Enhanced piezo-photocatalytic water splitting activity via engineering robust dipole moments in covalent organic frameworks

Domain: physics
Published: 2025-12-12
Editorial outcome: minor_revision

Abstract
Covalent organic frameworks with tunable optical band gaps and notable piezoelectricity enable sustainable piezo-photocatalytic production of H2 and H2O2 via pure water splitting. Strong dipole moments can significantly boost piezoelectric properties, necessitating a systematic exploration of their structure-property relationships. Here, we synthesize a series of β-ketoenamine/imine-linked covalent organic frameworks, such as TP-BT-0F/1F/2F-COF, BTA-BT-0F-COF, and TP-SB-COF, through molecular design engineering. The combination of symmetry-breaking benzothiadiazole units and in-plane polarized β-ketoenamine linkages creates a robust dipole moment in TP-BT-0F-COF. This material demonstrates a high piezoelectric coefficient and band gap narrowing, achieving H2 and H2O2 production rates of 1501.4 and 1435.8 μmol g−1 h−1 under co-exposure to ultrasound (60 W, 40 kHz) and visible light. Density functional theory identifies N8 and C6 sites on benzothiadiazole units as potential catalytic sites for H2 and H2O2 evolution, respectively. Molecular design of covalent organic frameworks with prominent dipole moments advances mechano-optical energy conversion technologies.

Introduction
Efficient water splitting to produce H2 and H2O2 is considered a potential green technology for converting renewable energy into other energy forms1. However, the photocatalytic water splitting reactions used for H2 production typically requires a sacrificial agents and/or co-catalysts2–4. Piezoelectric catalysis through the conversion of mechanical to chemical energy is a promising, environmentally friendly approach to the production of H2 and H2O2 5,6. In non-centrosymmetric crystalline materials, mechanical stress in piezoelectric catalysis generates polarized charges and built-in electric fields, driving charge carrier separation and inhibiting recombination7,8. Moreover, the piezoelectric-induced polarization drives band bending, promoting electron-hole separation, overcoming the thermodynamic barrier of water oxidation, and thus synergistically boosting catalytic efficiency9. A variety of piezoelectric materials, including BiFeO3, BaTiO3, and g-C3N4, have been utilized for water splitting to produce H2 and H2O2 via piezoelectricity10–14. However, classical inorganic piezoelectric materials are limited by poor structural tunability and complex polarization processes, which severely hamper their mechanically driven catalytic efficiency15. Consequently, the intrinsic structural tunability of framework materials establishes them as highly suitable candidates for emerging piezo-photocatalytic systems16–19.

Covalent organic frameworks (COFs) have emerged as a paradigm-shifting photocatalytic platform. Their ordered nanoporosity and atomically precise structural programmability, achieved through modular covalent assembly, demonstrate their potential as pivotal visible-light-responsive photocatalysts20–22. Capitalizing on these programmable attributes, recent advances have exploited this design versatility to create two-dimensional (2D) COFs with engineered non-centrosymmetric frameworks, where symmetry-breaking polarization motifs synergistically enhance piezoelectric responses, thus expanding the scope of piezoelectric research23,24. For example, recent work engineered twistable bicarbazole motifs or fluorinated alkyl chains into COFs, enhancing piezoelectric performance25,26. Our prior work first reported that a highly crystalline β-ketoenamine-linked COF (EA-COF) could exhibit efficient piezo-photocatalytic H2 and H2O2 production in pure water without any co-catalyst. The piezoelectric properties of EA-COF were mainly attributed to the in-plane polarization of the β-ketoenamine groups within the framework27. At present, the structure-performance relationships dictating piezoelectricity in COFs are poorly understood. Thus, deliberate architectural tuning of piezo-photocatalytically tailored COFs requires systematic study to unravel critical design principles.

In this study, we rationally design and synthesize a series of β-ketoenamine/imine-linked BT-COFs exhibiting high piezo-photocatalytic activity through meticulous molecular engineering. The two critical factors governing the performance, such as in-plane polarization units and dipole moment, are systematically explored (Fig. 1). β-Ketoenamine-linked COFs exhibit piezoelectricity from ketoenamine unit in-plane polarization, whereas imine-linked COFs lacking these units have lower performance (Fig. 1A). Adjusting the dipole moment by introducing fluorine atoms or changing the structural unit affects the piezo-photocatalytic performance (Fig. 1B). Among them, TP-BT-0F-COF exhibits the highest piezo-photocatalytic water splitting efficiency, producing H2/H2O2 at rates of 1501.4/1435.8 μmol g−1 h−1 under ultrasound (60 W, 40 kHz) and visible light irradiation, respectively, and also shows good stability with H2/H2O2 production amounts of 18.9/14.5 mmol g−1 within 24 h. The enhanced performance arises from the synergy between ultrasound-induced polarized electric fields and the donor-acceptor benzothiadiazole (BT) unit design, which breaks symmetry to yield strong local dipole polarization. The N8 and C6 positions of the BT unit are also confirmed as the potential sites of H2 and H2O2 production by theoretical calculations, respectively. Experimental and computational analyses reveal that the BT-driven charge polarization mechanism enhances both photocatalytic efficiency and piezoelectric response, establishing a robust structure-property relationship for improved piezo-photocatalysis.

Results
Synthesis and characterization of β-ketoenamine/imine-linked COFs
To facilitate the synthesis of β-ketoenamine-linked isoreticular COFs, 4,4′-diamino-substituted p-terphenyl (SB), 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BT-0F), 4,4′-(5-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dianiline (BT-1F) (Figs. S1 and S2) and 4,4′-(5,6-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dianiline (BT-2F)28 were employed as linking agents and subsequently reacted individually with 1,3,5-triformylphloroglucinol (TP) (Fig. 1C), yielding crystalline TP-SB-COF, TP-BT-0F-COF, TP-BT-1F-COF, and TP-BT-2F-COF. Additionally, upon replacing TP with 1,3,5-benzenetricarboxaldehyde (BTA), the imine-linked BTA-BT-0F-COF bearing the BT-0F ligand was synthesized.

The powder X-ray diffraction (PXRD) results showed a high crystallinity of the synthesized TP-BT-0F-COF (Fig. 2A), BTA-BT-0F-COF (Fig. S3), BT-1F-COF (Fig. S4), TP-BT-2F-COF (Fig. S5) and TP-SB-COF (Fig. S6), respectively, which was in good agreement (Rp and Rwp < 10%) with the Pawley refinement results in space group P-6 (Table S1), P-6 (Table S2), P3 (Table S3), P3 (Table S4), and P-3 (Table S5), respectively, and stacking mode AA. The chemical structures of all the resulting COFs were investigated using various analytical methods.

Fourier transform infrared (FT-IR) spectra revealed that the characteristic vibrational frequencies corresponding to the C=O, C=C, and C–N bonds were observed at 1619, 1451, and 1253 cm−1 for TP-BT-0F-COF, respectively (Figs. 2B and S7). These signals were attributed to the formation of β-ketoenamine subunits. These characteristic peaks were also observed in the other three COFs (TP-BT-1F-COF, TP-BT-2F-COF, and TP-SB-COF), demonstrating the effective synthesis of all four COFs (Figs. 1B and S8–S10). Meanwhile, BTA-BT-0F-COF was synthesized via an imine bond, and the peak at 1630 cm−1 was indicative of the imine bond (C=N) (Figs. 2B and S11). The C=O signals at 1694 cm−1 were observed corresponding to the terminal -CHO group on the surface of BTA-BT-0F-COF28–30.

The surface chemical states were elucidated by X-ray photoelectron spectroscopy (XPS). In the high-resolution C 1s region (Fig. S12A), TP-BT-0F-COF exhibited two distinct peaks at 284.8 and 286.0 eV, corresponding to carbon atoms in C=C and C–N bonds. The N 1s spectra (Fig. S12B) showed a single nuclear level peak at 399.8 eV for TP-BT-0F-COF, clearly attributed to pyrrolic nitrogen species (=NH), confirming the sp2 covalent C–N bond in TP-BT-0F-COF. In addition, typical S 2p signals were detected in TP-BT-0F-COF, depicting the sulfur atoms within the benzothiadiazole unit (Fig. S12C). A similar phenomenon was observed in the other four COFs, namely BTA-BT-0F-COF (Fig. S13), TP-BT-1F-COF (Fig. S14), TP-BT-2F-COF (Fig. S15), and TP-SB-COF (Fig. S16). Unlike TP-BT-0F-COF, a single nuclear level peak at 399.2 eV for BTA-BT-0F-COF was attributed to pyridinic nitrogen species (C=N–C), confirming the sp2 covalent C=N bond in BTA-BT-0F-COF. No S 2p peaks were found in TP-SB-COF without the benzothiadiazole group. In addition, the presence of a C–F bond was clearly observed in the structure of TP-BT-1F-COF and TP-BT-2F-COF, as indicated by the characteristic binding energy peak at 687.4 eV in the XPS analysis.

Nitrogen adsorption isotherms were measured at 77 K to assess the surface areas and pore structures of the five COFs. The Brunauer-Emmett-Teller (BET) surface areas of the five COFs were calculated to be 1525.4 m2/g (TP-BT-0F-COF) (Fig. S17A), 121.2 m2/g (BTA-BT-0F-COF) (Fig. S18A), 2925.2 m2/g (TP-BT-1F-COF) (Fig. S19A), 2902.3 m2/g (TP-BT-2F-COF) (Fig. S20A), and 312.2 m2/g (TP-SB-COF) (Fig. S21A). All COFs exhibited a characteristic type IV adsorption isotherm with H3 or H4 hysteresis, indicating their mesoporous nature. This phenomenon could be attributed to the high crystallinity of the β-ketoenamine-bound COFs, which enhanced the regularity of the pore structure. As a result, β-ketoenamine-linked COFs exhibited a significantly larger specific surface area. In contrast, imine-linked COFs had lower crystallinity, resulting in a reduced specific surface area. This observation suggested that the hydroxyl groups in the triformylphenyl-based building blocks significantly influenced the final accessible surface areas of the COFs. The pore size distributions were predominantly concentrated around a narrow range, with peak maxima around 2.8 nm, as determined by the non-local density functional theory (NLDFT) method (Figs. S17–S21B).

Prior to the photocatalytic studies, the light-harvesting properties and bandgaps of these COFs were evaluated using UV-vis diffuse reflectance spectra. TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF and TP-SB-COF exhibited comparatively restricted optical absorption with absorption onset at 599, 590, 585, 585, and 519 nm (Fig. 2C) and a direct optical band gap of 2.07, 2.13, 2.17, 2.17, and 2.39 eV (Fig. 2D), respectively. Among the five COFs, TP-BT-0F-COF exhibited a pronounced redshift of the absorption onset with a smaller optical band gap due to the conjugated donor-acceptor (D-A) structures where phenylamine acted as the electron donor and benzothiadiazole as the electron acceptor. This indicated that it exhibited a broader absorption spectrum within the visible light range, thereby suggesting the potential for enhanced photocatalytic performance. Mott-Schottky measurements and valence band X-ray photoelectron spectroscopy (VB-XPS) were carried out to determine the electronic band positions of as-prepared TP-BT-0F-COF. The curve obtained showed typical characteristics of an n-type semiconductor, with a corresponding Fermi level (Ef) of −0.79 V vs. normal hydrogen electrode (NHE). The VB-XPS results for TP-BT-0F-COF are shown in Fig. S22, where the measured value of 1.98 eV indicated the energy difference between the Ef level and the valence band (VB) level. Consequently, the VB energy level for TP-BT-0F-COF was determined to be 1.19 V vs. NHE, and its conduction band (CB) energy level was −0.88 V vs. NHE. The CB and VB of the other four COFs, BTA-BT-0F-COF (Fig. S23), TP-BT-1F-COF (Fig. S24), TP-BT-2F-COF (Fig. S25), and TP-SB-COF (Fig. S26) were derived using the same method. The results showed that the VB potentials for BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, and TP-SB-COF were measured to be 0.96, 1.17, 1.14, and 0.82 V vs. NHE, respectively. Similarly, the CB potentials were calculated to be −1.17, −1.00, −1.03, and −1.57 V vs. NHE for BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, and TP-SB-COF, respectively. The results indicated that the CB and VB positions were suitable for facilitating the H2 evolution and water oxidation reactions (Fig. 2D).

Scanning electron microscopy (SEM) images of all COFs clearly showed distinct nanosheet morphologies (Figs. 2E and S27–30A), suggesting a homogeneous and well-defined structure. In addition, high-resolution transmission electron microscopy (HRTEM) further verified the high crystallinity and systematically organized mesoporous architecture of TP-BT-0F-COF (Fig. 2F), reinforcing the precise synthesis and structural coherence of the material. Similar TEM images of TP-BT-0F-COF (Fig. S27B, C), TP-BT-1F-COF (Fig. S28B–D), TP-BT-2F-COF (Fig. S29B–E), TP-SB-COF (Fig. S30B–E) were obtained, confirming their high crystallinity.

The unusual piezoelectric properties of the five COFs were further investigated using piezoresponse force microscopy (PFM)31. The piezoelectric hysteresis loop with a change of 180° under ±5 V direct current bias electric field showed a clear hysteresis with a local switching behavior of polarization for TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, and TP-BT-2F-COF. From the amplitude-voltage butterfly loop, TP-BT-0F-COF had the maximum effective piezoelectric coefficient (d33) of approximately 0.92 nm/V (Fig. 3A), while the maximum effective d33 coefficients of BTA-BT-0F-COF, TP-BT-1F-COF and TP-BT-2F-COF were 0.56 nm/V (Fig. S31A), 0.43 nm/V (Fig. S31D) and 0.26 nm/V (Fig. S31G) respectively32,33. The results unambiguously indicated that the materials’ piezoelectric properties followed the sequence: TP-BT-0F-COF > BTA-BT-0F-COF > TP-BT-1F-COF > TP-BT-2F-COF. In addition, a discernible resonance peak around 112–201 kHz signified the voltage-induced piezoelectric vibrations within the COFs (Figs. 3B and S31B, E, H). The amplitude showed a significant linear correlation with the excitation voltage, confirming the linear piezoelectricity of TP-BT-0F-COF (Fig. 3C), BTA-BT-0F-COF (Fig. S31C), TP-BT-1F-COF (Fig. S31F), and TP-BT-2F-COF materials (Fig. S31I). The butterfly-shaped amplitude hysteresis loops and corresponding phase hysteresis loops dependent on the bias voltage indicated that the polarization of these COFs could be switched by an electric field. Moreover, lateral PFM amplitudes and phase images of TP-BT-0F-COF (Fig. S32), BTA-BT-0F-COF (Fig. S33), TP-BT-1F-COF (Fig. S34), and TP-BT-2F-COF (Fig. S35) were also collected. It was noticeable that no such phenomenon was observed on TP-SB-COF (Fig. S36), indicating that TP-SB-COF lacked piezoelectric properties, which was consistent with the performance tests mentioned below.

Furthermore, the direct piezoelectric effect of all COFs was determined by quasi-static method. High piezoelectric coefficient d33 was obtained for TP-BT-0F-COF (5.5 pC/N) (Fig. S37A), BTA-BT-0F-COF (5.1 pC/N) (Fig. S37B), TP-BT-1F-COF (4.7 pC/N) (Fig. S37C) and TP-BT-2F-COF (4.1 pC/N) (Fig. S37D), consistent with the results obtained from PFM measurements. Such a switchable nature of the domain structures provided robust evidence for the occurrence of piezoelectricity in TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, and TP-BT-2F-COF25.

The non-centrosymmetric nature of TP-BT-0F-COF, critical for its piezoelectricity, was also confirmed by second-harmonic generation. Under 1400 nm femtosecond laser irradiation, the COF exhibited a distinct 700 nm second-harmonic signal (Fig. S38A)34. Additionally, violation of the mutual exclusion rule was observed, with normally forbidden vibrational peaks (1366, 1189, and 855 cm−1) appearing concurrently in IR and Raman spectra (Fig. S38B). This breakdown of selection rules confirmed a non-centrosymmetric lattice.

In situ high-pressure FT-IR spectroscopy was used to further investigate the structural change of TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, and TP-SB-COF. It revealed systematic blue-shifting of β-ketoenamine C–N/C=C vibrational peaks in TP-BT-0F-COF (Fig. 3D), TP-BT-1F-COF (Fig. S39A), TP-BT-2F-COF (Fig. S39B), and TP-SB-COF (Fig. S39C) under pressure. Quantitatively, at about 15 GPa, TP-BT-0F-COF exhibited maximal shifts (C–N: 1257→1312 cm−1, Δ=55 cm−1, rate=3.27 cm−1 GPa−1; C=C: 1458→1506 cm−1, Δ=48 cm−1, rate=3.52 cm−1 GPa−1; Fig. 3E), surpassing fluorinated analogs TP-BT-1F (3.12/3.24 cm−1 GPa−1; Fig. S39D), TP-BT-2F (2.89/2.99 cm−1 GPa−1; Fig. S39E), and non-ketoenamine TP-SB-COF (1.01/1.87 cm−1 GPa−1; Fig. S39F). These blue shifts indicated bond shortening and elevated vibrational frequencies, evidencing pressure-induced dipole moment enhancement within the framework. This intensified structural polarization and intramolecular charge distribution asymmetry35. Consequently, TP-BT-0F-COF exhibited the most pronounced shifts, confirming its greater pressure-responsive polarization sensitivity attributable to its inherently larger dipole moment. This directly correlated with the ketoenamine unit being the key structural determinant of COF piezoelectricity.

