ORIGINAL PAPER

Influence of the magnitude and direction of electric fieldon the transport and growth property of depositedpolyaniline films

Kavita Pandey & Pankaj Yadav & Indrajit Mukhopadhyay

Received: 10 August 2013 /Revised: 20 September 2013 /Accepted: 22 September 2013 /Published online: 12 October 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Polyaniline (PANI) films were deposited by elec-trochemical polymerization of aniline monomer on a fluorine-doped glass substrate at room temperature under differentelectric field directions. The as-synthesized PANI films ob-tained at different growth cycles were characterized by ACimpedance spectroscopy and scanning electron microscopy(SEM). The results revealed that the film morphology, trans-port kinetics, and electrical properties are strongly dependenton the electric field direction and magnitude of the appliedfield during electropolymerization. The SEMmorphology andAC impedance (modulus spectroscopy) indicate that a morehomogeneous, high-porous, and conducting PANI film isinduced by horizontal electric field direction (HEFD) electro-deposition, whereas the modulus spectroscopy of the PANIfilm deposited by vertical electric field direction (VEFD)reveals that VEFD deposition favours two-dimensionalgrowth of PANI. The obtained polymer is more of dielectricin nature due to preferable dendritic growth which is support-ed by SEM analysis.

Keywords Electric field direction . PANI . AC impedancespectroscopy .Modulus spectroscopy .Morphology

Introduction

It has been seen that PANI can be grown in different morphol-ogies such as in tubular form or in fractal form depending onthe suitable condition of deposition [1]. Among the conductingpolymers, polyaniline (PANI) has elicited the most interest dueto its wide range of conductivity from insulating to metallic

regime, unique redox tenability, good environmental stability,low cost, ease of synthesis and promising applications [2–6].

The electrochemical growth and redoxmechanisms of PANIhave been widely investigated by means of numerous in situand ex situ techniques [7–11]. It is well recognized that redoxtransitions occur simultaneously with the exchange of ions,compensating the excess charge within the polymer [12–16].The ion mobility within the polymer matrix has been found tobe slower than the electron motion when the polymer is in theconducting state. Since impedance techniques provide the ki-netic models (equivalent circuits), allowing one to efficientlyattribute a clear physical meaning to the parameters of interest,this method is believed to be a good technique for decouplingthe electrochemical and mass transport processes in the redoxmechanisms of polymers [17]. The complex sequence of reac-tions which are expected to occur simultaneously in PANIredox states and benzoquinone/hydroquinone (BQ/HQ) redoxprocesses with different rates can cause interesting impedancecharacteristics. Electrochemical impedance spectroscopy (EIS)can be effectively used for obtaining important electrochemicalinformation about conducting polymers. The well-developedtheoretical background and established experimental proce-dures make EIS a convenient and readily available techniquefor obtaining important electrochemical information, such aselectrolyte resistance, charge transfer resistance, double-layercapacitance, faradic capacitance, etc. [18–20]. For films ofPANI and its derivatives, several reports are available to studythe diffusion of ions from the electrolyte to the electrode andacross the film using EIS. Suitable equivalent circuits havebeen proposed to interpret the electrochemical behaviour ofPANI [17, 21, 22].

The charge transport occurs via hopping of solitons, po-larons and bipolarons among localized states. Very often, thestructural disorder/inhomogeneity of the polymer and inhomo-geneous doping makes the transport properties of conductingpolymer very complicated, and the inherent macroscopic

K. Pandey : P. Yadav : I. Mukhopadhyay (*)School of Solar Energy, Pandit Deendayal Petroleum University,Gandhinagar 382007, Gujarat, Indiae-mail: sujindra_03@rediffmail.com

J Solid State Electrochem (2014) 18:453–463DOI 10.1007/s10008-013-2270-5

disorder of the system predominates the transport process.From the fundamental point of view, dielectric relaxation spec-troscopy has been widely used to realize the microscopic dy-namical relaxation process in complicated systems. The de-tailed investigation of the dielectric loss, electrode and interfa-cial polarization effects of polymers is of great importance [23].Majority of the studies in the past have focused on understand-ing the dielectric behaviour of PANI and its derivative at orbelow room temperature [24–26]. Low-frequency conductivityand dielectric relaxation measurements have proven to be valu-able in giving additional information on the conduction mech-anism which DC conductivity measurement is incapable ofproviding [27].

The present work directs its attention to use EIS to investi-gate the influence of applying different electric field directionsduring polymerization on the conducting properties of PANI atdifferent oxidation states. A possible correlation of the transportkinetics and morphological features of the PANI film obtainedunder different directions of electric field is presented.

