Introduction

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Thanks to Fujishima and Honda discovery related to the solar driven photocatalytic splitting of water on TiO₂ photoelectrodes in the early 1970s [1], photocatalytic and photoelectrochemical processes have been widely emerged as low cost and efficient environmental key solutions to tackle serious energy crisis and worldwide pollution issues [2,3]. Despite the outstanding features of TiO₂ including its eco-friendliness, cost-effectiveness, robustness, corrosion stability, high electrical and optical properties and strong photocatalytic activity [4,5], its wide band gap of 3.0eV for rutile and 3.2eV for anatase has severely affected its photoresponse which get limited to only 5% in the ultraviolet region of sunlight energy and resulted in lower photoconversion efficiency due to the high recombination rates of photogenerated eloctron-hole pairs [6]. In order to adress that issue, several strategies have been investigated to extend the light response field of TiO₂ to visible light and minimizing the recombination rates of the photoexcited charge carriers through doping [7-10], surface modification [11,12] and coupling TiO₂ with narrow band-gap semiconductors [13-15]. In particular, nanostructured II–VI semiconductors based on cadmium chalogenides are globally used as light harvesters such as cadmium telluride and cadmium selenide thanks to its high values of absorption coefficient.

Precisely, Cadmium Selenide, named also cadmoselite (CdSe) earned recognizable interest as noteworthy active material to alter TiO₂ thanks to its exclusive and special properties namely its reduced band gap (1.7 eV) near to visible spectrum, high absorption coefficient, n-type conductivity, low resistivity, luminescent properties and its direct and reduced band gap around 1.73 eV which is characteristic of the solar spectrum radiation [16]. To date, huge research efforts have been conducted on seeking for the best and low cost technology to sensitize TiO₂ with CdSe for improving photoelectrochemical performance. By doing a literature survey, several research groups have extensively proposed either chemical bath deposition strategy [17-26] or SILAR approach [26-30] for the CdSe sensitization of TiO₂ and reported its effect on photoelectrochemical performance (Table1). However, these two direct deposition techniques have restricted application in coating onto substrates and consume long deposition time. Thereby, it is necessary to establish a low cost and simple strategy for the design of highly efficient CdSe/ TiO₂ photoanodes. Interestingly, electrodeposition method has owned huge interest in recent years since it has numerous advantages including its low cost of synthesis, its environmentally friendly, its simple operating conditions, efficiently adjustability of different parameters like temperature, chemical composition, morphology, area and thichness, last but not least it produces CdSe films at a very high yield.

