Effect of Precursors Concentration on The Optical and Photoelectrochemical Properties of Bi ₂ S ₃ /TiO ₂ Nanotubes Arrays Photoanode Synthesized by the SILAR Technique

The use of robust solar energy-driven photocatalysis materials to address the global energy and environmental crisis has gained significant attention in recent years. However, the wide band gaps in many robust semiconductor photocatalysts hinder their absorption of visible light from the solar spectrum. To address this issue, the modification of the large band gap semiconductor with the lower band gap material using the Successive Ionic Layers Adsorption and Reaction (SILAR) technique has emerged as an economical, accessible


Introduction
The world is dealing with enormous challenges related to energy security and environmental sustainability [1].The use of fossil fuels contributes to environmental pollution by releasing carbon dioxide into the atmosphere, and the continuing depletion of fossil fuels seriously threatens the global community [2].In addition, the increasing amount of hazardous industrial waste being disposed of raises serious environmental concerns with industry advancement.As a result, efforts are being undertaken to investigate technology for enhanced ecological cleanup and renewable energy sources [3].It requires material that supports the objectives of obtaining renewable energy sources and environmental restoration.
Notably, TiO2 in nanotube arrays (NTAs) especially attracts interest due to its porous, one-dimensional structure, offering a higher specific surface area and improved electron-hole transfer efficiency [7].However, despite these advantages, pristine TiO2 NTAs exhibit limited photocatalytic activity outside the UV region due to their wide band gap (3.0-3.2 eV).To overcome this limitation, various modification strategies such as doping, dye sensitization, and junction formation with lower band gap materials are commonly employed.
Bi2S3 semiconductor is a significant semiconductor within the V-VI family of materials, drawing technological interest due to recent research advancements.It finds potential applications in various fields, such as supercapacitors, photocatalysis, and fluorescent markers [8].As an orthorhombic n-type semiconductor with a band gap of ~1.7 eV, Bi2S3 efficiently absorbs visible light in the range of 400 to 900 nm [9].Sensitizing TiO2 NTAs with Bi2S3 enhances the photocatalytic activity compared to their pristine counterparts due to the matched band potentials [10,11].Furthermore, the low cost, natural abundance, nontoxicity, and environmental friendliness of Bi2S3 further enhance its applicability [12,13].
Successive Ionic Layers Adsorption and Reaction (SILAR) is a method that can be employed to fabricate TiO2 NTAs sensitized with Bi2S3 material.SILAR is known for its straightforward process, cost-effectiveness, and shorter deposition time, making it advantageous for depositing thin films of binary semiconductors [12].Using the SILAR approach, weakly bound species are eliminated by immersing the substrate separately in two precursor solutions and then washing it between them with the appropriate solvent, allowing the formation of the well-dispersed nanomaterials on the substrate [14].Therefore, a SILAR cycle entails the following steps, i.e., adsorption of cation precursors, solvent flushing, adsorption of anion precursors, further reactions, and rinsing [15].Concisely, the SILAR approach prevents homogenous precipitation in the solution by successive flushing with the appropriate solvent between each immersion and utilizing the precursor solution's adsorption and ion reaction [16,17].
The synthesis of Bi2S3/TiO2 NTAs material reported previously was mainly conducted at a fixed concentration [12,14].However, it is recognized that the concentration of precursors significantly influences the character of the resulting precipitate in the SILAR method [17].Therefore, this study proposes a different set of cation and anion concentrations for the SILAR synthesis process.Specifically, we investigate how varying concentrations affect the optical and photoelectrochemical properties of Bi2S3/TiO2 NTAs.The concentration ratios used in this research were varied as follows: 1 mM: 1.5 mM, 2 mM: 3 mM, and 10 mM: 30 mM.

Synthesis of TiO2 NTAs
Titanium foil with a 0.2 mm thickness was cleaned by sonicating it at room temperature for 15 minutes in C2H5OH and C3H6O solutions, after which it was rinsed with distilled water and left to air dry.Every anodization experiment was conducted using a two-electrode electrochemical cell.Pt was utilized as the cathode, and the Ti plate (6 × 1.5 × 0.02 cm) was used as the anode.An electrolyte consisting of 2% H2O and 0.3% NH4F was a C2H6O2 solution.The two electrodes were spaced apart by about 1.5 centimeters.At a potential of 50 V, the anodization process was run for 60 minutes.Following the anodization procedure, the sample was immersed and dried, then calcined for 2 hours at a temperature of 450°C with a temperature rise rate of 5°C/min [18].