Experimental

Aniline (99.5 %) was purchased from Sigma-Aldrich andpurified by double distillation, and the resulting colourlesssolution was stored in a nitrogen glove box (with moistureand oxygen content <2 ppm) prior to use. Analytical reagentgrade HCl was used to make a monomer solution of aniline inMilli-Q water. A fluorine-doped glass substrate (area of 10×10 mm2 and 2 mm thick) (sheet resistance ∼12 Ω/□) waspurchased from Sigma-Aldrich. The electrochemical experi-ments were performed by using a potentiostat (CHI 633) withgeneral purpose electrochemical system software. A three-electrode cell assembly was utilized with an Ag/AgCl (1 NKCl) (CH Instruments) reference electrode. A Pt ring wasused as the counter electrode, and a fluorine-doped glass withan area of 1.0 cm2 was used as the working electrode. Forvertical electric field direction (VEFD) deposition of PANI,the same cell setup as described in our previous work [1] wasused. For horizontal field direction deposition, a different cellsetup was used in which the working electrode was heldvertically in a three-compartment cell with a Pt wire as thecounter electrode and Ag/AgCl as the reference electrode. Afluorine-doped glass with an area of 1.0 cm2 was used as theworking electrode. The cell and the counter electrode werecleaned with freshly prepared 1:1 (v/v) H2O2/H2SO4 solutionfollowed by ultra-sonication in Milli-Q water (resistivity18MΩ cm) prior to each experiment. For electropolymerization,2 % aniline in aqueous 1 M HCl (v/v) solution was used as themonomer [1].

Electropolymerization was performed by potentiodynamicsweeping from −0.2 to +0.76 Vat a scan rate of 0.015 V/s fordifferent number of cycles. It is important to mention here that

PANI was always deposited by terminating the applied poten-tial at the initial value. After each experiment, the remainingelectrolyte was soaked with a filter paper and then kept fordrying [1]. PANI films for different thickness values andelectric field directions were prepared by altering the numberof potential cycles in CV. The thickness (d ) of deposited filmswas estimated using the area under the first redox processfrom the charge necessary to convert the leucoemeraldineform of PANI to the emeraldine form. The thickness (d ) ofthe deposited PANI film is calculated by Eq. (1).

d ¼ QMw

zFAρð1Þ

where Q is the charge under the first cyclic voltammetricpeak, Mw is the molecular weight of aniline (93.13 g/mol),z =0.5 (no. of electrons/aniline unit), A is the area of electrode(1.0 cm2), ρ the specific density of aniline (1.02 g/cm3) and Fis Faraday’s constant [8]. The thickness of the VEFD-deposited PANI film deposited at different potential scans(34, 51 and 68 cycles) was estimated to be 0.98, 2.12 and3.29 μm, respectively. The thickness of the HEFD-depositedPANI film deposited at different potential scans (34, 51 and68 cycles) was estimated to be 0.30, 0.73 and 1.10 μm,respectively. The thickness of the electrodeposited PANI filmhas been determined by assuming 100 % faradic efficiency.Unless mentioned otherwise, all the potentials are referredwith respect to a standard Ag/AgCl/Cl− (saturated) electrode.

Electrochemical impedance spectroscopy (EIS)

A theoretical model has been presented to explore the trans-port kinetics of a homogeneous and porous PANI film by EIS.In AC impedance technique, the real resistive and imaginarycapacitive behaviour of device under test (DUT) as a functionof applied bias and frequency is tested, which can elaboratethe dynamic nature of DUT. The equivalent circuit corre-sponding to the impedance response of the PANI film isrepresented in Fig. 1.

The parallel association of charge transfer resistance (Rct)and constant phase element (CPE1) elucidates the non-equilibrium charge transfer across the solid electrode/electrolyte interfaces at high frequency. The CPE is selectedinstead of the capacitance in equivalent circuit analysis be-cause a solid electrode/electrolyte interface often reveals afrequency dispersion that cannot be described by simple ele-ments such as resistances, capacitances, inductances or con-vective diffusion (e.g. Warburg) impedance; under these cir-cumstances, “capacitance dispersion” at high frequency isexpressed in terms of a constant phase element (CPE). TheCPE behaviour is generally ascribed to distributed surfacereactivity, surface inhomogeneity, roughness or fractal geom-etry, electrode porosity and current and potential distributions

454 J Solid State Electrochem (2014) 18:453–463

associated with electrode geometry. The parallel combinationof R ct and CPE can also take care of the Randles circuitbehaviour, Warburg contribution; capacitive responses at highfrequencies and diffusion–migration transport of electronsand/or ions under the condition of non-equilibrium chargetransfer across the interfaces at the boundaries of the films.Themass transfer and its limitations due to diffusion processesadjacent to the electrode are well understood by using theWarburg impedance in series with the Rct and in parallel withthe constant phase element (CPE1). The complex equationwhich reveals the function of the represented circuit at highfrequency shown in Fig. 1 is given by Eq. (2)

1

Z1¼ 1

Rct þ ZWþ 1

ZCPE1ð2Þ

where ZW ¼ σ= jωð Þ12 and ZCPE=1/T (jω )n, respectively.The CPE has been considered to represent a circuit param-

eter with limiting behaviour: n =1 for a capacitor, n =0 for aresistor and n =−1 for an inductor. The complex impedance(Z1) function is further simplified and presented in Eq. (3).

