Multisite atomic-chlorine-passivation stabilizes perovskite interfaces for efficient H_2O_2 photosynthesis from seawater

Domain: biology
Published: 2026-03-15
Editorial outcome: major_revision

Abstract
Lead halide perovskites are promising for artificial photosynthesis but suffer from aqueous instability. Here, we stabilize CsPbI3 quantum dots within a hydrophobic chlorine-functionalized covalent organic framework through multisite atomic-chlorine passivation, forms dual Cl-Pb coordination and Cl-I halogen bonding at the interface. This suppresses ionic migration while creating a gas-liquid-solid triphase interface for enhanced O2 diffusion. The resulting S-scheme heterojunction spatially separates carriers to concurrently drive two-electron oxygen reduction and water oxidation for H2O2 synthesis without sacrificial agents. The system achieves production rates of 20.37 mmol h-1 g-1 in seawater, with a solar-to-chemical conversion efficiency of 1.38%, and operates stably for 20 h. Importantly, natural sunlight tests yield 11.7 mmol L-1 H2O2 in 10 h. Mechanistic studies confirm synergistic interfacial charge transfer and dual-reaction pathways via both oxygen reduction and water oxidation. This work demonstrates an approach for robust perovskite-based photocatalysts toward solar-driven chemical synthesis from seawater.

Introduction
Hydrogen peroxide (H2O2) is an indispensable chemical oxidant with burgeoning applications in water treatment, disinfection, and green synthesis1, 2. However, the incumbent anthraquinone process for H2O2 production is energy-intensive, generates significant waste, and relies on centralized facilities, limiting its accessibility and sustainability. Solar-driven photosynthesis of H2O2 directly from water (H2O) and oxygen (O2) offers a compelling decentralized and sustainable alternative3. Seawater, constituting over 96% of Earth's hydrosphere, presents an abundant aqueous resource for this process, yet its complex ionic composition and corrosivity pose formidable challenges to photocatalyst stability and performance4. The development of effective, seawater-compatible photocatalytic systems is therefore essential to unlock scalable solar H2O2 synthesis.

Lead halide perovskites, particularly CsPbI3 quantum dots (QDs), have emerged as light harvesters for artificial photosynthesis5, 6. This is attributed to their extensive visible-light absorption, suitable band edges, and high charge-carrier mobility7. However, their well-known vulnerability to ionic dissolution and structural degradation in aqueous environments, especially in ion-rich seawater, limits their practical applications8. Although conventional encapsulation strategies using polymers or metal oxides offer some degree of barrier protection, they often impede mass transport (such as O2 diffusion) or fail to completely suppress interfacial ion migration, which is a crucial degradation pathway9, 10. Covalent organic frameworks (COFs) have recently gained attention as protective matrices due to their crystallinity, designable porosity, and functional versatility11, 12. However, chlorine-functionalized COFs (COF-Cl) remain underexplored, despite the potential of atomic chlorine sites to form strong, directional bonds with perovskite surfaces through halogen interactions (Cl-I) and coordination (Cl-Pb)13, 14. Such atomic-scale passivation could simultaneously stabilize the perovskite and enhance interfacial charge dynamics.

Herein, we introduce a interfacial engineering strategy to simultaneously passivate CsPbI3 QDs against aqueous degradation and enhance its photocatalytic activity for H2O2 production directly from seawater (Figure 1). We design and synthesize COFs featuring precisely positioned, multisite atomic chlorine functionalities. The COF-Cl matrix stabilizes CsPbI3 QDs through dual halogen bonding (Cl-I) and coordination (Cl-Pb), effectively suppressing ionic migration while creating a hydrophobic triphase interface to enhance O2 diffusion. Additionally, the S-scheme heterojunction forms between the CsPbI3 QDs and COF-Cl, enabling efficient spatial separation of charge carriers: electrons localized on the COF-Cl drive the 2e⁻ ORR, while holes retained on the CsPbI3 QDs drive the WOR. This system achieves production rates of 20.37 mmol h-1 g-1 (natural seawater) with >90% stability over 20 hours in seawater and operates efficiently under natural sunlight. Through integrated experimental and theoretical analyses, we elucidate the synergistic interfacial mechanisms driving this performance. Our work provides a strategy for robust perovskite-based photosynthesis, unlocking scalable solar-driven H2O2 synthesis from abundant seawater.

Results
Synthesis and characterization of hydrophobic halogenated COFs and CsPbI3@COF-X.

A series of hydrophobic halogenated covalent organic frameworks (COF-X: Cl, Br, I) were synthesized via Schiff-base condensation between 4,4',4''-(1,3,5-triazine-2,4,6-triyl) trianiline and halogenated 2,5-dihaloterephthalaldehydes (Cl, Br, I) (Figure S1). Highly crystalline materials were obtained in o-dichlorobenzene/n-BuOH/6 M HAc (5:5:1, v/v) at 120 °C for 72 h. Structural analysis confirmed successful synthesis and high crystallinity. Powder X-ray diffraction (PXRD) patterns (Figure S2) showed prominent (100) peaks at 2.79°, 2.75°, and 2.70° for COF-Cl, COF-Br, and COF-I, respectively, matching simulated AA stacking models (Figure S3-S5, Table S1). Solid-state 13CNMR revealed the characteristic imine peak (~160 ppm, Figure S6-S9). Fourier transform infrared (FTIR) spectra (Figure S10) confirmed Schiff-base formation by the disappearance of precursor -NH2 (3213, 3327 cm−1) and aldehyde (1688 cm−1) stretches and the appearance of -C=N stretches (1633 cm−1) for all three COF-X15. COF-X exhibit mesoporosity with type IV isotherms (Figure S11). Brunauer-Emmett-Teller (BET) surface areas decreased with increasing halogen size: 2289.7 m2 g-1 (COF-Cl), 1757.0 m2 g-1 (COF-Br), 1150.9 m2 g-1 (COF-I), correlating with pore diameters of 3.53 nm, 3.09 nm, and 2.63 nm, respectively (Table S2). Scanning electron microscope (SEM) and transmission electron microscope (TEM) images (Figure S12 and S13) revealed uniform fibrous morphologies and layered structures, with visible pores in COF-X HR-TEM (Figure S13b). X-ray photoelectron spectra (XPS) confirmed elemental composition (Figures S14-S16). Water contact angles of 143.0°, 145.5°, and 147.5° (Figure S17) demonstrate increasing hydrophobicity with halogen size. The thermal stability was confirmed by TGA, showing negligible weight loss at 150 °C (Figure S18). These results confirm the successful synthesis of crystalline, mesoporous, hydrophobic halogenated COFs.

A sequential deposition route was employed to synthesize CsPbI3@COF-X composites (Figure S19). Concretely, COF-X matrices were immersed in a PbI2 precursor solution at 70 °C. Residual PbI2 was removed via centrifugation using DMF/ethanol. The resulting PbI2@COF-X intermediates were then dispersed in a CsI-containing DMF solution under ultrasonication. Finally, the reaction mixture was precipitated in toluene at room temperature (~ 25 ℃) to obtain a series of CsPbI3@COF-X systems, where CsPbI3 QDs in-situ nucleated and crystallized inside the COF-X matrix (for a full description of the methods, see the Supplementary Information).

Taking CsPbI3@COF-Cl system as an example, we demonstrate how the framework and topology of the COF-Cl matrix direct the confined growth of CsPbI3 QDs. Structural characterization confirmed the successful synthesis of CsPbI3@COF-Cl. SEM imaging (Figure S20) revealed that the external surfaces of the COF-Cl matrix remained clean without observable aggregates of CsPbI3 QDs, indicating their successful embedding within the matrix. TEM analysis (Figure 2a, 2b and S21) further shows CsPbI3 QDs uniformly dispersed within the mesoporous channels of the COF-Cl matrix. Isolated CsPbI3 QDs with an average diameter of 3.2 nm are clearly resolved (Figure 2a), which closely matches the theoretically predicted mesopore diameter range of 3.13-4.01 Å (Figure S3b, Table S1). This close size correlation is highly suggestive of size-constrained growth within the porous network. High-resolution TEM (HRTEM, Figure 2c) displays distinct lattice fringes (0.32 nm spacing), matching the (200) planes of α-CsPbI3 (space group Pm-3m). The crystalline structure is further confirmed by fast-Fourier transform (FFT) patterns (Figure 2d). STEM-HAADF imaging (Figure 2e) corroborates the uniform dispersion of 3.2 nm CsPbI3 QDs within the COF-Cl framework. Elemental mapping (EDX, Figure 2f) confirms homogeneous distribution of Cl, Cs, Pb, and I throughout the composite. Hydrophobicity, critical for aqueous applications, was evaluated via contact angle measurements (Figure 2g). The CsPbI3@COF-Cl system exhibits apparent hydrophobicity (contact angle of 139.5°), which originates from the hydrophobic COF-Cl matrix. This property allows the lightweight composite to float and spread uniformly on the water surface (Figure S22), establishing a distinct gas-liquid-solid (air-water-catalyst) triphase interface. To further verify the formation and function of this triphase architecture, we performed comparative electrochemical impedance spectroscopy (EIS) under two configurations: with the catalyst fully immersed (diphasic liquid-solid interface) and with it floating at the air-water interface (triphase interface). A clear reduction of ~70% in charge-transfer resistance was observed under the floating configuration (Figure S23), providing direct electrochemical evidence that the triphase interface enhances mass transport (particularly O2 diffusion) and interfacial charge-transfer kinetics (Figure S24). PXRD analysis (Figure 2h) verifies crystallinity, with peaks at 14.1°, 19.9°, 24.5°, and 28.4° corresponding to the (100), (110), (111), and (200) planes of α-CsPbI3 material (PDF No. 01-080-4039)16. N2 adsorption-desorption isotherms (Figure 2i) and the corresponding pore size distribution curves (Figure S25) provide compelling evidence for the confinement of CsPbI3 QDs within the COF-Cl mesochannels. The pore size distribution of pristine COF-Cl shows a predominant pore diameter centered at ~3.10 nm (range: 2-4 nm). For the CsPbI3@COF-Cl composite, a clear reduction in the pore volume signal within this characteristic range is observed. Synchronously, the BET specific surface area plummets from 2289.7 m2 g-1 (COF-Cl) to 1318.1 m2 g-1, and the pore volume decreases from 1.77 cm3 g-1 to 1.05 cm3 g-1 (Table S2). This substantial reduction in both accessible surface area and internal pore volume is a direct signature of pore filling or occupancy by the in-situ grown CsPbI3 QDs. Combined with the close match between CsPbI3 QD size and host pore dimensions, this data confirms that the CsPbI3 QD growth is effectively confined within the accessible mesoporous channels of the crystalline COF-Cl framework, rather than merely occurring on external surfaces or between loosely packed structures. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis after complete digestion of the composite confirmed a CsPbI3 QD loading of 21.2 wt% within the COF-Cl matrix. Collectively, these results confirm the successful construction of the CsPbI3@COF-Cl.

To investigate the interfacial interactions between the COF-Cl matrix and CsPbI3 quantum dots (QDs), we employed X-ray photoelectron spectroscopy (XPS), solid‑state ¹³C nuclear magnetic resonance (NMR) spectroscopy, and time‑resolved fluorescence decay spectroscopy (TRFDS) to probe the electronic and chemical environments of the constituent elements. XPS survey spectra confirmed the presence of C, N, Cl, Cs, Pb, and I in the CsPbI3@COF‑Cl composite (Figure S26), verifying its successful synthesis. In the deconvoluted Pb 4f spectrum of CsPbI3@COF‑Cl (Figure 3a and Table S3), the higher-binding-energy doublet (orange curves) is assigned to surface Pb2+ ions engaged in direct Cl-Pb coordination with the COF-Cl matrix, while the lower‑binding‑energy doublet (green curves) corresponds to bulk Pb2+ ions within the CsPbI3 QD core, which are shielded from direct interfacial contact. Similarly, the I 3d spectra (Figure 3b) show a higher‑binding‑energy doublet (orange curves) arising from surface I⁻ ions directly involved in Cl-I halogen bonding with COF-Cl, whereas the lower‑binding‑energy doublet (green curves) originates from bulk I⁻ ions in the CsPbI3 QD interior. Conversely, the Cl 2p peaks shift toward lower binding energy in the composite (Figure 3c), further evidencing clear chemical interaction.

A detailed comparative XPS analysis of the CsPbI3@COF-Br and CsPbI3@COF-I composites reveal fundamentally different interfacial chemistries that explain their inferior performance. In the CsPbI3@COF-Br system, the Pb 4f spectrum shows a large positive binding energy shift (Δ = +2.15 eV, Figure S27a, Table S4), indicating electron transfer due to Br-Pb coordination. However, the Br 3d binding energy shows only a minimal negative shift (Δ = -0.05 eV, Figure S27c, Table S3), suggesting a lack of effective electron compensation via halogen bonding. This unbalanced interaction likely leads to interfacial charge accumulation and reduced stability. Most critically, the CsPbI3@COF-I system reveals interfacial incompatibility and degradation. The deconvolution of its I 3d spectrum requires three distinct doublets (Figure S28, Table S5). Besides peaks for bulk CsPbI3 I⁻ and interfacial I⁻ interacting with COF-I, a third component at a very high binding energy (Δ = +3.75 eV) unequivocally confirms the oxidation of interfacial I⁻ to species such as I2 or IOx. This demonstrates that COF-I not only fails to passivate the CsPbI3 surface but actively induces detrimental interfacial reactions. In stark contrast, the CsPbI3@COF-Cl system exhibits a synergistic, bidirectional electron redistribution profile with a moderate positive shift for Pb (Δ = +1.17 eV, Figure 3a) and a concurrent negative shift for Cl (Δ = -0.22 eV, Figure 3c), which definitively proves the coexistence of stable Cl-Pb coordination and Cl-I halogen bonding. To further verify the specific chemical interaction between COF-Cl and the CsPbI3 QDs, we extended XPS analysis to the PbI2@COF-Cl composite. The spectra reveal a distinct positive shift in the binding energy for both Pb and I in the composite compared to pristine PbI2 (Figure S29, S30 and Table S6). This positive shift is highly similar to that observed in the CsPbI3@COF-Cl system. Most definitively, the physically mixed sample (PM-CsPbI3/COF-Cl) show no discernible peak splitting or significant binding-energy shifts relative to the pristine components (Figure S31 and Table S7), confirming that the spectral changes in the in-situ composites are not artifacts of physical proximity but arise from chemically bonded interfaces formed during synthesis. The established trend in Pb-X bond strength (Pb-Cl > Pb-Br > Pb-I) provides a fundamental chemical rationale for the favorable stability afforded by the Cl-based interface17‑19.

To further investigate the chemical environment changes, solid-state 13C NMR spectroscopy was performed. The spectrum (Figure 3d and S32) revealed a downfield shift of 0.3 ppm for the carbon atoms directly bonded to chlorine in CsPbI3@COF-Cl relative to pristine COF-Cl. This shift provides evidence for the formation of halogen-halogen (Cl-I) bonds and coordination interactions (Cl-Pb) at the interface. To directly assess the impact of these interactions on defect passivation, TRFDS measurements were conducted (Figure 3e, Table S3). Fluorescence decay curves were fitted to a tri-exponential model, where fast decay correlates with surface/crystal defects and slow decay with carrier transport. Pristine CsPbI3 QDs exhibited rapid decay (τ1 = 3.13 ns, τ2 = 13.68 ns, τ3 = 62.22 ns; τₐᵥ = 24.96 ns), attributed to non-radiative recombination from Pb-I antisite defects and vacancies20. In contrast, CsPbI3@COF-Cl showed slower decay (τ1 = 9.60 ns, τ2 = 57.52 ns, τ3 = 289.41 ns; τₐᵥ = 174.38 ns), demonstrating that COF-Cl passivates defects via halogen-halogen bonds and coordination interactions, thereby suppressing non-radiative pathways20. Moreover, TRPL decay dynamics provide direct evidence that the performance of CsPbI3@COF-Cl stems from its effective interfacial passivation. The measured lifetimes for the composites follow a definitive trend: CsPbI3@COF-Cl exhibits the longest decay components and an average lifetime greater than those of CsPbI3@COF-Br (τavg = 106.78 ns) and CsPbI3@COF-I (τavg = 24.68 ns) (Figure S33, S34 and Table S8).

To elucidate the multi‑site bonding mechanism depicted schematically in Figure 3f, we investigated in detail the adsorption of the halogen‑rich porous COF‑Cl matrix onto CsPbI3 QD surfaces, together with the corresponding binding and formation energies. Preliminary computational modeling (Figure S35 and S36), based on idealized periodic surface models, suggests that the periodic architecture of COF-Cl enables continuous multi-site bonding with the perovskite surface. Specifically, on a Pb-rich surface, adjacent-Cl groups can form a sequential chain of coordination bonds (Cl-Pb) with neighboring under-coordinated Pb2+ sites. Similarly, on an I-rich surface, they form a continuous network of halogen bonds (Cl-I) with consecutive I⁻ sites. Within the constraints of our simplified DFT models, this continuous multi-site passivation, saturating contiguous chains of surface defects rather than capping isolated sites sparsely, creates a robust interfacial network. To probe the advantage of this continuous multi-site mode over isolated passivation, we performed controlled Density Functional Theory (DFT) calculations. We compared the ideal continuous model against an artificially created discontinuous model, where specific Cl groups were removed from the framework to prevent contiguous bonding (Figure S37 and S38). The formation energy for establishing the continuous Cl-Pb network was profoundly more favorable (-2.76 eV) than for the discontinuous case (-1.28 eV). A similar trend was observed for Cl-I bonding (-2.56 eV vs. -0.80 eV) (Figure S39a). The average binding energy per site was also stronger in the continuous configuration (Figure S39b). This computational evidence suggests that, under the ideal conditions of our models, passivating a sequence of neighboring defects is energetically synergistic; interaction at one site favorably pre-configures the environment for the next, leading to collective stabilization and a more robust interface. Furthermore, limited molecular dynamics (MD) simulations under simplified conditions indicate that structural stability conferred by this continuous network. In these simulations, the root means square deviation (RMSD) of atomic positions for systems with continuous Cl-Pb/I passivation stabilizes quickly and remains consistently and lower than for systems with discontinuous passivation throughout the simulation (Figure S40), hinting at reduced interfacial atomic displacement and fluctuation.

Characterizations of the electronic properties and the band edge positions.

Based on the ultraviolet-visible diffuse reflection spectra (UV-vis DRS), the COF-X matrix exhibits adsorption peaks (Soret band) around 350-500 nm (Figure 4a), attributed to its periodic molecular framework and long-range ordered π-stacking structure. In contrast, CsPbI3 QDs show a broad absorption band extending to 700 nm, indicative of a narrow bandgap. Upon encapsulation of CsPbI3 QDs by COF-X to form the CsPbI3@COF-X system, the absorption edge of the CsPbI3@COF-X system undergoes a significant redshift beyond 700 nm, enabling more efficient solar energy utilization. This redshift clearly demonstrates the synergistic effect between COF-X and CsPbI3 QDs. Tauc plot analysis (Figure S41) determined the bandgap energies (Eg) of COF-Cl, COF-Br, COF-I, and CsPbI3 QDs to be 2.50 eV, 2.58 eV, 2.49 eV, and 1.77 eV, respectively, confirming their semiconductor nature. For efficient photocatalytic reactions, the electronic band structure must align with the relevant redox potentials. Valence band XPS spectra (VB-XPS, Figure S42) measured the valence band maximum (EVB) positions relative to the vacuum level (EVB, XPS). After conversion to the standard hydrogen electrode (NHE) scale, the EVB values are: COF-Cl (2.52 eV), COF-Br (2.39 eV), COF-I (2.27 eV), and CsPbI3 QDs (0.94 eV). The resulting band diagrams are plotted in Figure 4b. Importantly, this staggered band alignment satisfies the prerequisite for forming an S-scheme heterojunction. The results indicate that CsPbI3 QDs possess strong reducing capabilities, while the COF-X matrices exhibit high oxidizing capabilities, with COF-Cl showing the strongest oxidizing potential (EVB = +2.52 V vs. NHE) compared to COF-Br (+2.39 V) and COF-I (+2.27 V). This deeper valence band position of COF-Cl signifies a stronger oxidation capability and promotes the formation of a a larger built-in electric field at the heterojunction interface, providing a greater driving force for charge separation. This favorable energy level alignment within the CsPbI3@COF-Cl composite not only maximizes the redox capabilities of the individual components but also facilitates the construction of an efficient S-scheme heterojunction (Figure 4c). The CsPbI3@COF-Cl system enhances charge transfer and carrier separation. Photoluminescence (PL) spectroscopy and TRFDS show substantially quenched PL intensity and prolonged fluorescence lifetime in CsPbI3@COF-Cl compared to pure CsPbI3 QDs (Figure S43 and 2e), indicating suppressed electron-hole recombination due to spatial charge separation. These properties directly improve photochemical conversion activity. Photocurrent response tests (Figure S44) show faster charge transfer in CsPbI3@COF-Cl compared to COF-Cl alone. Pure CsPbI3 QDs were excluded due to their aqueous instability. Electrochemical impedance spectroscopy (EIS) further confirms reduced charge-transfer resistance in CsPbI3@COF-Cl, evidenced by a smaller Nyquist semicircle (Figure S45).

The interfacial electron transfer mechanism in the CsPbI3@COF-Cl heterojunction was demonstrated by Ultraviolet photoelectron spectroscopy (UPS). The results are displayed in Figure S46 and Figure S47. The work functions of CsPbI3 QDs and COF-Cl are 5.83 eV and 7.00 eV, respectively. Correspondingly, the Fermi levels of CsPbI3 QDs and COF-Cl are -5.83 eV and -7.00 eV (vs vacuum level), respectively. This work function disparity establishes a higher Fermi level in CsPbI3 QDs than in COF-Cl, driving electron migration from CsPbI3 QDs to COF-Cl upon heterojunction formation21. This initial electron transfer leads to charge redistribution at the interface, establishing a unified Fermi level, an internal electric field (IEF) 22, and causing upward band bending in CsPbI3 QDs and downward band bending in COF-Cl. Further validation through in-situ XPS under light illumination tracked binding energy shifts in key elements. As shown in Figure 4d, light exposure caused the Cl 2p peaks in CsPbI3@COF-Cl to shift from 202.2 eV and 200.6 eV to 202.6 eV and 201.0 eV, indicating electron loss from the COF-Cl component. Simultaneously, the Pb 4f peaks shifted from 144.29 eV/139.42 eV and 143.57 eV/138.70 eV to 143.92 eV/139.05 eV and 143.25 eV/138.38 eV (Figure 4e), while the I 3d peaks shifted from 632.12 eV/620.62 eV and 630.95 eV/619.45 eV to 632.46 eV/619.96 eV and 630.46 eV/618.96 eV (Figure 4f). These shifts collectively confirm electron gain by the CsPbI3 QDs component. Therefore, photogenerated electrons transfer from COF-Cl to CsPbI3 QDs under light excitation, directly evidencing the S-scheme charge transfer process. The combined UPS and in-situ XPS results provide direct experimental evidence for the charge transfer process and quantitatively support the proposed S-scheme mechanism initiated by the Fermi level offset (Figure S48). Initially, the Fermi level difference drives the flow of electrons from CsPbI3 QDs to COF-Cl, thereby forming the IEF. This leads to an upward band bending in CsPbI3 QDs and a downward band bending in COF-Cl at the interface. Under illumination, the IEF, Coulomb attraction, and band bending promote recombination of COF-Cl conduction band electrons with CsPbI3 valence band holes.

