U.S. patent application number 10/573902 was filed with the patent office on 2007-03-29 for thermal treatment of functionalized carbon nanotubes in solution to effect their functionalization.
This patent application is currently assigned to WILLIAM MARSH RICE UNIVERSITY. Invention is credited to Christopher A. Dyke, James M. Tour.
Application Number | 20070071667 10/573902 |
Document ID | / |
Family ID | 34619319 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070071667 |
Kind Code |
A1 |
Tour; James M. ; et
al. |
March 29, 2007 |
Thermal treatment of functionalized carbon nanotubes in solution to
effect their functionalization
Abstract
The present invention is directed towards methods of thermally
defunctionalizing functionalized (derivatized) carbon nanotubes
(CNTs) in solution or suspended in a liquid medium. Such
defunctionalization largely comprises the removal of sidewall
functionality from the CNTs, but can also serve to remove
functionality from the CNT ends. Such methods facilitate the
resuspension of such defunctionalized CNTs in various solvents and
permit the defunctionalization of functionalized CNTs that would
normally decompose (or partially decompose) upon thermal treatment.
Such methods of defunctionalization can typically lead to
defunctionalized CNTs that are essentially pristine (or nearly
pristine), and which, in contrast to prior art methods of thermal
defunctionalization, can be easily resuspended in a variety of
solvents.
Inventors: |
Tour; James M.; (Bellaire,
TX) ; Dyke; Christopher A.; (Humble, TX) |
Correspondence
Address: |
WINSTEAD SECHREST & MINICK P.C.
P.O. BOX 50784
DALLAS
TX
75201
US
|
Assignee: |
WILLIAM MARSH RICE
UNIVERSITY
6100 MAIN STREET
HOUSTON TEXAS
US
77005
|
Family ID: |
34619319 |
Appl. No.: |
10/573902 |
Filed: |
October 28, 2004 |
PCT Filed: |
October 28, 2004 |
PCT NO: |
PCT/US04/35894 |
371 Date: |
November 13, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60516392 |
Oct 31, 2003 |
|
|
|
Current U.S.
Class: |
423/447.1 ;
977/847 |
Current CPC
Class: |
C01B 2202/06 20130101;
C01B 32/168 20170801; C01B 2202/02 20130101; B82Y 30/00 20130101;
C01B 2202/04 20130101; C01B 2202/22 20130101; B82Y 40/00
20130101 |
Class at
Publication: |
423/447.1 ;
977/847 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Goverment Interests
[0002] The present invention was made with support from the
National Aeronautics and Space Administration, Grant Nos.
JSC-NCC-9-77 and URETI NCC-01-0203; the National Science
Foundation, Grant No. NSR-DMR-0073046; and the Air Force Office of
Scientific Research, Grant No. F49620-01-1-0364.
Claims
1. A method comprising the steps of: (a) suspending a quantity of
functionalized carbon nanotubes in a solvent to form a suspension
of functionalized carbon nanotubes; and (b) heating said suspension
to a temperature that will thermally defunctionalize the
functionalized carbon nanotubes yielding a defunctionalized
product.
2. The method of claim 1, wherein the carbon nanotubes are selected
from the group consisting of single-wall carbon nanotubes (SWNTs),
multi-wall carbon nanotubes (MWNTs), double-wall carbon nanotubes,
semiconducting carbon nanotubes, metallic carbon nanotubes,
semi-metallic carbon nanotubes, chiral carbon nanotubes,
buckytubes, carbon fibrils, and combinations thereof.
3. The method of claim 1, wherein the solvent is thermally stable
at the temperatures required for defunctionalization.
4. The method of claim 1, wherein the solvent is selected from the
group consisting of o-dichlorobenzene, benzene, toluene, water,
sulfuric acid, oleum, sulfuric acid with dissolved potassium
persulfate, liquid ammonia, liquid ammonia with dissolved alkali
metals, alkanes, parafins, thiophene, and combinations thereof.
5. The method of claim 1, wherein the suspension is completely
enclosed in a vessel.
6. The method of claim 1, wherein the suspension further comprises
a polymeric species.
7. The method of claim 1, wherein the suspension further comprises
a surfactant.
8. The method of claim 1, wherein the defunctionalized product is
selected from the group consisting of unfunctionalized carbon
nanotubes, partially functionalized carbon nanotubes, and
combinations thereof.
