Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 6 3 4 3e6 3 5 5
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Synthesis and characterization of TiO2 doped polyanilinecomposites for hydrogen gas sensing
Subodh Srivastava a,*, Sumit Kumar a, V.N. Singh b, M. Singh a, Y.K. Vijay a
aThin Film and Membrane Science Lab, Department of Physics, University of Rajasthan, Jaipur, IndiabThin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e i n f o
Article history:
Received 14 September 2010
Received in revised form
24 January 2011
Accepted 26 January 2011
Available online 16 March 2011
Keywords:
In-situ polymerization
TiO2/PANI composite
Chemiresistor sensor
H2 gas sensing
Transmission electron microscopy
(TEM)
Scanning electron microscopy (SEM)
* Corresponding author. Tel.: þ91 141 270245E-mail addresses: subodhphy@gmail.com
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.01.141
a b s t r a c t
The Polyaniline (PANI) and Titanium dioxide (TiO2)/PANI composite thin film based
chemiresistor type gas sensors for hydrogen (H2) gas sensing application are presented in
this paper. Pure PANI and TiO2/PANI composites with different wt% of TiO2 were synthe-
sized by chemical oxidative polymerization of aniline using ammonium persulfate in acidic
medium at 0e5 �C. Thin films of PANI and TiO2/PANI composites were deposited on copper
(Cu) interdigited electrodes (IDE) by spin coating method to prepare the chemiresistor
sensor. Finally, the response of these chemiresistor sensors for H2 gas was evaluated by
monitoring the change in electrical resistance at room temperature. It was observed that
the TiO2/PANI composite thin film based chemiresistor sensors show a higher response as
compared to pure PANI sensor. The structural and optical properties of these composite
films have been characterized by X-ray diffraction (XRD) and UVeVisible (UVeVis) spec-
troscopy respectively. Morphological and structural properties of these composites have
also been characterized by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) respectively.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction Today, hydrogen (H2) is an important industrial chemical.
Gas sensing instruments are required to meet increasingly
stringent legal restrictions, industrial health and safety
requirements as well as for environmental monitoring, auto-
motive applications and for manufacturing process control.
To meet these demands, the sensitivity, selectivity and
stability of conventional devices need to be drastically
improved [1]. Recent advances in the development of nano-
structured catalysts such as metal oxide nanoparticles,
nanowires, nanorods and nanobelts provide the opportunity
to greatly increase the response of these materials, as sensor
performance is directly related to granularity, porosity, and
ratio of surface area to volume in the sensing element [2e5].
7; fax: þ91 141 2707728.(S. Srivastava), vijayyk@
2011, Hydrogen Energy P
Recently, H2 has attractedmuch attention as a clean, efficient,
and sustainable energy source [6,7]. It can be used directly for
combustion or as a fuel in fuel cells, as cryogenic-fuel in
rockets and as a lift off gas in weather balloons, etc. In power
plants, gaseous H2 is used for removing friction-heat in
turbines. H2 as a fuel in futuristic automobiles using H2/O2 fuel
cell is a definite possibility; in these applications there is
a need for a H2 sensor to monitor the fuel (H2) leak.
However, the increasing use of H2 gas should not be
considered as one without disadvantages. In fact, a number of
problems arise involving the storage of this gas. H2 is the
smallest molecule and thus can leak easily. A H2 leak in large
quantity should be avoided because H2, when mixed with air
gmail.com (Y.K. Vijay).ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 6 3 4 3e6 3 5 56344
in the ratio of 4.65e93.9 vol % is explosive [8]. In addition it can
catch fire, for example, form spark. For this reason, there is
a big demand of reliable, flexible and inexpensive H2 gas
sensors to prevent accidents due to its leakage, thus, saving
lives and infrastructure. At present several H2 gas sensors are
commercially available in the market, but either they are very
costly or inaccurate.
