Novel magnetically separable ZnO/AgBr/Fe3O4/Ag3VO4 nanocomposites with tandem n-n heterojunctions as highly efficient visible-light-driven photocatalysts

Article abstract of DOI:10.1039/c5ra21356h

In the present work, novel magnetically separable ZnO/AgBr/Fe₃O₄/Ag₃VO₄ nanocomposites with different weight percentages of Ag₃VO₄ were successfully prepared. The preparation method is a fa

Full text of DOI:10.1039/c5ra21356h

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Novel magnetically separable ZnO/AgBr/Fe O /Ag VO nanocomposites with tandem
3
4
3
4
n-n heterojunctions as highly efficient visible-light-driven photocatalysts
Maryam ShekoftehꢀGohari, Aziz HabibiꢀYangjeh*
Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box
1
79, Ardabil, Iran.
Abstract
In the present work, novel magnetically separable ZnO/AgBr/Fe
with different weight percents of Ag VO were successfully prepared. The preparation method
3
4 3 4
O /Ag VO nanocomposites
3
4
is a facile large–scale method with a low temperature of 96 °C. Characterization techniques of
XRD, EDX, SEM, TEM, UV–vis DRS, FTꢀIR, PL, and VSM were applied to study
microstructure, purity, morphology, spectroscopic, and magnetic properties of the resultant
samples. Photocatalytic activity was evaluated by degradation of rhodamine B under visibleꢀ
light irradiation. The results indicated that the nanocomposite with 10% of Ag VO has the
3
4
best activity. Photocatalytic activity of this nanocomposite is about 2.8 and 3.5ꢀfold higher
than those of the ZnO/AgBr/Fe and ZnO/Ag VO /Fe nanocomposites, respectively.
The highly enhanced activity was attributed to formation of tandem nꢀn heterojunctions
between ZnO, AgBr, and Ag VO which suppresses recombination of the photogenerated
3
O
4
3
4
3 4
O
3
4
electronꢀhole pairs. Furthermore, the enhanced activity of the nanocomposite for degradation
of two more dye pollutants and a colorless pollutant was confirmed. Finally, the magnetic
photocatalyst was recycled using external magnetic field with reasonable activity after four
runs.
Keywords: ZnO/AgBr/Fe
3
O
4
/Ag
3
VO
4
; Magnetic photocatalyst; Visibleꢀlightꢀdriven;
Pollutants.
*
3
Corresponding author: Aziz HabibiꢀYangjeh, Tel: +98 045 33514702; Fax: +98 045
3514701; Email: ahabibi@uma.ac.ir
1

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Introduction
It is believed that heterogeneous photocatalytic processes, as green and costꢀeffective
technology, have potential to address different challenges in the field of environmental
pollution, energy shortage, and global warming facing the human being in the present
1
‒3
century. In order to utilize the solar energy as source of the photocatalytic processes, it is
essential that visibleꢀlightꢀdriven photocatalysts with reasonable activity are provided.
However, traditional photocatalysts such as TiO
Consequently, they can mainly absorb UV fraction of the solar radiation, accounting for about
% of the solar energy, which leads to their poor photocatalytic activities under visibleꢀlight
2
and ZnO have wide band gaps.
4
4‒6
irradiation. Moreover, owing to their fast recombination of electronꢀhole pairs, they have
low photocatalytic activity. Hence, preparation of highly enhanced visibleꢀlightꢀdriven
photocatalysts based on TiO
2
and ZnO has been the subject of many articles and at the
7‒20
present it is an interesting and attractive topic in the field of photocatalysis research.
In order to increase visibleꢀlight photocatalytic activity, two strategies can be used. The first
one is enabling photocatalysts to absorb more energy from the solar irradiation and
subsequently produce more photogenerated electron−hole pairs. The second one is preventing
the photogenerated charge carriers from undesirable recombination, resulting in enhancement
21‒24
of electron−hole separation efficiency.
However, finding oneꢀcomponent photocatalyst
with considerable activity under visibleꢀlight irradiation is almost impossible. Hence, we have
to design multiꢀcomponent photocatalysts with wide visibleꢀlight absorption range, low
recombination of charge carriers, nonꢀselective degradation ability for different pollutants,
and reasonable stability. Recently, it was found that by combining two nꢀtype
25‒28
semiconductors, nꢀn heterojunctions could be formed at the interface of them.
Due to
formation of internal electric field at the interface of these semiconductors, separation of the
2

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photogenerated electronꢀhole pairs could be greatly enhanced. Being as nꢀtype
semiconductors, by combining ZnO, AgBr, and Ag VO with each other, nꢀn heterojunctions
3
4
could be formed, resulting in enhanced photocatalytic activity.
On the other hand, after using photocatalysts in the reaction system, they cannot be entirely
recycled, leading to generation of secondary pollution. Consequently, a major problem in
largeꢀscale utilization of photocatalysts is removal of them from the treated solutions.
Magnetic photocatalysts, as new generation of photocatalysts, could tackle the above
2
9
mentioned problem. However, to the best of our knowledge, there is no report about the
preparation and photocatalytic activity of the magnetic ZnO/AgBr/Fe O /Ag VO
3
4
3
4
nanocomposites with tandem n‒n heterojunctions.
