Ac Impedance Spectroscopy of Polyanilines Having Various Molecular and Nano Structures

Singh P and Singh RA

Published on: 2021-10-05

Abstract

AC impedance spectroscopy of polyanilines having a variety of molecular, nanostructures and microstructures; synthesized under a variety of experimental conditions has been shown as a novel tool for the electrical characterization as a function of a range of frequencies similar to the other common spectroscopic techniques. A comparative study of different types of materials prepared using redox polymerization of aniline with a variety of oxidants and characterized by AC impedance, FT-IR and UV-Visible spectroscopy has been made to evolve the synthesis-structure-property correlations.  It has been shown that this technique is not only sensitive to various structural forms of polyanilines but also on different micro/nano- structures. Simultaneously it helps in assessing the electrical properties of the materials as well as separating contributions originating from various electrode processes such as grain/bulk, grain-boundary and contact/electrode. The advantages and limitations of the technique have been discussed.

Keywords

Polyaniline; Nanostructures; AC impedance spectroscopy; Rredox polymerization; Hybrid materials; Composites

Introduction

Electrochemical AC impedance spectroscopy has evolved in the recent years as one of the most common technique for determining the electrical properties of materials [1-3]. It is an electrochemical technique because one studies the current-voltage characteristics of the materials using alternate-current stimuli of small amplitude and it is a spectroscopic technique because the electrical behavior of materials are studied as a function of frequency in the range of mHz to MHz. The uniqueness and the novelty of technique lies in the fact that the experimental data in a limited range of frequency can be simulated over a much larger frequency range using complex non-linear least square (CNLS) fitting software and contributions to the total impedance/admittance from different processes such as bulk/grain, grain-boundary and electrodes/contacts could be separated with the help of corresponding equivalent circuits. No other spectroscopy technique has this advantage hence it is becoming a regular tools in the studies of electrical properties of materials.   Polyanilines, one of the most studied conducting polymers, have been known to exhibit a range of electrical properties from insulator to metallic regime [4,5]. The electrical conductivities of this single polymer varies from 10-12 to 102 Scm-1 depending on its molecular forms, level of proton doping- undoping and de-doping and the size of the polymer in the nanometer scale. For example, Leuco Emeraldine salt, Leuco Emeraldine base, Pernigraniline base, Pernigraniline salt and Emeraldine base have the electrical conductivities of the order of  10-12 Scm-1    whereas Emeraldine salt has a conductivity in the range of  10-4 - 102 Scm-1, [ 6 ].Structural and electronic transitions in well-defined oxidation states of polyaniline were studied using Fourier transform infrared spectroscopy [7]. On the basis of spectroscopic and electrochemically induced transitions in polyaniline in aqueous acidic and organic electrolytes, the nature of metallic state was proposed. AC impedance measurements on polyaniline blends containing equal amounts of HCl and DBSA were reported [8]. Low temperature synthesis of high molecular weight polyaniline using potassium dichromate as oxidant in acidic solution containing lithium chloride was made [9]. A solvent cast polyaniline-camphorsulfonate film was uniaxially stressed to give a conductivity of 848 ± 70 Scm-1 along the stretch direction and 118 ± 10 Scm-1 perpendicular to this direction. Dielectric spectroscopy of lightly doped polyanilines were studied over the frequency range of 100Hz to 1MHz and between 77 -444 K [10]. It was found that at 100K, the frequency dependent electrical conductivity (σ(ω)= A ω s  where s= 1)  decreased with the increasing temperature indicating the metal-like behavior. It was further shown that in the low temperature region, the electrical conductivity was mainly controlled by a process of dipolar origin and conduction occurred through thermal activation as in semiconducting materials. The inductive behavior of polyaniline was studied by using electrochemical impedance spectroscopy [11]. The impedance results clearly revealed the existence of inductive behavior of polyaniline at the upper potential limits of 0.75 and 0.90 V due to formation of benzoquinone/hydroquinone. The conduction behavior of material was described on the basis of equivalent circuit components of R (RC) elements. Impedance spectroscopic studies on doped polyanilines containing various amounts of HCl were performed at room temperature, over a wide range of frequencies from 1 kHz to 45 MHz [12]. It was shown that the resistance dominated the AC behavior of polyaniline confirming that the charge transports in these materials occur through a