A highly selective and sensitive H2S sensor at low temperatures based on Cr-doped α-Fe2O3 nanoparticles

Article abstract of DOI:10.1039/c8ra07365a

Cr-doped α-Fe₂O₃ nanoparticles were synthesized by a low-cost and environmentally friendly hydrothermal route. Their gas sensing properties were investigated and the sensor showed high sensitivity and selectivity to H₂S gas. Different Cr doping levels from 0 to 8.0 wt% were studied, and the sensor of 4.0 wt% Cr-doped α-Fe₂O₃ showed the largest response, with a response of 213 to 50 ppm H₂S at 100 °C. The incorporation of Cr ions within α-Fe₂O₃ nanocrystals increases the specific surface area, and promotes the oxidation of H₂S and oxygen adsorption in the air. Thus, the doping of Cr into α-Fe₂O₃ nanostructures would be a promising method for designing and fabricating high performance H₂S gas sensors.

Full text of DOI:10.1039/c8ra07365a

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A highly selective and sensitive H2S sensor at low
temperatures based on Cr-doped a-Fe2O3
nanoparticles
Cite this: RSC Adv., 2019, 9, 4150
*
Qiuhong Li
Dongyang Xue,
Rui Zhou, Xiaoping Lin, Xiaochuan Duan,
and Taihong Wang
Cr-doped a-Fe2O3 nanoparticles were synthesized by
a low-cost and environmentally friendly
hydrothermal route. Their gas sensing properties were investigated and the sensor showed high
sensitivity and selectivity to H2S gas. Different Cr doping levels from 0 to 8.0 wt% were studied, and the
sensor of 4.0 wt% Cr-doped a-Fe2O3 showed the largest response, with a response of 213 to 50 ppm
H2S at 100 ꢀC. The incorporation of Cr ions within a-Fe2O3 nanocrystals increases the specific surface
area, and promotes the oxidation of H2S and oxygen adsorption in the air. Thus, the doping of Cr into a-
Fe2O3 nanostructures would be a promising method for designing and fabricating high performance H2S
gas sensors.
Received 4th September 2018
Accepted 3rd January 2019
DOI: 10.1039/c8ra07365a
rsc.li/rsc-advances
sensitive and markedly selective gas sensors based on a-Fe2O3
nanostructures remains a challenge. Up to now, many efforts
1. Introduction
have been taken such as heterostructure constructing, element
doping, and decorating in order to improve its perfor-
mance.10,20,21 Among these methods, element doping has been
turned out to be a very simple and feasible method. Chromium,
as a member of transition metals, exhibits a wide range of
possible oxidation states. And it is a promising candidate for
efficient doping materials in sensor applications. There have
been some reports showing the performance of Cr dopant in
ammonia sensors,22–24 NO2 sensors,25,26 acetone sensors,27
oxygen sensor,28 and so on. Herein we report a facile one-step
hydrothermal method to synthesize Cr-doped a-Fe2O3 nano-
Recently, atmospheric environmental pollution has become
one of the most serious problems that all the countries of the
earth are facing.1 Many steps have been taken to resolve it, such
as enhancing control over pollutant discharge, researching
high-performance catalysts for the catalytic conversion of auto
exhausts, developing new technologies for the monitoring of
toxic gases, enhancing the environmental awareness of people
etc. Among these, controlling and monitoring poisonous gases
from sources of pollution are quite important so far. Accord-
ingly, the development of gas sensors with enhanced sensitivity
and selectivity is very urgent and important.
Hydrogen sulde (H2S) is a colorless, highly ammable and
extremely toxic gas with a characteristic foul odour of rotten
eggs.2–5 Furthermore, it is also produced in large quantity from
industrial activities and places such as petroleum natural gas
drilling and rening, wastewater treatment, coke ovens,
tanneries, kra paper mills, and landlls.6,7 Hence, it is neces-
sary to detect concentrations of H2S in work environments such
as oil and natural gas industries. To date, much effort has been
made to prepare a number of gas-sensing nanomaterials, such
as Fe2O3,8–13 WO3,14 SnO2,15 CuO,16 ZnO,3 and some
compounds.17–19 Among them, a-Fe2O3 is an excellent potential
H2S-sensitive material.