Based on the above considerations, we report novel magnetically separable
ZnO/AgBr/Fe /Ag VO nanocomposites with different weight percents of Ag VO
4
3
O
4
3
4
3
prepared using a facile refluxing method at a low temperature of 96 °C. The microstructure,
purity, morphology, spectroscopic, and magnetic properties of the prepared samples were
studied using Xꢀray diffraction (XRD), energy dispersive analysis of Xꢀrays (EDX), scanning
electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse
reflectance spectroscopy (UVꢀvis DRS), Fourier transformꢀinfrared spectroscopy (FTꢀIR),
photoluminescence spectroscopy (PL), and vibrating sample magnetometry (VSM)
techniques. Photocatalytic activity of the nanocomposites was investigated by degradations of
rhodamine B (RhB), methylene blue (MB), methyl orange (MO), and phenol under visibleꢀ
light irradiation. To further elucidate the photocatalytic degradation mechanism, a series of
scavengers for the reactive species were employed. It was demonstrated that weight percent
3 4
of Ag VO , refluxing time, and calcination temperature have considerable influence on the
degradation reaction. Finally, the nanocomposite was magnetically separated from the treated
solution after four successive cycles.
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Experimental
Materials
Zinc nitrate (Zn(NO
3
2
) .4H
2
3 2
O), ferric chloride (FeCl .6H O), ferrous chloride
2 2
(FeCl .4H O), silver nitrate, sodium bromide, ammoniumꢀmetavanadate, ammonia, and
sodium hydroxide were purchased from Loba chemie. RhB, MO, MB, 2ꢀpropanol,
ammonium oxalate, benzoquinone, and absolute ethanol were obtained from Merck and
employed without further purification. Deionized water was used throughout this study.
Instruments
The XRD patterns were recorded by a Philips Xpert Xꢀray diffractometer with Cu Kα
radiation (λ= 0.15406 nm), employing scanning rate of 0.04˚/sec in the 2θ range from 20˚ to
80˚. Surface morphology and distribution of particles were studied by LEO 1430VP SEM,
using an accelerating voltage of 15 kV. The purity and elemental analysis of the products
were obtained by EDX on the same SEM instrument. For SEM and EDX experiments,
samples mounted on an aluminum support using a double adhesive tape coated with a thin
layer of gold. The TEM investigations were performed by a ZeissꢀEM10C instrument with an
acceleration voltage of 80 kV. The DRS spectra were recorded by a Scinco 4100 apparatus.
The FTꢀIR spectra were obtained by a Perkin Elmer Spectrum RX I apparatus. The PL spectra
of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer
with an excitation wavelength of 300 nm. The conditions were fixed in order to compare the
PL intensities. UVꢀvis spectra for the degradation reaction were studied using a Cecile 9000
spectrophotometer. Magnetic properties of the samples were obtained using an alternating
gradient force magnetometer (model AGFM, Iran). The ultrasound radiation was performed
4

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using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz).
The pH of solutions was measured using a Metrohm digital pH meter of model 691.
Preparation of the nanocomposites
Nanoparticles of Fe
detailed procedure was described elsewhere. For preparation of the ZnO/AgBr/Fe
nanocomposite, 0.128 g of Fe nanoparticles was dispersed into 150 mL of water by
3 4
O were prepared by chemical coꢀprecipitation method that the
30
3
O
4
3
O
4
ultrasonic irradiation for 30 min in a cylindrical Pyrex reactor provided with water circulation
arrangement to maintain its temperature at 25 °C. Then, 1.305 g of zinc nitrate and 0.236 g of
3 4
silver nitrate were added to the formed dark brown suspension of Fe O with mechanical
stirring continued for 30 min. Afterwards, aqueous solution of sodium hydroxide (5 M) was
slowly added to the solution under stirring until pH of the solution reached 10. Then, aqueous
solution of potassium bromide (0.319 g dissolved in 20 mL of water) was dropwise added to
the dark brown suspension under stirring. Afterwards, the formed orange suspension was
stirred for 60 min and refluxed at about 96 °C for 60 min. The light brown product was
centrifuged to get the precipitate and washed two times with water and ethanol and the
product was magnetically separated. The precipitate was dried in an oven at 70 °C for 24 h.
In a typical procedure for preparation of the ZnO/AgBr/Fe
nanocomposite, where %10 is the weight percent of Ag VO , 0.450 g of ZnO/AgBr/Fe O
4
3
O
4
/Ag
3
VO
4
(10%)
3
4
3
nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 10 min.
Then, silver nitrate (0.058 g) was dissolved in the suspension and mechanically stirred for 60
min. After that, an aqueous solution of NaOH (5 M) was dropwise added into the solution
under stirring at room temperature until the pH of the solution reached to 10. Then, an
aqueous solution of ammonium metavanadate (0.013 g in 50 mL of water) was slowly added
5

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to the formed suspension. Finally the suspension was refluxed at 96 °C for 60 min. The
suspension was centrifuged to get the precipitate out and washed twice with water and
ethanol and dried in an oven at 60 °C for 24 h. The schematic diagram for preparation of the
nanocomposites can be illustrated in scheme 1.
[Please insert Scheme 1 about here]
Photocatalysis experiments
Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400
mL capacity. Temperature of the reactor was maintained at 25 °C using a water circulation
arrangement. The solution was mechanically stirred and continuously aerated by a pump to
provide oxygen and complete mixing of the reaction solution. An LED source with 50 W was
used as a visibleꢀlight source. The source was fitted on the top of the reactor. Prior to
illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL aqueous solution
ꢀ5
ꢀ5
ꢀ5
ꢀ5
of RhB (1×10 M), MB (1.3×10 M), MO (1.05×10 M), or phenol (5.0×10 M), was
continuously stirred in the dark for 60 min, to attain adsorption equilibrium. Samples were
taken from the reactor at regular intervals and the photocatalyst removed before analysis by
the spectrophotometer at 553, 664, 477, and 270 nm corresponding to the maximum
absorption wavelengths of RhB, MB, MO, and phenol, respectively.