Table 1

Thus far, plenty of research groups have investigated the impact of various deposition parameters during the electrolysis process on the possibility to achieve enhanced photoelectrochemical performance of high controlled electrodeposited CdSe/TiO₂ photoanodes such as the deposition time, electrolyte’s pH, type of electrolyte and the annealing treatment temperature (Table 2). In fact, Wang et al 2015 [31] reported potentiostatically electrodeposition of CdSe onto anodized TiO₂ nanotube arrays. (NTAs) at various deposition time (30s, 35, 25s, 20s and 60s). They concluded that the deposition time of 30 s displayed excellent PEC performance by exhibiting a maximum photocurrent intensity of ~1 mA/cm2 versus saturated calomel electrode (SCE) under visible light irradiation at 0.5 V. They ascribed this result to the high dispersity of CdSe nanoparticles on both inside and outside of the TiO₂ NTAs pore walls, the strong combination and heterojunctions between CdSe and TiO₂ through Cd-O bonds. In 2018, Ayal’s research group [32] have synthesized CdSe / TiO₂ heterostructure through pulse electrodeposition method and studied the effect of different duty cycles varying from 10 % to 90 % on photoelectrochemical properties. They found that duty cycles played a crucial role in the formation of CdSe nanoparticles. The maximum photocurrent was evaluated to be 1.94 mA/cm² vs Ag/AgCl which has been obtained at a duty cycle of 50 %. The reached photoconversion efficiency was 1.18% which is 59 times higher than nude TiO₂. In 2013, Xue and co-workers [33] have prepared photoabsorbing CdSe electrodeposited films onto anodized TiO₂ from an acidic bath and studied the effect of the SeO₂ concentration (0, 2, 4, 6, 8 and 10 mM). on the PEC performance. At a precise SeO₂ content of 4 mM shows a maximum photocurrent of 7.72 mA/cm² vs SCE under visible light irradiation with a V_oc value of 1.24 V and an optimum efficiency of 7.1 %. In 2016, the same group of Ayal [34] have potentiostatically electrodeposited CdSe layer onto TiO₂ filmsand reported the effect of various pH values (1.0, 2.0, 3.0 and 3.3) on its PEC performance. The maximum photocurrent density of 2.13 mA/cm² vs Ag/AgCl under halogen light irradiation was reported at a pH of 3. The highest photoconversion efficiency was 1.02 % which is 51 times higher than bare TiO₂ nanotube array. This enhanced photoresponse may be due to the formation of well crystallized CdSe nanocrystals which were well bonded with TiO₂. In 2014, Xue and his colleagues [35] have investigated the impact of electrolyte’s pH on PEC performance of electrodeposited CdSe on TiO₂ layers. They have varied the pH values from 1 to 4 and they found that pH value of 3 has exhibited the maximum photocurrent density around 4.0 mA/cm² at 0.5 V (vs SCE) which is 7 times higher than bare TiO₂ NTAs and a V_oc of -1.26 V. In 2017, Ayal’s team work [36] have investigated the effect of thermal annealing on the enhancement of PEC properties of electrodeposited CdSe onto anodized TiO₂. They deduced that annealing treatment play an important role in controlling the formation of CdSe nanoparticles on TiO₂. The highest photocurrent density and photoconversion efficiency achieved values were 2.98 mA/cm² vs Ag/AgCl and 1.67 % respectively obtained at an annealing temperature of 250 °C

Table 2

In 2019, Zhuang et al [37] were the first to publish the electrodeposition of CdSe photosensitizers onto TiO₂ films from an organic bath of N,N-dimethylformamide (DMF) and studying the effect of deposition time on the PEC properties of elaborated CdSe/TiO₂ photoanodes. At a deposition time of 500 s, enhanced photoelectrochemical (PEC) performance was achieved with a significant saturated photocurrent density of 2.1 mA/cm² at 0 V (vs Ag/AgCl) under visible light irradiation (λ ≥ 420 nm). They attributed this enhancement to the good distribution of CdSe nanoparticles on TiO₂ nanorod arrays, the favorable band alignment, and the intimate interfacial interaction between CdSe nanoparticles and TiO₂ nanorods.

Based on literature, the electrodeposition of CdSe onto TiO₂ films could be applied from various types of electrolytes including aqueous solutions [31-36], organic solutions [37] or ionic liquid medium. In fact, ionic liquids are molten salts with melting points often around 20–25 °C or below, composed only of ions. As compared to acqueous and organic electrolytes, ionic liquids are much preferred thanks to its high thermal and good electrochemical stability (> 300 ◦C) Which permitting the electrodeposition to be proceeded even at high temperatures allowing atom diffusion on the surface during growth. It has crucial positive impact on the quality of the resulted layers. ILs exhibit good solvating properties, high conductivity, nonvolatility, low toxicity and wide electrochemical potential window. The viscosity of ionic liquids causes a slowdown electrocoating giving a more homogeneous and compact thin films. Another important advantage of ILs based electrolyte is the absence of hydrogen evolution simultaneously with metal deposition that often occurs during electrodeposition in aqueous solution. This phenomenon has a deep effect not only on photocurrent efficiency but also on the final quality of deposited layer. Thus, Ionic liquids (ILs) were considered as an appropriate alternative electrolytes for the electrodeposition of CdSe onto TiO₂ thanks to their intrinsic physico-chemical properties [38,39]. Actually, There are no published work reported the effect of the ionic liquid based electrolyte or ionic liquid/ organic solvent based electrolyte medium on the PEC performance of electrodeposited CdSe films on to TiO₂ films.

