Microstructure and magnetic properties of tubular cobalt-silica nanocomposites

Article abstract of DOI:10.1016/j.jmmm.2006.11.126

Co-based (Co and Co₃O₄) nanoparticles were self-integrated into SiO₂ nanotubes with a methodology based on the use of a Co salt as a template structure for the formation of SiO₂ nanotubes. Within the confinement of tubular matrix of SiO₂, the nanofibres of cobalt precursor, i.e., [Co(NH₃)₆](HCO₃)(CO₃)·2H₂O, were treated in a H₂ atmosphere with different parameters. With a sufficient reduction on the cobalt precursor, sphere-like Co-based nanoparticles are obtained, being well aligned in the interior space of the SiO₂ nanotubes. With an insufficient reduction, platelet-like Co-based nanoparticles are formed, being arranged in a random manner inside the SiO₂ nanotubes. The sufficiently reduced Co-SiO₂ nanocomposite exhibits an open hysteresis loop in the low field region (<3 kOe) and a paramagnetic response in high field (>3 kOe) at 300 K. An observed wide separation between the zero-field-cooling (ZFC) and field-cooling (FC) curves over the whole temperature region has demonstrated a characteristic feature of ferromagnetism with a magnetically anisotropic barrier diverting the easy axis from the axis of the applied field. The predominant factor leading to this anisotropic potential barrier is attributed to the shape anisotropy native to the one-dimensional arrangement of Co-based nanoparticles within the tubular matrix, i.e. SiO₂ nanotubes.

Full text of DOI:10.1016/j.jmmm.2006.11.126

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 312 (2007) 405–409
www.elsevier.com/locate/jmmm
Microstructure and magnetic properties of tubular
cobalt–silica nanocomposites
Lirong Rena,c, Lin Heb, Chinping Chenb, Michael Warkc, Chunping Lia,
Ã
Ping Chea, Lin Guoa,
aSchool of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China
bDepartment of Physics, Peking University, Beijing, 100871, PR China
cInstitute for Physical Chemistry and Electrochemistry, University of Hanover, Callinstr. 3-3A, Hannover 30167, Germany
Received 12 July 2006; received in revised form 28 October 2006
Available online 30 November 2006
Abstract
Co-based (Co and Co3O4) nanoparticles were self-integrated into SiO2 nanotubes with a methodology based on the use of a Co salt as
a template structure for the formation of SiO2 nanotubes. Within the confinement of tubular matrix of SiO2, the nanofibres of cobalt
precursor, i.e., [Co(NH3)6](HCO3)(CO3) ꢀ 2H2O, were treated in a H2 atmosphere with different parameters. With a sufficient reduction
on the cobalt precursor, sphere-like Co-based nanoparticles are obtained, being well aligned in the interior space of the SiO2 nanotubes.
With an insufficient reduction, platelet-like Co-based nanoparticles are formed, being arranged in a random manner inside the SiO2
nanotubes. The sufficiently reduced Co–SiO2 nanocomposite exhibits an open hysteresis loop in the low field region (o3 kOe) and a
paramagnetic response in high field (43 kOe) at 300 K. An observed wide separation between the zero-field-cooling (ZFC) and field-
cooling (FC) curves over the whole temperature region has demonstrated a characteristic feature of ferromagnetism with a magnetically
anisotropic barrier diverting the easy axis from the axis of the applied field. The predominant factor leading to this anisotropic potential
barrier is attributed to the shape anisotropy native to the one-dimensional arrangement of Co-based nanoparticles within the tubular
matrix, i.e. SiO2 nanotubes.
r 2006 Elsevier B.V. All rights reserved.
Keywords: Magnetic properties; Silica nanotubes; Cobalt; Nanocomposite
1. Introduction
magnetic properties of the dispersive systems have been
extensively studied.
The dispersion and stabilization of freshly prepared
magnetic nanoparticles have become hot topics for
different systems and composites. Magnetic particles in
ferrofluids [1,2] are protected from oxidation and agglom-
eration by surfactant molecules covering the particle
surfaces. In Co–Au [3], CoFe–Au/Ag [4] or oxide-
passivated Co [5], Fe [6,7], CoFe [5,8] particles, the
core–shell morphology can effectively stabilize the dis-
persive system. In nanocomposites such as Co–Al2O3 [9],
Fe–Al2O3 [10,11] magnetic particles are embedded in
a chemically dissimilar matrix. The influences of the
dispersants, i.e., surfactants, shells or matrices, on the
Co–SiO2 nanocomposites with porous SiO2 as matrix are
not yet fully studied magnetically [12]. They are particu-
larly attractive as supported Co catalysts for Fischer–
Tropsch synthesis, i.e. the conversion of synthetic gas to
long-chain hydrocarbons [13–15]. The conventional pre-
paration of Co–SiO2 nanocomposites is mainly based on
aqueous impregnation of the porous oxide with solutions
of Co salts. The SiO2-supported catalysts are prepared
after the decomposition of the supported Co salt via
calcination in air and subsequent reduction process in an
atmosphere of H2 [13]. Recently, a novel Co–SiO2
nanocomposite consisting of aligned sphere-like nanopar-
ticles of Co within SiO2 nanotubes was developed by
employing accurately tailored nanofibres of a Co salt as
templates in a sol–gel process [16]. The calcination in H2
Ã
Corresponding author. Tel./fax:. +86 10 82338162.
E-mail address: guolin@buaa.edu.cn (L. Guo).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.11.126

