Solar hydrogen production through ambient-pressure seawater splitting

Domain: neuroscience
Published: 2026-02-16
Editorial outcome: major_revision

Abstract
Although polymeric carbon nitride holds promise for solar-driven hydrogen production, its scalability is constrained by reliance on costly vacuum conditions and scarce freshwater supplies. Herein, ultrathin carbon nitride nanosheets are covalently linked to electron-donating pyrene units via π-bridges. The resulting donor–π–acceptor frameworks, featuring a biphenyl π-bridge, exhibit reduced exciton binding energy and long-lived charge-separated states. Insitu spectroscopic and electrochemical analyses collectively demonstrate efficient intramolecular electron transfer and a strengthened built-in internal electric field. Theoretical calculations suggest that electron accumulation on heptazine units may enhance the adsorption of Na⁺/Mg2⁺–triethanolamine complexes, accelerating hole consumption in seawater. As a result, the optimal donor–π–acceptor catalyst shows efficient photocatalytic seawater splitting under ambient pressure and natural sunlight, achieving a hydrogen evolution rate of 134 mmol h−1 g−1. Here, we show a molecular design strategy that advances photocatalytic ambient-pressure seawater splitting and promotes the commercialization of green hydrogen production.

Introduction
Although particulate photocatalyst–promoted water splitting enables the sustainable conversion of solar energy into hydrogen as a clean fuel1–3, the related processes typically rely on scarce freshwater supplies and are therefore not easily upscalable4–6. Consequently, considerable attention has been drawn to the solar-driven photocatalytic splitting of seawater—which accounts for ~96.5% of Earth’s water resources—using advanced photocatalysts (e.g., TiO2, g-C3N4, and CdZnS)7–9. Given that these systems are typically operated under low-pressure conditions to suppress back reactions and are also not easily upscalable (Fig. 1a)10, photocatalysts enabling solar-driven hydrogen production through seawater splitting at ambient pressure—such as Domen’s 100m2 array of panel reactors for natural solar hydrogen production—are highly sought after11. Polymeric carbon nitride (CN) materials are well-suited for photocatalytic hydrogen evolution (PHE) because of their flexible chemical structures, tunable band gaps, and cost-effective syntheses12–14. The lone-pair electrons of nitrogen and sp2-hybridized orbitals of CN frameworks result in a high electron density, efficiently suppressing Cl− adsorption and the associated side reactions15. Although these properties make CN a promising platform for photocatalytic seawater splitting, CN-based photocatalysts still exhibit suboptimal PHE performances, largely because of the high binding energy of the singlet Frenkel excitons formed upon excitation. In turn, this high binding energy is due to the strong coulombic interaction in the photogenerated electron–hole pairs, which restricts their dissociation into free charge carriers16. Furthermore, the disordered π-conjugated structure of CN leads to the random migration and rapid recombination of charge carriers17. Rational design strategies reducing the exciton binding energy, enhancing electron delocalization, and improving charge separation in CN are therefore critical for realizing seawater splitting–based PHE under ambient pressure.

The construction of donor–acceptor (D–A)-structured organic semiconductors—such as covalent organic frameworks18, conjugated polymers19, and conjugated molecules20—is an effective strategy for reducing the exciton binding energy and promoting exciton dissociation. This effect arises from intramolecular charge transfer (ICT) due to the electron push-pull effect of the D and A units21. Consequently, various D (e.g., thiophene, triazine, and methoxyphenyl) and A (e.g., benzene, anthraquinone, and benzothiadiazole) groups have been incorporated into CN to construct D–A-structured frameworks. Such modifications facilitate exciton dissociation and enhance charge separation between CN and D or A units upon excitation, thereby improving photocatalytic activity (Fig. 1b)22,23. The electronegativity of CN promotes the adsorption of protons, because of the positive-negative electrostatic attraction7. This concurrent accumulation of electrons and protons favors rapid proton reduction and, hence, hydrogen production. Therefore, by incorporating electron-donating rather than electron-accepting groups into CN frameworks, one can not only preserve the hydrogen production sites but also promote the continuous transfer of excited electrons to the CN backbone and thus optimize the intermediate adsorption/desorption energy and reduce the energy barrier of the PHE reaction24. However, the electron-donating groups examined to date, such as thiophene, triazine, and methoxyphenyl moieties, exhibit weak electron delocalization ability because of their narrow aromatic hydrocarbon structures25, and are therefore inefficient in modulating electron delocalization within CN.

Pyrene, a well-known strongly electron-delocalized chromophore due to its four fused benzene rings of an alternating polycyclic aromatic hydrocarbon25, featuring a wide π-conjugated structure that ensures rapid electron transfer and efficient light harvesting, has been extensively explored in the construction of D–A-type covalent organic frameworks for photocatalytic hydrogen peroxide and hydrogen production26–29. However, the direct linking of CN with pyrene units results in low photocatalytic activities29–31, probably because the limited electron delocalization within the CN–pyrene structure restricts photoexcited charge migration between the two components. In the case of D–A-type covalent organic frameworks, the introduction of π-bridges between the D and A units effectively extends π-conjugation and thereby enhances electron delocalization and facilitates ICT32–34. Furthermore, larger delocalization systems and longer distances between the D and A units hinder the recombination of photogenerated charge carriers35. Hence, π-bridge-induced electron delocalization between CN and pyrene units can be leveraged to design D–π–A-structured CN frameworks with reduced exciton binding energies, extended π-conjugated systems, improved charge transport properties, and, hence, high PHE performances under ambient pressure.

In this work, we introduce D–π–A-structured CN frameworks by covalently incorporating electron-donating pyrene units into ultrathin CN nanosheet (UCN) using π-bridges (a single benzene ring, biphenyl moiety, or a benzene ring linked to an acetylene group; Fig. 1c, d). Through a combination of temperature-dependent photoluminescence (TD-PL), femtosecond transient absorption spectroscopy (fs-TAS), insitu X-ray photoelectron spectroscopy (XPS), and theoretical calculations, we quantitatively demonstrate that the biphenyl π-bridged UPy2 framework achieves a significantly reduced exciton binding energy, a prolonged charge-carrier lifetime, and a strengthened intramolecular electric field. Moreover, density functional theory (DFT) reveals that the enhanced electron accumulation on heptazine units promotes the adsorption of seawater cations and their complexes with triethanolamine, thereby accelerating hole-consumption. As a result, UPy2, featuring a biphenyl moiety as π-bridges, exhibits the highest PHE performance in natural seawater under simulated solar light (AM 1.5G) and ambient pressure, achieving a hydrogen production rate of 82 mmol h−1 g−1. A scalable 20 cm diameter disk reactor with this photocatalyst demonstrated a PHE rate of 134 mmol g−1 h−1 in natural seawater under natural sunlight, further demonstrating the scalability of this photocatalytic system and highlighting its potential for practical seawater hydrogen production.

Results
Design, synthesis, and characterization of the UPy frameworks
Three D–π–A-structured CN frameworks (Fig. 1d)—UPy1, UPy2, and UPy3—were obtained by covalently linking CN with π-pyrene-π molecules (Py1, Py2, and Py3) (Supplementary Fig. 1), each featuring a pyrene core (D unit), π-bridge (a single benzene ring, biphenyl moiety, or a benzene ring linked to an acetylene group), and four aldehyde groups as Schiff-base reactive sites. The electrostatic surface potential (ESP) distributions of these molecules (Supplementary Fig. 1 and Supplementary Data 1) suggested that negative potentials were primarily located around the pyrene units, favoring electron donation during excitation36. Among them, Py2 exhibited the largest molecular dipole moment (3.52 D), thus achieving the strongest electric-field polarization and potentially facilitating charge transfer37–39. The covalent linking of the heptazine rings and π-pyrene-π molecules yielded frameworks (UPy1, UPy2, and UPy3) with asymmetric ESP distributions indicative of the formation of localized electric fields that may promote electron delocalization and charge separation (Fig. 1d and Supplementary Data 2). This functionalization resulted in the potential at the center of the heptazine ring becoming more negative, suggesting an electron transfer from the pyrene unit to CN. The spatial distributions of the corresponding energy levels (Supplementary Figs. 2 and 3 and Supplementary Data 3 and 4) indicated that in CN and Py1–Py3, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were largely localized on the heptazine ring and pyrene units, respectively, which could hinder charge separation under excitation. In contrast, the UPy frameworks exhibited a clear HOMO–LUMO separation. This separation was most pronounced for UPy2, in which case the LUMO was distributed over the heptazine ring and π-bridge, and the HOMO was primarily located on the pyrene unit. This spatial orbital separation could facilitate electron transfer from the pyrene unit to the heptazine ring via the π-bridge upon excitation40–42.

The covalent integration of UCN with the π-pyrene-π molecules is illustrated in Fig. 2a. UCN was synthesized by the thermal oxidation of bulk CN obtained via the thermal polymerization of urea. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging revealed that bulk CN exhibited a thick layered structure (Supplementary Fig. 4), whereas UCN displayed a thin porous structure (Fig. 2b, c). Upon ultrasonication, UCN fragmented into small, irregular pieces, as displayed in the SEM image (Supplementary Fig. 5). These fragments were subsequently covalently linked into large blocky-like structures by Py2 molecules via a Schiff-base reaction, forming UPy2 framework (Fig. 2d, e). The rod-like Py1 and Py2 particles and lamellar Py3 particles, each several micrometers in size (Supplementary Fig. 6), were not observed in the UPy frameworks (Fig. 2b, c; Supplementary Fig. 7), which suggests the formation of uniform covalent links between the π-pyrene-π molecules and UCN.

The X-ray diffraction (XRD) patterns of UCN and the UPy frameworks (Supplementary Fig. 9) exhibited two characteristic peaks at approximately 13.0° and 27.4° corresponding to the (100) in-plane repeating units and (002) interlayer stacking of the conjugated motifs of CN, respectively43. The absence of π-pyrene-π molecule signals in the UPy framework patterns was ascribed to the uniform covalent bonding of these molecules with UCN. The Fourier transform infrared (FTIR) spectra of UCN, UPy1, UPy2, and UPy3 (Supplementary Fig. 10a) exhibited peaks corresponding to –NH groups (3000–3500 cm−1), aromatic C–N heterocycles (1300–1700 cm−1), and triazine rings in CN (810 cm−1)44. The –NH band intensities of UPy1, UPy2, and UPy3 were lower than that of UCN, indicating the consumption of –NH groups during covalent bonding with the π-pyrene-π molecules. No characteristic stretching vibrations of aromatic groups (–C=C–/–C–C–) in the π-pyrene-π molecules (typically 800–1700 cm−1) were observed in the UPy spectra, probably because of their overlap with imine (–C=N–) stretching bands. A weak –C≡C– peak at 2200 cm−1 was observed for UPy3 (Supplementary Fig. 10c). Additionally, the increased intensities and shifts of the peaks at 1407 and 1460 cm−1 observed for UPy1, UPy2, and UPy3 confirmed the formation of covalent links between UCN and the π-pyrene-π molecules45 (Supplementary Fig. 10b). In addition, the Raman spectra (Supplementary Fig. 11) revealed that UCN and UPy frameworks exhibited characteristic vibration modes of CN heterocycles at 470, 708, 770, 1213, 1232, 1310, and 1350 cm−1. The additional bands at ~1400 and 1560 cm−1 corresponded to C–C/C=C vibrations of the pyrene units in the UPy frameworks. Furthermore, the additional peaks near 1610 cm−1 in UPy frameworks were assigned to C=N bond vibrations, providing additional evidence for the successful covalent linkage of Py1, Py2, and Py3 to UCN46. The 13C nuclear magnetic resonance spectra of UCN, UPy1, UPy2, and UPy3 showed two prominent peaks at 165.4 and 156.5 ppm, assigned to the C(1) atoms of N=C–(N)2 and C(2) atoms of N=C–N(NHx) in heptazine units, respectively47 (Supplementary Fig. 12). The UPy spectra displayed additional peaks in the range of 120–140 ppm, attributed to sp2-bonded skeletal carbons, and weak peaks near 150 ppm, ascribed to covalent linkages between UCN and the π-pyrene-π molecules48.

The deconvoluted C 1s X-ray photoelectron spectrum of UCN (Supplementary Fig. 13a) featured peaks at 284.80 and 288.06 eV, which were assigned to sp2-C bonds originating from extrinsic carbon impurities (used as a calibration reference) and sp2-C bonds in the heptazine rings (N–C=N), respectively. The increased intensity of the former peak observed for UPy1, UPy2, and UPy3 indicated the incorporation of aromatic units from the π-pyrene-π molecules49, consistent with the higher C/N molar ratio observed for these frameworks by elemental analysis (Supplementary Table 1). Elemental analysis was further employed to estimate the π-pyrene-π contents in the UPy frameworks, yielding values of approximately 1.84 wt% for UPy1, 2.87 wt% for UPy2, and 2.26 wt% for UPy3. These values are lower than the nominal feeding ratio (0.1 g UCN vs. 5 mg π-pyrene-π molecules), likely because a fraction of the π-pyrene-π molecules did not successfully couple to the UCN. The new UPy peak at 286.60–286.50 eV was ascribed to C=N bonds and confirmed the formation of covalent linkages50. The deconvoluted N 1s spectra of UCN and the UPy frameworks featured four peaks corresponding to π* excitations, terminal N–H groups, tertiary nitrogen in N–(C)3 moieties, and sp2-hybridized nitrogen in C=N–C moieties44 (Supplementary Fig. 13b). The decrease in the fractional area of the N–H peak observed for the UPy frameworks was ascribed to the consumption of N–H groups during covalent bond formation with the π-pyrene-π molecules51.

The C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of UCN and UPy2 (Fig. 2f) displayed peaks at 288.5 and 288.7 eV, respectively, attributed to the π* (N–C=N) sp2-hybridized states of CN. The peak at ~285.6 eV observed for UPy2 was assigned to π*(C–C/C=C) transitions from the aromatic units of the Py2 moiety. The stronger peak at ~286.2 eV, attributed to π*(C–N=C), suggested the presence of imine linkages between UCN and Py252. The N K-edge NEXAFS spectra of UCN and UPy2 (Fig. 2g) featured peaks near 400.0, 402.0, and 403.0 eV, corresponding to nitrogen in C=N–C, N–(C)3, and C–NHx moieties, respectively53. The spectrum of UPy2 also showed a weak peak at 406.3 eV, which was assigned to the C–N σ* orbital and confirmed the presence of C=N bonds54. Together, these results corroborate the formation of covalent linkages in the UPy frameworks.

PHE performances
PHE tests were conducted in a glass-closed gas system under a negative pressure and visible-light illumination (λ>400 nm, 100 mW cm−2) at 281 K (Supplementary Figs. 14 and 15). Deionized (DI) water containing 10 vol% TEOA was used as a sacrificial reagent, and 3 wt% Pt was loaded onto the catalyst via photoinduced deposition as a hydrogen evolution cocatalyst (see “Methods” for details). As shown in Supplementary Fig. 16, the PHE rates of UPy1, UPy2, and UPy3 were markedly higher than those of pure CN, UCN, or the individual π-pyrene-π molecules. For each UPy framework, the catalysts prepared with the optimal π-pyrene-π molecules (UPy1-5, UPy2-5, and UPy3-3) achieved the highest PHE rates. Further increases in π-pyrene-π contents led to decreased PHE rates, likely due to aggregation of excess π-pyrene-π molecules, which partially shield surface active sites. Among all the UPy frameworks, the UPy2-5 exhibited the highest PHE rate of 17.85 mmol h−1 g−1—357, 64, and 40 times higher than those of Py2 (0.05 mmol h−1 g−1), CN (0.28 mmol h−1 g−1), and UCN (0.44 mmol h−1 g−1), respectively. Although UPy1 and UPy3 also exhibited enhanced PHE performances after Py1 and Py3 loading optimization, their PHE rates remained lower than that of UPy2, possibly because of the stronger electron transfer from pyrene to CN facilitated by the longer π-conjugated chain in UPy2.

Compared with the covalent linkage–containing UPy2, UPy2-π-π, which featured interfacial π-π interactions only, exhibited a lower PHE rate of 4.12 mmol h−1 g−1. This result highlights the critical role of well-regulated charge transfer at the covalent interface of the D-π-A-type UPy2 in achieving efficient PHE. Similarly, a control sample in which Py2 was covalently linked to the edges of bulk CN (CPy2) showed a low PHE rate of 4.94 mmol h−1 g−1, which confirmed the advantage of reassembling UCN via covalent bonding with Py2. During long-term stability testing, the PHE rate of UPy2 (Supplementary Fig. 17) decreased from 17.85 mmol h−1 g−1 in the first cycle to 14.20 mmol h−1 g−1 in the fourth cycle, probably because of the detachment of unstable Py2 molecules from the catalyst surface, and remained stable in the subsequent 11 cycles. This behavior indicates the high long-term stability of UPy2 for PHE applications.

PHE experiments under ambient pressure without inert-gas protection were conducted in a Pyrex vessel connected to a gas circulator using DI water, simulated seawater (3.5 wt% aqueous NaCl), and natural seawater as reaction media under simulated solar light illumination (AM 1.5G, 100 mW cm−2) at 298 K (Supplementary Figs. 18–21). TEOA (10 vol%) and Pt (3 wt%) were used as the sacrificial reagent and cocatalyst, respectively. All UPy frameworks, as well as UPy2-π-π and CPy2, exhibited notable PHE performances under ambient pressure (Fig. 3a and Supplementary Fig. 22). In contrast, neither CN nor UCN produced detectable amounts of hydrogen in any of the media, probably because the small amount of the generated hydrogen could not overcome surface tension to escape from the liquid phase. The π-pyrene-π molecules also exhibited negligible PHE activity, as evidenced by the absence of detectable H₂ signals in the gas chromatography (GC) spectra (Supplementary Fig. 23). UPy2 exhibited the highest activity among the tested catalysts, achieving a PHE rate of 44.50 mmol h−1 g−1 in seawater. The lower PHE rates observed in DI water and NaCl solution suggested the advantageous effect of metal cations present in seawater on PHE.

To further optimize the PHE performance of UPy2 in seawater, we varied the temperature, catalyst mass, Pt loading, TEOA concentration, and sacrificial reagent type. The PHE activity increased with the increasing temperature because of the concomitant acceleration of charge and mass transfer (Supplementary Fig. 24). The non-normalized PHE rate increased with the increasing catalyst mass up to 20 mg and then plateaued (Supplementary Fig. 25a). The catalyst mass–normalized PHE rate reached a maximum at 20 mg and then declined (Supplementary Fig. 25b). These results indicate that the absolute and catalyst mass–normalized hydrogen production rates were not strictly proportional to catalyst mass, probably because of limitations in light absorption and reactant/product mass transport55 (Supplementary Fig. 25c). Thus, the optimal catalyst mass was identified as 20 mg. Under this condition, the PHE rate increased with the increasing Pt content (Supplementary Fig. 26a), equaling 71.65 and 79.13 mmol h−1 g−1 at 5 and 10 wt% Pt, respectively. In addition, UPy2 without Pt loading exhibited negligible PHE activity, as shown in Supplementary Fig. 26b. Considering cost-effectiveness, the optimal Pt loading was selected as 5 wt%. Under these conditions, the PHE rate increased and then decreased with the increasing TEOA content, peaking at 5 vol% TEOA (82.00 mmol h−1 g−1) (Supplementary Fig. 27a). UPy2 achieved a competitive PHE performance compared with previously reported systems under similar conditions (Supplementary Table 2). Even at a low TEOA content of 1 vol%, the PHE rate remained high at 50.19 mmol h−1 g−1, which underscores the essential role of TEOA in PHE for seawater splitting. Unlike most ambient-pressure photocatalytic systems that require inert-gas protection6,56, the UPy2 system maintains high PHE activity under an air atmosphere, without N₂ or Ar purging, highlighting its strong potential for practical applications. In contrast, no hydrogen production was observed for pure seawater (Supplementary Fig. 27b, c) or in the presence of alternative sacrificial reagents, including 10 vol% methanol, 50 mM ascorbic acid, and 0.25 M Na2S/0.35 M Na2SO3 (Supplementary Fig. 27b). These results emphasize the effectiveness of TEOA, which may be due to its coordination with metal cations in seawater enabling efficient hole consumption.

The effect of irradiation wavelength on the PHE performance of UPy2 in seawater under ambient pressure was investigated using catalyst, Pt, and TEOA loadings of 20 mg, 5 wt%, and 5 vol%, respectively. The apparent quantum efficiency (AQE) followed the light absorption profile of UPy2 (Fig. 3b and Supplementary Table 3) and reached 4.77% at 450 nm in seawater. The lower values obtained in the NaCl solution (1.46%) and DI water (0.24%) highlight the high potential of UPy2 for efficient solar-driven seawater splitting under ambient pressure. During long-term stability tests at ambient pressure and 298 K, the PHE rate of UPy2 peaked during the third hour and gradually declined thereafter (Fig. 3c). Following evacuation at the ninth and seventeenth hours, the rate recovered and increased to ~30 mmol h−1 g−1. The reduced activity in the final two cycles compared with that in the first cycle was ascribed to the precipitation of Mg2+ from seawater as Mg(OH)2 on the UPy2 surface, which impeded catalytic activity.

The chemical composition and morphology of UPy2 after PHE in DI water and seawater were analyzed by XRD, FTIR, high-resolution XPS, SEM, and TEM. The XRD patterns and FTIR, C 1s, and N 1s spectra of the two samples were identical to those of pristine UPy2, revealing that the crystalline and molecular structures remained unchanged (Supplementary Figs. 28 and 29). Distinct Mg(OH)2 peaks and weak Ca(OH)2 peaks were observed in the XRD pattern of the seawater-processed sample, indicating the co-precipitation of Mg2+ and Ca2+ during the PHE reaction (Supplementary Fig. 28a). SEM and TEM imaging revealed that the morphology of UPy2 was preserved in both cases (Supplementary Figs. 30 and 31). Additional nanosheets were observed on the surface of the seawater-processed sample, confirming the formation of Mg(OH)2 and Ca(OH)2. The reduction of H+ to H2 during PHE increased the local alkalinity at the catalyst surface, promoting Mg(OH)2 and Ca(OH)2 precipitation. The predominance of Mg(OH)2, accompanied with trace amounts of Ca(OH)2, was further confirmed by Mg 1s, Ca 2p, and O 1s XPS spectra (Supplementary Fig. 32). The absorption spectra of UPy2 after PHE in DI water and seawater remained largely unchanged except for the enhanced absorption at 550–800 nm, which was attributed to photo-deposited Pt nanoparticles on the catalyst surface (Supplementary Fig. 33).

