Facile synthesis of Bi2MoO6-MIL-100(Fe) metal-organic framework composites with enhanced photocatalytic performance

Article abstract of DOI:10.1039/c6ra26110h

A novel Bi₂MoO₆-MIL-100(Fe) heterostructure photocatalyst was synthesized by a facile solvothermal method for the first time, and characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectra, transmission electron microscopy, and photoluminescence spectra. The as-prepared Bi₂MoO₆-MIL-100(Fe) composites exhibited excellent photocatalytic activity and stability towards the degradation of Rhodamine B (RhB) under visible light irradiation. The introduction of MIL-100(Fe) into semiconductor Bi₂MoO₆ improved the photocatalytic performance remarkably, and the optimal composite with 9% MIL-100(Fe) content showed the highest photocatalytic activity. According to the active radical trapping experiments, the h⁺ and &z.rad;O₂^- radicals were the two main active species in the photocatalytic reaction. The enhanced photocatalytic performance is mainly attributed to the formation of the Bi₂MoO₆-MIL-100(Fe) heterostructure, which is beneficial to the transfer and separation of photo-generated electron-hole pairs. The possible mechanism of the enhanced photocatalytic performance of the Bi₂MoO₆-MIL-100(Fe) composites was discussed.

Full text of DOI:10.1039/c6ra26110h

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Facile synthesis of Bi2MoO6–MIL-100(Fe)
metal–organic framework composites
Cite this: RSC Adv., 2017, 7, 2943
with enhanced photocatalytic performance
abcd
Jia Yang,abc Xiaojun Niu,
Shaorong An,abc Weiyi Chen,abc Jie Wangabc
*
and Wei Liuabc
A novel Bi2MoO6–MIL-100(Fe) heterostructure photocatalyst was synthesized by a facile solvothermal
method for the first time, and characterized by X-ray diffraction, scanning electron microscopy, X-ray
photoelectron spectroscopy, UV-vis diffuse reflectance spectra, transmission electron microscopy, and
photoluminescence spectra. The as-prepared Bi2MoO6–MIL-100(Fe) composites exhibited excellent
photocatalytic activity and stability towards the degradation of Rhodamine B (RhB) under visible light
irradiation. The introduction of MIL-100(Fe) into semiconductor Bi2MoO6 improved the photocatalytic
performance remarkably, and the optimal composite with 9% MIL-100(Fe) content showed the highest
photocatalytic activity. According to the active radical trapping experiments, the h+ and cO2 radicals
ꢀ
were the two main active species in the photocatalytic reaction. The enhanced photocatalytic
performance is mainly attributed to the formation of the Bi2MoO6–MIL-100(Fe) heterostructure, which is
beneficial to the transfer and separation of photo-generated electron–hole pairs. The possible
mechanism of the enhanced photocatalytic performance of the Bi2MoO6–MIL-100(Fe) composites was
discussed.
Received 1st November 2016
Accepted 4th December 2016
DOI: 10.1039/c6ra26110h
www.rsc.org/advances
2.8 eV), which makes it utilize more sunlight and displays
remarkable photocatalytic performance for organic pollutant
degradation under visible light irradiation.10 However, the
1. Introduction
The semiconductor photocatalysis technique has attracted exten-
sive attention in the splitting of water and the degradation of
organic pollutants using sunlight as the energy source.1 Among
the various photocatalysts, TiO2 is an attractive photocatalyst,
because of its low toxicity, high photocatalytic activity and long
term chemical stability.2–4 However, the applications of TiO2 are
severely limited, owing to it only being excited by ultraviolet (UV)
light which accounts for less than 5% of the solar spectrum.
Therefore, many reports are focused on developing visible light
responsive photocatalytic materials to replace TiO2, such as
Bi2MoO6,5 In2O3,6 Ag3PO4,7 and g-C3N4 (ref. 8) photocatalysts.
Bi2MoO6 is a typical Aurivillius oxide, exhibited excellent
photocatalytic activity under the visible light irradiation.9
Compared with TiO2, Bi2MoO6 has a lower band gap (2.5–
practical application of the pure phase Bi2MoO6 is limited by its
poor quantum yield, which is caused by the rapid recombina-
tion of photoinduced charge carriers. To further improve the
photocatalytic activity of Bi2MoO6, many feasible methods have
been explored, including designing textural properties, doping
composites11,12 and combining with other semiconductors,13–15
carbon materials16 or noble metals.17 Nevertheless, there is still
plenty of space to explore unfullled potentials for fabricating
Bi2MoO6-based photocatalysts.
