Degradation of Organic Dye Congo Red by Heterogeneous Solar Photocatalysis with Bi₂S₃, Bi₂S₃/TiO₂, and Bi₂S₃/ZnO Thin Films

Abstract

In this work, bismuth sulfide (Bi₂S₃) thin films were deposited by a chemical bath deposition (CBD) technique (called soft chemistry), while titanium dioxide (TiO₂) nanoparticles were synthesized by sol–gel and zinc oxide (ZnO) nanoparticles were extracted from alkaline batteries. The resulting nanoparticles were then deposited on the Bi₂S₃ thin films by spin coating at 1000 rpm for 60 s each layer to create heterojunctions of Bi₂S₃/ZnO and Bi₂S₃/TiO₂. These materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The optical and contact angle analyses were undertaken by UV–Vis spectroscopy and a contact microscopy angle meter, respectively. The calculated band gap values were found to be between 1.9 eV and 2.45 eV. The Bi₂S₃ presented an orthorhombic structure, the TiO₂ nanoparticles presented an anatase structure, and the ZnO nanoparticles presented a wurtzite hexagonal crystal structure. Furthermore, heterogeneous solar photocatalysis was performed using the Bi₂S₃, Bi₂S₃/ZnO, and Bi₂S₃/TiO₂ thin film combinations, which resulted in the degradation of Congo red increasing from 8.89% to 30.80% after a 30 min exposure to sunlight.

1. Introduction

Nowadays, water pollution has become a global problem due to the growth and development of modern industry [1]. Municipal and industrial wastewater is estimated to be about 2212 km³ in volume. The 2017 United Nations World Water Development Report estimates that 80% of all wastewater generated by industry is discharged into the environment without prior treatment [2]. These wastewater streams contain persistent organic pollutants; organic dyes; and heavy metals, such as cobalt, copper, iron, and mercury [3].

The textile industry is one of the largest dischargers of wastewater containing organic dyes, where it releases approximately 150 billion liters of untreated wastewater into aquifers [4,5]. In these 150 billion liters of wastewater, more than 70,000 to 100,000 tons (10% to 15% of the total organic dye wastewater discharges) of various organic dyes may be dissolved [6]. These, in turn, are filtered into the aquifers, which affect the environment with a darker appearance of the water, and thus, prevent the passage of sunlight and cause a reduction in oxygen in the aquatic environment [7].

There are more than 10,000 different organic dyes in various applications [8], which are classified into natural and synthetic dyes, with the latter being divided into three categories: anionic dyes, cationic dyes, and nonionic dyes [9]. They are also classified by their chemical structures into azo dyes, anionic dyes, and indigo dyes, all of which share a complex aromatic structure that makes them recalcitrant to the environment [10,11]. More than 60% of these dyes are aromatic azo compounds [12]. Some of these dyes are: Aniline Blue, Alcian Blue, Basic Fuchsin, Methylene Blue, Methyl Orange, Crystal Violet, Toluidine Blue, and Congo Red [13].

Congo red dye, which is an anionic diazo chromophore dye based on benzidine [14], is one of the most commonly used model dyes. It is a stable and non-biodegradable pollutant with a complex aromatic structure (see Figure 1), which makes it soluble and difficult to remove from water [15]. This dye may cause cancer, eye and skin irritation, central nervous system damage, liver damage, and drowsiness when humans are exposed to it [16].

To degrade organic dyes, scientists have turned to heterogeneous photocatalysis as an emerging technology that employs the advanced oxidation process (POA). This process begins with the irradiation of a solid semiconductor material, which becomes excited, thus creating electron/hole pairs [17]. It comprises four steps for the mineralization of these organic pigments: 1—absorption of light followed by the separation of the electron/hole pair, 2—adsorption of the reagents, 3—redox reaction, and 4—desorption of the products [18]. This technology is advantageous due to its high efficiency, low operating cost, operation at ambient pressure and temperature, a requirement of no chemical additives, and environmentally friendly and non-toxic nature [19,20,21,22].

To effectively conduct heterogeneous photocatalysis on any organic pollutant, it is necessary to utilize solid semiconductors, such as metal oxides and chalcogenides as interfaces [22].