Conversely, C=O peaks at ~1625 cm−1 in these samples underwent pressure-induced red-shifting. Most prominently in TP-BT-0F-COF, the β-ketoenamine C=O vibration shifted 25 cm−1 (1625→1600 cm−1 at 4.2 GPa) at rate of −5.54 cm−1 GPa−1, significantly exceeding the red-shift rates of other β-ketoenamine COFs (Fig. S39D–F). This systematic discrepancy quantitatively confirmed that enhanced bond polarization in TP-BT-0F-COF originated from its amplified piezoelectric response, where pressure-driven electron redistribution toward C=C units dispersed C=O electron density36,37, reducing bond order and inducing red-shifting.

Concurrently, BTA-BT-0F-COF exhibited pressure-induced blue-shifting of benzene ring vibrations (Fig. S40), attributed to enhanced π-conjugation strengthening C=C bonds. In stark contrast, its imine C=N peak (1630 cm−1) showed negligible shift under pressure, indicating minimal piezoelectric polarization contribution from this linkage. This fundamental disparity in bond responsiveness explained the diminished piezoelectric output of BTA-BT-0F-COF versus β-ketoenamine-based TP-BT-0F-COF, correlating with its inferior catalytic performance.

Density functional theory (DFT) was used to observe and compare the local polarization behavior of the as-synthesized COFs molecules. As shown schematically in Fig. S41, TP-BT-0F-COF exhibited dipole moments of 1.420 D, which were larger than those of BTA-BT-0F-COF (0.7476 D), TP-BT-1F-COF (0.5995 D), TP-BT-2F-COF (0.4416 D), and TP-SB-COF (0.013 D). By gradually increasing the content of F elements in the skeleton, the dipole moment showed a decreasing tendency. This phenomenon indicated that the introduction of F elements had a significant effect on the charge distribution and molecular polarity of the COF skeleton. During the continuous increase of F elements, the distribution of the electron cloud inside the molecule changed. Due to the relatively strong electronegativity of the F atoms, the electron cloud was more likely to be distributed around the F atoms, thus changing the overall polarity of the molecule and eventually leading to a gradual decrease in the dipole moment. The results indicated that the local dipole moment change was modulated by the introduction of BT derivative units, which enhanced the self-polarization and piezoelectric properties of the COFs. Furthermore, the dipole moment of BTA-BT-0F-COF was lower than that of TP-BT-0F-COF, confirming that β-ketoenamine subunits dominated over BTA motifs in enhancing molecular dipoles and polarization.

Under the current DFT model, our calculations tentatively suggested the presence of piezoelectricity in these COFs. Table S6 presents the calculated ion-clamped piezoelectric tensors of these COFs, which were derived from their response to finite electric fields (E=0.001 eV/Å) under the simulated conditions. The trace of the piezoelectric tensors was 2.626, 1.743, 1.178, and -0.351 eÅ for TP-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, and TP-SB-COF, respectively, which was in agreement with the in situ high-pressure FT-IR spectroscopy results. The origin of piezoelectricity was further investigated. Of all the elements in piezoelectric tensors, the in-plane elements were much larger than the out-plane elements. Tables S6–S10 show the Born effective charges of ions in COFs with charges greater than 2.0 |e| (ion index can be found in Fig. S42), all of which were near the β-ketoenamine group (Fig. S43). As the large piezoelectric elements were mainly in the COF plane, the in-plane norms were calculated to be 4.496, 4.243, 3.341, and 0.342 eÅ, respectively. The magnitudes of the trace and the in-plane norms of the tensors were suggestive of a potential correlation with the piezoelectric activities of the COFs. The piezoelectric response of the materials was strongly influenced by their structural components. TP-BT-0F-COF contained both β-ketoenamine and BT units, showing a stronger piezoelectric response due to the active participation of these groups in polarization generation. For BTA-BT-0F-COF, the absence of β-ketoenamine groups reduced the primary piezoelectric active units, resulting in a lower piezoelectric performance. In the case of TP-BT-1F-COF and TP-BT-2F-COF, the introduction of the strong electron-withdrawing fluorine (F) element gradually reduced the skeletal dipole moment, weakened the piezoelectric effect of the BT units, and resulted in similarly low piezoelectric performance. Finally, TP-SB-COF, without BT units, underwent changes in its overall structural symmetry. This altered symmetry was likely the reason for its lack of piezoelectric properties. In conclusion, by tuning the dipole moment of COFs, the polarization effect of COFs could be significantly modulated, thereby tuning their piezoelectric performance.

The piezoelectric potential in piezoelectric materials could generate an intrinsic electric field that modulated charge carrier energy, thereby bending energy bands and enhancing redox activity. In situ pressure-dependent UV-visible absorption spectroscopy was used to investigate the redox band positions of COFs, allowing the charge migration process to be monitored. With increasing pressure, the optical absorption spectra of the COF samples showed pronounced red shifts (Figs. S44–S48). TP-BT-0F-COF exhibited maximal band gap narrowing (2.11→1.64 eV, Δ=−0.47 eV; rate=−0.0534 eV GPa−1; Fig. 3F–H), surpassing fluorinated analogs BTA-BT-0F-COF (Δ=−0.40 eV, rate=−0.0496 eV GPa−1; Fig. S49A–C), TP-BT-1F-COF (Δ=−0.42 eV, rate=−0.0488 eV GPa−1; Fig. S49D–F), and TP-BT-2F-COF (Δ=−0.36 eV, rate=−0.0478 eV GPa−1; Fig. S49G–I), and non-ketoenamine TP-SB-COF (Δ=−0.32 eV, rate=−0.0358 eV GPa−1; Fig. S49J–L). This band gap compression enhanced visible-light harvesting and charge migration kinetics, with full reversibility confirmed during decompression (Fig. S50). The distinct response of TP-BT-0F-COF stemmed from its large dipole moment and ketoenamine moieties, which conferred high pressure sensitivity by shifting band positions to reduce electron excitation energy, generating amplified polarized charges under irradiation, and accelerating surface redox processes38, collectively enabling an enhanced piezo-photocatalytic performance.

The pronounced piezoelectricity in TP-BT-0F-COF could be traced to the molecular-level symmetry breaking within its framework. This non-centrosymmetry originated not from large torsional motions but from the synergistic interplay of two structural features: the robust in-plane polarization of the β-ketoenamine linkage and the strong electronic asymmetry of the donor-acceptor BT unit. The BT unit created a permanent local dipole, which was maximized in the non-fluorinated analog (TP-BT-0F-COF) as confirmed by its largest calculated dipole moment (1.420 D). The locked, planar conformation of the β-ketoenamine bond amplified this effect by facilitating efficient charge separation across the framework plane. This mechanistic picture was supported by the pronounced pressure-induced blue shifts of the β-ketoenamine C–N and C=C vibrations in TP-BT-0F-COF, indicating a highly polarizable structure, and was directly validated by the experimental observation of second-harmonic generation. Consequently, the integration of the polar BT unit within the β-ketoenamine-linked framework provides a powerful design strategy for breaking symmetry and engineering high piezoelectric responses in COFs.

Piezo-photocatalytic water splitting
To evaluate the piezo-photocatalytic performance of TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, and TP-SB-COF for H2 and H2O2 evolution from overall water splitting, the following tests were designed. Several regulated experiments for pure water splitting, including photocatalytic, piezocatalytic, and piezo-photocatalytic methods, were performed under optical irradiation, acoustic oscillation (60 W, 40 kHz), and simultaneous optical and acoustic co-irradiation. Water and a catalytic agent serve as the primary substances in an inert argon environment, with a circulating coolant regulating the reaction temperature, and without the use of any sacrificial reagents or noble metal co-catalysts.

Under the conditions of pure water and visible light irradiation, all COFs could hardly produce H2 and only produced H2O2. This may be because the COFs reduced the residual oxygen in the water to H2O2, as confirmed by other studies39,40. Among the four COFs, TP-BT-2F-COF showed the highest H2O2 production (335.4 μmol g−1 h−1), followed by TP-BT-0F-COF (242.2 μmol g−1 h−1), while BTA-BT-0F-COF (128.0 μmol g−1 h−1) and TP-SB-COF (140.7 μmol g−1 h−1) showed much lower performance (Fig. S51).

When ultrasonic irradiation was applied, H2/H2O2 production occurred on TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, and TP-BT-2F-COF. It was noteworthy that the oxidation product of H2O was value-added H2O2, illustrating that the piezo-potential induced positive charge (q+) oxidized OH− to H2O2 instead of O2. As shown in Fig. 4A, B, the H2/H2O2 evolution rate of TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, and TP-BT-2F-COF was 1115.2/1019.1, 708.1/642.1, 441.4/398.9, and 130.7/112.9 μmol g−1 h−1, respectively. The time-dependent H2 and H2O2 yields over different samples and control groups showed that TP-SB-COF did not exhibit H2 evolution due to its lack of piezoelectric properties, and the sonolysis of water was negligible. Under pure ultrasound conditions, the H2O2 yield of all samples was slightly lower than that of H2, rather than 1:1. This was mainly due to the fact that not all hydroxyl radicals were fully combined to form H2O2.

Subsequently, visible light (λ≥420 nm) was introduced into the ultrasound reaction system, which further enhanced H2 and H2O2 evolution for both samples. Under simultaneous irradiation by light and ultrasound, TP-BT-0F-COF exhibited a prominent H2/H2O2 production rate of 1501.4/1435.8 μmol g−1 h−1, which was competitive among reported COF-based catalysts and other piezocatalysts (Table S11). These values were 1.6/1.6, 2.3/2.5, and 5.9/4.2 times those of BTA-BT-0F-COF (938.3/914.4 μmol g−1 h−1), TP-BT-1F-COF (649.8/569.84 μmol g−1 h−1), and TP-BT-2F-COF (250.7/339.7 μmol g−1 h−1) (Fig. 4C–E). The synergistic contributions of their photo- and piezo-catalytic processes were quantified as 26% (photo): 74% (piezo) for TP-BT-0F-COF, 25% (photo): 75% (piezo) for BTA-BT-0F-COF, 32% (photo): 68% (piezo) for TP-BT-1F-COF, and 48% (photo): 52% (piezo) for TP-BT-2F-COF, respectively. The improved piezo-photocatalytic performance suggested the significant role of illumination in promoting the piezoelectric activity.

The long-term duration experiment of TP-BT-0F-COF was also performed. TP-BT-0F-COF showed good stability with H2/H2O2 production levels of 18.9/14.5 mmol/g over 24 h, and its performance in water splitting remained consistent after long piezo-photocatalytic testing (Fig. S52). Although the XRD peak intensities were diminished post-reaction, the nearly unchanged FT-IR spectra confirmed the robust structural integrity of the TP-BT-0F-COF framework throughout piezo-photocatalysis (Fig. S53).

Compared to the other COFs, the higher H2 and H2O2 production of TP-BT-0F-COF could be due to its greater piezoelectricity and more efficient vibration energy harvesting. Interestingly, this phenomenon differed from photocatalysis and was mainly attributed to structural differences between the COFs, which led to variations in their piezoelectric performance. Firstly, TP-SB-COF, which lacked the BT unit, completely lost its piezoelectric performance and showed a low hydrogen evolution rate, comparable to that of pure water under the same conditions. Moreover, the absence of β-ketoenamine groups in BTA-BT-0F-COF and the increase in F elements in TP-BT-1F-COF and TP-BT-2F-COF also resulted in a reduction in piezoelectric performance. In addition, under the combined effects of light irradiation and ultrasound, TP-BT-2F-COF was the only material where H2O2 production exceeded hydrogen evolution. This phenomenon was probably due to the introduction of two F atoms, which could promote the generation of H2O2 originating from the CB of TP-BT-2F-COF41.

The piezo-photocatalytic activity of TP-BT-0F-COF was systematically evaluated under ultrasonic powers of 30, 60, and 120 W while keeping all other reaction conditions identical (Fig. 4F). The evolution rates of H2 and H2O2 increased steadily with rising ultrasonic power, achieving a pronounced piezo-photocatalytic synergy at 120 W (2773.2 μmol g−1 h−1 for H2 and 2735.5 μmol g−1 h−1 for H2O2). This enhancement stemmed from higher ultrasonic power inducing stronger polarization within TP-BT-0F-COF, which generated more pronounced built-in electric fields that facilitated the separation of photogenerated carriers, thereby boosting catalytic efficiency. In sharp contrast, no discernible H2 and H2O2 production was detected under milder mechanical perturbations (e.g., 800 rpm stirring, Fig. S54), thereby emphasizing the critical role of ultrasound-driven piezoelectric activation. The following frequency-dependent studies showed a steady rise in H2 yield with increasing ultrasound frequency, peaking at 60 kHz. This optimal response was likely due to the proximity to the intrinsic resonant frequency of TP-BT-0F-COF, where maximal mechanical energy transfer occurred (Fig. 4G). Wavelength-dependent apparent quantum yield (AQY) measurements were performed on 2 mg of TP-BT-0F-COF using a series of band-pass filters as shown in Fig. 3H. In particular, the AQYs of P-BT-0F-COF were 15.50% (450 nm), 14.61% (500 nm), 14.30% (550 nm), 13.62% (600 nm), and 9.65% (650 nm), respectively. The observed gradual decline in AQYs with increasing wavelength correlated with the optical absorption edge of TP-BT-0F-COF, substantiating the notion that light-driven carrier generation played an essential role.

Mechanism of piezo-photocatalysis
Scavenger-assisted mechanistic studies elucidated the charge transfer pathways in piezo-photocatalysis (Fig. S55). Selective electron depletion with 10 mM AgNO3 suppressed H2 evolution by 90%, verifying the electron-mediated proton reduction (2H+ + 2e− → H2, −0.41 V vs. NHE). Concurrently, the 87% decrease in H2O2 yield upon hole scavenging (10 mM EDTA-2Na) and 91% reduction with hydroxyl radical quenching (10 vol% tert-butanol, TBA) collectively demonstrated that holes drove the indirect two-electron water oxidation pathway (H2O + h+ → •OH + H+, 2.32 V vs. NHE; 2•OH → H2O2, 0.74 V vs NHE)42,43. Isotopic labeling experiments using D2O (95%) in the reduction halves confirmed the origin of the produced H2. The molecular ion peak at m/z = 4.00 in the mass spectrum indicated D2 production from D2O (Fig. S56)34. To further investigate the water oxidation reaction (WOR) in piezo-photocatalytic water splitting, •OH generation was measured using terephthalic acid photoluminescence (TA-PL) assays10. As shown in Fig. S57, the TA-OH fluorescence intensities of all COFs followed the order: TP-BT-0F-COF > BTA-BT-0F-COF > TP-BT-1F-COF > TP-BT-2F-COF > TP-SB-COF, under both ultrasonic irradiation and combined light-ultrasound excitation. This trend aligned with their hierarchical piezo-photocatalytic performance, confirming that •OH was a key indicator of WOR efficiency. In contrast, significantly weaker fluorescence occurred under light irradiation alone (Fig. S58), as photogenerated electrons primarily drove the oxygen reduction reaction (ORR) to form •OH, rather than direct water oxidation. This was consistent with the VB positions of all COFs being significantly more positive than the thermodynamic potential for the indirect 2e− two-step WOR, precluding •OH generation via water oxidation under sole photocatalysis. In situ electron paramagnetic resonance (EPR) analysis revealed four characteristic signals of •OH radicals for TP-BT-0F-COF under combined ultrasound and light irradiation, with negligible signals observed in light-only controls (Fig. S59). This confirmed the occurrence of an indirect 2e− two-step WOR pathway exclusively under piezo-photocatalytic conditions, and its absence under photocatalytic conditions alone. To further investigate the origin of •OH, an isotopic labeling experiment was conducted using H218O (98%) as the reaction solution. Benzoic acid served as a •OH trap, oxidizing to p-hydroxybenzoic acid44. The observation of a molecular ion peak at m/z = 139.0296 (Fig. S60) corresponded to the formation of 18O-labeled p-hydroxybenzoic acid, confirming that the •OH originated from H218O. This result provided direct evidence that H2O2 originated from water oxidation during piezo-photocatalysis.

In situ pressure-dependent photoluminescence (PL) spectroscopy measurements were conducted to examine the charge recombination dynamics (Fig. 5A). Under increasing pressure (0.0→5.0 GPa), the four COFs initially exhibited a red coloration, which transitioned to a colorless state at higher pressures, as shown in Figs. S61 and S62. Fluorescence quenching occurred for the four COFs at about 2.0 GPa, accompanied by a pronounced redshift with increasing pressure. The emission quenching resulted from the significant enhancement of π-π interactions at elevated pressures, leading to a continuous redshift24,45. Consequently, 2D COFs with extensive conjugation and densely packed structures were more prone to fluorescence quenching. This implied that charge carriers within the COF separated and transferred at an increased rate under the influence of pressure. Ultimately, this promoted the generation of more electrons and holes and thus increased their accessibility for redox reactions. Notably, TP-BT-0F-COF exhibited the most suppressed PL intensity at equivalent pressures, directly correlating with its high piezo-photocatalytic carrier separation efficiency.

The charge conduction characteristics in the piezo-photocatalytic process were investigated by piezo-electrochemical measurements. The photocurrent-time curves were recorded under visible light, ultrasound, or a combination of both to confirm the charge conduction behavior of TP-BT-0F-COF46. TP-BT-0F-COF exhibited higher photocurrent densities when exposed to both ultrasound and light simultaneously compared to light/ultrasound alone, demonstrating that ultrasound/light enhanced the charge transfer efficiency of the material (Fig. 5B). As shown in Fig. 5C, the transient photocurrents under light and ultrasound exposure were in the order of TP-BT-0F-COF > BTA-BT-0F-COF > TP-BT-1F-COF > TP-BT-2F-COF > TP-SB-COF, consistent with the piezo-photocatalytic results above. TP-BT-0F-COF exhibited the higher photocurrent density compared to the other four COFs, suggesting that charge separation and transfer were more efficient in TP-BT-0F-COF. This improvement in charge transfer allowed a greater number of piezoelectric charges to be used to drive electrochemical reactions during the photopiezoelectric catalytic process.

