Phenomenon identification—theoretical prediction—activity evaluationObservations revealed that the decomposition of potassium iodide, mainly when used for an extended period, could occur (Supplementary Fig. 2a). Integrating these decomposed components into the g-C3N4 (CN) matrix drastically enhanced its catalytic performance (Supplementary Fig. 2b, c). A vital aspect of this improved performance was the formation of I3-, predominantly attributed to incorporating I2 (Supplementary Fig. 2a). Drawing inspiration from these findings, various amounts of I2 were introduced into the CN system. This addition aimed to simulate the metathesis process and facilitate the forming of intercalated layers comprising I-/I3- redox mediators, potentially enhancing the photocatalytic efficacy.To elucidate the impact and functionality of I- or I3- species as redox mediators in subsequent high-value chemical reactions, this study established three distinct model systems: the original CN, CN co-polymerized with KI (termed CN-I), and CN co-polymerized with a KI/I2 mixture (CN-I/I3) (Fig. 2a–c, Supplementary Fig. 3 and Supplementary Data 1). A comprehensive investigation utilizing density of states (DOS) analyses was conducted to assess the influence of I- or I3- incorporation on the electronic band structure and band gap of CN (Fig. 1e–g). The valence band (VB) of unmodified CN predominantly consisted of N’s 2p orbitals, while its conduction band (CB) was chiefly composed of the 2p orbitals of C and N. Integration of I- the CN matrix led to the emergence of mid-gap states within the CB, causing a downward shift in the CB edge and narrowing the band gap. In contrast, introducing I3- elicited an upward shift in CN’s VB. Notably, CN co-polymerized with both I- and I3- (CN-I/I3) exhibited the most significant reduction in the band gap. This narrower band gap enhanced visible light absorption and improved the driving force for ORR.Fig. 2: Qualitative and quantitative characterizations.a, b, c Optimized CN (a), CN-I (b), and CN-I/I3 (c) geometric structures with an isosurface value of 0.009 e/Bohr3. d TEM-EDS mappings of CN-I/I3. e–k XPS spectra (e, f, g), FT-IR spectra (h), XRD patterns (i), C K-edge (j), and N K-edge (k) XANES spectra of CN, CN-I, and CN-I/I3. l ESR spectra of CN and CN-I/I3. m, n, o TEM (inset), and HR-TEM images of CN (m), CN-I (n), and CN-I/I3 (o). p Relationship between H2O2 production and FWHM value of the (002) plane for samples with different CN-I-I2 ratios.Theoretical calculations revealed surface work functions for CN, CN-I, and CN-I/I3 as 5.150, 4.254, and 4.168 eV, respectively (Fig. 1d, h, l). Incorporating I- and the combined I-/I3- systems could result in a progressive elevation of the Fermi level, easing the constraints on free electron movement and concurrently reducing the work function. These changes suggested that the addition of I- or I3- facilitated the migration of photo-generated electrons, thereby increasing surface reaction rates29. The synergistic effect was more pronounced with the simultaneous introduction of I- and I3-. Furthermore, electrostatic potential (ESP) analyses and two-dimensional electron density mapping were used to investigate the charge distribution characteristics in CN post I- or I3- (Fig. 1b, c). While electronegative atoms presented negative electrostatic potential and conjugated surfaces displayed positive potential, creating electron-rich and electron-deficient areas, the symmetric structure of pristine CN led to a uniformly distributed charge, impeding charge transfer. The inclusion of I- or I-/I3- disrupted such uniformity, promoting charge separation. Furthermore, optical property evaluations based on a six-flux model indicated that CN-I/I3 showed better characteristics compared to CN and CN-I, implying enhanced photoactivity (Supplementary Fig. 4; Supplementary Tables 1 and 2). These findings confirm that embedding I- or I3- boosted light absorption and charge separation efficiency in CN.Taking into account the aforementioned considerations, we developed a dual high-value conversion system to validate the innovative design of the catalyst. In photocatalytic H2O2 production, benzyl alcohol (BzOH) was used as the organic substrate to evaluate the performance of a precisely engineered catalyst. This assessment was conducted within an optimized reaction system using the six-flux model (Fig. 1i and Supplementary Figs. 5–7). Incorporating KI into the catalyst notably amplified its capacity for H2O2 production. Furthermore, the subsequent addition of I2 to the CN-I framework resulted in a discernible secondary enhancement in H2O2 by CN-I production. Compared to the baseline CN and the CN-I