studies on tin oxide-intercalated polyaniline nanocomposite for ammonia gas

On exposure to ammonia gas 100–500 ppm in air at room temperature, it was found that the PANI film resistance increases, while that of the nanocomposite PANI + SnO2 film decreases from the

Contents lists available atScienceDirect Sensors and Actuators B: Chemical

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s n b

Studies on tin oxide-intercalated polyaniline nanocomposite for ammonia gas sensing applications

N.G Deshpandea,c, Y.G Gudagea, Ramphal Sharmaa,∗, J.C Vyasb, J.B Kimc, Y.P Leec

aThin Film and Nanotechnology Laboratory, Department of Physics, Dr B.A Marathwada University, Auranganbad 431004 (M.S.), India

bTechnical Physics and Prototype Engineering Division, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India

cQuantum Photonic Science Research Center and BK21 Program Division of Advanced Research and Education in Physics, Hanyang University, Seoul 133-791, Republic of Korea

a r t i c l e i n f o

Article history:

Received 12 August 2008

Received in revised form

22 December 2008

Accepted 2 February 2009

Available online 20 February 2009

Keywords:

Conducting polymer

Solution route technique

Nanocomposite PANI films

Surface morphology

Optical studies and gas sensor analysis

a b s t r a c t

Thin films of tin oxide-intercalated polyaniline nanocomposite have been deposited at room temperature, through solution route technique The as-grown films were studied for some of the useful physico-chemical properties, making use of XRD, FTIR, SEM, etc and optical methods XRD studies showed peak broadening and the peak positions shift from standard values, indicating presence of tin oxide in nanopar-ticles form in the polyaniline (PANI) matrix FTIR study shows presence of the Sn–O–Sn vibrational peak and characteristic vibrational peaks of PANI Study of SEM micrograph revealed that the composite par-ticles have irregular shape and size with micellar templates of PANI around them AFM images show topographical features of the nanocomposite similar to SEM images but at higher resolution Optical absorbance studies show shifting of the characteristics peaks for PANI, which may be due to presence

of tin oxide in PANI matrix On exposure to ammonia gas (100–500 ppm in air) at room temperature, it was found that the PANI film resistance increases, while that of the nanocomposite (PANI + SnO2) film decreases from the respective unexposed value These changes on removal of ammonia gas are reversible

in nature, and the composite films showed good sensitivity with relatively faster response/recovery time

© 2009 Elsevier B.V All rights reserved

1 Introduction

Metal oxide thin-film gas sensors are widely used for detecting

gas species by measuring changes in their physical properties on

exposure to specific gas, in particular making use of reversible redox

reactions in presence or absence of the specific gas media Usually,

the small change in chemical state of the film material may reflect as

measurable change in some physical properties, such as electrical

conductivity, which can be monitored by external electrical circuits

Pure tin oxide, SnO2is a remarkable n-type semiconductor material

having wide band gap (∼3.6 eV), and by making use of small

quan-tity of dopant into it’s matrix, thin films of this material find use in

several devices such as flat panel displays, gas sensors[1,2], etc to

name a few However, the sensors incorporating tin oxide require

an elevated temperature (≥200◦C) for their optimum operation.

This calls for a separate temperature controlled heater assembly to

operate the device, and requiring extra power for heating In

addi-tion, the sensor operation at elevated temperature in itself causes

gradual changes in the tin oxide film properties, which in turn

devi-ate gas sensing properties of the device with time Therefore, it is

highly desirable to have sensors, which can operate at room

tem-∗ Corresponding author Tel.: +91 9422793173; fax: +91 240 2403335/3115.

E-mail address:ramphalsharma@yahoo.com (R Sharma).

perature, but having comparable properties with that of tin oxide for gas sensing

Conducting polymers (CPs) are in use as an alternative to metal oxide materials for gas sensing applications Among the CPs, polyaniline (PANI) has become one of the technologically impor-tant CPs, because of it’s relatively easier synthesis, and for having excellent electronic and electro-chromic properties It has been used in making organic solar cell, as well as gas sensor applications [3–6] However, PANI is not as sensitive as metal oxides towards gas species, and its poor solubility in organic solvents limits its applica-tions In spite of these problems with PANI, efforts are being made

to improve its solubility by involving protonation with organic acids

or preparing it using emulsion polymerization in presence of sur-factants[7] There have been several reports on improving PANI’s sensitivity and selectivity by making use of new methods, such as its synthesis in nano-structured forms[8,9], or by addition of metal catalysts[10,11], and by combination with other polymers[12] Recently a new class of materials emerged, known as compos-ites, prepared by mixing suitably the organic and inorganic base materials in proper form The composite materials have special properties, but as seen in some of the cases, they can also have few desirable properties from both the parent organic and inorganic class of materials As a consequence, there are growing interests in combining both organic and inorganic materials for applications in electronics, optics, magnetism, etc.[13–15] In literature, there are 0925-4005/$ – see front matter © 2009 Elsevier B.V All rights reserved.

some reports concerning PANI/inorganic nanocomposite sensors

[16–18] However, very few researchers have studied the composite

SnO2/PANI for sensor application[19,20]