In this work, we report for the first time ever on the simple and low cost electrochemical deposition strategy of CdSe onto solgel prepared TiO₂ thin film from Tricaprylmethylammonium chloride/Formamide based electrolyte as eventual mixture of (Ionic Liquid/ Organic Solvent) medium. Then, the effect of TiO2 sensitization with the electrodeposition of CdSe layer on structural, optical, photoelectrochemical and photocatalytic properties has been deeply investigated.

Experimental Details

Materials

All chemicals were of analytical reagent grade, purchased from Sigma Aldrich and were used without further purification. Aqueous solutions were prepared using deionized water. Sol-gel via spin coating method was adopted to deposit the TiO₂ thin films onto ITO conductive glass substrates. Prior to use, all substrates were ultrasonically cleaned during 15 min with consecutively acetone and iso-propanol, rinsed with deionized water, and finally dried in air at room temperature. The CdSe was deposited by electrodeposition technique on TiO₂ electrode

Preparation method of TiO₂

TiO₂ thin films were prepared using sol–gel process and then deposited on ITO glass. Titanium isopropoxide Ti(OCH(CH₃)₂)₄ was used as a TiO₂ source. The sol was a mixture, in a molar ratio, of titanium isopropoxide, as a precursor, isopropanol CH₃CH(OH)–CH₃ and methanolCH3OH, as solvents and acetic acid CH3COOH as a catalyst. First, 1.5 mL of Ti(OCH(CH₃)₂)₄ was dissolved in 4.5 mL of isopropanol. The solution was homogenized for 30 min under magnetic stirring while heating up to 60 °C. Then 4 mL of acetic acid was added, with stirring for 30 min under the same heating. Finally, heating was stopped and 12 mL of methanol was added. The mixture was held under the magnetic stirring for 2 h. The TiO₂ was deposited onto (ITO)-coated glass substrates by using spin coating technique. The spinning speed was set at 3000 rpm and the deposition time to 30 s. The TiO2 solution was dropped one time onto the substrate and then dried in the oven at 100 °C for 10 min; The drying process was repeated three times and the resulted samples have been annealed at 450 °C for 2h.

Electrodeposition of CdSe onto TiO₂ thin film

CdSe sensitized layer was electrodeposited onto the annealed TiO₂ thin film at room temperature under agitation from an electrolyte composed of 30 % aliquat 336 (TOMAC) and 70 % formamide, 10^-2 M SeO2, 10-2 M CdCl₂ and 0.1 M KCl. The electrodeposition was carried out via three-electrodes system (Voltlab PGZ 100) containing TiO₂ film as working electrode, Ag/AgCl as a reference electrode, and Pt as a counter electrode. During the electrodeposition, the electrolyte was continuously stirred at a moderate speed with the help of a magnetic stirrer. The deposition potential was fixed at a constant voltage -1.1 volt for about 15 minutes. Finally, the resulted CdSe/TiO₂ heterostructure was annealed at 250 °C for 30 min in nitrogen atmosphere.

Characterization of bareTiO₂ and CdSe/TiO₂ thin films

The samples were characterized by X-ray diffraction (XRD) using Bruker D8 advance X-ray diffractometer with CuKα (λ= 1.541 Ǻ) radiation for 2θ values in the range of 10–80 °. A Raman scattering study was performed on a Jobin Yvon Lab RAM HR spectrometer using 632.8 nm irradiation from a He–Ne laser at (3 mW). Surface morphology of the prepared films was characterized via Atomic Force Microscopy (AFM)(Nano-scope III) in tapping configuration and using a fieldemission scanning electron microscope (FE-SEM, JSM-5410, JEOL). Accelerating voltage of SEM was set to 20 kV. Optical absorption A (λ) of the films was measured by a UV–vis– NIR (Lambda 950) spectrophotometer, equipped with an integrating sphere, in the wavelength range 300–800 nm. Bare TiO₂ and CdSe/TiO₂ thin films were also characterized by room-temperature photoluminescence spectra (PL) on a PL fluorescence spectrometer (Perkin-ElmerLS55) using a Xe lamp with an excitation wavelength of 300 nm.

