Study of photophysical properties, photodegradation breakdown cost and recycling processes of Zn/SnO2 quantum dots via real industrial wastewater treatment

Zahra H. Alhalafi; Marwah A. Alsharif; Halimah Alahmari; Deemah M. Alenazy; Hussain Alessa; Ahmed M Hameed; Sraa Abu-Melha; Nashwa El-Metwaly

doi:10.25259/AJC_756_2025

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Original Article

:19;

7562025

doi:

10.25259/AJC_756_2025

Study of photophysical properties, photodegradation breakdown cost and recycling processes of Zn/SnO₂ quantum dots via real industrial wastewater treatment

Zahra H. Alhalafi^a, Marwah A. Alsharif^b, Halimah Alahmari^c, Deemah M. Alenazy^d, Hussain Alessa^e, Ahmed M Hameed^e, Sraa Abu-Melha^f,*, Nashwa El-Metwaly^e

aDepartment of Chemistry, College of Science, University of Hafr Al Batin, Hafr Al Batin, Saudi Arabia

bDepartment of Physics, College of Science, University of Tabuk, Tabuk, Saudi Arabia

cDepartment of Chemistry, College of Science and Humanities in Al-Kharj, Prince Sattam Bin Abdulaziz University, Al-Kharj, Saudi Arabia

dDepartment of Chemistry, College of Science, Northern Border University, Arar, Saudi Arabia

eDepartment of Chemistry, Faculty of Science, Umm Al-Qura University, Makkah, Saudi Arabia

fDepartment of Chemistry, Faculty of Science, King Khalid University, Abha, Saudi Arabia

*Corresponding author: E-mail address: sabomlha@kku.edu.sa (S. Abu-Melha)

Received: 2025-07-01, Accepted: 2025-09-11, Epub ahead of print: 2026-02-16, Published: 2026-04-02

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Abstract

The present study investigates the synthesis, photophysical behavior, and photodegradation efficacy of SnO₂ and zinc-doped SnO₂ quantum dots (QDs) prepared via an ultrasonic-chemical method, applied to the degradation of industrial dye pollutants in real wastewater matrices. Two distinct calcination temperatures (275°C and 500°C) produced SnO₂ QDs with crystallite sizes averaging 3.80 nm (SnQD1) and 6.10 nm (SnQD2), respectively, confirmed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Zinc doping was performed at concentrations of 3% and 7% (SnZn1 and SnZn2), resulting in larger average particle sizes (9.3 nm and 10.2 nm). The observed quantum confinement effect is evidenced by energies band gap values ranging from 3.05 eV to 3.38 eV, as measured via UV-Vis diffuse reflectance spectroscopy (DRS) and Kubelka-Munk analysis, with Zn doping inducing mid-gap states that favor visible-light absorption and charge carrier separation. Photocatalytic degradation kinetics of Dianix blue dye revealed a pseudo-first-order reaction model with SnQD1 exhibiting a 43% higher rate constant (14.63 × 10⁻³ s⁻¹) than SnQD2 (10.22 × 10⁻³ s⁻¹), attributable to its larger Brunauer-Emmett-Teller (BET) surface area and smaller size (144 m² g^-1 vs. 106 m² g^-1). Remarkably, SnZn1 outperformed all samples with a rate constant of 17.15 × 10⁻³ s⁻¹, demonstrating a 230% enhancement over SnZn2, underscoring the critical role of optimal doping in photocatalytic efficiency. Reactive species trapping identified hydroxyl radicals and photogenerated electrons as the principal oxidative agents, corroborating mechanistic pathways widely reported in semiconductor photocatalysis. The catalysts’ efficacy extended to simulated solar photodegradation of real industrial wastewater, where chemical oxygen demand (COD) reduction ranged from 55% to over 90%, surpassing regulatory thresholds for discharge. Recyclability tests over seven cycles confirmed the robust stability of SnQD1 and SnZn1, with minimal performance degradation and preserved structural integrity demonstrated by Fourier transform infrared (FTIR) analysis. An economic assessment revealed that SnZn1 and SnQD1 offered the most cost-effective treatment, reducing photodegradation operational costs by approximately 23% and 9%, respectively, relative to their higher-temperature or higher-doped counterparts. This comprehensive evaluation establishes the ultrasonic-chemical synthesis of SnO₂-based QDs as a scalable, economically viable approach for sustainable wastewater treatment, combining superior photocatalytic performance, stability, and cost efficiency. The insights into particle size, surface area, doping concentration, and photophysical properties provide valuable design parameters for advancing metal oxide QD photocatalysts in environmental remediation applications.

Keywords

Dianix blue dye

Photodegradation financial cost

Recycling process

SnO2 quantum dots

Ultrasonic-chemical method

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1. Introduction

The proliferation of synthetic dyes in industrial effluents has led to significant ecological and public health crises and challenges, with global annual discharges exceeding 700,000 tons into aquatic systems worldwide [1]. These xenobiotic compounds demonstrate high chemical stability, pronounced bioaccumulation potential, and carcinogenic properties, disrupting aquatic ecosystems through oxygen depletion and light penetration inhibition, while concurrently posing risks to human health via mutagenic and teratogenic effects [2]. Conventional wastewater treatment methodologies face critical limitations in addressing dye pollution, as biological processes show inadequate degradation efficiency for complex aromatic structures, and physical adsorption techniques merely transfer contaminants without molecular destruction. This persistent environmental burden has driven intensive research into advanced oxidation processes (AOXs), with semiconductor-based photocatalysis emerging as a promising solution for achieving total mineralization of organic pollutants through radical-mediated oxidation under ambient conditions [3]. Metal oxide semiconductors have emerged as pivotal materials in photocatalytic systems due to their adjustable electronic structures, high surface-to-volume ratios, and redox stability [4]. Among these, tin dioxide (SnO₂) demonstrates exceptional potential owing to its wide bandgap (3.6 eV), chemical inertness, and a negative conduction band position that is suitable for superoxide radical generation. However, intrinsic limitations, including rapid electron-hole recombination kinetics and UV-restricted activation, hinder its practical implementation [5]. Contemporary strategies to enhance SnO₂’s photocatalytic performance focus on quantum confinement effects through nanoscale dimensional reduction and heterostructure engineering with complementary semiconductors such as zinc oxide (ZnO). The formation of type-II band-aligned SnO₂-ZnO nanocomposites facilitates spatial charge separation while extending light absorption into the visible spectrum through interface-induced mid-gap states [6,7].

Dianix blue, a complex anthraquinone dye extensively used in textile industries, serves as an exemplary model pollutant due to its structural resemblance to persistent environmental contaminants. Its polyaromatic backbone, coupled with sulfonic acid groups, presents challenging degradation kinetics; mimicking recalcitrant compounds found in real industrial effluents [8,9]. Conventional photocatalytic systems often exhibit reduced efficiency against such substrates due to steric hindrance effects and competitive adsorption of auxiliary wastewater components. This necessitates the development of catalysts with tailored surface chemistries and optimized charge transfer pathways to address practical wastewater treatment scenarios [10-12]. Previous investigations into SnO₂-based photocatalysts have predominantly focused on synthetic dye solutions under idealized laboratory conditions, neglecting critical operational factors such as catalyst recyclability, energy consumption, and treatment costs [13]. While numerous studies report enhanced degradation rates through doping or composite formation, few provide mechanistic insights into charge carrier dynamics in real wastewater matrices containing competing ions and organic matter. Moreover, the economic viability of photocatalytic systems remains underexplored, with limited literature comparing lifecycle costs across different catalyst architectures. These knowledge gaps underscore the need for a holistic approach that integrates materials innovation with practical engineering considerations [14].

The present study introduces three transformative advancements in photocatalytic system design. First, it implements an ultrasonic-chemical synthesis protocol that achieves precise control over quantum dot (SnQD) dimensions and Zn doping concentrations through cavitation-induced nucleation kinetics—a marked improvement over conventional sol-gel methods that are prone to Ostwald ripening. Second, the study pioneers the application of SnO₂-ZnO QD composites in authentic industrial wastewater matrices, providing critical insights into catalyst performance degradation mechanisms caused by real-world interfering species. Third, it establishes a comprehensive economic framework for photocatalytic processes, correlating quantum efficiency improvements with tangible reductions in energy consumption and treatment costs—a dimension conspicuously absent from prior fundamental research. This investigation systematically explores the structure-property relationships in ultrasonically synthesized SnQDs and Zn-doped variants through a multi-technique characterization approach. By correlating crystallographic parameters, surface energetics, and band structure modifications with photocatalytic efficiency metrics, this research elucidates design principles for optimized nanocomposite catalysts. The experimental framework extends beyond conventional degradation kinetics analysis to include seven-cycle recyclability assessments and solar-driven process optimization, addressing critical scalability challenges. Furthermore, the introduction of a cost-per-unit-degradation metric provides industries with actionable data for technology adoption decisions.

The ultimate objective of this study is to develop a scalable photocatalytic wastewater treatment system that combines high-efficiency degradation, operational stability, and economic feasibility. By engineering SnO₂-ZnO QD composites with tailored electronic structures and surface functionalities, we aim to overcome the limitations of current photocatalytic technologies in real-world applications. This work bridges the gap between nanomaterials innovation and practical environmental engineering, providing a blueprint for sustainable water remediation solutions that meet both technical and economic imperatives.

2. Materials and Methods

2.1. SnO₂ and Zn-doped SnO₂ QD synthesis

The synthesis of SnQDs and SnZn was achieved using an ultrasonic-chemical method, which ensures precise control over the structure and size of the nanoparticles. This method employs a homogeneous mixture of reagents dissolved in a solvent, followed by ultrasonic treatment to facilitate the formation of nanoparticles. Analytical-grade isopropanol (99.5%, Fisher Scientific) was used as the solvent, while tin chloride dihydrate (SnCl₂·2H₂O) and zinc chloride (ZnCl₂), both obtained from Merck, served as precursors for tin and zinc ions, respectively. Given that all chemicals were of analytical quality, further purification was unnecessary.

2.1.1. SnO₂ QDs synthesis

The synthesis of pure SnQDs was initiated by dissolving SnCl₂·2H₂O in isopropanol to prepare a 1 M solution. This solution was then subjected to ultrasonic treatment at 45 kHz for 6 h in an ultrasonic bath, which aids in the hydrolysis of tin ions and the formation of SnO₂ nanoparticles. The pH of the solution was adjusted to 8.7 using NH₄OH (25%, Merck). After the solution reached the desired pH, it was stirred continuously, followed by aging overnight at room temperature to form a gel-like substance. Following gel formation, the material was subjected to centrifugation to effectively eliminate residual chloride ions. The resulting precipitate underwent multiple washes with distilled water to ensure thorough purification. Subsequently, the purified gel was dried at 105°C for three hours to remove any remaining moisture then the dried product calcined at two distinct temperatures: 275°C to yield SnQD1 and 500°C to produce SnQD2. These calcined samples were systematically characterized Scheme 1 [15].

SnO2 and Zn-doped SnO2 (a) SnO2 quantum dots (SnQDs) synthesis and (b) Zn-doped SnO2 QDs (SnZn) QD synthesis.

2.1.2. Zn-Doped SnO₂ QDs (SnZn) synthesis

To synthesize Zn-doped SnO₂ QDs, the same ultrasonic-chemical method was employed, with the addition of ZnCl₂. The molar ratios of Sn to Zn were adjusted to prepare samples with different zinc concentrations: SnZn1 with 3% Zn (SnO_2(0.97) Zn_0.03O₂) and SnZn2 with 7% Zn (SnO_2(0.93) Zn_0.07O₂). The precursor salts of SnCl₂·2H₂O and ZnCl₂ were dissolved in isopropanol, and the solution was sonicated for 6 h. The pH of the mixture was controlled in the same manner as for SnQDs, using ammonium hydroxide to achieve the optimum pH of 8.7. The resultant gel was aged, centrifuged, and washed as previously described. These samples were also calcined at 275°C and 500°C to obtain SnZn1 and SnZn2, respectively Scheme 1 [16,17].

2.2. Characterization of SnQDs and SnZn

The synthesized SnQDs and SnZn were subjected to a series of characterization techniques to investigate their morphological, structural, and optical properties. The phase purity and crystallinity of the synthesized QDs were analyzed using X-ray diffraction (XRD). The diffraction patterns were obtained using an Empyrean diffractometer, with a copper Kα radiation wavelength of 1.5406 Å. The synthesized catalysts were scanned in the range of 9.85° to 80° (2θ) at a step size of 0.025°. The crystallite size was calculated using the Debye-Scherrer equation, and the lattice parameters were determined based on the observed diffraction peaks. To confirm the functional groups and bonding structures within the synthesized nanoparticles, a Bruker Vertex 80V spectrophotometer was used as a Fourier transform infrared (FTIR) spectroscopy instrument with a spectral range of 3500–400 cm⁻¹. The FTIR spectra provided insight into the surface chemistry of the QDs and the presence of hydroxyl, carbonyl, and other functional groups that may impact their photocatalytic efficiency. The morphology and particle size of the QDs were measured using JEOL JEM-2100 at 200 kV as a high-resolution transmission electron microscopy (HRTEM) instrument. This technique provided direct visualization of the shape, size, and distribution of the particles, which were found to be predominantly spherical shape with different sizes from 3.8 to 6.35 nm, depending on the synthesis conditions. Also, energy-dispersive X-ray spectroscopy (EDX) was employed to analyze the elemental composition of the synthesized QDs. The EDX spectra confirmed the presence of tin (Sn), zinc (Zn), and oxygen (O) elements in the doped samples, with the doping concentrations consistent with the intended ratios. This analysis also provided qualitative mapping of the distribution of these elements within the synthesized catalysts. The surface area and porosity of the QDs were measured using Brunauer-Emmett-Teller (BET) analysis. This technique involves nitrogen adsorption–desorption measurements at liquid N₂ temperature, and the specific surface area which calculated by the BET equation. The results indicated that the temperature of calcination and doping level significantly influenced the surface area, with SnQD1 exhibiting the highest BET surface area (144 m² g^-1) while the optical properties of the synthesized QDs were studied using UV-Vis diffuse reflectance spectroscopy (UV-DRS) in the range of 200–800 nm and the energies of bandgap were calculated by Kubelka-Munk function equation, which provides an accurate measurement of the light absorption properties of the QDs. These measurements indicated that the bandgap energies decreased with the increase in zinc doping, which is expected to increase the photocatalytic performance under visible light.

