Hydrogen generation from Red Sea water using CsSnI2Cl lead-free perovskite/porous CuO nanomaterials: Coast of Jeddah, Saudi Arabia

This study performed the preparation of inorganic CsSnI2Cl Lead-free perovskite material with a great optical behavior that qualifies these materials for photoelectrode application. CsSnI2Cl is prepared through the casting adding the perovskite solution on porous CuO that previously prepared under the combustion of Cu-foil. Through short heating, the CsSnI2Cl/CuO/Cu photoelectrode is prepared. This perovskite material is confirmed through the optical absorbance that has a great optical behavior with a band gap of 1.75 eV. The application of CsSnI2Cl/CuO/Cu for hydrogen generation is performed using red sea water as an electrolyte, in which the hydrogen generation rate is estimated using the produced current density (Jph) value. This Jph value is 20 mA/cm2 under a metal halide lamp. Moreover, this photoelectrode is estimated under various wavelengths, in which the optimum Jph (10.8 mA cm−2) is achieved at 340 nm, in which the incident photon to electron conversion efficiency is 33% at this wave lengths. This photoelectrode provides its qualified for hydrogen generation reaction under a wide optical range from 340 to 730 nm. Soon, our team is working on designing an electrochemical cell that can convert the red sea water into hydrogen gas directly.


Introduction
The creation of safe and effective materials for photoelectrodes in hydrogen generation is a significant challenge for researchers.This technology can address the global energy crisis, particularly in developing nations, and promote a cleaner environment for improved health.By reducing the toxic oxides generated from the burning of fossil fuels each year, these deadly gases can be prevented from causing harm to lives [1][2][3][4].
Hydrogen generation reactions involve two aspects: the development of a suitable photoelectrode and achieving high efficiency in hydrogen generation.Researchers are focused on both aspects, with a particular emphasis on developing a material that is highly sensitive to photons.Oxides and halides are among the materials that exhibit good light-responsive behavior, and their stability makes them attractive for use in various applications [5][6][7][8].By developing an efficient photoelectrode for hydrogen generation, or harmful effects, further solidifying its suitability for the study's objectives.
To execute the study, a CuO/Cu substrate serves as the foundation upon which the CsSnI 2 Cl lead-free perovskite material is deposited.This deposition process results in the creation of the CsSnI 2 Cl/CuO/Cu photoelectrode.The primary objective of this unique photoelectrode is to facilitate the hydrogen generation process, and it is employed within a three-electrode cell setup for this purpose.The researchers employ a variety of optical conditions and wavelengths to comprehensively investigate the performance of the CsSnI 2 Cl/ CuO/Cu photoelectrode.Specifically, they calculate key parameters such as the IPCE, the Half-Cell Solar to Hydrogen (HC-STH) conversion efficiency, and the amount of hydrogen generated in moles.By varying these optical conditions and wavelengths, the study aims to gain a comprehensive understanding of the photoelectrode's performance and its efficiency in driving the hydrogen generation reaction.This multifaceted approach aims to shed light on the photoelectrode's efficiency and its potential for applications in hydrogen generation that open the door for the industrial applications through designing an electrochemical cell that can convert the solar light into H 2 gas directly.

Materials and characterization
The chemicals used in the experiment include cesium chloride (CsCl, 99.9%), tin chloride (SnCl 2 , 99.9%), and dimethyl formamide (DMF, 99.9%).These chemicals are obtained from Sigma Aldrich, USA, while potassium iodide (KI, 99.9%) are obtained from Pio-Chem Co., Egypt.Additionally, copper foil with a thickness of 1.0 mm and purity of 99.9% is used in the experiment.

