Fabrication of 2D-2D Heterojunction Catalyst with Covalent Organic Framework (COF) and MoS2 for Highly Efficient Photocatalytic Degradation of Organic Pollutants

Article abstract of DOI:10.1021/acs.inorgchem.0c00422

In this work, for the first time, we fabricated a novel covalent organic framework (COF)-based 2D-2D heterojunction composite MoS₂/COF by a facile hydrothermal method. The results of photocatalytic degradation of TC and RhB under simulated solar light irradiation showed that the as-prepared composite exhibited outstanding catalytic efficiency compared with pristine COFs and MoS₂. The significantly enhanced catalytic efficiency can be ascribed to the formation of 2D-2D heterojunction with a well-matched band position between COF and MoS₂, which can effectively restrain the recombination of charge carriers and increase the light absorption as well as the specific surface area. Moreover, the fabricated 2D-2D layered structure can effectively increase the contact area with an intimate interface contact, which greatly facilitates the charge mobility and transfer in the interfaces. This study reveals that artful integration of organic (COFs) and inorganic materials into a single hybrid with a 2D-2D interface is an effective strategy to fabricate highly efficient photocatalysts.

Full text of DOI:10.1021/acs.inorgchem.0c00422

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Article
Fabrication of 2D−2D Heterojunction Catalyst with Covalent
Organic Framework (COF) and MoS2 for Highly Efficient
Photocatalytic Degradation of Organic Pollutants
Kyu Kyu Khaing, Dongguang Yin,* Yinggen Ouyang,* Songtao Xiao,* Bingqi Liu, Linlin Deng, Luqiu Li,
Xiandi Guo, Jun Wang, Jinliang Liu, and Yong Zhang*
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ABSTRACT: In this work, for the first time, we fabricated a novel covalent organic framework (COF)-based 2D−2D
heterojunction composite MoS2/COF by a facile hydrothermal method. The results of photocatalytic degradation of TC and RhB
under simulated solar light irradiation showed that the as-prepared composite exhibited outstanding catalytic efficiency compared
with pristine COFs and MoS2. The significantly enhanced catalytic efficiency can be ascribed to the formation of 2D−2D
heterojunction with a well-matched band position between COF and MoS2, which can effectively restrain the recombination of
charge carriers and increase the light absorption as well as the specific surface area. Moreover, the fabricated 2D−2D layered
structure can effectively increase the contact area with an intimate interface contact, which greatly facilitates the charge mobility and
transfer in the interfaces. This study reveals that artful integration of organic (COFs) and inorganic materials into a single hybrid
with a 2D−2D interface is an effective strategy to fabricate highly efficient photocatalysts.
Recently, two-dimensional covalent organic frameworks
(COFs), as an emerging class of porous crystalline materials
assembled with organic molecules through covalent bonds,
have attracted much attention. COFs have been testified to be
ideal candidates employed in catalysis, gas adsorption and
separation, proton conductivity, optoelectronics, and energy
applications14−19 due to their chemically and mechanically
robust, inherent porosity, uniquely ordered channel structure,
1. INTRODUCTION
Anthropogenic pollutions such as antibiotics and organic waste
in bodies of water have arisen serious threats to the
environment, human health, and ecosystems.1,2 Various
techniques such as membrane separation, adsorption, chem-
istry methods, and photocatalysis have been applied to
eliminate these pollutants.3−7 Compared to other techniques,
photocatalysis is recognized to be attractive methods due to it
being environmentally-friendly, solar energy utilization, and
cost-effective.8−11 However, plenty of developed catalysts are
limited in practical applications due to their drawbacks such as
poor visible-light absorption, rapid charges carrier recombina-
tion rate, and low quantum yield.12,13 Thus, it is imperative to
develop novel photocatalysts with strong visible-light absorp-
tion and outstanding photoactivity.