ꢀ
particles, for optimizing the gas sensing properties. At 100 C,
the sensor using 4 wt% Cr-doped a-Fe2O3 displayed the highest
response toward hydrogen sulde gases with a low cross-
selectivity and short response time. The enhanced sensing
response can be attributed to the increases in surface area and
catalysis effect by Cr doping.
2. Experimental details
2.1 Synthesis of Cr-doped a-Fe2O3
All of the reagents used in the experiments were of analytical
grade without further purication. Fe(NO3)3$9H2O and
Cr(NO3)3$9H2O were used as iron and chromium sources,
respectively. A series of Cr-doped a-Fe2O3 (0.0, 2.0, 4.0, 6.0, and
8.0 wt%) nanoparticles were synthesized by one-step hydro-
thermal reaction. In a typical experiment, Fe(NO3)3$9H2O
(0.8077 g), Urea (0.5 g) and Cr(NO3)3$9H2O (0.0168 g) were put
into 35 ml of distilled water to form homogeneous solution
under vigorous stirring. Subsequently 300 ml ammonia was
In recent years, a-Fe2O3 has attracted great attention for its
distinctive sensing properties toward H2S gas. Despite some
exciting results obtained, the development of more highly
Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University,
Xiamen, China. E-mail: liqiuhong@xmu.edu.cn; Fax: +86-0592-2187196; Tel: +86-
0592-2187198
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introduced into above solution by magnetic stirring. The The transmission electron microscopy images (TEM) were ob-
precursor solution was continuously stirred at room tempera- tained using JEOL JEM-2100. The Raman spectra were per-
ture to obtain a homogeneous mixture. Aerwards, the solution formed by using the IDSpec ARCTIC Raman system. X-ray
was transferred to a Teon-lined stainless steel autoclave with photoelectron spectroscopy (XPS) measurements were per-
a capacity of 50 ml and maintained at 180 ꢀC for 12 h. Aer the formed with a PHI Quantum 200 spectrophotometer with Mg K
hydrothermal procedure, the autoclave naturally cooled to room radiation.
temperature. The solid products were subjected to centrifuga-
tion, rinsed alternately with distilled water and ethanol several
times, and dried at 70 ꢀC in air for 24 h. Finally, the dry
3. Results and discussion
3.1 Characterization of sensor material
precipitates were annealed in a muffle furnace at 500 ꢀC for 2 h
in air.
The Raman spectrum of the a-Fe2O3 is presented in Fig. 2. In
the range of 200–1500 cmꢁ1, many peaks could be observed,
2.2 Sensor fabrication and sensing measurements
which were located at 225, 290, 405, 495, 611, 658, 825, 1081,
and 1322 cmꢁ1, respectively. All these peaks correspond to a-
In a typical experiment, a small amount of the sensing material
powder was mixed with terpineol followed by 2 h grinding to
Fe2O3 phase.29,30 The XRD patterns of the pure and Cr-doped a-
Fe2O3 samples are shown in Fig. 3a. Clearly, all the diffraction
peaks of all the samples could be indexed to the rhombohedral
structure of a-Fe2O3 phase (JCPDS le no. 33-0664). No other
peaks corresponding to Cr-related compounds were observed in
all the XRD patterns of 0–8 wt% Cr-doped a-Fe2O3 samples. In
addition, as shown in Fig. 3b, a high angle shi was detected
from the (110) peak via comparing the different Cr doping a-
form a paste, then coated uniformly onto a ceramic tube. Aer
the coating was air dried, the a-Fe2O3-coated Al2O3 microtube
was xed to a special pedestal with six poles by welding four Pt
electrodes to four poles of the pedestal. A Ni–Cr heating coil was
then inserted through the Al2O3 microtube, and its two ends
were welded to the other two poles of the pedestal. A schematic
representation and a micrograph image of gas sensor were
shown in Fig. 1. The measurements of hydrogen sulde gases
sensing were conducted on a ZhongKe NS-4003 Smart Sensor
Analyser (Beijing ZhongKe Micro-Nano Networking Technology
Co., Ltd.). The relative humidity at room temperature was about
52% during the whole experiment. The sensor response is
dened as response ¼ Ra/Rg, where Ra is the resistance in air
and Rg is the measured resistance in the presence of the test gas.