The presence of Pt on the catalyst surface was confirmed by XRD and high-resolution Pt 4f spectra. The XRD patterns (Supplementary Fig. 28a) featured a weak peak at 46° due to the (200) plane of cubic Pt nanoparticles. The Pt 4f spectra of the water- and seawater-processed samples featured three sets of doublets at 73.3/73.2, 70.3/70.2, and 76.2/75.5 eV, indicating the partial reduction of Pt4+ to Pt⁰ and Pt2+, respectively (Supplementary Fig. 29c). TEM imaging revealed the uniform distribution of Pt nanoparticles on the UPy2 surface (Supplementary Fig. 34). Particle size analysis revealed that the PHE reaction in seawater promoted the formation of smaller Pt particles (mean size = 6.7 nm, cf. 13.9 nm in DI water). These smaller Pt nanoparticles probably contributed to the enhanced performance by increasing the interfacial area and facilitating charge transfer between Pt and UPy2. This effect could have resulted from the uniform adsorption of metal cation–TEOA complexes on the catalyst surface, which rapidly captured photogenerated holes, accelerated charge separation, and promoted localized electron accumulation, driving the reduction of Pt4+ to smaller Pt nanoparticles.

When the suspension obtained after the PHE reaction in seawater was transferred to a beaker and illuminated with an Xe lamp, numerous bubbles formed within 2 min and adhered to the inner wall of the beaker (Fig. 3d and Supplementary Movie 1). Furthermore, the exposure of the beaker to natural sunlight also produced abundant bubbles (Fig. 3e and Supplementary Movie 2). A blank control experiment was also conducted by exposing the same reaction solution to outdoor sunlight in the absence of a catalyst, during which no bubble formation was observed (Supplementary Movie 3). A disk-shaped reactor (20 cm diameter, 10 cm height) was designed to accurately evaluate PHE activity using natural seawater under ambient pressure (Fig. 3f and Supplementary Fig. 35). In this setup, UPy2 produced ~170 mL of hydrogen within 1 h (10:00 a.m. to 11:00 a.m.; Supplementary Movie 4), achieving a PHE rate of 134 mmol h−1 g−1. To verify the gas composition, the gas collected from the reactor equipped with a gas collection bag (Supplementary Fig. 36) was analyzed using GC spectrum. A distinct hydrogen peak was observed in Supplementary Fig. 37, confirming that hydrogen evolution occurred under outdoor sunlight irradiation. Furthermore, the successful ignition of gas collected from the disk-shaped reactor (20 cm diameter, 3 cm height) connected to a gas-collecting bag, as shown in Supplementary Movie 5 and Fig. 38, confirms hydrogen generation from PHE in seawater under natural sunlight and ambient pressure57. These results confirm the potential of UPy2 for photocatalytic hydrogen generation from seawater under ambient pressure.

Optical properties and band structures
To further rationalize the high PHE performance of UPy2, we examined the light absorption properties and band structures of UCN, the π-pyrene-π molecules, and the UPy frameworks using ultraviolet–visible diffuse reflectance spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and valence band XPS (VB-XPS). Owing to the broadband absorption of visible light by the π-pyrene-π molecules, their covalent linkage to UCN substantially enhanced its visible-light absorption capacity (Fig. 4a and Supplementary Fig. 39). The spectrum of UPy2 exhibited a pronounced absorption increase in the 420–550 nm range compared with that of UCN. Additionally, the optical absorption edges of the UPy frameworks were red-shifted relative to those of UCN, which was attributed to the expansion of the π-conjugated structure due to the covalent integration of the π-pyrene-π molecules.

The band gaps (Eg) of UCN, UPy1, UPy2, and UPy3 were determined using the transformed Kubelka–Munk function as 2.97, 2.93, 2.81, and 2.89 eV, respectively (Fig. 4b and Supplementary Fig. 40). The valence band (VB) maximum (EVB) and conduction band (CB) minimum (ECB) of UCN were determined as +2.09 and −0.88 eV vs. the reversible hydrogen electrode (RHE), respectively14, based on the corresponding ultraviolet photoelectron spectrum (Supplementary Fig. 41). Based on the VB-XPS profiles of UPy1, UPy2, and UPy3 (Supplementary Fig. 42), the corresponding VB maxima were shifted to 2.31, 2.34, and 2.17 eV vs. RHE, respectively. The CB positions of UPy1, UPy2, and UPy3 were calculated as −0.62, −0.47, −0.72 eV vs. RHE, respectively, according to ECB = −(Eg − EVB) (Fig. 4b). The VB potentials of the UPy frameworks were more positive than that of UCN, suggesting a stronger driving force for the oxidation of TEOA.

Mechanism of charge separation
The photoluminescence quantum yield (PLQY) was employed as an indirect indicator of photogenerated carrier transport efficiency. As shown in Supplementary Fig. 43, pristine UCN exhibited a PLQY of 3.61%, whereas the lower PLQYs of UPy1 (2.12%) and UPy2 (0.39%) indicated that covalent coupling with Py1 or Py2 promoted charge transfer and suppressed radiative recombination58. In contrast, the higher PLQY of UPy3 (9.11%) arises from emissive charge transfer states near 600 nm that enhance radiative recombination but compete with charge separation. The high exciton binding energies (Eb) of organic photocatalysts substantially hinder the dissociation of excitons into free electrons and holes, limiting photocatalytic performance59. This restricted exciton dissociation is the primary reason for the low activity of polymeric UCN catalysts. Herein, TD-PL measurements were conducted to investigate exciton dissociation behavior and determine Eb. The PL intensity decreased with an increase in temperature from 83 to 298 K, indicating enhanced exciton dissociation (Fig. 4c–f). The corresponding Eb values were calculated as ~60 meV for UCN, ~50 meV for UPy1, ~28 meV for UPy2, and ~32 meV for UPy3 by fitting the PL spectra using the Arrhenius equation (see “Methods” for details). These results highlight the promotional effects of the UPy frameworks on exciton dissociation and their exceptional charge carrier generation abilities60.

Inefficient charge transfer and separation is another key bottleneck limiting PHE activity. To address this problem, we examined the effect of covalently linking UCN with the π-pyrene-π molecules on the photoinduced charge transfer and separation properties. The PL spectrum of UCN recorded under 365 nm excitation (Fig. 4g) exhibited a strong emission peak at ~460 nm corresponding to the radiative decay of the lowest excited state. The weakening of this peak in the spectra of the UPy frameworks indicated the suppression of radiative recombination due to enhanced charge transfer between UCN and the π-pyrene-π molecules, which enhanced non-radiative carrier recombination. Moreover, shortened lifetimes (i.e., accelerated PL decay) were observed for the UPy frameworks under 365 nm excitation: 7.22 ns for UCN, 5.46 ns for UPy1, 3.50 ns for UPy2, and 4.39 ns for UPy3 (Fig. 4h and Supplementary Table 4). These results suggest that photoinduced electron transfer from the electron-donating pyrene units to UCN accelerated charge transfer in the UCN matrix.

UPy2 exhibited the highest photocurrent response, the smallest arc radius and resistance (Supplementary Fig. 44 and Supplementary Table 5), which indicated that the covalent linkage of Py2 with UCN enabled charge transfer more efficiently than that in UCN, UPy1, and UPy323. The surface charge transfer properties were further investigated using surface photovoltage (SPV) measurements. UPy2 displayed strong positive SPV signals characteristic of an n-type semiconductor, including a pronounced peak in the 300–520 nm range (Supplementary Fig. 45). This enhanced SPV response indicates improved charge transfer efficiency and electron accumulation on the surface and can be attributed to electron transfer from Py2 to UCN61. To elucidate the driving force of charge transfer, the magnitudes of the built-in internal electric field (BIEF) were quantified using surface charge density (Supplementary Fig. 44b) and SPV measurements (Supplementary Fig. 45)62. As shown in Fig. 4i, all UPy frameworks exhibited higher BIEF intensities than UCN, with UPy2 displaying the strongest field, which is 8.2 times higher than that of UCN. The highly symmetric structure of UCN resulted in a weak BIEF, limiting efficient electron–hole separation63,64. In contrast, covalent incorporation of π-pyrene-π molecules into UCN induced a D–π–A configuration in the UPy frameworks, significantly enhancing electron delocalization and ICT. This structural design effectively strengthens the BIEF and contributes to the high charge-separation efficiency observed for UPy265. Photo-assisted Kelvin probe force microscopy (KPFM) provided additional insights into surface charge dynamics. UCN showed a surface potential decrease of 15 mV under illumination, which indicated a reduction in photogenerated electron density (Supplementary Fig. 46a–c). In contrast, UPy1, UPy2, and UPy3 exhibited surface potential increases of 4, 67, and 51 mV, respectively (Supplementary Fig. 46d–l), which suggested that the photoinduced electrons in the UPy frameworks mainly migrated to the surface, driven by enhanced electron transfer from the π-pyrene-π molecules to UCN66. Subsequently, we conducted electron paramagnetic resonance spin-trap experiments using 5,5-dimethyl-1-pyrroline n-oxide (DMPO) to detect the reactive oxygen species generated during the photocatalytic process. The signal intensity of the DMPO–•O2− adduct produced under illumination followed the order UPy2>UPy3>UPy1>UCN (Supplementary Fig. 47). The strongest signal was observed for UPy2, confirming its efficient charge separation and surface electron accumulation, indicative of an enhanced electron reduction capability61.

Insights into charge transfer kinetics
The charge transfer dynamics were further probed by fs-TAS, with measurements conducted using 325 nm laser pulses and a white-light probe (350–650 nm). Supplementary Fig. 48 and Fig. 5a, b present the two-dimensional contour plots and kinetic profiles of UCN and UPy2. Positive signals in the 500–750 nm range indicate excited-state absorption (ESA) from trapped photogenerated electrons67. The corresponding kinetic decay traces at 700 nm, a representative ESA wavelength, are shown in Fig. 5c, d. Pristine UCN exhibited three decay components, namely τ1 = 2.5 ps—corresponding to rapid electron capture by intrinsic shallow-trap (ST) states, τ2 = 31.6 ps—associated with slower band-edge exciton recombination, and τ3 = 443.0 ps—ascribed to long-lived charge separation via deep-trap (DT) states (Fig. 5e)68. The inclusion of a long-time offset constant (y0) further highlight the contribution of persistent photogenerated carriers that sustain photocatalytic activity. In contrast, UPy2 exhibited four decay components: τ1 = 4.1 ps, τ2 = 52.5 ps, τ3 = 742.1 ps, τ4 = 1683 ps, indicating that D–A covalent coupling substantially slows recombination and enhances charge separation. Specifically, τ1 is assigned to ultrafast ICT between the UCN and pyrene unit; τ2 corresponds to band-edge recombination; τ3 reflects deep-trap stabilization of separated charges; and τ4 represents a long-lived charge-separated (CS) state extending beyond the ps regime69,70. The emergence of these slower components (τ3 and τ4) confirms the markedly improved electron–hole separation efficiency in UPy271. The charge transfer dynamics of UPy1 and UPy3 were also evaluated using fs-TAS (Supplementary Fig. 49). The covalent linkage of pyrene to the CN framework through alternative π-bridges (a single benzene ring or a benzene-linked alkynyl group) also helped prolong the charge carrier lifetime. The longest average lifetime was observed for UPy2, which suggested that the incorporation of a biphenyl moiety π-bridge most effectively suppressed charge recombination by enhancing electron delocalization between CN and pyrene.

Given the strongly pump fluence-dependent kinetics on CN-based systems, fs-TAS measurements for both UCN and UPy2 were conducted at four pump energy densities: 0.24, 0.37, 0.96, and 1.94 mJ cm−2. As shown in Supplementary Fig. 50 and Fig. 5f, g, both UCN and UPy2 exhibited pronounced pump-fluence-dependent fs-TAS decay kinetics. The initial ΔA amplitudes at ~500 fs increased linearly with excitation fluence, with slopes close to unity (>0.96), indicating fluence-independent initial electron generation (Fig. 5h). All decay traces were well fitted by a power-law model, ΔA(t) = α·(t−t₀)−β, with β consistently falling within 0.20±0.02 (Supplementary Table 6)72. Under linear-response conditions, the transient absorption signal ΔA(t) was proportional to the population of the long-lived charge carriers contributing to the probed band. Accordingly, the parameter α reflects the carrier density at the onset of the power-law regime. Across all fluences, UPy2 displayed substantially larger α values than UCN, demonstrating that UPy2 sustained a higher density of long-lived charge-separated carriers immediately after excitation73. The extracted half-times (t0.5) further revealed distinct recombination dynamics. Both UCN and UPy2 displayed shorter t0.5 at higher fluences, consistent with fluence-enhanced bimolecular or higher-order recombination processes. However, under identical excitation conditions, UPy2 consistently showed longer t0.5 than UCN, indicating slower carrier recombination and more efficient stabilization of long-lived charge carriers74. Additional differences were observed in the residual electron population at ~2 ns. For UPy2, the fraction of long-lived electrons decreased from 31% at low fluence to 21.7% at high fluence, whereas UCN showed negligible variation across the same range. The stronger fluence dependence in UPy2 arises from its larger initial carrier densities, which accelerate bimolecular recombination at high fluence. Importantly, the reduced residual electrons at high fluence do not imply weaker charge separation but rather reflect redistribution of an initial larger carrier population into long- and short-lived recombination channels. In contrast, the weak fluence response of UCN was consistent with its inherently limited charge-separation capability and the absence of an efficient ICT pathway72. Collectively, the larger α values, longer decay half-times, and stronger fluence-dependent modulation of long-lived electrons in UPy2 confirmed its markedly enhanced electron–hole separation compared with UCN.

To further investigate how visible-light excitation influences the excited-state dynamics of the D–π–A UPy systems, the fs-TAS responses of the three π-pyrene-π molecules and the corresponding UPy frameworks under 450 nm excitation were compared (Supplementary Figs. 51 and 52). The π-pyrene-π molecules displayed a strong stimulated-emission (SE) band at ~600 nm and broad ESA extending to ~750 nm. These features were markedly suppressed in UPy frameworks. The reduced SE and ESA intensities indicated the rapid formation of an ICT state between the pyrene units and UCN. Correspondingly, the kinetic traces of the UPy frameworks exhibited significantly slower decay dynamics with prominent long-lived components, consistent with the presence of stabilized ICT or charge-separated state75. These findings demonstrated that covalent linkage of π-pyrene-π molecules to UCN enhances electron–hole separation and prolongs excited-state lifetimes under visible-light excitation.

The synchronous illumination XPS measurements were conducted to investigate the charge transfer dynamics of the UPy frameworks under illumination. Under visible-light irradiation, the N–C=N peak in the C 1s spectrum and all peaks in the N 1s spectrum of UCN noticeably shifted to higher binding energies, which indicated a reduction in the electron density around the heptazine ring (Supplementary Fig. 53). In contrast, only minimal shifts to lower binding energies were observed for the C 1s and N 1s spectra of UPy2, suggesting that photogenerated electrons were transferred from the pyrene units of Py2 to the heptazine ring of UCN, increasing the electron density at the ring76. The substantial negative shift in the π-excitation features of UPy2 indicated an increased π-electron density, attributed to the extended π-conjugated structure of this framework. To further highlight the mechanistic significance of covalent D–A linkage, the charge transfer properties of UPy2 and UPy2-π-π were compared. Evidence from PL quenching, faster PL decay, and significantly prolonged fs-TAS decay lifetime (Supplementary Fig. 54) confirmed that UPy2 suppresses radiative and non-radiative recombination far more effectively than UPy2-π-π. This strengthened electronic coupling and ICT efficiency enabled by covalent bonding are key factors underlying the improved PHE performance77.

The kinetic behavior of excited-state electrons and holes in UCN and UPy2 was investigated using time-dependent density functional theory (TD-DFT) calculations. The UCN model suggested a disordered distribution of excited electrons and holes, whereas the UPy2 model showed a more pronounced tendency toward spatial separation, with excited electrons and holes localized on the heptazine ring and pyrene units, respectively (Supplementary Fig. 55 and Supplementary Data 5). Furthermore, UPy2 model structures containing two or three heptazine ring segments covalently linked to one Py molecule (U2Py2 and U3Py2) were constructed, and their excited-state charge distributions were investigated in Supplementary Figs. 56 and 57, and Supplementary Data 6 and 7. These models exhibited similar trends of spatial separation of excited electrons and holes, localized on the heptazine ring and pyrene units, respectively. This spatial separation suggests electron transfer from the pyrene unit to the heptazine ring, accompanied by reverse hole transfer under excitation22. Thus, electron accumulation in the heptazine ring region may facilitate proton reduction and enhance the adsorption of metal cations in seawater. As displayed in Supplementary Table 7, the UPy2, U2Py2, and U3Py2 models showed larger oscillator strengths than the UCN model, which can be attributed to extended π-conjugation and D–A alignment. These features are expected to enlarge the transition dipole moment and contribute to stronger visible-light absorption, enhanced intramolecular fields, and more efficient charge separation, collectively leading to the observed improvement in PHE performance78.

Mechanism of PHE in seawater
To elucidate the origin of the high PHE performance of UPy2 in seawater, we examined its behavior in simulated seawater containing varying concentrations of NaCl and MgCl2 (Fig. 6a and Supplementary Fig. 58a, b). The volcano-shaped relationship between the hydrogen production rate and salt concentration indicates that the PHE performance was maximized at optimal NaCl and MgCl2 concentrations. Owing to their high concentration in natural seawater, Cl− ions can be oxidized to dissolved Cl2 and HClO, which are typically detected using their chromogenic reactions with KI5. However, no color change was observed upon the addition of KI to the post reaction mixture, which suggested the absence of these species, probably because of the suppressed electrostatic adsorption of Cl− on the UPy2 surface. Subsequently, Cl− was replaced with alternative anions. The comparable PHE rate obtained in a 5.1 wt% NaNO3 solution (which had a Na+ concentration equivalent to that of the 3.5 wt% NaCl solution) indicated the important role of Na+ in enhancing PHE (Supplementary Fig. 58c). Additionally, the PHE performance improved when DI water was spiked with metal cations at seawater-relevant concentrations (0.074 wt% KCl and 0.15 wt% CaCl2) (Supplementary Fig. 58d). This enhancement was ascribed to the coordination of these cations by the electron-deficient heptazine ring of UPy2, which promoted TEOA adsorption and accelerated hole consumption.

Given that cations tend to bind TEOA through ionic interactions with its –OH groups to form cation–TEOA complexes79, the adsorption behavior of these complexes on UCN and UPy2 was investigated using DFT calculations. The energies of Na+–TEOA (−1.24 eV) and Mg2+–TEOA (−2.27 eV) adsorption on UPy2 were substantially more negative than those observed for CN (−0.44 eV and −2.11 eV, respectively), indicating that the covalent linkage of Py2 to UCN enhanced the adsorption of both complexes (Fig. 6b, Supplementary Figs. 59–62, and Supplementary Data 8). This finding is consistent with an enhanced affinity of the heptazine units in UPy2 toward metal–triethanolamine complexes, which may be associated with increased electron density arising from intramolecular charge redistribution involving the pyrene moiety. The TD-DFT-based charge distribution analysis of Na+–TEOA and Mg2+–TEOA adsorbed on UPy2 indicated a tendency toward electron–hole separation, with electrons preferentially redistributed toward the heptazine ring under excitation (Fig. 6c, Supplementary Figs. 63 and 64, and Supplementary Data 9 and 10). This electron redistribution may enhance Na+–TEOA adsorption and promote charge separation. As a result, hole consumption by TEOA at both the Py2 and Na+–TEOA sites was facilitated, while proton reduction (and, hence, hydrogen generation) occurred at the UCN site. Collectively, these effects contributed to the high PHE performance of UPy2 in seawater. Because photogenerated holes are primarily localized on the pyrene core, the adsorption behavior of uncoordinated TEOA on Py2 and UPy2 was investigated. The adsorption energy of TEOA on UPy2 was slightly more negative than that of Py2 (Supplementary Figs. 65 and 66), suggesting that covalent linkage of Py2 to UCN enhanced hole accumulation at the pyrene core and facilitated TEOA adsorption. Moreover, TD-DFT-based charge distribution analysis of TEOA-adsorbed UPy2 showed a similar trend of excited-state charge redistribution, accompanied by an increase in local electron density around the heptazine ring (Supplementary Fig. 67 and Supplementary Data 11).

To further evaluate the TEOA adsorption capacity, we measured the zeta potentials of UCN and UPy2 in different solutions (DI water, DI–TEOA, and seawater–TEOA) at room temperature. UCN and UPy2 exhibited negative zeta potentials across all conditions (Supplementary Figs. 68 and 69). UPy2 showed a larger decrease in zeta potential (−9.5 mV) upon dispersion in DI–TEOA than UCN (−3.3 mV), thus featuring stronger TEOA adsorption despite having a two-fold lower specific surface area. When dispersed in seawater–TEOA, UPy2 exhibited a greater positive shift in zeta potential (+10.7 mV) than UCN (+5.5 mV), which confirmed the efficient ability of the former to adsorb metal cations from seawater. The enhanced TEOA adsorption was further supported by the results of total organic carbon (TOC) analysis (Supplementary Table 8), with the lower TOC content of UPy2 in seawater–TEOA indicating that the presence of salt ions enhanced TEOA adsorption80. Collectively, the theoretical and experimental results demonstrate that the covalent functionalization of CN with electron-donating pyrene units via π-bridges facilitates metal ion and TEOA adsorption, accelerates hole consumption, and enhances charge carrier separation in seawater.

