U.S. patent application number 10/169786 was filed with the patent office on 2003-01-16 for high yiel vapor phase deposition method for large scale sing walled carbon nanotube preparation.
Invention is credited to Liu, Jie.
Application Number | 20030012722 10/169786 |
Document ID | / |
Family ID | 22617167 |
Filed Date | 2003-01-16 |
United States Patent
Application |
20030012722 |
Kind Code |
A1 |
Liu, Jie |
January 16, 2003 |
High yiel vapor phase deposition method for large scale sing walled
carbon nanotube preparation
Abstract
An improved vapor phase deposition method for preparation of
single walled carbon nanotubes on an aerogel supported metal
catalyst. The total yield of SWCNTs often is at least about 100%,
based the weight of the catalyst, for a reaction time of at least
about 30 minutes.
Inventors: |
Liu, Jie; (Chapel Hill,
NC) |
Correspondence
Address: |
WILLIAMS MORGAN & AMERSON
7676 HILLMONT
SUITE 250
HOUSTON
TX
77040
US
|
Family ID: |
22617167 |
Appl. No.: |
10/169786 |
Filed: |
July 2, 2002 |
PCT Filed: |
January 5, 2001 |
PCT NO: |
PCT/US01/00335 |
Current U.S.
Class: |
423/447.3 |
Current CPC
Class: |
D01F 9/127 20130101;
B82Y 30/00 20130101 |
Class at
Publication: |
423/447.3 |
International
Class: |
D01F 009/12 |
Goverment Interests
[0001] This work is in part supported by Office of Naval Research
grant #00014-98-1-0597 through the University of North Carolina in
Chapel Hill, N.C., United States of America. Thus, the United
States government has certain rights in the invention.
Claims
What is claimed is:
1. A method of preparing single walled carbon nanotubes comprising
depositing a carbon-containing compound under vapor phase
conditions onto a supported catalyst comprising a metal catalyst
and an aerogel support while heating under reaction conditions of
temperature sufficient and time sufficient to form the single
walled carbon nanotubes on the aerogel supported catalyst.
2. The method of claim 1, where the carbon-containing compound has
a molecular weight of 28 or less.
3. The method of claim 2, where the carbon-containing compound is
selected from the group consisting of methane, carbon monoxide, and
combinations thereof.
4. The method of claim 1, where the carbon-containing compound has
a molecular weight greater than 28 and is mixed with hydrogen.
5. The method of claim 4, where the carbon-containing compound is
selected from the group consisting of ethylene, benzene, and
combinations thereof.
6. The method of claim 1, where the metal catalyst is selected from
the group consisting of Fe/Mo, Fe/Pt, and combinations thereof.
7. The method of claim 1, where the aerogel support is selected
from the group consisting of Al.sub.2O.sub.3 aerogel support,
Al.sub.2O.sub.3/SiO.sub.2 aerogel support, and combinations
thereof.
8. The method of claim 1, where said depositing is with a
sufficient flow rate of the carbon-containing compound, with
sufficient heat for a sufficient time, to obtain a yield of at
least about 100% based on the weight of the catalyst.
9. The method of claim 8, where the sufficient flow rate ranges
from about 900 sccm to about 1300 sccm.
10. The method of claim 1, where the aerogel supported catalyst has
a surface area of from about 500 m.sup.2/g to about 600
m.sup.2/g.
11. The method of claim 1, where the aerogel supported catalyst has
been dried by drying selected from the group consisting of
supercritical drying, freeze drying, and combinations thereof.
12. The method of claim 11, where the supercritical drying is
selected from the group consisting of CO.sub.2 supercritical
drying, ethanol supercritical drying, and combinations thereof.
13. The method of claim 11, where the freeze drying is freeze
drying using water.
14. The method of claim 1, where the sufficient heat provides a
temperature ranging from about 750.degree. C. to about 1000.degree.
C.
15. The method of claim 1, where the sufficient time is at least
about 0.25 hours.