We fabricated nanocomposites thin films of SnO2/PANI by

incor-porating SnO2particles in the form of colloidal suspensions in PANI

through solution route technique The as-grown composite films

were characterized using X-ray diffraction (XRD), Fourier transform

infrared (FTIR) spectroscopy, scanning electron microscopy (SEM),

atomic force microscopy (AFM), and optical absorbance studies The

as-grown films were exposed to NH3gas at room temperature and

the electrical response was noted For a comparison, thin films of

tin oxide, and PANI were also prepared separately, and evaluated

along with the tin oxide/PANI composite films for sensing

ammo-nia gas at room temperature We report our findings in this paper

and discuss a plausible mechanism for the formation and electronic

behaviour of such nanocomposites

2 Experimental

2.1 Synthesis

We employed solution-route technique, to synthesize tin

oxide/polyaniline nanocomposites In this technique, formation of

nanocomposites proceeds through an inorganic/organic interface

reaction Tin chloride (SnCl4·5H2O), hydrogen peroxide (H2O2),

aniline, ammonium peroxydisulphate (APS) [(NH4)2S2O8] and

hydrochloric acid (HCl) (all chemicals having AR grade), were

pur-chased from M/s Loba Chemie, Mumbai (India) Aniline monomer

was distilled under reduced pressure Initially, SnCl4·5H2O, was

hydrolyzed, using 2 g of SnCl4·5H2O in 50 ml of double distilled

water (DDW) with constant stirring, and it’s pH was maintained

at≤4, using dilute HCl Hydrogen peroxide was added in the above

solution, which oxidizes tin ions to tin oxide, and the solution turns

into a white colored suspension of SnO2and it serves as the starting

reaction mixture for further processing From this reaction mixture,

40 ml volume was taken and mixed with appropriate volume of

ani-line, and kept below 4◦C After 30 min, the APS solution was added

in the above mixture to make the reaction bath mixture In this

bath mixture pre-cleaned glass substrates were inserted vertically

It was found that after few minutes the solution color turns bluish

to green, which also mark the growth of film on the substrate

The reaction mechanism was studied by monitoring the changes

in pH and temperature of the reaction bath with time[21–24], for

both cases, i.e., for baths having with the tin oxide nanoparticles

suspension and without it The pH and temperature were measured

by digital␮-pH system 361, supplied by Systronics

The as-grown films were washed with DDW and dried

Simi-larly, the precipitate was washed thoroughly using DDW, dried and

casted into pellets In order to study the response of the above

films to ammonia gas, silver contacts were made on top of the

film surface, by vacuum evaporation technique and making use

of shadow masking For this purpose HIND-HIGHVAC system was

used The chamber pressure during silver evaporation was kept

around 0.5× 10−5Torr, and during metal evaporation, film

sub-strates were not heated

2.2 Characterization

The physical thickness of the as-grown nanocomposites film

was measured using Fizeau fringe technique, and it was about

191 nm The XRD patterns of these films were recorded on a Bruker

AXS (D8 Advanced, Germany) diffractometer in the scanning range

of 20–70◦ (2) using Cu K␣ radiation having a wavelength of

1.5405 Å The infrared spectrum of nanocomposite samples

pel-letized with KBr were measured using a Fourier transformed

infrared spectrometer (PerkinElmer’s Spectrum1 spectrometer)

The surface morphology was studied by field emission scanning electron microscopy (FESEM JEOL-JSM 6500F) The surface mor-phology was studied using an AFM (Nanoscope IIIa produced

by Vecco Digital Instruments) The root mean square (rms) sur-face roughness was determined using software provided with the microscope Absorbance spectra were recorded in a range of 300–1000 nm by means of a PerkinElmer Lambda 25 UV–VIS spec-trophotometer For evaluating the gas sensing properties of these films, a known concentration of ammonia gas (3N purity, sup-plied by M/s Chemtron Industries, Mumbai) was purged into a test chamber (made up of steel) kept at room temperature, by using micro-syringe The gas sensing behaviour of the as-grown films was

determined by measuring the current–voltage (I–V), characteristics

in absence/presence of NH3gas, and data was recorded online, using

a computer interfaced with the system

3 Results and discussion

3.1 Reaction mechanism

Variation in pH and temperature of reaction bath with respect to time for both cases, i.e., for bath containing tin oxide nanoparticles

Fig 1 Reaction kinetics for (a) polyaniline and (b) tin oxide/polyaniline

nanocom-suspension and without it, is shown inFig 1a and b It may be seen

that during the induction period (starting time of about 2–3 min),

small change in bath temperature noticed, but subsequently a

pro-nounced increase in bath temperature follows, indicating a faster

film growth rate, and mainly due to the exothermic nature of aniline

polymerization

A simple explanation to describe such a process may be

follow-ing The cation radicals, as the primary intermediates of aniline

oxidation, are produced in the homogeneous aqueous medium and

a fraction of them is adsorbed at available surfaces sites in random

way, as allowed by thermo-dynamical state of the system These

sites initiate growth of future PANI chains, in which the monomer

molecules join (with such chains) and in the process give out their

own kinetic energy (of free monomer state) to the system This

transfer of energy apparently raises the temperature of the bath

by an equivalent amount, and observed during the process of

poly-merization (Fig 1a and b)

