U.S. patent application number 12/435877 was filed with the patent office on 2010-03-25 for methods for purifying carbon materials.
This patent application is currently assigned to California Institute of Technology. Invention is credited to Channing Ahn, Anne Dailly, Brent T. Fultz, Rachid Yazami.
Application Number | 20100074832 12/435877 |
Document ID | / |
Family ID | 34994336 |
Filed Date | 2010-03-25 |
United States Patent
Application |
20100074832 |
Kind Code |
A1 |
Dailly; Anne ; et
al. |
March 25, 2010 |
METHODS FOR PURIFYING CARBON MATERIALS
Abstract
Methods of purifying samples are provided that are capable of
removing carbonaceous and noncarbonaceous impurities from a sample
containing a carbon material having a selected structure.
Purification methods are provided for removing residual metal
catalyst particles enclosed in multilayer carbonaceous impurities
in samples generate by catalytic synthesis methods. Purification
methods are provided wherein carbonaceous impurities in a sample
are at least partially exfoliated, thereby facilitating subsequent
removal of carbonaceous and noncarbonaceous impurities from the
sample. Methods of purifying carbon nanotube-containing samples are
provided wherein an intercalant is added to the sample and
subsequently reacted with an exfoliation initiator to achieve
exfoliation of carbonaceous impurities.
Inventors: |
Dailly; Anne; (Pasadena,
CA) ; Ahn; Channing; (Pasadena, CA) ; Yazami;
Rachid; (Los Angeles, CA) ; Fultz; Brent T.;
(Pasadena, CA) |
Correspondence
Address: |
GREENLEE WINNER AND SULLIVAN P C
4875 PEARL EAST CIRCLE, SUITE 200
BOULDER
CO
80301
US
|
Assignee: |
California Institute of
Technology
Pasadena
CA
Centre National De La Recherche Scientifique
Paris
|
Family ID: |
34994336 |
Appl. No.: |
12/435877 |
Filed: |
May 5, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
11081841 |
Mar 16, 2005 |
7537682 |
|
|
12435877 |
|
|
|
|
60553930 |
Mar 17, 2004 |
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Current U.S.
Class: |
423/447.1 ;
977/742; 977/845 |
Current CPC
Class: |
C01B 32/215 20170801;
D06M 2101/40 20130101; C01B 32/05 20170801; D01F 11/127 20130101;
D06M 11/83 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
423/447.1 ;
977/742; 977/845 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made, at least in part, with United
States governmental support awarded by Department of Energy Grant
DE-FC36-01GO11090. The United States Government has certain rights
in this invention.
Claims
1. A method of purifying a sample containing a carbon material
having a selected structure said method comprising the steps of:
providing said sample containing said carbon material having a
selected structure and impurities comprising carbonaceous
impurities, wherein said carbonaceous impurities comprising one or
more carbon layers having a structure different than that of said
selected structure; exfoliating at least a portion of the carbon
layers of said carbonaceous impurities, thereby generating
exfoliated carbonaceous material; and removing said impurities,
thereby purifying said sample containing a carbon material having a
selected structure.
2. The method of claim 1 wherein said step of removing said
impurities comprises oxidizing the exfoliated carbonaceous
material.
3. The method of claim 1 wherein said step of removing said
impurities comprises preferentially oxidizing said exfoliated
carbonaceous material such that the carbon material having a
selected structure is not substantially oxidized.
4. The method of claim 1 wherein said step of removing said
impurities comprises oxidizing said exfoliated carbonaceous
materials in air at a temperature selected from the range of about
250 degrees Celsius to about 450 degrees Celsius.
5. The method of claim 4 wherein said step of removing said
impurities comprises oxidizing said exfoliated carbonaceous
materials in the presence of air having a relative humidity
selected from the range of about 50 percent to about 100
percent.
6. The method of claim 1 wherein said impurities comprise metal
particles, wherein said metal particles are at least partially
enclosed by a plurality of said carbon layers, wherein said step of
exfoliating at least a portion of said multilayer carbonaceous
impurities exfoliates at least a portion of said carbon layers
enclosing said metal particles, thereby exposing the surface of
said metal particles.
7. The method of claim 6 wherein said step of removing said
impurities comprises oxidizing said metal particles, thereby
generating oxidized metal particles, wherein the volume of said
metal particles increases upon oxidation, thereby disrupting said
carbon layers enclosing said metal particles further exposing said
metal surface of said oxidized metal particles.
8. The method of claim 6 wherein said step of removing said
impurities comprises dissolving said metal particles.
9. The method of claim 6 wherein said step of removing said
impurities comprises dissolving said metal particles into an acidic
solution.
10. The method of claim 9 wherein said step of dissolving said
metal particles into an acidic solution comprises refluxing said
sample in concentrated hydrochloric acid.
11. The method of claim 6 wherein said metal particles comprise one
or more transition metals.
12. The method of claim 6 wherein said metal particles comprise one
or more metals selected from the group consisting of: nickel;
yttrium; iron; molybdenum; palladium copper; molybdenum; and
cobalt.
13. The method of claim 1 wherein said step of exfoliating at least
a portion of the carbon layers of said carbonaceous impurities
comprises adding an intercalant to said sample.
14. The method of claim 13 wherein said intercalant inserts into
interstitial sites in between multiple carbon layers comprising
said carbonaceous impurities, interstitial sites between carbon
layers comprising said carbonaceous impurities and said carbon
material having a selected structure or both, thereby generating
intercalated carbon layers.
15. The method of claim 14 wherein said carbon layers comprise a
material selected from the group consisting of: graphene layers;
graphite layers; and amorphous carbon layers.
16. The method of claim 14 wherein said step of exfoliating at
least a portion of the carbon layers of said carbonaceous
impurities further comprises increasing the temperature of said
sample.
17. The method of claim 13 wherein said intercalant is an electron
donor intercalant.
18. The method of claim 17 wherein said electron donor intercalant
is an alkali metal.
19. The method of claim 13 wherein said intercalant is an electron
acceptor intercalant.
20. The method of claim 19 wherein said electron acceptor
intercalant is selected from the group consisting of: a halogen; a
metal chloride; nitric acid; perchloric acid; sulfuric acid; and
formic acid.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/081,841, filed Mar. 16, 2005, which claims
benefit of U.S. Provisional Patent Application 60/553,930 filed
Mar. 17, 2004. These previous applications are hereby incorporated
by reference in their entireties to the extent not inconsistent
with the disclosure herein.
BACKGROUND OF INVENTION
[0003] Since their discovery in the early 1990s, carbon nanotubes
have been the subject of intense scientific research directed
toward developing techniques for synthesizing high quality nanotube
materials, and evaluating their physical and chemical properties.
Carbon nanotubes are allotropes of carbon comprising one or more
cylindrically configured graphene sheets. Carbon nanotubes
typically have small diameters (.apprxeq.1-10 nanometers) and large
lengths (up to several microns), and therefore may exhibit very
large aspect ratios (length to diameter ratio .apprxeq.10.sup.3 to
about 10.sup.5). Nanotube materials are often classified on the
basis of structure as either single walled carbon nanotubes (SWNTs)
or multiwalled carbon nanotubes (MWNTs). Research over the past
decade has demonstrated that carbon nanotube materials exhibit
extraordinary mechanical, electrical and chemical properties, which
has stimulated substantial interest in developing applied
technologies exploiting these properties.
[0004] Single walled carbon nanotubes (SWNTs) are one class of
carbon nanotubes which have been identified as potentially useful
materials for a number of applied technologies. SWNTs are made up
of a single, contiguous graphene sheet wrapped around and joined
with itself to form a hollow, seamless tube having capped ends
similar in structure to smaller fullerenes. SWNTs typically have
very small diameters (.apprxeq.1 nanometer) and are often present
in curled and looped configurations. The energy band structure of
SWNTs varies considerably, and SWNTs exhibit either metallic or
semiconductor electrical behavior depending on their precise
molecular structure and diameter. Under some experimental
conditions, SWNTs undergo efficient self assembly processes that
generate bundles (or ropes) of SWNTs aligned along their lengths
and strongly bound together by van der Waals forces. SWNTs are
chemically versatile materials which have been demonstrated as
capable of functionalization of their exterior surfaces and capable
of encapsulation of materials within their hollow cores, such as
gases and molten materials.
[0005] Research over the last decade has identified a number of
unique properties of SWNTs which make these materials particularly
promising candidate materials for a variety of device applications
ranging from a revolutionary class of new electronic devices to
composite materials having enhanced mechanical properties. First,
SWNTs are believed to have remarkable mechanical properties
suggesting their utility as structural reinforcement additives in
high strength, low weight and high performance composite materials.
For example, calculations and experimental results suggest that the
SWNTs have tensile strengths at least 100 times that of steel or
any known other known fiber. In addition, SWNTs are stiffer than
conventional reinforcement materials, such as carbon fibers, while
also exhibiting a very large Young's Modulus (as large as about 1
TPa) when distorted in some directions. Second, SWNTs exhibit
useful electrical properties which may serve the basis of a new
class of nanotube based electronic devices. For example, the
electron transport behavior in carbon nanotubes is predicted to be
essentially that of a quantum wire, which has stimulated interest
in fabricating ultrafast nanotube based devices. In addition, the
electrical properties of SWNTs have been observed to vary
significantly upon charge transfer doping and intercalation, which
has opened up new avenues for tuning the electrical properties of
these materials. Further, due to their nanometer size diameter,
mechanical robustness, chemical stability and high electrical
conductivity, SWNTs may provide enhanced field emitters in a range
of devices, including flat panel displays, AFM tips and electron
microscopes. Finally, SWNTs are also believed to possess useful
thermal, magnetic and optical properties which make them suitable
materials for a range of emerging applied technologies.
[0006] The unique chemical and physical characteristics of carbon
nanotubes is often severely attenuated or entirely lost when these
materials are present with substantial amounts of impurities.
