Improving CMTS Physical Properties Through Potassium Doping for Enhanced Rhodamine B Degradation

Abstract

This study investigated the enhancement of Cu₂MnSnS₄ (CMTS) thin films’ photocatalytic properties through potassium (K) doping for rhodamine B degradation under visible light. K-doped CMTS films synthesized using spray pyrolysis technology achieved a 98% degradation efficiency within 120 min. The physical property improvements were quantitatively validated through X-ray diffraction (XRD) analysis, which confirmed enhanced crystallinity. Scanning electron microscopy (SEM) revealed significant modifications in surface morphology as a function of potassium content, highlighting its influence on film growth dynamics. Optical characterization demonstrated a pronounced reduction in transmittance, approaching negligible values at 7.5% potassium doping, and a narrowed optical band gap of 1.41 eV, suggesting superior light absorption capabilities. Photocatalytic performance was significantly enhanced, achieving a Rhodamine B degradation efficiency of up to 98% at 7.5% doping. These enhancements collectively improved the material’s light-harvesting capabilities and charge separation efficiency, positioning K-doped CMTS as a highly effective photocatalyst compared to other ternary and quaternary materials.

1. Introduction

Rhodamine B (RhB), a synthetic organic dye, is extensively utilized in industries such as textiles, food processing, and cosmetics due to its vibrant fluorescence and chemical stability [1]. However, its discharge into aquatic environments poses significant environmental and health risks due to its non-biodegradable, toxic, and potentially carcinogenic properties, adversely affecting aquatic ecosystems and human health. Its resistance to conventional wastewater treatment methods has necessitated the development of advanced remediation technologies [2].

Photocatalysis emerged as a promising approach for RhB degradation, leveraging semiconductor materials to decompose organic pollutants under light irradiation [3]. Established photocatalysts, such as titanium dioxide (TiO₂) and zinc oxide (ZnO), exhibit limitations due to their wide band gaps, which restrict efficiency under visible light [4,5]. In contrast, copper-based chalcogenides, including Cu₂O and CuInS₂, offer narrower band gaps and enhanced visible-light absorption, making them more suitable for photocatalytic applications [6].

Cu₂MgSnS₄ (CMTS), a quaternary chalcogenide semiconductor, was investigated for its high absorption coefficient, tunable optoelectronic properties, and Earth-abundant composition [7]. Its exceptional light-harvesting capabilities position CMTS as a promising candidate for photocatalysis [8]. CMTS is gaining attention as an emerging quaternary chalcogenide material with favorable optoelectronic and environmental characteristics. Compared to other Cu-based chalcogenides like Cu₂ZnSnS₄ (CZTS) and Cu(In,Ga)Se₂ (CIGS), CMTS offers distinct advantages for photocatalytic applications. CMTS exhibits a high absorption coefficient (>10⁴ cm⁻¹) and a tunable direct band gap (1.5–1.6 eV), similar to CZTS, ideal for solar and catalysis-driven applications [9,10,11]. However, the substitution of Zn with Mg helps to reduce intrinsic cation disorder and the formation of detrimental secondary phases like ZnS, which are often observed in CZTS systems and can negatively impact carrier transport and device performance. Unlike CIGS, which relies on scarce and costly indium and gallium, CMTS is composed of Earth-abundant elements, enhancing its cost-effectiveness and environmental sustainability. Furthermore, the presence of Mg, a lightweight and non-toxic element, contributes to enhanced structural and thermal stability. These combined advantages make CMTS a promising and more sustainable alternative for optoelectronic and photocatalytic applications [12].

Despite its potential, CMTS remains underexplored for photocatalysis, with only one prior study reporting 94% methylene blue degradation under sunlight [7]. This scarcity of research points to the need to explore CMTS’s potential for degrading other pollutants, such as RhB.

A key challenge of using CMTS in photocatalysis is its high electron–hole recombination rate, which diminishes photocatalytic efficiency [9]. Doping with alkali metals, particularly potassium (K), is recognized as an effective strategy to enhance crystallinity, charge carrier mobility, and charge separation [13,14].

