room temperature liquefied petroleum gas (lpg) sensor based on p - polyaniline n - tio2 heterojunction

Sensors and Actuators B 134 (2008) 988–992
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Room temperature liquefied petroleum gas (LPG) sensor based on
p-polyaniline/n-TiO
2
heterojunction
D.S. Dhawale, R.R. Salunkhe, U.M. Patil, K.V. Gurav, A.M. More, C.D. Lokhande
∗
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
article info
Article history:
Received 31 March 2008
Received in revised form 4 July 2008
Accepted 7 July 2008
Available online 16 July 2008
Keywords:
Thin films
TiO
2
Polyaniline
Heterojunction
LPG sensor
abstract
In the present work, we report on the performance of a room temperature (300 K) liquefied petroleum
gas (LPG) sensor based on a p-polyaniline/n-TiO
2
heterojunction. The heterojunction was fabricated
using electrochemically deposited polyaniline on chemically deposited TiO
2

on a stainless steel substrate.
Both the methods (chemical bath deposition and electrodeposition) are simple, inexpensive and suit-
able for large-scale production. TiO
2
and polyaniline films were characterized for their structural as well
as surface morphologies and LPG response was studied. The XRD analysis showed formation of poly-
crystalline TiO
2
while polyaniline exhibited amorphous nature. Morphological analysis using scanning
electron microscopy (SEM) of the junction cross-section revealed formation of a diffusion free inter-
face. The heterojunction showed the maximum response of 63% upon exposure to 0.1 vol% LPG at room
temperature.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Many studies on various materials as gas sensors have been
reported in recent years. Gas sensing materials can be classified
mainly into two types, namely, organic and inorganic materials.
Semiconductor inorganic gas sensors like doped or undoped SnO
2,
ZnO or Fe
2
O
3
have been well studied to detect most of reduc-
ing gases and they are considered interesting for their low cost
and simple sensing methods [1–7]. Nevertheless, there still exist
some problems with them, for example high working temperature
of 423–623 K for SnO
2
and 673–723 K for ZnO [8,9]. Heterojunc-

tion sensors are mostly based on the interface between p-type
(p) and n-type (n) semiconducting ceramics [10–13]. Hazardous
gases, specifically liquefied petroleum gas (LPG), have been widely
used for several industrial and domestic applications. But, at cer-
tain low concentration of the gases, these metal oxide sensors show
poor performance with respect to the sensitivity, long term sta-
bility, selectivity, etc. Recently, conducting polymers have been
widely investigated as effective materials for room temperature
chemical sensors. Polyaniline is one of the most attractive mate-
rials among the variety of conducting polymers due to its unique
electrical properties, environmental stability and easy fabrication
process. Due to its interesting properties, polyaniline has been a
potential candidate in sensor applications [14,15], light emitting
∗
Corresponding author. Tel.: +91 231 2609229; fax: +91 231 260933.
E-mail address: l
(C.D. Lokhande).
diodes [16], and rechargeable batteries [17]. However, the prob-
lems with these conducting polymers are their low processing
ability, poor chemical stability and mechanical strength [18].As
an option, there is a room to fabricate heterojunctions between
organic and inorganic materials with enhancement of the sensor
characteristics and mechanical strength. By using electrochemi-
cal polymerization, polyaniline and its nanocomposite have been
fabricated in a bulk form. Pd-polyaniline nanocomposite was pre-
pared for a methanol gas sensor [19]. Tai et al. [20] fabricated a
polyaniline–titanium dioxide nanocomposite for NH
3
and CO sen-
sors and reported that the resistance of the composite increased

with increasing concentration of the gases. Nicho et al. [21] devel-
oped a polyaniline composite sensor for low concentration of NH
3
gas. A ZnO/polyaniline layer-by-layer assembly and heterostruc-
tured polyaniline/Bi
2
Te
3
nanowires were fabricated by Paul et
al. [22] and Xu et al. [23], respectively. Recently, Joshi et al.
developed n-CdSe/p-polyaniline and n-CdTe/p-polyaniline hetero-
junctions for a room temperature LPG sensor [24,25].
Among the inorganic materials, nanocrystalline TiO
2
is one of
the most attractive and extensively used materials for detection
of H
2
,NH
3
,NO
2
and LPG gases [26,27]. However, due to the long-
term instability at elevated temperature, it is desirable to develop
sensors that operate at room temperature.
In the present work, for the first time, we report fabrication of
a p-polyaniline/n-TiO
2
heterojunction with a good rectifying ratio
by adopting a simple and inexpensive chemical route. Specifically,

