In this report, the ZnO nanostructure films have been  deposited by ultrasonic spray pyrolysis technique (USP) on fluorine doped  tinoxide~(FTO) substrate using zinc acetate and aqueous 2-propanol as  reactants. The effects of solution spray rate on structural, morphological  and optical properties of ZnO nanostructure films were investigated by X-ray  diffraction (XRD), atomic force microscopy (AFM) and UV-Vis measurements.  ZnO films were crystallized in the hexagonal wurtzite phase and preferential  orientation changed with changing of spray rate. Grain size and roughness of  films depended on spray rate. Increasing spray rate made the films grown  with bigger grain size and higher roughness. Band gap energy was determined  from the UV-Vis. Its value was found to be of 3.25 eV and was almost  independent on the spray rate

Anh, Luu Thi Lan

Bac, Luong Huu

Hieu, Nguyen Duc

Hung, Pham Phi

Neto, Mateus Manuel

Son, Vo Thach

Thai, Tran Thanh

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 24, No. 3S1 (2014), pp. 108-112
DOI:10.15625/0868-3166/24/3S1/5236
INFLUENCE OF SPRAY RATE ON STRUCTURAL AND OPTICAL
PROPERTIES OF SPRAYED ZnO FILMS
LUU THI LAN ANH, LUONG HUU BAC, PHAM PHI HUNG, NGUYEN DUC HIEU,
MATEUS MANUEL NETO, VO THACH SON,
Hanoi University of Science and Technology
TRAN THANH THAI
Quy Nhon University
E-mail: phamthetansp@gmail.com
Received 20 June 2014
Accepted for publication 20 August 2014
Abstract. In this report, the ZnO nanostructure films have been deposited by ultrasonic spray pyrolysis technique
(USP) on fluorine doped tinoxide (FTO) substrate using zinc acetate and aqueous 2-propanol as reactants. The effects
of solution spray rate on structural, morphological and optical properties of ZnO nanostructure films were investigated
by X-ray diffraction (XRD), atomic force microscopy (AFM) and UV-Vis measurements. ZnO films were crystallized
in the hexagonal wurtzite phase and preferential orientation changed with changing of spray rate. Grain size and
roughness of films depended on spray rate. Increasing spray rate made the films grown with bigger grain size and
higher roughness. Band gap energy was determined from the UV-Vis. Its value was found to be of 3.25 eV and was
almost independent on the spray rate.
Keywords: ZnO, Ultrasonic Spray Pyrolysis, thin film, Solution Spray Rate.
I. INTRODUCTION
Zinc oxide (ZnO) has attracted much attention recently for potential applications due to its
wide band gap (∼3.4 eV) and large exciton binding energy (∼60 meV) at room temperature [1].
ZnO is one of the typical II-VI semiconductor materials which have various applications such as
light-emitting diodes, electroluminescent devices, electrooptic modulators and window layers in
photovoltaic cells and gas sensors [2–9]. ZnO thin film have been fabricated by various methods
such as electrochemical deposition [10], sol-gel method [11], chemical bath deposition [12], RF
magnetron sputtering [13], and chemical spray pyrolysis [14,15]. Among these methods, the spray
pyrolysis is a simple but efficient method for the rapid production of large-area metal oxide thin
films.
Many technological parameters affect on the properties of deposited ZnO films in spray
pyrolysis method such as zinc source and concentration, dopant type and concentration, solvent,
growth temperature, etc. [8–10, 16–18]. Among these parameters, the solution spray rate and its
effect on properties of sprayed ZnO thin films have been studied not much. The report of [19]
c©2014 Vietnam Academy of Science and Technology
LUU THI LAN ANH et al. 109
is consider as a first research dealing with the spray rate effect of solution. Besides, the solution
spray rate also affects the surface morphology and roughness of undoped ZnO thin films produced
by spray pyropysis on glass substrate [20]. The effect of solution srpay rate on the properties of
undoped ZnO thin films fabricated by spray pyrolysis onto fluorine doped tinoxide (FTO) have not
been studied yet. Therefore in this work we investigated the effect of the solution spray rate on
the structural, morphological, optical properties of undoped ZnO thin films deposited by chemical
spray pyropysis.
II. EXPERIMENTAL
ZnO thin films were grown from 30 ml of spray solution that was sprayed onto preheated
FTO substrate (10 × 25 × 2.2 mm3). The spray pyrolysis system is reported elsewhere [15]. The
zinc oxide precursor solution consisted of 0.1 M zinc acetate (Zn(CH3COO)2 ·2H2O (Merck,
analytical grade, 99.5%)) in a ratio of water and isopropanol mixture about 2:3 volume. A few
drops of acetic acid were added to the spray solution to avoid the precipitation of zinc hydroxide.
The starting solution was atomized at a frequency of 130 kHz by an ultrasonic nebulizer. Deposi-
tion temperature was kept at 420oC. The solution spray rate was changed at 0.5 ml/min, 1 ml/min
and 4 ml/min to investigate its effect on properties of fabricated ZnO thin films.
The structure of the films was characterised by X-ray diffraction (XRD) with monochro-
matic Cu Kα radiation (λ= 0.15406 nm). The FTO substrates were washed with soapy water,
rinsed with deionised water, and cleaned ultrasonically in ethanol. The morphology of the ZnO
nanostructures was investigated by using atomic force microscope. The optical transmission and
reflectance spectra of the ZnO thin films were measured at room temperature with a HP Agilent
8453 spectrometer, operated in air in the range of 300 ÷ 900 nm.
III. RESULTS AND DISCUSSION
III.1. The structure and morphology of ZnO films
20 30 40 50 60 70
0
1000
2000
3000
4000
5000
6000
(a)
(b)
*
*
(201)(103)(110)(102)
(101)
(002)
 