Synthesis of Bi2S3/TiO2 Nanotubes Arrays (Bi2S3/TiO2 NTAs) by SILAR Method
Bi2S3 on the surface of TiO2 NTAs was synthesized using the SILAR method with variations of SILAR cycles 1, 3, 5, and 7, as illustrated in Figure 1.In addition, this research was also compared to variations of concentrations for precursor cations and anions.The concentration variation (mM) used were 1: 1.5, 2: 3, and 10: 30.The proposed reaction mechanism for the formation of Bi2S3 is shown in Equations (1 to 3) [19].
where R is the diffuse reflectance value, h is Plank's constant, n is the frequency of vibration, α is the absorption coefficient, Eg is the band gap, A is the proportional constant, and n depends on the band structure of the sample is 2 for indirect allowed transition (n = 2), due to TiO2 in amorphous and anatase phase exhibiting indirect electron transfer [21].
The F(R) values were plotted versus energy photon values according to the Tauc plot, where the Eg was determined at the F(R) value equal to zero.
In this research report, the morphology of the resulting material is not presented.However, based on the anodization and SILAR techniques employed, previous studies have confirmed that TiO2 NTAs possess a diameter of 67.91 nm and a height of 4.4 µm [16].

Measurement of Photoelectrochemical Performance (PEC)
The preparation of photoelectrochemical cells was carried out using a potentiostats 3-electrode system.Working electrodes were TiO2 NTAs and Bi2S3/TiO2 NTAs.Meanwhile, the counter electrode was Pt, and the reference electrode was Ag/AgCl.A 40-watt Philips tungsten lamp was used as a visible light source.TiO2 NTAs and Bi2S3/TiO2 NTAs were tested utilizing 0.1 M Na2SO4 electrolyte.The test was carried out using the Multi Pulse Amperometry (MPA) in dark and light conditions for 10 seconds each other, and the applied potential was set at 0 V.

UV-Vis DRS Characterization of TiO2 Nanotube Arrays
The anodization of titanium metal plate in C2H6O2 containing water and fluoride ions produces an immobilized thick film of a highly ordered titanium oxide (TiO2 NTAs).The results of UV-Vis DRS characterizations are presented in Figure 2a and Figure 2b.The TiO2 NTAs show a typical reflectance spectrum in the 200-800 nm wavelength range [22].The step decrease in %R occurs at wavelengths under 400 nm, which indicates that the absorption area of TiO2 NTAs is in the region.It is important to note that the low %R values observed at wavelengths between 500 and 800 nm are attributed to refractive phenomena rather than absorption, as previously reported [23].Based on this spectrum, the Kubelka-Munk and Tauc equations were used to create the Tauc plot, and the band gap was determined to be 3.2 eV.

Synthesis of Bi2S3/TiO2 NTAs
The synthesized Bi2S3/TiO2 NTAs by the SILAR approach were used to deposit Bi2S3 nanoparticles onto the surface of TiO2 NTAs (Figure 1).The following protocols were put into place, i.e., separately dissolves of Bi(NO3)3 in 50 milliliters 0.1 M mannitol solution (solution A) and Na2S.3H2O in 50 milliliters of deionized water (solution B).Firstly, TiO2 NTAs were soaked in solution A for three minutes, rinsed with deionized water, and then dipped in solution B for three minutes before being rinsed with deionized water.For the first cycle, it was sufficient to precipitate Bi2S3 particles on TiO2 NTAs.The process was repeated several times to increase the loading of Bi2S3.The samples were labeled 1, 3, 5, and 7 cycles to distinguish them from one another.The thin films produced were dried at 60°C for an hour [24].

FTIR Characterization of Bi2S3/TiO2 NTAs
The FTIR spectra in Figure 3 confirmed the presence of the conventional Bi-S-Bi vibration at wavenumber ~1100-1200 cm -1 and SH group stretching, which is visible at wavenumber 1344 cm -1 , indicating the successful of Bi2S3 deposition onto TiO2 NTAs [16].In addition, the absorption of functional groups by OH bending and OH stretching are also observed at wavenumbers 1540 cm -1 and 3247 cm -1 [19].It also observed that the absorption of Bi-S at ~1130 cm -1 increases with the number of cycles and the concentration ratio, while shifting to a larger wavenumber.This indicates that the deposition of Bi2S3 onto TiO2 NTAs was successful.

Optical Band Gap of Bi2S3/TiO2 NTAs
Figure 4 (a, b, and c) displays the Tauc Plot of TiO2 NTAs and Bi2S3/TiO2 NTAs of each concentration variation, while the band gap calculation summary of every sample is presented in Figure 4d.The results confirm that the addition of Bi2S3 at varying concentrations reduces the band gap of TiO2 NTAs.An increase in SILAR cycles further decreases the band gap values, indicating that more Bi2S3 is deposited with additional cycles.As the concentration of precursors increases, the band gap reduction in each cycle becomes more pronounced, signifying higher Bi2S3 deposition in more concentrated precursors.This is characterized by the absorption shift to the visible region and the corresponding decrease in energy, resulting in a reduced band gap.

