dodeca tungstophosphoric acid nanocomposites: Enhancement of conductivity and humidity sensing

Physica B 405 (2010) 4313–4319

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Interfacially polymerized polyaniline/dodeca tungstophosphoric acid nanocomposites: Enhancement of conductivity and humidity sensing P. Chithra lekha a, S. Subramanian b, J. Philip c, D. Pathinettam Padiyan a,n a

Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627 012, India Department of Physics, The MDT Hindu College, Tirunelveli 627 010, India c Sophisticated Analytical Instrument Facility, Cochin University of Science and Technology, Cochin, India b

a r t i c l e in fo

abstract

Article history: Received 22 January 2010 Received in revised form 17 July 2010 Accepted 19 July 2010

Polyaniline (PAni) doped with dodeca tungstophosphoric acid (12TPA) nanomaterials prepared by interfacial polymerization are characterised via UV–vis spectroscopy, X-ray diffraction, thermogravimetry and ac impedance analysis. From the X-ray diffraction pattern, the crystallite size of the nanomaterials is found to be around 10–50 nm. The ac impedance studies on these materials in the temperature range 300–398 K show that the conductivity in PAni is due to the hopping of polarons following correlated barrier hopping model whereas in PAni doped with 12TPA (P12TPA) it is due to the tunneling of large polarons. It gives an understanding on the role of thermal annealing over the conductivity of the P12TPA, which arises from the presence of hydroxyl groups and is also evident from the thermogram. The humidity sensing behaviour of the nanomaterial elucidates the P12TPA as a good humidity sensor. The results of optical, structural and thermal analysis illustrate that PAni and 12TPA are not simply blended or mixed up, but a strong interaction is there between the polymer backbone and the polyanion. & 2010 Elsevier B.V. All rights reserved.

Keywords: Polyaniline Dodeca tungstophosphoric acid Ac impedance Humidity sensor

1. Introduction The development in the synthesis of nanostructured materials including polymer nanocomposites has attracted much attention across scientific and engineering disciplines. These nanostructured materials have become important candidates in replacing the conventional bulk materials in micro electronics and in chemical and biological sensors. Polyaniline (PAni) nanomaterials with high surface area and high porosity are considered as the best performance electrode material for redox super capacitors [1]. Keggin anion is one of the most relevant members of the polyoxometalates (POMs) family with the general formula [XW12O40]n  , where X can be a main-group element (P, Si, Al, Ge, etc.) or a transition-metal ion (Fe, Co, Cu, etc.). They are the most studied POMs, which are oxidants in aqueous solutions. The inclusion of the dodeca tungstophosphoric acid (12TPA) in a polymer matrix is shown to enhance the catalytic activity and the sensitivity towards gases relative to that of a heteropolyacid in its pure crystalline state [2,3]. Developing a general synthesis method that reliably produces high quality hybrid organic/ inorganic nanostructures in desired quantities is the key to studying its properties and applications. In conventional

n

Corresponding author. Tel.: +91 4622333887; fax: + 91 4622322973. E-mail address: [email protected] (D. Pathinettam Padiyan).

0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.07.033

polymerization, the overgrowth of PAni on the nanofiber scaffolds leads to the final irregularly shaped micron sized particulates, which is the barrier to be overcome to get nanocomposite [4]. Huang and Kaner [5] recently developed a simple and practical method using biphasic or ‘‘interfacial’’ polymerization for making uniform, template-free [6] nanofibers. In the interfacial polymerization, the overgrowth of polymer fibres is suppressed, which leads to the formation of ultra small nanofibers. Furthermore, Wei et al. [7] and others [8–12] have developed a template-free solution method in which the diameter of the tube can be controlled by the dopant functionality and amount. In this work, PAni and PAni doped with 12TPA (P12TPA) nanomaterials were synthesized by interfacial polymerization method. An enhancement in the conductivity and the humidity sensing property of these materials is achieved.

