Fabrication of novel ZnFe2O4/BiOI nanocomposites and its efficient photocatalytic activity under visible-light irradiation

Article abstract of DOI:10.1016/j.jallcom.2016.11.323

Novel ZnFe₂O₄/BiOI nanocomposites have been successfully synthesized by a simple and economical method and characterized by X-ray diffraction (XRD), energy dispersive X-ray spectra (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS), Fourier transform infrared spectra (FT-IR) and photoluminescence (PL). The ZnFe₂O₄particles were determined to be deposited on the surface of the BiOI successfully. The as-prepared ZnFe₂O₄/BiOI nanocomposites were further applied to degrade rhodamine B (RhB), methylene blue (MB) and methylene orange (MO) under visible-light irradiation. The photocatalytic test suggested that ZnFe₂O₄/BiOI nanocomposites exhibited a higher activity for degradation of these pollutants than pure BiOI and ZnFe₂O₄under visible-light irradiation. Among the as-prepared nanocomposites, 3%-ZnFe₂O₄/BiOI displayed the best photocatalytic activity of the degradation. The enhanced photocatalytic performance was mainly ascribed to the separation efficiency of the carriers, and the formation of highly active species [rad]O₂^?on the surface of ZnFe₂O₄/BiOI, which has the potential application for the treatment of various pollutants in wastewater utilizing solar energy effectively.

Full text of DOI:10.1016/j.jallcom.2016.11.323

Journal of Alloys and Compounds 696 (2017) 353e361
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of novel ZnFe O /BiOI nanocomposites and its efficient
2
4
photocatalytic activity under visible-light irradiation
Yawen Zhou , Shanshan Fang , Man Zhou , Guanqiu Wang c, Su Xue a c,
a
,
c
a
a
a
,
,
a
,
b
,
*
a, b, **
, Chao Yao a
Zhongyu Li
a
, Song Xu
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, 213164,
PR China
Jiangsu Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou, 213164, PR China
b
c
College of Hua Loogeng, Changzhou University, Changzhou, 213164, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 September 2016
Received in revised form
Novel ZnFe O /BiOI nanocomposites have been successfully synthesized by a simple and economical
method and characterized by X-ray diffraction (XRD), energy dispersive X-ray spectra (EDS), scanning
electron microscopy (SEM), transmission electron microscopy (TEM), UVevis diffuse reflectance spectra
2
4
13 November 2016
(
DRS), Fourier transform infrared spectra (FT-IR) and photoluminescence (PL). The ZnFe
were determined to be deposited on the surface of the BiOI successfully. The as-prepared ZnFe
nanocomposites were further applied to degrade rhodamine B (RhB), methylene blue (MB) and meth-
ylene orange (MO) under visible-light irradiation. The photocatalytic test suggested that ZnFe /BiOI
nanocomposites exhibited a higher activity for degradation of these pollutants than pure BiOI and
ZnFe under visible-light irradiation. Among the as-prepared nanocomposites, 3%-ZnFe /BiOI dis-
2
O
4
particles
Accepted 22 November 2016
Available online 23 November 2016
2 4
O
/BiOI
2 4
O
Keywords:
ZnFe O /BiOI nanocomposites
2 4
Visible-light irradiation
Photocatalytic activity
2
O
4
2 4
O
played the best photocatalytic activity of the degradation. The enhanced photocatalytic performance was
ꢀ
ꢁ
Visible-light driven photocatalyst
mainly ascribed to the separation efficiency of the carriers, and the formation of highly active species O
on the surface of ZnFe
lutants in wastewater utilizing solar energy effectively.
2
2 4
O /BiOI, which has the potential application for the treatment of various pol-
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
TiO
% of the solar spectrum energy caused by its wide band gap of
3.2 eV [18].
For the purpose of utilizing solar light, some visible-light sen-
sitive photocatalysts like BiOX (X ¼ Cl, Br and I) [19e21], Bi WO
[22], CaBi [23] and NaTaO [24] have been investigated.