Metal–organic frameworks (MOFs) are an extensive class of
crystalline materials formed by the self-assembly between metal
ions and organic ligands.18 Recently, they have obtained
extensive attention owing to their huge potential applications in
sensing,19 gas storage,20 drug delivery,21 optical properties22 and
catalysis. These MOFs composites can act as catalysts, using
unsaturated metal sites and catalytically active organic lik-
ers.23,24 Additionally, MOFs exhibit tunable photo-physical and
photo-chemical properties, it can work well as excellent candi-
dates for photocatalysis. For example, Dan-Hardi et al. synthe-
sized a new photoactive crystalline titanium dicarboxylate
(MIL-125(Ti)), which exhibits high photonic sensitivity under
UV-visible irradiation.25 Hou Wang et al. had reported that
In2S3@MIL-125(Ti) can remove the tetracycline by adsorption
and photocatalysis. These reports showed that some MOFs can
aSchool of Environment and Energy, College of Environment and Energy, South China
University of Technology, Guangzhou, 510006, China. E-mail: xjniu@scut.edu.cn; Fax:
+86-20-3934-9919; Tel: +86-20-3934-9919
bGuangdong Provincial Key Laboratory of Atmospheric Environment and Pollution
Control, South China University of Technology, Guangzhou Higher Education Mega
Centre, Guangzhou 510006, PR China
cThe Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,
Ministry of Education, South China University of Technology, Guangzhou Higher
Education Mega Centre, Guangzhou 510006, PR China
dThe State Key Laboratory of Lake Science and Environment, Nanjing Institute of
Geography and Limnology, Chinese Academy of Sciences, Nanjing, 210008, PR China
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be applied in photocatalysis.26 Compared with traditional to the method, as-synthesized composites with expected MIL-
semiconductor photocatalysts, the superiority of MOFs is their 100(Fe) contents of 3, 6, 9 and 12% were synthesized and
ultrahigh surface area and narrow micropore distribution named as Bi2MoO6–MIL-100(Fe)-3, Bi2MoO6–MIL-100(Fe)-6,
which may lead to the formation of monodisperse photoactive Bi2MoO6–MIL-100(Fe)-9 and Bi2MoO6–MIL-100(Fe)-12, respec-
species supported on MOFs.27 Recently, several semi- tively. For comparison, the Bi2MoO6–MIL-100(Fe) physical
conductor@MOF heterostructures like CdS@MIL-100(Fe),28 mixture composites were synthesized by simple mechanical
CdS@UiO-66(NH2),18 ZnO@ZIF-8,29 and GaN@ZIF-8 (ref. 30) mixture, and named as Bi2MoO6–MIL-100(Fe)-9(M). As
have been synthesized and exhibited excellent catalytic perfor- a control, pure Bi2MoO6 was also prepared in ethanol under the
mance due to their synergistic effect. However, the photo- same reaction conditions as that of Bi2MoO6–MIL-100(Fe)
catalytic performance of Bi2MoO6–MOF composites, to the best composites except for the addition of MIL-100(Fe).
of our knowledge, has not been investigated.
Herein, we report a novel Bi2MoO6–MIL-100(Fe) composite that
shows excellent photocatalytic activity in degradation organic
pollutants. A series of Bi2MoO6–MIL-100(Fe) composites with
different weight addition ratios of MIL-100(Fe) have been
successfully synthesized via a facile solvothermal method. The
photocatalytic performance was evaluated by the degradation of
Rhodamine B under visible light irradiation. The combination of
MIL-100(Fe) remarkably improved the photostability and the
photocatalytic efficiency of the photocatalyst for degradation RhB.
Furthermore, the possible mechanism of the enhanced photo-
catalytic performance of the Bi2MoO6–MIL-100(Fe) composite was
discussed.
