The effect of strontium doping on structural and morphological properties of zno nanofilms synthesized by ultrasonic spray pyrolysis method

Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

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Original Article

The effect of strontium doping on structural and morphological
properties of ZnO nanofilms synthesized by ultrasonic spray pyrolysis
method
A. Ouhaibi a, M. Ghamnia a, *, M.A. Dahamni a, V. Heresanu b, C. Fauquet b, D. Tonneau b
a
b

Laboratoire LSMC, D
epartement de Physique, Universit
e d'Oran 1 Ahmed Ben Bella, 31100 Oran, Algeria
Centre CINaM, Campus de Luminy, Universit
e d'Aix-Marseille, Marseille 13009, France

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 20 December 2017
Received in revised form
26 January 2018
Accepted 26 January 2018

Available online 16 February 2018

Pristine and strontium doped ZnO nanometric films were successfully synthesized on heated glass
substrates by the ultrasonic spray pyrolysis technique. The samples were characterized by means of X-ray
diffraction (XRD), Atomic Force Microscope (AFM), UVevisible spectroscopy and photoluminescence (PL).
X-ray diffraction patterns confirmed the hexagonal (wurtzite) structure, where the most pronounced
(002) peak indicates the preferential orientation along the c-axis perpendicular to the sample surface.
The intensity of this peak was increased rapidly from the first doping of 1% and its position was shifted
toward higher angles under Sr-doping effect. For the used doping range of 1e5%, the Sr-doping at 3%
attracted an especial attention. At this concentration, the particular transformation in the surface
morphology of doped ZnO films was observed. The surface became granular and rough by expanding the
crystallites' size. From optical measurements, transmittance and PL spectra were found to be sensitive to
Sr-doping, where two different behaviors were observed before and after 3% of Sr-doping.
© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
Ultrasonic spray pyrolysis
Sr-doped ZnO
Morphology study
Optical properties

1. Introduction
Pristine and doped zinc oxide (ZnO) is among the most studied
materials because of its interesting characteristics such as its easy
synthesis, its non toxicity, its chemical stability, its suitability for
doping with different metals. ZnO has several favourable properties
such as good transparency, strong room temperature luminescence,
high electron mobility. In materials science, ZnO is a n-type semiconductor with a wide direct bandgap (3.37 eV), a large excitation
binding energy (60 meV) and high transmission in the visible range.
For these important physical properties, ZnO is used successfully in a

variety of applications such as in electronics, in optoelectronic devices, in solar cells, in light emitter diodes [1e5].
ZnO thin films can be easily nanostructured and synthesized by
several techniques. The growth techniques must be physical as
sputtering, evaporation, pulsed laser deposition, … [6e8] or
chemical as solegel, chemical vapour deposition (CVD), metalorganic CVD, hydrothermal and spray pyrolysis … [9e13]. Among
these chemical synthesis methods, we explored in this paper, the

ultrasonic spray pyrolysis technique for its low cost and especially
for its simplicity to implement for fabricating oxide thin films of
good qualities. The crystalline quality, through the control in
composition and the synthesis in large scale on substrates, is easily
obtained by this technique. In order to improve some physical
properties of ZnO, lot of works has been carried out on the doping
of ZnO thin films. ZnO has often been doped with metal ions such as
Mn, Al, Ni [14e16]. It has been shown that the ferromagnetism, the
magnetism, the performance of organic solar cells or the conductivity related to the structural, optical and electrical properties are
improved after having doped ZnO.
It is in this context that the present paper is inscribed. It is about
the synthesizing and characterizing of strontium (Sr) doped ZnO. A
few work were carried on the strontium doped ZnO obtained
by chemical synthesis or physical growth. We can mention the
work of K. Pradeev Raj et al. [17] on SreZnO obtained by the
co-precipitation method, the work of Xu et al. [18] who used the
solegel method and the works of Raghavendra et al. [19,20] who
studied Sr-ZnO using the spray pyrolysis synthesize. Sr is an

* Corresponding author. LSMC Laboratory, Oran 1 University, 31000, Oran, Algeria.
E-mail address: (M. Ghamnia).
Peer review under responsibility of Vietnam National University, Hanoi.
/>2468-2179/© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license