Received: February 10, 2020
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Scheme 1. Schematic Diagram of the Synthesis Process for 2D−2D Heterojunction Composite MoS2/COF
high surface area, and tunable light harvesting properties.20−22
More recently, COFs have been recognized as a new type of
photocatalysts and some of the COFs have exhibited
fascinating properties for visible-light H2 production.23 The
excellent photocatalytic properties of COFs should be
attributed to their porous structure, wide light-harvesting
ability, and strong π−π interactions between the adjacent
layers.24 Moreover, crystalline COFs have emerged as ideal
platforms to be used as an effective support matrix for
stabilizing the homogeneously semiconducting materials to
synthesize versatile photocatalysts in terms of their high surface
area, porosity, and high stability.24,25 Particularly, the 2D
crystalline feature of COFs can enhance the charge mobility
and transfer between the interfaces, owing to its π−π
conjugation along the layers.26 Although COFs-based photo-
catalysts have been used for H2 production with considerable
efficiency, their potential for pollutant removal remains hardly
explored. Moreover, in most of the studies, noble metals as
cocatalysts are inevitably employed in COFs-based photo-
catalysis to realize a high catalytic efficiency, owing to their
high recombination rate of photoinduced electrons and
holes.27 Thus, it is crucial to seek an alternative to noble
metal cocatalysts that match well with COFs, suppress the
recombination of charge carrier, and consequently promote the
photocatalytic activity. Usually, the fabrication of a 2D
heterojunction is an efficient method to improve charge-
separation efficiency of a semiconductor and consequently
improve its photocatalytic activity. For example, Y. J. Yuan et
al. reported the g-C3N4/graphene/MoS2 two-dimensional
nanojunction, which exhibited a much higher photocatalytic
H2 production activity than the optimized Pt-loaded g-C3N4.28
F. M. Zhang et al. reported that when 2D TpPa-1 COF was
combined with NH2−UiO-66 MOF to form heterojunction, its
photocatalytic H2 evolution rate was enhanced 20 times.26 Z.
B. Han et al. reported a heterojunction catalyst PCN-222-Co@
TpPa-1 that can effectively catalyze the deacetalization−
Knoevenagel condensation reaction (yield of 99.3%).29
In this work, for the first time, a typical TpPa-1 COF was
synthesized and it was integrated with MoS2 to form a 2D−2D
heterojunction composite by a facile hydrothermal method.
The photocatalytic properties of the as-prepared products were
assessed via the degradation of tetracycline (TC) and
rhodamine B (RhB). The prepared MoS2/COF 2D−2D
heterojunction can greatly facilitate the charge mobility and
transfer between the interface and effectively suppress the
recombination of charge carriers, resulting in a remarkably
enhanced photocatalytic efficiency.
2. EXPERIMENTAL SECTION
2.1. Materials. 2,4,6-Trihydroxy-benzene-1,3,5-tricarbaldehyde
(C9H6O6), p-phenylenediamine (C6H8N2), 1,4-dioxine, 1,3,5-trime-
thylbenzene, acetic acid, sodium molybdate dihydrate, thiourea,
sodium sulfate (Na2SO4), 1,4-benzoquinone (BQ), triethanolamine
(TEOA), and isopropanol (IPA) were purchased from Sinopharm
Chemical Reagent Corp. Tetracycline (TC) was obtained from
Tianjin Seans Biochemical Technology Co., Ltd.
2.2. Synthesis of TpPa-1 COF. The TpPa-1 COF was
synthesized according to the reported method with a slight change.30
One milliliter of acetic acid (3 mol/L), 3 mL of 1,3,5
trimethylbenzene, and 3 mL of dioxane with a volume ratio of
1:3:3 were mixed in a round-bottom flask. Then, 63 mg of 1,3,5-
triformylphloroglucinol (0.3 mmol) and 48 mg of paraphenylenedi-
amine (0.45 mmol) were added into the mixture. The mixture was
continuously sonicated for 15 min until a homogeneous dispersion
was obtained. Then, the mixture was bubbled with N2 for 15 min and
heated at 120 °C for 3 days under a nitrogen atmosphere. After
cooling down to room temperature, the resultant product was,
respectively, washed with tetrahydrofuran, acetone, and hot DI water
three times. The obtained product was dried at 120 °C for 10 h in a
vacuum oven.
2.3. Synthesis of 2D−2D heterojunction composite MoS2/
COF. As shown in Scheme 1, 2D−2D heterojunction composite
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Figure 1. (a) XRD patterns of the as-prepared samples and (b) enlarged view of XRD patterns for the as-prepared samples.
