In Situ Polymerization and Characterization of HighlyConducting Polypyrrole Fish Scales for High-FrequencyApplications

NINAD B. VELHAL,1 NARAYAN D. PATIL,1 and VIJAYA R. PURI1,2

1.—Thick and Thin Film Device Lab, Department of Physics, Shivaji University, Kolhapur416004, India. 2.—e-mail: vijaypapuri1@gmail.com

Polypyrrole (Ppy) thin films on alumina were synthesized by an in situchemical oxidative polymerization method at 300 K with equal monomer-to-oxidant ratio. Fourier transform infrared spectroscopy (FTIR) and FT-Ramanspectroscopy confirmed the formation of Ppy. A thickness-dependent changefrom cauliflower to fish-scale morphology was observed. Microwave propertiessuch as transmission, reflection, shielding effectiveness, permittivity, andmicrowave conductivity are reported in the frequency range from 8 GHz to12 GHz. The direct-current (DC) conductivity varied from 9.45 9 10�3 S/cm to17.29 9 10�3 S/cm, whereas the microwave conductivity varied from 63.07 S/cm to 349.08 S/cm. The shielding effectiveness varied between 6.18 dB and10.39 dB.

Key words: Polypyrrole thin film, chemical bath deposition, thickness,morphology, conductivity

INTRODUCTION

A number of studies have indicated that con-ducting polymers have electromagnetic shieldingand absorption properties.1,2 Conducting polymersare relatively lightweight and corrosion-resistantmaterials, which may be easily produced on largescale especially at low cost. Among conductingpolymers (CPs), polypyrrole (Ppy) has attractedconsiderable attention, because it offers reasonablyhigh conductivity and fairly good environmentalstability and can be widely used in a variety ofapplications such as batteries,3,4 supercapacitors,5

sensors,6 anhydrous electrorheological fluids,7

microwave shielding, and corrosion protection.8,9

The chemical bath deposition technique is a well-known and cost-effective method for thin-filmdeposition. It has several advantages over othertechniques; e.g., it does not require any sophisti-cated equipment, can be operated at lower temper-ature, and has a nonpolluting nature, as well asoffering large-area deposition.10 Many reports are

available on microwave studies of chemically syn-thesized Ppy thin films on various substrates. Tothe best of our knowledge, this is the first report onoxidative-time-dependent morphological effects inchemically synthesized Ppy thin films on aluminasubstrate.

EXPERIMENTAL PROCEDURES

Materials

All chemicals used were of analytical reagent (AR)grade: pyrrole (99%) as monomer, sulfuric acid(98.08%) as electrolyte, and ammonium peroxy-disulfate (APS) as oxidant, used as received.

Polymerization of Ppy Thin Films

For deposition of Ppy thin films, alumina sub-strate was used. It was necessary to clean the sub-strate to achieve good adhesion. The aluminasubstrate (5 cm 9 1.1 cm) was cleaned with soapsolution (twice) and double-distilled water, thendried at room temperature for 20 min. The poly-merization solution consisted of ammonium persul-fate (0.57 g) dissolved in 25 ml double-distilled(Received November 20, 2014; accepted June 15, 2015)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-015-3907-1� 2015 The Minerals, Metals & Materials Society

water and pyrrole (0.1 mol) mixed with 0.1 M sul-furic acid separately in 25 ml distilled water. Then,pyrrole solution was added to APS solution in onestep. Simultaneously, the substrates were placedinto the solution in vertical position. The solutioncolor changed vigorously from green to black. Thepolymerization time was varied from 30 min to120 min. The whole reaction was carried out at roomtemperature. The molar ratio of Ppy to APS was keptat 1:1 for all depositions. After polymerization, thesamples were rinsed with double-distilled water,then dried in normal air for 1 h. Adherent black-colored Ppy thin films were obtained. Figure 1 pre-sents a schematic of the synthesis of Ppy thin films.The thickness of the synthesized thin films wasmeasured using the gravimetric weight differencemethod. The thickness of the film varied from 0.5 lmto 5 lm. The resistance of the Ppy thin film wasmeasured using the two-point probe method, and theresistivity and conductivity were calculated. Theconductivity of the Ppy varied from 9.45 9 10�3 S/cm to 17.29 9 10�3 S/cm.

