Enhancing photocatalytic performance of BiOI/TiO2 NTs nanocomposites through SILAR optimization for methylene blue degradation

BiOI nano-leaves were deposited on to TiO2 nanotubes (NTs) using the Successive Ionic Layer Adsorption and Reaction (SILAR) technique, which was developed using the electrochemical anodization method. Various SILAR cycle numbers (three, five, and seven cycles) were employed in the experiment. The as-prepared nanocomposites (NCs) were characterized by several technique, the morphology of the elaborated NCs samples was examined using a S − 4800 field emission scanning electron microscope (SEM), the Reflectance and diffuse reflectivity of the NCs samples were measured by a Shimadzu UV-3100S spectrophotometer in the spectral range [300 –1200 nm] and the XRD diffraction was used to identify the crystalline structure of the processed BiOI/TiO2 NTs nanocomposites. A diffractometer with a Cu Kα anode (λ = 0.1542 nm) operating at 40 kV and 30 mA was used. The as-prepared NCs, specifically BiOI/TiO2 NTs, were designed for the photocatalytic degradation of methylene Blue (MB). X-ray diffraction analysis revealed the formation of the TiO2 anatase phase and polycrystalline BiOI films in all processed NCs. UV/Vis measurements indicated a shift in the nanocomposite’s active region from UV to visible light. The highest absorption and the lowest bandgap energy (Eg value, ∼2 eV) were observed in the NCs with 5 BiOI cycles. The photocurrent density reached 27 μA cm−2, approximately three times higher than the photocurrent density exhibited by TiO2 nanotubes under similar conditions. The optimal photocatalytic rate was achieved with BiOI/TiO2 NCs processed after five SILAR cycles.

Yu-Long Xie and co-authors [36] successfully deposited BiOI nanosheets onto the surface of threedimensional porous TiO 2 nanotube arrays (NTs) substrate using the SILAR method.This coupling of BiOI and TiO 2 NTs significantly improved the photocatalytic activity of Rhodamine B (RhB) degradation under visiblelight irradiation.
Building up on many previous research and considering the specific surface characteristics, this study focuses on depositing BiOI nano-leaves on to TiO 2 nanotubes via the Successive SILAR technique, which were fabricated using electrochemical anodization.The main objective is to explore the impact of various SILAR cycle numbers on the photocatalytic degradation of Methylene Blue.The results reveal that the BiOI/TiO 2 NTs with 5 SILAR cycles exhibit the most efficient photocatalytic activity compared to pure TiO 2 NTs, accompanied by a higher photocurrent value.

Experimental section 2.1. NCs preparation
An electrochemical anodization method was used to elaborate on Titanium foil a TiO 2 NTs array at ambient temperature.Pieces of 2 × 1.5 cm was cut from Ti foil (Sigma Aldrich, 99.8% purity) then used as working electrodes.A Platinum foil of similar size was used as counter electrode.The electrolyte used was prepared by mixing 0.25 wt% ammonium fluoride in 2 ml of dionised water followed by addition of 75 ml of ethylene glycol (EG, 99.8% purity).The anodization of the Ti electrode NCs was carried at a constant voltage of 30 V for 4 h using a DC power supply (0-60 V, Consort, EV 245).Afterwards, the NCs were copiously rinsed with distilled water and then allowed to dry at room temperature.The NCs were then annealed at 450 °C for 2 h in a resistive furnace to improve crystallization.In a standard synthesis, the prepared NTs were first submerged in 20 ml of ethanol solution containing 1 M Bi (NO 3 ) 3 •5H 2 O for 30 s, Subsequently, they were rinsed in distilled water before being immersed in a 20 ml solution of 0.05 M NaI aqueous for another 30 s, immersed in 20 ml 0.05 M KI aqueous solution for 20 s and rinsed again in distilled water.The rinsing process were considered as an important step to remove the precursor solution on the film surface and prevent the blockage of tube mouths by BiOI nano-leaves.This SILAR cycle was then repeated three, five and seven times.The producing samples were designated as BiOI/ TiO 2 NTs_3, BiOI/ TiO 2 NTs_5 and BiOI/ TiO 2 NTs_7.

