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Sensors and Actuators B 125 (2007) 79–84
H
2
S sensing characteristics of Pt-doped ␣-Fe
2
O
3
thick film sensors
Yan Wang, Shurong Wang, Yingqiang Zhao, Baolin Zhu, Fanhong Kong,
Da Wang, Shihua Wu
∗
, Weiping Huang, Shoumin Zhang
Department of Chemistry, Nankai University, Tianjin 300071, China
Received 1 November 2006; received in revised form 23 January 2007; accepted 24 January 2007
Available online 30 January 2007
Abstract
␣-Fe
2
O
3
nanoparticles doped with different amounts of Pt were synthesized by chemical coprecipitation and characterized by X-ray diffraction
(XRD), transmission electron micrograph (TEM) and X-ray photoelectron spectrometer (XPS). The gas sensing properties of the thick film sensors
prepared by Pt-doped ␣-Fe
2
O
3
nanoparticles were investigated and compared with those of undoped ␣-Fe
2
O
3
sensors. Obtained results indicated
that the Pt-doped ␣-Fe
2
O
3
sensors presented much higher response, better selectivity and rather low optimum operating temperature to H
2
S than
the undoped ␣-Fe
2
O
3
sensors. The sensor of 2 wt% Pt/␣-Fe
2
O
3
still showed excellent response towards very low concentration H
2
S (10 ppm). The
sensing mechanism of the Pt/␣-Fe
2
O
3
sensor to H
2
S was also discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Gas sensor; H
2
S; Pt-doped ␣-Fe
2
O
3
1. Introduction
Hydrogen sulfide (H
2
S) is a toxic and malodorous gas as
main sources from gasoline, natural gases and city sewage. It
is badly harmful to human body and the environment. Accord-
ing to the safety standards established by American Conference
of Government Industrial Hygienists, the threshold limit value
(TLV) defined for H
2
S is 10 ppm. Meanwhile, the type of oil
or natural gas is correlative with the concentration of H
2
S. The
oil or natural gases mines can be found depending on the con-
centration of H
2
S. Therefore, the detection and monitoring of
H
2
S are of high importance for both resource exploitation and
human health. In the recent researches, a number of semicon-
ductor sensors have been found to be sensitive to H
2
S, including
SnO
2
,WO
3
,In
2
O
3
, ZnO
2
, and a few perovskite-type materials
like NdFeO
3
and NiFeO
4
[1–9]. Recently, Sun et al. have found
that ␣-Fe
2
O
3
exhibited is sensitive to H
2
S based on the catalytic
chemiluminescence at 360
◦
C [10,11]. However, the application
of these sensors is limited by some disadvantages, for instance,
poor selectivity, long response time, high operating temperature
or the limited detection range. As a consequence, a new sort of
∗
Corresponding author. Tel.: +86 22 2350 5896; fax: +86 22 2350 2458.
E-mail address: wushh@nankai.edu.cn (S. Wu).
H
2
S sensor, which can satisfy the requirement for application,
must be searched.
␣-Fe
2
O
3
is an n-type semiconductor oxide and has been
widely used as gas sensors. However, most of the researches
on ␣-Fe
2
O
3
sensors were focused on the sensing properties
towards alcohol, and the sensing characteristics of Pt-doped
␣-Fe
2
O
3
sensors to H
2
S have not been reported until now.
In this paper, the crystallographic characteristics of undoped
and Pt-doped ␣-Fe
2
O
3
and their gas sensing performance to
H
2
S are investigated. The sensing mechanism of them is also
discussed.
2. Experimental
Pt/␣-Fe
2
O
3
powders were prepared by a coprecipitation
method. A small quantity of polyglycol was added to the an
aqueous solution of H
2
PtCl
6
·6H
2
O and Fe(NO
3
)
3
·9H
2
O. The
aqueous mixture was then added dropwise to an aqueous solu-
tion of Na
2
CO
3
under vigorous stirring at 80
◦
C. The pH of the
solution was adjusted by a diluted Na
2
CO
3
aqueous solution in
the reaction process. After stirring for 1 h, a solid precipitate was
formed and kept digesting overnight at room temperature. Then
the precipitate was washed with deionized water, dried at 80
◦
C
and calcined at 400
◦
C for 1 h, a serious of 0.5, 1.0, 1.5, 2.0, 3.0
and 5.0 wt% Pt-doped ␣-Fe
2
O
3
powders were obtained.
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2007.01.037
80 Y. Wang et al. / Sensors and Actuators B 125 (2007) 79–84
Fig. 1. Schematic diagram of the Pt/␣-Fe
2
O
3
thick film sensor: (1) Pt wire; (2)
Ni–Cr heated wire; (3) Al
2
O
3
tube; (4) Pt/␣-Fe
2
O
3
thick film; (5) Au electrode.
