Formation of Hollow Magnetite Microspheres and Their Evolution into Durian-like Architectures

Hollow magnetite microspheres with a diameter of ca. 1 mu m have been successfully synthesized in aqueous medium by use of triblock copolymer F127 (PEO106PPO70PEO106) as capping and assembly reagents and aspartic acid (Asp, HOOCCH(NH2)CH2COOH) as a reductant. The products were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), FT-IR spectroscopy, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Brunauer-Emmett-Teller (BET) gas sorptometry, and vibrating sample magnetometer (VSM). These hollow microspheres are hierarchically assembled by hundreds of tiny magnetite nanoparticles. The time-dependent experiments unveil that the magnetite nanoparticles first aggregate into spherolites, then the spherolites develop into hollow microspheres. The in-depth investigations, based on control experiments and FT-IR spectrum analyses, reveal that at the stage of nanopaticles assembling, F127 molecules capping to the surface of individual nanoparticle play a crucial role in inhibiting nanoparticles regrowth and promoting nanopaticles aggregation. At the subsequent stage Ostwald ripening contributes to the formation of the hollow microspheres. With further increasing hydrothermal duration, hollow magnetite microspheres can evolve into solid durian-like architectures with compact surface and numerous octahedral vertexes, which can be attributed to the further growth of magnetite nanocrystals on the wall. Moreover, we also measured the magnetic properties of the synthesized products. The saturation magnetizations (M-s) of hollow and durian-like microspheres are 45.2 and 62.3 emu/g, respectively.