Synthesis, Characterization, Dielectric, and Electrical Conductivity Studies of Zinc Oxide Nanoparticles

A co-precipitation technique is used to synthesize of ZnO nanoparticles. The synthesized ZnO nanoparticles were characterized by UV-visible spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), photoluminescence, Fourier-Transform Infrared Spectroscopy (FTIR), and X-ray diffraction (XRD). The size of the nanoparticles was wide-ranging from 1 nm to 100 nm, corresponding to the HR-TEM analysis. The photoluminescence study of ZnO nanoparticle shows emission in the UV region. The particle dimension of ZnO nanoparticle has also been studied through XRD. Dielectric spectroscopy of synthesized ZnO NPs pellet has been studied at a wide frequency range 10−1 to 105 Hz. The capacitance and dielectric permittivity of ZnO nanoparticles drop continuously with frequency as dipoles have less time to align in the field. Dielectric permittivity of ZnO pellets increase up to 5 mm thickness and subsequently drop, perhaps due to raise in resistivity. The dielectric loss of ZnO pellet has been examined as a function of frequency. The electrical conductivity of ZnO nanoparticles rise exponentially with frequency. Based on the dielectric studies, the dielectric permittivity and electrical conductivity of ZnO are highly depending on thickness and frequency range. The percolation threshold of ZnO pellets has been found between 4-5 mm thicknesses.


Introduction
The bulk of studies in this area of nanomaterial synthesis methodology are focused on dealing with their shape, size, and content.Every one of such elements plays crucial role in shaping the characteristics of materials that give rise to various technological applications [1].ZnO nanoparticles (ZnO NPs) are a versatile substance with exceptional chemical along with physical properties such as excellent chemical stability, higher electrical coupling coefficient, in addition to broad radiation absorption spectrum and excellent photo stability [2][3].We must alter the nanometer-scale surfaces of these materials to create goods with novel features [4].Based on its relatively small dimensions as well as highly specialized surface shape, synthesized metal/metal oxide nanostructures exhibit significantly better activity of the catalyst compared to the bulk state.Centrifugation can successfully separate them as a result of reactions mixture and they can be recycled with no effect any of their effectiveness.
Several noble metal also metal oxide nanostructures work catalytically well in the breakdown of dyes [5].ZnO is a semiconductor of the n-type with a band gap of 3.27 eV at room temperature along with activation binding energies of 60 meV.For the reason of their bulky surface-to-volume percentage, ZnO nanostructures offer appealing chemical and physical aspects.Zinc oxide nanoparticles surface needs to be changed in order to increase dispersion.Vapor-liquid-solid (VLS) synthesis is one method for producing ZnO nanostructures [6].The composition and structure of ZnO have a significant impact on its electrical characteristics [7].Furthermore, there has lately been extensive research into metal oxide nanoparticles such as ZnO, In2O3, SnO2, as well as dope oxides.ZnO is one of these metal oxide nanomaterials that is attracting greater interest since, compared to other materials, it is plentiful, inexpensive, as well as environmentally friendly [8].Among NPs fillers, metal oxides NPs are essential for improving the dependability of electrical insulation and achieving a compact design in electric power apparatuses through polymer nano dielectrics [9].Numerous businesses, including those in research, engineering, medicine, and other fields are using nanotechnology today.Nanoparticles are subatomic particles that have properties dependent on their size, quantum confinement, and surface plasma resonance.Researcher's focus during the last 20 years has been on metal oxide nanoparticles because of their distinct electrical, optical, mechanical, magnetic, also chemical capabilities [10].Due to their innovative optical, transport characteristics, semiconductor compounds have grabbed significant a lot of interest since over the last few decades and hold tremendous promise for a many applications in optics.In the near ultra violet and visible ranges, the wide band gap semiconductor ZnO exhibits great optical transparency and luminous characteristics [11].Although the nanoparticles were created using a variety of physicochemical techniques, the environment is however negatively impacted by them.Now a day, most academics focus their attention on eco-friendly methods to sustain the planet as a result of environmental worries [12].ZnO NPs were used to decorate the surface of bacterial nano wires in an effort to increase their electrical conductivity due to their distinct physical, electrical, and structural features [13].The exceptional ability of these methods to produce metal and metal oxide based NPs to serve as catalysts and aid in several industrial, electrical and physical processes is the driving force behind the growing interest in doing so.These methods' oxidationreduction processes which are extremely sensitive but also very particular, describe their formational mechanisms [14].Han et al. have investigated the ZnO NPs single crystal's transverse optical phonon mode and low frequency dielectric characteristics [15].Zinc oxide nanoparticle was synthesized via the co-precipitation technique in present investigation.The morphological properties of ZnO nanoparticle were studied using high-Resolution transmission electron microscopy (HR-TEM) scanning electron microscopy (SEM).Its optical characteristics were investigated with UV-Vis spectroscopy.FTIR were used to explore the functional groups attached to the surface of ZnO nanoparticles.Dielectric characteristics of ZnO pellets have been measured in form of capacitance, dielectric permittivity, electrical conductivity, percolation behaviour for use in various fields.

