Dielectric and Optical Band Gap Studies of Nanostructured Manganese Nickel Oxide and Cobalt Nickel Oxide

Nano oxides of Manganese Nickel and Cobalt Nickel were synthesized by chemical coprecipitation method from the reaction of respective metal sulfides of manganese, nickel, cobalt and sodium carbonate using ethylene diamene tetra acetic acid as an effective capping agent. The carbonate precursors were heated at different temperatures so as to form their oxides. Through the X-ray line broadening technique, the mean particle sizes were calculated at different temperatures. The elastic micro strains versus particle size variations were thoroughly studied. The metal oxide formations of Manganese Nickel and Cobalt Nickel were confirmed with the help of FTIR spectra. EDX spectra of Manganese Nickel oxide and Cobalt Nickel oxide provide chemical composition of the samples. The crystallite shapes were studied using the scanning electron microscopy images. The optical direct band gap values of both Manganese Nickel and Cobalt Nickel oxides were calculated using Tauc’s relation. keywords: Nanoparticles, SEM, EDX, Optical band gap, Dielectric properties


INTRODUCTION
Oxides of cobalt are stable and are available in abundance which are cheaper than other noble metals.In the recent past, oxide nanomaterials have been drawing wide attention due to their comparatively excellent electrical, optical [1][2] or magnetic 3 properties.Generally nanoparticles have large surface to volume ratio.So their properties such as electrical, optical, etc. can be tuned by engineering size, morphology or composition.Nano oxides of Manganese Nickel and Cobalt Nickel with large surface area can be used in gas sensors.These nano oxide systems may have entirely different properties compared to their parent bulk materials 4 .The ultra fine oxide nanoparticles exhibit unique UV absorbing ability, excellent stability at elevated temperatures, very high hardness and reactivity as catalyst [5][6] .Manganese-Nickel oxide and Cobalt-Nickel oxide systems can be considered as potential candidates for electrodes in batteries, in super capacitor, in sensors, switches etc.

MATERIALS AND METHODS
The nano particles of both Manganese-Nickel oxide (MnNiO) and Cobalt-Nickel oxide (CoNiO) were synthesized by arrested precipitation from AR grade 0.4 M cobalt sulphate, 0.4 M nickel sulphate, 0.4 M manganese sulphate and 0.6 M sodium carbonate using ethylene diamene tetra acetic acid as an effective capping agent.The carbonate precipitates so formed were separated from the reaction combination and were washed with distilled water and ethanol to remove all impurities.The dried precipitates at room temperature were thoroughly grounded using an agate mortar to obtain their metal carbonate precursor in the form of fine powder.On heating to sufficient temperatures (400, 600 and 800 ºC), these carbonate precursor decomposes to their corresponding metal oxides.

Characterization
XRD is one of the ideal techniques used for the determination of crystallite size of the nano powders.Based on the broadening of diffraction peaks, a few techniques involving Scherrer equation, Integral breadth analysis or Hall-Williamson approach and Fourier method of Warren-Averbach etc. were developed [7][8][9] .Warren and Averbach proposed the first theory related with the broadening of diffraction peaks.XRD studies were done on XPERT-PRO powder diffractometer with Cu-K a radiation in the 2θ range 10° to 70° at 30mA, 40 kV.The surface morphological studies of the powder samples were evaluated by a scanning electron microscope.The energy dispersive analyses of X-rays were carried out on the nano samples to ascertain its precise composition.The ultra violet spectroscopic studies were carried out using Shimadzu UV-2550 UV visible spectrophotometer.

XRD Studies
The nano crystallanity of the powder samples are verified using XRD analysis.1A and 1B respectively.On analyzing the table, it is confirmed that when the temperature increases, the particle size also increases.So temperature of the reaction can be considered as one of the prime parameters on the crystallites size 11 .The reason for increasing   The elastic strain of MnNiO and CoNiO are found out using the values of full width at half maximum of XRD lines 13 .From the table 1A and 1B, it is confirmed that, as the nano particle size increases elastic strain decreases.This strain is one of the major reasons behind the broadening of the XRD peak of the nanomaterials.

