1. Introduction
The total demand of energy in the world increases with the demographic growth and the development of technology. Among all renewable energy sources, solar energy is extremely useful and promising as an abundant and clean energy source[1]. Photovoltaic conversion is one of the most advanced technologies, which consists in directly transforming solar energy into electric energy using a semiconductor and it has attracted attention for many decades. The high production cost of solar energy materials constitutes a serious drawback for the commercialization of photovoltaic cells. Furthermore, toxic substances are involved in the production and processing of most semiconductors, causing environmental problems. Thus, research efforts have been made, especially in the last decade, in order to develop materials which were able to guarantee optimal characteristics in terms of environmental compatibility, abundance and photoactivity[2]. Among the various metal oxide materials for photovoltaic applications, a promising material is zinc oxide (ZnO), one of the oldest known semiconductors. ZnO is an important binary Ⅱ-Ⅵ semiconductor compound because of its interesting properties such as resistivity control over the range 10
Various synthesis methods are usually used to prepare ZnO thin films, such as pulsed laser deposition[9], chemical vapor deposition[4], thermal oxidation[6], sol gel[10], photochemical deposition[11] and electrodeposition[12-15]. Besides these methods, electrodeposition provides several advantages over the other methods because of its simplicity, low equipment cost, the possibility of preparing large area thin films and the control of the film thickness[16]. ZnO thin films are usually electrodeposited from zinc precursor solution, such as zinc nitrate[17-20], zinc sulfate[21], zinc chloride[21-24]and zinc acetate[25, 26].
The growth mechanism in the electrochemical deposition of ZnO thin films is the reduction of an oxygen precursor at the interface of the electrode in the presence of zinc ions. Three main oxygen precursors have been described up to now: nitrate ions[12, 17, 18, 27, 28], dissolved molecular oxygen[25], and hydrogen peroxide[22, 29, 30]. Among them, the nitrate ion-based oxygen precursor has attracted considerable interest. Compared with other zinc precursors, the zinc nitrate precursor can act as both the zinc and oxygen precursor, which will simplify the electrolyte composition, and widen the adjustable range of oxygen concentration[31]. In order to improve the quality of the deposited films such as uniformity, adhesion and crystallinity, it is necessary to add a complexing agent into the electrolytic bath. Many researchers use various complexing agents such as polyvinylpyrolidone[19], lactic acid[20], tartaric acid[24], ethylene diamine tetra acetic acid[23, 26], citric acid[26] and sodium thiosulfate[11, 21, 32]. Furthermore, sodium thiosulfate shows a promising complexing agent of the deposited ZnO thin films because of its non-toxicity and low-cost compared to other complexing agents[19, 20, 23, 24, 26]. No work has been published concerning the electrodeposition of ZnO using sodium thiosulfate with zinc nitrate as precursor.
The aim of this work is devoted to the preparation of ZnO thin films onto Cu and ITO-coated glass substrates by electrodeposition technique in a solution of zinc nitrate with sodium thiosulfate. Cyclic voltammetry and chronoamperometry were utilized to study the electrochemical behavior of electrolyte bath containing zinc nitrate. The effect of sodium thiosulfate on the electrochemical deposition, structural and morphological of ZnO thin films was investigated. Deposited films were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), optical, photoelectrochemical (PEC) and electrical measurements.
2. Experimental details
2.1 Reagents
All chemical reagents used in the present work were of analytical grade. Zinc nitrate hexahydrate (Zn (NO
2.2 Preparation and characterization techniques of ZnO thin films
Electrodeposition of ZnO thin films was carried out using a three electrode electrochemical cell with a platinum (Pt) wire as a counter electrode, and copper (Cu) plates or indium tin oxide (ITO)-coated glass (8-10
Cyclic voltammetry and chronoamperometry studies were carried out using Princeton Applied Research Model 273 A Potentiostat/Galvanostat, coupled to a personal computer with Power Suite software for data acquisition and potential control. Thin films of zinc oxide were electrodeposited at −0.60 V for 30 min. Following the deposition, the deposited ZnO films were dried in air at 100 ℃.
