Note: Descriptions are shown in the official language in which they were submitted.
, CA 02795965 2012-11-16
GRAPHENE NANO-SHEETS AND METHODS FOR MAKING THE
SAME
DETAILED DESCRIPTION
Background
[0001] Graphene has attracted considerable interest in recent years
due
to extraordinary electronic, thermal, and mechanical properties. Potential
applications of graphene include transparent electrodes and semiconductors,
nano-
composite materials, batteries, supercapacitors, hydrogen storage, etc.
[0002] Conventional approaches for producing graphene sheets include a
bottom-up method of forming sp2-bonding between carbon atoms in a monolayer.
Chemical vapor deposition (CVD) and epitaxial growth from silicon carbide are
used
for this bottom-up method. Another conventional approach for producing
graphene
sheets includes a top-down method by exfoliating graphite through chemical
oxidation and reduction. This top-down method is also known as Hummer's
method. However, oxidative agents and toxic reducing agents must be used in
Hummer's method, which creates defects in the final graphene sheets.
[0003] It is therefore desirable to develop an easy, clean, and
effective
method for forming graphene products with high purity.
SUMMARY
[0004] According to various embodiments, the present teachings include
a
method of making expanded graphite. The expanded graphite can be formed by
first
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placing expandable graphite within a chamber and then applying vacuum to the
chamber. An electric current can then be applied to induction heat the
expandable
graphite under the vacuum to form the expanded graphite.
[0005] According to various embodiments, the present teachings also
include a method of making a graphene product. In this method, expandable
graphite can be placed within a chamber. The expandable graphite can include
an
intercalation agent capable of producing thermal expansion. A vacuum can then
be
applied to the chamber. By applying an electric current to induction heat the
expandable graphite under the vacuum, expanded graphite can be formed. The
expanded graphite can then be exfoliated in a solvent to form one or more
graphene
nano-sheets dispersed in the solvent.
[0006] According to various embodiments, the present teachings further
include graphene nano-sheets produced by induction heating expandable graphite
under a vacuum. The graphene nano-sheets can be substantially wrinkle free
with
a uniform thickness that is within plus or minus about 1 nm of a desired
thickness.
The graphene nano-sheets can be substantially free of impurity moieties. The
graphene nano-sheets can have a carbon content of at least about 99 % by
weight.
[0006a] According to an aspect, there is provided a method of making
graphene nano-sheets, comprising:
placing expandable graphite within a chamber, such that a vacuum
pressure in the chamber ranges from about 10-1 mbar to 10-7 mbar in the
chamber;
applying an electric current to induction heat the expandable
graphite within the chamber under the vacuum to form an expanded graphite; and
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exfoliating the expanded graphite in a solvent to form one or more
graphene nano-sheets dispersed in the solvent.
[0006b] According to another aspect, there is provided a method of
making
a graphene product comprising:
placing expandable graphite within a chamber, the expandable
graphite comprising an intercalation agent for producing thermal expansion;
applying vacuum to the chamber, wherein a vacuum pressure
ranges from about 10-1 mbar to 10-7 mbar in the chamber;
applying an electric current to induction heat the expandable
graphite under the vacuum to form expanded graphite; and
exfoliating the expanded graphite in a solvent to form one or more
graphene nano-sheets dispersed in the solvent.
[0007] It is to be understood that both the foregoing general
description
and the following detailed description are exemplary and explanatory only and
are
not restrictive of the present teachings, as claimed.
DESCRIPTION OF THE EMBODIMENTS
[0008] Reference will now be made in detail to embodiments of the
present teachings, examples of which are illustrated in the accompanying
drawings.
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Wherever possible, the same reference numbers will be used throughout the
drawings to refer to the same or like parts. In the following description,
reference is
made to the accompanying drawings that form a part thereof, and in which is
shown
by way of illustration specific exemplary embodiments in which the present
teachings may be practiced. The following description is, therefore, merely
exemplary.
[0009] Various embodiments provide materials and methods for forming
graphene products. The graphene products can include expanded graphite formed
of graphene nano-sheets, exfoliated graphene nano-sheets, and/or a graphene-
containing composite including graphene nano-sheets and an organic
semiconductor.
[0010] In one embodiment, graphene nano-sheets can be formed in a
vacuum environment using induction heating of expandable graphite. The
resultant
expanded graphite can then be exfoliated in a solvent (or a gas carrier) to
form a
graphene dispersion including graphene nano-sheets dispersed in the solvent.
In
embodiments, the graphene nano-sheets can be dispersed in a solution
containing
conjugated polymers or other organic semiconductors to form a graphene
dispersion and then form a graphene-containing composite.
