U.S. patent application number 11/475919 was filed with the patent office on 2010-09-23 for method of producing carbon nanotubes.
This patent application is currently assigned to Honda Motor Co., Ltd.. Invention is credited to Avetik Harutyunyan.
Application Number | 20100239491 11/475919 |
Document ID | / |
Family ID | 39661384 |
Filed Date | 2010-09-23 |
United States Patent
Application |
20100239491 |
Kind Code |
A1 |
Harutyunyan; Avetik |
September 23, 2010 |
Method of producing carbon nanotubes
Abstract
The present teachings are directed to methods of preparing
cylindrical carbon structures, specifically single-walled carbon
nanotubes, with a desired chirality. The methods include the steps
of providing a catalyst component on a substrate and a carbon
component, contacting the catalyst component and the carbon
component to produce a cylindrical carbon structure. Then, no
longer providing the carbon component and determining the chirality
of the cylindrical carbon structure. The catalyst component is then
cleaned and the process is repeated until the cylindrical carbon
structure fulfills a desired characteristic, such as, length. The
chirality of the single-walled carbon nanotube grown, after
cleaning of the catalyst component, has the same chirality as the
initially produced nanotube.
Inventors: |
Harutyunyan; Avetik;
(Columbus, OH) |
Correspondence
Address: |
Capitol City TechLaw, PLLC
113 S. Columbus St., Suite 302
Alexandria
VA
22314
US
|
Assignee: |
Honda Motor Co., Ltd.
Tokyo
JP
|
Family ID: |
39661384 |
Appl. No.: |
11/475919 |
Filed: |
June 28, 2006 |
Current U.S.
Class: |
423/447.3 ;
423/447.1; 977/742; 977/750; 977/751; 977/752; 977/773; 977/842;
977/843 |
Current CPC
Class: |
B01J 37/0234 20130101;
B01J 23/74 20130101; C01B 2202/06 20130101; B82Y 40/00 20130101;
C01B 32/162 20170801; C01B 2202/04 20130101; C01P 2004/13 20130101;
B01J 23/28 20130101; B01J 23/745 20130101; B01J 23/462 20130101;
Y02P 20/584 20151101; B01J 21/08 20130101; B01J 21/04 20130101;
B01J 38/12 20130101; B01J 23/881 20130101; B82Y 30/00 20130101;
B01J 37/0203 20130101; C01B 2202/02 20130101; B01J 23/94
20130101 |
Class at
Publication: |
423/447.3 ;
423/447.1; 977/742; 977/750; 977/751; 977/842; 977/773; 977/843;
977/752 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Claims
1. A method of preparing cylindrical carbon structures comprising:
a) providing a catalyst component on a substrate; b) providing a
carbon component; c) contacting the catalyst component and the
carbon component to produce a first cylindrical carbon structure;
d) stopping providing the carbon component; e) cleaning the
catalyst component; f) repeating steps b) through e), and g)
producing continued first cylindrical carbon structure having the
same chirality as the first cylindrical carbon structure.
2. The method according to claim 1, further comprising: determining
the chirality of the first cylindrical carbon structure after step
d).
3. The method according to claim 1, further comprising: repeating
step f) until the cylindrical carbon structure satisfies a desired
characteristic.
4. The method according to claim 3, wherein the desired
characteristic comprises at least one member selected from the
group consisting of length, electrical conductivity, thermal
conductivity, metallic character, semi-conductor character and
non-metallic character.
5. The method according to claim 3, further comprising: removing
the cylindrical carbon structure from the catalyst component.
6. The method according to claim 1, wherein the catalyst component
comprises nanoparticles containing at least one member selected
from the group consisting of iron, nickel, cobalt, molybdenum,
ruthenium and combinations thereof.
7. The method according to claim 1, wherein the carbon component
comprises carbon vapor produced by either a plasma enhanced
chemical vapor deposition method or a thermal chemical vapor
deposition method.
8. The method according to claim 7, wherein the carbon vapor is
produced from a carbon source comprising at least one element
selected from the group consisting of methane, ethylene, acetylene
and carbon dioxide.
