Localized polarons in in situ synthesized polyaniline nanocomposite improve the morphology and the thermal and electrical conductivity

Abstract

This paper reports the localized polarons improves the morphology, thermal and electrical conductivity of dodecyl benzene sulfonic acid (DBSA) surfactant based polyaniline (PANI) – cobalt oxide (Co₃O₄) nano have synthesized by using in situ chemical reaction method. The PANI nanocomposite characterizations were done by Fourier transform infrared (FT-IR), X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. The impedance measurements were carried out at different temperatures and frequencies. The FT-IR results confirmed the chemical interaction in the PANI nanocomposite. The XRD results provide the structural phase change in the PANI nanocomposite. The TGA and DSC results reveal the enhanced thermal stability with increasing nano Co₃O₄ concentration. The SEM image shows a substantial change in PANI morphology after incorporating nano Co₃O₄ composite and the TEM image shows the occurrence of nano rods in the PDC2 composites. The AC conductivity and dielectric constant results were enhanced with temperature and nanocomposite concentration. The DC electrical conductivity increases with increasing amounts of nano cobalt oxide in the composite and achieved maximum conductivity for PDC2. These results suggest that the polyaniline (PANI)/dodecyl benzene sulfonic acid (DBSA)/cobalt oxide (Co₃O₄) (PDC) nanocomposite.

---

1. Introduction

Nowadays, PANI nanocomposites are attracting attention as a new class of synthetic material owing to various potential applications, such as in supercapacitors, light emitting diodes (LED), sensors, photovoltaic cells, and optical devices. They have unique characteristics, like easy synthesis, low cost, high electrical conductivity and environmental stability. These applications require materials with high current density, power density, storage capacity and energy conversion efficiency. It means that the energy transfer per unit time, volume power density, specific capacitance and high power conversion efficiency are important for all storage devices like solid state batteries, supercapacitors and solar cells. Therefore, researchers have focused on identifying suitable materials with high power density, which may facilitate fast charging efficiency and electrochemical stability for a long discharge period. Recently, the reported physical properties of PANI nanocomposites with graphene,¹ aluminium,² and nano rare earth salts³ seem to be suitable for the above applications, but unfortunately it is not yet possible to use them in practical applications owing to many unanswered questions. Therefore, it is necessary to gain a deep understanding about the rate of storage of energy and the electrical conductivity to understand the motion of charges under an applied potential at different temperatures.⁴ The few results that have been reported are for bulk PANI composites, such as with cadmium sulfate,⁵ lithium cobalt oxide⁶ and zinc oxide.⁷ The nanoparticles favor the increase of the physical properties.^8–10 A few results on PANI with nano metal have been reported and showed improved chemical and physical properties.^11,12 The properties of PANI nanocomposites depend upon the synthesis conditions and the size of the nanocomposites directly influences the structure and electrical conductivity; similar results have been reported for TiO₂,¹³ ZnO,¹⁴ SiO₂.¹⁵

A few reports are available on a surfactant-based PANI nanocomposite, a PANI magnetic nanocomposite,¹⁶ a PANI silicon nanocomposite,¹⁷ a DBSA surfactant-based PANI BaTiO₃ nanocomposite that showed improved electromagnetic interference,¹⁸ a PANI/CSA solution that demonstrated enhanced electrochemical performance,¹⁹ and PANI–CSA–silica, which showed improved structure and electrical conductivity.²⁰ In the present work, the PANI nanocomposite was synthesized in the presence of DBSA as the surfactant; DBSA has a polar sulfonic acid (–SO₃H) group as the head and a long non polar dodecyl (–C₁₂H₂₅) chain as the tail, which are responsible for its surfactant characteristics. The chemical interaction was established between PANI and Co₃O₄ nanoparticles via the polar group of sulfonic acid, as shown in Scheme 1. This change in the chemistry of nanocomposite provided flexibility in the motion of the charge ratio between the surface and volume charges. We observed the appearance of nanorods and high electrical conductivity for a very low weight percentage of nanocomposite. In the present investigation, we synthesized a DBSA surfactant-based PANI nanocomposite using in situ method and characterized it to determine the chemical interactions, structural phase, thermal, morphology and microstructural properties. Furthermore, the temperature dependence of the electrical conductivity, AC conductivity and dielectric constant were also studied.

Scheme 1 Extended chain and compact coil and colorization of PANI nanocomposites.