The microwave properties of the films werestudied using an X-band waveguide reflectometerand the voltage standing wave ratio (VSWR) tech-nique. For insertion and removal of films, a sampleholder of nonconducting material was used. Initiallythe whole setup was calibrated and the transmit-tance and reflectance of alumina were measured forgreater accuracy.

Instruments

The Fourier transform infrared (FTIR) spectrumwas recorded using a PerkinElmer 1710 spec-trophotometer to identify the bonds of Ppy in the

range from 450 cm�1 to 4000 cm�1 at spectral res-olution of 2 cm�1. Vibrational study of Ppy wascarried out using an FT-Raman spectrometer with aNd-YAG laser at wavelength of 1664 nm. Surfacemorphological study was carried out by scanningelectron microscope (JSM-6360LA, JEOL, Japan).The microwave properties of the films were studied byusing an X-band (X-2051 model) waveguide reflec-tometer and the VSWR (X-6051 model) technique.

RESULTS AND DISCUSSION

Reaction Mechanism

It is well known that Ppy can be prepared byelectrochemical and chemical oxidation meth-ods.11,12 Both polymerization methods yield con-ducting Ppy which exhibits good stability underambient conditions. Chemical oxidative polymer-ization of pyrrole results in black powder, and thereaction mechanism has been known for manyyears.13–16 A radical cation mechanism is initiatedin the presence of a strong oxidizing agent such asammonium peroxydisulfate, ferric chloride, etc.,which also serves as a dopant.

Here, ammonium peroxydisulfate was used as theoxidant. During oxidative polymerization, pyrrole(Py) monomers lose a proton at a-position andbecome linked with other monomers of pyrrole toform long-chain Ppy polymer as shown in Scheme 1.The length of the polymer chain depends on theamount of monomer and oxidant used, i.e., on themonomer-to-oxidant ratio. Figure 1 shows a sche-matic of Ppy thin films obtained after different timeperiods. Different morphologies were obtained fordifferent thicknesses, and the color of the filmchanged from greenish black to black.

Fig. 1. Schematic of chemical bath deposition technique with different thicknesses for different time periods.

Velhal, Patil, and Puri

Infrared Spectroscopy

Figure 2 shows the FTIR spectrum of Ppy thinfilm in the range from 400 cm�1 to 4000 cm�1. Thecharacteristic peaks at 1554 cm�1 and 1199 cm�1

correspond to fundamental vibrations of the Ppyring and the Ppy ring breathing, respectively,17,18

whereas the peaks at 1300 cm�1 and 1116 cm�1

correspond to C–N stretching vibrations of thebenzoid and quinoid rings, and the out-of-planebending of C–H in substituted benzoid ring isobserved at 797 cm�1.19 The peak at 1047 cm�1 isdue to N–H in-plane deformation.20 The signal at919 cm�1 is attributed to C–H wagging.21 Theobserved peaks confirm the formation of Ppy struc-ture.

Raman Spectroscopy

The chemical structure and functional groupspresent in the chemically synthesized Ppy film werestudied with the help of FT-Raman spectroscopy, asshown in Fig. 3. The symmetric and asymmetricC–H in-plane bending and C–N stretching modeswere observed at 1081 cm�1 and 1378 cm�1,respectively.21,22 The intense band observed at936 cm�1 is due to the out-of-plane C–H deforma-tion of quinoid structure of the pyrrole ring. TheC–H bending mode is observed at 1232 cm�1.22

