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2008
Processable aqueous dispersions of graphene nanosheets Processable aqueous dispersions of graphene nanosheets
Gordon G. Wallace
University of Wollongong
, gwallace@uow.edu.au
Richard B. Kaner
University of California - Los Angeles
Marc Muller
University of Wollongong
Scott Gilje
University of California - Los Angeles
Dan Li
University of Wollongong
, danli@uow.edu.au
Follow this and additional works at: https://ro.uow.edu.au/scipapers
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Recommended Citation Recommended Citation
Wallace, Gordon G.; Kaner, Richard B.; Muller, Marc; Gilje, Scott; and Li, Dan: Processable aqueous
dispersions of graphene nanosheets 2008, 101-105.
https://ro.uow.edu.au/scipapers/1259
Research Online is the open access institutional repository for the University of Wollongong. For further information
contact the UOW Library: research-pubs@uow.edu.au
Processable aqueous dispersions of graphene nanosheets Processable aqueous dispersions of graphene nanosheets
Abstract Abstract
Graphene sheets offer extraordinary electronic, thermal and mechanical properties and are expected to
find a variety of applications. A prerequisite for exploiting most proposed applications for graphene is the
availability of processable graphene sheets in large quantities. The direct dispersion of hydrophobic
graphite or graphene sheets in water without the assistance of dispersing agents has generally been
considered to be an insurmountable challenge. Here we report that chemically converted graphene
sheets obtained from graphite can readily form stable aqueous colloids through electrostatic
stabilization. This discovery has enabled us to develop a facile approach to large-scale production of
aqueous graphene dispersions without the need for polymeric or surfactant stabilizers. Our findings make
it possible to process graphene materials using low-cost solution processing techniques, opening up
enormous opportunities to use this unique carbon nanostructure for many technological applications.
Keywords Keywords
Processable, aqueous, dispersions, graphene, nanosheets
Disciplines Disciplines
Life Sciences | Physical Sciences and Mathematics | Social and Behavioral Sciences
Publication Details Publication Details
Li, D., Muller, M. B., Gilje, S., Kaner, R. B. & Wallace, G. G. (2008). Processable aqueous dispersions of
graphene nanosheets. Nature Nanotechnology, 3 (2), 101-105.
This journal article is available at Research Online: https://ro.uow.edu.au/scipapers/1259
Processable aqueous dispersions of graphene nanosheets
Dan Li
1
*, Marc B. Müller
1
, Scott Gilje
2
, Richard B. Kaner
2
and Gordon G. Wallace
1
*
1
ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research
Institute, University of Wollongong, NSW 2522, Australia
2
Department of Chemistry and Biochemistry, Department of Materials Science and
Engineering, and California NanoSystems Institute, University of California, Los Angeles,
California 90095-1569, USA
*e-mail:
danli@uow.edu.au; gwallace@uow.edu.au
Abstract
Graphene sheets offer extraordinary electronic, thermal and mechanical properties and are
expected to find a variety of applications. A prerequisite for exploiting most proposed
applications for graphene is the availability of processable graphene sheets in large quantities.
The direct dispersion of hydrophobic graphite or graphene sheets in water without the
assistance of dispersing agents has generally been considered to be an insurmountable
challenge. Here we report that chemically converted graphene sheets obtained from graphite
can readily form stable aqueous colloids through electrostatic stabilization. This discovery
has enabled us to develop a facile approach to large-scale production of aqueous graphene
dispersions without the need for polymeric or surfactant stabilizers. Our findings make it
possible to process graphene materials using low-cost solution processing techniques,
opening up enormous opportunities to use this unique carbon nanostructure for many
technological applications.
Graphene, a new class of two-dimensional carbon nanostructure, has attracted tremendous
attention from both the experimental and theoretical scientific communities in recent years
1
.
This unique nanostructure holds great promise for potential applications in many
technological fields such as nanoelectronics, sensors, nanocomposites, batteries,
supercapacitors and hydrogen storage
1
. However, a lack of an efficient approach to producing
processable graphene sheets in large quantities has been a major obstacle to exploiting most
proposed applications.
Like carbon nanotubes and many other nanomaterials, a key challenge in the synthesis and
processing of bulk-quantity graphene sheets is aggregation. Graphene sheets, which have a
high specific surface area, unless well separated from each other, tend to form irreversible
agglomerates or even restack to form graphite through van der Waals interactions. This
problem has been encountered in all previous efforts aimed at large-scale production of
graphene through chemical conversion or thermal expansion/reduction
2–5
. The prevention of
aggregation is of particular importance for graphene sheets because most of their unique
properties are only associated with individual sheets. Aggregation can be reduced by the
attachment of other molecules or polymers onto the sheets
4–6
. However, the presence of
foreign stabilizers is undesirable for most applications. New strategies to produce relatively
clean graphene sheets in bulk quantity while keeping them individually separated are
required.
Graphite, consisting of a stack of flat graphene sheets, is inexpensive and available in large
quantities from both natural and synthetic sources. This ordinary carbon material is likely the
most readily available and least expensive source for the production of bulk graphene sheets.
Mechanical cleavage of graphite originally led to the discovery of graphene sheets
7
and is the
process currently used in most experimental studies of graphene
1
. However, the low
productivity of this method makes it unsuitable for large-scale use. Chemical conversion
from graphite appears to be a much more efficient approach to bulk production of graphene
sheets at low cost
2–5
.
