Magnetically separable Bi2MoO6/ZnFe2O4 heterostructure nanofibers: Controllable synthesis and enhanced visible light photocatalytic activity

Article abstract of DOI:10.1016/j.jallcom.2018.03.107

In this paper, Bi₂MoO₆/ZnFe₂O₄ heterojunctions were successfully obtained by combining electrospinning and a solvothermal technique. The morphology and loading of Bi₂MoO₆ nanosheets of Bi

Full text of DOI:10.1016/j.jallcom.2018.03.107

Accepted Manuscript
Magnetically separable Bi MoO /ZnFe O heterostructure nanofibers: Controllable
2
6
2 4
synthesis and enhanced visible light photocatalytic activity
Chengcheng Zhao, Changlu Shao, Xinghua Li, Xiaowei Li, Ran Tao, Xuejiao zhou,
Yichun Liu
PII:
S0925-8388(18)30975-7
10.1016/j.jallcom.2018.03.107
JALCOM 45337
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 19 November 2017
Revised Date: 5 March 2018
Accepted Date: 9 March 2018
Please cite this article as: C. Zhao, C. Shao, X. Li, X. Li, R. Tao, X. zhou, Y. Liu, Magnetically separable
Bi MoO /ZnFe O heterostructure nanofibers: Controllable synthesis and enhanced visible light
2
6
2 4
photocatalytic activity, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.107.
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ACCEPTED MANUSCRIPT

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Magnetically separable Bi2MoO6/ZnFe2O4 heterostructure
nanofibers: controllable synthesis and enhanced visible light
photocatalytic activity
Chengcheng Zhao, Changlu Shao*, Xinghua Li*, Xiaowei Li, Ran Tao, Xuejiao zhou
and Yichun Liu
Center for Advanced Optoelectronic Functional Materials Research, and Key
Laboratory of UV-Emitting Materials and Technology (Northeast Normal University),
Ministry of Education, 5268 Renmin Street, Changchun 130024, People's Republic of
China
Abstract
In this paper, Bi2MoO6/ZnFe2O4 heterojunctions were successfully obtained by
combining electrospinning and a solvothermal technique. The morphology and
loading of Bi2MoO6 nanosheets of Bi2MoO6/ZnFe2O4 heterojunctions could be
controlled by adjusting the concentration of molybdenum precursor during the
solvothermal process. Comparative experiments revealed that the resulting
Bi2MoO6/ZnFe2O4 heterojunctions exhibit a higher visible light photocatalytic activity
for the degradation of Rhodamine B (RB) than pure Bi2MoO6 or ZnFe2O4. The
photocurrent and PL measurements of the Bi2MoO6/ZnFe2O4 heterojunctions also
indicated an enhanced separation of photo-induced electron-hole pairs. Moreover, the
* Corresponding author.
E-mail address: clshao@nenu.edu.cn (C.L. Shao), lixh781@nenu.edu.cn (X.H. Li).
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heterojunctions could be easily recycled without a decrease in their photocatalytic
activity due to their good magnetic separation performance and excellent chemical
stability. Based on these findings, Bi2MoO6/ZnFe2O4 heterojunctions are a promising
visible-light-driven photocatalyst for converting solar energy to chemical energy for
environmental remediation.
Keywords: Bi2MoO6/ZnFe2O4 heterojunctions; nanofibers; magnetic separation;
photocatalytic activity; visible light.
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1. Introduction
Oxide semiconductor photocatalysts have been extensively applied in the field of
environmental remediation due to their unique physicochemical properties,
nontoxicity, and excellent photocatalytic activity [1-5]. In recent decades, titanium
dioxide (TiO2) has attracted considerable attention due to its low-cost, nontoxicity,
and chemical inertness [6,7]. Nevertheless, its use is limited to UV light irradiation
due to its wide band gap structure [7]. It is well known that UV light is just 4% of
solar radiation, while visible light is about 43%. Therefore, the design and synthesis
of highly efficient visible-light-driven photocatalysis is an important research focus
[8-11].
Bi2MoO6 semiconductor with a band gap of about 2.6 eV stands out as a
promising visible-light-driven photocatalysts due to its low cost, excellent chemical
stability and nontoxicity [12-16]. It possess a unique layered structure of perovskite
slabs that are corner-shared with distorted octahedra sandwiched between (Bi2O2)2+
layers. This is beneficial for the separation of photogenerated electron-hole pairs
[16-18]. Many studies have confirmed that Bi2MoO6 exhibits good photocatalytic
activity under visible-light irradiation [15,17,18].