Several modes observed in the 900 cm�1 to1150 cm�1 range are due to polarons and bipolaronsin the polymer backbone, confirming that the Ppy isin conducting form.23 The C=C backbone stretchingmode occurred at 1590 cm�1.22

Surface Morphology

Figure 4a–d shows scanning electron microscopy(SEM) images of Ppy thin films of different thick-nesses deposited on alumina substrate. The micro-graphs show cauliflower-like growth of Ppy ongranular-structured alumina substrate. As theoxidative time was increased, the thicknessincreased and the cauliflowers became agglomer-ated and stacked on one another to form a fish-scale-like morphology. With increasing thickness, thestructure became more compact and dense, butpores were still observed. This is due to the com-paratively slow growth of cauliflowers and due tothe bath temperature. The initial nucleation centerswere locked onto the substrate, due to which growthdid not occur at other locations on the substrate andcauliflowers grew on the initially grown material.As the deposition time was increased, nucleationtook place at other locations on the substrate,forming uniform large grains of fish scales with adiversity of orientations.24

Scheme 1. Reaction mechanism of pyrrole.

Fig. 2. FTIR spectrum of Ppy thin film.

Fig. 3. FT-Raman spectrum of Ppy thin film.

In Situ Polymerization and Characterization of Highly Conducting Polypyrrole Fish Scalesfor High-Frequency Applications

DC Conductivity

The resistance of the Ppy thin film was measuredby using the two-point probe method. The resistivityand direct-current (DC) conductivity were calcu-lated using the following relation:

q ¼ RA=l; (1)

where q is the resistivity of the Ppy thin film, R isthe resistance and A is the area of the film, and l isthe distance between the two probes. Table I pre-sents the resistivity, conductivity, and thickness ofthe Ppy thin films. From this table, it is observedthat the conductivity of the Ppy film increased from9.45 9 10�3 S/m to 17.29 9 10�3 S/m with thick-ness (0.5 lm to 5 lm). A possible reason is that, asthe polymerization time was increased, the thick-ness of the film increased, which increased thelength of the conjugated chains and the arrange-ments of the Ppy conjugated chains reached theiroptimal form with the least defects and highestcarrier mobility in the p-conjugated system.24 The

fish-scale-like morphology gives better contactbetween grains for easy flow of current.

Microwave Properties

Transmission and Reflection

Figure 5a and b show the microwave transmis-sion and reflection versus frequency for the Ppyfilms on alumina. It is seen that, as the thickness ofthe Ppy film increased, the transmittance decreasedand the reflectance increased. Generally, transmis-sion is observed when the film thickness is less thanthe skin depth, whereas reflection is the result ofinteraction between free electrons or vacancies inthe material and the electromagnetic field. Here, adecrease in transmittance is observed as the thick-ness of the film increased. This might be due to thefact that, as the thickness increased, the surfacebecame more irregular, being responsible forgreater reflection. In the case of the film with 0.5 lmthickness, the reflectance was less than for all theother films; here, well-grown cauliflowers were

Fig. 4. (a–d) SEM images of Ppy thin films of different thicknesses.

Table I. DC resistivity, conductivity, and thickness of Ppy

Serial No. Deposition Time (min) Thickness (lm) Resistivity (X cm) Conductivity (1023 S/cm)

1 30 0.5 1.0575 9.452 60 1 0.9321 10.723 90 1.5 0.7260 13.774 120 5 0.5781 17.29

Velhal, Patil, and Puri

observed, with multiple reflections within thembeing responsible for loss due to scattering.25

Shielding Effectiveness

The microwave shielding effectiveness is theattenuation of electromagnetic radiation by reflectionor absorption of a material. The microwave shieldingeffectiveness of the Ppy thin films was calculated fromthe transmission coefficient of the samples.

Shielding effectiveness (SE) ¼ 10 log Pt=Pi

� �; (2)

where Pt is the transmitted power and Pi is the incidentpower. Figure 5c shows that the observed shielding

effectiveness is thickness dependent. As the thicknessincreased, the SE also increased, varying between6.18 dB and 10.39 dB. These results are consistentwith reported values.20 The SE and electromagneticabsorption depend on the conductivity and permittivityof the material and their variations with frequency.

