Interface photo-charge kinetics regulation by carbon dots for efficient hydrogen peroxide production

Article abstract of DOI:10.1039/d0ta10231h

Hydrogen peroxide (H2O2) is a multi-functional chemical for a range of industries, but the present H2O2 production requires complex processes, and leads to environmental pollution, etc. Solar water-splitting is one of the potential avenues to combine H2O and O2 into H2O2 through a cheap and clean way. Most of the photocatalysts involve multiple components and interfaces to improve the catalytic activity and energy conversion efficiency. However, it is difficult to regulate the photo-charge kinetics between the multi-interface catalyst, which hinders the practical application of photocatalysts. Here, we report a SnS2/In2S3 type II heterostructure modified by carbon dots (SnS2/In2S3/CDs) to highly improve the stability of sulfides and realize generation of H2O2 by the oxygen reduction reaction (ORR). Notably, in situ transient photovoltage measurements (TPV) were carried out to analyze the charge transfer process among SnS2, In2S3 and CDs. The optimal SnS2/In2S3/CD composite (n(Sn):n(In) = 50%) displays a prominent H2O2 production rate of 1111.89 μmol h-1 g-1 without any sacrificial agent under the conditions of normal pressure and neutral solution (pH = 7). The quantum efficiency (QE) of H2O2 production was calculated to be 3.9% under light (λ = 535 nm), and the solar energy conversion efficiency (SCC) was up to 1.02%, which is the highest known production of H2O2 from sulfides as photocatalysts. Our work provides a new way to regulate the photo-charge kinetics of the multi-interface catalyst using CDs to achieve the extremely efficient production of H2O2 by photocatalytic water-splitting.

Full text of DOI:10.1039/d0ta10231h

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Interface photo-charge kinetics regulation by
carbon dots for efficient hydrogen peroxide
production†
Cite this: DOI: 10.1039/d0ta10231h
*
Yi Li, Yajie Zhao, Haodong Nie, Kaiqiang Wei, Jingjing Cao, Hui Huang,
*
*
Mingwang Shao,
Yang Liu and Zhenhui Kang
Hydrogen peroxide (H2O2) is a multi-functional chemical for a range of industries, but the present H2O2
production requires complex processes, and leads to environmental pollution, etc. Solar water-splitting
is one of the potential avenues to combine H2O and O2 into H2O2 through a cheap and clean way. Most
of the photocatalysts involve multiple components and interfaces to improve the catalytic activity and
energy conversion efficiency. However, it is difficult to regulate the photo-charge kinetics between the
multi-interface catalyst, which hinders the practical application of photocatalysts. Here, we report
a SnS2/In2S3 type II heterostructure modified by carbon dots (SnS2/In2S3/CDs) to highly improve the
stability of sulfides and realize generation of H2O2 by the oxygen reduction reaction (ORR). Notably, in
situ transient photovoltage measurements (TPV) were carried out to analyze the charge transfer process
among SnS2, In2S3 and CDs. The optimal SnS2/In2S3/CD composite (n(Sn):n(In)
¼
50%) displays
a prominent H2O2 production rate of 1111.89 mmol hꢀ1 gꢀ1 without any sacrificial agent under the
conditions of normal pressure and neutral solution (pH ¼ 7). The quantum efficiency (QE) of H2O2
production was calculated to be 3.9% under light (l ¼ 535 nm), and the solar energy conversion
efficiency (SCC) was up to 1.02%, which is the highest known production of H2O2 from sulfides as
photocatalysts. Our work provides a new way to regulate the photo-charge kinetics of the multi-
interface catalyst using CDs to achieve the extremely efficient production of H2O2 by photocatalytic
water-splitting.
