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Visible-light-driven photocatalytic and photoelectrochemical properties of
porous SnS
x
(x ¼ 1,2) architectures
Junfeng Chao,
a
Zhong Xie,
a
XianBao Duan,
c
Yuan Dong,
a
Zhuoran Wang,
a
Jing Xu,
a
Bo Liang,
a
Bin Shan,
c
Jinhua Ye,
b
Di Chen
*
a
and Guozhen Shen
*
a
Received 25th November 2011, Accepted 16th February 2012
DOI: 10.1039/c2ce06586j
By using a facile and template-free polyol refluxing process, we reported the successful synthesis of
porous SnS and SnS
2
architectures on a large scale. The as-synthesized samples were characterized by
using XRD, SEM, TEM, UV-vis DRS, Raman and N
2
adsorption–desorption analyses. Studies
revealed that the as-synthesized SnS and SnS
2
products mainly consist of porous flower-like
microstructures with reasonable BET surface areas of 66 m
2
g
1
and 33 m
2
g
1
, respectively.
Photocatalytic properties of trace amounts of samples were investigated by photodegradation of MB
and RhB under visible light irradiation. The photoelectrochemical properties of both samples were also
studied by configuring the samples as photoelectrochemical (PEC) cells, exhibiting excellent
photosensitivity and response with greatly enhanced I
on/off
as high as 1.4 10
3
, three orders of
magnitude higher than previous work. The results indicate the potential applications of the SnS
x
nanostructures in visible-light-driven photocatalysts, high response photodetectors and other
optoelectronic nanodevices.
Introduction
Porous semiconductor nanomaterials with a large surface area
have been widely investigated due to their important applications
in many fields such as photocatalysts, optoelectric devices,
semiconductor sensors, solar cells and electrode material for
batteries and so on.
1–7
Up to now, many synthetic methods have
been developed to prepare nanomaterials with porous micro-
structures. For example, 3D mesoporous carbon structures were
successfully fabricated from the soft chemical route using
commercially available colloidal silicas as templates.
8
Nanosized
flower-like nickel hydroxide, synthesized by a microwave-assis-
ted hydrothermal method, shows good electrochemical activity
for the electrochemical reduction of O
2
at room temperature.
9
Hierarchical WO
3
architectures including dendrites, spheres and
dumbbells were synthesized from different tungstates with
similar morphologies and show enhanced photocatalytic activi-
ties for the degradation of some organic dyes compared to the
commercial materials.
10
Thus, developing simple and effective
methods to fabricate semiconductors with porous structures is
important for future device applications.
The photocatalytic technique for the degradation of organic
contaminants in solution or in air has attracted extensive interest
to remedy the effects of environmental pollution because of its
simple and exhaustive decomposition process.
10–14
Among all
kinds of semiconductors, TiO
2
is the most investigated one in
view of its low cost, good stability, nontoxicity and so on.
However, with a wide band gap of 3.2 eV, TiO
2
is only active in
the ultraviolet light region and is not responsive to visible light.
Although some dyes could be degraded over TiO
2
by self-
photosensitization under visible light irradiation, the efficiencies
are relatively lower than that under UV light irradiation.
15–18
It is
well known that only about 4% of the solar spectra falls in the
UV region, thus it is appealing to develop efficient visible light
sensitive photocatalysts in view of the better utilization of solar
energy. Until now, many researchers have attempted to synthe-
size new catalysts with narrow band gaps in order to realize
visible-light-driven catalytic reactions.
19–25
Metal sulfides have
been reported to be promising photocatalysts in the visible
region. For instance, ultrathin beta-In
2
S
3
nanobelts prepared by
Qian’s group showed enhanced photocatalytic activity for the
decomposition of methyl orange under visible light irradiation.
26
From a two-step solution route, K. Domen and his coauthors
have synthesized nanostructured CdS with a high photoactivity
for hydrogen evolution when irradiated by visible light.
