SINGLE-WALL CARBON NANOTUBES

Abstract: A method of producing essentially pure single-walled carbon nanotubes (SWNTs), comprises the steps of: long-laser pulsing a graphite target, refluxing the pulsed material in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized; separating the acid from the material; and oxidizing the carbon coating. The invention further provides essentially pure single-walled carbon nanotubes (SWNTs) consisting essentially of, in percent by weight: SWNTs greater than 98; and metal less than 0.5.

Description: PURE SINGLE-WALL CARBON NANOTUBES The United States Government has rights in this invention pursuant to Contract No. DE-AC36-99GO10337 between the United States Department of Energy and the Midwest Research Institute. Technical Field. This invention relates to single-wall carbon nanotubes and, in particular, a method for the making of essentially pure single-wall carbon nanotubes from a starting material. Background Art. Single-wall carbon nanotubes ("SWNTs") are projected to have a variety of electronic applications and may also be useful in gas storage and purification processes as well as in the construction of strong, light composite materials. However, in order to realize any of these applications single- walled nanotube materials of high purity must be produced. Pure SWNT materials obtained by a scalable purification method, would greatly facilitate rapid advancement in all venues of SWNT research. Methods for a complete, simple, non-destructive purification technique are not currently available. See, e.g.. A.G. Rinzler, et al, Applied Physics A 1998, 6~, 29 (purification method yields materials of 90 wt % purity after more than 21 steps and several days of processing); S. Bandow, et al, J. Phys. Chem. B 1997, 101, 8839 (not suitable for large-scale application); E. Dujardin, T.W. Ebbesen, A. Krishnan & M.M.J. Treacy. Adv Materials 1998, 10, 611 (incomplete, destructive); K. Tohji, et al., Nature 1996, 383, 679 (incomplete). Disclosure of Invention. Therefore, it is an object of the invention to provide single-walled carbon nanotube materials having high purity. It is a further object of the invention to provide a non-destructive purification process that is readily scalable and still results in materials having high purity. It is yet another object of the invention to provide a method by which the single-walled nanotube wt % in a raw soot material may be accurately determined. SUΕSTITUTE SHEET (Rule 26 ) Briefly, the invention provides a method of producing essentially pure single- walled carbon nanotubes (SWNTs), comprising the steps of: long-laser pulsing a graphite target, refluxing the pulsed material in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized; separating the acid from the material; and oxidizing the carbon coating. The invention further provides essentially pure single-walled carbon nanotubes (SWNTs) consisting essentially of, in percent by weight: SWNTs greater than 98; and metal less than 0.5. Brief Description of Drawings. FIG. la is a transmission electron microscope image of crude 4.2 W laser generated SWNT soot. FIG. lb is a transmission electron microscope image of a crude material which was refluxed for 16 h in 3M HNO 3 . FIG. lc is a transmission electron microscope image of purified SWNTs produced by oxidizing the acid treated sample for 30 min. in air at 550 °C. FIG. Id is a transmission electron microscope image of purified tubes at high magnification after annealing to 1500 °C in vacuum. FIG. 2 is a thermal gravimetric analysis of 1-2 mg samples ramped from 25 - 875 °C at 5 degrees per minute in a platinum sample pan under 100 seem flowing air. FIG. 2a shows materials produced at a laser power of 4.2 W; fully purified, crude soot, and crude soot after a 16 h reflux in 3M HNO 3 . The data for the refluxed material was normalized to 100 wt % at 100 °C to compare dry weights. FIG. 2b shows materials produced with 6W of laser power. Samples were refluxed in 3M HNO 3 for 4, 16, and 48 h. These curves were normalized to 100 wt % at 100 °C to compare dry weights, and then re-normalized to account for the different weight losses in the HNO refluxes. FIG. 3 is a Raman spectra obtained at 488 nm with a resolution of 2-6 cm "1 for purified, crude, and crude material which was refluxed for 16 h in 3M HNO 3 acid. The inset of the figure shows the region from 1200 - 1500 cm "1 at an amplified intensity scale. Best Mode for Carrying Out the Invention A dilute HNO 3 reflux of a long-laser pulsed material enables the isolation of essentially pure SWNTs via air oxidation The reflux is performed for sufficient time to produce a carbon coating on the SWNTs which can be removed by oxidation, but which does not result in damage or digestion of SWNTs The invention provides a non-destructive, 4-step, growth and purification process that is readily scalable and results in materials with > 98 wt % purity The process is based on the fact that non-nanotube carbon fractions can be functionalized and reorganized into a reactive uniform