Received 20th October 2020
Accepted 27th November 2020
DOI: 10.1039/d0ta10231h
rsc.li/materials-a
photocatalysts involve multiple components and interfaces to
improve the catalytic activity and energy conversion effi-
ciency.13–18 Many multi-component and multi-interface catalysts
have been proved to possess excellent photocatalytic perfor-
mance compared to that of a single substance, such as hetero-
junctions, Z-schemes, co-catalysts, etc.19–25 However, it is
difficult to regulate the photo-charge kinetics of the multi-
interface catalyst, which seriously hinders the practical appli-
cation of photocatalysts.26–29
Carbon dots (CDs), with a size of less than 10 nm, have
become an attractive new material in the eld of catalysis due to
their abundant surface functional groups, unique electron
storage ability, photoinduced electron transfer properties and
efficient and stable reactive active centers.30–36 Herein, we report
that the SnS2/In2S3/CD composite can achieve the production of
H2O2 with high efficiency (1111.89 mmol hꢀ1 gꢀ1) and stability
under visible light under the conditions of normal pressure and
neutral solution (pH ¼ 7) without any sacricial agents. The
quantum efficiency (QE) of this photocatalyst is up to 3.9%
under a wavelength (l) of 535 nm, which is the highest known
production of H2O2 from suldes as photocatalysts.15,27,37,38 In
this system, CDs can store the electrons generated by SnS2 and
In2S3, regulate the photo-charge kinetics in the multi-interface
1. Introduction
Hydrogen peroxide (H2O2), as a multi-functional chemical, has
been widely applied in lots of elds, such as chemical produc-
tion, biomedicine, and water purication.1–3 However, H2O2 is
still synthesized by anthraquinone oxidation or the direct
synthesis from H2 and O2 in industry. These methods need vast
energy investment, multiple steps and may carry the risk of
explosion.4 One potential avenue is to combine solar water-
splitting and oxygen to form H2O2.5 During the photocatalytic
process, H2O can be oxidized to produce H2O2/O2 by photo-
holes. H2O2 can be produced simultaneously via the two-elec-
tron reduction of O2 which is from air and produced by the
catalyst.6–8
Over the past decade, a range of oxide and organic photo-
catalysts were developed for H2O2 production, such as Pt/TiO2,9
Au/BiVO4,10 rGO/TiO2/CoPi,11 and ZIF-8/C3N4.12 Most of the
Institute of Functional Nano and So Materials (FUNSOM), Jiangsu Key Laboratory for
Carbon-based Functional Materials and Devices, Soochow University, Suzhou, China.
E-mail: zhkang@suda.edu.cn; yangl@suda.edu.cn; hhuang0618@suda.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta10231h
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catalyst under visible light, and then further increase the elec-
tron–hole separation efficiency of SnS2/In2S3/CDs. On the other
hand, CDs are the photo-active sites for the oxygen reduction
reaction (ORR) to produce H2O2.14,39
2. Experimental
2.1 Synthesis of two-dimensional ultrathin SnS2
Two-dimensional SnS2 nanosheets were prepared through
a one-step hydrothermal process. Briey, 0.451 g (2 mmol)
SnCl2$2H2O and 1.127 g (15 mmol) thioacetamide were mixed
with 40 mL deionized water, and then stirred for 30 min. The
mixture was heated up to 160 ꢁC for 12 h and then the SnS2 was
ꢁ
collected and dried at 80 C for 12 h aer being washed with
ethanol and deionized water.
2.2 Synthesis of three-dimensional In2S3
For comparison, three-dimensional In2S3 was also synthesized
through a one-step hydrothermal route. Briey, 0.382 g (1.0
mmol) In(NO3)3$4.5H2O and 0.363 g (3 mmol) L-cysteine were
mixed with deionized water (60 mL) and stirred for 30 min. The
ꢁ
mixture was heated at 180 C for 12 h. The In2S3 was collected
and dried at 80 ꢁC for 12 h aer being washed with ethanol and
deionized water.
Fig. 1 (a) TEM image with size distribution and HRTEM image (inset) of
CDs. SEM images of (b) SnS2, (c) In2S3, and (d) SnS2/In2S3/CDs. (e)
HAADF-STEM image and the elemental mapping images of SnS2/
In2S3/CDs for (f) In, (g) Sn, (h) S and (i) C elements.
2.3 Preparation of CDs
CDs were synthesized by the typical electrochemical method.
Simply put, two graphite rods were placed in ultrapure water,
and 30 V DC power was employed in the reaction. Aer 20 days
of continuous stirring, the CD solution was acquired with a dark
brown colour. Aer ltering the large size CDs through a three-
layer lter paper device, the CD powder was obtained by freeze-
drying.
