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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 2

Avai lab le a t www.sc iencedi rec t .com

journa l homepage : www.e lsev ier . com/ loca te /he

Preparation and characterization of tantalum/polyanilinecomposite based chemiresistor type sensor for hydrogen gassensing application

Subodh Srivastava*, Sumit Kumar, Y.K. Vijay

Thin Film and Membrane Science Lab, University of Rajasthan, Jaipur-302055, India

a r t i c l e i n f o

Article history:

Received 19 February 2011

Received in revised form

11 April 2011

Accepted 12 April 2011

Available online 14 June 2011

Keywords:

Polyaniline (PANI)

Ta/PANI composite

Chemiresistor sensor

H2 gas sensing

Atomic force microscopy (AFM)

X-ray diffraction (XRD)

* Corresponding author. Tel.: þ91 (0) 141 270E-mail address: subodhphy@gmail.com (

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.155

a b s t r a c t

In the present work we have reported the effect of Shift heavy ion (SHI) irradiation on the

gas sensing properties of tantalum (Ta)/Polyaniline (PANI) composite thin film based

chemiresistor type gas sensor for hydrogen gas sensing application. PANI was synthesized

chemically by in situ oxidative polymerization method. The thin sensing films of PANI

were deposited onto finger type Cu-interdigited electrodes using spin cast technique and

a thin Ta layer was deposited on to PANI thin film to prepare Ta/PANI composite chem-

iresistor sensor. These chemiresistor sensing films were irradiated with energetic Auþ12

ions (150 MeV) at the different fluencies ranging from 1 � 109 to 1 � 1011 ions/cm2. The

structural and morphological properties of these composite thin films were characterized

by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements before and

after SHI irradiation. The electrical properties of these composite thin films were charac-

terized by IeV characteristic measurements. The changes in resistance of the composite

thin film sensor were utilized for detection of hydrogen gas. It was observed that after SHI

irradiation Ta/PANI composite sensor shows a high response value and sensitivity with

good repeatability in comparison to the pristine sample.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Generally, polymers are deposited as sensitive layer of

In recent years numerous sensors with polymeric materials

have been developed for detecting many analytes, ionic

species, organic vapours and gases [1,2]. The incorporation of

metal nanoparticles into conducting polymers has been

reported by a number of groups [3e6]. Generally, these efforts

were directed at the development of gas sensitivematerials in

order to improve the sensitivity, response time and stability of

gas sensors. It has been reported earlier that metal species in

the conducting polymer ensures high surface area, improved

conductivity and possible enhancement of the unique char-

acteristics of the composite [7,8].

2457; fax: þ91 (0) 141 270S. Srivastava).2011, Hydrogen Energy P

sensor and then metal thin film are deposited on the surface

of the sensitive polymer layer as active material to increase

the area/volume ratio and favour the adsorption of gases. The

deposition of active metal precursor can be made by thermal

evaporation or sputtering techniques. In case of hydrogen gas

platinum, palladium, silver and titanium have been widely

reported as active element therefore their composite with

polymer may have potential applications in gas sensors and

electrocatalysis [9e15]. Among the conducting polymer,

Polyaniline (PANI) has been preferred as a sensitive media

for hydrogen sensing, due to its environmental stability,

selectivity and sensitivity towards hydrogen gas at room

1149.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 23826

temperature. The further incorporation of sensitive nano-

particles makes the PANI composite more sensitive towards

hydrogen gas. Therefore PANI has also been used as a host

matrix for the active filers like palladium, platinum, gold,

silver and titanium [16e22].

1.1. Swift heavy ions (SHI)

It is worth noting that the sensitivity of gas sensors is strongly

affected by the structural and morphological parameter of

sensing materials [23].

The ion irradiation has been established as a potential tool

for material modification [24]. As an ion penetrates a solid, it

loses energy by two distinct interactions: (1) by elastic nuclear

collisions with the target atoms (nuclear stopping Sn) and (2)

by excitation or ionization of atoms by inelastic collisions,

(known as Electronic Stopping, and the energy spent in this

process is called electronic energy loss (Se).

