Effect of Au Plasmonic Material on Poly M-Toluidine for Photoelectrochemical Hydrogen Generation from Sewage Water

This study provides H2 gas as a renewable energy source from sewage water splitting reaction using a PMT/Au photocathode. So, this study has a dual benefit for hydrogen generation; at the same time, it removes the contaminations of sewage water. The preparation of the PMT is carried out through the polymerization process from an acid medium. Then, the Au sputter was carried out using the sputter device under different times (1 and 2 min) for PMT/Au-1 min and PMT/Au-2min, respectively. The complete analyses confirm the chemical structure, such as XRD, FTIR, HNMR, SEM, and Vis-UV optical analyses. The prepared electrode PMT/Au is used for the hydrogen generation reaction using Na2S2O3 or sewage water as an electrolyte. The PMT crystalline size is 15 nm. The incident photon to current efficiency (IPCE) efficiency increases from 2.3 to 3.6% (at 390 nm), and the number of H2 moles increases from 8.4 to 33.1 mmol h−1 cm−2 for using Na2S2O3 and sewage water as electrolyte, respectively. Moreover, all the thermodynamic parameters, such as activation energy (Ea), enthalpy (ΔH*), and entropy (ΔS*), were calculated; additionally, a simple mechanism is mentioned for the water-splitting reaction.


Introduction
Fossil fuels, such as petroleum, oil, and natural gas, represent an essential energy source. The leakage of these sources and the contaminations of gases exhausts, SOx, NOx, and Cox, push scientists worldwide to think about a replaceable energy source, which is represented by renewable energy sources [1][2][3]. These renewable energy sources have advantages represented in clean energy and sustainability [4][5][6].
Hydrogen gas represents the primary solar energy source produced under the watersplitting reaction [7,8]. Hydrogen gas has advantages in its high combustion and performance [9]. Moreover, water splitting is carried out using photocatalytic materials under

Electropolymerization of MT and Preparation of the Electrode
A potentiodynamic technique in the three-electrode cell was performed in the electropolymerization phase of MT to form PMT, as defined in our previous study [32]. The electropolymerization reaction was carried out on ITO glass (20 Ω) through the anodic polymerization reaction using Potentiostat/Galvanostat Wenking (PGS 95), Hubertusstr, Germany. ITO, PT, and calomel represent the working, counter, and reference electrodes, respectively. During the electropolymerization process, Na 2 SO 4 and H 2 SO 4 were used as electrolyte and acid medium, respectively. The electropolymerization reaction was carried out from 0.4 to +1500 mV (in the positive direction) with a scan rate of 40 mVs −1 . The thickness of the PMT was determined through 10 cyclic voltammetry runs on ITO, then the film was rinsed in water and dried at 60 • C for 12 h.
The sputter coating of the Au was carried out from Au disc (99.99%) using a sputter coating device for 1 and 2 min on the surface of PMT for PMT/Au-1 min and PMT/Au-2 min, respectively.
This electrode was then used for hydrogen generation from sewage water or Na 2 S 2 O 3 solution, and the results of both electrolytes were compared to each other. The hydrogen generation was carried out from a three-electrode cell, in which the PMT/Au-2 min was the working electrode. At the same time, graphite and calomel were the counter and reference electrodes, respectively.

Characterization Process
The prepared polymer and polymer/Au morphology were characterized using an Xray diffractometer (XRD) device (X'Pert Pro, Almelo, The Netherlands) Almelo, the Netherlands, worked at 40 mA and 45 kV. FTIR-340 Jasco spectrophotometer, Easton, USA, was used for FTIR analyses. The 1H-NMR measurement (the Varian EM 360 L NMR, Oberkochen, Germany) confirmed the functional groups in the polymer materials. Scanning electron microscopy was used to conduct morphological studies (ZEISS SUPRA Gemini Column, Oberkochen, Germany). Optical analyses of the prepared films were determined using the Shimadzu UV/Vis spectrophotometer (M160 PC, Waltham, MA, USA).

