Sensors and Actuators B 140 (2009) 623–628
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Nanoplates of ␣-SnWO
4
and SnW
3
O
9
prepared via a facile hydrothermal
method and their gas-sensing property
Hui Dong, Zhaohui Li
∗
, Zhengxin Ding, Haibo Pan, Xuxu Wang, Xianzhi Fu
∗
Research Institute of Photocatalysis, Fuzhou University, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, PR China
article info
Article history:
Received 19 March 20 09
Received in revised form 2 May 2009
Accepted 11 May 2009
Available online 19 May 2009
Keywords:
SnWO
4
SnW
3
O
9
Nanoplate

Hydrothermal
Gas-sensing
abstract
Nanoplates of ␣-SnWO
4
and SnW
3
O
9
were selectively synthesized in large scale via a facile hydrothermal
reaction method. The final products obtained were dependent on the reaction pH and the molar ratio of
W
6+
to Sn
2+
in the precursors. Theas-prepared nanoplates of ␣-SnWO
4
and SnW
3
O
9
were characterized by
X-ray powder diffraction (XRD), N
2
-sorption BET surface area, transmission electron microscopy (TEM),
high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS).
The XPS results showed that Sn exists in divalent form (Sn
2+
)inSnW
3

O
9
as well as in ␣-SnWO
4
. The gas-
sensing performances of the as-prepared ␣-SnWO
4
and SnW
3
O
9
toward H
2
S and H
2
were investigated.
The hydrothermal prepared ␣-SnWO
4
showed higher response toward H
2
than that prepared via a solid-
state reaction due to the high specific surface area. The gas-sensing property toward H
2
S as well as H
2
over SnW
3
O
9
was for the first time reported. As compared to ␣-SnWO

4
, SnW
3
O
9
exhibits higher response
toward H
2
S and its higher response can be well explained by the existence of the multivalent W (W
6+
/W
4+
)
in SnW
3
O
9
.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Since both SnO
2
and WO
3
are well-known materials in the
semiconductor gas-sensor field and have found applications in
commercial sensor devices, the studies on the gas-sensing prop-
erty of ternary Sn–W–O systems have also attracted a lot of interest
[1–3]. Most already reported Sn-based gas-sensing semiconduc-
tors contain Sn

4+
as in the case of SnO
2
[4–6]. The inclusions of
both Sn
4+
and Sn
2+
have also been reported in some Sn–W–O gas-
sensing semiconductors. For example, Solis and Lantto [3] reported
the gas-sensing property of Sn
x
WO
3+x
with the atomic ratio x
between 1.25 and 2.5. The only reported Sn
2+
-based gas-sensing
semiconductor is ␣-SnWO
4
. Since tin in the divalent form may
enable an electron transfer between the Sn
2+
lattice ions at the sur-
face and surface adsorbates, the developments of other Sn–W–O
gas-sensing materials with Sn in divalent state would be interest-
ing.
␣-SnWO
4
is an n-type semiconductor with an orthorhom-

bic crystal structure and both Sn and W atoms have distorted
octahedral oxygen coordinations [1,2]. Traditional methods in the
preparations of the ternary Sn–W–O mixed oxides, including ␣-
SnWO
4
, are solid-state reactions [3,7,8] or the direct redox reaction
between metal Sn and the tungstenic acid [9]. Since Sn
2+
can be
∗
Corresponding authors. Tel.: +86 591 83779105; fax: +86 591 83779105.
E-mail addresses: (Z. Li), (X. Fu).
easily oxidized to Sn
4+
, these reactions have to be carried out under
N
2
atmosphere. The other disadvantage of these reaction meth-
ods is the difficulty in the preparations of nanocrystalline products
with small particle size and high surface area. The as-obtained
products are therefore not favorable for the applications in the
gas-sensor field since a high “surface accessibility” is crucial in
obtaining a high sensitivity of the semiconductor material [10–12].
According to the generally accepted theory, the gas sensitivity of a
semiconductor material is generated by the gas–solid interactions,
i.e., the adsorption/desorption and reactions on the semiconduc-
tor surface [13]. Therefore nanocrystalline semiconductor materials
with small particle sizes and high active surface area are expected
to exhibit superior gas-response property to their bulk counter-
part since they can provide more active surface for the adsorbates