Recently, Bi-based compounds like BiOX (X ¼ Cl, Br and I) have
2
electrode in 1972 [9e17]. However, it only takes advantage of
4
With the rapid development of economy and society, water
contamination has been an increasing troublesome environmental
problem for human society [1,2]. To solve this problem, plenty of
efforts have been devoted to hunting for efficient methods of
environmental pollutants decomposing, such as chemical oxida-
tion, solvent extraction, filtration, adsorption, floatation and pho-
tocatalytic degradation [3e8]. Among those, photocatalytic
approaches have been extensively studied for environmental pu-
rification after the discovery of high photocatalytic efficiency of
2
6
2
O
4
3
3
þ
10
gained considerable attention because Bi and d configuration
are considered as one of the main factor for an oxide served as
visible-light photocatalysts [25,26]. Among them, BiOI show the
best photocatalytic activity under visible light thanks to its nar-
rowest band gap energy (1.74 eV) [27]. It is a potential sensitizer to
sensitize wide gap semiconductors. But there are still some dis-
advantages, such as the slow rate of charge transfer as well as low
efficiency of light absorption, limited its photocatalytic activity [28].
To improve its photocatalytic activity, some efforts have been made
*
Corresponding author. Jiangsu Key Laboratory of Advanced Catalytic Materials
and Technology, School of Petrochemical Engineering, Changzhou University,
Changzhou, 213164, PR China.
*
and Technology, School of Petrochemical Engineering, Changzhou University,
Changzhou, 213164, PR China.
* Corresponding author. Jiangsu Key Laboratory of Advanced Catalytic Materials
to find efficient inorganic semiconductors such as g-C
BiOI [29] and Fe /BiOI [30] to enhance the charge separation.
These nanocomposites can improve the photocatalytic efficiency to
3 4 3 4
N /Fe O /
2 3
O
E-mail addresses: zhongyuli@mail.tsinghua.edu.cn (Z. Li), cyanine123@163.com
(
S. Xu).
http://dx.doi.org/10.1016/j.jallcom.2016.11.323
925-8388/© 2016 Elsevier B.V. All rights reserved.
0

354
Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
a certain and they aroused our interest on Fe-based compounds.
2 4
2.3. Preparation of ZnFe O /BiOI nanocomposites
10
The spinel structured ZnFe
ration, has been proven to be very useful to utilize solar energy for
its suitable band gap structure [31,32]. At the same time, ZnFe
has a good photochemical stability and it can easily be made and
recovered [33]. Hence, ZnFe -based composites provide
advantage for repeated separation purposes and we believe that it
is meaningful to attempt to add ZnFe to BiOI nanocomopsites.
In this article, we reported the fabrication of ZnFe /BiOI
2 4
O , a metal oxide with d configu-
2 4
A typical synthesis of ZnFe O /BiOI nanocomposites is exhibited
2
O
4
in the following way: firstly, a stoichiometric amount of ZnFe O
2 4
was dissolved into moderate distilled water to obtain solution C
and then ultrasonicated for 30 min. Then, a measured amount of
BiOI was added into solution C. After 30 min ultrasonication and
24 h stirring, the samples were centrifuged and washed with
distilled water and absolute ethanol for three times. Finally, the
2
O
4
a
2 4
O
2 4
O
ꢂ
nanocomposites and investigated its photocatalytic performance
toward RhB under visible light.
obtained samples were dried in a baking oven at 60 C for 24 h
before further characterization. Weight contents of ZnFe
2 4
O in the
composite photocatalysts were 1, 3 and 5 wt% respectively.
2.4. Characterization
2
. Experimental
The X-ray diffraction (XRD) patterns of the samples was char-
acterized using a Rigaku D/Max-2500PC X-ray diffractometer
Rigaku Co., Japan) by Cu K radiation (
acceleration voltage of 40 kV and the applied current of 100 mA.
