Fe3O4 core/layered double hydroxide shell nanocomposite: Versatile magnetic matrix for anionic functional materials

Article abstract of DOI:10.1002/anie.200901730

Layering up: The title nanocomposite shows high anion loading capacity. The demonstration of the assembly of a W₇O₂₄^6- catalyst by this method may provide a general route to the facile and direct fabrication of composite s

Full text of DOI:10.1002/anie.200901730

Communications
DOI: 10.1002/anie.200901730
Magnetic Materials
Fe3O4 Core/Layered Double Hydroxide Shell Nanocomposite:
Versatile Magnetic Matrix for Anionic Functional Materials**
Liang Li,* Yingjun Feng, Yongsheng Li, Wenru Zhao, and Jianlin Shi*
Recently, micro- or nanospheres with magnetic cores and
functional shell structures have received much attention
because of their potential applications in catalysis, drug
storage/release, selective separation, chromatography, and
chemical or biosensors.[1–10] Until now, different shell frame-
works have been developed to pursue these aims. Some
groups fabricated mesoporous silica shell to load metal oxides
and fluorescence imaging materials and used them in catalysis
and drug storage/release systems.[1–3] Diat et al. discovered
that monodisperse and compactly packed onion phases could
be obtained in concentrated solutions of surfactants under
controlled mechanical shear. Their applications as carriers for
drug delivery or nanoreactors for the synthesis of metal
nanoparticles have been explored.[8] Wang et al. used dimer-
captosuccinimide acid modified Fe2O3 nanoparticles and
combined them with CdSe/ZnS quantum dots (QDs) to
prepare water soluble magnetic luminescent composites in an
organic/water two-phase mixture.[9] Bawendi and co-workers
simultaneously encapsulated both CdSe QDs and g-Fe2O3 in
silica microspheres to prepare a magnetic luminescent
composite.[10] All these exploitations of the shell structure
greatly expand the realm of applications of magnetic core/
shell materials. However, in all of these applications, the
method for loading functional materials into the shell
structure is largely different from each other, even for the
same shell structures. Is there a facile and versatile approach
to synthesize magnetic core/functional shell structures?
Layered double hydroxides (LDHs), consisting of stacked
brucite-type octahedral layers with anions and water mole-
cules occupying the interlayer space, are currently attracting
intense research interest.[11–13] Recently, we succeeded in total
delamination of LDH crystals in nitrate form upon treatment
with formamide, and used this product as a building block to
fabricate the LDH hollow shell structure.[14] The LDH
structure collapsed upon heat treatment at about 5008C to
yield mixed oxides of constituent elements, which underwent
restoration of the LDH structure in the presence of water and
anions.[15] This so-called memory effect property of LDHs has
been employed as an effective synthetic route for inserting
various functional inorganic and organic anions into LDHs
galleries.[16,17]
Herein, we report the synthesis and characterization of a
Fe3O4 core/oxide shell nanocomposite decomposed from a
layered double hydroxide, which combines both versatile
anion loading capability and high separation efficiency,
making it an ideal support for recoverable anionic functional
materials. As shown in Figure 1, the center of the structure is a
Figure 1. Synthesis of the magnetic core/anionic functionalized LDH
shell composite structure. The Fe3O4 core is first coated with a thin
layer of silica, then further coated with a multilayer CO32À–LDH
composite shell. Afterwards, a calcination process is performed to
remove CO32À, and the shell become amorphous. Finally, the LDH
structure can be restored and anions can be simultaneously interca-
lated in between the LDH galleries upon immersion in an aqueous
solution containing the desired anions.
[*] Prof. Dr. L. Li, Dr. Y. Feng, Dr. Y. Li, Dr. W. Zhao, Prof. Dr. J. Shi
Key Laboratory for Ultrafine Materials of Ministry of Education,
School of Materials Science and Engineering, East China University
of Science and Technology, Shanghai 200237 (China)
Fax: (+86)21-6425-0740
silica-coated magnetite (Fe3O4) core, which is composed of
many small Fe3O4 crystallites. The Fe3O4 cores strongly
interact with external magnetic fields and can be easily
separated from solution under a moderate magnetic field. The
outer surface of the structure is layered double hydroxide. It
can be expected that recoverable anionic functional materials
will be easily formed through the reconstruction of this
thermally decomposed Fe3O4 core/LDH shell composite.