Electropolymerization of M-Toluidine
The electropolymerization process was carried out through the optimization of the conditions of the preparation, represented in the current J P value. This Jp value represents the rate of the reaction [33].
The effect of the m-toluidine monomer 0.04 and 0.14 M study is shown in Figure 1A at 40 mVs −1 and 303 K. The optimum concentration was 0.12 M, with an optimum J p value. After the increase in the concentration to over 0.12 M, more collisions with the monomer take place, which cause the deactivation of the electrode surface [34]; this decreases the J p value to 0.14 M.
In the same manner, the effect of H 2 SO 4 from 0.1 to 0.6 M is shown in Figure 1B; the optimum concentration of H 2 SO 4 is 0.5 M. Additionally, the effect of Na 2 SO 4 from 0.025 to 0.1 Mis shown in Figure 1C; the optimum concentration is 0.075 M. The effect of temperature from 278 to 303 K is shown in Figure 1D; the optimum temperature is 298 K. These conditions confirm that the optimum conditions for the preparation of PMT are 0.12 M m-toluidine, 0.5 M H 2 SO 4 , and 0.075 M Na 2 SO 4 at 298 K.

PMT and PMT/Au Analyses
The chemical construction of the PMT nanomaterials is shown in Figure 2a, and the data are summarized in Table 1. From the Figure and the Table, it can be observed that the PMT function groups appeared well, in which the C-H, N-H, and C-N function groups appeared at 3106, 3429, and 1339 cm −1 , respectively. C=C, benzenoid and quinoid appear at 1465, 1407, and 1631 cm −1 , respectively. The vibration of C-H in/out of the plan is localized at 1051 and 595 cm −1 , respectively. After the PANI/Au composite formation,

PMT and PMT/Au Analyses
The chemical construction of the PMT nanomaterials is shown in Figure 2a, and the data are summarized in Table 1. From the Figure and the Table, it can be observed that the PMT function groups appeared well, in which the C-H, N-H, and C-N function groups appeared at 3106, 3429, and 1339 cm −1 , respectively. C=C, benzenoid and quinoid appear at 1465, 1407, and 1631 cm −1 , respectively. The vibration of C-H in/out of the plan is localized at 1051 and 595 cm-1, respectively. After the PANI/Au composite formation, there are shifts in most of the PANI peaks (Figure 2a, red curve), which is related to the interaction between the functional groups and the Au nanoparticles.
For more confirmation of the chemical structure of the PMT, an 1HNMR analysis was carried out, as shown in Figure 2b. The protons related to the methyl and N-H group are located at signals δ = 1.23 and 4.07 ppm, respectively. The benzene ring has signals from δ = 6.93 to δ = 7.27 ppm [35]. The summarized data are shown in Table 2. For more confirmation of the chemical structure of the PMT, an 1HNMR analysis was carried out, as shown in Figure 2b. The protons related to the methyl and N-H group are located at signals δ = 1.23 and 4.07 ppm, respectively. The benzene ring has signals from δ = 6.93 to δ = 7.27 ppm [35]. The summarized data are shown in Table 2.
To further confirm the chemical structure of PMT, PMT/Au-1 min, and PMT/Au-2 min, XRD analyses were carried out and the results are shown in Figure 2c. The PMT shows a broad peak at 2 θ = 25.55, indicating the semi-crystalline nature of the polymer [40,41]. After the Au coating, this peak is more enhanced due to the composite formation; sharp peaks appear in this region with high intensity, which increases from Au coating 1 to 2 min. Moreover, there is an additional peak that appears at 2 θ = 38.3 • at growth direction (111); this peak increases in intensity for 1 and 2 min coating.
This confirms the growth of the composite with high crystallinity after the Au coating process [41]; this crystalline nature confirms the availability of the composite for watersplitting reaction with high efficiency [42].
Scherrer's formula [41] (D = 0.9 λ/W cosθ) was used to calculate the crystal size. This formula depends on the angle (θ) in radian, wavelength (λ), and the full width half maximum (W). From this formula, we calculated that the crystalline size of PMT is 15 nm, and this crystalline size increases to 24 and 35 nm, after Au coating for 1 and 2 min, respectively. Additionally, the standard stick patterns for ITO/PMT/Au-1 min and ITO/PMT/Au-2 min are shown in Figure 2d,e, respectively.
The morphologies of the prepared PMT, PMT/Au-1 min, and PMT/Au-2 min are shown in Figure 3A-C, respectively. The PMT has a nanopore surface with a smooth lamellar behavior. The diameter of the particles is about 20-30 nm. After Au nanoparticle coating for 1 and 2 min (sputter coating), the porous nature of the surface increased, which is caused by the increase in the active sites of the PMT/Au composite. The average particles sizes of Au nanoparticles for 1 and 2 min are~15 and 38 nm, respectively. Increasing the Au coating to 3 min ( Figure 3D) makes the surface completely blocked with the Au metals. So, these properties prevent the role of the PMT for additional photocatalytic applications.   Aromatic rings para-disubstituted The protons of the benzene ring [39] Polymers 2022, 14, x FOR PEER REVIEW 7 of 18 porosity and roughness increase after the Au coating process for 1 and 2 min. Moreover, the cross-section of the film increases with a Au coating from 1 to 2 min. By increasing the Au coating to 3 min, the Au nanoparticles completely block the PMT surface. This modeling study confirms the SEM image's behavior.  The PMT/Au-2 min has the optimum features for additional photocatalytic applications from these surface morphology properties.
These morphological properties are confirmed using the ImageJ program modeling as shown in Figure 4A-C, for PMT, PMT/Au-1 min, and PMT/Au-2 min, respectively. This program can calculate the cross-section and show the morphology well. From these figures, it can be observed that the PMT morphology is porous with uniform nature; this porosity and roughness increase after the Au coating process for 1 and 2 min. Moreover, the cross-section of the film increases with a Au coating from 1 to 2 min.   By increasing the Au coating to 3 min, the Au nanoparticles completely block the PMT surface. This modeling study confirms the SEM image's behavior.
The optical properties of the PMT, PMT/Au-1 min, and PMT/Au-2 min nanomaterials are shown in Figure 5A, and their bandgaps are shown in Figure 5B. From the absorbance curve, it can be observed that the PMT has two absorbance peaks in the UV and Vis regions at 265 and 550 nm, respectively. These peaks are related to the electrons' transition from band to band and the polymer conjugations chains [41].

