Synthesis and stabilization of FeCo nanoparticles.

FeCo alloys are an important soft magnetic material because of their unique magnetic properties including large permeability and very high saturation magnetization. FeCo nanoparticles are ideal building blocks for nanostructured thin film or bulk magnetic materials1-3 and are also suitable for biomedical applications.4 However, synthesis of monodisperse FeCo nanoparticles remains a challenging task due to the poor chemical stability of the nanoparticles. Several attempts to synthesize FeCo nanoparticles have been made to obtain particles with desired size and composition.5-8 Desvaux et al.9 synthesized FeCo nanoparticles by co-decomposition of organometallic precursors under hydrogen. Superlattice multilayers of the FeCo nanoparticles were successfully obtained. Recently, Seo et al. have obtained FeCo/graphite core/ shell nanocrystals by chemical vapor deposition method and investigated their applications in magnetic resonance imaging in vivo.10 In this paper, we report a simple method to synthesize bimetallic FeCo nanoparticles with well-controlled particle size and size distribution.11 The synthesis is based on reductive decomposition of Fe(III) acetylacetonate (Fe(acac)3) and Co(II) acetylacetonate (Co(acac)2) in a mixture of surfactants and 1,2-hexadecanediol (HDD) under a gas mixture of 93% Ar + 7% H2 at 300 °C. X-ray diffraction (XRD) measurements showed that FeCo nanoparticles could also be synthesized under argon atmosphere, indicating that HDD was the reducing agent in the reaction. However, partial oxidation was observed from the samples synthesized in argon, which means that the presence of a fraction of hydrogen played an important role in protecting the particles from oxidation. Size of the nanoparticles can be well-controlled by varying the binding ligands. FeCo nanoparticles of average 20 nm size were obtained when a mixture of oleic acid and oleyl amine was used as surfactant. Using oleyl amine alone could also produce FeCo nanoparticles, but the shape of the particles was difficult to control and the particles were prone to aggregate in the dispersion. Similarly, particles with an average size of 10 nm with narrow size distribution (SD ) 12%) were obtained by using a combination of oleic acid and trioctylphosphine (TOP) as surfactants (Supporting Information). Figure 1A shows the transmission electron microscopy (TEM) image of the FeCo particles with average diameter of 20 nm. Analysis on the TEM images indicates that the nanoparticles are monodisperse with narrow size distributions (SD ) 7%). XRD patterns of the as-synthesized samples (Figure 1B) show typical bcc structure of the FeCo alloy phase, indicating crystal nature of the nanoparticles. The average particle diameter estimated from the XRD using the Scherrer equation12 was 19 nm, which is very close to the size determined by statistical analysis of the TEM images. The chemical stability of the FeCo nanoparticles in air is dependent on the nature of the binding ability of the surfactants as well as their chemical compositions. Due to their oxophilicity, oleic acid and TOP tend to form a stronger chemical bond with the nanoparticles than oleyl amine. As a result, the 10 nm FeCo nanoparticles, which were bonded by oleic acid and TOP, were found to be chemically more stable than the 20 nm FeCo particles. The chemical stability of the FeCo nanoparticles in air is also dependent on the chemical compositions of the particles. For both 10 and 20 nm particles, the Fe/Co ratio was controlled by varying the initial molar ratio of the metal precursors. For example, FeCo nanoparticles of atomic percentage 60 to 40 for Fe to Co were obtained by feeding an initial molar ratio of 1.5:1 for Fe and Co metal precursors, whereas atomic percentage 50 to 50 for Fe to Co was obtained when an initial feeding molar ratio was 1:1 for Fe to Co metal precursors. The final particle composition was confirmed by EDX. It was found that air stability of the FeCo nanoparticles with equal composition of Fe and Co was much higher compared to the Fe-rich nanoparticle. It can be clearly seen from the XRD patterns of 20 nm FeCo (Supporting Information) that, when the initial molar ratio of Fe/Co metal precursors is 1, only pure FeCo phase can be formed whereas an increase in Fe content resulted in slight oxidation of FeCo. All of the XRD measurements were made within 2 h of particle synthesis, and all samples were handled under air atmosphere. Similar results were obtained in the case of 10 nm particles, as well. It is hard to wash out the surfactants completely from the surface of as-synthesized nanoparticles. The presence of surfactants on the surface of nanoparticles causes difficulty in determining actual magnetization value. In order to determine the actual magnetization of the particles, atomic absorption spectroscopy (AAS) was used to quantitatively determine the content of Fe/Co metals and surfactants in the nanoparticles (Supporting Information). An AAS experiment was carried out on FeCo nanoparticles having a Fe/Co ratio of 1.5. The nanoparticles were initially washed four times with hexane and ethanol mixture. The AAS result showed that the presence of surfactants on the surface of nanoparticles contributed # University of Texas at Arlington. ‡ University of Texas Southwestern Medical Center at Dallas. § Brown University. Figure 1. (A) TEM bright-field image of the FeCo nanoparticles and (B) X-ray diffraction patterns of the as-synthesized 20 nm FeCo nanoparticles. Published on Web 05/12/2007