Fabrication of a novel MoO3/Zn–Al LDHs composite photocatalyst for efficient degradation of tetracycline under visible light irradiation

Article abstract of DOI:10.1016/j.jpcs.2020.109698

Herein, a novel MoO₃/Zn–Al LDHs composite photocatalyst was successfully prepared. Morphology, chemical composition, and photoelectrochemical properties of the prepared catalyst were explored by different characterization methods, including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), Brunauer Emmett-Teller (BET), UV–Vis diffuse reflectance spectrometry (DRS) and Electron Spin Resonance (ESR). Using tetracycline as a target pollutant, the photocatalytic activity of MoO₃ (10 wtpercent)/Zn–Al LDHs composite was tested by photodegradation of tetracycline under visible light irradiation. The experimental results indicate that the MoO₃ (10 wtpercent)/Zn–Al LDHs composite exhibited higher photocatalytic removal efficiency than pure Zn–Al LDHs and MoO₃. The removal efficiency of TC in 60 min could reach 90.5percent. It was confirmed that the optimal conditions were a catalyst dosage of 1.0 g/L, an initial pH of 9, and an initial concentration of TC of 40 mg/L. In addition, a possible mechanism for the photodegradation of TC with MoO₃ (10 wtpercent)/Zn–Al LDHs composite was proposed. The introduction of MoO₃ facilitates the separation of electron-hole pairs. It could be seen that the MoO₃ (10 wtpercent)/Zn–Al LDHs composite has great application prospects for the treatment of antibiotic wastewater.

Full text of DOI:10.1016/j.jpcs.2020.109698

Journal Pre-proof
Fabrication of a novel MoO /Zn–Al LDHs composite photocatalyst for efficient
3
degradation of tetracycline under visible light irradiation
Jun Wang, Xiaofang Lei, Cong Huang, Lian Xue, Wei Cheng, Qiang Wu
PII:
S0022-3697(20)31217-8
https://doi.org/10.1016/j.jpcs.2020.109698
PCS 109698
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 8 May 2020
Revised Date: 6 July 2020
Accepted Date: 8 August 2020
Please cite this article as: J. Wang, X. Lei, C. Huang, L. Xue, W. Cheng, Q. Wu, Fabrication of a
novel MoO /Zn–Al LDHs composite photocatalyst for efficient degradation of tetracycline under
3
visible light irradiation, Journal of Physics and Chemistry of Solids (2020), doi: https://doi.org/10.1016/
j.jpcs.2020.109698.
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CRediT authorship contribution statement
Jun Wang: Conceptualization, Methodology, Writing-original draft, Investigation.
Xiaofang Lei: Data curation, Investigation.
Cong Huang: Supervision.
Lian Xue: Supervision, Validation.
Wei Cheng: Resources, Validation.
Qiang Wu: Supervision, Writing- Reviewing and Editing.

Graphical Abstract
In this study, a novel MoO /Zn-Al LDHs composite photocatalyst was
3
successfully prepared for the first time and applied to the photocatalytic degradation
of tetracycline (TC). The results show that MoO (10 wt.%)/Zn-Al LDHs composite
3
has higher photocatalytic removal efficiency than pure Zn-Al LDHs and MoO . At the
3
same time, the MoO (10 wt.%)/Zn-Al LDHs photocatalyst showed beneficial
3
reusability and stability to TC degradation. This research have shown that MoO (10
3
wt.%)/Zn-Al LDHs composite has broad application prospects in the treatment of
antibiotic wastewater, which is expected to provide new insights for exploring
efficient and stable photocatalysts.