In recent years, sensitive photocatalysts, such as CuS, CdS, PbS, and Bi₂S₃, which are excited in the visible region with a narrow bandgap energy, have been employed to complement the more common semiconductors, like ZnO and TiO₂, which are activated in the ultraviolet region [23,24]. By combining these materials, various contaminants or organic dyes, such as methyl orange, rhodamine B, methylene blue, and Congo red, can be almost completely degraded [25,26,27].

Oxide semiconductors, such as TiO₂, produce an excellent photocatalyst oxidative decomposition of organic pollution under ultraviolet irradiation [27]; with a band gap energy of 3.2 eV in the anatase phase [22], this material has excellent properties, such as low cost, non-toxicity, stability chemical, and being environmentally friendly [28,29]. On the other hand, ZnO has a band gap energy of 3.3 eV [29].

Some researchers used Bi₂S₃ in different forms as an interface to degrade any pollutants dissolved in water. For example, Zhao [30] used Bi₂S₃ microspheres to degrade methyl orange (MO) at a concentration of 25 mg/L in an aqueous solution, where the experimentation consisted of irradiation with a 500 W high pressure mercury lamp using the photochemical reaction apparatus. This resulted in the degradation of MO by Bi₂S₃ microspheres up to 20% after 30 min.

Balachandran [31] complemented Bi₂S₃ with ZnO by forming a Bi₂S₃-ZnO nanosheet heterostructure, where this material was irradiated using four parallel medium-pressure mercury lamps that emitted a wavelength of 365 nm to degrade a concentration of 180 mg/L of AB (Acid Black); 200 mg of Bi₂S₃-ZnO nanosheets was placed in a reaction tube and irradiated at 365 nm to obtain 44% degradation after 30 min.

On the other hand, Bessekhouad [32] proposed complementing the Bi₂S₃ efficiency with TiO₂. For the photocatalytic degradation of an organic pollutant (Orange II), the solar box ATLAS Suntest CPS was used to simulate natural radiation, where 100 mL of 10 ppm of Orange II was mixed with 50 mg of Bi₂S₃-TiO₂, and after 30 min, the photocatalytic degradation of Orange II with Bi₂S₃-TiO₂ resulted in up to 60% removal of the pollutant. They proposed Bi₂S₃ as a thin film, where this nanomaterial can be used as an interface to form radicals (•OH⁻) to degrade Congo Red (CR). The advantage of this photocatalytic experiment is that the Sun was used as a source to irradiate the CR solution, where 30 min of Sun irradiation achieved up to 30.81% degradation of the CR.

For this research, metal oxide semiconductors, such as titanium dioxide (TiO₂) and zinc oxide (ZnO) were utilized in nanoparticles. Both are inorganic n-type semiconductors that absorb electromagnetic radiation in the ultraviolet region (UV, >400 nm) [33,34]. Additionally, metallic sulfide (Bi₂S₃) was employed, which is an anisotropic n-type semiconductor due to its physical properties [35]. This material is activated in the visible region (from 400 nm to 750 nm) of the electromagnetic spectrum, with a bandgap of 1.7 eV [36]. The advantage of the heterostructure is that it presents a greater efficiency when carrying out the advanced oxidation process or degradation of any contaminant [37]. Three different materials were synthesized using these materials: Bi₂S₃ thin films, Bi₂S₃ thin films coated with TiO₂ or ZnO nanoparticles, and Bi₂S₃/TiO₂ or Bi₂S₃/ZnO thin films.

Bi₂S₃ thin films were synthesized using chemical bath deposition, while TiO₂ was obtained using a sol–gel process assisted by a microwave and ZnO nanoparticles were extracted from alkaline batteries. These materials served as interfaces for heterogeneous solar photocatalysis, which allowed for the utilization of UV–visible regions of up to 55% of the solar radiation [38]. The resulting material’s photocatalytic activity was tested by degrading Congo red dissolved in deionized water at an initial concentration of 20 ppm, which demonstrated a reduction in this model organic pollutant.