SnS2 and In2S3 (Fig. 1b and c), it was found that SnS2 has
a typical two-dimensional sheet structure with a size of 200 nm,
and In2S3 has a three-dimensional spherical structure like
a dandelion with a diameter of about 2 mm. The morphology
and microstructure of the SnS2/In2S3/CD composite in Fig. 1d
show that the two-dimensional lamellar SnS2 and the spherical
structured In2S3 are tightly combined together. Fig. 1e–i show
the elemental mapping images of SnS2/In2S3/CDs, where the Sn,
In, S, C elements are evenly distributed.
The crystal structures of SnS2, In2S3, and SnS2/In2S3/CDs
were detected by X-ray diffraction (XRD). As shown in Fig. S1,†
the relative intensity and peak position of SnS2 and In2S3 are
consistent with the standard cards JCPDS No. 22-951 and JCPDS
No. 65-459, respectively, indicating the successful synthesis of
SnS2 and In2S3. In Fig. 2a, there are 6 distinct diffraction peaks
of SnS2 at 2q ¼ 15, 32.2, 28.4, 41.2, 50, and 52.6ꢁ, which are
attributed to (001), (101), (100), (102), (110) and (111) reec-
tions, and the sharp diffraction peaks of In2S3 at 2q ¼ 27.5, 28.8,
33.4 and 47.98ꢁ correspond to (311), (222), (400), and (440)
crystal planes. It is noteworthy that the composite material
SnS2/In2S3/CDs has the characteristic peaks of both SnS2 and
In2S3, indicating successful combination of SnS2 and In2S3. The
important thing to note here is that XRD peaks of CDs cannot be
measured in the XRD pattern of SnS2/In2S3/CDs because of the
small number of CDs and the weak diffraction intensity of CDs
in the composite.
2.4 Synthesis of SnS2/In2S3/CDs
In our experiments, 0.382 g (1 mmol) In(NO3)3$4.5H2O, 0.363 g
(3 mmol) L-cysteine, 0.09 g (0.5 mmol) SnS2, and 24 mg CDs
were added to deionized water (60 mL) and stirred for 30 min.
Then, the mixture was transferred into a Teon-lined autoclave,
which was heated for 12 h at 180 C. Next, the SnS2/In2S3/CDs
were collected and dried at 80 C for 12 h aer being washed
with ethanol and deionized water. For the preparation of SnS2/
In2S3/CDs, SnS2 and In2S3 with different amounts (n(Sn):n(In) ¼
10%, 30%, 40%, 50%, 60%, 70% and 100%) were added to the
compound. The detailed characterization test and experiment
of photocatalysis for SnS2/In2S3/CDs are included in the ESI.†
ꢁ
ꢁ
3. Results and discussion
The morphology of SnS2/In2S3/CDs was characterized by using
a scanning electron microscope (SEM) and transmission elec-
tron microscope (TEM). From Fig. 1a, the TEM image, HRTEM
image and particle size distribution of monodisperse CDs were
obtained, illustrating that CDs are particles with a diameter of
2–7 nm and lattice spacing of 0.21 nm, corresponding to the
(100) crystal planes of graphitic carbon. From SEM images of
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Fig. 3 (a) UV-Vis absorption spectra of SnS2/In2S3/CDs, In2S3, and
SnS2. (b) Plots of (ahn)2 versus energy (hn) for SnS2 and In2S3. (c) UPS
spectrum of SnS2 and In2S3. (d) Band structure diagram of SnS2 and
In2S3.
Fig. 2 (a) XRD patterns of SnS2, In2S3 and SnS2/In2S3/CDs (black, blue
and red traces, respectively). (b) XPS survey spectrum of SnS2/In2S3/
CDs. (c) FTIR spectra of SnS2/In2S3/CDs and CDs. (d) Raman spectra of
SnS2/In2S3/CDs, In2S3, SnS2, and CDs.
SnS2/In2S3/CDs. As depicted in Fig. 3b, the band gaps of SnS2
and In2S3 are 1.93 eV and 2.01 eV, respectively. Furthermore, the
valence bands of SnS2 and In2S3 can be determined by ultravi-
olet photoelectron spectroscopy (UPS). As shown in Fig. 3c, the
valence bands (VB) of SnS2 and In2S3 were calculated to be 1.84
V and 1.36 V, respectively, while the conduction band (CB)
values of SnS2 and In2S3 were also calculated to be ꢀ0.09 V and
ꢀ0.65 V (the specic calculation method is in S1†). According to
Fig. 3d, the band gap structures of SnS2 and In2S3 can be derived
from the positions of the CB and VB, suggesting that SnS2 and
In2S3 both meet the requirements of photocatalytic water
splitting.