( />

30

A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

element which has a large cationic radius (1.18 Å) and a heavy
atomic weight (87.62 g) in comparison with zinc (the ionic radius of
0.6 Å and the atomic weight of 65.4 g). Due to this size effect (the
radius ratio RSr =RZn ¼ 1.96), doping with Sr is somehow difficult to
obtain. It induces changes in structural, morphological and optical
properties of ZnO. In this work, we show that the strontium doping
of concentrations ranging from 1 to 5% caused a significant modification of the surface state.
2. Experimental
Pristine and Sr-doped ZnO nanofilms were synthesized using
the ultrasonic spray pyrolysis technique. As reported in reference
[21], this technique differs slightly from the classical spray pyrolysis. We dissolved zinc acetate di-hydrate (Zn (CH3OO)2, 2H2O) salt
as precursor of the ZnO particles in 100 ml of methanol for
obtaining a transparent solution concentred at 0.3 M LÀ1. To obtain
ZnO doped with strontium (Sr), we added to the 0.3 M LÀ1 solution
different amounts of strontium chloride hexahydrate (SrCl2, 6H2O).
In this way, we got the following doping: 1%, 2%, 3%, 4%, and 5%. The
resulting aqueous solution was stirred for 24 h before spraying it
onto heated glass substrates. Before spraying and in order to
eliminate residual contamination caused by air contact, the glass
substrates were previously cleaned in diluted acetone and rinsed in
deionised water for several cycles. After the chemical cleaning, the
samples were dried with nitrogen gas. In Fig. 1 we present a
simplified schematic of the ultrasonic spray pyrolysis assembled by
us. After having vaporized ultrasonically the solution, the vapour is

sprayed and deposited on glass substrates heated at 350  C and
held at a 20 cm from the spray nozzle. With the deposit time, ZnO
films were thus prepared.
The structural characteristics of the pristine and Sr-doped ZnO
films were examined by x-ray diffraction using Cu-Ka source of
wavelength of 1.54 Å. The state of the surface morphology was
characterized by AFM in a tapping mode. The optical properties
were studied at room temperature by using uvevisible spectroscopy and photoluminescence (PL).
3. Results and discussion
3.1. Structural and morphological characterization of ZnO and
Sr-ZnO thin films
The XRD patterns of pristine and Sr-doped ZnO are shown in
Fig. 2(a). From this figure, six orientations of different intensities

Fig. 1. Simplified scheme of the ultrasonic pyrolysis technique.

can be identified: (100), (002), (101), (102), (110) and (103). According to JCPDS 036-1451 card, these peaks indicate the hexagonal (wurtzite) structure of ZnO. As we can observe from these
spectra, the (002) and (101) planes are the most pronounced. As
the (002) peak is the most intense, the ZnO growth is preferentially made in this direction along the c-axis perpendicular to the
sample surface. But its intensity does not follow the increase in
Sr-doping concentration. It is observed to be increased rapidly
from the first doping (1%) and then it slowly decreases (Fig. 2(b))
till 3% of Sr-doping and increases again from 4 to 5%. This may be
explained by the size effect of strontium (RSr/RZn ratio ¼ 1.96)
which is probably at the origin of the formation of several ZnO
nanocrystallite phases. With Sr-doping, the (002) peak shifts towards high diffraction angles 2q as observed in Fig. 2(c). This shift
moves up till 3% of Sr-doping where D(2q) ¼ 0.12 and it returns
toward low angles for 4 and 5%. XRD signal shows no additional
peak which may suggest that Sr2ỵ ions go to the regular Zn sites
in the ZnO. The used concentrations of strontium from 1 to 5%

did not form a new compound and we attribute the shift of the
(002) peak, the instability of its intensity and its up and down
behaviour to the change of the crystallinity of Sr-ZnO. The
method of preparation may also contribute to this perturbation of
the (002) peak but its effect is not significant in this study. As
reported previously in references [22e25], the difference in
atomic size provokes changes in the density of defects, induces
stress, lattice distortion and leads to the reduction of oxygen
vacancies [25]. The effect of Sr-doping is also responsible for the
changes in the ZnO lattice parameters as shown in Table 1.
According to the values listed in Table 1 corresponding to the
(002) peak, c decreases slightly with increasing the Sr-doping concentration from 1 to 3% and increases for 4 and 5% Sr-doping. This
decrease/increase of the c lattice parameter is consistent with the
displacement of the (002) peak and the variation of its intensity. To
better understand these, we determine the average grain size (D)
from the XRD pattern of the (002) peak using the DebyeeScherrer's
formula [26].

