Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing

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: [email protected]

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.

r e f e r e n c e s

[1] Hooker SA. Nanotechnology advantages applied to gassensor development. In: Proceedings of the 5th Annual BCCNanoparticles Conference. New York, NY, USA; 2002. p. 1e7.

[2] Varghese OK, Gong D, Paulose M, Ong KG, Grimes CA.Hydrogen sensing using titania nanotubes. Sens Actuators B2003;93:338e44.

[3] Guidi V, Carotta MC, Ferroni M, Martinelli G, Paglialonga L,Comini E, et al. Preparation of nanosized titania thick andthin films as gas-sensors. Sens Actuators B 1999;57:197e200.

[4] Wang ZL. Nanobelts, nanowires, and nanodiskettes ofsemiconducting oxidesdfrom materials to nanodevices. AdvMater 2003;15:432e6.

[5] Choopun S, Hongsith N, Tanunchai S. Single crystalline ZnOnanobelts by RF sputtering. J Crystal Growth 2005;282:365e9.

[6] Dresselhaus MS, Thomas LL. Alternative energytechnologies. Nature 2001;414:332e7.

[7] Perry ML, Fuller TF. A historical perspective of fuel celltechnology in the 20th century. J Electrochem Soc 2009;149:59e67.

[8] Katsuki A, Fukui K. H2 selective gas sensor based on SnO2.Sens Actuators B 1998;52:30e7.

[9] Semancik S, Cavicchi RE. The growth of thin, epitaxial SnO2

films for gas sensing applications. Thin Solid Films 1991;206:81e7.

[10] Michel HJ, Leiste H, Schierbaum KD, Halbritter J. Adsorbatesand their effects on gas sensing properties of sputtered SnO2

films. Appl Surf Sci 1998;126:57e64.[11] Malyshev VV, Vasiliev AA, Eryshkin AV, Koltypin EA,

Shubin YI, Buturlin AI, et al. Gas sensitivity of SnO2 and ZnOthin-film resistive sensors to hydrocarbons, carbonmonoxide and hydrogen. Sens Actuators B 1992;10:11e4.

[12] Baik NS, Sakai G, Miura N, Yamazoe N. Hydrothermallytreated sol solution of tin oxide for thin-film gas sensor. SensActuators B 2000;63:74e9.

[13] Shin W, Matsumiya M, Qiu F, Izu N, Murayama N.Thermoelectric gas sensor for detection of high hydrogenconcentration. Sens Actuators B 2004;97:344e7.

[14] Maffei N, Kuriakose AK. A solid-state potentiometric sensorfor hydrogen detection in air. Sens Actuators B 2004;98:73e6.

[15] Okazaki S, Nakagawa H, Asakura S, Tomiuchi Y, Tsuji N,Murayama H, et al. Sensing characteristics of an optical fibersensor for hydrogen leak. Sens Actuators B 2003;93:142e7.

[16] Jakubik WP, Urbanczyk MW, Kochowski S, Bodzenta J.Palladium and phthalocyanine films for hydrogen detectionin a surface acoustic wave sensor system. Sens Actuators B2003;96:321e8.

[17] Wong YM, Kang WP, Davidson JL, Wisitsora-at A, Soh KL. Anovel microelectric gas sensor utilizing carbon nanotube forhydrogen gas detection. Sens Actuators B 2003;93:327e32.

[18] Baselt DR, Fruhberger B, Klaassen E, Cemalovic S,Britton Jr CL, Patel SV, et al. Design and performance ofa microcantilever-based hydrogen sensor. Sens Actuators B2003;88:120e31.

[19] Nakahara T, Koda H. In: Yamauchi S, editor. Chemical sensortechnology, vol. 3. Tokyo: Kodansha; 1992. p. 19e32. Chapter 2.

[20] Marsella MJ, Carroll PJ, Swager TM. Conductingpseudopolyrotaxanes: a chemoresistive response viamolecular recognition. J Am Chem Soc 1995;116:9347e8.

[21] Dhawale DS, Salunkhe RR, Patil UM, Gurav KV, More AM,Lokhande CD. Room temperature liquefied petroleum gas(LPG) sensor based on p-polyaniline/n-TiO2 heterojunction.Sens Actuators B Chem 2008;134:988e92.

[22] Mashat LA, Tran HD, Wlodarski W, Kaner RB, Zadeh KK.Conductometric hydrogen gas sensor based on polypyrrolenanofibers. IEEE Sens J 2008;8(4).

