In order to enhance the sensitivity of the sensor, the sensing film was fabricated with a porous shape by high temperature heat treatment and the TiO2layer was doped with WO3to improve th
Trang 1Gas sensing properties of WO 3 doped rutile TiO 2 thick film at high
operating temperature
School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
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
a b s t r a c t
A semiconductor gas sensor based on WO3doped TiO2having a rutile phase was fabricated on an Al2O3
substrate The sensing film of the sensor was deposited by using screen printing In order to enhance the sensitivity of the sensor, the sensing film was fabricated with a porous shape by high temperature heat treatment and the TiO2layer was doped with WO3to improve the gas selectivity The surface topography and inner morphological properties of the sensing film were characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) The gas sensing proper-ties of the fabricated sensor were evaluated by detecting NO2and other oxidizing gases (CO, O2and CO2)
at high operating temperature (600 °C)
Ó 2009 Elsevier B.V All rights reserved
1 Introduction
Semiconductor gas sensors can detect oxidizing and reducing
gases by measuring the conductance changes in the surface
phe-nomena in terms of gas adsorption and desorption[1,2] TiO2is
a well known metal oxide semiconductor It can be used for high
temperature gas sensors which are more suitable for controlling
many high temperature technological processes such as the
en-gine’s work of automobiles TiO2exists 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 TiO2
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 TiO2based gas sensors can tolerate high heat
treatment temperature levels Therefore, rutile TiO2 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, WO3doped rutile TiO2was used as the material of the sensing film to reduce the grain growth of the film during the heat treatment process The sensing film was deposited
on an Al2O3substrate by screen printing and was thermally treated
at high temperature (1100 °C) The surface and morphological properties of the fabricated sensor were studied, while its gas sensing properties were investigated by detecting NO2and other oxidizing gases (CO, O2and CO2) at high operating temperature
2 Experimental The sensor was fabricated on an alumina substrate The sensing film was deposited by screen printing on the IDT (interdigitated) electrode A printable paste for screen printing was prepared by using an organic vehicle based onaterpineol The paste consisted
of 16 wt.% nanopowders (average particle size of TiO2: 20 nm,
WO3: 30 nm) and 84 wt.% solvent (aterpineol, ethyl cellulose, dis-persing agent) The 1.5lm 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: yjk@yonsei.ac.kr (Y.-J Kim).
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Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys
Trang 2Fig 1 SEM images of the sensing films.
Fig 2 AFM images of the sensing films.
Fig 3 XRD patterns of undoped TiO 2 , 11 wt.% WO 3 doped TiO 2 and 15 wt.% WO 3 doped TiO 2
Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys
Trang 3To characterize the fabricated sensor, it was used to detect NO2
and other oxidizing gases (CO, CO2and O2) The sensor was
oper-ated at 600 °C The ratio of the WO3doping into TiO2was varied
among 0 (undoped), 11 and 15 wt.%
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
WO3doping did not induce any change
AFM micrographs of the undoped, and 11 wt.% and 15 wt.% WO3
doped TiO2films are shown inFig 2 The AFM micrographs
con-firmed that the WO3doping 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 inFigs 3 and 4 Although, the WO3doping
ef-fect on the surface topography was not clearly evident, the inner
morphological properties of the sensing film were affected by the
WO3doping.Fig 3revealed that the sensing films consisted of
al-most a single phase of rutile TiO2, whileFig 4revealed the (1 1 0)
peak of the rutile TiO2of each film The peaks became broader and
lower with increasing amount of doped WO3 The grain sizes of the
sensing films were calculated by the Scherrer equation(1)
In this study, we used TiO2 nanopowder with a particle size of
20 nm After heat treatment, the grain sizes of the undoped, and
11 wt.% and 15 wt.% WO3 doped TiO2 films were 102.2 nm,
68.1 nm and 62.9 nm, respectively Following the heat treatment,
