U.S. patent application number 11/669124 was filed with the patent office on 2008-11-13 for method and apparatus for growth of high quality carbon single-walled nanotubes.
Invention is credited to Avetik R. Harutyunyan.
Application Number | 20080279753 11/669124 |
Document ID | / |
Family ID | 38997607 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080279753 |
Kind Code |
A1 |
Harutyunyan; Avetik R. |
November 13, 2008 |
Method and Apparatus for Growth of High Quality Carbon
Single-Walled Nanotubes
Abstract
Method and processes for synthesizing single-wall carbon
nanotubes is 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 50 nm. The reaction temperature is selected such that it is
near the eutectic point of the mixture of metal catalyst particles
and carbon.
Inventors: |
Harutyunyan; Avetik R.;
(Columbus, OH) |
Correspondence
Address: |
HONDA/FENWICK
SILICON VALLEY CENTER, 801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Family ID: |
38997607 |
Appl. No.: |
11/669124 |
Filed: |
January 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60763813 |
Jan 30, 2006 |
|
|
|
Current U.S.
Class: |
423/447.2 ;
423/447.1; 977/750 |
Current CPC
Class: |
B82Y 40/00 20130101;
C01B 32/162 20170801; C01B 2202/02 20130101; B82Y 30/00
20130101 |
Class at
Publication: |
423/447.2 ;
423/447.1; 977/750 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Claims
1. A chemical vapor deposition method for the preparation of
single-wall carbon: nanotubes (SWNT), the method comprising:
contacting a carbon precursor gas with a catalyst on a support at a
temperature less than the melting point of the catalyst and about
5.degree. C. to about 150.degree. C. above the eutectic point of
the catalyst wherein SWNT are formed.
2. The method of claim 1, wherein the carbon precursor gas is
methane.
3. The method of claim 2, wherein the carbon precursor gas further
comprises an inert gas and hydrogen.
4. The method of claim 3, wherein the inert gas is argon, helium,
nitrogen, hydrogen, or combinations thereof.
5. The method of claim 1, wherein the catalyst is iron, molybdenum,
or combinations thereof.
6. The method of claim 1, wherein the catalyst has a particle size
between 1 nm to 10 nm.
7. The method of claim 6, wherein the catalyst has a particle size
of about 1 nm.
8. The method of claim 6, wherein the catalyst has a particle size
of about 3 nm.
9. The method of claim 6, wherein the catalyst has a particle size
of about 5 nm.
10. The method of claim 6, wherein the support is a powdered oxide
selected from the group consisting of Al.sub.2O.sub.3, SiO.sub.3,
MgO and zeolites.
11. The method of claim 10, wherein the powdered oxide is
Al.sub.2O.sub.3.
12. The method of claim 1, wherein the temperature is about
10.degree. C. to about 100.degree. C. above the eutectic point.
13. The method of claim 1, wherein the temperature is about
50.degree. C. above the eutectic point.
14. The method of claim 13, wherein the temperature is about
80.degree. C. above the eutectic point.
15. The method of claim 1, wherein the SWNTs have a diameter of
about 0.8 nm to about 2 nm.
16. A single-wall carbon nanotube (SWNT) produced by the process
of: contacting a carbon precursor gas with a catalyst on a support
selected from the group consisting of Al.sub.2O.sub.3, SiO.sub.3,
MgO and zeolite; and maintaining reaction temperature between the
melting point of the catalyst and the eutectic point of the
catalyst.
17. The process of claim 16, wherein the carbon precursor gas is
methane.
18. The process of claim 17, wherein the carbon precursor gas
further comprises an inert gas and hydrogen.
19. The process of claim 18, wherein the inert gas is argon,
helium, nitrogen, hydrogen, or combinations thereof.
20. The process of claim 16, wherein the catalyst is iron,
molybdenum, or combinations thereof
21. The process of claim 16, wherein the catalyst has a particle
size between 1 nm to 10 nm.
22. The process of claim 16, wherein the powdered oxide is
Al.sub.2O.sub.3.
23. The process of claim 16, wherein the catalyst and the support
are in a ratio of about 1:1 to about 1:50.
24. The process of claim 16, wherein the temperature is about
10.degree. C. to about 100.degree. C. above the eutectic point.