Z1 ¼ c−jdð Þ aþ jbð Þa2 þ b2

ð3Þ

To explore the transport kinetics of the deposited film athigh frequency, a plot of real (Z1′) and imaginary term (Z1

′′) asa function of frequency is plotted. The values of a , b , c and dcoefficients are defined by Eqs. (4), (5), (6), (7), respectively.

a ¼ 1þ RctT1ωn1cosθ1 þ σω−1

2T1ωn1 cosθ1−σω

−12T1ω

n1 sinθ1 ð4Þ

b ¼ σω−12T1ω

n1cosθ1 þ RctT1ωn1 sinθ1 þ σω−1

2T1ωn1 sinθ1 ð5Þ

c ¼ Rct þ σω−12 ð6Þ

d ¼ σω−12 ð7Þ

The faradic process in the bulk redox transition of the PANIfilms at low frequency is taken care by the parallel combina-tion of the pseudo capacitor (CF) in parallel with a kineticallycontrolled electron transfer resistor (Rct2). The complex func-tion (ZF) represents the bulk faradic impedance due to redoxtransition within the PANI film. Since the redox transitionswithin the PANI film should not be as reversible and fast as acapacitor, the faradic pseudo capacitor should be modelled byanother constant phase element (CPE2) [20].

1

Z F¼ 1

Rct2þ 1

ZCPE2ð8Þ

The complex impedance (ZF) function is further simplifiedand presented in Eq. (9).

Z F ¼ Rct2a0 þ jRct2b

0

a 02 þ b 02 ð9Þ

The values of a ′ and b ′ are defined by Eqs. (10) and (11),respectively.

a0 ¼ 1þ Rct2T2ω

n2cosθ2 ð10Þ

b0 ¼ 1þ Rct2T2ω

n2 sinθ2 ð11ÞIn practice, the surface of the electrodes is always rough

more or less. Thus, the equipotential surfaces are not parallelto the rough electrode/electrolyte interface but their shapes aregoverned by the frequency ω . At lower frequency, the finerand deeper recesses of electrode surface are exposed to theoscillatory drifting ion which makes the effective area of theinterface larger as compared to that at higher frequency. For an

Fig. 1 Equivalent circuit torepresent the electrochemicalprocess

J Solid State Electrochem (2014) 18:453–463 455

ideal flat interface, the real part of interfacial impedance (ZF)in parallel to pseudo capacitance (CF) is zero. But for a roughinterface, at low frequency, the electrolytes infiltrating into therecesses of an electrode will contribute to the resistance ofinterfacial impedance [28]. Hence, the respective equivalentcircuit to represent the electrochemical process occurringacross the PANI/electrolyte interface is a contribution of theindividual component represented in Fig. 1, which has beencorrelated by a complex function Z and given by Eq. 12

Z ¼ Re þ Z1ð ÞZ F

Re þ Z1 þ Z Fþ Rs ð12Þ

where R s is the uncompensated solution resistance and Re

represents the pore resistance. When the pore size (Re) of theconducting polymers is equal to or smaller than the solvatedion sizes, the transport of ions within the polymer will beimpeded by the solvated water. This phenomenon is mainlyattributed to cations and anions which can be considered asaggregates consisting of the central ions and the solvatedwater molecules. Attempts have been made to explain thenon-ideal impedance behaviour due to non-uniformity of film[29], slow charge leakage [30], ionic trapping in the polymerchains [31], swelling of the films [32] in homogeneousoxidation/reduction processes, etc.

Results and discussion

Electrochemical polymerization of aniline

For different electric field directions, the PANI films were de-posited on the fluorine-doped glass by using cyclic voltammetry,sweeping the potential from −0.2 to 0.76 V. Cyclic voltammetrywas selected to obtain a PANI film with lower backgroundcurrent and better electrochemical characteristic [33].

The cyclic voltammogram for the electropolymerization ofthe VEFD- and HEFD-deposited polyaniline at a sweep rateof 0.015 V/s is shown in Figs. 2 and 3, respectively [1, 34–36].

From Figs. 2 and 3, it can be seen that the voltammogramshows a prominent redox couple “a1–c 1” which may beattributed to the electroformation and reduction of theleucoemeraldine form of PANI. The current rise at the extremeanodic scan a3 is generally attributed to the formation ofpolyemeraldine. The weak redox couple at 0.5 V indicatesthat there is no significant formation of a benzoquinone-typedecomposition product due to restriction of terminal oxidationpotential to 0.76 V. The formation of benzoquinone as ahydrolysis product has been identified by several researchers[11, 37]. The initial cycle shows a very low peak current forthe polymer oxidation and reduction processes [1]. Thechange in the electric field direction results in different currentdensities (Figs. 2 and 3) for the same number of growth cycle

which in turn induces different morphologies of the PANI filmon the fluorine-doped glass substrate.

Electrochemical impedance spectroscopy of the PANI film

Vertical electric field deposition (VEFD)

To explore the influence of electric field magnitude on thetransport phenomena of the VEFD-deposited PANI films, theimpedance spectra were recorded at DC potentials of 0.10,0.30 and 0.50 V with a distance of 1 cm between the counterand working electrodes. Typical Nyquist diagrams of theVEFD-deposited PANI film for 68 growth cycles at differentapplied potentials are shown in Fig. 4.

In Fig. 4, the corresponding solid lines represent the best-fitcurves based on the equivalent circuit proposed in “Electro-chemical impedance spectroscopy (EIS)” for 68 growth

Fig. 2 Cyclic voltammogram for the electropolymerization of theVEFD-deposited polyaniline on a fluorine-doped glass substrate obtainedat a sweep rate of 0.015 V/s at room temperature. The highest currentpeak belongs to the 68th (the last) cycle

Fig. 3 Cyclic voltammogram for the electropolymerization of theHEFD-deposited polyaniline on a fluorine-doped glass substrate obtainedat a sweep rate of 0.015 V/s at room temperature. The highest currentpeak belongs to the 68th (the last) cycle

456 J Solid State Electrochem (2014) 18:453–463

cycles. Under different magnitudes of electric field, the EISdata showed good fitting with the proposed equivalent circuitmodel. The extracted parameter from the detailed analysis ofimpedance spectra is shown in Table 1.

