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Nanoscale Res Lett (****) *:*** ***
NANO EXPRESS
One-Pot Reaction and Subsequent Annealing to Synthesis Hollow
Spherical Magnetite and Maghemite Nanocages
Wei Wu Xiangheng Xiao Shaofeng Zhang
Hang Li Xiaodong Zhou Changzhong Jiang
Received: 24 April 2009 / Accepted: 5 May 2009 / Published online: 22 May 2009
to the authors 2009
Abstract Water-soluble hollow spherical magnetite superparamagnetic, low Curie temperature, and high
(Fe3O4) nanocages (ca. 100 nm) with high saturation coercivity, high susceptibility. [1]. Currently, magnetic
magnetization are prepared in a one-pot reaction by sol-gel nanoparticles are used in important biological applications,
method and subsequent annealing to synthesise the ma- mainly including magnetic bioseparation and detection of
ghemite (c-Fe2O3) nanocages with similar nanostructures. biological entities such as cell, protein, nucleic acids,
The nanocages have been investigated by powder X-ray enzyme, bacterials, and virus. [2, 3]. To this end, magnetic
diffraction (XRD), transmission electron microscope iron oxide nanoparticles have become strong candidates,
(TEM), high-resolution transmission electron microscope and the application of small iron oxide nanoparticles in
(HRTEM), and superconducting quantum interference in vitro diagnostics has been practiced for nearly half a
device (SQUID). The results indicated that glutamic acid century [4, 5]. In addition, magnetite (Fe3O4), maghemite
(c-Fe2O3) and hematite (a-Fe2O3) are promising and pop-
played an important role in the formation of the cage-like
nanostructures. ular candidates since biocompatibility has been obtained.
Usually, different biological applications require different
Keywords Magnetite Maghemite Sol-gel growth morphologies and size of magnetic nanoparticles. More-
Nanocages over, magnetic colloid particles offer attractive possibilities
in bioseparation or biodetection and they should be made at
dimensions comparable to those of a virus (20 500 nm), a
protein (5 50 nm), or a DNA (10 100 nm) [6 10].
Introduction
The internal structure and the external morphology of
Magnetic nanoparticles are of great interest to researchers iron oxide nanoparticles have a signi cant in uence on their
for their wide range a board of applications, including practical applications. Particularly, the polymorphic nature
magnetic uid, data storage, catalyst and biotechnology, of iron oxides and phase-transition studies in the nanoscale
owing to their unique magnetic properties such as regime have attracted much attention due to its widely
applications. Therefore, it is important to develop facile
methods to regular both their surface morphology and size.
W. Wu C. Jiang However, it is still a technical challenge to control the size,
Key Laboratory of Acoustic and Photonic Materials and Devices
shape, dispersibility and stability of iron oxide nanoparti-
of Ministry of Education, Wuhan University, 430072 Wuhan,
People s Republic of China cles. Several preparation methods have also been reported
e-mail: *******@***.***.**
on the synthesis of high quality of iron oxide nanoparticles,
including co-precipitation, thermal decomposition, micro-
W. Wu X. Xiao S. Zhang H. Li X. Zhou C. Jiang
emulsion, hydrothermal synthesis, and sono-chemical
Department of Physics, Wuhan University, 430072 Wuhan,
People s Republic of China method. [11, 12]. In these methods, co-precipitation was
often employed for obtaining water-soluble and biocom-
W. Wu X. Xiao C. Jiang
patible iron oxide nanoparticles, but this method presents
Center of Electron Microscopy, Wuhan University,
low control of the particle shape, generates broad size
430072 Wuhan, People s Republic of China
123
Nanoscale Res Lett (2009) 4:926 931 927
distributions, and cannot avoid aggregation. Thermal separation stand (MSS), purchased from Promega (Z5333),
decomposition and microemulsion are generally stabilized was used to separate magnetic particles using washing and
in an organic solvent by surfactants. The hydrothermal selecting steps.
