Enhanced Photo-Catalytic Degradation of MB Dye over Hydrothermal Grown-Up of ZnO/SnO2 Catalyst

Abstract

In this work, pure ZnO, SnO₂, and ZnO-SnO₂ nanostructured composites (Z-S heterostructure) are fabricated by a facile and environmental savvy hydrothermal method at 230°C for 12 hours and followed by high-temperature annealing. The synthesized samples were examined to analyse the structural and optical characteristics using XRD, TEM, XPS, and UV-DRS techniques. Investigate the photocatalytic activity (PCA) of pure ZnO, SnO₂, and Z-S heterostructure for the decolorization of methylene blue (MB) model pollutant in wastewater using visible light irradiation. XRD analysis reveals that ZnO and SnO₂ exhibit hexagonal wurtzite structure and tetragonal rutile structure. TEM micrographs infer the nanorod-like structure of ZnO, nanoflake-like structure of SnO₂, and a thin nanosheet-like structure of Z-S. XPS analysis evidence for the presence of constitutes elements and Zn⁺², Sn⁺², O⁻² valance states along with environmental peaks in Z-S heterostructure. The optical band gaps of ZnO (3.37 eV), SnO₂ (3.6 eV), and Z-S heterostructure (2.84 eV) are inferred by UV-DRS analysis, together with evidence for the red shift. Under two hours of visible light illumination, the photocatalytic degradation efficiency of MB dye in pure ZnO, SnO₂, and Z-S heterostructure is 38.55, 32.68, and 82.45%. The formation of a unique Z-S heterostructure and the maximum departure of charge carriers are the reasons for obtaining robust degradation activity than pristine ZnO and SnO₂. MB dye degradation process in the presence of Z-S heterostructure photocatalyst, the reusability test findings, and the suggested charge separation technique for customizing Z-S heterostructure photocatalyst under visible light irradiation is presented.

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.

1. Introduction

In recent times, significant global environmental issues have been brought about by the hazardous wastes associated with industrial operations, such as organic wastewater, soil inadequacy, and poisoned air [1]. At present, major part of the water resources have been contaminated due to the action of human community across the globe in the form of countless industry effluents, agroproducts, pharmaceutical waste, and textile products [2]. The emissions of different industries are amalgamated with fresh water bodies and made them contaminated, subsequently leading to severe health hazardous to people and the animal world [3]. Fresh water bodies are contaminated due to the arrivals of different industrial syntactic dyes and mineral pollutant dyes such as methyl blue (MB), congo-red, acid orange, acid yellow, methyl orange (MO), rhodamine B (RhB), potassium, sodium, and copper butyl xanthate. Hence, to mitigate the water pollution and control the toxicity of the water bodies, abundant approaches have been put forth such as filtration, adsorption, chemical methods, biological approach, stripping method, oxidation, and photocatalysis [4]. Out of all mentioned approaches, photocatalysis is erected as the best method to detoxify the various pollutants and/or dyes in the water and made them decontaminate under the influence of light irradiation. The strategy and fabrication of extremely efficient visible light-induced photocatalysts has attracted a lot of attention from the academic community in recent decades because of their potential use in solar energy conversion and environmental clean-up [5]. In photocatalysis mechanism, the separated photogenerated charge carriers, i.e., as electrons and holes are reacted with dissolved oxygen to form super oxide (O₂^.−) and hydroxyl ions (OH^.−) free radicals with CO₂ and H₂O as harmless degradation products. In this way, these strong free radicals are degrading the MB dye molecules effectively based on the oxidation/reduction process. Moreover, the degrading efficiency of the organic dyes in a photocatalytic mechanism is primarily determined by the effectiveness of charge transport, efficient solar light gathering, and charge creation [6].

At present, a superior work was done on the progress of new kind of photocatalyst materials such as oxides, sulphides, graphene-based materials, nitrates, binary/ternary composite metal oxides, oxyhalides, and Ag/Au-based composites in order to detoxify the water bodies [7]. Among all, oxide materials associated with the semiconducting material have grabbed much interest by the researchers owing to their outstanding features such as electronic and optical properties, cheap in cost, detoxicity, thermal stability, high solar conversion efficiency, and more oxidizing power [8, 9]. Furthermore, combining several semiconductors (ZnO/SnS, ZnO/SnO₂, ZnS/SnS, ZnO/CdS, ZnO/CdO, ZnO/TiO₂, and ZnS/TiO₂) has shown to be effective in creating photocatalysts with higher performance of degradation compared to single semiconductors (ZnO, SnS, TiO₂, CdS, NiO, SnO₂, ZnS, and CdO) [10]. It has been discovered that integrating two kinds of semiconductors with distinct bandgaps to form an ordered heterostructure will boost the photoexcitation energy range and enhance photocatalytic activity because there will be less electron-hole recombination (optimal separation). In addition to above significant features, combined semiconductors (as mentioned above) in their nanophased regime exhibit maximal surface area, strange morphology, quantum confinement effect, less conductivity, easier charge separation, tunable bandgap, and thermodynamical conditions; hence, semiconducting materials with tunable bandgap are targeted by the researchers for the effective removal of organic contaminates/pollutants/dyes from waste water bodies [11]. An overview of studies highlights the significance of temperature conditions, energy bandgap characteristics, preparation of optimized composition, morphological features, and environmentally conscious synthetic settings as factors to achieve a novel nanostructured semiconductor heterostructure with improved photocatalytic degradation effectiveness under light irradiation [12]. At this time, novel combinational photocatalyst materials that show improved photocatalytic activity and stability in the decolorization of organic dyes under visible light irradiation need to be examined. As such, the creation of hierarchical structure in the arrangement of semiconductor/semiconductor (p-n, p-p or n-n) NCs was demonstrated as an astonishing strategy for obtaining better photocatalyst than single semiconductor [13].

Hence, keeping in view of the above literature, a novel zinc oxide (ZnO)/tin oxide (SnO₂) (Z-S) heterostructure is synthesized by a facetious single-step hydrothermal method and trailed by a high-temperature calcination process [14]. As it is familiar that ZnO is a wide bandgap material and passive under visible light, it is not suitable for photocatalytic mechanism [15]. To overwhelm this difficulty, combination of ZnO with other semiconductor material would consequence for the better removal of organic contaminates in wastewater, i.e., attain robust photocatalytic efficiency in the presence of light irradiation. Further, coupling of ZnO with SnO₂ to make a novel heterojunction, i.e., (ZnO/SnO₂) for the validation of attaining idyllic nanocomposite with high solar energy captivation and maximal separation of photocharge carriers (e⁻ and h⁺) in order to achieve robust photocatalytic activity [16]. The fabrication of ZnO (n-type)-based heterostructure through selective band alignment techniques and their further coupling with SnO₂ (n-type) semiconductor would render better efficient photocatalytic composites to degrade pollutants. Design of a semiconductor (n-type)/semiconductor (n-type) type of hetero junction nanocomposite seems to be an effective approach to increase the charge separation and enhanced absorption in the vicinity of a visible range of solar spectrum, than that for the cases of semiconductor/metal and semiconductor/insulator heterojunction etc., composites. While tunable nature of bandgap of semiconductor material is expected to usher a significant change in the photocatalytic activity, which in turn is anticipated to increase the separation of photogenerated charge carriers, i.e., electron-hole pairs. In current investigations, in order to encounter the requirements in high energy range, degradation efficiency of MB dye is observed in the manifestation of prepared ZnO/SnO₂ heterostructures in visible light illumination, and also an environmental savvy hydrothermal method is adopted for the preparation of ZnO/SnO₂ nanocomposite at 230°C for 12 hours and followed by calcination in order to anticipate the better photocatalytic degradation efficiency using HIPR-MP400 UV-Vis annular type photoreactor as shown in Figure 1.