Two impedance arcs in the high- and low-frequency regionare found in all spectra. A vertical line arises with a slope ofapproximately 45° in the low-frequency region. The extent ofthe deviation in the vertical line is contributed to the irregular

geometry in the deposited PANI films. TheWarburg diffusionregion at low frequency is attributed to the semi-infinitediffusion of protons in the polymer/electrolyte interface [8].An insignificant signature of the inductive characteristic in thelow-frequency region is noticed for the PANI film at allapplied potentials. This is because least amount of benzoqui-none formation occurs due to a termination potential of 0.76 V[9]. The obtained value of n1 is less than 1 which representsthe non-ideal capacitive behaviour of double layer.

A small impedance arc in the high-frequency region wasobserved for all deposited films at different applied potentials.From Fig. 4, it can be seen that all impedance curves have adepressed semicircle in the mid- and high-frequency region.The impedance arc in the high-frequency region is attributableto the process at the polymer/electrolyte interface, which isexpected to be the double-layer capacitance in parallel withthe ionic charge transfer resistance (Rct). From the best fitextracted parameters listed in Table 1, the increase in ioniccharge transfer resistance R ct with the positive shift in appliedpotential is due to the transformation of PANI from theemeraldine to leucoemeraldine or pernigraniline forms. Thisspeculation is further supported by the fact that the resistanceof the pore (R e) increases with the applied potential whichcauses the hindrance to the entrance of ions [20]. The weakdependence of the impedance responses in the high-frequencyregion on the applied DC potentials implies that the double-layer charging and discharging behaviour of bare fluorine-doped tin oxide (FTO) should be much less sensitive to thepotential domain investigated.

Horizontal electric field direction (HEFD)

To explore the influence of electric field magnitude on thetransport phenomena of the HEFD-deposited PANI films, theimpedance spectra were recorded at DC potentials of 0.10,0.30 and 0.50 V. Typical Nyquist diagrams of the HEFD-

Fig. 4 Nyquist impedance spectra of the VEFD-deposited PANI filmwith 68 growth cycles of CV measured at different bias in 1 M HCl

Table 1 Values of the electrochemical parameters derived from the bestfit of equivalent circuit in Fig. 1 to the impedance data for the PANI filmsat different growth cycles with vertical electric field deposition. Thefitting error for the VEFD-deposited PANI film is 1.35–2.15 %

Element Growth cycle Bias (V)

0.1 0.3 0.5

Rs/Ω cm2 34 4.7 5.1 6.9

51 4.4 4.95 5.94

68 4.08 4.45 5.55

Rct/Ω cm2 34 23 29 35

51 3.5 2.5 2.4

68 2 2.8 2.8

T1/(mF cm−2) 34 1.50 0.45 0.30

51 2.50 1.50 5.00

68 1.50 1.90 1.50

Rct2/Ω cm2 34 29 31 39

51 11.4 6.5 11

68 5.2 6.3 6.9

T2/(mF cm−2) 34 0.60 0.38 0.09

51 2.80 1.02 0.80

68 1.02 0.60 0.30

Re/Ω cm2 34 6.9 7.2 8.63

51 6.2 7.03 7.54

68 6.01 6.95 7.23 Fig. 5 Nyquist impedance spectra of the HEFD-deposited PANI filmwith 68 growth cycles of CV measured at different bias in 1 M HCl

J Solid State Electrochem (2014) 18:453–463 457

deposited PANI film for 68 growth cycles at different appliedpotentials are given in Fig. 5.

A significant change in the impedance responses for theHEFD-deposited PANI film with different fields is observedin Fig. 5. This indicates that the electrochemical properties ofthe HEFD-deposited PANI film is having a distinct variationwith the applied field. The impedance spectra for the HEFD-deposited PANI film at different applied potentials do notreveal the presence of a Warburg diffusion element whichappears at low frequency in the impedance spectra of theVEFD-deposited PANI film. However, the experimental plotcan be fitted perfectly only when the Warburg impedance istaken into account even though it is physically not seen. Thisis because the data were not acquired at frequencies lower than1 Hz due to the sluggish electron transfer kinetics.

The semicircle observed at high frequency is significantlydepressed, suggesting that the semicircle must result frommore than one electron transfer kinetics [38]. The change inRct is related to the change in conductivity of the PANI film.From the obtained data listed in Table 2, a decrease trend inthe conductivity of the PANI film is observed with appliedbias. The obtained difference in R ct magnitude for the HEFD-and VEFD-deposited PANI films signifies the dependence ofR ct on the magnitude and direction of electric field. FromFig. 5, it can be observed that a semicircle is developed at

0.3 V bias in the mid-frequency range. This semicircle is dueto the formation of a new interface between the insulatingphase of PANI and electrolyte, developed at 0.3 V. At 0.5 V,the semicircle at the mid-frequency range is again depressedwhich can be attributed to the formation of a more conductingphase of PANI.