When the energy of incoming ion is very high, the elec-

tronic stopping is dominant, where the displacement of atoms

due to elastic collisions is insignificant. Such heavy ions, with

energies so high that the electronic loss process dominates,

are referred to as Swift heavy ions (SHI) [25]. The direct

interaction between energetic ion and target atoms can lead to

structural changes such as generation of point defects,

amorphization of crystalline materials or phase trans-

formations in amorphous atomic networks, surface modifi-

cation, mixing of materials and formation of new compounds

[26e31]. The SHI induces themixing at the interface in order to

produce novel composite materials and phases. The mixing

takes place due to transfer of energy, by the energetic ions in

the electronic subsystem, subsequently transfer to atomic

subsystem via electron-phonon coupling which results in

a rise in the lattice temperature up to 104 K. Therefore the

material within few nanometers from the ion path melts for

the duration of 10�12 e10�11 s and then quenches at very fast

rate, forming the latent tracks and this induces inter-mixing

in bilayer systems [32,33]. On irradiation with SHI,

a dramatic change in the structural and electrical properties of

polyaniline composites was also observed [34e37]. The SHI

irradiated polyvinylchloride polyethylene terephthalate (PVC-

PET) composites have been tested for hydrogen gas and

ammonia gas sensitivity [38,39].

Theuseof tantalum, tantalumalloysandtantalumoxidehas

already been suggested for sensor purposes [40,41]. Tantalum

(Ta) is an active element for hydrogen gas and has been studied

to investigatediffusionofhydrogen in it [42e45].There isalways

afiniteprobability forhydrogenordeuteriumatomtooccupyan

interstitial site in its metal lattice. Two phases, designed a and

b are known to exist in the tantalum-hydrogensystemand their

properties determine, to a large extent, the kinetics of the

diffusion process [46e48].. The presence of hydrogen, or media

containing hydrogen compounds (which can liberate free

hydrogen) can lead to change in mechanical and structural

properties of Ta surface [49].

It has been reported earlier that Ta and Niobium react with

carbon, nitrogen, oxygen, and hydrogen at room temperature

[50,51]. It has also been reported that Ta, with a properly acti-

vated surface, may be a suitable element for hydrogen sepa-

rationmembrane [52e54]. Therefore it is impotent to use Ta as

an active material with PANI for hydrogen gas sensing. Sepa-

rately PANI has been widely tested for hydrogen gas sensing

application [55e58]. However, to the best of our knowledge

Ta/PANI composite films have not yet been used as a sensitive

layer in chemiresistor type sensor for hydrogen gas sensing.

In the present work PANI was synthesized chemically by in

situ oxidative polymerization of aniline using ammonium

persulfate in acidic medium at low temperature. The thin

sensing films of PANI were deposited onto finger type cu-

interdigited electrodes using spin cast technique and a thin

Ta layerwas deposited on to PANI thin film to prepare Ta/PANI

composite chemiresistor sensor. These chemiresistor sensing

films were irradiated with Auþ12 ions at the different fluencies

and the effect of SHI irradiation on the gas sensing properties

of Ta/PANI composite films was studied for hydrogen gas.

2. Experimental

2.1. Materials

PANI was synthesized by in-situ chemical oxidative poly-

merization method as described elsewhere [59,60]. Tantalum

sheet (99.9% pure, 1 mm thick and 2 inch diameter) was used

as sputtering target for thin film deposition as shown in Fig. 1.

2.2. Sensor preparation

CSA-PANImixture was dissolved in 30ml chloroform solution

using magnetic stirrer. Thin films of this solution were

deposited on cleaned Cu-IDE epoxy substrates using the spin

coating technique and a thin tantalum layer was deposited

onto spin coated PANI thin film using DC magnetron sput-

tering system under high vacuum of the order of 10�5 torr.

In the sputtering process the Ta target is normally fixed at

10e12 cm apart from the substrate holder and positioned in

front of the sample surface. The substrate was rotated during

depositionyieldingauniformthickness throughout the sample.

Argongaswas inserted as the sputtering gas through theneedle

valve at a constant pressure of 1 � 10�1e2 � 10�1 torr. The

schematic diagram of Ta coated PANI Chemiresistor sensor is

shown in Fig. 2.

