U.S. patent application number 13/429309 was filed with the patent office on 2012-09-20 for catalyst for the growth of carbon single-walled nanotubes.
This patent application is currently assigned to THE OHIO STATE UNIVERSITY RESEARCH FOUNDATION. Invention is credited to Avetik R. Harutyunyan, Elena Mora, Toshio Tokune.
Application Number | 20120237436 13/429309 |
Document ID | / |
Family ID | 38997608 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120237436 |
Kind Code |
A1 |
Harutyunyan; Avetik R. ; et
al. |
September 20, 2012 |
Catalyst For The Growth Of Carbon Single-Walled Nanotubes
Abstract
Methods and processes for synthesizing single-wall carbon
nanotubes are provided. A carbon precursor gas is contacted with
metal catalysts deposited on a support material. The metal
catalysts are preferably nanoparticles having diameters less than
about 3 nm. The reaction temperature is selected such that it is
near the eutectic point of the mixture of metal catalyst particles
and carbon. Further, the rate at which hydrocarbons are fed into
the reactor is equivalent to the rate at which the hydrocarbons
react for given synthesis temperature. The methods produce carbon
single-walled nanotubes having longer lengths.
Inventors: |
Harutyunyan; Avetik R.;
(Columbus, OH) ; Tokune; Toshio; (Columbus,
OH) ; Mora; Elena; (Columbus, OH) |
Assignee: |
THE OHIO STATE UNIVERSITY RESEARCH
FOUNDATION
Columbus
OH
HONDA MOTOR CO., LTD.
Tokyo
|
Family ID: |
38997608 |
Appl. No.: |
13/429309 |
Filed: |
March 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11669128 |
Jan 30, 2007 |
8163263 |
|
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13429309 |
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60763943 |
Jan 30, 2006 |
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Current U.S.
Class: |
423/447.3 ;
977/750; 977/843; 977/902 |
Current CPC
Class: |
B82Y 30/00 20130101;
B01J 37/0211 20130101; B01J 35/0013 20130101; B01J 23/74 20130101;
C01B 32/162 20170801; C01B 2202/02 20130101; B01J 23/28 20130101;
Y10S 977/843 20130101; B01J 37/343 20130101; B82Y 40/00 20130101;
B01J 23/44 20130101; B01J 21/185 20130101 |
Class at
Publication: |
423/447.3 ;
977/750; 977/843; 977/902 |
International
Class: |
D01F 9/127 20060101
D01F009/127 |
Claims
1.-20. (canceled)
21. A chemical vapor deposition method for the preparation of
single-wall carbon nanotubes (SWNT), the method comprising the
steps of: providing supported catalyst nanoparticles, wherein the
catalyst has a particle size between 1 nm and 10 nm; heating the
supported catalyst nanoparticles to a temperature below the melting
point of the metal phase of the catalyst; contacting the supported
catalyst nanoparticles with a carbon precursor gas, wherein the
carbon precursor gas comprises methane, an inert gas, and hydrogen;
forming SWNT by contacting the supported catalyst nanoparticles
with the carbon precursor gas at a rate equivalent to the rate at
which the carbon precursor gas reacts.
22. The method of claim 21, wherein the inert gas is argon, helium,
nitrogen, or combinations thereof.
23. The method of claim 21, wherein the catalyst is iron,
molybdenum, or combinations of iron and molybdenum.
24. The method of claim 21, wherein the support is a powdered
oxide.
25. The method of claim 24, wherein the powdered oxide is selected
from the group consisting of Al.sub.2O.sub.3, SiO.sub.2, MgO and
zeolites.
26. The method of claim 25, wherein the powdered oxide is
Al.sub.2O.sub.3.
27. The method of claim 21, wherein the catalyst and the support
are in a ratio of about 1:1 to about 1:50.
Description
RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of U.S.
Provisional Application No. 60/763,943, filed on Jan. 30, 2006,
which is incorporated by reference herein in its entirety.
FIELD OF INVENTION
[0002] The present invention relates to methods for the preparation
(synthesis) of carbon single-walled nanotubes using chemical vapor
deposition method.
INTRODUCTION
[0003] Catalyst features are of central importance for carbon
single-walled nanotubes growth. Combination of catalyst activity
and the melting point evolution during nanotube growth, with Raman
studies bolster the "V"-shape behavior of the nanocatalyst-carbon
binary phase liquidus line as a critical feature. In fact, the
"L"-shape is the ideal feature, which implies low activation energy
of diffusion, high carbon solubility, excludes the carbide
formation and, thereby possess long "lifetime" for nanotube growth.