Preparation of CsSnI 2 Cl/CuO and electrochemical testing
CuO is synthesized through a hydrothermal method, achieved by oxidizing copper foil at a temperature of scientists hope to provide a solution to the global energy crisis while also reducing the harmful effects of fossil fuel combustion on the environment and human health.
Improving the morphology of the materials is a strategy to enhance their efficiency, particularly through developing nanomaterials with different surface morphologies, such as sheets and wires, which can greatly impact their optical properties.However, these materials still face limitations, including low efficiency for hydrogen generation or high costs associated with complex preparation techniques [2,9,10].
Perovskite materials are an attractive option for photoelectrodes due to their excellent optical properties, particularly their high sensitivity to photons [11,12].Among these materials, lead-free perovskites are considered to be safe and highly sensitive to light, making them a promising option for use in various applications [13].The second challenge in the hydrogen generation reaction is to develop an electrolyte that can serve as a source of hydrogen, and reducing the use of a sacrificing agent is a promising approach to reduce the costs of this process.
Rabia et al., have undertaken extensive research in the field of hydrogen generation through the electrolysis of sewage water, achieving commendable levels of efficiency, primarily measured by the J ph value, which signifies the rate of the hydrogen generation reaction.In their research endeavors, they have reported noteworthy results, including a J ph value of − 1.6 mA cm −2 , which was achieved by employing poly-3-methyl aniline in conjunction with wastewater as electrolyte [14].Another study yielded a J ph value of − 0.1 mA.cm −2 through the use of a TiN/TiO 2 /Alumina membrane [15], and a separate investigation resulted in a J ph value of 0.13 mA cm −2 when utilizing polyaniline/PbS for sewage water splitting [16].Furthermore, Rabia et al. have conducted additional studies focusing on hydrogen generation from sewage water, underscoring their commitment to advancing this area of research [17,18].
In this study, the study capitalizes on the utilization of Red Sea water as an electrolyte for the hydrogen generation process.The rationale behind this choice stems from the fact that Red Sea water boasts a high concentration of ions, rendering it a natural sacrificial agent.Additionally, this choice is underscored by the cost-effectiveness and eco-friendly nature of Red Sea water, making it a desirable option for experimental use.Importantly, it is worth noting that this water source carries no adverse 400 °C in an air environment.Following this, the formation of CsSnI 2 Cl is carried out by drop-casting a solution that contains KI, CsCl, and SnCl 2 dissolved in N,N-dimethylformamide (DMF) with a ratio of 3:1:1 onto the CuO foil.Subsequently, the layered structure is subjected to a drying process at 150 °C for a duration of 5 min, resulting in the creation of a perovskite layer that overlays the CuO material.For experimental purposes, the CsSnI 2 Cl/CuO photoelectrode is employed as the working electrode within a three-electrode cell setup.In this configuration, the generated current density, denoted as J ph , is measured utilizing a CHI608 workstation.This measurement serves as a crucial indicator for determining the rate of hydrogen generation.The electrochemical cell is also equipped with a reference electrode, specifically a calomel electrode, and a counter electrode made of graphite.
The photocurrent, representing the rate of hydrogen gas production, is quantified through electrochemical assessments performed under light illumination sourced from a 400 W metal halide lamp.It is important to note that the magnitude of J ph is contingent upon the characteristics of the incident light, including its wavelengths and other illumination conditions.A visual representation of the photoelectrochemical cell's setup is provided in Fig. 1.