The heterogeneous catalytic film growth requires formation of

initial seeding on the fresh substrate In the reaction bath without a

substrate into it, aniline monomers are in random thermo-dynamic

state, and depending upon thermo-chemical conditions can join

with each other to form small size (2 and higher monomer units)

polymer molecules (such as dimer, trimer, etc.) known as oligomers

In normal conditions, the reverse of this process, i.e., the breaking

of oligomers into smaller units also goes on simultaneously in the

reaction bath, at about similar rate At a fresh surface (based on its

electro-phobic or electro-philic nature), the reactivity of adsorbed

entities (monomer and oligomers) can be substantially enhanced

compared with free species, allowing a relatively larger probability

of surface attachment[25] Once initial nucleation takes place on

the fresh surface, the activation energy for the next layers (steps)

in the polymerization slowly decrease as number of steps increase

Finally, after some time there is no net growth of the film when

equilibrium between the joining rate and detaching rate become

almost equal

The polymerization at the surface, producing a PANI film, and

the polymerization in the bulk, giving rise to a PANI precipitate,

proceed in succession, the former having relatively larger rates in

the start, but after some finite time it equals with the latter

How-ever, when tin oxide nanoparticles are also present in the reaction

bath, these SnO2particles impede the growth rate of the PANI film,

and we see different times for the induction, and oxidative

poly-merization periods Similarly the pH, and the temperature of the

bath are also different for above two cases Therefore, tin oxide

composite film formation should take somewhat larger time, and

indeed we observed this difference experimentally as shown in

Fig 1a and b It was found that for PANI reaction to occur the

induction time was about 2 min, polymerization time was about

5 min, the maximum temperature evolved was 39.4◦C and pH

was∼1.17 In case of tin oxide-intercalated polyaniline reaction to

occur the induction time was 2 min 40 s, polymerization time was

8 min, maximum temperature was 34.9◦C and pH was 1.01

3.2 Structural analysis

The XRD patterns for tin oxide, PANI and tin oxide/PANI

nanocomposites, are shown inFig 2a–c, respectively.Fig 2a reveals

that the material deposited is SnO2 of polycrystalline in nature

On comparing the observed XRD peaks and corresponding planes

with the standard (h k l) planes a good matching was seen between

the two sets, confirming that the deposited films consist of SnO2

having primitive tetragonal structure (JCPDS DATA CARD 41-1445).

The XRD pattern for tin oxide thin films showed diffraction peaks

along (1 1 0), (1 0 1), (2 0 0), (2 1 1), (3 1 0) and (3 0 1), respectively

The films were preferentially oriented along (2 0 0) plane The

aver-age value of lattice parameters was found to be a = b = 4.755 Å and

Fig 2 XRD patterns for (a) tin oxide, (b) polyaniline and (c) tin oxide/polyaniline

nanocomposite.

c = 3.205 Å, while the standard bulk value for tin oxide crystalline

structure is respectively, a = b = 4.738 Å and c = 3.187 Å This suggests

that the tin oxide grains in thin film form are strained, may be due

to the smaller average physical size of the grains themselves The average crystallite size found using the standard Scherer’s formula was equal to 40 nm.Fig 2b shows the XRD pattern for PANI films, which suggests that the film has amorphous structure.Fig 2c is the XRD patterns for tin oxide intercalated in the PANI matrix, and one can see the presence of peaks corresponding to tin oxide nano-crystallites However, these peaks are slightly shifted, from their respective standard positions, may be due presence of PANI matrix

In addition, we observed reduced intensity of the peaks, and rela-tively larger peak broadening, compared with XRD of pure SnO2 film This indicates still smaller average size of tin oxide nano-crystallites in composite film, compared that for pure SnO2 film

The lattice constant was found to be a = b = 4.716 Å and c = 3.24 Å;

while the average crystallite size was found to be nearly 23 nm The (2 0 0) peak of tin oxide is seen in XRD of composite material shown

inFig 2c, along with some other peaks However, intensity of (3 0 1) peak is suppressed in the composite film compared to XRD of pure tin oxide This suggests that tin oxide is present in the PANI matrix, and presence of PANI has influenced the preferred orientation of tin oxide grains in the film to some extent

3.3 Fourier transform infrared analysis

In order to find the nature of bonding in the film material we studied FTIR spectrum of tin oxide/PANI precipitate.Fig 3shows the FTIR spectrum for SnO2/PANI nanocomposites, having peaks at wave numbers 1579, 1490, 1446, 1288, 1367, 1160, and 738 cm−1,

Fig 3 FTIR spectrum for tin oxide/polyaniline nanocomposite.

