HYDROGEN GAS SENSORS BASED ON ZnO THIN FILMS

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International Journal of Semiconductor
Science & Technology (IJSST)
ISSN(P): 2250-1576; ISSN(E): 2278-9405
Vol. 4, Issue 5, Oct 2014, 5-14
© TJPRC Pvt. Ltd.
HYDROGEN GAS SENSORS BASED ON ZnO THIN FILMS
SABAH IBRAHIM ABBAS
Department of Physics, College of Science, University of Wasit, Iraq
ABSTRACT
Nanocrystalline (ZnO) thin films were synthesized by chemical spray pyrolysis technique, a 0.1 M Spray solution
is prepared by dissolving Zinc Chloride (ZnCL2) in 100 ml distilled water were deposited on glass substrates heated 450
. The crystal structure and morphology of synthesized films were characterized by X-ray diffraction (XRD) and Atomic
Force Microscope (AFM). The crystallite size was evaluated to be 48.599 nm by using Scherrer’s equation and the optical
absorption spectra were taken to determine the band gap 3.2 nm eV increase in band gap energy is revealed that
nanostructure films. The gas sensing properties of the ZnO films were investigated towards the reducing gas such as H2.
The sample showed the maximum gas sensitivity (64.65%) at operating temperature 300 with fast response time 6 s and
recovery time 57 s respectively.
KEYWORDS: Spray Pyrolysis, ZnO Thin Films, H2 Gas Sensing, Sensitivity
INTRODUCTION
Zinc Oxide is one of the compound semiconductors of the II-VI family with a direct band gap of 3.37 eV at room
temperature, and a large excitation binding energy (60 meV) [1]. ZnO Posses potential material for many devices
application such as gas sensor, solar cells, high density data storage devices, UV detectors, optoelectronic devices,
photoluminescent devices [2-4]. Several deposition techniques have been used to grow ZnO thin films. These include
sputtering, spray pyrolysis, sol- gel process, pulse laser deposition, plasma enhanced chemical vapour deposition, vacuum
evaporation, electron beam evaporation, low pressure metal organic chemical vapour deposition (MOCVD) [5,6]. In 1962
Seiyama et al discovered that the electric conductivity of ZnO changed dramatically by the presence of reactive gases in
the air [7]. Thin films of ZnO are expected to exhibit a high degree of gas sensitivity, because the sensing mechanism
involves chemisorptions followed by charge transfer at the surface leading to a change resistance of the sensor element [8].
Generally, ZnO sensors provide a wide variety of advantages, such as low cost, short response time, easy manufacturing,
and small in size, compared with the traditional analytical instruments [9]. Among the semiconductor metal oxides ZnO
was one of the earliest discovered and is the most widely applied oxide gas sensing materials due to its high mobility of
conduction and good chemical and thermal stability under operating condition. ZnO gas sensor have been fabricated in
various forms, including single crystals, sintered pellets, thick films, thin films, and heterojunctions which were studied to
detect H2, NO2, NH3, CH4, O2, CO, and ethanol [10].
In this work, a hydrogen gas sensing device has been designed, fabricated by using chemical spray pyrolysis
deposition technique on glass substrates and tested for H2 gas sensing. X- Ray diffraction (XRD) and Scanning Probe
Microscope (SPM) and resistance measurement are used to characterize the microstructure and electrical properties of ZnO
gas sensor.