one-dimensional hopping process. Electrochemical characterization of polyaniline fibers as electrodes in acid solutions was made using low scan rate cyclic voltametry and impedance spectroscopy [13]. The electrochemical behavior of polyaniline fibers was found to be similar to that of films obtained by electro polymerization on metallic substrates. The chemical polymerization of aniline in presence of different linear dicarboxylic acids was carried out using a variety of oxidants such as K2Cr2O7, KMnO4, K2S2O8, KIO3 and FeCl3 [14]. The highest yield and conductivity were observed with K2Cr2O7. Potassium dichromate initialed polymerization of aniline in acidified aqueous medium was carried out under different condition such as pH, aniline/ Oxidant ratio, temperature and time [15].The room temperature conductivity of 2.5-2.6 Scm-1 was reported for all the products prepared  under different conditions. Electrical impedance spectroscopic study of polyaniline prepared by electro polymerization on platinum, glassy carbon and carbon fiber microelectrodes has been reported [16]. The results indicated that the change of the electrodes had a little effect on anodic and cathodic peak potentials during electro polymerization process. An equivalent circuit R (Q(R(C(R(C(RW))))(CR) was proposed for PAni Prepared on three different electrodes. Chemical and electrochemical oxidative synthesis of polyaniline using K2Cr2O7  as an oxidant and H2SO4 as dopant was done [17]. The structural assignments were done with the help of UV-visible and FT-IR spectroscopy which were consistent with the literature data. The electrical conductivity values for both the methods were comparable 0.58 and 0.27 Scm-1 respectively.  AC electrical conductivity of polyaniline prepared in different acidic medium was found to vary as ω s in the frequency range (100 Hz – 10 MHz) with value of s <1[18]. Effects of the nature of oxidant and synthesis conditions on the properties of nanocomposites of polyaniline and carbon nanotubes were studied [19]. KMnO4 and (NH4)2S2O8 were used as oxidants. It was found that structure and morphology of materials were determined by the conditions of the synthesis.  An electrochemical impedance spectroscopy study of electrochemically induced ageing of polyaniline was reported [20]. It was shown that changes in impedance response could be associated with the decrease of the electrochemically active area of the polymer/electrolyte interface. AC impedance spectroscopic studies on polyaniline nano-structures prepared via interfacial polymerization were reported recently [21]. It was shown that the nature of impedance plots and corresponding equivalent circuits differ significantly depending on experimental conditions. These results indicated that AC-impedance spectroscopy can be used as a characterization technique similar to the other spectroscopic techniques. Impedance spectroscopic studies on Pani/CeO2 composites were reported recently [22]. The frequency dependent conductivity and dielectric behaviour of these composites have been studied in the frequency range of 50 Hz to 5 MHz. The variation of AC-conductivity was small in all composites in the frequency range of 50Hz to 10 KHz. But large variations in conductivity were observed in the frequency range of 10 KHz to 5 MHz. Synthesis, characterisation and ac- impedance spectroscopy studies on Pani /PEG nano-composites prepared by interfacial polymerization method was recently reported [23-24]. It was found that the electrical conductivity of interfacially synthesized PAni-PEG composite was 3.96 x 10-2 Scm-1. The above ideas motivated us to use the AC impedance spectroscopy as a characterization tool for materials containing various molecular and structural forms of polyaniline, prepared under a variety of conditions with the purpose of learning whether this technique can be used as a differentiator for various molecular structures, similar to UV-visible or FT-IR spectroscopy. This technique will additionally yield the electrical properties of materials. Polyanilines have been prepared by using a variety of oxidants and acids; with water-soluble polymers eg. PVA/PEG as composites and hybrids with inorganic oxides such as Al2O3, TiO2, ZnO. Their nanostructures have been prepared by using interfacial and biomimetic synthesis methods. It has been shown that AC-impedance spectroscopy does help in determining structure-properties correlations as each material has a unique ac impedance spectrum. The data from other techniques such as UV-visible and FT-IR spectroscopy has been used as supporting evidences.

Material and Methods

Aniline (Merck, A.R. Grade) was distilled before use. TiO2, ZnO, Al2O3 (all from Merck), polyvinyl alcohol (S.D.S. chemicals, A.R. grade), acids (HCl, H2SO4, H3PO4) all from BDH were used as received. Triple distilled water obtained from a quartz double distillation unit was used for the synthesis. Ammonium peroxodisulphate (Merck), potassium permagnet (Spectrochem. Labs AR), potassium dichromate (Nice laboratory) were used as received.

