Sustainable Method for the Large-Scale Preparation of Fe3O4 Nanocrystals

Article abstract of DOI:10.1111/jace.14262

In this work, a facile synthetic process is reported for the large-scale synthesis of Fe₃O₄ nanocrystals. Thermal decomposition of Fe(acac)₃ (100 g) in 1-hexadecanol produced Fe₃O₄ nanocrystals with well-controlled sizes and morphologies. The nanocrystals were spherically shaped with average diameters of 7.8 ± 0.6, 6.5 ± 0.4, and 5.9 ± 0.2 nm when prepared at 300°C, 270°C, and 250°C, respectively. Mechanisms of crystal formation were elucidated on the basis of gas chromatography-mass spectroscopy analysis, enabling the large-scale preparation of Fe₃O₄ nanocrystals. To provide an environmentally benign route, Fe₃O₄ nanocrystals were prepared with recycled solvent which was recovered from the initial experiment. The resulting porous Fe₃O₄ nanocrystals had larger average sizes than those of the initial nanocrystals. Structural characterization was performed using transmission electron microscopy and powder X-ray diffraction.

Full text of DOI:10.1111/jace.14262

J. Am. Ceram. Soc., 1–7 (2016)
DOI: 10.1111/jace.14262
© 2016 The American Ceramic Society
ournal
J
Sustainable Method for the Large-Scale Preparation of Fe3O4 Nanocrystals
SungWoo Lee,‡ Jae-Sik Yoon,§ Sungkyoung Kang,¶ Kihyun Kwon,k Ki Soo Chang,§ SangGap Lee,§
Sang-Il Choi,‡‡ Jong-Ryul Jeong,‡,† Gaehang Lee,§,† and Ki Min Nam§§,†
‡Department of Materials Science and Engineering and Graduate School of Energy Science and Technology, Chungnam
National University, Daejeon 305-764, Korea
§Korea Basic Science Institute(KBSI), Daejeon 305-806, Korea
¶Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea
kR&D Center, Kumho Petrochemical, Daejeon 305-348, Korea
‡‡Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University,
Daegu 702-701, Korea
§§Department of Chemistry, Mokpo National University, Jeonnam 534-729, Korea
In this work, a facile synthetic process is reported for the
large-scale synthesis of Fe3O4 nanocrystals. Thermal decompo-
sition of Fe(acac)3 (100 g) in 1-hexadecanol produced Fe3O4
nanocrystals with well-controlled sizes and morphologies. The
nanocrystals were spherically shaped with average diameters of
7.8 ꢀ 0.6, 6.5 ꢀ 0.4, and 5.9 ꢀ 0.2 nm when prepared at
300°C, 270°C, and 250°C, respectively. Mechanisms of crystal
formation were elucidated on the basis of gas chromatography-
mass spectroscopy analysis, enabling the large-scale prepara-
tion of Fe3O4 nanocrystals. To provide an environmentally
benign route, Fe3O4 nanocrystals were prepared with recycled
solvent which was recovered from the initial experiment. The
resulting porous Fe3O4 nanocrystals had larger average sizes
than those of the initial nanocrystals. Structural characteriza-
tion was performed using transmission electron microscopy and
powder X-ray diffraction.
Iron oxide nanocrystals have been studied extensively
because of their catalytic and magnetic properties.2,7 Previ-
ously, our group also demonstrated the synthesis and charac-
terization of Fe3O4 nanocrystals using nuclear magnetic
resonance (NMR) spectroscopy and X-ray diffraction
(XRD).8,9 However, elucidation of the formation mecha-
nisms in a nonhydrolytic system remains challenging. The
development of plausible synthetic mechanisms should enable
the preparation of targeted compounds with controlled sizes
in a predicted manner.
In this work, a facile synthetic process is reported for the
large-scale synthesis of Fe3O4 nanocrystals. Thermal decom-
position of Fe(acac)3 (acac = acetylacetonate) in 1-hexadeca-
nol produced Fe3O4 nanocrystals with controlled sizes and
morphologies. Crystal formation mechanisms were elucidated
based on gas chromatography-mass spectroscopy (GC-MS)
analysis, enabling the large-scale production of Fe3O4
nanocrystals. Structural characterization was performed
using transmission electron microscopy (TEM) and XRD.
Moreover, porous Fe3O4 nanocrystals were prepared using
recycled solvent, which was recovered from a previous pro-
cess to demonstrate the feasibility of a practical application.