synthetic route often requires high temperature and pressure
[13]. Moreover, it is important to note that using these Preparing Hollow Spherical Magnetite Nanocages
methods at is dif cult to obtain [50 nm iron oxide nano-
particles in a one-pot reaction without extra coating or seed- For the synthesis of hollow spherical magnetite nanocages,
mediate processes [14]. in a typical synthesis, solution A was prepared by dis-
Furthermore, hollow iron oxide nanoparticles have large solving 2.02 g KNO3 and 0.28 g KOH in 50 mL double
surface area and low material density, and these nanopar- distilled water; solution B was prepared by dissolving
0.070 g FeSO4 7H2O in 50 mL double distilled water.
ticles could be potential lightweight structure materials and
can be utilized for catalysis or drug-delivery. Titirichi and Then, the two solutions were mixed together under mag-
co-workers reported the diameter of ca. 500 nm hollow netic stirring at a rate of ca. 400 rpm. Two minutes later,
iron oxide microspheres by the hydrothermal approach solution C [prepared by dissolving 0.18 g glutamic acid
[15], and recent progress has shown that hollow magnetite (Gla) in 25 mL double distilled water] were added drop-
nanoparticles can be synthesized by controlling oxidation wise into the mixed solution. The reaction temperature was
raised incrementally to 90 C and kept for 3 h under argon
of Fe-Fe3O4 nanoparticles [16]. Yu et al. also reported that
cage-like Fe2O3 hollow spheres were fabricated by the (Ar) atmosphere. Meanwhile, the brown solution was
template route [17]. Herein, we report a facile and con- observed to change black. After the mixture was cooled to
trolled synthesis of ca. 100 nm hollow magnetite and room temperature, the precipitate products were magneti-
maghemite nanocages with uniform spherical morphology cally separated by MSS, washed with ethanol and water
by a one-pot reaction via the sol-gel method (Fig. 1). To two times, respectively, and then redispersed in ethanol
the best of our knowledge, there have been, so far, a few (sample 1, S1). The same preparing process without added
reports using nely controlled synthesis of magnetite any Gla was used to obtained the sample 3 (S3).
nanocages with spherical morphology and using a one-pot
sol-gel technology. Such a synthetic route is expected to Preparing Hollow Spherical Maghemite Nanocages
have potential applications in other materials.
Precipitate S1 was subjected to a series of isochronal
annealing at 500 C (sample 2, S2) for 2 h in oxygen
atmosphere, and the heating rate was 5 C/min.
Experimental
Materials Characterization
Ferrous sulfate (FeSO4 7H2O, AR) and potassium XRD patterns of the samples were recorded on a D8
Advance X-ray diffractometer using Cu Ka radiation (k =
hydroxide (KOH, AR) were purchased from Tianjin Ker-
mel Chemical Reagent CO., Ltd., potassium nitrate (KNO3, 0.1542 nm) operated at 40 kV and 40 mA. For TEM
AR) was purchased from Beijing Hongxing Chemical observations, S1 and S3 (powder samples redissolved in
Reagent CO., Ltd., ethanol (C2H5OH, 95%, AR) and L - ethanol) were dropped on copper grids and observed on a
glutamic acid (C5H9NO4, BR) were purchased from Sin- JEOL JEM-2010 (HT) transmission electron microscope
opharm Chemical Reagent CO., Ltd., and all were used as at an acceleration voltage of 150 kV. For HRTEM obser-
received. The MagneticSphere Technology magnetic vations, S1 and S3 (the annealed powders redissolved in
ethanol) were dropped on copper grids and observed on a
JEOL JEM-2010 (FEF) eld-emission transmission electron
microscope at an acceleration voltage of 200 kV. Magnetic
measurements were performed using a Quantum Design
MPMS XL-7 SQUID magnetometer. The powder sample
was lled in a diamagnetic plastic capsule, and the packed
sample was then put in a diamagnetic plastic straw and
impacted into a minimal volume for magnetic measure-
ments. Background magnetic measurements were checked
for the packing material. Fourier transform infrared spec-
trum (FT-IR) measurement was carried out on a Nicolet 5700
Fig. 1 Schematic gure depicting the synthesis processes of hollow
FT-IR Spectrometer. Vacuum-dried S1 samples were mixed
iron oxide nanocages
123
928 Nanoscale Res Lett (2009) 4:926 931
and compressed with KBr to obtain pellets for FT-IR
analysis.