The mechanism underlying the high PHE performance of UPy2 in seawater was further investigated by measuring its charge transfer and separation properties in different metal cation–containing solutions (NaCl, KCl, MgCl2, and CaCl2). Compared with that in DI water, UPy2 in metal cation–containing solutions exhibited a decreased PL intensity and shorter PL lifetimes (Fig. 6d, e and Supplementary Table 9), which indicated that metal cations facilitated the separation of photogenerated charge carriers. Additional PL measurements were conducted for UPy2 in DI water, seawater, and seawater containing TEOA. The PL intensity and lifetime decreased upon going from DI water to seawater, which was attributed to the adsorption of metal cations passivating radiative recombination centers on the UPy2 surface (Fig. 6f, g; Supplementary Table 10). The presence of TEOA in seawater further suppressed radiative recombination and enhanced charge separation in UPy2.

Finally, given the notably enhanced PHE performance of UPy2 in the MgCl2 solution, we examined the role of Mg2+ in the PHE process. The TEM imaging of UPy2 after PHE in seawater revealed the formation of ultrathin Mg(OH)2 nanosheets during the reaction (Supplementary Figs. 26 and 29). Although the Mg(OH)2 precipitate partially covered the PHE active sites of UPy2, the observed distribution of Pt nanoparticles on Mg(OH)2 suggests its electron-donating capability, as supported by the energy shift in the insitu Mg 1s spectrum after illumination (Supplementary Fig. 70). These findings indicate that Mg(OH)2 may also provide active sites for the PHE reaction. The role of Mg(OH)2 in charge transfer was examined using PL measurements under 365 nm excitation. Seawater-processed UPy2 exhibited the lowest PL intensity and fastest PL decay rate, which indicated that Mg(OH)2 precipitation promoted electron transfer and separation, thereby suppressing charge carrier recombination and reducing carrier lifetimes (Fig. 6h, i; Supplementary Table 11).

Methods
Materials
Urea (CO(NH2)2, 99.5%), anhydrous ethanol (C2H6O, 99.8%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Pt 37.5% min), sodium chloride (NaCl, 99.8%), magnesium chloride (MgCl2, 99%), potassium chloride (KCl, 99.99%), and calcium chloride anhydrous (CaCl2, 94%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. 4,4′,4′′,4′′′-(Pyrene-1,3,6,8-tetrayl)tetrabenzaldehyde (Py1, 97%), 4′,4′′′,4′′′′′,4′′′′′′′-(Pyrene-1,3,6,8-tetrayl)tetrakis(([1,1′-biphenyl]-4-carbaldehyde)) (Py2, 98%), and 4,4′,4′′,4′′′-(Pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzaldehyde (Py3, 95%) were obtained from Bide Pharmatech Co., Ltd. Acetic acid (C2H4O2, 99.8%), triethanolamine (C6H15NO3, 99%), and sodium nitrate (NaNO3, 99.0%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All chemicals were used as received without further purification. Deionized (DI) water with a resistivity of 18.25 MΩ cm was used throughout the experiments.

Synthesis of ultrathin CN nanosheets (UCN)
CN was synthesized via thermal polymerization of urea. Typically, 30 g of urea powder was placed in a covered ceramic crucible (100 mL) and calcined in air at 823 K for 4 h with a ramping rate of 10 K min−1. The resulting yellow powder was collected as bulk CN. Ultrathin CN nanosheets (denoted as UCN) were then obtained by direct thermal oxidation etching of bulk CN (0.5 g) at 793 K for 6 h with a ramping rate of 10 K min−1. The final product was collected with an approximate yield of 20%.

Synthesis of UPy frameworks
UCN (0.1 g) was dispersed in 40 mL of anhydrous ethanol and ultrasonicated for 6 h. A solution of x mg Py2 in 10 mL of absolute ethanol was added dropwise to the suspension under stirring. Subsequently, 0.1 mL of acetic acid was added, and the resulting mixture was transferred to a thermostatic oil bath and refluxed at 353 K for 24 h under continuous stirring. After naturally cooling to room temperature, the product was centrifuged, washed with ethanol, and then freeze-dried for 48 h. The resulting samples were denoted as UPy2-x, where x indicates the mass of Py2. The UPy1-x and UPy3-x samples were synthesized following the same procedure as UPy2-x, using Py1 and Py3, respectively. For comparison, CPy2-3 was synthesized using the same method as UPy2-3, except that bulk CN was used instead of UCN. The UPy2-π-π composite, featuring interfacial π-π interaction, was synthesized by adding 5 mg of Py2 to the UCN suspension and stirring at room temperature for 24 h. The suspension was then centrifuged and freeze-dried for 48 h.

Characterization
XRD patterns were recorded using a Rigaku Ultima Ⅳ X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA). FTIR spectra were acquired on a Vertex 70-Hyperion 3000 FTIR spectrometer using the KBr pellet technique. Solid-state 13C nuclear magnetic resonance (NMR) spectra were measured on a Bruker Ascend 600 MHz NMR instrument. Elemental analysis was performed using a Vario EL Cube elemental analyzer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained using a JEOL JSM-IT200A SEM and a Thermo Fisher Talos 200X G2 TEM (200 kV), respectively. Synchrotron-based X-ray spectroscopic measurements at the C and N K-edge were conducted at the Catalysis and Surface Science Endstation, beamline BL11U, at the National Synchrotron Radiation Laboratory, Hefei, China. Nitrogen adsorption-desorption isotherms were recorded using a Micromeritics ASAP 2460 3.01 instrument at 77 K, after degassing the samples at 393 K for 6 h. Specific surface area and pore volume were analyzed using the Brunauer-Emmett-Teller method. UV-vis diffuse reflectance spectra were recorded using a Techcomp UV-2600 spectrometer equipped with an integrating sphere. UPS spectra were acquired using a Thermo Fisher ESCALAB Xi+ spectrometer. Insitu XPS spectra were measured using a Thermo-VG Scientific ESCALAB 250 photoelectron spectrometer under λ≥400 nm light excitation. The C 1s signal at 284.80 eV was used as a reference for binding energy calibration. All XPS analyses were performed under high vacuum (P<10−9 mbar) with a pass energy of 100 eV for survey scans and 30 eV for high-resolution scans. TOC was measured by a Total Organic Carbon Analyzer (Shimadzu TOC-L). 10 mg of catalyst was dispersed into a 20 mL solution (DI water or seawater) containing 5 vol% TEOA by ultrasonication for 60 min. The resulting suspension was filtered by a 0.22 μm pore size membrane. 100 μL of filtrated solution was added to 30 mL DI water for TOC measurement. Photoluminescence (PL) emission quenching, PLQY, PL decay, and TD-PL spectra were collected using an Edinburgh FLS-1000 spectrofluorometer. The average lifetime (τavg), PLQY, and exciton binding energy (Eb) were calculated using the following Eqs. (1–3):

τavg = (A1τ1^2 + A2τ2^2) / (A1τ1 + A2τ2) (1)

PLQY = (RL / (ES - ER)) × 100% (2)

I(T) = I0 / (1 + k exp(-Eb / kBT)) (3)

where L is the emission spectrum of the sample; and ES and ER are the excitation light with and without the sample in the integrating sphere, respectively. I0 is the PL intensity at 0 K, kB is the Boltzmann constant. fs-TAS was conducted using a Helios pump-probe system (Ultrafast Systems LLC), coupled with a femtosecond laser system (Coherent). A 325 nm pump pulse (~0.1 μJ/pulse) was generated via a TOPAS-800-fs optical parametric amplifier, excited by a Ti:sapphire regenerative amplifier (Legend Elite-1K-HE, 800 nm, 35 fs, 7 mJ/pulse, 1 kHz) and seeded by a mode-locked Ti:sapphire laser system (Micra 5) and a Nd: YLF laser (Evolution 30). White-light continuum (WLC) probe pulses (350–650 nm) were generated by focusing 800 nm beams (400 nJ/pulse) onto a CaF2 plate. A reference beam was used to compensate for pulse-to-pulse WLC fluctuations. A motorized optical delay line controlled the pump-probe delay (0–8 ns). The instrument response function was determined to be 100 fs via cross-correlation. A mechanical chopper (500 Hz) modulated the pump pulses to acquire spectra alternately with and without excitation. For testing, 0.1 g L−1 samples were dispersed in ethylene glycol and loaded into quartz cuvettes with a 2 mm path length. SPV spectra were measured using a CEL-SPS1000 spectrometer. The BIEF magnitude was calculated using the Kanata model as follows (Eq. 4):

Fs = (ρ / εε0) * 2Vs (4)

where Fs denotes the magnitude of the BIEF, Vs stands for the surface potential, ρ stands for the surface charge density, ε is the low-frequency dielectric constant, and ε0 is the permittivity of free space. Because both ε and ε0 are constants, the magnitude of BIEF is governed by the values of ρ and Vs81. Electron spin resonance spectra of spin-trapped radicals (with 5,5-dimethyl-1-pyrroline n-oxide, DMPO, in methanol) were recorded using a Bruker EMX plus-6/1 spectrometer. KPFM was performed on a Bruker atomic force microscope equipped with a light irradiation system to assess surface morphology and potential. Zeta potentials of aqueous suspensions were measured via dynamic light scattering on a Malvern Zetasizer Nano ZS90.

Photocatalytic hydrogen evolution (PHE) test
PHE under negative pressure. PHE under negative pressure was conducted in a Pyrex reaction vessel maintained at 281 K, connected to a closed gas circulation system (Labsolar IIIAG, Beijing Perfectlight Technology Co., Ltd.; Supplementary Fig. 14). In a typical experiment, 10 mg of photocatalyst was dispersed in 100 mL of an aqueous solution containing 10 vol% triethanolamine (TEOA) as a hole scavenger. A 3 wt% Pt cocatalyst was deposited via insitu photo-deposition using H2PtCl6. A 300 W Xe lamp (Microsolar 300, Beijing Perfectlight Technology Co., Ltd.) equipped with a 400 nm cut-off filter was employed as the visible light source, with the irradiation intensity calibrated to ~100 mW cm−2. The amount of evolved H2 was quantified using a gas chromatograph (GC7900, TCD, Ar carrier gas). The H2 evolution curve under negative pressure is shown in Supplementary Fig. 15a.

PHE under ambient pressure. PHE under ambient pressure was carried out in a Pyrex vessel connected to a gas circulation system (Supplementary Fig. 18). The reaction temperature was maintained at 298 K using a cooling chiller. A 300 W Xe lamp with an AM 1.5G filter (Microsolar 300, Beijing Perfectlight Technology Co., Ltd.) served as the simulated solar light source, and the irradiation intensity was calibrated to ~100 mW cm−2. The corresponding simulated solar spectrum is provided in Supplementary Fig. 19. Gas samples (1 mL) were extracted hourly using a syringe and injected into the closed gas system for H2 quantification via gas chromatography (GC7900, TCD, Ar carrier). H2 evolution was measured under ambient pressure conditions to construct a calibration curve based on peak area, which was then used to determine H2 production during PHE measurements. The gas chromatography spectrum of H2, O2, and N2 is displayed in Supplementary Fig. 21. The H2 evolution curve under ambient pressure is shown in Supplementary Fig. 15b. Natural seawater was collected from the South China Sea (19°37.421′ N, 114°12.334′ E). The wavelength-dependent H2 evolution was investigated using a Microsolar 300 Xe lamp equipped with bandpass filters (λ=380, 420, 450, 475, and 500 nm). The light intensity was measured using a PL-MW2000 optical power meter and calculated by the five-point method. The distance between the probe and the xenon lamp was adjusted to match the distance between the Xe lamp and the liquid surface during the photocatalytic reaction (Supplementary Fig. 20)77. For AQE measurements, 20 mg of photocatalyst and a trace amount of H2PtCl6 were dispersed in 100 mL of DI water, 3.5 wt% NaCl solution, or natural seawater, each containing 5 vol% TEOA. The AQE was determined from a single experiment under steady-state conditions and calculated using the following Eq. (5); therefore, no statistical error analysis is provided.

AQE(%) = (2 × Number of H2 molecules) / (Number of incident photons) = (2 × RH2 × NA) / (S × P × t × λ / hc) × 100% (5)

RH2 represents the amount of produced H2 (μmol), NA is Avogadro’s constant, S denotes the illumination area, P and λ correspond to the intensity (W cm−2) and wavelength (nm) of the monochromatic light, respectively, t is the irradiation time (s), h is Planck’s constant, and c is the speed of light82.

PHE test in natural seawater under ambient pressure and natural sunlight was performed in a custom-built disk reactor made entirely of quartz with a top quartz window. The reactor had a diameter of 20 cm and a height of 10 cm. Approximately 40 mg of photocatalyst was dispersed in 200 mL of seawater containing 5 vol% TEOA and transferred into either the panel or disk reactor. On November 26, 2024, at 10:00 a.m., the solar irradiance was measured to be 95.5 mW cm−2. The evolved H2 gas was collected via the upward water displacement method (Supplementary Fig. 35).

Photoelectrochemical measurements. Photoelectrochemical measurements, including linear sweep voltammetry, transient photocurrent response, and electrochemical impedance spectroscopy (EIS), were performed using a CHI660C electrochemical workstation with a standard three-electrode configuration. A KCl-saturated Ag/AgCl electrode served as the reference electrode, and a platinum plate was used as the counter electrode. The working electrode was prepared as follows: 5 mg of the solid catalyst and 5 μL of 0.5% Nafion solution were ultrasonically dispersed in a mixture of 0.5 mL ethanol and 0.5 mL ultrapure water for 2 h. Subsequently, 5 μL of the resulting catalytic ink was drop-cast onto a L-type glassy carbon electrode and dried under an infrared lamp. A 0.5 M Na2SO4 aqueous solution (30 mL) was used as the electrolyte with the pH=6.8±0.2. Illumination (λ>400 nm) was provided by a 300 W Xe lamp (Microsolar 300, Beijing Perfectlight Technology Co., Ltd.). The transient photocurrent response was recorded under chopped light irradiation at 20-s intervals without applying an external bias. The linear sweep voltammograms curves were referred to the reversible hydrogen electrode (RHE): ERHE = EAg/AgCl + 0.197 + 0.0591·pH. EIS measurements were conducted in the dark over a frequency range of 0.1 to 105 Hz at the corresponding open-circuit potential. For EIS testing, the electrolyte consisted of a 1:1 mixture of 0.1 M K3Fe(CN)6 and K4Fe(CN)6 aqueous solution.

Theoretical calculations. The theoretical calculations were performed via the Gaussian 16 suite of programs. The geometric structures of the target molecules (CN, Py1, Py2, Py3, UPy1, UPy2, and UPy3) were optimized using the B3LYP-D3BJ functional with the 6-31G(d) basis set. Vibrational frequency calculations were performed at the same level of theory to confirm that all frequencies are real, thereby verifying that each structure corresponds to a local minimum on the potential energy surface. The molecular dipole moments were evaluated using the larger 6-311+G(d) basis set83. The Visual Molecular Dynamics program was used to plot the color-filled iso-surface graphs to visualize the molecular electrostatic potential. The HOMO and LUMO energy levels of CN and UPy models were calculated using DFT calculations with B3LYP functionals and a 6-31G (d, p) basis set. It should be noted that these DFT calculations are based on simplified molecular models and do not fully capture the structural complexity and dynamic catalytic environment of the actual materials. Therefore, the theoretical results are intended to provide qualitative insights into possible charge redistribution and electronic structure trends rather than a quantitative description under real catalytic conditions. Excited-state calculations were carried out using TD-DFT at the B3LYP/6-31G(d) level. Hole-electron analysis for the S1–S10 states was conducted using the Multiwfn program. The adsorption energy (ΔEads) was defined as the following Eq. (6):

ΔEads = Ead/sub − Ead − Esub (6)

where Ead/sub is the total energy of the optimized adsorbate/substrate system, Ead is the energy of the adsorbate in its isolated structure, and Esub corresponds to the energy of the clean substrate. A more negative Eads value generally indicates stronger adsorption.

The Gibbs free energy change (ΔG) was calculated according to Eq. (7):

ΔG = ΔE + ΔZPE − ΔTS (7)

where ΔG, ΔE, ΔZPE, and ΔTS represent the Gibbs free energy, the total energy change obtained from DFT calculations, the zero-point energy correction, and the entropic contributions (with T=298 K), respectively84. The change in Gibbs free energy of a molecule is calculated according to the following Eq. (8):

ΔG(ab) = ΔG(a) − ΔG(b) (8)