[14–16]. Although sometimes the instability of the sensitivity is
observed due to the evolution of the fine microstructure during
the working of the sensor at high temperature, the application of
the nanomaterials for gas-sensing still attracted much recent inter-
est. For example, a recent report showed that the sensor made of
hierarchical Cu
2
O microspheres with hollow and multilayered con-
figuration exhibited much higher gas-sensing property than bulk
Cu
2
O [15].
The applications of low temperature hydrothermal method in
the preparations of pure crystalline nanomaterials with small par-
ticle size, narrow grain size-distribution and large specific surface
area without high temperature treatment have been well docu-
mented. To make the hydrothermal method more attractive is that
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.05.010
624 H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628
it can give products with tunable morphology and particle size by
simply adjusting the reaction temperature, time and reaction pH as
well as using the additives [17].
Herein, we report the selective syntheses of nanoplates of ␣-
SnWO
4
and SnW
3
O
9

in large scale via a facile low temperature
hydrothermal route. The reaction pH and the molar ratio of W
6+
to Sn
2+
in the precursors played important roles in the final prod-
ucts obtained. Although the hydrothermal method hasbeen applied
in the preparations of many tungstates [18–20], to the best of
our knowledge, it has never been reported in the preparations of
ternary Sn–W–O mixed oxides. The gas-sensing performances of
the as-prepared ␣-SnWO
4
and SnW
3
O
9
toward H
2
S and H
2
were
also investigated. SnW
3
O
9
represents another ternary Sn(II)–W–O
semiconductor which shows gas-sensing property. Our result also
revealed that the hydrothermal prepared ␣-SnWO
4
showed higher

sensitivity toward H
2
than that prepared via a solid-state reaction.
2. Experimental
2.1. Syntheses
Nanocrystalline ␣-SnWO
4
and SnW
3
O
9
were prepared by the
hydrothermal method. All of the reactants and solvents were ana-
lytical grade and were used without further purifications.
In a typical procedure for the preparation of ␣-SnWO
4
,
SnCl
2
·2H
2
O (1.128 g, 5 mmol), (NH
4
)
5
H
5
[H
2
(WO

4
)
6
]·H
2
O (1.335 g,
0.833 mmol) (molar ratio of Sn
2+
to W
6+
at 1:1)were added to 65 mL
aqueous solution. The pH of the resulting mixture was adjusted to
1, 3, 5, 7, 9 and 11 with sodium hydrate solution (2 mol L
−1
) under
vigorous stirring. The resulting suspension was transferred to a
100 mL Teflon-lined stainless steel autoclave and sealed tightly. The
autoclaves were kept at 200
◦
C for 48 h. After cooling to room tem-
perature, the precipitate was collected, washed with distilled water
and then dried in air at 80
◦
C. The procedure for the preparation of
pure SnW
3
O
9
is similar to that of ␣-SnWO
4

except that the molar
ratio of Sn
2+
to W
6+
is 1:2 and the pH value is lower than 1.
For comparison, bulk ␣-SnWO
4
sample was prepared from SnO
and WO
3
using a conventional solid-state synthesis route [3].To
prevent the oxidation of Sn
2+
to Sn
4+
, SnO and WO
3
was heated in
an argon atmosphere at 600
◦
C for 15 h to obtain the sample.
2.2. Characterizations
X-ray diffraction (XRD) patterns were collected on a Bruker D8
Advance X-ray diffractometer with CuK
˛
radiation. The accelerating
voltage and the applied current were 40 kV and 40 mA, respec-
tively. Data were recorded at a scanning rate of 0.02
◦