2. Experiment 2.1. Sample preparation Aniline monomer, chloroform, HCl and ammonium peroxydisulphate used were of analytical grade from E. Merck. Aniline was distilled prior to use and the double distilled water was used throughout the experiment. 12TPA was prepared by the ether extraction method. The organic solution was prepared by

P. Chithra lekha et al. / Physica B 405 (2010) 4313–4319

dissolving 0.625 M aniline monomer in chloroform. The aqueous solution containing 0.16 M ammonium peroxydisulphate in 1 M HCl was added to the organic solution, and kept undisturbed for 2 h. At the interface between the two liquids, a thin layer of green PAni nanomaterial appears. During the oxidative polymerization of aniline, HCl acts as proton source and provides necessary counter ions to the developing charged polyaniline [13]. Heeger [14] has shown that the interface between the immiscible organic/aqueous system acts as a template that preferentially organizes the aniline monomer prior to polymerization. As the aniline monomer is protonated on the surface, its head to tail coupling forms a low-energy helical structure when polymerized and precipitates into aqueous phase of the biphasic system. After 2 h, the PAni was filtered using a Buchner funnel, washed with ethanol and water to remove unreacted chemicals and aniline oligomers, oven dried and the nano PAni was collected. The preparation of P12TPA nanocomposite was made by replacing the protonic acid HCl with the subsequent 12TPA. Here the 12TPA acts as a proton source as well as a dopant.

0.6 PAni

0.4 ABS

4314

0.2 P12TPA

0.0 300

600 Wavelength (nm)

900

Fig. 1. UV–visible spectra of PAni and P12TPA.

2.2. Measurement The UV–visible spectra of PAni and P12TPA nanomaterials were recorded using a Varian Carry 5000 UV–vis–NIR Spectrometer. X-ray diffraction (XRD) patterns were taken for PAni and P12TPA nanomaterials using a Bruker AXS D8 Advance with CuKa radiation. The thermo gravimetric analysis (TGA) was studied using the Perkin Elmer Diamond thermal analysis system in the nitrogen atmosphere at a constant heating rate of 5 1C/min in non-isothermal condition. The spectrometer used to measure the thermal diffusivity consists of a 120 mW He–Cd laser of wavelength 442 nm, a variable-frequency optical chopper (Stanford Research Systems, Model SR 540), a small-volume photoacoustic (PA) cell and a lock-in amplifier (Stanford Research Systems, Model SR 830) to analyze the signal detected by the high-sensitivity electret microphone (Knowles, Model BT 1753) kept inside the PA cell. The variation in the PA signal amplitude with chopping frequency is measured to determine the thermal diffusivity of each sample. The ac impedance analysis was done with Lock-in Amplifier SR830 in the frequency range 10 Hz– 100 kHz at temperatures ranging from room temperature to 398 K. The material was placed in an indigenous setup with silica gel and calcium chloride as dehumidifiers to vary the relative humidity (% RH). The resistivity was measured at various RH values using a Keithley source measure meter Model 2400.

3. Results and discussions 3.1. UV–visible spectral analysis The UV–visible spectra of PAni and P12TPA taken by dissolving the nanomaterials in N-methyl pyrrolidinone (NMP) are shown in Fig. 1. In PAni, the absorption band at 350 nm is assigned to p–p* transition of the benzenoid rings, indicating the charge delocalization. The shoulder seen at 432 nm is due to the n–p* transition associated with bipolarons. In the optical spectra, the peak at 432 nm is found to be overlapped with the peak at 350 nm. Such an observation is reported in the spectroscopic studies on PAni doped with HCl by Kulkarni et al. [15]. The band observed at 780 nm confirms the conducting emaraldine salt form of PAni [16]. Both the bands at 432 and 780 nm originating from the formation of delocalized polarons and bipolarons are consistent with the green color emaraldine salt form [17–19]. The increased absorption can be explained by the increase in

Fig. 2. XRD pattern for (a) PAni and (b) P12TPA.