Theoretically, the formation of heterojunctions can hinder the recombination of
photogenerated electron-hole pairs more effectively. Therefore, constructing
heterojunctions of Bi2MoO6 can further enhance their photocatalytic performance
versus pure Bi2MoO6. The energy band gap of the second semiconductor strongly
affects the photocatalytic performance of the Bi2MoO6-based heterojunctions, and a
variety of Bi2MoO6-based heterojunctions have been developed, such as
Bi2MoO6/Fe2O3, Bi2MoO6/TiO2, Bi2MoO6/ZnO, and others [6,19,20]. More recently,
zinc ferrite (ZnFe2O4) has been reported to be a promising visible light photocatalyst
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with low cost, good photochemical stability, magnetic properties and nontoxicity
[21–23]. Its conduction and valence band position are both higher than that of
Bi2MoO6 [23,24]. It has a narrow band gap of 1.9 eV, which provides a wider range of
visible light absorption [21,22]. Therefore, constructing Bi2MoO6/ZnFe2O4
heterojunctions might not only hinder the charge recombination but also extend the
visible light absorption, both of which would enhance their photocatalytic activity.
Furthermore, considering the good magnetic properties of ZnFe2O4, the resulting
Bi2MoO6/ZnFe2O4 heterojunctions might also be effectively recycled and reused by
applying an appropriate external magnetic field [21,22,25]. To the best of our
knowledge, there are no reports on the preparation and photocatalytic properties of
Bi2MoO6/ZnFe2O4 heterojunctions.
Herein, in our work, we attempt to construct Bi2MoO6/ZnFe2O4 heterojunctions
via electrospun ZnFe2O4 nanofibers as hard templates. The one-dimensional (1D)
nanostructure of ZnFe2O4 nanofibers might provide an efficient channel for quick
charge transfer to improve the separation of photogenerated electron-hole pairs. Their
3D open structure and large surface areas could provide more active sites for the
assembly of secondary nanostructures [26,27]. After the hydrothermal process,
Bi2MoO6 nanostructures can be controllably grown on ZnFe2O4 nanofibers. The
results show that the 1D Bi2MoO6/ZnFe2O4 heterojunctions exhibit excellent
photocatalytic activity in the degradation of RB under visible-light irradiation and
could be easily recycled without a decrease in their photocatalytic activity due to their
good magnetic separation performance and excellent chemical stability.
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2. Expertimental Section
2.1. Preparation of ZnFe2O4 nanofibers
First, the 1.7 g of Polyvinylpyrrolidone (PVP, Mw = 1,300,000) powder was
dissolved in 10 mL of N,N-dimethylformamide (DMF) solution. After stirring for 6 h,
a homogeneous solution was obtained. Then, Fe(NO3)·9H2O and Zn(NO3)2·6H2O
were added, and the mixture was magnetically stirred for 6 h at room temperature to
form a homogeneous precursor solution for electrospinning. Finally, the brown-red
solution was loaded into a syringe-equipped steel needle that was connected to a
direct current high voltage of 12 kV. The collector was placed 15 cm away from the
tip of the steel needle. The as-collected nanofibers were calcined at 600 °C for 2 h at a
heating rate of 1 °C /min to obtain ZnFe2O4 nanofibers.
2.2. Fabrication of Bi2MoO6/ZnFe2O4 heterojunctions
In a typical procedure, 0.1 mmol Bi(NO3)3·5H2O and 0.05 mmol Na2MoO4·2H2O
were dissolved in 2.5 mL of ethylene glycol under magnetic stirring. Then, 15 mL
ethanol was slowly added into the solution followed by stirring for 10 min. Finally, 10
mg ZnFe2O4 nanofibers was added. The resulting solution was transferred into a 25
mL Teflon-lined stainless autoclave, sealed, and held at 160 °C for 24 h. It was then
cooled to room temperature. The resulting composite nanofibers were washed with
deionized water and ethanol to remove any ionic residual and then dried in an oven at
60 °C for 12 h. The as-fabricated sample was denoted as BMO/ZFO-1. By tuning the
precursor concentration for synthesizing Bi2MoO6, two other samples of
Bi2MoO6/ZnFe2O4 were fabricated and were denoted as BMO/ZFO-2 and
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BMO/ZFO-3. And Bi2MoO6 was synthesized in the absence of ZnFe2O4 nanofibers.
The detailed experimental conditions for the fabrication of BMO/ZFO-1,
BMO/ZFO-2 and BMO/ZFO-3 were listed in Table 1.