Various types of H2 gas sensors with different operating
principles have been explored including the resistive type
[9e18]. Among the various types of gas sensors, semi-
conductingmetal oxides have been studied extensively for the
detection of reducing gases such as H2, CO and hydrocarbons
[9e12,19]. The high temperature operation of these sensors
makes the lifetime of the sensor shorter and thus requires
more power for operation. Other problem associated with
metal oxide thin film sensor is their poor performance
regarding the sensitivity, stability and selectivity at certain
low concentration of the gases in ambient conditions, there-
fore the sensitivity of pure metal oxide is not satisfactory for
detecting a trace amount of H2. In addition, the high operating
temperature (200e400 �C) of these sensors may be inadequate
for measuring high H2 concentrations due to the danger of
explosions. Several strategies have been explored to fabricate
the sensorswith a high efficiency, sensitivity and selectivity at
lower operating temperatures. The organic materials, partic-
ularly conducting polymers receive a growing attention in
field of H2 gas sensors. The conducting polymers have
improved many aspects of the gas sensors especially in
lowering the operating temperature to around room temper-
ature. In addition to this, the ability to incorporate specific
binding sites into conducting polymers promises to improve
both selectivity and sensitivity [20]. Conducting polymers are
easy to be synthesized through chemical or electrochemical
processes and theirmolecular chain structure can bemodified
by copolymerization or structural derivations [21]. Mashat et
al. report the fabrication of a hydrogen gas sensor from pol-
ypyrrole nanofibers deposited on to conductometric trans-
ducers and they obtained a resistance shift of 312 U in the
presence of a H2 with a 43 s response time [22].
Polyaniline (PANI) has been investigated as a potential
material for H2 gas sensing applications, due to its controllable
electrical conductivity, environmental stability and inter-
esting redox properties associated with the chain nitrogen’s
[23,24]. It is a unique type of conducting polymer in which the
charge delocalization can, in principle, offer multiple active
sites on its backbone for the adsorption and desorption of
H2 gas.
However, pure conducting polymers are chemically
sensitive and exhibit low conductivity and poor mechanical
properties. For that reason, hybridization of metal oxide and
conducting polymer could improve the properties of pure
metal oxides or conducting polymers gas sensor.
Titanium dioxide (TiO2) was chosen due to its unique
physical and chemical properties such as large energy gap,
dielectric constant, and environmental-friendliness and easy
to synthesis. In particular, TiO2 films have been investigated
as sensors for H2 gas [25]. The reducing gas like H2 reacts with
the negatively charged oxygen adsorbed on the surface of TiO2
nanoparticles and supplies electrons to the conduction band,
leading to a decrease in electric resistance. Grimes and
coworkers [26] have suggested that a H2 sensor with a high
sensitivity can be achieved using TiO2 with various nano-
dimensional architectures.
Sazek et al. have developed the TiO2 and Au nanocrystal-
doped TiO2 thin films based surface acoustic wave (SAW)
sensor devices and the measured sensor response was
approximately 6.5 kHz and 7.4 kHz towardH2 at 230e260 �C for
the TiO2 and AueTiO2 sensors, respectively [27].
Therefore the combination of conducting polymer (PANI)
and TiO2 due to their unique properties presents a fascinating
structure, which has shown promising application in the field
of hydrogen gas sensor. It has been reported that doping of
TiO2 further improves the gas sensing performance of PANI
for various gases [28,29].
In the present work, we have investigated structural,
optical and electrical properties of chemically synthesized
TiO2 doped PANI composites. The effect of TiO2 doping on the
morphological and gas sensing properties of composite films
have also been investigated for hydrogen gas. For this purpose
a chemiresistor type gas sensor containing several pairs of Cu-
interdigited electrodes (Cu-IDE) coated with a sensing film of
composite material has been fabricated.
2. Experimental methods
2.1. Materials
The aniline (99% pure) and hydrochloric acid (HCl) (35%
concentrated) were purchased from Merck specialties, India.
Aniline was used after double distillation. TiO2 (99.9%, P-25)
nanoparticles with an average particle size of about 10e15 nm
in rutile phase was obtained from Degussa, Germany.
Ammonium peroxide sulfate (APS) (99%) was purchased from
Qualigens Fine Chemicals, India. Chloroform (99.9%) and
ammonia solution (25% concentrated) were purchased from
Fisher scientific. 10-Camphor sulfonic acid (CSA) was obtained
from Hi media chemicals and used as received.
