Morphological Characterization by HRTEM and STEM of Fe3O4 hollow nano-spheres

A IBARRA; G F GOYA; J ARBIOL; ENIO LIMA JR.; H R RECHENBERG; J M VARGAS; R D ZYSLER; M RICARDO IBARRA

Aachen, Germany

Simposio; EMC 2008 14th European Microscopy Congress; 2008

Morphology, surface and finite size effects in magnetic nanoparticles have been the subject of growing interest in recent years from both experimental and theoretical point of view. The magnetic properties are strongly associated with the morphological and structural homogeneity of the nanoparticles. Interparticle interactions also play an important role in the magnetic behaviour of an ensemble of nanoparticles, which differs from that of non-interacting systems. The aim of this work is the characterization, by means of transmission electron microscopy (TEM), of the morphology and interparticle interactions of Fe3O4 nanoparticles prepared by chemical route in order to understand the magnetic behaviour observed in the reported samples. TEM specimens were prepared dispersing the nanoparticles in toluene and dropping this colloidal solution onto a carbon-coated copper grid. TEM analyses were performed in a JEOL 1010 (200 kV). By General TEM view of the sample, the projection of nanoparticles reveals a toroidal-like shape and weakly interaction between them is observed. The EELS analyses show that the particles are constituted by Fe3O4, which is corroborated by a chemical analysis where the ~60 % wt. of the final powder corresponds to the Fe3O4 nanoparticles, and ~40 % wt. corresponds to the organic cap of oleic acid that covers the particles and avoid their agglomeration. A more detailed analysis by High Resolution TEM (HRTEM) shows that depending of defocus; new crystallographic planes appear “inside” the nanostructures. Therefore, the microstructure observed corresponds with hollow spheres instead of toroidal particles. Moreover EFTEM Fe Map, it is possible to observe the homogeneity of the Fe around the whole surface of the sphere. The presence of this new morphology for magnetic nanoparticles, as hollow spheres do not reported before in the literature, can explain the interesting magnetic properties observed in the samples elaborated by this chemical route.