2.4 Characterization
X-ray diffraction (XRD) patterns of all samples were carried out
using an Empyrean diffractometer (PANalytical B.V., Nether-
lands), with Cu Ka radiation (l ¼ 1.54184 A) in the 2q range of
˚
10–80ꢁ. The morphologies and microstructures of the products
were characterized by a Merlin eld emission scanning electron
microscope (FE-SEM, Zeiss). Transmission electron microscopy
(TEM) images and high-resolution transmission electron
microscopy (HRTEM) images were obtained by FEI Tecnai G2
F20 microscope at an accelerating voltage of 200 kV. The Bru-
nauer–Emmett–Teller (BET) surface area was examined by
nitrogen adsorption–desorption isotherm at 77 K on Micro-
meritics Instrument 3Flex adsorption analyzer. X-ray photo-
electron spectroscopy (XPS) was performed on a Thermo Escalb
250Xi spectrometer with a monochromatic Al (Ka) radiation.
The UV-vis diffuse reectance spectra (DRS) of samples were
measured with Hitachi U-4100 UV-vis spectrophotometer. The
photoluminescence (PL) spectra were tested with an FLS 920
FluorescencLe spectrometer at room temperature.
2. Experimental
2.1 Materials
Bismuth nitrate pentahydrate, sodium molybdate dehydrate
were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd.
(Tianjin, China), benzene-1,3,5-tricarboxylic acid (H3btc) and Fe
powder were purchased from Aladdin Industrial Corporation
(Shanghai, China). Ethylene glycol (EG), and ethanol (EtOH)
were purchased from Sinopharm Chemical Reagent Co., Ltd.
2.5 Photocatalytic evaluation
(Shanghai, China). All chemicals were analytic grade and used The photocatalytic activity of samples was evaluated by the
without further purication.
degradation of Rhodamine B (RhB). In experiments, 0.1 g
samples were added to 100 mL aqueous solution of RhB (10 mg
Lꢀ1) in a BL-GHX photochemical reactor (Shanghai Bilon
Instrument. ltd. China). A 500 W Xe lamp was used as the visible-
light source with 420 nm cut-off lter. Before irradiation, the
suspension was kept in the dark under stirring for 30 min to
ensure an adsorption–desorption equilibrium. Aer a given
irradiation, about 4 mL suspension was collected, centrifuged
and analyzed immediately. The RhB concentration were analyzed
by a UV-vis spectrometer (Agilent 8453, USA) at 554. The TOC
concentration was analyzed by Shimadzu TOC-LCPN analyzer.
The degradation efficiency (%) was calculated as follows:
2.2 Preparation of MIL-100(Fe)
According to the previous literature,31,32 MIL-100(Fe) was
prepared by solvothermal method. In detail, a mixture of Fe
powder with H3btc, HF, HNO3 and H2O with a molar ratio of
1 : 0.67 : 2 : 0.6 : 2.77 was synthesized in a Teon-lined stain-
less steel autoclave at 150 ꢁC for 24 h. Then the orange solid was
centrifuged, washed with ethanol and hot water for several
ꢁ
times, and nally dried overnight 60 C under vacuum.
2.3 Synthesis of Bi2MoO6–MIL-100(Fe) composite
ðC0 ꢀ CÞ
Degradation ð%Þ ¼
ꢂ 100%
(1)
The as-prepared MIL-100(Fe) with the desired weight addition
was dispersed in 10 mL ethanol by ultrasonication to gain the
homogeneous MIL-100(Fe)–EtOH dispersion. Next, 2 mmol
Bi(NO3)3$5H2O and 1 mmol Na2MoO4$2H2O dissolved in 20 mL
ethylene glycol were dropwised into MIL-100(Fe)–EtOH disper-
sion. Aer being stirred for 30 min at room temperature, the
mixed solution was transferred into a 50 mL Teon-lined
stainless steel autoclave and kept at 160 ꢁC for 12 h. Finally,
C0
where C0 is the initial concentration of RhB, and C is the time-
dependent concentration of RhB upon irradiation.
3. Results and discussion
3.1 XRD analyses
the obtain samples were collected, washed with deionized water The crystal structures of the as-synthesized samples were
and ethanol, dried overnight at 60 ꢁC under vacuum. According examined by XRD patterns. Fig. 1 shows the XRD patterns of
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100(Fe)-9 composite revealed the existence of Bi, Mo, C, O and
Fe. The tight interaction between the Bi2MoO6 and MIL-100(Fe)
can enhance the photocatalytic activity.