2.2. Synthesis of TiO2/PANI composite
Pure PANI was synthesized by in-situ chemical oxidative
polymerization method at 0e5 �C, which has been reported
elsewhere [30,31]. The TiO2/PANI composite was prepared by
an in-situ chemical oxidation polymerization of aniline using
APS as an oxidant in presence of colloidal TiO2 nanoparticles
at 0e5 �C in air. In a typical procedure, the TiO2 nanoparticles
were suspended in 1 M HCl solution and sonicated for 1 h to
reduce aggregation of TiO2 nanoparticles. The 0.1 M of aniline
was dissolved in 100 ml of 1 M HCl solution and then mixed
with 10ml of sonicated colloidal TiO2 nanoparticles by further
sonication for 30 min. The 100 ml of 1 M HCl solution con-
taining the APS ((NH4)2S2O8) with an equal molar ratio to
aniline was then slowly added drop wise to well dispersed
suspension mixture for 2 h with a continuous stirring at
0e5 �C. After 3 h, a good degree of polymerization is achieved
and the dark green precipitate was recovered. The solution
was left in undisturbed position for a night for the completion
of chemical reaction. The precipitate produced in the reaction
was removed by filtration, washed repeatedly with 1 M HCl
Fig. 1 e Schematic diagram of step by step synthesis process of TiO2/PANI composite using in-situ chemical oxidative
polymerization method.
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Fig. 2 e (a) Schematic diagram and (b) prepared Cu-IDE configuration on epoxy glass substrate.
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and dried under vacuum for 24 h. The composite powder
thus obtained was conductive emeraldine salt (ES) form of
TiO2/PANI. The different contents TiO2/PANI composites were
synthesized using 5, 10, 20, 30 wt % of TiO2 with respect to
aniline monomer. The schematic diagram of step wise
synthesis process is shown in Fig. 1. For emeraldine base (EB),
TiO2/PANI (ES) powder was kept in 0.1 M ammonia solution
and stirred for 6 h at room temperature. The precipitate was
filtered and washed with deionized water until filtered solu-
tion became neutral and then dried in vacuum for 24 h to
obtain emeraldine base TiO2/PANI composite.
2.3. Sensor preparation
The chemiresistor type sensor containing interdigited elec-
trode (IDE) geometry and a layer of sensing material used in
this work was prepared as follows:
Fig. 3 e The Schematic representation and prepared chemires
2.3.1. Interdigited electrode (IDE) substrateIt consists of finger type Cu electrodes electrochemically
patterned onto 1 � 1 cm2 area of epoxy glass substrate. The
width of overlap electrode and the gap between two succes-
sive electrodes was 0.5 mm. Before use, the IDE-epoxy glass
substrates were cleaned by ultrasonic treatment in acetone
and by hot soap solution, then rinsed thoroughly with pure
water and dried in vacuum. The schematic diagram of IDE
configuration is shown in Fig. 2.
2.3.2. Film depositionNormally PANI and its composite in emeraldine salt (ES) form
are not soluble directly in any organic solvent and it is difficult
to process it in conducting form. Therefore, firstly PANI (ES)
and its composites were converted into emeraldine base (EB)
form and then protonated with CSA to make it conducting
processable solution for film casting.
istor sensor containing Cu-IDE on epoxy glass substrate.
Fig. 6 e UVeVis spectrum for spin coated PANI and TiO2/
PANI composite films on glass substrate.Fig. 4 e X-ray diffraction (XRD) pattern of (a) PANI (EB) and
(b) PANI (ES).
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In a typical procedure, the 0.3 g of PANI (EB) and the TiO2/
PANI (EB) composite powders were separately mixed with of
CSA by grinding in a smooth agate mortar. The PANI/CSA and
PANI/TiO2/CSA (with 5, 10, 20 and 30 wt % of TiO2) mixtures
were separately added in 30 ml chloroform to prepare the
conducting solution. The solution preparation requires 4e5
days of continuous stirring to make the solution homoge-
nous. Thin films of these prepared conducting homogenous
solutions were deposited onto cleaned Cu-IDE-epoxy glass
substrates using the spin coating technique (Apex instru-
ments) at a speed of 2000 rpm for 2 min. These sensor films
were dried at 60 �C in vacuum for 24 h. The chemiresistor
type gas sensors prepared using Cu-IDE configuration is
shown in Fig. 3.
Fig. 5 e The XRD pattern for PANI and TiO2/PANI
composites.