Synthesis of PAni

Polyaniline was synthesized by IUPAC protocol [25] by oxidation of aniline in acidic medium using ammonium peroxodisulphate as an oxidizing agent. 5.0 mL aniline (0.05 mol) was added in 100 mL hydrochloric acid and cooled below 50 C. In this solution, 14.0 g ammonium peroxodisulphate dissolved in 100 mL water; cooled below 50 C was added slowly for about half an hour with continuous stirring. The reaction was done at low temperature, i.e. below 50 C as it is an exothermic reaction. A bluish green precipitate was obtained. The reaction mixture was placed in refrigerator overnight. Then, it was filtered and dried in an oven at 600C for 12 hours and then over anhydrous CaCl2.Yield in (g) = 3.4 g

Synthesis of Pani Using Different Oxidizing Agents:

Ammonium Peroxodisulphate (APS): 5 mL aniline was taken in 100 mL HCl. In this 14.0 g APS dissolved in 100 mL distilled H2O was added slowly with continuous stirring. The reaction temperature was maintained in between 0-5 0C. A bluish green mixture was obtained. The resulting solution was stirred for about one hour and then kept in refrigerator overnight. It was filtered and dried in oven.

Yield (g): PAni (APS) at 0-50 C = 3.4 g

Potassium Permanganate (PPM): 5 mL aniline (0.05 mol) was taken in 100 mL HCl. To maintain the temperature of solution below 50 C, it was placed in ice cooled water bath. Pre-cooled solution of KMnO4 (1.5g dissolved in 100 mL H2O) was added slowly with continuous stirring in aniline solution. Bluish colour appeared initially and then becomes deep blue at last. The resulting solution was stirred for about two hour and placed in freezer overnight. The solution was filtered and washed with water several times and then the precipitate .was dried in oven at 600 C.

Yield (g): PAni (PPM) = 1.6 g

Potassium Dichromate (PDC): 5 mL aniline was taken in 100 mL HCl. In this 3.0 g K2Cr2O7¬ dissolved in 100 mL distilled water was added slowly with continuous stirring for half an hour. The above reaction was done in between 0-50C. Bluish green solution was obtained. The solution was filtered and washed with water to remove the unreacted salts. The precipitate was dried in oven and over anhydrous CaCl2.

Yield (g): PAni (PDC) = 3.5 g

Synthesis Of Polyaniline Using Different Acids (Hcl, H2SO4, H3PO4) And APS As An Oxidizing Agent: 5 mL aniline was taken in 100 mL 1M of acids (HCl, H2SO4, H3PO4). In these solutions of aniline, 14.0 g APS dissolved in 100 mL water was added slowly respectively with continuous stirring. The reaction was done at low temperature i.e. below 5 0C. Bluish green colour was obtained. The reaction mixture was kept in refrigerator overnight. It was filtered and washed with HCl, H2SO4 and water several times. The precipitate was dried in oven and over anhydrous CaCl2.

Yield of the products are:

PAni (HCl) = 3.4 g, PAni (H2SO4) = 4.7 g PAni (H3PO4) = 4.9 g

Preparation of Pani Composites with Different Metal Oxides (Tio2, Al2O3, Zno): PAni/ZnO Composite 5 mL aniline (0.05 mol) was added in 100 mL HCl (1M). In this 0.5g of ZnO was added and stirred for about 5 minutes. Its temperature was maintained in between 0-50 C. In the resulting solution, 14.0 g APS dissolved in 100 mL distilled H2O was added slowly for about half an hour with continuous stirring. Bluish green solution was obtained. The solution was filtered and washed several times with distilled water. The precipitate was dried in oven between 50-600C and then over anhydrous CaCl2. Yield:  PAni/ZnO (0.5g) = 2.5 g.

Table 1:  Measured AC impedance data of polyaniline using ammonium peroxodisulphate as an oxidizing agent.

υ (Frequency) (Hz)

Cp×10-8(Capacitance) (Farad)

‌‌Z ? ‌? (Impedance) (Ohm)

G x 10-3 (Conductance) (Siemen)

σ ×10-1 (Conductivity) (S/cm)

4  ×  101

0

1.58

630.2

1.41

6  ×  101

0

1.58

633.8

1.42

8  ×  101

0

1.57

636.6

1.42

1  ×  102

0

1.56

639.7

1.43

2  ×  102

10

1.55

644.6

1.44

3  ×  102

10

1.54

647.5

1.45

4  ×  102

10

1.54

648.6

1.45

6  ×  102

8

1.53

651.3

1.46

7 ×  102

8

1.53

652.3

1.46

9 ×  102

8

1.53

653.9

1.46

1 ×  103

8

1.53

654.9

1.47

2 ×  103

7

1.52

655.8

1.47

3 ×  103

6

1.52

656.8

1.47

5 ×  103

6

1.52

657.8

1.47

7 ×  103

5.5

1.52

658.5

1.47

9 ×  103

5.7

1.52

659.9

1.48

1 ×  104

5

1.51

661.2

1.48

2 ×  104

4

1.51

660.7

1.48

4 ×  104

3

1.51

660.4

1.48

6 ×  104

2.44

1.51

659.8

1.48

8 ×  104

2.13

1.51

659.2

1.48

1 ×  105

1.9

1.51

658.6

1.48

Table 2: Simulated AC impedance data of polyaniline with using ammonium peroxodisulphate as an oxidizing agent.

