U.S. patent application number 10/502320 was filed with the patent office on 2005-06-02 for plasma synthesis of hollow nanostructures.
This patent application is currently assigned to Cambridge University Technical Services Limited. Invention is credited to Cash, Stephen, Kinloch, Ian, Mackinnon, Ian, Shaffer, Milo, Windle, Alan.
Application Number | 20050118090 10/502320 |
Document ID | / |
Family ID | 9929660 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050118090 |
Kind Code |
A1 |
Shaffer, Milo ; et
al. |
June 2, 2005 |
Plasma synthesis of hollow nanostructures
Abstract
A method is described for the continuous production of nanotubes
comprising forming a plasma jet, introducing into the plasma jet a
metal catalyst or metal catalyst precursor to produce vaporised
catalyst metal, directing one or more streams of quenching gas into
the plasma to quench the plasma and passing the resulting gaseous
mixture through a furnace, one or more nanotube forming materials
being added whereby nanotubes are formed therefrom under the
influcence of the metal catalyst and are grown to a desired length
during passage through the furnace, and collecting the nanotubes so
formed.
Inventors: |
Shaffer, Milo; (Cambridge,
GB) ; Kinloch, Ian; (Histon, GB) ; Cash,
Stephen; (Noth Alberton, GB) ; Mackinnon, Ian;
(Darlington, GB) ; Windle, Alan; (Cambridge,
GB) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
Cambridge University Technical
Services Limited
16 Mill Lane
Cambridge
GB
CB2 1SB
|
Family ID: |
9929660 |
Appl. No.: |
10/502320 |
Filed: |
January 24, 2005 |
PCT Filed: |
January 24, 2003 |
PCT NO: |
PCT/GB03/00249 |
Current U.S.
Class: |
423/447.1 ;
423/447.3 |
Current CPC
Class: |
C01B 2202/06 20130101;
D01F 9/133 20130101; D01F 9/127 20130101; B82Y 30/00 20130101; B01J
2219/0894 20130101; C30B 29/02 20130101; B82Y 40/00 20130101; C30B
25/105 20130101; B01J 19/088 20130101; B01J 2219/0009 20130101;
C01B 32/162 20170801; B01J 2219/0875 20130101; B01J 2219/0892
20130101; C30B 29/605 20130101; H05H 1/30 20130101; C01B 32/164
20170801 |
Class at
Publication: |
423/447.1 ;
423/447.3 |
International
Class: |
D01F 009/12; D01C
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2002 |
GB |
0201600.4 |
Claims
1. A method for the continuous production of nanotubes comprising
forming a plasma jet, introducing into the plasma jet a metal
catalyst or metal catalyst precursor to produce vaporised catalyst
metal, directing one or more streams of quenching gas into the
plasma to quench the plasma and passing the resulting gaseous
mixture through a furnace, one or more nanotube forming materials
being added whereby nanotubes are formed therefrom under the
influence of the metal catalyst and are grown to a desired length
during passage through the furnace, and collecting the nanotubes so
formed.
2. A method as claimed in claim 1, wherein the nanotube forming
material is a carbon containing material and the product is carbon
nanotubes.
3. A method as claimed in claim 2, wherein the nanotube forming
material is carbon containing material and the product is
predominantly carbon nanotubes.
4. A method as claimed in claim 1, wherein the nanotube forming
material is a carbon containing material and the product contains
multi wall nanotubes.
5. A method as claimed in claim 2, wherein the nanotube forming
material is carbon monoxide, carbon particulates, a normally liquid
or gaseouse hydro carbon, or an oxygen containing hydrocarbon
derivative.
6. A method as claimed in claim 1, wherein the catalyst precursor
material acts as a nanotube forming material and vice versa.
7. A method as claimed in claim 2, wherein the nanotube forming
material further comprises non-carbon dopant elements.
8. A method as claimed in claim 1, wherein the nanotube forming
material comprises borazine, boron powder plus nitrogen gas,
boranes plus nitrogen gas, tunsten oxide powder plus hydrogen
disulphide gas, or tungsten disulphide powder.
9. A method as claimed in claim 1, wherein the nanotube forming
material is added upstream of the plasma jet.
