SINGLE-WALL CARBON NANOTUBES FOR HYDROGEN STORAGE OR SUPERBUNDLE FORMATION

Abstract: A method of processing single-walled carbon nanotubes (SWNTs) in the formation of superbundles or for use in hydrogen storage, or both, is provided comprising the steps of mixing a SWNT substrate in a solvent solution into a suspension, and agitating the suspension using an ultrasonic energy means.

Description: Single- Wall Carbon Nanotubes For Hydrogen Storage Or Superbundle Formation Technical Field This invention relates to single-wall carbon nanotubes ("SWNTs"), and in particular to a method of processing SWNTs for use in hydrogen absorption or superbundle formation. Background Art As is well known in the materials science art, there has been interest in the mechanical, electrical, physical, and optical properties of SWNTs. This interest is not surprising when consideπng the broad impact that these materials will make in the areas of science and technology, whether as applied in the form of super-strong composites, nanoelectronics, or to the safe storage of hydrogen which is vital in the development of hydrogen fuel cells or combustion engines. Indeed, the shape of SWNTs suggests that these composite materials would serve as an ideal hydrogen storage container. In order to further these applications, one must evaluate the intrinsic mechanical and electrical properties of SWNTs. A distinct disadvantage of the prior art has heretofore been the inability in evaluating these properties when using the current synthesis and purification procedures, which result in SWNTs having a random oπentation, diameter, and length distribution. Moreover, many electπcal and mechanical applications further require an orderly cutting or alignment of the individual nanotubes which comprise SWNT composite materials. For hydrogen storage, one must further resolve the problem of processing SWNTs, to either open, reorganize, cut, or functiona ze them, without consuming the nanotubes in the process. The ability to form organized SWNT superbundles is also important because of the possibility of performing macroscopic analyses on well-defined samples. Of particular interest would be polarized Raman spectroscopy studies that would allow unambiguous assignment of observed spectral bands to specific types of SWNTs. Superbundle formation may also be a first step towards the production of longer. Control over the superbundle length and diameter at either the formation step or subsequently through the use of other means may provide a route to electπcal connectors, or perhaps superbundle crystals and films that would be useful as hydrogen adsorbents or gas transport membranes. It is also desirable in some applications to obtain high levels of hydrogen storage in SWNT materials at ambient conditions in a matter of minutes or seconds. In 1997 single- walled carbon nanotubes were first shown to adsorb hydrogen at near ambient temperature and pressure following heating in vacuum to 973K(1). It was subsequently reported that graphite nanofibers could adsorb up to 67 wt% hydrogen at room temperature and 120 atm(2), however, these results could not be corroborated(3). Hydrogen adsorption on puπfied, cut SWNTs was then shown to exceed 8 wt% at -160 atm and 80 K(4). Lithium doped carbon nanotubes were shown to adsorb hydrogen at ambient pressure to -20 wt% between 473-673 K while potassium doped samples adsorbed -14 wt% at room temperature. However, the potassium doped nanotubes were found to be oxidized rapidly upon exposure to aιr(5). Recently, acid treated large diameter (1.85 nm) SWNTs were shown to adsorb 4.2 wt hydrogen at room temperature and about 100 atm. The samples could be charged to 70% of the whole adsorption capacity in - 1 hr (6). Disclosure of Invention The present invention is intended to provide a method for processing SWNTs for use in high density hydrogen storage or in electπcal or mechanical applications It is further intended that the present invention provide a process of realignment and collapse of the small SWNT bundles into much larger superbundle configurations It is further intended that the present invention provide a process of forming SWNT mateπals which are capable of at least 6-7 wt% hydrogen adsorption at