Direct Z-Scheme Heterojunction of SnS2/Sulfur-Bridged Covalent Triazine Frameworks for Visible-Light-Driven CO2 Photoreduction

Article abstract of DOI:10.1002/cssc.202000712

Solar-driven reduction of CO₂ into renewable carbon forms is considered as an alternative approach to address global warming and the energy crisis but suffers from low efficiency of the photocatalysts. Herein, a direct Z-Scheme SnS₂/sulfur-bridged covalent triazine frameworks (S-CTFs) photocatalyst (denoted as SnS₂/S-CTFs) was developed, which could efficiently adsorb CO₂ owing to the CO₂-philic feature of S-CTFs and promote separation of photoinduced electron–hole pairs. Under visible-light irradiation, SnS₂/S-CTFs exhibited excellent performance for CO₂ photoreduction, yielding CO and CH₄ with evolution rates of 123.6 and 43.4 μmol g^?1 h^?1, respectively, much better than the most catalysts reported to date. This inorganic/organic hybrid with direct Z-Scheme structure for visible-light-driven CO₂ photoreduction provides new insights for designing photocatalysts with high efficiency for solar-to-fuel conversion.

Full text of DOI:10.1002/cssc.202000712

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Direct Z-Scheme Heterojunction of SnS2/Sulfur-bridged Covalent
Triazine Frameworks for Visible-light-driven CO2 Photoreduction
Authors: Shien Guo, Peng Yang, Yanfei Zhao, Xiaoxiao Yu, Yunyan
Wu, Hongye Zhang, Bo Yu, Buxing Han, Michael George, and
Zhimin Liu
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To be cited as: ChemSusChem 10.1002/cssc.202000712
Link to VoR: https://doi.org/10.1002/cssc.202000712
0
1/2020

ChemSusChem
10.1002/cssc.202000712
COMMUNICATION
Direct Z-Scheme Heterojunction of SnS
2
/Sulfur-bridged Covalent
Triazine Frameworks for Visible-light-driven CO
2
Photoreduction
Shien Guo,[a,b] Peng Yang,[a,b] Yanfei Zhao, Xiaoxiao Yu,
[b]
[a,b]
Yunyan Wu,[a,b] Hongye Zhang, Bo Yu,
[b]
[b]
Buxing Han,[a,b,c] Michael George, and Zhimin Liu*[a,b,c]
[d]
Abstract: Solar-driven reduction of CO
2
into renewable carbon
reported.
forms is considered as an alternative approach to address global
warming and energy crisis, but suffering from low efficiency of the
Recently, polymer semiconductors have emerged as
promising alternatives to inorganic photocatalysts because they
can be designed with multiple functions. Covalent triazine
photocatalysts. Herein, we report a direct Z-scheme SnS
bridged covalent triazine frameworks (S-CTFs) photocatalyst
denoted as SnS /S-CTFs), which can efficiently adsorb CO due to
the CO -phillic feature of S-CTFs and promote separation of
photoinduced electron–hole pairs. Under visible light irradiation,
2
/sulfur-
frameworks (CTFs) as a typical organic polymer have been
capture and separation,[ heterogeneous
7]
(
2
2
applied in CO
2
catalysis[ becuase they can efficiently adsorb and/or activate
8]
2
CO
2
molecules. Especially, as fully conjugated organic
SnS
2
/S-CTFs exhibits excellent performance for CO
2
photoreduction, structures, CTFs have also been explored as photocatalysts for
−
1
[9]
[10]
yielding CO and CH
4
with evolution rates of 123.6 and 43.4 μmol g
hydrogen evolution,
water splitting
and chemical
−
1
[11]
h , respectively, much better than the most catalysts reported. This
inorganic/organic hybrid with direct Z-scheme structure for visible-
synthesis, but suffering from weak electronic conductivity and
low efficiency caused by rapid recombination of charge carriers.
In addition, their stability is a general concern due to their low
light-driven CO
2
photoreduction provides new insights for designing
photocatalyst with high efficiency for solar-to-fuel conversion.
tolerance
to
photo-oxidation.
