Unraveling Charge-separation and oxidation mechanisms in Fluorenone-COF/CdS S-Scheme Heterojunction for Photocatalytic benzaldehyde and H 2 Generation

Unraveling Charge-separation and oxidation mechanisms in Fluorenone-COF/CdS S-Scheme Heterojunction for Photocatalytic benzaldehyde and H 2 Generation | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Energy & Environmental Materials This is a preprint and has not been peer reviewed. Data may be preliminary. 17 July 2025 V1 Latest version Share on Unraveling Charge-separation and oxidation mechanisms in Fluorenone-COF/CdS S-Scheme Heterojunction for Photocatalytic benzaldehyde and H 2 Generation Authors : Boning Feng , Bin Qi , Song Wang , Peng Zhang , Rongchen Shen , Youji Li , and Xin Li 0000-0002-4842-5054 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175278079.99476226/v1 343 views 186 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Photocatalytic hydrogen evolution coupled with benzyl alcohol oxidation to val-ue-added chemicals offer an efficient route enabled by CdS. However, the charge recombination and photocorrosion of CdS towards the low productivity of H 2 and benzalde-hyde. Herein, a novel fluorenone-based covalent organic framework (COF) is coupling with a CdS S-scheme heterojunction. The suitable band level and active center of fluorenone moiety endows the fluorenone-based COFs with strong oxidative capability towards benzyl alcohol and a low reaction energy barrier. Furthermore, the built-in electric field created by the constructed S-scheme heterojunction effectively enhances the separation and migration of photogenerated charge carriers while suppressing carrier recombination within each sem-iconductor, thereby reducing the corrosive effect of photogenerated holes on CdS. Conse-quently, the heterojunction significantly improved the productivity of benzaldehyde and hy-drogen production activity. In the presence of Pt as a co-catalyst, the production rates of H 2 and benzaldehyde reached 23.38 and 17.36 mmol g -1 h -1 , respectively. This work not only addresses the challenges associated with the utilization of electron holes but also paves an effective green and low-carbon pathway to overcome the issues of low efficiency and high costs in photocatalytic hydrogen production. 1. Introduction Hydrogen energy, due to its zero carbon emissions and high efficiency in energy conversion, is widely regarded as an ideal energy carrier for addressing the energy crisis and environmental challenges [1-3] . Among various methods, solar-driven photocatalytic water splitting for hydrogen production has attracted significant attention from both academia and industry because it directly utilizes renewable resources [4-6] . In addition, by coupling photocatalytic water splitting for hydrogen production with selective organic oxidation reactions driven by semiconductor-based catalysts, it is possible to substitute the energy-intensive oxygen evolution reaction with the oxidation of organic molecules, thus enabling the efficient utilization of photogenerated holes and reducing the reaction energy barrier [7,8] . This approach is a highly promising functional strategy. Notably, this strategy has demonstrated significant effectiveness in the field of fine chemical synthesis; for instance, the selective oxidation of benzyl alcohol (BA), which is the simplest aromatic alcohol model molecule, into benzaldehyde (BAD). BAD serves as a key intermediate in the synthesis of pharmaceuticals, fragrances, and pesticides [9-11] . However, this technology faces critical bottlenecks. Thermodynamically, as the oxygen evolution half-reaction in photocatalytic water splitting involves a four-electron transfer process, leading to excessively high activation energy, slow reaction kinetics, and low quantum efficiency [12,13] . Kinetically, the separation and transfer of photogenerated carriers (electrons and holes) remain inefficient [14] . Moreover, although sulfides exhibit high