synthesis and growth of hematite nanodiscs through a facile hydrothermal approach

RESEARCH PAPER
Synthesis and growth of hematite nanodiscs through a facile
hydrothermal approach
X. C. Jiang Æ A. B. Yu Æ W. R. Yang Æ
Y. Ding Æ C. X. Xu Æ S. Lam
Received: 28 November 2008 / Accepted: 7 April 2009
Ó Springer Science+Business Media B.V. 2009
Abstract This study reports a facile hydrothermal
method for the synthesis of monodispersed hematite
(a-Fe
2
O
3
) nanodiscs under mild conditions. The
method has features such as no use of surfactants, no
toxic precursors, and no requirements of high-temper-
ature decomposition of iron precursors in non-polar
solvents. By this method, a-Fe
2
O
3
nanodiscs were
achieved with diameter of 50 ± 10 nm and thickness
of *6.5 nm by the hydrolysis of ferric chloride. The
particle characteristics (e.g., shape, size, and distribu-
tion) and functional properties (e.g., magnetic and
catalytic properties) were investigated by various
advanced techniques, including TEM, AFM, XRD,
BET, and SQUID. Such nanodiscs were proved to
show unique magnetic properties, i.e., superparamag-
netic property at a low temperature (e.g., 20 K) but

ferromagnetic property at a room temperature
(*300 K). They also exhibit low-temperature
(\623 K) catalytic activity in CO oxidation because
of extremely clean surfaces due to non-involvement of
surfactants, compared with those spheres and ellip-
soids capped by PVP molecules.
Keywords Hematite nanoparticles Á
Nanodiscs Á Hydrothermal synthesis
Introduction
Hematite (a-Fe
2
O
3
) nanoparticles have been widely
studied because of their attractive properties, includ-
ing stability in air, n-type semiconducting, non-
toxicity, and corrosion-resistance. These properties
have driven them for potential applications in catal-
ysis, gas sensing, pigment, nonlinear optic, and field-
effect transistor (Shin et al. 2004; Schertmann and
Cornell 1991; Wang and Willey 1998, 1999). A
variety of synthesis methods have been employed
for shape and size control, such as ball milling,
co-precipitation, sol-gel method, micelle template,
thermal decomposition of precursors in non-polar
solvents, and hydrothermal methods (Ozaki et al.
1984; Matijevic
´
1985; Matijevic
´

and Hamada 1982;
Matijevic
´
and Scheiner 1978; Woo et al. 2003;
Vayssieres et al. 2005; Cao et al. 2005; Yin et al.
2005; Raming et al. 2002). Some methods focused on
X. C. Jiang Á A. B. Yu (&)
School of Materials Science and Engineering, University
of New South Wales, Sydney, NSW 2052, Australia
e-mail:
W. R. Yang
Australian Key Centre for Microscopy and Microanalysis
(AKCMM), Electron Microscopy Unit, University
of Sydney, Sydney, NSW 2006, Australia
Y. Ding Á C. X. Xu
School of Chemistry and Chemical Engineering,
Shandong University, Jinan, Shandong 250100, China
S. Lam
CSIRO Materials Science and Engineering, P.O. Box 218,
Lindfield, NSW 2070, Australia
123
J Nanopart Res
DOI 10.1007/s11051-009-9636-8
the thermal decomposition of organometallic precur-
sors (e.g., Fe(CO)
5
, Fe(acac)
3
, Fe(oleate)
3

, or their
dual source systems) in non-polar solvents for
generating monodispersed iron oxide nanoparticles,
which, however, may limit their applications in
aqueous system (Yin and Alivisatos 2005; Casula
et al. 2006; Cheon et al. 2004). For example, the iron
oxide nanoparticles obtained by thermal decomposi-
tion in non-polar solvents are difficult to be transferred
directly into aqueous solution because of the surface-
coated surfactants. The removal of such surfactants
may lead to particle aggregation, and hence affect the
covalently binding other surfactants such as poly(eth-
ylene)glycol (PEG) spacer with hydrophilic groups for
further dextran coating that is targeted toward solid
tumor treatment (Sonvico et al. 2005).
Among the achieved hematite nanocrystals
obtained, non-spherical particles have become attrac-
tive because of their anisotropic properties. To date,
one-dimensional (1D) iron oxide nanoparticles (e.g.,
rods and wires) have been widely studied, but only a
few studies were reported on two-dimensional (2D)
ones (e.g., plates and discs). Their growth mecha-
nisms are not well understood. A representative
example in this area was reported by Casula et al.
(2006) who demonstrated the preparation of iron and/
or iron oxide nanodiscs through a thermal decompo-
sition (at *293°C) of iron pentacarbonyl in the
presence of an oxidizer and surfactants (tridecanoic
acid or 3-Chloro peroxybenzoic acid). Niederberger
et al. (2002) reported the fabrication of hematite

disclike particles with outer diameter of *1 lm and
the thickness of *250 nm through a hydrolysis and
subsequent hydrothermal approach. Chen and Gao
(2004) prepared the crystalline a-Fe
2
O
3
nanodiscs by
a hydrothermal method at 150°C and through aging
for 24 h in the presence of surfactants. The above
synthesis methods suggested that the utilization of
surfactants is necessary and important for shape
control. This is also evidenced by other wet-chem-
istry methods in the past. However, drawbacks
resulting from surface-adsorbed surfactants have the
unpredictable influence on the surface functionality
of nanoparticles and the diminished accessibility to
particle surface. A small amount of residual may
significantly reduce the functionalities in catalysis or
gas sensing. For example, the pyrolysis of oleic acid
could introduce some reducing agents such as carbon
(C), carbon monoxide (CO) and hydrogen (H
2
),
which can cause a negative effect on the particle
performances (Kim et al. 2007). Therefore, to
develop facile synthesis methods to produce mon-
odispersed iron oxide nanodiscs with extremely clean
surfaces is still a challenging task.
In this study, we demonstrate a facile hydrother-

mal approach to generate monodispersed a-Fe
2
O
3
nanodiscs in the absence of surfactants under mild
conditions. The particle characteristics (e.g., shape,
size, crystallization) and physicochemical properties
(e.g., magnetic, catalytic properties) of the as-
prepared nanoparticles are investigated by various
advanced techniques. The influence of a few exper-
imental parameters (e.g., pH, temperature, time, and
concentration of Fe
3?
) on the particle growth in the
surfactant-free system is then investigated. The
catalytic CO oxidation, as one of typical functional-
ities of hematite nanoparticles, is also examined.
Experimental work
Synthesis of iron oxide nanoparticles
The iron oxide nanoparticles could be prepared by the
hydrolysis of FeCl
3
salt in an acid solution under mild
conditions. This approach is similar to the previous
studies (Matijevic
´
and Scheiner 1978; Raming et al.
2002), and the modification of experimental para-
meters has been adopted to prepare plate-like nano-
particles. In a typical procedure, three steps were

involved. First, 0.5 g FeCl
3
Á 6H
2
O (Sigma-aldrich,
99.9%) was put in 10 ml of water, followed by
vigorous stirring to ensure that all the powders got
dissolved completely. Second, the transparent yellow-
brown solution was quickly injected into a conical
flask containing 90 ml of hot water (*90°C) and
0.75 ml of dilute HCl (1.0 M), followed by vigorous
stirring to ensure that the reaction system was
homogeneous. Finally, the mixed solution was
refluxed heating at 90°C for around 5 min before
being transferred into an oven for heating at 90°C. In
order to avoid water evaporation, the flask was sealed
by aluminum foil and a glass lid. After heating for
48 h, the solution turned deep red color. The particles
were found homogeneously dispersed in this solution.
In the surfactant-assisted synthesis, poly(vinyl pyr-
rolidone) (PVP, M
w
= 55,000, Sigma-aldrich, 99.9%)
was used to control particle shape and size, but other
J Nanopart Res
123
parameters were kept constant. Ultra-pure water was
used in all the synthesis processes. All the glasswares
were cleaned with aqua regia, thoroughly rinsed with
ultra-pure water and alcohol prior to use.

Characterization
Various techniques were used to characterize the
particle characteristics and properties in this study, as
described below:
(i) Particle characteristics such as shape, size, and
size distribution were checked using Philips
CM200 field emission gun transmission electron
microscope (TEM) operated at an accelerated
voltage of 200 kV. The specimen was prepared
by dropping the solution onto a Formvar-coated
copper grid and dried in air naturally. The data
for particle-size distribution were collected
based on TEM analysis, and also assisted by
Image Processing and Analysis Program (ImageJ
1.37v, 2006);
(ii) The composition of the as-synthesized sample
was identified by powder X-ray diffraction
(XRD), and recorded using Siemens D5000 at
a scanning rate of 0.5°/min in the 2h range of
20–80°;
(iii) The atomic force microscope (AFM) image
was obtained by a Molecular Imaging Picoscan
II instrument in tapping mode. The sample was
prepared by depositing a few drops of a dilute
solution of the nanoparticles onto a mica disc
and then dried in air naturally. Analysis of the
AFM image was performed using the WSxM
software (version 3, Nanotec Electronica S.L.,
Spain);
(iv) The Brunauer–Emmett–Teller (BET) surface

area of the as-prepared particles was measured
at 77 K (liquid nitrogen) on a Quantachrome
Autosorb-6B Surface Area & Pore Size Ana-
lyzer. Before BET measurements, the sample
was degassed at 150°C for 3–4 h to ensure that
no gas molecules adsorbed on the particle
surfaces;
(v) The magnetic properties were investigated on
a Quantum Design MPMS XL-5 (SQUID)
magnetometer. The sample was put in a low-
susceptibility plastic sample holder for mea-
surements. The magnetic moment from the
sample holder was found to be at least three
orders of magnitude smaller than the signal
from the sample and thus can be ignored;
(vi) The catalytic oxidation of CO gas was per-
formed in a home-built fixed bed microreactor.
A reactant gas containing CO (6,000 ppm) in
O
2
atmosphere (CO/O
2
= 1/10) was buffered
with N
2
gas and with a total flow rate of 60 ml/
min. An appropriate amount of hematite pow-
ders (*50 mg) was used in the measurement.
The heating and cooling cycles were monitored
in a temperature range of 30–450° C.

Results and discussion
Microstructure of iron oxide nanoparticles
The microstructure of the as-prepared nanoparticles
obtained by the proposed synthesis strategy was
checked by TEM technique. Figure 1a shows the
TEM image of one representative sample. The
particles were found nearly monodispersed with
diameters of 50 ± 10 nm based on their size distri-
bution (Fig. 1b). In order to confirm the crystalliza-
tion, the selected area electron diffraction (SAED)
was carried out under TEM operations. Several clear
diffraction rings doped with spots were recorded and
shown in a pattern (inset of Fig. 1a), suggesting that
the nanoparticles are of crystalline structure. The
further confirmation on single crystalline or poly-
crystalline particles needs other techniques like XRD
and HRTEM. These diffraction rings could be
assigned to (104), (110), (113), (024), (116), and
(300) crystallographic planes of rhombohedral phase
a-Fe
2
O
3
, respectively, based on the standard JCPDS
card (No. 02-915) (Cullity and Stock 2001).
A close inspection on the particle shape and
crystallization was conducted by various techniques.
Figure 2a shows a magnification TEM image reveal-
ing that the as-prepared particles are spherical parti-
cles, and some of them overlapped, as pointed by

arrows. Further evidence could be directly obtained
from the AFM image shown in Fig. 2d. The curve
plotted in Fig. 2d reveals that the particle is of plate-
like structure with a thickness of *6.5 nm. Combin-
ing the structural analysis of TEM and AFM, the
as-prepared hematite particles are of nanodiscs in
J Nanopart Res
123
shape. The lattice fringes of the individual nanodisc
could be clearly seen in the high-resolution TEM
(HRTEM) image (Fig. 2b), indicating that the nano-
discs are well crystallized under the reported condi-
tions. Measuring the distance between two adjacent
planes gives a value of *0.411 nm, corresponding to
the lattice spacing of {110} facets of rhombohedral
a-Fe
2
O
3
. The electron diffraction (ED) pattern could
be indexed to the [001] zone of rhombohedral hematite
(inset of Fig. 2b). The crystallization of the nanodiscs
was also evidenced by the well-resolved peaks in
XRD pattern (Fig. 2c), in which all the diffraction
peaks could be assigned to rhombohedral phase
a-Fe
2
O
3
(a = b = 5.028 A

˚
and c = 13.728 A
˚
,
JCPDS 02-915) (Cullity and Stock 2001). This is also
supportive to the indexed diffraction rings in the
SAED pattern (inset of Fig. 1a). These results revealed
that the as-prepared nanodiscs are pure rhombohedral
a-Fe
2
O
3
with single crystalline structure.
Particle nucleation and growth
The particle nucleation and growth occurred while
the ferric ions solution was mixed with hot water
(90°C), accompanied by a rapid color change from
yellow to red. In order to trace the growth, the
colloids were isolated from the heated suspension at
different times for statistic analysis. Owing to the
limitations in in-site observing the nucleation of
colloids, several representative samples were chosen
here to illustrate the growth process. After heating for
*1 min, small colloids formed, but they are difficult
to be clearly distinguished in shape and size as shown
in Fig. 3a. After heating for *5 min, the solution
turned a bit dark red. The colloids isolated from this
dark-red solution were checked by TEM technique.
Figure 3b shows the TEM image that the shapes of
colloids are still difficult to distinguish, but the

particle size becomes larger (20 ± 10 nm) than those
obtained at the reaction time of *1 min. This result
suggested that the nucleation is fast, as the so-called
‘‘burst-nucleation’’ happens in this reaction system.
Although the nucleation and the growth may be
overlapping each other at the initial stage, the particle
size increasing with time could be clearly observed
after 5 min (Fig. 3b–f). The fast nucleation and the
subsequent slow growth obtaining well-crystallized
nanodiscs could also be confirmed by the relationship
between reaction time and particle sizes as described
in Fig. 4, the corresponding data for which were
collected and compared on the basis of TEM images
obtained at different times. This is different from the
nucleation-delayed mechanism that occurred in the
formation of iron oxide nanodiscs through the
thermal decomposition of Fe(CO)
5
precursor in
non-polar solvent (Casula et al. 2006).
After 5-min heating at 90°C, the reaction system
was transferred into an oven with the heating
continued further. In order to further understand the
particle growth, the particles produced at different
times (e.g., 1, 6, 12, and 24 h) were separately
isolated for TEM characterization. Figure 3c shows
the TEM image that 1-h heating merely resulted in
the formation of irregular-shaped particles, but par-
ticle size increased with time. The 6-h heating was
found to result in small particles to grow to a

diameter of 50–70 nm, and some of them were still
irregular in shape (Fig. 3d). Again, the aggregation of
small particles or the clusters could be observed in
Fig. 1 a TEM image of hematite (a-Fe
2
O
3
) nanodiscs with
inset of SAED pattern; and b The size distribution of the
a-Fe
2
O
3
nanodiscs
J Nanopart Res
123
this sample. With continuous heating up to 12 h,
more well-shaped particles formed with diameter of
*70 nm, although the size distribution was a bit
wide. In the meantime, it was found that almost the
smaller irregular particles disappeared at this stage
(Fig. 3e). This process could be consistent with
Ostwald ripening, i.e., smaller particles continue to
shrink, while larger particles continue to grow
(Ostwald 1896). The 24-h heating could lead to the
formation of nearly monodispersed particles with a
mean diameter of 55 nm (Fig. 3f). A close look at the
particles reveals that they have a slight shrinkage in
20 30 40 50 60 70 80
(306)

(217)
(1010)
(208)
(300)
(214)
(018)
(116)
(024)
(113)
(110)
(104)
(012)
Intensity (a.u.)
2 θ (degree)
C
200 300 400 500 600
0
2
4
6
8
10
Thickness (nm)
Diameter (nm)
D
Fig. 2 a A high
magnification TEM image
of a-Fe
2
O

3
nanodiscs with
overlapping as pointed by
arrows; b HRTEM image
showing the lattice fringe of
{110} planes with spacing
between two adjacent
planes of 0.411 nm; c XRD
pattern of the nanodiscs
showing that the particles
are of rhombohedral phase;
d AFM image of an
individual nanodisc and the
curve showing that the
thickness is *6.5 nm
J Nanopart Res
123
size relative to those obtained by heating for a short
time (e.g., 12 h). This was probably due to by atomic
reconstruction on particle surfaces to minimize
surface energy. Moreover, the electron diffraction
rings recorded in the SAED patterns become clear,
indicating that the particles crystallized better with
longer heating duration.
In order to clearly describe the time-dependent
growth, the relationship between particle size and
heating time was plotted and shown in Fig. 4.It
could be seen from Fig. 4 that the nucleation is fast
but the particle size increase slowly with time. The
particle size increases up to the maximum *70 nm

around 12 h. After that, more and more well-shaped
Fig. 3 Time dependence of
iron oxide nanoparticles
formed in aqueous solution:
a 1 min; b 5 min; c 1h;
d 6h;e 12 h; f 24 h
J Nanopart Res
123
nanodiscs formed, and the particle size gradually
decreased to *55 nm due to the possible recon-
struction of the surface atoms with extended heating
duration.
In order to understand the formation and growth
mechanism of nanodiscs, a few possibilities have been
proposed previously. A typical example was reported
by Casula et al. (2006) who supposed a delayed
nucleation mechanism for the formation of iron oxide
nanodiscs during the high-temperature decomposition
process, which results in the occurrence of nanopar-
ticle crystallization well separated in time from the
injection of the precursors. They suggested a burst-
like nucleation at a certain delayed time and
subsequent fast nanocrystal line growth at a high iron
monomer concentration that promoted the kinetically
induced formation of anisotropic discs. The retarda-
tion of the nucleation was induced by the surfactant
(e.g., fatty acid) used as a coordinating agent, which
strongly stabilizes the monomer in solution. On the
other hand, Redl et al. (2004) and Hyeon et al. (2001)
reported that when the reaction was carried out under

conditions that favor gradual and slower monomer
release into the solution, thermodynamically sta-
ble iron oxide nanospheres were produced. For
those nanodiscs obtained in aqueous solution,
Niederberger et al. (2002) supposed that the use of
an iron–polyolate complex of [N(CH
3
)
4
]
2
–[OFe
6
(H
-3
thme)
3
(OCH
3
)
3
Cl
6
] Á MeOH as a precursor mate-
rial can produce disclike hematite particles by a
procedure involving the hydrolysis and subsequent
hydrothermal treatment at 150°C over 24 h. They
found that each of the large particles (outer diameter
*1 lm and thickness *250 nm) was made up of
many small plate-like particles. Chen and Gao (2004)

also suggested that the presence of surfactants, such as
poly(oxyethylene)(20)-sorbitan monooleate (Tween
80) and pluronic amphiphilic triblock copolymer
(P123), played an important role in the formation of
crystalline a-Fe
2
O
3
nanodiscs during hydrothermal
treatment at 150°C for a period of 24 h. The
abovementioned methods revealed that the surfactants
played a key role in formation and growth of iron
oxide nanodiscs.
However, these proposed mechanisms seem to be
unsuitable for our case. On the one hand, no delayed
nucleation occurred on the basis of the time-depen-
dent nucleation and growth processes (Figs. 3, 4).
That is, the burst-like nucleation and the subsequent
anisotropic growth do not significantly occur in this
case under the reported conditions. On the other hand,
no surfactants or complex precursors were used in our
synthesis, and thus the surfactant-assisted growth by
selective face adsorption could not be considered.
Therefore, it is believed that the particle morphology
may be determined by other factor(s) in this system.
Let us now consider chloride (Cl
-
) ions first. In the
reaction solution, the Cl
-

ions are excessive due to the
addition of HCl for pH adjusting below 2 (it was
measured that the pH is *2 without addition of acid in
this case), which is believed to cause the Cl
-
ions to
play dual possible roles: to retard hydrolysis of the
ferric ions and to reduce surface energy on a certain
crystal plane to promote preferential growth. Some
investigators have also studied the particle preferential
growth in the systems without surfactants. For exam-
ple, Wang et al. (2008) suggested that the forced
hydrolysis of ferric chloride under acidic pH could
result in the direct transformation from amorphous
iron oxide to crystalline hematite when aged at 100°C
for a period of 48 h, and these hematite nanocrystals
could assemble into large-size disclike particles via
solvent evaporation. Moreover, Matijevic
´
and Scheiner
(1978) and Matijevic
´
(1985) investigated the influ-
ence of inorganic ions such as chloride, nitrate
(NO
3
-
), and perchlorate (ClO
4
-

) on the shape and
size of hematite in the hydrothermal reaction carried
out at 100°C. They found these inorganic ions could
lead to different shapes and sizes of hematite particles.
Raming et al. (2002) reported a similar approach
by using iron chloride salt and being carried out at
01020304050
0
20
40
60
80
Particle diameter (nm)
Reaction time (h)
Fig. 4 The plotted curve showing the time-dependent nucle-
ation and growth of the colloids
J Nanopart Res
123
90–100°C for different times (e.g., 1–6 days) to
prepare hematite colloids; however no disclike parti-
cles were formed. In other systems, the effect of
inorganic ions on particle growth was also studied.
Both Livage et al. (1988) and Reeves and Mann
(1991) groups have demonstrated the influence of
inorganic ions such as chloride, phosphate (PO
4
3-
),
sulphate (SO
4

2-
), and perchlorate on the shape and
size of hematite and other transition metal oxides.
They reported that the presence of Cl
-
ions could
result in rhombohedral hematite crystals comprising
10
"
14 faces that exhibited relatively high energy. The
formation of such high-energy 10
"
14 faces indicates
that the Cl
-
anion has a profound influence on the
stability of these faces because the 10
"
14 face has an
open structure that may be able to accommodate Cl
-
(ionic radius 1.8 A
˚
) and thereby stabilize the bonding
within the surface plane. The selective surface effect
of Cl
-
anion could finally affect the morphology and
size of particles. Similar effects have been observed
for nanocrystals grown in the presence of fluoride

(F
-
), Cl
-
, and bromide (Br
-
) for copper nanorods
(Filankembo et al. 2003; Filankembo and Pileni
2000), hydroxide (OH
-
) for silver nanowires (Caswell
et al. 2003), as well as F
-
and Cl
-
anions for titania
nanosheets (Yang et al. 2008; Penn and Banfield
1999), through selective surface adsorption.
As a further confirmation, the replacement of HCl
by dilute HNO
3
, HClO
4
, and H
2
SO
4
was carried out
to adjust solution pH in this study. Figure 5 shows the
corresponding TEM images of nanoparticles obtained

by addition of various acids as mentioned above.
When NO
3
-
ions were added, the irregular-shaped
particles were obtained (Fig. 5a). The precise size of
particles was difficult to measure. The addition of
SO
4
2-
ions could result in flocculation rapidly
(B3 min) during heating at 90°C. The particles
obtained under such conditions were unshaped, and
the size is difficult to estimate (Fig. 5b). While ClO
4
-
ions were used, cube-like particles formed with edge
lengths of 30–70 nm (Fig. 5c), consistent with the
previous literature. Unfortunately, the addition of
these acids could not produce disclike hematite
particles under the reported conditions. This sug-
gested that the Cl
-
ions indeed played a crucial role
in the formation of hematite nanodiscs under the
conditions considered, which is consistent with the
observations reported by Matijevic
´
and Scheiner
(1978) that the inorganic ions could lead to shape

and size change of hematite particles.
Effects of experimental parameters
In order to better understand, the effects of other
experimental parameters were also investigated
including pH, reaction temperature, and concentra-
tion of Fe
3?
ions, as discussed in the following
context.
pH
The hydrolysis of Fe
3?
ions is closely related to the
pH of the solution. The pH is normally adjusted by
addition of acid or base. Here the effect of pH on
particle growth was further investigated. Figure 6
shows the TEM images of the particles obtained at
different pH values. At pH = 1.5, the particles were
found to be well crystallized with an average
Fig. 5 Effect of the inorganic anions on the shape and size of nanoparticles: a NO
3
-
; b SO
4
2-
; c ClO
4
-
J Nanopart Res
123

diameter of 77 nm (Fig. 6a), confirmed by their size
distribution shown in Fig. 6d. When the pH value
was increased to *2.5 by addition of an appropriate
amount of NaOH solution (5 M), the average size of
nanoparticles decreased a bit to *70 nm in diameter
(Fig. 6, panels b and e). On further increasing pH to
*3.0, the particles continuously reduced in average
size to *63 nm (Fig. 6, panels c and f). However,
when pH was further altered by adding HCl or
NaOH, it was found that a low pH (\1.0) could not
produce any particle but a clear solution, whereas a
high pH ([3.5) could lead to some precipitates (e.g.,
Fe(OH)
3
) prior to any reflux heating. Further inspec-
tion of the particles (Fig. 6, panels a–c) suggested
that the slight shrinkage in size with pH increasing
was probably caused by the fast hydrolysis rate at a
high pH (*3.0). The diffraction rings in the SAED
patterns (inset of Fig. 6, panels a–c) revealed that the
as-prepared nanoparticles are of crystalline structure.
In this system, the H
?
ions may have two
functions: to slow down the hydrolysis rate of Fe
3?
ions and to stabilize the oxygen-terminated crystal
planes such as a-Fe
2
O

3
{0001} (Cotton and Wilkinson
1988). At a low pH (1–3), a particle prefers to grow
along other planes such as a-Fe
2
O
3
{01ı¯1}, beneficial
for the anisotropic growth of particles (Schertmann
and Cornell 1991; De Leeuw and Cooper 2007;
Goldschmidt 1913/1923). After many tests, we found
that the most suitable pH value for disc formation is
1–3. Lower pH (\1.0) could not produce any particle
due to their rapid dissolution, whilst higher pH ([3.5)
could lead to precipitates (e.g., Fe(OH)
3
) directly
under the reported conditions. This is in agreement
with those previously reported that a-Fe
2
O
3
was
Fig. 6 Effect of solution
pH on the shape and size of
iron oxide nanoparticles and
the corresponding size
distributions: a, d
pH = 1.5; b, e pH = 2.5;
c, f pH = 3.0

J Nanopart Res
123
merely obtained at pH \4 (Weiser and Milligan
1935; Mackenzie and Meldau 1959). Due to the
complicated processes involving hydrolysis (Fe
3?
)-
nucleation and (Fe(OH)
3
)-phase transformation (from
b-FeOOH to a-Fe
2
O
3
), it is believed that a further
study needs to be performed to understand the
particle growth.
Reaction temperature
The particle growth is also affected by the reaction
temperature. Figure 7 shows the TEM images of the
prepared nanoparticles at different temperatures (e.g.,
70, 80, and 100°C). Other experimental parameters
were maintained the same as those for the temperature
of 90°C. When heated at 70°C for 48 h, small particles
formed with irregular shape (Fig. 7a). Particle size
was in the range of 5–10 nm, and most of the particles
aggregated together. The weak diffraction rings in the
SAED pattern suggested that these particles were not
well crystallized (inset of Fig. 7a). When heated at
80°C, some bigger particles were formed (diameter of

40–80 nm), along with some smaller ones (Fig. 7b).
The diffraction rings (Fig. 7b) became clear, indicat-
ing that the particles crystallized further with increas-
ing temperature. While at 100°C, the as-produced
particles were spindle or multi-armed nanostructures
with diameters of 20–50 nm and length up to several
hundred nanometers (Fig. 7c). Similar scenarios were
observed as reported by Raming et al. (2002) con-
firming that the same particles were present after
heating at 100°C for 1 day and for 1 week if the ferric
chloride salt was added directly into the preheated
hydrochloric acid solution (method 1). They also
described that a mixture of two particle types (i.e.,
spindle and oval shapes) was produced if the ferric
chloride was not added directly to the preheated
hydrochloric acid solution, which, however, first
dissolved in cold water before heating to 100°C
(method 2). In particular, the XRD analysis from
Raming et al. (2002) showed the presence of two
phases, hematite and akagane
´
ite, if the reaction was
carried out by method 2.
Moreover, such spindle or multi-armed nanostruc-
tures show rough surfaces and no well-defined
crystalline faces, similar to those prepared by addi-
tion of phosphate ions during the hydrolysis and
growth processes. Reeves and Mann (1991) sug-
gested that the interaction of phosphate with hematite
crystals was not specific to a single set of symmetry-

related faces in forming spindle-shaped iron oxide
particles. Our observations are also consistent with
the previous studies that were performed at a high
temperature (e.g., 100°C), although the shape and the
size distribution of particles are slightly different. The
precursor of [Fe(OH)
2
(OH
2
)
5
]
?
does not form a
polycation but nucleates directly into a-Fe
2
O
3
parti-
cles, which, however, may result in various morphol-
ogies (Matijevic
´
and Scheiner 1978).
Concentration of Fe
3?
ions
In order to investigate the effect of concentration of
ferric salt ([Fe
3?
]) on particle shape and growth, the

concentrations tuned from 0.038 to 5.55 mM were
tested. During all the tests, the reaction temperature
and heating time were kept constant. The pH value of
the solution was adjusted carefully and kept around 2.
Fig. 7 Temperature dependence of iron oxide nanoparticles formed in aqueous solution: a 70°C; b 80°C; c 100°C
J Nanopart Res
123
Figure 8 shows the TEM images indicating that the
shape and size of particles are quite different under
different concentrations of ferric ions. When [Fe
3?
]
was fixed to 5.55 mM, short nanorods were formed
with diameter of *70 nm and aspect ratio [2
(Fig. 8a). While [Fe
3?
] was decreased to 4.6 mM,
nanospheres were formed with a diameter of
*100 nm (Fig. 8b). When decreased [Fe
3?
]to
0.46 mM, monodispersed nanocubes or tilted cubes
were formed with an edge length of *45 nm
(Fig. 8c). When further decreased [Fe
3?
] to 0.30
and 0.15 mM, irregular-shaped particles were
obtained (Fig. 8, panels d and e). While at a very
low concentration of [Fe
3?

] (e.g., 0.038 mM), hollow
nanostructures were prepared and shown in Fig. 8f,
but the size was difficult to determine. Further
observation from the magnified TEM image in the
inset of Fig. 8f reveals that the hollow or porous
structures are composed of small particles, which
possibly self-assemble into big spheres with pores
through the electrostatic and/or van der Waals forces.
The formation process is similar to the literature, in
which the porous hematite nanostructures (e.g., rings,
tubes, porous rods) were prepared through template-
free solution-based synthesis, and the investigators
addressed the key role of the ions in the formation of
hollow/porous structures, such as H
2
PO
4
-
,SO
4
2-
,
and Cl
-
(Wen et al. 2005; Wu et al. 2006; Jia et al.
2007; Gou et al. 2008). In this case, the Cl
-
ions
probably play an important role of in the formation of
such nanostructures. However, the nature is still not

clear. Therefore, further study needs to be performed
for understanding the hollow nanostructures.
As could be seen from the TEM images in Fig. 8,
the particle shape was not well defined at low
concentrations of 0.038–0.3 mM. In comparison,
only when the [Fe
3?
] was fixed to *1.85 mM,
nearly monodispersed iron oxide nanodiscs could
Fig. 8 Concentration dependence of iron oxide nanoparticles formed under the reported concentration of FeCl
3
: a 5.55; b 4.60; c
0.46; d 0.30; e 0.152; f 0.038 mM
J Nanopart Res
123
form (Figs. 1 and 2). This is probably because the
different concentrations of ferric chloride would
result in different concentration of Cl
-
ions that play
a key role in the shape control, consistent with the
discussion in previous studies (Matijevic
´
and Scheiner
1978; Raming et al. 2002). Despite different mor-
phologies of the products, their corresponding SAED
patterns (inset of Fig. 8) still reveal that they are of
crystalline structure.
Functional properties of hematite nanodiscs
The properties of nanoparticles are heavily dependent

on the morphology and size, particle surface, and
crystallinity. As characterized above, the hematite
nanodiscs achieved by a surfactant-free hydrothermal
method show a few features including monodisperse
in shape and size, extremely clean surfaces, and well-
crystallized structure. These features are anticipated
to result in unique functionalities. As an example,
two representative functionalities, e.g., magnetic
property and catalytic ability in CO oxidation were
investigated.
Magnetic property
The anisotropic nanoparticles can result in unique
physiochemical properties. The magnetic properties
of the hematite nanodiscs, including hysteresis loops,
coercivity, and blocking temperature (T
B
), were
investigated by using a SQUID magnetometer.
Figure 9a shows the magnetization of a-Fe
2
O
3
nano-
discs versus the applied magnetic field at 5 and 300 K
by cycling the field of ±50 kOe. The data show a
hysteresis loop with coercivity (H
c
), which is indic-
ative of the presence of ferromagnetic components.
The inset of Fig. 9a shows H

c
= 2,016 and 758 Oe at
5 and 300 K, respectively. Note that the remanent
magnetization at 5 K is slightly larger than the value
at 300 K but the coercive force is significantly
different. This may be caused by the energy barrier
for the spin alignment of individual nanodisc when
using the external magnetic field. Moreover, it was
noted that the magnetization at a high field
([10 kOe) is almost linear and almost the same as
at 5 and 300 K, which is similar to the superantif-
erromagnetic behaviors of ‘ferritin’ with a spherical
protein shell of external diameter 13 nm surrounding
an antiferromagnetic iron oxyhydroxide core of
diameter 7 nm, and nickel oxide nanoparticles with
size larger than *10 nm (Kilcoyne and Cywinski
1995; Khadar et al. 2003).
Figure 9b displays the temperature-dependent
field-cooled (FC) and zero-field-cooled (ZFC) mag-
netization curves of the a-Fe
2
O
3
nanodiscs in a field of
10 Oe. At room temperature (*300 K), these nano-
discs exhibit weak ferromagnetic behavior that may
be contributed from both the canting of the sublattice
magnetization directions and the uncompensated
spins as observed for a-Fe
2

O
3
nanodiscs; whereas at
a low temperature, they show superparamagnetic
behavior with the Morin temperature (T
M
)of
*20 K. This is quite different from other hematite
nanoparticles that have a clear transition temperature
(i.e., Morin temperature) between the weak ferro-
magnetic and the uniaxial antiferromagnetic states
(Amin and Arajs 1987). Such a difference is probably
caused by unique morphology and dimension of
hematite nanodiscs (e.g., diameter of 50 ± 10 nm and
Fig. 9 a Magnetization as a function of field for a-Fe
2
O
3
nanodiscs at temperatures of 5 and 300 K; the inset showing
the data around zero field with an expanded scale ranging from
-20 to ?20 kOe; and b low-field susceptibility as a function of
temperature measured after zero-field cooled (ZFC) and filed
cooled (FC) measurements in a 10 Oe field
J Nanopart Res
123
thickness of *6.5 nm). This is consistent with the
previous studies; for example, Kim et al. (2006)
reported that a-Fe
2
O

3
nanowires show a T
M
of 125 K
and unique magnetic anisotropy due to the easy axis of
the magnetocrystalline anisotropy perpendicular to
the nanowire axis. Zhang et al. (2004) suggested that
polycrystalline wires (diameter of *200 nm) were
reported to have T
M
= 80 K and T
N
= 350 K. Jin
et al. (2004) described that the nanorods (diameter of
20–40 nm and length of 100–140 nm) show weak
ferromagnetic behavior at room temperature. Hema-
tite nanoparticles usually exhibit much lower T
B
and
T
M
values, e.g., T
B
= 145 K and T
M
\ 5 K for
d = 3 nm, and T
B
= 150 K but T
M

is not significant
for d = 20 nm (Zysler et al. 2001; Hansen et al.
2000), respectively. This is consistent with that the
Morin temperature reduces as the particle size
decreases, tending to vanish for particles smaller than
8–20 nm (Amin and Arajs 1987). In addition, for
spherical hematite it has been shown that there is a
threshold diameter of *8 nm below which the
particles become superparamagnetic (Fiorani et al.
1985; Zysler et al. 1994). Other values for this
threshold diameter have been reported by Ku
¨
ndig
et al. (1966) as well, e.g., 14 nm. Therefore, the as-
prepared nanodiscs showing superparamagnetic
behavior at T
M
= *20 K may be contributed from
the anisotropic geometry.
Moreover, the large coercivity and non-saturated
magnetization (up to 50 kOe) behaviors are similar to
the superantiferromagnetic nanoparticles (e.g., ferri-
tin) below its blocking temperature (Kilcoyne and
Cywinski 1995). The magnetic properties are heavily
dependent on the uncompensated spin on the surface.
The diameter of our nanodiscs is *50 nm but
thickness is only 6.5 nm. Therefore, a large distribu-
tion of the surface area exists in the bulk sample,
which is probably the main reason on the observa-
tions of weak superparamagnetic property (Fig. 9b).

Furthermore, the surface spinning of the magnetic
moment could be strong compared with the thermal
energy and hence, weak ferromagnetic effect still
exists at high temperatures (Zhang et al. 2004). To
our knowledge, the unique magnetic properties
displayed by the hematite nanodiscs had not been
reported, albeit such discshaped hematite nanoparti-
cles had been studied by other investigators (Casula
et al. 2006; Chen and Gao 2004; Niederberger et al.
2002).
Catalytic property
Characterized with extremely clean surfaces, the
hematite nanodiscs produced have great potential in
catalysis. In order to confirm this, the catalytic
activity of a-Fe
2
O
3
nanodiscs in CO oxidation was eva-
luated and examined under the conditions where CO
concentration is 6,000 ppm and CO/O
2
= 1:10 buf-
fered with N
2
gas, with a total flow rate of 60 ml/min.
Figure 10a (triangle spotted line) shows the temper-
ature dependence of catalytic CO oxidation by
a-Fe
2

O
3
nanodiscs. It was found that the catalytic
CO oxidation mainly took place below 623 K. This
catalytic reaction temperature is also lower than those
reported in the previous studies (Seiyama and Kag-
awa 1966; Geatches et al. 1991), in which the
investigators found that such a pure material detects
CO and other gases (e.g., CO
2
,CH
4
,O
2
, and H
2
)in
the temperature range of 723–1,348 K. In order to
achieve low-temperature iron oxide catalysts
50 100 150 200 250 300 350 400 450
0
20
40
60
80
100
cooling of PVP-
α
-Fe
2

O
3
heating of PVP-
α
-Fe
2
O
3
heating of
α
-Fe
2
O
3
discs
cooling of
α
-Fe
2
O
3
discs
CO Conversion (%)
Temperature (
o
C)
A
0.0 0.2 0.4 0.6 0.8 1.0
0
30

60
90
120
150
α-Fe
2
O
3
discs SA= 22.96 m
2
/g
PVP-α-Fe
2
O
3
SA= 14.91 m
2
/g
Volume (cm
3
/g)
Relative Pressure (P/P
0
)
B
Fig. 10 a Heating–cooling cycles (triangle spotted lines)in
catalytic CO oxidation showing that the a-Fe
2
O
3

nanodiscs are
of high catalytic activity and stable structure, and the circle
spotted lines showing the catalytic properties of hematite
elliptic particles; b The measured curves illustrating that the
hematite nanodiscs have a BET surface area of *23 m
2
/g, and
*15 m
2
/g for elliptic nanoparticles
J Nanopart Res
123
(\723 K), metal- and/or oxide-doped a-Fe
2
O
3
(e.g.,
Pd, Pt, RuO
2
, or SnO
2
) were usually prepared
(Ryabtsev et al. 1999; Tan et al. 2000). In this case,
the low-temperature (\623 K) catalytic ability
observed for a-Fe
2
O
3
nanodiscs could be attributed
to a few possibilities. First, the high surface-to-

volume ratio (diameter of *50 nm and thickness of
*6.5 nm) will benefit in the surface adsorption of O
2
and other gases (e.g., CO). Second, the a-Fe
2
O
3
could
be partially or fully reduced to Fe
3
O
4
by the reducing
gas of CO in heating treatment. The newly formed
spinel Fe
3
O
4
shows high conductivity at room
temperature, which facilitates the low-temperature
catalytic reaction (Henrich and Cox 1994; Xu et al.
1990; Han et al. 1999; Kalinkin et al. 1999). Third,
the defective or rough surfaces (inset of Fig. 2a)
could also improve the interaction with adsorbed gas
molecules and hence enhance the performance in
catalysis. In addition, the nearly overlapped heating–
cooling cycles (Fig. 10a, triangle spotted lines)
suggested that the a-Fe
2
O

3
nanodiscs are stable in
structure during the catalytic test.
In order to verify the availability and applicability
of the proposed synthesis approach, we prepared
PVP-capped iron oxide particles for catalytic prop-
erty examination. This approach was carried out by
slightly modifying the procedures for preparing
nanodiscs. The molar ratio of PVP to Fe
3?
ions was
kept at 2:1, and other experimental parameters were
kept constant. Figure 11a shows the TEM image that
the nanoparticles are ellipsoids with a length of
*100 nm and a width of *70 nm. The composition
of such particles was further checked by XRD
technique. Figure 11b shows the XRD pattern (b)
that the composition can be identified as hematite,
which is consistent with the pattern (a) of the
hematite nanodiscs. All the diffraction peaks could
be indexed to rhombohedral phase. The PVP-capped
nanoparticles show a slightly high temperature
(*673 K) in catalytic CO oxidation (circles spotted
curves), compared to those of as-prepared nanodiscs
(\623 K) (Fig. 10a, triangles spotted curves). Fur-
thermore, the separation of the heating and the
cooling cycles was probably caused by the removal of
all or partial PVP molecules from the particle
surfaces or the residues from PVP thermal decom-
position during heating (Millan et al. 2002). The

effect of the residues needs further investigations.
Nevertheless, the above analysis suggests that the
a-Fe
2
O
3
nanodiscs with extremely clean surfaces
might result in lower temperature (\623 K) catalytic
activity than those of ellipsoids or spheres capped by
PVP.
A further confirmation involves measuring the
surface areas of those nanoparticles by BET tech-
nique, as shown in Fig. 10b. The measured curves for
calculating BET surface area of particles were plotted
and analyzed. It can be seen that the surface area of a-
Fe
2
O
3
nanodiscs (triangle spotted lines) was esti-
mated to be * 23 m
2
/g, but *15 m
2
/g for those
ellipsoids capped by PVP (circle spotted lines), which
is slightly higher than those for hematite colloids
(*10 m
2
/g) obtained by sol–gel method using ferric

acetylacetonate as a precursor (Wang and Willey
1998, 1999). Moreover, no porous structure was
identified for our cases. The high surface area of
nanodiscs than that of PVP-capped particles is also a
20 30 40 50 60 70 80
(a) α-Fe
2
O
3
discs
(b) PVP-
α-Fe
2
O
3
(306)
(217)
(1010)
(208)
(300)
(214)
(018)
(116)
(024)
(113)
(110)
(104)
(012)
b
a

Intensity (a.u.)
2 θ (degree)
A
B
Fig. 11 a The TEM image of PVP-capped hematite ellipsoids;
b The XRD pattern of the PVP-capped ellipsoids showing the
similar composition as that of hematite nanodiscs
J Nanopart Res
123
possible reason why the a-Fe
2
O
3
nanodiscs have low
temperature (\623 K) catalytic ability in CO oxida-
tion. Here, the PVP molecules may occupy some
active sites on the particles surface to block or reduce
the adsorption of CO and/or O
2
molecules and then
reduce the catalytic ability. On the other hand, the
residues from PVP decomposition in the heating
process might also affect the catalytic property of
nanoparticles (Millan et al. 2002; Kim et al. 2007).
Although these nanodiscs show relatively high sur-
face area, there is always room for improvement,
which will be directly related to the catalytic activity
enhancement. Nonetheless, the features such as high
surface area, extremely clean surface, and well-
crystallized structure will make hematite nanodiscs

them be a promising low-temperature (\623 K)
catalyst.
Conclusions
We have developed a facile hydrothermal method to
synthesize monodispersed hematite nanodiscs in the
absence of surfactants under mild conditions, and no
need of toxic precursors and high-temperature
decomposition in organic solvents. The nanodiscs
(diameters of *50 nm and thickness of *6.5 nm)
were characterized with well-crystallized structure
and extremely clean surfaces because of non-involve-
ment of surfactants. The optimized experimental
parameters for synthesis of such nanodiscs are
summarized as follows: ferric chloride salt as a
precursor with concentration of *1.85 mM, HCl
used for adjusting solution with pH of 1–3, reaction
time being over 24 h by heating at *90°C. These
nanodiscs show unique magnetic properties, namely,
superparamagnetic property at a low temperature
(e.g., 20 K) but ferromagnetic property at room
temperature (*300 K). The availability and applica-
bility of the proposed synthesis method were also
confirmed by the nanodiscs showing low temperature
(\623 K) catalytic activity in CO oxidation com-
pared with the PVP-capped hematite ellipsoids and
spheres reported in previous studies. Discussion of
the underlying principle by considering the selective
surface adsorption of chloride ions would be helpful
for further understanding iron oxide nanodiscs and
other anisotropic structures. These findings would be

useful for shape-controlled synthesis of metal oxide
nanoparticles with desirable functional properties.
Acknowledgments We gratefully acknowledge the financial
support from the Australia Research Council (ARC) through
the ARC Centre of Excellence for Functional Nanomaterials,
Natural Science Foundation of China (NSF50671019), and
China Postdoctoral Science Foundation (No. 2005038252).
X. J. gratefully thanks Miss K.Y. Koh at UNSW for her help in
measurement of BET surface area, and Mr. S. Gnanarajan for
his technical assistance in using the SQUID Magnetometer in
CSIRO (Australia).
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