Magnetic nanocomposite spinel and FeCo core-shell and mesoporous systems

Article abstract of DOI:10.1016/j.jmmm.2012.01.025

The fabrication of condensed silica and mesoporous silica coated spinel CoFe₂O₄ and FeCo alloy magnetic nanocomposites are reported. The encapsulation of well-defined 5 nm thick uniform silica layer on CoFe₂O₄ magnetic nanoparticles was performed. The formation of mesopores in the shell was a consequence of removal of organic group of the precursor through annealing. The NiO nanoparticles were loaded into the mesoporous silica. The mesoporous silica shells leads to a larger coercivity than that of pure CoFe₂O₄ magnetic nanoparticles due to the decrease of interparticle interactions and magneto-elastic anisotropy. In addition, the FeCo nanoparticles were coated by condensed and mesoporous silica. The condensed silica can protect the reactive FeCo alloy from oxidation up to 300 °C. However, saturation magnetization of FeCo nanoparticles coated by silica after 400°C annealing is dramatically decreased due to the oxidation of the FeCo core. The mesoporous silica coated magnetic nanostructure loaded with NiO as a final product could be used in the field of biomedical applications.

Full text of DOI:10.1016/j.jmmm.2012.01.025

Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Magnetic nanocomposite spinel and FeCo core-shell and
mesoporous systems
Kai Zhang a, O. Amponsah a, M. Arslan a, T. Holloway a, David Sinclair a, Wei Cao b,
n
M. Bahoura b, A.K. Pradhan a,
a Center for Materials Research, Norfolk State University, Norfolk, VA 23504, USA
b Applied Research Center, 12050 Jefferson Avenue, Newport News, VA 23606, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The fabrication of condensed silica and mesoporous silica coated spinel CoFe2O4 and FeCo alloy
magnetic nanocomposites are reported. The encapsulation of well-defined 5 nm thick uniform silica
layer on CoFe2O4 magnetic nanoparticles was performed. The formation of mesopores in the shell was a
consequence of removal of organic group of the precursor through annealing. The NiO nanoparticles
were loaded into the mesoporous silica. The mesoporous silica shells leads to a larger coercivity than
that of pure CoFe2O4 magnetic nanoparticles due to the decrease of interparticle interactions and
magneto-elastic anisotropy. In addition, the FeCo nanoparticles were coated by condensed and
mesoporous silica. The condensed silica can protect the reactive FeCo alloy from oxidation up to
300 1C. However, saturation magnetization of FeCo nanoparticles coated by silica after 400 1C annealing
is dramatically decreased due to the oxidation of the FeCo core. The mesoporous silica coated magnetic
nanostructure loaded with NiO as a final product could be used in the field of biomedical applications.
& 2012 Elsevier B.V. All rights reserved.
Received 25 October 2011
Received in revised form
14 January 2012
Available online 3 February 2012
Keywords:
Magnetic nanocomposite
Core-shell
Magneto-elastic anisotropy
Mesopore
Oxidation
Magnetic property
1. Introduction
properties of moderate saturation magnetization, relatively large
magnetic anisotropy, high mechanical hardness and chemical
Silica based mesoporous nanostructures with high surface area
(ꢀ1000 m2/g), high pore volume (4 1 cm3/g), high ordered pore
structures and uniformly distributed pore sizes have been exten-
sively investigated due to their industrial and scientific applications
in the field heterogeneous catalysis, separation of large molecules,
polluted environmental controls and also in the field of Li-ion
battery due to spinel structure [1–5]. Currently, mesoporous silica
nanostructures have also attracted considerable research attention
to become the potential candidate in the field of biotechnology and
biomedicine since silica is nontoxic and offers the possibility of
attaching the antibodies, proteins, medical drugs and in vivo bio-
medical applications [6–8]. However, it is very difficult for meso-
porous silica nanostructure to be applied in drug delivery and
protein purification owing to their intrinsic limitations.
Magnetic nanoparticles are of current interest due to their
biomedical applications such as magnetic resonance imaging
(MRI) enhancement, magnetic separation of DNA, cells and pro-
teins, hyperthermia and magnetic targeted drug delivery [9–11].
Among many magnetic materials, spinel Cobalt ferrite (CoFe2O4)
nanoparticles received a lot of attention due to their remarkable
stability [12]. In addition, soft magnetic nanoparticles, such as
FeCo, are of interest in this context due to their very high Curie
temperature, low coercivity, low anisotropy, high permeability,
and high saturation magnetization (Ms) [13]. They can be synthe-
sized with a narrow size distribution. However, FeCo magnetic
nanoparticles are very reactive and oxidized easily in the presence
of air environment such that FeCo nanoparticles subsequently lose
their magnetic properties and limit their applications. Moreover,
these two magnetic nanoparticles are very toxic and is not
biocompatible [14] and restrict them to be used in the field of
biomedical applications.
Encapsulating the magnetic nanoparticle with silica nanostruc-
tures is able to produce core shell nanocomposites with high surface
area which enables this material a potential candidate for magnetic
separation process. Furthermore, the silica shell is selected to reduce
the agglomeration of nanoparticles. The silica shell improves the
chemical and thermal stability and solubility of magnetic nanopar-
ticles [15]. It is also very important to note that the silica coating on
magnetic nanoparticles can reduce the toxicity of nanoparticles and
provide the core shell structures with an additional function. The
shell can also prevent the oxidation of the core materials, such as in
case of FeCo nanoparticles, it becomes a necessity. Recent research
interest in both type of nanoparticles [16–18] have raised further
motivation for studying these materials in details.
n Corresponding author.
E-mail address: apradhan@nsu.edu (A.K. Pradhan).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.01.025

K. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
1939
Fig. 1. Schematic representation of fabrication procedure of magnetic core shell structure nanocomposites.
On the other hand, Ni ions are widely used to separate proteins
2.2. CoFe2O4/NiO/silica and FeCo/silica composites fabrication
attached with polyhistidine affinity tag (His-tag) from the protein
complex systems, since His-tagged proteins have a high affinity
for nickel (Ni2þ) ions. However, separation systems used until
now have constraints due to the complication, long time opera-
tion, and poor magnetic behavior and recyclability problems
[19–21]. Therefore, it is necessary to develop new magnetic
systems which can be used into protein separation effectively
with high performance.
The nanoparticles of CoFe2O4 and FeCo as the cores were
encapsulated with a dense and thin silica shell by adopting a
base-catalyzed sol-gel process. This condensed silica layer is used
to prevent the magnetic core from erosion or oxidation.100 mg of
CoFe2O4 and FeCo were dispersed in 20 ml of 2-propanol solution
and sonicated for 30 min. 75
ml of tetra ethoxysilane (TEOS) and
25 L of 25% NH3H2O solution were injected into the above
m
In order to satisfy the purification of His-tagged proteins, the
magnetic nanocomposites with magnetic core and mesoporous
shell impregnated with NiO nanoparticle were designed and
developed, the synthesis process is illustrated as shown in
Fig. 1. At first, the magnetic core of CoFe2O4 or FeCo nanoparticles
are synthesized through facile wet chemical precipitation routes.
A thin and condensed layer of silica was coated on the surface of
original magnetic cores. Then the mesoporous silica layer was
generated by sol-gel polymerization. Finally, NiO was loaded into
the mesopores for the protein separation.
mixture and sonicated for 30 min at 60 1C. Secondly, (3-amino-
propyl) trimethoxysilane (APS) were added into the previous
formed core-condensed shell solution and sonicated about
30 min. the mixture of TEOS and n-octadecyl trimethoxysilane
(C18TMS) was subsequently added and achieved the mesoporous
layer of silica out of the condensed silica shell via the simulta-
neous sol-gel polymerization. The prepared magnetic composites
were annealed at different temperature to get rid of organic
residuals and form the mesopores which can be utilized to load
the NiO for protein purification. Finally, Nickel acetate was
transported in the mesopores of the magnetic composites through
wet impregnation route. The resulting product was washed and
centrifugated by water and ethanol for several time followed by
drying at 75 1C for overnight. the dried composites were annealed
at different temperature for 5 hours under mixture of Ar and H2
(7:3), such that reduce the Ni2þ to element Ni, once Ni nanopar-
ticles attached in the mesopores are exposed of atmosphere, they
were oxidized into NiO nanoparticles. The schematic illustration
of the fabrication of magnetic core shell structure nanocompo-
sites is shown in Fig. 1.
2. Experimental
2.1. Core-shell CoFe2O4 nanoparticles synthesis
CoFe2O4 nanoparticles were synthesized by a modified co-
precipitation process from their high-purity chloride precursors
(Sigma-Aldrich, USA) described in ref. [22]. In this process, 0.2 M
aqueous solution was prepared by dissolving cobalt chloride
hexahydrate and iron chloride hexahydrate in an aqueous solu-
tion with the ratio of Co to Fe as 1:2 in deionized H2O stirred for
30 minutes. 3 M NaOH aqueous solution was prepared and sub-
sequently added into the chloride solution drop wise with the
same volume under stirring to produce the brown color precipi-
tate. During the precipitate process, a magnetic field of 0.5 T was
applied. The obtained products were washed and collected by
centrifugation, and dried at 75 1C for overnight to evaporate all
the liquid residual in the precipitate powders. The as-prepared
samples were ground and sintered at 600 1C for duration of
2 hours in the furnace to obtain the CoFe2O4 of about 42 nm.
2.3. Characterization techniques
The phase purity and crystallinity of the obtained nanoparti-
cles and nanocomposites were performed on a Rigaku powder
X-ray diffractometer with Copper
˚
Ka incident radiation
l¼1.5405 A). The morphology and the microstructure of the
(
samples were measured using a high-resolution transmission
electron microscopy (HRTEM) of JEOL JEM 2100 field-emission
electron microscope systems. For TEM observations, the resulting
magnetic nanoparticles and magnetic nanocomposites were dis-
persed in the ethanol and sonicated. The suspensions were
dropped on carbon-coated copper grids and dried at about 80 1C
for 12 hours. The magnetic properties were measured by Quan-
tum Design (MPMS, XL) superconducting quantum interference
device (SQUID) magnetometer.
FeCo nanoparticles were prepared by
a co-precipitation
method through reduction reaction under Ar atmosphere proce-
dure. Briefly, cobalt and iron chloride hydrate precursors were
mixed with deionized water in a three-neck, round bottom flask
and stirred magnetically protected by Ar inert gas. 0.15 M sodium
borohydride aqueous solution was prepared and then injected
into the solution of iron and cobalt. The ratio of Fe and Co salts
during the reduction is 1:1, and the concentration related to
sodium borohydride NaBH4 is 0.5 M. The black-colored product
was generated immediately. The reactant products were stirred
for about 1 hour. The precipitate produced in this solution was
washed with DI water and acetone for several times. The FeCo
nanoparticles were purged with Ar gas until the sample was
dried.
3. Results and discussion
XRD peaks of CoFe2O4 magnetic nanoparticle samples calcined
at 600 oC are shown in Fig. 2. The XRD results can be indexed to
the cubic inverse spinel structure of CoFe2O4 phase (JCPDS Card
No. 22-1086). The sharp and stronger diffraction peaks indicate
the high crystallinity of CoFe2O4 nanoparticles owing to thermal

