U.S. patent application number 12/628718 was filed with the patent office on 2010-06-10 for methods of preparing and purifying carbon nanotubes, carbon nanotubes, and an element using the same.
This patent application is currently assigned to SONY CORPORATION. Invention is credited to Liping Huang, Hisashi Kajiura, Yongming Li, Yunqi Liu, Yu Wang, Dacheng Wei, Hongliang Zhang.
Application Number | 20100143234 12/628718 |
Document ID | / |
Family ID | 42231309 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100143234 |
Kind Code |
A1 |
Kajiura; Hisashi ; et
al. |
June 10, 2010 |
METHODS OF PREPARING AND PURIFYING CARBON NANOTUBES, CARBON
NANOTUBES, AND AN ELEMENT USING THE SAME
Abstract
A method of preparing carbon nanotubes (CNT), a method of
purifying carbon nanotubes, carbon nanotubes, and an element using
said carbon nanotubes are provided. The method includes preparing
carbon nanotubes by arc-discharge and employs a coordination
chemistry process to remove a catalyst and/or optional promoter
used in arc-discharge.
Inventors: |
Kajiura; Hisashi; (Shanghai,
CN) ; Li; Yongming; (Beijing, CN) ; Zhang;
Hongliang; (Beijing, CN) ; Wang; Yu; (Beijing,
CN) ; Liu; Yunqi; (Beijing, CN) ; Wei;
Dacheng; (Beijing, CN) ; Huang; Liping;
(Beijing, CN) |
Correspondence
Address: |
K&L Gates LLP
P. O. BOX 1135
CHICAGO
IL
60690
US
|
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
42231309 |
Appl. No.: |
12/628718 |
Filed: |
December 1, 2009 |
Current U.S.
Class: |
423/447.2 ;
423/447.1; 977/742; 977/750; 977/842; 977/845 |
Current CPC
Class: |
B82Y 40/00 20130101;
C01B 2202/28 20130101; C01B 32/162 20170801; B82Y 30/00 20130101;
C01B 2202/02 20130101; C01B 32/17 20170801; C01B 32/174
20170801 |
Class at
Publication: |
423/447.2 ;
423/447.1; 977/742; 977/750; 977/842; 977/845 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 2008 |
CN |
200810178857.7 |
Claims
1. A method for preparing carbon nanotubes, the method comprising:
producing carbon nanotubes by an arc-discharge method in presence
of a catalyst and optionally a promoter; coordinating the metal
elements present in the catalyst and/or the optional promoter with
a substance capable of forming a complex with the metal elements to
produce a complex; and removing the complex.
2. The method of claim 1, wherein the promoter is employed.
3. The method of claim 2, wherein the promoter is FeS.
4. The method of claim 1, wherein the catalyst is selected from the
group consisting of lanthanum metal oxide, transition metal, the
mixture of nickel and a rare earth element, and mixtures
thereof.
5. The method of claim 1, wherein the catalyst is selected from the
group consisting of Y--Ni alloy, Fe--Ni alloy, Fe--Co alloy, Co--Ni
alloy, Rh--Pt alloy, and Ce--Ni alloy.
6. The method of claim 1, wherein coordinating the metal elements
includes: converting the metal elements present in the catalyst
and/or the optional promoter into ions; and coordinating the ions
with the substance capable of forming a complex with the metal
elements present in the catalyst and/or the optional promoter to
produce a complex.
7. The method of claim 6, wherein converting the metal elements
includes: oxidizing the catalyst and/or the optional promoter to
produce the oxides thereof.
8. The method of claim 7, wherein corresponding metal ions are
obtained from the oxide by using the substance capable of forming a
complex with the metal elements present in the catalyst and/or
optionally the promoter, and are coordinated with the substance to
produce a complex.
9. The method of claim 8, wherein the substance capable of forming
a complex with the metal elements present in the catalyst and/or
optionally promoter is selected from aminopolycarboxylic acids.
10. The method of claim 9, wherein the aminopolycarboxylic acid is
selected from the group consisting of ethylenediaminetetraacetic
acid (EDTA), trans-1,2-diaminocyclohe-xane-N,N,N',N'-tetracetic
acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA),
and triethylenetetraaminehexaacetic acid (TTHA).
11. The method of claim 9, wherein the aminopolycarboxylic acid is
triethylenetetraaminehexaacetic acid (TTHA).
12. The method of claim 1, wherein removing the complex includes
converting the complex into a salt form and removing the complex in
the salt form.
13. The method of claim 7, wherein converting the metal elements
further comprises: reacting the oxides with an acid to produce ions
of the metal elements present in the catalyst and/or the optional
promoter.
14. The method of claim 6, wherein the substance capable of forming
a complex with the metal elements present in the catalyst and/or
optionally the promoter is selected from the group consisting of
tetrahydrofuran, trialkyl phosphine, .epsilon.-caprolactone,
.epsilon.-caprolactam, dimethyl formamide, and dimethyl
sulfoxide.
15. The method of claim 6, wherein the complex is selected from the
group consisting of
{M[(NC).sub.2CC(OCH.sub.2CH.sub.2OH)C(CN).sub.2].sub.2(4,4'-bpy)(H.sub.2O-
).sub.2}, Dinuclear
[{M'(phen).sub.2}.sub.2V.sub.4O.sub.12]C.sub.6H.sub.12O.H.sub.2O
and [Ni(L)(H.sub.2O).sub.3]2H.sub.2O, wherein M is selected from
Ni, Fe and Co; M' is selected from Ni and Co; bpy is bipyridine;
phen is phenyl; L is
(2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic
acid.
16. The method of claim 7, wherein oxidizing the catalyst comprises
oxidizing the catalyst and/or optionally the promoter with an
oxygen containing gas.
17. The method of claim 16, wherein the oxidation time and the
oxidation temperature of the oxygen containing gas are sufficient
to convert the catalyst and/or the optional promoter into
oxides.
18. The method of claim 16, wherein the oxygen containing gas is
air.
19. The method of claim 17, wherein the oxidation temperature is
about 80.degree. C. to about 300.degree. C.
20. The method of claim 17, wherein the oxidation time is about 1
hour to about 20 hours.
21. The method of claim 1, further comprising centrifugation after
removing the complex.
22. The method of claim 21, wherein centrifugation is carried out
at a speed of about 5000 rpm to about 30000 rpm for about 1 hour to
about 20 hours.
23. The method according to claim 1, wherein the carbon nanotubes
are single-walled carbon nanotubes.
24. A method for purifying carbon nanotubes produced by an
arc-discharge method in the presence of a catalyst and optionally a
promoter, the method comprising: coordinating the metal elements
present in the catalyst and/or the optional promoter with a
substance capable of forming a complex with the metal elements to
produce a complex; and removing the complex.
25. The method of claim 24, wherein coordinating the metal elements
includes: converting the metal elements present in the catalyst
and/or the optional promoter into ions; and coordinating the ions
with the substance capable of forming a complex with the metal
elements present in the catalyst and/or the optional promoter to
produce a complex.
26. The method of claim 25, wherein converting the metal elements
includes: oxidizing the catalyst and/or the optional promoter to
produce the oxides thereof.
27. The method of claim 26, wherein corresponding metal ions are
obtained from the oxide by using the substance capable of forming a
complex with the metal elements present in the catalyst and/or the
optional promoter, and are coordinated with the substance to
produce a complex.
28. The method of claim 27, wherein the substance capable of
forming a complex with the metal elements present in the catalyst
and/or the optional promoter is selected from aminopolycarboxylic
acids.
29. The method of claim 28, wherein said aminopolycarboxylic acid
is selected from the group consisting of ethylenediaminetetraacetic
acid (EDTA), trans-1,2-diaminocyclohe-xane-N,N,N',N'-tetracetic
acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA) and
triethylenetetraaminehexaacetic acid (TTHA).
30. The method of claim 28, wherein the aminopolycarboxylic acid is
triethylenetetraaminehexaacetic acid (TTHA).
31. The method of claim 24, wherein removing the complex comprises
converting the complex into a salt form and removing the complex in
the salt form.
32. The method of claim 26, wherein oxidizing the catalyst and/or
optional promoter comprises: reacting the oxides with an acid to
produce ions of the metal elements present in the catalyst and/or
the optional promoter.
33. The method of claim 24, wherein said catalyst is selected from
Y--Ni alloy, Fe--Ni alloy, Fe--Co alloy, Co--Ni alloy, Rh--Pt
alloy, and Ce--Ni alloy.
34. The method of claim 25, wherein the substance capable of
forming a complex with the metal elements present in the catalyst
and/or the optional promoter is selected from the group consisting
of tetrahydrofuran, trialkyl phosphine, .epsilon.-caprolactone,
.epsilon.-caprolactam, dimethyl formamide, and dimethyl
sulfoxide.
35. The method of claim 25, wherein the complex is selected from
M[(NC).sub.2CC(OCH.sub.2CH.sub.2OH)C(CN).sub.2].sub.2(4,4'-bpy)(H.sub.2O)-
.sub.2}, Dinuclear
[{M'(phen).sub.2}.sub.2V.sub.4O.sub.12]C.sub.6H.sub.12O.H.sub.2O
and [Ni(L)(H.sub.2O).sub.3]2H.sub.2O, wherein M is selected from
Ni, Fe and Co; M' is selected from Ni and Co; bpy is bipyridine;
phen is phenyl; L is
(2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic
acid.
36. The method of claim 26, wherein the catalyst and/or the
optional promoter are oxidized with an oxygen containing gas.
37. The method of claim 36, wherein an oxidation time and an
oxidation temperature of the oxygen containing gas are sufficient
to convert the catalyst and/or the optional promoter into
oxides.
38. The method of claim 36, wherein the oxygen containing gas is
air.
39. The method of claim 37, wherein the oxidation temperature is
about 80.degree. C. to about 300.degree. C.
40. The method of claim 37, wherein the oxidation time is about 1
hour to about 20 hours.
41. The method of claim 24, further comprising a centrifugation
step after removing the complex.