9. The method according to claim 1, wherein cleaning the catalyst
component comprises heating the catalyst component to about
750.degree. C. under a reductive atmosphere.
10. The method according to claim 9, wherein cleaning the catalyst
component comprises utilizing a method sufficiently active to
remove, to the extent that cleaning allows production of the
continued cylindrical carbon structure, any coating present on the
catalyst component.
11. The method according to claim 9, wherein cleaning the catalyst
component comprises utilizing a cleaning method that does not react
with the cylindrical carbon structure.
12-13. (canceled)
14. The method according to claim 9, wherein heating comprises
exposing the coating to at least one member selected from the group
consisting of electromagnetic radiation, laser radiation and
microwave radiation.
15. The method according to claim 10, wherein the coating comprises
at least one member selected from the group consisting of amorphous
carbon, multilayer carbon, metal carbide and combinations
thereof.
16. The method according to claim 1, wherein the cylindrical carbon
structure comprises at least one member selected from the group
consisting of single-walled carbon nanotubes, double-walled carbon
nanotubes and multi-walled carbon nanotubes.
17. The method according to claim 1, wherein the cylindrical carbon
structure comprises single-walled carbon nanotubes.
18. The method according to claim 1, wherein the catalyst component
is heated to a temperature ranging from about 600.degree. to about
1000.degree. C. during the contacting step.
19. (canceled)
20. A method of preparing single-walled carbon nanotubes
comprising: a) providing a catalyst component on a substrate; b)
providing a carbon component; c) contacting the catalyst component
and the carbon component to produce a first single-walled carbon
nanotube having a chirality; d) stopping providing the carbon
component; e) cleaning the catalyst component; f) repeating steps
b) through e) to produce a continued first single-walled carbon
nanotube with the same chirality as the first single-walled carbon
nanotube; g) repeating step f) until the continued first
single-walled carbon nanotube satisfies a desired characteristic;
and h) removing the single-walled carbon nanotube from the catalyst
component, wherein cleaning the catalyst component comprises
heating the catalyst component in a reductive atmosphere.
21. The method according to claim 20, further comprising:
determining the chirality of the first produced single-walled
carbon nanotube after step d).
22. The method according to claim 20, wherein the desired
characteristic comprises at least one member selected from the
group consisting of length, electrical conductivity, thermal
conductivity, metallic character, semi-conductor character and
non-metallic character.
23. The method according to claim 20, wherein the catalyst
component comprises nanoparticles containing at least one member
selected from the group consisting of iron, nickel, cobalt,
molybdenum, ruthenium and combinations thereof.
24. The method according to claim 20, wherein the carbon component
comprises carbon vapor produced by either a plasma enhanced
chemical vapor deposition method or a thermal chemical vapor
deposition method.
25. The method according to claim 24, wherein the carbon vapor is
produced from a carbon source comprising at least one element
selected from the group consisting of methane, ethylene, acetylene
and carbon dioxide.
26. The method according to claim 20, wherein cleaning the catalyst
component comprises heating the catalyst component to about
750.degree. C.
27. The method according to claim 26, wherein cleaning the catalyst
component comprises utilizing a method sufficiently active to
remove, to the extent that cleaning allows production of the
continued single-walled carbon nanotube, any coating present on the
catalyst component.
28. The method according to claim 26, wherein cleaning the catalyst
component comprises utilizing a cleaning method that does not react
with the single-walled carbon nanotube.
29-30. (canceled)
31. The method according to claim 26, wherein heating comprises
exposing the coating to at least one member selected from the group
consisting of electromagnetic radiation, laser radiation and
microwave radiation under a reductive atmosphere.
32. The method according to claim 27, wherein the coating comprises
at least one member selected from the group consisting of amorphous
carbon, multilayer carbon and metal carbide.
33. The method according to claim 20, wherein the catalyst
component is heated to a temperature ranging from about 600.degree.
to about 1000.degree. C. during the contacting step.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present teachings relate to methods of producing carbon
nanotubes from initially produced nanotubes so that the
subsequently produced nanotubes have the same chirality as the
initially produced nanotubes.
