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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 11689–11691 11689
Cite this: Chem. Commun., 2011, 47, 11689–11691
Oxidizing metal ions with graphene oxide: the in situ formation
of magnetic nanoparticles on self-reduced graphene sheets for
multifunctional applications
Yuhua Xue,wab Hao Chen,wa Dingshan Yu,b Shuangyin Wang,b Michal Yardeni,b
Quanbin Dai,bMingming Guo,
cYong Liu,
aFan Lu,
aJia Qu*
aand Liming Dai*
ab
Received 3rd August 2011, Accepted 14th September 2011
DOI: 10.1039/c1cc14789g
Fe2+
cations in FeCl2 or FeSO4 were oxidized by graphene
oxide, leading to an in situ deposition of Fe3O4 nanoparticles
onto the self-reduced graphene oxide (rGO) sheets. The
resultant Fe3O4/rGO sheets were demonstrated to possess
interesting magnetic and electrochemical properties attractive
for a large variety of potential applications.
The Noble-Prize-winning discovery of graphene1 has created
an entirely new branch of materials science and technology.
Being a single-atom-thick sheet of carbon atoms packed in
two-dimensional (2D) honeycomb lattices, graphene possesses
a large surface area and high electrical/thermal conductivity as
well as excellent mechanical properties.1,2 These interesting
properties make graphene attractive for a variety of potential
applications, including electronic devices, solar cells, super-
capacitors, batteries, fuel cells, sensors, and actuators.3–8
However, graphene sheets without functionalization are
insoluble and infusible, and the poor processability has limited
their large-scale practical applications. Recent effort has led to
solution-processable graphene oxides (GOs) from exfoliation
of graphite by acid oxidation.9,10 Subsequent reduction of
GOs yields reduced graphene oxide (rGO) or graphene nano-
sheets (GNs).11 The availability of solution-processable GOs
has not only allowed for the formation of GN films through
various solution processing methods8–12 but also facilitated
functionalization of GNs.8,12
Having a large number of oxygen-containing functional
groups (e.g., carboxyl, hydroxyl, epoxy groups), GOs could
be used as strong oxidizing reagents. While the oxygen-
containing groups of GOs have often been removed via
chemical reduction to produce GNs or used as functional sites
for chemically bonding other moieties,2 there is little discussion
on potential use of GOs as oxidizing reagents (catalysts).13
Along with the recent intensive effort in developing metal-free
catalysts based on carbon nanomaterials (e.g., N-doped
carbon nanotubes, N-doped graphene),7,14 we found that GOs
could act as strong oxidizing reagents to effectively oxidize
Fe2+ into Fe3O4 nanoparticles, which simultaneously deposited
on the self-reduced GO surface. As we shall see later, the
resultant Fe3O4-nanoparticle-decorated reduced graphene
oxide (Fe3O4/rGO) shows interesting magnetic and electro-
chemical behaviors useful for potential energy storage, catalytic,
and even biomedical applications (e.g., supercapacitors, magnetic
bioimaging and targeted drug delivery). As indicated by the
present work, GO can be used to oxidize many metal and even
non-metal ions. Therefore, the methodology developed in this
study could be regarded as a facile, but effective and versatile,
approach toward the fabrication of reduced graphene sheets
decorated with many other metal oxide nanoparticles of practical
significance.
In a typical experiment, GO was prepared by acid oxidation
of graphite powder according to the modified Hummers
method (ESIw).9,10 Instead of reducing the resultant GO with
those widely-used highly toxic/explosive reduction reagents,
such as hydrazine11 and NaBH4,15 we used the GO as an
efficient oxidizing reagent to oxidize Fe2+ from FeCl2 or
FeSO4 to form Fe3O4/rGO via the spontaneous in situ deposi-
tion of Fe3O4 nanoparticles onto the self-reduced GO surface
in this study.
As shown in Scheme 1a, the Fe3O4/rGO can be prepared via a
redox reaction between GO and Fe2+. The redox reaction was
evident by a color change from yellow (Scheme 1b), characteristic
of GO in an aqueous solution of NH4OH (pH = 9), to dark
black (Scheme 1c, left panel) upon addition of a predetermined
amount of FeCl2 (weight ratio of FeCl2�4H2O to GO = 10 : 1).
These Fe3O4-nanoparticle-decorated rGO showed strong attrac-
tion towards an external magnet, leading to an almost full
separation of the Fe3O4/rGO out of the solution (Scheme 1c,
right panel). This clearly indicates that Fe3O4 nanoparticles have
imparted useful magnetic properties to the rGO.