The resulting Fe3O4 nanocrystals had larger average sizes
and pores. Because 1-hexadecanol could be used repeatedly
without the generation of waste, the entire process economi-
cally consumed reagents. This synthetic method had a high
yield, good reproducibility, and convenient processing proce-
dures, enabling large-scale industrial applications.
Keywords: large-scale preparation; economical method; iron
oxides; nanomaterials
I. Introduction
ETAL oxide nanocrystals have attracted great interest
M
because they can be used as catalysts, battery materi-
als, electrical devices, and gas sensors.1–3 The wet chemical
method for the production of monodisperse nanocrystals,
involving nucleation and growth of particles in solution, has
been well-established for several decades.4 For practical use,
many metal oxides have been synthesized via hydrolysis and
condensation of reagents in an aqueous solution.5,6 These
processes have also been successfully adapted for nanocrystal
synthesis. However, hydrolytic processes suffer from a few
major drawbacks, such as low crystallinity and poor size con-
trol of particles despite extensive efforts for improvement
because of the complexity of the aqueous reaction conditions.
A large-scale process with high product quality is still
required for future technological applications as well as fun-
damental scientific interest.
II. Experimental Procedure
(1) Materials
Fe(acac)3 (97%) and 1-hexadecanol (99%) were purchased
from Sigma Aldrich (St. Louis, MO) and used without fur-
ther purification. Ethanol (99.5%) and toluene (99.5%) were
purchased from Daejung Chemicals (Shiheung, Korea).
(2) Preparation of Fe3O4 Nanocrystals for GC-MS
Analysis
P. Davies—contributing editor
Fe(acac)3 (2 g) and 1-hexadecanol (8.18 g) were vigorously
stirred in a 50 mL glass vial at 80°C for 1 h in an ambient
air atmosphere. The reaction mixture was heated to 300°C
(with a 30 min ramp time) and held at this temperature for
30 min. The resulting black-brown reaction mixture was
cooled to room temperature before ethanol (30 mL) was
Manuscript No. 38092. Received January 29, 2016; approved March 21, 2016.
S. W. Lee and J.-S. Yoon contributed equally to this work.
†Authors to whom correspondence should be addressed. e-mails: namkimin.
chem@gmail.com, ghlee@kbsi.re.kr and jrjeong@cnu.ac.kr
1

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Journal of the American Ceramic Society—Lee et al.
added. The suspension was then centrifuged, and the super-
natant was collected to study the mechanism of the forma-
tion of Fe3O4 (magnetite) nanocrystals.
were performed using the iTEM software package (Olympus
€
Soft Imaging Solutions, Munster, Germany). Images were
recorded with a charge-coupled device camera (MegaView,
Olympus Soft Imaging Solutions).
Quantitative analysis of the solvent (1-hexadecanol) and
supernatants from the reaction mixtures was performed using
a gas chromatograph coupled with a mass spectrometer (GC
Clarus 600/MS Clarus 600T, Perkin Elmer). The gas chro-
matograph was equipped with a capillary column, Elite-5MS
(Perkin Elmer, Waltham, MA, 30 m length, 0.25 mm inner
diameter, and 0.25 lm film thickness). The mass spectrome-
ter was configured for electron impact ionization mass spec-
trometry (EI-MS) at 70 eV. The interface and source
temperature were both 250°C. The temperature of the col-
umn was controlled by following a temperature program
such that the column temperature was ramped at 10°C/min
from 50°C to 280°C and maintained at 280°C for 2 min.
Magnetic properties of the products were measured at room
temperature with a vibrating sample magnetometer (VSM,
7004, Lake Shore, Westerville, OH) with an applied field
between ꢁ10000 and 10000 Oe.
(3) Large-Scale Synthesis of Fe3O4 Nanocrystals
A mixture of Fe(acac)3 (100 g) in 1-hexadecanol (409 g) was
prepared in a 2 L glass beaker. This mixture was stirred at
80°C for 1 h in an ambient air atmosphere. The solution was
then heated to 300°C (with a 30 min ramp time) and held at
this temperature for 30 min, yielding a black-brown disper-
sion (See Figs. S1 and S2). The resulting solution was cooled
down to room temperature and centrifuged to collect the
supernatant for recycling the solvent. Then, the precipitates
were washed three times with toluene (50 mL) and ethanol
(150 mL) to remove excess reactants and byproducts. The
nanocrystals were spherically shaped with an average diame-
ter of 7.8 ꢀ 0.6 nm. Synthetic and purification procedures
analogous to those reported above resulted in Fe3O4
nanocrystals with diameters of 6.5 ꢀ 0.4 and 5.9 ꢀ 0.2 nm
at reaction temperatures of 270°C and 250°C, respectively.