[figure(s) omitted; refer to PDF]

In order to illustrate the yield and degradation efficiency, various techniques are employed to characterize the yields of as-synthesized composites. These techniques include X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis absorption spectrophotometer (UV-DRS), photocatalytic activity (PCA), degradation pathway, reusability test, free radical capture test, and charge separation mechanism under visible light.

2. Materials and Methodology

2.1. Chemicals and Materials

The materials (AR grade, purchased from Merck and Sigma-Aldrich Company) used for the synthesis of ZnO, SnO₂, and ZnO-SnO₂ composites are chemically very clean and no need any further purification. Zinc sulfate (ZnSO₄), sodium hydroxide (NaOH), tin chloride (SnCl₄), ammonium hydroxide (NH₄OH), methylene blue (MB) dye, ethanol, and deionized water.

2.2. Synthesis Method (Preparation of ZnO-SnO₂ Nanocomposite)

A facile and eco-savvy hydrothermal method is preferred to synthesise ZnO-SnO₂ nanocomposite. In this typical synthesis, same mole percentage of Zn and Sn, i.e., (1 : 1) is preferred. The schematic diagram of the preparation of ZnO-SnO₂ nanocomposite via hydrothermal method is presented in Figure 2. 0.2 mol% of zinc sulfate (ZnSO₄) is dissolved in 50 mL deionized water and stirred for 30 minutes at 800 rpm (at 70°C), and sodium hydroxide (NaOH) is also dissolved in the same volume (50 mL) of deionized water. Now, this NaOH solution is added drop-wise to zinc solution for again 30 minutes of continuous stirring at 800 rpm (at 70°C) and adjusts to pH value to 10.6 using ammonium hydroxide additive which indicates alkaline nature. After specified time of stirring, a white solution is formed called as ZnO solution. Further, 0.2 mol% of tin chloride (SnCl₄) is dissolved in the 50 mL of deionized water is allowed to release drop wise into ZnO solution for 40 minutes of continuous stirring of 800 rpm at 70°C. Now, ammonium hydroxide is added in to drop wise to this solution till adjust pH value 10.7 (alkaline nature). The hydroxide precipitates formed at given pH value will have high solubility in the alkaline medium at high pressures (hydrothermal conditions) and a continuous dissolution of precipitates and recrystallization of oxide materials exist in equilibrium condition. Finally, white-silver colored precipitation is formed to indicate ZnO-SnO₂ nanocomposite. Further, above solution is transferred into different volumes (100 and 150 mL) of Teflon-coated autoclaves and kept in high-temperature hydrothermal reaction at 230°C for 12 hours, and further, impurities are separated from the resultant solution and proceed for centrifuge (5000 rpm for 10 minutes) process. Further, the precipitation is washed with deionized water and dried in an air-oven at 100°C for 12 hours. Finally, to obtain better crystalline structure, the dried powder is annealed at 700°C for 8 hours. The fabrication of ZnO-SnO₂ nanocomposite by a simple hydrothermal method involves the below chemical reactions:$$\begin{array}{c}\text{(1)}& {\text{ZnSO}}_4+{\text{SnCl}}_4+6\text{NaOH}⟶\frac{\text{Zn}{\text{OH}}_2}{\text{Sn}{\text{OH}}_4}+4\text{NaCl}+{\text{Na}}_2{\text{SO}}_4\frac{\text{Zn}{\text{OH}}_2}{\text{Sn}{\text{OH}}_4}⟶\frac{\text{ZnO}}{{\text{SnO}}_2}+3{\mathrm{H}}_2\mathrm{O}\uparrow \end{array}$$

[figure(s) omitted; refer to PDF]

2.3. Characterization Techniques

To gather powder X-ray diffraction (XRD) data, Cu K radiation and an X-Pert Pro PAN analytical diffractometer (λ = 0.15406 nm) were utilized. TEM micrographs were obtained with HITACHI H-7650 equipment, which had a 120 kV accelerating voltage. To ascertain the oxidation (valance) states of the component elements present in the sample, an XPS analysis was performed using a Thermo Scientific K-alpha surface analysis device. To find the optical energy bandgap, DRS spectra in the 200–900 nm wavelength range were acquired using a JASCO V-670 spectrophotometer. Using a HIPR-MP 400 UV-Vis annular type photoreactor, the photocatalytic degradation of the MB model pollutant dye was investigated.

2.4. Photocatalytic Activity (PCA) Technique

MB aqueous solution is exposed to 300 W induced light source in order to evaluate the photocatalytic dye degradation using HIPR-MP 400 annular photoreactor. Further, 10 mg of the synthesized photocatalyst and 100 mL of MB aqueous solution (10 ppm) were added to each sample. Before visible light illumination, the solution was magnetically stirred for 60 minutes in the dark condition in order to preserve the adsorption and/or desorption steadiness between MB and the photocatalyst. After then, the final mixture was exposed to visible light. 5 mL aliquots of the MB dye solution were taken out in each regular interval of 15 minutes, and the degradation of MB dye was examined using a UV-Vis-NIR spectrophotometer. The degradation process is conducted in two ways, i.e., under blank condition (pure MB dye) and in the presence of a prepared photocatalyst (Z-S heterostructure). The standard MB dye solution is taken before the visible light irradiation to know the basic concentration of MB dye (C₀), and after illumination of light, MB dye concentration is noted for every 15-minute intervals of time (C). Further, the ratio between standard and time illumination concentration of MB dye follows first-ordered pseudo kinetic relation. Finally, the efficiency of dye degradation pertaining MB dye is associated with absorption and/or intensity of the major band wavelength at 663 nm. For further use of prepared photocatalyst in various fields/sectors, a reusability/recycling test was preferred against visible light irradiation. The photodegradation capacity of MB dye in the presence of a prepared photocatalyst material follows the following equation:$$\begin{array}{}\text{(2)}& \text{MB\hspace{0.17em}Degradation\hspace{0.17em}efficiency}\%=1-\frac{C}{C_0}\times 100\end{array}$$

Here, C and $C_0$ denote concentrations of MB dye initially and during time intervals.

