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

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

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

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

(93%) at an operating temperature of 60 °C.

1 INTRODUCTION

Hydrogen sulfide (H

S) is a colorless, poisonous,

flammable gas with the characteristic foul odour of

rotten eggs. H

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

S can

cause death (Struve at al., 2001). Health effect also

have found in human longer exposed to low-level

concentration of H

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

S and act as a signalling molecules.

Moreover, H

S produced endogenously in mammals,

including human and has various physiological

effect on the human body. For example, H

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

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

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

S level.

However, semiconductor metal oxide is one of the

most alternative ways for H

S detection applications.

Semiconductor metal oxides such as ZnO, CuO,

SnO

, and In

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

(NiO). In this work, we describe the fabrication,

characterization and application of semiconductor

NiO nanopowder and different weight % Pd doped

NiO nanopowder for H

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

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

S (93%) at an

operating temperature of 60 °C. The selectivity of

the sensor elements for H

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

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

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

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

/g and that of the 5

weight percent Pd doped NiO nanopowder is 56.35

/g. The prepared nanopowders were subjected to

gas sensor studies with test gas like H

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

S gas was higher

(93%) than that of NiO, meaning that the 5 wt% Pd

doped NiO material was highly reactive for H

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

S gas. When the H

S gas was inleted, the overall

reaction of H

S and chemisorbed oxygen species as

follows (Balamurugan and Lee, 2015);

(ads)

+ 3O

(ads)

→ 2H

(g)

+ 2SO

(g) + 3e¯

(1)

(ads)

+ 4O¯

(ads)

→ 2H

(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

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

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

nanopowder as p-type semiconductor gas sensor for

S detection’, Sensors and Actuators B Chemical,

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Struve, MF. (2001), ‘Neurotoxicological effects associated

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hydrogen sulfide’, Neurotoxicology, 22 (3), pp. 375-

385.

Yu, YG. (2010), ‘The first-principle calculation of H

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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