Shape Evolution of 'Multibranched' Mn-Zn Ferrite Nanostructures with High Performance: A Transformation of Nanocrystals into Nanoclusters

摘要

Monodisperse magnetic Mn-Zn ferrite nanostructures with various morphologies have been successfully synthesized via high-temperature decomposition of metal acetylacetonate (acac) in the presence of oleic acid (OA) and oleyamine (OAm). In a classical crystal nucleation/growth process, differential stabilization of OA on specific crystal facets may alter relative crystal growth rates, resulting in the formation of zero-dimensional (0-D) spherical, cubical, and starlike nanocrystals (ca. 9, 11, 16 nm), respectively. Furthermore, shortening nucleation duration might bring a deficient nucleation and a rapid increase in monomer concentration, which accelerates the subsequent growth process of nanocrystals, leading to the formation of the starlike nanocrystals with larger size (ca. 19-23 nm). They are further oriented to assemble reciprocally, gradually forming initial three-dimensional (3-D) "branched" nanoclusters (ca. 30-40 nm) to minimize the magnetostatic energy, owing to their size-dependent magnetic dipolar interaction. In addition, the surface-defect-induced secondaiy growth of the "branched" nanoclusters may considerably improve their uniformity, accompanied by the size increase in the presence of the monomers, resulting in the final "multibranched" nanoclusters with formation of sharp or obtuse edges (ca. 45-50 nm). Our study reveals the transformation of 0-D nanocrystals to 3-D nanoclusters as well as the shape evolution mechanism, which provide a versatile synthetic strategy for shape-controlled nanostructure. The multibranched nanoclusters have the higher magnetization and magnetically induced heating efficiency in an alternating current magnetic field, which can be used as promising heating agents for biomedical application.