2. Results and Discussion

2.1. Characterization of Bi₂S₃ Thin Films Deposited with Two and Three Layers

2.1.1. Morphology of Bi₂S₃ Thin Films

The morphologies of the Bi₂S₃ thin films that consisted of two and three layers (Figure 2) resembled spherical shapes of entangled sea urchins, which were referred to as such by some authors [39], while others described them as flower-shaped microspheres [30]. In Figure 2B, crystal growth was observed compared with Figure 2A due to the third layer of the chemical bath. The methodology followed by Carrillo and collaborators [39] was entirely reproducible and resulted in the same morphology.

2.1.2. X-ray Diffraction of Bi₂S₃ Thin Films

Figure 3 displays various diffraction patterns, where the characteristic peak of the Bi₂S₃ thin films appears on the crystallographic plane (2 2 1), which corresponds to the orthorhombic structure. This structure aligns with the reference codes 00-017-0320 and 01-089-8965 of the X’Pert HighScore Plus calculation program, which is consistent with previously reported results [36,39,40,41]. Figure 3 shows that the three-layer Bi₂S₃ thin-film peaks were more intense due to the increased number of chemical bath depositions, which led to an increase in the crystal size from 19.4 nm to 22 nm.

2.1.3. Optical Characterization

The absorption spectra in Figure 4 show a shift to the right (red shift) as the number of layers increased from two to three. Also, in the range of 750 nm to 550 nm, the absorption in the two different layers showed a higher value, which indicates the possibility of excitation in the visible range. Carrillo and co-workers replicated the synthesis of such thin films, and thus, the results are similar [39,40]. The transmittance spectra can be seen in FS1.

Regarding the band gap energy, Tauc’s method (see Figure 4) [42] was used and reported values of 1.9 eV and 1.95 eV were obtained, shown with blue arrows. Other authors that reproduced the same material also obtained a change in the band gap energy when increasing the number of chemical baths [43].

2.1.4. Contact Angle of Bi₂S₃ Thin Films

The study of the surface wettability of the thin films indicated contact angles that ranged between 123° and 117° for the two-layer Bi₂S₃ thin film (Figure 5A) and three-layer Bi₂S₃ thin film (Figure 5B), respectively. While classified as hydrophobic films since they had contact angles with water greater than 90° [43], as the number of chemical baths increased, the angle decreased.

Thin films with contact angles classified as hydrophobic allow for higher flow and prevent saturation, which facilitates an increased formation of oxidizing radicals on the catalyst surface [44].

2.2. Characterization of Bi₂S₃/TiO₂ Thin Films

2.2.1. Morphology of Bi₂S₃/TiO₂ Thin Films

Figure 6 shows the micrographs of Bi₂S₃ thin films with TiO₂ nanoparticles deposition. To obtain additional insight into the topographies of the TiO₂ nanoparticles, the EDX analysis of the sample was performed from the same area, as shown in Figure 6A,B. The EDX analysis (Figure 6C,D) confirmed the presence of titanium oxide nanoparticles on Bi₂S₃ thin films. The elemental analysis of the two-layered Bi₂S₃/TiO₂ thin films gave ~30.28% of bismuth, ~12.01% of sulfur, ~0.22% of titanium, and ~57.49% of oxygen, and for the three-layered Bi₂S₃/TiO₂ thin films, the analysis gave ~30.56% of bismuth, ~11.73% of sulfur, ~0.74% of titanium, and ~56.96% of oxygen, which proved that the deposited TiO₂ nanoparticles were in the Bi₂S₃ thin films. Also, it was observed by means of EDAX analysis, as shown in Figures S2 and S3, that the elemental distributions of Bi, S, Ti, and O were homogeneous.

2.2.2. X-ray Diffraction of Bi₂S₃/TiO₂ Thin Films

Figure 7 shows distinct intensity peaks in the diffractograms that indicate characteristic orientations of the TiO₂ nanoparticles. The (1 0 1) orientation corresponded to the anatase phase according to the TiO₂ Nps reported in [45,46,47] while the (2 0 2) orientation belonged to the rutile phase [46], which was located at 56.82°. These crystallographic planes are typical of TiO₂. The characteristic peak of Bi₂S₃ remained unaffected by the presence of TiO₂ nanoparticles, with its peak located in the (2 2 1) crystallographic plane, which corresponded to the orthorhombic structure [37].