[0011] The disclosed methods of using vacuum induction heating can
provide an easy, clean, and effective process, because no conventional
oxidation
and reduction chemicals are used. Additionally, due to use of a vacuum,
gaseous
chemical such as oxygen are not involved. Graphene products with high purities
can then be generated. Further, induction heating is a fast heating process
with
high heating rates. Desired high temperatures can be reached in a short time.
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Furthermore, expandable graphite can be heated homogeneously by induction
heating, since the expandable graphite is conductive. Conventional thermal
expansion methods using a thermal oven are limited by thermal diffusion,
especially
when graphite has a large size. Therefore, the expansion is not as efficient
as
induction heating under vacuum. For this reason, the disclosed vacuum
induction
heating method can form thinner product with more uniform thicknesses, as
compared to conventional oven-based thermal expansion processes.
[0012] As used herein, the term "graphite" refers to a three
dimensionally
(3D) ordered array of carbon atoms with planar sheets of arrayed atoms stacked
in
a defined, repeating pattern. The term "expandable graphite" or "thermally
expandable graphite" refers to graphite intercalation compound where
molecules,
for example an intercalation agent, are incorporated in between the planar
sheets.
The intercalation agent can produce an expansion or a thermal expansion of the
graphite upon heating which changes the intercalation agent from a liquid or
solid
phase into a gas phase. Increase of volume of the intercalation agent upon
phase
change can force the adjacent graphene layer within the expandable graphite to
separate.
[0013] In embodiments, the thermally expandable graphite can be an
intercalation compound of graphite that expands and/or exfoliates when heated.
Intercalation is a process whereby an intercallant agent is inserted between
the
planar sheets of a graphite crystal or particle. The term "expandable
graphite" can
also be referred to as "intercalated graphite". A variety of chemical species
or
intercalation agents can be used to intercalate graphite materials. These
intercalation agents can include halogens, alkali metals, sulfate, nitrate,
various
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organic/inorganic acids such as H2SO4 or HCI, aluminum chloride, ferric
chloride,
other metal halides, arsenic sulfide, thallium sulfide, etc. In one example, a
graphite
intercalation compound can include the "sulfate" intercalation compound
sometimes
referred to as "graphite bisulfate". This material can be manufactured by
treating
highly crystalline natural flake graphite with a mixture of sulfuric acid and
certain
other oxidizing agents which aid in "catalysis" of the sulfate intercalation.
The
resultant product can be a highly intumescent form of graphite, which is
referred to
herein as "expanded graphite".
[0014] As used herein, the term "expanded graphite" refers to a
graphite
product in a highly intumescent form, which can be obtained by processing the
expandable graphite. In one embodiment, the expanded graphite can be formed by
induction heating the expandable graphite under a vacuum. The intercalation
agent(s) in the expandable graphite can be removed during the process. As a
result, the expanded graphite can be free of intercalation agent(s). In
embodiments,
the expanded graphite can further be exfoliated to form graphene nano-sheets.
[0015] As used herein, the term "graphene nano-sheet" refers to a
graphene product including one or a few atomic monolayers of sp2-bonded carbon
atoms. The disclosed graphene nano-sheet can have an average thickness, for
example, ranging from about 0.3 nm to about 15 nm, or ranging from about 0.3
nm
to about 10 nm, or ranging from about 0.3 nm to about 6 nm. Alternatively, the
disclosed graphene nano-sheet can have from about 1 graphene layer to about 30
graphene layers, or ranging from about 1 to about 20 graphene layers, or
ranging
from about 1 to about 10 graphene layers. Formed by vacuum induction heating
methods, graphene nano-sheets can be wrinkle-free and can provide a uniform
CA 02795965 2012-11-16
thickness across each graphene nano-sheet. For example, the graphene nano-
sheets can have a uniform thickness that is within plus or minus about 1 nm of
a
desired thickness, or within plus or minus about 0.5 nm, or within plus or
minus
about 0.3 nm. It should be noted that graphene or graphene nano-sheets
produced
by conventional methods such as Hummer's methods usually contain wrinkles,
which are most likely caused by defects in the graphene nano-sheets as known
to
one of ordinary skill in the art.
[0016] In
embodiments, expanded graphite and/or graphene nano-sheets
generated by vacuum induction heating can be composed substantially of carbon,
for example, having at least about 98 % or at least about 99 % of carbon by
weight,
including from about 99 % to about 99.99 (21/0 by weight. In some embodiments,
the
expanded graphite and/or graphene nano-sheets generated by vacuum induction
heating can be substantially free of S, Cl, N, or other impurity atoms, which
are
normally detected in graphene nano-sheets produced using Hummer's methods.