a nanocrystalline TiO
2
thin film was deposited on a stainless
steel substrate by chemical bath deposition (CBD), followed by
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2008.07.003
D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992 989
Fig. 1. A schematic representation of a p-polyaniline/n-TiO
2
heterojunction.
polyaniline film by an electrodeposition (ED) method. These films
were characterized using XRD and SEM techniques. The sensing
performance at different concentrations of LPG (0.04–0.12 vol%)
was studied at room temperature (300 K) by current–voltage (I–V)
characteristics under the forward bias condition.
2. Experimental details
2.1. Fabrication of p-polyaniline/n-TiO
2
heterojunction
Preparation of a TiO
2
thin film by the CBD method is based on
the heating of an acidic solution of titanium (III) chloride contain-
ing a substrate immersed in it. The titanium (III) chloride solution
was mixed with double distilled water in appropriate quantities.
Specifically, 2.5ml of TiCl
3
(30 wt% in HCl, Loba Chemie, India) was
added to 50 ml of double distilled water. The pH of the solution
was adjusted to ∼1 using urea (NH

2
CONH
2
) while constantly stir-
ring at room temperature for 30 min. A stainless steel substrate was
immersed vertically in the above bath and the bath was heated. At
353 K, the precipitation was started in the bath. During the pre-
cipitation, heterogeneous reaction occurred and deposition of TiO
2
took place on the substrates. The substrate coated with TiO
2
thin
film were removed after 2 h, washed with double distilled water,
and dried in air. Further the film was annealed at 673 K for 2 h. The
deposited film was specularly reflecting, uniform andwelladherent
to the substrate.
For fabrication of a p-polyaniline/n-TiO
2
heterojunction, a
polyaniline film was deposited onto a previously chemically
deposited TiO
2
film by an electrodeposition (ED) method using a
galvanostatic mode by applying a constant current of 4 mA/cm
2
.
The electrodeposition (ED) cell employed a standard three elec-
trode configuration comprising a TiO
2
thin film based stainless

steel substrate, a graphite rod and a saturated calomel electrode as
working, counter and reference electrodes, respectively. To deposit
a polyaniline film, a solution containing 0.5 M H
2
SO
4
+ 0.45 M ani-
line (C
6
H
5
NH
2
) was used. The thickness of the film was calculated
by a weight-difference method, employing a sensitive microbal-
ance. The optimized thicknesses of TiO
2
and polyaniline films
were 0.55 and 0.9 ␮m, respectively, and roughness of the sur-
face was 0.869 ␮m. The forward biased junction current–voltage
(I–V) characteristic was examined by making front aluminium foil
press contact and back stainless steel contact to a heterojunc-
tion sample of area 1 cm × 1cm. The schematic diagram of the
p-polyaniline/n-TiO
2
heterojunction is depicted in Fig. 1. It consists
of a stainless steel substrate, onto which TiO
2
and polyaniline films
were subsequently deposited by the chemical bath deposition and

electrodeposition methods.
2.2. Characterization techniques
The structural characterization of the TiO
2
and polyaniline films
was carried out using a Philips (PW 3710) X-ray diffractometer with
CuK
␣
radiation ( = 1.5406Å) in a 2Â rangefrom10
◦
to 1 00
◦
. The
surface morphological study of the TiO
2
, polyaniline and cross-
sectional interface of a p-polyaniline/n-TiO
2
heterojunction was
carried out using scanning electron microscopy (JEOL-6360). For
this, the films were coated with a 10nm platinum layer using a
polaron scanning electron microscopy (SEM) sputter coating unit
E-2500 before taking the image.
2.3. LPG sensing properties p-polyaniline/n-TiO
2
heterojunction
The LPG sensing properties of the p-polyaniline/n-TiO
2
het-
erojunction were studied by using a home-made gas sensor unit,

described elsewhere [24]. Through the external connections, junc-
tion I–V characteristics were recorded using a potentiostat (EG&
G Princeton Applied Research Model 262-A). The forward biased
I–V characteristics of the junction before and after exposure to
LPG were recorded at different concentrations in the range of
0.04–0.12 vol% in a voltage range of 0–2 V. From the plot, maximum
current change was recorded at a fixed voltage (+2 V). The electri-
cal currents of a p-polyaniline/n-TiO
2
heterojunction in air (I
a
) and
in the presence of LPG (I
g
) were measured and using the following
relation the gas response was calculated.
S (%) =
I
a
− I
g
I
a
× 100 =
I
I
a
× 100 (1)
The response and recovery times of the junction to various concen-
trations of LPG were determined by holding the junction to a fixed