Characterization
The morphology of as prepared BiOI/ TiO 2 NT NCs was examined using a S − 4800 field emission scanning electron microscope (SEM).The Reflectance and diffuse reflectivity of these NCs samples were measured by a Shimadzu UV-3100 S spectrophotometer in the spectral range [300-1200 nm].Additionally, XRD diffraction was used to identify the crystalline structure of the processed BiOI/TiO 2 NTs nanocomposites.A diffractometer with a Cu Kα anode (λ = 0.1542 nm) operating at 40 kV and 30 mA was used.Over an angular range of 20 to 80 we collected the diffraction patterns, given by the spectrometer, with a step size of 0.05 per step.

Photoelectrocatalytic and photoelectrochemical measurements
Photoelectrocatalytic (PEC) activity of the NCs samples were given by using an aqueous solution of methylene blue (V = 20 ml, 0.5 M) prepared at ambient temperature.Firstly, the prepared samples (2 × 2.5 cm 2 ) were dipped in a 15 ml MB solution for 1 h to achieve the adsorption-desorption equilibrium.Secondly, The obtained BiOI/TiO 2 NTs nanocomposite were dipped in the solution and exposed under light irradiation every 30 mn.The photo-electrocatalytic decolorization of the solution, during exposure time on solar irradiation, is a pseudo first-order reaction solution.Finally, the concentration of the change solution was measured by using an UV visible absorption spectroscopy working in a transmission mode (UV-2550) following the methylene blue absorption peak intensity decrease at 660 nm.The decolorization kinetics may be expressed by the following relation : Ln(C 0 /C) = kt, where k, C0 and C are the apparent rate constant, the initial and reaction concentrations of the MB aqueous solution, respectively.The visible-light photoelectrocatalytic activity of all NCs was as well measured in the same conditions.
The photoelectrochemical performance measurements of BiOI/TiO 2 NTs nanocomposites were carried out in a quartz electrolytic cell.The prepared samples (1 cm 2 ) were employed as the working electrode.Pt plate (1 cm 2 ) an Ag/AgCl mesh were used as counter and reference electrodes, respectively.An aqueous Na 2 SO 4 (0.5 M) solution, prepared using deionized water, was employed as the electrolyte.All measurements were given at room temperature.Current densities, as a function of applied potential (−1 to +0.8 V versus Ag/AgCl electrode), were recorded under front-side illumination with a computer-controlled Potentiostat/Galvanostat PGSTAT3 (Eco Chemie BV) for all PEC experiments.The samples were illuminated with a solar simulator equipped with a 300 W Xenon short arc lamp (Perkin Elmer Model PE300BF) with white light intensity of 200 mW cm −2 .The intensity of incident light from the Xenon lamp was defined using a Photometer Model 70310 from Spectraphysics.