To further investigate the charge transfer resistance, electrochemical impedance spectroscopy (EIS) measurements of the TP-BT-0F-COF electrodes were performed under different conditions, and the corresponding Nyquist plots and fitting results are shown in Fig. S63B and Table S12. As depicted in the equivalent circuit diagram, R2 corresponds to the charge transfer resistance at the sample/electrolyte interface, whereas R1 represents the resistance of the electrolyte and other related system resistances. Notably, when both light and ultrasound were applied simultaneously, the charge transfer resistance was significantly reduced compared to either light or ultrasound alone, suggesting an improved electronic interaction between piezoelectric charges and electroactive materials under these combined conditions47. In addition, the semicircular radius of the EIS Nyquist plots and R2 of the five COFs decreased in the order of TP-BT-0F-COF (4392 Ω) < BTA-BT-0F-COF (5513 Ω) < TP-BT-1F-COF (9767 Ω) < TP-BT-2F-COF (15,310 Ω) < TP-SB-COF (26,326 Ω) (Fig. 5D, Table S13), indicating that the solid-phase interfacial resistance and surface charge transfer barrier were lower in TP-BT-0F-COF compared to the other materials.

The surface potentials of the COFs were measured both in the dark and under irradiation using Kelvin probe force microscopy (KPFM). As shown in Fig. 5E, F, TP-BT-0F-COF exhibited a surface potential of 68 mV in the dark, which decreased to −30 mV upon exposure to light. The observed decrease of 98 mV was probably due to the effect of the inward piezoelectric potential field48,49. The similar trend was also observed for the other three COFs, but their potential differences of 58 mV for BTA-BT-0F-COF (Fig. S64A, B), 80 mV for TP-BT-1F-COF (Fig. S64C, D), and 76 mV for TP-BT-2F-COF (Fig. S64E, F) were all lower than that of TP-BT-0F-COF. These results could be attributed to the piezo-induced polarization charge being partially depleted by photogenerated carriers, which was beneficial for the separation of photogenerated charges and improvement of catalytic activity11,32.

To reveal the overall water splitting processes, DFT calculations were then performed to reveal possible reactions such as the hydrogen (HER), oxygen (OER), and H2O2 evolution reactions in the piezo-photocatalytic reactions. Normally, HER occurs at the cathode, which provides electrons for the H+ to form H2. Therefore, the reaction active site should be close to the region where the conduction band is distributed. For the same reason, the active site of the OER should be close to the valence band region. Figure 6A, B shows the orbitals of the conduction band minimum and the valence band maximum of TP-BT-0F-COF, and the corresponding band structures are shown in Fig. S65. For the HER, the possible active sites were indexed by the number of O1, N2, C3, C4, C6, N8 and S9 (Fig. 6C), and the calculated corresponding free energy penalties for H+ adsorption on these sites are shown in Fig. 6D. In general, the N ion in the BT unit (site N8) has the lowest free energy requirement of 0.65 eV.

For OERs and H2O2 evolution reactions, the reaction path became very complicated due to multiple steps and reaction intermediates. Finding reaction paths on very large unit cells, such as COFs, was computation-consuming. Therefore, a reasonable simplified model was required, such as the simplified molecular model28. Here, we simplified the linking unit by a double carbon bond and obtained a one-dimensional chain (Fig. S66A), and then tested the free energy changes of the HER compared to the original model (Figs. 6D and S66B). It was found that the energy errors near the linker (sites N2, C3, and C4) were higher by 0.2 eV. It became minimal for the sites on the BT unit (site C6, N8, and S9) where the OER occurred according to the orbital distribution of the VB maximum. Therefore, the simplified model was used to search for OER paths for acceleration. The error of this model should be less than 0.1 eV. Figures 6E and S67 show the possible OER pathways of TP-BT-0F-COF starting from C6. The first step was OH− adsorption, which was found to be the most difficult step. The free energy required was 2.14 eV, resulting in an overpotential of 0.91 eV. The OER could also start by adsorbing OH− on the S9 and N8 sites of the BT unit (Fig. 6F). For example, when OH− was adsorbed on the S9 ion, the free energy increase was 2.29 eV, which was 0.15 eV higher than when adsorbed on the C6 ion. The adsorption on the N8 ion was much higher, which was unlikely to occur. We focused here on the reactions starting from the C6 ion. After an OH− adsorption, there were four possible subsequent reaction pathways (Fig. S67). Path 1: The transfer of an H to the neighboring C5 site led to a decrease in energy. However, the next reaction step of adsorbing another OH− required an incoming energy of 1.66 eV, followed by the removal of the H and release of an O2 with energy increases of 1.15 and 0.82 eV, respectively. Path 2: Removing an H from the adsorbed OH− increased the energy by about 0.56 eV. The free energy increase for the next OH− adsorption was 1.46 eV. Removing O had an energy increase of 1.17 eV. The final step of releasing O2 would occur spontaneously with an energy decrease of 0.41 eV. Path 3: Adsorption of another OH− had to absorb 0.63 eV. The coupling of two OH− would produce H2O2, costing only 0.68 eV. Path 4: Same as pathway 3, adsorption of another OH− would have to absorb 0.63 eV, then continuous removal of Hs from the two adsorbed OH− would release O2 with a maximum energy penalty of 1.39 eV. The possible OERs were fully investigated on other sites and are shown in Fig. S68. It was found that OERs on COF were not able to form the *O-OH bond (* is for the adsorption site), which was the rate-determining step for most OERs catalyzed by inorganic catalysts50. The O-O bond in COF-catalyzed OERs was formed by the linkage of *O-O*. The most favorable reaction path for O2 production after initial OH− adsorption required a minimum energy cost of 1.39 eV, which was twice that for H2O2 production.

Based on the current DFT-derived insights and simulated conditions, OERs appeared to be inactive in this piezo-photocatalytic system, while the H2O2 evolution reactions might preferentially follow pathway 3. This pathway started with an OH− absorption at the C6 ion site with an energy cost of 2.14 eV, and continued with two low-energy cost steps of further OH− adsorption and H2O2 release. Combining the experimental and DFT calculation results, we proposed a possible mechanism for piezo-photocatalytic water splitting using TP-BT-0F-COF as an example (Fig. 6G). Upon visible light and ultrasound stimulation, TP-BT-0F-COF underwent deformation and bending, inducing in-plane stretching and compression. The mechanical strain changed the state of the local dipoles, inducing spontaneous polarization and generating a variable piezoelectric field that modulated the energy band structure of TP-BT-0F-COF. As a result, the free electrons in N8 site of the BT unit were captured by adsorbed H+ to produce H2, while holes were consumed by adsorbed OH− and H2O at C6 site of the BT unit to produce oxidation products such as •OH and H2O2, thereby facilitating the water splitting reaction.

Methods
Materials
All chemicals and reagents were of analytical grade and were used as received. The 1,2-dichlorobenzene (o-DCB, 99%), anhydrous n-butanol (99.4%), dioxane (99%), mesitylene (99%), and pyrrolidine (Py, 98%) were sourced from Macklin Chemicals. 2,4,6-tris(4-aminophenyl)triazine (TP, 97%), 4,4″-diamino-substituted p-terphenyl (SB, >97.0%), 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BT-0F, 98%), and benzene-1,3,5-tricarbaldehyde (BTA, >98.0%) were obtained from Jilin Chinese Academy of Sciences-Yanshen Technology Co. Ltd.

Characterization
PXRD patterns were collected on a PANalytical Empyrean diffractometer using Cu Kα radiation (λ=1.54 Å) operated at 45 kV and 35 mA. FT-IR spectra were acquired with a Thermo Fisher Nicolet Avatar 6700 FT-IR spectrometer. UV-Vis absorption spectra were recorded on a Shimadzu UV-2700 spectrophotometer. Nitrogen sorption isotherms were measured at 77 K using an iPore 400 analyzer. Prior to measurement, samples were degassed at 120°C under dynamic vacuum (<10−3 Torr) for 10 h. Specific surface areas were determined using the BET method, and pore size distributions were derived from the sorption isotherms via NLDFT. XPS was performed on a VG ESCALAB 250 system. TEM was carried out on a JEOL JEM-2010 instrument. The piezoelectric response was characterized by PFM using a Bruker Dimension Icon atomic force microscope. Photo-enhanced KPFM measurements were conducted using an AFM system equipped with an SKPM module and a xenon lamp source.

Piezo-photocatalytic H2 and H2O2 evolution
The photocatalytic reaction was conducted in a 40 mL cylindrical optical reactor (outer diameter: 27.5 mm, height: 95 mm) containing 2 mg of catalyst dispersed in 20 mL of ultrapure water. The suspension was simultaneously irradiated with a 300 W Xe lamp (CEL-PF300-T9, Beijing Zhongjiao Jinyuan) and ultrasonicated (JP101T cleaner, 40 kHz, 60 W; Shenzhen Jiemen). The lamp was equipped with a visible light cut-off filter (λ≥420 nm) to provide an incident light intensity of 1000 W m−2, as calibrated by a solar power meter (Model SM206-SOLAR). The reaction temperature was maintained at 20.0 ± 0.5°C by a circulating cooling water system.

Prior to reaction, the system was purged by three alternating cycles of evacuation and argon purging (each 5 min) to ensure an anaerobic environment. During the reaction, 100 μL of headspace gas was periodically extracted with a gas-tight syringe for hydrogen analysis by gas chromatography (GC, FULI GC9790 PLF-01, FULI instruments) equipped with a thermal conductivity detector (TCD). The H2 yield was determined using a standard calibration curve (Fig. S69A).

The concentration of hydrogen peroxide (H2O2) was quantified via an enzymatic method using horseradish peroxidase (HRP) to catalyze the reaction between H2O2 and 3,3’,5,5’-tetramethylbenzidine (TMB).

The piezo-photocatalytic production of H2 and H2O2 was investigated under varying ultrasonic intensities and frequencies. The experiments employed a multi-frequency ultrasonic cleaner (YNLH0203-204060, Shenzhen Yunyi) with selectable frequencies (20, 40, 60 kHz) and corresponding powers (30, 60, 120 W). All other conditions were consistent with those described above.

Measurement of H2O2
The H2O2 concentration was quantified using a 3,3′,5,5′-tetramethylbenzidine (TMB)-based enzymatic assay, in which horseradish peroxidase (HRP) catalyzes the oxidation of TMB by H2O2.

The TMB stock solution was prepared by first dissolving 15 mg of TMB in 0.3 mL of dimethyl sulfoxide (DMSO). This solution was then mixed with 5 mL of glycerol and 45 mL of an aqueous solution containing 20 mg of ethylenediaminetetraacetic acid (EDTA) and 95 mg of citric acid. The mixture was finally diluted to a total volume of 500 mL with deionized water. The HRP solution was prepared by dissolving 2 mg of the enzyme in 10 mL of deionized water.

For the calibration, 200 μL of the TMB solution and 10 μL of the HRP solution were added to a known concentration of H2O2 (100 μL). After 3 min of reaction, HCl (10 μL, 12 M) was added to stop the reaction, and the absorbance at 450 nm was measured by UV-Vis spectroscopy. A linear standard curve was established from these measurements and used to determine the H2O2 concentration in the experimental samples. The testing method for the actual samples was identical to that used for the reference standard. The H2O2 yield was calculated based on these calibration curves (Fig. S69B, C).

Apparent quantum yields (AQYs) measurements
Apparent quantum yields (AQYs) were determined using 5 mg of samples dispersed in 20 mL of water, with a 300 W Xe lamp (CEL-PF300-T9, Beijing Zhongjiao Jinyuan Technology Co., Ltd.) equipped with band-pass filters (λ=450, 500, 550, 600 and 650 nm) as the incident light source (irradiation area: 0.25 cm2), under the combined conditions of irradiation and ultrasonic assistance for 30 min. The light intensity was measured with a solar power meter (Model SM206-SOLAR). The AQY was calculated using the following formula (1):

η = (2 × M × NA × h × c) / (S × P × t × λ) × 100% (1)

Where M is the amount of H2 produced (mol), NA is the Avogadro constant (6.022 × 1023/mol), h is the Planck constant (6.626 × 10−34 Js), c is the speed of light (3 × 108 m/s), S is the irradiation area of the incident light (m2), P is the intensity of the incident light (W/m2), t is the photoreaction time (s), and λ is the wavelength of the monochromatic light (m).

By substituting the known constants, we obtained the following simplified formula (2) for the AQY:

η = (2 × M × 2.66 × 109) / (P × λ) × 100% (2)

Where M is the amount of H2 produced (mol), P is the intensity of the incident light (W/m2), and λ is the wavelength of the monochromatic light (nm). Specific values are provided in Table S14.

Trapping experiments of hydroxyl radicals
An aqueous terephthalic acid (TA) solution (40 mL, 0.5 mM) was formulated in a 2 mM NaOH medium and then transferred into a quartz reactor. A 5 mg portion of the photocatalyst was uniformly dispersed in the aforementioned solution. The resulting mixture was exposed to visible light irradiation (300 W Xe lamp, λ>420 nm, light intensity: 1000 W/m2) for 1 h at 20°C under constant magnetic stirring. Following the irradiation process, the suspension was subjected to centrifugation at 10,000 rpm for 10 min. The fluorescence emission intensity of the supernatant was determined at 425 nm (excitation wavelength: 315 nm) via a fluorescence spectrometer.

Piezo-photoelectrochemical measurements
Piezo-photoelectrochemical measurements were performed using a standard three-electrode setup on a CHI660E electrochemical workstation, under visible light (λ≥420 nm) and ultrasonic irradiation. The working electrode was prepared by dispersing 2 mg of catalyst in a mixture of 0.5 mL ethanol and 10 μL Nafion solution, followed by sonication for 5 min to obtain a homogeneous suspension. Next, 50 μL of the suspension was drop-cast onto a pre-cleaned ITO glass substrate (0.5 × 0.5 cm2) fixed with double-sided tape. A proton exchange membrane (1 × 1 cm2) was then placed on the catalyst-coated region and hot-pressed at 100°C under 350 psi pressure for 3 min, thereby forming the working electrode (Fig. S70).

Mott-Schottky analysis was carried out using a three-electrode configuration, where the COF/ITO glass served as the working electrode, a platinum (Pt) plate functioned as the counter electrode, and a saturated Ag/AgCl electrode acted as the reference. The electrolyte consisted of 30 mL deionized water with 0.20 M Na2SO4. This study involved applying a polarization potential from −1.2 to 1.6 V (vs. SCE), with alternating potential frequencies of 500, 1000, and 1500 Hz employed throughout the study.

Photocurrent tests were conducted under the conditions of irradiation, sonication, or their simultaneous application. EIS tests were conducted at a bias voltage of −0.4 V, covering irradiation, sonication, or their combined action, with a frequency range of 0.001–100000 Hz. For EIS tests performed under ultrasound, the electrochemical cell was secured in an ultrasonic bath. The acquired EIS data were fitted with a suitable equivalent circuit17.

In situ high-pressure PL, IR, and UV-Vis absorption measurements
High-pressure experiments utilized a symmetric diamond anvil cell (DAC) (Fig. S71) equipped with ultra-low fluorescence diamond anvils (culet diameter: 400 μm). Samples were loaded into a 150 μm-diameter chamber laser-drilled in a pre-indented T301 stainless steel gasket (thickness: 45 μm), with silicon oil (PL/UV-Vis) or KBr (IR) selected as pressure-transmitting media. The ruby fluorescence technique was utilized for pressure determination, relying on the spectral shift of the R1 line in ruby fluorescence. All the aforementioned measurements were performed at 298 K.

Spectroscopic measurements. Photoluminescence (PL): excited with a 355 nm semiconductor laser (with power stabilized to avoid intensity-related artifacts), and the corresponding spectra were collected by an Ocean Optics QE65000 fiber spectrometer (Fig. S72).

UV-Vis absorption: a deuterium-halogen excitation source was coupled to the same spectrometer, with a focus on excitonic absorption bands (Fig. S73).

Infrared (IR): recorded using a Nicolet iN10 microscope spectrometer (equipped with a liquid N2-cooled MCT detector), where KBr was used as the pressure-transmitting medium (Fig. S74).

In situ EPR monitoring experiments
EPR spectra were recorded at 298 K using a Magnettech ESR 5000 spectrometer (X-band, 9.85 GHz) (Fig. S75) with the adoption of the following instrumental parameters: modulation amplitude of 1.000 G, modulation frequency of 100 kHz, center field of 3500 G, and sweep width of 100 G. For in situ radical detection, 10 mg of the COF composite was dispersed in 20 mL of an aqueous solution supplemented with 200 μL of 0.1 M DMPO as the spin trap, and then exposed to simultaneous ultrasonic treatment (40 kHz, 60 W) and visible-light irradiation (300 W Xe lamp, λ≥420 nm) in a 40 mL quartz reactor. The resulting reaction mixture was continuously circulated through the EPR sample cell by a peristaltic pump at a flow rate of 6 mL/min, facilitating real-time detection at 30-s intervals.

