Electrical and gas-sensing properties of WO
3
semiconductor material
Wang Yu-De
*
, Chen Zhan-Xian, Li Yan-Feng, Zhou Zhen-Lai, Wu Xing-Hui
Department of Materials Science and Engineering, Yunnan University, Kunming 650091, People's Republic of China
Received 28 November 2000; received in revised form 22 January 2001; accepted 19 February 2001
Abstract
In this paper, the electrical and gas-sensing properties of calcined tungsten trioxide semiconductor materials were
investigated. X-ray diraction, scan electron microscopy and infrared were used to characterize structure and perfor-
mance of WO
3
semiconductor material. The average grain size of WO
3
was 22 nm after calcination at less than 800°C
and 24±26 nm at more than 900°C for 1 h. The sensors of indirect heating type were fabricated. The eects of calcining
temperature and operating temperature on electrical resistance and sensitivity, and sensitivity-gas concentration
properties of the WO
3
-based sensors were investigated. The sensor based on WO
3
exhibited high sensitivity and good
response characteristics to ethanol gas. The electrical properties of WO
3
were analyzed and the sensitive mechanism was
discussed. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: WO
3
; Gas-sensing properties; Calcination temperature; Operating temperature; Sensitivity
1. Introduction

gas sensor was ®rst reported for detection of H
2
by
Shaver [5], who showed that the conductivity of WO
3
®lms changed greatly upon the exposure to the H
2
am-
bient. Following this pioneering work, many works have
been performed on the structural, electrical properties
and sensing characteristics of WO
3
®lms. It was dem-
onstrated by dierent authors that WO
3
-based thin and
thick ®lms were both sensitive to NO
x
gas [1,4,6±10]. It
has been reported that WO
3
materials have good sensi-
tivity for low concentration of NO
x
gas [6]. However,
most reports focused on the NO
x
gas sensors, and the
study of sensing to other gases was rare. In this paper,
we have investigated the electrical and gas-sensing

samples were analyzed
using X-ray diraction (XRD, Rigaku D/MAX-3B
powder diractometer). In order to obtain high resolu-
tion and to minimize the signal to noise ratio, we have
performed the measurements with ®xed slits. The mean
crystallite sizes (D) were measured from XRD peaks that
were obtained at a scan rate of 2° min
À1
. D is based on
the Scherrer's equation: D  k=DW cos h. Where k is
the wavelength of X-ray (k  1:5418

A), h the Bragg's
diraction angle, and DW the true half-peak width. The
microstructure of powder was characterized by scan
electron microscopy (SEM, CSM950). The conductance
type of the WO
3
was measured with the hot probe.
2.3. Fabrication of sensor elements
In order to prepare series of sensors, we have chosen
the indirect heating type as the structure of sensor. The
sensor were fabricated according to the literature [11].
WO
3
semiconductor materials with SiO
2
(4 wt.%) were
fabricated on an alumina tube with Au electrodes and
Platinum wires. The SiO

a
) to that in gas (R
g
).
3. Results
3.1. Structure of samples
The average grain size of WO
3
was 22 nm after cal-
cination at less than 800°C and 24±26 nm at more than
900°C for 1 h. The X-ray powder diraction patterns of
the WO
3
powder calcined at 500°C are indexed as
monoclinic and triclinic WO
3
compounds, which have
shown high degree of crystallinity (Fig. 1). Their particle
sizes based on Scherrer's equation are 21 nm. The av-
erage particle sizes of SEM show consistency with the
results of XRD. With the increasing of calcining tem-
perature, WO
3
powder was crystallized with sharp
peaks, the size of grains and macro pores gradually in-
creased. According to the examined results by the hot
probe, WO
3
is n type semiconductive material. This re-
sult is in good accordance with the literature [4].