10. A method as claimed in claim 1, wherein the nanotube forming
material is added as or with the quenching gas, or is added
downstream of the plasma jet separately from the quenching gas.
11. A method as claimed in claim 1, wherein the metal catalyst or
catalyst precursor is or contains copper, chromium, molybdenum,
tungsten, iron, cobalt, nickel, ruthenium, rhodium, palladium,
osmium, iridium, platinum, yttrium, a lanthanide or an actinide, or
a mixture of two or more thereof.
12. A method as claimed in claim 1, wherein the temperature within
the furnace is from 700 to 1200.degree. C.
13. A method as claimed in claim 1, wherein the introduced
materials have a residence time within the furnace of from 5 to 30
seconds.
14. A method as claimed in claim 1, wherein the plasma is generated
by an inductively coupled plasma 15 torch.
15. A method as claimed in claim 1, wherein said quenching gas is
directed radially into said plasma jet from multiple directions to
induce full mixing of the plasma and quenching gas to produce
uniform conditions within the mixture thereof.
16. A method as claimed in claim 12, where the symmetric array of
nozzles are directed at an angle to the radial direction to induce
a turbulent vortex where mixing of the plasma and quenching gas
occurs.
Description
[0001] The present invention relates to methods for the continuous
production of carbon or other nanotubes.
[0002] Carbon nanostructures, and nanotubes in particular, are very
promising candidates for a wide range of applications. One of the
major limitations to further development is the volume of material
that can presently be produced. A number of different synthesis
routes have been reported with the vast majority falling into one
of the following three categories:
[0003] 1) Electric carbon arc: typically a large current is passed
between carbon electrodes in a low pressure inert atmosphere,
leading to the evaporation of carbon species in the high
temperature arc discharge. Products may be deposited on the counter
electrode or on the chamber walls. Single-wall nanotubes are grown
through the introduction of catalyst metal powders (typically
transition metals such as Ni, Co, or Y) into the graphite
electrodes. This method yields nanotubes with a high crystallinity
but the purity of the product is low due to the instability of the
arc and the non-uniformity of the growth conditions. Despite
improvements allowing motorised insertion of electrodes this
approach is essentially a batch process yielding only a few grams
of material per run, with relatively little scope of
improvement.
[0004] 2) Laser ablation: a high power laser, usually pulsed,
sometimes continuous is used to ablate a graphite target,
containing metal catalyst particles, into an inert atmosphere.
Single-wall nanotubes condense from the mixed carbon and metal
vapour. This method can produce high quality material but the
yields and overall energy efficiency are low. Non-uniform ablation
of the target means that this approach must also be run as a batch
process.
[0005] 3) Catalytic vapour deposition: hydrocarbon gas is
decomposed on a transition metal catalyst at relatively low
temperatures (500-1200 C). The catalyst may either be pre-produced
or formed in situ by the decomposition of a metal-containing
compound. A closely related process uses the disproportionation of
carbon monoxide as the carbon source. Material produced via this
route, can be very pure, but usually has a high concentration of
crystalline defects. Radio-frequency and microwave plasmas are
sometimes used to enhance the growth process at low temperatures
(e.g. 500-700 C) and pressures (<<1 atm).
[0006] There have also been a number of proposals for the
continuous production of fullerenes using a plasma torch. Thus,
WO94/04461 discloses passing a particulate carbon or liquid or
gaseous hydrocarbon feedstock (e.g. acetylene or naphthalene)
through an induction plasma torch and quenching the vaporised
carbon with a quenching gas such as helium or argon. A soot is
produced containing a few percent of fullerenes. No carbon nanotube
production is disclosed. The products are described as being
C.sub.60, C.sub.70 and higher molecular weight fullerenes of
similar structural configuration.
[0007] U.S. Pat. No. 5,395,496 uses carbon halides as a plasma
forming gas in a plasma torch to form a vaporised carbon cloud
which is condensed to produce a soot containing fullerenes.
Optionally, hydrocarbons or an inert gas are added. Again, no
production of nanotubes is reported.
[0008] In WO93/23331, carbon powder is supplied to a plasma flame
produced between a cathode and an anode and the reaction products
are annealed as they flow down a pathway lined with a refractory
insert, which may be heated. Again, the product is a fullerene
containing soot.