9. The method of claim 1, wherein the defunctionalized product is
functionally uniform.
10. The method of claim 1, wherein the defunctionalized product is
resuspendable in a solvent.
11. The method of claim 1, wherein the functionalized carbon
nanotubes are selectively defunctionalized according to different
(n,m) types, said types displaying differential propensity for
defunctionalization.
12. A method comprising the steps of: (a) dispersing a quantity of
functionalized carbon nanotubes in a polymer matrix to form a first
blended material comprising functionalized carbon nanotubes in a
polymer host; and (b) heating said first blended material to a
temperature that will thermally defunctionalize the functionalized
carbon nanotubes with the polymer host to yield a second blended
material comprising defunctionalized or partially defunctionalized
carbon nanotubes in a polymer host.
13. The method of claim 12, wherein the carbon nanotubes are
selected from the group consisting of single-wall carbon nanotubes
(SWNTs), multi-wall carbon nanotubes (MWNTs), double-wall carbon
nanotubes, semiconducting carbon nanotubes, metallic carbon
nanotubes, semi-metallic carbon nanotubes, chiral carbon nanotubes,
buckytubes, carbon fibrils, and combinations thereof
14. The method of claim 12, wherein the defunctionalized carbon
nanotubes are selected from the group consisting of
unfunctionalized carbon nanotubes, partially functionalized carbon
nanotubes, and combinations thereof.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This Application claims priority to U.S. Provisional Patent
Application Ser. No. 60/516,392, filed Oct. 31, 2003.
FIELD OF THE INVENTION
[0003] The present invention relates generally to carbon nanotube
materials. More specifically, the invention relates to methods of
defunctionalizing previously functionalized carbon nanotubes.
BACKGROUND OF THE INVENTION
[0004] Carbon nanotubes (CNTs), comprising multiple concentric
shells and termed multi-wall carbon nanotubes (MWNTs), were
discovered by Iijima in 1991 [Iijima, Nature 1991, 354, 56-58].
Subsequent to this discovery, single-wall carbon nanotubes (SWNTs),
comprising single graphene sheets rolled up on themselves to form
cylindrical tubes with nanoscale diameters, were synthesized in an
arc-discharge process using carbon electrodes doped with transition
metals [Iijima et al., Nature 1993, 363, 603-605; and Bethune et
al., Nature 1993, 363, 605-607]. These carbon nanotubes (especially
SWNTs) possess unique mechanical, electrical, thermal and optical
properties, and such properties make them attractive for a wide
variety of applications. See Baughman et al., Science, 2002, 297,
787-792.
[0005] Methods of making CNTs include the following techniques: arc
discharge [Ebbesen, Annu. Rev. Mater. Sci. 1994, 24, 235-264];
laser oven [Thess et al., Science 1996, 273, 483-487]; flame
synthesis [Vander Wal et al., Chem. Phys. Lett. 2001, 349,
178-184]; and chemical vapor deposition [U.S. Pat. No. 5,374,415],
wherein a supported [Hafner et al., Chem. Phys. Left. 1998, 296,
195-202] or an unsupported [Cheng et al., Chem. Phys. Lett. 1998,
289, 602-610; Nikolaev et al., Chem. Phys. Lett. 1999, 313, 91-97]
metal catalyst may also be used.
[0006] Techniques of chemically functionalizing CNTs have greatly
facilitated the ability to manipulate these materials, particularly
for SWNTs which tend to assemble into rope-like aggregates [Thess
et al., Science, 1996, 273, 483-487]. Such chemical
functionalization of CNTs is generally divided into two types: tube
end functionalization [see, e.g., Liu et al., Science, 1998, 280,
1253-1256; Chen et al., Science, 1998, 282, 95-98], and sidewall
functionalization [see, e.g., PCT publication WO 02/060812 by Tour
et al.; Khabashesku et al., Acc. Chem. Res., 2002, 35, 1087-1095;
and Holzinger et al., Angew. Chem. Int. Ed., 2001, 40, 4002-4005],
and can serve to facilitate the debundling and dissolution of such
CNTs in various solvents. Scalable chemical strategies have been,
and are being, developed to scale up such chemical manipulation
[Ying et al., Org. Letters, 2003, 5, 1471-1473, Bahr et al., J. Am.