To further explore carrier transfer mechanisms, femtosecond transient absorption (fs-TA) spectroscopy was employed to analyze the dynamics of photogenerated charge carriers using a 400 nm pump pulse. The fs-TA spectra of CsPbI3 QDs (Figure S49a and S49b) exhibit characteristic ground-state bleaching (GSB) at 460~600 nm and photoinduced absorption (PIA) at 600~700 nm, with decay kinetics (Figure S49c) fitting to τ1 = 13.19 ps and τ2 = 243.50 ps. For COF-Cl (Figure S50a and S50b), two distinct GSB signals are observed at 460~560 nm and 630~700 nm, with kinetics (Figure S50c) described by τ1 = 5.95 ps and τ2 = 184.90 ps. In the CsPbI3@COF-Cl system (Figure S51a and S51b), key spectral changes confirm the S-scheme mechanism: the electron-related signal of COF-Cl (630~700 nm) is completely quenched, and the hole-associated PIA signal of CsPbI3 QDs (600~700 nm) is strongly suppressed. This simultaneous quenching provides direct evidence that electrons from COF-Cl recombine with holes from CsPbI3 QDs at the interface. The kinetic analysis (Figure S51c) further supports this model, revealing a shortened decay component (τ2 = 67.55 ps) indicative of accelerated interfacial recombination, and the emergence of a long-lived component (τ3 = 934.40 ps) corresponding to the stabilized useful charges—electrons in CsPbI3 QDs and holes in COF-Cl. Moreover, in-situ irradiated Kelvin Probe Force Microscopy (KPFM) was employed to monitor the surface photovoltage (SPV) changes of the heterojunction material (Figure 4g and 4h). Upon light irradiation, the surface potential of CsPbI3@COF-Cl exhibited a notable increase from 198.6 mV to 276.1 mV, corresponding to an SPV change of 77.5 mV (Figure 4i). This increase indicates effective suppression of charge carrier recombination. In contrast, pristine COF-Cl alone showed negligible SPV change (Figure S52). More specifically, the accumulation of photo-generated holes in the valence band of the surface-exposed COF-Cl component provides evidence for an S-scheme heterojunction mechanism, wherein electrons from COF-Cl recombine with holes from the CsPbI3 QDs. This interpretation is further supported by control experiments on a deliberately synthesized CsPbI3-COF-Cl composite, in which perovskite was grown on the outer surface of COF-Cl (Figure S53). In this configuration, a clear SPV increase on COF-Cl accompanied by a reduced SPV response from the CsPbI3 was observed, reinforcing those holes accumulate in COF-Cl while electrons are retained within the CsPbI3 QDs under illumination. This mechanism retains useful electrons in the CB of CsPbI3 QDs and holes in the VB of COF-Cl, maximizing their redox potential for enhanced photocatalytic oxygen reduction.

Full reaction photosynthesis of H2O2 on a three-phase interface.

The CsPbI3@COF-Cl system demonstrates significant potential for the photocatalytic synthesis of H2O2, as its energy band structure is sufficiently capable of driving both essential half-reactions: the oxidation of H2O (E(H2O2 / H2O) = +1.76 V vs. NHE, pH = 0) and the reduction of O2 (E( H2O2 /O2) = +0.68 V vs. NHE, pH = 0)23. This indicates its theoretical suitability as an effective photocatalyst for the full photosynthetic reaction producing H2O2. Cyclic voltammetry (CV) further elucidates these redox capabilities. As shown in Figure S54, the CV curve identifies a reduction potential of -0.94 V (vs. Ag/AgCl, pH = 7), corresponding to -0.33 V vs RHE (pH = 0), assigned to the reduction of O2 to •O2⁻. Simultaneously, an oxidation potential of +1.15 V (vs. Ag/AgCl, pH = 7), equivalent to +1.76 V vs RHE (pH = 0), is observed for the oxidation of H2O to H2O2. Therefore, the combined theoretical and electrochemical data confirm that the CsPbI3@COF-Cl system can effectively drive the photochemical synthesis of H2O2 using only water and atmospheric oxygen.

Photocatalytic H2O2 generation was evaluated in both pure water and seawater under visible-light irradiation and ambient air, without sacrificial agents, employing the iodometric method for quantitative determination (Figure S55). Benefiting from its lightweight nature and hydrophobicity, the CsPbI3@COF-Cl system readily spreads across the water surface, facilitating redox reactions at this air-liquid-solid interface (Figure S22). The reduction of charge transfer resistance by over 70% under the floating configuration provides direct evidence that the formation of an efficient gas-liquid-solid triphase interface enhances the reaction kinetics (Figure S23). Under air in pure water, the H2O2 yield showed clear accumulation over time, exhibiting a near-linear relationship between production and irradiation duration for both the COF-Cl matrix alone and the CsPbI3@COF-Cl system (Figure 5a). This indicates sustained high photosynthetic rates even under prolonged illumination. After 1 h, H2O2 yields reached 15.57 mmol g-1 h-1 for COF-Cl and 25.29 mmol g-1 h-1 for CsPbI3@COF-Cl, respectively (Figure S56). This significant enhancement in the CsPbI3@COF-Cl system is attributed to optimized charge separation efficiency achieved through interfacial band engineering, coupled with efficient O2 diffusion in the three-phase (gas-liquid-solid) reaction system. Conversely, when evaluated in a diphase (liquid-solid) system, the H2O2 production rate showed a decrease of more than 68% (Figure S57), underscoring that limited O2 supply becomes a critical bottleneck when the advantageous three-phase interface is absent. Given this promising performance, we systematically evaluated H2O2 production under seawater and air conditions (Figure 5b). After 1 h of photochemical conversion, H2O2 yields of 12.52 mmol g-1 h-1 and 20.37 mmol g-1 h-1 were achieved for the COF-Cl matrix and the CsPbI3@COF-Cl system, respectively (Figure S58). During the photocatalytic process, Cl2 was not monitored by online mass spectrometry (MS) (Figure S59). The progressive accumulation of oxidative chlorine species, as quantified by the DPD (N,N-diethyl-p-phenylenediamine) method (Figure S60) and shown by the increase from 5.2 ppm to 6.4 ppm over the reaction period (Figure S61), confirms chloride oxidation as a competing side reaction.

The apparent quantum yield (AQY) action spectrum of the CsPbI3@COF-Cl system was measured across the visible range (Figure 5c, and tabulated in Table S9) under ambient atmospheric conditions, yielding values of 15.45% at 420 nm and 15.76% at 450 nm. The trend of photocatalytic efficiency closely follows the composite’s own absorption profile (Figure 5c), rather than that of the individual CsPbI3 QDs or COF-Cl components. This correlation provides direct evidence that the high activity originates from the integrated light-harvesting and charge generation of the formed S-scheme heterojunction as a whole. We note that obtaining a reliable AQY for pristine CsPbI3 QDs in aqueous media was not feasible due to their rapid degradation, which itself underscores the critical stabilizing role of the COF-Cl matrix.

Cycling stability tests confirmed the system's durability. The CsPbI3@COF-Cl system maintained stable H2O2 production performance over five consecutive photocatalytic cycles in seawater under air (Figure S62 and S63). TEM imaging and XRD analysis revealed no significant structural changes or deposition of insoluble salts (e.g., Mg/Ca hydroxides/carbonates) after cycling in seawater (Figures S64 and S65), highlighting its robustness against fouling. Post-cycling XPS analysis confirms the chemical stability of CsPbI3@COF-Cl, as the characteristic dual-doublet peaks of Pb, I, and Cl remain intact after five catalytic runs (Figure S66), supporting its stable cycling performance. To further verify operational stability in seawater, a continuous 20-h photocatalytic reaction was conducted (Figure 5d), demonstrating stable H2O2 production. PXRD measurements during operation further confirmed structural stability under prolonged seawater exposure (Figure 5e). To assess the environmental safety and practical viability, the reaction solution from the 20-hour stability test was analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The leaching concentrations of Pb2+ and Cs+ were quantified as 0.64 ppm and 1.52 ppm, respectively. The Pb2+ concentration falls below the 1.0 ppm limit of China’s Integrated Wastewater Discharge Standard (GB 8978-1996), which is attributed to the effective stabilization via Cl-Pb coordination and the hydrophobic barrier of the COF-Cl matrix. Neat CsPbI3 QDs rapidly degrades in water (Figure S67), while the control composite CsPbI3-COF-H without halogen modification, with perovskite poorly encapsulated and predominantly in the inactive δ-phase on the surface (Figure S68), exhibits negligible performance (Figure S69).

To demonstrate practical solar-to-chemical energy conversion, the H2O2 production performance of CsPbI3@COF-Cl was evaluated under natural sunlight in Lanzhou City, China (May 1, 2025; Figure 5f and S70). Real-time monitoring was carried out to track solar irradiance at 450 nm, system temperature, and H2O2 yield. After 10 hours, an accumulated H2O2 concentration of 11.7 mmol L-1 was achieved, confirming efficient photocatalytic activity under real-world conditions. Combined with its high efficiency, these findings indicate that the CsPbI3@COF-Cl system holds significant promise for large-scale, solar-driven H2O2 production from pure water and natural seawater. This performance is competitive among reported photocatalysts (Figure S71 and Table S10)24-45. The CsPbI3@COF-Cl system achieved solar-to-chemical conversion (SCC) efficiency of 1.38%, while competitive among direct photosynthesis systems, is primarily limited by the suboptimal utilization of the full solar spectrum, optical losses at the triphase interface, and the kinetic overpotentials required to drive the dual 2e⁻ ORR and WOR pathways simultaneously.

To elucidate the reaction mechanism of hydrogen peroxide (H2O2) generation, electron paramagnetic resonance (EPR) spectroscopy was employed to detect key intermediates during photochemical conversion. Using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMP) as a spin trap for singlet oxygen (1O2) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) for superoxide anion radicals (•O2⁻), distinct signals for DMPO-•O2⁻ and TEMP-1O2 adducts were observed (Figures 6a, 6b)46. The CsPbI3@COF-Cl system exhibited stronger intermediate signals than COF-Cl alone. Superoxide (•O2⁻), a key intermediate in H2O2 generation via oxygen reduction reactions (ORR), forms through O2 + e⁻ → •O2⁻ (electrode potential: −0.33 V vs. RHE, pH = 0) and subsequently reacts via •O2⁻ + e⁻ + 2H⁺ → H2O2. Singlet oxygen (1O2) is generated by •O2⁻ oxidation by photogenerated holes (h⁺): •O2⁻ + h⁺ → 1O2, a pathway consistent with prior reports on photocatalytic H2O2 production47. Furthermore, 1O2 contributes to H2O2 formation through a two-electron reduction: 1O2 + 2e⁻ + 2H⁺ → H2O2. Alternatively, H2O2 can form directly via a two-electron ORR: O2 + 2e⁻ + 2H⁺ → H2O2 (0.68 V vs. RHE, pH = 0). The significant EPR signals for both 1O2 and •O2⁻ observed in the CsPbI3@COF-Cl system demonstrate that H2O2 generation proceeds via both pathways, contributing to its enhanced photosynthetic efficiency. We also explored the contribution of the water oxidation reaction (WOR) to H2O2 production. Previous studies indicate that water can react with holes to form hydroxyl radicals (H2O + h⁺ → •OH + H⁺), which dimerize to produce H2O2 (2•OH → H2O2)22. This process has a standard electrode potential of 2.72 V vs. RHE (pH = 0). However, no •OH signals are detected in the CsPbI3@COF-Cl system, as their VB positions are more negative than the standard redox potential of H2O/•OH, rendering them thermodynamically incapable of oxidizing H2O to generate •OH radicals21. Alternatively, H2O2 can be generated directly from water and holes (2H2O + 2h⁺ → H2O2 + 2H⁺; E = 1.76 V vs. RHE, pH = 0), suggesting WOR also contributes to H2O2 formation. Isotopic labeling using H218O under anaerobic conditions confirmed the direct formation of H218O2 (Figure S72), providing definitive evidence that H2O2 originates from water oxidation, while MS analysis simultaneously verified the absence of competing O2 or H2 evolution (Figure S73).

To validate this hypothesis and explore alternative H2O2-production pathways, a series of control experiments were performed over CsPbI3@COF-Cl system. As shown in Figure 6c, p-benzoquinone (p-BQ), tryptophan (Trp), AgNO3, and ethanol (EtOH) were employed as scavengers for •O2⁻, 1O2, e-, and h+, respectively48. The addition of p-BQ drastically reduced the H2O2 yield, underscoring the critical role of •O2⁻ in H2O2 generation. Similarly, Trp moderately suppressed H2O2 production, confirming the participation of 1O2 in the formation process. Furthermore, while AgNO3 and EtOH also diminished H2O2 yields, a significant amount of H2O2 persisted, suggesting that both e- and h+ contribute to but are not solely responsible for H2O2 production. Moreover, controlled anaerobic/aerobic experiments quantitatively show that the water oxidation pathway contributes ~29.3% to the total H2O2 yield (Figure 6c), confirming its role as a significant secondary channel.

Supporting evidence for surface intermediates was obtained through in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under visible-light irradiation in the presence of H2O and O2 (Figure 6d). The appearance of a C-OH vibration peak (1077 cm-1) confirms the presence of adsorbed *OH species, likely formed by H2O dissociation49. Furthermore, characteristic vibration signals of *OOH (1647 and 1013 cm-1) were detected, demonstrating the activation of adsorbed O2 by photogenerated electrons (e⁻) to form *OOH intermediates50. Moreover, the signals of O-O stretching mode (890-921 cm-1) gradually increased51-53.

To gain theoretical insight into possible reaction pathways, density functional theory (DFT) calculations were performed to estimate the Gibbs free energy (ΔG) for the adsorption of intermediates (*O2, *OOH, *OH) at distinct sites on the CsPbI3@COF-Cl system during both the oxygen reduction reaction (ORR) and water oxidation reaction (WOR) pathways for photocatalytic H2O2 synthesis (Figure S74). These pathways involve intermediates and reaction steps. For the ORR process, O2 undergoes conversion to *O2 and *OOH, followed by *OOH protonation to yield H2O2. As visualized in Figure S74a, the model predicts that both *O2 and *OOH adsorb preferentially on CsPbI3 QDs. The computed energy barriers for these steps are -0.44 eV and -0.88 eV, respectively, suggesting a possible synergistic role of the S-scheme heterojunction in reducing the overall reaction energy barrier in silico. Conversely, the WOR pathway involves H2O dehydrogenation to *OH (the rate-determining step) followed by *OH dimerization to form H2O2. Figure S74b illustrates that, according to the calculation, *OH adsorbs exclusively on COF-Cl sites, with estimated energy barrier of 1.72 eV for the initial dehydrogenation step. These theoretical results are consistent with the interpretation that the CsPbI3@COF-Cl system may favor adsorptions of *OOH (ORR) and *OH (WOR) intermediates. Under visible light, photogenerated electrons and holes separate via the S-scheme heterojunction and migrate to CsPbI3 QDs and COF-Cl, respectively, enabling simultaneous ORR and WOR. If the computed pathway is operative, on CsPbI3 QDs, ORR proceeds through *O2 hydrogenation to *OOH and subsequent protonation to H2O2. Meanwhile, WOR on COF-Cl involves H2O dehydrogenation to *OH (ΔG = 1.72 eV) followed by *OH coupling. Critically, protons released during WOR are consumed in ORR, maintaining charge balance. Taken together, the computational models suggest that the S-scheme heterojunction in CsPbI3@COF-Cl could accelerate charge separation, potentially facilitating O2 reduction by electrons and H2O oxidation by holes to generate reactive intermediates, thereby contributing to efficient H2O2 production.

Methods
Materials and Chemicals. 2,5-Dichloroterephthalaldehyde (PDA-Cl, 95%), 2,5-dibromoterephthalaldehyde (PDA-Br, 95%), and 2,5-diiodoterephthalaldehyde (PDA-I, 95%) were obtained from Zhongke Yanshen Technology Co., Ltd. (Jilin, China). Lead(II) iodide (PbI2, 99.99%) were procured as perovskite-grade reagents from Tokyo Chemical Industry (TCI). Cesium iodide (CsI, 99.99%) was obtained from Aladdin Industrial Corporation. Nafion (5 wt%), Oleic acid (OA, 90%) and Toleylamine (OAm, 98%) were purchased from Sigma-Aldrich. Terephthalaldehyde (TPA, 98%), 4-Aminobenzonitrile (C7H6N2, 98%), trifluoromethanesulfonic acid (CF3SO3H, 98%), 2,2,6,6-tetramethylpiperidine (TEMP, 99%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97%), Heavy-oxygen water (H218O, 97 atom % 18O), N,N-diethyl-p-phenylenediamine sulfate (C10H18N2O4S, 98%) and potassium iodide (KI, 99.99%) were acquired from Macklin Biochemical Co., Ltd. Potassium iodide (KI, analytical grade) was supplied by Cologne Chemicals Co., Ltd. Potassium hydrogen phthalate (KHC8H4O4, 99.9%) was purchased from Tianjin Chemical Reagent Research Institute. N-butanol (n-BuOH), o-dichlorobenzene, and acetic acid were purchased from Energy Chemical, Beijing Innochem Technology, and Shanghai Bide Pharmaceutical Technology. The anhydrous potassium dihydrogen phosphate (KH2PO4), disodium ethylenediaminetetraacetate (Na2EDTA), anhydrous disodium hydrogen phosphate (Na2HPO4) and hydrogen peroxide (H2O2) were purchased from Chron Chemicals (Chengdu, China). All electrolytes were prepared fresh prior to each experiment and used immediately. All chemicals were used as received without any further purification.

Characterization. The sample morphology was examined using a field emission scanning electron microscope (FE-SEM; FEI Sirion 200). Transmission electron microscopy (TEM) analyses were conducted on an FEI Tecnai G2-F30 electron microscope at an accelerating voltage of 200 kV. Powder X-ray diffraction (PXRD) patterns were obtained using an AXS D8-Advanced diffractometer with Cu-Kα radiation. Nitrogen adsorption-desorption measurements were performed on a fully automated surface area and porosity analyzer (ASAP 2020M); the specific surface area was calculated based on the Brunauer-Emmett-Teller (BET) model, and the pore size distribution was evaluated using the non-local density functional theory (NLDFT) model. X-ray photoelectron spectroscopy (XPS), valence band XPS (VB XPS), and ultraviolet photoelectron spectroscopy (UPS) measurements were carried out using a PHI5702 multifunctional spectrometer with Al-Kα radiation. The light source used for the in-situ XPS measurements was a 300 W xenon lamp (HSX-UV300, Beijing NBeT Technology Co., Ltd.), identical to the illumination source employed in our photocatalytic H2O2 synthesis experiments. Fourier-transform infrared (FTIR) spectra at room temperature were recorded on a Nicolet FT-170SX spectrometer. Thermogravimetric (TG) analysis was performed using a TGA/DSC3+/Sartorius/MCA125S-2CCNE-1 thermogravimetric/differential thermal analyzer. Contact angle measurements were conducted using an SZ-CAMB3 instrument. Ultraviolet-visible (UV-vis) spectroscopy was carried out using a spectrophotometer (Thermo Fisher Scientific Genesys 10S UV-vis) with a 1 cm path length cuvette. Photoluminescence (PL) spectra and time-resolved fluorescence decay spectra (TRFDS) were collected using an Edinburgh Instruments FLS920 steady-state fluorescence spectrometer and a transient fluorescence spectrometer, respectively. Solid-state UV-Vis diffuse reflectance spectra (DRS) were measured using a UV-2006 ultraviolet spectrophotometer. The light source used in our experiments was a 300 W xenon lamp (HSX-UV300) from Beijing NBeT Technology Co., Ltd. Electron paramagnetic resonance (EPR) spectroscopic measurements were performed at room temperature using a CIQTEK EPR100 spectrometer. The ultrafast transient absorption (fs-TA) measurements were conducted using a laser system consisting of a Coherent Legend Elite regenerative amplifier (1 kHz, 800 nm), seeded by a Coherent Chameleon oscillator (120 fs, 80 MHz). The pump pulses included the fundamental 800 nm output from the amplifier and its 400 nm second harmonic generated via a BBO crystal. Additional pump wavelengths across 290-2600 nm were provided by a Light Conversion Oper A-Solo optical parametric amplifier. A broadband probe beam spanning from UV to NIR (350-1600 nm) was employed.The Kelvin probe forcemicroscopy (KPFM) tests were conducted on the BrukerDimension Icon, using a 300 W xenon light source with awavelength of 420nm. Inductively coupled plasma optical emission spectrometry (ICP-OES) was employed to quantitatively determine the concentrations of metal ions. Products were monitored in real time using mass spectrometry (MS) (Omnistar, PFEIFFER). The photoelectrochemical properties of the samples were tested using a CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, China). Photocatalytic tests were performed using a xenon lamp (HSXF/UV 300) equipped with 400-800 nm filters. Proton nuclear magnetic resonance (¹H NMR and 13C NMR) spectra were acquired on a JEOL ESC 400 M instrument using tetramethylsilane (TMS) as the internal standard. Time-resolved spectra were fitted with a triexponential function, and the average lifetime was calculated using the following formula. Standard silica particles dispersed in water were used as the reference.

t t t
A(t) = A1 exp(- ) + A2 exp(- ) + A3 exp(- ) (1)
t1 t2 t3

τ = A1 τ1^2 + A2 τ2^2 + A3 τ3^2 (2)
av A1 τ1 + A2 τ2 + A3 τ3

A'1 = A1 τ1 (3)
A1 τ1 + A2 τ2 + A3 τ3

A'2 = A2 τ2 (4)
A1 τ1 + A2 τ2 + A3 τ3

A'3 = A3 τ3 (5)
A1 τ1 + A2 τ2 + A3 τ3

Synthesis of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT). A solution was prepared by adding 2.0 g (17.0 mmol) of 4-aminobenzonitrile to a 50 mL round-bottom flask and cooling it in an ice bath at 0 °C for 30 minutes. Under a nitrogen atmosphere, 5.0 mL (55.5 mmol) of trifluoromethanesulfonic acid was slowly added dropwise, maintaining the temperature at 0 °C throughout the addition. Upon completion, the reaction mixture was poured into 50 mL of deionized water. The mixture was then neutralized to pH 7 using a 2 mol·L-1 NaOH solution, resulting in the formation of a pale yellow precipitate. This precipitate was collected by filtration and purified through multiple washes with distilled water to afford the TAPT product.

Synthesis of COF-H Framework. The COF-H frameworks were synthesized by dissolving TAPT (0.16 mmol, 56.7 mg) and TPA (0.24 mmol, 32.2 mg) in a pressure-resistant glass tube containing a 1:1 (v/v) mixture of o-dichlorobenzene and n-butanol (1.0 mL each). The mixture was sonicated for 5 minutes to enhance dissolution. Subsequently, 0.2 mL of 6 mol·L-1 acetic acid solution was added quickly, followed by brief sonication 10 seconds. The glass tube was then subjected to three freeze-pump-thaw cycles under liquid nitrogen and vacuum, sealed, and heated in an oil bath at 120 °C for 72 hours. After cooling to room temperature(~ 25 ℃), the resulting yellow product was collected by centrifugation, thoroughly washed with tetrahydrofuran and methanol, and purified by Soxhlet extraction to yield the COF-H framework.

