Non-monotonic size change of monodisperse Fe₃O₄ nanoparticles in the scale-up synthesis

A non-monotonic size change of monodisperse Fe₃O₄ nanoparticles (NPs) with a diameter of 3-20 nm is observed in the scale-up organic-phase synthesis. The crystal structures and the valence state of the Fe ions of the as-prepared NPs were determined by X-ray diffraction (XRD) and Mössbauer spectroscopy, respectively. It is interestingly observed that particle size does not decrease monotonously with either the increase of the molar ratio of oleic acid (OA) to FeO·OH, or the decrease of precursor concentration. Furthermore, the reaction process was investigated via the time-dependent Fourier transform infrared spectra (FTIR) and the transmission electron microscopy (TEM) images, which reveal that the non-monotonic size change results from the different influence of OA on the three reaction stages including monomer formation, nucleation, and growth with increasing precursor amounts.