World Journal of Condensed Matter Physics, 2011, 1, 49-54
doi:10.4236/wjcmp.2011.12008 Published Online May 2011 (http://www.SciRP.org/journal/wjcmp)
Copyright © 2011 SciRes. WJCMP
49
Synthesis and Characterization of
Superparamagnetic Fe3O4@SiO2 Core-Shell
Composite Nanoparticles
Meizhen Gao, Wen Li, Jingwei Dong, Zhirong Zhang, Bingjun Yang
Key Lab for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou, China.
Email: liwen3311@126.com
Received January 9th, 2011; revised March 8th, 2011; accepted March 15th, 2011.
ABSTRACT
The Fe3O4@SiO2 composite nanoparticles were obtained from as-synthesized magnetite (Fe3O4) nano particles through
the modified Stöber method. Then, the Fe3O4 nanoparticles and Fe3O4@SiO2 composite nanoparticles were character-
ized by means of X-ray diffraction (XRD), Raman spectra, scanning electron microscope (SEM) and vibrating sample
magnetometer (VSM). Recently, the studies focus on how to improve the dispersion of composite particle and achieve
good magnetic performance. Hence effects o f the volume ratio of tetraethyl orthos ilicate (TEOS) and magnetite colloid
on the structural, morphological and magnetic properties of the composite nanoparticles were systematically investi-
gated. The results revealed that the Fe3O4@SiO2 had better thermal stability and dispersion than the magnetite
nanoparticles. Furth ermore, the particle size and magn etic property of the Fe 3O4@SiO2 composite nan opa rticles can be
adjusted by changing the vo lume ratio of TEOS and magnetite colloid.
Keywords: Magnetite Nanoparticles, Fe3O4@SiO2 Composite Nanoparticles, Dispersion, Thermal Stability, Particle
Size, Magnetic Property
1. Introduction
Magnetite nanoparticles have attracted a great deal of
attention because of their unique physicochemical prop-
erties and great potential use in various biomedical ap-
plications, such as contrast agents in magnetic resonance
imaging (MRI), carriers for targeted drug delivery, the
magnetic separation in microbiology, biochemical sens-
ing [1-4], etc.
However, the magnetite nanoparticles are unstable in
air and easily agglomerated after synthesis. The surface
coatings and functionalization could effectively solve
these problems [5-9]. Silica surfaces are chemically sta-
ble, biocompatible and can be easily functionalized for
bioconjugation purpose. Hence silica-coated magnetite
composite nanoparticles (Fe3O4@SiO2/core-shell) have
been synthesized by many groups [10-12]. Recently, sil-
ica coated magnetite functionalized with γ-mercaptopro-
pyltrimethoxysilane have been successfully applied to
extract Cd2+, Cu2+, Hg2+, and Pb2+ from water in a wide
pH range [13].
In this work, the silica-coated magnetite nanoparticles
are synthesized through two steps. The magnetite nano-
particles are firstly prepared by coprecipitation method
[5]. Then the magnetite nanoparticles are used to synthe-
size the Fe3O4@SiO2 composite nanoparticles through
the modified Stöber method [12]. The thermal stability
and morphologies of Fe3O4 and Fe3O4@SiO2 are studied.
Afterward, the effects of experimental parameters, such
as the volume of TEOS and magnetite colloid on the
properties of Fe3O4@SiO2 composite nanoparticles are
also systematically investigated.
2. Experiment
2.1. Materials
The iron (II) chloride tetrahydrate (FeCl2·4H2O, 99.7%),
iron (III) chloride hexahydrate (FeCl3·6H2O, 99.0%),
hydrochloric acid (HCl, 35 wt.% - 37 wt.%), sodium hy-
droxide (NaOH, 96 %), polyethylene glycol (PEG, Mw=
2000), tetraethyl orthosilicate (TEOS, 28%), ammonia
(NH3·H2O, 25 wt.% - 28 wt.%), and ethanol (C2H5OH,
99.7%) are all commercially available. Distilled water is
Synthesis and Characterization of Superparamagnetic FeO@SiO Core-Shell Composite Nanoparticles
50 3 42
also used for preparation of the solutions.
2.2. Preparations of the Magnetite Nanoparticles
An aqueous solution of Fe ions with a molar ratio of
Fe(II)/Fe(III) ~ 0.5 was prepared by dissolving 5.46 g
FeCl3·6H2O, 2.00 g FeCl2·4H2O and 0.6 g PEG-2000 in
60 ml of aqueous acid of 50 ml distilled water with 10 ml
of 1 M HCl, and then added dropwise into 100 ml of 1 M
NaOH with 1.0 g PEG-2000 solution under vigorous
stirring at 60˚C. The reaction was carried out in an inert
atmosphere by purging the reactor with high purity argon
(99.9%) all through. After all of the Fe ions solution was
added, the mixture was stirred for a further 2 h. Then the
colloid solution was washed by distilled water for several
times until it is neutral. A part of colloid is dried in fridge.