As recently demonstrated by Ruoff and co-workers
2,4
, the solution-based route involves
chemical oxidation of graphite to hydrophilic graphite oxide, which can be readily exfoliated
as individual graphene oxide (GO) sheets by ultrasonication in water (Fig. 1). Graphene
oxide, which is electrically insulating, can be converted back to conducting graphene by
chemical reduction, for example, using hydrazine. Unfortunately, previous work
2,4
has shown
that, unless stabilized by selected polymers, chemically converted graphene (CCG) sheets
obtained through this method precipitate as irreversible agglomerates owing to their
hydrophobic nature. The resulting graphene agglomerates appear to be insoluble in water and
organic solvents
2
, making further processing difficult.
It is well known that exfoliated graphite oxide (or GO) can form well-dispersed aqueous
colloids
8–12
. Our study on the surface charge (zeta potential) of as-prepared GO sheets shows
that these sheets are highly negatively charged when dispersed in water (Fig. 2a), apparently
as a result of ionization of the carboxylic acid and phenolic hydroxyl groups that are known
to exist on the GO sheets
13,14
. This result suggests that the formation of stable GO colloids
should be attributed to
electrostatic repulsion, rather than just the hydrophilicity of GO
previously presumed2. Given that car
hydrazine under the given
reaction conditions2, these groups should therefore remain in
reduced product as confirmed by our FT
acid groups suggests that
the surface of the graphene sheets in aqueous solution
be charged after reduction. We surmised that
makes GO colloids
stable could also enable the formation of well
colloids.
Figure 1
Scheme showing the chemical route to the synthesis of aqueous graphene dispersions. 1,
Oxidation of graphite (black blocks) to graphite oxide (lighter coloured blocks) with greater interlayer
distance. 2, Exfoliation of graphite oxide in water by son
stabilized by electrostatic repulsion. 3, Controlled conversion of GO colloids to conducting graphene
colloids through deoxygenation by hydrazine reduction.
Figure 2
Surface properties of GO and CCG.
aqueous dispersions at a concentration of
absorption band at around
1,700 cm
at this r
ange is observable but not as prominent as that observed for GO, likely due to the
of the strong absorption of graphene sheets in this region.
As demonstrated in many colloid experiments
stabilizer-free conducting
polymer aqueous colloids
stabilized dispersion is strongly dependent on pH,
dispersed
particles. By controlling these parameters, we n
sheets are indeed able to form
electrostatic repulsion, rather than just the hydrophilicity of GO
previously presumed2. Given that car
boxylic acid groups
are unlikely to be reduced by
reaction conditions2, these groups should therefore remain in
reduced product as confirmed by our FT
-IR analysis
(Fig. 2b). The presence of carboxylic
the surface of the graphene sheets in aqueous solution
be charged after reduction. We surmised that
the electrostatic repulsion mechanism that
stable could also enable the formation of well
-
dispersed
Scheme showing the chemical route to the synthesis of aqueous graphene dispersions. 1,
Oxidation of graphite (black blocks) to graphite oxide (lighter coloured blocks) with greater interlayer
distance. 2, Exfoliation of graphite oxide in water by son
ication to obtain GO colloids that are
stabilized by electrostatic repulsion. 3, Controlled conversion of GO colloids to conducting graphene
colloids through deoxygenation by hydrazine reduction.
Surface properties of GO and CCG.
a, Zeta potential of GO and CCG
as a function of pH, in
aqueous dispersions at a concentration of
~0.05 mg ml
-1
. b, FT-
IR spectra of GO and CCG. The
1,700 cm
-1
is attributed to carboxyl groups. The absorption of CCG sheets
ange is observable but not as prominent as that observed for GO, likely due to the
of the strong absorption of graphene sheets in this region.
As demonstrated in many colloid experiments
15
, including our
previous work on the synthesis of
polymer aqueous colloids
16
, the colloidal stability of an
stabilized dispersion is strongly dependent on pH,
the electrolyte concentration, and the content of
particles. By controlling these parameters, we n
ow find that
chemically converted graphene
sheets are indeed able to form
stable colloids through electrostatic stabilization. Graphene oxide
electrostatic repulsion, rather than just the hydrophilicity of GO
as
are unlikely to be reduced by
reaction conditions2, these groups should therefore remain in
the
(Fig. 2b). The presence of carboxylic
the surface of the graphene sheets in aqueous solution
should still
the electrostatic repulsion mechanism that
dispersed
graphene
Scheme showing the chemical route to the synthesis of aqueous graphene dispersions. 1,
Oxidation of graphite (black blocks) to graphite oxide (lighter coloured blocks) with greater interlayer
ication to obtain GO colloids that are
stabilized by electrostatic repulsion. 3, Controlled conversion of GO colloids to conducting graphene
as a function of pH, in
IR spectra of GO and CCG. The
is attributed to carboxyl groups. The absorption of CCG sheets
ange is observable but not as prominent as that observed for GO, likely due to the
overlapping
previous work on the synthesis of
, the colloidal stability of an
electrostatically
the electrolyte concentration, and the content of
chemically converted graphene
stable colloids through electrostatic stabilization. Graphene oxide
