Research Article

Received: 16 March 2009, Revised: 1 May 2009, Accepted: 3 May 2009, Published online in Wiley InterScience: 27 May 2009

(www.interscience.wiley.com) DOI: 10.1002/pat.1470

Nanofibrous polyaniline thin film prepared byplasma-induced polymerization technique fordetection of NO2 gas

Ashutosh Tiwaria*,y, Rajendra Kumarb, Mani Prabaharanc, Ravi R. Pandeya,Premlata Kumarid, Anurag Chaturvedie and A. K. Mishrab

A nanofibrous polyaniline (PANI) thin film was fabrica

Polym. Adv

ted using plasma-induced polymerization method and exploredits application in the fabrication of NO2 gas sensor. The effects of substrate position, pressure, and the number ofplasma pulses on the PANI film growth rate were monitored and an optimum condition for the PANI thin filmpreparation was established. The resulting PANI film was characterized with UV–visible spectrophotometer, FTIR,scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The PANI thin film possessednanofibers with a diameter ranging from 15 to 20nm. The NO2 gas sensing behavior was studied by measuring thechange in electrical conductivity of PANI filmwith respect to NO2 gas concentration and exposure time. The optimizedsensor exhibited a sensitivity factor of 206 with a response time of 23 sec. The NO2 gas sensor using nanofibrous PANIthin film as sensing probe showed a linear current response to the NO2 gas concentration in the range of 10–100ppm.Copyright � 2009 John Wiley & Sons, Ltd.

Keywords: plasma polymerization; polyaniline; nanofiber; thin film; NO2 gas sensor

* Correspondence to: A. Tiwari, Division of Engineering Materials, NationalPhysical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India.E-mail: ashunpl@gmail.com; tiwaria@mail.nplindia.org

a A. Tiwari, R. R. Pandey

Division of Engineering Materials, National Physical Laboratory, Dr. K. S.

Krishnan Marg, New Delhi 110012, India

b R. Kumar, A. K. Mishra

Department of Physics, Nanak Chand Anglo-Sanskrit College, Meerut 250001,

India

c M. Prabaharan

Department of Mechanical Engineering, University of Wisconsin-Milwaukee,

Milwaukee, WI 53211, USA

d P. Kumari

Applied Science and Humanities Department, S.V. National Institute of

Technology, Surat 395007 Gujarat, India

e A. Chaturvedi

Department of Physics, University of South Florida, 4202 East Fowler Avenue,

Tampa FL 33620, USA

y Present address: Department of Mechanical Engineering, University of

Wisconsin-Milwaukee, Milwaukee WI 53211, USA.

Contract/grant sponsor: Department of Science and Technology, Government

of India. 6

INTRODUCTION

There is a growing need to detect hazardous environmentalgases such as nitrogen dioxide in the atmosphere, which areemitted from the combustion processes, in order to effectivelymonitor air quality and prevent adverse health problems. NO2 gassensors based on chemical luminescence as well as IR absorptionhave been commonly used; however, they are expensive, large insize, and sometimes cannot operate at room temperature.[1]

Thus, much attention has been paid recently to the developmentof compact, low-priced sensors that can detect NO2 gas in realtime at room temperature. There have been considerableinterests in utilizing the organic substances, such as pentacene,[2]

porphyrin,[3] phthalocyanines,[4,5] and doped conductive poly-mers,[6] for sensing purpose. The conducting polymers are foundto be good candidates for the fabrication of chemical or elec-trochemical sensors.[7] The sensors based on conducting polymers,mostly of the conductometric, potentiometric, and amperometric,could providemore precise data in a limited concentration range. Itis difficult to build sensors with enhanced sensitivity and the abilityto detect specific gas in a complex gas environment and reset themquickly for the next sensing cycle. With the advent ofnanotechnology, nanostructured materials with novel character-istics provide new opportunities to address these challenges.Gas sensors based on nanostructured materials have attracted

much attention because of their increased sensitivity due to thehigh surface-to-volume ratio.[8] Recent progress of nanostruc-tured polymers with every imaginable combination of physicaland chemical characteristics has led to the fabrication of efficientgas sensors[9–11] that can be used for a wide range of applica-tions.[12] These polymers do not suffer from sensing complications

. Technol. 2010, 21 615–620 Copyright � 200

and synthesis complexities, and they possess high efficiency witha long shelf-life; however, most of the polymers used to makesensors are not specific and do not have quick response to theanalyte.[13] These problems can be overcome by developing athree-dimensional nanostructed conducting polymer as a NO2

sensor using plasma-induced polymerization technique.