The purity of prepared nanocomposites was investigated by energy
dispersive X-ray spectra (EDX) analysis. Surface morphology and
distribution of samples were performed by TEM (JEM-2100) and
SEM (JSM-6360LA, JEOL, Japan). Fourier transform infrared spectra
2
.1. Preparation of BiOI
(
a
l
¼ 1.54056 Å) with the
All the reagents were of analytical grade and used without
further purification. The BiOI sample was prepared by solvothermal
method. Firstly, 1.94 g Bi(NO $5H O and 0.83 g KI were dissolved
in 80 mL of ethylene glycol respectively to obtain two kinds of
solutions. Then, solution contained with KI was added into the
solution with Bi(NO ) slowly, and the mixture was sonicated and
3 3
stirring for 10 min to get a stable solution. Then the stable solution
was transferred into a Teflon-lined stainless steel autoclave. After a
solvothermal reaction of 160 C for 6 h, it was cooled down to room
temperature naturally. After three times wash with distilled water
and absolute ethanol, the samples were dried at 120 C for 8 h for
3
)
3
2
(FT-IR)with the wavelength range from 400 to 4000cm-1 were
obtained by Nicolet (PROT eꢀ G eꢀ 460) spectrometer. The Raman
spectra were conducted employing a LabRAM XploRA Raman
spectrometer (Horiba Jobin Yvon, France) equipped with a laser
(532 nm) focused on a spot about 3 nm diameter. The UVevis
diffuse reflectance spectroscopy (DRS) spectra were recorded by a
UVevis scanning spectrophotometer (Shimadzu UV-2550). The
photoluminescence (PL) spectra of samples were studied by an F-
ꢂ
ꢂ
the next step.
280 spectrometer.
2 4
2.2. Preparation of ZnFe O
2
.5. Photocatalytic experiments
The ZnFe
method. After 1 mmol Zn(NO
dissolved in 30 ml distilled water, 5 ml of NaOH (3 mol/L) solution
was added to the mixture. Then, the mixture was stirred at room
temperature for 10 min and transferred into a 50 mL Teflon-lined
2
O
4
nanoparticles was prepared by solvothermal
2
$6H O and 2 mmol FeCl $6H O
3
)
2
2
3
The photocatalytic activities of the ZnFe
2 4
O /BiOI nano-
composites were evaluated by the photodegradation of rhodamine
B (10 mg/L), methylene blue (15 mg/L) and methylene orange
(10 mg/L) under visible light irradiation. Firstly, 25 mg of photo-
stainless autoclave. The autoclave was sealed and maintained at
catalyst was added into 50 mL of pollutants aqueous solution and
the suspensions were magnetically stirred in a dark condition for
3
ꢂ
1
80 C for 15 h and then cooled down it at room temperature. After
that, the product was filtered, washed with distilled water and
absolute ethanol for three times, and dried at 120 C for 8 h.
0 min to attain adsorption/desorption equilibrium between pol-
ꢂ
lutants and photocatalysts. After sampled for the first time and
ultrasonicated to disperse samples, the suspensions were irradiated
Fig. 1. XRD patterns of pure ZnFe
2
O
4
, BiOI and ZnFe
2
O
4
/BiOI nanocomposites.
2 4
Fig. 2. EDX spectra for the 3%-ZnFe O /BiOI samples.

Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
355
by a 1 kw Xe lamp with continuously stirred by a dynamoelectric
samples. All diffraction peaks in XRD patterns for ZnFe
2
O
4
/BiOI
stirrer. In the process, the suspension containing the sample pow-
der and pollutants was sampled at a regular interval times (20 min).
composites are corresponding to both ZnFe
the existence of two components.
2 4
O and BiOI, reflecting
2 4
The ZnFe O /BiOI nanocomposites were taken from the solution
with centrifugation at 10000 rpm for 5 min. The clarified solution
was analyzed by a UV759 UVevis spectrometer (Shanghai Precision
3.2. EDX spectra
&
Scientific Instrument Co., Ltd., China) to obtain the absorbance of
the target contaminant at its maximum absorption wavelength.
The decomposition efficiency of the target contaminant was
2 4
The purity of prepared ZnFe O /BiOI nanocomposites was
investigated by energy dispersive X-ray spectra (EDX) analysis and
the results are presented in Fig. 2. The ZnFe /BiOI photocatalyst is
calculated by D ¼ ð1 ꢁ C
t
=C Þ ꢃ 100%, where C
o t
and C are the
0
2 4
O
equilibrium concentration of the target contaminant before and
after visible light irradiation, respectively.
consists of Zn, Fe, O, Bi and I elements. As showed in the spectra, the
peaks were ascribed to the elements of photocatalyst and no other
elements can be found. It indicates the high purity of ZnFe
2 4
O /BiOI
samples.
