Magnetic and rheological properties of monodisperse Fe₃O₄ nanoparticle/organic hybrid

Abstract

Fe₃O₄ nanoparticle/organic hybrids were synthesized via hydrolysis using iron (III) acetylacetonate at ∼80 °C. The synthesis of Fe₃O₄ was confirmed by X-ray diffraction, selected-area diffraction, and X-ray photoelectron spectroscopy. Fe₃O₄ nanoparticles in the organic matrix had diameters ranging from 7 to 13 nm depending on the conditions of hydrolysis. The saturation magnetization of the hybrid increased with an increase in the particle size. When the hybrid contained Fe₃O₄ particles with a size of less than 10 nm, it exhibited superparamagnetic behavior. The blocking temperature of the hybrid containing Fe₃O₄ particles with a size of 7.3 nm was 200 K, and it increased to 310 K as the particle size increased to 9.1 nm. A hybrid containing Fe₃O₄ particles of size greater than 10 nm was ferrimagnetic, and underwent Verwey transition at 130 K. Under a magnetic field, a suspension of the hybrid in silicone oil revealed the magnetorheological effect. The yield stress of the fluid was dependent on the saturation magnetization of Fe₃O₄ nanoparticles in the hybrid, the strength of the magnetic field, and the amount of the hybrid.

Introduction

Inorganic particle/organic hybrid materials are composed of nanoparticles and chemically bound organic matrices; these materials have beneficial properties due to the mixed inorganic and organic phases [1]. In particular, magnetic nanoparticle/organic hybrids have attracted considerable attention as they can be used to produce materials with magnetic properties, good functionality, and biocompatibility. Such materials have many potential applications: they can be used as ferrofluids [2], and can be employed in several applications such as magnetic separation [3], magnetic resonance imaging [4], and hyperthermia [5]. A magnetic particle/organic hybrid can be synthesized in situ by the hydrolysis of metal-organics. The advantage of this method is that the crystallinity and size of the magnetic particles can be controlled by an appropriate selection of chemical bonds; thus, the magnetic properties of the hybrid can be controlled. Moreover, the organic matrices prevent agglomeration of nanoparticles by magnetic moment and van der Waals force [6], [7], [8]; as a result, the synthesis can be carried out at low temperatures and the magnetic particles are uniformly dispersed.

Magnetite (Fe₃O₄) is a well-known magnetic oxide; it has a high Curie temperature (850 K) and high saturation magnetization (92 A m²/kg at 300 K) [9]. Fe₃O₄ undergoes a phase transition from the cubic form to the low-symmetry crystal form at the Verwey temperature (∼125 K) [10]. Above the Verwey temperature, Fe₃O₄ becomes an inverse spinel-type ferrite. Below the Curie temperature, Fe³⁺ ions occupy the tetrahedral sites (A sites), and Fe³⁺ and Fe²⁺ ions occupy the octahedral sites (B sites) in antiparallel arrangement, yielding ferrimagnetic spinel.

Future applications of Fe₃O₄ to high-performance magnetic nanodevices and biomedical materials will necessitate the synthesis of monodisperse Fe₃O₄ nanoparticles with diameters less than 20 nm and a narrow size distribution (less than 10% standard deviation). To this end, syntheses of monodisperse Fe₃O₄ nanoparticles by the thermal decomposition of metal-organics at high temperatures ranging from 200 to 300 °C have been recently reported [11], [12], [13]. The authors reported the syntheses of spinel ferrite particle/organic hybrids below 100 °C by using precursors with chelated Fe–O and M–O bonds, which were hydrolyzed with methylhydrazine (MH) [14], [15], [16]. However, the spinel particles in the hybrids had sizes of less than 10 nm, and the particles were superparamagnetic. Therefore, in the case of hybrids containing particles with sizes greater than 10 nm, low-temperature synthesis methods are required to utilize their ferrimagnetic properties.

A ferrofluid typically consists of ferrimagnetic particles such as Fe₃O₄ (approximately 10 nm in size) suspended in a nonmagnetic carrier fluid. Ferrofluids exhibit a remarkable property: the rheological properties of ferrofluid change on application of an external magnetic field [17]. Typical host fluids include hydrocarbons, water, fluorocarbons, esters, and silicones. A ferrofluid is a suspension of nano-sized particles. A magnetorheological (MR) fluid, in contrast, is a suspension of micron-sized ferromagnetic particles such as α-iron [18]. Although MR fluids exhibit larger viscosity changes and yield stresses than do ferrofluids, they have a tendency to exhibit irreversible particle agglomeration and settling. Ferrofluids, on the other hand, are stable due to the Brownian motion, the balance of van der Waals attraction, and the steric (or Coulomb) repulsion. Magnetic nanoparticle/organic hybrid materials contain organic matrices and are thus free from the above-mentioned problems. In order to produce ferrogels [19] and medical materials containing superparamagnetic particles, it is important to study the response of these particles to the magnetic field. However, very few studies have investigated the relation between the sizes of the particles around the transition region from superparamagnetic to ferrimagnetic and their MR effects.

In this paper, we describe the in situ synthesis of a monodisperse Fe₃O₄ nanoparticle/organic hybrid using iron (III) acetylacetonate, Fe(acac)₃, at low temperatures of around 80 °C. The synthesis conditions for the size-controlled Fe₃O₄ particles were investigated. The magnetic properties of the hybrid containing Fe₃O₄ particles with sizes of around 10 nm were studied. In addition, the MR properties of suspensions containing the hybrid and silicone oil were evaluated.