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Impact Factor (JCC): 3.8869 Index Copernicus Value (ICV): 3.0
Experimental Work
The ZnO thin films were prepared by chemical spray pyrolysis technique. The films were deposited on glass
substrates heated to (450°C). A 0.1 M Spray solution is prepared by dissolving Zinc Chloride (ZnCL2) of 98% purity in
100 ml distilled water. The above mixture solution was placed in the flask of the atomizer and spread by controllable
pressurized nitrogen gas flow on the heated substrats. The spraying time was 4 seconds, which is controlled by adjustable
solenoid valve. The heated substrate was left for 20 sec after each spraying run to give time for the deposited (ZnO) layer.
The optimum experimental conditions for obtaining homogeneous ZnO thin film at (450°C) were determined by the
spraying time, the drying time and the flashing gas pressure [11]. The preparation of Zinc Oxide thin films depends on
surface hydrolysis of metal chloride according to the following chemical reaction equation [12].
(1)
Optical interferometer method is used for measuring the thickness of the film by employing He –Ne laser
(0.632 µm) wavelength with incident angle 450
. This method depends on the interference of the laser beam reflected from
thin film surface and the substrate. The structural of the thin films were examined by X-ray diffractometer
(6000-Shimadzu) using CuKα radiation with a wavelength, λ=1.54060 Å, voltage 30 kV, current 15 mA.
The morphological of the films were analyzed using scanning Probe Microscope (SPM, model AA3000 Angstrom
Advanced. Inc). The optical absorbance of the films was measured using UV-visible spectrophotometer (SP-3000 Optima)
in the wavelength range 200-1200 nm at room temperature. Interdigitated Aluminum ohmic metal contacts are deposited
on the ZnO films using vacuum evaporation technique. The mask of (1mm) electrode spacing was used to deposit the
electrical electrode on the ZnO layer surface. The gas sensing chamber had been employed for testing of these films to
gases. A fixed voltage of 6 V was applied across the films. The current was measured using a Scope digital multimeters
(UT81).
RESULTS AND DISCUSSIONS
The X-ray diffraction (XRD) pattern of the 400 nm thick ZnO thin films were deposited on glass substrates heated
to (450°C) is shown in figure 1. From this figure, the well –crystallized diffraction peaks were observed with crystal planes
(002), (101), and (102). The film is crystallized in the hexagonal wurtzite phase and presents a preferential orientation
along the c-axis indicated by the plane (002). The result is a good agreement with data mentioned in the literature
(JCPDF card no 36-1451) [13]. The strongest peak, observed at can be attributed to the (002) plane of
the hexagonal ZnO. Another major orientations like (101) and (102) at and . The mechanism
of formation of the c-axis preferentially oriented ZnO thin films can be suggested that the value of surface energy is
minimum for the ZnO (002) plane at the growth stage [14]. The grain size (D) of film is 48.599 nm were calculated by
using the Scherrer’s equation [15].
(2)
Where is the wavelength of the X-Ray radiation used, θ the Bragg diffraction angle of the XRD
peak and β is the measured broadening of the diffraction line peak at an angle of 2θ, at half its maximum intensity
(FWHM) in radian.

3. Hydrogen Gas Sensors Based on ZnO Thin Films 7
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Figure 1: XRD Patterns of ZnO Thin Films (d= 400 nm) Deposition at Temperature 450 °C
The surface morphology of the samples was investigated using AFM and shown in Figure 2. The image
(micrograph) is taken over scale of 2 х 2 . It is observed that particles on the surface are irregular and inhomogeneous
nature, with some voids which arises from the aggregation of grains due to the movement of grain boundaries [16].
The root mean square (rms) of the film surface roughness is 0.206 nm, the surface of ZnO film becomes smoother again.
The higher sensitivity results from an increase in the roughness of the sensing film [17]. The grain value 89.93 nm is found
to be larger than the grain size calculated from XRD data. This may be due to the aggregation of smaller grains which from
bigger clusters on the surface of the films.
Figure 2: AFM Surface Morphology of ZnO Film (Scan Area 2 х 2 ) Deposited on Glass Substrat
Figure 3 Shows the transmission spectra in the wavelength range of 350 to 1000 nm. From the spectra, it is clear
that film exhibit a transmission of about 85% in the visible range. Transmission however falls very sharply in the UV
region due to onset of fundamental absorption. The optical transmittance of film is known to depend strongly on its surface

4. 8 Sabah Ibrahim Abbas
Impact Factor (JCC): 3.8869 Index Copernicus Value (ICV): 3.0
morphology [16].
The optical energy bandgap of the films were derived assuming a direct transition between the edges of the
valence and the conduction band, using the tauc relationship as follows [18].
( (3)
Where α is the absorption coefficient, A is a constant, h is planck’s constant, ν is the photon frequency, the value
of n is 1/2 or 2 depending on presence of the direct and indirect allowed transitions. Figure 4. Shows the plot of
on the y–axis versus photon energy (hν) on the x-axis gives the value of band gap . An extrapolation of the linear part of
the above plot (αhν) 2
= 0 gives the energy gap value of ZnO film, which was found to be about (3.2 eV), at room
temperature, Which is agreement with Shingo et al [19].
Figure 3: Optical Transmittance Spectra of ZnO Thin Film on the Glass Substrate
However, the band- edge of ZnO nano particle was blue -shifted by quantum confinement effect.
Figure 4: Plots of versus hν of ZnO Thin Film on the Glass Substrate

