Monodispersed triangular Ni nanoplates were successfully synthesized based on a facile thermal decomposition method. The Ni nanoplates have an average edge length of 15.4 nm, and their thickness is about 6 nm. Owing to high anisotropic structures, the Ni nanoplates exhibit typical ferromagnetic behaviors at room temperature. The blocking temperatures of the Ni nanoplates are over 400 and 226 K when the applied field is 100 and 500 Oe, respectively. Compared with bulk Ni, the Ni nanoplates exhibit significant increase in magnetic anisotropy due to the presence of shape anisotropy and the reduction in particle size. These nickel nanoplates are expected to bring new opportunities in application of magnetic storage and catalysis.

For the first time, single crystalline Ni nanosheets have been successfully synthesized with the aid of iron species. The as-prepared nanosheets are mainly triangular and hexagonal in shape, with edge lengths ranging from several tens to several hundreds of nanometres. The exposed sheet planes are assigned to be (111) planes of a face-centred cubic nickel crystal. The well defined geometry enhances the anisotropic energy of Ni nanosheets, and therefore increases its blocking temperature (TB) to room temperature. Notably, the coercive force of the Ni nanosheets is 172 Oe at 300 K, which is significantly higher than that of the bulk one (ca. 0.7 Oe at room temperature). A possible mechanism is proposed to explain the formation of the thermodynamically unfavorable morphology of nanosheets. We suggest that crystal twinning, which is formed by etching of the introduced iron species with oleic acid, lowers the system energy, and leads to the growth of these Ni nanosheets.

A solution phase method has been used to synthesize triangular and hexagonal Ni nanosheets with different edge lengths by controlling the reaction kinetics. This procedure is realized by introducing Fe(CO)5 into the reaction system to slow the formation rate of Ni(0). The introduced Fe(CO)5 exists as Fe(III) ions in the solution, which could oxidize Ni(0) back to Ni(II). By controlling the nucleation density, the sheet edge lengths could be changed from 19 nm to several hundreds of nanometres. The Ni nanosheets exhibit the transition from superparamagnetism to ferromagnetism with increasing sheet edge lengths. Their blocking temperature decreases with applied field and increasing sheet edge lengths. The Ni nanosheets also exhibit a surface plasmon resonance (SPR) feature, which is quite different from that of the Ni nanoparticles.