Microstructural studies
For the surface morphological studies, the nano powders heated at 600 ºC were transferred in to the SEM chamber.SEM images along with EDX of both MnNiO and CoNiO nanoparticles are shown in the Fig. 2A, 2B, 3A and 3B respectively.The nanoparticles are found to be almost spherical with some agglomerations.

FTIR Studies
The FTIR spectra of both MnNiO and CoNiO are shown in Fig. 4A and 4B respectively.The absor ption bands found in the regions 3420 cm -1 for MnNiO and 3430 cm -1 for CoNiO are broad and are due to the presence of co-ordinated or entrapped water in the samples.The bands in the regions 1640 and 1630 cm -1 for both MnNiO and CoNiO correspond to carboxylate ions [14][15] .There are some bands around 600cm -1 which might be due to the bending modes of the metal oxides in the nanoparticles.

Uv spectral studies
The ultra violet spectra of both MnNiO and CoNiO sintered at 400, 600 and 800 ºC are taken in the wavelength range 200 to 800 nm are shown in fig 5A and 5B respectively.The optical band gaps of the nanoparticles are found out using the UV spectra.
On analyzing the UV spectra, there is a decrease in absorbance with increase in wavelength which can be considered as an indication of the presence of optical band gaps in the materials.This decrease corresponds to excitation of surface plasmons in the nano composite.The Tauc's relation connects energy band of the material and the absorption coefficient by the formula ahυ = A(hυ -Eg) n , where A is a constant, hυ is the photon energy, Eg is the band gap energy; for an allowed direct transition the value of n is 1/2.(ahυ) 2 versus hυ graphs in Fig 6A and 6B gives the of direct band gap values 16 of MnNiO and CoNiO sintered at 600 ºC.
When the sintering temperature increases, the nano particle size increases.However the band gap decreases.The reason for decreasing band gap with increase in sintering temperature may be due to the surface defects on the nanocomposite.So it is concluded that the band gap has an

Dielectric studies
To study the dielectric constant values of both MnNiO and CoNiO, the nanopowders were first pelletized and then studied about the variations of dielectric constant with frequency of the applied field.Fig 7A and 7B represents the dielectric constant versus frequency graphs of both MnNiO and CoNiO at 600 ºC.The dielectric constant decreases very fast in the low frequency region but after that it reaches a constant value which shows that it is independent of frequency at high frequency region.The dielectric nature of the nanoparticles is mainly due to space charge and rotational polarizations 17 .Nanoparticles possess a large number of interfaces and the defects in these interfaces can cause a change of positive as well as negative space charge distribution.In the presence of an electric field these space charges can move and may be trapped by the defects which results in the formation of dipole moments.Interfaces found in nanoparticles contain several oxygen ion vacancies, and are equivalent to positive charges giving dipole moments.When exposed to an electric field, these dipoles can rotate, giving a resultant dipole moment in the direction of the field.These are the reasons for the high value of dielectric constant at low frequencies 18 .
CoNiO nanoparticles are found to be better dielectric material than MnNiO nanoparticles at all frequencies especially in the lower regime.

CONCLUSIONS
Nanoparticles of Manganese-Nickel Oxide and Cobalt-Nickel oxide were prepared by arrested precipitation.The X-ray diffractograms compared with JCPDS data confirms the presence of the cubic phases with prominent peaks at (311).From the XRD analysis, there is a direct dependence of sintering temperature and particle size.The nanoparticles are found to be almost spherical in shape with agglomerations to a certain extend.Optical absorption studies indicate that Manganese-Nickel Oxide and Cobalt-Nickel oxides have the direct band gaps which decrease with increase in temperatures.Variations in the dielectric constant with frequency of the applied field are thoroughly studied for both the samples so as to recommend CoNiO as a better dielectric material compared to MnNiO nanoparticles.
The nano particle sizes are calculated using Debye-Scherrer equation, d = 0.9l/bCosθ 10 , where b represents the full width at half maximum of XRD lines, l = 1.54060 [Å]. Figure 1A and 1B represents the XRD patterns of MnNiO and CoNiO sintered at 600 ºC.The most intense peaks are obtained from the (311) planes.The crystallite sizes of MnNiO and CoNiO at 400, 600 and 800 ºC using Debye-Scherrer equation are as recorded in Table