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Accepted Manuscript
Wide Spectral Response Photothermal Catalysis-Fenton Coupling Systems with
3D Hierarchical Fe3O4/Ag/Bi2MoO6 Ternary Hetero-superstructural Magnetic
Microspheres for Efficient High-Toxic Organic Pollutants Removal
Ziyuan Xiu, Yan Cao, Zipeng Xing, Tianyu Zhao, Zhenzi Li, Wei Zhou
PII:
DOI:
Reference:
S0021-9797(18)30961-5
https://doi.org/10.1016/j.jcis.2018.08.047
YJCIS 23980
To appear in:
Journal of Colloid and Interface Science
Received Date:
Revised Date:
Accepted Date:
13 July 2018
11 August 2018
15 August 2018
Please cite this article as: Z. Xiu, Y. Cao, Z. Xing, T. Zhao, Z. Li, W. Zhou, Wide Spectral Response Photothermal
Catalysis-Fenton Coupling Systems with 3D Hierarchical Fe3O4/Ag/Bi2MoO6 Ternary Hetero-superstructural
Magnetic Microspheres for Efficient High-Toxic Organic Pollutants Removal, Journal of Colloid and Interface
Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.08.047
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Wide
Spectral
Response
Catalysis-Fenton
Coupling
Hierarchical
Fe3O4/Ag/Bi2MoO6
Photothermal
Systems
with
3D
Ternary
Hetero-superstructural Magnetic Microspheres for
Efficient High-Toxic Organic Pollutants Removal
Ziyuan Xiua, Yan Caoa, Zipeng Xinga,*, Tianyu Zhaoa, Zhenzi Lib,*, Wei Zhoua,*
a
Department of Environmental Science, School of Chemistry and Materials Science,
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of
the People?s Republic of China, Heilongjiang University, Harbin 150080, P. R. China,
Tel: +86-451-8660-8616, Fax: +86-451-8660-8240,
Email: xingzipeng@hlju.edu.cn; zwchem@hotmail.com
b
Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin
150086, P. R. China
Email: zhenzhenlee2014@163.com
1
Abstract: 3D hierarchical Fe3O4/Ag/Bi2MoO6 magnetic microspheres are fabricated
through hydrothermal-photoreduction strategy, which fabricate an advanced
photocatalytic-Fenton coupling system. The H2O2 produced by photocatalysis could
form the photo-Fenton system when combined with Fe3O4 under light illumination.
The introduction of Ag nanoparticles could induce the localized surface plasmon
resonance (LSPR), which provides ?hot electrons? for promoting photocatalysis. The
addition of Fe3O4 achieve the successful coupling of photocatalysis-Fenton, and
enhanced the broad spectrum response of the final composite sample. The 3D
hierarchical Fe3O4/Ag/Bi2MoO6 exhibit excellent UV-Vis-NIR-driven photocatalytic
performance for mineralization of high-toxic Aatrex and Bisphenol A. The addition of
magnetic Fe3O4 is conducive to the magnetic separation, which favors practical
application. The strategy provides thought-provoking insights for constructing new
types of high-performance photocatalytic system.
Keywords: photocatalysis; Fe3O4/Ag/Bi2 MoO6 hetero-superstructure; Fenton reaction;
localized surface plasmon resonance; magnetic separation
2
1 Introduction
In recent years, serious residues of toxic pollutants pose a great threat to the
natural environment. The traditional biochemical treatment of toxic pollutants is
unsatisfactory. Photocatalysis, as one of the most promising environmentally friendly
technology, is widely used in the degradation of environmental pollutants [1-4].
Bi-based semiconductors, such as Bi2O3, BiOCl, Bi2MoO6, and Bi2WO6, have
attracted extensive attention due to their unique sheet-like structure and high
photocatalytic activity [5-9]. In especial, Bi et al. revealed that Bi2MoO6, as a n-type
semiconductor and a typical Aurivillius oxide, is a promising visible light-driven
photocatalyst because of its narrow band gap of ~2.5 eV and its suitable conduct
band/valance band (CB/VB) edge positions [8,10]. Bi2 MoO6 belongs to Aurivillius
oxide family and consists of stacking of [Bi2O2] layers and [MoO4] layers [11].