42. The method of claim 41, wherein the centrifugation step is
carried out at a speed of about 5000 rpm to about 30000 rpm for
about 1 hour to about 20 hours.
43. The method according to claim 24, wherein the carbon nanotubes
are single-walled carbon nanotubes.
44. A carbon nanotube material comprising carbon nanotubes produced
by arc-discharge in presence of a catalyst and optionally a
promoter, wherein metal elements present in the catalyst and/or the
optional promoter are coordinated with a substance capable of
forming a complex with the metal elements to produce a complex, and
wherein the complex is removed.
45. An element of carbon nanotubes comprising a carbon nanotube
material including carbon nanotubes produced by arc-discharge in
presence of a catalyst and optionally a promoter, wherein metal
elements present in the catalyst and/or the optional promoter are
coordinated with a substance capable of forming a complex with the
metal elements to produce a complex, and wherein the complex is
removed.
46. The element of carbon nanotubes of claim 45, wherein the
element of carbon nanotubes is selected from the group consisting
of conductive film of carbon nanotubes, field emission source,
transistor, conductive wire, nano-electro-mechanic system, spin
conduction device, nano cantilever, quantum computing device,
lighting emitting diode, solar cell, surface-conduction
electron-emitter display, filter, drug delivery system, thermal
conductive material, nano nozzle, energy storage system, space
elevator, fuel cell, sensor, and catalyst support material.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application claims priority to Chinese Patent
Application No. CN 200810178857.7 filed in the Chinese Patent
Office on Dec. 4, 2008, the entire content of which is hereby
incorporated by reference.
BACKGROUND
[0002] The present application relates to a method of preparing
carbon nanotubes (CNT), a method of purifying carbon nanotubes,
carbon nanotubes obtained by said methods, and an element using
said carbon nanotubes. More specifically, the present application
relates to a method for preparing carbon nanotubes by an
arc-discharge method, a method of purifying carbon nanotubes,
carbon nanotubes obtained by said methods, and an element using
said carbon nanotubes. The preparation method or the purification
method of the present application employs a coordination chemistry
process to remove the catalyst and/or optional promoter used in an
arc-discharge method.
[0003] As one-dimension carbon nanomaterials, carbon nanotubes
(CNT) have attracted increasing attention for their superior
electrical, mechanical and chemical properties. Further study on
nanomaterials brings great potential application of carbon
nanotubes in a wide range of fields such as electron source for
field emission, nano field effect transistor, hydrogen storage
materials and high-strength fiber, and the like.
[0004] Carbon nanotubes can be classified as single-walled carbon
nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) according
to the number of the layers of the carbon atoms to form the wall.
Multi-walled carbon nanotubes may be also considered as
multi-layers by encasing several single-walled carbon nanotubes
with different diameters. In practical research and application,
single-walled carbon nanotubes and multi-walled carbon nanotubes
with fewer layers are important due to their unique electrical,
thermal, mechanical and chemical properties.
[0005] Conventional methods for preparing carbon nanotubes include
an arc-discharge method, chemical vapor deposition (CVD), laser
evaporation, and the like. To date, the arc-discharge method is one
of the most efficient techniques for large-scale production of high
quality carbon nanotubes.
[0006] Nevertheless, impurities normally are formed during the
preparation of carbon nanotubes by an arc-discharge method, such as
graphite particles, amorphous carbon, carbon nanoparticles in other
forms and metal catalyst particles. The mixture of said impurities
with carbon nanotubes greatly hampers the further study and
application of carbon nanotubes. Therefore, various physical and
chemical processes are applied to purify the primary product of
carbon nanotubes as-prepared so as to obtain carbon nanotubes with
higher purity. The commonly used purification processes include
liquid phase oxidation and gas phase oxidation. For example, K.
Tohji et al. disclosed hydrothermal treatment in J. Phys. Chem.
1997, 101, 1974. Z. Shi et al. developed gaseous oxidation, see Z.
Shi et al. Solid State Commun. 1999, 112, 35. E. Mizoguti et al,
reported catalytic oxidation in Chem. Phys. Lett. 2000, 321, 297.
Additionally, many researches have been made on nitric acid reflux
method, for example, see J. L. Zimmerman et al. Chem. Mater. 2000,
12, 1361. Furthermore, the purification based on an initial
selective oxidation to remove amorphous carbon, followed by a
reflux in concentrated nitric acid has been found effective in
removing metals from the reaction products (see K. Tohji et al.
Nature, 1996, 383, 679).
[0007] Said purification processes are known in the art, by which
the impurities can be removed to achieve the purification based on
the fact that carbon nanotubes are more stable and difficult to be
oxidized than the impurities such as amorphous carbon, metal
catalyst particles, and the like. According to the different
oxidizing atmospheres, gas phase oxidation can be classified into
oxygen (or air)-oxidation, carbon dioxide-oxidation and the like.
The commonly used liquid oxidants in liquid phase oxidation include
potassium permanganate, nitric acid solution or potassium
dichromate, and the like. In addition, physical processes such as
centrifugation and micro-filtration can also be used to separate
carbon nanotubes. The above processes can be used singly or in a
combination of one or more process. For example, gas phase
oxidation such as air-oxidation can be used to remove impurities
like amorphous carbon that are easy to remove from carbon
nanotubes; liquid phase oxidation such as nitric acid oxidation can
be used to remove the impurities like metal catalyst particles that
are difficult to remove from carbon nanotubes. Meanwhile, the
purified carbon nanotubes can also be obtained by employing the
centrifugation process in combination.
[0008] Nevertheless, the main challenge is to purify the prepared
carbon nanotubes without damaging the carbon nanotubes. As known,
liquid phase oxidation, such as a purification method by refluxing
in nitric acid, can induce the damage in sidewalls of the
nanotubes. Even if gas phase oxidation is employed, carbon
nanotubes are also damaged since the oxidation temperature used is
high, usually about 470.degree. C.
[0009] In addition, when a catalyst such as Y--Ni alloy are
employed in the arc discharge process, said catalyst present inside
the reaction product or on its surface is difficult to remove by a
mineral acid reflux method. Although most of the catalyst particles
can be removed after a long time reflux, carbon nanotubes are
damaged synchronously. As a result, the quality of carbon
nanotubes, especially in the electrical conductivity, is
degraded.
[0010] Furthermore, strong acid reflux induces the following
defects in the carbon nanotubes as prepared:
[0011] five- or seven-membered rings in the carbon framework,
instead of the normal six-membered ring, leads to a bend in the
tube;
[0012] Sp.sup.3-hybridized defects (R=H and OH);
[0013] carbon framework damaged by oxidative conditions, which
leaves a hole lined with --COOH groups; and
[0014] open end of the carbon nanotubes, terminated with --COOH
groups.
[0015] The above defects are shown in FIG. 1.
[0016] As a consequence, it is essential to find methods to prepare
and purify carbon nanotubes without damaging the carbon
nanotubes
SUMMARY
[0017] Carbon nanotubes can be prepared and purified by a method of
the present application without damaging the carbon nanotubes,
especially without damaging the sidewalls of the carbon nanotubes
according to an embodiment.
[0018] In the first aspect, the present application provides a
method for preparing carbon nanotubes, the method includes:
[0019] producing carbon nanotubes by an arc-discharge method in the
presence of a catalyst and an optional promoter;
[0020] coordinating the metal elements present in the catalyst
and/or the optional promoter with a substance capable of forming a
complex with said metal elements to produce a complex; and
[0021] removing the complex.
[0022] In one embodiment, a promoter is employed during
art-discharge. Furthermore, the preferred promoter is FeS.
[0023] In another embodiment, the catalyst is selected from the
group consisting of lanthanum metal oxides, transition metals, the
mixture of nickel and a rare earth element, and mixtures thereof.
The catalyst is preferably selected from the group consisting of
Y--Ni alloy, Fe--Ni alloy, Fe--Co alloy, Co--Ni alloy, Rh--Pt
alloy, and Ce--Ni alloy.
[0024] In another embodiment coordinating the metal elements
includes:
[0025] converting the metal elements present in the catalyst and/or
the optional promoter into ions; and
[0026] coordinating the ions with the substance capable of forming
a complex with the metal elements present in the catalyst and/or
the optional promoter to produce a complex.
[0027] In an embodiment, converting the metal element includes:
[0028] oxidizing the catalyst and/or the optional promoter to
produce the oxides thereof.
[0029] In an embodiment, the substance capable of forming a complex
with the metal elements present in the catalyst and/or the optional
promoter is selected from aminopolycarboxylic acids.
[0030] In an embodiment, the aminopolycarboxylic acid is selected
from the group consisting of ethylenediaminetetraacetic acid
(EDTA), trans-1,2-diaminocyclohexane-N,N,N',N'-tetracetic acid
hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA), and
triethylenetetraaminehexaacetic acid (TTHA).
[0031] In an embodiment, the aminopolycarboxylic acid is
triethylenetetraaminehexaacetic acid (TTHA).
[0032] Furthermore, the complex produced from aminopolycarboxylic
acid is converted into a salt form to facilitate removing the
complex in an embodiment.
[0033] In one embodiment converting the metal elements
includes:
[0034] reacting the oxides with an acid to produce ions of the
metal elements present in the catalyst and/or the optional
promoter.
[0035] In another embodiment, the substance capable of forming a
complex with the metal elements present in the catalyst and/or the
optional promoter is preferably selected from the group consisting
of tetrahydrofuran, trialkyl phosphine, .epsilon.-caprolactone,
.epsilon.-caprolactam, dimethyl formamide, and dimethyl
sulfoxide.
[0036] Furthermore, in one embodiment, the complex is preferably
selected from
{M[(NC).sub.2CC(OCH.sub.2CH.sub.2OH)C(CN).sub.2].sub.2(4,4'-bpy)(H.s-
ub.2O).sub.2}, Dinuclear
[{M'(phen).sub.2}.sub.2V.sub.4O.sub.12]C.sub.6H.sub.12O.H.sub.2O
and [Ni(L)(H.sub.2O).sub.3]2H.sub.2O, wherein M is selected from
Ni, Fe and Co; M' is selected from Ni and Co; bpy is bipyridine;
phen is phenyl; L is
(2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic
acid.
