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Supporting information
Co-catalyst-free ZnS-SnS2 porous nanosheets for clean and
recyclable photocatalytic H2 generation
Lijing Wang,a Gan Jin,b Yanhong Shi,a Hao Zhang,b Haiming Xie,a Bai Yang*b and
Haizhu Sun*a
a College of Chemistry, National & Local United Engineering Laboratory for Power
Batteries, Northeast Normal University Changchun 130024, People’s Republic of
China.
b State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, People’s Republic of China.
To whom correspondence should be addressed. E-mail: [email protected];
byangchem@ jlu.edu.cn. Tel: +86-431-85099667. Fax: +86-431-85099667.
S1
Photoelectrochemical measurements
The photoelectrochemical measurements were conducted by a SP-200
electrochemical workstation from Bio-logic of China, using a three-electrode system
with 0.5 M Na2SO4 as electrolyte. A Pt wire was for the counter electrode and
saturated calomel electrode was used as the reference electrode. ZTSx and SnS2 were
coated on the ITO as the working electrodes according to the literature [28]. The
working electrodes were prepared as follows: First, certain amount of photocatalysts
powder was dispersed in water with the concentration of 10 mg mL -1 under
ultrasonication. Then, dropped the dispersion onto indium tin oxide (ITO)-coated
glass electrode (1.5 cm × 1.5 cm) followed by a drying process at 60 oC for 12 h. The
obtained working electrodes were slowly heated to 200°C and keep this temperature
for 2h in the tube furnace. The photocurrents were measured at 1.0 V with the 300 W
Xe-lamp (CEL-SPH2N, Beijing) on and off. The electrochemical impedance spectra
(EIS) data were obtained with the above three-electrode system (the frequency range
is 0.1-200 Hz with the amplitude of 10 mV).
Photocatalytic H2 production activity
The photocatalytic hydrogen evolution activity was tested by the CEL-SPH2N
photocatalytic activity evaluation system. A 300 W Xe lamp with a light cutoff at 420
nm was employed as the light source. Generally, 20 mg photocatalyst was dispersed
into 50 mL 0.35 M Na2S and 0.25 M Na2SO3 solution under ultrasound. Before
irradiation, the whole system was sealed and vacuumed with a mechanical pump for
20 min to remove gas impurities. Then, the obtained gas products were extracted and
measured every 30 min with an on-line gas chromatograph about seven times. The
generated amount of hydrogen was evaluated according to the fitted standard curve.
To test the stability of the catalyst, cycling experiments were carried out by
centrifuging and washing the catalyst after each experiment.
S2
Powder X-ray diffraction patterns (XRD) spectra of ZTSx porous nanosheets
The powder X-ray diffraction patterns (XRD) spectra in Fig. S1 shows the phase
changes of ZTSx porous nanosheets with the increasing of Sn content. A mixed ZnS
phase of hexagonal (JCPDF#36-1450, marked with black *) and wurtzite (JCPDF#05-
0566 marked with yellow *) can be observed. The seven main peaks at 26.9, 28.5,
30.5, 39.6, 47.5, 51.7 and 56.3 degree are attributed to the {100}, {002}, {101},
{102}, {110}, {103} and {112} planes of hexagonal ZnS, while the peak at about 34
degree is from the wurtzite ZnS. For comparison, pure SnS2 is prepared, its XRD
spectrum (JCPDF#21-1231, marked with #) also contains seven peaks at 15.0, 29.2,
32.1, 41.9, 50.1, 52.5 and 58.3 degree, respectively, corresponding to the {002},
{101}, {102}, {104}, {110}, {112} and {200} planes. For the ZTS-0.05 sample, none
of obvious diffraction peak belonging to SnS2 is observed due to the weak
crystallinity, high dispersity and small amount of SnS2. As the Sn content further
increases, the peaks of SnS2 at 15.0, 32.1, 41.9 and 50.1 degree become more obvious
for the ZTS-0.2 and ZTS-0.5 samples, indicating the successful preparation of ZnS-
SnS2.
Fig. S1. XRD curves of SnS2 and ZTSx porous nanosheets: ZnS, ZTS-0.05, ZTS-0.2
and ZTS-0.5, indicating the successful preparation of ZnS-SnS2.