Synthesis of COF-X (X = Cl, Br, I) Frameworks. The COF-X frameworks were synthesized by dissolving TAPT (0.16 mmol, 56.7 mg) and the appropriate dialdehyde monomer (0.24 mmol) in a pressure-resistant glass tube containing a 1:1 (v/v) mixture of o-dichlorobenzene and n-butanol (1.0 mL each). The dialdehyde monomers used were: 2,5-dichloroterephthalaldehyde (PDA-Cl, 48.7 mg) for COF-Cl, 2,5-dibromoterephthalaldehyde (PDA-Br, 70.1 mg) for COF-Br, and 2,5-diiodoterephthalaldehyde (PDA-I, 92.6 mg) for COF-I. The mixture was sonicated for 5 minutes to enhance dissolution. Subsequently, 0.2 mL of 6 mol·L-1 acetic acid solution was added quickly, followed by brief sonication (10 seconds). The glass tube was then subjected to three freeze-pump-thaw cycles under liquid nitrogen and vacuum, sealed, and heated in an oil bath at 120 °C for 72 hours. After cooling to room temperature (~ 25 ℃), the resulting yellow product was collected by centrifugation, thoroughly washed with tetrahydrofuran and methanol, and purified by Soxhlet extraction to yield the COF-X framework (The atomic coordinates of COF-X are listed in Table S11, S12 and S13).

Preparation of CsPbI3@COF-X Artificial Photosynthetic Systems. For each CsPbI3@COF-X system, 50 mg of COF-X (X = Cl, Br, I) powder was immersed in 2.5 mL of a 0.04 mol·L-1 PbI2 solution in N,N-dimethylformamide (DMF). The suspension was sonicated for uniform dispersion and reacted at 70 °C for 2 hours. The resulting solid, designated as PbI2@COF-X, was collected and washed repeatedly with DMF and ethanol to remove surface-adsorbed PbI2. This intermediate was then immersed in 2.5 mL of a 0.02 mol·L-1 CsI solution in DMF for 1 hour. Following this, 10 μL of oleic acid (OA) and 5 μL of oleylamine (OM) were added to stabilize the precursor mixture. Toluene was then added to induce the formation of CsPbI3 quantum dots (QDs) within the pores of the COF-X framework. The final CsPbI3@COF-X material was collected by centrifugation, washed thoroughly with toluene three times, and dried.

Photochemical H2O2 Production Test under Simulated Sunlight. The photocatalytic performance for H2O2 production was evaluated using a 5 mg sample of the CsPbI3@COF-X artificial photosynthetic system dispersed in a double-layered reaction beaker containing 20 mL of either deionized water or seawater (0.25 g·L-1). The reaction mixture was gently shaken to ensure uniform dispersion of the photocatalyst on the water surface. A thermostatic circulating water system maintained the reaction temperature at 25 °C. Photoreactions were conducted in air using a quartz and Pyrex glass hybrid reaction cell (Figure S75). Irradiation was provided by a 300 W xenon lamp. Throughout the reaction, the temperature was maintained at 25 °C using a stirrer and the circulating water system. At 10-minute intervals, a 1.5 mL aliquot of the reaction solution was extracted, centrifuged, and filtered through a 0.22 μm membrane to remove the photocatalyst. The hydrogen peroxide concentration in the filtrate was quantified using the iodometric method.

Catalyst Recovery Procedure for Cyclic Stability Tests. The highly hydrophobic CsPbI3@COF-Cl composite spontaneously forms a continuous film at the air-water interface during photocatalytic operation. For catalyst recovery in the cyclic stability tests, a sequential separation protocol was employed to ensure near-quantitative retrieval. First, the majority of the floating catalyst was gently collected from the water surface using a fine-mesh stainless steel scoop. Subsequently, the remaining reaction mixture was passed through a polyvinylidene fluoride (PVDF) microporous membrane (pore size: 0.22 μm) under mild vacuum to capture any residual catalyst particles. The recovered catalyst was then rinsed lightly with deionized water and dried at 60 °C under vacuum for 2 hours before being used in the next catalytic cycle. This combined approach achieves efficient catalyst recovery without the need for high-speed centrifugation, underscoring a practical advantage of the interfacial reaction system.

H2O2 Detection. The hydrogen peroxide (H2O2) content was determined using the iodometric method[54]. Briefly, 1 mL of potassium iodide solution (0.4 mol·L-1) and 1 mL of potassium hydrogen phthalate solution (0.1 mol·L-1) were mixed, followed by the addition of 0.5 mL of the centrifuged reaction supernatant. The mixture was allowed to stand for 30 min to ensure the complete reaction of H2O2 with I⁻ under acidic conditions, generating I3⁻ (H2O2 + 3I⁻ + 2H+ → I3⁻ + 2H2O). The absorbance of I3⁻ was then measured at 350 nm using UV-visible spectroscopy, and the H2O2 concentration was quantified based on a standard curve.

Total Chlorine Detection. The total chlorine content was determined using the N,N-diethyl-p-phenylenediamine (DPD) spectrophotometric method. Specifically, 2.00 mL of the sample was mixed with 0.1 mL of phosphate buffer (pH ~5.8) and 0.1 mL of DPD indicator solution. Then, 0.2 mL of potassium iodide solution (100 g·L-1) was added. The mixture was allowed to stand for 2 min to ensure complete oxidation of iodide by chlorine species to iodine, which subsequently reacted with DPD to form a red-colored complex. The absorbance of the complex was measured at 515 nm using a UV-Vis spectrophotometer. The total chlorine concentration was quantified based on a standard curve prepared with chlorine standards.

Apparent quantum yield (AQY) measurement. The AQY of H2O2 over CsPbI3@COF-Cl was measured under irradiation through different wavelength bandpass filters (420, 450, 500, 550 and 650 nm). All measurements were conducted in ambient air at standard atmospheric pressure. The photon flux of incident light was measured by a NBT 300UV photo radiometer. The AQY was calculated according to the following equation (6)[55]:

AQY (%) = Nh / Np * 100% = 2 * NA * M * h * c / (P * S * λ * t) * 100% (6)

Where N is the amount of reaction holes, N is the number of incident photons, N is the Avogadro constant (6.022 × 1023/mol), M is the molar amount of H2O2 (mol), h is the Planck constant (6.626 × 10-34 J s), c is the speed of light (3 × 108 m/s), P is the intensity of the irradiation (W m-2), S is the irradiation area (m2), λ is the irradiation wavelength (m), and t is the irradiation time (s). The amounts of H2O2 produced and the light intensity of monochromatic light illumination are tabulated in Table S9.

Solar-to-chemical conversion. The solar-to-chemical conversion (SCC) efficiency was determined using a 300 W xenon lamp was used as light source and calculated according to the following equation [55]:

SCC efficiency (%) = [ΔG for H2O2 generation (J mol-1)] * [H2O2 formed (mol)] / [Total input power (W)] * [Reaction times (s)] * 100% (7)

The free energy for H2O2 formation is 117 kJ mol−1.

