U.S. patent application number 10/110082 was filed with the patent office on 2002-10-17 for single-wall carbon nanotubes for hydrogen storage or superbundle formation.
Invention is credited to Dillon, Anne C., Gennett, Thomas, Heben, Michael J..
Application Number | 20020150529 10/110082 |
Document ID | / |
Family ID | 22331135 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020150529 |
Kind Code |
A1 |
Dillon, Anne C. ; et
al. |
October 17, 2002 |
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.
Inventors: |
Dillon, Anne C.; (Boulder,
CO) ; Gennett, Thomas; (Pittsford, NY) ;
Heben, Michael J.; (Denver, CO) |
Correspondence
Address: |
Paul J White
Senior Counsel
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden
CO
80401
US
|
Family ID: |
22331135 |
Appl. No.: |
10/110082 |
Filed: |
April 3, 2002 |
PCT Filed: |
January 17, 2001 |
PCT NO: |
PCT/US01/01698 |
Current U.S.
Class: |
423/460 |
Current CPC
Class: |
B82Y 30/00 20130101;
D01F 9/12 20130101 |
Class at
Publication: |
423/460 |
International
Class: |
D01F 009/12 |
Claims
1. A method of processing single-walled carbon nanotubes (SWNTs)
for use in the formation of superbundles or hydrogen storage or
both comprising 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 comprising 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 comprises approximately
20 minutes at an ultrasonic power of about 90 Watts 1 cm.sup.2.
7. The process of claim 1 wherein agitating further comprises
applying ultrasonic energy to the suspension in a flowing
state.
8. The process of claim 1 wherein agitating further comprises
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 comprises greater
than 98% SWNTs by weight.
11. The process of claim 1 wherein the substrate is an impure SWNT
material.
12. The process of claim 1 wherein the substrate is a multi-walled
carbon nanotube material.
13. The process of claim 2 wherein the dilute acid consists
essentially of HNO.sub.3.
14. The process of claim 2 further comprising 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
0.4-1 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 SM HNO.sub.3.
18. The process of claim 14 wherein agitation comprises 16 hours at
90 W/cm.sup.2 and the degass proceeds at a temperature of about 825
K, the hydrogen adsorption comprising approximately 6.5 wt %.
19. The process of claim 14 wherein the hydrogen is adsorbed on at
least two adsorption sites, the sites capable of desorption 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.sup.2, the final absorption capacity of the
bundles having a range of 2-7 wt %.
Description
TECHNICAL FIELD
[0001] 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
[0002] 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
considering 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.
[0003] 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 orientation, diameter, and length distribution. Moreover,
many electrical 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 functionalize them, without consuming the
nanotubes in the process.
[0004] 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 electrical connectors, or
perhaps superbundle crystals and films that would be useful as
hydrogen adsorbents or gas transport membranes.
[0005] 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 singlewalled 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
purified, cut SWNTs was then shown to exceed 8 wt % at .about.160
atm and 80 K(4). Lithium doped carbon nanotubes were shown to
adsorb hydrogen at ambient pressure to .about.20 wt % between
473-673 K while potassium doped samples adsorbed .about.14 wt % at
room temperature. However, the potassium doped nanotubes were found
to be oxidized rapidly upon exposure to air(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 .about.1 hr
(6).
DISCLOSURE OF INVENTION
[0006] The present invention is intended to provide a method for
processing SWNTs for use in high density hydrogen storage or in
electrical or mechanical applications.
[0007] 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.
[0008] It is further intended that the present invention provide a
process of forming SWNT materials which are capable of at least 6-7
wt % hydrogen adsorption at ambient conditions.
[0009] Additional advantages of the present invention will be set
forth in part in the description 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.
[0010] 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
[0011] FIG. 1a is a transmission electron microscope image of crude
4.2 W laser generated SWNT soot.
[0012] FIG. 1b is a transmission electron microscope image of a
crude material which was refluxed for 16 h in 3M HNO.sub.3.
[0013] FIG. 1c is a transmission electron microscope image of
purified SWNTs produced by oxidizing the acid treated sample for 30
min. in air at 550.degree. C.