However, a drawback of Bi2MoO6 is the low quantum yield, which is caused by the
rapid recombination of photogenerated electron-hole (e--h+) pairs, limiting its
practical application [12]. Noble metals, as doping elements, have attracted great
focus due to the ability to extend the photoresponse to visible-near-infrared (NIR)
region and the high photocatalytic activity [13-14]. Among various noble metals,
silver is the most suitable choice to composite with Bi2 MoO6 because of its relatively
low cost and high stability [15]. Photogenerated hot electron (e-) can transfer from Ag
to conduction bands of Bi2MoO6, thus considerably increasing the separation
efficiency of photogenerated charge carriers, favoring the improvement of
3
photocatalysis [16]. Studies also have proven that the Ag nanoparticles could
effectively absorb visible light due to localized surface plasmon resonance (LSPR)
effect [17-18]. When the plasma photocatalyst irradiated by visible or near infrared
light, due to LSPR, "hot electrons" overflowed from Ag could promote the
photocatalytic activity for protons reduction by the abundant hydrogen active sites
and high charge mobility of the neighboring Bi2MoO6. Therefore, construction of
Ag/Bi2MoO6 will be a good choice for improving the visible-NIR-driven
photocatalytic performance. Besides, the addition of Ag contributes to the acceleration
of photothermal catalysis.
However, the photocatalytic performance of the isolated photocatalytic system is
still unsatisfactory for practical applications [19-21]. Therefore, coupled with
advanced oxidation technology in physical chemistry, Fenton technology, has become
the best choice to solve this problem [22]. The principle of Fenton reaction is to
oxidize organic compounds, such as carboxylic acids, alcohols, esters, and so on, to
inorganic state by the mixture of H2O2 and Fe2+ [23]. The mixed solution of H2O2 and
Fe2+ has strong oxidation property. Magnetite, as a member of Fenton catalysts, has
attracted great attention. Fe3O4 contains Fe2+ cation, which has higher decomposition
ability than Fe3+ on the effect of H2O2 [24-25]. In addition, the Fe3O4 has wide
spectral response to improve the utilization efficiency of light. It is well-known that
the Fenton reaction can produce a large number of oxygen related species through the
following reactions:
4
Fe2+ + H2O2 ? Fe3+ + .OH + OH-
(1)
Fe3+ + H2O2 ? Fe2+ + .OOH + H+
(2)
In the photocatalytic system, the automatically generated H2O2 on the interface
can replace the addition of H2O2, because the photogenerated electrons
instantaneously capture the dissolved oxygen in the water medium, resulting in the
formation of H2O2 [23]. It is also one of the advantages of photocatalytic-Fenton
coupling system. At the same time, the magnetic separation is another advantage of
taking Fe3O4 as Fe2+ source in Fenton technology [26]. The strategy enables the
catalyst to be reused readily [27-29]. Furthermore, Fe3O4 materials with narrow
bandgap can extend the photoresponse to near-infrared region and immensely make
efficient use of the sunlight, thus promoting the formation of photo-thermal catalysis
[30-32]. Hence, combining photocatalysis and Fenton technology to fabricate a
photocatalysis-Fenton system maybe further improve the degradation efficiency.
In this paper, 3D hierarchical Fe3O4/Ag/Bi2MoO6 ternary hetero-superstructural
magnetic microspheres are fabricated through hydrothermal-photoreduction method.
The prepared Fe3O4/Ag/Bi2MoO6 catalysts with a narrow band gap of ~1.6 eV present
excellent photocatalytic property. The multilayer 3D spherical morphology of
Fe3O4/Ag/Bi2 MoO6 also contributes to the light transmission and effective absorption
of the catalyst. The introduction of Fe3O4 and Ag extends the absorption to visible
light and NIR regions. The photocatalytic activities have been evaluated under
simulated sunlight irradiation by removing Bisphenol A (BPA) and Aatrex. Besides,
5
the cycling stability of the catalyst is also measured. Finally, the possible
photocatalytic degradation mechanism of the synergistic photocatalytic-Fenton
coupling system is proposed.
2 Experimental section
2.1 Materials
Ferric chloride hexahydrate (FeCl3�2O), sodium molybdate dihydrate
(Na2MoO4�2O), propanediol, glycol, silver nitrate (AgNO3), and sodium hydroxide
(NaOH) were produced by Tianjin Kermel Chemical Reagent Co. Ltd, China.
Bismuth nitrate pentahydrate (Bi(NO3)3�2O) was purchased from Tianli Fuchen
Chemical Reagent Co. Ltd, China. Natrium acetate was purchased from Tianjin
Guangfu Technology Development Co. Ltd, China. Absolute ethanol (EtOH) was
purchased from Tianli Chemical Reagent Co. Ltd, China. These chemicals were used
in this study without further refined.