Photoelectrochemical and Electrochemical Characterization. Photoelectrochemical and electrochemical properties were characterized using a standard three-electrode system with an Ag/AgCl reference electrode, a platinum sheet counter electrode, and an FTO conductive glass working electrode coated with the catalyst. The working electrode was prepared by dispersing 5 mg of the photocatalytic material in 0.5 mL of an ethanol/isopropanol mixture (3:1 v/v), sonicating for 30 minutes, adding 20 μL of Nafion solution, and sonicating for an additional 30 minutes. A 50 μL aliquot of the resulting catalyst slurry was uniformly coated onto the FTO glass and dried at 60 °C for 2 hours. A 300 W xenon lamp with a 400 nm cutoff filter served as the light source. Transient Photocurrent Response: Measured in 0.1 mol·L-1 Na2SO4 electrolyte under light illumination. Mott-Schottky (MS) Analysis: Conducted in 0.1 mol·L-1 Na2SO4 electrolyte at frequencies of 500 Hz, 1000 Hz, and 1500 Hz, scanning the potential from -1.6 V to +1.6 V vs. Ag/AgCl. Electrochemical Impedance Spectroscopy (EIS): Performed in 0.1 mol·L-1 Na2SO4 electrolyte over a frequency range of 106 to 103 Hz.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have developed a COF@perovskite dot hybrid for photocatalytic application. Certain issues need to be\naddressed:\nFigure S1 is not mentioned in the manuscript…\nIn manuscript, the authors state that COF synthesis was in DCB/BuOH, yet, in Figure S2, it is shown Dioxane/BuOH. Why\nthis inconsistency?\nWhy this large difference in surface area between chlorine- and iodine-modified COFs?\nRegarding Fig. S18, the authors refer to “clean surfaces”. What they mean by that?\nThe pore size of chlorine-COF was measured 3.09 nm and the dots are mentioned to have a mean diameter of 3.2 nm. The\nauthors state: “…shows CsPbI3 QDs uniformly dispersed in the mesopores of COF-Cl matrix.” Do the dots really insert into\nthe ring structure or they are just adsorbed inbetween the ribbon structures. For me, Scheme 1 illustrations do not provide\nthe exact configuration of components. Please explain.\nThe survey scans of neat perovskite dots contains an intense C1s peak (Fig. S21). Why is that? Could the presence of\ncarbon impurity play some vital role to the studied properties?\nXPS analysis (Pb 4f) of COF@PbI2 gives similar shift of the binding energy peak as that of COF@CsPbI3? (compare with\nFig. 2a). Please add xps deconvoluted spectrum of Pb 4f of the aforementioned intermediate adduct. In Fig. 2a, in which\nspecies the authors assign each deconvoluted peak (blue and red curve)? Add discussion in manuscript.\nIn Fig. 2b, in which species the authors assign each deconvoluted peak (blue and red curve)?\nPlease add the full name of TRFDS in the manuscript.\nPlease add details about the light source for in situ XPS measurements.\nPlease acknowledge if Figures 4a,b are correct, regarding the coloration of each sample. COF is red-colored in one fig. and\nblue-colored in the other.\nThe works reported in Table S4 and Fig 4g are all triphase systems? Please make clear for each case. Ideally, all reported\ncases should be under similar conditions.", "reviewer_2": "The manuscript by Meng et al. presents a highly innovative approach to stabilizing CsPbI₃ quantum dots within a chlorine-\nfunctionalized covalent organic framework (COF-Cl) for photocatalytic H₂O₂ production. The concept of \"multisite atomic-\nchlorine passivation\" directly targets the perennial challenge of perovskite instability in water, and the reported performance\nmetrics—including remarkable production rates and stability in seawater—are impressive. The work is comprehensive and\nhas the potential to be a significant contribution to the field. However, to firmly establish the proposed mechanisms and meet\nthe high evidentiary standards of Nature Communications, the following critical points require addressing through additional\nexperiments and clarifications.\n1. The proposed S-scheme charge transfer is central to explaining the system's high performance, but the current evidence\n(UPS, XPS shifts) remains indirect and circumstantial. A Type-II heterojunction could similarly explain the observed charge\nseparation without invoking the unique charge recombination pathway of an S-scheme. Femtosecond transient absorption\n(fs-TA) spectroscopy is essential to directly probe the ultrafast interfacial charge transfer. The decisive evidence would be\nthe simultaneous quenching of the electron signal in the COF-Cl and the hole signal in the CsPbI₃ QDs, confirming the\nproposed charge recombination pathway. Kelvin probe force microscopy (KPFM) could provide direct spatial mapping of the\nbuilt-in electric field at the heterojunction interface, offering solid evidence for the band bending required for the S-scheme.\n2. While the synthesis of COF-Br and COF-I controls is commendable, the data do not conclusively prove that the superior\nperformance of COF-Cl stems from its unique chemical passivation rather than a general halogen or hydrophobic effect. A\ncomparative analysis of XPS peak shifts (for Pb, I, and the halogens) and time-resolved photoluminescence (TRPL) \nlifetimes\nfor the CsPbI₃@COF-Br and CsPbI₃@COF-I\ncomposites must be provided. This will directly test the correlation between\nspecific halogen interactions (Cl–Pb coordination, Cl–I bonding) and enhanced stability/charge separation.\n3. The scavenger experiments in Figure 5c qualitatively suggest both the oxygen reduction reaction (ORR) and water\noxidation reaction (WOR) pathways are active. However, a quantitative deconvribution of their contributions is lacking.\nPerform photocatalytic H₂O₂ production under strictly anaerobic conditions. The yield under these conditions directly\nquantifies the contribution from the WOR pathway. Comparison with the yield in aerated/oxygenated conditions will allow for\na quantitative assessment of each pathway's role. To provide incontrovertible evidence for direct water oxidation, an isotopic\nlabeling experiment using H₂¹⁸O is strongly recommended, with mass spectrometry detection of ¹⁸O-labeled H₂O₂ or O₂.\n4. For a system touted for seawater operation and practical application, the potential leaching of toxic Pb²⁺ is a critical\nconcern that must be quantified. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the reaction solution\nafter the long-term (20 h) stability test is necessary to report the leaching concentrations of Pb²⁺ and Cs⁺.\n5. The proposal of a gas–liquid–solid triphase interface is plausible but lacks direct visual evidence. Contact angle\nmeasurements alone are insufficient. Cryo-SEM or other suitable techniques could be employed to directly visualize the\nspatial distribution of the solid catalyst, liquid water, and gas phases within the porous structure.\n6. The performance comparison in Figure 4g is valuable, but the conditions of the benchmarked systems must be clarified.\nThe caption or main text must explicitly state whether all compared catalysts were tested without sacrificial agents.\nFurthermore, the discussion should more prominently highlight the system's leading solar-to-chemical conversion (SCC)\nefficiency, as this is a more comprehensive metric for practical potential than production rate alone.\n7. The AQY of 15.76% at 450 nm is excellent, but its reporting can be improved. The atmospheric conditions (O₂ partial\npressure) during the AQY measurement must be specified, as this critically influences the ORR pathway. Providing the AQY\naction spectrum at multiple wavelengths would be highly valuable to correlate the photocatalytic activity with the absorption\nprofiles of the individual components, further supporting the synergistic operation of the heterojunction.\n8. The manuscript should briefly describe the method for recovering the floating powder catalyst for the cyclic stability tests,\nas this has practical implications for real-world application.", "reviewer_3": "In this work, Meng et al. constructed CsPbI3 quantum dots (QDs) within a hydrophobic chlorine-functionalized covalent\norganic framework (COF-Cl) through multisite atomic-chlorine passivation route. Within the QDs@COF-Cl catalyst, a gas-\nliquid-solid triphase interface was created with enhanced O2 diffusion and suppressed ionic migration to concurrently drive\ntwo-electron oxygen reduction and water oxidation for artificial photosynthesis H2O2 in seawater. There are several\nquestions that require further clarifications as follows. I recommend acceptance after major revisions.\n1. The prevailing SEM and TEM images cannot support the growth of QD inside the pores of COF. High-quality HRTEM\nimages should be provided to support this key conclusion.\n2. The QD loading amount should be quantified.\n3. The pore size distribution curves of QDs@COF-Cl and COF-Cl should be supplemented.\n4. The underlying reason of enhanced performance by chlorine modification against other halogens is unclear.\n5. The mechanism of \"continuous multi-site passivation\" route is unclear and should be further elaborated.\n6. Is H2O2 the only product of WOR? Provide experimental evidence.\n7. In seawater, whether the chlorine evolution reaction occurs as the side reaction should be studied.\n8. The promising seawater performance would benefit from a deeper discussion of its practical implications. To strengthen\nthe work, it is important to clarify the origin of the reduced yield in seawater compared to pure water, and to evaluate the\npotential competing effects of chloride ions on the atomic passivation stability. Besides, the impact of co-existing cations\nsuch as Mg²⁺ and Ca²⁺ should also be explored.\n9. The XPS spectra of physically mixed sample should be presented to support the specific Cl–Pb/Cl–I chemical interaction\nbetween COF-Cl and QD.\n10. A pristine COF without halogen modification should also be used to load QD for comparison, aiming to demonstrate the\nsuperiorities of COF-Cl.\n11. The Ef position of CsPbI₃ QDs and COF-Cl should be clearly indicated, as it is critical for understanding the initial band\nalignment and the subsequent formation of the S-scheme heterojunction.\n12. The evidence of S-scheme charge transfer process is insufficient. Please provide more characterization, such as Kelvin\nProbe Force Microscopy (KPFM) and in-situ XPS.\n13. The XPS of CsPbI3@COF-Cl after photocatalytic reaction should be supplemented for assessing the cycling stability.\n14. The H2O2 production rate is impressive (high up to 25.29 mmol h-1 g-1) while the solar-to-chemical conversion (SCC)\nefficiency is only 1.38%. The key factors limiting the SCC efficiency should be discussed.\n15. The format of references should be unified. Several entries (e.g., Refs. 32, 39, 40) are incomplete, lacking volume\nnumbers, page ranges, or full DOI identifiers."}, "author_response": "Dear Reviewers:\nWe are grateful to the reviewers for their insightful comments on our manuscript\nentitled “Multisite atomic-chlorine-passivation stabilizes perovskite interfaces for\nefficient H O photosynthesis from seawater” (Full Paper, NCOMMS-25-70275-T).\n2 2\nWe have now completed the invited revision, which includes significant additional\nexperimental data. These comments are all valuable and very helpful for revising and\nimproving our paper. We have carefully addressed each one in the revision (Blue italic:\nreviewer’s remarks; Black type: Our response), which we hope meets with approval.\nAll changes have been marked in red in the manuscript, and our detailed responses to\neach comment are provided as follows.\nReviewer #1 (Remarks to the Author):\nThe authors have developed a COF@perovskite dot hybrid for photocatalytic\napplication. Certain issues need to be addressed.\nResponse: We sincerely thank the reviewer for their thorough evaluation of our\nmanuscript and for the constructive feedback provided. We appreciate the positive\nrecognition of our work on the CsPbI @COF-Cl system for photocatalysis and are\n3\ngrateful for the opportunity to address the issues raised. In response to the reviewer’s\ncomments, we have carefully revised the manuscript to clarify and strengthen the\nrelevant aspects. The specific modifications made are detailed point-by-point in the\nfollowing sections.\n1. Figure S1 is not mentioned in the manuscript…\nResponse: We sincerely thank the reviewer for this observation. We have now\nexplicitly cited Figure S55 (originally in Figure S1) in both the Supplementary\nInformation and the main manuscript, and have re-written this part as shown below.\n“Photocatalytic H O generation was evaluated in both pure water and seawater under\n2 2\nvisible-light irradiation and ambient air, without sacrificial agents, employing the\niodometric method for quantitative determination (Figure S55).…”\n2. In manuscript, the authors state that COF synthesis was in DCB/BuOH, yet, in Figure\nS2, it is shown Dioxane/BuOH. Why this inconsistency?\nResponse: We sincerely thank the reviewer for their careful observation. The\ninconsistency raised is indeed due to an error in the original version of Figure S1\n(originally in Figure S2), for which we apologize. We have now corrected Figure S1 to\naccurately reflect that the synthesis was performed in the DCB/BuOH solvent system,\nas stated in the manuscript. The corrected version of Figure S1 is provided below:\nFigure S1. Schematic illustration of synthesis of COF-X matrixes.\n3. Why this large difference in surface area between chlorine- and iodine-modified\nCOFs?\nResponse: We appreciate the reviewer’s insightful question regarding the significant\nvariation in surface area among our halogen-functionalized COFs. This observed trend,\nwhere the BET surface area decreases substantially from COF-Cl (2289.7 m2 g-1) to\nCOF-Br (1757.0 m2 g-1) and further to COF-I (1150.9 m2 g-1), is a key finding of our\nstudy. We believe this can be attributed to a combination of intrinsic atomic properties\nand their consequential impact on the material’s microstructure.\nThe primary contributing factor is the inherent physical difference between chlorine,\nbromine, and iodine atoms. As the atomic mass and van der Waals radius increase from\nCl to I, two direct effects emerge. Firstly, the greater mass of iodine increases the overall\ndensity of COF-I. Since surface area is normalized per unit mass (m2 g-1), a heavier\nframework naturally results in a lower specific surface area.\nThis “mass effect” is a recognized phenomenon in porous materials. For instance,\na related study noted that a hydrogenated COF exhibited a higher surface area than its\nfluorinated counterpart, partly due to the significant mass difference between fluorine\nand hydrogen (Figure R1 and R2, Han et al., Angew. Chem. Int. Ed. 64, e202517545\n(2025).). A parallel trend of decreasing surface area with increasing halogen mass was\nexplicitly reported for a TpPaX (X = F, Cl, Br) series (Figure R3, Wang et. al. Chem.\nSci. 16, 12906-12916 (2025).). Secondly, the larger size of the iodine atom can lead to\nframework dilation or subtle pore blockage, which is reflected in our measured pore\nvolumes: COF-Cl (1.77 cm3 g-1), COF-Br (1.53 cm3 g-1), and COF-I (0.76 cm3 g-1). A\nreduced pore volume directly limits the internal surface area available for gas\nadsorption.\nBeyond these inherent properties, the increase in atomic (van der Waals) radius\nfrom Cl to I plays a decisive role by impairing the framework’s crystallinity and\nstacking efficiency. As shown in our XRD data (Figure S2), the diffraction peaks\nprogressively broaden and lose intensity from COF-Cl to COF-I, indicating a reduction\nin long-range structural order. We propose that the larger iodine atom introduces greater\nsteric hindrance at the reaction sites. This steric bulk can kinetically hinder the dynamic,\nreversible bond-forming processes that are essential for error correction and the growth\nof a highly ordered crystalline framework. Consequently, the synthesis of COF-I likely\nproceeds with more structural defects, lattice distortions, and potentially a higher\nfraction of amorphous content. A lower degree of crystallinity invariably leads to fewer\nwell-defined, accessible micropores, thereby causing a sharp decline in the effective\nsurface area. This rationale is well-supported in the literature; for example, in the study\nby Wang et al. (Figure R3, Chem. Sci. 16, 12906-12916 (2025)), the poorest\ncrystallinity (indicated by very weak and broad XRD peaks) in a TpPaH COF was\ndirectly correlated with its lowest surface area within a comparative series.\nIn summary, the large difference in surface area between COF-Cl and COF-I is not\ndue to a single cause but results from a synergistic combination of factors. The\nincreased atomic mass and size of iodine directly reduce the specific surface area and\npore volume. Simultaneously, these same properties impede the formation of a highly\ncrystalline structure during synthesis. The cumulative effect of these physical and\nstructural factors satisfactorily explains the observed trend. We hope this detailed\nexplanation addresses the reviewer’s query.\n[FIGURE REDACTED]\nFigure R1. The expeimental PXRD patterns of a) H-COF and b) F-COF are presented with\ncomparative refinement analysis. Each plot shows experimental profiles (blue and red line), Pawley-\nrefined patterns (black dotted curve), the Bragg positions (green bar), and the refinement differences\n(pink line). (cite from Angew. Chem. Int. Ed. 64, e202517545 (2025).)\n[FIGURE REDACTED]\nFigure R2. The N sorption isotherms and the pore size distributions (inset) of c) H-COF and d) F-\n[FIGURE REDACTED]\nFigure R3. a) Schematic illustration of the synthesis of TpPaX (X = F, Cl, Br, H). b) Lattice structure\nin top and side views (orange: carbon; blue: nitrogen; red: oxygen; yellow: chlorine) for the AA-\nstacking model of TpPaCl. c-f) Experimentally observed (ink blue) and Pawley refined (magenta)\nPXRD patterns with their refinement difference (gray), the reflections, and the simulated ones (blue)\nfor the eclipsed AA-stacking model. g) N sorption isotherms of TpPaX. (cite from Chem. Sci. 16,\n2\n12906-12916 (2025).)\nFigure S2. (a) Experimental (red dot) and simulated (blue line) PXRD patterns of the COF-Cl\nmatrix. (b) Experimental (red dot) and simulated (orange line) PXRD patterns of the COF-Br matrix.\n(c) Experimental (red dot) and simulated (red line) PXRD patterns of the COF-I matrix. Insets are\nthe modeled crystal structures.\n4. Regarding Fig. S18, the authors refer to “clean surfaces”. What they mean by that?\nResponse: We sincerely thank the reviewer for this question, which allows us to clarify\nthis point. The term “clean surfaces” in this context refers to the absence of observable\nfree-standing CsPbI QD aggregates or other particulate contaminants on the fibrous\n3\nstructures of the COF-Cl matrix, as revealed by the SEM image (Figure S20). The\nmicrograph predominantly shows the intrinsic fibrous morphology of the COF-Cl\nmatrix. This indicates that the CsPbI QDs are primarily successfully encapsulated\n3\nwithin the lumen of the COF-Cl matrix, rather than being physically adsorbed on the\nexternal surfaces in an uncontrolled manner. For example, Zhao et al. (Angew. Chem.\nInt. Ed. 59,23100-23106 (2020).) observed a clean surface of the AlPO-5 template,\nindicating that the MAPbBr NCs are embedded within it (Figure R4).\nthat the external surfaces of the COF-Cl matrix remained clean without observable\naggregates of CsPbI QDs, indicating their successful embedding within the matrix.”\n3\n[FIGURE REDACTED]\nFigure R4. d) SE-STEM image of an AlPO-5 zeolite filled with MAPbBr NCs. (cite from Angew.\n5. The pore size of chlorine-COF was measured 3.09 nm and the dots are mentioned to\nhave a mean diameter of 3.2 nm. The authors state: “…shows CsPbI QDs uniformly\n3\ndispersed in the mesopores of COF-Cl matrix.” Do the dots really insert into the ring\nstructure or they are just adsorbed inbetween the ribbon structures. For me, Scheme 1\nillustrations do not provide the exact configuration of components. Please explain.\nResponse: Thank you for this excellent and critical question regarding the precise\nlocation and configuration of the CsPbI QDs relative to the COF-Cl matrix. We\n3\nappreciate the opportunity to clarify this important structural point. Our\ncharacterization data collectively support a model where the CsPbI QDs are\n3\npredominantly confined within the accessible mesoporous channels of the crystalline\nCOF-Cl framework, rather than merely adsorbed on external surfaces or loosely\nsituated between stacked layers/ribbons. We base this conclusion on the following\nconvergent evidence:\nPore Size and QD Dimension Correlation: The pore size of the COF-Cl matrix,\nas estimated from NLDFT analysis of N adsorption data, is in the range of 2~4 nm\n2\n(Figure S25), with a peak around ~3.1 nm. This is consistent with the theoretically\nsimulated pore diameter range of 3.13~4.01 nm for our model structure (Figure S3 and\nTable S1). Statistical analysis of TEM images for the CsPbI @COF-Cl composite\n3\nshows that the CsPbI QDs have a size distribution of 2.4~4.0 nm, with a mean diameter\n3\nof ~3.2 nm (Figure 1b, e). The close correspondence between the QD size and the host's\npore dimensions is highly suggestive of size-constrained growth within the porous\nnetwork. It is important to note that the QDs are formed via in-situ nucleation and\ncrystallization inside the COF matrix during synthesis (described in Methods and\nFigure S19). Therefore, their final size is likely templated and limited by the confining\ngeometry of the mesopores, explaining the close match.\nDirect Textural Property Changes: The most compelling physical evidence\ncomes from the dramatic changes in the textural properties of the composite. After the\nin-situ formation of CsPbI QDs, the BET specific surface area of the material\n3\nplummets from 2289.7 m2 g-1 (pristine COF-Cl) to 1318.1 m2 g-1 (CsPbI @COF-Cl)\n3\n(Figure 1i). Simultaneously, the pore volume decreases from 1.77 cm3 g-1 to 1.05 cm3\ng-1 (Table S2). This substantial reduction in both the accessible surface area and the\ninternal pore volume is a classic signature of pore filling or occupancy (e.g., Yu et al. J.\nAm. Chem. Soc. 143, 14253-14260 (2021)). If the QDs were primarily located on the\nexternal surface or between loosely packed ribbons, we would expect a much smaller\ndecrease in the N -accessible surface area and pore volume.\ne) show the CsPbI QDs as uniformly dispersed dark contrasts embedded within the\n3\nlighter COF-Cl matrix, without significant aggregation on the exterior surfaces. The\nhomogeneous distribution of all elements (Cs, Pb, I, Cl) in EDX mapping (Figure 1f)\nfurther supports a well-integrated composite structure rather than a simple surface\nmixture.\nRegarding the reviewer’s insightful query about “insertion into the ring structure\nversus adsorption between ribbons”, we acknowledge that Scheme 1 is a simplified\nschematic intended to illustrate the general concept of confinement and interfacial\npassivation within a porous matrix, not an atomically precise depiction. Our\ninterpretation of “dispersion in the mesopores” refers to confinement within the\nperiodic, accessible nanochannels formed by the stacked 2D layers of COF-Cl. We\npropose that during the sequential synthesis, PbI precursors first infiltrate these\n2\nchannels, and subsequent reaction with CsI leads to the nucleation and growth of CsPbI\nQD diameter (~3.2 nm) compared to the maximum pore diameter (~4 nm) allows for\ntheir accommodation within the channels, likely in a templated or slightly confined\nmorphology. This model is consistent with all the observed data: uniform dispersion,\nsignificant textural property changes, and the formation of the intimate interfacial bonds\n(Cl-Pb and Cl-I) crucial for stability and charge transfer, as detailed in the main text.\nIn summary, while direct, atomically resolved imaging of an individual CsPbI QD\n3\nsitting within a single pore remains technically challenging, the convergence of\nmultiple lines of evidence, (1) the close size match between CsPbI QDs and host pores,\n3\n(2) the dramatic reduction in surface area and pore volume upon composite formation,\nand (3) the uniform internal dispersion observed via microscopy, provides a robust case\nfor effective confinement of the CsPbI QDs within the mesochannels of the COF-Cl\n3\nmatrix. We sincerely thank the reviewer for raising this point, which has allowed us to\nclarify and strengthen the discussion of our composite’s structure.\nRevised Sections in the Manuscript:\nWe have revised the manuscript to provide a clearer and more precise description\nof the location and configuration of the CsPbI QDs within the COF-Cl matrix, directly\n3\naddressing the reviewer’s insightful question. The key revisions are in the “Results”\nsection, under the subsection “Synthesis and characterization of hydrophobic\nhalogenated COFs and CsPbI @COF-X.”\n3\n1. Revised Paragraph:\n“Taking CsPbI @COF-Cl system as an example, we demonstrate how the framework\n3\nand topology of the COF-Cl matrix direct the confined growth of CsPbI QDs.\nremained clean without observable aggregates of CsPbI QDs, indicating their\n3\nsuccessful embedding within the matrix. TEM analysis (Figure 1a, 1b and S21) further\nshows CsPbI QDs uniformly dispersed within the mesoporous channels of the COF-\n3\n(Figure 1a), which closely matches the theoretically predicted mesopore diameter range\nof 3.13-4.01 Å (Figure S3b, Table S1). This close size correlation is highly suggestive\nof size-constrained growth within the porous network. High-resolution TEM (HRTEM,\nFigure 1c) displays distinct lattice fringes (0.32 nm spacing), matching the (200)\nplanes\nof α-CsPbI (space group Pm-3m). The crystalline structure is further confirmed by fast-\ncorroborates the uniform dispersion of 3.2 nm CsPbI QDs within the COF-Cl\n3\nframework. Elemental mapping (EDX, Figure 1f) confirms homogeneous distribution\nof Cl, Cs, Pb, and I throughout the composite.”\n2. Revised Paragraph (referencing Figure 1i and Table S2):\n“N adsorption-desorption isotherms (Figure 1i) and the corresponding pore size\n2\ndistribution curves (Figure S25) provide compelling evidence for the confinement of\nCsPbI QDs within the COF-Cl mesochannels. The pore size distribution of pristine\nthe CsPbI @COF-Cl composite, a clear reduction in the pore volume signal within this\n3\ncharacteristic range is observed. Most notably, the BET specific surface area plummets\nfrom 2289.7 m2 g-1 (COF-Cl) to 1318.1 m2 g-1, and the pore volume decreases from\n1.77 cm3 g-1 to 1.05 cm3 g-1 (Table S2). This substantial reduction in both accessible\nsurface area and internal pore volume is a direct signature of pore filling or occupancy\nby the in-situ grown CsPbI QDs. Combined with the close match between CsPbI QD\n3 3\nsize and host pore dimensions, this data robustly confirms that the CsPbI QD growth\n3\nis effectively confined within the accessible mesoporous channels of the crystalline\nCOF-Cl framework, rather than merely occurring on external surfaces or between\nloosely packed structures.”