XRD spectra of the samples were measured with a Japan
Rigaku D/max 2500 X-ray diffractometer with Cu K␣ radiation
(λ = 1.5418
˚
A). The morphology of the sample was observed by
a Holland Philips T200ST transmission electron microscopy.
X-ray photoelectron spectra (XPS) were recorded on a PHI-
1600 spectrometer (USA) equipped with a Mg K␣ radiation for
exciting photoelectrons.
An alumina substrate tube with a 4 mm length was used for
the heater and sensing base. The schematic diagram of a typi-
cal gas sensor is shown in Fig. 1. A small Ni–Cr alloy coil was
placed through the tube to supply the operating temperatures of
100–500
◦
C. Electrical contacts were made with two platinum
wires attached to each gold electrode. The Pt/␣-Fe
2
O
3
powder
was mixed with terpineol to form a paste. Then the paste was
coated onto the outside surface of the alumina tube, and the thick-
ness of the coated film was around 50 m. In order to improve
their stability and repeatability, the gas sensors were sintered at
300
◦
C for 10 days in air. Gas properties of the sensors were
tested in a glass chamber with a volume of 15 L. The test gases
were injected into the closed chamber by a microinjector. The
gas response S is defined as the ratio R
a
/R
g
, where R
a
and R
g
are
the resistances measured in air and in a test gas, respectively.
Fig. 2. XRD pattern of 2 wt% Pt/␣-Fe
2
O
3
.
3. Result and discussion
3.1. Structure characterization
Fig. 2 shows the XRD pattern of ␣-Fe
2
O
3
doped with
2 wt% Pt additions. The diffraction pattern of ␣-Fe
2
O
3
(2 wt%
Pt) matched perfectly with the standard ␣-Fe
2
O
3
reflections
(JCPDS No. 33-664). However, no obvious Pt peaks were
observed, which may be due to high dispersion of Pt particles.
The sharp peaks suggest that the crystalline of ␣-Fe
2
O
3
is per-
fect. The mean size of the crystalline is around 25 nm, calculated
by the Deby–Scherrer equation.
TEM images of 2 wt% Pt/␣-Fe
2
O
3
are shown in Fig. 3. From
Fig. 3a, it can be seen that the dopant is highly dispersed on
the surface of spherical ␣-Fe
2
O
3
particles in the form of very
small grains. The spherical ␣-Fe
2
O
3
particles are 20–50 nm in
diameter, which is larger than the result from XRD pattern, and
indicates the presence of some polycrystalline nanostructures
Fig. 3. TEM images of 2 wt% Pt/␣-Fe
2
O
3
with different magnification.
Y. Wang et al. / Sensors and Actuators B 125 (2007) 79–84 81
Fig. 4. XPS spectra of 2 wt% Pt/␣-Fe
2
O
3
(insets pictures are (a) Fe 2p and (b)
Pt 4f spectrum, respectively).
in the products. As shown in Fig. 3b, the clear fringe spacing
(0.37 nm) is very close to the space between (0 1 2) lattice planes
of ␣-Fe
2
O
3
, implying the high crystallinity of the sample.
The component analysis of Pt/␣-Fe
2
O
3
was investigated by
X-ray photoelectron spectrum. Fig. 4 shows the XPS spectra
of 2 wt% Pt/␣-Fe
2
O
3
. It reveals that the surface of the sam-
ple contains Fe, Pt, O and C elements. The existence of C
element may be caused by the residual solvent. Higher res-
olution spectra of Fe and Pt are shown in the inset pictures
(a and b). The bending energies of Fe 2p
3/2
and Fe 2p
1/2
are
710.7 and 724.3 eV, respectively, which agree well with the lit-
erature values of Fe
3+
in ␣-Fe
2
O
3
[12]. The binding energies
of Pt 4f
7/2
and Pt 4f
5/2
are 74.48 and 77.60 eV, respectively,
which indicate that the state of Pt in the two samples is +4
[13]. Surface elemental analysis reveals that the atomic ratio
of Pt/Fe is 3/100. This ratio is higher than the theoretical one,
which indicates the Pt dopant is well dispersed on the surface of
␣-Fe
2
O
3
.
3.2. Gas sensing properties
The gas sensing properties of pure ␣-Fe
2
O
3
and Pt-doped
␣-Fe
2
O
3
were studied. It is well known that the gas sensitiv-
ity is greatly influenced by the operating temperature and the
amounts of additives. In order to determine the optimum operat-
ing temperature and additive amount, responses of all the sensors
to 100 ppm H
2
S gas at different operating temperatures were
examined. The results are shown in Fig. 5. It can be seen that the
response of the sensors to H
2
S varies with not only the amount
of Pt but also the operating temperature. Each curve reveals a
maximum at an optimum operating temperature. All the doped
sensors have the maximum gas response at 160
◦
C, while the
undoped sensor has the maximum response at 200
◦
C. The lower
operating temperature of the sensors would result in the lower
energy consumption. Furthermore, all the doped sensors exhibit
much higher response than the undoped one. Especially, the sen-
sor doped with 2 wt% Pt exhibits the largest response to H
2
Sat
160
◦
C.