2. Experimental methods

2.1. Synthesis of PANI and its nanocomposite

Aniline (C₆H₇NH₂; distilled under reduced pressure), dodecylbenzenesulfonic acid (DBSA, C₁₈H₂₉ NaO₃S; used as a surfactant), and cobalt oxide (Co₃O₄) nanoparticles (<50 nm) were purchased from Sigma-Aldrich USA. Ammonium peroxydisulfate ((NH₄)₂S₂O₈; used as an oxidant) and hydrochloric acid (HCl) of analytical reagent (AR) grade were purchased from Merck India.

PANI was synthesized by using in situ chemical reaction method.²¹ 4.6 ml of double distilled aniline monomer was dissolved in 100 ml of 1 M HCl and 2.8 g of (NH₄)₂S₂O₈ (APS) was dissolved in 100 ml of 1 M HCl solution. The APS solution was added into the aniline solution drop wise with constant stirring at freezing temperature of 0–3 °C maintained using ice bath and the mixture was continuously stirred for about 24 hours. The dark green residue was filtered using Whatman 100 paper and exhaustively washed with distilled water in order to eliminate the unwanted contents, and it was then dried in a hot oven at 55 °C for about 24 hours, then ground to a fine powder.

2.2. Synthesis of PANI/Co₃O₄ nanocomposite

DBSA acts as a surfactant and different wt% of Co₃O₄ (i.e., 1 wt%, 2 wt% and 5 wt%) were added to the aniline prior to the addition of APS. The remaining process is the same as stated above. Using stoichiometry, polyaniline, PANI/DBSA : 1% (Co₃O₄), PANI/DBSA : 2% (Co₃O₄), PANI/DBSA : 5% (Co₃O₄) were synthesized and coded as PANI, PDC1, PDC2 and PDC5, respectively.

3. Characterization techniques

The chemical changes were examined by Fourier transform infrared spectrometer (FT-IR, ALPHA Bruker, spectral range of 61 600 cm⁻¹). The XRD characterization was done using a Rigaku Miniflex-II X-ray diffractometer with Ni filtered CuKα radiation of wavelength λ = 1.5406 Å and scanned from 10 to 60° at a scanning rate of 5° per minute. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out using a TA Instruments Q-600 heating from 25 to 700 °C and from 30 to 300 °C with a heating rate of 10 °C min⁻¹ under a nitrogen flow rate of approximately 20 ml min⁻¹. The surface morphology field emission scanning electron microscope (FESEM) images were obtained using a Sigma Zeiss with an operating voltage of 15 kV (with a magnification of 2 μm) in order to limit the charging effects and volume contribution to the image contrast. The TEM images were recorded using a JEOL, JEM-2100 transmission electron microscope (TEM). The electrical conductivity and dielectrics were studied using a Wayne Kerr 6500B impedance analyzer.

4. Results and discussion

4.1. FT-IR analysis

The FT-IR spectra of the PANI and PANI/DBSA/Co₃O₄ nanocomposites are as shown in Fig. 1. The PANI exhibits absorption bands at 1555 and 1470 cm⁻¹, which are associated with the C=N and C=C stretching modes of vibration related to the quinoid and benzenoid structure of PANI.

Fig. 1 (a) FT-IR spectra of PANI, PDC1, PDC2 and PDC5 nanocomposites, and (b) pure Co₃O₄.

The other peaks observed at 1290 and 1232 cm⁻¹ are assigned to the N–H bending and C–N stretching modes of the benzenoid ring. The peak observed at 793 cm⁻¹ is attributed to the out of plane C–H bending vibration.²² The band at 1032 cm⁻¹ is assigned to the absorption of the –SO₃H group of DBSA in the nanocomposite.^23,24Fig. 1(b) shows FT-IR spectrum of Co₃O₄ nanoparticles and the peak observed at 660 cm⁻¹ is related to the stretching vibration in the Co²⁺ tetrahedral hole.^22,25 These results confirm the chemical interaction between PANI and nano cobalt in the presence of the DBSA surfactant.

4.2. XRD analysis

The XRD patterns of the PANI, PDC1, PDC2 and PDC5 nanocomposites are shown in Fig. 2. The distinctive peaks of PANI were observed at 2θ = 14.83°, 20.24° and 25.38° for (011), (020), and (022), respectively, and the reflection corresponds to the crystal domain of PANI.^26,27

Fig. 2 XRD patterns of PANI, PDC1, PDC2 and PDC5 nanocomposite.