2.3. Swift heavy ions (SHI) irradiation

In the present work, a 15 UD Pelletron Accelerator facility

located at IUAC New Delhi was used for SHI irradiation

[61e63]. It is basically a Van de Graff type tandem electrostatic

accelerator in vertical configuration, having maximum

terminal voltage up to 16 MV.

The prepared samplesweremounted on target assembly in

material science chamber under high vacuum (10�6 Torr) The

SHI irradiation was performed at room temperature using

Auþ12 ions having energy of 150 MeV at different fluencies

ranging from 1� 109 to 1� 1011 ions/cm2 depending on time of

bombardment. The beam current was kept 1 pnA and moni-

tored intermittently with a Faraday cup. The ion beam was

defocused using magnetic scanning system, so that an area of

1 � 1 cm2 was uniformly irradiated. The irradiated samples

were stored at room temperature in air.

Fig. 3 e XRF spectra of Ta/PANI composite before and after

SHI irradiation.

Fig. 1 e Ta sputtering target mounted in target holder.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 2 3827

3. Result and discussion

3.1. X-ray fluorescence (XRF)

The XRF has been used to analyze the elemental composition

of composite materials. Fig. 3 shows the XRF spectra of

Ta/PANI composite, which exhibits two characteristic X-ray

energy peaks at Ka ¼ 8.1 keV and Kb ¼ 9.2 keV corresponding to

tantalum (see inset in Fig. 3) and confirms the presence of Ta

in PANImatrix. It has also been observed that after irradiation,

the intensity of energy peak has been decreased with

increasing ion fluence. The decreased in peak intensity is due

to two reasons: theremay be small variation in thickness of Ta

layer during sputtering process due to the difference in target

to source distance (10e12 cm) which influence the number of

Ta atoms deposited at the surface of PANI and hence the

intensity of corresponding x-ray energy peak of Ta decreased.

Secondly theremay bemixing of tantalumwithin PANImatrix

during irradiation process therefore the relative XRF counts

coming from the surface are decreased.

Fig. 2 e Schematic diagram of Ta coated PANI thin film

chemiresistor sensor and (b) prepared sensor.

3.2. X-ray diffraction (XRD)

The XRD pattern of Ta/PANI composite films before and after

the SHI irradiation is shown in Fig. 4. The XRD pattern of PANI

(ES) exhibits two broad amorphous peaks, observed at

2q z 20.1� and 2q z 25.2� (Fig. 4a), which may be attributed to

periodicity parallel and perpendicular to PANI conjugation

chains, respectively [64e66]. It has been observed that all Ta/

PANI composite films exhibit a broad peak appeared at

2q z 25.2� corresponding to the amorphous nature of PANI

and two sharp crystalline peaks, centered at 2q z 38� and

2q z 70.1� corresponding to the crystalline nature of Ta. The

Peak observed at 2qz 38� can be ascribed to either (110) of bcc-

Ta or (200) of b-Ta, while the peak at 2q z 70� corresponds to

the (400) of b-Ta [67e69]. It was observed that after irradiation

the peaks become more sharpen as the ion fluence increases.

This indicates that after SHI irradiation the crystallinity of Ta

in PANI matrix has been increases. No new structural order

has found to be generated within composites after the irra-

diation as shown in Fig. 4ced. It has been earlier reported that

Fig. 4 e X-ray diffraction patterns of pure PANI and Ta/PANI

composite films before and after SHI irradiation.

Fig. 5 e AFM images of Ta/PANI composite film (a) before irradiation, (b) after irradiated at 1 3 109 ion/cm2 and (c) after

irradiated at 1 3 1011 ion/cm2.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 23828

the density of the polymer increases during the SHI irradiation

making the polymer more compact which results in more

crystalline regions in polymer films resulting in an increase in

the degree of crystallinity [30,43] of composite.

3.3. Atomic force microscopy (AFM)

Surface morphology of pristine and irradiated Ta/PANI

composite films has been examined by AFM measurements.

Fig. 7 e Current-Voltage (IeV) characteristics curve of (a)

unirradiated and (b) Irradiated Ta/PANI composite films

with temperature.