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors report the synthesis of donor–π–acceptor (D–π–A) structured carbon nitride (UPy) frameworks by covalently linking ultrathin CN nanosheets to electron-donating pyrene units via various π-bridges. This design aims to achieve efficient photocatalytic seawater splitting under ambient pressure and natural sunlight. The study demonstrates that employing biphenyl moieties as π-bridges in D–π–A frameworks significantly reduce exciton binding energy and enhances electron delocalization and charge separation, thereby leading to an impressive hydrogen evolution rate of 134 mmol h⁻¹ g⁻¹. This result highlights the potential of the developed materials for practical solar-driven hydrogen production in natural seawater. Furthermore, the authors systematically investigate the influence of metal ions in seawater on the catalytic mechanism through a combination of control experiments, spectroscopic studies, and theoretical calculations. The work is representative and insightful in the field of photocatalytic water splitting. I recommend the publication of the manuscript in Nature Communications after minor revisions.\n1. Electron delocalization is beneficial for the formation of the build-in electric field. Please quantify the intensity of the built-in electric field toward UCN and UPy frameworks in this work (Angewandte Chemie, e202507028; Applied Catalysis B: Environment and Energy 362, 124724).\n2. In Figure S23, the UPy2 catalyst exhibits strong dependence on Pt loading, with optimal performance at 5 wt%. This is notably higher than the typical optimal loading (<3 wt%) for Pt in carbon nitride or other organic semiconductor-based photocatalytic systems. Why such a high Pt loading is effective here without causing performance degradation, the reasons should be specified.\n3. To better understand the role of metal ions, it would be helpful to include additional DT-DFT charge distributions for Na⁺–TEOA and Mg²⁺–TEOA adsorbed on UPy2 (ACS Catalysis 14 (18), 13768-13780).\n4. Hydrophilicity plays a crucial role in photocatalytic activity. The discussion should be provided, if possible, quantify the hydrophilicity differences between UCN and the UPy frameworks.\n5. In Supplementary Figures 28 and 29, the authors confirm the formation of Mg(OH)₂ on UPy2 after PHE in seawater. Was Ca(OH)₂ also observed?\n6. What are the PHE performances of the pure pyrene derivatives Py1–3 under ambient pressure in seawater? The data is needed for a better understanding.", "reviewer_2": "In this manuscript (#NCOMMS-25-66364-T), the authors synthesized a series of highly conjugated UPy frameworks through covalent incorporation of pyrene units and π-bridges into a carbon nitride backbone. Among these materials, UPy2 demonstrated the highest photocatalytic hydrogen evolution performance under ambient conditions, with rates of 82 mmol h-1 g-1 under simulated sunlight and 134 mmol h-1 g-1 under natural sunlight. Time-resolved photoluminescence (PL) and femtosecond transient absorption (fs-TA) spectroscopy revealed that UPy2—which features a biphenyl π-bridge—exhibits a significantly reduced exciton binding energy and promotes more efficient charge separation between the carbon nitride and pyrene units compared to both UCN and the other UPy frameworks. The study is interesting; however, the following critical concerns must be satisfactorily addressed before it can proceed to the next stage of evaluation.\n1. The PL quantum yields (PLQYs) of all samples should be provided, as a low PLQY may lead to unreliable exciton binding energy (Eb) values derived from temperature-dependent photoluminescence measurements.\n2. The analysis of the fs-TA kinetics for UCN (Fig. 5c) should be refitted, as the current fitting parameters provided in the figure fail to accurately reproduce the data due to the absence of a long-time constant (or offset). Without this component, a physically meaningful kinetic model cannot be properly established. Additionally, the kinetic scheme proposed for UCN in Fig. 5e is incorrect and requires revision.\n3. The analysis of the fs-TA kinetics for UPy2 (Fig. 5d) also appears to lack a long-time constant (or offset), indicating that the current fitting parameters may be inadequate.\n4. The pump energy density employed in the fs-TA kinetic measurements must be clearly specified, as PCN-based systems are known to exhibit strongly pump fluence-dependent kinetics (J. Am. Chem. Soc. 2019, 141, 11219). Given this dependence, the kinetic differences observed between UCN and UPy2 at high pump energy densities may not stem solely from structural differences. The authors should either conduct the fs-TA study under a relatively low pump energy density or provide pump-fluence-dependent kinetic data for both UCN and UPy2.\n5. Although the authors employed FTIR, XPS, NMR, and NEXAFS to confirm the successful linkage of Py1, Py2, and Py3 to CN, additional Raman spectroscopy characterization would provide further supportive evidence.", "reviewer_3": "Li and coworkers introduced pyrene (Py) structural units into carbon nitride (CN) via covalent linkage, achieving a maximum hydrogen production rate of 134 mmol h⁻¹ g⁻¹ in photocatalytic seawater splitting, which is an excellent result among g-C3N4-based photocatalysts. The author also discussed the promoting effects of metal ions in seawater on the catalytic process. These results are interesting. However, there are some main issues with the mechanism discussion in the manuscript that need to be addressed before I can recommend publication in Nature Communication.\n1.The percentage content of the Py in UPy frameworks should be determined and stated. The naming (UPy-x) in the Synthesis Method section is related to the mass of Py, but no relevant discussion is found in the manuscript—maybe I am mistaken. In addition, the effect of different Py contents on the catalytic activity should be discussed.\n2.The absorption edges of the UPy1-3 frameworks seem to exhibit a blue-shift compared with those of the single-component Py1-3. How to explain this? No significant emergence of new peaks or disappearance of old peaks is observed in the absorption spectra of UPy, which seems insufficient to support the donor-acceptor (D-A) charge transfer process claimed by the authors.\n3.The hydrogen production activity of UPy is significantly higher than that of the single-component CN and Py. What is the significance of the covalent linkage between the two components in UPy for the enhanced activity? Would simply compositing Py and CN also lead to an improvement in catalytic activity?\n4.For FIG. 5C and FIG. 5D, the corresponding R² values are not provided. From the graphs alone, the double-exponential fitting in FIG. 5C does not seem to be the most appropriate; a triple-exponential fitting might lead to other conclusions.\n5.Could the transient absorption (TA) spectra of the three small molecules be provided?\n6.The pump light wavelength for transient absorption spectroscopy is selected as 365 nm. Would the photophysical process under excitation at this wavelength differ from that under visible light excitation? Have experiments been conducted pumped by visible light?\n7.DFT calculations provide the electron distribution of the excited states based on a model structure consisting of one heptazine ring segment and one Py molecule. The oscillator strength data of the corresponding excited states should be provided to understand the contribution of different excited states. In addition, have calculations been performed for the case where one Py molecule is linked to multiple CN segments?\n8.Since the photogenerated holes are distributed on the pyrene core, shouldn't the adsorption of TEOA on pyrene be studied, rather than on the heptazine ring?", "reviewer_4": "This paper covalently linked ultrathin CN nanosheets to electron-donating pyrene units via different π-bridges. The optimal catalyst efficiently promoted photocatalytic seawater splitting under ambient pressure and natural sunlight, achieving a record hydrogen evolution rate of 134 mmol h−1 g−1. But, the core innovations of this paper raise several fundamental concerns. There is a suspicion that the narrative was constructed to fit the data rather than the other way around. Recommend rejection by considering the following issues:\n1.A comparison between UCN (Fig. 2b) and UCN fragments (Fig. 2d) in SEM images should be conducted at the same magnification to more intuitively assess whether ultrasonication can effectively fragment the nanosheets. Otherwise, the core premise of this paper—which involves fragmenting CN and then linking the fragments with π-pyrene-π small molecules—would be invalidated.\n2.​From a processing perspective, are the air-heating preparation method for CN and the oxygen etching method for UCN fundamentally different?​ If not, why is CN's preparation method simply called \"air heating,\" while UCN's method is given a more academic-sounding name like \"oxygen etching\"? If they share the same essence, shouldn't urea air heating directly yield UCN? Did the authors artificially introduce catalyst preparation steps for the sake of narrative in the paper?\n3.The BET data often exhibits significant variability—each measurement may yield different results, which does not necessarily imply structural differences between CN and UCN.\n4.The video of Fig. 3g is quite interesting, but the observed bubbles could result from water vapor evaporation or air expansion due to light heating. A hydrogen test tube 'pop' experiment should be conducted to verify. If this system can indeed produce hydrogen at ambient pressure and under natural light, this work would be highly publishable in top-tier journals like Nature or Science.\n5.UPy frameworks should also exhibit decent performance even without sacrificial agents and without expensive Pt loading, right?\n6.How does the performance of CN or ucn composite with π-pyrene-π small molecules loading? Could it potentially match the excellence of UPy systems as reported in the literature?\n7.The experimental data of XPS, BET, PL, FTIR, etc. data may inherently vary due to experimental conditions, and selective reporting risks overinterpretation. No comment."}, "author_response": "Response to the Comments of Editor and Referees\nManuscript ID: NCOMMS-25-66364-T\nTitle: Solar-driven hydrogen production through ambient-pressure photocatalytic\nseawater splitting over donor-π-acceptor carbon nitride frameworks\nDear Editor and Reviewers:\nWe sincerely appreciate the Editor’s letter and the reviewers’ insightful comments\nand constructive suggestions. Their valuable feedback has greatly helped us improve\nthe overall quality of the manuscript. We have carefully addressed all the issues raised\nby the reviewers and revised the manuscript accordingly. For ease of review, all changes\nin the revised manuscript and Supporting Information are highlighted in yellow. In\naddition, we have provided detailed, point-by-point responses to all comments in blue\ntext, as detailed below.\nReviewer #1 (Remarks to the Author):\nIn this work, the authors report the synthesis of donor–π–acceptor (D–π–A) structured\ncarbon nitride (UPy) frameworks by covalently linking ultrathin CN nanosheets to\nelectron-donating pyrene units via various π-bridges. This design aims to achieve\nefficient photocatalytic seawater splitting under ambient pressure and natural sunlight.\nThe study demonstrates that employing biphenyl moieties as π-bridges in D–π–A\nframeworks significantly reduce exciton binding energy and enhances electron\ndelocalization and charge separation, thereby leading to an impressive hydrogen\nevolution rate of 134 mmol h⁻¹ g⁻¹. This result highlights the potential of the developed\nmaterials for practical solar-driven hydrogen production in natural seawater.\nFurthermore, the authors systematically investigate the influence of metal ions in\nseawater on the catalytic mechanism through a combination of control experiments,\nspectroscopic studies, and theoretical calculations. The work is representative and\ninsightful in the field of photocatalytic water splitting. I recommend the publication of\nthe manuscript in Nature Communications after minor revisions.\nResponse to the general comment: We sincerely thank the reviewer for this positive\nevaluation of our work and for the constructive suggestions. Accordingly, we calculated\nand compared the built-in internal electric field (BIEF) intensities of UCN and the UPy\nframeworks. We evaluated the PHE activity of pure pyrene derivatives, verified the co-\nprecipitation of Mg(OH)₂ and Ca(OH)₂ during seawater splitting, and quantified\ncatalyst hydrophilicity. The reviewer’s insightful feedback has been invaluable in\nrefining our study, and we believe these revisions have significantly strengthened the\nquality of the paper.\nComment 1. Electron delocalization is beneficial for the formation of the build-in\nelectric field. Please quantify the intensity of the built-in electric field toward UCN and\nUPy frameworks in this work (Angewandte Chemie, e202507028; Applied Catalysis B:\nEnvironment and Energy 362, 124724).\nin Fig. 1d, the UCN model exhibits high structural symmetry, which typically results in\na weak BIEF intensity and consequently limits efficient separation of electron-hole\npairsR1,2. In contrast, the donor–π–acceptor (D–π–A) architecture of the UPy\nframeworks exhibits promoted electron delocalization and intramolecular charge\ntransfer (ICT), both of which are highly favorable for generating a strong BIEFR3.\nFig. 1d. Molecular structures and electrostatic surface potential distributions of UCN\nand UPy frameworks (UPy1, UPy2, and UPy3).\nTo address the reviewer’s request, we have quantitatively evaluated the BIEF\nintensities of UCN and UPy frameworks. The BIEF magnitude was calculated using\nthe Kanata model according to the following equationR4:\n-2V ρ\ns\nF =√\ns ε ε\nS S\ndensity,  is the low-frequency dielectric constant, and  is the permittivity of free space.\n0\ncharge density (ρ) and the surface voltage (V ).\nS\nThe surface charge density ρ was measured by integrating the transient\nphotocurrent density (minus the steady state values of photocurrent) over time, which\nis proportional to the number of accumulated surface positive charges. The calculated\nρ values for UCN, UPy1, UPy2, and UPy3 were 0.07, 0.09, 0.30, and 0.10 C cm-2,\nrespectively (Supplementary Fig. 43b). The surface voltage V was obtained by\nS\naveraging the surface photovoltage spectra in the range of 370-525 nm (Supplementary\nFig. 44), yielding 0.0014, 0.0015, 0.022 and 0.0022 mV for UCN, UPy1, UPy2, and\nUPy3, respectively. As a result, all UPy frameworks exhibited higher ρ and Vs values\n3\nthan UCN, indicating significantly enhanced BIEF intensities. Notably, UPy2 showed\nthe highest ρ and Vs values, corresponding to the strongest BIEF intensity, which was\n8.2 times higher than that of UCN (Fig. 4i). This pronounced enhancement aligns well\nwith the superior charge-separation efficiency and improved photocatalytic\nperformance of UPy2 framework.\nSupplementary Fig. 43. Linear sweep voltammetry (LSV) curves (a), transient\nphotocurrent densities and calculated surface charge densities (b), and electrochemical\nimpedance spectroscopy (EIS) spectra (c) of UCN and UPy frameworks.\nSupplementary Fig. 44. SPV spectra of UCN and UPy frameworks.\nreflectance spectra (a) and band structures (b) of UCN and the UPy frameworks. c–f,\nIntegrated photoluminescence (PL) emission intensity as a function of temperature (80–\ntemperature-dependent PL spectra). g–i, PL emission spectra (g), PL decay spectra (h),\nand relative built-in internal electric field (BIEF) intensity (i) of UCN and the UPy\nframeworks.\nThe following descriptions have been revised in the manuscript:\nPage 16, lines 364-372: “To elucidate the driving force of charge transfer, the\nmagnitudes of the built-in internal electric field (BIEF) were quantified using surface\ncharge density (Supplementary Fig. 43b) and SPV measurements (Supplementary Fig.\n44)62. As shown in Fig. 4i, all UPy frameworks exhibited higher BIEF intensities than\nUCN, with UPy2 displaying the strongest field, which is 8.2 times higher than that of\nUCN. The highly symmetric structure of UCN resulted in a weak BIEF, limiting\nefficient electron–hole separation63,64. In contrast, covalent incorporation of π-pyrene-\n5\nπ molecules into UCN induced a D–π–A configuration in the UPy frameworks,\nsignificantly enhancing electron delocalization and ICT. This structural design\neffectively strengthens the BIEF and contributes to the superior charge-separation\nefficiency observed for UPy265.”\nComment 2. In Figure S23, the UPy2 catalyst exhibits strong dependence on Pt loading,\nwith optimal performance at 5 wt%. This is notably higher than the typical optimal\nloading (<3 wt%) for Pt in carbon nitride or other organic semiconductor-based\nphotocatalytic systems. Why such a high Pt loading is effective here without causing\nperformance degradation, the reasons should be specified.\nResponse #2: Thank you for this thoughtful comment. As shown in Supplementary Fig.\n26, UPy2 exhibited the highest PHE rate at a Pt loading of 5 wt%, which exceeded the\ntypical optimal Pt loading (<3 wt%) reported for carbon nitride and other organic\nsemiconductor-based photocatalytic systems. This higher optimal loading can be\nattributed to the smaller particle size and more favorable spatial distribution of Pt\nnanoparticles formed on the UPy2 surface. Particle size analysis (Supplementary Fig.\n34) revealed that the PHE reaction in seawater induced the formation of smaller Pt\nnanoparticles (mean size: 6.7 nm) compared with those formed in DI water (mean size:\n13.9 nm). The smaller and more uniformly distributed Pt nanoparticles effectively\nincrease the metal–semiconductor interfacial area, thereby facilitating more efficient\nelectron transfer from UPy2 to Pt and enhancing overall photocatalytic activity. This\neffect likely arises from the uniform adsorption of metal cation–TEOA complexes on\nthe UPy2 surface, which rapidly capture photogenerated holes, accelerate charge\nseparation, and promote localized electron accumulation. This localized charge\nenvironment favors the reduction of Pt4+ into smaller Pt nanoparticles. As a result, UPy2\nmaintains efficient charge separation and high catalytic activity even at higher Pt\nloadings, enabling an optimal cocatalyst content of 5 wt%.\npressure over UPy2 with different amount of Pt cocatalyst.\nSupplementary Fig. 34. TEM images (a, c) and size distributions of Pt particles (b, d)\nover UPy2 after PHE in DI water and seawater.\nComment 3. To better understand the role of metal ions, it would be helpful to include\nadditional TD-DFT charge distributions for Na⁺–TEOA and Mg²⁺–TEOA adsorbed on\nUPy2 (ACS Catalysis 14 (18), 13768-13780).\nwe provided additional TD-DFT charge distribution analyses for Na⁺–TEOA (S →S and\n0 1\nS →S ) and Mg²⁺–TEOA (S →S and S →S ) adsorbed on UPy2, as presented in\n0 10 0 3 0 5\nSupplementary Figs. 62 and 63. These results further confirm the occurrence of\nphotoinduced electron–hole separation from the pyrene unit toward the heptazine ring\nupon excitation, resulting in increased local electron density around the CN framework.\nThe enhanced charge separation provides deeper insight into the role of metal ions in\npromoting interfacial charge transfer and facilitating efficient photocatalytic processes.\nSupplementary Fig. 62. Charge distribution of Na⁺–TEOA adsorbed on UPy2 (S →S\n0 1\nand S →S ) (blue and green isosurfaces represent hole and electron distributions,\n0 10\nrespectively).\nSupplementary Fig. 63. Charge distribution of Mg2⁺–TEOA adsorbed on UPy2\n(S →S and S →S ) (blue and green isosurfaces represent hole and electron\n0 3 0 5\ndistributions, respectively).\ndiscussion should be provided, if possible, quantify the hydrophilicity differences\nbetween UCN and the UPy frameworks.\nResponse #4: We sincerely thank the reviewer for this valuable suggestion. The\nhydrophilicity of the catalyst surfaces was evaluated by measuring the static contact\nangles of water droplets. As shown in Fig. R1, UCN exhibited a slightly larger contact\nangle than the UPy frameworks, indicating a modest increase in surface hydrophilicity\nfor UPy1, UPy2, and UPy3. This enhancement likely originates from the higher density\nof amino groups in the UPy frameworks. During the preparation process of UPy\nframeworks (Fig. 2a), UCN was firstly fragmented under ultrasonication, exposing\nmore amino groups due to the formation of small and irregular nanofragments, as\nconfirmed by SEM image (Supplementary Fig. 5). These unbonded amino groups\ncontribute to the increased hydrophilicity of the UPy frameworks. However, the\nrelatively small differences in contact angle between UCN and the UPy frameworks\nsuggests that hydrophilicity does not play a dominant role in influencing their PHE\nperformance. Instead, the superior activity of the UPy frameworks is primarily\nattributed to their enhanced light-harvesting capability and more efficient charge\nseparation and transfer processes.\nFig. R1. Comparison of the contact angles of water droplets on the surface of UCN,\nUPy1, UPy2, and UPy3.\nSupplementary Fig. 5. SEM image of UCN fragments.\nComment 5. In Supplementary Figures 28 and 29, the authors confirm the formation\nof Mg(OH)₂ on UPy2 after PHE in seawater. Was Ca(OH)₂ also observed?\nResponse #5: We thank the reviewer for this important question. To investigate whether\nCa(OH)₂ is also formed on UPy2 after the PHE reaction in seawater, we conducted\nXRD, XPS, and TEM–EDX elemental mapping analyses. As shown in Supplementary\nFig. 28a, in addition to the strong diffraction peaks assigned to Mg(OH) in the XRD\n2\npattern of UPy2-PHE in seawater, the weak signals at approximately 18.1° and 36.0°\nwere detected, corresponding to the (001) and (102) characteristic peaks of Ca(OH) ,\n2\nrespectively. The high-resolution Ca 2p XPS spectrum of UPy2-PHE in seawater\n(Supplementary Fig. 32c) shows a weak peak at 343.8 eV, which is attributed to the Ca\n2p from Ca(OH) . In addition, elemental mapping of Ca confirms the presence of\nPHE reaction in DI water and seawater.\nSupplementary Fig. 32. High-resolution XPS spectra of Mg 1s (a), O 1s (b), and Ca\n2p (c) over UPy2 after PHE in seawater.\nmapping images of C (d), N (e), O (f), Mg (g), Ca (h), and Pt (i) over UPy2 after PHE\nin seawater.\nThese results collectively demonstrate that both Mg(OH)₂ and Ca(OH)₂ formed on\nUPy2 surface during the PHE reaction in seawater. However, Mg(OH)₂ is the\npredominant species, which can be explained by its lower solubility product constant\n(K = 5.61 × 10⁻¹²) compared with Ca(OH)₂ (K = 5.50 × 10⁻⁶), favoring Mg²⁺\nsp sp\nprecipitation under alkaline conditions generated during photocatalysis.\nCorresponding details can be found in the revised manuscript:\nPage 11, lines 263-265: “Distinct Mg(OH) peaks and weak Ca(OH) peaks were\n2 2\nobserved in the XRD pattern of the seawater-processed sample, indicating the co-\nprecipitation of Mg2+ and Ca2+ during the PHE reaction (Supplementary Fig. 28a).”\nPage 11, lines 266-270: “Additional nanosheets were observed on the surface of\nthe seawater-processed sample, confirming the formation of Mg(OH) and Ca(OH) .\n2 2\nThe reduction of H+ to H during PHE increased the local alkalinity at the catalyst\n2\nsurface, promoting Mg(OH) and Ca(OH) precipitation. The predominance of\n2 2\nMg(OH) , accompanied with trace amounts of Ca(OH) , was further confirmed by Mg\n2 2\n1s, Ca 2p, and O 1s XPS spectra (Supplementary Fig. 32).”\nunder ambient pressure in seawater? The data is needed for a better understanding.