16. The method of claim 1, where the yield is at least about
100%.
17. The method of claim 1, further including separating the single
walled carbon nanotubes from the aerogel supported catalyst.
18. A method of preparing single walled carbon nanotubes comprising
depositing a carbon-containing compound under vapor phase
conditions onto a supported catalyst comprising a metal catalyst
and an aerogel support where the carbon-containing compound is
selected from the group consisting of methane, carbon monoxide, and
combinations thereof, where the metal catalyst is selected from the
group consisting of Fe/Mo, Fe/Pt, and combinations thereof, where
the aerogel support is selected from the group consisting of
Al.sub.2O.sub.3 aerogel support, Al.sub.2O.sub.3/SiO.sub.2 aerogel
support, and combinations thereof, where the aerogel supported
catalyst has been dried by drying selected from the group
consisting of supercritical drying, freeze drying, and combinations
thereof, and where said depositing is with a sufficient flow rate
of the carbon-containing compound, with sufficient heat for a
sufficient time, to obtain a yield of at least about 100% based on
the weight of the catalyst.
Description
TECHNICAL FIELD
[0002] The present invention relates, in general, to a vapor phase
deposition method for the preparation of single walled carbon
nanotubes, where the method employs a metal catalyst on a support.
More particularly, the present invention relates to an improved
method where the support comprises an aerogel, such as an
Al.sub.2O.sub.3 aerogel or an Al.sub.2O.sub.3/SiO.sub.2 aerogel, as
compared to prior art methods that employed supports that are
powders. The improved method results in far higher yields of single
walled carbon nanotubes than the prior art methods.
1 Abbreviations ASB aluminum tri-sec-butoxide AFM atomic force
microscope (acac).sub.2 bis(acetylacetonato) cm centimeter C
centrigrade CVD chemical vapor deposition EtOH ethanol g gram kg
kilogram kV kilovolt m meter ml milliliter MW molecular weight
MWCNT multi-walled carbon nanotube nm nanometer psi pounds per
square inch SEM scanning electron microscope SWCNT single walled
carbon nanotube sccm standard cubic centimeter per minute STP
standard temperature and pressue Tpa tera pascal TGA thermal
gravimetric analyzer TEM transmission electron microscope
BACKGROUND OF THE INVENTION
[0003] Ever since the discovery by Iijima in 1991 of the carbon
nanotube, it has been one of the most actively studied materials in
today's research. See, Iijima, Vol. 354, Nature, pp. 56-58 (1991).
This active study is not very surprising given the outstanding
chemical and physical properties that this material possesses and
its potential applications in many different fields.
[0004] For example, depending on the number of concentric walls of
a graphene sheet and the ways that a graphene sheet is rolled into
a cylinder, carbon nanotubes can be conductive behaving like metals
or can be semi-conductive. See, Dresselhaus et al., Science of
Fullerenes and Carbon Nanotubes, Academic Press, San Diego
(1996).
[0005] Furthermore, experiments have shown that individual carbon
nanotubes can behave as quantum wires and can even be made into
room temperature transistors. See, Tans et al., Vol. 386, Nature,
pp. 474477 (1997) vis--vis quantum wires and Tans et al., Vol. 393,
Nature, pp. 49-52 (1998) vis--vis transistors.
[0006] In addition, carbon nanotubes have been shown to possess
superior mechanical properties and chemical stability. Experimental
measurements of carbon nanotubes for Young's moduli by AFM and for
thermal vibrations afforded respective values of 1.3 Tpa and 1.8
Tpa, which are higher than the values for any other known material.
See, Wong et al. Vol. 277, Science, pp. 1971-1975 (1997) vis--vis
AFM and Treacy et al., Vol. 381, Nature, pp. 678-680 (1996)
vis--vis thermal vibrations.
[0007] Consequently, the chemical stability, the superior
mechanical properties, the ballistic transport property of
metallic-like behavior, and the rich variation in electronic
properties due to different helicities make carbon nanotubes ideal
candidates for high strength composite materials, and for
interconnections and functional devices in molecular
electronics.