scattering of light due to the presence of smaller particles [20]. In P12TPA, a small absorption band is present at 280 nm, which may be due to the presence of 12TPA. Encapsulation of nano-titanium oxide in the conducting polymer as mentioned by Li et al. [21] also results in the formation of an additional absorption peak at 290 nm. The blue-shift of the absorption maximum from 433 nm in PAni to 409 nm in P12TPA is most likely caused by the dopant. The absorption peak around 335 nm is the contribution of excitations of amine nitrogen of the benzenoid segment of PAni. There is a blue-shift in the characteristic absorption band at 772 nm that elucidates after doping with 12TPA, PAni gets protonated and is in the conducting emaraldine salt form. The presence of chemically non-equivalent benzenoid and quinoid rings in the polymer chain is confirmed by these two peaks in the P12TPA.

3.2. XRD X-ray diffraction patterns for PAni and P12TPA are shown in Fig. 2a and b, respectively. The PAni nanomaterial displays a broad peak at 2y ranging from 151 to 321, which is reported as the characteristic distance between the ring planes of benzene ring in

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adjacent chain. A prominent peak observed at a low angle of 2y E6.31 is assigned to the scattering along orientation parallel to polyaniline chain in one dimensional structures [22]. In P12TPA, the observed diffraction lines are indexed to the 12tungstophosphoric acid, which agrees well with the JCPDS ICDD card no. 76-1815 reported [23]. The crystallite size (D) was calculated using the Scherrer formula D¼

0:94l b cos y

where l is the wavelength of X-ray, b the full width at half maximum (FWHM) of the peak in radians and y the Bragg angle of the peak and was found to be between 10 and 50 nm. In Fig. 2b, the broad peak of PAni due to the ring planes of benzene is noticed as a higher background in the region 18–351. The observed broad and sharp peaks corresponding to those of PAni and 12TPA indicate that there is no structural change but only conformational change in the polymer backbone due to the addition of 12TPA. The low angle peak observed in PAni is also present in Fig. 2b but it is sharp due to the overlapping of 12TPA peak.

3.3. Thermal studies The TG-DTA thermogram for PAni given in Fig. 3a shows a three step weight loss as follows: the loss of water molecules (25–85 1C), the loss of HCl and aniline oligomers (143–315 1C) and the destruction of the skeletal backbone of PAni (318–632 1C). In

0.00

4315

the DTA of PAni, there are three endothermic peaks at 54, 309 and 510 1C indicating the consecutive weight loss stages as discussed in TG. In the case of P12TPA, the complete process of weight loss is composed of four stages, namely, the dehydration from PAni and 12TPA, decomposition of aniline oligomers and/or dehydration of the water of crystallization of 12TPA, decomposition of anions of 12TPA and decomposition of P12TPA complex. The percentage of weight loss in P12TPA is higher than the pristine PAni. In P12TPA (Fig. 3b) the four step weight loss (I: 35–70 1C, II: 165–195 1C, III: 240–355 1C and IV: 355–700 1C) process is quite different from the thermal behaviour of the other PAniPOMs. In P12TPA, the DTA has six endothermic peaks at 45, 182, 226 and 742 1C, and a doublet at 311 and 348 1C. Rosenheim’s dehydration experiment [24] showed that water of crystallization of polyoxometalates was easily expelled from crystals at moderate temperatures, but the appropriate number of constitutional water molecules was expelled only at much higher temperatures accompanied by the disintegration of the complexes [25]. According to Nakamura et al. [26], in 12-molybdophosphoric acid (12MPA), TGA shows a dehydration of all the molecules (29H2O) of water of crystallization around 100 1C, whereas the tungsten analogue dehydrates 23 water molecules around 100 1C and the remaining six water molecules dehydrate around 200 1C. Usually, at moderate temperatures, the 12TPA has the highest thermal stability than the molybdenum analogue, but the thermally decomposed molybdenum POMs can be reconstructed under exposure to water vapor. However, it is not possible in the case of its tungsten analogue due to the contribution of these six water molecules. In the TGA of P12TPA, the additional shoulder seen at 195–240 1C is attributed to the expulsion of these six water molecules contributing to the crystallization of 12TPA and the proceeding degradation is the disintegration of other anions.

