Conversion of Sewage Water into H2 Gas Fuel Using Hexagonal Nanosheets of the Polyaniline-Assisted Deposition of PbI2 as a Nanocomposite Photocathode with the Theoretical Qualitative Ab-Initio Calculation of the H2O Splitting

This study is very promising for providing a renewable enrgy (H2 gas fuel) under the elctrochemical splitting of the wastwater (sewage water). This study has double benefits: hydrogen generation and contaminations removel. This study is carried out on sewage water, third stage treated, from Beni-Suef city, Egypt. Antimony tin oxide (ATO)/polyaniline (PANI)/PbI2 photoelectrode is prepared through the in situ oxidative polymerization of PANI on ATO, then PANI is used as an assistant for PbI2 deposition using the ionic adsorption deposition method. The chemical structural, morphological, electrical, and optical properties of the composite are confirmed using different analytical tools such as X-ray diffreaction (XRD), scanning electron microscope (SEM), transmision electron microscope (TEM), Fourier-transform infrared spectroscopy (FTIR), and UV-Vis spectroscopy. The prepared PbI2 inside the composite has a crystal size of 33 nm (according to the peak at 12.8°) through the XRD analyses device. SEM and TEM confirm the hexagonal PbI2 sheets embedded on the PANI nanopores surface. Moreover, the bandgap values are enhanced very much after the composite formation, in which the bandgap values for PANI and PANI/PbI2 are 3 and 2.51 eV, respectively. The application of ATO/PANI/PbI2 nanocomposite electrode for sewage splitting and H2 generation is carried out through a three-electrode cell. The measurements carreid out using the electrocehical worksattion under th Xenon lamp (100 mW.cm−2). The produced current density (Jph) is 0.095 mA.cm−2 at 100 mW.cm−2 light illumination. The photoelectrode has high reproducibility and stability, in which and the number of H2 moles is 6 µmole.h−1.cm−1. The photoelectrode response to different monochromatic light, in which the produced Jph decreases from 0.077 to 0.072 mA.cm−2 with decreasing of the wavelengths from 390 to 636 nm, respectively. These values confirms the high response of the ATO/PANI/PbI2 nanocomposite electrode for the light illuminaton and hydrogen genration under broad light region. The thermodynamic parameters: activation energy (Ea), enthalpy (ΔH*), and entropy (ΔS*) values are 7.33 kJ/mol, −4.7 kJ/mol, and 203.3 J/mol.K, respectively. The small values of ΔS* relted to the high sesnivity of the prepared elctrode for the water splitting and then the hydrogen gneration. Finally, a theoretical study was mentioned for calculation geometry, electrochemical, and thermochemistry properties of the polyaniline/PbI2 nanocomposite as compared with that for the polyaniline.


