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$^{\mathrm{-3}}$-10$^{\mathrm{5\thinspace }}\Omega \cdot$cm, transparency in the visible range, high electrochemical stability, direct band gap (3.3 eV) with large exciton binding energy of 60 meV at room temperature, absence of toxicity and good abundance in nature and high absorption coefficient suitable for solar cell applications. ZnO crystallizes preferentially in the hexagonal wurtzite structure and presents typically n-type electrical conductivity due to residual donors^[3]. Thus it is promising in various applications, including laser diode^[4], electronic and optoelectronic^[5], phosphors^[6], gas sensors^[7]. They also can be used as transparent electrodes in dye-sensitized solar cells^[8].

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$_{\mathrm{3}})_{\mathrm{2}}$$\cdot$6H$_{\mathrm{2}}$O, 98%, Sigma Aldrich) was used as zinc and oxygen sources. However, sodium thiosulfate pentahydrate (Na$_{\mathrm{2}}$S$_{\mathrm{2}}$O$_{\mathrm{3}}$$\cdot$5H$_{\mathrm{2}}$O, 99.5%, Sigma Aldrich) was added as complexing agent. All the aqueous solution was prepared using distilled water.

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 $\Omega$/square, Sigma Aldrich) substrates were used as working electrode. A saturated calomel electrode (SCE) was used in all experiments as a reference electrode. Before the deposition, the substrates were ultrasonically rinsed in 0.1 M HCl, distilled water, and acetone and then rinsed in distilled water^{[31, 24]}. Zinc oxide thin films were electrodeposited in an aqueous solution of 0.1 M Zn (NO$_{\mathrm{3}})_{\mathrm{2}}$$\cdot$6H$_{\mathrm{2}}$O and 1.2 mM Na$_{\mathrm{2}}$S$_{\mathrm{2}}$O$_{\mathrm{3}}$$\cdot$5H$_{\mathrm{2}}$O at 90 ℃. The pH of the solution was adjusted to 5.74 by NaOH additions. Sodium thiosulfate is used to stabilize Zn$^{\mathrm{2+}}$ ions in the solution^{[11, 21]}.

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 CuK$_{\alpha}$ radiation ($\lambda = 1.5406$ Å). FTIR spectra were recorded on a Shimadzu FTIR-8000 series spectrophotometer in the wavelength range of 400-4000 cm$^{\mathrm{-1}}$. Scanning electron microscopy imaging was performed using a VEGA3 TESCAN scanning electron microscope operating at 20 kV accelerating voltage. Optical transmittance of thin films was measured by a Perkin Elmer Lambda 950 UV-vis-NIR spectrometer in the wavelength range 200-1100 nm. Photoelectrochemical measurements were carried out by cyclic voltammetry technique in a three electrode electrochemical cell using deposited ZnO films as a working electrode, platinum wire as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode from an aqueous solution containing 0.1 M sodium sulfate (Na$_{\mathrm{2}}$SO$_{\mathrm{4}})$. A tungsten halogen lamp (100 W) was used to illuminate an active area of 1.5 cm$^{\mathrm{2}}$ of ZnO electrode. Electrical properties of deposited ZnO films were determined by a HMS 3000 Hall measurement system at room temperature. Ag-paste was used at the corners of the films to make four good contacts with the probes. Thickness of the films was measured using Dektak stylus profilometer.

