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G. He et al. / Materials Research Bulletin 48 (2013) 1885–1890
2. Experimental
2.1. Materials
3. Results and discussion
3.1. Characterization of Fe3O4@GO composite
Natural graphite powder (99.9%, 500 mesh), FeCl3Á6H2O,
FeSO4Á7H2O, ammonia (22–28%), hydrazine hydrate (80%),
nitrobenzene, o-nitrotoluene, o-, m-, p-nitroaniline and other
materials were purchased from Sinopharm Chemical Reagent Co.,
Ltd. (China). All chemicals were of analytical grade and used
as received.
The phase structure of the sample was obtained through X-ray
diffraction (XRD) measurement. As shown in Fig. 1, the original GO
exhibits a sharp peak at 2u = 11.28, which can be ascribed to the
(0 0 1) plane of GO. However, no diffraction peak of GO is
observable for the as-prepared Fe3O4@GO composite, suggesting
that the layer stacking of the GO sheets was destroyed by the
loading of Fe3O4 NPs. The diffraction peaks at 2u = 30.188, 35.528,
2.2. Preparation of Fe3O4@GO composite
43.368, 53.708, 57.308 and 62.788 can be indexed to (2 2 0), (3 1 1),
(4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of cubic Fe3O4 (JCPDS 89-
2355). The broad diffraction peaks indicate a small size of the Fe3O4
NPs, which is about 11.2 nm according to the Scherrer equation.
The FTIR spectra of GO and the Fe3O4@GO composite are shown
in Fig. 2. As expected, the FTIR spectrum of GO is in good agreement
with previous works [21,22]. The broad and intense band observed
at 3402 cmÀ1 is ascribed to the stretching vibration of O–H. The
band at 1090, 1380 and 1716 cmÀ1 corresponds to the C–O
stretching vibration, the C–O–H deformation vibration, and the
C55O stretching vibration, respectively. The peak at 1618 cmÀ1 can
be assigned to the aromatic skeletal C55C stretching vibration of the
unoxidized graphitic domains. Compared with that of GO, a new
prominent absorption band appeared at about 580 cmÀ1 in the
FTIR spectrum of the Fe3O4@GO composite, which corresponds to
the stretching mode of Fe–O [23].
Graphite oxide was synthesized using a modified Hummers
method [19,20] and exfoliated to afford an aqueous dispersion
of graphene oxide (GO) under ultrasonication. Fe3O4@GO
composites with different Fe3O4 to GO mass ratios (2, 5, 10,
15 and 20, respectively) were synthesized. Taken as an example,
the Fe3O4@GO composite with Fe3O4 to GO mass ratio of 10
was synthesized as follows. 15 mL of GO (2.72 mg mLÀ1
)
was dispersed into 40 mL ethanol with stirring. 0.9522 g of
FeCl3Á6H2O (3.5228 mmol) and 1.0520 g of FeSO4Á7H2O
(3.7842 mmol) were dissolved in 10 mL of distilled water under
sonication, then the solution was injected dropwise into the GO
suspension and stirred for 30 min. The resulting mixture was
heated to 68 8C before ammonia solution was added to adjust
the pH to 10. The mixture was stirred at 68 8C for 2 h and then
cooled to room temperature. The Fe3O4@GO composite was
separated from the mixture using a permanent magnet, and
rinsed three times with ethanol and distilled water respectively
before being dried at 65 8C for 12 h. For comparison, the bare
Fe3O4 NPs was prepared following the same procedure in the
absence of GO.
The TEM image of the Fe3O4@GO composite (Fig. 3) showed that
the GO sheets with a lateral dimension of about 1
mm were
decorated with large quantity of Fe3O4 NPs. The transparency of
the GO sheet indicated that the graphite oxide was well exfoliated
into few-layer GO sheets. As shown in the inset (a) of Fig. 3, a
narrow size distribution of Fe3O4 NPs was obtained with a mean
size of around 12 nm, which is in accordance with the XRD result.
The homogeneous distribution of the Fe3O4 NPs was expected to
offer an enhanced catalytic activity. The crystal lattice fringes with
d-spacing of 0.25 nm (inset b of Fig. 3) can be assigned to the (3 1 1)
plane of Fe3O4.
N2 adsorption–desorption isotherms were conducted to inves-
tigate the porous structure and surface area of the bare Fe3O4 NPs
and the Fe3O4@GO composite. As shown in Fig. 4, the surface area
of the Fe3O4@GO composite calculated by the Brunauer–Emmett–
Teller method is 137 m2 gÀ1, which is much higher than that of the
bare Fe3O4 NPs (58 m2 gÀ1 as shown in Fig. S1). The N2 isotherm of
the Fe3O4@GO composite is close to Type IV with an evident
hysteresis loop in the 0.4–1.0 range of low relative pressure,
indicating that the existence of GO prevented the Fe3O4 NPs from
2.3. Characterization
X-ray diffraction (XRD) measurements were carried out using
a Bruker D8 Advance diffractometer with Cu-K
˚
a radiation
(
l
= 1.54 A). FTIR spectra were recorded on a Nicolet 370 FT/IR
spectrometer (Thermo Nicolet, USA) using pressed KBr pellets.
Transmission electron microscopy (TEM) images were taken with
a JEOL JEM-2100 microscope (JEOL, Japan). The BET surface area of
the as-synthesized samples was measured using an ASAP2010 C
surface aperture adsorption instrument (Micromeritics Instru-
ment Corporation, USA) by N2 physisorption at 77 K. The reaction
process was monitored using an Agilent 6890N gas chromatogra-
phy equipped with an FID detector (Agilent, USA). The
product was analyzed using an Agilent 6890N/5973N GC-MS
(Agilent, USA).
2.4. Catalytic reduction of nitroarenes over Fe3O4@GO composite
A mixture of an aromatic nitro compound (e.g., nitrobenzene,
2.4600 g, 20 mmol), ethanol (12 mL) and as-prepared Fe3O4@GO
composite (0.0773 g, 3.1 wt% of nitrobenzene) was heated to
reflux with stirring, followed by dropwise addition of hydrazine
hydrate (72 mmol, 3.6 equiv.). The reaction mixture was kept at
reflux with stirring and the reaction progress was monitored by
gas chromatography. The Fe3O4@GO composite was separated
by applying an external magnetic field at the end of the
reaction and washed with ethanol three times before being
vacuum dried. Then, the composite was weighed and recycled
for the reduction reaction. Quantitative and qualitative analyses
of the reactions were performed using GC and GC–MS, and
the products were identified by comparison with authentic
samples.
Fig. 1. XRD patterns of GO and Fe3O4@GO composite.