Fabrication of a novel MoO /Zn-Al LDHs composite
3
photocatalyst for efficient degradation of tetracycline under
visible light irradiation
a
a
a
a
a
a,
Jun Wang , Xiaofang Lei , Cong Huang , Lian Xue , Wei Cheng , Qiang Wu *
a
Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,
Shanghai University of Electric Power, Shanghai, 200090, China.
*
Email address: qiangwu@shiep.edu.cn (Q. Wu).
Fax: +86-21-35303544; Tel: +86-21-35303544
Abstract
Herein, a novel MoO /Zn-Al LDHs composite photocatalyst was successfully
3
prepared. Morphology, chemical composition, and photoelectrochemical properties of
the prepared catalyst were explored by different characterization methods, including
X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-
ray photoelectron spectroscopy (XPS), Brunauer Emmett-Teller (BET), UV-Vis
diffuse reflectance spectrometry (DRS) and Electron Spin Resonance (ESR). Using
tetracycline as a target pollutant, the photocatalytic activity of MoO (10 wt.%)/Zn-Al
3
LDHs composite was tested by photodegradation of tetracycline under visible light
irradiation. The experimental results indicate that the MoO (10 wt.%)/Zn-Al LDHs
3
composite exhibited higher photocatalytic removal efficiency than pure Zn-Al LDHs
and MoO . The removal efficiency of TC in 60 min could reach 90.5%. It was
3
confimed that the optimal conditions were a catalyst dosage of 1.0 g/L, an initial pH
1

of 9, and an initial concentration of TC of 40 mg/L. In addition, a possible mechanism
for the photodegradation of TC with MoO (10 wt.%)/Zn-Al LDHs composite was
3
proposed. The introduction of MoO facilitates the separation of electron-hole pairs. It
3
could be seen that the MoO (10 wt.%)/Zn-Al LDHs composite has great application
3
prospects for the treatment of antibiotic wastewater.
Keywords: Tetracycline; Photocatalytic degradation; Zn-Al LDHs; MoO3
1
. Introduction
Over the years, with the development of social economy and the pharmaceutical
industry, tetracycline, an important antibiotic drug, has been widely used in human
therapy and animal infections [1-3]. More than 85% of tetracycline (TC) has been
detected in the environment [4]. However, TC does not easily degrade in the
environment, it poses huge threat to the ecological environment, aquatic plants,
animals, and human health [5, 6]. To solve these tricky problems, many advanced
technologies have been developed in the field of antibiotic degradation, such as
adsorption [7], biodegradation [8], electrochemical oxidation [9], membrane filtration
[
10], and photocatalytic degradation [11]. It is worth noting that the photocatalytic
degradation is the most advantageous method for TC removal, due to its low cost,
strong oxidising ability, and no pollution to the environment [12, 13].
Layered double hydroxides (LDHs) consist of positively charged brucite
hydroxide sheets and charge-balanced interlayer anions [14]. Their special layered
structure and the property of anion exchangeability make them attract a wide range of
2

attention in ion exchange, adsorption, catalysis, and biomedical applications [15-18].
In recent years, LDHs have been explored as effective photocatalysts for degradation
of some organic pollutants. Nonetheless, LDHs usually have a wide band gap and
poor response to visible light, and thus possess unsatisfactory photocatalytic
performance in most cases [19-21]. To resovle the above issues, many researchers
have tried to enhance the photocatalytic performance of LDHs by coupling them with
other semiconductors like Ni-Ti LDHs/RGO [22], Bi O /CuNiFe LDHs [23],
2
3
Bi S /Zn-Al LDHs [24], and Co(OH) /ZnCr LDHs [25].
2
3
2
Molybdenum trioxide (MoO ) is a n-type semiconductor which has a wide band
3
gap of 2.92 eV, good chemical stability, and abundant sources [26]. However, the
weak absorption of visible light and low charge transfer ability have limited its
photocatalytic efficiency [27]. In order to overcome the previously stated problems, it
is promising to develop MoO -based composite photocatalysts such as Z-scheme
3
Bi O /MoO [28], Ag PO /MoO p-n heterojunction [29], and Z-scheme g-
2
3
3
3
4
3
C N /MoO [30]. It has been proved that the MoO -based composites photocatalysts
3
4
3
3
have potential application for organic pollution degradation.
Herein, we report a novel MoO /Zn-Al LDHs composite for the first time, which
3
was synthesized via an uncomplicated solvent evaporation route. Compared with pure
MoO and Zn-Al LDHs, the as-prepared MoO /Zn-Al LDHs composite photocatalyst
3
3
has significant photocatalytic activity for the removal of TC wastewater. Furthermore,
the effects of MoO content, TC initial concentration, and reaction pH were studied.
3
Finally, a possible photocatalytic mechanism was proposed. The results have clearly
3

