A general strategy for synthesizing FePt nanowires and nanorods.

Synthesis of FePt nanoparticles with controlled shape and magnetic alignment has become an important goal in developing nanocrystal arrays for applications in information storage, permanent-magnet nanocomposites, and catalysis. FePt alloys are chemically stable owing to the spin–orbit coupling and the hybridization between Fe 3d and Pt 5d states, and their magnetic properties can be tuned by simply controlling the atomic ratio of Fe and Pt in the alloy structure. Recent syntheses have shown that spherical FePt nanoparticles are readily made by simultaneous reduction of platinum acetylacetonate ([Pt(acac)2]) and thermal decomposition of iron pentacarbonyl ([Fe(CO)5]). [1a,5] Thermal annealing results in hard magnetic FePt nanoparticle assemblies with coercivity reaching 30 kOe. These small FePt nanoparticles are also very active in formic acid oxidation under fuel cell reaction conditions. Despite these synthetic progresses, aligning these nanoparticles magnetically has constantly been a problem, and the magnetic easy axes of the nanocrystals in the assemblies are randomly oriented in three dimensions. Previous work on the synthesis and selfassembly of FePt nanocubes suggests that elongated nanocrystals may be used to achieve texture and magnetic alignment. This controlled alignment of FePt nanoparticles is essential for the fabrication of single-particle recording media with ultrahigh density, magnetic nanocomposites with maximum energy product, and magnetotransport devices with optimum magnetoresistivity. Herein we report a general strategy for synthesizing FePt nanowires (NWs) and nanorods (NRs). We refer to the onedimensional nanostructures with a length of 100 nm or longer as NWs, and those below 100 nm as NRs. The diameters of both nanostructures are controlled to be 2–3 nm. The nanostructures were synthesized by reduction of [Pt(acac)2] and thermal decomposition of [Fe(CO)5] in a mixture of oleylamine (OAm) and octadecene (ODE) at 160 8C with the length readily tuneable. Our synthesis is fundamentally different from the very recent report on the preparation of FePt nanorods, for which the reaction was performed in oleic acid and oleylamine in a closed autoclave reaction system without stirring, and offers much better control of both the dimensions and composition of the NWs/NRs. Owing to the structure confinement in the elongated shapes, these NWs and NRs show partial structural and magnetic alignment in thermally annealed self-assemblies. This study indicates that well-controlled NWs or NRs are likely the future choice for controlling texture and magnetic alignment in self-assembled nanomagnet arrays to support high-density magnetic information and as building blocks for fabricating highly sensitive magnetotransport devices. The length control of the FePt NWs/NRs was realized by tuning the volume ratio of OAm/ODE, reaching from over 200 nm for NWs down to 20 nm for NRs. For example, FePt NWs with a length of over 200 nm were made when only OAmwas used as both surfactant and solvent, while an OAm/ ODE ratio of 3:1 gave FePt NWs of length 100 nm, and a 1:1 volume ratio of OAm/ODE led to FePt NRs of length 20 nm. Notably, using a greater proportion of ODE (OAm/ODE 1:3) led to the formation of spherical FePt nanoparticles of diameter 3 nm (see Figure S1 in the Supporting Information). With the amount of [Pt(acac)2] fixed (see the Experimental Section), the compositions of these FePt nanostructures were controlled by varying the amount of [Fe(CO)5] added to the reaction mixture and were measured by energy-dispersive spectroscopy (EDS). For example, for the FePt NWs of length 200 nm, using 0.15 mL [Fe(CO)5] led to about 55% Fe in the final product, while using 0.1 mL [Fe(CO)5] yielded the product with about 45% Fe. Note that CoPt NWs can also be made by reduction of [Pt(acac)2] and decomposition of [Co2(CO)8] under similar reaction conditions (see Figure S2 in the Supporting Information). Transmission electron microscope (TEM) images of the representative NWs and NRs are given in Figure 1. The images in Figure 1a–c show NWs of length 200 nm and NRs of length 50 nm and 20 nm. The diameter of these NWs and NRs is about 2–3 nm. Figure 1d is a high-resolution TEM (HRTEM) image of two single NWs of empirical formula Fe55Pt45. In one NW, the lattice fringes are oriented approximately 558 from the wire-growth direction. The interfringe distance was measured to be 0.214 nm, which is close to the lattice spacing of the (111) planes (0.22 nm) in the facecentered cubic (fcc) FePt structure. This result indicates that the [100] direction is parallel (or perpendicular) to the wiregrowth direction, which is further confirmed by the image showing the lattice fringe in the second NW with clearly discernible (100) planes (0.198 nm interfringe spacing). The synthesis and TEM analyses imply that OAm, a common organic surfactant, induces the one-dimensional growth of FePt under the current synthetic conditions. It is likely that OAm self-organizes into an elongated reversemicelle-like structure within which the FePt nuclei are formed. This type of formation is similar to what has been proposed in the synthesis of Au NRs in the presence of [*] C. Wang, Dr. Y. Hou, J. Kim, Prof. S. Sun Department of Chemistry Brown University Providence, RI 02912 (USA) Fax: (+1)401-863-9046 E-mail: [email protected] [email protected]