3
. Results and discussion
3.1. XRD patterns
3.3. FE-SEM images
XRD (X-ray powder diffraction) technique was used to evaluate
the phase composition of the synthesized samples. Fig. 1 shows
XRD patterns of pure ZnFe , pure BiOI and ZnFe /BiOI photo-
catalysts. The original ZnFe sample shows strong peaks centered
at 35.3 and 62.4 , corresponding to the (311) and (440) crystal
planes of cubic ZnFe (JCPDS No. 82-1049), respectively [34]. The
BiOI exhibited main diffraction peaks centered at 29.73 , 31.72 ,
The surface morphologies and microstructures of pure ZnFe
BiOI and ZnFe /BiOI samples were examined by scanning elec-
tron microscopy. As can be seen in Fig. 3a, pure ZnFe nano-
particles are graininess. The shape of pure BiOI is spheres with
polyporous and loosened surface, which size varied from 3.5 to
2 4
O ,
2
O
4
2
O
4
2 4
O
2
O
4
2 4
O
ꢂ
ꢂ
2 4
O
ꢂ
ꢂ
9.0
mm (Fig. 3b). Fig. 3c and d shows the FE-SEM images of the 3%-
ꢂ
ꢂ
4
5.46 and 55.23 , conforming to the (012), (110), (014) and (212) of
2
ZnFe O
4
/BiOI nanocomposites. The ZnFe particles were tightly
2 4
O
the BiOI (JCPDS No. 10-0445), respectively. These two diffraction
adhered attached on the surface of BiOI which was considered to
advance the efficiency of charge carrier transfer. In brief, the
ZnFe O nanoparticles were introduced on the surface of BiOI
2 4
successfully and it was thought to be favorable for photocatalytic
performance.
ꢂ
ꢂ
peaks of the compound at 35.3 and 62.4 increased in the intensity
gradually with the increasing content of ZnFe , and finally can be
observed obviously in the curve of 5%-ZnFe /BiOI. No other peaks
were found, indicating the high purity and crystallization of the
2
O
4
2
O
4
2 4 2 4
Fig. 3. FE-SEM images of the pure ZnFe O (a), BiOI (b) and as-prepared 3%-ZnFe O /BiOI nanocomposites (c, d).

356
Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
BiOI
ZnFe O4
2
1%-ZnFe O /BiOI
2 4
3%-ZnFe O /BiOI
2
4
5%-ZnFe O /BiOI
2 4
300
400
500
600
700
800
Wavelength/nm
Fig. 5. UVevis absorption spectra of pure ZnFe
2
O
4
,
BiOI and ZnFe
2
O
4
/BiOI
nanocomposites.
180
170
160
150
140
130
120
110
100
ZnFe O
2
4
5%-ZnFe O /BiOI
2 4
3%-ZnFe O /BiOI
2 4
1%-ZnFe O /BiOI
2 4
BiOI
90
80
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber(cm
)
2 4 2 4
Fig. 6. FT-IR spectra of pure ZnFe O , BiOI and ZnFe O /BiOI nanocomposites.
Fig. 4. TEM images of the pure ZnFe
composites (c).
2
O
4
(a), BiOI (b) and 3%-ZnFe
2
O
4
/BiOI nao-
absorption in the range of wavelength around 570 nm. With the
increase of ZnFe (from 1% to 5%), the spectra of ZnFe /BiOI
exhibited a blue shift in the visible-light region compared with
pure BiOI, which could be ascribed to the ZnFe 4, a sensitizer to
extend the optical response, assembled on the surface of BiOI. The
enhanced light absorption may accelerate the production of
electron-hole pairs, thus improve the photocatalytic performance.
2
O
4
2 4
O
2
O
3.4. TEM images
The transmission electron microscope (TEM) images of the pure
ZnFe , BiOI and ZnFe /BiOI nanocomposites are shown in
2 4
Fig. 4. As presented in Fig. 4a, morphology of the pure ZnFe O
2
O
4
2 4
O
The result indicated that ZnFe
abilities than pure samples to utilize visible-light which benefit
from the synergetic effects of the ZnFe and BiOI.
2 4
O /BiOI nanocomposites have better
nanoparticles is composed of a small cubic structure with an
average size of about 10 nm, which is in accordance with previously
reported results. It can be seen clearly from Fig. 4b that the sphere-
like structure of BiOI has a loosened surface. Fig. 4c shows that the
2 4
O
3.6. FTIR analysis
ZnFe
mation of ZnFe
2
O
4
microspheres lie on the BiOI surface, indicating the for-
/BiOI heterostructures.