[0014] FIG. 1d is a transmission electron microscope image of
purified tubes at high magnification after annealing to
1500.degree. C. in vacuum.
[0015] FIG. 2a is a thermal gravimetric analysis of 1-2 mg samples
ramped from 25-875.degree. C. at 5 degrees per minute in a platinum
sample pan under 100 sccm flowing air. The figure 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.sub.3. The data for the
refluxed material was normalized to 100 wt % at 100.degree. C. to
compare dry weights.
[0016] FIG. 2b is a thermal gravimetric analysis of 1-2 mg samples
ramped from 25-875.degree. C. at 5 degrees per minute in a platinum
sample pan under 100 sccm flowing air. The figure shows materials
produced with 6W of laser power. Samples were refluxed in 3M
HNO.sub.3 for 4, 16, and 48 h. These curves were normalized to 100
wt % at 100.degree. C. to compare dry weights, and then
re-normalized to account for the different weight losses in the
HNO.sub.3 refluxes.
[0017] FIG. 3 is a Raman spectra obtained at 488 nm with a
resolution of 2-6 cm.sup.-1 for purified, crude, and crude material
which was refluxed for 16 h in 3M HNO.sub.3 acid. The inset of the
figure shows the region from 1200-1500 cm.sup.-1 at an amplified
intensity scale.
[0018] FIG. 4a is a is a transmission electron microscope image of
purified tubes at high magnification.
[0019] FIG. 4b is a is a transmission electron microscope image of
purified tubes formed into "superbundles" of 0.5 to 2 microns in
width.
[0020] FIG. 4c is a transmission electron microscopy image of a
superbundle from FIG. 4b at higher magnification.
[0021] FIG. 5a is a transmission electron microscope image of SWNT
superbundles extracted from water at low magnification illustrating
the length of the fiber.
[0022] FIG. 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.
[0023] FIG. 6 is a transmission electron microscope image of
purified SWNTs following ultrasonication in 5M HNO.sub.3. The
apparently endless ropes seen after purification have been "cut"
into compact bundles .about.1-5 microns in length.
[0024] FIG. 7 shows temperature programmed desorption spectroscopy
data from both SWNTs and microcrystalline graphite after a 10 min.
hydrogen exposure at 500 torr. Both samples had been sonicated in
dilute HNO.sub.3 for 16 hours.
[0025] FIG. 8a displays the hydrogen desorption signal from a
HNO.sub.3 sonicated SWNT sample following a 500 torr hydrogen
exposure at room temperature. The sample remained at room
temperature while the vessel was evacuated.
[0026] FIG. 8b displays the CO.sub.2 TPD signal after a hydrogen
dose as in FIG. 8a was followed by a 10 min. CO.sub.2 dose at 500
torr.
[0027] FIG. 8c displays the TPD signal for the hydrogen which also
evolved along with the CO.sub.2 shown in FIG. 8b.
[0028] FIG. 9a is a Raman spectrum from purified SWNTs.
[0029] FIG. 9b is a Raman spectrum obtained after purified SWNTs
had been sonicated at 90 W/cm.sup.2 for 4 hours in 5M
HNO.sub.3.
[0030] FIG. 9c is a Raman spectrum obtained after purified SWNTs
were sonicated at 90 W/cm.sup.2 in SM HNO.sub.3 for 4 hours and
then degassed in vacuum to 973 K.
[0031] FIG. 9d is a Raman spectrum obtained after purified SWNTs
were sonicated at 90 W/cm.sup.2 in SM HNO, for 4 hours, degassed in
vacuum to 973 K, and then exposed to hydrogen at 500 torr.
[0032] FIG. 9e is a Raman spectrum obtained after purified SWNTs
were sonicated at 90 W/cm.sup.2 in 5M HNO.sub.3 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
[0033] 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.
EXAMPLE 1
[0034] Referring now to the drawing figures, SWNT materials were
synthesized by a laser vaporization 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. 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 11/8 inch dye.