[0008] Although carbon nanotube materials possess many unique and
technically important properties, lack of a way to produce a
sufficient amount of materials has limited not only the study of
the fundamental properties but also the development of more
practical applications. The discovery of a low cost, high yield
method for preparation of SWCNT material will certainly solve one
of the biggest problems facing this field in the past and open new
opportunities for a wide variety of applications.
[0009] Currently, carbon nanotubes are synthesized by three
different techniques: (1) arc discharge between two graphite
electrodes, (2) CVD through catalytic decomposition of a
hydrocarbon or of CO, and (3) laser evaporation of the carbon
target. With respect to CVD, see, International Publication No. WO
89/07163 to Synder et al.; U.S. Pat. No. 4,663,230 (issued in 1987)
to Tennent et al.; M. Terrones et al., Nature 388, 52-55 (1997); Z.
F. Ren et al., Science 282, 1105-1107 (1998); J. Kong, A. Cassell,
and H. Dai, Chemical Physics Letters 292, 4-6 (1998); J. H. Hafner
et al., Chemical Physics Letters 296, 195-202 (1998); E. Flahaut et
al., Chemical Physics Letters 300, 236-242 (1999); S. S. Fan et
al., Science 283, 512-514 (1999); H. J. Dai et al., Chemical
Physics Letters 260, 471-475 (1996); H. M. Cheng et al., Applied
Physics Letters 72, 3282-3284 (1998); and A. M. Cassell, J. A.
Raymakers, J. Kong, and H. J. Dai, Journal of Physical Chemistry B
103, 6484-6492 (1999).
[0010] Both the laser method and the arc method yield high quality
SWCNTs. However, both techniques suffer from the problem that it is
hard to increase the production volume of the nanotube materials
from laboratory scale to industrial scale.
[0011] On the other hand, based on published reports, the CVD
method appears to represent the best hope for large scale
production of nanotube materials. This method has been reported in
the 1980's (by Tennent et al. in U.S. Pat. No. 4,663,230 (issued in
1987) and by M. S. Dresselhaus, G. Dresselhaus, K. Sugihara, I. L.
Spain, and H. A. Goldberg in Graphite Fibers and Filaments. M.
Cardona, et al., Eds., Springer Series in Materials Science 5
Springer-Verlag, New York (1988) vol. 5) for preparation of various
carbon materials such as carbon fibers and multi-walled carbon
nanotubes with a yield higher and on a scale larger than those
reported for the laser method and the arc method.
[0012] More recently in the 1990's, there have been reports of
SWCNT preparation by CVD (carbon monoxide or methane) and reports
of SWCNT preparation, mixed with a substantial amount of MWCNT
preparation, by CVD (benzene or ethylene). With respect to carbon
monoxide CVD, see, H. J. Dai et al., Chemical Physics Letters 260,
471-475 (1996) and P. Nikolaev et al., Chemical Physics Letters
313, 91 (1999). With respect to methane CVD, see, A. M. Cassell, J.
A. Raymakers, J. Kong, and H. J. Dai, "Large Scale CVD Synthesis of
Single-Walled Carbon Nanotubes", Journal of Physical Chemistry B
103,6484-6492 (1999) and E. Flahaut et al., Chemical Physics
Letters 300, 236-242 (1999). With respect to benzene CVD, see, H.
M. Cheng et al., Applied Physics Letters 72, 3282-3284 (1998). With
respect to ethylyne CVD, see J. H. Hafner et al., Chemical Physics
Letters296, 195-202(1998). Hence, although the report for ethylene
and the report for benzene each mentions SWCNTs, they have the
drawback that they are always mixed with a substantial amount of
MWCNTs.