For the catalyst with diameter less than 1.2 nm the choice of
support material may results significant modification of features.
The catalyst features and corresponding synthesis parameters
required for nanotube growth are presented.
[0004] The elucidation of catalyst particle specific features
favorable for carbon single-walled nanotubes (SWNTs) growth will
help to control over the characteristics of grown nanotubes and
eventually will promote exploitation of their unique properties (R.
H. Baughman, A. A. Zakhidov, W. A. De Heer, Science 297, 787
(2002)). There are a large number of studies regarding the
influence of catalyst composition on SWNTs growth, where most often
3d metals and their combinations have been considered (K. B. K.
Teo, C. Singh, M. Chhowalla, W.I . Milne, Encyclopedia of
Nanoscience and Nanotechnology X, 1-22, (2003), A-C. Dupuis, Prog.
In Materials Science 50, 929-961 (2005)). It is already established
that catalyst preparation methods, pretreatments, diameters,
crystallographic and electronic structures, and abilities of
carbide and oxide formations also have remarkable influence on
nanotube growth (A-C. Dupuis, Prog. In Materials Science 50,
929-961 (2005)). These studies are extended by reports about the
essential role of catalyst-supports coupling (more common supports
Al, Zr, Mg, Si based oxides). In addition, the synthesis parameters
have a crucial impact on the thermodynamics and kinetics of the
growth (A-C. Dupuis, Prog. In Materials Science 50, 929-961
(2005)). Nevertheless, intense research to reveal the common
features of the catalysts favorable for nanotube growth from
described complexity is still underway. We have undertaken a
systematic in-situ parametrical study of the catalyst activity
evolution during nanotubes growth, by using an enhanced synthesis
CVD technique. We show that, by combining the results with
differential scanning calorimetry (DSC) and Raman spectroscopy
studies, it is possible to elucidate the hidden common features of
catalysts responsible for nanotube growth and their relationship
with synthesis parameters and, as A RESULT, TO PREDICT THE FEATURES
FOR THE IDEAL CATALYST.
SUMMARY
[0005] The present invention provides methods and processes for
growing single-wall carbon nanotubes. In one aspect, a carbon
precursor gas and metal catalysts on supports are heated to a
reaction temperature near the eutectic point (liquid phase) of the
metal-carbon phase. Further, the reaction temperature is below the
melting point of the metal catalysts.
BRIEF DESCRIPTION OF THE FIGURES
[0006] FIG. 1. A) Hydrogen concentration evolution during carbon
SWNTs growth Insets (a1) sequential introduction of C.sup.12 and
C.sup.13 isotopes, respectively for (a1) 3 min and 17 min; (a2) 7
min and 13 min; and (a3) 13 min and 7 min B) Evolution of melting
point of Fe catalyst during carbon nanotube growth measured by DSC
technique. C) Variation of Raman I.sub.G/I.sub.D intensities ratio
and carbon uptake during carbon SWNTs growth
[0007] FIG. 2. The radial breathing and tangential modes for Raman
spectra of carbon SWNTs synthesized by sequential introduction of
carbon with C.sup.12 and C.sup.13 isotopes
[0008] FIG. 3. A) Hydrogen concentration evolution at 820.degree.
C. for Al.sub.2O.sub.3; Fe:Al.sub.2O.sub.3 (1:15 molar ratio);
Mo:Al.sub.2O.sub.3 (0.21:15 molar ratio); and Fe:Mo:Al.sub.2O.sub.3
(1:0.21:15 molar ratio) simples B) Time dependence of Raman spectra
for carbon SWNTs growth on base of Fe:Mo:Al.sub.2O.sub.3
(1:0.21:15) and C) on base of Fe:Al.sub.2O.sub.3 (1:15)
catalyst
DESCRIPTION OF THE INVENTION
[0009] Unless otherwise stated, the following terms used in this
application, including the specification and claims, have the
definitions given below. It must be noted that, as used in the
specification and the appended claims, the singular forms "a," "an"
and "the" include plural referents unless the context clearly
dictates otherwise. Definition of standard chemistry terms may be
found in reference works, including Carey and Sundberg (1992)
"Advanced Organic Chemistry 3.sup.rd Ed." Vols. A and B, Plenum
Press, New York, and Cotton et al. (1999) "Advanced Inorganic
Chemistry 6.sup.th Ed." Wiley, N.Y.