MoS2/COF was prepared by a facile hydrothermal method. Typically,
8 mg of as-prepared COF, 48.4 mg of sodium molybdate dihydrate
(Na2MoO4·2H2O), and 30.5 mg of thiourea were mixed in 60 mL of
DI water with magnetic stirring for 1 h. The resulting mixture was put
into Teflon-lined stainless steel autoclave and heated at 200 °C for 12
h. After cooling down to the room temperature, the resulting product
was collected by centrifugation, washed with DI water three times,
and finally was dried in a vacuum oven at 80 °C overnight. Different
weight ratios of MoS2/COF ranging from 10% to 30% (referred to
COF proportion) were synthesized. The obtained MoS2/COF
composites with different weight ratios including 10, 20, and 30 wt
% were donated as MoS2/COF (10), MoS2/COF (20), and MoS2/
COF (30), respectively. The MoS2/COF (20) was chosen for
characterizations. Pure MoS2 was prepared with the same
experimental method but without the addition of COF.
adsorption−desorption isotherm, which was collected by using
Quantachrome instruments (Autosorb-iQ-MP-AG). Photolumines-
cence spectra (PL) of samples were recorded on a FS5 fluorescent
spectrophotometer (Edinburgh Instruments). FTIR spectra were
conducted on a Nicolet iS10 spectrophotometer.
2.5. Photoelectrochemical Measurements. The photocurrent
of the as-prepared samples was recorded by a CHI 660A workstation
with a three-electrode cell, where a Pt wire electrode was regarded as
the counter electrode, the as-prepared sample as the working
electrode, and a saturated calomel electrode as the reference
electrode. A Na2SO4 (0.2 M) solution was employed as the
supporting electrolyte. A 300W Xe lamp was used as the light source.
2.6. Evaluation of Photocatalytic Activity. Photocatalytic
performances of as-prepared catalysts were evaluated by the
degradation of tetracycline (TC) and Rhodamine B (RhB). The
photocatalytic degradation experiments were carried out at room
temperature using a SGY-IB multifunction photochemical reactor
(Hao Bin Co. Ltd., Nanjing, China) equipped with a 300W Xeon
lamp. In a typical experiment, 10 mg of as-prepared sample and 20
mL of TC (20 mg/L) or RhB (20 mg/L) aqueous solution were
mixed in a quartz cuvette. The suspension was first stirred in the dark
for 30 min to get an adsorption−desorption equilibrium between the
catalyst and dye. Then, the photoreaction vessels were exposed to
Xenon lamp irradiation with continuous stirring. The wavelength
distribution of the 300 W Xe lamp (PL-X500D) is similar to that of
solar light. After the photodegradation started, 1 mL of the sample
was taken at a given time intervals and filtered with 0.22 μm PVDF
disposable filters. The sample was analyzed by measuring UV−visible
absorption at the maximum absorption bands of RhB (λmax = 554 nm)
and TC (λmax = 357 nm) with a TU-1810DPC UV−vis
spectrophotometer.
3. RESULTS AND DISCUSSION
Figure 2. FTIR spectra of the as-prepared samples.
3.1. Characterizations. The X-ray diffraction patterns of
pure MoS2, COF, and MoS2/COF are shown in Figure 1. The
diffraction peaks of MoS2 at 14.4°, 33.45°, 39.54°, and 58.35°
can be assigned to (002), (101), (103), and (110) planes of
the hexagonal phase of MoS2 (JCPDS 75-1539).31,32 The
intense peak at 4.7° for COF corresponds to the reflection of
its (100) plane.26,30 The peak at a 2θ value ∼27° of COF
corresponds to its (001) plane ascribed to their π−π stacking
structure.33,34 The obtained pattern is similar to a previous
report.26 As shown in Figure 1a, the composites display similar
diffraction patterns of MoS2, suggesting that the coupling with
COF does not change the crystal structure of MoS2. However,
its diffraction peak at 39.54° slight decreases after coupling
with COF and with an increasing amount of COF due to their
2.4. Characterizations. The crystal phases of the samples were
characterized by powdered X-ray diffraction (XRD) on a Rigaku D/
max-2500 diffractometer (Rigaku) using Cu Kα radiation operated at
18 kV and 80 mA in the range from 4° to 85°. The morphologies of
the samples were investigated by a transmission electron microscope
(TEM) with a HT7700 electron microscope (HITACHI) and by a
scanning electron microscope (SEM) (Hitachi S-4800). The structure
specification and chemical states of the sample were investigated by
XPS spectra analysis on a Thermo ESCALAB 250XI spectrometer.