Crude soot containing SWNTs was produced at 800-1300.degree. C.,
with 500 Torr Ar flowing at 100 sccm. The transmission electron
microscope ("TEM") image in Figure 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 .about.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. Conf Proc.
1998, 526, 403. Inductively coupled plasma spectroscopy ("ICPS")
indicated the laser-generated crude material has the same metal
content as the targets (.about.6 wt %).
[0035] Approximately 80 mg of crude soot produced at 4.2 W was
refluxed in 60 ml 3M HNO.sub.3 for 16 h at 120.degree. C. The
solids were collected on a filter that allows ready separation of
the nanotubes, such as 0.2 .mu.m polytetrafluoroethylene coated
polypropylene, 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 .about.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. 1b. The material was then oxidized in stagnant air inside a
tube furnace at 550.degree. C. for 30 minutes. In this manner, the
carbon coating was completely removed leaving behind pure SWNTs,
corresponding to .about.20 wt % of the crude material (FIG. 1c).
FIG. 1d displays the purified tubes at high magnification. Thermal
gravimetric analysis ("TGA") revealed the purity of the isolated
SWNTs. The decomposition temperature (Td) is 735.degree. 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.degree. C., and <1
wt % remains above 850.degree. C. The metal content of these pure
SWNTs was measured to be below 0.5 wt % by ICPS.
[0036] 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 slight
increase in weight at low temperatures due to the oxidation of the
Ni and Co metals. The carbonaceous fractions begin to combust at
.about.370.degree. C. and are mostly removed by oxidation below
600.degree. C. A small final weight loss at .about.650.degree. C.
can be attributed to oxidation of surviving SWNTs (.about.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.degree. C. corresponds to the weight expected for
the oxidized metals (.about.8 wt %).
[0037] The TGA data is different after crude materials are refluxed
for 16 h in 3M HNO.sub.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.degree. 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.degree. 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.
[0038] The combustion of non-nanotube fractions is essentially
complete at the inflection point in the TGA curve of the refluxed
soot at 560.degree. C. At this point, the sample consists of only
pure SWNTs which amount to .about.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.degree. C. in stagnant air (.about.20 wt %), and
considerably higher than the tube content determined by TGA
analysis of the crude material (.about.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.degree. C. (up to 1 h) nor holding at
550.degree. 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.degree. C., and the TEM image of Figure lc shows only
SWNTs.
[0039] 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.sub.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 .about.450.degree. C., and a plateau
associated with SWNT stability is observed at 540.degree. 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.degree. 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.degree. C. of
material refluxed for 16 h in 3M HNO.sub.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.
[0040] 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 .about.625.degree. 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. 1b, 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 .about.8 wt %, or
<50% of the tubes known to be present, are found at the
inflection point in the curve at 625.degree. C. (FIG. 2b).
[0041] Raman spectroscopy further elucidates the HNO.sub.3 reflux
process. The Raman spectra displayed in FIG. 3 for purified and
crude materials both exhibit a strong feature at 1593 cm.sup.-1
with shoulders at 1567 and 1609 cm.sup.-1 as expected for the SWNT
tangential C-atom displacement modes. However, the broadened
feature at 1349 cm.sup.-1 in the crude spectrum indicates the
presence of impurities and a contribution from the disordered
sp.sup.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. Alsmeyer & R. L., Mccreedy, Cheli. Mater. 1990, 2,
557. Unlike other reports, G. Rinzler, et al., Applied Physics A
1998, 67, 29; S. Bandow, et al., J. Phys. Chem. B 1997, 101, 8839,
no spectral evidence for C.sub.6. 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.sub.2g mode of
disordered graphite is observed where the SWNT modes are expected.
The disordered graphite coating (FIG. 1b) evidently prohibits
observation of the resonantly enhanced SWNT modes. This is striking
since the SWNT content in the 16 h HNO.sub.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).
[0042] Conclusively, the 16 h 3M HNO.sub.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-surfacearea, decreased domain size,
degree of functionalization, and uniformity of the carbon film
produced by the 16 h 3M HNO.sub.3 reflux that allows
non-destructive purification of SWNTs with air oxidation.
