Synthesis of iron oxide nanocubes via microwave-assisted solvolthermal method

Article abstract of DOI:10.1016/j.jallcom.2010.05.020

Iron oxide nanocubes were prepared by thermal decomposition of iron oleate complex in the presence of oleic acid via microwave-assisted solvolthermal method, followed by Ostwald ripening procedures. X-ray powder diffraction and transmission electron microscopy were used to characterize the structure and morphology of the products. The results revealed that the primary nanoparticles synthesized by microwave heating were low crystalline spheres with an average diameter of about 6 nm. After aging at 180°C, these iron oxides transform to crystalline a-Fe₂O₃ and trace amount of Fe ₃O₄. The influence of aging time on the size and morphology of a-Fe₂O₃ nanocrystals was studied. Room temperature magnetization curves measured to study the magnetic properties of as-synthesized nanoparticles.

Full text of DOI:10.1016/j.jallcom.2010.05.020

Journal of Alloys and Compounds 503 (2010) L31–L33
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Synthesis of iron oxide nanocubes via microwave-assisted
solvolthermal method
F.Y. Jiang∗, Ch.M. Wang, Y. Fu, R.C. Liu
School of Environment and Materials Engineering, Yantai University, Qingquan Road, Laishan Zone, Yantai, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron oxide nanocubes were prepared by thermal decomposition of iron oleate complex in the presence
of oleic acid via microwave-assisted solvolthermal method, followed by Ostwald ripening procedures.
X-ray powder diffraction and transmission electron microscopy were used to characterize the structure
and morphology of the products. The results revealed that the primary nanoparticles synthesized by
microwave heating were low crystalline spheres with an average diameter of about 6 nm. After aging at
180 ◦C, these iron oxides transform to crystalline ␣-Fe2O3 and trace amount of Fe3O4. The influence
of aging time on the size and morphology of ␣-Fe2O3 nanocrystals was studied. Room temperature
magnetization curves measured to study the magnetic properties of as-synthesized nanoparticles.
© 2010 Elsevier B.V. All rights reserved.
Received 7 December 2009
Received in revised form 4 May 2010
Accepted 4 May 2010
Available online 11 May 2010
Keywords:
Magnetic materials
Nanomaterials
Solvolthermal
Ostwald ripening
1. Introduction
et al. reported the synthesis of monodisperse ␣-Fe2O3 nanocrystals
with continuous aspect-ratio tuning and fine shape control by such
In recent years, magnetic nanoparticles have attracted consider-
able attention not only for fundamental research, but also for broad
range of applications, such as magnetic fluids [1], catalysis [2,3],
biotechnology and biomedicine [4], magnetic resonance imaging
[5–7], etc. Among these magnetic materials, iron oxides (Fe2O3
and Fe3O4) have been extensively investigated due to its excellent
performance. The synthesis of uniform iron oxide nanoparticles is
important because the properties of these nanoparticles depend
strongly on their dimensions [7].
Various chemical routes to synthesize iron oxide nanoparticles
have been developed, such as coprecipitation [8,9], hydrolysis of
iron salt [10,11], decomposition of organic iron precursor [12–15],
microemulsion [16], hydrothermal and solvolthermal methods
[17–19]. However, the relatively poor size uniformity and low
crystallinity of obtained nanoparticles strongly deteriorate their
performance. Hence, the synthesis of monodisperse iron oxide
nanoparticles with uniform size and high crystallinity remains a
challenge.
a method [21].
Herein, we report a simple solvolthermal method to prepare
uniform ␣-Fe2O3 nanocubes with high crystallinity. In our exper-
iments, a mild microwave-assisted solvolthermal method is used,
and an economic and environmentally friendly reagent, FeCl3
serves as Fe source.
