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: sunhz335@nenu.edu.cn;

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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