Results and Discussion
Figure 2 shows the X-ray diffraction (XRD) patterns of
Fe3O4 nanocages (S1, curve a) and c-Fe2O3 nanocages (S2,
curve b). The (220), (311), (400), (422), (511), and (440)
diffraction peaks observed at curves can be indexed to the
cubic spinel structure, and all peaks are in good agreement
with the Fe3O4 phase (JCPDS card 19-0629 is also shown
in the bottom of Fig. 2, blue line) and the c-Fe2O3 phase
(JCPDS card 39-1346 is also shown in the bottom of Fig. 2,
red line), respectively. Maghemite can be prepared by the
oxidation of magnetite under air at T = 523 K. This result
reveals that the phase changes are in the direction of
magnetite to maghemite, and the width of the diffraction
line of S2 increases, owing to the annealing treatment [18].
The obvious electron-density differences between the
dark edge and pale center of Fig. 3 further con rms the
hollow interiors clearly. Figure 3a, b displays the TEM
images of the Fe3O4 and c-Fe2O3 nanocages . It was
found that the Fe3O4 nanocages had a hollow structure and
the overall diameter of the nanocages is around 100 nm,
which indicated an oriented aggregation of small Fe3O4
nanoparticles. One can see that the shape and size of the
c-Fe2O3 nanocages are similar to those Fe3O4 nanocages.
However, the size of the central hole of nanocages
becomes smaller after annealing, owing to the thermal
diffusion of the small nanoparticles. The selected-area
electron diffraction (SAED) pattern in the insert of Fig. 3a,
b reveals the polycrystal-like feature of the samples, and
their pattern agree well with the structure planes of iron
oxide nanocages. When FeSO4 and KOH mixed together,
Fig. 3 TEM images of a S1, b S2 and c S3; the inset is the
corresponding SAED pattern
the solution generates Fe(OH)2 gels; subsequently upon
addition of KNO3 to this mixture, many of small magnetite
nanoparticles (1# as shown in Fig. 3c) were formed
through homogeneous nucleation. In contrast, if the Gla
Fig. 2 XRD pattern of Fe3O4 nanocages (a) and c-Fe2O3 nanocages
was not added, these small magnetite nanocages neither
(b)
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Nanoscale Res Lett (2009) 4:926 931 929
demonstrate obvious growth nor do they aggregate due to
the gel network structure in the gel solution, when several
nucleation take place, the gels begin to occulate during
the aging period and form large Fe3O4 nanoparticles, the
size range from 50 to 100 nm (2# as shown in Fig. 3c)
[19]. Further detailed studies on the formation mechanism
of the nanocages are currently under investigation.
Information on high-resolution morphologies and
structures of the iron oxide nanocages can be gleaned from
Fig. 4, and the HRTEM images obtained near the center
region of hollow nanocages. Figure 4a shows that the
Fe3O4 nanocages include three single crystalline with an
Fig. 5 FT-IR spectra of S1 (signi cant IR bands (1) 577.2 cm-1; (2)
1,147.4 cm-1 (3) 1,633.0 cm-1)
interplanar spacing of 0.296 nm for the {220} plane,
0.210 nm for the {400} plane, and 0.172 nm for the {422}
plane, respectively. Figure 4b shows that the c-Fe2O3
nanocages include two single crystalline obviously with an
interplanar spacing of 0.374 nm for the {210} plane and
0.295 nm for the {220} plane, respectively. The results
further con rmed that the nanocages consist of randomly
small iron oxide nanocrystals, and the nanocages present
polycrystalline feature on the whole.