s
−1
in the 2Â
range of 10–70
◦
. It was used to identify the phase present and their
crystallite size. The crystallite size was calculated from X-ray line
broadening analysis by Scherer equation: D = 0.89/ˇcos Â, where D
is the crystal size in nm,  is the CuK
˛
wavelength (0.15406 nm), ˇ is
the half-width of the peak in rad, and  is the corresponding diffrac-
tion angle. The Brunauer–Emmett–Teller (BET) surface area was
measured with an ASAP2020M (Micromeritics Instrument Corp.).
The transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HRTEM) images were measured
by JEOL model JEM 2010 EX instrument at the accelerating voltage
of 200 kV. The powder particles were supported on a carbon film
coated on a 3 mm diameter fine-mesh copper grid. A suspension
in ethanol was sonicated and a drop was dripped on the sup-
port film. X-ray photoelectron spectroscopy (XPS) measurements
were carried out using a VG Scientific ESCA Lab Mark II spectrom-
eter equipped with two ultra-high vacuum 6 (UHV) chambers. All
binding energies were referenced to the C 1s peak of the surface
adventitious carbon at 284.8 eV.
2.3. Gas-sensing property measurements
The sensor structure and the testing principle were similar to
that reported previously [10,11,21]. The electrode for measurement
was composed of a pair of four-fingered gold electrodes of 120 ␮m
width and 40 ␮m spacing between fingers on an alumina substrate.

The overlap distance of the fingers was 400 ␮m, and a Ni–Cr heater
(37 ) was made on the opposing face of the substrate. The thick
film was coated with a layer of sensor materials of about 10 ␮m
thick. After drying at 150
◦
C for 2 h in air to improve the stabil-
ity, the electrical contact was made through connecting the four
platinum wires with the instrument base by silver paste. During
the measurement, the sensors were hosted in a closed plastic tube
equipped with appropriate inlets and outlets for gas flow. A given
amount of gas such as H
2
SorH
2
was injected into the chamber by
a micro-injector. The resistance of a sensitive material is measured
in air (R
air
) and in air–gas mixtures (R
gas
) under the same operating
current. The gas response magnitude (S) was defined as the ratio of
R
air
to R
gas
(S =R
air
/R
gas

).
3. Results and discussion
3.1. Syntheses
The pH value plays an important role in controlling the compo-
sition of the final products. Fig. 1 shows the XRD patterns for the
products obtained from the hydrothermal treatment of the precur-
sors with a 1:1 molar ratio of W
6+
to Sn
2+
at 200
◦
C for 48 h under
different pH values. It is observed that pure phase of ␣-SnWO
4
(JCPDS no. 29-1354) can only be obtained at aneutralpH value (from
6 to 9) although the formation of the phase of ␣-SnWO
4
starts at pH
of 2. The products obtained in the acidic condition (pH from 2 to 5)
are a mixture of ␣-SnWO
4
, SnW
3
O
9
(JCPDS no. 86-628) and SnO
2
.
With pH decreasing to 1, the phase of ␣-SnWO

4
cannot be obtained
and the product obtained is a mixture of SnW
3
O
9
and SnO
2
. With
pH increasing to basic condition, a mixture of SnO
2
and SnO can be
obtained.
The molar ratio of W
6+
to Sn
2+
also plays an important role in
the final product. Fig. 2 shows the representative XRD patterns of
the products obtained when treated hydrothermally at 200
◦
Cata
pH value lower than 1 under different molar ratio of W
6+
to Sn
2+
.
It is found that pure SnW
3
O

9
can only be obtained when the molar
ratio of W
6+
to Sn
2+
is 2:1. A lower molar ratio of W
6+
to Sn
2+
(<2:1)
only gives a mixture of SnO
2
and SnW
3
O
9
. It is not strange that pure
Fig. 1. XRD patterns of the samples prepared at 200
◦
C for 48 h with different pH
values, (a) pH 1; (b) pH 3; (c) pH 5; (d) pH 7; (e) pH 9; (f) pH 11. (᭹) SnW
3
O
9
; (*)
SnO
2
;() ␣-SnWO
4

