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• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y40/00—Manufacture or treatment of nanostructures
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
  • B82B3/00—Manufacture or treatment of nanostructures by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
  • B82B3/0009—Forming specific nanostructures
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/152—Fullerenes
  • C01B32/156—After-treatment
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/159—Carbon nanotubes single-walled
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/16—Preparation
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/16—Preparation
  • C01B32/162—Preparation characterised by catalysts
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/02—Single-walled nanotubes
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/20—Nanotubes characterized by their properties
  • C01B2202/36—Diameter

Definitions

• the invention relates to methods for preparing single walled carbon nanotubes. More specifically, the invention relates to methods for preparing single walled carbon nanotubes from a metal layer which is in contact with fullerenes on one side and in contact with a solid carbon source on the other side. Alternatively, instead of a solid carbon source, the metal layer may be saturated with carbon atoms or may be in contact with a non- solid carbon source.
• Carbon nanotubes are vermicular carbon deposits having diameters less than 1.0 ⁇ , preferably less than 0.5 ⁇ , and even more preferably less than 0.2 ⁇ .
• Carbon nanotubes can be either multi walled (i.e., have more than one graphite layer on the nanotube axis) or single walled (i.e., have only a single graphite layer on the nanotube axis).
• Other types of carbon nanotubes are also known, such as fishbone fibrils (e.g., resembling nested cones), etc.
• carbon nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes (i.e., dense, microscopic particulate structure comprising entangled or bundled carbon nanotubes) or a mixture of both.
• Carbon nanotubes are distinguishable from commercially available continuous carbon fibers. For instance, the diameter of continuous carbon fibers, which is always greater than 1.0 ⁇ and typically 5 to 7 ⁇ , is far larger than that of carbon nanotubes, which is usually less than 1.0 ⁇ . Carbon nanotubes also have vastly superior strength and conductivity than carbon fibers.
• Carbon nanotubes also differ physically and chemically from other forms of carbon such as standard graphite and carbon black.
• Standard graphite because of its structure, can undergo oxidation to almost complete saturation.
• carbon black is an amorphous carbon generally in the form of spheroidal particles having a graphene structure, such as carbon layers around a disordered nucleus.
• carbon nanotubes have one or more layers of ordered grapheme carbon atoms disposed substantially concentrically about the cylindrical axis of the nanotube.
• Multi walled and single walled carbon nanotubes differ from each other.
• multi walled carbon nanotubes have multiple layers of graphite along the nanotube axis while single walled carbon nanotubes only have a single graphitic layer on the nanotube axis.
• the methods of producing multi walled carbon nanotubes also differ from the methods used to produce single walled carbon nanotubes. Specifically, different combinations of catalysts, catalyst supports, raw materials and reaction conditions are required to yield multi walled versus single walled carbon nanotubes. Certain combinations will also yield a mixture of multi walled and single walled carbon nanotubes.
• Processes for forming multi walled carbon nanotubes are well known. E.g.,
• This band is commonly referred to as the 'D-band' and the position of this band has been shown to depend strongly on the laser excitation wavelength.
• "Radial breathing modes (RBM)” typically below 300 cm “1 ) were observed with single-walled nanotubes, where all the carbon atoms undergo an equal radial displacement. A small change in laser excitation frequency produces a resonant Raman effect. Investigation in the RBM has shown it to be inversely proportional to the SWCNT diameter. This relationship is expressed in the following equation,
• diameter of a nanotube is defined as the distance between the nuclei of carbon atoms at opposite ends of a tube diameter. It is to be understood that this diameter differs from distance of closest approach by a second nanotube which is greater because of the repulsion of the respective ⁇ clouds as often defined by TEM.