ambient conditions Additional advantages of the present invention will be set forth in part in the descπption that follows and in part will be obvious for that description or can be learned from practice of the invention. The advantages of the invention can be realized and obtained by the method particularly pointed out in the appended claims Briefly, the invention provides a method of processing single-walled carbon nanotubes (SWNTs) in the formation of superbundles or for use in hydrogen storage, or both, comprising the steps of mixing a SWNT substrate in a solvent solution into a suspension, and agitating the suspension using an ultrasonic energy means Brief Description of Drawings Figure la is a transmission electron microscope image of crude 4.2 W laser generated SWNT soot. Figure lb is a transmission electron microscope image of a crude matenal which was refluxed for 16 h in 3M HNO 3 . Figure lc is a transmission electron microscope image of puπfied SWNTs produced by oxidizing the acid treated sample for 30 min. in air at 550 °C. Figure Id is a transmission electron microscope image of puπfied tubes at high magnification after annealing to 1500 °C in vacuum. Figure 2a is a thermal gravimetnc 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. The figure shows mateπals produced at a laser power of 4.2 W; fully puπfied, crude soot, and crude soot after a 16 h reflux in 3M HNO The data for the refluxed material was normalized to 100 wt % at 100 °C to compare dry weights. Figure 2b is a thermal gravimetnc 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 an . The figure shows matenals produced with 6W of laser power. Samples were refluxed in 3M HNO, 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. Figure 3 is a Raman spectra obtained at 488 nm with a resolution of 2-6 cm ' for punfied, crude, and crude matenal which was refluxed for 16 h in 3M HNO-, acιd. The inset of the figure shows the region from 1200 - 1500 cm ' at an amplified intensity scale Figure 4a is a is a transmission electron microscope image of purified tubes at high magnification. Figure 4b is a is a transmission electron micioscope image of puπfied tubes formed into "superbundles" of 0.5 to 2 microns in width Figure 4c is a transmission electron microscopy image of a superbundle from figure 4b at higher magnification Figure 5a is a transmission electron microscope image of SWNT superbundles extracted from water at low magnification illustrating the length of the fiber. Figure 5b is a transmission electron microscope image of SWNT superbundles extracted from water at high magnification illustrating the width and dense packing of the superbundle. Figure 6 is a transmission electron microscope image of punfied SWNTs following ultrasonication in 5M HNO The apparently endless ropes seen after puπfication have been "cut" into compact bundles - 1 - 5 microns in length. Figure 7 shows temperature programmed desorption spectroscopy data from both SWNTs and microcrystal ne graphite after a 10 min. hydrogen exposure at 500 torr. Both samples had been sonicated in dilute HNO 3 for 16 hours. Figure 8a displays the hydrogen desorption signal from a HNO 3 sonicated SWNT sample following a 500 torr hydrogen exposure at room temperature. The sample remained at room temperature while the vessel was evacuated. Figure 8b displays the CO 2 TPD signal after a hydrogen dose as in Figure 8a was followed by a 10 min. CO 2 dose at 500 torr. Figure 8c displays the TPD signal for the hydrogen which also evolved along with the CO 2 shown in Figure 8b Figure 9a is a Raman spectrum from puπfied SWNTs. Figure 9b is a Raman spectrum obtained after puπfied SWNTs had been sonicated at 90 W/cm 2 for 4 hours in 5M HNO Figure 9c is a Raman spectrum obtained after puπfied SWNTs were sonicated at 90 W/cm 2 in 5M HNO 3 for 4 hours and then degassed in vacuum to 973 K. Figure 9d is a Raman spectrum obtained after puπfied SWNTs were sonicated at 90 W/cm 2 in 5M HNO 3 for 4 hours, degassed in vacuum to 973 K, and then exposed to hydrogen at 500 torr. Figure 9e is a Raman spectrum obtained after punfied SWNTs were sonicated at 90 W/cm 2 in 5M HNO-, for 4 hours, degassed in vacuum to 973 K, exposed to hydrogen at 500 torr, and then heated again to 973 K in vacuum. Best Mode for Carrying Out the Invention 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 mateπals similar or equivalent to those descπbed herein can be used in the practice or testing of the present invention, the preferred methods and matenals are now descπbed. Example 1 Referπng now to the drawing figures, SWNT matenals were synthesized by a laser vaponzation method. 