Results and discussion
3.1.Morphology of BiOI/TiO 2 NT heterostructures Figure 1(a) illustrates the morphology of the porous uniform arrays of TiO 2 NTs.This typical SEM image show a regular arrangement of the nanotubes, with an average diameter of approximately 100 nm.Zhiyuan Liu et al [6] were prepared an uniform and regular tube structures with an average diameter and wall thickness of TiO 2 NTs nearly 150 and 15 nm, respectively.Subsequently, the morphology and the microstructure of the BiOI/TiO 2 NTs nanocomposite processed at various SILAR cycle numbers (three, five, and seven cycles) were analyzed using SEM.
As shown in figures 1(b)-(d) BiOI nano-leaves with a thin thickness were covered on the microstructure surface by the SILAR method.BiOI nano-leaves increased with the SILAR cycles, and they were randomly deposited over the TiO 2 nanotubes surface without blocking tube entrances.When the SILAR cycles were increased a nanosheets were massively produced and covered the surface of TiO 2 NTs [6].BiOI/TiO 2 NTs_5 NCs exhibits a few thin nanosheets standing on the nanotubes surface, whereas BiOI/TiO 2 _7 NCs displays numerous thick nanosheets covering the entire nanotube structures.As they shown previously [21,22] the presence of tetragonal layer structures containing [Bi 2 O 2 ] 2+ slabs facilitate the formation of nanosheets.In figure 2, the TiO 2 anatase phase peaks (JCPDS card # 21-1272 ) are represented by A symbol.It has been pointed out that high crystalline photoactive materials exhibit greater diffusion length of charge carriers leading to improved charge separation in heterojunctions and then achieving superior photoactivity [37,38].
The crystallites size (D) and strain (ε) were calculated from XRD data analysis using Williamson-Hall (W-H) plot method.In XRD data, the broadening T b of the peaks is due to the combine effect of crystallites size D b ( ) and micro-strain (β ε ): XRD peak broadening is directly related to crystallite size through the Debye-Scherer equation [39,40].
Where β D is the FWHM (Full Width at Half Maximum) in radians, K = 0.9 is the shape factor, λ = 0.15406 nm is the wavelength of the incident X -ray, D is the crystallite size and q is the peak position in radians.Similary, the XRD peak broadening due to micro strain is given by: Where βε is broadening due to strain, e is the strain and q is the peak position in radians.
Combining equations (2) and (3) in equation (1) we get the following equation [41,42]: )for the prepared BiOI/TiO 2 NCs.The crystalline size (D) and the micro-strain of the prepared nanostructures were calculated and given in table 1.
The crystallite size of BiOI/TiO 2 NTs nanocomposite compared to TiO 2 NTs can offer valuable insights into the structural properties and performance of the composite material.As we noticed, the crystallite size attributed to the BiOI/TiO 2 NTs nanocomposite prepared with five cycles had the lowest values for both BiOI crystalline size and micro-strain, measuring 24 nm and 0.32, respectively.This suggests a low defect density in these composites, despite the small size of the BiOI crystals.The integration of BiOI nanoparticles in to TiO 2 nanotubes results in finer crystallites, which increases the surface area available for chemical reactions.This enhancement provides more active sites for photocatalytic reactions, thereby improving efficiency in processes such as pollutant degradation and hydrogen production [43].Smaller crystallites are known to exhibit better charge carrier transport properties due to reduced diffusion distances and fewer defects.In the case of BiOI/TiO 2 NTs prepared with five cycles, the smaller crystallite size promotes more efficient charge separation and migration within the composite structure, leading to enhanced photocatalytic activity and electrical conductivity.Moreover, smaller crystallites contribute to more pronounced light scattering effects, enhancing light trapping within the composite material and improving photon absorption efficiency.

Diffuse reflectance spectroscopy analysis
The UV-Vis diffuse reflectance spectra of elaborated BiOI/ TiO 2 NTs nanocomposite was recorded in the wavelength range of 300-800 nm as shown in figure Where F(R) is the absorption obtained from the Kubelka-Munk function, h is Planck's constant, υ is the incident light frequency, A is a constant and Eg is the band gap width.In figure 5, the extrapolation of the Tauc plot to the x-axis provides the band gaps for the corresponding NC.Table 2 gives Eg values calculated by Tauc's model.The NC bandgap of the composite material was found to be minimum for the 5 cycles NCs which is nearly 2 eV.This result is in agreement with the high absorption in the visible wavelength range.TiO 2 nanotubes show an absorption edge at a wavelength of 386 nm which corresponds to a band gap of 3.27 eV.However, BiOI/TiO 2 NTs − 5 NCs shows a narrow band gap nearly 2 eV.The band gap gets reduced from UV active region (3.27 eV) to visible active region (∼2 eV).This enhancement was previously discussed by Aprizal et al [47].They showed that the TiO 2 -nanotubes/BiOI with repeat deposition (five times) had bandgap energy of 2.26 eV and better photocatalytic activity than bare TiO 2 -nanotubes, where TiO 2 -nanotubes/BiOI were active in the visible region and gave a higher current density response.