• Table A presents sample diameter and CO RBM correlations as previously reported in Tables I and II of Jorio, A, et al., "Structural (n,m) Determination of Isolated Single- Wall Carbon Nanotubes by Resonant Raman Scattering," Physical Review Letters, The American Physical Society, Vol. 86, No. 6, pp. 1118-21 (February 5, 2001), herein incorporated by reference:
• the present invention provides novel methods of preparing single walled carbon nanotubes from an arrangement comprising a metal layer, fullerenes in contact with one side of said metal layer, and a solid carbon source in contact with the other side of said metal layer.
• the fullerene/metal layer/solid carbon source assembly Once the fullerene/metal layer/solid carbon source assembly has been prepared, it is heated to a temperature below where said fullerenes sublime.
• the solid carbon source and fullerenes are permitted to dissolve at least in part at the metal layer interface and single walled carbon nanotubes are grown on the fullerene side of the metal layer.
• the temperature may be increased after the fullerenes have nucleated nanotubes to permit greater growth of single walled carbon nanotubes (e.g., 700-1100 0 C).
• any type of fullerenes may be used (e.g. , C60,
• the fullerenes may be deposited in one or more layers of closely packed arrangements onto the metal layer.
• the metal layer may be comprised of a metal catalytic for the growth of single walled carbon nanotubes, such as Fe, Co, Mn, Ni, Cu and Mo.
• the metal layer is preferably of a thickness which permits the diffusion of carbon from the solid carbon source from one side of the metal layer to the other side of the metal layer (e.g., 1-20 nm, 2-20 nm, 3-5 nm, etc.).
• the solid carbon source may be carbon fibers or any other solid carbon source known in the art.
• the single walled carbon nanotubes are prepared from a metal layer in contact with fullerenes and a non-solid carbon source using similar methods as described for a metal layer in contact with a solid carbon source.
• the single walled carbon nanotubes are prepared from a metal layer in contact with fullerenes and saturated with carbon atoms using similar methods as described for a metal layer in contact with a solid carbon source.
• the methods of the present invention grow a multiplicity of single walled carbon nanotubes, wherein at least 80% of said single walled carbon nanotubes in said multiplicity have a diameter within + 5% of a single walled carbon nanotube diameter D present in the multiplicity. Diameter D may range between 0.6-2.2 nm.
• Figure 1 is an illustration of the fullerene/metal layer/solid carbon source arrangement in accordance with an exemplary embodiment of the present invention.
• Figure 2 is an illustration of the dissolving of the fullerenes and the beginning of single walled carbon nanotube growth in accordance with an exemplary embodiment of the present invention.
• Figure 3 is an illustration of the growth of single walled carbon nanotubes in accordance with an exemplary embodiment of the present invention.
• Figures 4 A and B are SEM micrographs of carbon nanotubes grown on
• Figures 5 A and B are transmission electron microscopic images of CNTs grown from a sandwich catalyst.
• the present invention provides a new method for preparing single walled carbon nanotubes from an arrangement of fullerenes, a metal layer, and a solid carbon source.
• a metal layer is formed or placed onto the surface of the solid carbon source, resulting in one side of the metal layer being in contact with and supported by the solid carbon source.
• Fullerenes are placed or deposited onto the other side of the metal layer.
• the metal layer is said to be in contact with fullerenes on one side and a solid carbon source on the other side.
• the assembly of this arrangement can be done in any order.
• fullerene/metal layer/solid carbon source arrangement or sandwich is then heated in an inert atmosphere to a temperature just below ⁇ e.g., within 1O 0 C or within 5 0 C) that at which the fullerenes sublime. It will be appreciated that this is a dynamic system: fullerenes are simultaneously vaporizing and dissolving into the metal layer.
• the "apparent" sublimation temperature ⁇ e.g., about 65O 0 C for C60 fullerenes at atmospheric pressure), is best determined by thermogravimetric analysis of an actual sandwich.