Chem. Soc., 2001, 123, 6536-6542; and Kamaras et al., Science,
2003, 301, 1501].
[0007] Carbon nanotube chemistry has been described using a
pyramidization angle formalism [Niyogi et al., Acc. of Chem. Res.,
2002, 35, 1105-1113]. Here, chemical reactivity and kinetic
selectivity are related to the extent of s character due to the
curvature-induced strain of the sp.sup.2-hybridized graphene sheet.
Because strain energy per carbon is inversely related to nanotube
diameter, this model predicts smaller diameter nanotubes to be the
most reactive, with the enthalpy of reaction decreasing as the
curvature becomes infinite. While this behavior is most commonly
the case, the role of the electronic structure of the nanotubes in
determining their reactivity is increasingly important--especially
when desiring selectivity among a population of similar-diameter
CNTs (such as is often the case with SWNT product).
[0008] The diameter and chirality of individual CNTs are described
by integers "n" and "m," where (n,m) is a vector along a graphene
sheet which is conceptually rolled up to form a tube. When
|n-m|=3q, where q is an integer, the CNT is a semi-metal (bandgaps
on the order of milli eV). When n-m=0, the CNT is a true metal and
referred to as an "armchair" nanotube. All other combinations of
n-m are semiconducting CNTs with bandgaps typically in the range of
0.3 to 1.0 eV. See O'Connell et al., Science, 2002, 297, 593. CNT
"type," as used herein, refers to such electronic types described
by the (n,m) vector (i.e., metallic, semi-metallic, and
semiconducting).
[0009] All known preparative methods lead to polydisperse materials
of semiconducting, semimetallic, and metallic electronic types. See
M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of
Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996;
Bronikowski et al., Journal of Vacuum Science & Technology
2001, 19, 1800-1805; R. Saito, G. Dresselhaus, M. S. Dresselhaus,
Physical Properties of Carbon Nanotubes, Imperial College Press,
London, 1998. As such, a primary hurdle to the widespread
application of CNTs, and SWNTs in particular, is their manipulation
according to electronic structure [Avouris, Acc. Chem. Res. 2002,
35, 1026-1034]. Recently, however, methods to selectively
functionalize CNTs based on their electronic structure (i.e.,
electronic type) have been reported [Strano et al., Science, 2003,
301, 1519-1522; commonly assigned co-pending International Patent
Application PCT/US04/24507, filed Jul. 29, 2004]. In such reports,
metallic CNTs are seen to react preferentially with diazonium
species, permitting a separation or fractionation of metallic
(including semimetallic) and semiconducting CNTs via partial
functionalization of a mixture of metallic and semiconducting CNTs.
For a detailed discussion of CNT types and their optical
identification, see Bachilo et al., Science, 2002, 298,
2361-2366.
[0010] Regeneration of the pristine-like nanotube structure becomes
of paramount importance if covalent functionalization is used as a
handle for separation or for controlled manipulation of material,
particularly when the original extended .pi.-electron-derived
optical or electronic properties are required for the ultimate
desired function. It has been shown that functionalized material
treated thermally, in the dry state, in an inert atmosphere,
defunctionalizes to regenerate the pristine-like SWNT structure
[Dyke et al., J. Am. Chem. Soc. 2003, 125, 1156-1157; Dyke et al.,
Nano Lett. 2003, 3, 1215-1218; Bahr et al., J. Am. Chem. Soc. 2001,
123, 6536-6542; and Bahr et al., Chem. Mater. 2001, 13, 3823-3824].
However, material treated thermally in the dry state is
intractable; it can not be resuspended in organic solvent even with
extended sonication. Two possible mechanisms exist for explaining
the loss of solubility. First, covalent cross-linking of multiple
nanotubes might ensue during thermolysis via radical reactions.
Secondly, as the material is annealed, defects are moved to the
ends of the nanotube thereby providing bundled material that is
energetically difficult to unbundle. This second possibility could
be the reason that even thermally treated pristine (unreacted)
material is difficult or impossible to solublize. Note that
nanotubes have a reported van der Waals attraction of 0.5 eV per
nanometer of tube-tube contact [O'Connell et al., Chem. Phys. Lett.
2001, 342, 265-271; and Thess et al., Science 1996, 273,
483-487].
