Abstract
In this study, the synthesis of nickel oxide nanostructure by hydrothermal route is presented along with its thermal and structural characterizations. Thermogravimetric (TG) measurements were performed to calculate the activation energy of phase transition from beta nickel hydroxide with hexagonal crystal structure to cubic nickel oxide nanostructure which found as 84 +/- 5 kJ/mol. Structural evolutions were investigated by performing in situ X-ray diffraction measurements in the temperature range between 573 K and 1023 K. Lattice constant of the cubic NiO phase found to decrease from 4.1825 to 4.1758 angstrom with temperature indicating lattice contraction due to lattice defects and oxygen vacancies whereas crystallite size calculated from X-ray diffraction peaks showed an increase from 9 nm to 40 nm. In addition, the surface area of the NiO nanostructure found to decrease from 98.28 to 2.01 m(2)/g as the morphology changed from nanosheet to nanoparticles. Both strain and dislocation density decreased with the increase of temperature indicating an improvement of the degree of crystallinity and reduces the content of the lattice defects, oxygen vacancies, dislocation and lattice strain with temperature. Hence, the densification of the NiO nanostructure improved as indicated by the increase of density from 6.78 to 6.82 g/cm(3). Grain growth study of NiO nanostructure exhibited an ideal grain growth with growth exponent n = 0.5 in the early stages followed by abnormal growth (n = 0.3) as T > 837 K that attributed to the decrease of the interfacial energy due to further grain growth.