[0009] U.S. Pat. No. 5,593,740 discloses passing metal powder (e.g.
Fe, Co, Ni, Cu or Al) into an inert gas plasma jet, which extends
into a region in which a carbon containing gas is present. The
plasma is quenched by a feed of nitrogen and the product is
collected. Rather than carbon nanotubes, the product is ultrafine
metal particles encased in a shell of carbon.
[0010] The present invention now provides a method for the
continuous production of nanotubes comprising forming a plasma jet,
introducing into the plasma jet a metal catalyst or metal catalyst
precursor to produce vaporised catalyst metal, directing one or
more streams of quenching gas into the plasma to quench the plasma
and passing the resulting gaseous mixture through a furnace, one or
more nanotube forming materials being added whereby nanotubes are
formed therefrom under the influence of the metal catalyst and are
grown to a desired length during passage through the furnace, and
collecting the nanotubes so formed.
[0011] The nanotube forming material may be a carbon containing
material and the product is then carbon nanotubes.
[0012] Generally, the nanotube forming material may be carbon
monoxide, carbon particulates, a normally liquid or gaseous
hydrocarbon, or an oxygen containing hydrocarbon derivative.
Suitable carbon-containing compounds for use as the carbon source
include carbon monoxide and hydrocarbons, including aromatic
hydrocarbons, e.g. benzene, toluene, xylene, cumene, ethylbenzene,
naphthalene, phenanthrene or anthracene, non-aromatic hydrocarbons,
e.g. methane, ethane, propane, butane, pentane, hexane,
cyclohexane, ethylene, propylene or acetylene, and
oxygen-containing hydrocarbons, e.g. formaldehyde, acetaldehyde,
acetone, methanol or ethanol, or a mixture of two or more thereof.
Carbon in particulate form entrained in a suitable carrier gas may
be used. In preferred embodiments, the carbon-containing compound
is carbon monoxide (CO), methane, ethylene or acetylene.
[0013] The carbon source may be mixed with one or more gases acting
as a diluent such as inert gases, e.g. argon. It may also be mixed
with non carbon containing gases which play no direct role in the
nanotube forming reaction but which play a contributory role, for
instance by reacting with amorphous carbon as it is formed (as a
by-product) and so keeping the reaction sites on the catalyst clean
and available for nanotube formation.
[0014] Gases which may be mixed with the carbon source include
argon, hydrogen, nitrogen, ammonia, carbon dioxide or helium. The
nanotube forming material may further comprise non-carbon dopant
elements such as nitrogen (suitably as nitrogen gas) or boron
(introduced using for example boranes or carboranes) or sulphur
(introduced using for example thiophene).
[0015] The method is not however restricted to the production of
carbon nanotubes and the nanotube forming material optionally
comprises or consists of borazine, boron powder plus nitrogen gas,
boranes plus nitorgen gas, tunsten oxide powder plus hydrogen
disulphate gas, tungsten disulphide powder, etc.).
[0016] The nanotube forming material may be added upstream of the
plasma jet or it may be introduced into the plasma jet or after the
quenching gas, or at two or more of these stages. Thus, in a first
mode of operation which is essentially a pure chemical vapour
deposition method, the only active ingredient passed through the
plasma is the metal catalyst and all of the structural material of
the nanotube product is introduced with the quenching gas or
thereabouts. In this case, the plasma is used to generate suitably
sized clusters of catalyst metal atoms, generally with diameters
equivalent to the nanotubes to be produced. The metal nanoclusters
catalyse the breakdown of the nanotube forming material to allow
growth of the nanotubes on the metal particles.
[0017] In an alternative mode, which might suitably be termed
`mixed mode growth`, nanotube forming material is introduced with
the metal catalyst to pass through the plasma so that both the
metal and the structural material of the nanotube (typically
carbon) are vaporised in the plasma and nanotube nuclei condense
from the plume of the plasma consisting of slightly larger metal
nanoparticles (typically 3-5 nm) with relatively short nanotubes or
`fullerenic caps` attached that then lengthen during passage
through the furnace.
[0018] The nanotube forming material may serve as the quenching gas
or it may be co-added with the quenching gas, or it may be added
downstream of the plasma jet separately from the quenching gas.