the grain sizes of the fabricated sensing films increased But, WO3
doping disturbed the grain growth of the sensing films That is,
the WO3 doping clearly affected the grain growth of the TiO2
particles
Fig 5shows the results of NO2(500 ppm) detection at 600 °C
The undoped and two WO3 doped TiO2 sensors exhibited a gas
sensing property similar to that of a p-type, semiconductor gas
sensor The sensitivity to gases is defined as (Ra Rg)/Rg, where
Raand Rg are the steady state resistances of the device in null
(N2) and oxidizing gas, respectively The two WO3doped TiO2
sen-sors had a higher sensitivity to NO2than that of a pure TiO2sensor
Generally, the sensing film with a smaller grain size has a larger
surface area of the sensing film and higher sensitivity[1] There-fore, at 600 °C, the 15 wt.% WO3doped TiO2sensor had the highest sensitivity (145.4) due to its relatively small grain size
Due to its highest sensitivity to NO2, the 15 wt.% WO3 doped TiO2sensor was used to verify the selectivity by detecting three other individual oxidizing gases, CO, O2and CO2, at 600 °C, and a gas mixture (NO2+ oxidizing gases) with CO, O2and CO2 concen-trations of 1000 ppm, 15% and 10%, respectively The detection re-sults of the fabricated sensor for the various oxidizing gases and the gas mixture are shown inFigs 6 and 7 InFig 6, sensitivities
of the sensor (15 wt.% WO3doped TiO2) to CO, O2and CO2were very small And inFig 7, the resistance of the sensor decreased very fast in the NO2condition but exhibited very small changes
in the gas mixtures (NO2+ other oxidizing gases) These sensing re-sults demonstrated the high sensitivity and good selectivity to NO2
of the WO3doped TiO2sensor
The potential energy barrier eVSbetween particles of the sensor
is proportional to Nt,
2Nt
where Ntis the surface density of adsorbed oxygen ions such as O,
O2, O
2 Andere0is the permittivity of the semiconductor, and Nd 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.
Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys
Trang 4dizing gases such as NO2are reacted with the oxygen species at the
sensor surface as shown in Eq.(3)
The presence of other metal oxides (WO3) in the lattice of the
TiO2film can cause a change of its electrical property and the inner
morphological properties of the TiO2film, which made the smaller
particle size of the sensor It consequently affected the area of the
sensor surface which related to the number of adsorbed oxygen
ions and the gas sensing properties
Also, we assumed that the high heat treatment temperature of
the sensor induced an electrical interaction between WO3 and
TiO2, which in turn affected the electrical properties of the sensing
film The WO3 may occupy the interstitial sites in TiO2 lattice
according to the following:
WO3TiO!2W00
Since the predominant defect is an oxygen vacancy, this equilibrium
can be described by Eq.(5),
OxO!1
2O2ðgÞ þ 2e
0þ V00
where V00
Ois the oxygen vacancy and OxOis neutral oxygen in lattice
That is, WO3doping induces an increase of the oxygen partial
pres-sure at the sensor surface So, at high working temperature such as
600 °C, there is large number of adsorbed oxygen ions at the
sur-face And it makes the high surface density of adsorbed oxygen ions
and high sensitivity to specific gas such as NO2
4 Summary
A WO3 doped TiO2 sensor, with a sensing film deposited by
screen printing, was fabricated successfully The use of rutile
TiO2 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 WO3doping did not exert a dominant effect on the surface topography but it did af-fect the inner morphological properties of the TiO2 film The
15 wt.% WO3 doped TiO2 sensor showed the best sensitivity to
NO2at 600 °C The proposed sensor had good sensitivity and selec-tivity to NO2, in comparison with its sensitivity to other oxidizing gases such as CO, O2, and CO2
Acknowledgements This work was supported in part by the Information Technology Research Center support program (IITA-2005-C1090-0592-0012), and in part by Korea Science and Engineering Foundation (KOSEF) Grant funded by the Korea government (MEST) (2008-8-1253)
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Fig 7 The results of gas mixture detection at 600 °C.
Please cite this article in press as: S.-E Jo et al., Gas sensing properties of WO doped rutile TiO thick film at high operating temperature, Curr Appl Phys