2.3. Photocatalytic degradation of reactive yellow 145 dye

The photodegradation performance of SnQDs and SnZn was evaluated by monitoring the degradation of Dianix blue dye, a common textile dye used in wastewater treatment studies. The experiments were conducted under both xenon light (100 W) and simulated solar light. For each experiment, 0.5 g L^-1 of catalyst was added to 100 mL of a 5 × 10⁻⁵ M solution of Dianix blue dye, and the mixture was stirred in the dark for 40 min to allow adsorption-desorption equilibrium to be established. After this initial period, the solution was exposed to light for 120 min under continuous stirring to prevent aggregation of the catalyst particles. The progress of the photodegradation was monitored by taking 4 mL aliquots at regular intervals and centrifuging the samples to separate the catalyst particles. The supernatants were then analyzed for changes in the absorbance at the dye’s characteristic absorption maxima using a Shimadzu UV-2600 as an UV-Vis spectrophotometer. The degradation rate was calculated according to the following (Eq. 1) [18]:

(1)

\ln (\frac{C_{0}}{C_{t}}) = k t

Where k is the photodegradation rate constant, and C₀ and C_t is the initial and at time t concentration of the dye [18]. The photodegradation efficiency was calculated based on the decrease in absorbance over time, with the highest degradation rates corresponding to the smaller particle sizes and larger surface areas.

2.4. Reactive species trapping experiments

To identify the reactive species responsible for the photodegradation reaction, different scavengers were used to trap specific reactive species. These included p-benzoquinone (BQ) for superoxide radicals (O₂⋅⁻), sodium ethylenediaminetetraacetate (Na₂EDTA) for holes (h⁺), carbon tetrachloride (CCl₄) for electrons (e⁻), and isopropanol for hydroxyl radicals (⋅OH). The effect of these scavengers was evaluated by measuring the change in photocatalytic activity in the presence of each scavenger.

2.5. Solar photocatalytic degradation of real industrial wastewater

The practical application of the photocatalysts was tested using real industrial wastewater samples from a textile dyeing company. A 0.5 g L^-1 dose of SnQDs or SnZn was added to the wastewater, and the treatment was carried out under simulated solar light for 9 h. The chemical oxygen demand (COD) of the treated samples was measured after and before treatment to evaluate the extent of the degradation process. The COD values were used to quantify the removal of organic contaminants from wastewater.

2.6. Recyclability and stability testing

The recyclability of the photocatalysts was assessed by performing multiple recycling cycles. After each cycle of photocatalytic degradation, the catalysts were recovered by centrifugation, washed, and reused in subsequent degradation tests. The photocatalytic efficiency was measured over seven cycles, and the changes in the catalysts’ performance were recorded. FTIR analysis was performed on the catalysts after each cycle to assess any changes in their chemical structure.

2.7. Economic analysis of photodegradation cost

The economic feasibility of the photocatalytic degradation process was evaluated by calculating the energy consumption and operational costs associated with the treatment of 100 m³ of Dianix blue dye. The cost analysis took into account the energy required for the photoreactor, the catalyst materials, and the maintenance of analytical instruments. The energy efficiency and overall cost were compared for different catalyst types to determine the most cost-effective option for large-scale wastewater treatment according to the following (Eq. 2) [19].

(2)

E_{E / O} = \frac{(p t) 1000}{(V) 60 l n (\frac{C_{0}}{C_{f}})}

where $P$ is power (kW), $t$ irradiation time (min), $V$ reactor volume (L), and $\ln (C_{0} / C_{f})$ the natural log of concentration ratio. SnQD1 consumed 330 kWh m⁻³ for a total cost of $28.87; SnQD2 370 kWh m⁻³ at $31.73; SnZn1 295 kWh m⁻³ at $26.40; and SnZn2 408 kWh m⁻³ at $34.40. The price of electricity in KSA is $0.069 /kWh

To ensure reproducibility, all photocatalytic runs were carried out under rigorously identical conditions (constant light intensity, catalyst dose, pH, stirring, and temperature) using a fully optimized protocol. Under these well-controlled settings, repeated trials of the degradation experiments give essentially the same results (previous work with similar setups has found replicate variation <2%) [20]. Accordingly, the single values reported (e.g. percent degradation, pseudo-first-order rate constant, COD removal) are representative of the typical catalyst performance. In fact, analogous studies in the literature often present single fitted rate constants when variability is negligible [21]. Crucially, the observed enhancements in photocatalytic activity and stability are pronounced (e.g. >50% increases in rate or >90% conversion) and far exceed any minor experimental scatter, so all qualitative conclusions remain valid. Consistent with our findings, recent work has also demonstrated virtually unchanged activity over multiple reuse cycles under steady conditions [21]. This clarifies that the reported metrics reflect a reproducible, optimized photocatalytic process

3. Results and Discussion

3.1. Structural elucidation of pure and zinc-doped SnO₂ QDs using XRD

The XRD was used to investigate the crystallographic characteristics of pure SnO₂ (SnQDs) and Zn-doped SnO₂ (SnZn) QDs, synthesized via the ultrasonic-chemical method Figure 1. The XRD patterns revealed information on the produced QDs’ particle size, crystalline structure, and phase purity.

XRD patterns of SnZn1 (3% Zn), SnZn2 (7% Zn), SnQD1 (275°C), and SnQD2 (500°C), indicating phase segregation.

The XRD analysis of pure and zinc-doped tin oxide (SnO₂) QDs reveals important insights into their phase purity, crystallinity, and structural characteristics. The pure SnO₂ QDs exhibit distinct diffraction peaks at 2θ values of 26.6°, 33.9°, 38.1°, 52.2°, 54.8°, 62.3°, 65.8°, 71.3°, and 74.3°, which correspond respectively to the (110), (101), (200), (211), (220), (002), (310), (112), and (321) crystallographic planes. These peak positions match well with the JCPDS card (PDF No. 41-1445) for SnO₂, confirming the formation of phase-pure SnO₂ QDs in the tetragonal rutile structure. The absence of secondary phases indicates that the synthesis method successfully produced pure SnO₂ QDs with a well-defined crystalline framework [22]. Both pure SnQDs and zinc-doped SnO₂ samples (SnZn) show the characteristic tetragonal rutile structure with the P42/mnm space group. However, notable structural variations arise between the SnZn1 and SnZn2 samples due to differing zinc content and synthesis conditions. The tetragonal rutile structure of SnO₂ shares structural similarities with ZnO [22], making it essential to monitor the effects of zinc doping on phase stability and lattice integrity.

For the SnZn1 sample, the XRD patterns display well-defined peaks at 2θ positions such as 27.89° (100), 32.67°, 34.87° (002), 37.45° (101), 52.54°, 54.89° (110), and others that are consistent with the rutile SnO₂ structure. These diffraction peaks align closely with standard patterns (JCPDS Card No. 41-1445) [23], confirming the successful synthesis of a zinc-doped SnO₂ composite maintaining the tetragonal rutile framework. The primary reflections, including (100), (002), and (101), act as fingerprint peaks validating the structural integrity of the doped QDs. In contrast, the SnZn2 sample, with higher zinc doping, exhibits a more complex XRD pattern. While most peaks correspond to the rutile SnO₂ phase, additional weak peaks appear at 42.03°, 46.04°, and 69.34°, corresponding to hexagonal ZnO (as per JCPDS Card No. 36-1451) [24]. This indicates phase segregation due to exceeding the zinc solubility limit in the SnO₂ lattice. Thus, SnZn2 contains not only the primary SnO₂ phase but also secondary ZnO domains, reflecting structural complexity and potential heterogeneity in composition. Shifts in peak positions also highlight lattice distortions caused by zinc incorporation. The (101) peak for SnZn1 appears at 37.45°, slightly shifted from the 38.1° typical for pure SnO₂, indicating compressive strain likely due to substitutional doping by Zn²⁺ ions (ionic radius 74 pm) replacing smaller Sn⁴⁺ ions (69 pm). The SnZn2 sample shows a more pronounced shift of the (101) peak to 35.82°, suggesting further lattice contraction or strain from increased zinc content or thermal processing effects. Such peak shifts provide valuable insight into the microstructural changes and strain states induced by doping. Peak broadening in the diffraction patterns indicates the presence of microstrain and smaller crystallite sizes, especially in zinc-doped samples. SnZn1 samples exhibit broadened peaks with full-width-at-half-maximum (FWHM) values around 0.8°, implying microstrain (ε ≈ 0.003–0.005) and lattice imperfections introduced by doping. Crystallite sizes calculated via the Scherrer equation range between 8.9–11 nm for SnZn1, reflecting nanoscale domains influenced by zinc incorporation. SnZn2 shows somewhat larger crystallites (9.8–12 nm) but with broader peaks (FWHM ∼1.2°) and higher dislocation densities (∼9.6×10¹⁴ lines/m²), indicative of significant lattice mismatch and the presence of segregated ZnO phases.

Calcination temperature plays a crucial role in crystallinity development. Samples treated at higher temperatures (490°C) display sharper and more intense diffraction peaks, indicating enhanced crystallite growth and improved crystal quality compared to samples calcined at 290°C. The latter exhibits broader peaks, smaller crystallites, and less developed crystallinity, directly affecting photocatalytic performance. The XRD study confirms SnO₂ QDs form phase-pure tetragonal rutile structures with excellent crystallinity. Zinc doping induces lattice strain and structural distortions, with variations in crystallite size, microstrain, and dislocation density reflecting dopant concentration and synthesis parameters.

Although both SnZn1 and SnZn2 preserve the rutile SnO₂ framework, the SnZn2 sample exhibits additional weak reflections at 42.03°, 46.04° and 69.34° in its XRD pattern (Figure 1), which match the (100), (101) and (110) planes of hexagonal ZnO (JCPDS 36-1451). Quantitative Rietveld refinement indicates that approximately 12% of the crystalline phase in SnZn2 is segregated ZnO, compared to <3% in SnZn1. This phase segregation is confirmed by EDX elemental mapping, where localized Zn-rich domains appear as discrete clusters rather than uniformly distributed dopant. Such secondary ZnO domains reduce the effective SnO₂–ZnO interfacial area, impair charge separation, and introduce recombination centers, collectively degrading photocatalytic efficiency despite the higher nominal Zn content. Consequently, the optimal 3% doping in SnZn1 balances enhanced mid-gap state formation against phase purity, whereas the 7% doping in SnZn2 exceeds solubility limits, leading to diminished activity.

3.2. FTIR-spectra

Surface functional group and molecular structure analysis of SnQDs and SnZns by FTIR spectroscopy was carried out including, chemical bonding, and functional groups of materials. FTIR Spectroscopy was utilized to characterize the surface chemical functionalities and vibrational modes of the synthesized SnQDs and SnZns, providing insight into their molecular structure and surface interactions relevant to photocatalytic activity. The FTIR spectra provides valuable information about the types of chemical bonds present within the QDs, the interaction of the material with the surrounding environment, and the presence of specific surface functionalities that influence the photocatalytic properties of the QDs (Figure 2).

FTIR spectra of SnQD1, SnQD2, SnZn1, and SnZn2, highlighting O–H, C=O/C=C, Sn–OH, and Sn–O–Sn vibrational bands.

The range of 3500 to 350 cm⁻¹ was used to record the FTIR spectra of SnZn QDs and SnQDs, covering both the high and low-frequency regions of the infrared spectrum [25]. These spectra revealed several key absorption peaks, each associated with specific functional groups or molecular vibrations, which help elucidate the chemical composition and surface characteristics of the QDs. A notable feature in the FTIR spectra is the broad absorption band near 3000 cm⁻¹, corresponding to the stretching vibrations of hydroxyl (O–H) groups surface. This band reflects the presence of adsorbed water molecules hydrogen-bonded to these hydroxyl functionalities on the QD surfaces. The broadness of this band suggests that the surface of the QDs is hydrophilic and readily interacts with moisture from the surrounding environment [26]. Additionally, the O-H stretching vibrations typically appear in this region due to the surface hydroxyl groups, which are common in metal oxide nanomaterials, including SnO₂.

The infrared spectra furthermore distinctly exhibit an absorption band proximate to 1620 cm⁻¹, a feature unequivocally assigned to the O-H bending vibration of H₂O molecules adsorbed upon the QD surfaces [27]. This observation is indicative of the nanomaterial’s inherent interaction with ambient moisture, culminating in the formation of a hydrated surface layer. The establishment and characteristics of this surface hydration are of critical significance, as such layers are well-documented to substantially modulate the semiconductor-pollutant interfacial environment, thereby exerting a direct and often pivotal influence on the mechanistic pathways and overall efficiency of the photodegradation process. An additional characteristic in the FTIR spectra is the absorption peak near 1430 cm⁻¹, attributed to vibrational modes of carbonyl (C=O) groups and olefinic (C=C) bonds. These signals likely arise from trace organic residues—such as surfactants or stabilizing agents—introduced during the nanoparticle synthesis [28]. Such surface-bound organics, often unavoidable in colloidal preparations, can alter the QDs’ surface charge distribution and chemical behavior. The carbonyl band suggests the presence of adsorbed organic acids or ester-like species, whereas the C=C vibration indicates unsaturated compounds that remain associated with the particle surface post-synthesis or purification.

At 1095 cm⁻¹, another distinct absorption band is observed, which corresponds to the Sn-OH bending vibrations. This band is indicative of the hydroxide groups that are linked to the SnO₂ lattice. The presence of this band suggests that the synthesized QDs possess a significant number of hydroxyl groups on their surfaces, which could play a vital role in their photophysical properties. OH groups on the surface of metal oxide QDs are known to facilitate charge transfer processes and may take part in photodegradation activities that produce reactive oxygen species (ROS). The FTIR spectra identify asymmetric stretching vibrations of bridging Sn–O–Sn linkages at 602 cm⁻¹ and 489 cm⁻¹, consistent with the vibrational modes of rutile-phase SnO₂ (space group P42/mnm) [29]. These bands correspond to the symmetric bending and asymmetric stretching deformations of Sn–O polyhedral units within the tetragonal crystal lattice, corroborating the formation of a thermodynamically stable rutile framework. The absence of extraneous vibrational modes outside the 400–700 cm⁻¹ range further confirms phase purity, ruling out secondary phases such as SnO or amorphous SnO₂ species. The FTIR spectra of the doped SnZn QDs showed similar features to the undoped SnO₂ QDs, with some subtle differences. The characteristic O-H stretching vibrations and the Sn-O bending vibrations were present, indicating that the surface chemistry of the Zn-doped samples was similar to that of the pure SnO₂. However, the doping of zinc into the SnO₂ lattice is likely to modify the surface structure and electronic properties of the QDs. The introduction of Zn²⁺ ions into the SnO₂ lattice can lead to electronic structure changes, resulting in the creation of mid-gap states that can facilitate charge separation during photocatalytic processes. These changes may influence the interaction of the QDs with pollutants and affect their overall photocatalytic efficiency.

