Self-regulation of Lewis acid sites on FeOCl toward piezo-self-Fenton reaction for continuous hydroxyl radicals generation

Domain: physics
Published: 2026-03-11
Editorial outcome: major_revision

Abstract
Clarifying the structure-property relationship is critical to the design of high-performance piezocatalysts. Herein, with iron oxychloride (FeOCl) as the model piezocatalyst, a systematic investigation into the evolution of surface properties by the induced piezopotential is performed. Except for the promotion in carrier dynamics, the induced piezopotential also achieve in-situ regulation of the surface Lewis acid sites. The surface Lewis acidity of FeOCl, as well as other classic piezocatalyst such as BaTiO3, BiTiO3, and BiFeO3, is dynamically enhanced under the application of mechanical stress. Consequently, the activation anSd consecutive conversion of the O2 molecule, whose Lewis basicity is weaker than H2O2, is realized on the FeOCl piezocatalyst. The heterogeneous Fenton activity of FeOCl catalyst is upgraded into a more advanced piezo-self-Fenton activity, producing hydroxyl radicals efficie ntly under ultrasonic vibration (556.8 µmol g−1 h−1). The FeOCl piezo-self-Fenton system exhibits exceptional broad-spectrum degradation efficiency, operational stability, and scaElability for the treatment of antibiotic wastewater. This work highlights the in-situ regulatCion of surface property by the induced piezopotential as a new feature for the piezocatalysts, which contributes to the enhancement of piezocatalytic activity and even changes the reaction pathway.

Introduction
Piezocatalysis has emerged as a sustainable and cost-efficient strategy for environmental remediation.1,2 This technique converts the ambient mechanical energy (sound, wave, and vibration) into chemical energy, facilitating the on-demand and on-site energy utilization without the constraint of location and time.3-5 During piezocatalysis, the catalyst with piezoelectricity will generate an induced surface potential in response to the external mechanical stress, which triggers the catalytic reaction.6-8 Improving the piezocatalytic activity is critical to the practical application of this technology. In recent years, researchers have made great efSforts in exploring catalysts with strong piezoelectricity and in constructing active sites through material engineering. Nevertheless, an in-depth investigation of the structure-property relationship of the piezocatalyst is still needed, which may guide the rational design of the high-pe rformance piezocatalyst.9,10

The piezocatalytic activity is the consequence of the induced piezopotential acting on the surface active sites.11-13 Mechanical energy, as wEell as the surface energy, compensates for the activation barrier of the target reactant anCd facilitates the catalytic reaction. The positive effect of piezopotential mainly reflects on the promotion of the carrier dynamics.14 For instance, Ding et al. confirmed that the heterogeneous Fenton activity of iron oxychloride (FeOCl) catalyst for bisphenol A degradation was enhanced by 18 times with the induced piezopotential under ultrasonic vibration.15 Our recent study also highlighted the importance of constructing active sites on the surface where the induced piezopotential was located, which may effectively connect the process of piezoresponse and surface catalysis. In addition to facilitating the carrier dynamics, the induced piezopotential can also realize the in-situ regulation of the surface properties, representing another key issue to improving the piezocatalytic activity.16-18 Wang et al. reported the phenomenon of an increase in surface Lewis acid/base sites on the hydroxyapatite piezocatalyst under ultrasonic vibration, providing new insights into the mechanism of piezocatalysis.19 However, the exploration of the in-situ regulation behavior by the piezopotential is still in its infancy. Clarifying the relevant mechanisms will help guide the modification and optimization of piezocatalysts.

Proceeding from the insights above, FeOCl is herein selected as the model catalyst with intrinsic heterogeneous Fenton activity and piezoelectricity. This study focuses on the evolution of surface properties on FeOCl during the application of mechanical stress. Surprisingly, in-situ Lewis acid (LA) site engineering by the induced piezopotential is validated and extended to the common piezocatalysts. More importantly, the in-situ regulation of surface LA sites over FeOCl switches the reaction pathway from heterogeneous Fenton reaction (H2O2 → HO•) to heterogeneous piezo-self-Fenton reaction (O2 → H2O2 → HO•). The activation and coSnsecutive conversion of the O2 molecule, whose Lewis basicity is weaker than H2O2, is achieved. As a result, efficient HO• production (556.8 µmol g−1 h−1) over FeOCl from the pRiezo-self-Fenton process is realized under ultrasonic vibration without H2O2 input. This piezo- self-Fenton activity is comparable to the state-of-the-art homogeneous and heterogeneous Fenton activity, showing significant potential in the treatment of antibiotic-laden wastewatEer. The FeOCl piezo-self-Fenton system demonstrates unparalleled broad-spectrum degCradation efficiency against diverse antibiotic pollutants, as validated through both simulated wastewater models and pharmaceutical wastewater. This work emphasizes the in-situ regulation of surface property by the piezoelectric effect of the catalyst as a general phenomenon, offering insightful viewpoints for the optimization of high-performance piezocatalytic systems.

Results
FeOCl catalyst and its intrinsic piezoelectricity
FeOCl nanobelts, as the representative heterogeneous Fenton catalysts, were synthesized via the pyrolysis of FeCl3·6H2O.20-22 Supplementary Fig. 1 presents the X-ray diffraction (XRD) pattern of the as-prepared FeOCl. The pattern closely aligns with the standard reference data (JCPDS No. 39-0612)23, confirming the successful preparation of the FeOCl catalyst. The transmission electron microscopy (TEM) image in Fig. 1a shows that the length and width of the FeOCl nanobelt are approximately 400 and 50 nm, respectively. Energy-dispersive X-ray spectrometry (EDS) mapping images of the sample further confirm that Fe, O, and Cl are uniformly distributed within the nanobelt (Supplementary Fig. 2). Additionally, the atomic force microscopy (AFM) image (Supplementary Fig. 3) indicates a thickness of ~200 nm for the FeOCl nanobelt. The piezoresponse behavior of FeOCl was assessed using piezoresponse force microscopy (PFM). Fig. 1b shows a characteristic ~180° phase loop and a butterfly-shaped amplitude-voltage curve collected from the FeOCl nanobelts, demonstrating the intrinsic piezoelectricity. The piezopotential generated by FeOCl was further corroborated by theS Kelvin probe force microscopy (KPFM) (Fig. 1c). The surface potential of 18.3 mV provided additional support for the intrinsic piezoelectricity of FeOCl. Electrochemical impedance spectroscopy (EIS) further revealed that the charge‑transfer resistance of FeOCl decreased significantly under ultrasonic vibration (Supplementary Fig. 4), confirming that the induced piezopotential effectively facilitates interfacial electron transfer. The significEant transient current response of FeOCl under ultrasonic vibration (Supplementary Fig. 5) Calso suggests the effective charge migration achieved by the induced piezopotential. These findings indicate that the intrinsic piezoelectricity of FeOCl contributes to the enhancement of carrier dynamics, coinciding with the promotion of the heterogeneous Fenton activity under ultrasonic vibration as previously reported.24-26

Validation of in-situ LA site engineering
The influence of induced piezopotential on the surface property of FeOCl catalyst was then studied. Based on the crystal structure, the surface coordinatively unsaturated Fe atoms with Lewis acidity on FeOCl contribute to the intrinsic heterogeneous Fenton activity.27,28
Focusing on the evolution of LA sites, a series of measurements with pyridine as the probe molecule were performed (Supplementary Fig. 6). For pyridine adsorption over FeOCl (Fig. 1d), a remarkable 8.1-fold increase was attained under ultrasonic vibration compared to the static adsorption. Partial desorption of the adsorbed pyridine was detected when the vibration was stopped, but the adsorption capacity remained higher than that of static adsorption.19 This result indicates a significant increase in the concentration of surface LA sites on FeOCl under the application of mechanical stress. Infrared spectroscopy of pyridine adsorption (Py-IR) was employed to probe the surface acid sites on FeOCl before and after ultrasonic vibration (Fig. 1e). The characteristic absorption bands near at 1450 cm-1 and 1603 cm-1 are attributed to pyridine adsorbed on LA sites.29 Compared to the fresh sample, the post-treated FeOCl exhibited stronger peak intensities under identical desorption temperatures, suggesting an overall increase in LA site density. As the temperature was raised, the intensity of these bandSs gradually decreased. Since the site with stronger Lewis acidity usually requires a higher temperature for pyridine desorption, this trend reveals that there were abundant acid sites with different Lewis acidity on the FeOCl catalyst. Quantitative analysis based on the band near 145 0 cm-1 further discloses the effect of induced piezopotential on FeOCl (Fig. 1f). For the fresh FeOCl catalyst, the density of LA sites at 50, 100, and 200 °C was 24.87, 14.01, and 5.95 μEmol g-1, respectively. After being subjected to ultrasonic vibration, these values increased tCo 28.32, 16.80, and 7.34 μmol g-1. Furthermore, the NH3-TPD profile of FeOCl after ultrasonic vibration reveals an intensified desorption signal compared to the fresh sample, accompanied by a distinct shift of the NH3 desorption peak from 147 °C to 172 °C (Supplementary Fig. 7). These results collectively affirm that the induced piezopotential not only in-situ increases the concentration of surface LA sites on FeOCl but also strengthens the acidity.30 Consistently, O2-TPD indicates enhanced O2 adsorption after treatment. Elevated desorption intensity reflects increased adsorption capacity, and the shift of the desorption peak to higher temperatures signifies strengthened O2-surface interactions (Supplementary Fig. 8). These results demonstrate that the enhanced Lewis acidity directly facilitates O2 adsorption and activation.
Furthermore, the XRD pattern of FeOCl after ultrasonic vibration exhibits no notable changes compared with the fresh sample (Supplementary Fig. 9), indicating that the observed enhancements in surface properties are not due to phase transformation or loss of structural integrity.
To provide more insights for in-situ LA site engineering, In-situ Raman spectra was employed to investigate the bonding configuration of FeOCl under external mechanical forces. As shown in Fig. 2a, the vibration mode of FeOCl at 224 cm⁻1 under atmospheric pressure is attributed to the stretching vibration of the Fe-Cl bond, whereas the modes at 291 and 410 cm⁻1 correspond to the stretching vibration of the Fe-O bond.31,32 As the applied pressure increases, the Raman signals of FeOCl shift toward higher frequencies, although the overall profile of the spectra remains unchanged. When the applied pressure is above 2.2 GPa, a new Raman signal at approximately 300 cm⁻1 is gradually detected, indicating a reduced symmetry of SFeOCl under mechanical forces. The change of lattice symmetry may be the key to the in-situ regulation of surface properties. As shown in Fig. 2b, the magnetic hysteresis loop of FeOCl exhibits negligible changes after ultrasonic vibration. This result implies that the spi n state of the Fe center was not affected under the application of mechanical stress, pointing the essence of in-situ LA site engineering to the evolution of electronic structure caused bEy the induced piezopotential.33
Based on the results above, the Cvalence state of Fe on the FeOCl catalyst is then characterized. The Fe 2p X-ray photoelectron spectroscopy (XPS) spectra of the FeOCl catalysts before and after being subjected to ultrasonic vibration are shown in Fig. 2c. The spectra show three deconvoluted doublet peaks, corresponding to the Fe3+ 2p3/2, Fe3+ 2p1/2, and oscillating satellite signals. Following exposure to ultrasonic vibration, only a slight shift in binding energy is observed, reflecting minor changes in the surface electronic structure. X-ray absorption spectroscopy (XAS) provides additional details of the electronic structure and coordination environment of the FeOCl catalyst at the atomic level. In the Fe K-edge X-ray absorption near-edge structure (XANES) spectra (Fig. 2d), the curve of FeOCl closely aligns with that of Fe2O3, indicating that the oxidation state of Fe atoms is predominantly +3.34,35 After ultrasonic vibration, the absorption edge of FeOCl shifts to a lower energy. The result of linear combination fitting (Fig. 2e) confirms that the average oxidation state of Fe on FeOCl varies from +2.89 to +2.88 along with the application of mechanical stress.36 The subtle changes observed in the XPS and XANES spectra reflect a reversible modulation of the surface electronic structure induced by the piezopotential, rather than persistent changes in material properties. The results indicate that mechanical stress can dynamically modulate surface properties. Crucially, this minimal change in formal oxidation state indicates that the enhancement in Lewis acidity does not arise from the conventional redox process. Instead, density-of-states (DOS) analysis reveals its fundamental origin in the electronic structure by showing that the d‑band center of the surface Fe atoms shifts upward from -1.72 eV at 0 GPa to -1.47 eV at 2 GPa (Supplementary Fig. 10). This upshift of the d‑band center, a key descriptor in catalysis, shifts the energy of Fe 3d orbitals closer to the Fermi leSvel. Consequently, the elevated d states strengthen interactions between Fe sites and electron-donating adsorbates, thereby increasing the Lewis acidity of the Fe sites.37,38
To evaluate the universality of in-situ LA site e ngineering by the induced piezopotential, we extended our investigation to three classic piezocatalysts BaTiO3, BiTiO3, and BiFeO3. The intrinsic piezoelectricity of these piezocEatalysts has been verified by PFM (Supplementary Fig. 11). Similar to FeOCl, all three maCterials exhibited a significant increase in the capacity of pyridine adsorption (by approximately 5.3, 1.3, and 2.2 times, respectively) under ultrasonic vibration (Supplementary Fig. 12 and Supplementary Fig. 13). Systematic Py-IR analysis (Supplementary Fig. 14) revealed a notable enhancement in the Lewis acidity for all three materials after ultrasonic vibration. Quantitative analysis based on the characteristic peak at 1450 cm-1 (Supplementary Fig. 15) further elucidated the change in LA sites. Following exposure to the mechanical stress, the derived LA site density on BaTiO3 and BiTiO3 across the desorption temperatures from 50 °C to 200 °C was all higher than that of the fresh samples. In the case of BiFeO3, although the change in LA site density was negligible from the Py-IR spectrum at 200 °C, obvious increases were observed from that at 50 °C and 100 °C for the sample after ultrasonic vibration. These results demonstrate the general outcome of in-situ LA site engineering when the piezocatalyst is subjected to mechanical stress.