Results and discussion
Figure 1 exhibits XRD patterns of the Fe
ZnO/Ag VO /Fe , and ZnO/AgBr/Fe /Ag VO nanocomposites with different weight
percents of Ag VO . For pure Fe , the diffraction peaks well match with the (220), (311),
400), (331), (422), (333), and (531) planes of the faceꢀcentered cubic structure (JCPDS file
3 4 3 4 3 4
O , ZnO/Fe O , ZnO/AgBr/Fe O ,
3
4
3
O
4
3
O
4
3
4
3
4
3 4
O
(
6

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3
1
number of 75ꢀ1610). In the case of the ZnO/Fe
ascribed to faceꢀcentered cubic Fe and wurtzite hexagonal crystalline ZnO (JCPDS file
number of 65ꢀ3411). For the ZnO/AgBr/Fe and ZnO/Ag VO /Fe nanocomposites,
3 4
O nanocomposite, the diffraction peaks are
3
O
4
3
2
3
O
4
3
4
3 4
O
the diffraction patterns verify coꢀexistence of all counterparts of the nanocomposites.
Furthermore, in the ZnO/AgBr/Fe O /Ag VO nanocomposites, diffraction peaks of Ag VO
4
3
4
3
4
3
33
are clearly observed that well matched with its monoclinic phase (JCPDS No. 43–0542).
Please insert Fig. 1 about here]
[
Purity of the samples was verified by EDX spectra and the results for the ZnO/Fe
3 4
O ,
ZnO/AgBr/Fe O , ZnO/Ag VO /Fe O , and ZnO/AgBr/Fe O /Ag VO (10%) samples are
3
4
3
4
3
4
3
4
3
4
shown in Fig. 2a. It is evident that the samples have reasonable purity and all of the peaks are
simply ascribed to elements of the resultant samples. Other peaks in these spectra are related
to the elements applied for sputter coating of the samples on the EDX stage. Weight percents
of Zn, O, Ag, Br, Fe, and V elements in the ZnO/AgBr/Fe
are 37.5, 23.8, 17.0, 3.8, 11.4, and 6.4, respectively. Furthermore, EDX mapping was
employed to display the element distributions in the ZnO/AgBr/Fe /Ag VO (10%)
3 4 3 4
O /Ag VO (10%) nanocomposite
3
O
4
3
4
nanocomposite and the results are shown in Fig. 2bꢀh. It is evident that there is homogeneous
distribution for the elements. Hence, it was concluded that the counterparts of the
nanocomposite have uniformly combined to each other.
[Please insert Fig. 2 about here]
Morphology and junction between constituents of the ZnO/AgBr/Fe O /Ag VO (10%)
3
4
3
4
nanocomposite was investigated by SEM and TEM techniques and the results are shown in
Fig. 3. For this nanocomposite, particles of Fe , AgBr, and Ag VO are clearly seen on
O
3 4
3
4
ZnO sheets (Fig. 3a). Moreover, the TEM images of the nanocomposite confirm distribution
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of Fe
3
O
4
, AgBr, and Ag
3
VO
Please insert Fig. 3 about here]
It is believed that photocatalytic activity is closely related to extent of generation,
4
particles on the ZnO plates (Figs. 3b and 3c).
[
34
separation, and transfer of the photogenerated charge carriers. UVꢀvis spectra of
photocatalysts give useful information about extent of electronꢀhole pairs generated under the
light irradiation. Hence, UVꢀvis DRS spectra of the resultant samples were provided and the
results are shown in Fig. 4a. As can be seen, the ZnO/AgBr/Fe
3 4 3 4 3 4
O , ZnO/Ag VO /Fe O , and
ZnO/AgBr/Fe /Ag VO nanocomposites have intense absorption in the visibleꢀlight range.
3
O
4
3
4
Consequently, under visibleꢀlight irradiation, more electronꢀhole pairs are generated on the
nanocomposites, resulting in enhanced photocatalytic activity.
[
Please insert Fig. 4 about here]
To characterize structural properties of the resultant samples, FTꢀIR spectra of the Fe
ZnO/AgBr/Fe , ZnO/Ag VO /Fe , and ZnO/AgBr/Fe /Ag VO (10%) samples were
provided and the spectra are shown in Fig. 4b. For the all samples, the broad absorption bands
3 4
O ,
3
O
4
3
4
3
O
4
3
O
4
3
4
ꢀ1
around 3200ꢀ3400 cm are related to the OꢀH stretching vibration of adsorbed water. For the
ꢀ1
Fe
3 4
O nanoparticles, two characteristic peaks at 435 and 610 cm are related to the vibrations
29
ꢀ1
of Fe–O bond. For the samples with ZnO, the peak at 560 cm is ascribed to the stretching
2
8
vibration of the ZnꢀO bond. In the case of the samples containing Ag
AgꢀV and VꢀO bonds were appeared about at 770, 880, and 920 cm . For the
ZnO/AgBr/Fe /Ag VO (10%) nanocomposite, the peaks for FeꢀO, ZnꢀO, AgꢀV, and VꢀO
3 4
VO , the peaks for
ꢀ1 24
3
O
4
3
4
bonds are clearly seen in the wavenumbers similar to its counterparts.
It is well known that magnetic properties of photocatalysts affect on magnetic separability
of them. Hence, magnetization of the pure Fe
3 4 3 4 3 4
O nanoparticles and ZnO/AgBr/Fe O /Ag VO
(10%) nanocomposite were investigated and the results are shown in Fig. 5. As can be seen,
8

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saturated magnetization for the Fe
5.5 and 6.26 emu/g, respectively. Decrease of the magnetization of the nanocomposite is
simply related to the presence of the nonmagnetic ZnO, AgBr, and Ag VO particles along
with the magnetic Fe O nanoparticles. As can be seen in inset of Fig. 5, saturated
3 4
O nanoparticles and the nanocomposite at 8500 Oe are
5
3
4
3
4
magnetization of the nanocomposite is high enough to separate it from the solution after
decontamination processes.
