U.S. patent application number 12/463555 was filed with the patent office on 2010-11-11 for new class of tunable gas storage and sensor materials.
This patent application is currently assigned to HONDA PATENTS & TECHNOLOGIES NORTH AMERICA, LLC. Invention is credited to Avetik Harutyunyan.
Application Number | 20100284903 12/463555 |
Document ID | / |
Family ID | 43062434 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100284903 |
Kind Code |
A1 |
Harutyunyan; Avetik |
November 11, 2010 |
New Class of Tunable Gas Storage and Sensor Materials
Abstract
The electronic structure of nanowires, nanotubes and thin films
deposited on a substrate is varied by doping with electrons or
holes. The electronic structure can then be tuned by varying the
support material or by applying a gate voltage. The electronic
structure can be controlled to absorb a gas, store a gas, or
release a gas, such as hydrogen, oxygen, ammonia, carbon dioxide,
and the like.
Inventors: |
Harutyunyan; Avetik;
(Columbus, OH) |
Correspondence
Address: |
HONDA/FENWICK
SILICON VALLEY CENTER, 801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Assignee: |
HONDA PATENTS & TECHNOLOGIES
NORTH AMERICA, LLC
Raymond
OH
|
Family ID: |
43062434 |
Appl. No.: |
12/463555 |
Filed: |
May 11, 2009 |
Current U.S.
Class: |
423/648.1 ;
502/413; 977/750; 977/752; 977/962 |
Current CPC
Class: |
C01B 21/24 20130101;
C01B 2202/22 20130101; B01D 2257/404 20130101; B01D 2257/7025
20130101; B82Y 40/00 20130101; C01B 3/0031 20130101; Y10S 977/932
20130101; B82Y 15/00 20130101; C01B 3/0078 20130101; C01B 32/40
20170801; Y02C 20/20 20130101; B01D 2253/102 20130101; B01J 20/3085
20130101; B01J 37/10 20130101; Y02P 20/156 20151101; B01J 35/0006
20130101; B01D 2253/25 20130101; Y02E 60/325 20130101; B01D 2253/34
20130101; B01J 37/0236 20130101; B82Y 30/00 20130101; Y02E 60/32
20130101; B01J 37/04 20130101; C01B 3/0026 20130101; G01N 27/125
20130101; B01J 20/3295 20130101; C01B 13/02 20130101; Y02C 10/08
20130101; C07C 7/12 20130101; Y02C 20/40 20200801; B01D 2253/304
20130101; B01J 20/20 20130101; Y02E 60/327 20130101; B01D 53/04
20130101; B01D 2257/102 20130101; C01B 2202/02 20130101; C01B
3/0021 20130101; B01J 21/04 20130101; C01B 3/0042 20130101; B01D
2257/504 20130101; C01C 1/006 20130101; Y10S 977/75 20130101; B01D
2257/502 20130101; C01B 32/50 20170801; B01J 23/745 20130101; Y02P
20/151 20151101; B01J 20/324 20130101; B01D 2257/108 20130101; B01D
53/32 20130101; B01J 20/3441 20130101; Y10S 977/843 20130101; B01J
20/205 20130101; Y02P 20/152 20151101; Y10S 977/957 20130101; B01D
53/02 20130101; B01D 2257/104 20130101; C01B 32/162 20170801 |
Class at
Publication: |
423/648.1 ;
502/413; 977/750; 977/752; 977/962 |
International
Class: |
B01J 20/20 20060101
B01J020/20; C01B 3/02 20060101 C01B003/02 |
Claims
1. A gas storing device, the device comprising: a support; and a
carbon-containing material deposited on the support wherein the
support is in electrical communication with the carbon-containing
material such that a voltage is applied to tune gas absorption by
the carbon-containing material.
2. The device of claim 1, wherein the support is selected from the
group consisting of silicon oxide.
3. The device of claim 2, wherein the support is silicon oxide.
4. The device of claim 1, wherein the carbon-containing material is
selected from the group consisting of carbon nanotubes and
nanowires.
