single-walled carbon nanotubes.

The choice of substrate is an important aspect of the invention. Substrates that could be used with the present invention include silicon; silicon having a top layer of silicon dioxide; silicon carbide; silicon carbide with a top layer of silicon dioxide; silicon nitride; silicon nitride with a top layer of silicon dioxide; quartz; and glass. Attempts at growing long carbon nanotubes on a substrate of boron nitride (BN) were unsuccessful.

According to the invention, the carbon source for CNT growth was alcohol vapor. Alcohol vapor, preferably ethanol vapor, is used because alcohol is a clean burning material that tends not to produce amorphous carbon, and the use of a clean burning source of carbon is an important aspect of this invention. Other input gases that can be used with alcohol vapor include hydrogen (H ₂ ), inert gases such as argon, helium, and nitrogen; mixtures of

hydrogen and these inert gases. These other input gases were used during the initial heating stages to provide an inert and/or reducing atmosphere, so that the solution of transition metal catalyst species, when heated to remove the solvent, would release finely divided metal catalyst particles. It has been determined that while the use of hydrogen can be used to provide this reducing atmosphere, the use of hydrogen is not a critical part of the invention; inert gas such as argon can be used instead.

The temperature used in the demonstration example for decomposing the alcohol was about 900 degrees Celsius. It is expected that carbon nanotubes can be formed according to the invention when the substrate is heated to a temperature of from about 600 degrees Celsius to about 1200 degrees Celsius.

Because of the length of the CNTs that are capable of being produced, the invention is expected to have a significant impact for applications in which shorter carbon nanotubes are inadequate. It is expected that the relatively long carbon nanotubes produced according to the present invention can be used to make fibers that are much stronger than any current engineering fibers, and that the carbon nanotubes and fibers could be used for applications that include, but are not limited to, neuronal growth, micro electric motors, neuronal implants, biological and chemical sensors, optical and electronic cables, and micro electromechanical systems.

The foregoing description 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, and obviously many modifications and variations are possible in light of the _r above teaching.

The embodiment(s) were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.