potential (+2 V) and the junction current change was recorded with
time.
3. Results and discussion
3.1. Crystal structural studies
Figs. 2(a) and(b) shows the X-ray diffraction patterns of TiO
2
and
polyaniline films, respectively. From Fig. 2(a), the presence of broad,
small and well distinct peaks indicates the nanocrystalline nature of
the TiO
2
film. The planes corresponding to (11 0), (1 0 1), (11 1) and
(2 1 0) are in good agreement with the Joint Committee on Powder
Diffraction Standard (JCPDS) (no. 21-1276), confirming the forma-
tion of nanocrystalline TiO
2
. The same kind of result was reported
Fig. 2. X-Ray diffraction patterns of (a) TiO
2
annealed at 673K and (b) polyaniline
thin film.
990 D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992
Fig. 3. Scanning electron micrographs of (a) TiO
2
annealed at 673K, (b) polyaniline, and (c) an interface cross-section of a p-polyaniline/n-TiO
2
heterojunction.
elsewhere [28] for TiO
2
thin films deposited by a hydrothermal

route. Fig. 2(b) declares the absence of any sharp diffraction lines,
indicating that the deposited polyaniline film is amorphous, sim-
ilar to the results reported by Joshi and Lokhande [29]. The peaks
marked by triangles are due to the contribution from the stainless
steel substrate.
3.2. Surface morphological studies
Figs. 3(a) and (b) shows the scanning electron micrographs of
TiO
2
and polyaniline films at × 10,000 magnification, respectively.
It is seen that the TiO
2
(Fig. 3(a)) film is more compact and has
strong adhesion with the stainless steel substrate. The SEM image
of the polyaniline film (Fig. 3(b)) exhibits a fibrous structure with
many pores and gaps among the fibers. Fig. 3(c) shows interface
cross-sectional SEM image of a p-polyaniline/n-TiO
2
heterojunc-
tion at high magnification of ×40,000, which clearly indicates the
formation of a diffusion free interface. It is evident that there are
many pores on the polyaniline surface, which seem to contribute
to the short response and recovery times. Due to the porous struc-
ture, LPG diffusion as well as reaction between gas molecules and
the interface occurs more easily.
3.3. LPG sensing properties of p-polyaniline/n-TiO
2
heterojunction
Fig. 4 represents the typical forward biased I–V characteristics of
the p-polyaniline/n-TiO

2
heterojunction in the absence and pres-
ence of LPG at room temperature (300 K). Curve (a) in Fig. 4 shows
the I–V characteristic in the absence of LPG and curves (b–e) are
in the presence of LPG for the concentrations ranging from 0.04 to
0.12 vol%. As the heterojunction was exposed to LPG, the forward
current drastically decreased with an increase in concentration of
LPG up to 0.1 vol%. A similar type of result is also observed by Tai et
al. [20] for NH
3
and CO gases. The decrease in current has been
attributed due to an increase in resistance of polyaniline or an
increase in potential barrier height at the interface when exposed
to LPG, in contrast to hydrogen gas sensors based on Pd/TiO
2
[30].
The LPG response of the p-polyaniline/n-TiO
2
heterojunction at
an applied potential of +2V is depicted in Fig. 5. From the figure, it
D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992 991
Fig. 4. Forward biased I–V characteristics of a p-polyaniline/n-TiO
2
heterojunction
at various concentrations of LPG (a) in air, (b) 0.04 vol%, (c) 0.06 vol%, (d) 0.1 vol% and
(e) 0.12vol% LPG.
is concluded that the gas response is a function of LPG concentra-
tion. The gas response increased from 15 to 63% with an increase
in concentration of LPG from 0.04 to 0.1 vol%. The maximum gas
response of 63% was observed at 0.1 vol%. At 0.12 vol% of LPG, the