D¼

0:9l
bcosq

(1)

where l is the wavelength of X-ray, b is the full-width half
maximum (FWHM) of the XRD peak, and q is the diffraction angle.
The estimated values of ZnO particle sizes are summarized in
Table 1. It is clearly seen that the grain size of ZnO increases from 1
to 3% of Sr-doping and decreases for 4e5%. In general, the doping

reduces the surface roughness and consequently the size of ZnO
particles must decrease; it is not the case here and indeed, the Srdoping was observed to play an important role in the ZnO structured films. The 3% Sr-doping particularly attracts our attention
where we can say that the Sr-ZnO films have two behaviours
delimited by the 3% doping: a behaviour for a doping situated between 0 and 3% (where c was decreased and the grains size
increased) and a second behaviour for 4 and 5% of Sr-doping (c was
increased and the grains size decreased). This is clearly observed in
the AFM analysis whose results were discussed just below (Fig. 4),
showing that the roughness present also two behaviours delimited
by the 3% Sr-doping.
Surface morphology was characterized with atomic force microscope (AFM, Model Dimension Edge of Bruker) operating at
room temperature in a tapping mode, and the images were treated
using WSxM software [27]. We have used the scanning area
3 mm  3 mm for the study of surface morphology. AFM images were
acquired with a resolution of 512 Â 512 pixels. The AFM analysis
allows us to determine the surface roughness. The roughness is
defined either by the mean square roughness s (rms) or the average


A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

.

.

.

.

.

31

.

.

.

Fig. 2. (a) XRD spectra of pristine and Sr-doped ZnO films. (b) Profile of the (002) peak intensity. (c) Shift of the (002) peak under Sr-doping content variations.
Table 1
Determination of the lattice parameters a, c and the grain size from XRD patterns for pristine and Sr-doped ZnO films.
Sr-doping concentration (%)

Parameter a (Å)

Parameter c (Å)

Grain size (Å)a

Cluster size (nm)b

0
1
2
3
4
5

3.2506
3.2486

3.2466
3.2474
3.2516
3.2520

5.2128
5.2084
5.2010
5.1996
5.2090
5.2098

23.39
24.53
26.23
28.22
27.57
27.81

40
150
180
200
172
153

a
b

From DebyeeScherrer's formula.

From AFM measurements.

roughness sa. These two roughnesses are dened by the following
expressions [28]:

srmsị ẳ
and

PN

iẳ1 Zi

Zavgị2
N

!1
2

(2)

sa ẳ

PN

iẳ1 ðZi

À ZavgÞ
N

(3)

where N is the number of points, Zi is the number of ith point of Z and
Zavg is the average value of Z. These two expressions of the roughnesses were treated by WsXM software and their profiles were
extracted from AFM images as shown in Fig. 3(a) and (b). This figure
shows the examples of pristine and 5% Sr-doped ZnO films where
the roughness srms is determined to be 4 and 75 nm, respectively.


32

A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

Fig. 3. AFM images and roughness profile. (a) Pristine ZnO films, (b) roughness profile for pure ZnO films with srms ¼ 4 nm. (c) Sr-doped ZnO at 5%, (d) roughness profile with
srms ¼ 75 nm.

The AFM characterization of the pristine ZnO film revealed homogeneous and continuous surface uniformly distributed nanometre sized grains where the surface roughness srms is
determined to be ~4 nm. The state of the surface changed under Srdoping effect and the ZnO films became granular with irregular
ZnO particles and somewhat porous. As shown in Fig. 4(c), the 3%
Sr-concentration affected noticeably the surface morphology
where ZnO nanoparticles agglomerated on the surface and formed
flower-like clusters. The clusters grew with increasing Sr concentration from 1 to 3% and became large-sized grains covering
partially the surface and reducing thus the roughness for 3% Srdoping. The increase and decrease of the roughness with Srdoping (Fig. 5) are in agreement with what we have observed on
the grain size determined from the (002) XRD peak and especially
on the shift of this peak at the 3% Sr-doping. Overall, the change on
the surface morphology of ZnO was observed as a result of the Srdoping effect. In order to complete the study of the surface
morphology, the particle size of pristine and doped ZnO thin films
were evaluated by the WSxM software analysis. According to the
calculated grain size and as reported in Table 1, the cluster size is