[23] Heeger AJ. Semiconducting and metallic polymers: thefourth generation of polymeric materials. Synth Met 2002;125:23e42.

[24] MacDiarmid AG. A novel role for organic polymers. SynthMet 2002;125:11e22.

[25] Mather GC, Marques FMB, Frade JR. Detection mechanism ofTiO2-based ceramic H2 sensors. J Eur Ceramic Soc 1999;19:887e91.

[26] Varghese OK, Gong D, Paulose M, Ong KG, Dickey EC,Grimes CA. Extreme changes in the electrical resistance oftitania nanotubes with hydrogen exposure. Adv Mater 2003;15:624e7.

[27] Sadek AZ, Buso D, Martucci A, Mulvaney P, Wlodarski W,Kalantar-zadeh K. Titanium dioxide-based 64� YX LiNbO3

surface acousticwave hydrogen gas sensors. J Sens; 2008:1e5.[28] Huiling Tai, Yadong Jiang, Guangzhong Xie, Junsheng Yu,

Xuen Chen. Fabrication and gas sensitivity of polyaniline-titanium dioxide nanocomposite thin film. Sens Actuators B2007;125:644e50.

[29] Huiling Tai, Yadong Jiang, Guangzhong Xie, Junsheng Yu,Xuen Chen, Zhihua Yu. Influence of polymerizationtemperature on NH3 response of Pani/TiO2 thin film gassensor. Sens Actuators B Chem 2007;129:319e26.

[30] Subodh Srivastava, Sharma SS, Sumit Kumar,Shweta Agrawal, Singh M, Vijay YK. Characterization of gassensing behavior of multi walled carbon nanotubepolyanniline composite films. Int J Hydrogen Energy 2009;34:8444e50.

[31] Subodh Srivastava, Sharma SS, Shweta Agrawal,Sumit Kumar, Singh M, Vijay YK. Study of chemiresistor typeCNT doped polyaniline gas sensor. Synth Met 2010;160:529e34.

[32] MacDiarmid AG, Epstein AJ. Secondary doping in polyaniline.Synth Met 1995;69:85e92.

[33] Yan H, Ohta T, Toshima N. Stretched polyaniline films dopedby (^)-10-camphor sulfonic acid: anisotropy andimprovement of thermoelectric properties. Macromol MaterEng 2001;286:139e42.

[34] Han MG, Im SS. Electrical and structural analysis ofconductive polyaniline/polymides blends. J Applied PolymSci 1999;71:2169e78.

[35] Barthet C, Armes SP, Lascelles SF, Luk SY, Stanley HME.Synthesis and characterization of micrometer-sized,polyaniline-coated polystyrene latexes. Langmuir 1998;14:2032e41.

[36] Zhang LJ, Wan MX. Polyaniline/TiO2 composite nanotubes.J Phys Chem B 2003;107(28):6748e53.

[37] Pouget JP, Jozefowicz ME, Epstein AJ, Tang X,MacDiarmid AG. X-ray structure of polyaniline.Macromolecules 1991;24:779e89.

[38] Kwanghee Lee, Shinuk Cho, Sung Heum Park, Heeger AJ,Chan-Woo Lee, Suck-Hyun Lee. Metallic transport inpolyaniline. Nat Lett 2006;441:65e8.

[39] Tang H, Prasad K, Sanjines R, Schmid PE, Levy F. Electricaland optical properties of TiO2 anatase thin films. J Appl Phys1994;75:2042e7.

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 6355

[40] Youn-Gyu Han, Takafumi Kusunose, Tohru Sekino. A studyof conductive elastomer composites reinforced with sulfonicacid doped polyaniline coated titanium dioxide. J CeramicProcess Res 2009;10:208e11.

[41] Xingwei Li, Wei Chen, Chaoqing Bian, Jinbo He, Ning Xu,Gi Xue. Surface modification of TiO2 nanoparticles bypolyaniline. Appl Surf Sci 2003;217:16e22.

[42] Ram MK, Yavuz O, Lahsangah V, Aldissi M. Co gas sensingfrom ultra thin nano composite conducting polymer film.Sens Actuators B 2005;106:750e7.

[43] Ma X, Wang M, Li G, Chen H, Bai R. Preparation of polyanilineeTiO2 composite film with in situ polymerization approachand its gas sensitivity at room temperature. Mater ChemPhys 2006;98:241e7.