Mg–Al LDH powder (mole ratio Mg/Al = 3:1) in carbon-
ate form was obtained from Kyowa Chemical Industry Co.,
Ltd. Mg–Al LDH was converted into a nitrate form by anion
exchange in a typical salt/acid treatment, and subsequently
E-mail: liliang@ecust.edu.cn
Prof. Dr. J. Shi
State Laboratory of High Performance Ceramic and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy
of Sciences, Shanghai 200050 (China)
E-mail: jlshi@sunm.shcnc.ac.cn
[**] This work was supported by the National Nature Foundation of
China (grant no. 20633090), Shanghai Pujiang Program (grant no.
08PJ14035), and Shanghai Nature Science Foundation (grant no.
07ZR14028).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901730.
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Chemie
delaminated in formamide.[14a] A typical synthetic procedure
of versatile magnetic matrix for anionic functional materials is
as follows. Magnetic Fe3O4 crystalline particles (ca. 400 nm)
were synthesized using a solvent–thermal method.[18] After
being coated by a layer of silica following a sol–gel process
reported previously by Shi et al.,[3] silica-coated magnetite
core composites (0.5 g) were dispersed in a formamide
suspension (100 mL) containing LDH nanosheets (0.05 g)
and then ultrasonically agitated for 20 minutes to promote the
adsorption of LDH nanosheets onto the silica surface. The
sample was recovered by centrifugation (6000 rpm, 10 min)
and washed with ultrapure water. In the next step, the sample
was redispersed in an aqueous solution of Na2CO3 (100 mL,
2 gLÀ1). The product was recovered by centrifugation. Fe3O4/
SiO2 cores coated with 20 layer pairs of carbonate and LDH
nanosheets ((CO32À/LDH)20) were synthesized by repeating
the above procedures 20 times. The obtained sample was
heated to 4808C at a ramp rate of 208ChÀ1 under N2
atmosphere and kept at this temperature for 4 h to remove
CO32À and water. Finally, the calcined material was dispersed
into aqueous solution to recover its original LDH structure
and in the meantime absorb the functional anions into the
LDH galleries.
the LDH nanosheet architecture remained intact in the layer-
by-layer assembly process.
This kind of LDH core/shell structure possesses versatile
anion loading capability after calcination because of its
memory effect, which makes them ideal supports for recov-
6À
erable anionic functional materials. Herein, we chose W7O24
as an example and investigated the photodegradation behav-
ior of aqueous organochlorine pesticide by using this Fe3O4
core/W7O246À LDH as a model system to demonstrate the use
of our core/shell structure as a versatile recoverable catalyst
support.
The original carbonate LDH shell structure was destroyed
after heating at 4808C under N2 atmosphere for 4 h. The XRD
pattern (Figure 2c) does not show any peak other than that
from the Fe3O4 core, suggesting the transformation of the
LDH shell to amorphous metal oxides such as MgO and
Al2O3. When this Fe3O4 core/amorphous shell material was
dispersed into the prepared aqueous solution at 508C to
6À
absorb the W7O24 for 24 h, the sample became crystalline
again (Figure 2d). The four major peaks (at 2q values of 7.28,
14.58, 21.68, and 29.08) can be ascribed to a basal diffraction
series with an interplanar spacing of 1.2 nm. The additional
peak at 60.58 can be identified as two-dimensional diffraction
peaks of the 110 plane for a hexagonal cell with a = 0.31 nm.
The XRD data strongly suggest an anion inserting process
and reconstruction of the LDH structure by the layer memory
effect. The basal spacing of 1.2 nm is characteristic of LDHs
Figure 2 depicts X-ray diffraction (XRD) data for the
sample at various stages of the fabrication of Fe3O4 core/LDH
shell nanocomposite. Characteristic peaks from the Fe3O4
in W7O24 form.[17] Furthermore, compared with the original
6À
calcined sample, there is also a broad diffraction peak around
a 2q value of 108 (indicated by the asterisk), which is the
fingerprint of polyoxometalates pillared LDH.[20] This result
6À
is further confirmation that W7O24 ions have been success-
fully inserted into the LDH gallery. In addition, the average
shell thickness of the reconstructed Fe3O4 core/W7O246À LDH
shell composite is calculated by using Sherrerꢀs equation on
the (003) peak to be around 20 nm.
