Effect of air annealing on properties of maghemite nanoparticles produced by radiation-chemical method

Amorphous-crystalline nanoparticles (NPles) of maghemite (gamma-Fe2O3) were successfully prepared by a radiation-chemical method using a precursor-heptahydrate of iron (II) sulfate. Synthesized NPles were quasi-spherical agglomerates 200-300 nm in size with an uneven surface covered with thin, porous, randomly oriented, elongated plates of irregular shape, with sharp vertices. The amorphous component of NPles included ultra-fine NPles maghemite about 2 nm in size. Stepwise annealing of the original nanopowder (NP) in air showed a phase transformation of NPles along the following path: maghemite + amorph (RT-150 degrees C) -> amorph (200-500 degrees C) -> hematite (570-1050 degrees C). The hematite NP produced after annealing at a temperature of 575 degrees C showed good texture characteristics: the specific surface area (SSA) increased from 21 to 36.8 m(2)/g, the pore volume increased from 0.04 to 0.15 cm(3)/g, while maintaining the particle nanosize of not more than 38 nm. X-ray photoelectron spectroscopy (XPS) showed the presence of the following elements on the surface of the synthesized NPles -S, C, O, Fe and N (N only in the original sample S0) and increased O-Fe bond with an increase in annealing temperature. The paramagnetic properties of the studied NPles indicate the prospect of their use as contrast agents in magnetic resonance imaging (MRI).