Materials Chemistry and Physics 111 (2008) 438–443
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Facile route to the synthesis of porous ␣-Fe
2
O
3
nanorods
Saikat Mandal, Axel H.E. M
¨
uller
∗
Makromolekulare Chemie II and Bayreuther Zentrum f
¨
ur Kolloide und Grenzfl
¨
achen, Universit
¨
at Bayreuth, D-95440 Bayreuth, Germany
article info
Article history:
Received 17 September 2007
Received in revised form 15 April 2008
Accepted 21 April 2008
Keywords:
Iron oxide
Template synthesis
Porous materials
Magnetic materials
abstract

© 2008 Elsevier B.V. All rights reserved.
1. Introduction
An important area of research in nanotechnology is the develop-
ment of reliable synthesis protocols for nanostructured materials
over a range of chemical compositions, shapes and sizes. Over
the past few years, the synthesis of inorganic nanoscale materials
with special morphologies has been of great interest in mate-
rial science [1,2] because the intrinsic properties of nanoscale
materials are mainly determined by their composition, structure,
crystallinity, size, and morphology [3]. Compared with nondi-
mensional nanoparticles, one-dimensional (1D) nanomaterials are
more interesting because of their potential high technological
applications for electronics, photonics, and magnetic materials [4].
In recent years, the preparation of magnetic nanomaterials is
under scrutiny for potential applications in information storage [5],
color imaging [6], magnetic refrigeration [7] bioprocessing [8],gas
sensors [9], ferrofluids [10], and so on. In particular, hematite (␣-
Fe
2
O
3
), the most stable iron oxide, with n-type semiconducting
properties under ambient conditions, is of scientific and techno-
logical importance because of its usage in catalysts [11], sensors
∗
Corresponding author.
E-mail address: (A.H.E. M
¨
uller).
[12], and lithium-ion batteries [13]. Because of their nontoxicity,

¨
uller / Materials Chemistry and Physics 111 (2008) 438–443 439
capability [31–35]. Many synthesis methods have been developed
for generating 1D ␣-Fe
2
O
3
nanostructures, such as nanorods [36],
nanowires [37], nanobelts [38], and nanotubes [39] using various
methods such as vapor–solid (VS) reaction [40], vapor–liquid–solid
growth technique [41], metalorganic chemical vapor deposition
(MOCVD) technique [42], sol–gel process [43], hard porous tem-
plates [44], ␥-irradiation method [45]. However, all these reported
methods either produced solid nanorods, nanowires or hollow nan-
otubes but without pores on the wall.
To the best of our knowledge, very few reports on the synthe-
sis of porous ␣-Fe
2
O
3
nanorods have been published to date [46].
Owing to their specific characteristics and promising applications
exploring proper methods for the synthesis of nanoscale porous ␣-
Fe
2
O
3
rods proves to be stimulating and valuable. Therefore, it is
important to develop the methods to regulate both the pore and
particle morphology in a one-dimensional structure of these mate-

2
atmosphere
and followed by vigorous stirring for 2 h. After 2 h stirring, 5 mL of 3M NaOH solu-
tion was introduced into the mixed solution under N
2
atmosphere with vigorous
stirring for 2 h more. After adding the NaOH solution into the reaction mixture a
brownish black-colored reaction mixture appears instantaneously. The next step for
the hydrothermal treatment, 2.5 mL of the brownish black-colored reaction mix-
ture was transferred into a 25 mL Teflon-lined autoclave and the autoclave was
sealed and heated at 120
◦
C for 24 h without shaking or stirring during the heat-
ing period and allowed to cool to room temperature naturally. After the reactions
were completed, the final yellow solid products were centrifuged and washed with
distilled water and absolute ethanol several times, and then dried at 40
◦
C under a
vacuum for 4 h. The obtained yellow solid products were collected for the follow-
ing experiments and characterization. To prepare porous hematite nanostructures,
the as-prepared rod-like iron oxide nanostructures were treated/stirred with acidic
(hydrochloric)–ethanolic solution at temperature 65
◦
C for 24 h, followed by calci-
nation at 500
◦
C with a ramping rate of 5 K min
−1
and then maintained at 500
◦