2.2 Synthesis
The hierarchical Bi2MoO6 microspheres were synthesized via a solvothermal
approach [33]. Typically, 1.406 g of Bi(NO3)3�2O and 0.412 g of Na2MoO4�2O
were dissolved in 7 mL of glycol, respectively. Then, the two solutions were mixed
together, and 30 mL of ethanol was added into the mixture. After stirring for 30 min,
the mixture was hydrothermally treated in a 50 mL Te?on-lined autoclave at 160 oC
for 12 h. The resulting solution was dried at 60 oC for 10 h in an oven, followed by
washed 6 times with deionized (DI) water and absolute ethanol. The dried powder
6
was calcined at 400 oC for 3 h (2 oC min-1 heating rate) and the final Bi2MoO6
microspheres were obtained. Ag/Bi2MoO6 nanocomposites were prepared by
photoreduction method. 0.5 g of Bi2 MoO6 was dissolved into 30 mL ethanol. Then, 3
mL of AgNO3 solution (A1=1 mg mL-1, A2=3 mg mL-1, A3=5 mg mL-1) was added to
the above solution. The purpose of adding different amounts of AgNO3 to is to find
the best proportion of Ag nanoparticles, so that the final sample has the best
photocatalytic performance. The mixed solution was irradiated with PL-XQ500 W Xe
lamp for 30 minutes and washed with ethanol for 3 times to remove the AgNO3 which
was not reflected. Afterwards, the solution was dried at 40 oC for 2 h to obtain
Ag/Bi2MoO6
microspheres.
3D
hierarchical
Fe3O4/Ag/Bi2 MoO6
magnetic
microspheres were prepared in an alkaline condition. 1.25 g of Ag/Bi2MoO6, 0.8 g of
FeCl3�2O, and 2.85 g of natrium acetate were added into 40 mL of propanediol,
ultrasonic treatment for 10 min. Then, NaOH (1 mol/L) was added into the solution
with fully stirred, so that the pH value was adjusted to about 9. After stirring for 30
min at 90 oC, the solution was hydrothermally treated in a 50 mL Te?on-lined
autoclave at 160 oC for 12 h to disperse Fe3O4 nanoparticle on the Ag/Bi2 MoO6
surface. The suspension was dried at 60 oC for 5 h in an oven, followed by washed for
several times with DI water and absolute ethanol. Finally, Fe 3O4/Ag/Bi2MoO6
multi-effect composite material was obtained (as shown in Fig. 1). For comparison,
the Fe3O4 was synthesized under the same conditions in the absence of Ag/Bi2MoO6.
7
Figure 1. Schematic diagram for the formation of 3D hierarchical Fe3O4/Ag/Bi2MoO6
magnetic microspheres.
2.3 Characterization
The crystal structure of the Fe3O4/Ag/Bi2 MoO6 (A2) was measured using X-ray
diffraction (XRD, Bruker D8 Advance di?ractometer, Gobel mirror monochromated
Cu K? radiation, ? = 1.54056 �). The morphologies of these as-prepared samples
were observed with a Hitachi S-4800 ?eld emission scanning electron microscope
(SEM) with a Philips XL-30-ESEM-FEG instrument operating (20 kV). For
transmission electron microscopy (TEM) and high-resolution TEM (HRTEM)
observation, final sample was dispersed in absolute ethanol by sonication and then
dropped onto a TEM grid. TEM was carried out at 200 kV using JEM-2100
microscope. X-ray photoelectron spectroscopy (XPS) was used to detect the
8
distribution of Fe, Ag, Bi, Mo and O in the Fe3O4/Ag/Bi2 MoO6 (A2) composite
material and carried out with a Thermal Scientific K-Alpha instrument. The UV-vis
diffuse reflectance spectra (DRS) of the samples were recorded on a Lambda 750
UV-Vis-NIR spectrophotometer (Perkin-Elmer, USA), using BaSO4 as the
background ranging from 190 to 2000 nm. The total organic carbon (TOC) removal
was measured using the TOC analysis equipped with analytic jena multi NIC 2100
analyzer.
The
.
OH
radicals
were
measured
by RF-5301PC
fluorescence
spectrophotometer at room temperature. The magnetization was measured under room
temperature on a vibrating sample magnetometer (VSM, Lake Shore 7404, USA).
Scanning Kelvin probe (SKP) test (SKP5050 system, Scotland) was executed to
evaluate the work function at ambient atmosphere. Photoluminescence (PL)
measurements were taken using a FL920 spectrofluorometer (Edinburgh Instruments)
with the photoexcitation wavelength set at 350 nm.
2.4 Photoelectrochemical measurement
Electrochemical impedance spectroscopy (EIS) and Mott-Schottky curve were
performed on an electrochemical analyzer (CHI660E, Shanghai) equipped with a
standard three-electrode system and using Na2SO4 (0.1 M) as electrolyte solution, Pt
foil as the counter electrode, Ag/AgCl as a reference electrode.
