Synthesis, characterization, AC conductivity,and diode properties of polyaniline–CaTiO3compositesAashis S. Roya, Shruti Gopalkrishna Hegdeb and Ameena Parveenc*

Polyaniline–CaTiO3 composites of different weight percentages were prepared by in situ polymerization. The preparedcomposites were characterized by Fourier transform infrared spectroscopy for structural studies, and a morphologyanalysis was carried out by scanning electron microscopy studies. Real and imaginary parts of the complex impedancewere determined for given samples as a function of frequency. Current–voltage and capacitance–voltage measure-ments were also carried out. The carrier mobility μ values of neat polyaniline and polyaniline–CaTiO3 composites werefound to be 5.37×10�3 and 2.73×10�2, respectively, and a significant enhancement, as compared with the reporteddata, was observed. Therefore, this study may provide a better route for technological applications in all fields inthe near future and can also be represented by a pure electronic model. Copyright © 2013 John Wiley & Sons, Ltd.

Keywords: conductivity; impedance spectroscopy; bulk resistance; grain boundaries; J–V measurements

INTRODUCTION

1The electrical conduction properties of metal oxide-dopedconducting polymer composites are very important. Most of thehigh-frequency applications related to electrical properties areconcerned with dielectrics and frequency dependence onresistivity of the composites. The properties and characteristics ofthematerials are approached from twomain points of view.[1] Mate-rials engineers consider them primarily as components in electricalcircuits having specified property values and characteristics with re-gard to electrical measurements. Physicists consider these proper-ties in terms of a quantitative understanding of electronic andionic behaviors.[2] The transition metal oxide and ceramics take anintermediate position, considering both the problems of the ulti-mate user and the ability to understand the effects of composition,structure, and environment on properties. Thus, conductingpolymers have recently attracted prominence as candidates forelectronic materials. The physics and chemistry of conductingpolymer polyanilines (PANI) have been the subject of intense studyfor more than a decade,[3–5] because of their fundamental andimportant technological properties with their applications for solarenergy conversion, sensor preparation, rechargeable batteries, gasseparationmembranes, nonlinear optical devices,[6] semiconductingdevices, electrochromic displays, light emitting devices, and othertechnological systems. Literature studies reveal that conductingpolymers can be used as an active layer material and metal contactas an electrode in solar cell. Contact properties have been studied insome detail in the case of poly(sulfur nitride), (SN)x, andpolyacetylene, (CH)x.[7] Metal contacts to polyacetylene in thesemiconducting regime have been shown to be similar to those ofclassical semiconductors. Ohmic contacts are formed with high-work-function metals, and blocking Schottky barriers are formedwith low-work-function metals on p-type CHx. Earlier, there werereports on applications of photo-electrochemically generatedconducting polymers as contacts[8] and corrosion protection[9] onn-type semiconductors.

A particularly important topic in this search for electronic applica-tions is the nature of metal/conducting polymer junctions. Theapplications of these new materials require incorporation into theexisting electronic infrastructure. The results of electronic studiesindicated that the polymer under investigation, PANI, formsSchottky barriers on n-type semiconductors and that the workfunction of conducting PANI is 5 eV. The results of the study ofmetal/polymer junctions reported here indicate that in the case ofpolymer/metal interfaces, charge transport is limited by the forma-tion of interphases of insulating materials. With inert metals, nointerphases are found, and the contacts are ohmic.In this present paper, PANI is selected as an active layer

because PANI seems to be the most important class ofconducting polymers at present from the viewpoint of scientificand technological application-oriented research, owing to itswide variety, ease of synthesis, and efficiency.