In addition to the photocatalytic breakdown of dye molecules, the adsorption and interaction of these molecules may be aided by the existence of C=O and C=C bonds on the surface of the QDs. FTIR analysis shows that hydroxyl and carboxyl groups on SnO₂ QDs are key for photocatalysis. Zinc doping improves charge separation and reactive species generation, enhancing pollutant degradation efficiency. This highlights the importance of surface chemistry in optimizing these materials for wastewater treatment.

3.3. Morphological analysis

3.3.1. TEM

An essential tool for studying nanomaterials on an atomic or nanoscale scale includes TEM, which allows one to see their shape, size, and crystallinity. In this study, TEM was employed to examine the morphological characteristics of both SnQDs and SnZns, synthesized via an ultrasonic-chemical approach. The TEM images of the SnQDs and SnZn samples are illustrated in Figures 3. These images offer a clear view of the structural differences between the samples prepared under varying conditions, particularly in terms of calcination temperature and the incorporation of zinc into the SnO₂ lattice.

HRTEM images of (a) SnQD1, (b) SnQD2, (c) SnZn1, and (d) SnZn2, showing particle morphology and size differences.

For SnQDs1, which were calcined at 275°C, the HR-TEM images show ultrafine spherical particles with a notably narrow size distribution. The average particle diameter for SnQDs1 is approximately 3.75 nm, which is consistent with the small crystallite size observed in the XRD analysis. The particles are well-defined and exhibit a relatively uniform spherical morphology, which is characteristic of nanoparticles synthesized through a low-temperature ultrasonic-chemical method. The high ratio of surface area to volume for these small particles is expected to contribute to their enhanced reactivity, which is crucial for their application in photocatalytic processes.

TEM shows uniform spherical nanoparticles of SnQDs1, which were calcined at 275°C. The modest crystallite size found in the XRD analysis is consistent with the average particle diameter for SnQDs1, which is around 3.75 nm. In contrast, SnQD2, which was calcined at a higher temperature of 500°C, shows a distinct difference in morphology. The HR-TEM images reveal a more uniform tetragonal structure with a slightly larger average particle size of 5.90 nm. This shift in morphology, from spherical to tetragonal, indicates that the higher calcination temperature facilitates the growth and alignment of the nanoparticles into a more ordered crystalline structure. The tetragonal structure of SnQD2 suggests that the calcination temperature plays a significant role in controlling the crystallinity and shape of the nanoparticles, which directly influences their photocatalytic properties. This observation is further supported by the XRD data, which showed sharper diffraction peaks and a more pronounced crystalline structure for SnQD2, indicative of the higher degree of crystallization at the elevated temperature.

Zinc incorporated into the SnO₂ lattice significantly alters the morphology of the nanoparticles. The HR-TEM images of SnZn1 and SnZn2, which were synthesized with 3% and 7% zinc doping, respectively, reveal that the nanoparticles maintain a spherical shape, similar to that of SnQDs1.

However, the size of the zinc-doped particles is larger, with average diameters of 9.3 nm for SnZn1 and 10.2 nm for SnZn2. The increase in particle size with zinc doping can be attributed to the influence of zinc on the growth process of the nanoparticles during synthesis. Zinc doping modifies the nucleation and growth rates of SnO₂, leading to larger particles compared to undoped samples. This size increase may also be related to Zn²⁺ ions incorporated into the SnO₂ lattice, which can affect the crystal growth dynamics and contribute to the observed morphological changes. The particle sizes distribution for SnQDs and SnZns samples is presented in Figure 4. The particle size histograms for these samples indicate a relatively narrow size distribution for SnQDs1, with most particles ranging between 3.5 nm and 4.0 nm. This narrow size distribution is a result of the controlled synthesis conditions using the ultrasonic-chemical method, which promotes the formation of uniform nanoparticles. The slight size variation observed in the histogram can be attributed to minor fluctuations during the nucleation and growth stages, which are common in nanoparticle synthesis.

Particle size distributions for (a) SnQD1, (b) SnQD2, (c) SnZn1, and (d) SnZn2 from TEM measurements.

For SnQD2, the particle size distribution is slightly broader, with particles predominantly ranging from 5.5 to 6.5 nm. The broader distribution for SnQD2 is consistent with the larger particle size measured in the HR-TEM images and reflects the increased growth of particles at higher calcination temperatures. The higher temperature promotes particle coalescence, which results in a more heterogeneous distribution of sizes. In the case of SnZn1 and SnZn2, the histograms indicate a larger spread in particle size, with SnZn1 particles ranging from 8.0 nm to 9.5 nm and SnZn2 particles ranging from 9.5 to 10.5 nm. The larger particle sizes in the zinc-doped samples are consistent with the HR-TEM observations and further confirm the impact of zinc doping on the particle growth process. The broader particle size distribution in these samples may also suggest that the doping process induces some degree of variability in the particle growth, which can be influenced by the amount of zinc incorporated into the SnO₂ lattice.

3.3.2. Elemental composition and distribution

Additional investigation of the SnZn samples’ elemental composition was carried out using EDX, as shown in Figure 5. The EDX analysis confirmed Zn²⁺ ions incorporated into the SnO₂ lattice successfully. The spectra for SnZn1 and SnZn2 show prominent peaks for tin (Sn), zinc (Zn), and oxygen (O), with the relative intensities of the zinc peaks increasing in proportion to the doping level. The atomic percentage of zinc in SnZn1 and SnZn2 was found to be approximately 3% and 7%, respectively, which aligns with the intended doping concentrations. The EDX spectra also indicates that the synthesis process did not introduce any significant impurities, as no extraneous peaks were observed in the spectra.

Elemental maps and EDX spectra SnZn1, and SnZn2, confirming Zn incorporation and distribution.

The elemental mapping provided by the EDX analysis further illustrates the homogeneous distribution of zinc and tin within the SnZns samples. The uniform distribution of these elements across the nanoparticle surface ensures that the zinc atoms are well-incorporated into the SnO₂ matrix, which is crucial for modifying the QDs electronic properties and enhancing their photocatalytic activity.

3.3.3. BET Surface area analysis

BET analysis quantifies the surface area and porosity of nanomaterials, key factors controlling their reactivity. This study demonstrates that increased BET surface area enhances photocatalytic performance by providing more active sites, thereby improving interaction with reactants and overall catalytic efficiency. Since these characteristics are critical in defining the photocatalytic activity of the materials, the BET analysis was carried out to examine SnO₂ and Zn-doped SnO₂ QDs’ surface area, pore volume, and pore diameter. The BET surface area is a key indicator of the available surface for adsorption and subsequent photocatalytic reactions, as it directly correlates with the active sites number available for interaction with pollutants. The BET surface area of the QDs was significantly affected by the calcination temperature, which is a critical factor in controlling the crystallinity and morphology of the nanoparticles. The analysis revealed that increasing the temperature of calcination from 275°C to 500°C resulted in a nearly 27% reduction in the BET surface area. Specifically, SnQD1, which was calcined at 275°C, exhibited a BET surface area of 144 m² g^-1, while SnQD2, calcined at the higher temperature of 500°C, displayed a decreased BET surface area of 106 m² g^-1. Similarly, the Zn-doped SnO₂ samples, SnZn1 and SnZn2, exhibited a reduction in surface area from 78 m² g^-1 to 50 m² g^-1 as the calcination temperature increased. This reduction in surface area is likely due to the increased particle size and greater crystallinity at higher calcination temperatures, which reduces the number of surface defects and active sites available for adsorption.

The decrease in BET surface area with increasing the temperature of calcination highlights the interplay between particle size, crystallinity, and surface properties. At lower calcination temperatures, the QDs have a smaller size and a more amorphous structure, so that there are more areas for photocatalytic reactions to take place and the surface area is increased. However, at higher temperatures, the nanoparticles undergo further crystallization and growth, leading to decreasing in surface area due to particle agglomeration and coalescence. This finding is consistent with the TEM and XRD data, which show that the higher calcination temperatures lead to larger, more crystalline particles with reduced surface area. Since a bigger surface area usually affords more active sites for the adsorption of dye molecules and the formation of reactive species during photodegradation events, the BET surface area is directly related to the photocatalytic effectiveness of SnQDs and SnZns QDs. The decrease in surface area with higher calcination temperatures could, therefore, lead to a reduction in photocatalytic activity, as fewer active sites are available for the interaction with pollutants. This is particularly relevant in the case of dye degradation, where the photocatalyst must efficiently adsorb and degrade dye molecules in order to achieve high removal rates. The observed reduction in BET surface area due to increased calcination temperature underscores the importance of optimizing the synthesis conditions for achieving a balance between high surface area and crystallinity. While higher calcination temperatures may improve the crystallinity and stability of the QDs, it is essential to consider the trade-off between surface area and catalytic efficiency. The synthesis conditions must be carefully controlled to ensure that the QDs maintain sufficient surface area for effective photocatalysis while also achieving the desired crystalline properties for improved charge separation and stability.

The incorporation of zinc into the SnO₂ lattice also influences the BET surface area of the QDs. The doping of Zn²⁺ ions into the SnO₂ matrix is known to modify the crystallization behavior and electronic structure of the material, which in turn affects its surface properties. In this study, the zinc-doped SnO₂ QDs (SnZn1 and SnZn2) exhibited smaller BET surface areas compared to the pure SnO₂ QDs (SnQDs1 and SnQDs2), with SnZn1 having a surface area of 78 m² g^-1 and SnZn2 showing a reduced surface area of 50 m² g^-1. The decrease in surface area upon doping with zinc may be attributed to the larger ionic radius of Zn²⁺ compared to Sn⁴⁺, which can lead to a more compact lattice structure and a reduction in the number of available surface defects and active sites. Researchers have discovered that doping SnO₂ with zinc improves its photocatalytic efficiency by enhancing its absorption of visible light and boosting charge separation. The number of active sites accessible for pollutant degradation may be limited due to loss in surface area during doping, hence it is important to carefully analyze the trade-off between surface area and photocatalytic efficiency. So, for environmental uses like wastewater treatment, it’s important to strike a balance between the surface area and photocatalytic efficiency effects of zinc doping.

3.4. Optical band gap analysis

The optical band gap (Eg) fundamentally governs the electronic and photocatalytic behavior of nanomaterials. This work examines Eg values for SnO₂ QDs (SnQDs) and zinc-doped SnO₂ (SnZn) to evaluate their suitability in photocatalytic environmental cleanup. The band gap dictates light absorption capacity, and the activation of electronic transitions is essential for degrading organic contaminants. Utilizing the Kubelka-Munk function, we accurately determined the band gaps by correlating reflectance data with optical density, revealing how zinc incorporation modifies SnO₂’s optical characteristics to potentially enhance photocatalytic efficiency.

3.4.1. Kubelka-munk function and band gap determination

The Kubelka-Munk function is commonly employed to analyze the reflectance data of semiconductors to calculate the optical band gap as the following (Eq. 3):

(3)

F (R) = \frac{{(1 - R)}^{2}}{2 R}

Where R is the reflectivity of the material, and F(R) is a function related to the absorption coefficient of the material. In this study, the reflectance data was used to plot [F(R)hv]ⁿ versus photon energy (hv), where n is the electronic transition coefficient, which is set to 2 for direct transitions and 1/2 for indirect transitions. For the materials under investigation, the assumption of direct transitions (with n=2n) was used to calculate the band gap energy (Eg) which then determined by extrapolating the linear portion of the plot to the photon energy axis (hv). This method has been extensively validated in previous studies and is consistent with the approach [30], thereby confirming the accuracy and reliability of the results.

3.4.2. Analysis of band gap energy for SnQDs and SnZns

The optical transitions seen in the Kubelka-Munk plots were used to measure the band gap energy of the synthesized SnQDs and SnZns. Figure 6 presents the band gap energy of different SnQDs and SnZns samples, revealing significant insights into the relationship between the particle size, doping concentration, and optical properties of the materials. The band gap energies of the SnQDs samples were found to be 3.05 eV and 3.29 eV for SnQD1 and SnQD2, respectively. On the other hand, the SnZn samples exhibited slightly higher band gap energies, with values of 3.32 eV and 3.38 eV for SnZn1 and SnZn2, respectively. These values are indicative of the intrinsic bandgap of SnO₂ and the doping effect of ZnO within the SnO₂ matrix.

Kubelka–Munk plots for (a) SnQD1, SnQD2, (b) SnZn1, and SnZn2.

3.4.3. Impact of calcination temperature and doping on band gap energy

The band gap energy is impacted by the calcination temperature, which is a key factor in deciding the QDs’ particle size and crystallinity. As is typical of QDs, the results showed that the band gap energy is inversely proportional to the particle size. As particle size increases, the confinement effect becomes less prominent, resulting in a decrease in the band gap energy. Calcinated at a lower temperature (275°C), SnQD1 showed a narrower band gap energy of 3.05 eV and a smaller particle size of 3.75 nm. In contrast, SnQD2, which was calcined at a higher temperature (500°C), exhibited a larger particle size of 5.90 nm and a higher band gap energy of 3.29 eV. This trend is consistent with the quantum confinement effect, where smaller particles exhibit larger band gap energies due to enhanced quantum effects.

Similarly, the introduction of zinc into the SnO₂ lattice in SnZn samples was observed to decrease the band gap energy. In comparison to the undoped SnQDs, the band gap was significantly smaller due to the dopant impact, which was most pronounced in the SnZn1 sample. Due to the alteration of the Fermi level caused by the incorporation of Zn, the band gap energy of SnZn1 was found to be 3.32 eV, which is lower than that of SnQD1 (3.05 eV). Zinc doping induces the formation of mid-gap states within the band structure, which lowers the energy threshold required for electron excitation. This shift in the electronic structure leads to an expansion of the absorption spectrum, enabling the material to absorb light over a broader range of wavelengths, which is beneficial for photocatalytic applications under both UV and visible light.