[Please insert Fig. 5 about here]
To investigate visibleꢀlight photocatalytic activity of the prepared samples, RhB, a widely
used dye, was selected as a target pollutant. The degradation reaction was performed over the
Fe O , ZnO/Fe O , ZnO/AgBr/Fe O , ZnO/Ag VO /Fe O , and ZnO/AgBr/Fe O /Ag VO
3
4
3
4
3
4
3
4
3
4
3
4
3
4
nanocomposites with different weight percents of Ag
3
4
VO and the results are displayed in
Fig. 6a. Furthermore, photolysis of RhB under the visibleꢀlight irradiation (in absence of any
photocatalyst) was carried out and the results are shown in this figure. It is evident that RhB
has reasonable stability under the light irradiation for 360 min. It is observed that
photocatalytic activity of the ZnO/AgBr/Fe O /Ag VO nanocomposites with different
3
4
3
4
weight percents of Ag
and ZnO/Ag VO /Fe
ZnO/AgBr/Fe /Ag VO
3
VO
4
is higher than those of the Fe
3
O
4
, ZnO/Fe
3
O
4
, ZnO/AgBr/Fe
3 4
O ,
3
4
3
O
4
samples. Among the
prepared
samples, the
3
O
4
3
4
(10%) nanocomposite has the superior activity in the degradation
reaction. After the light irradiation for 105 min, about 21, 16, 69, 65, and 98% of RhB were
degraded over the Fe O , ZnO/Fe O , ZnO/AgBr/Fe O , ZnO/Ag VO /Fe O , and
3
4
3
4
3
4
3
4
3
4
ZnO/AgBr/Fe
absorbance versus wavelength for degradation of RhB over the ZnO/Fe
ZnO/AgBr/Fe , ZnO/Ag VO /Fe , and ZnO/AgBr/Fe /Ag VO (10%) samples at
3 4 3 4
O /Ag VO (10%) samples, respectively. Figures 6bꢀe shows plots of
3
4
O ,
3
O
4
3
4
3
O
4
3
O
4
3
4
various irradiation times. It is evident that intensity of the absorption peaks continuously
decreases and there are not any blue‐shifts for RhB absorption peaks centered at 554 nm.
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Therefore, it was concluded that degradations of RhB over the nanocomposite takes place by
35,36
aromatic ringꢀopening mechanism.
[
Please insert Fig. 6 about here]
In order to describe quantitatively photocatalytic activity of the resultant samples,
observed firstꢀorder rate constants (k ) of the degradation reaction over the samples was
obs
1
1
calculated using the slope of equation ln[RhB] = ln[RhB] − k t
,
in which [RhB] and
are concentrations of RhB in time of t and zero, respectively and the results are shown
in Fig. 7a. As can be seen, the degradation rate constant increases with weight percent of
Ag VO up to 10 and then decreases. The degradation rate constants of RhB over the Fe
ZnO/Fe
o
obs
[
RhB]
o
3
4
3 4
O ,
3
O
4
, ZnO/AgBr/Fe
3
O
4
3 4 3 4 3 4 3 4
, ZnO/Ag VO /Fe O , and ZnO/AgBr/Fe O /Ag VO (10%)
ꢀ4
ꢀ4
ꢀ4
ꢀ4
ꢀ4
ꢀ1
samples 4.92×10 , 4.74×10 , 102×10 , 82.6×10 , and 290×10 min , respectively. Hence,
activity of the nanocomposite is about 59, 61, 2.8, and 3.5ꢀfold larger than those of the Fe
3 4
O ,
ZnO/Fe , ZnO/AgBr/Fe , and ZnO/Ag VO /Fe samples, respectively. The emission
3
O
4
3
O
4
3
4
3 4
O
intensity in a PL spectrum is closely related to recombination of photogenerated electronꢀhole
pairs. Consequently, PL spectra have useful information about separation of photogenerated
charge carriers. To disclose the enhanced activity of the ZnO/AgBr/Fe O /Ag VO (10%)
3
4
3
4
nanocomposite relative to the ZnO/Fe
ZnO/AgBr/Fe /Ag VO (5%), and ZnO/AgBr/Fe
spectra were provided and the results are shown in Fig. 7b. As can be seen, compared with
the other samples, intensity of the PL spectrum for the ZnO/AgBr/Fe /Ag VO (10%)
3
O
4
,
ZnO/AgBr/Fe
3
O
4
,
3 4 3 4
ZnO/Ag VO /Fe O ,
3
O
4
3
4
3
O
4
/Ag VO (20%) samples, their PL
3
4
3
O
4
3
4
nanocomposite shows significant decrease, suggesting efficiently separation of the charge
carriers in this nanocomposite relative to the other samples. This result clearly indicates that
hybridization of the ZnO/Fe
3
O
4
with AgBr and Ag
3
VO
4
particles promotes the separation of
electron–hole pairs in the ZnO/AgBr/Fe
3
O
4
/Ag VO
3
4
nanocomposites. However, similar to
1
0

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3
7ꢀ39
many multiꢀcomponent photocatalysts
leads to a decrease in the degradation rate constant. This may be due to agglomeration of the
excess Ag VO particles on the nanocomposites, which reduces the heterojunctions between
Ag VO and ZnO/AgBr/Fe O counterparts, thereby separation of the charge carriers
3 4
, further increase in weight percent of Ag VO
3
4
3
4
3
4
decreases. Hence, the amount of Ag VO in the nanocomposites needs an optimum value.