photocatalytic activity, irreversible sulfur ion photooxidation induced by excited holes result to a gradual decrease in catalytic activity until deactivation, which seriously hampers their practical application [15] . Taking the typical semiconductors such as CdS as an example, while it is widely used in photocatalytic hydrogen production and the oxidation of BA to BAD [16-20] , it faces inherent issues: (1) a rapid recombination rate of photogenerated charge carriers; (2) unavoidable photocorrosion phenomena; and (3) a high activation energy barrier for the activation of the αC─H bond in the BA molecule, coupled with insufficient photon utilization. These deficiencies collectively hinder the enhancement of reaction efficiency—non-productive recombination of charge carriers inhibits surface redox reactions, while the high activation energy barrier further limits the kinetics of BA conversion. To overcome these bottlenecks, the construction of heterojunctions has been demonstrated as an effective strategy to optimize energy barriers and accelerate charge carrier separation (e.g., systems such as CdS/MoS 2 [21] , CdS/PFC-8 [22] , CdS /Co 9 S 8 [23] and CdS/U6N [24] ). This approach extends the lifetime of charge carriers through interface engineering, theoretically enhancing both hydrogen production activity and BA productivity simultaneously [25] . However, existing heterojunction systems still exhibit notable limitations: (1) the strong oxidizing nature of the oxidation-end semiconductor can lead to the over-oxidation of BA, thereby reducing the productivity towards BAD; and (2) the activation energy barrier for the αC─H bond remains fundamentally high, resulting in limited improvements in productivity [26] ; (3) the difficulty in regulating the structure of inorganic semiconductors leads to the inability to better improve the yield of benzaldehyde. Consequently, the design of active sites in oxidation semiconductors and precise tuning of their band structures, aiming to suppress charge carrier recombination while selectively reducing the activation energy barrier for C─H bond activation [27] . This approach could achieve a synergistic optimization of hydrogen production efficiency and BAD productivity. Covalent organic frameworks (COFs) are porous materials composed of light elements such as carbon, hydrogen, nitrogen, and oxygen, characterized by their uniform pore size, tunable pore dimensions, and customizable functionalities [28] . COFs allow for precise modulation of the band structure by selecting specific building units. In COF-based photocatalysts for the photocatalytic production of hydrogen peroxide, benzyl alcohol is frequently utilized as a hole sacrificial agent to enhance photocatalytic activity. By tailoring the bandgap and energy level positions to match the oxidation of benzyl alcohol BA, the confined pore structures of COFs can regulate the residence time of reaction intermediates, thereby preventing the over-oxidation of BA to BAD [29] . Therefore, COFs have potential as oxidation-end semiconductors in the oxidation of BA. However, to date, there have been no reports on COF-based heterojunctions for photocatalytic hydrogen production coupled with the conversion of BA. In this study, we synthesized a 1D-2D S-scheme heterojunction by integrating 1D CdS nanorods with a novel fluorenone-based 2D COFs. The CdS nanorods were synthesized via a modified solvothermal method in our previous works. The TBADF was synthesized by solvothermal copolymerization method of 2,7-Dinitro-9-Fluorenone (DF) and 2,4,6-tris(4-benzaldehyde)-1,3,5-triazine (TBA). Subsequently, we mixed the CdS with the 5% of TBADF in EtOH to form a S-scheme heterojunction. As we expected, the CdS-TBADF has exhibited superior photocatalytic hydrogen evolution performance compared to pure CdS nanorods. In the presence of Pt as a co-catalyst, the production rates of H₂ and benzaldehyde reached 23.38 and 17.36 mmol g -1 h -1 , respectively. Mechanistic study indicates that the formation S-scheme not only facilitate photocarrier separation but also prevented photocorrosion of CdS. This work provides a new strategy for achieving efficient hydrogen production while providing a green synthesis pathway for the synthesis of industrial chemicals. 