[0037] In another embodiment the catalyst and/or the optional
promoter are oxidized with an oxygen containing gas.
[0038] It is desirable that the oxidation time and the oxidation
temperature of the oxygen containing gas are sufficient to convert
the catalyst and/or the optional promoter into oxides. More
preferably, the oxygen containing gas is air. The oxidation
temperature is further preferably about 80.degree. C. to about
300.degree. C., and the oxidation time is preferably about 1 hour
to about 20 hours.
[0039] In one embodiment, the method further comprises a
centrifugation step after removing the complex. The centrifugation
step is preferably carried out at a speed of about 5000 rpm to
about 30000 rpm for about 1 hour to about 20 hours.
[0040] In an embodiment, the carbon nanotubes are single-walled
carbon nanotubes.
[0041] In another embodiment, the present application provides a
method for purifying carbon nanotubes produced by an arc-discharge
method in the presence of a catalyst and an optional promoter, and
the method includes:
[0042] coordinating the metal elements present in the catalyst
and/or the optional promoter with a substance capable of forming a
complex with said metal elements to produce a complex; and
[0043] removing the complex.
[0044] In one embodiment, a promoter is employed in the
arc-discharge method. The preferred promoter is FeS.
[0045] In another embodiment, the catalyst is selected from
lanthanum metal oxides, transition metals, the mixture of nickel
and a rare earth element, and the mixtures thereof. The catalyst is
preferably selected from the group consisting of Y--Ni alloy,
Fe--Ni alloy, Fe--Co alloy, Co--Ni alloy, Rh--Pt alloy, and Ce--Ni
alloy.
[0046] In another embodiment, coordination of the metal elements
include:
[0047] converting the metal elements present in the catalyst and/or
the optional promoter into ions; and
[0048] coordinating the ions with the substance capable of forming
a complex with the metal elements present in the catalyst and/or
the optional promoter to produce a complex.
[0049] It is preferred that converting the metal elements
includes:
[0050] oxidizing the catalyst and/or the optional promoter to
produce the oxides thereof.
[0051] It is further preferred that the substance capable of
forming a complex with the metal elements present in the catalyst
and/or the optional promoter is selected from aminopolycarboxylic
acids.
[0052] In an embodiment the aminopolycarboxylic acid is selected
from ethylenediaminetetraacetic acid (EDTA),
trans-1,2-diaminocyclohe-xane-N,N,N',N'-tetracetic acid hydrate
(CYDTA), diethylenetriaminepentaacetic acid (DTPA) and
triethylenetetraaminehexaacetic acid (TTHA).
[0053] In an embodiment, the aminopolycarboxylic acid is
triethylenetetraaminehexaacetic acid (TTHA).
[0054] In an embodiment, the complex produced from
aminopolycarboxylic acid is converted into a salt form to
facilitate removing the complex.
[0055] In another embodiment converting the metal elements
include:
[0056] reacting said oxides with an acid to produce ions of the
metal elements present in the catalyst and/or the optional
promoter.
[0057] In one embodiment, the substance capable of forming a
complex with the metal elements present in the catalyst and/or the
optional promoter is preferably selected from tetrahydrofuran,
trialkyl phosphine, .epsilon.-caprolactone, .epsilon.-caprolactam,
dimethyl formamide, and dimethyl sulfoxide.
[0058] Furthermore, in another embodiment, the complex is
preferably selected from
{M[(NC).sub.2CC(OCH.sub.2CH.sub.2OH)C(CN).sub.2].sub.2(4,4'-bpy)(H.sub.2O-
).sub.2}, Dinuclear
[{M'(phen).sub.2}2V4O12]C.sub.6H.sub.12O.H.sub.2O and
[Ni(L)(H.sub.2O).sub.3]2H.sub.2O, wherein M is selected from Ni,
Fe, and Co; M' is selected from Ni and Co; bpy is bipyridine; phen
is phenyl; and L is
(2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic
acid.
[0059] In an embodiment, the catalyst and/or the optional promoter
is oxidized with an oxygen containing gas.
[0060] It is desirable that the oxidation time and the oxidation
temperature of the oxygen containing gas are sufficient to convert
the catalyst and/or the optional promoter into oxides. More
preferably, the oxygen containing gas is air. The oxidation
temperature is further preferably about 80.degree. C. to about
300.degree. C., and the oxidation time is preferably about 1 hour
to about 20 hours.
[0061] In one embodiment, the method further comprises a
centrifugation step after removing the complex. The centrifugation
step is preferably carried out at a speed of about 5000 rpm to
about 30000 rpm for about 1 hour to about 20 hours.
[0062] In an embodiment, the carbon nanotubes are single-walled
carbon nanotubes.
[0063] In another embodiment, the present application provides
carbon nanotubes prepared by a method according to embodiments
previously discussed and further discussed below in greater detail.
The carbon nanotubes are not damaged in sidewalls compared with
carbon nanotubes prepared by a conventional methods.
[0064] In a further embodiment, the present application provides an
element of carbon nanotubes, preferably carbon nanotubes that are
single-walled carbon nanotubes.
[0065] Additional features and advantages are described herein, and
will be apparent from the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0066] FIG. 1 is a schematic drawing of carbon nanotubes purified
by acid reflux.
[0067] FIG. 2 is a schematic drawing of an arc furnace for
preparing carbon nanotubes according to an embodiment.
[0068] FIG. 3 is a schematic drawing of the purification of carbon
nanotubes by using CYDTA.
[0069] FIG. 4 shows Raman spectra of carbon nanotubes purified by
using EDTA and CYDTA, and also shows that of P3 carbon nanotubes
(commercially available from Carbon Solutions Inc., and its purity
is higher than 85%).
[0070] FIG. 5 is XPS spectrum of carbon nanotubes purified by using
EDTA.
[0071] FIG. 6a is the SEM image of pristine carbon nanotubes; FIG.
6b is the SEM image of carbon nanotubes after the purification by
TTHA.
[0072] FIG. 7 shows TEM images of carbon nanotubes after the
purification by TTHA, wherein the difference between FIG. 7a and
FIG. 7b is just magnification.
[0073] FIG. 8 shows Raman spectra of carbon nanotubes before and
after the purification by TTHA.
[0074] FIG. 9 shows XPS spectra of carbon nanotubes purified by
using TTHA and by conventional acid treatment, respectively.
[0075] FIG. 10 shows a carbon nanotube film made from carbon
nanotubes purified with TTHA.
[0076] FIG. 11 shows sheet resistances of the carbon nanotube film
made in Film Fabrication Example 1 and the film made in Comparative
Example 1.
[0077] FIG. 12(a) shows a vapor generator used in the present
application; FIG. 12(b) shows the cross section of glass casing in
the vapor generator of (a).
DETAILED DESCRIPTION
[0078] The present application will be described below in detail
with reference to the drawings according to an embodiment.
[0079] In an embodiment, the present application provides a method
for preparing carbon nanotubes, the method includes:
[0080] producing carbon nanotubes by an arc-discharge method in the
presence of a catalyst and an optional promoter;
[0081] coordinating the metal elements present in the catalyst
and/or the optional promoter with a substance capable of forming a
complex with said metal elements to produce a complex; and
[0082] removing the complex.
[0083] Each step will be described in detail as follows.
[0084] (1) Arc-Discharge Process
[0085] An arc-discharge method is one of the earliest techniques
for preparing carbon nanotubes. There is no specific restriction on
the arc-discharge method utilized to prepare carbon nanotubes
according to the present application. Conventional arc-discharge
methods may be used to prepare application carbon nanotubes in the
present application. The device, conditions and materials in the
arc-discharge process will be described briefly hereafter.
[0086] FIG. 2 shows a drawing of an arc furnace 100 for preparing
carbon nanotubes. Said arc furnace includes a vacuum chamber 160, a
cathode connection 110, a cathode 120, an anode 130, an anode
connection 140 and a linear motion feedthrough 150. Cathode 120 is
normally a graphite rod with large diameter (e.g. about 13 mm), or
a metal electrode such as copper. Anode 130 is a graphite rod with
small diameter (e.g. about 6 mm).
[0087] In one embodiment, the anode graphite rod used for anode 130
is prepared as follows. A hole is drilled in the center of the
anode graphite rod, in which filled with an anode mixture by
uniformly mixing graphite powders and powders of catalyst and
optional promoter, and then being compacted so as to form an anode
130 to generate arc. Alternatively, said anode 130 can be formed as
an anode graphite rod by mixing the catalyst and the optional
promoter with graphite to obtain the anode mixture and then molding
the mixture.
[0088] Before the arc discharging, the vacuum chamber 160 is
vacuumed, and then filled with protective inert gas (e.g. Helium or
argon gas), hydrogen gas, nitrogen gas or the mixture thereof. When
connected to the power supply, an arc can be stably generated
between anode 130 and cathode 120 by adjusting the distance (the
distance is generally held as a predetermined constant value, e.g.
about 1-5 mm) therebetween using a linear motion feedthrough 150.
Cathode 120 and anode 130 will not be connected at first so that no
initial current is generated, and then anode 120 was gradually
moved towards cathode 130 until an arc is generated. During the arc
discharging, the high-speed plasma flow was generated between anode
130 and cathode 120 so that the surfaces of cathode 120 and anode
130 reached very high temperature, e.g. about 3000.degree. C. and
5000.degree. C. or more, respectively and anode 130 rapidly
evaporates as carbon clusters and is gradually consumed. In high
temperature region between cathode 120 and anode 130, said carbon
clusters evaporated from anode 130 can form carbon nanotubes and
they fulfill the whole vacuum chamber to deposit on the wall of the
vacuum chamber 160 and/or cathode 120. Normally, anode is consumed
out for merely 10 min to complete arc-discharge, and then the
vacuum chamber is cooled.
[0089] After reaction and fully cooling, the following substances
may be collected in the vacuum chamber 160: cloth-like soots which
adhere on the wall of vacuum chamber 160; web-like soots which hang
between the chamber wall and cathode; deposits which adhere on one
end of the cathode; and collar-like soots surrounding the deposit.