2 4
O
Fig. 6 shows the FTIR spectra of ZnFe
2
O
4
2 4
, BiOI and ZnFe O /BiOI
heterojunctions with different content of ZnFe
spectra of BiOI, the peak at about 3450 cm is attributable to the
2 4
O
. In the FTIR
ꢁ1
3.5. UVevis diffuse reflectance spectra
stretching vibrations of eOH of absorbed water molecules [35],
ꢁ
1
The absorption of BiOI, ZnFe
composites were characterized by UVevis diffuse reflectance
spectrometer (Fig. 5). It was found that BiOI showed strong
2
O
4
and ZnFe
2
O
4
/BiOI nano-
while those at 1587 cm assigned to deformation vibration of
physisorbed or chemisorbed water molecules [36,37]. Moreover,
ꢁ
1
the band at about 500 cm for both samples of pure BiOI and

Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
357
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
6000
(a)
BiOI
ZnFe O4
2
t=0
5000
1
%-ZnFe O /BiOI
t=10min
t=20min
t=30min
t=40min
t=50min
t=60min
t=70min
2 4
3%-ZnFe O /BiOI
2
4
4000
3000
2000
1000
0
5
%-ZnFe O /BiOI
2
4
400
450
500
550
600
650
700
750
500
550
600
650
700
Wavelength/nm
Wavelength/nm
Fig. 8. PL spectra of pure ZnFe
2
O
4
, BiOI and series of ZnFe
2
O
4
/BiOI nanocomposites.
1
0
0
0
0
0
.0
(
b)
2 4 2 4
ZnFe O of the ZnFe O /BiOI nanocomposites [38]. All the absorp-
.8
.6
.4
.2
.0
2 4
tion indicated that ZnFe O were deposited on the surface of BiOI
successfully.
3
.7. Photocatalytic activity of ZnFe
2
O
4
/BiOI nanocomposites
/BiOI samples was tested
The photocatalytic activity of ZnFe
2 4
O
by the degradation of RhB under visible-light irradiation at room
temperature. RhB is very difficult to be decomposed for its chem-
ically stability and it shows a maximum adsorption peak at 553 nm
BiOI
Zn Fe O
2
2
4
1
3
5
%-Zn Fe O /BiOI
2 2 4
[39]. Fig. 7a shows the changes in the absorbance profiles of RhB
%-Zn Fe O /BiOI
2
2
4
solution in the presence of ZnFe /BiOI nanocomposites under
2 4
O
%-Zn Fe O /BiOI
2
2
4
visible-light irradiation. The spectrum at t ¼ 0 min was sampled
from the solution of RhB after 30 min stirring in a dark condition.
With the time of irradiation increasing, the peaks at 553 nm were
reduced quickly. At the same time, the blue shift for absorption
maximum at 553 nm can be seen obviously which can be related to
de-ethylation process forwarded by the destruction of the skeleton
0
10
20
30
40
50
60
70
Time/min
2.5
(c)
BiOI
k=0.0049
k=0.00063
%-Zn Fe O /BiOI k=0.024
[
40,41].
Fig. 7b displays the photodegradation behavior of RhB in the
presence of pure BiOI, pure ZnFe and ZnFe /BiOI samples. For
the pure ZnFe and pure BiOI, very little photodegradation of RhB
can be found in 70 min. As expected, the ZnFe /BiOI nano-
composites exhibited efficient photocatalytic activity under visible-
light irradiation. When the content of ZnFe located at 3%, the
best photocatalytic activity of ZnFe /BiOI can be obtained and the
degradation of RhB is up to 86.54% in 70 min. With increasing of
Zn Fe O
2
2
4
2.0
1.5
1.0
0.5
0.0
1
3
2
2
4
2
O
4
2 4
O
%-Zn Fe O /BiOI k=0.027
2
2
4
2 4
O
5%-Zn Fe O /BiOI k=0.012
2 2 4
2 4
O
2 4
O
2 4
O
ZnFe
ZnFe
2
O
O
4
4
content from 1% to 5%, the photocatalytic activity of
/BiOI nanocomposites first increased and then decreased. It
2
2 4
suggested that the optimum content of ZnFe O could be well
dispersed on the surface of BiOI and could improve the photo-
catalytic activity.