\nResponse #6: We thank the reviewer for this helpful suggestion. We evaluated the PHE\nperformances of the pure pyrene derivatives Py1–3 under ambient pressure in seawater.\nAs shown in Supplementary Fig. 23, all three molecules exhibited negligible hydrogen\nevolution activity, and no characteristic H₂ peaks were detected in the corresponding\ngas chromatography (GC) spectra. These results confirm that the pyrene units alone do\nnot contribute to photocatalytic hydrogen evolution under the tested conditions.\nSupplementary Fig. 23. GC spectra of gas products for PHE test in seawater under\nambient pressure over Py1, Py2, and Py3.\nCorresponding details have been incorporated into the revised manuscript:\nPage 10, lines 225-226: “The π-pyrene-π molecules also exhibited negligible PHE\nactivity, as evidenced by the absence of detectable H₂ signals in the gas chromatography\n(GC) spectra (Supplementary Fig. 23).”\nIn this manuscript (#NCOMMS-25-66364-T), the authors synthesized a series of highly\nconjugated UPy frameworks through covalent incorporation of pyrene units and π-\nbridges into a carbon nitride backbone. Among these materials, UPy2 demonstrated the\nhighest photocatalytic hydrogen evolution performance under ambient conditions, with\nrates of 82 mmol h-1 g-1 under simulated sunlight and 134 mmol h-1 g-1 under natural\nsunlight. Time-resolved photoluminescence (PL) and femtosecond transient absorption\n(fs-TA) spectroscopy revealed that UPy2—which features a biphenyl π-bridge—\nexhibits a significantly reduced exciton binding energy and promotes more efficient\ncharge separation between the carbon nitride and pyrene units compared to both UCN\nand the other UPy frameworks. The study is interesting; however, the following critical\nconcerns must be satisfactorily addressed before it can proceed to the next stage of\nevaluation.\nResponse to the general comment: We sincerely appreciate the reviewer’s\nconstructive comments and valuable suggestions, which have substantially\nstrengthened the quality of our manuscript. Accordingly, we provided the PLQY data\nfor UCN and UPy frameworks. Furthermore, we updated the fs-TAS kinetics and pump-\nfluence-dependent analyses to further validate the charge-transfer dynamics. To\nconfirm the successful covalent linkage of Py1, Py2, and Py3 to UCN, additional\nRaman spectroscopy characterizations were included. Detailed responses to each\ncomment are provided below.\nComment 1. The PL quantum yields (PLQYs) of all samples should be provided, as a\nlow PLQY may lead to unreliable exciton binding energy (E ) values derived from\nb\ntemperature-dependent photoluminescence measurements.\nResponse #1: We sincerely thank the reviewer for this important and constructive\nsuggestion. We fully agree that low PLQY values may introduce uncertainty into the\ndetermination of exciton binding energy (E ) derived from temperature-dependent\nb\nphotoluminescence (PL) measurement. To address this concern, we measured the\nUCN exhibited a PLQY of 3.61%. Upon covalent coupling with π-pyrene-π molecules,\nthe PLQYs decreased to 2.12% for UPy1 and 0.39% for UPy2, indicating that covalent\nlinkage promoted efficient charge transfer between UCN and the pyrene units,\nsuppressed radiative recombination, and enhanced charge-carrier utilization during\nphotocatalysis. In contrast, UPy3 showed a higher PLQY of 9.11%, likely attributable\nto the strong emission centered around ~600 nm arising from emissive charge transfer\nstates that enhance radiative recombination and compete with the charge separation\nrequired for effective photocatalysisR5.\nThe low PLQY of UPy2 (0.39%) is physically consistent with its lower exciton\nbinding energy (E ) and more efficient exciton dissociation. Because PLQY reflects the\nb\nfraction of photogenerated excitons undergoing radiative decay, a lower PLQY\nindicates that fewer excitons recombine radiatively and that a larger proportion\ndissociate into free carriers participating in interfacial redox reactionsR6. Such behavior\ntypically correlates with enhanced charge separation and transport efficiency, as free\ncarriers are preferentially consumed in photocatalytic processes rather than lost through\nradiative emission. Therefore, the lower E of UPy2 facilitates exciton dissociation into\nb\nfree carriers, leading to more efficient charge transfer, consistent with the observed high\nPHE performanceR7.\nWe acknowledge that reduced PLQY can also originate from trap-mediated non-\nradiative recombination, which might affect the measurement reliability of E\nb\nestimation. However, the temperature-dependent PL spectra for all samples exhibited\nwell-defined, reproducible emission peaks with excellent signal-to-noise ratios. These\nresults support that the low PLQY of UPy2 primarily arises from efficient exciton\ndissociation rather than trap-mediated quenching, ensuring the reliability of the derived\nE values.\nb\nUPy3 (d). Detected apparent quantum yield was measured at a 365 nm excitation\nwavelength, BaSO was used as a reference.\nPage 15, lines 332-337: “The photoluminescence quantum yield (PLQY) was\nemployed as an indirect indicator of photogenerated carrier transport efficiency. As\nshown in Supplementary Fig. 42, pristine UCN exhibited a PLQY of 3.61%, whereas\nthe lower PLQYs of UPy1 (2.12%) and UPy2 (0.39%) indicated that covalent coupling\nwith Py1 or Py2 promoted charge transfer and suppressed radiative recombination58. In\ncontrast, the higher PLQY of UPy3 (9.11%), arising from emissive charge transfer\nstates near 600 nm that enhance radiative recombination but compete with charge\nseparation.”\nthe current fitting parameters provided in the figure fail to accurately reproduce the data\ndue to the absence of a long-time constant (or offset). Without this component, a\nphysically meaningful kinetic model cannot be properly established. Additionally, the\nkinetic scheme proposed for UCN in Fig. 5e is incorrect and requires revision.\nResponse #2: We thank the reviewer for this professional suggestion regarding the fs-\nTAS kinetic fitting of UCN (Fig. 5c) and the associated kinetic scheme (Fig. 5e).\nFollowing your recommendation, we re-fitted the kinetic decay trace of UCN at 700\nnm and induced a constant offset to account for the residual signal component.\nAccordingly, the kinetic fitting model was revised from a biexponential to a\ntriexponential function:\nΔA(t) = A e−(t/τ1) + A e−(t/τ2) + A e−(t/τ3) + y\n1 2 3 0\nwhere y represents the constant offset corresponding to time-independent residual\n0\nsignal arising from long-lived speciesR8.\nAs displayed in Fig. 5e, pristine UCN exhibited three decay components, namely\nτ = 2.5 ps—corresponding to rapid electron capture by intrinsic shallow trap (ST) states,\n1\nτ = 31.6 ps—associated with slower band-edge exciton recombination, and τ = 443.0\n2 3\nps—assigned to long-lived charge separation via deep-trap (DT) statesR9,10. This\ntriexponential model provides a physically meaningful and comprehensive description\nof the sequential charge relaxation processes in UCN. The updated kinetic scheme (Fig.\n5e) has been updated accordingly to reflect these three processes: shallow trapping,\nband-edge recombination, and deep trapping.\nCorresponding details can be found in the revised manuscript:\nPages 16-17, lines 388-392: “Pristine UCN exhibited three decay components,\nnamely τ = 2.5 ps—corresponding to rapid electron capture by intrinsic shallow-trap\n1\n(ST) states, τ = 31.6 ps—associated with slower band-edge exciton recombination, and\n2\nτ = 443.0 ps—ascribed to long-lived charge separation via deep-trap (DT) states (Fig.\n3\n5e)68. The inclusion of a long-time offset constant (y ) further highlights the\n0\ncontribution of persistent photogenerated carriers that sustain photocatalytic activity.”\nand decay kinetic curves (c, d) of UCN and UPy2. e, Charge trapping models proposed\nfor UCN and UPy2. f, g, Fluence dependent electron decay kinetics after excitation at\n325 nm of UCN (f) and UPy2 (g). h, Dependence of the initial amplitude (blue), decay\nhalf time (red), and % of residual signal relative to initial signal (yellow) for the\nphotogenerated electron signal at ~700 nm as a function of excitation energy\ndetermined from the data in Fig. 5f, g.\nComment 3. The analysis of the fs-TA kinetics for UPy2 (Fig. 5d) also appears to lack\na long-time constant (or offset), indicating that the current fitting parameters may be\ninadequate.\nResponse #3: We appreciate the reviewer’s professional suggestion. Following your\ncomment, we incorporated a long-time constant into the fs-TAS kinetic analysis of\nUPy2, as shown in Fig. 5d. In contrast to UCN, which exhibited three decay\ncomponents, UPy2 displayed a four-component decay (τ₁ = 4.1 ps, τ₂ = 52.5 ps, τ₃ =\n18\n742.1 ps, τ₄ = 1683 ps). As illustrated in Fig. 5e, τ₁ is assigned to ultrafast electron\ninjection from the pyrene unit to UCN and hole transfer from UCN to the pyrene unit;\nτ₂ corresponds to the band-edge recombination; τ₃ reflects deep-trap (DT) stabilization\nof separated charges; and τ₄ represents the long-lived charge-separated (CS) stateR11,12.\nThe emergence of the slower components (τ₃, τ₄), together with the larger residual offset\n(y ), provides evidence for the substantially enhanced electron–hole separation\n0\nefficiency achieved in UPy2.\nFig. 5 | Insights into charge transfer kinetics. a–d, Transient absorption spectra (a, b)\nand decay kinetic curves (c, d) of UCN and UPy2. e, Charge trapping models proposed\nfor UCN and UPy2. f, g, Fluence dependent electron decay kinetics after excitation at\n325 nm of UCN (f) and UPy2 (g). h, Dependence of the initial amplitude (blue), decay\nhalf time (red), and % of residual signal relative to initial signal (yellow) for the\nphotogenerated electron signal at ~700 nm as a function of excitation energy\ndetermined from the data in Fig. 5f, g.\nPage 17, lines 392-398: “In contrast, UPy2 exhibited four decay components: τ₁\n= 4.1 ps, τ₂ = 52.5 ps, τ₃ = 742.1 ps, τ₄ = 1683 ps, indicating that D-A covalent coupling\nsubstantially slows recombination and enhances charge separation. Specifically, τ₁ is\nassigned to ultrafast ICT between the UCN and pyrene unit; τ₂ corresponds to band-\nedge recombination; τ₃ reflects deep-trap stabilization of separated charges; and τ₄\nrepresents a long-lived charge-separated (CS) state extending beyond the ps regime69,70.\nThe emergence of these slower components (τ₃ and τ₄), together with the increased\nresidual offset (y ), confirms the markedly improved electron–hole separation\n0\nefficiency in UPy271.”\nComment 4. The pump energy density employed in the fs-TA kinetic measurements\nmust be clearly specified, as PCN-based systems are known to exhibit strongly pump\nfluence-dependent kinetics (J. Am. Chem. Soc. 2019, 141, 11219). Given this\ndependence, the kinetic differences observed between UCN and UPy2 at high pump\nenergy densities may not stem solely from structural differences. The authors should\neither conduct the fs-TA study under a relatively low pump energy density or provide\npump-fluence-dependent kinetic data for both UCN and UPy2.\nResponse #4: We sincerely thank the reviewer for this valuable suggestion. In our\noriginal submission, all femtosecond transient absorption spectroscopy (fs-TAS) kinetic\nmeasurements were conducted under a pump energy density (0.21 mJ cm⁻²), which\nrepresents the lowest excitation energy density that still provides a measurable fs-TAS\nsignal. To further clarify the pump fluence-dependent kinetics, we conducted additional\nfs-TAS measurements for both UCN and UPy2 at four pump energy densities: 0.24,\n0.37, 0.96, and 1.94 mJ cm⁻². The corresponding decay kinetics, probed at\napproximately 700 nm (the absorption band of photogenerated electrons), are presented\nin Fig. 5f, g, Supplementary Fig. 49, and Supplementary Table 4.\nand decay kinetic curves (c, d) of UCN and UPy2. e, Charge trapping models proposed\nfor UCN and UPy2. f, g, Fluence dependent electron decay kinetics after excitation at\n325 nm of UCN (f) and UPy2 (g). h, Dependence of the initial amplitude (blue), decay\nhalf time (red), and % of residual signal relative to initial signal (yellow) for the\nphotogenerated electron signal at ~700 nm as a function of excitation energy\ndetermined from the data in Fig. 5f, g.\nSupplementary Fig. 49. 2D mapping TAS spectra of UCN (a-d) and UPy2 (e-h) at\nfour pump energy densities.\nthe fluence dependent recombination kinetics in Fig. 5f, g.\nPulse energy\nα  t\n(mJ cm-2)\n0.24 0.00235 0.196 -1.57 0.68\n0.37 0.00480 0.187 -1.06 0.92\nUCN\n0.96 0.00681 0.222 -1.65 0.85\n1.94 0.0126 0.193 -0.576 0.96\n0.24 0.00271 0.166 -2.06 0.62\n0.37 0.00444 0.172 -0.981 0.94\nUPy2\n0.96 0.00765 0.179 -0.450 0.92\n1.94 0.0129 0.199 -0.991 0.94\nAs shown in Supplementary Fig. 49 and Fig. 5f, g, both UCN and UPy2 showed\nclear pump-fluence-dependent decay kinetics. The initial ΔA amplitudes at ~500 fs\nincreased linearly with excitation fluence, with slopes close to unity (>0.96), indicating\nthat the efficiency of initial electron generation is independent of excitation fluence (Fig.\n5h)R13. Consistent with previous results, all decay kinetics can be well-fitted by a\npower-law model:\nΔA(t) = α·(t-t )-\n0\nwhere  values lying within 0.20 ± 0.02 across all fluences (Supplementary Table 4)R14.\nUnder linear-response conditions, the transient absorption signal ΔA(t) was\nproportional to the population of the long-lived charge carriers contributing to the\nprobed band. Accordingly, the parameter α reflects the density of these charge carriers\nat the onset of the power-law regime. Across all fluences, UPy2 exhibited significantly\nhigher α values than UCN, indicating a higher density of long-lived charge-separated\n22\ncarriers immediately after excitationR15. The effective decay half-times (t ) extracted\n0.5\nfrom the power-law fits decreased with increasing excitation fluence for both UCN and\nUPy2, indicating that the recombination dynamics become faster at higher\nphotogenerated carrier densities, in line with the enhanced probability of bimolecular\nand higher-order recombination under such conditions. Notably, at any given fluence,\nUPy2 exhibited systematically longer t values than UCN. These extended decay times\n0.5\nsignified slower charge recombination and more effective stabilization of\nphotogenerated carriers in UPy2, consistent with the improved charge separation\nenabled by the covalent D-A coupling between the pyrene unit and the UCN\nframeworkR16. Differences in the long-lived electron population at 2 ns further support\nthis conclusion. For UPy2, the residual signal decreased from 31% at low fluence to\n21.7% at high fluence, whereas UCN maintained minimal variation over the same range.\nThe stronger fluence dependence in UPy2 arises from its larger initial carrier densities,\nwhich promote faster bimolecular recombination at high excitation levels. Importantly,\nthe reduction in residual electrons at high fluence does not indicate diminished charge\nseparation; rather, it reflects redistribution of an initially larger carrier population\ninto both long-lived and short-lived recombination channels under high-density regimes.\nIn contrast, the weak fluence dependence observed in UCN is consistent with its\nintrinsically limited charge-separation capability and absence of an efficient\nintramolecular charge-transfer pathwayR13,14.\nOverall, the larger α values, longer decay half-times, and stronger fluence-\ndependent modulation of long-lived carriers in UPy2 collectively confirm its\nsubstantially enhanced electron–hole separation efficiency compared with UCN.\nThe donor–acceptor interactions in UPy2 facilitates efficient ICT and promote stabilization\nof a larger population of separated charge carriers, which remain long lived until\nbimolecular recombination pathways become dominant at high fluence.\nCorresponding details can be found in the revised manuscript:\nPages 17-18, lines 404-428: “Given the strongly pump fluence dependent kinetics\non CN-based systems, fs-TAS measurements for both UCN and UPy2 were conducted\n23\nat four pump energy densities: 0.24, 0.37, 0.96, and 1.94 mJ cm⁻². As shown in\nSupplementary Fig. 49 and Fig. 5f, g, both UCN and UPy2 exhibited pronounced pump-\nfluence-dependent fs-TAS decay kinetics. The initial ΔA amplitudes at ~500 fs\nincreased linearly with excitation fluence, with slopes close to unity (>0.96), indicating\nfluence-independent initial electron generation (Fig. 5h). All decay traces were well\nfitted by a power-law model, ΔA(t) = α·(t − t₀)⁻ᵝ, with β consistently falling within 0.20\n± 0.02 (Supplementary Table 4)72. Under linear-response conditions, the transient\nabsorption signal ΔA(t) was proportional to the population of the long-lived charge\ncarriers contributing to the probed band. Accordingly, the parameter α reflects the\ncarrier density at the onset of the power-law regime. Across all fluences, UPy2\ndisplayed substantially larger α values than UCN, demonstrating that UPy2 sustained a\nhigher density of long-lived charge-separated carriers immediately after excitation73.\nThe extracted half-times (t ) further revealed distinct recombination dynamics. Both\n0.5\nUCN and UPy2 displayed shorter t at higher fluences, consistent with fluence-\n0.5\nenhanced bimolecular or higher-order recombination processes. However, under\nidentical excitation conditions, UPy2 consistently showed longer t than UCN,\n0.5\nindicating slower carrier recombination and more efficient stabilization of long-lived\ncharge carriers74. Additional differences were observed in the residual electron\npopulation at ~2 ns. For UPy2, the fraction of long-lived electrons decreased from 31%\nat low fluence to 21.7% at high fluence, whereas UCN showed negligible variation\nacross the same range. The stronger fluence dependence in UPy2 arises from its larger\ninitial carrier densities, which accelerate bimolecular recombination at high fluence.\nImportantly, the reduced residual electrons at high fluence does not imply weaker\ncharge separation but rather reflects redistribution of an initial larger carrier population\ninto long- and short-lived recombination channels. In contrast, the weak fluence\nresponse of UCN was consistent with its inherently limited charge-separation capability\nand the absence of an efficient ICT pathway72. Collectively, the larger α values, longer\ndecay half-times, and stronger fluence-dependent modulation of long-lived electrons in\nUPy2 confirmed its markedly enhanced electron–hole separation compared with UCN.”\nconfirm the successful linkage of Py1, Py2, and Py3 to CN, additional Raman\nspectroscopy characterization would provide further supportive evidence.\nResponse #5: We sincerely thank the reviewer for this constructive suggestion. To\nfurther confirm the successful covalent linkage of the π-pyrene-π molecules to UCN,\nRaman spectroscopy was performed for UCN and UPy frameworks. As shown in\nSupplementary Fig. 11, all samples exhibited the characteristic vibration modes of CN\nheterocycles at 470, 708, 770, 1213, 1232, 1310, and 1350 cm-1. Notably, additional\nbands observed at approximately 1400 and 1560 cm-1 in the UPy spectra, corresponding\nto the C–C/C=C stretching vibrations of Py1, Py2, and Py3. Moreover, the distinct\npeaks around 1610 cm-1 in UPy frameworks are attributed to C=N bond vibrationsR17,\nproviding evidence for successful covalent linkage of the π-pyrene-π molecules to UCN.\nSupplementary Fig. 11. Raman spectra of UCN and UPy frameworks.\nCorresponding details can be found in the revised manuscript:\nPage 8, lines 159-164: “In addition, the Raman spectra (Supplementary Fig. 11)\nrevealed that UCN and UPy frameworks exhibited characteristic vibration modes of\nCN heterocycles at 470, 708, 770, 1213, 1232, 1310, and 1350 cm⁻¹. The additional\nbands at ~1400 and 1560 cm⁻¹ corresponded to C–C/C=C vibrations of the pyrene units\nin the UPy frameworks. Furthermore, the additional peaks near 1610 cm⁻¹ in UPy\nframeworks were assigned to C=N bond vibrations, providing additional evidence for\nthe successful covalent linkage of Py1, Py2, and Py3 to UCN48.”\nLi and coworkers introduced pyrene (Py) structural units into carbon nitride (CN) via\ncovalent linkage, achieving a maximum hydrogen production rate of 134 mmol h⁻¹ g⁻¹\nin photocatalytic seawater splitting, which is an excellent result among g-C3N4-based\nphotocatalysts. The author also discussed the promoting effects of metal ions in\nseawater on the catalytic process. These results are interesting. However, there are some\nmain issues with the mechanism discussion in the manuscript that need to be addressed\nbefore I can recommend publication in Nature Communication.\nResponse to the general comment: We greatly appreciate the reviewer’s valuable and\nconstructive comments on the mechanism discussion, which have significantly\nimproved the clarity and depth of our manuscript. In response, we have made several\nkey revisions. First, we quantified the Py contents in the UPy frameworks and clarified\ntheir effect on photocatalytic performance. We also elaborated on the correlation\nbetween the absorption edges and the D–A charge transfer process in the UPy\nframeworks, highlighting how covalent linkage enhances photocatalytic activity.\nSecond, we refined the fs-TAS analysis and incorporated additional fs-TAS data for\nboth π-pyrene-π molecules and UPy frameworks under excitation of 450 nm to better\nelucidate the charge transfer dynamics. Finally, we extended the DFT calculations to\ninclude models in which a single Py molecule is covalently linked to multiple CN\nsegments, and we evaluated the adsorption energy of TEOA on the pyrene unit in UPy2.\nCollectively, these revisions have substantially strengthened the mechanistic discussion\nand improved the overall comprehensiveness of the manuscript.\nComment 1. The percentage content of the Py in UPy frameworks should be\ndetermined and stated. The naming (UPy-x) in the Synthesis Method section is related\nto the mass of Py, but no relevant discussion is found in the manuscript—maybe I am\nmistaken. In addition, the effect of different Py contents on the catalytic activity should\nbe discussed.\nResponse #1: We sincerely thank the reviewer for this insightful comment. In our\n26\nsystem, covalently coupling occurs through a Schiff-base reaction between the\naldehyde groups (–CHO) of the π-pyrene-π molecules and the amine groups (–NH₂)\nlocated at the edges of UCN. However, accurately controlling and evaluating the extent\nof this coupling remains challenging due to the irregular molecular structure of UCN,\nwhich is formed by thermal condensation of urea and subsequently subjected to\nsecondary calcination. As a result, the abundance and spatial distribution of –NH₂\ngroups at UCN edges are nonuniform, making precise determination of the coupling\nratio difficult.\nNevertheless, we employed elemental analysis to indirectly estimate the\npercentage content of the π-pyrene-π molecules in UPy frameworks. As shown in\nSupplementary Table 1, the increase in carbon weight fraction serves as an indicator of\nthe amount of incorporated π-pyrene-π molecules. For example, the carbon content\nincreases from 34.20 wt% in UCN to 36.09 wt% in UPy2, corresponding to an\nestimated Py2 content of approximately 2.87 wt%. This value is lower than the initial\nfeeding ratio (0.1 g UCN vs. 5 mg Py2), likely because a portion of Py2 molecules dose\nnot successfully form covalent bonds with UCN.