A comparative study of potentiostatic and pulsed Electrodeposition of ZnS nanostructures
(1)K. Ghezali, (1)B. Boudine
(1)Laboratory of Crystallography, Department of Physics, University of Constantine, Road Ain El bey 25000,
Constantine, Algeria
ghezali_khaoula1991@yahoo.com , b_boudine@yahoo.fr
(2)L. Mentar, (2)A.Azizi
(2)Laboratoire de Chimie, Ingénierie Moléculaire et Nanostructures, Université Ferhat Abbas-Sétif 1, 19000
Sétif, Algeria
menter.loubna@yahoo.fr , aziziamor@yahoo.fr
Abstract— Electrochemical, structural and optical properties of ZnS thin films deposited by either potentiostatic or pulsed electrodeposition were investigated. The grown thin films of ZnS from the both processes were studied by means of Mott-Schottky (MS), AFM, XRD, Raman and UV-Vis spectrometry. The deposits of ZnS were grown using an aqueous solution containing 10-3M Na2S2O3 and 10-4M ZnSO4 at pH=2,4. The electronic properties using Mott-Schottky confirm the n type conductivity, the AFM shown a strongly changement on the surface of the obtained film for both method. The XRD and Raman show that the deposited films are polycrystalline in nature and crystallize with blende structure in both processes, while the optical properties of the deposited ZnS thin films have a band gap equal to 3.5eV.
Keywords—ZnS; Mott-schottky; pulse; potentiostatic; XRD.
Introduction
The production of thin film solar cells has recently been of interest since they provide a potential enhancements for either low cost, low-energy requirements, high efficiency and obtained materials of good crystalline quality [1]. In this work we deposit Zinc sulfide (ZnS), which is an n-type semiconductor with a good transparency in the visible region, it had been widely used as transparent electrodes in optoelectronic devices [2],
In particular, ZnS is believed to be one of the most promising materials for blue light emitting laser diodes [3] and thin film electroluminescent displays. It is an important device material for the detection, emission and modulation of visible and near ultra-violet light. ZnS is a direct wide band gap 3.5eV semiconductor [1]. So ZnS thin films have been widely investigated due to the variation on size and shape and are considered to be promising alternative thin films in many application sectors [4].
The electrochemical deposition had widely employed to elaborate thin films due to low cost, low temperature of
fabrication, the possibility to control the film thickness and simplicity of process with potential for large-scale production.
Different electrochemical method was used to elaborate thin films such as pulse [5], potentiostatic [2] and galvanostatic [1].
In the present study, a comparative study of pulse and potentiostatic electrodeposition was investigated. The Electrochemical study was employed by Mott-schottky (MS) mesure it confirm n type conductivity when the Atomic force microscopy show a marked changement in the surface of the obtained films, XRD and Raman shows a cubique structure and the gap energy was estimated, the results are clearly discussed below.
I. EXPERIMENTALES DETAILS
A three-electrode cell equipped with a saturated calomel electrode (SCE) placed as a reference electrode, platinum plate as a counter electrode and Indium tin oxide (ITO) coated glass substrates as the working electrodes. was carried out to elaborate ZnS thin films. the ITO substrates were ultrasonically cleaned with acetone, methanol and distillate water for 10 min, respectively and then dried in the air. The deposition solution contained 10-4M of ZnSO4 and 10-3M of Na2S2O3. The pH of the solution was adjusted using diluted H2SO4 acid to be pH=2.4. The samples were deposited by pulsed and potentiostatic electrochemical deposition using Potentiostat/Galvanostat (PGZ301, Radiometer Analytical).
the deposition potential was about -1.3 V/ ECS. The obtained films were rinsed with distillate water and dried in the air.
The films was analyzed by a Bruker AXS D8 Discover diffractometer using Cu Kα1 radiation at 40 kV (Cu Kα1, λ = 0.15406 nm).The surface was observed by Atomic Force Microscopie (AFM) Asylum research (MFP-3D SPM). The Raman spectroscopy was performed using a Raman spectrometer (SENTERRA) with a 532 nm (20 mW) Ne laser line as an excitation source. Optical transmittance spectra were obtained with a SHIMADZU 2401PC spectrophotometer in the UV-visible region. The spectra were corrected for glass substrates. All measurements were conducted at room temperature.
II. RESULTS AND DESCUSION A. Mott-Schottky :
Electrochemical impedance spectroscopy (EIS) on the electrode/electrolyte was also employed to determine the flat band (Efb) and carrier density (Nd), which can be obtained in a Mott–Schottky (MS) plot with 1/C2 versus potential at a fixed frequency of 800 Hz. Mott-Schottky plots 1/C2 vs E is presented for ZnS thin films obtained for (a) pulse and (b) potentiostatic mode according to the following equation [6]:
(1) With C2 represents the space charge capacitance, ε is the dielectric constant (8.3 for ZnS [7]), ε0 is the permittivity of free space (8.85*10-14 F/cm), ND is the carrier concentration, Efb is the flat band potential, k is Boltzmann's constant (1.38*10-23 J/K), T is the absolute temperature (298 K), and q is the elementary electron charge (1.6021*10-19 C).
-800 -400 0 400 800
50 100 150 200 250 300 350 400
1/C² *109 (cm4 F2 )
E(V) (a)
-800 -400 0 400 800
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
1/C² *109 (cm4 F2 )
E(mV) (b)
Fig. 1: Mott-Schottky plots of (a) pulsed and (b) potentiostatic ZnS thin films
The capacitance potential measurements was presented in Fig.01. A positive straight line was 0observed indicate n type conductivity of the obtained thin films for both method. The extrapolating of the line on x axis give the potential flatband values (recorded in Table I), when the slope yields carrier concentration, it’s 4.17*1019cm-3 for pulse and 1.94*1022 cm-3 for potentiostatic ZnS thin films this augmentation might related to ZnS vacancies.
