Solventless nanoparticles synthesis under low pressure

Article abstract of DOI:10.1021/ic701570z

In the present study, metal nanocrystals were obtained by the very easy, economical, and nontoxic thermal decomposition method and stabilized by coating oleate without any solvent. These nanocrystals have a highly crystalline structure due to a high decomposition temperature (～563-573 K) at low pressure and very narrow distribution. The prepared Fe₃O₄ nanocrystals were controlled by the annealing time and vacuum pressure. A TEM image of monodispersed Fe₃O₄ nanocrystals showed the 2D assembly of nanocrystals, demonstrating their uniformity. The particle size is 10.6 ± 1.2 nm. TEM images of silver nanocrystals a showed 2D assembly with 9.5 ± 0.7 nm. An electron diffraction image and X-ray diffraction of the nanocrystals showed the highly crystalline nature of metal nanocrystals.

Full text of DOI:10.1021/ic701570z

Inorg. Chem. 2008, 47, 121−127
Solventless Nanoparticles Synthesis under Low Pressure
Hyun Gil Cha,† Don Keun Lee,‡ Young Hwan Kim,† Chang Woo Kim,† Chung Sup Lee,§ and
Young Soo Kang*,
†
Department of Chemistry, Pukyong National UniVersity, Namgu, Busan 608-737, South Korea,
Cheil Industries Inc., Gocheon-Dong, Uiwang-Si, Gyeonggi-Do 437-711, South Korea, and
Department of Physics, Pukyong National UniVersity, Namgu, Busan 608-737, South Korea
Received August 7, 2007
In the present study, metal nanocrystals were obtained by the very easy, economical, and nontoxic thermal
decomposition method and stabilized by coating oleate without any solvent. These nanocrystals have a highly
crystalline structure due to a high decomposition temperature (
distribution. The prepared Fe nanocrystals were controlled by the annealing time and vacuum pressure. A TEM
image of monodispersed Fe nanocrystals showed the 2D assembly of nanocrystals, demonstrating their uniformity.
The particle size is 10.6 1.2 nm. TEM images of silver nanocrystals a showed 2D assembly with 9.5 0.7 nm.
∼
563−573 K) at low pressure and very narrow
3 4
O
3 4
O
±
±
An electron diffraction image and X-ray diffraction of the nanocrystals showed the highly crystalline nature of metal
nanocrystals.
Introduction
addition, monodispersed magnetic nanocrystals such as
12
13
14
15,16
iron, cobalt, R-Fe
2
O
3
,
and Fe
3
O
4
have been synthe-
The synthesis of monodispersed nanocrystals is of key
importance because the properties of these nanocrystals
depend strongly on their dimensions. For example, the color
sharpness of semiconductor nanocrystal-based optical devices
is strongly dependent on the uniformity of the nanocrystals,
and monodisperse magnetic nanocrystals are critical for the
next-generation multi-terabit magnetic storage media. For
these monodisperse nanocrystals to be used, an economical
mass-production method needs to be developed. Unfortu-
nately, in most syntheses reported so far, only sub-gram
quantities of monodisperse nanocrystals were produced.
Uniform-sized nanocrystals of CdSe8 and gold
produced using colloidal chemical synthetic procedures. In
1
7
sized using various synthetic methods. As these syntheses
are generally carried out in solution and it is impossible in
practice to completely eliminate coagulation. Korgel et al.
have pioneered the synthesis of nanoparticles in the absence
1
-4
1
8-21
of solvent.
Scheme 1 shows a schematic drawing of the
5-7
nontoxic synthesis of functional nanoparticles without any
solvent. The metal-oleate complex serves as the molecular
precursor and provides the capping ligand to control particle
growth. In the solventless reaction environment, interparticle
collisions rarely occur, and particle growth proceeds prima-
,9
10,11
have been
(
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1
(
*
To whom correspondence should be addressed. E-mail: yskang@
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pknu.ac.kr.
†
Department of Chemistry, Pukyong National University.
Cheil Industries Inc.
Department of Physics, Pukyong National University.
‡
§
(
(
(
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2000, 30, 545.