III. Results and Discussion
(4) Economical Preparation of Porous Fe3O4 Nanocrystals
Fe(acac)3 (100 g) and recycled solvent from the experimental
procedure (3), the first supernatant, were mixed in a 2 L flask
at 80°C for 1 h. The reaction mixture was then heated to
300°C (with a 30 min ramp time) and maintained at this
temperature for 30 min. The resulting black-brown reaction
mixture was cooled to room temperature and centrifuged to
collect the supernatant for the second recycling of the sol-
vent. The precipitates were washed three times with toluene
(50 mL) and ethanol (150 mL) to provide porous Fe3O4
nanocrystals. The resulting porous nanocrystals were quasi-
cube shaped with an average diameter of 25 ꢀ 2.7 nm. When
this recycling procedure was conducted again with Fe(acac)3
(100 g) and recollected solvent, the second supernatant, por-
ous Fe3O4 nanocrystals were formed with a diameter of
36 ꢀ 3.2 nm.
(1) Reaction Mechanism for the Formation of Fe3O4
Nanocrystals
Synthetic processes for the preparation of monodisperse
nanocrystals are relatively well-established.14 Unfortunately,
most of the synthetic methods considered to date have only
focused on the quality of the nanocrystals produced. An
understanding of synthetic mechanisms would enable the
preparation of targeted compounds with controlled sizes in a
predictable fashion.15,16 The Fe3O4 nanocrystals have been
prepared by using 1-hexadecanol not only as a reactant but
also as a surfactant. Analysis of the side products helps us to
understand the detailed formation mechanism of Fe3O4
nanocrystals. GC-MS was used to investigate the mechanism
of the chemical reaction of Fe(acac)3 and 1-hexadecanol
under ambient atmospheric conditions. When the synthesis
of Fe3O4 was complete, the reaction mixture was centrifuged.
The supernatant was collected and analyzed by GC-MS.
Four side products were identified by comparison to the ref-
erence peaks shown in Fig. 1, which included (1) 16-hydroxy-
hexadecanoic acid, (2) 4-hydroxy-5-methyl-pentan-2-one, (3)
1-hexadecanol, (4) 1-hexadecanyl acetate. Species (1) is an
(5) Material Characterization
The crystal phase of Fe3O4 nanocrystals was characterized
by powder XRD using a Bruker AXS D8 ADVANCE
diffractometer equipped with a Linxeye 1-D detector (Bruker
AXS GmbH, Karlsruhe, Germany), with a CuKa radiation
source (40 kV and 40 mA), 2h = 20° and 80°, a divergence
angle of 0.2°, a step size of 0.02°, and an acquisition time of
two-seconds per step. Rietveld refinement was performed
using the TOPAS software package (ver. 4.2, Bruker AXS
2005) while implementing the fundamental parameters
approach (FPA).10,11 The FPA was based on a convolution
profile-fitting process. Profile shape calculations were per-
formed, and intensity profiles were generated based on the
standard instrumental parameters using the FPA imple-
mented in the TOPAS program. The results were fit to a
LaB6 standard measurement, which was synthesized from
the emission profile, the specimen-dependent parameters, and
the instrument parameters. The background was fit using the
user-specified number of coefficients of a Chebychev polyno-
mial, and “Crystallite size” and/or “Microstrain” parameters
were used to model microstructure-controlled line broaden-
ing.12,13 The Rietveld analysis generates R factors, e.g., Rexp,
which were found to be large. The
R factors were
Rexp = 10.54%, Rwp = 11.05, Rp = 8.77, and GOF = 1.05
(Table S1). A low value of v2 (goodness of fit) was refined,
justifying the reasonable refinement value. TEM images,
high-resolution TEM (HR-TEM) images, and electron-dif-
fraction patterns of Fe3O4 nanocrystals deposited on a car-
bon-coated Cu grid were observed using a transmission
electron microscope, TEM, (JEOL JEM-2100, Tokyo, Japan)
operated at 200.0 kV. Image acquisition and size calculations
Fig. 1. Gas chromatogram of the supernatant from the reaction
mixture for the preparation of Fe3O4 nanocrystals. The products are
as follows: (1) 16-hydroxyhexadecanoic acid, (2) 4-hydroxy-5-methyl-
pentan-2-one, (3) 1-hexadecanol, (4) 1-hexadecanyl acetate. The
enlarged chromatogram is shown in the inset.