2 Results and discussion 2.1. Structure and morphology characterization Figure 1. (a) Schematic representation of CdS-TBADF S-scheme heterojunction. (b) XRD patterns of CdS and CdS-TBADF. (c) XRD pattern of TBADF. A comparison between the experimental (blue curves) and Pawley refined (red curve) profiles, the Bragg positions (green bar), and the refinement differences (black line). (d) Solid-state 13 C NMR spectra of TBADF. Figure 2. (a) FT-IR image of TBADF. XPS spectra of CdS, TBADF and CdS-TBADF: (b) Cd 3d, (c) S 2p, (d) C 1s, (e) O 1s. (f) N 2 adsorption and desorption isotherms of TBADF . SEM of (g) TBADF (h) CdS (i) CdS-TBADF. Initially, TBADF was synthesized through copolymerization of 2,7-Diamino-9-Fluorenone and 2,4,6-tris(4-benzaldehyde)-1,3,5-triazine via a Schiff base reaction. Subsequently, CdS and TBADF were sonicated to obtain a CdS-TBADF S-scheme heterojunction about simultaneous photocatalytic reaction. Figure 1a is the schematic representation of CdS-TBADF S-scheme heterojunction. The crystal structure and degree of crystallinity of the prepared samples were characterized using X-ray diffraction (XRD) patterns. As displayed in Figure 1b , CdS nanorod shows high crystallinity. The peaks located around 25.5, 26.8, 28.9, 43.8, 48.5 and 52.5° correspond to the (100), (002), (101), (102), (110), (103) and (112) planes of hexagonal CdS (PDF#89-2944), respectively. The peaks of TBADF at 6.7°, 14.4° and 25.8° correspond to the (300), (250) and (001) (Figure 1c ). No obvious peak for TBADF could be found in the XRD spectrum of CdS-TBADF due to the low content of TBADF. In the solid-state 13 C-NMR spectrum of TBADF. the peak at about 192.2 ppm was attributed to the carbon of C=O. Moreover, the peaks of 150.1 and 169.6 ppm were considered as the existence of the C=N and C-N bond, which improved the combination of TBADF through imine bond ( Figure 1d ). Fourier transform infrared (FT-IR) spectroscope showed the functional groups of TBADF in Figure 2a . It could observe the characteristic vibrations of C=N, C-N, C=O bonds. The absorption bands at 1623 cm -1 were considered as the C=N stretching bands of the imine linkage and the absorption bands at 1261 cm -1 , 1701 cm -1 and 3348 cm -1 correspond to the C-N, C=O and C=N stretching bands [30-32] . The formation of CdS-TBADF was future confirmed by X-ray photoelectron spectroscopy (XPS). The full XPS spectra of CdS, TBADF and CdS-TBADF is shown in Figure S1 and S2 . As displayed in Figure 2b , CdS exhibited the Cd 3d5/2 and 3d3/2 peaked at 404.53 eV and 411.32 eV, respectively. Meanwhile, the S 2p3/2 and 2p1/2 located at 160.94 eV and 162.12 eV, respectively ( Figure 2c ). In the N 1s and O 1s spectrum of TBADF, the peaks located at about 398.32, 398.92, 531.88 and 533.28 eV could be attributed to C-N and C=N bonds, C=O and C-O bonds, respectively ( Figure S3 and 1e ). Three obvious peaks at about 284.70, 286.60 and 288.76 eV could be found in the C 1s spectrum of TBADF, which corresponding to the C-C, C-O and C=O bonds ( Figure 1d ). N 2 adsorption/desorption measurements at 77 K were conducted to evaluate the architectural rigidity and permanent porosity of TBADF. The TBADF displayed a BET surface area of 98.82 m 2 g -1 ( Figure 2f ). Figure 3. (a) and (b) TEM and HTEM of TBADF. (c) and (d) TEM and HTEM of CdS. (e) and (f) TEM and HTEM of CdS-TBADF. (g) elemental mapping of CdS-TBADF. Through the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and HRTEM, the morphology of as prepared samples is displayed in Figure 2g, 2h, 2i and 3 . Pure TBADF displayed Two-dimensional layered structure. The nanorod structure of CdS with a length of approximately 300-600 nanometers was observed