Experimental observations and nucleation and growth theory of polyhedral magnetic ferrite nanoparticles synthesized using an RF plasma torch

We have used transmission electron microscopy (TEM) to observe the morphologies of ferrite nanoparticles synthesized using a radio frequency plasma torch. These nanoparticles were faceted, having cuboctahedral polygonal growth forms with exclusively the {1 1 1} and {1 0 0} type faces. A model for the critical nucleus shapes and the growth forms of small cuboctahedral particles is proposed considering the Helmholtz free energies for nucleation and growth. The critical nucleus shape is predicted to be either a perfect cube or a perfect octahedron, while the equilibrium growth forms are the truncated shapes. High-resolution TEM observations of individual nanoparticles reveal that the smallest nanoparticles exhibit perfect octahedral shapes, while larger particles have truncated octahedral shapes. These observations are consistent with the model predictions. The morphology observations are also consistent with 0.95<γ100γ111<1.14, where γ1 0 0 and γ1 1 1 are the surface energies of the (1 0 0) and (1 1 1) surfaces, respectively.