Fig. 6 e Current-Voltage (IeV) characteristics curve of

unirradiated and Irradiated Ta/PANI composite films at

room temperature.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 2 3829

The Fig. 5 shows AFM images of Ta/PANI composite films

before and after SHI irradiation. The pristine Ta/PANI

composite film shows a highly dense random shaped uneven

granular structure on the surface (Fig. 5a).

The composite film irradiated at low fluencies

(1 � 109d1 � 1010 ions/cm2) shows a typical polycrystalline

clustered structure of irregular grains aligned in small

crystallized domains (Fig. 5b). The film irradiated at

1 � 1011 ions/cm2 shows more compact, closely packed

interconnected rope like structure at the surfacewhich results

in an increased crystallinity of the composite film and thus

show good agreement with XRD measurement results after

irradiation. The AFM images revealed a continuous increase in

the granular cluster in composite film with the increase of ion

fluence. The cluster formation may be attributed to large

amount of electronic energy loss induced collision cascades

therefore particles agglomerated due to the partially melted

Ta layer. This took place near the surface and is responsible

for the displaced atoms forming clusters.

3.4. IeV characteristics

Fig. 6 shows the IeV characteristics of Ta/PANI composite film

before and after SHI irradiation at room temperature. From

the IeV characteristics curve it has been observed that at low

voltages, the current is proportional to the applied voltage

corresponding to an ohmic regime, which extends almost up

to 0.3 V. With increasing bias voltage beyond 0.3 V, an

increasing trend in the current was observed showing the non

linear region.

IeV curve of pristine Ta/PANI thin film shows almost

similar characteristics as that of pure PANI thin film. This

suggests that Ta thin filmonto PANI surface does not affect the

density as well as transportation of charge carrier within PANI

matrix. In case of irradiated Ta/PANI composite thin films, it

was observed that from the ohmic regime to the non linear

region, the current increases slightly with increasing fluence.

This implies that the background free charge carrier density

increases slightly with increasing ion fluence [70]. Also, due to

irradiation, Ta melts and diffuses into PANI matrix, which

provides more conducting path for easy charge transport

between consecutive PANI chains and hence current increase

with increasing ion fluence. B. Scrosati andHussein et. al. have

reported that the increase in the crystallinity of the composite

films upon SHI irradiation may also contribute to the increase

in conductivity of the films [30,71], which is in good agreement

with our XRD and AFMmeasurements.

Fig. 7 shows the IeV characteristics of unirradiated and

irradiated Ta/PANI composites thin film with increasing

temperature. The increasing trend in currentwas observed for

both unirradiated and irradiated composite samples with

increasing temperature. It may be attributed to the increase in

the number of thermally activated charge carriers with

temperature, which indicates that composite thin films are

semiconductor in nature.

3.5. Gas sensing measurements

Fig. 8 shows the variation in the resistance of pristine and

irradiated Ta/PANI composite sensors towards hydrogen gas

at room temperature. This Figure clearly reveals that resis-

tance of all irradiated composite films decreases very rapidly

by introduction of hydrogen gas and become stable after few

seconds. This may be attributed to the reducing nature of

Fig. 10 e Variation in %sensitivity of Ta/PANI composite

sensors irradiated at different ion fluencies.

Fig. 8 e Change in resistance of unirradiated and irradiated

Ta/PANI composite sensors with time after exposed to

hydrogen gas at room temperature.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 23830

hydrogen gas. Introduction of hydrogen gas in the composite

thin films injected electrons to the film, and thus significantly

increase the number of charge carrier in the film. As a result,

more electrons flowed in the film and at the same time

reduced the resistance of the film. An exception was observed

in the pristine Ta/PANI composite film, where the resistance

change is very small or almost negligible. Fig. 9 shows the

response of pristine and irradiated Ta/PANI composite

sensors towards the hydrogen gas at the room temperature. It

was observed that unirradiated Ta/PANI composite sensor

shows almost negligible response in comparison to irradiated

sensors. It may be due to the Ta layer coated over the PANI

surface, which does not react with hydrogen at room

temperature and inhibited the hydrogen to diffuse in to the

PANI matrix. Therefore at room temperature pristine Ta/PANI

Fig. 9 e Response versus time plot for unirradiated and

Irradiated Ta/PANI composite sensors after hydrogen

exposure at room temperature.

sensor dose not shows any response for hydrogen. While

upon irradiation, it was observed that Ta/PANI composite

sensor show a higher response and the response increases

slightly with increasing ion fluence.

