Sensors and Actuators B 120 (2006) 338–345
Review
Ethanol and ozone sensing characteristics of WO
3
based
sensors activated by Au and Pd
A. Labidi
a,b
, E. Gillet
a
, R. Delamare
a
, M. Maaref
b
, K. Aguir
a,∗
a
L2MP (CNRS UMR 6137), Service 152, FST St J´erˆome, Universit´e Paul CEZANNE Aix-Marseille III, 13397 Marseille Cedex 20, France
b
Unit´e de Recherche de Physique des Semiconducteurs et Capteurs, IPEST, BP 51 La Marsa 2070, Tunis, Tunisia
Received 29 November 2005; received in revised form 7 February 2006; accepted 7 February 2006
Available online 29 March 2006
Abstract
The sensitivity towards ethanol (C
2
H
6
O) and ozone (O
3
)ofWO
3

3. Results and discussion 340
3.1. DC measurements 340
3.2. AC measurements 341
4. Conclusion 343
References 344
Biographies 344
1. Introduction
Numerous metal oxide semiconductor materials were
reported to be usable in conductometric gas sensors, such as
ZnO, SnO
2
,WO
3
,TiO
2
, ␣-Fe
2
O
3
and so on. These candidates
have non-stoichiometric structures, so free electrons originating
∗
Corresponding author. Tel.: +33 4 91 28 89 73; fax: +33 4 91 28 89 70.
E-mail address: [email protected] (K. Aguir).
from oxygen vacancies contribute to electronic conductivity
when the composition of the surrounding atmosphere is altered
[1–6].
Actually WO
3
is one of promising material for gas sensor

In addition to the previous cited strategies, the modification
of the metal oxide active layer by adding small amount of noble
metals has been recently emerged as a promising way for the
improvement of sensors selectivity. The metals that have been
used frequently as surface dopants for these purposes are Au,
Pt, Pd and Ag [19–23].
The main objective of our work was the understanding of
the mechanisms which explain the activation of WO
3
thin
film sensors by noble metals. For that, two metals have been
chosen Pd and Au, and their effect on the sensor sensitivity
towards ozone and ethanol was studied. The DC transient and
AC responses were analyzed in order to discriminate the sensor
parts which are crucial for the sensing mechanisms (grains–grain
boundaries–metal/oxide interfaces ). In order to avoid bulk
parameters variation the tests were carried out at the same tem-
perature T
work
= 300
◦
C. So the conductance changes that we
measured could be attributed to surface or near surface phe-
nomena.
2. Experimental
2.1. Sensing device preparation
The WO
3
sensitive layer has been grown by vapor deposi-
tion at room temperature on SiO

The morphology of each sensor was controlled by AFM
(Nanoscope III-Digital Instruments) and SEM (Phillips XL30
S5). Fig. 1 is an image of the bare WO
3
layer, which is formed
from small grains with a mean diameter of 40 nm. The mean
roughness calculated in a 1.5 ␮m ×1.5 ␮m area is 0.598 nm, it
remains the same after some hours of working time. It was dif-
ficult to distinguish by AFM the metals particles from the oxide
grains, so we have analyzed the activated layers by SEM. Fig. 2a
and b presents micrographs of Au/WO
3
and Pd/WO
3
surfaces,
respectively. The mean diameter of gold particles is 5 nm, the
population is homogeneous with a 6 ×10
11
cm
−2
number in
density. The size of palladium particles is larger (9 nm) with
a4×10
11
cm
−2
number in density. The three samples were
observed after the sensing tests, no changes were visible in the
metal layers as long as the working temperature (T
work

