In Situ Polymerization and Characterization of Highly ... - Springer Link

Journal of ELECTRONIC MATERIALS, Vol. 44, No. 12, 2015

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

In Situ Polymerization and Characterization of Highly Conducting Polypyrrole Fish Scales for High-Frequency Applications 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, Kolhapur 416004, India. 2.—e-mail: [email protected]

Polypyrrole (Ppy) thin films on alumina were synthesized by an in situ chemical oxidative polymerization method at 300 K with equal monomer-tooxidant ratio. Fourier transform infrared spectroscopy (FTIR) and FT-Raman spectroscopy confirmed the formation of Ppy. A thickness-dependent change from cauliflower to fish-scale morphology was observed. Microwave properties such as transmission, reflection, shielding effectiveness, permittivity, and microwave conductivity are reported in the frequency range from 8 GHz to 12 GHz. The direct-current (DC) conductivity varied from 9.45 9 103 S/cm to 17.29 9 103 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 and 10.39 dB. Key words: Polypyrrole thin film, chemical bath deposition, thickness, morphology, conductivity

INTRODUCTION A number of studies have indicated that conducting polymers have electromagnetic shielding and absorption properties.1,2 Conducting polymers are relatively lightweight and corrosion-resistant materials, which may be easily produced on large scale especially at low cost. Among conducting polymers (CPs), polypyrrole (Ppy) has attracted considerable attention, because it offers reasonably high conductivity and fairly good environmental stability and can be widely used in a variety of applications 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 wellknown and cost-effective method for thin-film deposition. It has several advantages over other techniques; e.g., it does not require any sophisticated equipment, can be operated at lower temperature, and has a nonpolluting nature, as well as offering large-area deposition.10 Many reports are (Received November 20, 2014; accepted June 15, 2015; published online August 4, 2015)

available on microwave studies of chemically synthesized Ppy thin films on various substrates. To the best of our knowledge, this is the first report on oxidative-time-dependent morphological effects in chemically synthesized Ppy thin films on alumina substrate. EXPERIMENTAL PROCEDURES Materials All chemicals used were of analytical reagent (AR) grade: pyrrole (99%) as monomer, sulfuric acid (98.08%) as electrolyte, and ammonium peroxydisulfate (APS) as oxidant, used as received. Polymerization of Ppy Thin Films For deposition of Ppy thin films, alumina substrate was used. It was necessary to clean the substrate to achieve good adhesion. The alumina substrate (5 cm 9 1.1 cm) was cleaned with soap solution (twice) and double-distilled water, then dried at room temperature for 20 min. The polymerization solution consisted of ammonium persulfate (0.57 g) dissolved in 25 ml double-distilled 4669

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Velhal, Patil, and Puri

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

water and pyrrole (0.1 mol) mixed with 0.1 M sulfuric acid separately in 25 ml distilled water. Then, pyrrole solution was added to APS solution in one step. Simultaneously, the substrates were placed into the solution in vertical position. The solution color changed vigorously from green to black. The polymerization time was varied from 30 min to 120 min. The whole reaction was carried out at room temperature. The molar ratio of Ppy to APS was kept at 1:1 for all depositions. After polymerization, the samples were rinsed with double-distilled water, then dried in normal air for 1 h. Adherent blackcolored Ppy thin films were obtained. Figure 1 presents a schematic of the synthesis of Ppy thin films. The thickness of the synthesized thin films was measured using the gravimetric weight difference method. The thickness of the film varied from 0.5 lm to 5 lm. The resistance of the Ppy thin film was measured using the two-point probe method, and the resistivity and conductivity were calculated. The conductivity of the Ppy varied from 9.45 9 103 S/ cm to 17.29 9 103 S/cm. The microwave properties of the films were studied using an X-band waveguide reflectometer and the voltage standing wave ratio (VSWR) technique. For insertion and removal of films, a sample holder of nonconducting material was used. Initially the whole setup was calibrated and the transmittance and reflectance of alumina were measured for greater accuracy. Instruments The Fourier transform infrared (FTIR) spectrum was recorded using a PerkinElmer 1710 spectrophotometer to identify the bonds of Ppy in the