Compared
to
organic
semiconductors, inorganic photocatalysts are more stable. Thus,
the combination of organic semiconductor with inorganic
photocatalyst may provide an effective way to improve the
activity and the photostability of photocatalysts. As an inorganic
photocatalyst, SnS has attracted numerous attention due to its
2
cheapness, abundance, non-toxicity, narrow band gap (2.2-2.4
eV) and short average diffusion length of photocarriers.[12] It has
The excessive emission of CO
fuels has caused severe global warming and environmental
problems. Photocatalytic CO conversion into fuels and
chemicals is regarded as a promising way to address both
environmental issues and energy-related concerns.[1] During the
past decades, considerable efforts have been devoted to the
2
due to the depletion of the fossil
2
2
been proved experimentally and theoretically that SnS coupled
2
development of efficient photocatalysts for CO photoreduction,
with semiconductors can enhance electron-hole separation
which are generally required to be capable of adsorbing CO
2
efficiency.[ Therefore, coupling CO
13]
2 2
-philic CTFs with SnS is
[2]
and have appropriate band gaps. A variety of strategies such
envisaged to provide an opportunity to improve the efficiency for
CO photoreduction.
[3]
creating surface defects,[4] and
as bandgap engineering,
2
[5]
nanostructuring design has been exploited to improve the
efficiency of photocatalysts, and various photocatalysts have
been reported. Alternatively, direct Z-scheme system composed
of two semiconductors has attracted great interest because it
enables boosting separation of photogenerated electrons and
holes, simultaneously coupling respective advantages of each
component, thus showing enhanced activity.[6] However, the
direct Z-scheme system for CO
2
photoreduction is seldom
[a]
S. Guo, P. Yang, X. Yu, Y. Wu, Dr. H. Zhang, Dr. B. Yu, Dr. Y.
Zhao, Prof. B. Han, and Prof. Z. Liu
Beijing National Laboratory for Molecular Sciences, Key Laboratory
of Colloid, Interface and Thermodynamics, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
E-mail: liuzm@iccas.ac.cn
[
b]
c]
S. Guo, P. Yang, X. Yu, Y. Wu, Prof. B. Han, and Prof. Z. Liu
University of Chinese Academy of Sciences, Beijing 100049, China.
Prof. B. Han, and Prof. Z. Liu
[
Physical Science Laboratory, Huairou National Comprehensive
Science Center
[d]
Prof. M. George
School of Chemistry, University of Nottingham, Nottingham NG7
Scheme 1. (a) Schematic illustration of the formation process for the SnS
CTFs hybrids. (b) Schematic diagram of the photogenerated electron−hole
separation and transfer in SnS /S-CTFs.
2
/S-
2RD, UK.
2
Supporting information for this article is given via a link at the end of
the document.
1
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COMMUNICATION
Herein, we report a direc Z-scheme system composed of
sulfur-bridged CTFs (S-CTFs) nanospheres and in situ formed
composed of nanocrystals with size <5 nm. The lattice fringes of
0.27 and 0.31 nm correspond to the (101) and (100) planes of
SnS
2
nanoparticles on S-CTFs, denoted as SnS
2
/S-CTFs, for
crystalline SnS
direct evidence on the formation of SnS
2
particles, respectively.[
15]
These results give
visible-light-driven photoreduction of CO
2
(Scheme 1). The S-
2
nanocrystals, which are
CTFs nanospheres were prepared via the nucleophilic
intimately attached onto the surface of the S-CTFs spheres.
From the energy-dispersive X-ray (EDX) elemental mapping
images of the selected area, the homogeneous distribution of C,
substitution coupling of cyanuric chloride and trithiocyanuric acid
14]
(
Figure S1)[ , which showed energy gap (Eg) of 2.73 eV with
conduction band (CB) at -0.93 eV and good ability to adsorb
CO . The SnS nanoparticles were in situ formed on S-CTFs via
treating the S-CTFs spheres in SnCl ethanol solution at 180 C,
which exhibited Eg of 2.40 eV with CB at -0.27 eV. It was found
that the transfer of the photo-induced electron-hole pairs in
N, S, and Sn elements in SnS
(Figure 1d-i), further confirming the formation of heterojunction
2
/S-CTFs can be clearly observed
2
2
2
structure between the SnS
For comparison, the SnS
SnCl
plate-like nanostructures with size of 200-300 nm (Figure S4).