9 John Wiley & Sons, Ltd.

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Plasma-induced polymerization has been widely used for thepreparation of conducting polymeric thin films includingpolyaniline (PANI).[14,15] The thin films produced by this methodshowed a good opacity, uniform thickness, and adhesionconformability with the substrates.[16] The composition of aplasma polymerized film can be tailored with the appropriateprocessing conditions and the properties of the resulting thinfilm are often unique and unobtainable by wet syntheticmethods.[17] In the plasma-induced polymerization, though theexcited plasma species are mainly high energy electrons that arerelatively indiscriminate in rupturing chemical bonds, thesubstrate can be kept at the ambient temperature.[18]

In this study, we explore nanofibrous PANI thin film preparedby plasma-induced polymerization technique for potential NO2

gas sensing application. The effects of substrate position,pressure, and the number of plasma pulses on the depositionof PANI film were optimized. The morphology of the thin filmwas analyzed by SEM and TEM. The NO2 gas sensing behaviorwas studied by measuring the change in electrical conductivityof PANI film with respect to NO2 gas concentration andexposure time. The major advantages of this new type of sensorare low-cost, high sensitivity and specificity to NO2 gas at ppmlevel.

EXPERIMENTAL

The reagent grade aniline (Aldrich; 99.5%) monomer was usedafter purification by triple distillation over zinc granules andstored in a dark bottle under nitrogen. The NO2 gas standardswere prepared by mixing two parts of NO gas (Zhuo Zheng gaslimited company, 99.9%) with one part of oxygen (De-Luxe, 99%)in the gas bottles.[19] The gas bottles were kept for two weeks toallow any residual oxygen to react with the blended NO.

Instrumentation

A hand-made assembled plasma reactor was used for thepreparation of nanofibrous PANI thin film. In the system,thermocouple gauge (GIC-11-B, Veeco Instruments Inc.) andcapacitance manometer (627A01TBC, MKS Baratron) wereattached to monitor the reactor pressure. The calibration ofmanometer was performed in triplicate from 0 to 1000mTorrwith air, aniline vapor, and a mixture of aniline vapor andhydrogen. Once enough monomer was injected into the plasmareactor at the desired deposition pressure, the plasma wasrepetitively activated through the discharge of a 1.8mF capacitorinitially held at 23 kV, the RF coil excitation being with a dampedsinusoid of 290 kHz and a decay time constant of 10msec. After 10plasma shots, the reactor was completely evacuated and refilledwith fresh monomer vapor. Plasma pulses ranging from 50 to 100were used to grow PANI films at the reactor pressure in the rangeof 13 to 40 Pa. The substrate holder was movable and could bepositioned at various distances from the RF coil. The glasssubstrate, with inter digited gold and glass slides, was used assubstrate to deposit the PANI. The thickness of the representativeplasma-polymerized PANI films was measured using profilometer(SPN Technology).

Preparation of nanofibrous PANI thin film

Nanofibrous PANI thin film was prepared by an inductivelycoupled pulsed-plasma reactor at different RF plasma pulsing,

www.interscience.wiley.com/journal/pat Copyright � 2009 John

monomer injection, and substrate positions. The custom builtautomotive injector with an oscilloscope was used to control theinjection of vaporized aniline monomer under pressure rangingfrom 13 to 40 Pa. As a pulse of the aniline entered into theevacuated reactor, most of the aniline immediately vaporized byflash boiling and the remaining aniline disintegrated intodroplets that were collected by mesh separators placed about2 cm from the injector nozzle. The aniline was injected with10msec pulses at different reactor pressure into the plasmareactor. All the PANI depositions were performed with a static fillof aniline vapor at the electrical pulses ranging from 50 to 100over the substrate.