Introduction
The huge demand for energy sources all over the world drives scientists to concentrate all efforts on renewable energy sources [1][2][3][4]. These renewable energy sources provide replaceable energy sources for people to overcome the problem related to fossil fuels. These fuels have limited energy sources with additional fetal hazardous effects on the livings and the environment. There are famous hazardous gases are confirmed such as SO X , CO X , NO X usually release from these fossil fuels [5][6][7].
One of the most important renewable energy sources is solar energy. Through this solar energy, the scientists worked on the photocatalytic materials that can use this energy for performing additional chemical reactions through which the H 2 gas is evolved. This H 2 gas is very important as a fuel for factories and companies, in addition to its use as a fuel for the usual working inside the home such as warming and cooking.
This H 2 gas resulted from the water-splitting reaction by receiving electrons from the photocatalytic semiconductor nanomaterials such as metal oxides and nitrides [8,9]. The enhancements in these materials are carried out by increasing the active sites in their surface area, this is carried out through the preparation of these materials in nanoscale with great surface morphologies such as nanowire, nanotube, and nanosheets [10][11][12].
One of the great challenges is the application of polymer nanomaterials as semiconductor photocatalytic for the replacement of the previous semiconductor materials. The polymer materials have great properties such as large surface area, mass production, and low cost. PANI and its derivatives are considered semiconductor materials with high electrical and optical properties qualified them for photocatalytic applications. Moreover, this category of the polymer has additional properties such as high safety, stability, compatibility, redox state, and low bandgap [13,14].
Recently, few studies have concentrated on using PANI or conductive polymers as a photocatalytic material for water-splitting reactions [15][16][17][18][19][20][21]. Belabed et al. [21] prepared PANI/TiO 2 as catalytic material for water splitting under artificial light. Zhang et al. [22] studied PANI/MoS 2 for H 2 generation by using H 2 SO 4 as sacrificing agent, in which the J ph value was 0.09 mA.cm −2 . Corte et al. [20] fabricated Ni/PANI composite for water-splitting reaction by H 2 SO 4 as an electrolyte, in which the J ph value was 0.091 mA.cm −2 . In addition to that, there were studies carried out on poly(3-aminobenzoic acid) frame as photocatalyst for H 2 generation through using H 2 SO as an electrolyte, the J ph was 0.08 mA.cm −2 [15]. In addition to that, the applications of metal oxides and nitrides have a great advantage of high stability. These metals are deposited by high complexed techniques with high costs such as physical vapor deposition, RF sputtering, and laser techniques [16,17].
There are many drawbacks to the previous studies related to the water-splitting reaction. This literature depended on using sacrificing agents for water spitting reactions such as Na 2 SO 3 , Na 2 S 2 O 3 , HCl, and NaOH [18][19][20]. These sacrificing agents have a great role in electrode corrosion and then decreasing the lifetime of the electrode. Moreover, the previous studies have very small J ph values released from the splitting reaction, this confirmed that these studies have very small efficiency for the splitting reaction [21][22][23][24]. In addition to that, these previous studies usually used freshwater as a source of H 2 , this was a big problem due to the leakage in freshwater that is used as drinking water.
In this study, wastewater is used as a source for renewable energy production (H 2 gas). PANI/PbI 2 composite is prepared using a very cheap method, in which PANI is used as an assistant for the deposition of PbI 2 through the ionic adsorption deposition method on the supporter ATO glass. The ATO/PANI/PbI 2 nanocomposite is used as a working electrode for the wastewater-splitting reaction, in which sewage water (the third stage treated) is used as an electrolyte without using any additional sacrificing agent.
This study provides H 2 gas from wastewater, at the same time, this study decreases using of fossil fuels with their harmful fetal effects related to their hazardous gases.
Here, ATO/PANI/PbI 2 nanocomposite is used as a working electrode in a threeelectrode cell, in which graphite and saturated calomel represent the counter and references electrodes. The effects of light wavelengths on/off chopped light and reproducibility are studied. The number of H 2 moles is calculated through Faraday's laws. The thermodynamic parameters are calculated using Eyring equations. Moreover, the simple mechanism for the water-splitting reaction is mentioned.
Soon, we will work on synthesis an industrial model of an electrochemical cell for the industrial applications. This electrochemical cell can be used inside the home for converting the sewage water into H 2 gas directly that can be used as fuel for warming and cooking directly. Moreover, this model can be used inside the economic companies and factories with high financial returns.

Preparation of PANI/PbI 2 Nanocomposite
The preparation of PANI occurred under the in situ oxidation polymerization of aniline on the antimony tin oxide (ATO) glass (Sigma Aldrich, St. Louis, MO, USA). A total of 50 mL (0.1 M) of aniline (El Nasr co., Cairo, Egypt) was dissolved under the ultrasonic for 30 min in the presence of (0.5 M) CH 3 COOH (El Nasr co., Cairo, Egypt) as acid medium and solvent.
In a parallel flask, 50 mL (0.15 M) of (NH 4 ) 2 S 2 O 8 (Piochem co., Cairo, Egypt) was dissolved well that represents the oxidant. (NH 4 ) 2 S 2 O 8 is added suddenly over the aniline solution, through this process the polymerization of aniline to PANI took place and led to the formation of green color indicated the formation of PANI. Finally, ATO/PANI thin film was washed with distillated water and dried at 60 • C for 6 h.
The deposition of PbI 2 over the ATO/PANI thin film occurred using the ionic adsorption precipitation method. ATO/PANI was immersed in (0.05 M) Pb(NO 3 ) 2 (Piochem co., Cairo, Egypt) solution for 2 h at 298 K. through this process, the adsorption of Pb 2+ ions occur and led to the formation of ATO/PANI/Pb 2+ . This thin film was dried well and immersed in (0.01 M) iodine solution at 25 • C for 15 min. The reaction between I − and Pb 2+ was completed and led to the deposition of PbI 2 on the ATO/PANI thin film and then the formation of ATO/PANI/PbI 2 thin film.
From this reaction, the deposition of PbI 2 occurred on the surface and inside the polymer chains, as shown in the schematic diagram ( Figure 1a).

