Sensors and Actuators B 96 (2003) 219–225
Sensitivity properties of a novel NO
2
gas sensor based
on mesoporous WO
3
thin film
L.G. Teoh
a
, Y.M. Hon
a
, J. Shieh
b
, W.H. Lai
a
, M.H. Hon
a,∗
a
Department of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan, ROC
b
National Nano Device Laboratories, 1001-1 Ta-Hsueh Road, Hsinchu 30050, Taiwan, ROC
Received 19 December 2002; received in revised form 20 May 2003; accepted 27 May 2003
Abstract
Mesoporous WO
3
thin films micro-gas sensor was fabricated and the NO
2
gas-sensing as well as electrical properties have been
investigated. The film had nano-sized grains, porous structure with a relative surface area of 143 m
2
/g as calcined at 250

mental pollution resulting from combustion or automotive
emissions [1]. Existing gas sensor materials include semi-
conducting metal oxides [1], silicon [2,3] and organic mate-
rials [4,5]. Semiconducting metal oxides such as WO
3
and
SnO
2
had been widely used for NO
2
detection [1,6]. These
sensors have to operate at 200–500
◦
C in order to improve
the sensitivity by enhancing the chemical reaction between
gas and the sensor material [7,8]. Obviously, it would be
desirable for many applications if the sensor could operate
at temperatures <100
◦
C or even at room temperature, es-
pecially for battery-operated devices. Recently it also has
been reported that ZrO
2
–SnO
2
[9] and ZnO [10] materials
can be used as H
2
S and NH
3

been reported to have good selectivity for low concentration
NO
x
gas [12]. In this study, we developed a novel NO
2
gas
sensors based on mesoporous WO
3
thin film to detect small
concentration of NO
2
at low operating temperatures.
Li and Kawi [13] have shown that a linear relationship
was found between the surface areas of SnO
2
sensors and
their sensitivities to 500 ppm of H
2
. Accordingly, meso-
porous WO
3
with a higher surface area provides more
surface adsorption sites for the reaction of NO
2
gas, which
is beneficial to the operating temperature and sensitivity of
the sensor. Mesoporous materials are generally prepared
by amphiphilic self-assembling surfactants as templates
[14,15]. In this study, the mesoporous WO
3

ethanol solution with vigorously stirring for 1 h. The result-
ing sol solution was gelled in an open Petri dish at 60
◦
C
in air. Alternatively, the sol solution can be used to prepare
thin films on Al
2
O
3
substrate that was coated with Pt elec-
trode by dip coating. The thin films can be dried within sev-
eral hours at 60
◦
C. The as-made bulk samples or thin films
were calcined at 250
◦
C for 5 h and then washed by ethanol
to remove the residual block copolymer.
X-ray powder diffraction (XRD) patterns were obtained
on a Rigaku D/max-X-diffractometer using Cu K␣ radiation
with Ni filter. Transmission electron microscopy (TEM)
studies were carried out on a Hitachi Model HF-2000 elec-
tron microscope operating at 200 keV. The samples for
TEM were prepared by directly dispersing the fine powders
of the product onto 200 mesh Cu grids. The morphology of
mesoporous WO
3
films was observed by scanning electron
microscope (SEM, Philips XL-40 FEG). The nitrogen ad-
sorption and desorption isotherms at 77 K were measured

thin film calcined at 250
◦
C for 5 h.
Brunauer–Emmett–Teller (BET) surface areas were esti-
mated over a relative pressure (P/P
0
) range from 0 to 1.0.
Pore size distribution was obtained from the analysis of
the adsorption branch of the isotherms using the Barrett–
Joyner–Halenda (BJH) model. The pore volume was taken
at the P/P
0
= 0.983 signal point.
The resistance of the films was obtained by measuring
the current through the film at a constant voltage of 1 V
and recorded by a multimeter (HP 3458 A). The samples
under test were placed in a quartz chamber (85 cm
3
) and
exposed to 3 ppm NO
2
gas and 4000ppm H
2
, respectively.
Gas-sensing properties of the films were studied at various
operating temperatures T
g
in the range of 35
◦
C <T

thin film calcined at 250
◦
C
for 5 h.
After employing Scherrer’s formula, the calculated grain
size of WO
3
is approximately 3.8 nm. These grains contact
contributes to the gas-sensing properties of the mesoporous
WO
3
films (the smaller grain size increases gas sensitivity
since the diameter is comparable with or less than the space
charge region of the grain).
Fig. 2 shows the SEM micrographs of the WO
3
thin films
calcined at 250
◦
C for 5 h. The sample exhibits porous struc-
ture with a spherical powder of approximately 1.5 ␮m. It
means that such a structure of film is likely to facilitate the
adsorption process of NO
2
molecules because of the cap-
illary pore and large surface area. This implies the conclu-
sion that this type of film will offer a good sensitivity to
NO
2
gas.