The surface chemical and elemental states of SnS2, In2S3,
and SnS2/In2S3/CDs were further studied using XPS technology.
From the XPS full spectrum in Fig. 2b, it can be found that SnS2/
In2S3/CDs contain three elements of In, Sn, and S. At the same
time, the full spectrum of SnS2/In2S3/CDs can simultaneously
identify the peaks of SnS2 and In2S3, indicating that SnS2 and
In2S3 are combined successfully. The high-resolution XPS
spectra of Sn 3d, In 3d, S 2p and C 1s for the SnS2/In2S3/CD
composite are shown in Fig. S2a–d.† As depicted in Fig. S2a,†
two peaks of SnS2/In2S3/CDs at 486.4 eV and 494.8 eV corre-
spond to Sn 3d5/2 and Sn 3d3/2. Notably, the binding energies of
Sn 3d5/2 and Sn 3d3/2 in SnS2/In2S3/CDs both show a 0.5 eV
negative shi compared to that of SnS2, implying that SnS2 has
got the electrons in the composite. Meanwhile, according to
Fig. S2b,† the peaks of In 3d5/2 and In 3d3/2 in SnS2/In2S3/CDs
(452.3 eV and 444.7 eV) show a 0.3 eV positive shi compared to
that of In2S3, indicating that In2S3 has lost the electrons in the
composite. As depicted in Fig. S2c,† the S 2p3/2 and 2p1/2 in
SnS2/In2S3/CDs correspond to 161.4 eV and 162.6 eV, respec-
tively. Besides, the C 1s spectrum of the SnS2/In2S3/CDs
composite (Fig. S2d†) centered at 288.7 eV, 285.8 eV, and 284.5
eV can be attributed to the C]O, C–O, and C–C of CDs. As
displayed in Fig. 2c, the FTIR spectrum proves the combination
In Fig. 4a, the TPV overall diagrams of SnS2, In2S3 and CDs
are displayed. As shown in Fig. 4b, the charge extraction process
of SnS2/In2S3 and CDs. The peaks at 3433 cmꢀ1, 1631 cmꢀ1
,
1384 cmꢀ1 and 1331 cmꢀ1 of SnS2/In2S3/CDs belong to the
stretching vibrational peaks of the O–H, C]O, C]C, and C–O–
C bonds in the CDs, respectively.33 Besides, in Fig. 2d, the peaks
at 1329 cmꢀ1 and 1567 cmꢀ1 of SnS2/In2S3/CDs belong to the
CDs and the peaks at 288 cmꢀ1 and 1433 cmꢀ1 of SnS2/In2S3/
CDs belong to the In2S3, indicating that Raman spectra further
conrm the combination of SnS2, In2S3, and CDs.
Fig. 4 (a) Comparison of the TPV with SnS2, In2S3, and CDs. (b)
Maximum extraction rate of the charge of SnS2, In2S3 and CDs. (c)
Charge extraction process of SnS2, In2S3 and CDs. (d) Charge
recombination process of SnS2, In2S3 and CDs.
Fig. 3a shows a typical absorption range in the 420–800 nm
region of SnS2, In2S3, SnS2/In2S3/CDs, indicating that the CDs
can obviously enhance the visible light absorption capacity of
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of CDs (tmaxc) is the fastest compared to that of SnS2 and In2S3, of time attenuation is caused by the secondary discharge
indicating that CDs have the fastest electron transmission process of CDs as good charge acceptors and donors.
