Cobalt hydrate and doped binary Co0.9M0.1OOH (M = Ni, Mn, Fe) nanorings of 100–300 nm were fabricated in solution through a facile ambient oxidation method. A transformation from Co0.9Ni0.1(OH)2 nanodiscs to hollow Co0.9Ni0.1OOH nanorings was observed with prolonged reaction time. Core-shell nanodiscs have elemental segregation with a Co(OH)2 core and Ni(OH)2 shell. Co0.9Ni0.1OOH nanorings displayed a higher electrochemical capacitance than Mn and Fe doped nanorings materials or materials with disc-like geometries.

Octahedral Co3O4 nanoparticles were synthesised in an aqueous ammonia solution using hexagonal β-Co(OH)2 nanoplates as starting materials. Electron microscopy analysis indicates that the Co3O4 particles have diameters of 20-40 nm and adopt a well-crystallized cubic spinel structure. The octahedral habit was verified by high angle annular dark field imaging. High resolution electron microscopy results revealed that the long axis of the octahedral Co3O4 nanoparticles coincides with crystallographic <111> direction and the facets are the {111} planes. Magnetization measurements reveal antiferromagnetic ordering below 10 K, with a paramagnetic Curie temperature of 3 K and a paramagnetic susceptibility that is double that expected for high-spin Co2+. The results show that a substantial fraction of the B-site Co3+ in the nanoparticles is in a high-spin state.

The preparation and analysis of Ni doped Co1–x Ni x (OH)2 (x = 0.6–0.8) nanorings are reported as produced using core–shell structured Co0.9Ni0.1(OH)2 nanodiscs as a starting template material. The resulting structure and elemental distribution were analyzed to investigate the transformation from discs to rings. A representative example involving the doping of Cu in the Co–Cu hydrate system is also presented.

Topotactic transformation was one of the most frequently related processes in the synthesis process of cobalt oxide nanomaterials. Microstructural evolution studies of this process inform and enable the process of controlling the shape of the final products. One such topotactic transformation was investigated here, the transition from Co(OH)2 hexagonal nano platelets to Co3O4 octahedra by a hydrothermal method in an ammonia solution. Evolution of phase and microstructure was studied during different stages of the reaction to reveal the criteria which prompt this transformation. A phase transformation from Co(OH)2 to CoOOH and hence to Co3O4 was observed. High resolution transmission electron microscopy and image simulation were utilised to disclose the role of defects in the topotactic transformation. A high density of defects including edge dislocations and displacements was found on the surface of dried Co(OH)2 nano platelets. The coexistence of CoOOH domains embedded in the Co(OH)2 surface indicates the absence of the hydrogen bonds of the starting materials. The breaking of hydrogen bonds leads to bonding variation and the further creation of defects, finally prompting the transformation from Co(OH)2 to CoOOH. The breaking of hexagonally arranged hydrogen bonds creates seeds for the formation of Co3O4. DOI: http://dx.doi.org/10.5755/j01.ms.25.3.22005