High piezoelectric coefficient (d33) test
The powder sample was subjected to a pressure of 4 tons for tableting to prepare a flake-shaped specimen, which was then placed in an oil bath for polarization treatment. The polarization conditions were as follows: the polarization voltage was 1400 V, the leakage current was controlled at 0.05 mA, the polarization temperature was room temperature, and the polarization duration was set to 0.5 h. The test sample was further tested between the upper and lower probes. The electromagnetic drive part in the test head generated a low-frequency alternating force of approximately 0.25 N at a frequency of 110 Hz. The two piezoelectric signals generated by the positive piezoelectric effect were then processed by the instrument, which displayed the d33 value.

Calculation method
All DFT calculations in this work were calculated with the Vienna ab initio simulation package (VASP, version 6.3.0), implemented with plane waves and the projector augmented wave method. The Perdew-Burke-Ernzerhof (PBE) functionals were coupled with van der Waals (vdW) dispersion corrections of the zero-damping DFT-D3 method of Grimme51,52. All structures of these COFs (see Supplementary Data 1–4) were optimized until all Hellmann-Feynman forces on atoms were less than 0.005 eV/Å with cell optimization and an energy cutoff of 550 eV and a Γ-centered k-mesh with a k-point distance of <2π/36 Å−1. The piezoelectric tensors, effective Born charges of these COFs were determined from the response to finite electric fields (E=0.001 eV/Å). Geometry optimizations and electronic structure characterizations of the molecules were performed at the B3LYP/6-311G level using the Gaussian 1653. Our DFT calculations employed idealized COF models, which cannot fully capture the inherent structural complexity and dynamic nature of the real systems.

Synthesis experiments
Synthesis of 4,4′-(5-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dianiline (BT-1F). The reaction was conducted in a mixture of toluene (45 mL), ethanol (12 mL), and water (9 mL) containing 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole (0.80 g, 2.55 mmol), 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1.34 g, 6.12 mmol), and K2CO3 (2.90 g, 27.30 mmol). Under an argon atmosphere, established by 30 min of purging, Pd(PPh3)4 (0.29 g, 0.25 mmol) was added to initiate the Suzuki coupling. The reaction proceeded at 100°C over 24 h. Standard workup included cooling, dilution with dichloromethane (20 mL), and removal of the catalyst by filtration through a Celite pad. The crude product obtained after solvent evaporation was subjected to column chromatography, yielding the desired compound as an orange solid (617.0 mg) with a yield of 72%. 1H NMR (400 MHz, DMSO-d6) δ (ppm) 7.85 (d, J=8.6 Hz, 2H), 7.73 (d, J=12.5 Hz, 1H), 7.48 (d, J=8.5, 1.6 Hz, 2H), 6.81–6.62 (m, 4H), 5.52 (s, 2H), 5.41 (s, 2H).

Synthesis of 4,4′-(5,6-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dianiline (BT-2F)28. The reaction was conducted in a mixture of toluene (45 mL), ethanol (12 mL), and water (9 mL) containing 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole (0.84 g, 2.55 mmol), 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1.34 g, 6.12 mmol), and K2CO3 (2.90 g, 27.30 mmol). Under an argon atmosphere, established by 30 min of purging, Pd(PPh3)4 (0.29 g, 0.25 mmol) was added to initiate the Suzuki coupling. The reaction proceeded at 100°C over 24 h. Standard workup included cooling, dilution with dichloromethane (20 mL), and removal of the catalyst by filtration through a Celite pad. The crude product obtained after solvent evaporation was subjected to column chromatography, yielding the desired compound as an orange solid (722.0 mg) with a yield of 80%. 1H NMR (400 MHz, CDCl3) δ (ppm) = 7.62 (d, J=8.8 Hz, 4H), 6.83 (d, J=8.8 Hz, 4H), 3.94 (s, 4H).

Synthesis of TP-BT-0F-COF54. 2,4,6-Trihydroxybenzene-1,3,5-tricarbaldehyde (TP) (10.5 mg, 0.05 mmol), 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BT-0F) (23.9 mg, 0.08 mmol), o-1,2-dichlorobenzene (0.75 mL), n-butanol (0.25 mL), and Py (0.10 mL) were combined in a Pyrex tube. After 10 min of sonication for homogenization, the mixture was flash-frozen in liquid nitrogen (77 K) and subjected to three freeze-pump-thaw cycles. The tube was then evacuated to 100 mTorr, sealed, and heated at 120°C for 3 days. The brown precipitate that formed was collected via centrifugation and washed sequentially with tetrahydrofuran and anhydrous acetone. The resulting powder was activated by stirring in dimethyl sulfoxide for 6 h. Subsequent washing with tetrahydrofuran and drying at 60°C afforded a crimson powder (26 mg, 75% yield).

Synthesis of TP-BT-1F-COF. 2,4,6-Trihydroxybenzene-1,3,5-tricarbaldehyde (TP) (10.5 mg, 0.05 mmol), 4,4′-(5-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dianiline (BT-1F) (25.3 mg, 0.08 mmol), o-1,2-dichlorobenzene (0.75 mL), n-butanol (0.25 mL), and Py (0.10 mL) were combined in a Pyrex tube. After 10 min of sonication for homogenization, the mixture was flash-frozen in liquid nitrogen (77 K) and subjected to three freeze-pump-thaw cycles. The tube was then evacuated to 100 mTorr, sealed, and heated at 120°C for 3 days. The brown precipitate that formed was collected via centrifugation and washed sequentially with tetrahydrofuran and anhydrous acetone. The resulting powder was activated by stirring in dimethyl sulfoxide for 6 h. Subsequent washing with tetrahydrofuran and drying at 60°C afforded an orange powder (28 mg, 80% yield).

Synthesis of TP-BT-2F-COF. 2,4,6-Trihydroxybenzene-1,3,5-tricarbaldehyde (TP) (10.5 mg, 0.05 mmol), 4,4′-(5,6-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dianiline (BT-2F) (26.5 mg, 0.08 mmol), o-1,2-dichlorobenzene (0.75 mL), n-butanol (0.25 mL), and Py (0.10 mL) were combined in a Pyrex tube. After 10 min of sonication for homogenization, the mixture was flash-frozen in liquid nitrogen (77 K) and subjected to three freeze-pump-thaw cycles. The tube was then evacuated to 100 mTorr, sealed, and heated at 120°C for 3 days. The brown precipitate that formed was collected via centrifugation and washed sequentially with tetrahydrofuran and anhydrous acetone. The resulting powder was activated by stirring in dimethyl sulfoxide for 6 h. Subsequent washing with tetrahydrofuran and drying at 60°C afforded an orange powder (30 mg, 80% yield).

Synthesis of SB-COF. 2,4,6-Trihydroxybenzene-1,3,5-tricarbaldehyde (TP) (16.8 mg, 0.08 mmol), 4,4″-diamino-substituted p-terphenyl (SB) (32.1 mg, 0.12 mmol), dioxane (0.67 mL), mesitylene (0.33 mL), and Py (0.10 mL) were mixed in a Pyrex tube. After 10 min of sonication for homogenization, the mixture was flash-frozen in liquid nitrogen (77 K) and subjected to three freeze-pump-thaw cycles. The tube was then evacuated to 100 mTorr, sealed, and heated at 120°C for 3 days. The brown precipitate that formed was collected via centrifugation and washed sequentially with tetrahydrofuran and anhydrous acetone. The resulting powder was activated by stirring in dimethyl sulfoxide for 6 h. Subsequent washing with tetrahydrofuran and drying at 60°C afforded a deep yellow powder (40 mg, 80% yield).