The response time is dened as the period in which the sensor
output change reached 90% of the stabilized value, similarly,
the recovery time is dened as the time required for the sensor
to reach 10% of the initial steady-state value aer the sensor was
taken out of the test chamber.
Fe2O3. This is due to larger ionic radius of Fe3+ (0.67 A) as
˚
compared to Cr3+ (0.64 A), which according to Bragg's law shis
˚
the peak position to high diffraction angles. The incorporation
2.3 Characterization
X-ray powder diffraction (XRD) data of the as synthesized
materials were collected on a Philips X'pert pro diffractometer
using Cu-Ka radiation at 40 kV and 30 mA. Scanning electron
microscopy (SEM) images and EDX data were recorded on
a scanning electron microscope (SEM, Zeiss SUPRA 55) equip-
ped with an energy dispersive X-ray Spectroscopy (EDS, Oxford).
Fig. 2 Raman scattering spectra of pristine a-Fe2O3.
Fig. 3 (a) XRD patterns and (b) high-resolution of (110) peak of the
Fig. 1 (a) A schematic representation and (b) a micrograph image of pure, 2.0 wt%, 4.0 wt%, 6.0 wt% and 8.0 wt% Cr-doped a-Fe2O3
gas sensor.
nanoparticles.
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of atom with low ionic radius atom results in shrink of unit cell
that decreases the lattice parameter and decreases d-spacing
thus shiing peak positions to higher angles. Therefore, the
substitution of Fe3+ by Cr3+ induced the high angle shi of
diffraction peaks, suggesting that Cr element is incorporated
into the a-Fe2O3 lattice.
Morphology of the pristine and Cr-doped a-Fe2O3 structures
were also discerned by SEM observations. As shown in Fig. 4a,
spherical particles coexisted with irregular particle aggregates.
As the amount of Cr3+ was increased to 4 wt%, the obtained
product consisted of small nanospheres with good mono-
dispersity, as shown in Fig. 4c. However, particle aggregates
without an orderly shape were collected when the amount of
Cr3+ increased to 8 wt%, as shown in Fig. 4e. a-Fe2O3 nano-
particles can be obtained by using a one-step hydrothermal
process at an appropriate concentration of ammonia. And Cr3+
may control the morphology of a-Fe2O3 because of its acid
nature.23,31 The detailed mechanism of Cr on Fe2O3 nanoparticle
is not clear now and needs further study. The EDS patterns of
4 wt% Cr-doped a-Fe2O3 nanoparticles shown in Fig. 3f indicate
that the as-prepared 4 wt% Cr-doped a-Fe2O3 nanoparticles are
composed of Fe, O and Cr, where the x-axis represents X-ray
energy (in keV) and the y-axis represents the intensity. And it
is evident that the peaks associated with chromium in the
doped samples conrm the presence of chromium in a-Fe2O3
nanoparticles.
Fig. 5 (a) Low magnification TEM image of the 4 wt% Cr-doped a-
Fe2O3. (b and c) HRTEM images and (d) SAED pattern of the 4 wt% Cr-
doped a-Fe2O3.
the lattice distances were about 0.368 nm and 0.222 nm, which
can be corresponded to the (012) and (113) lattice plane of a-
Fe2O3. The diffraction pots in a SAED pattern (Fig. 5d) matched
well with (012), (110), (113) and (024) planes of a-Fe2O3.