9 3.4. Photoacoustic spectroscopy

8

-0.04

7

-0.06

6 5

-0.08

4 -0.10 200

400

600

800 4.0

0.00 3.5

-0.01 -0.02

3.0

-0.03

2.5

-0.04 2.0 -0.05 1.5

-0.06 -0.07 200

400 600 Temperature (°C)

Fig. 3. TG-DTA for (a) PAni and (b) P12TPA.

1.0 800

Weight (mg)

Derivative weight (mg/min)

-0.02

The method involved is for the determination of the characteristics frequency fc. Above fc the PA signal does not depend on the thermal properties of the backing material used. For this the sample is made sufficiently thin so that ec lies well within the operating frequency range of the optical chopper used in the measurements. The sample is fixed on an aluminum disc, which acts as a thermally thick backing material. It is ensured that proper thermal contact is established by applying carbon black between the sample and the backing medium. The characteristic frequency is determined from a plot of the PA amplitude vs. chopping frequency. At the characteristic frequency, fc, the sample goes from a thermally thin to a thermally thick regime, or from an o  3/2 dependence to an o  1 dependence, which is manifested as a slope change in the log frequency vs. PA amplitude plot. Once the characteristic frequency is determined, the thermal diffusivity of the sample can be evaluated from the expression [27,28] a ¼ pfcl2, where a is the thermal diffusivity of the sample, fc the characteristic frequency and l the thickness of the sample. Fig. 4 shows the PA amplitude as a function of chopping frequency for PAni and P12TPA. From the slope of these plots the critical frequency (fc) is found to be 43.65 Hz for PAni and 54.95 Hz for P12TPA. The corresponding thermal diffusivity values for PAni and P12TPA are 0.641(3) and 0.676(1) cm2/s, respectively, which are comparable to the earlier reports on camphor sulphonic acid doped PAni (0.760 cm2/s) [29]. Thermal diffusivity value essentially determines the rate of heat diffusion through the sample and the inverse of thermal diffusivity yields a measure of the time required to establish equilibrium in systems for which a transient temperature change has occurred. The accurate measurement of thermal diffusivity and the study of influence

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-1500 -0.45

-1000 -0.50

-500 0

log amplitude

-jZ''

-0.55

0

1000

2000

3000

4000

300 K 323 K 348 K 373 K 398 K

-300

-0.60 1.4

1.6

1.8

2.0

5000

-200 -100

-3.30

0 0

-3.35

200

400

600

800

1000

Z' Fig. 5. The Nyquist plot for (a) PAni and (b) P12TPA at different temperatures. Solid lines were obtained by CNLS fitting of experimental data.

-3.40

Table 1 The conductivity at different temperatures for PAni and P12TPA.

-3.45 1.6

1.8 log frequency

2.0

2.2

T (K)

s  10  3 (S/m)

s

PAni

P12TPA

PAni

P12TPA

PAni

P12TPA

1.48 2.92 3.53 4.31 5.98

7.20 12.41 13.10 10.75 8.14

0.90 0.87 0.70 0.61 0.54

0.60 0.59 0.55 0.60 0.63

0.2012 0.2128 0.2820 0.1712 0.1368

0.1480 0.1190 0.2183 0.2285 0.1973

a

Fig. 4. The photoacoustic spectra of (a) PAni and (b) P12TPA.

of dopant on the thermal diffusivity value have significant role. According to de Albuquerque et al. [30], in the conducting polymers the thermal and electrical conduction have different mechanisms rather than most of the inorganic materials. The energy transfer in thermal conduction involves not only electrons but also phonons in contrast to the electrical current where the transport is due to the movement of charge carriers. Due to the disordered structure of the polymer, the electronic contribution to the thermal conductivity is negligible [31]. In this work, the inorganic polyoxometalate is doped with polyaniline and thus the contribution of electrons and holes is also considerable in the thermal conduction mechanism of P12TPA. On doping with 12TPA, the increase in electrical conductivity is followed by thermal diffusivity. Hence there is a correlation between electrical and thermal conduction mechanism [30,31]. Here the doping of 12TPA induces interchain coupling and enhances ordering between crystalline regions as well as on the chains of PAni bridging the metallic regions. The ordering of these regions increases the localisation length, which in turn allows the transport of heat and electricity.