[0003] 2. Discussion of the Related Art
[0004] The desire to produce cylindrical carbon structures,
specifically carbon nanotubes, and more specifically, single-walled
carbon nanotubes (hereinafter "SWNT"), with a specific chirality
has been an unfilled desire since it was realized that the
chirality of the nanotube influences or controls numerous nanotube
properties.
[0005] Smalley et al. have described a method of "cloning" SWNT
grown by a CVD based method by growing SWNT fibers with open ends,
reductively docking nanosized transition metal particles to the
open ends of the SWNT fibers and restarting growth of the SWNT on
the exposed metal particles. The SWNT growth from the docked
nanocatalysts is said to have the same diameter and chirality (n,m)
as the base SWNT. See Nanoletters, Vol. 5, No. 6, June 2005, pp.
997-1002.
[0006] The total amount of SWNT that could be grown by prior
methods of growing SWNT using metal catalysts was limited by the
build-up and coating of the metal catalyst with a layer composed
of, among other compounds, amorphous carbon and metal carbides.
Additionally, the methods of growing the SWNTs did not offer means
of controlling the chirality of the SWNT produced.
[0007] SWNTs have attracted attention because of their unique
chemical and physical properties. A carbon nanotube can be
described as a rolled-up graphite sheet in which hexagonal-shaped
units of carbon atoms are bound to each other with very strong
bonds between the carbon atoms. SWNTs have minimum diameters of
about 0.4 nm with lengths ranging as long as several hundred
micrometers with extremely small dimensional fluctuations. The
electrical conductivity of carbon nanotubes range from a
semiconductor to a metal depending upon the chirality of
nanotube.
[0008] Chirality of a nanotube is denoted by a double index (m,m)
where n and m are integers that describe how a single strip of
hexagonal "chicken-wire" graphite is cut so it forms a tube that
wraps perfectly onto the surface of a cylinder. When the two
indices are the same, that is n=m, the resultant tube is said to be
of the "arm-chair" (or n,n) type, since when that type of tube is
cut perpendicular to the tube axis, only the sides of the hexagons
are exposed and their pattern around the periphery of the tube edge
resembles the arm and seat of an arm chair repeated n times. Due to
their metallic nature, with extremely high electrical and thermal
conductivity, the arm-chair tubes are a preferred form of SWNT.
[0009] Metallic nanotubes can exhibit ballistic conduction,
conduction by non-scattered charge carriers. With ballistic
conduction, the resistance value becomes independent of length, and
the so-called quantum resistance (6.5 k.OMEGA.) is observed.
[0010] Arc discharge, laser ablation, thermal chemical vapor
deposition (hereinafter "CVD") and plasma enhanced CVD are several
of the known methods for manufacturing carbon nanotubes. Both SWNT
and multi-walled nanotubes can be produced by the arc discharge and
laser ablation methods.
[0011] Catalysts supported on a variety of suitable supports can be
utilized in the CVD methods to produce carbon nanotubes. A complete
understanding of the effects of catalyst formulation, for instance,
transition metals (Ni, Co, Fe, etc.), support material,
catalyst/support interaction, synthesis temperature and hydrocarbon
gas on the diameter and chirality of the carbon nanotubes produced
by CVD methods is still being developed. See, for example,
Harutyunyan et al, Nanoletters, Vol. 2, No. 5, 2002, pp. 525-530
and U.S. Patent Application Publication No. US 2003/0124717 A1.
SUMMARY
[0012] The present teachings satisfy the need for a method of
producing cylindrical carbon structures from initially produced
cylindrical carbon structures so that the subsequently produced
cylindrical carbon structures have the same chirality as the
initially produced cylindrical carbon structures.
[0013] A method of preparing cylindrical carbon structures by
providing a catalyst component on a substrate and a carbon
component, and contacting the catalyst component and the carbon
component to produce a first cylindrical carbon structure is taught
by the present disclosure. The method further includes stopping
providing the carbon component, cleaning the catalyst component,
and then again providing the carbon component to produce more of
the cylindrical carbon structure.