27
With narrow band gaps, tin sulfides, one of the IV–VI group
semiconductors, include a variety of phases such as SnS, SnS
2
,
Sn
2
S
3
,Sn
3
S
4
and Sn
4
S
5
. Among them, SnS and SnS
2
, with
a
Wuhan National Laboratory for Optoelectronics (WNLO) and College of
Optoelectronic Science and Engineering, Huazhong University of Science
and Technology (HUST), Wuhan, 430074, China. E-mail: gzshen@mail.
hust.edu.cn; dichen@mail.hust.edu.cn; Fax: +86-27-87792225
b
International Center for Materials Nanoarchitectonics (MANA)
National Institute for Materials Science (NIMS), 1-2-1 Sengen,
Tsukuba, Ibaraki, 305-0047, Japan. E-mail: Jjinhua.YE@nims.go.jp;
Fax: (+)81-29-859-2301
c
School of Materials Science and Engineering, Huazhong University of
Science and Technology (HUST), Wuhan, 430074, China
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indirect band gaps of 1.3 eV and 2.2 eV, have received more
attention due to their possible applications in optoelectronics,
solar cells, lithium-ion batteries, near-infrared detectors, photo-
catalysts and photoluminescence.
28–35
In this paper, we reported
the synthesis of porous SnS and SnS
2
architectures from a simple
and template-free polyol refluxing route. Photocatalytic prop-
erties of both samples were investigated by photodegradation of
MB and RhB under visible light irradiation. The electronic
structures of the porous SnS and SnS
2
samples were also inves-
tigated by band structure calculations. The photoelectrochemical
properties of both samples were also studied by configuring the
samples as photoelectrochemical (PEC) cells, exhibiting
a reasonable photosensitivity and response with greatly
enhanced I
on/off
ratios.
Experimental
Materials
All reagents, such as SnCl
2
$2H
2
O, SnCl
4
$5H
2
O, thiourea (Tu)
and ethylene glycol were analytical grade and used directly
without further purification.
Synthesis of SnS and SnS
2
architectures
Porous SnS and SnS
2
architectures were successfully prepared
from the simple polygol refluxing process, respectively. In
a typical procedure for the SnS sample, appropriate amounts of
SnCl
2
and Tu were first added into 50 mL ethylene glycol in
a round bottom flask. After stirring for 5 min, the reaction
system was heated to 175
C and kept at that temperature for 1 h
and then cooled to room temperature naturally. Finally, black
precipitates were collected, washed with absolute ethanol and
distilled water and then dried at 60
C. The synthetic process for
porous SnS
2
nanostructures was similar to that of SnS products
except that SnCl
4
was used as the raw material instead of SnCl
2
and the refluxing system was kept at 160
C for 1.5 h.
Material characterization
The crystal structures of the as-prepared samples were confirmed
by the X-ray diffraction pattern (X’Pert PRO, PANalytical B.V.,
the Netherlands) with radiation of a Cu target (Ka, l ¼ 0.15406
nm). The morphologies and sizes of the samples were charac-
terized by scanning electron microscopy (SEM, JSM-6700F) and
transmission electron microscopy (TEM, JEM-2010). UV-vis
diffuse reflectance spectra were recorded on a UV/vis spec-
trometer (UV-2500, Shimadzu) and were converted from reflec-
tion to absorbance by the standard Kubelka–Munk method. The
surface areas were measured on a surface area analyzer (Micro-
meritics, Shimadzu) by nitrogen absorption at 77 K using the
Brunauer–Emmett–Teller (BET) method.
Results and discussion
SnS and SnS
2
are two kinds of important layered structured
semiconductor materials and the corresponding ball-and-stick
models are shown in Fig. 1. Typically, a SnS crystal with
orthorhombic structure consists of double layers perpendicular
to the c-axis in which Sn and S atoms are tightly bound
(Fig. 1, left). While the SnS
2
with hexagonal cadmium iodide
(CdI
2
) structure is composed of sheets of tin atoms sandwiched
between two close-packed sheets of sulfur atoms (Fig. 1, right).