respectively These peaks correspond to most of the

characteris-tic peaks for PANI, as described in literature[26,27] The peaks

at wave numbers 1579 and 1490 cm−1 are attributed to C N and

C C stretching mode for the quinoid and benzenoid rings; while

the peak at wave number 1446 cm−1is attributed to C–C aromatic

ring stretching of the benzenoid diamine unit The peaks at wave

numbers 1288 and 1367 cm−1are attributed to C–N stretching; and

peak at wave number 1160 cm−1is considered to be due to N Q N

stretching The peak at the wave number 738 cm−1 is attributed

to C–H out of plane bending vibrations However it may be noted

that these peaks are slightly shifted with respect to their normal

positions as seen for pure PANI films Once again these peak

shift-ings might be due to the presence of tin oxide in the PANI matrix

Furthermore, we observed a strong peak at wave number 615 cm−1,

which is due to the antisymmetric Sn–O–Sn mode in SnO2as shown

in literature[28–30], and in a way confirms presence of tin oxide

in the PANI matrix Dutta and De[28]have observed similar results

for tin oxide/PANI nanocomposites

3.4 Surface morphological analysis

The SEM micrographs of as-grown films of tin oxide,

polyani-line and tin oxide/PANI nanocomposites, are shown inFig 4a–c,

respectively The SEM profile shown inFig 4a, indicates fine

gran-ular surface of tin oxide, covering the entire glass substrate, with

some agglomeration of finer particulates to form bigger clusters

Such agglomerations result in case of metal oxide films deposited

by chemical methods[31,32] The average grain size was∼120 nm

In case of pure PANI, the film growth appears to be of dendritic

nature, with some part of it having growth of amorphous phase

(Fig 4b) In case of tin oxide/PANI nanocomposites films (Fig 4c),

Fig 4 FESEM micrographs for (a) tin oxide, (b) polyaniline and (c) tin

oxide/polyaniline nanocomposite (inset is a high-resolution magnified image of nanocomposite).

the composite particles are highly dispersed, with less amount of agglomeration The average grain size was∼80 nm, with dispersion

of±5 nm The observed difference in the measurement of the grain size by XRD and SEM would be due to the fact that two or more

crystallites may be fused together to form a particle (not resolved

by SEM profile, but XRD can figure out easily)[33,34]

The grain growth of nanoparticles in such films may be

understood through two basic mechanisms of aggregation These

mechanisms depend on the dispersion of colloidal particles at

low-solid volume fractions (ϕ0→ 0) having with

(a) diffusion-limited cluster aggregation (DLCA), and

(b) reaction-limited cluster aggregation (RLCA)

In DLCA every collision between two clusters results in the

for-mation of a new cluster, the aggregate of the two colliding clusters

In RLCA only a small fraction of all the collisions leads to the

forma-tion of a new aggregate[35] InFig 5, a simple schematic is shown,

which provides reaction mechanism for formation of such kind of

structures In the present case, formation of polymer shell around

the nano-crystalline particle/s can easily be seen in the magnified

image inset of Fig 4c, assisting the growth and further

aggre-gates formation indicating a DLCA type mechanism; but it appears

that for SnO2/PANI film formation, the RLCA mechanism preferably

dominates This is because, the reaction kinetics as reflected from

Fig 1a and b, show a relatively lower temperature of polymerization

at low-pH values, and gives rise to limited aggregation Secondly,

the final thickness of the film is limited to around less than 200 nm

In contrast to this, with the DLCA mode, a much larger film thickness

can be achieved, but not observed by us

The concentration of surface states has correlation with the

roughness and grain size via the surface-to-volume ratio, and the

gas sensitivity has a proportional relationship with the film

rough-ness In order to study the surface roughness, the film samples were

characterized using AFM.Fig 6a–c show respective AFM profiles

for tin oxide, PANI and SnO2/PANI nanocomposites The rms

sur-face roughness was found to be 46.1, 22.7 and 31.5 nm for tin oxide,

PANI and SnO2/PANI nanocomposites, respectively Notice that the

surface roughness of nanocomposites films 31.5 nm, is in between

that of the pure tin oxide and pure PANI films

3.5 Optical analysis

In case of conducting polymers, optical spectroscopy is an

important technique to understand the conducting states

corre-sponding to the absorption bands of inter-gap and intra-gap states

[36] Usually PANI-HCl shows three characteristic peaks of

absorp-tion in wavelength bands 306–324, 402–420 and 828–835 nm,

respectively The peak in wavelength band 306–324 nm is due to

the␲–␲*transition of benzenoid ring; the peak of wavelength band

402–420 nm, is due to the polaron–␲*transition and the peak in

wavelength band 828–835 nm, is attributed to the␲–polaron

tran-sition In addition, the peaks in wavelength bands 402–420 and

828–835 nm, arise owing to the doping level and the formation of

polarons[37–39] InFig 7, we show optical absorbance with respect

to wavelength, for pure PANI and tin oxide/PANI nanocomposite

thin films The observed absorption peak positions in present case

were found at∼324, ∼430, and ∼828 nm, for pure PANI; whereas in

case of tin oxide/PANI nanocomposites these peaks were at∼303,

∼430, and ∼800 nm, respectively It is interesting to note that the

characteristic peaks of the doped PANI appear in the SnO2/PANI

nanocomposite thin films, but with some shift in their positions

(especially for 324 and 828 nm peaks) compared with the pure film

Such shifts in the characteristic peak positions of one or both of the

composite forming species are related with surface modifications,

and similar shift of peak positions in CdS/PANI nanocomposites

films, has been observed by Pethkar et al.[14] In addition, there is

an increase in the absorption at lower wavelengths in the SnO2/PANI

nanocomposites case This is characteristic property of oxides,

indi-cating the presence of tin oxide

Fig 6 AFM images for (a) tin oxide, (b) polyaniline and (c) tin oxide/polyaniline

nanocomposites.

