Chemically Modified NiO Gas Sensor for Environmental Monitoring
Yun-Jin Jeong, Chandran Balamurugan and Dong-Weon Lee
MEMS and Nanotechnology Laboratory, School of Mechanical Systems Engineering, Chonnam National University,
Gwangju 500757, Republic of Korea
Keywords: H
2
S Gas Sensor, Pd-doped NiO, Nanocrystalline.
Abstract: In this work, we report semiconducting nanocrystalline NiO and Pd-doped NiO sensor with high sensitivity
and excellent selectivity for H
2
S gas. These nanomaterials were synthesized by the solution based technique.
Related structural and electrical properties of doped and pure NiO thick films were studied used to XRD,
XPS, SEM, EDX, BET/BJH and impedance technique. The gas sensing characteristics of pure NiO and Pd-
doped NiO were compared using a homemade gas sensing measurement system. The sensitivity, operating
temperature, and response/recovery time were systematically investigated based on the change in electrical
resistance of the materials in the presence of reduced gas (H
2
S, CO, LPG and ethanol) environment.
Experimental results confirmed that gas sensitivity was enhanced by doping different concentration of Pd in
NiO thick films. The 5 wt% Pd doped NiO thick film sensor showed a maximum response to 20 ppm H
2
S
(93%) at an operating temperature of 60 °C.
1 INTRODUCTION
Hydrogen sulfide (H
2
S) is a colorless, poisonous,
flammable gas with the characteristic foul odour of
rotten eggs. H
2
S gas takes part in many industrial
processes, which is a highly reducing gas and
heavier than air, and it is produced as a by-product
in more than 70 industries (Balamurugan and Lee,
2015). Human expose of high levels of H
2
S can
cause death (Struve at al., 2001). Health effect also
have found in human longer exposed to low-level
concentration of H
2
S will lead to chronic poisoning
symptoms, such as hypoesthesia, losing weight,
headache, fatigue and so on (Yu, Ning and Qian,
2010). In addition, human bodies produced small
amount of H
2
S and act as a signalling molecules.
Moreover, H
2
S produced endogenously in mammals,
including human and has various physiological
effect on the human body. For example, H
2
S is
physiologically produced by cystathionine-
γ-lyase
(CSE) and cystathionine-β-synthase (CBS). These
are dependent on pyridoxal-5'-phosphate enzymes,
which are expressed in the liver, kidney, brain,
thoracic aorta, ileum, pancreatic islets, uterus, and
placenta, among other locations, are crucial in the
synthesis of H
2
S. CBS is predominantly expressed in
the brain and the nervous system. However,
expression of CSE proteins has been mainly
observed in vascular smooth muscle cells and in the
heart (Zhang at al., 2013) Therefore, monitoring and
detection of low level H
2
S is a very important
requirement in various fields, such as industrial area,
human body and biological environment. The high
cost of the sophisticated analytical instruments
systems (e.g., spectroscopic gas sensor, optical gas
sensor, mass chromatography and mass spectrograph)
limits the control and monitoring of the H
2
S level.
However, semiconductor metal oxide is one of the
most alternative ways for H
2
S detection applications.
Semiconductor metal oxides such as ZnO, CuO,
SnO
2
, and In
2
O
3
are widely used as gas sensors
based on the change in their electrical conductivity
on exposure to the test gases. Beyond the most
investigated metal oxides, NiO metal oxides have
attracted considerable interest due to their unique
structural and electrical properties. Many attempts
were presented to significantly enhance the NiO
sensing performance by design and implementation
of novel structures, which are determinative for the
absorption/desorption, charge-transport path, surface
area, and electrical conductivity. However, they
have some disadvantages, such as a high working
temperature, poor selectivity and limited time
stability. Therefore, by introducing the noble metal
nanoparticales is one of the ways to enhance the
sensitivity and selectivity of the base materials
176
Jeong, Y., Balamurugan, C. and Lee, D.
Chemically Modified NiO Gas Sensor for Environmental Monitoring.
DOI: 10.5220/0005700801760178
In Proceedings of the 9th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2016) - Volume 1: BIODEVICES, pages 176-178
ISBN: 978-989-758-170-0
Copyright
c
2016 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
(NiO). In this work, we describe the fabrication,
characterization and application of semiconductor
NiO nanopowder and different weight % Pd doped
NiO nanopowder for H
2
S gas sensing applications.
NiO and doped metal oxide nanopowder were
synthesized via the metal-citrate complex method.
Related structural and electrical properties of doped
and pure NiO nanopowder were studied using to
various characterization technique. The H
2
S sensing
characteristics of pure NiO and Pd-doped NiO
nanopowder based sensor were compared using a
homemade gas sensing measurement system. The
experimental results confirmed that the 5 wt% Pd
doped NiO nanopowder based sensor showed a
maximum response to 20 ppm H
2
S (93%) at an
operating temperature of 60 °C. The selectivity of
the sensor elements for H
2
S against different
interfering gases such as CO, ethanol and LPG was
studied.