Photoelectrochemical measurements

The PEC splitting performance measurements were carried out in a quartz electrolytic cell. The sample (with an average area of 1 cm₂), a Pt plate (with an average area of 1 cm₂), and an Ag/AgCl electrode were employed as the working electrode, counter electrode and reference electrode respectively. The electrolyte was composed of 0.5 M sodium sulfate aqueous solution. Current densities, as a function of applied potential (-1 to + 0.5 V vs. Ag/AgCl electrode) for the samples, were recorded under front-side illumination with a computer-controlled Potentiostat/Galvanostat PGSTAT30 (Eco Chemie BV) for all PEC experiments. A 300 W Xe short arc lamp passing through an AM 1.5 G filter Model-SS80AA with white light intensity of 100 mW/cm² was employed to simulate solar light. The intensity of incident light from the Xe lamp was measured using a Photometer Model 70310 from Spectra-Physics.

Key Findings

Electrochemical Growth

In order to determine the deposition potential of CdSe, a systematic cyclic voltammetry study was undertaken on ITO and TiO2/ITO working electrodes from ionic liquid based electrolyte (Figure 1). The potential started at the open circuit potential and was initially swept towards the negative direction at a scan rate of 20 mV/s. From Figure 1(a), three cathodic peaks are clearly seen in the voltammogram at about -0.6 V, -0.9 V, and -1.1V versus Ag/AgCl reference electrode

Figure 1

The deposited cadmium selenide was attributed to the following reactions:

At the beginning, the H₂SeO₃ (aq) was reduced into H2Se(aq) soluble species. The reduction of H2 Se (aq) to Se0 was started from -0.75 V and reached the peak value at about -0.9 V. Finally, the electrodeposition mechanism of CdSe thin film onto ITO coated glass substrate took place at a potential of -1.1 V versus Ag/AgCl wherein Cd²⁺ was reduced to Cd on the surface of the substrate. Hence, the CdSe alloy was formed according the potential deposition mechanism [40].

Figure 1 (b) displays the cyclic voltammogram (CV) of electrodeposited CdSe onto TiO₂/ITO working electrode from ionic liquid based electrolyte. Obviously, only one main cathodic peak at about -1.1 V versus Ag/AgCl was noted. The change of the cyclic voltammogram CV was attributed to the change in the working electrode’s surface. As compared with Figure 1(a), this main reduction peak was certainly assigned to the electrodeposition of CdSe thin film.

In order to confirm this assumption, the potentiostatic cathodic electrodeposition was conducted at the potential of -1.1 V versus Ag/AgCl onto sol-gel coated TiO₂/ITO film. The deposited film was therefore annealed at 250 °C for 30 min in N2 atmosphere and was analyzed by XRD and RAMAN measurements.

Structural study

Figure 2 exibits the XRD patterns of bare TiO₂ and CdSe/TiO₂ samples. The sol-gel prepared TiO₂ layer onto ITO glass substrate via spin coating approach shows a main peak at 25.35 ° which was assigned to (101) plane and attributed to the anatase phase with tetragonal crystal structure of TiO₂ (JCPDS #89-4921). This result is in agreement with those published in the literature [41]. The electrodeposition of CdSe onto TiO₂/ITO substrate at an applied potential of -1,1 V from the ricaprylmethylammonium chloride/Formamide based electrolyte, additional characteristic peaks have appeared at 2θ = 25.5 °, 35.2 °, and 41.9 ° which were attributed to (002), (102) and (110) planes respectively. These noticeable planes were ascribed to the hexagonal structure of CdSe according to the card number (JCPDS # 65-3415) verified by JCPDS (Joint Committee on Powder Diffraction Standards) database. This structural study reveals that the hybride CdSe/TiO₂ nanostructure has been successfully prepared by the low cost and facile electrodeposition approach at the applied potential of -1.1 V.

