X. Hou et al. / Journal of Alloys and Compounds 638 (2015) 214–220
215
Hitachi) with an accelerating voltage of 100 kV. Fourier transform infrared (FT-IR)
spectra were collected in the range of 4000–400 cmꢀ1 on
FT-IR spectrum
structure is conducive to charge transfer, and thereby could sup-
press the recombination of photogenerated carriers. What is more,
Bi2MoO6 dispalys a typical band gap of about 2.60 eV [25]. These
excellent structural and spectral properties imply that Bi2MoO6
should be a promising photocatalyst [26–29]. Not surprisingly,
recently studies confirmed that Bi2MoO6 has excellent visible-
light-driven photocatalytic activities for water splitting and the
degradation of organic pollutants [30,31]. Despite these advan-
tages of Bi2MoO6, it is still challenging to simply recover nanopow-
ders from the solution.
Herein, magnetically separable Fe3O4/SiO2/Bi2MoO6 composite
was prepared via a facile hydrothermal method. In the obtained
Fe3O4/SiO2/Bi2MoO6 composite, flower-like three dimensional
(3D) Bi2MoO6 microspheres were decorated with Fe3O4/SiO2 mag-
netic nanoparticles, and thus formed a hierarchical structure. The
visible light photocatalytic tests show that the present Fe3O4/
SiO2/Bi2MoO6 composite possess excellent photocatalytic activity
for degrading RhB. After the photocatalytic reaction has been com-
pleted, the composite can be easily collected for reuse by applying
an external magnetic field.
a
(AVATAR 360, Nicolet) using the KBr pellet method at room temperature.
Magnetic properties at room temperature were measured by a vibrating sample
magnetometer (VSM, Lake Shore7407) in a maximum field of 15kOe. UV–Vis
absorption spectrum was investigated by UV–Vis spectrometer (Perkin Elmer,
lambda 950) with the wavelength ranging from 200 to 800 nm.
2.5. Photocatalytic tests
Photocatalytic activities of the as-prepared samples were evaluated by degrada-
tion of Rhodamine B (RhB) under visible light irradiation. A 300 W xenon lamp with
a cut-off filter was used as the visible light source. Briefly, 100 mg of photocatalyst
was suspended in 100 mL of RhB solution (10 mg Lꢀ1) with constant stirring. Before
illumination, the suspension was stirred in the dark for 1 h to establish adsorption–
desorption equilibrium. The temperature of the system was controlled at room
temperature by circulating cooing water. At the given time intervals, 3 mL of mix-
ture was collected and separated by a magnet. The concentration of the resulting
supernatant was monitored by checking the absorbance at 553 nm using Lambda
950 spectrophotometer. In the recycle reaction, the catalyst was separated by an
external magnetic field, the recycled catalyst was washed with ethanol and deion-
ized water several times before being re-dispersed in the dye solution(100 mL,
10 mg Lꢀ1)for the next cycling.
3. Results and discussions
2. Materials and methods
All chemicals were analytical grade and used without further purification.
Deionized water (18 M
3.1. Characterization of Fe3O4/SiO2/Bi2MoO6 composite
X
cmꢀ1) was used for all experiments.
The crystal structure of Fe3O4, Fe3O4/SiO2 and Fe3O4/SiO2/
Bi2MoO6 composite were characterized by XRD, as shown in
Fig. 1. The pure Fe3O4 nanoparticles (Fig. 1a) exhibit feature diffrac-
tion peaks at about 30.1°, 35.4°, 43.1°, 53.4°, 57.0° and 62.5° that
match well with the (220), (311), (400), (422), (511) and (440)
crystal planes of the face-centered cubic (fcc) Fe3O4 phase
(a = b = c = 8.397 Å) as identified using the standard data JCPDS
No. 19-0629 [34,35]. No impurity phase is emerged, indicating
high purity of Fe3O4 nanoparticles. After coating with a SiO2 layer,
in Fig. 1b no new diffraction peaks are observed owing to amor-
phous phase of the prepared SiO2. As for Fe3O4/SiO2/Bi2MoO6 com-
posite, all of the diffraction peaks can be readily indexed to the
orthorhombic Bi2MoO6 (a = 5.500 Å, b = 16.240 Å, c = 5.490 Å,
JCPDS No. 76-2388) [36], with respect to the invisible diffractions
of Fe3O4 in the composite, this is because the content of Fe3O4/
SiO2 is too low.