2.3. Characterization
X-ray diffraction (XRD) measurements were carried out using a D/max 2500
XRD spectrometer (Rigaku) with Cu Kα line of 0.1541 nm. Fourier transform
infrared (FTIR) spectra were recorded on a Nicolet Instruments Research Series 5PC
FTIR. Scanning electron microscopy (SEM; Quanta 250 FEG) and transmission
electron microscopy (TEM; JEOL JEM-2100) were used to characterize the
morphologies of the products. X-ray photoelectron spectroscopy (XPS) was
performed on a VG-ESCALAB LKII instrument with Mg Kα ADES (hν = 1253.6 eV)
source at a residual gas pressure of below 1 × 10−8 Pa. The UV-Vis diffuse reflectance
(DR) spectra of the samples were recorded on a Cary 500. Photoluminescence (PL)
spectra of the samples were detected with a Jobin Yvon HR800 micro-Raman
spectrometer using a 325 nm line from a He-Cd laser. The Mott-Schottky (M-S)
curves of the samples were measured by using an electrochemical analyzer (PRSTAT
2273).
2.4. Photocatalytic tests
Photocatalyst (0.05g) was added into 50 mL of RB solution (10 mg·L−1). The
solution was stayed in the dark for 30 min to reach adsorption-desorption equilibrium
between the organic molecules and the photocatalyst surface. Then the mixture
aqueous solution was irradiated by a 150 W xenon lamp with a cutoff filter (λ > 420
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nm) and continuous stirring. At given intervals of illumination, 2 mL reacted solutions
in series were taken out and analyzed. The concentrations of RB in the reacting
solutions were analyzed by a UH4150 spectrophotometer (PerkinElmer) at λ = 554
nm. And the total organic carbon (TOC) of the degraded dyes was determined by a
TOC analyser (Vario TOC).
3. Results and Discussion
3.1. Structure and Morphology
The XRD patterns of ZnFe2O4 nanofibers, BMO/ZFO-1, BMO/ZFO-2,
BMO/ZFO-3 and Bi2MoO6 are shown in Fig. 1. For the ZnFe2O4 nanofibers,
diffraction peaks at 29.9°, 35.3°, 42.9°, 53.2°, 56.7°, 62.2°, and 73.5° can be
attributed to the (2 2 0), (3 1 1), (4 0 0), (4 2 0), (5 1 1), (4 4 0), and (5 3 3) facets of
the spinel ZnFe2O4 (JCPDS No. 22-1012), respectively [22]. The pure Bi2MoO6
prepared hydrothermally showed several diffraction peaks at 28.2°, 32.3°, 46.7°,
55.3°, and 58.2°. These can be indexed to the characteristic peaks of the orthorhombic
Bi2MoO6 structure (JCPDS No. 76-2388) [15]. For samples of BMO/ZFO-1,
BMO/ZFO-2, and BMO/ZFO-3 prepared via a combination of electrospinning and
hydrothermal techniques, all peaks could be perfectly indexed to the spinel ZnFe2O4
and the orthorhombic structure of Bi2MoO6, no other signals were observed,
suggesting that no impurity species were formed between BMO/ZFO-1, BMO/ZFO-2,
and BMO/ZFO-3. Of note, the ratio in diffraction peaks for Bi2MoO6 to ZnFe2O4
increased with increasing concentrations of molybdenum precursor in the
hydrothermal process, suggesting that the Bi2MoO6/ZnFe2O4 heterojunctions with
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different amounts of Bi2MoO6 loading were successfully prepared.
Fig. 2 shows the FT-IR spectra of the samples. For pure ZnFe2O4 nanofibers, the
absorption bands at 540 cm−1 is due to Fe-O stretching vibration, while the weak peak
at 410 cm−1 is due to a Zn-O stretching vibration of the ZnFe2O4 [28,29]. As for the
patterns of samples BMO/ZFO-1, BMO/ZFO-2 and BMO/ZFO-3, in addition to
characteristic peaks of ZnFe2O4, the peak appearing at 841cm-1 is ascribed to Mo=O
stretching vibration, while the peaks at 732 and 544 cm−1 are attributed to Mo(VI)-O
tetrahedral stretching and Bi(III)-O octahedral stretching vibrations, respectively
[30,31]. All the absorption peaks of ZnFe2O4 and Bi2MoO6 appeared in the
Bi2MoO6/ZnFe2O4 heterojunctions further testified the XRD results.