Figure 2

Raman study

The Raman spectra of bare TiO₂ and CdSe/TiO₂ films in the range of 100 cm^-1 to 550 cm^-1 are shown in Figure 3. Solgel coated TiO₂ spectrum reveals three symmetric vibration modes; A1g, B1g and Eg attributed to the tetragonal anatase phase of TiO₂. They were identified at 144 cm^-1 for Eg mode, 396 cm^-1 for B1g mode and 516 cm−1 for A1g vibrational mode respectively [42]. The electrodeposition of CdSe onto TiO₂ film resulted in the decrease of anatase characteristic peak intensity of TiO₂ (Eg) due to the surface recovery of TiO₂. In addition, two new peaks have appeared at 203 cm^-1 and 406 cm^-1 corresponding to the longitudinal optical (LO) and to the second harmonic (2 LO) respectively of CdSe [43]. Those Raman results confirm those obtained by XRD study.

Figure 3

Morphological studies

Figure 4 shows the morphology of bare TiO₂ and CdSe/TiO₂ films by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The surface of sol-gel coated TiO₂ film was dense, homogeneous and covered with well distributed small spherical shaped grains showing the general characteristic morphology of the anatase phase [44]. However, the surface topography has changed after the electrodeposition of CdSe which could be attributed to the CdSe deposited layer onto TiO₂ thin film. The root mean square (RMS) surface roughness of nude TiO₂ thin film has increased from 1.08 nm to 38.833 nm after electrodeposition of CdSe. The increase in the film roughness has proved the growth of electrodeposited CdSe onto nanostructured TiO₂ based thin film. In order to confirm this AFM results, SEM topography has also been investigated in Figure 4.

Figure 4

SEM image of bare TiO₂ layer reveals that the surface was clearly filled with granular shapes. After CdSe electrodeposition, the morphology has changed and consists of agglomerated small grains well distributed on the TiO₂ surface resulted in the growth of CdSe film

Optical properties

UV-Visible absorption spectra of TiO₂ and CdSe/TiO₂ thin films are shown in Figure 5. The bare TiO₂ has an absorption edge at around ∼375 nm, showing a fundamental absorption edge which corresponds to the TiO₂ band gap energy of anatase structure (3.2 eV) in the ultraviolet region [45, 46]. The electrochemical deposition of CdSe onto TiO₂ has obviously extended the absorption spectrum to the visible light region. The narrow band-gap of photoabsorber CdSe was responsible for the improved absorption capability of TiO₂ thin film in the visible-light region and consequently enhances its photocatalytic and photoelectrochemical performances [47].

Figure 5

The charge separation and transfer mechanism was revealed by measuring the electron injection efficiency by photoluminescence (PL) spectroscopy as shown in Figure 6. Through an excitation wavelength of 300 nm, the CdSe based photosensitizer exhibited a characteristic emission peak situated at 600 nm which corresponds to the recombination of the excited electrons. Whereas, the PL signal disappears with sensitization of TiO₂ with CdSe. This PL quenching reveals the decrease in the rate of excited electrons which get recombined radiatively with the holes situated in CdSe valance band. Therefore, it can be concluded that most of the excited electrons are successfully injected into the TiO₂ conduction band [48-51].

Figure 6

According to the literature, photoluminescence spectroscopy confirms the presence of the type-II interfacial transition between the valence band of CdSe and the conduction band of TiO₂ [52, 53] thanks to its energy levels shown in Figure 7. [52- 54]. This PL results suggest that electrodeposition of CdSe onto TiO₂ photoelectrode enhances the charge separation

Figure 7

Photoelectrochemical properties

The linear sweep voltammogram (LSV) is a useful technique evaluate the charge carrier characteristics at the semiconductor/ electrolyte interface by evaluating the separation efficiency of photogenerated electron-hole pairs. In order to investigate the impact of the photosensitization of TiO₂ with CdSe on the separation efficiency of photogenerated electron–hole pairs, we expose the elaborated photoanodes to a photocurrent response measurements. Figure 8 (A) and Figure 8(B) show current density-voltage (J−V) measurements of TiO₂ and CdSe/TiO₂ photoanodes in dark and under illumination with 100 mW/cm2 from Xenon lamp in aqueous solution containing 0.5 M Na₂SO₄ (pH=7) with applied potential ranging from -1 to 0.5 V (vs. Ag/AgCl).