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 - 8 W. Matenal 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 800 - 1300 °C, with 500 Torr Ar flowing at 100 seem. The transmission electron microscope ("TEM") image in Figure la reveals the components of the laser-generated matenal Bundles of SWNTs span between large agglomerations of amorphous and micro-crystalline carbon which contain metal nanoparticles. Typical raw mateπals were estimated to contain - 20 - 30 wt % SWNTs by a detailed analysis of numerous different TEM images, A.C. Dillon, P. A. Panlla, 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 matenal has the same metal content as the targets (~ 6 wt %). 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 polytetrafluoroethylene coated polypropylene, and nnsed with deiomzed 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 matenal The reflux caused the non-nanotube carbon fractions to be redistπbuted as a uniform coating on the SWNTs as seen in Figure lb The matenal 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 (Figure lc). Figure 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 Figure 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 (Figure 2a) shows a slight 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 (Figure 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 punfication. 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 matenal, or -21 wt % of the pre-reflux weight. This latter value is in excellent agreement with the yield after refluxed matenal was heated to 550 °C in stagnant air (-20 wt %), and considerably higher than the tube content determined by TGA analysis of the crude matenal (-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 duπng TGA expeπments 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 Figure lc shows only SWNTs. To determine if tubes are damaged or consumed dunng the dilute acid reflux, TGA was performed on matenals which were refluxed for 4, 16, and 48 h in 3M HNO, (Figure 2b) Mateπals for these expeπments were produced with 6 W of laser power. The TGA data was adjusted for the dry-weight lost duπng reflux so that the y-axis represents the wt % remaining of the initial crude matenal 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 7c The data sets are virtually identical at the higher temperatures despite the difference in the matenal weights which were lost duπng 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 matenal refluxed for 16 h m 3M HNO 3 . It is interesting to note that the SWNT content in the 6 W matenal is lower than in the 4.2 W matenal. Such a quantitative compaπson was not possible pπor to the punfication and assessment methods disclosed herein. Unlike the 16 h process, the 4 h reflux did not always permit good punfication by oxidation. In these cases, a TGA curve very similar to that of the crude matenal was observed. The oxidation reactions are no longer well-separated after a 48 h reflux (Figure 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, hydrophihc carbon coating produced after 16 h of refluxing and thought to be necessary for punfication 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 Figure 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 (Figure 2b). Raman spectroscopy further elucidates the HNO 3 reflux process. The Raman spectra displayed in Figure 3 for punfied and crude mateπals both exhibit a strong feature at 1593 cm ! with shoulders at 1567 and 1609 cm ' as expected for the SWNT tangential C-atom displacement modes. However, the broadened feature at 1349 cm ' in the crude spectrum indicates the presence of impunties and a contnbution from the disordered sp 2 carbon "D band" of non-nanotube graphite components, P.C. Eklund, J.M. Holden & R.A. Jishi, Carbon 1995, 33,959; Y Wang, D C. Als eyer & R.L., McCreedy, Chem. Mater. 