The peak appearing at around 20–30° is attributed to the overlapping of DBSA with PANI.²⁸

The X-ray diffractograms of the DBSA surfactant-based PANI/Co₃O₄ show distinct crystalline peaks attributed to the nanocomposite. The Co₃O₄ nanoparticles show strong reflection peaks at 2θ = 31.24°, 36.78°, 44.59° and 59.30° corresponding to reflection planes (220), (311), (400) and (511), respectively.²⁷ It becomes clear that the new peak at 36.78° indicates the dispersion of Co₃O₄ nanoparticles into PANI in the presence of DBSA. The intensities of the peaks varied with increasing Co₃O₄ and we clearly noticed the change in the crystallinity and amorphous phase.²² The intensity of the PANI peak reduced with increasing nanocomposite, which may signify an increase in the amorphousity of the composite.

4.3. FESEM images

Fig. 3(a)–(d) shows the SEM images of the PANI, PDC1, PDC2 and PDC5 nanocomposites. The FESEM image of PANI clearly depicts a cloud-like morphology with a highly agglomerated granular uniform shape and size. Fig. 3(b) shows a smaller granular shape with increased porosity owing to nanocomposite effects through the surfactant. PDC2 shows a uniform cloud-like morphology with reduced porosity and size, as shown in Fig. 3(c), indicating the formation of polarons.^29,30

Fig. 3 FESEM images of (a) PANI, (b) PDC1, (c) PDC2 and (d) PDC5 nanocomposites.

The PDC5 image in Fig. 3(d) shows grains that are highly dense and a number of cobalt nano ions closely associated like a compact coil reveals the formation of bipolarons. The appearance of polarons and bipolarons in the PANI nanocomposite clearly represents Scheme 1.

4.4. Elemental EDAX analysis

The elemental composition of the PANI, PDC1, PDC2 and PDC5 nanocomposites as obtained by EDAX as shown in Fig. 4 and results are shown in Table 1. The results are consistent with the presence of surfactant and Co₃O₄. The analysis shows the homogeneous dispersion of the nanocomposite in the presence of DBSA and the corresponding percentages of elements are given in Table 1.

Fig. 4 EDAX analysis of (a) PANI, (b) PDC1, (c) PDC2 and (d) PDC5 nanocomposites.

Table 1 Yield and elemental composition of the PANI, PDC1, PDC2 and PDC5 nanocomposites

Elements	PANI		PDC1		PDC2		PDC5
	Weight%	Atom%	Weight%	Atom%	Weight%	Atom%	Weight%	Atom%
C	0.00	16.72	0.00	4.18	0.03	17.98	0.02	17.35
O	—	—	—	—	0.06	24.95	0.06	31.36
S	0.01	16.38	0.01	6.02	0.02	4.58	0.01	3.38
Cl	0.03	66.90	0.04	16.74	0.11	22.00	0.03	6.69
N	—	—	0.06	65.08	0.05	23.89	0.04	24.37
Co	—	—	0.03	7.98	0.05	6.61	0.12	16.85

---

The scheme represents the formation of polarons and bipolarons in the nanocomposite. It can also interpret that the electrical conductivity increases because of the polarons and bipolarons or they have extended bulk of linear polymer chain like extended coil (localized chain) where they are more flexible are called localized polarons.

Further increasing the nano concentration increases protonation so that more cobalt ions interact with the PANI via the surfactant. The localized polarons come close and their unpaired electrons overlap to form a bipolaron; as a result of the compact coil, they are unable to move freely, hence the conductivity decreases. These results exactly correlate with the TEM images. The PANI colorization changes from light brown color to dark blue with the addition of nano Co₃O₄ in the presence of DBSA, which also signifies the effect of nano Co₃O₄ on the physical properties of the PANI nanocomposite.

4.5. TEM analysis

The TEM images of the PANI, PDC1, PDC2 and PDC5 nanocomposites are shown in Fig. 5. Fig. 5(a) shows the TEM image of PANI; the thick dark region on the white surface reveals the amorphous nature.

Fig. 5 TEM images of (a) PANI, (b) PDC1, (c) PDC2 (inset (e) SAED) and (d) PDC5 nanocomposites. (c) Shows the appearance of a nano rod.

