Impact of Rhodamine B solution on charge carrier generation and photocatalytic performance in CdS/TiO2 heterostructures synthesized by CBD.

This article presents the study of Rhodamine B solution effects on the generation of charge carriers and the photocatalytic performance of CdS/TiO2 heterostructure films. CdS/TiO2 heterostructure films were synthesized using the chemical bath deposition technique (CBD). The structure and morphology of the heterostructures were studied by X-ray diffraction (XRD), and Scanning Electron Microscope (SEM), respectively. The optical properties of the CdS/TiO2 heterostructures were characterized using a UV–vis spectrophotometer. The results showed that the CdS/TiO2 optical band gap energy (Eg) was 2.75 eV, between the TiO2 band gap energy (3.38 eV) and the CdS band gap energy (2.57 eV). The electrochemical characterization indicated that the photocurrent of the CdS/TiO2 heterostructure decreased from 29 mA to 12.4 mA when the UV-vis light source was turned off. The density of charge carriers generated on the surface of the CdS/TiO2 was simulated employing the WXAMPS software. The results suggest that the thickness of the TiO2 layer should be optimized to allow the activation of CdS. Finally, this study concludes that the presence of rhodamine B in the solution attenuates light transmission and affects the generated electron-hole pairs, resulting in poor photocatalytic behavior.

Due to the needs of society for affordable and fashionable clothing, the textile industry has exponentially increased the production of clothes.Such an event has produced a phenomenon known as "fast fashion" which has contributed to water pollution with different types of dyes.Dye molecules are stable chemical structures that are difficult to degrade [1].In the last decade, photocatalysis has been part of the socalled advanced oxidation process (AOP) and has been widely studied [2], [3], [4].In photocatalysis, complex organic molecules can be reduced through redox reaction processes using different materials [5].A variety of semiconductors, such as nitrides, oxides, sulfides, and selenides; ternary compounds like vanadates, bismuths, titanates, ferrites; and niobates, have been widely employed in photocatalysis processes [6] [7].Among these materials, TiO 2 stands out due to its high photoactivity, low cost, toxicity, corrosion resistance, and chemical and thermal stability.Besides, TiO 2 has a wide bandgap of 3.2 eV that enables the semiconductor to work under ultraviolet (UV) irradiation.However, this radiation only constitutes 3 to 5% of the solar spectrum.Different studies have reported that the absorption bandgap of TiO 2 can be shifted towards the visible light region using various synthesis methods.The most representative include doping processes with metals (e.g., V [9], Eu [10]) and a combination with semiconductors (e.g., CdS [11] and MoS 2 [12]).Cadmium sulfides (CdS) is an n-type semiconductor with a narrow band gap of 2.42 eV.Its band gap can change depending on the size of its particles, allowing it to have an absorption window around 424 -543 nm [13,14].CdS has a redox potential in hydrogen evolution [13] [14], CO 2 reduction [15], and water treatment, mainly in the degradation of organic dyes [16].Specifically, CdS/TiO 2 heterostructures have been synthesized with different morphologies as nanorods [17], nanospheres [18], and nanofibers [19].In particular, the photocatalyst powders are difficult to remove from the solution after the photoreaction, which makes its reuse difficult and causes secondary contamination.This fact decreases the viability of its use on an industrial scale.Nevertheless, the use of immobilized catalysts as heterostructure films is an alternative option to address this issue.CdS/TiO 2 heterostructure films has been synthesized by chemical bath deposition.The photocatalytic performance of semiconductor materials is usually attributed to factors like bandgap energy, surface area, and the capacity of electro-hole pair charges generated.However, no studies have investigated how the optical properties of the dye in solution impact the photocatalytic effect of the semiconductor.In this work, we present the study of the photocatalytic performance of CdS/TiO 2 heterostructure films on the degradation of Rhodamine B solution under UV-vis light irradiation.In addition, we introduce the use of wxAMPS to simulate the results of the role played by the optical properties of the dye solution on the photocatalytic performance of the heterostructure.

