CuFeO 2 /Cu Photoelectrode Modified with SrTiO3 Perovskite for Efficient Hydrogen Generation from Sanitation Water

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Introduction
Renewable energy sources are critical to human life's actuality. The bulk of these sources are based on photocatalytic materials used in the fabrication of photodetectors, solar cells, and photocatalytic electrodes [1][2][3][4]. Hydrogen is one of the most ecofriendly and sustainable energy carriers available, and it may replace fossil fuels. Currently, most of the H 2 is generated from methane or fossil fuels with a significant carbon footprint. Extensive research is being performed to develop new technologies for economically producing hydrogen from renewable materials [5][6][7]. Photoelectrochemical (PEC) water splitting is regarded as one of the most promising methods for hydrogen production. The creation of H 2 gas by a water-splitting process is a vital energy source for aircraft, vehicles, and industries that utilize H 2 fuel. The synthesized photoelectrode must have high water-splitting efficiency and qualities that qualify it for numerous applications, such as low cost, simple preparation, easy operation, and stability [8]. The electrode can be utilized in sunlight or with another type of artificial light. Perovskite oxide substances are currently considered promising photocatalysts for a variety of potential photocatalytic applications due to their promising qualities, including compositional flexibility, excellent electronic, optical, and magnetic properties, overall resistance to photocorrosion, and excellent thermal stability properties [6,9]. These characteristics are anticipated to improve their stability and desired photocatalytic activity. The single perovskite oxides can be represented as ABO 3 , where A and B are metal cations. Alkaline-earth metal cations, transition metal cations, or alkali metal cations are the three options for A and B [10][11][12].
The morphologies and surface properties of several perovskite oxides, including SrTiO 3 , KTaO 3 , NaTaO 3 , KNbO 3 , Zn 1-x Ba x Fe 2 O 4 , ZnFe 1.96 La 0.04 O 4 , BiFeO 3 , and NaNbO 3 , were observed to have a significant impact on the photocatalytic activities in this context [13][14][15]. The main characteristics of perovskite oxide-based photocatalysts in solar-driven photocatalytic hydrogen evolution reaction (HER) have been particularly emphasized in several great academic studies over the past few years. Recent studies have proven the insertion of transition metal causes additional enhancement in these perovskite materials that appear as enhancements in the optical and electrical properties [16][17][18][19][20].
Recent advances in single perovskite oxide ABO 3 -based photocatalysts, perovskite materials, and SrTiO3-based photocatalysts for use in HER via water splitting were also discussed [21]. Certainly, photocatalysts based on perovskite oxide will play a vital role in solar-driven HER during the next years. It is found that the current H 2 evolution rate is still low and will require further enhancement in the future. This has motivated the scientific community to focus on understanding the existing state of affairs and ideas for improving the overall quality, productivity, and efficiency of perovskite material or oxide catalysts for HER [22][23][24].
The main goal of this study was to develop a SrTiO 3 / CuFeO 2 /Cu photoelectrode for hydrogen generation through water splitting. To achieve this, the CuFeO 2 and Cu-SrTiO 3 layers were fabricated using combustion and chemical bath deposition methods, respectively. The efficiency of the photoelectrode for H 2 production was evaluated by analyzing its current-voltage relationship under various light intensities, wavelengths, and temperatures. Additionally, various thermodynamic parameters were calculated along with quantum efficiency (QE) and incident photon-to-current conversion efficiency (IPCE). The photoelectrode showed high stability and has promising potential for utilization in the industrial sector.