The peak at 28.51° was shared by both Bi₂S₃ and TiO₂, but in different crystallographic planes. For TiO₂, this peak represented a (1 0 1) crystallographic orientation, while for Bi₂S₃, it corresponded to the (2 3 0) crystallographic plane. In the three-layered thin-film Bi₂S₃/TiO₂, the peaks were more closely aligned, where the orientation remained at (1 0 1) for TiO₂ [47] and a crystallographic (2 1 1) plane for Bi₂S₃.

It is worth noting that the structural composition of neither material was compromised. In the case of Bi₂S₃, the (2 2 1) crystallographic plane appeared more intense due to the increased number of chemical bath depositions, which attested to the purity of Bi₂S₃ [39]. For TiO₂, the anatase and rutile phases were visible through their respective characteristic peaks and crystallographic orientations, which confirmed the successful formation of a heterojunction [48].

2.2.3. Optical Characterization of Bi₂S₃/TiO₂ Thin Films

The absorption spectrum of the two-layer Bi₂S₃/TiO₂ exhibited a blue shift relative to the three-layer Bi₂S₃/TiO₂ thin film, which enabled excitation with longer wavelengths in the visible region, which was associated with electron/hole pair generation [49] The transmittance spectra can be seen in FS4.

The TiO₂ induced a shift toward the UV region, as shown by an absorption edge at 400 nm [28].

Both heterostructures can initiate the redox process of pollutants due to strong absorption in the visible region [48].

Figure 8 shows the bandgaps obtained using Tauc’s method for the two-layer and three-layer Bi₂S₃/TiO₂ films, which resulted in 2.3 eV and 2.35 eV respectively, shown with blue arrows. These results indicate well-defined absorption edges and lower bandgaps compared with the findings of Serentuya and colleagues [49]. Notably, the absorption edge extended up to 650 nm, which indicates visibility in the visible region for both materials.

2.2.4. Contact Angle of Bi₂S₃/TiO₂ Thin Films

The contact angles of the two-layer and three-layer thin films (Figure 9A,B) showed contact angles of 95.3° to 86.0°. This means that the TiO₂ nanoparticles influenced the affinity to water, which shifted the three-layer Bi₂S₃/TiO₂ film from hydrophobic to hydrophilic. This reduction resulted in fewer hydroxyl groups being formed due to the wettability of the film surface [50].

In general, the synthesized films had poor wettability, which was derived from the chemical compositions and morphologies of the materials, and thus, a similar formation of oxidizing groups was expected and a very similar degradation on the same pollutant was derived [51].

2.3. Characterization of Bi₂S₃/ZnO Thin Films

2.3.1. Morphology of Bi₂S₃/ZnO Thin Films

Micrographs captured at 30k magnification (Figure 10) depicted the morphology of the Bi₂S₃ thin films, where the nanospheres remained unaltered upon deposition of the ZnO nanoparticles. The EDX spectra of the Bi₂S₃/ZnO samples are shown in Figure 10C,D. The names and percentages of the elements for two layers of Bi₂S₃/ZnO and three layers of Bi₂S₃/ZnO are shown in the labeling. Additionality was observed by means of EDAX analysis, as shown in Figures S5 and S6, that the elemental distributions of Bi, S, Zn, and O were homogeneous.

2.3.2. X-ray Diffraction of Bi₂S₃/ZnO Thin Films

Figure 11 displays the diffractogram peaks of the material separately, as it was considered a heterojunction, where the ZnO was deposited by spin coating and neither material was structurally modified.

Characteristic peaks in the crystallographic planes (1 0 1) and (1 0 0) are shown for ZnO, which correspond to the hexagonal wurtzite crystal structure [52,53] according to the ZnO Nps reported in [54].

The hexagonal crystalline structure of ZnO is advantageous for photocatalysis due to its chemical stability and high refractive index [55], which leads to increased hydroxyl ion production and photoactivity [51].