The TEM and HRTEM images of Bi2MoO6–MIL-100(Fe)-9
sample were shown in Fig. 3a and b, respectively. The TEM
image shows the Bi2MoO6 nanoparticles on the MIL-100(Fe)
surface, and the microstructures of the Bi2MoO6–MIL-100(Fe)-
9 sample. The HRTEM images clearly indicated the resolved
lattice fringes of 0.315, 0.406 and 0.192 nm, which correspond
to the (131), (040) and (062) planes of orthorhombic phase of
Bi2MoO6, respectively. The results indicate that the formation of
composites of Bi2MoO6 and MIL-100(Fe) can promote faster
separation of photoinduced charge carriers and improve pho-
tocatalytic activities.
3.3 XPS analysis
The surface chemical composition of Bi2MoO6–MIL-100(Fe)-9
was analyzed by XPS. Fig. 4 shows XPS survey spectra and
high-resolution XPS spectra of the Bi 4f, Mo 3d, O 1s, Fe 2p and
C 1s regions for the sample of Bi2MoO6–MIL-100(Fe)-9. The
survey spectra indicated that Bi, Mo, O, Fe and C exist on the
surface of Bi2MoO6–MIL-100(Fe)-9, and no impurities were
detected, in accordance with EDS result. In the Bi 4f spectrum,
two peaks at 159.2 eV and 164.5 eV are attributed to Bi 4f7/2 and
Bi 4f5/2 of Bi3+, respectively.33 In the Mo 3d spectrum, peaks at
232.5 eV and 235.6 eV were ascribed to Mo 3d5/2 and Mo 3d3/2 of
Mo6+, respectively.34 The O 1s peak of the composite at 530.1 eV
is observed, which was relate to Mo–O.35 The Fe 2p peak was
associated with the binding energy at 712.1 eV, which are
characteristic of Fe(III) in MIL-100(Fe).31 In addition, the C 1s
peaks at 288.8 and 284.7 eV arises from MIL-100(Fe) and
external carbon contamination, respectively.36 In summary, the
XPS spectra analysis conrms that Bi2MoO6–MIL-100(Fe)
composites have been synthesized successfully.
Fig. 1 XRD patterns of (a) MIL-100(Fe), (b) MIL-100(Fe)-simulated, (c)
Bi2MoO6, (d) Bi2MoO6–MIL-100(Fe)-3, (e) Bi2MoO6–MIL-100(Fe)-6, (f)
Bi2MoO6–MIL-100(Fe)-9, (g) Bi2MoO6–MIL-100(Fe)-12.
Bi2MoO6, and Bi2MoO6–MIL-100(Fe) composites with different
contents of MIL-100(Fe). For comparison, the patterns of pure
MIL-100(Fe), Bi2MoO6 as well as simulated MIL-100(Fe) are also
shown. These distinct diffraction peaks at 2q ¼ 10.85, 28.2, 32.5,
46.7 and 55.5ꢁ, can be attributed to the (020), (131), (200), (062)
and (133) crystal planes of pure Bi2MoO6 (JCPDS 76-2388). The
diffraction peaks of pure MIL-100(Fe) sample can match with
simulated MIL-100(Fe), the XRD result indicated the structure
of pure MIL-100(Fe) sample is intact. For the Bi2MoO6–MIL-
100(Fe) samples (Fig. 1d–g), all diffraction peaks indexed to
the orthorhombic Bi2MoO6 phase are clearly observed.
However, no peak in samples assigned to MIL-100(Fe) was
observed, possibly due to the low content and high dispersion
of MIL-100(Fe). In addition, no impurity was observed. With the
increase of MIL-100(Fe) contents, the intensities of peaks for
Bi2MoO6 get reduced.
3.4 BET analysis
Brunauer–Emmett–Teller (BET) surface area of the as-
synthesized samples were examined by nitrogen adsorption–
desorption isotherms at 77 K. It can be found that the
introduction of MIL-100(Fe) can signicantly affect the BET
surface area of samples. Compared with pure Bi2MoO6, the BET
surface area (SBET) of Bi2MoO6–MIL-100(Fe) composites obvi-
ously increased, from 9.3172 m2 gꢀ1 to 109.6347 m2 gꢀ1. The
larger surface area of photocatalysts can provide more surface
active sites and promote charge carriers transport easily,
contributing to an enhanced photocatalytic performance.37
Therefore, MIL-100(Fe) in synthesized composites may play the
key role in enhancing the photocatalytic activity.