Figure 3c shows a scanning electron microscopy (SEM)
2À
image of the obtained Fe3O4/SiO2 core/CO3 LDH shell
composite. The spherical morphology of monodisperse Fe3O4
core/silica shell beads was preserved well after the deposition
of the 20 layers of carbonate LDH shell. The LDH nanosheets
can rarely be identified in SEM images because of their high
flexibility. The only noticeable difference between the spheres
with or without shells was the surface roughness. The
spherical morphology was well preserved and there is no
visible unwrapped core or separate irregular particles in the
SEM image. The CO32À–LDH shell thickness is estimated
from Figure 3d to around 15 nm.
Figure 2. XRD patterns of Fe3O4 core alone (a); after deposition of 20
LDH/CO32À layer pairs (b); after calcination at 4808C for 4 h (c); and
6À
after treatment in W7O24 aqueous solution (d). (Diffraction peaks of
:Fe3O4; : carbonate–LDH; : W7O246À–LDH; : polyoxometalate–
~
&
*
*
LDH).
When calcined at 4808C, the Fe3O4 core/carbonate shell
sample lost 4.2% of its weight in two steps (Figure 4). These
weight losses were accompanied by huge endothermic peaks,
suggesting the decomposition of the carbonate LDH shell.
The endothermic peak at 2148C can be assigned to dehy-
dration of the carbonate LDH layer, and two peaks at about
3208C and 4128C can be assigned as dehydroxylation or the
collapse of the hydroxide layers, which overlaps the decom-
position of CO32À to CO2, as shown in Figure 5. These data are
consistent with those in the literature.[18] Slow heating at a rate
of 28CminÀ1 was essential to retain the shell structure. Rapid
core in an angular range of 18–658 remained intact during
the complete process. The sample coated with 20 layer pairs of
2À
LDH nanosheets/CO3 shows two major peaks at 2q values
of 11.3 and 22.78 (indicated by triangles), which can be
ascribed to the formation of carbonate LDH shell nano-
structure with a repeating distance of approximately 0.78 nm;
a similar value has been reported many times for carbonate
LDH in power form.[19] One additional new peak at 60.58 can
be assigned to intrasheet reflections of 110 peaks from a two-
dimensional hexagonal cell (a = 0.31 nm),[20] confirming that
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Figure 4. Thermogravimetry/differential thermal analysis (TG-DTA) dia-
gram of Fe3O4 core/carbonate LDH shell composite.
Figure 5. FTIR spectra of Fe3O4/SiO2 core initially (a), after layer-by-
layer coating with carbonate LDH (b), after calcination at 4808C for
6À
*
:
4 h (c), and after reconstruction in W7O24 aqueous solution (d).
adsorption bands of water molecules; : W7O246À ions.
~
process. In addition to these peaks, there were strong
adsorption bands at 3700–3000 and at 1630 cmÀ1, which are
attributable to the stretching and bending vibrations of water
molecules. This result suggests that the LDH/CO32À shell has
been hydrated. In contrast, the spectrum of the calcined
sample was rather featureless as a consequence of the
removal of interlayer ions and the collapse of LDH layers.
The reconstruction of the LDH structure after mixing with
the W7O246À aqueous solution is clear from the bands at 3400
and 1630 cmÀ1 (labeled with black dots). Furthermore, some
additional peaks at 950, 905, 833, 661, and 581 cmÀ1 (indicated
by triangles) appeared. All these bands can be ascribed to the
Figure 3. SEM images of Fe3O4 core (a), Fe3O4 core covered with SiO2
layer (b), Fe3O4/SiO2 core/CO32À–LDH shell composite (c), the core
6À
shell structure after calcination (e), and the Fe3O4/SiO2 core/W7O24
LDH shell structure after reconstruction (g); TEM images of Fe3O4/
–
SiO2 core/carbonate LDH shell composite (d), the core shell structure
after calcination (f), and Fe3O4/SiO2 core/W7O246À–LDH shell structure
after reconstruction (h).
heating of the sample could disrupt the shell structure as a
result of violent evolution of steam and carbon dioxide.
The FTIR data shown in Figure 5 are additional evidence
for the chemical and structural modifications of the sample
described above. The as-prepared silica covered Fe3O4 core/
carbonate LDH shell nanocomposite showed sharp adsorp-
6À
vibration mode of W7O24 ions,[17] again indicating the
6À
formation of LDH in W7O24 form.