capacity. Meanwhile, because it is easier for the lamellar
In Fig. 6a, the i–t curve was used to evaluate the photocurrent
structure of SnS2 to transmit electrons than the three-dimen- response capability under six light-on and light-off cycles for
sional spherical structure of In2S3, the charge extraction process SnS2/In2S3/CDs, SnS2/In2S3, In2S3, and SnS2. Generally, the
of SnS2 (tmax1) is faster than that of In2S3 (tmax2). According to response intensity of photocurrent is closely related to photo-
Fig. 4c, the time attenuation constants of SnS2 can be divided charge. Obviously, the photocurrent intensity of In2S3 is
into two sections, namely (0.099) and (0.66), because the elec- signicantly stronger than that of SnS2, indicating that In2S3 is
tron transport between the layers of SnS2 can decrease the mainly providing photogenerated charge in the photocatalytic
recombination rate of carriers. In contrast, the time attenuation process of SnS2/In2S3/CDs, which is consistent with the result of
constant of In2S3 was (0.123), suggesting that In2S3 has only the maximum charge extraction efficiency of SnS2 and In2S3 in
a period of attenuation and the overall electron–hole recombi- TPV. At the same time, the photocurrent intensity of SnS2/In2S3
nation rate is faster compared to SnS2. As displayed in Fig. 4d, is signicantly stronger than that of SnS2 and In2S3, which
the maximum charge extraction efficiency of In2S3 is greater proves that the heterojunction structure can increase the rate of
than that of SnS2 and CDs, suggesting that the main source of photo-charge separation and the stability of suldes. Aer
electrons and holes in the photocatalysis of SnS2/In2S3/CDs is adding CDs, the photocurrent intensity of SnS2/In2S3/CDs is
In2S3.
further enhanced, suggesting that the CDs further enhance the
As shown in Fig. 5a, the charge extraction process of SnS2/ charge separation efficiency and stability of suldes based on
CDs (tmax3) is faster than that of SnS2 (tmax1), indicating that CDs heterojunctions.
can increase the rate of charge transport in layer-to-layer of
Normally, a lower electron-transfer resistance inside the
SnS2. Meanwhile, the maximum charge extraction efficiency of catalyst leads to a smaller semicircle arc in the EIS spectrum. As
SnS2/CDs (0.017) is greater compared to SnS2 (0.014), suggesting shown in Fig. 6b, SnS2/In2S3/CDs has the smallest arc radius
that CDs can promote the separation of carriers of SnS2. In because of the presence of heterojunctions and CDs, indicating
Fig. 5b, the time attenuation constants of SnS2/CDs are (0.118) that the photo-electrons of SnS2/In2S3/CDs have faster electron
and (1.22), respectively. And (0.118) > (0.099), (1.22) > (0.66) transmission capacity than that of SnS2, In2S3 and SnS2/In2S3.
mean that CDs can slow down the recombination rate of Meanwhile, the semicircle diameter of SnS2 is smaller than that
carriers from the angle of time attenuation constants. As of In2S3, suggesting that it is easier for the two-dimensional
depicted in Fig. 5c, the charge extraction process of In2S3 (tmax2
)
lamellar structure of SnS2 to transmit electrons than that of
is a little faster than that of In2S3/CDs (tmax4), indicating that In2S3. As depicted in Fig. 6c, the charge extraction process of
CDs slightly delay the transport rate of charges due to the SnS2/In2S3 (tmax5) is faster than that of SnS2/In2S3/CDs (tmax6),
charge storage function of CDs. Similarly, the maximum charge indicating that the CDs are good electron donors and acceptors
extraction efficiency of In2S3/CDs (0.03) is bigger than that of in SnS2/In2S3/CDs. The continuous storage and release of elec-
In2S3 (0.025), suggesting that CDs can promote the separation trons by the CDs during the electron transport process slow
of carriers of In2S3. In Fig. 5d, the time attenuation constants of down the charge extraction rate. Meanwhile, the maximum
In2S3/CDs are (0.119) and (0.683), respectively. The second stage
Fig. 6 (a) The photocurrent curves of the SnS2/In2S3/CDs composite,
Fig. 5 (a) Maximum extraction rate of the charge and charge extrac- SnS2/In2S3 heterojunction, In2S3, and SnS2. (b) The EIS curves of the
tion process of SnS2 and SnS2/CDs. (b) Charge recombination process SnS2/In2S3/CDs composite, SnS2/In2S3, In2S3, and SnS2. (c) Maximum
of SnS2 and SnS2/CDs. (c) Maximum extraction rate of the charge and extraction rate of the charge and charge extraction process of SnS2/
charge extraction process of In2S3 and In2S3/CDs. (d) Charge recom- In2S3 and SnS2/In2S3/CDs. (d) Charge recombination process of SnS2/
bination process of In2S3 and In2S3/CDs.