[0014] The present teachings further provide single-walled carbon
nanotubes prepared by a process including providing a catalyst
component on a substrate, providing a carbon component and
contacting the catalyst component and the carbon component to
produce a first single-walled carbon nanotube having a chirality.
Then stopping providing the carbon component, cleaning the catalyst
components, and again providing the carbon component to produce a
continued first single-walled carbon nanotube, such that the
continued first single-walled carbon nanotube has the same
chirality as the first single-walled carbon nanotube.
[0015] Another method disclosed by the present teachings of
preparing single-walled carbon nanotubes includes providing a
catalyst component on a substrate, providing a carbon component,
contacting the catalyst component and the carbon component to
produce a first single-walled carbon nanotube having a chirality
and stopping providing the carbon component. The catalyst component
is then cleaned, and the carbon component is again provided to
produce a continued first single-walled carbon nanotube with the
same chirality as the first single-walled carbon nanotube. This
procedure is repeated until the continued first single-walled
carbon nanotube satisfies a desired characteristic, at which time,
the single-walled carbon nanotube is removed from the catalyst
component.
DETAILED DESCRIPTION
[0016] The present teachings provide a method of preparing
cylindrical carbon structures, specifically SWNT, by providing a
catalyst component on a substrate, providing a carbon component,
contacting the catalyst component and the carbon component to
produce a first cylindrical carbon structure, and then stopping the
provision of the carbon component. At this point in the method, the
catalyst component can be cleaned, and after cleaning, the carbon
component can be reintroduced to produce additional cylindrical
carbon structure.
[0017] The chirality of the first cylindrical carbon structure can
be determined after the provision of the carbon component is
stopped. The preparation can then be continued by repeating the
steps of providing carbon component, contacting the catalyst and
carbon components to produce a continued cylindrical carbon
structure, stopping the provision of the carbon component, and
cleaning the catalyst component, until the cylindrical carbon
structure satisfies a desired characteristic.
[0018] While it is not presently feasible to produce a cylindrical
carbon structure, or SWNT, with a predetermined chirality, in the
present disclosure, the chirality of the continued cylindrical
carbon structure produced has the same chirality as the first
cylindrical carbon structure. The presently disclosed process
provides that where given an initial cylindrical carbon structure,
preferably an SWNT, with a certain chirality, that cylindrical
carbon structure can be, for instance, increased in length with the
additional cylindrical carbon structure having the same chirality
as the initial cylindrical carbon structure.
[0019] The desired characteristic can include, for example, at
least one member selected from the group consisting of length,
electrical conductivity, thermal conductivity, metallic character,
semi-conductor character and non-metallic character. Upon
satisfying the desired characteristic the cylindrical carbon
structure can be removed from the catalyst component.
Alternatively, the production process can be ceased when the
efficiency of the process decreases due to build-up of a coating on
the catalyst component as described in more detail herein.
[0020] The catalyst component can include nanoparticles containing
at least one member selected from the group consisting of
transition metals, such as, for example, iron, nickel, cobalt,
molybdenum, ruthenium and combinations thereof. Of particular
interest are catalyst formulations of transition metals and
combinations thereof which exhibit resistance to or decreased
formation of coatings on the catalyst itself. Typically, the
coatings are composed of amorphous carbon, multilayer carbon and
metal carbides.
[0021] The present method of producing cylindrical carbon
structures can utilize either a plasma enhanced CVD method or a
thermal CVD method to produce the carbon component as a carbon
vapor produced from a carbon source, such as, for example, methane,
ethylene, acetylene or carbon dioxide. In the present method, the
catalyst component can be heated to a temperature ranging from
about 60.degree. C. to about 100.degree. C.
[0022] In the CVD methods that can be utilized according to the
present disclosure, the catalyst nanoparticle utilized in the
method can, after exposure for a period of time to a carbon source,
develop a coating or layer of non-reactive material. Various
cleaning processes are presented in the present disclosure which
clean the catalyst component by reducing any coating present on the
catalyst component.