The as-prepared carbon nanotubes normally are bound with each other
by van der Waals force, and arranged in a hexagonal crystal
structure. Carbon nanotubes, especially single-walled carbon
nanotubes mainly exist in three parts: cloth-like soots, web-like
soots and collar-like soots. Among them, the purity of carbon
nanotubes, especially single-walled carbon nanotubes in the
web-like soot is the highest, the purity of those in the cloth-like
soot is the lowest, and the purity of those in the collar-like soot
is between them. Many impurities, such as amorphous carbon and
metal catalyst particles, may be present together with the carbon
nanotubes. The impurities can be removed by subsequent purification
process, which will be specifically set forth hereafter.
[0090] In the preparation of carbon nanotubes by an arc-discharge
method according to an embodiment, it is required to use catalyst.
Catalyst exert important effect on the growth of carbon nanotubes,
especially single-walled carbon nanotubes. The catalyst used in the
present application can be transition metals or the oxides of
lanthanum metals. In addition, catalyst can be the mixtures of
nickel and a rare earth element such as Y, Ce, Er, Tb, Ho, La, Nd,
Gd, Dy or the mixture thereof. In one embodiment, catalyst are
preferably selected from Y--Ni alloy, Fe--Ni alloy, Fe--Co alloy,
Co--Ni alloy, Rh--Pt alloy, and Ce--Ni alloy.
[0091] In the method of preparing carbon nanotubes according to an
embodiment, it is optional to use a promoter, which also exerts
important effect on the growth of carbon nanotubes, especially
single-walled carbon nanotubes, and in particular, on improving the
purity and controlling the diameter distribution of carbon
nanotubes. Therefore, it is preferred that FeS is used as the
promoter in the present application.
[0092] In an embodiment, the catalyst and the promoter can be used
at any ratio, provide that the ratio used will not negatively
affect the properties such as the growth, purity and diameter
distribution of carbon nanotubes.
[0093] Normally, the weight ratio of catalyst and promoter
(catalyst/promoter) is in a range from 1:1 to 20:1, preferably in a
range from 5:1 to 15:1, and more preferably 10:1. However, other
weight ratios outside the above range are also useful if
necessary.
[0094] During the preparation of carbon nanotubes, it is also
required to use carbon source. Preferred carbon source is graphite.
In an embodiment, the catalyst and carbon source can be used at any
ratio, providing that said ratio will not negatively affect the
properties such as the growth, purity and diameter distribution of
carbon nanotubes. In one embodiment, the mole ratio of said carbon
source and catalyst is in a range from 1:1 to 50:1, preferably in a
range from 5:1 to 30:1 and more preferably 15:1.
[0095] In an embodiment, the carbon source is graphite, the
catalyst is Y--Ni alloy, and the promoter is FeS.
[0096] Protective gas normally is applied during arc discharging,
such as inert gas (e.g. Helium, argon gas, or the mixture thereof),
hydrogen gas, nitrogen gas or the mixture thereof, and the like.
Helium is a conventional protective gas. If the hydrogen gas is
applied, its pressure may be lower than that of Helium gas. Since
hydrogen gas has greater heat conductivity, and can form a C--H
bond with carbon and etch the amorphous carbon, carbon nanotubes
with higher purity can be produced. The pressure of protective gas
may be about 6.67 kPa to 203, preferably about 13.3 kPa to 160 and
more preferably about 66.7 kPa to 120 such as about 80.0 kPa to
93.3.
[0097] In order to achieve the arc discharge between the anode and
the cathode, the current is generally about 30-200 amperes (A),
preferably about 70-120 A, e.g. about 100 A. If the current is too
low, the stable arc cannot be achieved, whereas if the current is
too high, the impurities such as amorphous carbon and metal
particles will increase and render the subsequent purification
process difficult. The direct voltage used is about 20-40V, e.g.
about 30V. Since carbon nanotubes may be integrated with other
by-products such as amorphous carbon and metal particles by
sintering, they are difficult to be separated and purified. Thus,
the water-cooling is normally used to reduce the temperature of the
cathode of graphite so as to prepare carbon nanotubes with perfect
structure and higher purity. For example, the cathode of graphite
can be fixed on the copper base cooled with water to reduce its
temperature. In addition, metals with superior heat conductivity to
dissipate heat such as copper (Cu) can be used as cathode to
facilitate the formation of carbon nanotubes. During the
arc-discharge process, the temperature controlling apparatus can be
additionally used to control the temperature of vacuum chamber 160
to further avoid the increase of impurities such as amorphous
carbon and the like due to the low temperature.
[0098] Furthermore, although in the furnace 100 of FIG. 2, the arc
is generated between the opposite end sides of cathode and anode,
the cathode and anode can be placed in the same side to form a
certain angle, then the discharge between the anode and cathode is
performed in a way of point to point, and the resultants formed as
sheets adhere to the wall of vacuum chamber 160 and somewhere else.
The yield of carbon nanotubes can increase thereby.
[0099] After the arc-discharge process, web-like soot is usually
collected for the following purification process since the purity
of carbon nanotubes, especially single-walled carbon nanotubes in
the web-like soot is the highest.
[0100] (2) The Purification Process--Forming a Complex and Removing
the Complex
[0101] As previously described, oxidation at a high temperature or
oxidation in a strong acid (reflux) is generally employed to remove
impurities in carbon nanotubes, especially to remove the residual
catalyst and/or optional promoter. But said methods damage the
carbon nanotubes, especially in the sidewalls of the carbon
nanotubes.
[0102] In order to eliminate the damage caused by oxidation at a
high temperature or oxidation in a strong acid in carbon nanotubes,
the inventor has found a method based on coordination chemistry to
remove the residual catalyst and/or optional promoter in carbon
nanotubes without damaging the carbon nanotubes through in-depth
researches.
[0103] Compared with the conventional purification process, the
purification process based on coordination chemistry is gentle and
does not damage the sidewalls of the carbon nanotubes. Hence, the
quality of carbon nanotubes as prepared according to the present
application is superior to that of conventional methods,
particularly in the electric conductivity.
[0104] The inventor has found that a substance capable of forming a
complex with metal elements present in the catalyst and/or the
optional promoter to produce a complex can be employed to
coordinate said metal elements to facilitate removing the catalyst
and/or the optional promoter.
[0105] As used herein, the term "a substance capable of forming a
complex with metal elements present in the catalyst and/or the
optional promoter to produce a complex" refers to a substance
capable of forming a complex with metal elements present in the
catalyst or the promoter to produce a complex, or a substance
capable of forming a complex with metal elements present in the
catalyst and the promoter to produce a complex. As previously
described, the catalyst used in the present application may be
transition metals or lanthanum metal oxides. In addition, the
catalyst may also be the mixture of the metal nickel (Ni) and rare
earth elements such as Y, Ce, Er, Tb, Ho, La, Nd, Gd, Dy or a
mixture thereof. Therefore, a substance capable of forming a
complex with said transition metal elements and rare earth metal
elements to produce a complex should be preferably selected as "a
substance capable of forming a complex with metal elements present
in the catalyst and/or the optional promoter to produce a complex"
to facilitate removing the catalyst and/or the optional promoter in
the prepared carbon nanotubes.
[0106] Since the catalyst in an arc discharge are typically
selected from Y--Ni alloy, Fe--Ni alloy, Fe--Co alloy, Co--Ni
alloy, Rh--Pt alloy, and Ce--Ni alloy, a substance capable of
coordination with Y, Ni, Fe, Co, Rh, Pt or Ce is preferably
selected as "a substance capable of forming a complex with the
metal elements present in the catalyst and/or the optional
promoter".
[0107] When a promoter is employed in the arc discharge to produce
carbon nanotubes, the amount of the promoter is negligible compared
with that of the catalyst. Likewise, the amount of the promoter in
the prepared carbon nanotubes is also negligible compared with that
of the catalyst. Hence, it is suitable that only the metal elements
present in the catalyst is taken into account for selecting the
substances to form a complex.
[0108] Even so, it is more preferred that both the metal elements
present in the catalyst and the metal elements present in the
promoter are taken into account. A substance capable of forming
complexes with all the metal elements present in the catalyst and
the promoter is further preferred so that all the residual catalyst
and promoter can be removed by just a substance.
[0109] Although some substances can coordinate with elementary
metals (0 valent metals) to form a complex, it is difficult to
directly form a complex between substances and metals employed in
an arc discharge process since the metals are present as alloys in
the arc discharge process. Therefore, it is preferred that the
metal elements present in the catalyst and/or the optional promoter
are converted into ions for the coordination.
[0110] Various methods of converting the metal elements into ions
can be used without any specific restriction. For example, strong
acid oxidation can be used to convert the metal elements into ions.
However, In order to minimize the damage in the prepared carbon
nanotubes, metal ions can be obtained by suitable acids treatment
after suitable oxidation of the metal elements into metal oxides.
Since the reaction condition of the oxidation and the acid
treatment is gentler than that of conventional purification
methods, the quality of the resulting carbon nanotubes is not
significantly degraded.
[0111] In an embodiment, since "a substance capable of forming a
complex with metal elements present in the catalyst and/or the
optional promoter to produce a complex" is an acid such as an
aminopolycarboxylic acid which can not only convert the metal
oxides into metal ions but also coordinate with the metal ions to
form complexes. No other acid is required to achieve the metal
ions.
[0112] In an embodiment, in order to improve their solubility in a
solvent (typically in water) to facilitate removing the complexes,
the formed complexes may be converted into other suitable forms
such as salt forms. Said conversion improves the solubility of
complexes and facilitates separating insoluble carbon nanotubes
from complexes with the residual catalyst and/or optional promoter
minimized by a method such as filtration. Any filtration medium,
such as Polytetrafluoroethylene filter membrane, can be used in the
above filtration.
[0113] For example, when aminopolycarboxylic acids are used as
substances, it is preferred that the complexes formed by the
aminopolycarboxylic acid are converted into the salt forms. A basic
solution such as NaOH and KOH may be added to adjust the pH into
basic and convert the complexes into the salt forms.
