Novel <i>In Situ</i> Fabrication of Fe-Doped Zinc Oxide/Tin Sulfide Heterostructures for Visible-Light-Driven Photocatalytic Degradation of Methylene Blue

Dharmana, Govinda; Gurugubelli, Thirumala Rao; Viswanadham, Balaga; Bathula, Babu; Yoo, Kisoo

doi:https://doi.org/10.1155/2023/1407395

Journal of Chemistry

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 1407395 | https://doi.org/10.1155/2023/1407395

Novel In Situ Fabrication of Fe-Doped Zinc Oxide/Tin Sulfide Heterostructures for Visible-Light-Driven Photocatalytic Degradation of Methylene Blue

Govinda Dharmana,¹Thirumala Rao Gurugubelli,¹Balaga Viswanadham,¹Babu Bathula,²and Kisoo Yoo²

Academic Editor: Liviu Mitu

Received21 Mar 2023

Revised06 May 2023

Accepted13 May 2023

Published23 May 2023

Abstract

Using a hydrothermal synthesis process, Fe-doped ZnO/SnS nanostructures were created and a variety of analytical methods were used to describe their characteristics. X-ray diffraction patterns were employed to confirm the hexagonal and orthorhombic crystal structures of ZnO and SnS, respectively. Nanorods and nanoparticle clouds were visible in TEM pictures, and XPS investigation verified that the dopant Fe ions were in the 3+ oxidation state. Additionally, absorption spectroscopy revealed a decrease in the energy bandgap with an increase in Fe content, and photoluminescence analysis demonstrated that the ZSF3 sample significantly reduced the rate of recombination of charge carriers. Impressively, the optimized sample (ZSF3) displayed 95.8% more photocatalytic activity during the 120 min degradation of MB dye. This study demonstrated that an easy hydrothermal procedure, carried out at 220°C for 12 hours, may be used to create iron-doped ZnO/SnS nanocomposites. The tunable energy bandgap characteristics of heterogeneous semiconducting materials and the effective charge carrier separation were thought to be the causes of the increased photocatalytic activity. Furthermore, the heterostructure of charge carriers was proposed to facilitate photocatalytic activity when exposed to light.

1. Introduction

Currently, a major obstacle to human growth and well-being worldwide is the poisoning of clean water and aquatic habitats. The discharge of effluents from various sources, including cement, jute, oil refineries, leather, paint, agro-processing, and textile industries, has resulted in substantial contamination of fresh water [1]. Organic dyes, such as methylene blue (MB), methyl orange (MO), Congo red, acid orange (AO), and rhodamine B (RhB), are typical pollutants released by these industries that pose a serious threat to freshwater ecosystems and human communities [2, 3]. Consequently, considerable efforts have been directed towards the removal of these organic pigments from freshwater bodies. Several techniques, such as air stripping, photocatalysis, biological treatment [4], oxidation, adsorption [5], ultrafiltration [6], electrochemical techniques [7], reverse osmosis, and oxidation [8], have been employed to remove heavy metals and organic dyes from fresh water. However, the advanced oxidation process, utilizing p/n-type semiconductor nanostructured materials mediated photocatalytic degradation, has emerged as a robust technique for degrading toxic levels of organic dyes in water bodies [9]. When the semiconductor medium is transformed into its nanostructure, dye degradation will proceed effectively. Although many different approaches are used to construct p/n-type semiconductors, green chemical techniques are projected to perform more effectively. Effective charge transfer, efficient charge generation, and efficient solar light harvesting are the primary determinants of the photocatalytic system’s degradation efficiency [10]. The trends of photocatalytic behaviour are addressed by bulk semiconductors (ZnO, SnS₂, BiVO₄, TiO₂, SnS, SnO₂, Sb₂O₃, WO₃, and Na₂WO₄, etc.) and their doped (Cr, Cu, V, Al, Pt, Pd, Co, and Fe as a cocatalyst) variants. However, composites have a larger surface-to-volume ratio in their nanophased state, and it is anticipated that they will exhibit increased photocatalytic activity [11]. Nanostructured semiconductors as composites/heterostructures aroused much interest due to their unique trends of morphology, mobility, bandgap, conductivity, and fluorescence, etc. [12]. In recent years, semiconductor nanocomposite materials have been recognized as viable replacements for conventional materials because of their superior photocatalytic performance and photovoltaic responsiveness. Utility of semiconductor nanocomposites involves optical waveguides, biosensors, spintronic, biomedical appliances, photovoltaic cells, light emitting diode, and drug delivery mechanism [13].

