Sonochemical synthesis of functionalized amorphous iron oxide nanoparticles

Sonochemical decomposition of Fe(CO) was carried out in the presence of different surfactants. The reactions give stable colloids of undecenoate, dodecyl sulfonate, and octyl phosphonate coated FeO nanoparticles of 5-16 nm in diameter. The ionic binding of the surfactants to the nanoparticle surfaces was confirmed by FTIR spectroscopy. Electron paramagnetic resonance measurements, magnetization curves, and zero-field cooled and field cooled studies indicate that the as-prepared amorphous nanoparticles are superparamagnetic. These studies show that the phosphonate-coated nanoparticles behave in a strikingly different manner from the other particles. It is proposed that the extra negative charge on the phosphonate, as compared to the carboxylate and sulfonate groups, makes it a strong bridging bidentate ligand, resulting in the formation of strong ionic bonds to the surface Fe ions, which decreases the number of unpaired spins, possibly through a double superexchange mechanism through a Fe-O-P-O-Fe pathway.