[0114] When aminopolycarboxylic acids are employed in the method of
the first aspect of the present application, there is no any
restriction on the aminopolycarboxylic acids. For example,
ethylenediaminetetraacetic acid (EDTA),
trans-1,2-diaminocyclohexane-N,N,N',N'-tetracetic acid hydrate
(CYDTA), diethylenetriaminepentaacetic acid (DTPA), or
triethylenetetraaminehexaacetic acid (TTHA) may be used.
[0115] When aminopolycarboxylic acids are used for coordination
with Y to form a complex, the preferred aminopolycarboxylic acids
are ethylenediaminetetraacetic acid (EDTA),
trans-1,2-diaminocyclohexane-N,N,N',N'-tetracetic acid hydrate
(CYDTA), diethylenetriaminepentaacetic acid (DTPA) and
triethylenetetraaminehexaacetic acid (TTHA). The structures are as
follows:
##STR00001##
[0116] The structures of the complexes formed by Y and EDTA, CYDTA,
DTPA or TTHA are as follows:
##STR00002##
[0117] The structure of the complex formed by Y and EDTA
##STR00003##
[0118] The structure of the complex formed by Y and CYDTA
##STR00004##
[0119] The structure of the complex formed by Y and DTPA
##STR00005##
[0120] The structure of the complex formed by Y and TTHA
[0121] In addition, TTHA can also be coordinated with Ni to form
the following complex:
##STR00006##
[0122] Among them, triethylenetetraaminehexaacetic acid (TTHA) is
most preferred since the purity and the transparence of the carbon
nanotubes purified with TTHA is superior to that of the carbon
nanotubes purified with other aminopolycarboxylic acids.
[0123] FIG. 3 is a schematic drawing of the purification of carbon
nanotubes by using CYDTA. It can be seen from FIG. 3 that CYDTA is
coordinated with Y present in the carbon nanotubes to form a
complex and separated from the carbon nanotubes.
[0124] In addition, suitable substances that can be coordinated
with Y to form complexes include, but are not limited to,
tetrahydrofuran, trialkyl phosphine, .epsilon.-caprolactone,
.epsilon.-caprolactam, dimethyl formamide, and dimethyl sulfoxide.
The substances that can be coordinated with Y to form complexes and
the complexes are described in Shashank Mishra "Anhydrous scandium,
yttrium, lanthanide and actinide halide complexes with neutral
oxygen and nitrogen donor substances" Coordination Chemistry
Review, 2008, 252, 1996-2025, which is incorporated herein by
reference. All the substances that can be coordinated with Y to
form complexes listed in that document may be used to remove the
catalyst such as Y in the Y--Ni alloys.
[0125] One exemplary example of the complexes of Ni, Co or Fe
according to the present application is
{M[(NC).sub.2CC(OCH.sub.2CH.sub.2OH)C(CN).sub.2].sub.2(4,4'-bpy)(H.sub.2O-
).sub.2}, wherein M is selected from Ni, Fe and Co; bpy is
bipyridine, see Inorganica Chemica Acta, 2008, 361, 3856-3862. The
compounds and substances listed in the document to form said
complexes are suitable for the present application. Another
exemplary example of the complexes of Ni, Co or Fe according to the
present application is Dinuclear
[{M'(phen).sub.2}.sub.2V.sub.4O.sub.12]C.sub.6H.sub.12O.H.sub.2O,
wherein M' is selected from Ni and Co; phen is phenyl, see
Inorganica Chemica Acta, 2008, 361, 3681-3689. The compounds and
substances listed in the document to form said complexes are also
suitable for the present application.
[0126] Complexes of Ni suitable for the present application also
include [Ni(L)(H.sub.2O).sub.3]2H.sub.2O, wherein L is
(2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid;
Ni[P(Ph.sub.2)--N(H)--CH.sub.2Py].sub.4, wherein Ph is phenyl, Py
is pyridine. As for suitable substances and compounds to form
complexes, please refer to Inorganica Chemica Acta, 2008, 361,
3723-3729; Journal of Organometallic Chemistry, 2008, 693,
2171-2176; and Inorganic Chemistry Communications, 2008, 11,
1023-1026 herein incorporated by reference.
[0127] As for suitable substances and compounds to form complexes,
please refer to in Inorganica Chimica Acta, 2008, 361, 3926-3930;
Coordination Chemistry Reviews, 2002, 229, 27-35; Coordination
Chemistry Reviews, 2002, 232, 151-171; and Coordination Chemistry
Reviews, 2002, 234, 273-287 herein incorporated by reference.
[0128] Some non-limiting examples of suitable the substances that
can be coordinated with Fe, Co or Ni to form complexes are
described in Coordination Chemistry Reviews, 1974, 12, 151-184;
Coordination Chemistry Reviews, 1973, 11, 343-402; and Coordination
Chemistry Reviews, 1967, 2, 173-193.
[0129] All the above documents are incorporated herein by
reference.
[0130] The substances can be provided in forms such as alloy,
elementary substance and compound, of the catalyst and/or the
optional promoter and the species of the metal elements
present.
[0131] Furthermore, the coordination step is carried out in a
manner according to the specific catalyst and/or optional promoter.
For example, it is dependent on the specific catalyst and/or
optional promoter whether the catalyst and/or optional promoter
need to be converted into ions. When oxidation is employed to
convert the catalyst and/or optional promoter into ions, it further
needs to be determined whether an acid other than the substances is
necessary for the conversion.
[0132] As previously described, in order to convert metal elements
present in the catalyst and/or optional promoter into ions, the
metal elements may be converted into oxides by moderate oxidation
at first. It is preferred that an oxygen-containing gas such as air
is employed to oxidize the catalyst and/or optional promoter. The
oxidation condition of said oxygen-containing gas is gentler than
that of conventional purification by gas phase oxidation.
Generally, there is no specific restriction on the oxidation time
and the oxidation temperature for the oxygen-containing gas to
oxidize the catalyst and/or optional promoter, provided that the
oxidation time and the oxidation temperature are sufficient to
convert the catalyst and/or the optional promoter into oxides. The
selected oxidation temperature is typically about 80.degree. C. to
about 300.degree. C., more preferably about 100.degree. C. to about
200.degree. C., most preferably about 150.degree. C. to about
200.degree. C. The oxidation time can vary according to the
selected oxidation temperature. The selected oxidation time is
typically about 1 hour to about 20 hours, more preferably about 5
hours to about 15 hours, most preferably about 8 hours to about 10
hours. It can be seen that the oxidation employed in the
preparation of carbon nanotubes according to the present invention
is at low temperature compared with the high temperature oxidation
employed in conventional methods (typically at about 470.degree.
C.). Therefore, the low temperature oxidation does not damage the
carbon nanotubes.
[0133] The process for preparing carbon nanotubes can further
include centrifugation to remove the residual amorphous carbon
incorporated in the carbon nanotubes after removing the complex,
thereby so as to further improve the purity of the carbon
nanotubes. Although any speed may be used for the centrifugation
step, a higher centrifugation speed is preferred. For example, the
centrifugation step may be carried out at a speed of about 5000 rpm
to about 30000 rpm, preferably about 10000 rpm to about 20000 rpm.
The selected time for centrifugation actually depends on the
selected speed for centrifugation, and the centrifugation step is
carried out typically for about 1 hour to about 20 hours,
preferably about 2 hours to about 10 hours, for example, 3
hours.
[0134] The preferred carbon nanotubes are single-walled carbon
nanotubes in an embodiment, including metallic single-walled carbon
nanotubes (M-SWNT), semiconductor single-walled carbon nanotubes
(S-SWNT) and the combinations thereof.
[0135] In another embodiment, the present application provides a
process for purifying carbon nanotubes, wherein the carbon
nanotubes are synthesized using an arc-discharge method in the
presence of a catalyst and optional promoter. The process
includes:
[0136] coordinating metal elements present in the catalyst and/or
the optional promoter with a substance capable of forming a complex
with said metal elements to produce a complex; and
[0137] removing the complex.
[0138] According to an embodiment, carbon nanotubes produced by an
arc discharge method can be purified, so as to improve the purity
of the carbon nanotubes and properties thereof.
[0139] There is no specific restriction on carbon nanotubes,
provided that the carbon nanotubes are produced by an arc discharge
method.
[0140] In general, the catalyst used is a transition metal, the
oxide of lanthanum metal, or the mixture thereof. In addition, the
catalyst can be the mixture of nickel and a rare earth element such
as Y, Ce, Er, Tb, Ho, La, Nd, Gd, Dy or mixture thereof. In one
embodiment, the catalyst is preferably selected from Y--Ni alloy,
Fe--Ni alloy, Fe--Co alloy, Co--Ni alloy, Rh--Pt alloy, or Ce--Ni
alloy.
[0141] A promoter such as FeS is typically employed in the arc
discharge process.
[0142] The residual catalyst and promoter can be efficiently
removed from the carbon nanotubes produced by an arc discharge
method by the purification process according to an embodiment
without damaging the quality of the carbon nanotubes, particularly
in the electric property.
[0143] According to an embodiment, "a substance capable of forming
a complex with the metal elements present in the catalyst and/or
the optional promoter" is employed to coordinate with the metal
elements to form complexes which are removed for purifying carbon
nanotubes.
[0144] In an embodiment, the substance capable of forming a complex
with said transition metal elements and rare earth metal elements
should be selected as "the substance capable of forming a complex
with the metals present in the catalyst and/or the optional
promoter" to facilitate removing the catalyst from the carbon
nanotubes.
[0145] Since the catalyst in an arc discharge are typically
selected from Y--Ni alloy, Fe--Ni alloy, Fe--Co alloy, Co--Ni
alloy, Rh--Pt alloy, and Ce--Ni alloy, a substance capable of
coordination with Y, Ni, Fe, Co, Rh, Pt or Ce is selected as "a
substance capable of forming a complex with the metal elements
present in the catalyst and/or the optional promoter".
[0146] When a promoter is employed in the arc discharge to produce
carbon nanotubes, the amount of the promoter is negligible compared
with that of the catalyst. Likewise, the amount of the promoter in
the prepared carbon nanotubes is also negligible compared with that
of the catalyst. Hence, it is suitable that only the metal elements
present in the catalyst is taken into account for selecting the
substances to form a complex.
