Seed size-dependent formation of Fe3O4/MnO hybrid nanocrystals: Selective, magnetically recyclable catalyst systems

Article abstract of DOI:10.1021/cm2027724

A family of Fe₃O₄/MnO hybrid nanocrystals differing in heterojunction structure and number of MnO domains were synthesized through the seed-mediated growth process of MnO at the surface of Fe₃O ₄ nanocrystal. In this process, the resulting heterostructures, including core/shell, dumbbell-like, and flowerlike structures, were found to be largely influenced by the size of initially injected Fe₃O ₄ seeds. The further transformation of Fe₃O ₄/MnO heterodimer through a nanoscale etching process within silica nanosphere generated the nanoreactor framework composed of a superparamagnetic Fe₃O₄ nanocrystal, a functionalized cavity with a catalytically active Mn₃O₄ layer, and a porous silica shell. The newly developed nanoreactor successfully catalyzed the cyanosilylation reaction of aromatic aldehydes in a size selective manner and could be recovered magnetically and reused without loss of catalytic activity even after ten successive cycles.

Full text of DOI:10.1021/cm2027724

Article
pubs.acs.org/cm
Seed Size-Dependent Formation of Fe3O4/MnO Hybrid Nanocrystals:
Selective, Magnetically Recyclable Catalyst Systems
Kyung Sig Lee,† Rahman Md Anisur,† Ki Woong Kim,† Won Sun Kim,† Tae-Joon Park,‡ Eun Joo Kang,†
and In Su Lee*,‡
†Department of Applied Chemistry, Kyung Hee University, Gyeonggi-do 446-701, Korea
‡Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
ABSTRACT: A family of Fe3O4/MnO hybrid nanocrystals differing in
heterojunction structure and number of MnO domains were synthesized through
the seed-mediated growth process of MnO at the surface of Fe3O4 nanocrystal. In
this process, the resulting heterostructures, including core/shell, dumbbell-like, and
flowerlike structures, were found to be largely influenced by the size of initially
injected Fe3O4 seeds. The further transformation of Fe3O4/MnO heterodimer
through a nanoscale etching process within silica nanosphere generated the
nanoreactor framework composed of a superparamagnetic Fe3O4 nanocrystal, a
functionalized cavity with a catalytically active Mn3O4 layer, and a porous silica shell.
The newly developed nanoreactor successfully catalyzed the cyanosilylation reaction
of aromatic aldehydes in a size selective manner and could be recovered
magnetically and reused without loss of catalytic activity even after ten successive
cycles.
KEYWORDS: hybrid nanocrystal, manganese, iron, nanocatalyst, recyclable catalyst
process within silica nanospheres.14−17 The employment of
such a transformation generated the hollow nanoreactor
INTRODUCTION
The synthesis and fabrication of hybrid nanocrystals containing
■
framework for a selective, magnetically recyclable catalyst
two or more chemically and/or functionally different species is
an important achievement in nanochemistry.1−3 The most
composed of a superparamagnetic Fe3O4 nanocrystal, a
functionalized cavity with a catalytically active Mn3O4 layer,
studied hybrid nanocrystals have been of the core/shell
and a porous silica shell.
structural type, in which a nanocrystalline core is encased in
We report our findings on the formation of various Fe3O4/
a shell composed of a different material. In recent years, phase-
MnO hybrid nanocrystals through the seed-mediated growth
segregated heterostructures, in which two or more inorganic
process and the influence of seed size on the resulting
domains are joined through a small interfacial area, have
heterostructures. We also report the successful employment of
attracted increased attention. In particular, heterodimers
Fe3O4/MnO hybrid nanocrystals for the fabrication of a
composed of magnetic components including M/Fe3O4 (M =
Au, Ag, Ni, Pd, Pt), CdS/FePt, CdSe/Fe3O4, and γ-Fe2O3/
metal sulfides have received significant scrutiny due to their
nanoreactor system and which selectively catalyzed the
cyanosilylation reaction of aromatic aldehydes and could be
recovered via magnetic separation and reused in multiple
novel properties and potential applications that cannot be
achieved solely with single component nanocrystals.4−11 In this
iterations.