2.5 Photocatalytic degradation
The liquid phase photodegradation of Aatrex and BPA was used to evaluate the
performance of photodegradation under simulated sunlight irradiation at room
9
temperature, under a 300 W Xeon-lamp equipped with a 420 nm cut-off filter. In a
typical degradation test, 35 mg samples (Fe3O4/Ag/Bi2MoO6, Ag/Bi2MoO6, Bi2MoO6,
Fe3O4) were dispersed in 50 mL BPA (10 mg L-1) and Aatrex (10 mg L-1) aqueous
solution, respectively, and magnetically stirred for 30 min in the dark to keep
adsorption equilibrium. The reaction tube was placed at 20 cm from the Xe light
source. After photocatalytic reaction for 120 min, the reaction solution was
immediately centrifuged and filtrated to separate the suspended catalysts. After
photo-degradation, the samples were collected and filtered, tested by total organic
carbon (TOC) analyzer.
3 Results and discussion
The crystalline structures of the hierarchical Bi2MoO6 microspheres, Fe3O4
nanoparticles, Ag/Bi2MoO6 (A2), and Fe3O4/Ag/Bi2MoO6 (A2) are identi?ed by
X-ray diffraction (XRD). As shown in Fig. 2a, five peaks for Fe 3O4 nanoparticles
located at 30.1, 35.4, 43.1, 57.0, and 62.5o are well matched with the (220), (311),
(400), (333), and (440) plane of cubic Fe3O4, respectively. The purity of Fe3O4
nanoparticles is high because no other impurity phases are detected. The diffraction
peaks of orthorhombic Bi2 MoO6 can be indexed to (131), (002), (060), (202), (062),
(331), (191), and (262) planes. Compared with pure Bi2MoO6, all diffraction peaks of
Ag/Bi2MoO6 composite perfectly attributed to the characteristic peaks of Bi2 MoO6.
Obviously, the characteristic peaks of Ag are not observed because of the small
amount of Ag. The additive amount of Fe3O4 is little because the excess Fe3O4 will
10
block the active site of Bi2MoO6. In diffraction peaks of Fe3O4/Ag/Bi2MoO6 (A2)
composite materials, except for the typical Bi2MoO6 peaks, five characteristic peaks
of Fe3O4 can be detected, confirming the efficient construction of Fe3O4/Ag/Bi2 MoO6
(A2) composite materials.
Figure 2. XRD patterns (a) of Bi2MoO6, Fe3O4, Ag/Bi2MoO6 (A2), and
Fe3O4/Ag/Bi2 MoO6 (A2), respectively. SEM image (b) of Bi2MoO6, SEM (c), TEM
(d), and HRTEM images (e) of Fe3O4/Ag/Bi2MoO6 (A2).
As shown in Fig. 2b, the Bi2MoO6 microspheres are formed by coating irregular
flakes, showing similar hierarchical morphology, as reported [11]. The Yellow dashed
11
line in Fig. S1a is a surface broken Bi2MoO6 microsphere. Unlike traditional solid
catalysts, this hierarchical structure promotes the light to pass through the sample in
the process of photocatalytic degradation and improve the sample utilization. One of
the main reasons for the low catalytic performance of traditional catalysts is their low
utilization of light [17]. However, the 3D hierarchical structure overcomes the
shortcoming of low utilization of light. Light enters the next layer through the gap
between flake and flake, as shown in Fig. S1b. The diameter of Bi2MoO6 hierarchical
microspheres is about 2 ?m. The Fe3O4/Ag/Bi2MoO6 (A2) exhibits a globular
structure similar to that of Bi2MoO6 (Fig. 2c), except that the surface is coated with
Fe3O4 nanoparticles. The amount of Fe3O4 is relatively small and does not completely
cover the Bi2 MoO6 microspheres, because excessive Fe3O4 will block the active sites
of Bi2MoO6 and reduce the photocatalytic degradation. Fig. 2d is an edge map of
composite microspheres enlargement. In order to clearly observe the Fe 3O4
nanoparticles and Ag nanoparticles dispersed on surface of Bi2 MoO6 uniformly, one
nanosheet is selected to enlarge (yellow box illustration). The detailed structure is also
characterized by HRTEM (Fig. 2e). The lattice fringes with spacing of 0.315 nm is
ascribed to (131) plane of Bi2MoO6, while those with spacing of 0.24 and 0.26 nm
correspond to (111) facets of Ag and (311) facets of Fe3O4, respectively. The regions
of blue and red circles in Fig. 2e are Ag nanoparticle and Fe 3O4 nanoparticle,
respectively. It can be seen from the electron micrograph that Ag, Bi2MoO6, and
Fe3O4 are successfully combined. Energy Dispersive X-Ray (EDX) is also verified the
12
above result, as shown in Fig. S2.
Figure 3. Molecular structure of Bi2MoO6 (a), and single crystal cell molecular
structure of Fe3O4 (b), Ag (c), Bi2MoO6 (d). XPS spectra of Bi 4f (e), Mo 3d (f), Ag
3d (g), and Fe 2p (h) for Fe3O4/Ag/Bi2 MoO6 (A2).