confirmed that the as-prepared MoO /Zn-Al LDHs composite could be developed as a
3
potentially effective photocatalyst for degradation of tetracycline wastewater.
2
. Experimental section
2
.1. Reagents
The main reagents used in the experiment such as zinc nitrate (Zn(NO ) ·6H O),
3
2
2
aluminium nitrate (Al(NO ) ·9H O), molybdenum trioxide (MoO3), and
3
3
2
hexamethylenetetramine (HMT) were purchased from Sinopharm. Other reagents
used in the experiment were analytical grade and were purchased from Aladdin. All
reagents were used in this research without further purification.
2
2
.2 Synthesis of photocatalyst
.2.1. Fabrication of Zn-Al LDHs
Zn-Al LDHs were prepared by a simple hydrothermal method [14]. Firstly,
Zn(NO ) ·6H O (8 mmol), Al(NO ) ·6H O (4 mmol), and HMT (16 mmol) were
3
2
2
3 3
2
added to 80 mL of deionised water. Secondly, the suspension was transferred to a
stainless steel autoclave lined with Teflon and held at 120°C for 12 h. The resulting
white precipitate was washed three times with deionised water and ethanol.
Ultimately, the samples were collected and dried in an oven at 60� overnight,
grinded for later use.
2
.2.2. Fabrication of MoO /Zn-Al LDHs composites
3
4

MoO /Zn-Al LDHs composites were synthesized by solvent evaporation. In
3
simple terms, 0.5 g of Zn-Al LDHs powder and various contents of MoO (5 wt.%, 10
3
wt.%, 15 wt.%) were added to 40 mL of ethanol. Then, the suspension was
ultrasonicated for 2h to achieve uniform dispersion, followed by stirring at 80°C.
Finally, a series of target MoO /Zn-Al LDHs composites photocatalysts were
3
obtained.
2
.3. Characterization
The composition and structure of the as-synthesized samples were characterized
by X-ray diffraction (XRD) on a BRUKER-D8 diffractometer with Cu-Kα radiation.
The morphology of the as-prepared photocatalyst was analysed by field emission
scanning electron microscopy (FE-SEM), which was measured on a Hitachi SU-1500.
X-ray photoelectron spectrometer (XPS) was characterized with a PH 1500 C XPS
system. The specific surface area measurement was recorded by N adsorption-
2
desorption isotherm using a Brunauer Emmett-Teller (BET) instrument (Gem-ini VII
2
390). UV-Vis diffuse reflectance spectrometry (DRS) was used to test the optical
performance of the photocatalyst with a Shimadzu UV-2550 spectrophotometer.
Electron Spin Resonance (ESR) spectra was carried out on a Bruker EPR A300-10/12.
2
.4. Photodegradation activity test
The photocatalytic performance of the as-prepared MoO /Zn-Al LDHs
3
composites were examined by the degradation of TC under visible light irriadiation.
In a typical photocatalytic reaction, 100 mg of the photocatalyst was put in a 100 mL
5

TC aqueous solution. Before Xe lamp was opened, the suspension was subjected to a
dark reaction for 30 min to reach adsorption-desorption equilibrium. Afterwards, the
above solution was exposed by a 300 W Xe arc lamp (PerfectlightCo., PLS-SXE 300)
equipped with an optical cutoff filter. 4 mL of the reaction solution were collected
every 10 min, and followed by detecting on a UV-Vis spectrophotometer. High
performance liquid chromatography mass spectrometry (HPLC-MS) was used to
analyze the degradation intermediates and pathway of TC.
2
.5. Photoelectrochemical properties
The photoelectrochemical performance measurement of the MoO /Zn-Al LDHs
3
composites were measured on a CHI660E electrochemical workstation with a three-
electrode system. The specific process of preparing the working electrode was as
follows: 20 mg of photocatalyst was dissolved in a solution which contained 450 μL
of ethanol and 50 μL of Nafion, followed by sonication for 30 min. Next, a 20 μL
2
suspension was coated onto the FTO glass electrode with an active area of 1 cm . All
photoelectrochemical measurements were performed in 0.5M Na SO . Saturated
2
4
calomel electrode (SCE) and carbon rod were used as reference electrode and counter
electrode, respectively. Under the irradiation of a 300W Xe lamp with a filter, the
current i~t curve was measured at the open circuit potential. The Mott-Schottky (M-S)
curve was recorded at a frequency of 1000 Hz. The electrochemical impedance
spectroscop (EIS) test was conducted at an open-circuit voltage overfrequency of 0.1
5
Hz to 10 Hz with an amplitude of 5 mV.
6