2.4. Characterization
The X-ray diffraction (XRD) measurements of PANI and TiO2/
PANI composite materials were performed with PANalytical
make X’pert PRO MPD system, (USA) having CueKa radiation
source of wavelength l ¼ 1.540 A, and the scattered radiation
diffraction patterns were collected within the range of
2q z 5�e65�. The Ultravioletevisible (UVevis) absorption
spectra were recorded using a Hitachi-330. The surface
morphology of the prepared thin films was investigated and
evaluated by means of Scanning Electron Microscopy (SEM)
(MIRA//TESCAN-FESEM scanning electron microscope). In the
present study high resolution transmission electron micros-
copy (HRTEM, Tecnai G20-Stwin (200 kV)), was used to char-
acterize the structural properties of synthesized composite
materials. The H2 gas sensing behavior of PANI and TiO2/PANI
nanocomposite films was analyzed by measuring the change
Fig. 7 e Current-voltage (IeV) characteristics of spin coated
thin films of PANI and MWNT/PANI-IP composites.
Fig. 9 e SEM images for (a) Pure TiO2 nanoparticles, (b) pure PAN
image of 20% TiO2/PANI composite.
Fig. 8 e Variation in conductivity of TiO2/PANI composite
thin films with different concentrations of TiO2
nanoparticles at room temperature.
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of an electrical resistance of the films after H2 exposure in air
at room temperature. The electrical resistance was measured
using programmable LCR meter (100 kHz, APLAP LCR meter).
3. Results and discussion
3.1. X-ray diffraction (XRD)
The structural order of synthesized materials has been
studied by X-ray powder diffraction. Fig. 4 shows the XRD
pattern of (a) PANI (EB) and (b) PANI (ES). The PANI (EB) shows
an amorphous nature with broad and weak amorphous peaks
at 2qz 15� and 22�. This indicates that the chains are strongly
disordered [32e34]. While in case of PANI (ES), XRD pattern
reveals a single crystalline peak appeared at 2q ¼ 25.2� with
two broad amorphous peaks centered at 2q ¼ 15� and 20.5�.The peaks observed at 20.5� and 25.2� can be ascribed to
periodicity parallel and perpendicular to PANI conjugation
chains, respectively [35e37]. The crystalline peak is more
prominent than the amorphous peaks indicating that HCl
I, (C) 20% TiO2/PANI composite and (d) higher magnification
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doped PANI is not fully amorphous as well as pep interchain
stacking has improved [38].
Fig. 5 presents the XRD patterns of pure TiO2 nanoparticles
and TiO2/PANI composites. It is apparent that the pure TiO2
nanoparticles are crystalline and the positions of all the sharp
peaks reveal that the TiO2 is in rutile phase and have in good
agreement with earlier reported data [39,40].
Adopting the Scherrer formula, the calculated size of TiO2
nanoparticles is z10e30 nm, consistent with the observed
result of TEM measurements. The XRD patterns of PANI/TiO2
composites show the characteristic peaks not only for the
PANI but also for the TiO2 nanoparticles, proving the existence
of TiO2 nanoparticles within the composite. For the lowest
concentration of TiO2 (5 wt %) in PANI, the diffraction pattern
is almost identical with PANI (ES), consisting of a sharp peak at
2q ¼ 25.2� with two weak peaks centered at 15� and 20.5�.However, for TiO2/PANI composite samples having higher
concentration of TiO2 nanoparticles (10 wt % to 30 wt %) the
broad amorphous peaks have been disappeared, whereas the
Fig. 10 e TEM images for (a) Pure TiO2 nanoparticles, (b) pure P
magnification image of 20% TiO2/PANI composite.
peaks related to the TiO2 nanoparticles centered at 2q z 25�,27.4�, 36.9�, 37.8�, 38.6�, 41.2�, 48�, 53.8� and 55� became more
sharp and crystalline. This confirmed that the TiO2/PANI
composites become more crystalline as the concentration of
TiO2 is increased and PANI deposited on the surface of TiO2
particles has no effect on the crystallization behavior of TiO2
particles in the composite [41].
3.2. Ultravioletevisible (UVevis) spectroscopy
Fig. 6(a) depicts the UVevis absorption spectra of pure PANI
and TiO2/PANI composite thin films deposited on the clean
glass substrate. The UVevis absorption spectra for CSA doped
PANI and TiO2/PANI composites thin films exhibit three
absorption bands at 380, 440 and 700e800 nm. The bands at
380 nm and 440 nm are attributed to the pep* and polaronep*
transition in the conducting PANI (ES) [42,43]. The localized
polaron band around 700e800 nm indicates a compact coiled
conformation of PANI [44,45]. From the Fig. 6(bee) it can be
ANI, (C) 20% TiO2/PANI composite and (d) higher
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seen that for TiO2/PANI composite thin films, the absorption
intensity increases as the concentration of TiO2 increases. It
may be due to the good absorption property of TiO2 nano-
particles. In addition, it can be noted that there are some shifts
in the peaks for TiO2/PANI composites thin films as compared
to the pure PANI thin film. It may be due to that the encap-
sulation of TiO2 nanoparticles has the effect on the doping of
conducting PANI or coordinate complex formation between
TiO2 nanoparticles and PANI chains [43,46,47].