EXPERIMENTAL

Synthesis of polyaniline

We dissolved 0.1mol of aniline in 1 MHCl to form aniline hydro-chloride. To this reaction mixture, 0.1M of ammonium persulfate

* Correspondence to: Ameena Parveen, Department of Physics, Government FirstGrade College, Gurmatkal, Yadgir, Karnataka, India.E-mail: Aparveen981@gmail.com

aa A. S. RoyDepartment of Materials Science, Gulbarga University, Gulbarga, Karnataka,India

bb S. G. HegdeChemical Engineering, Dayananda Sagar College of Engineering, Bangalore,Karnataka, India

cc A. ParveenDepartment of Physics, Government First Grade College, Gurmatkal, Yadgir,Karnataka, India

Research article

Received: 5 December 2012, Revised: 1 September 2013, Accepted: 24 September 2013, Published online in Wiley Online Library: 24 October 2013

(wileyonlinelibrary.com) DOI: 10.1002/pat.3214

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[(NH4)2S2O8], which acts as an oxidant, was added slowly withcontinuous stirring for 6 h at 0–5°C. Furthermore, the reactionsolutions were kept for 24 h in order to complete the polymeriza-tion. The precipitate powder recovered was then vacuum filteredand washed with water and acetone to remove the ammoniumpersulfate and unreacted monomers from the solution. Finally,the resultant precipitates were dried in an oven for 24 h toachieve a constant weight.[10]

Synthesis of polyaniline–CaTiO3 composites

Synthesis of PANI–CaTiO3 composites is carried out by using0.1mol of aniline by dissolving 1M HCl to form aniline hydro-chloride. Calcium titanate (CaTiO3) was added to this solutionin the weight percentages of 10, 20, 30, 40, and 50 to formaniline hydrochloride solution with vigorous stirring in order tokeep the calcium titanate (CaTiO3) suspended in the solution.To this reaction mixture, 0.1M of ammonium persulfate [(NH4)2S2O8], which acts as an oxidant, was added slowly with contin-uous stirring for 4–6 hr at 0–5°C. The precipitate was filteredunder dynamic vacuum and washed with water and acetone.Finally, the resultant precipitate was dried in an oven at 60°Cfor 24 hr to achieve a constant weight. The synthesized compos-ites obtained were crushed into fine powder in an agate mortarin the presence of acetone medium. Lastly, the powders of PANIand PANI–CaTiO3 composites so obtained from the synthesistechniques discussed earlier were crushed and ground finely inthe presence of acetone medium in an agate mortar. Thispowder was pressed to form pellets of 10-mm diameter andthickness that varies from 1 to 2mm by applying a pressure of90MPa in a hydraulic press. Finally, the pellets of PANI and itscomposites so obtained from the aforementioned techniqueswere coated with silver paste on either side of the surfaces toobtain a better contact.

Fourier transmission infrared spectroscopy

The Fourier transmission infrared (FTIR) spectra of all the sampleswere recorded on a Perkin Elmer (model 783) infrared spectrometer(Massachusetts 02451, USA) in KBr medium at room temperature.For recording FTIR spectra, powders were mixed with KBr in aratio of 1:25 by weight to ensure uniform dispersion in KBr pellets.The mixed powders were pressed in a cylindrical dye to obtainclean disks of approximately 1-mm thickness.

Scanning electron microscopy

The surfacemorphology of PANI and its composites were studied byusing Phillips XL30 ESEM scanning electronic microscope (Reston,Virginia). The powder samples were dispersed on the surface ofa carbon tape mounted on an aluminum tab, and conductinggold was sputtered on the sample so as to avoid charging on thesample surfaces, and hence, selected areas were photographed.

Dielectric spectroscopy

The dielectric tangent loss and dielectric constant were stud-ied by sandwiching the pellets of these composites betweenthe silver electrodes and were studied in the frequencies of102–106 Hz, using Hioki LCR meter (Koizumi, Ueda, Nagano).

Current density–voltage (J–V) Characteristics

The sourcematerialswere aluminum shots purchased fromAlfa Aesarwith 99.999%purity and a surface area of 4.30m2g�1 purchased fromNeo-Ecosystems and Software Pvt. Ltd. A conical-shape tungsten (W)coil was used for evaporating the metal over indium tin oxide (ITO)-coated glass slides purchased from Blue Star product.

Fabrication of Schottky device

The aluminum thin film was deposited at room temperature bythermal vacuum evaporation. The working pressure of thevacuum chamber is ≈10�6mbar. A tungsten (W) coil was usedfor evaporating the source material, and the substrates werecleaned by acetone, isopropanol, and distilled water, respec-tively, at the end dried with ultra-pure argon gas (Ar). Asource–substrate distance of 12 cm was maintained for all thesamples. The thickness and evaporation rate were monitoredby a quartz crystal monitor.