The response value has been found z1.1 (i.e. %

Sensitivityz 9.2%) for Ta/PANI composite sensor irradiated at

fluence 1 � 109 ion/cm2, which was increased up to 1.42 (i.e. %

Sensitivity z 30%) for composite sensor irradiated at fluence

1 x 1011 ion/cm2. The % Sensitivity of unirradiated and irra-

diated Ta/PANI composite sensors is shown in Fig. 10.

In case of irradiated Ta/PANI composites sensor the

interaction of hydrogen with PANI is predominantly res-

ponsible for higher response of sensor towards hydrogen gas.

It may suggest that due to the SHI irradiation Ta melts and

diffuses into PANI matrix, which provides comparatively

Fig. 11 e Reproducibility of Ta/PANI composite sensor

exposed to hydrogen gas at room temperature.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 2 3831

rough, and higher surface area for hydrogen adsorption

and rapid diffusion, therefore more interaction sites are

available for hydrogen sensing and hence the sensing

response is increased. It has been reported that rough and

fiber like structure of PANI shows a faster and higher

response for hydrogen than conventional PANI film, because

the three dimensional porous structure of a PANI nanofibers

allows for easy and rapid diffusion of hydrogen gas into PANI

[57,58].

Although, a few work has been published on effects of

hydrogen on the mechanical properties of tantalum material

[50,51,72,73], but the exact mechanism of direct interaction

between hydrogen and tantalum is not yet fully understood. It

may also suggest that hydrogen molecules could be absorbed

between the interstitial sites within the tantalum lattice and

then dissociate into hydrogen atoms. The following forma-

tion of new NeH bonds between the hydrogen atoms and

nitrogen atoms of PANI can reduce the resistance of

Composite sensor [74].

Fig. 11 shows the response e recovery property of Ta/PANI

composite sensor upon SHI irradiation at 1 � 1011 ion/cm2.

Over long periods of hydrogen exposure it was observed that

composite film sensors exhibited a good stability and repeat-

ability as gas sensors. It was also observed that after first cycle

the sensor takes longer time to reach at the stable value of

response magnitude. It may be due to the slow diffusion rate

of hydrogen gas with time.

4. Conclusion

The Ta/PANI composite based chemiresistor type gas sensors

were fabricated on interdigitated electrodes and irradiated

with 150 MeV Auþ12 ions at different fluence ranging from

1 � 109 to 1 � 1011 ions/cm2. XRD measurements revealed

that Ta/PANI composite film exhibit both amorphous and

crystalline nature due to presence of PANI and Ta respec-

tively. Upon irradiation, the crystalline nature of Ta/PANI

composite films increased with increasing ion fluence due to

the mixing of tantalum atoms in PANI matrix and hence the

resistance of composite film decreased. AFM study shows

that the ion beam irradiation leads to formation of clusters

and craters in Ta/PANI composite films. The response

behavior was monitored in terms of resistance change of

unirradiated and irradiated Ta/PANI composite sensors

towards H2 gas in air at room temperature. The irradiated

Ta/PANI composite sensors showed high response value and

sensitivity with good repeatability than pristine one. The role

of tantalum as per its contribution in higher response of

irradiated TA/PANI composite sensor towards H2 is not clear

at present and some more work is required to explore the use

of tantalum as sensitive material in hydrogen gas sensing

application.

Acknowledgements

Authors are grateful to the UGC, New Delhi, for the financial

support in the form of a research project.

r e f e r e n c e s

[1] Adhikari B, Majumdar S. Prog Polym Sci 2004;29:699e766.[2] Wang L, Fine D, Sharma D, Torsi L, Dodabalapur A. Anal

Bioanal Chem 2006;384:310e21.[3] Cadena GJ, Riu J, Rius FX. Analyst 2007;132:1083e99.[4] Janata J, Josowicz M. Nat Mater 2003;2:19e24.[5] David WH, Josowicz M. Chem Rev 2008;108:746e69.[6] Baibarac M, Romero PG. J Nanosci Nanotechnol 2006;6:1e14.[7] Muraviev DN, Macanas J, Farre M, Munoz M, Alegret S. Sens

Actuators B 2006;118:408.[8] Muraviev DN, Pividory MI, Soto JLM, Alegret S. Solv Extract

Ion Exchange 2006;24:731.[9] Kinyanjui JM, Hanks J, Hatchett DW, Smith AJ, Josowicz M.

