ARTICLE
pubs.acs.org/JPCC
On the Gelation of Graphene Oxide
Hua Bai,†,‡ Chun Li,† Xiaolin Wang,‡ and Gaoquan Shi*,†
†
Key Laboratory of Bio-organic Phosphorous Chemistry and Chemical Biology, Tsinghua University, Beijing, 100084,
People's Republic of China
‡
Department of Chemical Engineering, Tsinghua University, Beijing, 100084, People's Republic of China
S
b Supporting Information
ABSTRACT: Graphene oxide (GO) has been recognized as a unique twodimensional building block for various graphene-based supramolecular
architectures. In this article, we systematically studied the three-dimensional
self-assembly of GO sheets in aqueous media to form hydrogels. The gelation
of GO can be promoted by different supramolecular interactions, including
hydrogen bonding, π-stacking, electrostatic interaction, and coordination.
Furthermore, the lateral dimensions of GO sheets also have strong influences
on GO gelation. The resulting GO hydrogels exhibited low critical gelation
concentrations and good reversibility upon chemical stimulations. These findings indicate that GO has rich supramolecular
properties, and its hydrogels may have a variety of technological applications.
’ INTRODUCTION
In the past half decade, graphene oxide (GO), a precursor of
graphene,1À4 has attracted a great deal of attention due to its
unique structure and outstanding physical and chemical
properties.5À8 Particularly, GO behaves like an amphiphilic
macromolecule with hydrophilic edges and a more hydrophobic
basal plane,9À11 which makes it an attractive building block for
the construction of various supramolecular architectures.12,13
Furthermore, the two-dimensional (2D) structure of GO sheets
provides them with various new supramolecular behaviors
compared with conventional low-dimensional counterparts.
Although GO sheets have been assembled into various macrostructures, such as LangmuirÀBlodgett (LB) films and paper-like
films,14À17 their 3D assembly behavior has not yet been clearly
revealed.18 We have first reported the 3D assembly of GO sheets
in water solution by adding poly(vinyl alcohol) (PVA) as a crosslinker, forming a pH-sensitive supramolecular hydrogel.19 Hydrogen bonding between GO sheets and PVA chains is believed
to be responsible for the formation of the hydrogel. Recently,
single-stranded DNA was also found to be a good cross-linker for
preparing a GO/DNA composite hydrogel, in which πÀπ
interaction was the dominant driving force.20 Similarly, hydrogels
based on chemically concerted graphene (CCG) have also been
reported by us and other groups.21À25 These examples reflect
that GO and CCG are good gelators. However, the gelation of
GO sheets has not yet been studied extensively and the fundamental roles behind gelation phenomena have also not been
clearly revealed. Here, we report a systematical study on GO
gelation. The GO-based hydrogels were prepared by acidification
or adding small organic molecules, polymers, or ions as crosslinkers. The effects of different driving forces (e.g., hydrogen
bonding, electrostatic interaction, and coordination) and lateral
dimensions of GO sheets on GO gelation are discussed.
r 2011 American Chemical Society
’ EXPERIMENTAL SECTION
Natural graphite powders were bought from Qingdao Huatai
lubricant sealing S&T Co. Ltd. (Qingdao, China). Poly(vinyl
pyrrolidone) (PVP), poly(ethylene oxide) (PEO), polydimethyldiallylammonium chloride (PDDA), polyethylenimine
(PEI), cetyltrimethyl ammonium bromide (CTAB), tetramethylammonium chloride (TMAC), and melamine were the products
of Alfa Aesar. Ethylene diamine tetraacetic acid disodium salt
(Na2EDTA) was purchased from Biosharp Co. Ltd. All the
inorganic salts, including NaCl, KCl, AgNO3, CaCl2, MgCl2,
CuCl2, Pb(NO3)2, CrCl3, and FeCl3, were purchased from
Sinopharm Chemical Reagents Co. Ltd. (Beijing, China). All
the chemicals were used as received without further purification.
GO was prepared from natural graphite powder by a modified
Hummers method26,27 and purified by dialysis for 1 week to
remove any impurities. Natural graphite with average particle
sizes of 325 and 12 000 mesh were used for synthesizing GO
sheets with larger and smaller lateral dimensions, respectively. To
prepare GO hydrogels, a certain volume GO dispersion was
mixed with the solution of acid or other cross-linkers. The blend
was then shaken violently for several seconds to form a hydrogel.
The formation of the hydrogel was examined by a tube inversion
method, in which a polypropylene tube with an inner diameter of
8 mm was used.
