Study of Optical Properties of Non Stoichiometric Nickel Oxide

In order to create non-stoichiometric nickel oxide, the precursor was thermally decomposed at various sintering temperatures up to 1100 °C. Structure of synthesis compound was characterization by X-ray diffraction method along with ritvield refinement. In UV-visible spectroscopy the band gap of the non-stoichiometric nickel oxide due to excitation of electrons can be obtained. The presence of Ni3+ ions in the oxide lattice shows charge transfer transition, with the consequent absorption in the visible region, which is mainly due to different ratio of Ni3+ and Ni2+ ions in the sample. These data were analysed, and it was discovered that by altering the decomposition temperature, the defects present in the non-stoichiometric get held up, causing neighbouring divalent nickel ions to become Ni3+ due to charge transfer process.


Introduction
Nickel oxide (NiO) is one of the most extensively studied transition metal oxide for the long period of time [1].It is one of the potential and ecological semiconductors having holes as charge carriers having band gap energy in the rang from 3.6to 4 eV (bulk form).The band structure of NiO is complex due to which its study is very interesting.At room temperature it shows conductivity less than 10 −13 S cm −1 hence it is Mott-Hubbard insulator in character.The small conductutivty is attributed to hopping of charge carriers associated with Ni 2+ vacancies [2].NiO has been considered as efficient material for various applications, such as, chromic window [3], fuel cells [4], batteries [5], sensors [6,7], auto emission catalyst [8], etc. Insulating behavior of nickel oxide can be explained by numerous reasons but the conductivity of nickel oxide can be altered by Creating Ni vacancies by substitution at nickel side.Hence the composition ratio of Ni and O present in NiO changes by which NiO becomes nonstoichiometric.Due to this metal deficiency or excess in oxygen level of non-stoichiometric nickel oxide various properties including optical property changes.The band gape of the samples changes as the stoichiometry changes [9].

Experimental Procedure
Nickel nitrate hexa-hydrate [Ni(NO3)2.6H2O]heated in the muffle furnace at different temperature [10,11].The precursor decomposed to produce nickel oxide of different stoichiometries [12].This decomposition takes place in presence of air for 3 hours at 400 o C and 700 o C. Hence two samples are obtained by heating precursor at 400 o C and 700 o C which in this paper now onward refers as NiO400 and NiO700.Bruker D8 Advance X-ray diffractometer is used for x-ray diffraction analysis, this instrument uses Cu Kα radiation (0.154 nm) in the angle range 10 o -90 o .The optical band gap of non-stoichiometric nickel oxide was deduced from transmittance and reflectance spectra using a JASCO, V-670 UV-visible spectrophotometer in the range 300-800 nm.

Result and Analysis
To identify the crystalline structure of as prepared samples x-ray diffraction analysis was conducted.This method is useful in the sense that it provides the information about purity and crystalline nature of the crystals.Figure 1(a) show the XRD data of NiO samples prepared at 400 and 700 o C. We find different peaks for these samples at 2θ in the range of 10 o -90 o .These Peaks shows similarity with the JCPDF File 47-1049.Which indicates the nickel oxide samples are crystalline in nature and with the help of Scherrer formula the crystal size was estimated which is given in Table 1.These Bragg's reflection peaks are corroborates with analytical analysis data and the structural data, shoes that samples obtained by thermal decomposition of nickel nitrate hexa-hydrates are FCC phase having lattice constant a of cubic unit cell, 0.4177 nm.The samples are poly-nuclear coordination compounds shows no deviation within the limit of accuracy of structural and analytical method.Nickel oxide prepared from this method is stable phase because no other peaks were identified in the graph.The crystallite size of nickel oxide samples can be determined by applying the Sherrer formula: L = kλ/βcos(θ), where L is the crystallite size, k is the Sherrer constant, which is typically set at 0.89, λ is the wavelength of the x-ray radiation (0.154056 nm for CuKα), and β is the full width at half maximum (FWHM) of diffraction peak measured at 2 θ.From Table 2, in which crystallite size of nickel oxide The X-ray diffraction graph and its fitted curves by Rietveld refinement matches and their data is given in Table 2. Our previous study reveals that both this samples contain different excess oxygen [13].
As Nickel oxide is a semiconductor which have very high band gap, and it behavior is almost as insulator.While exposing the samples to UV-visible spectroscopy it shows absorption in the ultra violet region and found no absorption in the visible region.Ultraviolet (UV) radiation get absorption by the nonstoichiometric nickel oxide samples and the excitation of electrons takes place, from lower to higher energy levels.Specific amount of ultra violet light will be required for transitions of NiO atoms from one energy level to another energy level, because these energy levels are quantized.Hence small amount of light will be absorbed due to which band gap of the nonstoichiometric nickel oxide can be obtained.Following the absorption in visible range, the charge transfer transitions takes place due to presence of Ni 3+ ions in the oxide lattice [14].This is mainly due to stoichiometry of the samples and the presence of Ni 3+ ions and Ni 2+ ions in different ratio.Another reason may be due to presence of adjacent divalent nickel ions become Ni 3+ due to charge transfer process caused by presence of nickel vacancy.

Conclusion
By using the thermal decomposition approach, non-stoichiometric nickel oxide with varying amounts of oxygen can be created.This form of nickel oxide, which occurs naturally as a crystalline cubic phase, has been shown to have many applications.The shift in the optical band gap reveals that when the oxygen content changes due to the presence of neighbouring divalent nickel ions, the charge transfer mechanism causes the nickel ions to become Ni 3+ .
samples are given, it is clear that as the temperature of sample preparation increases the crystallite size of nickel oxide samples also inctresses.Full-Prof software package [http://www-llb.cea.fr/fullweb/] is used for Rietveld refinements for both the samples.Plotted are the experimental data (circles), the estimated pattern (continuous line), and the difference between them (continuous line at the bottom of the figure).In Fig.1(b), the positions of the Bragg reflections are shown by vertical bars for each sample.

Figure 2 (
Figure 2(a).Absorbance of nonstoichiometric Ni1-δO for NiO400.The satellite shows the Tauc plot to calculate the band gap of both samples.

Figure 2 (
Figure 2(b).Absorbance of nonstoichiometric Ni1-δO for NiO700.The satellite shows the Tauc plot to calculate the band gap of both samples.

TABLE 1 .
Shows the crystallographic data using analytical analysis for NiO samples.

TABLE 2 .
Rietveld parameters of different NiO samples.