UV−vis diffused reflectance spectra were recorded on a Cary-5000
Ultraviolet−visible near-infrared spectrophotometer by using BaSO4
as a reference. The specific surface area was calculated by the
Brunauer−Emmett−Teller (BET) method according to a N2
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Figure 3. TEM images of (a) COF, (b) MoS2, and (c) MoS2/COF.
Figure 4. (a−e) EDX elemental mapping and (f) EDX spectrum of MoS2/COF (20).
Figure 5. (a) UV−vis diffuse reflectance spectra and (b) band gap energies for the as-synthesized samples.
interaction (Figure 1b). No obvious diffraction peaks of COF
can be detected in the hybrids, owing to its inherently poor
diffraction peak intensity in comparison to that of MoS2.
In the Fourier transform infrared (FTIR) spectrum of COF
(Figure 2), the peaks at 1580 and 1250 cm−1 are assigned to
the characteristic peaks of CC and CN bonds,
respectively.34 These characteristic peaks remain unaltered
after hybrid formation, indicating that the COF is highly stable
to the hydrothermal conditions on the course of the formation
of heterojunction. MoS2 shows its characteristic peaks at 1400
and 473 cm−1, which are attributed to the bridging MoS
bond (MoS) vibration perpendicular to the basal plane.35,36
All the patterns of the MoS2/COF hybrids exhibit the
characteristic peaks of both MoS2 and COF, indicating that
the composites are successful assembled. Moreover, the
characteristic diffraction peak intensities (1400 cm−1) of
MoS2 increase with increasing of the mass ratios of MoS2/
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Figure 6. XPS survey spectra of (a) whole spectrum, (b) C 1s spectra, (c) N 1s spectra, (d) O 1s spectra, (e) Mo 3d spectra, and (f) S 2p spectra
for MoS2/COF (20).
COF, which is in good accordance with the rising MoS2
contents in the synthesis process.
charge transfer in the interfaces and consequently contribute to
the enhancement of photocatalytic activity. As shown in Figure
S1, the SEM image of the TpPa-1-COF displays a kind of
flower-like cluster consisting of nanorods, while the SEM
image of MoS2/COF (20) shows that MoS2 nanosheets are
distributed on the TpPa-1-COF. Furthermore, the elemental
mapping analysis (Figure 4a−e) displays that Mo, S, C, and N
are evenly distributed. EDX spectroscopy was also applied to
confirm the chemical composition of the product. As shown in
Figure 4f, all of the elements including C, O, N, Mo, and S are
detected in the composite. All of the above results confirm that
The morphologies of the samples were detected by TEM
images. As shown in Figure 3a, the COF displays an irregular
sheet-like structure, which can be ascribed to the π−π stacking
of COF layer. Interestingly, the morphology of MoS2 is similar
to that of the COF, with a sheet-like two-dimensional layer
structure (Figure 3b). Figure 3c shows that the composite
MoS2/COF (20) displays a 2D−2D heterojunction structure
with an intimate interfacial contact. It is worth noting that the
2D−2D structure can effectively increase the contact area with
the intimate interface contact, which can greatly facilitate the
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the 2D−2D heterojunction composite MoS2/COF is success-
fully prepared.
defects or edges of MoS2.42 The weak peak located at 226.18
eV can be ascribed to a S 2s band.51 The S 2P spectra (Figure
6f) can be resolved into two peaks at 161.8 and 163.04 eV,
corresponding to S 2p3/2 and S 2p1/2 of S2− in MoS2,
respectively.40,42 The XPS results verified that the MoS2/COF
composite is successfully synthesized.
The light absorptions of the samples were measured by the
UV−vis diffuse reflectance spectra (DRS). As shown in Figure
5a, the COF exhibits a broad absorption band over the visible
region with an edge around 620 nm, while the MoS2 displays a
full absorbance range from 200 to 1200 nm. When the COF is
coupled with MoS2, its absorbance edge has a remarkable red
shift and its light absorption in visible-light region exhibits an
obvious enhancement. In addition, the red shift and enhanced
light absorption in the visible-light region raise with increasing
of MoS2 contents. The red shift and enhanced visible-light
absorption suggest that the composite can absorb more visible
light to generate more electron−hole pairs. The band gap
energies (Eg) of pure MoS2, COF, and MoS2/COF are
determined according to the following equation:37
TG analysis was carried out to determine the stabilities of as-
synthesized photocatalysts (Figure 7). According to the TGA
(αhv) = A(hv − Eg)n/2
Where, α, hv, Eg, and A are the absorption coefficient, photon
energy, band gap energy, and Planck’s constant, respectively.