3.2 Morphology
Fig. 2 shows the SEM images of Bi2MoO6, MIL-100(Fe) and their
composites Bi2MoO6–MIL-100(Fe)-9, respectively. The rough
microspheres composed of nanoparticles with dozens of
nanometers are observed for Bi2MoO6 (Fig. 2a). And the diam-
eter of the microsphere is 1–3 mm. In the Fig. 2b, the pure MIL-
100(Fe) exhibited regular octahedron morphology with diam-
eter ranging from 2 to 4 mm. As for the Bi2MoO6–MIL-100(Fe)-9
composite (Fig. 2c and d), it can be observed that MIL-100(Fe)
serves as a support and the nanoparticles of Bi2MoO6 grow on
the surface of MIL-100(Fe). Meanwhile, there is no aggregation
of Bi2MoO6 nanoparticles on MIL-100(Fe), indicating that the
3.5 PL analysis
interaction between Bi2MoO6 and MIL-100(Fe) is strong. Photoluminescence (PL) spectra were used to investigate the
Besides, the regular octahedron morphology of MIL-100(Fe) was transfer and separation efficiency of photogenerated electron–
not changed in the composite, the result demonstrated the hole pairs.38 As we all known, the lower PL intensity indicated
structure of MIL-100(Fe) was not destroyed in the synthesized a lower recombination of photoinduced charge carriers. The PL
process of Bi2MoO6–MIL-100(Fe) composite. The Energy spectra of Bi2MoO6, MIL-100(Fe) and Bi2MoO6–MIL-100(Fe)-9
Dispersive Spectroscopy (EDS) patterns of Bi2MoO6–MIL- for excitation wavelength 605 nm are shown in Fig. 5. The
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Fig. 2 SEM images of (a) pure Bi2MoO6, (b) pure MIL-100(Fe), (c) and (d) Bi2MoO6–MIL-100(Fe)-9 composites, and (e) EDS patterns of the
Bi2MoO6–MIL-100(Fe)-9 composites.
main emission peaks around 460–500 nm appear for three photocatalytic samples are located in visible light region. The
photocatalysts, and the emission peak of Bi2MoO6–MIL-100(Fe)- absorption edge of Bi2MoO6 is 485 nm, and MIL-100(Fe)
9 is lower than the Bi2MoO6 and MIL-100(Fe). The resulting of exhibits absorption edge in 550 nm. As the composites of two
the PL spectra indicated the addition of MIL-100(Fe) into different semiconductors, the Bi2MoO6–MIL-100(Fe)-9 exhibits
semiconductor Bi2MoO6 is contributed to the separation of absorption edges at 510 nm. Furthermore, the band gap ener-
photo-generated electron–hole pairs.
gies of Bi2MoO6 and MIL-100(Fe) could be calculated by the
following equation:39
a(hv) ¼ A(hv ꢀ Eg)1/2
(2)
3.6 UV-vis DRS
The UV-vis diffusive reectance spectra (UV-vis DRS) of
Bi2MoO6, MIL-100(Fe) and Bi2MoO6–MIL-100(Fe)-9 photo-
catalysts were shown in Fig. 6a. The adsorption of three
where a, v, Eg, and A are the absorption coefficient, the light
frequency, the energy band gap of semiconductor, and
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Fig. 3 (a) TEM and (b) HRTEM images of Bi2MoO6–MIL-100(Fe)-9.
a constant, respectively. According to eqn (2), the band-gap of
photocatalysts can be estimated from plots of (ahv)2 versus
energy (hv). In the Fig. 6b, the band gaps of as-synthesized
Bi2MoO6, MIL-100(Fe) and Bi2MoO6–MIL-100(Fe)-9 are esti-
mated to be 2.58, 2.50 and 2.38 eV, respectively. The resulting
conrmed that Bi2MoO6 can match up with MIL-100(Fe)
Fig.
5
PL spectra of Bi2MoO6, MIL-100(Fe) and Bi2MoO6–MIL-
100(Fe)-9, recorded at an excitation wavelength of 605 nm.
