RESEARCH ARTICLE
Copyright © 2008 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 8, 1–5, 2008
Hybrid Cross-Linked Polyaniline-WO
3
Nanocomposite
Thin Film for NO
x
Gas Sensing
Ajeet Kaushik
1 2
, Raju Khan
2
, Vinay Gupta
3
, B. D. Malhotra
2 ∗
, Sharif Ahmad
1 ∗
, and S. P. Singh
2
1
Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India
2
Biomolecular Electronics and Conducting Polymer Research Group, National Physical Laboratory,
New Delhi 110012, India
3
Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
Nanocomposite thin film of cross-linked polyaniline [CLPANI, derived from polyaniline and aniline
formaldehyde condensate (AFC)] and WO
3
has been fabricated using vacuum thermal evaporation
technique. X-ray diffraction pattern of as grown film shows broad reflections of polymer along with
mixed reflection of nonhydrated and hydrated WO
3
nanoparticles. The scanning electron microscopy
studies reveal the uniform dispersion of WO
3
nanoparticles in CLPANI network. The broadening of
the absorption band at 320 nm corresponding to PANI and AFC-WO
3
nanocomposite is attributed
to the cross-linking between the polymeric units. The NO
x
gas sensing characteristics of vacuum
deposited CLPANI-WO
3
thin film has been studied by measuring the change in resistance with
respect to time. The sensor has been operated at room temperature, which could extend the shelf
life of the sensor. The response time and recovery time of the sensor operated at room temperature
have been experimentally determined as 30 second and 11 minute respectively.
Keywords: Cross-Linked Polyaniline, Tungsten Oxide, Nanocomposite Thin Film, NO
2
Gas
Sensors.
1. INTRODUCTION
Harmful gases, such as nitrogen oxide, carbon monox-
ide and hydrocarbons are of serious concern because of
increasing atmospheric pollution as a result of industrial-
ization and increasing automobile density. The monitoring
of these gases is important to control their emission from
automobiles and industrial plants. The nitrogen oxides
(NO
x
) sensor that can be attached to the exhaust systems
of automobiles can play an important role in monitoring
the level of NO
x
.
1
Nitrogen oxides are involved in the for-
mation of ground-level ozone in atmosphere, acid rains,
and an active ingredient to global warming. They are also
involved in various nitration processes with biomolecules,
causing cancers and other diseases.
2
Among several oxides
of nitrogen involved in the air pollution, NO and NO
2
are
the most important in terms of both health risk and envi-
ronmental damage.
4
NO
2
is produced together with NO
by oxidation of nitrogenous compounds in fuel combus-
tion. Initially, in a combustion process the concentration
of NO dominates; NO
2
is formed during the reaction with
atmospheric oxygen close to the source of combustion
∗
Authors to whom correspondence should be addressed.
i.e., 2NO + O
2
→ 2NO
2
. The increase level of ozone in
the environment is also a serious concern and NO act as
a cofactor for the formation of ground level ozone, the
process of formation of ozone is governed by the reac-
tions NO
2
+ hv → NO + O and O + O
2
→ O
3
+ M Where,
M is third species taking part in the reaction that absorb
the energy from the formation of ozone molecules and hv
represents the photochemical reaction sunlight.
2 3
There-
fore, efficient sensors for detection of NO
x
in the ambient
are required.
A promising approach to develop efficient sensors is
to use novel materials based on semiconducting metal
oxides, conducting polymers and nanocomposites for gas
sensing applications.
4–7
Due to simple design, metal oxide
semiconductor sensors are generally prepared for sensing
application. Among the metal oxide, tungsten oxide is con-
sidered as one of the best materials for NO
x
sensing due to
its good selectivity to low concentration NO
x
gases. The
sensing mechanism lies in the changes in the resistance
of WO
3
films resulting from physisorption, chemisorption
and catalytic reaction of gas phase species with the film
surface.
8–10
WO
3
thin film activated by noble metals cata-
lyst (Pt, Au and Ag) have been found to be more selective
J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 12 1533-4880/2008/8/001/005 doi:10.1166/jnn.2008.417 1