Fig. 1a andb showatomic forcemicroscopic (AFM,Agilent 5500
AFM) images of theGOandFe3O4/rGO.As can be seen in Fig. 1a,
a Institute of Advanced Materials for Nano-Bio Applications,School of Ophthalmology & Optometry, Wenzhou Medical College,270 Xueyuan Xi Road, Wenzhou, Zhejiang325027, China.E-mail: jqu@wzmc.edu.cn
bDepartment of Macromolecular Science and Engineering,Case Western Reserve University, Cleveland, Ohio 44106, USA.E-mail: liming.dai@case.edu
c Polymer Science Institute, University of Akron, Akron, Ohio 44325,USA. E-mail: ldai@mail.eye.ac.cnw These authors contributed equally.
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11690 Chem. Commun., 2011, 47, 11689–11691 This journal is c The Royal Society of Chemistry 2011
the GO surface is smooth with a thickness of about 1 nm and
free from any particulate contamination. In contrast, the
corresponding AFM image for the Fe3O4/rGO given in
Fig. 1b clearly shows the formation of apparently spherical
particles with size of 4–10 nm characteristic of Fe3O4. The
presence of Fe3O4 nanoparticles in Fe3O4/rGO was further
evident by scanning electron microscopic (SEM, Nova nano-
SEM 600) imaging of GO (Fig. 1c) and Fe3O4/rGO (Fig. 1d). A
high-resolution transmission electron microscopic (TEM,
JEOL JEM-2100) image given in the ESIw (Fig. S1) shows a
nanoparticle with 0.29 nm lattice spacing, attributable to Fe3O4
(100). While the microstructure of the rGO supporting layer
could not be clearly seen in Fig. S1 (ESIw) due possibly to the
electron beam damage during the microscopic imaging, an
X-ray diffraction (Miniflex Desktop X-ray Diffractometer)
profile from Fe3O4/rGO (Fig. 1e) unambiguously shows all
the characteristic peaks for Fe3O4 single crystal particles. The
XRD characteristic peaks of Fe3O4 disappeared after washing
Fe3O4/rGOwith an aqueous solution of HCl (10 wt%, Fig. 1e),
indicating a complete removal of Fe3O4 nanoparticles from the
rGO substrate. The appearance of a broad band over low
diffraction angles (2y E 241) in Fig. 1e for the sample after
HCl washing suggests some degree of reaggregation of the
newly-released rGO sheets. The weight percentage of Fe3O4
in the Fe3O4/rGOhybridwas determined by thermogravimetric
analysis (TGA, TA instrument) to be B82% (Fig. 1f). The
initial B15% weight loss seen for GO up to B100 1C was
associated with the thermal desorption of water molecules
physically adsorbed onto the hydrophilic GO surface. This
was followed by another significant weight loss of B35% at
B200 1C, presumably due to the loss of those oxygen-containing
groups, before the complete oxidative decomposition of the
graphitic substrate over 550–700 1C. Interestingly, the thermo-
gravimetric profiles given in Fig. 1f show amuch better thermal
stability for the Fe3O4/rGO hybrid, either before or after the
HCl washing, with respect to GO.
Fourier transform infrared (FTIR, Perkin Elmer), UV/Vis
(Beckman DU 640), X-ray photoelectron spectroscopic (XPS,
VG Microtech ESCA 2000), and nuclear magnetic resonance
(NMR, Varian VNMR 500 MHz) measurements provide
further evidence for the oxidation of Fe2+ into Fe3O4 by using
GO as the oxidizing reagent. As shown in Fig. 2a, the strong
band at 580 cm�1 in the IR spectrum of the as-prepared
Fe3O4/rGO is due to the Fe–O vibration of Fe3O4 nano-
particles, which indicated that Fe2+ has been oxidized
into Fe3O4 nanoparticles by GO. The IR bands of hydroxyl
(3400 cm�1), along with epoxy (1228 cm�1) and carbonyl
(1731 cm�1) groups, associated with GO were significantly
reduced upon its reduction to produce Fe3O4/rGO (Fig. 2a
and b, see also Scheme 1a). Subsequent HCl washing caused
the disappearance of the Fe–O vibration peak at 580 cm�1
from the rGO spectrum in Fig. 2a, indicating, once again, a
complete removal of the Fe3O4 nanoparticles. As expected, the
optical absorbance at 230 nm of GO red shifted for both
Fe3O4/rGO and rGO, indicating the restoration of the
graphitic conjugated structure via reduction of GO with Fe2+.
Fig. 2c and d show XPS spectra of GO before and after the
redox reaction with Fe2+, respectively. While Fig. 2c shows
only the C and O peaks for GO, the corresponding XPS
spectrum for the Fe3O4/rGO in Fig. 2d reveals the presence
of carbon, oxygen, and iron, arising from Fe3O4 nanoparticles
and the underlying rGO sheet. The inset of Fig. 2c reproduces
the high-resolution C1s spectrum for GO, which exhibits the
presence of C–C (284.5 eV), C–O (286.5 eV), CQO (288.1 eV)
and COOH (289.0 eV) groups. Upon the reduction by Fe2+,
the peak intensities for most of the oxygen-containing groups,
particularly C–O, decreased dramatically (inset of Fig. 2d) due
to the redox reaction shown in Scheme 1a. The presence of
Scheme 1 (a) A schematic representation of the preparation route to
Fe3O4/rGO via redox reaction between GO and Fe2+. Photos showing
a water/NH4OH (pH = 9) solution of Fe3O4/rGO (b) before and
(c, left panel) after the redox reaction with Fe2+, and (c, right panel)
with an applied magnet.