Piezo-self-Fenton activity for HO• generation
Given above, the induced piezopotential on FeOCl nanobelt not only promotes the carrier dynamics but also enhances the Lewis acidity of surface Fe sites. Beyond the improvement of heterogeneous Fenton activity as previously reported, the in-situ regulation of LA sites on FeOCl also affect the reaction pathway. The activation and conversion of the O2 molecule with weaker Lewis basicity than H2O2 is expected to be achieved on the FeOCl piezocatalyst.
The piezocatalytic activity of FeOCl for H2O2 generation was examined in pure water under ultrasonic vibration (100 W, 40 kHz). As shown in Fig. 3a, the H2O2 yield reached 1618.8 μmol g⁻1 h⁻1, which was comparable to the performance of some classic piezocatalysts.39,40 However, as the reaction progressed, a slowdown in H2O2 accumulation was observed. The H2O2 concentration reached the equilibrium at 1.3 mmol L⁻1, with the H2O2 generation rate declining from 1618.8 μmol g⁻1 h⁻1 in the first hour to 853.6 μmol g⁻1 h⁻1 over the next eight hours. This result validates O2 conversion into H2O2 on the FeOCl piezocatalyst, meanwhile suggesting the simultaneous H2O2 conversion. During the reaction, tChe solution pH decreased from ~6.7 to ~4.3 and then slightly increased to ~4.5 under ultrasonic vibration (Supplementary Fig. 16). The intrinsically acidic surface of FeOCl establishes a local proton-rich microenvironment, facilitating the protonation steps during the O2 to H2O2 reduction.41 With terephthalic acid (TA) as a fluorescent probe, the production of HO• radicals by the FeOCl piezocatalyst was further quantified (Fig. 3b).42 Continuous HO• generation (556.8 µmol g−1 h−1) was confirmed along with the application of mechanical stress. The performance represents the highest level to date (Fig. 3c). The long-term stability of the FeOCl piezo-self-Fenton system was confirmed through cyclic and extended-duration experiments. Under alternating ultrasound (ON for 20 minutes) and static (OFF for 20 minutes) conditions, HO• were generated only during ultrasound exposure and ceased immediately when the ultrasound was turned off, confirming the reversible, piezopotential-driven radical generation. The total HO• yield accumulated over three cycles (60 minutes in total) closely matched that obtained under continuous ultrasonic vibration for 1 hour, demonstrating the consistent performance under intermittent operation (Supplementary Fig. 17). Furthermore, during the 4 h continuous ultrasonic vibration experiment, the concentration of HO• exhibited sustained growth, verifying persistent catalytic performance (Supplementary Fig. 18). To be noted, Fe leaching during ultrasonic vibration was also monitored (Supplementary Fig. 19). The concentration was approximately 135 μg L⁻1, which was insufficient to generate HO• radicals at such a level through the homogeneous Fenton reaction. To further confirm that the enhanced activity arises from piezopotential-induced surface modulation rather than other ultrasound-related contributions, control experiments were conducted. HO• generatioSn under ultrasound alone (20.05 μmol L-1), using leached Fe3+ (27.72 μmol L-1), and from the pyridine-poisoned FeOCl (20.92 μmol L-1) were all markedly lower than that R achieved on FeOCl (111.36 μmol L-1) (Supplementary Fig. 20). These results confirm tha t the enhanced activity originates from the in-situ modulation of Lewis acid sites on FeOCl by the piezopotential, rather than from cavitation or Fe leaching. In-situ electron paramagneEtic resonance (EPR) analysis was also performed using 5,5-dimethyl-1-pyrroline N-oxide C(DMPO) as a trapping agent to monitor the evolution of HO• radicals (Fig. 3d and Supplementary Fig. 21). As shown in Fig. 3e, Fig. 3f, and Supplementary Fig. 22, the signal of DMPO-HO• adduct emerged at about 36 s and became stable under ultrasonic vibration. Considering the limited lifetime of spin adducts, these results offer additional evidence for the sustained production of HO• radicals over the FeOCl piezocatalyst with LA sites.
The heterogeneous Fenton activity of FeOCl was also evaluated. For H2O2 decomposition over FeOCl (Supplementary Fig. 23), the concentration of H2O2 decreased from 5 mM to 1.1 mM over ten hours. This process was accelerated under ultrasonic vibration. The Fenton activity was assessed using 3,3,5,5-tetramethylbenzidine dihydrochloride (TMB) as the substrate. As shown in Supplementary Fig. 24, under ultrasonic vibration, the increasing absorbance of oxidized TMB (ox-TMB) at 652 nm implied the continuous Fenton reaction in the absence of H2O2.43,44 Supplementary Fig. 25 shows that the reaction in the presence of H2O2 was significantly enhanced under ultrasonic vibration, with the kinetics of the reaction accelerating, further supporting the promotion of the heterogeneous Fenton activity by mechanical stress.45
Collectively, efficient O2 conversion into H2O2 and the subsequent heterogeneous Fenton reaction were realized over FeOCl under ultrasonic vibration. With the in-situ regulation of surface LA sites by the induced piezopotential, the heterogeneous Fenton activity of FeOCl catalyst was upgraded into a more advanced piezo-self-Fenton activity.46 Bypassing the H2O2 input endows the FeOCl piezo-self-Fenton system with more possibilities in practical application.

Mechanism of the piezo-self-Fenton reaction
Tracing the reaction pathway elucidated the catalytic mechanism driven by the piezopotential induced regulation of LA sites. Various scavengers were employed to probe the generation of H2O2 and HO• by the FeOCl piezo-self-Fenton system. Specifically, p-benzoquinone (BQ) was used to scavenge the superoxide radicals (O2•⁻), tert-butanol (TBA) was used to scavenge the HO• radicals, and ethylenediaminetetraacetic acid disoEdium salt (EDTA-2Na) was used to scavenge holes (h+). As shown in Supplementary Fig. 26, H2O2 generation in the FeOCl piezo-self-Fenton system was inhibited by 87% in the presence of BQ but slightly affected by the introduction of TBA and EDTA-2Na. Furthermore, the experiment under N2 atmosphere revealed significant inhibition of H2O2 production. These results indicated that the two-electron reduction of O2 was responsible for H2O2 generation. In-situ EPR analysis monitoring the O2•⁻ radicals were also performed. Under ultrasonic vibration, the signal of DMPO- O2•⁻ adduct was readily detected, increasing rapidly and reaching the equilibrium (Supplementary Fig. 27 and Supplementary Fig. 28). Thus, the O2•⁻ radicals are critical intermediates in O2 conversion and the subsequent Fenton reaction. Driven by the enhanced LA sites, the activation of the Lewis basic O2 molecule represents a key initial step in this pathway, promoting its electron transfer and reduction. The result of quenching experiments focusing on the production of HO• radicals is shown in Fig. 3g. Similar to the case of H2O2 generation, the production of HO• radicals was significantly inhibited in the presence of BQ or under the N2 atmosphere. When MeOH was introduced into the FeOCl piezo-self-Fenton system, the efficiency was barely affected. However, HO• production was suppressed by 83.88% along with the addition of TBA. This result may be ascribed to the different affinities of MeOH and TBA toward the surface of FeOCl. TBA with a strong affinity to the surface may quench the newly-produced HO• radicals at the solid-liquid interface, while MeOH with an affinity poorer than TBA may eliminate the HO• radicals diffused into the bulk solution. The result suggests that the HO• radicals generated by the FeOCl piezo-self-Fenton system were centered at the solid-liquid interface. Since most of the HO• radicals in heterogeneous Fenton systems undergo spontaneous deactivation during diffusion, the near-surface HO• radicals prodSuced by the FeOCl piezo-self-Fenton system may effectively participate in the oxidation of pollutants (Fig. 3h) .47,48
Systematic DFT calculations provided additional insights into the role of LA sites in piezocatalytic mechanism. Starting from the interac tion between the FeOCl piezocatalyst and the O2 molecule, the evolution of charge density difference and adsorption energy is exhibited in Fig. 4a and Fig. 4b. At ambient pressure, the interaction between FeOCl piezocatalyst and the O2 molecule resulted in a 0.17 e of electron transfer from FeOCl to O2 as well as an O2 adsorption energy of -0.69 eV. Along with the application of stress (2 GPa), a more significant electron transfer (0.25 e) was observed, accompanied by a more favorable adsorption energy (-2.55 eV), demonstrating the enhanced capability of LA sites under piezopotential to activate O2. These results imply that the electron donation from FeOCl to O2 could be enhanced by the application of mechanical stress, facilitating O2 adsorption and activation. Crystal Orbital Hamiltonian Population (COHP) analysis provides additional support (Fig. 4c). The interaction between FeOCl and the O2 molecule yielded a more negative ICOHP under 2 GPa than at ambient pressure compared to the ambient pressure, which suggests a stronger Fe-O interaction, further corroborating the role of LA sites in strengthening the adsorbate-catalyst interaction. Similar promotion in the adsorption and activation of H2O2 was also verified. For the interaction between FeOCl and the H2O2 molecule (Supplementary Fig. 29 and Supplementary Fig. 30), an electron transfer of 0.06 e from FeOCl to H2O2, as well as a H2O2 adsorption energy of -0.12 eV, was obtained at ambient pressure. These values changed into 0.1 e and -0.98 eV, respectively, under 2 GPa. Meanwhile, corresponding COHP calculations (Supplementary Fig. 31) also yielded a more negative ICOHP under 2 GPa compared to the ambient pressure.49 Based on these findings, the Gibbs free energy diagram for the consecutive O2 conversion into H2O2 and HO• radical over the FeOCl piezocatalyst was simulated (Fig. 4d). Key elementary steps include: (i) O2 adsorption; (ii) *O2 protonation and reduction into *OOH; (iii) *OOH protonation and reduction into *H2O2; and (iv) *H2O2 reduction into HO• radical.50 Under the application of mechanical stress, consecutive O2 conversion on the FeOCl piezocatalyst was thermodynamicallSy favorable, exhibiting reaction barriers lower than those under ambient pressure in each elementary step. Overall, steps (i)-(iv) define a consecutive three-electron reduction pathway from O2 to HO•. Further electrochemical validation using rotating ring‑disk electrode (RRDE) measurements (Supplementary Fig. 32) shows that the treatment of ultrasonic vibration substantially enhances the O2 reduction current on FeOCl and shifts the electron transfer number toward 3, confirming the feasibility of efficient O2 to HO• conversion via this pathway. Based on the analyses above, the LA sites on FeOCl modified in-situ by the induced piezopotential achieved efficient O2 activation and consecutive conversion (Fig. 4e).

