International Conference on Advanced Materials and Nanotechnologies (ICAMN)
Hanoi, 2012
Effect of Synthesis Pathways on Morphology and Gas-sensing Properties of
Nanoporous Hematite Nanoparticles
Nguyen Duc Hoa1, Nguyen Duc Cuong1,2, Nguyen Van Hieu1*
1
International Institute for Materials Science, Hanoi University of Science and Technology, Hanoi, Vietnam
2
Faculty of Hospitality and Tourism, Hue University, 22 Lam Hoang, Vy Da Ward, Hue City, Vietnam
*Corresponding author: hieu@itims.edu.vn
Abstract—The development of a low-cost and scalable gas sensor for the detection of toxic and flammable gases with fast response and
high sensitivity is extremely important for monitoring environmental pollution. In this work, we introduce two different synthesis
pathways for the preparation of scalable Fe2O3 nanoparticles for gas sensor applications. One is co-precipitation and the other is
hydrothermal method. The gas sensing properties of the Fe2O3 nanoparticles (NPs) fabricated by different synthesis pathways were
studied and compared. The performance of the NPs in the detection of toxic and flammable gases such as carbon dioxide, ammonia,
liquefied petroleum gas, ethanol, and hydrogen was evaluated. The Fe 2O3 NP-based gas sensors exhibited high sensitivity and a
response time of less than a minute to analytic gases. However, the NPs fabricated by the one-step direct method exhibited higher
sensitivities than those generated by the Fe2O3 NPs obtained by co-precipitation synthesis possibly because of their nanoporous
structure.
Keywords: Nanoporous, Nanoparticles, Hydrothermal, Gas Sensor
INTRODUCTION
Air pollution caused by toxic and flammable
gases, such as CO, NH3, NO2, and H2, is one of the
critical problems contributory to global warming,
climate change, and damage to human health [1].
Therefore, the development of gas sensors for the
detection of such gases has received considerable
interest in recent years [2]. Essentially, resistive gas
sensors based on different forms of metal oxides,
organic compounds, and polymers have been
developed for monitoring air quality; the studies
focused mainly on the enhancement of gas sensor
performance, as well as the realization of fast
response and recovery time, low cost, low power
consumption, and high sensitivity [3].
I.
The working mechanism of a resistive metal
oxide-based gas sensor relies on variations in the
electrical conductivity of a sensing layer; these
variations are caused by surface reactions, such as
oxidation or reduction, upon exposure to different
gases. Because these surface reactions depend on the
active centers and defects existing on the surface
layer of materials, sensor response is usually
determined by the grain size and surface-to-volume
ratio of materials [4]. Therefore, the nanostructures
of metal oxides have been investigated for gas
sensor applications [5], in which both n- and p-type
semiconductors such as Fe2O3 [6], WO3 [7], SnO2
[8], and CuO [9] exhibit a significant resistance
change upon exposure to trace concentrations of
reducing or oxidizing gases. Of these materials,
different iron oxides (Fe2O3) phases [i.e., -Fe2O3
(hematite), -Fe2O3,-Fe2O3 (maghemite), and ϵFe2O3] have also been studied for different
applications [10-11]. Hematite (-Fe2O3) is one of
the most stable phases with n-type semiconducting
properties (Eg: 2.1 eV); it has been widely used as
a catalyst, pigment, gas sensor, and electrode
material owing to its low cost, high resistance to
corrosion, and environment-friendly properties [12–
13]. Given the excellent physical and chemical
properties of -Fe2O3, considerable attention has
been
directed
to
its
controlled-synthesis
nanostructures, such as nanopropellers [14],
nanorods, nanoporous, nanoleaflets [15], and
nanospheres [16], as well as nanocubic [17] and
hexagonal platelets [18]. Furthermore, because of its
affordability, good stability, and reversibility, Fe2O3 has been proven an important semiconducting
material for gas sensor applications. Gas sensors
based on -Fe2O3 nanostructures have been
explored by many researchers. However, the
conventional materials have low specific surface
areas and low sensitivity to target gases, creating the
need for developing -Fe2O3 nanostructures with
high surface areas.