variant, CN co-polymerized with both KI and I2 (CN-I/I3) demonstrated a stronger photocatalytic activity, achieving an apparent quantum yield of 34.61% at 400 nm. This performance largely surpassed similar reported systems for the artificial photosynthesis of H2O2 (Supplementary Fig. 8). The catalytic activity for H2O2 decomposition was not notably inhibited (Supplementary Fig. 9), suggesting that the observed improvement was likely attributable to the enhanced crystallinity and the formation of I-/I3- redox mediators within its layered structure. To optimize the yield of photocatalytically produced H2O2, a strategy involving segmented addition of the organic substrate was implemented during the reaction process (Supplementary Figs. 11–15). This approach increased H2O2 yield from 12.34 mM to 15.63 mM, indicative of more efficient utilization of photo-generated carriers. The successful H2O2 production using various organic substrates corroborates the effectiveness of this enhanced photocatalytic strategy.In this study, the efficacy of H2O2 photo-production and concurrent oxidation for benzaldehyde (BA) synthesis was meticulously investigated (Fig. 1j, k; Supplementary Figs. 16 and 17). Co-polymerization with both KI and KI/I2 markedly improved H2O2 production. Significantly, the CN-I/I3 sample exhibited exceptional performance in H2O2 generation, reaching 62.52 mmol g−1 h−1, which was approximately 143.0 times higher than that of pristine CN (0.436 mmol g−1 h−1), and displayed optimal BzOH oxidation efficiency. The conversion rate of BzOH using CN-I/I3 w was around 17.18%, with a high BA selectivity of 92.65%, in stark contrast to the 0.94% conversion rate and 14.06% selectivity observed with unmodified CN.Interestingly, an analysis of the functional relationship between photocatalytic rates of H2O2 and BA across different samples revealed high correlation coefficients, up to 0.970 (Supplementary Fig. 19). This strong correlation underscores a synergistic effect between photocatalytic reduction and oxidation processes, enabling dual-efficient production H2O2 and BA while fully utilizing generated electrons and holes. Additionally, the specifically engineered CN-I/I3 system demonstrated effective production of H2O2, 2,5-diformylfuran (a selective oxidation product of hydroxymethylfurfural, achieving 82.59% selectivity), and dihydroxyacetone (a selective oxidation product of glycerol, with 74.88% selectivity). This result was achieved even when using diverse reactants such as 5-hydroxymethylfurfural and glycerol. These results highlight the system’s prominent adaptability in coupling the selective oxidation of organic substrates with the photo-production of H2O2 (Supplementary Fig. 21 and Supplementary Table 3). After five cycles of reuse, CN-I/I3 retained its photocatalytic activity with minimal change (Fig. 1m). In the fifth cycle, H2O2 yield remained above 50 mmol g−1 h−1, and BA yield reached 0.77 mol g−1 h−1, indicating its excellent durability in both H2O2 generation and selective BzOH oxidation. In various complex environments, including different pH values and the presence of assorted ionic species, CN-I/I3 consistently achieved high H2O2 yields (Supplementary Fig. 22). It also maintained significant activity in continuous H2O2 production without alterations in its structural and chemical properties, as evidenced by Scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR) analyses after repeated reaction cycles (Supplementary Figs. 23–25), further confirming its stability and resistance to interference.Notably, cyclic utilization of BzOH increased the conversion rate from 17.18% to 95.60% after five reactions, accompanied by significantly enhanced yields of H2O2 (91.32 mmol g−1) and BA (1.72 mol g−1) (Fig. 1n), suggesting a higher potential for practical application. The effectiveness of the catalyst was further validated by substantial H2O2 production in real-world samples, including Yangtze River water (9.16 mM) and Yellow Sea seawater (7.13 mM) (Fig. 1o), highlighting its applicability.Physicochemical characterizations of target catalystsTo verify the successful synthesis of the required catalyst, a comprehensive chemical structure analysis of the prepared catalysts was performed using XPS, X-ray absorption near-edge structure spectroscopy (XANES), FT-IR, and XRD (Fig. 2e–k; Supplementary Figs. 26–36, and Supplementary Tables 4–7). Deconvolution of high-resolution XPS spectra for C 1 s and N 1 s revealed characteristic peaks at ~284.8, 286.0, 288.0, 398.0, 399.0, and 400.0 eV, corresponding to C=C, C-NHx/C, N-C=N, C-N=C, N-C3, and C-N-H functionalities within the CN framework30 (Supplementary Figs. 28, 29, 