Photocurrent response
The transient photocurrent responses of the TiO 2 NTs and BiOI/TNTs nanocomposites processed after 3,5 and 7 cycles were measured under intermittent visible-light irradiation, as shown in figure 6.As we observed, the photocurrent curves exhibited a spike at the initial time of irradiation for all obtained nanocomposites and then decayed and kept at a constant value, and the photocurrents rapidly drop to zero when the light was turned off.The initial spike photocurrent is attributed to the generation and separation of photoinduced electron-hole pairs at the semiconductor/electrolyte interface : With the electric driving forces (a positive bias, 0.2 V) the holes will move to the surface of the semiconductor, and then be captured by the reduced species in the electrolyte, while the electrons are transferred along walls of TiO 2 nanotubes to the back contact Ti substrate [48,49] The decay of the photocurrent to a constant value indicated that a fraction of holes can reach the semiconductor surface, instead of capturing electrons from the electrolyte or recombining with the photoinduced electrons during the migration, and finally the generation and recombination of electron-hole pairs gradually achieved an equilibration [50].Figure 6 illustrates the photocurrent density behavior of BiOI/TiO 2 NTs nanocomposites with three different BiOI deposition SILAR cycles.In comparison to a single layer of TiO 2 NTs, the nanocomposites exhibit a higher photocurrent value under visible light irradiation.This enhancement might be  ascribed to the collective effects of several factors.Firstly, the internal electrostatic field in the p-n junction reduces the recombination of photogenerated charge carriers (electrons and holes).Secondly, the photocurrent is mainly enhanced by the strong visible absorption, increasing the photogenerated charge carriers and there by improving both electron diffusion to the back contact and hole transfer at the semiconductor/electrolyte interface [51][52][53][54].Third, the efficient separation and transfer of photogenerated charge carriers are facilitated by the external electrostatic field, further enhancing the performance of BiOI/TiO 2 NTs.The highest photocurrent density (27 μA cm −2 ) is achieved with the BiOI/TiO 2 NTs − 5 cycles nanocomposites as photoanodes.This behavior is attributed to the improvement brought about by the enhanced separation efficiency of photogenerated charge carriers.The incorporation of BiOI nano-leaves into the nanocomposite structure, particularly under visible light, renders it more photoactive, resulting in a remarkable enhancement of photoelectrical properties.
It is noteworthy that the highest efficiency in photoelectric transformation was observed for BiOI/TiO 2 NTs with 5 deposition cycles.However, when processed with a higher or lower number of BiOI deposition cycles than 5, BiOI/TiO 2 NTs displayed a diminished photocurrent response.As shown in figure 6, the BiOI/TiO 2 NTs NC processed with 5 cycles, provide a photocurrent density value nearly 27 μA, approximately three times higher than the photocurrent density exhibited by TiO 2 nanotubes (under similar conditions.The optimal photocatalytic rate was achieved with BiOI/TiO 2 NCs processed after five SILAR cycles.An appreciable enhancement in photocurrent density (0.52 mA cm −2 was given previously [55] using an hybrid electrode formed by ZnFe 2 O 4 micro crystals that deposited over electrochemical anodized TiO 2 nanotube array.

Photocatalytic propriety analysis
The as prepared BiOI/TiO 2 NTs photocatalysts presented high photocatalytic activities though methylene bleu (MB) degradation (figure 7).The most efficient photocatalytic degradation of MB was observed in samples prepared with 5 cycles of BiOI.Specifically, the efficiency of these samples reached 97% after solar irradiation for 180 min, nearly four times more active than pure TiO 2 NT layers.This heightened photocatalytic performance is mainly attributed to the formation of heterojunction between TiO 2 and BiOI that promotes the separation of the photoexcited electron-hole pairs and their efficient transfer as well.
Figure 8 illustrated the photocatalytic degradation of MB over various photocatalysts as a function of time under visible light irradiation.Notably, TiO 2 NTs display an incapability for MB degradation due to their limited absorption of visible light.In contrast, the p-semi-conductor BiOI, characterized by its narrow band gap, facilitates the swift recovery of electrons and holes upon visible light exposure.So, The elaborated TiO 2 NTs adorned with BiOI nano-leaves through the SILAR method improved the photocatalytic behavior.We noticed that BiOI/TiO 2 NTs nanocomposite after 5 cycles exhibits the highest photocatalytic activity.
According to this equation: Where C, C 0 are the concentrations of the analyte molecule at time t and initially, respectively.k is the first-order rate constant.The first-order rate constants for the photocatalytic decomposition of methylene blue are given in table 3.
The first-order rate constants for various nanocomposites were presented.So, results were adequate with the efficiency shown previously.We observed that the photo-electrocatalytic degradation of Methylene Blue (MB) reached the optimal efficiency with BiOI/TiO 2 NTs − 5 samples.Notably, the last one exhibited an optimal rate constant of approximately 27.5 ×10 −3 min −1 compared with TiO 2 NTs sample.The BiOI/TiO 2 NTs nanocomposite elaborated facilitates the separation and migration of photogenerated electron-hole pairs.This separation reduces recombination rates, leading to more efficient charge transfer and utilization in photocatalytic processes.Consequently, BiOI/TiO 2 NTs exhibit enhanced photocatalytic activity compared to TiO 2 NTs alone.Sheng-Zhe et al [32] demonstrated that BiOBr/TiO 2 nanotubes have significantly enhanced separation efficiency of photo-generated carriers and photocatalytic decomposition compared with pure TiO 2 nanotubes.This enhancement is attributed to the growth of BiOBr nanosheets leading to a substantial increase of the specific surface area and the internal electric field between the heterojunction of BiOBr/TiO 2 .Other studies showed [33] that the presence of BiOI nanoparticles on the surface of TiO 2 NTs provides additional active sites for higher photocatalytic activity and stability for the degradation of dye.