An increase in the pore resistance (R e) with the applied biasis observed for the HEFD-deposited PANI films. The increaseof pore resistance is attributed to the decrease in the pore sizewith the applied field. A similar increasing trend for poreresistance is observed for the VEFD-deposited films. Theobserved magnitude of R e for the HEFD-deposited PANI filmis much higher than the VEFD-deposited PANI film. Theobserved result signifies that the VEFD-deposited PANI filmsgrown for longer time is more porous and open structured, andthus, the movement of ions through the pores becomes easier.

The obtained value of n2 is less than 1, revealing that CF

does not exhibit the ideal capacitor impedance responses. Thehigh-frequency part of the impedance spectra shows a signif-icant deviation from pure resistive impedance responses in allthe spectra. At high frequency, the source of distortion may bethe reference electrode, counter electrode, geometrical ar-rangement of electrodes and the measuring instrumentationand wirings [39].

Effect of growth cycles on EIS for different electric fielddirections

The coverage as well as thickness of conducting polymer filmscan be controlled by varying the number of cycles (potentialcycling method) or the deposition time (potentiostatic orgalvanostatic methods) of electrochemical polymerization [20].In this work, the numbers of cycles are varied to increase thethickness of the PANI film. The effect of thickness as a functionof electric field direction is investigated in this section.

Table 2 Values of the electrochemical parameters derived from the bestfit of equivalent circuits in Fig. 1 to the impedance data for the PANI filmsof different growth cycles with HEFD deposition. The fitting error for theHEFD-deposited PANI film is 1.65–2.2 %

Element Growth cycle Bias (V)

0.1 0.3 0.5

Rs/Ω cm2 34 23 23.2 24.7

51 9 9.1 9.4

68 4.7 5 5.64

Rct/Ω cm2 34 8 5 3.6

51 20 32 26

68 6.5 2.3 1.8

T1/(mF cm−2) 34 10.0 1.02 0.35

51 1.40 0.51 0.20

68 1.20 0.03 0.02

Rct2/Ω cm2 34 – – –

51 28 35 31

68 10.2 6 6.7

T2/(mF cm−2) 34 – – –

51 0.05 1.50 1.52

68 0.85 0.02 0.1

Re/Ω cm2 34 29.2 30.9 31.3

51 12.7 13.5 16.1

68 9.8 11.4 13.9 Fig. 6 Nyquist plot of the VEFD-deposited PANI film at an applied biasof 0.10 V with different growth cycles of CV in 1 M HCl

458 J Solid State Electrochem (2014) 18:453–463

Typical Nyquist diagrams for the VEFD- and HEFD-deposited PANI films for 34, 51 and 68 growth cycles mea-sured at electric field corresponding to 0.10 V are shown inFigs. 6 and 7, respectively. The best fit extracted parameters

for the VEFD- and HEFD-deposited PANI films is given inTables 1 and 2, respectively.

The deviation from the semicircle in the complex imped-ance plots at lower frequencies in Figs. 6 and 7 indicates thepresence of distribution of relaxation time and conductionlosses [40].

From the observed complex impedance plot shown in Figs. 6and 7, an impedance arc is found in the high-frequency regionfor both the VEFD- and HEFD-deposited PANI films. Asimilar decreasing trend of the impedance arc for the VEFD-and HEFD-deposited PANI films with the number of growingcycles is observed. A significant contribution of Rct2 and CF isobserved for the VEFD- and HEFD-deposited PANI films inthe low-frequency region. The presence of R ct2 and CF isattributed to the electrochemical reactions on the electroactivesites of FTO. A decreasing trend of Rct2 with the number ofgrowing cycles is observed for the VEFD- and HEFD-deposited PANI films. A lower value of Rct2 observed forVEFD as compared to the HEFD film signifies the highercoverage of PANI over the electroactive sites of FTO.

Fig. 7 Nyquist plot of the HEFD-deposited PANI film at an applied biasof 0.10Vwith different growth cycles of CVin 1MHCl. The inset showsthe impedance spectra for the PANI film deposited at 34 growth cycles

Fig. 8 Frequency dependence ofthe real part of M ′: a VEFD-deposited PANI film and bHEFD-deposited PANI film with34 (circles), 51 (squares) and 68(triangles) growth cycles

J Solid State Electrochem (2014) 18:453–463 459

The Rct signifies the conductivity of the deposited PANIfilm. The decreasing value of Rct with the number of growthcycle signifies that the entrance of pores becomes wider withthe growing cycles, favouring the exchange of ions [20]. Thepositive favouring of exchange of ions with the number ofgrowing cycles is attributed to an increase in the effectivesurface area of the polymer electrode. Dependence of Rct onthe film thickness was observed for the different electric fielddirections. The extracted and listed value of Rct in Tables 1and 2 shows a severe decrease for the VEFD film when thegrowth cycle is increased from 34 to 51. For the growth cyclesabove 55, a negligible change in Rct value is found for theVEFD film. An opposite behaviour of Rct is observed for theHEFD-deposited film. Thus, the thickness of the PANI filmdeposited is influenced by the combined effect of the numberof growth cycles and the electric field direction. The imped-ance values can be considered to be more accurate as theycorrespond to dynamic state conditions. Structural factors(chain conformation, film morphology) appear to influencemany characteristics of the PANI film electrodes, particularlyat redox potentials where hysteresis in many electrical param-eters is observed.