range from 450 cm1 to 4000 cm1 at spectral resolution of 2 cm1. Vibrational study of Ppy was carried out using an FT-Raman spectrometer with a Nd-YAG laser at wavelength of 1664 nm. Surface morphological study was carried out by scanning electron microscope (JSM-6360LA, JEOL, Japan). The microwave properties of the films were studied by using an X-band (X-2051 model) waveguide reflectometer and the VSWR (X-6051 model) technique. RESULTS AND DISCUSSION Reaction Mechanism It is well known that Ppy can be prepared by electrochemical and chemical oxidation methods.11,12 Both polymerization methods yield conducting Ppy which exhibits good stability under ambient conditions. Chemical oxidative polymerization of pyrrole results in black powder, and the reaction mechanism has been known for many years.13–16 A radical cation mechanism is initiated in the presence of a strong oxidizing agent such as ammonium peroxydisulfate, ferric chloride, etc., which also serves as a dopant. Here, ammonium peroxydisulfate was used as the oxidant. During oxidative polymerization, pyrrole (Py) monomers lose a proton at a-position and become linked with other monomers of pyrrole to form long-chain Ppy polymer as shown in Scheme 1. The length of the polymer chain depends on the amount of monomer and oxidant used, i.e., on the monomer-to-oxidant ratio. Figure 1 shows a schematic of Ppy thin films obtained after different time periods. Different morphologies were obtained for different thicknesses, and the color of the film changed from greenish black to black.

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

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Scheme 1. Reaction mechanism of pyrrole.

Fig. 2. FTIR spectrum of Ppy thin film. Fig. 3. FT-Raman spectrum of Ppy thin film.

Infrared Spectroscopy Figure 2 shows the FTIR spectrum of Ppy thin film in the range from 400 cm1 to 4000 cm1. The characteristic peaks at 1554 cm1 and 1199 cm1 correspond to fundamental vibrations of the Ppy ring and the Ppy ring breathing, respectively,17,18 whereas the peaks at 1300 cm1 and 1116 cm1 correspond to C–N stretching vibrations of the benzoid and quinoid rings, and the out-of-plane bending of C–H in substituted benzoid ring is observed at 797 cm1.19 The peak at 1047 cm1 is due to N–H in-plane deformation.20 The signal at 919 cm1 is attributed to C–H wagging.21 The observed peaks confirm the formation of Ppy structure. Raman Spectroscopy The chemical structure and functional groups present in the chemically synthesized Ppy film were studied with the help of FT-Raman spectroscopy, as shown in Fig. 3. The symmetric and asymmetric C–H in-plane bending and C–N stretching modes were observed at 1081 cm1 and 1378 cm1, respectively.21,22 The intense band observed at 936 cm1 is due to the out-of-plane C–H deformation of quinoid structure of the pyrrole ring. The C–H bending mode is observed at 1232 cm1.22

Several modes observed in the 900 cm1 to 1150 cm1 range are due to polarons and bipolarons in the polymer backbone, confirming that the Ppy is in conducting form.23 The C=C backbone stretching mode occurred at 1590 cm1.22 Surface Morphology Figure 4a–d shows scanning electron microscopy (SEM) images of Ppy thin films of different thicknesses deposited on alumina substrate. The micrographs show cauliflower-like growth of Ppy on granular-structured alumina substrate. As the oxidative time was increased, the thickness increased and the cauliflowers became agglomerated and stacked on one another to form a fish-scalelike morphology. With increasing thickness, the structure became more compact and dense, but pores were still observed. This is due to the comparatively slow growth of cauliflowers and due to the bath temperature. The initial nucleation centers were locked onto the substrate, due to which growth did not occur at other locations on the substrate and cauliflowers grew on the initially grown material. As the deposition time was increased, nucleation took place at other locations on the substrate, forming uniform large grains of fish scales with a diversity of orientations.24

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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)

30 60 90 120

0.5 1 1.5 5

1.0575 0.9321 0.7260 0.5781

9.45 10.72 13.77 17.29

1 2 3 4

DC Conductivity The resistance of the Ppy thin film was measured by using the two-point probe method. The resistivity and direct-current (DC) conductivity were calculated using the following relation: q ¼ RA=l;