Herein, we report the controlled synthesis of
scalable nanoporous -Fe2O3 NPs for gas sensor
International Conference on Advanced Materials and Nanotechnologies (ICAMN)
applications. The nanoporous -Fe2O3 NPs were
synthesized by an one-step facile and scalable
hydrothermal method. Their gas sensing properties
were investigated and compared with those of
condensed -Fe2O3 NPs fabricated by post
synthesis, in which the fabrication included the coprecipitation of Fe3O4 and calcinations to generate
-Fe2O3 NPs. The effects of synthesis pathways on
the morphologies and gas sensing properties of the
materials were investigated for the detection of CO,
NH3, and H2, among others. Our findings reveal
that the nanoporous NPs exhibited higher sensitivity
compared with that of the condensed counterpart.
The developed sensors could detect low
concentrations of analytic gases at the ppm level.
II.
EXPERIMENTAL
Co-precipitation synthesis of Fe2O3
nanoparticles
The Fe2O3 NPs were synthesized by a coprecipitation method and subsequent calcinations at
high temperature. First, the Fe3O4 NPs were
fabricated by a co-precipitation method using ferric
chlorides (Fe3+) and ferrous chlorides (Fe2+) as
precursors [19]. In brief, ferric chlorides (2 mmol)
and ferrous chlorides (1 mmol) (molar ratio, 2:1)
were dissolved in 100 ml HCl (pH=2). The chemical
precipitation was achieved by slowly adding a 0.1 M
solution of NaOH with vigorous stirring for 30 min
at 80 °C. The precipitated Fe3O4 products were
recovered by filtering, washing, and drying at 60 °C.
The obtained Fe3O4 NPs were loaded in an alumina
boat and inserted into a tube furnace for calcinations
at 600 °C for about 5 h.
A.
B.
Hydrothermal synthesis of nanoporous Fe2O3
nanoparticles
Nanoporous Fe2O3NPs were synthesized by
a facile and scalable hydrothermal method. In a
typical experiment, ferric nitrate (2 mmol),
cetyltrimethylammonium bromide (1 g) and urea
(15 mmol) were dissolved in 35 ml of distilled
water by magnetic stirring at room temperature for
2 h to obtain a slurry solution. The slurry solution
was then transferred into a 200 ml Teflon-lined
autoclave for hydrothermal processing. The
hydrothermal processing was carried out at 80 °C
Hanoi, 2012
for 36 h and then naturally cooled to room
temperature. The solid products were collected and
washed with distilled water and ethanol several
times by centrifugation to ensure the total removal
of the un-reacted inorganic ions and surfactant. The
collected powders were dried at 60 °C in air.
The microstructures and morphologies of
the
as-synthesized
Fe3O4
and
-Fe2O3
nanostructures were characterized by X-ray
diffraction (XRD, D8 Advance, Brucker, Germany),
scanning electron microscopy (SEM), and
transmission electron microscopy (TEM). For SEM
characterization, the powders of the synthesized
materials were attached onto an SEM holder by a
carbon tape, and then 10 nm Pt was coated onto it to
prevent electrostatic charge generation. The SEM
images were recorded at an acceleration voltage of
15 kV. The TEM samples were prepared by
dispersing the synthesized materials in ethanol
solution using an ultrasonic cleaner. Thereafter, the
solution was dropped onto a carbon-coated Cu grid
for TEM characterization
Gas sensor fabrication and characterization
The gas sensors were fabricated by a thick film
technique, in which the synthesized NPs were
dispersed in ethanol solution, and then drop-casted
onto an interdigitated electrode substrate. Thereafter,
the sensors were heat treated at high temperature to
increase the adhesion between the sensing materials
and substrates. The gas sensing properties of the
sensors were investigated for the detection of
different gases such as H2 (25–500 ppm), CO (10–
100 ppm), C2H5OH (50–500 ppm), and NH3 (50–
5000 ppm) at different temperatures (300–400 °C)
using a homemade setup system with a high speed
switching gas flow (from/to air to/from balance gas).