Photoelectrochemical Performance Using Multi
Pulse Amperometry (MPA) Method Under Visible Light Irradiation The performance of the prepared photoanode (Bi2S3/TiO2 NTAs) in generating the electron-hole pair under a visible light source was evaluated by the MPA techniques.It can be observed that the photocurrent density remains a constant high value when the light is turned on and then quickly decreases to zero mA/cm 2 as long as the light is turned off, indicating that the photocurrent is generated due to the photoelectric conversion of the Bi2S3/TiO2 NTAs photoelectrode, and the electron transport rate is very fast.In addition, spikes in all Bi2S3/TiO2 NTAs photoelectrodes can be observed when the light is intermittent, which is attributed to the accumulation of charge carriers.
Figure 5 shows a considerable photocurrent density that evolved when the visible light was switched ON.The results of this measurement showed the maximum electrochemical performance for each change in concentration and the cycles that vary with variation.optimum electrochemical performance arises at 5 SILAR cycles (0.12 mA/cm 2 ) for concentration variation of 1 mM: 1.5 mM (Figure 5a), the optimum electrochemical performance occurs at 3 SILAR cycles (0.05 mA/cm 2 and stable) for concentration ratio of 2 mM: 3 mM (Figure 5b), and the optimum electrochemical performance occurs at 1 SILAR cycle (0.14 Ma/cm 2 and unstable) for concentration ratio of 10 mM: 30 mM (Figure 5c).Compared with the report conducted by Wang et al. [10], the best current density in 5 SILAR cycles with a ratio of cation and anion concentrations of 0.01 M: 0.01M using distilled water as a solvent.In addition, when compared to other research reports, maximum current density was obtained at 3 SILAR cycles with a ratio cation and anion concentrations of 0.01 M: 0.01 M. In that research report, the current density in 3 SILAR cycles is the maximum compared to other cycles; when compared to pure TiO2 NTAs, the increase is fourfold [16].In this research, the maximum current density is in 5 SILAR cycles for a ratio of 1 mM: 1.5 mM; when compared to pure TiO2 NTAs, the increase is fifteenfold.This is a consequence of the amount of Bi2S3 being deposited, which will also affect the material's surface.The photocurrent evolved due to the ability of Bi2S3 to absorb visible light to create exited and free electrons, which were subsequently injected into the TiO2 NTAs conduction band and then produced photocurrent, which induced photocatalytic activity [25].Based on PEC performance (MPA method), material TiO2 NTAs before being deposited with Bi2S3 have a bad response to visible light compared with TiO2 NTAs after being deposited with Bi2S3.It also confirmed the UV-DRS data of TiO2 NTAs; the more bandgap energy of the TiO2 shifts towards visible light, the more the current density increases.However, the resulting current density is also not maximized when it shifts to the visible area in optical properties.

Conclusion
This research applied the SILAR method to deposit Bi2S3 nanoparticles on TiO2 NTAs as a photosensitizer as part of its evaluation.The results indicate that increasing the precursor concentration leads to greater deposition of Bi2S3, evidenced by a reduction in the band gap value.However, the over-deposited Bi2S3 impedes electronhole mobility within the photoanode, consequently lowering the photocurrent performance.The best photoelectrochemical performance was shown by the Bi2S3/TiO2 NTAs photoanode synthesized at a cationanion ratio of 1 mM: 1.5 mM at 5 SILAR cycles with an energy band gap of 1.71 eV when compared to pristine TiO2 has a fifteenfold increase in current density.For other ratios, the current density for a cation-to-anion concentration ratio of 2 mM: 3 mM at 3 SILAR cycles yields a steady-state photocurrent of ~0.01 mA cm -2 .Similarly, a concentration ratio of 10 mM: 30 mM at 1 SILAR cycle.

Figure 1 .
Figure 1.Successive Ionic Layers Adsorption and Reaction (SILAR) illustration on the synthesis of Bi2S3 deposited on the TiO2 nanotubes array

Figure 4 .Figure 5 .
Figure 4.The results of characterization using UV-DRS for TiO2 NTAs and Bi2S3/TiO2 NTAs prepared by SILAR at different precursor concentrations and different cycles (a) Tauc plot for 1 mM of cationic and 1.5 mM of anionic, (b) Tauc plot for 2 mM of cationic and 3 mM of anionic, (c) Tauc plot for 10 mM of cationic and 30 mM of anionic, (d) diagram for summary band gap energy of Bi2S3/TiO2 NTAs for different precursor concentrations and different cycles