The remnant of the colloid was dispersed in distilled wa-
ter by ultrasonic, and then was ready for coating process.
2.3. Preparations of the Fe3O4@SiO2 Composite
Nanoparticles
Placed a certain volume of Fe3O4 colloid (2 wt.%) and
distilled water (total volume is 19 ml) into a 250 ml
three-neck flask, then added 80 ml ethanol, a certain
amount of TEOS and ammonia under vigorous stirring
(800 rpm) at room temperature for 12 h. The obtained
Fe3O4@SiO2 colloid was washed by repeated cycles of
distilled water and ethanol. Then, the nal products were
dried in an oven at 60°C for 24 h.
2.4. Characterization
The phase identification and crystalline structures of the
nanoparticles was characterized by X-ray powder dif-
fraction of the dried samples using a D/Max-2400 X-ray
diffractometer equipped with a Cu Kα monochromatic
radiation source (λ = 1.54187 Å).The Raman spectra of
nanoparticles were collected using a Laser Confocal Ra-
man Spectroscopy (LCRS). The morphologies were ob-
served using a scanning electron microscope Hitachi
S4800 operating at an accelerating voltage of 15 kV.
Magnetic measurements were performed using a vibrat-
ing sample magnetometer Lake Shore 7304.
3. Result and Discussion
3.1. Thermal Stability of Fe3O4 Nanoparticles
and Fe3O4@SiO2 Composite Nanoparticles
Phase identification is one of the most important uses of
XRD. Firstly, we obtained XRD pattern of the materials,
then, we compare data with known standards in the
JCPDS file to preliminary identify the materials.
As shown in Figure 1, XRD patterns of Fe3O4 after
heat treatments are shown. For the nanoparticles without
(Figure 1(a)) and with heat treatment at 200˚C (Figure
Figure 1. XRD patterns of Fe3O4 a nanoparticles under
different temperature heat treatments for 3 h. (a: without
treatment; b: 200˚C; c: 400˚C; d: 600˚C; e: 800˚C).
1(b)); 400˚C (Figure 1(c)), the XRD patterns (marked )
are well indexed to the cubic spinel phase of magnetite
(JCPDS No. 89-43191). No other significant peaks can
be observed in Figures 1(a) and (b), however, because
the diffraction peaks of γ-Fe2O3 are similar with Fe3O4,
we can only confirm that the nanoparticles with and
without 200˚C heat treatment do not contain impurities
except γ-Fe2O3. A further investigation will be obtained
by Raman. What’s more, the characteristic peaks of
spinel structure Fe2O3 (Marked *) can be simultaneously
observed in Figure 1(c). We can infer that Fe3O4 can be
completely transformed into Fe2O3 when Fe3O4 experi-
ences a heat treatment at 400˚C for enough time. Upon
heating at 600˚C for 3 h (Figure 1(d)), all of Fe3O4
nanoparticles transform into Fe2O3.
Similarly, Raman spectra are also used to identify the
phase of materials. To verify whether there is a phase
transition from Fe3O4 to γ-Fe2O3 for the nanoparticles
with and without 200˚C heat treatment, the correspond-
ing Raman spectra were also obtained. Figure 2 illus-
trates the Raman spectra of magnetite nanoparticles be-
fore (Figure 2(a)) and after (Figure 2(b)) heat treatment
at 200˚C. As we can see the peaks at 350 cm–1, 550 cm–1
and 670 cm–1 can be observed (Figure 2(a)), which can
be attributed to T2g,3, T2g,2, A1g vibration mode of Fe3O4
respectively. At higher wavenumber (~ 1378 cm–1), there
are no apparent peak. Hence, this result gives the obvious
evidence for the existence of Fe3O4 other than γ-Fe2O3
[14].
In Figure 2(b), the peak at 1400 cm–1 as well as peaks
at 350 cm–1, 550 cm–1 and 670 cm–1 indicates part of
Fe3O4 transforms into γ-Fe2O3. We can confirm Fe3O4
will completely transform into γ-Fe2O3 under a 200˚C
heat treatment for a long enough time.
In general, the Fe3O4 nanoparticles will partly trans-
form into γ-Fe2O3 under a 200˚C heat treatment for 3 h.