Modulus spectroscopy of the deposited PANI films

Electric modulus spectroscopy is done to analyse the conduc-tion relaxation property of the developed PANI film. Theelectric modulus represents the real dielectric relaxation pro-cess which corresponds to the relaxation of the electric field inthe material when the electric displacement remains constant.The advantage of adopting the electric modulus (M*) is tointerpret the relaxation behaviour of the developed films undersuppressed DC conductivity at low frequency. The electricmodulus (M*) also explores the dynamic aspects of the chargemotion in conductors in terms of relaxation in electric fieldwithout any molecular polarization phenomena [38]. Electricmodulus (M*) of the VEFD- and HEFD-deposited PANIfilms with different thickness values can be explored by usingEq. (13).

M � ¼ jωC0Z� ð13Þ

Figure 8 shows the variation of the real part of electricmodulus (M ′) with frequency for both the VEFD and HEFD-deposited films at different growth cycles.

From Fig. 8a, b, it can be seen that M ′ approaches zero atlow frequencies, indicating that the electrode polarizationgives a negligible low contribution to M ′ and can be ignoredwhen the permittivity data are expressed in this form [41, 42].

From Fig. 8a, b, it can be seen that the trend of onsetfrequency for the VEFD- and HEFD-deposited PANI filmswith increasing growth cycles is different. This can be attribut-ed to the different growth mechanisms involved for both the

electric field directions. The value of onset frequency at whichM ′ starts increasing shifts to higher frequency with the numberof growth cycles for the VEFD-deposited film as shown inFig. 8a. This can be attributed to the formation of nanorods ofPANI on FTO for lower growth cycles as it can be seen from ascanning electron microscopy (SEM) image (Fig. 9a). As thenumber of growth cycle increases, swelling of PANI nanorodstakes place which shifts the onset frequency to a higher value.This can be observed from the SEM image shown in Fig. 9b.

The inset of Fig. 9 shows the thickness measurement of theVEFD-deposited PANI films by surface profiling method(Dektak 150, Veeco). From the profilometry, the thickness ofthe deposited PANI film is estimated to be 0.959 and 3.624 μmfor 34 and 68 growth cycles, respectively. The average surfaceroughness is 0.32 and 0.63 μm for 34 and 68 growth cycles,respectively. The experimental and calculated thickness of thePANI film is within the acceptable limit of 5 %.

An opposite behaviour for the variation of M ′ is observedfor the HEFD-deposited film as shown in Fig. 8b. At lowergrowth cycles, i.e. when the polymer is deposited for only 34cycles, the onset frequency forM ′ is very high as compared to

Fig. 9 SEM image of the PANI film deposited by VEFD: a 34 growthcycles and b 68 growth cycles

460 J Solid State Electrochem (2014) 18:453–463

the polymer grown for much longer time (for 51 and 68cycles). This may be attributed to the effective polymer grainsize. At lower growth time, the average presence of smallPANI grains is maximum as it can be seen from the SEMimage of HEFD PANI deposited on FTO in Fig. 10a. Hence,the onset frequency for lower growth cycles of the HEFD filmis larger compared to that for the higher growth cycles. Athigher growth cycles, the grains of PANI get accumulatedwhich leads to the formation of continuous path of PANI onFTO (Fig. 10b).

Hence, the HEFD-deposited PANI films exhibit moreconducting nature than the VEFD films. An unexpected peakis obtained for both the VEFD and HEFD films due to theeffect of cumulative dipole moments in the polymer chain.

The different conduction conditions such as conjugatestates, chain structure and doping level can be explored for

the deposited PANI film using the imaginary part of complexmodulus (M″ ). Figure 11 shows the variation of the imaginarypart of electric modulus (M″) with frequency for both theVEFD- and HEFD-deposited films at different growth cycles.

The peak maximum of the M″ curve seems to be locatedbeyond the interval of frequencies, used in the measurements.This is mainly attributed to the large number of free chargecarrier concentration and their motion within the material. Athigh frequency, the period of the AC signal is much lower thanthe lifetime of the interface states. Hence, the interface statescannot follow the reversal of AC signal, so electrical modulusreaches a maximum value due to the relaxation process.

The peak frequency in the M″ vs. log f spectra is anindication of a transition from short-range to long-range mo-bility at decreasing frequency along conductivity paths. Theregion to the left of the peak is where the ions are mobile overlong distances, and the region to the right is where the ions arespatially confined to their potential wells. From Fig. 11a, b,the appearance of the M″ shifts to higher frequency with thenumber of growth cycles which indicates the enhanced DC

Fig. 11 Frequency dependence of the imaginary part of M″: a VEFD-deposited PANI film and b HEFD-deposited PANI film with 34 (circles),51 (squares) and 68 (triangles) growth cycles

Fig. 10 SEM image of the PANI film deposited by HEFD: a 34 growthcycles and b 68 growth cycles. The inset shows the thickness measure-ment of the HEFD-deposited PANI films by surface profiling method(Dektak 150, Veeco). From the profilometry, the thickness of the depositedPANI film is estimated to be 0.356 and 0.985 μm for 34 and 68 growthcycles, respectively. The average surface roughness is 0.047 and 0.031 μmfor 34 and 68 growth cycles, respectively. The experimental and calculatedthickness of the PANI film is within the acceptable limit of 5 %

J Solid State Electrochem (2014) 18:453–463 461

conductivity [43]. The decrease in relaxation times with thenumber of growth cycles obtained from the modulus spectrareveals a decrease in the carrier hopping length, therebyfacilitating charge transport and increasing the conductivity.This is due to the reduced interchain distance on PANI surface,leading to an increased carrier hopping [44]. The same kind ofbehaviour is observed for both the VEFD- and HEFD-deposited PANI films.

