Construction of heterogenous s-c-s mos2/sns2/r-go heterojunction for efficient co2 photoreduction

Article abstract of DOI:10.1021/acs.inorgchem.9b02676

Photocatalytic reduction of CO₂ by semiconductors is of great significance in generating value-added fuels. Here, we construct a novel S-C-S heterojunction constituted of MoS₂/SnS₂/r-GO by a simple solvothermal method. The prepared MoS₂/SnS₂/r-GO showed significant photoexcitation of photosensitive oxygen (ROS) by electron spin resonance spectroscopy, demonstrating that superoxide radicals (^a￠O₂^-), pores, and hydroxyl radicals (^a￠OH) are the main active species. The constructed S-C-S heterojunction has a multilevel electron transport mechanism and synergistic effect, which provides the possibility of producing more organic fuel. The photocatalytic materials were characterized by XRD, XPS, SEM, TEM, PL, etc. As a result, the atomic layer MoS₂/SnS₂/r-GO heterojunction exhibited a CO formation rate of 68.53 μmol g^-1 h^-1 and a CH₄ formation rate of 50.55 μmol g^-1 h^-1, respectively. This work opens up new prospects for the formation of heterojunctions of chalcogenide transition-metal sulfides.

Full text of DOI:10.1021/acs.inorgchem.9b02676

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Construction of Heterogenous S−C−S MoS2/SnS2/r-GO
Heterojunction for Efficient CO2 Photoreduction
Shikang Yin,† Jinze Li,† Linlin Sun,† Xin Li,† Dong Shen,† Xianghai Song,† Pengwei Huo,*,†
Huiqin Wang,*,‡ and Yongsheng Yan†
†Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University,
Zhenjiang 212013, People’s Republic of China
‡School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China
S
* Supporting Information
ABSTRACT: Photocatalytic reduction of CO2 by semi-
conductors is of great significance in generating value-added
fuels. Here, we construct a novel S−C−S heterojunction
constituted of MoS2/SnS2/r-GO by a simple solvothermal
method. The prepared MoS2/SnS2/r-GO showed significant
photoexcitation of photosensitive oxygen (ROS) by electron
spin resonance spectroscopy, demonstrating that superoxide
radicals (•O2 ), pores, and hydroxyl radicals (•OH) are the
−
main active species. The constructed S−C−S heterojunction
has a multilevel electron transport mechanism and synergistic
effect, which provides the possibility of producing more organic
fuel. The photocatalytic materials were characterized by XRD,
XPS, SEM, TEM, PL, etc. As a result, the atomic layer MoS2/
SnS2/r-GO heterojunction exhibited a CO formation rate of 68.53 μmol g−1 h−1 and a CH4 formation rate of 50.55 μmol g−1
h−1, respectively. This work opens up new prospects for the formation of heterojunctions of chalcogenide transition-metal
sulfides.
other excellent properties.9 In addition, MoS2 materials with
unique structures and excellent properties are expected to be
used in the fields of semiconductors, linear photoconductors,
1. INTRODUCTION
Carbon dioxide (CO2) is the main contributor to the
greenhouse effect in the atmosphere, the rising concentration
of which causes the radiation of infrared energy on the surface
transistors, light-emitting devices, solar cells, supercapacitors,
and the like to exhibit p-type or n-type conductivity.10−12
of the earth and brings about a series of problems: the increase
Nevertheless, MoS2 still has shortcomings such as poor
in surface temperature rise causes global warming and sea level
conductivity, low stability, and inclination for photocorrosion
rise, the surrounding ecological balance and biodiversity has
that have limited its application in artificial photosynthesis. On
been destroyed, and there have been outbreaks of various
diseases.1,2 Therefore, it is urgent to find a low-cost, pollution-
the basis of the above analysis, it is necessary to further
improve and optimize molybdenum disulfide via adjusting the
free, and sustainable new energy to alleviate the consumption
of fossil fuels.3 Biomass, wind, and solar energy are new
forbidden bandwidth, further enhancing the responsiveness to
visible light, and improving stability to meet the practical needs
sustainable energy sources. Among them, photocatalytic
of artificial photocatalysts. At present, some articles have been
reduction technology is considered to be one of the most
promising methods for solving the energy crisis due to its
published on the latest optimization and improvement of
MoS2. For example, Yu et al.13 reported a MoS2/TiO2/Pt
environmental friendliness, warm reaction conditions, low
price, and clean and efficient advantages.4 An increasing
number of researchers have invested a great deal of energy in
researching efficient, renewable, and environmentally friendly
ternary composite with good photocatalytic hydrogen
evolution activity, Fan et al.14 studied one kind of RP-MoS2/
r-GO photocatalyst with good ability for removal of Cr (VI)
and organic contaminants, and Zhang et al.15 successfully
prepared a MoS2/Ti3C2Tx hybrid which has high electro-
catalytic hydrogen evolution activity. Meanwhile, different
dimensions of MoS2 have been successfully prepared, such as
MoS2 quantum dots,16 three-dimensional MoS2 nanoflower
photocatalysts.5 For the past few years, various promising
semiconductors have been applied to photocatalytic reduction
of CO2, such as transition-metal oxides,6 polysulfides,7 etc. In
order to capture CO2, it is converted into valuable organic fuels
such as methane (CH4), carbon monoxide (CO), methanol
(CH3OH), etc.8 Among them, molybdenum disulfide (MoS2)
has attracted a great deal of attention from researchers due to
its large specific surface area, plentiful abundance, low cost, and
Received: September 5, 2019
© XXXX American Chemical Society
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Scheme 1. Schematic Illustration of MoS2/SnS2/r-GO Nanocomposite Synthesis Process
balls,17 and two-dimensional ultrathin MoS2 nanosheets.18
However, it is well-known that all of these photocatalysts have
the disadvantages of short photogenerated carrier lifetime and
high recombination rate.