25. The process of claim 16, wherein the temperature is about
50.degree. C. above the eutectic point.
26. The process of claim 16, wherein the temperature is about
80.degree. C. above the eutectic point.
Description
RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of U.S.
Provisional Application 60/763,813, 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.
BACKGROUND
[0003] Carbon nanotubes are hexagonal networks of carbon atoms
forming seamless tubes with each end capped with half of a
fullerene molecule. They were first reported in 1991 by Sumio
Tijima who produced multi-layer concentric tubes or multi-walled
carbon nanotubes by evaporating carbon in an arc discharge. They
reported carbon nanotubes having up to seven walls. In 1993,
Iijima's group and an IBM team headed by Donald Bethune
independently discovered that a single-wall nanotube could be made
by vaporizing carbon together with a transition metal such as iron
or cobalt in an arc generator (see Iijima et al Nature 363:603
(1993); Bethune et al, Nature 363: 605 23085-12560 (1993) and U.S.
Pat. No. 5,424,054). The original syntheses produced low yields of
non-uniform nanotubes mixed with large amounts of soot and metal
particles.
[0004] Presently, there are three main approaches for the synthesis
of single- and multi-walled carbon nanotubes. These include the
electric arc discharge of graphite rod (Journet et al. Nature 388:
756 (1997)), the laser ablation of carbon (Thess et al Science 273:
483 (1996)), and the chemical vapor deposition of hydrocarbons
(Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science
274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on
a commercial scale by catalytic hydrocarbon cracking while
single-walled carbon nanotubes are still produced on a gram
scale.
[0005] Generally, single-walled carbon nanotubes are preferred over
multi-walled carbon nanotubes because they have unique mechanical
and electronic properties. Defects are less likely to occur in
single-walled carbon nanotubes because multi-walled carbon
nanotubes can survive occasional defects by forming bridges between
unsaturated carbon valances, while single-walled carbon nanotubes
have no neighboring walls to compensate for defects. Defect-free
single-walled nanotubes are expected to have remarkable mechanical,
electronic and magnetic properties that could be tunable by varying
the diameter, number of concentric shells, and chirality of the
tube.
[0006] Single-walled carbon nanotubes have been produced by
simultaneously evaporating carbon and a small percentage of Group
VIII transition metal from the anode of the arc discharge apparatus
(Saito et al. Chem. Phys. Lett. 236: 419 (1995)). Further, the use
of mixtures of transition metals has been shown to increase the
yield of single-walled carbon nanotubes in the arc discharge
apparatus. However, the yield of nanotubes is still low, the
nanotubes can exhibit significant variations in structure and size
(properties) between individual tubes in the mixture, and the
nanotubes can be difficult to separate from the other reaction
products. In a typical arc discharge process, a carbon anode loaded
with catalyst material (typically a combination of metals such as
nickel/cobalt, nickel/cobalt/iron, or nickel and transition element
such as yttrium) is consumed in arc plasma. The catalyst and the
carbon are vaporized and the single-walled carbon nanotubes are
grown by the condensation of carbon onto the condensed liquid
catalyst. Sulfur compounds such as iron sulfide, sulfur or hydrogen
sulfides are typically used as catalyst promoter to maximize the
yield of the product.
[0007] A typical laser ablation process for producing single-walled
carbon nanotubes is disclosed by Andreas Thess et al. (1996). Metal
catalyst particle such as nickel-cobalt alloy is mixed with
graphite powder at a predetermined percentage, and the mixture is
pressed to obtain a pellet. A laser beam is radiated to the pellet.
The laser beam evaporates the carbon and the nickel-cobalt alloy,
and the carbon vapor is condensed in the presence of the metal
catalyst. Single-wall carbon nanotubes with different diameters are
found in the condensation. However, the addition of a second laser
to their process which give a pulse 50 nanoseconds after the pulse
of the first laser favored the (10,10) chirality (a chain of 10
hexagons around the circumference of the nanotube). The product
consisted of fibers approximately 10 to 20 nm in diameter and many
micrometers long comprising randomly oriented single-wall
nanotubes, each nanotube having a diameter of about 1.38 nm.
[0008] Many researchers consider chemical vapor deposition as the
only viable approach to large scale production and for controllable
synthesis of carbon single walled nanotubes (Dai et al. (Chem.