Therefore, the successful development of nanotube based
technologies taking full advantage of their extraordinary
properties depends critically on the availability of sources of
substantially pure nanotube materials. Currently available methods
for synthesizing SWNTs, however, do not directly result in
substantially pure samples containing these materials. Rather,
conventional synthesis processes, such as techniques utilizing arc
discharge, laser ablation, and chemical vapor deposition, yield a
complex reaction product comprising a mixture of SWNTS,
carbonaceous impurities and noncarbonaceous impurities. The
impurity component of the reaction product generated using many of
these techniques is very significant and SWNTs often comprise less
than half of the reaction product by weight. Carbonaceous
impurities generated in conventional synthesis processes are
present in both single layer and multilayer configurations and
include amorphous carbon, graphene sheets, graphite, incomplete and
complete fullerenes and multiwalled nanotubes. Noncarbonaceous
impurities commonly present in SWNT containing samples include
residual metal catalyst particles, such as particles comprising
nickel, yttrium, iron, molybdenum, palladium, and cobalt, and
catalyst support materials, such as ceramic materials.
[0007] As a result of associative intermolecular interactions, such
as van der Waals interactions, impurities and SWNTs in samples
prepared via conventional synthesis techniques are present in
highly coupled physical states. For example, the outer surfaces of
SWNTs and bundles of SWNTs are typically heavily coated with a
variety of single and multilayer carbonaceous impurities. In
addition, metal catalyst particles unavoidably generated in
catalytic synthesis methods are often entirely or partially
encapsulated in high stable multilayers comprising carbonaceous
impurities. Associative intermolecular interactions involving these
materials present a unique challenge for purifying and isolating
SWNTs in samples prepared by conventional synthesis methods. For
example, the carbon multilayers surrounding metal catalyst
particles severely reduce the effectiveness of purification via
dissolution of the particles in acids provided to the sample. In
addition, associative interactions between carbonaceous impurities
and SWNTs pose significant problems for positioning, aligning
and/or integrating SWNTs into desired device configurations.
[0008] Significant research has been direct toward developing
methods of isolating and purifying SWNTs, due to the inability of
conventional synthesis methods to directly produce substantially
pure samples of high quality carbon nanotubes. Effective
purification methods are capable of removing the wide range of
different impurities that exhibit markedly different chemical and
physical properties. In addition, effective purification methods
are capable of selectively removing a majority of these impurities
without causing significant damage to or destruction of the SWNTs.
Furthermore, effective purification methods are capable high
throughout processing of significant quantities of SWNTs in a
relatively small number of efficient purification steps, thereby
avoiding unreasonably long processing times.
[0009] A number of different approaches have been pursued in recent
years for purifying SWNT containing samples. First, several
techniques have been developed that are based on chemically
modifying impurities to enhance their removal, including selective
oxidation processes, such as gas phase oxidation, catalytic
oxidation and acid oxidation, and nitric acid and hydrogen peroxide
reflux methods. Although chemical modification techniques have been
demonstrated as capable of enhancing the purity of SWNT containing
samples, these treatments invariably destroy a significant portion
of the SWNTs present in the sample, and are not effective at
removing metal particulate impurities enclosed in carbon
multilayers. Second, purification methods have been pursued based
on microfiltration and cross flow filtration techniques. These
techniques require a relatively large number of repeated filtration
and suspension processing steps, however, making the procedures
relatively slow and inefficient. Finally, purification of SWNTs via
size exclusion chromatography has also been demonstrated. However,
this approach requires use of surfactants for suspension of the
SWNTs in the sample undergoing chromatographic separation, which
can result in residual surfactant in the purified sample that can
deleteriously affect the chemical and physical properties of the
purified nanotubes.
[0010] The following references relate generally to methods of
synthesizing and purifying carbon nanotubes: (1) A. M. Cassell, J.
A. Raymakers, J. Kong, H. J. Dai, J. Phys. Chem. B 103(31), (1999),
pp. 6484-6492; (2) M. Su, B. Zheng, J. Liu, Chem. Phys. Lett.
322(5), (2000), pp. 321-326; (3) B. Kitiyanan, W. E. Alvarez, J. H.
Harwell, D. E. Resasco, Chem. Phys. Lett. 317(3-5), (2000), pp.
497-503; (4) J. H. Hafner, M. J. Bronikowski, B. R. Azamian, P.
Nicolaev, A. G. Rinzler, D. T. Colbert, K. A. Smith, R. E. Smalley,
Chem. Phys. Lett. 296(1-2), (1998) pp. 195-202 (5) H. M. Cheng, F.
Li, G. Su, H. P. Pan, L. L. He, X. Sun, M. S. Dresselhaus, Appl.
Phys. Lett. 72(25), (1998), pp. 3282-3284; (6) B. Zheng, Y. Li, J.
Liu, Applied Physics A 74, 345-348 (2002); (7) C. Journet, W. K.
Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant,
R. Lee, J. E. Fischer, Nature 388, 756-758 (1997); (8) A. Thess, R.
Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S.
G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanek,
J. E. Fischer, R. E. Smalley, Science 273, (1996) 483-487; (9) M.
Holzinger, A. Hirsch, P. Bernier, G. S. Duesberg, M. Burghard,
Applied Physics A 70, (2000) 599-602; (10) G. S. Duesberg, J.
Muster, V. Krstic, M. Burghard, S. Roth, Appl. Phys. A 67, (1998),
117-119; (11) J. Liu, A. G. Rinzler, H. Dai, J. H. Hafner, R. K.
Bradley, P. J. Boul, A. Lu, T. Iverson, K. Shelimov, C. B. Huffman,
F. Rodriuez-Macias, Y. S. Shon, T. R. Lee, D. T. Colbert, R. E.
Smalley, Science 280, (1998) 1253-1256; (12) A. G. Rinzler, J. Liu,
H. Dai, P. Nikolaev, C. B. Huffman, F. J. Rodriguez-Macias, P. J.
Boul, A. H. Lu, D. Heymann, D. T. Colbert, R. S. Lee, J. E.
Fischer, A. M. Rao, P. C. Eklund, R. E. Smalley, Appl. Phys. A 67,
(1998) 29-37; (13) Y. Feng, G. Zhou, G. Wang, M. Qu, Z. Yu, Chem,
Phys, Lett, 375, (2003) 645-648; (14) T. Takenobu, M. Shiraishi, A.
Yamada, M. Ata, H. Kataura, Y. Iwasa, Synthetic Metals 135-136,
(2003) 787-788; (15) I. W. Chiang, B. E. Brinson, A. Y. Huang, P.
A. Willis, M. J. Bronikowski, J. L. Margrave, R. E. Smalley, R. H.
Hauge, J. Phys. Chem. B 105, (2001) 8297-8301.
[0011] It will be appreciated from the foregoing that there is
currently a need in the art for improved methods for purifying
carbon nanotube materials, particularly SWNTs, such that these
materials can be effectively integrated into a range of applied
technology settings. Specifically, methods are needed for purifying
SWNT containing samples generated via conventional SWNT synthesis
methods, such as samples comprising mixtures of SWNTS, a range of
carbonaceous impurities and metal or metal oxide catalyst
particles. In addition, purification methods are needed that
provide high yields of high quality nanotube materials, thereby
minimizing loss or damage to SWNTs in a sample undergoing
purification. Further, low cost and versatile purification methods
are needed that are compatible with high throughput processing of
large amounts of SWNTs generated by the conventional synthesis
techniques.
SUMMARY OF THE INVENTION
[0012] The present invention provides processes for purifying
samples comprising one or more desired forms of carbon by removing
impurities from the sample. Purification methods of the present
invention are capable of preferentially removing undesired
impurities, thereby increasing the purity a desired carbon material
having a selected structure in a sample. Methods of the present
invention are capable of effectively removing impurities without
substantially damaging or destroying the desired carbon material in
a sample undergoing processing. In some embodiments, therefore, the
present methods provide a means of purifying a selected carbon
material such that the purified selected carbon material has a
structure and composition that does not different significantly
from its structure and composition in the starting material subject
to processing or has a structure that is enhanced after processing
via the present methods, such as a structure exhibiting enhanced
extent of crystallinity. The present purification methods are
versatile and, thus capable of generating a range of substantially
pure carbon materials, including carbon nanotubes (SWNTS and
MWNTs), fullerenes, and other carbon allotropes such as carbon
fibers, carbon films and diamond, from samples generated by a range
of synthesis methods, including, catalytic and noncatalytic
processes. In one embodiment, the methods of the present invention
provide a source of substantially pure, high quality carbon
nanotubes, are compatible with high throughput processing of
samples comprising large quantities of carbon nanotubes, and
require substantially less processing steps than conventional
methods of purifying carbon nanotubes.
[0013] The present purification methods selectively remove
carbonaceous impurities, noncarbonaceous impurities and multiphase
impurities having both a carbon-containing portion and a
noncarbon-containing portion by exfoliating single and multilayer
carbonaceous impurities having structures that differ from the
structures of selected carbon materials in the sample. Exfoliation
processes useful in the present methods disrupt the structure and
arrangement of single and multilayer carbon impurities thereby
making these materials more susceptible to removal via subsequent
processing steps. Exfoliation of carbon impurities comprising a
plurality of layers of graphene, graphite and/or amorphous carbon,
for example, makes these materials susceptible to removal by gas
phase oxidation at temperatures lower than those corresponding to
the gas phase oxidation of three materials in their unexfoliated
forms. In embodiments of the present invention providing high
yields of selected carbon materials, exfoliation is carried out
under conditions wherein undesired carbonaceous impurities are at
least partially exfoliated, while the desired, selected forms of
carbon do not undergo substantial exfoliation and/or chemical
degradation. Any exfoliation process capable of selectively
disrupting the structure and arrangement of single and multilayer
carbonaceous impurities may be used in the present purification
methods, including methods wherein one or more intercalants are
added to the sample and insert into interstitial sites between
multilayers of carbonaceous impurities and/or into interstitial
sites located between a selected carbon material and a carbonaceous
impurities coating its outer surface.
[0014] The present invention provides methods that are particularly
useful for purifying samples prepared by processes that yield a
mixture of a desired carbon material, including carbon nanotubes
(SWNTS and/or MWNTs), fullerenes, and/or other selected allotropes
of carbon, and impurities comprising residual metal catalyst
particles or catalyst support materials, such as transition metal
or metal oxide particles partially or completely enclosed in
multilayer carbonaceous impurities. In one embodiment, the present
invention provides methods for at least partially exfoliating
carbonaceous multilayers enclosing metal catalyst particles in a
sample. This aspect of the present invention enhances subsequent
removal of metal particles because exfoliation ruptures the
structure of the carbonaceous layers thereby making the particles
susceptible to removal via dissolution. Optionally, methods of this
aspect of the present invention may further comprise the step of
selectively removing the exfoliated carbonaceous multilayers, for
example via wet oxidation, which further exposes the outer surfaces
of the metal particles and enhances subsequent removal via
dissolution. This aspect of the present invention provides
versatile purification methods useful for increasing the purity of
a desired carbon material in samples generated by virtually any
catalytic synthesis techniques employing metal or metal based
catalysts such as metal carbonyls.