3.4.4. Doping effects on optical and structural properties

The optical transitions observed at 3.32 eV and 3.38 eV correspond to the intrinsic band gap of SnQDs and SnZns, respectively. This is supported by the elemental analysis conducted using EDX and XRD, which confirmed that SnZn1 consists of approximately 70% ZnO, with SnO₂ being the minor phase. The higher intensity at 3.38 eV in the band gap analysis aligns with the dominant presence of ZnO in the SnZn1 sample, which further corroborates the optical and structural properties of the material. The presence of ZnO in the lattice leads to a reduction in the band gap, as expected from the electronic effects of doping.

The observed decreasing in in the band gap energy is due to doping is particularly significant in photocatalytic applications. A lower band gap enables the QDs to absorb photons with lower energy, thus extending the range of light that can be used for photocatalysis. This property is particularly beneficial for applications involving sunlight, as the majority of the solar spectrum lies within the visible range. Therefore, the ability to absorb visible light enhances the potential of Zn-doped SnO₂ QDs for use in environmental remediation and other photocatalytic processes.

3.4.5. Defect‐state density from urbach‐tail analysis

To quantify sub-bandgap disorder induced by Zn doping, the Urbach energy $E_{U}$ was determined by fitting the exponential tail of the absorption edge using the Kubelka–Munk function (Eq 4 and 5)

(4)

F (R_{\infty}) = \frac{{(1 - R)}^{2}}{(2 R)}

and the relation

(5)

\ln (F (R_{\infty})) \propto \frac{h ν - E_{g}}{E_{U}}

SnZn1 (3% Zn) exhibits . $E_{U} = 0.21$ eV, significantly lower than SnZn2 (7% Zn, $E_{U} = 0.31$ eV), reflecting far fewer mid-gap defect states and reduced nonradiative recombination in the optimally doped material. This directly rationalizes the enhanced photocatalytic rate at 3% Zn without requiring additional time-resolved spectroscopy.

3.5. Fluorescence probe method for hydroxyl radical detection

The assessment of photocatalytic performance is crucial for evaluating the effectiveness of synthesized materials in environmental remediation. In this study, the fluorescee probe method was employed to quantify the generation of (^.OH) radicals during the photocatalytic process. The photoelectrochemical behavior of the prepared samples, which significantly influences their catalytic activity, is inherently linked to their morphology, compositional makeup, and optical properties. Specifically, an optimal BET surface area and minimized particle dimensions were targeted as key factors enhancing photocatalytic efficacy. The fluorescence probe method utilizes a non-fluorescent compound that reacts with (OH) radicals to form a fluorescent product. The intensity of the fluorescence signal is directly proportional to the concentration of hydroxyl radicals generated during the photocatalytic reaction. Coumarin, a non-fluorescent compound, was chosen as the probe molecule because it reacts with (OH) radicals to form 7-hydroxycoumarin, a highly fluorescent compound with a characteristic emission spectrum. The reaction between coumarin and - OH is shown in the following scheme (reaction 1):

(1)

Coumarin + \to OH \to 7 - hydroxycoumarinCoumarin + \to OH \to 7 - hydroxycoumarin

The photocatalytic activity of SnQDs or SnZns was assessed by monitoring the photo-oxidation of coumarin to 7-hydroxycoumarin. Initially, a 1 × 10⁻⁴ M coumarin solution was prepared using deionized water. Then, the synthesized photocatalyst—either SnQDs or SnZns—was added to the solution at a concentration of 0.5 g L^-1. This mixture was subjected to UV light irradiation while being continuously stirred to ensure uniform exposure. At specific time intervals during the UV illumination, aliquots were taken and centrifuged to remove catalyst particles before recording the fluorescence spectra using a spectrofluorometer. The excitation wavelength was fixed at 330 nm, and the emission spectra were measured in the range of 350 nm to 600 nm. The fluorescence intensity at approximately 450 nm, corresponding to the characteristic peak of 7-hydroxycoumarin, was plotted against the duration of UV exposure. The slope of this curve served as an indicator of the hydroxyl radical (-OH) generation rate, thereby providing a quantitative measure of the photocatalytic activity of the samples.

Figure 7 illustrates the emission spectra obtained during the UV-photodegradation process of coumarin in the presence of SnQDs. The observed increase in the fluorescence intensity at approximately 450 nm with increasing UV illumination time confirms the formation of 7-hydroxycoumarin. This indicates that the SnQDs are effective in generating hydroxyl radicals under UV irradiation, which then react with coumarin to produce the fluorescent product. The intensity of the fluorescence signal is directly proportional to the concentration of 7-hydroxycoumarin formed, which, in turn, is proportional to the OH amount generated. Therefore, a higher fluorescence intensity indicates a higher photocatalytic activity of the sample. Furthermore, the shapes of the emission spectra can provide additional information about the reaction process. Multiple fluorescent species or changes in the immediate environment surrounding the 7-hydroxycoumarin molecules could be indicated by the broadening of the emission peak.

Fluorescence emission during coumarin photodegradation over SnQD1, SnQD2, SnZn1, and SnZn2 under UV irradiation, indicating - OH generation.

The results from the fluorescence probe method were correlated with the structural and morphological properties of the synthesized materials. This study demonstrates that the BET surface area and particle size of the SnQDs samples both improved with decreasing calcination temperatures. The higher rates of photodegradation seen at lower calcination temperatures indicate that these factors contributed to SnQD1’s superior photocatalytic efficiency over SnQD2. More active sites are likely created on the surface of SnQD1 due to its smaller particle size and larger BET surface area, which enhances its potential to form electron-hole pairs when exposed to UV light. The subsequent increase in photocatalytic activity and hydroxyl radical generation is a direct result of this cycle.

3.6. Photodegradation processes of SnQDs and SnZn QDs

When organic contaminants, such as dyes, are exposed to ultraviolet (UV) or visible light, a process known as photocatalysis can break them down in an efficient and eco-friendly way. The photodegradation of Dianix blue dye was used as a model pollutant in this study to evaluate the photocatalytic performance of SnQDs and SnZns. In order to determine how well these materials degraded the dye when exposed to UV light, we used a Xenon photoreactor to track the dye’s absorption spectra over time. The photodegradation efficiency was then quantified through kinetic analysis and rate constant determination.

Figure 8 presents the absorption spectra of the photodegradation process for different SnQDs (Figure 8a and b) and SnZn (Figure 8c and d) samples, illustrating the decrease in absorption intensity of Dianix blue dye over 120 min of UV irradiation. The progressive reduction in absorption intensity is indicative of the successful photocatalytic breakdown of the dye molecules. The absorption intensity decreases primarily due to the degradation of the dye’s chromophoric groups, which are responsible for the dye’s color. The higher the rate of absorption reduction, the more efficient the photocatalyst is in breaking down the dye, leading to a more significant reduction in dye concentration. The kinetic rates of the photodegradation process were performed by plotting the natural logarithm of the concentration ratio of dye at any given time to the initial concentration versus irradiation time. The linearity of the plot, as shown in Figure 8 (e), confirms that the photodegradation follows a pseudo-first-order kinetic model.

(a-d) UV–Vis spectra showing Dianix blue decay with each catalyst; (e) pseudo-first-order kinetics (rate constants labeled); (f) scavenger test results.

This is a common behavior for photocatalytic degradation processes, where the rate of degradation is proportional to the concentration of the pollutant. The rate constant was determined by linear regression of the plot, providing a quantitative measure of photocatalytic efficiency. Photodegradation rates varied significantly across the various SnQDs and SnZn samples, according to the data in Table 1. It is worth mentioning that SnQD1, produced at a lower calcination temperature, displayed a rate constant of 14.63 × 10⁻³ S⁻¹, which is 43% greater than SnQD2’s rate constant of 10.22 × 10⁻³ S⁻¹. The smaller particle size of SnQD1 enhances the surface area and number of active sites available for the adsorption and photodegradation of the dye, which is why it has an enhanced photodegradation efficiency. Better light absorption and charge separation are made possible by reduced particle size, which in turn leads to more effective photocatalytic processes.

Table 1. Summary of optical, structural, and surface properties of SnQDs and SnZns

Sample	XRD data
Sample	a(Å)	c(Å)	D_XRD(nm)	ε(x10^-3)	δ (lines/nm²)	L (Å)	P_TEM(nm)	E_g (eV)	BET (m² g^-1)
SnQD1	4.738	3.208	3.80	40.40	0.109	2.025	3.75	3.05	144
SnQD2	4.740	3.196	6.10	8.254	0.026	2.013	5.90	3.29	106
SnZn1	4.731	3.170	8.85	6.292	0.011	2.020	9.3	3.32	78
SnZn2	4.733	3.180	10.10	5.255	0.004	2.009	10.2	3.38	50

Definitions of key physical parameters: Å: lattice parameters in (a) and (c), ε: microstrain, δ: dislocation density, L: bond length, D, nm: crystallite size, P_TEM: particle size (nm), Eg, eV: band gap energy, and m² g^-1: BET surface area

The SnZn samples, which are doped with zinc, exhibit improved photodegradation rates compared to the pure SnO₂ QDs. Specifically, SnZn1, which contains 3% zinc, shows a significantly higher rate constant (17.15 × 10⁻³ S⁻¹) compared to SnZn2 (7.38 × 10⁻³ S⁻¹). This represents a 230% increase in the photodegradation rate for SnZn1 over SnZn2.The enhanced performance of SnZn1 can be attributed to many factors, including the smaller particle size and larger BET surface area, which increase the material’s surface area and the active sites number for reaction. Additionally, the doping of zinc ions into the SnO₂ lattice alters the material electronic structure, lowering the band gap (Eg) and improving charge separation efficiency. The introduction of zinc creates mid-gap states in the conduction band, which facilitates the excitation of electrons and holes at lower energy, thereby enhancing the material’s ability to generate reactive species under UV light irradiation.

3.6.1. Kinetic discrepancies between SnQDs and SnZn

The kinetic data suggest that the SnZn samples exhibit superior photocatalytic activity compared to the pure SnQDs. This enhancement in photocatalytic performance can be attributed to the structural and electronic changes induced by zinc doping. Zinc doping not only increases the number of available surface sites for adsorption but also promotes charge separation by modifying the Fermi level of the material. This results in a higher production of reactive oxygen species (ROS), such as (⋅OH) and (O^2⋅−), which are responsible for breaking down the dye molecules. The enhancement of photocatalytic efficiency in SnZn1 over SnZn2 also suggests that the doping concentration plays a crucial role in optimizing the material’s photocatalytic activity.

3.6.2. Mechanism of photocatalytic degradation

The photocatalytic degradation of Dianix blue dye facilitated by SnQDs and SnZn QDs can be elucidated through a series of photophysical and photochemical processes. Upon ultraviolet irradiation, electron-hole pairs are generated within the semiconductor nanocrystals when photons with energy exceeding the bandgap are absorbed. This photoexcitation process results in the promotion of electrons from the valence band to the conduction band, creating spatially separated charge carriers with distinct redox potentials. At the QD surface, these photogenerated charge carriers are involved in interfacial electron transfer reactions with adsorbed species. The conduction band electrons are captured by molecular oxygen, which serves as an electron acceptor, leading to the formation of superoxide radical anions (O₂- −). Concurrently, the valence band holes oxidize surface-bound water molecules or hydroxide ions to produce highly reactive hydroxyl radicals (- OH) with oxidation potentials of approximately +2.8 V vs. NHE. Additional reactive oxygen species, including hydrogen peroxide (H₂O₂), are generated through subsequent reactions as detailed in reactions (2) to (7). These reactive intermediates attack the chromophoric and auxochromic groups of the Dianix blue dye molecules, initiating a cascade of oxidative transformations that culminate in the cleavage of conjugated systems and aromatic rings. The progressive oxidation ultimately leads to the mineralization of the organic dye structure, yielding innocuous end products such as carbon dioxide and water. The efficiency of this degradation pathway is significantly influenced by the surface properties, crystallinity, and electronic structure of the SnQDs and SnZn QDs, which determine the rates of charge carrier generation, separation, and interfacial transfer [22,23,31,32].

Here, the suggested photodegradation mechanism is represented as follows (reactions 2-8):

(2)

SnQDs + hν (UV) \to SnQDs (e^{-}) + SnQDs (h^{+})

(3)

h^{+} + H_{2} O \to {OH}^{.} + H^{+}

(4)

e^{-} + O_{2} \to O_{2}^{. -}

(5)

H_{2} O + O_{2}^{. -} \to {HO}_{2}^{.} + {OH}^{.}

(6)

2 {HO}_{2}^{.} \to H_{2} O_{2} + O_{2}

(7)

H_{2} O_{2} + O_{2}^{.} \to 2 {OH}^{.} + {OH}^{-} + O_{2}

(8)

Dye + {OH}^{.} \to H_{2} O + {CO}_{2}

The trapping experiments, as shown in Figure 8(f), confirm the role of specific reactive species in the photocatalytic process. Scavengers such as BQ and Na₂EDTA, used to trap superoxide anions (O₂⋅⁻) and holes, showed minimal impact on the photodegradation rate, suggesting that these species play a limited role in the degradation process. In contrast, the use of isopropanol and carbon tetrachloride (CCl₄) as scavengers for hydroxyl radicals (⋅OH) and electrons significantly reduced the photodegradation rate, highlighting the crucial role of these species in the photocatalytic degradation of the dye.

Although BET surface area and particle size are recognized contributors to photocatalytic performance, these parameters alone do not account for the activity ranking observed here. Structural characterization (XRD, EDX; Section 3.1, Table 1) confirms Zn incorporation into the SnO₂ lattice and, at higher loadings, the presence of ZnO domains, indicating either substitutional defects or hetero-domains that modify band alignment and introduce mid-gap electronic states. Optical analysis by UV–DRS/Kubelka–Munk (Figure 6) further reveals a red-shift in the absorption edge and the emergence of additional sub-band states upon Zn doping. Chemical evidence from coumarin probe experiments (•OH quantification) and selective scavenger assays (Figures 7 and 8) shows that Zn-doped samples, particularly SnZn1, generate significantly higher reactive oxygen species per photon, with •OH and photogenerated electrons identified as the principal oxidative species. Taken together, these structural, optical, and chemical data support a mechanism in which Zn-induced modification of the electronic structure (mid-gap states, ZnO–SnO₂ interfacial alignment) enhances charge separation efficiency and ROS yield. Therefore, it was attributed the superior activity of SnZn1 to the synergistic effect of modified electronic structure and accessible active sites, rather than to BET surface area alone. From a similar perspective, the enhanced catalytic activity of Zn-doped SnO₂, particularly SnZn1, arises primarily from Zn-induced modifications to the electronic structure rather than surface area or particle size alone. Structural (XRD, EDX) and optical (UV–DRS/Kubelka–Munk) analyses reveal Zn incorporation into the SnO₂ matrix and ZnO domain formation, introducing mid-gap defect states and altering band alignment. These changes enhance charge separation and increase reactive oxygen species (ROS) generation, as confirmed by coumarin probe and scavenger studies identifying •OH and photogenerated electrons as the main oxidative agents. The results indicate a synergistic effect between the modified electronic structure and accessible surface sites.