Characterizations

The absorbance of nanofiborous PANI thin film was determinedusing UV–visible spectrophotometer (Ocean optics HR 4000). FTIRspectrum was collected on a Perkin-Elmer (Spectrum BX II)spectrometer. The surface morphology of the thin film wasexamined with a LEO-440 SEM operated at 5 kV. The specimenswere sputter-coated with a thin layer of gold (�20 nm) prior toexamination. The morphology of PANI nanofibers was furtherstudied by transmission electron microscopy (TEM, FEI-Morgagni-268D) operated at 75 kV. A TEM sample was preparedby depositing 6 mL solution of PANI (ultrasonically dispersed inTHF) on a copper grid coated with formbar and a carbon filmusing phosphotungstic acid as a negative staining agent. Theelectrical conductivity measurements were carried out at 208Cwith a Keithley electrometer (comprising a fast x-y-t recorder of220 programmable current source) having 181 nanovoltameter,and 195A digital multimeter.

Fabrication of nanostructured sensing probe

An inter-digited glass electrode attached with copper wires wasplaced in a plasma reactor. Next, a nanofibrous PANI thin film wasdeposited in between the inter-digited space of electrode andthe remaining portion of electrode was masked. In the plasmareactor, the electrode was placed at 15.5 cm from the RF coil andan average deposition rate of 3.15 nm/pulse at 40 Pa pressure wasapplied.

Design and construction of NO2 gas sensor

The PANI thin film was kept in a glass chamber, wherein NO2 gaswas injected in a chamber using an automatic gas-tight syringe.During study, NO2 gas of desirable concentration was simul-taneously supplied and unused NO2 gas was taken out fromchamber as exhaust gas. The current passed through theinter-digited PANI probe was quantitatively measured aselectrical conductivity with respect to the varying concentrationof NO2 gas ranging from 10 to 100 ppm.

RESULTS AND DISCUSION

Nanofibous PANI thin film: optimization plasma condition

The PANI thin films were prepared using plasma polymerizationtechnique and the thickness of the films was varied by changingthe plasma conditions. Figure 1 shows the profile-meter data ofPANI thin films prepared by various plasma conditions includingthe pressure of plasma reactor and number of plasma pulses. The

Wiley & Sons, Ltd. Polym. Adv. Technol. 2010, 21 615–620

ThicknessDeposition rate

13/500

50

100

150

200

250

300

350

1

2

3

4

5

Thi

ckne

ss (n

m)

Pressure (Pa)/pulse number

Dep

ositi

on r

ate

(nm

/pul

se)

13/100 27/50 27/100 40/50 40/100

Figure 1. Film thickness and average deposition rate of nanofibrousPANI thin film.

NANOFIBROUS POLYANILINE THIN FILM

thickness of PANI thin film increases with the increase in appliedpressure and number of pulses of the reactor. The film depositionrate was initially constant and then increased with the increase inpressure from 27 at pulse number 100 to 40 Pa at pulse number50. However, the deposition rate was dramatically reduced at40 Pa when using 100 pulse numbers. At threshold pressure,higher pulse number may cause depolymerization of PANI thatmay be responsible for this observation. The initial constantdeposition rate observed was due to the incubation effects.Hence, thin film surface defects were strongly influenced by thenumber of pulses as reported previously.[20] Moreover, thepolymerization of aniline using plasma polymerization methodwas a function of distance between the RF coil and substrate inthe plasma reactor. In order to find the optimum distancebetween RF coil and substrate for film growth, the glass substratewas placed at various distances such as 15.5, 24, 31, and 39 cmfrom the center of the RF coil under identical plasma conditions(40 Pa and 50 plasma pulses). The sample placed at 15.5 cm wasfound to be at an ideal distance for the deposition of nanofibrousPANI film. At this distance, RF coil may exhibit the highest

300 400 500 600 700 800 9000.0

0.3

0.6

0.9

1.2

1.5

Abs

orba

nce

(au)

Wavelength (nm)

(A) (B

Figure 2. (A) UV–Vis absorption spectra and (B) FTIR spectrum of n

Polym. Adv. Technol. 2010, 21 615–620 Copyright � 2009 John Wiley

retention of anilinemonomer functionality for the polymerizationreaction. The PANI film prepared on the glass substrate placed ata distance of 15.5 cm from the RF coil under 40 Pa pressure and 50plasma pulses was used for further studies.