ment operated at an accelerating voltage at 200 kV. The magnetic properties of the
porous nanorods were examined using SQUID (superconducting quantum interfer-
ence device) (Quantum Design, MPMS-7).
3. Results and discussion
Fig. 1A shows the XRD patterns recorded in the 2␪ range 20–70
◦
of the samples before (curve 1) and after calcination (curve 2).
Well-defined XRD patterns were observed and all diffraction peaks
were perfectly indexed, which are in agreement with the data of
␣-FeOOH (curve 1) (JCPDS 29-713) and ␣-Fe
2
O
3
(hematite, curve
2) (JCPDS 33-664). The strong and sharp peaks indicate that the
␣-FeOOH and ␣-Fe
2
O
3
powders are highly crystalline.
Fig. 1B shows FTIR spectra of as-prepared sample, before
solvent extraction and calcination (curve 1) and sample after sol-
vent extraction and calcination (curve 2) in the spectral region
2700–3100 cm
−1
. The C–H symmetric and antisymmetric stretch-
Fig. 1. (A) The XRD patterns recorded from the as-prepared sample (curve 1) and after calcination sample (curve 2). (B) The FTIR spectra of as-prepared sample, (curve 1)
and sample after solvent extraction and calcination (curve 2).
440 S. Mandal, A.H.E. M
¨

exhibit the porous surface after calcination, while the shape (length
and diameter) of the nanorods remain almost same in both the
cases (before and after template removal). The porous structure
is much more clearly seen in the FE-SEM image of the nanorods
(Fig. 2B) where the presence of relatively homogeneous pores of
9–12 nm sizes on the surface of the nanorods are observed after
calcination.
The transmission electron microscopy image recorded from the
as-prepared sample is shown in Fig. 2C and it clearly shows the
Fig. 3. The TEM image recorded from the as-prepared sample synthesized without
surfactant in control experiment.
S. Mandal, A.H.E. M
¨
uller / Materials Chemistry and Physics 111 (2008) 438–443 441
Fig. 4. (A) Magnetic hysteresis loop of the porous ␣-Fe
2
O
3
nanorods at 300 K. The inset of this figure shows a magnified view of the hysteresis loop highlighting the residual
magnetization and the coercivity. (B) Temperature dependence of ZFC and FC magnetization of the porous ␣-Fe
2
O
3
nanorods under an applied magnetic field of 500 Oe.
rod-like structure with smooth surface. The inset of Fig. 2C shows
a HRTEM image of the nanorods. The lattice planes are clearly seen
and the interplanar spacing 4.26
˚
A correspond to the (1 0 0) planes,
which reveal the crystalline nature of the as-obtained ␣-FeOOH

co-exist with a small percentage of rod-shaped particles. The mor-
phologies obtained from the as-prepared sample in the control
experiment are thus totally different than that obtained from the
experiment in presence of SDS surfactant as template. It is most
likely that the rod-like micelle of the surfactant helps in the mor-
phology selectivity during the growth process.
The magnetic properties of the porous ␣-Fe
2
O
3
nanorods were
further investigated using SQUID. To investigate the magnetic
properties of the porous ␣-Fe
2
O
3
nanorods, magnetic hystere-
sis measurement was carried out in an applied magnetic field at
300 K (room temperature) with the field sweeping from −60 to
60 kOe. Fig. 4A shows the hysteresis loop of the porous nanorods.
It can be seen that saturation is not reached up to the maximum
applied magnetic field. The inset of this figure shows a magni-
fied view of the hysteresis loop recorded for the porous ␣-Fe
2
O
3
nanorods and shows weak ferromagnetic behavior with a rema-
nent magnetization of 0.02 emu g
−1
and a coercivity of 250 Oe at

the FC and ZFC magnetization curves split significantly; the ZFC
magnetization decreases sharply, while the FC magnetization rises
significantly. The Morin transition temperature for the porous ␣-
Fe
2
O
3
nanorods (223 K, calculated from differential ZFC curve) is
lower than that for bulk ␣-Fe
2
O
3
(263 K), which may be related to
the decrease in diameters for 1-D nanohematite, agreeing with the
theory that T
M
decreases with decreasing particle size. Because of
nanoscale confinement, nanomaterials can exhibit unusual mag-
netic behavior that is quite different from that of conventional bulk
materials.
In Scheme 1 we illustrate a possible mechanism that could
explain the formation of the rod-like morphology of iron oxide
and the formation of pores during calcination. SDS is known to
form cylindrical micelles at high concentration in solution and
it is an ionic compound, which ionizes completely in water to
form a negatively charged molecule with a long hydrophobic tail.
The first step results in a possible electrostatic interaction exist-
ing between the added Fe
3+
/Fe