;() SnO.
H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 625
Fig. 2. XRD patterns of the samples prepared with the different molar ratio of Sn
2+
to
W
6+
under the strong acidity condition (pH < 1) at 200
◦
C for 48 h, (a) 1:1; (b) 1:1.5;
(c) 1:2; (d) 1:2.5; (e) 1:3. (*) SnO
2
; (#) H
2
W
1.5
O
5.5
·H
2
O.
phase of SnW
3
O
9
cannot be obtained at the stoichiometric molar
ratio of W
6+
to Sn
2+

at 3:1 since part of Sn and W are engaged in the
redox reaction as evidenced from the following XPS result. Instead,
a mixture of SnW
3
O
9
and H
2
W
1.5
O
5.5
·H
2
O is obtained. The reason
why pure SnW
3
O
9
can only be obtained when the molar ratio of
W
6+
to Sn
2+
is 2:1 is not very clear. We proposed that Sn
2+
and NH
4
+
in (NH

4
)
5
H
5
[H
2
(WO
4
)
6
]·H
2
O are responsible for the reduction of
partofW
6+
to give W
4+
and lead to the formation of SnW
3
O
9
. The
exact reactions occurring may be very complicated. However, the
involvement of NH
4
+
in the formation of SnW
3
O

9
can be confirmed
by the fact that no SnW
3
O
9
can be obtained when Na
2
WO
4
instead
of (NH
4
)
5
H
5
[H
2
(WO
4
)
6
]·H
2
O is used as the starting material.
3.2. Characterizations
XPS analyses were carried out on the as-prepared SnW
3
O

9
and
␣-SnWO
4
. XPS spectra of both SnW
3
O
9
and ␣-SnWO
4
in the Sn 3d
region show binding energies of Sn 3d
5/2
and Sn 3d
3/2
at around
486.6 and 495.1 eV respectively and suggest that in both samples
Sn exist in the chemical states of Sn
2+
[22] (Fig. 3a). The high-
resolution XPS spectra of the W 4f region for ␣-SnWO
4
show peaks
around 35.5 eV for W 4f
7/2
and 37.8 for W 4f
5/2
, which indicates that
WexistasW
+6

in ␣-SnWO
4
[23] (Fig. 3b). For SnW
3
O
9
, the high-
resolution XPS spectra of the W 4f
7/2
region can be deconvoluted
into two peaks around 34.2 and 35.6 eV respectively and suggests
that W exist in multi-valency in SnW
3
O
9
. The binding energy at
34.2 eV can be ascribed to W
4+
while the other peak at 35.6 eV orig-
inates from W
6+
[24,25]. The atomic ratio of W
4+
/W
6+
as evidenced
from the XPS result is around 1/2. This indicates that part of the W
is reduced from W
6+
to W

4+
. It is possible that Sn
2+
and NH
4
+
in
(NH
4
)
5
H
5
[H
2
(WO
4
)
6
]·H
2
O are responsible for the reduction of W
6+
to W
4+
and the formation of SnW
3
O
9
. This can also explain that

the stoichiometric 3:1 molar ratio of W
6+
to Sn
2+
cannot give the
pure phase of SnW
3
O
9
. The high-resolution XPS spectra of the O 1s
peaks can be deconvoluted into oxygen in lattice (O
2−
) at binding
energy of 530.5 eV and surface adsorbed oxygen (O
−
) at 532.0 eV
[26] (Fig. 3c).
The TEM image shows that the as-prepared ␣-SnWO
4
sample
consists of thin irregular nanoplates with the dimension range
from several tens of nanometers to several hundred nanometers.
(Fig. 4a) The HRTEM image (Fig. 4b) shows clear lattice fringes. The
fringes of d = 0.577 and 0.375 nm correspond to (0 2 0) and (1 0 1)
crystallographic plane of ␣-SnWO
4
, respectively. The typical TEM
image of SnW
3
O

