U.S. patent application number 15/899807 was filed with the patent office on 2018-07-05 for highly purified carbon nanotubes and method of their preparation.
The applicant listed for this patent is YAZAKI CORPORATION. Invention is credited to Satyabrata RAYCHAUDHURI, Yongan YAN.
Application Number | 20180186644 15/899807 |
Document ID | / |
Family ID | 61633418 |
Filed Date | 2018-07-05 |
United States Patent
Application |
20180186644 |
Kind Code |
A1 |
YAN; Yongan ; et
al. |
July 5, 2018 |
HIGHLY PURIFIED CARBON NANOTUBES AND METHOD OF THEIR
PREPARATION
Abstract
Highly purified carbon nanotubes (CNT) having virtually no
carbonaceous impurities (amorphous carbon) nor inorganic impurities
(metal and metal oxides), and methods of their preparation are
described. The purified CNT feature excellent electrical,
mechanical, and thermal properties due to the near total absence of
detrimental impurities. The CNT starting material is preferably in
the form of wafer, film, or buckypaper for efficient diffusion of
purifying media. The highly pure CNT are prepared by heat treating
a CNT starting material in a specified amount of oxygen, then
treating the CNT in a solution comprising water and acid, or
further heat treating the CNT in an atmosphere comprising chlorine
(Cl.sub.2). Extremely low levels of inorganic impurities may be
achieved by treating sequentially with a treatment solution
followed by chlorine. Removal of chloride from purified CNT may be
achieved by further treating the chlorine-treated material in an
atmosphere comprising hydrogen (H.sub.2).
Inventors: |
YAN; Yongan; (Thousand Oaks,
CA) ; RAYCHAUDHURI; Satyabrata; (Thousand Oaks,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
YAZAKI CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
61633418 |
Appl. No.: |
15/899807 |
Filed: |
February 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14663339 |
Mar 19, 2015 |
9926200 |
|
|
15899807 |
|
|
|
|
61970821 |
Mar 26, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 32/17 20170801 |
International
Class: |
C01B 32/17 20170101
C01B032/17 |
Claims
1. A method for purifying carbon nanotubes, comprising the steps
of: (a) obtaining a carbon nanotube starting material that contains
impurities; (b) removing amorphous carbon from the carbon nanotube
starting material by heat treating the carbon nanotube starting
material between 300 and 550.degree. C. for 20 to 80 hours in a
heating chamber, wherein the heating chamber contains a specified
amount of oxygen and inert gas, and contains less than 1 ppm of
water vapor and other reactive species; and (c) removing inorganic
impurities from the carbon nanotube starting material by treating
the carbon nanotube in an atmosphere comprising chlorine
(O.sub.2).
2. The method of claim 1, wherein in step (b), the molar ratio of
the amount of oxygen provided during the heat treating to the
amount of carbon removed is between 1 and 8.
3. The method of claim 1, wherein in step (a), the carbon nanotube
starting material is in the form of a wafer, film, or buckypaper
having a thickness less than 50 micrometers.
4. The method of claim 1, wherein the treating in an atmosphere
comprising chlorine is conducted at a temperature between
600.degree. C. and 1200.degree. C.
5. The method of claim 1, wherein after treating in an atmosphere
comprising chlorine (Cl.sub.2), the carbon nanotube starting
material is further treated in an atmosphere comprising hydrogen
(H.sub.2).
6. The method of claim 5, wherein the treating in an atmosphere
comprising hydrogen is conducted at a temperature between
600.degree. C. and 1200.degree. C.
7. A method for purifying carbon nanotubes, comprising the steps
of: (a) obtaining a carbon nanotube starting material that contains
impurities; (b) removing amorphous carbon from the carbon nanotube
starting material by heat treating the carbon nanotube starting
material between 300 and 550.degree. C. for 20 to 80 hours in a
heating chamber, wherein the heating chamber contains a specified
amount of oxygen and inert gas, and contains less than 1 ppm of
water vapor and other reactive species; (c) removing inorganic
impurities from the carbon nanotube starting material by treating
it with a treatment solution comprising water and one or more
acids; and (d) removing inorganic impurities from the carbon
nanotube starting material by treating it in an atmosphere
comprising chlorine (Cl.sub.2).
8. The method of claim 7, wherein in step (c), the one or more
acids comprise hydrochloric acid (HCl) and/or hydrofluoric acid
(HF).
9. The method of claim 7, wherein in step (c) the treatment
solution further comprises an organic solvent.
10. The method of claim 9, wherein the organic solvent is an
alcohol selected from the group consisting of methanol, ethanol,
propanol, or any combination thereof.
11. The method of claim 7, wherein the treating in an atmosphere
comprising chlorine is conducted at a temperature between
600.degree. C. and 1200.degree. C.
12. The method of claim 7, wherein after treating in an atmosphere
comprising chlorine (Cl.sub.2), the carbon nanotube starting
material is further treated in an atmosphere comprising hydrogen
(H.sub.2).
13. The method of claim 12, wherein the treating in an atmosphere
comprising hydrogen is conducted at a temperature between
600.degree. C. and 1200.degree. C.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 14/663,339, filed Mar. 19, 2015; which claims the benefit of
U.S. Provisional Application No. 61/970,821, filed Mar. 26, 2014;
the above applications are incorporated herein by reference in
their entireties.
TECHNICAL FIELD
[0002] This invention relates generally to carbon nanotube
materials having extremely high purity with regard to amorphous
carbon and inorganic impurities, i.e. metals and metal oxides. This
invention also relates to methods of preparing extremely pure
carbon nanotubes without excessive loss of nanotube material, and
without measurable damage to the material. The highly purified
carbon nanotubes are useful in various electronics applications
requiring high conductivity and durability,
BACKGROUND
[0003] Carbon nanotubes (CNT) are of substantial interest for
numerous potential applications due to their unique electrical,
thermal, and mechanical properties. For example, certain nanotubes
are believed to have strength on the order of 100 s of GPa, or more
than 100 times stronger than steel. They also have unique
electrical properties that make them attractive for use in solar
cells, capacitors, batteries and other energy storage devices, as
conductive coatings, in gas sensors, etc.
[0004] Impurities in CNT materials appear in multiple forms and are
often introduced during the synthesis of CNT. In a typical
manufacturing process called alcohol catalytic chemical vapor
deposition (ACCVD), evaporated methanol or ethanol vapors come in
contact with catalyst particles such as nickel or iron, embedded on
magnesium oxide or silica as catalyst support, at high temperatures
inside a furnace. At such conditions, ethanol or methanol molecules
break down, and CNT start growing around the catalyst. However,
this process also results in the generation of amorphous carbon,
which can be located randomly on the outer surfaces of CNT.
Amorphous carbon is the most common impurity and the hardest to
remove, due to bonding on certain carbon atoms. Other types of
impurities include catalyst residue such as iron (Fe), nickel (Ni),
cobalt (Co), molybdenum (Mo), etc., and catalyst support materials
such as magnesium oxide (MgO), aluminum oxide (Al.sub.2O.sub.3),
and silicon dioxide (silica, SiO.sub.2).
[0005] Amorphous carbon and inorganic impurities in CNT materials
are detrimental to the electrical, thermal, and mechanical
properties of the material. The presence of impurities can affect
the properties to an extent that renders the CNT material
unsuitable for many applications. For this reason, significant
effort has been undertaken to produce purified CNT.
[0006] A common method of evaluating CNT for the presence of
amorphous carbon is by visual observation at high magnification
(greater than about 20,000.times.). This can be accomplished using
commercially available instruments such as a scanning electron
microscope (SEM) or transmission electron microscope (TEM). The
visual appearance of amorphous carbon under high magnification is
quite distinct from that of CNT and CNT bundles, and good
qualitative evaluations of amorphous carbon content in a CNT sample
can be achieved by this method.
[0007] Evaluation of inorganic impurity contact in a CNT material
can be accomplished by a variety of means to different levels of
accuracy and precision. Common methods include Energy Dispersive
X-Ray Spectroscopy, which is usually conducted in concert with SEM
or TEM, and Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
The former is a semi-quantitative method that is most useful for
identifying, rather than quantifying, the impurity elements present
in a CNT sample. The latter can be used to determine precise
amounts of those elements in the material.
[0008] Another technique useful for evaluating the quality of CNT,
i.e., the concentration of structural defects and amorphous carbon
impurities included therein, is by measuring the intensity ratio of
two characteristic Raman spectral peaks, called the G/D ratio. The
G-band is a tangential shear mode of carbon atoms that corresponds
to the stretching mode in the graphite plane. The D-band is a
longitudinal optical (LO) phonon and is known as the disordered or
defect mode, as it is a typical sign for defective graphitic
structures in CNT. The comparison of the ratios of these two peaks'
intensities gives a measure of the quality of the CNT samples.