from the TEM images. In the HRTEM image of CdS-TBADF, the lattice spacing value of 0.335 nm was attributed to the (101) plane of CdS. CdS nanorods adhere on the surface of TBADF nanosheets, forming a composite structure with intimate contact [33] . Figure 3g and S4 show the elemental mappings of the CdS with COFs composite and they clearly demonstrate that the distribution of Cd, S, C, O and N elements are uniform. Figure 4. In-situ XPS of CdS-TBADF: (a) Cd 3d and S 2p, (b) C 1s and O 1s. Work Function of (c) CdS and (d) TBADF. (e) KFPM of CdS and TBADF. (f) Kubelka-Munk function vs. the energy of incident light plots. (g) Band structures of CdS and TBADF. We used Valence-band XPS and UV-visible spectrum measurements analyses in order to verify the mechanism and impact of the electronic band structures on the activity of H 2 evolution ( Figure 4f, S5 ). The determination of the conduction band potential close to the flat band potential involved the utilization of the following equation: E CB = E (Ag/AgCl) + 0.197 + 0.0591 · pH (pH=7) and according to the equation E VB = φ (Instrument work function) + E VB, XPS -4.44 + 0.0591 · pH (pH=7) [34] . The calculation of band gap for CdS and TBADF was presented in Figure 4g . The UV-vis diffuse reflectance spectra of the photocatalysts were shown in Figure 5a . Pure CdS shows narrow light-absorption edge shorter than 550 nm. In contrast, the COF exhibits a broad range of light absorption, exceeding 800nm. Meanwhile, after the formation of the S-scheme heterojunction, the light absorption range of CdS is significantly enhanced. Therefore, the bandgap of CdS and TBADF were 2.37 and 1.88 eV, respectively. The VB level of CdS and TBADF were confirmed by VB-XPS. The VB level of CdS and TBADF were 1.34 and 2.23 eV, respectively. By combining the bandgap, the CB level of CdS and TBADF were -0.66 and 0.78 eV, respectively. Through the analysis of the band structure, it is found that CdS and TBADF can thermodynamically form an S-scheme heterojunction. In addition, the VB level of TBADF exhibits strong oxidation capabilities. Figure 5. (a) PL spectra of CdS, TBADF and CdS-TBADF. (b) UV-vis diffuse reflection spectra of CdS, TBADF and CdS-TBADF. (c) ESR spectra under visible light (DMPO-·O 2- ). (d) Photocurrent response. (e) Nyquist plots of EIS. (f) Comparison of the hydrogen evolution rates of CdS, TBADF, CdS-TBADF and Pt-CdS-TBADF under AA sacrificial agent. (g) The recycled photocatalytic hydrogen evolution experiments of CdS and CdS-TBADF under AA sacrificial agent. (h) H 2 evolution coupling benzyl alcohol oxidation. (i) The photocatalytic H 2 and BAD production rate of CdS-TBADF without Pt as the cocatalyst reported photocatalysts. The formation of S-scheme heterojunction was further confirmed by the electron resonance spin (ESR) trap technique with DMPO as the spin trap reagent. CdS-TBADF S-scheme heterojunctions shows the strongest signals of ·O 2− compared to single CdS and TBADF, respectively ( Figure 5b ) [35] . Furthermore, we calculated the work function of the CdS and TBADF using Kelvin probe. As displayed in Figure 4e , the work function of the CdS and TBADF were 4765.4 and 5016.9 meV, respectively. Moreover, through the DFT simulations, the bonding at the interfaces of the S-scheme heterojunction and the mechanisms of charge transfer are greatly examined. As the work function (Φ) shows ( Figure 4c and 4d ), the Φ values for the CdS (100) surface and TBADF (001) surface are 5.27 eV and 5.84 eV, which prove the interface charge transfer TBADF to CdS. Therefore, when CdS is in close contact with TBADF, electrons in CdS will spontaneously transfer to TBADF until reaching the equilibrium Fermi level. This process establishes an internal electric field between CdS and TBADF. Under visible light irradiation, electrons in both CdS and TBADF are generated from the VB to the CB, while the holes remain in the VB. Induced by the internal electric field, the photoelectrons of TBADF will