[0023] Cleaning the catalyst component refers to using a cleaning
method sufficiently active to remove or deactivate, to the extent
that cleaning allows subsequent continued production of the
cylindrical carbon structure, any coating or build-up present on
the catalyst component. Preferably, cleaning the catalyst component
includes a cleaning method that does not react, or does not react
substantially, with the cylindrical carbon structure.
[0024] Oxidation, reduction, dissolution, radiative heating,
chemical treatment, plasma treatment and combinations thereof are
examples of suitable cleaning methods for removal of the coating on
the catalyst component. Examples of chemical treatment include
contacting the coating with, for example, water, peroxides and
acids. Radiative heating includes exposing the catalyst component
and coating to radiation of a wavelength capable of heating
primarily the coating and/or the catalyst component to thereby
induce oxidation of the coating. Preferably, the radiative heating
does not adversely affect either of the catalyst component or the
cylindrical carbon structure. Examples of suitable radiation
methods include electromagnetic radiation, laser radiation and
microwave radiation.
[0025] The coating present on the catalyst component typically
consists of amorphous carbon, multilayer carbon, metal carbide and
combinations thereof. According to present theory, without being
limited thereby, as the CVD process continues, non-nanotube forming
carbon arrives at the catalyst component and can form, for example,
amorphous carbon, multilayer carbon and metal carbide. Each of
these formations results in decreased access to the catalyst
component for the incoming carbon component and eventually leads to
decreased or ceased nanotube growth. According to present theory,
these coating components arise in a variety of ways, including
incomplete combustion of the supplied hydrocarbon, incomplete
formation of cylindrical carbon structures, formation of metal
carbides with the metallic elements of the catalyst component, and
layering of either or both of incompletely combusted hydrocarbons
or incompletely formed cylindrical carbon structures.
[0026] The catalyst component can also become less active through
the formation of metal oxides on the catalyst. Reduction of the
metal oxides back to the metallic state can also improve the
catalyst performance, and can in some cases be accomplished during
the cleaning of the catalyst component.
[0027] The cylindrical carbon structures produced by the present
methods can include single-walled carbon nanotubes, double-walled
carbon nanotubes and multi-walled carbon nanotubes. Preferably, the
present method produces single-walled carbon nanotubes.
[0028] The substrate utilized in the presently disclosed methods is
not generally restricted, and can include any commonly used
substrate. Suitable examples of substrates include, without
limitation, silicon substrates, glass substrates, alumina
substrates and quartz substrates.
[0029] According to the present disclosure, single-walled carbon
nanotubes can be prepared by providing a catalyst component on a
substrate, providing a carbon component and contacting the catalyst
component and the carbon component to produce a first single-walled
carbon nanotube having a chirality. After a sufficient amount of
the initial SWNT is formed, the carbon component is no longer
provided, and the catalyst component can be cleaned. After
cleaning, the carbon component can again be provided to produce a
continued single-walled carbon nanotube which has the same
chirality as the first single-walled carbon nanotube.
[0030] This process can further include determining the chirality
of the first single-walled carbon nanotube at any point after the
provision of the carbon component has ceased. The process can be
repeated until single-walled carbon nanotubes satisfying a desired
characteristic are produced, or until the catalyst component after
cleaning can no produced the continued first single-walled carbon
nanotube.
[0031] The desired characteristic can be, for instance, length,
electrical conductivity, thermal conductivity, metallic character,
semi-conductor character and non-metallic character.
[0032] The present disclosure further includes a process of
preparing single-walled carbon nanotubes by providing a catalyst
component on a substrate and a carbon component, then contacting
the catalyst component and the carbon component to produce a first
single-walled carbon nanotube having a chirality. Stopping the
provision of the carbon component can be the next step and allows
for the cleaning the catalyst component. Repeating the provision of
the carbon component and contacting it with the catalyst component
produces a continued first single-walled carbon nanotube with the
same chirality as the first single-walled carbon nanotube. This
process can be repeated until the continued first single-walled
carbon nanotube satisfies a desired characteristic, and then
removing the single-walled carbon nanotube from the catalyst
component.