The image of PDC1 Fig. 5(b) is dark and cloudy owing to the effect of agglomerated nanoparticles. PDC2 shows the occurrence of a nanorod like structure, as shown in Fig. 5(c), and the selected-area electron diffraction pattern (SAED) exhibits a spotty ring structure (inset Fig. 5(e)). The fine particles ring structure represents the crystalline nature of the Co₃O₄ nanoparticles. PDC5 shows a thick dark region embedded on the surface of a white fiber background, indicating that more metal ions closely associate and overlapped to form the bipolaron like a compact coil.

4.6. TGA analysis

Fig. 6 shows the TGA curves of the PANI, PDC1, PDC2 and PDC5 nanocomposites. It is observed that the thermal behavior changes in three stages. The first stage from room temperature to 145 °C is attributed to the release of the moisture and the residual organic solvent entangled in the polymer nanocomposite.

Fig. 6 TGA curve of PANI, PDC1, PDC2 and PDC5 nanocomposite.

The second stage between 200 and 370 °C is caused by the loss of the doping acid.³¹ It would need more energy for the acid to be removed from the polymer chains if the PANI chains doped with acid are well arranged in a more crystalline structure. The second stage weight loss will shift to higher temperature and overlap with the third weight loss stage. The third stage weight loss ranging from 360 to 520 °C is due to the breakdown of the structural backbone.³² The increase in the concentration of the nanocomposite in the PANI matrix leads to a decrease in the degradation temperature. The lower degradation rate and relatively larger residual rate indicate the protective effect in the PANI nanocomposites.

4.7. DSC analysis

The DSC study gives the nature of the heat flow in the PANI nanocomposite employed by differential scanning calorimetry (DSC) method.

The DSC plots show endothermic and exothermic behaviors. Fig. 7 shows the DSC thermograms of the PANI and PDC nanocomposites. From Fig. 7 an improvement in the thermal stability owing to polar groups present in the chemical structures of PANI nanocomposites was observed.

Fig. 7 DSC curve of PANI, PDC1, PDC2 and PDC5 nanocomposite.

4.8. Differential thermogrammetric analysis (DTA)

Fig. 8 shows the DTA curves of the PANI and PDC nanocomposites. The DTA curves show two exothermic peaks with maxima at 94.46 °C and 490 °C, which confirm that the removal of external water molecules and the structural transformation were clearly shifted to higher temperatures than that of PANI.

Fig. 8 DTA curve of PANI, PDC1, PDC2 and PDC5 nanocomposite.

This increase in the thermal stability of the PANI with increasing nanocomposites may be owing to the barrier effect or the interactions between PANI and Co₃O₄via the surfactant.³³ In our present study, the thermal stabilities of PANI chains increased gradually with the increase in the Co₃O₄ content, it exactly correlates with SEM as well as Lee's results.³³

4.9. Dielectric studies

Dielectric real (ε′) and imaginary parts (ε′′) as a function of frequency and temperature are shown in Fig. 9 and 10. The real and imaginary parts were calculated using the equations ε′ = c_pd/ε₀A and ε′′ = ε′ tan δ. The AC conductivity σ_(ac) was calculated from σ_ac = ωc_pd tan δ/A, where d is the thickness of the sample, A is the electrode area, ε₀ is the dielectric permittivity in vacuum (8.85 × 10⁻¹² F m⁻¹) and ω is the angular frequency.²¹ The dielectric constant (ε′) and dielectric loss (ε′′) are in the temperature region 303–393 K and the frequency range from 20 kHz to 1 MHz.

Fig. 9 Dielectric constant versus frequency of (a) PANI, (b) PDC1, (c) PDC2 and (d) PDC5 nanocomposites at different temperatures.

Fig. 10 Dielectric loss versus frequency of (a) PANI, (b) PDC1, (c) PDC2 and (d) PDC5 nanocomposites at different temperatures.

From Fig. 9(a)–(d) we observed that the dielectric constant is more pronounced at lower frequency because of the interfacial polarization. This polarization will arise only when the phases have different electrodes. At higher frequency, the induced dipoles are not able to orient themselves in the direction of the applied field, hence it remains constant. The dielectric constant found increases with increasing the concentration of cobalt nano due to large number of metals ions closely localized or overlapped tightly to form the bipolaron (compact coil) as a result dielectric constant high. The increase in the dielectric constant with increasing temperature reveals that this is a thermally activated process.