[0015] In one aspect, the present invention provides a method of
purifying a sample containing a carbon material having a selected
structure wherein at least partial exfoliation of undesired single
and multilayer carbonaceous impurities present in the sample allows
for effective removal of impurities having a range of chemical
compositions and physical states. A sample is provided containing
the carbon material having a selected structure and impurities. The
impurities present in the sample comprise carbonaceous impurities
comprising one or more carbon layers having structures different
than that of the selected carbon material undergoing purification.
Optionally, the impurities present in the sample comprise
noncarbonaceous impurities, such as metal particles and/or metal
derivative particles such as metal oxide particles, and multiphase
impurities, such as metal particles and/or metal derivative
particles such as metal oxide particles partially or completely
enclosed in multilayer carbonaceous impurities.
[0016] At least a portion of the carbon layers of the carbonaceous
impurities are exfoliated, and the impurities are removed thereby
purifying the sample. Exfoliation of at least a portion of the
carbon layers in this method of the present invention allows for
effective removal of carbonaceous impurities, noncarbonaceous
impurities and multiphase impurities. In one embodiment, removal of
exfoliated carbonaceous materials directly purifies the sample. Any
method of removing exfoliated carbonaceous material may be used in
the present methods provided that it does not substantially damage
or destroy the selected carbon material undergoing purification,
including but not limited to chemical transformation methods (e.g.
wet oxidation and dissolution), filtration (e.g. cross flow or
microfiltration methods) and chromatographic methods (e.g. size
exclusion chromatography).
[0017] In another embodiment, exfoliation processing steps
exfoliate at least a portion of carbon multilayers enclosing metal
particles in the sample undergoing processing. In this embodiment,
exfoliation at least partially exposes the surface of the metal
particles, thereby allowing for their effective removal by a wide
variety of removal methods. Removal of the exposed metal particles
may be provided by any technique known in the art including, but
not limited to, dissolution in a solution added to the sample, for
example by refluxing the sample in a concentrated inorganic and/or
organic acid, and/or treatment with an appropriate gas, for example
by treatment with a halogen gas, such as chlorine at high
temperatures (e.g. at or above about 900 degrees Celsius).
Optionally, the purification methods of the present invention may
further comprise the step of annealing the sample after the
impurities have been removed. This optional step is beneficial in
some applications as it may repair any damage to the carbon
material having a selected structure that occurs during processing.
It also removes residual reactant and solvant(s) and/or enhances
the crystalline structure of the selected carbon material.
[0018] Purification methods of the present invention are capable of
enriching the purity of a single selected carbon material or
plurality of different selected carbon materials. Selectivity in
the present invention is provided by processing steps that are
capable of exfoliating a selected set of materials corresponding to
undesired impurities, while avoiding exfoliating or damaging to the
desired carbon material(s) in the sample undergoing purification.
In one embodiment, for example, the present invention provides
methods for selectively enriching the purity of single walled
carbon materials, such as SWNTs and/or single walled fullerenes, by
selectively exfoliating carbonaceous materials that comprise a
plurality of carbon multilayers, such as graphene multilayer
structures, graphite multilayers, amorphous carbon multilayers,
multiwalled fullerenes, MWTNs or any combination of these
materials. Selective exfoliation in this aspect of the present
invention allows for multilayer carbonaceous impurities to be
efficiently removed, for example by selective oxidation, without
significant loss of or damage to the desired single walled carbon
materials. Alternatively, the present invention provides methods
for selectively enriching the purity of one or more selected
multilayer carbon structures, such as MWNTs, multiwalled
fullerenes, diamond, carbon fibers, carbon films or any combination
of these materials. In this embodiment, the exfoliation technique
employed and experimental conditions of the sample (e.g.
temperature, sample composition, composition of exfoliation
initiator added to the sample etc.) are chosen such that undesired
multilayered carbonaceous impurities are selectively exfoliated and
subsequently removed, resulting in enhancement of the purity of the
selected multilayer carbon structures in the sample.
[0019] Exfoliation of multilayer carbonaceous impurities can be
carried out in any manner which promotes effective removal of
impurities, while not substantially damaging or destroying the
carbon materials having a selected structure in the sample.
Exfoliation processes preferred for some application are rapid
exothermic chemical reactions or physical changes that exfoliate
carbonaceous materials without damaging desired components of a
sample. In some embodiments, exfoliation is carried out by addition
of one or more intercalants to the sample that insert into
interstitial sites between carbon multilayers of multilayer
carbonaceous impurities and/or insert into interstitial sites
between a selected carbon material, such as SWNTs or MWNTs, and a
carbonaceous impurity coating the outside of the selected carbon
material.
[0020] In one embodiment of the present invention, addition of an
intercalant generates intercalated carbon materials having
intercalants present in interstitial sites between carbon
multilayers comprising carbonaceous impurities. The presence of
intercalants between the multilayers of carbonaceous impurities
makes these materials susceptible to various selective exfoliation
processes. For example, some intercalants weaken or disrupt the
intermolecular or intramolecular forces between adjacent
multilayers resulting in more selective and more complete
exfoliation. Insertion of some intercalants of the present
invention between multilayers results in an expansion of the
interlayer distance separating adjacent multilayers and/or may
provide a reactant for chemical reactions that initiate selective
exfoliation of multilayer carbonaceous impurities.
[0021] Alternatively, addition of intercalants in the present
invention generates intercalated carbon materials having
intercalants present in interstitial sites between a selected
carbon material and an impurity comprising a carbonaceous outer
layer coating the selected carbon material. This aspect of the
present invention is useful for disrupting associative
interactions, such as van der Waals interactions, between
impurities and desired carbon materials in a sample. In one
embodiment, insertion of intercalants into interstitial sites
between a selected carbon material and a layer comprising
carbonaceous impurity provides a means of separating the selected
carbon material from impurities that associatively interact with
it. This aspect of the present invention is particularly useful for
separating and removing single or multiple layer carbonaceous
impurities coating the outer surfaces of SWNTs and MWNTs. This
aspect of the present invention is useful for removing coatings of
carbonaceous impurities on carbon nanotubes which affect their
physical, electrical and chemical properties and present
difficulties in integrating these materials into desired device
configurations.
[0022] In one embodiment of the present invention, exfoliation is
achieved by adding an exfoliation initiator to a sample containing
one or more intercalants which reacts with intercalants in
intercalated carbon layers, thereby initiating selective
exfoliation. Exemplary exfoliation initiators react exothermically
with intercalants in intercalated carbon layers and, optionally
initiate an expansion of material present between carbon layers,
for example by generating a gaseous reaction product. In another
embodiment, exfoliation is achieved by subsequently raising the
temperature of the intercalated carbon layers, for example by
heating the sample or exposing the sample to electromagnetic
radiation. Rapidly raising the temperature of intercalated samples
provides sufficient energy to selectively exfoliate the carbon
layers of carbonaceous impurities.
[0023] As many carbonaceous impurities are amphoteric in nature,
intercalants useful in this aspect of the present invention
include, electron donor intercalant, such as alkali metals (e.g.
Li, K, Na, Rb and Cs), and electron acceptor intercalants, such as
halogens (e.g. fluorine, bromine and iodine), metal chlorides and
acids (e.g. lewis acids such as nitric acid, perchloric acid,
sulfuric acid; and formic acid). The present invention includes
methods wherein more than one type of intercalant is added to the
sample undergoing purification, for example methods in which carbon
layers are bi-intercalated. The present invention includes
intercalation processing of samples via electrochemical methods,
including but not limited to, intercalation initiated via
chronoamperometric or chronovoltametric methods. Intercalants may
be provided as liquids, gases and in solutions added to samples
undergoing purification. Preferred intercalants for applications
requiring high yields of high quality carbon materials do not
significantly damage or disrupt the desired carbon material
undergoing purification.
[0024] The present methods are capable of processing samples
comprising complex mixtures of materials having a wide range of
chemical compositions, structures and physical states. For example,
the present methods are useful for purifying sample generated by
virtually all catalytic and noncatalytic synthesis methods for
making SWNTs, MWNTs, catalytically grown carbon fibers,
catalytically grown carbon films and diamond, including but not
limited to, arc-discharge methods, chemical vapor deposition
methods, plasma enhanced chemical vapor deposition methods, high
pressure carbon monoxide disproportionation methods; pyrolytic
methods, flame synthesis methods, electrochemical synthesis
methods; and laser ablation methods. The versatility with respect
to the type of carbon material that is purified in the present
methods largely arises from the use of selective exfoliation
wherein impurities having structures similar to, but not the same
as, a desired carbon material may be exfoliated and removed without
substantial loses or damage to the desired carbon material. In
addition, this versatility arises from the use of selective
exfoliation to render noncarbonaceous impurities in a state
susceptible to removal via dissolution.
[0025] The present invention provides a method of purifying a
sample containing SWNTS by selectively removing graphene layers,
graphite, amorphous carbon, incomplete fullerenes, metal particles
or any combination of these materials. In one embodiment, methods
of this aspect of the present invention are capable of providing
purified samples corresponding to a yield of SWNTs selected over
the range of about 15% to about 20% and a purity selected from
about 90 to about 98% by weight. In the context of this
description, the term yield refers to the weight ratio (in percent)
of the achieved purified SWNT to the initial amount of carbon
material before purification (e.g. 1 gram of carbon will provide
150 milligram of purified SWNT in a process providing a 15% yield).
In another embodiment, the present invention provides a method of
purifying a sample containing MWNTS by selectively removing
graphene layers, graphite, amorphous carbon, incomplete fullerenes,
metal particles or any combination of these materials. Further, the
methods of the present invention are applicable to purifying
nanotubes made from other materials, such as boron nitride
nanotubes. The present methods are not limited to purifying carbon
nanotube containing samples, and are capable of isolating and
purifying other useful allotropes of carbon, such as single walled
fullerenes, multiwalled fullerenes, diamond, carbon fibers, carbon
films, and carbon whiskers.