[0147] Even so, it is more preferred that both the metal elements
present in the catalyst and the metal elements present in the
promoter are taken into account. A substance capable of forming
complexes with all the metal elements present in the catalyst and
the promoter is further preferred so that all the residual catalyst
and promoter can be removed by just a substance.
[0148] Although some substances can coordinate with elementary
metals (0 valent metals) to form a complex, it is difficult to
directly form a complex between substances and metals employed in
an arc discharge process since the metals are present as alloys in
the arc discharge process. Therefore, it is preferred that the
metal elements present in the catalyst and/or the optional promoter
are converted into ions for the coordination according to an
embodiment.
[0149] Various methods for converting the metal elements into ions
can be used without any specific restriction. For example, strong
acid oxidation can be used to convert the metal elements into ions.
However, in order to minimize the damage in the prepared carbon
nanotubes, metal ions can be obtained by suitable acids treatment
after suitable oxidation of the metal elements into metal oxides.
Since the reaction condition of the oxidation and the acid
treatment is gentler than that of conventional purification
methods, the quality of the resulting carbon nanotubes is not
significantly degraded.
[0150] In an embodiment, since "a substance capable of forming a
complex with metal elements present in the catalyst and/or the
optional promoter to produce a complex" is an acid such as an
aminopolycarboxylic acid which can not only convert the metal
oxides into metal ions but also coordinate with the metal ions to
form complexes, no other acid is required to achieve the metal
ions.
[0151] In an embodiment, in order to improve their solubility in a
solvent (typically in water) to facilitate removing the complexes,
the formed complexes may be converted into other suitable forms
such as salt forms. The conversion improves the solubility of
complexes and facilitates separating insoluble carbon nanotubes
from complexes with the residual catalyst and/or optional promoter
minimized by a method such as filtration. Any filtration medium,
such as Polytetrafluoroethylene filter membrane, can be used in the
above filtration.
[0152] For example, when aminopolycarboxylic acids are used as
substances, it is preferred that the complexes formed by the
aminopolycarboxylic acid are converted into the salt forms. A basic
solution such as NaOH and KOH may be added to adjust the pH into
basic and convert the complexes into the salt forms.
[0153] When aminopolycarboxylic acids are employed, there is no
specific restriction on the aminopolycarboxylic acids. For example,
ethylenediaminetetraacetic acid (EDTA),
trans-1,2-diaminocyclohexane-N,N,N',N'-tetracetic acid hydrate
(CYDTA), diethylenetriaminepentaacetic acid (DTPA), or
triethylenetetraaminehexaacetic acid (TTHA) may be used, according
to an embodiment.
[0154] When aminopolycarboxylic acids are used for coordination
with Y to form a complex, the preferred aminopolycarboxylic acids
are ethylenediaminetetraacetic acid (EDTA),
trans-1,2-diaminocyclohexane-N,N,N',N'-tetracetic acid hydrate
(CYDTA), diethylenetriaminepentaacetic acid (DTPA) and
triethylenetetraaminehexaacetic acid (TTHA), according to an
embodiment. The structures are as follows:
##STR00007##
[0155] The structures of the complexes formed by Y and EDTA, CYDTA,
DTPA or TTHA are as follows:
##STR00008##
[0156] The structure of the complex formed by Y and EDTA
##STR00009##
[0157] The structure of the complex formed by Y and CYDTA
##STR00010##
[0158] The structure of the complex formed by Y and DTPA
##STR00011##
[0159] The structure of the complex formed by Y and TTHA.
[0160] In addition, TTHA can also be coordinated with Ni to form a
complex:
##STR00012##
[0161] Among them, triethylenetetraaminehexaacetic acid (TTHA) is
most preferred since the purity and the transparence of the carbon
nanotubes purified with TTHA is superior to that of the carbon
nanotubes purified with other aminopolycarboxylic acids, according
to an embodiment.
[0162] In addition, suitable substances that can be coordinated
with Y to form complexes include, but are not limited to,
tetrahydrofuran, trialkyl phosphine, .epsilon.-caprolactone,
.epsilon.-caprolactam, dimethyl formamide, and dimethyl sulfoxide.
The substances that can be coordinated with Y to form complexes and
the complexes are described in Shashank Mishra "Anhydrous scandium,
yttrium, lanthanide and actinide halide complexes with neutral
oxygen and nitrogen donor substances" Coordination Chemistry
Review, 2008, 252, 1996-2025, which is incorporated herein by
reference. All the substances that can be coordinated with Y to
form complexes listed in aforesaid document may be used to remove
the catalyst such as Y in the Y--Ni alloys.
[0163] One exemplary example of the complexes of Ni, Co or Fe
according to an embodiment is
{M[(NC).sub.2CC(OCH.sub.2CH.sub.2OH)C(CN).sub.2].sub.2(4,4'-bpy)(H.sub.2O-
).sub.2}, wherein M is selected from Ni, Fe and Co; bpy is
bipyridine; see Inorganica Chemica Acta, 2008, 361, 3856-3862. The
compounds and substances listed in the document to form the
complexes are suitable for the present application. Another
exemplary example of the complexes of Ni, Co or Fe according to the
present application is Dinuclear
[{M'(phen).sub.2}.sub.2V.sub.4O.sub.12]C.sub.6H.sub.12O.H.sub.2O,
wherein M' is selected from Ni and Co; phen is phenyl, see
Inorganica Chemica Acta, 2008, 361, 3681-3689. The compounds and
substances listed in the document to form said complexes are also
suitable for the present application.
[0164] Complexes of Ni suitable for the present application also
include [Ni(L)(H.sub.2O).sub.3]2H.sub.2O, wherein L is
(2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid;
Ni[P(Ph.sub.2)--N(H)--CH.sub.2Py].sub.4, wherein Ph is phenyl, Py
is pyridine. Other suitable substances and compounds to form
complexes provided, for example in, Inorganica Chemica Acta, 2008,
361, 3723-3729; Journal of Organometallic Chemistry, 2008, 693,
2171-2176; and Inorganic Chemistry Communications, 2008, 11,
1023-1026. Inorganica Chimica Acta, 2008, 361, 3926-3930;
Coordination Chemistry Reviews, 2002, 229, 27-35; Coordination
Chemistry Reviews, 2002, 232, 151-171; and Coordination Chemistry
Reviews, 2002, 234, 273-287.
[0165] Non-limiting examples of suitable substances that can be
coordinated with Fe, Co or Ni to form complexes are described in,
for example, Coordination Chemistry Reviews, 1974, 12, 151-184;
Coordination Chemistry Reviews, 1973, 11, 343-402; and Coordination
Chemistry Reviews, 1967, 2, 173-193.
[0166] All the above documents are incorporated herein by
reference.
[0167] The substances can be selected according to the forms (such
as alloy, elementary substance and compound) of the catalyst and/or
the optional promoter and the species of the metal elements
present.
[0168] Furthermore, the coordination step is carried out in a
manner according to the specific catalyst and/or optional promoter.
For example, it depends on the specific catalyst and/or optional
promoter whether the catalyst and/or optional promoter need to be
converted into ions. When oxidation is employed to convert the
catalyst and/or optional promoter into ions, it further needs to be
determined whether an acid other than said the substances is
necessary for the conversion.
[0169] As previously described, in order to convert metal elements
present in the catalyst and/or optional promoter into ions, the
metal elements may be firstly converted into oxides by moderate
oxidation. It is preferred that an oxygen-containing gas such as
air is employed to oxidize the catalyst and/or optional promoter.
The oxidation condition of said oxygen-containing gas is gentler
than that of conventional purification by gas phase oxidation.
Generally, there is no specific restriction on the oxidation time
and the oxidation temperature for the oxygen-containing gas to
oxidize the catalyst and/or optional promoter, provided that the
oxidation time and the oxidation temperature are sufficient to
convert the catalyst and/or the optional promoter into oxides. The
selected oxidation temperature is typically about 80.degree. C. to
about 300.degree. C., more preferably about 100.degree. C. to about
200.degree. C., most preferably about 150.degree. C. to about
200.degree. C. The oxidation time can vary according to the
selected oxidation temperature. The selected oxidation time is
typically about 1 hour to about 20 hours, more preferably about 5
hours to about 15 hours, most preferably about 8 hours to about 10
hours. It can be seen that the oxidation employed in the
purification of carbon nanotubes according to the present invention
is at low temperature compared with the high temperature oxidation
employed in conventional methods. Therefore, said low temperature
oxidation does not damage the carbon nanotubes.
[0170] The process for purifying carbon nanotubes can further
include centrifugation to remove the residual amorphous carbon
incorporated in the carbon nanotubes after removing the complex,
thereby so as to further improve the purity of the carbon
nanotubes. Although any speed may be used for the centrifugation
step, a higher centrifugation speed is preferred. For example, the
centrifugation step may be carried out at a speed of about 5000 rpm
to about 30000 rpm, preferably about 10000 rpm to about 20000 rpm.
The selected time for centrifugation actually depends on the
selected speed for centrifugation, and the centrifugation step is
carried out typically for about 1 hour to about 20 hours,
preferably about 2 hours to about 10 hours, for example, 3
hours.
[0171] In a further embodiment, the present application provides
carbon nanotubes as prepared and purified according to an
embodiment described in the present application.
[0172] According to the number of the layers of the carbon atoms
forming the wall, the carbon nanotubes may include single-walled
carbon nanotubes, multi-walled carbon nanotubes and the
combinations thereof. According to their electrical property, the
carbon nanotubes may include metallic carbon nanotubes,
semiconductor carbon nanotubes and the combination thereof.
However, the preferred carbon nanotubes are single-walled carbon
nanotubes, including metallic single-walled carbon nanotubes
(M-SWNT), semiconductor single-walled carbon nanotubes (S-SWNT) and
the combinations thereof according to an embodiment.