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K. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
treatment at 600 1C. The lattice parameters of annealed nanopar-
ticles were calculated taking into account the cubic symmetry
formula, 1/d2¼(h2þk2þl2)/a2, where d is the interplanar distance,
h, k, and l are the miller indices, and a is the lattice parameter.
Based on the (311) crystallographic planes, the lattice parameter
is a¼0.8380 nm, which closely matches with the reference value
of a¼ 0.8377 nm. The average crystallite size is calculated with
Scherrer’s equation and the particle size is determined to be
42 nm for nanoparticles annealed at 600 1C, respectively.
Fig. 3 shows the representative low and high magnification
TEM images of CoFe2O4 nanoparticles after annealing at 600 1C.
The mean particles size is in the range of 40 to 50 nm. The size
distribution of all these samples is rather narrow. It is noted that
the size of the nanoparticles annealed at 600oC measured in TEM
is consistent with the result calculated from the XRD data.
Fig. 4 demonstrates the TEM micrograph of condensed silica
coated CoFe2O4 core shell nanocomposites. It reveals that the
CoFe2O4 magnetic nanoparticles were encapsulated by well
defined silica shell with a uniform thickness of 5 nm and all the
magnetic particles are coated by the silica. The silica layer around
the magnetic nanoparticles has an effect on the magnetic cou-
pling interaction between the nanoparticles.
Fig. 5 (a) and (b) exhibits the TEM micrographs of porous silica
coated CoFe2O4 core shell nanocomposites. The nanostructures pre-
sent the mesoporous characteristic in the shell and the thickness of
Fig. 2. X-ray diffraction of CoFe2O4 nanoparticles annealed at 600 1C.
Fig. 3. Transmission electron microscopy image of CoFe2O4 nanoparticles calcined
at 600 1C.
Fig. 4. TEM images of core shell CoFe2O4/SiO2 nanoparticles.

K. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
1941
the porous layer can be controlled through changing the amount of
the precursors and reacting conditions. The sol gel polymerization of
TEOS with C18TMS at 80 1C was carried out around the magnetic
nanoparticles. Once annealing the product of TEOS and C18TMS, the
mesopores in the shell were produced as a consequence of the
organic group of the precursor removed. The Ni2þ ions in the
precursor of nickel acetate were loaded into the mesopores by wet
impregnation process followed by drying and reduced under the
mixture of H2 and N2 at high temperature, the Ni elements were
generated in the silica mesopores and subsequently oxidized into
the NiO at ambient condition as shown in Fig. 5 (c) and (d). The
mesoporous silica coated magnetic nanostructure loaded with NiO
as a final product could be used in the protein separation for their
potential drug manufacturing applications.
Fig. 6 shows the magnetic properties of (a) magnetic CoFe2O4
o
nanoparticles annealed at 600 C and (b) CoFe2O4 core/mesoporous
silica shell structures. One major difference clearly observed is the
increase in coercivity of silica coated CoFe2O4 as compared to the
core free CoFe2O4 magnetic samples. For core-shell magnetic nanos-
tructures the coercivity was found to be 2000 Oe whereas for 600 1C
annealed nanoparticles the coercivity magnitude was significantly
lower. This may be attributed due to decrease of interparticle
Fig. 5. TEM images of (a) and (b) porous silica coated CoFe2O4 nanostructures and (c) and (d) NiO nanoparticle loaded silica coated CoFe2O4 nanostructures.
Fig. 6. Magnetic loops of (a) CoFe2O4 nanoparticles annealed at 600 1C and (b) CoFe2O4 core/mesoporous silica shell structures.