2.3.3. Optical Characterization of Bi₂S₃/ZnO Thin Films

The absorption edges of the two- and three-layer Bi₂S₃/ZnO thin films ranged from 750 nm to 450 nm (see Figure 12). A blue shift, which indicates a shift toward shorter wavelengths, was observed compared with the Bi₂S₃ thin films without ZnO. Additionally, the three-layer Bi₂S₃/ZnO films showed a red shift toward the visible region [56]. The addition of ZnO resulted in enhanced absorption in the visible region because it absorbed a small part of visible light near 400 nm, but it was not enough because this is the boundary of the UV and visible regions; therefore, material that absorbs light toward the visible region is required to take advantage of the source of solar radiation [53], which is one reason for expanding the photocatalytic activity of Bi₂S₃ thin films. The transmittance spectra can be seen in FS7.

The addition of ZnO enhanced the photocatalytic activity of the Bi₂S₃ thin films in the ultraviolet–visible spectrum such that the range of OH radical formation was broader. Hence, sunlight became a viable radiation source since it falls within the range of 450 to 700 nm [57]. Thus, photons with energies greater than those depicted in Figure 12 (2.45 eV and 2.0 eV, shown with blue arrows.) can induce radical formation, which leads to pollutant reduction. The bandgap increased significantly when ZnO was added to the Bi₂S₃ thin films, as the metal oxide exhibited a bandgap of 3.3 eV [54]. Al-Zahrani showed different band gaps of Bi₂S₃/ZnO, where the cationic concentration (the amount of positive charges present on the surfaces of materials) caused the band gap energy blue shift; in this case, this was caused by a heterostructure with anionic concentration because the materials were type n, and therefore, shifted toward the red [54].

2.3.4. Contact Angle of Bi₂S₃/ZnO Thin Films

The introduction of ZnO nanoparticles decreased the contact angle (Figure 13A,B) compared with the Bi₂S₃ thin films alone, which indicates a hydrophobic nature. This reduced wettability facilitates the flow of OH radicals. The contact angle values observed in this study were higher than those reported by Yu and colleagues [55], which is beneficial for ensuring effective OH radical flow.

2.4. Photocatalytic Activity of Bi₂S₃, Bi₂S₃/TiO₂, and Bi₂S₃/ZnO Thin Films

The photocatalytic activity of the thin films was evaluated with the degradation of the contaminant Congo red dissolved in deionized water at initial concentrations of 20,011 ppm, 20,495 ppm, and 20,373 ppm in the first, second, and third replicas, respectively.

Table 1. Congo red degradation replicas through heterogeneous solar photocatalysis using thin films.

Congo Red
	1st Replica		2nd Replica		3rd Replica
Material	Concentration	% Degradation	Concentration	% Degradation	Concentration	% Degradation
C initial	20.011	0	20.495	0	20.373	0
Two-layer Bi2S3	15.727	21.408	16.696	18.53622835	17.510	14.05291317
Three-layer Bi2S3	15.904	20.523	16.58	19.10222005	18.560	8.899033034
Two-layer Bi2S3/ZnO	13.846	30.808	15.225	25.71358868	17.475	14.22470917
Three-layer Bi2S3/ZnO	15.237	23.856	15.264	25.52329837	17.312	15.02478771
Two-layer Bi2S3/TiO2	15.46	22.742	16.654	18.74115638	17.237	15.392922
Three-layer Bi2S3/TiO2	15.691	21.588	15.728	23.25933154	17.378	14.70082953

shows the summary of each of the replicas of the photocatalytic activity exposed for 30 min with each of the different materials.

Figure 14 represents a graph of the three replicas showing the two Bi₂S₃/ZnO films with higher efficiency in the degradation of the dye; it also showed a higher standard deviation, which indicates the variability of the degradation attributed to the incidence of solar radiation. The average for the three replicas showed a degradation from 20.33 ppm to 15.87 ppm for an efficiency of 21.93%.

The first replica was performed on 17 May 2023 and had the highest radiation and efficiency due to the incident solar radiation of the three replicas. The material that obtained the highest degradation in the first replicas was the two-layer Bi₂S₃/ZnO thin film with 30.808% (applying Equation (1)) in 30 min. Figure 14 shows the decrease in absorbance and degradation kinetics.

Figure 15 shows the absorbance related to the organic contaminant degradation, which reveals a primary absorption edge at 497 nm. As the organic molecule transformed and its concentration decreased, this edge diminished. Notably, the absorbance edges showed a decreasing trend, which was characterized by a π→π* electronic transition with a bathochromic shift (red shift), which indicates absorption at longer wavelengths [58]. The material exhibited no signs of wear or chemical reactions on the film, which was attributable to its hydrophobic nature. This stability suggests its efficacy in combating organic contaminants with complex molecular structures under natural climatic conditions.