The morphology of the samples was analyzed by SEM. The ZnFe2O4 nanofibers
in Fig. 3a had a smooth and uniform surface without secondary nanostructures and
their diameter ranged from 150 to 250 nm. However, Fig. 3b shows that a small
number of Bi2MoO6 nanosheets uniformly grew on the surface of ZnFe2O4 nanofibers
after solvothermal reaction for 24 h. When the amount of Bi(NO3)3·5H2O was
increased from 0.1 mmol (for BMO/ZFO-1) to 0.2 mmol (for BMO/ZFO-2) in the
hydrothermal process, the density of Bi2MoO6 nanosheets increased significantly
compared with for BMO/ZFO-1 (Fig. 3c). When the amount of Bi(NO3)3·5H2O was
increased to 0.4 mmol, the density of the Bi2MoO6 nanosheets on the surface of
ZnFe2O4 nanofibers for BMO/ZFO-3 increased dramatically. The high magnification
SEM (Fig. 3d inset) showed that the diameter of BMO/ZFO-3 is obviously larger than
that of pure ZnFe2O4 nanofibers. Notably, although the density of Bi2MoO6
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nanosheets increased significantly, no obvious aggregation could be observed on the
surface of the compsite nanofibers. The homogeneous heterostructure is not only
favorable for charge separation under visible-light irradiation but also advantageous to
increase the interfacial contact area for surface reaction, both of which are beneficial
for improving their photocatalytic activity [7,24,26,27].
Transmission electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) were used to obtain more detailed information about
the morphology and crystalline structure of the Bi2MoO6/ZnFe2O4 heterojunctions.
Fig. 4 shows typical TEM images of BMO/ZFO-2. There were numerous Bi2MoO6
nanosheets uniformly distributed on the ZnFe2O4 nanofibers. The Bi2MoO6
nanosheets were about 8 nm in thickness and 40 nm in size. The HRTEM image from
the heterojunctions displayed two types of clear lattice fringes (Fig. 4b). One set of
the fringe spacing is ca. 0.255 nm and corresponds to the (3 1 1) plane of the spinel
ZnFe2O4 [22,25]. Another set of the fringes spacing measures ca. 0.275 nm and
corresponds to the (2 0 0) lattice spacing of the orthorhombic phase of Bi
2
MoO
6
[32].
The TEM image of the BMO/ZFO-2 revealed that very dense secondary Bi
2
MoO
6
nanosheets were successfully grown on the primary ZnFe2O4 nanofiber templates
coinciding with the XRD and SEM observations.
3.2. X-ray Photoelectron Spectroscopy (XPS) Spectra
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical
compositions and chemical states of samples. The typical high resolution XPS
spectrum of Bi 4f is shown in Fig. 5a. The peaks at 159 eV and 164.3 eV correspond
to Bi 4f7/2 and Bi 4f5/2, respectively, which indicates a normal state of Bi3+ in Bi2MoO6
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[33]. BMO/ZFO-2 has a corresponding binding energy located near 159.2 and 164.5
eV. This is about 0.2 eV higher than that of the pure Bi2MoO6. Fig. 5b reveals the high
resolution XPS spectrum of Mo 3d, the binding energies of Mo 3d3/2 and Mo 3d5/2 are
at 235.3 eV and 232.2 eV for pure Bi2MoO6, but 235.7 eV and 232.6 eV for
BMO/ZFO-2 [33,34]. This shifts toward the higher value of about 0.4 eV. These
obvious binding energy shifts occurring on the XPS spectra may result from the
interaction of ZnFe2O4 and Bi2MoO6 in the heterojunctions [35]. Fig. 5c shows the
XPS spectrum of Zn 2p, the spectra of ZnFe2O4 and the BMO/ZFO-2 are very similar
to each other. The two peaks centered at 1044.6 eV and 1021.6 eV correspond to the
binding energy of Zn 2p1/2 and Zn 2p3/2, respectively, indicating the oxidation state of
Zn2+ in the samples [28,37]. Fig. 5d shows Fe 2p spectrum of ZnFe2O4 and the
BMO/ZFO-2 , two distinguishable main peaks are observed at 726.4 eV and 712.5 eV,
which can be assigned to Fe 2p1/2 and Fe 2p3/2, respectively [28,37]. Further, the
shoulder peaks at 733.6 and 719.7 eV originate from shakeup process of 2p1/2 and
2p3/2 and are marked as satellite peaks in Fig. 5d [37]. These analyses are very
consistent with the literature and indicate that the Fe exists mainly as Fe3+ [28,37].
The O 1s spectra of the various samples are presented in Fig. 5e. Usually, the O 1s
peaks are composed of two peaks for metal oxide nanostructures. The peak at the
lower binding energy corresponds to lattice oxygen while the peak at the higher
ꢀ
binding energy originates from the surface-adsorbed groups, such as OHꢀor O2 [38].