Generally, the G/D ratio is used to quantify the structural quality
of carbon nanotubes. Thus, CNT having a higher G/D indicate a lower
amount of defects and a higher level of quality.
[0009] A G/D ratio is typically determined using a Raman
spectroscopy technique. Any of various commercially available
instruments may be used to measure the G and D band intensities and
to calculate the G/D ratio. One example of such equipment is
available from HORIBA Jobin Yvon Inc., Edison, N.J., under the
model name LabRAM ARAMIS.
[0010] The G/D ratio usually changes after a purification is
applied to a sample of CNT. When purified of amorphous carbon, the
G/D ratio of the purified CNT is typically greater than the G/D
ratio of the starting CNT, indicating that the purified CNT has
fewer structural defects and/or carbonaceous impurities with
different carbon bond types than that of CNT. For removal of
inorganic impurities, prior art methods, such as reacting in highly
concentrated acids at elevated temperatures, typically result in
significant decrease in G/D ratio, indicating that the purification
process imparted damage to the CNT structure.
[0011] Various methods of removing amorphous carbon and inorganic
impurities are known in the literature, including thermal
oxidation, various solution treatments, and various gas treatments.
However, existing methods tend to damage CNT (as mentioned above),
cause significant loss of CNT, or result in only partial
purification. Known commercial methods typically entail treatment
of CNT with concentrated acid, such as nitric acid, often at
elevated temperatures, followed by a slow heat treatment. Although
this protocol has been proven to reduce both amorphous carbon and
inorganic content, it is unsafe, and a substantial amount of
contamination can still remain on the surface. Furthermore,
treatment with concentrated acids is somewhat counterproductive, as
it also introduces structural defects while removing superficial
ones.
[0012] Therefore, there exists a need for an efficient and safe
process for preparing purified CNT; the method should efficiently
remove all or nearly all amorphous carbon and inorganic impurities
without damaging or destroying the CNT.
SUMMARY OF THE INVENTION
[0013] The present invention is directed to a carbon nanotube (CNT)
material having extremely high purity with regard to both
carbonaceous impurities (amorphous carbon), and inorganic
impurities (metal and metal oxides), as well as high Raman G/D
ratio.
[0014] The present invention is also directed to a method for
preparing a CNT material having extremely high purity with regard
to both carbonaceous impurities (amorphous carbon), and inorganic
impurities (metal and metal oxides).
[0015] In a first embodiment, the method comprises the steps of (a)
obtaining a CNT starting material that contains impurities, (b)
selectively removing amorphous carbon from the CNT starting
material by heat treating in the presence of a specified amount of
oxygen, and (c) selectively removing inorganic impurities from the
CNT material by treating it with a treatment solution comprising
water and acid.
[0016] In a second embodiment, the method comprises the steps of
(a) obtaining a CNT starting material that contains impurities, (b)
selectively removing amorphous carbon from the CNT starting
material by heat treating in the presence of a specified amount of
oxygen, and (c) selectively removing inorganic impurities from the
CNT material by treating it in an atmosphere comprising
chlorine.
[0017] In a third embodiment, the method comprises the steps of (a)
obtaining a CNT starting material that contains impurities, (b)
selectively removing amorphous carbon from the CNT starting
material by heat treating in the presence of a specified amount of
oxygen, (c) selectively removing inorganic impurities from the CNT
material by treating it with a treatment solution comprising water
and acid, and (d) further selectively removing inorganic impurities
from the CNT material by treating it in an atmosphere comprising
chlorine.
[0018] In a preferred embodiment, the CNT starting material is in
the form of wafer, film, or buckypaper. In another preferred
embodiment, the treatment solution comprises organic solvent in
addition to water and acid. In a further preferred embodiment, CNT
material that has been purified of inorganic impurities by treating
in chlorine, is further treated in an atmosphere comprising
hydrogen to remove residual chloride.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows a flow diagram illustrating the CNT
purification method of the present invention.
[0020] FIG. 2 shows scanning electron microscope (SEM) images of
SWCNT powder as-received from a commercial supplier, and the same
material after being formed into a wafer, as described in Ex.
3.
[0021] FIG. 3 shows charts of temperature and oxygen content in a
controlled-atmosphere tube furnace vs. time, during a heat
treatment to remove amorphous carbon from a CNT material, as
described in Ex. 4.
[0022] FIG. 4 shows a SEM image of a CNT starting material after
heat treatment to remove amorphous carbon, as described in Ex.
4.
[0023] FIG. 5 shows the results of energy dispersive x-ray
spectroscopy (EDS) analysis of a CNT material after heat treatment
to remove amorphous carbon, as described in Ex. 4. Iron (Fe) and
silicon (Si) are the primary impurity elements present in the
material.
[0024] FIG. 6 shows a SEM image of a CNT material after solution
treatment to remove inorganic impurities, as described in Ex.
5.
[0025] FIG. 7 shows a SEM image of a CNT material after solution
treatment to remove inorganic impurities, as described in Ex.
6.
[0026] FIG. 8 shows a SEM image of a CNT material after treatment
in chlorine and hydrogen atmospheres, as described in Ex. 7.
[0027] FIG. 9 shows a SEM image of a CNT material after treatment
in chlorine and hydrogen atmospheres, as described in Ex. 8.
[0028] FIG. 10 shows a chart of Raman G/D ratio and percent yield
of CNT material vs. the ratio of oxygen supplied to carbon
consumed, in the heat treatment of CNT starting material to remove
amorphous carbon, as described in Ex. 9.
DETAILED DESCRIPTION
[0029] A novel method is herein described for preparing purified
carbon nanotubes, or CNT. The method allows the removal of all (or
nearly all) amorphous carbon, as well as nearly all inorganic
impurities (metals and their oxides), without degrading or damaging
the CNT. The method results in CNT having extremely high purity,
and Raman G/D ratio greater than that of the starting material.
[0030] In a first embodiment, the method comprises the steps of:
(a) obtaining a carbon nanotube (CNT) starting material that
contains impurities; (b) selectively removing amorphous carbon from
the CNT starting material by heat treating it in the presence of a
specified amount of oxygen; and (c) selectively removing inorganic
impurities from the material by treating it with a treatment
solution comprising water and acid. This embodiment is illustrated
by flow diagram (1), in FIG. 1. The purified CNT obtained by this
embodiment of the invented method is designated "grade 1".
[0031] In a second embodiment, the method comprises the steps of
(a) obtaining a CNT starting material that contains impurities, (b)
selectively removing amorphous carbon from the CNT starting
material by heat treating in the presence of a specified amount of
oxygen, and (c) selectively removing inorganic impurities from the
CNT starting material by treating it in an atmosphere comprising
chlorine. This second embodiment is illustrated by flow diagram
(2), in FIG. 1. The purified CNT material obtained by this
embodiment of the invented method is designated "grade 2".
[0032] In a third embodiment, the method comprises the steps of (a)
obtaining a CNT starting material that contains impurities, (b)
selectively removing amorphous carbon from the CNT starting
material by heat treating in the presence of a specified amount of
oxygen, (c) selectively removing inorganic impurities from the CNT
material by treating it with a treatment solution comprising water
and acid, and (d) further selectively removing inorganic impurities
from the CNT material by treating it in an atmosphere comprising
chlorine. This third embodiment is illustrated by flow diagram (3),
in FIG. 1. The purified CNT material obtained by this embodiment of
the invented method is designated "grade 3".
[0033] In all embodiments of the invention described herein, the
sequence of steps is important to achieving the maximum benefit in
terms of impurities removed from the CNT starting material. In
particular, it is important that the heat treatment in the presence
of oxygen, to remove carbonaceous impurities, is performed prior to
any of the described treatments used to remove inorganic
impurities. The main reason for this is that carbonaceous
impurities such as amorphous carbon tend to form in the areas
immediately adjacent to inorganic impurities in the CNT, such as
catalyst and catalyst support residue materials. These inorganic
impurities can be shielded, surrounded, or encapsulated by the
carbonaceous impurities, thereby preventing their removal by the
described techniques such as solution treatment or exposure to
chlorine gas. Once the carbonaceous impurities are removed, the
residual catalyst material is exposed and readily removed by the
subsequent step(s).
[0034] The first step of the invented method comprises obtaining a
carbon nanotube (CNT) starting material that contains impurities.