Fig. 10(a) and (b) shows the dielectric loss versus frequency at different temperatures. It is seen that dielectric loss decreases with frequency and temperature. As the temperature increases, the dielectric loss varies because of the chain motion of the PANI nanocomposite system. It can also be interpreted that decreasing the dielectric loss with frequency suggests the motion of ions satisfying the Debye type relaxation mechanism.

Generally, it can be emphasized that the dielectric constant (ε′) and loss (ε′′) reduce with frequency and increase with temperature, which is analogous to the work of Makhlouf.^34–37 The dielectric process can occur owing to displacement of charges under the applied field; it means dipoles are responding to the field, causing the dipole moment. In lower frequency the diploes are unable to respond because tight bounded and higher frequency dipoles move along direction of field, some time dipole may behaves as a free charge (conduction electron) whenever the dielectric medium breakdown occurs.^38,39

5. AC conductivity

Fig. 11(a)–(d) shows AC conductivity versus frequency of the PANI and the nanocomposites at various temperatures. It is observed that in the lower frequency (up to 1 kHz) region AC conductivity is independent of frequency, which was attributed to the long-range translational movement of charges adding to DC conductivity, which was clarified by Funke et al.⁴⁰ It demonstrates that at higher frequencies (beyond 1 kHz) the AC conductivity slightly increases because of the aggressive impact of AC and DC conductivities. There is maximum improvement in the AC conductivity for the PDC2 nanocomposite owing to the effects of Co₃O₄ nanoparticles via the surfactant, clearly evidenced from the TEM image (Fig. 5(c)), which favors strong hopping. It can also be interpreted that there may be charge carriers that are easily transported by hopping through the defect sites along the polymer chain through the surfactant; the observed results agree well with the reported results.⁴¹

Fig. 11 Variation of log AC as a function of log f of (a) PANI, (b) PDC1, (c) PDC2 and (d) PDC5 nanocomposites at different temperatures.

In heterogeneous systems like polymer–metal nano composites the accumulation of mobile charges at the interfaces very easily because of conjugated amino group of PANI may link to cobalt nano via surfactant as a result formation of more flexible large number of dipoles around metal particles or clusters. Several theoretical models have been proposed for the AC conduction in amorphous semiconductors, such as classical hopping and quantum mechanical tunneling.

The characteristics of amorphous semiconductors and disordered systems the frequency dependent conductivity σ(ω) obeys a power law.⁴² The total conductivity σ(ω) at a particular temperature over a wide range of frequencies can be expressed as:

σ(ω) = σ_dc + Aω^S (1)

where σ_dc is the DC conductivity and A is a constant dependent on temperature. The frequency exponent S lies between 0 and 1. Such a universal dynamic response is found in a variety of disordered materials.

The charge transport in the complex inhomogeneous system is established by classical hopping and quantum mechanical tunneling over the potential barrier separating two energetically favorable centers in a random distribution.⁴² The temperature dependence of exponent S decides the genuine charge conduction process in the PANI nanocomposite. The estimation of frequency exponent S diminishes with increasing temperature. This behavior is consistent with the correlated barrier hopping model.⁴² The temperature dependence of S based on this model is:

where W_H is the effective barrier height at boundless intersite separation, Γ₀ is the characteristic relaxation time and κ is the Boltzmann constant.

5.1. Temperature-dependent electrical conductivity

The DC conductivity for PANI and PDC nanocomposites at different temperatures is shown in Fig. 12. It is seen that Arrhenius plot of DC conductivity shows straight line behavior. The DC conductivity of PANI increased exponentially with increasing nanocomposite.⁴³

Fig. 12 Temperature dependence of DC conductivity for PANI, PDC1, PDC2 and PDC5 nanocomposite.

The conductivity as a function of temperature can be represented by the relation:⁴⁴

where E_a is the activation energy for the DC conduction mechanism, κ is the Boltzmann constant and σ₀ is the pre-exponential factor.

The conductivity of the polyaniline nanocomposites was calculated using eqn (3). The conductivity of PDC2 was the highest among nanocomposites; this is because of the effects of the cobalt nanocomposite through the surfactant or localized polarons. PDC2 was more prominent in the graph, as shown in Fig. 12. The increase in DC conductivity is due to the shift in the Fermi level.⁴⁵

It represent that the conduction mechanism through the carrier concentration at the Fermi level. However, the activation energy (E_a) alone does not provide any information as to whether the conduction takes place in extended states or by hopping in localized states.

