microstructure control of wo3 film by adding nano-particles of sno2 for no2 detection in ppb level

Procedia
Chemistry
www.elsevier.com/locate/procedia

Proceedings of the Eurosensors XXIII conference
Microstructure control of WO
3
film by adding nano-particles of
SnO
2
for NO
2
detection in ppb level
Kengo Shimanoe
a
*, Aya Nishiyama
b
, Masayoshi Yuasa
a
, Tetsuya Kida
a
,
Noboru Yamazoe
a
a
Faculty of Engineering Sciences,Kyushu University
b
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

Abstract
To fabricate more excellent NO
2
sensor with high sensor response and good linearity between the sensor response and NO
2

concentration, the microstructure of WO
3
lamellae was controlled by adding nano-particles of SnO
2
. It was found that the
sintering of WO
3
lamellae was inhibited by adding nano-particles of SnO
2
. The device using WO
3
lamellae added a small amount
of SnO
2
nano-particles had the highest sensor response, exhibiting a high sensor response (S = 60-540) even to dilute NO
2
(100-
1000 ppb) in air at 200°C.

Keywords: Gas sensors, Microstructure control, Lamellar, WO
3
, NO
2
, SnO

2

1. Introduction
It is well known that WO
3
is a semiconductor material to detect NO
2
gas and that the morphology and size of
particles composing the sensing layers play an important role in determining the sensing properties. Previously we
reported that NO
2
sensor using nano-sized WO
3
lamellae shows a high NO
2
sensitivity [1-5]. It was found that the
sensor response was significantly increased with a decrease in the thickness of the WO
3
lamellae and was well-
correlated with its thickness. Another important feature of the devices was the porous microstructure of the sensing
layer packed with WO
3
lamellae with a high anisotropic shape. A sufficiently high sensor response was obtained,
even to 10 ppb NO
2
in air, when WO
3
lamellae with ca. 30 nm in thickness and 1 μm in lateral dimension were used
for the sensing film. In addition, the acidification of NaWO
4

with a strong acid solution produced lamellar-
structured WO
3
particles with 100-350 nm in lateral size and 20-50 nm in thickness, resulting in excellent NO
2

sensing properties (S = 150 against 500 ppb NO
2
in air) at the low temperature of 200°C. On the other hand,
however, the linearity between the sensor response and the NO
2
concentration was not well understood. It is
sometimes observed that the sensor response showed a tendency to be saturated with increasing NO
2
concentration.
Such saturation seems to be owing to that the lamellae particles agglomerated heavily by sintering were dispersed

* Corresponding author. Tel.:+81-92-583-7876; fax:+81-92-583-7538.
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1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.proche.2009.07.053
Procedia Chemistry 1 (2009) 212–215

Fig. 1 FE-SEM images of the surface for thick films of WO
3
(a), WO
3
-SnO
2

(1:0.01) (b) and WO
3
-SnO
2
(1:0.1) (c) calcined at 300°C.

C.

into the sensing film. In this study, in order to extend the detectable concentration range by improving the sensor
resp
onse at high NO
2
concentration, we investigated the microstructure control of WO
3
film by adding nano-
particles of SnO
2
.
2. Experimental
Sol of WO
3*
2H
2
O with lamellar-structure was prepared by the acidification of NaWO
4
with a strong acid solution
(H
2
SO
4

at pH = -0.8) [5]. On the other hand, sol of crystalline SnO
2
with mean grain (crystallite) size of 7 nm was
prepared by hydrothermal treatments [6]. Both sols were mixed together with W:Sn=1:0-0.1 in molar ratio and
stirred for 24 h. The mixed sols were washed with distilled water by centrifugation. The obtained precipitates were
mixed with water to form a paste. The resulting paste was screen-printed on an alumina substrate equipped with a
pair of comb-type Au microelectrodes (line width: 180 μm; distance between lines: 90 μm; sensing layer area: 64
mm
2
). The paste deposited on the substrates was calcined at 300-500°C for 2 h in air to form a sensing layer of
SnO
2
-dispersed WO
3
via the dehydration of the precursor, WO 2H
3* 2
O.
The surface morphology of the samples was analyzed with a field emission scanning e
lectron microscope (FE-
SEM). The thickness of the films was estimated to be 15-25 μm by FE-SEM observations. The crystal structure
and specific surface area of the samples were measured using an X-ray diffractometer (XRD)
with copper Kα
radi
ation and a BET surface area analyzer, respectively. The NO
2
sensing properties of the devices were examined
at an operating temperature of 200°C in a concentration range of 50 to 1000 ppb in air. Measurements were
performed using a conventional gas flow apparatus equipped with an electric furnace at a gas flow rate of 0.1 dm
3
/

min. The sensor response (S) was defined as the ratio of resistance in air containing NO
2
(R
g
) to that in dry air (R
a
)
(S = R
g
/R ).
a
3. Results and Discursion
Figure 1 shows FE-SEM images of the surface for thick films of WO
3
(a), WO
3
-SnO
2
(1:0.01) (b) and WO
3
-SnO
2