Piezo-self-Fenton system for antibiotic-laden wastewater treatment
The potential of the H2O2-free FeOCl piezo-self-Fenton system in wastewater treatment was fully examined, using sulfamethazine (SMZ) as a representative model pollutant. As shown in Fig. 5a, the H2O2-free FeOCl piezo-self-Fenton system demonstrated 99% SMZ degradation within 60 minutes under ultrasonic vibration. This performance was even better than that of the FeOCl/H2O2 heterogeneous Fenton system, in which the H2O2 concentration of 15 and 20 mM resulted in SMZ degradation of 60% and 93%, respectively. This superior activity underscores the efficiency of the LA sites in driving the H2O2-free reaction pathway. For SMZ degradation, the FeOCl piezo-self-Fenton system outperformed the heterogeneous Fenton systems employing classic Fe-based catalysts (Fe2O3, Fe3O4, CoFe2O4, NiFe2O4, ZnFe2O4, CuFe2O4, Fe-BC, Fe-C3N4) and was comparable to the Fe2+/H2O2 homogeneous Fenton system (Fig. 5b and Supplementary Fig. 33). The level of total organic carbon (TOC) removal by the FeOCl piezo-self-Fenton system was also considerable in comparison with that of the FeOCl/H2O2 heterogeneous Fenton system and the Fe2+/H2O2 homogeneous Fenton system (Fig. 5c). This superior performance originates from the piezopotential-driven self-regulation of surface Lewis acid sites, which facilitates the efficient consecutive conversion of O2 to H2O2 and then to hydroxyl radicals (HO•). The generated HO• radicals are primarily responsible for the rapid and complete degraSdation of SMZ (Supplementary Fig. 34).
The comparable SMZ degradation performance of the FeOCl piezo-self-Fenton system to that of the traditional homogenous and heterogeneo us Fenton system highlights its remarkable potential for practical application. To further assess its sustainability and environmental impacts, a life cycle assessment (LCA) was coEnducted, and the FeOCl piezo-self-Fenton process was quantitatively compared with the Fe2+/H2O2 homogeneous Fenton system and the classic nanoscale zero-valent iron (nZVI) heterogeneous Fenton system (nZVI/H2O2) (Supplementary Fig. 35 and Supplementary Table S3). The entire wastewater treatment process at the laboratory scale and the key impacts relevant to current wastewater treatment processes, such as water consumption, resource scarcity, ecotoxicity, and eutrophication, were considered.51,52 As depicted in Fig. 5d, the FeOCl piezo-self-Fenton system demonstrated the lowest environmental impact scores across all impact categories among three systems. In addition, transportation can also be reduced since the process does not require H2O2 input. Therefore, the FeOCl piezo-self-Fenton system provides distinct environmental advantages over the FeOCl/H2O2 heterogeneous Fenton system and the Fe2+/H2O2 homogeneous Fenton system.
The practicability of the FeOCl piezo-self-Fenton system was further evaluated. After five cycles, the system maintained an SMZ degradation efficiency of 90% (Supplementary Fig. 36). The performance of the FeOCl piezo-self-Fenton system was barely affected by the anions commonly found in realistic scenarios (Supplementary Fig. 37), showing robust tolerance to the interfering anions even at high concentrations. The pH sensitivity of the system was also validated across a wide pH range of 3-9 (Supplementary Fig. 38). Particularly in near-neutral aqueous solutions, the system demonstrated superior degradation performance. Additionally, the versatility of the system was evident in the degradation of various organic micropollutants, including tetracycline (TC), diclofenac (DCF), phenol (PhOH), sulfamethoxazole (SMX), carbamazepine (CBZ), ibuprofen (IBU), norfloxacin (NOR), and ciprofloxacin (CIP) (Fig. 5e and Supplementary Table S4). In the control experiments, the degradation achieved uSnder ultrasonic vibration alone was minimal, in contrast to the high efficiencies of the FeOCl piezo-self-Fenton system (Supplementary Fig. 39 and Supplementary Fig. 40). Scalable preparation of FeOCl-loaded granular activated carbon (FeOCl@GAC) catalyst s was then performed (Supplementary Fig. 41 and Supplementary Fig. 42), and a continuous-flow reaction device was constructed to evaluate the potential of this system for practical applications. Fig. 5f shows that the FeOCl@GAC piezo-self-Fenton system achieved a gradCual reduction in the chromaticity of pharmaceutical wastewater after treatment, with a high TOC removal percentage of 30.17% (Supplementary Fig. 43). As shown in Fig. 5g, the degradation efficiency of SMZ remained above 99% for 13 hours with a 5 L input of SMZ solution. In the treatment of pharmaceutical wastewater, the excitation-emission matrix (EEM) spectra (Fig. 5h) revealed that the soluble microbial byproducts and humic acid-like organic matter were effectively removed, with a significant reduction in the fluorescence intensity of the detected peaks. These results demonstrated the great potential of the piezo-self-Fenton system with in-situ LA site engineering in practical wastewater purification.53
The self‑regulating phenomenon reported here is fundamentally driven by mechanical stress. This mechanism offers opportunities for implementation using widely available, low-frequency mechanical energy, such as water flow or ambient vibration. Future efforts will therefore focus on designing flow-or pressure-driven reactor systems to achieve this piezo-self-Fenton process for practical, energy-efficient water treatment applications.

Methods
Piezocatalytic H2O2 production
All the procedures were consistent if not specified. Generally, an open 300 mL beaker was used as the reactor, positioned above the ultrasonic transducer in a KS-2200DB ultrasonic cleaner (Kunshan Jielimei, China). The power and frequency of the equipment were 100 W and 40 kHz, respectively. The temperature of the water bath was controlled at room temperature by running tap water, and the water level was equal to the liquid level in the beaker. The system was stirred at 400 rpm by a JJ-1B-200W agitating machine (Xinrui, China) during the whole test. 20 mg of FeOCl catalyst was dispersed into 100 mL of deionized water under stirring. The initial pH of the solution was ~6.7 (unadjusted). Then the ultrasonic vibration was exerted on the suspension. The concentration of H2O2 was determined by the iodide spectrophotometric method. For details, 1 mL of freshly prepared KI reagent A (0.4 M KI, 0.05 M NaOH, 1.6×10−4 M (NH4)6Mo7O24·4H2O) and 1 mL of reagent B (0.1 M KHC8H4O4) were mixed with 1 mL of the above samples.

HO• measurement and quantification
The HO• radicals were quantified using a fluorescence methodS with terephthalic acid (TA) as the probe, which could react with HO• to generate 2-hydroxyterephthalic acid, displaying strong fluorescence at 430 nm. The 2-hydroxyterephthalic acid concentration was measured by a fluorescence spectrophotometer (Hitachi F-4600) w ith an excitation wavelength of 315 nm.
All the procedures were consistent if not specified. Generally, an open 300 mL beaker was used as the reactor, positioned above the ulEtrasonic transducer in a KS-2200DB ultrasonic cleaner (Kunshan Jielimei, China). The poCwer and frequency of the equipment were 100 W and 40 kHz, respectively. The temperature of the water bath was controlled at room temperature by running tap water, and the water level was equal to the liquid level in the beaker. The system was stirred at 400 rpm by a JJ-1B-200W agitating machine (Xinrui, China) during the whole test. 20 mg of FeOCl catalyst was dispersed into 100 mL of 10mM terephthalic acid solution under stirring. The reaction was conducted at the initial solution pH of ~5.6 (unadjusted). Then the ultrasonic vibration was exerted on the suspension. The concentration of HO• was determined by a fluorescence spectrophotometer (Hitachi F-4600) with an excitation wavelength of 315 nm.

Piezo-self-Fenton degradation of SMZ
All the procedures were consistent if not specified. Generally, an open 300 mL beaker was used as the reactor, positioned above the ultrasonic transducer in a KS-2200DB ultrasonic cleaner (Kunshan Jielimei, China). The power and frequency of the equipment were 100 W and 40 kHz, respectively. The temperature of the water bath was controlled at room temperature by running tap water, and the water level was equal to the liquid level in the beaker. The system was stirred at 400 rpm by a JJ-1B-200W agitating machine (Xinrui, China) during the whole test. 50 mg of FeOCl catalyst was dispersed into 100 mL 10 mg L−1 SMZ solution under stirring. The initial pH of the SMZ solution was ~6.3 (unadjusted). Samples were periodically withdrawn, quenched with excess Na2S2O3 to terminate the catalytic reaction, and filtered using a 0.22 µm fiber filter. The concentration of organic pollutants was determined by a liquid chromatograph (HPLC, Shimadzu LC-16 with a C18 column, Poroshell 120 SB-C18, 100 mm × 2.1S mm, 2.7 µm) equipped with a UV detector. The removal efficiency was represented by the relative concentration (C/C0). C and C0 are the initial and reaction concentrations.

Trials for wastewater treatment
20 g of FeOCl@GAC catalyst was pre-loaded into the U-tube of the continuous-flow reaction device, and the collected pharmaceutical wastewater was pumped through a peristaltic pump. After that, ultrasonic vibration was turneCd on, and 5 mL of treated wastewater was withdrawn at 1-hour intervals, and filtered through a 0.22 m membrane needle filter for further analysis. Finally, the TOC and EEM spectra of wastewater were used to evaluate the practicality of the FeOCl piezo-self-Fenton system.