Atomic force micrographs (AFMs) were recorded on a
Nanoscope III MultiMode SPM (Digital Instruments) with an
AS-12 (“E”) scanner operated in tapping mode in conjunction
with a V-shaped tapping tip (Applied Nanostructures SPM
model: ACTA). The images were taken out at a scan rate of
Received: December 17, 2010
Revised:
January 27, 2011
Published: March 14, 2011
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The Journal of Physical Chemistry C
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Scheme 1. Chemical Structure of GOa
Hydrophilic groups are colored in red (carboxyl groups) and blue
(hydroxyl and epoxy groups).
a
2 Hz. X-ray diffraction (XRD) was carried out using a D8
Advance (Bruker) X-ray diffractometer with Cu KR radiation
(λ = 1.5418 Å). All the GO solutions and hydrogels were
lyophilized for at least 24 h before XRD measurement. Scanning
electron micrographs (SEMs) were taken out by the use of a FEI
Quanta 200 scanning electron microscope. Rheological studies
were performed on an MCR 300 (Paar Physica) rheometer. All
the viscosity data were obtained by using a 25 mm diameter
parallel plate with a 1 mm plateÀplate gap.
’ RESULTS AND DISCUSSION
A GO sheet can be regarded as a single-layer graphite that
brings various hydrophilic oxygenated functional groups
(Scheme 1).28À30 Thus, GO sheets can be dispersed in water
to form a stable colloidal dispersion. It was also believed that the
electrostatic repulsion between GO sheets, resulting from their
ionized carboxyl groups, prevented their aggregation in aqueous
medium.31 Therefore, acidification of a GO dispersion would
weaken the electrostatic repulsion and lead to the formation of a
graphite oxide flocculent. This phenomenon was observed by us
as well as other researchers. The ζ potential of a GO dispersion
increases with the decrease of its pH value, indicating the
protonation of carboxyl groups (Figure 1A). Therefore, GO
sheets are instable in a strong acidic aqueous medium because of
insufficient mutual repulsion.32 However, as the concentration of
GO (CGO) was sufficiently high (>4 mg mLÀ1), a hydrogel
instead of an amorphous precipitation formed upon acidification.
Figure 1A also gives the zero-shear viscosities (η0) of a 5
mg mLÀ1 GO solution at different pH values. Because of the
ionization of carboxyl groups, the initial GO solution is acidic
(pH = 4.6). Increasing the pH value of the GO solution using
NaOH slightly decreased its η0, owing to the increase of the
repulsion force between GO sheets. However, when the pH
value of the GO solution was lowered from 4.6 to 0.6 by adding
hydrochloric acid, its η0 increased dramatically from 290 to
10 850 Pa s. As a result, a hydrogel quickly formed within several
seconds, indicating the generation of a 3D infinite network in the
solution. The formation of the hydrogel was tested to be
independent of the types of acids used for acidification. Thus,
the 3D GO network must have resulted from the self-assembly of
acidified GO sheets. In this case, the electrostatic repulsion
between GO sheets was weakened and their hydrogen-bonding
force was enhanced due to the protonation of carboxyl groups.
Further lowering the pH value (2
μm) and small ( 3
mg mLÀ1. At 4 mg mLÀ1, a stable hydrogel was formed, as
confirmed by the tube inversion method. This is a rather low
critical gel concentration (CGC), indicating that GO falls into
the concept of “super gelator”.33
An essential difference between GO sheets and conventional
amphiphilic molecules is that GO sheets have much larger lateral
dimensions. Furthermore, we found that the lateral dimensions
of GO sheets have strong effects on their gelation behavior. For
example, as the lateral dimensions of most GO sheets are as large
as several micrometers (Figure 2A), a stable hydrogel can be
easily formed by acidification (Figure 1A). Decreasing the lateral
dimension of GO sheets is unfavorable for GO gelation. Actually,
if the lateral dimensions of most GO sheets are smaller than 1 μm
(Figure 2B), no gelation occurred at any pHs even if the CGO was
as high as 9 mg mLÀ1 (Figure 1B). In this case, the acidified
solutions of small GO sheets turned cloudy in several minutes,
indicating the occurrence of GO precipitation. Therefore, the
lateral dimension of GO sheets is a key factor for deciding their
gelating ability.
The size effect described above can be explained as follows. To
some extent, GO can be regarded as an amphiphilic macromolecule. However, the sizes of large GO sheets (several micrometers) are much larger than those of conventional polymers.