Fig. 4 XPS survey spectra of Bi2MoO6–MIL-100(Fe)-9 and high-resolution XPS spectra of the Bi 4f, Mo 3d, O 1s, Fe 2p and C 1s.
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Fig. 6 (a) UV-vis diffuse reflectance spectra (DRS) of Bi2MoO6, MIL-
100(Fe) and Bi2MoO6–MIL-100(Fe)-9; (b) the plots of (ahv)n/2 vs.
photon energy (hv) for the band-gap energies of Bi2MoO6, MIL-
100(Fe) and Bi2MoO6–MIL-100(Fe)-9.
Fig. 7 (a) Photodegradation of as-prepared samples for RhB. (b) The
kinetic constants (k) of photocatalytic for RhB.
However, the addition of MIL-100(Fe) improved the photo-
catalytic activities of Bi2MoO6–MIL-100(Fe) composites signi-
cantly. The photocatalytic activities of Bi2MoO6–MIL-100(Fe)
composites enhanced, with the increase of content of MIL-
100(Fe). When the addition of MIL-100(Fe) reached 9%,
Bi2MoO6–MIL-100(Fe) composites demonstrated greater degra-
perfectly, which contributes to the separation of the electron–
hole pairs.
3.7 Photocatalytic activity
The photocatalytic activities of as-prepared photocatalysts were dation performance, up to 88.46% RhB removed in 120 min.
evaluated by the degradation of RhB under visible light irradi- Meanwhile, the absorption of RhB in Bi2MoO6–MIL-100(Fe)-9
ation. The RhB adsorption and photodegradation process of increased to 22.25% compared with pure Bi2MoO6, indicating
different photocatalysts are shown in Fig. 7a. Before the visible that the introduction of MIL-100(Fe) promoted the increase of
light irradiation, the absorption–desorption balance has been adsorption ability of catalysts. Besides, the physical mixture
reached between the catalyst and RhB for 30 min absorption Bi2MoO6–MIL-100(Fe)-9(M) composite exhibited lower activity
process. Fig. 7a shows that RhB was hardly decomposed by than Bi2MoO6–MIL-100(Fe)-9, it may be due to the instable
photolysis without any catalysts under visible light irradiation. Bi2MoO6–MIL-100(Fe) heterojunctions. The result indicated
The adsorption rate of pure Bi2MoO6 and MIL-100(Fe) are that the presence of MIL-100(Fe) enhanced light absorption,
15.36% and 44.39% in the dark, respectively. The adsorption separation of photo-generated electron–hole pairs, and
rate of MIL-100(Fe) is very high, but it has low photocatalytic increased surface area. The enhanced photocatalytic perfor-
performance for RhB under the visible-light irradiation. mance is contributed to the heterostructure between Bi2MoO6
Meanwhile, pure Bi2MoO6 shows low photocatalytic activity and MIL-100(Fe). In addition, Fig. 7b shows the corresponding
for RhB degradation, the photodegradation rate is 62.56%. rst-order kinetics plot by the equation:40
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and hydroxyl radical (cOH). The p-benzoquinone (BQ)42 and N2
(ref. 43) are used as the scavenger of superoxide radical (cO2ꢀ). As
shown in Fig. 9, the photocatalytic degradation of RhB over
Bi2MoO6–MIL-100(Fe)-9 was slightly affected by the addition of
TBA (1 mL). However, the photocatalytic activity of Bi2MoO6–
MIL-100(Fe)-9 was signicantly inhibited by the addition of
EDTA-2Na (0.5 mmol). Similarly, the degradation rate was obvi-
ously decreased aer the addition of BQ (0.5 mmol). In the word,
active species including h+ and cO2ꢀ play the crucial roles in the
photodegradation of pollutants.