In Fig. 5e, the O 1s spectrum of Bi2MoO6 can be fitted to two characteristic peaks.
The peaks located at 529.7 eV and 531.2 eV are attributed to the lattice oxygen
(Mo/Bi-O), and surface oxygen, respectively. For the ZnFe2O4 nanofibers, the O 1s
from the Zn/Fe-O bond appears at 530.2 eV, and the surface-related peak is at 532.2
eV. For the BMO/ZFO-2, the binding energies at 529.7 eV, 530.4 eV and 532.1 eV
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are attributed to the Mo/Bi-O, Fe/Zn-O and surface-related bonds, respectively. The
XPS results suggest the coexistence of Bi2MoO6 and ZnFe2O4 in the
Bi2MoO6/ZnFe2O4 heterojunctions.
3.3. Optical Properties
To investigate the light absorption properties of the Bi2MoO6/ZnFe2O4
heterojunctions, the UV–Vis absorption spectra is presented in Fig. 6a. Pure Bi2MoO6
absorbs from the UV to the visible light region with a clear absorption edge at 460 nm.
Versus Bi2MoO6, the curves of BMO/ZFO-1, BMO/ZFO-2, and BMO/ZFO-3 had a
significant red-shift toward the visible light region. Obviously, the visible light
absorption of Bi2MoO6/ZnFe2O4 heterojunctions is better than that of Bi2MoO6, which
possibly result good visible-light utilization.
3.4. Photocatalytic Activity
To demonstrate the photocatalytic activity of the obtained Bi2MoO6/ZnFe2O4
heterojunctions for the degradation of organic pollutants, we carried out the
experiments of the photocatalytic degradation of Rhodamine B (RB) under
visible-light irradiation as a model pollutant. However, two comparative experiments
were performed before studying the activities of the Bi2MoO6/ZnFe2O4
heterojunctions under visible light: (a) in the presence of photocatalysts but in the
dark, and (b) with visible-light irradiation but in the absence of the photocatalysts as
the blank test (Fig. 7a and Fig. 7b). These results show that the adsorption-desorption
equilibrium of RB in the dark was established within 30 min. The blank test showed
that the concentration of RB remained the same after 240 min confirming that RB was
very stable and its photolysis could be ignored under visible-light irradiation without
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photocatalysts [39]. Fig. 7c shows the photocatalytic activities of pure ZnFe2O4
nanofibers, Bi2MoO6, Bi2MoO6/ZnFe2O4 heterojunctions, and mechanically mixed
Bi2MoO6 and ZnFe2O4 nanofibers (MBZ) for the degradation of RB aqueous solution
under visible light. It is clearly found that pure ZnFe2O4 nanofibers have the lowest
photocatalytic efficiency because of the rapid recombination of photogenerated
electrons and holes, the degradation was only about 20 % in 240 min. In addition,
Bi2MoO6 had about 47 % degradation in 240 min. Versus ZnFe2O4 nanofibers and
Bi2MoO6, the Bi2MoO6/ZnFe2O4 heterojunctions exhibited a much better
photocatalytic activity under visible-light irradiation. Fig. 7c shows that after
visible-light irradiation for 240 min, the degradation efficiency of RB were about
81.3 %, 99.9 % and 63.8 % for BMO/ZFO-1, BMO/ZFO-2 and BMO/ZFO-3,
respectively. The BMO/ZFO-2 had the highest photocatalytic efficiency compared to
the BMO/ZFO-1 and BMO/ZFO-3. The EDAX data of BMO/ZFO-2 showed that
equal amounts of Bi2MoO6 nanosheets were loaded on the ZnFe2O4 nanofibers. The
same amount of Bi2MoO6 and ZnFe2O4 were mechanically mixed to degrade RB
under visible light as a validation test. This clearly showed that BMO/ZFO-2 exhibits
much higher photocatalytic efficiency than mechanically mixed Bi2MoO6 and
ZnFe2O4, suggesting that charge separation efficiency might be improved due to the
effects of the heterojunction.