3.6 Gas sensor analysis

The as-grown films of tin oxide, polyaniline, and SnO2/PANI composites were tested for ammonia gas at room temperature For this films having metallic contacts were kept in the test chamber

of known volume with electrical leads taken out for electrical mea-surements A fixed amount (corresponding to 100 ppm) of NH3gas was injected into the test chamber, and film resistance measured with respect to time (for every 10 s interval), until it reached a steady value This procedure was followed once again after

remov-Fig 7 Optical absorbance versus wavelength for tin oxide/polyaniline

nanocom-posites.

ing NH3 and exposing the test chamber to clean air These steps

were repeated for all three different films and for different NH3

gas concentrations (100–500 ppm) InFig 8a–c, we show typical

current–voltage characteristics taken for pure tin oxide, pure PANI,

and the SnO2/PANI composite films kept at room temperature (RT),

respectively It is seen from Fig 8a that no appreciable change

noticed in the film resistance for the case of pure tin oxide film, on

exposure to different concentrations of NH3gas, and tin oxide films

remained insensitive to this gas at RT However, in case of pure PANI

films (Fig 8b), we see large changes in the film resistance on NH3

gas exposure The film resistance increases by more than an order of

magnitude from its original value within a minute, indicating that

the electrical resistance of PANI films is a sensitive parameter in the

presence of ammonia gas, as reported earlier in literature[40,41]

The I–V characteristics of the composite films show a different but

more interesting phenomenon as may be seen fromFig 8c, that

the composite SnO2/PANI film resistance decreases on exposure to

ammonia (∼300 ppm) Furthermore, the I–V characteristics of

com-posite SnO2/PANI films show a diode-like exponential behaviour,

a characteristic of percolation in disordered systems, wherein the

electrical conductance is through hopping mechanism Kukla et al

[41]proposed that the sensitivity and reversibility of pure PANI

lay-ers to NH3gas exposure is a deprotonation–reprotonation process,

and the film resistance show an exponential rise with increase in

NH3concentration, this mechanism seems to fit with our

obser-vations However, the decrease in resistance of composite film on

exposure to ammonia gas needs further explanation

It is well known that tin oxide is an n-type semiconductor, while

PANI films are normally of p-type semiconductor This is due to the

fact that during the polymerization process of aniline, acids (such as

HCl) are used, which acts as dopant for PANI molecules, and usually

bound with the central N atom of aniline (monomer) molecule, like

H+ N Cl−(other bonds on sides of N atom are left here for want

of clarity, and more details are provided in literature, see Fig 4 of

Ref.[41]) In equilibrium at room temperature, the positive charge

of bonded hydrogen shifts on N atom, making the structure looks

like H N+ Cl− While the negative charge on Cl−is retained with

it and remains localized, the positive charge on nitrogen becomes

mobile charge in PANI matrix, via its other bonds, making the PANI

as a p-type semiconductor[41]

In presence of SnO2crystallites, the PANI matrix gets a modified

structure electronically The PANI molecules encapsulate each SnO2

crystallite, similarly toFig 5 The SnO crystallites being an n-type

Fig 8 I–V curves (in the presence of ammonia gas) for (a) tin oxide, (b) polyaniline

and (c) tin oxide/polyaniline nanocomposites.

surrounded by p-type PANI molecules make a p–n junction like formation locally, immersed within PANI matrix of the composite film The n-type nature of SnO2crystallites annihilate the holes of PANI molecules, near its boundary making a depletion layer like region, which in turn makes the overall PANI matrix electrically more insulating in nature

A tentative explanation of change in electrical resistance of com-posite film may be following On exposing the comcom-posite film with ammonia (which can be permeated into the PANI matrix freely), some of the NH3molecules might reach into the depletion region, which is surrounding the SnO crystallite and act as a dielectric

Fig 9 Sensitivity versus concentration plot for tin oxide, polyaniline and tin

oxide/polyaniline nanocomposites.