Balance gases (0.1% in air) were purchased from
Air Liquid Group (Singapore). The system employs
a flow-through with a constant rate of 200 sccm, as
reported in ref. [20]. During sensing measurement,
the resistance of the nanosensors was automatically
recorded through Keithley controlled by a computer
via a software program. The sensor response was
defined as S=R0/R, where R0 and R was the
resistance of sensor measured in air and in analytic
gas, respectively.
C.
International Conference on Advanced Materials and Nanotechnologies (ICAMN)
III.
RESULTS AND DISCUSSION
Material characteristics
The phase formation, and crystal structure of the
as-synthesized and calcinated NPs fabricated by the
co-precipitation synthesis pathway and investigated
by XRD are shown in Fig. 1(A, B). The XRD
pattern of the as-synthesized NPs exhibit a facecentered cubic profile typical of the Fe3O4 crystal
structure (JCPDS No. 65-3107). The main peaks
can be indexed to the (220), (311), (400), (422),
(422), (511), and (440) reflections (Fig. (A)).
However, after calcination at 600 °C for 5 h (Fig.
1(B)), the cubic Fe3O4 was converted into -Fe2O3
phase. The main peaks of -Fe2O3 are indexed to a
rhombohedral profile characteristics of the -Fe2O3
crystal structure (JCPDS No. 81–2810). No
detectable peak of impurities and other phases was
observed, indicating the formation of single-phase
-Fe2O3. The average crystalline sizes of the Fe3O4
and -Fe2O3 NPs calculated from the XRD data
using the Scherrer equation (d = 0.9λ/(β cos θ)) are
about 15 and 20 nm, respectively [17].
A.
Hanoi, 2012
confirms that the -Fe2O3 NPs have a condensed
structure without any observation of nanopores
inside the NPs. The particle size estimated by TEM
image was about 150 nm, which is larger than the
value calculated from the XRD data. This result
indicates that the NPs are not single crystals but are
polycrystalline in nature.
Figure 2.
(A) SEM and (B) TEM images of the Fe2O3
NPs fabricated by two-step post-synthesis
Figure 1.
XRD patterns of the as-synthesized Fe3O4
(A) and -Fe2O3 (B) NPs fabricated by two-step postsynthesis.
The morphologies of the -Fe2O3 NPs fabricated by
co-precipitation post-synthesis were characterized
by SEM and TEM (Fig. 2). The -Fe2O3 NPs are
irregularly shaped and they are aggregated because
of the grain growth that occurred at a high
calcination temperature. The TEM image also
In contrast to the NPs fabricated by the coprecipitation synthesis pathway, the as-synthesized
and calcinated NPs prepared by the one-step direct
hydrothermal method have a rhombohedral -Fe2O3
structure, as confirmed by the XRD patterns (Fig.
3). No impurity peak in the calcinated NPs were
observed, indicating that calcination does not
change the crystal structure of -Fe2O3.
International Conference on Advanced Materials and Nanotechnologies (ICAMN)
Hanoi, 2012
The gas sensing properties of the
nanoporous
Fe2O3
NPs
synthesized
by
hydrothermal synthesis pathway are shown in Fig.
5(B). The nanoporous Fe2O3 NP-based sensor
exhibited sensing properties similar to those of the
NPs fabricated by co-precipitation synthesis (the
condensed one). The sensor response (Ro/R) to 10
ppm CO was approximately 5, which is 3.8-fold
higher than the response of the condensed Fe2O3
NPs. The sensitivities of the nanoporous Fe2O3 NPs
fabricated by direct synthesis were much higher
than those of the NPs prepared by the coprecipitation method.
Figure 3.
XRD patterns of the Fe2O3 NPs fabricated
by the one-step direct hydrothermal pathway: (A) assynthesized; (B) calcinated NPs)
The Fe2O3 NPs fabricated by hydrothermal
synthesis have very homogenous morphologies with
nearly spherical shapes, as shown in Fig. 4. The
synthesized NPs are well separated but not as
aggregated as the products obtained by coprecipitation synthesis. The Fe2O3 NPs fabricated
by hydrothermal synthesis have an average particle
size of about 100 nm [Fig. 4(A)]. The TEM image
indicates that the Fe2O3 NPs have nanopores of less
than about 10 nm distributed randomly inside the
NPs [Fig. 4(B)].