Detection of these atoms can be performed using any suitable methods
including,
for example SEM EDS analysis. In some embodiments, the expanded
graphite/graphene nano-sheets generated by vacuum induction heating can be
substantially free of oxygen. That means, the oxygen contents can be for
example
less than about 2 % by weight, including less than about 1 `)/0, or less than
about 0.5
cY0, or less than about 0.1% by weight of the entire product generated from
vacuum
induction heating. Although the expanded graphite/ graphene nano-sheets
prepared according to various embodiments can be substantially free of
impurity
species such as 0, S, Cl, N, the graphene nano-sheets can be, for example,
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CA 02795965 2012-11-16
=
subsequently modified such as surface modified, so that the modified graphene
can
contain these species for certain applications.
[0017] Various embodiments provide methods for forming expanded
graphite/ graphene products by using thermally expandable graphite. Exemplary
expandable graphite can be that manufactured by treating flake graphite with
various intercalation reagents that migrate between the graphene layers in a
graphite crystal and remain as stable species, for example, as supplied by
Carbon
Asbury Inc. (Asbury, NJ).
[0018] The thermally expandable graphite can be placed in a
chamber.
The chamber can be a vacuum chamber for conducting induction heating. In
embodiments, the vacuum applied to the thermally expandable graphite can have
a
vacuum pressure in the vacuum chamber ranging from about 1 mbar to about 10-7
mbar, or from about 0.1 mbar to about 10-6 mbar, or from about 10-4 mbar to
about
10-7 mbar.
[0019] Under the vacuum environment, electric current can then be
applied to induction heat the thermally expandable graphite in the vacuum
chamber
until a heating temperature is achieved. In embodiments, the heating
temperature
can be from about 500 C to about 1100 C, or from about 600 C to about 1050
C,
or from about 700 C to about 1000 C. The thermally expandable graphite can
be
heated at the heating temperature for a time length of from about 0.1second to
about 5 minutes, or from about 1second to about 5 minutes, or from about 30
seconds to about 2 minutes. The heating temperature, the heating time, and
their
combination are not limited. Any suitable known devices for applying vacuum
and/or induction heating can be used. The thermally expandable graphite can
then
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be expanded. Optionally, the expanded graphite can be cooled to room
temperature.
[0020] In embodiments, the expandable/expanded graphite can have an
expansion rate of about 200 % or more by volume, or ranging from about 200 %
to
about 1000 % by volume, or ranging from about 300 % to about 800 % by volume
of
the original thermally expandable graphite prior to the vacuum induction
heating
process. In embodiments, the expanded graphite can be formed free of
intercalation agents, which are contained in the original expandable graphite.
[0021] After optionally cooled to room temperature, the expanded
graphite
can be exfoliated, for example, by dispersing in a solvent. In embodiments, a
sonicator or other mechanical mixing techniques can be used to facilitate
exfoliating
and/or dispersing of the expanded graphite. Various solvents including, but
not
limited to, toluene, chlorobenzene, dichlorobenzene, trichlorobenzene,
chlorotoluene, xylene, mesitylene, chloroethane, chloromethane,
dimethylformamide
(DMF), N-Methyl-2-pyrrolidone (NMP), and/or the like, can be used. A graphene
dispersion can then be formed having one or more graphene nano-sheets
dispersed
in the solvent.
[0022] In embodiments, a stabilizer can be included in the solvent for
exfoliating/dispersing the expanded graphite and forming the graphene
dispersion.
Exemplary stabilizers can include surfactants, insulative polymers such as
polystyrene, PMMA, polyurethane, and the like, conjugated compounds including
organic semiconductors such as small molecular compounds and/or semiconducting
polymers.
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[0023] In embodiments, graphene nano-sheets can be present in an
amount ranging from about 0.0001 % to about 0.5 /0, or from about 0.0005 % to
about 0.1 %, or from about 0.001 % to about 0.06 % by weight of the graphene
dispersion regardless of whether the organic semiconductors are present. In
embodiments, when organic semiconductors are used, they can be present in an
amount ranging from about 0.001 % to about 20 %, or from about 0.1 % to about
10
%, or from about 0.1 % to about 5 %, by weight of the graphene dispersion.
[00024] Exemplary organic semiconductors can include those described in
co-pending U.S. Patent Application Ser. No. 12/575,739, filed Oct. 8, 2009 and
entitled "Electronic Device."