porous ␣-Fe
2
O
3
nanorods were obtained. The detailed morphology,
crystallinity and magnetic properties of the resulting porous-
nanorods were determined using combined SEM, XRD, HRTEM and
SQUID measurements. Crystalline porous ␣-Fe
2
O
3
was obtained
after heat-treating the as-obtained ␣-FeOOH nanorods, which
retain the same nanorod morphology, even at 500
◦
C. The proposed
method has great advantages in large-scale industrial manufactur-
ing for a simple hydrothermal process, such as inexpensive raw
materials, high purity, and a high morphology yield of the products.
The surfactant SDS plays a key role in controlling the nucleation
and growth of the nanorods, and the possibility of using the ionic
surfactants as rod-like template is exciting. It is our hope that this
convenient and efficient synthesis route can be applied as a gen-
eral method for the preparation of porous 1D nanostructure of other
metal and oxides with possible applications in catalysis and novel
optical materials.
Acknowledgements
SM thanks the Alexander von Humboldt Foundation for a
research fellowship. We thank Mr. Benjamin Balke (Inorganic
Chemistry, University of Mainz, Germany) and Mr. Ram Sai Yela-

(b) J. Chen, L. Xu, W. Li, X. Gou, Adv. Mater. 17 (2005) 582.
[14] J. Wang, W.B. White, J.H. Adair, J. Am. Ceram. Soc. 88 (2005) 3449.
[15] T. Ohmori, H. Takahashi, H. Mametsuka, E. Suzuki, Phys. Chem. Chem. Phys. 2
(2000) 3519.
[16] S.W. Kim, M. Kim, W.Y. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642.
[17] (a) Z. Kang, E. Wang, L. Gao, S. Lian, M. Jiang, C. Hu, L. Xu, J. Am. Chem. Soc. 125
(2003) 13652;
(b) Z. Kang, E. Wang, B. Mao, Z. Su, L. Chen, L. Xu, Nanotechnology 16 (2005)
1192.
[18] Y.G. Sun, B. Mayers, Y. Xia, Adv. Mater. 15 (2003) 641.
[19] Z.Y. Jiang, Z.X. Xie, X.H. Zhang, S.C. Lin, T. Xu, S.Y. Xie, R.B. Huang, L.S. Zheng,
Adv. Mater. 16 (2004) 904.
[20] J.J.E. Lee, J. Lee, J.H. Yu, B.C. Kim, K. An, Y. Hwang, C.H. Shin, J.G. Park, J. Kim, T.
Hyeon, J. Am. Chem. Soc. 128 (2006) 688.
[21] M. Lal, L. Levy, K.S. Kim, G.S. He, X. Wang, Y.H. Min, S. Pakatchi, P.N. Prasad,
Chem. Mater. 12 (2000) 2632.
[22] K. Sharma, S. Das, A. Maitra, J. Colloid Interface Sci. 284 (2005) 358.
[23] X. Tan, S. Liu, K. Li, J. Membr. Sci. 188 (2001) 87.
[24] N.E.Botterhuis, Q. Sun, P.C.M.M. Magusin, R.A. van Santen, N.A.J.M. Sommerdijk,
Chem. Eur. J. 12 (2006) 1448.
[25] Z.Z. Li, S.A. Xu, L.X. Wen, F. Liu, A.Q. Liu, Q. Wang, H.Y. Sun, W. Yu, J.F. Chen, J.
Controlled Release 111 (2006) 81.
[26] J.F. Chen, H.M. Ding, J.X. Wang, L. Shao, Biomaterials 25 (2004) 723.
[27] A.P.R. Johnston, B.J. Battersby, G.A. Lawrie, M. Trau, Chem. Commun. (2005) 848.
[28] Z.Z. Li, L.X. Wen, L. Shao, J.F. Chen, J. Controlled Release 98 (2004) 245.
[29] S.B. Yoon, J.Y. Kim, J.H. Kim, S.G. Park, J.Y. Kim, C.W. Lee, J S. Yu, Curr. Appl. Phys.
6 (2006) 1059.
[30] F. Caruso, R.A. Caruso, H. M
¨
ohwald, Science 282 (1998) 1111.

Commun. 129 (2004) 485;
(c) P C. Wu, W S. Wang, Y T. Huang, H S. Sheu, Y W. Lio, T L. Tsai, D B. Shieh,
C S. Yeh, Chem. Eur. J. 13 (2007) 3878.
[47] Z. Jing, S. Wu, Mater. Lett. 58 (2004) 3637.
[48] M. Sorescu, R.A. Brand, D.M. Tarabasanu, L. Diamandescu, J. Appl. Phys. 85
(1999) 5546.
[49] (a) H. Naono, K. Nakai, T. Sueyoshi, H. Yagi, J. Colloid Interface Sci. 120 (1987)
439;
(b) H. Naono, R. Fujiwara, J. Colloid Interface Sci. 73 (1980) 406.