\nCorresponding details can be found in the revised manuscript:\nPage 8, lines 175-179: “Elemental analysis was further employed to estimate the\nπ-pyrene-π contents in the UPy frameworks, yielding values of approximately 1.84 wt%\nfor UPy1, 2.87 wt% for UPy2, and 2.26 wt% for UPy3. These values are lower than the\nnominal feeding ratio (0.1 g UCN vs. 5 mg π-pyrene-π molecules), likely because a\nfraction of the π-pyrene-π molecules did not successfully couple to the UCN.”\nSupplementary Table 1. Elementary analysis of UCN, UPy1, UPy2, and UPy3.\nSample C (wt%) N (wt%) H (wt%) C/N (molar ratio) Estimated content of Py\n(wt%)\nUCN 34.20 60.04 1.70 0.66 -\nUPy1 35.41 56.92 1.75 0.73 1.84\nUPy2 36.09 56.91 1.78 0.74 2.87\nUPy3 35.69 56.78 1.74 0.73 2.26\nπ content on the catalytic activity. A detailed discussion of this effect was incorporated\ninto the revised manuscript.\nPage 9, lines 198-203: “As shown in Supplementary Fig. 16, the PHE rates of\nUPy1, UPy2, and UPy3 were markedly higher than those of pure CN, UCN, or the\nindividual π-pyrene-π molecules. For each UPy frameworks, the catalysts prepared with\nthe optimal π-pyrene-π molecules (UPy1-5, UPy2-5, and UPy3-3) achieved the highest\nPHE rates. Further increases in π-pyrene-π contents led to decreased PHE rates, likely\ndue to aggregation of excess π-pyrene-π molecules, which partially shield surface active\nsites.”\nComment 2. The absorption edges of the UPy1-3 frameworks seem to exhibit a blue-\nshift compared with those of the single-component Py1-3. How to explain this? No\nsignificant emergence of new peaks or disappearance of old peaks is observed in the\nabsorption spectra of UPy, which seems insufficient to support the donor-acceptor (D-\nA) charge transfer process claimed by the authors.\nResponse #2: We sincerely thank the reviewer for pointing out the blue-shift of the\nabsorption edges in UPy1-3 relative to the single-component Py1-3. As displayed in\nFig. R2, comparison of the UV–vis DRS spectra of the π-pyrene-π molecules\n(Supplementary Fig. 39) and the UPy frameworks (Fig. 4a) confirms that the UPy\nmaterials exhibit slightly blue-shifted absorption edges relative to their molecular\ncounterparts. This behavior can be primarily attributed to the conformational restriction\nand increased orbital localization induced by grafting the π-pyrene-π molecules onto\nUCN. In their free molecular state, π-pyrene-π molecules possess considerable\nconformational flexibility that allows partial π-electron delocalization, thereby\nlowering the π→π* excitation energy. Covalent attachment of the π-pyrene-π molecules\nto the UCN restricts their rotational and vibrational degrees of freedom, leading to\nenhanced π-electron localization. This increased localization raises the π→π* excitation\nenergy, resulting in the observed blue-shift in the absorption edgesR18.\n(right).\nThe absence of distinct new absorption bands or the disappearance of existing\nbands in the UPy spectra does not contradict the presence of donor–acceptor (D–A)\ncharge transfer interactions. It is well-established that charge transfer states in D–A\nsystems often exhibit very small oscillator strengths and strongly overlap with local π–\nπ* transitions, so that no separate charge transfer absorption band can be resolved in\nsteady-state UV–vis spectra, particularly when the loading of π-pyrene-π molecules on\nthe UCN is relatively low (~2 wt%)R19. In the UPy frameworks, the red-shifted\nabsorption edges and modestly increased absorption intensities relative to UCN are\nfully consistent with electronic coupling between the π-pyrene-π molecules and the\nUCN framework. These observations support the presence of D–A charge transfer, even\nthough the spectral signatures are not strongly pronounced. This scenario closely\nresembles low-loading acceptor systems, where clear charge transfer features emerge\nonly at higher loadings, while weak charge transfer character at low concentrations\nmanifests mainly as subtle spectral shifts and intensity changes rather than distinct new\nbandsR20. Thus, despite the absence of a distinct charge transfer band, the observed\nspectral shifts provide reliable evidence supporting D–A charge transfer interactions in\nthe UPy frameworks.\nof the single-component CN and Py. What is the significance of the covalent linkage\nbetween the two components in UPy for the enhanced activity? Would simply\ncompositing Py and CN also lead to an improvement in catalytic activity?\nResponse #3: We sincerely thank the reviewer for this insightful question. The\nmarkedly higher PHE activity of the UPy frameworks arises from the synergistic effects\nenabled by covalent linkage between the π-pyrene-π units and the UCN framework.\nThese effects cannot be achieved by simply mixing the two components.\nFor single-component UCN, the low PHE performance mainly results from its\nlimited light-harvesting capability (<400 nm) and high charge recombination rate, as\ndemonstrated by the UV-vis DRS spectra (Fig. 4a) and PL spectra (Fig. 4g, h). Similarly,\nthe weak PHE performances of π-pyrene-π molecules are mainly attributed to their high\ncharge recombination, as demonstrated by the fs-TAS decay kinetics (Supplementary\nFigs. 50, 51). In the UPy frameworks, Py units were uniformly covalently anchored to\nthe edges of UCN, which leads to several key improvements. First, the light absorption\ncapability of the UPy frameworks is enhanced due to the expansion of the π-conjugated\nstructure upon covalent incorporation of the π-pyrene-π molecules (Fig. 4a). Second,\nthe donor–π–acceptor architecture of the UPy frameworks enabled facile\nintramolecular pyrene-to-CN electron transfer through an electron push-pull effect,\nsignificantly lowering the exciton binding energy and promoting electron delocalization\nand charge separation, supported by the DFT calculations (ESP distributions in Fig. 1d,\nHOMO and LUMO distributions in Supplementary Figs. 2, 3), exciton dissociation (Fig.\n4c-f), PL analysis (Fig. 4g, h), built-in internal electric field intensity (Fig. 4i), and\ncharge transfer kinetics (Fig. 5). Collectively, these results demonstrate that covalent\nlinkage is essential for activating and sustaining efficient intramolecular charge transfer\nand separation, thereby accounting for the superior photocatalytic performance of the\nUPy frameworks.\nreflectance spectra (a) and band structures (b) of UCN and the UPy frameworks. c–f,\nIntegrated photoluminescence (PL) emission intensity as a function of temperature (80–\ntemperature-dependent PL spectra). g–i, PL emission spectra (g), PL decay spectra (h),\nand relative built-in internal electric field (BIEF) intensity (i) of UCN and the UPy\nframeworks.\nexcitation. (a-c) 2D mapping TAS spectra of Py1, Py2, and Py3 collected over a 0-2 ns\ntime window. (d-f) Representative kinetic traces at selected probe wavelengths (Py1:\n630 nm; Py2: 600 nm; Py3: 613 nm).\nSupplementary Fig. 51. fs-TAS spectra of the UPy1, UPy2, and UPy3 under 450 nm\nexcitation. (a-c) 2D mapping TAS spectra of UPy1, UPy2, and UPy3 collected over a 0-2 ns\ntime window. (d-f) Representative kinetic traces at selected probe wavelengths\n(UPy1: 630 nm; UPy2: 600 nm; UPy3: 613 nm).\nand UPy frameworks (UPy1, UPy2, and UPy3).\nSupplementary Fig. 2. The lowest unoccupied molecular orbital (LUMO) and highest\noccupied molecular orbital (HOMO) distributions of CN, Py1, Py2, and Py3 molecules.\nSupplementary Fig. 3. LUMO and HOMO distributions of UPy1, UPy2, and UPy3\nmodels.\nand decay kinetic curves (c, d) of UCN and UPy2. e, Charge trapping models proposed\nfor UCN and UPy2. f, g, Fluence dependent electron decay kinetics after excitation at\n325 nm of UCN (f) and UPy2 (g). h, Dependence of the initial amplitude (blue), decay\nhalf time (red), and % of residual signal relative to initial signal (yellow) for the\nphotogenerated electron signal at ~700 nm as a function of excitation energy\ndetermined from the data in Fig. 5f, g.\nTo further clarify the role of covalent linkage, we compared UPy2 with the simply\ncompositing Py2 and UCN, denoted as UPy2-π-π, which featured interfacial π-π\ninteractions. Under negative pressure and visible-light illumination at 281 K, UPy2-π-\nπ exhibited a lower PHE rate of 4.12 mmol h−1 g−1 than covalently linked UPy2 (17.85\nmmol h−1 g−1) (Supplementary Fig. 16d). A similar trend was observed under ambient-\npressure seawater conditions at 298 K, where UPy2-π-π achieved a PHE rate of 10.84\nmmol h⁻¹ g⁻¹—substantially lower than that of UPy2 (44.50 mmol h⁻¹ g⁻¹) (Fig. 3a).\nThe substantial performance difference between UPy2 and the UPy2-π-π highlights the\n34\nmechanistic significance of covalent D–A linkage. In UPy2-π-π, the Py2 and UCN are\nassembled via weak π–π interactions, resulting in limited orbital overlap, inefficient\ncharge delocalization, and rapid exciton recombination. By contrast, the covalent\nbonded framework in UPy2 establishes a rigid, electronically coupled interface that\nfacilitates ultrafast electron transfer from the Py2 to the UCN and stabilizes the resulting\ncharge-separated stateR5. Consistent with this interpretation, PL and fs-TAS analyses\n(Supplementary Fig. 53) show that UPy2 exhibited markedly lower PL intensity, faster\nPL decay, and significantly prolonged charge-carrier lifetimes compared with UPy2-π-\nπ. These results confirm that covalent linkage dramatically enhances charge separation\nand charge-transfer efficiency, thereby achieving the superior photocatalytic\nperformance of UPy2.\nSupplementary Fig. 16. Time-dependent PHE curves (a-c) and H2 production rates (d)\nof CN, UCN, π-pyrene-π molecules, UPy frameworks, UPy2-π-π, and CPy2 under\nnegative pressure.\nSupplementary Fig. 53. PL emission spectra (a) and PL decay spectra (b) of UPy2 and\nUPy2-π-π. 2D mapping TAS spectrum (c) and decay kinetic curve of UPy2-π-π (d).\nCorresponding details can be found in the revised manuscript:\nPages 18-19, lines 447-451: “To further highlight the mechanistic significance of\ncovalent D–A linkage, the charge transfer properties of UPy2 and UPy2-π-π were\ncompared. Evidence from PL quenching, faster PL decay, and significantly prolonged\nfs-TAS decay lifetime (Supplementary Fig. 53) confirmed that UPy2 suppresses\nradiative and non-radiative recombination far more effectively than UPy2-π-π. This\n36\nstrengthened electronic coupling and ICT efficiency enabled by covalent bonding are\nkey factor underlying the improved PHE performance77.”\nComment 4. For Fig. 5C and Fig. 5D, the corresponding R² values are not provided.\nFrom the graphs alone, the double-exponential fitting in Fig. 5C does not seem to be\nthe most appropriate; a triple-exponential fitting might lead to other conclusions.\nResponse #4: We thank the reviewer for this insightful comment. In the revised\nmanuscript, we provided the corresponding R2 values for the kinetic fits in Fig. 5c and\n5d. In addition, following your suggestion, we re-fitted the kinetic decay trace of\npristine UCN using a triple-exponential model. As displayed in Fig. 5e, pristine UCN\nexhibited three decay components, namely τ = 2.5 ps— corresponding to rapid electron\n1\ncapture by intrinsic shallow trap (ST) states, τ = 31.6 ps—associated with slower band-\n2\nedge exciton recombination, and τ = 443.0 ps—assigned to long-lived charge\n3\nseparation via deep-trap (DT) statesR9,10. This triexponential model provides a\nphysically meaningful and comprehensive description of the sequential charge\nrelaxation processes in UCN. The updated kinetic scheme (Fig. 5e) has been updated\naccordingly to reflect these three processes: shallow trapping, band-edge recombination,\nand deep trapping.\nCorresponding details can be found in the revised manuscript:\nPages 16-17, lines 388-392: “Pristine UCN exhibited three decay components,\nnamely τ = 2.5 ps—corresponding to rapid electron capture by intrinsic shallow-trap\n1\n(ST) states, τ = 31.6 ps—associated with slower band-edge exciton recombination, and\n2\nτ = 443.0 ps—ascribed to long-lived charge separation via deep-trap (DT) states (Fig.\n3\n5e)68. The inclusion of a long-time offset constant (y ) further highlights the\n0\ncontribution of persistent photogenerated carriers that sustain photocatalytic activity.”\nand decay kinetic curves (c, d) of UCN and UPy2. e, Charge trapping models proposed\nfor UCN and UPy2. f, g, Fluence dependent electron decay kinetics after excitation at\n325 nm of UCN (f) and UPy2 (g). h, Dependence of the initial amplitude (blue), average\ndecay time (yellow), and % of residual signal relative to initial signal (red) for the\nphotogenerated electron signal at ~700 nm as a function of excitation energy\ndetermined from the data in Fig. 5f, g.\nComment 5. Could the transient absorption (TA) spectra of the three small molecules\nbe provided?\nResponse #5: We thank the reviewer for this valuable suggestion. The femtosecond\ntransient absorption spectroscopy (fs-TAS) spectra of the Py1, Py2, and Py3 small\nmolecules under the excitation of 450 nm have been added in Supplementary Fig. 50.\nAll three small molecules exhibited a pronounced negative ΔA band centered at ~600\nnm, which overlapped well with their steady-state emission and was therefore assigned\n38\nto stimulated emission (SE) from the S₁ state, which is in line with the general TAS\nsignature of SE in π-conjugated organic chromophores. In addition, positive ΔA bands\nin the 700–800 nm region, observed for Py1 and Py2, were characteristic of singlet\nexcited-state absorption (S₁ → Sₙ) commonly observed in π-conjugated donor\nmoleculesR21,22. The kinetic traces at representative probe wavelengths (Py1 at 630 nm,\nPy2 at 600 nm, and Py3 at 613 nm) showed multi-exponential decay behavior, revealing\nfast decay of SE on the picosecond timescale. Notably, Py2 exhibited a substantial long-\nlived offset in its fs-TA signal, which did not return to baseline within the experimental\ntime window. This offset arises from the combined effects of weak SE and overlapping\nESA contributions. As a result, the kinetic trace of Py2 suffered from severe parameter\ncorrelation during multiexponential fitting. To avoid introducing artefactual lifetimes,\nwe therefore presented the raw kinetic trace of Py2 without imposing a multi-\nexponential fit.\nSupplementary Fig. 50. fs-TAS spectra of the Py1, Py2, and Py3 under 450 nm\nexcitation. (a-c) 2D mapping TAS spectra of Py1, Py2, and Py3 collected over a 0-2 ns\ntime window. (d-f) Representative kinetic traces at selected probe wavelengths (Py1:\n630 nm; Py2: 600 nm; Py3: 613 nm).\nComment 6. The pump light wavelength for transient absorption spectroscopy is\nselected as 325 nm. Would the photophysical process under excitation at this\nwavelength differ from that under visible light excitation? Have experiments been\n39\nconducted pumped by visible light?\nResponse #6: We sincerely thank the reviewer for this constructive suggestion. The\npump wavelength used in our original fs-TA measurements (325 nm) was selected\nbased on the UV–vis DRS spectra of UCN and the UPy frameworks, as all samples\nexhibit strong absorption in this region. However, because UCN absorbs only weakly\nin the visible region, fs-TAS measurements under visible excitation cannot be\nperformed for UCN. In response to the reviewer’s request, we conducted additional fs-\nTAS measurements for the UPy frameworks and three π-pyrene-π molecules (Py1, Py2,\nPy3) under 450 nm excitation. The resulting spectra were added in Supplementary Figs.\n50 and 51.\nCompared with π-pyrene-π molecules, the UPy frameworks exhibited markedly\nsuppressed stimulated-emission (SE) band near ~600 nm and diminished excited-state\nabsorption (ESA) features near ~750 nm. The weakened SE intensity reflected reduced\nradiative S₁→S₀ transition probability due to rapid intramolecular charge transfer (ICT)\nbetween the pyrene units and the UCN, which decreased the locally excited (LE) state\nresponsible for SER23. The suppressed ESA amplitude suggested that the ICT states in\nUPy frameworks possessed a different S₁→Sₙ transition compared with the LE-\ndominated S₁ state in π-pyrene-π moleculesR24. Furthermore, kinetic traces of UPy\nframeworks at ~630 nm (Supplementary Fig. 51) revealed that UPy frameworks\ndisplayed significantly slower decay dynamics and a dominant long-lived component,\nin contrast to the fast LE-state relaxation observed in π-pyrene-π molecules.\nThese features were characteristic of the formation of a stabilized ICT or charge-separated\nstate in the UPy frameworks.\nCollectively, these results show that visible-light excitation (450 nm) effectively\nprobes the charge separation dynamics in UPy frameworks and leads to photophysical\nbehavior fundamentally distinct from that of the π-pyrene-π molecules. The observed\nprolonged excited-state lifetimes highlight the role of the D-A electronic structure in\npromoting electron–hole separation.\nexcitation. (a-c) 2D mapping TAS spectra of Py1, Py2, and Py3 collected over a 0-2 ns\ntime window. (d-f) Representative kinetic traces at selected probe wavelengths (Py1:\n630 nm; Py2: 600 nm; Py3: 613 nm).\nSupplementary Fig. 51. fs-TAS spectra of the UPy1, UPy2, and UPy3 under 450 nm\nexcitation. (a-c) 2D mapping TAS spectra of UPy1, UPy2, and UPy3 collected over a 0-2 ns\ntime window. (d-f) Representative kinetic traces at selected probe wavelengths\n(UPy1: 630 nm; UPy2: 600 nm; UPy3: 613 nm).\nCorresponding details can be found in the revised manuscript:\nPage 18, lines 429-438: “To further investigate how visible-light excitation\ninfluences the excited-state dynamics of the D–π–A UPy systems, the fs-TAS responses\n41\nof the three π-pyrene-π molecules and the corresponding UPy frameworks under 450\nnm excitation were compared (Supplementary Figs. 50 and 51). The π-pyrene-π\nmolecules displayed strong stimulated-emission (SE) band at ~600 nm and broad\nexcited-state absorption (ESA) extending to ~750 nm. These features were markedly\nsuppressed in UPy frameworks. The reduced SE and ESA intensities indicated the rapid\nformation of an ICT state between the pyrene units and UCN. Correspondingly, the\nkinetic traces of the UPy frameworks exhibited significantly slower decay dynamics\nwith prominent long-lived components, consistent with the presence of stabilized ICT\nor charge-separated state75. These findings demonstrated that covalent linkage of π-\npyrene-π molecules to UCN enhances electron–hole separation and prolongs excited-\nstate lifetimes under visible-light excitation.”\nComment 7. DFT calculations provide the electron distribution of the excited states\nbased on a model structure consisting of one heptazine ring segment and one Py\nmolecule. The oscillator strength data of the corresponding excited states should be\nprovided to understand the contribution of different excited states. In addition, have\ncalculations been performed for the case where one Py molecule is linked to multiple\nCN segments?\nResponse #7: We sincerely thank the reviewer for this valuable suggestion. To address\nthis point, we added the oscillator strength data for the relevant excited states obtained\nfrom TD-DFT calculations (Supplementary Table 6). These values clarify the relative\ncontributions of individual excited states to the overall absorption profile. Compared\nwith the UCN model, the UPy2 models containing one, two, or three heptazine rings\nexhibited systematically larger oscillator strengths. This increasement originates from\nthe extended π-conjugation and strengthened donor–acceptor alignment introduced by\ncovalently linking Py2 to the heptazine core, which increases the transition dipole\nmoment and thus the oscillator strengthR25. Larger oscillator strengths in UPy2, U2Py2\nand U3Py2 directly account for their stronger visible-light absorption and, together with\nthe donor–acceptor architecture, intensify the built-in intramolecular electric field,\n42\nthereby facilitating more efficient exciton dissociation and charge separation. These\nresults mechanistically support the observed improvement in photocatalytic\nperformance.\nWe also agreed that the original UPy2 model, consisting of a single heptazine ring\nsegment and one Py molecule, may not fully signify the accurate electron distribution\nin the excited states. To address this, we constructed additional UPy2 models containing\ntwo and three heptazine ring segments and one Py molecule (U2Py2 and U3Py2). As\nshown in Supplementary Fig. 55 and 56, similar to the excited-state charge distributions\nof UPy2 model in Fig. 5h, these extended U2Py2 and U3Py2 models exhibited\npronounced spatial separation of excited electrons and holes localized on the heptazine\nring and pyrene units, respectively. This reinforces the conclusion that the donor–\nacceptor architecture facilitates robust charge separation across different structural\nconfigurations.\nCorresponding details can be found in the revised manuscript:\nPages 19-20, lines 463-466: “Furthermore, UPy2 model structures containing two\nor three heptazine ring segments covalently linked to one Py molecule (U2Py2 and\nU3Py2) were constructed, and their excited-state charge distributions were investigated\nin Supplementary Figs. 55 and 56. These models exhibited pronounced spatial\nseparation of excited electrons and holes, localized on the heptazine ring and pyrene\nunits, respectively.”\nPage 20, lines 469-473: “As displayed in Supplementary Table 6, the UPy2,\nU2Py2, and U3Py2 models showed systematically larger oscillator strengths than the\nUCN model. These increases result from extended π-conjugation and D–A alignment,\nwhich enlarges the transition dipole moment and contributes to stronger visible-light\nabsorption, enhanced intramolecular fields, and more efficient charge separation,\ncollectively leading to the observed improvement in PHE performance78.”\nSample Excited state Oscillator strength\nS →S 0.0008\n0 1\nUCN S →S 0.0002\n0 6\nS →S 0.0010\n0 7\nS →S 1.3043\n0 1\nUPy2\nS →S 0.0010\n0 4\nS →S 0.3794\n0 1\nS →S 0.2284\n0 2\nU2Py2\nS →S 0.0053\n0 6\nS →S 0.0030\n0 7\nS →S 0.5040\n0 1\nS →S 0.0304\n0 3\nU3Py2\nS →S 0.0028\n0 9\nS →S 0.0170\n0 10\nS →S 0.1172\n0 1\nUPy2-TEOA-Na+ S →S 0.0796\n0 3\nS →S 0.0207\n0 10\nS →S 0.0901\n0 3\nUPy2-TEOA-Mg2+\nS →S 0.0262\n0 5\nS →S 0.2053\n0 1\nS →S 0.0511\n0 2\nUPy2-TEOA\nS →S 0.0097\n0 6\nS →S 0.0005\n0 10\nrings (S →S , S →S , S →S , and S →S ) (blue and green isosurfaces represent hole\n0 1 0 3 0 9 0 10\nand electron distributions, respectively).\nSupplementary Fig. 56. Charge distribution of U3Py2 model consisting three\nheptazine rings (S →S , S →S , S →S , and S →S ) (blue and green isosurfaces\n0 1 0 2 0 6 0 7\nrepresent hole and electron distributions, respectively).\nshouldn't the adsorption of TEOA on pyrene be studied, rather than on the heptazine\nring?\nResponse #8: We sincerely thank the reviewer for this constructive comment. In UPy2,\nefficient intramolecular charge transfer between the heptazine rings and pyrene units\nleads to the accumulation of photogenerated electrons on the heptazine rings and holes\non the pyrene core. Although the adsorption of TEOA on the heptazine rings within\nUPy2 was shown in Fig. 6b, c, this is based on the adsorption of cation–TEOA\ncomplexes (Na⁺–TEOA and Mg²⁺–TEOA) formed through ionic interactions between\ncations and the –OH groups of TEOA. We fully agree that examining the adsorption of\nuncoordinated TEOA on the pyrene units (holes accumulation sites) is necessary.\nTherefore, we investigated the adsorption behavior of TEOA on both the Py2 molecule\nand UPy2. As shown in Supplementary Figs. 64 and 65, the adsorption energy of TEOA\non UPy2 was slightly more negative than that of Py2, indicating that the covalent\nlinkage of Py2 to UCN enhanced hole accumulation at the pyrene core and strengthened\nTEOA adsorption. This result highlights the key role of the pyrene units in facilitating\nhole capture and hole-driven oxidation processes in UPy2.