Synthesis of CdS films
The CdS films were synthesized by chemical bath deposition (CBD) using 25,3 mM of CdCl 2 solution as a Cd 2+ ion source, and 200 mM of thiourea solution as S 2-ions.The films were synthesized on glass substrates of 4 cm x 4 cm side.Before their use, the substrates were previously cleaned with piranha solution and placed vertically on container walls.The CdCl 2 solution was mixed with a 462,2 mM KOH solution and 1,37 mM NH 4 NO 3 solution with a volumetric relation of 1:2, and 5:1, respectively.After being stirred and heated at 85 °C for 15 minutes, a 100 ml solution of thiourea was added, resulting in a

Fabrication of TiO 2 films
The TiO 2 films were synthesized by adding C1 2 H 28 O 4 Ti, C 2 H 6 O, and C 3 H 8 O in a volumetric ratio of 1:4:8 to a glass container at room temperature.The substrates were immersed vertically in the solution for 5 minutes.Then, the films were dried at 100°C for 10 minutes to evaporate the solvents.The process was performed three more times.The crystallization of the TiO 2 films was achieved by annealing at 350°C for 1 hour in extra-dry air.

Fabrication of CdS/TiO 2 heterostructures films
The CdS/TiO 2 heterostructure films were an ensemble depositing a CdS film as the first layer and a TiO 2 film as the second layer.The heterostructure was annealed at 350 °C for 1 hour under an extra-dry air atmosphere; the samples were labeled as CdS/TiO 2 .The samples were deposited onto titanium substrates following the same process.

CHARACTERIZATION
The morphology of the films was observed by Scanning Electron Microscopy (FE-SEM) in an Auriga model from Zeiss, operating with at 1 KV to secondary electrons detector.The structural characterization was performed by X-ray diffractometer using the CuK α radiation (D8 ECO, Bruker); all samples were analyzed at 40KV and 40 mA, in the range of 5° to 80° 2-theta degrees with a step size of 0.05° and a step time of 0.5 s.Transmittance and absorption spectra were collected in an Ocean Optics spectrophotometer model USB4000-XR1-ES coupled to UV/Vis/NIR light source model DH-2000.

CdS/TiO 2 heterostructure simulation.
The software WXAMPS [20] was used to calculate the number of charge carriers generated from the surface of the CdS/TiO 2 heterostructure film when it was excited by a light source.As a UV-vis light source, an AM1.5:G:A solar simulator was used.The physical properties values were extracted from CdS [21] and TiO 2 [22].The absorption spectra of Rhodamine B were taken from the PhotoChemCAD3 software to be like the dye's effect on the current density generated on the heterostructure CdS/TiO 2 surface.

Electrochemical characterization
A chronoamperometry characterization was performed to study the variations in photocurrent produced by the sample under UV and Vis light illumination.The test was performed in a VersaSTAT 3 Autolab computarized potentiostat/galvanostat operating with Versa Studio®.The study was performed in a conventional three-electrode cell with a platinum mesh of 1 cm 2 as a counter electrode, an Ag/AgCl(sat.)as a reference electrode, and the samples deposited on titanium substrates as working electrodes.The cell employed had a capacity of 18 ml, and the electrolyte used was 0.1 M of KOH solution.A Xenon lamp (model 66901, Oriel Instruments) and an OPS-A500, operating at 150 W, were used as light sources.To simulate the solar spectrum, an AM1.5:G:A optic filter was placed between light sources and a window of the electrochemical cell.In addition, an optical filter of 400 nm was used to cut the ultraviolet irradiation, and the illumination was interrupted for 50s cycles.

Attenuation of light transmitted.
The intensity attenuation of the light transmitted from the lamp to the sample and the dye solution was measured by placing in a cuvette holder 3 ml of Rhodamine B solution at 1 cm from the light source, equal to the distance that there was between the samples to the lamp inside the photoreactor.The optical spectrum was collected by a fiber and sent to the UV-Vis spectrometer.