Permittivity

From the position of minima as measured by theVSWR method and reflection coefficient, the permit-tivity was calculated using the following equation:

e0 ¼ 1 þ D/k0

360d

� �2

and e00 ¼ DAk0

ffiffiffie0

p

8:686pd; (3)

Fig. 5. (a) Transmittance, (b) reflectance, and (c) shielding effectiveness.

In Situ Polymerization and Characterization of Highly Conducting Polypyrrole Fish Scalesfor High-Frequency Applications

where DU is the difference of the phase shift withand without the sample, k0 is the wavelength of thecorresponding frequency, DA is the difference of theattenuation with and without the sample, and d isthe thickness of the sample.

The graph of the real (e¢) and imaginary parts (e¢¢)of the permittivity versus frequency is shown inFig. 6a and b. As the frequency increases, both e¢and e¢¢ decrease: e¢ from 124.45 at 8.2 GHz to 14.88at 12 GHz, and e¢¢ from 523.41 at 8.2 GHz to 94.57at 12 GHz. At higher frequencies, side groups orsmall units of main chains gradually losetheir ability to follow the field and their contribution

to the polarization ceases. Thickness-dependenteffects are more prominent at higher compared withlower frequencies.

Microwave Conductivity

The microwave conductivity of the Ppy thin filmsdeposited on alumina was calculated by using thefollowing equation:

r ¼ 2pf e0e00; (4)

where f is the frequency, r is the microwave con-ductivity, e0 is the permittivity of free space

Fig. 6. (a) Real and (b) imaginary parts of permittivity, and (c) microwave conductivity.

Velhal, Patil, and Puri

(e0 = 8.854 9 10�12 F/m), and e¢¢ is the imaginarypart of the dielectric constant.

A plot of the microwave conductivity versus fre-quency for the Ppy thin films is shown in Fig. 6c. Itis seen that the microwave conductivity is severaltimes larger in magnitude than the DC conductiv-ity. This is due to the formation of excess chargecarriers, i.e., polarons and bipolarons, at higherfrequencies; this is called microscopic conductivity,increasing due to the increase in order of the chains.Order increases the compactness and molecularorientation, leading to increased microscopic con-ductivity, which improves the weak links betweencauliflowers and fish-scale grains and results instronger coupling through fish-scale grain bound-aries. Also, the microwave conductivity is directlyrelated to the dielectric loss factor, which arises dueto localized motion of charge carriers. As the fre-quency is increased, the inertia of molecules and thebinding forces become dominant, being the basis forhigh dielectric loss at higher frequencies. Thedielectric loss factor leads to conductivity relax-ation, and it decreases with frequency. The micro-wave conductivity varied between 63.07 S/cm and349.08 S/cm, being in the range obtained by otherworkers on stainless-steel substrates.26

CONCLUSIONS

Ppy thin films with cauliflower or fish-scale-likemorphology, depending on the thickness, were suc-cessfully synthesized on alumina by a chemicaloxidative polymerization method. The electromag-netic shielding effectiveness depended on the levelof the conductivity and varied with frequency andthickness. The microwave conductivity of the Ppythin films was several times larger in magnitudethan the DC conductivity. For application inmicrowave antennas, conducting and reflecting Ppythin films are important, so this fish-scale-likestructure on alumina could be very useful.

ACKNOWLEDGEMENTS

V.R.P. gratefully acknowledges UGC India forAward of Research Scientist ‘C’. N.B.V. acknowledges

DAE-BRNS (2012/34/36/BRNS/1034) for their finan-cial support. The authors also thank UGC-SAP andDST-FIST for their assistance.

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In Situ Polymerization and Characterization of Highly Conducting Polypyrrole Fish Scalesfor High-Frequency Applications