response decreased to 25%.
The response/recovery time is an important parameter used for
characterizing a sensor. It is define d as the time required to reach
90% of the final change in current, when the gas is turned on and
off, respectively. The device response vs. time is shown in Fig. 6 for
0.1 vol% of LPG. From the plot, it is seen that the response time is
140 s and the recovery time is 180 s. Fig. 7 shows the heterojunction
response and recovery times for different vol% of LPG. It is revealed
that the response time decreased from 200 to 140 s when LPG con-
centration increased from 0.02 to 0.1vol%. This may be due to the
presence of sufficient gas molecules at the interface of the junction
for reaction to occur. From the same graph, it is found thatfor higher
concentrations of LPG, the recovery time was long. This may prob-
ably be due to the heavier nature of LPG and the reaction products
are not leaving from the interface immediately after the reaction.
Fig. 5. Gas response (%) vs. LPG concentration of a p-polyaniline/n-TiO
2
heterojunc-
tion.
Fig. 6. Gas response (%) vs. time (s) of a p-polyaniline/n-TiO
2
heterojunction at a
fixed voltage of +2 V and at a concentration of 0.1 vol% LPG.
Fig. 7. Variation of response and recovery time of the heterojunction sensor with
LPG concentration.
4. Conclusions
In the present work, for the first time, we have succeeded in
fabrication of a p-polyaniline/n-TiO
2
heterojunction for a room

temperature (300 K) liquefied petroleumgas (LPG) sensor. Morpho-
logical analysis using SEM of the junction cross-section revealed
the formation of a diffusion free interface. The gas sensing prop-
erties of heterojunction to LPG indicated that the thin film of
p-polyaniline/n-TiO
2
heterojunction is a candidate for LPG detec-
tion. The maximum gas response of 63% was achieved upon
exposure to 0.1 vol% LPG.
Acknowledgement
Authors are grateful to the Department of Science and Tech-
nology, New Delhi for financial support through the scheme no.
SR/S2/CMP-82/2006.
992 D.S. Dhawale et al. / Sensors and Actuators B 134 (2008) 988–992
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Biographies
D.S. Dhawale received his B.Sc.degree (2005) in general physics, M.Sc. degree (2007)
in materials science and presently doing Ph.D. in liquefied petroleum gas sensor per-
formance of polyaniline based heterojunctionsfrom the Shivaji University, Kolhapur,
India (M.S.). His present research interest includes synthesis of polyaniline based
heterojunctions and their application in gas sensor at room temperature (300 K).
R.R. Salunkhe received his B.Sc. (2003) in general physics, M.Sc. (2005) in solid-state
physics and presently he is doing his Ph.D. (2007) in chemical preparation of CdO
thin films and application in gas sensors, from Shivaji University, Kolhapur, India.
His present research interests include mainly the synthesis of nanocrystalline metal
oxide thin films and their applications in gas sensor.
U.M. Patil received his B.Sc. degree (2004) in general physics, M.Sc. degree (2006) in

Solid state Physics and presently doing Ph.D. in supercapacitive behavior of synthe-
sized RuO
2
–TiO
2
thin films from the Shivaji University, Kolhapur, India (M.S.). His
present research interest includes synthesis of TiO
2
and RuO
2
thin films by chemical
methods and their application in supercapacitor.
K.V. Gurav received his B.Sc. degree (2004) in general physics, M.Sc. degree (2006)
in Solid state Physics and presently doing Ph.D. in nanostructured ZnO: synthesis
and application in LPG sensors from the Shivaji University, Kolhapur, India (M.S.).His
present research interest includes synthesis of ZnO thin films by chemical methods
and their application in sensor.
A.M. More received his B.Sc. (2003) in general physics from Shivaji University, Kol-
hapur (India), M.Sc. (2005) in general physics from Pune University, Pune, India
(M.S.). Presently, he is working as a Ph.D. scholar in Thin Film Physics Laboratory,
Department of Physics, Shivaji University, Kolhapur. His research interests include
mainly the synthesis of nanocrystalline TiO
2
thin films by chemical methods and
their applications in dye sensitized solar cells and gas sensor.
C.D. Lokhande received his Ph.D. in 1984. He was a Humboldtian (Hahn-Meitner
Institute Berlin Germany). He is fellow of Institute of Physics. He is currently a reader
in the Department of Physics, Shivaji University, Kolhapur, India (M.S.). He has been
continuously engaged in the research field more than last 30 years. His research
interest includes the synthesis of thin films of metal chalcogenides, metal oxides,

conducting polymers and ferrites by chemical, electrochemical methods and their
applications in dye sensitized solar cells, gas sensors, energy storage devices, etc.