Photocatalysis test
The photocatalytic test was performed on the discoloration of 200 ml of Rhodamine B at a concentration of 5 mg/l.The test was performed in a glass reactor with a water recirculation system around the lamp to keep the solution at room temperature and cut the infrared radiation.A 100 W halogen lamp with an operation range of 350 nm to 2500 nm was used as the light source (USHIO, USA).The power lamp was modulated by a SORENSEN variable power supply until a potential of 11.5V and a current of 7.6A was achieved.Four samples with a total surface of 80 cm 2 were placed vertically around the reactor walls and radial irradiated with 87.4 W for 6 h.During the test, the solutions were constantly stirred to maintain constant contact with the samples.The residual concentration (C/Co) of the Rhodamine B solution was monitored each hour by the variation intensity of the absorption band at 551 nm.Before the photocatalysis test, the dye solution was stirred in the dark for one hour to reach the adsorptiondesorption equilibrium.

RESULTS AND DISCUSSION 4.1. Morphology
The SEM micrographs in figure 1 show the morphology of the deposited of CdS/TiO 2 heterostructure films, two layers are observed.In the lower side, a uniform coverage corresponding to the CdS film is observed, while the upper side exhibits TiO 2 cluster structures.The TiO 2 film grows as popcorn-like clumps, creating a rough surface that improves the photocatalytic activity by increasing the contact area between the photocatalyst and the contaminant in the degradation process.

Energy band gap of Heterostructures
The Tauc plot for the TiO 2 , CdSe, and CdS/TiO 2 heterostructure films is shown in figure 3. The calculated band gap of the TiO 2 was 3.38 eV, which is higher than the commonly stated band gap of 3.2 eV.The calculated band gap value for CdS was 2.57 eV, which falls between 2.28 eV and 2.92 eV on average.In both cases, it has been reported that the TiO 2 and CdS band gap varies with the particle size on the deposited film [26], [27], [28].In the case of CdS/TiO 2 heterostructure, the band gap is between the TiO 2 and CdS values, according to reported by M. A. Basit, which induces a transfer of electrons from the CdS film to the conduction band of TiO 2 [29].The obtained results demonstrate that the CdS/TiO 2 heterostructure can work in the visible light spectrum.

Electrochemical behavior
Figure 4 shows the photocurrent response versus the time from the chronoamperometry test when the samples were irradiated with sunlight (a) or visible light (b).Initially, all the samples generated a current peak and then tended to equilibrium.Under sunlight, the average photocurrent generated for TiO 2 , CdS, and CdS/TiO 2 , their first cycle of 100 s, were 7.7 µA, 13.3 µA, and 29 µA respectively.The AM1:5:G:A sunlight filter has a maximum intensity of around 500 nm.The values obtained are according to the ban gap of samples.We suggest that the photocurrent generated in the TiO 2 film is from the UV light, and the CdS photocurrent is from the visible light principally.In the case of the CdS/TiO 2 heterostructure, we consider there are also contributions by the UV and visible light.The above was verified by placing a UV filter as shown in figure 4b.In all cases the photocurrent generated decreased by around half.The photocurrent generated was 12.4 µA to CdS/TiO 2 heterostructure and 6.15 µA to CdS, whereas the TiO 2 did not generate photocurrent.This can be explained due to its absorption edge being at 336 nm.
According to chronoamperometry studies, the CdS/TiO 2 heterostructure is best suited for the photoresponse, since it induces twice the number of electron-hole pairs than the CdS film when exposed to visible light.

