Cobalt Ferrite as a Novel Catalyst for Synthesizing Co3Fe7@BN Microsphere Composites

The cobalt ferrite (CoFe₂O₄) nanocatalyst, with an average diameter of 25 nm, was synthesized via an optimized aqueous coprecipitation method utilizing cobalt nitrate, iron nitrate, and ammonium carbonate as precursors. This catalyst demonstrates effective catalytic activity in facilitating the formation of boron nitride (BN) microspheres with a diameter of 3 μm, assembled from nanosheets. Systematic studies on the phase composition and microstructural evolution of BN products were conducted as functions of the annealing temperature and catalyst content, revealing the vapor–liquid–solid (V–L–S) mechanism as the dominant pathway for microsphere formation. Specifically, the high-temperature reaction between CoFe₂O₄ and B generates Co₃Fe₇/CoB and gaseous B₂O₂; subsequently, B₂O₂ along with nitrogen derived from NH₃ decomposition adsorbs onto the surface of Co–B droplets, initiating heterogeneous nucleation of BN crystals. Under appropriate conditions, including annealing temperature and catalyst content, the anisotropic diffusion rate of B–N species within the BN nuclei preferentially occurs along their (002) crystallographic planes, thereby driving the oriented precipitation of 2D nanosheets that spontaneously assemble into 3D BN microspheres via interfacial energy minimization. This work not only deciphers the fundamental growth mechanism but also establishes a versatile platform for morphology-engineered BN nanostructures.