\nSupplementary Fig. 66. Molecular structure models of Py2 and UPy2 adsorbing\nTEOA.\nFurthermore, TD-DFT-based charge distribution analysis of TEOA-adsorbed\nUPy2 revealed distinct charge separation, with photogenerated electrons localized on\nthe heptazine ring and holes on the pyrene unit (Supplementary Fig. 66). This\nredistribution increased the local electron density around the heptazine ring, supporting\nmore efficient charge transfer between heptazine rings and pyrene units.\nSupplementary Fig. 66. Charge distribution of TEOA adsorbed on UPy2 (S →S ,\n0 1\nS →S , S →S , and S →S ) (blue and green isosurfaces represent hole and electron\n0 2 0 6 0 10\ndistributions, respectively).\nPage 22, lines 509-516: “Because photogenerated holes are primarily localized on\nthe pyrene core, the adsorption behavior of uncoordinated TEOA on Py2 and UPy2\nwere investigated. The adsorption energy of TEOA on UPy2 was slightly more negative\nthan that of Py2 (Supplementary Figs. 64 and 65), indicating that covalent linkage of\nPy2 to UCN enhanced hole accumulation at the pyrene core and facilitated TEOA\nadsorption. Moreover, TD-DFT-based charge distribution analysis of TEOA-adsorbed\nUPy2 revealed distinct electron–hole separation from the pyrene unit toward the\nheptazine ring upon excitation, accompanied by an increase in local electron density\naround the heptazine ring (Supplementary Fig. 66).”\nThis paper covalently linked ultrathin CN nanosheets to electron-donating pyrene units\nvia different π-bridges. The optimal catalyst efficiently promoted photocatalytic\nseawater splitting under ambient pressure and natural sunlight, achieving a record\nhydrogen evolution rate of 134 mmol h−1 g−1. But, the core innovations of this paper\nraise several fundamental concerns. There is a suspicion that the narrative was\nconstructed to fit the data rather than the other way around. Recommend rejection by\nconsidering the following issues:\nResponse to the general comment: We sincerely thank the reviewer for these\nprofessional evaluation, constructive comments, and valuable suggestions on our\nmanuscript. We greatly appreciate the reviewer’s concerns and have made substantial\nefforts to address them carefully and transparently.\nIn response, we added SEM images of UCN and UPy2 at the same magnification.\nWe discussed the preparation process of CN and UCN in detail. To minimize\nmeasurement variability in BET surface area data, we repeated the BET measurements\nthree times. Hydrogen evolution under ambient pressure and natural sunlight was\nfurther verified through ignition tests and gas chromatography (GC) analyses. In\naddition, we added a detailed discussion of the PHE performance of UPy2 without\nsacrificial agents or without Pt cocatalyst, as well as the performance of composited\nUCN/Py2 systems (UPy2-π-π), to provide a more comprehensive understanding of the\ncatalytic behavior. Finally, we carefully revised the manuscript to ensure that the\ninterpretation of results remains fully supported by the experimental data, avoiding any\noverextension of conclusions. We believe that these revisions have strengthened the\nclarity, accuracy, and overall rigor of the work. All specific issues raised by the reviewer\nhave been addressed in detail below.\nComment 1. A comparison between UCN (Fig. 2b) and UCN fragments (Fig. 2d) in\nSEM images should be conducted at the same magnification to more intuitively assess\nwhether ultrasonication can effectively fragment the nanosheets. Otherwise, the core\n49\npremise of this paper—which involves fragmenting CN and then linking the fragments\nwith π-pyrene-π small molecules—would be invalidated.\nResponse #1: We sincerely thank the reviewer for this valuable suggestion. A direct\ncomparison of SEM images between UCN (Fig. 2b) and UPy2 (Fig. 2d) has been\nconducted at the same magnification, as displayed in Fig. R3. The UPy2 exhibits the\nmorphology of spliced nanosheets, which is consistent with its TEM image (Fig. 2e).\nFig. R3. SEM images of UCN and UPy2 at the same magnification.\nTo further invalidate that ultrasonication effectively fragments the nanosheets,\nSEM image of UCN fragments were provided and shown in Supplementary Fig. 5.\nCompared with the thin, porous structure of UCN, the ultrasonicated UCN fragments\nclearly exhibited smaller, irregularly shaped pieces. These fragments provided more\nactive edges and sites for covalent bonding with π-pyrene-π molecules, facilitating the\nformation of the spliced nanosheet architecture observed in UPy2.\nSupplementary Fig. 5. SEM image of UCN fragments.\nCorresponding details can be found in the revised manuscript:\nirregular pieces, as displayed in the SEM image (Supplementary Fig. 5). These\nfragments were subsequently covalently linked into large blocky-like structures by Py2\nmolecules via a Schiff-base reaction, forming UPy2 framework (Fig. 2d, e).”\nComment 2. From a processing perspective, are the air-heating preparation method for\nCN and the oxygen etching method for UCN fundamentally different? If not, why is\nCN's preparation method simply called \"air heating,\" while UCN's method is given a\nmore academic-sounding name like \"oxygen etching\"? If they share the same essence,\nshouldn't urea air heating directly yield UCN? Did the authors artificially introduce\ncatalyst preparation steps for the sake of narrative in the paper?\nResponse #2: We sincerely thank the reviewer for this insightful question regarding the\nnaming and fundamental distinction between the synthetic process of CN and UCN.\nAlthough both processes involve thermal treatment in the presence of air, their reaction\nnature and objectives are fundamentally different.\nThe preparation of CN is a polymerization process, in which urea is thermally\npolymerized at 823 K. During this process, small molecular precursors (urea)\nunderwent condensation to form bulk CN. The air atmosphere here only provides a\nbackground environment and does not play a significant reactive role. As shown in\nSupplementary Fig. 4a, b, the resulting CN product is a dense bulk material with low\nsurface area and limited exposed active sites. In contrast, the preparation of UCN is an\noxidative etching process. It is carried out by subjecting pre-synthesized bulk CN to a\nsecond heat treatment at 793 K. At this stage, oxygen in air actively participates in a\nselective oxidative etching reaction, removing surface CN layers, thinning the CN into\nultrathin porous nanosheets, and increasing specific surface area, as proved in SEM and\nTEM images in Supplementary Fig. 4c, d. This process is fundamentally destructive\nand structure-modifying, in contrast to the constructive polymerization process of CN.\nThe effect of oxygen etching of CN is clearly reflected in the physicochemical\nproperties: the yield of UCN decreases to approximately 20%, the BET specific surface\narea increases markedly from 68 m² g⁻¹ (CN) to 210 m² g⁻¹ (UCN), and SEM/TEM\nanalyses confirm the formation of a porous ultrathin nanosheet structure. These\nstructural transformations cannot be achieved by direct urea air heating alone.\nTherefore, the term “oxygen etching” is not used for narrative embellishment but\naccurately reflects the underlying reaction mechanism and structural evolution of CN\nunder oxidative conditions. Direct heating of urea in air does not produce UCN because\nthe polymerization and etching processes occur at different stages and require different\nreaction conditions and material states. This second step is essential for tailoring the\nmorphology and surface chemistry of CN to achieve the desired ultrathin porous\nstructure, and it represents a well-established and mechanistically distinct modification\nstrategy rather than an artificial addition to the synthesis procedure.\nmay yield different results, which does not necessarily imply structural differences\nbetween CN and UCN.\nResponse #3: We sincerely thank the reviewer for this constructive comment. We fully\nagree that BET specific surface area measurements may exhibit some variability due to\ninstrumental and operational factors. To ensure the reliability of our data, we\nindependently repeated the N₂ adsorption–desorption measurements for both CN and\nUCN samples three times. The mean values and standard deviations were then\ncalculated and reported as error bars. The measured specific surface areas and pore\nvolumes were 72 ± 1 m² g⁻¹ and 0.35 ± 0.01 cm3 g⁻¹ for CN, and 184 ± 0.6 m² g⁻¹ and\n0.52 ± 0.05 cm3 g⁻¹ for UCN, respectively (Supplementary Fig. R4). These values are\nconsistent with those reported in the manuscript (68 m² g⁻¹ and 0.33 cm3 g⁻¹ for CN;\n210 m² g⁻¹ and 0.53 cm3 g⁻¹ for UCN). The increase in specific surface area of UCN\nafter oxygen etching is therefore well beyond the expected measurement uncertainty,\nconfirming that the observed difference arises from genuine structural modification\nrather than random experimental fluctuations.\nFig. R4. Measured N adsorption-desorption isothermal curves and pore size\n2\ndistribution of CN, and UCN for three times.\nresult from water vapor evaporation or air expansion due to light heating. A hydrogen\ntest tube 'pop' experiment should be conducted to verify. If this system can indeed\nproduce hydrogen at ambient pressure and under natural light, this work would be\nhighly publishable in top-tier journals like Nature or Science.\nResponse #4: We sincerely appreciate the reviewer’s valuable and constructive\nsuggestion. Following this recommendation, we repeated the outdoor photocatalytic\nseawater-splitting experiment and performed the classic hydrogen “pop” ignition test.\nThe evolved gas was collected directly from the reactor using a syringe (Supplementary\nFig. 36). However, the collected gas did not ignite. This outcome is most likely due to\ndilution by residual air within the reactor at the start of the experiment; as hydrogen is\ngenerated, it mixes with the pre-existing air, resulting in a hydrogen concentration\nbelow the threshold required for ignition.\nTo unambiguously verify the identity of the evolved gas, we subsequently\nanalyzed the collected sample using gas chromatography (GC). A distinct hydrogen\npeak was observed (Supplementary Fig. 37). The retention time of this peak matches\nthat of the H₂ signal obtained during the PHE measurements conducted under ambient\npressure (Supplementary Fig. 20), confirming that hydrogen is indeed produced during\noutdoor sunlight irradiation.\nSupplementary Fig. 36. The reactor equipped with a gas collection bag for outdoor\nPHE experiment in seawater.\nunder ambient pressure.\nIn our PHE measurements, ambient-pressure PHE experiments were conducted in\na Pyrex reactor connected to a closed gas-circulation system (Supplementary Fig. 18).\nA calibration curve correlating the H₂ peak area with known amounts of H₂ under\nambient pressure was first established, and an excellent linear relationship was obtained\n(Supplementary Fig. 15b). Using this calibration, the PHE rates of the UPy frameworks\nwere quantified and similarly exhibited linear behavior (Supplementary Fig. 22),\nconfirming the accuracy and reliability of the ambient-pressure PHE measurements.\nFurthermore, when the reaction suspension collected after seawater PHE was\ntransferred to a beaker and exposed to natural sunlight, vigorous bubble evolution was\nobserved (Supplementary Video 2 and Fig. 3f). In contrast, a blank control experiment\nperformed under identical conditions but without photocatalyst produced no bubbles\n(Supplementary Video S4), unequivocally confirming that the observed gas evolution\narises from photocatalytic hydrogen production.\ntest under ambient pressure.\nSupplementary Fig. 15. H amount-peak area curves calibrated by a certain amount of\n2\npure H under negative pressure (a) and ambient pressure (b).\n2\nseawater under ambient pressure for CN, UCN, UPy frameworks, UPy2-π-π, and CPy2.\nThese results confirm that the bubbles observed in Fig. 3g originate from PHE,\neven though the hydrogen concentration in the reactor was insufficient to trigger\nignition in the pop test. In future work, we aim to optimize the outdoor seawater-\nsplitting setup to achieve higher hydrogen purity and concentration, which may enable\nsuccessful ignition and allow more precise gas quantification.\nCorresponding details can be found in the revised manuscript:\nPage 13, lines 298-303: “To verify the gas composition, the gas collected from the\nreactor equipped with a gas collection bag (Supplementary Fig. 36) was analyzed using\nGC spectrum. A distinct hydrogen peak was observed in Supplementary Fig. 37,\nconfirming that hydrogen evolution occurred under outdoor sunlight irradiation. A\nblank control experiment was also conducted by exposing the same reaction solution to\noutdoor sunlight in the absence of a catalyst, during which no bubble formation was\nobserved (Supplementary Video 4).”\nsacrificial agents and without expensive Pt loading, right?\nResponse #5: We sincerely thank the reviewer for this insightful comment. As noted,\nin the absence of both sacrificial agents and Pt co-catalysts, the UPy frameworks\nexhibited negligible PHE activity. When triethanolamine (TEOA) was introduced as a\nsacrificial agent, the PHE rate first increased and then decreased with rising TEOA\nconcentration, reaching a maximum of 82.00 mmol h⁻¹ g⁻¹ at 5 vol% TEOA\n(Supplementary Fig. 27a). Even at a low concentration of 1 vol% TEOA, the PHE rate\nremained high (50.19 mmol h⁻¹ g⁻¹), underscoring the essential role of TEOA in driving\nhydrogen evolution during seawater splitting. In contrast, no hydrogen production was\ndetected in pure seawater or when alternative sacrificial agents were used, including 10\nvol% methanol, 50 mM ascorbic acid, and 0.25 M Na₂S/0.35 M Na₂SO₃\n(Supplementary Fig. 27b). These results highlight the exceptional efficiency of TEOA,\nowing to its ability to coordinate with metal cations in seawater, thereby facilitating\nefficient hole consumption and suppressing charge recombination.\nSupplementary Fig. 27. (a) Time-dependent PHE curve in seawater under ambient\npressure and different concentrations of TEOA over UPy2. (b) H production rates of\nFor the Pt co-catalyst, the PHE rate increased with rising Pt loading\n(Supplementary Fig. 26), reaching 71.65 mmol h⁻¹ g⁻¹ and 79.13 mmol h⁻¹ g⁻¹ at 5 wt%\nand 10 wt% Pt, respectively. Considering both catalytic performance and material cost,\n5 wt% Pt was identified as the optimal loading. The absence of a Pt co-catalyst severely\n58\nlimits electron extraction efficiency during hydrogen evolution, as the intrinsic charge\nseparation within the UPy frameworks, while beneficial, are insufficient to sustain\nefficient PHE on their own.\nSupplementary Fig. 26. Time-dependent PHE curves in seawater under ambient\npressure over UPy2 with different amount of Pt cocatalyst.\nTherefore, under optimized conditions combining Pt loading and TEOA as a\nsacrificial agent, the UPy frameworks exhibited markedly enhanced PHE performance\ncompared with conventional CN systems. These findings underscore the pivotal role of\nUPy functionalization in promoting effective charge utilization and accelerating\nreaction kinetics.\nComment 6. How does the performance of CN or UCN composite with π-pyrene-π\nsmall molecules loading? Could it potentially match the excellence of UPy systems as\nreported in the literature?\nResponse #6: We sincerely thank the reviewer for this insightful question. To address\nthis, we prepared a photocatalyst by combining Py2 small molecules with UCN through\nπ–π interactions (denoted as UPy2-π-π in the manuscript). As shown in Fig. 3a, the PHE\nperformance of UPy2-π-π was significantly lower than that of the covalently bonded\nUPy2 system. The substantial performance difference between UPy2 and the UPy2-π-\nπ highlights the mechanistic significance of covalent donor–acceptor linkage. In UPy2-\nπ-π, the Py2 and UCN were assembled via weak π–π interactions, resulting in limited\n59\norbital overlap, inefficient charge delocalization, and rapid exciton recombination. By\ncontrast, the covalent framework in UPy2 produced a rigid, electronically coupled\ninterface that facilitated ultrafast electron transfer from the Py2 to the UCN acceptor\nand stabilizes the ensuing charge-separated state. We compared the charge transfer\nproperties between UPy2 and UPy2-π-π by PL and fs-TAS analyses. In Supplementary\nFig. 53, the UPy2 exhibited lower PL intensity, faster PL decay, and longer charge\ncarrier lifetime compared with UPy2-π-π, indicating more efficient charge separation\nand transfer in the covalently linked system. These findings highlight the critical role\nof the covalent interface in regulating charge dynamics and achieving high\nphotocatalytic performance.\nFig. 3a. PHE performances in deionized (DI) water, 3.5 wt% NaCl, and seawater.\nUPy2-π-π. 2D mapping TAS spectrum (c) and decay kinetic curve of UPy2-π-π (d).\nCorresponding details can be found in the revised manuscript:\nPages 18-19, lines 447-451: “To further highlight the mechanistic significance of\ncovalent D–A linkage, the charge transfer properties of UPy2 and UPy2-π-π were\ncompared. Evidence from PL quenching, faster PL decay, and significantly prolonged\nfs-TAS decay lifetime (Supplementary Fig. 53) confirmed that UPy2 suppresses\nradiative and non-radiative recombination far more effectively than UPy2-π-π. This\n61\nstrengthened electronic coupling and ICT efficiency enabled by covalent bonding are\nkey factor underlying the improved PHE performance77.”\nComment 7. The experimental data of XPS, BET, PL, FTIR, etc. data may inherently\nvary due to experimental conditions, and selective reporting risks overinterpretation.\nNo comment.\nResponse #7: We sincerely thank the reviewer for highlighting this important point.\nWe fully acknowledge that measurements such as XPS, BET, PL, and FTIR inevitably\n61\nexhibit some degree of variation due to factors including sample preparation,\ninstrument calibration, measurement environment, and operator handling. Such\nvariability is inherent to experimental characterization and reflects the realistic range\nof material behavior. To ensure the reliability and representativeness of the reported\nresults, each characterization was performed through multiple independent\nmeasurements, all of which showed consistent trends across replicates. The figures and\ntables in the manuscript present representative datasets that accurately capture the\noverall material properties, thereby avoiding any selective reporting. When interpreting\nthe data, we exercised particular caution to avoid conclusions that exceed what the\nmeasurements directly support. For instance, in discussions of visible-light absorption,\nBIEF intensity, and photoluminescence behavior, we emphasize comparative trends\nrather than absolute values and explicitly indicate experimental uncertainties where\nappropriate. 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Unveiling charge dynamics of visible light absorbing oxysulfide\nfor efficient overall water splitting. Nat. Commun. 12, 7055 (2021).\nR16 Michael, S. et al. Evaluation of surface state mediated charge recombination in\nanatase and rutile TiO . J. Phys. Chem. Lett. 7, 3742-746 (2016).\ninto emissive polymers: insights into emission quenching. Chem. Mater. 37, 8048-\n058 (2025).\nR18 Wei, Z. W. et al. Rigidifying fluorescent linkers by metal-organic framework\nformation for fluorescence blue shift and quantum yield enhancement. J. Am.\nChem. Soc. 136, 8269-276 (2014).\nR19 Wan, P. et al. Direct observation of the charge transfer states from a non-fullerene\norganic solar cell with a small driving force. J. Phys. Chem. Lett. 12, 10595-0602\n(2021).\nR20 Veaceslav, C. et al. Charge-transfer electronic states in organic solar cells. Nat. Rev.\nMater. 4, 89-07 (2019).\nR21 Krishna, P. K. et al. Ultrafast dynamics in flavocytochrome C by using transient\nabsorption and femtosecond fluorescence lifetime spectroscopy. J. Phys. Chem. B\n129, 3731-739 (2025).\nR22 Natalie, B. et al. Sensitization of fullerenes by covalent attachment of a\ndiketopyrrolopyrrole chromophore. J. Mater. Chem. 22, 13286-13294 (2012).\nR23 Ji, L. et al. Thermodynamic equilibrium between locally excited and charge-\ntransfer states through thermally activated charge transfer in 1-(pyren-2′-yl)-o-\ncarborane. Chem. Sci. 13, 5205-5219 (2022).\nR24 Arvind, B. et al. Excited-state intramolecular charge-transfer dynamics in 4-\ndimethylamino-4′-cyanodiphenylacetylene: an ultrafast Raman loss spectroscopic\nperspective. ChemPhotoChem 6, e202200146 (2022).\nR25 Cleber, F. N. M. et al. Tuning the photocatalytic properties of porphyrins for\nhydrogen evolution reaction: an in-silico design strategy. J. Power Sources Adv. 15,\n100090 (2022).\n64"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors improved the paper significantly and have addressed all my comments.", "reviewer_2": "The authors have addressed the majority of concerns raised in my earlier review. The revised manuscript has been significantly improved. Once the following issues are corrected, the manuscript will be acceptable for publication. No further review is necessary.\n1. Please add the unit (OD) to y0 in Figs. 5c and 5d. Without it, the parameters cannot be used to reproduce the data shown in these figures. Same question for Supplementary Figs. 48c and 46f. In addition, the value of y0 in Fig. 5d seems incorrect.\n2. In Supplementary Figs. 50(d–f) and 51(d–f), it is evident that all deltaA values shift from negative to positive over time, which raises substantial concerns regarding the validity of the applied multi-exponential decay fitting model. Please explain the physicochemical mechanism responsible for this sign reversal in deltaA.\n3. Please add the unit (OD) for alfa and the unit (ps) for t0 in Supplementary Table 4.", "reviewer_3": "The authors have addressed all my comments. Thus, I'd like to recommend its publication in the current form.", "reviewer_4": "In my opinion, the narrative aspect of this paper outweighs its scientific value. More importantly, the optimal catalyst described in the paper achieves a photocatalytic hydrogen production rate of 134 mmol h⁻¹ g⁻¹ under ambient pressure and natural light conditions, which is consistent with the abundant bubbles observed in the video. However, it is highly unusual that the tube ignition test could not be successfully conducted. This is quite perplexing. Under normal circumstances, by extending the reaction time and utilizing the low density of hydrogen gas, it should be possible to collect hydrogen of sufficient purity for ignition using a funnel or similar apparatus. Additionally, the authors did not address Comment 5, in which I requested an investigation into the performance of UPy frameworks without sacrificial agents and without expensive Pt loading. Furthermore, Figure 2d has not been replaced with an image at the same magnification as Figure 2b. Therefore, I still recommend rejecting the manuscript."}, "author_response": "Response to the Comments of Editor and Referees\nManuscript ID: NCOMMS-25-66364-A\nTitle: Solar hydrogen production through ambient-pressure seawater splitting\nDear Editor and Reviewers:\nWe sincerely appreciate the Editor’s letter and the constructive comments\nprovided by Reviewers #2 and #4. Their thoughtful suggestions have significantly\nimproved the quality and clarity of our manuscript. All changes in the revised\nmanuscript and Supplementary Information are highlighted in yellow. In addition, we\nprovide a detailed point-by-point response addressing each comment and describing the\ncorresponding revisions.\nThank you again for your time, consideration, and kind assistance.