[0013] Among these reported CVD methods, only the methane CVD
method has been reported to produce high purity and high quality
SWCNT materials. However, the reported yield of the methane CVD
method is low, with the best results so far giving a total yield of
40% based on the weight of catalyst for a reaction time of 10 to 45
minutes, where the catalyst was supported on Al.sub.2O.sub.3 powder
or on Al.sub.2O.sub.3/SiO.sub.2 powder and the catalyst/support had
a surface area of about 100 m.sup.2/g. See, Cassell et al.,
supra.
[0014] Thus, a CVD method that affords high quality SWCNTs in a far
higher yield (such as at least about 100% for a reaction time of
about 30 minutes) would be desirable.
SUMMARY AND OBJECTS OF THE INVENTION
[0015] Accordingly, the present invention provides vapor phase
method that employs a metal catalyst supported on an aerogel, for
instance on Al.sub.2O.sub.3 aerogel and/or on
Al.sub.2O.sub.3/SiO.sub.2 aerogel. The catalyst/support employed in
the present invention was made by solvent-gel synthesis with
subsequent removal of the liquid solvent by drying selected from
the group consisting of supercritical drying, freeze drying and
combinations thereof, with supercritical drying being preferred.
The inventive method involves vapor phase depositing on the
catalyst/support a carbon-containing compound. The compound should
have a molecular weight of 28 or less, and if the compound has a
higher molecular weight, then the compound should be mixed with
H.sub.2. The vapor phase depositing is with sufficient heat for a
sufficient time, in order to produce SWCNTs on the aerogel
supported catalyst. Then, the SWCNTs may, if desired, be removed
from the aerogel supported catalyst. Typically, the SWCNTs are
produced in high yield, for instance, about 100% or greater, based
on the weight of the catalyst.
[0016] Thus, it is an object of the invention that in a preferred
embodiment SWCNTs are obtained in high yields heretofore
unobtainable. This yield is far higher than that of the prior art
CVD method, which resulted, at best, in a yield of about 40% based
on the weight of the catalyst.
[0017] Hence, it is an advantage that this inventive discovery
affords a way to prepare SWCNT materials on a large scale, i.e.,
industrial scale, with low cost.
[0018] Some of the objects and advantages of the invention having
been stated, other objects will become evident as the description
proceeds, when taken in connection with the laboratory examples and
drawings described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a graph showing typical TGA yield curves for (a)
as-prepared and (b) purified SWCNT materials in air, made in
accordance with the inventive method.
[0020] FIG. 2 is a graph showing weight gain versus reaction time
at 900.degree. C. with a methane flow at 1158 sccm for a SWCNT
material prepared by the inventive method.
[0021] FIGS. 3a and 3b are, respectively, photographs taken through
a microscope showing (a) a SEM image and (b) a TEM image of a SWCNT
sample prepared by the inventive method on an Al.sub.2O.sub.3
aerogel supported Fe/Mo catalyst. The sample was prepared at about
900.degree. C. under a CH4 flow. The flow rate was 1158 sccm. The
reaction time was 30 minutes.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present invention provides single walled carbon
nanotubes using a novel vapor phase method in which a particular
catalyst/support is employed in deposition of a carbon-containing
compound. In a preferred embodiment, the present invention provides
a dramatic increase in the yield of single walled carbon nanotubes
as compared to the prior art method that uses powder for a
support.
[0023] By the term single walled carbon nanotubes is meant what is
conventionally known in the art. Moreover, with the inventive
method, it is not intended to exclude that a minor amount, for
instance <1%, of multi-walled carbon nanotubes may be
concurrently produced.
[0024] A suitable carbon-containing compound may be one that is
vapor at STP or may be one that is capable of being converted into
vapor at reaction conditions. Preferably, the compound is one that
has a molecular weight of 28 or less. Examples are CO, CH.sub.4,
and combinations thereof. If the compound has a molecular weight
greater than 28, for instance benzene (MW=78) or ethylene (MW=30),
then the compound should be admixed with H.sub.2, for instance, 50%
by volume H.sub.2.
[0025] To effect the preferred embodiment of a high yield of about
100% or more, a sufficient flow rate of the carbon-containing
compound should be employed, and may range from about 900 sccm to
about 1300 sccm.