3
4
[Please insert Fig. 7 about here]
To describe the enhanced activity of the nanocomposite, a hypothesized mechanism for
separation of the charge carriers is presented (Fig. 8). The conduction band (CB) and valence
band (CB) energies for ZnO, AgBr, and Ag VO at the point of zero charge were calculated
3
4
4
0
using Butler and Ginley model and the results are shown in Table 1. Being as nꢀtype
semiconductors, the Fermi levels for ZnO, AgBr, and Ag VO are close to their CB levels.
The Fermi level of ZnO is more negative than those of AgBr and Ag VO . Hence, after
contacting of ZnO with AgBr, electrons simply flow from ZnO to AgBr until their Fermi
3
4
3
4
41, 42
levels coincide.
As a result, negative and positive charges are accumulated on AgBr and
ZnO counterparts of the nanocomposite, respectively. Hence, an nꢀn heterojunction with the
same Fermi level is formed (Fig. 8). Separation of the charge carriers by nꢀn heterojunction
produces an internal electrostatic field directed from the ZnO to AgBr. Moreover, similar nꢀn
3 4
heterojunction is formed after contacting ZnO with Ag VO . In this region, electrons transfer
from ZnO to Ag VO . After reaching to equilibrium state, an electric field is formed at the
3
4
interface area, directed from ZnO to Ag VO . As a result, two nꢀn heterojunctions are formed
3
4
in the contacting region between ZnO with AgBr in one side and ZnO with Ag
3 4
VO in the
other side. Hence, tandem nꢀn heterojunctions between these nꢀtype semiconductors are
28
formed. Due to their narrow band gaps, under the visibleꢀlight irradiation, electronꢀhole
pairs are generated on AgBr and Ag VO counterparts of the ZnO/AgBr/Fe O /Ag VO
3
4
3
4
3
4
nanocomposites. The photogenerated electrons on the CB levels of AgBr and Ag VO simply
3
4
1
1

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transfer to the CB of ZnO with the help of the internal electrostatic field produced by tandem
nꢀn heterojunctions. On the other hand, the VB energies of AgBr and Ag VO are less
3
4
positive than that of the ZnO. Consequently, the holes cannot transfer from the VB levels of
AgBr and Ag VO to that of the ZnO. Finally, after transfer of the electrons, holes and
3
4
electrons are accumulated on the narrow band gap semiconductors and ZnO, respectively. As
a result, due to formation of tandem nꢀn heterojunctions between ZnO with AgBr and ZnO
3 4
with Ag VO , the charge carriers are spatially separated from each other, resulting in an
increased lifetime for the electronꢀhole pairs. Afterwards, the separated charge carriers in the
ZnO/AgBr/Fe O /Ag VO nanocomposites effectively migrate to surface of the photocatalyst
3
4
3
4
and participate in the degradation reactions, leading to enhanced activity relative to their
counterparts. It is noteworthy that separation of the photogenerated charge carriers in AgBr
and Ag
the CB levels of AgBr and Ag
photogenerated electrons cannot transfer from AgBr and Ag VO to ZnO, resulting in more
3
VO
4
could not take place without formation of tandem nꢀn heterojunctions; because,
3
VO are less negative than the CB of ZnO. Hence, the
4
3
4
recombination of the photogenerated electronꢀhole pairs and high intensity of the PL
spectrum. However, separation of the charge carriers in the nanocomposite was demonstrated
by the PL spectra. Hence, formation of the tandem nꢀn heterojunctions between the three nꢀ
type semiconductors is confirmed.
[Please insert Fig. 8 about here]
In order to explore the role of reactive species responsible for degradation of RhB over the
photocatalyst, the degradation reaction was conducted in the presence of a series of scavengers
and the results are shown in Fig. 9. In the present work, benzoquinone, ammonium oxalate, and
•
–
+
•
43
2
ꢀPrOH were used as scavengers of O
2
, h , and OH, respectively. It is evident that decrease
of the rate constant in the presence of scavengers is as benzoquinone > ammonium oxalate > 2ꢀ
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PrOH. Hence, it is concluded that the effect of superoxide ions and holes on the degradation
reaction is higher than that of hydroxyl radicals.
[
Please insert Fig. 9 about here]
It is well known that preparation time of photocatalysts can considerably affect morphology,
crystallinity, aggregation, and size of particles. Hence, the preparation time could remarkably
affect photocatalytic activity. To find optimum preparation time, the ZnO/AgBr/Fe
3 4 3 4
O /Ag VO
(10%) nanocomposite prepared by refluxing for 15, 30, 60, 120, and 240 minutes and the
results are shown in Fig. 10a. It can be seen that the degradation rate constant increases with
increasing the preparation time up to 60 min and then decreases. Decrease of the photocatalytic
activity for the samples prepared with higher refluxing time could be related to more
aggregation of the particles, resulting in decreased surface area of the photocatalyst.