Life cycle assessment
Life cycle assessment (LCA) is a standardized approach for evaluating and comparing environmental performance among various products and systems.54,55 Here, we applied LCA using the openLCA (version 2.2.0) to compare the environmental impacts of three Fenton systems (i.e., FeOCl piezo-self-Fenton system, nZVI heterogeneous Fenton system, homogeneous Fenton system) for treating 1 kg SMZ. The foreground inventory of materials (e.g., FeSO4, H2O2, HCl, FeCl3·6H2O), electricity, transport, and waste emissions in the three systems were estimated based on our experimental data and literature.56 The background inventory was selected from the ecoinvent database (version 3.8).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript investigates a piezoelectric FeOCl catalyst capable of generating hydroxyl radicals through an H2O2-free piezo-self-Fenton reaction. The authors attribute the improved reactivity to a “self-regulating” effect of piezopotential on the Lewis acid sites of FeOCl under ultrasonic excitation. The topic of piezocatalytic activation for Fenton-like processes is interesting. However, the present study suffers from several critical conceptual, methodological, and interpretational weaknesses. The claims are overstated relative to the supporting evidence, the mechanistic argument lacks rigor, and several statements are ambiguous or speculative. Therefore, I cannot recommend publication of this work in its current form.\n1. The central claim of “self-regulating Lewis acid sites” on FeOCl is not convincingly demonstrated. The data show enhancement of hydroxyl radical generation under ultrasound, but such enhancement could arise from cavitation effects, increased mass transfer, or Fe leaching rather than true piezopotential-driven site modulation. Furthermore, the result shows a limited enhancement in Lewis acid sites density under ultrasound in both FeOCl and BaTiO3, BiTiO3, and BiFeO3. Thus, the authors have not provided sufficient evidence to distinguish among these mechanisms. The idea of “Self-regulating of Lewis acid site-driving H2O2-free self-Fenton” remains speculative.\n2. The manuscript compares FeOCl with BaTiO₃, BiTiO₃, and BiFeO₃ to support the superiority of FeOCl. However, these materials differ fundamentally in crystal structure, band structure, and catalytic mechanism. Their piezoresponse and surface chemistry are not comparable. The observed performance differences may simply reflect distinct redox chemistries rather than a universal “self-regulation” mechanism. Such comparisons should be avoided or carefully qualified.\n3. The evidence regarding the Fe active site’s electron-transfer process in FeOCl is debatable. The 0.3 eV shift of Fe 2p in XPS spectra, as well as the 0.01 change of Fe oxidation state before and after used in EXANES spectra, cannot be considered valid evidence. Thus, these outcomes can’t support the notion of “FeOCl enable O2 molecule activation and consecutive conversion via Fe donate electron process”. Additional experimental validation is required, such as rotating ring-disk electrode (RRDE) measurement.\n4. The description of the DFT simulations is insufficient to reproduce the results. The paper mentions “charge-density difference” analysis and simulations “under 2 GPa pressure,” yet it is not explained how the mechanical stress or piezopotential was introduced in the calculation. Simply applying pressure in a periodic DFT model does not reproduce a real piezoelectric field. The authors should explicitly describe how charge redistribution and Lewis acidity were quantified, and how these changes correlate with the proposed piezoelectric polarization.\n5. The authors claim the induced piezopotential enables in-situ regulation of the surface Lewis acid sites, endowing the FeOCl catalyst with advanced H2O2-free piezo-self-Fenton activity. More experiments should be conducted to illustrate the underlying mechanism of this transition. Additionally, this result is not supported by the similar reaction energy barriers at the rate-determining steps for both samples in Figure 4d.\n6. The authors claim continuous production of •OH radicals, but no long-term stability or recyclability data are provided. To support this statement, at least several on–off cycles or extended sonication experiments with quantitative radical detection would be necessary.\nMinor points:\n1. Figures lack clear axis labels and scale bars in several cases; the axis labels in font size are inconsistent and even contain errors, such as Figure 5c.\n2. The schematic illustrations are qualitative and do not convey quantitative relationships.\n3. Experimental details on FeOCl synthesis, surface area, and electron transfer process of active site are too brief.\n4. The discussion section should separate observed phenomena from hypothesized mechanisms.\n5. The manuscript contains numerous grammatical and typographical errors that obscure meaning and detract from the professionalism of the presentation. The abstract in particular is poorly worded and should be rewritten for clarity and precision. The abstract and main text use speculative phrasing such as “may also achieve in-situ regulation” or “might enhance charge transfer”, which weakens scientific clarity. Claims should be written in definitive terms only when supported by direct evidence; otherwise they should be clearly marked as hypotheses. And many typo mistakes, for example, “Figure 3a” in Line 179", "reviewer_2": "Reagent-free AOPs are an interesting research direction given their sustainability characteristics. The manuscript NCOMMS-25-38873A-Z presents an iron oxychloride (FeOCl) piezocatalyst system in which H2O2 is converted from DO for Fenton-like degradation of antibiotics. The construction of Lewis acid sites has facilitated the O2 conversion to H2O2 which is dissociated into OH radicals in the Fenton-like reactions. It is interesting to see this self-Fenton system achieved the highest OH radical yield compared with previous reports. Experimental evidence including EPR, scavenger tests, DFT calculations have been demonstrated to proof the proposed reaction pathways, which seems reasonable. Overall, the experiments are well designed and the results are promising. I support the publication of this work, provided it undergoes some improvements.\nHere are specific comments that could improve the clarity and quality of the manuscript.\n1. It is not clear if, as a control, the ultrasonication itself also contributes to the degradation of tested antibiotics?\n2. In the figure captions, please specify which pH has been used for each experiment, as this is one of the most important factor for Fenton reactions.\n3. In DFT calculations, it is not clear if the solvation model is integrated into the calculations?\n4. Can the authors further explain why up to 200 °C was tested for the density of LA sites? This high temperature is irrelevant for water treatment processes.\n5. In Figure 3(a), the H2O2 yield decreased with time. Is it due to catalyst inactivation or other reasons? It is important to address the stability and reusability of the catalyst.\n6. I also notice the degradation performance of the self-Fenton system is quite close to the homogeneous Fenton treatment. Applying ultrasound to a large amount of wastewater can be challenging. What’s the possible application case for the proposed method?", "reviewer_3": "Major revisions are required before further consideration. Please address the following queries.\n1. Please incorporate an appropriate reference in the Results section (line 92) to substantiate the statement regarding intrinsic piezoelectricity and its effect on carrier dynamics\n2. Clarify the total number of atoms used in the FeOCl surface model employed for the DFT simulations.\n3. Please present the density of states (DOS) for the FeOCl surface before and after O₂ adsorption (0 GPa, and 2 GPa), as this comparison will provide insight into the electronic structure changes induced by the adsorption process.\n4. Please provide the bond-length values before and after O₂ adsorption (at both 0 GPa and 2 GPa) on the FeOCl surface, as obtained from your DFT calculations.\n5. One electron oxygen reduction is a critical part of the proposed mechanism. Where is the electron coming from and how is it being replenished in the catalyst?\n6. It is not clear how superoxide radical anions are being converted to H2O2. Generally, the superoxide pathway to H2O2 is known to be non-selective because of multiple possibilities. Why does this catalyst exhibit such specificity?\n7. Moreover, where are H+ ions coming from? What is the pH? Trace the change in pH with progress of the reaction.\n8. It would be helpful to include a schematic illustration of the in-situ Fenton mechanism involved in SMZ degradation over the FeOCl catalyst. This will improve the clarity and overall understanding of the proposed reaction pathway."}, "author_response": "Response to the Reviewers of NCOMMS-25-38873A-Z\nFor Reviewer: 1\nDear Reviewer,\nWe sincerely appreciate your time and valuable feedback on our manuscript (Ms. Ref. No.: NCOMMS-25-38873A-Z). Your comments have greatly helped us strengthen the work. We have revised the manuscript accordingly and hope it now meets the journal’s standards. Our point-by-point responses follow below, with your original comments (in black), our replies (in blue), and revised text excerpts (in red).\nYour comments: The manuscript investigates a piezoelectric FeOCl catalyst capable of generating hydroxyl radicals through an H O -free piezo-self-Fenton reaction. The authors attribute the improved reactivity to a “self-regulating” effect of piezopotential on the Lews acid sites of FeOCl under ultrasonic excitation. The topic of piezocatalytic activation for Fenton-like processes is interesting. However, the present study suffers from several critical conceptual, methodological, and interpretational weaknesses. The claims are overstated relative to the supporting evidence, the mechanistic argument lacks rigor, and several statements are ambiguous or speculative. Therefore, I cannot recommend publication of this work in its current form.\nOur reply: We appreciate your constructive suggestions for improving our manuscript.\nYour comments: 1. The central claim of “self-regulating Lewis acid sites” on FeOCl is not convincingly demonstrated. The data show enhancement of hydroxyl radical generation under ultrasound, but such enhancement could arise from cavitation effects, increased mass transfer, or Fe leaching rather than true piezopotential-driven site modulation. Furthermore, the result shows a limited enhancement in Lewis acid sites density under ultrasound in both FeOCl and BaTiO , BiTiO , and BiFeO . Thus, the authors have not provided sufficient evidence to distinguish among these mechanisms. The idea of “Self-regulating of Lewis acid site-driving H O -free self-Fenton” remains speculative.\nOur reply:\nWe sincerely thank the reviewer for these thoughtful and constructive comments. Our key finding demonstrates that the piezopotential induced by mechanical stress directly modulates the electronic structure and chemical properties of Lewis acid sites. This self-regulating behavior was experimentally verified. Py-IR and NH -TPD measurements confirm that ultrasonic vibration enhances the acid strength of the surface sites (Fig. 1f and Supplementary Fig. 7). In dynamic pyridine adsorption experiments (Fig. 1d), ultrasonic vibration led to an 8.1-fold enhancement in adsorption capacity on FeOCl compared with static conditions. Quantitative Py-IR analysis reflects the evolution of the acid site density. More importantly, dynamic pyridine adsorption experiments directly capture a real-time, reversible increase in adsorption capacity under mechanical stress, highlighting the sensitivity of the approach to transient, piezoelectricity-induced changes in surface properties. These results collectively confirm an increase in Lewis acid site content under mechanical stress and demonstrate the in-situ regulation of surface reactivity, consistent with the concept of “self-regulation.”\nWe appreciate the reviewer’s observation regarding the limited increase in site density. The dynamic adsorption experiments provide compelling evidence for piezopotential-induced enhancement of surface Lewis acid sites. Besides, the enhancement in catalytic activity arises not only from the increase in Lewis acid site density, but more critically from the piezopotential-induced enhancement in acid strength and functionality. The shift to higher temperature in the NH ‑TPD profile of the post‑reaction FeOCl, together with the marked increase in O adsorption energy revealed by DFT calculations (from -0.69 eV at 0 GPa to -2.65 eV at 2 GPa) (Fig. 4b), collectively demonstrates a significant strengthening of Lewis acidity and adsorption capability. These enhanced properties serve as the primary driving force underlying the promoted reaction pathway.\n(1) Distinguishing piezopotential-driven activity from cavitation, mass transfer, and leaching effects in piezocatalysis.