X-ray photoelectron spectroscopy (XPS) was performed to
illuminate the surface composition of the studied 4 wt% Cr-
doped a-Fe2O3 nanoparticles. The spectra of Fe 2p and Cr 2p
are shown in Fig. 6. In Fig. 6a, the bending energies of Fe 2p3/2
and Fe 2p1/2 are 710.7 and 724.3 eV, respectively, which are in
good agreement with the literature values of Fe3+ in a-Fe2O3.32–34
The binding energy of Cr 2p3/2 is 577.8 eV and Cr 2p3/2–Cr 2p1/2
binding energy separation is 9.6 eV, that is typical for Cr3+
centers dispersed in an oxide environment (Fig. 6b).35–37
Microstructures of the as-prepared products were studied by
selected area electron diffraction (SAED) and high resolution
transmission electron microscopy (HRTEM) characterizations.
As shown in Fig. 5a, 4 wt% Cr-doped a-Fe2O3 nanoparticles were
observed by low magnication transmission electron micros-
copy. In the HRTEM images shown in Fig. 5b and c, we found
3.2 Gas sensing properties
3.2.1 Gas-sensing measurement. Gas sensing experiments
to 50 ppm hydrogen sulde gases at different temperatures are
performed as shown in Fig. 6a. The responses of pure, 2.0 wt%,
4.0 wt%, 6.0 wt%, and 8.0 wt% Cr-doped a-Fe2O3 samples are
tested at the temperatures from 60 to 160 ꢀC which are found to
increase with the increasing temperature, reach the maximum
at a certain temperature, and then decrease with a higher
Fig. 4 (a–e) Typical SEM images of the pure, 2.0 wt%, 4.0 wt%, 6.0 wt%
and 8.0 wt% a-Fe2O3 nanoparticles, (f) the EDX spectrum of the Fig. 6 XPS spectra of 4.0 wt% Cr-doped a-Fe2O3 nanoparticles (a) Fe
4.0 wt% a-Fe2O3 nanoparticles.
2p, (b) Cr 2p.
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operating temperature. It is a common behavior on the surface Cr-doped a-Fe2O3 nanoparticles showed excellent selectivity to
of a-Fe2O3 or similar semiconducting metal oxides which can be H2S as opposed to other test gases.
explained by the kinetics and mechanics of gas adsorption and
The concentrations of hydrogen sulde were varied from 20
desorption. When the heating temperature is low, the chemical to 200 ppm. Transient response curves of the sensor (composed
activation of nanoparticles is accordingly low, and it causes of pure, 2.0 wt%, 4.0 wt%, 6.0 wt%, and 8.0 wt% Cr-doped a-
a low response. The increasing temperature leads to positive Fe2O3) at the optimal temperature of 100 ꢀC are shown in
chemical activation, and the sensor gets the maximum response Fig. 7c. The nearly rectangle-shaped response and recovery
at a certain heating temperature. The further increasing oper- curves indicate the excellent hydrogen sulde response and
ating temperature results in some adsorbed gas molecules rapid reaction rate of the sensor. Exposing the sensor to H2S
escaping from surface before gas reaction, and then the signicantly increases conductance until it reaches saturation.
response decreases correspondingly.
Aer the removal of H2S, the conductance approaches its initial
Meanwhile, Fig. 7a shows that the Cr-doped a-Fe2O3 gas value.