We also carried out a kinetic analysis of the degradation of RB for a better
comparison of the photocatalytic efficiency of photocatalysts. Fig. 7d shows the
kinetic linear fitting curves over different photocatalysts. This indicates that the
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photocatalytic degradation of RB followed a Langmuir-Hinshelwood apparent
first-order kinetics model:
ꢀꢁ
ln
= ꢂꢃꢄ = ꢂꢅꢆꢆꢄ
ꢀ
where C0 is the initial concentration (mg/L) of the reactant; C is the concentration
(mg/L); t is the visible-light irradiation time; k is the reaction rate constant
(mg/(L.min)); and K is the adsorption coefficient of the reactant (L/mg); and kapp is
the apparent first-order rate constant (min-1). The kapp and the relative coefficient (R2)
of different samples can be seen in Table 2. As seen from Table 2, the values of R2 of
different samples are relatively high, close to 1, implying that the photocatalytic
process follows the Langmuir-Hinshelwood apparent first-order kinetics model well.
And this indicated that the photocatalytic activities is on the order of BMO/ZFO-2 >
BMO/ZFO-1 > BMO/ZFO-3 > MBZ > Bi2MoO6 > ZnFe2O4. This indicates that the
construction of Bi2MoO6/ZnFe2O4 heterojunctions can effectively enhance the
photocatalytic properties.
It is known that the total organic carbon (TOC) analysis could be used to
determine the extent to which the dye molecules are broken down during
photocatalysis. The TOC removal in Fig. 8 suggest that the mineralization of RB after
240 min photocatalytic reaction could reach 15.20%, 75.19%, 88.97%, 53.08%, 46.21%
and 40.67% for the ZnFe2O4, BMO/ZFO-1, BMO/ZFO-2, BMO/ZFO-3, MBZ and
Bi2MoO6, respectively. Obviously, BMO/ZFO-2 exhibits best activity for dye
mineralization. The residual organic materials may require extended irradiation times
to ensure their complete mineralization [40]. The above results indicate that the
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Bi2MoO6/ZnFe2O4 heterojunctions are potential catalysts for the mineralization of the
organic dye molecules being mineralised to be carbon dioxide, water, and other
inorganic species.
3.5. Possible Mechanism for Photocatalytic Reactions
The enhanced photocatalytic performance of BMO/ZFO-2 may be due to the
following correlated factors: first, the nanosheet structure of Bi2MoO6 could improve
the specific surface area of the BMO/ZFO-2. The Brunauer-Emmett-Teller (BET)
surface area of the BMO/ZFO-2 is measured to be about 30.89 m2 g−1, which is much
higher than that of the pure ZnFe2O4 nanofibers (15.49 m2 g−1). The high surface area
of the heterojunctions could provide more active catalytic sites, resulting better
photocatalytic performance. Second, their rough surface character also enhances the
light absorbance and utilization in the photocatalytic reaction due to multiple light
scattering effects. Third, the uniform contacts of the heterojunctions could
theoretically induce internal electric fields for effective suppression of electron-hole
recombination.
PL was used to further investigate the charge carrier transfer of the photoinduced
electrons and holes in the heterojunctions of Bi2MoO6/ZnFe2O4 (Fig. 9a). Versus
Bi2MoO6/ZnFe2O4 heterojunctions, pure ZnFe2O4 nanofibers had the strongest
intensity indicating high radiative recombination. Moreover, all Bi2MoO6/ZnFe2O4
heterojunctions have a similar peak shape and peak position, but much lower PL
intensities. Of these, BMO/ZFO-2 has the lowest intensity suggesting that the sample
possessed the highest separation efficiency of photogenerated electrons and holes.
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The results indicated that the Bi2MoO6/ZnFe2O4 heterojunction is favorable at
suppressing radiative recombination. This led to better utilization of electron and hole
pairs induced by light irradiation. The photocurrent is another useful method for
studying the separation efficiency of photogenerated carriers in the photocatalysts. Fig.
9b shows that ZnFe2O4, Bi2MoO6, and BMO/ZFO-2 electrodes had a rapid
photocurrent response when the visible light illumination was turned on and off.
However, the photocurrent of BMO/ZFO-2 electrode was obviously higher than that
of ZnFe2O4 and Bi2MoO6 electrode demonstrating that their photogenerated charge
carriers have higher separation efficiency. All the above results show that the
construction of heterojunctions could definitely improve the charge separation due to
effect of internal electric field, which contributes to their enhancement of
photocatalytic activity under visible light.