3
. Results and discussion
3
.1 Characterization
Fig. 1 presents the XRD patterns of MoO , Zn-Al LDHs, and MoO /Zn-Al LDHs
3
3
composites. It was found that the as-preared Zn-Al LDHs showed the typical
diffraction peaks of (003), (006), (012), (018), (110) and (113), which perfectly
corresponded to 2θ = 11.7°, 23.6°, 34.6°, 46.8°, 60.2°, and 61.6° (JCPDS#48-1023).
Furthermore, the shape of the diffraction peaks was narrow and sharp, indicating that
the hydrothermally synthesized Zn-Al LDHs had high crystallinity and no impurity
peaks. In addition, the XRD patterns of MoO can be well indexed to the standard
3
card of MoO (JCPDS#05-0508). As expected, the XRD patterns of the as-prepared
3
MoO /Zn-Al LDHs match with the polycrystalline structures of MoO and Zn-Al
3
3
LDHs, and no third phase was formed. This result indicates the successful synthesis
of MoO /Zn-Al LDHs composites in the present study.
3
The morphology of MoO , Zn-Al LDHs, and MoO (10 wt.%)/Zn-Al LDHs
3
3
composite were observed by FE-SEM. As shown in Fig. 2a, MoO exhibited irregular
3
micron-level block-like morphology, with obvious edges and a smooth surface. Fig.
2
b shows that the sheet of Zn-Al LDHs was very thin, and mostly had a hexagonal
layer structure, which was a typical hydrotalcite-type material structure. However,
due to the large size and small surface area, a large part of the hydrotalcite nanosheets
had no contact with MoO . As for the as-prepared MoO (10 wt.%)/Zn-Al LDHs
3
3
composite, it could be observed that a portion of the hydrotalcite nanosheets was
7

supported on the MoO . Meanwhile, MoO accumulated in a large number of
3
3
hydrotalcite nanosheets (Fig. 2c and 2d). The FE-SEM analysis aslo confirms the
formation of MoO /Zn-Al LDHs composites in the present study. It was worth
3
mentioning that the close contact between MoO and Zn-Al LDHs was beneficial to
3
the migration of photogenerated carriers.
Further, the elemental composition of MoO (10 wt.%)/Zn-Al LDHs was tested
3
by XPS spectrum. The survey spectrum of MoO (10 wt.%)/Zn-Al LDHs composite is
3
displayed in Fig. 3a. As observed, the survey spectrum demonstrates the co-existence
of Zn, Al, Mo, C, and O elements in the composites. As shown in Fig.3b, the peaks of
1
021.6 and 1044.6 eV were mainly due to the binding energy of Zn 2p, indicating the
2
+
high-spin Zn state [31]. The Mo 3d high-resolution spectrum (Fig 3c) showed that
the binding energy of Mo 3d5/2 and Mo 3d were approximately 232.4 and 235.5eV,
3/2
6
+
respectively, confirming the presence of Mo in the composites [32]. Note that O 1s
high-resolution spectrum exhibited three characteristic peaks, the peak at 530.6 eV
was consistent with the lattice O bound by the metal cations (Zn, Al, and Mo) in the
composites, and the other two peaks at 531.7 and 532.8 eV were associated with
surface hydroxyl groups and adsorbed water molecules, respectively [33].
Furthermore, the peak around 74.1 eV was attributed to the binding energy of Al 2p
(
Fig. 3e) [34]. The XPS results provide good proof that MoO /Zn-Al LDHs
3
composites were formed in the present study, which well consisted with the above
XRD and FE-SEM results.
8

