Gas sensing properties of WO
3
doped rutile TiO
2
thick film at high
operating temperature
Sung-Eun Jo, Byeong-Geun Kang, Sungmoo Heo, Soonho Song, Yong-Jun Kim
*
School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea
article info
Article history:
Received 31 March 2009
Received in revised form 29 June 2009
Accepted 29 June 2009
Available online xxxx
PACS:
51.50.+v
Keywords:
WO
3
doped rutile TiO
2
High temperature heat treatment
Grain growth
High temperature gas sensor
abstract
A semiconductor gas sensor based on WO
3
doped TiO
2
having a rutile phase was fabricated on an Al

gine’s work of automobiles. TiO
2
exists in several crystalline mod-
ifications, the common forms being anatase and rutile [3]. Anatase
is transferred to rutile at above 600 °C [4], so that anatase TiO
2
based sensors have relatively low heat treatment temperature lev-
els [5,6]. However, low heat treatment temperature can induce
grain growth of the sensing films in sensors operating at high
temperature ($500 °C) and decreases the sensors’ sensitivity [7].
It is known a specific surface area of the sensing film is decreased
with increase of grain size [1]. So, the grain growth of the sensing
film induces low sensitivity of the sensor. On the other hand, ther-
mally stable, rutile TiO
2
based gas sensors can tolerate high heat
treatment temperature levels. Therefore, rutile TiO
2
based gas
sensors can detect gases at high operating temperature with no
grain growth. Although, the high heat treatment temperature
ensures stable operation of the sensors at high temperature, it
causes grain growth of the sensing films during the heat treat-
ment process.
Therefore, in this study, WO
3
doped rutile TiO
2
was used as the
material of the sensing film to reduce the grain growth of the film

terpineol, ethyl cellulose, dis-
persing agent). The 1.5
l
m thick sensing film was heat treated at
1100 °C for 1.5 h in dry air condition to define a porous shaped
sensing film and enhance the thermal stability of the sensor.
The surface topography of the screen printed sensing film was
investigated with an S4800 scanning electron microscope (SEM)
operated at 15 keV and an XE150 atomic force microscope (AFM).
The structural analysis was carried out by using a D/MAX2500H
X-ray diffractometer (XRD).
1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cap.2009.06.053
* Corresponding author. Tel.: +82 2 2123 2844.
E-mail address: (Y J. Kim).
Current Applied Physics xxx (2009) xxx–xxx
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ARTICLE IN PRESS
Please cite this article in press as: S E. Jo et al., Gas sensing properties of WO
3
doped rutile TiO
2
thick film at high operating temperature, Curr. Appl. Phys.
(2009), doi:10.1016/j.cap.2009.0 6.053
Fig. 1. SEM images of the sensing films.
Fig. 2. AFM images of the sensing films.
Fig. 3. XRD patterns of undoped TiO
2

3. Results and discussion
Fig. 1 shows SEM images of the sensing films deposited by
screen printing. The sensing film exhibited the typical microstruc-
ture of screen printed sensing films. A porous sensing film surface
was obtained due to the high heat treatment temperature. The
WO
3
doping did not induce any change.
AFM micrographs of the undoped, and 11 wt.% and 15 wt.% WO
3
doped TiO
2
films are shown in Fig. 2. The AFM micrographs con-
firmed that the WO
3
doping was not a dominant factor in defining
the porous and rough surface of the sensing films.
The crystal structure of the sensing films was revealed in the
XRD patterns shown in Figs. 3 and 4. Although, the WO
3
doping ef-
fect on the surface topography was not clearly evident, the inner
morphological properties of the sensing film were affected by the
WO
3
doping. Fig. 3 revealed that the sensing films consisted of al-
most a single phase of rutile TiO
2
, while Fig. 4 revealed the (1 1 0)
peak of the rutile TiO

The undoped and two WO
3
doped TiO
2
sensors exhibited a gas
sensing property similar to that of a p-type, semiconductor gas
sensor. The sensitivity to gases is defined as (R
a
À R
g
)/R
g
, where
R
a
and R
g
are the steady state resistances of the device in null
(N
2
) and oxidizing gas, respectively. The two WO
3
doped TiO
2
sen-
sors had a higher sensitivity to NO
2
than that of a pure TiO
2
sensor.

the gas mixture are shown in Figs. 6 and 7.InFig. 6, sensitivities
of the sensor (15 wt.% WO
3
doped TiO
2
) to CO, O
2
and CO
2
were
very small. And in Fig. 7, the resistance of the sensor decreased
very fast in the NO
2
condition but exhibited very small changes
in the gas mixtures (NO
2
+ other oxidizing gases). These sensing re-
sults demonstrated the high sensitivity and good selectivity to NO
2
of the WO
3
doped TiO
2
sensor.
The potential energy barrier eV
S
between particles of the sensor
is proportional to N
t
,

d
the volumetric density of the electron donors. From Eq. (2),we
can know that the sensitivity of the sensor is dominated by the
number of adsorbed oxygen ions at the sensor surface. Because oxi-
Fig. 4. XRD patterns – (1 1 0) peak of undoped TiO
2
, 11 wt.% WO
3
doped TiO
2
and
15 wt.% WO
3
doped TiO
2
.
Fig. 5. The results of NO
2
(500 ppm) detection at 600 °C.
Fig. 6. The results of CO, O
2
and CO
2
detection at 600 °C.
S E. Jo et al. / Current Applied Physics xxx (2009) xxx–xxx
3
ARTICLE IN PRESS
Please cite this article in press as: S E. Jo et al., Gas sensing properties of WO
3
doped rutile TiO

3
and
TiO
2
, which in turn affected the electrical properties of the sensing
film. The WO
3
may occupy the interstitial sites in TiO
2
lattice
according to the following:
WO
3
!
TiO
2
W
00
Ti
þ V
00
O
þ O
x
O
ð4Þ
Since the predominant defect is an oxygen vacancy, this equilibrium
can be described by Eq. (5),
O
x

3
doped TiO
2
sensor, with a sensing film deposited by
screen printing, was fabricated successfully. The use of rutile
TiO
2
as the sensing film material enabled the heat treatment to
be performed at high temperature, which facilitated the fabrication
of a sensitive and porous shaped sensing film. The WO
3
doping did
not exert a dominant effect on the surface topography but it did af-
fect the inner morphological properties of the TiO
2
film. The
15 wt.% WO
3
doped TiO
2
sensor showed the best sensitivity to
NO
2
at 600 °C. The proposed sensor had good sensitivity and selec-
tivity to NO
2
, in comparison with its sensitivity to other oxidizing
gases such as CO, O
2
, and CO

(2009), doi:10.1016/j.cap.2009.06.053