Sensors and Actuators B 111–112 (2005) 45–51
Gas sensing properties of nanoparticle indium-doped WO
3
thick films
V. Khatko
a,∗
, E. Llobet
a
, X. Vilanova
a
, J. Brezmes
a
, J. Hubalek
b
, K. Malysz
b
, X. Correig
a
a
Department d’ Enginyeria Electronica, Campus Sescelades, Universitat Rovira i Virgili, 43007 Tarragona, Spain
b
Department of Microelectronics, Technical University of Brno, 60200 Brno, Czech Republic
Available online 11 August 2005
Abstract
The gas sensing properties of pure and indium-doped nanoparticle WO
3
thick films were studied. Sensors were prepared using commercial
WO
3
nanopowders and powder mixtures with different concentrations of In (1.5, 3.0 and 5.0wt.%). The gas sensing properties of the sensors
to nitrogen dioxide, carbon monoxide, ammonia and ethanol were investigated. It was observed that the pure nanoparticle WO

oxide sensors [7,8]. Additionally, the inclusion of differ-
ent doping metals in the sensing films has been shown to
increase their sensitivity to specific gases. In very recent
papers, the effects of doping either with In or indium oxide
on the response of tin oxide to hydrogen [9,10], methanol and
carbon monoxide [11] have been investigated. An important
reduction in the operating temperature of the sensors for opti-
mal sensitivity to these species has been reported. The aim
of this work is to study the sensing properties of nanoparticle
WO
3
gas sensors doped with In. A mechanism of response
to NO
2
and CO will be presented and discussed.
2. Experimental
Sensors were fabricated by screen-printing onto alumina
substrates. A heating element and a temperature sensitive
∗
Corresponding author. Tel.: +34 977558653; fax: +34 977559605.
E-mail address: (V. Khatko).
meander on the backside substrate and interdigited gold elec-
trodes on the front side of substrate were prepared by using
commercial platinum (Heraeus C3657) and conductive (ESL
8884) pastes, according to the sensor fabrication procedures
reported in [12]. The commercial WO
3
nanopowder (Aldrich
55,008-6) with calculated spherical diameter up to 33.1 nm
was mixed with InCl

the presence of either pure air (R
air
) or the differentpollutants
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2005.06.060
46 V. Khatko et al. / Sensors and Actuators B 111–112 (2005) 45–51
(R
gas
) at different concentrations was monitored and stored
in a PC.
The morphology of the sensing layers was determined by
JSM 6400 scanning electron microscope. In order to obtain
high-resolution SEM pictures and chemical element distri-
bution, the samples were coated with ultra thin gold and
carbon layers, respectively. The phase composition of the
sensing layers was determined by X-ray diffraction (XRD)
measurements using a Siemens D5000 diffractometer oper-
ated at 40 kV and 30mA with Cu K␣ radiation.
3. Results and discussion
3.1. Gas sensitivity studies
For the first set of experiments, the sensing layers based
on pure tungsten oxide nanopowders were used. The WO
3
sensors were operated in a working temperature range from
room temperature to 350
◦
C. The gas sensing properties of
the sensors to nitrogen dioxide and carbon monoxide were
investigated. WO
3

the response of the former is much faster than the response
of the latter. At working temperatures higher than 150
◦
C the
sensor resistance increases monotonously (Fig. 1d) and the
sensitivity to nitrogen dioxide decreases.
In a second set of experiments, the level of In doping
in the response of tungsten oxide based sensors to nitrogen
dioxide and carbon monoxide was investigated. The sensors
were operated in a temperature range from 150 to 350
◦
C.
Figs. 2 and 3 show the effects of indium concentration in the
detection of nitrogen dioxide and carbon monoxide, respec-
Table 1
Responsiveness of pure nanopowder WO
3
sensing layers to NO
2
(1 ppm) as
a function of the working temperature and thick film composition
Thick film composition Working temperature
Room temperature 100 (
◦
C) 150 (
◦
C)
WO
1
3

), 3.0 wt.% (WIn
2
) and 5.0 wt.%
(WIn
3
), operated at 150
◦
C (a) and 200
◦
C (b).
tively. The In-doped WO
3
sensors were more sensitive to
NO
2
when operated at 200
◦
C and more sensitive to CO
when operated at 300
◦
C. The sensors showed the highest
responsiveness to NO
2
when indium concentration was set at
3.0 wt.%. When operated at 200
◦
C in the presence of 1ppm
of NO
2
, the responsiveness of the In-doped tungsten oxide

sensors with
indium concentrations of 1.5 wt.% (WIn
1
), 3.0 wt.% (WIn
2
) and 5.0 wt.%
(WIn
3
), operated at 250
◦
C (a) and 300
◦
C (b).
Fig. 4. Responses to ethanol of indium-doped WO
3
sensors with indium
concentrations of 1.5 wt.% (WIn
1
), 3.0 wt.% (WIn
2
) and 5.0 wt.% (WIn
3
),
operated at 300
◦
C.
48 V. Khatko et al. / Sensors and Actuators B 111–112 (2005) 45–51
Table 2
Responsiveness of In doped WO
3