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Weller, H. AdV. Funct. Mater. 2002, 12, 653.
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Magn. Mater. 1993, 122, 42.
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J. Am. Chem. Soc. 2003, 125, 5638.
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(
6) Speliotis, D. E. J. Magn. Magn. Mater. 1999, 193, 29.
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(
8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993,
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1
0.1021/ic701570z CCC: $40.75
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 1, 2008 121
Published on Web 12/07/2007

Cha et al.
Scheme 1. Procedure of the Synthesis of Nanocrystallites Using
M-oleate Complex (M ) Ag , Fe , etc.)
X’Pert-MPD System with a Cu KR radiation source (λ ) 0.154056
nm). The size and shape of the nanoparticles were obtained by
transmission electron microscopy (TEM). TEM measurements were
carried out on Hitachi H-7500 (low-resolution) and a JEOL
JEM2010 (high-resolution) transmission electron microscopes.
Samples for TEM study were prepared on 300 mesh copper grids
coated with carbon. A drop of metal nanocrystal solution was
carefully placed on the copper grid surface and dried. The size
distributions of the nanocrystals were measured from enlarged
+
2+
a
3 4
photographs of the TEM images. The magnetization curve of Fe O
nanocrystals was characterized with the Lake Shore 7300 vibrating
sample magnetometer (VSM). M o¨ ssbauer spectra were recorded
using a conventional M o¨ ssbauer spectrometer of the electrome-
chanical type with a 20 mCi 57Co source in a rhodium matrix. To
produce uniform thickness over the area of the M o¨ ssbauer absorber,
a sample was mixed with boron nitride powder. The area density
a
The precursors were prepared from the reaction of metal chlorides and
sodium oleate in water. The thermalysis of the complex was performed
under low pressure at about 593 K without any solvent.
2
of iron for the flattened sample was 10 mg/cm . The size distribution
and characterization of the nanocomposite film of the surface coated
Fe O nanocrystals was done with TEM analysis. The optical
3 4
rily by monomer addition to the particle surface, leading to
22
monodispersed size and shape distributions. The advantages
of this method is that one does not need to make a separation
nor to evaporate solvent.
property of the silver nanoparticles has been characterized by
absorption spectroscopy. UV-vis absorption spectra of the syn-
thesized silver nanocrystals were taken using a Varian, Cary 1C.
Thermal analysis was performed under vacuum conditions with a
heating rate of 10 K/min using a PerkinElmer TGA 7 connected
with a vacuum system.
2. Experimental Section
2
.1. Materials. FeCl ‚4H
2
2
O (99+%), AgNO (99+%), and
3
sodium oleate (C17 33COONa, 98%) were used as supplied from
Sigma-Aldrich. Hexanes was purchased from Jusei Co and used
as received. Water was doubly distilled and deionized.
H
3
. Results and Discussions
To synthesize monodispersed metal nanoparticles, we
2
3
modified the published procedure using thermal decom-
position of the metal-oleate complex in the absence of
surfactants and solvent. When pure metal carboxylates are
heated, the organic moiety in the metal-oleate complex was
decomposed at temperatures near to or higher than 300 °C.
The thermal decomposition reaction of transition metal
carboxylates occurs via the formation of thermal free radicals,
as shown in previous work. As the temperature increases,
the breakage of M-O and MO-C bonds in metal carboxy-
late results in radical species according to eqs 1 and 2.
2
.2. Synthesis of Fe
3
O
4
Nanocrystals. To prepare iron-oleate
complex, 2 g of FeCl ‚4H
2
2
O (10 mmol) was dissolved in deoxy-
genated water, and the resulting solution was added into 6.09 g of
sodium oleate (20 mmol) under vigorous stirring for 2 h. The
precipitate was separated by filtration and washed with doubly
deionized water to be free of sodium and chlorine ions. After drying,
+2
2
the Fe -oleate complex was transferred into the pyrex tube. The
complex was first flushed with nitrogen, and the tube was sealed
under 300 mTorr. The sample was slowly heated from room
temperature to 593 K at 1 K/min. After reaching the desired
temperature, it was held at 593 K for 2 h and cooled to room
temperature. The complex color was changed to black, indicating
2
4
M-OOCR f M* + *OOCR
M-OOCR f MO* + OC*R
(1)
(2)
that Fe
nanocrystals can be easily redispersed in hexane.