[0011] In light of the above-described advances in the chemical
functionalization of CNTs, and the disadvantages of current thermal
defunctionalization methods, a better method of regenerating
pristine carbon nanotubes from functionalized carbon nanotubes is
clearly needed.
BRIEF DESCRIPTION OF THE INVENTION
[0012] The present invention is directed towards methods of
thermally defunctionalizing functionalized (derivatized) carbon
nanotubes (CNTs) in solution or while suspended in a liquid medium.
Such defunctionalization largely comprises the removal of sidewall
functionality from the CNTs, but can also serve to remove
functionality from the CNT ends. Such methods allow for the
resuspension of such defunctionalized CNTs in various solvents and
permit the defunctionalization of functionalized CNTs that would
normally decompose (or partially decompose) upon thermal treatment.
Such methods of defunctionalization can typically lead to
defunctionalized CNTs that are essentially pristine (or nearly
pristine), and which, in contrast to prior art methods of thermal
defunctionalization, can be easily resuspended in a variety of
solvents.
[0013] The methods of the present invention generally comprise the
steps of: (a) suspending/dissolving a quantity of functionalized
CNTs in a solvent to form a suspension/solution of functionalized
CNTs; and (b) heating said suspension/solution to a temperature
that will thermally defunctionalize the functionalized CNTs
yielding a defunctionalized product. Temperatures exceeding the
atmospheric pressure boiling point of the solvent are easily
achieved by sealing the mixture in a closed pressure vessel.
[0014] The methods of the present invention are flexible in that
they work with a variety of different kinds of functionalized CNTs,
and they can employ a variety of different solvents. In some
embodiments, the functionalized CNTs have been partially and/or
selectively functionalized (e.g., by electronic type). In some
embodiments, the solvent selection is directed by the kinds of
functionalized CNTs being defunctionalized.
[0015] In some embodiments, the defunctionalization is used to
render a partially defunctionalized product. By carrying out the
defunctionalization in solution, the defunctionalization can be
homogeneous. Such partial defunctionalization can lead to
functionalized CNTs with stoichiometries that might otherwise be
unattainable with direct functionalization methods.
[0016] In some embodiments, one or more analytical techniques are
used to evaluate the defunctionalized product. Such techniques can
be used to determine the extent of defunctionalization and the
extent to which the defunctionalized CNTs have been returned to
their pristine (original) state.
[0017] While many of the exemplary embodiments described herein
utilize single-wall carbon nanotubes (SWNTs), it should be
understood that such methods are generally applicable to all
functionalized carbon nanotubes.
[0018] The foregoing has outlined rather broadly the features of
the present invention in order that the detailed description of the
invention that follows may be better understood. Additional
features and advantages of the invention will be described
hereinafter which form the subject of the claims of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0020] FIG. 1 illustrates two reaction schemes by which SWNTs can
be functionalized by diazonium species, where in reaction 1 the
diazonium species is generated in situ under solvent free
conditions, and where in reaction 2 the diazonium species is added
directly to a surfactant-aided suspension of SWNTs;
[0021] FIG. 2 depicts a thermogravimetric analysis (TGA) plot of
the thermal defunctionalization of heavily functionalized
(4-chlorophenyl) SWNTs in the dry state;
[0022] FIGS. 3 A-E depict Raman spectra (taken with 633 nm
excitation) of (A) pristine (unreacted) SWNTs, (B) heavily
functionalized SWNTs containing 4-chlorophenyl addends by treatment
of micelle-coated SWNTs with 4-chlorobenzenediazonium
tetrafluoroborate and corresponding to the material used for the
TGA in FIG. 2, (C) the same material as in 3B, but after neat
thermal treatment at 10.degree. C./min to 650.degree. C. in Ar, (D)
the same material as in 3B, but after neat thermal treatment at
10.degree. C./min to 450.degree. C. and holding at 450.degree. C.
for 2 hours, and (E) the same material as in 3B, but after thermal
defunctionalization in ortho-dichlorobenzene (ODCB) at 450.degree.
C. for 3 hours; and
[0023] FIG. 4 illustrates the thermolysis of
4-tert-butylphenyl-functionalized SWNTs (prepared by the SDS-coated
SWNT/H.sub.2O protocol) in ODCB (solvent) that affords
defunctionalized SWNTs and two discernable volatile products, the
biphenyls 1 and 2, as determined by a GC-MS analysis of the
reaction mixture.