5. The device of claim 4, wherein the carbon-containing material is
carbon nanotubes.
6. The device of claim 5, wherein the carbon nanotubes are
single-walled carbon nanotubes (SWNTs) or multi-walled carbon
nanotubes (MWNTs).
7. The device of claim 1, wherein the carbon-containing material is
about 2 .mu.m in depth.
8. The device of claim 1, wherein the gas is hydrogen, oxygen,
carbon dioxide, carbon monoxide, methane or ammonia.
9. The device of claim 8, wherein the gas is hydrogen.
10. The device of claim 1, further comprising releasing the gas by
decreasing the voltage.
11. The device of claim 10, wherein the voltage is decreased by
about 5% to about 50%.
Description
FIELD OF INVENTION
[0001] The present invention relates to compositions and methods
for reversibly storing gases, in particular by changing the
electronic structure of a material thereby tuning the material for
storing or sensing a particular gas.
BACKGROUND
[0002] A common technique for storing gases is via a liquefaction
process where the gas is compressed and cooled from a gas phase
into a liquid phase. For example, hydrogen gas liquefies at 20 K at
atmospheric pressure, and approximately 70 g/L of the hydrogen gas
can be stored in the liquid phase. However, the liquefaction
process is very energy intensive and the liquid gas needs to be
maintained at the lower temperature requiring specially designed
insulated containers and very careful handling.
[0003] Another common technique for storing gases is to compress
the gas into a suitable vessel. For example, a gas tank pressurized
to 35 MPa can store 15 g/L of hydrogen. However, a pressurized-gas
tank is heavy, cumbersome, and difficult to transport.
[0004] Gases can also be stored by chemically bonding the gas to an
appropriate host material. Several types of materials have been
studied as hosts, including metals, metal hydrides, glass
microspheres and carbon nanotubes. However, the materials
investigated so far all have low gas storage capacity. Further,
high temperatures are required for releasing the gas, such as from
a metal hydride, make these methods unsuitable for commercial
use.
[0005] Recently, LC resonant sensors have been combined with carbon
nanotube materials for utilization as gas sensors. For example,
Ong, et al. IEEE Sensors Journal, 2: 82 (2002) described a gas
sensor formed of a responsive multi-wall carbon nanotube/silicon
dioxide composite layer deposited on a planar LC resonant circuit.
The permittivity and/or conductivity of the MWNT/SiO.sub.2
composite changes with adsorption of CO.sub.2, O.sub.2, or NH.sub.3
which changes the resonant frequency of the sensor, which can be
remotely monitored through a loop antenna. The sensors showed
reversible response to O.sub.2 and CO.sub.2, and an irreversible
response to NH.sub.3.
[0006] Hydrogen can also be stored in carbon nanostructures, such
as graphite and carbon nanofibers (A. Dillon et al. Nature 386: 377
(1997), A. Chambers et al. J. Phys. Chem. B 102: 3378 (1998), and
U.S. Pat. No. 5,653,951 "Storage of hydrogen in layered
nanostructures" to N. Rodriguez and R. Baker). Nanostructures can
be defined as atomic structures that have a spatial extent of less
than a few hundred nanometers in one, two, or all three dimensions.
A class of nanostructures is formed by planar networks, sometimes
referred to as layered compounds. The stored hydrogen, however, is
not easily released from the carbon nanostructures.
[0007] J.P. Patent Publication No. 2003225561A2, published Dec. 8,
2003 "Gas Adsorption Element" by Mitsubishi Heavy Ind. Ltd.
discloses that surface of a metal foil can be coated with a carbon
material. The carbon material has the capacity for hydrogen
occlusion and has high thermal conductivity. The carbon material
can be carbon nanotube, carbon nanofiber, or other carbon
materials.
[0008] The known methods of storing gases are not convenient,
require specialized equipment or handling, or high pressures or
temperatures to release the trapped gasses. Accordingly, the
present invention provides compositions, methods, and processes for
the storing or sensing of particular gas where the gas can be
easily released.