[0026] A sufficient time may range from about 0.25 hours to about 7
hours. A sufficient temperature may range from about 750.degree. C.
to about 1000.degree. C. The yield may be about 200%, about 300%,
or even higher.
[0027] A suitable catalyst is any metal catalyst known in the art
for making nanotubes. A preferred metal catalyst may be Fe/Mo,
Fe/Pt, and combinations thereof. A suitable support is any aerogel
as that term is conventionally adopted in the art to mean a gel
with air as dispersing agent prepared by drying. The aerogel
support could be a powdered support converted to an aerogel by
known methods. As discussed in more detail below, the drying may be
supercritical drying or may be freeze drying, but it is not
intended to include drying that results in a xerogel. A preferred
aerogel support may be Al.sub.2O.sub.3 aerogel support,
Al.sub.2O.sub.3/SiO.sub.2 aerogel support, and combinations
thereof.
[0028] As shown in FIG. 1, the yield of the SWCNT material was
measured by heating up the prepared SWCNT material under flowing
air in a TGA. The total yield of SWCNT material, which yield is
shown on the vertical axis as a % weight gain, was calculated by
the weight loss between 300.degree. C. and 700.degree. C., which
temperature is shown on the horizontal axis, where the SWCNT
material burned in air, divided by the weight left at 700.degree.
C., which was presumably the weight of the catalyst and support
materials.
[0029] Purification of the material prepared in the inventive
method was also studied. Because of the highly amorphous nature of
the aerogel support prepared in the inventive method, removing the
catalyst and support from the SWCNT material turned out to be quite
easy. The support can be removed by stirring in dilute HF acid,
refluxing in another dilute acid (such as HNO.sub.3), or refluxing
in dilute base, such as NaOH solution. FIG. 1 shows the TGA result
of the material refluxed in 2.6 M HNO.sub.3 for about 4 hours,
followed by filtration.
[0030] As shown in FIG. 2, for a typical growth time of about 60
minutes at about 900.degree. C., the average yield using this
catalyst/support was about 200%. The maximum yield (weight gain)
was found to be about 600% for about 6.5 hours of growth. The
inventive method showed a yield of significantly better than the
values previously reported values by A. M. Cassell, J. A.
Raymakers, J. Kong, H. J. Dai, Journal of Physical Chemistry B 103,
6484-6492 (1999) Kong, Cassell, and Dai, Chemical Physics Letters
292, 4-6 (1998).
[0031] As shown in FIGS. 3a and 3b, the quality of the prepared
SWCNT was characterized by SEM imaging and TEM imaging.
[0032] More particularly, as depicted in FIG. 3a, the SEM image of
the as-prepared SWCNT material showed a tangled web-like network of
very clean fibers. The diameters of the fibers appeared to be in
the range of about 10 to about 20 nanometers. It is noted that the
SEM image was of as-grown materials; no purification was performed
before the imaging.
[0033] Furthermore, as depicted in FIG. 3b, the TEM image of the
SWCNT material showed that the fibers observed in the SEM image
were actually bundles of single walled carbon nanotubes. The
diameters of the nanotubes measured from the high resolution TEM
images were between about 0.9 and about 2.7 nm.
[0034] Both the SEM and the TEM images showed the SWCNT materials
possessed characteristics similar to those of high quality single
walled carbon nanotube materials prepared in the laser method (see,
A. Thess et al., Science 273, 483487 (1996) and T. Guo, P.
Nikolaev, A. Thess, D. T. Colbert, and R. E. Smalley, Chemical
Physics Letters 243, 49-54 (1995)) and the arc method (see, M.
Wang, X. L. Zhao, M. Ohkohchi, and Y. Ando, Fullerene Science &
Technology 4, 1027-1039 (1996) and C. Journet et al., Nature 388,
756-758 (1997)).