3.5. AC conductivity The ac impedance spectra for PAni and P12TPA measured from 300 to 398 K in the frequency range 10 Hz–100 kHz along with the complex nonlinear least square (CNLS) fitted [32] plots for real part of impedance (Z0 ) vs. imaginary part of impedance (  jZ00 ) are given in Fig. 5a and b, respectively. The capacitive and resistive components in the ac impedance data contribute to the semicircles of the Nyquist diagram [33]. In PAni, the Nyquist plot shows a single semi-circle indicating a single relaxation process at room temperature. A slight deviation from the ideality occurred causing depression of the semi-circle and the measured

300 323 348 373 398

points from the semi-circle. These deviations from ideality are attributed to surface roughness at the electrodes. At higher temperatures a shift of the low frequency region towards the lower resistance is observed in the Nyquist plot of PAni. It shows that the resistivity of PAni decreases with the rise in temperature. The imaginary component reaches to zero, which shows that the electrodes are non-blocking type. The Nyquist plot of P12TPA shows a single semi-circle with a slight stretch at lower frequency region at room temperature. On increasing the temperature this stretch becomes normal and then it folds inward. This is due to the contribution of spatial charges. The resistivity of P12TPA decreases on rising the temperature up to 348 K and then increases. This shows that at higher temperature the loss of hydroxyl group increases the resistivity of P12TPA and it is also confirmed in TGA studies. The higher room temperature conductivity in P12TPA compared to PAni is due to the protons and other ions in the 12TPA. The relaxation frequency for PAni and P12TPA at room temperature is found to be 1.5 kHz, which gives the relaxation time as 6.67  10  4 s. From the Nyquist plot the bulk electrical conductivity (sb) is obtained using the equation sb ¼l/RbA, where l is the thickness and A is the area of the electrode is given in Table 1 for all temperatures. In the figure, the centre of the semi-circle is displaced below the real axis because of the presence of distributed elements in the material–electrode interface. The angle a by which a semicircle is depressed below the real axis is related to the width of the relaxation time distribution and is as such an important parameter. The equivalent circuit shown in Fig. 6, corresponding to the impedance spectrum, consists of a frequency dependant

P. Chithra lekha et al. / Physica B 405 (2010) 4313–4319

Fig. 6. A schematic complex impedance diagram, showing a semicircle whose centre lies below the real axis.