X-ray diffraction (XRD) patterns of SnS /S-CTFs (Figure S5)
indicate that the SnS /S-CTFs composite possesses the
features of both S-CTF and SnS , indicating its heterojunction
structure. The Fourier transform infrared (FT-IR) spectrum of
2
particles and the S-CTFs sphere.
2
nanoparticles were prepared from
2
and thiourea in ethanol solution at 180 C, which showed
SnS
2
/S-CTFs follows direct Z-scheme mechanism. That is,
nanoparticles
photo-induced electrons in the CB of the SnS
2
2
tend to combine with holes in the valence band (VB) of S-CTFs,
achieving effective separation of photo-generated carries. As a
2
2
result, SnS
photoreduction using TEOA as a sacrifice reagent under visible-
light irradiation, yielding CO and CH with evolution rates of
23.6 μmol g−1 h−1 and 43.4 μmol g−1 h−1, respectively, much
higher than those obtained over pristine S-CFTs and other
reported photocatalysts. Notably, the SnS /S-CTFs hybrid
displayed excellent stability for photoreduction of CO . To the
2 2
/S-CTFs displayed high efficiency for catalyzing CO
−
1
SnS
2
/S-CTFs (Figure S6) exhibits a band at 545 cm assigning
to vibration of Sn-S,[
16]
and the absorption peaks to the
4
1
characteristic stretching vibration of triazine ring shifting to
higher wavenumbers in comparison with that of pristine S-CTFs.
These results indicate the strong interface interaction between
the SnS
2
particles and the S-CTFs support.[
2
17]
2
best of our knowledge, this is the first example of
inorganic/CTFs hybrid with direct Z-scheme structure for
photocatalytic CO reduction.
2
Figure 1. (a) SEM and (b) TEM images of SnS
2
/S-CTFs, (c) HRTEM image of
SnS /S-CTFs (corresponding to the marked square in red line). (e-i)
Corresponding mapping images and that of C, N, S, and Sn elements along
with their superimposition in panel (d).
2
Figure 2. High-resolution X-ray photoelectron spectra of (a) N 1s for S-CTFs
and SnS /S-CTFs, (b) Sn 3d for SnS and SnS /S-CTFs. (c) UV–vis diffuse
reflectance spectra and the corresponding band gaps (inset) and (d) CO
adsorption isotherms of S-CTFs, SnS and SnS /S-CTFs.
2
2
2
2
2
2
The formation of S-CTFs was confirmed by solid-state 13C
NMR CP/MAS spectroscopy and FT-IR analysis (Figure S2),
and the S-CTFs powders showed spherical morphology with
relatively smooth surface and size around 400 nm (Figure S3).