To test the role of dissolved O2 in the reaction process,
enough nitrogen (N2) was bubbled into the suspension to
ensure that the degradation system was operated under anoxic
conditions. In the presence of N2, the photocatalytic activity of
catalyst was inhibited under the same condition. This result
means that dissolved O2 is a vital factor in RhB removal over
Bi2MoO6–MIL-100(Fe) composites, which affects the formation
of cO2ꢀ via direct reduction of O2 and the formation of cOH via
indirect reduction of O2.44
3.8.2 The proposed photocatalytic mechanism. It is well-
known that the photocatalytic performances of catalysts rely
on the transfer, separation and generation of electron–hole
pairs. In our study, the interaction between Bi2MoO6 and MIL-
100(Fe) can promote the separation of photogenerated charge
carries and improve the photodegradation rate under visible
light. In order to understand the photocatalytic mechanism of
the Bi2MoO6–MIL-100(Fe) composites in depth, the initial
energy band structures of Bi2MoO6 and MIL-100(Fe) were esti-
mated. The band edges of Bi2MoO6 and MIL-100(Fe) were
calculated by the following empirical formulas:45
ꢀln(C/C0) ¼ kt
(3)
where C0 and C are the RhB concentrations in solution at
times 0 and t, respectively, and k is the apparent rst-order
rate constant. According to the equation, the rates of RhB
degradation are 0.00881 minꢀ1, 0.01248 minꢀ1, 0.01314 minꢀ1
,
0.02174 minꢀ1, 0.01494 minꢀ1 and 0.01201 minꢀ1 for pure
Bi2MoO6, Bi2MoO6–MIL-100(Fe)-3, Bi2MoO6–MIL-100(Fe)-6,
Bi2MoO6–MIL-100(Fe)-9, Bi2MoO6–MIL-100(Fe)-12 and Bi2MoO6–
MIL-100(Fe)-9(M). The rate of RhB degradation of Bi2MoO6–MIL-
100(Fe)-9 is higher than other photocatalysts. The result indi-
cated that the interaction between Bi2MoO6 and MIL-100(Fe) can
improve the photocatalytic performance, and the photocatalytic
activity is signicantly enhancing with the increase of the mass
ratio of MIL-100(Fe). The photocatalytic activity reached the
maximum value, when the mass ratio of MIL-100(Fe) is 9%.
The mineralization rate of RhB over photocatalysts was
detected by Total Organic Carbon (TOC) test. The TOC results
(Fig. 8) indicated that only 67.42% of RhB is mineralized over
Bi2MoO6–MIL-100(Fe)-9 aer 120 min and more irradiation
time is needed to make the substrate completely mineralized.
Aer 210 min, the mineralization rate of RhB over Bi2MoO6–
MIL-100(Fe)-9 can only reach 87.29%. But the mineralization
rate of RhB over Bi2MoO6–MIL-100(Fe)-9 is higher than that over
Bi2MoO6 and Bi2MoO6–MIL-100(Fe)-9(M). The above results
show that Bi2MoO6–MIL-100(Fe) composites exhibited much
higher photocatalytic performance than pure Bi2MoO6 under
visible light irradiation.
3.8 The proposed photocatalytic mechanism
3.8.1 Analysis of active species. The active species including
holes (h+), superoxide radical (cO2ꢀ), and hydroxyl radicals (cOH)
are possibly impact factors for the photodegradation of organic
pollutants. It is important to carry out the trapping experiments
for conrming the main active species generated over Bi2MoO6–
MIL-100(Fe) composites. In the detection, EDTA-2Na (ref. 33) and
tert-butyl alcohol (TBA)41 are used as the scavengers of holes (h+)
EVB ¼ X ꢀ Ee + 0.5Eg
ECB ¼ EVB ꢀ Eg
(4)
(5)
where EVB is the valence band (VB) edge potential, ECB is
the conduction band edge potential, Eg is the band gap
energy of the semiconductor, X is the electronegativity of the
Fig.
9 Photocatalytic degradation of RhB over Bi2MoO6–MIL-
Fig. 8 TOC removal process of RhB solution.
100(Fe)-9 with different scavengers.
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ꢀ
MIL-100(Fe)(eꢀ) + O2 / cO2 + MIL-100(Fe)
(9)
semiconductor, which is the geometric mean of the electro-
negativity of the constituent atoms, and the X values for
Bi2MoO6 and MIL-100(Fe) were calculated to be 5.55 and
5.10 eV, respectively. Ee is the energy of free electrons on the
hydrogen scale (about 4.5 eV). The Eg of Bi2MoO6 and
MIL-100(Fe) are estimated as 2.58 and 2.50 eV. According to
above formulas, the VB and CB edge potentials of Bi2MoO6 are
located at ꢀ0.24 and 2.34 eV, respectively. The VB and CB edge
potentials of MIL-100(Fe) are determined to be ꢀ0.65 and
1.85 eV, respectively.
cO2 + RhB/RhB+c / other products / CO2 + H2O (10)
cOH + RhB/RhB+c / other products / CO2 + H2O (11)
ꢀ
h+ + RhB/RhB+c / other products / CO2 + H2O
(12)
3.9 Regeneration and reusability
On the basis of above band energy level analysis and active
species trapping detection, the possible proposed mechanism
was discussed as illustrated in Scheme 1. Under the visible light
irradiation, both Bi2MoO6 and MIL-100(Fe) can be excited for
generation of electrons (eꢀ) and holes (h+) in their CB and VB.