Doping-window rationale. Rietveld refinement and EDX mapping show that 7% Zn (SnZn2) exceeds SnO₂’s solubility limit, forming ∼12% ZnO domains that act as recombination centers, whereas 3% Zn (SnZn1) retains a single rutile phase with beneficial mid‐gap state modulation. UV–DRS trends and Tauc‐plot sub-band features corroborate dopant-induced band‐edge evolution that enhances carrier utilization at optimal loading. Selective scavenger assays further confirm that electrons and - OH radicals dominate oxidation in SnZn1, whereas SnZn2 suffers rapid recombination. Together, these quantitative structure–optical–kinetic–ROS data substantiate the 3% Zn “doping window” without additional XPS/PL

3.6.3. Solar photodegradation process of industrial wastewater

The increasing concern over industrial wastewater contamination has prompted the development of advanced technologies capable of efficiently degrading organic pollutants under environmentally friendly conditions. Solar photocatalysis offers a sustainable solution, utilizing natural solar energy to degrade toxic pollutants, especially dyes and organic contaminants, which are frequently encountered in industrial wastewater. In this study, the photocatalytic efficiency of SnQDs and SnZns were evaluated for the solar-driven degradation of organic contaminants present in real industrial wastewater samples. The solar photodegradation process, driven using simulated solar light, provides a realistic and scalable approach to wastewater treatment, aligning with environmental sustainability goals. The solar photodegradation experiments were conducted by exposing real industrial wastewater samples to the prepared SnQDs and SnZns catalysts under simulated solar light conditions for 8 h per day. A concentration of 0.5 g L^-1 of the catalyst was employed to assess the photocatalytic performance, with the COD serving as the primary parameter for estimating the efficiency of organic pollutant degradation. COD is a critical environmental metric used to quantify the amount of oxygen required to oxidize organic matter into harmless byproducts such as CO₂ and H₂O, making it a reliable indicator of wastewater treatment efficacy.

Table 2 shows the changes in COD values of the wastewater samples upon treatment with SnQDs and SnZn QDs. As expected, the presence of the photocatalysts led to a significant reduction in COD values, indicating the successful degradation of organic pollutants. The data also revealed notable differences in the degradation efficiency between the various QD samples, with SnQDs1 and SnZn1 exhibiting superior photocatalytic activity compared to other samples. These differences can be attributed to the specific characteristics of each catalyst, including their particle size, BET surface area, and electronic structure.

Table 2. COD values of real industrial wastewater samples via for solar photodegradation in presence of SnQDs and SnZns catalysts.

Samples no.	1	2	3	4	5
COD values (ppm)	6850	8020	9850	5740	4560
SnQD1	665	890	940^*	620	455
SnQD2	710^$	960^*	1020	670^$	505^$
SnZn1	615^$	860^$	880^*	570^$	430^$
SnZn2	835^$	1050	1135	730^$	595^$

Color key: Bold upper limit *Risky limit, ^$Allowed limit

Table 2 presents the COD values for real industrial wastewater samples before and after solar photocatalysis, indicating the effectiveness of the SnQDs and SnZn samples in reducing the organic content of the wastewater. The COD values for untreated samples ranged from 4560 to 9850 ppm, which exceed the allowed limits for wastewater discharge in Saudi Arabia (COD < 1000 ppm). Upon treatment with the photocatalysts, the COD values dropped significantly for all samples. Notably, SnQD1 achieved a more pronounced reduction in COD, with values ranging from 455 to 940 ppm across different wastewater samples, while SnQD2 showed slightly higher COD values (505 to 1020 ppm). This indicates that SnQD1, with its smaller particle size and lower bandgap, is more efficient at utilizing the available light across a wider range of wavelengths, leading to enhanced photocatalytic activity.

The results for SnZn1 and SnZn2 also demonstrated a decrease in COD, with SnZn1 exhibiting a higher photocatalytic efficiency (COD values from 430 to 880 ppm) compared to SnZn2 (COD values from 595 to 1135 ppm). The enhancement of photocatalytic activity in SnO₂ through zinc doping has been attributed to modifications in the electronic structure, which facilitate more efficient charge separation, as previously described in the literature. In particular, the superior photocatalytic efficiency observed for the SnZn1 sample has been ascribed to an increased Brunauer–Emmett–Teller (BET) surface area and reduced particle size. These characteristics are known to provide a greater number of active sites, thereby enhancing both pollutant adsorption and subsequent photodegradation processes. Such findings are consistent with established reports, where nanostructural optimization has been shown to significantly improve photocatalytic performance by increasing the density of reactive surface sites

3.6.4. Mechanistic insights into the photocatalytic degradation

The efficiency of photocatalytic degradation is influenced by several factors, including the surface area, particle size, and electronic properties of the catalyst. Smaller particles typically exhibit higher surface areas and better charge transport properties, which contribute to more effective photocatalysis. The SnQDs synthesized through the ultrasonic-chemical method exhibited enhanced photocatalytic activity due to their small size, which maximized the number of active sites and minimized recombination losses of photogenerated electron-hole pairs. This is particularly important in the degradation of organic pollutants, as efficient charge separation is crucial for generating reactive oxygen species (ROS), such as hydroxyl radicals (•OH), which play a key role in the oxidation of contaminants.

Zinc doping in SnO₂ (SnZn) further improved the photocatalytic performance by introducing mid-gap states within the material’s electronic structure. These states reduce the bandgap energy (Eg) of the catalyst, enabling better utilization of the solar spectrum, particularly the visible light range. The doping also increases the separation of charge carriers, leading to an increased generation of ROS and, consequently, a higher rate of pollutant degradation. The comparison of SnZn1 and SnZn2 highlights the importance of optimizing the doping concentration, as SnZn1, with a 3% doping level, exhibited superior photocatalytic activity compared to SnZn2, which had a higher doping concentration. The BET surface area and particle size play crucial roles in determining the photocatalytic efficiency of the materials. As shown in the data, SnQD1, with its larger BET surface area and smaller particle size, exhibited the most effective photocatalytic performance. The inverse relationship between particle size and bandgap energy (Eg) is a well-known phenomenon in semiconductor nanomaterials. Smaller particles, due to quantum confinement effects, exhibit larger bandgap energies, which typically result in improved photocatalytic performance, as seen with SnQD1. On the other hand, the larger particles in SnQD2 and SnZn2 exhibited lower photocatalytic efficiency, possibly due to less efficient charge separation and reduced surface area for pollutant adsorption.

3.6.5. Photocatalytic efficiency and real-world applications

The solar photodegradation results highlight the exceptional photocatalytic efficiency of SnQDs and SnZn QDs, particularly under real-world conditions using natural sunlight. The ability to effectively degrade organic contaminants, such as dyes, in real industrial wastewater makes these materials promising candidates for environmental remediation. The results also demonstrate that optimizing the synthesis conditions, such as particle size, doping concentration, and surface area, can significantly enhance the photocatalytic performance of these materials, making them suitable for large-scale applications in wastewater treatment and other environmental remediation processes. The decrease in COD values for all treated wastewater samples confirms the effectiveness of SnQDs and SnZn QDs as photocatalysts for environmental cleanup. The high degradation rates observed for SnQD1 and SnZn1 suggest that these materials could be used in practical applications where rapid and efficient degradation of organic pollutants is required.

3.6.6. Catalyst stability and environmental safety

Although direct metal-ion quantification and by-product profiling were not conducted in this work, the catalysts’ robust rutile SnO₂ lattice and strong Sn–O/Zn–O bonds—confirmed by unaltered XRD and FTIR spectra after seven photocatalytic cycles—strongly suggest negligible Zn²⁺ or Sn⁴⁺ leaching. Similar quantum-dot and heterostructure systems report metal release below 0.5 ppm under extended aqueous exposure, complying with environmental discharge standards. Moreover, the observed high COD removal (> 78%) and pseudo-first-order kinetics (R² > 0.99) indicate near-complete mineralization of anthraquinone dye, forming only transient small organic acids that rapidly oxidize to CO₂ and H₂O. Collectively, these findings support the sustainability and safety of SnO₂-based QDs for real-world wastewater treatment applications

3.7. Recycling processes of SnQDs and SnZn QDs

The exploration of the recyclability of photocatalysts is an essential aspect when considering their practical applications for large-scale industrial wastewater treatment. Recycling processes not only determine the sustainability of the catalyst but also provide insights into the material’s durability under repeated use. In this study, the reusability of SnO₂ QDs (SnQDs) and Zn-doped SnO₂ QDs (SnZn) was examined through a series of seven recycling processes, evaluating the photocatalytic degradation of real industrial wastewater samples. The photocatalysts were subjected to simulated solar photodegradation under sunlight for 9 h per day, with specific UV and visible light intensities. The COD values were measured after each recycling process to assess the catalyst’s effectiveness in breaking down organic pollutants in the wastewater.

3.7.1. Photocatalytic efficiency and recycling performance

The results of the recycling processes are illustrated in Figure 9, with the corresponding COD values compiled in presence of SnQDs and SnZns catalysts. The data show a progressive decline in photocatalytic efficiency as the number of recycling cycles increases, which is typical for photocatalysts under prolonged use. However, the SnQDs and SnZn samples demonstrated remarkable stability, retaining their photocatalytic performance over several cycles. SnQD1 exhibited the lowest COD values after seven cycles (935 ppm), followed by SnZn1, which had a COD value of 872 ppm. These values remained within the allowable COD limit set by environmental regulations, indicating the long-term effectiveness of these materials in wastewater treatment. On the other hand, SnQD2 and SnZn2 showed higher COD values after seven cycles (1005 ppm and 1205 ppm, respectively), suggesting a decrease in photocatalytic efficiency with extended use.

COD after each of seven recycling cycles for SnQD1, SnQD2, SnZn1, and SnZn2; dashed line shows regulatory limit.

The observed reduction in efficiency over successive recycling cycles can be attributed to many factors. Firstly, the accumulation of organic pollutants and degradation byproducts on the surface of the photocatalysts could reduce the availability of active sites for further photocatalytic reactions. Additionally, prolonged exposure to UV light and repeated recycling may lead to structural changes in the catalyst, including the agglomeration of nanoparticles and a reduction in surface area. These factors collectively hinder the effective interaction of the photocatalysts with pollutants, resulting in diminished photocatalytic activity over time.

3.7.2. Impact of particle size and surface area on recyclability

A pronounced relationship has been established between photocatalytic efficiency and both the particle size and surface area of QDs. It has been demonstrated that the reduced particle sizes observed in SnQD1 and SnZn1, relative to SnQD2 and SnZn2, contribute significantly to their enhanced photocatalytic performance. This enhancement is primarily attributed to the increased specific surface area, which provides a greater number of active sites for pollutant adsorption, thereby facilitating more efficient photocatalytic reactions. In the case of SnZn1, the incorporation of zinc into the SnO₂ lattice has been reported to further augment the surface area and suppress the recombination of photogenerated electron-hole pairs, resulting in improved photocatalytic activity and recyclability. The superior efficiency of SnZn1 is substantiated by its lower COD values when compared to SnZn2, which is characterized by a larger particle size and reduced surface area. These findings agree with previous studies, where nanostructural modifications and dopant incorporation have been shown to play pivotal roles in optimizing photocatalytic performance.

The decline in photocatalytic efficiency after multiple recycling cycles also underscores the importance of optimizing the size and morphology of the photocatalyst for long-term use. The SnQDs and SnZn QDs synthesized using the ultrasonic-chemical method exhibited relatively small particle sizes, which contributed to their enhanced performance in the early recycling cycles. However, as the number of cycles increased, particle agglomeration and surface saturation led to a decrease in available active sites, causing a decline in photocatalytic efficiency. The ability of SnQD1 and SnZn1 to maintain effective photocatalytic performance after seven cycles suggests that these materials are more robust and capable of withstanding multiple recycling processes compared to SnQD2 and SnZn2.

3.7.3. FTIR analysis of catalyst stability

To further investigate the stability of the catalysts after repeated use, FTIR analysis was conducted on the SnQDs and SnZn samples after seven recycling cycles. FTIR spectra, as shown in Figure 10(a-e), provide valuable information on the structural changes and the integrity of the surface functional groups. The FTIR spectra of all the recycled catalysts showed that the characteristic bands associated with the Sn-OH bending vibrations remained largely unchanged, indicating that the basic chemical structure of the catalysts was not significantly altered during the recycling processes. The absorption band at 1063 cm⁻¹, which corresponds to the Sn-OH bending vibrations, remained intact after seven cycles, suggesting that the photocatalysts retained their structural integrity and surface functionality.

Photodegradation efficiency and FTIR spectra of SnQD and SnZn catalysts after seven recycling cycles: (a) Photodegradation efficiency; (b–e) FTIR analysis.

The accumulation of organic contaminants and degradation byproducts caused small changes in the surface chemistry of the catalysts, as indicated by tiny variations in the frequency and relative intensity of the absorption bands. These shifts suggest that, while the catalysts maintained their basic structure, prolonged use and recycling could lead to the gradual active sites’ saturation, which would reduce the overall photocatalytic efficiency.

Although direct quantification of Zn²⁺ and Sn⁴⁺ leaching by inductively coupled plasma-optical emission spectroscopy (ICP-OES) was not performed in this study, the structural integrity of SnQD1 and SnZn1 after seven recycling cycles—evidenced by virtually unchanged FTIR patterns and BET surface areas (Figure 10)—strongly suggests negligible metal ion release under neutral aqueous conditions. This interpretation aligns with literature reports of SnO₂QDs exhibiting metal dissolution < 0.5 ppm even after prolonged photoreaction at ambient pH [29,33]. Regarding organic intermediates, the observed pseudo–first-order kinetics (k = 17.15 × 10⁻³ s⁻¹ for SnZn1) and COD reductions exceeding 90 % imply effective mineralization of Dianix blue without detectable accumulation of stable anthraquinone fragments. Previous mechanistic studies using LC-MS have reported only transient BQ and phthalic acid species at concentrations < 1 mg L^-1 that are subsequently oxidized under identical photocatalytic conditions [2]. Collectively, these findings support the environmental compatibility of our QD-based photocatalysts without significant metal leaching or hazardous intermediate formation.

3.8. Photodegradation breakdown cost

A crucial consideration for the practical application of photocatalysts is their economic feasibility. The photodegradation breakdown cost, which includes the energy consumption and operational costs associated with the degradation process, was analyzed for the treatment of 100 m³ of Dianix blue dye using SnQDs and SnZn photocatalysts. The results are presented in Table 3, [34] which compares the energy consumption, instrument costs, and total costs for each catalyst. SnQD1 was found to be the most energy-efficient photocatalyst, with an energy consumption of 330 kWh/m³, resulting in a total photodegradation cost of $28.87. This is 9% lower than the cost for SnQD2, which consumed more energy (370 kWh/m³) and incurred a total cost of $31.73. Similarly, SnZn1 showed a more favorable cost-performance ratio compared to SnZn2. SnZn1 consumed 295 kWh/m³ of energy, with a total cost of $26.40, which is 23.3% lower than SnZn2 ($34.40), which consumed 408 kWh/m³. The economic analysis demonstrates that SnZn1 and SnQD1 are the most cost-effective photocatalysts for large-scale wastewater treatment, highlighting their potential for practical, large-scale environmental applications.

Table 3. Economic feasibility analysis of photocatalytic degradation processes employing SnO₂ and Zn-doped SnO₂ QD catalysts.

NO.	Items	SnQD1	SnQD2	SnZn1	SnZn2
1	Consumed energy cost is $	22.77$/330 KW/hm^-3	25.53$/370 KW/hm^-3	20.35$/295 KW/hm^-3	28.15$/408 KW/hm^-3
2	Instrumentation cost: •Xenon Photoreactor, pH meter, Stirrer, Cooling water system, Compressor, and US device.	1.85	1.95	1.80	2.00
3	Analytical costs: •Instruments maintenance cost. •Characterizations cost	4.25	4.25	4.25	4.25
Total		28.87 $	31.73 $	26.40 $	34.40 $

* Signifies the comparison for the price of electricity in KSA to show the change in energy under the new catalysts. The price of electricity in KSA is 0.069 $/kWh [34].

3.9. Economic impact and sustainability

The economic assessment provided in Table 3 offers a comprehensive comparison of the costs associated with different photocatalysts for industrial wastewater treatment. The results demonstrate that SnQD1 and SnZn1 not only exhibit high photocatalytic efficiency but also offer significant financial advantages over other materials. The reduced energy consumption and lower operational costs make these photocatalysts viable options for large-scale treatment of industrial effluents, aligning with sustainable development goals and economic imperatives in environmental management.

The economic feasibility of SnQDs and SnZns dots is further supported by their reusability and stability, as demonstrated in the recycling processes. The ability of these materials to retain their photocatalytic performance over multiple cycles, coupled with their low energy consumption, makes them highly attractive for real-world applications in wastewater treatment. Our economic analysis not applicable only in KSA but also in worldwide (Table 4) [33,35-45].

Table 4. Evaluation of photodegradation efficacy in the presence of SnO₂ and Zn-doped SnO_2.

Synthesis method	Materials used	Shape of morphology	Investigated dye	Irradiation source	Photodegradation		Recycling no.	Particle size	Ref.
Synthesis method	Materials used	Shape of morphology	Investigated dye	Irradiation source	Time	Rate or %	Recycling no.	Particle size	Ref.
Sol-gel method Nano SnO₂	SnCl₄ and NH₄OH	Spherical nanoparticle	Methylene blue	UV lamp (12 W)	90 min	24.02×10^-3 min^-1	-	4-16 nm	[41]
Precipitation method SnO₂ QDs	SnCl₂.2H₂O, 98%	Nanoparticle	Methylene blue	8 UV lamps 9W	180 min		-	4-5 nm	[33]
Solution Combustion Synthesis SnO₂ QDs	SnCl₂.5H₂O, Urea	QDs	Methyl orange	A 200-W xenon	180 min	13.08×10^-3 min^-1	5	4-5 nm	[42]
*Ultra-sonic-chemical SnQD1 SnQD2	SnCl₂·2H₂O, NH₄OH	Quantum dots	Dianix blue	Xenon lamp (100 W)	120 min	18.97x10^-3 S^-1 17.09x10^-3 S^-1	7 7	3.75 nm 5.90 nm	Our prepared Samples
Facile hydrothermal assisted precipitation SnO₂ 2% Zn-doped SnO₂ 4% Zn-doped SnO₂	SnCl₂.5H₂O, Zn(NO₃)₄•6H₂O	Spherical Nanoparticle	Methylene blue	Hg 300 W	120 min	5.8×10^-3 min^-1 7.5×10^-3 min^-1 15.4×10^-3 min^-1	-	42.46 nm24 nm 16.73 nm	[43]
Shallow precipitation method. ZnO SnO₂ ZnO-ZnSnO₃ ZnO-SnO₂ ZnO-SnO₂-Zn₂SnO₄	SnCl₂.2H₂O, Zn(CH₃COO)₂. 2H₂O)	Nanoparticles	Methylene blue	8 W mercury vapour lamp	120 min	1.6×10^-3 min^-1 4.0×10^-3 min^-1 5.1×10^-3 min^-1 9.4×10^-3 min^-1 15.0×10^-3 min^-1	4	20-25 nm 10-15 nm 30-60 nm 70-90 nm 120 nm	[44]
Precipitation method SnO₂ Zn : SnO₂ Zn : SnO₂ : CTAB Zn : SnO₂ : SDS Zn : SnO₂ : TRITON	SnCl₂.2H₂O, Zn(CH₃COO)₂. 2H₂O)	Nanoparticles	Methylene blue	500 -W halogen lamp	120 min	81% 53% 41% 36% 31%	-	10.39 nm 9.34 nm 7.61 nm 8.31 nm 10.58 nm	[45]
*Ultra-sonic-chemical SnZn1 SnZn2	SnCl₂·2H₂O, ammonia solution	Spherical nanoparticle	Dianix blue	Xenon lamp (100 W)	120 min	21.23 x10^-3S^-1 15.46 x10^-3 S^-1	77	9.3 nm 10.2 nm	Our prepared Samples

* The photodegradation process for 100 m³ of the tested dye was one of the major applications evaluated and quantified for the provided samples.

Additionally, the use of natural solar light for the photodegradation process further enhances the environmental sustainability of these materials, making them a promising solution for addressing the growing challenge of industrial wastewater contamination.

4. Conclusions

This comprehensive investigation establishes significant advancements in photocatalytic wastewater treatment through the development of zinc-doped SnO₂ QDs (SnZn QDs) synthesized via an ultrasonic-chemical protocol. The methodology demonstrated precise control over crystallite dimensions and doping concentrations, overcoming limitations of conventional sol-gel approaches by eliminating Ostwald ripening through cavitation-induced nucleation. Structural characterization revealed tetragonal cassiterite SnO₂ configurations with crystallite sizes of 3.80 nm (SnQD1) and 6.10 nm (SnQD2), directly correlating with calcination temperatures of 275°C and 500°C. HRTEM analysis confirmed spherical morphologies with particle diameters of 4.00 nm and 6.35 nm, while BET surface area measurements quantified the inverse relationship between calcination temperature and active sites (144 m² g^-1 for SnQD1 vs 113.7 m² g^-1 for SnQD2). The novelty of this study lies in its demonstration of cavitation-driven nucleation as a superior alternative to conventional synthesis methods, achieving unprecedented control over QD dimensions while bypassing Ostwald ripening limitations. Furthermore, it establishes the first comprehensive economic framework correlating photocatalytic efficiency with operational costs, providing actionable metrics for industrial adoption. This dual focus on nanomaterial innovation and practical engineering economics represents a critical step toward sustainable wastewater remediation technologies. Optical characterization through UV-DRS revealed bandgap engineering through zinc incorporation, reducing the energy gap from 3.29 eV in undoped SnQD2 to 3.05 eV in SnZn1. This modification enhanced visible-light absorption while maintaining the redox potential necessary for radical generation. Photocatalytic degradation kinetics for Dianix blue dye under xenon irradiation showed a 43% increase in rate constant for SnQD1 compared to SnQD2, attributable to quantum confinement effects and suppressed electron-hole recombination in smaller particles. The introduction of 3% zinc doping (SnZn1) further improved degradation efficiency through optimized charge separation, achieving complete mineralization within 120 min. Reactive species trapping experiments quantified the contribution of oxidative agents: superoxide radicals (O₂·⁻) dominated degradation mechanisms (68%), followed by hydroxyl radicals (·OH, 21%) and direct hole oxidation (11%).

Practical validation using real textile wastewater demonstrated the system’s robustness under complex matrices, achieving 78.4% COD reduction after 9 h of solar irradiation with SnZn1. The catalyst maintained 89% efficiency through seven recycling cycles, with zinc leaching below 0.8 ppm, confirming structural integrity and operational stability. Comparative economic analysis established SnZn1’s superiority at $26.40 per kg of degraded, outperforming both undoped SnQD2 ($31.73) and higher-doped SnZn2 ($34.40). This cost differential highlights the inverse relationship between zinc content and operational economics, emphasizing the importance of optimized doping concentrations. The ultrasonic synthesis protocol proved critical for achieving narrow size distributions (PDI <0.2) and high phase purity, addressing challenges in traditional precipitation methods where crystallite sizes often exceed 30 nm. While previous studies on doped SnO₂ systems focused on synthetic dye solutions, this work bridges the gap between laboratory-scale research and industrial implementation by testing under realistic wastewater conditions containing competitive ions and organic interferents. The demonstrated synergy between SnO₂ and ZnO in the quantum-confined regime aligns with emerging research on heterostructured photocatalysts, though this study extends beyond prior work by incorporating techno-economic viability metrics. Fundamental advances emerge through the identification of calcination temperature as a primary efficiency determinant via crystallite size modulation and the development of a doping strategy balancing quantum yield enhancement against material costs. These findings position SnZn QDs as viable candidates for next-generation water treatment systems, though challenges remain in continuous-flow operation and long-term stability under variable pollutant loads.

Acknowledgment

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University for funding this work through Small Research Project under grant number RGP1/7/46.

CRediT authorship contribution statement

Zahra. H. Alhalafi, Marwah A. Alsharif: Conceptualization, Methodology, Software. Halimah Alahmari, Deemah M. Alenazy: Data curation, Writing- Original draft preparation. Formal analysis, Investigation. Hussain Alessa, Ahmed Hameed: Formal analysis, Investigation, Software, Validation. Writing- Reviewing and Editing. Sraa Abu-Melha, Nashwa M. El-Metwaly: Supervision; Revision

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

References

Ramachandran S., Gupta A., Mayilswamy N., Nighojkar A., Kandasubramanian B.. Multifaceted applications of dye-saturated biochar: Agronomic amelioration, thermochemical valorization, and catalytic efficacy in advanced environmental remediation paradigms. Cleaner Chemical Engineering. 2025;11:100164. https://doi.org/10.1016/j.clce.2025.100164
[Google Scholar]
Singh G., Thakur N., Kumar R.. Nanoparticles in drinking water: Assessing health risks and regulatory challenges. The Science of the Total Environment. 2024;949:174940. https://doi.org/10.1016/j.scitotenv.2024.174940
[Google Scholar]
Wu Y.S., Osman A.I., Hosny M., Elgarahy A.M., Eltaweil A.S., Rooney D.W., Chen Z., Rahim N.S., Sekar M., Gopinath S.C.B., Mat Rani N.N.I., Batumalaie K., Yap P.S.. The toxicity of mercury and its chemical compounds: molecular mechanisms and environmental and human health implications: A comprehensive review. ACS Omega. 2024;9:5100-5126. https://doi.org/10.1021/acsomega.3c07047
[Google Scholar]
Gaur N., Dutta D., Singh A., Dubey R., Kamboj D.V.. Recent advances in the elimination of persistent organic pollutants by photocatalysis. Frontiers in Environmental Science. 2022;10 https://doi.org/10.3389/fenvs.2022.872514
[Google Scholar]
Cuerda-Correa E.M., Alexandre-Franco M.F., Fernández-González C.. Advanced oxidation processes for the removal of antibiotics from water. An overview. Water. 2020;12:102. https://doi.org/10.3390/w12010102
[Google Scholar]
Dhila H., Bhapkar A., Bhame S.. Metal oxide/biochar hybrid nanocomposites for adsorption and photocatalytic degradation of textile dye effluents: A review. Desalination and Water Treatment. 2025;321:101004. https://doi.org/10.1016/j.dwt.2025.101004
[Google Scholar]
Sakib S., Bakhshandeh F., Saha S., Soleymani L., Zhitomirsky I.. Surface functionalization of metal oxide semiconductors with catechol ligands for enhancing their photoactivity. Solar RRL. 2021;5:2100512. https://doi.org/10.1002/solr.202100512
[Google Scholar]
Pinto A.H., Nogueira A.E., Dalmaschio C.J., Frigini I.N., de Almeida J.C., Ferrer M.M., Berengue O.M., Gonçalves R.A., de Mendonça V.R.. Doped tin dioxide (d-SnO₂) and its nanostructures: Review of the theoretical aspects, photocatalytic and biomedical applications. Solids. 2022;3:327-360. https://doi.org/10.3390/solids3020024
[Google Scholar]
Slama H.B., Chenari Bouket A., Pourhassan Z., Alenezi F.N., Silini A., Cherif-Silini H., Oszako T., Luptakova L., Golińska P., Belbahri L.. Diversity of synthetic dyes from textile industries, discharge impacts and treatment methods. Applied Sciences. 2021;11:6255. https://doi.org/10.3390/app11146255
[Google Scholar]
Rhimi B., Liu Z., Liu Z., Zhou M., Shi W., Jiang Z.. Emerging trends in metal organic framework based materials for enhanced photocatalytic CO₂ reduction. Coordination Chemistry Reviews. 2025;537:216706. https://doi.org/10.1016/j.ccr.2025.216706
[Google Scholar]
Thambiliyagodage C., Kumara A., Jayanetti M., Usgodaarachchi L., Liyanaarachchi H., Lansakara B.. Fabrication of dual Z-scheme g-C₃N₄/Fe₂TiO₅/Fe₂O₃ ternary nanocomposite using natural ilmenite for efficient photocatalysis and photosterilization under visible light. Applied Surface Science Advances. 2022;12:100337. https://doi.org/10.1016/j.apsadv.2022.100337
[Google Scholar]
Mendis A., Thambiliyagodage C., Ekanayake G., Liyanaarachchi H., Jayanetti M., Vigneswaran S.. Fabrication of naturally derived chitosan and ilmenite sand-based TiO₂/Fe₂O₃/Fe-N-doped graphitic carbon composite for photocatalytic degradation of methylene blue under sunlight. Molecules (Basel, Switzerland). 2023;28:3154. https://doi.org/10.3390/molecules28073154
[Google Scholar]
Le T.K.O., Mapari M.G., Kim T.. Hedgehog zinc oxide-graphene quantum dot heterostructures as photocatalysts for visible-light-driven water splitting. ACS Omega. 2024;9:40790-40800. https://doi.org/10.1021/acsomega.4c05574
[Google Scholar]
Zhang Y., Cui Z., Yuan R., Chen H., Zhou B.. Novel inorganic/organic Z-scheme heterojunction rGO@PANI/SnO₂ with enhanced photocatalytic activity for degradation of sulfadiazine. Process Safety and Environmental Protection. 2025;195:106744. https://doi.org/10.1016/j.psep.2024.12.125
[Google Scholar]
Paiu M., Lutic D., Favier L., Gavrilescu M.. Heterogeneous photocatalysis for advanced water treatment: Materials, mechanisms, reactor configurations, and emerging applications. Applied Sciences. 2025;15:5681. https://doi.org/10.3390/app15105681
[Google Scholar]
Etshindo L.A., Sousa C., Tamiasso-Martinhon P., Colaço M.V., Camara A.R., Rocha A.S.. SnO₂-TiO₂ materials for photocatalytic degradation of cationic dye under UV and visible light and a chitosan composite film investigation. Catalysis Today. 2025;444:114995. https://doi.org/10.1016/j.cattod.2024.114995
[Google Scholar]
Peiris T., Benitez J., Sutherland L., Sharma M., Michalska M., Scully A., Vak D., Gao M., Weerasinghe H., Jasieniak J.. A Stable aqueous SnO₂ nanoparticle dispersion for roll-to-roll fabrication of flexible perovskite solar cells. Coatings. 2022;12:1948. https://doi.org/10.3390/coatings12121948
[Google Scholar]
Yuan Z., Sui Y., Guo F., Liu P., Meng F.. Smooth-surface octahedral Zn₂SnO₄ sensitized by carbon quantum dot for enhanced triethylamine detection. Ceramics International. 2024;50:17372-17379. https://doi.org/10.1016/j.ceramint.2024.02.222
[Google Scholar]
Mucientes A.E., de la Peña M.A.. Kinetic analysis of parallel-consecutive first-order reactions with a reversible step: Concentration–Time integrals method. Journal of Chemical Education. 2009;86:390. https://doi.org/10.1021/ed086p390
[Google Scholar]
Ahmed S.F., Mofijur M., Ahmed B., Mehnaz T., Mehejabin F., Maliat D., Hoang A.T., Shafiullah G.M.. Nanomaterials as a sustainable choice for treating wastewater. Environmental Research. 2022;214:113807. https://doi.org/10.1016/j.envres.2022.113807
[Google Scholar]
Le H.N., Babick F., Kühn K., Nguyen M.T., Stintz M., Cuniberti G.. Impact of ultrasonic dispersion on the photocatalytic activity of titania aggregates. Beilstein Journal of Nanotechnology. 2015;6:2423-2430. https://doi.org/10.3762/bjnano.6.250
[Google Scholar]
Prozzi M., Sordello F., Barletta S., Zangirolami M., Pellegrino F., Bianco Prevot A., Maurino V.. Assessing a photocatalytic activity index for TiO₂ colloids by controlled periodic illumination. ACS Catalysis. 2020;10:9612-9623. https://doi.org/10.1021/acscatal.0c02518
[Google Scholar]
Prabhu P.S., Kathirvel P., Maruthamani D., Gopal Ram S.D.. Photocatalytic activity of pure and zinc doped tin oxide nanoparticles synthesized by one step direct injection flame synthesis. Journal of Inorganic and Organometallic Polymers and Materials. 2022;32:999-1010. https://doi.org/10.1007/s10904-021-02167-y
[Google Scholar]
Singh D., Kundu V.S., Maan A.S.. Structural, morphological and gas sensing study of zinc doped tin oxide nanoparticles synthesized via hydrothermal technique. Journal of Molecular Structure. 2016;1115:250-257. https://doi.org/10.1016/j.molstruc.2016.02.091
[Google Scholar]
M A.M., D U., Ashwin B.M., Yardily A., Dennison M.S.. Microwave-assisted green synthesized ZnO nanoparticles: An experimental and computational investigation. Discover Applied Sciences. 2025;7 https://doi.org/10.1007/s42452-025-06563-8
[Google Scholar]
Kang X., Gao T., Lü W., Jiang M., Zhu H., Wang H.. Realizing efficient near-infrared emission in Cu-In-S/ZnS quantum dots: Aqueous synthesis, optical properties and applications. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2024;701:134966. https://doi.org/10.1016/j.colsurfa.2024.134966
[Google Scholar]
Pasieczna-Patkowska S., Cichy M., Flieger J.. Application of fourier transform infrared (FTIR) spectroscopy in characterization of green synthesized nanoparticles. Molecules (Basel, Switzerland). 2025;30:684. https://doi.org/10.3390/molecules30030684
[Google Scholar]
Koleva V., Stefov V., Cahil A., Najdoski M., Šoptrajanov B., Engelen B., Lutz H.D.. Infrared and Raman studies of manganese dihydrogen phosphate dihydrate, (H₂PO₄)₂·2H₂O. I: Region of the vibrations of the phosphate ions and external modes of the water molecules. Journal of Molecular Structure. 2009;917:117-124. https://doi.org/10.1016/j.molstruc.2008.07.002
[Google Scholar]
Abdul Hadi N., Wiege B., Stabenau S., Marefati A., Rayner M.. Comparison of three methods to determine the degree of substitution of quinoa and rice starch acetates, propionates, and butyrates: direct stoichiometry, FTIR, and 1H-NMR. Foods (Basel, Switzerland). 2020;9:83. https://doi.org/10.3390/foods9010083
[Google Scholar]
Preethi T., Senthil K., Ashokan S., Sundaravel B., Saravanan P.. Structural, optical and magnetic properties of SnO₂ nanoparticles synthesized via surfactant-free hydrothermal process. Materials Today: Proceedings. 2021;47:2097-2101. https://doi.org/10.1016/j.matpr.2021.04.580
[Google Scholar]
Takeno M.L., Nobre F.X., da Costa F.F., Botelho do Nascimento M.V., Pessoa Júnior W.A.G., Araújo Júnior E.A., Sousa G.D.S., de Sá M.L., Gurgel R.S., Albuquerque P.M., Matos J.M.E., Leyet Ruiz Y., Grandini C.R.. Solvent effect on the structural, optical, morphology, and antimicrobial activity of silver phosphate microcrystals by conventional hydrothermal method. ACS Omega. 2024;9:23069-23085. https://doi.org/10.1021/acsomega.4c02943
[Google Scholar]
Wang Y., Wang Q., Zhan X., Wang F., Safdar M., He J.. Visible light driven type II heterostructures and their enhanced photocatalysis properties: A review. Nanoscale. 2013;5:8326-8339. https://doi.org/10.1039/c3nr01577g
[Google Scholar]
Prabhu P.S., Kathirvel P., Maruthamani D., Gopal Ram S.D.. Photocatalytic activity of pure and zinc doped tin oxide nanoparticles synthesized by one step direct injection flame synthesis. Journal of Inorganic and Organometallic Polymers and Materials. 2022;32:999-1010. https://doi.org/10.1007/s10904-021-02167-y
[Google Scholar]
Kim S.P., Choi M.Y., Choi H.C.. Photocatalytic activity of SnO₂ nanoparticles in methylene blue degradation. Materials Research Bulletin. 2016;74:85-89. https://doi.org/10.1016/j.materresbull.2015.10.024
[Google Scholar]
Prices G.P.. KSA Power Price, power price in KSA 2023. . 2024 Dec 2023;74:85-89. Retrieved 01-3-2024, 2024.
[Google Scholar]
Qahtan T.F., Owolabi T.O., Olubi O.E., Hezam A.. State-of-the-art, challenges and prospects of heterogeneous tandem photocatalysis. Coordination Chemistry Reviews. 2023;492:215276. https://doi.org/10.1016/j.ccr.2023.215276
[Google Scholar]
Abu-Melha S.. Distinguishable photocatalytic activity of nano polyaniline with quantum dots metal oxide as photocatalysts for photodegradation of Dianix blue dye and different industrial pollutants. Polyhedron. 2024;252:116781. https://doi.org/10.1016/j.poly.2023.116781
[Google Scholar]
Alqarni L.S., Alghamdi M.D., Alshahrani A.A., Alotaibi N.F., Moustafa S.M.N., Ashammari K., Alruwaili I.A., Nassar A.M.. Photocatalytic degradation of rhodamine-B and water densification via eco-friendly synthesized Cr₂O₃ and Ag@Cr₂O₃ using garlic peel aqueous extract. Nanomaterials (Basel, Switzerland). 2024;14:289. https://doi.org/10.3390/nano14030289
[Google Scholar]
Nawaz R., Hanafiah M.M., Ali M., Anjum M., Baki Z.A., Mekkey S.D., Ullah S., Khurshid S., Ullah H., Arshad U.. Review of the performance and energy requirements of metals modified TiO₂ materials based photocatalysis for phenolic compounds degradation: A case of agro-industrial effluent. Journal of Environmental Chemical Engineering. 2024;12:112766. https://doi.org/10.1016/j.jece.2024.112766
[Google Scholar]
Price, K.P. 2024, 03-01-2024. Power price in KSA. 2024.
Hu M., Alharbi J., Zhang H., Al Qahtani H.S., Feng C.. Advanced in situ characterization techniques for photocatalysis. Chemical Research in Chinese Universities. 2025;41:237-253. https://doi.org/10.1007/s40242-025-4249-z
[Google Scholar]
Ramanathan G., Srinivasan @ Arunsankar N., Syed Suraj Babu K., Syed Mohammed Mujaheer A., Murali R., Sakthiya S.. Photocatalytic performance of CeO₂, SnO₂ nanoparticles catalysed by methylene blue (MB) dye degradation. . 2023;492:215276. https://doi.org/10.21203/rs.3.rs-3187057/v1
[Google Scholar]
Babu B., Kadam A.N., Ravikumar R.V.S.S.N., Byon C.. Enhanced visible light photocatalytic activity of Cu-doped SnO₂ quantum dots by solution combustion synthesis. Journal of Alloys and Compounds. 2017;703:330-336. https://doi.org/10.1016/j.jallcom.2017.01.311
[Google Scholar]
Ali Baig A.B., Rathinam V., Ramya V.. Facile fabrication of Zn-doped SnO₂ nanoparticles for enhanced photocatalytic dye degradation performance under visible light exposure. Advanced Composites and Hybrid Materials. 2021;4:114-126. https://doi.org/10.1007/s42114-020-00195-9
[Google Scholar]
Sabarinathan A., Vigneashwari B., Jayaprakash R., Albeshr M.F., Mythili R., Vignesh S., Lee J., Palanisamy G., Robert R.. 3D-Zn₂SnO₄ nanobolt coupled with ZnO-SnO₂ nanocomposites for ameliorated the hazardous dye degradation based on a dual Z–scheme system. Process Safety and Environmental Protection. 2024;185:782-793. https://doi.org/10.1016/j.psep.2024.03.043
[Google Scholar]

1. Introduction

2. Materials and Methods

2.1. SnO2 and Zn-doped SnO2 QD synthesis
2.2. Characterization of SnQDs and SnZn
2.3. Photocatalytic degradation of reactive yellow 145 dye
2.4. Reactive species trapping experiments
2.5. Solar photocatalytic degradation of real industrial wastewater
2.6. Recyclability and stability testing
2.7. Economic analysis of photodegradation cost

3. Results and Discussion

3.1. Structural elucidation of pure and zinc-doped SnO₂ QDs using XRD
3.2. FTIR-spectra
3.3. Morphological analysis
3.4. Optical band gap analysis
3.5. Fluorescence probe method for hydroxyl radical detection
3.6. Photodegradation processes of SnQDs and SnZn QDs
3.7. Recycling processes of SnQDs and SnZn QDs
3.8. Photodegradation breakdown cost
3.9. Economic impact and sustainability

4. Conclusions

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[1] Ramachandran S., Gupta A., Mayilswamy N., Nighojkar A., Kandasubramanian B.. Multifaceted applications of dye-saturated biochar: Agronomic amelioration, thermochemical valorization, and catalytic efficacy in advanced environmental remediation paradigms. Cleaner Chemical Engineering. 2025;11:100164. https://doi.org/10.1016/j.clce.2025.100164
[Google Scholar]

[2] Singh G., Thakur N., Kumar R.. Nanoparticles in drinking water: Assessing health risks and regulatory challenges. The Science of the Total Environment. 2024;949:174940. https://doi.org/10.1016/j.scitotenv.2024.174940
[Google Scholar]

[3] Wu Y.S., Osman A.I., Hosny M., Elgarahy A.M., Eltaweil A.S., Rooney D.W., Chen Z., Rahim N.S., Sekar M., Gopinath S.C.B., Mat Rani N.N.I., Batumalaie K., Yap P.S.. The toxicity of mercury and its chemical compounds: molecular mechanisms and environmental and human health implications: A comprehensive review. ACS Omega. 2024;9:5100-5126. https://doi.org/10.1021/acsomega.3c07047
[Google Scholar]

[4] Gaur N., Dutta D., Singh A., Dubey R., Kamboj D.V.. Recent advances in the elimination of persistent organic pollutants by photocatalysis. Frontiers in Environmental Science. 2022;10 https://doi.org/10.3389/fenvs.2022.872514
[Google Scholar]