Experimental Methodology
2.1. Preparing of CuFeO 2 Perovskite Layer. Cu foil with a 10 mm × 5 mm dimension was immersed in 3 ml of Conc. H 2 SO 4 for 15 min on each side, then washed with deionized water. Then, the copper substrate was immersed in 0.1 FeCl 3 , dissolved in 50 ml of deionized water for 20 min on each side, left hanging to dry in the air for 15 min, then heated over a hot plate at 50°C for 10 min, and then annealed inside a tube at 500°C for 10 min using a CVD furnace. 3 Perovskite Layer. 0.1 M of strontium chloride (SrCl 2 , 99.6%) and 0.02 M of copper chloride (CuCl 2 , 99.7%) were dissolved in 10 ml of isopropanol (99.0%) using ultrasonic for 1 min. Then 5 ml of this solution was added to 0.1 M of titanium isopropoxide (99.9%). Cast 100 μl of Cu-SrTiO 3 solution over the CuFeO 2 layer, then let it dry in the air and then over a hot plate at 50°C for 15min, and then anneal inside a tube at 500°C for 10 min using a CVD furnace. Ag paste is diluted by adding a couple of drops of acetone (98.9%), then deposited on one side of the cell drop using cotton bud and then dried it over a hot plate at 70°C for 2 min.

Samples Characterization.
The prepared perovskite layers were characterized using different devices. The chemical structure was confirmed using an X-ray diffractometer (XRD, Bruker/Siemens D5000, Aubrey, TX, USA) by Kα radiation of Cu (λ = 1.5405 Å) at 30 mA and 40 kV. Moreover, the chemical structure was confirmed using X-ray photoelectron spectroscopy (XPS) models (advance diffractometer, Bruker D8) and K-ALPHA, USA. Besides, the elements inside the materials and morphologies were determined using a scanning electron microscope (SEM, Auriga Zeiss FIB, Zeiss Company, Oberkochen, Germany). Also, the morphologies were confirmed using transmitted electron microscope (TEM) models Hitachi, S-4800, and JEOL JEM-2100, respectively. The optical properties were characterized using a double-beam spectrophotometer (Perkin Elmer Lambda 950, Perkin Elmer Company, Waltham, MA 02451, USA).

2.4.
Water-Splitting Test. The water splitting reaction happened under a solar simulator from a three-electrode cell, in which the Cu-SrTiO 3 /CuFeO 2 /Cu represented the working electrode as a cathode, whereas graphite and calomel electrodes were worked as counter and reference electrodes, respectively. These electrodes are partially immersed in 30 ml of filtered sanitation water (third-stage treated, company of drinking water and wastewater, Beni-Suef City, Egypt). The measurements were carried out using an electrochemical workstation (CHI660E) at a scan rate of 100 mV.s -1 . The effects of light intensity (100 to 400 mW.cm -2 ), incident wavelength (390 to 636 nm), temperature reaction (30 to 70°C), and time stability were studied for water splitting for H 2 generation ( Figure 1). XPS signal attributed to the oxygen crystal lattice was detected at 528.4 eV. Surface oxygen was assigned a peak position of 531.8 eV due to hydroxyl groups [26,27]. On the other hand, the peak at 531.26 eV was attributable to a surface-coordinated water molecule. Figure 3(c) exhibits two significant peaks at around 932.6 and 952.4 eV that may be attributed to Cu2p3/2 and Cu 2p1/2, respectively, indicating that the synthesized material has a monovalent copper cation (Cu+). From Figure 3(d), the Sr 3d5/2 peaks at 134.04 eV and the Sr3d 3/2 peaks at 135.65 eV. As well, Ti2p peaks at 458.17 eV and 464.19 eV are illustrated in Figure 3(e).