[0010] The terms "single-walled carbon nanotube" or
"one-dimensional carbon nanotube" are used interchangeable and
refer to cylindrically shaped thin sheet of carbon atoms having a
wall consisting essentially of a single layer of carbon atoms, and
arranged in an hexagonal crystalline structure with a graphitic
type of bonding.
[0011] The term "multi-walled carbon nanotube" as used herein
refers to a nanotube composed of more than one concentric
tubes.
[0012] The terms "metalorganic" or "organometallic" are used
interchangeably and refer to co-ordination compounds of organic
compounds and a metal, a transition metal or metal halide.
[0013] The term "eutectic point" refers to the lowest possible
temperature of solidification for an alloy, and can be lower than
that of any other alloy composed of the same constituents in
different proportions.
[0014] The catalyst composition may be any catalyst composition
known to those of skill in the art that is routinely used in
chemical vapor deposition processes. The function of the catalyst
in the carbon nanotube growth process is to decompose the carbon
precursors and aid the deposition of ordered carbon. The method,
processes, and apparatuses of the present invention preferably use
metal nanoparticles as the metallic catalyst. The metal or
combination of metals selected as the catalyst can be processed to
obtain the desired particle size and diameter distribution. The
metal nanoparticles can then be separated by being supported on a
material suitable for use as a support during synthesis of carbon
nanotubes using the metal growth catalysts described below. As
known in the art, the support can be used to separate the catalyst
particles from each other thereby providing the catalyst materials
with greater surface area in the catalyst composition. Such support
materials include powders of crystalline silicon, polysilicon,
silicon nitride, tungsten, magnesium, aluminum and their oxides,
preferably aluminum oxide, silicon oxide, magnesium oxide, or
titanium dioxide, or combination thereof, optionally modified by
addition elements, are used as support powders. Silica, alumina and
other materials known in the art may be used as support, preferably
alumina is used as the support.
[0015] The metal catalyst can be selected from a Group V metal,
such as V or Nb, and mixtures thereof, a Group VI metal including
Cr, W, or Mo, and mixtures thereof, VII metal, such as, Mn, or Re,
Group VIII metal including Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, and
mixtures thereof, or the lanthanides, such as Ce, Eu, Er, or Yb and
mixtures thereof, or transition metals such as Cu, Ag, Au, Zn, Cd,
Sc, Y, or La and mixtures thereof. Specific examples of mixture of
catalysts, such as bimetallic catalysts, which may be employed by
the present invention include Co--Cr, Co--W, Co--Mo, Ni--Cr, Ni--W,
Ni--Mo, Ru--Cr, Ru--W, Ru--Mo, Rh--Cr, Rh--W, Rh--Mo, Pd--Cr,
Pd--W, Pd--Mo, Ir--Cr, Pt--Cr, Pt--W, and Pt--Mo. Preferably, the
metal catalyst is iron, cobalt, nickel, molybdeum, or a mixture
thereof, such as Fe--Mo, Co--Mo and Ni--Fe--Mo.
[0016] The metal, bimetal, or combination of metals are used to
prepare metal nanoparticles having defined particle size and
diameter distribution. The metal nanoparticles can be prepared
using the literature procedure described in described in
Harutyunyan et al., NanoLetters 2, 525 (2002). Alternatively, the
catalyst nanoparticles can be prepared by thermal decomposition of
the corresponding metal salt added to a passivating salt, and the
temperature of the solvent adjusted to provide the metal
nanoparticles, as described in the co-pending and co-owned U.S.
patent application Ser. No. 10/304,316, or by any other method
known in the art. The particle size and diameter of the metal
nanoparticles can be controlled by using the appropriate
concentration of metal in the passivating solvent and by
controlling the length of time the reaction is allowed to proceed
at the thermal decomposition temperature. Metal nanoparticles
having particle size of about 0.01 nm to about 20 nm, more
preferably about 0.1 nm to about 3 nm and most preferably about 0.3
nm to 2 nm can be prepared. The metal nanoparticles can thus have a
particle size of 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 nm, and up
to about 20 nm. In another aspect, the metal nanoparticles can have
a range of particle sizes. For example, the metal nanoparticles can
have particle sizes in the range of about 3 nm and about 7 nm in
size, about 5 nm and about 10 nm in size, or about 8 nm and about
16 nm in size. The metal nanoparticles can optionally have a
diameter distribution of about 0.5 nm to about 20 nm, preferably
about 1 nm to about 15 nm, more preferably about 1 nm to about 5
nm. Thus, the metal nanoparticles can have a diameter distribution
of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15
nm.