The Schottky device fabrications by using PANI–CaTiO3

composites over ITO-coated glass are shown in Fig. 1. Thealuminum (acting as a cathode) was deposited over the PANI–CaTiO3 composite ITO-coated glass (acting as an anode)employing a thermal evaporator. PANI–CaTiO3 composites of30wt% were coated over the ITO glass by a spin coater at3000 rpm for 5min between electrodes behaving as activematerial. The devices fabricated on ITO-coated glass slides werecleaned by an RAC-1 method successively. The active material solu-tion was obtained by dissolving 3wt% PANI–CaTiO3 composites(30wt%) in a N,N′-dimethyl propylene urea (1.016gcm�3) solventand was kept stirring for 3days and then filtered using 0.45-μmsyringe filter. The filtered composite solution was used for coatingover cleaned ITO substrates by employing a spin coater, annealedunder vacuum at 100°C at a pressure of 60mmHg for 30min. TheAl electrode contacts were vacuum deposited over the compositelayer by using an outline mask having a spherical shaped activecontact area of 0.04 cm2. The electrical transport measurementswere performed on fabricated structures for measuring the I–Vcharacteristics at various Al contacts.

RESULTS AND DISCUSSION

Fourier transmission infrared spectroscopy

Figure 2(a) shows the FTIR spectra for pure PANI. The absorp-tion peaks are found to be at 1561 cm�1, corresponding tothe C=N stretching of quinoid ring, 1480 cm�1 correspondingto the C–H stretching of a benzoid ring, 1302 cm�1 correspond-ing to the C–N stretching of a benzenoid ring, 1140 cm�1

corresponding to the characteristic vibrating mode of a quinoidring, and 801 cm�1 corresponding to the blending of the C–Hbond in an aromatic ring. These important peaks confirm theformation of PANI.[11]

Figure 2(b) shows the FTIR spectra of pure CaTiO3. It isobserved from the figure that there is a broad peak at583 cm�1, which is due to the Ti–O vibration, and another at450 cm�1, which is a characteristic feature of CaTiO3. Anotherabsorption peak is also observed at 1490 cm�1, which can beattributed to carbonate ion impurities.[12] The FTIR spectra ofthe PANI–CaTiO3 composite (50wt% CaTiO3 in PANI) are shownin Fig. 2(c). The prominent peaks in the PANI–CaTiO3 compositeare observed at 2922, 1571, 1482, 1303, 1142, 802, and 582 cm�1.

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By careful observation of infrared spectroscopy, the characteris-tic stretching frequencies are considerably shifted toward ahigher-frequency side. These data suggest that there is a Vander Waals kind of an interaction between the polymer chainand the CaTiO3 and confirm the formation of composites.

Scanning electron microscopy

The SEM micrograph image of pure PANI is shown in Fig. 3(a). Itis observed from the SEM image that PANI shows clusters ofgranular morphology and an average grain size of 0.3μm.Figure 3(b) shows the SEM image of PANI–CaTiO3 composites,a clear view of an oriented structure wherein almost all CaTiO3

particles are seen as embedded in the polymer matrix. Hence,it is observed that fine grains of CaTiO3 when coated with PANItend to coalesce and form agglomerates of size 0.2μm, whichin turn results in a homogeneous distribution of CaTiO3 in PANI.Because the structure property correlation plays a significantrole, there is a correlation between dimensions of the CaTiO3

used for composite preparation and its effect on electric andmagnetic properties.