Materials
(NH 4 ) 2 S 2 O 8 and aniline were obtained from Winlab (UK) and Rankem (India) companies, respectively. CH 3 COOH, Iodine (I 2 ), Pb(NO 3 ) 2 , and KI were purchased from ElNaser company (Egypt). The wastewater, i.e., sewage water, was obtained from the drinking water sanitation company, in which this wastewater was treated three stages (the third stage treatment), this company located in Beni-Suef city, Egypt.

Characterization and Analyses
X-ray diffractometer (PANalytical Pro, Holland, Almelo, The Netherlands) was used for chemical structure determination and crystal size calculation. Fourier transform infrared was used for confirming the chemical structure, FTIR 340 Jasco spectrophotometer (Easton, WA, USA). A scanning electron microscope was used for determining the morphology of the prepared samples (SEM) (ZEISS, Gemini, Column, Oberkochen, Germany). In the same manner, a transmitted electron microscope was used for determining the internal morphology of the samples (TEM) (JEOL JEM-2100, Oberkochen, Germany). The optical absorption and then the bandgap calculation was determined through the Shimadzu UV/Vis spectrophotometer, Waltham, MA, USA. ImageJ software was used for the calculation of the surface morphology and cross-section.

The Electrochemical Test
The electrochemical measurements were carried out using a power station (CHI660E), Austin, USA, under a Xenon lamp, Waltham, USA, as shown in Figure 1b. The measurements were carried out through a three-electrodes cell. ATO/PANI/PbI 2 , graphite, and satureated calomel were used as working, counter, and references electrodes, respectively. Sewage water was used as an electrolyte without using any additional sacrificing agents.

The Theoretical Calculation
All the calculations were performed using Orca software [25] with def2-SVP (Karlsruhe basis def2-SVP Split valence polarization [26] basis sets and def2/J auxiliary sets for all atoms except Pb and I for which def2-TZVP basis was assigned during the structural geometry optimizations. The calculation employs the atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ [26]. We used the conductor-like polarizable continuum model (CPCM) [27] solvation model properties to emulate the water medium for the reaction. The default self-consistent (SCF) setting for the energy calculations was employed. To further accelerate the geometry and frequency (for infra-red (IR) calculations) convergence, we adopted the "sloppy SCF" settings.