context, this study initially intended to integrate super-
paramagnetic iron oxide and Lewis acidic manganese oxide
RESULTS AND DISCUSSION
■
Synthesis of Fe3O4/MnO Hybrid NCs with Various
Heterojunction Structures. For the preparation of Fe3O4/
MnO hybrid nanocrystals, we adopted the previously reported
seed-mediated growth method. First, 5, 11, and 21 nm sized
Fe3O4 nanocrystals were prepared via thermal decomposition
of an Fe(III)-oleate complex. The seed Fe3O4 nanocrystals
were injected into a trioctylamine solution of Mn(CH3CO2)2
and oleic acid to initiate the growth process.18,19 In all of the
syntheses, the initial concentration of Mn(CH3CO2)2 was
nanoparticles into a single hybrid nanocrystal. In the course of
synthesizing hybrid nanocrystals through the seed-mediated
growth method, a family of Fe3O4/MnO hybrid nanocrystals
differing in heterojunction structure and number of MnO
domains were produced. Importantly, in this process, the
hybrid nanocrystal morphology type, core/shell, heterodimer,
or flowerlike nanocrystal, was found to be largely influenced by
the size of the initially injected seed nanocrystal.12,13 In addition
to providing fundamental knowledge, this finding offers a novel
basis for the fabrication of nanoreactor systems. The newly
synthesized Fe3O4/MnO heterodimer could be further trans-
formed into a heterodimer consisting of solid Fe3O4 and hollow
Mn3O4 grains through a recently developed nanoscale etching
Received: September 15, 2011
Revised: January 27, 2012
Published: February 9, 2012
© 2012 American Chemical Society
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constant (0.61 mM). The reaction temperature was then slowly
increased to 320 °C, thereby allowing nucleation and growth of
MnO at the Fe3O4 surface to occur. The reaction nanoparticles
were isolated by addition of excess ethanol, followed by
centrifugation. The shapes of the resulting Fe3O4/MnO hybrid
nanocrystals, observed using transmission electron microscopy
(TEM), were found to vary depending on the size of the
initially injected Fe3O4 nanocrystal seeds (Scheme 1). The
Scheme 1. Seed Size-Dependent Formation of Fe3O4/MnO
Hybrid Nanocrystals
reaction utilizing 5 nm sized Fe3O4 seed nanoparticles provided
uniform growth of a 3 nm thick MnO shell layer around the
Fe3O4 seed core to produce a spherical core/shell structured
Fe3O4(5 nm)/MnO NP (Figure 1a). The formation of a core/
shell structure is analogous with structures previously observed
from the synthesis of MFe2O4@MnO (M = Co, Zn)
nanocrystals using 5-nm-sized MFe2O4 seeds.19 On the other
hand, in the reaction with 11 nm sized Fe3O4 seed nanocrystals,
MnO was found to nucleate only at a single surface site on the
Fe3O4 seed. Further MnO growth on the nuclei resulted in the
formation of a dumbbell-shaped heterodimer, Fe3O4(11 nm)/
MnO NP, in which 16 nm sized MnO and 11 nm sized Fe3O4
grains were joined through a small interfacial area (Figure 1b).
The fringe distances, measured from a high resolution TEM
(HRTEM) image, were 2.22 and 2.42 Å for the two types of
grains, which are well-matched with the known lattice distances
for MnO and Fe3O4 phases, respectively. Based on the energy
dispersive X-ray spectroscopic (EDS) map, these two grain
types were identified as being composed of Mn and Fe (insets
in Figure 1b,c). The X-ray diffraction (XRD) pattern also
confirmed the formation of a MnO phase that preserved the
crystalline phase of the initially injected Fe3O4 seed nanocryst-
als (Figure 2). When 21 nm sized Fe3O4 nanocrystals were
injected into the reaction suspension, MnO nucleation
occurred at several sites on the Fe3O4 surface. The
simultaneous growth of multiple MnO grains on a single seed
nanocrystal generated a flowerike heterostructure, Fe3O4(21
nm)/MnO, with several MnO petals of a 16 nm average size
arranged on a central Fe3O4 particle (Figure 1c).
