Electrochemical behavior of spray deposited nickel oxide (NiO) thin film in Alkaline electrolyte

Nickel oxide (NiO) Thin film was successfully deposited on the glass substrate using an inexpensive spray pyrolysis (SP) technique. The structural, morphological, and optical properties have been studied, thus the electrochemical behavior of NiO film in Alkaline electrolytes has been investigated. The X-ray diffraction (XRD) analysis showed that NiO thin film exhibit a polycrystalline cubic rock-salt structure with a preferential orientation on the plane (111). This result was confirmed using Raman spectroscopy. The Scanning Electron Microscopy (SEM) images exhibit a smooth and dense surface without major cracks. Optical analysis shows an average transmission of about 55% in the visible light range, and the optical band gap energy was estimated by Tauc’s method and showed a value of 3,71 eV. Electrochemical properties as specific capacitance (Csp), optical density variation (ΔOD), and Coloration efficiency (CE) were studied using cyclic voltammetry in 1M KOH and 1M NaOH electrolytes. The results indicated that the behavior of the NiO layer in KOH is more effective than in NaOH electrolytes.


Introduction
For several years, transition metal oxides have attracted a noticeable attention of researchers due to their promising characteristics for various application fields. Among these materials, nickel oxide (NiO) is an attractive material because of its good magnetic, thermal, optical, mechanical and, electrical performance [1], [2], [3]. The stoichiometric NiO crystal is an insulator, whereas the conductivity can be enhanced in pure NiO by creating Ni vacancies [4]. The most attractive properties of NiO are; low material cost, wide bandgap semiconductor (3.6-4.0 eV) [5], excellent electrochemical stability [6], and finally the possibility of synthesis by a variety of techniques [7].
In this work, a thin film of NiO was synthesized by the spray pyrolysis method. The structural, optical, and morphological properties of the NiO layer have been studied. Thus, the electrochemical behavior in Alkaline electrolytes has been demonstrated using cyclic voltammetry (CV).

Experimental procedure
Before depositing the film, the glass substrates were cleaned chemically in Acetone, Ethanol, and dilute hydrochloric acid (HCl) respectively for 15 minutes in each solution, separated by rinsing in distilled water. Then several tests were made to find the optimal conditions for the preparation of the specimen using the spray pyrolysis method. A detailed description of this technique has been explained in [26]. After the optimization of parameters, a NiO thin layer was deposited on an amorphous glass substrate preheated at 450°C. This temperature was chosen during the optimization of the deposit parameters because, at other temperatures such as 350, 400 and 500, impurities have appeared in the thin films such as H 2 O and metallic nickel. 0,025 mol/l of hexahydrate nickel chloride (NiCl 2 •6H 2 O) was used as a precursor and dissolved in 50 ml of distilled water. The deposit parameters are summarized in Table 1

Characterization techniques
X-ray diffraction analysis (XRD) was performed using a D8 ADVANCE X-ray diffractometer (λKα1, Cu). To confirm the crystalline phase a micro-Raman spectroscopy SENTERRA II, with a laser source set at 532 nm was used. The SHIMADZU Ultraviolet-Visible Near Infrared (UV-Vis-NIR) spectrometer was adopted to inspect the optical properties. The morphological characteristics were explored by scanning electron microscopy (SEM), with an accelerated voltage at 10 kV, a magnification of x40K, and an emission current of 120 μA.
For the electrochemical study, NiO thin film was deposited on Indium Tin Oxide (ITO) substrate. This film was immersed in two different electrolytic solutions. The first is 1M KOH and the second is 1M NaOH. The cyclic voltammetry measurements were based on a three-contact electrode system; saturated calomel (SCE) as the reference, a platinum sheet as a back-contact electrode, and NiO/ITO as the working electrode.

X-ray diffraction results
The XRD diagram of the NiO layer deposited on a glass substrate is shown in Figure 1. The pattern shows a polycrystalline film with a cubic structure. The diffraction peaks observed at the 2θ diffraction angles 37,26°; 43,31° and 79,36° are attributed to the crystallographic planes (111), (200) and (222), respectively with a strong preferred orientation corresponding to the direction (111). Using the International Center for Diffraction Data (ICDD) card number (04-0835), these peaks are indexed in a cubic rock-salt structure. These results match other studies [27], [28]. The structural parameters of the sprayed thin film are listed in Table 2. The average size of the crystallite (D hkl ) corresponding to NiO is estimated from the XRD results using the Scherrer equation (Eq. (1)): Where θ hkl corresponds to Bragg's diffraction angle, λ is the wavelength and β hkl is the full width at Half-maximum intensity (FWHM) of the diffraction peak.
The lattice constant was determined using the cubic structure formula (Eq. (2)).
Where hkl are the Miller indices and d hkl represents the inter-plenary distance.
The strain ε in the film can be estimated using the following equation (Eq. (3)).
Film 2θ  Figure 2 shows the Raman spectra of the NiO thin film in the range from 60 cm −1 to 1600 cm −1 . There are two prominent peaks around 550 cm −1 and 1100 cm −1 , suggesting the one-phonon first-order and two-phonon second-order longitudinal-optical modes respectively [29]. These results confirm that the nanocrystalline NiO is successfully deposited.

