STRUCTURAL AND FREQUENCY DEPENDENT DIELECTRIC PROPERTIES OF ZnO NANOPARTICLES AND PANI / ZnO NANOCOMPOSITE BY MICROWAVE-ASSISTED SOLUTION METHOD

C. Thenmozhi 1 , V. Manivannan 1 , E. Kumar 2* and S. Veera Rethina Murugan 3 . 1. PRIST University, Thanjavur. 2. Dept of Physics, School of Science, Tamil Nadu Open University, Chennai. 3. Dept of Physics, R.S. Govt. College, Thanjavur. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History


ISSN: 2320-5407
Int. J. Adv. Res. 4 (10), 572-578 573 synthesis [10] and thermal evaporation [11]. Since, microwave assisted synthesis is a rapid technique and also cost effective compared to other methods, this method is chosen for producing ZnO nanoparticles. Zinc oxide (ZnO) is a common material that is readily used in semiconductor fabrication especially in solar cells and gas sensors due to its special properties. Zinc oxide, a versatile semiconductor material has been attracting attention because of the commercial demand for optoelectronic devices operating at blue and ultraviolet regions [12].
Experimental:-Materials and Characterization Techniques:-All the chemicals involved in this microwave assisted method were used as received from the chemical suppliers without any further purification and processing. Aniline (99.5%), Zinc Nitrate Hexahydrate (ZnNO 3 .6H 2 O), Sodium Hydroxide (NaOH) and Ethylene Glycol were procured from E. Merck and Ammonium persulfate (98%) was purchased from Hi-media and used as received. All the chemicals were of analytical grade and solutions were prepared with double distilled water. Aniline monomer was distilled using cubic condenser for purification.
The prepared ZnO nanoparticles were characterized by X-Ray Diffraction technique (XRD) and Scanning Electron Microscope (SEM) to find the structure and morphology Crystallographic studies were carried out using a X-Ray diffractometer (Brucker D8 with Nickel filtered Cu -K radiation), in the scanning range of 2θ from10° -80° using Cu -K radiations of wavelength 1.5406Å. SEM images of the sample were recorded using the model HITACHI SEM, to study the morphology of the samples and their elemental analysis. HRTEM micrographs of the prepared samples were taken using the Model JEOL -J2000. FT-IR spectra of the sample were recorded in the range of 400 -4000cm -1 using Shimadzu 8400S FT-IR Spectrometer.

Synthesis of ZnO Nanoparticles -Microwave Assisted synthesis:-
Zinc Nitrate Hexahydrate (ZnNO 3 .6H 2 O) and Sodium hydroxide (NaOH) were taken in 1:4 molar ratio and dissolved completely in de-ionized water separately. Then the dissolved ZnNO 3 .6H 2 O was added with Ethylene Glycol. Further, NaOH solution was added drop wise into the above mixture under vigorous stirring. Then the prepared mixture solution was kept in the microwave oven (900 W, 2450 MHz) for about 30 minutes. Finally, the as-prepared sample was centrifuged several times in double distilled water, ethanol and dried at 150 ˚C for 24hours, resulting in the formation of ZnO nanoparticles.

Synthesis of PANI / ZnO Nanocomposites:-
To prepare Polyaniline / ZnO nanocomposite, Aniline was injected into 2M HCL containing different wt% of ZnO nanoparticles under ultrasonic action to reduce the aggregation of ZnO nanoparticles. After 12hrs, Ammonium Peroxydisulphate as an oxidant(APS), was dropped into the solution with constant stirring for 10 min. The polymerization was allowed to proceed for 3hrs at 30 0 C. Reaction mixture was filtered and washed with 2M HCL and de-ionized water and then dried at 90 0 C for 12hrs in vacuum. PANI / ZnO nanocomposite with fine tint green colour was obtained.

