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Nickel oxide nanoparticles (NiO NPs) were synthesized through template free sol–gel method. Synthesized particles were calcined at 300∘C, 500∘C and 700∘C for 2[Formula: see text]h and further the particles were characterized for its thermal, functional, structural, morphological and optical properties. The peak broadening analysis was undertaken in order to calculate the variation in size and strain components of the particles with the calcinations temperature. The Debye–Scherer, Williamson–Hall and Size-strain plot methods were employed in peak broadening analysis and were compared with TEM results. All the NiO NPs crystallite size and lattice strain value were found to be in good agreement with all the models. The TEM analysis revealed that the calcinations at 700∘C NiO NPs have sphere like structure of less than 20[Formula: see text]nm particles size value. The strong peak observed at 360.5[Formula: see text]nm bandgap emission relates to the bandgap of 3.44[Formula: see text]eV which is because of the near band edge emission.

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In most of the chemical methods, the as-prepared NPs have metal hydroxide form. The calcination temperature is very important to prepare the metal oxide from metal hydroxide. On the other hand, calcinations is a commonly used treatment method to increase the crystallinity and activity of the prepared NPs. Thus, in the present work, the chemical precipitation and sol-gel routes were employed to produce the Nickel hydroxide nanoparticles and analyse their functional, structural and thermal behavior of prepared NPs using FTIR, XRD and TGA/DTG. The formation of Ni(OH)2 is confirmed by FTIR analysis including both techniques. The crystallite size values are 15.55 ± 1.65 and 4.11 ± 1.37 nm for chemical precipitation and sol-gel respectively. The kinetic factors of thermal activation energy (Ea), order of decomposition (n), and frequency factor (Z) were calculated using Sharp–Wentworth, Freeman-Carroll, MacCallum-Tanner, Coats-Redfern, Horowitz–Metzger, and Broido. The thermo dynamical factors were identified from thermal activation energy. The Phadnis–Deshpande model is employed to identify the mechanism of solid state kinetic reaction. The thermal activation energy are 10.23 KJmol− 1 for both methods. Solid state kinetic model observe a nucleation and nuclei growth (Avrami-Erofeev nuclei growth) and 2D diffusion mechanism for chemical precipitation and Sol-gel respectively. For both procedures, a temperature of 300°C is regarded the minimum appropriate temperature for further calcinations. From the kinetic parameters, we may also conclude that the decomposition of Ni(OH)2 to NiO is thermally stable, slow reaction and spontaneous process at the reported temperature.