2D magnetic frames obtained by the microwave-assisted chemistry approach

Article abstract of DOI:10.1002/ejic.201101244

Microwave chemistry is becoming a very attractive synthesis technique in many areas of synthetic chemistry. In particular, the utilization of this method to fabricate nanostructured materials is a fast growing research area with immense potential. Similarly, the use of sacrificial scaffolds has been demonstrated as an effective route to achieve intricate 2D and 3D porous architectures. Here, we present an extremely fast and versatile synthetic approach based on microwave heating to fabricate complex macroporous magnetic frames using sacrificial templates. In just a few minutes, a stoichiometric and homogeneous conformal nanometric coating of superparamagnetic nanoparticles was grown onto a 2D monolayer formed by self-assembled polystyrene colloids. No post-treatment was required, the sacrificial polystyrene template was removed simultaneously as the magnetic nanoparticles formed, and large-scale structural order was preserved.

Full text of DOI:10.1002/ejic.201101244

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SHORT COMMUNICATION
DOI: 10.1002/ejic.201101244
2
D Magnetic Frames Obtained by the Microwave-Assisted Chemistry
Approach
[a]
[a]
[a]
[a]
Oana Pascu, Martí Gich, Gervasi Herranz, and Anna Roig*
Keywords: Nanoparticles / Colloids / Microwave chemistry / Superparamagnetism
Microwave chemistry is becoming a very attractive synthesis
technique in many areas of synthetic chemistry. In particular,
the utilization of this method to fabricate nanostructured ma-
terials is a fast growing research area with immense poten-
fabricate complex macroporous magnetic frames using sacri-
ficial templates. In just a few minutes, a stoichiometric and
homogeneous conformal nanometric coating of superpara-
magnetic nanoparticles was grown onto a 2D monolayer
tial. Similarly, the use of sacrificial scaffolds has been demon- formed by self-assembled polystyrene colloids. No post-treat-
strated as an effective route to achieve intricate 2D and 3D
porous architectures. Here, we present an extremely fast and
versatile synthetic approach based on microwave heating to
ment was required, the sacrificial polystyrene template was
removed simultaneously as the magnetic nanoparticles
formed, and large-scale structural order was preserved.
Introduction
nanometric coating of manganese ferrite on a polystyrene
template over an area of half a square centimeter. No post-
treatment was required – the sacrificial template is removed
simultaneously with the formation of magnetic nanopar-
ticles.
Fabrication of nanostructured materials by using sacrifi-
cial scaffolds, against which another material is deposited
by mimicking the original template, has been demonstrated
as effective and versatile approach towards achieving com-
plex 2D and 3D porous architectures.[
1–2]
Among those, 2D
or 3D arrays of inorganic hollow spheres can display inter-
esting optical, optoelectronic, magnetic, electric, or catalytic
Results and Discussion
[
3]
The fabrication of complex nanostructures by microwave
heating is becoming an area of intense research and great
potentiality in synthetic chemistry. In addition to drastically
decreasing the time for the synthesis, the technique offers a
combination of kinetics and selectivity characteristics that
can be used advantageously, such as: (i) the acceleration of
the chemical reaction rate, (ii) the heating selectivity – polar
substances absorb electromagnetic radiation and are inten-
sively heated while the non-polar ones do not absorb it, and
(iii) the high diffusivity of the molecular species in solution
facilitates the conformal deposition onto intricate frames.
We have recently demonstrated some of those features on
the fabrication of three-dimensional magnetic photonic
properties. For such applications, the preservation of the
long-range order of the nanostructure is essential. Templat-
ing over polymer colloids is a commonly used strategy to
achieve long-range order, although it presents the drawback
of having to remove the template either by solvent dissol-
ution, etching, or calcination, which in turn could damage
the intended hollow structure.