X-ray photoelectron spectroscopy (XPS) analysis was
employed to investigate the surface chemical compositions. The
survey XPS spectrum of SnS /S-CTFs indicate the presence of
2
However, the in-situ growth of SnS
surface of S-CTFs spheres altered their morphology. As shown
in Figure 1a, the SnS /S-CTFs sphere showed flower-like
2
nanoparticles onto the
C, N, S, Sn and trace amount of Cl elements (Figure S7), and
the elemental contents are listed in Table S1. In the C 1s
spectrum of SnS /S-CTFs (Figure S8a), three peaks with binding
2
2
surface composed of vertically aligned nanosheets with
thickness of ≈6 nm, and average diameter of the spheres
increased to about 550 nm. High-resolution transmission
energies (BEs) at 284.8, 286.1 and 288.6 eV are attributed to C-
C, C-S and C=N-C, respectively.[9c] The high-resolution
2
spectrum of S 2p for SnS /S-CTFs can be deconvoluted into four
peaks with BEs at 161.6 and 162.8 eV assigning to S2- species,
and at 163.9 and 164.9 eV to S-C bond, respectively (Figure
electron microscopy (TEM) image of the SnS
2
/S-CTFs
composites (Figure 1c) clearly shows the nanosheets are
2
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ChemSusChem
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COMMUNICATION
S8b).[18] In the N 1s spectrum of S-CTFs (Figure 2a), two peaks
with BEs of 398.8 and 400.0 eV prove the existence of triazine
SnS
SnS
(1.05 wt%) and S-CTFs (2.60 wt%) could adsorb CO
, and
2
2
2
/S-CTFs showed an enhanced CO
2
uptake capacity of 3.43
ring in S-CTFs. Notably, the BEs of N species in SnS
display a shift of 0.5 eV to higher values relative to that of S-
CTFs, suggesting the electrons transfer from S-CTFs to SnS
nanoparticles. As illustrated in Figure 2b, the BEs of Sn 3d in
SnS /S-CTFs shift negatively with 0.4 eV as compared with that
of SnS , which may result in the formation of an interfacial
internal electric field with the orientation from S-CTFs to SnS
All above results robustly demonstrate the successful
construction of SnS /S-CTFs heterojunction nanocomposite.
2
/S-CTFs
wt% at 1.0 bar and 273.15 K. The DFT calculation results
(Figure S11) indicate that SnS /S-CTFs exhibits the most
negative adsorption energy (Eads=-0.148 eV) compared to SnS
(Eads=-0.128 eV) and S-CTFs (Eads=-0.136) eV. This suggests
that new surface created by the introduction of the SnS
nanoparticles have stronger ability to attract CO molecules than
both separate SnS nanoparticles and S-CTFs support. The
above results manifest that the introduction of SnS onto S-CTFs
not only improves the surface area of the catalyst, but also
2
2
2
2
2
2
2
2
.
2
2
2
To determine the charge-transfer path and energy band of
the heterojunction, the band edge configuration of the as-
synthesized samples was investigated using UV–vis diffuse
reflectance spectra and Mott−Schottky (MS) plots. As shown in
enables stronger ability to adsorb CO
favorable to the transformation of CO
2
molecules, which may be
2
.
Given proper band alignments and favorable CO
capacity, SnS /S-CTFs together with SnS and S-CTFs were
evaluated to photocatalyze CO reduction under visible light
irradiation. As shown in Figure 3a, both SnS and S-CTFs are
effective for CO photoreduction using triethanolamine (TEOA)
as an electron donor. SnS could afford only CH with an
average evolution rate of 5.9 μmol h g−1 without CO detectable,
while S-CTFs could offer both CO and CH with evolution rates
of 17.1 and 8.3 μmol h−1 g−1, respectively. Excitingly, the SnS
/S-
CTFs hybrid displayed greatly improved performance for CO
photoreduction, achieving average evolution rates of CO and
CH
up to 123.6 and 43.4 μmol h−1 g−1, respectively, higher than
those obtained over the most reported photocatalysts (Table S2).
In order to verify the carbon origin of CO and CH , control
experiments and an isotopic experiment with 13CO
conducted. No appreciable amounts of CO and CH
detected in absence of photocatalyst or CO
Meanwhile, using 13CO
instead of CO under identical other
conditions, 13CO and 13CH
with m/z signals at 29 and 17 were
detected by gas chromatography−mass spectrometry analysis
Figure S12). These results robustly confirm that CO and CH
were indeed generated from photocatalytic reduction of CO
over SnS /S-CTFs. The time course for the evolution of CO and
CH is depicted in Figure 3b, which shows that the yields of CO
and CH increases almost linearly with irradiation time. Moreover,
the photocatalytic activity had no noticeable attenuation after
four cycles, demonstrating the high reusability of SnS /S-CTFs.