[0026] The present methods are also suitable for purification of
diamond materials. Conventional diamond synthesis methods, such as
high temperature--high pressure processing of molten graphite and
chemical vapor deposition methods, generate diamond-containing
samples having significant carbonaceous impurities, such as
graphite. Methods of the present invention are useful for
exfoliating these carbonaceous impurities, thereby allowing for
more easy removal of the exfoliated carbonaceous impurities.
[0027] In another aspect, the present invention provides a method
of purifying a sample containing a carbon material having a
selected structure comprising the steps of: (1) providing the
sample containing the carbon material having a selected structure
and impurities comprising carbonaceous impurities, wherein the
carbonaceous impurities comprising one or more carbon layers having
a structure different than that of the selected structure; (2)
exfoliating at least a portion of the carbon layers of the
carbonaceous impurities, thereby generating exfoliated carbonaceous
material; and (3) removing the impurities, thereby purifying the
sample containing a carbon material having a selected structure.
Optionally, methods of this aspect of the present invention may
further comprise the step of annealing the purified sample to
repair any damage incurred during purification processing, remove
residual exfoliation reactants and solvent and/or enhance the
crystal structure of the carbon material having a selected
structure.
[0028] In another aspect, the present invention provides a method
of exfoliating at least a portion of a plurality of carbon
multilayers at least partially enclosing metal particles in a
sample generated in the synthesis of carbon nanotubes comprising
the steps of: (1) providing the sample generated in the synthesis
of carbon nanotubes containing the metal particles, wherein each
metal particle is at least partially enclosed by the carbon
multilayers; (2) adding an intercalant to the sample, wherein the
intercalant inserts between at least a portion of the carbon
multilayers, thereby generating intercalated carbon multilayers
having intercalant present between the carbon multilayers; and (3)
adding an exfoliation initiator to the sample which reacts with
intercalant present between the carbon multilayers, thereby
exfoliating at least a portion of the carbon multilayers.
[0029] In another aspect, the present invention provides a method
of removing carbonaceous coatings from the outer surfaces of carbon
nanotubes comprising the steps of: (1) providing a sample
comprising the carbon nanotubes, wherein at least a portion of the
outer surfaces of the nanotubes are partially or completely coated
with the carbonaceous coatings, and wherein the carbonaceous
coatings have a structure different from that of the nanotubes; (2)
adding an intercalant to the sample, wherein the intercalant
inserts into interstitial sites between the nanotubes and the
carbonaceous coating; and (3) adding an exfoliation initiator to
the sample which reacts with the intercalant, wherein the reaction
between the intercalant present between the carbon nanotubes and
the carbonaceous coating and the exfoliation initiator exfoliates
the carbonaceous coating, thereby removing the carbonaceous
coatings from the outer surfaces of the carbon nanotubes.
Optionally, methods of this aspect of the present invention may
further comprise the step of annealing the purified sample to
repair any damage incurred during purification processing, remove
residual exfoliation reactants and solvent and/or enhance the
crystal structure of the carbon material having a selected
structure.
[0030] In another aspect, the present invention provides a method
of purifying a sample containing single walled nanotubes comprising
the steps of: (1) providing the sample containing the single walled
nanotubes and impurities comprising metal particles, wherein the
metal particles are at least partially enclosed by a plurality of
carbon multilayers having a structure different from that of the
carbon nanotubes; (2) adding an intercalant to the sample, wherein
the intercalant inserts between at least a portion of the carbon
multilayers enclosing the metal particles, thereby generating
intercalated carbon multilayers having intercalant present between
the carbon multilayers; and (3) adding an exfoliation initiator
which reacts with intercalant present between the carbon
multilayers, thereby exfoliating at least a portion of the carbon
multilayers and exposing the surface of the metal particles; and
(4) refluxing the sample in concentrated hydrochloric acid, thereby
dissolving the metal particles and purifying the sample containing
single walled nanotubes. Optionally, methods of this aspect of the
present invention may further comprise the step of annealing the
purified sample to repair any damage incurred during purification
processing, remove residual exfoliation reactants and solvent
and/or enhance the crystal structure of the carbon material having
a selected structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1A provides a schematic diagram showing processing
steps in a purification method of the present invention useful for
removing carbonaceous impurities and metal catalyst particles from
a sample containing one or more desired carbon materials having
selected structures. FIG. 1B provides a schematic flow diagram
showing processing steps in a purification method using potassium
intercalation for purifying SWNTs generated by catalytic synthesis
techniques, such as arc-discharge, gas phase catalysis (e.g. metal
carbonyls) and laser ablation synthesis methods.
[0032] FIG. 2 shows a transmission electron microscope (TEM) image
of the as-received sample containing SWNT that was purified using
the present methods. The inset in FIG. 2 shows a region of the
as-received material that comprises of the nanotube rope structure.
The metal containing catalyst appears as darker, sphere shaped
domains.
[0033] FIGS. 3A and 3B show TGA data recorded in an Ar/O.sub.2 flow
for the two studies of metal catalyst removal, where FIG. 3A shows
data for the conventional reflux purification methods. The plots in
FIG. 3A correspond to: (1) As-received Carbolex, (2) after
processing in HNO.sub.3 and H.sub.2O.sub.2, (3) after heating at
various temperatures in wet air (3) 350.degree. C., (4) 425.degree.
C., (5) 480.degree. C., (6) 510.degree. C., (7) 520.degree. C. and
finally (8) after heat treatment in N.sub.2/H.sub.2 forming gas at
750.degree. C. The plots in FIG. 3B correspond to: (1) As-received
Carbolex, (2) after doping with K, reacting with EtOH and HCl
reflux, (3) after 350.degree. C. wet oxidation, (4) after
425.degree. C. wet oxidation and (5) after annealing at 750.degree.
C. in N.sub.2/H.sub.2 forming gas. Each oxidation step was followed
by HCl reflux.
[0034] FIG. 4 provides Raman spectra of the sample after the
different stages of purification using the present methods and
provides reference spectra useful for interpreting the Raman
spectra of the sample. The plots in FIG. 4 correspond to: (a)
as-received Carbolex, (b) after doping with K, (c) after reacting
with EtOH, (d) after HCl reflux, 350.degree. C. wet oxidation and
HCl reflux, (e) after 425.degree. C. wet oxidation and HCl reflux
(f) after N.sub.2/H.sub.2 forming gas treatment at 750.degree. C.,
(g) graphite powder, (h) graphite powder after K intercalation
(KC.sub.8).
[0035] FIG. 5 presents X-ray diffraction (XRD) patterns from
samples after different steps of purification using the present
methods. The plots in FIG. 5 correspond to: (a) SWNT material
purified with the HNO.sub.3 process, (b) As-received Carbolex
material, (c) after doping with K, (d) after reacting in EtOH, HCl
reflux, wet oxidation at 350.degree. C., HCl reflux, (e) after wet
oxidation at 425.degree. C., HCl reflux, (f) after annealing in
N.sub.2/H.sub.2 forming gas at 750.degree. C. The peak indexing is
shown.
[0036] FIG. 6 provides transmission electron microscope (TEM)
images of the SWNT-containing sample after purification by the
present methods.
[0037] FIG. 7 provides X-ray diffraction (XRD) patterns of the
starting carbon fiber-containing sample and the sample after
different purification processing steps of the present
invention.
[0038] FIG. 8 shows a transmission electron microscope (TEM) image
of the as-received sample containing carbon fibers that was
purified using the present methods.
[0039] FIGS. 9A, 9B and 9C provide TEM images of the carbon
fiber-containing sample after purification by the present
methods.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Referring to the drawings, like numerals indicate like
elements and the same number appearing in more than one drawing
refers to the same element. In addition, hereinafter, the following
definitions apply:
[0041] "Carbon material" and "carbonaceous materials" are used
synonymously in the present description and refer to a class of
compounds comprising carbon atoms. Carbon materials include
allotropes of carbon such as SWNTs, MWNTs, single walled and
multiwalled fullerenes, graphite, graphene, amorphous carbon,
carbon fibers, carbon films, carbon whiskers, and diamond, and all
derivatives thereof. Carbon materials in the present invention may
be entirely comprised of carbon atoms or may include carbon in
combination with other elements, such as dopants, intercalants and
functional groups conjugated to the carbon structures. Depending on
the composition of the sample and intended outcome of the
purification method, carbon materials may be classified as desired
carbon materials or undesired carbonaceous impurities. Desired
carbon materials have a selected structure (which depends on the
precise application of the method) and are purified by the present
methods. Carbonaceous impurities, in contrast, have a structure
different from that of the desired carbon materials and are subject
to removal by the present methods. Carbon materials, including both
desired carbon materials and carbonaceous impurities, may exist in
single or multilayer forms.
[0042] The term "carbon layer" refers to a structure of wherein
certain atoms are held together by directed covalent, ionic or
other strong intramolecular forces. Layers may be planar or
contoured. Interactions between different carbon layers may be via
intermolecular forces, such as van der Waals interactions. Examples
of carbon layers include, but are not limited to, graphite layers,
graphene sheets, walls of a multiwalled or single walled
fullerenes, MWNTs, SWNTs, layers comprising carbon fiber and carbon
films. Carbon layer may also refer to a carbonaceous impurity
coating the outer surface of a particle, such as a residual metal
catalyst particle, metal derivative particles such as metal oxide
particles or particle comprising catalyst support material, and a
carbonaceous impurity coating the outer surface of desired a carbon
material, such as a carbon nanotube (SWNT and MWNT), a fullerene,
other desired carbon allotrope. The terms "carbon layer" and
"carbon layers" does not limit the size, shape or orientation of
the layers.