Microwave energy is becoming a very attractive tool in
all areas of synthetic chemistry. It has been used not only
[
4]
[5]
for organic and inorganic nanoparticle synthesis but
also for the fabrication of high-quality nanocrystals and
large-scale defect-free complex nanostructures with control-
lable morphologies and surfaces.[6] Other intricate nanoar-
[7]
crystals by microwave synthesis, where direct SiO
verse Al photonic crystals have been uniformly infil-
trated with in situ synthesized magnetic oxide nanopar-
2
and in-
chitectures, such as nanoporous materials, superstruc-
[
6]
[8]
O
2 3
tures, or 3D magneto photonic opals have also been re-
ported.
[
8]
ticles.
In this communication, we describe a facile and fast mi-
crowave-assisted sol–gel chemical approach to produce two-
dimensional hollow magnetic frames using sacrificial tem-
plates. The method has allowed us to grow, in just a few
minutes, a stoichiometric and homogeneous conformal
Here, we describe the conformal coverage with small su-
perparamagnetic MnFe O nanoparticles of a 2D structure
2
4
resulting from self-assembled polystyrene (PS) spheres
coated with a thin Al O layer (10 nm). A schematic repre-
2
3
sentation of the synthesis steps and the resulting magnetic
[
a] Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) frame is displayed in Scheme 1.
Campus UAB,
The two-dimensional polystyrene colloidal template, ver-
tically immersed in a solution containing the solvent and
the organometallic precursors, was subjected to microwave
0
8193 Bellaterra, Spain
Fax: +34-93-5805729
E-mail: roig@icmab.es
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O. Pascu, M. Gich, G. Herranz, A. Roig
SHORT COMMUNICATION
0035). The magnetic properties of the nanoparticles were
evaluated by measuring the magnetization vs. the magnetic
field strength at 300 K (Figure 1d) and the zero-field-cooled
(ZFC), field-cooled (FC) magnetization curves (Figure 1d
inset) vs. temperature. The absence of remnant magnetiza-
tion at room temperature indicates the superparamagnetic
character of the nanoparticles. The low blocking tempera-
ture at 26 K corresponds to very small nanoparticles, which
is in agreement with the TEM data.
Scheme 1. Schematic representation of the fabrication process.
radiation for a few minutes. The solvent (benzyl alcohol),
being a polar substance, absorbs the microwave radiation
and increases its temperature, which promotes the decom-
position of the organometallic precursor and nanoparticle
formation. By coating the PS spheres with a thin layer of
an oxide (Al O in this case), we gain two additional advan-
2
3
tageous effects. Firstly, because PS has a glass transition at
[
9]
about 95–100 °C and starts melting above this tempera-
[
10]
ture, the PS template melts during the microwave synthe-
sis at 160 °C and thus the Al O frame keeps the mechani-
2
3
cal stability of the structure. Secondly, the hydroxy surface
terminations of the alumina efficiently absorb the micro-
wave energy, which activates the surface and selectively pro-
motes nucleation and growth of the magnetic nanoparticles
on it. Thus, a final 2D hollow structure with a conformal
magnetic coating is obtained. When the 2D template is
made only of PS spheres, a non polar compound, the coat-
ing is not effective, and the template is not mechanically
strong during particle deposition.
Prior to the discussion of the characteristics of the mag-
netic frames, the characterization of the nanoparticles is re- Figure 1. MnFe O nanoparticles stabilized in organic media. (a)
2
4
ported. As explained in the Experimental Section, non-at- TEM image of monodisperse nanoparticles stabilized with oleic
acid; the inset shows the powder X-ray diffratogram of these nano-
tached MnFe O nanoparticles could be separated from the
2
4
paricles, (b) size distribution fitted to a Gaussian function, (c) the
corresponding electron diffraction pattern, (d) magnetometry data;
M(H) at room temperature and ZFC–FC curves at 50 Oe.
reaction media by adding oleic acid once the reaction had
taken place and thus stable colloidal dispersions in hexane
[
11]
were obtained after centrifugation and washing steps.
