Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic Liquid

We report on the development of a facile, one-pot synthesis of single-crystalline magnetite (Fe₃O₄) nanoparticles (NPs) based on the thermal decomposition of the nontoxic iron precursor iron(III)acetylacetonate within the ionic liquid trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P_6,6,6,14][Tf₂N]) using 1,2-hexadecanediol as a polyol reducing agent in an "iono-polyol" process. In this expedient approach, the [P_6,6,6,14][Tf₂N] acts both as a low-volatility, thermostable solvent and as the colloid-stabilizing agent, eliminating the requirement for additional surface-capping agents. Performing the synthesis at 300 or 350 °C yielded quasi-spherical, monodispersed Fe₃O₄ NPs with a mean size of 14 nm. Evidence from thermogravimetry, X-ray fluorescence, and infrared analysis is consistent with nanocrystal coverage by a partial bilayer of [P_6,6,6,14][Tf₂N], accounting for the excellent dispersibility of the Fe₃O₄ NPs in solvents such as hexane, toluene, and methylene chloride. Time-dependent thermogravimetric analysis reveals that [P_6,6,6,14][Tf₂N] is transiently stable at 300 °C for 30 min (sufficient for nanocrystal formation) but rapidly degrades at 350 °C or higher. By employing a reaction temperature of 300 °C, the [P_6,6,6,14][Tf₂N] can be recycled and reused multiple times for the subsequent preparation of Fe₃O₄ NPs with no ill effects in terms of particle size, uniformity, or agglomeration. Finally, we demonstrate that the Fe₃O₄ NPs can be dispersed into [P_6,6,6,14][Tf₂N] as a solventless carrier fluid to produce an "iono-ferrofluid" responsive to an external magnetic field.