Figure 15 shows the degradation time with the highest efficiency that was calculated and the degradation kinetics of the organic pollutant. The two-layer Bi₂S₃/ZnO material degraded 100% at 300 min. Equation (1) was used to describe the degradation kinetics:

$$C\left(t\right)=C_0{e}^{kt}$$

(1)

Once Equation (1) was obtained, the values obtained from the measurements were substituted to obtain the constant k using Equation (2):

$$k=-{\displaystyle \frac{\mathrm{l}\mathrm{n}\left(\frac{C_t}{C_0}\right)}{t}}$$

(2)

The two-layer Bi₂S₃/ZnO thin films obtained the highest efficiency of 30.808% (as shown in

Table 2. Conditions of photocatalytic reaction (Ciudad Juarez Chihuahua, Mexico).

Conditions/Number of Replicas	1	2	3
Date and time	17 May 2023 (11:30 a.m. to 12:00 p.m.)	19 July 2023 (12:20 p.m. to 12:50 p.m.)	9 August 2023 (11:50 a.m. to 12:20 p.m.)
Solar radiation (W/m2)	1000	910	870
Solar UV index	7.12	11	10
Temperature (°C)	25	39	33
Humidity (%)	24	13	26
Initial concentration (Co) (mg/L)	20.011	20.495	20.373

) in 30 min of solar irradiation, which indicates that the highest amount of radicals were generated to degrade the pollutant.

The absorption edges provide insight into a potential degradation pathway, as depicted in Figure 16, commencing with the following: (1) The attack of hydroxyl radicals on amines (NH₂), which leads to deamination [59]. (2) Subsequently, degradation of sodium atoms occurs [14], with reactive oxygen species, such as OH and ^*O₂ radicals [60], represented at a wavelength of 235 nm, which vary according to the UV–Vis spectroscopy graph of dye degradations [61]. (3) The hydroxyl then separates, and nitrogen double bonds are broken by radical attacks [58], which form amine functional groups, while p-dihydroxyl biphenyl is concurrently generated [59]. (4) Hydroxyl radical attacks further lead to the formation of hydroquinone and two molecules of 3-aminonaphthalene-1-sulfonic acid [59]. (5) Following this, benzene rings begin to break down, which lead to the formation of carboxylic acids, malonic acid, acetic acid, aldehydes, alkanes, etc [62,63], and ultimately result in the mineralization of the Congo red molecule [62]. These degradation routes attest to the discoloration of Congo red to a lighter shade in water.

Various authors conducted similar experiments using different materials and methods. Hokonya and colleagues [15], for instance, degraded Congo red under controlled parameters and artificial light, which achieved a degradation of 15.87% in 30 min at an initial concentration of 25 ppm and 43.17% at an initial concentration of 15 ppm. They used P-ZrO₂CeO₂ZnO nanoparticles in suspension as catalysts and demonstrated comparable efficiencies in the same time frame, but with the added benefit of easy separation from the waste medium.

Hitkari and collaborators [63] also conducted controlled parameter experiments to ensure direct radiation toward the material for optimal activation. They utilized ZnO nanoparticles in suspension with copper, which achieved a degradation of 69% in 30 min using 50 mg of the synthesized sample in 50 mL of aqueous solution containing the pollutant. However, this approach led to saturation, where electromagnetic waves did not directly impact or impinge on the materials [63]. To address this, the present study employed a smaller amount of photocatalyst to enable effective degradation under normal climatic conditions.

Habibi and coworkers [64] degraded Congo red with CdS/ZnO (metal oxide chalcogenide) using a 250 W mercury lamp as a source to activate the photocatalytic material and obtained a decolorization time lapse of 105 min at a neutral pH in solution [64]. In this work, the solution was not modified because it was considered deionized water with a neutral pH. During the photocatalysis, the pH of the solution varied since the formation of the oxidizing radicals was directly related to the change in the pH of the solution.