In order to show the internal structure of the three substances more distinctly, the
layered molecular structure of Bi2MoO6, and the molecular structure of the single
crystal cell of Fe3O4, Ag, Bi2MoO6 are displayed in Fig. 3a-d. X-ray photoelectron
spectroscopy (XPS) is used to determine the surface chemical composition and
chemical state of the as-prepared Fe3O4/Ag/Bi2MoO6 (A2). The full-scale XPS
spectrum is displayed in Fig. S3, demonstrating that the final sample is mainly
composed of Bi, Mo, Fe, O, and Ag. The atom percentages of Fe 3O4/Ag/Bi2MoO6 (A2)
are shown in the illustration of Fig. S2. For Bi 4f, the peaks are characteristic of two
main peaks located around 159.0 and 164.6 eV (Fig. 3e), corresponding to Bi 4f7/2 and
Bi 4f5/2 orbitals, respectively. As shown in Fig. 3f, two peaks at 232.9 (Mo 3d 5/2) and
235.9 eV (Mo 3d3/2) are observed, which are linked to Mo-O bond. Fig 3g gives the
13
peaks located at 367.6 and 373.6 eV for Ag 3d5/2 and Ag 3d3/2. A spin-orbit separation
of 6.0 eV illustrates the characteristic of metallic silver (Ag 0). The characteristic peak
of Ag+ was not detected, indicating that AgNO3 was fully reduced to Ag nanoparticles.
As shown in Fig. 3h, the XPS of Fe 2p regions can be fitted into two contributions.
Peaks at around 711.5 and 724.7 eV are assigned to Fe 2p 3/2 and Fe 2p1/2, respectively,
which are linked to Fe2+ and Fe3+. The common effect of Ag, Fe3O4 and Bi2 MoO6 can
improve the performance of the original Bi2 MoO6, which applies to the above
discussions.
Figure. 4 The IR images of Bi2MoO6 (a), Fe3O4/Ag/Bi2MoO6 (b), respectively.
The influence of surface temperature change on the photo-thermal effect was
investigated by IR thermal driver. The direct heat radiation of xenon lamp and the
heat generated by the photo-thermal effect of the prepared sample are the two parts of
the thermal energy provided in the photo-thermal catalytic reaction. As shown in the
scaleplate in Fig. 4, the color of the infrared image varies with temperature. Before
irradiation, the temperature of the reactor is controlled at about 27 oC. After 1 minutes
14
of irradiation, different samples showed different temperature changes, corresponding
to different photothermal effects. As shown in Fig. 4a and 4b, after 1min of irradiation,
the surface temperature of the final sample Fe3O4/Ag/Bi2 MOO6 is greatly improved
compared with that of pure Bi2OMO6, which is attributed to the wide spectrum
response of Fe3O4 and the surface plasmon resonance effect produced by the addition
of Ag. In addition, the IR images of Ag/Bi2MoO6, Fe3O4/Bi2MoO6, and
Fe3O4/Ag/Bi2 MoO6 are analyzed under 30 seconds radiation of xenon lamp. The
results show that the addition of Fe3O4 and Ag enhances the absorption of light and
promotes the enhancement of photothermal response. It can be seen from the graph
that the temperature has a small amplitude increase with the increase of irradiation
time.
In order to further evaluate the outstanding photocatalytic activity of
Fe3O4/Ag/Bi2 MoO6, the degradation of Aatrex and BPA are carried out under
simulated sunlight irradiation. In order to detect the best conditions for degradation of
toxic pollutants, it is necessary to make a group of Fe3O4/Ag/Bi2 MoO6 (n)
degradation experiments in different pH value pollutants in advance (Fig. S5a-d).
According to the results of four contrastive tests, the acid condition of solution (pH=3)
is the best condition for degradation. The above results are attributed to .OH with high
oxidation potential under low pH value, so Fenton process is more effective under
acidic conditions [37]. In addition, the best degradation effect in 4 solution
environments is Fe3O4/Ag/Bi2MoO6 (A2), which shows that the particle size increases
15
with the increase of the Ag nanoparticles content [4, 13, 17, 25], thus reducing the
photocatalytic performance of the sample. According to the best conditions, the
following discussion on degradation of pollutants is Fe3O4/Ag/Bi2 MoO6 (A2).