Photocatalytic evaluation and simulation
The photocatalytic degradation of Rhodamine B solution for the CdS, TiO 2 films, and CdS/TiO 2 heterostructure film under UV-vis light is shown in figure 5.It is observed that samples show low degradation capacity in discoloration of Rhodamine B solution.The behavior of TiO 2 and CdS films was similar throughout the entire experiment.After 6 hours, the degradation percentages for TiO 2 and CdS films were approximately 20.2 % and 22.4 %, respectively.When CdS and TiO 2 were combined to form the heterostructure CdS/TiO 2 , the photocatalytic degradation improved, resulting in a 34.2 % discoloration.It is known that the absorption window of TiO 2 is in the UV, and it represents around 4 % of the solar spectrum.The performance of the CdS/TiO 2 heterostructure is lower than expected.This must be a consequence of the fact that the bandgap of the film coincides with the absorption range of Rhodamine B. When radiation passes through the solution it is attenuated, and only a fraction of the lamp's intensity falls on the sample, as illustrated in figure 6.The results are according to the observed in the chronoamperometry test and heterostructure simulation shown in Figure 6.This fact supports what was observed in the chronoamperometry measurement and the heterostructure simulation shown in figure 6. Photocatalysis is a surface phenomenon, and the number of active sites is proportional to the sample's surface area.Each set of samples had a total surface area of 80 cm².When simulating the density of charge carriers on the surface of the materials, the following experimentally measured thicknesses were used: 0.329 µm for CdS, 1.35 µm for TiO 2 , and 1.65 µm for CdS/TiO 2 .The calculations gave the following current density values 0.167 mA/cm 2 for CdS, 0.452 mA/cm 2 for TiO 2 , and 0.464 mA/cm 2 for the CdS/TiO 2 heterostructure.TiO 2 /CdS is a type II heterostructure, CdS absorbs the lower energy photons of visible light and generates electron-hole pairs to transfer them to TiO 2 , and generates electron-hole pairs in TiO 2 .Although TiO 2 is transparent to shortwave photons, the photons lose energy on the way to reach the CdS surface (bottom heterostructure layer).The TiO 2 layer in the heterostructure had a thickness of 1.3 µm, which made CdS activation difficult.As a result, the photocurrent generated in TiO 2 and CdS/TiO 2 is very similar.Figure 6a shows the transmission spectra of rhodamine B solution with a concentration of 5 mg/L.It can be noted that the percentage of light transmitted between 580 and 800 nm and below 450 nm was about 95 %.
Rhodamine B has an absorption window between 450 and 580 nm.At 551 nm, the transmitted light decreases to 30%.

CONCLUSIONS
The CdS/TiO 2 heterostructure was synthesized, shifting the gap towards the visible region relative to TiO 2 .In this heterostructure, the photocurrent decreased by half when the ultraviolet light was turned off.Furthermore, the heterostructure showed little efficiency in decolorizing the Rhodamine B solution under UV-vis light irradiation because the dye solution itself absorbs light in the same range as the bandgap of CdS.Therefore, the number of interacting photons with the surface of the heterostructure is low.The thickness of the TiO 2 layer is such that it prevents the CdS from being activated correctly.

Figure 4 .
Figure 4. Photocurrent generated for the CdS, TiO 2 films and CdS/TiO 2 heterostructure film under sun light (a) and visible light (b).

Figure 5 .
Figure 5. Photocatalytic degradation of RhB for different photocatalysts under visible-light irradiation.

Figure 6 (
b) the simulation of the attenuation of light transmitted caused by the light absorption in the Rhodamine B solution on the current density of the heterostructure surface.The current density produced was 0.1987 mA/cm 2 , suggesting a 57 % reduction in electron-hole pairs generated when the heterostructure is not directly irradiated.As a result, there are limited electron-hole pairs for oxidation-reduction reactions to interact with the colored solution.The above results are explained because the CdS/TiO 2 heterostructure shows very low photocatalytic degradation.

Figure 6 .
Figure 6.Transmision spectra of RhB at 5 mg/l (a) Simulation of attenuation of light transmitted (b).
yellow color indicating the formation of CdS.The substrates were removed, rinsed with bidistilled water, and dried for 45 minutes. 3