SUMMARY
[0009] The present invention provides compositions, methods, and
processes for gas storage and gas sensing. Advantageously, the
present invention also provides methods wherein the storage of gas
can be reversibly performed under ambient or higher pressure and
ambient or higher temperature.
[0010] In one aspect, the invention provides methods for modifying
materials which can to be used as sorbents in gas storage systems,
wherein the material can be modified by changing the potential
energy of the surface of the material. The material can be any
material capable of storing gas, such as, for example,
one-dimensional materials such as carbon nanotubes, carbon
nanowires, carbon nanofibers, and the like, or two-dimensional
materials such as films. Thus, the material can be selected from
carbon, activated carbon, carbon powder, amorphous or disordered
carbon, carbon fibers, carbon nanofibers and graphite, films, and
the like, as well as metal nanowires and thin films, such as Al,
Ni, Ga, As, and their alloys. The invention comprises the doping of
the material with electrons or holes thereby changing the
structural and electronic properties of the material. The term
"doping" refers to an application of a voltage, optionally with the
addition of one or more metals to the materials, with the result
that the structural and electronic properties of the material are
changed. In another aspect, the structural and electronic
properties of the material is changed by doping with electrons or
holes created by applying gradient of the potentials between the
materials and its support.
[0011] In another aspect of the invention, the electronic structure
of the materials can be modified by doping or by varying the
support material such that it has the optimal chemical potential
for storing a particular gas or sensing the selected gas. The gas
molecules can be released by turning off the gate voltage at
ambient temperatures and pressures.
[0012] These and other aspects of the present invention will become
evident upon reference to the following detailed description. In
addition, various references are set forth herein which describe in
more detail certain procedures or compositions, and are therefore
incorporated by reference in their entirety.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 illustrates an apparatus for carrying out the present
invention.
DETAILED DESCRIPTION
I. Definitions
[0014] 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.
II. Overview
[0015] The present invention discloses compositions, methods, and
processes for storing gases and sensing gases. One- or two
dimensional materials, such as single-walled carbon nanotubes
(SWNTs), multi-walled carbon nanotubes, carbon nanofibers, films,
and the like, as well as metal nanowires and thin films, such as
Al, Ni, Ga, As, and their alloys are deposited on a support, and
doped with electrons by applying a gradient of potentials between
the support and the material. The electronic structure of the
material can be tuned to be optimal for a particular gas by varying
the applied gate voltage and varying the support material. The gas
molecules can be released at ambient temperature and ambient
pressure by changing the gate voltage.
III. Selection and Synthesis of Material
[0016] The material for use in the present invention can be
one-dimensional or two-dimensional. Thus, the material can be
carbon nanotubes, activated carbon, carbon powder, amorphous or
disordered carbon, carbon fibers, carbon nanofibers, graphite and
thin-films. In addition, metal nanowires and thin films, such as
Al, Ni, Ga, As, and their alloys can be used in the practice of
this invention. The material chosen can be bought from a commercial
source or synthesized using known methods. It should be understood
that the specific method of forming the material is not critical to
the invention, and the described methods are merely exemplary, and
not meant to be limiting in any way to the invention.
[0017] Graphite is commercially available and has a layered
structure, high crystallinity and low surface area. The typical
graphite interplanar distance is 0.335 nm.
[0018] Carbon fibers are commercially available and made of carbon
with a graphite-like structure. Carbon fibers can be commercially
made by catalytic decomposition of hydrocarbons. The diameter of
carbon fibers is on the order of microns up to centimeters.
[0019] Active carbon is commercially available. The activity of
activated carbon is related to its large surface area, porosity and
low crystallinity. Amorphous carbon is commercially available
carbon with low crystallinity.
[0020] The single-walled carbon nanotubes (SWNTs) are commercially
available, or can be fabricated according to a number of different
techniques familiar to those in the art. For example, the SWNTs can
be fabricated by the laser ablation method of U.S. Pat. No.
6,280,697, the arc discharge method of Journet et al. Nature 388:
756 (1997), the chemical vapor deposition method where supported
metal nanoparticles 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), and the like.