Some of the nanotube forming material may also be added to pass
through the plasma with the metal catalyst at the same time. The
proportions of nanotube forming material added to pass through the
plasma jet and added downstream of it may be varied as desired.
[0019] The catalyst or catalyst precursor is suitably a transition
metal catalyst or precursor, particularly one comprising copper
(Cu), or a Group VIB transition metal (chromium (Cr), molybdenum
(Mo), tungsten (W)) or a Group VIIIB transition metal (iron (Fe),
cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium
(Pd), osmium (Os), iridium (Ir) and platinum (Pt)) or a mixture of
two or more thereof. Metals from the lanthanide and actinide series
may also be used. Preferred transition metal catalysts comprise a
mixture of two or more of the listed metals. Particularly preferred
transition metal catalysts comprise Fe, Ni, Co, Mo or a mixture of
two or more thereof such as a 50/50 mixture (by weight) of Ni and
Co, or a mixture of Fe and Ni, or a mixture of Fe and Mo. Any of
these transition metals individually or in combination with any of
the other transition metals listed may be used as a catalyst for
carbon nanotube growth.
[0020] The catalyst may be added as metal but is preferably a metal
containing compound from which metal atoms are freed in the plasma.
Such a precursor is preferably a plasma decomposable compound of
one or more metals listed above.
[0021] Preferably, the catalyst precursor is an organometallic
compound comprising a transition metal and one or more ligands. The
ligands of the catalyst precursor preferably contain the elements
C, H and O only, and preferably are simple molecules which
decompose without poisoning the catalyst metal or interfering with
the synthesis pathway.
[0022] Ligands of the catalyst precursor may include one or more
functional groups selected from carboxylates, alkoxides, ketones,
diketones, amines, amides, alkyls and aryls. Suitable ligands
include methyl, cyclohexyl, carbonyl, cyclopentadienyl,
cyclooctadiene, ethylene beta-diketones, phosphines,
organophosphorous ligands, polyethers, dithiocarbamates,
macrocyclic ligands (e.g., crown ethers) or benzene ligands, or a
mixture or two or more of thereof.
[0023] The catalyst precursor may be a multimetal atom cluster,
such as triiron dodecylcarbonyl.
[0024] Optionally, a finely divided substrate material may be
introduced at the level of the quenching gas or thereabouts. The
substrate material will help to nucleate and stabilise small metal
clusters. In the most straightforward case, the substrate particles
are simply finely ground powders, for example of silica or alumina.
Finer materials may be generated by a range of methods known to
those skilled in the art, such as fuming, colloidal processing,
spray-drying, hydrothermal processing and so on. Particular benefit
for the production of nanotubes may be derived by, using structured
substrate particles, particularly mesoporous silicas, anodised
alumina membranes, or zeolites. These materials have channels of
similar dimensions to nanotubes, and can further guide both the
deposition of catalyst and synthesis of nanotubes.
[0025] Alternatively, substrate particles may be introduced into
the furnace separately, for example in their own carrier.
[0026] A particularly preferred approach is to use so-called POSS
(polyhedral oligomeric silsequioxane) compounds as the
catalyst-substrate particles. In this case the distinction between
catalyst and substrate is rather blurred, as POSS compounds are
themselves molecular silica-based materials. A POSS molecule can
act as a site for catalyst formation in situ.
[0027] The advantages of using a POSS are numerous. They have a
very high surface area. Their diameters are around 1 nm (the same
size as single wall nanotubes) but are tuneable as different POSS
molecules have different sizes. They can be monodisperse (have
specific molecular weights) and hence have the potential to
generate well-defined products. As they have molecular character,
they may be liquid or may be dissolved in the supercritical fluid
or a suitable liquid carrier (and may potentially even be
evaporated directly) for injection into the furnace. They have
excellent thermal stability in themselves. They have the potential
to form well-defined derivatives that potentially add catalytic
metallic particles (for example iron). Generally a single POSS
molecule will constitute a particle of substrate for nanotube
growth.