Doping with potassium (K⁺) plays a crucial role in material science, particularly in enhancing catalytic performance. Due to its large ionic radius (1.38 Å), K⁺ can effectively distort the crystal lattice of host materials, leading to the formation of defect sites such as vacancies and interstitials [15]. These defects, especially sulfur vacancies in chalcogenide systems, are known to act as active sites that facilitate catalytic reactions by lowering activation energies and increasing the density of reactive centers. Moreover, the monovalent nature of K⁺ introduces local charge imbalances that modify the electronic environment, thereby enhancing surface reactivity and promoting faster electron transfer—both of which are vital for efficient catalysis. Potassium also offers practical benefits: it is Earth-abundant, cost-effective, and environmentally benign, which makes it an attractive dopant from a sustainability perspective. Numerous studies have reported the positive effects of alkali metal doping, especially K⁺. Importantly, Tong et al. [16] have demonstrated that K⁺ doping in CZTS thin films enhances the (112) preferred orientation, increases grain size, and reduces the presence of the undesirable ZnS secondary phase. These structural modifications contribute to improved film crystallinity and homogeneity, which are essential for achieving high-performance catalytic behavior [16].

This study presents potassium-doped CMTS thin films synthesized via spray pyrolysis, a technology that shows promise for producing supported photocatalysts due to its ability to create uniform, adherent thin films with controlled composition on various substrates. Through systematic investigation, we assessed the impact of various potassium concentrations on the structural, morphological, and optical properties of CMTS. The primary objective was to optimize doping levels to maximize RhB degradation efficiency, thereby establishing CMTS as a cost-effective and efficient material for wastewater treatment applications.

2. Materials and Methods

Potassium-doped copper magnesium tin sulfide (CMTS) thin films were deposited on glass substrates using spray pyrolysis technology. The precursor solution was prepared by dissolving copper dichloride (CuCl₂, 10 mM), magnesium dichloride (MgCl₂, 5 mM), tin dichloride (SnCl₂, 5 mM), and thiourea (SC(NH₂)₂, 40 mM) in 300 mL of bi-distilled water.

Glass substrates were ultrasonically cleaned with bi-distilled water and dried at 80 °C prior to deposition. The precursor solution was atomized using compressed air through a spray nozzle positioned 20 cm above the substrate, maintained at 210 °C.

Potassium doping was achieved by adding potassium chloride (KCl) to the precursor solution at molar concentrations of 2.5%, 5%, 7.5%, and 10% relative to copper content. The doping levels (2.5%, 5%, 7.5%, and 10%) were chosen to systematically explore potassium’s effects within a range that balances significant influence without causing secondary phases. Levels below 1% show negligible impact, while above 10% risk mixed phases, so this range optimizes material quality and performance [17].

Structural properties were characterized using X-ray diffraction (XRD) with CuKα radiation. Surface morphology was examined via scanning electron microscopy (SEM) at 30 kV with a 3 nm resolution. Optical properties were analyzed using a UV–VIS–NIR spectrophotometer over a wavelength range of 200–2000 nm at room temperature.

3. Results and Discussions

3.1. X-Ray Diffraction Analysis

Figure 1 presents the X-ray diffraction (XRD) patterns of undoped and potassium-doped CMTS thin films with doping concentrations of 2.5%, 5%, 7.5%, and 10%. All samples exhibit broad diffraction peaks characteristic of the kesterite crystal structure, consistent with the International Centre for Diffraction Data (ICDD, PDF 98-017-1983) [18]. The sharp, well-defined peaks reflect high crystallinity, underscoring the structural quality achieved through the spray pyrolysis technique [19]. The polycrystalline nature of the films is evident from multiple diffraction peaks corresponding to various crystallographic planes. In undoped CMTS, the (200) plane is the most intense, indicating a preferred orientation. However, potassium doping significantly alters this orientation, with the (112) plane becoming dominant, suggesting that potassium incorporation modifies crystal growth dynamics by favoring the (112) direction [20].

Crystallinity increases markedly with higher potassium content, peaking at 7.5% doping. This is evidenced by sharper, more intense diffraction peaks, indicative of enhanced grain growth and reduced structural defects. At 10% doping, slight peak broadening occurs, likely due to strain or defect formation from excessive potassium incorporation. These findings highlight the beneficial role of potassium doping in improving the structural quality of CMTS thin films, which enhances their optoelectronic and catalytic properties [21].

