Catalytic growth of single-wall carbon nanotubes from metal particles

[0001] 1. Field of the Invention

[0002] This invention relates generally to methods of producing single-wall carbon nanotubes, and to catalysts for use in such methods.

[0003] 2. Description of Related Art

[0004] Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.

[0005] Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen I), Large-Scale Synthesis Of Carbon Nanotubes, Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), Carbon Nanotubes, Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having multiple walls have been described in the prior art. Ebbesen II; Iijima et al., Helical Microtubules Of Graphitic Carbon, Nature, Vol. 354, p. 56 (Nov. 7, 1991).

[0006] Another known way to synthesize nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then diffuse through the particle and precipitate as part of nanotubes growing from one side of the particle. This procedure typically produces imperfect multi-walled nanotubes in high yield. See C. E. Snyder et al., International Patent Application WO 89/07163 (1989), hereby incorporated by reference in its entirety. Its advantage is that it is relatively simple and can be scaled to produce nanotubes by the kilogram.

[0007] Single-wall carbon nanotubes have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., Single-Shell Carbon Nanotubes of 1 nm Diameter, Nature, Vol. 363, p. 603 (1993); Bethune et al., Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls, Nature, Vol. 63, p. 605 (1993); Ajayan et al., Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis, Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., Single-Walled Carbon Nanotubes Growing Radially From YC2 Particles, Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al., Single-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters, Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., Carbon Nanocapsules Encaging Metals and Carbides, J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., Extrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source, Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., Improving Conditions Toward Isolating Single-Shell Carbon Nanotubes, Chem. Phys. Lett., Vol. 226, p. 364 (1994). While this arc discharge process can produce single-wall nanotubes, the yield of nanotubes is low and the tubes exhibit significant variations in structure and size between individual tubes in the mixture. Individual carbon nanotubes are difficult to separate from the other reaction products and purify.

[0008] High quality single-wall carbon nanotubes have also been generated by arc evaporation of a graphite rod doped with Y and Ni. See C. Journet et al., Nature 388 (1997) 756, hereby incorporated by reference in its entirety. These techniques allow production of only gram quantities of single-wall carbon nanotubes.

[0009] An improved method of producing single-wall nanotubes is described in U.S. Ser. No. 08/687,665, entitled Ropes of Single-Walled Carbon Nanotubes incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably wall nanotubes on the catalyst particles of less than 2-nanometer diameter but is formed into graphitic layers that encapsulate the larger catalyst particles, deactivating them as catalysts. Catalyst particles of greater than about 2 nanometers in diameter are more likely to form multiwall nanotubes, and since they are rendered ineffective by the process, the only remaining active catalyst particles support growth of primarily single-wall nanotubes. In a preferred embodiment, the method of this invention provides for treatment of supported catalyst material to deactivate catalyst particles that do not support growth of the desired nanotube types, with subsequent change of the feedstock composition or density to accelerate growth of the desired form of single-wall nanotubes. The method of this invention is capable of producing-material that is >50% SWNT, more typically >90% SWNT, or even >99% SWNT.

[0010] This invention also provides a catalyst/support system structured so that access of the feedstock gas to the catalyst is enhanced by that structure. Preferably, the catalyst is deposited so that there is clear distance between catalyst locations by dispersion of small catalyst particles on the substrate surface or other methods of deposition known to those skilled in the art.

[0011] The production of high quality single-wall carbon nanotubes, in some cases including double-wall carbon nanotubes, in yields much larger than previously achieved by catalytic decomposition of carbon-containing precursor gases is disclosed. The nanotubes formed are connected to and grow away from the catalyst particles affixed to the catalyst support surface. If the growth time is short, the tubes can be only a fraction of one micron long, but if the growth time is prolonged, single-wall carbon nanotubes in this invention can grow continuously to arbitrary lengths. The present invention demonstrates a means for nucleating and growing nanotubes only from the smallest of the supported catalyst particles, which produce single-wall carbon nanotubes, while deactivating the larger particles so that no multi-walled nanotubes are produced. This allows the growth exclusively of single-wall carbon nanotubes from catalyst systems previously thought to produce only larger diameter multi-walled nanotubes.