[0025] Exemplary small molecular compounds can include pentacene
and pentacene derivatives (pentacene precursors and pentacene analogs),
oligothiophenes, phthalocyanines, naphthalene-bisimides, and/or other fused-
ring
aromatic compounds.
[0026] Exemplary semiconducting polymers can include, for example, a
polythiophene of Formula (I):
A
wherein A is a divalent linkage; each R is independently selected from
hydrogen,
alkyl, substituted alkyl, aryl, substituted aryl, alkoxy or substituted
alkoxy, a
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heteroatom-containing group, halogen, ¨ON, or ¨NO2; and n is from 2 to about
5,000. In some embodiments, R is not hydrogen.
[0027] The term "alkyl" refers to a radical composed entirely of
carbon
atoms and hydrogen atoms which is fully saturated and of the formula CnH2n-f1.
The
term "aryl" refers to an aromatic radical composed entirely of carbon atoms
and
hydrogen atoms. The term "alkoxy" refers to an alkyl radical which is attached
to an
oxygen atom.
[0028] The substituted alkyl, substituted aryl, and substituted alkoxy
groups can be substituted with, for example, alkyl, halogen, ¨ON, and ¨NO2. An
exemplary substituted alkyl group can be a perhaloalkyl group, wherein one or
more
hydrogen atoms in an alkyl group are replaced with halogen atoms, such as
fluorine,
chlorine, iodine, and bromine. The term "heteroatom-containing group" refers
to a
radical which is originally composed of carbon atoms and hydrogen atoms that
forms a linear backbone, a branched backbone, or a cyclic backbone. This
original
radical can be saturated or unsaturated. One or more of the carbon atoms in
the
backbone can then be replaced by a heteroatom, generally nitrogen, oxygen, or
sulfur, to obtain a heteroatom-containing group. The term "heteroaryl" refers
generally to an aromatic compound containing at least one heteroatom replacing
a
carbon atom, and may be considered a subset of heteroatom-containing groups.
[0029] In particular embodiments, both R groups are alkyl having from
about 6 to about 18 carbon atoms. In certain examples, both R groups are the
same. In further desired embodiments, both R groups are alkyl, particularly ¨
Ci2H25 =
CA 02795965 2012-11-16
[0030] The divalent linkage A can form a single bond to each of the
two
thienyl moieties in Formula (I). Exemplary divalent linkages A can include:
Rt
0 Se
R; R'
R' R'
R'
R'
/-=1 100
AH-1/
R'
R`
Os,
R'
R'
\ /
R R'
R'
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R'
R'
N R'
\ 0I
N 0
R'
R' R'
R'
\ SOO S\
R'
R'
O NO
0 N 0
R' R'
R'
R'
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CA 02795965 2012-11-16
R' 000
R'
R'
and combinations thereof, wherein each R' is independently selected from
hydrogen,
alkyl, substituted alkyl, aryl, substituted aryl, alkoxy or substituted
alkoxy, a
heteroatom-containing group, halogen, ¨CN, and/or ¨NO2.
[0031] In embodiments, the semiconducting polymers can have a weight
average molecular weight of from about 1,000 to about 1,000,000, or from about
5000 to about 100,000.
[0032] In embodiments, a graphene-containing composite can be formed
from the graphene dispersion containing graphene nano-sheets. For example, the
disclosed graphene dispersion can be applied (e.g., coated or printed) to a
substrate
and then dried or otherwise cured to remove solvent from the graphene
dispersion
and form the graphene-containing composite. The substrate may or may not be
removed after the formation of the graphene-containing composite. Any
substrates,
rigid or flexible, can be used including, for example, semiconductors, metals,
ceramics, plastics, glass, paper, and/or wood.
[0033] In one embodiment, the graphene-containing composite can
include a plurality of graphene nano-sheets dispersed within a matrix of
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semiconducting polymer(s). The graphene nano-sheets can be present in an
amount ranging from about 0.001 % to about 5.0 %, or from about 0.01 % to
about
3.0 %, or from about 0.01 % to about 0.5 % by weight of the graphene-
containing
composite.
[0034] Examplel
[0035] Thermally expandable graphite flakes were obtained from Asbury
Carbon Inc. (Asbury, NJ). The graphite flakes were added into a tungsten
vacuum
evaporation boat, which was then placed within a vacuum chamber (Edwards Auto
306 evaporation system). The vacuum chamber was subsequently pumped down
to a pressure of about -2X10-6 m bar. About 70% of the power (220 V, 7 Am
input)
was then used to heat up the boat for about 1 minute. The temperature was
reached about 850 C. The graphite was expanded significantly as a black
twisted
solid to form the expanded graphite.
