Size-tunable synthesis of iron oxide nanocrystals by continuous seed-mediated growth: role of alkylamine species in the stepwise thermal decomposition of iron(ii) oxalate

Abstract

The properties of inorganic nanoparticles (NPs) are governed by their size. Therefore, tuning the size of NPs is a fundamental technique in nanoscience. However, the size-tunable synthesis of inorganic NPs is generally carried out in a dilute solution, which produces large quantities of waste. Herein, we report the predictable size-tunable synthesis of Fe₃O₄ NPs by the stepwise thermal decomposition of iron(II) oxalate (Fe(ox)). Monodisperse Fe₃O₄ seed crystals were synthesized by the thermal decomposition of oleylamine-coordinated iron oxalate (Fe(ox)-OAm) in a small amount of oleylamine, followed by continuous seed-mediated growth of Fe₃O₄ NPs. The thermal decomposition behavior of Fe(ox) in oleylamine with and without N,N-diethyl-1,3-diaminopropane (dedap) revealed the important role of dedap in the stepwise thermal decomposition of Fe(ox). The size of the Fe₃O₄ NPs was easily tuned via the stepwise thermal decomposition of Fe(ox) by controlling the amount of decomposed Fe(ox) in a small amount of an alkylamine mixture. The particle diameter was predicted from the size of the Fe₃O₄ seed crystals and the amount of decomposed Fe(ox). Finally, the size dependency of magnetic properties of the synthesized Fe₃O₄ NPs was studied. This continuous seed-mediated growth method based on the stepwise thermal decomposition of metal oxalate can be applied to control the size of a variety of metal and metal oxide NPs.