\nEfficient HO• generation is achieved through the piezo-self-Fenton mechanism, wherein the piezopotential-enhanced Lewis acid sites activate O and promote its consecutive reduction first to H O and subsequently to HO•. To clarify the individual contributions, a series of control experiments was conducted (Fig. R1). The results show that all control scenarios yielded only limited HO• radicals relative to the piezo-self-Fenton system. Specifically, ultrasound alone (without catalyst) produced 20.05 μmol L-1 (~18% of the system), leached Fe3+ contributed 27.72 μmol L-1 (~25%), and the pyridine-poisoned FeOCl generated 20.92 μmol L-1 (~19%). In each case, the activity remained well below that of the complete piezocatalytic system. These results confirm that the observed enhancement stems primarily from the piezopotential‑driven regulation of surface acid sites.\n(2) Regarding the limited enhancement in Lewis acid site density under ultrasound.\nExperiments on several representative piezocatalysts further support the generality of piezopotential-mediated surface modulation. For BaTiO , BiTiO , and BiFeO , dynamic pyridine adsorption measurements reveal pronounced increases in adsorption capacity under ultrasonic vibration, corresponding to approximately 5.3-, 1.3-, and 2.2-fold enhancements, respectively (Supplementary Fig. 11 and Supplementary Fig. 12). This result indicates an enhancement in surface Lewis acid site content under mechanical stress, consistent with that observed for FeOCl. As noted by the reviewer, quantitative Py‑IR analysis for these materials shows only a limited increase in the acid site density. This pattern, defined by substantial functional enhancement in dynamic adsorption alongside a modest rise in quantified acid site density, further supports our central conclusion that the primary effect of the piezopotential on catalytic activity lies not merely in generating new sites, but in dynamically and substantially enhancing the reactivity and adsorption functionality of existing surface sites. The consistent trend across multiple materials demonstrates that piezopotential-induced in-situ modulation of surface adsorption properties is a general phenomenon, extending beyond the specific case of FeOCl.\nIn this work, while the \"piezo‑self‑Fenton\" activity originates from the intrinsic Fenton property of FeOCl, this study elucidates a more general mechanism in which the piezopotential not only facilitates charge‑carrier separation, but also dynamically modulates the physicochemical properties of surface active sites. In FeOCl, this coupling among mechanical, electrical, and surface properties enables the H O ‑free self‑Fenton pathway.\nWe have strengthened the discussion of these points in the revised manuscript. Thank you again for the valuable comments, which have significantly improved the rigor and clarity of our work.\nThe new data has been added as (Supplementary Fig. 19) in the Supplemental Information and added in the revised manuscript lines 214-221. “To further confirm that the enhanced activity arises from piezopotential-induced surface modulation rather than other ultrasound-related contributions, control experiments were conducted. HO• generation under ultrasound alone (20.05 μmol L-1), from ultrasound, using leached Fe3+ (27.72 μmol L-1), and from the pyridine-poisoned FeOCl (20.92 μmol L-1) were all markedly lower than that achieved on FeOCl (111.36 μmol L-1) (Supplementary Fig. 19). These results confirm that the enhanced activity originates from the in-situ modulation of Lewis acid sites on FeOCl by the piezopotential, rather than from cavitation or Fe leaching.”\nYour comments: 2. The manuscript compares FeOCl with BaTiO , BiTiO , and BiFeO to support the superiority of FeOCl. However, these materials differ fundamentally in crystal structure, band structure, and catalytic mechanism. Their piezoresponse and surface chemistry are not comparable. The observed performance differences may simply reflect distinct redox chemistries rather than a universal “self-regulation” mechanism. Such comparisons should be avoided or carefully qualified.\nOur reply:\nWe thank the reviewer for the insightful observation regarding the inclusion of different piezoelectric materials. The primary goal of this study was not to systematically compare FeOCl with these materials, nor to claim its “superiority” in Fenton-like performance. As noted, such a comparison is challenging due to fundamental differences in crystal structures, electronic band structures, and intrinsic catalytic mechanisms. Instead, FeOCl was used as the model piezocatalyst to investigate the core phenomenon, namely the in-situ modulation of surface properties by the induced piezopotential. Py‑IR and dynamic adsorption measurements were also performed on BaTiO , BiTiO , and BiFeO to test whether piezopotential-induced enhancement of surface adsorption capacity occurs in other classic piezoelectric materials, suggesting a degree of generality.\nWe fully agree that comparing their catalytic activities in reactions such as the piezo-self-Fenton process is inappropriate and was not the aim of this study. We will revise the manuscript to elucidate this point more clearly, avoiding any potential misinterpretation and keeping the emphasis on the mechanism established for the FeOCl model. Future efforts will be made to systematically examine how this general phenomenon operates across different piezoelectric materials.\nYour comments: 3. The evidence regarding the Fe active site’s electron-transfer process in FeOCl is debatable. The 0.3 eV shift of Fe 2p in XPS spectra, as well as the 0.01 change of Fe oxidation state before and after used in EXANES spectra, cannot be considered valid evidence. Thus, these outcomes can’t support the notion of “FeOCl enable O molecule activation and consecutive conversion via Fe donate electron process”. Additional experimental validation is required, such as rotating ring-disk electrode (RRDE) measurement.\nOur reply:\nWe thank the reviewer for this valuable suggestion. While the XPS and EXANES spectra show only a minor change in the nominal Fe valence, DFT simulations reveal a significant reconfiguration of the electronic structure that underlies the enhanced acidity. The density of states (DOS) calculations reveal the upward shift in the d-band center of surface Fe sites from -1.72 eV at 0 GPa to -1.47 eV at 2 GPa (Fig. R2). The upward shift of the d-band center, a key descriptor in heterogeneous catalysis, shifts the energy of Fe 3d orbitals closer to the Fermi level. This result enhances the interaction between Fe and electron-donating adsorbates, which in turn elevates the Lewis acidity of the Fe sites (Angew. Chem. Int. Ed. 2021, 60, 26592-26598; Chem. Soc. Rev. 2024, 53, 7392). Thus, the primary role of the piezopotential is not to induce a classical redox change. Instead, the electronic structure of the Fe center was modulated, thereby enhancing the efficiency of electron transfer to O and initiating the catalytic reaction.\nAs requested, rotating ring‑disk electrode (RRDE) experiments were performed to compare the FeOCl catalyst before and after ultrasonic vibration. As shown in Fig.R3, the FeOCl after ultrasonic vibration exhibits substantially higher current density than the pristine sample, confirming a significantly enhanced capacity for O adsorption and activation. Quantitative analysis based on the Koutecky‑Levich plot further reveals that the oxygen reduction reaction over the treated FeOCl proceeds with an electron transfer number approaching 3. This result indicates a consecutive three-electron pathway for the conversion of O to HO•, which proceeds through the following elementary single-electron transfer steps:\nO + e− → O •− (1)\n2 2\nO •− + e− + 2H+ → H O (2)\n2 2 2\nH O + e− → HO• + OH− (3)\n2 2\nThe stepwise progression of these reactions directly explains the observed electron-transfer number of approximately three, confirming the dominance of this multistep reduction pathway.\nCollectively, these electrochemical results demonstrate that the piezopotential‑induced enhancement of Lewis acidity at Fe sites not only promotes the initial activation of O but also facilitates the rapid conversion of intermediates, thereby enabling efficient and sustained HO• generation. This finding strongly supports the proposed reaction mechanism. The corresponding discussion and data have been incorporated into the revised manuscript. We thank the reviewer again for the insightful comments. The new data have been added as (Supplementary Fig. 9) and (Supplementary Fig. 31) in the Supplemental Information and added in the revised manuscript lines 156-166, “The subtle changes observed in the XPS and XANES spectra reflect a reversible modulation of the surface electronic structure induced by the piezopotential, rather than persistent changes in material properties. The results indicate that mechanical stress can dynamically modulate surface properties. Crucially, this minimal change in formal oxidation state indicates that the enhancement in Lewis acidity does not arise from the conventional redox process. Instead, density-of-states (DOS) analysis reveals its fundamental origin in the electronic structure by showing that the d‑band center of the surface Fe atoms shifts upward from -1.72 eV at 0 GPa to -1.47 eV at 2 GPa (Supplementary Fig. 9). This upshift of the d‑band center, a key descriptor in catalysis, shifts the energy of Fe 3d orbitals closer to the Fermi level. Consequently, the elevated d states strengthen interactions between Fe sites and electron-donating adsorbates, thereby increasing the Lewis acidity of the Fe sites.”\nLines 296-301, “Overall, steps (i)-(iv) define a consecutive three-electron reduction pathway from O to HO•. Further electrochemical validation using rotating ring‑disk electrode (RRDE) measurements (Supplementary Fig. 31) shows that the treatment of ultrasonic vibration substantially enhances the O reduction current on FeOCl and shifts the electron transfer number toward 3, confirming the feasibility of efficient O to HO• conversion via this pathway.”\nYour comments: 4. The description of the DFT simulations is insufficient to reproduce the results. The paper mentions “charge-density difference” analysis and simulations “under 2 GPa pressure,” yet it is not explained how the mechanical stress or piezopotential was introduced in the calculation. Simply applying pressure in a periodic DFT model does not reproduce a real piezoelectric field. The authors should explicitly describe how charge redistribution and Lewis acidity were quantified, and how these changes correlate with the proposed piezoelectric polarization.\nOur reply:\nThanks for the valuable suggestion. The primary goal of introducing pressure in the calculations is to simulate the local mechanical strain induced by ultrasonic cavitation and to evaluate its direct impact on the catalyst lattice and electronic structure. In piezoelectric materials, such charge redistribution constitutes the microscopic origin of the macroscopically observed piezopotential. By modeling pressure-induced lattice distortion, the DFT calculations reveal corresponding changes in the electronic structure of active Fe sites, including increased Bader charge transfer, significantly enhanced adsorption energies, and electron redistribution visualized via charge-density difference plots. These results demonstrate that mechanical strain effectively regulates the Lewis acidity of Fe sites and strengthens their interaction with reactants such as O .\nAt the atomic scale, these results clarify the mechanism of piezopotential-driven surface regulation. The applied pressure of 2 GPa was chosen based on reported estimates that local impact pressures from cavitation bubble implosion under ultrasonic conditions (40 kHz, 100 W) can reach 0.1-2 GPa (Nature 2005, 434, 52-55; Chinese J. Catal. 2025, 74, 130-143). This value provides a reasonable approximation of the mechanical stimulus, enabling the calculations to capture the characteristic electronic-structure changes induced by the piezoelectric effect. We have explicitly described the pressure parameter and the charge-density difference analysis in the revised manuscript. The theoretical calculations method is detailed in the Supplementary Information, section Text S5. “The Charge Density Difference was calculated using the equation:\n𝛥𝜌 = 𝜌 − 𝜌 − 𝜌\n𝑎𝑑/𝑠𝑢𝑏 𝑎𝑑 𝑠𝑢𝑏\nwhere ρ is the electron density of the combined system with the adsorbate on the FeOCl surface; ρ\nad/sub sub\nis the electron density of the pure FeOCl surface; ρ is the electron density of the adsorbate molecule in its free state.