The magnitude of the M″ peak represents the dielectricconstant [28]. From Fig. 11a, b, it can be seen that the magni-tude of M″ at lower growth cycle for the VEFD-depositedPANI film is much less than the HEFD-deposited film. Hence,the VEFD-deposited PANI film exhibits more dielectric naturethan the HEFD-deposited PANI film.

Conclusion

The PANI films were deposited by electrochemical polymeri-zation of aniline monomer on a fluorine-doped glass substrateat room temperature under different electric field directions.The AC impedance spectroscopy results revealed that the filmmorphology, transport kinetics and electrical properties arestrongly dependent on the electric field direction and magni-tude of the field. The observation of SEMmorphology and ACimpedance (modulus spectroscopy shows that the formation ofa more homogeneous PANI film with high-porosity andconducting nature is induced by HEFD deposition). Modulusspectroscopy reveals that VEFD deposition favours two-dimensional growth of the PANI film which is more dielectricin nature.

Acknowledgments The authors acknowledge Dr. Puyam Singh, Cen-tral Salt and Marine Chemical Institute (CSMCSIR), Bhavnagar forcarrying out the scanning electron microscopy of our samples. We arethankful to the Department of Science and Technology (DST), Govern-ment of India for the financial support under the INSPIRE fellowshipscheme for PhD studies of Kavita Pandey.

References

1. Bhattacharjya D, Mukhopadhyay I (2012) Controlled growth ofpolyaniline fractals on HOPG through potentiodynamic elec-tropolymerization. Langmuir 28:5893–5899

2. Wessling B (1998) Dispersion as the link between basic research andcommercial applications of conductive polymers (polyaniline). SynthMet 93:143–154

3. Wang Y, Jing X (2004) Preparation of an epoxy/polyaniline compositecoating and its passivation effect on cold rolled steel. Polym J 36:374–379

4. Wang Y, Jing X (2005) Intrinsically conducting polymers for elec-tromagnetic interference shielding. Polym Adv Technol 16:344–351

5. Chandrakanthi N, CareemMA (2000) Thermal stability of polyaniline.Polym Bull 44:101–108

6. Persaud K (2005) Polymers for chemical sensing. Mater Today 8:38–44

7. Tang Z, Liu S, Wang Z, Dong S, Wang E (2000) Electrochemicalsynthesis of polyaniline nanoparticles. Electrochem Commun 2:32–35

8. Chen WC, Wen TC, Gopalan A (2002) Negative capacitance forpolyaniline: an analysis via electrochemical impedance spectroscopy.Synth Met 128:179–189

9. Chen WC, Wen TC, Hu CC, Gopalan A (2002) Identification ofinductive behavior for polyaniline via electrochemical impedancespectroscopy. Electrochim Acta 47:1305–1315

10. Kim YT, Yang H, Bard AJ (1991) Electrochemical control ofpolyaniline morphology as studied by scanning tunneling microsco-py. J Electrochem Soc 138:L71–L74

11. Stilwell DE, Park SM (1988) Electrochemical studies on growthproperties of polyaniline. J Electrochem Soc 135:2254–2262

12. Gholamian M, Contractor AQ (1990) Oxidation of formic acid atplatinum microparticles dispersed in a polyaniline matrix: influenceof long-range order and metal-polymer interaction. J ElectroanalChem 289:69–83

13. Ohsaka T, Ohnuki Y, Oyama N, Katagiri G, Kamisako K (1984) IRabsorption spectroscopic identification of electroactive andelectroinactive polyaniline films prepared by the electrochemicalpolymerization of aniline. J Electroanal Chem 161:399–405

14. Kobayashi T, Yoneyama H, Tamura H (1984) Electrochemical reac-tions concerned with electrochromism of polyaniline film-coatedelectrodes. J Electroanal Chem 177:281–291

15. Genies EM, Tsintavis C (1985) Redoxmechanism and electrochemicalbehaviour or polyaniline deposits. J Electroanal Chem 195:109–128

16. Kitani A, Yano J, Sasaki K (1986) ECD materials for the threeprimary colors developed by polyanilines. J Electroanal Chem 209:227–232

17. Genies EM, Lapkowski M (1987) Spectroelectrochemical study ofpolyaniline versus potential in the equilibrium state. J ElectroanalChem 220:67–82

18. Gabrielli C, Keddam M, Nadi N, Perrot H (2000) Ions and solventtransport across conducting polymers investigated by acelectrogravimetry. Application to polyaniline. J Electroanal Chem485:101–113