Synthesis of the BTA-BT-0F-COF29. A mixture containing benzene-1,3,5-tricarbaldehyde (BTA, 8.1 mg, 0.05 mmol) and 4,4′-(benzo-2,1,3-thiadiazole-4,7-diyl)dianiline (BT-0F) (24 mg, 0.08 mmol) in o-dichlorobenzene/mesitylene/aqueous acetic acid (6 M) (0.50:0.50:0.30 mL) was sonicated for 10 min to achieve a homogeneous solution. The mixture was flash-frozen in liquid nitrogen (77 K) and subjected to three freeze-pump-thaw cycles. The tube was then evacuated to 100 mTorr, sealed, and heated at 120°C for 3 days. The brown precipitate that formed was collected via centrifugation and washed sequentially with tetrahydrofuran and anhydrous acetone. The resulting powder was activated by stirring in dimethyl sulfoxide for 6 h. Subsequent washing with tetrahydrofuran and drying at 60°C afforded a yellow powder (22 mg, 70%).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present a promising approach to boosting piezo-photocatalytic water splitting through strategic dipole moment engineering in β-ketoenamine/imine-linked covalent organic frameworks (COFs). By precisely controlling molecular architecture, they developed TP-BT-0F-COF, demonstrating remarkable piezoelectric coefficients and record-breaking H2/H2O2 generation rates under simultaneous ultrasound and light irradiation. This study systematically establishes the relationship between dipole moment manipulation, piezoelectric polarization, and catalytic performance, revealing the crucial influence of symmetry-disrupted benzothiadiazole moieties and in-plane polarization of ketoenamine linkages. DFT calculations further pinpoint N⁸ and C⁶ as active catalytic sites, providing robust mechanistic understanding. Representing a major leap in mechano-optical energy conversion, this work offers a molecular-level design strategy for multifunctional COF catalysts. The comprehensive experimental evidence combined with theoretical analysis delivers deep insights into dipole-mediated piezoelectric enhancement. The work is recommended for publication in Nature Communications after addressing minor revisions:\n1. The observed discrepancy between the solid-state UV-Vis absorption spectrum (Figure 1C) and the calculated bandgap (Figure 1D) necessitates either re-evaluation of the experimental measurements or revision of the computational methodology.\n2. Regarding the Figures shown in the article, some logical or formatting issues are pointed out as follows: 1) Figures 1C and 1D show that the bandgap of TP-SB-COF is the largest among the involved COF materials. However, a rough analysis of the UV absorption spectra indicates a contradiction between this conclusion and the information presented in the spectra; 2) The pressure values in Figure 2 have inconsistent decimal places and need to be standardized\n3. In Figure 4D and Figure S55, the Nyquist plots of the EIS should be fitted with appropriate equivalent circuits in order to allow for a quantitative analysis of the interface resistance.\n4. More citations should be added to provide readers with a more comprehensive background on the application of crystalline porous materials in piezoelectric photocatalysis. Some references: 10.1002/adma.202419023, 10.1002/anie.202419867, 10.1002/adma.202106308, 10.1038/s41467-024-53834-w, 10.1002/anie.202420218.\n5. This study systematically investigates the piezo-photocatalytic performance of dipole-engineered COFs. However, the dipole moment calculations appear limited to the TP-based COF series (TP-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, TP-SB-COF), while omitting the BTA-BT-0F-COF counterpart. Could the authors clarify: 1) The scientific rationale for selectively calculating dipole moments only for TP-based frameworks? 2) Whether the dipole moment analysis was conducted for BTA-BT-0F-COF, and if not, whether its exclusion might influence the established structure-property relationships?\n6. Elaborate on the light-only experimental configuration, specifying whether agitation (e.g., magnetic stirring) was employed during the illumination period. Include a control experiment featuring stirring in the absence of light to decouple mechanical effects from photochemical contributions. Further distinguish the roles of ultrasound and mechanical stirring by clarifying their distinct influences on reaction kinetics-such as cavitation effects for ultrasound versus bulk mixing for mechanical stirring-to precisely define the action mechanisms of different energy inputs.", "reviewer_2": "This work presents an interesting study on piezo-photocatalytic water splitting using a series of β-ketoenamine/imine-linked covalent organic frameworks (COFs). The TP-BT-0F-COF catalyst with symmetry-breaking benzothiadiazole units and in-plane polarization of ketoenamine linkages achieves remarkable production rates of H2 and H2O2 at 1501.4/1435.8 μmol g-1 h-1 under simultaneous ultrasound and visible light irradiation. The mechanism and principle are investigated via advanced characterizations and theoretical calculations. Some of the observed phenomenon and results are intriguing but require further research and analysis. So, with appropriate revisions, I recommend its publication in this journal. Here are some of the problems in the manuscript:\n1. Some parts of the manuscript are lengthy to read clearly. The authors need to improve the language expression.\n2. In line 111, the text read “the Pawley refinement results”, which is inconsistent with the expression (Le Bail refined) in figure caption of Figure 1A and S3-6. What is the true refinement method?\n3. The ATR-FTIR spectrum of BTA-BT-0f-COF is abnormal (Figure 2B and S11). The vibration peak at near 1700 cm-1 is ascribed to stretching vibration of C=O of BTA, according to the chemical structures of the monomers. After polymerization, this peak should be substantially attenuated like other COFs in Figure S7-S10, because of the transformation from C=O to C=N. It indicates that the polymerization of BTA-BT-0f-COF is insufficient, and the other characterizations and activity of BTA-BT-0f-COF are lack of reliability.\n4. The in-situ high-pressure FT-IR spectrum of TP-SB-COF (lack of piezoelectric properties) should be provided to figure out whether the change of bond’s vibration can be ascribed to the piezoelectric polarization.\n5. Line 251-252: Why was the distinctive keto-enamine unit structure the main source of the piezoelectric properties of the COFs? In the part of PFM, the piezoelectric properties are more clealrly related to the changes in unit BT.\n6. The in-situ pressure-dependent UV-visible absorption spectrum of TP-SB-COF (lack of piezoelectric properties) is also highly wanted to figure out whether the change of light absorption and band structure can be correlated with the dipole moment and ketoenamine moieties.\n7. Under the conditions of pure water and visible light irradiation, all COFs could hardly produce H2. While under ultrasonic irradiation, hydrogen can be produced. What causes this difference? The phenomenon is intriguing and an in-depth discussion should be provided.\n8. Under both visible light irradiation and ultrasonic irradiation, TP-SB-COF showed smaller H2O2 production compared to the activity under only ultrasonic irradiation and TP-BT-0F-COF showed same amount of H2O2 production compared to the activity under only visible light irradiation. Where is the photocatalytic effect of TP-SB-COF and where is the piezocatalytic effect of TP-BT-0F-COF?\n9. The addition of AgNO3 and EDTA-Na, which are separately electron and hole scavengers, should robustly facilitate the reaction at the other side. But with the addition of AgNO3 or EDTA-Na, the production of H2O2 or H2 is close to the pristine production (Figure S57). What causes this phenomenon?\n10. The bandgap required for the formation of H2 and H2O2 is calculated as 2.73 eV, because the formation of H2O2 through ·OH path revealed by authors (2H+ + 2e- → H2 (0 V vs. RHE); H2O + h+ → ·OH + H+ (2.73 V vs. RHE)). How do the COFs fulfill this requirement simultaneously under visible light irradiation and ultrasonic irradiation?\nMinor problems:\nScheme 1: The name of imine-linked COF is incorrect.\nLine 298, 301 and 391: Hydroxyl is inexistent in TP-BT-0F-COF.", "reviewer_3": "Liu and co-workers have demonstrated photo-piezocatalytic water splitting using covalent organic frameworks (COFs), in their work entitled- “Enhanced piezo-photocatalytic water splitting activity via engineering robust dipole moments in covalent organic frameworks”. The study is interesting, especially since photo-piezocatalysis in COFs has been rarely documented. However, there are major experimental concerns that the authors must address.\nPlease find the comments given here.\n1. The biggest drawback of the work is perhaps the fact that no experimental evidence has been provided in the manuscript to confirm the non-centrosymmetric structure of the COFs utilised in the study. The authors state that the COFs crystallize in P-1 space group which is a centrosymmetric space group. The authors should re-evaluate the space groups of the materials used from their Pawley refinement patterns and provide additional proof for non-centrosymmetry with experiments like second harmonic generation. Please refer to and cite these recent related papers for these analyses: Chem. Mater. 2021, 33, 818, Adv. Funct. Mater. 2025, 2502787, ACS Catal. 2024, 14, 7853. Additionally, the framework structures seem planar and symmetric. The authors must explain why and how symmetry breaking occurs in the framework, giving rise to piezoelectric polarization.\n2. The PFM studies need to be rechecked. For an artifact free dataset, the switching potentials observed for the phase loop and amplitude loop should match. In this case, in the positive region, the potentials do not match, which raises significant concern regarding the contribution of charging or electrochemical reactions to the PFM data. Please check relevant references: Nat. Commun., 2019, 10, 1661, Adv. Mater. 2022, 34, 2206237. Also, the authors don’t mention whether the provided data is the ‘off-state’ or ‘on-state’ data. Also, the sentence, “From the amplitude loop, TP-BT-0F-COF had an amplitude of 3.0 nm” doesn’t quite make much sense. The authors should evaluate the piezoelectric coefficient of the materials from the amplitude data following previous reports.\n3. The authors should explore the photo-piezocatalytic activity in more detail. Analyses must be done by varying the ultrasonication power and frequency to check the catalytic activity. Furthermore, the apparent quantum efficiency of the process must be evaluated under a single ultrasonication frequency (preferably the best one) by varying the light wavelength. The synergy % of the photo- and piezocatalytic processes must be evaluated and compared.\n4. Isotope labelling experiments for both the redox halves need to be carried out to confirm the origin of the products. The authors should also carry out charge trapping experiments for both the redox halves. Please refer to: Nano Lett. 2022, 22, 8867, Adv. Funct. Mater. 2025, 2502787.\n5. KPFM gives an idea of the surface potential of a material which is also often correlated with its surface charge and work function. The authors claim that TP-BT-0F-COF exhibited a surface potential of 68 mV in the dark, which decreased to -30mV upon exposure to light due to the effect of the inward piezoelectric potential field. This is rather surprising. A strong justification with suitable references must be provided. A decrease can be anticipated but such a huge difference with charge in potential sign seems difficult. Also, the topography image of KPFM should reflect the topography/height of the material. The image presented in Fig. 4E shows a rather flat, blank surface with no material present.\n6. In Fig. 4B-D, how do the authors make sure that the material coating doesn’t come off the electrode surface upon rigorous ultrasonication?\n7. Finally, the authors should comment on the ferroic properties of the materials used. Are they ferroelectric, piezoelectric, or ferrielectric? What is the mechanism behind their piezo-tunable dipole generation? These are crucial aspects to be looked at for photo-piezocatalyst design, especially in emerging materials like COFs.\nBased on all these concerns, I am unable to recommend this manuscript for publication in Nature Communications.", "reviewer_4": "The authors Liang et al. synthesized four COFs based on the monomer 1,3,5-triformylphloroglucinol (TP) and applied them for piezoelectric photocatalytic overall water splitting to produce H2 and H2O2. I do not recommend this paper for publication in Nature Communications, and I will elaborate on my reasons through the following points:\n1. Innovation: In the introduction, the authors state that \"classical inorganic piezoelectric materials are limited by poor structural tunability and complex polarization processes, which severely hamper their mechanically driven catalytic efficiency,\" while highlighting the advantages of COFs. They also note recent reports on piezoelectric COFs. However, their core innovation claim—\"current research lacks systematic tuning of COF ligand structures to modulate dipole moments and establish polarization centers, which are essential for achieving precise piezoelectric control\"—is invalid.\nMultiple studies have already addressed these aspects:\n• Li et al. (Angew. Chem. Int. Ed. 2025, 64, e202420218) demonstrated how ethenyl, F, Br, and OMe groups modulate dipole moments and piezoelectric of COFs.\n• Gu et al. (Angew. Chem. Int. Ed. 2024, 63, e202409708) constructed piezoelectric COFs using fluorinated alkyl chains and investigated the effects of benzene rings and triazine groups on dipole moments and piezoelectric properties.\n• Chen et al. (ACS Catal. 2024, 14, 7853−7866) synthesized two piezoelectric conjugated microporous polymers (CMPs) with Zn-Salen, discussing similar structural influences. Notably, their Zn-Salen-TEPT and Zn-Salen-TEPB systems achieved higher H2 and H2O2 production rates than the authors' COFs under ultrasound alone. (Note: When COFs lose crystallinity, they become conjugated porous polymers; thus, comparison with CMPs is valid.).\nAdditional examples of ligand structure tuning in organic frameworks include: Adv. Mater. 2021, 33, 2106308; Chem. Commun. 2025, 61, 2536–2539; Angew. Chem. Int. Ed. 2025, 64, e202425656; ChemCatChem 2025, 17, e202402105; Chem. Eng. J. 512 (2025) 162529, et. al. Thus, the authors' claim that \"current research lacks systematic tuning of COF ligand structures\" is unsupported.\nSide Notes:\n• Reference 21 is unrelated to piezoelectricity but is cited to support piezoelectric COFs.\n• References 23–24, while piezoelectric, do not substantiate the authors' assertion that \"These elements are essential for achieving precise piezoelectric control.\n• The authors’ assertion in the abstract claiming \"outperforming state-of-the-art COF-based piezo-photocatalysts\" lacks supporting evidence. For reference, the following studies demonstrate superior performance: ACS Catal. 2024, 14, 7853–7866; Angew. Chem. Int. Ed. 2025, 64, e202420218; Angew. Chem. Int. Ed. 2024, 64, e202419867. These works already report COF-based or related systems with higher catalytic efficiency, further undermining the novelty and competitiveness of the authors’ claims.\nRegarding the second innovation claim—\"establish polarization centers\"—the authors themselves previously attributed the piezoelectricity of EA-COF to the in-plane polarization of TP groups in ACS Catal. 14, 10447–10461 (2024). The current work fails to provide new insights beyond reiterating this phenomenon.\nWhile the authors conducted extensive experimental work, the paper lacks significant original innovation. I will now address technical concerns in subsequent sections.\n2. Technical:\n1. Numerous studies have demonstrated that introducing fluorine (F) to modulate dipole moments in organic materials often results in positive regulation.\nDipole moments quantify the degree of charge separation between positive and negative charges, reflecting asymmetry in charge distribution at molecular or atomic scales. Mathematically, the dipole moment (μ) is defined as:\nμ= q⋅d, where q is the charge magnitude and d is the displacement vector from the negative to the positive charge.\nThe authors’ DFT computational model adopts a symmetric structure centered on the BT molecule. However, since their ligand modifications are solely based on the BT moiety, the inherent symmetry of this design diminishes the impact of molecular adjustments. For instance, in TP-SB-COF, the dipole moments from opposing sides are equal in magnitude but opposite in direction, leading to negligible overall dipole moments in DFT calculations (Fig. 1). Using the molecular fragment within the red box in Figure 2 (one TP molecule connected to three BT molecules) for calculations can avoid the aforementioned issues.\nFig.1 Dipole polarization ability of TP-SB-COF\nFig.2 The recommended molecular fragment model for dipole polarization\n2. The water oxidation reaction to produce H2O2 should involve quenching ·OH radicals, not holes (h⁺).\n3. While terephthalic acid is widely used to detect ·OH radicals, its instability under light-ultrasound conditions compromises reliability. Additionally, the authors claim: \"This may be because the COFs reduced the residual oxygen in the water to H2O2.\" This introduces ambiguity in experimental reproducibility and challenges the validity of ·OH detection via terephthalic acid. Proper control experiments, such as isotopic labeling with H218O, are critical to confirm that H2O2 originates from water oxidation rather than oxygen reduction.\n4. Lacking cyclic tests and characterization of the materials after reaction.\n5. Powdered materials cannot be directly tested for d33 coefficients using standard d33 meters. Supplementary Fig. S37 merely displays a photograph without detailing sample preparation or measurement protocols.\n6. The authors employed diamond anvil cells for characterization, but it remains unclear whether such extreme pressure conditions are relevant to practical piezoelectric catalysis. While similar methods are used in piezofluorochromism studies (e.g., Angew. Chem. Int. Ed. 2024, e202409099), the connection between these fields and piezoelectric catalysis remains unexplained.\n3. Conclusion\nThe paper suffers from insufficient innovation and logical rigor. Based on the above analysis, I do not recommend this paper for publication in Nature Communications."}, "author_response": "Dear Editor,\nThanks very much for your comments. We have revised our manuscript according to the reviewers’ comments. The detailed responses to the comments are listed as follows.\nReviewers’ comments:\nReviewer 1:\nThe authors present a promising approach to boosting piezo-photocatalytic water splitting through strategic dipole moment engineering in β-ketoenamine/imine-linked covalent organic frameworks (COFs). By precisely controlling molecular architecture, they developed TP-BT-0F-COF, demonstrating remarkable piezoelectric coefficients and record-breaking H /H O generation rates under simultaneous ultrasound and light irradiation. This study systematically establishes the relationship between dipole moment manipulation, piezoelectric polarization, and catalytic performance, revealing the crucial influence of symmetry-disrupted benzothiadiazole moieties and in-plane polarization of ketoenamine linkages. DFT calculations further pinpoint N⁸ and C⁶ as active catalytic sites, providing robust mechanistic understanding. Representing a major leap in mechano-optical energy conversion, this work offers a molecular-level design strategy for multifunctional COF catalysts. The comprehensive experimental evidence combined with theoretical analysis delivers deep insights into dipole-mediated piezoelectric enhancement. The work is recommended for publication in Nature Communications after addressing minor revisions:\n1. The observed discrepancy between the solid-state UV-Vis absorption spectrum (Figure 1C) and the calculated bandgap (Figure 1D) necessitates either re-evaluation of the experimental measurements or revision of the computational methodology.\nResponse: We sincerely thank the reviewer for highlighting this important discrepancy. We have re-prepared the TP-SB-COF sample and re-measured its solid-state UV-Vis absorption spectrum, obtaining a revised experimental bandgap of 2.39 eV.\nIn our revised manuscript:\nPrior to the photocatalytic studies, the light-harvesting properties and band gaps of these COFs were evaluated using UV-vis diffuse reflectance spectra. TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF and TP-SB-COF exhibited comparatively restricted optical absorption with absorption onset at 599, 590, 585, 585, and 519 nm (Fig. 1C) and a direct optical band gap of 2.07, 2.13, 2.17, 2.17, and 2.39 eV (Fig. 1D).\n2. Regarding the Figures shown in the article, some logical or formatting issues are pointed out as follows: 1) Figures 1C and 1D show that the bandgap of TP-SB-COF is the largest among the involved COF materials. However, a rough analysis of the UV absorption spectra indicates a contradiction between this conclusion and the information presented in the spectra; 2) The pressure values in Figure 2 have inconsistent decimal places and need to be standardized.\nResponse: We sincerely appreciate the reviewer meticulous observations. 1) As recommended by reviewers, we remeasured the solid-state UV-vis absorption spectrum of TP-SB-COF. Subsequent Tauc plot analysis yielded a revised optical bandgap of 2.39 eV. 2) Pressure values in Figure 2 have been standardized to one decimal place.\nIn our revised manuscript:\nPrior to the photocatalytic studies, the light-harvesting properties and band gaps of these COFs were evaluated using UV-vis diffuse reflectance spectra. TP-BT-0F-COF, BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF and TP-SB-COF exhibited comparatively restricted optical absorption with absorption onset at 599, 590, 585, 585, and 519 nm (Fig. 1C) and a direct optical band gap of 2.07, 2.13, 2.17, 2.17, and 2.39 eV (Fig. 1D).