Results in Physics 10 (2018) 706–713
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Results in Physics
journal homepage: www.journals.elsevier.com/results-in-physics
Effect of deposition runs on the physical properties of In2S3 chemically
synthesized for photocatalytic application
Yassine Ben Salem ⇑, Mouna Kilani, Najoua Kamoun
Laboratoire de Physique de la Matière condensée, Faculté des Sciences de Tunis, Tunis El Manar 2092, Tunisia
a r t i c l e
i n f o
Article history:
Received 9 January 2018
Received in revised form 28 February 2018
Accepted 28 February 2018
Available online 17 March 2018
a b s t r a c t
Polycrystalline In2S3 thin films have been prepared using low cost chemical bath deposition technique on
SnO2/Pyrex substrate. In this work, we have investigated the effect of multilayer deposition on physical
proprieties of the In2S3 thin films from 1 to 6 deposition runs with the aim of optimizing them for photocatalytic application. The structure, the surface morphology and the optical properties of In2S3 films
were studied by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Atomic force microscopy (AFM) and spectrophotometer. The crystallinity seems to be improved as the film thickness
increases. After three deposition runs, a remarkable change in the shape of the grains is observed and
the band gap energy is found to be about 2.52 eV. On the other hand, In2S3 thin film prepared using
5 deposition run exhibits an excellent performance as sensing film along with a whole degradation of
methylene blue (MB) dye.
Ó 2018 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction
In2S3 can be synthesized with high reproducibility by using various methods including chemical vapor deposition [1], atomic layer
epitaxy [2], spray pyrolysis [3] and chemical bath deposition (CBD)
[4–6]. Among these methods, chemical bath deposition (CBD) technique, which is well known as not using vacuum technology,
requiring simple equipment and producing large-area thin films
at relatively low temperature.
b-In2S3 material is one of the most promising for optoelectronic
and photovoltaic applications. Indeed this material is a promoter
candidate in this field such as photovoltaic devices [7], optical
material [8], optical mass memories [9] and solar cell devices
[10]. Due to its chemical stability, not toxic [11–13], interesting
structural characteristics, high transmittance and electronical
properties, b-In2S3 can be used as a buffer layer nontoxic by substitution of the cadmium sulfide (CdS) in Cu(In,Ga)Se2 based solar
cells [14].
Recently, many attempts have been carried out to test In2S3
material under several forms such as nanocomposite and by means
of various chemical as well as physical processes as an efficiency
compound in sensing. Indeed, Jinz Li et al. [15] obtained fast electron transfer and enhanced visible light photocatalytic activity
using multi-dimensional components of carbon quantum dots@3D
daisy-like In2S3/single-wall carbon nanotubes. Also, Calcium
⇑ Corresponding author.
E-mail address: b.salem.yassine@gmail.com (Y.B. Salem).
doped b-In2S3 hierarchical structures have been used for photocatalytic hydrogen generation and Rhodamine B (RhB) organic
dye degradation under visible light irradiation [16].
Moreover, Tetragonal b-In2S3 has been tested to degrade the
Nitrate under the irradiation of a Xe lamp [17].
The thickness ‘e’ of b-In2S3 is quite low and it is desirable to
increase it for the better use of the b-In2S3 as an optical window
in a solar cell. In order to increase the thickness and to obtain a
good window layer in solar cell, multi-layer deposition can be processed by repeating the same experimental conditions.
Usually CuInSe2 based photovoltaic solar cells have been made
on glass coated by transparent conductive oxides (TCO) such as tin
oxide (SnO2) [18,19] or indium tin oxide (ITO) over which a thin
layer In2S3 is deposited.
In the present work, we have deposited polycrystalline In2S3
films on SnO2 by chemical bath deposition.
Studies of the multilayer films In2S3/glass have been reported
by several authors [20] in our knowledge, there is no study work
devoted on the multilayer deposition of In2S3 on SnO2/Pyrex prepared by CBD reporting the effect of the preparation method and
the number of layers on the physical properties of this compound.
In fact SnO2 acts as a transparent conductive oxide (TCO) in solar
cells were In2S3 is an optical window.