Figure 1. TEM and HRTEM images of (a) core/shell type Fe3O4(5
nm)/MnO, (b) dumbbell-shaped Fe3O4(11 nm)/MnO, and (c)
flower-shaped Fe3O4(21 nm)/MnO NCs and histograms showing the
size distribution of the Fe3O4 and MnO grains. Insets in the left
images: TEM images of the Fe3O4 nanocrystals used as seeds. Insets in
the right images: STEM line profiling and EDS maps showing the
distribution of Fe and Mn elements.
Figure 2. XRD patterns of core/shell type Fe3O4(5 nm)/MnO,
dumbbell-shaped Fe3O4(11 nm)/MnO, and flower-shaped Fe3O4(21
nm)/MnO NCs. Peaks corresponding to Fe3O4 and MnO are marked
with green squares and red circles, respectively. The lines below show
the position of the reflections corresponding to cubic MnO phase
(JCPDS Card No. 07−0230) and the tetragonal Fe3O4 phase (JCPDS
Card No. 88−0315).
Although an exact mechanism to explain the formation of
these differing nanoparticle composites requires further
research, the generation of the core/shell type structure only
from the 5 nm sized Fe3O4 seed can be rationalized based on
the higher surface chemical potential of the smaller nanocrystals
with a high radius of curvature.20,21 Because the surfaces of 5
nm sized nanocrystals have a high degree of reactivity,
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Chemistry of Materials
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nucleation and growth of the secondary MnO phase is allowed
all over the surface, leading to development of a spherical shell
around the Fe3O4 core. This is also consistent with the previous
observation on the preferential growth of the secondary grain
onto the high-curvature surface and the lower formation
temperature of the shell around the smaller nanocrystal.22,23 In
contrast, for larger nanocrystals with a relatively stable surface,
growth of the secondary phase occurs only at special sites,
which can facilitate nucleation and/or allow better accom-
modation of an interfacial system.24 Therefore, in the cases of
11 and 21 nm sized seed nanocrystals, the number of exposed
defects or facets with high reactivity becomes highly influential
in determining the number of phase-segregated MnO
grains.25,26
Synthesis of Nanoreactor Framework from the Fe3O4/
MnO Heterodimer NC. Concerning prospective applications
of these newly synthesized hybrid nanocrystals, the Fe3O4/
MnO heterodimer NC was further developed as a nanoreactor
to function as a selective and magnetically recyclable
catalyst.27−33 The strategy used to fabricate the nanoreactor
from the heterodimer NC is illustrated in Scheme 2. In this
Scheme 2. Fabrication of the Nanoreactor, Fe3O4/HMON@
h-SiO2, from the Fe3O4/MnO Heterodimer NC
Figure 3. TEM images of (a) Fe3O4/MnO@SiO2 and (b) Fe3O4/
HMON@h-SiO2 nanospheres. Histograms show the size distribution
of the silica sphere and Fe3O4 and MnO grains and shell thickness of
HMON. Inset: HRTEM images. (c) XRD patterns of Fe3O4/MnO@
SiO2 (black line) and Fe3O4/HMON@h-SiO2 spheres (magenta line).
Peaks corresponding to Fe3O4, MnO, and Mn3O4 phases are marked
with green squares, red circles, and blue triangles, respectively. The
lines below show the position of the reflections corresponding to the
tetragonal Fe3O4 phase (JCPDS Card No. 88−0315), the cubic MnO
phase (JCPDS Card No. 07−0230), and the tetragonal Mn3O4 phase
(JCPDS Card No. 24−0734). (d) Field-dependent magnetization
curves at 293 K, showing superparamagnetic characteristics of Fe3O4/
MnO@SiO2 and Fe3O4/HMON@h-SiO2. (e) N2 sorption isotherm
and pore size distribution (inset) of Fe3O4/HMON@h-SiO2 derived
by using HK model.
