Lattice distortion SnS₂ piezoelectric self-Fenton system for efficient degradation and detoxification of pollutants

Organic pollutants are pervasive in aquatic environments and are challenging to remove because degradation can generate more toxic intermediates. Advanced oxidation processes (AOPs) that generate reactive oxygen species (ROS) can mineralize organics to nontoxic small molecules. Piezoelectric catalysis harnesses mechanical energy (ultrasound, shear, deformation) to separate charges and produce ROS without added chemicals, light, or electricity. SnS₂, a layered TMD, has been explored in piezocatalysis due to high surface area and abundant active sites, but electron–hole recombination limits its performance. Modifying symmetry via doping can enhance internal electric fields and reduce recombination. SnS₂ can also generate H₂O₂ under piezo conditions; however, efficient utilization typically requires Fenton chemistry to convert H₂O₂ into •OH. Conventional Fenton needs added Fe²⁺ and acidic pH, limiting applicability. Self-Fenton systems overcome these drawbacks by using intrinsic catalytic centers to activate in situ H₂O₂. This work constructs an Fe-doped SnS₂ (Sn₁₋ₓFexS₂) piezoelectric self-Fenton system, clarifies Fe-induced crystal and piezoelectric changes, evaluates RhB degradation and detoxification, analyzes intermediates and toxicity (ECOSAR, zebrafish), and elucidates the mechanism via DFT, ESR, H₂O₂/•OH quantification, and electrochemical/KPFM analyses.

Prior studies report SnS₂-based piezoelectric devices for energy harvesting and environmental remediation, with performance enhanced by compositing (e.g., SnS₂/CNFs) and metal doping (Ag, Cu). Piezoelectric catalysts with non-centrosymmetric structures generate internal fields that mitigate e⁻–h⁺ recombination. Under ultrasonic/piezo conditions, aqueous systems and SnS₂ can form H₂O₂, but effective ROS generation benefits from Fenton activation. Conventional Fenton requires added Fe²⁺ and acidic conditions, whereas emerging self-Fenton systems utilize intrinsic catalytic centers to activate H₂O₂, improving practicality for water treatment.

Synthesis: Sn₁₋ₓFeₓS₂ was prepared hydrothermally by dissolving 8 mmol thioacetamide (CH₄N₂S) and 2 mmol SnCl₄·5H₂O in 78 mL ethanol, adding X mmol Fe₂(SO₄)₃·9H₂O (dissolved in 2 mL ultrapure water), then treating at 180 °C for 18 h. Products were washed with water and ethanol. X = 0.02, 0.06, 0.1, 0.2 mmol yielded Sn₀.₉₈Fe₀.₀₂S₂, Sn₀.₉₇Fe₀.₀₃S₂, Sn₀.₉₅Fe₀.₀₅S₂, Sn₀.₉₀Fe₀.₁₀S₂; SnS₂ was synthesized without Fe. Characterization: XRD (phase, peak shifts), Raman (A1g), XPS (Fe 2p deconvolution to Fe²⁺/Fe³⁺), SEM/TEM/HRTEM/SAED (morphology, lattice spacing changes), HAADF-STEM mapping (elemental distribution). Piezoelectric properties: PFM amplitude/phase imaging and hysteresis/butterfly loops to estimate d33; finite-element simulations (COMSOL) of surface potential under cavitation pressures. Charge transport: KPFM surface potential mapping; transient piezocurrent responses, current density measurements, and EIS (Nyquist plots). Piezocatalytic tests: Ultrasonic cleaner (40 kHz, 100 W) at 25 °C; 15 mg catalyst dispersed in 50 mL RhB (20 mg L⁻¹); dark adsorption 1 h prior to sonication; aliquots every 10 min; RhB concentration by UV-vis at 554 nm. Power-dependence and recyclability (4 cycles) assessed. Reactive species analysis: Scavengers—TEOA (h⁺), tert-butanol (•OH), Ar purging (O₂/•O₂⁻)—to infer contributions via kinetic constants. ESR spin-trapping for •O₂⁻ (DMPO–O₂⁻), h⁺ (TEMPO–h⁺), and •OH (DMPO–OH). H₂O₂ quantification over time; •OH quantified by fluorescence (terephthalic acid/2-hydroxyterephthalic acid). Toxicity: ECOSAR predictions (acute and chronic toxicity) for RhB and LC–MS/MS-identified intermediates; zebrafish embryo exposure—survival, hatchability, and locomotor activity under alternating light/dark at 28 °C. Computations: DFT (VASP) for geometry optimization, electron density differences, bond lengths/angles, adsorption energies (Eads) for RhB on SnS₂ and Sn₀.₉₇Fe₀.₀₃S₂; Bader charge analysis; Gaussian/Multiwfn for ESP, HOMO/LUMO, Fukui functions (f⁺, f⁻, f⁰) and condensed double descriptor (CDD) to identify reactive sites.