\nThe results showed that the VB potentials for BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF and TP-SB-COF were measured to be 0.96, 1.17, 1.14 and 0.82 V vs. NHE, respectively. Similarly, the CB potentials were calculated to be -1.17, -1.00, -1.03 and -1.57 V vs. NHE for BTA-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF and TP-SB-COF, respectively. The results indicated that the CB and VB positions were suitable for facilitating the H evolution and water oxidation reactions (Fig. 1D).\n3. In Figure 4D and Figure S55, the Nyquist plots of the EIS should be fitted with appropriate equivalent circuits in order to allow for a quantitative analysis of the interface resistance.\nResponse: Thanks for the reviewer’s comment. We have added the appropriate equivalent circuits to Figure 4D and Figure S55.\nIn our revised manuscript:\nTo further investigate the charge transfer resistance, electrochemical impedance spectroscopy (EIS) measurements of the TP-BT-0F-COF electrodes were performed under different conditions, and the corresponding Nyquist plots are shown in Fig. S55. As depicted in the equivalent circuit diagram, R corresponds to the charge transfer resistance at the sample/electrolyte interface, whereas R represents the resistance of the electrolyte and other related system resistances.\n4. More citations should be added to provide readers with a more comprehensive background on the application of crystalline porous materials in piezoelectric photocatalysis. Some references: 10.1002/adma.202419023, 10.1002/anie.202419867, 10.1002/adma.202106308, 10.1038/s41467-024-53834-w, 10.1002/anie.202420218.\nResponse: We sincerely appreciate the reviewer insightful suggestion. To strengthen the background and highlight recent advances in crystalline porous materials for piezoelectric photocatalysis, we have included key references in the revised manuscript.\nIn our revised manuscript:\n(1) Consequently, the intrinsic structural tunability of framework materials establishes them as highly suitable candidates for emerging piezo-photocatalytic systems16-19.\n(2) For example, recent work engineered twistable bicarbazole motifs or fluorinated alkyl chains into COFs, enhancing piezoelectric performance25, 26.\n(3) From the amplitude-voltage butterfly loop, TP-BT-0F-COF had maximum effective d coefficients of approximately 0.92 nm/V (Fig. 2A), while the maximum effective d coefficients of BTA-BT-0F-COF, TP-BT-1F-COF and TP-BT-2F-COF were 0.56 nm/V (Fig. S31A), 0.43 nm/V (Fig. S31D) and 0.26 nm/V (Fig. S31G) respectively32, 33.\n(4) The observed decrease of 98 mV was probably due to the effect of the inward piezoelectric potential field48, 49.\n5. This study systematically investigates the piezo-photocatalytic performance of dipole-engineered COFs. However, the dipole moment calculations appear limited to the TP-based COF series (TP-BT-0F-COF, TP-BT-1F-COF, TP-BT-2F-COF, TP-SB-COF), while omitting the BTA-BT-0F-COF counterpart. Could the authors clarify: 1) The scientific rationale for selectively calculating dipole moments only for TP-based frameworks? 2) Whether the dipole moment analysis was conducted for BTA-BT-0F-COF, and if not, whether its exclusion might influence the established structure-property relationships?\nResponse: We sincerely thank the reviewer for their insightful observation. 1) Our initial focus on the TP-based COFs (TP-BT-0F/1F/2F-COF and TP-SB-COF) was driven by their systematically tunable dipole moments via directional fluorine substitution-a core design principle of our dipole engineering strategy. This series provides a well-controlled molecular platform to isolate the effects of incremental dipole modulation (0F→2F) and linker variation (SB vs. BT) within comparable frameworks. 2) We fully agree, however, that incorporating BTA-BT-0F-COF significantly strengthens the generalizability of our conclusions. In response to this valuable suggestion, we have computed dipole moments for all COFs in this study, including BTA-BT-0F-COF.\nIn our revised manuscript:\nAs shown schematically in Fig. S41, TP-BT-0F-COF exhibited dipole moments of 1.420 D, which were larger than those of BTA-BT-0F-COF (0.7476 D), TP-BT-1F-COF (0.5995 D), TP-BT-2F-COF (0.4416 D) and TP-SB-COF (0.013 D). By gradually increasing the content of F elements in the skeleton, the dipole moment showed a decreasing tendency. This phenomenon indicated that the introduction of F elements had a significant effect on the charge distribution and molecular polarity of the COF skeleton. During the continuous increase of F elements, the distribution of the electron cloud inside the molecule changed. Due to the relatively strong electronegativity of the F atoms, the electron cloud was more likely to be distributed around the F atoms, thus changing the overall polarity of the molecule and eventually leading to a gradual decrease in the dipole moment. The results indicated that the local dipole moment change was modulated by the introduction of BT derivative units, which enhanced the self-polarization and piezoelectric properties of the COFs. Furthermore, the dipole moment of BTA-BT-0F-COF was lower than that of TP-BT-0F-COF, confirming that β-ketoenamine subunits dominated over BTA motifs in enhancing molecular dipoles and polarization.\n6. Elaborate on the light-only experimental configuration, specifying whether agitation (e.g., magnetic stirring) was employed during the illumination period. Include a control experiment featuring stirring in the absence of light to decouple mechanical effects from photochemical contributions. Further distinguish the roles of ultrasound and mechanical stirring by clarifying their distinct influences on reaction kinetics-such as cavitation effects for ultrasound versus bulk mixing for mechanical stirring-to precisely define the action mechanisms of different energy inputs.\nResponse: We thank the reviewer for this constructive suggestion. In light-only experiments, no agitation (e.g., magnetic stirring) was employed during illumination to isolate photonic effects. Responding to the reviewer's request, we now include dark control experiments with magnetic stirring. These confirm mechanical agitation alone induces no observable reaction progress without light. Furthermore, we have clarified the distinct roles of ultrasound and mechanical stirring in the revised manuscript: 1) Ultrasound accelerates kinetics via acoustic cavitation, generating transient high-temperature/-pressure microenvironments; 2) Magnetic stirring facilitates bulk mixing, enhancing macroscopic mass transport. These mechanistic differences are explicitly addressed in the revised Discussion section to delineate energy input-specific action mechanisms.\nIn our revised manuscript:\nIn sharp contrast, no discernible H and H O production was detected under milder mechanical perturbations (e.g., 800 rpm stirring, Fig. S52), thereby emphasizing the critical role of ultrasound-driven piezoelectric activation.\nReviewer 2:\nThis work presents an interesting study on piezo-photocatalytic water splitting using a series of β-ketoenamine/imine-linked covalent organic frameworks (COFs). The TP-BT-0F-COF catalyst with symmetry-breaking benzothiadiazole units and in-plane polarization of ketoenamine linkages achieves remarkable production rates of H and H O at 1501.4/1435.8 μmol g-1 h-1 under simultaneous ultrasound and visible light irradiation. The mechanism and principle are investigated via advanced characterizations and theoretical calculations. Some of the observed phenomenon and results are intriguing but require further research and analysis. So, with appropriate revisions, I recommend its publication in this journal. Here are some of the problems in the manuscript:\n1. Some parts of the manuscript are lengthy to read clearly. The authors need to improve the language expression.\nResponse: We sincerely thank the reviewer for this valuable suggestion. In response, we have comprehensively revised the manuscript to enhance linguistic clarity and conciseness. Particular attention was given to streamlining verbose passages and eliminating redundancies while preserving scientific accuracy. We believe these edits significantly improve the work’s readability and presentation.\n2. In line 111, the text read “the Pawley refinement results”, which is inconsistent with the expression (Le Bail refined) in figure caption of Figure 1A and S3-6. What is the true refinement method?\nResponse: We thank the reviewer for their insightful observations. The term \"Le Bail refinement\" in Figure 1A and Figures S3-S6 captions was a terminological error; all refinements used exclusively Pawley methodology. This has been corrected throughout the manuscript (including Line 111 and relevant captions). We emphasize this is solely a nomenclature correction-refinement methodology and structural interpretations remain unchanged. We apologize for any confusion caused and appreciate this clarification opportunity.\n3. The ATR-FTIR spectrum of BTA-BT-0f-COF is abnormal (Figure 2B and S11). The vibration peak at near 1700 cm-1 is ascribed to stretching vibration of C=O of BTA, according to the chemical structures of the monomers. After polymerization, this peak should be substantially attenuated like other COFs in Figure S7-S10, because of the transformation from C=O to C=N. It indicates that the polymerization of BTA-BT-0f-COF is insufficient, and the other characterizations and activity of BTA-BT-0f-COF are lack of reliability.\nResponse: We thank the reviewer for noting the residual C=O vibration (~1700 cm⁻¹) in BTA-BT-0F-COF's ATR-FTIR spectrum. Following this observation, we re-synthesized the material and confirmed persistent aldehyde signals (Fig. 1B, S11, revised). This feature stems from terminal aldehyde groups at crystalline edges and surface-bound monomers—a well-documented phenomenon in imine-linked COFs attributable to finite crystal dimensions (e.g., (1) ACS Catal. 2023, 13, 7, 4316-4329. (2) ACS Catal. 2021, 11, 2098-2107. (3) Angew. Chem. Int. Ed. 2020, 59, 16902-16909.). Crucially, PXRD-confirmed crystallinity (matching simulated AA stacking) robustly validates successful polymerization. Thus, while present, the residual peak does not indicate compromised material reliability.\n4. The in-situ high-pressure FT-IR spectrum of TP-SB-COF (lack of piezoelectric properties) should be provided to figure out whether the change of bond’s vibration can be ascribed to the piezoelectric polarization.\nResponse: We thank the reviewer for their insightful comments. In situ high-pressure FT-IR spectroscopy of TP-SB-COF reveals only subtle β-ketoenamine peak shifts under pressure, significantly smaller than those in piezoelectric COFs (TP-BT-0F/1F/2F-COF, BTA-BT-0F-COF). This differential response confirms bond vibration changes originate primarily from piezoelectric polarization, not mechanical compression alone. Crucially, peak shift magnitude correlates directly with dipole moment modulation: TP-BT-0F-COF's BT units enable enhanced dipole alignment under stress, amplifying local electric fields and bond polarization. Conversely, TP-SB-COF's structural symmetry precludes such dipole-ordering capability.\n5. Line 251-252: Why was the distinctive keto-enamine unit structure the main source of the piezoelectric properties of the COFs? In the part of PFM, the piezoelectric properties are more clearly related to the changes in unit BT.\nResponse: We thank the reviewer for their insightful comments. The observed piezoelectricity originates from keto-enamine units – the primary polarization centers that generate intrinsic dipoles under mechanical stress. Crucially, their polarization efficiency is synergistically enhanced by BT units, which optimize dipole alignment as evidenced by localized PFM responses. This complementary mechanism operates through: 1) Keto-enamine units acting as polarization sources and 2) BT units facilitating dipole-field coupling, collectively amplifying macroscopic piezoelectric output. Thus, PFM signal dominance near BT regions reflects complementary dipole-field cooperation, not contradictory behavior.\n6. The in-situ pressure-dependent UV-visible absorption spectrum of TP-SB-COF (lack of piezoelectric properties) is also highly wanted to figure out whether the change of light absorption and band structure can be correlated with the dipole moment and ketoenamine moieties.\nResponse: We thank the reviewer for their insightful comments. In situ pressure-dependent UV-vis spectroscopy of non-piezoelectric TP-SB-COF (Fig. S49J-K) reveals a key distinction: while all COFs exhibit pressure-induced red shifts from orbital compression, bandgap narrowing magnitude quantitatively correlates with piezoelectric activity. TP-BT-0F-COF shows the most pronounced shift (-0.0534 eV GPa-1) versus TP-SB-COF's minimal reduction (-0.0358 eV GPa-1). This divergence confirms ketoenamine moieties function as polarization centers only when coupled with aligned dipoles (e.g., in BT units), where piezoelectric strain amplifies charge transfer integrals, enhancing band distortion and light absorption beyond mechanical effects.\n7. Under the conditions of pure water and visible light irradiation, all COFs could hardly produce H . While under ultrasonic irradiation, hydrogen can be produced. What causes this difference? The phenomenon is intriguing and an in-depth discussion should be provided.\nResponse: We thank the reviewer for their insightful comments. The critical distinction lies in ultrasound-driven piezoelectric polarization, which overcomes rapid carrier recombination in visible-light catalysis. Ultrasonic stress generates strong internal electric fields within COFs, effectively separating electron-hole pairs and enabling long-lived charge separation. In contrast, visible light alone relies on diffusion-limited carrier migration, suffering near-total recombination losses in pure water without sacrificial agents or built-in fields. We have expanded this mechanistic discussion in the revised manuscript.\n8. Under both visible light irradiation and ultrasonic irradiation, TP-SB-COF showed smaller H O production compared to the activity under only ultrasonic irradiation and TP-BT-0F-COF showed same amount of H O production compared to the activity under only visible light irradiation. Where is the photocatalytic effect of TP-SB-COF and where is the piezocatalytic effect of TP-BT-0F-COF?\nResponse: We appreciate the reviewer’s insightful query regarding the mechanistic distinctions. The key points are clarified below:\n(1) For TP-SB-COF: 1) Under sole visible light, it generates minimal H O (140.7 μmol g-1 h-1) exclusively through oxygen reduction (O + 2e- + 2H+ → H O ); 2) Under dual irradiation, its apparent H /H O production (141.1/131.2 μmol g-1 h-1) stems entirely from sonolytic water splitting rather than piezocatalysis of TP-SB-COF, as confirmed by near-identical yields to ultrasound-only controls, confirming negligible piezocatalytic contribution.\n(2) For TP-BT-0F-COF: The 5.9-fold higher H O yield under dual irradiation (1435.8 μmol g-1 h-1) than under sole visible light (242.2 μmol g-1 h-1) unambiguously demonstrates piezoelectric enhancement. Visible light alone drives weak O reduction, whereas ultrasound activates hole-dominated in-direct water oxidation, synergistically amplified by light-induced carrier mobility.\n9. The addition of AgNO and EDTA-2Na, which are separately electron and hole scavengers, should robustly facilitate the reaction at the other side. But with the addition of AgNO or EDTA-2Na, the production of H O or H is close to the pristine production (Figure S57). What causes this phenomenon?\nResponse: We sincerely appreciate the reviewer’s insightful observations, which prompted rigorous experimental validation. Repeated tests confirmed that the addition of electron/hole scavengers (AgNO or EDTA-2Na) maintains H₂O₂/H₂ production near pristine levels—a reproducible and intrinsic feature of our system.\nThis behavior originates from the following mechanisms, supported by consistent experimental results: Under ultrasonic irradiation, the designed COFs generate exceptionally strong piezoelectric polarization. This field not only promotes charge separation but also induces dynamic band bending, enabling simultaneous utilization of electrons and holes at dedicated sites (electrons at reductive centers for H₂ evolution; holes at oxidative centers for •OH generation and subsequent H₂O₂ formation). Thus, the intrinsic separation efficiency is so high that external scavengers exhibit limited impact. For example, while Ag⁺ (from AgNO₃) thermodynamically outcompetes H⁺ reduction at N⁸ sites (Ag⁺/Ag⁰: +0.8 V vs. NHE; 2H⁺/H₂: −0.41 V vs. NHE), diverting electrons toward Ag⁰ deposition, it does not significantly disrupt the hole-mediated H₂O₂ formation pathway.\n10. The bandgap required for the formation of H and H O is calculated as 2.73 eV, because the formation of H O through •OH path revealed by authors (2H+ + 2e- → H , 0 V vs. RHE); H O + h+ → •OH + H+, 2.73 V vs. RHE). How do the COFs fulfill this requirement simultaneously under visible light irradiation and ultrasonic irradiation?\nResponse: We sincerely appreciate the reviewer's insightful inquiry regarding the thermodynamic bandgap requirement. As illustrated in Fig. 5G, while the conduction band position thermodynamically satisfies proton reduction for H evolution, the valence band potential under pure photocatalysis cannot drive water oxidation to •OH radicals (H O → •OH + H+, 2.32 V vs NHE, pH = 7). Critically, ultrasound-induced piezoelectric fields overcome this limitation through two synergistic mechanisms.\n(1) Dynamic Band Engineering: Non-centrosymmetric dipoles in BT-COFs generate localized piezoelectric potentials under strain. This dynamically tilts energy bands at β-ketoenamine sites, transiently elevating the effective valence band maximum to potentials >2.32 V vs NHE.\n(2) Field-Driven Charge Separation: Concurrently, the built-in electric field forcibly separates carriers, directing electrons to N8 sites for H+ reduction and holes to C6 sites for H O oxidation.\nThis dual function of mechanical energy compensating electronic deficits enables visible-light-driven overall water splitting exclusively under piezo-photocatalysis, as verified by •OH detection (Fig. S59) and isotopic labeling (Fig. S56, S60).\nMinor problems:\nScheme 1: The name of imine-linked COF is incorrect.\nResponse: Thanks for the reviewer’s suggestions. The name of the imine-linked COF has been revised.\nLine 298, 301 and 391: Hydroxyl is inexistent in TP-BT-0F-COF.\nResponse: Thanks for the reviewer’s suggestions. ‘Hydroxyl’ in TP-BT-0F-COF has been revised as ‘β-ketoenamine groups’.\nReviewer 3:\nLiu and co-workers have demonstrated photo-piezocatalytic water splitting using covalent organic frameworks (COFs), in their work entitled- “Enhanced piezo-photocatalytic water splitting activity via engineering robust dipole moments in covalent organic frameworks”. The study is interesting, especially since photo-piezocatalysis in COFs has been rarely documented. However, there are major experimental concerns that the authors must address.\nPlease find the comments given here.\n1. The biggest drawback of the work is perhaps the fact that no experimental evidence has been provided in the manuscript to confirm the non-centrosymmetric structure of the COFs utilised in the study. The authors state that the COFs crystallize in P-1 space group which is a centrosymmetric space group. The authors should re-evaluate the space groups of the materials used from their Pawley refinement patterns and provide additional proof for non-centrosymmetry with experiments like second harmonic generation. Please refer to and cite these recent related papers for these analyses: Chem. Mater. 2021, 33, 818, Adv. Funct. Mater. 2025, 2502787, ACS Catal. 2024, 14, 7853. Additionally, the framework structures seem planar and symmetric. The authors must explain why and how symmetry breaking occurs in the framework, giving rise to piezoelectric polarization.\nResponse: We sincerely appreciate the reviewer's rigorous critique regarding the non-centrosymmetric validation. The following revisions and analyses address these concerns comprehensively:\n(1) Space Group Correction: The cited P-1 space group in the original manuscript was an inadvertent error. Re-refinement of XRD data unequivocally confirms crystallization in the non-centrosymmetric P1 space group, consistent with piezoelectricity prerequisites.\n(2) Experimental Verification of Non-centrosymmetry: Second Harmonic Generation (SHG): TP-BT-0F-COF exhibits a distinct SHG signal at 700 nm under 1400 nm excitation (Fig. S38A). Violation of Mutual Exclusion Rule: Simultaneous activation of IR and Raman-forbidden modes provides definitive evidence for symmetry breaking (Fig. S38B).\n(3) The recent related papers for these analyses (Adv. Funct. Mater. 2025, 2502787, ACS Catal. 2024, 14, 7853.) have been cited.\n(4) Piezo-Photocatalytic Viability: The synergistic interaction between BT unit-mediated symmetry breaking and β-ketoenamine-driven polarization enables efficient piezo-photocatalytic water splitting into H and H O , as demonstrated by piezoelectric coefficients (d = 5.5 pC/N) and production rates (1501.4 μmol g-1 h-1 H and 1435.8 μmol g-1 h-1 H O ).\n2. The PFM studies need to be rechecked. For an artifact free dataset, the switching potentials observed for the phase loop and amplitude loop should match. In this case, in the positive region, the potentials do not match, which raises significant concern regarding the contribution of charging or electrochemical reactions to the PFM data. Please check relevant references: Nat. Commun., 2019, 10, 1661, Adv. Mater. 2022, 34, 2206237. Also, the authors don’t mention whether the provided data is the ‘off-state’ or ‘on-state’ data. Also, the sentence, “From the amplitude loop, TP-BT-0F-COF had an amplitude of 3.0 nm” doesn’t quite make much sense. The authors should evaluate the piezoelectric coefficient of the materials from the amplitude data following previous reports.