DETAILED DESCRIPTION OF THE INVENTION
[0024] In the following description, specific details are set forth
such as specific quantities, sizes, etc. so as to provide a
thorough understanding of embodiments of the present invention.
However, it will be obvious to those skilled in the art that the
present invention may be practiced without such specific details.
In many cases, details concerning such considerations and the like
have been omitted inasmuch as such details are not necessary to
obtain a complete understanding of the present invention and are
within the skills of persons of ordinary skill in the relevant
art.
[0025] Referring to the drawings in general, it will be understood
that the illustrations are for the purpose of describing a
particular embodiment of the invention and are not intended to
limit the invention thereto.
[0026] The present invention is directed towards methods of
thermally defunctionalizing functionalized (derivatized) carbon
nanotubes (CNTs) in solution or suspended in a liquid medium. Such
defunctionalization is largely directed to the removal of sidewall
functionality from the CNTs, but can also serve to remove
functionality from the CNT ends. Such methods allow for the
resuspension of such defunctionalized CNTs in various solvents and
permit the defunctionalization of functionalized CNTs that would
normally decompose (or partially decompose) upon thermal treatment.
Such methods of defunctionalization can typically lead to
defunctionalized CNTs that are essentially pristine (or nearly
pristine), and which, in contrast to prior art methods of thermal
defunctionalization, can be easily resuspended in a variety of
solvents. Additionally, such solvent-based defunctionalization can
partially defunctionalize functionalized CNTs in a generally
homogeneous manner.
[0027] The methods of the present invention generally comprise the
steps of: (a) dissolving or suspending a quantity of functionalized
CNTs in a solvent to form a solution/suspension of functionalized
CNTs; and (b) heating said solution/suspension to a temperature
that will thermally defunctionalize the functionalized CNTs to
yield a defunctionalized product.
[0028] CNTs, according to the present invention, include, but are
not limited to, single-wall carbon nanotubes (SWNTs), multi-wall
carbon nanotubes (MWNTs), double-wall carbon nanotubes (DWNTs),
buckytubes, fullerene tubes, tubular fullerenes, graphite fibrils,
and combinations thereof. Such CNTs can initially be of a variety
and range of lengths, diameters, number of tube walls, chiralities
(helicities), etc., and can generally be made by any known
technique. The terms "carbon nanotube" and "nanotube" will be used
interchangeably herein. Such CNTs are often subjected to one or
more purification steps [see, e.g., Chiang et al., J. Phys. Chem.
B, 2001, 105, 1157-1161; Chiang et al., J. Phys. Chem. B 2001, 105,
8297-8301] and/or methods for separating by length [see, e.g.,
Farkas et al., Chemical Physics Letters, 2002, 363, 111-116] and/or
electronic type [see, e.g., Chattopadhyay et al., J. Am. Chem.
Soc., 2003, 125, 3370; Zheng et al., Science, 2003, 302, 1545-1548;
Weisman, Nat. Mater., 2003, 2, 569-570; and commonly assigned,
co-pending U.S. patent applications Ser. Nos. 10/379,022 and
10/379,273, both filed Mar. 4, 2003]. While many of the exemplary
embodiments and examples described herein utilize single-wall
carbon nanotubes, it should be understood that such methods are
generally applicable to all functionalized carbon nanotubes.
[0029] Functionalized CNTs, according to the present invention, can
be chemically functionalized derivatives of any of the
above-mentioned kinds or types of CNTs. Chemically functionalized,
according to the present invention, is the chemical attachment
(typically via covalent bonding) of functional moieties to the
sidewalls and/or ends of CNTs. Examples of suitably functionalized
CNTs include, but are not limited to, those described in the
following references: Liu et al., Science, 1998, 280, 1253-1256;
Chen et al., Science, 1998, 282, 95-98; Holzinger et al., Angew.
Chem. Int. Ed., 2001, 40, 4002-4005; Khabashesku et al., Acc. Chem.
Res., 2002, 35, 1087-1095; International Patent Publication Number
WO 02/060812, filed Jan. 29, 2002; Bahr et al., J. Am. Chem. Soc.
