ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1609–1611
Ordered iron nanowires in the mesoporous silica matrix Andrei A. Eliseev*, Kirill S. Napolskii, Alexei V. Lukashin, Yuri D. Tretyakov Department of Materials Science, Moscow State University, Moscow 119992, Russia
Abstract We report a novel synthetic method for preparation of ordered magnetic iron nanowires in mesoporous silica matrix. The method is based on the incorporation of a hydrophobic metal compound, Fe(CO)5, into the hydrophobic part of a freshly prepared mesoporous silica–surfactant composite. Shape and size of obtained iron nanowires is consistent with the dimensions of porous framework. Particles are uniform and well ordered in the silica matrix. Thus, mesoporous silica serves as nanoreactor for the formation of Fe-nanoparticles. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30.C; 81.05.Y; 75.50.T Keywords: Nanowire; Iron; Magnetic; Mesoporous silica; Ordered magnetic nanocomposites
Modern information technologies require development of novel high-density data storage devices due to colossal growth of digital information volume. The special role in creation of the components for such devices belongs to high-quality nanostructures and nanocomposites . However, usually very small (10– 1000 nm3) magnetic nanoparticles shows para- or superparamagnetic properties, with very low blocking temperatures and no coercive force at room temperatures . One possible solution of this problem is the preparation of highly anisotropic nanostructures. On the other hand, the use of purely nanocrystalline systems is limited because of their low stability and addiction to form aggregates. These problems could be solved by encapsulation of nanoparticles in a chemically inert matrix. It enables one to avoid aggregation of nanoparticles and protect them from the effect of external factors, and, therefore, makes it possible to take an advantage of the speciﬁc properties of nanomaterials . One of the promising matrices for preparation of highly anisotropic magnetic nanoparticles is mesoporous silica. Mesoporous silica is an amorphous SiO2 with a highly ordered uniform pore structure (the pore *Corresponding author. Tel.: +7-095-939-5931; fax: +7095-939-0998. E-mail address: [email protected]
diameter can be varied from 2 to 50 nm) . This pore system is a perfect reactor for synthesis of nanocomposites due to the limitation of reaction zone by the pore walls. One could expect that size and shape of nanoparticles incorporated into mesoporous silica to be consistent with the dimensions of the porous framework. Recently, several attempts have been made to prepare metal nanowires in mesoporous silica matrix by simple soaking mesoporous SiO2 in an aqueous solution of the metal salt with subsequent reduction . However, it was found that the size of metal particles exceeds the size of the pores and the particle size distribution is not uniform. The reason for the formation of nanoparticles outside the pores is probably the hydrophobic nature of the pore walls , which prevents ﬁlling the pores by an aqueous solution. Here we suggest a novel method of synthesis of ordered magnetic iron nanowires in the mesoporous silica matrix based on the introduction of a hydrophobic metal compound, into the hydrophobic part of silica–surfactant composite. Mesoporous silica (MCM-41) was prepared by the common technique described everywhere .This method is based on polycondensation of a silica source in the presence of template. 1D hexagonal porous structure with the lattice parameter, a=4.10 nm, was observed for prepared mesoporous silica/surfactant composite using small angle X-ray scattering.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.742
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A.A. Eliseev et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1609–1611
Intercalation of iron was performed using iron pentacarbonyl because this non-polar molecule can be expected to dissolve well in the hydrophobic part of the SiO2/surfactant micelles and can be easily decomposed to give elemental iron . Dried mesoporous silica–surfactant matrix (B1 g) was soaked in 10 ml of liquid Fe(CO)5 for 2 days. After ﬁltration the sample was washed with heptane in order to get rid of Fe(CO)5 absorbed on the external surface. Decomposition of Fe(CO)5 to amorphous iron was carried out under UV-irradiation (DRT-1000 lamp, 1000 W) in vacuum (105 bar) for 10 h. In order to achieve a formation of crystalline, anisotropic nanoparticles the sample was annealed in hydrogen ﬂow in temperature range from 260 C to 400 C for 3 h. Nanowires with characteristic width of 1–1.5 nm and length more than 100 nm were observed by TEM (Fig. 1). Electron diffraction studies conﬁrm the formation of metallic iron in the system. An overall quantity
of intercalated iron was measured by chemical analysis. In all samples it corresponds to the molar ratio SiO2 : Fe=9:1. Temperature dependence of magnetic susceptibility for all samples was studied with Cryogenics S-600 SQUID magnetometer (Fig. 2). Assuming cylindrical shape of the particles we can estimate their length from blocking temperatures (Eq. (1)) [2,8]. According to calculations the increase of reduction temperature up to 350oC results in increase of particle length. However, higher reduction temperatures gives the shorter particles, which indicates the crystallization process as similar to percolation one.
Fig. 1. TEM image and electron diffraction patterns for Fe/ SiO2 nanocomposites after annealing at 350 C.
Fig. 2. Temperature dependence of magnetic susceptibility for Fe/SiO2 nanocomposites.
DE ½0:25 Is2 ðNjj N> Þ þ K1 V : E k lnðt f0 Þ 25k
The calculations, shows that the average form factor attains the value of 38 (see Table 1). So, at the particle diameter of 1 nm the average domain length will be of 38 nm. At the same time the magnetic hysteresis measurements of samples indicates the coercive force values up to 650 Oe at 4 K and 80 Oe at room temperature, which is nearly sufﬁcient for modern magnetic data storage. The room temperature magnetic susceptibilities also were found relatively high.
Table 1 Magnetic data for Fe/SiO2 nanocomposites Annealing temperature, C — 260 350 400
Blocking temperature, K o3:5 50.8 100.5 89.1
o2:3 21 38 35
Coercive force, Oe
Saturation magnetization at 300 K, emu/g
0 397.6 563.1 665.0
0 11.6 28.9 84.4
— 2.56 3.63 3.68
ARTICLE IN PRESS A.A. Eliseev et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1609–1611
Thus, the proposed method enables the formation of highly anisotropic magnetic nanostructures in the mesoporous silica matrix, where the pore system serves as one-dimensional solid state nanoreactor. The main advantage of this system is the presence of ordered isolated nanodomains inside the diamagnetic matrix, which gives rise to possibility of the precision positioning of writing/reading head in information storage technologies. This paper was partially supported by RFBR program (No. 03-03-32182) and INTAS (No. 2001-03-204).
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