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

Rajarshi Krishna Nath, Subhash Debnath, Indira Dey
Gurucharan University, Silchar, Assam India
DOI: https://dx.doi.org/10.51584/IJRIAS.2025.1010000097
2430 Published: 11 November 2025
ABSTRACT
A spray pyrolysis technique have been used to fabricate Zinc Oxide (ZnO) thin films using Zn (CH
3
COO)
2
as a
precursor solution. The structural, optical and electrical properties of the films are explored and then tested for
ethanol sensing. Structural studies show that the films are polycrystalline in nature, possessing “hexagonal
wurtzite” structure. A decrease of FWHM(Full Width at Half Maximum) with an increase in film thickness is
observed, which confirms the increase of crystallite size with an increase in film thickness. The optical bad gap
of the films was studied and a “Blue Shift” was observed from 2.97eV to 3.12eV as the film thickness was
decreased from 441nm to 172nm. The crystallite sizes obtained from XRD and TEM studies shows an increase
with a increase in film thickness. The gas sensing properties of the films have been studied using ethanol,
methanol, acetone and LPG at different concentrations and at different operating temperatures. It is observed
that thinner films show higher response for all the test gas/vapours.
Keywords: ZnO thi films, spray pyrolysis, blue shift, gas sensing.
INTRODUCTION
Zinc oxide (ZnO) is one of the most prominent metal- oxide n-type semiconductors of hexagonal (wurtzite)
structure with a direct band gap of about 3.37eV at room temperature. Because of its good electrical and optical
properties, thermal/chemical stability, abundance in nature, low cost and absence of toxicity, this material has
got wide applications in electronic and optoelectronic devices such as transparent conductors, solar cell windows,
gas sensors etc.[1-6]. For gas sensing purposes, this material has been investigated in various forms such as
single crystals, sintered pellets, thick films, thin films and heterojunctions [7-12]. However, thin films are more
suitable for such sensors because gas sensing properties are related to the material surface where the gases are
adsorbed and the surface reactions occur. This reaction modifies the concentration of charge carriers in the
material giving rise to a change in its electrical resistance, which is used for the purpose of gas detection. In
addition, solid state thin film gas sensors offer certain advantages compared to sintered pellets or thick film
sensors, i,e their fabrication process is compatible with microelectronic fabrication processes. In this paper, we
report a study on thickness dependent structural, optical, electrical and gas sensing properties of ZnO thin films
prepared by chemical spray pyrolysis technique.
Experimental Details.
ZnO thin films are deposited on to the glass substrates, which are cleaned with freshly prepared chromic acid,
detergent solution and distilled water. The schematic representation of the spray system is described elsewhere
[13]. The deposition method involves the decomposition of a solution of 0.1 M concentration of high purity zinc
acetate dehydrate (Merck, India) prepared in distilled water. The resulting solution is subsequently sprayed onto
heated substrate at a constant temperature of (410 20
0
C), which is monitored by a chromel alumel
thermocouple fitted close to the substrate with the help of a Motwane Digital Multimeter (Model: 454). The
atomization of the solution into a spray of fine droplets is affected by the spray nozzle with the help of
compressed air as carrier gas.
The thickness of the films is determined by the weight difference method using an electronic precision balance
(Citizen, model: CY 204). Structural analysis of the films is carried out using a PANalytical X’Pert Pro X-ray
diffractometer with CuKα radiation (λ = 1.5418 Ǻ) as an X-ray source at 40kV and 30mA in the scanning angle
INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)
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(2θ) from 30 to 70
0
with a scan speed 0.02
0
/ s. The optical transmission spectra of the films are obtained in the
UV/VIS/near IR region up to 1100 nm using Perkin Elmer UV-VIS spectrometer (Model: Lamda 35). For
making ohmic contacts at both the ends of the film, high conducting silver paste is used and is dried at a
temperature of 150
0
C. The film is mounted on a homemade two-probe assembly placed inside a silica tube,
which is inserted co-axially inside a resistance-heated furnace. The electrical resistance of the film is measured
before and after exposure to ethanol using a Keithly System Electrometer (Model: 6514)
Table 1. Process parameters for the deposition of ZnO thin films.
