Effect of the oxygen content in the reaction environment on size and shape of CoFe2O4 nanoparticles: morphological analysis by aspect maps

Advanced synthesis approaches are necessary to achieve a strict control of the structural, morphological and chemical properties of nanomaterials, being at the basis of the reproducible manipulation of their physical behavior. In fact, any advanced application of nanoparticles (NPs) system will also rely on the achievement of such control, especially for magnetic nanoparticles that are complex physical objects, whose properties greatly differ from their parent massive material, being particularly sensitive to the particle size.[1] Within this context, the synthesis of spinel ferrite nanoparticles (MeFe 2 O 4 , Me = Fe 2+ , Co 2+ , Ni 2+ , Zn 2+ , ...) with controlled morpho-structural features represents an important issue due to the strong interest from both a fundamental and a technological point of view (e.g., MRI, hyperthermia, drug delivery, catalysis). The present study investigates the effect of residual oxygen dissolved in the reaction environment on the reaction kinetics and the morphological properties of CoFe 2 O 4 nanoparticles.[2] The amount of residual oxygen (RO) was controlled by degassing the reaction environment at a different extent and monitoring the values of total residual pressure. It will be demonstrated how the control of RO is a powerful tool to tune the size of nanoparticles up to ~19 nm, while retaining a narrow size distribution. Cobalt ferrite particles represent a model system for this study; furthermore, they are of interest in several fields, e.g., CoFe 2 O 4 NPs in the 19-25 nm size have attracted attention for the design of new exchange coupled nanocomposites for applications as permanent magnets.[3,4] Moreover, the specific attention on obtaining a narrow size distribution emerges from applications that demand the switching of the magnetization in a very small range of magnetic applied field (e.g., magnetic hyperthermia [5]), which can be obtained with a sharp size distribution.