Additionally, XRD analysis reveals a progressive leftward shift in diffraction peaks corresponding to the (111), (112), (200), and (220) planes with increasing potassium content. This shift indicates lattice expansion, likely due to the incorporation of potassium ions into the CMTS framework [22]. Notably, no secondary phases are detected, confirming that potassium is seamlessly integrated into the lattice, preserving phase purity.

Crystallite size (D), dislocation density (δ), and lattice strain (ε) were calculated using the following Equations [23]:

$$D={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }}}$$

$$\delta ={\displaystyle \frac{1}{D^2}}$$

$$\epsilon ={\displaystyle \frac{\beta cos\theta }{4}}$$

where λ represents the X-ray wavelength (1.5418 Å), K is a shape factor (0.94), β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg’s diffraction angle [13]. The calculated crystallographic parameters are summarized in

Table 1. Crystallite size (D), dislocation density (δ), and strain (ε) calculated along the preferred orientation (112) of undoped and K-doped CMTS films.

Samples	2 θ (°)	D (nm)	δ × 1014 (lines/m2)	ε (10−3)
CMTS undoped	29.1	25	8.3	10.2
CMTS–K2.5%	29.1	28	4.2	5.8
CMTS–K5%	29.2	32	0.6	3.2
CMTS–K7.5%	29.2	41	0.4	2.9
CMTS–K10%	29.3	35	1.8	3.9

and Figure 1b. With increasing potassium doping, crystallite size increases linearly, reaching a maximum of 41 nm at 7.5% K, accompanied by a significant reduction in dislocation density (0.4 × 10¹⁴ lines/m²) and lattice strain (2.9 × 10⁻³). These improvements reflect enhanced crystallinity and fewer structural defects in the doped films. Reduced defect density minimizes grain boundaries and dislocations, which act as charge carrier traps, thereby improving charge carrier mobility [24].

This enhancement in grain size and reduction in defect density is particularly advantageous for photocatalytic applications. Larger grains and lower defect densities facilitate charge carrier transport, reducing electron–hole recombination. This enables more charge carriers to reach the film surface, increasing the availability of active sites for photocatalytic reactions. Consequently, the photocatalytic performance of CMTS thin films is boosted, leading to the more effective degradation of organic pollutants. These results underscore the critical role of potassium doping, particularly at 7.5%, in tailoring the structural properties of CMTS thin films for superior photocatalytic performance [25].

Raman spectroscopy was employed to confirm the high purity and absence of secondary phases in K-doped CMTS thin films with varying potassium concentrations (7.5 at.% and 10 at.%). The Raman spectra (Figure 2) exhibited distinct peaks corresponding to the vibrational modes of the kesterite structure, with no detectable signals from impurity phases such as binary or ternary sulfides (e.g., SnS, Cu₂S). The primary peaks observed in the spectra were assigned as follows: the dominant mode near 320–330 cm⁻¹ is attributed to the A1 symmetry vibration of sulfur atoms in the tetrahedral coordination of CMTS, characteristic of its kesterite structure. Additional peaks in the range 250–300 cm⁻¹ and 350–400 cm⁻¹ correspond to mixed cationic (Cu/Mg/Sn) vibrational modes, further confirming the formation of a single-phase CMTS structure. The absence of extraneous peaks outside these ranges, particularly around 100–200 cm⁻¹ (associated with Sn-S bonds in SnS) or above 400 cm⁻¹ (typical of Cu-S bonds in Cu₂S), validates the phase purity and successful incorporation of K into the CMTS lattice without disrupting its crystallinity. The consistency of peak positions and the lack of broadening or shifting with K doping also indicate minimal lattice distortion, reinforcing the high quality of the synthesized films [26,27].

3.2. Morphological Properties

Scanning electron microscopy (SEM) was employed to examine the surface morphology of undoped, 7.5% K-doped, and 10% K-doped CMTS thin films, with the results shown in Figure 3. The SEM micrographs reveal that undoped CMTS films exhibit a highly agglomerated surface with densely packed clusters. In contrast, potassium doping induces significant morphological changes. At the optimal 7.5% K doping level, agglomeration decreases markedly, giving way to distinct, isolated grains.