Preferably, the SWNTs are produced by the chemical vapor deposition
method.
[0021] The chemical vapor deposition (CVD) method for the synthesis
of carbon nanotubes uses 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.
[0022] The catalyst composition for use in CVD can be any catalyst
composition known to those of skill in the art. Conveniently, the
particles will be of a magnetic metal or alloy, such as, for
example, iron, iron oxide, or a ferrite such as cobalt, nickel,
chromium, yttrium, hafnium or manganese. The particles useful
according to the invention will preferably have an average overall
particle size of up to 50 nm to about 1 .mu.m, although, in
general, the particle sizes for individual particles can be from
about 400 nm to about 1 .mu.m.
[0023] 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.
[0024] The metal, bimetal, or combination of metals can be used to
prepare metal nanoparticles having defined particle size and
diameter distribution. The catalyst nanoparticles can be prepared
by thermal decomposition of the corresponding metal salt added to a
passivating solvent, 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. The metal salt can be any
salt of the metal, and can be selected such that the salt is
soluble in the solvent and/or 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, nitrite, 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. 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.
[0025] In another aspect of the present invention, the material can
be multi-walled carbon nanotubes (MWNTs). MWNTs are commercially
available or they can be formed according to a chemical vapor
deposition method. Using known methods, highly aligned and high
purity MWNTs can be produced by the thermal decomposition of a
xylene-ferrocene mixture. The xylene serves as the hydrocarbon
source and ferrocene provides the iron catalyst nanoparticles that
can seed the nanotubes that are grown. According to one process,
ferrocene (approximately 6.5%) can be dissolved in xylene and fed
into a quartz tube at a flow rate of about 1 ml/hr. The mixture can
vaporize upon reaching the end of the pre-heater (maintained at
about 200.degree. C.), and the vapors can then be carried into the
furnace in an Ar/H.sub.2 flow. The furnace is maintained at a
temperature (e.g., about 750.degree. C.) that enables the
xylene/ferrocene mixture to decompose and form the MWNTs. The
nanotubes are harvested from the walls of the furnace and can have
a diameter of about 25 nm.
[0026] A nanowire refers to a wire having a diameter typically in
the range of about one nanometer (nm) to about 500 nm. Nanowires
are solid, and can have amorphous structure, graphite like
structure, or herringbone structure. The nanowires are periodic
only along their axis, and can therefore assume any energetically
favorable order in other planes, resulting in a lack of crystalline
order.
[0027] Nanowires are typically fabricated from a metal or a
semiconductor material, and some of the electronic and optical
properties of the metal or semiconductor materials are different
than the same properties of the same materials in larger sizes. For
example, metallic wires having a diameter of 100 nm or less display
quantum conduction phenomena, such as the survival of phase
information of conduction electrons and the obviousness of the
electron wave interference effect. Semiconductor or metal nanowires
have attracted considerable attention because of their potential
applications in mesoscopic research, the development of
nanodevices, for use as gas sensors and field emitters, and the
potential application of large surface area structures. For
example, U.S. Pat. No. 5,973,444 to Xu et al. discloses carbon
fiber-based field emission devices, where carbon fiber emitters are
grown and retained on a catalytic metal film as part of the device.
Xu et al. disclose that the fibers forming part of the device may
be grown in the presence of a magnetic or electric field, as the
fields assist in growing straighter fibers.
[0028] One technique for fabricating quantum wires utilizes a micro
lithographic process followed by metalorganic chemical vapor
deposition (MOCVD). This technique may be used to generate a single
quantum wire or a row of gallium arsenide (GaAs) quantum wires
embedded within a bulk aluminum arsenide (AlAs) substrate. One
problem with this technique, however, is that microlithographic
processes and MOCVD have been limited to GaAs and related
materials. Moreover, this technique does not result in a degree of
size uniformity of the wires suitable for practical
applications.