[0043] "Exfoliation" refers to a process whereby adjacent layers
are separated by a distance such that the strength of associative
interactions between adjacent layers is decreased. Exfoliation of a
multilayer material, such as a plurality of graphene sheets or
graphite layers, includes separation of a single layer comprising a
multilayer structure and includes involve separation of a plurality
of layers comprising a multilayer structure. Exfoliation may
involve separation of two different materials or separation of
different layers of a multilayer material by disruption of
associative interactions, such as van der Waals interactions, that
hold the materials together. For example, the present invention
provides methods of exfoliating carbonaceous impurities that coat a
desired carbon material such as a carbon nanotube (SWNT or MWNT),
fullerene, carbon fiber, diamond, and carbon film. In addition, the
present invention provides methods of exfoliating carbonaceous
impurities that coat particles, such as metal catalyst particles,
metal derivative particles such as metal oxide particles, and
particles comprising catalyst support materials. The present
invention provides methods wherein carbon layers comprising
carbonaceous impurities are either partially or entirely
exfoliated.
[0044] "Intercalant" refers to a material that inserts into
interstitial sites and/or endohedral sites in a single or
multilayer receiving material. Addition of an intercalant to a
receiving material comprising a carbon material (e.g. graphite,
graphene, amorphous carbon, MWNTs etc.), generates intercalated
carbonaceous material. In some cases, the intercalated carbonaceous
material has a well defined stoichiometry with respect to the
amount of intercalant associated with the carbon material. In one
embodiment of the present invention, intercalant is added to a
sample and inserts into interstitial sites between carbon layers in
multilayer carbonaceous impurities and/or inserts into interstitial
sites between a desired carbon material and a carbonaceous impurity
coating its outer surface.
[0045] "Exfoliation initiator" refers to a material that reacts
with intercalant in intercalated carbonaceous material, thereby
causing at least partial exfoliation of the intercalated
carbonaceous material. Exfoliation initiators may comprise
molecules, ions, atoms or complexes thereof, and may be in solid,
gas, liquid or solution phases.
[0046] "Metal catalyst particles" refer to metal particles
generated by catalytic synthesis methods for generating carbon
materials. Metal catalyst particles typically comprise transition
metals and mixtures thereof including, but not limited to, nickel,
molybdenum, palladium, yttrium, iron, copper, molybdenum; and
cobalt.
[0047] "Purification" refers to a process whereby the percentage
(or fraction) of a desired material in a sample is increased via
processing. Purification methods of the present invention may
increase the percentage by mass, percentage by weight, percentage
by volume and/or mole fraction of one or more selected carbon
materials in a sample undergoing processing. Purification methods
of the present invention useful for some applications are capable
of providing a purified selected carbon material having a structure
and composition that does not vary significant from its structure
and composition in the starting material subject to processing or
has a structure that is enhanced after processing via the present
methods, such as a structure exhibiting an enhanced extent of
crystallinity. For example, methods of the present invention are
capable of purifying a SWNT-containing sample such that the
purified SWNTs are not present in derivatized (i.e. oxidized) or
functionalized forms.
[0048] In the following description, numerous specific details of
the devices, device components and methods of the present invention
are set forth in order to provide a thorough explanation of the
precise nature of the invention. It will be apparent, however, to
those of skill in the art that the invention can be practiced
without these specific details.
[0049] This invention provides methods of purifying samples
containing a carbon material having a selected structure by
removing carbonaceous, noncarbonaceous and multiphase impurities
from the sample. Particularly, the present invention provides
purification methods for samples containing selected carbon
materials, such as carbon nanotubes (SWNTs and MWNTs), wherein
carbonaceous impurities comprising carbon layers having a different
structure than the selected carbon material are exfoliated, thereby
facilitating subsequent removal of impurities via chemical
transformation, for example by selective oxidation and/or
dissolution processes.
[0050] FIG. 1A provides a schematic diagram showing processing
steps in a purification method of the present invention useful for
removing carbonaceous impurities and metal catalyst particles from
a sample containing one or more desired carbon materials having
selected structures. As shown in process step 1 of FIG. 1A, a
sample is provided for processing that comprises a mixture of one
or more desired carbon materials having selected structures and
impurities. Impurities in the sample may have a range of different
chemical compositions and physical states, including carbonaceous
impurities comprising carbon materials having structures different
from that of the desired carbon material, such as impurities
comprising single or multilayer forms of graphene, graphite,
amorphous carbon, carbon fibers, carbon films, MWNTs and selected
combinations of these materials, noncarbonaceous materials such as
metal catalysts, metal derivatives including metal oxides, catalyst
support materials and aggregates and particles thereof, and
multiphase impurities, such as metal catalyst particles partially
or completely enclosed in multilayer carbonaceous impurities.
[0051] Referring again to FIG. 1A, an intercalant is added to the
sample in process step 2, thereby generating intercalated carbon
layers in the sample. In one embodiment, enough intercalant is
added to allow the formation of intercalated carbon layers having
the highest possible stoichiometry, and in some embodiments the
sample containing intercalant is allowed to react to substantial
completion. Intercalants provided to the sample may insert into
interstitial sites between adjacent layers in multilayer
carbonaceous impurities and/or insert into interstitial sites
located between a desired carbon material and a carbonaceous
impurity coating its outer surface. In addition, intercalants added
to the sample may insert into endohedral sites of carbonaceous
impurities. Intercalants may be added to the sample in liquid form,
gaseous form or in a carrier solution. In methods of the present
invention using highly reactive intercalants, such as alkali
metals, the sample is optionally evacuated and heated to remove
moisture and oxygen prior to the addition of the intercalants, and
the sample containing intercalant is optionally kept in an inert
atmosphere for the entire duration of the intercalation reaction.
Optionally, the sample containing intercalants is electrochemically
intercalated by application of a constant or variable electric
potential to the sample during intercalation, as also indicated in
process step 2.
[0052] As shown in process steps 3, 4 and 5 of FIG. 1A, the sample
having intercalated carbon layers is selective exfoliated, thereby
generating exfoliate carbonaceous material. In the context of this
description selective exfoliation refers to a process whereby
carbon materials comprising impurities undergo at least partial
exfoliation, while carbon materials undergoing purification (having
a selected structure) do not undergo substantial exfoliation. FIG.
1A shows two pathways (process steps 4 & 5) for achieving
selective exfoliation. In the embodiment shown in process step 4,
an exfoliation initiator is added to the sample that undergoes a
chemical reaction with intercalants in the intercalated carbon
layers. Preferably for some applications, the reaction between the
exfoliation initiator and intercalants in the intercalated carbon
layers is very a fast exothermic reaction and, optionally, a
reaction that results in an expansion of material between
intercalated layers, for example by forming a product in the gas
phase. The reaction between the exfoliation initiator and
intercalants in the intercalated carbon layers is selective such
that desired carbon materials having a selected structure are not
lost or damaged during the exfoliation process. An alternative
route to selective exfoliation is shown in process step 5. In this
embodiment, exfoliation of intercalated carbon layers is initiated
by raising the temperature of the sample, preferably for some
embodiments raising the temperature very rapidly.
[0053] Referring to process step 6 of FIG. 1A, exfoliated
carbonaceous material is selectively removed from the sample.
Removal of exfoliated carbonaceous material may be achieved by any
method that does not significantly destroy or damage the selected
carbon material undergoing purification. In some embodiments,
chemical transformation of exfoliated carbonaceous materials is
used to direct remove impurities, such as selective oxidation in
wet air at successively higher temperatures. In addition,
exfoliated carbonaceous material may be removed using separation
techniques, such as microfiltration and chromatography. As also
shown in process step 6 in FIG. 1A, in one embodiment wherein the
sample contains multiphase impurities comprising metal, metal
derivative or ceramic particles partially or completely enclosed in
a plurality of carbon impurity layers, exfoliation and subsequent
removal of exfoliated carbonaceous material exposes the outer
surface of the metal or metal derivative residual catalyst
particles. Metal particles having an exposed outer surface are
subsequently removed via dissolution, for example dissolution in
concentrated inorganic or organic acid added to the sample. As
shown in FIG. 1A, selective exfoliation steps (process steps 3, 4
and 5) and impurity removal steps (process steps 6 and 7) may be
repeated until a selected purity of the desired carbon material
having a selected structure is achieved.
[0054] FIG. 1B provides a schematic flow diagram showing processing
steps in a purification method using potassium intercalation for
purifying SWNTs generated by catalytic synthesis techniques, such
as arc-discharge and laser ablation synthesis methods. As shown in
process step 1 of FIG. 1B, a sample is provided for processing that
comprises a mixture of (i) SWNTs, (ii) carbonaceous impurities
having structures different than that of the SWNTs and (iii)
residual metal catalyst particles enclosed in multilayer
carbonaceous impurities carbonaceous impurities having structures
different than that of the SWNTs. The sample undergoing processing
is evacuated and heated, for example to a temperature of 300
degrees Celsius, to remove any trace of water and oxygen, as
illustrated in processing step 2 of FIG. 1B. Potassium intercalant
is added to the sample and allowed to react to completion in an
inert atmosphere (e.g. argon), thereby generating carbon layers
intercalated with potassium (see process step 3). In one embodiment
where carbonaceous impurities in the sample include graphite and/or
graphene layers, the amount of potassium added to the sample is
sufficient to form the most potassium rich intercalation material,
such as an intercalation material having the stoichiometry of one
potassium atom to eight carbon atoms (i.e. KC.sub.8).
[0055] Referring to process step 4 in FIG. 1B, the intercalated
sample is reacted with an exfoliation initiator comprising ethanol,
which initiates an exothermic reaction that selectively exfoliates
carbon layers intercalated with potassium. In one embodiment,
ethanol reacts with potassium present in intercalated carbon layers
to generate potassium alkoxide and hydrogen gas:
2C.sub.2H.sub.5OH+2K.fwdarw.2C.sub.2H.sub.5O.sup.-K.sup.++H.sub.2
[0056] In one embodiment, the sample is reacted with ethanol until
a neutral pH is observed, indicating that substantially all the
intercalated potassium has undergoing reaction. Exfoliation
generates exfoliated carbonaceous material and also ruptures the
multilayer structure of carbonaceous impurities enclosing the
residual metal catalyst particles. Exfoliation provided by this
processing step can disrupt and/or fracture the shell-like
multilayer structure of carbonaceous impurities enclosing the
residual metal catalyst particles resulting in at least partial
exposure of their outer surfaces.
