Synthesis of Polymer-based ZnO/TiO2 NCs Flexible Sheets as High Dielectric Materials

Polymer-based ZnO/TiO 2 NCs flexible sheets with high dielectric permittivity and low loss factor have numerous applications in light emitting and energy storage devices. In this work, polymer-based ZnO/TiO 2 NCs are synthesized by co-precipitation technique. The development of various diffraction planes (X-rays diffraction analysis) related to TiO 2 and ZnO phases confirms the synthesis of polycrystalline polymer-based ZnO/TiO 2 NCs. The crystallinity of various phases is associated with increasing ZnO nanofillers. The surface morphology (scanning electron microscopic analysis) comprising of nanoparticles of different shapes is associated with increasing amount of nanofillers. The EDX analysis confirms the presence of Zn, O and Ti in the synthesized polymer-based ZnO/TiO 2 NCs. Dielectric measurements demonstrating the sharp increase in dielectric permittivity with relatively low dissipation factor of synthesized polymer-based ZnO/TiO 2 NCs are associated with increasing amount of ZnO nanofillers. The static value of dielectric constant (  ’ ) at low frequency (100 Hz) is found to be 14.56 for sample having 10% ZnO nanofillers that is 2.11 times greater than pure PVDF and it shows relatively low value of dissipation factor. The observed σ ac of synthesized polymer-based ZnO/TiO 2 NCs at 3.0×10 5 Hz and 1.0×10 6 Hz are ranged from 3.75-9.79 and 15.61-42.72  S/m respectively. The decreasing complex impedance and increasing electric modulus further confirm that the synthesized polymer-based ZnO/TiO 2 NCs flexible sheets are the promising candidate for better capacitive performance showing high strength and flexibility.


Introduction
Polymeric nanocomposites (NCs) are more attractive due to their remarkable "dielctric, optical, mechanical and electrical" properties. The polymer-based NCs showed outstanding properties as compared to unalloyed polymer which may be due to microstrains, defects and residual stresses developed during synthesis process. The properties of polymer-based NCs depend on weight fraction of different nanofillers, microstructural features and interfacial areas. The large interfacial area between the polymer and nanofillers plays a vital role to enhance the various properties of polymer-based NCs because they have high aspect ratio as well as high surface area. The numerous surface properties of polymer-based NCs are related to used polymer, weight fraction of nanofillers and their uniform mixing viva synthesis process [1][2][3][4][5]. The more commonly used polymers to synthesized polymer-based NCs flexible sheets are polyvinyl alcohol (PVA), Polymethyl methacrylate (PMMA) and polyvinylidene difluoride (PVDF).
Mallick et al [6] have synthesized the polymer-based titanium dioxide (TiO2) NCs flexible sheets and they said that the humidity sensor has shown linear and stable response over the investigated range of frequencies. They pointed out that the response and recovery times of polymer-based sensors were found to be 45s and 11s respectively. Ishaq et al [7] have synthesized the polymer- Amid polymers, PVDF is a lightweight and hydrophobic polymer due to low-cost, mechanical flexibility, low-temperature processing, ferroelectricity, high dielectric constant, high chemical resistive, more thermally stable and biocompatible. It is known that the TiO2 a most promising semiconducting material with high optical energy band gap, excellent optical transmittance, high refractive index and good dielectric properties [9]. The zinc oxide (ZnO), a semiconducting material with remarkable properties is being used in solar cell applications [10]. One of the most important characteristics of polymer-based NCs flexible sheets is their dielectric properties like dielectric constant, dielectric loss, loss factor, AC conductivity, impedance, however these properties intensely are depended on the size, shape, distribution of nanofillers, formation of bonds between nanofillers and polymer hydroxyl groups, microstructural features, interfical area between nanofillers and polymer matrix. Moreover the conductivity of nanofillers also plays a vital role to improve the surface properties of polymer-based NCs flexsible sheets. The outstanding properties of polymers-based NCs flexible sheets are associated to the complex motion of nanoparticles through molecular relaxations creating significant transitions whereas the polymeric interfaces behave as charge carrier trapping sites. It is important to study the effect of interfaces on the generation, transportation and storage of charge carrier in polymer matrix [11].
According to our knowledge, few research work has been done on the synthesis of polymerbased ZnO-TiO2 NCs flexible sheets. Therefore, it is necessary to synthsize the said polymer- In this research work, the polymer-based ZnO-TiO2 NCs flexible sheets are synthesized via copricipitation method. The synthesized polymer-based ZnO-TiO2 NCs flexible sheets are characterized by X-rays diffraction (XRD), scanning electron microscopy (SEM) , energy dispersive X-rays spectroscopy (EDX) and LC spectrometer in order to investigate the crystal structure, surface morphology, elemental composition and dielectric properties respectively.

Experimental Setup
The flexible sheets of polymer-based ZnO-TiO2 NCs (dimensions; ~ 0.5 mm × 1 cm × 1 cm) are synthesized via co-precipitation technique. The source materiasl used to synthesis the flexible sheets of polymer-based ZnO-TiO2 NCs are PVDF, ZnO and TiO2 of analytical grade. The weight fractions of source materials are measured by using digital weight balance. Table 1 shows the composition of synthesized (A-1, A-2, A-3 and A-4) samples. Figure 1 demonstrates the schematic diagram of synthesis process of polymer-based ZnO-TiO2 NCs flexible sheets. The synthesis process consists of various steps (i) for sample A-1 (only PVDF), (ii) for sample A-2, A-3 and A-4, the ZnO and TiO2 nanofillers are dissolved into 20 ml solvent (dimethyleformamide: DMF) along with PVDF according to the composition given in table 1 and stirred them at ~ 55 °C for 6 hrs. A gel type solution is formed which is converted into particular shape by using specific dies which then placed into oven at ~ 55 °C for 10 hrs results in the formation of flexible sheets. These synthesized flexible sheets are characterized by using XRD, SEM, EDX and LC spectrometer in order to study the crystal structure, surface morphology, elemental composition, electrical and dielectric properties.