Electrochemical Hydrogen generation
The electrochemical H2 generation reaction was carried out through the three-electrode cell, in which the prepared electrode (ITO/PMT, ITO/PMT/Au-1 min, or ITO/PMT/Au-2 min) acts as the working electrode, while graphite and calomel are the After the Au sputter coating for 1 and 2 min, the optical absorbance increases, and the composite has three absorbance peaks, at 295 nm in the UV region and 430 and 605 nm in the Vis region. The peak at 605 nm is related to the plasmonic resonance of Au nanomaterials. After the formation of these three peaks, the composite has peaks that are over most of the optical regions in the UV, Vis, and near IR regions; these peaks have redshifts in wavelengths in comparison with the PMT absorbance.
The Au nanoparticles have a significant role in enhancing the optical properties of the composite. The main property of the Au NPs derives from dimensional confinement (sizes smaller than the wavelength of light), which leads to the alteration of their optical response following the appearance of a phenomenon called "surface plasmon resonance" (SPR), which fundamentally arises from the collective and coherent oscillation of the free conduction electrons in a continuous band structure, due to the resonant excitation caused by the incident photons or electromagnetic radiation [43,44]. This LPSR causes the free electrons of Au to oscillate coherently, creating a strong electric field on the Au nanoparticles. This field is transferred to the neighbor PMT semiconductor polymer for creasing an electron-hole pair [45]. From this process, the electrons are collected on the surface and ready for an additional photocatalytic reaction and to create J ph values. So, the PMT/Au-2 min nanocomposite is qualified to generate photoelectrochemical H2 gas under the water-splitting reaction.
From Tauc's equation and Figure 5B,C, it can be observed that the bandgap of ITO/PMT is 3.74 eV for the wavelength 329.12 nm; this value agrees well with the previous literature [48]. After the Au sputter coating and the formation of the PMT/Au-1 min and PMT/Au-2 min, these two composites have bandgaps for the absorbance peaks in the near IR region at 1.64 and 1.63 eV, respectively ( Figure 5B). Moreover, another bandgap is also related to the UV region peak at 3.8 and 3.70 eV ( Figure 5C) for the two composites, respectively.
These absorbance and bandgap values for the PMT/Au-2 min confirm that this composite has the optimum optical properties compared to PMT or PMT/Au-1 min.
So, the photoelectrode based on this composite is qualified for photocatalytic applications and H 2 generation through the water-splitting reaction.