[0173] As described before, the process of preparing carbon
nanotubes and the process of purifying carbon nanotubes do not
damage the sidewalls of the carbon nanotubes. Furthermore, the
properties, particularly electric conductivity of the carbon
nanotubes are not degraded.
[0174] As showed in FIG. 1, typically known carbon nanotubes are
defective due to the limitation of preparation and purification
conditions. However, carbon nanotubes according to an embodiment
are smooth in the sidewalls and not damaged, as shown in FIG. 7.
Furthermore, as shown in FIG. 8 and FIG. 9, the purity of carbon
nanotubes of the present application is desirably high, and the
quality is desirably good.
[0175] In another embodiment, the present application provides an
element of carbon nanotubes, including the carbon nanotubes as
described in the present application.
[0176] The elements of carbon nanotubes include but not limited to
conductive film of carbon nanotubes, field emission source,
transistor, conductive wire, electrode material (such as
transparent, porous or gas diffusion electrode material)
nano-electro-mechanic system (NEMS), spin conduction device, nano
cantilever, quantum computing device, lighting emitting diode,
solar cell, surface-conduction electron-emitter display, filter
(such as high frequency filter or optical filter), drug delivery
system, thermal conductive material, nano nozzle, energy storage
system (such as hydrogen storage material), space elevator, fuel
cell, sensor (such as gas, glucose or ion sensor), or catalyst
carrier.
[0177] The following examples are provided to illustrate the
elements of carbon nanotubes, whereas the present application is
not limited to those examples.
[0178] 1. Conductive Film of Carbon Nanotubes
[0179] Carbon nanotubes combine strength and flexibility and are
excellent candidates for flexible electronic components. In
particular, flexible, transparent, conductive thin films made of
CNTs have attracted much attention, partly because of the
applications in electroluminescent, photoconductor and photovoltaic
devices.
[0180] Although the optically transparent and highly conductive
indium tin oxide (ITO) has enjoyed widespread use in optoelectronic
applications, the inherent brittleness of ITO severely limits film
flexibility. The properties of CNT thin films are suitable to
replace ITO. For instance, CNT films can be repeatedly bent without
fracture. The thin films with low sheet resistance are also
transparent in the visible and infrared range. Furthermore, both
the low cost and tunable electronic properties offer additional
advantages for CNT thin films.
[0181] Conductive films of carbon nanotubes can be fabricated
according to the present application as follows:
[0182] Typically, 10 mg of carbon nanotubes are dispersed in 200 ml
of 1 wt. % aqueous octyl-phenol-ethoxylate (denoted Triton X-100)
solution for 20 min in an ultrasonic bath. The dispersion is
filtered out with a mixed cellulose ester (MCE) filter membrane
(Millipore, 0.2 .mu.m pore), and the resulting carbon nanotube film
is formed on the membrane in a vacuum filtration apparatus
(Millipore). Substantially all of the Triton X-100 on the obverse
of the carbon nanotube film is dialyzed against a
Tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffer (50
mM, PH 7.5) for two days. The Tris-HCl buffer is subsequently
washed away with purified water, then the carbon nanotube films are
transferred onto a quartz substrate. After drying the sample for 1
h at 90.degree. C., the filter membrane is removed by using acetone
vapor. Finally, the carbon nanotube films are dried in a vacuum at
100.degree. C. for 1 h.
[0183] The method for preparing the carbon nanotube films,
particularly employing acetone vapor to remove the filter membrane,
described in Chinese Patent Application No. 200810005631.7 filed on
Feb. 14, 2008, which is hereby incorporated by reference.
[0184] For example, the vapor generator shown in FIG. 12 may be
used. FIG. 12(a) shows a vapor generator used in the present
invention. FIG. 12(b) provides a schematic cross-section view of
glass casing of vapor generator shown in FIG. 12(a).
[0185] The vapor generator includes:
[0186] a glass casing with a condensing device, having a porous
support station in the inner of the casing for disposing a sample,
an inlet of a condensing media located on the lower portion of the
casing and an outlet of the condensing media located on the upper
portion of the casing, and the height of the station is nearly the
same as that of the inlet of the condensing media;
[0187] a vessel, such as a round-bottomed flask, to hold solvent
(such as acetone);
[0188] a heating device, such as a temperature-adjustable heating
jacket, to heat the solvent; and
[0189] an optional mixing device, such as a magnetic stirrer.
[0190] The porous support station is made of glass, for example.
There is no specific restriction on the pore size of said station,
provided that a sufficient amount of vapors can flow through the
station and a sample can be supported by the station. The size of
the station is determined by the inner diameter of the glass
casing.
[0191] 2. Carbon Nanotubes Field Effect Transistor
[0192] Single carbon nanotube and a bundle thereof can be used to
construct the basic element of nano-electro-element, that is,
carbon nanotubes field effect transistor (CNT FET). Carbon
nanotubes in the as-prepared products normally exist in bundles
other than separately. That is, several, even hundreds of carbon
nanotubes are combined parallel in the same axial orientation, to
form carbon nanotubes bundles with diameters varying from several
to decades of nanometers. However, in order to apply said CNT FET
to nano-electro-elements, single or small size carbon nanotubes
should be firstly separated from the carbon nanotubes bundles.
[0193] The diameters of carbon nanotubes can be the same with each
other in bundles, and arrange in a close packed form so that the
bundles themselves exhibit crystallization in some extent.
Normally, the separation of the carbon nanotubes bundle or carbon
nanotubes in bundles from each other is achieved by dispersing the
powder of carbon nanotubes into the organic solution, and then by
carrying out the ultrasonic treatment for a long time. The effect
depends on the type of solutions, the time of ultrasonic treatment
and so on. The commonly used solutions include alcohol,
isopropanol, acetone, carbon tetrachloride, dichloroethane,
dimethyl formamide (DMF) and the like.
[0194] 3. Transistor--Nano-Electro-Dynatron
[0195] There are two kinds of nano-electro-dynatrons at present:
single electron transistor (SET) and carbon nanotubes dynatron. The
latter is also called field effect transistor (FET), including
carbon nanotubes between source and drain electrodes, and the
transportation of electron (or hole) via carbon nanotubes is
controlled by gate voltage.
[0196] One typical FET preparation process is as follow: as
above-mentioned, the primary products of the carbon nanotubes
normally are bundles which are winded together. Firstly, they are
fully separated by ultrasonic treatment in organic solution (e.g.
alcohol), and then the liquid is dropped onto the chips of silicon
with SiO.sub.2 surface. Numerous metal electrodes are prepared on
the chips of silicon by traditional photoetching, metal
evaporation, or screen printing. Then, an atomic force microscope
(AFM) is used to detect whether single carbon nanotube or the
bundle thereof connecting two electrodes is present. Those two
electrodes will be used as source and drain electrodes of
as-prepared FET. The typical distance between the two electrodes is
about 100 nm, for example, varying from 0.1 micron to 1 micron.
Another electrode under SiO.sub.2 layer or doped silicon substrate
is used as a gate electrode of FET, to control the current through
carbon nanotubes by applying the gate voltage, and thereby the FET
thus prepared is a bottom gate FET. Of course, a top gate FET can
also be prepared as follows: firstly, carbon nanotubes or bundles
thereof are prepared on the substrate to connect source and drain
electrodes, and then the gate insulating layer is deposited
thereon, and then the gate electrode is prepared on the insulating
layer above the carbon nanotubes or bundles thereof by screen
printing. Alternatively, single carbon nanotube or the bundle
thereof can be firstly sputtered to the substrate in a given
orientation, and then the electrodes are deposited on two ends of
said carbon nanotubes or bundle thereof by electron beam. However,
the process might break the carbon nanotubes between two
electrodes.
[0197] The relation between transmission result and gate voltage
(I-V property) is detected at room temperature. In the detection,
the linear conductivity of metal carbon nanotubes are not or weakly
affected by the gate voltage, whereas semiconductor carbon
nanotubes show strong dependence on gate voltage.
EXAMPLES
[0198] The following examples further illustrate the present
application according to an embodiment. Unless otherwise indicated,
various raw materials and reagents used in the present application
are commercially available, or can be prepared in an conventional
manner.
[0199] The sources of the main raw materials are summarized as
follows:
[0200] Y--Ni alloy catalyst is purchased from General Research
Institute for Nonferrous Metals.
[0201] Graphite rod is purchased from Shanghai Carbon Works.
[0202] FeS is purchased from Beijing Yili Chemical Co., Ltd.
[0203] NaOH and o-dichlorobenzene are purchased from Beijing
Chemical Works.
[0204] Ethylenediaminetetraacetic acid (EDTA),
trans-1,2-diaminocyclohexane-N,N,N',N'-tetracetic acid hydrate
(CYDTA), diethylenetriaminepentaacetic acid (DTPA) and
triethylenetetraaminehexaacetic acid (TTHA) are purchased from Alfa
Aesar.
[0205] Triton X-100 is purchased from Acros.
[0206] Tri(hydroxymethyl)aminomethane is purchased from Acros
99%.
[0207] HCl aqueous solution is purchased from Beijing Chemical
Works, with a HCl content of 36-38%.
[0208] Characterization
[0209] As-prepared carbon nanotubes can be characterized as
follows:
[0210] Raman spectroscopy data are obtained with Renishaw 100
micro-Raman System.
[0211] X-ray photoelectron spectroscopy (XPS) data are obtained
with an ESCALab220i-XL electron spectrometer from VG Scientific
using 300 W Al k.alpha. radiation.
[0212] Scanning electron microscope data are obtained with JEOL
JSM-6700F.
[0213] Transmission electron microscope data are obtained with
JEOL-2010, 200 kV.
[0214] The sheet resistances of the carbon nanotube films are
measured by Loresta-EP MCP-T360 with a 4-pin probe, and the
transparency of the carbon nanotube films are measured by
UV-vis-NIR Spectrophotometer (JASCO V-570).
[0215] Raman spectroscopy is one of the useful methods to detect
carbon nanotubes, which not only shows the regularity and purity of
the sample, but also defines the diameter distribution of carbon
nanotubes. The sample can be treated as follows to preclude the
effect of carbon nanotubes in bundles imposed on the result of
Raman spectroscopy detection: ultrasonic treating for 5 min in
ethanol, then dropping the obtained suspension onto the glasses and
drying in air.