,
O
À
ads
,OH
À
ads
and O
2À
ads
. However, above 300°C their in-
trinsic defects, such as oxygen vacancies are responsible
for the conductance of the sensor. Generally, a higher
sintering temperature is needed during the fabrication of
gas-sensing elements, and gas sensors have to operate in
the temperature range from 200°C to 400°C for a long
time [15]. So it is important for gas sensor to have good
thermal stability. In Fig. 3, it can be seen that the eect
of operating temperature from 175°C to 225°C on re-
sistance of WO
3
sensor is smaller than that of the other
temperatures. So that sensor's resistance changed little
in this temperature range, and the sensor based on WO
3
has good thermal stability when their operating tem-
peratures are in this range. This thermal stability is of
signi®cance to apply the sensors to certain control and
monitoring.
3.3. Gas-sensing properties

temperature in ambient humidity air.
Fig. 4. The in¯uence of calcinations temperature on the sensi-
tivity gas concentration  100 ppm).
Fig. 5. The in¯uence of operating temperature on the sensi-
tivity of the sensor for ethanol and petrol 100 ppm, and for
butane and methane 1000 ppm.
W. Yu-De et al. / Solid-State Electronics 45 (2001) 639±644 641
sample gases. The maximum sensitivity to 100 ppm
ethanol gas occur at 80°C is about 40. However, sensi-
tivities to ethanol is reduced with the increasing of op-
erating temperature in the range of 80±200°C. On the
other hand, the sensitivity of sensor to other gases such
as petrol, butane and methane is very low, though gas
concentration is 1000 ppm.
Fig. 6 shows the relationship between sensitivity and
sample gases concentration for sensor operating at
200°C (because sensor has good thermal stability at this
temperature). When the sensor is operated at 200°C, the
sensitivity exhibits a good dependence on ethanol gas
concentration. Long time stability of the WO
3
sensor in
the whole investigated time rang is shown in Fig. 7.
4. Discussion
4.1. Electrical properties
The oxygen adsorbed on the surface of the material
in¯uences the conductance of the WO
3
-based sensor.
The oxygen adsorbed depends on the particle size, large

2À
ads
The oxygen species capture electrons from the ma-
terial, leading to increasing of the hole concentration
and decreasing of the electron concentration. WO
3
is a
kind of the acidic oxide and can react with the alkali.
Besides the state of oxygen adsorbed on the surface of
WO
3
, there is OH
À
that comes from water. The exis-
tence form of chemisorbed water on WO
3
is more
complicated. The reaction can be summarized as
W
lat
 H
2
O 6W
lat
À OH
À
H

ads
where W

chemisorbed water desorbs and transfers to O
À
ads
and
O
2À
ads
. The resistance starts to go up in the temperature
ranging from 250°C to 300°C, which may be attributed
to such electron depletive type mechanisms [15].
4.2. Gas-sensing mechanism
The gas-sensing mechanism is based on the changes
in the conductance of WO
3
. The reducing gas reacted
with oxygen adsorbed on the surface of the sensor and
the possibility of the reaction between reducing gas and
lattice oxygen was very small. The reducing gas acting
on the WO
3
sensor surface can be explained as [18]:
R  O
À
ads
6 RO  e
À
To maintain neutrality, the electrons release back WO
3
material, resulting in the increase of the electron con-
Fig. 6. The eects of gas concentration on the sensitivity of

.
The possible process of the reaction can be explained as
follows:
where O±CH
2
±CH
3
is negatively charged and H

ads
is
activated.
2H

ads
 O
À
ads
3 H
2
O
gas
H

ads
 OH
À
ads
3 H
2

can interact to produce ethyl ether:
Fig. 8. IR spectrum of ethanol after reacting with WO
3
material at (a) 145°C, (b)200°C, (c) 250°C, (d) 295°C, (e) 350°C, (f) 407°C, (g)
455°C, (h) 500°C.
W. Yu-De et al. / Solid-State Electronics 45 (2001) 639±644 643
All these reactions release electrons into the WO
3
ma-
terial, leading to the increase of the electron concentra-
tion, and the decrease of the resistance of WO
3
-based
sensor. This result is in good accordance with the above
analysis.
5. Conclusions
Monoclinic and triclinic WO
3
compounds can be
used as a gas-sensing material for ethanol gas. The
analysis of the electrical properties and gas-sensing
mechanism of WO
3
-based sensors revealed that the
calcining and operating temperature of sensor obviously
in¯uence on the resistance change and gas sensitive
characteristics of the WO
3
sensor. At calcining temper-
ature of 500°C and operating temperature of 200°C, the

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