C. The sensor response to ammonia follows an
involved pattern (see Fig. 5). The resistance of the different
sensing layers increased when 10 or 100 ppm of ammonia
were injected into the test chamber, and decreased sharply
when ammonia concentration was increased to 500ppm.
3.2. Structural characterisation
SEM and XRD investigations were used to explain the
experimental results obtained. Fig. 6 shows the surface
(Fig. 6a) and the morphology of the pure WO
3
thick film
(Fig. 6b) and In doped (5 wt.%) WO
3
thick film (Fig. 6c)
sensing layers. It can be seen that the film surface (Fig. 6a)
shows a large quantity of cracks. It is assumed that crack for-
mation is related to the high (up to 80
◦
C/min) temperature
increase rate at the initial stage of firing. The average gran-
ule size in the pure WO
3
sensing layers was up to 60 nm
(Fig. 6b). The morphology of the In doped sensing layer
did not depend on indium concentration. The tungsten oxide
layers were nanoparticular with average granule size around
Fig. 5. Responses to ammonia of indium-doped WO
3
sensors with indium
concentrations of 1.5 wt.% (WIn

V. Khatko et al. / Sensors and Actuators B 111–112 (2005) 45–51 49
Fig. 7. XRD patterns fromWO
3
nanopowders,and pure (WO
1
3
) and indium-
doped WO
3
thick films (WIn
2
and WIn
3
).
eters a = 7.3008, b =7.5389, c = 7.6896, β = 90.892
◦
and
a =5.2771, b=5.1569, c= 7.666, β = 91.742
◦
, respectively.
Fig. 7 presents the XRD patterns from 2θ =22
◦
to 25
◦
of
WO
3
nanopowder, both pure and In-doped WO
3
thick films.

thick films (wt.%)
Chemical elements
O In W In/W (%)
1.5 15.81 1.11 83.08 1.34
3.0 14.54 2.08 83.39 2.49
5.0 14.03 3.81 83.43 4.57
based on the Scherrer equation [13].
D
hkl
=
0.9λ
β
hkl
cos(θ
hkl
)
(1)
where λ is wavelength of the incident radiation, β
hkl
the full
width at half-maximum(FWHM)of the peak in the radiation,
and θ
hkl
the Bragg angle.Theaverage crystallitesizeofWO
3
nanopowder and pure WO
3
thick film is 30.4 and 45.5 nm,
respectively. In In-doped WO
3

traps for electrons.
The sensing properties of pure WO
3
thick films are con-
trolled by their surface defects and structure. A probable rea-
son for the high responsiveness to nitrogen dioxide shown by
pure tungsten oxide films at 100
◦
C is the desorption of O
2
−
[14]species.NO
2
replacesthedesorbed oxygenspeciesat the
surface of the WO
3
thick films, which causes a sharp growth
in their resistance. Increasing the working temperature of the
sensorschangesthe equilibriumof the adsorption–desorption
reaction with NO
2
. The decrease in the number of active
adsorption sites when the temperature of the sensor is raised
causes a reduction of the sensor response to NO
2
at work-
ing temperatures higher than 100
◦
C. The addition of glass
frit increases sensor sensitivity because of the presence of

. These energetic levels are the source of added
support of charge that changes the sensing properties of the
WO
3
-based sensors.
4. Conclusions
The gas sensing properties of pure and indium-doped
nanoparticle WO
3
thick films were studied. Sensors were
prepared using commercial WO
3
nanopowders and pow-
der mixtures with different concentrations of In (1.5, 3.0
and 5.0wt.%). Their response to different concentrations of
nitrogen dioxide, carbon monoxide, ammonia and ethanol
were investigated. It was found that the pure WO
3
sensors
responded to nitrogen dioxide even when operated at room
temperature. These sensors showed a maximum sensitivity
to NO
2
when working at 100
◦
C. The fact that pure WO
3
sensing layers show response to NO
2
at low temperatures is

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Biographies
Viacheslav Khatko graduated in nuclear physics from the Byelorussian
State University (Minsk, Belarus) in 1971. He received his PhD in mate-
rial science in 1985 and Dr.Sc. in electronic engineering in 2001. In
1975–2003 he worked at the Physical Technical Institute of National
Academy of Sciences of Belarus, Minsk, as a researcher, head of the
V. Khatko et al. / Sensors and Actuators B 111–112 (2005) 45–51 51
Laboratory of Electronic Engineering Materials, head of the Thin Film
Materials Department and then as Principal Investigator of the insti-
tute. He was Ford SABIT Intern and Ford Visiting Scientist in 1998
and 1999, respectively. From April 2003 he is Ram
´
on y Cajal profes-
sor in the Electronic Engineering Department at the Universitat Rovira
i Virgili (Tarragona, Spain). His current research interests include the
development and application of semiconductor thin and thick film gas
sensors.
Eduard Llobet graduated in telecommunication engineering from the
Universitat Polit
`
ecnica de Catalunya (UPC), (Barcelona, Spain) in 1991,
and received his PhD in 1997 from the same university. During 1998,
he was a visiting fellow at the School of Engineering, University of
Warwick (UK). He is currently an associate professor in the Electronic
Engineering Department at the Universitat Rovira i Virgili (Tarragona,
Spain). His main areas of interest are in the fabrication, and modelling,
of semiconductor chemical sensors and in the application of intelligent
systems to complex odour analysis. Dr. Llobet is a member of the Institute

dubice, faculty of Chemistry and Chemical Technology in 2000. Since
2000 he has been a PhD student at the Department of Microelectronics,
Brno University of Technology (Brno, Czech republic). His work focuses
on the preparation of new types of semiconductor gas sensors and gas
sensing materials.
Xavier Correig graduated in telecommunication engineering from the
Universitat Polit
`
ecnica de Catalunya (UPC), (Barcelona, Spain) in 1984,
and received his PhD in 1988 from the same university. He is a full
professor of Electronic Technology in the Electronic Engineering Depart-
ment at the Universitat Rovira i Virgili (Tarragona, Spain). His research
interests include heterojunction semiconductor devices and solid-state gas
sensors. Dr. Correig is a member of the Institute of Electrical and Elec-
tronic Engineers.