recombine with the holes in the VB of CdS. The electron transfer from TBADF to CdS was future confirmed by in-situ XPS. As presented in Figure 4a , the binding energy of Cd and S elements exhibited a negative shift under visible light irradiation comparing with the measured in the darkness, indicating a decreased electron density of CdS under light irradiation. Conversely, the binding energy of O and C element exhibited a positive shift under visible light irradiation, indicating enhanced electron density of COFs under light irradiation ( Figure 4b ). This shift of binding energy directly demonstrates the movement of electrons from COF to CdS. In comparison, the peak for Cd 3d5/2 and 3d3/2 were located at about 404.81 and 411.57 eV, respectively. Two obvious peaks could be found a 161.24 and 162.40 eV in the S 2p spectrum of CdS-TBADF, which corresponds to the S 2p3/2 and S 2p1/2. CdS-TBADF presented the C 1s peaks at about 284.74, 286.2 and 288.61 eV and O 1s peaks at about 531.73 and 533.14 eV, respectively. Unfortunately, the peaks of N 1s in the composite catalyst were covered by Cd 3d in reason of the minimal amount of TBADF ( Figure S6 ). Comparing the binding energy, in CdS-TBADF the Cd, S elements displayed a positive shift and the C, O elements displayed a negative shift, revealing that electrons are transferred from TBADF to CdS. 2.2. Photocatalytic activity and optical property The photoluminescence spectroscopy can also prove the raise of the light absorption ( Figure 5c ) [36] . The photocurrent response was used to further research the photocarriers separation and migration efficiency. As shown in Figure 5d , the CdS-TBADF displayed the strongest photocurrent density among the samples, which improved that the CdS-TBADF S-scheme heterojunction could accelerate the separation of photoexcited electrons and holes. Meanwhile, the electrochemical impedance spectra (EIS) Nyquist plots showed that CdS-TBADF had the minimal semicircle, which is benefit to the interfacial charge transfer and improve the photocatalytic property ( Figure 5e and S6 ). The photocatalytic hydrogen evolution activity of the prepared samples was tested under visible light. Firstly, we evaluated the photocatalyst by using sacrificial agents. Figure 5f illustrates the photocatalytic performance per unit mass of the prepared photocatalysts. As the irradiation time prolongs, the hydrogen evolution rate significantly increases. The pure CdS exhibits a hydrogen evolution rate of 2.45 mmol·g -1 ·h -1 . In contrast, pure TBADF shows no photocatalytic hydrogen production activity. After forming an S-scheme heterojunction, the photocatalytic hydrogen evolution activity of CdS-TBADF is notably enhanced, reaching 7.27 mmol·g -1 ·h -1 . Moreover, when Pt is loaded as a cocatalyst, the photocatalytic hydrogen evolution activity of CdS-TBADF further improves to 127.27 mmol·g -1 ·h -1 . Additionally, the formation of the heterojunction prevents the photodegradation of CdS, thereby ensuring stable photocatalytic performance. After 18 hours of cycling, a 15% loss in activity was observed which pure CdS resulted 37% loss after 6hours in comparison ( Figure 5g ). More different comparison of the HER which using sacrificial agents are shown in Figure S7 . Since using sacrificial agents is a wasting for high energy electron holes, we using benzyl alcohol instead of sacrificial agents which can make full use of high energy electron holes. Without cocatalyst, pure CdS exhibits hydrogen evolution rate of 7.96 mmol·g -1 ·h -1 and benzaldehyde production rate of 5.37 mmol·g -1 ·h -1 . On the other hand, CdS-TBADF reaches 16.81 mmol·g -1 ·h -1 hydrogen evolution rate and 12.63 mmol·g -1 ·h -1 benzaldehyde production rate. Moreover, after the Pt loading as a cocatalyst, the photocatalytic hydrogen evolution activity and benzaldehyde production of CdS-TBADF further improves to 23.38 mmol·g -1 ·h -1 and 17.36 mmol·g -1 ·h -1 ( Figure 5h ). Figure 5i shows that the CdS-TBADF has a better photocatalytic activity than many other photocatalysts [16,17,20,24,37-41] . 