NANO LETTERS
Aqueous Chemical Route to Ferromagnetic 3-D Arrays of Iron Nanorods
2002 Vol. 2, No. 12 1393-1395
Lionel Vayssieres,* Lew Rabenberg, and Arumugam Manthiram Texas Materials Institute, UniVersity of Texas at Austin, Austin, Texas 78712 Received October 11, 2002; Revised Manuscript Received October 22, 2002
ABSTRACT The fabrication of very large arrays of oriented ferromagnetic iron nanorods by aqueous chemical growth without template, surfactant, or applied electric or magnetic field is reported. The method involves the direct growth of β iron oxyhydroxide nanorods from an aqueous ferric chloride solution onto single or polycrystalline substrates, followed by reduction in hydrogen atmosphere at mild temperatures.
Nanomaterials with higher degree of engineering and more complex architectures such as 3-dimensional, oriented arrays consisting of 1-dimensional metal and metal oxide nanowires are currently under scrutiny for their potential high technological applications for electronics, photonics, and magnetic materials.1 The synthetic methods differ in the degree of complexity, cost, manufacturability, and environmental hazard. The current methods involve the use of templates2 and anodic membranes,3 metal evaporation,4 pyrolysis,5 molecular beam epitaxy,6 and photolithography.7 The template methods require the use of a strong basic medium to dissolve the membranes, which not only affects the purity of the materials but also introduces safety and environmental hazards. The other methods involve high temperatures and/ or vacuum technology, which increase cost and limit the choice of substrates. The ultimate challenge of nanotechnology is the large-scale manufacturing of thin film devices of large physical area at an affordable cost. In this regard, we are reporting here a novel templateless, lowtemperature fabrication of very large arrays (several tens of cm2) of oriented high-coercivity ferromagnetic iron nanowires using a simple aqueous chemical route, without the use of surfactant or external field. The strategy involves the control of the interfacial thermodynamics and kinetics of nucleation and growth by chemical and electrostatic monitoring of the interfacial tension to achieve tailored size, shape, and crystal engineering.8 The synthetic method consists of growing large arrays of oriented nanorods of akaganeite (βFeOOH) directly onto the substrates by heteronucleation from the hydrolysis-condensation of an aqueous metal salt solution, and then reducing such arrays to metallic iron (RFe) in hydrogen atmosphere (Scheme 1). Typically, the substrates (e.g., single-crystalline sapphire or polycrystalline * Corresponding author. E-mail:
[email protected]. 10.1021/nl025840l CCC: $22.00 Published on Web 11/02/2002
© 2002 American Chemical Society
Scheme 1
F-SnO2) are immersed in a bottle containing a 0.15 M aqueous solution of FeCl3 in 1 M NaNO3 at pH ) 1.5 and then heated at 95 °C for about 12 h in a laboratory oven. The nanoparticulate thin films formed onto the substrates are thoroughly washed with water and reduced in H2 at 300 °C for 10 h. Such relatively low temperature of reduction is due to the very narrow diameter of the individual akaganeite nanorods and overall porosity of the array, which facilitate diffusion of hydrogen. In addition, a simple crystallographic transformation pathway exits between tetragonal β-FeOOH
Figure 1. Electron micrographs of the oriented iron nanorod array: (a) SEM of the perpendicularly oriented nanorod bundles of iron, (b) TEM of a flake of oriented iron nanorods scratched off the array.
(001)-elongated nanorods and cubic R-Fe (110)-elongated nanorods, which contributes significantly in lowering the activation energy of the reduction process.
Electron microscopy (TEM and SEM) reveals that the thin films consist of nanorods oriented normal to the substrate surface (Figure 1). The iron nanorods consist of small anisotropic crystalline nanoparticles of about 5-10 nm in diameter and about 15-30 nm in length, stacked as columns and bundled as nanorods that are 30-40 nm in diameter and of 0.8-1 µm in length as determined by profilometry. The nanorods are homogeneously distributed onto the entire surface of the substrate, regardless of its crystallinity and physical surface area. EDS analysis reveals that the nanorods consist of elemental iron. X-ray and electron diffraction patterns (Figure 2) correspond to R-Fe (cubic system, space group Im3m) with a lattice constant of 2.86 Å as observed in bulk iron samples (e.g., JCPDS 06-0696). No evidence of strong crystallographic texture among the Fe grains is seen, and the high magnification TEM image suggests that the volume difference between the Fe rods and their precursor remains as voids between the Fe crystallites. DC magnetic measurements were carried out on a superconducting quantum interference device (SQUID) as a function of temperature (FC and ZFC protocol) and as a function of applied field H at 5 and 300 K for fields oriented perpendicular and parallel to the iron nanorods. The nanorod array shows a strong, temperature-independent magnetization, with a slightly higher saturation magnetization value for the parallel orientation compared to that for the perpendicular orientation. This would suggest that the easy axis lies along the nanorod axis. The samples show saturation magnetization (at field of 10 kOe) Ms values of 115 and 150 emu/g, respectively, for the samples oriented perpendicular and parallel to the magnetic field (Figure 3). The samples show high coercivity values of 1200 Oe at 5 K and 900 Oe at 300 K without significant difference for the two orientations. The remanent magnetization is as high as 50 emu/g, which gives the hysteresis loop a relative squareness and a high hysteresis loss (inset Figure 3). For a rod-like shape ferromagnetic material, the demagnetizing factor is zero when the field is parallel to the rod axis and up to 2π when the field is perpendicular to the rod axis, which yields
Figure 2. X-ray and electron diffraction patterns of an oriented nanorod array of iron grown on commercial sapphire substrate and of an individual rod, respectively. The pattern was indexed according to JCPDS 06-0696 for iron and 77-2135 for the sapphire substrate. The shoulder marked with an asterisk (*) corresponds to the (113) reflection of corundum. 1394
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Figure 3. Variation of the magnetization at 5 and 300 K with applied field perpendicular and parallel to the iron rods. The inset shows the hysteresis loop.
to a nonsquare hysteresis loop. However, the present nanorod arrays do not show evidence of such behavior. Indeed, the loop shape is similar regardless of the orientation and temperature. Since there is no strong crystallographic texture within each rod, the effects of magnetocrystalline anisotropy are not seen. This is consistent with the TEM observations, which show low aspect ratio single crystals of iron stacked along the rod axis resulting from the solid-state reduction of single crystalline bundles of akaganeite nanorods. Such nanoengineered large arrays of oriented iron nanorods with high remanence, high coercivity, and relative loop squareness could be of interest for magnetic recording as well as for magnetoelectronic sensor applications, considering their simple and cost-effective fabrication. In addition to the technological interest of oriented metallic and magnetic nanowires, a fundamental understanding of their electronic structure and magnetic properties (e.g., quantum confinement effects, metal-to-insulator transition as well as the effect of shape anisotropy on the magnetization reversal) is crucial for further device optimization. Such investigations are currently being pursued at synchrotron facilities by polarization-dependent X-ray absorption, resonant inelastic X-ray scattering, and circular dichroism. Acknowledgment. This work was supported by the Welch Foundation Grant F-1254. Nano Lett., Vol. 2, No. 12, 2002
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