.3. Synthesis of Silver Nanocrystals. To prepare the Ag+1
oleate complex, 1.7 g of AgNO (10 mmol) was dissolved in
deoxygenated water (300 mL, 18 MΩ, nitrogen gas bubbling for
0 min), and the resulting solution was added into 3.05 g of sodium
3 4
O nanoparticles were being formed. The iron oxide
2
-
3
From the reaction between the simultaneously formed
radical species, these species can combine, decompose into
smaller molecules, or react with other metal carboxylate
molecules to propagate to decompose metal carboxylates in
the metal-oleate complex. We controlled the morphologies
of metal nanoparticles by temperature and vacuum pressure.
3
oleate (10 mmol) under vigorous stirring for 2 h. The precipitate
was separated by filtration and washed with doubly deionized water
+
1
to be free of sodium and nitrate ions. After drying, the Ag
-
oleate complex was transferred into the pyrex tube. The complex
was first flushed with nitrogen, and the tube was sealed at 300
mTorr. The sample was slowly heated from room temperature to
3 4
As Fe O nanocrystals were studied as a model for different
morphologies of metal nanoparticles by thermal decomposi-
tion, we observed that the morphology of metal nanoparticles
depends on the annealing time and vacuum pressure.
563 K by 2 K/min, and it was annealed at 563 K for 1 h and cooled
to room temperature. The complex color was changed to black,
indicating that silver nanocrystals were being formed. The silver
nanocrystals can be easily redispersed in hexane.
3.1. Fe
3 4
O Nanocrystals. Part A of Figure 1 is a low-
resolution TEM image of monodisperse Fe O nanocrystal-
3
4
2.4. Characterization. To identify the structure and properties
3 4
lites. All of the Fe O particles are spherical. The mean size
of the synthesized metal nanoparticles, we performed various
experiments. The crystal structure of the synthesized nanocrystals
was identified by X-ray powder diffraction (XRD) with a Philips
of the Fe
3 4
O nanoparticles is 10.6 nm, with a standard
deviation of 1.2 nm, which was obtained from enlarged
(
23) Han, Y. C.; Cha, H. G.; Kim, C. W.; Kim, Y. H.; Kang, Y. S. J.
(
22) Teranishi, T.; Hasegawa, S.; Shimizu, T.; Miyke, M. AdV. Mater. 2001,
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122 Inorganic Chemistry, Vol. 47, No. 1, 2008

SolWentless Nanoparticles Synthesis under Low Pressure
3 4
nearest-neighbor Fe O particles can be inferred. The super-
paramagnetic behavior is documented by the hysteresis loop
measured at 300 K as shown in part B of Figure 1. There is
almost an immeasurable coercivity (1.7 Oe) at room tem-
3 4
perature. This indicates that Fe O particles are superpara-
2
6-28
magnetic and nanosized. The saturation magnetization,
3 4
, is 72.3 emu/g for Fe O , which is lower than that of
M
s
2
9,30
bulk magnetite particles (Mbulk ) 92 emu/g).
in M is due to the weight of oleate coated on the surface of
the Fe nanocrystals and shows the superparamagnetism
of magnetite particles, which occurs when the particle size
decreases below 30 or 20 nm. From the magnetization
curve, it can be seen that the magnetization achieves a
saturation of 72.3 emu/g at 2500 Oe. This result differs from
that of iron oxide nanocrystals synthesized by the hydrolytic
method. They do not need to be saturated by applying a
The decrease
s
3
O
4
3
1
3
0,31
magnetic field of more than 10 000 Oe.