The CNT starting material may comprise single-wall carbon nanotubes
(SWCNT), double-wall carbon nanotubes (DWCNT), multi-wall carbon
nanotubes (MWCNT), or any combination thereof. In a preferred
embodiment, the CNT starting material is selected from the group
consisting of SWCNT, DWCNT, and the combination thereof.
[0035] The CNT starting material may be in the form of powder,
particles, flakes, loose agglomerates, wafer, film, "buckypaper",
any combination of the preceding forms, or any of those forms
individually or in any combination, as an aqueous slurry or wet
cake.
[0036] In a preferred embodiment, the CNT starting material is in
the form of wafer, film, or buckypaper. Such structures may be
prepared by any of a number of known techniques, such as those
described in WIPO publication WO 2010/102250 A2, WIPO publication
WO 2012/177555 A2, or US patent application publication US
2013/0040229 A1, all incorporated herein by reference in their
entirety. Preferably, the thickness of the film, buckypaper, or
wafer is less than about 500 .mu.m, more preferably less than about
100 .mu.m, and still more preferably less than about 50 .mu.m.
[0037] The preferred embodiment of the starting material is a
wafer, film, or buckypaper, because this configuration allows the
most efficient and complete purification to take place via the
invented method. CNT materials, especially those typically provided
by commercial suppliers, are often in the form of powders or
granules having diameter or largest dimension in the range of
hundreds of micrometers to several millimeters. These shapes and
sizes effectively isolate the material in the cores of the powder
particles or granules, preventing the penetration of purifying
substances including gases and liquids. This results in only
partial purification when the CNT starting material is exposed to
the purifying treatments. In contrast, wafer, film, or buckypaper
allow diffusion of gases and liquids through their thicknesses,
resulting in exposure of all material to the purifying effects of
those media.
[0038] The impurities present in the CNT starting material may
comprise carbonaceous impurities including amorphous carbon,
inorganic impurities such as metals and their oxides, or both types
of impurities. The method of the invention is not limited as to the
amount of impurities present in the starting material, although it
is especially useful in purifying CNT that contains both types of
impurities, in amounts typically found in commercially available
CNT. In particular, the total amount of all types of impurities in
a CNT starting material is preferably less than 99%. The total
amount of inorganic impurities as metal and/or metal oxides is
preferably less than 90%, more preferably less than 50%, and still
more preferably less than 20%, in order to achieve the best
possible results via the method of the present invention.
[0039] CNT starting materials may be obtained from commercial
sources. Examples include SWCNT and MWCNT materials sold under the
trade name "Elicarb.RTM.", available from Thomas Swan & Company
(Consett, County Durham, United Kingdom), SWCNT and MWCNT available
from Southwest Nanotechnologies Inc. (Norman, Okla.), SWNT
materials available from Carbon Solutions, Inc. (Riverside,
Calif.), or any of various SWCNT, DWCNT, and MWCNT materials sold
under the trade name Nanocyl.TM., by Nanocyl S. A. (Auvelais,
Belgium).
[0040] In the second step of the invented method, amorphous carbon
is selectively removed from the CNT starting material by heat
treating it in the presence of a specified amount of oxygen. This
step may be conducted using any temperature-controlled chamber
capable of heating above 200.degree. C., having an internal space
sufficient to hold the CNT material. Preferably, this step is
conducted using an oven or furnace capable of providing a
controlled atmosphere with a gas supply, that can be sealed to
isolate the internal space from the outside environment. A vacuum
furnace, graphite furnace, or quartz tube furnace are examples of
appropriate heating chambers with which to conduct this step of the
method. Preferably, the heating chamber is connected to gas
supplies including oxygen and an inert gas such as helium,
nitrogen, or argon.
[0041] The heating and cooling rates used in this step are not
particularly limited, although the heating rate should be
sufficiently slow such that the maximum desired temperature is not
exceeded by "overshooting" the temperature set point. Cooling may
be accomplished as quickly as the heating chamber will allow,
although preferably no quenching (gas or liquid) should be used to
increase cooling rate.
[0042] Prior to heating the CNT starting material, it is loaded
into the heating chamber, the chamber is sealed, and the chamber is
then purged with inert gas to effectively remove any air or
moisture. The inert gas may be any chemically non-reactive gas such
as a noble gas or other non-oxidizing gas, or a mixture of such
gases. Nitrogen (N.sub.2), helium (He), and argon (Ar) are examples
of appropriate gases. Preferably, an ultra-high purity grade of
inert gas is used, such as are commercially available, or measures
are taken to ensure the supply of inert gas has been purified prior
to entering the chamber.
[0043] The main parameters used to control the removal of amorphous
carbon from the CNT starting material in this step are (i) the
temperature at which the material is heated; (ii) the duration of
heating at that temperature; and (iii) the amount or concentration
of oxygen supplied during the heating at that temperature.
[0044] After placing it into the heating chamber and sealing the
chamber, the CNT starting material is heated to a specified
temperature between 200.degree. C. and 1100.degree. C., or between
300.degree. C. and 800.degree. C., or between 300.degree. C. and
700.degree. C., or between 300.degree. C. and 400.degree. C., or
between 400.degree. C. and 500.degree. C., or between 500.degree.
C. and 600.degree. C., or between 600.degree. C. and 700.degree.
C., or between 350.degree. C. and 450.degree. C., or between
450.degree. C. and 550.degree. C., or between 550.degree. C. and
650.degree. C. The ideal temperature range is that which allows
complete or near complete removal of amorphous carbon while
minimizing combustion and collateral loss of CNT. This range may
vary depending on the type or source of CNT, initial amount of
amorphous carbon present, and the form of the CNT to be treated
(i.e. powder, wafer, etc.)
[0045] The duration of heating at the specified temperature is
typically between 0.2 and 200 hr, preferably between 2 and 100 hr,
and more preferably between 20 and 80 hr. It is especially
preferable for the duration of heating to be between 30 and 60 hr,
as this provides the best balance between precise process control
(to maximize purification and minimize CNT loss), and the economic
demands for expeditious material processing.
[0046] During the heat treatment, the atmosphere inside the heating
chamber comprises oxygen and inert gas. Preferably, contaminants
such as water vapor or other potentially reactive species are not
present in the atmosphere or are at least limited to trace levels
(less than 1 ppm). The atmosphere may be static or continuously
flow through the chamber. The latter case is preferable as this
promotes removal of reaction products from the chamber and exposure
of the CNT material to fresh oxygen.
[0047] In general, the amount of oxygen provided during the heat
treatment has an effect on the amount of amorphous carbon removed,
as well as the amount of CNT that is lost collaterally due to
combustion. The amount of oxygen provided during the heat treatment
is defined as the amount of oxygen supplied to the heating chamber
while the heating chamber is at the specified temperature.
[0048] The amount of oxygen provided during the heat treatment may
be specified as a percentage or ratio by moles or volume of the gas
inside the chamber, or of the supplied gas stream, for example in
terms of parts per million. In general, the oxygen content is
typically between 1 ppm and 200,000 ppm (20%), or between 1000 ppm
and 100,000 ppm (10%), or between 2000 ppm and 20,000 ppm (2%). Or,
the oxygen content is between 500 ppm and 50,000 ppm, or between
1000 ppm and 10,000 ppm. Or, the oxygen content is between 100 ppm
and 10,000 ppm, or between 500 ppm and 5000 ppm. Or, the oxygen
content is between 2000 ppm and 200,000 ppm, or between 4000 ppm
and 40,000 ppm.
[0049] The oxygen provided to the chamber may be controlled by any
appropriate available means, such as mass flow controllers or
rotameters. Further, the oxygen content of the gas exiting the
chamber may be continuously monitored using a commercially
available oxygen monitoring device, for example, Series 3000
instrument sold by Alpha Omega Instruments, Lincoln, R.I.
[0050] Alternatively, and preferably, the amount of oxygen provided
during the heat treatment step is specified in terms of the ratio
of total amount of oxygen supplied to the total quantity of carbon
removed. The total carbon removed includes both amorphous carbon
and CNT. That is, total carbon removed is the difference between
the total amount of carbon present in the CNT starting material,
including CNT and carbonaceous impurities, and the total amount of
carbon present in the material after the heat treatment, again
including CNT and any remaining carbonaceous impurities. For any
CNT starting material comprising amorphous carbon, there exists an
optimum range of this ratio, within which the amount of amorphous
carbon removed is maximized while the amount of CNT lost due to
combustion is minimized. This ratio of oxygen supplied to total
carbon removed (O.sub.2:C.sub.Rem) in terms of moles is typically
between 1 and 100, preferably between 1 and 10, more preferably
between 1 and 8, and still more preferably between 1 and 5.
