Synthesis of Highly Crystalline and Monodisperse Maghemite
Nanocrystallites without a Size-Selection Process
Taeghwan Hyeon,* Su Seong Lee, Jongnam Park, Yunhee Chung, and Hyon Bin Na
Contribution from the School of Chemical Engineering and Institute of Chemical Processes,
Seoul National UniVersity, Seoul 151-744, Korea
ReceiVed August 10, 2001
Abstract: The synthesis of highly crystalline and monodisperse γ-Fe2O3nanocrystallites is reported. High-
temperature (300 °C) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition
of iron pentacarbonyl in the presence of oleic acid at 100 °C, was found to generate monodisperse iron
nanoparticles. The resulting iron nanoparticles were transformed to monodisperse γ-Fe2O3nanocrystallites by
controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16
nm by controlling the experimental parameters. Transmission electron microscopic images of the particles
showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these
nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic
(TEM) images of the nanoparticles showed the highly crystalline nature of the γ-Fe2O3structures. Monodisperse
γ-Fe2O3nanocrystallites with a particle size of 13 nm also can be generated from the direct oxidation of iron
pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant.
The development of uniform nanometer sized particles has
been intensively pursued because of their technological and
fundamental scientific importance.1These nanoparticular ma-
terials often exhibit very interesting electrical, optical, magnetic,
and chemical properties, which cannot be achieved by their bulk
counterparts.2So far, the majority of nanoparticle research has
been focused upon II-VI semiconductors and noble metals.
Comparatively little work has been conducted upon the fabrica-
tion of uniform oxide nanoparticles despite their many important
technological applications.3The fabrication of patterned media
arrays of discrete single domain magnetic nanoparticles is very
important for their potential applications in multi-terabit/in2
magnetic memory devices.4Such magnetic nanoparticles could
also find applications in ferrofluids, refrigeration systems,
medical imaging, drug targeting, and catalysis.5The syntheses
of several uniform-sized magnetic metal nanoparticles have been
reported.6However, relatively little work has been done on the
fabrication of monodispersed and crystalline magnetic oxide
nanoparticles. Several magnetic oxide nanoparticles including
γ-Fe2O3and magnetite have been synthesized by using micro-
emulsion and other methods.7However, particle size uniformity
and the crystallinity of these nanoparticles are comparatively
poor. Although the syntheses of relatively uniform maghemite
and magnetite nanoparticles have been recently reported,
exhaustive size selection procedures were necessary.8Here we
report upon a novel non-hydrolytic synthetic method of fabricat-
ing highly crystalline and monodisperse γ-Fe2O3nanocrystalline
particles without a size selection process, which allows the
production of selected particle sizes from 4 to 16 nm.
Synthesis of γ-Fe2O3Nanocrystallites through the Oxidation of
Iron Nanoparticles. To prepare monodispersed iron nanoparticles, 0.2
mL of Fe(CO)5(1.52 mmol) was added to a mixture containing 10 mL
of octyl ether and 1.28 g of oleic acid (4.56 mmol) at 100 °C. The
resulting mixture was heated to reflux and kept at that temperature for
1 h. During this process, the initial orange color of the solution gradually
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J. Am. Chem. Soc. 2001, 123, 12798-12801
10.1021/ja016812s CCC: $20.00© 2001 American Chemical Society
Published on Web 11/29/2001
changed to black. The resulting black solution was cooled to room
temperature and 0.34 g of dehydrated (CH3)3NO (4.56 mmol) was
added. The mixture was then heated to 130 °C under an argon
atmosphere and maintained at this temperature for 2 h, whereupon it
formed a brown solution. The reaction temperature was slowly increased
to reflux and the reflux continued for 1 h; the color of the solution
gradually turned from brown to black. The solution was then cooled
to room temperature, and ethanol was added to yield a black precipitate,
which was then separated by centrifuging. The resulting black powder
can be easily re-dispersed in hydrocarbon solvents, such as hexane,
octane, and toluene.
Synthesis of 13 nm Sized γ-Fe2O3Nanocrystallites through the
Oxidative Decomposition of Iron Pentacarbonyl. Fe(CO)5(0.2 mL,
1.52 mmol) was injected into a solution containing 0.91 g of lauric
acid (4.56 mmol), 7 mL of octyl ether, and 0.57 g of (CH3)3NO (7.60
mmol) at 100 °C in an argon atmosphere, with vigorous stirring. As
soon as Fe(CO)5was injected into the mixture, the temperature rose to
120 °C and the solution became dark-red, which indicated the successful
oxidation of Fe(CO)5. The reaction mixture was then stirred for 1 h at
120 °C, and the solution was slowly heated to reflux. The solution
color gradually became black, indicating that nanoparticles were being
formed. After refluxing for 1 h, the solution was cooled to room
temperature, and a black precipitate was obtained upon adding excess
ethanol and centrifuging. The precipitate can be easily redispersed in
octane or toluene.