3. Results and Discussion

3.1. XRD Structural Analysis

The crystal phase and structure of all fabricated nanocomposites are examined by the powder XRD study. XRD patterns of all as-fabricated nanocomposites such as pure ZnO, pure SnO₂, and ZnO-SnO₂ are presented in Figure 3. Curve (a) depicted the diffraction planes (1 1 0) (0 0 2), (1 0 1), (1 0 2), (1 0 3), and (1 0 4) at different 2θ values of 27, 35, 36, 48, 63, and 72° for pristine ZnO are strictly matches with JCPDS card number 36–1451 to confirm hexagonal wurtzite structure [17]. Whereas curve (c) ascribed the diffraction planes of (1 1 0), (1 0 1), (2 0 0), (2 1 1), (0 0 2), (3 0 1), (2 0 2), and (3 2 1) at various 2θ values of 26, 36.5, 39, 52, 59, 65, 72, and 79° for SnO₂ are firmly matches with the JCPDS card number 41–1445 to confirm the tetragonal rutile structure [18]. Curve (b) endorsed the diffraction planes of (1 1 0), (0 0 2), (1 0 2), (2 1 1), (0 0 2), and (3 0 1) for ZnO-SnO₂ nanocomposite with mixer of the two different crystal structures, i.e., hexagonal ZnO and tetragonal SnO₂. In curve (b), intensity of SnO₂ planes dominates the ZnO planes owing to adequate deposition of SnO₂ nanoparticles in nanocomposite. The lattice parameters such as grain size (D), strain (ε), and dislocation-density (δ) of as-synthesized samples are evaluated using the following expressions [19]:$$\begin{array}{c}\text{(3)}& D=\frac{0.9\lambda }{\beta \cos \theta }\epsilon =\frac{\beta \cos \theta }{4}\\ \delta =\frac{1}{D^2}\end{array}$$

[figure(s) omitted; refer to PDF]

Further, it is found that mean grain size of ZnO nanoparticles is bigger than SnO₂ NPs owing to surface diffusion allows ZnO to prevent the SnO₂ grain size. The mean grain size of pure ZnO is 26.58 nm that of pure SnO₂ is 17.82 nm. Whereas in Z-S heterostructure, grain size of ZnO is 26.46 nm and that of SnO₂ is 17.12 nm. From the above statement, it is clear that grain size of SnO₂ NPs in Z-S heterostructure is much lower than pristine SnO₂ (perhaps difference is 0.7 nm), whereas crystal size of ZnO NPs in Z-S heterostructure is endured with the size of pure ZnO NPs (probably difference is 0.12 nm), which infers the strong crystalline structure of ZnO in the Z-S heterostructure. The data of mean grain size, FWHM, strain, and density of pure ZnO, SnO₂, Z-S heterostructure are depicted in Table.1. Functional behaviour of the different lattice parameters, i.e., d-spacing, crystallite size, microstrain, and dislocation density of the as-synthesized samples, is depicted in Figure 4. The trend of lattice parameters follows the steep rise or fall initially owing to the dominant role of hexagonal ZnO and self-organized behaviour of the prepared nanocomposites.

Table 1

The data of crystallite size, FWHM, microstrain, and dislocation density of all-synthesized samples.

Sample	Crystallite size (D nm)	FWHM	Microstrain (ε) × 10–3	Dislocation density (δ) × 1015 lines/m2
Pure ZnO	26.58	0.315	1.309	1.428
Pure SnO2	17.82	0.471	1.950	3.166
ZnO in Z-S NC	26.46	0.315	1.313	1.435
SnO2 in Z-S NC	17.12	0.486	2.023	3.408

[figure(s) omitted; refer to PDF]

3.2. TEM Analysis

We carefully examined the nanostructure trend of the Z-S heterostructure in order to comprehend their morphological backgrounds. Figure 5 depicts the TEM micrograph of the Z-S heterostructure. TEM microstructure (Figure 5(a)) infers that ZnO exhibits nanorod-like structure, whereas SnO₂ exhibits thin nanoflake-like structure (Figure 5(b)). Further, TEM image shows the partial agglomeration trend of ZnO and SnO₂ nanoparticles owing to overloading of secondary tin (Sn) nanoparticles and hydrothermal conditions, which indicates the formation of Z-S heterostructure. Moreover, every nanoparticle is associated with surrounding NPs and no individual ZnO and/or SnO₂ NPs remain, which also recommend the effective formation of ZnO-SnO₂ heterostructure with nanosheets/plate-like structure. Figure 5(c) confirms the formation of Z-S heterostructure. Since ZnO is an n-type semiconductor, as is widely known, it is expected that negative charge carriers accumulated on a nanorod’s surface will interact with the positive charge carriers of Sn⁺² ions [20]. Therefore, it is anticipated that a thin layer will form between the ZnO and SnO₂ structures. Nonetheless, the circumstance gives rise to the notion of H₂O molecule hydrolysis, resulting in hydroxyl (OH-) ions and moreover fosters enhanced photocatalytic activity [21].

[figure(s) omitted; refer to PDF]

3.3. XPS Study

Z-S heterostructure catalyst’s surface composition and the chemical states of the zinc (Zn) and tin (Sn) ions along with other elements are investigated using XPS analysis. The survey scan spectrum of Z-S heterostructure along with other constitutional elements is presented in Figure 6(a). As shown in figure, the main constitute elements in the Z-S heterostructure are Zn 2p, Sn 3d, and oxygen (O 1s) and environmental yield peaks such as nitrogen (N) and carbon(C).

[figure(s) omitted; refer to PDF]

The XPS spectrum full scan of ZnO-SnO₂ is depicted in Figure 6(a). Two incredible bands at binding energies (BE) of 1022.86 and 1044.68 eV, attributed to Zn 2p_3/2 and Zn 2p_1/2 states, respectively, are seen in the XPS survey spectrum (Figure 6(b)). Furthermore, Zn²⁺ valance state with spin-orbit splitting in the binding energy of 22.82 eV is confirmed by detected XPS peaks [22]. Furthermore, one conspicuous peak at BE of 531.32 eV in oxygen (O) XPS survey spectrum is linked to the appearance of oxygen (O) for 1s state as shown in Figure 6(c) [23] with an oxidation state of O^2–, indicating the effect of the O element in the Zn-O bond structure. The XPS survey spectrum of the Sn element is shown in Figure 6(d). It has two prominent peaks at BE of 486.36 eV and one smaller peak at 494.54 eV. The 3d_5/2 and 3d_3/2 states of Sn are assigned to these peaks, respectively, and their corresponding crests permit the occurrence of Sn²⁺ states with a binding energy shift of 8.18 eV [24]. Furthermore, the environment’s influence results in the production of extra shoulder peaks designated for the N and C components. The Z-S heterostructure component elements’ availability and chemical valance states are therefore thoroughly confirmed by XPS analysis.