Besides, As shown in Fig. 5a and 5e, the changes of Aatrex and BPA concentrations
under different degradation time are monitored. With the increase of irradiation time,
the peak intensity of characteristic of Aatrex and BPA decrease continuously. After
150 min, the absorption peak is almost disappeared. The line chart (Fig. 5b and 5f)
can clearly and intuitively show the degradation rate of the four contrast samples
every 30 min. The degradation efficiency of Fe3O4/Ag/Bi2 MoO6 (A2) is the best, and
the degradation efficiency of Aatrex and BPA are 2.3 and 3.8 times higher than that of
the pristine Bi2MoO6, respectively. The histogram of Fig. 5c and Fig. 5g shows the
final degradation rate after 150 min. The degradation rates of Aatrex for Fe 3O4,
Bi2MoO6, Ag/Bi2MoO6 (A2), and Fe3O4/Ag/Bi2 MoO6 (A2) are 22.2, 41.8, 54.3 and
98.9%, respectively. And for BPA, the degradation rates are 7.2, 29, 60.5 and 99.2%,
respectively. The results show that the Fe3O4/Ag/Bi2MoO6 (A2) has the highest
mineralization performance compared with others. The high mineralization properties
of the samples are attributed to the introduction of advanced oxidation techniques to
improve the degradation efficiency of highly toxic organic pollutants. The stability of
the catalyst is also the standard to verify its efficiency. After being reused for four
times of Aatrex and BPA, the photocatalytic activity has no obvious change, which
proves that the Fe3O4/Ag/Bi2MoO6 (A2) has good stability, as shown in Fig. 5d and
16
5h, implying the potential applications in environment.
Figure 5. UV absorbance at different times for the degradation of BPA and Aatrex in
the presence of Fe3O4/Ag/Bi2MoO6 (A2) (a and e), the test of photocatalytic
degradation BPA and Aatrex for as-prepared samples (b and f), TOC removal of
as-prepared samples (c and g), and four cycling tests of photocatalytic degradation
17
BPA and Aatrex for Fe3O4/Ag/Bi2MoO6 (A2) (d and h).
The reason why Fe3O4/Ag/Bi2MoO6 (A2) has such good degradation
performance is the coupling of Fe3O4 and Bi2MoO6 photocatalyst, which can form
H2O2 in the system, thus realizing the coupling of photocatalysis and Fenton
technology. At the same time, adding noble metal Ag in the system is helpful to the
formation of oxygen radicals. The photoexcited Fe3O4/Ag/Bi2 MoO6 (A2) produces
H2O2 via the oxidation of water with AgNO3 as an electron acceptor (Fig. S5) and the
reduction of O2 with EtOH as an electron donor. The variation trend of H 2O2
concentration is shown in Fig. S6. The concentration is up to about 45 ?M h-1.
However, it is significant to explore the active species which play a major role in
the photocatalytic oxidation degradation of BPA and Aatrex. Three kinds of scavenger
agents, tert-butanol, EDTA-2Na, and benzoquinone, are used as capture agent to
capture hydroxyl radical ( ?OH), photogenerated holes (h+), and superoxide
radicals(稯2-), respectively. As shown in Fig. 6a, the photocatalytic degradation rate of
BPA is reduced to about 0.6 times of that without benzoquinone degradation, which
indicated that O2- has an effect on the degradation of BPA. However, in the presence
of EDTA-2Na, the degradation rate of BPA shows no obvious downward trend,
indicating that the h+ has an extremely weak influence on the degradation of BPA.
Most important of all, when adding tert-butanol, the degradation of BPA is greatly
restrained, suggesting that the primary active substance in the process of BPA
degradation is .OH. ?OH are identified as the major species in the photocatalytic
18
Figure 6. Degradation of BPA for Fe3O4/Ag/Bi2MoO6 (A2) sample with addition of
different sacrificial agents (tert-butanol, benzoquinone and EDTA-2Na) (a),
fluorescence intensity under 1 h irradiation by using terephthalic acid (TA) solution
for four samples (b), UV-visible diffuse reflectance spectra (c), determination of the
indirect interband transition energies (d) of Bi2MoO6, Ag/Bi2 MoO6 (A2),
Bi2MoO6/Fe3O4, and Fe3O4/Ag/Bi2MoO6 (A2), Mott-Schottky plots (e), and PL decay
curves of different samples (f).
reaction. Terephthalic acid solution (TA) is used as a fluorescent probe because it can
19
react with .OH to produce 2-hydroxy terephthalic acid in basic solution. As shown in
Fig. 6b, the fluorescence signal appears at about 425 nm, and the fluorescence
intensity of Fe3O4/Ag/Bi2MoO6 (A2) is the highest in four contrast samples at 425 nm.
This is powerful evidence that Fe3O4/Ag/Bi2MoO6 (A2) can produce the most .OH
under simulated sunlight illumination. This result also confirms the results of previous
degradation experiments, in which the catalyst with the maximum .OH yield has the
best photocatalytic activity.
