Processable aqueous dispersions of graphene nanosheets

TL;DR: It is reported that chemically converted graphene sheets obtained from graphite can readily form stable aqueous colloids through electrostatic stabilization, making 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.

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.

Summary

Synthesis

• Graphite oxide was synthesized from natural graphite (SP-1, Bay Carbon) by a modified Hummers method as originally presented by Kovtyukhova and colleagues 10,30 .
• Ultrapure Milli-Q ® water was used in all experiments.
• As-purified graphite oxide suspensions were then dispersed in water to create a 0.05 wt% dispersion.
• In a typical procedure for chemical conversion of graphite oxide to graphene, the resulting homogeneous dispersion (5.0 ml) was mixed with 5.0 ml of water, 5.0 ml of hydrazine solution (35 wt% in water, Aldrich) and 35.0 ml of ammonia solution (28 wt% in water, Crown Scientific) in a 20- ml glass vial.
• Note that the concentration of hydrazine in the reduction mixture can be varied from 0.0175 wt% (used in the above procedure) to 1.75 wt%.

Characterization

• UV-vis absorption and/or transmission spectra were obtained using a Shimadzu UV 1601 spectrophotometer.
• Mechanical tensile tests were conducted with a Q800 Dynamic Mechanical Analyzer (TA Instruments).
• Szabo, T., Szeri, A. & Dekany, I. Composite graphitic nanolayers prepared by self-assembly between finely dispersed graphite oxide and a cationic polymer.

Figures

Figure 5 Examples demonstrating that fi CCG dispersions using various solution

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 range 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.

Figure 4 UV-vis absorption spectra showing the change of GO dispersions time. The absorption peak of the GO dispersion at absorption in the whole spectral region ( electronic conjugation within the graphene sheets is restored upon

Figure 3 Colloidal and morphological characterization of CCG dispersions.

Full text

University of Wollongong University of Wollongong
Research Online Research Online
Faculty of Science - Papers (Archive) Faculty of Science, Medicine and Health
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
Part of the Life Sciences Commons, Physical Sciences and Mathematics Commons, and the Social
and Behavioral Sciences Commons
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

Frequently asked questions

Q1. What have the authors contributed in "Processable aqueous dispersions of graphene nanosheets" ?

Here the authors report that chemically converted graphene sheets obtained from graphite can readily form stable aqueous colloids through electrostatic stabilization. 

Q2. What is the significance of graphene sheets?

Of particular significance is that, owing to the high aspect ratio of the graphene sheets, a very thin graphene coating, which is almost transparent, can result in the formation of a continuous conducting network. 

Q3. How do the authors increase the pH of graphene sheets?

In order to obtain maximal charge density on the resulting graphene sheets, ammonia is added to the reaction solution to increase the pH to around 10. 

Q4. What is the description of graphene sheets?

Graphene sheets should be superior to normal synthetic conducting polymers in terms of thermal and chemical stability and mechanical strength, and more competitive than carbon nanotubes in terms of production cost. 

Q5. What is the effect of the electrostatic assembly technique?

the highly charged state of the CCG sheets in water makes it possible to use the wellknown layer-by-layer electrostatic assembly technique 26–29 to build up complex and controllable graphene-based nanosystems with other functional molecules, polymers and nanostructures. 

Q6. What is the effect of graphene sheets on the surface of water?

Owing to their hydrophobic nature, of graphite or graphene sheets in water has first time, suggests that ordinary natural graphite, when treated readily disperse in water to generate stable graphene colloids without the need for any surfactant stabilizers. 

Q7. What is the effect of hydrazine reduction on graphene sheets?

by hydrazine reduction of GO sheets appear to act as a p conductivity exhibits a field-effect an exciting material for use in future nanoelectronics. 

Q8. What is the common method of producing graphene sheets?

consisting of a stack of flat graphene sheets, is inexpensive and available in largequantities from both natural and synthetic sources. 

Q9. What is the simplest way to obtain graphene?

Chemically converted graphene can now be viewed as a special water-soluble conducting macromolecule that can be simply obtained from graphite. 

Q10. What is the effect of the lyophobic colloids on graphene?

Like many other lyophobic colloids, once the graphene colloids are dried, they are not redispersible in water, rendering as-prepared graphene coatings water-resistant. 

Q11. What is the advantage of dispersant-free graphene sheets?

The dispersant-free feature offers a great deal of flexibility in the creation of novel graphene-based nanocomposites with many other molecules and nanostructures. 

Q12. What is the simplest explanation for the formation of graphene colloids?

As demonstrated in many colloid experiments stabilizer-free conducting polymer aqueous colloids stabilized dispersion is strongly dependent on pH, dispersed particles. 

Q13. How does the absorbance of graphene sheets change with the number of assembly cycles?

The absorbance increases linearly with an increase in the number of assembly cycles (denoted above each curve), indicative of the successful assembly of CCG sheets on the substrate. 

Q14. What is the reason for the agglomeration of graphene sheets?

previous work 2,4 has shownthat, unless stabilized by selected polymers, chemically converted graphene (CCG) sheetsobtained through this method precipitate as irreversible agglomerates owing to theirhydrophobic nature. 

Q15. What is the effect of the chemical conversion of graphene?

The resulting graphene agglomerates appear to be insoluble in water and organic solvents 2 , making further processing difficult. 

Q16. What is the effect of hydrazine on the particle size of graphene sheets?

The authors note that if GO dispersions with concentrations less than 0.5 mg ml -1 are reduced by hydrazine under appropriate conditions (see Methods), the particle size of the resulting CCG sheets does not increase after the reduction is complete (Fig. 3a). 

Q17. What is the effect of the electrostatic assembly technique on graphene sheets?

It would be reasonable to expect that the successful formation of graphene colloids will open up possibilities to use this powerful electrostatic assembly technique to manipulate graphene sheets for creating many new and potentially useful nanosystems. 

Q18. What is the zeta potential of the reduced graphene dispersions?

As shown in Fig. 2a, the zeta potential of the reduced graphene dispersion is pH dependent, which is consistent with the fact that the ionization of carboxylic acid groups is strongly related to pH. 

Q19. What is the zeta potential of graphene dispersions?

The feasibility of forming stable graphene dispersions through electrostatic stabilization is further supported by their zeta potential analysis. 

Q20. How much hydrazine is in the reduction mixture?

Note that the concentration of hydrazine in the reduction mixture can be varied from 0.0175 wt% (used in the above procedure) to 1.75 wt%. 

Q21. What is the colloidal nature of the resulting CCG dispersions?

The colloidal nature of the resulting CCG dispersions is further confirmed by two experiments typically conducted in colloid science: investigations of the Tyndall effect and the salt effect.