Structural analysis
The XRD analysis is used to study the crystal structure of flexible sheets of polymer-based ZnO-TiO2 NCs synthesized at various compostions of PVDF and ZnO nanofillers for fixed amount of TiO2. Table 1  The dislocation density (δ) is defined as the length of dislocation lines per unit volume which can be calculated by employing the following relation [12].    Table 2 demonstarets the different structural parameters like phase identification, h k l, FWHM, C. S, peak intensity and microstrains developed due to increasing wt.% of ZnO nanofillers in polymer-based ZnO-TiO2 NCs.

SEM analysis
The SEM analysis is used to study the microstructural features of PVDF and polymer-based ZnO-TiO2 NCs. Figure  Micro-

Dielectric and electrical analysis
The dielectric permittivity of real ( ′ ) and imaginary ( ′′ ) parts can be calculated by the following formulas.
Where C, d, o and A are the capacitance, thickness, permittivity of free space (8.85 × 10 -12 F/m) and area of flexible sheets polymer-based NCs respectively where j = (-1) 1/2 [13].   of ɛ′ and ε″ of polymer-based NCs is due to the reduction of particle size as well as increasing porosity of synthesized flexible sheets with increasing wt.% of nanofillers [18].
In order to observe the conducting behavior of synthesized polymer-based NCs then we have to determine the parameters that may control the conduction mechanism of polymer-based NCs.
The AC conductivity (σac) of synthesized polymer-based NCs is calculated by using the flowing relation [19].

= ′ ( )
Where ω = 2πf and tan is the dissipation factor. Nasir et al [20] have reported that the electrical conduction mechanism is related to the elelctrons and polaron hopping mechanism which can act as hopping channels. The increasing frequency facilitates the conductive channels and hence promots the hopping of charge carrier's. Therefore, the increasing σac of synthesized flexible sheets is due to conduction mechanism occured due to small polaron hopping.
Furthermore, the increasing behavior of σac with increasing frequency is devided into two regions (region I = up to 2.0×10 5 Hz and region II = from 2.0×10 5 to 1.0×10 6 Hz). In region I, the conductivity plot is frequency independent while in region II, it is frequency dependent, however, after region I, the conductivity starts to increase with increasing frequency [21]. The increasing conductivity of synthesized flexible sheets of polymer-based NCs may be decribed by Jonscher's Power Law [22].

= +
This law indicate that the first term is frequency independent while second term is frequency dependent at lower and higher frequencies respectively. In second term, A, and n are constant amplitude, angular frequency and slope of frequency dependent conductivity curve.
The value of n is associated with the increasing wt.% of nanofillers which can be found by  It has been reported that [24][25] the σac is increased with increasing temperature which is attributed to dislocations at interface of involved constituents. In our case, the mixing of nanofillers into PVDF matrix is responsible to generate dislocations at the interface of 0.0 3.0x10 5 6.0x10 5 9.0x10 5   increasing frequency and wt.% of nanofillers. The values of these parameters are sharply decreased at low frequency but there is a smaller change with increasing frequency (inset plot), however, it seems to becomes frequency independent at high frequency. Such type of variation in these parameters is agreed well with the literature. Atiq et al. have [28] reported that the availability of more non-traped charge carriers are responsible to increase the conduction in the synthesized polymer-based NCs with increasing frequency. It means that the impedence of polymer-based NCs is decreased with increasing frequency. Batoo et al. [29] have demonstrates that the decreasing values of Z′ and Z″ with increasing frequency is responsible to increase the conduction of synthesized flexible sheets which is due to hopping of electron between localized ions which are increased with increasing frequency. Singh et al. [30] have reported that the Z′ and Z″ parameters become frequency independent in high frequency region which is agreed well with our results. However, a small change in these parameters with increasing frequency may be due to mixing The electrical modulus is used to explain transport of electrical charges to with in the dielectric medium.The complex modulus is inversely related to complex dielectric constant calculated by the relations: * = ′ + ′′ =1/ * M′ = ε′/(ε′ 2 + ε" 2 ) where M′ is the real part and M′′ is the imaginary part of complex electric modulus. Figure 8 shows the variation of complex electric modulus for all compositions (ZnO, ZnO+TiO2) with increasing frequency. The value of M′ shows a significant dispersion in lower frequency which increases. This asymptotic value of M′ depicts that the synthesized polymer-based NCs flexible sheets are capacitive in nature [33,34]. This increasing behavior shows the relaxation processes existing over extensive range of frequencies. addition of various wt.% of nanofillers and hence responsible to strengthen the polymer-based NCs flexible sheets as well as increases the dielectric permittivity [35]. Additionally, the value of complex modulus (below 10 kHz for M′ and up to 1000 Hz for M′′) of polymer-based NCs at low frequency region indicates the removal of elelctrode polarization [36,37].