Fig. 2a–c displays the SEM images of various synthetic stages of
Fe3O4/SiO2/Bi2MoO6 composite. The pristine Fe3O4 nanoparticles
possess a mean diameter of about 300 nm and near-spherical mor-
phology. Compared with the Fe3O4 nanoparticles, although the
overall morphology of the obtained Fe3O4/SiO2 nanoparticles
(Fig. 2b) is no significant variation, these nanoparticles exhibit
more relatively smooth surface. After introducing Bi2MoO6,
Fe3O4/SiO2 nanoparticles immobilized on flower-like three dimen-
2.1. Preparation of monodisperse Fe3O4 nanoparticles
The magnetic Fe3O4 nanoparticle was synthesized according to a previous
report with a little modification [32]. In a typical process, 1.3 g of FeCl3ꢁ6H2O was
dissolved in 40 mL ethylene glycol to form a clear solution. Then 0.4 g of trisodium
citrate and 2.4 g of sodium acetate were added under vigorous stirring. After vigor-
ously stirring for 30 min, the resulting homogeneous dispersion was transferred
into a Teflon-lined stainless-steel autoclave with a capacity of 80 mL, sealed, heated
at 200 °C and maintained for 10 h. The as-prepared products were collected with a
magnet and washed with ethanol and deionized water for several times, then dried
at 60 °C under vacuum for further use.
2.2. Preparation of Fe3O4/SiO2 nanoparticles
The synthesis of Fe3O4/SiO2 nanoparticle was carried out by a modified sol–gel
method [33]. Typically, 0.1 g as-prepared Fe3O4 nanoparticles were treated with
0.1 M HCl aqueous solution (50 mL) by ultrasonication. After the treatment for
10 min, the magnetite particles were separated, washed with water, and then
homogeneously dispersed in the mixture of 40 mL ethanol and 10 mL deionized
water by ultrasonication for about 10 min. Subsequently, 1 mL ammonia solution
was added in the above solution under continuous mechanical stirring, and
followed 0.1 mL TEOS was added dropwise. Finally, the reaction was allowed to pro-
ceed at room temperature for 6 h. The resulting products were washed with ethanol
and dried at 60 °C under vacuum.
2.3. Preparation of Fe3O4/SiO2/Bi2MoO6 composite
A facile hydrothermal method was used for the immobilization of Fe3O4/SiO2
nanoparticles onto the surface of Bi2MoO6 to obtain Fe3O4/SiO2/Bi2MoO6 composite.
Briefly, 0.97 g (2 mmol) of Bi(NO3)3ꢁ5H2O and 0.03 g Fe3O4/SiO2 nanoparticles were
added in 5 mL of HNO3 (2 M) solution at room temperature, and then the mixture
was sonicated for 30 min. After aging for 2 h, 30 mL of Na2MoO4ꢁ2H2O (1 mmol)
solution was added. The mixture was sonicated at room temperature for another
30 min before being transferred to a stainless steel autoclave with a Teflon liner
of 80 mL capacity and maintained at 160 °C for 12 h. After the autoclave had been
cooled to room temperature, the products were harvested using a magnet, and sub-
ject to several cycles of magnetic separation/washing/redispersion before being
dried at room temperature. Pure Bi2MoO6 microspheres prepared without Fe3O4/
SiO2 were used for comparison.
sional (3D) Bi2MoO6 microspheres with sizes of ꢂ3
lm, thus the
hierarchical structure has been formed. In fact, the flower-like 3D
structure of Bi2MoO6 are constructed by numerous two dimen-
sional (2D) interlaced nanosheets. Fig. 2d is the EDX elemental
microanalysis of Fe3O4/SiO2/Bi2MoO6 composite, clearly suggesting
that Fe, Si, Bi, Mo and O are present in the composite.
The microstructure of Fe3O4/SiO2 nanoparticles and Fe3O4/SiO2/
Bi2MoO6 composite were observed by using TEM technique. In
Fig. 3a, the Fe3O4 nanoparticles are completely encapsulated into
SiO2 layer with thickness of ꢂ20 nm. The light area with circle
shape is silica shell, and the dark area with sphere shape is Fe3O4
core, clearly suggesting that the Fe3O4/SiO2 nanoparticles possess
a core–shell structure. However, such core–shell structures (small
black spheres in Fig. 3b) become invisible due to the large size
difference between Bi2MoO6 microspheres and Fe3O4@SiO2
nanoparticles. Therefore, the small black spheres should be
Fe3O4/SiO2 nanoparticles and the large black spheres are Bi2MoO6
2.4. Characterization
Powder X-ray diffraction (XRD) patterns of the prepared products were carried
out on a Dmax-rA powder diffractometer, Cu K
a as a radiation source with an
operating voltage of 40 kV and an operating current of 40 mA. The morphologies
and sizes of the as-prepared samples were characterized by scanning electron
microscopy (SEM, FEI Helios Nanolab 600i) equipped with an energy dispersive
X-ray analyzer (EDX) and Transmission Electron Microscopy (TEM, H-7650,
Hou, Xuemei