In order to clarify the role of different active species during the photocatalytic
reaction, in addition to BMO/ZFO-2 photocatalyst was added into RB aqueous
solution, scavengers of Cr(VI), ammonium oxalate (AO), Tert-butyl alcohol (TBA)
and benzoquinone (BQ) were employed to capture active electrons (e-), active holes
-
(h+), hydroxyl radical (·OH) and superoxide radical anion (·O2 ), respectively, under
visible-light irradiation [26,41,42,43]. In Fig. 10, the slight decrease of RB removal
efficiency in the presence of TBA suggested that ·OH did not contribute much to the
photocatalytic degradation of RB over Bi2MoO6/ZnFe2O4 heterojunctions. In contrast,
the addition of BQ had a dramatic negative effect on RB degradation, implying
-
that ·O2 played an indispensable role in photocatalytic reaction. Meanwhile, about 40 %
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of RB degradation was inhibited when h+ were captured by AO indicating that h+ also
played an important role in RB photocatalytic degradation. It is noteworthy that the
RB photocatalytic degradation was enhanced upon the addition of Cr(VI). This might
be attributed to the electron trapping role of Cr(VI) inhibiting the recombination of
electron-hole pairs, which left more holes to oxidize RB further confirming the crucial
function of holes in the photocatalytic removal of RB by the Bi2MoO6/ZnFe2O4
heterojunctions [44,45].
To determine the conduction band (CB) and valence band (VB) edges of
ZnFe2O4 and Bi2MoO6, Mott-Schottky curves were measured and shown in Fig.11.
The positive slope of Mott-Schottky plots indicate ZnFe2O4 and Bi2MoO6 are both
n-type semiconductors. Additionally, the corresponding flat band potentials (Vfb) can
be determined by extrapolating the lines to 1/C2 = 0. The Vfb of ZnFe2O4 and
Bi2MoO6 are found to be -0.71 V and -0.53V versus Ag/AgCl (at pH = 6.8),
respectively. Therefore, the Vfb of ZnFe2O4 and Bi2MoO6 are calculated to be -0.51 V
and -0.33 V versus the normal hydrogen electrode (NHE) at pH = 0, respectively. As
reported, the conduction band potential (ECB) was very close to the Vfb for n-type
semiconductors. Thus, the ECB values of ZnFe2O4 and Bi2MoO6 are determined as
-0.51 V and -0.33 V vs NHE. Moreover, the EVB values can be calculated according
to the formula:
ECB = EVB – Eg
From Fig. S1, the Eg of the pure ZnFe2O4 and Bi2MoO6 are calculated to be about
2.12 eV and 2.66 eV. Therefore, the EVB values are estimated to be 1.61 V and 2.33 V
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for ZnFe2O4 and Bi2MoO6. The calculated ECB and EVB for ZnFe2O4 and Bi2MoO6 are
very close to the previous reports [23,24].
These results allow us to propose mechanisms of light absorption, charge transfer,
and the reaction pathways for generating active radicals by Bi2MoO6/ZnFe2O4 and the
degradation of RB. As shown in Scheme 1, the conduction band (CB) and valence
band (VB) of ZnFe2O4 are both higher than that of Bi2MoO6. When ZnFe2O4 contacts
Bi2MoO6, the diffusion of electrons and holes creates an inner electric field where a
space charge region is formed at the interfaces of heterojunction. Under visible-light
irradiation, both ZnFe2O4 and Bi2MoO6 could be excited, and the generated holes in
Bi2MoO6 are driven to the valence band (VB) of ZnFe2O4. The electrons in ZnFe2O4
would be driven to the conduction band (CB) of Bi2MoO6 by internal electric field.
The holes on the VB edge of ZnFe2O4 are the important oxidizing species responsible
for the degradation of RB. The electrons on the CB edge of the Bi2MoO6 are good
-
reductants that can capture O2 to generate ·O2 because the CB edge potential of the
-
-
Bi2MoO6 is more negative than E (O2/·O2 ) (−0.046 V vs NHE) [46]. These ·O2
radicals are the most important oxidizing species responsible for the degradation of
RB.
3.6. Magnetic separation and recycled photocatalytic degradation performance
In fact, the reusability of photocatalysts is a noteworthy issue in practical
applications, especially for the separation property and their stability. The
Bi2MoO6/ZnFe2O4 heterojunctions have magnetism due to the presence of ZnFe2O4
nanofibers. To confirm the magnetic properties of the BMO/ZFO-2, a separation
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experiment was conducted as follows. Fig. 12a shows the BMO/ZFO-2 dispersion in
water solution and the changes under external magnetic field action for 5 min. Clearly,
under the applied magnetic field, the catalysts in the mixed solutions were gradually
adsorbed by magnet, and the solutions became clear, it is demonstrated that the
BMO/ZFO-2 has magnetic properties that can solve the difficult separation problem
of photocatalysts for the recycled use. We also reused the BMO/ZFO-2 three times to
test the stability of Bi2MoO6/ ZnFe2O4 heterojunctions on the photodegradation of RB.