[0051] In order to determine the amount of oxygen to provide that
will result in an O.sub.2:C.sub.Rem ratio within the preferred
range, it may be necessary to conduct a series of trial-and-error
heat treatments with a particular CNT starting material (for
example, a particular grade or catalog number of material from a
particular supplier, for example, SWCNT sold under the name
Elicarb.RTM. SW by Thomas Swan & Co., Ltd.). The series of
trial-and-error heat treatments represent an iterative process,
wherein the initial parameters such as heat treatment temperature
and oxygen concentration in the gas stream are selected based on
past experience with other CNT types.
[0052] In this iterative process, a first trial heat treatment with
the particular CNT starting material is conducted under the
selected conditions. During the trial heat treatment, the oxygen
content of the gas exiting the chamber is monitored. The exiting
oxygen content will decrease from an initial value when the
oxidation of amorphous carbon commences. For example, the O.sub.2
concentration in the exit gas may initially be about 10,000 ppm,
and then drop to about 4000 ppm when oxidation commences at the
selected temperature. After some time, the oxygen concentration in
the exit gas will start to increase, and will eventually return to
approximately its initial level. This indicates that oxidation of
carbon has ceased or at least substantially declined, and the trial
heat treatment is then concluded by cooling the chamber and
discontinuing the flow of oxygen.
[0053] The amount of oxygen supplied during the heat treatment is
calculated from the concentration, flow rate, and time of heat
treatment, and the amount of carbon removed is calculated by
subtracting the weight of the heat treated CNT material from the
initial weight of the CNT starting material. The amounts of oxygen
supplied and carbon removed are converted to moles and the molar
ratio of O.sub.2:C.sub.Rem is then determined. If this ratio falls
within the preferred range, then the parameters of the heat
treatment are optimized for the particular CNT starting material.
If the ratio is outside the preferred range, a second iterative
heat treatment may be conducted with the same CNT starting
material, with one or more process parameters varied in order to
bring the O.sub.2:C.sub.Rem ratio within the preferred range.
[0054] A ratio of O.sub.2:C.sub.Rem above the preferred range may
indicate that amorphous carbon removal was completed, but the heat
treatment was longer than necessary, resulting in excess oxygen
being supplied to the chamber. The extent of amorphous carbon
removal may be cross-checked by examination of the heat treated
material under high magnification, such as by electron microscopy.
Measurement of Raman G/D ratio may also be helpful in determining
the extent of amorphous carbon removal. If removal of amorphous
carbon was complete, as evidenced by its visible absence and/or an
attendant increase in G/D ratio to a value greater than about 10,
the amount of oxygen supplied or the duration of heat treatment may
be reduced, both of which will lower the O.sub.2:C.sub.Rem ratio.
If amorphous carbon is still present in the material, as evidenced
by its visible presence or a G/D ratio close to that of the
starting material, or lower than about 10, the heat treatment
temperature may be increased, in order to increase C.sub.Rem and
thereby lower the O.sub.2:C.sub.Rem ratio.
[0055] A ratio of O.sub.2:C.sub.Rem below the preferred range may
indicate that amorphous carbon removal was incomplete in the heat
treatment process. Again, this may be cross-checked by electron
microscopy and Raman measurements. If amorphous carbon is still
present, O.sub.2 concentration, duration of heat treatment, and/or
heat treatment temperature may be increased, in order to increase
the O.sub.2:C.sub.Rem ratio.
[0056] After ascertaining the O.sub.2:C.sub.Rem ratio, and
determining the extent of amorphous carbon removal, a second trial
heat treatment is conducted under process parameters modified as
described above, and the O.sub.2:C.sub.Rem ratio for the second
trial is determined. This iterative process is continued until the
experimental parameters are determined that will result in a ratio
of O.sub.2:C.sub.Rem within the preferred range, along with
complete removal of amorphous carbon as evidenced by visual
observation and Raman G/D ratio measurements.
[0057] In the third step of the invented method, inorganic
impurities are selectively removed from the CNT material by
treating it with a treatment solution comprising water and acid.
The type of acid used in the treatment solution is not particularly
limited, and may be any organic or inorganic acid. One or more than
one type of acid may be used. Inorganic acid is preferable because
it is typically more reactive with the inorganic impurities (metals
and their oxides) present in CNT and will not leave organic
residues in the CNT after the treatment. Commonly available acids
such as HCl, HNO.sub.3, and H.sub.2SO.sub.4 are preferred.
[0058] In a preferred embodiment, the acid is HCl, which is
commonly available as a concentrated aqueous solution containing
about 37% by weight HCl, or about 12N (for example, product number
258148, Sigma-Aldrich Corporation, St. Louis, Mo.). The molar
concentration of acid in the treatment solution is typically
between 0.01N and 20N, preferably between 0.1N and 10N, more
preferably between 1N and 8N, and still more preferably between 1N
and 5N.
[0059] In another preferred embodiment, the treatment solution
comprises hydrofluoric acid (HF), in addition to one or more other
acids. HF is commonly available as a concentrated aqueous solution
containing about 49% by weight HF, or about 29N (for example,
product number 339261, Sigma-Aldrich Corporation). The molar
concentration of HF in the treatment solution is typically between
0.01N and 20N, preferably between 0.05N and 5N, more preferably
between 0.1N and 1N, and still more preferably between 0.1N and
0.5N.
[0060] In an especially preferred embodiment, the treatment
solution comprises HCl at a concentration of between 1N and 5N, and
HF at a concentration of between 0.1N and 0.5N. A treatment
solution thusly prepared is particularly suitable for removing
inorganic impurities typically found in CNT, which typically
comprise catalyst residues such as Fe, Ni, Co, Mo, etc., and
catalyst support residues such as SiO.sub.2, Al.sub.2O.sub.3, etc.
The HCl present in the treatment solution effectively removes most
metallic and some metal oxide residues, while the HF present in the
treatment solution effectively removes SiO.sub.2 and assists in
removing metallic residues.
[0061] It is important to note that if the inorganic residue in the
CNT material includes Mg, MgO, or other element(s) for which an
insoluble fluoride compound exists, HF should not be used in the
treatment solution unless the CNT material has already been treated
to remove those elements. Otherwise, reaction with HF will produce
the fluoride compound(s), which are generally insoluble in water
and difficult to remove from CNT by washing. In such instances, the
CNT material may be sequentially treated with one treatment
solution capable of removing the fluoride-producing element(s),
such as 1N-8N HCl, then treated with another treatment solution,
such as 0.1N-0.5N HF.
[0062] In yet another preferred embodiment, the treatment solution
comprises organic solvent in addition to water and acid. The
presence of organic solvent in the treatment solution can
substantially enhance the effectiveness of the solution in removing
inorganic impurities. Certain CNT starting materials can be highly
hydrophobic and will not wet when brought in contact with an
aqueous solution. In particular, CNT wafer, film, and buckypaper
may be hydrophobic due to the effects of processes applied to
assemble the material into these forms. Similarly, certain types of
CNT in their as-synthesized form, or in the condition as-received
from a commercial supplier, may have certain types of functional
groups or chemical additives present that render the material
hydrophobic. Therefore, having both water and organic solvent in
the treatment solution can significantly improve wetting of the CNT
material to be purified.
[0063] The type of organic solvent used in the treatment solution
is not particularly limited, but miscibility with water and the
selected acid is a necessity. One or more type(s) of organic
solvent may he used, provided all are miscible with water. Lower
order alcohols such as methanol, ethanol, and propanols are
preferred, as they tend to be safer to handle and less expensive
than other solvents. In a preferred embodiment, the organic solvent
is reagent alcohol, which is typically commercially available as a
mixture of primarily (>90%) ethanol with propanol and sometimes
methanol as minor components (for example, catalog number A995,
Thermo Fisher Scientific, Waltham, Mass.).
[0064] The weight ratio of water:organic solvent in the treatment
solution is typically between 20:1 and 1:20, preferably between 4:1
and 1:4, and more preferably between 3:1 and 1:2.
[0065] To apply the treatment solution to the CNT material,
typically the material is immersed in the solution in a container.
The size and shape of the container are not particularly limited,
provided that the CNT material and the treatment solution can both
be accommodated, and the CNT material can be fully covered by the
solution. The container should be made of a material that will not
be attacked or otherwise damaged by the treatment solution.
Immersion of the CNT material into the treatment solution may be
accompanied by mechanical agitation, sonication, or a combination
of these techniques, although this is optional.