Analyses
The analysis involves the examination of the morphology and topography of two substances: CuO nanomaterials and CsSnI 2 Cl perovskite.For CuO nanomaterials, Fig. 2a, SEM analysis shows that the material has a great porous structure with pore diameter ranging from 150 to 250 nm.These pores can work as a trap for other materials, allowing the growth of additional material inside it.For CsSnI 2 Cl perovskite materials, Fig. 2b, the morphological characteristics are reminiscent of pyramid-shaped crystals with substantial crystal sizes, validating their excellent and densely packed surface attributes.This phenomenon further underscores the perovskite materials' proficiency in responding to light and generating electrons when exposed to illumination, as discussed in reference [19], The larger perovskite particles contribute to heightened light absorption through the reflection of light by these pyramid structures, which possess a remarkable capacity for capturing photons.The sides of these pyramid structures extend approximately 800 nm above the surface.Furthermore, the impressive compactness of these crystals collectively augments the stability of the thin films.
The theoretical modeling of CsSnI 2 Cl perovskite surface roughness is represented in Fig. 2c, where a 3D pyramid structure appears for the perovskite materials.These particles are surrounded by other smaller Fig. 1 The schematic diagram of the electrochemical system for hydrogen gas generation using Perovskite/CuO photocathode as a working electrode particles that consider incomplete pyramid-structure particles, but they have an advantages for the prevention of the formation of pores.Finally, the TEM analyses for CsSnI 2 Cl perovskite material are shown in Fig. 2d, where the 2D pyramid structure are confirmed again with a dark color, in which there are additional small particles surrounding it.
The XPS analysis of CsSnI 2 Cl is showcased in Fig. 3c, providing a comprehensive survey of the perovskite material's elemental composition.This analysis confirms the successful formation of the perovskite material, as all the expected elements are clearly detected.
The XPS survey reveals distinct peaks corresponding to various elements present in CsSnI 2 Cl.Notably, the Chlorine (Cl) and Iodine (I) spectra are Furthermore, the XPS analysis identifies the Nitrogen (N) and Oxygen (O) spectra, which appear at 404 and 534 eV, respectively.These spectra are associated with the 1s electron spectra of both N and O. Their detection underscores the presence of nitrogen and oxygen components within the perovskite composition.
The XPS analysis also captures the presence of Tin (Sn) in the perovskite material.The Sn atom is notably observed at 490 eV, corresponding to the 3d orbital spectra of Sn.This observation solidifies the inclusion of tin within the perovskite structure.
Additionally, the XPS survey registers the presence of Cesium (Cs) in the perovskite composition, with Cs spectra being identified at 719 eV.This detection reaffirms the existence of cesium as a constituent element of the CsSnI 2 Cl perovskite material.
Intriguingly, the XPS analysis also reveals a minor peak related to the Potassium (K) spectra, appearing at 297 eV.While this peak is relatively small, its presence suggests the possible inclusion or interaction of potassium within the perovskite material.
Figure 3d shows the optical absorbance of CsSnI 2 Cl perovskite material, indicating its absorption band in the UV, Vis, and near-IR region up to 680 nm.The incorporation of Cl and I atoms in the perovskite material leads to this enhancement in optical behavior, effectively spanning a wide range of the optical spectrum.This enhancement is attributed to the exceptional light-absorbing properties of halide materials, with particular emphasis on the effectiveness of iodine [22].To evaluate the superiority of this material, its bandgap is calculated using Tauc's Eqs. 1 and 2 [23].These equations utilize the density (d) and absorption coefficient of the material (α) to estimate the bandgap (E g ), which is found to be 1.75 eV.
The remarkable optical properties of perovskite materials are intricately linked to their impressive topography and morphological characteristics, as previously discussed in the analysis of their morphological behavior (as illustrated in Fig. 2).The pyramid-like structure inherent to perovskite materials plays a pivotal role in their ability to absorb light efficiently across a broad optical spectrum, ranging from the ultraviolet (UV) to the visible (Vis) range, while also exhibiting a promising bandgap.This distinctive morphological feature, resembling a pyramid, offers several advantages for light absorption such as the diverse absorption capability: The pyramid-shaped structure of perovskite materials enables them to capture light across a wide range of wavelengths.This extensive absorption capacity is crucial for harnessing a significant portion of the solar spectrum.Enhanced Also, this feature increases of the light-material interaction: The presence of multiple surfaces, each inclined at various angles, greatly increases the likelihood of incident photons being absorbed by the material.This geometric arrangement effectively enhances the interaction between incoming light and the perovskite material, promoting efficient absorption.Also, this geometry decreases of the reflectance: The complex geometry of perovskite materials results in reduced reflectance.When light strikes the material, it often undergoes multiple internal reflections before being absorbed.This characteristic minimizes the loss of light due to reflection or scattering.This behavior results of the extended optical path length: The internal reflections occurring within the pyramid-like structure result in a longer optical path length for incoming photons. (1 This elongated path allows for more extensive lightmatter interactions, facilitating efficient absorption.All these factors cause the effective electron-hole generation: Efficient light absorption is a prerequisite for the generation of electron-hole pairs (excitons) within the material.These excitons are the fundamental entities responsible for electricity generation in solar cells.The unique morphological behavior of perovskite materials aids in the creation and separation of excitons, ensuring a high degree of charge carrier generation [24,25].
Figure 3e provides insights into the optical characteristics of the CuO porous sheet.This material exhibits highly favorable optical properties, as evident from the reflectance values obtained.In particular, the reflectance values are exceedingly low across the optical spectrum and even extend into the near-infrared (IR) region.This low reflectance indicates that the material possesses exceptional photon absorbance capabilities, extending well into the near-IR regions.Consequently, this material is deemed an ideal substrate for the deposition of perovskite material.The optical behavior of the CuO porous sheet, as depicted in Fig. 3e, is noteworthy for several reasons.Firstly, the material demonstrates a strong tendency to absorb incident photons, as indicated by its low reflectance values.This characteristic makes it highly efficient in capturing photons across a wide range of wavelengths, including those within the near-IR spectrum.The material's suitability as a substrate for perovskite deposition is a key implication of its optical behavior.Its capacity to efficiently absorb photons suggests that it can serve as an excellent foundation for casting perovskite materials.This is particularly advantageous in the context of photovoltaic and optoelectronic applications where maximizing photon capture and conversion efficiency is crucial.To further elucidate the optical properties of the synthesized perovskite material, CsSnI 2 Cl, the absorption coefficient (α) is presented in Fig. 3f.According to the data in this figure, the calculated α value stands at 1.8 eV.This value has been determined utilizing Eq. 3, with the known film thickness value (t) set at 800 nm.