the carbon framework enables rapid carrier transfer, improving
the overall efficiency of the photocatalyst. What is more, as a
2D support, r-GO can provide a platform to connect other
semiconductors to form 3D nanostructures. This facilitates
photocatalytic reduction of CO2; the success can be attributed
to more reactive sites being produced in comparison to 1D and
2D nanostructured materials.24
In addition, a credible manufacturing mechanism is
proposed on the basis of the current assembly process analysis.
The 3D S−C−S heterojunction is formed by weak van der
Waals interactions, and the large specific surface area and
surface functional groups strengthen the synergistic effect of
the interface; in addition, the unique structure of the same
family of metal sulfides increases defects and reduces electron
transport barriers, thereby promoting the transfer of electrons
at the interface through electron tunneling and providing more
active sites, reducing the barrier to photocatalytic reduction of
CO2. In addition, the high efficiency produces more organic
fuel. The entire process of manufacturing 3D MoS2/SnS2/r-
GO is shown in Scheme 1.
Considering the problem of short lifetime and high
recombination rate of the photogenerated carrier, reduced
graphene oxide (r-GO) has attracted extensive attention from
researchers thanks to its high specific surface area, high
conductivity, and high carrier mobility.19 The introduction of
r-GO can effectively increase carrier mobility and activity due
to the pz orbital of every carbon atom that is perpendicular to
the layer plane can take shape entirely through the whole layer
and the large π bond of the atom constitutes a unique two-
position network structure with a carrier mobility of about
15000 cm2/(V s) at room temperature.20,21 In order to further
increase the activity of MoS2/r-GO, we expect to find an
outstanding material that combines with MoS2/r-GO that can
increase the performance of artificial materials more effectively.
A combination of the main-group and transition-metal sulfides
has rarely been reported in the photoreduction of CO2; thus,
the combination with main-group-metal sulfide semiconductor
groups is a new strategy.
2. EXPERIMENTAL SECTION
Atomic layer tin disulfide (SnS2) is considered to be an
extremely promising photocatalytic semiconductor thanks to
its large specific surface area, high absorption coefficient, and
good stability.22 In addition, the unique structure of the atomic
layer level SnS2 provides a rich functional active site.23
Furthermore, a good conduction band position and a suitable
band gap can both reduce CO2 to a greater extent and make it
more likely to absorb sunlight. Thus, it has received wide
attention in various fields: Zhang et al.24 successfully prepared
ultrathin 2D SnS2/C3N4 nanosheets for photodegradation of
organic pollutants, Qu et al.25 successfully prepared porous
coral-like SnS2 for the elimination of Cr(VI) in water, and
Parveen et al.26 successfully prepared SnS2 of different shapes
for high-performance supercapacitors. The storage perform-
ance of lithium batteries was studied by the synergistic effect of
MoS2/SnS2 heterojunctions by Man et al.27
Hence, we report a novel MoS2/SnS2/r-GO three-dimen-
sional (3D) nanocomposite prepared by a solvothermal
method, which photoreduced CO2 under ultraviolet light.
Interestingly, the MoS2 assembled into 3D uniform nanoflower
balls after a simultaneous coupling connection with r-GO and
SnS2.3,28 In our study, we have reduced the GO by
hydrothermal methods.29 Significantly, the introduction of
2.1. Materials. Tin chloride pentahydrate (SnCl4·5H2O),
thioacetamide (C2H5NS, TAA), sodium dodecyl benzenesulfonate
(SDBS), L-cysteine (C3H7NO2), potassium permanganate (KMnO4),
ethylene glycol (C2H6O2), hydrogen peroxide (H2O2, 30%), hydro-
chloric acid (HCl, 5%), graphite powder, sodium nitrate (NaNO3),
ethanol (C2H5OH), and sulfuric acid (H2SO4) were all purchased
from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Anhydrous sodium molybdate was purchased from Aladdin Reagent
(Shanghai) Co., Ltd. All reagents used for preparation and direct use
were of analytical grade and were not further purified. Deionized
water from local sources was used in all experiments.