[0066] The ratio of amount of treatment solution to amount of CNT
material to be treated is typically between 0.1 and 100 liters
solution/gram material (L/g), preferably between 0.3 and 10 L/g,
and more preferably between 0.5 and 6 L/g. The time of immersion in
the treatment solution is typically between 0.1 and 100 hr,
preferably between 0.2 and 50 hr, and more preferably between 0.5
and 25 hr. Mild mechanical agitation, sonication, or a combination
of the techniques may reduce the time needed to achieve maximum
effect of the solution treatment, but again, is not required.
[0067] After the solution treatment, the CNT material is rinsed
with a rinsing solution. The rinsing solution should not contain
any components that can re-introduce impurities into the CNT
material. The rinsing solution may consist essentially of water, or
it may consist essentially of a solution of water and organic
solvent. In the latter case, the ratio of water to organic solvent
is typically 50/50 (w/w), but is not particularly limited. The type
of organic solvent in the rinsing solution is also not particularly
limited, but is preferably the same solvent that is used in the
treatment solution, or a solvent that is miscible with the solvent
used in the treatment solution. Rinsing is typically repeated once
or twice, to ensure best possible removal of the treatment solution
and dissolved impurities from the CNT material.
[0068] After rinsing the purified CNT material with the rinsing
solution, the purified CNT may be dried by any practical means.
Typically, the material is dried at or above room temperature for
several hours. This may be accomplished under vacuum or at
atmospheric pressure, and under an inert atmosphere or in air. In
one embodiment, the material is dried in air at 200.degree. C. for
3 hr in a standard convection oven.
[0069] After applying the heat treatment and solution treatment
steps described above to the CNT starting material, the resulting
CNT material is typically characterized by the total or near-total
absence of amorphous carbon, and low levels of inorganic
impurities. The total amount of inorganic impurities (metals and
metal oxides) in a CNT material purified in this manner is
typically less than about 1.0 wt %, usually less than about 0.5 wt
%, and sometimes less than about 0.25 wt % (2500 ppm).
[0070] Qualitative determinations of amorphous carbon removal are
possible by observing the starting material and purified material
at high magnification under a scanning or transmission electron
microscope (SEM or TEM). Typically, the percent reduction in
amorphous carbon after applying the invented method to a CNT
starting material is greater than 90%, greater than 99%, or greater
than 99.9%. Typically, after applying the invented method to a CNT
starting material, amorphous carbon cannot be observed in the
purified CNT material at up to about 500,000.times.
magnification.
[0071] The extent of amorphous carbon removal may also be
qualitatively evaluated by measuring and comparing the Raman G/D
ratios of the CNT starting material and the purified CNT material.
Typically, G/D ratio increases substantially after applying the
purification method, and in particular, after applying the heat
treatment step. The increase in G/D ratio is typically by a factor
of at least 2, usually by a factor of at least 3, and often by a
factor of 4 or more.
[0072] The amount of inorganic impurities in the purified CNT
material is determined using any of appropriate known methods of
elemental analysis, such as x-ray fluorescence spectroscopy (XRF),
inductively coupled plasma optical emission spectrometry (ICP-OES),
or ICP mass spectrometry (ICP-MS). The primary inorganic impurity
elements present in a CNT starting material can be determined using
qualitative or semi-quantitative techniques such as energy
dispersive x-ray spectroscopy (EDS), or XRF. Once these elements
are identified, quantitative measurements on selected elements of
interest can be conducted using the prior-mentioned techniques.
After applying the invented purification method, the amount of a
particular element present in the CNT material decreases typically
to less than 50%, usually to less than 20%, and often to less than
5% of the original amount.
[0073] The purity of the CNT material with regard to inorganic
content can also be determined by measuring the total carbon amount
in the sample, or by thermogravimetric analysis. In the latter
case, the sample is heated in the presence of oxygen above at least
600.degree. C. for sufficient time that all combustible material is
removed. The remaining weight of material is assumed to be metals
and metal oxides, that is, the inorganic content of the sample. The
total inorganic content in the purified CNT material is typically
less than 1% and usually less than 0.5% by weight of the
material.
[0074] In a second embodiment of the present invention, the method
comprises the steps of (a) obtaining a carbon nanotube (CNT)
starting material that contains impurities; (b) selectively
removing amorphous carbon from the CNT starting material by heat
treating it in the presence of a specified amount of oxygen, and
(c) selectively removing inorganic impurities from the CNT material
by treating it in an atmosphere comprising chlorine gas
(Cl.sub.2).
[0075] In this embodiment, step (a), obtaining a carbon nanotube
(CNT) starting material that contains impurities, and step (b),
selectively removing amorphous carbon from the CNT starting
material by heat treating it in the presence of a specified amount
of oxygen, are conducted in the manner previously described
herein.
[0076] In this third step of this embodiment, inorganic impurities
are selectively removed from the CNT material by treating it in an
atmosphere comprising chlorine gas (Cl.sub.2). The treatment in
chlorine may be accomplished in any suitable heating chamber or
furnace with a controlled atmosphere that can be well-isolated from
the outside environment. For example, a commercially available
laboratory or industrial-scale furnace with a quartz tube chamber
and flanged or ground-glass end caps is appropriate. Such equipment
may be purchased, for example, from Mellen Company, Concord, N.H.,
and from GM Associates Inc., Oakland, Calif.
[0077] In a typical chlorine (Cl.sub.2) treatment, the CNT material
is loaded into the furnace or heating chamber and the chamber is
purged with an inert gas (e.g., nitrogen, argon, or helium) at a
sufficient flow rate, and for a sufficient time to remove moisture
and oxygen. Then, while continuing the flow of inert gas, the
chamber temperature is increased at a specified rate from room
temperature to an intermediate temperature (between about
200.degree. C. and 500.degree. C.). The chamber is held at that
temperature for a sufficient time to allow any absorbed moisture to
evolve from the CNT material and interior surfaces of the chamber,
and be purged from the chamber.
[0078] The chamber temperature is then further increased, still
under flow of inert gas, to the Cl.sub.2 treatment temperature,
which is typically between about 600.degree. C. and 1200.degree.
C., preferably between 800.degree. C. and 1100.degree. C., and more
preferably between 900.degree. C. and 1100.degree. C. When the
chamber temperature reaches the treatment temperature, the gas
supply is changed to a mixture of an inert gas and chlorine. The
gas mixture is typically comprised of between 1% and 50% (v/v)
Cl.sub.2, preferably between 3% and 30% Cl.sub.2, and more
preferably between 5% and 15% Cl.sub.2. The chamber is held at the
treatment temperature for between 0.1 hr and 100 hr, preferably
between 0.5 hr and 10 hr, more preferably between 1 hr and 5 hr.
During the exposure of the CNT material to chlorine, the gas flow
rate should be sufficient to provide at least 0.5 volume change per
hour, preferably at least 1 volume change per hour.
[0079] After exposing the CNT material to chlorine as specified,
the chamber is then purged again with inert gas sufficiently to
remove any remaining chlorine. The chamber is then cooled to room
temperature, and the CNT material is removed.
[0080] A CNT material purified via this particular embodiment, i.e.
heat treated under oxygen, then heat treated under chlorine, is a
highly pure material. A CNT material purified in this manner
contains virtually no amorphous carbon, and total inorganic
impurity content is typically lower than 0.5% (w/w), usually lower
than 0.2%, and sometimes lower than 0.1% (1000 ppm), as determined
by a quantitative analysis method such as ICP-MS.
[0081] In a third embodiment of the present invention, the method
comprises the steps of (a) obtaining a carbon nanotube (CNT)
starting material that contains impurities; (b) selectively
removing amorphous carbon from the CNT starting material by heat
treating it in the presence of a specified amount of oxygen; (c)
selectively removing inorganic impurities from the material by
treating it with a treatment solution comprising water and acid,
and (d) further removing inorganic impurities from the material by
treating it in an atmosphere comprising chlorine gas.
[0082] In this embodiment, an extreme high level of purity can be
achieved in the CNT material through the additive effect of
removing inorganic impurities through two sequential techniques,
i.e. treating with a treatment solution, and then treating in an
atmosphere comprising chlorine gas. A CNT material purified via
this particular embodiment, i.e. heat treated under oxygen, then
solution treated, and then heat treated under chlorine, is an
extremely pure material. A CNT material purified in this manner
contains virtually no amorphous carbon, and total inorganic
impurity content is typically lower than 0.5% (w/w), usually lower
than 0.1%, and often lower than 0.05% (500 ppm), as determined by a
quantitative analysis method such as ICP-MS.