On experiencing a 600˚C heat treatment for 3 h, the
Copyright © 2011 SciRes. WJCMP
Synthesis and Characterization of Superparamagnetic Fe3O4@SiO2 Core-Shell Composite Nanoparticles
Copyright © 2011 SciRes. WJCMP
51
Figure 3. XRD patterns of Fe3O4@SiO2 nanoparticles under
different temperature heat treatments for 3 h. (a: without
treatment; b: 200˚C; c: 400˚C; d: 600˚C; e: 800˚C).
Figure 2. Raman spectra of Fe3O4 nanoparticles before (a);
and after heat treatment at 200˚C for 3 h (b).
Fe3O4 nanoparticles will completely transform into
Fe2O3. content accumulates with the TEOS volume increasing.
Figure 6, the SEM images of Fe3O4 @SiO2 composite
nanoparticles with different ratio of TEOS, indicates that
the increasing TEOS volume ratio leads to a bigger size
of composite particle. This can be ascribed to the fact
that when the dosage of TEOS increases, the quantity of
SiO2 increases. Therefore, the Fe3O4 particles are ade-
quately coated, and the size of composite nanoparticles
increases with ratio of TEOS increasing. This result con-
sisted with the XRD result.
Thermal stability of Fe3O4@SiO2 composite nanopar-
ticles is also investigated. In Figure 3, XRD patterns of
Fe3O4@SiO2 after heat treatments are shown. The peaks
of Fe3O4 and SiO2 amorphous hump at 23° can be ob-
served for all samples. In addition, characteristic peaks of
Fe2O3 appear (Marked *) with the heat treatment tem-
perature of 800°C (Figure 3(e)). After 800°C heat treat-
ment for 3 h, the predominant phase of the Fe3O4@SiO2
composite nanoparticles is still Fe3O4(Figure 3(e)). But
the pure Fe3O4 has completely transformed into Fe2O3 at
600°C for 3 h (see Figure 1(d)). This suggests that the
Fe3O4@SiO2 has a significantly higher thermal stability
than the pure Fe3O4.
Room temperature magnetic properties of Fe3O4@
SiO2 composite nanoparticles with different ratio of
TEOS were measured using VSM, as shown in Figure 7.
It is manifest in Figure 7 that there is no hysteresis in
the magnetization curve, the coercivity eld and remnant
magnetization cannot be found from the curve. It
conrms that Fe3O4@SiO2 composite nanoparticles are
superparamagnetic. The magnetization does not saturate
at 11000 Oe, and the values are 12.7 emu/g, 6.3 emu/g,
4.3 emu/g, 3.1 emu/g and 2.4 emu/g for various TEOS
concentrations respectively, which shows a trend of
gradual decrease. The reason is that SiO2 coating grows
thicker with increasing concentration of TEOS, thus re-
duces their magnetism. In a word, the magnetism of the
composite particles can be controlled by adjusting the
concentration of TEOS.
3.2. The improvement in Dispersion by SiO2
Coating
The morphological characteristics of the Fe3O4 nanopar-
ticles and Fe3O4@SiO2 composite nanoparticle are
shown in Figure 4. The Fe3O4 nanoparticles are agglom-
erated seriously and the size is very small (Figure 4(a)).
But the composite nanoparticles are almost monodisperse
with uniform size (Figure 4(b)). It proves the dispersion
of Fe3O4@SiO2 composite nanoparticles is apparently
improved.
3.3. The effect of the Volume of TEOS on the
Structural, the Morphological and the Mag-
netic Properties
3.4. The Effect of the Volume of Magnetite
Colloid on the Structural, the Morphological
and the Magnetic Properties
While increased the volume ratio of TEOS, other condi-
tions, such as 4 ml Fe3O4 colloid, 2 ml ammonia at room
temperature, kept the same.
The influence of the concentration of magnetite colloid
on structure, morphology and magnetism were also stud-
ied here. During this reaction, the ammonia and TEOS
both were fixed to 2 ml, while the volume of magnetite
colloid was different.
Figure 5 summarizes the crystalline structure de-
pendence on the ratio of TEOS. The amorphous hump of
SiO2 around 23° and the characteristic peaks of Fe3O4
around 35.4° can be observed for all samples. The in-
crease in intensity of amorphous hump suggests that SiO2
Figure 8 shows XRD spectra of Fe3O4@SiO2 com-
posite nanoparticles with different ratio of magnetite
Synthesis and Characterization of Superparamagnetic FeO@SiO Core-Shell Composite Nanoparticles
52 3 42
Figure 4. SEM images of Fe3O4 nanoparticles (a) and Fe3O4@SiO2 composite nanoparticles (b).
Figure 5. XRD patterns of Fe3O4@SiO2 nanoparticles syn-
thesized with different TEOS volume (a: 1 ml TEOS; b: 2
ml TEOS; c: 3 ml TEOS; d: 5 ml TEOS; e: 8 ml TEOS).