[0028] The finely divided substrate particles preferably have a
size not smaller than 1 nm, e.g. not less than 5 nm. They may
contain not less than 10 atoms, e.g. not less than 15 to 20 atoms,
perhaps not less than 50 atoms or 75 atoms. The substrate is fed to
the zone in which the catalyst precursor is decomposed and
preferably is essentially unchanged in the step of generating
supported-catalyst particles, except for the deposition thereon of
the catalyst material. However, some chemical modification of the
substrate particles during the formation of the supported-catalyst
particles is permissible, e.g. the removal of surface chemical
groups or solvating chemical side chains. Preferably, the size of
the substrate particles remains substantially unchanged.
[0029] The presence of the substrate particles during the
decomposition of the catalyst precursor serves to lower the
nucleation energy of the catalyst atoms and to control the size and
shape of the catalyst cluster so formed.
[0030] Alternatively, a substrate precursor, for example a
silicon-containing material such as tetramethyl orthosilicate or
tetraethyl orthosilicate, may be contained in the solution. Such a
precursor will decompose in the furnace to form a finely divided
substrate material.
[0031] The plasma jet may be formed by numerous methods known in
the art, but preferably is an inductively coupled plasma jet.
Alternatives include the use of a cathode upstream from a tubular
anode as in WO93/23331 or a microwave generator as in WO94/04461.
Such inductively coupled plasma torch devices are well known as
means of atomising a wide range of materials and are commonly used
a means of preparing sample fragments within a mass spectrometer.
They are also known as a means of producing fine powders and
nanocrystalline films. Such torches consist of an inert gas stream
that is passed though a cooled conducting coil which is protected
by a cool gas sheath and a quartz tube. Once the plasma is ignited
(usually with a spark from a piezoelectric device), high frequency
alternating current flowing in the coils couples inductively with
the ions in the gas, thus increasing their kinetic energy.
Effective temperatures of over 10,000 K can be reached. The
material to be vaporised (which can be solid, liquid or gas) is
injected into the plasma where it decomposes into its constituent
atoms. In the present invention, the plasma torch is used to
generate a vapour containing at least the metal catalyst atoms.
[0032] The furnace provides an opportunity for the carbon or other
nanotube forming material feedstock to become progressively
incorporated into growing nanotubes and the residence time in the
furnace and its temperature will affect the length of the nanotubes
produced suitably, the temperature within the furnace is from 700
to 1200.degree. C. It may be uniform, or may decline toward the
outlet of the furnace. The introduced materials preferably have a
residence time within the furnace of from 5 to 30 seconds, e.g.
about 10 seconds.
[0033] To promote rapid and effective mixing said quenching gas may
be directed radially into said plasma jet from multiple directions
to produce uniform conditions within the mixture thereof. The
quenching gas may be inert (e.g. Ar), or may be reactive (e.g.
H.sub.2 which can help to etch away unwanted amorphous carbon, or
N.sub.2 which may dope the growing nanostructures). Most preferably
the quenching gas contains a carbon-containing gas (e.g. methane or
CO) that can contribute to the growth of the nanotubes. Without
wishing to limited by theory, we believe that the initial quenching
step particularly controls nucleation of the nanotubes, either by
determining the size of the metal clusters that are produced or by
determining the diameters of fullerenic caps that appear on the
surface of slightly larger metal clusters. Nanotube nuclei may be
carbon-metal structures or pure metal clusters, that grow during
subsequent thermal treatment.
[0034] The invention will be further described and illustrated with
reference to the accompanying drawing, in which the single FIGURE
shows apparatus suitable for the performance of the method of the
invention.
[0035] The apparatus comprises a plasma torch formed by a tube 10
surrounded by water cooled induction coils 12 and having at its
upper end a first inlet 14 for a carrier gas containing metal
catalyst precursor and optionally also a carbon or other nanotube
forming material containing feedstock. A second inlet 16 is
provided for a plasma forming gas which suitably is an inert gas.
Materials introduced through the inlets 14 and 16 pas inside an
cylindrical shield 18 upstream of the coils 12 and a sheath gas is
introduced into the annular space between the tube 10 and the
shield 18 via an inlet 20. As the introduced materials pass through
the tube 10 within the coils 12, a plasma jet is formed. The sheath
gas functions to separate the plasma formed by the coils from the
tube wall to reduce the heating of the wall of the tube 10.