J Electrochem Soc 2004;151:D113.[10] O’Mullane AP, Dale SE, Day TM, Wilson NR, Macpherson JV,

Unwin PR. J Solid State Electrochem 2006;10:792.[11] Niu L, Li Q, Wei F, Wu S, Liu P, Cao X. J Electroanal Chem

2005;578:331.[12] Mascaro LH, Goncualves D, Bulhoees LOS. Thin Solid Films

2004;461:243.[13] Kinyanjui JM, Hatchett DW, Smith AJ, Josowicz M. Chem

Mater 2004;16:3390.[14] Mallick K, Witcomb MJ, Dinsmore A, Scurrell MS. Macromol

Rapid Commun 2005;26:232.[15] Guo Z, Zhang Y, Huang L, Wang M, Wang J, Sun J, et al.

J Colloid Interface Sci 2007;309:518.[16] Mallick K, Witcomb MJ, Scurrell MS. J Phys Condens Matter

2007;19:1.[17] Huang K, Zhang Y, Long Y, Yuan J, Han D, Wang Z, et al.

Chem Eur J 2006;12:5314.[18] Do JS, Chang WB. Sens Actuators B 2001;72:101;

Do JS, Chang WB. Sens Acuators B 2004;101:97.[19] Hatchett DW, Wijeratne R, Kinyanjui JM. J Electroanal Chem

2006;593:203.[20] Park JE, Park SG, Koukitu A, Hatozaki O, Oyama N. Synth Met

2004;141:265.[21] Sharma S, Nirkhe C, Pethkar S, Athawale A. Sens Actuators B

2002;85:131.[22] Chowdhury AN, Islam MS, Azam MS. J Appl Polym Sci 2007;

103:321.[23] Lee D, Han S, Huh J, Lee D. Sens Actuat B-Chem 1999;60:

57e63.[24] Dhar S. Crit Rev Solid State Mater Sci 2007;32:1.[25] Avasthi DK. Currt Sci 2000;78:1297.[26] Kanjilal D. Curr Sci 2001;80:12.[27] Wang WM, Wan HH, Rong TW, Bao JR, Lin SH. Nucl Instrum

Meth Phys Res B 1991;61:466e71.[28] Wang WM, Lin SH, Bao JR, Rong TW, Wan HH, Sun J, et al.

Instrum Meth Phys Res B 1993;74:514e8.[29] Lin SH, Sheng KL, Rong TW, Bao JR, WangWM,Wan HH, et al.

Nucl Instrum Meth Phys Res B 1991;59/60:1257e62.[30] Hussain AMP, Kumar A, Singh F, Avasthi DK. J Phys D Appl

Phys 2006;39:750e5.[31] Venkatesan T. Meth Phys Res B 1984;7e8:461e7.[32] Szenes G. Phys Res B 1995;51:8026.[33] Chakraborty BR, Halder SK, Karar N, Kabiraj D, Awasthi DK.

J Phys D Appl Phys 2005;38:2836.[34] Sonar P, Sharma AL, Chandra A, Muellen K, Srivastava A.

Curr Appl Phys 2003;3:247.[35] Davenas J, Xu XL, Boiteux G, Sage D. Nucl Instrum Meth Phys

Res B 1989;39:754e63.[36] Tsukuda S, Seki S, Sugimoto M, Tagawa S, Idesaki A,

Tanaka S, et al. Fabrication of nanowires using high energyion beams. J Phys Chem B 2004;108:3407e9.

[37] Srivastava MP, Mohanty SR, Annapoorni S, Rawat RS. PhysLett A 1996;215:63e8.

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 7 ( 2 0 1 2 ) 3 8 2 5e3 8 3 23832

[38] Singh NL, Shrinet V, Pandaya NR, Sharma A, Patel NV,Avasthi DK. Nucl Instr Meth B 1999;156:191.