υ (Frequency) (Hz)

C (Capacitance) (Farad)

‌‌Z ? ‌? (Impedance) (Ohm)

G (Conductance) (siemen)

σ  ×10-2 (Conductivity) (S/cm)

1.00×10-3

3.25×10-6

1.55

0.65

1.1

2.15×10-3

3.26×10-6

1.55

0.65

1.1.

1.0×10-2

3.25×10-6

1.55

0.65

1.1

4.64×10-2

3.26×10-6

1.55

0.65

1.1

2.15×10-1

3.26×10-6

1.55

0.65

1.1

4.64×10-1

3.26×10-6

1.55

0.65

1.1

1.00×101

3.26×10-6

1.55

0.65

1.1

4.64×101

3.24×10-6

1.55

0.65

1.1

1.00×102

3.18×10-6

1.55

0.65

1.1

1.0×103

9.16×10-7

1.53

0.65

1.1

4.64×103

7.79×10-8

1.52

0.65

1.1

2.15×104

2.42×10-8

1.52

0.65

1.1

1.0×105

2.16×10-8

1.52

0.66

1.1

4.64x105

2.15x10-8

1.51

0.66

1.1

4.64×106

2.15×10-8

7.99×10-1

0.66

1.1

2.15×107

2.15×10-8

7.42×10-2

0.66

1.1

1.0×108

2.12×10-8

3.61×10-3

0.66

1.1

4.64×109

2.15×10-8

1.68 ×10-6

0.66

1.1

2.15×1010

2.15×10-8

7.80×10-8

0.66

1.1

1.00×1011

2.15×10-8

3.62×10-9

0.66

1.1

PAni/TIO2 Composite: 5 mL aniline (0.05mol) was added in 100 mL HCl. 0.5g TiO2 was added into it and stirred for few minutes. In this 14g APS dissolved in 100 mL H2O was added slowly with continuous stirring. The reaction was done at low temperature i.e. below 50C. Bluish green precipitate was obtained, which was filtered and washed with distilled water. The precipitate was then dried in oven at 600C and then over anhydrous CaCl2.

Table 3: AC components along with its values of polyaniline and its composite with different metal oxide (TiO2, Al2O3 and ZnO) and PAni/PVA using ammonium peroxodisulfate as an oxidizing agent.

Solvent

Electrical Properties

Grain

Electrode

Grain Boundary

R(Ohms)

C(Farad)

R(Ohms)

Q/C(Farad)

R(Ohms)

C(Farad)

PAni ( APS)

2.96 x 10-2

8.85 x 10-3

1.52 x 100

2.14 x 10-8

-----

-----

PAni (PPM)

3.70 x 104

3.60 x 10-10

2.55 x 104

2.49 x 10-8

6.93 x 104

3.09 x 10-11

PAni (PDC)

2.08 x 104

2.87 x 10-11

5.89 x 103

1.44 x 10-7

4.59 x 103

4.72 x 10-9

PAni-ZnO

1.37x100

5.69 x 10-8

----

----

------

-----------

PAni-TiO2

1.34x100

3.60 x 10-8

3.71x10-2

1.43 x10-2

--------

----------

PAni- Al2O3

4.47x100

5.71 x 10-9

-----

-----

----------

------------

Pani (Interfacial)

1.57 x 10-1

5.37 x 10-3

7.22 x 10-2

4.86 x 10-4

1.10 x 100

6.82 x 10-8

PAni-PVA

1.89 x 102

7.73x10-10

-----

-----

-----

-----

PAni-PVA (Interfacial)

4.71 x 100

1.48x10-9

1.04 x 10-1

7.80 x 10-4

-----

------

PAni/Al2O3 Composites: Aniline (5 mL) was dissolved in 100 mL hydrochloric acid (1M). Al2O3   (0.5g) was added into it and the resulting solution was stirred for about 5 minutes.  The temperature was maintained below 50 C.  Prepared APS solution (14.0 g APS dissolved in 100 mL of water) was added slowly for about half an hour with constant stirring. Bluish green solution was obtained, which was filtered and then dried in oven at 600C

Yield: PAni/Al2O3 (0.5g) = 3.5 g

Table 4: Conductivity data summary of polyaniline   and its composites with different metal oxides and composites.         