Results and Discussion:-X-ray Diffraction technique (XRD):-
The XRD pattern of pure ZnO nanoparticles is shown in Figure 1a. The XRD result shows that the sharp and the well-defined diffraction peaks formed at 33 0 and 58.7 0 confirms the formation of ZnO nanoparticles and also indicate the good crystallinity of synthesized material. The characteristics peaks of ZnO formed at 33 0 , 45.6 0 , 58.7 0 , 69 0 and 73 0 corresponds to the miller indices (0 0 2), (1 0 2), (1 1 0), (2 0 1) and (0 0 4) respectively, which can be indexed with hexagonal phase(JCPDS Card No. 80-0075) [13]. The crystalline size of ZnO nanoparticles was calculated using the value of FWHM from the most intense XRD peaks as around 45nm by using Scherrer formula(1) [14].
where D is the crystallite size, K is the shape factor(0.94),  is the wavelength of X-rays ( = 1.54059A 0 ),  is the full width at half maximum(FWHM) of the diffraction peaks and  is the angle of diffraction. The XRD pattern of PANI / ZnO nanocomposite ( Figure 2a) has a peak at 2θ = 25° belongs to PANI, representing the amorphous structure with low crystallinity. The XRD patterns of PANI/ZnO nanocomposite include the characteristic peaks of 574 both PANI and ZnO with the crystalline structure of hexagonal wurtzite, which confirms the formation of nanocomposite with lower crystallinity [15, 16a, b, c].  Figures 2a and 2b. The peak at low wave number region of 576 cm −1 is assigned to ZnO group [17,18]. The sharp peak at 1123cm -1 is due to C-H plane bending vibration [19]. The band appears at 1619 cm -1 may be attributed to C=C Stretching mode of quinoid ring [20]. The peak at 3278cm -1 is due to the absorption of water during the preparation of IR pellet [21].  (Figure 3b), it is observed that ZnO nanoparticles are surrounded by polyaniline matrix and hence it appears as agglomerated macromolecules.

Microstructural properties by HRTEM and SAED pattern:-
The morphology and particle size of Zinc Oxide nanoparticles were observed using HRTEM micrograph (Figure. 4a). These particles with particle size of 50nm were observed. The particle size of ZnO nanoparticles was observed as 50nm and it is same for PANI / ZnO nanocomposites. The presence of rod-like shaped PANI / ZnO nanocomposites was observed from HRTEM micrograph. The particle size observed from HRTEM micrograph matches with the particles size calculated from the XRD investigation. These results confirm the formation of crystalline Zinc Oxide nanoparticles. The corresponding SAED pattern of ZnO nanoparticles is shown in Figure 4b. SAED pattern provides rings made up of bright spots, denotes the crystalline nature of the prepared ZnO nano particles. The dielectric properties of ZnO Nanoparticles can be explained as a function of frequency of the applied electric field and temperature. Figure 5a and 5b. shows the variation of ac conductivity with varying frequency for ZnO nanoparticles and PANI / ZnO at different temperatures. The ac conductivity of ZnO Nanoparticles was calculated using the formula, ζ ac = ε 0 ε r ω tanδ where ε 0 is the permittivity of free space (8.85 × 10 −12 Farad/metre), ε r is the dielectric constant, ω(2πf) is the angular frequency and tanδ is the loss factor. It is observed from the figure that the ac conductivity remains almost constant upto 1 KHz. Above 1 KHz, it increases gradually upto 40 KHz and then increases rapidly from 40 -620 KHz. As T increases, ζ ac decreases. The result revealed that ZnO nanoparticles shows frequency dependence behaviour and the temperature independence behaviour of conductivity. The ε r decreases faster at lower frequency and slowly at higher frequency. The frequency dependence of electrical properties shows that the dielectric constant ε r decreases due to the increase of ac conductivity with increasing frequency. The rapid decrease of ε r in the low frequency region was due to space-charge contribution.

Conclusion:-
ZnO Nanoparticles were prepared using a microwave assisted hydrothermal method. The XRD analysis confired that pure ZnO nanoparticles were formed in this method. Crystallite size for ZnO nanoparticles synthesized in this method was found to be 50nm. The results obtained from TEM studies exactly matches with that of XRD results, which confirmed the formation of ZnO nanoparticles. FT-IR analysis also confirmed the formation of Zinc Oxide nanoparticles. XRD and FTIR of PANI / ZnO nanocomposite revealed that PANI undergoes interaction with ZnO Nanoparticles which are embedded in polymer matrix. SEM and TEM micrograph shows the uniform distribution of ZnO Nanoparticles in PANI / ZnO nanocomposite. Dielectric studies of ZnO nanoparticles and PANI / ZnO nanocomposite shows the frequency dependence dielectric behavior of ZnO nanoparticles. Results reveals that the dielectric constant decrease with the increasing frequency whereas the a.c conductivity of the sample increases with the increase in frequency. The decrease of dielectric constant ε r was due to the increase of ac conductivity with increasing frequency. The rapid decrease of the dielectric constant(ε r ) in the low frequency region was attributed to space-charge contribution.