32,33). These XPS findings, along with their fitting analyses, suggest that the co-polymerization with KI or KI/I2 successfully integrated K and I elements into the CN framework without altering its inherent chemical structure. The K 2p and I 3d3/2 spectra of CN-I and CN-I/I3 showed subtle shifts near 293.0/295.0 and 630.0 eV, respectively, compared to standard KI, affirming the incorporation of K and I into the CN lattice rather than existing as mere KI aggregates. Notably, in the I 3d3/2 spectrum of CN-I/I3, a new peak indicative of I3- emerged, contrasting with the CN/KI spectrum. This observation suggested that the addition of I2 resulted in the formation of I3- (KI + I2 → KI3), potentially acting as a redox mediator alongside I-, thereby enhancing the transport of photo-generated charge carriers during the photocatalytic process.The electronic environments of C and N in CN, CN-I, and CN-I/I3 were then examined using XANES (Fig. 2j, k). The C-K-edge XANES spectra of these samples exhibited characteristic resonances around 290.0 eV, corresponding to π*(N-C=N) transitions. In CN-I and CN-I/I3, these π*(N-C=N) transition peaks were negatively shifted by approximately 0.08 eV compared to pristine CN, a shift attributable to the increased electron density around carbon atoms following co-polymerization with KI and KI/KI3. Additionally, a distinct shoulder peak at 289.5 eV in CN-I and CN-I/I3, absent in CN, was observed, indicative of enhanced in-plane and interlayer interactions in these modified samples. The N-K-edge XANES spectra revealed similar electron transition peaks among all samples, including π*(C-N=C), π*(N-C3), π*(C-NHx), and π*(N-C=N). The only notable difference was the upward shift of the π*(C-N=C) transition peak in CN-I/I3 by about 0.07 eV compared to CN and CN-I, suggesting that I3- incorporation increased π* electron density, thereby providing additional electrons for reduction reactions.The XRD analyses of CN revealed characteristic diffraction peaks at 12.9° (100) and 27.7° (002), corresponding to the in-plane stacking and interfacial packing of heptazine units, respectively31. The KI or KI/I2 co-polymerization altered the crystal structure, leading to the disappearance or weakening of certain lattice plane diffraction peaks. The persistence of the (002) peak indicated the retention of heptazine units, while the absence of the (100) peak and a slight shift of the (002) peak towards higher angles suggested reduced interlayer spacing and a denser structure, generally indicative of enhanced crystallinity32,33.Compared to CN and CN-I, CN-I/I3 exhibited smaller average pore sizes and increased thickness, confirming the denseness of their structures (Supplementary Fig. 39). As anticipated, high-resolution transmission electron microscopy (HR-TEM) images of CN-I/I3 showed lattice stripes corresponding to the (002) plane, with the quantity and quality of these stripes increasing with the amount of I2 co-polymerized (Fig. 2o and Supplementary Fig. 40). In contrast, the HR-TEM results of CN and CN-I lacked lattice fringes (Fig. 2m, n), suggesting that the mixed salt (KI/KI3) in KI/I2 might enhance crystallinity by facilitating in-plane ‘sewing’ and interlayer ‘cutting’ of CN34. The slight transition of CN-I/I3 to a more crystalline PHI phase further substantiated this perspective35,36,37 (Supplementary Fig. 38).A functional relationship was established between the full width at half maximum (FWHM) value of the (002) plane, and the photocatalytic H2O2 production rate (Fig. 2p). A significant negative correlation between the FWHM value and H2O2 production underscored the critical role of enhanced crystallinity in improving the photocatalytic activity of CN-I/I3. This enhancement was further attributed to the potential formation of I-/I3- redox mediators induced by KI/I2 co-polymerization and the associated increase in unpaired electron density in CN-I/I3 compared to CN (Fig. 2l), facilitating more efficient separation of photo-generated charge carriers.Therefore, the qualitative characterization results affirmatively suggest the improved crystallinity of CN-I/I3 and the potential formation of the I-/I3- redox mediators. Furthermore, the observed configurations aligned with DFT predictions corroborated the precise synthesis of the catalyst.Electronic structure and carrier separation propertiesOptical properties are pivotal in assessing the catalytic performance of photocatalysts. To this end, UV–visible diffuse reflectance spectroscopy (UV–Vis DRS), Tauc plot analysis, and Mott-Schottky (MS) assessment were utilized to investigate the effects of KI or KI/I2 co-polymerization on the electronic and optical attributes of