3.3. IeV characteristics
The currentevoltage (IeV) characteristics of spin coated thin
films of PANI and TiO2/PANI composites are shown in Fig. 7. In
the IeV characteristics an ohmic current was observed with
a symmetric current plot on both sides in positive and nega-
tive voltage regions within the range of �1 V. An increasing
trend of current was observed as the concentration of TiO2
nanoparticles (up to 20 wt %) in PANI was increased, while
for 30 wt % of TiO2 in PANI the current has found to be
decreased.
In comparison to pure PANI the conductivity of 20 wt %
TiO2/PANI composite thin film has found to be increased from
z2 � 10�6 S/m to 2 � 10�5 S/m as shown in Fig. 8. This may be
attributed to that the doping of TiO2 nanoparticles within
PANI matrix form a more efficient network for charge trans-
port between different molecular chains of PANI, thus
enhancing the conductivity of composite [48]. The decrease in
conductivity for 30 wt % TiO2/PANI composite may be due to
the partial blockage of conductive path and reduction of
Fig. 11 e Schematic block diagram of gas
conjugation length between PANI chains by excess of TiO2
nanoparticles within the PANI matrix [49,50].
3.4. Scanning electron microscopy (SEM)
It is well reported that morphological parameters like particle
size, surface structure, surface to volume ratio, etc. may affect
the nature of sensitive films [51,52]. The surface morphology
of pure PANI and TiO2/PANI nanocomposite thin films is
determined by scanning electron microscopy (SEM) as shown
in Fig. 9. Fig. 9a shows the SEM image of pure TiO2 nano-
particles of about 10 nme30 nm in size. The SEM image of pure
PANI thin film exhibits a completely amorphous region as
shown in Fig. 9b. This image clearly reveals that surface of
PANI is not smooth and uneven lumps and holes are visible in
PANI thin film, which are suitable for gas adsorption.
It is found that the doping of TiO2 strongly affected the
morphology of the resulting TiO2/PANI composites thin films.
In case of TiO2/PANI composite, the SEM micrograph revealed
a honeycomb like structure with an interlocking arrangement
of granular particles (Fig. 9c). This suggests that the most of
TiO2nanoparticleswerecoatedwithPANI, and theexcessPANI
formed a nano-rod like network during the polymerization
process.A close investigationof the surfaceof thinfilm (Fig. 9d)
revealed that TiO2/PANI composite exhibited a porous open
structure and high surface area, which is suitable for gas
sensing applications. It has been pointed out that such porous
structure significantly enhanced the rapid diffusion of gas
molecules due to its larger exposure area. Therefore the reac-
tion between gas molecules and thin film occurs easily and
sensing setup used in present work.
Fig. 13 e Response versus time plot for PANI and TiO2/PANI
composite thin film sensors after H2 exposure at room
temperature.
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resulted in alteration of resistance of thin films. This observa-
tion is in good agreement with other reported findings [51,52].
3.5. Transmission electron microscopy (TEM)
The TEM images for TiO2 nanoparticles and pure PANI are
shown in Fig. 10a and b respectively. As seen in the micro-
graphs, the TiO2 nanoparticles were about 10e15 nm in size,
while synthesized PANI showed a disordered amorphous
structure. In case of TiO2/PANI composite, TiO2 nanoparticles
are not well dispersed and TiO2 grains exhibited a strong
agglomeration in PANI matrix as shown in Fig. 10c. In addi-
tion, The TiO2 rich regions in composite seem to be composed
of many small TiO2 nanoparticles of about 10e30 nm in size
(see inset in Fig. 10c). The Fig. 10d shows a TEM image of
composite at higher magnification and confirmed that TiO2
nanoparticles were coated with PANI.