6
O vapor in dry air was achieved using a two-arm
gas-flow device. Two mass flow controllers allowed the flow rate
of the dry air that act as the carrier gas to be controlled from 0 to
94lh
−1
in one arm (d
1
) in which the carrier gas passed through
a balloon flask containing the vapor equilibrated with 200 cm
3
of the liquid, and from 0 to 94 l h
−1
in the other arm (d
2
). The
balloon flask is put into a furnace and maintained at the fixed
temperature T
vap
=30
◦
C, in order to fix the partial pressure of
the C
2
H
6
O vapor.
In these conditions, a range of concentrations of the C
2
H

the atmospheric pressure. By varying d
1
and d
2
(d
1
+ d
2
was kept constant at 50 l h
−1
), different concentration
values for C
2
H
6
O in dry air can be obtained.
For the test under O
3
the two mass flow controllers used for
C
2
H
6
O vapor will be turned off. Once this condition is satisfied,
the O
3
gas was generated by oxidizing oxygen molecules of a
dry air controlled from 0 to 50 l h
−1
in arm d

6
O and 0.8 ppm of O
3
.
3. Results and discussion
3.1. DC measurements
In Table 1 are reported the conductances G
0
in dry air and
G
gas
under the target gases, for the sensors WO
3
bare, Au/WO
3
and Pd/WO
3
at T
work
= 300
◦
C, and resulting sensing response
“S
gas
” of the sensors calculated by using the relations:
S
gas
=
G
gas

0
(WO
3
)=5G
0
(Pd) and G
0
(Au) = 100G
0
(Pd). This indicates that
the activation mode of Au and Pd are different.
The transient responses of the three sensors towards pulses of
2% C
2
H
6
O and 0.8 ppm O
3
at 300
◦
C are compared in Fig. 4a and
b, respectively. The exposure time was kept constant at 15 min
for each test and the time between successive pulses was also
15 min. As expected under C
2
H
6
O (reducing gas) the conduc-
tance increases and it is clear that the Au/WO
3


−1
) G
Ozone
(×10
−9

−1
) S
Ethanol
S
Ozone
WO
3
bare 6.72 4.3 5.5 36 7.6
Au/WO
3
104 180 6.5 177 78
Pd/WO
3
1.5 0.025 11 0.76 0.15
S was calculated by using relations (3).
the largest response magnitude when bare WO
3
has the smaller
response and recovery times.
The kinetics of the responses to O
3
on bare WO
3

origin of conductivity changes on activated sensors.
3.2. AC measurements
The impedance sensors response versus frequency was stud-
ied in the AC impedance spectrum following the analysis proce-
dure that was established in our previous study [27]. The WO
3
modified sensitive layers were modeled by a serial association of
three parallel RC circuits, attributed to grains (b), grains bound-
aries (gb) and grains–electrodes interfaces (el). Each RC circuit
rises to a semicircle in the complex plan plot of Z

(ω) versus
Z

(ω) (Nyquist diagram). Under small amplitudes of sinusoidal
signal, the total impedance of sensor is given by:
Z
Total
(jω) =

i
Z
i
(jω) (4)
Z
i
(jω) = Z

i
(ω) + jZ

The results of modeling for C
2
H
6
O and O
3
were reported in
Tables 2 and 3, respectively; they confirm the DC analyses,
the best response was obtained by the sensor doped by gold
(Au/WO
3
) which became practically conductor under ethanol.
The RC modeling show the existence of two semicircles
under dry air, contrary to the sensor without metals (WO
3
bare)
that gives only one semicircle either under dry air or C
2
H
6
O.
The first semicircle is attributed to WO
3
surface and bulk phe-
nomena, the second one could be attributed probably to the
carrier exchanges in the Au/WO
3
grains boundaries, because
under dry air this circle appears only when Au is added to the
WO

−
(6)
CH
3
CHO
(ads)
+O
(lattice)
↔ CH
3
COOH
(vap)
+V
O
(7)
The electrons produced by this reaction are injected into the con-
duction band of WO
3
, which induces a decrease of the resistance.
The response to the C
2
H
6
O is improved for sensor (Au/WO
3
),
342 A. Labidi et al. / Sensors and Actuators B 120 (2006) 338–345
Fig. 5. Impedance measurements (symbols) and modeling (lines) at 300
◦
C. (a) 2% of C