Gas sensing properties
The CO gas sensing properties (measured at
different temperatures) of the sensor based on the
Fe2O3 NPs synthesized by co-precipitation synthesis
are shown in Fig. 5 (A). The sensor could detect CO
gas at low ppm concentrations at all measured
temperatures. However, the sensor exhibited the
highest sensitivity at 350 °C, in which the sensor
resistance decreased rapidly upon exposure to CO
gas and reached saturation within a minute. After
switching from CO to air, the sensor resistance
recovered to the initial resistance, indicating the
reversible interaction between CO molecules and
the surface of the Fe2O3 sensing layer. The sensor
response (Ro/R) increased from 1.3 to 2.45 when
CO concentration increased from 10 to 100 ppm.
B.
Figure 4.
(A) SEM and (B) TEM images of the
nanoporous Fe2O3 NPs fabricated by the one-step direct
hydrothermal synthesis pathway
International Conference on Advanced Materials and Nanotechnologies (ICAMN)
Hanoi, 2012
sensing sites for gas adsorption and resulted in
higher sensitivity.
Figure 5.
CO sensing properties of the Fe2O3 NPs
fabricated by two different synthesis pathways measured at
different temperatures: (A) two-step post-synthesis; (B) onestep direct hydrothermal method.
The H2 sensing properties of synthesized
nanosensors are shown in Fig. 6. Both nanosensors
exhibited a similar trend of response to that with
CO gas, where the sensor resistance decreased
rapidly upon exposure to H2 gas. The nanosensors
could detect H2 gas at a very low concentration of
ppm level, which is much lower than the “lower
explosive limit (~4%)”, suggesting a possibility of
using for practical application in monitoring of
hydrogen leak in air. The sensor response of the
porous Fe2O3 NPs was higher than that of the
condensed once, whereas the sensitivity to 500 ppm
H2 concentration measured at 300 oC was about 27
and 4.5 for the porous and condensed nanosensors,
respectively. The higher sensitivity of the porous
Fe2O3 nanosensor was possibly due to the porous
structure of Fe2O3 NPs which provided larger
Figure 6.
H2 sensing properties of the Fe2O3 NPs
fabricated by two different synthesis pathways measured at
different temperatures: (A) two-step post-synthesis; (B) onestep direct hydrothermal method.
The sensing mechanism of the Fe2O3 NP-based
gas sensors can be explained by the depletion
region. During the gas-sensing measurement, the
oxygen in the air captured the electrons from the


2
Fe2O3 crystal and ion-adsorbed ( O2 , O and O )
on the surface of the sensing layer; this
phenomenon resulted in the formation of the
electron-depletion region [21]. Upon exposure to
reducing gases, such as CO, the CO molecules
interacted with the pre-adsorbed oxygen and
International Conference on Advanced Materials and Nanotechnologies (ICAMN)
released
electrons,
according
to
the


equation CO  O ads   CO2  e . The release of
free electrons increased the total carrier and reduced
the electron-depletion region, resulting in a decrease
in sensor resistance. After the analytic gas flow was
discontinued, the adsorption of oxygen molecules
onto the surface of the sensing layer returned the
sensor resistance to the initial value.
[6]
CONCLUSION
For the practical application of gas sensors in the
detection of toxic and/or flammable gases, the
development of suitable sensing devices with low
cost, fast response, and high sensitivity is very
important. In this work, we introduced two different
synthesis pathways for the scalable synthesis of αFe2O3 NPs for effective gas sensor applications. The
effects of synthesis pathways on the morphologies
and gas sensing properties of NPs were investigated
and discussed. Our experiments demonstrated that
the nanoporousα-Fe2O3 NPs fabricated by the onestep direct hydrothermal method are highly
promising materials for the detection of toxic and
flammable gases, despite the selectivity of sensors
needed to be improved.
[9]
IV.
ACKNOWLEDGMENT
The current work was financially supported by
the Application-oriented Basic Research Program
(2009–2012, Code: 05/09/HÐ-DTÐL).
[7]
[8]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
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