It is clearly seen in Figure 8 (A) that there is no dark photocurrent for both photoelectrodes.

Figure 8

According to a literature survey, this result could be explained by the presence of the TiO₂ based compact film that plays an effective role in blocking the direct contact between the electrolyte and ITO substrate which suppresses the leakage photocurrent [54]. The use of TOMAC/FA as ionic electrolyte in the electrodeposition of CdSe onto TiO₂/ITO substrate has a crucial positive impact on the quality of the resulted photoanodes by improving the compactness of the film [54]. Under illumination, the increase at applied voltage (V vs.Ag/AgCl) promotes the separation of photo-generated charges and the photo-current density (JSC) has increased [55, 56]. All samples have a common behavior; a photocurrent flowed in the positive potentials and increased gradually which is the characteristic of n-type semiconductor.

Moreover, low photocurrent was observed for the both photoanodes. This may be explained by the fast recombination of the photogenerated electrons and holes. Similar behavior was also reported by others authors [57,58]. Under illumination, bare TiO₂ presents a photocurrent density of 0.04 mA/cm² at 0.5 V which is comparable to that reported by Ayal et al [34], in CdSesensitization of TiO₂ nanotube arrays through electrodeposition approach. They have found a 0.03 mA/cm2 as photocurrentdensity for bare TiO₂.

As compared to TiO₂ layer, the photocurrent density of hybrid CdSe/TiO₂ heterostructure was greatly enhanced and reached a value of 0.30 mA/cm2 at 0.5 V which is 7.5 times higher than bare TiO₂ We have also noticed from Figure 10(B) that CdSe/TiO₂ PA shows a more negative zero-current potential (-0.56V) than that of the bare TiO₂ thin film (-0.46 V). These results could be explained through the high absorption and the better charge separation efficiency of the photogenerated carriers [59]. These results are consistent with the Photoluminescence results. This suggests that the electrodeposition of CdSe on TiO₂ thin film has improved the generation and separation of photoinduced charges.

Figure 9 shows the PEC water splitting device for the CdSe/ TiO₂ PA. Through coupling the two semiconductors, most of the excited electrons have been successfully injected from the CdSe conduction band into that of TiO₂. This reduces the photogenerated carriers recombination rate in TiO₂. Consequently, it facilitates the transport of electrons to the platinum electrode through the external circuit to participate in the reduction of protons at the interface between the cathode and the electrolyte leading to the formation of hydrogen gas. Meanwhile, the holes generated vacancies and contribute to the oxidation of water by producing oxygen at the interface between the photoanode and the electrolyte. This PEC water splitting mechanism undergo the following steps:

According to the (J-V) curve under illumination, the corresponding photoconversion efficiency, the applied bias photon-to-current efficiency (ABPE) was calculated via the following equation [60] and presented in Figure 10.

where η is the photoconversion efficiency, J_sc is the photocurrent density (mA/cm²), I_Light is the incident light, E°_rev is the standard reversible potential which is 1.23 V RHE, and E_app is the applied potential which is E_app = E_meas − E_aoc, where Emeas is the electrode potential (vs Ag/AgCl) of the working electrode and E_aoc is the electrode potential (vs Ag/AgCl) of the same working electrode under open circuit and under illumination conditions.

Figure 9

Figure 10

The PEC parameters including open-circuit voltages (V_oc), shortcircuit current density (Jsc), fill factor (FF) and power conversion efficiency (η) were tabulated in Table 3.

Table 3

Typically, the CdSe/TiO₂ photoanode shows higher conversion efficiency of 0.13 % at -0.2 V (vs Ag/AgCl) which is ~ 4.3 times higher than that of pure TiO₂ layer (0.03 %). The enhanced photoconversion efficiency could be assigned to the improvement in incident light absorption and the ameliorated separation rate of photogenerated charges.

