Zinc Oxide Nanostructure Thick Films as H<sub>2</sub>S Gas Sensors at Room Temperature

doi:10.4236/jst.2013.33006

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Journal of Sensor Technology, 2013, 3, 31-35

http://dx.doi.org/10.4236/jst.2013.33006 Published Online September 2013 (http://www.scirp.org/journal/jst)

Zinc Oxide Nanostructure Thick Films as

H2S Gas Sensors at Room Temperature

Vinod S. Kalyamwar1*, Fulsingh C. Raghuwanshi2, Narayan L. Jadhao1, Anil J. Gadewar1

1Bharatiya Mahavidyalaya, Amravati, India

2Vidyabharati Mahavidyalaya, Amravati, India

Email: *[email protected]

Received August 29, 2012; revised September 29, 2012; accepted October 6, 2012

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

The ZnO nanostructures have been synthesized and studied as the sensing element for the detection of H2S. The ZnO

nanostructures were synthesized by hydrothermal method followed by sonication for different interval of time i.e. 30,

60, 90 and 120 min. By using screen printing method, thick films of synthesized ZnO nanostructure were deposited on

glass substrate. Gas sensing properties of ZnO nanostructure thick films were studied for low concentratio n H2S gas at

room temperature. The effects of morphology of synthesized ZnO nanostructure on gas sensing properties were studied

and discussed. ZnO nanostructure synthesized by this method can be used as a promising material for semiconductor

gas sensor to detect poisonous gas like H2S at room temperature with high sensitivity and selectivity.

Keywords: ZnO Nanostructure; Room Temperature Gas Sensor for H2S

1. Introduction

Nanostructured materials such as WO3, ZnO, SnO2, and

V2O5 have shown good sensing properties [1-7]. Among

these nanostructure-semiconducting materials, ZnO has

been studied extensively for gas sensing application. It

has been proved that Zn O is a good gas sen s itive material

for detection of both reducing and oxidizing gases [8-16].

Various gases have been tested for ZnO nanostructure

sensor studied including ethanol, acetone, NO2, NH3, H2

and CO2 and hydrogen [17-22].

H2S is a toxic gas produced from the coal, oil and

natural gas industries. In order to enhance the sensitivity

and selectivity o f H2S, many attempts were made to sys-

temize nanostructure ZnO with different morphologies

[23-27]. However, there are very few reports on ZnO

nanostructure based room temperature H2S sensors.

In the present work, efforts were made to synthesize

ZnO nanostructure with innovative morphology by hy-

drothermal route. The synthesized ZnO shows high sen-

sitivity with fast response and recovery for low concen-

tration of H2S ga s .

2. Experimental

2.1. Synthesis of ZnO Nanostructure

All chemicals were of analytical grade and used as pur-

chased with out further purification.

In present work, 5.2 g of Zinc acetate dehydrate was

dissolved in 480 ml of distilled water. Subsequently, 20

ml of 2 M NaoH aqueous solution was introduced into the

above aqueous solution drop by drop with constant stir-

ring. The obtained mixture was kept at room temperature

for 5 min. and transferred into 700 ml Teflon-lined

stainless steel reactor (autoclave), maintained at tem-

perature 120˚C. After 6 hr, allow it to cool to room tem-

perature naturally and the resultant white solution were

collected in a beaker. The obtained white solution were

sonicated (Ultrasonic wave treatment) for different in-

terval of time say 30, 60, 90 and 120 min with pulse rate

4 s and power 0.7 A. The resultant products were col-

lected by centrifugation, washed several times with dis-

tilled water and ethanol, dried at temperature 70˚C for 3

hr. The obtained powder which was sonicated for 30, 60,

90 and 120 min were termed as 30 min ZnO, 60 min ZnO,

90 min ZnO and 120 min ZnO respectively.

2.2. Preparation of Thick Films

Thick films of synthesized nanostructure ZnO were pre-

pared by using screen printing technique. In present

process, thixotropic paste was formulated by mixing the

synthesized ZnO powder with ethyl cellulose (a tempo-

rary binder) in a mixture of organic solvents such as butyl

*Corresponding autho

V. S. KALYAMWAR ET AL.

cellulose, butyl carbitol acetate and turpineol. The ratio

of ZnO to ethyl cellulose was kept at 95:05. The ratio of

inorganic to organic part was kept as 75:25 in formulat-

ing the pastes. The thixotropic pastes were screen printed

on a glass substrate in desired patterns. The films pre-

pared were fired at 500˚C for 12 hr. Prepared thick films

were called as pure ZnO thick films.