1990, 2, 557. Unlike other reports, G Rinzler, et al, Applied Physics A 1998, 67, 29, S. Bandow, et al, J. Pins. Chem. B 1997, 101, 8839, no spectral evidence for C 60 was ever detected in any of our matenals After the 16 h reflux, the D band intensity is significantly increased indicating decreasing domain size (Figure 3). Also, a signal denved from the fundamental E 3g mode of disordered graphite is observed where the SWNT modes are expected. The disordered graphite coating (Figure lb) evidently prohibits observation of the resonantly enhanced SWNT modes. This is stnking since the SWNT content in the 16 h HNO s treated sample is actually higher than in the crude matenal, and demonstrates that Raman spectroscopy can be poorly suited for determining SWNT contents in certain types of samples. The D-band is narrower in puπfied mateπals 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 nitπc acid reflux introduces reactive functional groups onto the surfaces of the non-nanotube carbon matenal These two effects serve to maximize the surface area of the nonnanotube carbon and provide for enhanced oxidation kinetics. Furthermore, since the functionahzed coating is oxidized at lower temperatures, and the coating is evenly distnbuted, the heat generated by the exothermic reactions does not initiate oxidation of SWNTs In contrast, SWNTs in raw matenals are consumed simultaneously with impunties because the oxidation of agglomerated impuπties generates local "hot spots". It is a combination of the high-surface- area, decreased domain size, degree of functionahzation, and uniformity of the carbon film produced by the 16 h 3M HNO 3 reflux that allows non-destructive punfication of SWNTs with air oxidation The puπfied SWNT soots were mixed in a aqueous 5M HNO-, solution Using an ultrasonic probe, a concentrated ultrasonic energy was directed into the solution The resultant cavitation produced microscopic domains of extremely high temperature The combination of the ultra sonic energy, high temperatures, and oxidative strength of the HNO 3 solvent provided a reproducible cut and an increase in bundle size or both of the SWNTs A hydrogen adsorption analysis for the SWNTs cut using the 5M HNO 3 solution demonstrated an extremely high SWNT hydrogen affinity, in the range of 7 % w/w The punfied bundles, used as the starting matenal and shown in Figure 4a. were 5 to 10 nm in width Example 2 A single Nd.YAG laser(1064 nm) was employed to synthesize the carbon nanotubes from 1.2 at % metal doped (50:50 Co/Ni) pressed graphite targets. The targets were placed in a quartz tube that was heated to a temperature of 1200 °C in a clam-shell furnace. With the laser operating at a frequency of 10 Hz, the laser power and beam size were adjusted to provide -20 J/pulse-cm 2 at a -450 ns pulse width. An argon flow of 100 seem at 500 torr was maintained through the reaction vessel for the duration of the synthesis. The raw soot was punfied by refluxing in 3M nitπc acid for 16 hr, filteπng and washing with dio zed water on a polytetraflouroethylene (PTFE) filter, and then heating the obtained paper in air for 30 min at 550 °C. This procedure results in tubes of greater than 98 wt.% punty when target matenal is not sputtered and trapped in vaponzed soot. A representative TEM image of the resultant pure tubes is shown in Figure 4a. The random onentation and small size of the long bundles are apparent. The SWNTs exist as a random tangle because of the conditions under which they are synthesized and puπfied. The tubes are formed within the high temperature plasma generated by the laser stnking the graphite target, and their formation is rapidly quenched as the tubes diffuse out of the plasma plume. The resulting SWNTs exist in small bundles and are accompanied by other graphitic and amorphous carbon fractions as well as metal nanoparticles. The puπfication process succeeds in removing the non-nanotube carbon fractions and the metals. However, the puπfication process still results in a random onentation of tube bundles which have - 5 - 20 nm diameters. A 1 0 mg sample of puπfied SWNTs was placed in a cylinder containing 10 ml of deiomzed water or other polar hydrophihc solvents or solvent mixtures A Heat Systems- Ultrasonics Inc. model W-220F