Synthesis of Zinc Oxide (ZnO) nanoparticles
ZnO nanoparticles were synthesized by a co-precipitation procedure using Zn(NO3)2 and NaOH as precursors.Throughout the experiment, a 0.1M aqueous solution of zinc nitrate (Zn(NO3)2.6H2O)was maintained during constantly stirring for one hour using a magnetic stirrer, and a 0.8M aqueous solution of sodium hydroxide was also prepared using the same method with one hour of stirring.After the zinc nitrate was completely dissolved, 0.8M NaOH aqueous solution was added drop by drop (slowly for 45 minutes) while constantly shaking at high speed.After adding all of the sodium hydroxide, the reaction was left to run for 2 hours.In this state, the beaker was sealed for 4 hours.The solution was permitted to rest overnight until the supernatant solution had been removed appropriately.Residual solution was centrifuged for 10 minutes for removing the precipitation.Precipitated ZnO NPs were rinsed three times with distilled water along with ethanol to remove impurities prior drying in an air environment at 60°C.Zn(OH)2 is completely converted into ZnO NPs after dry.The optical and nano structured substances characteristics of the produced ZnO nanoparticles were investigated [16].

Characterization Techniques
XRD spectrum shows the crystalline structure of the generated substance ZnO nanoparticle, whereas UV-Visible spectroscopy reveals optical properties of ZnO nanoparticles.A UV-Visible spectrophotometer was used to get the UV-Visible spectrum between 250 to 550 nm (Perkin Elmer, USA).At the surrounding temperature, the photoluminescence spectrum of zinc oxide nanoparticles has been studied with wavelengths of excitation ranging from 440 to 600 nm.FTIR spectroscopy was used to study the functional categories of zinc oxide nanoparticle pellets prepare with KBr.Zinc oxide spectrum were investigate in the 400-4000 cm -1 region.The morphology of ZnO was investigated using SEM and HR-TEM.

Dielectric Relaxation Spectra
The dielectric relaxation spectra (DRS) of ZnO NP pellets were measured using a Hi Tester LCR meter and a Hioki 3533, with silver foil acting as a blocking electrode.The item under investigation was round, with a diameter of 1 cm and a thickness of 2.0 mm.Using the connection, electrical conductivity (σ) was determine from dielectric data by following equation.σ AC =ωɛ0έtanδ (2) Where ω is 2πf (f is frequency), ℇ0 is vacuum permittivity and έ is dielectric permittivity or dielectric constant έ = Cp/C0 (3) The electrical conductivity (σ) was calculated by dielectric data [17].

Optical Properties
The optical absorption spectra of zinc oxide nanoparticles were recorded using UV-Visible spectrophotometer Model LT-2900 (Labtronics).The spectrophotometer's spectral bandwidth is 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 absorbances, and in the wavelength ranges from 200 to 600 nm.The UV-Vis absorption spectra of zinc oxide nanoparticles can be seen in Figure 1.Its absorption spectra have been measured between 250 and 550 nm.The absorbance peak at 400 nm corresponded to the typical band of zinc oxide nanoparticles in the spectra [18].

Photoluminescence Spectra
It is essential to note that the physical properties of semiconducting materials alter when their size reduces to the nanoscale, a phenomenon known as "quantum size effects."Photoluminescence shows that quantum confinement raises the band gap energy of ZnO.[19].
Figure 2 represents the photoluminescence spectrum of ZnO nano powder at room temperature.The wavelength range displays two emission peaks: one at 480 nm (UV region) due to close band gap excitonic emissions, and another at 550 nm due to the presence of single ionized oxygen vacancies.The atomic interactions of a photogenerated hole that an electron filling the oxygen vacancy causes the emission [20][21].Additionally, the scale indicates the powder's narrow size allocation of nanoparticles 4 with the luminescence peak full width half-maximum (FWHM) just a small number of nanometers wide [22].