Spray parameters
Optimum value/ Item
Nozzle
Nozzle- Substrate distance
Solution concentration
Solvent
Solution flow rate
Carrier gas
Gas pressure
Substrate temperature
Angle of spraying
Glass
50 cm
0.1 M
Distilled water
5 ml/ min
Compressed air
2.8 kg / cm
2
(410 ± 20)
0
C
30- 45
0
RESULTS AND DISCUSSION
Formation of Zinc oxide (ZnO) thin film
When aerosol droplets arrive close to the heated glass substrates, a pyrolytic (endothermic) process takes place
and a highly adherent film is formed on the glass substrates. Possible reaction mechanism in ZnO film formation
is as follows. [14]
heat
Zn(CH
3
COO)
2
[solid near substrate]
→ 4 Zn(CH
3
COO)
2
[gas near substrate]
+ H
2
O
Adsorption
→ Zn
4
O(CH
3
COO)
6
[adsorbed/substrate]
+ 2CH
3
COOH
[gas near substrate]
↑
and Zn
4
O(CH
3
COO)
6
[adsorbed/substrate]
+ 3 H
2
O → 4ZnO
[film/substrate]
+6CH
3
COOH
[gas]
↑
Structural analysis
The X-ray diffractograms of ZnO films of two different thicknesses namely (172±14) nm and (225±18) nm is
shown in fig. 1.
Fig.1. XRD patterns of ZnO film of two different thickness.
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Table 2.shows the different XRD data obtained for the two films.
Film thickness
(nm)
Diffraction
planes ( hkl )
Diffraction
angle,2θ (deg)
Average crystallite
Size(nm)
Lattice
Strain (%)
172 ±14
002
34.4
41.69 ± 0.36
0.28
101
36.25
102
47.5
225 ±18
100
32
48.72 ± 0.42
0.23
002
34.6
101
36.45
102
47.75
110
56.85
GIXRD data of ZnO film of two different thickness viz.172 nm and 225 nm
It is observed from the fig.1 that the intensity of the peak increases due to an increase in film thickness. Further
a decrease of FWHM with an increase in film thickness is observed, which confirms the increase of crystallite
size with an increase in film thickness as reported by other [15]. Since the peak intensity and grain size are
associated with the crystallinity of the film, poor crystallinity in a thinner film could be associated with the
incomplete growth of the crystallites as only few atomic layers of disordered atoms constitute the bulk of the
film [15].
Transmission Electron Microscopic studies
The TEM images of ZnO thin film of two different thicknesses are displayed below
Fig.2. TEM images of ZnO
Film thickness: 172 nm Film thickness: 225 nm
The crystallite size obtained from TEM for the two ZnO films of thicknesses 172 nm and 225 nm are listed in
table 3.
Table 3. Crystallite size from TEM
Film thickness (nm)
Crystallite size (nm)
172±14
42.5
225±18
49.8
It is observed that the crystallite sizes obtained by XRD and TEM for the two films are quite similar. Both the
XRD and TEM studies reveal that the crystallite size decreases with a decrease in film thickness.
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Optical Studies
The optical transmission spectra of zinc oxide (ZnO) thin films of different thicknesses are shown in fig.3.
Fig.3. Optical transmission spectra of ZnO films of different thickness
These spectra show that for film having 398 nm thickness, the average transmission over the range 500-1100
nm exceeds 80 % with a sharp fall near the fundamental absorption; whereas fall in transmission is gradual for
the other films.
The optical absorbance spectra of the films are shown in figure 4.
Fig.4. Absorbance spectra of ZnO thin films of different thickness.
These spectra reveal that films grown under the same parametric conditions have low absorbance in the visible/
near infrared region while absorbance is high in the ultraviolet region. The absorption coefficient (α) was found
to follow the relation [13].
Plots of (αhν)
2
versus photon energy (hv) in the absorption region near the fundamental absorption edge is
shown in figure.5. In this case n = 2 gives the best linear graph which indicates direct allowed transition in the
film material as mentioned earlier.
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Fig.5. Plots of (αhυ)
2
vs hυ for the ZnO films of different thicknesses.
The optical energy gap, estimated from the extrapolation of the linear portion of the graph to the photon energy
axis, and its dependence on film thickness is illustrated in figure.6.
Fig.6. Optical energy gap Vs thickness of the film.
It is observed that optical band gap (E
g
) decreases with increasing film thickness. This may be due to the
structural defects in the films arisen during the time of their preparation, which could give rise to the allowed
states near the conduction band in the energy band gap [13]. In case of much thicker films, these allowed states
could well merge with the conduction band resulting in the reduction of the energy band gap.
The variation of band gap with film thickness is listed in table 4.