Additionally, doped samples display void formation and increased surface roughness, which are advantageous for photocatalysis. These features expand the active surface area for contaminant adsorption and light absorption. The rougher texture also improves contact between the photocatalyst and the surrounding medium, facilitating efficient charge transfer. These morphological enhancements increase the number of reactive sites and reduce electron–hole recombination, thereby boosting photocatalytic performance [17]. Thus, potassium doping significantly refines the surface structure of CMTS thin films, enhancing their efficacy for environmental remediation.

Transmission electron microscopy (TEM) images, presented in Figure 4, provide further insight into the structural evolution of undoped and K-doped CMTS films. TEM analysis reveals a clear increase in crystallite size upon potassium doping, corroborating the XRD results. Specifically, the crystallite size increases from approximately 26 nm for the undoped film to 39 nm for the 7.5% K-doped sample—the highest value observed—before slightly decreasing to 33 nm at 10% doping. This trend highlights the role of potassium in enhancing crystallinity and promoting grain growth, with 7.5% being the optimal concentration. The presence of well-defined lattice fringes in the TEM images, particularly at 7.5% doping, indicates improved structural ordering and a reduction in defect density, emphasizing the beneficial influence of potassium on the microstructural quality of the CMTS films [28].

3.3. Optical Properties

The optical properties of undoped and potassium-doped CMTS thin films were analyzed using UV-Vis-NIR spectrophotometry over the 200–2000 nm range (Figure 5). The 7.5% potassium-doped CMTS thin film exhibits significantly reduced transmittance values, approaching near-zero across the ultraviolet, visible, and infrared regions, indicating enhanced light absorption. The 7.5% K-doped sample exhibited a transmittance of approximately 2.3% at 600 nm, highlighting its superior light absorption. This improved absorption arises from the increased interaction between incident light and the material, likely due to modifications in the electronic structure and a higher density of absorbing species introduced by potassium doping [29]. Transmission spectra reveal clear redshift and a substantial decrease in transmittance with increasing potassium concentration, with the 7.5% K-doped sample showing the strongest effect (Figure 5a). Such strong absorption promotes efficient photon capture and electron–hole pair generation, which are critical for achieving high photocatalytic activity [18]. The presence of absorption in the infrared region further suggests potential benefits for solar-driven catalytic applications by improving energy conversion and pollutant degradation efficiency [19]. The transmittance measurements were conducted over the full UV-Vis-NIR spectrum to thoroughly characterize the optical behavior of the films. While photocatalytic activity predominantly depends on visible light absorption under solar illumination, the infrared data provide additional insight into sub-bandgap transitions and overall film response, which may affect thermal and energy conversion processes [30].

Reflectance spectra (Figure 5b) showed minimal variation across all samples, with the overall profile remaining largely unaffected by potassium doping. However, slight reductions in reflectance intensity were observed at higher doping levels, indicating that potassium primarily influences absorption and transmission rather than reflectance.

Key optical parameters were derived from transmission (T) and reflectance (R) data. The optical bandgap was determined using the Tauc method (Figure 6), offering insights into the films’ light absorption and energy conversion capabilities [31].

Table 2. Band gap energy values of undoped and K-doped CMTS thin films.

Sample	Eg (eV)
CMTS undoped	2.40
CMTS–K 2.5 at.%	2.20
CMTS–K 5 at.%	1.61
CMTS–K 7.5 at.%	1.45
CMTS–K 10 at.%	1.69

highlights a significant reduction in the bandgap of potassium-doped CMTS thin films, offering a key advantage for photocatalytic applications. A narrower bandgap enables the material to absorb a broader range of the solar spectrum, particularly in the visible light region, enhancing photocatalytic efficiency under natural sunlight [20,21]. This increased light absorption generates more photogenerated charge carriers, which are essential for driving catalytic reactions [32]. Consequently, potassium-induced bandgap narrowing significantly enhances the photocatalytic performance of CMTS thin films, making them highly effective for environmental remediation and related applications [33].

The calculated bandgap values, summarized in Table 2, show a clear trend of decreasing from 2.4 eV for undoped CMTS to 1.45 eV at 7.5% K doping. This substantial reduction results from electronic structure modifications caused by potassium incorporation, which introduces intermediate energy states within the bandgap. These states facilitate electron excitation from the valence to the conduction band, improving the material’s ability to utilize visible light. At 10% K doping, a slight increase to 1.69 eV suggests possible structural strain.