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 $T = 90$ ℃. The potential sweep was ranged from $-0.2$ to $-1.2$ V with a scan rate of 50 mV/s. From Fig. 1(a), the cathodic peak is observed around $-0.61$ V corresponding to the reduction of nitrate to nitrite ions with water molecules converted to hydroxide ions. These hydroxide ions combined with Zn$^{\mathrm{2+}}$ ions leading to the formation of Zn (OH)$_{\mathrm{2}}$, which dehydrated and converted to ZnO by the following mechanism^[12]:

$\text{Zn}{{\left( \text{N}{{\text{O}}_{3}} \right)}_{2}}\to \text{Z}{{\text{n}}^{2+}}+2\text{N}{{\text{O}}_{3}}^{-},$

(1)

$\text{N}{{\text{O}}_{3}}^{-}+{{\text{H}}_{2}}\text{O}+2{{\text{e}}^{-}}\to \text{N}{{\text{O}}_{2}}^{-}+2\text{O}{{\text{H}}^{-}},$

(2)

$\text{Z}{{\text{n}}^{2+}}+2\text{O}{{\text{H}}^{-}}\to \text{Zn}{{(\text{OH})}_{2}}\to \text{ZnO}+{{\text{H}}_{2}}\text{O}.$

(3)

From $-1.1$ V, an abrupt increase of cathodic current takes place, which can be mainly ascribed to the deposition of zinc with release of hydrogen [Eqs. (4) and (5)].

$\text{Z}{{\text{n}}^{\text{2+}}}\text{+2}{{\text{e}}^{-}}\to \text{Zn, }$

(4)

$2{{\text{H}}^{+}}+2{{\text{e}}^{-}}\to {{\text{H}}_{2}}.$

(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$^{\mathrm{2+}}$ ions by the sodium thiosulfate, leading to acceleration of the electrochemical reaction kinetics for the formation of ZnO thin films. This phenomenon was observed in Refs. [21, 24, 34]. The ratio between cathodic peak current with and without sodium thiosulfate is about 3.18. This value is higher than that of other complexing agents^{[20, 24]}.

3.1.2   Chronoamperometry

Fig. 2 shows the chronoamperometry curve for the deposition of ZnO thin films onto Cu substrate with sodium thiosulfate at $T = 90$ ℃ and $E = -0.6$ V. The current starts with high value, which is assigned to a rapid nucleation growth. The decrease of cathodic current as function of time is due to a depletion of the metal-ion concentrations close to the electrode surface. The current stabilized at even longer time (1200 s). This confirms that the limited value of the stationary current is related to the formation of zinc oxide on the surface. A similar behavior was observed for different zinc salt solutions with other complexing agents^{[13, 19, 26]}.

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 ($a = b = 3.2909$ Å, $c = 5.1926$ Å) were slightly different than those of standard ZnO (JCPDS 36-1451, $a = b = 3.249$ and $c = 5.206$ Å)^[18].

The crystallite size ($D)$ of ZnO thin films was calculated from the major diffraction peaks of the base of (002) using Scherrer's formula (Eq. (6))^{[10, 35]}:

$D=0.89\lambda /\beta \cos \theta,$

(6)

where $\lambda$ is the wavelength of the incident beam (1.5406 Å), $\beta$ is the full width at half maximum of the diffraction peak (002) and $\theta$ is the Bragg diffraction angle of the XRD peak in rad. The average crystallite size of deposited ZnO with sodium thiosulfate is found to be 34.69 nm. This value is greater than that of ZnO films without sodium thiosulfate (33.83 nm).

The comparison of the observed $d$ values of ZnO films with JCPDS data (36-1451) are shown in Table 1. The value of $d$-spacing for ZnO films obtained was a little different than that of the $d$-spacing for standard hexagonal ZnO [JCPDS 36-1451], suggesting that all the ZnO films exhibited tensile strain and dislocation density, which is reported by others^[36-38].

The dislocation density ($\delta )$, defined as the length of dislocation lines per unit volume, was evaluated from Eq. (7)^[10]:

$\delta =1/D^{2}.$

(7)

Strain ($\varepsilon )$ of the thin films was calculated by Eq. (8)^[10]:

$\varepsilon =\beta \cos \theta /4.$

(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 ($\delta )$ and the strain ($\varepsilon )$ of the ZnO thin films from the (002) direction.