The absorption properties and band gaps of the samples are assessed by UV/Vis
diffuse reflectance spectra (DRS). As displayed in Fig. 6c, Bi2MoO6 exhibits light
absorption in visible light range, and the absorption edge of Bi2MoO6 are located
around 471 nm. The comparison between Bi2MoO6/Fe3O4 and Fe3O4/Ag/Bi2MoO6
(A2) shows that the characteristic absorption peaks (479 nm) in visible light region
can be observed after introducing silver into the system, which may be due to the
LSPR effect of silver. Compared with pristine Bi2 MoO6, the Fe3O4/Ag/Bi2MoO6 (A2)
gives a better light response in UV, visible light, and near infrared region owing to the
black color of coupling Fe3O4 and Ag. The absorption intensity of Fe3O4/Ag/Bi2 MoO6
(A2) is only lower than that of pristine Fe3O4. Furthermore, the values of band gap
energies for the five contrast samples are also calculated, as shown in Fig. 6b and Fig.
S7,
the
band
gaps
of
Bi2 MoO6,
Bi2MoO6/Fe3O4,
Ag/Bi2MoO6
(A2),
Fe3O4/Ag/Bi2 MoO6 (A2), and Fe3O4 are 2.48, 2.30, 2.02, 1.60, and 0.10 eV,
respectively. The band gap narrowing contributes to the enhancement of
20
visible-NIR-driven photocatalytic activity, so the Fe3O4/Ag/Bi2 MoO6 (A2) exhibits
the best photocatalytic performance.
The results of Mott-Schottky (M-S) analysis for five samples are presented in Fig.
6e. The result shows that Fe3O4/Ag/Bi2MoO6 (A2) shows a smaller slope in M-S plot
than those of other four samples, indicating a higher charge carrier density and better
conductivity. Carrier density can be calculated from the corresponding slope
according to Eq. (3):
Nd ?
2 / e0?? 0
d (1 / C 2 ) / dV
(3)
where Nd, e0, ?, ?0, refer to the carrier density, electron charge, dielectric constant, and
permittivity of vacuum, respectively [34-36]. Taking the dielectric coef?cient as 34, 9,
and 19 for Fe3O4, Ag, and Bi2MoO6, the electron densities of the Fe3O4, Bi2MoO6,
Ag/Bi2MoO6 (A2), Fe3O4/Bi2MoO6 and Fe3O4/Ag/Bi2MoO6 (A2) are 0.42�20,
0.52�20, 0.63�20, 0.86�20, and 3.3�20 cm-3, respectively. The results show
that the photogenerated carrier lifetime of the final sample Fe3O4/Ag/Bi2MoO6 (A2)
increases, thus increasing the charge carrier mobility and then enhance photocatalytic
and photoelectrochemical property. At the same time, the electrochemical impedance
spectroscopy (EIS) is used to analyze the electrical properties of the interface between
the electrode and the solution (Fig. S8). The diameters of the semicircles are
equivalent to the charge-transfer resistance of the four as-prepared samples. Fe3O4
shows
an
exciting
electrochemical
21
performance.
The
final
sample
(Fe3O4/Ag/Bi2MoO6 (A2)) also shows the electrochemical performance almost next to
pristine Fe3O4, which can be attributed to plasma resonance and photocatalytic
coupling with Fenton.
In order to further verify the reason that the final product has such a dramatic
degradation effect, the time-resolved PL studies of Bi2 MoO6 and Fe3O4/Ag/Bi2MoO6
(A2) are used to detect the carrier lifetime of initial and final sample. The insert of Fig.
6f summarizes positron lifetime and relative intensities of the prepared samples. The
photogenerated charge carriers lifetime of the Fe3O4/Ag/Bi2MoO6 (A2) (6.2�3 ns)
is significantly longer than that of Bi2 MoO6 (5.6�3 ns). The result shows that the
coupling of photocatalytic and Fenton technology and the surface plasmon resonance
effect can prolong the lifetime of the photogenerated charge carriers, thus favoring the
improvement of photocatalytic performance.
Figure 7. The Scanning Kelvin probe maps (A) of pristine Fe3O4 (a), Bi2MoO6 (b),
Bi2MoO6/Fe3O4 (c), Ag/Bi2MoO6 (A2) (d), and Fe3O4/Ag/Bi2MoO6 (A2) (e),
respectively. Magnetization hysteresis loops (B) of pure Fe3O4, Bi2MoO6, and
Fe3O4/Ag/Bi2 MoO6 (A2) (a). The insets are the photos of magnetic separation.
22
Scanning Kelvin probe (SKP) technique is carried out to characterize the work
function, which is an important parameter to characterize surface electron transfer and
chemical reaction. The SKP maps of pristine Fe3O4 (a), Bi2MoO6 (b), Bi2MoO6/Fe3O4
(c), Ag/Bi2MoO6 (A2) (d), and Fe3O4/Ag/Bi2MoO6 (A2) (e) are shown in Fig. 7A.