Results and Discussion
The surface types of Fe, Cu, O, and C atoms in CuFeO 2 / Cu are shown in the survey X-ray photoelectron spectrum in Figure 3(f), which displays the different elements and their corresponding binding energies (BE). The XPS plot comprises Cu2p, Fe2p, O1s, and C1s at 932.61, 711.25, 530.98, and 285.0 eV, respectively. The above findings suggest that the CuFeO 2 /Cu thin film is produced through the combustion approach.
3.1.2. SEM and TEM Studies. In Figure 4(a), the design of a homogenous highly crystalline surface is seen. The crystals are compacted and connected high together on one surface. The formed nano-/microparticle for the perovskite materials is expected to have high optical properties. The significant roughness and minimum porousness of the Cu-SrTiO 3 / CuFeO 2 /Cu surface qualify it for absorbing light in many directions in an area of 1531:9 * 1589 nm, as drawn with the ImageJ program (Figure 4(b)). The SEM of the CuFeO 2 /Cu thin film is presented in Figure 4(c). It could be seen that the highly crystalline polyhedral CuFeO 2 /Cu delafossite thin film is free of cracks with a smooth surface. This surface is used as a substrate for the deposition of the Cu-SrTiO 3 materials.
The TEM image of Cu-SrTiO 3 /CuFeO 2 /Cu is shown in Figure 4(d). This figure reveals the nano-/microstructured morphology of Cu-SrTiO 3 /CuFeO 2 with an average crystalline particle size of 200 nm.

Optical Analyses.
The optical characteristics of the prepared Cu-SrTiO 3 /CuFeO 2 and CuFeO 2 are shown in Figure 5(a). The refraction of CuFeO 2 is higher than that of Cu-SrTiO 3 /CuFeO 2 , indicating that the absorption intensity is increasing, in which the electron transition is the main factor for peak formation in the UV, Vis, and near IR regions [28][29][30][31].
The CuFeO 2 and Cu-SrTiO 3 /CuFeO 2 bandgap (Eg) values of 1.4 and 1.26 eV, respectively, show an improvement in optical characteristics (Figure 5(b)). The Kubelka-Munk equation   3 International Journal of Energy Research was used to obtain the Eg value, where S and K are the scattering and absorption coefficients, respectively; R is the diffuse reflectance; and FðRÞ is the Kubelka-Munk function [32].
3.2. Photoelectrochemical Activity. In the presence of a Xenon lamp (Newport, 66926-500HX-R07), the photoelectrochemical performance of the constructed electrode Cu-SrTiO 3 / CuFeO 2 /Cu for hydrogen generation is measured. The electrochemical workstation used for the measurements is CHI660E. The relationship between voltage and current density is revealed in Figure 6(a). It appears that the Cu-SrTiO 3 / CuFeO 2 /Cu electrode served as a photocathode for the formation of H 2 gas. Figure 6(a) shows that when exposed to light, the resultant current density (J ph ) is 1.292 mA.cm -2 at 0.5 V.
The result of light intensity on the prepared photoelectrode is shown in Figures 6(a) and 6(b). When the light intensity is increased from 25 to 100 mW.cm -2 , the produced J ph values increase from -1.02 to -1.292 mA.cm -2 . The J ph rises with increasing light intensity owing to enhanced electron-hole pair production [33,34]. The generated current is directly proportional to the amount of light absorbed. Because J ph indicates the current density generated in the cell as a result of water splitting, it can also measure the rate of hydrogen generation from sanitation water. As seen in Equation (2), the number of photons (N) is directly proportional to the intensity of light (P) [35]. This equation is also affected by other parameters including wavelength (λ), Planck constant (h), and light velocity (c). Applying this equation, the N per second is changed from photon/s under light intensity from 25 to 100, and mW·cm −2 is changed from 2 × 10 21 to 8 × 10 21 photon/s, respectively.   International Journal of Energy Research The stability of the prepared Cu-SrTiO 3 /CuFeO 2 /Cu photoelectrode was investigated, as shown in Figure 7(a). The relationship between time and the generated J ph value is discussed, and the produced J ph value under light and dark chopping is steady with time (500 s) at around -0.11 mA.cm -2 . According to the figure, the electrode has good stability and high light sensitivity. The variation in J ph values across on-and off-chopped light is clearly visible.