The result can
be explained from the difference in nanocrystals and the size
distributions of the iron oxide nanoparticles. Compared with
other iron oxide nanoparticles, produced Fe O nanoparticles
3 4
possess not only a more regular spherical shape but also
better monodispersity and crystallinity. The M o¨ ssbauer
3 4
spectrum of the Fe O nanocrystals is given in part C of
Figure 1 at room temperature. The solid line is a computer-
fitted M o¨ ssbauer spectrum, and the solid circles give the
experimental M o¨ ssbauer spectrum. The spectrum was con-
voluted to two sextets that correspond to iron ions in
tetrahedral (T
Fe can be written as Fe (Fe Fe )O
transfer process (electron hopping) between Fe and Fe
ions on the O B site takes place above 110-120 K. The
d h
) A and octahedral (O ) B sites of magnetite.
3
+
2+
3+
O
3 4
4
. A fast electron-
2
+
3+
h
spectrum shows clearly two hyperfine magnetic splitting,
which is clear evidence for magnetite. The XRD pattern of
3 4
the Fe O nanocrystals sample was shown in part D of Figure
1
. No impurity peak is observed in the X-ray diffraction
pattern. The diffraction peaks are broadened, which is a result
of the reduced particle size. The discernible peaks can be
indexed to (220), (311), (222), (400), (422), (333), and the
(
440) planes of a cubic unit cell, which corresponds to that
of the magnetite structure (JCPDS card no. 79-0418). Part
E Figure 1 shows lattice images of Fe nanoparticles. It
is perfectly matched with the result of the XRD study.
Figure 2 shows TEM images of Fe nanocrystals
prepared at different annealing times of 1 h (A), 2 h (B), 4
h (C), and 6 h (D). The mean sizes of Fe nanocrystals at
Figure 1. Evolution of shape, size, magnetic property, and crystal structure
after annealing of the Fe2+-oleate2 complex at 573 K under 300 mTorr
for 2 h. (A) TEM image of a drop of Fe3O4 nanocrystals/hexane solution.
3
O
4
(
B) Magnetization versus applied field at 300 K for Fe3O4 nanocrystals
57
using VSM. (C) Fe M o¨ ssbauer spectrum of synthesized Fe3O4 nanocrys-
tals. (D) X-ray diffraction pattern of synthesized Fe3O4 nanocrystals. (E)
High-resolution TEM characterizations of Fe3O4 nanocrystals.
3 4
O
3
O
4
images and collected statistics over 150 particles in part A
of Figure 1. A monolayer of nanoparticle was observed
without any multilayer. Part A of Figure 1 shows an example
of an extended area where particles are packed in a highly
organized manner, exhibiting a remarkable degree of long-
range order. Nearly monodisperse nanoparticles are arranged
in a 2D hexagonal closed packed way, demonstrating the
uniformity of the particle size. Especially the interparticle
573 K for 1 h and 2 h are 6.5 and 9.6 nm, respectively.
Their morphologies are almost spherical. When the annealing
time is above 3 h, the size of Fe O nanocrystals was
3 4
(
25) Kaneko, F.; Yamazaki, K.; Kitagawa, K.; Kikyo, T.; Kobayashi, M.;
Kitagawa, Y.; Matsuura, Y.; Sato, K.; Suzuki, M. J. Phys. Chem. B
1997, 101, 1803.
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Magn. Mater. 2001, 225, 30.
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Eng., A 2000, 286, 101.
3 4
spacing is very even. Here, as in most cases, adjacent Fe O
(
28) Zaitsev, V. S.; Filimonov, D. S.; Presnyakov, I. A.; Gambino, R. J.;
nanocrystals are separated by a region of approximately 2
nm, which does not exhibit any diffraction contrast. This
distance is considerably less than twice the expected oleate
length (1.75 nm);25 interdigitation of the alkyl chains from
Chu, B. J. Colloid Interface Sci. 1999, 212, 49.
(29) Sato, T.; Iijima, T.; Inagaki, N. J. Magn. Magn. Mater. 1987, 65, 252.
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(
1
Inorganic Chemistry, Vol. 47, No. 1, 2008 123

Cha et al.
Figure 2. TEM images of Fe3O4 nanocrystals. Each of the Fe3O4 nanocrystals was prepared at different annealing times (A, 1 h; B, 2 h; C, 4 h; and D,
6
h).
increased, and the shape was changed into shapelessness.