Phys. Lett 260: 471 (1996), Hafner et al., Chem. Phys. Lett. 296:
195 (1998), Su. M., et al. Chem. Phys. Lett., 322: 321 (2000)).
Typically, the growth of carbon SWNTs by CVD method is conducting
at the temperatures 550-1200.degree. C. by decomposition of
hydrocarbon gases (methane, ethylene, alcohol, . . . ) on metal
nanoparticles (Fe, Ni, Co, . . . ) supported by oxide powders. The
diameters of the single-walled carbon nanotubes vary from 0.7 nm to
3 nm. The synthesized single-walled carbon nanotubes are roughly
aligned in bundles and woven together similarly to those obtained
from laser vaporization or electric arc method. The use of metal
catalysts comprising iron and at least one element chosen from
Group V (V, Nb and Ta), VI (Cr, Mo and W), VII (Mn, Tc and Re) or
the lanthanides has also been proposed (U.S. Pat. No.
5,707,916).
[0009] Presently, there are two types of chemical vapor deposition
for the syntheses of single-walled carbon nanotubes that are
distinguishable depending on the form of supplied catalyst. In one,
the catalyst is embedded in porous material or supported on a
substrate, placed at a fixed position of a furnace, and heated in a
flow of hydrocarbon precursor gas. Cassell et al. (1999) J. Phys.
Chem. B 103: 6484-6492 studied the effect of different catalysts
and supports on the synthesis of bulk quantities of single-walled
carbon nanotubes using methane as the carbon source in chemical
vapor deposition. They systematically studied Fe(NO.sub.3).sub.3
supported on Al.sub.2O.sub.3, Fe(SO.sub.4).sub.3 supported on
Al.sub.2O.sub.3, Fe/Ru supported on Al.sub.2O.sub.3, Fe/Mo
supported on Al.sub.2O.sub.3, and Fe/Mo supported on
Al.sub.2O.sub.3--SiO.sub.2 hybrid support. The bimetallic catalyst
supported on the hybrid support material provided the highest yield
of the nanotubes. Su et al. (2000) Chem. Phys. Lett. 322: 321-326
reported the use of a bimetal catalyst supported on an aluminum
oxide aerogel to produce single-walled carbon nanotubes. They
reported preparation of the nanotubes is greater than 200% the
weight of the catalyst used. In comparison, similar catalyst
supported on Al.sub.2O.sub.3 powder yields approximately 40% the
weight of the starting catalyst. Thus, the use of the aerogel
support improved the amount of nanotubes produced per unit weight
of the catalyst by a factor of 5.
[0010] In the second type of carbon vapor deposition, the catalyst
and the hydrocarbon precursor gas are fed into a furnace using the
gas phase, followed by the catalytic reaction in a gas phase. The
catalyst is usually in the form of a metalorganic. Nikolaev et al.
(1999) Chem. Phys. Lett. 313: 91 disclose a high-pressure CO
reaction (HiPCO) method in which carbon monoxide (CO) gas reacts
with the metalorganic iron pentacarbonyl (Fe(CO).sub.5) to form
single-walled carbon nanotubes. It is claimed that 400 g of
nanotubes can be synthesized per day. Chen et al. (1998) Appl.
Phys. Lett. 72: 3282 employ benzene and the metalorganic ferrocene
(Fe(C.sub.5H.sub.5).sub.2) delivered using a hydrogen gas to
synthesize single-walled carbon nanotubes. The disadvantage of this
approach is that it is difficult to control particles sizes of the
metal catalyst. The decomposition of the organometallic provides
disordered carbon (not desired) the metal catalyst having variable
particle size that results in nanotubes having a wide distribution
of diameters and low yields.
[0011] In another method, the catalyst is introduced as a liquid
pulse into the reactor. Ci et al. (2000) Carbon 38: 1933-1937
dissolve ferrocene in 100 mL of benzene along with a small amount
of thiophene. The solution is injected into a vertical reactor in a
hydrogen atmosphere. The technique requires that the temperature of
bottom wall of the reactor had to be kept at between
205-230.degree. C. to obtain straight carbon nanotubes. In the
method of Ago et al. (2001) J. Phys. Chem. 105: 10453-10456,
colloidal solution of cobalt:molybdenum (1:1) nanoparticles is
prepared and injected into a vertically arranged furnace, along
with 1% thiophene and toluene as the carbon source. Bundles of
single-walled carbon nanotubes are synthesized. One of the
disadvantages of this approach is the very low yield of the
nanotubes produced.