coating by a dilute nitric acid reflux This enables the selective removal of the impurities by oxidation in air The invention further provides a technique by which the SWNT wt % in a raw material may be accurately determined Unless specifically defined otherwise, all technical or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described Referring now to the drawing figures, SWNT materials were synthesized by a laser vaporization method similar to that reported by A Thess, et al, Science 1996, 273. A distinct difference being, however, that a single Nd YAG laser was used which produced gated laser light ranging in duration from 300 to 500 ns The gated laser light contained numerous short laser pulses ranging in duration from 5 to 15 ns The emission wavelength was 1064 nm and at an average power of 4 - 6 W Although not necessary, a gating rate of Hz was employed Material was produced at rates of 75 - 150 mg / h Targets were made by pressing powdered graphite doped with 0 6 at % each of Co and Ni in a 1 1/8" inch dye Crude soot containing SWNTs was produced at 1200 °C, with 500 Torr Ar flowing at 100 seem The transmission electron microscope ("TEM") image in FIG. la reveals the components of the laser-generated material Bundles of SWNTs span between large agglomerations of amorphous and micro-crystalline carbon which contain metal nanoparticles Typical raw materials were estimated to contain ~ 20 - 30 wt % SWNTs by a detailed analysis of numerous different TEM images, A C Dillon, P A Parilla, K M Jones, G Riker & M J Heben, Mater. Res. Soc. ConfProc. 1998, 526, 403. Inductively coupled plasma spectroscopy ("ICPS") indicated the laser-generated crude material has the same metal content as the targets (~ 6 wt %) in contrast to previous studies where an enrichment of metal in the crude soot was observed, E Dujardin, T W Ebbesen, A Krishnan & M M J Treacy, Adv. Materials 1998, 10, 611 Approximately 80 mg of crude soot produced at 4 2 W was refluxed in 60 ml 3M HNO 3 for 16 h at 120 °C The solids were collected on a filter that allows ready separation of the nanotubes, such as 0 2 μm polytetrafluoro ethylene coated polypropylen, and rinsed with deionized water After drying, an 82 wt % yield was obtained The weight lost is consistent with the digestion of the metal and an additional ~ 12 wt % of the crude material The reflux caused the non-nanotube carbon fractions to be redistributed as a uniform coating on the SWNTs as seen in FIG. lb The material was then oxidized in stagnant air inside a tube furnace at 550 °C for 30 minutes In this manner, the carbon coating was completely removed leaving behind pure SWNTs, corresponding to ~ 20 wt % of the crude material (FIG. lc) FIG. Id displays the purified tubes at high magnification Thermal gravimetric analysis ("TGA") revealed the purity of the isolated SWNTs The decomposition temperature (Td) is 735 °C, as determined by the derivative of the TGA curve, for the pure SWNTs displayed in FIG. 2a The purified tubes are very stable presumably due to the lack of dangling bonds or defects at which oxidation reactions may initiate The final purity is estimated to be >98 wt % since <1 wt % is consumed below 550 °C, and <1 wt % remains above 850 °C The metal content of these pure SWNTs was measured to be below 0 5 wt % by ICPS TGA was also used to evaluate the crude and acid-refluxed materials to illuminate the key features of the purification process The data for the crude soot (FIG. 2a) shows a shght increase in weight at low temperatures due to the oxidation of the Ni and Co metals The carbonaceous fractions begin to combust at ~370 °C and are mostly removed by oxidation below 600 °C A small final weight loss at ~650 °C can be attributed to oxidation of surviving SWNTs (~4 wt %) The majority of SWNTs in the crude soot are combusted along with the other carbonaceous materials at lower temperatures The weight remaining at 875 °C corresponds to the weight expected for the oxidized metals (~ 8 wt %) The TGA data is different after crude materials are refluxed for 16 h in 3M HNO 3 (FIG. 2a) A first thing to note is that refluxed samples getter as much as 10 wt % water from lab air, while purified and crude samples remain relatively dry More importantly, the onset of non- nanotube decomposition occurs at a lower temperature for the refluxed material and is completed before the onset of SWNT combustion A plateau extending from 550 to 650 °C is clearly evident in the TGA data because the oxidation now occurs in two separate regimes The sample weight is reduced to approximately zero by 850 °C since all carbonaceous materials have been removed and essentially no metal is left The acid treatment not only removes the metal but also produces carboxyl, aldehyde, and other oxygen - containing functional groups on the surfaces of the non-nanotube carbonaceous fractions As a result, the coating is extremely hygroscopic and reactive towards oxidation, enabling efficient purification The combustion of non-nanotube fractions is essentially complete at the inflection point