between the PANI and SnO2border The depletion region field might

polarize the ammonia molecules, and in turn provide a positive

charge to PANI molecules, which can become mobile on its transfer

to the central N atom of PANI molecule So in all this process

cre-ates some free holes on PANI molecules, which increase the hopping

conductivity of the film, and therefore make the composite film

rel-atively more conducting electrically Once the process of polarizing

the ammonia molecules by p–n junction like formation is saturated,

this mechanism cannot generate additional holes in the composite

PANI film and therefore no additional change in the film

conductiv-ity even by further addition of ammonia to it In the present case this

saturation happened at around 300 ppm, as seen inFig 9 However,

it may be noted that ammonia gas within PANI regions of the

com-posite film, opens up another channel parallel to above mechanism

always present in case of pure PANI films, making them more

resis-tive on gas exposure In this channel, ammonia molecules exchange

the mobile hole charge with central N atom of PANI molecule and

make it localized This reduces the conductivity of the film, as found

in case of pure PANI films So on exposure with ammonia, both of the

channels compete with each other, and the dominating channels

dictate the direction of net change in resistance of the composite

film

Sensitivity (S%) is defined as the relative variation of the

resistance of the sensitive film in percent per ppm of applied

gas concentration, i.e., (|Rgas − Rair|/·Rair) × 100, whereas gas

response is defined as|Rgas − Rair|/Rair, ‘Rair’ is the resistance of

sensor in air, ‘Rgas’ is the steady resistance of sensor in the

pres-ence of a test gas and ‘’ is the concentration of gas (in ppm) In

Fig 9, we show sensitivity (S%) of pure tin oxide, pure PANI, and

the tin oxide/PANI nanacomposite film, on exposure to ammonia

for different concentrations (100–500 ppm) For the case of pure

tin oxide film, no response found (i.e., having response value 1, or

no change in film resistance) within explored range However, for

pure PANI films the response value increases linearly up to 300 ppm,

and saturate thereafter or slightly decrease for larger ammonia gas

concentrations In case of SnO2/PANI nanocomposite film, a smooth

increase of response was seen up to 300 ppm, and it remains same

thereafter It can be seen that at 300 ppm concentration of

ammo-nia gas, both pure PANI, and SnO2/PANI composite films had highest

response

We also studied response and recovery time of the films with

respect to ammonia gas exposure The response time, and the

recov-ery time are defined as the time required for a film resistance to

Fig 10 Sensing reproducibility and reversibility curves for (a) polyaniline and (b)

tin oxide/polyaniline nanocomposites.

reach 90% of its saturation value from the starting value on gas exposure, and on removal of the gas, respectively In our case, the PANI films had relatively faster response times∼8–10 s, but as usual the recovery times were relatively larger, around 160 s Notice that the larger recovery times are due to the slower out diffusion rate (concentration dependent) of the gas, which always decreases as time progress Furthermore, these diffusion rates are small at room temperature The SnO2/PANI nanocomposites films have response times of 12–15 s, and the recovery times around 80 s It may be seen that the SnO2/PANI nanocomposites films showed faster recovery time (a factor of 2) as compared to the PANI films InFig 10a and

b, we show typical response of the film with respect to time, for repeated exposure and removal of ammonia (300 ppm) gas, and it may be seen that both PANI and SnO2/PANI nanocomposites films showed good reproducible resistance change for a number of cycles

4 Conclusions

We synthesized tin oxide-intercalated polyaniline nanocompos-ites (SnO2/PANI) in thin film form, and compared the properties

of the composite films with that of the thin films made from the constituent base materials XRD studies were used to find particu-late size, while FTIR study showed presence of both SnO2and PANI molecules SEM micrograph of these nanocomposite films revealed that the constituent composite particles have irregular shape and size, and encapsulated by fibrous PANI matrix It was found that

pure SnO2films remain inert on NH3gas exposure at RT However,

presence of SnO2crystallites in the nanocomposites SnO2/PANI film

changes the electronic property of PANI matrix in drastic way While

pure PANI films become more resistive on exposure to NH3gas, the

composite film becomes less resistive on a similar exposure We

have provided a suitable explanation for such behaviour of these

films These SnO2/PANI nanocomposites films showed good

sensi-tivity, reproducibility with relatively faster response for ammonia

gas, at room temperature In addition, the nanocomposites films

showed faster recovery time (twice) as compared with the PANI

films However, there are still many other issues pertaining to gas

sensing activity which need more attention, such as long-term

sta-bility, selectivity with specific gas, etc and need further research in

this field

Acknowledgments

We are thankful to BRNS-DAE Project No 2005/34/1/BRNS/380

for financial assistance to carry out the research work We are

also thankful to Head, Department of Physics, Dr B.A.M

Univer-sity, Aurangabad for providing the lab facilities In addition, we

highly acknowledge the help rendered by Dr R.S Devan and Prof Y

Ma, Department of Physics, National Dong Hwa University, Taiwan

for doing SEM characterization of our samples as well as

help-ful discussions Authors especially, N.G Deshpande (currently), J.B

Kim and Y.P Lee were supported by the KOSEF through

Quan-tum Photonic Science Research Center, Seoul, Korea, and by MEST,

Korea

References

[1] M.J Madou, S.Y Morison, Chemical Sensing with Solid State Devices, Academic

Press, San Diego, 1989.

[2] N Barsan, U Weimar, Understanding the fundamental principles of metal oxide

based gas sensors; the example of CO sensing with SnO 2 sensors in the presence

of humidity, J Phys Condens Matter 15 (2003) R813–R839.

[3] M Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338–344.