[0035] The fact that the SEM image showed only nanotubes, but no
amorphous carbon overcoat, indicated that the catalyst/support
surface was substantially fully covered with nanotube materials.
However in the TEM image, for samples with a weight gain, i.e.,
yield, higher than about 300%, an amorphous carbon overcoat was
observed, which probably can be eliminated during production of
SWCNTs and/or removed after production of SWCNTs.
[0036] Moreover, the inventive method reflects that a drying
process of the wet gel, as discussed below in the laboratory
examples, is a necessary step in preparing the high performance
catalysts on aerogel supports, as employed in the inventive method.
The drying may be achieved by supercritical drying, such as by
CO.sub.2 supercritical drying, or by ethanol supercritical drying,
or alternatively, may be achieved by freeze drying, such as by
freeze drying using water, and combinations thereof.
[0037] However, it is not intended to include drying that results
in a xerogel. Fricke, Aerogels, Springer-Verlag, Berlin,
Heidelberg, New York, Tokyo (1986) and N. Husing, U. Schubert,
Angew. Chem. Int. Ed. 37, 2245 (1998) discuss that merely
evaporating the liquid solvent at ambient conditions (i.e., about
STP) would cause the gel to shrink due to the collapse of the
porous structures by the strong forces from surface tension at the
liquid/gas interfaces within the pores in the gel, and this
shrinkage would significantly reduce the total surface area and
pore volume of the dried material, which is normally called
xerogel.
[0038] On the other hand, in the supercritical drying process,
which is performed at a temperature well above STP and should also
be at a pressure well above STP, the liquid solvent in the wet gel
is put into the supercritical state, for instance, under a carbon
dioxide blanket. Therefore, there are substantially no liquid/gas
interfaces in the pores during drying. The original porous
structure in the wet gel is thus substantially maintained in the
resultant dried catalysts/aerogels.
[0039] Also, as more and more nanotubes were grown on the surface
of the aerogel supported catalyst, the diffusion of the
carbon-containing compound, i.e., methane or carbon monoxide in the
examples below, to the catalyst/support became more difficult.
Furthermore, since as noted in the above discussion of FIGS. 3a and
3b, an amorphous carbon deposition was observed on the nanotubes at
a longer growth time, this carbon overcoat probably further reduced
the diffusion rate of the carbon-containing compound to the
catalyst/support. This overcoat would explain why the growth rate
slowed down versus time as shown in FIG. 2.
[0040] In summary, discovered was a new method employing a form of
catalyst/support that can be used in a vapor phase deposition
method to prepare single walled carbon nanotubes, preferably in
yields greater than those obtainable by prior art processes. The
yield was typically improved by at least a factor of 2.5 and often
5, compared with a similar catalyst supported on Al.sub.2O.sub.3
powder.
Laboratory Examples
Materials
[0041] All materials used in the laboratory examples were research
grade materials purchased from different suppliers.
[0042] Aluminum tri-sec-butoxide (abbreviated below as ASB),
Fe.sub.2(SO.sub.4).sub.3.4H.sub.2O, and
bis(actylacetonato)dioxomolybdenu- m (abbreviated below as
MoO.sub.2(acac).sub.2) were purchased from Sigma/Aldrich
Chemicals.
[0043] Reagent grade nitric acid, ammonium hydroxide, and ethanol
were purchased from VWR Scientific Products.
[0044] High purity methane, carbon dioxide, and hydrogen were
supplied by National Welders Inc.
EXAMPLE I
Catalyst/Support Preparation
[0045] Catalysts/supports were prepared using the solvent-gel
technique , as reported in D. J. Suh and J. T. Park, Chemistry of
Materials 9, 1903-1905 (1997) followed by supercritical drying.
Optionally, some were dried by freeze drying.
[0046] In a typical experiment, 23 g of ASB were dissolved in 200
ml of ethanol as the liquid solvent in a round bottom flask under
reflux conditions. Then, 0.1 ml of concentrated HNO.sub.3, diluted
with 1 ml of water and 50 ml of ethanol, was added into the
mixture.