 
In
te
n
si
ty
 
(a.
u
)
2theta (degree)
(100) *
(c)
 Fig. 1. XRD patterns of ZnO thin films pre-
pared by spray pyrolysis at different spraying rate:
(a) 0.5 ml/min, (b) 1 ml/min and (c) 4 ml/min.
Fig. 1 presents the XRD patterns of
ZnO film prepared by spray pyrolysis at
different solution spray rates. All ZnO
films were indexed to the hexagonal wurtzite
structure. It can be seen that in the XRD
patterns, there are peaks at 2θ = 31.71,
34.36, 36.196, 47.52, 56.58, 62.82 and
68.02 corresponding to (100), (002), (101),
(102), (110), (103) and (201) diffraction
planes of ZnO, respectively (PDF 36-1451).
Peaks presented at 2θ = 26.42, 37.733 and
51.5 which correspond to SnO2 (substrates).
When spray rate was low (0.5 ml/min), the
film had the highest peak intensity at 2θ
= 36.25o, indicating that film was crystal-
lized with preferably oriented (101) plane.
110 INFLUENCE OF SPRAY RATE ON STRUCTURAL AND OPTICAL PROPERTIES OF SPRAYED ZnO FILMS
Communication in Physics, Vol…, No …, (201..), pp,… 
5 
 
decreased to 40 nm and 24 nm for the film obtained at a solution rate of 1 ml/min and 4 
ml/min, respectively.  
 
 
 
 
 
 
 
 
0.5 ml/min 
 
1 ml/min 
 4 ml/min 
(a) 
 