Based on the layered structure properties, research on SnS and
SnS
2
has attracted more and more attention.
XRD was used to study the phase purity of the as-synthesized
SnS and SnS
2
products as shown in Fig. 2. All of the peaks in the
XRD pattern in Fig. 2a can be readily indexed to the pure
orthorhombic SnS phase with space group P6nm (JCPDS card
No: 39-354). The pattern in Fig. 2b can be indexed to the
hexagonal SnS
2
phase with space group P-3m1 (JCPDS card No:
23-677). Clearly, no characteristic peaks from other crystalline
impurities were detected in these XRD patterns, indicating the
formation of pure products.
Raman spectroscopy is an appropriate technique to probe the
detailed structure of materials because the bonding states in the
coordination polyhedra of a material can be deduced directly
from the Raman vibrational spectrum. In orthorhombic SnS, 24
vibrational modes are represented by the following irreducible
representations at the center of Brillouin zone as:
Fig. 1 The ball-and-stick models of SnS (left) and SnS
2
(right) crystals.
Fig. 2 XRD patterns of the as-synthesized porous SnS and SnS
2
architectures.
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G ¼ 4A
g
+2B
1g
+4B
2g
+2B
3g
+2A
u
+4B
1u
+2B
2u
+4B
3u
Among them, there are 21 optical phonons, of which 12 are
Raman active modes (4A
g
,2B
1g
,4B
2g
and 2B
3g
), seven are
infrared active modes (3B
1u
,1B
2u
and 3B
3u
) and two are inactive
(2A
u
).
36
Fig. 3 shows the Raman spectra of the as-synthesized
SnS and SnS
2
products, respectively. The Raman modes shown
in Fig. 3a are observed at 69, 95, 165, 192 and 219 cm
1
,
respectively. Among the peaks, the peaks at 69 cm
1
,95cm
1
,
165 cm
1
corresponding to the B
1g
or B
2g
mode, A
g
mode and B
3g
mode, respectively, are in good agreement with the literature.
37
The peaks at 192 cm
1
and 219 cm
1
can be assigned to the A
g
mode. The Raman spectrum of the as-synthesized SnS
2
is shown
in Fig. 3b. A strong Raman peak located at about 313 cm
1
is
observed, which can be assigned to the A
1g
mode according to
the group theory analysis given by Lucovsky et al.
38
According to
the Raman results, it can be further confirmed that pure phase
SnS and SnS
2
products were prepared from the present process.
The morphology and microstructure of the as-prepared
sulfides from the simple polyol refluxing process were charac-
terized by SEM. Fig. 4a shows the low-magnification SEM image
of SnS products, which demonstrates that porous SnS samples
on a large scale were prepared and composed of many nano-
platelets with uniform size. Further observation (Fig. 4b) depicts
that large numbers of platelets interconnect with each other with
a thickness of several tens of nanometres to form a porous
architecture structure. In the present case, a similar porous
architecture structure was also observed in SnS
2
products as
shown in Fig. 4c, when the raw material of SnCl
4
was used in this
refluxing solution at 165
C. The high-magnification FESEM
image of the products in Fig. 4d indicates that the surfaces of the
architectures are highly porous in structure, suggesting that the
samples possess possibly large surface areas. The measurements
show that the BET surface areas of porous SnS and SnS
2
architectures are 66 m
2
g
1
and 33 m
2
g
1
, respectively, which are
favourable for potential applications in waste water treatment
such as photocatalysts.