\nUnder the ultrasonic conditions employed in this study (40 kHz, 100 W), the local impact pressure generated by cavitation bubble collapse is estimated to reach 0.1-2 GPa. The total pressure inside a cavitation bubble can be described by the following equation:\n𝑃 = 𝑝 +2𝜎/𝑅\n0 0\nwhere 𝑝 is the disturbed pressure, 𝜎 is the surface tension, and 𝑅 is the initial bubble radius. It has been reported that the pressure released inside a bubble in n‑dodecane can reach up to 2 GPa at an acoustic frequency of 19.5 kHz and an acoustic pressure amplitude of 1.3 atm under the conditions of T = 298 K, p = 1 atm, and R = 50 μm. Based on these reports and given the comparable or even stronger driving conditions in our experiments (40 kHz, 100 W), a pressure of up to 2 GPa can be reasonably expected. Therefore, a value of 2 GPa was selected for the variable‑pressure theoretical simulations to represent a credible upper‑bound estimate of the transient mechanical stimulus.”\nYour comments: 5. The authors claim the induced piezopotential enables in-situ regulation of the surface Lewis acid sites, endowing the FeOCl catalyst with advanced H O -free piezo-self-Fenton activity. More experiments should be conducted to illustrate the underlying mechanism of this transition. Additionally, this result is not supported by the similar reaction energy barriers at the rate-determining steps for both samples in Figure 4d.\nOur reply:\nThanks for the valuable suggestion. Our key finding is that the induced piezopotential enhances the Lewis acidity of surface Fe sites, thereby strengthening the adsorption and activation of the key reactant, O . To substantiate this causal link, we integrate evidence of surface modulation with new experimental data that directly demonstrates the resulting enhancement in O interaction.\n(1) Regarding the underlying mechanism of the transition and the evidence for enhanced Lewis acidity.\nSystematic verification of enhanced Lewis acidity on FeOCl surfaces under ultrasonic vibration is presented as follows. Dynamic pyridine adsorption experiments (Fig. 1d) reveal an approximately 8.1‑fold increase in adsorption capacity relative to static conditions, demonstrating real‑time surface responsiveness to mechanical stress. Quantitative Py‑IR analysis (Fig. 1e-f) further shows that after ultrasonic vibration, characteristic absorption peaks for Lewis acid sites (e.g., at 1450 cm-1) were intensified and the corresponding site density increased. In parallel, NH ‑TPD (Supplementary Fig. 7) shows a shift in the desorption peak from 147 °C to 172 °C, confirming a substantial increase in acid strength. Notably, only subtle changes in the Fe oxidation state are observed by XPS and XAS (Fig. 2c-e), indicating that the pronounced enhancement in Lewis acidity does not originate from a conventional redox pathway. Instead, DFT calculations attribute the effect to electronic-structure modulation, showing that the d-band center of surface Fe atoms shifts upward from -1.72 eV at 0 GPa to -1.47 eV at 2 GPa (Fig.R2). The upshift of the d-band center, a key descriptor in heterogeneous catalysis, moves the d-band center closer to the Fermi level, thereby strengthening interactions with adsorbates and accounting for the enhanced Lewis acidity at the electronic level.\nTo further corroborate the proposed mechanism, O -TPD measurements were conducted. The results demonstrate an enhanced O adsorption capacity for FeOCl after reaction (Fig. R4), providing direct experimental support for the increased activity of its surface Lewis acid sites under mechanical stress. Specifically, the O -TPD profile of the treated catalyst exhibits a more pronounced desorption signal and a higher desorption temperature, indicating a strengthened interaction between O molecules and the catalyst.\n(2) Regarding the Gibbs free energy in Figure 4d.\nAs shown by the Gibbs free energy in Fig. 4d, the pathway for O conversion to HO• follows the same elementary steps at both 0 GPa and 2 GPa. In contrast, applying a mechanical pressure of 2 GPa renders each step thermodynamically more favorable. This is directly evidenced by the strengthening of O adsorption from -0.69 eV to -2.65 eV. The reduced reaction barriers under pressure confirm that the piezopotential facilitates the critical steps of the reaction pathway. The new data have been added as (Supplementary Fig. 8) in the Supplemental Information and added in the revised manuscript lines 126~130, “Consistently, O -TPD indicates enhanced O adsorption after treatment. Elevated desorption intensity reflects increased adsorption capacity, and the shift of the desorption peak to higher temperatures signifies strengthened O -surface interactions (Supplementary Fig. 8). These results demonstrate that the enhanced Lewis acidity directly facilitates O adsorption and activation.”\nYour comments: 6. The authors claim continuous production of •OH radicals, but no long-term stability or recyclability data are provided. To support this statement, at least several on–off cycles or extended sonication experiments with quantitative radical detection would be necessary.\nOur reply:\nThanks for the suggestion. In response, on-off cyclic and extended-duration experiments were conducted. As shown in Fig. R5a, during alternating 20-minute “ON” (ultrasound) and 20-minute “OFF” (static) cycles, HO• was generated only in the ultrasound‑ON periods and ceased immediately when the ultrasound was off. The total HO• yield accumulated over three cycles (60 minutes in total) was virtually identical to that obtained under continuous ultrasonic vibration for 1 hour, confirming the efficiency and reproducibility of the piezoelectric activation. Furthermore, under continuous ultrasonic vibration for 4 hours (Fig. R5b), the concentration of HO• increased steadily, confirming sustained radical generation. Together, these results demonstrate the operational stability and reusability of the catalytic system. The new data have been included as Supplementary Fig. 16 and Supplementary Fig. 17 in the Supporting Information and discussed in the revised manuscript (lines 203-211). “The long-term stability of the FeOCl piezo-self-Fenton system was confirmed through cyclic and extended-duration experiments. Under alternating ultrasound (ON for 20 minutes) and static (OFF for 20 minutes) conditions, HO• were generated only during ultrasound exposure and ceased immediately when the ultrasound was turned off, confirming the reversible, piezopotential-driven radical generation. The total HO• yield accumulated over three cycles (60 minutes in total) closely matched that obtained under continuous ultrasonic vibration for 1 hour, demonstrating the consistent performance under intermittent operation (Supplementary Fig. 16). Furthermore, during the 4 h continuous ultrasonic vibration experiment, the concentration of HO• exhibited sustained growth, verifying persistent catalytic performance (Supplementary Fig. 17).”\nMinor points：\nYour comments: 1. Figures lack clear axis labels and scale bars in several cases; the axis labels in font size are inconsistent and even contain errors, such as Figure 5c.\nOur reply:\nThanks for the suggestion. All figures in the manuscript have been carefully checked and revised to address issues such as incomplete axis labels, inconsistent font sizing, and missing scale bars. Corrections have been applied where needed, including in Fig. 3b (revised as Fig. R6), Fig. 3c (revised as Fig. R7), Fig. 5b (revised as Fig. R8), and Fig. 5c (revised as Fig. R9).\nYour comments: 2. The schematic illustrations are qualitative and do not convey quantitative relationships.\nOur reply:\nThank you for your valuable suggestion. The schematic illustrations in Fig. 3d (depicting the setup for in-situ electron paramagnetic resonance detection) and Fig. 3h (illustrating the proposed reaction mechanism) are intended as descriptive aids to clarify experimental design and conceptual pathways; the schematic illustrations do not convey quantitative relationships. Additionally, this work reports and validates the general phenomenon of in situ surface regulation by the piezopotential, providing a qualitative understanding of piezocatalysis. Quantitative correlations between the contribution of in-situ regulation and the catalytic activity represent a meaningful direction for future investigation.\nYour comments: 3. Experimental details on FeOCl synthesis, surface area, and electron transfer process of active site are too brief.\nOur reply:\nThanks for the suggestion. The details about the synthesis procedure of FeOCl and the DFT calculations method involving electron transfer have been complemented in the Supplementary Information, section Text S2. Catalyst preparation. “After naturally cooling to room temperature, the resulting product was thoroughly ground, washed via centrifugation with a large amount of anhydrous acetone seven times, and finally dried in a vacuum oven at 60 °C for 8 h.” and section Text S5. Theoretical calculations method. “The Charge Density Difference was calculated using the equation:\n𝛥𝜌 = 𝜌 − 𝜌 − 𝜌\n𝑎𝑑/𝑠𝑢𝑏 𝑎𝑑 𝑠𝑢𝑏\nwhere ρ is the electron density of the combined system with the adsorbate on the FeOCl surface; ρ\nad/sub sub\nis the electron density of the pure FeOCl surface; ρ is the electron density of the adsorbate molecule in its free state.\nYour comments: 4. The discussion section should separate observed phenomena from hypothesized mechanisms.\nOur reply:\nThanks for the suggestion. We have checked and adjusted the description of results and discussion in each section to ensure the logicality and readability of the manuscript.\nYour comments: 5. The manuscript contains numerous grammatical and typographical errors that obscure meaning and detract from the professionalism of the presentation. The abstract in particular is poorly worded and should be rewritten for clarity and precision. The abstract and main text use speculative phrasing such as “may also achieve in-situ regulation” or “might enhance charge transfer”, which weakens scientific clarity. Claims should be written in definitive terms only when supported by direct evidence; otherwise they should be clearly marked as hypotheses. And many typo mistakes, for example, “Figure 3a” in Line 179\nOur reply:\nThanks for the suggestion. We have thoroughly checked the manuscript, corrected the grammatical and typographical errors, and reorganized the ambiguous descriptions. For detailed revision, please refer to the revised manuscript in Lines 16~17, “Except for the promotion in carrier dynamics, the induced piezopotential also achieve in-situ regulation of the surface Lewis acid sites.”; in Lines 30~31, “Piezocatalysis has emerged as a sustainable and cost-efficient strategy for environmental remediation.”; in Lines 185~187, “Beyond the improvement of heterogeneous Fenton activity as previously reported, the in-situ regulation of LA sites on FeOCl also affect the reaction pathway.” and in Lines 190~192, “As shown in Fig. 3a, the H O yield reached 1618.8 μmol g⁻1 h⁻1, which was comparable to the performance of some classic piezocatalysts such as BaTiO , BiOCl, and BiFeO .”\nAnyway, we thank you very much again for your insightful and professional suggestions on our manuscript, which encourage us to further improve the quality of the paper.\nThanks a lot, with our best wishes for your great efforts in this regard!\nSincerely yours,\nMingshan Zhu\nResponse to the Reviewers of NCOMMS-25-38873A-Z\nFor Reviewer: 2\nDear Reviewer,\nThank you for your thoughtful review and constructive suggestions on our manuscript (Ms. Ref. No.: NCOMMS-25-38873A-Z). Your insights have been instrumental in improving the quality of our work. We have carefully addressed each point in the revised manuscript and believe the changes have enhanced its clarity and rigor. Below, we provide a point-by-point response, with your original comments in black, our replies in blue, and excerpts from the revised text in red.\nYour comments: Reagent-free AOPs are an interesting research direction given their sustainability characteristics. The manuscript NCOMMS-25-38873A-Z presents an iron oxychloride (FeOCl) piezocatalyst system in which H O is converted from DO for Fenton-like degradation of antibiotics. The construction of Lewis acid sites has facilitated the O conversion to H O which is dissociated into OH radicals in the Fenton-like reactions. It is interesting to see this self-Fenton system achieved the highest OH radical yield compared with previous reports. Experimental evidence including EPR, scavenger tests, DFT calculations have been demonstrated to proof the proposed reaction pathways, which seems reasonable. Overall, the experiments are well designed and the results are promising. I support the publication of this work, provided it undergoes some improvements.\nOur reply: We appreciate your encouraging assessment and constructive suggestions for improving our manuscript.\nYour comments: 1. It is not clear if, as a control, the ultrasonication itself also contributes to the degradation of tested antibiotics?\nOur reply:\nThanks for the valuable suggestion. The degradation of tested antibiotics under ultrasound alone was limited, with efficiencies of 5.8% for tetracycline (TC), 9.3% for ciprofloxacin (CIP), 8.0% for ibuprofen (IBU), 6.2% for norfloxacin (NOR), 4.1% for phenol (PhOH), 6.5% for sulfamethoxazole (SMX), 5.7% for carbamazepine (CBZ), and 3.7% for diclofenac (DCF) (Fig. R10a). In contrast, the degradation efficiencies on the FeOCl piezo-self-Fenton system largely above 80% (Fig. R10b). These results confirm that the high performance originates from the piezocatalytic reaction on FeOCl. The corresponding data and discussion have been incorporated into the revised manuscript. For detailed revision, please refer to Lines 347~349, “In the control experiments, the degradation achieved under ultrasonic vibration alone was minimal, in contrast to the high efficiencies of the FeOCl piezo-self-Fenton system (Supplementary Fig. 39).”