The work function of Fe3O4/Ag/Bi2MoO6 (A2) (~5.06 eV) is significantly the lowest,
implying the approximate Fermi level of Fe3O4/Ag/Bi2MoO6 (A2) is higher than that
of other four contrast samples, which enhances the built-in electric field and surface
band bending. Thus, the separation of photogenerated electron-hole pairs can be
effectively improved. The results show that the noble metal deposition and
photocatalytic-Fenton coupling can effectively reduce the recombination rate of
photogenerated electron-hole, which is beneficial to improve the photocatalytic
activity.
Magnetic properties of pristine Fe3O4, Bi2 MoO6, and Fe3O4/Ag/Bi2MoO6 (A2)
are also detected by vibrating sample magnetometer (VSM). From the magnetic
hysteresis loop of Fig. 7B, it can be seen that both Fe3O4 and Fe3O4/Ag/Bi2MoO6 (A2)
have typical ferromagnetic behaviors with saturation magnetization of 88 and 60 emu
g?1, respectively. The magnetic properties of pristine Bi2MoO6 are negligible.
Although the final composite sample has a slight decrease in magnetic properties, the
composite photocatalyst still exhibits magnetic property. The introduction of magnetic
Fe3O4 can result in strong oxidation of the photocatalyst, and solve the problem of
poor separation of the catalyst. It can be seen from the inset of Fig. 7B that the
23
catalyst can be easily magnetically separated.
According to all above experimental results and discussions, the charge transfer
in Fe3O4/Ag/Bi2 MoO6 and the proposed mechanism for pollutants (Aatrex and BPA)
decomposition are schematically illustrated as shown in Fig. 8. The CB position of
Bi2MoO6 hierarchical microspheres is higher than that of Fe3O4 nanoparticles. The
addition of Ag nanoparticles is considered as ?hot electrons? to transfer electrons to
the conduction band of Bi2MoO6 and Fe3O4, respectively. Essentially, electron
transfer from Ag to Fe3O4 can promote the reduction of Fe3+ and Fe2+ in Fe3O4 to Fe2+
and Fe0, respectively. Electron (e-) in the Bi2MoO6 hierarchical microspheres instantly
captures dissolved oxygen in the aqueous medium, resulting in the formation of H2O2
in the system. H2O2 can also oxidize Fe2+ and Fe0 to Fe3+ and Fe2+, respectively.
Moreover, the access of e - is also opened to produce both 稯H and 稯2? through
Fenton process, which dramatically increases the amount of active radicals and
therefore greatly enhances photoactivity. At the same time, the pollutants can be
effectively decomposed into CO2 and H2O through the VB.
24
Figure 8. Schematic illustration of the visible-NIR-driven photocatalytic-Fenton
coupling mechanism for Fe3O4/Ag/Bi2MoO6 hierarchical microspheres.
4 Conclusions
In summary, novel Fe3O4/Ag/Bi2MoO6 hierarchical microspheres composite was
fabricated as a high-performance UV-Vis-NIR-driven photocatalyst. The degradation
rates of Aatrex and BPA (pH=3) for resultant Fe3O4/Ag/Bi2 MoO6 (A2) were the
largest under simulated sunlight irradiation, which were 2.3 and 3.8 times higher than
that of pristine Bi2MoO6, respectively. The cycle of Fe3O4/Ag/Bi2MoO6 (A2) was also
supernormal, indicating the high stability. Besides, the easily magnetic separation was
favorable for practical applications in environmental fields due to the magnetism of
Fe3O4. Fe3O4/Ag/Bi2MoO6, with high magnetic properties, wide spectrum response,
thermal effect, and high oxidation degradation performance, its preparation strategy
provides new insights for high-performance catalysts in future.
25
Acknowledgments
We gratefully acknowledge the support of this research by the National Natural
Science Foundation of China (51672073), the Natural Science Foundation of
Heilongjiang Province (B2018010 and H2018012), the Heilongjiang Postdoctoral
Startup Fund (LBH-Q14135), the University Nursing Program for Young Scholars
with Creative Talents in Heilongjiang Province (UNPYSCT-2015014 and
UNPYSCT-2016018), and the Postgraduate Innovative Science Research Project of
Heilongjiang University (YJSCX2018-165HLJU).
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Graphical Abstract
Efficient UV-Vis-NIR-Driven Photocatalytic-Fenton Coupling Systems with 3D
Hierarchical Fe3O4/Ag/Bi2MoO6 Ternary Hetero-superstructural Magnetic
Microspheres are fabricated via hydrothermal-photoreduction strategy and exhibit
excellent photocatalytic performance, which is ascribed to the introduction of Ag, the
coupling of photocatalytic-Fenton, the enhanced absorption of solar light, and the
increased organic pollutants oxidation ability in water.
33
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