\nFigure S25. Pore size distributions of COF-Cl Matrix and CsPbI @COF-Cl system calculation\n3\nbased upon NLDFT. Source data are provided as a Source Data file.\nFigure S3. The topology network of the COF-Cl matrix.\nTable S1. The crystallographic parameters of COF-X (X = Cl, Br and I) matrixes.\nSample a (Å) b (Å) c (Å) N-N（Å） X-X（Å）\nCOF-Cl 37.135900 37.135900 3.670495 40.1283 31.2480\nCOF-Br 36.977100 36.977100 3.746085 39.9460 30.7207\nCOF-I 36.973900 36.973900 3.957944 39.9380 30.2872\nFigure 1. Structural and Physicochemical Characterizations of the CsPbI @COF-Cl System.\n3\n(a, b) TEM images showing CsPbI QDs (black dots) confined within the COF-Cl matrix. (c)\n3\ntransform (FFT) pattern corresponding to the CsPbI QDs, confirming their high crystallinity. (e)\n3\nconfined CsPbI QDs.\n3\n6. The survey scans of neat perovskite dots contains an intense C1s peak (Fig. S21).\nWhy is that? Could the presence of carbon impurity play some vital role to the studied\nproperties?\nResponse: We thank the reviewer for this important question. The intense C1s peak\nobserved in the XPS survey scan of the neat CsPbI QDs (Figure S26) is an inherent\n3\nfeature of their standard colloidal synthesis. As widely reported in the perovskite QD\nliterature (e.g., Kovalenko et al. Nano Lett. 15, 3692 (2015); Deng et al. ACS Nano 10,\n3648 (2016).), these CsPbI QDs are synthesized using long-chain organic ligands like\n3\noleic acid and oleylamine. The carbon signal primarily originates from these surface-\nbound ligands, which are essential for passivating surface defects, controlling growth,\nand ensuring colloidal stability in organic solvents during and after synthesis.\nHowever, these carbon-containing ligands do not play a vital role in the exceptional\nphotocatalytic properties or the aqueous stability reported for our CsPbI @COF-Cl\n3\nsystem. Their function is largely limited to the initial synthesis and isolation steps.\nCritically, the protection offered by these native ligands is completely insufficient for\naqueous applications, which is the central challenge our work addresses. To\ndemonstrate this, we performed a control experiment dispersing the ligand-capped neat\nCsPbI QDs in pure water. As shown in Figure S67, their photoluminescence (PL) was\n3\nquenched by >99% within two minutes, confirming rapid and catastrophic degradation.\nThis unequivocally proves that the original carbon-based ligand shell cannot confer the\nstability required for photocatalysis in water.\nTherefore, the observed carbon signal is irrelevant to the performance of our\ndesigned CsPbI @COF-Cl composite. The pivotal, vital role in achieving\n3\nunprecedented stability and activity is played exclusively by the COF-Cl matrix. It\nfunctions through a dual mechanism: (i) its intrinsic hydrophobicity creates a protective\nmicroenvironment that physically shields the CsPbI QDs from water, and (ii) more\n3\nimportantly, its atomic chlorine sites form strong chemical interactions (Cl-Pb\ncoordination and Cl-I halogen bonds) with the perovskite surface. This multisite\npassivation provides far superior stabilization compared to the weak, dynamic binding\nof organic ligands and is integral to forming the efficient S-scheme heterojunction.\nIn summary, while the neat CsPbI QDs require carbon-based ligands for colloidal\n3\nstability, these ligands are ineffective in water. The breakthrough of our work is\nreplacing this inadequate protection with a multifunctional COF-Cl host that provides\nrobust chemical passivation, hydrophobic confinement, and enhanced charge transfer.\nThis engineered interface is the true origin of the system's high performance and\nstability, not the residual carbon from the initial synthesis.\nFigure S67. Photoluminescence (PL) spectra of CsPbI QDs in water recorded at (a) 0 min, (b) 1\n3\nmin, and (c) 2 min. (d) Time-dependent PL intensity of CsPbI QDs in water over 2 min.\n3\n7. XPS analysis (Pb 4f) of COF@PbI gives similar shift of the binding energy peak as\n2\nthat of COF@CsPbI ? (Compare with Fig. 2a). Please add xps deconvoluted spectrum\n3\nof Pb 4f of the aforementioned intermediate adduct. In Fig. 2a, in which species the\nauthors assign each deconvoluted peak (blue and red curve)? Add discussion in\nmanuscript.\nResponse: We sincerely thank the reviewer for this insightful suggestion, which has\nallowed us to provide more robust and comparative evidence for our proposed\ninterfacial interaction mechanism. Following this advice, we synthesized the\nintermediate PbI @COF-Cl composite and performed a detailed XPS analysis.\na highly informative parallel between the PbI @COF-Cl and final CsPbI @COF-Cl\n2 3\nsystems. For pristine PbI , the Pb 4f spectrum shows a single doublet (Pb 4f : 143.21\n2 5/2\neV; Pb 4f : 138.34 eV). Upon formation of the PbI @COF-Cl composite, this\n7/2 2\nspectrum resolves into two distinct doublets (Figure S30a): a higher binding energy\ncomponent (Pb 4f : 145.92 eV; Pb 4f : 141.05 eV) and a lower one (Pb 4f : 144.15\n5/2 7/2 5/2\neV; Pb 4f : 139.28 eV). The higher component exhibits a substantial positive shift (Δ\n7/2\n≈ +2.71 eV) relative to pristine PbI .\nCsPbI @COF-Cl system (Figure 2a). In CsPbI @COF-Cl, the Pb 4f spectrum also\n3 3\ndeconvolutes into two doublets, with the higher binding energy component showing a\nclear positive shift (Δ ≈ +1.17 eV for the Pb 4f peak) compared to pristine CsPbI\n7/2 3\nQDs. This parallel confirms that the strong chemical interaction between the COF-Cl\nhost and lead species is a general feature, driven by the specific affinity of the\nframework's Cl atoms for under-coordinated Pb2+ ions, and is established early in the\nsynthesis pathway.\nFurthermore, this interaction is corroborated by complementary shifts in the I 3d\nand Cl 2p spectra for both composites. In PbI @COF-Cl, the I 3d peak splits and shifts\n2\npositively (Figure S30b), while the Cl 2p peak shifts to lower binding energy (Δ = -0.37\neV, Figure S30c). An identical trend, positive shift for I, negative shift for Cl, is\nobserved in CsPbI @COF-Cl (Figures 2b and 2c). This consistent, multi-element\n3\nsignature provides direct spectroscopic evidence for the concurrent formation of Cl-Pb\ncoordination and Cl-I halogen bonds at the interface.\nRegarding Peak Assignment in Figure 2a: We have clarified this assignment in\nthe revised manuscript. “In the deconvoluted Pb 4f spectrum of CsPbI @COF-Cl\n3\n(Figure 2a and Table S3), the higher-binding-energy doublet (orange curves) is\nassigned to surface Pb2+ ions engaged in direct Cl-Pb coordination with the COF-Cl\nmatrix, while the lower-binding-energy doublet (green curves) corresponds to bulk Pb2+\nions within the CsPbI QD core, which are shielded from direct interfacial contact.”\n2 2\nCl composite.\nTable S6. High-resolution XPS spectra of COF-Cl, PbI and the PbI @COF-Cl composite.\n2 2\nSample Pb 4f Pb 4f I 3d I 3d Cl 2p Cl 2p\n5/2 7/2 3/2 5/2 1/2 3/2\nCOF-Cl / / / / 202.43 200.83\nPbI 143.21 138.34 630.61 619.11 / /\n2\n145.92 141.05 632.48 620.98\nPbI @COF-Cl 202.06 200.46\n2\n144.15 139.28 631.46 619.96\n8. In Fig. 2b, in which species the authors assign each deconvoluted peak (blue and red\ncurve)?\nResponse: We thank the reviewer for raising this point for clarification. We\nacknowledge that the assignment of the deconvoluted peaks in Figure 2b (I 3d spectra)\nwas not explicitly stated in the original manuscript and have revised the text accordingly.\nAs described in our response to Question #7 and now detailed in the manuscript, the\ndeconvolution of the I 3d spectrum in Figure 2b follows the same logic as for Pb 4f:\n“Similarly, the I 3d spectra (Figure 2b) show a higher-binding-energy doublet (orange\ncurves) arising from surface I⁻ ions directly involved in Cl-I halogen bonding with\nCOF-Cl, whereas the lower-binding-energy doublet (green curves) originates from bulk\nI⁻ ions in the CsPbI QD interior.”\n3\n9. Please add the full name of TRFDS in the manuscript.\nResponse: We thank the reviewer for this comment. The full name of TRFDS was\nactually provided in the first sentence of the paragraph immediately following Figure\n1, which states: “…To evaluate the interfacial interaction between COF-Cl and CsPbI\nmagnetic resonance (NMR) spectroscopy, and time-resolved fluorescence decay\nspectroscopy (TRFDS) to analyze the electronic structure of constituent elements.…”\n10. Please add details about the light source for in situ XPS measurements.\nResponse: We appreciate this comment regarding the light source details. We thank the\nreviewer for this comment. The light source used for the in situ XPS measurements in\nthis study was a 300 W xenon lamp (HSX-UV300, Beijing NBeT Technology Co., Ltd.),\nidentical to the illumination source employed in our photocatalytic H O synthesis\n2 2\nexperiments. This configuration ensures that the electronic structure of the material is\nprobed under illumination conditions directly relevant to the photocatalytic process.\nThe full details of the light source configuration have been added to the Supplementary\nMethods of the revised manuscript.\n11. Please acknowledge if Figures 4a,b are correct, regarding the coloration of each\nsample. COF is red-colored in one fig. and blue-colored in the other.\nResponse: We sincerely thank the reviewer for pointing out the color inconsistency\nbetween Figures 4a and 4b. We apologize for this graphical oversight. We have now\ncorrected Figure 4a in the revised manuscript to maintain a consistent color scheme for\nall components across both figures. The revised figure 4 is shown below:\nFigure 4. Photocatalytic H O production over COF-Cl and CsPbI @COF-Cl. (a) Kinetic\n2 2 3\ncurves of H O generation in pure water under air. (b) Kinetic curves of H O generation in seawater\n2 2 2 2\nunder air.\n12. The works reported in Table S4 and Fig 4g are all triphase systems? Please make\nclear for each case. Ideally, all reported cases should be under similar conditions.\nResponse: We sincerely thank the reviewer for this important point regarding the\nperformance comparison in our manuscript. We fully agree that clarity in benchmarking\nis crucial. In the revised version, we have explicitly clarified the reaction system for\neach cited work in Figure 4g and the accompanying table (now Table S9). We\nacknowledge that a direct comparison under identical conditions is ideal. However, the\nfield of sacrificial-agent-free photocatalytic H O production specifically in triphase\n2 2\nsystems remains in its early stages, with a limited number of reported studies available\nfor a head-to-head comparison.\nTo provide the most comprehensive and meaningful performance context, our\ncomparative analysis is structured in two specific dimensions to rigorously highlight\nthe advantages of our triphase system. First, we distinguish the reaction medium by\ncomparing performance in both pure water and seawater environments under\nsacrificial-agent-free conditions. This distinction is critical, as it directly showcases our\ncatalyst’s robustness and practical potential in complex media. Second, within each\nmedium category (pure water and seawater), we further classify the cited works based\non their reaction architecture: our core comparison focuses on the emerging domain of\ntriphase (gas-liquid-solid) systems, while we also include leading benchmarks from the\nestablished field of biphasic (liquid-solid) systems for broader context.\nThis refined, two-tiered comparative framework is now clearly delineated in the\nrevised Figure 4g and Table S9. Each entry is explicitly tagged with its specific reaction\nmedium (pure water/seawater) and system configuration (triphase/diphase). We believe\nthis approach significantly enhances the transparency and utility of the performance\ncomparison. It not only honestly reflects the current, developing state of triphase\ncatalysis but also precisely showcases the superior performance of our CsPbI @COF-\nand particularly within the challenging triphase architecture, our catalyst achieves a\nhighly competitive H O production rate, underscoring its efficiency in overcoming\n2 2\nmass-transfer limitations and its potential for practical solar-driven applications.\nSpecific Revisions in the Manuscript Text: The revision is primarily reflected in the\nlegend/caption of Figure 4g and the updated Table S9, where each cited work is now\nclearly annotated with its reaction conditions. Additionally, the following explanatory\nsentence has been added to the main text in the “Full reaction photosynthesis of H O\n2 2\non a three-phase interface” section (Figure 4g) to guide readers to the detailed\ncomparison:\n“This performance is significantly higher than that of most reported photocatalysts\n(Figure 4g and Table S9)24-45. The CsPbI @COF-Cl system achieved solar-to-chemical\n3\nconversion (SCC) efficiency of 1.38%, while competitive among direct photosynthesis\nsystems, is primarily limited by the suboptimal utilization of the full solar spectrum,\noptical losses at the triphase interface, and the kinetic overpotentials required to drive\nthe dual 2e⁻ ORR and WOR pathways simultaneously.”\nFigure 4. Photocatalytic H O production over COF-Cl and CsPbI @COF-Cl. (g) Performance\n2 2 3\ncomparison of the CsPbI @COF-Cl system with recently reported photocatalysts for H O\n3 2 2\nproduction in pure water and seawater without sacrificial agents. The solar-to-chemical conversion\n(SCC) efficiency of our system (1.38%) is highlighted as a key metric for practical solar energy\nconversion. Detailed conditions for all benchmarks are provided in Table S9.\nReviewer #2 (Remarks to the Author):\nThe manuscript by Meng et al. presents a highly innovative approach to stabilizing\nCsPbI quantum dots within a chlorine-functionalized covalent organic framework\n3\n(COF-Cl) for photocatalytic H O production. The concept of \"multisite atomic-\n2 2\nchlorine passivation\" directly targets the perennial challenge of perovskite instability\nin water, and the reported performance metrics—including remarkable production\nrates and stability in seawater—are impressive. The work is comprehensive and has\nthe potential to be a significant contribution to the field. However, to firmly establish\nthe proposed mechanisms and meet the high evidentiary standards of Nature\nCommunications, the following critical points require addressing through additional\nexperiments and clarifications.\nResponse: We sincerely thank the reviewer for their thoughtful assessment of our\nmanuscript and their positive comments regarding the innovation and potential\nsignificance of our work. We are also grateful for the recognition of our approach in\naddressing the critical challenge of perovskite stability in aqueous environments. The\nconstructive suggestions provided are greatly appreciated and have been invaluable in\nhelping us strengthen the evidence for our proposed mechanisms and improve the\noverall quality of the manuscript.\nIn the revised version, we have performed additional experiments and provided\ndetailed clarifications to address all the points raised. These revisions, which include\nnew spectroscopic data, controlled experiments, and more precise textual explanations,\nhave significantly bolstered our conclusions and we believe they now meet the high\nevidentiary standards required. Our point-by-point responses to the specific comments\nbelow detail all the changes made. We are confident that these revisions have\nsubstantially improved the manuscript and we thank the reviewer again for their crucial\nrole in this process.\n1. The proposed S-scheme charge transfer is central to explaining the system’s high\nperformance, but the current evidence (UPS, XPS shifts) remains indirect and\ncircumstantial. A Type-II heterojunction could similarly explain the observed charge\nseparation without invoking the unique charge recombination pathway of an S-scheme.\nFemtosecond transient absorption (fs-TA) spectroscopy is essential to directly probe\nthe ultrafast interfacial charge transfer. The decisive evidence would be the\nsimultaneous quenching of the electron signal in the COF-Cl and the hole signal in the\nCsPbI QDs, confirming the proposed charge recombination pathway. Kelvin probe\n3\nforce microscopy (KPFM) could provide direct spatial mapping of the built-in electric\nfield at the heterojunction interface, offering solid evidence for the band bending\nrequired for the S-scheme.\nResponse: We sincerely appreciate the reviewer’s insightful suggestion regarding the\nneed for more direct evidence to distinguish the proposed S-scheme mechanism from a\nconventional Type-II heterojunction. In response, we have conducted comprehensive\nfemtosecond transient absorption (fs-TA) spectroscopy and in situ irradiated Kelvin\nprobe force microscopy (KPFM) experiments. The following revisions have been made\nto the manuscript:\n(1) Addition of fs-TA data and analysis (Revised/Added Text):\n“To further explore carrier transfer mechanisms, femtosecond transient absorption (fs-\nTA) spectroscopy was employed to analyze the dynamics of photogenerated charge\ncarriers using a 400 nm pump pulse. The fs-TA spectra of CsPbI QDs (Figure S49a\n3\nand S49b) exhibit characteristic ground-state bleaching (GSB) at 460~600 nm and\nphotoinduced absorption (PIA) at 600~700 nm, with decay kinetics (Figure S49c)\nfitting to τ = 13.19 ps and τ = 243.50 ps. For COF-Cl (Figure S50a and S50b), two\n1 2\ndistinct GSB signals are observed at 460~560 nm and 630~700 nm, with kinetics\n(Figure S50c) described by τ = 5.95 ps and τ = 184.90 ps. In the CsPbI @COF-Cl\n1 2 3\nsystem (Figure S51a and S51b), key spectral changes confirm the S-scheme mechanism:\nthe electron-related signal of COF-Cl (630~700 nm) is completely quenched, and the\nhole-associated PIA signal of CsPbI QDs (600~700 nm) is strongly suppressed. This\n3\nsimultaneous quenching provides direct evidence that electrons from COF-Cl\nrecombine with holes from CsPbI QDs at the interface. The kinetic analysis (Figure\n2\nps) indicative of accelerated interfacial recombination, and the emergence of a long-\nlived component (τ = 934.40 ps) corresponding to the stabilized useful charges—\n3\nelectrons in CsPbI QDs and holes in COF-Cl.”\n3\n(2) Addition of KPFM data and analysis (Revised/Added Text):\n“Moreover, in-situ irradiated Kelvin Probe Force Microscopy (KPFM) was employed\nto monitor the surface photovoltage (SPV) changes of the heterojunction material\n(Figure 3g and 3h). Upon light irradiation, the surface potential of CsPbI @COF-Cl\n3\nexhibited a notable increase from 198.6 mV to 276.1 mV, corresponding to an SPV\nchange of 77.5 mV (Figure 3i). This pronounced increase indicates effective\nsuppression of charge carrier recombination. In contrast, pristine COF-Cl alone showed\nnegligible SPV change (Figure S52). More specifically, the accumulation of photo-\ngenerated holes in the valence band of the surface-exposed COF-Cl component\nprovides strong evidence for an S-scheme heterojunction mechanism, wherein electrons\nfrom COF-Cl recombine with holes from the CsPbI QDs. This interpretation is further\n3\nsupported by control experiments on a deliberately synthesized CsPbI -COF-Cl\n3\ncomposite, in which perovskite was grown on the outer surface of COF-Cl (Figure S53).\nIn this configuration, a clear SPV increase on COF-Cl accompanied by a reduced SPV\nresponse from the CsPbI was observed, reinforcing those holes accumulate in COF-Cl\n3\nwhile electrons are retained within the CsPbI QDs under illumination.”\n3 3\nPseudocolor plots of CsPbI QDs. All the samples are tested under 400 nm laser pulse excitation.\nPseudocolor plots of COF-Cl. All the samples are tested under 400 nm laser pulse excitation.\nFigure S51. (a) Fs-TA spectra of CsPbI @COF-Cl. (b) Normalized fs-TA decay curves\n3 3\nnm laser pulse excitation.\nFigure 3 (g). Surface potential maps of CsPbI @COF-Cl system in dark. (h) Surface potential maps\n3\nof CsPbI @COF-Cl system under light irradiation. (i) Potential difference istribution of the\nCOF-Cl system under light irradiation. (c) Potential difference istribution of the COF-Cl system in\nthe dark and light. Source data are provided as a Source Data file.\nFigure S53. (a) Surface potential maps of CsPbI -COF-Cl system in dark. (b) Surface potential\n3\nmaps of CsPbI -COF-Cl system under light irradiation. (c, d) Potential difference istribution of the\n3\n2. While the synthesis of COF-Br and COF-I controls is commendable, the data do not\nconclusively prove that the superior performance of COF-Cl stems from its unique\nchemical passivation rather than a general halogen or hydrophobic effect. A\ncomparative analysis of XPS peak shifts (for Pb, I, and the halogens) and time-resolved\nphotoluminescence (TRPL) lifetimes for the CsPbI @COF-Br and CsPbI @COF-I\n3 3\ncomposites must be provided. This will directly test the correlation between specific\nhalogen interactions (Cl-Pb coordination, Cl-I bonding) and enhanced stability/charge\nseparation.\nResponse: We sincerely appreciate the reviewer’s insightful comments regarding the\ncomplexity of interfacial interactions and the necessity to look beyond simple XPS shift\ninterpretations. This critical observation has prompted us to undertake a more\nsystematic and in-depth exploration of the interplay between interfacial chemistry and\nband structure in different halogenated COFs. In strict accordance with the reviewer’s\nsuggestion, we have conducted a comprehensive comparative analysis of\nCsPbI @COF-Br and CsPbI @COF-I composites. The collected XPS and TRPL data\n3 3\nform a complete, multi-faceted evidence chain that unequivocally demonstrates that the\nsuperior performance of COF-Cl originates from its specific, synergistic interfacial\nchemical interactions, rather than generic halogen or hydrophobic effects, and that its\ndeeper valence band position (2.52 eV) plays a pivotal role in constructing an efficient\nS-scheme heterojunction.\nFirst, high-resolution XPS analysis provides deep insights into the strength, nature,\nand net effect of the interfacial interactions between the three COF-X materials and\nCsPbI QDs. In CsPbI @COF-Cl, we observe a highly synergistic interfacial electron\n3 3\nredistribution profile. The binding energy of interfacial Pb2+ shifts from 143.12/138.25\neV to 144.29/139.42 eV (Δ = +1.17 eV) (Figure 2a), clearly indicating the formation of\na Cl-Pb coordination bond. More revealingly, the Cl 2p binding energy concurrently\nshifts from 202.43/200.83 eV to 202.21/200.61 eV (Δ = -0.22 eV) (Figure 2c). This\nopposing shift is crucial, as it definitively proves the coexistence and synergy of Cl-I\nhalogen bonding. This bidirectional electron transfer mechanism creates a charge-\nrecirculation pathway, resulting in a stable, balanced, and robust interfacial chemical\nenvironment, with the net effect manifested as a moderate Pb shift and a negative Cl\nshift.\nIn stark contrast, the interfacial chemistry in CsPbI @COF-Br presents a markedly\n3\ndifferent picture (Figure S27). Its interfacial Pb2+ exhibits a much larger positive\nbinding energy shift (from 143.12/138.25 eV to 145.27/140.40 eV, Δ = +2.15 eV),\nsuggesting a more pronounced electron transfer due to Br-Pb coordination. However,\nthe Br 3d binding energy shows only a minimal change from 71.96/70.93 eV to\n71.91/70.87 eV (Δ = -0.05 eV). This nearly negligible shift indicates that Br atoms lack\neffective electron compensation via halogen bonding during coordination. This\nseverely unbalanced interaction mode likely leads to interfacial charge accumulation\nand reduced stability. Most alarmingly, the CsPbI @COF-I system reveals fundamental\n3\ninterfacial incompatibility through the three distinct doublets in its I 3d spectrum\n(Figure S28): besides the peaks assigned to bulk CsPbI I- (630.51/619.01 eV) and the\n3\ninterfacial I- interacting with I sites of COF-I (632.50/621.00 eV, Δ = +2.16 eV),\nthe third component at very high binding energy (634.09/622.59 eV, Δ = +3.75 eV)\nunequivocally confirms the oxidation of interfacial I- to harmful species like I or IO .\n2 x\nThis finding reveals that COF-I not only fails to passivate the CsPbI surface effectively\n3\nbut actively induces interfacial degradation reactions. The well-established literature\nconsensus that “Pb-Cl bond strength is higher than that of Pb-Br and Pb-I” provides\nsolid theoretical support for our observations, explaining at the chemical bond level\nwhy COF-Cl forms the most stable and effective passivation interface (Hu et al. J. Am.\nChem. Soc. 145, 10, 5872-5879 (2023).).\nThese significant differences in interfacial chemical properties, combined with the\nband structure characteristics of each COF-X material, collectively determine the\npractical efficiency and carrier dynamics of their S-scheme heterojunctions. Although\nthe band positions of all three COF-X materials (COF-Cl: 2.52 eV, COF-Br: 2.39 eV,\nCOF-I: 2.27 eV) theoretically allow for the formation of an S-scheme heterojunction\nwith CsPbI (Figure 3b). COF-Cl, leveraging the strongest Pb-Cl bonding, synergistic\nscheme heterojunction with the best balance between built-in electric field strength,\ninterfacial bonding stability, and charge transfer kinetics. The deeper valence band of\nCOF-Cl (2.52 eV) signifies stronger oxidation capability and greater interfacial band\nbending, which significantly enhances the built-in electric field strength of the S-\nscheme heterojunction, providing a stronger driving force for charge separation. In\ncontrast, the shallower valence band of COF-Br (2.39 eV) results in insufficient driving\nforce that limits its charge separation efficiency, while COF-I, with the shallowest\nvalence band (2.27 eV), possesses the weakest driving force for its heterojunction.\nThe time-resolved photoluminescence (TRPL) decay dynamics provide direct\nevidence that the superior performance of CsPbI @COF-Cl stems from its uniquely\n3\neffective interfacial passivation and optimal electronic structure. The measured\nlifetimes for the composites follow a definitive trend: CsPbI @COF-Cl exhibits the\n3\nlongest decay components (τ = 9.60 ns, τ = 57.52 ns, τ = 289.41 ns) and an average\n1 2 3\nlifetime (τ = 174.38 ns) significantly greater than those of CsPbI @COF-Br (τ =\navg 3 avg\n106.78 ns) (Figure S33) and CsPbI @COF-I (τ = 24.68 ns) (Figure S34).