Evidence of direct crystal growth and presence of hollow microspheres in magnetite particles prepared by oxidation of Fe(OH)2.

We provide new information relevant to the crystallinity and growth mechanism of magnetite particles that were fabricated following the method of Sugimoto and Matijević [J. Colloid Interface Sci. 74 (1980) 227]. These authors observed that in a small excess of Fe(2+), particles grew by aggregation and recrystallization of smaller units, so that until now the resulting particles were thought to be polycrystalline. With the help of transmission electron microscopy (TEM) and selected area electron diffraction (SAED), we also detected the presence of monocrystalline particles, which are strong evidence of the occurrence of direct crystal growth. This growth mechanism seems to coexist with that of the aggregation of primary units proposed by Sugimoto and Matijević. Careful examination of electron microscopy micrographs also revealed the presence of many hollow polycrystalline microspheres.