The optical band gap of the CMTS thin films was found to decrease with increasing K doping, reaching a minimum value at 7.5% K doping before increasing again at 10% doping. This trend correlates well with the microstrain behavior observed in our XRD analysis, where the microstrain decreases with K doping up to 7.5%, indicating improved crystallinity and reduced lattice imperfections, and then increases at higher doping levels due to excessive dopant incorporation, leading to lattice distortion. The initial reduction in band gap can be attributed to the improvement in crystallinity and the reduction in defect states within the band structure, allowing for better orbital overlap and a slight narrowing of the band gap. However, at higher doping concentrations (10%), the increase in microstrain and defect density introduces localized states and lattice distortions, leading to band tailing effects that can widen the effective band gap.

This bandgap tuning enhances the light-harvesting and charge carrier generation capabilities of CMTS thin films. By increasing responsiveness to solar radiation, potassium doping boosts catalytic efficiency, particularly for degrading organic pollutants and other photocatalytic processes [34,35].

The influence of potassium (K) doping on the defect landscape and radiative recombination behavior of CMTS thin films was systematically explored through photoluminescence (PL) spectroscopy (Figure 7). K incorporation plays a crucial role in modulating the structural and electronic properties of CMTS by influencing the defect formation energetics. As the K content increases, a progressive decrease in PL intensity is observed, with the 7.5% K-doped sample exhibiting the most significant quenching of photoluminescence. This reduction in PL intensity is indicative of a suppressed radiative recombination rate, which can be attributed to a lower density of deep-level defect states such as Cu vacancies (V_Cu), Sn-on-Cu antisites (Sn_Cu), and Mg-related disorder. K⁺ ions, owing to their large ionic radius and potential to occupy interstitial or substitutional positions, may contribute to the passivation of native defects or promote defect reordering, thereby improving the local crystal symmetry and reducing defect-induced trap states [36,37]. These findings complement the XRD data, which showed enhanced crystallinity and reduced defect density with K-doping, particularly at 7.5 at.%.

Furthermore, the lower PL emission intensity in the K-doped samples suggests enhanced charge carrier lifetime and reduced non-radiative losses, which are critical for improving catalytic or photovoltaic efficiency. In particular, the 7.5% K-doped film appears to achieve an optimal doping level where defect suppression and charge separation are most favorable. This aligns well with the observed improvement in catalytic performance discussed elsewhere in the manuscript. Therefore, the PL analysis not only confirms the presence of deep-level defects in undoped CMTS, but also demonstrates the beneficial role of K doping in mitigating these defects, enhancing material quality, and optimizing its functional properties.

3.4. Photocatalytic Study

Organic dyes, such as Rhodamine B (RhB), are extensively used in industries like textiles, paper, plastics, cosmetics, and pharmaceuticals due to their vibrant colors, chemical stability, and affordability. RhB is also employed as a fluorescent tracer in research and a dye in textile and printing processes [24]. However, its discharge into wastewater poses significant environmental and health risks. RhB is toxic, non-biodegradable, and resistant to conventional treatment methods, leading to potential health issues such as skin irritation, respiratory problems, and possible carcinogenicity. Moreover, its persistence in aquatic environments contributes to significant pollution, disrupting ecosystems and reducing overall water quality [38], thus underscoring the need for effective treatment methods to degrade such pollutants into harmless byproducts. Photocatalytic processes using advanced materials offer a sustainable solution for wastewater treatment and environmental protection [39].

As shown in Figure 8, the absorption peaks of RhB in solution decreased significantly upon introducing potassium-doped CMTS thin films, with the most pronounced reduction observed for the 7.5 at.% K-doped sample. This indicates substantial dye degradation facilitated by the photocatalytic activity of the films.

The Langmuir–Hinshelwood kinetic model is commonly employed to describe the kinetics of heterogeneous photocatalysis, particularly for organic pollutant degradation [27], as it provides an effective framework for analyzing the reaction dynamics of these processes. Following pseudo-first-order kinetics, the degradation rate of organic pollutants is proportional to the dye concentration, expressed as [40,41]

$$-ln\left({\displaystyle \frac{C}{C_0}}\right)=k_{1}t$$

where k₁ is the first-order rate constant, while C₀ is the initial dye concentration and C is the final concentration at time t. This relationship is depicted in Figure 9.