2001, 123, 6536-6542; Bahr et al., Chem. Mater. 2001, 13,
3823-3824; Bahr et al., J. Mater. Chem. 2002, 12, 1952-1958; Dyke
et al., J. Am. Chem. Soc., 2003, 125, 1156-1157; Dyke et al., Nano
Lett. 2003, 3, 1215-1218; Strano et al., Science, 2003, 301,
1519-1522; Dyke et al., SynLett 2004, 155-160; Dyke et al., Chem.
Eur. J. 2004, 10, 812-817; Ying et al., Org. Letters, 2003, 5,
1471-1473; Kamaras et al., Science, 2003, 301, 1501; and Hudson et
al., J. Am. Chem. Soc. 2004, 126, 11158-11159.
[0030] Generally, solvents employed in the methods of the present
invention are thermally stable at the temperatures required for
defunctionalization of the functionalized CNTs. In some
embodiments, the solvent selection is directed by the kinds of
functionalized CNTs being defunctionalized. Suitable solvents
include, but are not limited to, o-dichlorobenzene (ODCB), benzene,
toluene, water, sulfuric acid, oleum (sulfuric acid with dissolved
sulfur trioxide), sulfuric acid with dissolved potassium persulfate
or other radical initiator such as peroxide, liquid ammonia, liquid
ammonia with dissolved alkali metals, alkanes, parafins, thiophene,
and combinations thereof.
[0031] In some embodiments, the functionalized CNTs are further
polymer-wrapped and/or surfactant suspended in the solvent or
liquid medium prior to being defunctionalized. In these or other
embodiments, such polymer wrapping and/or surfactants can serve to
keep the CNTs in suspension after they have been completely or
partially defunctionalized. See O'Connell et al., Chem. Phys.
Lett., 2001, 342, 265-271; and O'Connell et al., Science, 2002,
297, 593-596 for exemplary methods of polymer wrapping and
surfactant suspending CNTs, respectively.
[0032] In some embodiments, the solvent includes a polymer such
that the defunctionalization is carried out while in a polymer
matrix and thereby provides a blended polymer/pristine nanotube
sample after defunctionalization and upon removal of the solvent.
In some embodiments, the functionalized CNTs are dispersed directly
in a polymer matrix, then defunctionalized to yield a product blend
comprising unfunctionalized CNTs in a polymer host. Such product
blends benefit from the greater dispersability of the
functionalized CNTs (relative to unfunctionalized CNTs) in the
polymer host.
[0033] The temperatures required for thermal defunctionalization
vary depending on the type(s) of functionalized CNTs being
defunctionalized. Typically, such defunctionalization temperatures
range from about 100.degree. C. to about 700.degree. C., and more
typically from about 250.degree. C. to about 400.degree. C. In some
embodiments, a ramped or variable heating process is used.
[0034] Heating a solution or suspension of functionalized CNTs to a
temperature required for complete or partial thermal
defunctionalization can be accomplished via a variety of heating
methods. Suitable heating methods include, but are not limited to,
heating mantles, immersion heaters, microwave heating, and
combinations thereof.
[0035] In some embodiments, the heating is carried out with
stirring, or some other kind of agitation, to ensure homogeneous
thermolysis by minimizing thermal gradients within the
suspension.
[0036] Typically the thermal defunctionalization process entails a
defunctionalization duration, lasting between about 3 minutes and
about 2 days, and more typically between about 30 minutes and about
3 hours, during which time the functionalized CNTs are heated.
[0037] In some embodiments, the functionalized CNTs are only
partially defunctionalized. This permits product stoichiometries of
such partially defunctionalized CNTs that might otherwise not be
achievable. In some embodiments, partial defunctionalization is a
result of selective thermal defunctionalization according to (n,m)
type, with differing types having differing propensities to
defunctionalize. In some embodiments, upon being fully or partially
defunctionalized, the CNTs flocculate or fall out of
suspension/solution.
[0038] In some embodiments, but not all, the defunctionalization is
carried out in a sealed reaction vessel. In some embodiments, these
sealed reaction vessels permit the use of temperatures that exceed
the atmospheric pressure boiling point of the solvent. In some
embodiments, some of the defunctionalization products are volatile.
In some embodiments, the defunctionalization products react with
the suspension/solution medium.
[0039] In some embodiments, one or more analytical techniques are
used to evaluate the defunctionalized product and/or byproducts.