[16]. Barrett–Joyner–Halenda (BJH)
analyses show that the calcined mesoporous WO
3
exhibits
mean pore size of 5 nm (Fig. 4 inset). From the absolute
adsorption, we can calculate a specific surface of 143m
2
/g.
This underlines that most pores are really accessible from
Fig. 3. TEM images of mesoporous WO
3
thin film calcined at 250
◦
C for
5 h: (a) bright field TEM image; (b) dark field TEM image obtained on
the same area of (a); (a) inset: selected-area electron diffraction pattern
recorded on the sample.
the outside and the pore system is fully interconnected
(from adsorption and desorption lines of N
2
).
3.2. Gas-sensing properties
In order to check the sensitivity of WO
3
sensors for the
concentration of 3 ppm NO
2
,WO
3
sensors were maintained

0.8
1.2
1.6
Pore volume (cc/g)
Fig. 4. Nitrogen adsorption(+)–desorption(᭺
) isotherms and BJH pore size distribution curves for mesoporous WO
3
calcined at 250
◦
C for 5 h.
the sensor decreased. The time response of WO
3
sensors,
which shows good sensitivity to NO
2
, is shown in Fig. 6.
After initial resistance was stabilized, NO
2
was injected into
the closed chamber in the batch system and vented the gas
after being maintained for 5min. The sensors with operat-
ing temperatures >70
◦
C and operating temperatures <50
◦
C
have a 90% response time of 1–2 min and above 10 min, re-
spectively. Fig. 7 illustrates NO
2
gas sensitivities of WO

C for de-
tecting 100ppm of NO
2
with a sensitivity of ∼300 [4,18].
Thus, the mesoporous WO
3
sensors have the advantage of
100
◦
C temperature operation for detecting 3 ppm with sen-
sitivity up to 226 over these materials.
The experiment on the selectivity of the mesoporous gas
sensor was carried out by monitoring the electrical resis-
tance change in H
2
atmosphere, as shown in Fig. 5.Itcan
be seen that the sensor operating at 100
◦
C and 4000 ppm of
H
2
exhibits a sensitivity of ∼3. Although the sensitivity is
much smaller than that of NO
2
, the opposite response in re-
sistance (NO
2
increases the resistance, while H
2
decreases

8.0E+008
1.2E+009
Resistance(
Ω
)
0 1000 2000 3000
0 1000 2000 3000
↓
On
←
Off
100
o
C
70
o
C
50
o
C
35
o
C
NO
2
NO
2
(a)
Time (sec)
0.0E+000

Temperature (
o
C)
0
10
20
Time (min)
Fig. 6. Time response of WO
3
thin film sensor.
20 40 60 80 100 120
Operating temperature (
0
C)
0
50
100
150
200
250
Sensitivity (R
g
/R
a
)
Fig. 7. Sensitivity of mesoporous WO
3
thin film upon operating temper-
atures from 35 to 100
◦

2
molecules. The NO
2
ions adsorbed at low temperatures on oxide semiconductor
surfaces are thought to be ONO
−
(nitrito type adsorbates)
and dissociate into nitrosyl type adsorbates (NO
+
,NO
−
)
[23]. This enables to conclude that the normal response of
sensor for NO
2
might originate from the superior number of
NO
+
absorbates than NO
−
adsorbates, even at room temper-
ature. Consequently, the electron transfer to surface species
in connection with NO
2
chemisorption creates Schottky
energy barrier at the surface yielding a large resistance of
the film. It is clear that the response is related to a catalytic
reaction of WO
3
with the adsorbed NO

high sensitivity at a low temperature to 3 ppm NO
2
.
4. Conclusions
This study has shown that mesoporous WO
3
thin films
with unique microstructure lead to excellent sensing prop-
erties upon exposure to low concentration of NO
2
in air
at low temperatures and enabled the selective detection of
NO
2
and H
2
gases. A high surface area and small crystal-
lites present in the mesoporous WO
3
films are the factors
contributing to this behavior. Apart from small grain sizes,
the main feature of mesoporous WO
3
thin film sensors is
that they operate at low temperatures and low concentration
of NO
2
with sensitivity as high as 226. Thus, mesoporous
WO
3

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Biographies
Lay Gaik Teoh received her BS and MS degrees of Materials Science
and Engineering from National Cheng Kung University, Tainan, Taiwan
in 1997 and 1999, respectively. She has been a PhD candidate at National
Cheng Kung University, Tainan, Taiwan, 1999. Her major research has
related to mesoporous materials, semiconductor gas sensor, and PVD
Ba-Ti-Sn-O system thin films.
Yi Ming Hon received his BS, MS and PhD degrees of Materials Science
and Engineering from National Cheng Kung University, Tainan, Taiwan
in 1995, 1996 and 2001, respectively. He is now a postdoctoral fellow
at National Cheng Kung University, Tainan, Taiwan. His major research
has related to mesoporous materials, and battery materials.
Jiann Shieh received his BS, MS and PhD degrees of Materials Science
and Engineering from National Cheng Kung University, Tainan, Taiwan
in 1995, 1997 and 2002, respectively. He is now an associate researcher at
National Nano Device laboratories Hsinchu, Taiwan. His major research
has related to PECVD Ti-Al-C-N system nanocomposite thin films, semi-
conductor gas sensor, mesoporous materials, nanocrystal, and nanowire
materials.
Wei Hao Lai received his BS and MS degrees of Materials Science and En-
gineering from National Cheng Kung University, Tainan, Taiwan in 2000
and 2001, respectively. He has been a PhD candidate at National Cheng
Kung University, Tainan, Taiwan, 2001. His major research has related
to mesoporous materials, semiconductor gas sensor, and nanomaterials.