Fourier-Transform Infrared Spectroscopy (FTIR)
FT-IR experiments were carried out to validate the bond structure and identify related functional groups of synthesized ZnO nanoparticles utilizing optimal conditions.Figure 3 shows the infrared absorption spectra of ZnO nanoparticles in the 3500-500 cm -1 wave number region.The band at 437.71 cm -1 corresponds to the Zn-O stretching mode of the ZnO lattice.The bands at 3434.61 cm -1 correspond to the O-H mode of vibration.C=O exhibits a significant asymmetric mode of vibration between 1635.48 and 1635.48 cm -1 .Symmetric stretching arises between 1510.34 and 1337.66 cm -1 due to the existence of C-O.There is also a C-O-C peak there [23].The presence of multiple hydroxyl group-related vibration modes in this example is most likely due to ambient moisture absorption by the pellets sample during FTIR analysis.

Scanning Electron Microscopy (SEM)
A scanning electron microscope is a category of electron microscope that uses a high-intensity electron beam to generate features on the surface.Scanning is also used to agree on the external morphology (structure) along with size of crystalline particles.SEM was used to study the variance in particle size within the samples.The sample is composed up of electrons that combine through atoms to generate signals that give data regarding the material's constituent as well as surface shape.As shown in Figure 4, a considerable number of ZnO nanoparticles have a spherical form, and agglomerates are also changed into bigger particles.The SEM photomicrograph revealed individual ZnO nanoparticles as well as various aggregates.At a magnification of 25.00 KX, observable morphologies ZnO NPs are expected to have spherical forms and aggregated nanoparticle states.In SEM , a focused beam is swept over a large sample, and pixel by pixel, backscattered or secondary electrons are counted to generate an image of the material's surface.Variances in the process of synthesis of ZnO nanoparticles could lead to variations in particle shape, size, and dispersion [24].

High Resolution Transmission Electron Microscopy (HR-TEM)
HR-TEM has been utilized to evaluate the shape and size of the produced NPs.The minuscule powder of ZnO NPs was disseminated in methanol over a carbon-coated copper grid, as well as ultra high resolution HR-TEM pictures were obtain at an increasing current of 200 kV. Figure 5 represents the HR-TEM study of ZnO NPs, which are spherical in shape as well as evenly scattered with no agglomeration.The particle size distributions in the figure found in the range of 20-100 nm.The physical parameters (diameter, dispersion and morphology) of the nanostructures and crystallinity have been seen in Figure 5 [25].ZnO NPs possessed a spherical morphology as desired by the relative quantity fractions used in the synthesis as well as an exceptionally size distribution has limits with diameters ranging 20-50 nm [26].

Dielectric Permittivity
The Dielectric permittivity (ϵ′) of a material reveals its electrical polarization.Figure 8 shows the variations in dielectric permittivity as a function of frequency for various thicknesses (1 mm to 5 mm) of synthesized ZnO nanoparticles.Dielectric permittivity decreases with frequency due to the charge orientation and rotation of dipole moments.The presence of dipoles, interfaces, electric, and ions in the compounds allows in an increased dielectric permittivity value at low frequencies.Figure also shows that dielectric permittivity increases with increasing thickness of the ZnO pellets up to 4 mm beyond which it might be related to the complex confrontation due to decrease in capacitance [31].

Electrical Conductivity
Electrical conductivity (σ) is a measurement of a material's electrical current carrying capacity, as seen in Figure 9.The change in electrical conductivity with frequency for changing thicknesses and rising frequency of ZnO NPs pellets can be observed from the figure.The electrical conductivity of ZnO nanoparticles rises constantly with rising thickness of ZnO pellets up to 4 mm, after which the improvement or increase is very negligible due to increase in resistance.The increase in electrical conductivity confirms the conductive nature of zinc oxide nanoparticles.This is most likely owing to the information that zinc oxide nanoparticles have a higher stability as a function of frequency [32][33][34].As highlighted in Figure 10, the fluctuation of electrical conductivity of ZnO pellets because of rising thickness can be divided into three zones.The following are the regions: The conductivity of zinc oxide pellet increases gradually with thickness due to a small amount of charge particle passing through the system without the formation of continue conductive path; (ii) percolation region: an abrupt rise in conductivity of ZnO NPs thus a continuous conductive path is formed; and (iii) saturation region: the minor effect or electrical conductivity decreases precisely because of the formation of agglomerates or non-continuous conductive path [36].According to the Figure 11, the percolation threshold occurs at around 4 mm thickness based on the dielectric data and electrical conductivity of ZnO nanoparticles.