Characterizations

PANI thin film was characterized using UV–visible spectropho-tometer as shown in Fig. 2A. In the spectrum, the characteristricbands of PANI (emeraldine base) was observed at 320, 440, and620 nm due to the p�p� transition, polaron bands transition, andundoped quinoid unit, respectively.[21] These absorption bandsexhibit both excitations of amine nitrogen of the benzenoidsegments and imine nitrogen of the quinoid segments of PANI.[22]

This result clearly indicates the formation of PANI by the plasmapolymerization.The FTIR spectrum of the PANI thin film is shown in Fig. 2B. The

characteristic peaks of emeraldine base form of PANI wereobserved in the spectrum at 3261 cm�1 (N–H stretching withhydrogen bonded 28 amino groups); 3027 cm�1 (aromatic C–Hstretching); 1537 cm�1 (C––C stretching of quinoid rings);1482 cm�1 (C––C stretching vibration of benzenoid rings); and1286 cm�1 (C–N stretching).[23] The absorption band at1126 cm�1 is assigned to N––Q––N bending vibration shifttowards the lower wave number that corresponds to the PANIemeraldine base.[24] This result further supports the formation ofPANI using plasma polymerization technique.The morphology of the nanofibrous PANI film was analyzed

with SEM and TEM. As shown in Fig. 3A, the film exhibited aninterconnected fibrous topology with a diameter range of15–20 nm. The SEM images demonstrate the three-dimensionalnanostructured PANI fibers onto the substrate. The formation ofthree-dimensional nanostructured PANI probe can provide anexcellent surface to interact the NO2 gas during sensingmeasurement because of the increased surface to volume ratio.The TEM micrograph of PANI film showed interconnectednanofibers of PANI with a length of approximately 1000 nmand a diameter in the range of 15–20 nm (Fig. 3B). These studiesindicated that plasma polymerization at 40 Pa pressure producesuniform nanofibrous PANI thin film with interconnected polymernetwork.

500 1000 1500 2000 2500 3000 35000

20

40

60

80

Tra

nsm

ittan

ce (%

)

(cmWavenumber -1)

N-H

C-N

C=CC=C

N=Q=N

C-H

)

anofibrous PANI thin film prepared via pulsed plasma technique.

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Figure 3. (A) SEM images of the nanofibrous PANI thin film and (B) TEM micrograph of nanofiborous PANI deposited onto the glass substrate.

A. TIWARI ET AL.

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NO2 gas sensing

The NO2 gas sensing behavior of nanofibrous PANI thin film wasevaluated by the conductometric method. The change in theconductivity of the sensing PANI probe was recorded as afunction of NO2 concentration at room temperature. Thesensitivity of the sensor was estimated from the measured valueof conductivity in air and in presence of NO2 gas. The graph ofchange in conductivity of PANI probe at 3.5 V as a function of NO2

concentration is shown in Fig. 4. From this figure, it is observedthat the conductivity of the probe was increased by increasingthe concentration of NO2 gas from 10 to 100 ppm. The interactionof NO2 gas with the p-electron network of PANI captures theelectron from the polymer and decreases the resistance of probe.As PANI is an n-type semiconductor, it will create a space chargeregion at the PANI-NO2 interface. During NO2 gas adsorption, ahigh conductivity of sensing probe can be achieved because ofreduction in the space charge region. The insertion of fresh air inthe test chamber removes the adsorb gas molecules from theprobe surface and regains the original current. The modulation ofspace charge region at the interface of probe gives a high rangeof sensitivity for NO2 gas and hence nanofibrous PANI probe canoperate at room temperature. Furthermore, in this study, a

10 20 30 40 50 60 70 80 90 100

0

100

200

300

400

500

[NO2] ppm

Con

duct

ivity

-1cm

)-1

Figure 4. Effect of NO2 gas concentration on the conductivity of nano-

fibrous PANI thin film probe at 3.5 V.

www.interscience.wiley.com/journal/pat Copyright � 2009 John

relatively fast response (�23 sec) and recovery (2min) time wasobserved for NO2 gas using PANI sensor.Figure 5 shows the effect of thickness of PANI film on NO2 gas

sensitivity factor at 50 ppm of NO2 gas concentration. Thesensitivity factor of PANI film was found to be initially increasedwith the increase in film thickness from 30 to 100 nm and then itdramatically decreased. This observation indicates that thethickness of PANI film up to 100 nm is adequate for NO2 gassensor application. In general, during the sensing measurement,gas analyte adsorbs by the probe surface and then the adsorbedanalyte diffuses through the inter-domain space to interact withthe emeraldine state of PANI for electronic charge transfer. It isobvious that the increased thickness of film will increase theinter-domain space within the sensing probe that will result inpoor sensitivity of the probe.