Due to its beneficial qualities, such as its wide bandgap energy (3.36 eV), nontoxicity, low cost, and high exciton binding energy at ambient temperature, zinc oxide (ZnO) was chosen as a potential n-type direct bandgap semiconductor in this work. ZnO is severely constrained by its broad bandgap energy and fast rate of photogenerated electron-hole (e⁻/h⁺) pair recombination, which only allows for photocatalytic reaction in the UV area and results in a low light energy conversion efficiency [14]. Heterojunction nanostructures, which have significant potential for environmental safety, have emerged as very effective photocatalysts to meet this challenge. NiS, SnS₂, SnS, ZnS, and CuS are a few examples of the metal sulfide-based photocatalysts that can have their bandgap customized by size and shape without changing their chemical makeup. In this study, a heterojunction was made by combining ZnO, with its broad bandgap, with a low energy bandgap (1.3–1.6 eV) p-type direct bandgap material, such as SnS, to increase the photocatalytic activity (PCA) [15]. SnS has a number of benefits, including its abundance in the environment, nontoxicity, and wide range of uses, including photocatalysis, lithium-ion batteries, and solar cells. SnS also shows great potential as a photocatalytic material when exposed to visible light owing to its rapid charge transfer, cost-effective absorber layer, and high optical absorption coefficient. Thus, it has been determined that SnS is a good candidate for combining with ZnO to create a heterojunction that can effectively absorb solar energy and make it easier to separate e− and h+ couples [16]. Prior studies have extensively employed the technique of doping, using various materials such as rare earth metals, composite materials, transition metal oxides, semiconductors, noble metals, and reduced graphene-based oxides, to enhance the effectiveness of photocatalysts. The amazing photocatalytic performance of these dopant-based heterojunction nanocomposites in the reduction of specific dyes and heavy metals in water and waste has been demonstrated [17].

The Fe-doped ZnO/SnS nanocomposites used in this study give the ion diffusion and electron-transfer process a large surface area. Recent research has showed that nanocomposites doped with transition metal ions can also be employed for photocatalytic purposes. This motivated the researchers to create novel hybrid nanostructures for enhanced photocatalysis, i.e., transition metal ion doping ZnO/SnS nanocomposites [18]. Additionally, the ability to absorb light can be improved for visible light. As a result, the charge-transfer mechanism heavily depends on morphology, which greatly improves the photocatalytic efficacy. Therefore, the main variables impacting catalytic activity are surface area, shape, and doping influence [19]. Here, we try to comprehend how Fe-doping affects the characteristics of ZnO/SnS nanocomposites as part of our research. In the current study, we revealed that Fe-doped ZnO/SnS (ZSF) composites had improved visible light driven photocatalysis. Further, one of the main pollutants emitted from the textile and dye industries is MB, which affects marine life and mostly humans’ health in many ways such as [20, 21] difficulty in breathing, burning sensation, vomiting, diarrhoea, nausea, gastritis, abdominal and chest pain, severe headache, mental confusion, profuse sweating, urinary tract infection, and anemia and sometimes causes permanent injury to the eyes of people and their wellbeing. Therefore, the purpose of the current study is degradation of MB dye through the photocatalytic process under visible light illumination.

2. Materials and Methods

2.1. Materials

The precursors zinc acetate [Zn(CH₃COO)₂.2H₂O], sodium hydroxide (NaOH), crystal water containing tin chloride (SnCl₄.5H₂O), sodium sulfide (Na₂S), ferric oxide (Fe₂O₃), and ethanol-like solvents of analytical grade (99.9% in purity) are procured from Merck, for the synthesis of ZnO/SnS and Fe-doped ZnO/SnS nanocomposites. Deionised water is used for dilution.