Red Sea water as a source of hydrogen gas: electrochemical testing
This study used Red Sea water as a source of hydrogen gas.Table 1 provides the chemical composition of this water, which is a promising choice for generating hydrogen gas through electrochemical means.The heavy metals present in this water act as a sacrificing agent for the hydrogen gas production process, eliminating the need for an external electrolyte.This approach is cost-effective since the water is abundant and freely available.A three-electrode cell is used for their experiments, where the CsSnI 2 Cl/CuO/Cu photoelectrode was used as the working electrode, and the reference and auxiliary electrodes were connected to the cell to facilitate the electrochemical reactions.
The researchers applied photon illumination to the cell using the CHI608E instrument and a metal halide lamp with a power of 400 W.
Figure 4a shows the photosensitivity of the prepared CsSnI 2 Cl/CuO/Cu photoelectrode under light illumination.This photoelectrode receives photons that excite the surface of the semiconductor materials (CsSnI 2 Cl and CuO) for electron transition, and the resulting electrons are collected on the surface of the photoelectrode.These electrons are then transferred to the neighboring electrolyte for the hydrogen generation process through the J ph value.The J ph value produced by this photoelectrode is 20 mA/cm 2 in light, which indicates its high photosensitivity for the hydrogen generation reaction.
The small current density (J o ) observed in the dark indicates the semiconductive nature of the prepared material, which is significantly smaller than the J ph value produced under light illumination.This demonstrates the significant effect of incident photons on the photoelectrode for the hydrogen generation reaction.The curve is non-linear, indicating the generation of a Schottky barrier [26] during the electron transition from CuO to CsSnI 2 Cl.This is due to the difference in energy levels between the two materials.After the electrons are generated by the photoelectrode, they reach the surface of the perovskite material and combine with the electrons produced by the material itself.These electrons then move towards the seawater where they participate in the splitting reaction, leading to the production of hydrogen gas.
Figure 4b further supports this observation by testing the photoelectrode under both dark and light conditions till 2300 s.The significant increase in J ph value (by − 2.5 mA cm −2 ) and the decrease in J o value (by − 0.93 mA cm −2 ) in light compared to dark confirm the high sensitivity of the prepared CsSnI 2 Cl/CuO/Cu photoelectrode to light.
The response of the CsSnI 2 Cl/CuO/Cu photoelectrode to light is shown in Fig. 5a through the produced J ph values under different monochromatic lights.The J ph value at − 0.8 V is shown in Fig. 5b.As seen in Fig. 5a, there are significant enhancements in the J ph values as the light wavelength decreases from 730 to 440 nm, with the J ph value decreasing from − 10.8 to  − 4.9 mA cm −2 , respectively.This behavior is typical in the effect of light on a photoelectrode, where the high effect of light under low wavelength is due to the high frequency of light, which promotes electron transition from low to high energy levels [28,29].These electrons are collected on the upper level and transferred to the electrolyte solution for further reactions.In contrast, the J ph under low frequency (near IR) is related to bond vibrations that cause electron vibration.At V = 0, the high photocurrent value represents the efficiency of these materials for electron production, in this case, J ph = J SC .
The efficiency of the hydrogen generation reaction was estimated using Eq. 4 and is shown in Fig. 6a.The CsSnI 2 Cl/CuO/Cu photoelectrode achieved a high and broad efficiency across most of the optical region, with an optimal value of 33% at 340 nm.The efficiency decreased to 13% at 730 nm, indicating that the optimal generation of electrons occurred under photon incidence at 340 nm. Figure 6b presents the calculation of the number of H 2 moles produced from the CsSnI 2 Cl/ CuO/Cu photoelectrode based on the Faraday law [7] and using the constant Faraday constant (F, 9.65 × 104 C mol −1 ).The estimated H2 mole is 30 µmole/h cm 2 .This means that using a 10 cm 2 surface of this photoelectrode would result in the production of 0.3 m mol/h.Furthermore, the efficiency of the half cell solar to hydrogen (HC-STH) conversion process is determined through Eq. 5 and Fig. 6c, in which these calculations are performed at applied voltage (V APP ).This equation yields a maximum efficiency of 3.7% at a voltage of − 0.52 V.This result holds significant promise for the generation of hydrogen gas from seawater.Moreover, the efficiency of CsSnI 2 Cl/CuO/Cu photoelectrode in compared to the previous studies is mentioned in Table 2.In this diagram, an electron clouds accumulating on the surface of the CuO material is performed.These accumulated electrons subsequently establish a connection with the adjacent solution [30], which, in this case, is the Red Sea water.This interaction sets in motion a series of sequential steps, part of a mechanistic process.
As a result of this series mechanism, the culmination of these electron transfers catalyzes the formation of H 2 gas.In essence, the electrons, initially hosted on the CuO material's surface, initiate a chain reaction that ultimately results in the generation of hydrogen gas [31].This cascade of events underscores the intricate yet highly efficient electron transfer process that underlies the production of H 2 gas in this photoelectrode system.So, Fig. 7    the hydrogen generation reaction.It highlights the critical role of electron clouds accumulating on the CuO surface and their subsequent involvement in facilitating the series of reactions that culminate in the generation of hydrogen gas, a process of significant interest and importance in various energy and environmental applications.