[0017] The metal salt can be any salt of the metal, and can be
selected such that the melting point of the metal salt is lower
than the boiling point of the passivating solvent. Thus, the metal
salt contains the metal ion and a counter ion, where the counter
ion can be nitrate, nitride, perchlorate, sulfate, sulfide,
acetate, halide, oxide, such as methoxide or ethoxide,
acetylacetonate, and the like. For example, the metal salt can be
iron acetate (FeAc.sub.2), nickel acetate (NiAc.sub.2), palladium
acetate (PdAc.sub.2), molybdenum acetate (MoAc.sub.3), and the
like, and combinations thereof. The melting point of the metal salt
is preferably about 5.degree. C. to 50.degree. C. lower than the
boiling point, more preferably about 5.degree. C. to about
20.degree. C. lower than the boiling point of the passivating
solvent.
[0018] The metal salt can be dissolved in a passivating solvent to
give a solution, a suspension, or a dispersion. The solvent is
preferably an organic solvent, and can be one in which the chosen
metal salt is relatively soluble and stable, and where the solvent
has a high enough vapor pressure that it can be easily evaporated
under experimental conditions. The solvent can be an ether, such as
a glycol ether, 2-(2-butoxyethoxy)ethanol,
H(OCH.sub.2CH.sub.2).sub.2O(CH.sub.2).sub.3CH.sub.3, which will be
referred to below using the common name dietheylene glycol
mono-n-butyl ether, and the like.
[0019] The relative amounts of metal salt and passivating solvent
are factors in controlling the size of nanoparticles produced. A
wide range of molar ratios, here referring to total moles of metal
salt per mole of passivating solvent, can be used for forming the
metal nanoparticles. Typical molar ratios of metal salt to
passivating solvent include ratios as low as about 0.0222 (1:45),
or as high as about 2.0 (2:1), or any ratio in between. Thus, for
example, about 5.75.times.10.sup.-5 to about 1.73.times.10.sup.-3
moles (10-300 mg) of FeAc.sub.2 can be dissolved in about
3.times.10.sup.-4 to about 3.times.10.sup.-3 moles (50-500 ml) of
diethylene glycol mono-n-butyl ether.
[0020] In another aspect, more than one metal salt can be added to
the reaction vessel in order to form metal nanoparticles composed
of two or more metals, where the counter ion can be the same or can
be different. The relative amounts of each metal salt used can be a
factor in controlling the composition of the resulting metal
nanoparticles. For the bimetals, the molar ratio of the first metal
salt to the second metal salt can be about 1:10 to about 10:1,
preferably about 2:1 to about 1:2, or more preferably about 1.5:1
to about 1:1.5, or any ratio in between. Thus, for example, the
molar ratio of iron acetate to nickel acetate can be 1:2, 1:1.5,
1.5:1, or 1:1. Those skilled in the art will recognize that other
combinations of metal salts and other molar ratios of a first metal
salt relative to a second metal salt may be used in order to
synthesize metal nanoparticles with various compositions.
[0021] The passivating solvent and the metal salt reaction solution
can be mixed to give a homogeneous solution, suspension, or
dispersion. The reaction solution can be mixed using standard
laboratory stirrers, mixtures, sonicators, and the like, optionally
with heating. The homogenous mixture thus obtained can be subjected
to thermal decomposition in order to form the metal
nanoparticles.
[0022] The thermal decomposition reaction is started by heating the
contents of the reaction vessel to a temperature above the melting
point of at least one metal salt in the reaction vessel. Any
suitable heat source may be used including standard laboratory
heaters, such as a heating mantle, a hot plate, or a Bunsen burner,
and the heating can be under reflux. The length of the thermal
decomposition can be selected such that the desired size of the
metal nanoparticles can be obtained. Typical reaction times can be
from about 10 minutes to about 120 minutes, or any integer in
between. The thermal decomposition reaction is stopped at the
desired time by reducing the temperature of the contents of the
reaction vessel to a temperature below the melting point of the
metal salt.
[0023] The size and distribution of metal nanoparticles produced
can be verified by any suitable method. One method of verification
is transmission electron microscopy (TEM). A suitable model is the
Phillips CM300 FEG TEM that is commercially available from FEI
Company of Hillsboro, Oreg. In order to take TEM micrographs of the
metal nanoparticles, 1 or more drops of the metal
nanoparticle/passivating solvent solution are placed on a carbon
membrane grid or other grid suitable for obtaining TEM micrographs.