Samples

σ ac(S/cm)

σ ac(S/cm)

σ ac(S/cm)

σ ac (S/cm)

100Hz

1 KHz

10 KHz

100 KHz

Pani(APS)

1.44 x 10-1

1.47 x 10-1

1.48 x 10-1

1.48 x 10-1

PPM

5.59 x 10-7

7.04 x 10-7

8.08 x 10-7

10.62 x 10-7

PDC

2.37 x 10-6

2.79 x 10-6

3.13 x 10-6

3.60 x 10-6

PAni-ZnO

1.23 x 10-1

1.27 x 10-1

1.28 x 10-1

1.25 x 10-1

PAni-TiO2

1.17 x 10-1

1.19 x 10-1

1.19 x 10-1

1.19 x 10-1

PAni-Al2O3

2.18 x 10-2

2.20 x 10-2

2.30 x 10-2

3.40 x 10-2

Interfacial Synthesis of Polyaniline: 5.0 mL aniline (0.05mol) was added in 100 mL benzene. Its temperature was maintained in between 0-50 C. In this solution, 14.0 g APS dissolved in 100 mL hydrochloric acid (1M) was added slowly without stirring. A bluish green aqueous solution was obtained. The solution was kept in refrigerator overnight. It was filtered and then the precipitate obtained was dried in oven. Yield: PAni (benzene) = 2.4 g

Synthesis of Pani/PVA Composites: 5.0 mL aniline was taken in 100 mL HCl. 0.1 g PVA dissolved in 50 mL water was added into it. In the resulting solution 14.0 g APS dissolved in 100 mL water was added slowly with continuous stirring for about half an hour. The reaction was done in between 0-5 0C. Bluish green solution was obtained. The reaction mixture was placed in refrigerator overnight. The solution was filtered and dried in oven.

Yield in (g) = 2.1 g

Interfacial Synthesis of Pani/Pva Composite: 5.0 mL aniline was taken in 100 ml benzene. In this 0.2 g PVA dissolved in 50 mL water was added. 14.0 g APS dissolved in 100 mL hydrochloric acid was added slowly without stirring. The reaction was done at low temperature, i.e. below 50 C. Bluish green solution was obtained. The solution was placed in refrigerator overnight. It was filtered and the precipitate obtained was dried in oven and then over CaCl2. Yield in (g) = 2.0 g

Measurements: The FT-IR spectra of these materials were done in KBr medium in the range of 450-4000 cm-1 using Varian 3100 FT-IR Encalibur Series. UV-visible spectroscopy was done by using UV-1700 Pharma Spec. Simadzu corp. in nujol medium in the range of 300-1000 nm.  The AC conductance (G), capacitance (C) and resistance(R) of different samples were measured in the range of 40 Hz to 100 kHz using LCZ meter (Keithley, model-3330). The basic accuracy of the L C Z meter used for our measurements was 0.1 % for the impedances in the range of 0.1 m Ω to 19.999 MΩ ; capacitances in the range of 0.001 pF to 199.99 mF  and for conductance in the range of  0.001 µS to 199.99 S.  The measured data were simulated using CNLS (complex nonlinear least square fitting) software in the range of 10-3 – 1011 Hz by to determine the equivalent circuits. The relaxation times were evaluated from the products of R and C obtained from the equivalent circuit analysis.

Results and Discussion

FT-IR, UV-visible and AC Impedance Spectroscopy of PAni prepared by Different Oxidants and Acids (APS, PPM, PDC and HCl, H3 PO4 ,H2 SO¬4 )