\nYour comments: 2. In the figure captions, please specify which pH has been used for each experiment, as this is one of the most important factor for Fenton reactions.\nOur reply:\nThanks for the suggestion. The initial pH of the reaction solution has been specified in the relevant sections of the revised manuscript. Specifically, the H O generation experiments were conducted at an initial pH of ~6.7 (unadjusted), as noted in the “Piezocatalytic H O production” section. Hydroxyl radical generation was performed at an initial pH of ~5.6 (unadjusted), described in “HO• measurement and quantification.” The SMZ degradation experiments were carried out at an initial pH of ~6.3 (unadjusted), as indicated in “Piezo‑self‑Fenton degradation SMZ.” The corresponding discussion has been supplemented in the revised manuscript. For detailed revision, please refer to the revised manuscript in Lines 422~423, “The initial pH of the SMZ solution was ~6.3 (unadjusted).”, Lines 392~393, “The initial pH of the solution was ~6.7 (unadjusted).”, and Lines 403~412, “All the procedures were consistent if not specified. Generally, an open 300 mL beaker was used as the reactor, positioned above the ultrasonic transducer in a KS-2200DB ultrasonic cleaner (Kunshan Jielimei, China). The power and frequency of the equipment were 100 W and 40 kHz, respectively. The temperature of the water bath was controlled at room temperature by running tap water, and the water level was equal to the liquid level in the beaker. The system was stirred at 400 rpm by a JJ-1B-200W agitating machine (Xinrui, China) during the whole test. 20 mg of FeOCl catalyst was dispersed into 100 mL of 10mM terephthalic acid solution under stirring. The reaction was conducted at the initial solution pH of ~5.6 (unadjusted). Then the ultrasonic vibration was exerted on the suspension. The concentration of HO• was determined by a fluorescence spectrophotometer (Hitachi F-4600) with an excitation wavelength of 315 nm.”\nYour comments: 3. In DFT calculations, it is not clear if the solvation model is integrated into the calculations?\nOur reply:\nThanks for the suggestion. The DFT calculations in this study were performed using a vacuum slab model, without explicitly considering solvation effects. A vacuum layer of 20 Å was introduced to eliminate interactions between periodic images. Because this work focuses on the interaction between adsorbates and the catalyst surface under mechanical stress, this approach allows us to isolate and evaluate the piezopotential-induced electronic modulation. To avoid misunderstanding, the corresponding detail has been complemented in the revised manuscript. For detailed revision, please refer to the Supplementary Information, section Text S5. Theoretical calculations, “A 20 Å vacuum layer was introduced in the surface model to eliminate interactions between periodic images.”\nYour comments: 4. Can the authors further explain why up to 200 °C was tested for the density of LA sites? This high temperature is irrelevant for water treatment processes.\nOur reply:\nThanks for the suggestion. We would like to clarify that the elevated temperatures (e.g., 200 °C) cited here apply exclusively to the Py-IR characterization procedure, a well-established technique for quantifying surface Lewis acidity. These temperatures do not represent the actual water-treatment conditions. With the pyridine molecule as the Lewis basic probe, its interaction with the LA sites can be monitored by IR spectroscopy. The sample after pretreatment is subjected to the pyridine atmosphere for pyridine adsorption. In Py-IR measurements, spectra collected after pyridine adsorption at 50, 100, and 200 °C probe interactions between pyridine and Lewis acid sites of different strengths. Because stronger Lewis acid sites require higher temperatures to desorb pyridine, this temperature-dependent behavior indicates that the FeOCl catalyst contains a distribution of Lewis acid sites with varying acid strengths.\nYour comments: 5. In Figure 3(a), the H O yield decreased with time. Is it due to catalyst inactivation or other reasons? It is important to address the stability and reusability of the catalyst.\nOur reply:\nThanks for the valuable suggestion. This trend does not arise from catalyst deactivation but instead reflects the continuous consumption of in situ-generated H O within the established piezo-self-Fenton cycle. Under ultrasonic vibration, the Lewis acidic Fe sites on the FeOCl surface efficiently activate and decompose the accumulated H O as the reaction proceeds, converting it into hydroxyl radicals (HO•). This interpretation has supported by two key observations: (1) efficient H O decomposition over FeOCl under ultrasound (Supplementary Fig. 22), and (2) sustained accumulation of HO• over 4 hours (Fig. R5b). The continued generation of HO• throughout this extended reaction confirms that the catalytic sites remain active and do not deactivate.\nThus, the decline in H O concentration reflects a kinetic process where its consumption gradually exceeds its formation as the reaction proceeds. This behavior is an inherent feature of the consecutive O → H O → HO• pathway. The new data have been included as Supplementary Fig. 17 in the Supporting Information and discussed in the revised manuscript (lines 210-211). “Furthermore, during the 4 h continuous ultrasonic vibration experiment, the concentration of HO• exhibited sustained growth, verifying persistent catalytic performance (Supplementary Fig. 17).”\nYour comments: 6. I also notice the degradation performance of the self-Fenton system is quite close to the homogeneous Fenton treatment. Applying ultrasound to a large amount of wastewater can be challenging. What’s the possible application case for the proposed method?\nOur reply:\nWe appreciate the thoughtful suggestion. Indeed, the direct application of ultrasound to large volumes of wastewater remains challenging. The practical objective of piezocatalysis lies in harvesting and converting low-density mechanical energy rather than relying on ultrasound itself. Accordingly, investigations under ultrasonic vibration primarily aim to elucidate structure-property relationships that guide the design and optimization of high-performance piezocatalysts. Meanwhile, substantial efforts have been devoted to advancing the practical implementation of piezocatalytic technologies. In our previous work, we exhibited that mechanical energy from flowing water in urban drainage systems could be effectively harvested for piezocatalytic water purification. Using a spiral reactor to simulate pipeline conditions, water flow induced the piezoresponse of the MoS piezocatalyst, enabling the efficient degradation of multiple antibiotics with low energy consumption. This work established a proof of concept for self-powered, flow-driven piezocatalytic systems (ACS EST Engg. 2022, 2, 101-109). Notably, the scalability of such flow‑driven piezocatalysis was recently demonstrated by Xing et al., who implemented a hydrocyclone‑based reactor to trigger piezocatalytic hydrogen production from seawater at the scale of 20 L, demonstrating a realistic pathway toward practical implementation (Xing et al. PNAS 2023, 120, 7 e2218813120; Xing et al. Angew. Chem. Int. Ed. 2025, 64, e202504749). Thus, while the present study employs ultrasound as a well‑controlled mechanical stimulus to establish the H O ‑free piezo‑self‑Fenton system, the underlying mechanism of in situ surface regulation is broadly applicable for common piezocatalysts. Building on this mechanistic validation, the H O -free FeOCl piezo-self-Fenton system will be further optimized through engineering strategies toward real-world applications. The related discussions have been added to the manuscript lines 361-365. “The ‘self‑regulating’ phenomenon reported here is fundamentally driven by mechanical stress. This mechanism offers opportunities for implementation using widely available, low-frequency mechanical energy, such as water flow or ambient vibration. Future efforts will therefore focus on designing flow-or pressure-driven reactor systems to achieve this H O -free piezo-self-Fenton process for practical, energy-efficient water treatment applications.”\nWe are deeply grateful for the insightful guidance provided through your review. Your professional suggestions have been invaluable in elevating the quality of our manuscript, and we sincerely appreciate the time and expertise you dedicated to this process. Thank you once again for your thoughtful and constructive input.\nWith our best regards and high respect for your efforts,\nSincerely yours,\nMingshan Zhu\nResponse to the Reviewers of NCOMMS-25-38873A-Z\nFor Reviewer: 3\nDear Reviewer,\nWe are grateful for your careful evaluation and insightful comments on our manuscript (Ms. Ref. No.: NCOMMS-25-38873A-Z). Your suggestions have allowed us to substantially refine the study. The manuscript has been updated in response to your feedback, and we hope it now aligns with the journal’s expectations. In the following sections, we respond to each comment point by point, indicating your original text in black, our response in blue, and revised manuscript excerpts in red.\nMajor revisions are required before further consideration. Please address the following queries.\nYour comments: 1. Please incorporate an appropriate reference in the Results section (line 92) to substantiate the statement regarding intrinsic piezoelectricity and its effect on carrier dynamics\nOur reply:\nThanks for the suggestion. In response, the following key reference has been incorporated into the revised manuscript to substantiate the statement concerning intrinsic piezoelectricity and its influence on charge-carrier dynamics.\nReference:\n1. Pan, L. et al. Advances in Piezo-Phototronic Effect Enhanced Photocatalysis and Photoelectrocatalysis. Adv. Energy. Mater. 10, 25 (2020).”\n2. Guo, S. L., Lai, S. N. & Wu, J. M. Strain-Induced Ferroelectric Heterostructure Catalysts of Hydrogen Production through Piezophototronic and Piezoelectrocatalytic System. ACS Nano 15, 16106-16117 (2021).\n3. Ren, Z. R. et al. Efficient CO reduction to reveal the piezocatalytic mechanism: From displacement current to active sites. Appl. Catal. B-Environ. Energy 320, 11 (2023).\nYour comments: 2. Clarify the total number of atoms used in the FeOCl surface model employed for the DFT simulations.\nOur reply:\nThanks for the suggestion. In the DFT calculations, the total number of atoms used in the FeOCl model is 96. The corresponding detail has been complemented in the revised manuscript. For detailed revision, please refer to the Supplementary Information, section Text S5. Theoretical calculations, “In all the calculations, the total number of atoms used in the FeOCl model is 96.”\nYour comments: 3. Please present the density of states (DOS) for the FeOCl surface before and after O₂ adsorption (0 GPa, and 2 GPa), as this comparison will provide insight into the electronic structure changes induced by the adsorption process.\nOur reply:\nThanks for the suggestion. The calculated density of states (DOS) for the FeOCl surface reveals an upshift in the Fe d‑band center from -1.72 eV at 0 GPa to -1.47 eV at 2 GPa (Fig. R11a). The upshift of the Fe d‑band center strengthens the coupling between Fe 3d states and adsorbate orbitals (e.g., the molecular orbitals of O ), thereby promoting electron back‑donation from the Fe center to adsorbed O and thereby enhancing the surface activity toward O adsorption and activation (Angew. Chem. Int. Ed. 2021, 60, 26592-26598; Chem. Soc. Rev. 2024, 53, 7392).\nTo specifically characterize the adsorbate-surface interaction, Crystal Orbital Hamilton Population (COHP) analysis was performed. As shown in Fig. R11b, the integrated COHP (-ICOHP) for the Fe-O bond increases from -2.581 eV at 0 GPa to -2.651 eV at 2 GPa, confirming a strengthened covalent interaction and improved electron coupling under pressure.\nTogether, the upshifted d‑band center and the enhanced Fe-O bonding demonstrate that the piezopotential modifies the electronic structure of the active sites, thereby facilitating O adsorption and activation. The relevant DOS and COHP data are included in the Supplementary Information. For detailed revision, please refer to Lines 158-165, “Crucially, this minimal change in formal oxidation state indicates that the enhancement in Lewis acidity does not arise from a conventional redox process. Instead, density-of-states (DOS) analysis reveals its fundamental origin in the electronic structure by showing that the d‑band center of the surface Fe atoms shifts upward from -1.72 eV at 0 GPa to -1.47 eV at 2 GPa (Supplementary Fig. 9). This upshift of the d‑band center, a key descriptor in catalysis, shifts the the energy of Fe 3d orbitals closer to the Fermi level. Consequently, the elevated d states strengthen interactions between Fe sites and electron-donating adsorbates, thereby increasing the Lewis acidity of the Fe sites.”