S3
Chemical state of ZTSx
The surface chemical composition and valence state of ZnS and ZTS-0.2 are
studied by using X-ray photoelectron spectroscopy (XPS). The survey spectra in Fig.
S2a show the co-existence of Zn, S, C and O elements in ZnS and ZTS-0.2 sample,
while an additional peak at about 490 eV can be observed for the ZTS-0.2 sample,
which is attributed to the Sn 3d of SnS2[1, 2]. Its high resolution spectrum is shown in
Fig. S2b, from which the binding energies of 487.28 and 495.70 eV corresponding to
the Sn 3d5/2 and Sn 3d3/2 can be recognized.
Fig. S2. (a) Typical XPS survey spectra of ZnS and ZTS-0.2, (b) high-resolution XPS
spectra of Sn 3d in the ZTS-0.2 porous nanosheets.
Selection process of hole scavengers for ZTS-0.2 porous nanosheets.
To choose the most suitable hole scavenger for ZTS-0.2 porous nanosheets
photocatalyst, controlled experiments are carried out with five kinds of common
reagents, including methanol, glycerol, a mixture of 0.35 M Na2S and 0.25 M Na2SO3
aqueous solution, lactic acid, and triethanolamine. The corresponding results are
shown in Fig. S3. It is obvious that the Na2S and Na2SO3 mixture solution presents the
highest photocatalytic hydrogen evolution activity. Moreover, as a hole scavenger, it
possesses special advantages. On one hand, the mixture replenishes S2- during the
S4
photocatalytic process, which greatly suppresses the inherent photocorrosion caused
by ZTS-0.2 photocatalyst. On the other hand, it requires the least energy usage
compared to other scavengers, such as methanol, and ethanol, which are fuels
themselves. Thus, in the present study, the Na2S and Na2SO3 mixture is used as the
hole scavenger to assist the hydrogen production and restrain photocorrosion caused
by sulfide-based photocatalysts.
Fig. S3. The photocatalytic H2 evolution rate of ZTS-0.2 photocatalyst with different
hole scavengers: (a) 20%V methanol, (b) 20%V glycerol, (c) mixture of 0.35 M Na2S
and 0.25 M Na2SO3 aqueous solution, (d) 20%V lactic acid (e) 20%V triethanolamine,
indicating the most suitable hole scavenger is the mixture of 0.35 M Na2S and 0.25 M
Na2SO3 aqueous solution.
Pore size distribution of ZTSx porous nanosheets.
As shown in the Fig. S4, ZnS just contains small pore size of about 2-5 nm. For the
ZTS-0.5 sample, the excessive SnS2 may agglomerate together, leading to the
decreased BET value and a narrow pore size range of 2-8 nm. While for ZTS-0.05
and ZTS-0.2 sample, owing to the appropriate amount of SnS2 growth on the ZnS
surface, an extended pore size range of 2-50 nm are observed. Specially, the pore size
of ZTS-0.2 shows the widest distribution, the larger pore size with a wide range is
S5
more beneficial to the photocatalyst reaction, which will improve the photocatalystic
activity.
Fig. S4. Pore diameters of ZTSx: ZnS, ZTS-0.05, ZTS-0.2 and ZTS-0.5, indicating
the existence of mesopores in ZTSx samples.
Fluorescence decay curves of ZTSx porous nanosheets.