\nResponse: We have carefully addressed the concerns regarding our PFM data. In response to the observation that the switching potentials in the positive region did not match between the phase and amplitude loops in the original submission, we have re-measured the COF samples. The updated PFM hysteresis loops now demonstrate matching switching potentials for both phase and amplitude, confirming the robustness of our PFM data and minimizing concerns about contributions from charging or electrochemical artifacts.\n(1) All PFM data presented in the revised manuscript, including the amplitude values, correspond to the ‘on-state’ response.\n(2) We sincerely appreciate the reviewer's valuable comment pointing out the ambiguity in our original description. Following the reviewer's suggestion and referring to established methods in previous reports, we have re-evaluated the piezoelectric properties of the materials using the amplitude-voltage butterfly loop data and calculated the piezoelectric coefficients. The relevant sentence has been revised to: ‘From the amplitude-voltage butterfly loop, TP-BT-0F-COF had maximum effective d coefficients of approximately 0.92 nm/V (Fig. 2A), while the maximum effective d coefficients of BTA-BT-0F-COF, TP-BT-1F-COF and TP-BT-2F-COF were 0.56 nm/V (Fig. S31A), 0.43 nm/V (Fig. S31D) and 0.26 nm/V (Fig. S31G) respectively32, 33.’ This revision aims to more accurately reflect the piezoelectric performance of the materials, and we thank the reviewer for helping us improve the rigor and clarity of our presentation.\n3. The authors should explore the photo-piezocatalytic activity in more detail. Analyses must be done by varying the ultrasonication power and frequency to check the catalytic activity. Furthermore, the apparent quantum efficiency of the process must be evaluated under a single ultrasonication frequency (preferably the best one) by varying the light wavelength. The synergy % of the photo- and piezocatalytic processes must be evaluated and compared.\nResponse: We sincerely thank the reviewer for the valuable suggestions to deepen our mechanistic understanding of the piezo-photocatalytic process. We have now conducted systematic investigations as recommended:\n(1) Ultrasonication Power-Dependent Activity:\nThe piezo-photocatalytic evolution rates of H and H O for TP-BT-0F-COF were meticulously assessed under ultrasonic powers of 30, 60, and 120 W (with a fixed frequency of 40 kHz). A consistent and progressive enhancement in catalytic activity was observed as ultrasonic power increased. This phenomenon is attributed to the amplified polarization-induced carrier separation under heightened mechanical stress, which, in turn, intensifies the built-in electric field. Notably, control experiments conducted with mild stirring revealed negligible H and H O production, further confirming that catalytic activation is inherently dependent on ultrasound.\n(2) Frequency Optimization & Resonant Energy Transfer:\nThe catalytic performance was examined across various ultrasound frequencies (20, 40, and 60 kHz) at a fixed power of 60 W. The piezo-photocatalytic efficiency peaked at 60 kHz, indicating a resonant energy coupling between the ultrasonic wave and the intrinsic vibrational modes of TP-BT-0F-COF. This frequency corresponds to the optimal efficiency for mechanical energy transfer.\n(3) Wavelength-Resolved Apparent Quantum Yield (AQY):\nAt the optimized frequency (40 kHz) and power (60 W), the AQY was evaluated using bandpass filters (450-650 nm). The AQYs of P-BT-0F-COF were 15.54% (450 nm), 14.65% (500 nm), 14.31% (550 nm), 13.65% (600 nm) and 9.67% (650 nm), respectively. The observed gradual decline in AQY with increasing wavelength correlates with the optical absorption edge of TP-BT-0F-COF, substantiating the notion that light-driven carrier generation plays a pivotal role. These thorough analyses comprehensively demonstrate the intricate relationship between the mechanical, optical, and catalytic properties of TP-BT-0F-COF.\n(4) We appreciate the reviewer’s suggestion and have accordingly quantified and compared the synergistic contributions of the photo- and piezocatalytic components for each COF. As presented in the revised manuscript, the proportions are calculated as follows: 26% (photo) : 74% (piezo) for TP-BT-0F-COF, 25% (photo) : 75% (piezo) for BTA-BT-0F-COF, 32% (photo) : 68% (piezo) for TP-BT-1F-COF and 48% (photo) : 28% (piezo) for TP-BT-2F-COF, respectively. This comparative analysis highlights that the piezocatalytic contribution predominates in most cases, except for TP-BT-2F-COF, where the contributions of both processes are nearly equivalent. These results provide a clearer understanding of the interplay between the two mechanisms in enhancing the overall catalytic performance.\n4. Isotope labelling experiments for both the redox halves need to be carried out to confirm the origin of the products. The authors should also carry out charge trapping experiments for both the redox halves. Please refer to: Nano Lett. 2022, 22, 8867, Adv. Funct. Mater. 2025, 2502787.\nResponse: We deeply appreciate the reviewer’s perceptive suggestions, which have substantially enhanced the mechanistic understanding of our study. To address these insights, isotopic labeling experiments for both the redox halves and charge trapping studies have been systematically executed, as elaborated below:\n(1) Isotopic Labeling Confirms Product Origins:\n(a) Reduction Half-Reaction (H Origin):\nDeuterium labeling experiments utilizing D O (98%) as the solvent unequivocally demonstrated that H originates from the reduction of water. Mass spectrometry revealed a distinct molecular ion peak at m/z = 4.00 (Fig. S56), corresponding to D evolution, thereby providing irrefutable confirmation of the proton reduction pathway.\n(b) Oxidation Half-Reaction (H O and •OH Origins):\nIn order to investigate the water oxidation pathway, H 18O was employed as the reaction medium. The oxidation of benzoic acid produced p-hydroxybenzoic acid (p-HBA), with a characteristic mass peak at m/z = 139.0296 (Fig. S60). This signal corresponds to the 18O-labeled p-HBA adduct, substantiating that •OH radicals (and thus H O ) are derived from water oxidation. The reaction sequence is as follows: (H O + h+ → •OH + H+, 2.32 V vs NHE; 2•OH → H O , 0.74 V vs NHE)\n(2) Charge Trapping Experiments Elucidate Redox Pathways:\nTo distinguish the roles of electrons and holes in the redox processes, scavenger-assisted studies were carried out:\n(a) Electron Trapping (Reduction Suppression):\nThe addition of 10 mM AgNO (electron scavenger) led to a 90% suppression of H evolution, affirming that H production is governed by electron-driven proton reduction.\n(b) Hole Trapping and •OH Quenching (Oxidation Suppression):\nHole Scavenging: The introduction of 10 mM EDTA-2Na reduced H O yield by 87%, confirming that holes initiate the water oxidation step.\n•OH Quenching: The addition of 10 vol% tert-butanol (TBA) resulted in an 91% decrease in H O production, further corroborating the dominance of •OH coupling in H O formation. These findings are in harmony with in situ ESR data (Fig. S59), where •OH signals were exclusively observed under piezo-photocatalytic conditions (ultrasound and light), but were absent under light alone.\n(3) Summary of Mechanistic Insights\nThese integrated experiments provide definitive evidence that: H originates from water reduction via electron-mediated proton reduction. H O forms through an indirect two-step water oxidation process involving hole-generated •OH radicals. Piezoelectric fields are indispensable for activating the water oxidation reaction (WOR) pathway, as corroborated by isotopic labeling (in situ ESR and MS) and charge-trapping kinetics.\n5. KPFM gives an idea of the surface potential of a material which is also often correlated with its surface charge and work function. The authors claim that TP-BT-0F-COF exhibited a surface potential of 68 mV in the dark, which decreased to -30mV upon exposure to light due to the effect of the inward piezoelectric potential field. This is rather surprising. A strong justification with suitable references must be provided. A decrease can be anticipated but such a huge difference with charge in potential sign seems difficult. Also, the topography image of KPFM should reflect the topography/height of the material. The image presented in Fig. 4E shows a rather flat, blank surface with no material present.\nResponse: We appreciate the reviewer’s insightful critique regarding the KPFM results.\n(1). The observed surface potential shift from +68 mV (dark) to -30 mV (light) in TP-BT-0F-COF is indeed significant, and we provide the following clarifications backed by experimental and theoretical evidence: The values we measured are potential differences, which are independent of changes in the polarity (positive or negative) of the potential. Such changes in potential polarity are related to the work function of the probe. It is likely that the work function of TP-BT-0F-COF is relatively close to that of the probe used in our test, which is why differences in the polarity (positive or negative) of the potential are observed under dark and light conditions. Crucially, the 98 mV shift in contact potential difference magnitude aligns with reports on high-performance piezoelectric COFs: (a) Ma et al. observed a 281 mV shift in CH I-TpPa-1 (Nat Commun 15, 9576 (2024)), (b) Qiu et al. documented 71.57 mV in ZnO@COF hybrids (Nat Commun 16, 1078 (2025)). These reproducible potential shifts unambiguously reflect the strong piezoelectric polarization in TP-BT-0F-COF, directly correlating with its exceptional piezo-photocatalytic performance.\n(2). Topography Image Flatness: KPFM samples were cold-pressed into smooth pellets to minimize topography artifacts. The flat appearance in Fig. 4E validates this preparation, ensuring measured potentials reflect intrinsic electronic properties, not morphological variations.\n6. In Fig. 4B-D, how do the authors make sure that the material coating doesn’t come off the electrode surface upon rigorous ultrasonication?\nResponse: We sincerely appreciate the reviewer's valuable comments. The catalyst integrity during ultrasonication is ensured by a synergistic physical anchoring strategy:\n(1) Pre-adhesion of the ITO substrate with double-sided tape prevents lateral slippage;\n(2) Subsequent thermomechanical encapsulation via proton exchange membrane hot-pressing (100 °C, 350 psi, 3 min) forms a conformal composite armor that mechanically immobilizes catalyst particles against ultrasonic detachment. We have added this method to our revised manuscript.\n7. Finally, the authors should comment on the ferroic properties of the materials used. Are they ferroelectric, piezoelectric, or ferrielectric? What is the mechanism behind their piezo-tunable dipole generation? These are crucial aspects to be looked at for photo-piezocatalyst design, especially in emerging materials like COFs.\nResponse: We sincerely appreciate the reviewer's valuable comments. Through systematic validation using multiple characterization methods, our materials have been confirmed to exhibit piezoelectric properties, but they do not possess ferroelectric or ferrielectric characteristics.\n(1) Clarification of Piezoelectric Properties:\nThe piezoelectricity of the synthesized COFs has been comprehensively confirmed through multiple characterization methods. Specifically, piezoresponse force microscopy (PFM) measurements reveal switchable polarization behavior under a ±5 V bias, accompanied by a 180° phase reversal (Fig. 2A, S31), directly demonstrating their piezoelectric activity. Quasi-static measurements further verify high piezoelectric coefficients, with TP-BT-0F-COF exhibiting a d value of 5.5 pC/N (Fig. S37). Additionally, the presence of second harmonic generation (SHG) signals at 700 nm (Fig. S38A) and the violation of infrared (IR)/Raman mutual exclusion rules (Fig. S38B) collectively confirm the non-centrosymmetric structure of these COFs-a structural prerequisite for piezoelectricity. These results systematically validate the piezoelectric nature of the materials.\n(2) Ferroelectric or Ferrielectric (Excluded):\nThe materials were confirmed to lack ferroelectric and ferrielectric properties through systematic characterizations. Specifically, no hysteresis or remnant polarization was observed in the polarization-electric field (P-E) loops (As shown in the Figure R1 below), which directly rules out ferroelectric behavior. Additionally, polarization switching in PFM measurements is instantaneous and reversible, devoid of the characteristic hysteresis associated with ferroelectrics. For ferrielectricity, no evidence supporting this property was found either: there is no indication of antiparallel dipole domains or sub-lattice polarization asymmetry-features typical of ferrielectric materials. These results collectively demonstrate that the materials do not exhibit ferroelectric or ferrielectric behavior.\nReviewer 4:\nThe authors Liang et al. synthesized four COFs based on the monomer 1,3,5-triformylphloroglucinol (TP) and applied them for piezoelectric photocatalytic overall water splitting to produce H and H O . I do not recommend this paper for publication in Nature Communications, and I will elaborate on my reasons through the following points:\n1. Innovation: In the introduction, the authors state that ‘classical inorganic piezoelectric materials are limited by poor structural tunability and complex polarization processes, which severely hamper their mechanically driven catalytic efficiency,’ while highlighting the advantages of COFs. They also note recent reports on piezoelectric COFs. However, their core innovation claim- ‘current research lacks systematic tuning of COF ligand structures to modulate dipole moments and establish polarization centers, which are essential for achieving precise piezoelectric control’-is invalid.\nMultiple studies have already addressed these aspects:\n• Li et al. (Angew. Chem. Int. Ed. 2025, 64, e202420218) demonstrated how ethenyl, F, Br, and OMe groups modulate dipole moments and piezoelectric of COFs.\n• Gu et al. (Angew. Chem. Int. Ed. 2024, 63, e202409708) constructed piezoelectric COFs using fluorinated alkyl chains and investigated the effects of benzene rings and triazine groups on dipole moments and piezoelectric properties.\n• Chen et al. (ACS Catal. 2024, 14, 7853-7866) synthesized two piezoelectric conjugated microporous polymers (CMPs) with Zn-Salen, discussing similar structural influences. Notably, their Zn-Salen-TEPT and Zn-Salen-TEPB systems achieved higher H and H O production rates than the authors’ COFs under ultrasound alone. (Note: When COFs lose crystallinity, they become conjugated porous polymers; thus, comparison with CMPs is valid.).\nAdditional examples of ligand structure tuning in organic frameworks include: Adv. Mater. 2021, 33, 2106308; Chem. Commun. 2025, 61, 2536-2539; Angew. Chem. Int. Ed. 2025, 64, e202425656; ChemCatChem 2025, 17, e202402105; Chem. Eng. J. 512 (2025) 162529, et. al. Thus, the authors’ claim that ‘current research lacks systematic tuning of COF ligand structures’ is unsupported.\nSide Notes:\n• Reference 21 is unrelated to piezoelectricity but is cited to support piezoelectric COFs.\n• References 23-24, while piezoelectric, do not substantiate the authors' assertion that \"These elements are essential for achieving precise piezoelectric control.\n• The authors’ assertion in the abstract claiming \"outperforming state-of-the-art COF-based piezo-photocatalysts\" lacks supporting evidence. For reference, the following studies demonstrate superior performance: ACS Catal. 2024, 14, 7853-7866; Angew. Chem. Int. Ed. 2025, 64, e202420218; Angew. Chem. Int. Ed. 2024, 64, e202419867. These works already report COF-based or related systems with higher catalytic efficiency, further undermining the novelty and competitiveness of the authors’ claims.\nRegarding the second innovation claim-\"establish polarization centers\"-the authors themselves previously attributed the piezoelectricity of EA-COF to the in-plane polarization of TP groups in ACS Catal. 14, 10447-10461 (2024). The current work fails to provide new insights beyond reiterating this phenomenon.\nWhile the authors conducted extensive experimental work, the paper lacks significant original innovation. I will now address technical concerns in subsequent sections.\nResponse: We sincerely appreciate the reviewer's valuable comments, which provide important insights for refining our work. Regarding the concerns raised about the innovation of our research, we offer the following clarifications to highlight the distinctive aspects of our study.\n(1) Statement on Innovation\n(a) Context of Piezoelectric Framework Literature\nWe acknowledge the reviewer's attention to relevant piezoelectric material studies. Concerning the cited literature on piezoelectric COFs, we note that currently only three reports (Angew. Chem. Int. Ed. 2025, 64, e202420218, Angew. Chem. Int. Ed. 2024, 63, e202409708, ACS Catal. 14, 10447-10461 (2024)) exist:\nTwo reports focus on applications distinct from our work.\nThe third report is a prior publication from our own research group.\nAdditionally, one related study exists on piezoelectric conjugated microporous polymers (CMPs).\n(b) Distinction from Other Framework Materials (MOFs)\nThe cited framework materials by reviewer (Adv. Mater. 2021, 33, 2106308, Chem. Commun. 2025, 61, 2536-2539, Angew. Chem. Int. Ed. 2025, 64, e202425656, ChemCatChem 2025, 17, e202402105, Chem. Eng. J. 512 (2025) 162529, et. al.) primarily involve MOFs. While MOFs share the ‘framework’ designation with COFs, they differ fundamentally in design principles and structural characteristics-notably in their linkage chemistry, crystallinity, and organic-inorganic composition. Consequently, the structural regulation strategies and piezoelectric mechanisms observed in MOFs are not directly transferable to COF-based systems. Our work is explicitly centered on COF-specific structural engineering, representing a distinct research focus within piezoelectric framework materials.\n(2) Specific Analysis of Cited Articles\nWe have analyzed the three key piezoelectric COF articles identified by the reviewer – the only relevant studies to date.\n(Angew. Chem. Int. Ed. 2025, 64, e202420218): This work employs twisted bicarbazole groups to achieve non-centrosymmetry for piezo-photocatalysis. Crucially, its core building blocks, topology, and dipole modulation principles (focusing on group twistability) are fundamentally different from our COF structure and modification strategy (focused on dipole moment magnitude via element substitution). The universality discussion of bicarbazole groups does not involve deep comparison of dipole moments or overlap with our element modification approach.\n(Angew. Chem. Int. Ed. 2024, 63, e202409708): This study utilizes fluorinated alkyl chains on imine-linked COFs to induce non-centrosymmetry for piezoelectric nanogenerators (PENGs). Its structural design (side-chain functionalization) and application target (PENGs) are unrelated to the core focus of our work. It serves as complementary evidence for the potential of piezoelectric COFs rather than conflicting with our innovation.\n(ACS Catal. 2024, 14, 7853-7866 - CMP): This research investigates donor-acceptor (D-A) structured CMPs for piezo-catalytic water splitting. Being a polymer material (CMP) with a different structure and dipole modulation mechanism (D-A architecture), it does not pertain directly to crystalline COFs or our specific structural engineering approach. Its existence further supports the broader field but does not overlap with the novelty presented in our study.\nConclusion on Innovation:\nIn summary, the cited studies employ distinct methodologies (twisted groups, fluorinated side-chains, D-A polymers) to achieve piezoelectricity, differing fundamentally in their structural design, modification strategy, application focus, and/or material class (CMP) from our work. Our research introduces a novel and specific approach to piezoelectric COF design based on targeted element substitution within the core COF backbone to modulate dipole moment magnitude, addressing a unique aspect of COF structural engineering. Therefore, we believe the core innovation and contribution of our work remain valid and distinct.\n(3) Clarification on Innovation: Polarization Centers and Novelty\nWe appreciate the opportunity to further elaborate on our core innovation regarding polarization centers and material design:\n(a) Advancing beyond Prior Work\nWhile polarization centers were previously proposed via theoretical calculations, this study provides experimental validation through in situ high pressure techniques, offering deeper mechanistic insights.\n(b) First Introduction of BT Groups in Piezo-Photocatalysis\nOur work pioneers the strategic incorporation of benzothiadiazole (BT) functional groups into COFs for piezo-photocatalysis. This enables: Construction of non-centrosymmetric frameworks via BT’s inherent asymmetry, Precise dipole moment modulation through BT’s electron-withdrawing nature, enhancing polarization of TP units.\n(c) Novel Dipole-Gradient Optimization and Controlled Validation\nWe further demonstrate dipole-moment-enhanced polarization centers by systematically reducing dipole magnitude to achieve peak piezoelectric response. Crucially, we designed a control COF lacking polarization centers, confirming their necessity – an approach unreported in prior studies (including our own).\nConclusion: These combined strategies (BT integration, dipole engineering, experimental validation, and controlled comparison) represent a previously unexplored paradigm in piezoelectric COF design, solidifying the novelty of our contribution.\n(4) Performance Comparison and Scope Clarification\nWe address performance benchmarks and material scope as follows:\n(a) Direct Comparison under Matched Conditions\nWhile conjugated microporous polymers Zn-Salen-TEPT (ACS Catal. 2024, 14, 7853) reports higher piezocatalytic H and H O yield (3260.2 μmol g-1 h-1 and 1569.8 μmol g-1 h-1) at 200 W ultrasound, our material (TP-BT-0F-COF) achieves exceptional piezo-photocatalytic synergy (2773.2 μmol g-1 h-1 for H and2735.5 for H O ) under lower energy input (120 W, 40 kHz) and dual stimuli.