[0012] It is generally recognized that smaller catalyst particles
of less than 3 nm are preferred for the growth of smaller diameter
carbon nanotubes. However, the smaller catalyst particles easily
aggregate at the higher temperatures required for the synthesis of
carbon nanotubes. U.S. Patent Application No. 2004/0005269 to Huang
et al. discloses a mixture of catalysts containing at least one
element from Fe, Co, and Ni, and at least one supporting element
from the lanthanides. The lanthanides are said to decrease the
melting point of the catalyst by forming alloys so that the carbon
nanostructures can be grown at lower temperatures.
[0013] Aside from the size of the catalyst, the temperature of the
reaction chamber can also be important for the growth of carbon
nanotubes. U.S. Pat. No. 6,764,874 to Zhang et al. discloses a
method of preparing nanotubes by melting aluminum to form an
alumina support and melting a thin nickel film to form nickel
nanoparticles on the alumina support. The catalyst is then used in
a reaction chamber at less than 850.degree. C. U.S. Pat. No.
6,401,526, and U.S. Patent Application Publication No.
2002/00178846, both to Dai et al., disclose a method of forming
nanotubes for atomic force microscopy. A portion of the support
structure is coated with a liquid phase precursor material that
contains a metal-containing salt and a long-chain molecular
compound dissolved in a solvent. The carbon nanotubes are made at a
temperature of 850.degree. C.
[0014] Thus, it is well known that the diameter of the SWNTs
produced is proportional to the size of the catalyst particle. In
order to synthesize nanotubes of small diameter, it s necessary to
have catalyst particles of very small particle size (less than
about 1 nm). Catalysts of small particle size are difficult to
synthesize, and even with small catalyst particle sizes, a
distribution of catalyst sizes is obtained which results in the
formation of nanotubes with a range of diameters.
[0015] One solution to the synthesis of uniform diameter nanotubes
is to use a template, such as molecular sieves, that have a pore
structure which is used to control the distribution of catalyst
size and thereby the size of the SWNTs formed. Thus, the diameter
of SWNT can be changed by changing the pore size of the template.
These methods are not versatile. Thus, there is a need for methods
and processes for controllable synthesis of carbon single walled
nanotubes with small and narrow distributed diameters. Accordingly,
the present invention provides novel methods and processes for the
synthesis of SWNTs with small and narrow distributed diameters.
SUMMARY
[0016] 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.
[0017] In one aspect, the methods involve contacting a carbon
precursor gas with a catalyst on a support at a temperature near
the eutectic point of the catalyst-carbon phase wherein SWNT are
formed. The carbon precursor gas can be methane that can
additionally contain other gases such as argon and hydrogen. The
catalyst can be a V metal, a Group VI metal, a Group VII metal, a
Group VIII metal, a lanthanide, or a transition metal or
combinations thereof. The catalyst preferably has a particle size
between about 1 nm to about 50 nm. The catalyst can be supported on
a powdered oxide, such as Al.sub.2O.sub.3, SiO.sub.3, MgO and the
like, herein the catalyst and the support are in a ratio of about
1:1 to about 1:50. The SWNTs are produced by employing a reaction
temperature that is about 5.degree. C. to about 150.degree. C.
above the eutectic point.
[0018] In another aspect, the invention provides a carbon nanotube
structure produced by the process of contacting a carbon precursor
gas with a catalyst on a support at a temperature between the
melting point of the catalyst and the eutectic point of the
catalyst and carbon. The carbon precursor gas can be methane that
can additionally contain other gases such as argon and hydrogen.
The catalyst can be a V metal, a Group VI metal, a Group VII metal,
a Group VIII metal, a lanthanide, or a transition metal or
combinations thereof. The catalyst preferably has a particle size
between about 1 nm to about 15 nm. The catalyst can be supported on
a powdered oxide, such as Al.sub.2O.sub.3, SiO.sub.3, MgO and the
like, wherein the catalyst and the support are in a ratio of about
1:1 to about 1:50.