The TEM apparatus is then used to obtain micrographs of the
nanoparticles that can be used to determine the distribution of
nanoparticle sizes created.
[0024] The metal nanoparticles, such as those formed by thermal
decomposition described in detail above, can then be supported on
solid supports. The solid support can be silica, alumina, MCM-41,
MgO, ZrO.sub.2, aluminum-stabilized magnesium oxide, zeolites, or
other oxidic supports known in the art, and combinations thereof.
For example, Al.sub.2O.sub.3--SiO.sub.2 hybrid support could be
used. Preferably, the support is aluminum oxide (Al.sub.2O.sub.3)
or silica (SiO.sub.2). The oxide used as solid support can be
powdered thereby providing small particle sizes and large surface
areas. The powdered oxide can preferably have a particle size
between about 0.01 .mu.m to about 100 .mu.m, more preferably about
0.1 .mu.m to about 10 .mu.m, even more preferably about 0.5 .mu.m
to about 5 .mu.m, and most preferably about 1 .mu.m to about 2
.mu.m. The powdered oxide can have a surface area of about 50 to
about 1000 m.sup.2/g, more preferably a surface area of about 200
to about 800 m.sup.2/g. The powdered oxide can be freshly prepared
or commercially available.
[0025] In one aspect, the metal nanoparticles are supported on
solid supports via secondary dispersion and extraction. Secondary
dispersion begins by introducing particles of a powdered oxide,
such as aluminum oxide (Al.sub.2O.sub.3) or silica (SiO.sub.2),
into the reaction vessel after the thermal decomposition reaction.
A suitable Al.sub.2O.sub.3 powder with 1-2 .mu.m particle size and
having a surface area of 300-500 m.sup.2/g is commercially
available from Alfa Aesar of Ward Hill, Mass., or Degussa, N.J.
Powdered oxide can be added to achieve a desired weight ratio
between the powdered oxide and the initial amount of metal used to
form the metal nanoparticles. Typically, the weight ratio can be
between about 10:1 and about 15:1. For example, if 100 mg of iron
acetate is used as the starting material, then about 320 to 480 mg
of powdered oxide can be introduced into the solution.
[0026] The mixture of powdered oxide and the metal
nanoparticle/passivating solvent mixture can be mixed to form a
homogenous solution, suspension or dispersion. The homogenous
solution, suspension or dispersion can be formed using sonicator, a
standard laboratory stirrer, a mechanical mixer, or any other
suitable method, optionally with heating. For example, the mixture
of metal nanoparticles, powdered oxide, and passivating solvent can
be first sonicated at roughly 80.degree. C. for 2 hours, and then
sonicated and mixed with a laboratory stirrer at 80.degree. C. for
30 minutes to provide a homogenous solution.
[0027] After secondary dispersion, the dispersed metal
nanoparticles and powdered oxide can be extracted from the
passivating solvent. The extraction can be by filtration,
centrifugation, removal of the solvents under reduced pressure,
removal of the solvents under atmospheric pressure, and the like.
After evaporating the passivating solvent, the powdered oxide and
metal nanoparticles are left behind on the walls of the reaction
vessel as a film or residue. When the powdered oxide is
Al.sub.2O.sub.3, the film will typically be black. The metal
nanoparticle and powdered oxide film can be removed from the
reaction vessel and ground to create a fine powder, thereby
increasing the available surface area of the mixture. The mixture
can be ground with a mortar and pestle, by a commercially available
mechanical grinder, or by any other methods of increasing the
surface area of the mixture will be apparent to those of skill in
the art.
[0028] Without being bound by any particular theory, it is believed
that the powdered oxide serves two functions during the extraction
process. The powdered oxides are porous and have high surface area.
Therefore, the metal nanoparticles will settle in the pores of the
powdered oxide during secondary dispersion. Settling in the pores
of the powdered oxide physically separates the metal nanoparticles
from each other, thereby preventing agglomeration of the metal
nanoparticles during extraction. This effect is complemented by the
amount of powdered oxide used. As noted above, the weight ratio of
metal nanoparticles to powdered oxide can be between about 1:10 and
1:15, such as, for example, 1:11, 1:12, 2:25, 3:37, 1:13, 1:14, and
the like. The relatively larger amount of powdered oxide in effect
serves to further separate or `dilute` the metal nanoparticles as
the passivating solvent is removed. The process thus provides metal
nanoparticles of defined particle size.