[0033] The present process of preparing SWNT can utilize either a
plasma enhanced CVD method or a thermal CVD method to produce the
carbon component as a carbon vapor produced from a carbon source,
such as, for example, methane, ethylene, acetylene or carbon
dioxide. In the present process, the catalyst component can be
heated to a temperature ranging from about 60.degree. C. to about
100.degree. C.
[0034] The process can further include determining the chirality of
the first produced single-walled carbon nanotube after the
provision of the carbon component is ceased.
[0035] The desired characteristic exhibited by the continued first
SWNT can include, for example, length, electrical conductivity,
thermal conductivity, metallic character, semi-conductor character
and non-metallic character.
[0036] The catalyst component utilized to produce the SWNT can
include nanoparticles which contain transition metals, for
instance, iron, nickel, cobalt, molybdenum, ruthenium and
combinations thereof.
[0037] Cleaning the catalyst component can be accomplished by
reducing any coating present on the catalyst component. A cleaning
method sufficiently active to remove, to the extent that cleaning
allows production of the single-walled carbon nanotube, any coating
present on the catalyst component is preferable. Furthermore, any
cleaning method does not react, or at least does not substantially
react, with the single-walled carbon nanotube.
[0038] According to the present disclosure, oxidation, reduction,
dissolution, radiative heating, chemical treatment, plasma
treatment and combinations thereof can all be utilized as cleaning
methods. Chemical treatment includes contacting the coating with at
least one member selected from the group consisting of water,
peroxides and acids. Radiative heating includes exposing the
coating to, for example, electromagnetic radiation, laser radiation
or microwave radiation.
[0039] The chirality of the cylindrical carbon structures or SWNTs
can be determined by a variety of methods including Raman
characterization, micro Raman characterization, I-V
("current-voltage") characterization, and STM ("scanning tunneling
microscopy") measurement.
[0040] Electromagnetic radiation refers to radiation composed of
oscillating electric and magnetic fields and propagated at the
speed of light. Examples of electromagnetic radiation include,
without limitation, gamma radiation, X-rays, ultraviolet, visible,
infrared, microwave and radio waves.
[0041] All publications, articles, papers, patents, patent
publications, and other references cited herein are hereby
incorporated herein in their entireties for all purposes.
[0042] The foregoing detailed description of the various
embodiments of the present teachings has been provided for the
purposes of illustration and description. Many modifications and
variations will be apparent to practitioners skilled in this art.
The embodiments were chosen and described in order to best explain
the principles of the present teachings and their practical
application, thereby enabling others skilled in the art to
understand the present teachings for various embodiments and with
various modifications as are suited to the particular use
contemplated. The specific techniques, conditions, materials and
reported data set forth in the following examples to illustrate the
principles of the present teachings are exemplary and should not be
construed as exhaustive or limiting the scope of the present
teachings. It is intended that the scope of the present teachings
be defined by the following claims and their equivalents.
EXAMPLES
Example 1
[0043] Ferric nitrate (Fe(NO.sub.3).sub.3.9H.sub.2O) can be
dissolved in 2-propanol at an approximate concentration of 100
.mu.g/mL, and stirred for 15 minutes. A previously prepared silicon
dioxide substrate can then be immersed into the iron solution for
15 seconds, rinsed in hexane, and dried in air.
[0044] The substrate with the catalyst can then be placed in a tube
furnace and reduced under a helium/hydrogen (60/40) gas flow (200
sccm) at 500 C for one hour. The He/H.sub.2 gas mixture can then be
replaced with Ar gas, and the temperature increased to 750 C. Once
the higher temperature is reached, then methane gas can be added at
a flow rate of 20 sccm for 15 minutes, after which time the furnace
is cooled to room temperature under a flow of argon. An atomic
force microscopy ("AFM") image can be obtained of the
nanotubes.
[0045] The resulting supported iron nanoparticles with nanotubes
can be cleaned by exposing the sample to a dry air flow (100 sccm)
at a temperature of 200 C for thirty minutes.