2.2. Synthesis of r-GO. r-GO was synthesized by a classical ice−
water bath approach.30 The process used microwave heating: graphite
(1.0 g) and sodium nitrate (2.5 g) were dissolved in concentrated
sulfuric acid (30 mL), and then potassium permanganate was slowly
added. Then, 40 mL of deionized water and 10 mL of H2O2 (30 wt
%) were also added directly. The prepared samples were centrifugally
washed several times in water and ethanol and freeze-dried. The GO
solution was quantified as 1.2 mg/mL.29 The GO was treated by
ultrasound (100 kHz, 500 W) for 2 h at room temperature until the
solution was evenly dispersed. Then 0.5 mL of hydrazine hydrate and
20 mg of NaOH were added directly. After that, the solution was
transferred to a 100 mL autoclave at 200 °C for 12 h. The powder was
then freeze-dried.
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Figure 1. (a) XRD patterns of as-prepared r-GO, SnS2, MoS2, MoS2/r-GO, and MoS2/SnS2/r-GO. (b) XRD patterns of different solvent ratios of
as-prepared SnS2.
2.3. Synthesis of Pineapple-Shaped SnS2 with Hierarchical
Three-Dimensional Architecture. A slight change was made to a
typical modified water bath process:22 SnCl4·5H2O (0.25 mmol),
sodium dodecyl benzenesulfonate (0.6 mmol), and L-cysteine (2
mmol) were dissolved in a mixed solution of 20 mL of ethylene glycol
and 10 mL of deionized water, and the mixture was stirred for 10 min
to form a uniform solution. Then, it was transferred to a 50 mL
autoclave maintained at 160 °C for 12 h. An atomic layer of SnS2 was
prepared, and light yellow SnS2 samples were collected, washed
several times with deionized water and ethanol, and put in vacuum
freeze-drying equipment until lyophilization for further character-
ization.
2.4. Synthesis of MoS2/SnS2/r-GO Composites. The synthesis
of MoS2/SnS2/r-GO hybrid material was carried out by a
solvothermal process. In the typical experimental process,7 a certain
mass ratio of r-GO and ultrathin SnS2 was simultaneously introduced
in the preparation of MoS2. The remaining steps are the same as those
for making MoS2. Finally, MoS2/SnS2/r-GO nanocomposites with
different proportions of r-GO and SnS2 to MoS2 were prepared
successfully.
2.5. Characterization. The crystal structure was examined with
Ni-filtered Cu Kα radiation by X-ray diffraction (XRD; MAC Science,
Japan). The scan range 2θ was 5−80° and the scan speed 5°/min.
The measurements of X-ray photoelectron spectroscopy (XPS) were
analyzed on a PerkinElmer PHI 5300 instrument to check the element
status on the sample surface tested with a monochromatic Mg Kα
source. The high-resolution transmission electron microscopy (HR-
TEM) and transmission electron microscopy (TEM) images and
corresponding selected area electron diffraction (SAED) spectra were
observed by transmission electron microscopy (JEM-2010, Japan).
The scanning electron microscopy (SEM) images were characterized
by a field emission scanning electron microscope (FE-SEM, JSM-
7001F, Japan). The catalyst was subjected to ultraviolet−visible
diffuse reflectance spectroscopy (DRS) measurements using a
Shimadzu UV-3600 spectrometer, performed in the test zone of
200−800 nm and using BaSO4 as a reference. The photoluminescence
(PL) spectra for solid powders were carried out with an F-4500
instrument (Hitachi, Japan) and a xenon (Xe) lamp with an excitation
wavelength of 345 nm.
2.6. Photoelectrochemical Test. The electrochemical system
(CHI 660B, Shanghai, China) used a standard three-electrode quartz
cell containing 0.5 M Na2SO4 electrolyte solution, and a Pt wire and a
saturated calomel electrode (SCE) were used as a counter electrode
and a reference electrode, respectively. The process of working
electrode preparation was as in previous studies. Electrochemical
impedance spectroscopy (EIS) was performed in a 0.5 M Na2SO4
solution at a frequency from 0.1 Hz to 100 kHz and 0.5 V. The
sinusoidal potential amplitude applied in every case was 5 mV, which
was performed using an electrochemical workstation, and all
electrochemical signals were recorded with a CHI660B electro-
chemical analyzer (Chen Hua Instruments, Shanghai, China).
2.7. Photocatalytic Reduction CO2 Experiments. In order to
compare the photocatalytic reduction properties of pure MoS2,
different contents of synthetic MoS2/r-GO, and different contents of
synthetic MoS2/SnS2/r-GO composites, a series of CO2 photo-
reduction ability tests were carried out on the prepared samples.