2.2. Synthesis Method

In this experimental method, 0.2 mol of Zn(CH₃COO)₂.2H₂O is dissolved in 50 mL of an ethanol-water solution. Another ethanol-water mixture has NaOH dissolved in it. To determine the yield for ZnO, the zinc solution is gradually diluted with NaOH solution while being continuously stirred. A subsequent ethanol-water mixture is used to create 50 mL of SnCl₂.2H₂O solution. To the ZnO solution, this solution has now been added. When the mixture of solutions had taken on a colloid look after 15 minutes of continuous stirring, 50 mL of Na₂S solution was added dropwise. The creation of the ZnO/SnS nanocomposite structure is indicated by the white precipitate turning a light-yellow colour. The following is a description of the chemical processes involved in the preparation of ZnO/SnS nanocatalyst.

The ZnO/SnS nanocomposite was then added to 20 mL of a 1 mol% iron (III) nitrate nonahydrate, which was then continuously stirred for 4 hours. The mixture was then loaded into a Teflon-lined autoclave, which was then put into a high-temperature furnace and kept there for 12 hours at 220°C. The colloidal dispersion is repeatedly cleaned with ethanol and deionized water to eliminate the contaminants. The solution is next centrifuged for about 10 minutes at a speed of 5000 rpm. A 100°C hot-air oven is used to dry the sediment material for two hours before being mechanically milled into a fine powder. As a result, different mole percentages of Fe content (1%, 3%, and 5%) in optimal ZnO/SnS samples were used to create different concentrations of Fe-doped ZnO/SnS nanocomposites. The undoped sample is represented as ZS, whereas the doped samples are designated ZSF1, ZSF3, and ZSF5, respectively.

2.3. Characterization Techniques

X-ray diffraction (XRD) data were collected using a Cu-K radiation source and an XPert Pro PAN analytical diffractometer (λ = 0.15406 nm). Using HITACHI H-7650 equipment with an accelerating voltage of 120 kV, TEM micrographs were captured. Samples of powder were dissolved in an ethanol solution. The homogeneous dispersion was then dropped onto a copper grid that had been coated with carbon, where it was allowed to cure before being analysed. To ascertain the oxidation states of the component elements in the sample, an XPS analysis was performed using a Thermo Scientific K-alpha surface analysis device. To ascertain the energy bandgap, DRS spectra in the wavelength range of 200–900 nm were captured using a JASCO V-670 spectrophotometer. An HIPR-MP400UV-vis annular-type photo reactor was used to investigate the photocatalytic dye degradation. Furthermore, software used for the data analysis of various techniques are Xpert HighScorrer plus for XD, DigitalMicrograph for TEM, XPSPEAK4.1 for XPS, Thermo Scientific VISIONlite5 for UV-DRS, and Cary WinUV for photocatalytic analysis.

2.4. Photocatalytic Activity

At room temperature, MB aqueous solutions were exposed to visible light. An HIPR-MP400UV-vis annular-type photo reactor was used to investigate the dye’s photocatalytic breakdown. For each sample, 100 mL of a 10 ppm·MB aqueous solution and 10 mg of produced catalyst (Fe-doped ZnO/SnS) were combined. To maintain the adsorption/desorption equilibrium between MB and the catalyst before light illumination, the solution was magnetically agitated in the dark for one hour. The resulting formulation was then exposed to sun radiation. A UV-vis-NIR spectrophotometer was used to analyze the degradation of MB dye in 5 mL aliquots taken from the solution. The detailed photocatalytic degradation setup is presented in Figure 1. Each experiment was conducted twice: once under standard conditions (using only MB) and once with catalytic material (ZSF). The MB dye aqueous solution is collected for investigation just prior to the light illumination to measure the initial concentration of dye (C₀). The curve C/C₀ versus the illumination time (t) is drawn to derive the first order kinetics of the reaction. The MB dye degradation is correlated with the absorption and intensity fluctuation of the main band at 663 nm in the presence of visible light illumination.

The photocatalytic degradation efficiency of MB model dye is calculated by using the following expression:where C₀ is the real absorbance of MB dye and C is the concentration of MB during time intervals.

In view of reusability of the photocatalyst for other practical applications, cycling/reusability tests have been preferred for the optimized photocatalyst sample against its degradation efficiency.

