Abstract
A thermal decomposition route with different sintering temperatures was employed to prepare non-stoichiometric nickel oxide (Ni1−δO) from Ni(NO3)2·6H2O as a precursor. The non-stoichiometry of samples was then studied chemically by iodometric titration, wherein the concentration of Ni3+ determined by chemical analysis, which is increasing with increasing excess of oxygen or reducing the sintering temperature from the stoichiometric NiO; it decreases as sintering temperature increases. These results were corroborated by the excess oxygen obtained from the thermo-gravimetric analysis (TGA). X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) techniques indicate the crystalline nature, Ni–O bond vibrations and cubic structural phase of Ni1−δO. The change in oxidation state of nickel from Ni3+ to Ni2+ were seen in the X-ray photoelectron spectroscopy (XPS) analysis and found to be completely saturated in Ni2+ as the sintering temperature reaches 700 °C. This analysis accounts for the implication of non-stoichiometric on the magnetization data, which indicate a shift in antiferromagnetic ordering temperature (TN) due to associated increased magnetic disorder. A sharp transition in the specific heat capacity at TN and a shift towards lower temperature are also evidenced with respect to the non-stoichiometry of the system.
Highlights
As an antiferromagnetic[1] and Mott–Hubbard insulator,[2] stoichiometric nickel oxide (NiO) has been the most exhaustively investigated transition metal oxide
The unit cell volume decreases as the excess oxygen of the sample decreases X-ray diffraction (XRD) patterns show diffraction peaks associated only to NiO, indicating that nickel–nitrate hexahydrate has been transformed mostly into NiO
Nickel oxide samples of different stoichiometry were prepared by thermal route method and samples were thoroughly characterized by XRD indexed by full-proof re nement
Summary
Depending on the conditions of preparation, the sintering temperature, in particular, NiO samples of various surface areas, color, and degree of non-stoichiometry can be prepared. Present study, we have investigated the effect of oxygen concentration in nickel oxide prepared by the thermal decomposition method. It is found that the amount of excess oxygen or oxidation state of nickel in nickel oxide is closely related to synthesis temperature. Fourier transforms infrared (FTIR) spectroscopy is used to analyze the bonding of oxygen with metal ions as well as stoichiometry of the prepared samples. Using X-ray Photoelectron Spectroscopy (XPS) as a surface analytical method, binding information, chemical nature and valence states with compositional changes in the samples were investigated. Temperature dependence of magnetic susceptibility (c) and heat capacity (Cp) was studied to analyze the effect of non-stoichiometry on transition temperature in Ni1ÀdO samples
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