Cell Disrupter was submersed into the solution and the power was slowly increased to 90 watts/cm 2 . The ultrasonic agitation was continued for a maximum of 120 minutes Normally, SWNTs are not dissociated in aqueous solution without the use of surfactants With the use of the ultrasonic probe, however, the SWNT sample was almost immediately dispersed throughout the solvent. The SWNT matenal agglomerated in the solution when the ultrasonic agitation was turned off Gentle shaking of the solution redistnbuted the tube agglomerations into an apparently homogeneous suspension. If the sonicated solution was allowed to settle, a thin layer of SWNT superbundles were observed at the solvent interface. At this point the isolation of the superbundles was achieved via several different procedures and on a senes of substrates. For example, a TEM gnd could be used to lift off the SWNT interface layer to extract fibers. The resulting isolated SWNT superbundles were approximately 0.4 - 1 micron in diameter, 5-10 microns in length, and present in many locations on the TEM gnd Other collection strategies included the use of silver, platinum, silicon or highly ordered pyrolytic graphite substrates, sonication of the tubes in the presence of the substrate, or spin coating the sonicated solution onto a prepared substrate surface Preliminary results indicate that the polanty of the solvent greatly affects the size and density of the bundles after sonication. Solvents such as toluene, hexane, alcohols and water have been employed so far, and SEM images showed that sonication in toluene produces the smallest bundle diameters (=50 nm) while water produces the largest (~ lmicron). Figures 4b displays a TEM image of SWNTs on a gnd dipped into the film formed following sonication in a 50:50 methanol/water mixture. The large superbundles are readily apparent, and it is clear that some of these superbundles are completely isolated. An image of an isolated bundle at higher magnification is displayed in Fig. 4c. The superbundle configuration probably aπses from the minimization of the interactions between hydrophobic SWNT surfaces and the hydrophihc solvent maximizing the vVan der Waals interactions along the axial length of the tubes. The extent of collapse of the superbundle into a tight bundle depends upon solvent composition. Figure 5a displays a TEM image of a more collapsed bundle extracted from water. The dense bundle is displayed at higher resolution in Figure 5b. Some of the bundles extracted from water were so dense that a TEM image could not be obtained as the electron beam could not penetrate through the tight nanotube packing. However, these bundles were also shown to contain aligned tubes using scanning electron microscopy. Example 3 A single Nd:YAG laser (1064 nm) was rastered across 1.2 at % metal doped (50:50 Co/Ni) pressed graphite targets in a quartz tube that was heated to 1200sC. The laser was operated at a frequency of 10 Hz, at -10-30 J/pulse-cm2 with an - 450 ns pulse width Argon flowing at 100 seem at 500 torr was maintained through the reaction vessel for the duration of the synthesis The raw soot was punfied by refluxing in 3M nitnc acid for 16 hr, filtenng and washing with de-ionized water followed by air oxidation for 30 min at 825 K The H 2 adsoφtion capacity of vanous SWNT samples were probed by a previously descπbed temperature programmed desoφtion (TPD) technique Dillon, A.C. et al , Nature 1997, 386, 377. Initially, - 0 3 - 1 mg samples were degassed in vacuum up to 823-973 K atl K/s. Room temperature H : exposures between 10-500 torr for several seconds were then sufficient to achieve a maximum hydrogen adsoφtion Partial hydrogen coverage was obtained following H 2 exposure at only 10 mtorr The sample was generally cooled to 90 K pπor to evacuation of the TPD chamber in order to quickly obtain a base pressure of -5 x 10 8 torr The cooling process also ensured that hydrogen which could desorb from the sample near room temperature remained on the sample before acquinng TPD spectra. The hydrogen desoφtion signals from SWNTs were calibrated using known H 2 desoφtion signals from 0.3 - 1 mg samples of CaH 2 The calibration was confirmed by performing thermal gravimetnc analyses on a lmg SWNT