[0216] In the Raman spectra, there are three peaks or regions we
are concerned about, the radial breathing modes (RBM) (about
100-300 cm.sup.-1), D band (.about.1350 cm.sup.-1), and G band
(.about.1570 cm.sup.-1) (see M. S. Dresselhaus, et al., Raman
Spectroscopy of Carbon Nanotubes in 1997 and 2007, J. Phys. Chem.
C, 111(48), 2007, 17887-17893). The RMB peaks are the
characteristic peaks of carbon nanotubes, corresponding with the
diameters of carbon nanotubes. From the RBM peaks, we can tell the
distribution of carbon nanotubes diameters. According to the
relation (see Araujo, P. T., et al., Third and fourth optical
transitions in semiconducting carbon nanotubes. Phys. Rev. Lett.,
98, 2007, 067401.) .omega..sub.RBM=A/d.sub.t+B, with A=217.8.+-.0.3
cm.sup.-1 nm and B=15.7.+-.0.3 cm.sup.-1, where .omega..sub.RBM
refers to the wave number at the RBM peak in cm.sup.-1, and d.sub.t
refers to the diameter of carbon nanotubes in nm, we can infer the
diameter distribution of the as-prepared carbon nanotubes. The D
band and G band are corresponding to amorphous carbon and graphitic
carbon, respectively. We can estimate the purity of carbon
nanotubes by the intensity ratio of G band and D band (G/D). The
larger G/D is, and the more graphitic carbon are, and the less
impurities or defects are, so the purity is higher.
Preparation Example 1
[0217] In the arc furnace 100 as shown in FIG. 2, the anode 130 is
a 100 mm graphite rod with a diameter of 6 mm and the cathode 120
is a graphite rod with a diameter of 8 mm. A hole 4 mm in diameter
and 80 mm in length was drilled in one end of anode graphite rod,
which was filled with a powder mixture of high purity graphite
powders, YNi.sub.4.2 alloy powder as metal catalyst and FeS powder
as promoter wherein the mole ratio between carbon and the catalyst
is 15:1, the weight ratio between the catalyst and the promoter is
10:1. Then the powders filled in the hole were compacted. The
cathode was fixed on a water-cooled copper base. The arc furnace
100 was vacuumized to about 3.0 Pa, then the vacuum valve was
closed, and Helium gas was filled to reach about 0.07 MPa. After
the power supply was connected, the current and the voltage were
controlled at about 80-120 A and about 20-25V, respectively, and
the distance between two electrodes was maintained about 3 mm by
manually adjusting the cathode, so that a stably arc-discharge was
provided.
[0218] The three parts of the samples were collected: cloth-like
soot, which adhere on the wall of chamber, web-like soot, which
hang between the wall of the chamber and cathode, and collar-like
soot, which adhere on one end of the cathode. Among those three
parts, the purity of the web-like soot is highest, that of the
cloth-like soot is lowest, and that of the collar-like soot is
between them.
[0219] The web-like soot is used for the following examples as the
pristine sample.
Purification Example 1
[0220] Firstly, 10 mg of the pristine samples were annealed at
200.degree. C. using an air flow of 20 ml/min for 10 hours. The
samples as annealed were dispersed in deionized water and
ultrasonic treatment for 30 min. 0.5M EDTA aqueous solution was
prepared and then added into the carbon nanotube dispersion. After
the mixture was refluxed in 110.degree. C. for 18 hours, pH value
was adjusted to about 8 by using 1M NaOH solution. Subsequently the
dispersion was filtrated with 0.5 .mu.m porous
polytetrafluoroethylene filter and rinsed by hot water for many
times. Then the samples are dispersed in o-dichlorobenzene and
centrifuged at 15000 rpm for 3 hours. The supernatant was decanted
and collected via filtration with a mixed cellulose ester (MCE)
membrane filter.
Purification Example 2
[0221] The similar procedure as used in Purification Example 1 was
followed, except that 0.5M EDTA aqueous solution was replaced by
0.5M CYDTA aqueous solution.
Purification Example 3
[0222] The similar procedure as used in Purification Example 1 was
followed, except that 0.5M EDTA aqueous solution was replaced by
0.5M DTPA aqueous solution.
Purification Example 4
[0223] The similar procedure as used in Purification Example 1 was
followed, except that 0.5M EDTA aqueous solution was replaced by
0.5M TTHA aqueous solution.
[0224] Firstly, the samples purified with EDTA and CYDTA were
compared respectively with P3 nanotubes (available from Carbon
Solutions Inc., obtained with the acid reflux purification, with a
purity of higher than 85%). Their Raman spectra are showed in FIG.
4.
[0225] It can be seen from FIG. 4 that all three characteristic
spectral regions of carbon nanotubes are essentially retained
during the process: the RBM (150-250 cm-1), the D band (1330 cm-1),
and the G band (1520-1600 cm-1). Furthermore, it can be seen that
the G/D ratios from EDTA and CYDAT are larger than the one of P3,
which indicates that the purity of carbon nanotubes purified with
EDTA and CYDAT is higher than that of P3.
[0226] In addition, it also can be seen that the purity of carbon
nanotubes purified with CYDAT is higher than the purity of carbon
nanotubes purified with EDTA. This may be due to that the complex
of CYDAT and Y is more stable than the complex of EDTA and Y.
[0227] The carbon nanotubes purified with EDTA were analyzed with
XPS, and the result is showed in FIG. 5. It can be seen that the
peak of Na is appears in FIG. 5. This indicates that the sodium
salt of EDTA remains in the carbon nanotubes after the
purification, i.e., EDTA was not entirely eliminated.
[0228] Although it is not showed herein, the peak of Na 1 s also
appears in the XPS spectrum of carbon nanotubes purified with
CYDAT.
[0229] It can be seen that the catalyst impurities were not
entirely eliminated by CYDAT and EDTA. The possible reason is the
poor solubility of the sodium salts of CYDAT and EDTA in water.
[0230] The carbon nanotubes purified with TTHA were analyzed with
SEM, and the result is showed in FIG. 6b. The SEM image of pristine
carbon nanotubes is also showed in FIG. 6a.
[0231] The carbon nanotubes purified with TTHA were also analyzed
with TEM, and the result is showed in FIG. 7, wherein FIG. 7a
differs from FIG. 7b only in magnification.
[0232] By comparing FIG. 6a with FIG. 6b, it can be seen that most
of impurities are removed after the purification by TTHA with only
a little amorphous carbon adhered to the carbon nanotubes. The
residual impurities can be further removed by a combination of
sonication and ultracentrifugation.
[0233] It can be seen from FIG. 7a and FIG. 7b that the sidewalls
of the resulting carbon nanotubes are very smooth, which indicates
that the sidewalls are not damaged during the purification.
[0234] FIG. 8 shows Raman spectra of carbon nanotubes before and
after the purification by TTHA. By comparing the G/D ratios of the
curves, it is evident seen that the purity of carbon nanotubes is
significantly increased after the purification by TTHA. It is also
seen that the carbon nanotubes after the purification by TTHA are
of high quality and purity.
[0235] FIG. 9 shows XPS spectra of carbon nanotubes purified by
using TTHA and purified by conventional acid treatment (P3). It is
obviously seen from FIG. 9 that yttrium was entirely removed by
TTHA treatment, while more yttrium remains in the carbon nanotubes
after acid reflux treatment.
Film Fabrication Example 1
[0236] In this example, the carbon nanotubes purified by THHA were
used to fabricate films following a procedure based on the
filtration method. The process is described as follows.
[0237] 10 mg of CNTs were dispersed in 200 ml of 1 wt. % aqueous
octyl-phenol-ethoxylate (denoted Triton X-100) solution for 20 min
in an ultrasonic bath. The dispersion was filtered out with a mixed
cellulose ester (MCE) filter membrane (Millipore, 0.2 .mu.m pore),
and the resulting carbon nanotube film was formed on the membrane
in a vacuum filtration apparatus (Millipore). Substantially all of
the Triton X-100 on the obverse of the carbon nanotube film was
dialyzed against a Tris(hydroxymethyl)aminomethane hydrochloride
(Tris-HCl) buffer (50 mM, PH 7.5) for two days. The Tric-HCl buffer
was subsequently washed away with purified water, then the carbon
nanotube films were transferred onto a quartz substrate. After
drying the sample for 1 h at 90.degree. C., the filter membrane was
removed by using acetone vapor. Finally, the carbon nanotube films
were dried in a vacuum at 100.degree. C. for 1 h.
Comparative Example 1
[0238] The similar procedure as used in Film Fabrication Example 1
was followed, except that the carbon nanotubes purified by THHA
were replaced by the carbon nanotubes (P3) purified by the
conventional nitric acid reflux.
[0239] FIG. 10 shows a carbon nanotube film made from carbon
nanotubes purified with TTHA. The film was placed on a quartz
substrate with the word ICCAS written on. It can be seen from FIG.
10 that the word ICCAS can be clearly observed through the carbon
nanotube film (about 70 nm), which indicates the desirable
transparency of the film.
[0240] FIG. 11 compares sheet resistances of the carbon nanotube
film made in Film Fabrication Example 1 and the film made in
Comparative Example 1. It can be seen that sheet resistance of the
film made in Film Fabrication Example 1 drops significantly
compared with Comparative Example 1.
[0241] Since the damage of sidewalls in the carbon nanotubes is
avoided, the carbon nanotubes prepared or purified according to the
present application have superior properties and can be widely used
in optoelectronic applications.
[0242] It should be appreciated that additional process procedures
can be utilized and may include drying, washing and the like, as
long as there is no adverse impacts on the effects of the present
application.
[0243] "Optional` and "optionally" as used herein mean that the
subsequently described event or circumstance (such as treatment
step) may or may not occur, and that the description includes
instances where the event occurs and instances where it does
not.
[0244] All the references cited are incorporated by reference into
the present description to the extent applicable.
[0245] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its
intended advantages. It is therefore intended that such changes and
modifications be covered by the appended claims.
* * * * *