Fig. 5. Gas responses of undoped ␣-Fe
2
O
3
and Pt/␣-Fe
2
O
3
to 100 ppm H
2
S.
Selectivity is an important parameter of gas sensing proper-
ties, and the sensor has to have higher selectivity required for
its application. As far as we know from the recent researches,
the ␣-Fe
2
O
3
sensor also responds to other gases, such as ethanol
and acetone. Therefore, we examined the responses of the 2 wt%
Pt-doped ␣-Fe
2
O
3
sensor to other seven gases of 1000 ppm at
different operating temperatures. From Fig. 6, it can be observed
that the sensor exhibits the largest response to H
2
S, moderate
responses to ethanol and acetone, and negligible responses to
n-hexane, NH
3
,H
2
and CO. On the other hand, the optimum
operating temperature to ethanol and acetone is 200
◦
C, which
is higher than that to H
2
S. The selectivity to H
2
S is higher when
the operating temperature is below 160
◦
C. As a result, it is easier
to detect H
2
S at lower operating temperatures.
Gas responses of the undoped and 2 wt% Pt-doped ␣-Fe
2
O
3
sensors to various gases at their optimum operating temperatures
are compared in Fig. 7. The responses of 2 wt% Pt-doped ␣-
Fe
2
O
3
sensor to all of these gases are much higher than that of
the undoped one, especially to H
2
S.
Fig. 6. Gas responses of 2 wt% Pt-doped ␣-Fe
2
O
3
to various gases at different
operating temperature.
82 Y. Wang et al. / Sensors and Actuators B 125 (2007) 79–84
Fig. 7. Comparison of gas responses of 2 wt% Pt-doped ␣-Fe
2
O
3
to various
gases.
Fig. 8 shows the relationship between the response of the
2 wt% Pt-doped ␣-Fe
2
O
3
sensor to H
2
S and the H
2
S concen-
tration (10–200 ppm) at 160
◦
C. It can be seen that the response
increases with H
2
S concentration, and the concentration depen-
dence presents a good linearity basically, which suggests that
the Pt/␣-Fe
2
O
3
could meet the application demand. On the other
hand, the sensor still shows excellent response to low concen-
tration H
2
S, and its response to 10 ppm H
2
S is 147.7.
Response and recovery times are the basic parameters of
gas sensors, which are defined as the time required to reach
90% of the final resistance. Fig. 9 shows the response–recovery
characteristics of 2 wt% Pt-doped ␣-Fe
2
O
3
to H
2
S at differ-
ent concentrations. As can be seen from the curve, the sensor
responds very rapidly after introduction of H
2
S, but the recov-
ery time become longer and longer with an increase in H
2
S
concentration.
The reversibility of the Pt/␣-Fe
2
O
3
sensors was also investi-
gated. The sensors still have excellent responses to H
2
Seven
after 2 months. We believe that the Pt/␣-Fe
2
O
3
sensors find
promising applications to H
2
S in the future.
Fig. 8. Gas response vs. H
2
S concentration of 2 wt% Pt-doped ␣-Fe
2
O
3
.
Fig. 9. Response–recovery characteristic of 2 wt% Pt-doped ␣-Fe
2
O
3
to H
2
Sof
different concentrations.
3.3. Gas sensing mechanism
As has been generally reported, ␣-Fe
2
O
3
is a typical n-type
semiconductor, and its gas-sensing mechanism belongs to the
surface-controlled type. The change of resistance is dependent
on the species and chemisorbed oxygen on the surface. The
oxygen absorbed on the surface of an ␣-Fe
2
O
3
sensor cause
the electron depletion, consequently the resistance of the sen-
sor increases. The process can be expressed in the following
reactions:
O
2
(gas) → O
2
(ads) (1)
O
2
(ads) + e
−
→ O
2
−
(ads) (2)
O
2
−
(ads) + e
−
→ 2O
−
(ads) (3)
O
−
(ads) + e
−
→ O
2−
(ads) (4)
With introduction of H
2
S gas, it would be oxidized by these
chemisorbed oxygen species (O
2
−
,O
−
,O
2−
) on the surface
of the sensor. During this reaction, the electrons back into the
semiconductor, resulting in a decrease in resistance of the sensor.