Transmission electron microscopy (Figure 1a) reveals small
With regard to the 2D magnetic frames, the first observa-
nanoparticles with good polydispersity and a rather irregu- tion is that the thickness of the coating can be controlled
lar shape. A size distribution, fitted to a Gaussian function, by the duration of the chemical reaction. Scanning electron
gives an average size of 4.0Ϯ0.8 nm (Figure 1b). The low microscopy (SEM) was used to study the surface of the
polydispersity in the distribution of particle sizes is most magnetic frames at different reaction times (Figure 2). The
probably a consequence of the rather uniform formation of PS template covered by the thin Al O shell (about 10 nm)
2
3
nuclei as well as the uniform nanoparticle growth in the presents a very smooth surface (Figure 2a and inset). The
microwave reactor. The powder X-ray diffractogram (inset reaction at 160 °C for 2 min results in partial coverage of
of Figure 1a) and selected area electron diffraction pattern the template (bright spots on the sphere surface) (Figure 2b
(SADP) (Figure 1c) can be indexed with that of the cubic and inset). The reaction with the same precursor concentra-
inverse spinel manganese ferrite phase (ICDD PDF075- tion (0.1 m) at the same temperature (160 °C) but with a
2
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2D Magnetic Frames by the Microwave-Assisted Chemistry Approach
longer reaction time (5 min) produces a complete confor- time to 10 min, the coverage increases to such an extent
mal coating (Figure 2c). Higher magnification (inset of Fig- that the nanostructures are no longer evident and a con-
ure 2c) confirms that the layer is conformal, continuous and tinuous film is formed instead.
consists of small nanoparticles. By increasing the reaction
At that point, if the growth continues, every sphere starts
pushing against its neighbors, and cracks begin to appear
in the nanostructure, as clearly seen in Figure 2d.
The next issue addressed is what happens to the PS tem-
plate during the chemical reaction that forms the nanopar-
ticles. A close observation of the pristine template reveals
that the PS spheres are in contact, and, therefore, the alu-
mina will not be deposited on these contact areas (see
Scheme 1 and Figure 2a). Because PS is not very soluble in
[
12]
benzyl alcohol, we assume that the disappearance of the
PS during microwave heating results from a melting process.
Figure 2. The various coverage thicknesses depending on the dura-
tion of the chemical reaction are visualized by SEM micrographs.
Figure 3. SEM images of the transversal section across the spheres,
which reveals the morphology within the hollow spheres. (a) The
material with a bright Al O shell (160 °C/2 min); the white spots
2 3
(
(
a) The pristine opal; a higher magnification is included in the inset.
b) Partial magnetic coverage of the opal obtained at 160 °C/2 min;
the inset shows one sphere in more detail. (c) Homogeneous mag-
netic coverage of the 2D template obtained at 160 °C/5 min; the
inset shows in better detail the conformal uniform coating as a
single sphere. (d) A thick magnetic film is formed at 160 °C/10 min
over the 2D template; the inset shows a smaller magnification (scale
bar 20 μm) of the film surface.
uniformly distributed on the inner wall are nanoparticles. (b)
Spheres with a thicker deposition within the spheres (5 min reac-
tion). The cube inside is made of PS melted and recrystallized dur-
ing the reaction. (c) After a 10-min chemical reaction under micro-
waves MW, the PS spheres disappear completely. The white spots
inside the spheres are nanoparticle aggregates.