in the TGA curve of the refluxed soot at 560 °C At this point, the sample consists of only pure SWNTs which amount to ~26 wt % of the dry refluxed material, or -21 wt % of the pre-reflux weight This latter value is in excellent agreement with the yield after refluxed material was heated to 550 °C in stagnant air (~20 wt %), and considerably higher than the tube content determined by TGA analysis of the crude material (~4 wt%) The quantitative agreement between the bulk oxidation in stagnant air and the TGA measurements under dynamic conditions suggests that neither route consumes an appreciable amount of SWNTS In fact, neither longer times in stagnant air at 550 °C (up to 1 h) nor holding at 550 °C during TGA experiments produces further significant weight loss The final product is pure since the weight-loss proceeds as expected for oxidation of a single phase above 550 °C, and the TEM image of FIG. lc shows only SWNTs To determine if tubes are damaged or consumed during the dilute acid reflux, TGA was performed on materials which were refluxed for 4, 16, and 48 h in 3M HNO 3 (FIG. 2b) Materials for these experiments were produced with 6 W of laser power The TGA data was adjusted for the dry-weight lost during reflux so that the y-axis represents the wt % remaining of the initial crude material The data for the 4 and 16 h refluxes completely overlay at temperatures above ~450 °C, and a plateau associated with SWNT stability is observed at 540 °C and a SWNT content of 17 wt % The data sets are virtually identical at the higher temperatures despite the difference in the material weights which were lost during the refluxes Since the SWNT content is determined to be the same in both cases, neither reflux consumes a significant number of tubes As discussed earlier, tubes are not consumed by oxidation below 550 °C, so the 17 wt % value can be taken as an accurate assessment of the SWNT content in the crude soot Once again, this value was found to be in good agreement with the yield determined by batch oxidation at 550 °C of material refluxed for 16 h in 3M HNO 3 It is interesting to note that the SWNT content in the 6 W material is lower than in the 4 2 W material Such a quantitative comparison was not possible prior to the purification and assessment methods disclosed herein Unlike the 16 h process, the 4 h reflux did not always permit good purification by oxidation In these cases, a TGA curve very similar to that of the crude material was observed The oxidation reactions are no longer well-separated after a 48 h reflux (FIG. 2b), and there is only a slight indication of a SWNT stability plateau at ~ 625 °C The affinity for water is considerably less than in either the 4 or 16 h samples The thick, uniform, hydrophilic carbon coating produced after 16 h of refluxing and thought to be necessary for purification was not observed by TEM Instead, a generally thinner and patchy film was found along with occasional agglomerations In contrast to the TEM data of FIG. lb, SWNTs could be readily imaged and portions of tubes were observed to be sharply angled, cut and damaged The extended reflux digests most of the non-nanotube carbon and begins to attack the SWNTS These cut and defective tubes are more susceptible towards oxidation such that only ~8 wt %, or < 50 % of the tubes known to be present, are found at the inflection point in the curve at 625 °C (FIG. 2b) Raman spectroscopy further elucidates the HNO 3 reflux process The Raman spectra displayed in FIG. 3 for purified and crude materials both exhibit a strong feature at 1593 cm "1 with shoulders at 1567 and 1609 cm "1 as expected for the SWNT tangential C-atom displacement modes However, the broadened feature at 1349 cm "1 in the crude spectrum indicates the presence of impurities and a contribution from the disordered sp 2 carbon "D band" of non- nanotube graphitic components, P C Eklund, J M Holden & R A Jishi, Carbon 1995, 33,959, Y Wang, D C Alsmeyer & R L , McCreedy, Chem. Mater. 1990, 2, 557 Unlike other reports, G Rinzler, etal, Applied Physics A 1998, 67, 29, S Bandow, etal, J. Phys. Chem. B 1997, 101, 8839, no spectral evidence for Ceo was ever detected in any of our materials After the 16 h reflux, the D band intensity is significantly increased indicating decreasing domain size (FIG. 3) Also, a signal derived from the fundamental E 2g mode of disordered graphite is observed where the SWNT modes are expected The disordered graphite coating (FIG. lb) evidently prohibits observation of the resonantly enhanced SWNT modes This is striking since the SWNT content in the 16 h HNO 3 treated sample is actually higher than in the crude material, and demonstrates that Raman spectroscopy can be poorly suited for determining SWNT contents in certain types of samples The D-band is narrower in purified materials as observed by others (S Bandow, A M Rao, D A Williams, A Thess, R E Smalley, P C Eklund, J Phys, Chem B 1997, 101 (8839- 8842) presumably due to curvature-induced enhancement of electron-phonon coupling (J Kastner, T Pichler, H Kuzmany, S