The photodegradation efficiency is calculated using the following equation:

Efficiency = (C₀ − C)/C₀ × 100

The degradation efficiency of Rhodamine B as a function of time and potassium doping concentration is shown in Figure 10.

The enhanced photocatalytic performance observed in the Rhodamine B (RhB) degradation tests can be directly correlated with the structural improvements obtained in our XRD results. Specifically, the increase in crystallite size and the reduction in defect density, as revealed by the XRD analysis, minimize the density of grain boundaries and dislocations that typically act as charge carrier traps, thereby enhancing charge carrier mobility within the CMTS thin films. This structural optimization facilitates more efficient charge carrier separation and transport, effectively reducing the recombination rate of electron–hole pairs. Consequently, a greater number of photogenerated charge carriers can migrate to the film surface to participate in photocatalytic reactions, increasing the availability of active sites for pollutant degradation. This is evidenced by the significant enhancement in RhB degradation efficiency under illumination, confirming that the larger grains and lower defect densities achieved, as shown in our XRD results, directly contribute to the improved photocatalytic activity.

The degradation of RhB by K-doped CMTS films under visible light irradiation is shown in Figure 9a, which represents pseudo-first-order kinetic plots for the photocatalytic degradation of Rhodamine B under visible light using undoped and K-doped CMTS thin films at varying K concentrations (2.5, 5, 7.5, and 10 at.%). The linear fitting of ln(C/C₀) versus time aligns with the Langmuir–Hinshelwood model with high R² values (0.930–0.980), validating the kinetic analysis. The rate constant k increases with K doping up to 7.5 at.% and then decreases at 10 at.%, showing that optimal K incorporation enhances photocatalytic activity.

The degradation of RhB by K-doped CMTS films under visible light irradiation, as shown in Figure 9b, proceeds via a photocatalytic mechanism. Photons with energy equal to or greater than the material’s bandgap excite electrons (e⁻) from the valence band (VB) to the conduction band (CB), generating holes (h⁺) in the VB [42,43]:

CMTS (K-doped) + hν → e⁻(CB) + h+(VB)

Conduction band electrons reduce dissolved oxygen (O₂) to form reactive oxygen species (ROS), such as superoxide radicals (⋅O₂⁻):

O₂ + e⁻(CB) →⋅O₂⁻

Meanwhile, valence band holes oxidize water (H₂O) or hydroxide ions (OH⁻) to produce hydroxyl radicals (⋅OH):

h + (VB) + H₂O → ⋅OH + H⁺

h + (VB) + OH⁻ → ⋅OH

These ROS (⋅OH and ⋅O₂⁻) attack RhB molecules, breaking them into smaller, less toxic intermediates, which further mineralize into harmless products like CO₂, H₂O, and inorganic ions:

Dye + ⋅OH/⋅O₂⁻ → Intermediates → CO₂ + H₂O

The enhanced photocatalytic activity of K-doped CMTS is driven by bandgap narrowing, which increases light absorption, improves charge carrier separation, and reduces electron–hole recombination. This enables the efficient generation of ROS for dye degradation, making K-doped CMTS a promising material for wastewater treatment. Figure 10 shows that the 7.5% K-doped sample achieves the highest photocatalytic efficiency, reaching approximately 98% after 120 min of sunlight exposure.

Thus, potassium-doped CMTS, particularly at 7.5% doping, is recommended as a highly effective material for photocatalytic applications in environmental remediation.

To elucidate the photocatalytic degradation mechanism, a series of radical scavenger experiments were performed, and the results are presented in Figure 11a. Different scavengers were used to selectively quench specific reactive species: Na₂SO₄ for electrons (e⁻), isopropanol for hydroxyl radicals (⋅OH), ascorbic acid for superoxide radicals (⋅O₂⁻), and EDTA for photogenerated holes (h⁺). The degradation efficiency significantly increased in the presence of each scavenger compared to the control (no scavenger), confirming that all three species—⋅OH, ⋅O₂⁻, and h⁺—actively participate in the photocatalytic reaction. Among them, EDTA caused the most substantial suppression of activity, indicating that photogenerated holes are the dominant reactive species in the degradation process. Isopropanol and ascorbic acid also showed noticeable effects, highlighting the complementary roles of hydroxyl and superoxide radicals [44]. This comprehensive involvement of multiple radicals suggests a synergistic mechanism driving degradation, where both oxidation and reduction pathways contribute to photocatalytic efficiency.