Conclusion
The TiO 2 nanotube (NT) arrays, fabricated through anodization, were sensitized with multiple layers of BiOI nano-leaves using the Successive Ionic Layer Adsorption and Reaction (SILAR) technique.The SEM analysis confirmed the morphology of the resulting nanocomposites.The TiO 2 NTs exhibited an average diameter of approximately 100 nm.XRD analysis revealed that the nanocomposites prepared with 5 cycles (NC samples) had the lowest values for both BiOI crystalline size and micro-strain, measuring 24 nm and 0.32, respectively.This suggests a low defect density in these composites, despite the small size of the BiOI crystals.The 5 BiOI cycles NCs exhibited high absorption in the visible wavelength range, aligning with their lowest bandgap energy (Eg value) of nearly 2 eV, indicating the emergence of a type II p/n heterojunction sensitive to visible light.Consequently, it is anticipated that the BiOI/TiO 2 NTs −5 cycles NCs demonstrated the highest photocurrent density (27 μA cm −2 ).
As a result, the photo-electrocatalytic degradation of Methylene Blue (MB) reached optimal efficiency with BiOI/TiO 2 NTs − 5 samples.Specifically, these samples exhibited an optimal rate constant of approximately 27.5 ×10 −3 min −1 , and the final efficiency reached 97% after 180 min of solar irradiation.This study underscores the potential of BiOI/TiO 2 nanocomposites for the enhanced photocatalytic degradation of Methylene Blue in the context of ensuring safe drinking water.

)Figure 3
Figure 3 illustrate Williams -Hall plots 4 sin q ( ) versus cos T b q ()for the prepared BiOI/TiO 2 NCs.The crystalline size (D) and the micro-strain of the prepared nanostructures were calculated and given in table1.The crystallite size of BiOI/TiO 2 NTs nanocomposite compared to TiO 2 NTs can offer valuable insights into the structural properties and performance of the composite material.As we noticed, the crystallite size attributed to the BiOI/TiO 2 NTs nanocomposite prepared with five cycles had the lowest values for both BiOI crystalline size and micro-strain, measuring 24 nm and 0.32, respectively.This suggests a low defect density in these composites, despite the small size of the BiOI crystals.The integration of BiOI nanoparticles in to TiO 2

Figure 2 .
Figure 2. X-ray diffraction given for prepared films.

4 .
The TiO 2 NTs has high absorption in the UV range and lower absorption in visible light range.Comparatively, BiOI/TiO 2 NTs nanocomposites show a significant enhancement in the visible light region.The optical absorption edge red -shift along with the deposition of BiOI showing that the BiOI increases the absorption in visible range.Obviously, the BiOI/TiO 2 NTs − 5 NCs exhibits a red-shift of the absorbance.It is predicable that BiOI/TiO 2 NTs − 5 NCs should exhibit higher photoactivity under visible light due to their relatively high absorbance in the same visible wavelength range.BiOI is an indirect band gap semi -conductor and its band gap width could be obtained from equation (5) accordingly to the Tauc plot visualized in figure 5[44][45][46]

Figure 6 .
Figure 6.Comparison of transient photocurrent response of the TiO 2 NTs and BiOI/TiO 2 NTs nanocomposites in Na 2 SO 4 aqueous solution (1M) under visible-light irradiation at 0.5 V versus Ag/AgCl.

Figure 8 .
Figure 8. Photocatalytic degradation curves of Methylene Bleu over the BiOI/TiO 2 NTs samples under solar light irradiation.

Table 1 .
Crystalline size D and micro-strain e values of the processed nanostructures.

Table 2 .
Energy gap for the BiOI/TiO 2 -NT NCs corresponding to various cycles.