The N adsorption-desorption isotherm characterization of MoO , Zn-Al LDHs,
2
3
and MoO (10 wt.%)/Zn-Al LDHs composite are presented in Fig. 4. The nitrogen
3
adsorption-desorption isotherms of Zn-Al LDHs and MoO (10 wt.%)/Zn-Al LDHs
3
composite belong to type IV, indicating that they were mesoporous [35]. It is obvious
that the hysteresis loop of MoO (10 wt.%)/Zn-Al LDHs composite shifted slightly
3
after introducing MoO , which indicated change of pore texture. The BET specific
3
surface area and pore size of MoO , Zn-Al LDHs, and MoO (10 wt.%)/Zn-Al LDHs
3
3
are shown in Table S1. Although the BET specific surface area and pore size of MoO3
10 wt.%)/Zn-Al LDHs were reduced after adding MoO , which could still reach 32.3
(
3
m²/g and 11.1 nm. Note that the introduction of MoO did not interfere with the
3
channel mass transfer effect of mesoporous materials.
The UV-vis diffusion reflection spectra of MoO , Zn-Al LDHs and MoO (10
3
3
wt.%)/Zn-Al LDHs are shown in Fig. S1. After introducing MoO , the light-
3
harvesting ability of MoO (10 wt.%)/Zn-Al LDHs composite was significantly
3
higher than that of pure Zn-Al LDHs. This result demonstrates that the as-prepared
MoO (10 wt.%)/Zn-Al LDHs composite can act as a promising photocatalyst for
3
wastewater pollutant removal. As shown in Fig. 5, the optical band gap energy of Zn-
Al LDHs and MoO was obtained by UV-vis spectrophotometer. It was calculated
3
that the band gap of Zn-Al LDHs and MoO were about 4.80 and 3.12 eV,
3
respectively, which would be used in the subsequent mechanism analysis.
3
.2. Photocatalytic activity test
9

The photocatalytic activity test of the MoO /Zn-Al LDHs composites was
3
studied by the degradation rate of TC under the visible light irradiation. Several
parameters: initial TC concentration, reaction pH, and MoO decorating amount were
3
studied. In addition, the photostability and reusability were tested for practical
applications.
3
.2.1. Effect of MoO content
3
As shown in Fig. 6a, MoO , Zn-Al LDHs, and MoO /Zn-Al LDHs composites
3
3
were tested for photodegradation of TC. The influence of MoO decorating amount
3
was firstly studied. Note that TC (no photocatalyst) hardly photodegraded under the
visible light, so the self-degradation of TC (0.5% degradation rate after 60 min) was
almost negligible. Under the dark conditions, the adsorption-desorption equilibrium
could be reached in 30 min. As depicted in Fig. 6a, MoO (10 wt.%)/Zn-Al LDHs
3
composite exhibited the best photocatalytic performance among all the samples,
which was in the order of MoO (10 wt.%)/Zn-Al LDHs (79.0%) > MoO (5
3
3
wt.%)/Zn-Al LDHs (75.4%) > MoO (15 wt.%)/Zn-Al LDHs (74.1%) > pure Zn-Al
3
LDHs (57.1%) > pure MoO (25.8%). Obviously, with the increasing of MoO
3
3
content, the photocatalytic performance of MoO /Zn-Al LDHs composites gradually
3
improved and then decreased. This enhancement was due to the synergistic effect of
the appropriate amount of MoO and Zn-Al LDHs, which facilitated the rapid
3
separation of photogenerated carriers [27, 31]. When the MoO content was increased
3
to 15 wt.%, the photocatalytic performance decreased, which might be attributed to
1
0

