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.

ChemComm Dynamic Article Links

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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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4 D. Yu, K. Park, M. Durstock and L. Dai, J. Phys. Chem. Lett.,2011, 2, 1113.

5 D. Yu and L. Dai, Appl. Phys. Lett., 2010, 96, 143107.6 Y. Liu, D. Yu, C. Zeng, Z. Miao and L. Dai, Langmuir, 2010,26, 6158.

7 L. Qu, Y. Liu, J. B. Baek and L. Dai, ACS Nano, 2010, 4, 1321.8 X. Xie, L. Qu, C. Zhou, Y. Li, J. Zhu, H. Bai, G. Shi and L. Dai,ACS Nano, 2010, 4, 6050.

9 Y. Xu, L. Zhao, H. Bai, W. Hong, C. Li and G. Shi, J. Am. Chem.Soc., 2009, 131, 13490.

10 W. Hummers and R. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.11 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas,

A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff,Carbon, 2007, 45, 1558.

12 D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat.Nanotechnol., 2008, 3, 101.

13 D. R. Dreyer, H. P. Jia and C. W. Bielawski, Angew. Chem., Int.Ed., 2010, 49, 6813.

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.

16 Y. Xue, H. Wang, Y. Zhao, L. Dai, L. Feng, X. Wang and T. Lin,Adv. Mater., 2010, 22, 4814.

17 J. Fang, H. Wang, Y. Xue, X. Wang and T. Lin, ACS Appl. Mater.Interfaces, 2010, 2, 1449.

18 D. Yu and L. Dai, J. Phys. Chem. Lett., 2010, 1, 467–470.

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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