Further insight into the microstructural details of the porous
SnS and SnS
2
architectures were gained by using TEM and high-
resolution TEM (HRTEM). Fig. 5a shows a TEM image taken
from the porous SnS architectures. It is interesting that the SnS
products are composed of interconnected nanoplatelets, in good
accordance with the SEM observations. The representative
HRTEM image (Fig. 5b) taken from the edge of the micro-
structures exhibits the highly crystalline nature of the SnS
nanoplatelets. The lattice fringe in the image is clearly visible
with a spacing of 0.40 nm, corresponding to the (110) plane of
orthorhombic SnS. Fig. 5c displays the TEM image of the SnS
2
product. Nanoplatelets with a thickness of about several nano-
metres were found to be connected to each other to build the 3D
porous architectures. This is also in good agreement with the
SEM results. The distance between the lattice fringes of the
architectures can be indexed to the (100) planes of hexagonal
SnS
2
.
The optical properties of the products were measured and the
corresponding UV-vis absorption spectra are depicted in Fig. 6a.
A black-colored SnS sample (inset in Fig. 6a) exhibits broad and
strong absorption in the range from 200 to 800 nm. In contrast,
yellow-colored SnS
2
(inset in Fig. 6a) only absorbs light in the
region below 600 nm and it exhibits less intensive absorption
Fig. 3 Raman spectra of the as-synthesized porous SnS (a) and SnS
2
(b)
architectures.
Fig. 4 SEM images of the as-synthesized porous SnS (a, b) and SnS
2
(c, d) architectures from the simply polyol refluxing process.
Fig. 5 TEM and HRTEM images of the porous SnS (a, b) and SnS
2
(c, d) architectures from the simple polyol refluxing process.
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compared to the SnS sample. The optical band gaps of the two
samples could be evaluated from the following equation:
39–41
ahn ¼ A(hn Eg)
n/2
Here a, n, A, Eg and n are the absorption coefficient, incident
light frequency, constant, optical band gap, and an integer
(normally equal to 1, 2, 4 and 6), respectively. The n values for
the two observed optical transitions for the two samples should
be indirect. The a and n values at the steep edges of the
absorption spectra were used to construct the plots of (ahn)
2/n
against photon energy. As seen in Fig. 6b, the curves for the two
samples show a linear region with n ¼ 4 for the indirect band gap.
The optical band gaps of the samples could be determined by the
intersections of the extrapolated linear portions of the plots with
the energy axis. These values were calculated to be about 1.35 eV
for SnS and 2.18 eV for SnS
2
, which is in good agreement with
the previous reports.
42,32
To investigate the photocurrent properties of the as-synthe-
sized SnS and SnS
2
products, photoelectrochemical (PEC) cells
were prepared by using the as-synthesized products as the cor-
responding photoelectrode, respectively. The photoelectrodes of
the photoelectrochemical devices were prepared by the well-
known ‘‘doctor-blading’’ method widely used for dye-sensitized
solar cell (DSSC) fabrication. The corresponding SnS or SnS
2
product was first deposited onto a FTO substrate. The coun-
terelectrode was lightly platinized by coating with a drop of an
isopropanol solution of H
2
PtCl
6
, followed by firing at 400
C for
20 min. A solar simulator of AM 1.5G (100 mW cm
2
) was used
as the illumination source. In principle, when the external circuit
was closed, the illuminated PEC devices should generate
a photocurrent. Fig. 7 demonstrates the typical time-dependent
current change curves of the PEC devices made of SnS and SnS
2
architectures, respectively. From Fig. 7a, we can see that, upon
illumination, the current density rapidly increased from 0.052 mA
cm
2
to a stable value of 70.5 mAcm
2
. The significant
enhancement of the current density induced by electron-hole
generation is around 1.4 10
3
. Similar results were also obtained
for the SnS
2
architectures, as can be seen in Fig. 7b. Both values
are about three orders of magnitude higher than previous work
on the same materials.
43
Besides, there are current transients in
both the light-on state and the light-off state, which can be
attributed to a typical sign of surface recombination processes
according to previous reports.