[0012] According to one embodiment of the present invention, a process for producing single wall carbon nanotubes is disclosed. The process comprises the nanotubes) unless the process is carried out with excess hydrocarbon feedstock. The product of a typical process for making mixtures containing single-wall carbon nanotubes is a tangled felt, which can include deposits of amorphous carbon, graphite, metal compounds (e.g., oxides), spherical fullerenes, catalyst particles (often coated with carbon or fullerenes) and possibly multi-wall carbon nanotubes. The single-wall carbon nanotubes may be aggregated in ropes or bundles of essentially parallel nanotubes.

[0013] Nanotubes prepared using the catalytic method of this invention tend to be less contaminated with pyrolytic or amorphous carbon than nanotubes prepared by prior art methods. Furthermore, by using a catalyst with a narrow size distribution, the nanotubes produced consequently have a narrow size distribution. This will minimize the need for post-production activities to clean up the nanotube preparation. To the extent that the nanotube product contains pyrolytic carbon which needs to be removed, various procedures are available to the skilled artisan for cleaning up the product. Suitable processes for purifying carbon nanotubes prepared according to this invention include the processes described in International Patent Publication WO 98/39250.

[0014] According to the invention, predominantly single-wall carbon nanotubes, with a portion of double-wall carbon nanotubes under some conditions, are produced with diameters in the range from about 0.5 to about 3 nm. Typically, no 5 to 20 nm diameter multi-walled nanotubes are produced by supported catalyst particles. The key difference responsible for these effects is that the growth reaction rate is limited by the supply of carbon to the catalyst particles, whereas the multi-walled nanotube growth is thought to be limited by the diffusion of carbon through the catalyst particles.

[0015] The single-wall nanotubes of the present invention may have lengths exceeding one micron. The length may be controlled by lengthening or shortening the amount of time the catalyst is exposed to the feedstock gas at an appropriate temperature and pressure. In one embodiment, under proper conditions the single-wall nanotubes can grow continuously to an arbitrary length.

[0016] Single-wall nanotubes formed in the present invention are observed to form into organized bundles or ropes as they grow from catalyst particles in close proximity to each other. Examples of this behavior are shown in FIG. 4b. Such ropes of SWNT may be removed from the supported catalyst for subsequent processing and/or utilization, or they may be used as is while still attached to the catalyst particle. SWNT prepared according to this invention using a supported catalyst with widely dispersed catalytic particles may be recovered prior to aggregation of the individual nanotubes. These nanotubes may be collected in the form of a mat or felt with random orientation in two dimensions or individually for particular uses.

[0017] Using the product nanotubes

[0018] Carbon nanotubes, and in particular the single-wall carbon nanotubes of this invention, are useful for making electrical connectors in micro devices such as integrated circuits or in semiconductor chips used in computers because of the electrical conductivity and small size of the carbon nanotube. This invention provides a means of establishing a carbon nanotube directly in contact with a surface, but extending away from that surface. This occurs naturally in the present invention as the tube is grown from a catalyst particle in contact with the surface of a larger object (the catalyst support). This invention's provision of a simple means for creating structures that comprise a support surface with one or more nanotubes attached and extending away from that surface is particularly useful in known applications of nanotubes as probes in scanning tunneling microscopes (STM) and atomic force microscopes (AFM) and as field emitters of electrons for electronic applications. The carbon nanotubes are useful as antennas at optical frequencies, and as probes for scanning probe microscopy such as are used in scanning tunneling microscopes (STM) and atomic force microscopes (AFM). The carbon nanotubes are also useful as supports for catalysts used in industrial and chemical processes such as hydrogenation, reforming and cracking catalysts. The nanotubes may be used, singularly or in multiples, in power transmission cables, in solar cells, in batteries, as antennas, as molecular electronics, as probes and manipulators, and in composites.

[0019] In order to facilitate a more complete understanding of the invention, an Example is provided below. However, the scope of the invention is not limited to specific embodiments disclosed in this Example, which is for purposes of illustration only.

[0020] 1. Preparation

[0021] Single wall carbon nanotubes may be grown by passing carbon-containing gases (CO or C₂H₄) at elevated temperatures over nanometer-size metal particles supported on larger (10-20 nm) alumina particles. Two different metal catalysts may be used, one containing pure Mo, the other containing Fe and Mo. The ratio of FE to Mo may be 9:1. Both catalysts were made using a method known in the art.

