M.
Niranjana
,
L.
Yesappa
,
S. P.
Ashokkumar
,
H.
Vijeth
,
S.
Raghu
and
H.
Devendrappa
*
Department of Physics, Mangalore University, Mangalagangotri-574199, India. E-mail: dehu2010@gmail.com; Fax: +91-8242888289; Tel: +91-8242888707
First published on 5th December 2016
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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
Fig. 11 Variation of logAC as a function of logf 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) = σdc + AωS | (1) |
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:
(2) |
The conductivity as a function of temperature can be represented by the relation:44
(3) |
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 (Ea) 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 (σo) and other Mott and Davis parameters.46 The value of σo 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 logσdcvs. T−1/4 of the PANI nanocomposites to explain the conduction mechanism.
The explain that smaller value of σo indicating a wide range of localized states and the conduction taking place by the hopping process. In our case, the values of σo 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.
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