(1:0.1) (c) calcined at 300°C. The morphology of the lamellar particles seems to differ a little depending on amount
of adding SnO
2
. In the case of only WO
3
, comparatively large agglomerated particles are seen. However by adding
SnO

2
nano-particles, the particle size was still kept small although the thickness of lamellae was seen as it increased.
Table 1 shows specific surface area for each sample. By addition of a small amount of SnO
2
nano-particles, it is
found that the sintering of WO
3
lamellae was controlled and the porosity was kept as that result. It can be
considered that SnO
2
nano-particles were inserted between the thin WO
3
lamellae and they played a part in
inhibiting grain growth of WO
3
.
(a)
(b)
(c)

Table 1 Specific surface area of WO
3
-SnO
2
based samples calcined at 300°

K. Shimanoe et al. / Procedia Chemistry 1 (2009) 212–215
213


Figure 2 shows the sensor response as a function of NO
2
concentration at 200°C. In the figure, the properties of
sensor prepared through an ion-exchange method also indicated for comparison. These devices also responded to
dilute NO
2
and showed a sufficient ability to detect ppb level NO
2
in the atmosphere. Especially the device using
WO
3
lamellae added SnO
2
nano-particles indicates excellent sensor response. However, the sensor response of the
devices differed depending on amount of adding SnO
2
. The sensor fabricated with WO
3
-SnO
2
(1:0.01) showed the
best NO
2
response, but the device could not measure high concentration because the electric resistance was as high
as exceeding a measurement limit. Such high sensor response can be explained from the viewpoint of the specific
surface area as shown in Table 1. The sensor fabricated with WO

3
-SnO
2
(1:0.01) has more porous microstructure,
as compared with other devices. It is because the agglomeration of lamellae by sintering was inhibited by adding
nano-particles of SnO
2
. However the amount of addition of SnO
2
nano-particles seems to have the most suitable
value. The sensor response, when the amount of addition increased, lowered like a case of (b) in Fig. 2, although it
was more sensitive than the device without adding SnO
2
nano-particles. In addition, the excessive amount of
addition seems to make linearity between the sensor response and the NO
2
concentration poor.
In order to confirm the linearity between th
e sensor response and the NO
2
concentration for the sensor fabricated
with WO
3
-SnO
2
(1:0.01), the calcination temperature was elevated. Figure 3 shows the sensor response as a
function of NO
2
concentration at 300°C for the devices calcined at 400 and 500°C. The sensing properties were
measured at 300°C to restrain electric resistance in less than a measurement limit. It was found that the sensor

response decreased with increasing the calcination temperature. It can think about such a tendency that WO
3

particles grow due to the rise in calcination temperature. However though the calcination was made in high
temperature, the linearity between the sensor response and the NO
2
concentration was clearly observed for each
device. This result means that the sensor fabricated with WO
3
-SnO
2
(1:0.01) holds porous structure still and fully.
If Au electrodes for measurement can be optimized by using MEMS technology to reduce the electric resistance,
more excellent sensor, which can detect NO
2
of the wide concentration range, would be obtained at operating
temperature of 200°C.

Fig. 2 Sensor response as a function of NO concentration at 200°C for the devices using
2
-SnO (1:0.01), (b) WO -SnO (1:0.1), (c) WO by acidification method, and (a) WO
3 2 3 2 3
by ion-exchange method. These devices were calcined at 300°C. (d) WO
3



K. Shimanoe et al. / Procedia Chemistry 1 (2009) 212–215
214

Fig. 3 Sensor response as a function of NO concentration at 300°C for the devices
2
-SnO (1:0.01)) calcined at (a) 400 and (b) 500°C (WO
3 2
4. Conclusions
To extend the detectable concentration range by improving the sensor response at high NO
2
concentration, the
microstructure control of WO
3
film was investigated by adding nano-particles of SnO
2
. It was found that the
developed devices can detect NO
2

high-sensitively in a wide concentration range of 50-1000 ppb.
Acknowledgements
This work was financially supported in part by NISSAN SCIENCE FOUNDATION.
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