Organic pollutants pose a significant challenge to water treatment due to their complex structures and the potential formation of toxic byproducts during degradation. Advanced oxidation technologies, employing reactive oxygen species (ROS), offer effective decomposition of organic molecules into non-toxic smaller molecules. Piezoelectric catalysis, leveraging external mechanical energy to induce charge separation and ROS generation, presents an attractive approach requiring no additional chemicals, light, or electricity. SnS₂, a layered transition metal disulfide, is a promising piezoelectric material due to its high surface area and multiple reaction sites. However, its susceptibility to electron-hole recombination limits its efficiency. This study aimed to address this limitation by doping SnS₂ with Fe, creating a self-Fenton piezoelectric system. The self-Fenton system overcomes the limitations of traditional Fenton systems by not requiring added Fe²⁺ or acidic conditions. By combining the piezoelectric properties of SnS₂ with the self-Fenton catalytic activity of Fe, this research hypothesized that a highly efficient system for water purification could be constructed. The specific objective was to investigate the effect of Fe doping on the piezoelectric properties of SnS₂, evaluate its performance in degrading RhB, analyze the toxicity of degradation products, and elucidate the underlying mechanism.

Prior research has explored both piezoelectricity and self-Fenton catalysis for water purification independently. Studies have demonstrated the use of SnS₂ in piezoelectric nanogenerators and piezocatalytic systems for pollutant removal, but its performance has been limited by electron-hole recombination. Doping strategies, such as Ag or Cu doping, have shown promise in enhancing SnS₂'s photocatalytic properties. The generation of H₂O₂ under piezoelectric conditions is known, but its direct participation in reactions is challenging unless a Fenton system is employed to convert H₂O₂ into ROS. Self-Fenton systems have emerged as a promising alternative to conventional Fenton systems, offering advantages in terms of efficient H₂O₂ utilization and broader applicability. This study builds on this existing literature by integrating the principles of both piezoelectricity and self-Fenton catalysis within a single system based on Fe-doped SnS₂, aiming to achieve superior water purification capabilities.

The study involved the synthesis of Fe-doped SnS₂ (Sn₁₋ₓFexS₂) using a hydrothermal method. Different Fe doping levels (x = 0.02, 0.03, 0.05, 0.1) were investigated, along with undoped SnS₂ as a control. Material characterization employed XRD, Raman spectroscopy, XPS, SEM, TEM, and HAADF-STEM to analyze crystal structure, morphology, elemental composition, and lattice spacing. Piezoelectric properties were evaluated using PFM and COMSOL simulations. Piezocatalytic activity was assessed by measuring the degradation of RhB under ultrasonic conditions (40 kHz, 100 W), determining the rate constants, and performing active species scavenging experiments. DFT calculations were conducted to analyze the electronic structure, adsorption energies, charge transfer at the catalyst-pollutant interface, and bond angles/lengths. Toxicity assessment involved ECOSAR predictions and zebrafish embryo toxicity tests. ESR spectroscopy was used to analyze the generation of ROS, and fluorescence intensity measurements quantified ·OH production. Electrochemical experiments investigated charge transfer characteristics.

Fe doping significantly enhanced the piezoelectric properties of SnS₂, with Sn₀.₉₇Fe₀.₀₃S₂ exhibiting the best performance. This material demonstrated superior RhB degradation compared to undoped SnS₂, achieving 95.4% removal within 30 min. The degradation kinetics followed pseudo-first-order behavior, and the rate constant for Sn₀.₉₇Fe₀.₀₃S₂ was 14 times higher than that of SnS₂. Active species scavenging experiments revealed that h⁺, ·OH, and ·O₂⁻ all contributed to RhB degradation, with ·OH playing the most crucial role. DFT calculations showed that Fe doping introduced lattice distortion and altered the electron density distribution, leading to stronger adsorption of RhB and enhanced charge transfer. Analysis of RhB degradation products revealed a reduction in toxicity compared to the original RhB. Zebrafish embryo toxicity tests confirmed the reduction in toxicity of the degradation solutions. ESR measurements showed that Sn₀.₉₇Fe₀.₀₃S₂ generated significantly higher levels of ·O₂⁻, h⁺, and ·OH compared to SnS₂. The absence of detectable H₂O₂ in the Sn₀.₉₇Fe₀.₀₃S₂ system indicated the self-Fenton reaction mechanism, where Fe valence state changes facilitate ·OH generation. Electrochemical experiments confirmed the enhanced charge transfer ability of Sn₀.₉₇Fe₀.₀₃S₂ compared to SnS₂.

The findings confirm that Fe doping effectively enhances the piezoelectric and catalytic performance of SnS₂ for pollutant degradation. The combination of piezoelectric catalysis and the self-Fenton reaction leads to a synergistic effect, resulting in significantly improved efficiency in RhB removal and detoxification. The DFT calculations provide a mechanistic understanding of the enhanced performance, highlighting the role of lattice distortion, altered electron density, and enhanced charge transfer. The observed reduction in toxicity of degradation products demonstrates the effectiveness of the system in achieving complete mineralization and minimizing the environmental risks associated with intermediate products. These results contribute significantly to the advancement of water treatment technologies, showcasing the potential of combined piezoelectric and self-Fenton catalytic systems for efficient and environmentally friendly organic pollutant removal.

This study successfully demonstrated the construction and efficacy of a novel Sn₁₋ₓFexS₂ piezoelectric self-Fenton system for efficient water purification. The optimized Sn₀.₉₇Fe₀.₀₃S₂ composition displayed superior RhB degradation and detoxification capabilities compared to undoped SnS₂. The mechanistic studies revealed the synergistic effect of enhanced piezoelectricity due to lattice distortion and the self-Fenton catalytic activity of Fe, leading to increased ROS generation. Future research could explore different metal dopants, optimize synthesis parameters, and investigate the applicability of this system to a broader range of organic pollutants in real-world water matrices.

The study primarily focused on RhB degradation under controlled laboratory conditions. Further research is needed to evaluate the long-term stability and scalability of the system under diverse environmental conditions and with different types of pollutants. The toxicity assessment was primarily conducted with zebrafish embryos; broader toxicological studies using other organisms would strengthen the conclusions. The mechanism is comprehensively described, but additional experiments could support deeper mechanistic understanding.