the excessive MoO loading, which covered some active reaction sites of Zn-Al LDHs
3
[
32].
To further research the photocatalytic performance of MoO /Zn-Al LDHs
3
composites, the photocatalytic degradation kinetic curve of TC was fitted according to
1
1
the pseudo-second-order model (
−
= Kt ). Fig. 6b shows that the rate
Ct
C0
-1
-1
constant of MoO (10 wt.%)/Zn-Al LDHs was 0.03154 (mg/L) ·(min) , which was
3
2
.74 and 16.96 times higher than Zn-Al LDHs and MoO respectively. Meanwhile, all
3,
2
2
the values of the coefficient R were greater than 0.95 (R > 0.95) [36]. The results
show that the photocatalytic performance improved after combining with MoO3.
MoO (10 wt.%)/Zn-Al LDHs was used as the research object in the following
3
research.
3
.2.2. Effect of initial TC concentrations
The initial concentration was also one of the significant factors for the
photodegradation of TC. Fig. 6c showed the TC degradation rate over MoO (10
3
wt.%)/Zn-Al LDHs at different initial TC concentrations (30-60 mg/L). The results
demonstrate that with the increasing of TC concentration, the removal rate of TC first
escalated and then fell over MoO (10 wt.%)/Zn-Al LDHs. The high concentration of
3
TC would cause more adsorption over MoO (10 wt.%)/Zn-Al LDHs, which might
3
cause the active sites to be masked, thereby reducing the degradation rate [11, 37]. In
addition, the high concentration of TC might also prevent visible light from
irradiating the photocatalyst, so the photocatalytic performance was poor [38].
1
1

Therefore, 40 mg/L was selected as the optimal initial TC concentration for further
research.
3
.2.3. Effect of reaction pH value
To evaluate the effect of initial reaction pH value, the photocatalytic degradation
of TC over MoO (10 wt.%)/Zn-Al LDHs was tested after adding HCl or NaOH
3
solution. Fig. 6d displays the photocatalytic degradation of TC at different pH values
over MoO (10 wt.%)/Zn-Al LDHs. The results confirm that when the TC solution
3
was neutral and alkaline pH (pH = 7 and 9), the degradation performance was
significantly better than under acidic condition (pH = 3 and 5). This might be
attributed to the fact that a high pH TC solution tended to attract ·OH reactive species
because of its high electrical density on the ring system [39], which ultimately led to
increased degradation of TC under alkaline conditions. This was consistent with
previous reports in related literature [39, 40]. Thus, the optimal initial pH was
determined as pH = 9.
3
.2.4. Reusability and stability of MoO /Zn-Al LDHs composites
3
The reusability and stability of MoO (10 wt.%)/Zn-Al LDHs was investigated
3
by repeating photocatalytic degradation of TC for five times to assess the possibility
of the photocatalyst in practical applications. Fig. 7a reveals that the degradation rate
of MoO (10 wt.%)/Zn-Al LDHs composite only decreased by 8.1% after five cycling
3
runs, which showed that the photocatalyst had good photocatalytic stability. To
further prove the results, the photocatalyst was collected and tested by XRD. Fig. 7b
1
2

shows that the structure MoO (10 wt.%)/Zn-Al LDHs composite had no change after
3
the five cycling test. Therefore, the MoO (10 wt.%)/Zn-Al LDHs composite could be
3
used as a reusable and stable catalyst, which had a very important practical
application significance.
3
.3. Photoelectrochemical properties of MoO /Zn-Al LDHs composites
3
To study the electrochemical properties of the MoO /Zn-Al LDHs composites, a
3
transient photocurrent test was performed under the visible light. In Fig. 8a, the MoO3
10 wt.%)/Zn-Al LDHs composite had the highest photocurrent response compared
(
with pure MoO and Zn-Al LDHs, which indicates that the heterostructure formed
3
after the recombination of MoO and Zn-Al LDHs promoted the separation efficiency
3
of photogenerated carriers. Then, the EIS test was carried out in 0.5 M Na SO
2
4
solution. In general, the smaller the radius of the arc, the higher charge transfer
efficiency of the material. [41]. As evidenced by the EIS in Fig. 7b, the results
demonstrate that the MoO (10 wt.%)/Zn-Al LDHs composite has the smallest arc
3
radius, indicating that its photogenerated electron-hole pair separation efficiency was
the highest, which was consistent with the conclusion of photocurrent test.
3
.4. Possible degradation pathway of TC
The degradation intermediates and pathway of TC over MoO (10 wt.%)/Zn-Al
3
LDHs composite under visible light irradiation was conducted by HPLC-MS. Fig. S2
shows the mass spectra of TC over MoO (10 wt.%)/Zn-Al LDHs after 60 min of
3
photocatalytic reaction. Based on the results in Fig. S2, the possible degradation
1
3