[4] A.G MacDiarmid, Synthetic metals: a novel role for organic polymers (nobel

lecture), Angew Chem Int Ed 40 (2001) 2581–2590.

[5] A.G MacDiarmid, A.N Xia, J.M Wiesinger, Electrically Active Polymers, US Pat.

5,773,568, 1998.

[6] Y.S Negi, P.V Adhyapak, Development in polyaniline conducting polymers, J.

Macromol Sci Polym Rev 42 (2002) 35–53.

[7] G.G Wallace, G.M Spinks, A.P Kane-Maguire, P.R Tesdale, Conductive

Elec-troactive Polymers: Intelligent Materials Systems, CRC Press, 2002.

[8] J Huang, S Viriji, B.H Weiller, R.B Kaner, Nanostructured polyaniline sensors,

Chem Eur J 10 (2004) 1314–1319.

[9] J Huang, R.B Kaner, The intrinsic nanofibrillar morphology of polyaniline,

Chem Commun 4 (2006) 367–376.

[10] H.S Li, M Josowicz, D.R Baer, M.H Engelhard, J Janata, Preparation and

char-acterization of polyaniline–palladium composite films, J Electrochem Soc 142

(1995) 798–805.

[11] A.A Athawale, S.V Bhagwat, P.P Katre, Nanocomposite of Pd–polyaniline as a

selective methanol sensor, Sens Actuat B 114 (2006) 263–267.

[12] S.T McGovern, G.M Spinks, G.G Wallace, Micro-humidity sensors based on a

processable polyaniline blend, Sens Actuat B 107 (2005) 657–665.

[13] S.J Su, N Kuramoto, Processable polyaniline–titanium dioxide

nanocompos-ites: effect of titanium dioxide on the conductivity, Synth Met 114 (2000)

147–153.

[14] S Pethkar, R.C Patil, J.A Kher, K Vijaymohanan, Deposition and characterization

of CdS nanoparticle/polyaniline composite films, Thin Solid Films 349 (1999)

105–109.

[15] K.R Reddy, K.P Lee, A.I Gopalan, Self-assembly approach for the synthesis of

electro-magnetic functionalized Fe 3 O 4 /polyaniline nanocomposites: effect of

dopant on the properties, Colloid Surf A: Physicochem Eng Aspects 320 (2008)

49–56.

[16] J Wang, I Matsubara, N Murayama, S Woosuck, N Izu, The preparation of

polyaniline intercalated MoO 3 thin film and its sensitivity to volatile organic

compounds, Thin Solid Films 514 (2006) 329–333.

[17] T Taka, Humidity dependency of electrical conductivity of doped polyaniline,

Synth Met 57 (1993) 5014–5019.

[18] S.S Joshi, T.P Gujar, V.R Shinde, C.D Lokhande, Fabrication of

n-CdTe/p-polyaniline heterojunction-based room temperature LPG sensor, Sens Actuat.

B 132 (2008) 349–355.

[19] M.K Ram, O Yavuz, M Aldissi, NO 2 gas sensing based on ordered ultrathin films

[20] L Geng, Y Zhao, X Huang, S Wang, S Zhang, S Wu, Characterization and gas sensitivity study of polyaniline/SnO 2 hybrid material prepared by hydrothermal route, Sens Actuat B 120 (2007) 568–572.

[21] Y Fu, R.L Elsenbaumer, Thermochemistry and kinetics of chemical polymer-ization of aniline determined by solution calorimetry, Chem Mater 6 (1994) 671–677.

[22] T Sulimenko, J Stejskal, J Prokeˇs, Poly(phenylenediamine) dispersions, J Col-loid Interf Sci 236 (2001) 328–334.

[23] Y Fong, J.B Schlenoff, Polymerization of aniline using mixed oxidizers, Polymer

36 (1995) 639–643.

[24] P.M Beadle, Y.F Nicolau, E Banka, P Rannou, D Djurado, Controlled polymer-ization of aniline at sub-zero temperatures, Synth Met 95 (1998) 29–45 [25] P.W Atkins, Physical Chemistry, 3rd ed., Oxford University Press, Oxford, 1986,

pp 762–789.

[26] T Abdiryim, Z.X Gang, R Jamal, Comparative studies of solid-state synthesized polyaniline doped with inorganic acids, Mater Chem Phys 90 (2005) 367–372 [27] S Quillard, G Louarn, S Lefrant, A.G MacDiarmid, Vibrational analysis of polyaniline: a comparative study of leucoemeraldine, emeraldine, and perni-graniline bases, Phys Rev B 50 (1994) 12496–12508.

[28] K Dutta, S.K De, Optical and nonlinear electrical properties of SnO 2 –polyaniline nanocomposites, Mater Lett 61 (2007) 4967–4971.

[29] G Zhong, M Liu, Preparation of nanostructured tin oxide using a sol–gel pro-cess based on tin tetrachloride and ethylene glycol, J Mater Sci 34 (1999) 3213–3219.

[30] S Monredon, A Cellot, F Ribot, C Sanchez, L Armelao, L Gueneau, L Delattre, Synthesis and characterization of crystalline tin oxide nanoparticles, J Mater Chem 12 (2002) 2396–2400.