Conclusions
ZnO nanoparticles were successfully synthesized by co-precipitation technique.UV-visible spectrum of ZnO has a maximum absorption peak of around 400 nm.Based on the sharp and strong peaks, ZnO is crystalline.At normal temperature, the photoluminescence spectra of ZnO nanoparticles show excitation wavelength at 480 nm.The spectrum exhibits two emission peaks, one at 550 nm (UV region) associated with close band gap excitonic emission with the other at 560 nm associated with the occurrence of single ions in spaces in oxygen.X-ray diffraction (XRD) higher peak values for diffraction suggest that it is showing crystalline nature of the nanomaterial.The SEM image indicates thin sheets aggregated as well as spherical in shape, and the HR-TEM picture reveals that the particle size of ZnO can range from 1-100 nanometers, due the contribution of oxygen-containing functional groups with several other spherical structures.Capacitance value decreases with frequency due to charge orientation and variations in the coefficient of the group dipole moments of ZnO NPs.The capacitance value shows separate plot with increasing thickness of ZnO pellet and the maximum value is continuous up to 4 mm thickness.Because of charge orientation and dipole rotation, dielectric permittivity decreases as a function frequency.Dielectric permittivity values of ZnO nanoparticles increase with thickness and decrease with frequency as like capacitance.Electrical conductivity of ZnO nanoparticles pellets rises exponentially, indicating higher alternating current conductivity.The percolation behaviour based on electrical conductivity values of ZnO nanoparticles have been observed for various thicknesses of ZnO pellets.At all frequencies, electrical conductivity grows exponentially until 4 mm thickness, indicating the presence of a percolation limit, before saturation at 5 mm thickness of ZnO pellet.The characteristics of ZnO at the nanoscale enhance its applications in industrial, integration with other nanomaterials, electrochemical and in composite materials.Specifically, ZnO can also be used as accelerator and filler in elastomer composites to enhance its curing and strength.Also ZnO nanoparticles can be used as conducting materials in electrical and thermal applications.Hence, the features and characterizations of ZnO nanoparticles may be used in a variety of engineering applications.

Figure 4 .
Figure 4. SEM image of ZnO nanoparticles Figure 5. TEM image of ZnO nanoparticles

Figure 7 .
Figure 7. Capacitance plots of ZnO nanoparticles pellets measured as a function of frequency.

Figure 8 .
Figure 8. Dielectric permittivity plots of ZnO measured as a function of frequency.4.9.Electrical ConductivityElectrical conductivity (σ) is a measurement of a material's electrical current carrying capacity, as seen in Figure9.The change in electrical conductivity with frequency for changing thicknesses and rising frequency of ZnO NPs pellets can be observed from the figure.The electrical conductivity of ZnO nanoparticles rises constantly with rising thickness of ZnO pellets up to 4 mm, after which the improvement or increase is very negligible due to increase in resistance.The increase in electrical conductivity confirms the conductive nature of zinc oxide nanoparticles.This is most likely owing to the information that zinc oxide nanoparticles have a higher stability as a function of frequency[32][33][34].

Figure 9 .
Figure 9. Electrical conductivity plots of ZnO measured as a function of frequency.4.10.PercolationFigure10represents the percolation behaviour of the electrical conductivity of zinc oxide nanoparticles at various thicknesses.At all frequencies (1, 10, 100 Hz and 1, 10, 100 kHz), electrical conductivity grows exponentially until 4 mm thickness, indicating the presence of a percolation limit, before decreasing at 5 mm thickness of ZnO pellet.It could be because the nanoparticles contact each other and an ongoing pathway is formed throughout the capacity of sample for electrons to pass through up to 4 mm thickness, but at 5 mm thickness, the nanoparticles contact every single different as well as agglomerates form, and persistent path is reduced by the volume of sample for the movement of electron[35].

Figure 10 .
Figure 10.Percolation behavior of ZnO nanoparticles pellets at different frequencies.

Figure 11 .
Figure 11.Electrical conductivity of ZnO pellet as a function of thickness at 1 KHz frequency.