9
shows that it consists of hexagonal nanoplates
Fig. 3. XPS spectra of ␣-SnWO
4
and SnW
3
O
9
(a) Sn 3d; (b) W 4f; (c) O 1s.
with dimension in the range of 60–150 nm. (Fig. 4c) The clear lat-
tice fringes of d = 0.321 nm observed in the HRTEM image (Fig. 4d)
correspond to the (2 0 0) crystallographic plane of SnW
3
O
9
.
N
2
-sorption isotherm for both ␣-SnWO
4
and SnW
3
O
9
exhibits
stepwise adsorption and desorption (type IV isotherm), indicative
of porous solids (Fig. 5). Due to the smaller average crystallite
size (17.9 nm) as determined from the XRD result, ␣-SnWO
4
has

a higher BET specific surface area of 40.0 m
2
g
−1
as compared to
that of SnW
3
O
9
(27.2 m
2
g
−1
), which has a larger average crys-
tallite size of 35.7 nm. The BET surface area of the hydrothermal
prepared ␣-SnWO
4
is also much higher than that of ␣-SnWO
4
pre-
pared via a solid-state reaction (3.4 m
2
g
−1
). Therefore, compared
626 H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628
Fig. 4. (a) TEM and (b) HRTEM of ␣-SnWO
4
; (c) TEM and (d) HRTEM of SnW
3

O
9
.
to the conventional solid-state method, the hydrothermal method
is a practical method to prepare nanocrystalline SnWO
4
samples
with small particle size and large BET specific surface.
3.3. Gas-sensing property
The gas-sensing performance toward H
2
of the hydrothermal
prepared ␣-SnWO
4
was investigated. The operating temperature
100
◦
C was chosen in this study according to our previous study
that the response toward 1000 ppm H
2
is best when the operat-
ing temperature is 10 0
◦
C. Fig. 6 shows the response of both the
Fig. 5. N
2
-sorption isotherm and the pore size distribution plot for ␣-SnWO
4
and
SnW

3
O
9
. The pore size distribution was estimated from the desorption branch of
the isotherm.
hydrothermal prepared and solid-state prepared ␣-SnWO
4
toward
H
2
at a working temperature of 373 K. Since ␣-SnWO
4
is an n-type
semiconductor, the free carriers are originated from the oxygen
vacancies. Therefore ␣-SnWO
4
is expected to adsorb both moisture
in the form of hydroxyl groups and oxygen in the ambient envi-
ronment. The adsorbed O
2−
and HO
−
groups can trap the electrons
from the conduction band of ␣-SnWO
4
and induce the formation
of a depletion layer on the surface. When exposed to a test gas, gas
molecules are chemi-adsorbed at the active sites on the surface and
will be oxidized by the adsorbed oxygen and lattice O
2−

. During the
oxidation process, electrons will transfer to the surface of ␣-SnWO
4
Fig. 6. Response of the sensors made of the as-prepared ␣-SnWO
4(Hy)
and ␣-
SnWO
4(SSR)
samples toward H
2
S at an operating temperature of 100
◦
C.
H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 627
Fig. 7. Response of the sensors made of the as-prepared ␣-SnWO
4
and SnW
3
O
9
samples toward H
2
S at an operating temperature of 100
◦
C.
to lower the number of trapped electrons, inducing a decrease in the
resistance. Therefore the gas response for ␣-SnWO
4
can b e defined
as the ratio of the stationary electrical resistance of the sensor in air

(R
air
) and in the test gas (R
gas
), i.e., S = R
air
/R
gas
. As shown in Fig. 6,
the response of the hydrothermal prepared ␣-SnWO
4
nanocrys-
tals toward 100 ppm H
2
is estimated to be 2.8, which is nearly two
times as that of the ␣-SnWO
4
prepared via a solid-state reaction
(1.6). The higher specific surface area of the hydrothermal prepared
␣-SnWO
4
is responsible for the higher response since it can provide
more active site for the gas chemisorption.
Although the gas-sensing property of ternary Sn–W–O system
has well been documented, to the best of our knowledge, the gas-
sensing property of SnW
3
O
9
has never been reported previously.