[0083] In a preferred embodiment, after treatment in an atmosphere
comprising Cl.sub.2 gas, the CNT material is further treated in an
atmosphere comprising hydrogen (H.sub.2). The purpose of this
additional treatment is to remove chloride species that may remain
among the CNT. The hydrogen gas reacts with chlorides present in
the material to form HCl in gaseous form, which is then purged from
the chamber and scrubbed with (for example) sodium hydroxide (NaOH)
to form a neutral salt solution, such as Na.sup.+Cl.sup.- (aq).
[0084] Typically, the hydrogen treatment is conducted immediately
following the Cl.sub.2 treatment, at the same or similar
temperature as the O.sub.2 treatment, without cooling the treatment
chamber in between. However, it is not necessary for the H.sub.2
treatment to immediately follow the Cl.sub.2 treatment in this
manner. The two treatments may be conducted subsequently or
separately, and in the same or in different chambers.
[0085] In a typical hydrogen gas treatment, the chlorine-treated
CNT material is heated in the heating chamber to between about
600.degree. C. and 1200.degree. C., preferably between 800.degree.
C. and 1100.degree. C., and more preferably between 900.degree. C.
and 1100.degree. C., while purging the chamber with an inert gas.
If performing the H.sub.2 treatment immediately subsequent to the
Cl.sub.2 treatment, the chamber is purged with inert gas at a rate
and for a time sufficient to provide at least 0.5 volume change,
preferably at least 1 volume change. The gas supply is then changed
from inert gas to a mixture of hydrogen (H.sub.2) and inert gas.
The mixture is comprised of typically between 0.1% and 20% (v/v)
H.sub.2, preferably between 0.5% and 15% H.sub.2, and more
preferably between 1% and 10% H.sub.2. Commercially available
mixtures of 5% H.sub.2 in argon or helium are appropriate. The
chamber is further held at the treatment temperature under the flow
of hydrogen mixture for between 0.1 hr and 100 hr, preferably
between 0.5 hr and 10 hr, more preferably between 1 hr and 5 hr.
During the treatment with hydrogen, the gas flow rate should again
be sufficient to provide at least 0.5 volume change per hour,
preferably at least 1 volume change per hour. After the hydrogen
treatment, the gas supply is changed back to inert gas, the chamber
is cooled to room temperature, and the CNT material is removed.
[0086] CNT materials purified via the method of the present
invention feature significantly improved properties due to the
removal of impurities that are detrimental to those properties.
With the absence of such impurities, the intrinsically attractive
properties of CNT such as high electrical and thermal conductivity,
high mechanical strength, chemical stability, etc. may be utilized
to substantially greater benefit in many applications, compared to
the CNT starting materials.
[0087] Typically, the purified CNT materials show no trace of
amorphous carbon when observed via electron microscopy at up to
100,000.times. magnification. Also, CNT purified according to the
invented method typically have Raman G/D ratio greater than 10,
indicating that the invented method imparts no damage to the CNT
and in fact, improves its quality with regard to the presence of
bonding defects in the material.
[0088] Furthermore, CNT purified according to the invented method
contains low quantities of inorganic materials, i.e. metals and
metal oxides. Typically, the purified CNT contains less than about
0.5% total inorganic residue. When chlorine treated, the purified
CNT typically contains less than about 0.2% total inorganic
residue. When both solution treated and chlorine treated, the
purified CNT typically contains less than about 0.05% total
inorganic residue (i.e. <500 ppm).
[0089] In one embodiment, the present invention provides a purified
single wall or double wall carbon nanotubes prepared by the method
of the present invention. The purified carbon nanotubes have no
detectable amorphous carbon when observed at 100,000.times.
magnification via electron microscopy, have a total inorganic
impurity content as metals and metal oxides of less than 500 ppm,
and have a Raman G/D ratio of 10 or higher.
[0090] The invention is illustrated further by the following
examples that are not to be construed as limiting the invention in
scope to the specific procedures or products described therein.
EXAMPLE 1
Preparation of a Treatment Solution "AAA-1"
[0091] First, 240 grams de-ionized water e combined in a flask with
160 grams reagent alcohol (Catalog number A995-4, Thermo Fisher
Scientific, Waltham Mass.). Then, 100 grams of hydrochloric acid
(HCl, 35-38%) (Catalog number A508-500, Thermo Fisher Scientific)
are slowly added to the flask. The contents of the flask are mixed
thoroughly to form a uniform solution. The concentration of HCl in
this solution is about 1.9N. This solution is identified as
"AAA-1".
EXAMPLE 2
Preparation of a Treatment Solution "AAA-2"
[0092] First, 270 grams de-ionized water are combined in a flask
with 180 grams reagent alcohol.
[0093] Then, 300 grams of hydrochloric acid (HCl, 35-38%) are
slowly added to the flask. Lastly, 7.5 grams of hydrofluoric acid
(HF, 47-51%) (Catalog number A513-500, Thermo Fisher Scientific)
are slowly added to the flask. The contents of the flask are mixed
thoroughly to form a uniform solution. The concentrations of HCl
and HF in this solution are about 4.0N and 0.25N, respectively.
This solution is identified as "AAA-2".
EXAMPLE 3
Preparation of Single Wall Carbon Nanotube (SWCNT) Wafers
[0094] Cohesive carbon nanotube assemblies ("wafers") are prepared
according to a method described in detail in WIPO publication WO
2012/177555A2. Single wall carbon nanotube (SWCNT) powder is
obtained from Thomas Swan and Co., Ltd (Consett, County Durham,
United Kingdom), under the product name "Elicarb.RTM. SW". The
as-received SWCNT powder is examined using a scanning electron
microscope (SEM) (model JSM-7500F, JEOL Ltd, Tokyo) at up to
300,000.times. magnification. The material appears as randomly
oriented tubes and bundles with considerable portion of non-tubular
amorphous carbon, as seen in FIG. 2(a).
[0095] In a flask, 2.5 grams of this SWCNT powder are combined with
1250 ml toluene (Catalog number T324-4, Thermo Fisher Scientific).
A uniform dispersion of the SWCNT in the toluene is prepared
through a combination of mechanical agitation and sonication. The
uniform dispersion of SWCNT in toluene is then cast equally into 24
individual 9-cm diameter glass dishes, such that about 100 mg of
SWCNT are contained in each dish. The toluene is removed by
evaporation in a vacuum oven, resulting in the formation of 24
cohesive SWCNT assemblies. Each of the assemblies is about 35 .mu.m
thick and about 9 cm in diameter. The assemblies are free-standing
and are easily removed from the glass dishes. Four additional
batches of 24 SWCNT wafers each are prepared in the exact same
manner, to produce a total of 120 wafers.
[0096] A cohesive SWCNT assembly prepared as described above is
examined by SEM and, as for the as-received material, appears as
randomly oriented tubes and bundles surrounded by amorphous carbon,
as seen in FIG. 2(b).
EXAMPLE 4
Purification of SWCNT Wafers by Heat Treatment
[0097] 120 cohesive SWCNT assemblies prepared as described in Ex. 3
are loaded into a quartz tube furnace (Model S515-15x48M-3Z, Mellen
Company, Concord, N.H.), by placing them on perforated quartz
plates arranged in two horizontal levels (upper and lower) inside
the furnace tube. Seventy-two wafers are placed on the upper-level
plates, and 48 wafers are placed on the lower-level plates. Prior
to loading the wafers into the furnace, the weight of each wafer is
measured and recorded.
[0098] The furnace is sealed and then purged for 4 hours with
nitrogen flowing at 1.7 liter/min. The furnace gas supply is then
changed to argon with 1 wt % (10,000 ppm) oxygen (O.sub.2), flowing
at 1.5 liter/min. The furnace is purged for a further 6 hrs under
Ar/1% O.sub.2.
[0099] While continuing the flow of Ar/1% O.sub.2, the furnace is
heated from room temperature (25.degree. C.) to 500.degree. C. at a
rate of 250.degree. C./hr, then held at 500.degree. C. for 48 hrs.
During this period, the oxygen concentration in the gas exiting
from the furnace is measured using an oxygen monitor (Series 3000,
Alpha Omega Instruments, Lincoln, R.I.), and recorded at regular
intervals. After 48 hrs, furnace heating is discontinued and the
furnace is allowed to cool naturally. The flow of Ar/1% O.sub.2 is
discontinued when the furnace temperature decreases below
300.degree. C., and the heat treated SWCNT wafers are removed from
the furnace when the temperature decreases below 50.degree. C. The
weight of each wafer is again measured and recorded.