3. Results and Discussion

3.1. Powder X-Ray Diffraction (PXRD) Analysis

PXRD is an endorsed technique to know the structural analysis and phase determination of the prepared samples. Figure 2 depicts the XRD patterns of (a) ZS, (b) ZSF1, (c) ZSF3, and (d) ZSF5 nanocatalysts. The mixed phase occurrence of orthorhombic SnS and hexagonal ZnO was supported by the XRD patterns. The Miller planes (101), (202), (110), (103), (102), and (002) ascribed to the hexagonal ZnO, according to JCPDS card 36–1451. Similarly, the diffraction planes (131), (112), (120), (111), (212), (311), and (042) confirm the orthorhombic phase of SnS (JCPDS card: 39–0354). No significant diffraction peaks related to the dopant material are observed. Notable peak broadening nature was observed due to the increasing of Fe content, which may be further indicating the increased lattice strain. A gradual shift towards higher angles, i.e., smaller d spacing in the position of diffraction peaks, was argued that high interface width between heterostructures. Some symmetrical peaks were also detected due to the reflections from identical mirror planes. There is no trace of additional peaks regarding Fe-based composites that were observed corresponding to the impurity phase, indicating the high dispersion of Fe ions and purity of the crystal structure. Additionally, the high intensity of the plane (111) suggested that nanocomposite formation has accumulated favorably. The crystallite size (D) of the synthesized samples is evaluated by using the following expression [22]:

The terms wavelength, full width at half maxima, and diffraction angle are all used in the expression. The prepared samples’ microstrain (ε) and dislocation density (δ) were calculated using the methods listed as follows [23]:

The calculated values of the abovementioned parameters such as D, ε, and δ for undoped (ZS) and Fe-doped ZS (ZSF) nanocomposites are shown in Table 1.

3.2. TEM Analysis

Surface structure and particle appearance in the synthesized samples were tested by a TEM study. TEM images of (a) ZS, (b) ZSF1, (c) ZSF3, and (d) ZSF5 nanocatalysts are presented in Figure 3. The structural alteration brought on by the infiltration of Fe ions into core-level Zinc (Zn) ion lattice sites was linked to the change in morphology of ZSF nanocomposites [24]. Additionally, the trend of the nanophase regime is confirmed by the fact that the grain size of nanoparticles shrinks with increasing doping of Fe ions into ZS structures. The TEM analysis reveals that ZSF composites exhibit uneven morphology because of higher doping element (Fe) levels, but they also exhibit a noticeably wavy and heavily agglomerated surface. However, the surface structure of the produced nanocomposites changed as the amount of Fe doping increased (Figures 3(b)–3(d)). The enormous surface area and growth of nanoparticles along a fixed axis (a = b ≠c, hexagonal structure) are influenced by the interatomic distance between ZnO sites and Fe metal ions, which is argued to have changed because of this change in morphology [25]. The presence and influence of Fe ions quicken the ZnO hexagonal lattice structure and cause it to exhibit a high agglomeration trend. However, with increasing doping content, the ZS nanostructure is totally concealed by Fe ions, i.e., the ZS morphology seems to be highly embellished with a huge number of secondary nanoflake-shaped Fe ions, owing to the heavily doping of Fe content into the ZS nanostructure.

3.3. XPS Analysis

The condition of a transition metal’s surface on the ZnO/SnS nanocomposite must be accurately known in the case of heterogeneous formations. The chemical/valence position and constituent elemental composition of the treated samples are ascertained via a thorough XPS analysis [26]. In the binding energy range of 0–1300 eV, a high-resolution survey scan is performed. The XPS survey spectrum of a representative ZSF3 nanocomposite is depicted in Figure 4(a). Furthermore, the representative sample is additionally showing carbon (C) and oxygen (O) peaks which ascribed to the environmental effect and control groups [27]. The Zn, Sn, O, and S elements in the typical composition are likewise confirmed by these binding peaks.