40 nm for pristine ZnO, 150 nm for 1%, 180 nm for 2%, 200 nm for

3%, 172 nm for 4%, and 153 nm for 5% Sr-doping. We also observed
that the 3% Sr-doping is the limit between two different behaviours
of the ZnO films. In the doping range 0e3%, the cluster size
increased whereas it decreased for 4 and 5% Sr-doping. The increases in size of ZnO particles may be due to Sr ions that
substituted into Zn2ỵ sites. We recall that the Sr2ỵ radius is greater
than that of Zn2ỵ, so the incorporation of Sr2ỵ distorts the lattice
and creates supplementary structure defects that are responsible
for changes in the morphology and structure of ZnO surface probably composed of several phases. Fig. 4(b), (d) and (f) represent the
variation of the z-height as a function of the Sr doping. These
profiles reflect well the state of the surface and its roughness.
3.2. Optical properties
3.2.1. Uvevisible analysis
The optical properties of the pristine and Sr-doped ZnO films
were determined from transmission measurements in the range of
300e1400 nm. The transmittance spectra are shown in Fig. 6. It can


A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

33

Fig. 4. 2D AFM images and z-height profiles for pristine and Sr-doped ZnO films. (a) Pristine ZnO, (c) 3% Sr-ZnO, (e) 5% Sr-ZnO, (b), (d) and (f) represent the plots of the profile of zheight.


34

A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

1
d

a ¼ ln

 
1
T

(5)

T is the transmittance of the film and d is the film thickness.
In the plot of the relation (6) in Fig. 7, we determine the gap by
extrapolation of the linear part of the curve (ahn)2 and its intersection with the energy axis gives the value of the bandgap.
The examples are given for the undoped ZnO film and for the 5%
Sr-doped ZnO film. The determined gap values for the pristine
and Sr-doped ZnO films are listed in Table 2. As shown in Fig. 8, the
band gap of ZnO is found to decrease from 3.263 to 3.264 eV for 1 to

Fig. 5. Variations of the roughness srms and average sa with Sr doping.

be seen that all the ZnO films show a high transmittance in the
visible region. The visible transmission is ranging in the interval
88e92.5%. The increase of Sr-doping induces a displacement of the
absorption edge towards the lower wavelengths around 385 nm in
the uv region. The shift in the absorption threshold may be due
to the scattering of the light by the increasing of the roughness
surface from 1 to 3% Sr-doping concentration. For the 4% Sr-doping,
the decrease of the roughness was probably responsible for the
improvement of optical transmittance and for the change of the
optical gap discussed bellow. A slight decrease of the transmittance
yield was observed from pristine to Sr-doped ZnO. This decrease

can be ascribed to the effect of the incorporation of Sr-atoms which
induced changes in the homogeneity of the surface morphology
caused by the apparition of porous surface areas with agglomeration of some ZnO nanocrystallites as revealed from the AFM
analysis.
The analysis of the transmission spectra allows us to access the
calculation of the optical gap of pristine and Sr-doped ZnO films
using the following relation of Tauc [29]:
(ahn)2 ¼ A(hn e Eg),

(4)

where a is the absorption coefficient, A is a parameter depending
on the transition probability, h Planck constant, v the frequency of
the incident photons. a is determined from the relationship:

Fig. 7. Determination of the band gap value: (a) Pristine ZnO film and (b) 5% Sr-doped
ZnO film.

Table 2
Determination of the band gap values using the relation (ahn)2 ¼ A (hn e Eg).

Fig. 6. Transmittance spectra of pristine and Sr-doped ZnO films.

Sr-doping concentration (%)

Band gap value (eV)

Solid ZnO
0
1

2
3
4
5

3.370
3.267
3.263
3.264
3.285
3.286
3.285


A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

35

2% Sr-doping and it enhances rapidly to 3.285 eV when Sr-doping
reaches 3%. Eg decreases again for the 5% Sr-doping and stabilizes
at 3.285 eV. This behaviour at 3% Sr-doping may be attributed to the
modification of structural defects caused by the presence of Sr2ỵ in
the ZnO matrix. As discussed above, the substitution of Sr2ỵ for
Zn2ỵ creates non-linear defects due to the difference in atomic size
and reduces oxygen vacancies as confirmed below by the photoluminescence analysis.
3.2.2. Photoluminescence analysis
Photoluminescence (PL) is achieved in this study to complete
the optical investigation of Sr-doped ZnO films. It helps us to understand, analyze, and refine more effectively the effect of Sr
doping on the structure of these films. PL measurements were
recorded at room temperature in the wavelength range

200e1000 nm. Fig. 9 displays PL spectra with their deconvolutions
of pristine and Sr-doped ZnO films. PL spectra are composed of
three principal peaks for all the samples. One peak appearing in the
uv region is detected at 398 nm (3.11 eV) and is usually attributed to

Fig. 8. Plot of Eg versus Sr-doping concentration.