Concerning separate materials, it was found that they have a lower efficiency, as in the work of Bessekhouad et al., who degraded an organic pollutant using Bi₂S₃ with TiO₂ methyl oxide irradiated by a solar simulator. As a result, a degradation of the pollutant was presented and it was observed that the separated materials obtained a lower efficiency (similar to the present work); however, for this work, a smaller amount of photocatalyst was used [32].

3. Experimental Section

The preparation of Bi₂S₃ thin films utilized the following reagents: bismuth nitrate (III) pentahydrate (Bi(NO₃)₃ · 5H₂O, ≥98.0%, Sigma Aldrich Toluca, Edo. Mex., Mexico), triethanolamine (TEA)((HOCH₂CH₂)₃N, 99.80%, J.T. Baker, Phillipsburg, NJ, USA), sodium hydroxide (NaOH, 98.91%, CTR Scientific, Monterrey, México) and thiourea (NH₂CSNH₂, 99.2%, J.T. Baker).

For TiO₂ nanoparticles, the following reagents from Monterrey, Mexico, were used: titanium isopropoxide (C₁₂H₂₈O₄Ti, 97%, Sigma Aldrich), isopropanol (C₃H₈O, 99.8%, Fermont), and ethanol (C₂H₅OH, 99.5%, CTR).

ZnO nanoparticles were obtained from oxidized zinc (ZnO) extracted from the anode of a worn-out alkaline battery (type D Energizer brand).

The organic pollutant Congo red (C₃₂H₂₂Na₂N₆O₆S₂) from the HIMEDIA brand was employed for heterogeneous solar photocatalysis to degrade the pollutant.

3.1. Deposition of Bi₂S₃ Thin Films

The films were deposited on glass slide substrates (soda lime glass) previously washed with acetone, isopropanol, and deionized water for 10 min in each solvent sequentially under sonication (Branson 5800, Branson Ultrasonic, Brookfield, CT, USA). For the deposition of Bi₂S₃ thin films, the chemical bath deposition technique was used following the methodology of Carrillo [39]. This experiment used a mixture of 5 mL of TEA (1 M) with 40 mL of Bi(NO₃)₃5H₂O (0.1 M), 2.5 mL of TEA (C₆H₁₅NO₃) (0.5 M), 2.5 mL of sodium hydroxide (NaOH) (1 M), and 5 mL of thiourea (CH₄N₂S) (0.15 M). After obtaining the homogeneous mixture of the precursors, 3 substrates were introduced at a temperature of 60 °C +/− 2 °C for 80 min; this process was repeated for the second and third film layers to obtain a crystalline and homogeneous film growth.

Finally, and after 80 min of chemical bath deposition, the thin films were cleaned with methanol under ultrasound for 10 min and then in deionized water in ultrasound for 10 min to finally dry the films at room temperature.

3.2. Preparation of TiO₂ Nanoparticles

For the films, the microwave-assisted sol–gel method was used for the synthesis of TiO₂ nanoparticles, following the methodology of Mota-González [47]. A total of 2.72 g mL of titanium isopropoxide was poured into 40 mL of isopropanol and then stirred at 700 rpm for 1 min at 80 °C; after the minute, 0.52 mL of deionized water and 1 mL of isopropanol was added and then stirred at 700 rpm for 1 min to 80 °C. The solution was then allowed to precipitate for 24 h, during which time, the two phases were separated (sol–gel), and the supernatant was removed with a pipette from the solution. To dry the solution, a microwave was used for 15 min, with intervals of 5 s of drying with 60 s of rest outside the microwave. After drying, the nanoparticles were crushed in a mortar and then washed with deionized water.

3.3. Preparation of the ZnO Nanoparticles

For the synthesis of ZnO nanoparticles, Energizer brand D-type discharged alkaline batteries were used following the methodology of Diaz-León et al. [65]. Wasted zinc anodes were cleaned, dried, and then leached using 10 mL of nitric acid and 20 mL of hydrogen peroxide per gram of washed powder. The resulting zinc-rich solution was used to synthesize zinc oxide nanoparticles (ZnO Nps) via sol–gel methods. For starch-based synthesis, 10 g of rice starch was dissolved in 150 mL of water, while for dextrose-based synthesis, 21.4 g of dextrose was mixed with 150 mL of water. Thermogravimetric analysis (TGA) determined the calcination temperature (400–800 °C) to convert the dried gels into ZnO NPs [65].