[0046] The tube furnace can then be reheated to 750 C under a flow
(200 sccm) of an argon/hydrogen gas mixture. After the
nanoparticles reach a steady state temperature, methane can be
re-introduced into the tube furnace, at a flow rate of 20 sccm.
[0047] After fifteen minutes, the methane flow can be stopped and
the apparatus allowed to cool to room temperature under an argon
gas flow. The supported iron nanoparticles with nanotubes can then
be removed from the tube furnace.
[0048] A second AFM image can be obtained. The second AFM image can
show that the nanotubes have grown in length while maintaining the
same chirality as the initial nanotube.
Example 2
[0049] Ferric nitrate (Fe(NO.sub.3).sub.3.9H.sub.2O) and ammonium
molybdate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.4H.sub.2O) at a 1:0.17
Fe:Mo molar ratio can be dissolved in methanol, and then mixed with
a methanol suspension of alumina. The suspension can be deposited,
drop wise, onto a previously prepared silicon dioxide substrate,
and then dried in air.
[0050] The substrate with the bimetallic catalyst can then be
placed in a tube furnace and reduced under a helium/hydrogen
(60/40) gas flow (200 sccm) at 500 C for one hour. The He/H.sub.2
gas mixture can then be replaced with Ar gas, and the temperature
increased to 750 C. Once the higher temperature is reached, then
methane gas can be added at a flow rate of 20 sccm for 15 minutes,
after which time the furnace is cooled to room temperature under a
flow of argon. An atomic force microscopy ("AFM") image can be
obtained of the nanotubes.
[0051] The supported iron/molybdenum nanoparticles with nanotubes
can be cleaned by exposing the sample to a dry air flow (100 sccm)
at a temperature of 200 C for thirty minutes.
[0052] The tube furnace can then be reheated to 750 C under a flow
(200 sccm) of an argon/hydrogen gas mixture. After the
nanoparticles reach a steady state temperature, methane can be
re-introduced into the tube furnace, at a flow rate of 20 sccm.
[0053] After fifteen minutes, the methane flow can be stopped and
the apparatus allowed to cool to room temperature under an argon
gas flow. The supported Fe/Mo nanoparticles with nanotubes can then
be removed from the tube furnace.
[0054] A second AFM image can be obtained. The second AFM image can
show that the nanotubes have grown in length while maintaining the
same chirality as the initial nanotube.
Example 3
[0055] Ferric nitrate (Fe(NO.sub.3).sub.3.9H.sub.2O) can be
dissolved in methanol at an approximate concentration of 150
.mu.g/mL, and then mixed with a methanol suspension of alumina. The
alumina can have a BET surface area of 150 m.sup.2/g. The iron and
alumina suspension can be deposited, drop wise, onto a previously
prepared silicon dioxide substrate, and then dried in air.
[0056] The substrate with the catalyst can then be placed in a tube
furnace and reduced under a helium/hydrogen (60/40) gas flow (200
sccm) at 500 C for one hour. The He/H.sub.2 gas mixture can then be
replaced with Ar gas, and the temperature increased to 750 C. Once
the higher temperature is reached, then methane gas can be added at
a flow rate of 20 sccm for 15 minutes, after which time the furnace
is cooled to room temperature under a flow of argon. An atomic
force microscopy ("AFM") image can be obtained of the
nanotubes.
[0057] The supported iron nanoparticles with nanotubes can be
cleaned by exposing the sample to a dry air flow (100 sccm) at a
temperature of 200 C for thirty minutes.
[0058] The tube furnace can then be reheated to 750 C under a flow
(200 sccm) of an argon/hydrogen gas mixture. After the
nanoparticles reach a steady state temperature, methane can be
re-introduced into the tube furnace, at a flow rate of 20 sccm.
[0059] After fifteen minutes, the methane flow can be stopped and
the apparatus allowed to cool to room temperature under an argon
gas flow. The supported iron nanoparticles with nanotubes can then
be removed from the tube furnace.
[0060] A second AFM image can be obtained. The second AFM image can
show that the nanotubes have grown in length while maintaining the
same chirality as the initial nanotube.
* * * * *