[0036] Example 2
[0037] A small amount of the expanded graphite produced in Example 1
was added into dichlorobenzne solvent, and sonicated with a bath sonicator for
a
few minutes, followed by probe sonication (of about 50% power) for about 3
minutes
to better exfoliate and separate the individual graphene layers. After both
bath and
probe sonication procedures, the mixture was then centrifuged at about 3500
rpm
for about 10 min to remove large particles that were not successfully
exfoliated.
Following centrifugation, stable graphene dispersion containing graphene nano-
sheets was obtained, as revealed by a dark or gray color of the solution.
Concentration of graphene nano-sheets in the solution was calculated from UV-
Vis
absorbance spectra at a wavelength of about 660 nm using the Lambert-Beer law
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with the extinction coefficient. The UV-Vis spectroscopy, performed using a
Casey
UV-Vis-NIR spectrophotometer, depicted that the concentration of generated
graphene nano-sheets was up to about 0.017 mg/ml.
[0038] To examine the formed graphene nano-sheets, the graphene
dispersion was spin coated on a silicon wafer and examined. SEM images showed
graphene nano-sheets were flat on the silicon wafer without wrinkles. EDS
analysis
of graphene nano-sheets on silicon nitride substrate indicated that there were
no
impurity atoms such as S, N, Cl, etc. No oxygen atom was detected. These
results
depicted that the graphene nano-sheets produced by the vacuum induction
heating
methods had a high purity. The thickness of the graphene nano-sheets was
measured by AFM, which showed a thickness of less than about 10 nm in this
example. Stacked graphene nano-sheets and/or folded single graphene nano-sheet
were also produced and observed.
[0039] Example 3
[0040] The expanded graphite was exfoliated in a solvent containing
stabilizer(s). In this example, the expanded graphite was added into a 0.03
wt%
poly(3,3"'-didodecylquaterthiophene) (PQT) soultion in a solvent of 1,2-
dichlorobenzene. The expanded graphite had about 50 wt% loading of PQT. After
sonication and centrifugation, substantially no precipitation was observed.
That is,
the expanded graphite was exfoliated and stablized in the graphene dispersion.
The graphene dispersion containing graphene nano-sheets and PQT was then spin-
casted to form a film of a graphene-PQT composite on a silicon wafter. SEM
images showed that the graphene-PQT composite included graphene nano-sheets
dispersed in a polymer matrix of PQT.
CA 02795965 2014-06-13
[0041] Comparative Example 1
[0042] The same thermal expandable graphite used in Example 1
was
placed in a thermal oven for comparison. Under a forming gas (about 4.5 wt%
hydrogen in Nitrogen) flow, the graphite was heated at about 850 C for about
1
minute. Expanded graphite was then formed, taken out of the oven, and
sonicated
in the same way as depicted in Example 2. AFM images of the spin coated films
showed exfoliated graphene nano-sheets having a thickness of about 40 nm to
about 50 nm, which were significantly larger than graphene nano-sheets
produced
in Example 2. This indicated that vacuum induction heating method was more
_
efficient than conventional oven expansion.
[0043] Notwithstanding that the numerical ranges and
parameters setting
forth the broad scope of the disclosure are approximations, the numerical
values set
forth in the specific examples are reported as precisely as possible. Any
numerical
value, however, inherently contains certain errors necessarily resulting from
the
standard deviation found in their respective testing measurements. Moreover,
all
ranges disclosed herein are to be understood to encompass any and all sub-
ranges
subsumed therein.
[0044] While the present teachings have been illustrated with
respect to
one or more implementations, alterations and/or modifications can be made to
the
illustrated examples without departing from the scope of the appended claims.
In
addition, while a particular feature of the present teachings may have been
disclosed with respect to only one of several implementations, such feature
may be
combined with one or more other features of the other implementations as may
be
desired and advantageous for any given or particular function. Furthermore, to
the
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extent that the terms "including," "includes," "having," "has," "with," or
variants
thereof are used in either the detailed description and the claims, such terms
are
intended to be inclusive in a manner similar to the term "comprising."
Further, in the
discussion and claims herein, the term "about" indicates that the value listed
may be
somewhat altered, as long as the alteration does not result in nonconformance
of
the process or structure to the illustrated embodiment. Finally, "exemplary"
indicates the description is used as an example, rather than implying that it
is an
ideal.
[0045] Other embodiments of the present teachings will be apparent to
those skilled in the art from consideration of the specification and practice
of the
present teachings disclosed herein.
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