0

-2

ln σac

-2

-4

300 K 323 K 348 K 373 K 398 K

-4

-6

8

10

12

6 ln ω

8

10

12

4317

the fitting curves is shown in Fig. 7a and b for PAni and P12TPA, respectively. The conductivity varies slowly at low frequencies and then increases rapidly at higher frequency regions. The fractional dependence of ac conductivity upon frequency is found in many disordered materials and the above said exponential law arises from the many body interactions between hopping charges [35]. In such systems, the ac conductivity increases as the frequency of the electric field increases, due to the contribution of charge carriers moving along smaller distances that is confined inside clusters. Thus the frequency exponent s expresses the relative reduction of clusters upon frequency. In Fig. 7, a linear increase in conductivity with frequency is noticed in the region 100 Hz–20 kHz for PAni and 100 Hz–4.5 kHz for P12TPA. The s parameter values are determined from the linear slope of ln o vs. ln sac for PAni and P12TPA at different temperatures, and are given in Table 1. The frequency exponent is a diagnostic tool for characterizing the charge transport in conducting polymers. Exponents close to unity are associated with lattice response, while smaller values are due to impurities or dopants [36]. The electron tunneling model suggests that s is independent of T, but dependent on o. In the case of small polaron tunneling, s increases with temperature, whereas for overlapping large polaron tunneling (OLPT), s decreases up to a certain temperature and then increases for further increase of temperature. In the correlated barrier hopping model (CBH), the s parameter value is below unity and also decreases with increase in frequency. Here the charge carrier hops between the defect sites D + and D  over the potential barrier separating them [37]. According to Guintini et al. [38] each pair of D + and D  is assumed to form a dipole with relaxation energy. This type of energy can be attributed to the existence of potential barrier over which the carriers hop [39]. The temperature dependence of s is shown in Fig. 8 and can be explained by various theoretical models for ac conductivity. In PAni, at room temperature the value of s close to unity is the evidence for weak interaction of polarons with the polymer backbone. At high temperatures, the thermal treatment has reduced the value of frequency exponent and affected the grain size and the inter-grain separation. In Fig. 8a, the values of the frequency exponent for PAni are found to decrease with increasing temperature and the value of s parameter is below one, which obeys the correlated barrier hopping model (CBH).

Fig. 7. Log–log plot of the electrical conductivity at various temperatures in (a) PAni and (b) P12TPA. The dots represent the experimental points and the solid lines represent fitted curves.

where U and V are the distances of Z(o) from Zo and Z0, respectively. A plot of ln9V/U9 vs. ln o yields a straight line of slope (1  a). The limiting case of a ¼0 represents an equivalent circuit consisting of lumped R–C elements with a zero depression angle, while a ¼ 12 corresponds to a combination of resistors and Warburg impedance with 451 depression angle. The depression angle a determined from the Nyquist plot lies between 0 and 12 and is given in Table 1. The ac conductivity of conducting polymers consists of a frequency dependent behaviour in the limit of low frequencies and a sublinear response at higher frequencies [34]. The frequency dependence of ac conductivity in the high frequency region is obtained by the relation sac ¼Aos, where o is the angular frequency, s the frequency or fractional exponent and A is a temperature component. A plot of log o vs. log sac along with

0.9

PAni P12TPA

0.8 's' parameter

capacitor and two frequency independent resistors. It is found that it obeys the relation    V ¼ ð1aÞln oð1aÞln o0 ln U

0.7

0.6

0.5 300

320

360

340

380

400

T (K) Fig. 8. The temperature dependence of frequency exponent s for (a) PAni and (b) P12TPA.

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In P12TPA, the observed low value of the frequency exponent compared to PAni shows a strong interaction between polarons, polymer backbone and polyanion. In Fig. 8b, the s parameter for P12TPA decreases up to a particular temperature and then increases for further increase of temperature, suggesting the overlapping large polaron tunneling model. A mechanism of quantum mechanical tunneling of large polarons with overlapping polaron wells predicts that s(o) should behave in some respect in a similar manner to the CBH model, namely it should have a negative temperature dependence of s, at least at low temperatures. At higher temperatures the s has positive temperature dependence. This shows that the thermally activated carriers and thermal vibrations of the nanomaterial might have given rise to the tunneling between the polymer and the polyoxometalate. 3.6. Humidity sensor Fig. 9 shows the characteristic response of PAni and P12TPA as a function of relative humidity. In PAni it is observed that the conductivity varies linearly with humidity in two slopes. An inflection point is noted at 71% RH showing two different sensing responses. The decrease in resistivity with increasing humidity in the first region (40–71%RH) is attributed to the mobility of free charge carriers of the polymer chain. Usually the hydrophilic polymers are used as resistive type humidity sensors. In conventionally prepared hydrophilic PAni, the enhanced mobility of the charge carriers is due to the absorption of water molecules, which leads to swelling up and thus the uncurling of the polymer chain [40]. In the present work, in interfacially synthesized PAni nanomaterials, the folding of polymer chain is controlled and thus at low humidity levels, the mobility of charge carriers is less and thus the sensitivity is less. At higher humidity level the mechanism of humidity sensing is different. The presence of water molecule can transfer the PAni from reduced state to oxidized state by reacting with the NH2 in the polymer chain thus forming H3O + . This hydronium ion contributes to the enhanced conductivity at high humidity levels [41]. In Fig. 9b, the P12TPA shows two stage sensing responses like PAni but with modified slope. An increase in conductivity is observed in comparison with PAni at all humidity values. In 12TPA, as the water of crystallization increases, a mass of hydrogen bond is produced, which in turn increases the conductivity [42]. As the