BRIEF DESCRIPTION OF DRAWINGS
[0019] FIG. 1. A) Evolution of hydrogen concentration during carbon
SWNTs growth on Fe:Al.sub.2O.sub.3 (1:15 molar ratio) catalyst.
Insets: sequential introduction of C.sup.12 and C.sup.13 isotopes,
for 3 min and 17 min (a1); 7 min and 13 min (a2) and 13 min and 7
min (a3), respectively. B) Evolution of melting point of Fe
catalyst (solid circles) and Fe:Mo (1:0.21 molar ratio, open
squares) during carbon SWNTs growth measured by DSC technique. C)
Evolutions of Raman I.sub.G/I.sub.D ratios for carbon SWNTs growth
on Fe catalyst and carbon uptake dependence on synthesis duration.
Inset: Evolution of I.sub.G/I.sub.D ratios for the carbon SWNTs
growth on Fe:Mo (1:0.21 molar ratio) catalyst.
[0020] FIG. 2. Raman radial breathing and tangential modes for
carbon SWNTs synthesized on Fe and Fe:Mo catalysts by using
sequential introduction of C.sup.12 and C.sup.13 isotopes.
[0021] 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) samples.
[0022] FIG. 4. Hydrogen concentration evolution dependence on
reactor temperature for Al.sub.2O.sub.3 support material, for
Mo:Al.sub.2O.sub.3; Fe:Al.sub.2O.sub.3 and Fe:Mo:Al.sub.2O.sub.3
catalysts, respectively.
DETAILED DESCRIPTION
[0023] 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, New York.
[0024] 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 a hexagonal crystalline structure with a graphitic type
of bonding.
[0025] The term "multi-walled carbon nanotube" as used herein
refers to a nanotube composed of more than one concentric
tubes.
[0026] 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.
[0027] 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.
[0028] 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 the support,
preferably alumina is used as the support.
[0029] 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, molybdenum, or a mixture thereof,
such as Fe-Mo, Co-Mo and Ni-Fe-Mo.
[0030] The metal, bimetal, or combination of metals can be 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.
[0031] 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.
[0032] 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.2--O--(CH.sub.2).sub.3CH.sub.3, which will
be referred to below using the common name diethylene glycol
mono-n-butyl ether, and the like.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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, OR. 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.
[0038] 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.
[0039] 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, MA, or Degussa, NJ.
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.
[0040] 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.
[0041] 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.
For example, extraction includes heating the homogenized mixture to
a temperature where the passivating solvent has a significant vapor
pressure. This temperature can be maintained until the passivating
solvent evaporates, leaving behind the metal nanoparticles
deposited in the pores of the Al.sub.2O.sub.3. For example, if
diethylene glycol mono-n-butyl ether as the passivating solvent,
the homogenous dispersion can be heated to 231.degree. C., the
boiling point of the passivating solvent, under an N.sub.2 flow.
The temperature and N.sub.2 flow are maintained until the
passivating solvent is completely evaporated. 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] The growth technique, enhanced by an attached mass
spectrometer for in-situ parametrical studies, enables us to
elucidate the evolution of catalyst activity during carbon single
walled nanotubes (SWNTs) growth and in this manner reveal the
catalyst features and their relationship with the growth
conditions. Any changes of catalyst features due to the composition
modification, diameter variation or interaction with support
material we were detected by monitoring catalyst activity. By
variation of synthesis temperature, duration and carbon feedstock,
type of transport gas and pressure we exposed their relationship
with catalyst activity and in this manner with catalyst features
and thereby reveal the optimum condition for growth of high quality
carbon SWNTs.
[0050] 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. It is
already established that catalyst preparation methods,
pretreatments, diameters, crystallographic and electronic
structures, and abilities of carbide and oxide formations have
influence on nanotube growth. 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, it was found
that the synthesis parameters have an impact on the thermodynamics
and kinetics of the growth. Nevertheless, intense research to
reveal the common features of the catalysts and corresponding
synthesis parameters favorable for nanotube growth from described
complexity is still underway.