This can be explained on the basis of the pre-exponential factor (σ_o) and other Mott and Davis parameters.⁴⁶ The value of σ_o in the range 10³ to 10⁴ ohm cm⁻¹ indicates that the conduction takes place in the extended states. Fig. 13 shows a plot of log σ_dcvs. T^−1/4 of the PANI nanocomposites to explain the conduction mechanism.

Fig. 13 log σ_dcvs. T^−1/4 for PANI, PDC1, PDC2 and PDC5 nanocomposites.

The explain that smaller value of σ_o indicating a wide range of localized states and the conduction taking place by the hopping process. In our case, the values of σ_o were found to be in the order of 10⁻³ for PANI and 10⁻² for PDC2; therefore the conduction takes place by hopping process owing to the wide range of localized states in the PANI nanocomposite. The obtained DC electrical conductivity result exactly correlates with microstructure shown in the SEM and TEM images.

From the above results, it can be concluded that the hopping mechanism is responsible for the increase in the conductivity of the nanocomposites. The formation of polarons and bipolarons is used to explain the conduction mechanism.⁴⁷ The polarons and bipolarons play a leading role in determining the charge injection and transport properties of the PANI nanocomposites through the surfactant. These are self-localized particles like defects associated with characteristic distortions of the polymer backbone and with quantum states deep in the energy gap owing to strong lattice coupling.

6. Conclusions

In summary, DBSA surfactant-based PANI/Co₃O₄ nanocomposites have been synthesized by in situ chemical reaction method and nanorod formation achieved with a small amount of cobalt nanoparticles. FT-IR confirmed the chemical interaction in the nanocomposite via the surfactant and XRD signified the changes in the structural phase of the nanocomposite. SEM revealed proper dispersion of the cobalt nanoparticles via the surfactant in the PANI nanocomposites. TGA and DSC results showed the change in the decomposition temperature and melting point of the PANI nanocomposites. DTG results showed the improvement of the thermal stabilities for the nanocomposites as compared to that of PANI. The PDC2 nanocomposite showed high conductivity (10⁻² S cm⁻¹) and this is explained by Mott and Davis's model. The dielectric properties and AC conductivity increased with increasing nanoparticle concentration. These obtained results suggest that PDC nanocomposites are prominent candidates for electrode and supercapacitor applications.

Acknowledgements

The authors gratefully acknowledge the financial support from the SERB New Delhi for sanction project F.No. SREB/F/4506/2013-14 dated 11-10-2013 and also to the Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, KERALA for providing the transmission electron microscope facility.