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406
leads to the formation of almost spherical Co nanoparti-
cles, which are well aligned in rows due to magnetic dipolar
interactions between neighboring particles in combination
with the tube confinement. It is of great interest to
investigate the magnetic properties of such tubular
Co–SiO2 nanocomposite for both fundamental and prac-
tical purposes.
In this paper, detailed magnetic data on this novel
Co–SiO2 nanocomposite are reported. Samples of the
tubular nanocomposites were prepared using different H2
treatment and studied regarding their microstructures. The
influence of the dispersive system, i.e. the tubular matrix of
SiO2, on the temperature dependency of magnetization of
the tubular Co–SiO2 nanocomposite is discussed.
2. Experimental
The detailed preparation of tubular Co–SiO2 nanocom-
posite has been described previously [16]. In brief,
nanofibers of [Co(NH3)6](HCO3)(CO3) ꢀ 2H2O were pre-
cipitated from its aqueous solution by solvent modification
and then used as templates for the formation of SiO2 walls
in a sol–gel process. The as-prepared SiO2 nanotubes were
subjected to H2 (6 L/h) reduction with a heating rate of
5 1C/min at 400 1C for 4 h (sample S1) or at 300 1C for 1 h
(sample S2). The structure and composition of both
samples were characterized by transmission electron
microscopy (TEM) and selected area diffraction pattern
(SAED) mainly on a JEOL 2100F microscope. The
collective magnetic characterizations were performed on a
Quantum Design superconducting quantum interference
device (SQUID) magnetometer (MPMS-XL).
3. Results and discussions
The bright-field TEM image in Fig. 1a shows that the
sample S1 comprises SiO2 nanotubes (100–220 nm in
diameter) encapsulating a row of well-aligned, sphere-like
particles (ca. 10–70 nm). There is a distribution of rows
with different widths inside SiO2 nanotubes. The row width
may be related to the width of the templates, i.e. nanofibers
of Co precursor inside the tubes. Noteworthily, from the
TEM image of a bundle of tubes in Fig. 1b, neighboring
nanoparticles seem to be fused together to form short
nanowires (indicated with a white arrow). The particles are
polycrystalline on the basis of the SAED pattern exhibited
in Fig. 1c, being a mixture of hexagonal Co (JPCD5-727)
and cubic Co3O4 (JPCD80-1535). The ratio of metallic Co
inside S1, however, is difficult to be quantified by
techniques like X-ray photoelectron spectroscopy due to
the tube walls that is about 50-nm thick. The alignment of
Co-based particles occurs due to the tube confinement in
combination with the magnetic dipolar interactions be-
tween neighboring Co particles. SiO2 nanospheres, one of
which is indicated with a black arrow in Fig. 1b, are
occasionally present on the external surface of nanotubes.
Fig. 1. (a, b) TEM Images of the sufficiently reduced sample S1 and (c) a
selected area electron diffraction (SAED) pattern made on the site in Fig.
1b. Diffraction circles and points on the SAED pattern can be assigned to
Co (I) and Co3O4 (II).