[
Please insert Fig. 10 about here]
To investigate the effect of calcination temperature on the photocatalytic activity, the
ZnO/AgBr/Fe O /Ag VO (10%) nanocomposite was calcined at 200, 300, 400, 500, and 600
3
4
3
4
°C for 2 h and the degradation reactions were carried out over them and the rate constant are
shown in Fig. 10b. It can be seen that the degradation rate constant enhances with increasing
calcination temperature up to 500 °C and then promptly decreases. To explore structure of the
photocatalysts calcined at 500 and 600 °C, XRD patterns of the nanocomposite before and
after calcinations are shown in Fig. 11a. As can be seen, after calcination of the
nanocomposite at 500 °C, metallic silver is produced by decomposition of silver containing
4
4
semiconductors in the nanocomposite. The produced silver particles can increase transfer of
4
5, 46
electrons from the CB of Ag
3
VO
4
and AgBr to that of the ZnO
, resulting in more
separation of the charge carriers. However, activity of the nanocomposite calcined at 600 °C
is very lower than those of the other samples. Morphologies of the nanocomposite calcined at
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500 and 600 °C are compared in Figs. 11b and 11c. It is evident that there is a great changing
in morphology of the nanocomposite after calcination at 600 °C. As can be seen, after
calcination at 600 °C, plates of ZnO have tightly aggregated to each other and particles of
other counterparts of the nanocomposite have considerably separated from the ZnO plates,
resulting in decrease of the contact surface between counterparts of the nanocomposite.
Hence, the photogenerated electronꢀhole pairs could not be separated efficiently from each
other, leading to decrease the activity of the calcined photocatalyst at 600 °C.
[
Please insert Fig. 11 about here]
Generally, wastewaters contain mixtures of different pollutants. Hence, it is essential that
photocatalysts can degrade these pollutants nonꢀselectively. To show ability of the
nanocomposite calcined at 500 °C for degradation of different pollutants, degradations of
RhB, MB, MO, and phenol over the nanocomposite was studied under the visibleꢀlight
irradiation. Figure 12 shows plots of the degradation rate constants for RhB, MB, MO, and
phenol over the calcined ZnO/Fe O , ZnO/AgBr/Fe O , ZnO/Ag VO /Fe O , and
3
4
3
4
3
4
3
4
ZnO/AgBr/Fe
ZnO/AgBr/Fe
ZnO/AgBr/Fe
ZnO/AgBr/Fe
3
3
3
3
O
O
O
O
4
4
4
4
/Ag
3
3
VO
4
4
(10%) samples. It is evident that photocatalytic activity of the
(10%) nanocomposite is much higher than those of the ZnO/Fe
VO /Fe samples. Hence, it is confirmed that the magnetic
nanocomposites has considerable activity in degradations of three
/Ag
VO
3
4
O ,
, ZnO/Ag
3
4
3 4
O
/Ag VO
3
4
different dye pollutants and one colorless pollutant.
[
Please insert Fig. 12 about here]
For largeꢀscale utilization of photocatalysts, besides photocatalytic activity, stability is
another important factor from economical viewpoint. To investigate stability of the
photocatalyst, the degradation experiments were carried out and the results are shown in Fig.
13. In each cycle, after using the nanocomposite, it was magnetically separated, washed with
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ethanol and dried at 60 ºC for 24 h. It can be seen that the photocatalyst has reasonable
stability during the degradation reaction and magnetic recycling from the treated solution. To
investigate stability of the photocatalyst after fourth recycling, the XRD patterns of the
nanocomposite before and after using for four cycles are shown in Fig. 13b. It is evident that
very small amount of metallic silver is produced by photocorrosion of AgBr and Ag VO4. Hence,
3
it can be concluded that the magnetic nanocomposite has reasonable stability during the degradation
processes.
[
Please insert Fig. 13 about here]
Conclusions
Briefly, novel magnetically separable ZnO/AgBr/Fe
3
4 3 4
O /Ag VO nanocomposites, as highly
efficient visibleꢀlightꢀdriven photocatalysts, were successfully prepared by a facile largeꢀscale
procedure. Photocatalytic activity of the resultant samples was investigated by degradations
of three dye pollutants and phenol under visibleꢀlight irradiation. Among the prepared
samples, the ZnO/AgBr/Fe
Activity of this nanocomposite was about 3.5 and 2.8ꢀfold higher than those of the
ZnO/Ag VO /Fe and ZnO/AgBr/Fe nanocomposites in degradation of RhB,
3 4 3 4
O /Ag VO (10%) nanocomposite displayed the best activity.
3
4
3
O
4
3 4
O
respectively. The photocatalytic activity increases with increasing refluxing time up to 60 min
and then decreases. Moreover, the results showed that the nanocomposite calcined at 500 °C
has the best activity. Activity of the calcined nanocomposite in degradations of MB and MO
3 4
is about 4 and 10ꢀfold higher than that of the ZnO/AgBr/Fe O nanocomposite, respectively.
Hence, ability of the nanocomposite for efficiently degradation of different dye pollutants and
phenol is confirmed. To describe the enhanced activity, a hypothesized mechanism for
separation of the charge carriers was presented based on formation of tandem nꢀn
3 4
heterojunctions between ZnO with AgBr and ZnO with Ag VO . Finally, it was confirmed
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that the magnetic ZnO/AgBr/Fe
four successive runs.
3 4 3 4
O /Ag VO nanocomposites has considerable activity after
Acknowledgements
The authors wish to acknowledge University of Mohaghegh Ardabili, for financial support of
this work
.
Notes and references
1
K. Li, X. An, K.H. Park, M. Khraisheh and J. Tang, Catal. Today, 2014, 224, 3.
2
1
3
S. Dong, J. Feng, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun and J. Sun, RSC Advances, 2015, 5,
4610.
H. Ahmad, S.K. Kamarudin, L.J. Minggu and M. Kassim, Renewable Sustainable Energy
Rev., 2015, 43, 599.
M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop,
4
J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari and D.D. Dionysiou, Appl. Catal. B:
Environ., 2012, 125, 331.
5
4
6
7
8
9
1
1
S.ꢀM. Lam, J.ꢀC. Sin, A.Z. Abdullah and A.R. Mohamed, Desalin. Water Treatment, 2012,
1, 131.
