Different chemical approaches for the synthesis of polyaniline

nanofibers and its application in ammonia gas sensing

Vivek Talwar1,2,a, Ravi Chand Singh2,b

1Department of RIC,Punjab Technical University, Kapurthala, Punjab, India

2Department of Physics, Guru Nanak Dev University, Amritsar, Punjab, India

avivek_ptu@yahoo.in, bravichand.singh@gmail.com

Keywords: Polyaniline nanofibres, Conducting polymers, Rapid mixing, Gas sensing.

Abstract

Polyaniline nanofibers of varying morphology were synthesized using two different chemical

methods. The polyaniline samples were prepared through the oxidation of aniline in an ice bath. In

the first method, the oxidant is added drop wise in aniline solution whereas in other the samples

were prepared via rapid mixing of oxidant into aniline solution. The structural and morphological

analysis of prepared samples was carried out using XRD, FTIR and FESEM techniques. The thick

films of the synthesized powder were deposited on alumina substrate and their sensing response to

various volatile gases was investigated at room temperature. The morphology of synthesized

polyaniline powder depends upon method of synthesis and thus effect the sensing response and

selectivity of the fabricated sensor.

Introduction

Conducting polymers has been attraction to various research groups working the field of sensors

due to its unique electrical properties [1]. The relative ease of synthesis and room temperature

operation makes the conducting polymers promising candidates in the field of gas sensing.

polyaniline (PANI) has been found very promising material among the conducting polymers for the

detection of hazardous gases [2]. PANI can easily be synthesized by oxidation of aniline monomer

at room temperature. However its synthesis at low temperatures yields PANI with molecular weight

five to ten times higher than that synthesized at room temperature [3].Although it has easy to

synthesize polyaniline through different methods but it has very rich doping/dedoping chemistry

made it a promising material for gas sensors [4-5]. The selectivity and response of gas sensors

based upon PANI can be improved by controlling different parameters like doping, monomer to

oxidant ratio, reaction temperature [6-10].In this paper, we had reported the effect of method of

mixing of oxidant to gas sensing properties of polyaniline.

Experimental details

All the chemicals used for the synthesis of polyaniline (PANI) were of analytical grade and were

used without further purification Aniline, ammonium persulphate (APS), Hydrochloric acid (HCl),

m-cresol were procured from Spectrochem, India. To synthesize PANI, aniline was oxidized with

ammonium persulphate (APS) in aqueous acid solution. The solutions of aniline and APS with

monomer to oxidant molar ratio 1:1.25 were dissolved separately in 1 M HCl solution. Both the

solutions were placed in an ice bath (0-4o

C) and then oxidant was added drop wise to aniline

solution with constant stirring and kept in same ice bath for 4 hours. The obtained solution was kept

at room temperature for polymerization for 24 hours. The polymerized salt was filtered and washed

repeatedly with 1M HCl and double distilled water to remove excess acid. Finally filtrate was dried

in air and then in vacuum at 60°C. The final product was polyaniline emeraldine salt S1. Another

polyaniline sample was prepared following above said method except oxidant is added rapidly in

monomer solution without stirring in ice bath. After polymerization, filtration and washing we got

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polyaniline salt as residue S2. The thick films of the synthesized powder were deposited on alumina

substrate by dissolving it in m-cresol and after drying at 60o

C, response to various volatile gases

was investigated at room temperature.

Material Characterization

The structural and morphological analysis of synthesized samples S1 and S2 was done using X-ray

diffraction, Infrared spectroscopy and FESEM techniques. For crystal structure analysis, the

prepared samples were characterized by powder X-ray diffraction (XRD) using Cu Kα radiation

with Shimadzu 7000 Diffractometer and field emission scanning electron microscope (FESEM)

with Ziess, model Supra 55.

Results and Discussion

X-ray Diffraction (XRD) and Infrared (IR) Spectroscopy

Fig. 1 represents the x-ray diffraction pattern of PANI samples S1 and S2 prepared via

conventional and rapidly mixing methods. In both the samples the characteristic peaks of PANI

were observed. The change in the intensity of peaks was due to varied morphology of the

synthesized samples. From this we observe the sample S2 would be more crystalline than sample 1

Fig. 2 represents the infrared spectra of polyaniline samples S1 and S2. The IR peaks characteristics

of PANI were observed in both the samples.

10 20 30 40 50 60 70 80

Inte

nsi

ty (

a.u.)

<2Theta> (Degree)

(b)

(a)

4000 3500 3000 2500 2000 1500 1000 500

Tra

nsm

itta

nce

(a

.u.)