intermediates products were obtained. In our previous work, the anion characteristic
peak of deprotonated TC was detected at m/z=445 [11]. As shown in Fig. S3, TC
molecule (m/z=445) became molecule B (m/z=400) after a series of reactions such as
dealkylation, deamination and dehydroxylation. Secondly, molecule B was oxidized
by active species to form compound C (m/z=338). After that, compound D (m/z=277)
was formed by a ring-opening reaction. With the separation of methyl and hydroxyl
groups, compound D further generated compound E (m/z=246). The above results
prove that as the degradation progresses, TC molecule was decomposed into CO2,
H O and other intermediates products.
2
3
.5. Photocatalytic reaction mechanism
It was generally known that the essence of the photocatalytic reaction was that a
redox reaction occurred on the photocatalyst surface, thereby generating active
+
-
materials such as h , ·O , and ·OH. These active species could oxidise TC to CO and
2
2
H O. In order to know which reactive species played a major role in the
2
photocatalytic degradation of TC, a trapping experiment was tested by adding
different scavengers. As shown in Fig. 9a, when benzoquinone (BQ) and ethylene
diamine tetraacetic acid (EDTA) were added, the removal rate of TC declined from
9
0.5% to 41.6% and 33.2%, respectively. However, the addition of isopropanol (IPA)
-
did not have much effect on the photodegradation of TC. We could conclude that ·O2
+
and h were the main active free radicals during the reaction.
1
4

-
+
In Fig. 9b and 9c, the active free radicals ·O and h in the trapping experiment
2
were further verified by electron spin resonance (ESR) experiment. As expected, the
-
MoO (10 wt.%)/Zn-Al LDHs composite showed significant DMPO-·O and
3
2
+
-
+
TEMPO-h characteristic signals, which could infer that ·O and h were the main
2
reactive species playing a major role during the degradation. This was also in line
with the results of the trapping experiment.
Mott-Schottky plots of Zn-Al LDHs and MoO were obtained to determine flat
3
band potential using an electrochemical workstation (Fig. 10). In Fig. 10, the flat band
potential (E ) of Zn-Al LDHs and MoO were calculated as -0.36 and -0.05 V vs
FB
3
SCE, respectively. Because the tangent slope was negative, it could be seen that they
were all n-type semiconductors. It was known from the literature that the conduction
band (CB) potential of an n-type semiconductor was about 0.20 V lower than its EFB
[
42]. Therefore, the conduction bands of Zn-Al LDHs and MoO were calculated as -
3
0
.56 and -0.25 V. In the light of the formula, EVB = ECB + E , and the semiconductor
g
band gap obtained from Fig.5, the valence band (VB) edge positions could be
calculated as 4.24 and 2.87 V, respectively.
In summary, the possible mechanism and electron-hole separation schematic
diagram of the catalytic degradation of TC by the MoO /Zn-Al LDHs composites
3
under visible light are presented in Fig.11. The CB potential of Zn-Al LDHs and
MoO were -0.56 and -0.25 V, respectively. Because the CB potential of Zn-Al LDHs
3
was negative compared to MoO , and the CB potential of MoO was negative
3
3
-
compared to O /·O (-0.046 V) [11], the electrons on the CB of Zn-Al LDHs would
2
2
1
5