\n(b) Specific Focus on COF-Based Systems\nOur original statement explicitly references COF-based materials for piezo-photocatalytic both H and H O production. To date, only our prior work and this study reports such systems for both H and H O generation. Cited Angewandte studies (Angew. Chem. Int. Ed. 2025, 64, e202420218, Angew. Chem. Int. Ed. 2024, 64, e202419867) focus on H O production – a distinct application with different optimization targets (selectivity/yield/stability).\n(c) Enhanced Rigor via Benchmarking\nWe have revised contentious phrasing as suggested and included Table S6 (Supplementary Materials) providing comprehensive benchmarking against state-of-the-art systems to ensure precision.\n(5) Correction of Citation Issues\nWe sincerely apologize for citation inaccuracies and have implemented the following corrections:\n(a) Reference 21: Removed from the subsequent section where it was misplaced. It was originally intended to support ‘element-regulated COF performance’ but was incorrectly positioned.\n(b) References 23-24: Repurposed to support general principles of piezoelectric regulation (not our specific claims).\n(c) Text revisions\nThe relevant section has been rewritten for enhanced rigor, with appropriate references reinserted to accurately reflect the literature context.\n2. Technical:\n1. Numerous studies have demonstrated that introducing fluorine (F) to modulate dipole moments in organic materials often results in positive regulation.\nDipole moments quantify the degree of charge separation between positive and negative charges, reflecting asymmetry in charge distribution at molecular or atomic scales. Mathematically, the dipole moment (μ) is defined as: μ= q d, where q is the charge magnitude and is the displacement vector from the negative to the positive charge.\nThe authors’ DFT computational model adopts a symmetric structure centered on the BT molecule. However, since their ligand modifications are solely based on the BT moiety, the inherent symmetry of this design diminishes the impact of molecular adjustments. For instance, in TP-SB-COF, the dipole moments from opposing sides are equal in magnitude but opposite in direction, leading to negligible overall dipole moments in DFT calculations (Fig. 1). Using the molecular fragment within the red box in Figure 2 (one TP molecule connected to three BT molecules) for calculations can avoid the aforementioned issues.\nResponse: We sincerely appreciate the reviewer’s insightful suggestion to adopt an asymmetric molecular fragment model (one TP linked to three BT units) for dipole moment calculations. This revised approach effectively eliminates dipole cancellation artifacts inherent in symmetric full-unitcell models. Crucially, the new results validate our original findings:\n(1) Fluorine Substitution Reduces Dipole Moments:\nAs shown in Fig. S40, the dipole moments follow: TP-BT-0F-COF: 1.420 D, TP-BT-1F-COF: 0.5995 D, TP-BT-2F-COF: 0.4416 D, TP-SB-COF: 0.013 D. The inverse correlation between F-content and μ confirms that F atoms competitively withdraw electron density from BT units, diminishing their electron-accepting capability and thus reducing molecular polarity.\n(2) Mechanism of Dipole Suppression:\nThe strong electronegativity of F localizes electron density around itself, disrupting charge transfer along the BT-TP axis. This attenuates the overall charge separation vector (d in μ = q*d), explaining the reduction from 0F to 2F-COF.\n(3) Role of β-Ketoenamine Linkages:\nThe higher μ of TP-BT-0F-COF (1.420 D) vs. BTA-BT-0F-COF (0.7476 D) underscores that β-ketoenamine motifs enhance polarization more effectively than BTA units by promoting asymmetric charge distribution.\n2. The water oxidation reaction to produce H O should involve quenching •OH radicals, not holes (h⁺).\nResponse: We deeply appreciate the reviewer’s perceptive suggestions, which have substantially enhanced the mechanistic understanding of our study. To address these insights, isotopic labeling experiments for •OH trapping studies have been systematically executed, as elaborated below:\nIn order to investigate the water oxidation pathway, H 18O was employed as the reaction medium. The oxidation of benzoic acid produced p-hydroxybenzoic acid (p-HBA), with a characteristic mass peak at m/z = 139.0296 (Fig. S60). This signal corresponds to the 18O-labeled p-HBA adduct, substantiating that •OH radicals (and thus H O ) are derived from water oxidation. The reaction sequence is as follows: (H O + h+ → •OH + H+, 2.32 V vs NHE; 2•OH → H O , 0.74 V vs NHE)\n(1) Charge Trapping Experiments Elucidate Oxidation Pathways:\nTo distinguish the roles of holes in the oxidation processes, scavenger-assisted studies were carried out:\n(a) Hole Trapping and •OH Quenching (Oxidation Suppression):\nHole Scavenging: The introduction of 10 mM EDTA-2Na reduced H O yield by 87%, confirming that holes initiate the water oxidation step.\n•OH Quenching: The addition of 10 vol% tert-butanol (TBA) resulted in an 91% decrease in H O production, further corroborating the dominance of •OH coupling in H O formation. These findings are in harmony with in situ ESR data (Fig. S59), where •OH signals were exclusively observed under piezo-photocatalytic conditions (ultrasound and light), but were absent under light alone.\n(2) Summary of Mechanistic Insights\nThese integrated experiments provide definitive evidence that: H O forms through an indirect two-step water oxidation process involving hole-generated •OH radicals. Piezoelectric fields are indispensable for activating the water oxidation reaction (WOR) pathway, as corroborated by isotopic labeling (in situ ESR and MS) and charge-trapping kinetics.\n3. While terephthalic acid is widely used to detect •OH radicals, its instability under light-ultrasound conditions compromises reliability. Additionally, the authors claim: ‘This may be because the COFs reduced the residual oxygen in the water to H O .’ This introduces ambiguity in experimental reproducibility and challenges the validity of •OH detection via terephthalic acid. Proper control experiments, such as isotopic labeling with H 18O, are critical to confirm that H O originates from water oxidation rather than oxygen reduction.\nResponse: We sincerely appreciate the reviewer’s valuable concerns regarding 1) the suitability of terephthalic acid (TA) for •OH detection under combined light-ultrasound conditions and 2) the mechanistic ambiguity in H O generation. We have addressed these points as follows:\n(1) Reliability of TA Probe\nTA is a well-established •OH probe validated for piezo-photocatalytic systems. Previous studies confirm its stability under dual excitation within typical reaction duration (Adv. Mater. 2021, 33, 2101751). This is evidenced by: 1) Reproducible •OH quantification via TA fluorescence, 2) Direct correlation with in situ ESR signals (Fig. S59). 3) Isotopic labeling H 18O : Detection 18O-labeled p-hydroxybenzoic acid (m/z = 139.0296, Fig. S60). These three tests all confirm the generation of hydroxyl radicals, thereby further validating the reliability of the fluorescence produced by hydroxyl radical capture using TA. In addition, we conducted isotope tests with benzoic acid (which is similar to terephthalic acid), and the results can further demonstrate the stability of such substances after combining with hydroxyl radicals under piezo-photocatalytic conditions. This three-method validation ensures robust detection of water-oxidation-derived radicals.\n(2) Clarification of H O Origin\nWe apologize for the unclear phrasing and now explicitly distinguish mechanisms:\n(3) Photocatalysis (light only)\nH O originates from oxygen reduction (ORR) by photogenerated electrons, consistent with: Weak TA-PL signals (Fig. S58), and thermodynamically inaccessible water oxidation (all COF valence bands < +2.32 V vs. NHE).\n(4) Piezo-photocatalysis (light + ultrasound)\nH O derives from water oxidation, confirmed by:\n(a) Scavenger studies: H O production suppression with EDTA-2Na (hole scavenger) and TBA (•OH scavenger), indicating hole-mediated pathway: H O + h+ → •OH + H+, 2.32 V vs. NHE; 2•OH → H O , 0.74 V vs. NHE\n(b) Isotopic labeling H 18O : Detection 18O-labeled p-hydroxybenzoic acid (m/z = 139.0296, Fig. S60), confirming •OH (and thus H O ) originates from water.\n4. Lacking cyclic tests and characterization of the materials after reaction.\nResponse: We thank the reviewer for their valuable suggestion regarding cyclic stability testing and post-reaction characterization. In response, we have incorporated additional experimental data into the revised manuscript. While post-reaction XRD analysis revealed diminished peak intensities, the corresponding FTIR spectra (Fig. S53) remain nearly unchanged. This key observation confirms the robust structural integrity of the TP-BT-0F-COF framework throughout the piezo-photocatalytic process. Collectively, these results demonstrate the material's fundamental structural stability under reaction conditions.\n5. Powdered materials cannot be directly tested for d coefficients using standard d meters. Supplementary Fig. S37 merely displays a photograph without detailing sample preparation or measurement protocols.\nResponse: We appreciate the reviewer’s insightful comment regarding the challenges associated with measuring d coefficients in powdered materials. We acknowledge that standard d meters are optimized for dense, bulk samples and are not ideal for powdered specimens due to issues such as poor mechanical contact and non-uniform stress distribution. Regarding Supplementary Fig. S37, we agree that the original photograph alone was insufficient without detailed methodological context. To address this, we have now supplemented the manuscript with a comprehensive description of the sample preparation protocol.\n6. The authors employed diamond anvil cells for characterization, but it remains unclear whether such extreme pressure conditions are relevant to practical piezoelectric catalysis. While similar methods are used in piezofluorochromism studies (e.g., Angew. Chem. Int. Ed. 2024, e202409099), the connection between these fields and piezoelectric catalysis remains unexplained.\nResponse: We sincerely appreciate the reviewer's insightful comment, which prompts us to clarify the rationale for employing diamond anvil cells (DAC) in our study. Based on the experimental data presented in the study, the use of DAC for high-pressure characterization serves as a powerful proxy to elucidate the intrinsic structure-property relationships governing piezoelectric response, even though the absolute pressures (GPa range) exceed those in practical piezocatalytic conditions. The relevance lies in the following aspects, supported by the data:\n(1) Probing Polarization Sensitivity as a Signature of Piezoelectricity\nIn situ high-pressure FT-IR (Fig. 2D-E, S39) revealed that COFs with strong piezoelectricity exhibit significant blue shifts in β-ketoenamine units under pressure. Weakly piezoelectric/non-piezoelectric COFs (e.g., TP-SB-COF) showed minimal shifts. This demonstrates that pressure-induced bond polarization correlates directly with piezoelectric strength, as both arise from symmetry-breaking structural distortions.\n(2) Bandgap Narrowing Under Pressure Mirrors Piezo-Potential Effects\nIn situ UV-Vis (Fig. 2F-H, S44-S49) showed that TP-BT-0F-COF’s bandgap narrowed by 0.47 eV (at 15 GPa), while weaker piezoelectrics (e.g., TP-BT-2F-COF) narrowed by only 0.36 eV. Bandgap reduction under pressure simulates the piezoelectric field-driven band bending in actual catalysis (Fig. 5G), which facilitates charge separation under mechanical stress. This was experimentally validated: samples with larger pressure-induced band shifts showed higher H /H O yields under ultrasound (Fig. 3A-E).\n(3) Fluorescence Quenching Under Pressure Reflects Enhanced Charge Separation\nIn situ PL (Fig. 4A-B, S61-S62) revealed rapid fluorescence quenching in strongly piezoelectric COFs (e.g., TP-BT-0F-COF) under pressure, indicating suppressed charge recombination due to intensified π-π interactions and polarization. This parallels the ultrasound-driven charge separation in piezo-photocatalysis, where piezoelectric fields inhibit electron-hole recombination (Fig. 4B-C).\n(4) Connection to Piezofluorochromism Studies\nWhile piezofluorochromism focuses on pressure-induced emission shifts (e.g., Angew. Chem. Int. Ed. 2024, e202409099), this work extends the methodology to probe polarization-dependent electronic responses critical for catalysis. Both fields rely on high pressure to: (i) Amplify subtle structural distortions that induce dipole changes. (ii) Quantify how these distortions alter optoelectronic properties (bandgap, carrier mobility). The key difference: Piezofluorochromism tracks emission changes, whereas this study correlates pressure-driven structural/electronic responses with piezocatalytic activity.\n(5) Why High Pressure is Mechanistically Relevant\nPractical piezocatalysis operates at lower stresses, but the microscopic origin (symmetry-breaking → dipole alignment → charge separation) is identical. DAC experiments: Accelerate polarization effects to detectable levels. Rank materials by piezoelectric sensitivity (e.g., TP-BT-0F > TP-BT-2F). Validate DFT predictions of dipole moments (Fig. S41) and active sites (Fig. 5). Crucially, samples showing the largest high-pressure responses (TP-BT-0F-COF) delivered the best practical performance under ultrasound and light (Fig. 3C).\n(6) Conclusion\nHigh-pressure characterization is not a direct simulation of piezocatalytic conditions but an effective tool to decode the structure-polarization relationship. The study bridges these domains by showing that materials with high pressure sensitivity in bond polarization, bandgap narrowing, and carrier separation invariably exhibit superior piezocatalytic activity. This approach provides a molecular blueprint for designing high-performance piezocatalysts, where DAC experiments serve as a screening platform to identify candidates with robust dipole moments and polarization responses.\nThank you for your kind consideration.\nSincerely yours,\nJun-Min Liu\n\nResponse to Reviewers\nDear Reviewers,\nThanks very much for your comments. We have revised our manuscript “Enhanced piezo-photocatalytic water splitting activity via engineering robust dipole moments in covalent organic frameworks” according to your comments. The detailed point-by-point responses to the comments are listed as follows.\nReviewers’ comments:\nReviewer #1 (Remarks to the Author):\nAll my previous concerns have been satisfactorily resolved, and the manuscript is recommended to be accepted.\nResponse: We thank the reviewer for their positive feedback and endorsement.\nReviewer #2 (Remarks to the Author):\nI have no further questions.\nResponse: We thank the reviewer for their positive feedback and endorsement.\nReviewer #3 (Remarks to the Author):\nComments:\nThe authors have clarified most of the experimental concerns expressed in the previous round of review. The observation of the SHG peak at 700 nm for the COF does indicate its non-centrosymmetric nature. However, the authors must recheck the space group of the material still. P1 is the most basic space group with no symmetry operations, which can be used to refine absolutely any material. It seems unlikely from the COF structure that it would possess no symmetry elements at all. Also, the chemical rationale behind symmetry breaking, be it torsion or twisting or rotation of the motifs, is still missing, which makes the structure-property correlations incomplete still.\nResponse:\n1. We sincerely thank the reviewer for this insightful comment and for highlighting this critical point. The reviewer is absolutely correct that the P1 space group, due to its lack of symmetry, is often a starting point for refinement and may not reflect the true symmetry of a highly ordered COF structure. Following the reviewer’s suggestion, we have meticulously re-examined and re-refined the PXRD patterns for all COFs. The results conclusively demonstrate that a higher-symmetry space group provides an excellent fit to the experimental data. Specifically, the Pawley refinements were successfully performed in the following space groups, all of which yielded excellent agreement factors (Rwp and Rp < 10%): TP-BT-0F-COF and BTA-BT-0F-COF were refined in the P-6 space group. BT-1F-COF and TP-BT-2F-COF were refined in the P3 space group. The space groups of the above four COFs are all non-centrosymmetric. The space group P-3 of TP-SB-COF is centrosymmetric. This result is a directly quoted conclusion from relevant literature (Nature Chem 10, 1180-1189 (2018)). We have clearly marked the citation source in the subsequent supplementary materials to ensure the rigor of data traceability. Furthermore, fractional atomic coordinates for the unit cell of all COFs have been compiled and are presented in Tables S1-S5 in the Supporting Information. We believe that this revised structural assignment is now both physically reasonable and rigorously supported by the diffraction data. We are grateful to the reviewer for prompting this significant improvement to our work.\n2. We sincerely thank the reviewer for this insightful comment. We agree that a deeper mechanistic explanation of symmetry breaking is crucial for establishing robust structure-property relationships. In response, we have now revised the manuscript to provide a clearer chemical rationale, directly based on our experimental and computational data, which points to a combination of in-plane electronic polarization and bond-length alternation as the primary driver of symmetry breaking, rather than large-scale torsional or twisting motions. The key revisions and clarifications are as follows:\n1) Electronic Asymmetry from the Donor-Acceptor (D-A) Motif: The primary source of symmetry breaking is the intrinsic electronic asymmetry of the benzothiadiazole (BT) unit. When linked to the electron-rich tri-phenylamine (TP) unit, it creates a strong D-A pair. This is evidenced by: (1) DFT-Calculated Dipole Moments： TP-BT-0F-COF, with the unmodified BT unit, possesses the largest dipole moment (1.420 D). Systematic fluorination, which tunes the BT unit's electron-withdrawing strength, progressively reduces this moment (TP-BT-1F-COF: 0.5995 D; TP-BT-2F-COF: 0.4416 D), directly linking molecular polarity to structure. (2) Optical Properties: The pronounced red-shift and narrower bandgap of TP-BT-0F-COF confirm strong intramolecular charge transfer, a hallmark of electronic symmetry breaking.\n2) In-Plane Polarization from the β-Ketoenamine Linkage: The β-ketoenamine linkage enhances symmetry breaking not via torsion, but through its locked, planar conformation that enables inherent in-plane polarization. This is supported by: (1) In-situ High-Pressure FT-IR: Significant pressure-induced blue-shifts of C-N and C=C vibrations in β-ketoenamine-linked COFs (especially TP-BT-0F-COF) indicate bond shortening and intensified polarization. The absence of this effect in the imine-linked BTA-BT-0F-COF underscores its critical role. (2) Piezoelectric Tensor Analysis: DFT calculations confirm that the large piezoelectric tensor elements are primarily in-plane, with high Born effective charges (> |2.0 e|) localized near the β-ketoenamine group, identifying it as the key ‘piezoelectric active unit.’\n3) Direct Experimental Confirmation: The macroscopic non-centrosymmetry resulting from this molecular-level design is unambiguously confirmed by: (1) Second-Harmonic Generation (SHG): A distinct SHG signal from TP-BT-0F-COF. (2) Violation of the Mutual Exclusion Rule: The concurrent appearance of specific vibrational modes in both IR and Raman spectra.\nIn summary, the symmetry breaking in TP-BT-0F-COF is driven by the synergy between the strong electronic asymmetry of the D-A BT motif and the in-plane polarization of the rigid β-ketoenamine linkage, creating a robust dipole moment that is the direct origin of its superior piezoelectric and piezo-photocatalytic performance. We have revised the manuscript to articulate this rationale clearly, ensuring a complete structure-property correlation.\nReviewer 4 (Remarks to the Author):\nThe authors have addressed the majority of my questions and concerns, with the exception of Comment 4 (Lacking cyclic tests). Nevertheless, I find the manuscript suitable for acceptance.\nResponse: We are deeply grateful to the reviewer for their positive assessment and for recommending our manuscript for acceptance.\nRegarding Comment 4 on the lack of cyclic tests, we sincerely appreciate the reviewer’s insightful point. At the outset of this study, we employed long-term stability tests as an alternative yet rigorous approach to demonstrate the robustness of our COF materials. Specifically, as shown in Supplementary Fig. S52, the COF maintained a high level of catalytic activity over a continuous 24-hour reaction period without significant decay. These long-term stability results indicate that the COF exhibited no notable activity attenuation and maintained structural stability during the continuous piezo-photocatalytic process. This aligns with the goal of verifying activity stability pursued by cyclic tests and thus also confirms its stable piezo-photocatalytic activity. We believe that this demonstrated long-term operational stability effectively proves that the COF maintains its structural integrity and catalytic activity over time, which is a crucial indicator of its practical application potential.\nThank you for your kind consideration.\nSincerely yours,\nJun-Min Liu\nProfessor in Chemistry\nE-mail: liujunm@mail.sysu.edu.cn\nSchool of Materials Science and Engineering, Sun Yat-sen University"}, {"round": 2, "reviewer_comments": {"reviewer_3": "The authors have done a good job in clarifying the previous concerns. The structural aspects are very clear now and the manuscript can be accepted for publication in Nat. Commun. in its current state."}, "author_response": ""}]

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