The effect of catalyst dosage on photocatalytic performance was examined and is depicted in Figure 11b. Catalyst amounts ranging from 5 mg to 160 mg were evaluated to determine the optimal loading for maximum efficiency. An increase in degradation rate was observed with increasing catalyst dosage, attributed to the greater number of active sites and enhanced adsorption of the target molecules. The photocatalytic activity peaked at a dosage of 80 mg, which was identified as the optimal amount. Beyond this point, further increases in catalyst mass (100 mg and 160 mg) led to a slight decline in performance. This can be ascribed to the excessive turbidity and agglomeration of particles at high concentrations, which hinder light penetration and reduce the effective surface area exposed to irradiation. Additionally, too much catalyst can lead to recombination of charge carriers due to limited photon absorption per particle. Therefore, 80 mg strikes a balance between sufficient active surface area and effective light utilization, leading to optimal degradation efficiency [45].

Stability and reusability are critical parameters for evaluating the practical applicability of photocatalytic materials, especially in environmental and energy-related applications. A catalyst must maintain its efficiency over multiple cycles to be considered viable for long-term use. In this context, the 7.5% K-doped CMTS thin films—identified as the optimum composition—demonstrated excellent stability and reusability (Figure 12). Over six consecutive photocatalytic degradation cycles, the films retained a high level of activity, with only a slight decrease in degradation efficiency from 98% in the first cycle to 90% in the sixth. This minimal reduction underscores the structural robustness and surface durability of the films, confirming their potential for repeated use without significant loss of performance [46].

Our optimized 7.5% K-doped CMTS thin film demonstrates excellent photocatalytic activity for the degradation of rhodamine B under visible light, achieving 98% efficiency within 120 min. This performance surpasses many reported ternary, quaternary, and heterostructure chalcogenide materials (

Table 3. Comparison with previous studies [47,48,49,50,51,52].

No.	Catalyst Type	Material	Pollutant	Light Source	Time (min)	Efficiency (%)	References
1	Ternary	Citrate-CuFeS2	Bisphenol A	6 × 4 W Fluorescent	60	97.4	[53]
2	Ternary	Cu2SnS3/rGO	Rhodamine B	Xe lamp (300 W)	240	96.3	[54]
3	Quaternary	Cu2FeSnS4	Methylene blue	Sun & Xe lamp	—	81	[55]
4	Quaternary	Cu2ZnSnSe4	Rhodamine B	Xe lamp (300 W)	120	90	[56]
5	Heterostructure	Ag2S/AgInS2	Methyl orange	Visible light	—	80	[57]
6	This work	7.5% K-doped CMTS	Methylene blue	Visible light	120	98	This work

). For example, the quaternary Cu₂ZnSnSe₄ shows around 90% degradation in 120 min, while ternary materials like Cu₂SnS₃/rGO reach 96.3%, respectively, over longer durations (240 min). Heterostructures such as Ag₂S/AgInS₂ reach up to 80% degradation, remaining below our result. These comparisons clearly underline the high efficiency and competitiveness of our K-doped CMTS catalyst, making it promising material for visible-light-driven photocatalytic applications.

4. Conclusions

In the present study, potassium-doped Cu₂MgSnS₄ thin films were successfully synthesized using the spray pyrolysis method, and their structural, morphological, optical, and photocatalytic properties were comprehensively evaluated. The results revealed that crystallite size increased with potassium doping, peaking at 7.5 at.% K, where the films exhibited minimal dislocation density and lattice strain, indicating an optimal structural quality. Optical analysis demonstrated a significant bandgap reduction from 2.4 eV in undoped CMTS to 1.45 eV at 7.5% K doping, enhancing light absorption and suitability for photocatalytic applications.

Photocatalytic experiments confirmed that 7.5 at.% K-doped CMTS thin films achieved a remarkable 98% Rhodamine B degradation efficiency under sunlight irradiation, underscoring their superior performance. These findings highlight the potential of K-doped CMTS thin films for environmental remediation. Future research should focus on fine-tuning doping levels and assessing the long-term stability and reusability of these films to facilitate their application in scalable, sustainable wastewater treatment technologies.