M. Tahir and N.S. Amin, Energy Conver. Manag., 2013, 76, 194.
Y. Zang and R. Farnood, Appl. Catal. B: Environ., 2008, 79, 334.
K. Vignesh, A. Suganthi, M. Rajarajan and S.A. Sara, Powder Technol., 2012, 224, 331.
B. Krishnakumar, B. Subash and M. Swaminathan, Separ. Purif. Technol., 2012, 85, 35.
0 F. Xu, Y. Yuan, H. Han, D. Wu, Z. Gao and K. Jiang, Cryst. Eng. Comm., 2012, 14, 3615.
1 M. Pirhashemi and A. HabibiꢀYangjeh, Appl. Surf. Sci., 2013, 283, 1080.
1
6

Page 17 of 36
RSC Advances
View Article Online
DOI: 10.1039/C5RA21356H
12 C. An
,
W. Jiang
,
J. Wang
,
S. Wang
,
Z. Ma and Y. Li, Dalton Trans., 2013, 42 , 8796.
13 K. Dai, D. Li, L. Lu, Q. Liu, C. Liang, J. Lv and G. Zhu, Appl. Surf. Sci., 2014, 314, 864.
14 C. Dong, K.ꢀL.Wu, M.ꢀR. Li, L. Liu and X.ꢀW. Wei, Catal. Commun., 2014, 46, 32.
15 F. Chang, J. Zhang, Y. Xie, J. Chen, C. Li, J. Wang, J. Luo, B. Deng and X. Hu, Appl.
Surf. Sci., 2014, 311 , 574.
1
1
1
3
1
2
2
6 J. Yi, L. Huang, H. Wang, H. Yu and F. Peng, J. Hazard. Mater., 2015, 284, 207.
7 J. Ma, K. Wang, L. Li, T. Zhang, Y. Kong and S. Komarneni, Ceram. Int., 2015, 41, 2050.
8 S. ShakerꢀAgjekandy and A. HabibiꢀYangjeh, Mater. Sci. Semiconductor Process., 2015,
4, 74.
9 S. Yi, X. Yue, D. Xu, Z. Liu, F. Zhao, D. Wang and Y. Lin, New J. Chem., 2015, 39,
917.
0 C. Liu, C. Cao, X. Luo and S. Luo, J. Hazard. Mater., 2015, 285, 319.
2
2
2
2
2
1
Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar and J. He, Nanoscale, 2013, 5, 8326.
2 C.ꢀC. Shen, Q. Zhu, Z.ꢀW. Zhao, T. Wen, X. Wang and A.ꢀW. Xu, J. Mater. Chem. A,
015, 3, 14661.
3 W.ꢀS. Wang, H. Du, R.ꢀX. Wang, T. Wen and A.ꢀW. Xu, Nanoscale, 2013, 5, 3315.
4 Q. Zhu, W.ꢀS. Wang, L. Lin, G.ꢀQ. Gao, H.ꢀL. Guo, H. Du and A.ꢀW. Xu, J. Phys. Chem.
C, 2013, 117, 5894.
25 Y. Yan, H. Guan, S. Liu and R. Jiang, Ceram. Int., 2014, 40, 9095.
26 M. Xu, T. Ye, F. Dai, J. Yang, J. Shen, Q. He, W. Chen, N. Liang, J. Zai and X. Qian,
Chem. Sus. Chem., 2015, 8, 1218.
27 F. Kiantazh and A. HabibiꢀYangjeh, Mater. Sci. Semiconductor Process., 2015, 39, 671.
28 M. Pirhashemi and A. HabibiꢀYangjeh, Ceram. Int., 2015, 41, 14383.
29 S.ꢀQ. Liu, Environ. Chem. Lett., 2012, 10, 209.
1
7

RSC Advances
Page 18 of 36
View Article Online
DOI: 10.1039/C5RA21356H
3
3
3
3
0 R. Massart, Magnetics IEEE Trans., 1981, 17, 1247.
1 J. Safari and Z. Zarnegar, J. Mol. Struct.,2014, 1072, 53.
2 B.G. Mishra and G.R. Rao, J. Mol. Catal. A: Chem., 2006, 243, 204.
3 C.ꢀM. Huang, G.ꢀT. Pan, Y.ꢀC.M. Li, M.ꢀH. Li and T.C.ꢀK. Yang, Appl. Catal. A:
General, 2009, 358, 164.
3
4 H.H. Mohamed and D.W. Bahnemann, Appl. Catal. B: Environ., 2012, 128, 91.
5 X. Li and J. Ye, J. Phys. Chem. C, 2007, 111, 13109.
3
36 A. Martinezꢀdela Cruz and U.M. Garcia Perez, Mater. Res. Bull., 2010, 45, 135.
37 C. Xing, Z. Wu, D. Jiang and M. Chen, J. Colloid Interf. Sci., 2014, 433, 9.
38 S.ꢀW. Cao, X.ꢀF. Liu, Y.ꢀP. Yuan, Z.ꢀY. Zhang, Y.ꢀS. Liao, J. Fang, S.C.J. Loo, T.C.
Sum and C. Xue, Appl. Catal. B: Environ., 2014, 147, 940.
3
9 L. Zhang, X. Wang, Q. Nong, H. Lin, B. Teng, Y. Zhang, L. Zhao, T. Wu and Y. He,
Appl. Surf. Sci., 2015, 329, 143.
0 S.R. Morrison, Electrochemistry at semiconductor and oxidized metal electrode, Plenum,
New York, 1980.
4
4
1
4
1 C. Shifu, Z. Wei, L. Wei, Z. Huaye, Y. Xiaoling and C. Yinghao, J. Hazard. Mater., 2009,
72, 1415.