[0043] The purified SWNT soots were mixed in a aqueous 5M HNO.sub.3
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.sub.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.sub.3
solution demonstrated an extremely high SWNT hydrogen affinity, in
the range of 7% w/w. The purified bundles, used as the starting
material and shown in FIG. 4a. were 5 to 10 nm in width.
EXAMPLE 2
[0044] 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.degree. 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 .about.20
J/pulse-cm.sup.2 at a .about.450 ns pulse width. An argon flow of
100 sccm at 500 torr was maintained through the reaction vessel for
the duration of the synthesis. The raw soot was purified by
refluxing in 3M nitric acid for 16 hr, filtering and washing with
dionized water on a polytetraflouroethylene (PTFE) filter, and then
heating the obtained paper in air for 30 min at 550.degree. C. This
procedure results in tubes of greater than 98 wt. % purity when
target material is not sputtered and trapped in vaporized soot. A
representative TEM image of the resultant pure tubes is shown in
FIG. 4a. The random orientation and small size of the long bundles
are apparent.
[0045] The SWNTs exist as a random tangle because of the conditions
under which they are synthesized and purified. The tubes are formed
within the high temperature plasma generated by the laser striking
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 purification
process succeeds in removing the non-nanotube carbon fractions and
the metals. However, the purification process still results in a
random orientation of tube bundles which have .about.5-20 nm
diameters.
[0046] A 1.0 mg sample of purified SWNTs was placed in a cylinder
containing 10 ml of deionized water or other polar hydrophilic
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.sup.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
material agglomerated in the solution when the ultrasonic agitation
was turned off. Gentle shaking of the solution redistributed 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 series of substrates. For example, a
TEM grid 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 grid. 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 polarity 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 (.apprxeq.50 nm) while water produces the largest
(.apprxeq.1 micron).
[0047] FIGS. 4b displays a TEM image of SWNTs on a grid 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 arises from the
minimization of the interactions between hydrophobic SWNT surfaces
and the hydrophilic 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. FIG. 5a displays a TEM image of a more
collapsed bundle extracted from water. The dense bundle is
displayed at higher resolution in FIG. 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
[0048] 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 1200{haeck over (s)}C. The laser was operated at
a frequency of 10 Hz, at .about.10-30 J/pulse-cm2 with an
.about.450 ns pulse width. Argon flowing at 100 sccm at 500 torr
was maintained through the reaction vessel for the duration of the
synthesis. The raw soot was purified by refluxing in 3M nitric acid
for 16 hr, filtering and washing with de-ionized water followed by
air oxidation for 30 min at 825 K. The H.sub.2 adsorption capacity
of various SWNT samples were probed by a previously described
temperature programmed desorption (TPD) technique. Dillon, A. C. et
al., Nature 1997, 386, 377. Initially, .about.0.3-1 mg samples were
degassed in vacuum up to 823-973 K at 1 K/s. Room temperature
H.sub.2 exposures between 10-500 torr for several seconds were then
sufficient to achieve a maximum hydrogen adsorption. Partial
hydrogen coverage was obtained following H.sub.2 exposure at only
10 mtorr. The sample was generally cooled to 90 K prior to
evacuation of the TPD chamber in order to quickly obtain a base
pressure of .about.5.times.10.sup.-8 torr. The cooling process also
ensured that hydrogen which could desorb from the sample near room
temperature remained on the sample before acquiring TPD spectra.
The hydrogen desorption signals from SWNTs were calibrated using
known H.sub.2 desorption signals from 0.3-1 mg samples of
CaH.sub.2. The calibration was confirmed by performing thermal
gravimetric analyses on a 1 mg SWNT sample charged with hydrogen.
The hydrogen wt % measured by the two different methods was within
10%.
[0049] Previously a degas to 973 K in vacuum was sufficient to
activate hydrogen adsorption on arc-generated SWNTs. Dillon, A. C.
et al., Nature 1997, 386, 377. However in applying the same
experimental methods to the very long laser-generated nanotube
bundles no significant H.sub.2 adsorption was observed for either
the crude or purified materials. In order to activate the hydrogen
adsorption properties .about.1-3 mg of the purified laser-generated
material was placed in a cylinder containing 20 ml of 5M HNO.sub.3.