(a)

(b)

(c)

(d)

(e)

(f)

As shown in survey spectrum Figure 4(a), the additional peaks such as C1s and N1s appeared at the binding energy of 280 eV and 401.4 eV ascribed O-C bond formation and C-N and/or N-Fe bonds, respectively. The XPS spectrum of Zinc (Zn) element is depicted in Figure 4(b). Furthermore, the spectrum shows two significant binding energy peaks at 1021.62 and 1044.37 eV, which are argued to Zn2p_3/2 and Zn2p_1/2 orbital states [28], and these peaks tell the occurrence of Zn²⁺ valence position, i.e., Zn²⁺ in the representative sample with spin orbit separation of 22.75 eV. As displayed in Figure 4(c), the occurrences of high intense O1s XPS peaks at 530.02 and 531.53 eV are attributed to the establishment of metal oxide bond (Zn-O) with O²⁻ state and oxygen vacancies (O¹⁻) [29]. As represented in Figure 4(d), the noticeable peaks at binding energies of 486.18 eV and 494.58 eV argued to Sn 3d_5/2 and Sn 3d_3/2 orbital states [30, 31], respectively. Furthermore, these peaks provide evidence for the Sn²⁺ oxidation state, which has an 8.42 eV spin orbit separation. As shown in Figure 4(e), the spectrum of S element having the binding energy peak at 160.50 eV and another shoulder peak at 161.73 eV attributed to the orbital states S2p_3/2 and S2p_1/2, respectively, confirms the presence of S in −2 state, i.e., S²⁻ [31]. As depicted in Figure 4(f), the prominent peaks located at 716.80 eV and 726.11 eV attributed to Fe2p_3/2 and Fe2p_1/2 states, respectively, confirm the existence of the Fe³⁺ oxidation state [32, 33]. These results strongly confirm the existence of constitute elements (Zn²⁺, O²⁻, Sn²⁺, S²⁻, and Fe³⁺) in the ZSF nanocomposites.

3.4. Optical Absorption Study

UV-DRS is a needful study to know the energy bandgap and electronic band structure of crystalline materials. The absorption edge of the prepared samples is analysed to know the kind of band structure (p/n-type) and absorption coefficient [34]. The absorption diagram of ZS and ZSF samples is depicted in Figure 5(a). As represented in the figure, three prominent peaks (556, 554, and 551 nm) are traced for all nanocomposites in limit of the visible region. By analyzing Tau graphs, it is possible to determine the individual bandgap energy value (Eg) of nanocomposites. As depicted in Figure 5(b), the energy bandgap value for all the prepared composites such as ZS, ZSF1, ZSF3, and ZSF5 is noted as 3.28, 3.20, 2.28, and 2.74 eV, respective1y.

(a)

(b)

It is evident that the energy bandgap of undoped (ZS) composite steadily reduces as Fe content rises. Furthermore, defect energy levels formed inside the bandgap regime may operate as recombination centers of photogenerated charge carriers (electron and holes), and these defects and/or oxygen vacancies are responsible for the decreasing in bandgap [11]. However, observed energy bandgap was diminished for ZSF1 (3.20 eV) and ZSF3 (2.28 eV), and furthermore, it is increased for ZSF5 (2.74 eV) samples, owing to the intrusion of impurity energy levels, native point defects, and accumulation of Fe doping ions onto undoped (ZS) nanostructure. Low concentrations of Fe ions are coupled to the ZS structure in the ZSF1 sample. Maximum charge carrier separation (e⁻/h⁺ pairs) on the surface of the sample, formation of less dense interfacial defects/planer defects (i.e., external surfaces, stacking faults, and grain boundaries), and generation of new energy states in the bandgap region are the characteristics that lead to enhanced photocatalytic activity (PCA) for the composite ZSF3 [35]. Hence, ZSF3 composite is targeted as an ideal nanocomposite among all. Furthermore, in the ZSF5 composite, a fall in PCA was noticed owing to the heavy amount of Fe nanoparticles, which are identical for possessing greater density of planer defects, which encourages the concept of quenching of charge carrier recombination [36]. When the dopant concentration exceeds a particular (optimal) level, the charge zones become more condensed and the surface barrier increases, which lowers the photocatalytic performance. Hence, fall in PCA was achieved in the case of ZSF5 (91.3%) than ZSF3 (95.8%) optimized nanocomposite. The PL spectra of the prepared ZSF nanomaterials are presented in Figure 6. PL intensity is directly proportional to the recombination rate of photogenerated charge carriers. All the ZSF samples show a lower PL intensity than the pristine ZS sample, indicating a reduced rate of recombination of electron and hole pairs. Among the ZSF samples, ZSF3 shows the minimum PL intensity, indicating a significantly reduced rate of recombination of charge carriers. ZSF3 shows a lower bandgap and lower PL emission and is expected to show better photocatalytic degradation of methylene blue.