2. Experimental
The Fe(oleate)3 complex was synthesized by the reaction between ferric chlo-
ride and sodium oleate. In a typical synthesis, 19.00 g of sodium oleate was mixed
with 6.38 g of oleic acid, 10 mL of ethanol and 10 mL of water. Subsequently, 28.00 g
triethylene glycol was added to above mixture under vigorous stirring to form a
homogeneous solution. Then a mixture solution composed of 2.00 g anhydrous fer-
ric chloride (FeCl3, 98%), 10 mL of ethanol and 10 mL of water was added to the
above solution dropwise. The mixture was heated to 80 ◦C and maintained at this
temperature for 6 h under continuous stirring. Two phases occurred in the reaction
system. The upper layer was henna organic layer containing Fe(oleate)3 complex
and oleic acid, whereas the bottom layer was aqueous solution.
The decomposition of Fe(oleate)3 was conducted by a microwave digestion sys-
tem. The obtained mixture solution was transferred into a Teflon-lined digestion
vessel, and subjected to microwave treatment operating at 2.45 GHz with a power
of 400 W for 6 min, then cooled down to room temperature naturally. Fine iron
oxide nanoparticles can be collected by centrifugation and washed with ethanol
three times.
The Ostwald ripening procedure was carried out by transferring the microwave-
treated mixture into a Teflon-lined stainless autoclave, and aging in an oven at
180 ◦C for 10 h and 20 h respectively. When it was cooled to room temperature, the
precipitate was collected by centrifugation and washed with ethanol three times.
X-ray diffraction (XRD) patterns were collected on an X’Pert Pro X-ray diffraction
meter with Cu K␣ radiation (ꢀ = 0.154 nm) operating at 40 kV and 30 mA. The scan
range (2Â) was from 10◦ to 60◦ with a step of 0.08◦ and scan speed of 5.00◦/min.
Microwave can homogeneously heat the reaction medium
rapidly, resulting in a blooming of nucleation, which is favorable
for small particle size and narrow particle size distribution. The
use of microwave in conventional hydrothermal system developed
a new technique named microwave-hydrothermal method [20]. Yu
∗
Corresponding author. Tel.: +86 535 6706038; fax: +86 535 6706038.
E-mail address: fyjiang@ytu.edu.cn (F.Y. Jiang).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.05.020

L32
F.Y. Jiang et al. / Journal of Alloys and Compounds 503 (2010) L31–L33
Fig. 1. XRD patterns of iron oxide nanoparticles synthesized via microwave heating
(a), and microwave heating followed by aging at 180 ◦C for 10 h (b) and 20 h (c)
respectively.
Fig. 2. TEM image of iron oxide nanoparticles synthesized via microwave heating
(scale bar: 90 nm).
Transmission electron microscopy (TEM) observation was carried out on JEOL-
100CX operating at 100 kV. The samples for TEM observation were prepared by
dropping one or two droplets of cyclohexane dispersion of as-synthesized nanopar-
ticles on the carbon-coated copper grids and drying at room temperature. The
magnetization curves of as-synthesized nanoparticles were acquired on a LakeShore
vibrating sample magnetometer (VSM, model 7037/9509-P) at room temperature.
decomposition of iron oleate complex is responsible for the reduc-
tion of Fe3+ to Fe2+, that lead to the appearance of Fe3O4. The ratio of
integral intensity of Fe3O4 diffraction peaks to that of ␣-Fe2O3 for
aging 10 h and 20 h are about 0.078 and 0.082, respectively, which
indicates that ␣-Fe2O3 is the major product. The peak position and
relative intensity of the two ripened samples match well to each
other, suggesting that they have the same crystal structure. With
an aging time of 20 h, the intensity of peaks increased significantly
compared to that of 10 h, indicating the increase of crystallinity and
grain size of the nanoparticles.
Fig. 2 shows the TEM image of particles synthesized via
microwave heating. During microwave irradiation processes, the
rapid nucleation produced relatively uniform small spherical iron
oxide nanoparticles with average diameter of about 6 nm. These
small nanoparticles have low crystallinity as indicated by XRD
results (Fig. 1a).