Herein the gas-sensing performance of nanoplates of SnW
3
O
9
and
␣-SnWO
4
toward H
2
S was investigated. It is observed that both
semiconductors exhibit excellent gas-sensing performance toward
H
2
S(Fig. 7). The response of the nanoplates of SnW
3
O
9
toward
100 ppm H
2
S is estimated to be 20, while that of the hydrother-
mal prepared ␣-SnWO
4
is about 9. We note that even at a very low
H
2
S concentration of 20 ppm, both semiconductors still exhibit a
very impressive sensing response (8.0 for SnW
3
O

9
and 5.2 for ␣-
SnWO
4
). It is a little weird to observe that SnW
3
O
9
nanoplates, with
a lower specific surface area (27.2 m
2
g
−1
), show a higher response
toward H
2
S than ␣-SnWO
4
nanoplates with a higher specific sur-
face area (40.0 m
2
g
−1
). This relative higher response of SnW
3
O
9
can
be explained by the existence of multivalent W (W
6+

/W
4+
), which
can promote the chemi-adsorption of H
2
S and is beneficial to the
change of the resistance for the n-type semiconductor like SnW
3
O
9
.
The as-prepared SnW
3
O
9
also shows response to other gas, like
H
2
. The response of the as-prepared SnW
3
O
9
is estimated to be
2.30 toward 500 ppm H
2
. SnW
3
O
9
is another ternary Sn(II)–W–O

semiconductor which shows promising application as the gas sen-
sor.
4. Conclusions
In summary, nanoplates of ␣-SnWO
4
and SnW
3
O
9
can be selec-
tively synthesized in large scale via a facile hydrothermal reaction
method. The final products obtained are strongly dependent on
the pH and the molar ratio of W
6+
to Sn
2+
in the precursors.
Due to the high specific surface area, ␣-SnWO
4
nanoplates show
higher response toward H
2
than that prepared via a solid-state
reaction. The as-prepared SnW
3
O
9
haxagonal nanoplates show
gas-sensing performance for both H
2

S and H
2
. As compared to
␣-SnWO
4
, SnW
3
O
9
exhibits higher response toward H
2
S and its
higher response can be well explained by the existence of the mul-
tivalent W (W
6+
/W
4+
)inSnW
3
O
9
.
Acknowledgments
The work was supported by National Natural Science Foundation
of China (20537010, 20677009), National Basic Research Program
of China (973 Program: 2007CB613306, 2007CB616907), grant from
Fujian Province (E0710009). This work was also supported by Pro-
gram for Changjiang Scholars and Innovative Research Team in
University (PCSIRT0818). Z. Li thanks program for New Century
Excellent Talents in University (NCET-05-0572), State Education

Ministry of P.R. China.
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Biographies
Hui Dong received BSc degree in chemistry from Taishan University in 2006 and
MSc degree in inorganic Chemistry from Fuzhou University in 2009. His interest is
in the synthesis and application of nanomaterials.
Zhaohui Li received BSc degree in chemistry from Fudan University in 1990, MSc

degree from Fujian Institute of Research on the Structure of Matter, Chinese Academy
of Science in 1996, and PhD degree in chemistry from National University of Singa-
pore in 2000. She is currently a professor in Department of Chemistry and Chemical
Engineering of Fuzhou University. Her research interest includes nanostructured
materials and photocatalysis.
Zhengxin Ding received PhD degree in chemistry from Fuzhou University in 2003.
He is currently an associate professor in Department of Chemistry and Chemical
Engineering of Fuzhou University. His major research interests focus on photocatal-
ysis.
Haibo Pan received BSc degree in physics from Huaqiao University in 1984, MSc
degree from Shanghai University in 1990. He is currently an associate profes-
sor at Chemistry and Chemical Engineering Collage, Fuzhou University. His major
research interests include gas sensor by nanomaterials, computer simulation for
structure and properties of nanomaterials, and organic/inorganic compound by
nanomaterials.
Xuxu Wang received PhD degree in Department of chemistry from Ecole Supérieure
de Chimie Physique Electronique de Lyon, France, in 2000. He is currently a profes-
sor in Department of Chemistry and Chemical Engineering of Fuzhou University.
His major research interests focus mainly on photocatalytic mechanism and nano-
photocatalytic materials.
Xianzhi Fu received his BSc and PhD degree in Department of Chemistry from Beijing
University, China, in 1982 and 1991. He is currently a professor in Department of
Chemistry and Chemical Engineering of Fuzhou University. His research focused
mainly on photocatalytic mechanism and nano-photocatalytic materials.