the photocatalyst (Fig. 3a and Supplementary Figs. 41–43). Compared to CN, both CN-I and CN-I/I3 displayed a slight red shift in optical absorption spectra and a reduction in band gap width. This shift indicates that co-polymerization with KI or KI/I2 enhances the light-harvesting efficiency of the photocatalyst. Subsequent band edge position analyses revealed that this co-polymerization effectively modulated the conduction band (CB) position. The addition of I2 to CN-I/I3 resulted in a downward shift of its CB position, likely linked to increased crystallinity38. The modified CB positions (≤−0.73 eV) were strategically aligned above the reduction potentials of both single-electron (O2 + e- →·O2- + e- + 2H+ → H2O2, ENHE = −0.33 eV) and double-electron pathways (O2 + 2H+ + 2e- → H2O2, ENHE = 0.68 eV) in the ORR process. This indicates that the co-polymerization of KI or KI/I2, while facilitating the necessary reduction potential for efficient H2O2 generation, simultaneously maximized the valence band (VB) position, enhancing the oxidative capacity for selective oxidation of organic substrates. Also, by augmenting the H+ supply, the H2O2 production could be simultaneously improved.Fig. 3: Investigation of optical properties and carrier transfer kinetics.a, b, c UV–Vis DRS spectra, Inset: The corresponding band structure diagram (a), PL spectra, Inset: TR-PL spectra (b), and EIS spectra, Inset: photocurrent response (c) of CN, CN-I, and CN-I/I3. d, e, f, g, h, i Three-dimensional contour plots of fs-TAS (d, g), transient absorption kinetics at 375 nm within 1400 ps (e, h), and transient absorption intensity decay curves (f, i) within the 500–550 nm range for CN-I and CN-I/I3.Beyond band structure modifications, the co-polymerization of KI or KI/I2 was anticipated to influence the separation of photo-generated carriers. CN exhibited a broad and intense photoluminescence (PL) emission peak centered around 450 nm (Fig. 3b). Conversely, CN-I and CN-I/I3 markedly reduced PL intensities, with CN-I/I3 exhibiting almost negligible emission. Time-resolved PL spectra were employed to analyze the charge separation process and the average lifetimes of photo-generated charge carriers. The average lifetimes for CN, CN-I, and CN-I/I3 were 3.84 ns, 0.95 ns, and 0.9 ns, respectively. The substantial decrease in PL emission peaks and shorter carrier lifetimes in CN-I and CN-I/I3 suggest accelerated separation of electron-hole pairs. CN-I/I3, in particular, exhibited the fastest carrier separation rate, likely due to its significantly enhanced crystallinity and the formation of I-/I3- redox mediators. Additionally, CN-I/I3 exhibited the highest photocurrent response, the greatest open-circuit potential difference, the smallest electrochemical impedance spectroscopy (EIS) arc radius, and the most rapid electron accumulation (Fig. 3c and Supplementary Figs. 44–47). These findings collectively demonstrate its stronger efficiency in transferring photo-generated charge carriers compared to both CN and CN-I.To delineate the decay kinetics of photo-generated charge carriers, femtosecond transient absorption spectroscopy (fs-TAS) was utilized to probe the absorption characteristics of intermediate species. This analysis was aimed at shedding light on the impacts of augmented crystallinity and the introduction of I-/I3- redox mediators on the oxidation and reduction reactions occurring on the photocatalyst’s surface. Upon 375 nm excitation, the 3D contour plots and absorption profiles of CN and CN-I/I3 were delineated in Fig. 3d, g. The negative absorption features observed in CN are indicative of ground-state bleaching or stimulated emission (SE), while CN-I/I3 displayed a pronounced positive absorption feature in the excited-state absorption (ESA) spectrum, implying effective retention of photo-generated electrons, holes, or electron-hole pairs39,40. Representative dynamic decay traces captured by fs-TAS are presented in Fig. 3f, i, and the kinetic curves for CN and CN-I/I3 in the 500–550 nm wavelength range were accurately modeled using bi-exponential functions. The derived time constants, τ1 and τ2 are representative of photo-generated electron-hole pair recombination and shallow electron trapping processes, respectively41,42. Compared to CN (τ1 = 4.53 ps, τ2 = 72.22 ps), CN-I/I3 (τ1 = 7.56 ps, τ2 = 198.82 ps) exhibited prolonged durations for both photo-generated electron-hole recombination and shallow electron trapping.The observed retardation in the decay of photo-generated carriers and the amplified shallow electron trapping in CN-I/I3 suggest an increased likelihood and abundance of active electron transfers during photocatalysis. This facilitates