Figure 6. SEM images of Fe3O4@SiO2 nanoparticles syn-
thesized at different TEOS volume (a: 1 ml TEOS; b: 2 ml
TEOS; c: 3 ml TEOS; d: 5 ml TEOS; e: 8 ml TEOS).
colloid. The amorphous hump of SiO2 and characteristic
peaks of Fe3O4 can be observed for all samples. But the
width and the intensity of the amorphous hump decreases,
opposing to the increasing volume of Fe3O4 colloid.
As we can see the morphologies of Fe3O4@SiO2 com-
posite nanoparticles with different ratio of TEOS in Fig-
ure 9, the size of composite nanoparticles tends to reduce
gradually and the dispersion becomes worse with in-
Figure 7. Magnetic hysteresis curves of Fe3O4@SiO2
nanoparticles synthesized at different TEOS volume (a: 1
ml TEOS; b: 2 ml TEOS; c: 3 ml TEOS; d: 5 ml TEOS; e: 8
ml TEOS).
Figure 8. XRD pattern of Fe3O4@SiO2 nanoparticles with
different magnetite colloid volume (a: 1 ml magnetite col-
loid; b: 2 ml magnetite colloid; c: 4 ml magnetite colloid; d:
6 ml magnetite colloid; e: 8 ml magnetite colloid).
creasing amount of Fe3O4 colloid. The reason is just that
for increasing quantity of Fe3O4 colloid, the concentra-
tion of the SiO2 becomes diluted and consequently the
SiO2 is inadequate to coat Fe3O4 colloid. Therefore com-
posite particles become agglomerated, accompanied by
reduced size of theirs.
The magnetization curve of Fe3O4@SiO2 nanoparticles
with different volume of magnetite colloid are illustrated
in Figure 10. It is also without hysteresis, and no sign of
Copyright © 2011 SciRes. WJCMP
Synthesis and Characterization of Superparamagnetic FeO@SiO Core-Shell Composite Nanoparticles53
3 42
Figure 9. SEM images of Fe3O4@SiO2 nanoparticles with
different magnetite colloid volume (a: 1 ml magnetite col-
loid; b: 2 ml magnetite colloid; c: 4 ml magnetite colloid; d:
6 ml magnetite colloid; e: 8 ml magnetite colloid).
finite coercivity eld and remnant magnetization can be
found from the curve. From the magnetization curve, we
can also see that the magnetization does not saturate at
11000 Oe magnetic fields. The magnetization is 5.2 emu/
g, 8.3 emu/g, 16.7 emu/g, 20.2 emu/g and 23.8 emu/g for
increasing volume of colloid. The results are well coin-
cident with XRD spectrum and SEM results. This high-
lights proper concentration of magnetite colloid redounds
to good despersion and magnetism.
4. Conclusions
In summary, the Fe3O4 nanoparticles are seriously ag-
glomerated after synthesis. Additionally, the Fe3O4 nano-
particles completely transform into Fe2O3 after a 600°C
heat treatment for 3 h.
What’s more, the Fe3O4@SiO2 composite nanoparti-
cles are almost monodisperse. It proves the SiO2 coating
remarkably improves the dispersion of Fe3O4 nanoparti-
cles. When Fe3O4@SiO2 composite particles experience
an 800°C heat treatment for 3 h, the major phase is still
Fe3O4, which verifies that the composite particles exhibit
better thermal stability than magnetite nanoparticles. Be-
sides, silica surfaces are chemically stable, biocompatible
and can be easily functionalized for bioconjugation pur-
poses, Fe3O4@SiO2 composite nanoparticles have great
potential applications in various biomedical fields, such
as DNA purification, protein-separation, targeted drug
delivery, magnetic hyperthermia and magnetofection.
In modified Stöber method, the experimental parameters
such as volumes of TEOS and magnetite colloid are dis-
cussed. The particle size, dispersion and magnetic prop-
erties of the Fe3O4@SiO2 composite particles can be
controlled by changing the volume of TEOS and the
magnetite colloid.
Figure 10. Magnetic hysteresis curves of Fe3O4@SiO2
nanoparticles with different magnetite colloid volume (a: 1
ml magnetite colloid; b: 2 ml magnetite colloid; c: 4 ml
magnetite colloid; d: 6 ml magnetite colloid; e: 8 ml mag-
netite colloid).
5. Acknowledgements
This work was financially supported by the Key grant
Project of Chinese Ministry of Education (Grant No.
309027), and by the National Science Fund for Distin-
guished Young Scholars (Grant No. 50925103).
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