Each experiment was carried out under identical conditions (Fig. 12b). After three
experimental cycles, the photocatalytic activity of the BMO/ZFO-2 was nearly
unchanged. Fig. 12c and Fig. 12d shows the XRD patterns and SEM images of the
BMO/ZFO-2 before the photocatalytic reaction and after the photocatalytic reaction.
After three photocatalytic cycles, the position and the ratio of peaks were nearly the
same as that of fresh photocatalyst. The secondary Bi2MoO6 nanostructures were still
very complete, clearly indicating stability.
4. Conclusions
In summary, novel 1D Bi2MoO6/ZnFe2O4 heterojunctions are controllably
fabricated by using electrospinning and a solvothermal technique. Due to the
heterojunction effects and large specific surface area, the Bi2MoO6/ZnFe2O4
heterostrucuture nanofibers exhibited a higher visible light photocatalytic activity for
the degradation of RB than the pure Bi2MoO6 and ZnFe2O4. Furthermore, the
Bi2MoO6/ZnFe2O4 heterostructure nanofibers can be easily recycled without a
decrease in their photocatalytic activity because of their good magnetic separation
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performance and excellent chemical stability. The Bi2MoO6/ZnFe2O4 heterojunctions
with high photocatalytic activity will greatly promote their industrial applications to
eliminate organic and inorganic pollutants from wastewater.
Acknowledgments
The present work is supported financially by the National Natural Science
Foundation of China (Nos.51572045, 51272041, 61201107, 11604044, and
91233204), the National Basic Research Program of China (973 Program)
(No.2012CB933703), the 111 Project (No. B13013), the Natural Science Foundation
of Jilin Province of China (20160101313JC), the Fundamental Research Funds for the
Central Universities (2412017FZ009, 2412016KJ017) and the Opening Project of
Key Laboratory of Polyoxometalate Science of Ministry of Education.
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Figure captions:
Fig. 1 XRD patterns of pure ZnFe2O4, BMO/ZFO-1, BMO/ZFO-2, BMO/ZFO-3 and
Bi2MoO6.
Fig. 2 FT-IR patterns of pure ZnFe2O4, BMO/ZFO-1, BMO/ZFO-2, BMO/ZFO-3,
and Bi2MoO6.
Fig. 3 SEM images of (a) ZnFe2O4 nanofibers, (b) BMO/ZFO-1, (c) BMO/ZFO-2,
and (d) BMO/ZFO-3.
Fig. 4 (a) TEM image of sample BMO/ZFO-2 and (b) HRTEM image of the
heterojunction region.
Fig. 5 XPS spectra of (a) Bi 4f, (b) Mo 3d, (c) Zn 2p , (d) Fe 2p and (e) O 1s for
Bi2MoO6, ZnFe2O4 and BMO/ZFO-2.
Fig. 6 (a) UV-Vis diffuse reflectance spectra of ZnFe2O4, BMO/ZFO-1, BMO/ZFO-2,
BMO/ZFO-3 and Bi2MoO6.
Fig. 7 Degradation profiles of RB (a) in the presence of the photocatalysts but in the
dark, and (b) with visible-light irradiation but in the absence of the photocatalysts. (c)
Degradation curve of RB over different catalysts under visible light and (d) the
apparent first-order kinetics fitting over different catalysts under visible light.
Fig. 8 TOC removal of dyes with different samples after 240 min photocatalytic
reaction.
Fig. 9 (a) PL spectra of different samples, (b) photocurrent responses of ZnFe2O4,
Bi2MoO6, and BMO/ZFO-2 under visible light.
Fig. 10 Effect of different scavengers on the degradation of RB by the BMO/ZFO-2.
Fig. 11 Mott-Schottky plots for (a) ZnFe2O4 (b) Bi2MoO6 in 0.2 M Na2SO4 aqueous
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solution (pH = 6.8).
Fig. 12 (a) Magnetic responsiveness images of BMO/ZFO-2 under an external
magnetic field. (b) Degradation curves of RB over sample BMO/ZFO-2 for reusing
three times. (c) SEM images of the BMO/ZFO-2 before and after the photocatalytic
reaction. (d) XRD patterns of the BMO/ZFO-2 before and after the photocatalytic
reaction.
Scheme 1 Proposed mechanism for photocatalysis of Bi2MoO6/ZnFe2O4
heterojunctions under visible-light irradiation.
Table 1 Experimental conditions and the atom ratio of Zn and Mo.
Table 2 The first-order kinetic constants (kapp) and relative coefficient (R2) of
different samples.
28