• Operable temperature ranges can be between about 500 0 C to 700 0 C, at atmospheric pressure, depending on the fullerenes used. If the growth step is carried out at elevated pressures, even higher fullerene sublimation temperatures may be encountered. It is believed that changes in pressure may lead to changes in equilibrium partial pressure of the fullerene in the gas phase, and thus affect the driving force for vaporization. In any event, at the aforementioned temperature, the fullerenes and the solid carbon source dissolve into the metal layer until the thermodynamic activity of the dissolved carbons exceed that of carbons in single walled carbon nanotubes.
• the partially dissolved fullerenes in contact with the metal layer at this stage then nucleate or otherwise promote the growth of single walled carbon nanotubes since the thermodynamic activity of carbon in the walls of the single walled carbon nanotube is lower (e.g., more stable) than in the heated fullerene or solid carbon source.
• the partially dissolved fullerene would fittingly serve as an end cap for a single walled carbon nanotube of the same diameter, and thus is an excellent "seed" for single walled carbon nanotube growth.
• the single walled carbon nanotube may be of a different diameter from the original "seeding" fullerene end cap.
• the fullerene can alternatively also serve as a nucleation promoter. That is, the fullerenes serve to promote the nucleation and growth of single walled carbon nanotubes.
• a bundle of single walled carbon nanotubes having a uniform diameter of 1.6 nm may result under certain conditions from 0.7 nm fullerenes.
• the promotion effect of fullerenes can be seen from the narrow diameter distribution of grown single walled carbon nanotubes. This results in the Raman spectrum of such product usually presenting a single peak in the RBM region instead of multiple signals indicating several different diameter populations.
• a gaseous carbon source can be introduced.
• Useful gaseous carbon sources are CO, hydrocarbons and alcohols. It will be appreciated that introduction of a gaseous carbon source in principle allows the growth process to be carried out indefinitely, rather than being limited by the quantity of solid carbon source. A continuous process is thus feasible.
• the growth pattern of single walled carbon nanotubes can be influenced by the arrangement of fullerenes on the metal layer. For example, where the fullerenes are arranged in closely packed layers at the surface of the metal layer, single walled carbon nanotubes can grow as a close-packed quasi-crystalline rope or bundle to stabilize the metal-carbon interface. Single walled carbon nanotube growth continues as a result of carbon from the solid carbon source dissolving into one side of the metal layer and diffusing to the other side of the metal layer and into the nucleated tubes.
• the initial reaction temperature should be below that at which the fullerenes sublime in order to permit the fullerenes to partially dissolve into, for example, a hemisphere or hemispherical configuration which would be a fitting end cap for single walled carbon nanotubes and thus serve as a "seed" for the growth (or seed to promote the growth) of single walled carbon nanotubes.
• the reaction temperature may be increased in order to result in higher or faster growth rates (e.g., the lengthening or elongation of the nanotube itself). Preferred higher temperatures range between about 700 0 C to HOO 0 C.
• the single walled carbon nanotube growth is permitted to continue until a desired or usable length is attained.
• Fullerenes are a well known term of art used and recognized in the industry to refer to a form of carbon typically consisting of only carbon atoms bound together to make a roughly spherical ball (e.g., a "buckyball"). As such, the most commonly used fullerenes have sixty carbon and are known as C60 fullerenes. Any other forms of fullerenes which contain more or less than sixty carbon atoms, such as C70, ClOO, C36, etc., may also be used in accordance with the present invention.
• Fullerenes have an approximately spherical shape ("spheroidal").
• the end of single walled carbon nanotubes is typically in the form of a hemisphere.
• a half-dissolved fullerene (which resembles a hemisphere) would be a fitting end cap for a single walled carbon nanotube of the same diameter.
• a partially dissolved fullerene by its hemispherical nature, would be an excellent "seed" to facilitate single walled carbon nanotube growth because its hemispherical shape is consistent with the hemispherical shape of an end of a single walled carbon nanotube.
• bundles of single walled carbon nanotubes can be nucleated and grown from a plurality of fullerenes.
• the size of the fullerenes can be used to control the sizes of the single walled carbon nanotubes.