Characterization of Materials. Nanocrystallites were characterized
by low- and high-resolution TEM, electron diffraction, and X-ray
diffraction. Low-resolution transmission electron micrographs were
obtained on a JEOL EM-2000 EX II microscope. High-resolution
transmission electron micrographs were taken on a JEOL JEM-3000F
microscope. X-ray diffraction pattern was obtained with a Rigaku
D/Max-3C diffractometer equipped with a rotating anode and a Cu
KR radiation source (λ ) 0.15418 nm).
Results and Discussion
Two different approaches were applied in the synthesis. In
the first method, monodisperse iron nanoparticles were first
generated and were further oxidized to iron oxide. In the second
approach, Fe(CO)5was directly injected into a solution contain-
ing both the surfactant and trimethylamine oxide. In both
approaches, monodisperse iron oxide nanocrystallites were
directly obtained without a further size selection process;
however, the first method allowed better control of particle size
and reproducibility. By varying the experimental conditions, we
were able to get γ-Fe2O3nanocrystallites with different particle
sizes. The nanoparticles obtained were characterized by low-
resolution and high-resolution TEM, electron diffraction, X-ray
powder diffraction, X-ray photoelectron spectroscopy, and
In the following, a typical fabrication of iron oxide nanoc-
rystallites with a particle diameter of 11 nm through the first
synthetic method is described. At first, the iron oleate complex
was prepared by reacting Fe(CO)5 and oleic acid with a 1:3
molar ratio at 100 °C. The UV-visible absorption spectrum of
the metal complex revealed a single intense peak at 330 nm
(Supporting Information), and the FT-IR spectrum of the
complex exhibited no CO stretching peak. These spectroscopic
data and the Mo ¨ssbauer spectroscopic results on the decomposi-
tion of iron pentacarbonyl at 120 °C reported by Wonterghem
and co-workers9demonstrate that the iron(II) complex seemed
to be produced. However, the exact structure of the complex
was not elucidated. Iron nanoparticles were then generated by
aging iron complex at 300 °C. The TEM image of the iron
nanoparticles (Supporting Information) revealed that nanopar-
ticles are monodisperse. The electron diffraction pattern showed
that the nanoparticles are nearly amorphous. The XRD pattern
of the sample after being heat treated under argon atmosphere
at 500 °C revealed a bcc R-iron structure. These iron nanopar-
ticles were then transformed into γ-Fe2O3nanocrystallites by
oxidizing them at 300 °C with a mild oxidant, trimethylamine
oxide ((CH3)3NO). The TEM image (Figure 1) showed that
monodisperse nanoparticles of 11 nm diameter were arranged
in a 2-dimensional hexagonal closed packed way, demonstrating
the uniformity of the particle size. The high-resolution transmis-
sion electron micrograph (HRTEM) shown in Figure 2 illustrates
the highly crystalline nature of the nanoparticles. The electron
diffraction pattern exhibited a maghemite (γ-Fe2O3) structure
The X-ray powder diffraction pattern of the material also
proved its highly crystalline nature and the peaks matched well
with standard γ-Fe2O3reflections (Figure 4). The XRD peaks
of the nanocrystallites were compared with those of standard
maghemite and magnetite data (Supporting Information). In the
X-ray photoelectron spectrum, the positions of the Fe(2p3/2) and
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Figure 1. TEM image of a two-dimensional hexagonal assembly of
11 nm γ-Fe2O3nanocrystallites.
Figure 2. High-resolution TEM image of 2D hexagonally close-packed
11 nm γ-Fe2O3nanocrystallites.
Highly Crystalline and Monodisperse Maghemite NanocrystallitesJ. Am. Chem. Soc., Vol. 123, No. 51, 2001 12799
Fe(2p1/2) peaks are 711.3 and 724.4 eV, which are in good
agreement with the values reported for γ-Fe2O3in the literature
(Supporting Information).10The Raman spectrum, which was
often applied as a tool to differentiate between maghemite and
magnetite, matched well with the reported maghemite spectrum
(Supporting Information).11By manipulating the TEM sample
preparation condition, we could obtain a 3-dimensional close-
packed superlattice assembly (Figure 5).
When the starting reaction mixture containing 1:1 and 1:2
molar ratios of Fe(CO)5 and oleic acid were applied in the
synthesis, γ-Fe2O3nanocrystallites with particle sizes of 4 and
7 nm were obtained, respectively. The low-resolution and high-
resolution TEM image of the 7 nm sized nanocrystallites are
shown in Figure 6. The low-resolution TEM images of γ-Fe2O3
nanocrystallites with a particle diameter of 4 nm are shown in
To produce bigger nanocrystallites of >11 nm, a reaction
mixture with a 1:4 molar ratio of Fe(CO)5and oleic acid was
applied in the synthesis. The particle size of the resulting
nanocrystallites was, however, still around 11 nm. However,
we could get iron nanocrystallites with particle sizes bigger than
11 nm by adding more iron oleate complex into the previously
prepared 11 nm sized iron nanocrystallites, followed by aging
at 300 °C. The resulting bigger iron nanocrystallites were later
oxidized to get monodisperse γ-Fe2O3nanocrystallites through
a similar procedure as described above. Through this synthetic
procedure, we could tune the particle size of the nanocrystallites
from 11 to 16 nm. The TEM images of the γ-Fe2O3nanocrys-
tallites with particle sizes of 16 nm through the synthetic
procedure are shown in Figure 8.