Characterization of the Prepared Nanomaterials
The morphology of the prepared PANI is shown in Figure 2a, moreover the software Image J program modeling [28] is shown in Figure 2c. From both figures, the formation of a homogeneous nonporous surface appears clearly. The great roughness and small porousness in the PANI surface qualify it for composite well with additional nanomaterials. The morphology of the prepared PANI/PbI 2 is shown in Figure 2b. The hexagonal PbI 2 sheets cover the PNAI network, at the same time, these particles are embedded through the polymer network and cover its fibers. This feature was confirmed well through ImageJ software, as shown in Figure 2. The thickness of the film is greater in comparison with the PANI surface. The PbI 2 in the composite appears as an obelisk over the PANI surface. The average diameter of the PbI 2 sheets is about 300 nm.
The TEM image of the composite PANI/PbI 2 is shown in Figure 2e. The hexagonal PbI 2 shape appears well (dark color) through the PANI surface. For more confirmation, other SEM figures is inserted under different scale bars as shown in Figure S1a,b.
The great contact and homogenous morphology of the prepared composite qualify this composite for photocatalytic reactions very well. The composite will have the optical properties combined with its two materials.
The chemical structures of the prepared PANI and PANI/PbI 2 nanomaterials are confirmed using the FTIR as shown in Figure 3a. The summarized data is mentioned in Table 1   The XRD pattern of PANI and PANI/PbI 2 nanomaterials is shown in Figure 3b. For PANI, the XRD sharp peak at 20.7 • and semi-sharp peak at 25.5 • indicate the crystalline nature of PANI nanomaterial. These two peaks are located at the growth directions of (021) and (200), respectively.
After the PANI/PbI 2 composite, there is a shift in the peak at 25.5 • to 26.05 • with the appearance of a new peak at 20.7 • . Moreover, there is the appearance of three peaks at 12.8, 34.4, and 38.7 • corresponding to the growth directions of (001), (102), and (112), respectively. The standard XRD pattern is mentioned in Figure S2. These characteristic peaks are related to the PbI 2 nanomaterials inside the composite. The crystal size of the prepared nanomaterial is calculated using Scherrer's formula, Equation (1) [31,32]. This equation depends on many factors such as the width half maximum (W), the X-ray wavelength (λ), dimensionless factor (k), and Bragg angle (θ). From this Equation, the crystal size of the PbI 2 inside the composite is 33 nm according to the peak at 12.8 • .
The optical analyses of the prepared PANI and PANI/PbI 2 nanocomposite are shown in Figure 3c. From this Figure, there is more enhancement in the optical behavior after the composite formation. This is related to the enhancement in the optical absorption intensity and the position of the peaks. For both the PANI and PANI/PbI 2 , there is a peak in the UV region at 325 nm, but the intensity of this peak increases very much after the composite formation. This peak is related to the band-to-band electron transition process.
Moreover, there is a redshift from 585 nm to 610 nm after the composite formation, with the appearance of a great peak at 877 nm in the IR region. This peak is related to the electron vibration process.
Thus, there are more enhancements in the optical properties of the composite compared to the PANI material. This is related to the hexagonal sheets of PbI 2 that absorb and capture the photons. Then, they used these photons for hot electron generation through the formation of a hole electron bandgap.
The bandgap values of the PANI and PANI/PbI 2 are calculated from Tauc's equations, Equations This enhancement in the optical properties of the composite matched well with the good crystalline structure as shown by the XRD analysis before. So, the prepared PANI/PbI 2 composite is qualified for photocatalytic applications and water-splitting reactions.