65 PAni P12TPA

60

σ x 10-3 (S/m)

20

10

0 40

50

60 RH %

70

80

Fig. 9. A plot of relative humidity vs. conductivity.

90

relative humidity increases, the number of hydrogen bonds among the 12TPA, PAni and water molecule increases. The anions cannot move due to the large volume of the dopant and also due to the strength of the hydrogen bond when the relative humidity increases. But the conduction takes place by the transfer of protons from one water molecule to the next in the terminal water of 12TPA and the reaction sites of the polymer backbone. Since the 12TPA is added to the PAni nanomaterial, the increase in relative humidity results in the homogenization of the medium through which protons transport, due to the formation of an uninterrupted trajectory for proton migration. A possible interpretation of the humidity variation of conductivity in P12TPA is the involvement of Grotthuss mechanism [43]. The Grotthuss Mechanism is the mechanism by which an ‘excess’ proton or protonic defect diffuses through the hydrogen bond network of water molecules through the formation/cleavage of covalent bonds [44].

4. Conclusion Interfacially polymerized conducting polymer and its hybrid with grain sizes between 10 and 50 nm have discernible moisture content, which plays a vital role in conduction mechanism in the dehydration temperature. The temperature and frequency dependant ac conductivity of the nanomaterials obey CBH and OLPT models for PAni and P12TPA, respectively. The response to humidity in the high RH region is due to the Grotthuss mechanism.

Acknowledgements The authors acknowledge Department of Atomic Energy, Mumbai, for the financial assistance (Project no. 2006/34/28BRNS). One of the authors P.C. would like to thank CSIR, New Delhi, for the Senior Research Fellowship. References [1] A. Pron, Synth. Metals 46 (1992) 277. [2] S.A. Krutovertsev, O.M. Ivanova, S.I. Sorokin, J. Anal. Chem. 56 (2001) 1057. [3] P. Chithra lekha, E. Subramanian, D. Pathinettam Padiyan, Sens. Actuators B 122 (2007) 274. [4] Shabnam Virji, Jiaxing Huang, Richard B. Kaner, Bruce H. Weiller, Nano Lett. 4 (3) (2004) 491. [5] J. Huang, R.B. Kaner, J. Am. Chem. Soc. 126 (2004) 851. [6] C.R. Martin, R. Parthasarathy, V. Menon, Synth. Metals 55 (1993) 1165. [7] Z. Wei, Z. Zhang, M. Wan, Langmuir 18 (2002) 917. [8] J. Liu, M.J. Wan, J. Mater. Chem. 11 (2001) 404. [9] Y. Yang, M.J. Wan, J. Mater. Chem. 12 (2002) 897. [10] H.-J. Qiu, M. Wan, Chin. J. Polym. Sci. 19 (2001) 65. [11] H. Cao, C. Tie, Z. Xu, J. Hong, H. Sang, Appl. Phys. Lett. 78 (2001) 1592. [12] M. Delvaux, J. Duchet, P.-Y. Stavaux, R. Legras, C. Demoustier, Synth. Met. 113 (2000) 275. [13] A.R. Hopkins, D.D. Sawall, R.M. Villahermosa, R.A. Lipeles, Thin Solid Films 469–470 (2004) 304. [14] A.J. Heeger, Synth. Met. 57 (1993) 3471. [15] Milind V. Kulkarni, Annamraju Kasi Viswanath, R. Marimuthu, Tanay Seth, Polym. Engg. 44 (9) (2004) 1676Sci. 44 (9) (2004) 1676. [16] P. Passiniemi, K. Vakiparta, Synth. Met. 69 (1995) 237. [17] J. Maier, Ber. Bunsenges Phys. Chem. 90 (1986) 26. [18] S. Sunde, J. Electroceram. 5 (2000) 153. [19] Sanjay Chakane Shilpa Jain, A.B. Samui, V.N. Krishnamurthy, S.V. Bhoraskar, Sens. Actuators B 96 (2003) 124. [20] A. Bishop, P. Gouma, Rev. Adv. Mater. Sci. 10 (2005) 209. [21] Xingwei Li, Gengchao Wang, Xiaoxuan Li, Dongming Lu, Appl. Surf. Sci. 229 (2004) 395. [22] S. Goel, A. Gupta, K.P. Singh, R. Hehrotra, H.C. Kandpal, Mater. Sci. Engg. A. 443 (2007) 71. [23] A.J. Bradley, J.W. Illingworth, Proc. R. Soc. Lond. A 157 (1936) 113. [24] A. Rosenheim, H. Schwer., Z, Anorg. Chem. 89 (1914) 224. [25] L.C.W. Baker, D.C. Glick, Chem. Rev. 98 (1998) 3. [26] O. Nakamura, I. Ogino, T. Kodama, Solid State Ionics 3/4 (1981) 347. [27] P. Charpentier, F. Lepoutre, L. Bertrand, J. Appl. Phys. 53 (1982) 608.