3. Results and discussion
Fig. 1a shows the XRD pattern of as-synthesized products by
microwave heating. No clear peaks can be identified except weak
peaks located at 2Â values of 36◦ and 41◦, due to its low crystallinity
in the fast nucleation process. The broadening of peak widths indi-
cates the smaller size of the particles.
After aging at 180 ◦C, diffraction peaks in Fig. 1b and c show
the evolution of ␣-Fe2O3 with characteristic peaks of (0 1 2), (1 0 4),
(1 1 0), (1 1 3), (0 2 4), (1 1 6) and (1 2 2) (JCPDS 86-0550), which also
coincide with the appearance of cubic shaped nanoparticles shown
in TEM images. Impure peaks located at 2Â values of 31◦, 43◦ can be
assigned to magnetite (Fe3O4). Usually, the thermal decomposition
of metal carboxylates result in the formation of metal oxide along
with other byproducts such as CO, CO2, H2, ketones, esters, and
hydrocarbons with various chain lengths. Consequently, it seems
that a trace amount of CO, H2, and carbon produced by the thermal
After aging at 180 ◦C for 10 h, it has been observed that most
particles were cubic with the size of 13 nm and larded with small
spherical particles (Fig. 3a). During the aging progress, the small
spherical particles change their morphologies gradually through
Oswald ripening, in which smaller nanoparticles dissolved and
Fig. 3. TEM images of iron oxide nanoparticles synthesized via microwave heating followed by aging at 180 ◦C for (a) 10 h (scale bar: 90 nm) and (b) 20 h (scale bar: 200 nm).

F.Y. Jiang et al. / Journal of Alloys and Compounds 503 (2010) L31–L33
L33
procedures. Microwave-solvolthermal produced primary small
spherical particles with low crystallinity due to a fast nucleation
process. These small particles exhibit superparamagnetic charac-
ter at room temperature. At the early stage of aging process, cubic
shaped crystalline ␣-Fe2O3 with a trace amount of Fe3O4 formed,
larded with small spherical particles. With the prolonging of aging
time, small spherical particles transformed into cubic particles
mostly, and a small amount of large and irregular shaped parti-
cles appeared, the corresponding magnetic properties change from
superparamagnetic to ferromagnetic.
Acknowledgements
This work was supported by National Natural Science Foun-
dation of China (50972123) and Natural Science Foundation of
Shandong Province, China (Y2007F19).
References
Fig. 4. Room temperature magnetization curves of iron oxide nanoparticles synthe-
sized via microwave heating (a) and microwave heating followed by aging at 180 ◦C
for 20 h (b). Insets are closer look of traces (a) and (b).
[1] S. Chikazumi, S. Taketomi, M. Ukita, M. Mizukami, H. Miyajima, M. Setogawa,
Y. Kurihara, J. Magn. Magn. Mater. 65 (1987) 245–251.
[2] A.-H. Lu, W. Schmidt, N. Matoussevitch, H. BPnnermann, B. Spliethoff, B. Tesche,
E. Bill, W. Kiefer, F. SchVth, Angew. Chem. Int. Ed. 43 (2004) 4303–4306.
[3] S.C. Tsang, V. Caps, I. Paraskevas, D. Chadwick, D. Thompsett, Angew. Chem. Int.
Ed. 43 (2004) 5645–5649.
[4] A.K. Gupta, M. Gupta, Biomaterials 26 (2005) 3995–4021.
[5] S. Mornet, S. Vasseur, F. Grasset, P. Verveka, G. Goglio, A. Demourgues, J. Portier,
E. Pollert, E. Duguet, Prog. Solid State Chem. 34 (2006) 237–247.
[6] Z. Li, L. Wei, M.Y. Gao, H. Lei, Adv. Mater. 17 (2005) 1001–1005.
[7] Y.W. Jun, Y.M. Huh, J.S. Choi, J.H. Lee, H.T. Song, S. Kim, S. Yoon, K.S. Kim, J.S.
