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1. Introduction
Our current reliance on fossil fuels for energy generation can lead to significant ecological and economic issues. The use of renewable energy sources could help mitigate [1]. Fuel cells are one option because of their potential to turn chemical energy into electrical energy with minimal damage to the environment [2].
There are currently numerous types of fuel cells in use [2-4]. Methanol and ethanol are used as chemical fuels in direct alcohol fuel cells, which have been hailed as a promising renewable energy technology. It is supported by scholarly evidence [5] that the direct oxidation of alcohols has various benefits, including increased efficiency, a wider range of viable electrode materials, and reduced interference from the oxidation of other organic fuels. Methanol offers many advantages over ethanol, including being cheaper overall, having a higher energy density, being readily available from multiple sources, and being easy to store and transport. Since alcohols play such a crucial role in direct alcohol fuel cells, research into their electrochemical oxidation has received a lot of attention [6]. The primary difficulty is finding a suitable replacement for the expensive platinum and palladium alloys currently used in alkaline batteries. There has been significant work toward developing highly active electrocatalysts for alcohols [5] by employing metal oxides as support systems for catalytic metal sites, such as TiO2 [7], MnO2 [8], NiCO2O4 [9], etc.
Zinc oxide (ZnO) is a notable nontoxic n-type semiconductor with a wide direct band gap at the ambient temperature of 3.37 eV because of its chemical stability as well as its electrical and optical features, such as n-type conductivity and a wide range of visual transmittance. This is because ZnO has a wide range of visual transmittance [10]. Solar cells [11], sensors [12], batteries [13], smart windows [14], photocatalysts [15], etc., all make use of ZnO to varying degrees. Sputtering [16], chemical vapor deposition [17, 18], sol-gel using a spin coating or dip coating procedure [19, 20], electrodeposition [21], hydrothermal growth [22, 23], and spray pyrolysis [24] are only some of the methods that have been used to create ZnO thin films. Since it is straightforward, inexpensive, and can potentially be used for massive industrial production, the electrodeposition method stands out as a crucial and straightforward technology for the creation of thin films. In addition, in comparison to other methods, this one offers a number of advantages, the most notable of which is its low processing temperature, its adaptability in terms of substrate geometry, and its manageable film thickness [25]. Studies on modifying the morphology of electrodeposit-formed ZnO films have been conducted on multiple occasions [26]. Electrodeposition potential [27], annealing temperature [28], and growth rate [29] are just a few of the parameters that may be changed to produce a variety of morphologies. Particularly, it has been shown that pulsed electrodeposition (PE) encourages the nucleation and organization of small crystals [30] and improves attributes like porosity [31], ductility [32], hardness [33], and surface roughness [34]. Therefore, it should be feasible to optimize films for any desired morphology and attributes by adjusting the deposition settings.
Using a technique known as pulsed-pulse electrodeposition, a modified ZnO film electrode was created for the purpose of investigating electrocatalytic oxidation. It’s also worth noting that, to the best of our knowledge, no prior work has been done on the oxidation of methanol to determine the electrocatalytic oxidation of ZnO films using a process similar to the one we used.
2. Experimental
The ZnO thin films employed in the electrocatalytic oxidation of alcohols were synthesized using a pulsed electrodeposition (PE) method. Electrodeposition was carried out in a solution containing 0.1 M Zinc Nitrate (Zn (NO3)2: Sigma-Aldrich) as the Zn2+ source, 5.5 starting pH of deionized water and a synthesis temperature of 80 ± 1°C.
The conventional three-electrode cell was used to carry out the pulsed electrodeposition procedure. For the electrodes, we employed 2.2 cm2 of sputtered glass coated with indium tin oxide (ITO), a platinum mesh counter electrode, and a saturated calomel electrode (SCE) as the reference. Films were electrodeposited, then washed in deionized water, then annealed at 400°C for 1 h, with a 5°C·min -1 heating rate.
The acquired footage is analyzed using a number of various analysis techniques. To create the electrodeposition, a Voltalab PGZ 301 potentiostat-galvanostat was used. The crystal structure of the samples was confirmed by X-ray diffraction (XRD) using a CuK radiation (λ = 1.5406Å) Bruker D8 Advance diffractometer fitted with a linear Vantec super speed detector. Scanning Electron Microscopy (SEM) was used to examine the surface morphology at 10.00 KV. Quantitative atomic force microscopy (AFM) measurements were taken to delve deeper into the morphology’s finer points. Using a Shimadzu UV-1800 spectrophotometer, we measured the samples’ optical transmittance in the range of 200 to 1200 nm. In 0.1 M K2SO4 electrolyte, photocurrent was measured using a monochromatic UV-lamp at λ = 365 nm. To investigate electrochemical methanol oxidation, a ZnO/ITO (2.2 cm2 in the geometric area) working electrode was used. As a counter electrode, a platinum wire was employed, and as a reference electrode, a saturated calomel electrode (SCE) was put to good use. The conventional three configuration of an electrochemical quartz cell was used in this experiment. CV measurements were taken during electrochemical oxidation with a potentiostat/galvanostat (Volta-lab PGZ 301) throughout a 0-1 V potential range. The NaOH content in the starting solution was 0.1 M, and the amount of methanol was variable.