44–46
It was found that the photocurrent response can be repro-
ducibly switched from the ‘‘ON’’ state to the ‘‘OFF’’ state by
periodically turning the solar simulator on and off. When
switched from the dark to light condition, the photocurrent
response of the SnS-based PEC device changes abruptly and then
stably with time (in Fig. 7a), while the response curve of the SnS
2
-
based PEC device has a sharp-edged peak at first and then drops
slowly (in Fig. 7b). The response time and decay time of both
devices were around 0.5 s, indicating rapid photoresponse
characteristics. On the other hand, the relatively low dark current
also guarantees the outstanding optical switch performance for
the device. When the solar simulator illuminates, photons with
energy higher than the SnS
x
band gap are absorbed by the SnS
x
nanomaterials and a lot of electron–hole pairs are generated. The
holes accept electrons from the I
3
/I
redox electrolyte, and the
electrons move from the porous SnS
x
surface to the anode
contact. Along with the photocurrent flows, electrons pass
through the external closed circuit. Owing to the porous surface
area, the practical contact area of the device is much larger than
the flat light area, so the number of electron–hole pairs created
can be tremendously larger than that of the geometric device
area.
The photocatalytic activities of the as-synthesized porous SnS
and SnS
2
architectures were further evaluated for the degrada-
tion of some organic dyes such as methylene blue (MB) and
rhodamine B (RhB) under visible light irradiation at room
temperature. Since both samples possess large BET surface areas,
trace amounts of the SnS (10 mg) and the SnS
2
(20 mg) samples
were used in the present cases, respectively. First, an appropriate
Fig. 6 (a) UV-vis absorption spectra and (b) (ahn)
1/2
vs. hn plots for the
porous SnS and SnS
2
architectures.
Fig. 7 J–T curves of the porous SnS (a) and SnS
2
(b) architectures under
one sun illumination (AM 1.5G).
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catalyst was suspended into 100 mL MB or RhB aqueous solu-
tion in a pyrex reactor, respectively (the dye concentration was
8mgL
1
in both cases). The suspension was stirred in the dark
for a certain time to reach an adsorption–desorption equilibrium,
then the light was turned on. A 500 W Xe-lamp equipped with
a cutoff filter (l > 420 nm) and a water filter was used as the light
source. At given irradiation time intervals, appropriate reaction
suspension was collected and filtrated. The absorption spectra of
the filtrates were measured on a Shimadzu UV-2550 UV-vis
spectrometer and are shown in Fig. 8. Clearly, during the dark,
porous SnS architectures with a larger BET surface area of 66 m
2
g
1
showed higher adsorption abilities to MB and RhB than the
SnS
2
product. As shown in Fig. 8(a–d), porous SnS architectures
can adsorb about 75% of MB within 30 min. The dye concen-
tration adsorbed onto the surface of the catalyst further increases
with the prolonging of time (Fig. 8b). After 2 h, an adsorption–
desorption equilibrium in the solution was reached, since the
intensities of the main peaks located in the visible region and in
the UV region are similar to those for 2.5 h. When the light was
turned on, the main peaks decreased continuously with increased
irradiation time, indicating that the MB solution was decom-
posed completely in the present system. Fig. 8(c,d) shows the
great adsorption and degradation activities of the SnS architec-
tures when trace catalyst was dispersed into the RhB solution.
Due to the good adsorption and degradation effect of the SnS
architectures exhibited in the organic dye solution, we can
conclude that the obtained sample might be used in the waste-
water treatment in industry.
In addition, the photocatalytic abilities of the porous SnS
2
architectures were also investigated and Fig. 8(e,f) show the
photodegradation process of MB and RhB solutions in
the presence of SnS
2
catalysts, respectively. It was obvious that
the porous SnS
2
products with less BET surface area show
weaker adsorption and photocatalytic abilities compared to the
porous SnS architectures. However, in our experiments, we
found that trace amounts (10 mg or 20 mg) of the two samples
can accomplish good adsorption and degradation to the MB and
RhB when irradiated by visible light, which is better or compa-
rable to previous reports.