Conclusions
In summary, this study focused on the preparation and characterization of CsSnI 2 Cl/CuO/Cu photoelectrode for hydrogen generation.The perovskite material was prepared using a casting method on porous CuO, and the optical behavior of CsSnI 2 Cl was confirmed to have a bandgap of 1.75 eV.The photoelectrode showed a high J ph value of 20 mA/ cm 2 under a metal halide lamp, and an optimum J ph value of 10.8 mA/cm 2 was achieved at 340 nm.The IPCE efficiency was estimated to be 33% at this wavelength.The photoelectrode showed a wide optical range for hydrogen generation from 340 to 730 nm, with an estimated hydrogen generation rate of 30 µmole/h cm 2 .The results demonstrate the potential of CsSnI 2 Cl/CuO/Cu photoelectrode as a promising candidate for hydrogen generation using seawater as an electrolyte.

Fig. 2
Fig. 2 The topographical and morphological properties of the synthesized CsSnI 2 Cl perovskite/CuO: a SEM of CuO, while b SEM, c theoretical modeling images simulation, and d TEM of CsSnI 2 Cl perovskite material

Fig. 3 a
Fig. 3 a The XRD pattern of CuO.b The XRD pattern, c XPS, d optical absorbance (inserted bandgap) of CsSnI 2 Cl perovskite material, e reflectance of CuO material, and f absorption coefficient of CsSnI 2 Cl perovskite material

Fig. 4
Fig. 4 The behavior of CsSnI 2 Cl/CuO/Cu photoelectrode in dark and light conditions through the a current-voltage relation and b on/off light for stability confirmation

( 4 )Fig. 5 Fig. 6
Fig. 5 The behavior of CsSnI 2 Cl/CuO/Cu photoelectrode under various wavelengths through a currentvoltage relation and b the J ph value at − 0.8 V

Figure 7
Figure 7 illustrates a schematic representation of the electron transfer process within the fabricated CsSnI 2 Cl/CuO/Cu photoelectrode, specifically in the context of the hydrogen generation reaction.This visual representation elucidates how electron movement transpires throughout the system, ultimately leading to the production of H 2 gas.In this diagram, an electron clouds accumulating on the surface of the CuO material is performed.These accumulated electrons subsequently establish a connection with the adjacent solution[30], which, in this case, is the Red Sea water.This interaction sets in provides a visual representation of the electron transition pathway within the CsSnI 2 Cl/CuO/Cu photoelectrode during

Fig. 7
Fig. 7 The schematic diagram of the electron transition through the synthesized CsSnI 2 Cl/CuO/Cu photoelectrode for the hydrogen generation reaction

Table 2
The efficiency of CsSnI 2 Cl/CuO/Cu photoelectrode in compared to the previous studies