Under the irradiation of ultraviolet light, a 300 mL closed
photochemical reactor was used, adopting 8 W mercury lamp as the
light source for the photocatalytic reaction. In addition, 20 mg of the
photocatalyst, 100 mL of NaOH (0.1 M) solution, and a given mass
of triethanolamine (TEOA) were placed in the reactor system, and
the mixture was stirred under strong magnetic force for 10 min to
homogenize the solution. CO2 (99.999%) gas was bubbled for 20 min
to exhaust air, and then the system pressure was maintained at 1.4 ×
105 Pa. Photocatalytic materials were taken from the enclosed light at
60 min intervals. The type and content of the product were measured
every 1 h by a gas chromatography (GC-7920, China) detector. The
isotope-labeling experiments were performed using 13CO2 instead of
12CO2, and the products were analyzed using gas chromatography−
mass spectrometry (GC-MS, 7890A and 5975C, Agilent).
3. RESULTS AND DISCUSSION
3.1. Compositional and Crystal Structure Informa-
tion. Figure 1a shows the XRD patterns of SnS2, MoS2, MoS2/
r-GO, and MoS2/SnS2/r-GO with different SnS2 loading
amounts and different r-GO loading amounts. The distinct
peaks located at ca. 13.51, 33.95, 39.03, and 58.68° are indexed
to the (002), (100), (103), and (110) crystal planes of MoS2.
The distinct peaks located at ca. 26.49 and 51.61° are indexed
to the (100) and (110) crystal planes of the SnS2 phase with
lattice constants of a = 3.05 Å and c = 1.82 Å (JCPDS 40-
1467).14,23,31 The results show that all of the diffraction peaks
of MoS2 conform to the standard XRD pattern of the MoS2
structure, and the lattice constants are a = 6.09 Å, b = 2.63 Å,
and c = 1.581 Å (JCPDS 17-0744),32 whereas no typical
diffraction peaks of r-GO were detected on account of the
limited amounts of r-GO in the MoS2/r-GO composites, the
change in r-GO fraction, and the diffraction peak of MoS2
decreasing slightly; similar results have been reported else-
where.14 However, when the semiconductor SnS2 is added, the
intensity and position of other diffraction peaks are
significantly changed while the SnS2 diffraction peak appears,
indicating the successful synthesis of ternary composites.
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Figure 2. XPS spectra of (a) survey, (b) Mo 3d, (c) S 2p, (d) Sn 3d, (e) C 1s, and (f) O 1s of MoS2/SnS2/r-GO.
Figure 1b shows the XRD patterns of different solvent ratios of
SnS2.
peaks at 284.5, 285.7, and 287.8 eV can be assigned to C−S
bonds, C−O bonds, CO bonds or O−CO bonds,
revealing surface functional groups on r-GO.14,36 For the O
1s core level spectrum in Figure 2f, the two peaks fitted by the
O 1s spectrum are located at 531.78 and 533.41 eV.37,38 The
above XPS analysis further indicates that the S−C−S
heterojunction was successfully synthesized, and r-GO may
act as a bridge for transporting carriers of the electron channel.
This result is consistent with XRD.
To further determine the existence of MoS2 and SnS2, the
surface electronic state, elemental composition, and bonding
state of the synthesized samples were examined by XPS tests.
The measured full spectrum (Figure 2a) confirmed the
presence of Mo, S, Sn, C, and O elements in the sample,
and the peaks of Mo 3d, S 2p, Sn 3d, C 1s, and O 1s appeared
at their standard positions. As shown in Figure 2b, the Mo 3d
spectrum has three different peaks, with peaks centered at
3.2. Raman Analysis. The prepared photocatalysts were
characterized by Raman spectroscopy. The crystallinity, local
structure, and bonding state of the catalyst were studied. Figure
3 shows the Raman spectrum of the synthesized photo-
catalysts. All samples are well crystallized in a monoclinic
structure, which is consistent with the XRD results. As shown
in Figure 3, for pure SnS2, a distinct band appeared at 307
cm−1, corresponding to the typical Ag vibration mode of the
SnS2 material.34 Interestingly, the band of SnS2 moved upward
from 307 to 310 cm−1 in the composite. It is speculated that
the red shift may be due to the insertion of Mo4+.