International Journal of Energy Research
The remarkable influence of light on the electrode confirms the electrode's high sensitivity to light. Figure 7(b) depicts the repeatability of the electrode over two runs, with the voltage-current relationship exhibiting similar performance through four runs. This repeatability has been tested at 25°C and under natural light.
The temperature effect on the Cu-SrTiO 3 /CuFeO 2 /Cu photoelectrode for hydrogen generation because of water splitting is revealed in Figure 8(a), where the J ph rises from -1.25 to -1.91 mA.cm -2 as the temperature increases from 30 to 70°C, correspondingly. The overall rise in J ph values represents the rate of the reaction, which correlates to the rate of hydrogen generation [2,36]. The produced J ph values at various temperatures are mentioned in Figure 8(b). Relying on particle collision and the rate of water splitting, the activation energy (Ea) could be determined using the Arrhenius equation, Equation (3), where k, R, T, and A are the rate, universal gas, absolute temperature, and Arrhenius constants, respectively [34]. The Ea value defines the degree of reaction occurrence [37][38][39].  When analyzed with previously published results for different photocatalysts, the Ea value for this electrode is exceptionally low. As a result of the water-splitting reaction, the synthesized electrode is effective for H 2 evolution. Similarly, utilizing the Boltzmann constant (kB) and the Planck constant (h), the Eyring equation (4), maybe get the enthalpy (H * ) and entropy (S * ) To determine the ΔH * and ΔS * values, slope and intercept from Figure 8(d), respectively, are used. The ΔH value of Cu-SrTiO 3 /CuFeO 2 /Cu photoelectrode is 11.46 kJ·mol -1 , while ΔS * value is 34.9 kJ -1 ·mol -1 .
The photocurrent produced as a result of photon energy is defined as the incident photon-to-current conversion efficiency (IPCE). The wavelength value can be used to determine it. Equation (5) is used to compute the IPCE [18,[40][41][42].
In Figure 9, the IPCE is calculated at a light intensity of 100 mW.cm 2 and a temperature of 30°C. At 390 nm, the electrode has the optimum IPCE of 3.31%. This value is reduced by raising the wavelength to 500 nm, then rising until it reaches 636 nm. The IPCE value is obtained from the impact of the electrode's photocatalytic ability for sanitation water splitting without the presence of an additional   7 International Journal of Energy Research electrolyte. As a result, the device converts sanitation water to H 2 with more efficiency than earlier studies have shown.
Equation (6) was used to determine the amount of H 2 moles [43], where F is the Faraday constant and dt is the time change. Production of H 2 increases with time, corresponding to 10 ml/h for the sample with a dimension of 20 cm 2 for hydrogen production from a photoelectrode. Moreover, Table 1 illustrates a comparison of our study with previous literature.

Conclusions
A photodetector made of Cu-SrTiO 3 /CuFeO 2 /Cu was used to produce hydrogen gas from sanitation water. XRD analysis confirmed the formation of perovskite materials, including SrTiO 3 and Cu x SrTi 1-x O 3 , as well as CuFeO 2 and CuO. SEM analysis confirmed that the materials were highly crystalline with small surface pores. The materials' excellent optical properties were confirmed by their small bandgap values of 1.4 and 1.26 eV for CuFeO 2 and Cu-SrTiO 3 /CuFeO 2 , respectively. During hydrogen generation, J ph values were significantly enhanced under light compared to dark conditions. Moreover, increasing the light intensity from 25 to 100 mW/cm 2 increased the produced J ph values from -1.02 to -1.292 mA/ cm 2 . Temperature also had a significant effect, with the J ph rising from -1.25 to -1.91 mA/cm 2 as the temperature increased from 30 to 70°C. The study measured various thermodynamic parameters related to the rate of hydrogen gas evolution, including an Ea value of 14.14 kJ mol -1 and H * and S * values of 11.46 kJ mol -1 and 34.9 kJ mol -1 , respectively. These findings are promising for H 2 gas fuel production, and the team plans to design a prototype of cells that can directly convert wastewater into H 2 gas fuel.

Data Availability
The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.