An Environmentally Benign Route
3
impurity in 1-hexadecanol (Fig. S3). Species (2) and (4) are
side products of the decomposition of Fe(acac)3. Plausible
reaction pathways for the formation of Fe3O4 nanocrystals,
based on solvolysis and condensation, are given in Fig. 2.
1-hexadecanol undergoes a nucleophilic attack of a carbonyl
carbon atom in the acac ligand.17 The C–C bond between
the carbonyl carbon and the center carbon was broken,
forming species (4) and acetone through alcoholysis. Ace-
tones underwent an aldol reaction with the iron center, form-
ing species (2). The resulting Fe–OCH2R species reacted with
other Fe compounds to form Fe–O–Fe bonds, and the
continuous formation of the Fe–O bonds by the thermal
decomposition led to the generation of Fe3O4 seeds.18
During the thermal decomposition process of Fe(acac)3, 1-
hexadecanol decomposed to 4-hydroxy-5-methyl-pentan-2-
one and 1-hexadecanyl acetate, which produced reductive
environments.19–22 The reduction of M(acac)3 (M = Fe, Co,
Mn) induced by oxidative decomposition of surfactants was
studied well in the literature.19,20 Furthermore, the formation
mechanisms of oxygen-deficient metal oxides by heating in a
reductive environment were also well-known.15 Thus, the
reduction of Fe3+ to Fe2+ has been explained by oxidative
decomposition of 1-hexadecanol, i.e., dehydrogenative oxida-
tion of 1-hexadecanol,21,22 which is responsible for the for-
mation of Fe3O4 with a mixed valence ((Fe2+1/3Fe3+2/3)3O4,
magnetite phase) using only an Fe3+ precursor in these
synthetic conditions.
monodispersed nanocrystals with average diameters of
6.5 ꢀ 0.4 and of 5.9 ꢀ 0.2 nm, respectively (Figs. 3 and S4),
indicating that the scale-up processes were successfully
demonstrated.
In order to obtain a reliable structural description of this
material, Rietveld analysis was performed on nanocrystals
with a diameter of 7.8 ꢀ 0.6 nm (Fig. S6). The diffraction
pattern was indexed by the Fd-3m space group (magnetite,
JCPDS No. 71-6336) with a lattice parameter, a = 8.3842
ꢀ
(13) A, which confirmed the successful synthesis of Fe3O4
nanocrystals. Figure 4 shows the XRD patterns of the vari-
ous Fe3O4 nanocrystals assigned to the standard magnetite
phase. Crystallite sizes from the (311) diffraction peaks using
the Debye–Scherrer equation were also determined to be 7.9,
6.8, and 6.1 nm. These values were in agreement with the
particle sizes obtained from TEM images. Because Fe(acac)3
and 1-hexadecanol are only used in an ambient air atmo-
sphere, the entire process is economically facile. Overall syn-
thetic cost is comparable with that of the Fe-oleate
decomposition process.14
Moreover, it is more cost-effective to reuse solvents
rather than discard them when each batch of nanomaterials
is synthesized. The mechanistic study allowed for the recy-
cling of the solvent used for synthesizing various Fe3O4
nanocrystals. In this study, the supernatant from a previous
process was retained and then used for the subsequent
preparation of porous Fe3O4 nanocrystals composed of
small grains. When the nanoparticle is interconnected with
small grains, this nanoparticle is generally denoted as a por-
ous nanoparticle.23,24 A mixture of Fe(acac)3 (100 g) and
the first recycled solvent (the first supernatant) was heated
to 300°C and maintained at this temperature for 30 min.