The values of n is 1 for the direct and 4 for indirect transitions
of semiconductor, respectively.38 Therefore, the Eg values for
MoS2, COF, MoS2/COF (10), MoS2/COF (20), and MoS2/
COF (30) are calculated to be approximately 1.1, 2.11, 1.42,
1.59, and 1.72 eV, respectively. Compared with bare COF, the
band gap of MoS2/COF shows an obvious reduction. The
narrowing band gap of MoS2/COF is consistent with the
enhanced optical absorption (Figure 5b). The band gap energy
of the hybrid increases with an increase in COF mass content.
The results of Mott−Schottky measurements indicate that the
flat band position (Vfb) of TpPa-1-COF is approximately
−0.88 eV vs that of the saturated calomel electrode (SCE).
Generally, the bottom of the CB in many n-type semi-
conductors is more negative by about 0.10 V than the Vfb,26,39
thus the CB of TpPa-1-COF was estimated to be −0.78 eV.
Combined with the band gap energy, the VB position of TpPa-
1-COF was calculated as 1.33 eV.
The structure specification and chemical states of as-
prepared sample were investigated by XPS spectra analysis.
Figure 6a displays the XPS spectra of MoS2/COF(20), which
confirm the presence of C, N, O, Mo, and S in the prepared
sample. In the C 1s region (Figure 6b), the peak at 284.58 eV
can be attributed to the CC of COF40,41 and the peaks at
286.6 and 288.32 eV can be ascribed to the MoCO and C
OH (CO) bonds, respectively.39,42,43 The N 1s spectra
shown in Figure 6c can be resolved into three peaks at 394.5,
396.99, and 398.9 eV, respectively. The peak at 394.5 eV
corresponds to Mo2N (3p3/2), which may be caused by the
coordination between the Mo phase from MoS2 and N in
COF.44 The two other N 1s peaks at 396.99 and 398.9 eV refer
to a MoN bond and sp2-hybridized nitrogen CNC,
respectively.43−49 The O 1s spectra (Figure 6d) can be
deconvoluted into three peaks at 531.1, 532.6, and 533.3 eV,
which are ascribed to the CO, CO/HO, and MoO
bonds, respectively.42 As shown in Figure 6e, the Mo 3d
spectra can be resolved into three peaks at 229.02, 232.18, and
235.9 eV, respectively. Two strongest peaks centered at 229.02
and 232.18 eV are attributed to the Mo 3d5/2 (divulges from
MoN bond) and Mo 3d3/2 orbits of Mo4+.32,48,50
Furthermore, the peak centered at 235.9 eV can be assigned
to the Mo 3d3/2 orbit of characteristics state of Mo6+, which
may come from the slight partial oxidation of Mo atoms at the
Figure 7. TGA curves of MoS2, COF, and MoS2/COF (20).
curves, both the COF and MoS2/COF (20) show high thermal
stability up to 300 °C. The little weight loss around 100 °C can
be assigned to the absorbed water. The weight loss from 300
°C corresponds to the decomposition of COF and MoS2. The
mass ratio of COF in the MoS2/COF (20) was calculated to
be about 19.29 wt %, which is comparable to the experimental
value (20 wt %).
The surface areas and porous structure of the products were
measured by N2 adsorption−desorption experiments. As
shown in Figure 8 and Figure S2, all of the samples exhibit
type IV isotherms with hysteresis loops, suggesting a mesopore
Figure 8. N2 adsorption−desorption isotherm of the samples.
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structure of the materials. The calculated Brunauer−Emmett−
Teller surface areas (SBET) of the MoS2 and MoS2/COF (20)
were 3.159 and 18.984 m2/g, respectively. It is clear that the
SBET of MoS2/COF (20) is six times higher than that of pure
MoS2, suggesting that the formation of 2D−2D heterojunction
could increase specific surface area. As shown in Figure 8 and
Table 1, the SBET and pore volume of the composite increase
3.2. Phocatalytic Performances and Mechanism. The
photocatalytic performances of the prepared products were
accessed via the degradation of RhB and TC (Figure 10). In
Figure 10a, the removal efficiency of RhB is about 98% for
MoS2/COF (20) after simulated sun-light irradiation for 30
min, whereas it is only 37.5% and 35.6% for pure MoS2 and
COF, respectively. The apparent rate constant of MoS2/COF
(20) is 0.118 min−1, which is about seven and eight times
higher than those of the pristine MoS2 and COF, respectively.