In the following, we report on the structural, morphological and
optical properties of In2S3/SnO2 elaborated by CBD after one and
several deposition runs. Also, this work covers photocatalytic tests
of In2S3 synthesized samples for the photodegradation of MB dye
under sunlight irradiation.
https://doi.org/10.1016/j.rinp.2018.02.078
2211-3797/Ó 2018 Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Y.B. Salem et al. / Results in Physics 10 (2018) 706–713
Experimental details
Thin films of indium sulfide were coated on the SnO2/Pyrex by
CBD. The bath contains indium trichloride with constant concentration (0.025 M) and thioacetamide (TA) as sulfur precursor
(0.10 M). The pH of the solution is adjusted to 2.0 by adding acetic
acid into the reaction mixture. Bath temperature is kept constant
at 70 °C, the deposition time is td = 45 min [21]. The substrate is
kept vertically in the hermetically closed deposition bath mounted
on a heating magnetic agitator. We repeat the same bath up to four
times. At the end of each deposition run, the layer is rinsed with
bidistilled water.
The microstructure of sample was characterized using an FEI
Tecnai G2 TEM operating at 200 kV with a LaB6 filament. TEM
specimens were prepared by crushing the sample under butane.
A drop of the suspension was put on a holey carbon film supported
by a copper grid. Local chemical analysis was performed using the
energy dispersive X-ray spectroscopy (EDS) system attached to the
TEM. The crystal structure of the as-prepared film was examined
by X-ray diffraction (XRD) which are recorded with an automated
Bruker D8 advance X-ray diffractometer with CuKa radiations for
2h values over 10-80°.The wavelength, accelerating voltage and
current were respectively, 1.5418 Å, 40 kV and 20 mA. The film
surface morphology was studied by Atomic Force Microscopy
(AFM, standard Veeco Dimension 3100, used in tapping mode).
The optical transmission and reflection measurements were examined with Perkin-Elmer Lambda 950 spectrophotometer in the
wavelength range of 250–2500 nm at room temperature taking
air as reference. Photoluminescence spectra were characterized
with Perkin-Elmer LS 55 and the excitation was provided by a
diode at 320 nm. Moreover, the film thickness was measured by
Dektak 3 profilometer. All spectra were measured at room temperature. Finally, The photocatalytic degradation process of a 4 mg/L
solution of MB using In2S3 thin films is carried out in quartz cells
(4 2 2 cm3) with volume of 6 mL. Prior to irradiation, the solution was stirred in the dark during 30 min to reach the establishment of an adsorption–desorption equilibrium. Quartz cells have
been subjected to sunlight irradiation during various times (1–4
h). The concentration of MB was then detected by measuring the
maximum absorbance at 664 nm using SHUMATZU UV 3100 UV–
Vis–NIR spectrophotometer. According to the Beer–Lambert law,
the absorbance A and concentration C of MB are proportional, so
the degradation efficiency of MB is calculated using the following
formula:
g¼
A0 þ A
C0 þ C
100% ¼
100%
A0
C0
ð1Þ
Where C0, C and A0, A are the concentration and absorbance of MB
when the reaction time is 0 and t, respectively.
Results and discussion
Structural properties
The XRD patterns of the multi-layers films deposited by CBD on
SnO2 for the different runs 1D, 3D, 5D, 6D are shown on Fig. 1. Several sharp peaks due to SnO2 are observed (JCPDS N° 46-1088).
After one deposition run, only two peaks corresponding to the
cubic phase of the b-In2S3 compound (JCPDS N° 32-0456) are
observed at approximately 33.6° and 48° assigned respectively to
(4 0 0) and (4 4 0) reticular planes with the (4 0 0) peak as the preferred orientation. The intensity of XRD peaks corresponding to
SnO2 substrate decreases when the number of deposition
increases. The crystallinity seems to be improved as the film thickness increases after three; five and six deposition runs. The thick-
707
ness of multi-layers of In2S3 is summarized in Table 1. It can be
measured that In2S3 thin films have thickness closeto 510 nm in
the case of six deposition runs and 350 nm in the case of one deposition run. Above three deposition runs the XRD patterns showed
the apparent of three new diffraction peaks at 27.5°, 43.4° and
70° assigned respectively to the (1 0 9), (3 0 9) and (4 4 4) reticular
planes corresponding to the In2S3 tetragonal phase (JCPDS-731366). However after six deposition runs, it can be seen that the
(4 0 0) and (4 4 0) In2S3 diffraction peaks are much intense than
the ones corresponding to the substrate [22]. It is assumed that
the observed structural changes are related to the nanometric size
of crystallites. The mean crystallite size d, has been determined
from line width of the XRD patterns by using the Scherrer equation
[23,24]:
0:94k
d ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
b b20 cos h
ð2Þ
Where k is the X-ray wavelength of Cu Ka radiation (k = 1.5418 Å),
b0 is the width of the corresponding peak due to the instrumental
expansion [25,26]. b is the experimental full-width at halfmaximum (FWHM) of (4 0 0) diffracted peak measured in radians
and h is the Bragg’s angle. Values of d for multilayers films are presented in Table 1.