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[28] K.N. Madhusoodanan, J. Philip, G. Parthasarathy, S. Asokan, E.S.R. Gopal, Phil. Mag. B 58 (1988) 123. [29] Sajan D. George, S. Saravanan, M.R. Anantharaman, S. Venkatachalam, P. Radhakrishnan, V.P.N. Nampoori, C.P.G. Vallabhan, Phys. Rev. B 69 (2004) 235201. [30] J.E. de Albuquerque, W.L.B. Melo, R.M. Faria, Rev. Sci. Instrum. 74 (1) (2003) 306. [31] J.E. de Albuquerque, D.T. Balogh, R.M. Faria, Appl. Phys. A 86 (2007) 395. [32] /http://www.jrossmacdonald.com/levminfo.htmlS. [33] S. Lanfredi, A.C.S. Rodrigues, J. Appl. Phys. 86 (1999) 2215. [34] A.N. Papathanassiou, I. Sakellis, J. Grammatikakis, E. Vitoratos, S. Sakkopoulos, E. Dalas, Synth. Met. 142 (2004) 81. [35] Bukem Bilen, Yani Skarlatos, Gulen Aktas, J. Non-Cryst. Solids 351 (2005) 2153.

4319

[36] P. Extance, S.R. Elliott, E.A. Davis, Phys. Rev. B 32 (12) (1985) 8148. [37] A.M. Farid, A.E. Bekheet, Vacuum 59 (2000) 932. [38] J.C. Guintini, J.N. Zanchetta, D. Jullien, R. Enolie, P. Houenou, J. Non-Cryst. Solids 45 (1981) 57. [39] S. Abou El-Hassan, M. Hammad, Phys. Stat. Sol. A 185 (2001) 413. [40] Milind V. Kulkarni, Annamraju Kasi Viswanath, Sens. Actuators B 107 (2005) 791. [41] W.M. Sears, Sens. Actuators B 67 (2000) 161. [42] Qingyin Wu, Shouli Zhao, Jianming Wang, Jianqing Zhang, J. Solid State Electrochem. 11 (2007) 240. [43] V. Ramani, H.R. Kunz, J.M. Fenton, J. Membr. Sci. 31 (2004) 232. [44] Noam Agmon, Chem. Phys. Lett. 244 (1995) 456.