The I.R. spectra of polyaniline with different oxidizing agents and in presence of different acids have been taken in KBr medium. For polyaniline, the peaks at 1570 cm-1 and 1474 cm-1 have been assigned due to ring stretching mode of the quinoid and benzenoid rings respectively. The peak near 3200-3400 cm-1 was due to iminium ion,υ (N-H) stretching mode of vibration. The band at 2924 cm-1 and 799 cm-1 are due to υ (C-H) (Ph-H) stretching mode and υ (Ph-H) C-H out of plane bending vibration respectively. The peak at 1126 cm-1 was due to υ (C-C) stretching mode and at 1295 cm-1 was due to υ (C-N) stretching mode of vibration. These results are consistent with the results reported earlier [26].  The IR absorption bands for PAni with different oxidizing agent and acid ( APS, PPM, PDC and HCl, H3PO4  ,H2SO¬4) were observed at 1570 cm-1, 1474 cm-1 , 1296 cm-1 , 1126 cm-1 , 799 cm-1 , 1581 cm-1 , 1497 cm-1 , 1304 cm-1 , 1149 cm-1 , 813 cm-1 , 1584 cm-1 , 1484 cm-1 , 1299 cm-1 , 1134 cm-1 , 805  cm-1 and (H3PO4  ,H2SO¬4)  1569 cm-1 , 1483 cm-1 , 1304 cm-1 , 1128 cm-1 , 801  cm-1, 1574 cm-1 , 1482 cm-1 , 1305cm-1 , 1128 cm-1 , 801  cm-1  respectively. The analysis of I.R. spectra of polyaniline with different oxidizing agent showed a great change in the intensity of quinoid and benzenoid peak due to degree of oxidation [27]. The peak corresponding to υ (C-H) deformation showed red shift from APS to PDC to PPM, showing increasing extent of oxidation. UV-Visible spectra of polyaniline with different oxidizing agents and acids have been taken in nujol medium in the range of 300 nm to 1000 nm. For PAni the peaks around 383 nm and 427 nm were attributed to the π-π*and excitonic transition and peak around 825 nm was due to polaron formation. UV-visible absorption spectra of PAni with different oxidizing agents showed markedly change in absorption bands, number and position of λmax due to different extent of oxidation of PAni in the presence of different oxidizing agents e.g., with APS the electronic bands at 355 nm, 427 nm and 825 nm were observed whereas with PPM two peaks at 350 nm and 575 nm were observed. It indicates the presence of emeraldine salt (ES) and completely oxidized state of pernigraniline salt (PS) form respectively. For PDC, peaks at 367 nm, 435 nm and 815 nm (broad) were observed which indicates the presence of nigraniline salt (NS) of protonated PAni. In the UV-visible spectra of PAni with different acids i.e. with HCl, H2SO4, there were only minor changes in the position of three peaks due to strong nature of these acids having fully protonated form of PAni in both the cases.  However, in the electronic spectra of PAni (H3PO4), band at 369 nm, 420 nm and 810 nm were observed. For the AC impedance analysis of polyanilines with different oxidizing agents and acids, the measured data taken in the range of 40 Hz to 100 kHz were simulated using CNLS fitting software, in the frequency range of 10-3 Hz to 1011 Hz for best resolution using the parameters of the equivalent circuit. Three semicircles are expected for three components of system, grain, grain boundary, and electrode contributions. Two semicircles indicated contribution from grain and electrode only. Some of the representative plots have been given in Fig. 1-3 and related data given in Table 1-4. On observing the tables and figures PAni (APS), (R1C1) (R2C2) equivalent circuit was obtained having values R1 = 2.96 x 10-2 ?, C1 = 8.85 x 10-3 F, R2 = 1.52 x 100 ? and C2 = 2.14 x 10-8 F. The relaxation times of polyaniline for grain and electrode part were 2.62 x 10-4 s and 3.25 x 10-8 s showed large difference and therefore two fully resolved semicircles were obtained. For PAni (PDC & PPM), (R1C1)(R2C2)(R3C3) equivalent circuits were obtained and their values are R1 = 2.08 x 104 ?, C1 = 2.87 x 10-11 F, R2 = 5.89 x 103 ?, C2 = 1.44 x 10-7 F and R3 = 4.59 x 103 ?, C3 = 4.72 x 10-9 F and R1 = 3.70 x 104 ?, C1 = 3.60 x 10-10 F, R2 = 2.55 x 104 ?, C2 = 2.49 x 10-8 F  and R3 = 6.93 x 104 ?, C3 = 3.09 x 10-11 F respectively. In the impedance plot three semi circles were obtained in both the case indicating contributions from grain (R1C1), grain boundary (R2C2) and electrode (R3C3). On observing the Table 4, it was found that the best conductivity was obtained for APS as an oxidant and the worst conductivity for PPM which might be due to different molecular states of PAni as given by UV-visible spectra. As frequency increases, the conductivity also increases for all systems. It might be due to dependence of frequency on polarization process on the frequency such as space-charge polarization at low frequencies, orientation polarization at moderate frequencies and atomic/electronic polarization at high frequencies.