Fluorescence decay is closely related to photoexcited carrier lifetime [3,4]. Higher
fluorescence decay time relates to a more efficient carrier separation ability and better
photocatalytic activity[5,6]. The fluorescent intensities can be obtained according to
the following equation[7]:
(1)
Where, n (t) is the concentration of charge carrier, τ1, τ2, τ3 were the three radiative
lifetime of charge carriers, A, B1, B2, B3 are constant. The average life time is
calculated according to the equation:
(2)
Where, B1 and B2 are the percentages of τ1 and τ2, respectively. The corresponding
data of SnS2 and ZTSx are listed in Table S3. As shown in Fig. 5a, with an excitation
wavelength of 360 nm and emission wavelength of 430 nm, the average fluorescence
lifetimes of the relevant catalysts follow the order: ZTS-0.2 > ZTS-0.5 > ZTS-0.05 >
S6
2
21
1 τt-expB
τt-expBAFit =
2211
22
212
1
BτBτBτBτ
ZnS. Compared to ZnS, the average carrier lifetime of ZTSx are significantly
prolonged. Specifically, the average PL lifetime of ZTS-0.2 is 1.42 times longer than
that of ZnS by altering the contribution of each lifetime. The shorter (1.762 ns), and
longer lifetimes (9.862 ns) have the contributions of 63.23% and 36.77% to the total
PL emission intensity, respectively. All values are significantly different from those
of ZnS (1.151 ns, 65.46%, and 6.983 ns, 34.54%) or SnS2 (1.026 ns, 64.28%, 5.261
ns, 35.72%), suggesting the ZTSx samples have obviously distinguished emission
pathways from the pure ZnS and SnS2. That is, certain interface interaction as well as
charge transfer process occurs between ZnS and SnS2 by the diffusion of electrons on
the SnS2 surface and holes on the ZnS surface. This greatly enhances carrier transfer
ability and suppresses the carrier recombination strength because of the shortened
overlap between electrons and holes, improving the carrier lifetime[8]. Therefore, it is
concluded that the carrier separation and transfer ability of the as-prepared catalysts
follow the order: ZTS-0.2 > ZTS-0.5 > ZTS-0.05 > ZnS, which is consistent with
electrochemistry results.
The Zn vacancy in ZTSx
In Fig. 6, the high-resolution XPS spectrum of ZnS, the two peaks in Zn 2p at
1022.32 and 1045.39 eV are assigned to Zn 2p3/2 and Zn 2p1/2 respectively [9], while
the peak of S 2p in Fig. 6b at 162.29 and 163.68 eV are attributed to the S 2p3/2 and S
2p1/2 (black line). For ZTS-0.2, compared to pure ZnS, the binding energies of Zn 2p3/2
(1044.59 eV) and Zn 2p1/2 (1021.69 eV) shift towards lower binding energies by 0.63
and 0.80 eV, respectively, which may be from the existence of vacancy (VZn) in ZnS
structure[10, 11]. It is reported that the surface VZn may result from the reaction
between excess of Na2S and free Zn2+, which disturbs the ZnS lattice in the surface
and contributes to the zinc deficient-ZnS[12]. While in the S 2p spectra, left shift of
binding energies by 0.13 eV of 2p3/2 (162.12 eV) and 0.39 eV of S 2p1/2 (163.29 eV)
are also observed. It is inferred that the peaks of those S2− ions are close to the Zn
vacancy sites, which brings about increased electronic density around the Zn vacancy
sites and thus results in the decreased of the S2− ions binding energies. Since during
S7
the experiment, with increasing Sn content, increased amount of Na2S is added to
assist the sufficient growth of SnS2 on the surface of ZnS. So it is reasonable to infer
that ZTS-0.2 may have more VZn than pure ZnS, which results in lower binding
energies of Zn 2p and S 2p in ZTS-0.2. Furthermore, two additional peaks at lower
binding energies appear in ZnS (163.12 eV) and ZTS-0.2 (159.42 eV), further
indicating that the existence of the defect states in the ZnS and ZTS-0.2 structure. In
addition, two quite weak peak based on sulfur (S0) are found in Fig. 6b for both ZnS
(164.13 eV) and ZTS-0.2 (164.7 eV) [13], indicating a few amount of S0 exist in the
ZnS and ZTS-0.2. The S0 is probably from the oxidation of S2- ions by the dissolved
oxygen during the hydrothermal reaction process.
The photoluminescence (PL) spectra are used to further certify the VZn as well as
other defects in ZnS and ZTS-0.2. The corresponding data are shown in Fig. 6c, both
of ZnS and ZTS-0.2 samples excited at 320 nm present an intrinsic emission peak at
358.86 nm. Besides, a broad peak consist of several weak PL peaks can be found
between 430-530 nm, which are from the defects of ZnS. The four weak peaks at
443.82, 462.31, 477.23 and 487.52 nm are attributed to the point defects produced by
VZn[14-17], which are derived from the recombination of electrons in the conduction
band of ZnS with the holes from the VZn. For ZTS-0.2 sample, all of the emission
peak intensities are weakened. This is mainly because the charge transfer occurs
between ZnS and SnS2. That is, the electron in the CB of ZnS will transfer to the CB
of SnS2, while the hole in the VB of SnS2 will then transfer to the VZn of ZnS, which
restricts the recombination of the CB electron with the VZn in ZnS. Thus it can be
concluded that certain amount of the VZn exist in ZnS and ZTS-0.2. The VZn can be
also identified by the electron spin resonance (EPR) [18-20]. As shown in Fig. 6d,
there is no EPR signal for pure SnS2. However, both of ZnS and ZTS-0.2 samples
possess intensive multi-line EPR signals begin at g ≈ 2.0068, which is attributed to
VZn exist in them[21-23]. Besides, stronger EPR signal intensity can be observed by
ZTS-0.2, indicating higher concentration of VZn in ZTS-0.2, which is consistent with
the XPS and PL results.