the persistence of a substantial number of active electrons and holes, thereby enhancing the efficiency of H2O2 generation and the selective conversion of organic substrates.To corroborate the separation process of photo-generated carriers on the catalyst surface, Kelvin probe force microscopy (KPFM) was implemented to visualize the surface potential with a nanoscale spatial resolution (Fig. 4). The contact potential difference (CPD) was assessed based on the potential differential between the Kelvin probe tip and the sample. Under illumination, the surface potentials of CN, CN-I, and CN-I/I3 shifted negatively, a phenomenon attributed to interlayer charging owing to the segregation of photo-generated charge carriers43. The CPDs of CN, CN-I, and CN-I/I3 progressively increased to 48.98 mV, 95.21 mV, and 107.42 mV, respectively, under light exposure (Fig. 4d–f). The most pronounced ∆CPD signal in CN-I/I3 indicates an accelerated separation of photo-generated electron-hole pairs, facilitated by the newly formed I-/I3- redox mediators and improved crystallinity.Fig. 4: In-depth evaluation of charge transfer potential.a, b, c, d, e, f Surface morphology, height difference distribution, and surface potential difference distribution under illumination for CN (a, d), CN-I (b, e), and CN-I/I3 (c, f).Overall, under the combined influences of an internally generated electric field due to heightened crystallinity and the I-/I3- redox mediators, CN-I/I3 exhibited an enhanced capability for light absorption and prolonged participation of carriers in the reaction process, compared to CN and CN-I. These empirical observations are congruent with the increased yields of H2O2 and BA demonstrated by CN-I/I3.Synergistic photocatalytic reduction and oxidation reactionsTo elucidate the fundamental mechanism of H2O2 production, a series of experiments were conducted under varying atmospheric conditions and through active species trapping (Fig. 5a). The marked contrast in H2O2 yields under N2 and O2 atmospheres irrefutably confirmed the essential role of O2 as a reactant in H2O2 synthesis. The complete inhibition of H2O2 photo-production under K2Cr2O7 shielding, coupled with a progressive decline in free charge intensity in the presence of TEMPO as an electron scavenger, decisively indicated the crucial involvement of photo-generated electrons in H2O2 formation (Fig. 5c). Notably, the negligible H2O2 yield in the presence of p-benzoquinone (pBQ) underscored the pivotal role of superoxide radicals (•O2−) as intermediates in the production process. Electron paramagnetic resonance (EPR) experiments using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap and nitroblue tetrazolium (NBT) as a detection probe further corroborated the presence of •O2− (Fig. 5b and Supplementary Figs. 48–50). Compared to CN, the more pronounced •O2− EPR signal and greater NBT reduction observed with CN-I/I3, suggested a more efficient H2O2 generation in CN-I/I3. These findings robustly support the hypothesis that a two-step single-electron reduction pathway of O2 (O2 → •O2− → H2O2) was the predominant mechanism for H2O2 generation.Fig. 5: Synergistic mechanism for photocatalytic reduction and oxidation reactions.a Photocatalytic H2O2 production using CN-I/I3 with different reaction gases, electron capture agent (K2Cr2O7), and •O2- scavenger (p-BQ). b EPR signals of superoxide radicals for CN and CN-I/I3. c EPR signals of free charges for CN-I/I3. d, e, f In situ FT-IR spectra of CN-I/I3 under different reaction conditions and corresponding peak intensity changes with the light irradiation time. g Quasi-in situ cyclic voltammetry curves of CN and CN-I/I3. h, i Polarization curves (h), corresponding H2O2 selectivity, and average electron transfer number (i) for CN, CN-I, and CN-I/I3.In situ diffuse reflectance infrared Fourier-transform spectroscopy (in situ DRIFTS) was utilized to further delineate the mechanistic details of H2O2 production and BzOH oxidation under various testing conditions (Supplementary Fig. 51). The in situ FT-IR spectra of CN-I/I3 in both dry and BzOH aqueous solutions, as shown in Fig. 5d, e, revealed distinctive vibrational bands in the ranges of 908–1156 cm−1, 1300–1400 cm−1, 1600–1700 cm−1, and 3000–3300 cm−1. These bands were attributed to adsorbed O2, *OOH, *H2O2, and -OH groups (originating from the BzOH solution) on CN-I/I3, respectively44,45. In a dry environment, the increasing intensity of adsorbed O2 peaks over time indicated effective O2 adsorption by CN-I/I3. Conversely, in the BzOH aqueous solution, a decrease in these characteristic peaks with time was observed (Fig. 5f), suggesting that in the presence of a proton source, CN-I/I3 