After the shell structure recovered from the amorphous
metal oxides to the LDH structure, the spherical morphology
is still apparent (Figure 3e,f). Compared with the sample
before reconstruction, the shell thickness became somewhat
larger and the surface became rougher. The high-resolution
TEM (HRTEM) image shows a lamellar structure with a
repeating fringe of 1.2 nm (Figure 3h), and a shell thickness
about 20 nm, which is consistent with the XRD results.
2À
tion bands attributable to CO3 and SiO2. A strong peak at
1350 cmÀ1 can be assigned as a vibration mode of carbonate
ions and a peak at 1090 cmÀ1 can be assigned as a stretching
mode of Si-O-Si, indicating the formation of carbonate LDH
shell on the surface of Fe3O4/SiO2 core after the layer-by-layer
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6À
Table 1: Magnetic separation and recycling of the catalyst in the
photodegradation of HCH.
The content of W7O24 in the resulting catalyst is about
8.0 wt%, as determined by chemical analysis. The activity of
this magnetic recoverable catalyst was tested with the photo-
degradation of trace hexachlorocyclohexane (HCH) in aque-
ous solution (Figure 6). After the suspension was stirred in the
Cycle
1
2
3
4
5
6
Yield [%] of photodegradation 96.3 95.8 96.1 96.5 95.6 96.4
alkoxylation of mesitol with various alcohols (see the
Supporting Information).[21–23]
In summary, we have demonstrated the fabrication of
magnetic core/LDH shell structure and its use as recoverable
support for nanocatalysts. This composite structure possesses
high anion loading capacity and can be conveniently sepa-
rated. The success in the assembly of the W7O246À catalyst may
provide a general route to the facile and direct fabrication of
magnetic core/various anion-functionalized shell composite
structures. The magnetic nanocomposite catalyst system
demonstrated herein is expected to find many important
applications in catalysis. For example, the oxide shell could be
used as an absorbent to carry anionic drugs and used in a drug
storage/release system.
Figure 6. Mineralization of HCH as a function of irradiation time in
the presence of magnetic W7O246À catalyst.
Experimental Section
X-ray diffraction (XRD) data were collected using a Rigaku Rint-
2000 powder diffractometer with graphite monochromated Cu KR
radiation (l = 0.15405 nm) Transmission electron microscopy (TEM)
observations were performed on a field emission JEM-3000F (JEOL)
electron microscope operated at 300 kV equipped with a Gatan-666
electron energy loss spectrometer and energy-dispersive X-ray
spectrometer. Scanning electron microscopy (SEM) images were
obtained using a JEOL JSM-6700F electron microscope (accelerating
voltage of 10 kV). FTIR spectra were obtained on Nicolet 7000-C
spectrometer.
dark for 12 h or irradiated with a high pressure mercury lamp
(HPML) for 12 h in the absence of catalyst, there was no
apparent change in the concentration of HCH, and ClÀ and
CO2 were not detected. However, a significant transformation
of HCH into ClÀ and CO2 was observed after irradiating the
suspension containing synthesized catalyst with the HPML. In
the reaction system, the concentration of the product ClÀ ions
increased with the reaction time, suggesting that the degra-
dation of aqueous HCH was the result of photoexcited
catalysis rather than direct photolysis. Complete mineraliza-
tion of HCH was demonstrated by 96.3% chlorine recovered
as ClÀ for 10 mgLÀ1 HCH after 12 h irradiation of the
suspension. Photocatalytic activity was not observed with the
Fe3O4 core/carbonate LDH shell nanocomposite. The mag-
netic separation and recycling of the catalyst were also
investigated with the same reaction (Figure 7). The catalyst
could be effectively recycled and reused six times without any
apparent decrease in its catalytic activity (Table 1). This
catalyst also shows high catalytic activities in oxidative
Photocatalytic procedure: The temperature of the system was
maintained at 288C. The light source was a 125 W high-pressure
mercury lamp. A general photocatalytic procedure was carried out as
follows. The Fe3O4 core/layered double hydroxide shell nanocompo-
site (0.5 g) were suspended in a fresh aqueous solution of HCH. The
suspension was ultrasonicated for 5 min, stirred in the dark for
30 min, and then irradiated with the lamp. The suspension was
vigorously stirred during the irradiation process.
Received: March 31, 2009
Revised: May 20, 2009
Published online: July 2, 2009
Keywords: heterogeneous catalysis · layered materials ·
.
magnetic properties · nanostructures
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