Such techniques can be used to determine the extent of
defunctionalization and the extent to which the defunctionalized
carbon nanotubes have been returned to their pristine (original)
state. In some embodiments producing volatile defunctionalization
products, the liquid defunctionalization medium can be analyzed
with gas chromatography-mass spectrometry (GC-MS) or other suitable
analytical techniques.
[0040] In some embodiments, the defunctionalized product is only
partially defunctionalized, whereas in other embodiments it is
completely defunctionalized. The completely defunctionalized CNTs
are essentially in their pristine (or nearly pristine) state. An
important aspect of the methods described herein is that the
defunctionalized CNTs of the present invention (i.e., solvent
defunctionalized CNTs) can be redispersed much more easily than
CNTs that have been thermally defunctionalized in the dry
state.
[0041] In some embodiments, the wholly or partially
defunctionalized CNTs of the present invention are redispersed in
solvents with the aid of surfactants and/or polymers. In general,
such wholly or partially defunctionalized CNTs can be manipulated
in essentially any manner in which pristine CNTs can be
manipulated.
[0042] Using methods of the present invention, carbon nanotubes
(CNTs) chemically modified by bonding (e.g., covalently) functional
groups to their sidewalls and/or ends can be thermally
defunctionalized in solution, returning the previously
functionalized CNTs to their original state. In contrast to thermal
defunctionalization processes carried out in the dry state, wherein
functionalized CNTs are heated dry in a gaseous or vacuum
environment, the present invention does not render the
defunctionalized carbon nanotubes unsuspendable/insoluble from that
point on. While not intending to be bound by theory, it is believed
that solvent-based defunctionalization methods of the present
invention mitigate both the packing of the CNTs into ordered
bundles and, possibly, cross-linking which is thought to occur
between such funcetionalized CNTs when they are heated in the dry
state.
[0043] The following examples are provided to more fully illustrate
some of the embodiments of the present invention. It should be
appreciated by those of skill in the art that the techniques
disclosed in the examples which follow represent techniques
discovered by the inventors to function well in the practice of the
invention, and thus can be considered to constitute exemplary modes
for its practice. However, those of skill in the art should, in
light of the present disclosure, appreciate that many changes can
be made in the specific embodiments that are disclosed and still
obtain a like or similar result without departing from the spirit
and scope of the invention.
EXAMPLE 1
[0044] This Example serves to illustrate how CNTs can be
functionalized with diazonium chemistry in accordance with some
embodiments of the present invention.
[0045] Referring to FIG. 1, diazonium species can be generated in
situ (reaction 1), or added directly (reaction 2), to SWNTs to
render them functionalized. Reaction 1 is carried out without any
solvent [see Dyke et al., J. Am. Chem. Soc., 2003, 125, 1156-1157],
and in reaction 2, the SWNTs are first dispersed in water with
sodium dodecylsulfate (SDS) [see Dyke et al., Nano Lett. 2003, 3,
1215-1218]. For a detailed discussion on the mechanistic aspects of
such functionalization, see Dyke et al., SynLett, 2004, 1,
155-160.
[0046] In Examples 2-4 which follow, SWNTs were functionalized with
4-clorophenylene addends in accordance with reaction 2 in FIG.
1.
EXAMPLE 2
[0047] This Example serves to illustrate how functionalized CNTs
can be thermally defunctionalized in solution/suspension in
accordance with embodiments of the present invention.
[0048] Approximately 0.5 mg of SWNTs (purified HiPco, obtained from
Rice University's Carbon Nanotechnology Laboratory), unbundled and
functionalized with 4-clorophenylene addends, were dispersed in 5
mL of ortho-dichlorobenzene (ODCB) and placed in a thick-walled,
screw-cap tube. The reaction vessel was purged with nitrogen and
sealed with a TEFLON cap. The solution was heated in a sand bath
placed inside a heating mantle at approximately 450.degree. C. with
stirring for about 3 hours. After such time, the solution was
cooled to room temperature. The regenerated, defunctionalized SWNTs
were then filtered through a TEFLON membrane and collected.
Defunctionalization was confirmed by absorption and Raman
spectroscopies. Subsequent to their defunctionalization, these
solvent-defunctionalized SWNTs can be resuspended in any of a
number of different solvents (unlike those defunctionalized in the
dry state).