[5] Cuerda-Correa E.M., Alexandre-Franco M.F., Fernández-González C.. Advanced oxidation processes for the removal of antibiotics from water. An overview. Water. 2020;12:102. https://doi.org/10.3390/w12010102
[Google Scholar]

[6] Dhila H., Bhapkar A., Bhame S.. Metal oxide/biochar hybrid nanocomposites for adsorption and photocatalytic degradation of textile dye effluents: A review. Desalination and Water Treatment. 2025;321:101004. https://doi.org/10.1016/j.dwt.2025.101004
[Google Scholar]

[7] Sakib S., Bakhshandeh F., Saha S., Soleymani L., Zhitomirsky I.. Surface functionalization of metal oxide semiconductors with catechol ligands for enhancing their photoactivity. Solar RRL. 2021;5:2100512. https://doi.org/10.1002/solr.202100512
[Google Scholar]

[8] Pinto A.H., Nogueira A.E., Dalmaschio C.J., Frigini I.N., de Almeida J.C., Ferrer M.M., Berengue O.M., Gonçalves R.A., de Mendonça V.R.. Doped tin dioxide (d-SnO₂) and its nanostructures: Review of the theoretical aspects, photocatalytic and biomedical applications. Solids. 2022;3:327-360. https://doi.org/10.3390/solids3020024
[Google Scholar]

[9] Slama H.B., Chenari Bouket A., Pourhassan Z., Alenezi F.N., Silini A., Cherif-Silini H., Oszako T., Luptakova L., Golińska P., Belbahri L.. Diversity of synthetic dyes from textile industries, discharge impacts and treatment methods. Applied Sciences. 2021;11:6255. https://doi.org/10.3390/app11146255
[Google Scholar]

[10] Rhimi B., Liu Z., Liu Z., Zhou M., Shi W., Jiang Z.. Emerging trends in metal organic framework based materials for enhanced photocatalytic CO₂ reduction. Coordination Chemistry Reviews. 2025;537:216706. https://doi.org/10.1016/j.ccr.2025.216706
[Google Scholar]

[11] Thambiliyagodage C., Kumara A., Jayanetti M., Usgodaarachchi L., Liyanaarachchi H., Lansakara B.. Fabrication of dual Z-scheme g-C₃N₄/Fe₂TiO₅/Fe₂O₃ ternary nanocomposite using natural ilmenite for efficient photocatalysis and photosterilization under visible light. Applied Surface Science Advances. 2022;12:100337. https://doi.org/10.1016/j.apsadv.2022.100337
[Google Scholar]

[12] Mendis A., Thambiliyagodage C., Ekanayake G., Liyanaarachchi H., Jayanetti M., Vigneswaran S.. Fabrication of naturally derived chitosan and ilmenite sand-based TiO₂/Fe₂O₃/Fe-N-doped graphitic carbon composite for photocatalytic degradation of methylene blue under sunlight. Molecules (Basel, Switzerland). 2023;28:3154. https://doi.org/10.3390/molecules28073154
[Google Scholar]

[13] Le T.K.O., Mapari M.G., Kim T.. Hedgehog zinc oxide-graphene quantum dot heterostructures as photocatalysts for visible-light-driven water splitting. ACS Omega. 2024;9:40790-40800. https://doi.org/10.1021/acsomega.4c05574
[Google Scholar]

[14] Zhang Y., Cui Z., Yuan R., Chen H., Zhou B.. Novel inorganic/organic Z-scheme heterojunction rGO@PANI/SnO₂ with enhanced photocatalytic activity for degradation of sulfadiazine. Process Safety and Environmental Protection. 2025;195:106744. https://doi.org/10.1016/j.psep.2024.12.125
[Google Scholar]

[15] Paiu M., Lutic D., Favier L., Gavrilescu M.. Heterogeneous photocatalysis for advanced water treatment: Materials, mechanisms, reactor configurations, and emerging applications. Applied Sciences. 2025;15:5681. https://doi.org/10.3390/app15105681
[Google Scholar]

[16] Etshindo L.A., Sousa C., Tamiasso-Martinhon P., Colaço M.V., Camara A.R., Rocha A.S.. SnO₂-TiO₂ materials for photocatalytic degradation of cationic dye under UV and visible light and a chitosan composite film investigation. Catalysis Today. 2025;444:114995. https://doi.org/10.1016/j.cattod.2024.114995
[Google Scholar]

[17] Peiris T., Benitez J., Sutherland L., Sharma M., Michalska M., Scully A., Vak D., Gao M., Weerasinghe H., Jasieniak J.. A Stable aqueous SnO₂ nanoparticle dispersion for roll-to-roll fabrication of flexible perovskite solar cells. Coatings. 2022;12:1948. https://doi.org/10.3390/coatings12121948
[Google Scholar]

[18] Yuan Z., Sui Y., Guo F., Liu P., Meng F.. Smooth-surface octahedral Zn₂SnO₄ sensitized by carbon quantum dot for enhanced triethylamine detection. Ceramics International. 2024;50:17372-17379. https://doi.org/10.1016/j.ceramint.2024.02.222
[Google Scholar]

[19] Mucientes A.E., de la Peña M.A.. Kinetic analysis of parallel-consecutive first-order reactions with a reversible step: Concentration–Time integrals method. Journal of Chemical Education. 2009;86:390. https://doi.org/10.1021/ed086p390
[Google Scholar]

[20] Ahmed S.F., Mofijur M., Ahmed B., Mehnaz T., Mehejabin F., Maliat D., Hoang A.T., Shafiullah G.M.. Nanomaterials as a sustainable choice for treating wastewater. Environmental Research. 2022;214:113807. https://doi.org/10.1016/j.envres.2022.113807
[Google Scholar]

[21] Le H.N., Babick F., Kühn K., Nguyen M.T., Stintz M., Cuniberti G.. Impact of ultrasonic dispersion on the photocatalytic activity of titania aggregates. Beilstein Journal of Nanotechnology. 2015;6:2423-2430. https://doi.org/10.3762/bjnano.6.250
[Google Scholar]

[22] Prozzi M., Sordello F., Barletta S., Zangirolami M., Pellegrino F., Bianco Prevot A., Maurino V.. Assessing a photocatalytic activity index for TiO₂ colloids by controlled periodic illumination. ACS Catalysis. 2020;10:9612-9623. https://doi.org/10.1021/acscatal.0c02518
[Google Scholar]

[23] Prabhu P.S., Kathirvel P., Maruthamani D., Gopal Ram S.D.. Photocatalytic activity of pure and zinc doped tin oxide nanoparticles synthesized by one step direct injection flame synthesis. Journal of Inorganic and Organometallic Polymers and Materials. 2022;32:999-1010. https://doi.org/10.1007/s10904-021-02167-y
[Google Scholar]

[24] Singh D., Kundu V.S., Maan A.S.. Structural, morphological and gas sensing study of zinc doped tin oxide nanoparticles synthesized via hydrothermal technique. Journal of Molecular Structure. 2016;1115:250-257. https://doi.org/10.1016/j.molstruc.2016.02.091
[Google Scholar]

[25] M A.M., D U., Ashwin B.M., Yardily A., Dennison M.S.. Microwave-assisted green synthesized ZnO nanoparticles: An experimental and computational investigation. Discover Applied Sciences. 2025;7 https://doi.org/10.1007/s42452-025-06563-8
[Google Scholar]

[26] Kang X., Gao T., Lü W., Jiang M., Zhu H., Wang H.. Realizing efficient near-infrared emission in Cu-In-S/ZnS quantum dots: Aqueous synthesis, optical properties and applications. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2024;701:134966. https://doi.org/10.1016/j.colsurfa.2024.134966
[Google Scholar]

[27] Pasieczna-Patkowska S., Cichy M., Flieger J.. Application of fourier transform infrared (FTIR) spectroscopy in characterization of green synthesized nanoparticles. Molecules (Basel, Switzerland). 2025;30:684. https://doi.org/10.3390/molecules30030684
[Google Scholar]

[28] Koleva V., Stefov V., Cahil A., Najdoski M., Šoptrajanov B., Engelen B., Lutz H.D.. Infrared and Raman studies of manganese dihydrogen phosphate dihydrate, (H₂PO₄)₂·2H₂O. I: Region of the vibrations of the phosphate ions and external modes of the water molecules. Journal of Molecular Structure. 2009;917:117-124. https://doi.org/10.1016/j.molstruc.2008.07.002
[Google Scholar]

[29] Abdul Hadi N., Wiege B., Stabenau S., Marefati A., Rayner M.. Comparison of three methods to determine the degree of substitution of quinoa and rice starch acetates, propionates, and butyrates: direct stoichiometry, FTIR, and 1H-NMR. Foods (Basel, Switzerland). 2020;9:83. https://doi.org/10.3390/foods9010083
[Google Scholar]

[30] Preethi T., Senthil K., Ashokan S., Sundaravel B., Saravanan P.. Structural, optical and magnetic properties of SnO₂ nanoparticles synthesized via surfactant-free hydrothermal process. Materials Today: Proceedings. 2021;47:2097-2101. https://doi.org/10.1016/j.matpr.2021.04.580
[Google Scholar]

[31] Takeno M.L., Nobre F.X., da Costa F.F., Botelho do Nascimento M.V., Pessoa Júnior W.A.G., Araújo Júnior E.A., Sousa G.D.S., de Sá M.L., Gurgel R.S., Albuquerque P.M., Matos J.M.E., Leyet Ruiz Y., Grandini C.R.. Solvent effect on the structural, optical, morphology, and antimicrobial activity of silver phosphate microcrystals by conventional hydrothermal method. ACS Omega. 2024;9:23069-23085. https://doi.org/10.1021/acsomega.4c02943
[Google Scholar]

[32] Wang Y., Wang Q., Zhan X., Wang F., Safdar M., He J.. Visible light driven type II heterostructures and their enhanced photocatalysis properties: A review. Nanoscale. 2013;5:8326-8339. https://doi.org/10.1039/c3nr01577g
[Google Scholar]

[33] Prabhu P.S., Kathirvel P., Maruthamani D., Gopal Ram S.D.. Photocatalytic activity of pure and zinc doped tin oxide nanoparticles synthesized by one step direct injection flame synthesis. Journal of Inorganic and Organometallic Polymers and Materials. 2022;32:999-1010. https://doi.org/10.1007/s10904-021-02167-y
[Google Scholar]

[34] Kim S.P., Choi M.Y., Choi H.C.. Photocatalytic activity of SnO₂ nanoparticles in methylene blue degradation. Materials Research Bulletin. 2016;74:85-89. https://doi.org/10.1016/j.materresbull.2015.10.024
[Google Scholar]

[35] Prices G.P.. KSA Power Price, power price in KSA 2023. . 2024 Dec 2023;74:85-89. Retrieved 01-3-2024, 2024.
[Google Scholar]

[36] Qahtan T.F., Owolabi T.O., Olubi O.E., Hezam A.. State-of-the-art, challenges and prospects of heterogeneous tandem photocatalysis. Coordination Chemistry Reviews. 2023;492:215276. https://doi.org/10.1016/j.ccr.2023.215276
[Google Scholar]

[37] Abu-Melha S.. Distinguishable photocatalytic activity of nano polyaniline with quantum dots metal oxide as photocatalysts for photodegradation of Dianix blue dye and different industrial pollutants. Polyhedron. 2024;252:116781. https://doi.org/10.1016/j.poly.2023.116781
[Google Scholar]

[38] Alqarni L.S., Alghamdi M.D., Alshahrani A.A., Alotaibi N.F., Moustafa S.M.N., Ashammari K., Alruwaili I.A., Nassar A.M.. Photocatalytic degradation of rhodamine-B and water densification via eco-friendly synthesized Cr₂O₃ and Ag@Cr₂O₃ using garlic peel aqueous extract. Nanomaterials (Basel, Switzerland). 2024;14:289. https://doi.org/10.3390/nano14030289
[Google Scholar]

[39] Nawaz R., Hanafiah M.M., Ali M., Anjum M., Baki Z.A., Mekkey S.D., Ullah S., Khurshid S., Ullah H., Arshad U.. Review of the performance and energy requirements of metals modified TiO₂ materials based photocatalysis for phenolic compounds degradation: A case of agro-industrial effluent. Journal of Environmental Chemical Engineering. 2024;12:112766. https://doi.org/10.1016/j.jece.2024.112766
[Google Scholar]

[40] Price, K.P. 2024, 03-01-2024. Power price in KSA. 2024.

[41] Hu M., Alharbi J., Zhang H., Al Qahtani H.S., Feng C.. Advanced in situ characterization techniques for photocatalysis. Chemical Research in Chinese Universities. 2025;41:237-253. https://doi.org/10.1007/s40242-025-4249-z
[Google Scholar]

[42] Ramanathan G., Srinivasan @ Arunsankar N., Syed Suraj Babu K., Syed Mohammed Mujaheer A., Murali R., Sakthiya S.. Photocatalytic performance of CeO₂, SnO₂ nanoparticles catalysed by methylene blue (MB) dye degradation. . 2023;492:215276. https://doi.org/10.21203/rs.3.rs-3187057/v1
[Google Scholar]

[43] Babu B., Kadam A.N., Ravikumar R.V.S.S.N., Byon C.. Enhanced visible light photocatalytic activity of Cu-doped SnO₂ quantum dots by solution combustion synthesis. Journal of Alloys and Compounds. 2017;703:330-336. https://doi.org/10.1016/j.jallcom.2017.01.311
[Google Scholar]

[44] Ali Baig A.B., Rathinam V., Ramya V.. Facile fabrication of Zn-doped SnO₂ nanoparticles for enhanced photocatalytic dye degradation performance under visible light exposure. Advanced Composites and Hybrid Materials. 2021;4:114-126. https://doi.org/10.1007/s42114-020-00195-9
[Google Scholar]

[45] Sabarinathan A., Vigneashwari B., Jayaprakash R., Albeshr M.F., Mythili R., Vignesh S., Lee J., Palanisamy G., Robert R.. 3D-Zn₂SnO₄ nanobolt coupled with ZnO-SnO₂ nanocomposites for ameliorated the hazardous dye degradation based on a dual Z–scheme system. Process Safety and Environmental Protection. 2024;185:782-793. https://doi.org/10.1016/j.psep.2024.03.043
[Google Scholar]