[0029] As will be apparent to those of skill in the art, the
catalyst thus prepared can be stored for later use. In another
aspect, the metal nanoparticles can be previously prepared,
isolated from the passivating solvent, and purified, and then added
to a powdered oxide in a suitable volume of the same or different
passivating solvent. The metal nanoparticles and powdered oxide can
be homogenously dispersed, extracted from the passivating solvent,
and processed to increase the effective surface area as described
above. Other methods for preparing the metal nanoparticle and
powdered oxide mixture will be apparent to those skilled in the
art.
[0030] The metal nanoparticles thus formed can be used as a growth
catalyst for synthesis of carbon nanotubes, nanofibers, and other
one-dimensional carbon nanostructures by a chemical vapor
deposition (CVD) process.
[0031] The carbon nanotubes can be synthesized using carbon
precursors, such as carbon containing gases. In general, any carbon
containing gas that does not pyrolize at temperatures up to
800.degree. C. to 1000.degree. C. can be used. Examples of suitable
carbon-containing gases include carbon monoxide, aliphatic
hydrocarbons, both saturated and unsaturated, such as methane,
ethane, propane, butane, pentane, hexane, ethylene, acetylene and
propylene; oxygenated hydrocarbons such as acetone, and methanol;
aromatic hydrocarbons such as benzene, toluene, and naphthalene;
and mixtures of the above, for example carbon monoxide and methane.
In general, the use of acetylene promotes formation of multi-walled
carbon nanotubes, while CO and methane are preferred feed gases for
formation of single-walled carbon nanotubes. The carbon-containing
gas may optionally be mixed with a diluent gas such as hydrogen,
helium, argon, neon, krypton and xenon or a mixture thereof.
[0032] The methods and processes of the invention provide for the
synthesis of SWNTs with a narrow distribution of diameters. The
narrow distribution of carbon nanotube diameters is obtained by
activating small diameter catalyst particles preferentially during
synthesis by selecting the lowest eutectic point as the reaction
temperature.
[0033] In one aspect of the invention, the metal nanoparticles
supported on powdered oxides can be contacted with the carbon
source at the reaction temperatures according to the literature
methods described in Harutyunyan et al., NanoLetters 2, 525 (2002).
Alternatively, the metal nanoparticles supported on the oxide
powder can be aerosolized and introduced into the reactor
maintained at the reaction temperature. Simultaneously, the carbon
precursor gas is introduced into the reactor. The flow of reactants
within the reactor can be controlled such that the deposition of
the carbon products on the walls of the reactor is reduced. The
carbon nanotubes thus produced can be collected and separated.
[0034] The metal nanoparticles supported on the oxide powder can be
aerosolized by any of the art known methods. In one method, the
supported metal nanoparticles are aerosolized using an inert gas,
such as helium, neon, argon, krypton, xenon, or radon. Preferably
argon is used. Typically, argon, or any other gas, is forced
through a particle injector, and into the reactor. The particle
injector can be any vessel that is capable of containing the
supported metal nanoparticles and that has a means of agitating the
supported metal nanoparticles. Thus, the catalyst deposited on a
powdered porous oxide substrate can be placed in a beaker that has
a mechanical stirrer attached to it. The supported metal
nanoparticles can be stirred or mixed in order to assist the
entrainment of the catalyst in the transporter gas, such as
argon.
[0035] To evaluate catalyst activity we measured the evolution of
hydrogen concentration during carbon SWNTs growth, appeared as a
result of catalytic decomposition of hydrocarbon, by using a
mass-spectrometer (Thermo Star GSD 300T, with SEM Detector)
attached to the outlet of the gas stream of the CVD apparatus.
Thus, any changes of catalyst features which influence on catalyst
electronic structure and in this manner on hydrocarbon
decomposition efficiency and eventually on the kinetics of nanotube
growth were detected by monitoring of hydrogen concentration.
Analogously, influence of synthesis parameters was also
revealed.
[0036] The carbon SWNTs were grown by passing a mixture of methane
(60 cm.sup.3/min, Praxair, 99,999%)) diluted in argon (200
cm.sup.3/min) over the Fe catalyst particles (with molar ratio
Fe:Al.sub.2O.sub.3=1:15) at 820.degree. C. for 90 min as described
in the literature (A. R. Harutyunyan, B. K. Pradhan, U. J. Kim, G.