2.3. Mechanism of Photocatalytic Hydrogen Evolution and Biomass-derived alcohol Figure 6. Pseudocolor TA plot of (a) CdS, (d) CdS-TBADF and (g) TBADF. TA spectra of (b) CdS, (e) CdS-TBADF and (h) TBADF at indicated delay times from 1 to 1000 ps. fs-TA decay kinetics of (c) CdS and (f) CdS-TBADF at 500 nm, (i) TBADF at 715nm. In order to investigate the photogenerated carriers’ dynamics in S-scheme heterojunction, the femtosecond transient absorption (TA) technique of the samples was measured under 400 nm excitation [42] . The TA spectra of CdS and CdS-TBADF ( Figure 6b and 6e ) showed an obvious negative peak between 500 nm, which should be ascribed to ground-state bleaching (GSB), representing relaxation of the excited state that involves the change of intermolecular charge transport state to the excited state and further to the free charge carries. However, the TA spectra of CdS-TBADF ( Figure 6e ) display same signals but an enhanced GSB signal in comparison with pure CdS [43] which indicate that more electrons were transferred from TBADF to CdS. The features of the pseudocolor TA plots ( Figure 6a and 6d ) correspond to the TA spectra. The TA spectra and TA plots of TBADF was shown in Figure 6g and 6h . The TBADF exhibit excited-state absorption (ESA) signal, corresponding to electron transfer from the first excited singlet state (S1) to higher-energy excited singlet states (Sn). It indicates that the electrons can be excited and transform to the CdS. Moreover, the time profiles of samples probed at 500 nm were fitted to find out the mechanism of the electron trapping and the decay kinetics of photogenerated carries. By fitting decay profiles ( Figure 6c and 6f ), the decay traces are fitted to three-exponential model, which improved three pathways that can dominate the relaxation of photogenerated electrons. For the pure CdS and CdS-TBADF, the fast decay values (𝜏 1 ) represent the lifetime of electron diffusion. The second decay values (𝜏 2 ) represent the recombination of photogenerated electrons with the trapped holes. And the longest decay values (𝜏 3 ) represent the recombination of charge carriers. The fitting results showed that the form of the S-scheme heterojunction could contribute to the extension of interfacial carriers’ lifetime [44] and were corresponding to the TRPL ( Figure S10 ). Figure 7. (a) In situ DRIFT spectra of CdS-5%TBADF. (b) Schematic diagram of benzyl alcohol oxidation pathway. (c) Free energy profile for benzyl alcohol oxidation on CdS site and TBADF site in CdS-TBADF. (d) Schematic representation of interfacial electron transfer, E f equilibrium and migration of CdS and TBADF before contact, after contact, and under light irradiation. The in-situ DRIFT measurement was conducted to identify the oxidation pathway of the BA. Considering the spectra presented in Figure 7a , the peaks of benzyl alcohol showed a gradual decrease with the light irradiation was applied which mean a rapid benzyl alcohol consumption. And the peak of O-H bond in benzyl alcohol at bond (745 cm -1 ) and C-O bond (1023 cm -1 ) [45,46] , demonstrating the oxidation pathway of the benzyl alcohol. Following the reaction, new peaks ascribed to benzaldehyde showed and increased in intensity at 1200 cm -1 (aldehyde group of C-C bond in benzaldehyde), 1384 cm -1 (aldehyde group of C-H bond in benzaldehyde), 1681 cm -1 (aldehyde group of C=O bond in benzaldehyde) [47] . Furthermore, according to the benzyl alcohol oxidation pathway shows in Figure 7b , a detailed reaction pathway for benzyl alcohol oxidation was shown by free energy change (ΔG) calculation ( Figure 7c ). Comparing ΔG of CdS and TBADF, we turned the inorganic catalyst to organic catalyst in oxidation. In first step, the benzyl alcohol was adsorbed to the catalyst, the ΔG in TBADF site (ΔG = - 0.31 eV) is lower than CdS site (ΔG = - 0. 