Shin, J.S. Suh, J. Cheon, J. Am. Chem. Soc. 127 (2005) 5732–5733.
[8] D.K. Kim, M. Mikhaylova, Y. Zhang, M. Muhammed, Chem. Mater. 15 (2003)
1617–1627.
redeposited on the surface of bigger nanoparticles. The appear-
ance of cubic nanoparticles coincide with the evolution of ␣-Fe2O3
characteristic peaks in XRD pattern (Fig. 1b and c).
When the aging time was prolonged to 20 h, small particles
almost disappeared and the size of cubic particles increased to
28 nm with high crystalline nature (Fig. 3b). But a small amount
of large and irregular shaped particles also appeared. So a proper
aging time is necessary to obtain uniform nanocubes.
[9] R. Valenzuela, M.C. Fuentes, C. Parra, J. Baeza, N. Duran, S.K. Sharma, M. Knobel,
J. Freer, J. Alloys Compd. 488 (2009) 227–231.
The magnetization curves of as-synthesized iron oxide nanopar-
ticles measured at room temperature are shown in Fig. 4. The
saturation magnetization increases from 8 emu/g to 30 emu/g
when aging at 180 ◦C for 20 h. This can be attributed to the growth
of particles size and the improvement of crystallinity. Since the
hematite (␣-Fe2O3) is weakly magnetic at room temperature, the
relative higher saturation magnetization can be attributed to the
presence of Fe3O4, which was confirmed by XRD results (Fig. 1).
Due to the small particle size, the iron oxide nanoparticles syn-
thesized by microwave heating (Fig. 4(a)) is superparamagnetic
at room temperature, but when aging at 180 ◦C for 20 h, the iron
oxide nanoparticles become ferromagnetic that exhibit an obvious
hysteresis phenomenon. This is because the particle size already
exceeds the critical size of superparamagnetic.
[10] Zh.F. Pu1, M.H. Cao, J. Yang, K.L. Huang, Ch.W. Hu, Nanotechnology 17 (2006)
799–804.
[11] T. Ozkaya, M.S. Toprak, A. Baykal, H. Kavas, Y. Köseog˘lu, B. Akta, J. Alloys Compd.
472 (2009) 18–23.
[12] J. Rockenberger, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 121 (1999)
11595–11596.
[13] H.T. Song, J.S. Choi, Y.M. Huh, S. Kim, Y.W. Jun, J.S. Suh, J. Cheon, J. Am. Chem.
Soc. 127 (2005) 9992–9993.
[14] J. Kim, J.E. Lee, J. Lee, Y. Jang, S.W. Kim, K. An, H.H. Yu, T. Hyeon, Angew. Chem.
Int. Ed. 45 (2006) 4789–4793.
[15] R. Shi, X.H. Liu, G.H. Gao, R. Yi, G.Zh. Qiu, J. Alloys Compd. 485 (2009) 548–553.
[16] T. Nakanishi, H. Iida, T. Osaka, Chem. Lett. 32 (2003) 1166–1168.
[17] B.P. Jia, L. Gao, Cryst. Growth Design 8 (2008) 1372–1376.
[18] Sh.B. Wang, Y.L. Min, S.H. Yu, J. Phys. Chem. C 111 (2007) 3551–3554.
[19] J. Chen, F.B. Wang, K.L. Huang, Y.N. Liu, S.Q. Liu, J. Alloys Compd. 475 (2009)
898–902.
[20] G.J. Wilson, A.S. Matijasevich, D.R.G. Mitchell, J.C. Schulz, G.D. Will, Langmuir
22 (2006) 2016–2027.
[21] X.L. Hu, J.C. Yu, Adv. Funct. Mater. 18 (2008) 880–887.
4. Conclusions
Cubic iron oxide nanoparticles were prepared via microwave-
assisted solvolthermal method followed by Ostwald ripening