(b) 
(c) 
Fig. 2. AFM 3D images of ZnO films de-
posited on FTO substrate with different solu-
tion spray rates: (a) 0.5 ml/min, (b) 1 ml/min
and (c) 4 ml/min.
When spray rate increased to 1 ml/min, the in-
tensity of this peak decreased and peak at 34.42˚
became the strongest intensity, and the film pre-
ferred orientation in the (002). When the (002)
dominated the XRD diagram, axis mainly found
perpendicular to the membrane surface. However,
when the (101) dominated, c axis will tilt away
from substrate. Spray rate increased to 4 ml/min,
XRD pattern was similar to case of spray rate
1 ml/min. The high peak intensity of (002) of
these thin films exhibited the preferential orien-
tation of the [001] axis.
The microstructure of ZnO films was stud-
ied using AFM. As can be seen in AFM images
(Fig. 2), all sprayed ZnO films fabricated at differ-
ent spray rate exhibited a dense structure without
either cracks or pinholes. The grain size, surface
topology and roughness of the film were dramati-
cally affected by solution spray rate. The surface
of the film grown with the spray rate of 0.5 ml/min
consists of grains that vary in size from 100 nm to
200 nm (Fig. 2a). Besides, many small grains
gathered and formed mountains with size of ∼1.2
µm. These mountains size decreased to ∼900 nm
and to 500 nm in case of spray rate of 1 ml/min
and 4 ml/min, respectively. Using 4 ml/min spray
rate, the film surface had the fine grain size and
homogeneous structure. The formation of a big
mountain at solution spray rate 0.5 ml/min and 1
ml/min resulted in a high surface roughness. For
the film obtained at a solution rate of 0.5 ml/min,
mean roughness value estimated from AFM images was of 67 nm. This value decreased to 40 nm
and 24 nm for the film obtained at a solution rate of 1 ml/min and 4 ml/min, respectively.
III.2. The optical properties of ZnO films
Fig. 3 shows the optical transmission spectra of ZnO films prepared at different solution
spray rate. The obtained results show that spray rate had affected on the transmission properties
of the films. Transmitance decreased drastically in the visible region when spray rate was 0.5
ml/min. The absorption edge of the sample was not clearly seen on the spectra. Transmission
behavior of the film fabricated at 1 ml/min had the transmission spectrum increased rapidly and
had a sharp edge of absorption around 370 nm. When the spray rate increased to 4 ml/min, the
transmittance spectra of the film had a similar feature of one of the film fabricated at 1 ml/min. The
transmittance value reached over 80 % in visible region. The small transmitances of films at 0.5
LUU THI LAN ANH et al. 111
ml/min can be affected by high roughness of film surface. As seen above, the surface roughness
of films fabricated at low spray rate was higher than that of the film fabricated at high spray rate.
To determine the band gap of the ZnO film, one can use UV-Vis data. The absorption
coefficient (α) is calculated from the transmittance spectra using the relation [21]:
α =
ln(1/T )
d
(1)
where d is the film thickness and T is the transmittance. It is now well established that ZnO is a
direct band gap semiconductor. The absorption coefficient is related to the band gap Eg according
to the equation [21]:
(αhν)2 = A(hν−Eg) (2)
where A is a constant and h is the Planck constant. Band gap of the films is estimated from plots of
(αhν)2 vs. hν by extrapolating the straight line from height absorption region as shown in Fig. 3.
Communication in Physics, Vol…, No …, (201..), pp,… 
7 
 
where A is a constant and h is the Planck constant. Band gap of the films is estimated from 
plots of (αhν)2 vs. hν by extrapolating the straight line from height absorption region as 
shown in Fig.3. 
                             
     
 
 
 
 
 
The results of the determination of Eg from (αhν)2 Versus the ℎν plot is shown in Fig. 4. 
One can see that the both ZnO films fabricated with spray rate of 0.5 ml/min has a bandgap of 
about 3.0 eV. The bandgap slightly increased 3.25 eV with increase of spray rate to 1 ml/min 
and the same bandgap for spray rate of 4 ml/min. This is similar to reported in  [22] results for 
ZnO:In films. 
IV. CONCLUSION 
ZnO thin films on FTO subtrate have been prepared by using chemical spray pyrolysis 
from the solutions of Zn(CH3COO)2 0.1M  with spray changing rates from 0.5 ml/min to 4 
ml/min. All ZnO films have a hexagonal wurtzite structure. ZnO films have prefered 
orientation in the (101) plane at spray rate of 0.5 ml/min and in the (200) plane at spray rate of 
1 ml/min. Spray rate had drammatically affected on the roughness of film’s surface. 
Roughness of films increased with decrease of spray rate. ZnO thin film prepared at spray rate 
Fig. 4.  The plots of (αhν)2 versus hν for the     
calculation of  band gap Eg: 
           (a) 0.5 ml/min, (b) 1 ml/min and (c) 4 ml/min  
 
Fig. 3.  The optical transmission spectra of ZnO films    
deposited at different solution spray rate: 
          (a) 0.5 ml/min, (b) 1 ml/min and (c) 4 ml/min  
300 450 600 750 900 1050
0
20
40
60
80
100
(c)
(b)
 
 
Tr
an
sm
itt
a
n
ce
(%
)
Wavelength (nm)
(a)
1.5 2.0 2.5 3.0 3.5
0
100
200
300
400
500
hν (eV)
(α
ην
)2 ×
10
8  
(cm
-
2 .
eV
2 )
 (a) Eg=3.0eV
 (b) Eg=3.25eV
 (c) Eg=3.25eV
 
 
(a)
(b) (c)
Communication in Physics, Vol…, No …, (201..), pp,… 
7 
 
where A is a constant and h is t e Planck constant. Ba d gap of the films is estimated from 
plots of (αhν)2 vs. hν by extrapolating the straight line from height absorption region as 
shown in Fig.3. 
                             