33,47–50
The photocatalytic performances of sulfide photocatalysts
strongly depend on their electronic structures. In the present
work, the electronic structures of SnS and SnS
2
were further
investigated by plane-wave DFT calculations, respectively, and
Fig. 9 shows the energy structures and density of states (DOS)
calculated. As shown in Fig. 8a left, the conduction band
minimum (CBM) of SnS crystal is located in the area between the
G point and the Z point while the valence band maximum (VBM)
is located at the Y point of Brillouin zone, indicating that SnS is
indeed an indirect semiconductor. Similarly, the calculated energy
band of SnS
2
is shown in Fig. 9b left, it can be clearly seen that the
conduction band minimum (CBM) of SnS
2
crystal is located at the
M point of the Brillouin zone and the valence band maximum
(VBM) is located in the region between the K point and the G
point, indicating that SnS
2
is also an indirect semiconductor. For
an indirect semiconductor, the lifetime of the photoexcited elec-
tron–hole pair is comparably longer than that in the direct
samples, which is beneficial for the photocatalytic reactions.
The band gap of a given semiconductor is determined mainly by
the potential of the valence band top and the potential of the
Fig. 8 (a,b) UV-vis spectral changes of MB aqueous solution and the
magnification of the lit portion,(c,d) UV-vis spectral changes of RhB
aqueous solution and the magnification in the presence of SnS architec-
tures and (e) MB aqueous solution and (f) RhB aqueous solution in the
presence of SnS
2
architectures under visible light irradiation. Reaction
conditions: 100 mL, 8 mg L
1
, 500 W Xe-lamp, l > 420 nm.
Fig. 9 The calculated (left) energy bands and (right) density of states
(DOS) of as-synthesized porous (a) SnS and (b) SnS
2
samples.
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conduction band bottom. The total DOS displays (Fig. 9a and 9b
right) that the bands of SnS and SnS
2
are classified into two parts.
The bottom of the conduction band is mainly constituted by the
Sn 5p orbital, while the conduction band top is composed mainly
of the S 3p orbital. The band structure indicates that charge
transfer upon photoexcitation occurs from the S 3p orbital to the
empty Sn 5p orbital. The calculated band gaps of SnS and SnS
2
are
1.5 and 2.14 eV, respectively, which are slightly larger than the
experimental results based on the UV-vis absorption spectra.
Conclusions
In summary, we reported the synthesis of porous SnS and SnS
2
architectures from the simple polyol refluxing process on a large
scale. SEM investigations revealed that both the SnS and the
SnS
2
products are composed of many nanoplatelets inter-
connecting with each other to form the porous architecture. The
photoelectrochemical characteristics recorded under AM 1.5
illumination exhibited the increased photoresponse properties of
SnS and SnS
2
based devices. Greatly enhanced I
on/off
with a value
of about 1.4 10
3
, three orders of magnitude higher than
previous work, was obtained. Furthermore, during the photo-
catalytic degradation process, black SnS architectures with
a larger BET surface area and the narrower band gap of 1.35 eV
show much stronger adsorption and photodegradation abilities
for MB and RhB solutions compared to yellow SnS
2
products.
Our results indicate the potential applications of the SnS
x
nanostructures in visible-light-driven photocatalysts, high
response photodetectors and other optoelectronic nanodevices.
Acknowledgements
This work was supported by the National Natural Science
Foundation (21001046, 51002059), the 973 Program of China
(No.2011CB933300, 2011CBA00703), the Basic Scientific
Research Funds for Central Colleges (2010QN045), the Research
Fund for the Doctoral Program of Higher Education
(20090142120059, 20100142120053) and the Director Fund of
WNLO. We thank the Analytical and Testing Center of Huaz-
hong University Science & Technology, and the Center of Micro-
Fabrication and Characterization (CMFC) of WNLO for the
samples measurements.
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