3000

Intensity (a.u.)

Pristine ZnO

12000

6000

0

Intensity (a.u.)

1% Sr-doped ZnO
2000

1000

(b)

(a)
400

500

600

700

Wavelength (nm)

800

900

1000

500

(c)
400

500

600

700

Wavelength (nm)

800

Wavelength (nm)

900

(d)
400

500

600

700

800

900

4000

800

900

5% Sr-doped ZnO

Intensity (a.u.)

Intensity (a.u.)

(e)

700

800

Wavelength (nm)

900

600

700

600

0

900

1800

500

600

3% Sr-doped ZnO

4% Sr-doped ZnO

400

500

Wavelength (nm)

1200

2700

0

400

1800

2% Sr-doped ZnO

1500

0

0

Intensity (a.u.)

Intensity (a.u.)

18000

3000
2000

1000
0

(f)
400

500

600

700

Wavelength (nm)

Fig. 9. PL spectra: (a) pristine ZnO film, (b)e(f) Sr-doped ZnO films.

800

900


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A. Ouhaibi et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 29e36

the recombination of free excitons. It corresponds to the near-band
edge transition (NBE) of ZnO [28,29]. The other two peaks appear in
the visible region and are located toward 500 nm (2.48 eV) and
700 nm (1.77 eV). The blue emission (500 nm) may be due to the
oxygen vacancies and results from the recombination between the

electron localized at the oxygen defect and the hole in the valence
band. The large red emission peak detected around 700 nm is
probably related to stoichiometry defect due to the technique
synthesis of ZnO thin films.
From the PL spectra, we note that the incorporation of strontium
in the host ZnO matrix reduced 25% of the PL intensity signal for all
samples doped. At the 3% Sr-doping, the blue emission disappeared
completely and the red emission was at the lower intensity
(Fig. 9(d)). The intensity of PL signal enhanced again for 4% and 5%
Sr doping concentrations. This is in full agreement with the results
observed from the XRD, AFM and uvevis measurements. The Sr
doping concentration at 3% is the limit where the structural and
morphological changes of the system Sr-ZnO take place with
respect to reducing of the density defects related to the oxygen
vacancies and to the interstitial sites occupied initially by Zn2ỵ. The
crystalline quality of ZnO films was improved for the 3% Sr-doping.
4. Conclusion
The undoped and Sr-doped ZnO nanofilms were successfully
synthesized on glass substrates via the ultrasonic spray pyrolysis
technique. From the X-ray diffraction analysis, the preferred (002)
oriented hexagonal phase of ZnO was confirmed for all samples
studied. Sr-doped ZnO thin films showed an increase in intensity
for this peak for Sr-doping between 0 and 3%, whereas it decreased
beyond 3%. This peak was shifted toward the high diffraction angle
(2q). The behaviour of Sr-doping effect before and after the 3% Sr
doping were also revealed in the AFM analysis and the optical
study. The morphology of Sr-doped ZnO surface became rough and
composed of crystallite clusters of different sizes which were
enhanced in the Sr-doping range 1e3% and decreased for 4 and
5%. The transmittance signal was shifted toward low wavelengths,

while the photoluminescence intensity decreased. The PL peak
of the blue emission near 500 nm disappeared totally at a 3%
Sr-doping concentration. This doping concentration is considered
as a doping limit in the transformations of the Sr-doped ZnO
films and on its crystalline quality improvement.
Acknowledgements
The authors thank a lot A. Ranguis from CINaM of Aix-Marseille
University (France) for some experimental measurements and
R. Baghdad from Tiaret University (Algeria) for the samples' synthesis. The authors thank also the Algerian-French cooperation
through the Tassili 14MDU915 project for the funding support.
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