(1)

where q is the resistivity of the Ppy thin film, R is the resistance and A is the area of the film, and l is the distance between the two probes. Table I presents the resistivity, conductivity, and thickness of the Ppy thin films. From this table, it is observed that the conductivity of the Ppy film increased from 9.45 9 103 S/m to 17.29 9 103 S/m with thickness (0.5 lm to 5 lm). A possible reason is that, as the polymerization time was increased, the thickness of the film increased, which increased the length of the conjugated chains and the arrangements of the Ppy conjugated chains reached their optimal form with the least defects and highest carrier mobility in the p-conjugated system.24 The

fish-scale-like morphology gives better contact between grains for easy flow of current. Microwave Properties Transmission and Reflection Figure 5a and b show the microwave transmission and reflection versus frequency for the Ppy films on alumina. It is seen that, as the thickness of the Ppy film increased, the transmittance decreased and the reflectance increased. Generally, transmission is observed when the film thickness is less than the skin depth, whereas reflection is the result of interaction between free electrons or vacancies in the material and the electromagnetic field. Here, a decrease in transmittance is observed as the thickness of the film increased. This might be due to the fact that, as the thickness increased, the surface became more irregular, being responsible for greater reflection. In the case of the film with 0.5 lm thickness, the reflectance was less than for all the other films; here, well-grown cauliflowers were


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

observed, with multiple reflections within them being responsible for loss due to scattering.25 Shielding Effectiveness The microwave shielding effectiveness is the attenuation of electromagnetic radiation by reflection or absorption of a material. The microwave shielding effectiveness of the Ppy thin films was calculated from the transmission coefficient of the samples. Shielding effectiveness (SE) ¼ 10 log Pt=Pi ;

(2)

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

effectiveness is thickness dependent. As the thickness increased, the SE also increased, varying between 6.18 dB and 10.39 dB. These results are consistent with reported values.20 The SE and electromagnetic absorption depend on the conductivity and permittivity of the material and their variations with frequency. Permittivity From the position of minima as measured by the VSWR method and reflection coefficient, the permittivity was calculated using the following equation: pffiffiffi D/k0 2 DAk0 e0 0 00 ; (3) and e ¼ e ¼ 1þ 8:686pd 360d

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

where DU is the difference of the phase shift with and without the sample, k0 is the wavelength of the corresponding frequency, DA is the difference of the attenuation with and without the sample, and d is the thickness of the sample. The graph of the real (e¢) and imaginary parts (e¢¢) of the permittivity versus frequency is shown in Fig. 6a and b. As the frequency increases, both e¢ and e¢¢ decrease: e¢ from 124.45 at 8.2 GHz to 14.88 at 12 GHz, and e¢¢ from 523.41 at 8.2 GHz to 94.57 at 12 GHz. At higher frequencies, side groups or small units of main chains gradually lose their ability to follow the field and their contribution

to the polarization ceases. Thickness-dependent effects are more prominent at higher compared with lower frequencies. Microwave Conductivity The microwave conductivity of the Ppy thin films deposited on alumina was calculated by using the following equation: r ¼ 2pf e0 e00 ;

(4)

where f is the frequency, r is the microwave conductivity, e0 is the permittivity of free space


(e0 = 8.854 9 1012 F/m), and e¢¢ is the imaginary part of the dielectric constant. A plot of the microwave conductivity versus frequency for the Ppy thin films is shown in Fig. 6c. It is seen that the microwave conductivity is several times larger in magnitude than the DC conductivity. This is due to the formation of excess charge carriers, i.e., polarons and bipolarons, at higher frequencies; this is called microscopic conductivity, increasing due to the increase in order of the chains. Order increases the compactness and molecular orientation, leading to increased microscopic conductivity, which improves the weak links between cauliflowers and fish-scale grains and results in stronger coupling through fish-scale grain boundaries. Also, the microwave conductivity is directly related to the dielectric loss factor, which arises due to localized motion of charge carriers. As the frequency is increased, the inertia of molecules and the binding forces become dominant, being the basis for high dielectric loss at higher frequencies. The dielectric loss factor leads to conductivity relaxation, and it decreases with frequency. The microwave conductivity varied between 63.07 S/cm and 349.08 S/cm, being in the range obtained by other workers on stainless-steel substrates.26 CONCLUSIONS Ppy thin films with cauliflower or fish-scale-like morphology, depending on the thickness, were successfully synthesized on alumina by a chemical oxidative polymerization method. The electromagnetic shielding effectiveness depended on the level of the conductivity and varied with frequency and thickness. The microwave conductivity of the Ppy thin films was several times larger in magnitude than the DC conductivity. For application in microwave antennas, conducting and reflecting Ppy thin films are important, so this fish-scale-like structure on alumina could be very useful. ACKNOWLEDGEMENTS V.R.P. gratefully acknowledges UGC India for Award of Research Scientist ‘C’. N.B.V. acknowledges