Table 4.Variation of band gap with film thickness
Film thickness (nm)
Optical band gap (eV)
398 ±15
3.12 ±0.01
641 ± 17
3.05 ±0.01
887 ± 21
2.97 ±0.01
The decreasing band gap with increasing thickness indicates a decrease in barrier height with an increase in
crystallite size [16].
Electrical studies
The electrical resistance, in order of MΩ, observed for our ZnO films at room temperature (300 K) can be
attributed to the large density of extrinsic traps at the grain boundaries due to oxygen chemisorption. These traps
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deplete the grains and result in a charge carrier barrier at the grain boundaries [17]. This effect can be more
relevant for small grain size. The variations in sheet resistance (R
□
, expressed in MΩ/□) of the ZnO thin films
with temperature is shown in fig.7.
Fig.7.Variations in sheet resistance of the ZnO thin films with temperature.
For all the films, the sheet resistance is found to decrease with increasing temperature as well as film thickness
in the temperature range 300-500 K. The decrease in resistance is related to the increase in carrier concentration
resulting from activation of deep and shallow donors which may arise due to native defects such as interstitial
zinc atoms and oxygen vacancies [18].
The inverse absolute temperature dependence of the electrical conductivity of the films is shown in fig.3.16.The
activation energy was obtained using the relation [13].
σ = σ
O
exp (–E/kT)
where σ is the electrical conductivity at any temperature, σ
O
a constant, E the activation energy for conduction,k
the Boltzmann constant and T the absolute temperature
Fig.8. Plot of lnσ vs 1000/T for the ZnO films of different thickness
.
The conductivity studies on films show that all the films exhibit two activation energies at different temperature
regions. These activation energies vary with film thickness and are listed in table 5.
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Table .5. Variation of activation energy with thickness for ZnO thin films.
Film thickness (nm)
Activation energy
E
1
(eV)[300–400K]
E
2
(eV)[400–500K]
398 ± 15
0.034
0.468
641± 17
0.033
0.323
887± 21
0.012
0.226
The results indicate the presence of two donor levels, one deep and one shallow near the bottom of the conduction
band. Both of these levels depend on the film thickness, and are found to decrease with increasing thickness
[13].
Gas Sensing Properties
The basic principle of semiconductor gas sensors is the change in resistance of the sensor material that arises
from the change in electron concentration near the sensor surface by the reaction with gases or vapours. The
sensing characteristics of the ZnO thin film of two different thickness viz 225 nm and 172 nm as a function of
operating temperature for three different concentrations namely 100 ppm, 300 ppm and 500 ppm of ethanol is
shown in fig 9.
Fig. 9. Sensitivity characteristics of ZnO thin film of two different thickness (225 nm and 172 nm) as a function
of operating temperature for three different concentrations(100 ppm, 300 ppm and 500 ppm) of ethanol.
It is seen that for concentrations of ethanol in air, the sensitivity of ZnO thin film increases up to 550 K and then
decreases with further increase in temperature. It is observed that compared to the film of 225 nm thickess,
sensitivity increases highly in case of 172 nm thickness. The maximum sensitivity for 225 nm thickness is found
to be ~ 38% where as for 172 nm thickness, it is found to be ~98%. Thsis is atributted to the fact that film of 172
nm thickness has smaller crystallite size than that of 225 nm thickness as obsered in case of XRD and TEM
studies. As we know, smaller the crystallite size, the larger the specific surface area (S/V), which results in
greater oxygen adsorption and higher sensitivity. Similar phenomenon have been observed in case of other tested
gases /vapours namely LPG, methanol, acetone etc.
CONCLUSION
A study on the sprayed ZnO thin film has been carried out to inestigate the effect of thickness variation on
structrual, optical, electrical and gas sensing properties. XRD studies show that the films are polycrystalline in
nature with a decrease of FWHM with an increase of film thickness, which confirms the increease of crystallite
size with an increase in film thickness. The decrease in crystallite size with the decrease in film thickness has
also been cross checked by TEM studies. The UV-IS spectroscopic studies reveal that optical band gap (E
g
)
decreases with an increase in film thickness. It is also observed that the sensitivity of the film towards a particular
gas/apour increeases with a decrease in film thickness.
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ACKNOWLEDGEMENT
The authors wish to acknowledge Dr. P.P.Sahay, Department of Physics, MNNIT, Allahabad, UP, India for his
technical assistance in this work.
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