\n3 avg\nThis pronounced gradient in carrier lifetime is a direct spectroscopic measure of\ninterfacial quality. The exceptionally long lifetime in CsPbI @COF-Cl indicates highly\n3\neffective suppression of non-radiative recombination, which we attribute to a stable\nheterojunction interface where strong chemical passivation minimizes trap states. In\ncontrast, the notably shorter lifetime of the iodine-based composite is consistent with a\ndefective interface prone to rapid charge recombination, while the intermediate lifetime\nof the bromine-based system suggests only partial passivation efficacy.\nFurthermore, the deeper valence band position of COF-Cl (+2.52 eV vs. NHE)\ncompared to COF-Br (+2.39 eV) and COF-I (+2.27 eV) creates a stronger built-in\nelectric field in the S-scheme junction. This field not only enhances charge separation\nbut also stabilizes the separated carriers against recombination, directly contributing to\nthe prolonged lifetime observed. Therefore, the TRPL data independently confirm that\nthe chlorine-specific interaction is critical for establishing superior interfacial stability\nand charge-separation dynamics, correlating directly with the enhanced photocatalytic\nactivity of the CsPbI @COF-Cl system.\nRevised Paragraph (following the description of TRPL for CsPbI @COF-Cl):\n3\n“Moreover, TRPL decay dynamics provide direct evidence that the superior\nperformance of CsPbI @COF-Cl stems from its uniquely effective interfacial\n3\npassivation. The measured lifetimes for the composites follow a definitive trend:\nCsPbI @COF-Cl exhibits the longest decay components and an average lifetime\n3\nsignificantly greater than those of CsPbI @COF-Br (τ = 106.78 ns) and\n3 avg\nCsPbI @COF-I (τ = 24.68 ns) (Figure S33, S34 and Table S8).”\n3 avg\nNew Paragraphs:\n“A comprehensive comparative XPS analysis of the CsPbI @COF-Br and\n3\nexplain their inferior performance. In the CsPbI @COF-Br system, the Pb 4f spectrum\n3\nshows a large positive binding energy shift (Δ = +2.15 eV, Figure S27a, Table S4),\nindicating pronounced electron transfer due to Br-Pb coordination. However, the Br 3d\nbinding energy shows only a minimal negative shift (Δ = -0.05 eV, Figure S27c, Table\nS3), suggesting a lack of effective electron compensation via halogen bonding. This\nunbalanced interaction likely leads to interfacial charge accumulation and reduced\nstability. Most critically, the CsPbI @COF-I system reveals interfacial incompatibility\n3\nand degradation. The deconvolution of its I 3d spectrum requires three distinct doublets\n(Figure S28, Table S5). Besides peaks for bulk CsPbI I⁻ and interfacial I⁻ interacting\n3\nwith COF-I, a third component at a very high binding energy (Δ = +3.75 eV)\nunequivocally confirms the oxidation of interfacial I⁻ to species such as I or IO . This\n2 x\ndemonstrates that COF-I not only fails to passivate the CsPbI surface but actively\n3\ninduces detrimental interfacial reactions. In stark contrast, the CsPbI @COF-Cl system\n3\nexhibits a synergistic, bidirectional electron redistribution profile with a moderate\npositive shift for Pb (Δ = +1.17 eV, Figure 2a) and a concurrent negative shift for Cl (Δ\n= -0.22 eV, Figure 2c), which definitively proves the coexistence of stable Cl-Pb\ncoordination and Cl-I halogen bonding. The established trend in Pb-X bond strength\n(Pb-Cl > Pb-Br > Pb-I) provides a fundamental chemical rationale for the optimal\nstability afforded by the Cl-based interface17-19.”\nRevised Paragraph:\n“The resulting band diagrams are plotted in Figure 3b. Crucially, this staggered\nband alignment satisfies the prerequisite for forming an S-scheme heterojunction. The\nresults indicate that CsPbI QDs possess strong reducing capabilities, while the COF-\noxidizing potential (EVB = +2.52 V vs. NHE) compared to COF-Br (+2.39 V) and\nCOF-I (+2.27 V). This deeper valence band position of COF-Cl signifies a stronger\noxidation capability and promotes the formation of a more powerful built-in electric\nfield at the heterojunction interface, providing a greater driving force for charge\nseparation. This favorable energy level alignment within the CsPbI @COF-Cl\n3\ncomposite not only maximizes the redox capabilities of the individual components but\nalso facilitates the construction of an efficient S-scheme heterojunction (Figure 3c).”\nFigure 2. Evaluation of interaction mechanisms between the COF-Cl matrix and CsPbI QDs.\n3\n(a) High-resolution Pb 4f XPS spectra of CsPbI QDs and the CsPbI @COF-Cl system. (b) High-\n3 3\nresolution I 3d XPS spectra of CsPbI QDs and the CsPbI @COF-Cl system. (c) High-resolution Cl\n3 3\n2p XPS spectra of the COF-Cl matrix and the CsPbI @COF-Cl system.\n3 3\nSample Pb 4f Pb 4f I 3d I 3d Cl 2p Cl 2p\n5/2 7/2 3/2 5/2 1/2 3/2\nCOF-Cl / / / / 202.43 200.83\nCsPbI QDs 143.12 138.25 630.34 618.84 / /\n3\n143.57 138.70 630.95 619.45\nCsPbI @COF-Cl 202.21 200.61\n3\n144.29 139.42 632.12 620.62\nFigure S27. (a) High-resolution Pb 4f XPS spectra of CsPbI @COF-Br system. (b) High-resolution\n3 3\ncomposite.\nSample Pb 4f Pb 4f I 3d3 I 3d Br 3d Br 3d\n5/2 7/2 /2 5/2 3/2 5/2\nCOF-Br / / / / 71.96 70.93\nCsPbI QDs 143.12 138.25 630.34 618.84\n3\n145.27 140.40 633.38 621.88\nCsPbI @COF-Br 71.91 70.87\n3\n143.86 139.00 631.19 619.69\nFigure S28. (a) High-resolution Pb 4f XPS spectra of CsPbI @COF-I system. (b) High-resolution\n3 3\nSample Pb 4f Pb 4f I 3d3 I 3d\n5/2 7/2 /2 5/2\nCOF-I / / 632.78 621.28\nCsPbI QDs 143.12 138.25 630.34 618.84\n3\n144.95 140.08 634.09 622.59\nCsPbI @COF-I 143.36 138.49 632.50 621.00\n3\n630.51 619.01\nFigure 3．Characterizations of the optical properties for CsPbI QDs, COF-X matrix and\n3 3\nCsPbI @COF-X system. (b) Electronic band structures for CsPbI QDs and COF-X matrix (versus\n3 3\nNHE). (c) Schematics of photocatalytic H O production over CsPbI @COF-Cl system. (d) XPS Cl\n2 2 3\n2p spectra of CsPbI @COF-Cl system. (e) XPS Pb 4f spectra CsPbI @COF-Cl system. (f) XPS I\n3 3\n3d spectra CsPbI @COF-Cl system. (g) Surface potential maps of CsPbI @COF-Cl system in dark.\n3 3\n(h) Surface potential maps of CsPbI @COF-Cl system under light irradiation. (i) Potential\n3\ndifference istribution of the CsPbI @COF-Cl system in the dark and light.\n3\n3. The scavenger experiments in Figure 5c qualitatively suggest both the oxygen\nreduction reaction (ORR) and water oxidation reaction (WOR) pathways are active.\nHowever, a quantitative deconvribution of their contributions is lacking. Perform\nphotocatalytic H O production under strictly anaerobic conditions. The yield under\n2 2\nthese conditions directly quantifies the contribution from the WOR pathway. \nComparison with the yield in aerated/oxygenated conditions will allow for a\nquantitative assessment of each pathway’s role. To provide incontrovertible evidence\nfor direct water oxidation, an isotopic labeling experiment using H 18O is strongly\n2\nrecommended, with mass spectrometry detection of 18O-labeled H O or O .\n2 2 2\nResponse: We sincerely appreciate the reviewer’s constructive suggestion regarding\nthe quantitative assessment of the dual-pathway mechanism for H O photosynthesis.\n2 2\nIn direct response, we have performed controlled photocatalytic experiments under\nstrictly anaerobic (N -purged) and aerobic (O -saturated) conditions, along with\n2 2\nisotopic labeling studies, to quantitatively deconvolute the contributions of the oxygen\nreduction reaction (ORR) and water oxidation reaction (WOR) pathways.\nThe comparative yield analysis under N and O atmospheres provides clear\n2 2\nquantitative insight. Under anaerobic conditions, where dissolved O is rigorously\n2\nexcluded, the observed H O production originates exclusively from the WOR pathway.\n2 2\nComparison with the yield obtained under aerobic conditions indicates that the WOR\npathway contributes approximately 29.3% to the total H O yield, as summarized in the\n2 2\nrevised Figure 5c. This result provides a direct quantitative deconvolution,\ndemonstrating that while the ORR pathway is the dominant route, the WOR pathway\nconstitutes a significant and mechanistically relevant secondary channel.\nTo deliver unequivocal evidence for direct water oxidation, we conducted an\nisotopic labeling experiment using H 18O as the reactant under anaerobic conditions.\nH 18O (Figure S71). The detection of 18O-labeled hydrogen peroxide under an inert\n2 2\natmosphere provides definitive proof that oxygen atoms in the produced H O are\n2 2\nderived directly from water molecules, independently of any dissolved molecular\noxygen. This observation serves as direct and incontrovertible evidence for the\noperation of the WOR pathway in our system.\nFurthermore, to rule out the possibility of water splitting as a competing process,\nwe performed control experiments in pure water under illumination and carefully\nmonitored the gas products. No detectable amounts of O or H were observed (Figure\n2 2\nS72), confirming that the two-electron WOR pathway to H O proceeds selectively\n2 2\nwithout significant interference from competing four-electron or two-electron water\noxidation pathways leading to O or H evolution.\n2 2\nRevised Manuscript Text:\n(1) In the “Full reaction photosynthesis of H O on a three-phase interface” section, the\n2 2\nfollowing sentences have been added to present the quantitative analysis:\n“Controlled anaerobic/aerobic experiments quantitatively show that the water oxidation\npathway contributes ~29.3% to the total H O yield (Figure 5c), confirming its role as\n2 2\na significant secondary channel.”\n(2) In the same section, the following text has been added to incorporate the isotopic\nlabeling evidence:\n“Isotopic labeling using H 18O under anaerobic conditions confirmed the direct\n2\nformation of H 18O (Figure S71), providing definitive evidence that H O originates\n2 2 2 2\nfrom water oxidation, while MS analysis simultaneously verified the absence of\ncompeting O or H evolution (Figure S72).”\n2 2\nFigure 5. Mechanistic investigations of the photocatalytic H O production over the\n2 2\nCsPbI @COF-Cl system. (a) EPR spectra using DMPO as a spin-trapping agent for •O ⁻ under\n3 2\ndark conditions and visible light illumination. (b) EPR spectra using TEMP as a spin-trapping agent\nfor 1O under dark conditions and visible light illumination. (c) H O yields in the presence of O ,\n2 2 2 2\nN and specific scavengers: p-benzoquinone (p-BQ, •O ⁻ scavenger), L-tryptophan (Trp, 1O\n2 2 2\nscavenger), silver nitrate (AgNO , electron (e⁻) scavenger), and ethanol (EtOH, hole (h⁺) scavenger).\n3\n(d) Time-course in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) of the\nCsPbI @COF-Cl system under visible-light irradiation in an O atmosphere. Free energy diagram\n3 2\nfor photocatalytic H O synthesis via the (e) direct 2e⁻ oxygen reduction reaction (ORR) pathway\n2 2\nand (f) direct 2e⁻ water oxidation reaction (WOR) pathway on the CsPbI @COF-Cl system.\n2 2 3 2\nH 18O. Source data are provided as a Source Data file.\n2 2 2 3\nN -saturated H O. (b) The MS signals of the O , H O and H for CsPbI @COF-Cl after 2 h of\n2 2 2 2 2 3\nreaction in N -saturated H O. Source data are provided as a Source Data file.\n2 2\n4. For a system touted for seawater operation and practical application, the potential\nleaching of toxic Pb2+ is a critical concern that must be quantified. Inductively coupled\nplasma mass spectrometry (ICP-MS) analysis of the reaction solution after the long-\nterm (20 h) stability test is necessary to report the leaching concentrations of Pb2+ and\nCs+.\nResponse: We sincerely thank the reviewer for this essential comment. We fully agree\nthat quantifying the leaching of toxic ions, particularly Pb2+, is a critical step in\nassessing the practical viability of any perovskite-based material for aqueous or marine\napplications. As suggested, we performed a dedicated analysis to provide these crucial\ndata.\nFollowing the 20 h long-term stability test, the reaction solution was collected and\nanalysed via inductively coupled plasma mass spectrometry (ICP-MS). The results\nconfirm detectable leaching, with concentrations quantified at 0.64 ppm for Pb2+ and\n1.52 ppm for Cs+. We attribute the relatively low level of Pb2+ leaching, which is below\nthe 1.0 ppm discharge limit for lead specified in China’s stringent Integrated\nWastewater Discharge Standard (GB 8978-1996). As detailed in our manuscript, the\nchloride-functionalized COF-Cl matrix is not merely a passive encapsulant. The -Cl\ngroups can coordinate with Pb2+ ions, creating a stabilizing coordination bond that\neffectively suppresses the dissolution and release of Pb from the CsPbI QDs lattice.\nFurthermore, the overall hydrophobic nature of the COF-Cl shell provides a general\nbarrier against water ingress. While Cs+ ions, known for their high solubility in water\n(Wang et al. Chem. Mater. 30, 5846-5852 (2018); Zhang et al. Angew. Chem. Int. Ed.\n59, 2-8 (2020).), showed a higher leaching concentration (1.52 ppm), it is important to\nnote that conventional pollutant control frameworks do not specify emission limits for\nnon-radioactive cesium, reflecting its significantly lower ecological toxicity profile\ncompared to lead. The additional potential for halogen bonding (Cl-I) between the\nCOF-Cl and the CsPbI QDs may further contribute to the structural integrity of the\n3\ncomposite, working synergistically with Pb-Cl coordination and hydrophobicity to\nminimize ion dissolution.\nMoreover, we acknowledge the reviewer’s underlying concern. Despite our design\nyielding a Pb2+ leaching concentration within a strict regulatory limit, any detectable\nrelease of Pb remains an environmental concern and represents a central challenge for\nlead-halide perovskite applications. This result quantitatively defines the current state\nof our material and provides a clear benchmark. It powerfully motivates our ongoing\nand future research direction, which is precisely focused on developing lead-free\ncatalysts and further refining encapsulation strategies to ultimately eliminate heavy\nmetal leakage, moving towards truly environmentally benign photocatalytic systems\nfor seawater purification. These ICP-MS results and the accompanying discussion have\nbeen added to the revised manuscript.\nRevised Manuscript Text:\n“To assess the environmental safety and practical viability, the reaction solution\nfrom the 20-hour stability test was analyzed by inductively coupled plasma mass\nspectrometry (ICP-MS). The leaching concentrations of Pb2+ and Cs+ were quantified\nas 0.64 ppm and 1.52 ppm, respectively. The Pb2+ concentration falls below the 1.0 ppm\nlimit of China’s Integrated Wastewater Discharge Standard (GB 8978-1996), which is\nattributed to the effective stabilization via Cl-Pb coordination and the hydrophobic\nbarrier of the COF-Cl matrix.”\n5. The proposal of a gas-liquid-solid triphase interface is plausible but lacks direct\nvisual evidence. Contact angle measurements alone are insufficient. Cryo-SEM or other\nsuitable techniques could be employed to directly visualize the spatial distribution of\nthe solid catalyst, liquid water, and gas phases within the porous structure.\nResponse: We sincerely thank the reviewer for raising this important point regarding\nthe evidence for the proposed gas-liquid-solid triphase interface. We agree that direct\nvisualization of such interfaces can provide valuable insights. The suggestion to\nemploy Cryo-SEM or analogous techniques is excellent and represents a definitive\napproach we plan to adopt in our follow-up studies to obtain direct microstructural\nevidence. In our work, the formation of the triphase system is supported by multiple\nlines of experimental evidence:\n(1) Macroscopic Behavior: Due to its strong hydrophobicity (contact angle = 139.5°,\nFigure 1g), the CsPbI @COF-Cl powder uniformly floats at the air-water interface, as\n3\nshown in the optical images (Figure S22). This flotation is a clear macroscopic\nmanifestation of a stable triphase configuration, consistent with established reports on\ntriphase photocatalytic systems (e.g., Appl. Catal. B Environ. 377, 125527 (2025);\nAngew. Chem. Int. Ed. 64, e202515397 (2025)).\n(2) Functional Validation via Performance Contrast: A critical control experiment\nwas performed by mechanically submerging the catalyst to create a liquid-solid\n(diphase) system. The H O production rate under diphase conditions dropped by more\n2 2\nthan 68% compared to the triphase setup (Figure S57). This dramatic decrease\nunequivocally demonstrates that efficient O diffusion and direct access to gaseous\n2\noxygen at the triphase interface are essential for high photocatalytic activity.\n(3) Electrochemical Impedance Evidence: we performed additional electrochemical\nimpedance spectroscopy (EIS) measurements under a gas-liquid interfacial (“floating”)\nconfiguration. The results reveal that the charge transfer resistance, represented by the\ndiameter of the high-frequency semicircle in the Nyquist plot, decreased by more than\n70% compared to the fully immersed condition (Figure S23). This substantial reduction\nstrongly suggests that the floating configuration promotes the formation of an efficient\ngas-liquid-solid triphase reaction zone within the porous electrode, which drastically\nfacilitates gaseous reactant transport and reduces kinetic limitations.\n(4) Technical Considerations for Nanoscale Visualization: While techniques such as\ncryo-SEM could offer additional spatial information, the nanoscale dimensions of both\nthe COF pores (~3.2 nm) and the encapsulated QDs (~3.2 nm) pose significant\nchallenges for directly resolving the three-phase boundary within the pores. The\nevidence we present，combining visual flotation, quantitative wettability, and a decisive\nperformance comparison between triphase and diphase configurations, provides a\ncompelling, functionally relevant demonstration of the triphase interface in our system.\nWe believe this multifaceted approach robustly supports the existence and\nfunctional advantage of the gas–liquid–solid triphase interface in the CsPbI @COF-Cl\n3\nphotocatalytic system. We have revised the manuscript to more clearly articulate this\nevidence and its interpretation.\nThank you again for this insightful comment, which has allowed us to clarify and\nemphasize the evidence supporting this key aspect of our work.\nFigure S22. (a) Photographs of the floating triphase COF-Cl catalyst. (b) Photographs of the floating\ntriphase CsPbI @COF-Cl catalyst.\nFigure S57. The photosynthesis rates by the CsPbI @COF-Cl and COF-Cl matrix system for the\n3\ntwo systems under under pure water and air conditions.\n6. The performance comparison in Figure 4g is valuable, but the conditions of the\nbenchmarked systems must be clarified. The caption or main text must explicitly state\nwhether all compared catalysts were tested without sacrificial agents. Furthermore, the\ndiscussion should more prominently highlight the system’s leading solar-to-chemical\nconversion (SCC) efficiency, as this is a more comprehensive metric for practical\npotential than production rate alone.\nResponse: We sincerely thank the reviewer for their valuable comments regarding the\nperformance comparison. We have revised the manuscript to address both points\nthoroughly.\n(1) Clarification of Benchmarking Conditions: In the revised version, we have\nexplicitly stated in the caption of Figure 4g and in the Table S9 that all photocatalysts\nincluded in the comparison were tested without sacrificial agents to ensure a fair and\nrelevant benchmark. The comparison is further structured to distinguish between\nperformance in pure water and seawater, and between diphase (liquid-solid) and the\nmore challenging triphase (gas-liquid-solid) systems, providing clear context for our\nwork’s advancement.\n(2) Emphasis on Comprehensive Metrics: We fully agree that the solar-to-chemical\nconversion (SCC) efficiency is a more comprehensive metric for practical potential. In\nresponse, we have calculated and prominently reported the SCC efficiency (1.38%) and\napparent quantum yield (AQY, 15.76% at 450 nm) in the main text. Significantly\nexpanded the discussion in the Results section (paragraph preceding Figure 4g) to\nhighlight these values, noting that our SCC efficiency ranks among the highest for\nsacrificial-agent-free H O production, particularly from seawater. Revised the caption\n2 2\nof Figure 4g to include the SCC efficiency as a key point of comparison.\nThese revisions ensure the performance comparison is transparent, contextualized,\nand emphasizes the most relevant metrics for assessing practical solar energy\nconversion.\nRevised Manuscript Text:\n“This performance is significantly higher than that of most reported photocatalysts\n(Figure 4g and Table S9)24-45. The CsPbI @COF-Cl system achieved solar-to-chemical\n3\nconversion (SCC) efficiency of 1.38%, while competitive among direct photosynthesis\nsystems, is primarily limited by the suboptimal utilization of the full solar spectrum,\noptical losses at the triphase interface, and the kinetic overpotentials required to drive\nthe dual 2e⁻ ORR and WOR pathways simultaneously.”\nRevised Figure 4g Caption:\n“(g) Performance comparison of the CsPbI @COF-Cl system with recently reported\n3\nphotocatalysts for H O production in pure water and seawater without sacrificial\n2 2\nagents. The solar-to-chemical conversion (SCC) efficiency of our system (1.38%) is\nhighlighted as a key metric for practical solar energy conversion. Detailed conditions\nfor all benchmarks are provided in Table S9\nFigure 4. Photocatalytic H O production over COF-Cl and CsPbI @COF-Cl. (g) Performance\n2 2 3\ncomparison of the CsPbI @COF-Cl system with recently reported photocatalysts for H O\n3 2 2\nproduction in pure water and seawater without sacrificial agents. The solar-to-chemical conversion\n(SCC) efficiency of our system (1.38%) is highlighted as a key metric for practical solar energy\nconversion. Detailed conditions for all benchmarks are provided in Table S9.\n7. The AQY of 15.76% at 450 nm is excellent, but its reporting can be improved. The\natmospheric conditions (O partial pressure) during the AQY measurement must be\n2\nspecified, as this critically influences the ORR pathway. Providing the AQY action\nspectrum at multiple wavelengths would be highly valuable to correlate the\nphotocatalytic activity with the absorption profiles of the individual components,\nfurther supporting the synergistic operation of the heterojunction.\nResponse: We sincerely thank the reviewer for their valuable and constructive\nfeedback regarding the reporting of the apparent quantum yield (AQY). Their points\nare well-taken, and we agree that a more rigorous presentation of this key metric is\nessential. We have addressed both suggestions as detailed below.\n(1) Specification of Atmospheric Conditions: We fully agree on the importance of\nspecifying the O partial pressure, as it directly influences the oxygen reduction reaction\n2\n(ORR) kinetics. In direct response, we have now explicitly stated the conditions in the\nrevised Supplementary Methods section. All AQY measurements were conducted\nunder ambient air at standard atmospheric pressure (≈101.3 kPa), corresponding to an\noxygen partial pressure (pO ) of approximately 21.2 kPa. This clarification provides a\n2\npractical and relevant context for evaluating our catalyst's performance under\nconditions that utilize atmospheric oxygen directly.\n(2) AQY Action Spectrum: We thank the reviewer for this insightful suggestion. To\ncomprehensively correlate activity with light absorption, we had in fact measured the\nAQY across several key wavelengths. The complete dataset is now explicitly presented\nin the revised manuscript (and accompanying Supplementary Information), yielding\nvalues of 15.45% at 420 nm, 15.76% at 450 nm, 12.55% at 500 nm, 5.11% at 550 nm,\nand 1.88% at 650 nm.\nMore importantly, we have integrated this action spectrum with the optical analysis.\nAs suggested, plotting these AQY values against the absorption profile of the\nCsPbI @COF-Cl composite (originally Figure 3a, now referenced in the revised text)\n3\nreveals a compelling correlation: the trend of photocatalytic efficiency closely follows\nthe composite's own broad absorption profile, which is distinct from and extends\nbeyond the absorption of the individual CsPbI QDs or COF-Cl components. This\n3\nobservation provides direct experimental evidence that the high photocatalytic activity\noriginates from the integrated light-harvesting and effective charge\ngeneration/separation within the formed S-scheme heterojunction as a unified system.\nWe also note, regarding the performance of individual components, that obtaining\na reliable AQY for pristine CsPbI QDs in an aqueous environment was not feasible\n3\ndue to their instantaneous degradation—a fact that itself starkly underscores the critical\nstabilizing and enabling function of the COF-Cl matrix in our design.\nRevised Manuscript Text (for inclusion):\n“The apparent quantum yield (AQY) action spectrum of the CsPbI @COF-Cl\n3\nsystem was measured under ambient atmospheric conditions (pO ≈ 21.2 kPa) across\n2\nkey visible wavelengths (Figure 4c). The AQY values were determined to be 15.45%\nat 420 nm, 15.76% at 450 nm, 12.55% at 500 nm, 5.11% at 550 nm, and 1.88% at 650\nnm. Notably, the trend of photocatalytic efficiency closely mirrors the composite's own\nabsorption profile (Figure 3a), rather than that of the individual components. This\nstrong correlation provides direct evidence that the high activity stems from the\nintegrated light-harvesting and charge generation of the S-scheme heterojunction as a\nwhole. The rapid degradation of pristine CsPbI QDs in aqueous media precluded a\n3\nreliable AQY measurement for the QDs alone, further highlighting the indispensable\nstabilizing and functional role of the COF-Cl matrix.”\nWe believe these clarifications and the provided spectrum significantly strengthen\nthe discussion of our system's efficiency and its underlying mechanism. We thank the\nreviewer again for helping us improve the manuscript.\nFigure 4. Photocatalytic H O production over COF-Cl and CsPbI @COF-Cl. (c) Apparent\n2 2 3\nquantum yield (AQY) of the CsPbI @COF-Cl system.\n3\n8. The manuscript should briefly describe the method for recovering the floating\npowder catalyst for the cyclic stability tests, as this has practical implications for real-\nworld application.\nResponse: We thank the reviewer for raising this important practical consideration.\nThe recovery of the floating photocatalyst is indeed critical for assessing reusability\nand evaluating potential real-world application.\nDue to its strong hydrophobicity, the CsPbI @COF-Cl composite spontaneously\n3\naccumulates and forms a continuous film at the air-water interface during operation.\nThis characteristic enables a straightforward and low-energy recovery process.