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DAE-BRNS (2012/34/36/BRNS/1034) for their financial support. The authors also thank UGC-SAP and DST-FIST for their assistance. REFERENCES 1. S.K. Dhawan, N. Singh, and S. Venkatachalam, Synth. Met. 129, 261 (2002). 2. P. Chandrasekhar and K. Naishadham, Synth. Met. 105, 115 (1999). 3. J.U. Kim, I.S. Jeong, S.I. Moon, and H.B. Gu, J. Power Sources 97, 450 (2001). 4. J.H. Chen, Z.P. Huang, D.Z. Wang, S.X. Yang, W.Z. Li, J.G. Wen, and Z.F. Ren, Synth. Met. 125, 289 (2001). 5. K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, and E. Frackowiak, Chem. Phys. Lett. 347, 36 (2001). 6. B. Kumar, B.K. Kaushik, and Y.S. Negi, J. Mater. Sci. 25, 1 (2014). 7. J.W. Goodwin, G.M. Markham, and B. Vincent, J. Phys. Chem. B 101, 1961 (1997). 8. V.T. Truong, P.K. Lai, B.T. Moore, R.F. Muscat, and M.S. Russo, Synth. Met. 110, 7 (2000). 9. L.J. Buckley and M. Eashov, Synth. Met. 78, 1 (1996). 10. S.V. Jadhav and V. Puri, Synth. Met. 158, 883 (2008). 11. T.A. Skotheim, Handbook of Conducting Polymers (New York: Marcel Dekker, 1986). 12. M. Yamaura, T. Hagiwara, and K. Iwata, Synth. Met. 26, 209 (1988). 13. S.P. Armes, Synth. Met. 20, 365 (1987). 14. R.K. Bunting, K. Swarat, and D.J. Yan, Chem. Educ. 74, 421 (1997). 15. M.C. Henry, C.C. Hsueh, B.P. Timko, and M.S. Freund, J. Electrochem. Soc. 148, D155 (2001). 16. V. Shaktawat, N. Jain, R. Saxena, N.S. Saxena, K. Sharma, and T.P. Sharma, Polym. Bull. 57, 535 (2006). 17. A. Joshi and S.A. Gangal, DAE Solid State Physics Symposium, Mysore 297, 236 (2007). 18. R. Turcu, A. Darabont, and A.N. Nan, J. Optoelectron. Adv. Mater. 8, 643 (2006). 19. H. Gu, Y. Huang, and X. Zhang, Polymer 53, 801 (2012). 20. S.A. Jamadade, S.V. Jadhav, and V. Puri, J. Non-Cryst. Solids 357, 1177 (2011). 21. X. Liang, Z. Wen, Y. Liu, F. Zhang, H. Jin, M. Wu, and X. Wu, J. Power Sources 206, 409 (2012). 22. F. Chen, J. Zhang, F. Wang, and G. Shi, J. App. Polym. Sci. 89, 3390 (2003). 23. H. Javadi, K. Cromack, A. MacDiarmid, and A.J. Epstein, Phys. Rev. B 39, 3579 (1989). 24. X. Fan, J. Guan, W. Wang, and G. Tong, J. Phys. D Appl. Phys. 42, 075006 (2009). 25. K. Lakshmi, J. Honey, J. Rani, K.E. George, and K.T. Mathew, Microw. Opt. Techn. Lett. 50, 504 (2008). 26. N.B. Velhal, N.D. Patil, S.A. Jamdade, and V.R. Puri, Appl. Surf. Sci. 307, 129 (2014).