The final products obtained were dependent on the reaction pH and the molar ratio of W6+to Sn2+in the precursors.. The hydrothermal prepared␣-SnWO4showed higher response toward H2than tha
Trang 1Contents lists available atScienceDirect Sensors and Actuators B: Chemical
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b
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
a r t i c l e i n f o
Article history:
Received 19 March 2009
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
a b s t r a c t
Nanoplates of␣-SnWO4and SnW3O9were 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
W6+to Sn2+in the precursors The as-prepared nanoplates of␣-SnWO4and SnW3O9were characterized by X-ray powder diffraction (XRD), N2-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 (Sn2+) in SnW3O9as well as in␣-SnWO4 The gas-sensing performances of the as-prepared␣-SnWO4and SnW3O9toward H2S and H2were investigated The hydrothermal prepared␣-SnWO4showed higher response toward H2than that prepared via a solid-state reaction due to the high specific surface area The gas-sensing property toward H2S as well as H2 over SnW3O9was for the first time reported As compared to␣-SnWO4, SnW3O9exhibits higher response toward H2S and its higher response can be well explained by the existence of the multivalent W (W6+/W4+)
in SnW3O9
© 2009 Elsevier B.V All rights reserved
1 Introduction
Since both SnO2 and WO3 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 Sn4+as in the case of SnO2 [4–6] The inclusions of
both Sn4+and Sn2+have also been reported in some Sn–W–O
gas-sensing semiconductors For example, Solis and Lantto[3]reported
the gas-sensing property of SnxWO3+x with the atomic ratio x
between 1.25 and 2.5 The only reported Sn2+-based gas-sensing
semiconductor is ␣-SnWO4 Since tin in the divalent form may
enable an electron transfer between the Sn2+lattice ions at the
sur-face and sursur-face adsorbates, the developments of other Sn–W–O
gas-sensing materials with Sn in divalent state would be
interest-ing
␣-SnWO4 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
␣-SnWO4, are solid-state reactions[3,7,8]or the direct redox reaction
between metal Sn and the tungstenic acid[9] Since Sn2+can be
∗ Corresponding authors Tel.: +86 591 83779105; fax: +86 591 83779105.
E-mail addresses:zhaohuili1969@yahoo.com (Z Li), xzfu@fzu.edu.cn (X Fu).
easily oxidized to Sn4+, these reactions have to be carried out under
N2 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 Cu2O microspheres with hollow and multilayered con-figuration exhibited much higher gas-sensing property than bulk
Cu2O[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.
Trang 22 Experimental
2.1 Syntheses
Nanocrystalline␣-SnWO4 and SnW3O9 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 ␣-SnWO4,
SnCl2·2H2O (1.128 g, 5 mmol), (NH4)5H5[H2(WO4)6]·H2O (1.335 g,
0.833 mmol) (molar ratio of Sn2+to W6+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 SnW3O9is similar to that of␣-SnWO4except that the molar
ratio of Sn2+to W6+is 1:2 and the pH value is lower than 1
For comparison, bulk␣-SnWO4sample was prepared from SnO
and WO3 using a conventional solid-state synthesis route[3] To
prevent the oxidation of Sn2+to Sn4+, SnO and WO3was 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−1in the 2
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 , 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
in air (Rair) and in air–gas mixtures (Rgas) under the same operating
current The gas response magnitude (S) was defined as the ratio of
Rairto Rgas(S = Rair/Rgas)
3 Results and discussion
3.1 Syntheses
The pH value plays an important role in controlling the compo-sition of the final products.Fig 1shows the XRD patterns for the products obtained from the hydrothermal treatment of the precur-sors with a 1:1 molar ratio of W6+to Sn2+at 200◦C for 48 h under different pH values It is observed that pure phase of␣-SnWO4
(JCPDS no 29-1354) can only be obtained at a neutral pH value (from
6 to 9) although the formation of the phase of␣-SnWO4starts at pH
of 2 The products obtained in the acidic condition (pH from 2 to 5) are a mixture of␣-SnWO4, SnW3O9(JCPDS no 86-628) and SnO2 With pH decreasing to 1, the phase of␣-SnWO4cannot be obtained and the product obtained is a mixture of SnW3O9and SnO2 With
pH increasing to basic condition, a mixture of SnO2and SnO can be obtained
The molar ratio of W6+to Sn2+also plays an important role in the final product.Fig 2shows the representative XRD patterns of the products obtained when treated hydrothermally at 200◦C at a
pH value lower than 1 under different molar ratio of W6+to Sn2+
It is found that pure SnW3O9can only be obtained when the molar ratio of W6+to Sn2+is 2:1 A lower molar ratio of W6+to Sn2+(<2:1) only gives a mixture of SnO2and SnW3O9 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 (䊉) SnW 3 O 9 ; (*)
; () ␣-SnWO ; () SnO.