Chen, P. C. Eklund, Nano Letters, 2, 525 (2002)). The growth of
nanotubes was independently confirmed by transmission electron
microscopy and Raman measurements. The rapid increase of H.sub.2
concentration until t.about.7.+-.1 min (FIG. 1A) was followed by a
slowly return to the almost constant value, which consistent with
concentration of non catalytic decomposition of CH.sub.4 (FIG. 2A).
On the other hand, the DSC studies of the independent samples,
synthesized under analogical experimental conditions but different
synthesis durations (3; 5; 7; 20 and 90 min), revealed solid-liquid
(when t.ltoreq.7.+-.2 min) and liquid-solid (when t.gtoreq.20 min)
phase transitions of the catalyst induced by carbon atoms diffusion
into the catalyst, and formation of Fe--C phases, respectively
(FIG. 1B) (H. Kanzow, A. Ding, Phys. Rev. B 60, 11180 (1999); A. R
Harutyunyan, E. Mora, T. Tokune Applied Phys. Lett. 87, 051919
(2005)). Comparison of these results (FIGS. 1A and B) shows that
the increases of catalyst activity coincidences with liquefaction
process of catalyst, while the liquid-solid phase transition
initiates deactivation of catalyst. The Raman spectroscopy
(.lamda.=532 nm and 780 nm) studies show dramatic increases of the
ratio between the intensities of grown SWNT's G-band and D-band
(I.sub.G/I.sub.D), which is a measure of the graphitic order in the
carbon deposit (H. Cui, G. Eres, J. Y. Howe, A. Puretzky, M.
Varela, D. B. Geohegan, D. H. Lowndes, Chem. Phys. Lett. 374, 222
(2003)), in the same t<7 to 10 min interval, where the catalyst
is in liquid phase and possess high activity (FIG. 1C), and about
70 wt % of overall carbon yield (wt % carbon relative to the
Fe/alumina catalyst) also was gained in the same interval of time.
To help establish the relationship between observed evolution of
catalyst features and nanotube growth, along with C.sup.12H.sub.4
gas, we use sequential introduction of methane gas with C.sup.13
isotope (C.sup.13H.sub.4, 99.99%, Cambridge Isotope Lab. Inc.) in
the intervals of time when catalyst is liquefied and possess high
activity and as well as when catalyst begins to solidify and looses
the activity. A series of samples were prepared by using the
methane gas C.sup.12H.sub.4 for the first 3 min, 7 min (catalyst
still liquefied) and 13 min (catalyst solidified) with following
introductions of the C.sup.13H.sub.4 gas for 17 min, 13 min and 7
min respectively (insets in FIG. 1A: A1, A2, A3). Importantly, the
Raman spectra of carbon SWNTs obtained by using methane gas with
C.sup.13 isotope is identical to the spectra with C.sup.12 isotope.
The only principal difference is that the Raman shift frequency is
12/13 times smaller because the heavier carbon atoms result smaller
phonon energies. The Raman spectra for the sample synthesized using
the C.sup.12H.sub.4 for the first 3 min with following introduction
C.sup.13H.sub.4 for 17 min, contains significant contribution
corresponding to the SWNTs with C.sup.13 atoms, while for the
sample feed with C.sup.12H.sub.4 for first 7 min of growth duration
and then 13 min with C.sup.13H.sub.4, this contribution decreases
(FIG. 2B, C). Finally the spectrum for the sample with 13 min
duration of C.sup.12H.sub.4 source and 7 min C.sup.13H.sub.4 is
completely identical with the spectrum of nanotubes with only
C.sup.12 isotope. Comparison of these results with catalyst
activity and DSC measurements, (FIG. 1A,B,C) allow to conclude,
that the liquefied catalyst is favorable for carbon SWNTs growth.
Moreover, one of the reasons for growth termination is the
solidification of catalyst through the formation of stable carbide
phases.
[0037] It is known that the addition of Mo to Fe catalyst makes it
more efficient for SWNTs production (A-C. Dupuis, Prog. In
Materials Science 50, 929-961 (2005); Moisala, A. G. Nasibulin, E.
I. Kauppinen, Journ. of Condensed Matter 15, 53011 (2003)). The
above described experiments were conducted using
Fe:Mo:Al.sub.2O.sub.3 (with common molar ratio 1:0.21:15) catalyst
under analogically synthesis conditions. The first distinctive
feature observed is that detected activity of Fe: Mo:
Al.sub.2O.sub.3 catalyst was dramatically higher of that for
Fe:Al.sub.2O.sub.3 (FIG. 3A) for during all synthesis duration, and
the formation of carbon SWNTs occurs at earlier stages of synthesis
(FIG. 3B,C). Second, it is remarkable that the evolution behavior
of Fe:Mo:Al.sub.2O.sub.3 (1:0.21:15) catalyst is qualitatively and
quantitatively different compare with mathematical sum of
Fe:Al.sub.2O.sub.3 (1:15) and Mo:Al.sub.2O.sub.3 (molar ratio
0.21:15) catalysts. This fact was attributed to the intermetallic
interaction between Mo and Fe with formation of Fe--Mo alloy.