efficiently activates adsorbed O2 for H2O2 production. The rise in vibrational signals corresponding to *OOH and *H2O2 with prolonged exposure in the BzOH solution provided direct evidence of H2O2 formation. Additionally, the diminishing vibrational signal of hydroxyl (-OH) groups from BzOH over time confirmed the oxidative dehydrogenation of BzOH. The observed increase in the *H2O2 signal and decrease in the -OH signal indicated the concurrent occurrence of photocatalytic reduction (ORR) and oxidation (BzOH oxidation) processes. These concurrent reactions mutually facilitated each other by synchronously consuming photo-generated electrons and holes, efficiently utilizing photo-generated charge carriers.In quasi-in situ cyclic voltammetry (CV) experiments, the electrochemical active surface area (ECSA) of CN-I/I3 demonstrated a progressive increase with the reaction time, in contrast to pristine CN. Such an increase, coupled with the emergence of distinct oxidation (I- → I3-) and reduction (I3- → I-) peaks, provided solid evidence for the formation of an I-/I3- redox mediator within the catalyst structure46 (Fig. 5g and Supplementary Fig. 54). The parallel trends in ECSA enhancement and oxidation-reduction peak profiles observed in CN-I and CN-I/I3 during the reaction indicated their mutual engagement in converting the I-/I3- redox mediator. However, CN-I/I3 displayed a notably larger ECSA both pre- and post-reaction compared to CN-I and exhibited more pronounced changes in the oxidation-reduction peaks at equivalent reaction durations. This large disparity suggests that the incorporation of I2 effectively pre-implanted the I-/I3- redox mediator within the catalyst. The pre-formation of the redox mediator facilitated more rapid oxidation-reduction cycling in the photocatalytic process, accelerating the transfer of photo-generated charge carriers and enhancing overall photocatalytic activity.The rotating ring-disk electrode (RRDE) technique was further employed to evaluate the selectivity of O2 reduction and determine the number of electron transfers (N) during the ORR, based on the recorded reduction and oxidation currents (Fig. 5h, i and Supplementary Fig. 55). The estimated N values for CN, CN-I, and CN-I/I3 were 2.77, 2.49, and 2.06, respectively. Notably, the electron transfer characteristics of CN-I/I3 closely aligned with a 2e- O2 reduction process, favoring the selective production of H2O2 with a high selectivity of 96.62%47. This result suggests the enhanced suitability of CN-I/I3 for targeted H2O2 generation, positioning it as a potent candidate for selective photocatalytic applications.Thorough dissection of dual-channel high-value conversion mechanismsThe pivotal contribution of KI or KI/KI3 co-crystallization to the enhancement of photocatalytic activity was further elucidated through DFT calculations. Electronic density differences for CN-I and CN-I/I3 are depicted in Fig. 6a, b and Supplementary Fig. 56, with regions of electron increase and decrease represented by green and yellow, respectively. In CN-I, charge transfer predominantly occurred from I- to the CN framework. In contrast, CN-I/I3 exhibited more extensive charge redistribution due to concurrent charge transfer from the CN framework to both I- and I3-. These multiple charge transfer pathways augmented the electron exchange efficiency of CN-I/I3 and substantiated the existence of the I-/I3- redox shuttle mediator. The electron density distributions of the lowest unoccupied molecular orbitals (LUMO) and highest occupied molecular orbitals (HOMO), along with LUMO+1 and HOMO-1, for CN, CN-I, and CN-I/I3, provided detailed insights into their charge transfer characteristics (Supplementary Fig. 57). In CN, HOMOs were predominantly located on N atoms, while LUMOs were concentrated on C atoms. This uniform distribution led to efficient charge carrier recombination in CN48. The incorporation of I- facilitated aggregation of HOMOs, creating an uneven spatial distribution of HOMOs and LUMOs. The insertion of I3- intensified these spatial charge distribution disparities, effectively reducing e–h+ pairs recombination and enhancing charge separation, thus favoring photocatalytic reactions.Fig. 6: DFT assists in revealing the intrinsic mechanism.a, b, c Electron density difference diagrams of CN-I (a), CN-I/I3 (b), and CN-I/I3-BzOH (c) with an isosurface value of 0.009 e/Bohr3 (Regions of electron increase and decrease represented by green and yellow, respectively). d, f Energy diagrams of H2O2 evolution reaction on CN, CN-I, and CN-I/I3. e, g Energy diagrams of BzOH oxidation reaction on CN, CN-I, and CN-I/I3. h Adsorption structures of *O2, *OOH, *H2O2, *C7H8O, *C7H7O, *C7H6O