[0100] As seen in FIG. 3, the oxygen amount exiting the furnace is
initially stable at about 8500 ppm until the furnace temperature
reaches about 300.degree. C. (.about.1 hr), at which point it
decreases rapidly to a minimum amount of about 3000 ppm soon after
the temperature reaches 500.degree. C. This indicates that the
oxidation of amorphous carbon begins at about 300.degree. C., and
progresses more rapidly as the temperature increases to 500.degree.
C. After about 1 hour at 500.degree. C., the oxygen content
increases somewhat to about 4000 ppm, and remains there for about
17 hrs. After that, the oxygen content begins to rise steadily,
indicating that the oxidation of amorphous carbon is slowing down,
and nearing completion. Once furnace heating is discontinued, the
oxygen content in the exiting gas returns to its original level,
.about.8500 ppm.
[0101] The total weight of all 120 SWCNT wafers prior to the heat
treatment is 12,301 mg. The total weight of all 120 wafers after
heat treatment is 3,109 mg. Therefore, the overall yield of SWCNT
material for this heat treating process is about 25.4%. The average
weight of each wafer after heat treatment is about 21 mg.
[0102] Based on the flow rate of the gas stream, the concentration
of oxygen (10,000 ppm by weight), and the duration of the
experiment, the total quantity of oxygen supplied during this heat
treatment process is 1981 mmoles. The total amount of carbon
removed from the 120 SWCNT wafers is 9,192 mg or 766 mmoles.
Therefore, the molar ratio of oxygen supplied to carbon removed is
about 2.59.
[0103] A SWCNT wafer is examined after the heat treatment by SEM.
As seen in FIG. 4, the material consists almost entirely of carbon
nanotubes and bundles of nanotubes. The amorphous material observed
before the heat treatment, in both the as-received powder and the
wafer, is not observable by SEM after the treatment. A large number
of small white particles or clusters appear among the nanotubes and
bundles. These are presumed to be inorganic impurity residue such
as catalyst or catalyst support material.
[0104] A SWCNT wafer is examined after the heat treatment by energy
dispersive x-ray spectroscopy (EDS) (Noran System 6, Thermo Fisher
Scientific, Waltham, Mass.). As seen in FIG. 5, besides carbon, the
major elements present are iron (Fe), and silicon (Si), with some
oxygen, and traces of sodium, magnesium, phosphorus, and chlorine.
The material is then analyzed by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS) for Fe and Si (NSL Analytical Services Inc.,
Cleveland, Ohio). The Fe and Si contents in the SWCNT material are
6.3% and 4.8%, respectively (Table 1).
[0105] A sample of the as-received SWCNT is also analyzed by ICP-MS
for Fe and Si content. The Fe content is about 1.54%, and the Si
content is about 0.32% (Table 1). The increase in the percentages
of these impurities from before to after the heat treatment is due
to the removal of substantial amount of carbon (as amorphous
carbon) from the material. The successful removal of amorphous
carbon is also indicated by the increase in Raman G/D ratio from 5
in the as-received material to 22 in the heat-treated material.
EXAMPLE 5
Purification of SWCNT Wafers by Solution Treatment
[0106] SWCNT wafers prepared as described in Ex. 3, and purified by
heat treatment as described in Ex. 4, are further purified by
immersing them in treatment solution "AAA-1" prepared as described
in Ex. 1. Each wafer is placed in a separate glass dish having
diameter of 9 cm, and then 30 ml of solution AAA-1 are added to the
dish. The wafers are immediately wetted by the AAA-1 solution and
submerge in the solution. The dishes are visually checked to ensure
that the wafers are fully covered by the solution. The wafers are
allowed to soak in the solution for 16 hrs. Then, the solution is
discarded and the wafers are rinsed three times with a solution of
50% (w/w) reagent alcohol in de-ionized water, to remove the acid
and dissolved residue. The wafers are then dried in a standard
laboratory oven for 3 hrs. at 200.degree. C.
[0107] A wafer is examined by SEM after treatment with AAA-1
solution. As seen in FIG. 6, most of the small white particles or
clusters that are observed within the material after heat treatment
(FIG. 4) are now removed. A few white particles or clusters are
still visible.
[0108] A sample of SWCNT wafer material, after treating with AAA-1
solution, is analyzed by ICP-MS for Fe and Si. The Fe content is
about 1.1%, and the Si content is about 4.1% (Table 1). Treating
the SWCNT wafer with AAA-1 solution substantially reduces the
amount of Fe impurity, and partially reduces the amount of Si
impurity in the material, compared to the material that was only
heat treated to remove amorphous carbon (Ex. 4).
EXAMPLE 6
Purification of SWCNT Wafers by Solution Treatment
[0109] SWCNT wafers prepared as described in Ex. 3, and heat
treated as described in Ex. 4 are further purified by immersing
them in treatment solution "AAA-2" prepared as described in Ex. 2.
Each wafer is placed in a separate plastic dish, and then 40 ml of
solution AAA-2 are added to the dish. The wafers are immediately
wetted by the AAA-2 solution and submerge in the solution. The
dishes are visually checked to ensure that the wafers are fully
covered by the solution. The wafers are allowed to soak in the
solution for 16 hrs. Then, the solution is discarded and the wafers
are rinsed three times with a solution of 50% (w/w) reagent alcohol
in de-ionized water, to remove the acid and dissolved residue. The
wafers are then dried in a standard laboratory oven for 3 hrs at
200.degree. C.
[0110] A wafer is examined by SEM after treatment with AAA-2
solution. As seen in FIG. 7, essentially all of the small white
particles or clusters that are observed within the material after
heat treatment (FIG. 4) are now removed. Only one or two
white-colored features are visible, that might be residue
particles.
[0111] A sample of AAA-2 solution-treated wafer material is
analyzed by ICP-MS for the elements iron (Fe) and silicon (Si). The
Fe content in the heat-treated and AAA-2 solution-treated SWCNT
material is about 0.22% (.about.2200 ppm). The Si content in the
material is about 0.16% (.about.1600 ppm). Treatment of the SWCNT
in AAA-2 solution substantially reduces the amount of both Fe and
Si impurities in the material, compared to treatment in AAA-1
solution. This is due to the presence of hydrofluoric acid in the
AAA-2 treatment solution.
[0112] Furthermore, the AAA-2 solution treatment imparted little or
no damage to the purified CNT material, as shown by the Raman G/D
ratio of 18, which is only slightly lower compared to the same
material prior to the treatment in AAA-2 solution (Ex. 4, G/D ratio
of 22),
COMPARATIVE EXAMPLE 1
CNT Purification by Solution Treatment Followed by Heat Treatment
(Reverse Sequence of Example 6)
[0113] SWCNT wafers prepared as described in Ex. 3 are solution
treated by immersing them in treatment solution "AAA-2", prepared
as described in Ex. 2. The solution treatment, rinsing, and drying
are performed in the same manner as described in Ex. 6. Following
the solution treatment with AAA-2 solution, the SWCNT wafers are
heat treated in an atmosphere comprising oxygen, in the same manner
as described in Ex. 4. This sequence of purification treatments is
the reverse of the sequence described in Ex. 6.
[0114] A wafer is examined after the treatments by SEM. The
material closely resembles the material prepared as described in
Ex. 4, and shown in FIG. 4. That is, amorphous carbon is
substantially absent from the material, but inorganic impurities
are visible as numerous particles on the order of 1-100s of nm in
size. A sample is analyzed by ICP-MS and it is determined to
contain about 6% Fe and about 5% Si, also similar to the results
for the material prepared as described in Ex. 4 (Table 1).
[0115] In this comparative example, the solution treatment
performed prior to the heat treatment in oxygen is ineffective at
removing inorganic impurities. This is due to the carbonaceous
impurities, which have not as yet been removed, surrounding,
shielding, and/or encapsulating the inorganic impurity particles,
thereby preventing the treatment solution from having the desired
effect.
EXAMPLE 7
Treatment of Heat-Treated SWCNT in Chlorine and Hydrogen
Atmospheres
[0116] SWCNT wafers prepared as described in Ex. 3 and heat treated
as described in Ex. 4, are further purified of inorganic residue by
treating in atmospheres of chlorine gas, followed by hydrogen gas,
at elevated temperature. The wafers are not treated with either
solution AAA-1 or AAA-2.
[0117] The SWCNT wafers are first placed inside a Mellen quartz
tube furnace on two sets of perforated quartz plates arranged in
two horizontal levels. The wafers are arranged such that they are
evenly distributed within the hot zone of the furnace. The furnace
is sealed and then purged with nitrogen flowing at a rate of 2
liter/min for at least 3 hrs. The purge gas is then switched to
argon at a flow rate of 1.5 liter/min and purged for a further 2
hrs.