In2S3 and SnS2/In2S3/CDs.
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charge extraction efficiency of SnS2/In2S3/CDs (0.076) is 2.92 photovoltage in the 5 vol& water/ACN (v/v) solution saturated
times that of SnS2/In2S3 (0.026), suggesting that the addition of with N2, illustrating that H2O consumed the charge of Q1 ꢀ Q0
CDs can greatly promote the separation efficiency of carriers in (0.1133) in the WOR. According to Fig. 7b, SnS2/In2S3/CDs
SnS2/In2S3/CDs. As shown in Fig. S4b and d,† the time attenu- consumed a lot of charge under the conditions of an O2-satu-
ation constants of SnS2/In2S3 are (0.076) and (0.258), respec- rated atmosphere, and the amount of charge consumed by the
tively. And (0.076) < (0.099), (0.258) > (0.123) mean that aer the ORR is Q0 ꢀ Q2 (0.4817). Therefore, the ratio of (Q1 ꢀ Q0) and (Q0
formation of heterojunction between SnS2 and In2S3, the elec- ꢀ Q2) is the number of charges required for the SnS2/In2S3/CDs
tron–hole recombination rate of SnS2 is accelerated and the to undergo the WOR and ORR. The half reaction rate ratio l is
electron–hole recombination rate of In2S3 is slowed down due to 4.25 (0.4817/0.1133). At the same time, using the electro-
constant transfer of electrons from In2S3 to SnS2. In Fig. 6d, the chemical test method in S6,† the oxygen evolution reaction
time attenuation constants of SnS2/In2S3/CDs can be divided (OER) rate constant k1 (9.31 ꢂ 10ꢀ9) is also slower than that of
into three sections, namely (0.147), (0.177) and (0.752). And the ORR rate constant k2 (1.2 ꢂ 10ꢀ8), which is consistent with
(0.076) is less than (0.147) and (0.258) is greater than (0.177) the results obtained by the in situ TPV (the specic calculation
which illustrates that the addition of CDs slows down the method is in S7†).40 Besides, as displayed in Fig. 7c, the EPR
electron–hole recombination rate of SnS2 because of the ability spectra of SnS2/In2S3/CDs support the existence of cO2ꢀ without
of CDs to store electrons in the layers of SnS2, and the CDs also the presence of cOH, suggesting that the ORR is the only way for
decrease the electron–hole recombination rate of In2S3 due to H2O2 production by photocatalytic water splitting. In Fig. 7d,
the secondary discharge process of CDs.
the electron transfer number of SnS2/In2S3/CDs in the photo-
The in situ TPV test was performed in different atmospheres catalytic process can be calculated to be about 4, indicating that
shown in Fig. 7a and b. By integrating the curve of the in situ water is oxidized by photo-holes into O2 instead of H2O2 during
TPV, the amount of charge consumed during the oxidation and the photocatalytic reaction of SnS2/In2S3/CDs.
reduction reactions can be calculated. In the ACN solution with
Fig. 8a shows the H2O2 generation rate with various catalysts.
a N2-saturated atmosphere, the charge generated by SnS2/In2S3/ The production rate of SnS2/In2S3/CDs was calculated to be
CDs during light excitation and eventually consumed by the
entire circuit was dened as Q0. In the 5 vol& water/ACN (v/v)
solution saturated with N2, H2O consumed charge as a water
oxidation reaction (WOR), and the remaining charge was
consumed by the entire circuit, which was dened as Q1. Like-
wise, in the ACN solution saturated with O2, O2 as the ORR
reactant consumes part of the charge, and the remaining charge
was consumed by the entire circuit, which was dened as Q2. As
depicted in Fig. 7a, the SnS2/In2S3/CDs displayed a higher
Fig. 8 (a) Evolution of H2O2 catalysed by SnS2/In2S3/CDs, SnS2/In2S3,
In2S3 and SnS2, (b) the cycling stability of SnS2/In2S3/CDs for the
photocatalysis of H2O2 production and (c) comparison of H2O2
evolution catalysed by SnS2/In2S3/CDs with different molar ratios of Sn
Fig. 7 The in situ TPV pattern integral diagram of SnS2/In2S3/CDs and In (1 : 10, 3 : 10, 4 : 10, 5 : 10, 6 : 10, 7 : 10 and 10 : 10) during the
under (a) N2-saturated ACN solution versus N2-saturated 5 vol& synthesis process. (d) Quantum efficiency (QE) performance (red
water/ACN (v/v). (b) N2-saturated ACN solution versus O2-saturated points) versus UV-vis absorption spectrum of SnS2/In2S3/CDs.