strategy, the MnO phase was selectively removed from the
heterodimer NC, generating a Fe3O4/hollow Mn3O4 hetero-
dimer which combines magnetically controllable flocculation
and dispersion properties with the Lewis acid properties of each
grain. Fe3O4/MnO heterodimer nanocrystals were synthesized
by growing 30 nm sized MnO grains on 11 nm sized Fe3O4
nanocrystals, which were then encapsulated within a silica shell
using a modified emulsion technique. The silica nanosphere-
encased Fe3O4/MnO heterodimer, Fe3O4/MnO@SiO2, was
treated with a 1 M NH2OH solution in a procedure similar to
that previously applied to the synthesis of a hollow Mn3O4
nanoshell within a silica nanosphere.17 TEM, HRTEM, and
SEM analyses of nanospheres isolated after a 24 h reaction
showed that the MnO grains had been removed, leaving a
spherical silica shell with catalytically functionalized cavity by a
thin, 4.6-nm-thick Mn3O4 layer, whereas the Fe3O4 grains
maintained their original 11 nm size (Figures 3 and 4). The
XRD patterns of the spheres also revealed a significant decrease
in the MnO fraction, indicating its preferential dissolution
(Figure 3c). MnO dissolution was accompanied by partial
etching of the silica shell, resulting in the formation of a hollow
nanosphere, Fe3O4/HMON@h-SiO2, in which the nanocrystal-
line Fe3O4 particle with superparamagnetic properties and a
cavity functionalized with a catalytically active Mn3O4 layer
were coated by a porous silica shell. The pore size distribution
centered at 1 nm, which was derived by using HK model based
on the nitrogen adsorption/desorption isotherm, also supports
the formation of nanopores at the silica shell (Figure 3e). BET
surface was measured to be 73 m2 g−1. Magnetic measurements
Figure 4. Scanning electron microscopy (SEM) images of the (a)
dumbbell-shaped Fe3O4(11 nm)/MnO NC and (b) Fe3O4/MnO@
SiO2 and (c) Fe3O4/HMON@h-SiO2 nanospheres.
show that the Fe3O4/HMON@h-SiO2 had a saturated
magnetization value of 3.08 emu/g and superparamagnetic
behavior at 298 K (Figure 3d). By integrating the properties of
each component, it was concluded that the Fe3O4/HMON@h-
SiO2 could act as a nanoreactor with substrate size selectivity
that could be isolated magnetically and recycled.
Evaluation of Catalytic Performance of the Nano-
reactor Framework. To evaluate the effectiveness of Fe3O4/
HMON@h-SiO2 as a selective and recyclable catalyst, it was
examined in the cyanosilylation reaction of carbonyl substrates,
a reaction facilitated by Lewis acid catalysis. Benzaldehyde (1a)
was used to optimize suitable conditions, and the reaction
treated with cyanotrimethylsilane (1.5 equiv.) and 8 mol %
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a
Table 1. Cyanosilylation of Aryl Aldehydes with Fe3O4/HMON@h-SiO2 Catalyst
b
c
R
product
yield (%)
vol. (Å3)
phenyl (1a)
2a
2b
2c
2d
2e
2a
2a
2a
99
90
70
49
8
85
145
121
157
250
biphenyl (1b)
1-naphthyl (1c)
9-antracenyl (1d)
2-tert-butyldimethylsiloxy-5-methoxyphenyl (1e)
d
phenyl (1a) (10th run)
99
87
81
e
phenyl (1a)
d,e
phenyl (1a) (5th run)
a
Conditions: aldehyde (0.5 mmol), TMSCN (cyanotrimethylsilane, 0.75 mmol), catalyst (8 mol %, 0.040 mmol, based on Mn contents), CH2Cl2
b
c
d
(0.25 M), under N2. Determined by 1H NMR based on the carbonyl substrate. Molecular volume of aryl aldehydes. The recovered catalyst was
used in iterative cycles. Reaction with 4 mol % of catalyst
e
Fe3O4/HMON@h-SiO2 resulted in the cyanohydrin trimethyl-
1
silyl ether formation in quantitative yield. In the H NMR
spectrum to check the conversion yield, the peak at 10.02 ppm
(PhCHO) disappeared completely and a new peak at 5.50 ppm
(PhCHCN(OTMS)) was observed. Compared with that of
benzaldehyde, the same reaction performed with 1-naphthalde-
hyde (1c) and 9-anthraldehyde (1d) afforded the correspond-
ing cyanohydrins in much lower yield of 70 and 50% yields,
respectively. In particular, the reaction of 2-tert-butyldimethyl-
siloxy-5-methoxyphenyl carboxaldehyde (1e) reached only 8%
yield, representing a significant size-selective reaction in a
porous nanoreactor (Table 1).17,34,35 Molecular volume of aryl
Figure 5. Pictures of the product suspensions (a) before and (b) 10
min after placing a magnet and (c) the decanted product solution (left
vial) and the remained Fe3O4/HMON@h-SiO2 catalyst (right vial).