Wavelength (cm-1)

( S2)

(S1)

Fig. 1: XRD spectra of samples (a) S1 and (b) S2. Fig. 2: IR spectra of samples (a) S1 and (b) S2.

574 Materials and Applications for Sensors and Transducers III

Scanning Electron Microscopy

Fig. 3 represents SEM image of PANI samples S1 and S2 prepared by two different methods of

mixing of oxidant to monomer solution. Although the final product is polyaniline salt in both the

cases, but the morphology of synthesized samples was quite different. Fig. 3 clearly reflects the

formation of fine nanofibers of long length. The diameter of these fibers is about 55-65 nm whereas

the length of these fibers is few µm. It was found that the diameter of nanofibers prepared through

rapid mixing technique is about 90-100 nm but the length of these nanofibers is less than 1 µm, but

these are mophed in better way for enhancing surface area as shown in figure 4.

Fig. 3: SEM micrographs of sample S1 at different magnifications.

Fig. 4: SEM micrographs of sample S2 at different magnification.

100 200 300 400 500

1.0

1.5

2.0

2.5

3.0

3.5

Sen

sin

g R

esp

on

se

Ammonia (ppm)

Sam ple S1

Sam ple S2

0 200 400 600 800 1000 1200 1400

3.0

3.5

4.0

4.5

5.0

5.5

Sen

sor

Res

ista

nce

Rs

(KΩ

)

Time (seconds)

Fig. 5: Sensing response vs ppm of samples S1 and S2. Fig. 6: Sensing response vs time for sample S2

Key Engineering Materials Vol. 605 575

Gas sensing properties

Fig. 5 represents the gas sensing responses of PANI samples S1 and S2 for varying concetration of

ammonia gas at room temperature (27oC). The sensing response of sample S2 is more than that for

sample S1. Fig. 6 represents the actual sensor

resistance vs time for sample S2 at 27oC,

when exposed to 100 ppm of ammonia gas..

Fig. 7 represents the sensing response of

samples S1 and S2 for 100 ppm of ammonia,

ethanol, methanol and acetone at room

temperature.

The sample S1 had shown rapid increses in its

response than that for sample S2. The reason

for this might be favourable morphlogy and

more surface area for sample S2 as compared

with sample S1.It has easily predicted that

sample S2 is more selective as response is

more for ammonia but lesser for ethanol,

methanol and acetone.

Conclusion

From above we can conclude that the gas sensor made from sample prepared by rapid mixing

technique had better response and selectivity than prepared by conventional method of drop wise

addition. The reason for better sensing response and selectivity may be attributed to favourable

morphology of polyaniline samples prepared via rapid mixing technique of oxidant to monomer

solution than conventional method of drop wise mixing.

References

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[3] P.N. Adams, P.J. Laughlin, A.P. Monkme: Synth. Met.Vol. 76 (1996) p. 157-160.

[4] M. Ayad Mohamad, A. Zaki. Eman: Eur. Polym. J.Vol. 44 (2008) p. 3741-3747.

[5] J. Stejskal, I. Sapurina, M. Trchova: Prog. Polym. Sci. Vol. 35 (2010) p. 1420-1481.

[6] A.B. Samui, A. S. Patankar, R. S. Satpute, P. C. Deb: Synth. Met. Vol. 125 (2001) p. 423-427.

[7] J. Stejskal, M. Omastova’b, S. Fedorovac, M. Trchova: Polym. J. Vol. 44 (2003) p. 1353-1358.

[8] S. Adikari, J. Singh, R. Banerjee, P. Benerji: Sensor Lett. Vol. 9 (2011) p. 1807-1813.

[9] H. Kim, S. Hyun, H. Park: Sensor Lett. Vol. 9 (2011) p. 59-63.

[10] A. Roy, A. Kumar, M. Sasikala, M. Prasad: Sensor Lett. Vol. 9 (2011) p. 1342-1348.

Ammonia Ethanol methenol Acetone

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Sen

sin

g R

esp

on

se (

For

100

pp

m)

Gases

Sample S1

Sample S2

Fig. 7: Sensing response of S1 and S2 for different

gases

576 Materials and Applications for Sensors and Transducers III

Materials and Applications for Sensors and Transducers III 10.4028/www.scientific.net/KEM.605 Different Chemical Approaches for the Synthesis of Polyaniline Nanofibers and its Application in

Ammonia Gas Sensing 10.4028/www.scientific.net/KEM.605.573