EFFICIENT GAS SENSOR ZNO THIN FILMS: SYNTHESIS
AND CHARACTERIZATIONS
Department of chemistry
Annasaheb Dange college of engineering and technology,
Ashta
Abstract:
ZnO thin films were prepared by chemical bath deposition technique and
characterized by X-ray diffraction and UV-vis spectrophotometrically. The optical band
gap energy for ZnO was found to be 3.28eV. The crystallite size calculated from XRD
pattern, which was 25nm. The gas sensitivity of the ZnO thin films estimated using
ammonia as a model gas. The prepared ZnO showed better gas sensitivity at room
temperature.
KEYWORDS:
ZnO thin films, XRD, Gas sensitivity.
INTRODUCTION
Nanosized inorganic semiconductor materials have attracted considerable efforts in recent years
zinc oxide (ZnO) is an extensively studied material with a wide band-gap of 3.37 eV. ZnO is an important
multifunctional material with various applications such as gas sensors, catalyst, transparent conducting
oxide devices, UV protector, optoelectronic, magnetoelectronic devices, and electronic devices etc.
Moreover, ZnO is one of the well recognized gas sensing oxides, which has been widely considered for
detection of ignitable and poisonous gases, such as CH4, NO2, liquefied petroleum gas (LPG), acetone,
methanol, ethanol, toluene, xylene, propane, triethylamine, and CO [1]. A ZnO thin film shows a high
degree of gas sensitivity because of their high surface to volume ratio. Nanocrystalline ZnO is available in
the form of sol-gel, heterojunction, single crystals. The synthesis of nano particles with different shape and
morphology has a strong interest; due to the properties depend on their structure, shape, size, and phase.
Cho et al. reported a 1.8 fold decrease in the well dispersed ZnO nanorods at 1 ppm NO2[2]. ZnO nanorods
are also promising candidates for detecting extremely low concentrations of H2S was observed by Wang et
al.[3]. Ammonia is a poisonous gas that is naturally present in the atmosphere in sub-ppb levels. A large
amount of ammonia is produced by the chemical industry for the production of fertilizers or for use in
refrigeration systems. Ammonia sensors are needed for the early detection of possible leakages from such
systems.
Here, we have reported the synthesis of a cost effective, low temperature, and high sensitivity
methane gas sensor ZnO thin films by chemical bath deposition method. ZnO characterized by XRD and
UV-vis absorption. Gas sensing properties estimated using ammonia vapors present in the headspace of the
ammonium hydroxide solutions, as a testing model for the testing the gas sensing properties of ZnO thin
films.
Experimental
The ZnO nanoparticles were prepared by chemical bath deposition method at room temperature.
Zinc acetate (Zn(CH3COO)2.2H2O), and sodium dodecyl sulphate (SDS) were used as starting materials.
0.2 M zinc acetate solution was prepared by dissolving 1 g of zinc acetate in 25 mL of water. The solution
was stirred at room temperature, and then the 5mL SDS was added to the solution and again stirred for 3
hour at room temperature. ZnO thin films we prepared by dip-coating technique and then dried at room
temperature. The dried thin films were calcined at 673 K in an electric furnace.
Result and discussion
X-ray diffraction
Figure 1 shows the XRD pattern of ZnO thin film. The crystallographic structure of the synthetic
ZnO thin films were characterized by powder X-ray diffraction with a CuK source and 2è range of 30-80?.
The XRD pattern shows that all the peaks can be indexed to hexagonal wurtzite ZnO (JCPDS card No. 79-
2205). The average crystallite size were calculated from the full width at half maximum (FWHM) of the
peaks by using the Scherrer's formula, D = 0.94ë / (âcosè), where ë the X-ray wavelength, â the peak width
of half-maximum, and è is the Bragg diffraction angle. The average crystallite size calculated from (110)
peaks was approximately 25 nm.
Figure XRD pattern of ZnO thin films
UV-vis absorption measurement
The wavelength of 380 nm corresponds to the bulk band-edge of 3.2 eV for ZnO. Figure 2
shows the plot of (ahn)1/2 vs. photon energy hn of ZnO nanoparticles. The band gap energy value for
ZnO samples was calculated from this plot. The quantum confinement effect from the small particles
size of 25 nm as found in XRD analyses can be correlated with the blue shift in the optical absorption
wavelength at 360 nm.
Figure 2. Band gap energy of ZnO thin films
Gas sensing properties
The heater was fixed on the base plate for heating the sample. The current flowing through the
heating element was examined using a relay switch with adjustable ON and OFF time intervals. Here,
the temperature indicator was connected to output of the thermocouple. A Cr-Al thermocouple was used
to sense the operating temperature of the sensor. A valve was fitted at one of the ports of the base plate
for gas inlet. A constant voltage was applied to the sensor, and current was measured by a digital Picoammeter. After each NH3 gas exposure cycle air was allowed to pass into the glass dome.
Here, we have used ammonia vapors present in the headspace of the ammonium hydroxide
solutions, as a testing model for the testing the gas sensing properties of ZnO thin films. Oxygen
vacancies in ZnO function as n-type donors on oxide surfaces and are electrically and chemically
active. It was found that resistance change SR = increased linearly with ammonia gas concentration,
where Rair the resistance of the sensor in dry air and Rgas is resistance in the test gas. The gas
sensitivity of the ZnO thin films was obtained using relationship: S= SR/R. After the exposure to
ammonia, sensor was maintained for a recovering period in dry air. Figure 3 shows the room
temperature sensitivity of the ZnO thin films at about ~30000 ppm of ammonia. The sensor response
was also tested at different temperatures. The gas sensor shows three cycles reproducibility over
alternating exposure to ammonia in the headspace and dry air. These observations revealed that along
with the excellent performance reproducibility, the sensor also exhibited good shelf life.
Figure 3. Room temperature sensitivity of ZnO thin films for Nh3
Conclusion
ZnO thin films were synthesized by chemical bath deposition method and the sensing
properties of these ZnO thin films were studied by investigation their electrical response towards
ammonia vapours as a testing model. ZnO thin films shows better gas sensitivity at room temperature.
Refer?ences
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P.S. Cho, K.W. Kim, J.H. Lee, NO2 sensing characteristics of ZnO nanords prepared by hydrothermal method, Journal of Electroceramics, 17 (2006) 975.
C. H. Wang, X. F. Chu, and M. W. Wu, Detection of H2S down to ppb levels at room temperature using sensors based on ZnO nanorods, Sensors and Actuators B-Chemical, 113 (2006) 320. J. Buha (2007). Crystal Growth & Design 7. 113. ?L. Guo, Y. L. Ji, H. Xu, P. Simon, Z. Wu (2002). J. Am. Chem. Soc. 124. 14864. ?C. Xu, G. Xu, Y. Liu and G. Wang (2000). Solid State Commun. 122. 175. ?S. Chakarbarti and G. Chaudhari (2004). Mater. Chem. Phys. 87. 196.
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