aldehydes also shows good correlation with the conversion
yield, leading us to theorize that size of the substrates acts as a
crucial variable for determining the progress of Fe3O4/
ions from the catalyst. TEM images of the recovered catalyst
HMON@h-SiO2-catalyzed cyanosilylation reactions by influ-
also confirmed the preservation of the initial size and structure
of the Fe3O4/HMON@h-SiO2 even after ten reaction cycles
(Figure 6).
encing the diffusion rate through the pores of the silica shell.
The reaction with biphenyl carboxaldehyde, affording relatively
higher yield than expected correlation, can be understood that a
narrow, rod-shaped aldehyde could diffuse rapidly by
comparison with others of similar size. The control experiment
to check the reactivity of 1e toward the cyanosilylation reaction
was performed with Ti(OiPr)4 catalyst, and resulted in 98%
yield.
After the reaction with benzaldehyde was complete, the
Fe3O4/HMON@h-SiO2 catalyst was magnetically separated
from the product and was reused in consecutive reactions to
illustrate its recyclability. When a small magnet was placed on
the side of the reaction vessel, the catalyst was primarily
concentrated in close proximity to the magnet within 30 min.
Decantation of the product solution followed by nano-
particulate washing with CH2Cl2 allowed the catalyst to be
successfully isolated with no significant loss of material (Figure
5). The recovered catalyst was then subsequently reused in ten
further iterative cycles, furnishing the cyanosilylation product
with over 95% conversion in each cycle (Table 1). The
recycling experiments with 4 mol % of Fe3O4/HMON@h-SiO2
catalyst also showed reproducible yield of 81% in five iterative
reactions, demonstrating the excellent recyclability of the
Fe3O4/HMON@h-SiO2. Inductively coupled plasma atomic
emission spectroscopy (ICP-AES) analyses of the reaction
solutions magnetically separated from the catalyst after each
run indicated no loss of catalyst or any leaching of Mn or Fe
Figure 6. TEM images of the Fe3O4/HMON@h-SiO2 catalyst
recovered after ten consecutive reactions.
EXPERIMENTAL SECTION
■
General Considerations. All reagents, including FeCl3 (Aldrich),
Sodium Oleate (TCI), Oleic acid (Aldrich), Mn(CH3CO2)2 (Aldrich),
1-octadecene (Aldrich), hydroxylamine (Aldrich), cyclohexane,
NH4OH (Samchun chem.), tetraethylorthosilicate (Acros), and Igepal
CO-520 (Acros), were used as purchased without any further
purification. The transmission electron microscopy (TEM) analyses
were conducted with a JEOL JEM-2010. Scanning tunneling
microscopy (SEM) was carried out with a LEO SUPRA 55 (Carl
Zeiss, Germany). The magnetic properties of the nanoparticles were
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measured using a superconducting quantum interference device
(SQUID) magnetometer (Quantum Design, MPMS5XL) equipped
with a 5 T superconducting magnet. The X-ray diffraction patterns
were obtained using an X-ray Diffractometer (18 kW) (Mac Science,
Japan). The nitrogen adsorption and desorption isotherms were
measured at 77 K using a BELSORP-max (BEL Japan) gas adsorption
analyzer after the pretreatment at 120 °C in vacuo for 12 h. The1H
NMR spectra were recorded with a Jeol 300 MHz spectrometer and
referenced to CDCl3.