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407
They are the by-products following the rule developed by
Stober et al. [17].
¨
In the insufficiently reduced sample S2, tube fillings
mostly appear as platelets arranged in a random manner
through tube cavities (Fig. 2a). In few cases, (Fig. 2b) both
sphere-like and platelet-like particles are present. The fact
tells us that the platelet-like Co particles should be
precursor structures for the sphere-like nanoparticles.
Upon further calcination in a prolonged period and/or at
a higher temperature, the unstable platelets curl up and
fuse to sphere-like particles. Weak circles of electron
diffraction in Fig. 2c could be vaguely assigned to cubic
Co3O4 (JPCD80-1535). Due to the short treatment of S2 in
H2, the level of crystallization of Co3O4 in this sample is
very limited.
Fig. 3 shows the field and temperature dependence of
magnetization for S1. The temperature dependence of
magnetization has been studied by zero-field-cooling and
field-cooling (ZFC/FC) curves between 5 and 395 K. To
obtain ZFC curve, the procedure is to cool the sample
under zero applied field to 5 K, and then apply a small field
of 90 Oe. The magnetization of the sample is recorded in
the warming process. For the FC curve the procedure is the
same, except that the sample is cooled with the presence of
an applied magnetic field of 20 kOe.
In Fig. 3, the FC and ZFC curves separate from each
other with the temperature going up to 395 K. This
separation should be accounted for by the existence of
ferromagnetism with a magnetically anisotropic barrier
diverting the easy axis from the axis of the applied field.
For the FC measurement, the magnetic moment of each
particle is aligned along the orientation of the applied field
of 20 kOe with the descending temperature and thereafter
frozen at low temperature. After the cooling field is
removed, the magnetization, MFC, decreases as the
temperature ramps up. This is attributed to the thermal
activation effect reorienting the magnetic moment to the
orientation of the easy axis, which is along the orientation
of magnetic potential minimum. In this case, it is the axis of
the nanotube. On the other hand, with the ZFC, the
magnetic moments tend to be frozen in randomly
distributed orientations when the temperature drops to
the low-temperature limit of measurement. With increasing
temperature, MZFC increases due to thermal activation of
the magnetic moment. As a consequence, the magnetic
moments of the Co nanoparticles align more or less along
the field direction. The insets of Fig. 3a show the M–H data
for S1 measured at 300 K. In the low-field region (o3 kOe),
shown in the inset on the right, an open hysteresis loop is
apparent with a coercivity of 520 Oe, whereas in the high-
field region (43 kOe), shown in the inset on the left, a non-
saturated linear component reveals the existence of
paramagnetism. This notably large coercivity occurs due
to the single-domain state of Co particles (10–70 nm)
subject to the inter-particle, one-dimensional anisotropic
dipolar interaction in the nanotube [16]. The saturated
magnetization (MS) of S1 is determined by extrapolation of
Fig. 2. (a, b) TEM Images of the unsufficiently reduced sample S1 and (c)
a selected area electron diffraction (SAED) pattern made on the site in
Fig. 2a. The weak circles on SAED can be vaguely assigned to Co3O4 (II).