Moreover, DSC measurements show that the catalyst is still in
liquid state even for nanotube growth duration up to 90 min (FIG.
1B) and therefore is able to produce nanotubes. Indeed, in contrast
to Fe catalyst the Raman spectra for the nanotubes obtained by
using Fe/Mo catalyst and sequential introduction of C.sup.12H.sub.4
for 13 min with following introduction of C.sup.13H.sub.4 for 7 min
show clear contribution of C.sup.13 atoms (FIG. 2). However, the
catalyst does not show activity when t>30 min (FIG. 3A) and no
any contributions from C.sup.13 atoms were found in the Raman
spectra of nanotubes when C.sup.13H.sub.4 was introduced into the
reactor following C.sup.12H.sub.4 after t.gtoreq.20 min. So, even
though the catalyst was found liquefied till 90 min it does not
results nanotube growth when t.gtoreq.20 min. This fact attributed
to the formation of various form of disordered sp.sup.2 carbon
(M.S. Dresselhaus, G. Dresselhaus, M. Pimenta, P. C. Eklund,
Analytical Applications of Raman Spectroscopy; Pelletier, M., Ed.;
1999, 367) along with nanotube growth, which covers the surface of
catalyst and eventually deactivates it. Thus, the addition of Mo
importantly prevents the solidification because of carbide
formation, and as a result prolongs the catalyst lifetime favorable
for nanotube growth almost 2 times.
[0038] Even though the catalyst features are favorable for nanotube
growth still it is important to know appropriate synthesis
conditions. The activity of catalyst was evaluated during
synthesis. The variation of synthesis temperature for given other
parameters shows that at the temperature .about.680.degree. C. the
catalyst Fe/Mo can ignite the nanotube growth while in case of pure
Fe it requires temperature >720.degree. C. Taking into account
that growth of nanotube requires carbon induced liquefaction, the
min synthesis temperature is therefore limited by the temperature
of eutectic point. Without being bound to theory, this is in
agreement also with fact that the addition of Mo decreases the
temperature of eutectic point of bulk Fe--Mo--C according ternary
phase diagram. On the other hand the thermal decomposition
temperature of used particular carbon source limits the synthesis
temperature from the supreme. It is obvious that in case of
Tsynthesis>830.degree. C., the contribution of carbon atoms
formed because of thermal decomposition will be significant and may
rapidly poison the catalyst and affect on quality of tubes by
coating the catalyst surface and tubes walls, respectively. Finally
Tsynthesis>Tmelt (catalyst) results the intermetallic
interaction between Fe and Alumina support according to DSC
measurements, with further deactivation of catalyst.
[0039] Thus, as one can see from FIG. 1B, indeed the liquidus line
for supported nanoparticle Fe--C binary phases has a "V-shape" (is
not monotonous) dependence on carbon concentration. It is worth to
mention that the liquidus lines of corresponding bulk metal-carbon
binary phase diagrams of all common catalysts used for nanotube
growth have a "V"-shape, which assumes the liquefaction of catalyst
due to the carbon diffusion at appropriate temperatures. However,
it is crucial that the behavior of liquidus line depends on
interfacial interaction energy between nanocatalyst and support
material, which became more significant when E>0.1 ev. Moreover,
this interaction results significant increasing of melting
point/eutectic point for the smaller diameter .apprxeq.1 nm. The
expressions
T.sub.melting>T.sub.synthesis>T.sub.eutectic and
T.sub.decomp>T.sub.synthesis.sup.5
where T.sub.decomp. is a thermal decomposition temperature of
hydrocarbon, describe the relationship between catalyst features
and synthesis parameters favorable for SWNTs growth.
[0040] While the invention has been particularly shown and
described with reference to a preferred embodiment and various
alternate embodiments, it will be understood by persons skilled in
the relevant art that various changes in form and details can be
made therein without departing from the spirit and scope of the
invention. All printed patents and publications referred to in this
application are hereby incorporated herein in their entirety by
this reference.
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