Curran, W Blau, D N Weldon, M Delamesiere, S Draper, H Zandbergen, Chem Phys Lett 1994, 221, 53-58) Conclusively, the 16 h 3M HNO 3 reflux decreases the domain size of the disordered carbon and produces a uniform carbon coating on the SWNTs without damaging them Our own temperature programmed desorption studies show that the nitric acid reflux introduces reactive functional groups onto the surfaces of the non-nanotube carbon material These two effects serve to maximize the surface area of the nonnanotube carbon and provide for enhanced oxidation kinetics Furthermore, since the functionalized coating is oxidized at lower temperatures, and the coating is evenly distributed, the heat generated by the exothermic reactions does not initiate oxidation of SWNTs In contrast, SWNTs in raw materials are consumed simultaneously with impurities because the oxidation of agglomerated impurities generates local "hot spots" It is a combination of the high-surface-area, decreased domain size, degree of functionalization, and uniformity of the carbon film produced by the 16 h 3M HNO 3 reflux that allows non-destructive purification of SWNTs with air oxidation While the present invention has been illustrated and described with reference to particular structures and methods of fabrication, it will be apparent that other changes and modifications can be made therein with the scope of the present invention as defined by the appended claims

Claims: 8 We Claim 1 A method of producing essentially pure single- walled carbon nanotubes (SWNTs), comprising the steps of (a) long-laser pulsing a graphite target, (b) refluxing the pulsed material in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized, (c) separating the acid from the material, and (d) oxidizing the carbon coating 2 A method of purifying single- walled carbon nanotubes (SWNTs), comprising the steps of (a) refluxing the pulsed material in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized, (b) separating the acid from the material, and (c) oxidizing the carbon coating 3 A method of determining single-wall nanotubes (SWNTs) in a raw material, comprising the steps of (a) refluxing the pulsed material in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized, (b) separating the acid from the material, (c) oxidizing the carbon coating, the oxidation yielding an essentially pure SWNTs material, and (d) determining a weight percent of the essentially pure SWNTs in the raw material 4. The method of claim 3 wherein determining comprises gravimetric analysis 5. An essentially pure single-walled carbon nanotubes (SWNTs) consisting essentially of, in percent by weight SWNTs greater than 98, and metal less than 0 5 AMENDED CLAIMS [received by the International Bureau on 19 January 2001 (19.01.01); original claims 1 and 3 amended; remaining claims unchanged (1 page )] 1. A method of producing essentially pure single-walled carbon nanotubes (S NTs), comprising the steps of: (a) long-laser pulsing a graphite target (b) refluxing the pulsed material in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized; (c) separating the acid from the material; and (d) oxidizing the carbon coating. 2. A method of purifying single-walled carbon nanotubes (SWNTs), comprising the steps of: (a) refluxing the SWNTs in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized; (b) separating the acid from the SWNTs; and (c) oxidizing the carbon coating. 3. A method of determining single-wall nanotubes (SWNTs) in a raw material, comprising the steps of: (a) refluxing the SWNTs in dilute nitric acid for a time sufficient to remove an incorporated metal and produce a carbon coating on the SWNTs, the carbon coating capable of being oxidized; (b) separating the acid from the SWNTs; (c) oxidizing the carbon coating, the oxidation yielding an essentially pure SWNTs material; and (d) determining a weight percent of the essentially pure SWNTs in the raw material. 4. The method of claim 3 wherein determining comprises gravimetric analysis. 5. An essentially pure single-walled carbon nanotubes (SWNTs) consisting essentially of, in percent by weight: SWNTs greater than 98; and metal less than 0.5. STATEMENT UNDER ARTICLE 19(1 ) The amended claims 2 and 3 presented in page 8 accompanying the applicant's Amendment After Notification of Transmittal of the Intemational Search Report Under Article 19(1), Rule 46, as described in the accompanying Letter Describing Differences Between Replaced Sheets and Replacement Sheets submitted concurrently herewith, are made to amend the claims to provide antecedent basis. The Amendment claims the subject matter to provide anticident basis. This amendment impacts the description but not the drawings.

Inventors: DILLON ANNE C, GENNETT THOMAS, HEBEN MICHAEL J, DILLON, ANNE, C., GENNETT, THOMAS, HEBEN, MICHAEL, J.
Applicants: DILLON ANNE C, GENNETT THOMAS, HEBEN MICHAEL J, MIDWEST RESEARCH INST, DILLON, ANNE, C., GENNETT, THOMAS, HEBEN, MICHAEL, J., MIDWEST RESEARCH INSTITUTE
Application year: 2000
Main classification: D01F9
IPC codes: C01B31, D01F9