Synthesis and self-assembly of fcc phase FePt nanorods.

FePt nanocrystals with sizes ranging from 2 to 20 nm are an important class of magnetic nanomaterials.1 The promising applications in data storage, high performance permanent magnets, biomedicine, and catalysis promote extensive synthetic effort. To date, most of previous work has focused on the synthesis of spherical or cubic FePt nanocrystals.2 One-dimensional FePt nanoparticles (nanorods or nanowires) are more interesting because they are expected to have magnetic and structural anisotropy with unique magnetic properties.1 To this end, few efforts were made on the synthesis of CoPt nanorods, CoPt nanowires, and FePt nanowires.3 However, the resulting nanorods or nanowires were either ill-defined in shape or agglomerated, and therefore are not ideal for the formation of uniform magnetic anisotropy. Here for the first time, we report a synthesis of monodispersed FePt nanorods through confined decomposition of Fe(CO)5 and reduction of Pt(caca)2 in a surfactant liquid-crystal mesophase. Transmission electron micrscopy (TEM) in Figure 1A shows the monolayer self-assembled arrays of FePt nanorods with an average diameter of 2.1 nm and a length of 11.3 nm. HRTEM images (Figure 1B,C) show FePt nanorods with both {100} and {110} faces. X-ray diffraction (XRD) pattern of the as-prepared FePt nanorods shows the strongest (111) peak at ∼40.2° and a (200) peak at ∼47°, (Figure 2A), corresponding to the standard facecenter cubic (fcc) structure with a random crystalline orientation. Composition analysis using the energy dispersed X-ray spectroscopy EDS (Figure 2B) shows the Fe/Pt atomic ratio of ∼40/60. Annealing induced the phase transformation from fcc phase to long-range chemically ordered “fct” phase, with particles sintered after annealing. Alternative gradient magnetometry (AGM) measurement on annealed FePt nanorods shows the ferromagnetism with a coercivity of ∼5 kOe at room temperature (Figure 2C). We found that the uniformity of FePt nanorods was sensitive to heating rate. Reactions at a heating rate of 1 °C/min led to fairly monodisperse nanorods, while reactions at a heating rate of 10 °C/min generated a mixture of spherical and necklacelike FePt nanoparticles with an average diameter of 3 nm and a length of up to 80 nm (Figure S1). Elemental mapping of necklacelike FePt nanoparticles on HRTEM images shows uniform composition distribution along the c-axis with iron atomic percentages between 38 and 42% (Figure S2 and S3). The composition of FePt nanorods can be tuned by adjusting the molar ratio of Fe(CO)5/Pt(acac)2. The Fe(CO)5/Pt(acac)2 ratio of 2 gave Fe38Pt62 composition, as characterized by EDS. By adjusting the molar ratio of Fe(CO)5/Pt(acac)2 from 1 to 3, the atomic percentage of iron in the FePt nanorods changed from 22% to 40%. However, if the molar ratio of Fe(CO)5/Pt(acac)2 was larger than 3, spherical FePt nanocrystals began to appear in addition to nanorods. The rest of iron is in the form of Fe(II)-surfactant complex, as evidenced by the fact that the solution appeared as deep brown color after precipitation and centrifugation processes. In our experiments, we found that the formation of surfactant (OA and OAm) mesophase was critical on the controlled formation of FePt nanorods. Different from the previous synthetic process in which less concentrated surfactants were used for the synthesis of spherical and cubic FePt2a-c, we added more surfactants (OA and OAm) in the reaction solution. It is well-known that the spherical micelles of surfactants start to form in aqueous or organic solvent at a critical micelle concentration (CMC).4 When the concentration of surfactants further increases, the shape of micelles turns from spherical to cylindrical (or rodlike), or even ordered hexagonal packing of cylindrical in organic solvents.4 These shaped micelles have been consciously used as templates to synthesize spherical and rodlike nanocrystals.5 In our case, highly concentrated surfac† University of New Mexico. ‡ Sandia National Laboratories. # University of Texas at Arlington. Figure 1. TEM images of FePt nanoparticles: (A) TEM images of FePt nanorods; (B) high-resolution TEM of FePt nanorods with {110} faces; (C) FePt nanorods with {100} faces.