Theoretically, the Mo4+ radius is larger than the Sn4+ radius;
thus, the bonding force constant of Mo−S should be smaller,
which may result in a red shift of the SnS2 lattice vibration.39,40
27
229.1 and 232.3 eV attributed to Mo 3d5/2 and Mo 3d3/2
,
respectively. In addition, the binding energy of the Mo S2s
peaks is at 226.3 eV.12 In Figure 2c, the peaks at binding
energies centered at about 161.77 and 163.16 eV are ascribed
10,23,33
to S 2p3/2 and S 2p1/2
,
respectively. The binding energy
positions of Mo and S are consistent with those reported
previously. In Figure 2d, the two peaks at 486.7 and 495.3 eV
23,34
in the Sn spectrum belong to Sn 3d3/2 and Sn 3d5/2
,
respectively, further illustrating the coexistence of Sn4+ and
Mo4+ in the compound. The C 1s electron energy spectrum is
shown in Figure 2e. The high-resolution XPS spectral binding
energy of C 1s at 284.6 eV shows a main peak indicating C−C
or CC with an sp2 orbital.35 The binding energies of three
D
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Kubelka−Munk equation was applied to measure the optical
band gap (Eg), and then a plot was obtained via the
transformation based on [F(R∞)hν]1/2 − hν for direct band
gap semiconductors.37 The pure MoS2 sample showed strong
optical absorption in the UV light region, and an absorption
mutation occurred at about 347 nm, corresponding to a
forbidden bandwidth of about 1.78 eV.44 Different from the
pure MoS2 absorption behavior, the obtained MoS2/SnS2/r-
GO composite exhibits a strong absorption peak and a wide
visible light absorption range, which is caused by the
introduction of SnS2.14 The absorption shift occurred at
around 330−450 nm, which indicated its enhanced visible light
induced the photocatalytic activity.45 It is worth noting that the
visible light collection of MoS2/SnS2/r-GO composites is
significantly increased in comparison with that of pure MoS2
and MoS2/r-GO, obviously demonstrating that the enhanced
light absorption of MoS2/SnS2/r-GO composites can be
effectively utilized and thus result in the production of more
carriers under the similar illumination.46,47 In addition, the
effective mass was deduced at the top of the valence band and
the bottom of the conduction band from the band structure.48
In particular, the relative energy levels of the valence band
maximum (VBM) and the conduction band minimum (CBM)
play an important role in the redox capability of photocarriers
in the system.49 To further determine the band structure of the
prepared material, an XPS valence band spectrum was
obtained. As shown in Figure 4b, the highest occupied
electron state densities measured were 1.61 and 1.46 eV, which
matched values for the initial MoS2 and SnS2 by XPS. The
ternary composite material has a significant red shift
phenomenon, manifesting that the introduction of SnS2 does
alter the electronic density of the MoS2 matrix. This also
means that SnS2 doping into MoS2 may cause an electro-
negative defect state.50
Figure 3. Raman spectra of SnS2, MoS2, MoS2/r-GO, and MoS2/
SnS2/r-GO, respectively.
In addition, the other two main bands at 377 and 401 cm−1 can
correspond to the E21g and A1g modes of MoS2, respectively. It
is well-known that the E12g mode corresponds to the in-plane
opposite vibration of two S atoms with respect to Mo atoms,
while the A1g mode is related to the out-of-plane vibration of
the S atoms in the opposite direction,41 and they were both
highly sensitive to tensile or compressive strain. For the
composites, a slight red shift (1 cm−1) of the A1g mode was
also observed. The lattice spacing mismatch of MoS2/SnS2
between MoS2 and SnS2 may change the local strain on the
surface of the MoS2 nanoflower balls, resulting in a red shift (1
cm−1) of the A1g band.11 Moreover, the rest of the modes were
derived from the 1350.1 cm−1 defect-induced D band and the
1580.5 cm−1 symmetrically allowed G band. Furthermore, the
ID/IG ratios of MoS2/r-GO (1.28) and MoS2/SnS2/r-GO
(1.32) are both larger than that of GO (1.13);42,43 this
indicates that GO is accurately reduced to r-GO during the
synthesis. Obviously, this is consistent with the XRD.
3.4. Morphological Analysis. The crystal form and
morphology of the prepared samples were determined by
mapping images with TEM, HRTEM, SEM, and EDX
elements. Figure 5a,b shows TEM images of the obtained
MoS2 and r-GO, respectively. It is shown that the obtained
MoS2 is aggregated to form flowerlike spherical nanoparticles
and thin r-GO. As shown in Figure 5c, the carbon-incorporated
MoS2/r-GO clearly appears as petal-like shapes on r-GO.
Figure 5d shows a TEM image of the prepared MoS2/SnS2/r-
3.3. UV−Vis DRS Analysis. The light absorpion capacity
of photocatalysts was studied by UV−vis spectrophotometry.
Figure 4a shows the UV−vis diffuse reflectance spectra of SnS2,
MoS2, MoS2/r-GO, and MoS2/SnS2/r-GO samples. The
Figure 4. (a) UV−vis diffuse reflectance spectra of as-prepared SnS2, MoS2, MoS2/r-GO, and MoS2/SnS2/r-GO. Inset: forbidden bandwidths of
the corresponding samples. (b) XPS valence band spectra of pure MoS2 and pure SnS2.
E
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Figure 5. Morphology of the as-prepared MoS2/SnS2/r-GO blocks. TEM images of the MoS2 particles (a), r-GO (b), MoS2/r-GO (c), MoS2/
SnS2/r-GO (d). HRTEM image of the MoS2/SnS2/r-GO heterostructure (e). SEM image (f) and EDX elemental mapping (g−k) of MoS2/SnS2/r-
GO. EDX analysis (l) and SEAD pattern (m) of MoS2/SnS2/r-GO.