3.5. Photocatalytic Activity Studies

The photocatalytic degradation performance of undoped (ZS) and Fe-doped ZS (ZSF) nanocomposite materials is tested using the photodegradation of MB dye solution, which is initiated by visible-light irradiation at room temperature. In Figure 7, this is depicted. The absorbance spectra of Fe-doped ZS nanocomposites are measured every 15 min over a period of 120 min. In a control experiment, the dedying process is first shown without a photocatalyst or with just MB dye [37]. A prepared sample is magnetically spun in the dark to introduce the idea of saturation adsorption equilibrium and to comprehend the effects of photocatalyst dye degradation [38]. To understand the photocatalytic dye degradation process, the absorbance spectra of MB dye are computed as a function of illumination time. The absorbance peak at 663 nm wavelength consistently declines without moving with longer exposure times. Additionally, it has been discovered that compared to undoped (ZS) nanocomposites, doped Fe nanocomposites (ZSF1, ZSF3, and ZSF5) have higher photodegradation capabilities because Fe doping stimulates changing the photocatalytic dye degradation of doped (ZSF) nanocomposites by introducing impurity energy levels inside the bandgap domain [39].

(a)

(b)

The sequence of photocatalytic performance of produced nanocomposites is examined in Figure 7(a). Under visible light, the performance of nanocomposites deteriorates as ZS < ZSF1 < ZSF5 < ZSF3, in that order. The results obtained, ln (C₀/C_t.) = kt, are consistent with the Langmuir–Hinshelwood (L–H) model [40], and hence, they agree with the kinetic rate equation, as seen in Figure 7(b). By visualizing the relationship between ln (C₀/C_t) and irradiation time (t) and arguing that it should be a straight line, the L–H kinetic equation demonstrates that the photodegradation process complies to the first order kinetics link. Figure 8(a) displays the percentages of samples’ photodegradation efficiency under MB solution over a 120-minute illumination period. For the photocatalyst samples ZS, ZSF1, ZSF3, and ZSF5, the MB dye photodegradation rates are 10.4, 65.6, 84.7, 95.8, and 91.3%, respectively. Due to the substantial contact between nanoparticles and the creation of an efficient heterojunction, ZSF3 photocatalyst has the highest degradation efficiency of the MB dye of all nanocomposites or nearly 96% within the time of 120 minutes. The following first order kinetic relation will determine the kinetic constant of photocatalytic activity [41]:

(a)

(b)

The observed rate constant is given as 0.0042, 0.0086, 0.0124, and 0.0107 min⁻¹ for prepared samples such as ZS, ZSF1, ZSF3, and ZSF5. Also, the high kinetic rate constant of the optimized sample (ZSF3) makes it stand out from the other synthetic samples and shows that it destroys MB dye more efficiently than the others. Table 2 shows the dye’s photodegradation efficiency (%) and rate constant (k) values for all samples. The plot of the kinetic rate constant (k) for ready samples throughout the decolorization of MB dye under visible light irradiation is shown in Figure 8(b). Due to the incorporation of Fe ions into the ZS host lattice, all ZSF samples exhibit significant photocatalytic efficiency compared to undoped (ZS) samples, as shown in Figure 5(c). Additionally, Fe doping may generate a new energy level within the ZS lattice’s tunable bandgap, meaning that Fe ions are what that cause nanocomposites’ energy bandgap to decrease. Additionally, it has been found that the photocatalytic effectiveness of Fe-doped ZS nanocomposites steadily raise with Fe content. The ZSF3 photocatalyst reaches the highest degrading efficiency, while the ZSF5 composite finds that it decreases [23]. In the instance of the ZSF5 catalyst, unintentional overinclusion of Fe ions into ZS sites will result in narrowing of the bandgap and a reduction of scattered light. In comparison to other samples, the ZSF3 sample’s high surface area leads to a high number of active sites on the nanocomposite surface, which enhances the material’s ability for photodegradation.