3. Results and discussion
3.1. Film preparation
ZnO thin film pulsed electrodeposition (PE) curves onto ITO at two potentials are depicted in Fig. 1. The deposition potential is -1.1 V vs. SCE for the time-on (t on ) portion of each pulse. during the time-off (t off ) phase, an open-circuit potential (OCP = +0.27 V vs. SCE) is applied. The on-time for deposition was set at 5 s and the off time was set at 1 s.
Figure 1 Pulsed electrodeposition of ZnO thin films for the first five cycles in a 0.1 M Zn(NO 3)2 solution (t on = 5 s, t off = 1 s).
Whenever the time-on potential is applied, a negatively charged layer is created around the substrate (ITO) in PE. This layer thickens to a predefined threshold, blocking access to the substrate for the ions. This generated layer is then discharged under the time-off potential (OCP). So, the ions can more easily penetrate the layer and reach the substrate underneath. During time Off, the bath’s ions move to a deprived area. After a short delay, the pulse is turned on again, and this time there are more uniformly dispersed ions available for deposition into the substrate [35].
3.2. Crystalline structure
ZnO thin films deposited through pulsed electrodeposition on ITO glass substrates are shown as XRD patterns in Fig. 2. All the patterns’ peaks correspond well to the hexagonal wurtzite type (JCPDS 35 - 1451), with (002) being the preferred orientation of ZnO. It is well known that the growth of ZnO crystallites along the c-axis is congruent with the enhancement in (002) reflection, measured in terms of relative intensity [36]. Narrow diffraction peaks show and hint at the good crystalline quality of the deposited films [37].
The crystallite sizes ‘D’ of the films were calculated by the well-known Debye-Scherrer equation
where a and c lattice parameters, the volume of the lattice, d (hkl) is the interplanar spacing, D is the crystallite size in nm, (λ = 1.5406) is the wavelength of CuKα radiation, k is the shape factor, β is the full-width half maximum (FWHM) intensity and d (hkl) is the interplanar spacing. The lattice parameters a and c calculated from the diffractograms are found to be less than those typically seen for undoped ZnO (a = 3.249 and c = 5.207) [1] as shown in Table I, as well as the volume of the lattice (V = 47.62 Å) [1]. This difference is significant and may be due to several factors [2,3]. The grain size of the ZnO thin films prepared by PE is 90.65 nm. In addition, our film has a crystal orientation preference and a high (002) peak intensity. To improve this finding, we must determine the preferential orientation by calculating the texture coefficients using Eq. (8). Observe that the texture coefficient of the (002) plane (TC (002)) is greater than TC (103), indicating that the (002) peak is now preferentially oriented.
Table I Calculated structural parameters for ZnO electrode-posited by EP.
| sample | PE |
|---|---|
| 2θ (Å) | 34.58 |
| β | 0.1827 |
| d(Å) | 2.591 |
| c(Å) | 5.183 |
| a(Å) | 2.992 |
| V (Å3) | 41.10 |
| D (nm) | 90.65 |
| Dislocation Density (δ) (10 -3 line/m 2) | 0.73 |
| Tc(002) | 1.289 |
| Tc(103) | 0.763 |
| Strain(𝜖)(10 -3 ) | 0.93 |
Furthermore, strain (𝜖) and dislocation density (δ) were calculated using Eqs. (6) and (7), and the values are presented in Table I.
where I(hkl) and I 0(hkl) denote the measured intensity and the intensity of the standard powder diffraction pattern, respectively, and N is the number of considered reflections.
3.3. Morphological characterizations (SEM and AFM)
Scanning electron microscopy was used to examine the films’ morphology after deposition (SEM). In Fig. 3a) and b) display the SEM picture b). The results of pulsed electrodeposition (t on = 5 s, t off = 1 s) show that the ZnO nanostructures are spread evenly across the ITO surface, with hexagonal nanorods and some nanosheets of significant size having formed. However, it is observed that as the nanosheets assemble into longer blooms, their density decreases. Atomic force microscopy better revealed the ZnO growth in 2D and 3D directions (AFM). Figure 4 displays the results of the AFM analysis of ZnO thin films. Figure 4a) and b) display ZnO thin films with a surface topography consisting of uniformly sized and distributed hexagonal structures. The rms roughness of the films before oxidation is 11.9 nm and after oxidation, it is 22.6 nm. As shown in Fig. 4, the maximum roughness of a few ZnO sites reaches high values. Due to the rarity of these locations, however, their impact is greatly diminished when represented as rms roughness. Figure 5a) shows an AFM image showing the ZnO layer pre-oxidation, showing that it is produced by vast domains. After oxidation, as seen in Fig. 5b), oxide grains form on the ZnO layer’s surface, leading to an increase in RMS roughness.