• a skilled artisan seeking to have predominately larger sized single walled carbon nanotubes would use ClOO fullerenes instead of the smaller C36 fullerenes, as the diameter of the ClOO fullerenes is larger.
• the use of fullerenes as the seeds or nucleation points also permit greater control over the size/diameter distribution or variation of the single walled carbon nanotubes. For example, using all C60 fullerenes will result in a narrower distribution/variation of single walled carbon nanotube sizes/diameters as compared to other processes which do not control the size of the starting nucleation point or seed.
• the fullerenes are placed on a metal layer which helps to facilitate single walled carbon nanotube growth.
• the fullerenes are placed onto the metal later without initial contact with any possible contaminant sources.
• Known methods for accomplishing this task include sputtering and atomic deposition. Other conventional methods may be used.
• the number of fullerene layers on the metal layer is enough to substantially saturate the metal layer.
• the metal layer is comprised of a metal catalytic for the growth of single walled carbon nanotubes.
• the metal layer may comprise a metal selected from the group consisting of Fe, Co, Mn, Ni, Cu and Mo. Other metals which can catalyze single walled carbon nanotubes may be used as well.
• the metal layer may be in the form of a film, coating, sheet, membrane, etc.
• the metal layer be uniform in composition and smooth on its surface.
• the metal layer should be of a thickness that permits the diffusion of dissolved carbon from the carbon solid source (discussed below) on one side of the metal layer to the other side of the metal layer.
• the thickness of the metal layer may be between about 1 nm to 20 nm, preferably about 2 nm to 10 nm, or more preferably, about 3 nm to 5 nm.
• Different metals may result in different thickness limitations depending on its carbon solubility and mass transfer properties. For example, Fe is a preferred metal since its carbon solubility is high and permits more efficient mass transfer of carbon atoms from one side of the metal layer to the other side.
• a solid carbon source In contact with the side of the metal layer opposite the fullerenes is a solid carbon source.
• the solid carbon source provides the supply of carbon atoms which permits the single walled carbon nanotubes to grow. Specifically, the solid carbon source dissolves into the metal layer and diffuses to the other side of the metal layer to become a part of the single walled carbon nanotubes as they grow.
• the solid carbon source is free or substantially free of voids which may interrupt or distort the carbon/metal interface as the carbon dissolves into the metal.
• the solid carbon source is also preferably free or substantially free of non-carbon heteroatoms which may react with the metal layer to deactivate it or form gases which separate the metal layer from the solid carbon source. If there are heteroatoms present, it is preferred that they not participate in the carbon nanotube growth. For example, hydrogen would be a preferred heteroatom because it dissolves into the metal layer, diffuses through it and then leaves the metal/carbon nanotube interface as hydrogen gas.
• the surface of the solid carbon source in contact with the metal should have a high ratio of edge to basal plane carbon to stabilize the metal film.
• solid carbon sources there are a number of solid carbon sources that can be used in the present invention.
• glassy carbon is a viable source if it has not been graphitized to the extent that its thermodynamic activity is lower than that of the single walled carbon nanotubes.
• Pure carbon pitches such as those made by pyro lysis of poly cyclic aromatic hydrocarbons are also a viable solid carbon source, as are cross-linked carbon resins made by cyclotrimerizing or oxidative Iy coupling diethynylbenzenes. Needle-like crystals of polyparaphenylene made by anodic oxidation of benzene may also be used.
• Commercially available carbon fibers are preferred carbon sources.
• Pitch based as opposed to PAN based carbon fibers are preferred.
• the most useful carbon fibers are those having as many graphene layer edges as possible on the fiber surface. This can be determined by SEM. Vapor grown carbon nanofibers such as Pyrograf I and Pyrograf III from Applied Sciences Corp. Or VGCF from Showa Denka Corp. are also useful carbon sources.
• a non-solid carbon source such as a gaseous or liquid carbon source can be used in place of the solid carbon to provide the supply of carbon atoms which permit the single walled carbon nanotubes to grow.