When the sensor is exposed in air again, the gases are desorbed
as H
2
O and SO
2
. This reaction process may be as the following
equation:
H
2
S + 3O
2−
→ H
2
O + SO
2
+ 6e
−
(5)
The influence of Pt dopant on the gas sensing properties of
the sensor can be attributed to the spillover effect [9,14–16].Pt
can provide abundant sites to adsorb O
2
and test gas molecules.
Then, numerous chemisorbed oxygen species and gas molecules
spill over onto the surface of the ␣-Fe
2
O
3
support. Therefore,
we consider that the Pt dopant does not involve but accelerate
the electron exchange between the sensor and the test gas, which
may be the dominant reason for better gas sensing properties of
the Pt-doped ␣-Fe
2
O
3
.
To further ascertain the oxidation state of Fe and Pt in 2 wt%
Pt/␣-Fe
2
O
3,
XPS spectra were recorded after long-term expo-
Y. Wang et al. / Sensors and Actuators B 125 (2007) 79–84 83
Fig. 10. XPS spectra of 2 wt% Pt/␣-Fe
2
O
3
after long-term exposure to H
2
S
(insets pictures are (a) Fe 2p, (b) Pt 4f and (c) S 2p spectrum, respectively).
sure to H
2
S. The spectra are shown in Fig. 10. Higher resolution
spectra of Fe, Pt and S regions are shown in the insets (a–c).
The survey spectra reveal that the S element is introduced on the
surface after long-term exposure to H
2
S. The binding energy of
S 2p is about 168.3 eV, which indicates the S element exists as
SO
2
[17]. In addition, the binding energies of Fe 2p
3/2
and Fe
2p
1/2
are 710.9 and 724.5 eV, and the binding energies of Pt 4f
7/2
and Pt 4f
5/2
are 74.4 and 77.7 eV, respectively. The values are
all similar to those shown in Fig. 4, indicating that the oxidation
states of Fe and Pt are not changed after long-term exposure to
H
2
S. The results are in good conformity with the suggested H
2
S
gas sensing mechanism.
4. Conclusions
In summary, Pt-doped ␣-Fe
2
O
3
thick film sensors can be
prepared by a chemical coprecipitation method. The dopant is
highly dispersed on the surface of ␣-Fe
2
O
3
nanoparticles in the
form of PtO
2
. The Pt-dopping has led to a remarkable increase
in response to H
2
S and a decrease in optimum operating temper-
ature. Among all the tested sensors, the sensor based on 2 wt%
Pt/␣-Fe
2
O
3
exhibits the largest response to H
2
Sat160
◦
C, and
can detect low concentration H
2
S (10 ppm). A possible H
2
Sgas
sensing mechanism of the Pt-doped ␣-Fe
2
O
3
thick film sensors
is explored.
Acknowledgement
We gratefully appreciate the financial support of the 973
program of China (No. 2005Cb623607).
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Biographies
Yan Wang is studying in Shihua Wu’s group for her PhD degree in department
of chemistry, Nankai University in China. Her research focuses on the synthesis,
characterization and gas-sensing properties of metal oxide nanomaterials.
Shurong Wang is working at Nankai University. She received her MS degree in
chemistry from Nankai University in 2004. Her research covers nanomaterials,
catalysis and gas sensors.
Yingqiang Zhao received his Bachelordegree inchemistry from Tianjin Normal
University in 2004. He is currently a postgraduate in the department of chemistry
in Nankai University. His research is focused on the development and application
of gas-sensitive materials.
Baolin Zhu received her PhD degree in chemistry from Nankai University in
2006. Now, she is working at Nankai University. Her research is focused on the
preparation of nanomaterials.
Fanhong Kong is a graduate student in department of chemistry, Nankai
University now. Her interest is devoted to the preparation and application of
gas-sensitive materials.
84 Y. Wang et al. / Sensors and Actuators B 125 (2007) 79–84
Da Wang is an undergraduate student in department of chemistry, Nankai Uni-
versity now.
Shihua Wu received his degree in chemistry from Nankai University in 1970.
At present, he is Professor of chemistry at the Department of Nankai University,
where he has been working for many years in the field of preparation, character-
ization and catalytic and gas-sensing properties of metal oxides nanomaterials.
Weiping Huang is a professor of Chemistry Department at Nankai Univer-
sity. His researches are focused on the preparation and catalytic properties of
nanomaterials.
Shoumin Zhang is an associate professor of chemistry in Nankai University.
He received his PhD from Nankai University in 1999. His current research fields
are inorganic chemistry and materials.
. Sensors and Actuators B 125 (2007) 79–84
H
2
S sensing characteristics of Pt-doped ␣-Fe
2
O
3
thick film sensors
Yan Wang, Shurong. properties of the thick film sensors
prepared by Pt-doped ␣-Fe
2
O
3
nanoparticles were investigated and compared with those of undoped ␣-Fe
2
O
3
sensors. Obtained
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