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$^{\mathrm{-1}}$ associated with the characteristic vibrational mode of ZnO. The broad absorption band centered at 3699 cm$^{\mathrm{-1}}$ corresponds to the-OH group. Similar bands appeared in zinc acetate electrolyte for the deposition of ZnO thin films onto stainless steel substrate^{[10, 35]}.

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 ($\alpha)$ was determined in the order of >10$^{\mathrm{5}}$ cm$^{\mathrm{-1}}$.

The plot of ${(\alpha h\nu )}^{2}$ versus photon energy ($h\nu )$ of deposited thin films was presented in Fig. 7. It has been observed that the plots of ${(\alpha h\nu )}^{2}$ versus ($h\nu )\thinspace$are linear over a wide range of photon energies indicating the direct type of transitions. The band gap of the films was determined from Tauc's formula, which is as follows Eq. (9)^[40]:

${(\alpha h\nu )}^{2}=A\left( h\nu -E_{\rm g} \right),$

(9)

where $\alpha$ is absorption coefficient, $h$ is Planck's constant, $\nu$ is the photon energy, $A$ is a constant and $E_{\rm g}$ is the direct transition band gap. The optical band gap value was determined by extrapolating the linear portion of the plot of ${(\alpha h\nu )}^{2}$ versus photon energy ($h\nu )$, which is illustrated in Fig. 7. The band gap of ZnO thin films was estimated to be 3.28 eV, which is in good agreement with the values reported for ZnO films by others^{[41, 42]}. The obtained energy gap results make ZnO film a promising semiconductor material for fabrication of photovoltaic solar cells.

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 $-0.6$ V for the solution containing 0.1 M sodium sulfate (Na$_{\mathrm{2}}$SO$_{\mathrm{4}})$, in the dark and under illumination. In the dark, there is a negligible anodic photocurrent; however, under illumination of the surface of deposited zinc oxide, we observe an important anodic photocurrent compared with in the dark. These observations suggested that the excited minority carriers (h$+$*) diffuse to the surface to participate in the electrochemical reaction at the electrode interface with the ions present in the electrolyte^[21]. The anodic photocurrent generation represents the typical behavior of the n-type semiconductor of the ZnO thin films. This n-type semiconductor was observed when ZnO thin films were deposited onto various substrates by different zinc salt solutions^{[35, 43]}. n-type electrical conductivity of ZnO films will be also confirmed below by Hall effect measurements.

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 $\times$ 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$, mobility of 7.35 cm$^{\mathrm{2}}$V$^{-1}$s$^{-1}$ and a low electrical resistivity of 6.54 $\Omega$$\cdot$cm. These values are in agreement with the results reported by Chen^[44].

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 $-0.60$ V for 30 min with a film thickness of about 357 nm. X-ray diffraction analysis revealed that the synthesized ZnO thin films have a hexagonal wurtzite structure. The crystallite size was estimated to be 34.69 nm. FTIR results confirmed the presence of ZnO films at peak 558 cm$^{\mathrm{-1}}$. SEM images of ZnO films showed uniform, compact morphology without any cracks and consisting of flower-like ZnO agglomerates with star-shape. Photoelectrochemical measurements of ZnO thin films showed that this film behaves as a semiconductor of type n and presents high photo anodic-generated currents. These films exhibited a high optical transmission (> 80%) and high absorption coefficient ($\alpha >10^{\mathrm{5}}$ cm$^{\mathrm{-1}})$ in the visible region. The optical energy band gap was found to be 3.27 eV. Hall effect measurement results confirmed that the ZnO thin films have n-type conductivity with a low electrical resistivity of 6.54 $\Omega$$\cdot$cm, carrier concentration of $-1.3 \times 10^{\mathrm{17}}$ cm$^{\mathrm{-3 }}$ and mobility of 7.35 cm$^{\mathrm{2}}$V$^{-1}$s$^{-1}$. Taking account of its non-toxicity and low-cost, sodium thiosulfate could be used as a promising complexing agent to prepare ZnO thin films with suitable properties for fabrication of photovoltaic solar cells.