Trang 3Fig 2 XRD patterns of the samples prepared with the different molar ratio of Sn2+ 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 SnW3O9cannot be obtained at the stoichiometric molar
ratio of W6+to Sn2+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 SnW3O9and H2W1.5O5.5·H2O is obtained The reason
why pure SnW3O9can only be obtained when the molar ratio of
W6+to Sn2+is 2:1 is not very clear We proposed that Sn2+and NH4
in (NH4)5H5[H2(WO4)6]·H2O are responsible for the reduction of
part of W6+to give W4+and lead to the formation of SnW3O9 The
exact reactions occurring may be very complicated However, the
involvement of NH4 in the formation of SnW3O9can be confirmed
by the fact that no SnW3O9can be obtained when Na2WO4instead
of (NH4)5H5[H2(WO4)6]·H2O is used as the starting material
3.2 Characterizations
XPS analyses were carried out on the as-prepared SnW3O9and
␣-SnWO4 XPS spectra of both SnW3O9and␣-SnWO4in the Sn 3d
region show binding energies of Sn 3d5/2and Sn 3d3/2at around
486.6 and 495.1 eV respectively and suggest that in both samples
Sn exist in the chemical states of Sn2+[22] (Fig 3a) The
high-resolution XPS spectra of the W 4f region for␣-SnWO4show peaks
around 35.5 eV for W 4f7/2and 37.8 for W 4f5/2, which indicates that
W exist as W+6in␣-SnWO4[23](Fig 3b) For SnW3O9, the
high-resolution XPS spectra of the W 4f7/2region can be deconvoluted
into two peaks around 34.2 and 35.6 eV respectively and suggests
that W exist in multi-valency in SnW3O9 The binding energy at
34.2 eV can be ascribed to W4+while the other peak at 35.6 eV
orig-inates from W6+[24,25] The atomic ratio of W4+/W6+as evidenced
from the XPS result is around 1/2 This indicates that part of the W
is reduced from W6+to W4+ It is possible that Sn2+and NH4 in
(NH4)5H5[H2(WO4)6]·H2O are responsible for the reduction of W6+
to W4+and the formation of SnW3O9 This can also explain that
the stoichiometric 3:1 molar ratio of W6+to Sn2+cannot give the
pure phase of SnW3O9 The high-resolution XPS spectra of the O 1s
peaks can be deconvoluted into oxygen in lattice (O2−) 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␣-SnWO4sample
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␣-SnWO4, respectively The typical TEM
image of SnW O 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 SnW3O9
N2-sorption isotherm for both␣-SnWO4and SnW3O9exhibits 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,␣-SnWO4 has
a higher BET specific surface area of 40.0 m2g−1 as compared to that of SnW3O9 (27.2 m2g−1), which has a larger average crys-tallite size of 35.7 nm The BET surface area of the hydrothermal prepared␣-SnWO4is also much higher than that of␣-SnWO4 pre-pared via a solid-state reaction (3.4 m2g−1) Therefore, compared
Trang 4Fig 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 SnWO4 samples
with small particle size and large BET specific surface
3.3 Gas-sensing property
The gas-sensing performance toward H2 of the hydrothermal
prepared␣-SnWO4was investigated The operating temperature
100◦C was chosen in this study according to our previous study
that the response toward 1000 ppm H2 is best when the
operat-ing temperature is 100◦C.Fig 6shows the response of both the
Fig 5 N2 -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
hydrothermal prepared and solid-state prepared␣-SnWO4toward
H2at a working temperature of 373 K Since␣-SnWO4is an n-type semiconductor, the free carriers are originated from the oxygen vacancies Therefore␣-SnWO4is expected to adsorb both moisture
in the form of hydroxyl groups and oxygen in the ambient envi-ronment The adsorbed O2 −and HO−groups can trap the electrons
from the conduction band of␣-SnWO4and 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 O2 − During the
oxidation process, electrons will transfer to the surface of␣-SnWO4
Fig 6 Response of the sensors made of the as-prepared␣-SnWO 4(Hy) and ␣-SnWO samples toward H S at an operating temperature of 100 ◦ C.
Trang 5Fig 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␣-SnWO4can be defined
as the ratio of the stationary electrical resistance of the sensor in air
(Rair) and in the test gas (Rgas), i.e., S = Rair/Rgas As shown inFig 6,
the response of the hydrothermal prepared␣-SnWO4
nanocrys-tals toward 100 ppm H2is estimated to be 2.8, which is nearly two
times as that of the␣-SnWO4prepared via a solid-state reaction
(1.6) The higher specific surface area of the hydrothermal prepared
␣-SnWO4is 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 SnW3O9has never been reported previously
Herein the gas-sensing performance of nanoplates of SnW3O9and
␣-SnWO4 toward H2S was investigated It is observed that both
semiconductors exhibit excellent gas-sensing performance toward
H2S (Fig 7) The response of the nanoplates of SnW3O9 toward
100 ppm H2S is estimated to be 20, while that of the
hydrother-mal prepared␣-SnWO4is about 9 We note that even at a very low
H2S concentration of 20 ppm, both semiconductors still exhibit a
very impressive sensing response (8.0 for SnW3O9and 5.2 for
␣-SnWO4) It is a little weird to observe that SnW3O9nanoplates, with
a lower specific surface area (27.2 m2g−1), show a higher response
toward H2S than␣-SnWO4nanoplates with a higher specific
sur-face area (40.0 m2g−1) This relative higher response of SnW3O9can
be explained by the existence of multivalent W (W6+/W4+), which
can promote the chemi-adsorption of H2S and is beneficial to the
change of the resistance for the n-type semiconductor like SnW3O9
The as-prepared SnW3O9 also shows response to other gas, like
H2 The response of the as-prepared SnW3O9 is estimated to be
2.30 toward 500 ppm H2 SnW3O9is another ternary Sn(II)–W–O
semiconductor which shows promising application as the gas
sen-sor
4 Conclusions
In summary, nanoplates of␣-SnWO4and SnW3O9can 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 W6+ to Sn2+ in the precursors
Due to the high specific surface area,␣-SnWO4 nanoplates show
higher response toward H2 than that prepared via a solid-state
reaction The as-prepared SnW3O9 haxagonal nanoplates show
gas-sensing performance for both H2S and H2 As compared to
␣-SnWO4, SnW3O9 exhibits higher response toward H2S and its
higher response can be well explained by the existence of the
mul-tivalent W (W6+/W4+) in SnW O
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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