0
10
20
30
40
50
60
70
Fig. 7c reveals that the photocatalytic degradation of RhB on
Time/min
different photocatalysts fits pseudo-first-order kinetics, ln(C
) ¼ kappt, where kapp is the rate constant and t is the irradiation
time. As can be seen, ln(C /C ) exhibits a well linear relationship
with irradiation time. It indicated that the photocatalytic reaction
belongs to the pseudo-first-order reaction. Furthermore, the results
0
/
C
t
Fig. 7. (a) Absorption spectra of RhB aqueous solutions taken at different photo-
catalytic degradation times using 3%-ZnFe /BiOI nanocomposites; (b) photo-
degradation of RhB with pure ZnFe , BiOI and ZnFe /BiOI nanocomposites under
0
t
2 4
O
2
O
4
2 4
O
visible-light irradiation; (c) kinetic data for the degradation of RhB in the presence of
different photocatalysts.
implied that k values for all ZnFe
higher than pure ZnFe and pure BiOI. Among these composites,
the 3%-ZnFe /BiOI (0.027 min ) displayed the maximum value
of the degradation rate constant, which was superior to those of the
2 4
O /BiOI samples were much
2 4
O
ꢁ1
2 4
O
ZnFe
2 4
O /BiOI nanocomposites all can be ascribed to the vibration of
the BieO. Compared with the spectrum of pure BiOI, a weak ab-
ꢁ1 ꢁ1
2 4 2 4
O (0.0063 min ), BiOI (0.0049 min ), 1%-ZnFe O /
pure ZnFe
BiOI (0.024 min ) and 5%-ZnFe
ꢁ1
sorption at 489 cm
can be found which resulted from the
ꢁ1
ꢁ1
2 4
O /BiOI (0.012 min ). In a word,

358
Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
0.030
0.025
0.020
0.015
0.010
0.005
0.000
no scavenger
1.0
no scavenger
EDTA
BQ
0.8
0.6
0.4
IPA
0.2
0.0
0
10
20
30
40
50
60
70
Time/min IPA
EDTA
BQ
Scavengens
Fig. 9. Effects of different scavengers on the degradation of RhB in the presence of 3%-ZnFe2O4/BiOI under visible-light irradiation.
2 4
ZnFe O /BiOI samples, PL has been applied to reveal the efficiency
of charge carrier transfer, trapping and separation of the photoin-
duced electrons and holes in semiconductors. Fig. 8 shows the PL
2 4 2 4
spectra of pure ZnFe O , BiOI and ZnFe O /BiOI nanocomposites
with an excitation wavelength of 300 nm. Irradiation caused
electron-hole pair recombination of photocatalysts. At the same
time, photons were radiated which lead to photoluminescence. A
weaker intensity of peak indicated a lower recombination proba-
bility of photogenerated charge carries. It could be seen that all
ZnFe
peak position, while these samples exhibited different intensities.
Among them, 3%-ZnFe /BiOI has the lowest intensity, suggesting
2 4
O /BiOI nanocomposites have the similar peak shape and
2 4
O
the sample possessed the highest separation efficiency of photo-
generated electrons and holes. The result indicated that the doping
of ZnFe O is propitious for suppressing the recombination process,
2 4
lead to weak recombination of electron and hole pairs as well as
inhibiting high photon efficiency in the samples. It also can be
2 4
referred to that the 3%-ZnFe O /BiOI has a higher photocatalytic
activity than others, which is in accord with the result of photo-
catalytic experiment.
2 4
Fig. 10. Proposed mechanism for the photodegradation of RhB over ZnFe O /BiOI
nanocomposites under visibl-light irradiation.
3.9. Photocatalytic mechanism
the loading amount of ZnFe
2
O
4
has an obvious effect on the pho-
In order to ascertain the main active species for the photo-
tocatalytic activity and it could greatly enhance the photocatalytic
degradation of RhB in this study, some sacrificial agents, such as
efficiency of BiOI under visible-light irradiation.
iso-propyl alcohol (IPA), disodium ethylenediaminetetraacetate
(
EDTA) and 1,4-benzoquinone (BQ) were used as the scavengers of
ꢁ
the hydroxyl radical ($OH), holes (hþ) and superoxide radical ($O
2
),
3.8. PL analysis
respectively. As shown in Fig. 9, 1,4-benzoquinone seriously sup-
pressed the degradation of RhB while iso-propyl alcohol and
To understand the cause of enhanced photocatalytic activity of
disodium ethylenediaminetetraacetate also caused a distinct

Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
359
3
2
2
1
1
00
50
00
50
00
(
a)
5
0
0
BiOI
BiOI
BiOI
3%-ZnFe O /BiOI
ZnFe O
2 4
2
4
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
(
b)
3%-ZnFe O /BiOI
ZnFe O
2 4
2
4
9
0
5
0
5
0
5
0
(
c)
7
6
4
3
1
3%-ZnFe O /BiOI
2
4
ZnFe O
2 4
2 4 2 4
Fig. 11. The degradation rate constant of RhB (a), MB (b) and MO (c) over the 3%-ZnFe O /BiOI, pure ZnFe O and BiOI nanocomposites under visible-light irradiation.