on CN-I/I3. i Schematic diagrams of the enhanced oxidation and reduction conversions.Moreover, the Gibbs free energy changes in the ORR and BzOH oxidation processes intricately demonstrated the efficacy of KI/KI3 co-polymerization (Fig. 6d–g). The intercalation of I- or I-/I3- led to uneven charge redistribution, enhancing O2 adsorption via van der Waals interactions between the material’s fixed dipole and the induced dipole of O2 molecules. Specifically, the free energies for O2 adsorption on CN, CN-I, and CN-I/I3 were calculated to be +0.074 eV, +0.044 eV, and +0.03 eV, respectively, with corresponding O-O bond lengths post-adsorption of 1.24 Å, 1.26 Å, and 1.27 Å (Fig. 6h and Supplementary Fig. 58). Lower adsorption energies and longer O-O bond lengths implied the increased affinity for O2 adsorption and activation on CN-I/I3. Due to the synergistic effect of I- and I3-, the ORR pathway on CN-I/I3 exhibited a more favorable energy profile compared to CN and CN-I, facilitating the formation of the critical intermediate *OOH and its subsequent hydrogenation to H2O2. The concurrent selective oxidation of BzOH to BA was also analyzed. Following BzOH adsorption, the α-H atom was first removed, then the -O-H atom, leading to benzaldehyde formation. The thermodynamically rate-determining step involved the removal of the α-H atom. Notably, CN-I/I3 displayed a more pronounced uneven charge distribution, resulting in a longer C-H bond length in adsorbed BzOH (1.104 Å) compared to CN (1.099 Å) and CN-I (1.101 Å). This disparity significantly lowered the energy barrier for the rate-determining step (*C7H8O → *C7H7O). Furthermore, CN-I/I3-BzOH exhibited stronger electron transfer tendencies from BzOH to the material compared to CN-BzOH and CN-I-BzOH (Fig. 6c and Supplementary Fig. 59), indicating more pronounced charge accumulation at the adsorption site of benzyl alcohol, which facilitated charge transfer to the material, consuming more holes during the oxidation process. In summary, due to the incorporation of I-/I3- and enhanced crystallinity, both the ORR and BzOH oxidation pathways on CN-I/I3 were energetically more favorable than on CN and CN-I, significantly promoting the synergistic production of H2O2 and BA.Based on these findings, a likely mechanism for the dual-channel photosynthesis using CN-I/I3- was proposed (Fig. 6i). The I-/I3- redox shuttle mediator played a crucial role in speeding up charge separation and ensuring a continuous carriers supply. This mechanism allowed the photo-generated electrons to quickly interact with adsorbed O2 through a two-step, single-electron pathway, converting O2 into •O2− and subsequently into H2O2. Concurrently, the holes participated in the selective oxidation of BzOH, effectively producing BA and yielding excess protons for ORR.Does the substantial accumulation of H2O2 have a practical application potential?Leveraging its enhanced crystalline structure and the synergistic interaction of I-/I3- redox mediators, the CN-I/I3 exhibited exceptional photocatalytic performance, with its H2O2 yield largely surpassing those of other recently reported photocatalysts under analogous conditions (Fig. 7a, Supplementary Fig. 60 and Supplementary Tables 8–9). The photocatalytic efficacy of CN-I/I3 was further evaluated under natural sunlight. In various environments, including deionized water (H2O2, 158.48 mM, 1.58 mmol; BA, 2.55 mmol), authentic Yangtze River water (H2O2, 81.25 mM, 0.81 mmol; BA, 2.80 mmol), and Yellow Sea seawater (H2O2, 71.01 mM, 0.71 mmol; BA, 3.31 mmol), CN-I/I3 efficiently generated H2O2 and selectively produced BA over 8 h of natural sunlight exposure (Fig. 7b and Supplementary Fig. 61a). In an expanded reactor setup with a 100 mL working volume, CN-I/I3 consistently achieved a high H2O2 yield (33.16 mM).Fig. 7: Multi-dimensional and intensive application assessment.a Comparison with other recently reported photocatalysts for the production of H2O2. b CN-I/I3 under natural light irradiation: H2O2 production, BzOH conversion, and benzaldehyde selectivity in different solutions. c H2O2 production by continuous immobilization of CN-I/I3 under natural light, Inset: Continuous production of H2O2 physical diagrams. d Removal of ATZ in H2O2/O3 combined system. e Removal of TC-Cr(VI) by CN-I/I3 under intermittent light irradiation. f Efficiency of CN-I/I3 sample in activating PI for SMX removal under natural light, and OD600 values and bacterial counts of E. coli/cells in solution at different time intervals. g Comparison of disinfection effects using different samples on E. coli/cells and corresponding bacterial counts of E. coli. The error bars represent the