Photoelectrochemical Water-Splitting Reaction
The electrochemical measurements of the wastewater (sewage water, third treated) splitting were carried out using the PowerStation (CHI660E) under a Xenon lamp. The ATO/PANI/PbI 2 nanocomposite represents the working electrode, while graphite and saturated calomel represent the counter and reference electrodes, respectively.
The electrochemical splitting reaction was carried out through the sewage water without using any additional sacrificing agent, the chemical composition of the sewage water is mentioned in Table 2. The measurements were carried out at 25 • C with a sweep rate of 100 mV.s −1 . Under the photon incidence, there is charge transfer due to the splitting in the PANI levels, in which there is electron transfer from the LUMO to HOMO. The energy level of HOMO is higher than the conducting band of the PbI 2 , so there is energy transfer and collection of electrons over the conducting band of PbI 2 . Although there is a Schottky barrier [33] that affects the electron transfer from the PANI to PbI 2 and causes slow motion of electrons that appear in the behavior of the J ph -potential relation (Figure 4a). This depletion layer does not affect the electron transfer, in which the produced J ph value is 0.095 mA.cm −2 at 100 mW.cm −2 , and finally, the electrons reach the water molecules for the spitting process and H 2 generation reaction. The relation between the applied potential (−1 to +1V) and the produced J ph values for the prepared electrode is shown in Figure 4a. This relation is repeated five times under the same conditions, the J ph value is 0.095 mA.cm −2 with high reproducibility. The standard deviation is very small (at about 1%).
Under a very small bias voltage, the on/off chopped current is shown in Figure 4b. The J ph values change from 0.1 to 0.98 µA.cm −2 , these values indicate the response of the prepared electrode for light sensitivity. This high sensitivity is related to the role of PbI 2 that captures and traps the photons, in which these photons generate hot electrons that do oscillations and resonance on the PbI 2 surface. Moreover, the generated electrons on PANI combine with these electrons for creating a high flow of electrons that transfer to the neighbor H 2 O molecules for water-splitting reaction, and then H 2 and O 2 evolution reaction. The chopped current is repeated with high reproducibility, this indicates the high stability of the electrode for a long time [34][35][36]. This is related to the high stability of PANI chains, in which PANI is not dissolved in almost all the solvents [37]. The very small dark current (J d ) (almost zero) may indicate the full inhibition of the prepared electrode under dark conditions [38][39][40].
The number of H 2 moles is calculated from the Faraday law relation [5]. This law depends on the parameters; J ph , time change (dt), the molecular weight of H 2 gas, oxidation number (z), and Faraday constant (F, 9.65 × 10 4 C mol −1 ). Through this relation, 6 µmole.h −1 .cm −1 of H 2 gas evolved as small bubbles from the cell using the prepared photoelectrode.
The effects of light intensities from 25 to 100 mW.cm −2 on the ATO/PANI/PbI 2 appear clearly through the produced J ph values as shown in Figure 5a. The J ph values increase from 0.075 to 0.092 mA.cm −2 with increasing in the light intensities from 25 to 100 mW.cm −2 , respectively. This behavior is confirmed well in Figure 5b  The increases in J ph values are related to increasing of photons numbers (N) through increasing the light intensities (P) [7]. This relation is confirmed through Equation (5) using different parameters: wavelength (λ), light velocity (c), and Planck constant (h).
The photon flux is received through the photocatalytic material surface that activates the active sites [41]. Through this process, the splitting in the outer energy levels takes place and led to the production of hot electrons [42][43][44][45][46]. These electrons are collected on the surface of the photocatalytic material and cause the production of J ph [47]. Thus, with increasing of the hot electrons, the J ph value increase represents the rate of water-splitting reaction, and hence the rate of H 2 evolution.
The effect of monochromatic wavelengths on the ATO/PANI/PbI 2 photoelectrode is shown in Figure 6a. The prepared photoelectrode responds well to the various wavelengths, in which the produced J ph values decrease from 0.077 to 0.073 mA.cm −2 with increasing of the light wavelengths from 390 to 636 nm, respectively. The produced J ph values at 1.0 V under different monochromatic light are shown in Figure 6b. The decreasing of the J ph values with increasing of the wavelengths matches well with the optical absorption curve for the composite as mentioned before in Figure 3c. The high J ph values in the blue side are related to the high light frequency with high energy that causes electrons to transfer to the conducting band that appears as J ph values [37].
The response of the prepared ATO/PANI/PbI 2 photoelectrode for different temperatures (25 to 60 • C) is shown in Figure 7a. From this figure, the produced J ph values increase from 0.092 to 0.132 mA.cm −2 with increasing in the temperature from 25 to 60 • C, respectively. The high values of J ph at high temperature relate to the mobility of the increasing ions with temperature. This behavior indicates the increase of the water splitting, and hence the rate of H 2 generation with the increasing of the temperature [48].  The thermodynamic parameters (Ea, ∆S, and ∆H) are calculated through the Eyring equations: Equation (6) with Equation (7) [5,49,50] and using Figure 7b,c. The calculation is based on the constants: k B , h, k and R which are Boltzmann's, Planck's, reaction rate, and universal gas constants, respectively. From Equation (6) and Figure 7b, the Ea is 7.33 kJ/mol. While from Equation (7) and Figure 7c, the ∆H* and ∆S* values are −4.7 kJ/mol and 203.3 J/mol.K, respectively. The positive ∆S*, negative ∆H*, and small Ea values indicate the spontaneous H 2 generation reaction [51].
The comparison of the electrolyte used and the produced J ph of the prepared ATO/ PANOI/PbI 2 photoelectrode with the previous literature is mentioned in Table 3. From this comparison, although the previous literature usually uses sacrificing agents, their produced J ph values are still very small. The present study uses sewage water only, the produced J ph value is higher than the previous work. In addition to that, the prepared electrode has high advantages represented in the low cost, easily prepared, high stability, and reproducibility. These great properties qualify the prepared ATO/PANOI/PbI 2 photoelectrode for industrial applications.