It is noted that the diffraction data were collected over the
diffraction angle of 2h = 20–70° by step scanning with a width of
0.017° and a scanning step of 10 s. These scanning parameters
were found to provide accurate width of diffraction peaks for calculating the size of crystallites.
The average grain size (d) of In2S3 thin layers increased from
12.1 to 13.6 nm when the number of layer inc1reases from one
to six deposition runs.
Further, the layers prepared with a deposition number higher
than six could not show any significant change in the growth direction in the film.
In order to obtain more detailed information on the structure,
shape and size of In2S3 of 3D sample, transmission electron microscopy coupled with EDX was employed for imaging and selected
area diffraction as well as for microanalysis. EDX analysis was performed on various cristallites for sample. The EDX analysis resolution in the TEM observations is about 10 nm, so it is possible to get
the correct composition of these entities. EDX elemental analysis
shows that the cation ratio In/S is close to 3/2 witch is in agreement with a nominal composition of In2S3. Fig. 2b is a bright field
electron micrograph of the In2S3 region. This TEM micrograph
reveals the presence of nanometer scale particles with average size
around 12.6 nm, incorporated within the matrix. To get more information of In2S3 microstructure of 3D sample high resolution transmission electron microscopy is considered. The high resolution
TEM image in Fig. 2c corresponds to the electron diffraction pattern. This image shows that the interplanar distances are 0.31
nm and 0.27 nm corresponding respectively to (2 2 2) and (4 0 0)
planes and the angle value between these planes is about 54.8°
which is according to the cubic In2S3 system. So the results
obtained by TEM and XRD analysis prove that sample In2S3 which
having the important electrical properties has a cubic structure.
Morphological properties
In order to achieve a more direct insight into the thin film surface structural features, AFM is used. The evolution and the development of the morphology of the surface thin films is studied as a
function of deposition – runs. Plane view AFM images of the In2S3
film surface are shown on Fig. 3. These images show that the surface morphology of the films is strongly dependent on the layer
number. In fact, after one deposition run, the surface morphology
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Y.B. Salem et al. / Results in Physics 10 (2018) 706–713
Fig. 1. X-ray diffraction of In2S3 thin films elaborated at different deposition runs.
Table 1
Deposition runs effect on the thickness, grain size (d), RMS and Eg parameters of the
In2S3.
Sample
1D
3D
5D
6D
Thickness (nm)
Grain size d (nm)
RMS (nm)
Eg (eV)
350
12.1
30
2.97
382
12.7
34.4
2.80
450
13.1
37
2.48
510
13.6
39.3
2.48
is constituted by cluster on wich there are nanofibers distributed in
a random way and good connectivity all over the substrate surface
(SnO2). Moreover, after three, five and six deposition runs a
remarkable change in the shape of the grains is observed. This
can be due to the different crystalline structure which evolves from
cubic to tetragonal as observed by DRX.
We can observe that the surface roughness (RMS) increased
with increasing of film thickness. RMS increased from 30 nm for
1D to 39 nm especially for thin film prepared after six deposition
runs, these results are given in Table 1 and Fig. 4.
Optical properties
Optical transmission measurements in the wavelength range,
250–2500 nm are also performed in order to investigate the effect
of thickness on the optical performances of the In2S3 thin film.