In order to investigate the stability and the reproducibility of the device’s phtoresponse, the photocurrent-time (J-t) plots of TiO₂ and CdSe/TiO₂ photoanodes have been measured at an applied potential of 0 V and displayed in Figure 11. The photocurrent of elaborated samples appeared immediately upon irradiation then quickly returned to a steady state when the illumination was cut off. These results indicate the stability of the samples and the immediate generation and separation of electron (e-)/hole (h+) pairs in TiO₂ and CdSe/TiO₂ based photoanodes [61]. It is worth noty that the CdSe sensitized TiO₂ photoanode yields a photocurrent density of 0.28 mA/cm⁻² which is about 9.5 times higher than that reached with bare TiO₂ (0.029 mA/cm⁻²). These results indicated that the sensitization of TiO₂ with CdSe can clearly enhance the transient photocurrent responses which confirm the observed trends in Figure 8.

Figure 11

Photocatalytic activities

The discharge of untreated wastewater from various industries causes serious environmental pollution and threatens the human and ecosystem well-being. Thus, Methylene blue MB and methyl MO containing wastewater should be decolorized and detoxified before being discharged into the environment since they are potential carcinogen dyes [60-62].

The photocatalytic activities of synthesized photocatalysts were carried out by the degradation of MB and MO dyes under visible light irradiation and illustrated in Figure 12. It is clearly seen that the maximum absorbance of MB and MO dyes situated at 660 nm and 484 nm respectively have sharply decreased in the presence of TiO₂ and CdSe/ TiO₂ based catalysts. These results show that the photocatalytic degradation efficiency of TiO₂ has improved after photosensitization with CdSe. This improvement originated from the enhanced absorption capability and the efficient charge separation [65]. Figure 13.

Figure 12

Conclusion

In summary, the sensitation of TiO₂ with short band gap CdSe absorber has successfully provided through the low cost, facile and eco-friendly electrochemical deposition. It has been demonstrated that this electrochemical approach via the use of TOMAC/FA based electrolyte has a crucial role in the high stability and compactness of sol-gel synthesized TiO₂ layers.

Structural study proved the cyclic voltammetry synthesis of hybride CdSe/TiO₂ nanostructure at the applied potential of -1.1 V with well-defined phases. Meanwile, raman results confirm those obtained by XRD investigation by exhibiting the characteristic peaks for the anatase type TiO₂ and hexagonal phase of CdSe respectively. AFM morphological study of bare TiO₂ layer shows a dense, homogeneous and a surface covered with well distributed small spherical shaped grains. Its root mean square (RMS) surface roughness has rised from 1.08 nm to 38.833 nm after electrodeposition of CdSe which is a good proof for the successful growth of electrodeposited CdSe onto nanostructured TiO₂ based thin film via the TOMAC/FA medium.

In order to confirm this AFM results, SEM image of bare TiO₂ layer reveals that the surface was clearly filled with granular shapes.After CdSe sensitization, the morphology has been modified and consisting of agglomerated well distributed small grains on the surface of TiO₂ resulted in the growth of CdSe film. As compared to bare TiO₂, optical measurements show better charge separation efficiency for the CdSe/TiO₂ heterostructure. The sensitization with CdSe has resulted in enhanced photocurrent density of 0.30 mA/cm² at 0.5 V which is 7.5 times higher than bare TiO₂ when exposed to visible light irradiation. In terms of conversion efficiency, CdSe/TiO₂ photoanode shows higher conversion efficiency of 0.13 % at -0.2 V (vs Ag/AgCl) which is ~ 4.3 times higher than that of bare TiO₂ layer. Also, CdSe/TiO₂ PA shows a zero-current potential of -0.56 V which is more negative than that of bare TiO₂ thin film (-0.46 V). The sensitization process of TiO₂ with CdSe from the ionic liquid has improved photocatalytic degradation efficiency of TiO₂ towards MB and MO pollutant dyes thanks to the amelioration of its absorption capacity and charge separation rate of photogenerated carriers. The sensitization of TiO₂ with CdSe from TOMAC/FA based ionic liquid medium has resulted in low cost and high stable CdSe/TiO₂ photoanode for PEC solar cells and photocatalytic degradation applications and consequently overcome the serious environmental issues

Acknowledgments

This work was supported by the Ministry of Higher Education and Scientific Research of Tunisia.

Conflicts of Interest

The authors declare no conflict of interest.

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