sensors achieve a maximum sensitivity at 100 ꢀC, while the pure
Fig. 7d shows the relationship between the response of the
ꢀ
a-Fe2O3 sensor obtain the maximum sensitivity at 120 C. The sensor and H2S concentration from 20 to 200 ppm calculated
gas responses of the sensors based on the pure, 2.0 wt% 4.0 wt% from Fig. 7c. Increasing the concentration accelerates the
6.0 wt% and 8.0 wt% Cr-doped a-Fe2O3 to 50 ppm H2S at 100 ꢀC response. Among them, the response of the sensor based on the
is 112, 155, 213, 102 and 65, respectively. The result revealed 4.0 wt% Cr-doped a-Fe2O3 was apparently higher than the other
that sensor based on 4.0 wt% Cr-doped a-Fe2O3 showed the sensors. The response of Cr-doped Fe2O3 sensors showed
highest response to 50 ppm H2S and the value was about two almost no saturation with the variation of H2S concentrations
times higher than that of pure a-Fe2O3. According to the above between 20 and 200 ppm, indicating a wide response range of
test result, we could surmise that the Cr on sensor would play the Cr-doped sensor and its possible application in detecting
a crucial role in the governing mechanism. Therefore, further high H2S concentration environment.
measurements were carried out using this specic composition
From these, the Cr doping amount of 4.0 wt% was consid-
(4 wt%) and operating temperature (100 ꢀC) to explore the effect ered as the optimum doping concentration. Then the response
of other factors on the sensing properties of Cr-doped a-Fe2O3. and recovery characteristics of the sensor based on 4.0 wt% Cr-
Subsequently, variations in the electrical conductance were doped a-Fe2O3 nanoparticles were investigated to 50 ppm
measured on a-Fe2O3 nanoparticles. Butane, ammonia, ethanol hydrogen sulde gases at 100 ꢀC. As shown in Fig. 8a, the
and methane common gases (50 ppm) used as probe molecules conductance increased upon exposure to hydrogen sulde
to investigate the cross-sensitivity of the sensor at an operating gases, which was consistent with the gas sensing behavior of n-
temperature of 100 ꢀC (Fig. 7b). Compared with the conduc- type oxide semiconductor. The response and recovery times of
tance change in response to 50 ppm H2S at 100 ꢀC, the the sensors based on the 4.0 wt% Cr-doped a-Fe2O3 were 29 and
responses of the 4.0 wt% Cr-doped a-Fe2O3 sensor to other 80 s, respectively. Stability of the sensors is also investigated.
gases are negligible. The 2.0 wt% Cr-doped a-Fe2O3 sensor The 4.0 wt% Cr-doped a-Fe2O3 sensor was stored in air and kept
shows a low response to ammonia and ethanol. Consequently, working at 100 ꢀC for subsequent sensing property tests at
it was concluded that the sensor using as-synthesized 4.0 wt% a level of 50 ppm H2S aer aging for 30 days. It can be seen that
Fig. 7 (a) Gas responses of the sensors based on the pure, 2.0 wt%,
4.0 wt%, 6.0 wt%, and 8.0 wt% Cr-doped a-Fe2O3 nanoparticles vs.
operating temperatures to 50 ppm H2S. (b) Gas responses of five Fig. 8 (a) Dynamic response of the sensor based on 4 wt% Cr-doped
sensors to 50 ppm various target gases at 100 ꢀC. (c) Real-time a-Fe2O3 to 50 ppm H2S at 100 ꢀC. (b) Stability of the sensor to 50 ppm
response curves of five sensors to different H2S concentration at H2S. (c) Dynamic response of the sensors to different concentrations
100 ꢀC. (d) Gas responses of five sensors as a function of the H2S H2S from 1 ppm to 10 ppm. (d) Responses of the sensor to different
concentration at 100 ꢀC.
concentrations of H2S.