Electrochemical Hydrogen Generation
The electrochemical H 2 generation reaction was carried out through the three-electrode cell, in which the prepared electrode (ITO/PMT, ITO/PMT/Au-1 min, or ITO/PMT/Au-2 min) acts as the working electrode, while graphite and calomel are the counter and reference electrodes, respectively. The measurements were carried out from sewage water to electrolyte without using any additional electrolyte solution; then, these measurements were compared with Na 2 S 2 O 3 as a reference electrolyte. The measurements were carried out at room temperature (25 • C) under a 400 W artificial light, metal-halide lamp.
As shown in Figure 6a, the measurements were carried out in the dark and under light using the Na 2 On the other hand, using the sewage water as an electrolyte without using any additional electrolyte is very promising for the H2 generation reaction. The chemical construction of the sewage water is shown in Table 3. The Jph value is enhanced after using the sewage water by the electrode PMT/Au-2 min, as shown in Figure 6e. The Jph value reaches 1.09 mA⋅cm −2 , which is an enhanced value compared with the previous standard electrolyte Na2S2O3 that had a Jph value of 0.33 mA⋅cm −2 (Figure 6a). Moreover, the number of the produced H2 moles increased highly, as the produced H2 moles reaches 33.1 mmol⋅h −1 ⋅cm −2 , as shown in Figure 6f.  From this comparison using Na 2 S 2 O 3 as an electrolyte, ITO/PMT/Au-2 min electrode has the optimum behavior for the water splitting and H 2 generation process with the lowest photogeneration voltage (0.56 V).
The stability of the ITO/PMT/Au-2 min electrode was determined, at an applied bias voltage of 0.75 V, through the relation between the time and the produced J ph values, as shown in Figure 6b. The produced J ph values decrease smoothly from 0.3 to 0.1 mA cm −2 on the first time and then become constant until reaching 2000 s. The decrease in the first period is related to the limited corrosion process in the electrode surface under the presence of the electrolyte [49]. The presence of Au nanoparticles protects the PMT layer and decreases the corrosion behavior in this electrode. The stability and reproducibility of the electrode were measured for 7 days through the I-V relation, as shown in Figure 6c. The figure confirms the high stability of the electrode with time in relation to J ph , which decreases only from 0.32 to 0.275 mA cm −2 from the first to the seventh day, respectively.
The number of hydrogen mole was determined for the electrode, ITO/PMT/Au-2 min, by using the Na 2 S 2 O 3 electrolyte, as shown in Figure 6d. The hydrogen moles were calculated using Faraday's law (Equation (3)). This equation depends on the J ph and time (dt) and the Faraday constant (F; 9.65 × 10 4 C mol −1 ). The produced H 2 moles are 8.4 mmol/cm −2 h; this hydrogen gas evolves as bubbles from the electrolyte, in which the Au nanoparticles enhance the number of moles evolved.
On the other hand, using the sewage water as an electrolyte without using any additional electrolyte is very promising for the H 2 generation reaction. The chemical construction of the sewage water is shown in Table 3. The J ph value is enhanced after using the sewage water by the electrode PMT/Au-2 min, as shown in Figure 6e. The J ph value reaches 1.09 mA cm −2 , which is an enhanced value compared with the previous standard electrolyte Na 2 S 2 O 3 that had a J ph value of 0.33 mA cm −2 (Figure 6a). Moreover, the number of the produced H 2 moles increased highly, as the produced H 2 moles reaches 33.1 mmol h −1 cm −2 , as shown in Figure 6f. The effect of temperature of 25-70 • C on the electrode ITO/PMT/Au-2 min for the water-splitting reaction and H2 generation using the Na 2 S 2 O 3 electrolyte is shown in Figure 7a. The increasing in temperature from 25 to 70 • C causes the J ph values to increase from 0.32 to 0.88 mA/cm 2 , respectively, at 0.9 V. This is related to the role of the temperature in the increase in ion mobility and then the H 2 generation rate [50]. The effect of monochromatic light wavelengths between 390 and 636 nm on H2 generation using the ITO/PMT/Au-2 min electrode is shown in Figure 8a. From this figure, the produced Jph values decrease from 0.30 to 0.20 mA cm −2 , increasing the wavelengths from 390 to 500 nm, respectively. Then, the Jph increases again until it reaches 0.28 mA.cm −2 at 636 nm. The wavelength 500 nm has the minimum Jph value; this behavior is matched well with the optical absorbance spectrum in Figure 5a. Moreover, the good values of Jph in the broad wavelength region confirm the solar absorption for the ITO/PMT/Au-2 min and H2 generation in these light regions. Moreover, the Au nanoparticles play a good role in enhancing light absorption, and they overlap in the response of Au and PMT nanomaterials to produce Jph values at lower potential. Figure 8b represents the photon-to-current conversion efficiency (IPCE) for the electrode ITO/PMT/Au-2 min under the light for the water-splitting reaction and H2 generation using the Na2S2O3 electrolyte. This IPCE can be calculated using Equation (6) [52]. This equation depends on the Jph, wavelength ( ) and light intensity (ρ). The IPCE value is 2.3% at 390 nm. These values are compared with the previous literature as shown in Table 4.
. 100 (6) The activation energy (E a ) can be calculated from the Arrhenius and Eyring equations, Equations (4) and (5), respectively [46,51]. These equations depend on the following factors: the Arrhenius constant (A), the universal gas constant (R), the temperature (T), the rate constant (k), the Boltzmann constant (B), and the Planck constant (h). From the Arrhenius equation and Figure 7b, the activation energy (E a ) is 31.49 KJ mol −1 .
The t ∆H* and ∆S* values are calculated from the Eyring equation and Figure 7c, in which the values are 114.49 Jmol −1 and 160.46 JK −1 mol −1 , respectively: On the other hand, after using the sewage water as an electrolyte, there is more enhancement in the produced J ph values in comparison with the standard Na 2 S 2 O 3 . The increase in temperature from 25 to 60 • C causes the J ph values ro increase from 1.09 to 11.2 mA cm −2 , respectively. This confirms the superiority of sewage water as an electrolyte for the water-splitting and H 2 generation reaction.
The effect of monochromatic light wavelengths between 390 and 636 nm on H 2 generation using the ITO/PMT/Au-2 min electrode is shown in Figure 8a. From this figure, the produced J ph values decrease from 0.30 to 0.20 mA cm −2 , increasing the wavelengths from 390 to 500 nm, respectively. Then, the J ph increases again until it reaches 0.28 mA cm −2 at 636 nm. The wavelength 500 nm has the minimum J ph value; this behavior is matched well with the optical absorbance spectrum in Figure 5A. Moreover, the good values of J ph in the broad wavelength region confirm the solar absorption for the ITO/PMT/Au-2 min and H 2 generation in these light regions. Moreover, the Au nanoparticles play a good role in enhancing light absorption, and they overlap in the response of Au and PMT nanomaterials to produce J ph values at lower potential.     Figure 8b represents the photon-to-current conversion efficiency (IPCE) for the electrode ITO/PMT/Au-2 min under the light for the water-splitting reaction and H 2 generation using the Na 2 S 2 O 3 electrolyte. This IPCE can be calculated using Equation (6) [52]. This equation depends on the J ph , wavelength (λ) and light intensity (ρ). The IPCE value is 2.3% at 390 nm. These values are compared with the previous literature as shown in Table 4.
Using sewage water as an electrolyte for H 2 generation using the electrode ITO/PMT/Au-2 min, there are more enhancements in the produced J ph values as shown in Figure 8c. The variation of the produced J ph values with the monochromatic light has the same behavior by using Na 2 S 2 O 3 as an electrolyte. The J ph values decrease from 0.98 to 0.91 mA cm −2 , increasing the wavelength from 390 to 500 nm. Then, the J ph value increases to 0.97 mA cm −2 at 636 nm. The IPCE for the H 2 generation is 3.6%. These values are greater than the previous values by using the Na 2 S 2 O 3 electrolyte.