19. Bobacka J, Lewenstam A, Ivaska A (2000) Electrochemical imped-ance spectroscopy of oxidized poly (3,4-ethylenedioxythiophene)film electrodes in aqueous solutions. J Electroanal Chem 489:17–27

20. Hu CC, Chu CH (2001) Electrochemical impedance characterizationof polyaniline-coated graphite electrodes for electrochemical capac-itors—effects of film coverage/thickness and anions. J ElectroanalChem 503:105–116

21. Yang HJ, Bard AJ (1992) The application of fast scan cyclicvoltammetry. Mechanistic study of the initial stage ofelectropolymerization of aniline in aqueous solutions. J ElectroanalChem 339:423–449

22. Deslouis C,Musiani MM, Tribollet B (1994) AC impedance study oftransport processes in polyaniline membranes. J Phys Chem 98:2936–2940

23. Popkirov GS, Barsoukov E, Schindler RN (1997) Investigation ofconducting polymer electrodes by impedance spectroscopy duringelectropolymerization under galvanostatic conditions. J ElectroanalChem 425:209–216

24. Vorotyntsev MA, Badiali JP, Inzelt G (1999) Electrochemical imped-ance spectroscopy of thin films with two mobile charge carriers:effects of the interfacial charging. J Electroanal Chem 472:7–19

25. Gazotti WA, Matencio T, Depaoli MA (1997) Electrochemical im-pedance spectroscopy studies for chemically prepared poly (O -methoxyaniline) doped with functionalized acids. Electrochim Acta43:457–464

26. Lang G, Inzelt G (1999) An advanced model of the impedance ofpolymer film electrodes. Electrochim Acta 44:2037–2051

462 J Solid State Electrochem (2014) 18:453–463

27. Mansfeld F, Lin S, Chen YC, Shih H (1988) Minimization of high–frequency phase shifts in impedance measurements. J ElectrochemSoc 135:906–907

28. Venkataraman BH, Varma KBR (2005) Microstructural, dielectric,impedance and electric modulus studies on vanadium-doped andpure strontium bismuth niobate (SrBi2Nb2O9) ceramics. J Mater SciMater Electron 16:335–344

29. Lang G, Inzelt G (1991) Some problems connected withimpedance analysis of polymer film electrodes: effect of thefilm thickness and the thickness distribution. Electrochim Acta36:847–854

30. Lang G, Bacskai J, Inzelt G (1993) Impedance analysis of polymerfilm electrodes. Electrochim Acta 38:773–780

31. Tanguy J, Mermilliod N, Hoclet M (1987) Capacitive charge andnoncapacitive charge in conducting polymer electrodes. JElectrochem Soc 134:795–802

32. Komura T, Yamaguti T, Takahasi K (1996) Impedance study of thecharge transport at poly-O -phenylenediamine film electrodes. JElectrochim Acta 41:2865–2870

33. Rubinstein I, Sabatani E, Rishpon J (1987) Electrochemical imped-ance analysis of polyaniline films on electrodes. J Electrochem Soc134:3078–3083

34. Pruneanu S, Veress E, Marian I, Oniciu L (1999) Characterization ofpolyaniline by cyclic voltammetry and UV-Vis absorption spectros-copy. J Mater Sci 34:2733–2739

35. Nunziante P, Pistoia G (1989) Factors affecting the growth of thickpolyaniline films by the cyclic voltammetry technique. ElectrochimActa 34:223–228

36. Matsushita M, Kuramitz H, Tanaka S (2005) Electrochemical oxida-tion for low concentration of aniline in neutral pHmedium: applicationto the removal of aniline based on the electrochemical polymerizationon a carbon fiber. Environ Sci Technol 39:3805–3810

37. Kobayashi T, Yoneyama H, Tamura H (1984) Polyaniline film-coated electrodes as electrochromic display devices. J ElectroanalChem 161:419–423

38. Radosevic VH, Kvastek K (2007) Analysis of high-frequency dis-tortions in impedance spectra of conducting polyaniline film modi-fied Pt-electrode measured with different cell configurations.Electrochim Acta 52:5377–5391

39. Ravikiran YT, Lagare MT, Sairam M, Mallikarjuna NN, Sreedhar B,Manohar S, MacDiarmid AG, Aminabhavi TM (2006) Synthesis,characterization and low frequency AC conduction of polyaniline/niobium pentoxide composites. Synth Met 156:1139–1147

40. Grzeszczuk M, Olszak GZ (1993) Ionic transport in polyaniline filmelectrodes: an impedance study. J Eelctroanal Chem 359:161–174

41. Zhu D, Zhang J, Xu C, MatsuoM (2011) The frequency-dependenceconduction of polyaniline based on their para-crystalline structures.Synth Met 161:1820–1827

42. Lee HT, Chuang KR, Chen SA, Wei PK, Hsu JH, Fann W (1995)Conductivity relaxation of 1-methyl-2-pyrrolidone-plasticizedpolyaniline film. Macromolecules 28:7645–7652

43. Ray DK, Himanshu AK, Sinha TP (2007) Structural and low fre-quency dielectric studies of conducting polymer nanocomposites.Indian J Pur Appl Phys 45:692–699

44. Han MG, IM S (2001) Dielectric spectroscopy of conductivepolyaniline salt films. J Appl Polym Sci 82:2760–2769

J Solid State Electrochem (2014) 18:453–463 463