[0029] Another method of fabricating nanowire systems involves
using a porous substrate as a template and filling naturally
occurring arrays of nanochannels or pores in the substrate with a
material of interest. However, it is difficult to generate
relatively long continuous wires having relatively small diameters
because as the pore diameters become small, the pores tend to
branch and merge, and because of problems associated with filling
long pores having small diameters with a desired material.
[0030] The nanowires for use in the present invention can be
synthesized by providing a substrate, depositing a metalorganic
layer on the substrate, and heating the substrate with the
metalorganic layer to form nanowires on the substrate. The
substrate can be silicon oxide, aluminum oxide, magnesium oxide,
glass, mica, silicon, fiberglass, Teflon, ceramics, plastic, or
quartz or mixtures thereof. The metalorganic layer can be metal
phthalocyanine, such as iron phthalocyanine or nickel
phthalocyanine. The metalorganic can be deposited on the substrate
as a thin film, and heated under air to form the metal
nanowires.
[0031] In particular, the nanowires for use in the invention can be
synthesized by providing a substrate, depositing a metalorganic
layer on the substrate, wherein the metalorganic layer is iron
phthalocyanine, nickel phthalocyanine or mixtures thereof, and
heating the substrate with the metalorganic layer to form nanowires
on the substrate.
[0032] In another aspect of the invention, the two-dimensional
material can be used, such as thin films. The thin film for use in
the present invention preferably contain carbon as a main
component. Thus, the carbon thin film can be fullerene, SWNT, or
MWNT having a film thickness of 0.5 nm to about 100 nm, preferably
a film thickness of about 5 nm to about 80 nm, or even more
preferably, a film thickness of Oat least about 10 nm. The film can
contain elements other than carbon, such as boron, nitrogen, Cs,
Rb, K, Pd, Li, Al, Co, Fe, Ni, Cu, CrC, MoC, MoO.sub.3, WC.sub.x,
WO.sub.3, TiC, SiC, or the like. Preferably, the other element is
present at a concentration of about 50 atom percent or less, and
more preferably 30 atom percent or less.
[0033] The films for use in the invention can be thin amorphous
silicon, micro crystalline silicon and amorphous silicon film.
These films can be obtained from commercial sources or amorphous
silicon film, thin micro crystalline silicon film, thin silicon
nitride film can be manufactured using plasma enhanced chemical
vapor deposition (PECVD). Typically, the substrate is mounted on
the stage inside the vacuum reaction chamber and SiH.sub.4 is
supplied to the chamber through the gas inlet nozzles of the gas
supplying unit. Silicon source gases other than SiH.sub.4 such as
Si.sub.2H.sub.6, SiH.sub.2Cl.sub.2, etc. can also be used, usually
at a flow of 0.5 SCCM and a pressure of 70 mTorr. RF power at 40 W
is applied to the spiral antenna placed adjacent to the chamber to
form inductively coupled plasma. After the substrate temperature
reaches 250.degree. C., thin amorphous silicon film is deposited on
the substrate.
[0034] In another aspect, the thin film can be metal or metal
alloys, such as those of palladium, titanium, and the like. Alloys
of PdTi can be prepared which exhibit greater changes in electrical
resistivity when exposed to concentration of a gas, such as
hydrogen. The palladium-titanium alloy can have relative
concentration of each in the range from above 0 to below 100% such
as 1-99:99-1%. The exact alloy ratio used will depend on the
application. For example, if the gas is hydrogen and if the sensor
will be exposed to high concentrations of hydrogen, then the amount
of Ti in the alloy will be increased. Preferably, the alloy
contains between 50 or 60 and 99 atomic % Pd, or more preferably
between 70 and 98 atomic % Pd, or even more preferably, between 90
and 98 atomic % Pd. Thin films of the PdTi alloy can be formed by
sputtering. Atomic particles of palladium and titanium can be shot
onto a substrate. The sputtering rates can be varied to vary the
amount of each material present in the alloy. The Pd can be
sputtered at a power between 50 W and 450 W. In the preferred form,
the Pd is sputtered at a power between 75 W and 300 W. In the more
preferred form, the Pd is sputtered a power between 100 W and 200
W. Additional materials may be present in the alloy. These
additives include elements such as Cr, Ru, Ag, Au, Zr, Cu, Ir, Al,
Hf, Pt and Ni and can be present up to 20 atomic %. Other additives
including may also be used. Alloys containing these additives may
have less than or greater than 20 atomic % of Pt or Ni. The
sputtered particles adhere to the substrate and form a thin film
layer on a surface of the substrate.