[0051] 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. Then
current corresponding to the mass m=2 was measured which is
prortional to the molecules concentration. The catalytic
decomposition of methane results the proportional numbers of
hydrogen molecules and carbon atoms. Of course there are the
possibilities of formation also other hydrocarbons CxHy. However,
the analogical measurements by using in parallel also Gas
Chromatography (GC-17A, SHIMADZU), with monitoring the presents of
the other molecules C.sub.2H.sub.2, C.sub.2H.sub.6, C.sub.2H.sub.4,
CO and CO.sub.2 in the stream have shown that the main product of
the decomposition was H molecules. Therefore, the evolution of
carbon atoms concentration is analogical to the evolution behavior
of formed hydrogen molecules.
[0052] Before introduce into the reactor the gases were passed
through a purification cartridge of specific purifier (Praxair) in
order to trap residual O.sub.2 and H.sub.2O. After catalyst
reduction the reactor was thoroughly purged about 3 hours by Ar gas
in order to remove residual H.sub.2 and He from the reactor. For
calibration and comparison of independent measurements the exit
stream of the reactor was fitted by N.sub.2 as a standard gas.
Along with the hydrogen also CH.sub.4, H.sub.2O, gases were
monitored.
[0053] 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.
[0054] 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=1:1 5) 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.+-.11 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. Interestingly, the Raman
spectroscopy (.lamda.=785 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, 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, methane gas with C.sup.13 isotope
(C.sup.13H.sub.4, 99.99%, Cambridge Isotope Lab. Inc.) was
sequentially introduced 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). 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, (FIGS. 1A,B,C) allow
to conclude, that the liquefied catalyst is favorable for carbon
SWNTs growth and the catalyst lifetime, favorable for growth for
this particular synthesis conditions, is .tau..apprxeq.7-10 min.
Moreover, one of the reasons for growth termination is the
solidification of catalyst through the formation of stable carbide
phases.
[0055] It is well known that the addition of Mo to Fe catalyst
makes it more efficient for SWNTs production. The combined studies
described above were repreated 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. 3) for during all
synthesis duration, and the formation of carbon SWNTs occurs at
earlier stages of synthesis. According to previous studies, the
nanotube growth does not begin immediately after the introduction
of hydrocarbon gas, but require certain carbon concentration to be
dissolved in and diffuse through the particle before the growth can
start. The temperature and concentration gradients are the main
driving forces for this process. In CVD reactions the temperature
gradient appears because of exothermal hydrocarbon decomposition
reaction, which is not the case in our experiment because we use
the CH.sub.4 gas. So, the threshold concentration gradient of
carbon atoms has been reached rapidly in case of Fe/Mo catalyst due
to the high catalyst activity and probably less activation energy
of diffusion. Second, 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 possible 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 carbon 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.
[0056] Even though the catalyst features are favorable for nanotube
growth still it is important to know appropriate synthesis
conditions. The activity of catalyst on synthesis parameters was
evaluated. The evolution of hydrogen concentration dependence on
reactor temperature for the Al.sub.2O.sub.3; Fe:Al.sub.2O.sub.3;
and Fe:Mo:Al.sub.2O.sub.3 samples for fixed other parameters is
shown in FIG. 4. As one can see that the thermal decomposition
temperature of used particular carbon source (CH.sub.4) 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. On the
other hand adding the Mo results the higher decomposition rate of
carbon formation (increases the catalyst activity) for given
temperature compare with pure Fe catalyst.
[0057] Without being bound to theory, mass spectrometer attached
into the CVD technique may offer opportunities for parametrical
studies of catalyst features during carbon SWNTs growth, by in-situ
evolution of catalyst activity. By monitoring hydrogen
concentration occurred because of hydrocarbon decomposition it is
possible to reveal the a) catalyst lifetime for nanotube growth; b)
to find the appropriate support material for given catalyst
composition and diameter; c) establish the optimal catalyst
composition and size for growth of carbon nanotube; d) establish
the optimal synthesis temperature which leads growth of high
quality carbon nanotube by excluding of formation of amorphous
carbon; e) establish the carbon feedstock rate for growth of high
quality carbon SWNTs; f) establish the appropriate pressure of
gases inside the reactor favorable for carbon SWNTs growth; g)
establish the composition of transport gases, including the
oxidizers and reducers.
[0058] 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.
* * * * *