Figure 3 SEM image of ZnO thin films deposited by PE (t on = 5 s, t off = 1 s) a) before oxidation, b) after oxidation.
Figure 4 AFM surface images (10 × 10 μm2) of ZnO thin films deposited by PE (t on = 5 s, t off = 1 s) a) before oxidation, b) after oxidation.
3.4. Optical properties
As a function of wavelength, the optical transmittance curves are displayed in Fig. 6a). In the range of 300-900 nm, researchers measured the optical transmittance spectra of pure ZnO. The transmittance of the ZnO film is around 53%. AFM micrographs show that the growth of ZnO crystallites has become more uniform in both its repartition and orientation, which accounts for the higher value. Indeed, it has been reported that the films are suitable candidates as transparent conductive oxide (TCO) for electrochemical and photoelectrochemical (PEC) applications since the optical transparency greatly depends on the surface irregularity and crystallinity of the films.
Figure 6 Optical transmittance a) and the estimation of band gap b) of ZnO thin films deposited by PE (t on = 5 s, t off = 1 s) c) ZnO films after oxidation.
The optical band gaps of the samples were calculated from the transmission data using the Tauc equation is given as follows:
where α, h, υ, B, and E g respectively define the absorption coefficient, the Planck constant, the frequency, a constant, and the band gap. A straight band gap exists in ZnO. Figure 6 depicts the transmission spectrum and the band gap obtained by extrapolating the plot of (αhυ)2 vs. h (Tauc’s relationship) to the x-axis (b). So, the film had a band gap of roughly 3.26 eV. Our results for ZnO thin films correlate well with those found in the literature [38].
After oxidation, the transmittance of ZnO films increases dramatically to 70% at 600 nm, and their absorption edge shifts to longer wavelengths, indicating a slight reduction in the band gap. It is the increased catalytic oxidation of the surface layer of the film during oxidation that causes the increase in transmittance. The shrinkage of the grain size during catalytic oxidation may account for the decrease in the band gap.
3.5. Photoelectrochemical (PEC) properties
A photoelectrochemical analysis was performed to determine the conduction type of ZnO, as described above. The photocurrent responses of ZnO films are shown in Fig. 7 under 5-second ON-OFF cycles of UV light. The films were deposited at times of 5 s and 1 s. Under light, ZnO films exhibit an anodic (positive) current, revealing their n-type semiconductor conductivity [39-41]. Photo-responses were measured to be 305.17 A·cm -2 for ZnO films deposited at t on = 5 s, t off = 1 s when exposed to a constant potential of +0.8 V.
3.6. Electrocatalytic oxidation of alcohols
Using cyclic voltammetry in 0.1 M NaOH with an organic molecule of methanol, ZnO thin films were deposited onto an ITO substrate from a solution of 0.1 M Zn(NO3)2 for pulsed electrodeposition time-On value of 450 s. This allowed for an investigation of the ZnO’s electrochemical behavior in basic media.
The ITO substrate was electrochemically silent in the potential range of interest, as evidenced by the lack of any appreciable current at the surface of the ITO electrode in the presence of organic molecules [Fig. 8a)]. In addition, no appreciable current is measured at an ITO electrode when the organic molecules are present, suggesting that the ITO substrate lacks electrocatalytic activity for the oxidation of these compounds. The oxygen evolution reaction is shown to begin at a potential of 650 mV in the absence of alcohols [Fig. 8b), curve 1], and to increase toward a more positive potential of 700 mV in the presence of organic molecules [Fig. 8b)], curves 2, 3, and 4).
Figure 8 a) is the cyclic voltammograms in 0.1 M NaOH of ITO in the presence of alcohols. Figure 8b) presents cyclic voltammograms of ZnO thin Films in 0.1 M NaOH in the absence of methanol (1) and the presence of differences volumes of methanol 1 mL (2); 2 mL (3) and 3 mL (4).
The irreversible oxidation of alcohol is reflected as a single oxidation peak at the ZnO electrode. Rapidly increasing over time, the methanol oxidation current reaches its maximum at a voltage of around 580 mV. ZnO thin films produce an anodic current [Fig. 8b), curve 1] that is much lower than the currents obtained in solutions containing organic molecules [Fig. 8b), curves 2, 3, and 4]. The anodic current and the potential value of the oxidation peak in the anodic direction both rise with increasing alcohol volume. Since more molecules will be oxidized on the ZnO thin films as the volume increases, the oxidation current will also rise.
4. Conclusion
To produce ZnO thin films on an ITO substrate using pulsed electrodeposition (PE), it was necessary to optimize the conditions based on the data presented in this contribution. Produced ZnO films possessed a wurtzite structure with a pronounced orientation preference (002). Because the films exhibit large photogenerated currents when exposed to light, the process of pulsed electrodeposition offers potential as a way for preparing expensive window material for electrochemical and photoelectrochemical cells. This is because of the nature of the films. In light of the findings that were obtained, it was determined that the electrocatalytic oxidation of methanol could be performed with the assistance of zinc oxide that had been produced through the technique of pulsed electrodeposition.