[0022] For each growth experiment, a quartz boat containing a carefully weighed amount (typically 20 mg) of the catalyst powder was placed in the center of a 1 inch quartz tube furnace. The system was purged with Ar, then heated under flowing reactant gases to an elevated temperature for a controlled time. The resulting catalyst material, which now also contains reaction products dominated by single-wall carbon nanotubes, was removed from the boat and weighed again. The yield is defined as the mass increase divided by the original catalyst mass. Samples were prepared for TEM imaging by sonicating this material in methanol and drop-drying the resulting suspension onto TEM grids.

[0023] 2. Production of single-wall carbon nanotubes

[0024] The production of single-wall carbon nanotubes by the disproportionation of CO over alumina-supported Mo particles is greatly improved. The catalyst is 34:1 alumina:Mo by mass. The reaction is carried out at 850 C. under a flow of 1200 sccm of CO at 900 Torr. The resulting material, which consists of single-wall carbon nanotube very monodisperse in diameter (0.8 to 0.9 nm), is shown in FIG. 1. Particles of the fumed alumina support, 10 to 20 nm in size, are also visible in this and subsequent TEM images. The yield of nanotubes is plotted as a function of reaction time in FIG. 2. The yield continues to increase even for very long reaction times.

[0025] CO also forms nanotubes with a second catalyst. The second catalyst is prepared with 90:9:1 alumina:Fe:Mo by mass. The reaction, when carried out exactly as described above for the alumina:Mo catalyst, yields nanotubes of a wider diameter distribution, 0.5 to 3 nm, with single-wall carbon nanotubes and some double-wall carbon nanotubes. A representative TEM image is shown in FIG. 3. For this catalyst, the yield increases with time initially, but is limited to about 40% after one hour of exposure. No additional mass increase is observed even for much longer exposures (up to 20 hours).

[0026] Single-wall carbon nanotubes from C₂H₄ have been grown using this technique. The 90:9:1 alumina:Fe:Mo catalyst is first reduced by exposing the catalyst to 1000 sccm Ar and 0.33 sccm H₂ at 800 C. for 30 minutes. The growth reaction then proceeds at the reaction temperature by adding 0.66 sccm C₂H₄ to the gas flow. The resulting product is nanotube bundles containing single-wall carbon nanotubes and double-wall carbon nanotubes, shown in FIGS. 4 and 5. One hundred nanotube cross sections were observed at several reaction temperatures to count the relative number of single- to double-walled nanotubes. The amount of double-wall carbon nanotubes increases from 30% at 700 C. to 70% at 850 C. Outer diameters of the individual tubes in a bundle range from 0.5 to 3 nm. There appears to be no correlation between outer diameter and number of walls, other than that the smallest nanotubes (<1 nm diameter) are never double-walled.

[0027] The mass yield of nanotubes increases at a similar rate for reaction temperatures from 700 C. to 850 C., but the termination is temperature dependent. For reactions at 850 C., the yield increases until it reaches 7%, at which point the growth terminates. As the reaction temperature is lowered, the yield reaches higher levels before growth termination. At 700 C., the growth does not terminate, but its rate decreases as shown in FIG. 2.

[0028] The present invention demonstrates the ability to grow nanotubes by catalytic decomposition of C₂H₄ and CO only from the small particles in a supported catalyst system, leading to the growth of single-wall carbon nanotubes and deactivation of multi-walled nanotube growth by encapsulation of larger particles. For certain conditions, nanotubes can be grown to arbitrary length, but become limited by the diffusion of reactants to the catalyst particles. This problem has been solved for the production of multi-walled nanotubes from this catalyst by using flat alumina flakes, as opposed to fumed alumina particles, so that the nanotubes grow aligned in large bundles, keeping their growing ends exposed to the gaseous feedstock. Similar modifications to the current technique may allow the bulk production of single-wall carbon nanotubes.

[0029] While the invention has been described in connection with preferred embodiments, it will be understood by those skilled in the art that other variations and modifications of the preferred embodiments described above may be made without departing from the scope of the invention. Other embodiments will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification is considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.