[0118] Continuing the flow of argon, the furnace is heated from
room temperature to 200.degree. C. at a rate of 300.degree. C./hr
and held at 200.degree. C. for 2 hr. Then, the furnace is heated
from 200.degree. C. to 1050.degree. C. at a heating rate of
300.degree. C./hr. When the furnace reaches 1050.degree. C., the
gas supply to the furnace is changed from argon to a mixture of 10%
(v/v) chlorine (Cl.sub.2) in argon, at a flow rate of 1.7
liter/min. The furnace is held at 1050.degree. C. for 2 hr while
supplying the Cl.sub.2/Ar mixture.
[0119] The flow of chlorine is stopped and the gas supply is
changed back to pure argon at 1.5 liter/min. The furnace is held at
1050.degree. C. for 2 hrs while purging with argon to remove any
remaining chlorine. Then, the gas supply is changed to a mixture of
5% (v/v) hydrogen (H.sub.2) in argon, at a flow rate of 1.5
liter/min. The furnace is held for 2 hr at 1050.degree. C. while
supplying the H.sub.2/Ar mixture. The furnace is then cooled to
room temperature, initially at a rate of -600.degree. C./hr, until
the rate is reduced by natural cooling. After furnace cooling
starts, the gas is changed back to pure argon flowing at a rate of
1.5 liter/min. When the furnace temperature drops below 550.degree.
C., the gas supply is changed to 2 liter/min nitrogen until the
furnace is below 50.degree. C., at which point the nitrogen supply
is turned off and the SWCNT samples are removed from the
furnace.
[0120] After the Cl.sub.2/H.sub.2 treatment, a SWCNT wafer is
examined by SEM. As seen in FIG. 8, the material is largely free of
impurity residue, but some white particles and clusters are
visible. The material appears qualitatively similar to the
materials prepared as described in Ex. 5 and Ex. 6.
[0121] A sample of this material is analyzed by ICP-MS for Fe and
Si content. The Fe content is about 0.028% (.about.280 ppm) and the
Si content is about 0.10% (.about.1000 ppm) (Table 1). The
combination of heat treatment under oxygen, followed by
high-temperature heat treatment under chlorine followed by
hydrogen, results in a material having a high level of purity with
regard to both amorphous carbon and inorganic residue. Inorganic
impurity residue in the CNT material of about 0.1 wt % (1000 ppm)
is achieved after applying this purification process.
EXAMPLE 8
Further Purification of Heat-Treated and Solution-Treated SWCNT in
Chlorine and Hydrogen Atmospheres
[0122] SWCNT wafers prepared as described in Ex. 3, heat treated as
described in Ex. 4, and solution-treated as described in Ex. 6, are
further purified of inorganic residue by treating at elevated
temperature in atmospheres of chlorine gas, followed by hydrogen
gas, as described in Ex. 7.
[0123] A SWCNT wafer is examined by SEM after the Cl.sub.2/H.sub.2
treatment at 1050.degree. C. As seen in FIG. 9, the material
consists almost entirely of carbon nanotubes and nanotube bundles.
The material's appearance is similar to that of the material prior
to Cl.sub.2/H.sub.2 treatment (FIG. 7), with possibly even fewer
visible particles or clusters.
[0124] A sample of Cl.sub.2/H.sub.2 treated SWCNT wafer is analyzed
by ICP-MS for Fe and Si content. The Fe content is about 0.013%
(.about.130 ppm), and the Si content is about 0.032% (.about.320
ppm) (Table 1). The ICP-MS results demonstrate that the combination
of solution treatment followed by Cl.sub.2/H.sub.2 treatment
provides the highest purity among all procedures described herein,
with regard to Fe and Si content in the material. Inorganic
impurity residue in the CNT material of less than 0.05% (500 ppm)
is achieved after applying this purification process.
[0125] Furthermore, treatment in chlorine and hydrogen gases for
additional removal of inorganic residue did not impart damage to
the purified CNT, as indicated by the Raman G/D) ratio of 18, the
same ratio found in the material prior to the Cl.sub.2/H.sub.2
treatment (Ex. 6).
COMPARATIVE EXAMPLE 2
CNT Purification by Chlorine Treatment, Oxygen Heat Treatment, and
Solution Treatment
[0126] SWCNT wafers prepared as described in Ex. 3, are first
treated in an atmosphere comprising chlorine, then treated in an
atmosphere comprising hydrogen, in the same manner as described in
Ex. 7. Following that, the wafers are heat treated in an atmosphere
comprising oxygen, in the same manner as described in Ex. 4, and
then solution treated in the same manner as described in Ex. 6
using AAA-2 treatment solution. In this procedure, the same
treatment steps are applied as described in Ex. 8, except the
chlorine and hydrogen treatments are applied prior to the oxygen
heat treatment and solution treatment.
[0127] A wafer is examined after the treatments by SEM. The
material closely resembles the material prepared as described in
Ex. 6, shown in FIG. 7. Essentially all amorphous carbon is removed
from the material, and only a few clusters or particles of
inorganic impurity residue are visible. A sample of the material is
analyzed by ICP-MS, and is determined to contain about 0.2% Fe and
about 0.1% Si, also comparable to the results for the material
prepared as described in Ex. 6.
[0128] In this comparative example, the chlorine treatment applied
prior to the oxygen heat treatment and solution treatment, is only
partially effective in removing inorganic impurities. As in
Comparative Example 1, this is due to the carbonaceous impurities,
which have not as yet been removed, surrounding, shielding, and/or
encapsulating inorganic impurity particles, thereby preventing the
treatment solution from having the desired effect. In particular,
removal of residual catalyst materials such as iron by reaction
with chlorine gas is hampered by the presence of carbonaceous
impurities.
TABLE-US-00001 TABLE 1 Fe and Si content of SWCNT materials
purified by various methods. Fe (%), Si (%), Raman Example
Treatment Method ICP-MS ICP-MS G/D As-rec'd (4) None 1.54 0.32 5 4
Heat treatment w/O.sub.2 6.3 4.8 22 5 Ex. 4 + AAA-1 solution 1.1
4.1 -- treatment 6 Ex. 4 + AAA-2 solution 0.22 0.16 18 treatment 7
Ex. 4 + Cl.sub.2/H.sub.2 treatment 0.028 0.10 -- 8 Ex. 6 +
Cl.sub.2/H.sub.2 treatment 0.013 0.032 18 Comp. 1 AAA-2 solution
treatment + ~6 ~5 -- Ex. 4 Comp. 2 Cl.sub.2/H.sub.2 treatment + Ex.
6 ~0.2 ~0.1 --
EXAMPLE 9
Heat Treatments of SWCNT with Various Ratios of O.sub.2 Supplied/C
Consumed
[0129] The purification heat treatment of SWCNT wafers, as
described in Ex. 4, is repeated several times. For each individual
heat treatment procedure, a different ratio of oxygen supplied to
carbon consumed is employed. Over five discrete procedures, the
ratio is varied from a minimum of 2.59 to a maximum of 3.86.
[0130] For each procedure, the total yield of SWCNT is determined
by weighing all the wafers before they are loaded into the heat
treating furnace, and then weighing them again after the procedure,
and dividing the total final weight of the wafers by the total
initial weight of the wafers. Over the five discrete procedures,
the yield percent varies from a minimum of 16.1% to a maximum of
25.4%.
[0131] For each procedure, a sample of heat treated SWCNT wafer is
analyzed by Raman spectroscopy (LabRAM ARAMIS, Horiba Jobin Yvon
Inc.) and its G/D ratio determined. Over the five discrete
procedures, the G/D ratio varies from a minimum of about 22, to a
maximum of about 48.
[0132] Yield percentage of SWCNT, and Raman G/D ratio, are plotted
as functions of the ratio of oxygen supplied/carbon consumed
(O.sub.2/C), as shown in FIG. 10. Clearly, as the O.sub.2/C ratio
is increased, Raman G/D ratio increases as well, while yield
percentage of SWCNT decreases. These trends are due to more
complete removal of amorphous carbon from the SWCNT wafer, and
possibly due as well to some repair of defects in the SWCNT. At
O.sub.2/C ratio of about 3.5, the yield percentage and Raman G/D
ratio appear to level off (at values of about 16% and 45-50,
respectively). That is, further increasing the O.sub.2/C ratio does
not appear to have much effect on either yield or G/D ratio. This
suggests that an optimum O.sub.2/C ratio for removal of amorphous
carbon from this SWCNT material might be about 3.5.
* * * * *