COMPARATIVE EXAMPLE 3
[0049] This Example serves to illustrate prior art methods of
thermally defunctionalizing functionalized CNTs in the dry
state.
[0050] Referring to FIG. 2, thermogravimetric analysis (TGA) of
heavily functionalized (4-chlorophenyl) nanotubes (the same
material as used in Example 2) showed 49% weight loss, which
corresponds to 1 in 9 carbons on the nanotube bearing an aryl
moiety. The addends appear to be removed in two separate thermal
regions, one at 200-400.degree. C. and a second at 475-550.degree.
C. While not intending to be bound by theory, this might be
indicative of compressed bands of functionalization vs. dispersed
addend regions, or of the defunctionalization temperatures needed
for addend expulsion on semiconducting vs. metallic tubes.
[0051] The defunctionalized SWNTs resulting from such thermal
defunctionalization in the dry state could not be re-suspended in
solvent to any significant extent--even with vigorous
agitation.
EXAMPLE 4
[0052] This Example serves to illustrate how Raman spectroscopy can
be used to probe the defunctionalization of functionalized CNTs in
both the solvent-based and dry state methods.
[0053] Comparing the Raman spectrum (obtained using 633 nm
excitation) of the pristine SWNTs (FIG. 3A) with that of the
4-chlorophenylene-functionalized material (FIG. 3B), the
characteristic spectroscopic details for covalent functionalization
can be seen [Dyke et al., SynLett, 2004, 1, 155-160]. Upon neat
thermal treatment in Ar at either 650.degree. C. or 450.degree. C.
for extended periods, the addends can be removed, and the Raman
spectra, FIGS. 3C and 3D, respectively, resembled that of the
pristine SWNTs shown in FIG. 3A. However, attempts to disperse
these defunctionalized SWNTs in DMF were unsuccessful, even with
extended sonication. Using this same heavily functionalized
material (0.5 mg), but dissolving it in ortho-dichlorobenzene
(ODCB, 5 mL) prior to thermal treatment and heating the solution to
450.degree. C. for 3 hours in a screw-cap tube (see Example 2) gave
defunctionalized material with a corresponding Raman spectrum shown
in FIG. 3E. Raman spectra of this material confirmed significant
defunctionalization, albeit not completely pristine-like. However,
unlike thermal treatment in the dry state, this protocol yields
material with solubility that is better than that of material
obtained by thermally defunctionalizing in the dry state.
EXAMPLE 5
[0054] This Examples serves to illustrate how byproducts from some
of the solvent-based defunctionalization method of the present
invention can be analyzed.
[0055] While not intending to be bound by theory, and as discussed
generally above, the thermal treatment of functionalized CNT
material while dispersed in a solvent such as ODCB possibly
prevents nanotube radicals from combining to form nanotube dimers;
instead, two radicals on the same nanotube might combine by
extended conjugation to regenerate the C--C double-bond.
Conversely, extensive rebundling might be minimized under such
solvent-based conditions. To gain further insight into such
solvent-based thermal defunctionalization, the byproducts of a
ODCB-thermalized reaction of 4-tert-butylphenyl functionalized
SWNTs were examined by GC-MS analysis of the ODCB solution.
Referring to FIG. 4, two biphenyls, 1 and 2, were generated by the
addends coupling with the solvent, which substantiates (a) the
tert-butylphenyl attachment to the nanotube and (b) the reactive
nature of the moiety as it is evolved from the nanotube.
Interestingly, there was no discernible di(tert-butyl)biphenyl
present in the reaction mixture, so the addends did not evolve in
pairs. If the addends had evolved as aryl anions, reaction with
solvent would likely have led to chloride substitution on the ODCB.
Expulsion as the cation would have led to a highly reactive species
that could afford the products observed in FIG. 4. However,
thermolytic cleavage most likely proceeds via a homolytic
process.
[0056] All patents and publications referenced herein are hereby
incorporated by reference. It will be understood that certain of
the above-described structures, functions, and operations of the
above-described embodiments are not necessary to practice the
present invention and are included in the description simply for
completeness of an exemplary embodiment or embodiments. In
addition, it will be understood that specific structures,
functions, and operations set forth in the above-described
referenced patents and publications can be practiced in conjunction
with the present invention, but they are not essential to its
practice. It is therefore to be understood that the invention may
be practiced otherwise than as specifically described without
actually departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *