Facile Synthesis of Porous r-Fe
2
O
3
Nanorods and Their Application in Ethanol Sensors
Yan Wang, Jianliang Cao, Shurong Wang, Xianzhi Guo, Jun Zhang, Huijuan Xia,
Shoumin Zhang, and Shihua Wu*
College of Chemistry, Nankai UniVersity, Tianjin 300071, P.R. China
ReceiVed: June 2, 2008; ReVised Manuscript ReceiVed: September 12, 2008
A facile solution approach was employed to synthesize R-FeOOH nanorods by using FeSO
4
· 7H
2
O and
CH
3
COONa without templates at low temperature (40 °C). The porous R-Fe
2
O
3
nanorods were successfully
obtained by calcining the R-FeOOH precursors at 300 °C for 2 h. The as-prepared products were characterized
by thermogravimetry-differential thermal analysis, X-ray powder diffraction, transmission electron microscopy
(TEM), high-resolution TEM, and N
2
adsorption-desorption analysis techniques. The as-prepared porous
R-Fe
2
O
3
nanorods have a tiny crystal size (5 nm) and a pore size distribution of 1-10 nm, resulting in a high

those bulk materials.
1-4
Currently, one-dimensional (1-D)
nanostructures, such as nanorods, nanowires, nanobelts, and
nanotubes, have become the focus of intensive research not only
for their peculiar properties but also for many potential
applications in catalysis, electronics, photonics, drug delivery,
medical diagnostics, sensors, and magnetic materials.
5-8
Hematite (R-Fe
2
O
3
) is the most stable iron oxide with n-type
semiconducting properties (E
g
) 2.2 eV) under ambient
conditions. It has been intensively investigated because of its
wide applications in catalysts, pigments, magnetic materials, gas
sensors, and lithium ion batteries.
9-15
For its excellent properties,
much attention has been directed to the controlled synthesis of
one-dimensional (1-D) R-Fe
2
O
3
, such as nanospindles,
16,17
nanofibers,

through a hydrothermal process at 120 and 100 °C, respec-
tively.
28,29
The preparation of R-Fe
2
O
3
nanotubes with alumina
membranes as the substrates was also employed by many
researchers.
30-33
However, the gas-solid reaction usually
requires special equipment and high temperatures, the methods
employing templates or substrates often suffer from disadvan-
tages related to the high cost and the removal of impurities,
and the hydrothermal process usually needs tedious reaction
times. It is still a challenge to develop simple, low-cost, and
environmentally friendly approaches for the synthesis of 1-D
structural R-Fe
2
O
3
.
Recently, the concern over environmental protection and
increasing demands to monitor hazardous gases in industry and
the home has attracted considerable attention to developing gas
sensors for various polluting and toxic gases. Due to its low
cost, good stability, and reversibility, R-Fe
2
O

Herein, we report a facile route for the preparation of porous
R-Fe
2
O
3
nanorods without any templates via a low-temperature
(40 °C) solution approach. First, the precursor of R-FeOOH
nanorods was prepared by using FeSO
4
· 7H
2
O as the iron source
material in the presence of CH
3
COONa in an aqueous solution.
The CH
3
COONa was used as a source of hydroxide ions during
the hydrolysis of iron salts to form iron oxyhydroxide (FeOOH).
Then the porous R-Fe
2
O
3
nanorods were obtained by the
calcination of as-prepared R-FeOOH at 300 °Cfor2h.The
as-obtained porous R-Fe
2
O
3
nanorods have a tiny crystal size

COONa were
dissolved in 50 mL of deionized water under magnetic stirring.
After stirring vigorously for a period at 40 °C, a yellow slurry
was formed. The products were collected and washed with
* Corresponding author. Phone: +86 22 2350 5896. Fax: +86 22 2350
2458. E-mail: [email protected].
J. Phys. Chem. C 2008, 112, 17804–1780817804
10.1021/jp806430f CCC: $40.75  2008 American Chemical Society
Published on Web 10/23/2008
distilled water several times by vacuum extraction filtering with
two sheets of medium speed qualitative filter paper (pore
diameter 30-50 µm) and then dried at 40 °C under vacuum
for 2 h. The porous R-Fe
2
O
3
nanorods were obtained by
calcining the as-prepared R-FeOOH nanorods precursor at 300
°Cfor2hinair. The color of the samples changed from yellow
to red. The whole preparation process for the porous R-Fe
2
O
3
nanorods can be finished in no more than 6 h. The short
production process would be helpful for the large-scale industrial
manufacture of porous R-Fe
2
O
3
nanorods.

34,35
The porous R-Fe
2
O
3
nanorod samples were
mixed with terpineol to form a paste and then coated onto the
outside surface of an alumina tube 4 m m in length. The thickness
of the coated sensing layer is around 50 µm. A small Ni-Cr
alloy coil was placed through the tube to supply the operating
temperatures from 100 to 500 °C. Electrical contacts were made
with two platinum wires attached to each gold electrode. To
improve their stability and repeatability, the gas sensors were
sintered at 300 °C for 10 days in air. Here, the sensing properties
of the gas sensors were measured under a steady-state condition
in a chamber with a volume of 15 L at a working temperature
of 250 °C and 40% relative humidity (RH). An appropriate
amount of ethanol vapor was injected into the closed chamber
by a microinjector, and the sensor was exposed to air again by
opening the chamber when the test was completed.
3. Results and Discussion
TG-DTA measurement was performed to study the conver-
sion process of the as-prepared R-FeOOH during calcination
in air, and the result is shown in Figure 1. From the TG curve
of Figure 1, it can be seen that the total weight loss is about
12%, which is a little larger than the theoretical value (10.1%),
indicating that about 2% adsorbed water is present in the as-
prepared R-FeOOH. The abrupt weight loss (about 10.5%) that
occurred at the temperature range of 250-300 °C is attributed
to the decomposition of R-FeOOH precursors. Correspondingly,

O
3
(JCPDS No. 33-0664). No characteristic peaks
are observed for impurities such as γ-Fe
2
O
3
and Fe
3
O
4
,
indicating that the R-FeOOH precursor was completely trans-
formed into hematite at 300 °C, which is also consistent with
the results of TG-DTA.
Figure 1. TG-DTA curves of as-prepared FeOOH nanorods.
Figure 2. XRD patterns of (a) R-FeOOH and (b) R-Fe
2
O
3
nanorods.
Porous R-Fe
2
O
3
Nanorods J. Phys. Chem. C, Vol. 112, No. 46, 2008 17805
The morphologies of the as-prepared R-FeOOH and R-Fe
2
O
3

regular lattice fringes with a spacing of 0.37 nm, which
corresponds to the (012) plane of R-Fe
2
O
3
.
To investigate the formation processes of the R-FeOOH
nanorods and the porous R-Fe
2
O
3
nanorods, time-dependent
experiments were carried out, and the resultant products were
investigated by TEM (see Figure S1 in the Supporting Informa-
tion). At a shorter reaction time of only 5 min, there are almost
no nanorods formed, and the average diameter of the nanopar-
ticles is about 5 nm. As the reaction time increased to 10 min,
part of the nanoparticles began to combine with each other, and
the rodlike structure appeared. Upon prolonging the reaction
time to 30 min, the products were totally transformed to rodlike
nanostructures. If the reaction time was further increased to 2 h,
as seen in Figure 3, well-structured nanorods were obtained,
and the length of the nanorods increased with the increase in
the reaction time. On the basis of the above results, a growth
mechanism of the porous R-Fe
2
O
3
nanorods can be proposed.
The schematic diagram of the process is described in Figure 4.

3
COOH + OH
-
(1)
4Fe
2+
+8OH
-
+O
2
f 4FeOOH + 2H
2
O (2)
2FeOOH f Fe
2
O
3
+H
2
O (3)
The porosity of the porous R-Fe
2
O
3
nanorods was further
confirmed by nitrogen adsorption-desorption analysis, and the
results are shown in Figure 5. The isotherm indicates that the
R-Fe
2
O

18.31 m
2
· g
-1
, which is reported in our previous work.
37
The
high surface area of the porous R-Fe
2
O
3
nanorods may be
attributed to their tiny crystal size and porosity structure.
Prompted by the high specific surface area, we forecast that
the sensor based on the as-prepared porous R-Fe
2
O
3
nanorods
should have enhanced gas sensitivity.
As an n-type semiconductor, one of the most important
applications of R-Fe
2
O
3
material is in gas sensors. It has been
reported by many researchers that an R-Fe
2
O
3

3
nanorod.
Figure 4. Schematic diagram of the formation mechanism of the
porous R-Fe
2
O
3
nanorods.
Figure 5. N
2
adsorption-desorption isotherm and BJH pore-size
distribution plot (inset) of porous R-Fe
2
O
3
nanorods.
17806 J. Phys. Chem. C, Vol. 112, No. 46, 2008 Wang et al.
the crystallites in the sensing layer can result in a considerable
increase in sensitivity.
43
Thus, the as-prepared porous R-Fe
2
O
3
nanorods, which possess a tiny particle size (5 nm) and a high
surface area (221.9 m
2
· g
-1
), are expected to have a good gas-

3
nanoparticles under
the same ethanol concentration. This result indicates that the
gas-sensing property of the as-prepared porous R-Fe
2
O
3
nano
-
rods is much better than that of the previously reported R-Fe
2
O
3
nanoparticles. A comparison study between the nanorods and
the nanoparticles in sensitivities to ethanol of different concen-
trations is shown in Table 1. From Table 1, we can see that the
sensitivities of the porous R-Fe
2
O
3
nanorods are almost several
decade times greater than that of R-Fe
2
O
3
nanoparticles for all
the ethanol vapor of different concentrations. The gas sensitivity
is defined as the resistance ratio R
air
/R

the surface of materials is strongly dependent on the micro-
structure of the materials; namely, the specific area, particle size,
and the porosity. The main reason for the above result is that
the conventional R-Fe
2
O
3
nanoparticle sensor has a poor surface
area and a relatively large particle size, whereas the sensor based
on porous R-Fe
2
O
3
nanorods has a high surface area and tiny
crystal size, which can provide more adsorption-desorption sites
for gas molecules. Moreover, the abundant pores on the surface
of the R-Fe
2
O
3
nanorods can facilitate the diffusion of the gas
molecules and enable them to access all surfaces of the nano-
particles contained in the sensing unit.
As is known, response and recovery times, which are defined
as the time required to reach 90% of the final resistance, are
the basic parameters for gas sensors. It can also be seen from
Figure 6 that the porous R-Fe
2
O
3

has also been investigated. The sensitivity of the sensor to
ethanol at different relative humidities is shown in Figure S2
in the Supporting Information. The result reveals that it is no
problem for the sensor of porous R-Fe
2
O
3
nanorods to detect
ethanol under 60% relative humidity. Furthermore, the sensor
exhibited a nearly constant response to ethanol under the same
conditions, even after 6 months, illustrating the good reversibility
of the porous R-Fe
2
O
3
nanorod sensor.
4. Conclusions
In summary, we have presented a facile route for preparing
porous R-Fe
2
O
3
nanorods via a template-free solution approach
at low temperature (40 °C). This method is feasible for large-
scale industrial manufacture of porous hematite nanorods due
to the advantages of the simple production process, low cost,
and environmental friendliness. The as-prepared porous R-Fe
2
O
3

/g)
50 100 200 500 1000
porous R-Fe
2
O
3
nanorods
221.9 43.6 60.7 82.8 127.3 174.9
R-Fe
2
O
3
nanoparticles
18.31 1.9 2.2 2.9 4.8 11.8
Figure 7. Sensitivities of porous R-Fe
2
O
3
nanorods to various gases
of 50-1000 ppm.
Porous R-Fe
2
O
3
Nanorods J. Phys. Chem. C, Vol. 112, No. 46, 2008 17807
porous R-Fe
2
O
3
nanorods exhibited a much higher sensitivity

O
3
nanorods to ethanol at different relative
humidity. This material is available free of charge via the
Internet at http://pubs.acs.org.
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