2 W. Wang, J. Wang, Z. Wang, X. Wei, L. Liu, Q. Ren, W. Gao, Y. Liang and H. Shi,
Dalton Trans., 2014, 43, 6735.
43 Z. He, Y. Shi, C. Gao, L. Wen, J. Chen and S. Song, J. Phys. Chem. C, 2015, 118, 389.
44 M. ShekoftehꢀGohari and A. HabibiꢀYangjeh, Solid State Sci., 2015, 48, 177.
45 C. Liu, C. Cao, X. Luo and S. Luo, J. Hazard. Mater., 2015, 285, 319.
46 W. Cao, L. Chen and Z. Qi, J. Mol. Catal. A: Chem., 2015, 401, 81.
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3 4 3 4
Scheme 1. The schematic diagram for preparation of the ZnO/AgBr/Fe O /Ag VO
nanocomposites.
Caption for the figures:
Fig. 1 XRD patterns for the Fe
3
4
O , ZnO/Fe
3
O
4
, ZnO/AgBr/Fe
3
O
4
, ZnO/Ag
3
VO
4
/Fe
3
4
O , and
ZnO/AgBr/Fe /Ag VO nanocomposites with different weight percents of Ag
3
O
4
3
4
3
4
VO .
Fig. 2 (a) EDX spectra for the ZnO/Fe O , ZnO/AgBr/Fe O , ZnO/Ag VO /Fe O , and
3
4
3
4
3
4
3
4
ZnO/AgBr/Fe
ZnO/AgBr/Fe
3
3
O
O
4
4
/Ag
3
3
VO
4
4
(10%) samples. (b)ꢀ(h) EDX mapping of the
(10%) nanocomposite.
/Ag
VO
Fig. 3 (a) SEM image of the ZnO/AgBr/Fe
3
O
4
3 4
/Ag VO (10%) nanocomposite. (b) and (c)
TEM images of the ZnO/AgBr/Fe
magnifications.
O /Ag VO (10%) nanocomposite with two
3 4 3 4
Fig. 4 (a) UVꢀvis DRS spectra for the ZnO/AgBr/Fe
ZnO/AgBr/Fe /Ag VO nanocomposites with different weight percents of Ag
IR spectra for the Fe , ZnO/AgBr/Fe , ZnO/Ag VO /Fe , and
ZnO/AgBr/Fe /Ag VO (10%) samples.
3
O
4
, ZnO/Ag
3
VO
4
/Fe
3
O
4
, and
3
O
4
3
4
3
VO
4
. (b) FTꢀ
3
O
4
3
O
4
3
4
3 4
O
3
O
4
3
4
Fig. 5 Magnetization curves for the Fe O nanoparticles and ZnO/AgBr/Fe O /Ag VO (10%)
3
4
3
4
3
4
nanocomposite.
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Fig. 6 (a) Photodegradation of RhB over the Fe
ZnO/Ag VO /Fe , and ZnO/AgBr/Fe /Ag
percents of Ag VO . UV–Vis spectra for degradation of RhB under visibleꢀlight irradiation
over the (b) ZnO/Fe O , (c) ZnO/AgBr/Fe O , (d) ZnO/Ag VO /Fe O , and (e)
3
4
O , ZnO/Fe
3 4 3 4
O , ZnO/AgBr/Fe O ,
3
4
3
O
4
3
O
4
3
VO nanocomposites with different weight
4
3
4
3
4
3
4
3
4
3
4
ZnO/AgBr/Fe O /Ag VO (10%) nanocomposite.
3
4
3
4
Fig. 7 (a) The degradation rate constants of RhB over different samples. (b) PL spectra for the
ZnO/Fe , ZnO/Ag VO /Fe , ZnO/AgBr/Fe , ZnO/AgBr/Fe /Ag VO (5%),
ZnO/AgBr/Fe /Ag VO (10%), and ZnO/AgBr/Fe /Ag VO (20%) nanocomposites.
3
O
4
3
4
3
O
4
3
O
4
3
O
4
3
4
3
O
4
3
4
3
O
4
3
4
Fig. 8 Hypothesized mechanism for separation of the charge carriers in the
ZnO/AgBr/Fe O /Ag VO nanocomposites via tandem nꢀn heterojunctions.
3
4
3
4
Fig. 9 The degradation rate constant of RhB over the ZnO/AgBr/Fe
3
O
4
/Ag
3
VO
4
(10%)
nanocomposite in presence of various scavengers.
Fig. 10 (a) The degradation rate constants of RhB over the ZnO/AgBr/Fe
3
O
4
3 4
/Ag VO (10%)
nanocomposite prepared at different refluxing times. (b) The degradation rate constants of
RhB over the ZnO/AgBr/Fe
temperatures.
3 4 3 4
O /Ag VO (10%) nanocomposite calcined at different
3 4 3 4
Fig. 11 (a) XRD patterns for the ZnO/AgBr/Fe O /Ag VO (10%) nanocomposite before and
after calcinations at 500 and 600 °C. (b) SEM images of the ZnO/AgBr/Fe
nanocomposite calcined at 500 and 600 °C.
3 4 3 4
O /Ag VO (10%)
Fig. 12 The degradation rate constants of RhB, MB, MO, and phenol over the ZnO/Fe O ,
3
4
ZnO/AgBr/Fe
3 4 3 4 3 4 3 4 3 4
O , ZnO/Ag VO /Fe O , and ZnO/AgBr/Fe O /Ag VO (10%)
nanocomposites calcined at 500 °C.
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Fig. 13 (a) Reusability of the ZnO/AgBr/Fe
3 4 3 4
O /Ag VO (10%) nanocomposite for four
successive runs. (b) The XRD patterns of the nanocomposite before and after fourth recovery.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
2
9