[0057] As shown in process steps 5 and 6 in FIG. 1B, the sample is
refluxed in concentrated hydrochloric acid to dissolve metal
particles and subjected to low temperature oxidation under wet air
to selectively remove exfoliated carbonaceous materials. Removal of
the exfoliated carbonaceous materials further exposes the outer
surface of the metal particles, thereby enhancing their removal via
subsequent additional processing steps refluxing the sample in
concentrated hydrochloric acid. Oxidation in wet air may also at
least partially convert the metal particles to metal derivative
particles such as metal oxide or metal hydroxide particles. This
oxidation process results in an expansion of the volume of the
particle due to the lower density of the oxide and/or hydroxide
product that further disrupts the shell-like multilayer structure
of carbonaceous impurities enclosing the residual metal catalyst
particles and also increases the efficiency of subsequent removal
of metal particles via dissolution. Process steps 5 and 6 are
optionally repeated until a desired purity of the SWNTs is
achieved. In one embodiment, the sample is subjected to wet air
oxidation at successively higher temperatures each time that
process steps 5 and 6 are repeated. As indicated in process step 7
of FIG. 1B, the purified sample is optionally annealed in nitrogen
to repair any damage to the SWNTs which occurs during
processing.
[0058] The methods of the present invention may be realized using a
wide variety of instrumentation and reagents well known in the art.
Use of a two bulb glass reactor and a two zone furnace is
beneficial for adding gas phase potassium to the sample in a manner
which minimizes the likelihood of condensing potassium on the
sample undergoing processing. In this embodiment, the sample
undergoing processing is held in a first bulb maintained at a
slightly larger temperature and in fluid communication with a
second bulb containing the potassium intercalant. Manipulation of
samples undergoing purification in a glove box filled with any
inert gas, such as argon, is helpful for avoiding inadvertent
premature oxidation of intercalants comprising highly reactive
alkali metals (e.g. K and Na) during intercalation of the sample. A
variety of methods and instrumentation well known in the art may be
used for electrochemically intercalating samples including
voltammetric intercalation and electrolysis. Any intercalants
and/or exfoliation initiators can be used in the present methods
including electron donor intercalants, electron acceptor
intercalants and combinations of both electron donor intercalants
and electron acceptor intercalants. Exemplary intercalants useful
for preparing intercalated carbon layers include, but are not
limited to, Li, Na, K, Rb, Cs, F.sub.2, Cl.sub.2, Br.sub.2,
I.sub.2, chromium oxides such as CrO.sub.3, metal chlorides such as
CdCl.sub.2, YCl.sub.3, AlCl.sub.3, MoCl.sub.5, ZnCl.sub.2,
FeCl.sub.3, sulfuric acid, nitric acid, perchloric acid, formic
acid.
[0059] The terms and expressions which have been employed herein
are used as terms of description and not of limitation, and there
is no intention in the use of such terms and expressions of
excluding any equivalents of the features shown and described or
portions thereof, but it is recognized that various modifications
are possible within the scope of the invention claimed. Thus, it
should be understood that although the present invention has been
specifically disclosed by preferred embodiments and optional
features, modification and variation of the concepts herein
disclosed may be resorted to by those skilled in the art, and that
such modifications and variations are considered to be within the
scope of this invention as defined by the appended claims. The
specific embodiments provided herein are examples of useful
embodiments of the present invention and it will be apparent to one
skilled in the art that the present invention may be carried out
using a large number of variations and additional processing steps
including, but not limited to, sample transfer steps, temperature
variation steps and cycles, product purity analysis, additional
separations including phase separations, chromatographic
separation, separation by filtration techniques such as
microfiltration, removal of excess reagents such as process steps
removing intercalants and/or exfoliation initiators added to the
sample undergoing processing, and solution phase oxidation of
carbonaceous impurities.
[0060] The following references relate generally to methods of
exfoliating carbon materials: (1) "Exfoliation of carbon fibers",
M. Toyoda and M. Inagaki, Journal of Physics and Chemistry of
solids, 65 (2004) 109-117; (2) Review of the doping of carbon
nanotubes (multiwalled and single-walled)", L. Duclaux, Carbon, 40
(2002), 1751-1764; (3) "Exfoliation process of graphite via
intercalation compounds with sulfuric acid", M. Inagaki, R.
Tashiro, Y. Reactino and M. Toyoda, Journal of Physics and
Chemistry of solids, 65 (2004) 133-137; (4) "Graphite exfoliated at
room temperature and its structural annealing" M. Inagaki and M.
Nakashima, Carbon, Vol. 32, No. 7 (1994), 1253-1257; (5)
"Exfoliation process and elaboration of new carbonaceous
materials", Fuel, Vol 77, No. 6, (1998) 479-485; and (6)
"Electrochemical intercalation of ZnCl.sub.2--CrO.sub.3-GIC
(graphite intercalation compound) with sulphuric acid", J. M.
Skowronski and J. Urbaniak, Polish J. Chem. 78 (2004)
1339-1344.
[0061] All references cited in this application and all references
cited in these references are hereby incorporated in their
entireties by reference herein to the extent that they are not
inconsistent with the disclosure in this application. It will be
apparent to one of ordinary skill in the art that methods, devices,
device elements, materials, procedures and techniques other than
those specifically described herein can be applied to the practice
of the invention as broadly disclosed herein without resort to
undue experimentation. All art-known functional equivalents of
methods, devices, device elements, materials, procedures and
techniques specifically described herein are intended to be
encompassed by this invention.
Example 1
Purification of Carbon Single Walled Nanotubes Produced by
Arc-Discharge Methods
[0062] The ability of methods of the present invention using
potassium intercalation to purify SWNTs was experimentally verified
by purifying SWNT containing samples generated using arc discharge
synthesis methods. Thermogravimetric analysis (TGA), Raman
spectrometry, X-ray diffraction (XRD) and transmission electron
microscopy (TEM) were used to characterize the results of each
processing step, and to evaluate the effectiveness of the
purification treatment. The purified SWNT products generated by the
present methods were compared to SWNT samples purified via
conventional acid reflux purification methods. This potassium
intercalation step was found to be particularly useful for
exfoliating the graphitic shell structure that typically surround
residual metal catalyst particles present in samples prepared by
many catalytic synthesis methods. By exfoliating the shell
structure, subsequent treatments are more efficacious for removing
the metal catalyst particles.
1.a Introduction
[0063] There have been many investigations of single-wall carbon
nanotubes (SWNTs) since these materials were first reported in the
early Nineties. The unique physical, chemical and mechanical
properties of SWNTs make them candidate materials for service in
electronic devices, energy storage systems and structural
composites, for example. To obtain behavior in these applications
that can be unambiguously and solely attributed to nanotubes,
purification of the SWNTs is an essential step, since raw materials
are typically composed of less than half SWNTs. The efficiency of
any purification method is strongly dependent on the raw materials,
which differ for the three methods typically used for the
production of SWNTs: chemical vapor deposition (CVD), electric
arc-discharge and laser ablation.
[0064] It has proved a serious challenge to remove multiwalled
nanotubes and shells, incomplete fullerenes, graphites, and
disordered carbon impurities in the raw materials without also
destroying the SWNTs, since all these components are similar
chemically. In this example, we concentrate on the purification of
SWNTs prepared by arc-discharge and laser oven/ablation because
these methods produce SWNTs with a narrow distribution of tube
diameters. Important impurities in materials prepared by
arc-discharge and laser oven/ablation methods also include metal
catalyst particles, which are typically surrounded by shells of
polyaromatic carbons. The shell structure protects the metal
particles from dissolution in inorganic acids.
[0065] Many procedures for purifying SWNTs are based on
chromatography and filtration methods, or on selective oxidation
processes (i.e. acid oxidation and/or gas oxidation), or a
combination of these methods. For any method of purification, a
compromise is typically made between impurity removal and the final
yield of SWNTs. Another concern is that some purification processes
can damage the nanotubes. Refluxing in nitric acid, for example, is
effective for removing metallic impurities, but also produces
defects in the tubes.
[0066] Method of the present invention using a potassium
intercalation processing step was used to purify arc-derived carbon
SWNTs, which are generally recognized as the most challenging to
purify. The present purification methods procedure comprise the
steps of: (a) intercalating the SWNT containing sample with
potassium (b) reacting/exfoliating the resulting powder with
ethanol and (c) wet air oxidation at successively higher
temperatures, with each step followed by a reflux in concentrated
hydrochloric acid. Low temperature wet air oxidation has generally
been thought to remove the more disordered carbon layer coatings on
metal catalyst particles, allowing the dissolution of the metal in
an acid. Oxidation treatments at higher temperatures remove the
more stable carbon layer graphitic shells surrounding the catalyst
metal particles. During these wet air oxidation treatments, the
metal is converted to an oxide and/or hydroxide. The expansion of
the metal due to the lower density of the oxide (densities for Ni
and NiO are respectively 8.90 and 6.67) cracks the carbon shells
and the metal oxide, thus is exposed and is subsequently dissolved
by the proper acid.
[0067] The step of potassium intercalation followed by reacting
with ethanol causes more complete exfoliation of the intercalated
layers, weakening the carbon shell structure covering the metallic
particles. These structures are therefore more easily cracked by
wet air oxidation, more completely exposing the metal catalyst.
Each of these steps contributes to the purification, and the
combination of potassium intercalation, wet air oxidation and
hydrochloric acid treatments produces a high-purity SWNT containing
purified sample. By using transmission electron microscopy, Raman
spectrometry, x-ray diffractometry, and thermogravimetric analysis,
we show that the methods of the present invention provide better
yields and purities than another process involving a more
established purification process involving a combination of
refluxing in HNO.sub.3, centrifugations, refluxing in
H.sub.2O.sub.2 and wet oxidations in air at different temperatures
(350 to 520.degree. C.).
1.b Experimental
[0068] Single-wall carbon nanotube materials produced by the
arc-discharge method with a Ni--Y catalyst (5 at. %) were obtained
from Carbolex Inc. According to the manufacturer the as-received
sample purity may vary from 70 to 90 vol. %, and it contains
residual catalyst impurities. FIG. 2 shows a transmission electron
microscope (TEM) image of the as-received sample containing SWNT
that was purified using the present methods. The as-received
Carbolex soot has a large volume of metal catalyst of various
sizes, which appear as dark spheroids. X-ray microanalysis showed
these particles to be composed of Ni and Y. Also seen are different
types of carbon including: carbon shells, amorphous carbon, and
free single-wall carbon nanotubes. The inset in FIG. 2 shows a
region of the as-received material that comprises of the nanotube
rope structure. Note the hexagonal packing of the tubes seen end-on
at the top of the inset.