\nFor the cyclic stability tests, we employed a sequential recovery protocol to ensure\nnear-quantitative retrieval: First, the bulk of the floating catalyst was gently collected\nfrom the water surface using a fine-mesh scoop. The remaining reaction mixture was\nthen filtered through a microporous membrane (0.22 μm) to capture any residual\nparticles. This combined method achieves efficient catalyst recovery without the need\nfor high-speed centrifugation—a common and energy-intensive step for hydrophilic\ncolloidal systems. The detailed procedure has been added to the Experimental Section\nin the revised manuscript.\nThis simple recovery strategy, enabled by the material's inherent interfacial activity,\nrepresents a practical advantage for scalable operation.\nRevised Text for the Experimental Section:\n“The highly hydrophobic CsPbI @COF-Cl composite spontaneously forms a\n3\ncontinuous film at the air-water interface during photocatalytic operation. For catalyst\nrecovery in the cyclic stability tests, a sequential separation protocol was employed to\nensure near-quantitative retrieval. First, the majority of the floating catalyst was gently\ncollected from the water surface using a fine-mesh stainless steel scoop. Subsequently,\nthe remaining reaction mixture was passed through a polyvinylidene fluoride (PVDF)\nmicroporous membrane (pore size: 0.22 μm) under mild vacuum to capture any residual\ncatalyst particles. The recovered catalyst was then rinsed lightly with deionized water\nand dried at 60 °C under vacuum for 2 hours before being used in the next catalytic\ncycle. This combined approach achieves efficient catalyst recovery without the need\nfor high-speed centrifugation, underscoring a practical advantage of the interfacial\nreaction system.”\nReviewer #3 (Remarks to the Author):\nIn this work, Meng et al. constructed CsPbI quantum dots (QDs) within a hydrophobic\n3\nchlorine-functionalized covalent organic framework (COF-Cl) through multisite\natomic-chlorine passivation route. Within the QDs@COF-Cl catalyst, a gas-liquid-\nsolid triphase interface was created with enhanced O diffusion and suppressed ionic\n2\nmigration to concurrently drive two-electron oxygen reduction and water oxidation for\nartificial photosynthesis H O in seawater. There are several questions that require\n2 2\nfurther clarifications as follows. I recommend acceptance after major revisions.\nResponse: We sincerely thank the reviewer for their positive assessment of our work\nand for recommending acceptance after major revisions. We are particularly grateful\nfor the recognition of our multisite atomic-chlorine passivation strategy and the creation\nof a triphase interface for enhanced photocatalytic H O production. The constructive\n2 2\ncomments and questions raised have been invaluable in helping us strengthen the\nmanuscript.\nBelow, we provide a point-by-point response to each specific comment, detailing\nall the changes made in the revised manuscript.\n1. The prevailing SEM and TEM images cannot support the growth of QD inside the\npores of COF. High-quality HRTEM images should be provided to support this key\nconclusion.\nResponse: We sincerely appreciate the key question raised by the reviewers. To\nprovide more direct evidence, we conducted additional HRTEM characterizations and\nsuccessfully captured images showing clear lattice fringes of CsPbI within the COF\n3\nstructure (Fig. 1c and S21), demonstrating that crystalline QDs exist within the matrix.\nMore importantly, we have integrated multiple complementary lines of evidence to\njointly confirm the confined growth of QDs within the channels.\nThe most direct physical evidence: After the composite formation, the BET specific\nsurface area of the material sharply decreased from 2289.7 to 1318.1 m² g⁻¹, and the\npore volume dropped from 1.77 to 1.05 cm³ g⁻¹ (Figure 1i and Table S2). Such a\nsignificant decline is a classic feature of pore filling.\nThe rationality of size matching: The pore size of COF-Cl (~3.1-4.0 nm, Figure S3\nand Table S1) is highly consistent with the average size of the observed QDs (~3.2 nm),\nproviding a prerequisite for size-restricted growth.\nElemental distribution and stability evidence: STEM-EDX elemental mapping\n(Figure 1f) shows that Cs, Pb, and I elements are uniformly co-distributed with Cl\nelements (from the COF framework), rather than being concentrated on the surface of\nCOF particles. Meanwhile, the stability of encapsulated CsPbI₃ in seawater for up to 20\nhours (Figure 4d) contrasts sharply with the immediate deactivation of bare QDs upon\ncontact with water (Figure S67), indirectly but strongly indicating that the QDs are\neffectively encapsulated within the hydrophobic COF channels, isolated from water\nerosion.\nThe stability performance is further corroborated by the outstanding stability of the\nencapsulated composites in aqueous environments (Fig. 4d, e), in sharp contrast to the\ninstantaneous degradation of bare QDs (Fig. S67), which is consistent with the model\nwhere they are protected within hydrophobic channels.\nMoreover, we also note that such structural configurations, where perovskite\nnanocrystals are encapsulated within a porous host, align well with established\nprecedents in related material systems. For instance, Yu et al. (Angew. Chem. Int. Ed.\n63, e202319996 (2024).) reported the confinement of CsPbBr nanocrystals within a\nwere uniformly dispersed throughout interrupted nanospaces of the zeolite (Figure R5).\nSimilarly, Zhao et al. (Angew. Chem. Int. Ed. 59, 23100-23106 (2020).) demonstrated\nthe incorporation of MAPbBr nanocrystals into an AlPO-5 framework, with TEM\n3\nimaging clearly showing the distribution of nanocrystals within the host, and a\nsignificant reduction in specific surface area confirming pore filling (Figure R6). In both\ncases, porous encapsulation provided critical stabilization, paralleling our observations.\nTherefore, the combined evidence from direct HRTEM and HAADF-STEM image\nvisualization, specific surface area and pore volume, stability performance, and\nanalogous literature strongly supports that the quantum dots are successfully hosted\nwithin the protective porous architecture of COF-Cl.\nFigure 1. Structural and Physicochemical Characterizations of the CsPbI @COF-Cl System.\n3\n(a, b) TEM images showing CsPbI QDs (black dots) confined within the COF-Cl matrix. (c)\n3\ntransform (FFT) pattern corresponding to the CsPbI QDs, confirming their high crystallinity. (e)\n3\nconfined CsPbI QDs. (f) EDX elemental mapping showing the spatial distribution of Cl, Cs, Pb,\n3\nand I within the same region. (g) Contact angle measurements on a pressed tablet of the\nCsPbI @COF-Cl composite. (h) PXRD pattern of the CsPbI @COF-Cl system. (i) N sorption\n3 3 2\nisotherms at 77 K and corresponding Brunauer-Emmett-Teller (BET) surface area plots for pristine\nCOF-Cl and the CsPbI @COF-Cl composite.\nTable S1. The crystallographic parameters of COF-X (X = Cl, Br and I) matrixes.\nSample a (Å) b (Å) c (Å) N-N（Å） X-X（Å）\nCOF-Cl 37.135900 37.135900 3.670495 40.1283 31.2480\nCOF-Br 36.977100 36.977100 3.746085 39.9460 30.7207\nCOF-I 36.973900 36.973900 3.957944 39.9380 30.2872\nTable S2. Specific surface areas calculated with BET method.\nSpecific Surface Area Pore size\nSample Pore volume (cm3 g-1)\n(m2 g-1) (nm)\nCOF-Cl 2289.7 1.77 3.09\nCOF-Br 1757.0 1.53 3.23\nCOF-I 1150.9 0.76 2.63\nCsPbI @COF-Cl 1318.1 1.05 -\n3\nconfirming structural stability.\nFigure S67. Photoluminescence (PL) spectra of CsPbI QDs in water recorded at (a) 0 min, (b) 1\n3\nmin, and (c) 2 min. (d) Time-dependent PL intensity of CsPbI QDs in water over 2 min. Source\n3\ndata are provided as a Source Data file.\n[FIGURE REDACTED]\nFigure R5. (b) HAADF-STEM image (inset: size distribution of CsPbBr NCs). (cite from Angew.\n[FIGURE REDACTED]\nFigure R6. (a) TEM image of MAPbBr @AlPO-5 composite; the black dots are MAPbBr NCs.\n3 3\nInset:the particle size distribution histograms with Gaussian fitting. (b) HRTEM image with lattice\nspacingof the confined MAPbBr NCs. (cite from Angew. Chem. Int. Ed. 59, 23100-23106 (2020).)\n3\n2. The QD loading amount should be quantified.\nResponse: We sincerely thank the reviewer for raising this important point regarding\nthe quantification of the CsPbI QD loading. We have performed a precise\n3\nquantification of the CsPbI QD content within the COF-Cl matrix, and the results are\n3\nnow included in the revised manuscript.\nThe quantification was conducted as follows: A 50 mg sample of the synthesized\nCsPbI @COF-Cl composite was subjected to complete digestion using aqua regia. The\n3\nresulting solution was analyzed by inductively coupled plasma optical emission\nspectrometry (ICP-OES) to determine the absolute concentration of lead. Based on the\nstoichiometry of CsPbI , the mass of the CsPbI QD component was calculated and\n3 3\nreferenced against the initial mass of the composite to determine the mass fraction.\nOur ICP-OES analysis determined the CsPbI QD loading to be 21.2 wt% in the\n3\nthe material's performance and has been added to the \"Results\" section in the revised\nmanuscript.\nThe following sentence has been added to the “Results” section:\n“Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis\nafter complete digestion of the composite confirmed a CsPbI QD loading of 21.2 wt%\n3\nwithin the COF-Cl matrix.”\n3. The pore size distribution curves of QDs@COF-Cl and COF-Cl should be\nsupplemented.\nResponse: Thank you for your valuable suggestion. In response, we have supplemented\nthe nitrogen adsorption-desorption isotherms and the corresponding pore size\ndistribution curves for both pristine COF-Cl and the CsPbI @COF-Cl composite as\nbeen added to the main text.\nSpecifically, in the “Results” section, the following sentences have been\nrevised/added:\n“N adsorption-desorption isotherms (Figure 1i) and the corresponding pore size\n2\ndistribution curves (Figure S25) provide compelling evidence for the confinement of\nCsPbI QDs within the COF-Cl mesochannels. The pore size distribution of pristine\nthe CsPbI @COF-Cl composite, a clear reduction in the pore volume signal within this\n3\ncharacteristic range is observed. Most notably, the BET specific surface area plummets\nfrom 2289.7 m2 g-1 (COF-Cl) to 1318.1 m2 g-1, and the pore volume decreases from\n1.77 cm3 g-1 to 1.05 cm3 g-1 (Table S2). This substantial reduction in both accessible\nsurface area and internal pore volume is a direct signature of pore filling or occupancy\nby the in-situ grown CsPbI QDs. Combined with the close match between CsPbI QD\n3 3\nsize and host pore dimensions, this data robustly confirms that the CsPbI QD growth\n3\nis effectively confined within the accessible mesoporous channels of the crystalline\nCOF-Cl framework, rather than merely occurring on external surfaces or between\nloosely packed structures.”\nFigure S25. Pore size distributions of COF-Cl Matrix and CsPbI @COF-Cl system calculation\n3\nbased upon NLDFT.\n4. The underlying reason of enhanced performance by chlorine modification against\nother halogens is unclear.\nResponse: We are grateful to the reviewer for raising this insightful question regarding\nthe mechanistic origin of the superior performance of the chlorine-modified system.\nOur detailed analysis, combining interfacial spectroscopy, charge dynamics, and band\nstructure considerations, reveals that the enhanced performance of CsPbI @COF-Cl\n3\narises from the synergistic interplay of the most robust interfacial chemical passivation\nand the optimal electronic band alignment, which together establish a highly stable and\nefficient S-scheme heterojunction.\nThe underlying reason is multifaceted, rooted in the specific atomic-scale\ninteractions and their consequences on stability and charge carrier dynamics:\n(1) Superior Interfacial Chemical Passivation and Stability: High-resolution XPS\nanalysis provides direct evidence for the strength and nature of the interfacial\ninteractions. The CsPbI @COF-Cl system exhibits a synergistic and balanced\n3\ninterfacial electron redistribution. The binding energy (BE) of interfacial Pb2+ shifts\npositively (Δ ≈ +1.17 eV, Figure 2a and Table S3), indicating electron transfer from Pb\nto Cl, consistent with Cl-Pb coordination. Crucially, the Cl 2p BE shifts negatively (Δ\n≈ -0.22 eV, Figure 2c and Table S3), confirming concurrent electron acceptance by Cl\nvia Cl-I halogen bonding. This bidirectional electron transfer creates a stable, charge-\nbalanced interface.\nIn contrast, for CsPbI @COF-Br, the interfacial Pb2+ shows a much larger positive BE\n3\nshift (Δ ≈ +2.15 eV, Figure S27a and Table S4), suggesting a stronger but potentially\nless balanced Br-Pb coordination. The Br 3d BE shows a negligible shift (Δ ≈ -0.05 eV,\nFigure S27c and Table S4), indicating a lack of effective compensatory halogen\nbonding, which may lead to interfacial charge imbalance and reduced stability.\nMost critically, the CsPbI @COF-I system shows signs of interfacial degradation. The\nTable S5), assigned to the oxidation of interfacial I⁻ to I or IO species. This indicates\n2 x\nthat COF-I not only fails to passivate but actively promotes perovskite surface oxidation.\nThis is consistent with the established chemical trend where the Pb-Cl bond is stronger\nthan Pb-Br and Pb-I bonds (supported by literature: J. Am. Chem. Soc. 145, 5872-5879\n(2023).), making Cl the most effective passivating agent for stabilizing\nundercoordinated Pb sites and suppressing ion migration and interfacial degradation.\n(2) Optimal Charge Carrier Dynamics: Time-resolved photoluminescence (TRPL)\ndecay measurements directly correlate with this stability trend. The CsPbI3@COF-Cl\ncomposite exhibits the longest average carrier lifetime (τ = 174.38 ns, Figure 2e and\navg\nTable S8), significantly longer than CsPbI @COF-Br (τ = 106.78 ns) and\n3 avg\nCsPbI @COF-I (τ = 24.68 ns, Figure S33, S34, Table S8). This demonstrates that the\n3 avg\nCl-based passivation most effectively suppresses non-radiative recombination\npathways, preserving photogenerated charges for catalysis.\n(3) Most Favorable Band Alignment for an Efficient S-scheme Heterojunction:\nWhile all COF-X materials form an S-scheme heterojunction with CsPbI QDs in\n3\ntheory (Figure 3b), the depth of the valence band (VB) is crucial. COF-Cl has the\ndeepest VB maximum (2.52 eV vs. NHE, Figure 3b and S40), compared to COF-Br\n(2.39 eV) and COF-I (2.27 eV). A deeper VB in COF-Cl signifies a stronger oxidation\ncapability and, upon contact with CsPbI QDs, leads to greater interfacial band bending.\ninterface, which provides a more powerful driving force for the separation of useful\ncharge carriers (electrons in CsPbI , holes in COF-Cl) and the recombination of\n3\nwasteful charges. The stronger IEF in the Cl-system is corroborated by the most\npronounced surface photovoltage change observed via KPFM (Figure 3g-i). The\nshallower VB of COF-Br and especially COF-I results in a weaker IEF, limiting charge\nseparation efficiency.\nTherefore, the exceptional performance of CsPbI @COF-Cl is not due to a single\n3\nfactor but is the result of a unique combination: (i) the strongest and most synergistic\ninterfacial passivation via durable Pb-Cl coordination and Cl-I halogen bonding, which\nensures exceptional aqueous and photochemical stability; (ii) the resulting most\neffective suppression of charge recombination; and (iii) the most favorable electronic\nband alignment (deepest VB of COF-Cl), which generates the strongest driving force\nfor directional charge separation via the S-scheme mechanism. This trifecta is uniquely\nachieved with chlorine functionalization, explaining its superior performance over\nbromine and iodine analogues.\nRevisions Made in the Manuscript Text:\n“Moreover, TRPL decay dynamics provide direct evidence that the superior\nperformance of CsPbI @COF-Cl stems from its uniquely effective interfacial\n3\npassivation. The measured lifetimes for the composites follow a definitive trend:\nCsPbI @COF-Cl exhibits the longest decay components and an average lifetime\n3\nsignificantly greater than those of CsPbI @COF-Br (τ = 106.78 ns) and\n3 avg\nCsPbI @COF-I (τ = 24.68 ns) (Figure S33, S34 and Table S8).”\n3 avg\n“A comprehensive comparative XPS analysis of the CsPbI @COF-Br and\n3\nexplain their inferior performance. In the CsPbI @COF-Br system, the Pb 4f spectrum\n3\nshows a large positive binding energy shift (Δ = +2.15 eV, Figure S27a, Table S4),\nindicating pronounced electron transfer due to Br-Pb coordination. However, the Br 3d\nbinding energy shows only a minimal negative shift (Δ = -0.05 eV, Figure S27c, Table\nS3), suggesting a lack of effective electron compensation via halogen bonding. This\nunbalanced interaction likely leads to interfacial charge accumulation and reduced\nstability. Most critically, the CsPbI @COF-I system reveals interfacial incompatibility\n3\nand degradation. The deconvolution of its I 3d spectrum requires three distinct doublets\n(Figure S28, Table S5). Besides peaks for bulk CsPbI I⁻ and interfacial I⁻ interacting\n3\nwith COF-I, a third component at a very high binding energy (Δ = +3.75 eV)\nunequivocally confirms the oxidation of interfacial I⁻ to species such as I or IO . This\n2 x\ndemonstrates that COF-I not only fails to passivate the CsPbI surface but actively\n3\ninduces detrimental interfacial reactions.”\n“This favorable energy level alignment within the CsPbI @COF-Cl composite not only\n3\nmaximizes the redox capabilities of the individual components but also facilitates the\nconstruction of an efficient S-scheme heterojunction (Figure 3c).”\nFigure 2. Evaluation of interaction mechanisms between the COF-Cl matrix and CsPbI QDs.\n3\n(a) High-resolution Pb 4f XPS spectra of CsPbI QDs and the CsPbI @COF-Cl system. (b) High-\n3 3\nresolution I 3d XPS spectra of CsPbI QDs and the CsPbI @COF-Cl system. (c) High-resolution Cl\n3 3\n2p XPS spectra of the COF-Cl matrix and the CsPbI @COF-Cl system.\n3 3\nSample Pb 4f Pb 4f I 3d I 3d Cl 2p Cl 2p\n5/2 7/2 3/2 5/2 1/2 3/2\nCOF-Cl / / / / 202.43 200.83\nCsPbI QDs 143.12 138.25 630.34 618.84 / /\n3\n143.57 138.70 630.95 619.45\nCsPbI @COF-Cl 202.21 200.61\n3\n144.29 139.42 632.12 620.62\nFigure S27. (a) High-resolution Pb 4f XPS spectra of CsPbI @COF-Br system. (b) High-resolution\n3 3\ncomposite.\nSample Pb 4f Pb 4f I 3d3 I 3d Br 3d Br 3d\n5/2 7/2 /2 5/2 3/2 5/2\nCOF-Br / / / / 71.96 70.93\nCsPbI QDs 143.12 138.25 630.34 618.84\n3\n145.27 140.40 633.38 621.88\nCsPbI @COF-Br 71.91 70.87\n3\n143.86 139.00 631.19 619.69\nFigure S28. (a) High-resolution Pb 4f XPS spectra of CsPbI @COF-I system. (b) High-resolution\n3 3\nSample Pb 4f Pb 4f I 3d3 I 3d\n5/2 7/2 /2 5/2\nCOF-I / / 632.78 621.28\nCsPbI QDs 143.12 138.25 630.34 618.84\n3\n144.95 140.08 634.09 622.59\nCsPbI @COF-I 143.36 138.49 632.50 621.00\n3\n/ / 630.51 619.01\n5. The mechanism of “continuous multi-site passivation” route is unclear and should\nbe further elaborated.\nResponse: We sincerely thank the reviewer for this insightful comment, which allows\nus to elaborate on the foundational “continuous multi-site passivation” mechanism\ncrucial for the stability of our CsPbI @COF-Cl system. As suggested, we have\n3\nsignificantly expanded the explanation of this mechanism in the revised manuscript,\nsupported by detailed theoretical modeling.\nThe core of this mechanism is the synergistic structural match between the periodic\nCOF-Cl host and the defective surfaces of the CsPbI QDs guest. Unlike small\n3\nmolecular ligands passivating isolated sites, the 2D porous COF-Cl framework, with its\nwell-defined channels and an interlayer spacing of 0.367 nm, presents a high density of\nregularly spaced -Cl groups. These groups can engage with multiple adjacent under-\ncoordinated ions on the CsPbI QD surface in a coordinated, continuous manner.\nrevised text now reads:\n“Computational modeling (Figure S35 and S36) reveals that the periodic architecture\nof COF-Cl enables continuous multi-site bonding with the perovskite surface.\nSpecifically, on a Pb-rich surface, adjacent-Cl groups can form a sequential chain of\ncoordination bonds (Cl-Pb) with neighboring under-coordinated Pb2+ sites. Similarly,\non an I-rich surface, they form a continuous network of halogen bonds (Cl-I) with\nconsecutive I⁻ sites. This continuous multi-site passivation, saturating contiguous\nchains of surface defects rather than capping isolated sites sparsely, creates a\ncomprehensive and robust interfacial network”\nTo demonstrate the critical advantage of this continuous mode, we performed\ncomparative DFT calculations, now detailed in the revised results section alongside the\ndiscussion of formation/binding energies (related to Figure S37). The added text states:\n“To unequivocally demonstrate the advantage of this continuous multi-site mode\nover isolated passivation, we performed controlled Density Functional Theory (DFT)\ncalculations. We compared the ideal continuous model against an artificially created\ndiscontinuous model, where specific Cl groups were removed from the framework to\nprevent contiguous bonding (Figure S37, S38). The formation energy for establishing\nthe continuous Cl-Pb network was profoundly more favorable (-2.76 eV) than for the\ndiscontinuous case (-1.28 eV). A similar trend was observed for Cl-I bonding (-2.56 eV\nvs. -0.80 eV) (Figure S39a). The average binding energy per site was also stronger in\nthe continuous configuration (Figure S39b). This quantitative evidence indicates that\npassivating a sequence of neighboring defects is energetically synergistic; interaction\nat one site favorably pre-configures the environment for the next, leading to collective\nstabilization and a more robust interface.”\nMoreovre, the conclusion from the molecular dynamics (MD) simulations, supporting\nthe structural stability conferred by continuous bonding, has been strengthened. In the\nparagraph discussing the RMSD analysis (related to Figure S40), we now conclude:\n“Furthermore, molecular dynamics (MD) simulations confirm the superior\nstructural stability conferred by this continuous network. The root means square\ndeviation (RMSD) of atomic positions for systems with continuous Cl-Pb/I passivation\nstabilizes quickly and remains consistently and significantly lower than for systems\nwith discontinuous passivation throughout the simulation (Figure S40), demonstrating\nmarkedly reduced interfacial atomic displacement and fluctuation.”\nIn summary, the “continuous multi-site passivation” mechanism describes the\nformation of an extended, energetically stabilized interfacial network of chemical\nbonds (Cl-Pb and Cl-I), arising from the geometric and electronic match between the\nperiodic -Cl array on COF-Cl and chains of under-coordinated ions on the perovskite.\nThis cooperative anchoring creates a rigid and durable host-guest interface, which is\nthe fundamental reason for the unprecedented aqueous stability of our composite. We\nbelieve these detailed additions to the manuscript provide a clear and compelling\nrationale for the proposed mechanism.\nFigure S35. Differential charge density map illustrating the passivation of adjacent defects between\nCOF-Cl matrix and lead-rich CsPbI perovskite surface (a: front view; b: side view).\nCl/CsPbI perovskite iodine-rich interface (a: front view; b: side view).\n3\nperovskite lead-rich surface and COF-Cl matrix with partial Cl functional group removal (a: front\nview; b: side view).\nFigure S38. Atomic configuration of meta-defect passivation between iodine-rich CsPbI\n3\nperovskite surface and partially Cl-depleted COF-Cl matrix (a: front view; b: side view).\nFigure S39. (a) Simulated formation energies for continuous and discontinuous COF-Cl binding on\nthe PbI-terminated CsPbI QD surface. (b) Simulated binding energies for continuous and\n3\ndiscontinuous COF-Cl binding on the PbI-terminated CsPbI QD surface.\n6. Is H O the only product of WOR? Provide experimental evidence.\n2 2\nResponse: We sincerely thank the reviewer for raising this critical point regarding\nthe selectivity of the water oxidation reaction (WOR). Our study already provides direct\nexperimental evidence confirming that H O is the primary and dominant product of\n2 2\nthe WOR pathway in our system, with no detectable competitive O or H evolution.\n2 2\nSpecifically, we conducted isotopic labeling experiments using H 18O under\n2 2\nstrictly anaerobic conditions (nitrogen atmosphere). The mass spectrometry (MS)\nresults, presented in Figure S71, definitively show the formation of H 18O , directly\n2 2\nproving that H O originates from water oxidation. Crucially, concurrent online MS\n2 2\nanalysis during these experiments (Figure S72) confirmed the complete absence of\ngaseous O (m/z = 32) or H (m/z = 2) evolution throughout the reaction period.\n2 2\nThese findings are explicitly stated in the main text of the section “Full reaction\nphotosynthesis of H O on a three-phase interface”:\n2 2\n“Isotopic labeling using H 18O under anaerobic conditions confirmed the direct\n2\nformation of H 18O (Figure S71), providing definitive evidence that H O originates\n2 2 2 2\nfrom water oxidation, while MS analysis simultaneously verified the absence of\ncompeting O or H evolution (Figure S72).”\n2 2\nTherefore, the combined evidence from isotopic tracing and real-time gas\nmonitoring strongly supports that the two-electron water oxidation to H O (2 H O →\n2 2 2\nH O + 2H⁺ + 2e⁻) is the principal operative pathway, with no significant side reactions\n2 2\nleading to molecular O or H under our photocatalytic conditions\n2 2\nFigure S72. (a) The MS signals of the O , H O and H for CsPbI @COF-Cl after 1 h of reaction in\n2 2 2 3\nN -saturated H O. (b) The MS signals of the O , H O and H for CsPbI @COF-Cl after 2 h of\n2 2 2 2 2 3\nreaction in N -saturated H O. Source data are provided as a Source Data file.\n2 2\n7. In seawater, whether the chlorine evolution reaction occurs as the side reaction\nshould be studied.\nResponse: We sincerely thank the reviewer for raising the critical question regarding\nthe potential chlorine evolution reaction as a side pathway during seawater\nphotocatalysis. We have systematically investigated this issue through gas-phase\nproduct monitoring and quantitative solution analysis, as detailed in the revised\nmanuscript, to provide a clear and evidence-based response.\nWe explicitly state the absence of detected chlorine gas: “During the photocatalytic\nprocess, Cl was not monitored by online mass spectrometry (MS) (Figure S59).” This\n2\nindicates Cl does not accumulate as a gas under our experimental conditions, likely\n2\ndue to its high solubility and rapid subsequent reactivity in the aqueous phase.\nFurthermore, we provide direct experimental evidence for chloride oxidation as a\nparallel pathway by tracking the accumulation of oxidative chlorine species in solution:\n“The progressive accumulation of oxidative chlorine species, as quantified by the DPD\nmethod (Figure S60) and shown by the increase from 5.2 ppm to 6.4 ppm over the\nreaction period (Figure S61), confirms chloride oxidation as a competing side reaction.”