[0035] In one aspect of the invention, the one- or two-dimensional
material can be doped with a metal. The metal for doping the
material can be an alkali metal such as, for example, Li, Na, K, Rb
or Cs, or mixtures of the alkali metals. For example, two or three
different metals can be used, preferably a mixture of Li and one
additional alkali metal. An exemplary mixture is of Li and K.
[0036] The alkali metal salts can include carbonates, nitrates,
hydroxides, halogenides, acetates, hydrides, nitrites, or the like.
The molar ratio of alkali metal to the carbon materials in the
reaction is preferably from about 1:50 to 1:1, more preferably from
1:10 to 1:1, or even more preferably about 1:20 to 1:5.
[0037] The doping of alkali metals to the carbon materials can be
achieved by solid state reaction between the carbon materials and
alkali metal salts. The solid state reaction method preferably
involves thoroughly mixing the carbon materials with the alkali
metal salt, then subjecting the mixture to high temperature
treatment under inert gases, such as helium, nitrogen, argon, and
the like, or reductive gases such as hydrogen.
IV. Support
[0038] The one- or two-dimensional material can preferably be
placed on a support material. The support can be silica, alumina,
MCM-41, MgO, ZrO.sub.2, aluminum-stabilized magnesium oxide,
zeolites, or other supports known in the art, and combinations
thereof. For example, Al.sub.2O.sub.3--SiO.sub.2 hybrid support
could be used. In one aspect of the invention, the synthesis of the
one- or two-dimensional material can be carried out in the presence
of the support material. The support material can be powdered
thereby providing small particle sizes and large surface areas. The
powdered support material 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 support material 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. For example, 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. The weight ratio of
metal nanoparticles to powdered oxide can be between about 1:1 and
1:10, such as, for example, 1:1, 2:3, 1:4, 3:4, 1:5, and the
like.
V. Storing and Releasing Gases
[0039] The supported material synthesized above can be used to
store or detect a selected gas. In one aspect, the support is
provided with a plurality of through holes (FIG. 1) allowing for
the movement gas molecules. The shape of the holes is not
restricted to a circle but can be shaped as ovals, polygons or
slits.
[0040] As shown in FIG. 1, the lower part of the support can be
connected to the (-) pole of a power supply. The upper part of the
support can be connected to the (+) pole of the same power supply,
with the one- or two-dimensional material, such as SWNT, MWNT,
nanowire, or films deposited on the support. The ability of the
material to sense a gas or store a gas can be tuned by selecting
the support or by varying the voltage.
[0041] The carbon-containing material can be coated on the surface
of the surface of the support material, or the carbon-containing
material can be directly deposited on the support material. For
instance, in one embodiment, the carbon nanotube-containing
material can be directly deposited on the support material during
the nanotube formation process such that the carbon nanotubes can
be directly grown on the surface of the support material. The depth
and purity of the nanotube-containing layer is not critical to the
invention. For example, in one embodiment, the adsorptive
nanostructure-containing layer can be about 0.1 .mu.m to about 100
.mu.m thick, preferably about 0.5 .mu.m to about 10 .mu.m thick, or
about 2 .mu.m thick.
[0042] In one aspect of the invention, the support can be Si or
SiO.sub.2 and the carbon-containing material can be SWNTs. A gate
voltage is applied to tune the electronic structure of SWNTs such
that the selected gas preferentially adsorbs onto the SWNTs. The
selected gas can by oxygen (O.sub.2), nitrogen (N.sub.2), ammonia
(NH.sub.3), carbon dioxide (CO.sub.2), carbon monoxide (CO),
methane (CH.sub.4), nitrous oxide (NO) and the like. The gate
voltage can be from about -100 V to about +100 V, preferably about
-50 V to about +50 V, or more preferably about -20 V to about +20
V, and values in between. Thus, for example, if the selected gas is
oxygen, the device can be held at a temperature of about 20.degree.