\nYour sincerely,\nXinlong Tian\nReviewer #1 (Remarks to the Author):\nThe authors improved the paper significantly and have addressed all my comments.\nResponse: Thank you very much for your positive comment on the revisions we made\nto our manuscript.\nReviewer #2 (Remarks to the Author):\nThe authors have addressed the majority of concerns raised in my earlier review. The\nrevised manuscript has been significantly improved. Once the following issues are\ncorrected, the manuscript will be acceptable for publication. No further review is\nnecessary.\nResponse: Thank you very much for your positive comments on the revisions we made\nto our manuscript. We are also grateful for your new comments and suggestions. We\nhave revised the manuscript accordingly and implemented all of the requested\ncorrections as detailed below.\nComment #1. Please add the unit (OD) to y in Figs. 5c and 5d. Without it, the\n0\nparameters cannot be used to reproduce the data shown in these figures. Same question\nfor Supplementary Figs. 48c and 48f. In addition, the value of y in Fig. 5d seems\n0\nincorrect.\nResponse #1: Many thanks for the valuable comment. We have added the unit (OD) to\ny in Fig. 5c, d and Supplementary Fig. 49c, f. In addition, the value of y in Fig. 5d has\n0 0\nbeen corrected.\nand decay kinetic curves (c, d) of UCN and UPy2. e, Charge trapping models proposed\nfor UCN and UPy2. f, g, Fluence dependent electron decay kinetics after excitation at\n325 nm of UCN (f) and UPy2 (g). h, Dependence of the initial amplitude (blue), decay\nhalf time (red), and % of residual signal relative to initial signal (yellow) for the\nphotogenerated electron signal at ~700 nm as a function of excitation energy\ndetermined from the data in Fig. 5f, g.\ncorresponding TAS intensity decay curves (c, f) for UPy1 and UPy3. Schematic\nrepresentation of proposed charge trapping models for UPy1 (g) and UPy3 (h).\nComment #2. In Supplementary Figs. 50(d–f) and 51(d–f), it is evident that all ΔA\nvalues shift from negative to positive over time, which raises substantial concerns\nregarding the validity of the applied multi-exponential decay fitting model. Please\nexplain the physicochemical mechanism responsible for this sign reversal in ΔA.\nResponse #2: Thank you for your professional comment. In Supplementary Figs. 50(d-\nf) and 51(d-f), the probe wavelengths (630, 600, and 613 nm) locate in the spectral\nregion where ground-state bleach (GSB) and stimulated emission (SE) of the electron-\ndonating π-pyrene-π molecules (negative ΔA) overlap with the excited-state absorption\n(ESA) of the charge transfer (CT) states (positive ΔA). Immediately after\nphotoexcitation, the signal is dominated by GSB/SE, resulting in a negative ΔA.\nWith increasing delay time, the GSB/SE components decay, while the population of long-\n4\nlived CT states increases and their ESA contribution becomes dominant. Consequently,\nthe net ΔA changes from negative to positive. We describe this kinetic using a multi-\nexponential function,\nΔA(t)=∑Ae−t/τi+y\ni i 0\nwhere different amplitudes A (positive and negative) represent the decay of GSB/SE,\ni\nESA, and long-lived species. Such a model is commonly used in fs-TAS analysis and\ncan naturally account for non-monotonic kinetics and signal sign reversals.\nComment #3. Please add the unit (OD) for α and the unit (ps) for t in Supplementary\nResponse #3: We thank you for the valuable suggestion. We have added the unit (OD)\nfor α and the unit (ps) for t in Supplementary Table 4.\nthe fluence dependent recombination kinetics in Fig. 5f, g.\nPulse energy\nα (OD)  t (ps) R2\n0\n(mJ cm-2)\n0.24 0.00235 0.196 -1.57 0.68\n0.37 0.00480 0.187 -1.06 0.92\nUCN\n0.96 0.00681 0.222 -1.65 0.85\n1.94 0.0126 0.193 -0.576 0.96\n0.24 0.00271 0.166 -2.06 0.62\n0.37 0.00444 0.172 -0.981 0.94\nUPy2\n0.96 0.00765 0.179 -0.450 0.92\n1.94 0.0129 0.199 -0.991 0.94\nThe authors have addressed all my comments. Thus, I'd like to recommend its\npublication in the current form.\nResponse: Thank you very much for your positive comment on the revisions we made\nto our manuscript.\nReviewer #4 (Remarks to the Author):\nIn my opinion, the narrative aspect of this paper outweighs its scientific value. More\nimportantly, the optimal catalyst described in the paper achieves a photocatalytic\nhydrogen production rate of 134 mmol h⁻¹ g⁻¹ under ambient pressure and natural light\nconditions, which is consistent with the abundant bubbles observed in the video.\nHowever, it is highly unusual that the tube ignition test could not be successfully\nconducted. This is quite perplexing. Under normal circumstances, by extending the\nreaction time and utilizing the low density of hydrogen gas, it should be possible to\ncollect hydrogen of sufficient purity for ignition using a funnel or similar apparatus.\nAdditionally, the authors did not address Comment 5, in which I requested an\ninvestigation into the performance of UPy frameworks without sacrificial agents and\nwithout expensive Pt loading. Furthermore, Figure 2d has not been replaced with an\nimage at the same magnification as Figure 2b. Therefore, I still recommend rejecting\nthe manuscript.\nResponse: We sincerely thank you for the thoughtful and detailed comments provided\nin this round of review. We greatly appreciate the careful reading of our work and the\nopportunity to further improve the manuscript. We have carefully addressed each\nconcern and conducted additional experiments to strengthen the scientific clarity and\ncompleteness of the study. Our detailed responses to each point are provided below.\n(1) “The narrative aspect of this paper outweighs its scientific value.”\nWe sincerely thank you for raising this concern. We fully recognize the importance\n6\nof maintaining a clear distinction between descriptive narrative and experimentally\nverifiable scientific evidence. In our study, we presented a mechanistically resolved,\nmulti-layer experimental, and theoretical framework, supported by quantitative\nspectroscopy, charge carrier-dynamics analysis, in-situ characterizations, and DFT/TD-\nDFT simulations. These complementary datasets collectively establish a coherent and\nexperimentally validated mechanism for ambient-pressure seawater photocatalysis. To\naddress this concern more directly, we have substantially included direct experimental\nevidence rather than descriptive interpretation to ensure that all mechanistic\nconclusions, such as the complete mechanistic evidence chain, including fs-TAS,\nTD-PL, SPV, KPFM, in-situ XPS, NEXAFS, EIS, DFT, adsorption-energy calculations,\nthe quantitative comparison for exciton binding energies, charge lifetimes, BIEF\nintensities, PLQYs, and metal–TEOA adsorption energies. More importantly, we\nstrengthened the data–mechanism correlation, reorganized the Abstract and Introduction\nsection for clarity, and significantly reduced narrative description.\nWe believe that these revisions fully address your concerns and make the\nscientific value clear and transparent.\nCorresponding details can be found in the revised manuscript:\nPage 2, lines 23-31: “Quantitative spectroscopic analyses reveal that the\ndonor–π–acceptor architecture of the resulting CN (UPy) frameworks, featuring a\nbiphenyl π-bridge, reduces the exciton binding energy from 60 meV (UCN) to 28\nmeV and generates long-lived charge-separated states, as evidenced by\ntemperature-dependent photoluminescence and femtosecond transient absorption\nspectroscopy. In-situ X-ray photoelectron spectroscopy, photovoltage\nmeasurements, and Kelvin probe force microscopy collectively demonstrate\nefficient intramolecular electron transfer from pyrene to heptazine rings and a\nsubstantially strengthened built-in internal electric field in UPy frameworks.\nDensity functional theory further shows that the resulting electron accumulation on\nheptazine units significantly enhances the adsorption of Na⁺/Mg²⁺–triethanolamine\ncomplexes, thereby accelerating hole-consumption in seawater.”\nphotoluminescence, femtosecond transient absorption spectroscopy, in-situ X-ray\nphotoelectron spectroscopy, and theoretical calculations, we quantitatively\ndemonstrate that the biphenyl π-bridged UPy2 framework achieves a significantly\nreduced exciton binding energy, a prolonged charge-carrier lifetime, and a\nstrengthened intramolecular electric field. Moreover, density functional theory\nreveals that the enhanced electron accumulation on heptazine units promotes the\nadsorption of seawater cations and their complexes with triethanolamine, thereby\naccelerating hole-consumption.”\n(2) “Tube ignition test.”\nWe sincerely appreciate your valuable and constructive suggestion. We\napologize that, in our previous response, we were unable to demonstrate a\nsuccessful hydrogen “pop” ignition test, which may have caused confusion\nregarding the nature of the observed bubbles. In our initial outdoor trial, the\ncollected gas did not ignite. We believe this negative result was mainly caused by\n(i) substantial dilution by residual air in the reactor headspace at the beginning of\nthe experiment, and (ii) strong interference from water vapor/entrained moisture at\nthe elevated outdoor temperature (~38℃) under outdoor irradiation, both of which\ncan reduce the effective hydrogen concentration below the ignition threshold. To\ncollect hydrogen at sufficiently high concentration and free of water vapor for\nignition, we replaced the large reactor with a smaller reaction vessel, as displayed\nin Supplementary Fig. 18. In a typical experiment, 20 mg of UPy2 catalyst was\ndispersed in 100 mL of seawater containing 5 vol% TEOA. After PHE reaction for\n6 h at 25 ℃ under ambient pressure, approximately 5 mL of gas was collected from\nthe reaction using an injection syringe once the reactor was cooled to 5 °C. The\nsuccessful ignition of the collected gas, as shown in Supplementary Video 5 and\nFig. 38, confirms hydrogen generation and demonstrates that photocatalytic\nhydrogen evolution from seawater proceeds efficiently under ambient-pressure\nconditionsR1.\nSupplementary Fig. 18.\nCorresponding details can be found in the revised manuscript:\nPage 13, lines 310-312: “Furthermore, the ignition of collected gas from the\nreactor shown in Supplementary Fig. 18, as demonstrated in Supplementary Video 5\nand Fig. 38, confirms the successful generation of hydrogen from PHE in seawater\nunder ambient pressure58.”\n(3) “The performance of UPy frameworks without sacrificial agents and without\nexpensive Pt loading.”\nWe apologize for the previous lack of completeness and have now fully\naddressed this point. The PHE performances of the UPy2 framework without\nsacrificial agents and without expensive Pt loading have been added in\nSupplementary Fig. 27a and 26a, respectively. The corresponding GC spectra of\ncollected gases (Supplementary Fig. 26b and 27c) show negligible hydrogen\nproduction under both conditions, confirming that the sacrificial agent and Pt\ncocatalyst are essential for measurable PHE activity.\npressure over UPy2 with different amount of Pt cocatalyst. (b) GC spectrum of gas\nproducts for PHE test over UPy2 without Pt loading.\nSupplementary Fig. 27. (a) Time-dependent PHE curve in seawater under ambient\npressure and different concentrations of TEOA over UPy2. (b) H production rates of\nproducts for PHE test over UPy2 in pure seawater.\nCorresponding details can be found in the revised manuscript:\nPage 11, lines 244-245: “In addition, UPy2 without Pt loading exhibited\nnegligible PHE activity, as shown in Supplementary Fig. 26b.”\npure seawater (Supplementary Fig. 27b, c) or in the presence of alternative sacrificial\nreagents, including 10 vol% methanol, 50 mM ascorbic acid, and 0.25 M Na S/0.35 M\n2 3\n(4) “The image in Fig. 2d.”\nThank you for pointing this out. We apologize for not replacing the image in\nthe previous revised manuscript. Fig. 2d has now been replaced with a new SEM\nimage taken at the same magnification as Fig. 2b, ensuring an accurate comparison\nbetween UCN and UPy2.\nFig. 2 | Synthesis and characterization of the UPy frameworks. a, Synthesis of the\nUPy frameworks. b-e, Scanning electron microscopy and transmission electron\nmicroscopy images of UCN (b, c) and UPy2 (d, e). f, g, C (f) and N (g) K-edge near-\nedge X-ray absorption fine structure (NEXAFS) spectra of UCN and UPy2.\nabout the verifiability and potential ambiguity of the outdoor bubble observation, and\nwe have therefore added direct and reproducible validation (successful pop test, GC\nconfirmation, and essential controls). We hope these additions address your key\nreservations, and we appreciate your valuable time and effort.\nReference\nR1. Li, W. et al. 0.68% of solar-to-hydrogen efficiency and high photostability of\norganic-inorganic membrane catalyst. Nat. Commun. 15, 6763 (2024).\n12"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have addressed my comments. I recommend the revised manuscript for acceptance.", "reviewer_4": "This work can be accepted."}, "author_response": "Response to the Comments of Editor and Referees\nManuscript ID: NCOMMS-25-66364-B\nTitle: Solar hydrogen production through ambient-pressure seawater splitting\nDear Editor and Reviewers:\nWe sincerely thank you for your letter and for the valuable and constructive\ncomments provided by Reviewers #2 and #4. We have carefully addressed all raised\nissues and have revised the manuscript and Supplementary Information accordingly.\nThe detailed, point-by-point responses to each comment are provided below.\nThank you again for your time, consideration, and kind assistance.\nYour sincerely,\nXinlong Tian\nReviewer #2 (Remarks to the Author):\nThe authors have satisfactorily addressed my comments 1 and 3 in the revised\nmanuscript. However, my concerns regarding comment 2 remain unresolved. The\nfitting parameters presented in the supplementary figures appear to be inconsistent with\nthe explanation offered in the authors’ response. Specifically, all fitted amplitude values\nfor the distinct time constants share the same sign, which contradicts the physical\ninterpretation suggested by the authors. Additionally, the goodness of fit, as indicated\nby the reported R2 values, is considerably low (far from 1). This raises questions about\nthe suitability of the chosen model.\nResponse: Thank you for the careful assessment of the fitting results in Supplementary\nFigs. 51 and 52. We acknowledge that our previous explanation was not sufficiently\nrigorous. We have therefore re-analyzed the kinetic traces and revised both the fitting\nstrategy and the corresponding discussion.\nBased on a thorough re-analysis of kinetic traces of the π-pyrene-π molecules,\nwe identify that the origin of inconsistency arises from the choice of fitting time window.\nThe excited-state relaxation processes of the π-pyrene-π molecules occur on an ultrafast\ntimescale of only a few picoseconds. Applying multi-exponential fitting to kinetic\ntraces over an extended 0-2 ns time window therefore extends far beyond the ultrafast\nphysical processes of interest and can over-constrain the fit, which can lead to\ninconsistencies between the fitted amplitudes and their physical interpretation, and\nreduced goodness-of-fit values, as correctly pointed out by the reviewer. To address this\nissue, we have redefined the fitting window to 0-50 ps, which more accurately captures\nthe intrinsic excited-state dynamics of the π-pyrene-π molecules. Within this time scale,\nthe signal at 600 nm is dominated by a monotonic stimulated-emission (SE) decay,\ntherefore, the fitted amplitudes are expected to share the same sign and should be\ninterpreted as multiple decay components/channels rather than a rise-decay mixture.\nThe fitting quality improves, as evidenced by increased R2 values (Supplementary Fig.\n51d-f). The revised fitting results are now fully consistent with the expected ultrafast\n2\nrelaxation behavior of the π-pyrene-π molecules. Accordingly, Supplementary Fig. 51\nhas been updated to show the 0-50 ps 2D maps and the corresponding ~600 nm kinetics\nused for fitting.\nSupplementary Fig. 51. fs-TAS spectra of the Py1, Py2, and Py3 under 450 nm\nexcitation. (a-c) 2D mapping TAS spectra of Py1, Py2, and Py3 collected over a 0-50\nps time window. (d-f) Representative kinetic traces at probe wavelength of ~600 nm.\nWe further note that, under 450 nm excitation, the transient absorption signals of\nUPy frameworks are weaker than those of π-pyrene-π molecules, resulting in lower\nsignal-to-noise ratios in the kinetic traces. While this inevitably affects the R2 values,\nthe extracted time constants remain reliable, as confirmed by the small standard\ndeviations and well-constrained fitting parameters. Importantly, the residuals are\nrandomly distributed around zero without large deviation, and the standard deviation of\nthe residuals is comparable to the experimental noise level and exhibits no systematic\nbias (Supplementary Fig. 52d-f). These observations indicate that the applied kinetic\nmodel adequately captures the dominant decay dynamics. Supplementary Figs. 51 and\n52 have been updated accordingly using the revised fitting window and analysis. We\nbelieve that this revised treatment resolves the reviewer’s concerns regarding the\nconsistency of the fitting parameters, the goodness of fit, and the physical validity of\nthe kinetic model.\nFinally, we sincerely appreciate the reviewer’s valuable and insightful suggestions\n3\nregarding the transient absorption kinetic analysis. These suggestions have\nsignificantly improved the physical consistency of our fitting approach and have\nstrengthened the reliability of our conclusions. We are grateful for these constructive\ncomments, which will also inform our future studies of charge-carrier dynamics in\nphotocatalytic systems.\nSupplementary Fig. 52. fs-TAS spectra of the UPy1, UPy2, and UPy3 under 450 nm\nexcitation. (a-c) 2D mapping TAS spectra of UPy1, UPy2, and UPy3 collected over a\n0-2 ns time window. (d-f) Representative kinetic traces at probe wavelength of ~600\nnm. The inset is the corresponding standard deviation.\n1. Many H production test was carried out with ambient pressure N as blanket gas in\n2 2\nan atmospheric pressure reactor. Liu Naixu's research group from Southeast University\nin China has publish some related papers. So the “ambient pressure” bright spot remain\nto be discussed.\nResponse: Thank you for this insightful comment. We agree that many H production\n2\nstudies have been conducted under atmospheric pressure using N or Ar as a blanket\n2\ngas. We also note that we could not locate relevant publications by a researcher named\n“Liu Naixu”. We believe the reviewer may be referring to Prof. Naixu Li (Southeast\nUniversity, China), and we have now revised the manuscript to cite representative\nrelated studies accordingly.\nWe would like to clarify that the term “ambient pressure” specifically refers to a\nreaction environment without any inert-gas protection. This distinction is important\nbecause oxygen is a strong electron scavenger and commonly suppresses photocatalytic\nH2 evolution by consuming photogenerated electrons and quenching excited states. In\nour study, all PHE experiments under ambient pressure were performed directly under\nair atmosphere, without N or Ar purging. Notably, despite the presence of oxygen,\n2\nefficient PHE was still achieved. This distinction is critical, as oxygen is well known to\nseverely suppress PHE by competing with protons for photogenerated electrons and by\nquenching excited states. Consequently, some previously reported “ambient-pressure”\nsystems rely on inert-gas protection to eliminate oxygen interferenceR1-6. In contrast,\nthe ability of our UPy frameworks to sustain PHE under oxygen-containing condition\ndemonstrates a fundamentally enhanced tolerance to realistic environments.\nCorresponding details can be found in the revised manuscript:\nPage 2, lines 31-33: “Benefiting from these synergistic effects, the optimal UPy\ncatalyst efficiently promoted photocatalytic seawater splitting under ambient pressure\nwithout inert-gas protection and natural sunlight, achieving a record hydrogen\nevolution rate of 134 mmol h−1 g−1.”\nPage 10, line 223: “PHE experiments under ambient pressure without inert-gas\n5\nprotection were conducted in a Pyrex vessel connected to a gas circulator using DI\nwater”\nPage 11, lines 251-253: “Unlike most ambient-pressure photocatalytic systems\nthat require inert-gas protection56,57, the UPy2 system maintains high PHE activity\nunder an air atmosphere, without N or Ar purging, highlighting its strong potential for\n2\npractical application.”\n2. Under the full-spectrum lights used in the laboratory (Video 5), it is certain that a\nlarge amount of hydrogen gas can be produced. This is doesn't require proof. I mean\nthat “The hydrogen ignition experiment should be performed under natural sunlight\n(same as fig. 3g)”.\nResponse: Thank you for the constructive suggestion regarding the hydrogen ignition\nexperiment under natural sunlight. We agree that demonstrating hydrogen ignition\nunder real solar irradiation, consistent with those used in Fig. 3g, is essential to\nsubstantiate the practical relevance of the ambient-pressure PHE. In response to this\ncomment, we redesigned and assembled an outdoor PHE reactor. Specifically, a reactor\nwith the same diameter as that in Fig. 3g was fabricated, while the reactor height was\nreduced from 10 cm to 3 cm (Supplementary Fig. 38a). This modification significantly\ndecreased the internal reactor volume, thereby effectively overcoming gas-dilution\neffects that previously prevented successful ignition. Using this newly assembled\nsystem, PHE in seawater was conducted outdoors under natural sunlight and ambient\npressure. Upon cooling of the reactor, the accumulated gas was withdrawn using a\nsyringe and successfully ignited, as displayed in Supplementary Video 5 and Fig. 38b.\nThis result directly demonstrates hydrogen production from PHE in seawater under\nnatural sunlight and ambient pressure, fully addressing the reviewer’s concern and\nstrengthening the practical significance of the reported system.\nCorresponding details can be found in the revised manuscript:\nPage 13, lines 305-308: “Furthermore, the successful ignition of gas collected\nfrom the disk-shaped reactor (20 cm diameter, 3 cm height) connected to a gas-\ncollecting bag, as shown in Supplementary Video 5 and Fig. 38, confirms hydrogen\ngeneration from PHE in seawater under natural sunlight and ambient pressure58.”\ncm height) connected with the gas-collecting bag. (b) Ignition of collected gas from the\nreactor following the PHE in seawater under natural sunlight and ambient pressure.\nReferences\nR1. Zhu, D. S. et al. Porous microreactor chip for photocatalytic seawater splitting over\n300 hours at atmospheric pressure. Nano-Micro Letters 17, 188 (2025).\nR2. Ge, Y. et al. Sulfur and carbon co-doped g-C N microtubes with enhanced\n3 4\nphotocatalytic H production activity. Front. Energy 18, 110-121 (2023).\nsplitting. Adv. Mater. 36, 2406088 (2024).\nR4. Yue Q. H. et al. In-situ polarization of covalent organic frameworks in seawater\nenables enhanced photocatalytic hydrogen evolution under visible-light irradiation.\nNano Res. 16, 6251-6259 (2023).\nR5. Chang C. L. et al. Synergistic effect of crown ether and main-chain engineering for\nboosting hydrogen evolution of polymer photocatalysts in seawater. Adv. Energy\nMater. 13, 2300986 (2023).\nR6. Zhang J. N. et al. Photocatalytic hydrogen production from seawater under full\nsolar spectrum without sacrificial reagents using TiO nanoparticles. Nano Res. 15,\n2\n2013-2022 (2022).\n7"}]

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