be transferred to the MoO . Subsequently, the electrons of MoO were captured by the
3
3
-
+
O to generate ·O . h could stay on the VB of Zn-Al LDHs to directly oxidise TC
2
2
molecules due to its strong oxidising property [37]. Overall, MoO was introduced
3
into Zn-Al LDHs, which could promote the photocatalytic activity for TC removal.
4
. Conclusion
In this article, we used a simple hydrothermal method to prepare a Zn-Al LDHs
parent material, and the MoO (10 wt.%)/Zn-Al LDHs composite was synthesised by
3
solvent evaporation for the first time. The photocatalytic activity of the as-prepared
MoO (10 wt.%)/Zn-Al LDHs composite was evaluated by photocatalytic degradation
3
of TC under visible light. The degradation performance of the MoO (10 wt.%)/Zn-Al
3
LDHs composite for TC was significantly better than that of MoO and Zn-Al LDHs.
3
The experimental results demonstrate that with the catalyst dosage of 1.0 g/L, an
initial TC concentration of 40 mg/L, reaction pH 9, and a light time was 60 min, the
photocatalytic removal rate of TC with MoO (10 wt.%)/Zn-Al LDHs was the highest,
3
which could reach 90.5%. Meanwhile, the MoO (10 wt.%)/Zn-Al LDHs
3
photocatalyst displayed beneficial reusability and stability to TC degradation. In
-
+
addition, ·O and h were proved to be the main active species in the photocatalytic
2
reaction. Importantly, the enhanced photocatalytic property of the composites was due
to the migration of photogenerated charges, which reduced the electron-hole
recombination rate. This study also proves that MoO (10 wt.%)/Zn-Al LDHs
3
composite has broad application prospects in the treatment of antibiotic wastewater.
1
6

Acknowledges
This work was financially supported by Science and Technology Commission of
Shanghai Municipality (19DZ2271100)
1
7

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2
5

Figure captions
Fig.1. XRD patterns of MoO , Zn-Al LDHs, and various MoO /Zn-Al LDHs
3
3
composites.
Fig.2. FE-SEM images of (a) MoO , (b) Zn-Al LDHs, and (c,d) MoO (10 wt.%)/Zn-
3
3
Al LDHs composite.
Fig.3. XPS spectrum of MoO (10 wt.%)/Zn-Al LDHs composite (a) Survey spectrum,
3
(b) Zn 2p, (c) Mo 3d, (d) O 1s, and (e) Al 2p.
Fig.4. Nitrogen adsorption-desorption isotherm of the as-prepared MoO , Zn-Al
3
LDHs and MoO (10 wt.%)/Zn-Al LDHs composite.
3
Fig.5. The corresponding band gap obtained via UV-vis spectrophotometer (a) Zn-Al
LDHs and (b) MoO3.
Fig.6. (a) Photocatalytic degradation of TC in the presence of different photocatalysts;
(b) the corresponding kinetic curves; Effects of (c) initial TC concentrations; (d)
initial pH on the degradation of TC over MoO (10wt.%)/Zn-Al LDHs composite
3
under visible light irradiation.
Fig.7. (a) Stability of MoO (10 wt.%)/Zn-Al LDHs in photodegradation of TC under
3
visible light irradiation. (b) XRD patterns of MoO (10wt.%)/Zn-Al LDHs before and
3
after five cycling experiment.
2
6

Fig.8. (a) Transient photocurrent responses and (b) Electrochemical impedance
spectroscopy (EIS) chart of MoO , Zn-Al LDHs and MoO (10 wt.%)/Zn-Al LDHs
3
3
composite.
Fig.9. (a) Effects of different scavengers on the photocatalytic activity of MoO (10
3
wt.%)/Zn-Al LDHs composite. (b) DMPO spin-trapping ESR spectrum for MoO (10
3
-
wt.%)/Zn-Al LDHs in alcohol dispersion for DMPO-·O . (c) ESR spectrum of the
2
MoO (10 wt.%)/Zn-Al LDHs under visible light irradiation.
3
Fig.10. Mott-Schottky plots (potential vs. SCE) of (a) Zn-Al LDHs , and (b) MoO3.
Fig.11. Photocatalytic degradation mechanism for TC over MoO /Zn-Al LDHs
3
composites under visible light irradiation.
2
7