3.6. Reusability Test

Various practical/instrumental applications make use of the nanocomposite reusability. As a result, in the current study, the optimized sample (ZSF3) was put through a four-cycle reusability test while being exposed to visible light. The results of the visible light reusability test for the MB dye over ZSF3 composite are shown in Figure 9(a). According to the results, there was a decrease in the amount of MB dye degradation after four cycles. Results showed that the proposed ZSF3-optimized sample could withstand 91% of MB degradation, making it strongly recommended for usage in future practical applications. The characteristics suggest that Fe-doped ZnO/SnS nanocomposites (ZSF) display excellent photocatalytic activity response and good light absorption under visible light illumination. The created photocatalyst is also expected to be used to purge organic pollutants from water and marine life as much as feasible. The current work will be refocused on providing society with safe drinking water and preserving the aquatic ecosystem’s cycle.

(a)

(b)

3.7. Proposed Charge Carrier Transport Mechanism

The proposed photodegradation of MB dye in the presence of the ZSF3-optimized nanocomposite is shown in Figure 9(b). Hence, doping Fe ions entirely controls the trend of the energy bandgap of ZSF3. Some of the Zn active lattice locations in ZS nanocomposite also include Fe ions as doping agents. When the surface of the composite is irradiated by visible light, photogenerated holes diffuse in one direction, from the new heterojunction (Zn-O-Fe) CB to the CB of the SnS material.

Photogenerated electrons diffuse in the opposite way. As a result, an internal electric field is created in the depletion zone between the interfaces by the Zn-O-Fe/SnS heterojunction dispersed charge carrier transition [42, 43]. The best way to achieve the best photocatalytic performance is through the most thorough charge separation method. The superoxide anion radicals (O²⁻) produced by the adsorbed oxygen molecules when the excited electrons migrate to the surface of the ZSF3 nanocomposite are effective at degrading wastewater. Additionally, the holes are immediately forming hydroxide-free scavengers by interacting with water molecules. The superoxide and hydroxide ions (O²⁻ and OH) are several powerful free scavengers that efficiently break down MB molecules into harmless CO₂ and H₂O. Degradation is thought of as a complimentary oxidation or reduction process. In order to successfully remove organic contaminants, the photogenerated charge carriers are depicted as being sumptuously separated on the surface of ZSF3 nanocomposite. As a result, when exposed to visible light, a new heterojunction may be able to efficiently separate the photogenerated charge carriers that are present on the surface of nanocomposite materials and exhibit potent photocatalytic activity. The proposed scheme of degradation of dye is shown in Figure 10. Methylene blue is the initial cleavage of the N-CH₃ group followed by C-N bond, which is degrading to phenol and then CO₂ and water are formed. The comparison study of photodegradation efficiency of various ZnO based nanocomposites with reference to MB dye is presented in Table 3.

4. Conclusions

In a nutshell, Fe-doped ZnO/SnS nanocomposites were made using a simple hydrothermal technique at 220°C for 12 hours while being illuminated by visible light. Assimilated phase occurrence of wurtzite hexagonal structure of ZnO and orthorhombic SnS was noticed. On the surface of the undoped (ZS) structures, Fe doping nanoparticles were seen to have been deposited by TEM micrographs and a trend toward more agglomeration was seen as the Fe concentration rose. The XPS study ascribed the oxidation state of the elements in the representative sample and successful coalition of Fe ions into the ZS nanocomposite. According to the UV-DRS investigation, the energy bandgap of the nanocomposites shrank as the Fe concentration rose. The photodegradation efficiency of the ZSF3 catalyst was estimated as 95.8% in 120 min. Improved photocatalytic activity against MB dye was attained under visible light irradiation due to the maximum charge carrier separation and heterostructure formation.

Data Availability

The data that support the findings of this study are available within this article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors (Govinda Dharmana and Gurugubelli Thirumala Rao) wish to express their gratitude to the GMR Institute of Technology, Rajam, for providing financial assistance through the SEED grant for the research study.

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Copyright © 2023 Govinda Dharmana et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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