Wet chemical synthesis and magnetic properties of core-shell nanocolumns of Ni(OH)₂@Co(OH)₂ and their oxides

Ni(OH)₂@Co(OH)₂ core-shell nanocolumns have been synthesized by an easy wet chemical process. Characterizations reveal that the layer structure is formed by stacking of several hexagonal nanosheets along the [001] direction. Each sheet is found to be of good crystallinity according to high-resolution transmission electron microscopy, with Ni(OH)₂ in the central and Co(OH)₂ in the peripheral region. The diagonal length of the hexagon is about 700-900 nm and the height of the nanocolumn is about 140-200 nm, with each nanosheet of about 20 nm in thickness. Upon calcination, the Ni(OH)₂@Co(OH)₂ nanocolumns become a porous NiO@Co₃O₄ nanostructure. At low temperature, Ni(OH) ₂@Co(OH)₂ exhibits an antiferromagnetic (AFM) transition at T_N = 12 K. This is from Co(OH)₂ while the AFM transition of Ni(OH)₂, expected at 25 K, is not observed. A freezing temperature is also observed at T_F = 7.5 K most likely arising from the randomly oriented moments in the outermost surface of Co(OH)₂. After the calcination, the AFM transition of Co₃O₄ in the porous NiO@Co₃O₄ nanostructure is observed at 28 K, which is much reduced from the bulk value of T_N = 40 K, probably arising from the finite size effect. The characteristic dimension with the porosity is thus determined as about 8 nm.