These results suggest that the average size of nanocrystals
synthesized by thermal decomposition increased over opti-
mized reaction time and also morphology changed. In
general, particle size increases with increasing reaction time,
as more material adds to particle surfaces, and with increasing
temperature, as the rate of addition of materials to the existing
nuclei increases. The distribution kinetics approach with
single monomer addition and dissociation is reversible and
is generally applicable to growth, dissolution, or ripening
phenomena known as Ostwald ripening, in which the high
surface energy of the smaller particles promotes their
dissolution, whereas material is redeposited on the larger
particles.32
of physically adsorbed oleic acid, either free or weakly bound
2
+
to Fe ions at the first DTG peak; then the loss of 26.97%
(539-597 K) is owing to the oleate at the surface of
nanoparticles, and the residual weight of 62.46% should be
3 4
that of oleate-capped Fe O nanoparticles. These results can
explain that the yield between [Fe]decomposed/[Fe]complex is
100%. As we changed the pressure from 300 to 500 mTorr,
the decomposition temperature of the iron-oleate complex
increased almost 7 K, and no difference was observed
between the shapes of the TGA/DTG curves (Supporting
Information, Figure S1).
As shown in Figure 3, low-resolution TEM images (A,
B) and a high-resolution image (C) of polymorphologies of
Fe O nanocrystals were produced at 593 K after sealing
3
4
To obtain more detailed information on the thermal
decomposition process under different pressure, we carried
out a thermalgravimetry analysis connected with a vacuum
system. The iron-oleate complex under 300 mTorr started
to thermally decompose from 410 K and showed a maximal
rate of weight loss at 573 K, which is similar to solution-
phase synthesis.33 TGA/DTG patterns revealed that the loss
of 10.54% in the range of 428-524 K is due to the removal
under 500 mTorr as well as lattice images of spherical-shaped
(D), regular triangular-shaped (E), short rod-shaped (F),
diamond-shaped (G), and long rod-shaped (H) Fe O nanoc-
3
4
rystals. It indicated that various shaped nanocrystals were
highly crystallized as a Fe O crystal structure. From TGA
3
4
and TEM results, we suggest that the decomposition tem-
perature is increasing and decreasing the velocity of mono-
(33) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park,
(32) Zhang, X. L.; Kang, Y. S. Inorg. Chem. 2006, 45, 4186.
J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891.
124 Inorganic Chemistry, Vol. 47, No. 1, 2008

SolWentless Nanoparticles Synthesis under Low Pressure
Figure 3. TEM images show a various particle shape of Fe3O4 nanocrystals produced at 573 K after sealing under 500 mTorr. (A, B) TEM Images and
(C) high-resolution image of differently shaped Fe3O4 nanocrystals. (D) Spherical-shaped, (E) regular triangular-shaped, (F) short rod-shaped, (G) diamond-
shaped, and (H) long rod-shaped Fe3O4 nanocrystals. These lattice images revealed the highly crystalline nature of the nanocrystals.
mer addition to the particle surface, which nucleates nanoc-
rystals as pressure increases in the pyrex tube. It occurs that
the nucleation of iron nanocrystals was slow. Generally, a
faster nucleation leads to more particles of smaller size,
whereas a slower nucleation gives fewer particles of larger
size. Particles consist with more than one crystal structure
or polymorphs. Polymorphs with different shapes have
different surface properties, which influences the growth rate
of crystal faces and shape the crystal habit.34
3.2. Silver Nanocrystals. Part A of Figure 4 shows a low-
resolution TEM image of silver nanocrystals, which are
perfectly spherical. The mean size of the silver nanocrystals
is 9.5 nm with a standard deviation of 0.7 nm, which was
obtained from enlarged images and collected statistics over
1
50 particles in part A of Figure 4. Part B of Figure 4 shows
the UV-vis spectrum of silver nanocrystals (λmax ) 420 nm)
dispersed in hexane. The silver nanocrystals display an
optical absorption band peaked at 420 nm (∼3 eV), typical
of the absorption of metallic silver nanocrystals, as a result
of the surface plasmon resonance (SPR). The colloidal
(
34) Wang, Y.; Xu, X.; Tian, Z.; Zong, Y.; Cheng, H.; Lin, C. Chem.s
Eur. J. 2006, 12, 2542.