[0069] The as-received SWNT-containing sample was first evacuated
and heated to 300.degree. C. to remove any trace of moisture and
O.sub.2. Using an argon-filled glove box, samples were then loaded
into a two-bulb reactor of the type used to synthesize graphitic
potassium intercalation compounds. The carbon material and the
potassium metal intercalant are put into each bulb of a glass
reactor, evacuated, and then placed into a two-zone furnace. The
potassium zone temperature was maintained at 250.degree. C.,
whereas the nanotube zone temperature was adjusted to higher
temperature to minimize the possibility that potassium metal might
condense on the sample. The masses of potassium and carbon material
were chosen to allow the formation of the most potassium-rich
intercalation compound, KC.sub.8. After reaction for two days, the
potassium-doped nanotubes were reacted with ethanol until a neutral
pH was obtained. The sample was then refluxed in concentrated
hydrochloric acid (37%) for 24 hours: a typically green acid color
was observed due to the dissolved Ni.sup.2+. The materials were
then subjected to a low temperature oxidation (350.degree. C.
during 24 hours) under flow of wet air, and again refluxed with HCl
to further remove exposed catalyst particles. A final wet air
oxidation at 450.degree. C. with a shortened oxidation time of 2.5
hours was subsequently performed, followed by an HCl treatment. As
a final step, to repair possible damage to the tube walls, the
sample was annealed in nitrogen forming gas at 750.degree. C. for 2
hours.
[0070] Thermogravimetric analysis (TGA), Raman spectroscopy, X-ray
diffraction (XRD) and transmission electron microscopy (TEM) were
used to evaluate the effectiveness of the purification treatments
and to determine the relative purity of the SWNTs.
[0071] For TGA analysis, the nanotube samples were loaded into an
alumina crucible, and a flow of argon was maintained at 40 mL/min
while heating at 10.degree. C./min from room temperature to
200.degree. C. This temperature was held for 20 minutes under the
argon flow and then for another 20 minutes under a 50:50 Ar/O.sub.2
mixture. The chamber was then maintained under a continuous
Ar/O.sub.2 flow while the temperature was incrementally raised at
5.degree. C./min to 700.degree. C. or 1000.degree. C. The time (t)
dependence of the sample mass (m) was recorded.
[0072] To investigate possible SWNT damage from the steps of the
purification process, Raman spectra were acquired at room
temperature with a Renishaw Micro Raman spectrometer using a 514.5
nm argon ion laser with spectral resolution of 1 cm.sup.-1. The
samples were encapsulated in argon-filled glass ampoules, and to
prevent the loss of potassium from the samples or other
instabilities, Raman measurements were made with low laser
power.
[0073] The crystallinity of the nanotube rope structure and the
evolution of the impurity phases during purification were studied
by x-ray diffractometry (XRD) and transmission electron microscopy
(TEM). X-ray powder diffraction was performed with a Cu K.alpha.
X-ray source (Philips PW3040, .lamda. Cu=1.54056 .ANG., 45 kV, 40
mA). Morphological observations of the as-received SWNTs were
obtained by TEM measurements using a Philips EM420 operated at 100
kV.
1.c Results and Discussion
1.c.(i) Thermogravimetric Analysis
[0074] When heated in the presence of oxygen gas, carbon materials
undergo oxidation, forming the gases CO or CO.sub.2. Any material
remaining after all of the carbon has been consumed is the initial
metal catalyst in oxide form. FIGS. 3A and 3B show TGA data
recorded in an Ar/O.sub.2 flow for the two studies of metal
catalyst removal, where FIG. 3A shows data for the conventional
reflux purification methods [See, e.g. A. G. Rinzler, J. Liu, H.
Dai, P. Nikolaev, C. B. Huffman, F. J. Rodriguez-Macias, P. J.
Boul, A. H. Lu, D. Heymann, D. T. Colbert, R. S. Lee, J. E.
Fischer, A. M. Rao, P. C. Eklund, R. E. Smalley, Appl. Phys. A 67,
(1998) 29-37], and FIG. 3B shows data for the present methods using
an initial potassium intercalation/exfoliation step. Each set of
runs was performed with successive wet air oxidations at increasing
temperatures, followed by an HCl reflux step after each
oxidation.
[0075] For the data in FIG. 3A, a nitric acid reflux was performed
on the as-received raw soot material, followed by an H.sub.2O.sub.2
reflux. The sample was subsequently oxidized at temperatures of
350, 425, 480, 510 and 520.degree. C. Using a series of
temperatures in this way is necessary because amorphous carbon and
multi-shell carbon phases in the as-received soot cannot be removed
selectively with a single oxidation step. The oxidation temperature
range of amorphous carbon and multi-shell carbons (350.degree. C.
and 420.degree. C. respectively) overlap with the oxidation range
for SWNTs (400.degree. C.). For the data in FIG. 3B, the SWNT
material was reacted with potassium using a graphite intercalation
procedure, and then reacted with ethanol (EtOH). The number of
oxidation steps was reduced to two, performed at temperatures of
350.degree. C. and 425.degree. C.
[0076] For both sets of procedures, the samples underwent a final
anneal at 750.degree. C. under flowing forming gas consisting of
nitrogen plus hydrogen. In comparing the TGA curves after the first
step of purification for each method, FIGS. 3A and 3B shows that
the potassium reaction followed by EtOH reacting is more effective
as a first step in the purification process. The decomposition
temperature shifts to a lower temperature, consistent with the
formation of amorphous or turbostratic polyaromatic carbon phases
by graphite exfoliation. More significantly, a subsequent mild
treatment with HCl causes a larger mass loss after this
intercalation step, because the carbon shells around the metal
particles were more pervious to acid. In the second and third steps
the oxidation processes were carried out at 350.degree. C. and
425.degree. C. respectively, and the samples were reacted again in
HCl. With each reacting step, more metal is removed and the onset
oxidation temperature is seen to increase. Such oxidation processes
burn the carbon shell around the metal catalyst particles, exposing
the metal particles to the acid. Because the nanotubes are more
stable against oxidation than amorphous carbon, the nanotubes are
not affected by oxidation at these temperatures. Starting from the
second stage of purification, the TGA curves for both purification
processes show the same behavior, but the potassium intercalation
allows a smaller number of oxidation steps for the same amount of
catalyst removal.
[0077] Yields were measured corresponding to the present
purification process and the conventional process examined. The
SWNT content remaining in the sample at the end of the present
purification process involving potassium intercalation/exfoliation
is about 18 wt. % whereas the total yield of carbon nanotubes for
the more established purification process combining refluxing in
nitric acid, refluxing in H.sub.2O.sub.2 and wet oxidations in air
is less than a few weight percent.
1.c (ii) Micro Raman Spectroscopy
[0078] FIG. 4 compares the Raman spectra of the sample after the
different stages of purification using the present methods. For
comparison, typical Raman spectra of pure graphite are shown
before, plot (g) in FIG. 4, and after potassium intercalation, plot
(h) in FIG. 4. Graphite reacts with potassium to form graphite
intercalation compounds KC.sub.n. Intercalation compounds are
distinguished by their staging, which is defined as the number of
graphene layers separating the intercalated potassium layer. For
example, the compound KC.sub.8 is a stage 1 binary compound in
which all van der Waals gaps between the graphene layers are
occupied by potassium. In the spectrum of graphite, the sharp
feature at 1584 cm.sup.-1 is attributed to the E.sub.2g first order
mode. The symmetry of the E.sub.2g mode restricts the motion of the
atoms within the plane of the carbon atoms. An additional band at
1355 cm.sup.-1 is attributed to finite size effects, resulting from
smaller domains that are present among larger ones. This
corresponds to a `breathing mode` with A.sub.1g symmetry. A
different Raman profile is observed for the potassium intercalated
graphite compound. A broad asymmetric feature around 1500 cm.sup.-1
is observed, characteristic of stage 1 KC.sub.8.
[0079] The spectrum from as-received AP Carbolex material (plot (a)
in FIG. 4) has two main bands at low (100-200 cm.sup.-1) and high
(1500-1600 cm.sup.-1) frequencies, characteristic of SWNTs. The
high-frequency bands can be decomposed into two main peaks at 1590
and 1570 cm.sup.-1, which are associated with tangential modes
related to C--C bond stretching motions. At low wave number,
another characteristic tube mode is observed, where the carbon
atoms are displaced radially outward in an in-phase, breathing
mode. Superimposed on this spectrum is the contribution from
amorphous sp.sup.2 carbon, or possibly a thin coating on the
nanotubes with a peak around 1350 cm.sup.-1. Plot (b) in FIG. 4
shows the changes in the Raman spectrum after potassium doping, and
has features similar to the spectrum from KC.sub.8, plot (h) in
FIG. 4. There are significant shifts in the frequencies of both
radial and tangential modes, are evidence of the complete reaction
of the nanotubes with potassium. After reacting in EtOH, clean
SWNTs are recovered as shown by the spectrum of plot (c) in FIG. 4.
Potassium intercalation followed by EtOH reacting leaves the
nanotubes intact, while exfoliating the graphitic carbon coating
that surrounds the metal catalyst particles. This is consistent
with the increased intensity of Raman band from the turbostratic
polyaromatic carbon at 1350 cm.sup.-1. Further steps such as HCl
reflux and wet air oxidation are then more efficacious in removing
the catalyst than in removing disordered and graphitic sp.sup.2
carbons. Evidence for this is noted by the presence of the residual
line at 1350 cm.sup.-1. However, the intensity of this peak does
progressively diminish with processing. Raman spectra from the
samples after wet air oxidation and subsequent HCl reflux and the
last forming gas treatment (plots (d), (e) and (f) in FIG. 4,
respectively) show the peaks characteristic of SWNTs. They are also
consistent with at most minor nanotube damage during the
purification.
1.c (iii) X-Ray Diffraction
[0080] FIG. 5 presents X-ray diffraction (XRD) patterns from
samples after different steps of purification using the present
methods. The XRD pattern of the as-received material shown in plot
(b) of FIG. 5 is dominated by the 111 and 200 diffraction peaks
from fcc nickel at 44.degree. and 52.degree., and includes a
diffuse background attributed to synthesis byproducts such as
amorphous carbon, which coexist with the SWNTs. After reaction with
potassium, some metal particles remain and an increase in the
intensity of the background is observed in plot (c) in FIG. 5.