Fig. 5 shows the optical transmittance of the as-grown and after
multi deposition runs films elaborated on SnO2. As-deposited film
exhibits a good transparency in the visible and near infrared
regions (75%) which is suitable for the application in solar cells
as the window layer. The optical properties of these compounds
are influenced by the thickness. We note a decrease in the transmission values for the 6D compared to the as deposited layer 1D
which may be due to densification and to the thickness increase
with the deposition runs (Table 1). It is usually presumed that
the transmittance of the films decreases with grain size in the visible region of spectrum due to light scattering on their rough surface. Hence, the higher transmittance of the as deposited can be
attributed to more voids and so lower packing density of the films.
In the area of high absorption range [300–600] we see a shift in
the intrinsic absorption edge. This can be attributed to the onset of
the tetragonal phase, which appears for the 3D film, and the intensity of the preferential orientation (1 0 9) increases with the number of deposit, resulting a reduction in the band gap value.
Based on the optical transmission and reflection measurements,
(aht)2 is plotted as a function of photon energy (ht) in Fig. 6 for bIn2S3 layers grown on SnO2 substrate for different number of
deposit runs. The following equation can be applied for a direct
band gap transition [27]:
Y.B. Salem et al. / Results in Physics 10 (2018) 706–713
709
Fig. 2. (a) Transmission electron micrograph of the In2S3 showing nonentities imbedded in the matrix. Inset: EDX spectrum taken in the nanoscale entity; (b) High resolution
TEM image taken along the [0 0 1] direction, showing the (2 2 2) and (4 0 0) plans of the cubic crystal; (c) Electron diffraction patterns along [0 0 1]* zone corresponding to the
high resolution TEM image.
Fig. 3. AFM images of In2S3thin films grown for the different runs 1D, 3D, 5D, 6D.
ðahtÞ ¼ Aðah EgÞ
2
ð3Þ
where A is a constant. The band gap energy Eg is obtained by
extrapolating the linear portion of the plot to the crossing with
ht axis [28]. Eg decreases monotonically from 2.97 eV (for 1D) to
2.48 eV (for 5D). Generally in polycrystalline semiconductors, the
energy band gap can be affected by the stoichiometric deviations,
by quantum size effect, by dislocation density and by disorder at
the grain boundaries. Thus, in the present study, the decrease of
band gap at higher thickness can be produced by the coexistence
of the preferred orientation (4 0 0) for cubic phase and (1 0 9) corresponding to tetragonal phase. The energy band gap decreases
with increasing film thickness i.e. for films constituted by a mixture
of both crystalline phases. A similar result is observed by N.Revathy
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Y.B. Salem et al. / Results in Physics 10 (2018) 706–713
Fig. 4. The variation of the average surface roughness with the different runs 1D, 3D, 5D, 6D.
Fig. 5. Transmission spectra of In2S3 thin films deposited on SnO2/Pyrex substrates, for the different deposition runs 1D, 3D, 5D, 6D.
et al. [29] who reported that an improvement in the particle size of
evaporated In2S3 could contribute to the decrease of energy band
gap in the films with increasing film thickness [30].
PL spectroscopy
The room temperature photoluminescence investigation was
carried out on the multilayer films of In2S3 grown on SnO2 substrate by CBD method. PL spectrum of 1D, 3D, 5D and 6D samples
exhibited strong peaks intensity at 485 and 529 nm and two peaks
at 420 and 462 nm with slight visible intensity.
The emission at 420 nm was probably due to the defect level
related to the interface traps at the grain boundaries which was
due to the transition between this level and valence band. For 1D
sample, a broadening green peak emission at 529 nm and a strong
bleu at 485 nm are observed while for 3, 5 and 6 depositions all
samples show a single green emission centered at about 429 nm.
The broadness of this peak is related to the transitions involving
Y.B. Salem et al. / Results in Physics 10 (2018) 706–713
711
Fig. 6. Optical band gap energy (Eg) of indium sulfide (In2S3) grown for different runs.
different electronic levels in close vicinity to the bottom of the lowest empty band and the top of the highest one. Fig. 7 shows clearly
that in samples, the green emission position and shape do not vary
significantly following the number of films. The disappearance of
the bleu peak after 3D, 5D and 6D can be due to structural changes
during film deposition resulting in a reduction in defect concentration, sizes of grains, cluster density and crystallinity. In our case, it
was found that the grain size was increased in the case of one
deposition run with the increase of the number of films, which in
turn, increased the grain boundary area, causing an increase in
density of the interface traps. So it is interesting to note that after
3D, 5D and 6D deposition run the multi films In2S3 emit a single
green luminescence emission accompanied with deep level
emission.
