An easy fabrication of monodisperse oleic acid-coated Fe₃O₄ nanoparticles

Abstract

An easy route is described for the synthesis of monodisperse oleic acid-coated Fe₃O₄ nanoparticles with uniform size and shape via a thermal decomposition of Fe(acac)₃ in the presence of oleic acid (OA). The prepared Fe₃O₄ samples are characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectrometry, and vibrating sample magnetometer. The results show that the resulting OA-coated Fe₃O₄ nanoparticles have an average diameter of about 24 nm and an OA layer, around 3 nm in thickness. The magnetic saturation value of the prepared OA-coated Fe₃O₄ nanoparticles is determined to be 78.68 emu/g, indicating a well-established superparamagnetic property.

Introduction

The development of nanoparticles has been intensively pursued not only for the fundamental scientific interest, but also for many technological applications [1]. The synthesis of monodisperse nanoparticles is of key importance, because the electrical, optical, and magnetic properties of these nanoparticles depend strongly on their dimensions. For example, monodisperse magnetic nanoparticles have drawn much attention due to their numerous applications, e.g., magnetic storage, magnetic resonance imaging, immunoassay, and drug delivery [2], [3], [4], [5]. Magnetite (Fe₃O₄) is a common magnetic iron oxide that has a cubic inverse spinel structure with oxygen forming a fcc closed packing and Fe cations occupying interstitial tetrahedral sites and octahedral sites [6]. Over the past decades, there have been a number of synthetic routes to the Fe₃O₄ nanoparticles, such as coprecipitation, reverse micelle method, hydrothermal method, and thermal decomposition of organometallic or coordination compounds [7], [8], [9], [10], [11], [12]. Among these, thermal decomposition of iron complex (e.g., FeCup₃, Fe(CO)₅, Fe(acac)₃, or Fe(oleate)₂) was demonstrated to be a very promising route to the monodisperse Fe₃O₄ nanoparticles with uniform and controllable size and shape [13], [14], [15]. However, some expensive agents such as 1,2-hexadecanediol or 1-octadece, were often used in these syntheses, which will add cost to the overall fabrication process of the Fe₃O₄ nanoparticles. Therefore, a large-scale, economic preparation of the monodisperse Fe₃O₄ nanoparticles without using external expensive reagents will be highly desirable.

One problem faced in the formation of the Fe₃O₄ nanoparticles is that nanoscale particles with a large surface-to-volume ratio will cause aggregation during particle formation through van der Waals interparticle attractions [16]. To address this issue, stabilizing agents which can attach to the surface of the particles and provide spatial isolation in the synthesis system were used. Wooding et al. prepared stable aqueous magnetic nanoparticles dispersions with various saturated and unsaturated fatty acids as primary and secondary surfactants [17]. Zhao et al. [18] synthesized the Fe₃O₄ nanoparticles in an aqueous solution coated with bilayer oleic acid (OA). Recently, Lan et al. produced Fe₃O₄ nanoparticles coated with OA bilayer (a diameter about 12 nm), which are stable over a period of more than 3 months [19]. In this study, we developed a facile, low-cost synthesis of the monodisperse OA-coated Fe₃O₄ nanoparticles with uniform size and shape via a thermal decomposition of Fe(acac)₃ in the presence of OA. Furthermore, the process does not require a size-selection procedure and is readily scaled up for the mass production of magnetic nanoparticles.