Fig. 1 AFM images of (a) GO and (b) Fe3O4/rGO. SEM images of
(c) GO and (d) Fe3O4/rGO. (e) XRD profiles of Fe3O4/rGO and
rGO obtained by HCl washing to remove Fe3O4 from the Fe3O4/rGO
composite. (f) TGA (in air, scanning rate of 10 1C min�1) of GO,
Fe3O4/rGO, and rGO obtained by HCl washing to remove Fe3O4 from
Fe3O4/rGO.
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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 11689–11691 11691
surface COOH groups facilitates the diffusion of Fe2+ cations
towards the GO surface, and hence the efficient redox process
from Fe2+ to Fe3+ to produce Fe3O4 nanoparticles on the
oxygen-rich GO substrate under the slight base condition
(pH = 9).16,17 The formation of Fe3O4 nanoparticles is also
confirmed by the high-resolution Fe 2p spectrum (Fig. 2e), in
which the peaks at 724 and 710 eV are characteristic of
Fe 2p1/2 and Fe 2p3/2 of Fe3O4, which also agrees with the O
1s spectrum in Fig. 2f showing a significant downshift upon
reducing the oxygen-containing groups in GO into the O
atoms in Fe3O4. Further evidence for the oxidation of Fe2+
by GO comes from the solid state 13C NMR spectra given in
Fig. S2 (ESIw), which, in a good agreement with FTIR and
XPS, shows a significant loss of C–O groups. These spectro-
scopic results are well consistent with the scenario shown in
the following reactions (see also, Scheme 1a):
The cyclic voltammetric (CV) measurement (Fig. S3, ESIw)shows a much higher electric double-layer capacitance
(EDLC),18 along with additional Faradic pseudo-capacitance,
for the Fe3O4/rGO hybrid than that of the rGO obtained from
Fe3O4/rGO by HCl washing to remove Fe3O4.
In summary, we have demonstrated that graphene oxide
could be used as a green and efficient oxidizing reagent to
oxidize Fe2+ (e.g., FeCl2, FeSO4) cations into Fe3O4 nano-
particles. The resultant Fe3O4 nanoparticles were found to
simultaneously deposit onto the self-reduced graphene oxide
sheets, leading to the in situ formation of Fe3O4-nanoparticle-
decorated reduced graphene oxide (Fe3O4/rGO). While the
redox reactions were confirmed by various spectroscopic (e.g.,
FTIR, UV/vis, XPS, NMR) and microscopic (e.g., SEM,
TEM, AFM) measurements, the as-synthesized Fe3O4/rGO
has been shown to possess interesting magnetic and electro-
chemical properties useful for a wide range of potential
applications, including new nanomaterials in energy storage
and even magnetic bioimaging and targeted drug delivery
systems. Since GO can be used as a general oxidizing reagent
to prepare reduced graphene oxide sheets decorated with many
other metal oxide and even non-metallic nanoparticles, we
believe that the newly-developed approach and the interesting
magnetic and electrochemical characteristics of Fe3O4/rGO
demonstrated in this study will have both fundamental and
practical significance for the development of a large variety of
novel graphene-based hybrid materials for multifunctional
applications.
We thank the support fromWenzhouMedical College, Case
Western Reserve University, the Zhejiang Innovation Team
from Department of Education (T200917), the Zhejiang
Department of Science and Technology (2009C13019), the
Ministry of Science and Technology of China (2009DFB30380),
the Ministry of Education of China (IRT1077 and 211069),
and the National ‘‘Thousand Talents Program’’ of China.
Notes and references
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9 Y. Xu, L. Zhao, H. Bai, W. Hong, C. Li and G. Shi, J. Am. Chem.Soc., 2009, 131, 13490.
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14 K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009,323, 760.
15 H.-J. Shin, K. K. Kim, A. Benayad, S.-M. Yoon, H. K. Park,I.-S. Jung, M. H. Jin, H.-K. Jeong, J. M. Kim, J.-Y. Choi andY. H. Lee, Adv. Funct. Mater., 2009, 19, 1987.
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Fig. 2 (a) FTIR and (b) UV-vis spectra of GO, Fe3O4/rGO, and rGO
obtained by washing Fe3O4/rGO with HCl solution. (c, d) XPS survey
spectra of GO and Fe3O4/rGO, respectively (insets showing the
corresponding high-resolution XPS C 1s spectra). (e) High-resolution
XPS Fe 2p spectrum of Fe3O4/rGO. (f) High-resolution O1s spectra of
GO and Fe3O4/rGO.
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