The good photo-stability of the SnS /S-CTFs may be mainly
2
capture
2
2
Figure 2c, S-CTFs and SnS
and 517 nm, respectively, corresponding to the band gaps of
.73 and 2.40 eV. SnS /S-CTFs presents the hybrid absorption
features of SnS and S-CTFs with band gap of 2.45 eV. Since
the MS plots of S-CTFs and SnS (Figure S9) show typical
2
exhibit absorption edges at 454
2
2
2
2
2
2
2
4
−1
2
characters of n-type semiconductors, their flat band potentials
are approximately equal to CB bottoms.[19] Hence, the CB edges
4
2
of S-CTFs and SnS
2
were calculated to be -0.93 and -0.27 V (vs
2
NHE), respectively. Accordingly, their VB potentials were
estimated to be 1.80 and 2.13 eV, respectively. The CB and VB
positions of S-CTFs and S-CTFs are staggered as illustrated in
Scheme 1b, which strongly agree with the requirements for a
direct Z-scheme system.
4
4
2
were
were
4
2
or in dark.
2
2
4
(
4
2
2
4
4
2
2
attributed to the efficient charge separation and transfer aroused
by the heterojunction. In contrast, S-CTFs showed obvious
deterioration after recycling test (Figure S13), probably due to
photo-corrosion on the catalyst under light irradiation.
To probe interfacial charge transfer within SnS
steady and time-resolved photoluminescence (PL) spectra were
collected. As shown in Figure S14, the emission peak of SnS /S-
2
/S-CTFs, the
2
CTFs significantly quenched, suggesting that the recombination
of the photogenerated electron-hole pairs was effectively
inhibited. The lifetime of the electron-hole pair was estimated
from time-resolved transient PL spectroscopy. The emission
Figure 3. (a) Average gas production rates of SnS
CTFs. (b) Stability of SnS /S-CTFs during the photocatalytic CO
Transient photocurrent responses under visible light irradiation (λ>420nm),
and (d) time resolved PL decay curve of S-CTFs, SnS and SnS /S-CTFs.
2
, S-CTFs and SnS
2
/S-
2
2
reduction. (c)
2
2
decay data of SnS
biexponentially (Figure 3c), and the calculated average lifetimes
are summarized in Table S3. The average PL lifetime of SnS /S-
CTFs reached 7.18 ns, which was distinctly enhanced compared
to those of SnS (5.60 ns) and S-CTFs (4.28 ns). The
photoinduced charge carriers with prolonged lifetime in SnS /S-
CTFs boosts the possibility of their participation in the
subsequent photocatalytic process, leading to improvement in
photocatalytic activity.
2 2
, S-CTFs and SnS /S-CTFs were fitted
2
2
From the N sorption isotherms (Figure S10), the Brunauer–
Emmett–Teller (BET) specific surface area of SnS
2
/S-CTFs was
2
2
calculated to be 122 m /g, higher than that of the pristine S-
2
2
CTFs (98 m /g). The increased BET surface area can be
ascribed to the contribution of the SnS
adsorption ability is a critical factor affecting its performance for
photocatalyzing CO conversion. As depicted in Figure 2d, both
2 2
nanoparticles. The CO
2
3
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COMMUNICATION
The migration of charge carriers in photocatalysts were
monitored by photoelectrochemical measurements. As shown in
Figure 3d, all samples displayed repeatable photocurrent
to combine with the photoinduced holes at the VB of S-CTFs
through the interface electronic field, achieving the separation of
the electrons at the CB of S–CTFs and the holes at the VB of
response during light on-off cycling, and SnS
enhanced current intensity compared to individual SnS
S-CTFs component. These results are consistent with their
photocatalytic performances, suggesting more effective
2
/S-CTFs exhibited
and to
SnS
the basic sites of S-CTFs, the electrons on the CB of S-CTFs
would efficiently reduce the activated CO molecules to produce
CO and CH , meanwhile the sacrificial agent is oxidized by the
holes retained at the VB of SnS . The high activity of SnS /S-
CTFs for catalyzing CO photoreduction can be mainly ascribed
to the following two aspects: the CO -phillic feature and porous
structure of SnS /S-CTFs facilitates the CO adsorption and
diffusion; the direct Z-scheme heterojunction structure of
SnS /S-CTFs not only enables the efficient charge separation
2 2
. Since CO molecules can be adsorbed and activated at
2
2
4
separation of photoinduced charge carriers in SnS
From the Nyquist plots shown in Figure S15, it is clear that
SnS /S-CTFs shows the smallest semicircle arc at high
2
/S-CTFs.