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408
1.5
1.0
0.5
4. Conclusions
3
2
2
1
Within the confinement of the tubular matrix of SiO2,
1
0
-1
-2
0
different Co-based (Co and Co3O4) nanostructures were
formed by different H2 treatments, including well-aligned,
sphere-like nanoparticles after a sufficient H2 treatment at
400 1C for 4 h (S1) and randomly dispersed platelet-like
nanoparticles after an insufficient H2 treatment at 300 1C
for 1 h (S2). A strong ferromagnetic behavior is exhibited
in the sufficiently reduced sample S1. The wide separation
between the ZFC and FC curves over the whole
temperature region demonstrates a characteristic feature
of ferromagnetism with a magnetically anisotropic barrier
diverting the easy axis, which is the axis of the nanotube,
from the axis of the applied field. The interparticle dipolar
interaction native to the one-dimensional arrangement of
Co-based nanoparticles within the tubular matrix is the
predominant factor leading to this anisotropic potential
barrier.
-1
-2
-3
-4 -3 -2 -1
0 1 2 3 4
-40 -20
0
20 40
H (KOe)
H (KOe)
FC
ZFC
0
100
200
T (K)
300
400
Fig. 3. Magnetic properties of the sufficiently reduced sample S1. The FC
and ZFC curves are recorded in a field of 90 Oe with ascending
temperature. For the FC, the applied field in the cooling mode is
20 kOe. The insets show the hystereses taken at 300 K.
Acknowledgments
M versus H to H ¼ 0, i.e., ca. 2.0 emu/g (see inset in
Fig. 3a). Accordingly, MS of Co in the tubular Co–SiO2
nanocomposite (molar ratio of 1:10) can be calculated as
22.3 emu/gCo, or 0.24 mB per Co atom, which is roughly
14% of the bulk value. The small MS value may be
attributed to the small-sized particles exhibiting the
paramagnetism or superparamagnetism, as is obviously
revealed in the high-field region of the hysteresis loop
shown in the left inset of Fig. 3a. The remnant magnetiza-
tion (Mr) is approximately 6.36 emu/gCo and the Mr/MS
ratio is roughly 0.29.
During the thermal treatment in the presence of H2, the
Co precursor, [Co(NH3)6](HCO3)(CO3) ꢀ 2H2O, easily de-
composes, leading to the formation of Co3O4. In the
sufficient reduction process, it is supposed that the initially
formed Co3O4 would be totally reduced to metallic Co.
However, fresh nanoparticles of Co would be unstable
upon exposure to air and a layer of Co3O4 could form
around Co particles, hindering further oxidation. Due to
the limited level of crystallization, the existence of Co3O4
was not detected by X-ray diffraction in the previous paper
[16]. The cubic Co3O4 possesses a very similar diffraction
pattern as orthorhombic Co2SiO4 (JPCD29-0507). The
presence of Co2SiO4 in the sample, however, can be ruled
out because it is much easier to reduce Co3O4 to metallic
Co than to reduce Co2SiO4 [13]. Also the endothermic
peaks observed in thermogravimetric analysis of the Co
precursor [18] support this assertion, since it is reported
that the endothermic decomposition of Co(NO3)2 leads to
a reducible Co3O4, while the high exothermicity of
Co(CH3COO)2 induces the solid state reaction between
SiO2 and Co oxide and produces a barely reducible
Co2SiO4 [13]. According to the magnetic study, the
antiferromagnetic behavior inherent to Co3O4 [19,20] was
disguised by the strong ferromagnetic signal of metallic Co.
The authors thank Dr. Armin Feldhoff, Dr. Frank
Krumeich, Dr. Xiangcheng Sun, Dr. Xuedong Bai,
Dr. Shishen Yan, Dr. Huayong Pan and Dr. Yong Wang
for their invaluable help to this work. Support from the
National Natural Science Foundation of China (Grant no.
20673009), the Program for New Century Excellent Talents
in Universities (NCET-04-0164), Specialized Research
Fund for the Doctoral Program of Higher Education
(SRFDP-2006006005) as well as Engineering Research
Institute, Peking University (ERIPKU-204030) and
Deutsche Forschungsgemeinschaft (DFG, WA 1116/8) is
gratefully acknowledged.
References
[1] I. Hrianca, C. Caizer, Z. Schlett, J. Appl. Phys. 92 (2002) 2125.
[2] A.B. Pakhomov, Y.P. Bao, K.M. Krishnan, J. Appl. Phys. 97 (2005)
10Q305.
[3] Y. Bao, A.B. Pakhomov, K.M. Krishnan, J. Appl. Phys. 97 (2005)
10J317.
[4] J.M. Bai, J.P. Wang, Appl. Phys. Lett. 87 (2005) 152502.
[5] F.J. Darnell, J. Appl. Phys. 32 (1961) 186S.
[6] L.T. Kuhn, A. Bojesen, L. Timmermann, K. Fauth, E. Coering,
E. Johnson, M. Meedom Nielsen, S. Morup, J. Magn. Magn. Mater.
272–276 (2004) 1485–1486.
[7] S. Yu, G.M. Chow, J. Appl. Phys. 98 (2005) 114306.
[8] Y.W. Zhao, T. Zhang, J.Q. Xiao, J. Appl. Phys. 93 (2003) 8014.
[9] G.A. Nicklasson, C.G. Granquist, J. Appl. Phys. 55 (1984) 3382.
[10] J.L. Dormann, C. Djega-Mariadassou, J. Jove, J. Magn. Magn.
Mater. 104 (1992) 1567.
[11] C. Djega-Mariadassou, J.L. Dormann, M. Nogues, G. Villers,
S. Sayouri, IEEE Trans. Magn. 26 (1990) 1819.
[12] S.R. Mishra, I. Dubenko, J. Losby, K. Marasinghe, M. Ali, N. Ali,
J. Nanosci. Nanotechnol. 5 (2005) 2082.
[13] J.S. Girardon, A.S. Lermontov, L. Gengembre, P.A. Chernavskii,
A. Griboval-Constant, A.Y. Khodakov, J. Catal. 230 (2005) 339.
[14] P. Dutta, N.O. Elbashir, A. Manivannan, M.S. Seehra, C.B. Roberts,
Catal. Lett. 98 (2004) 203.

ARTICLE IN PRESS
L. Ren et al. / Journal of Magnetism and Magnetic Materials 312 (2007) 405–409
409
[15] Y. Wang, M. Noguchi, Y. Takahashi, Y. Ohtsuka, Catal. Today 68
(2001) 3.
[18] L.R. Ren, Ph.D. Thesis, 2004, University of Hanover, Germany.
[19] R.M. Wang, C.M. Liu, H.Z. Zhang, C.P. Chen, L. Guo, H.B. Xu,
S.H. Yang, Appl. Phys. Lett. 85 (2004) 2080.
[20] S.A. Makhlouf, J. Magn. Magn. Mater. 246 (2002) 184.
[16] L.R. Ren, L. Guo, M. Wark, Y. Hou, Appl. Phys. Lett. 87 (2005)
212503.
[17] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62.
¨