3. Materials Characterization

3.1. Thickness Measurement

Thickness of all ZnO thick films were measured by using

technique “Marutek film Thickness Measurement Sys-

tem” with the help of provided equipment. The thick-

nesses of all films were observed in the range from 31 to

35 µm. Thick films of approximately uniform thick-

nesses we re used for f urther c ha r a ct e ri z a ti on.

3.2. X-Ray Diffraction Studies

The crystallographic structure of the all synthesized ZnO

nanostructure was characterized by powder X-ray diffrac-

tion (Philips X-ray diffractometer) with cu source and 2θ

range of 20˚ - 80˚. Figure 1 shows the XRD pattern of

the 90 min ZnO nanostructure. The recorded XRD pat-

tern confirmed that synthesized ZnO are high crystalline

in nature. The corresponding X-ray diffraction peak for

(100), (002), (101) and (102) planes confirm the forma-

tion of hexagonal wurtzite structure of ZnO (JCPDS card

no. 36 - 1451)). Similarly, XRD pattern of 30 min ZnO,

60 min ZnO and 120 min ZnO shows similar result with

different half width full maxima (not shown in this arti-

cle).

The domain size of the crystal can be estimated from

the full width at half maximum (FWHM) of the peaks by

means of the Scherrer formula,

sin









where λ is the wavelength of incident beam (1.5406 Å), β

is the FWHM of the peak in radians, θ is the diffraction

angle and K is Scherrer constant. The average crystallite

size was calculated from (101) peak of 90 min ZnO is

found to be 17 nm.

3.3. Transmission Electron Microscope

By using Transmission Electron Microscope (TEM), the

morphology and structure of the synthesized powders

were investigated. TEM images show that the synthesi-

zed ZnO consists of flower like structure. Figure 2(c)

shows the ZnO sonicated for 90 min, shows that the cry-

stallite size is very less as compared to the other ZnO

sonicated for different time interval (Figures 2(a), (b)

and (d)).

4. Gas Sensing Properties

The gas response of the sensor was defied as the ratio of

the change in conductance of a sample upon exposure to

the target gas to the original conductance in air. Figure 3

shows the gas responses of ZnO thick films to 25 ppm

H2S at operating temperature. Th is high response of ZnO

thick film to H2S may be due to the interaction of ZnO

with H2S, forming ZnS [28]. ZnS exhibits higher elec-

Position [˚2 Theta]

30 40506070

Count s

100

400

(100) (002)

(101)

(102)

(110)

(103)

(200)

(112)

Zn O

Zn O Zn O

14 - 6 h

Figure 1. Powder XRD pattern of 90 min ZnO nanostructure synthesized by hydrothermal route.

V. S. KALYAMWAR ET AL. 33

(a)

(d)(c)

(b)

Figure 2. TEM image (a) 30 min ZnO; (b) 60 min ZnO; (c) 90 min ZnO; (d) 120 min ZnO.

Figure 3. Gas responses of ZnO nanostructure thic k films.

tronic conductivity as compared to ZnO.

Figure 3 also indicates that 90 min ZnO have maxi-

mum gas response (433) whereas 30 min ZnO has mini-

mum gas response (21) to low concentration H2S. The

higher response of 90 min ZnO nanostructure upon ex-

posure to H2S may be attributed to the decrease in con-

centration of oxygen ad so rben ts () and a resulting in-

O

crease in concentration of electron.

The gas response was mainly dependent upon two

factors. The first was the amount of active sites for oxy-

gen and the reducing gases on the surface of the sensor

materials. It is seen form TEM images (Figure 2(c)) that

the surface of 90 min ZnO rougher than that of other

thick films. The surfaces of 90 min ZnO contain more

active sites than of other thick films. This could explain

why the response of 90 min ZnO thick films was higher

than other t hi c k fi lms.

5. Conclusion

In summary, sensors were fabricated with ZnO nanos-

tructures, which were synthesized by a hydrothermal

method followed by sonication, and their gas sensing

properties were measured. The results demonstrated that

90 min ZnO is very sensitive to low concentration H2S.

Such nanomaterials with innovative structure can be used

V. S. KALYAMWAR ET AL.

for gas sensors to monitor hazards gas like H2S.

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