Mechanism
The prepared electrode PMT/Au-2 min mechanism for H2 generation from Na 2 S 2 O 3 or sewage-water electrolyte was carried out using two steps; interfacial charge transfers take place due to the electron-hole transfer. In addition to the localized surface plasmonic resonance (LSPR), this process causes the oscillation of the electron on the surface of the semiconductor material. These two phenomena can appear well through the optical analyses ( Figure 5A,B) and the electrochemical curve and H2 moles produced (Figure 6a-f). Under the light incident, there is an electron transfer from the LUMO to HUMO for the PMT [60]; then, these electrons oscillate on the surface of the PMT and transfer the energy from the Au to PMT. The wide range of absorbance related to the PMT/Au composite increases the electron generation on the surface that is finally collected on the PMT surface under the LSPR process [61]. More electrons are generated with this high electromagnetic coupling between Au and PMT. Then, more H2 generation reactions occur [62]. The electron transfer depends on the work function of Au and the electron affinity of PMT.
Moreover, the presence of the Schottky barrier limited the electron transfer under the presence of the internal electric field. This barrier push transfers the photoexcited electrons. At the same time, it prevents electron-hole pair recombination. This process causes an enhancement in the produced J ph and then water splitting for the H 2 generation reaction [63]. The hot electrons pass to the PMT, while the cold electrons still suffer the internal barrier, as shown in Figure 9. Finally, these hot electrons are collected on the PMT surface for water splitting and H 2 generation reaction.
Moreover, the presence of the Schottky barrier limited the electron transfer under the presence of the internal electric field. This barrier push transfers the photoexcited electrons. At the same time, it prevents electron-hole pair recombination. This process causes an enhancement in the produced Jph and then water splitting for the H2 generation reaction [63]. The hot electrons pass to the PMT, while the cold electrons still suffer the internal barrier, as shown in Figure 9. Finally, these hot electrons are collected on the PMT surface for water splitting and H2 generation reaction. Figure 9. The mechanism of water splitting reaction using the PMT/Au-2 min electrode.