S8
Table S1. Sn, Zn and S (mol.%) content, BET surface area, average pore size and rate
of H2 production of ZTSx samples.
Samples
ZTSx
Sn (mol.%)
from feed
ratio (x)
Sn (mol.%)
from EDS
Zn (mol.%)
from EDS
S (mol.%)
from EDS
BET
surface area
(m2 g-1)
Average pore
size (nm)
Rate of H2
production
(μmol h-1g-1)
ZTS-0 (ZnS) 0 0 48.5 51.5 20.5 3.2 50
ZTS-0.05 5 1.2 44.6 54.2 86.2 4.3 239
ZTS-0.2 20 8.8 32.9 58.3 246.7 12.5 536
ZTS-0.5 50 16.8 22.6 60.6 120.6 6.8 423
The obtained ZnS-SnS2 porous nanosheets are labeled as ZTSx (x = 0, 0.05, 0.2, 0.5)
according to the Sn/Zn feed ratio.
S9
Table S2. Comparison of photocatalytic performance in other references with this
work.
Sample
Rate of H2
production
(μmol h-1g-
1)
Surface
areaStability
Co-catalyst
or
surfactant
Light
sourceReferences
ZTS-0.2
nanosheets536 246.7
At least
four runsNone
Visible
lightThis work
ZnS-CuS
nanoflower
spheres
5152 92Not
givenCuS
Visible
light24
ZnS-CuS
nanosheets4147 37.5
Not
givenCuS
Visible
light25
Cu-ZnS 35 90Not
givenCuS
Visible
light26
(CuIn)xZn2(1-x)S2 2280Not
given
Not
givenPt
Visible
light27
CdS-ZnS core-
shell particles792 35
At least
six runsNone
Visible
light28
ZnS-CdS
nanorod239000
Not
given
At least
four runsNone
Visible
light29
Zn0.999Ni0.001S 280Not
given
Not
givenNiS
Visible
light30
Porous ZnS:Ag2S
nanosheets104.9 250.6
At least
three
runs
NoneVisible
light31
RGO-TiO2
nanosheets287.4 56
Not
givenNone
Visible
light32
S10
Sample
Rate of H2
production
(μmol h-1 g-
1)
Surface
areaStability
Co-catalyst
or
surfactant
Light
sourceReferences
Exfoliated
g-C3N4
5.44Not
given
Not
givenPt
Visible
light33
g-C3N4
nanosheets1070 186.3
At least
36hPt
Visible
light34
Holey g-C3N4
nanosheets
8920196
At least
9hPt
Visible
light35
RGO-ZnIn2S4 1680 92At least
12hNone
Visible
light36
CdxZn1-xS
nanosheets1700 56.3
At least
15hNone
Visible
light37
TiO2-CdS
nanosheets1425 1550
At least
16hPt
Visible
light38
N-doped TiO2 /g-
C3N4 nanosheets8931 16
Not
givenPt
Solar
light39
CdS-decorated
Cd Nanosheets16800 25
Not
givenPt
Visible
light40
RGO-oxide-
ZnxCd1–xS
Nanocomposite
1824Not
given
Not
givenPt
Solar
light41
Table S3. The radiative fluorescence lifetimes and their relative percentages of
photoexcited charge carriers of ZTSx photocatalysts powder.
S11
Sample τ1 (ns) Rel (%) τ2 (ns) Rel (%) τ (ns) χ2
ZnS 1.151 65.46 6.983 34.54 5.591 1.263
ZTS-0.05 1.452 64.55 8.520 35.45 6.832 1.125
ZTS-0.2 1.763 63.23 9.862 36.77 7.962 1.315
ZTS-0.5 1.562 64.08 8.983 35.92 7.223 1.216
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