[31] T.P Niesenand, M.R De Guire, Review: deposition of ceramic thin films at low temperatures from aqueous solutions, J Electroceram 6 (2001) 169–207 [32] G Korotcenkov, V Tolstoy, J Schwank, Successive ionic layer deposition (SILD)

as a new sensor technology: synthesis and modification of metal oxides, Meas Sci Technol 17 (2006) 1861–1869.

[33] D.S Sutrave, G.S Shahane, V.B Patil, L.P Deshmukh, Micro-crystallographic and optical studies on Cd 1−x ZnxSe thin films, Mater Chem Phys 65 (2000) 298–305 [34] C.M Shen, X.G Zhang, H.L Li, Influence of different deposition potentials on morphology and structure of CdSe films, Appl Surf Sci 240 (2005) 34–41 [35] P Sandkuhler, J Sefcik, M Morbidelli, Kinetics of gel formation in dilute dis-persions with strong attractive particle interactions, Adv Colloid Interf Sci 108–109 (2004) 133–143.

[36] M.K Ram, O Yavuz, V Lahsangah, M Aldissi, CO gas sensing from ultrathin nano-composite conducting polymer film, Sens Actuat B 106 (2005) 750–757 [37] A.G Macdiarmid, A.J Epstein, The concept of secondary doping as applied to polyaniline, Synth Met 65 (1994) 103–116.

[38] H Jiang, Y Geng, J Li, F Wang, Organic acid doped polyaniline derivatives, Synth Met 84 (1997) 125–126.

[39] B.J Kim, S.G Oh, M.G Han, S.S Im, Synthesis and characterization of polyaniline nanoparticles in SDS micellar solutions, Synth Met 122 (2001) 297–304 [40] D.S Sutar, N Padma, D.K Aswal, S.K Deshpande, S.K Gupta, J.V Yakhmi, Prepa-ration of nanofibrous polyaniline films and their application as ammonia gas sensor, Sens Actuat B 128 (2007) 286–292.

[41] A.L Kukla, Y.M Shirshov, S.A Piletsky, Ammonia sensors based on sensitive polyaniline films, Sens Actuat B 37 (1996) 135–140.

Biographies

Mr N.G Deshpande is currently working for his PhD degree (from 2008) in

Depart-ment of Physics, Hanyang University, Seoul, South Korea under the supervision

of Prof YoungPak Lee Current interest of research work is 1D and 2D magnetic photonic crystals and their applications Earlier worked as Senior Research Fellow (SRF) in BRNS-DAE project related to oxides, polymers and hybrid materials for gas sensor application (2005–2008) He published nearly 18 international research papers and attended/presented (research work) at various international/national conferences.

Mr Y.G Gudage is currently working for his PhD degree (from 2006) in

Depart-ment of Physics, Dr B.A Marathwada Univeristy, Aurangabad (M.S.), India under the supervision of Dr Ramphal Sharma Current research interest is photoelectrochem-ical solar cells He worked as Senior Research Fellow (SRF) in BRNS-DAE project on gas sensor applications He has published nearly 12 international research papers and attended various conferences.

Dr Ramphal Sharma received his PhD in 1991 from Rajasthan University, Jaipur,

India Currently, he is Associate Professor at Department of Physics, Dr B.A.M Uni-versity, Aurangabad (M.S.), India Currently, he is a Brain Pool Fellow in Department

of Chemistry, Hanyang University, Seoul, Korea He has more than 15 years of expe-rience in teaching field; while 20 years of expeexpe-rience in research, i.e., in thin film technology He has published more than 80 international and national papers in reputed journals His main interest of research is gas sensor, photosensor and solar cells He was visiting fellow of ICTP, Trieste, Italy in 1999–2001.

Dr J.C Vyas postgraduated in Physics from University of Rajasthan, Jaipur, and

received PhD from Bombay University, Mumbai He joined BARC in 1980, and over years worked in several different fields of technical interests, such as fabrication

single crystals, oriented thin films growth using MBE and their characterization,

high-temperature superconducting thin films based weak links for device

applica-tions, and thin film based gas sensors He is a member of Indian Thermal Analysis

Society, Material Research Society of India, etc.

Mr JinBae Kim received his BS and MS degrees in Department of Physics of

Sunmoon University, Korea, in 2000 and 2002, respectively He has been a PhD

candidate in Department of Physics from Hanyang University from 2002 He is

cur-rently focused on the physics and applications of magnetic nanostructures and

magnetic photonic crystals He has published nearly 20 papers in international

journals and attended/presented his work at various reputed international/national conferences.

Prof YoungPak Lee is currently Director of Quantum Photonic Science Research

Center and Distinguished Professor in Department of Physics, Hanyang University, Seoul, Korea He received his PhD degree in Condensed-Matter Physics, Iowa State University, Ames, Iowa, U.S.A (1987) Besides this he has worked at various reputed posts and has been awarded many honors from Ministry of Science and Technology, Korea and others His research interest is magnetic photonic crystals, meta-materials, nanomagnetism.