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Table 1 Comparison of the sensing performance of Fe2O3 sensors
Material
H2S concentration (ppm)
Temperature (ꢀC)
Response
Detection limit (ppm)
Reference
Porous a-Fe2O3
Ag loaded Fe2O3
Pd decorated Fe2O3
Au modied Fe2O3
Fe2O3@WO3
50
100
50
10
10
350
160
160
250
150
100
66.7
220
75
6.4
190
213
1
50
10
1
0.5
1
12
8
10
9
14
Cr-doped a-Fe2O3
50
This work
the sensor presented a nearly constant response to H2S during
However, a-Fe2O3 presented high catalyst for H2S oxidation,
the test, showing that the sensor exhibited long-term stability and the dominant product was SO2. Steijns et al. studied the
(Fig. 8b). We also investigated the detection limit of H2S by the H2S oxidation reaction, they showed that the SO2 formation rate
developed sensor. As shown in Fig. 8c, we detected dynamic was very slow at low temperatures. It was attributed to the high
response curve of sensor based 4 wt% Cr-doped a-Fe2O3 to H2S activation energy of SO2 formation, which would be about
ranging from 1 ppm to 10 ppm at 100 C. Fig. 8d showed the 125 kJ molꢁ1. Whereas the activation energy for sulfur forma-
ꢀ
relationship between the response of the sensor and H2S tion was between 8 and 50 kJ molꢁ1. So, Berben et al. prepared
concentration from 1 to 10 ppm. The detection of 4 wt% Cr- a-Fe2O3-based catalysts, with chromium as additives. It showed
doped a-Fe2O3 to 1 ppm H2S had the responses of about 2.5. good activity and good selectivity, which directly and selectively
When we tested H2S below 1 ppm at optimal temperature, the oxidized H2S with O2 into elemental sulfur.42,43
sensor did not have response.
Fig. 9 shows in a double logarithmic plot the variation of the
A comparison of the sensing performances between the response of a-Fe2O3 sensor versus H2S concentration. The linear
sensor in this work and other H2S sensors based on a-Fe2O3 trend can be related to the conductance model limited by the
reported previously is summarized in Table 1. From the table, electron transport across the intergranular Schottky barrier24,44
comprehensively considering the gas response and operating where
temperature, it is obvious that 4.0 wt% Cr-doped a-Fe2O3
s f exp(ꢁeVs/kT)
(6)
nanoparticles showed a high gas response at a low operating
temperature. This was conducive to practical application and
energy conservation. Therefore, the sensor based on 4.0 wt% Cr-
doped a-Fe2O3 nanoparticles had more superiority than those
reported in the literature.
3.2.2 Gas sensing mechanism. It is known that metal oxide
gas sensing depends on the interaction between the test gas and
the adsorbed oxygen species on the surface of the material.38,39
a-Fe2O3 is a n-type semiconductor. Therefore, when the a-Fe2O3
are exposed in an air atmosphere, oxygen molecules will adsorb
on the a-Fe2O3 surface and capture electrons from a-Fe2O3 to
generate the chemisorbed oxygen species at higher tempera-
tures, leading to an increase in resistance. The main chemical
equations involved in the process are shown as follows (eqn (1)–
(4)):40,41
(s is the conductance and eVs represents the surface barrier
height at the intergranular contact).
According to this model, the response of the a-Fe2O3 nano-
particles is explained by using the following equation:45,46
log s/s0 ¼ Ag + b log Pg
(7)
where s0 denotes the conductance in the absence of the target
gas, Ag is a prefactor, Pg is the gas partial pressure, and b is the
response order. Mechanisms were formulated to describe the
O2(gas) / O2(ads)
O2(ads) + eꢁ / O2ꢁ(ads)
O2ꢁ(ads) + eꢁ / 2Oꢁ(ads)
Oꢁ(ads) + eꢁ / O2ꢁ
(1)
(2)
(3)
(4)
When the a-Fe2O3 sensor is exposed to H2S atmosphere, the
surface oxygen ions react with the H2S molecule and release
a number of electrons back into the semiconductor, resulting in
a decrease in resistance of the sensor. This reaction process may
be as eqn (5)
Fig. 9 Response variation for the pure, 2.0 wt%, 4.0 wt%, 6.0 wt% and
8.0 wt% Cr-doped a-Fe2O3 nanoparticles.