Photocatalytic tests
As shown in Fig. 8 the increasing time exposure under sunlight
irradiation increases the percentage removal until 70% for solution
of dye with In2S3 film during 4 h. This results show that sunlight
irradiation plays an important role in degradation of MB. Such irradiation provides more energy to excite the electron of In2S3 and
creates more electron-hole pairs, thus improve the photocatalytic
process, a similar result is found else where [31,32].
Fig. 7. PL spectra of In2S3 thin films on SnO2/Pyrex substrate for different runs 1D, 3D, 5D, 6D.
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Y.B. Salem et al. / Results in Physics 10 (2018) 706–713
Fig. 8. Photocatalytic spectra for 5 deposition run of In2S3 thin films on SnO2/Pyrex substrate for different times (1–4 h).
The photocatalytic degradation of MB does not result from a
direct redox reaction between In2S3 thin films and the MB molecules. When absorbing light with photon energy (hm) equal or
exceeding the band gap, these films generate electron–hole
(e-/h+) pairs at the surface, as indicated in the following equation:
In2 S3 þ hm ! e CB þ h VB
ð4Þ
Also, oxygen vacancies can acts as a deep donor with an energy
level located at 2.52 eV (495 nm) above the top of the valence
band. Therefore its energy position is favorable to the generation
of holes in the valence band under visible light irradiation:
þ
Vþ Ox þ e ! VOX þ h VB
ð5Þ
The photogenerated holes (h+VB) on the surface of photocatalyst
thin films can react with adsorbed water (H2O) and hydroxide ions
(OH) to produce highly reactive hydroxyl radicals (OH)
þ In2 S3 h VB þ H2 O $ Hþ þ OH ads ! H þ OH
ð6Þ
At the same time, adsorbed oxygen acts as an electron acceptor
by forming a superoxide radical anion O2. Then the suspension of
superoxide radical anions may act as oxidizing agents or as an
additional source of hydroxyl radicals via the subsequent formation of hydrogen peroxide.
ðO2 Þads þ e CB ! O
2
ð7Þ
þ
O
2 þ H ! HO2
ð8Þ
HO
2 ! H2 O2 þ O2
ð9Þ
that justify the use of sunlight irradiation to degrade MB dye. In
fact, AFM images displayed rough surfaces having nanofibers
which are distributed in a random way. This may lead to a high
specific surface of the film and hence improved indeed the photocatalytic property of such film. This study regarding the degradiation of MB seems so interesting since a cost-effective CBD process
has been used to achieve this goal.
Conclusion
In2S3 thin films have been prepared on SnO2 substrates by CBD.
The number of deposition runs influences the properties of deposited films. The crystallinity seems to be improved as the film thickness increases. Above three deposition runs the XRD patterns
showed the apparent of tetragonal phase. The optical transmittance was found to decrease with the number of deposition,
corresponding to the decrease in layer thickness. The wider bad
gap (Eg 3.2) were obtained for the one run deposition. PL curves
of the In2S3 multilayers showed green and blue peaks for 1D sample while for 2, 3 and 5 deposition runs, all samples show a single
green emission. As the CBD technique allows the synthesis of good
b-In2S3 thin films, we could also improve the physical properties of
In2S3 deposited on the thin films without affecting the optical
properties by doping the layer with an appropriate element followed by heat treatment. Further studies are in progress to use
such films in other sensitivity applications such as gas sensors
and so on.
Appendix A. Supplementary data
H2 O2 ! 2OH
ð10Þ
The high oxidants associated with hydroxyl radicals react with MB
dyes and make the blue solution colorless:
RHðMBÞ þ OH ! R þ H2 O:
ð11Þ
It is worth noting that In2S3 thin film prepared with 5 deposition runs is selected to carry out the photodegradiation of MB dye.
This is due to the fact that this film has a relatively high RMS
roughness of the order of 40 nm which means that such film has
relatively high specific surface. Also, this film shows a PL response
signal situated in 500–550 nm domain, centered in visible range
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.rinp.2018.02.078.
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