Conclusions
PMT preparation was carried out by the electropolymerization of m-toluidine on the ITO using the anodic polymerization method. Then, Au under different sputter coating times (0, 1, 2, and 3 min) was deposited on the PMT, in which the ITO/PMT, ITO/PMT/Au-1 min and ITO/PMT/Au-2 min electrodes were prepared. The full characteristic analyses were carried out to confirm the chemical structure and morphology of the prepared nanomaterials. From the different analyses, the electrode ITO/PMT/Au-2 min has the optimum optical properties to be applied as an electrode for water splitting and H2 generation.

Conclusions
PMT preparation was carried out by the electropolymerization of m-toluidine on the ITO using the anodic polymerization method. Then, Au under different sputter coating times (0, 1, 2, and 3 min) was deposited on the PMT, in which the ITO/PMT, ITO/PMT/Au-1 min and ITO/PMT/Au-2 min electrodes were prepared. The full characteristic analyses were carried out to confirm the chemical structure and morphology of the prepared nanomaterials. From the different analyses, the electrode ITO/PMT/Au-2 min has the optimum optical properties to be applied as an electrode for water splitting and H 2 generation. The thermodynamic parameters were calculated for the H2 production from water using Na 2 S 2 O 3 as an electrolyte, in which E a , ∆S*, ∆H* values were 31.49 KJ mol −1 , 160.46 JK −1 mol −1 , and 114.49 J mol −1 , respectively. The electrode has high stability and reproducibility for H 2 generation reaction, in which the J ph was decreased from 0.32 to 0.27 mA/cm 2 during the 7 days. The H 2 generation under sewage water splitting was carried out using the PMT/Au-2 min with high IPCE 3.6% at 390 nm, and the produced H 2 moles evolved were 33.1 mmol h −1 cm −2 . These results were compared with the previous data of the Na 2 S 2 O 3 electrolyte with maximum IPCE 2.3% and H 2 moles of 8.4 mmol/cm 2 .h.
We will continue this research by working on the synthesis of an electrochemical cell for sewage water splitting directly. Sewage water can be used as electrolyte inside the cell directly for H 2 gas production that is used as fuel inside homes, factories, and companies. This cell will be promising for renewable energy production, especially in remote regions, such as deserts and spacecraft.

Institutional Review Board Statement:
The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Review Board.

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