[0047] The resultant was refluxed for 2 hours, until a clear
solution was formed, followed by adding 1.38 g of
Fe.sub.2(SO.sub.4).sub.3.4H.sub.2O and 0.38 g of
MoO.sub.2(acac).sub.2 into the mixture. The amount of Fe and Mo
were chosen so that the molar ratio of Mo:Fe:Al=0.16:1:16. After
refluxing for 2 more hours, the mixture was cooled to room
temperature and then 5 ml of concentrated NH.sub.4OH, diluted with
5 ml of water, was added into the mixture under vigorous stirring,
in order to enhance that the dissolved metal salts would form nm
sized hydroxide particles and would attach to the aerogel. Within a
few minutes, a gel formed.
[0048] The resultant was left to age for about 10 hours before the
supercritical drying step was performed under the following
conditions.
[0049] First, the catalyst/support wet gel was sealed in a
high-pressure container, which was then cooled to about 0.degree.
C. and pressurized to fill the container with liquid CO.sub.2, at
about 830 psi (about 59.4 kg/cm.sup.2). A solvent exchange step
followed, in order to exchange the ethanol liquid solvent in the
gel with liquid CO.sub.2, by flushing the container with liquid
CO.sub.2 a few times.
[0050] Then, the container was warmed up to between about
50.degree. C. and about 200.degree. C., which is above the critical
temperature (31.degree. C.) of CO.sub.2, and the pressure was kept
between about 1500 psi and 2500 psi (between about 106.4
kg/cm.sup.2 and 176.8 kg/cm.sup.2), which is above the critical
pressure (1050 psi, 74.8 kg/cm.sup.2) of CO.sub.2. The system was
held at these conditions for a short time before the pressure was
slowly reduced while the temperature was kept the same.
[0051] Finally, the temperature was reduced to room temperature.
Then, each catalyst (in metal hydroxide form) on aerogel support
was calcined at 500.degree. C. for 30 minutes, to effect conversion
to the metal oxide form. Then before being used for SWCNT growth,
conversion to the metal form was effected by reduction under
H.sub.2 for 30 minutes at 900.degree. C. The pressure at that stage
was about 830 psi (about 59.4 kg/cm.sup.2). Each catalyst/support
prepared this way was a catalyst supported on a highly porous, very
fine, free-flowing aerogel with a surface area of from about 500
m.sup.2/g to about 600 m.sup.2/g.
[0052] Alternatively, instead of CO.sub.2, some samples were
supercritically dried with ethanol or dried with freeze drying as
follows.
[0053] Ethanol supercritical drying: A 100 ml high pressure and
high temperature container was used. At least 35 ml of the wet gel
was added in the container. Before heating, N.sub.2 was used to
flush the system to drive the air out. Then the whole system was
sealed and heating was started. After the temperature reached
260.degree. C., the system was maintained at that temperature for
about 30 minutes before the EtOH was released slowly. The releasing
process took about 15 minutes. The, the system was cooled down
gradually and the aerogel supported catalyst taken out. The yield
of nanotubes for this was similar to the one dried with
CO.sub.2.
[0054] Freeze drying: The ethanol in the wet gel was replaced by
water through solvent exchange. Then, the sample was frozen with
liquid nitrogen and put in a freeze dryer (Freezone Plus 6,
Labconco, Kansas City, Mo., United States of America). It took a
few days to dry the sample totally and the yield of this was lower
than the one dried with CO.sub.2.
SWCNT Growth
[0055] SWCNTs were prepared in a simple vapor phase deposition
setup made of a tube furnace and gas flow control units. In a
typical growth experiment, about 50 mg of a catalyst/support sample
were put into an alumina boat inside a quartz tube. Each sample was
individually heated to reaction temperature, under an Ar flow at a
flow rate of about 100 sccm, and then, the Ar was switched to
H.sub.2 (about 100 sccm flow rate) for 30 minutes, before switching
to a methane flow (about 1000 sccm) for 30 minutes. An individual
sample was heated for each temperature of about 800.degree. C.,
about 850.degree. C., about 900.degree. C., and about 950.degree.