The attachment of Gla on the nanocage surface was
con rmed by FT-IR spectroscopy (Fig. 5). The Fe O
stretching vibration is observed at 577.2 cm-1, and C O
stretching vibration at 1,100 1,200 cm-1; the bands that can
be assigned to vibration of C O are observed at
1,147.4 cm-1, and the bands that can be assigned to vibration
of COO- group are observed at 1,633.0 cm-1 [20 22].
The magnetic properties of iron oxide nanocages were
also investigated by SQUID. Hysteresis loop (Fig. 6A)
measurements demonstrate that both samples exhibiting
magnetization curves for the magnetic nanocages show no
hysteresis; the forward and backward magnetization curves
overlap completely and are almost negligible. Moreover,
the magnetic nanocages have zero magnetization at zero
applied eld, indicating that they are superparamagnetic at
room temperature. No remnant magnetism was observed
when the magnetic eld was removed [23]. This magnetic
hysteresis phenomenon is also in agreement with previous
reports by other groups [24 26]. The result reveals that
these large nanocages are superparamagnetic is owing to
the fact that they are composed of many small nanoparti-
cles that show oriented aggregation into a hollow structure
[27]. The saturation magnetization (MS) of Fe3O4 nano-
cages was found to be 87 emu g-1 at 300 K and the MS of
Fig. 4 HRTEM images of the center region of hollow Fe3O4
c-Fe2O3 nanocages was found to be 27 emu g-1 at 300 K.
nanocages (a) and c-Fe2O3 nanocages (b)
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930 Nanoscale Res Lett (2009) 4:926 931
Fig. 6 Hysteresis loops (A) of
Fe3O4 nanocages (a) and c-
Fe2O3 Nanocages (b);
Photographs (B) of the Fe3O4
nanocages and c-Fe2O3
nanocages (dispersed in ethanol
solution) before and after
magnetic separation by an
external magnetic eld (this
magnet takes from the MSS)
It is noteworthy that the MS of Fe3O4 nanocages is close to In this method, Gla plays an important role in the formation
that of bulk magnetite (92 emu g-1) [28]. Moreover, in the of magnetite nanocages with hollow structure. The sub-
case of no hysteresis, the average size of the magnetic sequent annealing will decrease the size of the central hole
particle can be estimated from the initial susceptibility (vi), of hollow nanocages. The iron oxide nanocages prepared
vi = (dM/dH)H?0 coming mainly from the largest parti- can be well dispersed in aqueous solution and show good
cles. An upper limit for the magnetic size, Dm, may be stability. The magnetic property measurements of Fe3O4
estimated using the following formula [29]: nanocages show superparamagnetism with very high sat-
uration magnetization close to the value of bulk Fe3O4
18kB T vi
Dm (92 emu g-1). The synthetic strategy developed in this
2
p qMs
study may also be extended to the preparation of other
where q is the density of iron oxide nanoparticles, and magnetic nanoparticles, which also opens up new potential
Fe3O4 is 5.18 g/cm3, and c-Fe2O3 is 5.24 g/cm3 and kB is avenues for the nanostructural controlling and promising
the Boltzmann constant. Thus the value of vi can be applications in various elds of nanotechnology.
determined approximately. Using the values of saturation
Acknowledgments The author thanks the National Nature Science
magnetization, MS, obtained from the magnetization curve,
Foundation of China (No. 10775109), the Specialized Research Fund
the values of Dm were estimated: namely, the diameter of
for the Doctoral Program of Higher Education (No. 200********), and
Fe3O4 nanocages was about 28 nm, and that of c-Fe2O3 the Young Chenguang Project of Wuhan City (No. 200***-******) for
nanocages was about 42 nm, the result reveals that the iron nancial support. The author thanks Associate Prof. H. -Y. Zhang of
oxide nanocages are composed of small nanoparticles Tsinghua University for assistance with the SQUID measurements.
and the particle size as estimated by magnetization in
agreement with the TEM images (Fig. 3) and HRTEM
results (Fig. 4).
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