2 EXPERIMENTAL
2.1 Synthesis of 5 Wt% Pd Doped NiO
Nanopowder
All chemicals used in our experiments were reagent
grade and used without further purification. In a
typical our experiment, initial solutions were
prepared by dissolving stoichiometric mole ratios of
nickel nitrate, palladium nitrate and citric acid into
100 mL of deionized water. The mixture was stirred
at room temperature for about 1 h until the mixture
become homogeneous. Then, catalytic amount of
CTAB, PVA and ethylene glycol were added, and
the pH value of the mixture was adjusted to 7 using
NH
3
solution with vigorous stirring for 1 h. The
resulting mixture was heated on a hot plate with
continuous stirring; the solution boiled and
underwent dehydration, followed by decomposition,
with the evolution of large amounts of gases. The
produced powder was calcined by a gradual increase
of temperature up to 600 °C and was kept in air at
that temperature for 2 h to obtain 5 wt% Pd doped
NiO nanopowder.
2.2 Fabrication of Sensor Element
The fabrication of the H
2
S gas sensor was performed
as follows. The prepared nanopowder was mixed
with a suitable amount of adhesive (ethyl cellulose
and terpinol) and hand-ground in an agate mortar to
form a paste. The prepared paste was then laid
uniformly on the surfaces of ceramic tubes, with a
coating thickness of approximately 50 µm. The
sensor response S defined as S= (R
gas
-R
air
)/R
air
X
100% where, R
air
is the resistance of the sensor in
the air and R
gas
is the resistance of the sensor in the
presence of the test gas.
3 RESULT AND DISCUSSION
XRD patterns of the NiO and 5 wt% Pd doped NiO
precursor samples calcined at 600°C for 2 h, are
shown in Fig. 1 (a, b). As observed, all the
diffraction peaks can be well assigned to the cubic
structure with a space group of Fm3m, which are in
agreement with the standard JCPDS data (Card No.
73-1523). In Fig. 1 (b) the peaks emerged at 2θ
values of 34.62° and 55.43°, for the 5 wt% Pd doped
sample, indicates the Pd metal atoms are efficiently
dissolved in the NiO host lattice. The calculated
lattice parameter, a = 4.180 °A was well matched
with the standard lattice parameter values. Moreover,
the sharpness of the patterns corresponding to the
NiO and 5 wt% Pd doped NiO nanopowder
indicated that high levels of crystallization occurred.
The calculated average grain size well coincides
with the broadening of XRD peaks. The average
crystallite size of the 5 wt% Pd doped NiO
nanopowder is lower than the grain size for the NiO
nanopowder. Furthermore, BET surface area of the
NiO nanopowder is 45.25 m
2
/g and that of the 5
weight percent Pd doped NiO nanopowder is 56.35
m
2
/g. The prepared nanopowders were subjected to
gas sensor studies with test gas like H
2
S by
measuring sensor response as a function of various
operating temperature as shown in Fig.2. The
response increased linearly with increasing
temperature. The response of the 5 wt% Pd doped
NiO based sensor to 20 ppm of H
2
S gas was higher
(93%) than that of NiO, meaning that the 5 wt% Pd
doped NiO material was highly reactive for H
2
S gas.
Furthermore, the sensor based on NiO and 5 wt% Pd
doped NiO nanopowder exhibited a typical p-type
semiconducting nature, as there was a decreased in
resistance across sensor on exposure to the reducing
H
2
S gas. When the H
2
S gas was inleted, the overall
reaction of H
2
S and chemisorbed oxygen species as
follows (Balamurugan and Lee, 2015);
2H
2
S
(ads)
+ 3O
2
¯
(ads)
2H
2
O
(g)
+ 2SO
2
(g) + 3e¯
(1)
2H
2
S
(ads)
+ 4O¯
(ads)
2H
2
O
(g)
+ 2SO
2(g)
+ 4e¯
(2)
Chemically Modified NiO Gas Sensor for Environmental Monitoring
177
Figure 1: X-ray diffraction patterns of NiO precursor
calcined at 500 °C for 2h (a) and 5 wt% Pd doped NiO
nanopowder.
Figure 2: Response of NiO and 5 wt% Pd doped NiO
based sensor to H
2
S gas as a function of operating
temperature.
4 CONCLUSIONS
In this work, Pd-doped NiO nanopowders have been
presented as suitable semiconductor materials for
selective H
2
S detection. The sharp and single
diffraction peaks of XRD confirm the formation of
single-phase polycrystalline cubic NiO
nanomaterials. The gas sensing behaviour of NiO is
strongly dependent on the amount of Pd doped in
NiO. The sensitivity increased with increasing Pd
content and attained the maximum (93%) at 5 wt%
Pd doped NiO calcined at 600 °C for 2 h.
ACKNOWLEDGEMENTS
This work was supported by the International
Collaborative R&D Program through a KIAT grant
funded by the MOTIE (N0000894) and the National
Research Foundation of Korea (NRF) grant funded
by the Korea government (MSIP) (No.
2015R1A4A1041746) and the National Research
Foundation (NRF) grant
(No.2015R1A2A2A05001405) funded by the Korea
government.
REFERENCES
Balamurugan, C. (2015) ‘Perovskite hexagonal YMnO
3
nanopowder as p-type semiconductor gas sensor for
H
2
S detection’, Sensors and Actuators B Chemical,
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Struve, MF. (2001), ‘Neurotoxicological effects associated
with short-term exposure of Sprague-Dawley rats to
hydrogen sulfide’, Neurotoxicology, 22 (3), pp. 375-
385.
Yu, YG. (2010), ‘The first-principle calculation of H
2
S
adsorption and decomposition on the ZnO (0001)
surface’, Chinese Journal of Structural Chemistry,
29(8), pp. 1139-1146.
Zhang, Y. (2013), ‘Hydrogen sulfide, the next potent
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33(6) pp. 1104-1113.
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