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.