References

1. Q. Yang, Z. Quan, S. Wu, B. Du, M. Wang, P. Li, Y. Zhang and X. Wang, Tetrahedron, 2015, 71, 6124–6134.
2. F. Gmati, A. Fattoum, A. Manaii and A. B. Mohamed, J. Phys. D: Appl. Phys., 2011, 44, 315405.
3. K. Gupta, G. Chakraborty, S. Ghatak, P. C. Jana, A. K. Meikap and R. Babu, J. Appl. Phys., 2010, 108, 073701.
4. M. Čulo, E. Tafra, M. Basletić, S. Tomić, A. Hamzić, B. Korin-Hamzić, M. Dressel and J. A. Schlueter, Phys. B, 2015, 460, 208–210.
5. K. Dutta, J. Appl. Phys., 2007, 101, 093711.
6. K. Ferchichi, S. Hbaieb, N. Amdouni, R. Kalfat and Y. Chevalier, Mater. Chem. Phys., 2013, 142, 138–147.
7. S. P. Sharma, M. V. S. Suryanarayana, A. K. Nigam, A. S. Chauhan and L. N. S. Tomar, Catal. Commun., 2009, 10, 905–912.
8. W. Puin and P. Heitjans, Nanostruct. Mater., 1995, 6, 885–888.
9. P. Marquardt, Phys. Lett. A, 1987, 123, 365–368.
10. C. Mo, Z. Yuan, L. Zhang and C. Xie, Nanostruct. Mater., 1993, 2, 47–54.
11. S. B. Kondawar, S. R. Thakare, V. Khati and S. Bompilwar, Int. J. Mod. Phys. B, 2009, 23, 3297.
12. S. Chaudhari, A. B. Mandate, K. R. Patil and S. R. Sainkar, J. Appl. Polym. Sci., 2007, 106, 220–229.
13. Z. Liu, W. Guo, F. Daguang and W. Chen, Synth. Met., 2006, 156, 414.
14. S. K. Shukla, M. Vamakshi, G. Minakshi, A. Bharadavaja, A. Shekhar and A. Tiwari, Adv. Mater. Lett., 2012, 3, 421.
15. S. Sarmah and A. Kumar, Bull. Mater. Sci., 2013, 36, 31.
16. S. E. Jacobo, J. C. Aphesteguy, R. L. Anton, N. N. Schegoleva and G. V. Kurlyandskaya, Eur. Polym. J., 2007, 43, 1333–1346.
17. K. Dutta and S. K. De, Phys. Lett. A, 2007, 361, 141–145.
18. P. Saini, V. Choudhary, N. Vijayan and R. K. Kotnala, J. Phys. Chem. C, 2012, 116, 13403–13412.
19. S. Cho and K. H. Shin, ACS Appl. Mater. Interfaces, 2013, 5, 9186–9193.
20. H. T. Lee and C. C. Wang, Polym. Eng. Sci., 2008, 107, 840–845.
21. V. Mini, K. Archana, S. Raghu, C. Sharanappa and H. Devendrappa, Ind. Eng. Chem. Res., 2014, 53, 16873–16882.
22. V. Mini, K. Archana, S. Raghu, C. Sharanappa and H. Devendrappa, Mater. Chem. Phys., 2016, 170, 90–98.
23. M. Babazadeh, J. Appl. Polym. Sci., 2009, 113, 3980–3984.
24. S. Ashokan, V. Ponnuswamy and P. Jayamurugan, J. Alloys Compd., 2015, 646, 40–48.
25. D. B. Mahesh, R. Deshpande, B. Salimath and V. Abbaraju, Am. J. Mater. Sci., 2012, 3, 39–43.
26. X. Zhang and Z. Jiahua, Polymer, 2012, 53, 2109–2120.
27. V. Mini and H. Devendrappa, Mater. Res. Express, 2016, 3, 015502.
28. M. Babazadeh, J. Appl. Polym. Sci., 2009, 113, 3980–3984.
29. L. Haibao, G. Jihua, L. Jinsong and D. Shanyi, Appl. Phys. Lett., 2011, 98, 174105.
30. H. Lu, F. Liang and J. Gou, Soft Matter, 2011, 7, 7416.
31. X. Zhang and J. Zhu, Polymer, 2012, 53, 2109–2120.
32. T. Chen, C. Dong and L. X. Gao, Polym. Degrad. Stab., 2009, 94, 1788–1794.
33. D. Lee and K. Char, Polym. Degrad. Stab., 2002, 75, 555–560.
34. S. A. Makhlouf, Thin Solid Films, 2008, 516, 3112–3116.
35. C. G. Koops, Phys. Rev. Lett., 1951, 83, 121–124.
36. C. M. Mo, L. Zhang and G. Wang, Nanostruct. Mater., 1995, 6, 823–826.
37. N. Rezlescu and E. Rezlescu, Phys. Status Solidi A, 1974, 23, 575–582.
38. J. Maier, S. Prill and B. Reichert, Ionics, 1988, 28, 1465–1469.
39. B. Ramesh and D. Ravinder, Mater. Lett., 2008, 62, 2043–2046.
40. K. Funke, Prog. Solid State Chem., 1993, 22, 111.
41. A. Choudhury, Sens. Actuators, B, 2009, 138, 318–325.
42. S. R. Elliott, Adv. Phys., 1987, 36, 135.
43. M. S. Reda, M. Sheikha and A. Ghannam, Adv. Mater. Phys. Chem., 2012, 2, 75–81.
44. Z. H. Khani, M. M. Malik, M. Zulfequar and M. Husain, J. Phys.: Condens. Matter, 1995, 7, 8979–8991.
45. G. B. Shumaila, V. S. Lakshmi, M. Alam, A. M. Siddiqui, M. Zulfequar and M. Husain, Curr. Appl. Phys., 2010, 11, 217–222.
46. N. F. Mott and E. A. Davis, Properties of Amorphous Chalcogenides, Clarendon Press, Oxford, 1979.
47. G. B. Shumaila, V. S. Lakshmi, M. Alam, A. M. Siddiqui, M. Zulfequar and M. Husain, Curr. Appl. Phys., 2010, 11, 217–222.

---