The crystalline phase of ZnO thin films was investigated by X-ray diffraction using a Bruker Discover D8 Diffractometer with CuKα radiation (
3. Results and discussion
3.1 Electrochemical studies
3.1.1 Cyclic voltammetry
Fig. 1 shows the cyclic voltammograms of the solution containing 0.1 M zinc nitrate without (Fig. 1(a)) and with 1.2 mM sodium thiosulfate (Fig. 1(b)) at the same pH = 5.74 and
Zn(NO3)2→Zn2++2NO3−, |
(1) |
NO3−+H2O+2e−→NO2−+2OH−, |
(2) |
Zn2++2OH−→Zn(OH)2→ZnO+H2O. |
(3) |
From
Zn2++2e−→Zn, |
(4) |
2H++2e−→H2. |
(5) |
During reverse anodic scan, an oxidation peak is observed at 0.8 V, which can be attributed to the anodic dissolution of zinc metal[27, 29, 33]. From Fig. 1(b), we can observe that in the presence of sodium thiosulfate, the cathodic peak current of the formation of ZnO is much higher which indicates an increase in the deposition rate of ZnO thin films, whereas the oxidation peak of zinc metal is decreased compared to that without sodium thiosulfate (Table 1). This difference can be attributed to a complexing effect of the Zn
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3.1.2 Chronoamperometry
Fig. 2 shows the chronoamperometry curve for the deposition of ZnO thin films onto Cu substrate with sodium thiosulfate at
3.2 Characterization of ZnO thin films
3.2.1 XRD analysis
The thickness of the deposited films was measured to be 357 nm. Fig. 3 shows the XRD patterns of ZnO thin films deposited onto ITO-coated glass substrates without (Fig. 3(a)) and with sodium thiosulfate (Fig. 3(b)). From this figure, X-ray diffraction patterns indicate that the obtained ZnO thin films have a hexagonal wurtzite-type structure with preferable (002) growth direction and all the peaks of ZnO thin films correspond to the peaks of standard ZnO (Zincite phase JCPDS 36-1451). For the films prepared from the nitrate zinc solution containing sodium thiosulfate, the intensity of the (002) diffraction peak of ZnO is significantly higher than that of the films without sodium thiosulfate. It is also interesting to note that the sodium thiosulfate improved the good crystallinity of deposited ZnO thin films. The values calculated for lattice constant parameters of the ZnO films (
The crystallite size (
D=0.89λ/βcosθ, |
(6) |
where
The comparison of the observed
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The dislocation density (
δ=1/D2. |
(7) |
Strain (
ε=βcosθ/4. |
(8) |
The evaluated structural parameters of deposited ZnO with sodium thiosulfate are regrouped in Table 3, which represents the values of full width at half maximum, the crystallite size, the dislocation density (
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3.2.2 FTIR analysis
The ZnO thin films deposited onto Cu substrate with sodium thiosulfate were also examined by FTIR spectroscopy, the spectrum of which is shown in Fig. 4. From this spectrum, it can be observed apparently that there is a strong absorption band at 558 cm
3.2.3 SEM analysis
The surface morphologies of ZnO films deposited onto ITO-coated glass substrates obtained without and with sodium thiosulfate are shown in Fig. 5. The surface morphologies of deposited ZnO films prepared without sodium thiosulfate (Figs. 5(a) and 5(b)) shows non homogeneity and cracked growth surface. When sodium thiosulfate was added (Figs. 5(c) and 5(d)), we observed that the film structure is dense and uniform over a wide surface and composed of flower-like ZnO agglomerates with star-shape. Such type of star-shape and flower-like are also observed when ZnO thin films are deposited onto various substrates by different zinc salt solutions[13, 39].
3.2.4 Optical properties
The optical transmittance spectrum of ZnO thin films deposited with sodium thiosulfate from wavelength range of, 200-1100 nm taken at room temperature is shown in Fig. 6. It was shown that the films present a high optical transmission (>80%) in the visible wavelength range, which confirms the good optical quality of the electrodeposited ZnO thin films. The absorption coefficient (α) was determined in the order of >10
The plot of
(αhν)2=A(hν−Eg), |
(9) |
where
3.2.5 Photoelectrochemical measurements
Fig. 8 shows photoelectrochemical (PEC) response of zinc oxide (ZnO) deposited with sodium thiosulfate onto ITO-coated glass substrates at
3.2.6 Electrical properties
The Hall effect measurement results showed that the ZnO thin films deposited with sodium thiosulfate have n-type conductivity with carrier concentration of-1.3
4. Conclusion
Zinc oxide (ZnO) thin films have been successfully electrodeposited onto Cu and ITO-coated glass substrates from an aqueous zinc nitrate solutions with addition of sodium thiosulfate at 90 ℃. We found that the addition of sodium thiosulfate has a strong effect on the electrochemical reaction kinetics, crystallinity and uniformity of ZnO thin films. ZnO thin films were deposited at