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K. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
Fig. 7. FE-SEM images of as synthesized FeCo nanoparticles.
an effective way to improve the crystallinity. The XRD results for
annealed nanostructures are shown in Fig. 8. The evolution of (110)
diffraction peak at about 2
¼461 with annealing temperature from
y
200 to 300 1C is very distinct. This is attributed to the condensed silica
shell which protects the FeCo core from oxidation. However, while
the (110) peak disappears on annealing FeCo nanocomposites at
400 1C, other diffraction peak, such as (311) peak corresponding to
CoFe2O4, emerges. This indicates that the cracks may be formed in the
condensed shell, allowing diffusion of oxygen which oxidizes the
reactive FeCo nanoparticles, transforming to other related oxides.
Fig. 9 (a) and (b) exhibit the TEM micrographs of porous silica
coated FeCo core shell nanocomposites. The nanostructures present
spherical shape and the mesoporous silica layer is about 15–20 nm
in thickness, which can be controlled through changing the ratio of
precursors. During the reaction of TEOS with C18TMS, the sol gel
polymerization was formed around the magnetic nanoparticles.
The mesoporous structures can be formed by calcination to
remove the organic group in the sol gel product. Then the nickel
acetate was impregnated into the mesopores of the core shell
structures by flowing in Ar and N2 at high temperature to inhibit
reduction reaction. The resulting Ni nanoparticles were oxidized
to obtain the NiO after the sample is exposed to the atmosphere
as shown in Fig. 9 (c) and (d).
Fig. 8. (a) XRD patterns of FeCo nanoparticles coated with silica annealed with
The EDS measurements were performed on magnetic nano-
composites loaded with NiO. The elemental analysis shows the
presence of Ni (5.01%) in the core shell structure of nanocompo-
sites. This indicates that the Ni2þions were successfully impreg-
nated into the mesopores in the silica shell and reduced to Ni
nanoparticles by hydrogen. However, once nickel nanoparticles
were exposed to the ambient, NiO is readily formed. This can be
used to separation of His-tagged proteins, and make this as useful
mesoporous core shell composites for protein purification [23].
Magnetic property measurement of the nanostructures was
performed at room temperature. It should be stressed that
these data are original, where the magnetization is expressed in
units of emu/gram, which includes the diamagnetic shells. It can
be observed that magnetization of the core shell structure still
keeps very high saturation magnetization as annealed at 200 to
300 1C as shown in Fig. 10 (a) and (b). The decrease of the
saturation magnetization results from the increase in the mass
of the magnetic nanoparticle coated with silica shell. However,
saturation magnetization of FeCo coated with silica annealed at
400 1C is dramatically decreased as shown in Fig. 10 (c) due to the
fact that the FeCo nanoparticles were oxidized in the air due to
cracking of the shell. As an example, TEM morphological image of
FeCo nanoparticles annealed in air at 400 1C is shown in Fig. 10 (d).
It can be noted that the core of the FeCo nanocomposite disap-
peared after annealing at 400 1C. The contrast of the image is lost
and only a slightly darker area is visible around the center of the
core shell, which supports the fact the FeCo nanoparticles are
oxidized at high annealing temperature.
different temperature.
interactions and magneto-elastic anisotropy. As the silica shell
encapsulated the magnetic core of CoFe2O4, the stress produced
on the surface of the magnetic nanoparticles is enhanced, leading to
an increase in the magneto-elastic anisotropy. It is noted that the
saturation magnetization of CoFe2O4 nanoparticles is lower than
that of its bulk counterpart, and this is generally understood due the
surface defects and surface strain due to their smaller particle size.
Fig. 7 (a) and (b) shows the FE-SEM image of the uncoated
FeCo nanoparticles by coprecipitation method. The FeCo nano-
particles are spherical in shape with about 20 nm in size. The size
distribution is very narrow and uniform, indicating the product of
high-quality of the nanoparticles by coprecipitation reaction
process. The obtained FeCo nanoparticles are agglomerated as a
consequence of strong magnetization of the FeCo alloy and the
high surface energy of nanoparticles.
Since FeCo magnetic alloy is so reactive with oxygen in the
atmosphere, especially when the size of the sample in nano-scale,
that the magnetization is dramatically reduced to limit it potential
application. Therefore, it is essential to explore the technique to
increase the nanoparticle chemical stability. One preferred method is
the coating of FeCo nanoparticle with silica as an insulating shell to
stop the oxidization and keep the magnetic property of the samples.
In order to investigate the effect of the silica shell on the magnetic
nanoparticles, the resulting nanocomposite in this work were
annealed at 200, 300, and 400 1C for 2 h in ambient condition. It is
also well known that the annealing of nanoparticles at temperature is

K. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
1943
Fig. 9. TEM images of (a) and (b) porous silica coated FeCo nanostructures and NiO loaded silica coated FeCo nanostructures (c) and (d).
Fig. 10. Magnetic loops of porous silica coated FeCo nanostructures annealed at (a) 200 1C, (b) 300 1C and (c) 400 1C and TEM micrograph of porous core/shell annealed at
400 1C.

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K. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1938–1944
4. Conclusions
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In summary, we have successfully synthesized condensed silica
and mesoporous silica coated CoFe2O4 magnetic nanocomposites.
The CoFe2O4 magnetic nanoparticles were coated by well defined
silica shell with a uniform thickness of 5 nm. The mesopores in the
shell were fabricated as a consequence of removal of organic group
of the precursor. The NiO nanoparticles were loaded into the
mesoporous silica coated core shell structures. The mesoporous
silica shells result in a larger coercivity than the pure CoFe2O4
magnetic nanoparticles due to the decrease of interparticle interac-
tions and magneto-elastic anisotropy. On the other hands, the FeCo
nanoparticles were encapsulated by condensed silica and mesopor-
ous silica. The silica can protect the reactive FeCo alloy from
oxidation up to 300 1C and keep the magnetic nanoparticles high
magnetization. However, saturation magnetization of FeCo coated
by silica annealed at 400 1C is dramatically decreased due to the
oxidation of the FeCo core. The mesoporous silica coated magnetic
nanostructure loaded with NiO as a final product could be used in
the protein separation as well as other biomedical applications.
[14] K. Zhang, T. Holloway, M. Bahoura, A.K. Pradhan, R. Prabakaran, J. Pradhan,
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This work is supported by the NSF-CREST Center (CNBMD)
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