- Structure and defects: XRD showed weakening and slight right-shift of (001) and (101) planes upon Fe doping; Raman A1g retained structural motif. XPS Fe 2p revealed mixed Fe²⁺/Fe³⁺ states (e.g., 714.8/730.0 eV for Fe²⁺; 716.0/732.9 eV for Fe³⁺). TEM/HRTEM indicated lattice spacing variations and missing peaks consistent with S vacancies and lattice distortion; Fe uniformly distributed in flower-like nanosheets. - Piezoelectric enhancement: PFM maximum amplitudes increased from ~70 pm (SnS₂) to ~150 pm (Sn₀.₉₇Fe₀.₀₃S₂); d33 rose from 46.7 pm V⁻¹ to 108 pm V⁻¹ (~2.28×). COMSOL showed linear increase of surface piezopotential with cavitation pressure (1–10 MPa). - Piezocatalytic performance: Sn₀.₉₇Fe₀.₀₃S₂ removed 83% RhB in 10 min and 95.4% in 30 min at 100 W and pH 7.0; no significant degradation without catalyst. Kinetic rate constant was ~14× that of SnS₂. Higher ultrasonic power improved rates (100 W ≈ 7.5× 40 W). Stability preserved over 4 cycles with unchanged XRD patterns. - Reactive species: Scavenging and ESR indicated roles of h⁺, •OH, and O₂/•O₂⁻, with •OH dominant (contribution ~94.2%; h⁺ ~86.7%; O₂ ~90.7%). ESR showed stronger signals for •O₂⁻, h⁺, and •OH in Sn₀.₉₇Fe₀.₀₃S₂. - H₂O₂/•OH dynamics: Under ultrasound, SnS₂ produced measurable H₂O₂, whereas H₂O₂ was not detected in Sn₀.₉₇Fe₀.₀₃S₂ due to rapid consumption via self-Fenton. •OH concentration in Sn₀.₉₇Fe₀.₀₃S₂ was about twice that in SnS₂. - Charge transport: KPFM surface potential increased from ~96 mV (SnS₂) to ~165 mV (Sn₀.₉₇Fe₀.₀₃S₂); stronger transient piezocurrent and higher current density observed; smaller Nyquist arc implied reduced charge-transfer resistance. - DFT insights: Fe doping concentrated electron density near Fe, induced S vacancies and lattice distortion, decreased bond angles (e.g., S–Sn–S, Sn–S–Sn) and increased certain bond lengths (largest change at Fe–S₄), enhancing polarization. RhB adsorption was stronger on Sn₀.₉₇Fe₀.₀₃S₂ (Eads ≈ −1.618 eV) than on SnS₂ (≈ −0.658 eV). Bader charges indicated greater interfacial electron transfer (−0.878 e vs −0.708 e) and Fe acting as active center. - Degradation pathway/toxicity: DFT Fukui/CDD identified vulnerable sites (e.g., C11 and N atoms), consistent with LC–MS/MS intermediates (m/z 444, 416, 388, 230, 208, 163). ECOSAR predicted reduced acute/chronic toxicity for intermediates (notably P5 lowest). Zebrafish assays showed RhB was most toxic; toxicity decreased with degradation time; locomotor activity recovered to near-blank by 30 min.

Fe doping introduces S-vacancy-driven lattice distortion and nonuniform charge distribution in SnS₂, strengthening internal electric fields and piezoelectric polarization. This accelerates charge separation/transfer, suppresses e⁻–h⁺ recombination, and enhances ROS generation under ultrasound. The doped catalyst exhibits stronger adsorption of RhB and greater interfacial charge transfer, particularly at Fe sites, facilitating redox reactions. While pristine SnS₂ generates H₂O₂ under piezocatalysis, Fe centers in Sn₀.₉₇Fe₀.₀₃S₂ activate in situ H₂O₂ via a self-Fenton cycle to produce additional •OH, explaining the absence of detectable H₂O₂ and the higher •OH levels. The synergy between enhanced piezoelectricity and self-Fenton chemistry yields rapid RhB degradation and detoxification under neutral pH and without added reagents. Electrochemical (transient currents, EIS) and KPFM data corroborate improved charge dynamics, and ESR/scavenger tests confirm the dominant role of •OH alongside h⁺ and •O₂⁻/O₂. Toxicity assessments (ECOSAR, zebrafish) verify that intermediate products are less toxic and that overall solution toxicity decreases as degradation proceeds.

An Fe-doped SnS₂ piezoelectric self-Fenton system (optimal Sn₀.₉₇Fe₀.₀₃S₂) was developed for efficient degradation and detoxification of organic pollutants. Fe-induced lattice distortion and S vacancies increased piezoelectric coefficients, boosted interfacial charge transfer, and enabled self-Fenton activation of in situ H₂O₂ to generate abundant •OH. The system achieved fast RhB removal with strong stability and significantly reduced toxicity, as validated by chemical analyses and zebrafish bioassays. This approach offers a reagent-free, neutral-pH strategy for water treatment. Future work could explore broader pollutant scopes, long-term stability in complex waters, optimization of dopant levels/defect engineering, and scale-up under various mechanical energy inputs.