Sensors and Actuators B 115 (2006) 297–302
Structural and gas sensing properties of nanocrystalline
TiO
2
:WO
3
-based hydrogen sensors
G.N. Chaudhari
a,∗
, A.M. Bende
a
, A.B. Bodade
a
, S.S. Patil
a
, V.S. Sapkal
b
a
Gas Sensor and Thin Films Laboratory, Department of Chemistry, Shri Shivaji Science College, Amravati, Maharashtra-444602, India
b
Department of Chemical Technology, Amravati University, Amravati, Maharashtra-444601, India
Received 18 March 2005; received in revised form 24 August 2005; accepted 20 September 2005
Available online 17 October 2005
Abstract
Nanocrystalline TiO
2
:WO
3
-based hydrogen sensors have been developed by using a sol-precipitation method. Structural and gas-sensing char-
acteristics were performed by using XRD and resistivity measurements. XRD of TiO
2
calcined at 600
◦
C for 6 h showed good crystalline quality
having a 54 nm grain size. Although these elements were rather slow in response rate, this could be improved significantly by loading the elements
further with a noble metal. The operating temperature of the sensors was optimized under different operating conditions. A TiO
2
:15 wt.% WO
3
sensor doped with 0.5 wt.% Pt showed the maximum response to H
2
with a good selectivity at an operating temperature of 200
◦
C. The response
time of the TiO
2
:15 wt.%WO
3
sensor decreased from about 20 min to 1 min with the Pt-loading. The effect of platinum was seen not only in
decreasing the response rate but also in increasing considerably the response to H
2
. The electronic interaction between the additive and the oxide
semiconductor is proposed to account for the sensitization effects.
© 2005 Elsevier B.V. All rights reserved.
Keywords: TiO
2
; WO
3
;H
2
gas sensor; Selectivity; Response time
1. Introduction
The growing demand of fast, accurate and low cost air quality
analysis techniques for domestic and industrial environmental
monitoring, automotive applications, air conditioning and sen-
sors networks is tailoring the research toward new materials and
techniques to solve the problems related to the commercial sen-
sors. Metal-oxide semiconducting layers are the most promising
conductometric chemical sensors among solid-state devices, due
to their low dimension, price and power consumption [1–12].
The sensing properties are based on reactions between semi-
conductor oxides and gases in the atmosphere. These reactions
produce changes in electrical properties of semiconductors.
There are many possible reactions; the most common reac-
tion that leads to changes in conductivity is the adsorption of
gases on the semiconductor surface. It is known that the resis-
tance of semiconducting films is strongly influenced by the
presence of oxidizing or reducing gases [13]. Oxygen in the
∗
Corresponding author. Tel.: +91 7212551060.
E-mail addresses: gnc4@indiatimes.com, cgnroa@yahoo.com
(G.N. Chaudhari).
atmosphere can be adsorbed on the semiconductor surface in
different species (O
2
-
,O
2-
and O
-
). The charge exchange
between the adsorbed molecules and the semiconductor oxide
layers modifies the energy barrier eV
s
for grain-to-grain current
percolation and in turn the electrical conductance of the layers.
Accordingly, the conductance of the layer can be expressed by
G = G
o
exp(-eV
s
/kT) where G
o
is a constant. The interaction
of an oxidizing (reducing) gas occurs with either the film sur-
face or the oxygen adsorbates. It results in donating (capturing)
electrons at the semiconductor surface, which leads to a positive
(negative) conductance variation for an n-type semiconductor.
The opposite behavior occurs for a p-type semiconductor. In
both cases, the variation in conductance, G, depends on gas
concentration [gas], according to the empirical formula [14].
|G|
G
= A[gas]
B
where A and B are constants determined by temperature, grain
size, film porosity and specifically gas adsorption. The effects
of the microstructure are well recognized, including the ratio
of surface area to volume, grain size and pore size of the metal
oxide particles as well as film thickness of the sensor. Lack of
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2005.09.014