GO, which has excellent dispersibility, and the prepared
sample retains a favorable small-sized petal-like morphology
after r-GO modification.42,51 The HRTEM image is as shown
in Figure 5e, and the crystal structure and heterojunction of
the prepared MoS2/SnS2/r-GO can be clearly observed, so that
it is well matched with MoS2 and SnS2; the lattice spacings are
0.62 and 0.31 nm, respectively. They correspond to the (002)
crystal plane of the typical MoS2 and the (100) crystal plane of
SnS2.24,52 The EDX analysis and SEM images of MoS2/SnS2/r-
GO are shown in Figure 5f−l. The distributions of Mo, S, Sn,
C, and O elements (Figure 5g−l) are shown in the EDX
mapping images to further confirm the successful preparation
of the nanocomposite. Furthermore, the SAED character-
ization of MoS2/SnS2/r-GO with a scattering diffraction
pattern and concentric rings (Figure 5m) shows that MoS2/
SnS2/r-GO exhibits a nanoscale polycrystalline structure,
which is consistent with the XRD results.14 The successful
preparation of the S−C−S heterojunction was further
confirmed.
3.5. PL Spectra. The photocatalytic activity of the sample
was investigated by photoluminescence spectroscopy, and
light-induced electron−hole pair migration and recombination
processes on each sample were found. Because PL emission is a
free vector recombination,37,53,54 Figure 6 shows the steady-
state PL spectra of pure MoS2, pure SnS2, MoS2/r-GO, and
MoS2/SnS2/r-GO samples. The pure MoS2 material exhibits
strong light emission at 429 nm excitation, which mainly
results from the the high recombination of electrons of
MoS2.55 In addition, the PL signal strength of pure MoS2 is
higher than that of the MoS2/r-GO composite, indicating that
the composite probability of the MoS2/r-GO photogenerated
carrier recombination rate is lower. After the introduction of
SnS2, the PL intensity of MoS2/SnS2/r-GO composites is
significantly lower than that of MoS2/r-GO, the lifetime of the
electron−hole pairs is extended, and the recombination rate of
the electron−hole pair is reduced; this lowere rate means that
the MoS2/SnS2/r-GO composite has higher photocatalytic
activity. This phenomenon is attributed to the high electron
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The arc radius of the MoS2/SnS2/r-GO electrode is smaller
than the arc radius of the original MoS2, SnS2, and MoS2/r-GO
electrodes, indicating that the surface reaction speed of the
working electrode is faster, the charge recombination rate is
lowered, and the effective migration of electrons and holes is
improved;54,60 the results corresponded to the results of the
photoluminescence spectrum. The entire composite photo-
catalytic system is more efficient.
3.7. Analysis of Reactive Species. To determine the
possible mechanism, the samples were tested for ESR.
Moreover, the most directly validated detection technique
for detecting short-lived free radicals and paramagnetic
materials is the use of ESR spin capture and spin labeling
techniques for MoS2, MoS2/r-GO, and MoS2/SnS2/r-GO
nanomaterials under UV−vis light illumination.50 Each ROS
and electron/hole generated under excitation conditions is
identified. Here, we chose 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) and water (H2O) as solvents for the detection of
Figure 6. PL emission spectra of MoS2, SnS2,MoS2/r-GO, and MoS2/
SnS2/r-GO.
transport of the spatially extended p-bond conjugated system
of SnS2.22,56 At the same time, r-GO significantly accelerates
the generation of charge-transfer hybrid materials through an
effective interface. The electron transfer (ET) process reduces
the recombination of e/h+ pairs in the MoS2/SnS2/r-GO
interface layer.51,57,58 On the basis of the above analysis, the
MoS2/SnS2/r-GO composite photocatalyst exhibited excellent
photocatalytic activity.
3.6. Electrochemical Properties Analysis. In order to
investigate the photoelectrochemical properties of the obtained
samples, photoelectrochemical analysis was carried out with an
electrochemical workstation to detect the separation efficiency
of the photogenerated charge carriers. Photoelectric response
and EIS measurements are shown in Figure 7a,b. The transient
photocurrent response of pure MoS2, pure SnS2, MoS2/r-GO,
and MoS2/SnS2/r-GO in several open−close cycles under
visible light irradiation (λ >420 nm) were measured in 0.5 M
Na2SO4 solution. MoS2/10% SnS2/3% r-GO exhibits a strong
transient photocurrent response, which was approximately 2
times that of MoS2/3% r-GO and 4 times that of MoS2 (Figure
7a). The results show that r-GO plays a crucial role in effective
charge transport. In addition, the good photoresponse of SnS2
is one of the reasons for the higher photocurrent response,
resulting in a significant increase in the separation of light-
generated e/h+ pairs in composites.59 The EIS Nyquist test
(frequency 0.1 Hz to 100 kHz, 0.5 V) as shown in Figure 7b.
