Transition Metal (Fe, Co, Ni) Oxide Micro/Nanostructures: Phase, Morphology, and Decomposition of Precursors.

Transition metal oxides nano/micro structures including α-Fe₂O₃, Fe₃O₄, Co3O₄ and NiO were fabricated through controlling the decomposition of corresponding precursors. It is found that the morphology and phase composition of iron oxide depended strongly on heat-treatment approaches and conditions of the precursor. Furthermore, the iron oxide precursor created from FeCl₂, FeCl₃ and N₂H₄ in N,N-dimethylformamide solution with polyvinyl pyrrolidone via a hydrothermal synthesis was an unknown phase according to the X-ray diffraction (XRD) data base. Fe₃O4 nanoparticles (NPs) with several nanometers were obtained when the rod precursor was dissolved in water at room temperature, In contrast, when the precursor was calcined in an air atmosphere, porous α-Fe₂O₃ rods were acquired. Interestingly, the rod precursor can be transformed into perfect Fe₃O₄ rhombic dodecahedrons in the case of keeping the solvothermal reaction for a long time. For comparison, cobalt and nickel oxide precursors were prepared. After calcination, porous rod-like Co₃O₄ crystals comprised of tiny particles and flower-like NiO hierarchical structures comprised of porous sheets were obtained.