Interference study

The effect of interference (NH3, NO, and CO2) was studied on theconductometric responses of the sensor employing thenanofiborous PANI sensing probe. These three substances wereadded into the gas sensing chamber at their normal physical

0 50 100 150 200 250 300 350170

180

190

200

210

220

Sens

itivi

ty fa

ctor

(S)

Thickness (nm)

Figure 5. Plot of sensitivity factor versus thickness of PANI thin film at

50 ppm NO2 gas.

Wiley & Sons, Ltd. Polym. Adv. Technol. 2010, 21 615–620

Table 1. Effect of interference on the nanofiborus PANI thin film based NO2 gas sensor at 3.5 V

Sl. no. Analyte/interference Conductivity response (m V�1/cm)

1 NO2 (20 ppm) 1162 NO2 (20 ppm)þNH3 (5 ppm) 1133 NO2 (20 ppm)þNO (5 ppm) 1184 NO2 (20 ppm)þCO2 (10 ppm) 115

Table 2. Comparison between different materials tested for the NO2 gas sensors

Sensing probe Linearity with [NO2]Responsetime (sec) Shelf-life Sensitivity factor Reference

Nanofiborous PANI thin film 10–100 ppm 23 6 months 206 Present workPANI/polystyrenesulfonic acidcomposite film

20–100 ppm 10 — — [29]

PANI–SnO2 composite film 10–800 ppb 1000 — — [30]

PANI nanofibers 10–200 ppm 100 �4 months 10 [31]

NANOFIBROUS POLYANILINE THIN FILM

concentration, i.e. NH3 (5 ppm); NO (5 ppm); and CO2 (10 ppm).Table 1 shows the effect of interference on the sensing probe. Itwas found that the presence of interferences had a negligibleeffect on the conductivity obtained at a fixed concentration ofNO2 gas. This observation indicates that PANI probe can be usefulto detect NO2 gas without any interference. Table 2 compares thecharacteristics of PANI based NO2 gas sensors as reported in theliterature. From this comparison, it is observed that nanofiborousPANI sensing probe exhibited a longer shelf life, higher selectivity,and moderate response time in a limited NO2 gas concentrationrange.

Sensing mechanism

In earlier studies, it was reported that NO2 gas can quantitativelyincrease the conductivity of polymer films.[25,26] It is interpretedthat a charge transfer complex is formed between a PANI filmdonor and NO2 gas acceptor, resulting in the charge transfers byholes in the nanofibrous PANI film matrix. NO2 gas is p-electronacceptor, and accepted electron would delocalize over the NO2

planar structure. The high selectivity towards the NO2 gas may beexplained on the basis of charge transfer complex formedbetween the PANI film donor and NO2 acceptor molecules tocause fluctuation in terms of conductivity.[27,28] The evidencesupported that the present probe has ability to specifically senseNO2 gas and generate electrical signals that can be significantlyexplored for the fabrication of NO2 gas sensing device.

6

CONCLUSIONS

Nanofibrous PANI thin film was prepared by plasma polymeri-zation technique as a NO2 gas sensor. The effects of substrateposition, pressure, and the number of plasma pulses on the PANIfilm growth rate were optimized. The sensor response wasmeasured with the change of conductivity that increased linearly

Polym. Adv. Technol. 2010, 21 615–620 Copyright � 2009 John Wiley

with an increased NO2 gas concentration in a range from 10 to100 ppm at room temperature. The sensitivity factor ofnanofibrous PANI probe was found to be dependent on thethickness of the PANI film. The optimum sensitivity factor wasfound at the film thickness of 100 nm. The response time andrecovery time of the sensor were found to be �23 sec and 2min,respectively, which shows that the nanofibrous PANI sensor couldbe reused more frequently thus extending the shelf life of thesensor. The nanostructured PANI film could be an excellent probefor NO2 gas sensor application because of its large surface tovolume ratio.

Acknowledgements

Authors are thankful to the Director, National Physical Laboratory,New Delhi, India for providing infrastructure facilities to carry outthis work.

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