In this work, we report a very simple template-free hydrothermal approach to prepare hollow Ni(OH)₂ and NiO microspheres. The hollow Ni(OH)₂ microspheres were prepared via the hydrothermal reaction of NiCl₂ with aqueous ammonia at 140 ℃ for 12 h. Subsequently, the similar microstructured NiO hollow microspheres were obtained by calcining the above precursor at 400 ℃ for 2 h. The as-synthesized products were characterized by means of powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the hollow Ni(OH)₂ microspheres with the diameters of 3～4 μm are constructed from 2-dimensional Ni(OH)₂ nanosheets with the hexagonal structure and dimensions of 1.1～1.3 μm. The hollow NiO microspheres are co-constructed from NiO nanosheets and porous nanosheets with the cubic structure. Magnetic properties of the hollow NiO microspheres have been detected by a vibrating sample magnetometer at room temperature. It was found that the NiO microspheres exhibit weak ferromagnetism. The coercive force and remanent magnetization are 583 Oe and 0.213 emu/g respectively. The role of the ammonia in the growth and self-assembly of Ni(OH)₂ nanosheets was investigated, and a possible mechanism was also proposed to account for the formation and conversion of Ni(OH)₂ nanosheet-based hollow microspheres into NiO hollow microspheres.

Key words: NiO, Ni(OH)₂, hollow microsphere, hydrothermal route, ferromagnetism