B. AFM
The surface and roughness of the deposited ZnS thin films on ITO glass substrat by pulsed and normal electrochemical deposition were study by AFM analysis. Fig.01 (a), is the AFM image of the pulsed ZnS thin film and Fig.01 (b) is that of potentiostatic ZnS thin film. Both images show the surface of the Substrat had totally covered by the film. Also AFM images of the pulsed ZnS thin film has a greater surface than potentiostatic deposited thin films. The grain size of the pulsed ZnS thin film was smaller than that deposited by potentiostatic it caused by the increase in current density [8].
Fig. 2: AFM images of the deposited (a) pulse and (b) potentiostatic ZnS thin films
The Root mean square (RMS) roughness obtained from the AFM data. The surface roughness depend on the
(a)
(b)
electrochemical elaboration method, RMS for the obtained films was about 11.77 and 33.11 nm for pulsed and potentiostatic ZnS thin films respectively.
C. XRD
X-ray diffraction mesearment were performed to examinate the structure of the electrodeposited ZnS thin films for the pulsed (a) and potentiostatic (b), the obtained diffractogramme were shown in Fig.03. It exhibit a zinc blend structure for both films (from JCPDS card file No. 05- 0566).only one pic at around 33.04° attribuated to (200) plan, the diffraction peaks with asteriks are rlated to the ITO conductive glass substrat, the thin film elaborated by pulse has no record of impurty compared to that elaborated by potentiostatic we observe the presence of two peak d’impurty at 35.89° and 39.4° related to zinc metallic and sulfur respectively. From AFM images and XRD It is clear that the deposition films depend strongly on the electrochemical method. Both crystal nucleation and the growth rate was controled.
20 30 40 50 60 70
Intensity(ua)
2 (a) (b)
* *
ZnS Zn S
Fig. 3: XRD patterns of (a) pulse and (b) potentiostatic ZnS thin films.
The average crystallites size was calculated using Debye Scherer’s Formula [9]:
(2) D is crystallites size, λ is the wavelength of the X-ray (1.5456 A), β is the full width half maximum of the diffraction peak in radians and θ is Bragg diffraction angle. The lattice parameter a was calculated from [10]:
(3)
is interplanar distance and (h k l) is Miller index.
Crystallites size and lattice parameter.
The obtained crystallites size and lattice parameter were record in Table I, as a comparison between pulse and potentiostatic we obtained a very small crystallites size compared to that obtained by potentiostatic this result confirmed that obtained by AFM. When the lattice parameter are similar to other study [1,5].
D. Raman
Raman spectroscopy is used in condensed matter physics and chemistry to study low-frequency modes in a system. The laser light interacts with phonons or other excitations in the system, which results in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. In addition it can be used to observe other excitation like plasmon and magnons[11]. The Raman spectra of the pulse (a) and potanstiostatic (b) are given in Fig.04, the first-ordre raman phonon was observed at around 481.18 cm-1 correspond to [TOl+LA] ∑ . The second-order raman phonon observed at 553.75 cm-1 might originate from 2TOΓ and The third-order Raman phonons observed at 1090.59 cm-1 can be assigned as 3LO [11,12]. The first raman-order was observed only for pulsed ZnS thin films.
200 400 600 800 1000 1200
Intensité (ua)
Déplacement raman (cm-1) (b) 481,18 (a)
[TOl+LA]
553,75 2TO
1090,59 3LO
Fig. 4: Raman spectre of (a) pulse and (b) potentiostatic thin films
E. Gap energy:
In order to study the effect of pulse and potenstiostatic mode the optical band gap energy of ZnS thin films were determined through tauc plots according to the equations [13]:
hhEg)n (4)
Where is the absorption coefficient, h is Planck Constant, A is a constant, Eg is the band gap of the material, and n = 1/2 which is a characteristic of the direct band gap absorption.
The optical band gap was calculated from the transmittance curves Fig.05 shows the plot of hversus the photon energy (h), the extrapolating of the straight lines at the horizontal axis hgives Eg values, which are 3.53 and 3.55 eV for pulse and potentiostatic ZnS thin films. These values are near to that obtained in electrochemical process.
2 3 4
(h)²(ua)
Eg(eV) (a)
1,5 2,0 2,5 3,0 3,5 4,0 4,5
Eg (eV)
(h)²(ua)
(b)
Fig. 5: hvs Eg dependence for the determination of the optical band gap energy
TABLE I. RECORDED CRYSTALYTTES SIZE, LATTICE PARAMETTRE,
CARRIER CONCENTRATION, POTENTIAL FLAT BAND AND GAP ENERGY VALUES OF THE (A) PULSE AND (B) POTENTIOSTATIC ZNS THIN FILMS.
2θ(°) D
(nm) dhkl
a Nd Efb(V) Eg(eV)
(a) 32.98 7.78 2.71 5.42 4.17
*1019
-0.39 3.55 (b) 32.93 115.61 2.72 5.43 1.94
*1022
-0.11 3.53
Conclusion
In this paper, we have described a comparison study of pulse with a potentiostatic method to elaborate ZnS thin films. The film deposited by pulsed electrochemical has a good crystalinity compared with that obtained by potentio- static method, it has had no record of Zn or S impurity. the carrier concentration of pulse is more favorable than the other obtained by potentiostatic. When the band gap value is found to be 3.5eV this value are comparable with similar study.
Those properties make the obtained thin film obtained by pulsed electrochemical good than other obtained by potentiostatic.
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