Exploring the dielectric behavior of Co doped ZnO nanoparticles synthesized by wet chemical route using impedance spectroscopy
Introduction
Nanocrystalline oxide materials have drawn a great deal of attention due to their excellent properties such as large surface area, quantum confinement effect, good catalytic activity, etc. Wide band gap semiconducting nanostructures such as zinc oxide (ZnO), indium oxide (In2O3), tin oxide (SnO2) and doped oxides have been extensively investigated in recent years [1], [2], [3], [4], [5], [6], [7], [8]. Zinc Oxide is gaining more interest due to its fascinating properties. It is also an abundant, cost effective and non-toxic material as compared to others. ZnO is a II–VI semiconductor material with a wide band gap (3.37 eV) and the large exciton binding energy (60 meV) at room temperature [9]. ZnO nanoparticles have been used in a variety of applications such as UV absorption, antibacterial treatment [10], catalyst [11], photo catalyst [12] and additive in many industrial products. It is also used in the fabrication of solar cells [13], gas sensors [14], [15], luminescent materials [16], transparent conducting oxide [17], and coatings [18]. Different physical methods such as pulse laser deposition [19], [20], vapor phase transparent process [21] and chemical vapor deposition [22] have been developed for the preparation of nanostructures of ZnO. Gel combustion method is one of the most important wet chemical methods for the preparation of metal oxide nanoparticles [23]. Physical properties, such as electrical conductivity, piezoelectricity and defect structure, are greatly tailored by the amount of dopant in the parent system [24]. There are different methods to modify the dielectric and other properties of metal oxide nanostructures. It has also been reported in literature that several additives (Fe, Cu, Co, Cr, Al, Mn, Mg, P, S) can lead to an increase in the surface area of ZnO nanostructures. The doped impurity stabilizes the ZnO surface and promotes a decrease in grain size. It has been found that Fe+3 doped ZnO nanoparticles with lower crystallinity and high surface area have the higher catalytic activity and sensor signal than both pure ZnO and Fe2O3 system [25]. In many cases metal oxides such as SnO2, TiO2, CuO, CeO2, Fe2O3, ZnFe2O4 were often used as dopants [26]. There are many papers in the literature reporting the change in optical and magnetic properties of ZnO nanoparticles with the doping of transition metal ions. Arshad et al. have reported the increase in optical band gap of Co doped ZnO [27]. Faheem et al. showed room temperature ferromagnetism in Co doped ZnO [28]. But the dielectric behavior of ZnO is less explored area compared to its optical and magnetic properties. Dielectric properties are developed due to the defects of zinc excess at the interstitial position and the lack of oxidation. As pure ZnO is sensitive to oxidation, absorption of oxygen (O2) is inclined to decrease its dielectric properties. In cases where ZnO is doped with different dopants, the dielectric properties are changed by extrinsic defects [29]. In the present investigation we have reported the dielectric and electrical properties such as dielectric constant, ac conductivity, loss tangent and impedance of pure and Co doped ZnO nanoparticles.
Section snippets
Materials and methods
ZnCl2⋅2H2O and CoCl2⋅6H2O were used as starting materials for the synthesis of Zn1−xCoxO series. During synthesis, citric acid was added to 100 ml of distilled water with magnetic stirring, until pH becomes 1.5. Calculated amounts according to stoichiometric ratio of ZnCl2⋅2H2O and CoCl2⋅6H2O with (x = 0, 0.01, 0.03 and 0.05) were added to the solution and dissolved. Then 10 ml of ethylene glycol was added to the above solution and stirred for 20 min. Ample amount of aqueous ammonia was added drop
Structural properties
The XRD spectra of the pure and Co-doped ZnO samples annealed at 400 °C are shown in Fig. 1. The XRD spectra of each sample revealed the wurtzite structure of ZnO which were confirmed from the ICDD card No. 80-0075. No extra impurity peak was observed in the XRD pattern confirming the single phase sample formation. The crystallite size was calculated using Scherrer’s formula [30]
where λ is the wavelength of X-ray radiation, β is the full width at half maximum (FWHM) of the peaks at
Conclusion
Co-doped ZnO nanoparticles have been successfully synthesized using wet chemical route. The XRD spectra exhibit the wurtzite structure of all the samples and no impurity phase has been observed in XRD. Crystallanity, crystallite size and lattice constant were observed to decrease with cobalt doping. Cole–Cole plots demonstrate the existence of single semicircle for doped samples, suggesting the dominance of grain boundary resistance over grain resistance. Furthermore, the grain boundary
Acknowledgements
Authors are grateful to the Council of Science & Technology, Govt. of UP, India for financial aid in the form of Center of Excellence in Materials Science (Nanomaterials). Mr. Mohd. Arshad is also thankful to CSIR, New Delhi for providing the financial support in the form of SRF.
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