2
2
2
2
2
frequencies, implying the lowest electron-transfer resistance
across the electrode/electrolyte. Therefore, it can be concluded
2
2
that the SnS
of photoinduced electrons in SnS
2
modification on S-CTFs favors to rapid migration
/S-CTFs.
2
2
and migration, but also maintains the reduction ability of
photoinduced electrons.
In summary, a direct Z-scheme photocatalytic system
composed of inorganic semiconductor SnS
polymer semiconductor S-CTFs is presented. The direct Z-
scheme heterojunction in SnS /S-CTFs not only improves the
2
and organic
2
separation efficiency of electron–hole pairs, but also preserves
the strong reduction ability of photoinduced electrons,
meanwhile the CO
adsorption on the catalyst. As
demonstrates excellent activity towards photocatalytic CO
2
-phillic nature of S-CTFs enhances the CO
2
a
result, SnS /S-CTFs
2
2
reduction. This work provides a new strategy on developing
polymeric/inorganic heterojunction photocatalysts with effective
charge separation for photocatalytic CO
sustainable energy fuels.
2
reduction to
−
Figure 4. DMPO spin-trapping ESR spectra recorded for •OH (a) •O
2
(b)
under visible light for SnS , S-CTFs and SnS /S-CTFs.
2
2
To enclose the mechanism of charge-transfer path in Acknowledgements
2
/S-CTFs, electron spin resonance (ESR) analysis was
SnS
carried out. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was
This work was financially supported by National Natural Science
Foundation of China (21673256), Beijing Municipal Science &
Technology Commission (Z181100004218004), and Chinese
Academy of Sciences (QYZDY-SSW-SLH013).
−
employed as a spin trap to detect the reactive •OH and •O
2
species.[
20]
As shown in Figures 4a-b, after visible light
illumination for 30 seconds, the characteristic ESR signals
corresponding to DMPO-•OH adduct were detected only in the
presence of SnS
2
, while characteristic ESR signals of DMPO-
−
•
O
2
adduct were detected in the case of S-CTFs as the
/S-CTFs hybrid displayed
characteristic ESR signals corresponding to both DMPO-•OH
and DMPO-•O
−, with enhanced intensity. These results indicate
2
Keywords: photocatalytic CO reduction • covalent triazine
photocatalyst. Interestingly, the SnS
2
framework • SnS • Z-scheme heterojunction •
2
2
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2
2
(−0.33 V vs NHE), the photoinduced
−
2
to generate •O
2
radicals.
Similarly, •OH radicals cannot be thermodynamically generated
over S-CTFs because the VB potential of S-CTFs (1.80 V vs
2
NHE) is more negative than the potential of H O/•OH (1.99 V vs
[
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direct Z-scheme mechanism, as illustrated in Scheme 1b. That
is, a direct Z-scheme heterojunction is constructed between
SnS
photocatalytic CO
Under visible-light irradiation, both S-CTFs and SnS
to generate electrons and holes on each CB and VB edges,
respectively. Then, the excited electrons at the CB of SnS tend
2
and S-CTFs. Therefore,
a
plausible mechanism of
/S-CTFs is proposed.
are excited
2
reduction over SnS
2
[5]
2
2
4
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[
6]
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ChemSusChem
10.1002/cssc.202000712
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We develop a direct Z-scheme photocatalyst system comprised of sulfur-bridged covalent triazine frameworks (S-CTFs) and SnS
2
nanosheets, which shows excellent photocatalytic activity for CO
2
4
converting into CO and CH under visible light irradiation.
6
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