Morphology
The surface morphology of the NiO layer was analyzed using The Scanning Electron Microscopy at different magnifications (7500 and 40.000 magnification). Figure 3 represents the morphological properties of the sprayed film on the glass substrate. The images of the NiO layer exhibit a smooth and dense surface suggesting a poor surface area of the film which is not helpful for a good electrochemical performance, However, the porous structure of the NiO layer provides a large reaction surface [30].

Optical properties
The measurement of optical spectrum for NiO thin film was carried out using a UV-VIS-NIR spectrometer. Figure 4 shows the transmission spectra of NiO thin film grown on the glass substrate in the 300-1500 nm wavelength range. Figure.4 indicates that the transmittance is increased from low wavelength to higher wavelength region and the average transmission is about 55% in the visible light range. This relatively low value of the transmission may be due to the technique used. The optical bandgap of the film was estimated using Tauc‫׳‬s equations: Where, Eg the optical bandgap energy, A is a constant, and n = 2 for direct bandgap. From the plots of (αhυ) 2 versus hυ, Eg was calculated by extrapolating the linear portion of the curve to the energy axis for (αhυ) 2 = 0 ( Figure 5). The calculated energy bandgap value is 3,71 eV.

Electrochemical measurements
For the electrochemical study, a new sample is reprepared with the same conditions cited in the experimental details section except for the glass substrate which is replaced by ITO substrate to ensure the conductivity of the elaborated NiO thin film (NiO/ITO). The electrochemical properties of spray deposited NiO thin film are investigated in two different electrolytes such as 1M KOH and 1M NaOH by cyclic voltammetry. The measurement is investigated in the three-electrode system, consisting of NiO/ITO working electrode, Standard calomel electrode (SCE) reference electrode, and platinum (Pt) counter-electrode. As observed, the shape of all (a) and (b) CV curves are almost the same with two redox peaks. Oxidation peak related to charging process and reduction peak for discharging process [31]. All CV peaks show that the current density increases with an increase in scan rate which proves the direct relationship between CV current and scan rate, suggesting an ideal capacitive characteristic [32]. Also, the small separation between redox peaks suggests a fast electron transfer behavior, which is very important for energy storage systems [31].
Specific capacitance 'Csp' is calculated from the relation [33]: where, v is the potential scan rate (mV.s − 1 ), (Vc − Va) is a working potential window, (I) is the current response (mA) of the NiO electrode for the unit area (1 cm 2 ).
From Figure 7. it is observed that the value of specific capacitance decreases with an increase in scan rate for both electrolytes. This may be due to the fact that at a low scan rate there is sufficient time for transfer of charges between electrolyte and electrode interface. Maximum specific capacitance is found to be 24 F.g −1 at a scan rate of 5 mV.s −1 for KOH electrolyte. The general reaction or charge storage for nickel oxide electrode in the KOH or NaOH electrolytes is [34], [35]: NiO + xOH -↔ xNiOOH + e -Where x represents K or Na metal elements.
The transition from NiO to NiOOH after intercalation of OH − ions provoke charge transfer from Ni 2+ to Ni 3+ . Due to this transfer, the films get colored. During the cathodic scan, the reduction of Ni 3+ to Ni 2+ causes the bleaching of the film [36].
The transmittance spectra of the NiO layer in the colored and bleached states were recorded in the wavelength range of 350 to 850 nm, as shown in ( Figure  8). The values are obtained after subtracting the transmittance of the ITO substrates using a UV-Vis-NIR spectrometer. This result is further used to calculate the coloration efficiency (CE) using the equation [14]: where (ΔOD) is the change in optical density at λ = 630 nm and Q i is the intercalated charge (mC/cm 2 ).
The electrochromic parameters for the sample are given in Table 3. The value of CE is found to be a maximum of 15,15 cm 2 /C for the electrolyte KOH. These calculated values are relatively low compared to the previous works, this may be due to the smooth morphology of the thin film, which does not allow good intercalation of OH − ions [30].  The stability of the film is one of the important parameters to be taken into account for electrochemical applications [37]. The CV stability of NiO film has been investigated by recording 50 cycles of the oxydo/reduction operation at a scan rate of 20 mV/s (Figure 9). From the stability measurements, it is observed that NiO film is stable in both electrolytes.

Conclusion
In this work, Nickel oxide thin film was successfully elaborated using the spray pyrolysis technique on the glass substrate. The structural, morphological, optical, and electrochemical properties were studied using X-ray diffraction, scanning electron microscope, UV-visible-NIR spectroscopy, and cyclic voltammetry techniques.
The crystallographic study showed a polycrystalline cubic structure with preferential orientation on the plane (111). Surface morphology revealed a smooth and dense surface. Optical analysis showed an average transmission of about 55% in visible light range and gives an average direct band gap of 3,71 eV. Cyclic voltammograms of the sprayed film presented almost the same curves with two redox peaks. The Specific capacitance and the coloration efficiency of the NiO layer in 1M KOH were 24 Fg -1 (at the scan rate of 5 mv/s) and 15 cm 2 /C respectively. These values decreased in 1M NaOH electrolyte to become 20 Fg -1 and 7,8 cm 2 /C, which demonstrates the better performance of KOH than NaOH electrolyte. The stability measurement revealed that NiO film is stable in both electrolytes indicating its capability to insert/extract the ions for many cycles.