Monodisperse magnetite nanoparticles with nearly ideal saturation magnetization

Abstract

We present a scalable thermolysis and high temperature oxidation procedure for synthesizing monodisperse magnetite nanoparticles with saturation magnetization of up to 80 emu g⁻¹ (412 kA m⁻¹), 92% of bulk magnetite. Diameters in the 15–30 nm size range are produced from iron oleate via the thermolysis method at 324 °C and varying oleic acid ratios for size control (6.7–7.6 equivalents per Fe). The influence of the iron oleate synthesis procedure on the quality of resulting nanoparticles is examined and the structure of the iron oleate is proposed to have a triironoxonium core [Fe₃O⁺] based on magnetic susceptibility measurements. The thermolysis method is shown to initially give wüstite nanoparticles, which are oxidized in situ at 318 °C using 1% oxygen in argon to form highly magnetic magnetite nanoparticles. The use of 1% oxygen offers broad application as a safe and efficient reagent for the high temperature oxidation of nanoparticles. Special consideration to the reproducibility of nanoparticle diameter and monodispersity has uncovered critical factors. Additionally, the reduction of Fe(III) to Fe(II) is shown to occur during the heat up stage of thermolysis, beginning at less than 180 °C and being complete by 320 °C. Evidence for the reduction occurring by the oxidative decarboxylation of oleic acid is presented. Decomposition of the remaining oleic acid is shown to occur by a ketonization reaction producing oleone. The nucleation event and growth of particles is examined by TEM. Comparison of the solvents 1-octadecene and octadecane are presented demonstrating the effect on the reduction of Fe(III) during heat up, the large difference in particle size, and effects on the oxidation rate of iron oxide nanoparticles. Determination of Fe(II) content in magnetic iron oxide nanoparticles by titration is presented.