We have also used a second synthetic procedure, employing
the direct oxidative decomposition of Fe(CO)5in the presence
of trimethylamine oxide and a surfactant, to fabricate uniform
γ-Fe2O3nanocrystallites. The low- and high-resolution TEM
images of γ-Fe2O3nanocrystallites (Figure 9) demonstrated that
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Figure 3. Electron diffraction pattern of 11 nm γ-Fe2O3nanocrystal-
Figure 4. X-ray diffraction pattern of 11 nm γ-Fe2O3nanocrystallites.
Figure 5. TEM image of a three-dimensional superlattice of 11 nm
Figure 6. Low-resolution TEM image and high-resolution TEM image
of a single nanocrystallite (inset) of 7 nm γ-Fe2O3nanocrystallites.
Figure 7. TEM image of 4 nm γ-Fe2O3nanocrystallites.
12800 J. Am. Chem. Soc., Vol. 123, No. 51, 2001Hyeon et al.
the particle size is very uniform and highly crystalline, with a Download full-text
diameter of 13 nm.
Preliminary magnetic measurements were performed on the
γ-Fe2O3nanocrystallites with use of a superconducting quantum
interference device (SQUID). The temperature dependence of
the magnetization was measured with use of zero-field cooling
(ZFC) and field cooling (FC) procedures in an applied magnetic
field of 100 Oe between 5 and 300 K. The plot of temperature
versus magnetization for 4, 13, and 16 nm γ-Fe2O3nanocrys-
tallites with zero-field cooling (ZFC) is presented in Figure 10.
The blocking temperatures of the γ-Fe2O3nanocrystallites with
particle diameters of 4, 13, and 16 nm were found to be 25,
185, and ∼290 K, respectively. Detailed magnetic studies of
the γ-Fe2O3nanocrystallites are currently underway.
The highly crystalline and monodisperse γ-Fe2O3nanocrys-
tallites were fabricated from the controlled oxidation of uniform
iron nanoparticles which were generated from the thermal
decomposition of iron complex. The synthetic procedures
developed in the present study offer several very important
advantageous features for the fabrication of oxide nanoparticles.
First, they allow highly crystalline and monodisperse nanopar-
ticles to be obtained directly without a further size-selection
process. Second, particle size can be easily and reproducibly
altered by changing the experimental parameters. Third, the
nanocrystallites can be easily dispersed in many hydrocarbons
without particle aggregation. In addition, the yield of the current
process is over 80% and the indications are that scale-up can
be achieved relatively easily.
Acknowledgment. We are grateful to the Korea Institute
of Science and Technology (KIST) for financial support. We
also would like to thank Prof. Kwan Kim and Dr. Sang Woo
Han for the Raman spectroscopic studies, Prof. Zheong G. Khim,
Suyoun Lee, and Joonsung Lee for the magnetic studies, and
Mee Jeong Kang for the TEM studies.
Supporting Information Available: UV-visible absorption
spectrum of iron oleate complex, TEM image of 11 nm iron
nanoparticles, XRD pattern of 11 nm iron nanoparticles after
heating under Ar at 500 °C, a table showing the comparison of
d spacing values of the synthesized iron oxide nanocrystallites
with standard JCPDS γ-Fe2O3and Fe3O4data, Raman spectrum
of 11 nm γ-Fe2O3, X-ray photoelectron spectrum of 11 nm
γ-Fe2O3, plots of magnetization versus temperature of 4, 13,
and 16 nm γ-Fe2O3with field cooling and zero-field cooling at
the applied magnetic field of 100 Oe (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 8. TEM image of 16 nm γ-Fe2O3nanocrystallites.
Figure 9. Low-resolution and high-resolution TEM image of a single
nanocrystallite (inset) of 13 nm γ-Fe2O3nanocrystallites.
Figure 10. Magnetization versus temperature for 4 (triangles), 13
(squares) , and 16 nm (circles) γ-Fe2O3nanocrystallites with zero-field
cooling at the applied magnetic field of 100 Oe. The magnetic studies
were conducted with a Quantum Design MPMS SQUID magnetometer.
Highly Crystalline and Monodisperse Maghemite NanocrystallitesJ. Am. Chem. Soc., Vol. 123, No. 51, 2001 12801