For TC, its degradation ratio by MoS2/COF (20) is 85.9%
after 60 min irradiation, while it is only 47% and 37.8% for
pure MoS2 and COF, respectively. The apparent rate constant
of the TC degradation for MoS2/COF (20) is 0.0316 min−1,
which is about three and four times higher than those of the
pristine MoS2 (0.0103 min−1) and COF (0.0083 min−1),
respectively.
Table 1. Surface Areas, Pore Sizes, and Pore Volumes of the
Prepared Samples
surface area
(m2/g)
pore size
(nm)
pore volume
(cm3/g)
sample
MoS2
COF
3.159
474.5
4.001
1.52
0.019
0.2
MoS2/COF (10)
MoS2/COF (20)
MoS2/COF (30)
10.87
18.984
31.385
3.983
3.906
2.64
0.023
0.062
0.091
The superior photocatalytic performance of the prepared
MoS2/COF composite should be ascribed to the formation of
2D−2D heterostructure, which facilitates the transfer and
separation of the charge carriers and increases light absorption
and specific surface area. The MoS2/COF (20) exhibits the
best photocatalytic performance of all of the samples. The
mass ratio (MoS2/COF) impacts the photocatalytic perform-
ance of the composites. The photocatalytic efficiency increases
to a certain extent (up to 20 wt % COF) and then eventually
decreases on a further increase of the amount of COF. The
detriment of the photocatalytic activity is likely a consequence
of shielding effect. Due to its relatively low visible-light
absorption in contrast to MoS2, an excess of COF covers the
visible-light-active MoS2, resulting in a decrease of photo-
catalytic activity. The results of degradation of TC is similar to
that of RhB, further demonstrating that the as-prepared
composite possesses high photocatalytic activity and exhibits
potential capability for highly efficient degradation of organic
pollutants.
A cycling experiment was conducted to explore the stability
of the as-prepared photocatalyst. The used catalyst was
collected after each cycle test by centrifuging, washing, and
vacuum-drying at 60 °C overnight. As shown in Figure 11a, the
degradation efficiency of the catalyst does not obviously
decrease after three cycles, suggesting that the as-prepared
catalyst MoS2/COF (20) is quite stable and reusable in the
photocatalytic degradation process. Furthermore, XRD of the
hybrid after photocatalytic experiments was measured. As
shown in Figure S3, the XRD patterns between the samples
with an increase in the content of COF. It is well-known that a
larger surface area can provide more active sites for
photocatalytic reactions, resulting in enhanced photocatalytic
activity. Surface area, pore size, and pore volume for MoS2 and
MoS2/COF heterojunctions are displayed in Table 1.
The photoluminescence (PL) emission spectra were
conducted to determine the recombination rate of photo-
induced charge carriers. As shown in Figure 9a, the PL
intensity of MoS2/COF is much weaker than that of pure COF
and MoS2, indicating a great decrease of the recombination
rate of the photoinduced charge carriers in the MoS2/COF
composite. In addition, the emission PL peaks shift toward a
longer wavelength after the introduction of COF, due to their
interaction and ring-strain of COFs52,53
Transient photocurrent responses and electrochemical
impedance spectroscopy (EIS) were conducted to further
study the separation efficiency of the charge carriers. As shown
in Figure 9b, the photocurrent density of MoS2/COF is
significant higher than that of the pure COF and MoS2,
confirming the formation of heterojunction contributing to
accelerating the separation of charge carriers. As shown in
Figure 9c, the Nyquist semicircles diameter of the MoS2/COF
is smaller than that of the MoS2, indicating its lower interface
charge transport resistance. All of these results demonstrate
that the formation of 2D−2D MoS2/COF heterojunction can
effectively improve the interface charge transfer and separation.
Figure 9. (a) Photoluminescence (PL) emission spectra, (b) transient photocurrent responses, (c) and Nyquist plot of electrochemical impedance
spectra (EIS) of MoS2 and MoS2/COF composites.
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Figure 10. Photocatalytic degradation curves for (a) RhB and (b) TC over various as-prepared samples. The kinetic rate constants for degradation
of (c) RhB and (d) TC.