standard deviation of three replicate tests. h, i, j, k Scenario diagrams for the applications of CN-I/I3 system.For practical applications, the used CN-I/I3 could be readily recovered through techniques such as centrifugation and filtration. However, implementing a continuous flow system with CN-I/I3 immobilized on a membrane (containing 5 mg CN-I/I3) simplifies this process. This system demonstrated a high H2O2 yield of 1.82 mmol under 8 h of natural sunlight at a flow rate of 0.8 mL min−1, underscoring the potential for solar panel-level H2O2 photosynthesis using CN-based photocatalysts (Fig. 7c and Supplementary Fig. 61d).To better align with practical industrial development, we developed an integrated process system for the photocatalytic production, concentration, and separation of products (Supplementary Fig. 63). By loading the photocatalyst onto hydrophobic melamine foam, we further eliminated the need for solid-liquid separation while establishing a gas-liquid-solid catalytic interface to enhance mass transfer rates, further promoting the co-production of BA and H2O2. In a natural light photocatalytic production trial lasting up to 21 h (12 h on the first day, 9 h on the second), the conversion efficiency of BzOH reached 94.99%, with a BA yield of 6.29 mmol. The produced H2O2 and BA mixture could be easily concentrated through low-temperature (50 °C) evaporation. In the first concentration process, the H2O2 concentration increased from 47.28 mM to 97.39 mM, and in the second concentration process, it increased from 254 mM to 496.67 mM, achieving a yield of 10.93 mmol. Due to the difference in phase properties, the concentrated mixture solution was separated using a simply hydrophobically modified stainless steel mesh. The higher-density BA flowed out from beneath the hydrophobic membrane, while the H2O2 aqueous solution remained above it (Supplementary Fig. 62). The successful implementation of this integrated process system provided a promising feasibility for the practical application of photocatalytic simultaneous production of two high-value chemicals.Also, CN-I/I3’s dual enhancement mechanism was highly effective in water purification, activating periodate (PI) under natural sunlight. The system achieved complete decolorization of 100 mg/L rhodamine B (RhB) within 14 min (Supplementary Fig. 65) and complete degradation of 20 mg/L sulfamethoxazole (SMX) in 6 min (Fig. 7f). The treated water exhibited a higher density of E. coli survival, healthier wheat growth, and normal development of zebrafish embryos, accompanied by a reduction in the toxicity of the intermediates. These results indicate that the CN-I/I3-PI system possesses strong purification and detoxification capabilities for organics-contaminated water after activation under natural sunlight (Supplementary Figs. 66–70).The efficiently accumulated H2O2 in CN-I/I3 could be effectively utilized for in situ applications. The H2O2 generated by CN-I/I3 was harnessed for E. coli disinfection, exhibiting outstanding antibacterial efficacy. Specifically, CN-I/I3 achieved an impressive 99.38 ± 0.04% inactivation rate of E. coli within 15 min of exposure to visible light irradiation (Fig. 7g), demonstrating significant potential for practical applications in medical and health-related fields. Additionally, in the H2O2/O3 system, the combined treatment process significantly outperformed the individual processes in degrading atrazine, illustrating a prominent synergistic effect (Fig. 7d).Considering the well-documented memory photocatalytic effect associated with CN-based materials, the post-irradiation catalytic activity of CN-I/I3 was assessed for the removal of tetracycline and hexavalent chromium (TC-Cr(VI)) (Fig. 7e and Supplementary Fig. 71a). After 40-min continuous illumination, TC-Cr(VI) was completely removed, a process attributed to the oxidative breakdown of TC-Cr(VI) bonds and the oxidation of TC by photo-generated holes. The reduction of Cr(VI) was presumably driven by photo-generated electrons. Interestingly, under intermittent light irradiation, nearly equivalent TC-Cr(VI) removal to that achieved with continuous 40-min exposure was accomplished in 20 min. The substantial removal of both TC and Cr(VI) during dark periods suggests the release of electrons even in the absence of light, underscoring the material’s effective utilization of stored energy.Therefore, the well-designed CN-I/I3, under both natural and visible light irradiation, has demonstrated prominent potential in simulated environmental and antibacterial scenarios. These findings position CN-I/I3 as a promising and yet relatively unexplored photocatalyst with broad application prospects.