3.4. UV-DRS Analysis

UV-DRS analysis is used to calculate optical bandgap and study the structure of electronic bands in solid materials. UV-Vis spectra are described in terms of the amount of light reflected at a certain wavelength regime. UV-DRS spectra of pristine ZnO, SnO₂, and Z-S heterostructure and Tauc plot of Z-S heterostructure are presented in Figure 7. Absorbance spectra are recorded at ambient temperature conditions in the wavelength range of 200 to 700 nm. As shown in Figure 7(a), absorption edge of light is identified for the as-prepared samples (pure ZnO and SnO₂) by extrapolating the curve and noted the respective optical bandgap using the below expression [25], and Figure 7(b) ascribed the Tauc plot for knowing the optical bandgap of Z-S heterostructure.$$\begin{array}{}\text{(4)}& \text{Using\hspace{0.17em}absorption\hspace{0.17em}edge},\text{Optical\hspace{0.17em}bandgap}{\mathrm{E}}_{\mathrm{g}}=\frac{1240}{\lambda }\text{eV}\end{array}$$where λ ⟶ absorption wavelength (in nm)

[figure(s) omitted; refer to PDF]

As shown in Figure 7(a), ZnO, SnO₂, and Z-S heterostructure absorption edges are located around at 368 nm, 344 nm, and 436 nm, respectively, and calculated bandgaps are 3.37 eV, 3.6 eV, and 2.84 eV, respectively. Further, it is observed that ZnO and Z-S heterostructure absorption edges are shifted in the direction of higher wavelength side called as red shift. This shift concludes the presence of Sn-flake-like nanoparticles on the ZnO nanorod-like structure, which confirms the substantial formation of Z-S heterostructure [26]. From DRS-spectra, it is clear that a significant peak is observed around at 370 nm due to near band-edge emission (NBE) which impacts the recombination rate of photogenerated charge carriers, and noticeable peak is located around at 510 nm ascribed to oxygen vacancies (V_Ox). For Z-S heterostructure, the greater PCA is maintained due to the less density of interfacial defects and a suitable separation of charge carriers, i.e., electrons and holes (e⁻-h⁺) pairs [27]. Thus, Z-S heterostructure is perceived to display the utmost PCA to show its augmented structure and also observed that presence of O-vacancies (V_Ox) is anticipated to increase the capability of absorption and PCA under visible light irradiation.

3.5. Photocatalytic Activity Studies

Protecting human health and aquatic habitats requires the decontamination of organic contaminants like MB dye in wastewater contaminated via various industrial effluents. At dosage greater than 5 mg/kg, the monoamine oxidase inhibitory characteristics of MB dye can induce fatal serotonin toxicity in humans, apart from being a threat to fauna in the aquatic ecosystem. Thus, it is highly imperative to eliminate MB dye from wastewaters. MB dye is toxic, cancer-causing, and nonrecyclable that creates problems to human well-being and environmental security. Hence, our target in the present investigation is to accomplish strong photocatalytic performance under presence of visible light irradiation in order to breakdown MB model dye in waste water in a time period of 120 minutes. Initially, the time dependent UV-Vis absorption spectrum for MB dye is found to exhibit maximum absorption at 663 nm as presented in Figure 8(a). It is also observed that the absorbance corresponding to the MB molecule is appearing as the centred main peak is found to continuously decrease without any shift. However, it is found to be diminished after completion of the total irradiation time. Further, the first-order pseudokinetic behaviour of the as-prepared samples is depicted in Figure 8(b).

[figure(s) omitted; refer to PDF]

As described in figure, ZOS NC exhibits better kinetic behaviour rather than bare samples in 120 minutes of illumination time due to the active participation of the respective NC in speeding up the reaction as well superior degradation of the dye molecule. Figure 9 depicts the variation of bandgap and photocatalytic performance of as-prepared ZnO, SnO₂, and Z-S heterostructure. Figure 9(a) shows the trend of energy bandgap for the ZnO, SnO₂, and Z-S heterostructure is found to be 3.37, 3.60, and 2.84 eV, respectively. It is witnessed that the bandgap of the SnO₂ is found to be greater than that of ZnO. Moreover, observed increase of energy bandgap is claimed to approve the quantum detention due to less particle size (17.82 nm) [28]. The high density of interfacial defects (i.e., grain boundaries, stacking faults, and external surfaces) and an electron-hole recombination in SnO₂ leads to fall in photocatalytic activity when compared to ZnO [29]. Further, in Z-S heterostructure, the enhanced PCA is observed due to the less bandgap (2.84 eV), less density of interfacial defects/O-vacancies, and maximal separation of photogenerated charge carriers [30]. Further, Figure 9(b) represents the photocatalytic degradation efficiency of as-synthesized samples in MB dye under visible light irradiation against the illumination time of 120 minutes. The order of percentage of degradation efficiency of prepared NCs is Z-S heterostructure (82.45) > ZnO (38.55) > SnO₂(32.68) in presence of visible light irradiation. In case of absence of catalyst (i.e., blank condition), no discernible degradation (i.e., only 11.2%) of MB dye in aqueous solution is observed. For ZnO, moderate MB dye degradation (i.e., 38.55%) is observed when compared to blank condition due to the ability of ZnO nanoparticles which attributes to damage the molecular bondage of MB dye (through hydrolysis process) in waste water which leads to generation of strong free radicals such as hydroxyl and super oxide ions [31]. The strongest MB dye degradation (i.e., 82.45%) is observed for Z-S heterostructure catalyst due to the expanded, i.e., a stretched surface area which acquires by the catalyst in the nanophased regime. In the nanophased region, materials retain large count of active catalyst spots which are escorted by the composite nanoparticles (NPs) which debated to lead to robust degradation efficiency; hence, composite materials in the nanophased structure would be favourable for the degradation of organic contaminates in wastewater [32]. Further, the enhancement of degradation for Z-S heterostructure is owing to extreme separation of charge carriers, i.e., electron and hole pairs on the surface catalyst material and deposition of SnO₂ nanoflakes on the surface of ZnO nano rod-like structure (as shown in TEM micrographs) which leads to the development of a novel ZnO/SnO₂ heterostructure that facilitate the greater photocatalytic MB dye degradation under visible light irradiation and also in Z-S heterostructure huge number of catalyst sites are interact with incident light (called plasmonic effect) which results in minimizing the interfacial defects on the surface of catalyst composite would be evidence for obtain greater photodegradation efficiency [33]. First-order pseudokinetic expression elucidates the kinetic behaviour (speed-up the reaction) of as-synthesized samples in terms of $C_0$, C, k and t is as follows [34]:$$\begin{array}{c}\text{(5)}& \mathrm{C}={\mathrm{C}}_0\exp -\text{kt}\text{Rate\hspace{0.17em}constant\hspace{0.17em}}k=\frac{-\log C/C_0}{t}{\min }^{-1}\end{array}$$where ${\mathrm{C}}_0$ = initial MB absorbance, C = MB absorbance during time intervals and t = time of illumination.

[figure(s) omitted; refer to PDF]

The kinetic rate constant values of as-prepared samples are presented in Figure 9(c). Predictable kinetic rate constants of synthesized samples such as ZnO, SnO₂, and Z-S heterostructure are 0.0024, 0.0018, and 0.0072 min⁻¹, respectively. Further, Z-S heterostructure is found to display a high kinetic rate constant to conclude its superior efficiency to degrade the MB dye than other all-synthesized samples. High kinetic rate value of Z-S heterostructure infers to the presence of oxygen and increase the light intensity (i.e., effect of wavelength is associated with the structure MB dye) which may lead to increase the photodegradation ability of catalyst material [35]. The statistics related to absorption edge (in nm), bandgap (in eV), rate constant (min⁻¹), and degradation efficiency (in %) of as-synthesized samples are depicted in Table.2.

Table 2

The statistics related to absorption edge (nm), bandgap (eV), rate constant (min⁻¹), and degradation efficiency (%) of as-synthesized samples.

Sample	Absorbance edge (nm)	Optical bandgap (eV)	Rate constant (min−1)	Degradation efficiency (%)
ZnO	367	3.37	0.0024	38.55
SnO2	344	3.60	0.0018	32.68
Z-S	436	2.84	0.0072	82.45

The reusability test result for Z-S heterostructure is displayed in Figure 9(d). Numerous practical applications make use of nanocomposites’ reusability and recycling. For this reason, Z-S heterostructure is subjected to a three-cycle recycling test in the presence of visible light in the current investigation. The findings of the reusability test suggested that the MB dye’s degradation efficiency in the Z-S heterostructure decreased progressively from the first to the third cycle, eventually stabilizing around at 81%. These findings firmly support the notion that the Z-S heterostructure continued to be beneficial in a range of real-world uses. The aforementioned findings suggest that when exposed to visible light, Z-S heterostructure has excellent light absorption capabilities and a remarkable reaction to photocatalytic activity [36]. Additionally, it is expected that the synthesized photocatalyst composite will be used to remove as many organic pollutants/dyes/pigments from wastewater and marine organisms as possible. As a result, the current investigation will now be concentrated on preserving the aquatic ecosystems cycle and providing civilization with safe drinking water. In this way, prepared Z-S heterostructure shall promote the degradation efficiency of MB dye as comparable as and/or higher than the previous reports. Further, the distinguished reported data pertaining to the Z-S heterostructure and their allied 2D-heterostructure against the degradation of MB dye are presented in Table 3.

Table 3

The distinguished reported data pertaining to the Z-S heterostructure and their allied 2D-heterostructure against the degradation of MB dye.

Photocatalyst	Preparation Method	Mode Dye	Light Source	Illumination time (min)	Photodegradation Efficiency (%)	References
SnO2/ZnO	Sol-gel	MB	Visible	120	100	[37]
SnO2/ZnO	Wet chemical technology	MB	Visible	120	99	[38]
ZnO doped SnO2	Hydrothermal	MB	UV	90	75	[39]
SnO2 doped ZnO	Electrospinning	MB	Visible	360	49	[40]
ZnO-SnO2	Sol-gel	MB	Visible	180	88	[41]
ZnO-SnO2	Hydrothermal	MB	UV	240	30	[42]
Ag doped SnO2	Sol-gel	MB	UV	120	90	[43]
ZnO-SnO2	Coprecipitation	MB	UV	60	96	[44]
ZnO-SnO2	Precipitate decompose	MB	UV	60	100	[45]
SnO2/ZnO/TiO2	Sol-gel and solid state	MB	UV	40	85	[46]
ZnO-SnO2 nanocomposite	Hydrothermal	MB	Visible	120	82.4	Present Work

The results presented in the above table are well balanced when compared with our synthesized catalyst material. Particularly, our proposed ZOS catalyst materials display superior degradation efficiency than that of other reported papers such as [40–42]. The enhancement in efficiency is owing to hydrothermal conditions such as high temperature and high pressure followed by calcination. Also, the hydrothermal method can generate nanomaterials with high vapour pressures with negligible material damage, and no residual contaminates during this process. Hydrothermal conditions also stimulate the electrons and holes in a semiconductor heterostructure towards reduced bandgap and optimal separation, which are favourable conditions to obtain better degradation efficiency. Further, in this method, final yield can be very pure and exhibit superior structural, morphological, and optical properties along with photocatalytic activity. The novelty in the current investigation is anticipation of novel heterostructure (p/p-type) with enhanced catalytic activity under hydrothermal conditions, and also, annihilation will also evidence for the generation of large number of induced charge carriers and subsequently generation of internal electric field. These factors highly influence the ZOS NC towards achieving superior photocatalytic performance in presence of light irradiation. Hence, our proposed ZOS heterostructure catalyst plays a substantial role in the process of degradation of hazardous pollutants immersed in the wastewater upon light irradiation.

3.6. Mechanism of Charge Carrier Transformation (Reaction Process)

The proposed charge transfer mechanism of MB dye in occurrence of Z-S heterostructure is presented in Figure 10. The ability of charge carriers to separate on the surface of the Z-S heterostructure and the photocatalyst bandgap are the primary factors influencing the degradation reaction process [47]. As depicted in the figure, in presence of visible light, irradiation photogenerated charge carriers are transferred between conduction band (CB) and valance band (VB) of ZnO and SnO₂ materials based on the fermi energy level position and their tunable bandgap nature. Predominantly in presence of light, electrons in CB of ZnO are migrated to CB of SnO_2, and holes get diffused from VB of SnO₂ to VB of ZnO. Due to these carrier transformations, a small magnitude of electric field (charge free region) is generated at the interface of Z-S heterostructure, which is ascribed to thermal stability and creation of Z-S heterostructure photocatalyst heterostructure [48]. Thus, the generation of a novel Z-S heterostructure encourages the less recombination of photogenerated charge carriers (i.e., optimal separation of charge carriers) which is much beneficial to obtain superior photocatalytic dye degradation efficiency.

[figure(s) omitted; refer to PDF]

Further, the step-wise chemical reaction procedure of MB degradation under Z-S heterostructure photocatalyst in presence of visible light radiation is presented as follows [49]:

  ZnO/SnO₂ + hν (light) ⟶ ZnO (e⁻) (CB) + SnO₂ (h⁺) (VB)

  ZnO (e⁻) (CB) + O₂ (Oxygen) ⟶ O₂^•− (super oxide scavengers)

  O₂^•− + H₂O ⟶ HO₂^• + OH^•− (hydroxyl scavengers)

  HO₂^• + H₂O ⟶ OH^• + H₂O₂

  H₂O₂ ⟶ 2OH^•

  SnO₂ (h⁺) (VB) + OH⁻ ⟶ OH^• (hydroxyl scavengers)

  OH^• + MB (dye) ⟶ H₂O + CO₂

  h⁺ (holes) + MB (dye molecule) ⟶ CO₂ + H₂O (nonharmful yields)

As depicted in the above reaction process, electrons and holes are separated due to involvement of light irradiation, and electrons in CB of SnO₂ will react with the oxygen molecule in water and generates super oxide scavengers (O₂^•−) as well holes react with water molecules to produce hydroxyl scavengers (OH^•−) [26]. Further, these strong O₂^•− super oxide free radicals react with Mb dye molecule to decontaminate the water, and hydroxyl ions (OH^•−) also react with dye molecule through hydrolysis process to form nonharmful degradation yields such as carbon dioxide (CO₂) and water (H₂O) at the end of reaction process [50]. Further, the role of free radicals in the process of degradation is examined through the scavenger test and the same is also presented in Figure 11. As presented in figure, significant dye degradation is observed with isopropanol (IP) agent, i.e., hydroxyl ions play a vibrant participation in the degradation process than that of other agent benzoquinone (BQ) which infer the passive role of super oxide ions.

[figure(s) omitted; refer to PDF]

In this way, the synthesized Z-S heterostructure is efficiently degraded the MB dye molecule in wastewater under visible light illumination, and also, the current work is a small attempt towards supplying the safe drinking water to people and aquatic world. Further, Figure 12 represents the degradation pathway of MB dye towards decontaminating the water by producing nonharmful products (CO₂ and H₂O) in presence of visible light irradiation.

[figure(s) omitted; refer to PDF]

4. Conclusions

In summary, 2D-ZnO/SnO₂ heterostructure synthesized by a facile one step hydrothermal method at 230°C for 12 hours followed by high-temperature calcination is derived following findings: XRD analysis reveals that ZnO and SnO₂ exhibit hexagonal wurtzite structure and tetragonal rutile structure with a mean crystallite size of 26.58 and 17.82 nm, respectively. TEM micrographs infer the nanorod-like structure of ZnO, nanoflake-like structure of SnO₂, and a thin nanosheet-like structure of Z-S heterostructure, and more agglomeration trend is also observed. XPS analysis evidence for the presence of constitute elements and Zn⁺², Sn⁺², O⁻² valance states along with environmental peaks in optimized Z-S heterostructure. UV-DRS analysis infers the optical bandgap of ZnO (3.37 eV), SnO₂ (3.6 eV), and Z-S heterostructure (2.84 eV), and also evidence for the red shift. Optical band gap of Z-S heterostructure decreases than that of pure ZnO and SnO₂, which is due to the incursion of defects and/or oxygen vacancies. The photocatalytic degradation efficiency of MB dye in the pure ZnO, SnO₂, and Z-S heterostructure under visible light illumination for 2 hours is 38.55, 32.68, and 82.45%. Hydroxyl ions play a crucial role in the degradation process of MB dye against visible light illumination. Enhanced photocatalytic activity in Z-S heterostructure is owing to greatest separation of charge carriers and interface of a novel heterostructure.

Acknowledgments

The authors gratefully acknowledge the GMR Institute of Technology, Rajam.

References

[1] H. Kumari, Sonia, A. Kumar, Parmar, R. Suman, R. Ranga, S. Chahal, S. Devi, S. Sharma, S. Kumar, P. Kumar, S. Kumar, R. Parmar, "A review on photocatalysis used for wastewater treatment: dye degradation," Water, Air, and Soil Pollution, vol. 234 no. 6,DOI: 10.1007/s11270-023-06359-9, 2023.

[2] U. L. Usman, B. K. Allam, S. Sajad, N. B. Singh, S. Banerjee, "Plant leaves extract assisted eco-friendly fabrication of ZnO-SnO 2 @Chitosan for UV-induced enhanced photodegradation of single and ternary mixtures of Rhodamine B," Nano-Structures & Nano-Objects, vol. 37,DOI: 10.1016/j.nanoso.2023.101072, 2024.

[3] S. Lotfi, M. E. Ouardi, H. A. Ahsaine, A. Assani, "Recent progress on the synthesis, morphology and photocatalytic dye degradation of BiVO4 photocatalysts: a review," Catalysis Reviews, vol. 66 no. 1, pp. 214-258, DOI: 10.1080/01614940.2022.2057044, 2022.

[4] G. Murugadoss, S. Muruganandam, M. R. Kumar, A. Pugazhendhi, "Ultrafast photodegradation of textile industry dyes using efficient sulfide based ternary nanocomposites," Chemosphere, vol. 336,DOI: 10.1016/j.chemosphere.2023.139100, 2023.

[5] K. Selvakumar, T. H. Oh, T. Vijayaraj, K. Gokul Raja, M. Swaminathan, "Facile construction of sandwich-like composited Sm 2 MoO 6 /ZnO/rGO and its activity in photodecomposition of ibuprofen," Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 650,DOI: 10.1016/j.colsurfa.2022.129545, 2022.

[6] E. Kowsari, M. R. Ghezelbash, "Ionic liquid-assisted, facile synthesis of ZnO/SnO 2 nanocomposites, and investigation of their photocatalytic activity," Materials Letters, vol. 68, pp. 17-20, DOI: 10.1016/j.matlet.2011.10.028, 2012.

[7] P. Junploy, S. Thongtem, T. Thongtem, A. Phuruangrat, "Photocatalytic activity of Zn 2 SnO 4 -SnO 2 nanocomposites produced by sonochemistry in combination with high temperature calcination," Superlattices and Microstructures, vol. 74, pp. 173-183, DOI: 10.1016/j.spmi.2014.06.015, 2014.

[8] P. Dammala, J. Machado, B. Rani, S. Murali, S. Devi, M. Niraj Luwang, N. K. Sahu, "Synthesis of biphasic nanomaterials based on ZnO and SnO 2 : application towards photocatalytic degradation of acid red dye," Nano-Structures & Nano-Objects, vol. 18,DOI: 10.1016/j.nanoso.2019.100292, 2019.

[9] S. Sabbaghi, F. Doraghi, "Photo-Catalytic degradation of Methylene blue by ZnO/SnO 2 nanocomposite," Journal of Water and Environmental Nanotechnology, vol. 1, pp. 27-37, 2016.

[10] S. Kappadan, S. Thomas, N. Kalarikkal, "BaTiO 3 /ZnO heterostructured photocatalyst with improved efficiency in dye degradation," Materials Chemistry and Physics, vol. 255,DOI: 10.1016/j.matchemphys.2020.123583, 2020.

[11] M. Li, Y. Hu, S. Xie, Y. Huang, Y. Tong, X. Lu, "Heterostructured ZnO/SnO 2-x nanoparticles for efficient photocatalytic hydrogen production," Chemical Communications, vol. 50 no. 33, pp. 4341-4343, DOI: 10.1039/c3cc49485c, 2014.

[12] A. Kumar, B. Subash, B. Krishnakumar, A. J. F. N. Sobral, K. R. Sankaran, "Synthesis, characterization and excellent catalytic activity of modified ZnO photocatalyst for RR 120 dye degradation under UV A and solar light illumination," Journal of Water Process Engineering, vol. 13,DOI: 10.1016/j.jwpe.2016.07.009, 2016.

[13] S. Harish, J. Archana, M. Navaneethan, A. Silambarasan, K. D. Nisha, S. Ponnusamy, C. Muthamizhchelvan, H. Ikeda, D. K. Aswal, Y. Hayakawa, "Enhanced visible light induced photocatalytic activity on the degradation of organic pollutants by SnO nanoparticle decorated hierarchical ZnO nanostructures," RSC Advances, vol. 6 no. 92, pp. 89721-89731, DOI: 10.1039/c6ra19824d, 2016.

[14] A. Enesca, Y. Yamaguchi, C. Terashima, A. Fujishima, K. Nakata, A. Duta, "Enhanced UV-Vis photocatalytic performance of the CuInS 2 /TiO 2 /SnO 2 heterostructure for air decontamination," Journal of Catalysis, vol. 350, pp. 174-181, DOI: 10.1016/j.jcat.2017.02.015, 2017.

[15] A. Raja, N. Son, M. Kang, M. Kang, "Fabrication of graphene-incorporated tin-niobate-doped zinc oxide for highly enhanced photocatalytic hydrogen evolution and degradation," Journal of Alloys and Compounds, vol. 987,DOI: 10.1016/j.jallcom.2024.174129, 2024.

[16] K. Gupta, B. Sharma, V. Garg, P. P. Neelratan, V. Kumar, D. Kumar, S. K. Sharma, "Enhanced photocatalytic degradation of methylene blue and methyl orange using biogenic ZnO NPs synthesized via Vachellia nilotica (Babool) leaves extract," Hybrid advances, vol. 5,DOI: 10.1016/j.hybadv.2024.100160, 2024.

[17] G. T. Rao, R. V. S. S. N. Ravikumar, "Novel Fe-doped ZnO-CdS nanocomposite with enhanced visible light-driven photocatalytic performance," Materials Research Innovations, vol. 25 no. 4, pp. 215-220, DOI: 10.1080/14328917.2020.1774726, 2020.

[18] L. F. da Silva, J. C. M’Peko, A. C. Catto, S. Bernardini, V. R. Mastelaro, K. Aguir, C. Ribeiro, E. Longo, C. Ribeiro, E. Longo, "UV-Enhanced ozone gas sensing response of ZnO-SnO 2 hetero-junctions at room temperature," Sensors and Actuators B: Chemical, vol. 240, pp. 573-579, DOI: 10.1016/j.snb.2016.08.158, 2017.

[19] S. Khanchandani, S. Kundu, A. Patra, A. K. Ganguli, "Band gap tuning of ZnO/In2S3 core/shell nanorod arrays for enhanced visible-light-driven photocatalysis," Journal of Physical Chemistry C, vol. 117 no. 11, pp. 5558-5567, DOI: 10.1021/jp310495j, 2013.

[20] B. Babu, V. V. N. Harish, J. Shim, C. V. Reddy, "Solution combustion synthesis of SnO2–NiO p–n heterojunction nanocomposite for photocatalytic application," Journal of Materials Science: Materials in Electronics, vol. 29 no. 19, pp. 16988-16996, DOI: 10.1007/s10854-018-9794-5, 2018.

[21] N. T. Hanh, N. Le Minh Tri, D. Van Thuan, M. H. Thanh Tung, T. D. Pham, T. D. Minh, H. T. Trang, M. T. Binh, M. V. Nguyen, "Monocrotophos pesticide effectively removed by novel visible light driven Cu doped ZnO photocatalyst," Journal of Photochemistry and Photobiology A: Chemistry, vol. 382,DOI: 10.1016/j.jphotochem.2019.111923, 2019.

[22] H. Wang, J. Yu, X. Zhan, L. Chen, Y. Sun, H. Shi, "Direct 2D/2D Z-scheme SnNb 2 O 6 /ZnO hybrid photocatalyst with enhanced interfacial charge separation and high efficiency for pollutants degradation," Applied Surface Science, vol. 528,DOI: 10.1016/j.apsusc.2020.146938, 2020.

[23] Y. Liu, Y. Zhou, X. Zhou, X. Jin, B. Li, J. Liu, G. Chen, "Cu doped SnS 2 nanostructure induced sulfur vacancy towards boosted photocatalytic hydrogen evolution," Chemical Engineering Journal, vol. 407,DOI: 10.1016/j.cej.2020.127180, 2021.

[24] M. M. Sajid, N. A. Shad, Y. Javed, S. B. Khan, Z. Zhang, N. Amin, H. Zhai, "Preparation and characterization of Vanadium pentoxide (V 2 O 5 ) for photocatalytic degradation of monoazo and diazo dyes," Surfaces and Interfaces, vol. 19,DOI: 10.1016/j.surfin.2020.100502, 2020.

[25] G. T. Rao, R. J. Stella, B. Babu, K. Ravindranadh, C. Venkata Reddy, J. Shim, R. V. S. S. N. Ravikumar, "Structural, optical and magnetic properties of Mn 2+ doped ZnO-CdS composite nanopowder," Materials Science and Engineering: B, vol. 201, pp. 72-78, DOI: 10.1016/j.mseb.2015.06.012, 2015.

[26] J. Zhou, Z. Zhang, X. Kong, F. He, R. Zhao, R. Wu, T. Wei, L. Wang, J. Feng, "A novel P-N heterojunction with staggered energy level based on ZnFe 2 O 4 decorating SnS 2 nano sheet for efficient photocatalytic degradation," Applied Surface Science, vol. 510,DOI: 10.1016/j.apsusc.2020.145442, 2020.

[27] A. B. Makama, A. Salmiaton, E. B. Saion, T. S. Y. Choong, N. Abdullah, "Microwave-Assisted synthesis of porous ZnO/SnS 2 heterojunction and its enhanced photoactivity for water purification," Journal of Nanomaterials, vol. 2015,DOI: 10.1155/2015/108297, 2015.

[28] S. Jayswal, R. S. Moirangthem, "Construction of a solar spectrum active SnS/ZnO p–n heterojunction as a highly efficient photocatalyst: the effect of the sensitization process on its performance," New Journal of Chemistry, vol. 42 no. 16, pp. 13689-13701, DOI: 10.1039/c8nj02098a, 2018.

[29] G. Dharmana, P. S. R. Masabattula, D. M. Potukuchi, "Enhanced photocatalytic activity in hydro-thermally grown nano structured ZnO/SnS core-shell composites," Zeitschrift für Naturforschung A, vol. 77 no. 2, pp. 153-169, DOI: 10.1515/zna-2021-0209, 2022.

[30] G. Dharmana, M. P. Srinivasa Rao, D. M. Potukuchi, "Visible light driven robust photocatalytic activity in vanadium doped ZnO/SnS core-shell nanocomposites for decolorization of MB dye towards wastewater treatment," Inorganic and Nano-Metal Chemistry, vol. 52 no. 8, pp. 1059-1076, DOI: 10.1080/24701556.2022.2075386, 2022.

[31] T. Munawar, F. Iqbal, S. Yasmeen, K. Mahmood, A. Hussain, "Multi metal oxide NiO-CdO-ZnO nanocomposite-synthesis, structural, optical, electrical properties and enhanced sunlight driven photocatalytic activity," Ceramics International, vol. 46 no. 2, pp. 2421-2437, DOI: 10.1016/j.ceramint.2019.09.236, 2020.

[32] M. Masjedi-Arani, M. Salavati-Niasari, "Metal (Mn, Co, Ni and Cu) doped ZnO-Zn 2 SnO 4 -SnO 2 nanocomposites: green sol-gel synthesis, characterization and photocatalytic activity," Journal of Molecular Liquids, vol. 248, pp. 197-204, DOI: 10.1016/j.molliq.2017.10.055, 2017.

[33] A. H. Elsheikh, J. Yu, R. Sathyamurthy, M. M. Tawfik, S. Shanmugan, F. A. Essa, "Improving the tribological properties of AISI M50 steel using SnS/ZnO solid lubricants," Journal of Alloys and Compounds, vol. 821,DOI: 10.1016/j.jallcom.2019.153494, 2020.

[34] S. Khanchandani, S. Kundu, A. Patra, A. K. Ganguli, "Shell thickness dependent photocatalytic properties of ZnO/CdS core-shell nanorods," Journal of Physical Chemistry C, vol. 116 no. 44, pp. 23653-23662, DOI: 10.1021/jp3083419, 2012.

[35] G. Dharmana, T. R. Gurugubelli, P. S. R. Masabattula, B. babu, K. Yoo, "Facile synthesis, characterization, and photocatalytic activity of hydrothermally grown Cu 2+ doped ZnO-SnS nanocomposites for MB dye degradation," Catalysts, vol. 12 no. 3,DOI: 10.3390/catal12030328, 2022.

[36] L. Xu, F. Xian, Y. Zhang, W. Wang, K. Qiu, J. Xu, "Synthesis of ZnO-decorated SnO2 nanopowder with enhanced photocatalytic performance," Optik, vol. 194,DOI: 10.1016/j.ijleo.2019.162965, 2019.

[37] M. Ali, O. Qreshah, A. A. Ismail, F. A. Harraz, H. Algarni, M. Faisal, W. S. Chiu, "Influence of annealing temperature on photocatalytic and electrochemical sensing properties of SnO 2 /ZnO nanocomposites," International Journal of Electrochemical Science, vol. 13 no. 7, pp. 6626-6642, DOI: 10.20964/2018.07.200, 2018.

[38] M. Faisal, A. A. Ibrahim, F. A. Harraz, H. Bouzid, M. Al-Assiri, A. A. Ismail, "SnO2 doped ZnO nanostructures for highly efficient photocatalyst," Journal of Molecular Catalysis A: Chemical, vol. 397, pp. 19-25, DOI: 10.1016/j.molcata.2014.10.027, 2015.

[39] R. Lamba, A. Umar, S. K. Mehta, S. K. Kansal, "ZnO doped SnO2 nanoparticles heterojunction photo-catalyst for environmental remediation," Journal of Alloys and Compounds, vol. 653, pp. 327-333, DOI: 10.1016/j.jallcom.2015.08.220, 2015.

[40] P. Pascariu, A. Airinei, N. Olaru, L. Olaru, V. Nica, "Photocatalytic degradation of Rhodamine B dye using ZnO-SnO2 electrospun ceramic nanofibers," Ceramics International, vol. 42 no. 6, pp. 6775-6781, DOI: 10.1016/j.ceramint.2016.01.054, 2016.

[41] D. Torres Martínez, R. Castanedo Pérez, G. Torres Delgado, O. Zelaya Ángel, "Structural, morphological, optical and photocatalytic characterization of ZnO–SnO 2 thin films prepared by the sol–gel technique," Journal of Photochemistry and Photobiology A: Chemistry, vol. 235, pp. 49-55, DOI: 10.1016/j.jphotochem.2012.03.009, 2012.

[42] L. F. Da Silva, O. F. Lopes, A. C. Catto, W. Avansi, M. I. B. Bernardi, M. S. Li, C. Ribeiro, E. Longo, "Hierarchical growth of ZnO nanorods over SnO2 seed layer: insights into electronic properties from photocatalytic activity," RSC Advances, vol. 6 no. 3, pp. 2112-2118, DOI: 10.1039/c5ra23824b, 2016.

[43] M. A. Ahmed, M. A. Messih, E. El-Sherbeny, S. F. El-Hafez, A. M. M. Khalifa, "Synthesis of metallic silver nanoparticles decorated mesoporous SnO2 for removal of methylene blue dye by coupling adsorption and photocatalytic processes," Journal of Photochemistry and Photobiology A: Chemistry, vol. 346, pp. 77-88, DOI: 10.1016/j.jphotochem.2017.05.048, 2017.

[44] Y. J. Chiang, C. C. Lin, "Photocatalytic decolorization of methylene blue in aqueous solutions using coupled ZnO/SnO2 photocatalysts," Powder Technology, vol. 246, pp. 137-143, DOI: 10.1016/j.powtec.2013.04.033, 2013.

[45] V. Kuzhalosai, B. Subash, A. Senthilraja, P. Dhatshanamurthi, M. Shanthi, "Synthesis, characterization and photocatalytic properties of SnO2-ZnO composite under UV-A light," Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, vol. 115, pp. 876-882, DOI: 10.1016/j.saa.2013.06.106, 2013.

[46] G. Yang, Z. Yan, T. Xiao, "Preparation and characterization of SnO2/ZnO/TiO2 composite semiconductor with enhanced photocatalytic activity," Applied Surface Science, vol. 258 no. 22, pp. 8704-8712, DOI: 10.1016/j.apsusc.2012.05.078, 2012.

[47] Y. Shavisi, S. Sharifnia, Z. Mohamadi, "Solar-light-harvesting degradation of aqueous ammonia by CuO/ZnO immobilized on pottery plate: linear kinetic modeling for adsorption and photocatalysis process," Journal of Environmental Chemical Engineering, vol. 4 no. 3, pp. 2736-2744, DOI: 10.1016/j.jece.2016.04.035, 2016.

[48] H. R. Pant, C. H. Park, B. Pant, L. D. Tijing, H. Y. Kim, C. S. Kim, "Synthesis, characterization, and photocatalytic properties of ZnO nano-flower containing TiO2 NPs," Ceramics International, vol. 38 no. 4, pp. 2943-2950, DOI: 10.1016/j.ceramint.2011.11.071, 2012.

[49] S. J. Lee, H. J. Jung, R. Koutavarapu, S. H. Lee, M. Arumugam, J. H. Kim, M. Y. Choi, "ZnO supported Au/Pd bimetallic nanocomposites for plasmon improved photocatalytic activity for methylene blue degradation under visible light irradiation," Applied Surface Science, vol. 496,DOI: 10.1016/j.apsusc.2019.143665, 2019.

[50] T. R. Gurugubelli, R. V. S. S. N. Ravikumar, R. Koutavarapu, "Enhanced photocatalytic activity of ZnO-CdS composite nanostructures towards the degradation of rhodamine B under solar light," Catalysts, vol. 12 no. 1,DOI: 10.3390/catal12010084, 2022.