ACN solution. (c) EPR spectra of SnS2/In2S3/CDs, measuring the Comparison of H2O2 evolution produced by SnS2/In2S3/CDs in (e)
presence of cO2ꢀ and cOH under light-on and light-off. (d) i–t curves different atmospheres (N2/air/O2) and using (f) different scavengers
of SnS2/In2S3/CDs using a rotating ring-disk electrode (RRDE) under (AgNO3/no scavenger/CH3OH). All of the photocatalysis reactions
light-on and light-off with N2 saturation (0.1 M Na2SO4) and a rotating were measured under visible light irradiation (l $ 420 nm) for 3 h, with
speed of 1600 rpm.
the system containing 20 mg powder dispersed in 15 mL H2O.
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1111.89 mmol hꢀ1 gꢀ1, while that of SnS2/In2S3 was 471.14 mmol
hꢀ1 gꢀ1, that of In2S3 was 340.48 mmol hꢀ1 gꢀ1, and that of SnS2
was 170 mmol hꢀ1 gꢀ1 in a 3 h reaction. SnS2/In2S3/CDs exhibits
the optimal photocatalytic ability, and the H2O2 production rate
is 6.5 and 3.3 fold higher than that of pure SnS2 and In2S3. The
heterojunction formed by SnS2 and In2S3 facilitates the sepa-
ration of carriers, while the addition of CDs not only improves
the light capture ability of the photocatalyst, as good electron
acceptors and donors but also further facilitates the separation
of carriers. As displayed in Fig. 8b, the catalyst exhibits excellent
stability for ve cycles with each cycle being performed for 3 h,
indicating that the heterojunction structure and CDs enhance
the stability of SnS2/In2S3/CDs. As depicted in Fig. 8c, the effects
of different molar ratios of Sn and In of the composite on the
photocatalytic reaction were also explored. Clearly, the optimal
content of molar ratios of Sn and In is 50%, and the optimal
SnS2/In2S3/CDs displays the highest photocatalytic activity with
the H2O2 evolution rate of 1111.89 mmol hꢀ1 gꢀ1. The QE as
a function of the absorption spectrum of SnS2/In2S3/CDs is
displayed in Fig. 8d, and is calculated to be 3.9% at the irradi-
ation wavelength of 535 nm for 3 h (20 mg catalyst in 15 mL
H2O, the detailed calculations are displayed in S2†).
Fig. 9 (a) Schematic of photo-charge transfer between SnS2 and CDs
under visible light. (b) Schematic of photo-charge transfer between
In2S3 and CDs under visible light. (c) ORR performance of CDs, SnS2,
In2S3, SnS2/In2S3 and SnS2/In2S3/CDs in O2 saturation (0.1 M Na2SO4)
with the rate of 1600 rpm. (d) The mechanism diagrams of solar water-
splitting for the production of H2O2 over SnS2/In2S3/CDs.