5. Hydrogen Gas Sensors Based on ZnO Thin Films 9
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For gas sensing was used a vacuum –tight stainless steel cylindrical test chamber of diameter 21.8 cm and of
height 16.4 cm with the bottom base made removable and of O – ring sealed. The sensor was inserted in the central portion
of the cylinder with a 6 V bias voltage was applied across the films. The gas sensitivity tests at room temperature by
introducing 2% H2: air mixing volume ratio show no variation on the film resistance, the increase in the operation
temperature leads to an improvement of the films sensitivity. Figure 5. It is seen that the film maximum conductance goes
through a maximum on changing Temperature, with the best operating temperature at around 300 . The maximum peak
values at certain temperature called optimal temperature and then decreases with further increase in temperature.
The increase of conductance (the left of the maximum) results from an increase in the rate of surface reaction of the target
gas, while the decrease of conductance (the right side) results from a decrease in the utility of the gas sensing layer. At the
maximum conductance (response),
Figure 5: The Variation of Sensitivity with the Operating Temperature of ZnO Gas Sensors with 2% H2: Air
Mixing Volume Ratio and 6 Volt Bias Voltage
The target gas molecules have optimum penetration depth into the gas sensing grain i.e., optimum reactivity for
diffusion in the whole sensing layer, as well as for exerting sufficiently large interaction with surface. This explains the
correlations between conductance or sensitivity and temperature take a volcano shape for semiconductor gas sensors [20].
Figure 6. Shows that the gas sensing characteristics of the as-sprayed ZnO films are carried out for H2 reducing gas which
was kept constant at 2% in atmospheric air with different operating temperatures. The sensitivity (S) of the sensor was
calculated after the response had reached steady state condition by using equation [3].
(4)
Where, Ra is the resistance of the film in the air and Rg is the resistance of the film under exposed gas. It is evident
from the figure that the maximum value of sensitivity is (64.65%) at 300 with a fast response time of 6s and a baseline
recovery time of 57s.

6. 10 Sabah Ibrahim Abbas
Impact Factor (JCC): 3.8869 Index Copernicus Value (ICV): 3.0
The response time was defined as the time taken for sensor to attain 90% of maximum change in resistance or
Conductance upon exposure to H2. The time taken by the sensors to get back 90% of original resistance or conductance is
the recovery time, ( [21].
Figure 6: The Variation of Sensitivity versus Time at Different Operating Temperature of ZnO Gas Sensors with
2% H2: Air Mixing Volume Ratio and 6 Volt Bias Voltage
The gas sensing mechanism depends on the surface reaction between chemisorbed oxygen and reducing gases.
Adsorption of O2 on ZnO films the oxygen molecules extracts electrons from the conduction band to form oxygen ions.
That may lead to the formation of an electron depletion region near surface, which can greatly increase the resistance due
to the decrease of net carrier density. In the presence of a reducing gas as H2 a chemical reaction between gas molecules
and negatively charged adsorbed oxygen species ( leads to electron transfer back into the surface, the released
electrons will reduce the thickness of the depletion region, and decrease the resistance of the semiconductors.
The fundamental sensing mechanism of metal –oxide based gas sensor relies upon this change in electrical conductivity in
ambient gases. These processes are generically expressed by the reactions below [22].
(5)
(6)
Figure 7 Shows the response of ZnO thin film towards hydrogen gas of different mixing ratios has been explored.
It is apparent from figure, the sensor current to hydrogen gas increases linearly with H2 test gas mixing ratio.
The sensitivity tends to saturate in the high gas concentration. This may be due to a saturation of adsorption of H2 atoms at
the Al electrode / ZnO nanofilm interface and lack of adsorbed oxygen ions at the nanofilm surface to react with gas
molecules [23]. The variation of sensor sensitivity S, with test mixing ratio is illustrated in figure 8. The figure displays
that the maximum sensitivity obtained is 58.55% at 3% H2 gas concentration.

7. Hydrogen Gas Sensors Based on ZnO Thin Films 11
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Figure 7: Sensing Behavior of ZnO Gas Sensor to Different H2: Air Mixing Ratios (1%, 2%, 3%), the Tests were
Performed at 300 Temperature and the Bias Voltage 6 Volt
Figure 8: Sensitivity versus H2 Gas Concentration of ZnO Thin Film
CONCLUSIONS
ZnO thin films were successfully synthesized by chemical spray pyrolysis. The XRD studies reveled the
formation of nanocrystalline in the hexagonal wurtzite phase and presents a preferential orientation along the c-axis
indicated to the plane (002). AFM investigations that the average grain size is found to be larger than the grain size
calculated from XRD data. This may be due to the aggregation of smaller grains which from bigger clusters on the surface
of the films. The hydrogen gas sensing results show the sensitivity of the films increases with temperature, reaches a
maximum value at 300 and then decreases.
ACKNOWLEDGMENTS
The author would like to thank the members of Material Physics & Chemistry Research Establishment labs at the
Ministry of Science and Technology of Iraq.

8. 12 Sabah Ibrahim Abbas
Impact Factor (JCC): 3.8869 Index Copernicus Value (ICV): 3.0
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