\nMoreover, the observed photocatalytic performance in seawater versus pure water is\npresented in a context that acknowledges this competition: “Given this promising\nperformance, we systematically evaluated H O production under seawater and air\n2 2\nconditions (Figure 4b). After 1 h of photochemical conversion, H O yields of 12.52\n2 2\nmmol g-1 h-1 and 20.37 mmol g-1 h-1 were achieved for the COF-Cl matrix and the\nCsPbI @COF-Cl system, respectively (Figure S58).” This rate in seawater is\n3\nmoderately lower than that in pure water (25.29 mmol g-1 h-1), which can be rationally\nattributed to the competitive consumption of a fraction of photogenerated holes by the\nchloride oxidation pathway.\nFinally, the valence band potential of the COF-Cl component in our CsPbI @COF-\nsignificantly more positive than the standard potential for Cl /Cl⁻ (+1.36 eV vs. NHE),\n2\nproviding a substantial thermodynamic driving force for Cl⁻ oxidation by\nphotogenerated holes. However, as our solution analysis shows, the primary fate of the\noxidized chlorine is the formation of dissolved hypochlorous acid/hypochlorite\n(HClO/ClO⁻) species rather than gaseous Cl .\nevolution of Cl gas is not observed, the oxidation of chloride ions to form dissolved\n2\noxidative chlorine species (e.g., HClO/ClO⁻) does occur as a parallel side reaction in\nseawater. This side reaction competitively consumes a portion of the photogenerated\nholes, which is consistent with the observed, slightly reduced H O production rate in\n2 2\nseawater compared to pure water. Nevertheless, the primary H O production pathway\n2 2\nremains highly efficient and dominant, as evidenced by the record production rate\nmaintained in the seawater environment.\nFigure S59. The MS signals of the 35Cl , 36Cl and 37Cl for CsPbI @COF-Cl after 1 h of reaction\n2 2 2 3\nin seawater. Source data are provided as a Source Data file.\nFigure S61. Total chlorine generation in seawater under air.\n8. The promising seawater performance would benefit from a deeper discussion of its\npractical implications. To strengthen the work, it is important to clarify the origin of\nthe reduced yield in seawater compared to pure water, and to evaluate the potential\ncompeting effects of chloride ions on the atomic passivation stability. Besides, the\nimpact of co-existing cations such as Mg²⁺ and Ca²⁺ should also be explored.\nResponse: We sincerely thank the reviewer for raising these critical points regarding\nthe practical implications of our seawater performance. We have conducted a deeper\nanalysis based on the electronic structure of our catalyst and the electrochemistry of\nseawater, which provides a coherent explanation for the observed phenomena and\nunderscores the system's practical robustness.\nThe origin of the slightly reduced H O yield in seawater compared to pure water\n2 2\nis definitively attributed to the competitive oxidation of chloride ions (Cl⁻). The valence\nband potential of the COF-Cl component in our CsPbI @COF-Cl S-scheme\n3\nheterojunction is +2.52 eV vs. NHE (Figure 3b). This is substantially more positive\nthan the thermodynamic potential required for Cl⁻ oxidation to Cl /HClO (+1.36 V vs.\nabundant Cl⁻ in seawater a highly favorable and competing side reaction. We have\nverified this experimentally through the detection and quantification of oxidative\nchlorine species (hypochlorite) using the DPD method, which showed an increase from\n5.2 ppm to 6.4 ppm over the reaction period (Figures S56 and S57). This side reaction\nnecessarily diverts a portion of the oxidative capacity away from the direct water\noxidation (WOR) pathway to H O , leading to the modest decrease in the overall H O\n2 2 2 2\nyield.\nConcerning the impact of coexisting cations such as Mg2+ and Ca2+, a\nthermodynamic analysis confirms their influence on the primary catalytic process is\nnegligible. The conduction band potential of the CsPbI QDs component is -0.83 eV vs.\nseawater cations, given their extremely negative standard reduction potentials (e.g.,\nMg2+/Mg: -2.37 V vs. NHE; Ca2+/Ca: -2.87 V vs. NHE). Therefore, the photogenerated\nelectrons are exclusively and selectively channeled toward the two-electron oxygen\nreduction reaction (ORR) for H O production. Experimentally, post-reaction\n2 2\ncharacterization (TEM/EDX, XRD) revealed no significant deposition of Mg/Ca-based\nprecipitates (e.g., hydroxides or carbonates) on the catalyst surface after prolonged use\nin seawater (Supplementary Figures S60 and S61). We attribute this stability to the\nlocally acidic and oxidative microenvironment created by the in-situ generated HClO\nfrom Cl⁻ oxidation, which effectively suppresses the hydrolysis and precipitation of\nthese cations that commonly foul catalysts in neutral/alkaline seawater electrolysis.\nFinally, we have evaluated the potential competing effects of chloride ions on the\natomic passivation stability. The chloride oxidation (Cl⁻ → HClO/ClO⁻) is a hole-\nconsuming process that occurs on the COF-Cl surface, operating independently of the\nelectron-modulating passivation sites (Cl-Pb coordination and Cl-I halogen bonds) at\nthe CsPbI /COF-Cl interface. The consistent photocatalytic performance over five\n3\nconsecutive cycles and the unchanged surface chemical states of Pb, I, and Cl confirmed\nby high-resolution XPS analysis after the reaction (Figure S66) provide strong evidence\nthat the multisite atomic-chlorine passivation layer remains intact and stable against the\ncomplex seawater electrolyte.\nIn summary, the complex composition of seawater creates a uniquely selective\nenvironment for our system. The thermodynamic landscape ensures electrons are used\nsolely for efficient H O production via ORR. Concurrently, the hole-driven chloride\n2 2\noxidation, while causing a minor yield reduction, concurrently confers critical practical\nadvantages of in-situ self-disinfection (via HClO) and anti-scaling. This comprehensive\nmechanistic understanding, supported by both thermodynamic reasoning and\nexperimental evidence, significantly strengthens the case for the practical viability and\nrobustness of our CsPbI @COF-Cl system for sustained solar H O production in real\n3 2 2\nmarine environments.\nKey Revisions and Corresponding Text/Figure References in the Manuscript:\n(1) Explicitly linked the thermodynamic driving force (valence band position of COF-\nCl) to the chloride oxidation side reaction and cited the experimental proof (Figures\nS56 and S57): “The progressive accumulation of oxidative chlorine species, as\nquantified by the DPD method (Figure S60) and shown by the increase from 5.2 ppm\nto 6.4 ppm over the reaction period (Figure S61), confirms chloride oxidation as a\ncompeting side reaction.”\n(2) Clarified the thermodynamic impossibility of their reduction and provided the\nexperimental observation of no precipitate formation (Figures S60 and S61):\n“TEM imaging and XRD analysis revealed no significant structural changes or\ndeposition of insoluble salts (e.g., Mg/Ca hydroxides/carbonates) after cycling in\nseawater (Figures S60 and S61), highlighting its robustness against fouling.”\n(3) Explicitly stated that chloride oxidation and interfacial passivation are independent\nprocesses. Cited the key evidence of stable surface chemistry from post-cycle XPS\n(Figure S66) as proof of passivation integrity: “Post-cycling XPS analysis confirms the\nchemical stability of CsPbI @COF-Cl, as the characteristic dual-doublet peaks of Pb,\ncycling performance.”\nReframed the chloride oxidation not merely as a yield-reducing factor but as a source\nof beneficial in-situ self-disinfection and anti-fouling effects, enhancing the argument\nfor practical application.\n9. The XPS spectra of physically mixed sample should be presented to support the\nspecific Cl-Pb/Cl-I chemical interaction between COF-Cl and QD.\nResponse: We sincerely thank the reviewer for raising this important point. In direct\nresponse to your suggestion, we have now conducted XPS characterization on a\nphysically mixed sample of CsPbI QDs and COF-Cl (denoted as PM-CsPbI /COF-Cl).\n3 3\nThe corresponding data have been added as Figure S31 and Table S8.\nThe high-resolution XPS spectra of the PM-CsPbI /COF-Cl sample provide a\n3\ncritical control. The binding energies for Pb 4f and I 3d in this physical mixture (Figure\nS31a, b) are nearly indistinguishable from those of pristine CsPbI QDs, showing no\n3\ndiscernible peak splitting or significant shifts.\nThis result stands in stark contrast to the clear and unambiguous binding energy\nshifts observed for the in-situ synthesized CsPbI @COF-Cl heterostructure (Figure 2a\n3\nand 2b). The direct comparison conclusively demonstrates that the specific spectral\nchanges in the heterostructure, namely, the positive shifts for Pb 4f and I 3d, are not\nartifacts of simple physical proximity. Instead, they arise from chemically specific\ninteractions (Cl-Pb coordination and Cl-I halogen bonding) that occur only at the\nintimately bonded interface formed during the in-situ encapsulation and passivation\nprocess. This control experiment therefore provides strong and direct support for our\nproposed mechanism of multisite atomic-chlorine passivation.\nWe appreciate the reviewer’s suggestion, which has allowed us to substantiate our\nclaims more rigorously. The relevant discussion has been incorporated into the revised\nmanuscript.\nThe following sentences have been added to the “Results” section, within the\nparagraph discussing XPS analyses: “Most definitively, the physically mixed sample\n(PM‑CsPbI /COF‑Cl) shows no discernible peak splitting or significant binding‑energy\n3\nshifts relative to the pristine components (Figure S29 and Table S7), confirming that\nthe spectral changes in the in‑situ composites are not artifacts of physical proximity but\narise from chemically bonded interfaces formed during synthesis.”\nFigure S31. (a) High-resolution Pb 4f XPS spectra of PM-CsPbI /COF-Cl. (b) High-resolution I 3d\n3 3\nTable S7. High-resolution XPS spectra of PM-CsPbI /COF-Cl.\n5/2 7/2 /2 5/2 3/2 5/2\nPM-CsPbI /COF-Cl 143.56 138.69 630.94 619.44 202.45 200.85\n3\n10. A pristine COF without halogen modification should also be used to load QD for\ncomparison, aiming to demonstrate the superiorities of COF-Cl.\nResponse: We sincerely thank the reviewer for this critical suggestion. To isolate the\nunique role of halogen functionalization, we synthesized a pristine, non-halogenated\nCOF (COF-H) with an identical framework and pore size. Using it to encapsulate\nCsPbI3 QDs yielded the control composite CsPbI3@COF-H.\nCharacterization revealed a fundamental failure: CsPbI QDs predominantly\n3\naggregated on the external surface of COF-H rather than being incorporated into the\npores. More critically, the perovskite primarily existed in the non-photoactive\northorhombic δ-phase, as confirmed by XRD and optical spectroscopy (Figure S68).\nThis demonstrates that mere physical confinement by a porous COF is insufficient. The\nspecific halogen-mediated interactions (Cl-Pb coordination and Cl-I halogen bonding)\nin COF-Cl are essential for guiding confined QD growth within the pores and\nstabilizing the metastable, photoactive perovskite phase.\nConsequently, the photocatalytic H O production rate of CsPbI @COF-H was only\n2 2 3\n1.33 mmol g⁻¹ h⁻¹, over an order of magnitude lower than that of CsPbI @COF-Cl\n3\n(25.29 mmol g⁻¹ h⁻¹) (Figure S69).\nTherefore, this control experiment decisively proves that the superiority of our\nsystem is intrinsically tied to the chlorine functionality. The halogen atoms are crucial\nfor successful pore-directed loading, phase stabilization, and the creation of a\nchemically passivated interface that enables exceptional photocatalytic efficiency and\nstability.\nThe following sentences have been added to the “Results” section: “Neat CsPbI\npredominantly in the inactive δ-phase on the surface (Figure S68), exhibits negligible\nperformance (Figure S69).”\nFigure S68. (a, b) The TEM of CsPbI -COF-H. (c) PXRD of COF-H and CsPbI -COF-H.\n3 3\nFigure S69. Kinetic curves of H O generation of the CsPbI -COF-H in pure water under air.\n2 2 3\nSource data are provided as a Source Data file.\n11. The E position of CsPbI QDs and COF-Cl should be clearly indicated, as it is\nf 3\ncritical for understanding the initial band alignment and the subsequent formation of\nthe S-scheme heterojunction.\nResponse: We sincerely thank the reviewer for this critical comment. The Fermi level\n(E) position is indeed fundamental for understanding the initial band alignment and the\nf\ndriving force behind S-scheme heterojunction formation. In direct response to your\npoint, we have performed ultraviolet photoelectron spectroscopy (UPS) to determine\nthe E of the pristine components. The results, now added to the manuscript, show the\nf\nwork functions (Φ) and corresponding Ef values versus the vacuum level:\nCsPbI QDs: Φ = 5.83 eV, thus E = -5.83 eV.\n3 f\nCOF-Cl: Φ = 7.00 eV, thus E = -7.00 eV.\nf\nThis measured offset confirms that the Fermi level of CsPbI QDs is higher than\n3\nthat of COF-Cl. Upon contact, electrons spontaneously flow from CsPbI3 QDs to COF-\nCl until their Fermi levels equilibrate. This initial redistribution establishes a built-in\nelectric field (IEF) directed from CsPbI QDs toward COF-Cl and causes upward band\n3\nbending in CsPbI and downward band bending in COF-Cl at the interface.\nband bending, drives the recombination of photogenerated electrons in the COF-Cl\nconduction band with photogenerated holes in the CsPbI valence band at the interface.\neffectively preserves the more potent charge carriers: the electrons retained in the\nconduction band of CsPbI QDs and the holes retained in the valence band of COF-Cl,\n3\nthereby maximizing the redox potential for reactions.\nSpecific revisions in the manuscript text are as follows:\n“The interfacial electron transfer mechanism in the CsPbI @COF-Cl heterojunction\n3\nwas demonstrated by Ultraviolet photoelectron spectroscopy (UPS). The results are\ndisplayed in Figure S46 and Figure S47. The work functions of CsPbI QDs and COF-\n“This work function disparity establishes a higher Fermi level in CsPbI3 QDs than\nin COF-Cl, driving electron migration from CsPbI QDs to COF-Cl upon\n3\nheterojunction formation. This initial electron transfer leads to charge redistribution at\nthe interface, establishing a unified Fermi level, an internal electric field (IEF), and\ncausing upward band bending in CsPbI3 QDs and downward band bending in COF-Cl.”\n“The combined UPS and in-situ XPS results provide direct experimental evidence\nfor the charge transfer process and quantitatively support the proposed S-scheme\nmechanism initiated by the Fermi level offset (Figure S48).”\nFigure S48. Mechanism diagram of photogenerated electron transfer at the interface of S-scheme\nCsPbI @COF-Cl system.\n3\n12. The evidence of S-scheme charge transfer process is insufficient. Please provide\nmore characterization, such as Kelvin Probe Force Microscopy (KPFM) and in-situ\nXPS.\nResponse: We sincerely appreciate the reviewer’s insightful suggestion regarding the\nneed for more direct evidence to evidence the proposed S-scheme mechanism. In\nresponse to this comment, we have performed a series of complementary\ncharacterizations, including in situ irradiated Kelvin probe force microscopy (KPFM)\nand femtosecond transient absorption (fs-TA) spectroscopy. Combined with the already\npresented in situ X-ray photoelectron spectroscopy (XPS) under light illumination,\nthese results provide comprehensive and multi-faceted evidence that conclusively\nvalidates the S-scheme charge transfer pathway in the CsPbI @COF-Cl heterojunction.\n(1) Addition of fs-TA data and analysis (Revised/Added Text):\n“To further explore carrier transfer mechanisms, femtosecond transient absorption (fs-\nTA) spectroscopy was employed to analyze the dynamics of photogenerated charge\ncarriers using a 400 nm pump pulse. The fs-TA spectra of CsPbI QDs (Figure S49a\n3\nand S49b) exhibit characteristic ground-state bleaching (GSB) at 460~600 nm and\nphotoinduced absorption (PIA) at 600~700 nm, with decay kinetics (Figure S49c)\nfitting to τ = 13.19 ps and τ = 243.50 ps. For COF-Cl (Figure S50a and S50b), two\n1 2\ndistinct GSB signals are observed at 460~560 nm and 630~700 nm, with kinetics\n(Figure S50c) described by τ = 5.95 ps and τ = 184.90 ps. In the CsPbI @COF-Cl\n1 2 3\nsystem (Figure S51a and S51b), key spectral changes confirm the S-scheme mechanism:\nthe electron-related signal of COF-Cl (630~700 nm) is completely quenched, and the\nhole-associated PIA signal of CsPbI QDs (600~700 nm) is strongly suppressed. This\n3\nsimultaneous quenching provides direct evidence that electrons from COF-Cl\nrecombine with holes from CsPbI QDs at the interface. The kinetic analysis (Figure\n2\nps) indicative of accelerated interfacial recombination, and the emergence of a long-\nlived component (τ = 934.40 ps) corresponding to the stabilized useful charges—\n3\nelectrons in CsPbI QDs and holes in COF-Cl.”\n3\n(2) Addition of KPFM data and analysis (Revised/Added Text):\n“Moreover, in-situ irradiated Kelvin Probe Force Microscopy (KPFM) was employed\nto monitor the surface photovoltage (SPV) changes of the heterojunction material\n(Figure 3g and 3h). Upon light irradiation, the surface potential of CsPbI @COF-Cl\n3\nexhibited a notable increase from 198.6 mV to 276.1 mV, corresponding to an SPV\nchange of 77.5 mV (Figure 3i). This pronounced increase indicates effective\nsuppression of charge carrier recombination. In contrast, pristine COF-Cl alone showed\nnegligible SPV change (Figure S52). More specifically, the accumulation of photo-\ngenerated holes in the valence band of the surface-exposed COF-Cl component\nprovides strong evidence for an S-scheme heterojunction mechanism, wherein electrons\nfrom COF-Cl recombine with holes from the CsPbI QDs. This interpretation is further\n3\nsupported by control experiments on a deliberately synthesized CsPbI -COF-Cl\n3\ncomposite, in which perovskite was grown on the outer surface of COF-Cl (Figure S53).\nIn this configuration, a clear SPV increase on COF-Cl accompanied by a reduced SPV\nresponse from the CsPbI was observed, reinforcing those holes accumulate in COF-Cl\n3\nwhile electrons are retained within the CsPbI QDs under illumination.”\n3\n(3) Further validation through in-situ XPS under light illumination tracked binding\nenergy shifts in key elements. As shown in Figure 3d, light exposure caused the Cl 2p\npeaks in CsPbI @COF-Cl to shift from 202.2 eV and 200.6 eV to 202.6 eV and 201.0\n3\neV, indicating electron loss from the COF-Cl component. Simultaneously, the Pb 4f\npeaks shifted from 144.29 eV/139.42 eV and 143.57 eV/138.70 eV to 143.92 eV/139.05\neV and 143.25 eV/138.38 eV (Figure 3e), while the I 3d peaks shifted from 632.12\neV/620.62 eV and 630.95 eV/619.45 eV to 632.46 eV/619.96 eV and 630.46 eV/618.96\neV (Figure 3f). These shifts collectively confirm electron gain by the CsPbI QDs\n3\ncomponent. Therefore, photogenerated electrons transfer from COF-Cl to CsPbI QDs\n3\nunder light excitation, directly evidencing the S-scheme charge transfer process.\nThese three independent techniques collectively offer unambiguous and\nmultidimensional proof for the S-scheme charge transfer mechanism in the\nCsPbI @COF-Cl heterojunction, effectively distinguishing it from a conventional\nstrengthens the mechanistic foundation of our work.\nFigure. S49 (a) Fs-TA spectra of CsPbI QDs. (b) Normalized fs-TA decay curves CsPbI QDs. (c)\n3 3\nPseudocolor plots of CsPbI QDs. All the samples are tested under 400 nm laser pulse excitation.\nPseudocolor plots of COF-Cl. All the samples are tested under 400 nm laser pulse excitation.\nFigure. S51 (a) Fs-TA spectra of CsPbI @COF-Cl. (b) Normalized fs-TA decay curves\n3 3\nnm laser pulse excitation.\nFigure 3. (g) Surface potential maps of CsPbI @COF-Cl system in dark. (h) Surface potential maps\n3\nof CsPbI @COF-Cl system under light irradiation. (i) Potential difference istribution of the\nCOF-Cl system under light irradiation. (c) Potential difference istribution of the COF-Cl system in\nthe dark and light. Source data are provided as a Source Data file.\nFigure S53. (a) Surface potential maps of CsPbI -COF-Cl system in dark. (b) Surface potential\n3\nmaps of CsPbI -COF-Cl system under light irradiation. (c, d) Potential difference istribution of the\n3\n13. The XPS of CsPbI @COF-Cl after photocatalytic reaction should be supplemented\n3\nfor assessing the cycling stability.\nResponse: We sincerely thank the reviewer for this valuable suggestion. We have now\nperformed detailed X-ray photoelectron spectroscopy (XPS) analysis on the\nCsPbI @COF-Cl composite after five consecutive photocatalytic reaction cycles in\n3\nseawater (corresponding to the stability test in Figure S62). The high-resolution spectra\nof the key interfacial elements (Pb 4f, I 3d, and Cl 2p) are presented in the revised\nSupplementary Information as Figure S66.\nThe corresponding data and discussion are already presented in the manuscript: “Post-\ncycling XPS analysis confirms the chemical stability of CsPbI @COF-Cl, as the\n3\ncharacteristic dual-doublet peaks of Pb, I, and Cl remain intact after five catalytic runs\n(Figure S66), supporting its robust cycling performance.”\nFigure S66. The XPS analysis on the CsPbI @COF-Cl composite after five photocatalytic cycles.\n3\n14. The H O production rate is impressive (high up to 25.29 mmol h-1 g-1) while the\n2 2\nsolar-to-chemical conversion (SCC) efficiency is only 1.38%. The key factors limiting\nthe SCC efficiency should be discussed.\nResponse: We sincerely thank the reviewer for the positive acknowledgment of the\nH O production rate and the insightful query regarding the solar-to-chemical\n2 2\nconversion (SCC) efficiency. We agree that further improvement of the SCC efficiency\nis an important goal, and we appreciate the opportunity to discuss the key limiting\nfactors.\nOur reported SCC efficiency of 1.38% was measured under standard conditions (pure\nwater, air, visible light), which provides a fundamental baseline for performance. This\nvalue is comparable to, and in many cases favorable against, current state-of-the-art\nsystems for direct photocatalytic H O synthesis without sacrificial agents, as\n2 2\nbenchmarked in Figure 4g and the associated references (now listed as Table S9 in the\nrevised manuscript). The high production rate (25.29 mmol h-1 g-1) confirms that our S-\nscheme heterojunction is highly effective in suppressing carrier recombination and\nenhancing charge separation, as evidenced by the quenched PL intensity (Figure S42),\nprolonged TRPL lifetime (Figure 2e), and in-situ KPFM results (Figure 3g-i).\nHowever, several inherent factors in the overall photochemical energy conversion\nprocess collectively limit the SCC efficiency:\n(1) Suboptimal Solar Spectrum Utilization: While the CsPbI @COF-Cl system exhibits\n3\na broad absorption edge beyond 700 nm (Figure 3a), a significant portion of the infrared\nspectrum remains unutilized due to the intrinsic bandgaps of the components (CsPbI\nsolar energy that can be harvested.\n(2) Optical Losses at the Triphase Interface: The system operates at a gas-liquid-solid\ntriphase interface to enhance O diffusion (Scheme 1c, Figure S24). While crucial for\n2\nmass transport, this multiphase interface can lead to reflection and scattering losses of\nincident light, reducing the photon flux effectively absorbed by the photocatalyst.\n(3) Kinetic Overpotentials of the Dual-Reaction Pathway: The system concurrently\ndrives the two-electron oxygen reduction reaction (ORR) and water oxidation reaction\n(WOR) without sacrificial agents. Although DFT calculations show favorable\nenergetics for key steps (e.g., *OOH formation on CsPbI at -0.88 eV, Figure 5e),\n3\nthe overall reaction kinetics are governed by the slower step, which is the water oxidation\nto form *OH on COF-Cl with a higher energy barrier (ΔG = 1.72 eV, Figure 5f). This\nkinetic overpotential necessitates a higher driving force (i.e., more absorbed photons)\nthan the thermodynamic minimum, thereby lowering the energy conversion efficiency.\nWe have integrated this discussion into the revised manuscript in the paragraph\ndiscussing the SCC efficiency, stating: “The CsPbI @COF-Cl system achieved solar-\n3\nto-chemical conversion (SCC) efficiency of 1.38%, while competitive among direct\nphotosynthesis systems, is primarily limited by the suboptimal utilization of the full\nsolar spectrum, optical losses at the triphase interface, and the kinetic overpotentials\nrequired to drive the dual 2e⁻ ORR and WOR pathways simultaneously.\n15. The format of references should be unified. Several entries (e.g., Refs. 32, 39, 40) are incomplete, lacking volume\nnumbers, page ranges, or full DOI identifiers.\nResponse: We thank the reviewer for their meticulous attention to detail regarding the\nreference formatting. We have carefully reviewed and unified the format of all\nreferences in the manuscript according to the journal's required style. The specific\nentries mentioned (References 32, 39, 40) and others have been corrected to include the\ncomplete publication information, including volume numbers and full-page ranges\nidentifiers where applicable. The reference list in the revised manuscript now fully\ncomplies with the journal's formatting guidelines.\n32. Tang B, et al. Photocatalytic Synthesis of Hydrogen Peroxide by Rhenium Modified Metal-\nOrganic Frameworks Incorporating Bianthracene Ligands. J. Am. Chem. Soc. 174, 24326-24335\n(2025).\n39. Zhou S, et al. Chloromethylation Modified Pyranonitrile-Based Conjugated Microporous\nPolymers for Selective One-Step Two-Electron O Reduction to H O . Angew. Chem. Int. Ed. 64,\n2 2 2\ne202508436 (2025).\n40. Wang W, et al. Graphene Quantum Dot‐Modified Mn Cd S for Efficient Overall\n0.2 0.8\nPhotosynthesis of H O . Adv. Funct. Mater. 35, 2422307 (2025).\n2 2\n** See Nature Portfolio’s author and referees' website at www.nature.com/authors for information\nabout policies, services and author benefits."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately responded to all the queries in an efficient manner. From my side, I would recommend\nacceptance", "reviewer_2": "In the revised manuscript, the authors have answered in detail all questions raised by the referees. Therefore, I recommend\nthe paper for publication without the further revision.", "reviewer_3": "The authors have submitted an extensively revised manuscript. The comments have been addressed appropriately. The\nquality of this work has been improved. I recommend acceptance of this version in Nature Communications."}, "author_response": ""}]

Decision letter