• the non-solid carbon source need not be limited to contacting the metal layer on the side opposite the fullerenes. All that would be required is that the non-solid carbon source diffuse into and/or through the metal layer to become a part of the single walled carbon nanotubes as they grow.
• gaseous carbon sources include hydrocarbons, CO and alcohols.
• single walled carbon nanotubes may be grown from a metal layer that is saturated with carbon atoms. Any known methods and physical state of the carbon source (e.g., solid, liquid, gasesous) may be used to saturate the metal layer since all that is important in this embodiment is that there be a supply of carbon atoms to permit growth of single walled carbon nanotubes.
• the Resulting Single Walled Carbon Nanotubes e.g., solid, liquid, gasesous
• the methods of the present invention grow a multiplicity of single walled carbon nanotubes, wherein at least 80% of said single walled carbon nanotubes in said multiplicity have a diameter within + 5% of a single walled carbon nanotube diameter D present in the multiplicity.
• the diameter D represents the diameter of a particular single walled carbon nanotube present in the multiplicity by which at least 80% (preferably 80-90%, more preferably 80-95%, even more preferably 80-99%) of the remaining single walled carbon nanotubes within the multiplicity have diameters within + 5% of D.
• the diameter D may be measured using Raman spectroscopy and is preferably in the range between 0.6-2.2 nm, more preferably 1.0 to 1.8 nm, even more preferably 1.2 to 1.6 nm.
• Example 1 Preparation of solid carbon source through carbonization of polymers
• Solid carbon source was first prepared via carbonization of polymeric compound.
• a solution containing 10-30% polymer such as PAM-3k, phenolic resin, polyvinyl chloride and pitch was prepared by dissolving corresponding amount of polymer in a suitable solvent such as water, alcohol, ketone, ester, etc.
• a platinum wire was then immersed into such solution and a polymer coating was formed on the surface of metal wire after solvent evaporation. The thickness of formed polymer coating was estimated in the range of 1-3 mm.
• the coated Pt wire was placed mounted inside a metal evaporator, MEM-010 manufactured by Balzers Union Ltd. By passing current through the Pt wire, the Pt wire was heated through its resistance and the polymer was carbonized. The process was monitored by vacuum pressure until no pressure increase was recorded.
• Example 1 The catalysts made in Example 1, 2 and 3 were heated via resistance heating and controlled at 500-1000 0 C via electric current inside the metal evaporator under vacuum.
• the treated samples were examined by SEM ( Figures 4 A and 4 B) and TEM ( Figures 5 A and 5 B) and multiwalled carbon nanotubes with diameter of 6-10 nm were observed
• a sandwich- structured catalyst precursor was prepared similar to the description in Example 1-3.
• a silicon wafer was first deposited with phenolic resin emulsion via dip coating. Then the coated sample was heated in argon at 1000-1200 0 C in order to carbonize the polymer into solid carbon. After the carbon formation, the coated Si wafer was placed in a metal evaporator, e.g. MED-OlO, and a metal such as Fe, Co, Ni or Cu was deposited on the wafer surface via physical vapor deposition. The thickness of metal coating was monitored by a quartz positioner and controlled at 1-5 nm. Without taking the wafer outside the vacuum chamber, another Ceo coating was placed on top of metal coating subsequently as described in earlier examples. The thickness of C 6O was approximately 5-10 nm.
• the final catalyst format was C6o/[Fe, Co, Ni or Cu]/solid carbon/Si.
• the Si wafer supported catalyst is placed in a 1-inch quartz reactor that has been purged by argon for 10 minutes. Then the reactor is sealed at both ends and the temperature is raised quickly to 800 0 C and the sample is allowed to react for 10 minutes under argon. After cooled to room temperature, the sample is examined using Raman, and exhibits characteristic features of single -wall carbon nanotubes with diameter of 1 A ⁇ 0.2 nm.

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