−
hydroxyl radicals (•OH) and superoxide radicals (•O2 ),
respectively.61 Figure 8 gives ESR spectra of various traps in
MoS2, SnS2, MoS2/r-GO, and MoS2/SnS2/r-GO hybrid
materials before and during simulated sunlight exposure. For
comparison, the same experiment was performed on different
materials. It is obvious that there is no peak in Figure 8 before
the light is on. However, when light is irradiated, the
characteristic peaks appear rapidly in the spectra of MoS2,
SnS2, MoS2/r-GO, and MoS2/SnS2/r-GO in Figure 8, such as
a typical 1:2:2:1 quadruple feature spectrum and observed
distinct features corresponding to hydroxyl radicals
(DMPO-•OH) and superoxide radicals (DMPO-•O2 ),
−
respectively, and the intensities of hydroxyl radicals and
superoxide radicals change significantly.62,63 This result
signifies that photogenerated electrons can react with oxygen
to generate O2 radicals.17,64 Next, these samples detected
that DMPO-•OH radicals were acquired by further reaction of
•
−
−
•O2 radicals with electrons and H+, where H+ was derived
from the ionization process of H2O.65 The strengths of the
peaks of these two radicals are obviously enhanced by the
introduction of r-GO and SnS2,66 respectively, This is indirect
proof that the photocatalytic activity of MoS2/SnS2/r-GO
composite is better than that of pure MoS2, SnS2 and MoS2/r-
GO.
Figure 7. Transient photocurrent response (a) and EIS Nyquist plots (b) of MoS2, SnS2, MoS2/r-GO, and MoS2/SnS2/r-GO nanocomposites
under visible light irradiation.
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Figure 8. (a) DMPO spin-trapping ESR spectra of MoS2, SnS2, MoS2/r-GO, and MoS2/SnS2/r-GO in an aqueous dispersion and (b) in a
methanol dispersion.
13CO) and m/z 16, 28 (12CH4, 12CO) were clearly observed in
the mass spectrum of the photocatalytic product, confirming
that the detected reduction product was derived from MoS2/
SnS2/r-GO photocatalytic CO2 reduction. The results of the
analysis are consistent with those in other literature.69
3.8. Photocatalytic Reduction of CO2. The photo-
catalytic activity of these samples was examined by using
photoreduction equipment, with CO2 as the simulated
reactant. Gas-phase products were identified and quantitatively
analyzed by gas chromatography (GC). Prior to the photo-
catalytic test, the adsorption and desorption equilibria of all
samples were achieved by stirring in the dark for 20 min
without air and magnetic force. Control experiments showed
that CO and CH4 were not detected in the dark or in the
absence of carbon dioxide, indicating that CO2 produced CO
and CH4 under ultraviolet light under the action of the
catalysts. With the addition of different proportions of SnS2
semiconductors, the photocatalytic activity is significantly
improved due to better synergistic effects. The optimal
photocatalytic activity of MoS2/SnS2/r-GO has been deter-
mined to be quantitatively 3% r-GO and 10% SnS2. It can be
seen from Figure 9a−d that the yields of CO and CH4 under
ultraviolet light reach the maximum value, respectively.6,67
Amounts of r-GO and SnS2 over 3% and 10%, respectively,
lead to the degradation of photoreduction properties; this can
be ascribed to excessive r-GO and SnS2 adhesion on the MoS2
surface will limit the photocatalytic semiconductor photo-
absorption and reduce the contact area in the reactor−
photocatalyst system.68 In addition, the special structure of the
MoS2/SnS2/r-GO van der Waals heterojunction can provide
lower diffusion resistance and faster ion diffusion channels, and
the interface synergy can improve electrochemical perform-
ance.27 The same conclusion was obtained from ESR and PL
analyses (Figures 6 and 8). Therefore, the enhanced
photocatalytic properties with MoS2/SnS2/r-GO are attributed
to the presence of r-GO and SnS2, thanks to increasing charge
carriers and usable exciters. According to the above analysis, it
is indicated that MoS2/10% SnS2/3% r-GO has the highest
activity for photoreduction of CO2 under ultraviolet
irradiation. The catalyst yields of CO and CH4 after 5 h are
shown in Table 1.