Figure 11. (a) Cycle experiments for degradation of RhB over MoS2/COF (20). (b) Trapping experiments by using different scavengers on the
RhB degradation over MoS2/COF (20).
before and after photocatalytic experiments showed no obvious
differences, further confirming the high stability of the hybrid.
The trapping experiments were carried out to explore the
photocatalytic mechanism. To test the role of these reactive
species, triethanolamine (TEOA), isopropylalcohol (IPA), and
ρ-benzoquinone (BQ) were employed as scavengers for h+,
degradation ratios for RhB are all reduced after the addition of
the three trapping reagents, indicating that the h+, •OH, and
•O2− are all responsible for the degradation process. When the
TEOA is added into the reaction system, the degradation
efficiency is remarkably decreased, suggesting that h+ takes a
crucial part in the degradation of organic pollutants. Figure 11b
−
•OH, and •O2 , respectively. As can be seen in Figure 11b, the
H
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Inorganic Chemistry
pubs.acs.org/IC
Article
demonstrates that the influence sequence of the active radicals
heterojunction and the 2D−2D layered structure. This work
provides new insights into the fabrication of COF-based
composites as high-performance photocatalysts and facilitates
their application in environmental protection and hydrogen
energy issues using solar light.
•
•
−
for RhB degradation is h+ > OH > O2 .
Based on the experimental results, the tentative mechanism
for the superior photocatalytic performance of MoS2/COF is
illustrated in Figure 12. Both COF and MoS2 are excited and
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00422.
■
sı
Figures of SEM images, N2 adsorption−desorption
isotherms, and XRD patterns (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Dongguang Yin − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China;
orcid.org/0000-0003-3878-8027; Email: ydg@shu.edu.cn
Yinggen Ouyang − China Institute of Atomic Energy, Beijing
102413, China; Email: 13911262687@139.com
Songtao Xiao − China Institute of Atomic Energy, Beijing
102413, China; Email: 13331157677@189.cn
Yong Zhang − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China;
Email: yongzhang@shu.edu.cn
Figure 12. Schematic diagram for the proposed photocatalytic
reaction mechanism of MoS2/COF.
engender photogenerated electron−hole pairs under the
simulated solar-light irradiation. The generated electrons on
the CB of COF rapidly transfer to the MoS2 since the CB edge
potential of COF is more negative than the MoS2, meanwhile
the holes on the VB of MoS2 transfer to the COF since the VB
edge potential of MoS2 is more positive than the COF. This
can effectively restrain the recombination of photoinduced
electron−hole pairs for COF and MoS2. The accumulated
electrons on the CB of MoS2 can react with the adsorbed O2 to
Authors
Kyu Kyu Khaing − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China
Bingqi Liu − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China
Linlin Deng − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China
Luqiu Li − School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200444, China
Xiandi Guo − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China
Jun Wang − School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200444, China
•
−
form O2 radicals, which react with H+ and e− to generate
H2O2 and finally result in the production of •OH. The
•
generated OH and h+ are powerful to oxidize the organic
pollutants into small molecules, e.g., CO2 and H2O. The
photocatalytic reaction processes are illustrated below.
MoS2 + hν = h+ + e−
COF + hν = h+ + e−
Jinliang Liu − School of Environmental and Chemical
Engineering, Shanghai University, Shanghai 200444, China
COF (e−CB) → MoS2 (e−
)
CB
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00422
•
−
MoS2 (e−)+O2 = O2
−
•O2 + 2H+ + e− = H2O2
Notes
The authors declare no competing financial interest.
•
H2O2 +e− = OH− + OH
ACKNOWLEDGMENTS
•
•
−
h+ + OH + O2 + pollutants
■
The authors acknowledge the Innovative Research Team of
High-level Local Universities in Shanghai, the National Natural
Science Foundation of China (No. 21271126), the Program
for Innovative Research Team in University (No. IRT 13078),
and the National 973 Program (No. 2010CB933901).
= CO2 +H2O + degradation intermediate
4. CONCLUSION
In summary, a novel 2D−2D nanocomposite MoS2/COF has
been synthesized via a facile hydrothermal method. The
prepared products are characterized in detail. The results of
photocatalytic degradation of TC and RhB under simulated
solar-light irradiation show that the MoS2/COF composite
with an optimized weight ratio of 4:1 exhibits a significantly
enhanced catalytic efficiency in contrast to those of the pristine
MoS2 and COF. The enhanced catalytic activity can be
assigned to the formation of band position well-matched
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