Therefore, they can contact with each other even in a dilute
solution. This assumption was confirmed by the fact that the
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Figure 2. AFM images of two GO samples with different average lateral
dimensions. Scale bar = 5 μm.
freeze-dried sample of a 3 mg mLÀ1 GO dispersion can maintain
the original volume of the solution. In contrast, lyophilizing a 3
mg mLÀ1 PVA (Scheme 2) produced a shrunken sample (Figure
S2, Supporting Information). Furthermore, the lyophilized GO
dispersion shows a 3D network composed of GO sheets, as
shown by its SEM image (Figure 3A). Therefore, it is reasonable
to conclude that a loose dynamic GO network existed in the
original dispersion of GO sheets owing to a force balance
between electrostatic repulsion and binding interactions
(hydrogen bonding, π-stacking, hydrophobic effect, etc.). Gelation occurs as the GO network in solution is reinforced by
enhancing the bonding force or weakening the repulsion force.
Acidification is an effective way for this purpose, as described
above. On the other hand, both gelation and precipitation of GO
sheets can be induced by reinforcing their bonding interaction.
The difference between a GO hydrogel and graphite oxide
precipitation is the stacking states of their GO sheets. GO sheets
are randomly orientated in a hydrogel, whereas they adopt a
parallel arrangement in their precipitation. The latter staking
mode is energetically favorable because of the larger contacting
area between GO sheets. However, the mobility of large GO
sheets in solution is strongly limited. As a result, it is difficult to
adjust their orientation to be parallel with each other. Comparing
with the gelation process, precipitation is a kinetically slower
process. Moreover, the large conjugated basal planes make the
GO sheets stiff and form a stable network. The conversion of the
GO network to the energetically more stable state (precipitation)
is rather slow. In fact, a very slow precipitation process was
observed after 2 weeks of the formation of hydrogels. However, if
the sizes of GO sheets were reduced, for example, to be smaller
than 1 μm in our experiment, they can change their conformations and positions in solution more easily. Consequently,
acidified GO sheets trend to aggregate in an energetically
favorable layer-by-layer manner to form a precipitation. The
stability of a hydrogel is decided by the strength ratio of repulsion
and bonding forces between GO sheets. For example, overacidification of the dispersion of large GO sheets would also lead
to the formation of irregular aggregates because of too weak
repulsion forces.
To reveal the gelation behaviors of GO sheets more extensively, we used the large GO sheets for preparing hydrogels in the
following studies. According to the analyses described above, the
addition of a cross-linker can increase the bonding force between
GO sheets and consequently promote gelation. This deduction
Figure 3. SEM images of lyophilized GO solution and three typical GO
hydrogels: (A) GO solution, (B) GO/PVP hydrogel with 1 mg mLÀ1 PVP,
(C) GO/PDDA hydrogel with 0.1 mg mLÀ1 PDDA, and (D) GO/Ca2þ
hydrogel with 9 mM Ca2þ. CGO = 5 mg mLÀ1, and scale bar = 10 μm.
was first examined by introducing a polymer component to
provide additional hydrogen-bonding interaction. In a previous
work, we have reported that a small amount of PVA ( 3 mg LÀ1). The formation of the 3D network is because
of the large and flexible 2D structure of GO sheets and the force
balance between their static repulsion and bonding interaction.
Increasing the bonding force or decreasing the repulsion force
between GO sheets in solution will reinforce the 3D GO network
and induce GO gelation or participation. These purposes can be
realized by adding cross-linkers (polymers, small ammonium
salts, and metal ions) or acidizing the GO solution. During the
gelation and participation processes, many supramolecular interactions, such as hydrogen bonds, static or hydrophobic interactions, and coordination, are recognized as the possible driving
forces. Careful control of the balance between electrostatic
repulsion and bonding force is an important factor that governs
the formation of GO hydrogels. All the GO hydrogels showed
low critical gelation concentrations, and some of them exhibited
environmental responsive behavior. This work indicates that GO
is a good candidate for preparing “super” and “smart” hydrogels
and will enlighten further studies on the supramolecular chemistry of graphene and its derivatives.
’ ASSOCIATED CONTENT
S
b
Supporting Information. Dynamic rheological behavior
of GO hydrogels and SEM images, XRD patterns, and photo
images of lyophilized GO hydrogels. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: þ86-10-6277-3743. Fax: þ86-10-6277-1149. E-mail:
gshi@tsinghua.edu.cn.
’ ACKNOWLEDGMENT
This work was supported by the Natural Science Foundation
of China (50873052 and 91027028) and the China Postdoctoral
Science Foundation (20090460027 and 201003100).
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