The Theoretical Study
The geometrically optimized PANI/PbI 2 composite structure is shown in Figure 8. The computational procedure of the H 2 O splitting on the Pb site of the composite structure involves fixing the composite's atoms while freeing the H 2 O molecule. Table 4 shows the calculated electrochemical and thermochemistry properties of the PANI/PbI 2 nanocomposite as compared with that for the PANI. Interesting, an increase in the molecular energy gap between the frontier orbitals (HOMO and LUMO for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively) is observed due to the composite formation. The gap might be exaggerated due to the default high Hartree Fock portion used in the Hamiltonian during the SCF calculation. Nevertheless, our calculated effects of the composite formation on the electrochemical properties are expected to convey the essential information for comparison with the experiments. Table 4 also shows that electronegativity (χ), global hardness (η), and electrophilicity (ω) are enhanced due to composite formation. The negative difference between the Gibbs free energies for the composite and polymer indicates the spontaneous formation of the composite. Additionally, we have calculated the binding energy between PbI 2 and the polyaniline at −0.05 Ha, suggesting a stable composite structure. The composite exhibits a significant increase in the dipole moment as compared to the polymer. This may effectively influence the catalytic effect of the composite for splitting H 2 O into H 2 .  We point out that the infrared (IR) calculations for the composite revealed few imaginary frequency modes that could have been ruled by using more stringent calculation settings such as hybrid functionals and larger basis sets [56]. However, these are computationally expensive and may not affect our current qualitative conclusions. Figure 9 shows the energetics for the reaction path with reactants (R) formed by a free H 2 O molecule and the composite, and a product (P) representing a free H 2 and a non-bonded structure between O and the composite. The transition state structure (TS) is shown with higher energy than that for the P and R. Investigation of the IR frequency modes indicates that the TS structure is less stable compared to the P structure. The structures along the reaction path are indicated in Figure 10. Again, since we focused on the qualitative results, we did not employ the systematic procedure of the Nudged Elastic Band (NEB) for finding the TS state [57]. Moreover, we have qualitatively demonstrated the possible reaction towards water splitting into H 2 by using the bare composite. A more realistic and systematic (catalytic-like) reaction may be achieved via supporting the composite on a suitable metallic layer. We will postpone this to future investigations.

Conclusions
ATO/PANI/PbI 2 nanocomposite photoelectrode was prepared and used for H 2 generation from sewage water. The sewage water was related to Beni-Suef city, Egypt.
The preparation of PANI was carried out through in situ polymerization on the ATO electrode. This film was used as an assistant for the deposition of PbI 2 through the ionic adsorption deposition method. From the characterization devices, the crystal size of the composite was 33 nm, with a bandgap of 2.46 eV. The PbI 2 has hexagonal sheets embedded in the PANI nanopores surface.
The ATO/PANI/PbI 2 photoelectrode was applied for H 2 generation from sewage water through a three-electrodes cell. The rate of H 2 generated is estimated through J ph values. The J ph was 0.095 mA.cm −2 at 100 mW.cm −2 . The response of the electrode to various wavelengths was carried out, in which the J ph values decreased from 0.077 to 0.073 mA.cm −2 with decreasing of the wavelengths from 390 to 636 nm, respectively. The on/off chopped current confirmed the high sensitivity and reproducibility of the prepared photoelectrode. The thermodynamic parameters were calculated and confirmed the high efficiency of the electrode for H 2 generation reaction, in which Ea, ∆H*, ∆S* values were 7.33 kJ/mol, are −4.7 kJ/mol, and 203.3 J/mol.K, respectively. Finally, a theoretical study was mentioned for showing the geometry of the nanocomposite and calculation of some parameters such as dipole moment, HOMO, and LUMO energy for the PANI/PbI 2 composite as compared to the PANI. Soon, our team working on synthesis an industrial model of elctrochemical cell that can convert the sewage water into hydrogn gas direclty. This idea is very promising for providing hydrigen gas fuel for people in houses for warming and cooking. Moreover, providing hydrogen fuel for people in remote places or inside the deserts.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.