3
+Au
a
WO
3
+Pd
a
Dry air
b
Ethanol
b
Dry air
b
Ethanol
b
Dry air
b
Ethanol
b
R
s-b
(×10
6
) 15.77 0.2 1.46 0.005 59.17 37.23
C
b
(×10
−10
F) 1.65 1.87 1.72 1.22 1.65 1.78
n 0.993 0.986 0.991 0.971 0.986 0.975

WO
3
a
WO
3
+Au
a
WO
3
+Pd
a
Dry air
b
O
3
b
Dry air
b
O
3
b
Dry air
b
O
3
b
R
s-b
(×10
6

and CeO
2
[30–32].
Under O
3
the sensor Au/WO
3
gives also the best response
with a more pronounced grain boundaries effect. The transient
responses of Fig. 4b, suggested that one of the two steps of the
reaction (8) resulting of the O
3
dissociation is enhanced by the
presence of the metal particles [33–35].
p
2
O
2
+∗↔O
p(ads)
adsorption step
O
p(ads)
+ qe ↔ O
q−
p(ads)
electron transfer step
(8)
The AC analysis evidenced that it is the electron transfer step
which is improved, in effect when O

gas. The origin of such a behavior should be the adverse
effect of the PdO formation on the free carrier density.
4. Conclusion
Au has been found a good sensing activator for WO
3
thin
films. The sensitivities of Au/WO
3
sensors to ethanol and ozone
are in the 2/1 ratio; therefore, at a working point of 300
◦
C they
can provide a stable, sensitive element for ethanol gas. On the
contrary Pd/WO
3
sensors are practically insensitive in this tem-
perature range to the tested gases and in these senses could be
used as selective elements against ozone. The characteristics
of the dynamic responses of the activated WO
3
thin films sug-
gest complex phenomena which depend on the strength of the
metal–substrate interaction and consequently could be induced
by the formation of oxide or bimetallic species on the metal par-
ticles. In the actual knowledge state in the behavior of doped
344 A. Labidi et al. / Sensors and Actuators B 120 (2006) 338–345
oxide layers one needs an understanding of activation processes
at an atomic level if one want to progress in the design of
predictable sensing properties. In this way we have initiated a
research by XPS and synchrotron radiation photoelectron spec-

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¨

at Paul C
´
ezanne University – Aix-Marseille (France). She worked in the area
of Surface Science. In particular she studied chemisorption on transition metal
nanoparticles (model catalysts). Actually she is involved in a research devoted to
the electrical properties of nanostructured metal oxide semiconductor thin films
and nanorods for applications to new sensing devices.
Romain Delamare, was born in 1973. He is professor assistant at Paul
CEZANNE, Aix Marseille III University (France). He was awarded his PhD
degree in semiconductors physic from University of Orl
´
eans (France) in 2003.
His principal research interests are now directed towards WO
3
gas sensors and
selectivity enhancement strategies including noise spectroscopy and modelling
of sensor responses.
A. Labidi et al. / Sensors and Actuators B 120 (2006) 338–345 345
Mhamed Ali Maaref, was born in 1955. He is professor in Solid State Physics
at the Engineering Institute of Tunis (INSAT) in 2000. He was awarded his
Doctorat d’Etat in semiconductors from University of Tunis (Tunisia) in 1994.
He is head of research group in Semiconductor Physics and Sensors at the
Institute of Science and Technology (IPEST) University 7 Novembre (Tunisia).
His principal research activities are involved on III/V semiconductors materials:
Study by reflectivity and luminescence of GaAs/AlAs Super-lattices, Quantum
Wells and self organized InAs/GaAs Quantum dots elaborated by MBE.
Khalifa aguir, was born in 1953. He is professor at Paul CEZANNE – Aix Mar-
seille III University (France). He was awarded his Doctorat d’Etat
`
es Science