Synthesis of Fe3O4/MnO Hybrid Nanocrystals. The Fe3O4
nanocrystals with sizes of 5, 11, and 21 nm were prepared through
the previously reported procedure including the thermal decom-
position of Fe-oleic acid complexes at high temperature.36 Twenty
milligrams of powder of Fe3O4 nanocrystals, 150 mg of Mn-
(CH3CO2)2, and 1.28 mg of oleic acid were mixed in 20 mL of
trioctylamine solvent. The mixture suspension was slowly heated up to
320 °C. The reaction mixture was maintained at this temperature for 1
h and then cooled to room temperature. Fifteen ml of hexane and 20
mL of acetone were injected into the reaction suspension and the
resulting solids were isolated by centrifugation. The purification of the
Fe3O4/MnO hybrid nanocrystals was carried out by repeating the
procedure including the dispersion in hexane, addition of acetone, and
centrifugation processes three times.
Synthesis of Fe3O4HMON@h-SiO2. Fe3O4/MnO dumbbell
nanoparticles containing 12 nm sized Fe3O4 and 30 nm sized MnO
components were synthesized by injecting 12 nm Fe3O4 nanocrystals
into a solution containing 250 mg of Mn(CH3CO2)2, 1.28 mg of oleic
acid and 20 mL of trioctylamine. The silica coated dumbbell
nanoparticles (Fe3O4/MnO@SiO2) were prepared by a modified
version of the previously reported reverse microemulsion technique.
Polyoxyethylene(5)nonylphenyl ether (0.77 g, 1.74 mmol, Igepal CO-
520, containing 50 mol % hydrophilic groups) was dispersed in a
round-bottom flask containing cyclohexane solvent (17 mL). Next, a
cyclohexane suspension (6 mL) of Fe3O4/MnO nanoparticles (6 mg)
and an ammonium hydroxide solution (30%, 0.13 mL) were
successively added with vigorous stirring to form a translucent
suspension. Lastly, tetraethylorthosilicate (TEOS, 0.15 mL) was added
and stirred for 12 h. The resulting Fe3O4/MnO@SiO2was precipitated
from the reaction suspension by the addition of methanol (1 mL) and
retrieved by centrifugation. The crude Fe3O4/MnO@SiO2s were
purified by repeating the dispersion of the retrieved particles in ethanol
and centrifugation several times. The purified Fe3O4/MnO@SiO2
were redispersed in deionized water and stored for further use. For
the synthesis of Fe3O4/HMON@h-SiO2, 1 mg/mL of the Fe3O4/
MnO@SiO2 nanoparticles were treated with 1 M NH2OH solution at
room temperature for 72 h. The resulting Fe3O4/HMON@h-SiO2
nanoparticles were isolated from the reaction suspension by
centrifugation and purified by repeating the redispersion in water
and centrifugation procedures.
application of the new hybrid nanocrystal synthesis method-
ology, we fabricated a novel nanocomposite containing a
catalytically functionalized nanocavity, superparamagnetic
nanocrystal, and porous, hollow shell. We also demonstrated
the performance of this material as a nanoreactor that catalyzed
the cyanosilylation reaction of aromatic aldehydes in a size
selective manner and their ability to be recovered magnetically
and reused without loss of catalytic activity even after ten
successive cycles.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: 82-54-279-2103. Fax: 82-54-279-3399. E-mail:
insulee97@postech.ac.kr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research foundation
of Korea (KRF) grant funded by the Korea government
(MEST) (2011-0017377).
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1
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CONCLUDING REMARKS
■
In this study, we synthesized several Fe3O4/MnO hybrid
nanocrystals through a seed-mediate growth process and found
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