The photo-generated electrons in the CB of MIL-100(Fe) transfer
to the CB of Bi2MoO6 via the heterojunction structure. At the
same, the holes in the VB of Bi2MoO6 can migrate to the VB of
MIL-100(Fe), hence promoting the separation of photoinduced
electron–holes pairs. O2 molecules in solution adsorbed onto
catalysts form superoxide radical (cO2ꢀ) via electron transfer
from MIL-100(Fe) to O2, since the conduction band potential of
MIL-100(Fe) (ꢀ0.65 eV versus NHE) was more negative than the
standard redox potentials of (E(O2/cO2ꢀ) ¼ ꢀ0.33 eV versus
NHE). Similarly, the standard redox potentials of (E(H2O/cOH) ¼
+2.8 eV versus NHE) was more positive than the VB of the
Bi2MoO6 (2.34 eV) and MIL-100(Fe) (1.85 eV), thus it is difficult
to oxidize the adsorbed H2O into cOH for photo-generated. The
part h+ radicals reacted with H2O molecules in solution to
generate reactive hydroxyl (cOH) and H+ ions. Moreover, the
adsorbed RhB onto Bi2MoO6–MIL-100(Fe) can be excited to
RhB* under visible-light irradiation.46 Then the electrons from
RhB* are transformed into photocatalyst to form RhB+c. The
RhB (or RhB+c) is nally photodegraded by the reactive species
(O2, cOH) or h+ to generate H2O and CO2. According to above
discussion, the process can be described as follows:
It is signicant to evaluate the reusable performance and
stability of photocatalysts for practical applications.47 To
investigate the stability of Bi2MoO6–MIL-100(Fe)-9, we carried
out 4 times recycle experiment to degrade RhB under the same
conditions. Aer each cycle, the photocatalyst was centrifuged,
ltered, washed and dried at 60 ꢁC for 12 h. As shown in
Fig. 10a, aer four cycles, the degradation rate of RhB decreased
from 88.46% to 84.91%, the photocatalyst removed organic
pollutants with only a slight loss of activity. Aer being used
four times, as shown in Fig. 10b, the XRD patterns of Bi2MoO6–
MIL-100(Fe)-9 composite before and aer catalytic reaction
further demonstrated that the structure of the Bi2MoO6–MIL-
100(Fe) composites remain intact. Besides, the ICP-AES
analysis showed that the leaching Fe and Bi ions from the
photocatalysts in the reaction solution were 0.143 mg Lꢀ1 and
less than 0.01 mg Lꢀ1 aer cycles reaction respectively. The
leaching ions concentration is very low compared with the
concentration of catalysts (1000 mg Lꢀ1), which is negligible.
Therefore, the as-prepared Bi2MoO6–MIL-100(Fe)-9 composites
show excellent stability and reusability in the visible light
photochemical applications.
4. Conclusions
In summary, Bi2MoO6–MIL-100(Fe) photocatalysts have been
successfully fabricated by facile solvothermal method in this
work for the rst time. The characterizations of Bi2MoO6–MIL-
100(Fe) composites were conrmed by XRD, SEM/EDS, TEM,
and XPS. The as-synthesized Bi2MoO6–MIL-100(Fe) photo-
catalyst exhibited excellent photocatalytic performance in
degradation of RhB under visible light. The optimal mass ratio
MIL-100(Fe) + hv / MIL-100(Fe)(eꢀ + h+)
RhB + hv / RhB* / RhB+c + eꢀ
Bi2MoO2 + eꢀ / Bi2MoO2(eꢀ + h+)
(6)
(7)
(8)
Scheme 1 Photocatalytic degradation mechanism over Bi2MoO6–MIL-100(Fe) heterostructure under visible light irradiation.
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