FT-IR, UV-Visible and AC Impedance Spectroscopy of Pani and Its Composites (Pani/Tio2, Pani/Al2O3 and Pani/Zno)

 The I.R. spectra of polyaniline and its composites with different metal oxides have been taken in KBr medium. The IR absorption bands for polyaniline and its composites with different metal oxides were observed at 1570 cm-1, 1474 cm-1 , 1295 cm-1 , 1126 cm-1 , 799 cm-1 , 1567 cm-1 , 1477 cm-1 , 1295 cm-1 , 1130 cm-1 , 812 cm-1 , 1577 cm-1 , 1484 cm-1 , 1300 cm-1 , 1135 cm-1 , 798  cm-1 and 1569 cm-1 , 1484 cm-1 , 1296 cm-1 , 1136 cm-1 , 797  cm-1 respectively. On comparing the I.R. spectra of PAni and PAni/TiO2 composites, it was found that the main characteristic peaks of PAni appear in the I.R. spectra of PAni/TiO2 composites. It was also found that the peak at 625 cm-1 corresponded to Ti-O streching in the composites. On comparing the corresponding peaks of PAni to the peaks of PAni/TiO2 composite, all the peaks shifted slightly due to some interaction at the interface of PAni and TiO2 [28]. In case of composites of PAni with ZnO, all these peaks shifted slightly indicating that there is some interaction between PAni and ZnO particles. Similarly, in case of PAni/Al2O3 composite all the characteristics peaks of PAni appear in the I.R. spectra of PAni/Al2O3 composite but peaks below 800 cm-1 become stronger as the Al2O3 loading was increased. UV spectra of polyaniline and its composites with different metal oxides have been taken in nujol medium in the range of 300 nm to 1000 nm. The electronic spectra of PAni composite with ZnO bands at 380 nm and 423 nm were due to π-π*and excitonic transition, and band at 802 nm was due to polaron band transition respectively. For PAni/TiO2 the electronic band at 353 nm, 411 nm and at 780 nm were obtained respectively [29]. For PAni/Al2O3 peaks at 337 nm and 444 nm were obtained due to π-π*and exciton band transition whereas band at 800 nm was due to polaron transition. On observing these spectra of composites of PAni with different metal oxides (ZnO, TiO2, Al2O3), it is evident from the Fig. 7 that the shape of UV-visible absorption bands and their position λmax markedly affected indicating that in the presence of metal oxides the polymerization occurs at the surface of metal oxides and there is some interaction between PAni and metal oxides. There were no white colored particles of ZnO, TiO2 and Al2O3 present in the precipitate  containing polyaniline and ZnO, TiO2, Al2O3 indicating good coating of these metal oxides nanoparticles by polyaniline. The AC impedance analysis of polyaniline and its composites with different metal oxides have been shown in Fig.4. For PAni/ZnO (RC) equivalent circuit was found having values R = 1.37 x 100 ?, C = 5.69 x 10-8 F and the relaxation time for grain part was 7.79 x 10-8 s. In the impedance plot, only one   big semicircle was obtained.  For PAni/TiO2 (R1C1)(R2C2) equivalent circuit were obtained having values R1 = 1.34 x 100 ?, C1 = 3.60 x 10-8 F, R2 = 3.71 x 10-2 ?, C2 = 1.43 x 10-2 F  The relaxation time for PAni-TiO2 4.81 x 10-8 s,  5.30 x 10-4 s respectively indicating large difference and for that fully resolved semi circles were obtained. In the impedance plot, two semi-circle were obtained indicating contribution from both grain and electrode part. However, in case of PAni/Al2O3 composites, (RC) equivalent circuits were found having values R = 4.47 x 100 ?,, C = 5.71 x 10-9 F respectively. In the impedance plot one semicircle were found.

FT-IR, UV-Visible and AC Impedance Spectroscopy of Interfacial Synthesized Pani and It’s Composite with PVA

