Assembly of non-crystalline cobalt particles into crystalline tricobalt tetroxide nanowires under an external magnetic field

One-dimensional (1D) tricobalt tetroxide (Co3O4) nanowires with an average diameter of 40 nm have been successfully prepared via a magnetic-field-induced hydrothermal route. The morphological evolution of the particles, the effects of external magnetic field and temperature have been investigated in detail. It is found that the mechanism by which these nanowires form is analogous to orient attachment. At the beginning of the reaction, under the induction of an external magnetic field, non-crystalline cobalt nanoparticles self-assemble to 1D nanochains and then crystallize under the hydrothermal environment, however, in the next stage, different from the common scenario in orient attachment, these nanochains are oxidized, leading to the formation of Co3O4 and the local diffusion of cobalt ions to intraparticle gaps, which further results in the extinction of nanochains and the formation of Co3O4 nanowires in the following crystallization process. The oxidation of cobalt is thought to facilitate the formation of nanowires. This method has inherited advantages of orient attachment and has strong potential applications in the synthesis of other compound nanowires.