Precise quantitative details that delineate the effect of the
alkali metal reaction are difficult to discern from the XRD data
because of the inhomogeneity of this material and because of the
nano-scale dimensions of the particles constituting the soot. Plot
(d) in FIG. 5 shows that subsequent EtOH reacting, wet air
oxidation at 350.degree. C. and HCl reflux remove large amounts of
catalyst, as evidenced by the change in relative peak height of the
Ni diffractions with respect to remnant graphitic 002 diffraction
peak. As a result of the exfoliation of the graphitic shell around
the metal particles, the particles are more easily dissolved in
acid. The first wet air oxidation at low temperature removes a
large amount of amorphous carbon. This causes an increase in the
graphitic particle diffraction peak-to-background, seen in the XRD
profile as a doublet peak from turbostratic polyaromatic carbon
(d.sub.002*=340 nm) and graphite (d.sub.002=335 nm). Plot (e) in
FIG. 5, obtained after the second wet air oxidation at 425.degree.
C. followed by HCl reflux, shows well-defined diffractions from a
hexagonal molecular crystal with a large unit cell, consistent with
a close-packed rope structure of SWNTs. The forming gas treatment
greatly improves the crystallinity of the rope packing as evidenced
by the low-angle diffraction peaks in plot (f) of FIG. 5. The
2-dimensional nanotube-to-nanotube spacing is measured to be 1.712
nm. The X-ray diffraction pattern obtained after the first
purification and followed by wet air oxidation at 510.degree. C.
shows diffraction peaks from the SWNT rope structure and a small
111 diffraction peak from the remaining catalyst. This observation
again confirms the effectiveness of the purification process using
potassium.
1.c (iv) TEM Images of Sample after Purification
[0081] FIG. 6 provides transmission electron microscope (TEM)
images of the SWNT-containing sample after purification by the
present methods. These figures show that the present purification
methods do not substantially degrade or damage the SWNTs in the
sample undergoing processing, and indicate that the structure and
composition of the purified SWNTs are similar to their structure
and composition in the as-received sample. In addition, comparison
of FIG. 6 and FIG. 2 indicates that the abundance of metal
particles in the sample is significantly reduced after
purification.
1.d Conclusion
[0082] The present methods are effective for purifying carbon
single-wall nanotubes synthesized by the arc-discharge process. The
processing step of potassium intercalation and subsequent reacting
with EtOH exfoliates the graphite impurities, and exfoliates the
graphitic shells around the metal catalyst particles. After
intercalation, the metal catalyst particles are more susceptible to
removal by subsequent wet oxidation treatments coupled with HCl
reflux steps. It is demonstrated that purification methods using
this intercalation step decreases the catalyst content to levels
that are obtained by a conventional purification process, however,
using fewer purification steps. Evidence for the efficacy of this
potassium intercalation step was provided by TGA measurements and
TEM observations. Raman spectroscopy indicated that the structural
integrity of the nanotubes was not compromised by the various
stages of purification. XRD showed good crystallinity of the
material after purification.
Example 2
Purification of Carbon Nanofibers Synthesized by Catalytic
Methods
[0083] The present invention provides versatile methods for
purifying a range of carbon materials in samples generated by
catalytic synthesis techniques. The ability of methods of the
present invention using potassium intercalation to purify carbon
materials other than carbon nanotubes was experimentally verified
by purifying samples containing carbon nanofibers generated by
catalytic synthesis. The synthesis and characteristics of theses
fibers has been reviewed by K. P. De Jong and J. W. Geus in
Catalysis reviews-Science and Engineering, 42 (2000) pp 481-510.
X-ray diffraction (XRD) is used to characterize the composition of
the starting carbon fiber-containing sample and the composition of
the sample after important processing steps of the present methods.
The experimental results provided in this example illustrate that
the present methods provide an effective, nondestructive pathway
for purifying samples containing carbon nanofibers. Similar to the
experimental results obtained for SWNTs (See e.g., Example 1),
potassium intercalation followed by reacting in ethanol was
observed to be particularly useful for enhancing the removal of
residual metal catalyst particles by dissolution in an acid
solution.
[0084] Carbon fiber materials subject to purification were vapor
grown via catalytic growth techniques and, therefore, had a
significant impurity comprising residual cobalt-iron catalyst
particles having a composition of about 50 percent iron by mass and
about 50 percent cobalt by mass. (see for example: M. Audier et al.
in J. Crystal Growth 55 (1981) 549-556) The starting carbon
fiber-containing sample was first evacuated and heated to
300.degree. C. to remove any trace of moisture and O.sub.2. Using
an argon-filled glove box, samples were then loaded into a two-bulb
reactor of the type used to synthesize graphitic potassium
intercalation compounds. The carbon material and the potassium
metal intercalant are put into each bulb of a glass reactor,
evacuated, and then placed into a two-zone furnace. The potassium
zone temperature was maintained at 250.degree. C., whereas the
nanotube zone temperature was adjusted to higher temperature to
minimize the possibility that potassium metal might condense on the
sample. Intercalation of the carbon fiber-containing sample
proceeded under these conditions for two days. After intercalation,
the potassium-doped nanotubes were reacted with ethanol to initiate
exfoliation. The sample was then refluxed in concentrated
hydrochloric acid for 24 hours. The materials were then subjected
to a low temperature oxidation (350.degree. C. during 24 hours)
under a flow of wet air, and again refluxed with HCl for 24 hours
to further remove exposed catalyst particles. A final wet air
oxidation at 450.degree. C. with a shortened oxidation time of 2.5
hours was subsequently performed, followed by another treatment
with concentrated HCl for 24 hours. As a final step, the sample was
annealed in nitrogen-hydrogen forming gas at 750.degree. C. for 2
hours to repair any damage to the carbon fibers and improve
crystallinity.
2.a X-Ray Diffraction Results
[0085] FIG. 7 provides X-ray diffraction (XRD) patterns of the
starting carbon fiber-containing sample and the sample after
different purification processing steps of the present invention.
The XRD pattern of the starting material provided in plot (1) of
FIG. 7 indicates a peak at about 45 degrees corresponding to the
cobalt-iron particles. Also shown in plot (1) of FIG. 7 are the 002
and 101 graphite peaks at about 26 degrees and 44 degrees,
respectively, which correspond to graphite layers comprising the
carbon fibers. Plot (2) of FIG. 7 shows the diffraction pattern
observed after intercalation with potassium. The cobalt-iron peak
at about 45 degrees is clearly observable in plot (2). The 002 and
101 graphite peaks at 26 degrees and 44 degrees, however, are
entirely missing and replaced with a series of peaks at about 16
degrees, about 23 degrees and about 34 degrees. These peaks
corresponding to the 004 (at .apprxeq.16 degrees), hkl (at
.apprxeq.23 degrees) and 008 (at .apprxeq.34 degrees) peaks of
intercalated graphite having a stoichiometry of one potassium to
eight carbons (i.e. KC.sub.8). Plot (3) provides the XRD pattern
observed after reacting the intercalated sample with ethanol and
refluxing with concentrated HCl for 24 hours. The cobalt-iron peak
at about 45 degrees is not present in the diffraction pattern
provided in plot (3), which indicates this series of process steps
effectively removes most of the metal catalyst particles. The
reemergence of 002 and 101 graphite peaks at 26 degrees and 44
degrees may be attributed to the presence of unintercalated carbon
fibers and indicates that the carbon fibers in the sample are not
lost or significantly damaged during this processing step. The
similarity of the shape and position of 002 and 101 graphite peaks
in plots (1) and (3) of FIG. 7 indicates that the composition and
structure of the fibers after the reacting with ethanol and HCl
reflux processing step is similar to the composition of the carbon
fibers in the as-received sample. Plot (4) of FIG. 7 provides the
diffraction pattern obtained after wet oxidation for 24 hours at
350 degrees Celsius and refluxing with concentrated HCl for 24
hours. Similar to plot (3), plot (4) of FIG. 7 shows that the
carbon fibers are maintained in the sample during this processing
step. Plot (5) of FIG. 7, provides the diffraction pattern obtained
after wet oxidation for 24 hours at 450 degrees Celsius, refluxing
with concentrated HCl for 24 hours and annealing at 750 degrees in
a nitrogen-hydrogen forming gas for two hours. Comparison of plots
(3), (4) and (5) of FIG. 7 indicate that further sample processing
via wet oxidation at successively higher temperatures and annealing
improves the crystallinity of the purified carbon fibers.
[0086] The experimental results provided in this example
demonstrate the applicability of the present methods for purifying
carbon fibers materials generated by catalytic synthesis. The
processing step of potassium intercalation and subsequent reacting
with ethanol exfoliates carbonaceous impurities that form shells
around residual metal catalyst particles. After intercalation, the
residual metal catalyst particles are more susceptible to removal
by subsequent wet oxidation treatments coupled with HCl reflux
steps. XRD patterns obtained before, during and after sample
processing, indicate the present methods provide an effective
pathway for removing metal catalyst impurities and that the present
methods provide a source of purified carbon fibers exhibiting good
crystallinity.
2.b TEM Images of Sample Before and after Purification
[0087] FIG. 8 shows a transmission electron microscope (TEM) image
of the as-received sample containing carbon fibers that was
purified using the present methods. As shown in FIG. 8, the
as-received sample has a significant component comprising
cobalt-iron catalyst particles, which appear as dark spheroids.
Also seen are different types of carbon including: carbon shells,
amorphous carbon, and carbon fibers.
[0088] FIGS. 9A, 9B and 9C provide TEM images of the carbon
fiber-containing sample after purification by the present methods.
FIG. 9A is a lower resolution image than the images shown in FIGS.
9B and 9C. These figures show that the present purification methods
do not substantially degrade or damage the carbon fibers in the
sample undergoing processing, and indicate that the structure and
composition of the purified carbon fibers are similar to their
structure and composition in the as-received sample. In addition,
comparison of FIGS. 9A, 9B and 9C and FIG. 8 indicates that the
abundance of metal particles in the sample is significantly reduced
after purification by the present methods.
* * * * *