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O. Pascu, M. Gich, G. Herranz, A. Roig
SHORT COMMUNICATION
Since the microwave experiment is a very fast process – the small fraction of the PS recrystallizes at the center of the
[
13]
full cycle does not take longer than 15 min – we can also spheres in the form of small cubes (Figure 3b). For a 10-
safely assume that the melting of the PS and the nucleation min reaction time, the majority of the PS is melted, and
and growth of the nanoparticles occur simultaneously. large aggregates of nanoparticles accumulate at the center
When the PS melts, the contact areas between two PS of the sphere (Figure 3c). To certify the coverage of the PS
spheres become contact windows, which allows the dif- colloidal template with magnetic material, elemental analy-
fusion of the organometallic precursors inside the spheres sis by energy dispersive X-ray spectroscopy (EDX) of the
and consequently the formation of nanoparticles at the in- different materials was performed (Figure 4). The spectrum
ner side of the alumina takes place. This could be verified of the pristine template on a glass substrate is displayed
by the images taken from a transversal section across the in Figure 4a; naturally, iron and manganese peaks are not
templates (Figure 3). In the case of a 2-min reaction, it can present. Figure 4b shows the spectrum, obtained at 160 °C
be seen that the PS is only partially melted, and a small over 10 min, of a thick magnetic film that completely covers
quantity of nanoparticles uniformly distributed (bright the template, in which the Fe and Mn peaks are clearly
spots) inside the spheres can be observed (Figure 3a). By visible. Figure 4c-A corresponds to the spectra (the scanned
increasing the reaction time to 5 min, more nanoparticles area represented by the square A of the template covered
are deposited on the outer shell and inner shell, a large by a thinner ferrite layer, prepared at 160 °C over 5 min,
fraction of the PS is melted, which leaves big voids, and a and the intensity of the Fe and Mn peaks is smaller than
in the previous case. Finally, Figure 4c-B corresponds to the
spectrum of a cube inside the sphere (marked with a white
cross), which reveals that no Mn and Fe is present in this
area.
Conclusions
We have reported on a facile, rapid, and inexpensive mi-
crowave-assisted approach to fabricate large areas of high–
quality, hollow, two-dimensional magnetic frames using
sacrificial colloidal templates. We have proved that in ad-
dition to short reaction times, the technique offers other
characteristics in terms of reaction kinetics, molecular dif-
fusion, and selectivity, which can be used to enhance the
potentiality of such a synthetic chemistry approach. The
method has allowed the growth, in just a few minutes, of
a stoichiometric and homogeneous conformal nanometric
manganese ferrite coating of tenths of square millimeters.
The coating thickness can be controlled by controlling the
time of the reaction. No post-treatment was required – the
sacrificial template is removed simultaneously as the mag-
netic nanoparticle coating is formed.
Experimental Section
Microwave Synthesis: The microwave experiments were carried out
by using a CEM Discover reactor (Explorer 12-Hybrid) operating
at a frequency of 2.45 GHz and with a power of 200 W. The tem-
2 3
plate, polystyrene beads covered with a thin Al O layer grown by
atomic layer deposition, self-assembled in one monolayer with an
area of 5 ϫ5 mm onto a glass slide substrate, were vertically im-
mersed in the reaction solution by using a 10-mL closed pressurized
vessel. The reaction media contained the organometallic precur-
3
sors, iron(III) acetylacetonate [Fe(acaca) ] and manganese(II) acet-
ate [Mn(ac) ] (molar ratio of Fe/Mn = 2:1) with a total concentra-
2
tion of 0.1 m in anhydrous benzyl alcohol (1.5 mL). During a typi-
cal run, the power was automatically adjusted to heat the sample
to 160 °C, and the temperature was kept stable for 2, 5, or 10 min.
After the reaction took place, the solution was automatically co-
oled to 50 °C by compressed nitrogen. The temperature and the
pressure were controlled by a volume-independent infrared sensor.
Figure 4. EDX spectra for different materials: (a) the template of
pristine PS + Al on a glass substrate; (b) magnetic film com-
pletely covering the template obtained at 160 °C/10 min; (c-A) spec-
tra corresponding to the template covered by a ferrite layer at
2
O
3
160 °C/5 min. (c-B) spectra for the cube inside the sphere, no Mn
and Fe are present.