A Heat Systems-Ultrasonics Inc. model w-220F Cell Disrupter
ultrasonic probe was then employed to agitate the solution for time
periods ranging from 20 minutes to 16 hrs at powers ranging from
25-250 W/cm2. The final adsorption capacity of the SWNT samples
varied between 2-7 wt % depending on the material, the sonication
power, the sonication time and the sample degas temperature. Once
the unique hydrogen adsorption properties were obtained, the
samples remained activated for at least 8 months. The adsorption
was maximized following sonication of purified SWNTs for 24 hrs at
90 W/cm.sup.2 with a subsequent degas to 825 K.
[0050] FIG. 6 displays a typical TEM image of purified SWNTs
following ultrasonication in 5M HNO.sub.3. Here the as-synthesized
seemingly endless ropes of SWNTs have been "cut" into compact
bundles .about.1-5 microns in length. The average bundle diameter
has also been increased and the tubes have been reorganized and
functionalized. Upon degassing the cut SWNT samples in vacuum, NO,
H.sub.2O, CO, CO.sub.2, H.sub.2 and low molecular weight
hydrocarbons are observed. The desorption of NO is consisted with
the intercalation of nitric 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. FIG. 7 displays the
H.sub.2 TPD spectrum of a degassed SWNT sample following a brief
room temperature H.sub.2 exposure at 500 torr. The sample was
sonicated in dilute HN03 for 16 hrs and degassed to 825 K resulting
in hydrogen adsorption of 6.5 wt %. The spectrum is characterized
by two separate desorption signals peaked at 370 and 630 K
indicating at least two unique hydrogen adsorption 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 .about.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. FIG. 7 also displays the hydrogen
desorption signal from micro crystalline graphite following the
same pre-treatment and H.sub.2 exposure as the D optimized SWNT
sample. The graphite sample apparently has two H.sub.2 adsorption
sites that are very similar to the SWNT sample as there are two
desorption peaks at .about.380 and 610 K. It is interesting however
that the graphite hydrogen adsorption is only .about.19% of the
H.sub.2 adsorption observed for an SWNT sample of the same mass
following the same preparation.
[0051] Both of the SWNT hydrogen adsorption sites are effectively
"capped" by the adsorption. This phenomena was revealed in an
experiment where a standard hydrogen exposure to cut SWNTs was
followed by a room temperature 10 min. 500 torr exposure to
CO.sub.2. The sample was cut for 4 hrs at 90 W/cm.sup.2 resulting
in a total hydrogen adsorption capacity of .about.3 wt %. FIG. 8
displays the hydrogen desorption 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 .about.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. FIG. 8b and c display TPD desorption signals of
CO.sub.2 and H.sub.2 respectively when an identical hydrogen
exposure was followed by a CO.sub.2 exposure for 10 min. at 500
torr. The small CO.sub.2 desorption signal peaked at 400 K
demonstrates the existence of a small population of CO.sub.2
adsorption sites on the SWNT sample. Surprisingly, however the
H.sub.2 desorption signals are both shifted to higher temperature
by .about.50 K. It appears that the hydrogen does not begin to
desorb from the SWNTs until a significant portion of the CO.sub.2
has already evolved suggesting that the CO.sub.2 in fact blocks the
hydrogen desorption process. In a subsequent experiment the sample
was exposed to CO.sub.2 for 10 min at 500 torr at room temperature
and then exposed to H.sub.2 under the same conditions. Subsequent
TPD revealed only a CO.sub.2 desorption signal, similar to that
seen in FIG. 5b with no hydrogen absorption, demonstrating that
carbon dioxide completely blocks hydrogen adsorption sites.
CO.sub.2 also apparently "caps" the SWNTs upon exposure to air.
Samples charged with hydrogen and exposed to atmosphere were shown
to desorb first CO.sub.2 and then H.sub.2 several weeks later
without a significant loss in the stored hydrogen. This effective
"capping" of the SWNTs by CO.sub.2 may prove significant to the
commercial development of SWNTs for a hydrogen storage system.