The I.R. spectra of samples have been taken in the KBr medium for pure polyaniline, the peak shown near 3200-3400 cm-1 was due to υ (N-H) stretching mode. The peaks at 1570 and 1474 cm-1 were assigned due to ring stretching mode of the quinoid υ (N=Q=N) and benzenoid υ (N-B-N) rings respectively. For interfacial synthesis of PAni using benzene as an organic solvent, the peak corresponding to quinoid and benzenoid ring stretching vibration were observed at 1567 and 1487 cm-1 respectively confirming the oxidation state of PAni emeraldine salt. For interfacially synthesized polyaniline using benzene the peak corresponds to υ (C-C) stretching  1134 cm-1, υ (C-N) stretching  1295 cm-1 and υ (Ph-H) C-H out of plane bending vibration  803 cm-1show red shift. The band for υ (C-H) (Ph-H) in plane stretching 2924 cm-1are shows blue shift.  This shift may be due to the formation of nano-sized polyaniline or different length assignments compare well with those reported earlier [30]. For PAni/PVA composite (interfacial synthesis), band at 1115 cm-1 was due to υ (C-O-C) stretching mode and intense peaks at 1297 cm-1 and 1135 cm-1  were due to  υ (C-N) and υ (C-C)  stretching mode. The peaks at 1567 and 1474 cm-1 corresponds to quinoid and benzenoid stretching mode of vibration. This shifts indicated that there may be some hydrogen bonding between PAni and PVA. Electronic spectra of polyaniline and its composite (interfacial synthesis) have been taken in nujol medium in the range of 300 to 1000 nm. For polyaniline (bulk polymerization), the peak around 380 nm and 425 nm were attributed to the π-π* and excitonic transitions. The peak around 825 nm was due to polaron formation. From figures, it was evident that the polymerization of polyaniline at the interface of two immiscible liquids markedly affects the UV-visible absorption positions indicating that the particle size and morphology changed due to interfacial polymerization. For interfacial synthesis of PAni using benzene as organic solvent, the electronic absorption bands, obtained at 363 nm, 421 nm and 815 nm show that they are in emeraldine oxidation state [31]. For PAni/PVA composite, all the band of PAni were shifted slightly and a broad band around 349 nm is observed. This shift might be due to interaction between PAni and PVA through hydrogen bonding. This shift was in accordance with previously reported [32]. The electronic bands for PAni/PVA (bulk polymerization) were obtained at 349 nm, 431 nm and 740 nm due to π-π*, excitonic and polaron formation respectively.  UV-visible band at 347 nm, 416 nm and 730 nm were observed for PAni/PVA (interfacial polymerization) due to π-π*, excitonic and polaron formation. AC impedance measurements for some representative samples of interfacial synthesized polyaniline using organic solvent (benzene) and composite of PAni/PVA having benzene as organic phase at various frequencies have been given in Fig.5. The regression analysis of conductance data as a function of temperature for this system indicated three–dimensional variable range hopping charge transport in these materials. It was found that polyaniline synthesized by interfacial polymerization had their   room temperature conductivities lower (around 10-2 Scm-1) than the bulk synthesized polyaniline using the standard IUPAC conjugation method (around 10-1Scm-1). Addition of polyvinyl Alcohol in the interfacial synthesis of polyaniline using benzene as organic phase leads to slightly lower conductivity. These results indicate that conducting polyaniline nanostructures and their composites can be prepared by this route. The equivalent circuit elements for various systems are given as follows: For PAni (bulk), (R1C1) (R2C2) equivalent circuit was obtained having values R1 = 2.96 x 10-2? , C1 = 8.85 x 10-3  F, R2 = 1.52 x 100 ? and C2 = 2.14 x 10-8 F respectively indicating contribution from grain and electrode part only and two semicircle were obtained whereas for PAni (Interfacial), (R1C1)(R2C2)(R3C3) circuit was obtained having values R1 = 1.57 x 10-1 ?, C1 = 5.37 x 10-3 F, R2 = 7.22 x 10-2 ? , C2 = 4.86 x 10-4  F, R3 = 1.10 x 100 ?, C3.=6.82 x 10-8 F. It means that the polyaniline polymerized in this case was not homogeneous and three semicircles were obtained, indicating significant contribution from   grain, grain boundary and electrode. For PAni/PVA composite (interfacial synthesis), (R1C1) (R2C2) circuit were obtained having values R1= 4.71 x 100 ?, C1= 1.48 x 10-9 F, R2= 1.04 x 10-1 ?, C2= 7.80 x 10-4 F respectively. In the impedance plots, two semicircles were obtained one big and other small indicating that there was grain and electrode contribution which was comparatively greater than grain boundary contribution.  Pure polyaniline and its composite with PVA have two similar arcs and circuit elements. However, interfacially synthesized polyaniline using benzene as an organic phase had three arcs indicating significant contributions from grain, grain-boundary and electrode. It was evident that our data were resolved better in the impedance rather than the modulus plot. Two peaks in the Bode plots (imaginary impedance vs. frequency), indicate two different types of relaxation processes for grain and electrode contributions. The conductivities calculated from the grain resistances match very well with the values for different samples. The relaxation times determined from respective R & C values lie in the range of 3.67 x 10-9 s to 4.58 x 10-8 s for grain, and 8.80 x 10-5 s to 2.62 x 10-3 s for electrode and 3.51 x 10-5 s for grain boundary processes. In all the systems, conductivity increased with temperature indicating the semiconductor behavior of materials.

Conclusions

A novel approach, i.e., AC Impedance spectroscopy has been successfully observed the preparation of polyaniline nanostructures under various conditions. It helps in assessing the electrical properties of the materials as well as separating contributions originating from various electrode processes- grain/bulk, grain-boundary and contact/electrode. Each material has a characteristic AC impedance spectrum which correlates well with data from UV-visible and FT-IR technique.

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