Inorganic Chemistry, Vol. 47, No. 1, 2008 125

Cha et al.
Figure 4. Analysis of silver nanoparticles synthesized by annealing of the Ag+1-oleate complex at 573 K under 300 mTorr for 2 h. (A) Low-resolution
TEM image of monodispersed silver nanocrystals was identified. (B) UV-vis spectrum of silver nanocrystals synthesized at 563 K (λmax ) 420 nm) dispersed
in hexane solution. (C) The selected area diffraction pattern of silver nanocrystals synthesized by solventless thermolysis. (D) XRD pattern of synthesized
nanocrystals.
suspensions of our silver particles gave a bright yellow-
greenish color due to the intense bands around the excitation
of SPR. Although the conduction and valence bands of
semiconductors are separated by a well-defined band gap,
metal nanocrystals have close-lying bands and electrons
move quite freely. The free electrons give rise to a surface
plasmon absorption band in metal clusters, which depends
on both the cluster size and chemical surroundings. This
absorption band is a little blue-shifted compared with the
plasmon absorption band of silver colloids prepared by the
citrate reduction method (λmax ) 434 nm). This is caused
because the silver nanocrystals have a narrower size distribu-
tion and smaller diameter than silver nanocrystals prepared
by the citrate reduction method. Part C of Figure 4 shows
that the electron diffraction pattern exhibits a silver crystal
structure. The diffraction rings can be indexed to (111), (200),
(220), and (311) planes of a silver nanocrystal. Part D of
Figure 4 illustrates the XRD pattern of the annealed Ag -
oleate complex at 563 K under 300 mTorr. The signature of
silver is observed. Peaks are broadened due to the nanocrystal
nature of silver (JCPDS card no. 04-0783). Three peaks at
three values of 38.2, 44.5, and 64.5° correspond to (111),
(200), and (220) planes of silver, respectively. No impurity
peak is observed in the X-ray diffraction pattern. The
+
1
126 Inorganic Chemistry, Vol. 47, No. 1, 2008

SolWentless Nanoparticles Synthesis under Low Pressure
formation of a silver nanocrystal by thermal decomposition
was investigated by TGA (Supporting information, Figure
S2).
synthetic method is a generalized process that can be easily
extended to other metals and to alloys of two or more metal
nanocrystals.
Conclusion
Acknowledgment. The author would like to thank to Dr.
S. B. Kang at the Cooperative Laboratory Center, Pukyong
National University, for assisting in the TEM measurement
and Dr. H. J. Ryoo for conducting XRD measurements of
our samples. Financial support is from the Program for the
Training of Graduate Students in Regional Innovation, which
was conducted by the Ministry of Commerce, Industry and
Energy of the Korean Government, Brain Korea 21 project
in 2007 and Nano R&D Program (Korea Science &
Engineering Foundation, Grant 2007-02628). D.K.L. is
supported by the Korea Research Foundation Grant funded
by the Korean Government (MOEHRD, KRF-2005-214-
C00206).
In conclusion, an interesting experimental finding related
to a very simple synthetic method of metal nanocrystals and
their morphological control has been observed. This work
has shown the following results: (1) monodispersed metal
nanoparticles were prepared by metal-oleate thermal de-
composition without any solvent. (2) Different shaped metal
nanocrystals were prepared by annealing time and vacuum
pressure and each have a highly crystalline structure. This
synthetic procedure developed in the present study offers
several very important advantages over the previously
reported method for the synthesis of monodispersed nanoc-
rystals. First, this process is easily scaled up for industrial
purposes in a single reaction. When the reactor volume is
set large, several kilograms of monodispersed nanocrystal
can be readily obtained. Second, the synthetic process is
environmentally friendly and economical because it uses
nontoxic and inexpensive reagents and solvents. Third, the
33
Supporting Information Available: TGA results of Fe and
Ag-oleate complex (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
IC701570Z
Inorganic Chemistry, Vol. 47, No. 1, 2008 127