[0052] Raman spectroscopy was employed to elucidate the mechanism
of the unique room temperature hydrogen adsorption on SWNTs. Raman
spectra were obtained using a 50 mW Ar ion laser (488 nm) with a
resolution of .about.4-6 cm.sup.-1. FIG. 9 displays a series of
Raman spectra between 1500-1700 cm.sup.-1 collected from SWNT
samples throughout the pretreatment and H.sub.2 adsorption process.
The initial spectrum of the purified SWNTs (FIG. 9a) exhibits two
strong features at 1597 and 1571 cm.sup.-1. These features are
slightly shifted from their characteristic occurrence at 1593 and
1567 cm.sup.-1. FIG. 9b displays the Raman spectrum of the purified
sample following sonication in SM HNO.sub.3 for 4 hr at 90
W/cm.sup.2. The two features are now broadened and further shifted
to 1599 and 1575 cm.sup.-1 with their intensity quite dramatically
reduced. Following degassing in vacuum to 1000 K however the Raman
intensity returns, and the features are shifted back to their
characteristic values of 1593 and 1567 cm.sup.-1 (FIG. 9c).
Surprisingly, FIG. 93d reveals that subsequent exposure to H.sub.2
at 500 torr again results in a loss in Raman intensity and a shift
to 1596 and 1570 cm.sup.-1. The intensity of the Raman signal is
again restored by heating in vacuum to 100OK 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 attributed to the radial breathing modes between
162-203 cm.sup.-1.
[0053] The reversible shift to higher frequency in Raman spectral
bands observed in FIG. 9 is indicative of charge transfer to an
acceptor species. The SWNT purification process results in some
degree of intercalation and functionalization as indicated by the
desorption of NO, H.sub.2O, CO, CO.sub.2, H.sub.2 and hydrocarbons
versus heating in vacuum to .about.1500 K. The purified sample had
been annealed to 825 K in air. This should result in the desorption
of the H.sub.2O, CO.sub.2 and NO species. However, the desorption
of chemisorbed hydrogen from purified uncut SWNTs occurs between
.about.900-1500 K. Apparently, the chemisorbed hydrogen is acting
as an electron acceptor and produces the shift in the Raman
spectrum of the purified SWNTs to 1597 cm.sup.-1. The further shift
to 1599 cm.sup.-1 following cutting the SWNTs in HNO.sub.3 is
consistent with an increase in the chemisorbed hydrogen population.
The broadening and dramatic loss in intensity indicates that the
nitric 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.sup.-1 upon heating to 1000 K. At this
temperature a significant portion of the chemisorbed hydrogen
resulting from the purification and cutting procedures has
desorbed. The subsequent intensity loss and shift to higher
frequency for the SWNT sample following exposure to H.sub.2 at 500
torr again indicates charge transfer. These results therefore
suggest that some of the 6.5 wt % adsorbed H.sub.2 undergoes an
interaction with the SWNTs which is stronger than physisorption.
Surprisingly the majority of the process is reversible as indicated
by the regeneration of the Raman intensity upon degassing to 1000 K
as displayed in FIG. 9e. The fact that the curve displayed in FIG.
9e is still slightly blue shifted may be attributed to a small
population of hydrogen from the H.sub.2 exposure still remaining on
the sample.
[0054] The exact location and nature of the SWNT H.sub.2 adsorption
sites is difficult to determine. However, collectively the results
of this study give clues to the unique adsorption interactions. The
shift to higher frequency and the decrease in Raman intensity
following ultrasonication in HNO.sub.3 is consistent with nitric
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.sup.2 pi character
of the nanotubes indicating an interaction which is again stronger
than physisorption. It is likely that this strong H.sub.2
interaction with the SWNTs is the source of the high temperature
TPD peak, and it is somewhat surprising that such a reaction is
reversible. Since the low temperature At hydrogen desorption peak
evolves completely simply by sitting at room temperature overnight
in vacuum, it most likely stems from a very weak interaction or
physisorption.
[0055] 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.
* * * * *