360
Y. Zhou et al. / Journal of Alloys and Compounds 696 (2017) 353e361
suppression, indicating that hydroxyl radical, holes and superoxide
radical were involved in the degradation of RhB while superoxide
radical have the most crucial role. On the basis of the band gap
degrade a wide variety of pollutants, degradation of RhB, MB and
MO was carried out and compared with the pure counterparts
[49,50]. As can be seen clearly from Fig. 11, photocatalytic activity of
structure of ZnFe
mechanism for the dye photodegradation over ZnFe
nanocomposites is proposed as follows:
2
O
4
/BiOI and the effects of scavengers, a possible
the 3%-ZnFe
ZnFe and BiOI in three kinds of degradation under visible-light
irradiation. This increase can be ascribed to the establishment of
heterojunctions in the ZnFe /BiOI nanocomposites. It is believed
that the heterojunction not only can accelerate the separation ef-
ficiency of charge carriers, but also increase the absorption of
2 4
O /BiOI nanocomposite is much higher than pure
2
O
4
/BiOI
2 4
O
2 4
O
ZnFe
2
O
4
4
/BiOI þ hv / ZnFe
2
O
4
/BiOI (h þ eꢁ)
þ
(1)
(2)
(3)
(4)
(5)
ꢁ
ꢀ
ꢁ
2
ZnFe
2
O
/BiOI (e ) þ O
2
/ ZnFe
2
O
4
/BiOI þ O
2 4
visible light. Hence, ZnFe O /BiOI photocatalyst may have wide-
spread application in the degradation of various pollutants.
2ꢀ
O
ꢁ
þ 2H
ꢁ
O
þ O
2
2
O / 2OH þ H
2
2
2
4. Conclusion
ꢁ
ꢀ
ꢁ
H
2
O
2
þ ZnFe
2
O
4
/BiOI (e ) / OH þ OH þ ZnFe
2 4
O /BiOI
2 4
In summary, the ZnFe O /BiOI nanocomposites were success-
ꢀ
ꢁ
, ꢀOH, h / degradation products
þ
RhB þ O
2
fully prepared by a simple and economical method. SEM and TEM
analysis indicated ZnFe were evenly dispersed on the surface of
2 4
O
ꢁ
CB electrons (e ) were accumulated on the surface of ZnFe
BiOI. Some of them are then scavenged by oxygen on the surface of
2
O
4
/
sphere-like BiOI and the heterostructures were formed, which had
outstanding photocatalytic activity in degradation of RhB under
visible-light irradiation. Among these as-prepared nano-
ꢁ
the photocatalyst to form superoxide radicals ($O
react with H O to form H
to form hydroxyl radical ($OH). These three active species
were involved in the photocatalytic process.
2
), which can
2
2 2
O . Some of them were scavenged by the
2 4
composites, the 3%-ZnFe O /BiOI exhibited the highest photo-
2 2
H O
catalytic efficiency which attributed to the efficient separation of
photogenerated electron and holes. This study is believed to be
useful for developing effective photocatalysts in pollutant
decomposition.
3.10. Possible photocatalytic activity enhancement mechanism
The efficiency of photocatalytic activity is controlled by the four
Acknowledgments
steps [42e44]: (1) the semiconductor catalyst photoabsorption, (2)
the generation of photogenerated electron and holes, (3) the charge
carriers transfer, (4) the charge carriers utilization. In this study, the
enhancement of photocatalytic activity mainly caused by the effi-
cient generation, separation and electron-hole pairs transfer, which
depend on the band structure of the photocatalyst [45]. Band gap
This work was financially supported by Natural Science Foun-
dation of Jiangsu Province, China (BK20150259) and Natural Sci-
ence Foundation of Changzhou City, China (CJ20140053).
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