As shown in Fig. 8e, the photocatalytic performance experi-
ments of SnS2/In2S3/CDs were performed under N2, O2, or air
atmosphere conditions. It can be seen that the H2O2 produced by
N2 saturation (370.38 mmol gꢀ1 hꢀ1) was not completely inhibited
indicates that the electrons generated by SnS2 will transfer to the
CDs and enhance the electron transport process between the
layers of SnS2 (Fig. 9a). Similarly, the charge transfer process
between In2S3 and CDs is displayed in Fig. 9b, suggesting that the
electrons produced by In2S3 will also be transferred to the CDs and
stored on the CDs for secondary discharge during the catalysis
process. As shown in Fig. 9c, CDs display a more positive onset
potential and a higher diffusion-limited current density compared
with other suldes, suggesting that CDs are the active sites in
SnS2/In2S3/CDs for the ORR due to its strong ORR process.41,42
As depicted in Fig. 9d, when the SnS2/In2S3/CDs was irradi-
ated with visible light, SnS2 and In2S3 will generate electron–
hole pairs, respectively. And the electrons generated by SnS2
and In2S3 will be transferred to the CDs to undergo reduction
reaction with O2 to produce H2O2. On the other hand, water will
oxidize with holes on In2S3 to produce O2. The WOR can further
provide O2 for ORR. In brief, the CDs in the multi-interface
SnS2/In2S3/CDs act as both the electron carriers of SnS2 and
In2S3 and the active centres of the ORR. The charge regulation
of the CDs on the multi-interface further increases the efficiency
of carrier separation between SnS2 and In2S3. Meanwhile, it also
prevents the damage of H2O2 to suldes and greatly enhances
the stability of suldes.
and H2O2 produced by O2 saturation (1751.41 mmol gꢀ1 hꢀ1
)
showed an obvious increase compared with air conditions
(1111.89 mmol gꢀ1 hꢀ1), suggesting that oxygen is involved in the
production of hydrogen peroxide from SnS2/In2S3/CDs and the O2
source of H2O2 produced by SnS2/In2S3/CDs is composed of two
parts from the air and the SnS2/In2S3/CD WOR. Meanwhile, we
have observed the dependence of kinetics on the concentration of
O2 in Fig. S5.† It shows that the H2O2 production of SnS2/In2S3/CDs
is related to Po2, and with the increase of the reactant concentra-
tion, the rate of H2O2 generation gradually increases until it
approaches the limit value which is caused by the limited number
of active sites for the SnS2/In2S3/CDs to adsorb and reduce O2 when
Po2 is large enough. As depicted in Fig. S6,† before the detection of
H2O2, we have detected the generation of H2 using a GC-7900.
There is no H2 produced when SnS2/In2S3/CDs was used as
a photocatalyst under a N2 or air atmosphere, indicating that SnS2/
In2S3/CDs only produces H2O2 even under a N2 atmosphere. As
shown in Fig. 8f, the photocatalytic performance of SnS2/In2S3/CDs
was displayed with different scavengers (AgNO3/no scavenger/
CH3OH). AgNO3 solution as an electron sacricial agent (242.15
mmol gꢀ1 hꢀ1) and CH3OH solution as a hole sacricial agent
(1426.56 mmol gꢀ1 hꢀ1) were added into the system also to verify
the mechanism of hydrogen peroxide production by SnS2/In2S3/
CDs compared with the system without a scavenger (1111.89 mmol
gꢀ1 hꢀ1), suggesting that electrons are involved in the reaction
process of SnS2/In2S3/CDs producing hydrogen peroxide. These are
consistent with the mechanism process by which SnS2/In2S3/CDs
produces H2O2.
Note that, CDs have adjustable characteristics in composi-
tion, size and surface functional groups, and their electronic
structure and surface functional groups can be adjusted by
further doping as well as further surface modication. There is
plenty of room to achieve CDs to regulate photo-charge kinetics
in multi-interfaces for efficient photocatalyst design.
4. Conclusions
The mechanism of the photocatalytic reaction based on SnS2/
In2S3/CDs is depicted in Fig. 9. The charge transfer process The SnS2/In2S3/CDs composite was prepared by a facile two-step
between SnS2 and CDs under the irradiation of visible light hydrothermal treatment. The as-prepared photocatalyst
J. Mater. Chem. A
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Journal of Materials Chemistry A
displays superior activity with a yield of 1111.89 mmol hꢀ1 gꢀ1
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work is supported by the National MCF Energy R&D
Program (2018YFE0306105), National Key Research and Devel-
opment Project of China (2020YFA0406104), the Innovative
Research Group Project of the National Natural Science Foun-
dation of China (51821002), the National Natural Science
Foundation of China (51725204, 21771132, 51972216 and
52041202), the Natural Science Foundation of Jiangsu Province
(BK20190041), the Key-Area Research and Development
Program of GuangDong Province (2019B010933001), the
Collaborative Innovation Center of Suzhou Nano Science &
Technology, the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD), and the 111
Project.
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