Structural regulation of NiFe2O4 colloidal nanocrystal assembly and their magnetic and electrocatalytic properties

In-situ assembly of colloidal nanocrystals provides us an effective way to synthesize unique functional assemblies. Submicrometric spheres with tunable structures, which are assembled by nickel ferrite colloidal nanocrystals, were synthesized through solvothermal synthesis. The morphologies and structures of NiFe2O4 colloidal nanocrystal assemblies (CNAs) were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. One of the NiFe2O4 CNAs adopt an inverse spinel structure with a size of around 300 nm from the sodium acetate-contained synthesis system, in which the crystallite size of NiFe(2)O(4 )nanocrystals is about 11 nm. However, provided that the urea were added into the synthesis system, the as-prepared NiFe2O4 CNAs has a normal spinel structure, which these clusters assembled by 31 nm nanocrystals are 600 nm in size. The magnetometric measurement results showed that the small NiFe2O4 CNAs displayed nearly superparamagnetic behavior at room temperature while the large NiFe2O4 CNAs performed ferromagnetic behavior mainly owing to the effect of crystallite size. In particular, electrochemical sensing measurements showed that the size of the NiFe2O4 nanocrystals played an important role in the electrochemical reduction of H2O2. Based on the experimental results, the formation mechanisms of both NiFe2O4 CNAs as well as the relationship between their structures and properties were analyzed and discussed in this paper.