Temperature-induced structural phase transformation in samples of Co3O4 and Co3-xNixO4 for CoO

Abstract

Undoped and Ni-doped cobalt oxide (Ni=4%) powders were prepared by hydrothermal synthesis and subsequently examined. The powders obtained were subjected to thermal annealing at temperatures ranging between 600 °C and 1000 °C. The experimental results indicate that the annealing affects the structural properties with the phase change Co3O4 to CoO. X-Ray diffraction (XRD) analysis reveals the presence of pure spinel phase up to 800 °C in powders obtained with and without the addition of nickel. From 900 °C, the CoO phase was observed. Rietveld refinement was conducted with main GOF values ​​between 1.09 and 1.26 and also x2 values ​​between 1.28 and 1.57, indicating a good correspondence between the simulated and the experimental pattern. In addition, SEM measurements evidenced that particle size increased with temperature; however, samples doped with Ni exhibited a slightly smaller size compared to pure samples. The synthesized powders showed shifted and broad bands Raman, with increasing temperature, mainly for doped samples. These changes in the position of Raman vibrational modes are related to defects and distortions in the crystal lattice of the material as a result of the increase in temperature and the addition of Ni. Furthermore, such changes may also indicate loss of crystallinity in the material, or the beginning of the phase change.