3.9. Proposed Mechanism of Photoreduction. In the
results discussed above, it is clear that the MoS2/SnS2/r-GO
hybrid nanomaterials show a significant enhancement in the
production of superoxide and hydroxyl and photoinduced
cavities in comparison to MoS2, SnS2, MoS2/r-GO photo-
activity, which is consistent with the dispersibility of SnS2 and
the carbon-containing r-GO. It is worth noting that the r-GO
provides both a fast channel for electron transport and a 2D
structure of carbon for the MoS2/SnS2/r-GO heterojunction.44
The platform plays a vital role in the photocatalytic
performance. The empirical formulas for calculating the energy
band position of the relevant materials MoS2 and SnS2 are EVB
= X − Ee + 0.5Eg and ECB = EVB − Eg.37 On the basis of the
band gap energy calculated by DRS spectroscopy, the band gap
energies of pure MoS2 and pure SnS2 (Eg) are estimated on the
basis of the intercept of the tangent of the curve of (αhν)1/2
(or (αhν)2) and light energy. In Figure 4a the band gaps of
pure MoS2 and pure SnS2 are 1.78 and 2.05 eV, respectively. In
Figure S1a,b, the flat band potentials of pure MoS2 and pure
SnS2 were tested by the Mote Nitro Curve, which were −0.15
and −0.55 eV, respectively, and the valence band positions of
pure MoS2 and pure SnS2 were calculated by the formula as
1.63 and 1.50 eV, respectively. In addition, as shown in Figure
4b, the valence band position is further confirmed by XPS. The
valence band spectrum shows that the valence band positions
of pure MoS2 and pure SnS2 are 1.61 and 1.46 eV, respectively,
which are basically consistent with the empirical calculation
formula. Moreover, the VB and CB values of the semi-
conductor material determine the form and intensity of ROS
generated by photoexcitation.59,70 MoS2/SnS2/r-GO has a
−
lower CB value than E0(O2/•O2 ) (−0.33 eV vs NHE) due to
Further, in order to determine the source and content of the
product during CO2 photoreduction, an isotope tracer
experiment was carried out by gas chromatography−mass
spectrometry (GC-MS) for photoreduction of 13C and 12C on
MoS2/SnS2/r-GO. The labeled CO2 is analyzed as shown in
Figure 9e,f. The overwhelming signals at m/z 17, 29 (13CH4,
the enhanced superoxide radical strength of the composite.
However, the CB of pure MoS2 cannot produce superoxide;
therefore, electrons can only migrate from the MoS2
conduction band to the valence band of SnS2. In addition,
since MoS2 and SnS2 have overlapping band potentials, they
can form effective Z-type heterojunctions.28 Figure 10 shows a
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Figure 9. Reduction of CO2 by MoS2, MoS2/r-GO, and MoS2/SnS2/r-GO under ultraviolet light: (a, c) CO production at 5 h; (b, d) CH4
production at 5 h; (e, f) gas chromatography−mass spectrometry (GC-MS) analysis of CO and CH4 produced by MoS2/SnS2/r-GO at 3 h.
detailed plot. Under ultraviolet light, electrons are easily
excited from the MoS2 conduction band (CB) to the valence
band (VB) in SnS2, producing photoelectron hole pairs.44
Then, a part of the photogenerated electrons in SnS2
undergoes reduction of CO2, and another part of the
photogenerated electrons and the electrons of the MoS2
conduction band migrate to the surface of the r-GO to reduce
CO2 to CO and CH4, thereby forming multilevel electron
transport.71 At the same time, the holes of the MoS2 valence
band combine with the sacrificial agent EDTA to inhibit the
recombination of photogenerated carriers, so that more
photogenerated electrons participate in the process of
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Article
way to extend transition metal semiconductor based photo-
reduction technology.
Table 1. Yield of CO and CH4 (5 h) in Every Experiment
with Different Catalysts
photocatalyst
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02676.
■
MoS2 MoS2/3% r-GO MoS2/10% SnS2/3% r-GO
S
yield of CO
(μmol/g)
yield of CH4
(μmol/g)
1.98
113.97
342.66
1.15
81.61
252.74
Flat band potentials of pure MoS2 and pure SnS2 tested
by the Mote Nitro Curve (PDF)
photoreduction of CO2, producing more organic compound
fuel. In addition, the transfer of electrons and holes in the high-
efficiency Z-type heterojunction can efficiently improve the
separation of photoelectron hole pairs, inhibit their recombi-
nation, and prolong the lifetime, thereby improving the
photocatalytic performance.72 Since r-GO has a short
electronic transmission path and good electrical conductivity,
electron−hole recombination is extremely limited. In addition,
the uniform dispersion of MoS2 and SnS2 on r-GO provides
more photocatalytic active sites for CO2 gas reduction.
Therefore, the performance of MoS2/SnS2/r-GO photocatalyst
has been remarkably improved.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for P.H.: huopw.@mail.ujs.edu.cn.
*E-mail for H.W.: hqwang@mail.ujs.edu.cn.
ORCID
■
Pengwei Huo: 0000-0001-9647-7871
Yongsheng Yan: 0000-0001-7083-3292
Notes
The authors declare no competing financial interest.
4. CONCLUSION
ACKNOWLEDGMENTS
■
In summary, we have experimentally demonstrated the
formation of MoS2/SnS2/r-GO by coupling during a synthesis.
Representative detection of the photoexcitation process of
MoS2/SnS2/r-GO has been fairly analyzed by photolumines-
cence spectroscopy. From the effective adjustment of the
energy band structure of the r-GO-based material, the prepared
MoS2/SnS2/r-GO showed significant optical excitation in
photoactive oxygen (ROS), demonstrating that the superoxide
We gratefully acknowledge the financial support of the
National Natural Science Foundation of China (Grant Nos.
21776117 and 21576125), China Postdoctoral Science
Foundation (Grant Nos. 2017M611716 and 2017M611734),
and Six talent peaks project in Jiangsu Province (Grant No.
XCL-014).
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DOI: 10.1021/acs.inorgchem.9b02676
Inorg. Chem. XXXX, XXX, XXX−XXX