3.4. Preparation of Bi₂S₃/TiO₂ and Bi₂S₃/ZnO Thin Films

For the heterojunction of the materials, TiO₂ and ZnO nanoparticles were deposited on Bi₂S₃ films by a spin-coating technique. For the preparation of TiO₂ and ZnO nanoparticles, 0.02 g was dissolved in 10 mL of ethanol and then deposited at a speed of 1000 rpm for 60 s on Bi₂S₃ thin films. This process was repeated 4 times; afterward, the Bi₂S₃/TiO₂ and Bi₂S₃/ZnO thin films with the nanoparticles were dried at 85 °C for 15 min.

3.5. Characteritation of Materials

The morphology of the materials was characterized using a Hitachi SU5000 scanning electron microscope (Hitachi, Tokyo, Japan) at a voltage of 15,000 V. The elemental compositions were analyzed by EDAX quantitative analysis with a JEOL 6010 Plus (Tokyo, Japan). The crystalline structure of the materials was studied with X-ray diffraction using PANalytical (Malvern, UK) with CuKα(λ) = 1.54 Å operated at 35,000 V and 23 Ma, while scanning over 2θ in a range from 10 to 80°. Optical absorption measurements were performed using a Jeneway 6850 V/Vis spectrophotometer (Sapulpa, OK, USA) in the range of 300 nm to 1100 nm and PerkinElmer Lambda 25 UV–Vis spectrometer (Shelton, CT, USA) in a range of 300 to 750 nm, with a scan of 0.2 nm. A Kruss model DSA 30 microscope was used for the contact angle.

3.6. Photocatalytic Degradation of Congo Red

The photocatalytic activity of Bi₂S₃/TiO₂ and Bi₂S₃/ZnO thin films was evaluated through the degradation of the organic dye Congo red model by exposing it to solar radiation for 30 min, where three replicas were made under different climatic conditions. Table 2 shows the initial concentrations of each of the replicas and the climatic conditions when each of the replicas was performed. To prepare the aqueous solution, 80 mL of Congo red at an initial concentration of 20 ppm was prepared in a beaker while maintaining an agitation of 500 rpm [66]. At the beginning of the photocatalytic study, the thin films were introduced in such a way that they were in contact with the solution at a certain shrinkage.

After 30 min of activity, the sample was extracted with a syringe and then the concentration was evaluated in a PerkinElmer Lambda 25 UV–Vis spectrometer in a range from 300 to 750 nm. Derived from the spectrometer measurement, the percentage of degradation was evaluated (Equation (3)) [5]:

$$\mathrm{\%}degradation={\displaystyle \frac{C_0-C_t}{C_t}}\times 100$$

(3)

Additionally, the degradation time of the material was calculated from the spectrophotometer measurements using the first-order differential equation presented in Equation (4) [66]. The initial conditions were obtained to solve and calculate the maximum time required to degrade 100% of the pollutant under the same environmental conditions (Table 2) as the photocatalytic activity carried out with nanomaterials.

$${\displaystyle \frac{dc}{dt}}=kC$$

(4)

4. Conclusions

In this study, it was established that a stable material was synthesized for use as a photocatalyst against complex organic molecules, owing to the stable structures inherent in each of the materials employed. The chemical, structural, and morphological integrity of the Bi₂S₃ thin film remained unaffected during the deposition of the metal oxides.

Optical characterization of the Bi₂S₃/TiO₂ and Bi₂S₃/ZnO thin films revealed an increase in the band gap compared with the Bi₂S₃ thin film. Furthermore, the absorption edges exhibited a red shift upon deposition of the metal oxide nanoparticles, which indicates a broadening of the reaction spectrum relative to the Bi₂S₃ thin film.

Regarding the assessment of photocatalytic activity, the two-layer Bi₂S₃/ZnO material demonstrated an efficiency of up to 30.80% in degrading Congo red, which is a model pollutant, within 30 min. This novel material exhibited the capability to break down molecules that contained complex organic compounds, which suggests degradation into simpler molecular forms based on UV–Vis spectroscope evaluations.