Ordered iron nanowires in the mesoporous silica matrix

Article abstract of DOI:10.1016/j.jmmm.2003.12.742

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.

Full text of DOI:10.1016/j.jmmm.2003.12.742

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) , into the hydrophobic part of a
5
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 develop-
ment 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
diameter can be varied from 2 to 50 nm) [4]. This pore
system is a perfect reactor for synthesis of nanocompo-
sites 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 frame-
work. 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 [5].
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 forma-
tion of nanoparticles outside the pores is probably the
hydrophobic nature of the pore walls [4], 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–surfac-
tant composite.
nanocomposites [1]. However, usually very small (10–
1
3
000 nm ) magnetic nanoparticles shows para- or super-
paramagnetic properties, with very low blocking tem-
peratures and no coercive force at room temperatures
[
2]. 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 [3].
One of the promising matrices for preparation of
highly anisotropic magnetic nanoparticles is meso-
porous silica. Mesoporous silica is an amorphous SiO2
with a highly ordered uniform pore structure (the pore
Mesoporous silica (MCM-41) was prepared by the
common technique described everywhere [6].This meth-
od 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.
*Corresponding author. Tel.: +7-095-939-5931; fax: +7-
095-939-0998.
E-mail address: eliseev@inorg.chem.msu.ru (A.A. Eliseev).
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.742

ARTICLE IN PRESS
A.A. Eliseev et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1609–1611
1610
Intercalation of iron was performed using iron
of intercalated iron was measured by chemical analysis.
In all samples it corresponds to the molar ratio SiO2 :
Fe=9:1.
pentacarbonyl because this non-polar molecule can be
expected to dissolve well in the hydrophobic part of the
SiO /surfactant micelles and can be easily decomposed
2
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 give elemental iron [7].
Dried mesoporous silica–surfactant matrix( B1 g)
was soaked in 10 ml of liquid Fe(CO) for 2 days. After
5
filtration the sample was washed with heptane in order
to get rid of Fe(CO) absorbed on the external surface.
5
o
Decomposition of Fe(CO)
carried out under UV-irradiation (DRT-1000 lamp,
5
to amorphous iron was
to 350 C results in increase of particle length. However,
higher reduction temperatures gives the shorter parti-
cles, which indicates the crystallization process as similar
to percolation one.
5
1
000 W) in vacuum (10ꢀ bar) for 10 h. In order to
achieve a formation of crystalline, anisotropic nanopar-
ticles the sample was annealed in hydrogen flow in
2
s
DE
k lnðt Á f0Þ
½0:25 I ðNjj ꢀ N>Þ þ K1ꢂV
25k
ꢁ
ꢁ
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
TB ¼
E
:
ð1Þ
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
(Fig. 1). Electron diffraction studies confirm the forma-
tion of metallic iron in the system. An overall quantity
3
8 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.
Fig. 1. TEM image and electron diffraction patterns for Fe/
ꢁ
Fig. 2. Temperature dependence of magnetic susceptibility for
Fe/SiO2 nanocomposites.
SiO
2
nanocomposites after annealing at 350 C.
Table 1
2
Magnetic data for Fe/SiO nanocomposites
Annealing
temperature,
Blocking
temperature, K
Form factor
Coercive force, Oe
Saturation magnetization
at 300 K, emu/g
ꢁ
C
4
0
K
300 K
—
o3:5
50.8
o2:3
21
38
0
—
2
3
4
60
50
00
397.6
563.1
665.0
11.6
28.9
84.4
2.56
3.63
3.68
100.5
89.1
35

ARTICLE IN PRESS
A.A. Eliseev et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1609–1611
1611
Thus, the proposed method enables the formation of References
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 position-
ing of writing/reading head in information storage
technologies.
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[
This paper was partially supported by RFBR program
No. 03-03-32182) and INTAS (No. 2001-03-204).
1978, 5–27 (in Russian).
[8] D.L. Leslie-Pelecky, et al., Chem. Mater. 8 (1996) 1770.
(