sample charged with hydrogen The hydrogen wt% measured by the two different methods was within 10% Previously a degas to 973 K in vacuum was sufficient to activate hydrogen adsoφtion on arc-generated SWNTs Dillon, A C et al , Nature 1997, 386, 377 However in applying the same expenmental methods to the very long laser-generated nanotube bundles no significant H 2 adsoφtion was observed for either the crude or punfied matenals In order to activate the hydrogen adsoφtion properties - 1-3 mg of the punfied laser-generated matenal was placed in a cylinder containing 20 ml of 5M HNO-, A Heat Systems- Ultrasonics Inc. model W-220F Cell Disrupter ultrasonic probe was then employed to agitate the solution for time penods ranging from 20 minutes to 16 hrs at powers ranging from 25 - 250 W/cm2 The final adsoφtion capacity of the SWNT samples vaned between 2-7 wt% depending on the matenal, the sonication power, the sonication time and the sample degas temperature Once the unique hydrogen adsoφtion properties were obtained, the samples remained activated for at least 8 months The adsoφtion was maximized following sonication of puπfied SWNTs for 24 hrs at 90 W/cm 2 with a subsequent degas to 825 K Figure 6 displays a typical TEM image of punfied SWNTs following ultrasonication in 5M HNO 3 . Here the as-synthesized seemingly endless ropes of SWNTs have been "cut" into compact bundles - 1 - 5 microns in length. The average bundle diameter has also been increased and the tubes have been reorganized and functiona zed Upon degassing the cut SWNT samples in vacuum, NO, H 2 O, CO,CO 2 , H 2 and low molecular weight hydrocarbons are observed. The desoφtion of NO is consisted with the intercalation of nitnc acid in the nanotube bundles. The presence of the other species may be explained by the presence of carboxyl or hydroxyl groups and hydrogen at the nanotube ends or at defects generated by the cutting procedure. Figure 7 displays the H 2 TPD spectrum of a degassed SWNT sample following a bnef room temperature H 2 exposure at 500 torr. The sample was sonicated in dilute HNO 3 for 16 hrs and degassed to 825 K resulting in hydrogen adsoφtion of 6.5 wt%. The spectrum is characteπzed by two separate desoφtion signals peaked at 370 and 630 K indicating at least two unique hydrogen adsoφtion sites. The hydrogen corresponding to the signal peaked at 370 K could be evolved by holding the sample at room temperature overnight in vacuum. Holding the SWNT sample at 423 K for only 8 minutes also resulted in complete evolution from the low temperature site which corresponds to -2.5 wt%. In order to desorb all of the hydrogen from the higher temperature site it was necessary to heat the sample between 475-850 K. Figure 7 also displays the hydrogen desoφtion signal from micro crystalline graphite following the same pre-treatment and H 2 exposure as the optimized SWNT sample. The graphite sample apparently has two H 2 adsoφtion sites that are very similar to the SWNT sample as there are two desoφtion peaks at -380 and 610 K It is interesting however that the graphite hydrogen adsoφtion is only -19 % of the H 2 adsoφtion observed for an SWNT sample of the same mass following the same preparation Both of the SWNT hydrogen adsoφtion sites are effectively "capped" by the adsoφtion. This phenomena was revealed in an expenment where a standard hydrogen exposure to cut SWNTs was followed by a room temperature 10 min. 500 torr exposure to C0 2 . The sample was cut for 4 hrs at 90 W/cm 2 resulting in a total hydrogen adsoφtion capacity of - 3 wt% Figure 8 displays the hydrogen desoφtion signal from the SWNT sample following a 500 torr hydrogen exposure at room temperature without any subsequent cooling of the sample. The spectrum is very similar to the spectrum observed for the 16 hr cut (Fig. 7) with two peaks at - 420 K and 602 K. However, the lower temperature peak is slightly smaller and shifted to a higher temperature due to the absence of cooling while the hydrogen exposure was pumped out of the chamber. Figure 8b and c display TPD desoφtion signals of CO 2 and H 2 respectively when an identical hydrogen exposure was followed by a CO 2 exposure for 10 mm. at 500 torr. The small CO 2 desoφtion signal peaked at 400 K demonstrates