[44] Sainz R, Small WR, Young NA, Valles C, Benito AM,Maser WK, et al. Synthesis and properties of optically activepolyaniline carbon nanotube composites. Macromolecules2006;39:7324e32.

[45] Cao Y, Smith P, Heeger AJ. Spectroscopic studies ofpolyaniline in solution and in spin-cast films. Synth Met1989;32:263e81.

[46] Cruz-Silva R, Romero-Garcıa J, Angulo-Sanchez JL, Flores-Loyola E, Farıas MH, Castillon FF, et al. Comparative study ofpolyaniline cast films prepared from enzymatically andchemically synthesized polyaniline. Polymer 2004;45:4711e7.

[47] Li X, Wang G, Lu D. Surface properties of polyaniline/nano-TiO2 composites. Appl Surf Sci 2004;229:395e401.

[48] Annes SP, Gottesfeld S, Beery JG, Garzon F, Agnew SF.Conducting polymer-colloidal silica composites. Polymer1991;32:2325e30.

[49] Su Shi-Jian, Kuramoto Noriyuki. Processable polyaniline-titanium dioxide nanocomposites: effect of titanium dioxideon the conductivity. Synth Met 2000;114:147e53.

[50] Phang SW, Tadokoro M, Watanabe J, Kuramoto N. Synthesis,characterization and microwave absorption property ofdoped polyaniline nanocomposites containing TiO2

nanoparticles and carbon nanotubes. Synth Met 2008;158:251e8.

[51] Sadek AZ, Wlodarski W, Shin K, Kaner RB, Kalantar-zadeh K.A layered surface acoustic wave gas sensor based on

a polyanniline/In2O3 nanofiber composite. Nanotechnology2006;17:4488e92.

[52] Matsuguchi M, Io J, Sugiyama G, Sakai Y. Effect of NH3 gas onthe electrical conductivity of polyanniline blend films. SynthMet 2002;128:15e9.

[53] Su SJ, Kuramoto N. Polyaniline e titanium dioxidenanocomposite: effect of titanium oxide on the conductivity.Synth Met 2000;114:147e53.

[54] Feng W, Bai XD, Lian YQ, Liang J, Wang XG, Yoshino K. Wellaligned polyaniline/carbon nanotube composite film grownby in-situ aniline polymerization. Carbon 2003;41:1551e7.

[55] Tandon RP, Tripathi MR, Arora AK, Hotchandani S. Gas andhumidity response of iron oxide epolypyrrolenanocomposite. Sens Actuators B 2006;114:768e73.

[56] Sadek AZ, Wlodarski W, Zadeh KK, Baker C, Kaner RB. Dopedand dedoped polyaniline nanofiber based conductometrichydrogen gas sensors. Sens Actuators A Phys 2007;139:53e7.

[57] Sadek AZ, Baker CO, Powell DA, Wlodarski W, Kaner RB,Zadeh KK. Polyaniline nanofiber based surface acoustic wavegas sensorsdeffect of nanofiber diameter on H2 response.IEEE Sens J 2007;7(2).

[58] Sadek AZ, Trinchi A, Wlodarski W, Zadeh KK, Galatsis K,Baker C, Kaner RB. A room temperature polyanilinenanofibers hydrogen gas sensor. Proc. of IEEE Sensors Conf.USA, Nov. 2005.

[59] MacDiarmid AG. Presentation at DOE center of excellence oncarbon-based H2 storage, USA [Online] Available: http://www.hydrogen.energy.gov/pdfs/reviewO5/stp-42-macdiarmid.pdf; May 24, 2005.

[60] MacDiarmid AG. Conducting as new materials for hydrogenstorage. U.S. Department of Energy Presentation [online]Available: http://www.hydrogen.energy.gov/pdfs/review05/stp_42_macdiarmid.pdf; May 24, 2005.

[61] Virji S, Kaner RB, Weiller BH. Hydrogen sensors based onconductivity changes in polyaniline nanofibers. J Phys ChemB 2006;110:22266e70.

[62] Sadek AZ, Wlodarski W, Zadeh KK, Shin K, Kaner RB. A roomtemperature polyaniline/In2O3 nanofiber composite basedlayered ZnO/64� YX LiNbO3 SAW hydrogen gas Sens. IEEESens J; 2006:687e90.