Dielectric studies

Figure 4 shows the variation of ε′ as a function of frequency forPANI–CaTiO3 composites (10, 20, 30, 40, and 50wt%). In all thecases, it is observed that the dielectric constant is quite high ata lower frequency and decreases with an increase in appliedfrequency. The observed behavior may be due to space chargepolarization and electrode polarization of PANI–CaTiO3 compos-ites taking place in these materials, i.e., positive charges aredisplaced along the field and negative charges shift in theopposite direction, creating an internal electric field that partlycompensates the external field inside the composites. Amongall the weight percentages used, the 30wt% PANI–CaTiO3

composites showed the lowest real permittivity.[13,14]

The dielectric tangent loss (tanδ) as a function of frequency forPANI and PANI–CaTiO3 composites is shown in Fig. 5 for thevarious weight percentages. It is observed that the dielectric lossdecreases as a function of frequency. PANI and PANI–CaTiO3

composites exhibit a small value of dielectric loss at higherfrequencies, which suggests that these materials are losslessmaterials at frequencies beyond 103 Hz. The observed behavior

Figure 1. Schematic diagram of diode fabrication using polyaniline over indium tin oxide (ITO)-coated glass.

Figure 2. (a–c) Fourier transmission infrared transmission spectra of purepolyaniline (PANI), pure CaTiO3, and PANI–CaTiO3 composites of 50wt%.

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is consistent with the conductivity and dielectric constant resultsin these composites. A change in behavior is observed at 30wt%because of the low relaxation time. At this stage, the chargecarriers will not reorient with respect to the field. Once thereorientation is completed, relaxation time decreases; hence,tanδ also decreases.[15] The tangent loss at 20 and 40wt% isfound to be almost similar as a result of the alternation ofpolarity between the two opposite charges, and these chargesmust be displaced through the dielectric first in one directionand then in the other, and overcoming the opposition that theyencounter leads to more loss at 40wt%, resulting in a loss that isnearly equal to that at 20wt%.

Figure 6 shows the σac conductivity of pure PANI and PANI–CaTiO3 composites at room temperature at various frequencies.The conductivity of pure PANI and its composites increases withincreasing frequencies, obeying the universal power law. Theconductivity is almost constant up to 105 Hz and then suddenlyincreases with an increase in frequency, which is a characteristicproperty of disordered materials. Among all composites, ones at30wt% show a high conductivity of 1.40 × 10�3 S cm�1 at 104 Hzowing to interfacial as well as electrode polarization.

Impedance spectroscopy

Figure 7 shows a complex plane impedance Cole–Cole plots forthe different weight percentages of PANI and PANI–CaTiO3

composites. It is observed through the graph that the compleximpedance plots show a single semicircular arc passing throughthe origin. This arc is due to the electrical properties of theparallel combination of bulk grain resistance and bulk graincapacitance of PANI and PANI–CaTiO3, composites, and the othersemicircles are not completely resolved at a higher-frequencyregion. This indicates that the resistance of the grain boundaryis very high in comparison with that of the bulk. Thus, the graindominates conductivity of the sample, and the role of the grain

Figure 3. Scanning electron microscopy of (a) pure polyaniline (PANI)and (b) PANI–CaTiO3 composites of 50wt%.

Figure 4. Variation of є′ as a function of frequency of polyaniline (PANI)and PANI–CaTiO3 composites.

Figure 5. Variation of tanδ as a function of frequency for polyaniline(PANI) and PANI–CaTiO3 composites.

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boundary is negligible in conduction at higher frequencies. It isinteresting to note that the center of each arc lies either on thereal axis or very close to the real axis, i.e., the angle of disper-sion is negligible. This clearly highlights that this composite iselectrically homogeneous. Therefore, the intercept made bythe semicircle at the higher-frequency end corresponds to theresistance offered by the bulk grain, and the intercept dueto the lower-frequency arc corresponds to the combinedresistance of grain boundaries.[16]

Current density–voltage (J–V) character

The change in the J–V characteristics of fabricated devices wasmeasured by varying the applied voltage varied from �5 to+5 V with an increment of 0.1 V using a Keithely 2420C sourcemeter recorded with Lab tracer software as shown in Fig. 8.The linear J–V addiction of ITO/composites/Al indicates a sym-metric and non-ohmic behavior of this device in the tested

voltage range, independent of electrical polarity connected tothe device electrode. This result illustrates that both ITO andaluminum having a work function ≈4.2 eV form Schottkycontact with CaTiO3-doped PANI composites having a workfunction nearly similar to that of PANI, i.e., ≈5.3 eV.[17,18] Thespecific contact resistance was derived from the reciprocal ofthe derivative of the current density with respect to the voltageacross the interface. The J–V characteristics were analyzedusing the following equation:

R ¼ dJdV

� ��1

The nature of the J–V trace implies that aluminum contactformed over the active layer is Schottky. The current density Jfor PANI–CaTiO3 composites lies above that of the PANI. Thisincreased current density may be due to the interface effectthat is more effective in the area of the metal contact inPANI–CaTiO3 composites. Moreover, aluminum contact withthe PANI–CaTiO3 composites film is better because CaTiO3

improved packing at the interface of PANI and the metalelectrode. Because of improved interfacial contact betweenmetal and conducting polymers, the charge injection alsoenhances at the interface. Hence, the charges are easilyinjected, and thus, the current increases.

Capacitance–voltage character

The capacitance–voltage (C–V) characteristics of ITO/composites/Al electrode have been studied as shown in Fig. 9. It is observedthat each junction has a specific capacitance because of a spacecharge in the depletion layer, which depends on the junctionvoltage. The plots of 1/C2 versus the reverse-bias voltage arelinear, which indicates the formation of the Schottky junction.Therefore, it follows a standard Mott–Schottky relationship,

1

C2 ¼2 Vbi � V � kT

q

� �qε0εSA2Nd

where C is the diode capacitance, Vbi the built-in voltage, εS thepolymer relative dielectric constant, ε0 the permittivity in vacuum,V the applied voltage, q the charge, A the diode active area, kT/q

Figure 6. Variation of AC conductivity as a function of frequency forpolyaniline (PANI) and PANI–CaTiO3 composites.

Figure 7. Complex impedance Cole–Cole plots Z″ versus Z′ forpolyaniline (PANI)–CaTiO3 composites.

Figure 8. Current density–voltage (J–V) measurements of polyanilineand polyaniline–CaTiO3 composites.

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the thermal voltage at 303 K, and Nd the charge carrier concentra-tion. The charge carrier concentration can be determined from theslope of 1/C2 versus V plots. From the extrapolated intercept onthe voltage axis, Vbi can be estimated. C–V plots of PANI andPANI–CaTiO3 composites show that the capacitance is stronglydependent on dopants as well as bias voltage. The capacitancedecreases with a decrease in the applied voltage. These resultsshow that the barrier height decreases as the dopant increasesup to 30wt% in the PANI and that the number of charge carrierconcentration increases simultaneously. The order of chargecarrier concentration for pure PANI was 1017, whereas for thecomposite, it was 1015. These values are comparable withthe values reported earlier.[19,20]

The carrier mobility μ was determined from the relation ofconductivity σ=Ndeμ, as all the ionized charge carriers take partin the conductivity. The carrier mobility μ values of PANI andPANI–CaTiO3 composites were found to be 5.37 × 10�3 and2.73 × 10�2, respectively, and this is a significant increment fromthe reported data.[21,22] The number of charge carrier concentra-tions and mobility of these carriers increase with the increase ofdopant concentration as well increase in applied voltage; theseresults suggest that the tunnel process may be equally asimportant in these devices as the thermionic emission process.

CONCLUSION

The PANI–CaTiO3 composites of different weight percentageswere prepared by an in situ polymerization method. Infraredspectroscopy reveals that the characteristic stretching frequencieswere considerably shifted toward a higher-frequency side, whichsuggests that there is a Van der Waals force of interactionbetween the polymer chain and CaTiO3. It is observed throughSEM that fine grains of CaTiO3 when coated with PANI tend tocoalesce and form agglomerates with an average size of 0.2μm.

The observed behavior consistent with the conductivity anddielectric constant results in PANI and PANI–CaTiO3 compositesexhibiting a small value of dielectric loss at higher frequencies,which suggests that these materials are lossless materials atfrequencies beyond 106Hz. The dielectric constant is quite highat a lower frequency and decreases with an increase in appliedfrequency. The observed behavior may be due to a Debye-likerelaxation mechanism taking place in these materials. Frommodulus studies of PANI and PANI–CaTiO3 composites, arcs arewith their centers at the origin, and the radii of the arcs seem tobe same. Thus, the radius of the arc of a complex plane diagramis dependent on the electrical conductivity of the samples, thatis, a larger arc means a lower conductivity. It is observed thatthe complex impedance plots show a single semicircular arcpassing through the origin, which clearly highlights that thesecomposites are electrically homogeneous. The electrical propertiesof these interphase control the charge transport through the junc-tion, generally giving symmetric and non-ohmic characteristics.