the existence of a small population of CO 2 adsoφtion sites on the SWNT sample. Suφnsmgly, however the H 2 desoφtion signals are both shifted to higher temperature by - 50 K. It appears that the hydrogen does not begin to desorb from the SWNTs until a significant portion of the CO 2 has already evolved suggesting that the CO, in fact blocks the hydrogen desoφtion process. In a subsequent expenment the sample was exposed to CO 2 for 10 min at 500 torr at room temperature and then exposed to H 2 under the same conditions. Subsequent TPD revealed only a CO 2 desoφtion signal, similar to that seen in Figure 8b with no hydrogen absoφtion, demonstrating that carbon dioxide completely blocks hydrogen adsoφtion sites. CO 2 also apparently "caps" the SWNTs upon exposure to air. Samples charged with hydrogen and exposed to atmosphere were shown to desorb first CO 2 and then H 2 several weeks later without a significant loss in the stored hydrogen. This effective "capping" of the SWNTs by CO 2 may prove significant to the commercial development of SWNTs for a hydrogen storage system. ) Raman spectroscopy was employed to elucidate the mechanism of the unique room temperature hydrogen adsoφtion on SWNTs. Raman spectra were obtained using a 50 mW Ar ion laser (488 nm) with a resolution of - 4-6 cm '. Figure 9 displays a senes of Raman spectra between 1500 - 1700 cm ' collected from SWNT samples throughout the pretreatment and H 2 adsoφtion process. The initial spectrum of the puπfied SWNTs (Fig 9a) exhibits two strong features at 1597 and 1571 cm ' These features are slightly shifted from their charactenstic occurrence at 1593 and 1567 cm '. Figure 9b displays the Raman spectrum of the punfied sample following sonication in 5M HNO 3 for 4 hr at 90 W/cm 2 . The two features are now broadened and further shifted to 1599 and 1575 cm ' with their intensity quite dramatically reduced Following degassing m vacuum to 1000 K however the Raman intensity returns, and the features are shifted back to their charactenstic values of 1593 and 1567 cm ' (Fig. 9c). Surpnsingly, Figure 93d reveals that subsequent exposure to H 2 at 500 torr again results in a loss in Raman intensity and a shift to 1596 and 1570 cm ' The intensity of the Raman signal is again restored by heating in vacuum to 1000K resulting in the evolution of the adsorbed hydrogen (Fig. 9e). A similar somewhat less reversible loss in intensity and slight shifting was observed for features attnbuted to the radial breathing modes between 162-203 cm "1 . The reversible shift to higher frequency in Raman spectral bands observed in Figure 9 is indicative of charge transfer to an acceptor species. The SWNT punfication process results in some degree of intercalation and functionahzation as indicated by the desoφtion of NO, H 2 O, CO, CO 2 , H 2 and hydrocarbons versus heating in vacuum to -1500 K The puπfied sample had been annealed to 825 K in air This should result in the desoφtion of the H 2 O, CO 2 and NO species. However, the desoφtion of chemisorbed hydrogen from puπfied uncut SWNTs occurs between - 900-1500 K. Apparently, the chemisorbed hydrogen is acting as an electron acceptor and produces the shift in the Raman spectrum of the puπfied SWNTs to 1597cm '. The further shift to 1599 cm ' following cutting the SWNTs in HNO 3 is consistent with an increase in the chemisorbed hydrogen population The broadening and dramatic loss in intensity indicates that the nitπc acid intercalation interferes with the resonance enhancement of the SWNT Raman modes. The process is reversible, however, as indicated by the return of intensity and appearance of the Raman modes at 1593 and 1567 cm ' upon heating to 1000 K. At this temperature a significant portion of the chemisorbed hydrogen resulting from the punfication and cutting procedures has desorbed. The subsequent intensity loss and shift to higher frequency for the SWNT sample following exposure to H 2 at 500 torr again indicates charge transfer. These results therefore suggest that some of the 6.5wt% adsorbed H 2 undergoes an interaction with the SWNTs which is stronger than physisoφtion. Suφπsingly the ma)onty of the process is reversible as indicated by the regeneration of the Raman intensity upon degassing