C.
[0056] The reaction was carried out for the desired time before the
methane flow was turned off and the Ar flow turned on and the
temperature reduced to room temperature. Each resultant was then
weighed and characterized.
Characterization
[0057] SWCNT samples were fully characterized using TEM imaging and
SEM imaging.
[0058] TEM imaging was performed on a Philip CM-12 microscope
operating at 100 kV. The samples for TEM imaging were prepared by
sonicating about 1 mg of material in 10 ml of methanol for 10
minutes and drying a few drops of the suspension on a holy-carbon
grid.
[0059] SEM imaging was performed on a Hitachi S-4700 microscope
with a beam energy of 4 kV by placing the as-grown materials on
conductive carbon tape.
[0060] The yield of the SWCNT material with respect to the catalyst
was measured on a thermal gravimetric analyzer (model SDT 2960,
purchased from TA Instruments) under flowing air with a heating
rate of 5.degree. C./minute. The observed yield, measured by TGA,
was 100.2% as depicted in FIG. 1.
EXAMPLE II
[0061] The procedure of Example I was substantially repeated,
except this time with a methane flow for about 60 minutes (instead
of about 30 minutes) and a temperature of about 900.degree. C.
(instead of various temperatures of about 800.degree. C. about
850.degree. C., about 900.degree. C., and about 950.degree. C.) and
a flow rate of about 1158 sccm (instead of about 1000 sccm), during
SWCNT growth. The yield measured by TGA was about 200%.
EXAMPLE III
Comparison
[0062] Also, a catalyst/support made from the same Al.sub.2O.sub.3
wet gel, but dried differently to make xerogel, was compared. The
aerogel supported catalyst showed a yield of about 200% of high
purity SWCNT under a methane flow at about 900.degree. C. for about
60 minutes, as reported by Example I. On the other hand, the
xerogel supported catalyst showed a weight gain of <5% under the
same conditions.
EXAMPLE IV
[0063] The procedure was repeated as per the Fe/Mo catalyst
supported on Al.sub.2O.sub.3 aerogel, but this time with the Fe/Mo
catalyst instead supported on SiO.sub.2 aerogel prepared by a
similar method.
[0064] The weight gain of the catalyst on SiO.sub.2 aerogel under
the same conditions, about 900.degree. C. under a methane flow for
about 60 minutes, was almost 10%. Thus, it appears that although a
SiO2 aerogel support works (i.e., about 10%), it is preferred with
the inventive method to employ an Al.sub.2O.sub.3 aerogel support
or an Al.sub.2O.sub.3/SiO.sub.2 aerogel support to obtain
improvements that are far superior (i.e., weight gain of about 100%
or greater).
EXAMPLE V
[0065] The procedure of Example I was substantially repeated,
except this time with CO instead of CH.sub.4. Also, the temperature
of the CO flow was about 850.degree. C., with a CO flow rate of
about 1200 sccm for about 200 minutes. The result was a yield of
about 150%.
EXAMPLE VI
[0066] The procedure of Example I was substantially repeated,
except this time with Al.sub.2O.sub.3/SiO.sub.2 as the aerogel
support, instead of Al.sub.2O.sub.3 as the aerogel support.
Substantially the same results were obtained, except there was more
amorphous carbon.
EXAMPLE VII
Comparison
[0067] It is believed that more amorphous carbon resulted in
Example VI since in a comparison, Al.sub.2O.sub.3/SiO.sub.2 aerogel
(without any metal catalyst) was tried with methane for 30 minutes
at 900.degree. C. and this converted the methane to amorphous
carbon.
[0068] It will be understood that various details of the invention
may be changed without departing from the scope of the invention.
Furthermore, the foregoing description is for the purpose of
illustration only, and not for the purpose of limitation--the
invention being defined by the claims.
* * * * *