\nYour comments: 4. Please provide the bond-length values before and after O adsorption (at both 0 GPa and 2 GPa) on the FeOCl surface, as obtained from your DFT calculations.\nOur reply:\nThanks for the suggestion. According to our DFT calculations, the O-O bond length of the free O molecule is 1.234 Å. Upon adsorption on the FeOCl surface, this bond elongates to 1.257 Å at 0 GPa and increases further to 1.265 Å at 2 GPa. Meanwhile, the Fe-O bond between the Fe site of FeOCl and the adsorbed O shortens from 1.921 Å at 0 GPa to 1.910 Å at 2 GPa. These results shows that O activation is enhanced under mechanical pressure, consistent with the improved charge transfer and adsorption identified in our study.\nYour comments: 5. One electron oxygen reduction is a critical part of the proposed mechanism. Where is the electron coming from and how is it being replenished in the catalyst?\nOur reply:\nThanks for the suggestion. Piezocatalysis is a technique for mechanical-to-chemical energy conversion, based on the piezoelectric effect and surface catalysis. Under applied mechanical stress, the non-centrosymmetric structure of FeOCl generates an induced piezopotential, which drives directional charge separation and initiates redox reactions. The piezopotential induces spatial carrier separation, enabling reduction reactions at electron-rich regions and oxidation reactions at hole-rich regions. (Angew. Chem. Int. Ed. 2012, 51, 5962-5966; Adv. Mater. 2023, 35, 2208256). Along with continuous and periodic mechanical stress, consecutive energy conversion from mechanical energy to electrical energy and finally chemical energy can be realized on the piezocatalyst. In summary, the macroscopic electrical energy arises from the piezoelectric conversion of mechanical stress, whereas the microscopic electrons driving reduction originate from the migration of internal charge carriers within the catalyst.\nYour comments: 6. It is not clear how superoxide radical anions are being converted to H O . Generally, the superoxide pathway to H O is known to be non-selective because of multiple possibilities. Why does this catalyst exhibit such specificity?\nOur reply:\nWe thank the reviewer for the insightful question regarding the selective conversion of superoxide radical anions (O •⁻) to H O in this system. We acknowledge that, in many systems, O •⁻ undergoes nonselective transformations through multiple pathways, reduction to H O (O •⁻ + e⁻ + 2H+ → H O ) and disproportionation to O . However, the disproportionation pathway typically exhibits slow kinetics in pure water (k < 0.3 M-1 s-1) (J. Phys. Chem. Ref. Data 1985, 14, 1041; J. Am. Chem. Soc. 1982, 104, 1296-1299). Many studies have demonstrated that, during the oxygen reduction reaction, the selective adsorption and activation of key intermediates (e.g., O •⁻) on catalyst surfaces promote the H O formation pathway while suppressing competing side reactions (Angew. Chem. Int. Ed. 2024, 63, e202414417; Chem. Soc. Rev. 2020, 49, 6605-6631). In the FeOCl system, piezopotential-induced surface modulation enhances the adsorption and activation of molecular O , thereby directing selectivity toward efficient H O generation.\nUnder ultrasonic vibration, the mechanical stress induces a piezopotential in FeOCl that enhances the Lewis acidity of surface Fe sites and strengthens their interaction with oxygen species. DFT calculations reveal that at 2 GPa, O adsorption becomes stronger and electron transfer increases, providing a favorable electronic environment for O •⁻ conversion. The Gibbs free energy profile (Fig. 4d) further shows that the transformation from adsorbed *O to the *OOH intermediate is thermodynamically spontaneous (ΔG < 0) under stress, confirming a favorable reduction route. The intrinsically acidic surface of FeOCl (pH ≈ 1.75) acts as a complementary, creating and maintaining a proton-rich microenvironment at the catalyst-water interface (Process Saf. Environ. 2023, 172, 652-658). This persistent acidic interface supplies protons that facilitate the selective conversion of O •⁻ to H O .\nThe proposed pathway is supported by several experimental observations. Quenching experiments indicate that HO• originates from the sequential conversion of O (O → H O → HO•); in-situ EPR directly detects O •⁻ as an intermediate; and the measured kinetics of H O accumulation and consumption align with this stepwise mechanism. Rotating ring-disk electrode (RRDE) measurements on FeOCl after ultrasonic treatment (Fig.R3) further show an electron transfer number approaching 3 for oxygen reduction, corresponding to the consecutive reduction sequence from O₂ to O •⁻, H O , and finally to HO•.\nStepwise reduction of O to H O via O •⁻ is a common pathway in photocatalysis and piezocatalysis, often emerging as the dominant route on active catalysts without extensive discussion of its selectivity. The key finding here is that the piezopotential, by cooperatively enhancing surface Lewis acidity, elevates the efficiency of this pathway sufficiently to sustain a H O -free piezo-self-Fenton process.\nYour comments: 7. Moreover, where are H+ ions coming from? What is the pH? Trace the change in pH with progress of the reaction.\nOur reply:\nThanks for the valuable suggestion. In the proposed piezo-self-Fenton pathway, H+ ions are essential for the conversion of O to H O , which involves sequential reduction and protonation steps, as outlined in the mechanistic discussion (e.g., *O → *OOH and *OOH → *H O ) (Nat. Catal. 2021, 4, 374-384; Nat. Catal. 2024, 7, 195-206). The H+ involved in the reaction originates from H O.\nAs requested, the pH throughout the catalytic process was monitored. The results show that adding FeOCl to the initially solution (pH ≈ 6.7) reduces the pH to approximately 4.3, followed by a slight increase to around 4.5 during 1 h of ultrasonic vibration (Fig. R12). This pH decrease is consistent with the reported acidifying behavior of FeOCl; for example, the pH of a solution has been shown to decrease from 9.0 to 4.4 upon FeOCl addition. This effect is attributed to the point of zero charge of FeOCl (pH ≈ 1.75) and results in the pzc formation of a locally acidic environment that is favorable for Fenton-like reactions (Process Saf. Environ. 2023, 172, 652-658). The resulting locally acidic interface creates a proton-rich microenvironment that promotes proton transfer from water to adsorbed oxygen species, thereby facilitating the stepwise reduction of O to H O and ultimately to HO•. The relevant pH data are included in the Supplementary Information.\nFor detailed revision, please refer to Lines 196~199, “During the reaction, the solution pH decreased from ~6.7 to ~4.3 and then slightly increased to ~4.5 under ultrasonic vibration (Supplementary Fig. 15). The intrinsically acidic surface of FeOCl establishes a local proton-rich microenvironment, facilitating the protonation steps during the O to H O reduction.”\nYour comments: 8. It would be helpful to include a schematic illustration of the in-situ Fenton mechanism involved in SMZ degradation over the FeOCl catalyst. This will improve the clarity and overall understanding of the proposed reaction pathway.\nOur reply:\nThank you for your important suggestion. The schematic diagram (Supplementary Fig. 34) has been added to illustrate the proposed piezo‑self‑Fenton mechanism for SMZ degradation on FeOCl. The corresponding discussion has been supplemented in the revised manuscript. For detailed revision, please refer to the revised manuscript in Lines 318~322, “This superior performance originates from the piezopotential-driven self-regulation of surface Lewis acid sites, which facilitates the efficient consecutive conversion of O to H O and then to hydroxyl radicals (HO•). The generated HO• are primarily responsible for the rapid and complete degradation of SMZ (Supplementary Fig. 34).”\nOnce again, we extend our sincere appreciation for your thorough and perceptive comments. Your expertise has substantially strengthened the clarity and impact of our work, and we are truly thankful for the careful attention you gave to our manuscript. We are honored to have benefited from your review.\nThank you very much for your valuable time and dedication.\nSincerely yours,\nMingshan Zhu"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have addressed my concerns and it can be published at this point.", "reviewer_2": "Comments are addressed. Ok to publish.", "reviewer_3": "There is a significant change in pH (6.7 to 4.3) during the reaction. Several reports in the literature indicate that FeOCl is not stable in aqueous environments, as it often undergoes iron leaching and phase transformations upon reuse, forming species such as FeOOH and FeO. It is unclear whether this decrease in pH occurs in every reaction cycle or only during the first cycle. If the synthesized FeOCl is claimed to remain stable up to the fifth cycle, the authors should provide appropriate experimental evidence (such as XRD, XPS) to support this claim."}, "author_response": "Response to the Reviewers of NCOMMS-25-38873B\nFor Reviewer: 1\nYour comments: Authors have addressed my concerns and it can be published at this point.\nOur reply:\nWe appreciate for your recommendation of acceptance of this manuscript.\nResponse to the Reviewers of NCOMMS-25-38873B\nFor Reviewer: 2\nYour comments: Comments are addressed. Ok to publish.\nOur reply:\nWe appreciate for your recommendation of acceptance of this manuscript.\nResponse to the Reviewers of NCOMMS-25-38873B\nFor Reviewer: 3\nDear Reviewer,\nWe sincerely appreciate your careful evaluation and insightful comments on our manuscript (Ms. Ref. No.: NCOMMS-25-38873B). Your comments have greatly helped us strengthen the work. We have revised the manuscript accordingly and hope it now meets the journal’s standards. Our point-by-point responses follow below, with your original comments (in black), our replies (in blue), and revised text excerpts (in red).\nYour comments: There is a significant change in pH (6.7 to 4.3) during the reaction. Several reports in the literature indicate that FeOCl is not stable in aqueous environments, as it often undergoes iron leaching and phase transformations upon reuse, forming species such as FeOOH and FeO. It is unclear whether this decrease in pH occurs in every reaction cycle or only during the first cycle. If the synthesized FeOCl is claimed to remain stable up to the fifth cycle, the authors should provide appropriate experimental evidence (such as XRD, XPS) to support this claim.\nOur reply:\nWe sincerely thank the reviewer for the constructive comments. In response, we have performed additional characterizations to address these concerns.\nThe evolution of the pH value was monitored. With the addition of FeOCl piezocatalyst, the pH value dropped from 6.7 to around 4.5 in every cycle (Fig. R1). This behavior aligns with the inherently acidic surface properties of FeOCl.\nStructural stability of the FeOCl piezocatalyst after cycling tests was verified. The XRD pattern of FeOCl after cycling tests shows no notable changes compared to the fresh sample (Fig. R2), indicating that the crystal structure of FeOCl remains well preserved, with no formation of additional phases such as FeOOH or FeO. In the XPS spectra of FeOCl piezocatalyst after reaction (Fig. R3), slight shifts in binding energy are observed, reflecting minor changes in the surface electronic structure, in coordination with the XANES characterization (Fig. 2d and 2e). The Fe (III) chemical state was well maintained. In the O 1s XPS spectra, the signal can be deconvoluted as lattice oxygen (530.0 eV), surface hydroxyl groups and/or adsorbed oxygen species (531.5 eV), and adsorbed water (533.1 eV). FeOOH is characterized by a dominant signal at around 531.5 eV, more intense than that of the lattice oxygen (Nat. Commun. 2017, 8, 14264). For the FeOCl piezocatalyst after cycling tests, slight changes in peak area percentage are observed, ruling out the formation of FeOOH. In combination with the limited Fe leaching (0.13 mg L-1, Supplementary Fig. 19) during test, the results above validate the structural stability of FeOCl in the piezo-self-Fenton system.\nThe new data has been added as Supplementary Fig. 9 in the Supplemental Information and added in the revised manuscript lines 130-133. “Furthermore, the XRD pattern of FeOCl after ultrasonic vibration exhibits no notable changes compared with the fresh sample (Supplementary Fig. 9), indicating that the observed enhancements in surface properties are not due to phase transformation or loss of structural integrity.”"}]

Decision letter
A version of this paper was originally rejected for publication by Nature Communications, however that decision was reconsidered after appeal by the authors.