to 1000 K as displayed in Figure 9e The fact that the curve displayed in Figure 9e is still slightly blue shifted may be attnbuted to a small population of hydrogen from the H2 exposure still remaining on the sample. The exact location and nature of the SWNT H 2 adsoφtion sites is difficult to determine. However, collectively the results of this study give clues to the unique adsoφtion interactions. The shift to higher frequency and the decrease in Raman intensity following ultrasonication in HNO 3 is consistent with nitnc acid intercalation resulting in charge transfer and a loss of SWNT resonance enhancement. The Raman data (Fig. 9d) also suggests a hydrogen SWNT interaction which disrupts the sp 2 pι character of the nanotubes indicating an interaction which is again stronger than physisoφtion. It is likely that this strong H 2 interaction with the SWNTs is the source of the high temperature TPD peak, and it is somewhat suφπsing that such a reaction is reversible. Since the low temperature hydrogen desoφtion peak evolves completely simply by sitting at room temperature overnight in vacuum, it most likely stems from a very weak interaction or physisoφtion. While the present invention has been illustrated and descnbed with reference to particular structures and methods of fabπcation, 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: Claims 1. A method of processing single-walled carbon nanotubes (SWNTs) for use in the formation of superbundles or hydrogen storage or both compnsmg the steps of: (a) mixing a SWNT substrate in a solvent solution into a suspension; and (b) agitating the suspension using an ultrasonic energy means. 2. The process of claim 1 wherein the solvent is a dilute acid or an oxidizing solution 3. The process of claim 1 further compnsmg settling the suspension for a time sufficient to form an agglomeration of the bundles 4. The process of claim 1 wherein the solvent is selected from a group consisting of deionized water, toluene, hexane, or alcohol 5. The process of claim 1 wherein the solvent consists essentially of 50 50 methanol and water mixture. 6. The process of claim 1 wherein agitating compnses approximately 20 minutes at an ultrasonic power of about 90 Watts / cm 2 . 7. The process of claim 1 wherein agitating further compnses applying ultrasonic energy to the suspension in a flowing state. 8 The process of claim 1 wherein agitating further compnses applying ultrasonic energy to the suspension in a static state 9 The process of claim 1 wherein the substrate is a micro crystalline graphite 10 The process of claim 1 wherein the substrate compnses greater than 98% SWNTs by weight 11. The process of claim 1 wherein the substrate is an impure SWNT matenal 12 The process of claim 1 wherein the substrate is a multi-walled carbon nanotube matenal 13 The process of claim 2 wherein the dilute acid consists essentially of HNO 3 14. The process of claim 2 further compnsmg degassing at an elevated temperature and absorbing hydrogen at an ambient temperature and pressure. 15 The process of claim 3 wherein the bundles range in size from 04 -1 microns in diameter and 5- 10 microns in length 16. The process of claim 3 wherein the bundles comprise an essentially pure SWNTs, consisting essentially of, in percent by weight: SWNT's greater than 98 %; and metal less than 0.5 % wherein the SWNT's adsorb 2.0 to 7.0 % hydrogen. 17. The process of claim 13 wherein the dilute acid solution is about 5M HNO 3 . 18. The process of claim 14 wherein agitation comprises 16 hours at 90 W / cm 2 and the degass proceeds at a temperature of about 825 K, the hydrogen adsoφtion comprising approximately 6.5 wt %. 19. The process of claim 14 wherein the hydrogen is adsorbed on at least two adsoφtion sites, the sites capable of desoφtion at about 370 and 630 K. 20. The process of claim 14 wherein ambient conditions comprise room temperature and 500 torr. 21. The process of claim 14 wherein agitation comprises a time in the range of 20 minutes - 24 hours using a sonication power in the range of 25 - 280 W/cm 2 , the final absoφtion capacity of the bundles having a range of 2 - 7 wt %.

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: 2001
Main classification: C01B31
IPC codes: C01B3, C01B31