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).

Automated summary

Amorphous-crystalline nanoparticles of maghemite were synthesized using a radiation-chemical method. The nanoparticles had a quasi-spherical agglomerate structure with an uneven surface covered with thin, porous, randomly oriented, elongated plates. The annealing process resulted in a phase transformation from maghemite to amorphous to hematite. The hematite nanoparticles showed improved texture characteristics and could potentially be used as contrast agents in MRI.