4
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2D Magnetic Frames by the Microwave-Assisted Chemistry Approach
The Al
2 3
O layer was deposited by ALD (Savanah 100 system by
Cambridge Nanotech). Reactants for alumina growth, i.e. trimeth- [1] a) M. H. Kim, J. Y. Choi, H. K. Choi, S. M. Yoon, O. O. Park,
ylaluminum (TMA) and water were kept at room temperature. The
PS monolayer was placed in the ALD chamber at 100 °C, under a
N flow of 20 sccm and exposed to continuous TMA and water
2
pulses (0.15 s each). Every TMA/water cycle was followed by a
waiting time of 8 s, to ensure the complete reaction of the species
on the exposed surface of the sample. Hundred cycles were per-
formed as the rate of alumina growth is about 0.1 nm per cycle,
which yields an alumina layer of 10 nm. Non-attached nanopar-
ticles were also separated from the reaction media. Briefly, the reac-
tion solution was mixed with excess EtOH (in a volume ratio of
D. K. Yi, S. J. Choi, H. J. Shin, Adv. Mater. 2008, 20, 457–461;
b) Y. Wang, A. S. Angelatos, F. Caruso, Chem. Mater. 2008,
20, 848–858.
[
2] a) Y. Xia,Y. Lu, K. Kamada, B. Gates, Y. Yin in The Chemistry
of Nanostructured Materials (Ed.: P. Yang), Worl Scientific,
Singapore, 2003, pp. 69–101; b) M. Agrawal, S. Gupta, M.
Stamm, Mater. Chem. 2011, 21, 615–627; c) R. J. White, R.
Luque, V. L. Budarin, J. H. Clark, D. J. Macquarrie, Chem.
Soc. Rev. 2009, 38, 481–494.
[
3] a) J. Hu, M. Chen, X. Fang, L. Wu, Chem. Soc. Rev. 2011, 40,
5
472–5491; b) J. M. Campelo, D. Luna, R. Luque, J. M. Mari-
1:3) and surfactant (20 μL, oleic acid for organic dispersion) fol-
nas, A. A. Romero, ChemSusChem 2009, 2, 18–45.
lowed by centrifugation at 6000 rpm for 30 min. The supernatant
was discharged, and the precipitate redispersed in hexane (2 mL)
containing oleic acid (10 μL) followed by centrifugation at
[4] a) L. Perreux, A. Loupy, Tetrahedron 2001, 57, 9199–9223; b)
V. Polshettiwar, M. N. Nadagouda, R. S. Varma, Aust. J.
Chem. 2009, 62,16–26.
[5] a) M. Tsuji, S. Hikino, M. Matsunaga, Y. Sano, T. Hashizume,
H. Kawazumi, Mater. Lett. 2010, 64, 1793–1797; b) T. Yamau-
chi, Y. Tsukahara, T. Sakata, H. Mori, T. Yanagida, Nanoscale
6000 rpm for 15 min. No precipitate was separated, and the stable
dark brown solution was used for further analysis.
Material Characterization: TEM micrographs were obtained with
a JEOL JEM-1210 electron microscope, operating at 120 kV. The
selected area diffraction patterns were captured by using a JEOL
JEM-2011, operating at 200 kV. The samples were prepared by de-
positing a drop of diluted nanoparticle dispersion in hexane onto
a TEM grid and by letting the solvent evaporate. The mean dia-
meter and polydispersity of each system were determined by fitting
a particle size histogram of over 200 counts measured from TEM
images to a Gaussian distribution by using the imageJ software.
Template SEM images were acquired by using a Quanta FEI 200F
microscope in low vacuum mode with a cone LOW kV P.L.A. with
a 500-μm aperture. The working conditions were: acceleration volt-
age 5 kV, electron beam spot 2, pressure 40 Pa, and distance 5–
2
010, 2, 515–523; c) L. Xiao, H. Shen, R. von Hagen, J. Pan,
L. Belkoura, S. Mathur, T. Kawaic, Y. Wada, Chem. Commun.
010, 46, 6509–6511; d) P. Nekooi, M. Akbari, M. K. Amini,
2
Int. J. Hydrogen Energy 2010, 35, 6392–6398; e) I. Bilecka, M.
Kubli, E. Amstad, M. Niederberger, J. Sol-Gel Sci. Technol.