[39] Srivastava A, Singh V, Dhand C, Kaur M, Singh T, Witte K,et al. Sensors 2006;6:262e9.

[40] Gretillat MA, Lindner C, Dommann A, Staufert G, de Rooij NF,Nicolet MA. J Micromech Microeng 1998;8:88e90.

[41] Olthuis W, Sprenkels AJ, Bomer JG, Bergveld P. SensorsActuators B 1997;43:211e6.

[42] Hayward DO & Taylor N. 1968 Proc. 4th Int. VacuumCongress (Manchester: IPPS 1968);1;115e119.

[43] Gill JM, Hayward DO, Taylor N, Proc R. Soc Lond A 1973;335:141e61.

[44] Nakamura K, Uchida H, Fromm E. J Less Common Met 1981;80(1):19e29.

[45] Miranda L, Vargas P, Ceron H, Lagos M. Phys Lett A 1988;131(7e8):445e8.

[46] Stalinski B. Bull Acad Pol Sci 1954;2:245e7.[47] Waite TR, Wallace WE, Craig RS. J Chem Phys 1956;24:634.[48] Wallace WE. J Chem Phys 1961;35:2156e64.[49] Asakawa T, Nagano D, Denda S, Miyairi K. Jpn J Appl Phys

2008;Vol. 47:649e52.[50] Miller GL. Tantalum and Niobium. London: Butterworth

Scientific Publications; 1959. 488.[51] Miller GL. Tantalum and Niobium. London: Butterworth

Scientific Pub; 1959. 459, Table 10.9.[52] Veleckis E, Edwards RK. J Phys Chem 1969;73:683e92.[53] Steward, SA: Lawrence Livermore National Laboratory,

Report UCRL e 53441; August 15 1983.[54] Makrides A, Wright M, McNeill R. Final report for contract

DA-49-189-AMC-136(d). Waltham, MA: Tyco Lab; 1965.[55] Heeger AJ. Synth Met 2002;125:23e42.[56] MacDiarmid AG. Synth Met 2002;125:11e22.

[57] Sadek AZ, Baker CO, Powell DA, Wlodarski W, Kaner RB,Zadeh KK. IEEE Sensor J 2007;7:213.

[58] Huang SJ, Virji S, Weiller BH, Kaner RB. J Am Chem Soc 2003;125:314.

[59] Srivastava S, Sharma SS, Sumit K, Agrawal S, Singh M,Vijay YK. Int J Hydrogen Energy 2009;34:8444e50.

[60] Srivastava S, Sharma SS, Agrawal S, Kumar Sumit, Singh M,Vijay YK. Synth Metals 2010;160:529e34.

[61] Govil IM. JAPS 1999;1:295.[62] Blewett JP. Particle accelerators. New York: McGraw Hill;

1961.[63] Agrawal S, Srivastava S, Kumar Sumit, Sharma SS,

Tripathi B, Singh M, et al. Bull Mater Sci 2009;32:1e5.[64] Barthet C, Armes SP, Lascelles SF, Luk SY, Stanley HME.

Langmuir 1998;14:2032e41.[65] Zhang LJ, Wan MX. J Phys Chem B 2003;107(28):6748e53.[66] Pouget JP, Jozefowicz ME, Epstein AJ, Tang X,

MacDiarmid AG. Macromolecules 1991;24:779.[67] Hoon MK, Chang CK, Bum KK. J Vac Sci Technol B 1996;14(5):

3263e9.[68] Nie HB, Xu SY, Wang SJ, You LP, Yang Z, Ong CK, et al. Appl

Phys A 2001;73:229e36.[69] Face DW, Prober DE. J Vac Sci Technol A 1987;5(6):3408e11.[70] Virk HS, Chadi PS, Srivastava AK. Radiat Eff Defects Solids

2001;153:325e34.[71] Scrosati B, editor. Applications of electroactive polymers.

London: Chapman and Hall; 1993.[72] Data Survey on tantalum. 2nd ed. Chicago, IL: Fansteel.[73] Vakst VG, Zinenkoi VI. J.Phys Condens Matter 1991;3:

4533e45.[74] Virji S, Kaner RB, Weiller BH. J Phys Chem B 2006;110:

22266e70.