3.6. Gas sensing measurements
For gas sensing test, the sensor device called as chemiresistor
and measurement systemwere designed using the concept of
resistance variation in the sensingmaterials afterH2 exposure.
The gas sensing behavior of chemiresistor sensorswas studied
by calculating change in the surface resistance of sensing film
Fig. 12 e The change in resistance of (a) PANI and (b) TiO2/
PANI composite thin film sensors with time after exposed
to H2 gas at room temperature.
with time towardH2 exposureat pressureof about 30 psi andat
room temperature. The resistance variation was measured by
Aplab MT 4080D LCR meter at 10 kHz. The chemiresistor type
sensors were mounted on Al hot plate (filament heater) which
was coupled with ceramic base stand. The two leads of thin
copper wire were attached to interdigited electrodes by silver
paste for electrical connections. Finally this sensor setup was
fixed into the homemade iron gas sensing chamber. The elec-
trical connections for gas sensing measurements, thermo-
couple and temperature variation were made using
instrumentation feed through.Theschematicblockdiagramof
gas sensor setup is shown in Fig. 11.
During the sensingmeasurement, initially sensing chamber
wascleanedbyconnecting itwith lowvacuumrotarypump.The
sensor was exposed to pure air until the constant baseline
resistance was achieved and then H2 gas was introduced in the
Fig. 14 e Variation in % sensitivity with concentration of
TiO2 in TiO2/PANI composite sensor.
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chamber and the resistance of the sensor was recorded for
z300 s every time. During this time the steady resistance had
been achieved, and then the sensing chamberwas flushedwith
pureairconsecutively toallowthesurfaceofthesensitivefilmto
regainatmosphericconditionandthe resistance reachedsteady
and kept stable.
The response of sensor was monitored in terms of the
normalized resistance calculated by Response ¼ R0/Rg and the
Fig. 15 e Reproducibility of PANI and TiO2/PANI composite
sensitivity factor was monitored in terms of the % sensitivity
calculated by % sensitivity ¼ DR/R0. Where DR is the variation
in resistance of composite films from baseline after exposure
to H2 gas, Rg is the resistance of the sensor in presence of H2
gas and R0 is the initial baseline resistance of the films.
Fig. 12a and b shows the variation in the resistance of PANI
and TiO2/PANI composite thin films respectively toward H2
gas at room temperature. Both these figures reveal that the
sensor exposed to hydrogen gas at room temperature.
Table 1 e The shift in resistance, response value andresponse time for TiO2/PANI composites sensors.
Compositethin film sensor
Shift inresistance (U)
Responsevalue (R0/Rg)
Responsetime (s)
Pure PANI 249 1.386 268
5% TiO2/PANI 82 1.423 210
10% TiO2/PANI 84 1.590 180
20% TiO2/PANI 68 1.650 230
30% TiO2/PANI 162 1.76 140
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resistance of all composite films decreases rapidly upon
introduction of H2 gas and becomes stablewithin few seconds.
This may be attributed to the reducing nature of the H2 gas.
The introduction of H2 gas toward the composite thin films
injected electrons to the film, and thus significantly increased
the number of charge carriers in the film. As a result, more
electrons flowed in the film and at the same time reduced the
resistance of the film. In contrast, TiO2/PANI thin film sensors
exhibited a shorter time to achieve a stable resistance value in
comparison to pure PANI.
The initial resistance of PANI thin film sensorwas observed
higher than the lower initial resistance of TiO2/PANI thin film
sensor. This may be due to the increase of conjugation length
in PANI chains and the effective charge transfer between the
PANI and TiO2 may result in increase in conductivity [53,54].
Furthermore improved contact between each grain via grain
boundary andmore uniform coating of TiO2 particles with the
polymer may also enhance the conductivity [55]. The corre-
sponding room temperature response versus time plots for
PANI and TiO2/PANI composites films are shown in Fig. 13.
This figure clearly indicates that the TiO2/PANI composite
films show a higher and faster response in comparison to pure
PANI thin film sensor.
Sadek et al. have earlier reported the sensing performance
of dedoped and doped PANI nanofiber thin film conducto-
metric sensors and the sensitivity wasmeasured to be 1.11 for
doped and 1.07 for dedoped PANI nanofiber sensors upon
exposure to H2 [56]. It have also been predicted in literature
that sulphonic acid doped PANI nanofiber based sensors have
higher sensitivity toward H2 in comparing to the HCl doped
PANI with relatively fast response time of about 1 min and
a recovery time of about 4 min at room temperature [57,58].