     
 
 
 
 
 
The results of the determination of Eg from (αhν)2 Versus the ℎν plot is shown in Fig. 4. 
One can see that the both ZnO films fabricated with spray rate of 0.5 ml/min has a bandgap of 
about 3.0 eV. The bandgap slightly increased 3.25 eV with increase of spray rate to 1 ml/min 
and the same bandgap for spray rate of 4 ml/min. This is similar to reported in  [22] results for 
ZnO:In films. 
IV. CONCLUSION 
ZnO thin films on FTO subtrate have been prepared by using chemical spray pyrolysis 
from the solutions of Zn(CH3COO)2 0.1M  with spray changing rates from 0.5 ml/min to 4 
ml/min. All ZnO films have a hexagonal wurtzite structure. ZnO films have prefered 
orientation in the (101) plane at spray rate of 0.5 ml/min and in the (200) plane at spray rate of 
1 ml/min. Spray rate had drammatically affected on the roughness of film’s surface. 
Roughness of films increased with decrease of spray rate. ZnO thin film prepared at spray rate 
Fig. 4.  The plots of (αhν)2 versus hν for the     
calculation of  band gap Eg: 
           (a) 0.5 ml/min, (b) 1 ml/min and (c) 4 ml/min  
 
Fig. 3.  The optical transmission spectra of ZnO films    
deposited at different solution spray rate: 
          (a) 0.5 ml/min, (b) 1 ml/min and (c) 4 ml/min  
300 450 600 750 900 1050
0
20
40
60
80
100
(c)
(b)
 
 
Tr
an
sm
itt
a
n
ce
(%
)
Wavelength (nm)
(a)
1.5 2.0 2.5 3.0 3.5
0
100
200
300
400
500
hν (eV)
(α
ην
)2 ×
10
8  
(cm
-
2 .
eV
2 )
 (a) Eg=3.0eV
 (b) Eg=3.25eV
 (c) Eg=3.25eV
 
 
(a)
(b) (c)
Fig. 3. The optical transmission spectra of ZnO
films deposited at different solution spray rate:
(a) 0.5 ml/min, (b) 1 ml/min and (c) 4 ml/min.
Fig. 4. The plots of (αhν)2 versus hν for the
calculation of band gap Eg: (a) 0.5 ml/min,
(b) 1 ml/min and (c) 4 ml/min.
The results of the determination of Eg from (αhν)2 versus the hν plot is shown in Fig. 4.
One can see that the both ZnO films fabricated with spray rate of 0.5 ml/min has a band gap of
about 3.0 eV. The band gap slightly increased 3.25 eV with increase of spray rate to 1 ml/min and
the same bandgap for spray rate of 4 ml/min. This is similar to results reported in [22] for ZnO:In
films.
IV. CONCLUSION
ZnO thin films on FTO subtrate have been prepared by using chemical spray pyrolysis from
the solutions of Zn(CH3COO)2 0.1M with spray changing rates from 0.5 ml/min to 4 ml/min. All
ZnO films have a hexagonal wurtzite structure. ZnO films have prefered orientation in the (101)
plane at spray rate of 0.5 ml/min and in the (200) plane at spray rate of 1 ml/min. Spray rate
had drammatically affected on the roughness of film’s surface. Roughness of films increased with
decrease of spray rate. ZnO thin film prepared at spray rate of 4 ml/min has a highest transmittance,
namely 80 ÷ 90% in visible range. The optical band gap energy of ZnO films was found about of
3.0 ÷ 3.25eV depends on the spray rate.
112 INFLUENCE OF SPRAY RATE ON STRUCTURAL AND OPTICAL PROPERTIES OF SPRAYED ZnO FILMS
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support of the project KC.05.06/11-15.
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Influence of Spray Rate on Structural and Optical Properties of Sprayed ZnO Films

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Influence of Spray Rate on Structural and Optical Properties of Sprayed ZnO Films

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