C. to about 25.degree. C. at a pressure of about 0.95 atmospheres
to about 1.05 atmospheres for about 2 h to about 48 h while
applying a gate voltage of about 20 V to about +20 V volts.
[0043] In one aspect of the invention, the support can be Si or
SiO.sub.2 and the carbon-containing material can be nanowires. A
gate voltage is applied to tune the electronic structure of
nanowires such that the selected gas preferentially adsorbs onto
the nanowires. Thus, for example, if the selected gas is hydrogen,
the device can be held at a temperature of about 20.degree. C. to
about 25.degree. C. at a pressure of about 0.95 atmospheres to
about 1.05 atmospheres for about 2 h to about 48 h while applying a
gate voltage of about -10 v to about +15 v volts.
[0044] The gas thus stored can be released by changing the voltage.
In one aspect, the voltage is decreased or increased by about 1% to
about 50% over the optimal voltage in order to controllably release
the trapped gas. For example, the adsorbed gas can be released over
a time period of hours to days by decreasing the voltage by about
10%, or more rapidly by decreasing the voltage by about 25%, or the
gas can be released over a time period of a few seconds to a few
hours by completely turning off the voltage.
[0045] In another aspect of the invention, the ability of the
carbon-containing material to sense a gas or store a gas can be
tuned by including metals in the carbon-containing material. For
example, the carbon materials can be doped with Li, Na, or K. The
carbon materials thus doped can adsorb hydrogen, oxygen, carbon
monoxide or carbon dioxide, for example in the temperature range of
about 0.degree. C. to about 40.degree. C., preferably at about
20.degree. C. to about 25.degree. C., and at pressures of about 0.5
atmospheres to about 3 atmospheres, preferably at about 0.9
atmospheres to about 1.5 atmospheres, when a voltage is
applied.
EXAMPLES
[0046] Below are examples of specific embodiments for carrying out
the present invention. The examples are offered for illustrative
purposes only, and are not intended to limit the scope of the
present invention in any way. Efforts have been made to ensure
accuracy with respect to numbers used (e.g., amounts, temperatures,
etc.), but some experimental error and deviation should, of course,
be allowed for.
Example 1
Preparation of the Supported Catalyst
[0047] Catalysts were prepared by impregnating support materials in
metal salt solutions. In a typical procedure, Fe(NO.sub.2).sub.2
was used at a molar ratio of Fe:Al of 1:2. Under a nitrogen
atmosphere, Fe(NO.sub.2).sub.2 was added to water in the molar
ratio of 1 mM:20 mM. Then aluminum nitrite was added to the metal
salt containing aqueous solution. The reaction mixture was mixed
using a mechanical stirrer under the nitrogen atmosphere, and
heated under reflux for 90 minutes. The reaction was cooled to
about 60.degree. C. while flowing a stream of N.sub.2 over the
mixture to remove the solvent. A black film formed on the walls of
the reaction flask. The black film was collected and ground with an
agate mortar to obtain a fine black powder.
Example 2
Synthesis of Carbon Nanotubes
[0048] Carbon nanotubes were synthesized by using the experimental
setup described in Harutyunyan et al., NanoLetters 2, 525 (2002).
CVD growth of bulk SWNTs used the catalysts prepared in Example 1
and methane as a carbon source (T=800.degree. C., methane gas flow
rate 60 sccm). The carbon SWNTs were successfully synthesized with
a yield of about 40 wt % (wt % carbon relative to the iron/alumina
catalyst). Analysis of transmission electron microscopy (TEM)
images of SWNTs produced showed bundles were produced. Raman
spectra of carbon SWNTs produced using produced by the method above
were obtained using .lamda.=532 nm and .lamda.=785 nm laser
excitation.
[0049] 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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