2011, 57, 313–322; f) C. Gonzales-Arellano, A. M. Balu, R.
Luque, D. J. Macquarrie, Green Chem. 2010, 12, 1995–2002.
6] a) Z. Ai, K. Deng, Q. Wan, L. Zhang, S. Lee, J. Phys. Chem.
C 2010, 114, 6237–6242; b) X. Hu, J. C. Yu, J. Gong, J. Phys.
Chem. C 2007, 111, 11180–11185; c) X. Hu, J. C. Yu, Chem.
Mater. 2008, 20, 6743–6749; d) S. Q. Chen, Y. Wang, J. Mater.
Chem. 2010, 20, 9735–9739; e) C. Gonzales-Arellano, R.
Luque, D. J. Macquarrie, Chem. Commun. 2009, 30, 4581–
[
4
583.
6
mm. For the EDX analysis, 10–15 kV, a spot of 2.5–3, and a
[
7] a) G. A. Tompsett, W. C. Conner, K. S. Yngvesson, ChemPhys-
Chem 2006, 7, 296–319; b) C. Yacou, A. Ayral, A. Giroir-
Fendler, M.-L. Fontaine, A. Julbe, Microporous Mesoporous
Mater. 2009, 126, 222–227.
8] O. Pascu, J. M. Caicedo, M. Lopez-Garcia, V. Canalejas, A.
Blanco, C. Lopez, J. Fontcuberta, A. Roig, G. Herranz, Nano-
scale 2011, 3, 4811–4816.
9] J. Rieger, J. Therm. Anal. 1996, 46, 965–972.
10] K. Y. Suh, H. Yoon, H. H. Lee, A. Khademhosseini, R.
Langer, Appl. Phys. Lett. 2004, 85, 2643–2645.
11] O. Pascu, E. Carenza, M. Gich, G. Herranz, A. Roig, submit-
ted.
12] S. Gündüz, S. Dinçer, Polymer 1980, 21, 1041–1046.
13] a) X. Gao, R. Liu, Y. Huang, O. Starykov, W. Oppermann,
Macromolecules 2008, 41, 2554–2560; b) H. Wu, W. Wang, H.
Yang, Z. Su, Macromolecules 2007, 40, 4244–4249; c) E. M.
Woo, Y. S. Sun, M. L. Lee, Polymer 1999, 40,4425–4429.
Received: November 7, 2011
distance of 10 mm were used. Magnetic characterization was per-
formed with a quantum interference device (SQUID) magnetome-
ter (Quantum Design MPMS5XL). The sample was prepared by
using a gelatin capsule filled with compacted cotton impregnated
with 150 μL of a hexane dispersion of nanoparticles, which gave a
mass of 1.2 mg (magnetic material without surfactant).
[
[
[
Acknowledgments
[
We thank A. Blanco (ICMM-CSIC) for providing the polystyrene
template. We acknowledge funding from the Spanish Government
[
[
(CONSOLIDER Nanoselect-CSD2007-00041, MAT2009-08024,
MAT2009-06885-E, MAT2011-29269-C03, and RyC contract of
M. Gich), the Generalitat de Catalunya (2009SGR-203,
2009SGR376, and FI. of O. Pascu), and the European Commission
(Marie Curie Actions, PCIG09-GA-2011–294168).
Published Online: ᭿
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O. Pascu, M. Gich, G. Herranz, A. Roig
SHORT COMMUNICATION
Porous Magnetic Nanoparticles
O. Pascu, M. Gich, G. Herranz,
A. Roig* ............................................ 1–7
2D Magnetic Frames Obtained by the
Microwave-Assisted Chemistry Approach
The fabrication of porous magnetic
nanoarchitectures by microwave heating is
reported. In addition to drastically decreas-
ing the time for the synthesis, other charac-
teristics of the technique, i.e. reaction kin-
etics, diffusivity and selectivity, have been Keywords: Nanoparticles
/
Colloids
Superpara-
/
exploited.
Microwave
magnetism
chemistry
/
6
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