[0036] Below the coils 12, eight radially directed inlets 22 are
provided for a quenching gas. The eight inlets 22 are directed
towards the centre of the tube 10 at equal angles of 45.degree..
This ensures rapid mixing of the quenching gas with the material of
the plasma jet to produce a uniform mixture from this point on.
[0037] Below the inlet 22 is provided a tubular furnace 24
comprising a heated refractory tubular wall allowing control of the
temperature therein. At the outlet end of the furnace 24 is an
inlet 26 for additional cooling gas. A centrifugal separator 28 is
attached to the outlet of the furnace 22 and a gas exhaust outlet
30 from the separator 28 passes through a filter 32 to waste.
[0038] In optional modifications of what is shown, some of the
quenching gas inlets 22 may be used to introduce carbon or other
nanotube forming feedstock or a separate inlet for such feed stock
may be provided just upstream or just downstream from the quenching
gas inlets.
[0039] In use in the preferred embodiments of the invention, the
condensation process is controlled using a rapid turbulent
injection of gas to quench the material exiting the plasma torch.
This injection is carefully designed in order to ensure full mixing
of the plasma and the quenching gas and to ensure that uniform
conditions exist throughout the plume. Conveniently, the quenching
gas may be injected from a cylindrical array of angled nozzles as
shown. Nanotube elongation occurs over a longer period of time and
can be favoured by allowing a suitable residence time (eg 10 sec)
at moderate temperatures (e.g. 700-1200 C) with the option of
introducing additional carbon feedstock. To achieve the desired
quenching rate and elongation temperature the flow rate and
temperature of the quenching gas may be varied. The elongation
temperature can be maintained by passing the gas along a tube
furnace before the product is allowed to cool freely. Once
sufficiently cool, the product may be collected by the centrifugal
separator and filter unit. The invention is primarily aimed at the
production of hollow carbon nanotubes, whether single or
multiwalled, which may be doped (eg with N or B), but is also
applicable to analogous cage structures formed from other elements
(eg BN, WS.sub.2, etc) The following example is performed using the
apparatus as illustrated.
[0040] The plasma is generated using n 2 kW three phase 27 MHz RF
induction system, using Ar as the plasma and sheath gas. 10 wt %
ferrocene in toluene solution is introduced into the plasma using a
nubulizer at a rate of approximately 0.01 g/min. 50:50
Methane:hydogen at 750 C is used as the quenching gas and the
furnace is held at 850 C. The total flow rate of gas through the 90
cm long, 68 mm diameter tube furnace is 2 L/min. Products,
including single wall nanotubes, are deposited at the cold exit to
the furnace and on a fine filter placed in across the exhaust gas
stream.
[0041] The approach described has a number of advantages over the
existing techniques:
[0042] 1) The process uses a high temperature vapour to initiate
nanotube growth, which has been shown to produce high quality
materials in other approaches.
[0043] 2) The system may be operated continuously as feedstock is
injected into the plasma torch and the product collected by, for
example, a centrifugal separator.
[0044] 3) The option to add carbon feedstock after the plasma phase
reduces running costs because the total percentage of material that
must be raised to very high temperature is reduced. This option
also allows independent control of nanotube length and can, for
example, enable the production of very long tubes which may be
useful for mechanical reinforcement.
[0045] 4) The rate of quenching is critical in determining the
diameter of the nanotubes that are produced. Hence accurate control
of the quenching process allows excellent control over nanotube
diameter.
[0046] 5) The uniformity of mixing during the turbulent quenching
phase ensures that all the material experiences similar conditions
and hence a narrow diameter distribution and a high purity product
can be obtained. Current high temperature methods do not include a
deliberate turbulent stage and cooling is likely to be highly
uneven, as evidenced by the distribution of nanotube diameters
produced.
[0047] 6) The process can use of wide range of feedstocks, from
graphite and metallic powders to organometallic molecules and
simple hydrocarbons. Thus the process can make use of cheap
feedstocks that are conveniently handled. The most straightforward
method of introducing the required materials into the plasma is by
injection of a nebulised metal-containing liquid or solution.
[0048] Many modifications and variations of the illustrated
embodiments are possible within the general scope of the
invention.
* * * * *