H2S + 3O2 / H2O + SO2 + 3eꢁ
(5)
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surface reaction and predict the b value in eqn (7). This is nanoparticles with different doping levels. The results of X-ray
usually 1 or 1/2, depending on the charge state of the surface diffraction, EDS and XPS data proved that Cr ion successfully
species, the stoichiometry of the elementary surface reactions, entered into the crystal lattice of a-Fe2O3 without deteriorating
and the microstructure of the material. These properties the original crystal structure. Furthermore, an appropriate
contribute to the uctuations of the response order value by amount of Cr-doped a-Fe2O3 (4.0 wt% Cr-doped a-Fe2O3 sensor
different weights.
in this work) display, upon exposure to H2S, outstanding
In the case of pristine a-Fe2O3 nanoparticles (Fig. 9), the sensing properties even at relatively low operating temperature,
calculated b value (i.e., the slope of the curve described by eqn and the sensitivity resulted as much higher than that of the
(7)) was about 0.98. Differently, 2.0 and 4.0 wt% Cr-doped a- pristine sensor. This behaviour could be attributed to the
Fe2O3 sensors show b values about 1.17 and 1.26. This can be nanometer size effect and catalysis effect. Hence, we can
reasonably ascribed to the presence of Cr element which may conrm that the doping of Cr into a-Fe2O3 is a promising
give rise to a different sensing mechanism at the microscale strategy for improving the gas sensing performance of the a-
level.
Therefore, a mechanism was suggested to explain the
Fe2O3 based gas sensor.
excellent gas response of Cr-doped a-Fe2O3. First, when H2S gas
ows in, the H2S molecules are oxidized into elemental sulfur
and H2O by the Cr3+ catalyst in a-Fe2O3. The adsorption process
is as follows (eqn (8)):
Conflicts of interest
There are no conicts to declare.
Cr3þ
H2S þ 1=2 O2
1=nSn þ H2O
(8)
ƒ!
Acknowledgements
This work was partly supported by the National Natural Science
Foundation of China (Grant No. 61574118), and the Key Project
of Science and Technology Plan of Fujian Province (Grant No.
2015H0038).
In the meantime, some of the chemisorbed oxygen species
reacted with elemental sulfur (eqn (9))
1/nSn + 2O2 / SO2 + 2eꢁ
(9)
ꢁ
The ability of a reacting species to overcome a free energy
barrier that is preventing a reaction is commonly modeled by
the Arrhenius equation. For most of the chemical reactions,
including the primitive reaction and primitive action, the
Arrhenius equation gives the dependence of rates, K, of
a chemical reaction on the absolute temperature T (in Kelvin) as
shown in eqn (10):
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K ¼ A exp(ꢁEa/RT)
(10)
This equation shows that the rate constant exponentially
increases as activation energy (Ea) decreases. For the pristine a-
Fe2O3 sensor, higher activation energy is required for produce
SO2. Therefore, the operating temperature of the reaction is
higher. In the presence of Cr3+, the a-Fe2O3 sensor provides
a reaction path with lower activation energy, providing a high
response at a low temperature. For these reasons, the working
temperature of the Cr-doped a-Fe2O3 sensors can be reduced to
low temperature while the sensitivity was still kept high.
Finally, the further increasing of Cr doping content, the size
of a-Fe2O3 gas sensing material became larger and the
agglomeration of the nanoparticles became more obvious
(Fig. 4d and e), which would reduce surface-to-volume ratio and
efficient diffusion of the gas molecules. This may be the reason
for the poor performance of 6.0 wt%, and 8.0 wt% Cr-doped a-
Fe2O3 sensors.
ˇ
J. P. Froning, M. Otyepka, R. Zboril, F. Besenbacher and
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4. Conclusions
In summary, we have developed a simple one-step hydro- 13 Z. Sun, H. Yuan, Z. Liu, B. Han and X. Zhang, Adv. Mater.,
thermal method for the synthesis of Cr-doped a-Fe2O3
2005, 17, 2993–2997.
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