It was observed that the % sensitivity of TiO2/PANI
composite thin films linearly increased as the concentration
of TiO2 in PANI matrix is increased as shown in Fig. 14.
In case of pure PANI the H2 may form a bridge between
nitrogen atoms on two adjacent chains or theremay be partial
protonation of some imine nitrogen atoms [59]. These reac-
tions lead to the protonation of PANI nitrogen atoms, resulting
in more delocalized charge carriers (polarons and bipolarons)
on the backbone for conduction and an increase of film
conductivity i.e. resistance of thin films decrease. MacDiarmid
(2005) has presented similar mechanism for the interaction of
H2 with PANI [60].
Virji et al. reported that H2 can be adsorbed on the positive
charged nitrogen atoms of PANI, and then dissociate into H2
atoms. The following formation of new NeH bonds between
the H2 atoms and nitrogen can reduce the resistance of PANI
[61].
The gas sensing mechanism of TiO2/PANI composite thin
film sensor is not very clear at present, but several postulates
can be made as follows:
The PANI and TiO2 nanoparticles may form a semi-
conductor heterojunction [51]. Therefore the observed
increased response may be due to the creation of a positively
charged depletion layer on the surface of TiO2 nanoparticles,
which promotes the inter-particle electron transition between
PANI and TiO2 at the heterojunction, this would cause
a reduction of activation energy and enthalpy of physisorption
for H2 gas.
An interaction of H2 with the PANI backbone possibly
facilitated by TiO2 nanoparticlesmay induce dissociation of H2
leading to either a doping type response or chain alignment.
According to the SEM measurements, the surface of TiO2/
PANI composite thin films exhibited a porous open structure
and high surface area, which seems to contribute to short
response time and good reversibility of the composite sensor
in comparison to pure PANI sensor. This is due to the fact that
gas diffusion occurs more easily in the porous structure and
the reaction between gas molecules and the thin film there-
fore occurs more easily.
Sadek and his coworkers examined the H2 sensing perfor-
mance for a PANI/In2O3, PANI/PtO2, PANI/MoO3 and PANI/WVO3
nanocomposites based sensors. They reported thatmetal oxide
doped PANI sensors show a repeatable and large response
towardH2 incomparisonto thatofpurePANI,howeverthePANI/
In2O3 nanocomposite seems to be themost promising [62,51].
The suitability of TiO2/PANI composite films as H2 gas
sensor was also investigated. Fig. 15 shows the respon-
seerecovery property of composite film sensor. Over a long
period of hydrogen exposure it was observed that TiO2/PANI
composite film sensor exhibited a good stability and repeat-
ability as gas sensor with consistent pattern and response
magnitude. Table 1 summarizes the shift in resistance, rela-
tive response value and response time of TiO2/PANI composite
sensors exposed to the H2 gas at room temperature.
4. Conclusion
We have synthesized TiO2/PANI nanocomposite with different
wt % of TiO2 by in-situ chemical oxidative polymerization
method at low temperature. Emeraldine base form of TiO2/
PANI nanocomposite can be doped with CSA to prepare its
viscous and homogeneous suspension in chloroform, which
gave good coverage of sensor substrates under spin coating
technique. It is found that the morphology, size, crystallinity,
electrical properties, and gas sensing behavior of the TiO2/PANI
composite thin films are affected by the concentration of TiO2
in PANI. It is observed that during the polymerization, most of
TiO2 nanoparticles are coated with PANI and forms a honey-
comb like open porous structure having large surface area,
which is suitable for H2 gas sensing applications. The electrical
conductivity of TiO2/PANI composite films slightly increased
with increase in the TiO2 content upto 20 wt % and then
decreased with further additions of TiO2 in PANI. The decrease
in conductivity may be due to the partial blockage of conduc-
tive path and reduction of conjugation length between PANI
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 6 3 4 3e6 3 5 56354
chains by excess of TiO2 nanoparticles within the PANI matrix.
It is observed that in comparison to the pure TiO2 and PANI
based sensors reported earlier, the TiO2/PANI sensor in the
present study exhibits the faster response, and higher sensi-
tivity. Furthermore, the sensitivity of TiO2/PANI nano-
composite thin film based sensors toward hydrogen gas is
increased with increasing the TiO2 concentration in PANI
matrix.
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