As much as 20.9 g of nanocrystals were produced at a yield
of 95%. Figure 5(a) shows a TEM image of the quasicubic
Fe3O4 nanocrystals with an average size of 25 ꢀ 2.7 nm,
and the porous Fe3O4 nanocrystals are constructed from
interconnected nanograins. The lattice spacing of the Fe3O4
shown in the HRTEM image [Fig. 5(a) inset] was approxi-
mately 0.295 nm ((220) plane). The XRD patterns of the
Fe3O4 nanocrystals are shown in Fig. 5(b). The diffraction
peaks were also indexed to the magnetite phase (JCPDS
No. 71-6336). An analogous synthetic procedure was used
in a subsequent preparation using the second recycled sol-
vent (the second supernatant), which was the solvent col-
lected from the quasicubic Fe3O4 nanocrystals, which also
afforded porous Fe3O4 nanocrystals composed of many
interconnected nanograins. Figure 5(c) shows a TEM image
of Fe3O4 nanocrystals with an average size of 36 ꢀ 3.2 nm,
regular pores, and multigrain nanocrystals. The Fe3O4
(2) Large-Scale and Economical Preparation of Fe3O4
Nanocrystals
To scale-up the preparation of Fe3O4 nanocrystals, Fe(acac)3
(100 g) and 1-hexadecanol (409 g) were mixed in a 2 L bea-
ker, which was heated to 300°C (with a 30 min ramp time)
and maintained at this temperature for 30 min. The amount
of nanocrystals produced was as large as 20.3 g with a yield
of 93% compared to the theoretical yield of 21.9 g [Fig. S2
(a)]. The nanocrystals were well dispersed in toluene and
easily attracted to a magnet [Figs. S2(b) and (c)]. Figure 3(a)
and S4(a) show TEM images of the monodispersed Fe3O4
nanocrystals, which were spherically shaped with an average
size of 7.8 ꢀ 0.6 nm. The fast flourier transformation (FFT)
and selected area electron-diffraction (SAED) patterns were
identical to those of the magnetite phase [Figs. 3(a) and S5].
The lattice spacing of Fe3O4, which is shown in the high-
resolution HRTEM image in Fig. 3(b), was approximately
0.254 nm ((311) plane), which corresponded to that of a
highly crystalline nature with a well-defined nanocrystal mor-
phology. Reaction temperatures of 270°C and 250°C yielded
Fig. 2. Schematic view of reaction pathways for the formation of Fe3O4 nanocrystals in the presence of 1-hexadecanol (RCH2OH).

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Journal of the American Ceramic Society—Lee et al.
Fig. 3. TEM micrographs and FFT patterns of the Fe3O4 nanocrystals. [(a), (c) and (e)] TEM and [(b), (d) and (f)] HR-TEM images of Fe3O4
nanocrystals with an average size of [(a) and (b)] 7.8 ꢀ 0.6 nm, [(c) and (d)] 6.5 ꢀ 0.4 nm, and [(e) and (f)] 5.9 ꢀ 0.2 nm. FFT patterns (inset)
obtained using a Digital Micrograph with the corresponding areas in (b), (d), and (f).
shown in the HR-TEM image in Fig. 5(c) (inset) had lattice
spacing of approximately 0.291 nm ((220) plane) and
0.251 nm ((311) plane) and displayed small grain bound-
aries. The only experimental difference in this durable pro-
cedure was the presence of (4) in the solvent used, which
resulted in the formation of nanocrystals with larger diame-
ters and pores. The GC-MS results in Fig. 6 show a grad-
ual increase in the ratio (4)/(3) from 2.8 to 22.4 during
the reaction process, which matched well with increasing
particle sizes. The color of the reaction mixture changed
from red color for the iron precursor to black for the
Fe3O4 nanocrystals during the synthesis of Fe3O4 nanocrys-
tals. This color change was a good indicator of the reac-
tion’s progress. As the amount of (4) increased in the
mixture, the color change from red to black proceeded
more slowly, suggesting that the rate of Fe3O4 seed forma-
tion was significantly diminished. This phenomena indicate
that the presence of (4) diluted the reaction solvent (surfac-
tant), 1-hexadecanol, resulting in the formation of larger
Fe3O4 nanocrystals. Generally, lower concentration of sur-
factant tends to produce bigger nanopartices.21 However, it
is not clear whether (4) inhibited the formation of Fe3O4

An Environmentally Benign Route
5
Fig. 4. XRD patterns of Fe3O4 nanocrystals synthesized at reaction
temperatures of (a) 300°C, (b) 270°C, and (c) 250°C.
Fig. 6. Gas chromatograms of (a) the first supernatant, (b) the
second supernatant, and (c) the third supernatant. The inset shows a
ratio of (4)/(3).
seeds or played a more active role in the thermal decompo-
sition kinetics. A more quantitative analysis is necessary to
make this determination.