REFERENCES[1] T.-W. Kim, D. F. Zeigler, O. Acton, H.-L. Yip, H. Ma, K.-Y. Alex, J. Adv.

Mat. 2012, 24, 828–833.[2] A. S. Roy, K. R. Anilkumar, M. V. N. A. Prasad, J. Appl. Polym. Sci. 2011,

121, 675–680.[3] T. Xiao, W. Cui, J. Anderegg, J. Shinar, R. Shinar, Org. Electro. 2011,

12, 257–262.[4] M. Jorgensen, K. Norrman, F. C. Krebs, solar cells. Sol. Energy Mater.

Sol. Cells. 2008, 92, 686–714.[5] A. K. Singh, R. Prakash, RSC Advances 2012, 2, 5277–5283.[6] A. S. Roy, A. Parveen, R. Deshpande, R. Bhat, K. R. Anilkumar,

J. Nanopart. Res. 2013, 15, 14–1337.[7] S. Schaur, P. Stadler, B. Meana-Esteban, H. Neugebauer, N. Serdar

Sariciftci, Org. Electro. 2012, 13, 1296–1301.[8] W. Gaynor, J. Y. Lee, P. Peumans, ACS Nano 2010, 4, 30–34.[9] N. Espinosa, H. F. Dam, D. M. Tanenbaum, J. W. Andreasen, M.

Jorgensen, F. C. Krebs, Materials 2011, 4, 169–182.[10] Y. Lee, J.-r. Choi, K. J. Lee, N. E. Stott, D. Kim, Nanotechnology 2008,

19, 415604, 7.[11] K. E. Lilja, H. S. Majumdar, F. S. Pettersson, R. Osterbacka,

T. Joutsenoja, App. Mat. & Inter. 2011, 3, 7–10.[12] A. Parveen, A. S. Roy, M. Shakeel, Chem. Papers. 2013, 67, 350–356[13] S. Ameen, M. Song, D.-G. Kim, Y.-B. Im, H.-K. Seo, Y. S. Kim, H.-S. Shin,

Macr. Res. 2012, 20, 30–36.[14] M. Wan, Macromol. Rapid Commun. 2009, 30, 963–975.[15] T. Griesser, S. V. Radl, T. Koepplmayr, A. Wolfberger, M. Edler,

A. Pavitschitz, M. Kratzer, C. Teichert, T. Rath, G. Trimmel,G. Schwabegger, C. Simbrunner, H. Sitter, W. Kern, J. Mater. Chem.2012, 22, 2922–2928.

[16] S. M. Ebrahim, J. Polym. Res. 2009, 16, 481–487.[17] R. K. Gupta, R. A. Singh, J. Polym. Res. 2004, 11, 269–273.[18] L. V. Bejbouj, J. L. Miane, M.-T. Dang, E. M. Oualim, M. Harmouchi,

A. Mouhsen, Sol. Energy Mater. Sol. Cells 2010, 94, 176–181.[19] Q. Tang, L. Lin, X. Zhao, K. Huang, J. Wu, Langmuir 2012, 28, 3972–8.[20] A. D. G. Del Mauro,1 G. Nenna,1 V. Bizzarro, I. A. Grimaldi,1,3

F. Villani,1 C. Minarini, J. App. Polym. Sci. 2011, 122, 3618–3623.[21] B. Fan, F. A. de Castro, B. T.-T. Chu, J. Heier, D. Opris, R. Hany,

F. Nüesch, J. Mater. Chem. 2010, 20, 2952–2955.[22] W. H. Kim, G. P. Kushto, H. Kim, Z. H. Kafafi, J. Polym. Sci.: Polym. Phy.

2003, 41, 2522–2528.

Figure 9. Capacitance–voltage (C–V) measurements of polyaniline andpolyaniline–CaTiO3 composites.

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