The physicochemical properties are related to many ele-
ments such as morphology, crystallinity, and crystal
defects.25,26 The magnetic properties of Fe3O4 nanocrystals
depending on the morphologies were characterized by the
vibrating sample magnetometer (VSM) at 300 K (Fig. 7). As
expected, the Fe3O4 nanocrystals showed typical ferromag-
netic behaviors. The coercivity values for the Fe3O4
nanocrystals of 7.8 ꢀ 0.6 nm and porous Fe3O4 nanocrystals
(25 ꢀ 2.7 and 36 ꢀ 3.2 nm samples) were 67.32, 81.23, and
91.54 Oe, respectively. Besides, the Fe3O4 nanocrystals have
Fig. 5. [(a) and (c)] TEM images and [(b) and (d)] XRD patterns of porous Fe3O4 nanocrystals prepared using recycled solvent with average
sizes of [(a) and (b)] 25 ꢀ 2.7 nm and [(c) and (d)] 36 ꢀ 3.2 nm.

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Journal of the American Ceramic Society—Lee et al.
Fe(acac)3 (100 g) and 1-hexadecanol (409 g) in 2 L glass bea-
ker at room temperature, and (b) the set-up for thermal
decomposition reaction (300°C).
Fig. S2. Photographs of (a) the produced nanocrystals of
20.3 g and attraction of (b) the powder and (c) nanoparticles
dispersed in toluene by external magnetic field.
Fig. S3. Gas chromatogram of the solvent (1-hexadecanol)
from Sigma-Aldrich without further purification. The prod-
ucts are as follows: (1) 16-hydroxyhexadecanoic acid, and (3)
1-hexadecanol.
Fig. S4. TEM images and particle size distribution his-
tograms of Fe3O4 nanocrystals with an average size of (a)
7.8 ꢀ 0.6 nm, (b) 6.5 ꢀ 0.4 nm, and (c) 5.9 ꢀ 0.2 nm.
Fig. S5. SAED pattern of the 7.8 nm-sized Fe3O4
nanocrystals.
Fig. S6. Rietveld refinement patterns of the 7.8 nm-sized
Fe3O4 nanocrystals using X-ray powder diffraction. Black
solid line represent the observed pattern, and the red dot
mark(o) is calculated result. The difference plot (blue) is
shown at bottom. Tick marks indicate the reflection posi-
tions, which are identified by Rietveld analysis. The pub-
lished crystal structure of Fe3O4 was employed as a starting
structural model (ICSD # 6534).
Table S1. Refined crystal structural parameters of Fe3O4
nanocrystals obtained from the Rietveld refinement using
X-ray powder diffraction data at room temperature. The sym-
bols, O and Biso, represent the occupation and isotropic ther-
mal parameters, respectively. The numbers in parentheses are
the estimated standard deviations of the last significant figure.
Fig. 7. Magnetic hysteresis loops of (a) Fe3O4 nanocrystals
(average size of 7.8 ꢀ 0.6 nm), and porous Fe3O4 nanocrystals of
average size of (b) 25 ꢀ 2.7 and (c) 36 ꢀ 3.2 nm.
low remanence values of about 4.78, 5.97, and 10.5 emu/g.
Generally, the saturation magnetization (MS) is expected to
increase with an increasing particle size.22,25,26 However, the
MS value of porous Fe3O4 nanocrystals decreased with the
increasing particle size of the nanocrystals. The MS values of
porous Fe3O4 nanocrystals (25 ꢀ 2.7 and 36 ꢀ 3.2 nm sam-
ples) roughly accounts for 51% (47.22 emu/g), and 27%
(25.25 emu/g) of the corresponding bulk value (92 emu/g).
This unusual behavior seems to be attributable to the porous
nature of our Fe3O4 nanocrystals. The MS values were com-
monly reported to be smaller (by 20%–25%) for hollow
nanoparticles than those for the bulk.15 Ongoing work will
involve an investigation of the detailed formation mecha-
nisms and magnetic properties of porous Fe3O4 nanocrystals.
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We acknowledge KBSI grants (no. C36957) for financial support of this
project. This work was supported by Small and Medium Business Administra-
tion (S2060417) and the Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of
Science, ICT & Future Planning (NRF-2015R1C1A1A02037373, NRF-
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h