22eV). This calculation result means that the TBADF has a better adsorption than CdS. After adsorption, the following generation of oxidation pathways (PhCH 2 OH* + h + → PhCHO + 2H + ) requests a lower energy input in TBADF site [19] . The photocatalytic reaction mechanism of CdS-TBADF, as depicted in Figure 7d , leverages the compatibility of its band structure to successfully form an S-scheme heterojunction upon contact under irradiation. This configuration significantly enhances charge migration and separation, thereby boosting photocatalytic hydrogen evolution. 3. Conclusion In summary, we used a simple electrostatic self-assembly way for synthesizing 1D-2D S-scheme heterostructure in order to enhance photocatalytic H 2 evolution. By using XPS, ESR, fs-TA spectra, we discovered the S-scheme charge migration mechanism in detail by using DFT simulations. TBADF can reduce the interfacial charge distance and enhance light absorption effectively. Most importantly, in comparison to the pure CdS nanorods, the CdS-5%TBADF composite exhibits a significant increasement in photocatalytic H 2 evolution coupling the BA oxidation, achieving HER at 23.38 mmol·g -1 ·h -1 and benzaldehyde production rate at 17.36 mmol·g -1 ·h -1 . This work discovers an oxidative fluorenone-based COFs to form a 1D-2D S-scheme heterojunction and elucidates the criticality of dynamics of the photogenerated carries for the construction of CdS-TBADF S-scheme heterojunction. 4. Experimental Sections Preparation of the CdS nanorods : Using a modified solvothermal method to synthesis CdS nanorods [48] . 4.62 g of CdCl 2 ·2.5H 2 O and 4.62 g of CH 4 N 2 S were dissolved in 60 mL of ethylenediamine. Then, the mixture was transferred to a Teflon-lined autoclave and maintained at 160 °C for >24 h. After cooling down to room temperature, the resulting yellow solid products were collected by centrifugation, and washed with distilled water and ethanol three times. The product was then dried at 60 °C. Preparation of the 2,7-Diamino-9-Fluorenone : A stirred suspension of 2,7-Dinitro-9-Fluorenone (9.45 g, 35 mmol) in ethanol (375 mL) was added into a solution of sodium sulfide nonahydrate (37.85 g, 157.5 mmol) and sodium hydroxide (15 g, 375 mmol) in water (650 mL). The mixture was heated at reflux for 5 h and left to stand overnight. The mixture was cooled to 0–5 °C and the resulting precipitate was collected by filtration. The crude product was washed successively with water (2 ×100 mL), aqueous NaOH (2 ×100 mL, 5% w/v), water (3 ×100 mL), cold EtOH (2 ×50 mL). Then leave to stand overnight. Synthesis of the TBADF-COF : 15.75 mg (0.075 mmol) of 2,7-Diamino-9-Fluorenone (DF) and 19.65 mg (0.05 mmol) of 2,4,6-tris(4-benzaldehyde)-1,3,5-triazine (TBA) were charged into a Pyrex tube (10 × 1 cm) and mixed with 4 mL given solvent (3 mL o-DCB and 1 mL Trimethylbenzene). Then, 0.1mL 3M of Ac was added. After sonication for 5 min, the tube was flash-frozen at 77 K (liquid N 2 bath) and degassed through three freeze-pump-thaw cycles by evacuating through an oil pump and then sealed under a vacuum. The reaction proceeded at 120 °C for 3 days. Afterwards, the product was filtered and washed with tetrahydrofuran (THF) and acetone several times, respectively. The obtained red solids were dried at 80 °C in a vacuum for 8 h. Synthesis of the CdS-TBADF composite photocatalysts : Respectively, measuring 5%TBADF of CdS-TBADF composite and CdS in centrifuge tubes. After uniformly dispersing, mixing them in EtOH for 4h and letting it stand overnight. Afterwards, the CdS-TBADF composite was collected by filtration. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements The authors thank National Natural Science Foundation of China (22378148, 52472110, 22308113) and Natural Science Foundation of Guangdong Province (2024A1515012433) for their support. Graphical Abstract This graphical abstract depicts an S-scheme heterojunction formed by CdS and Fluorenone-COF. The diagram illustrates the charge transfer mechanism across the interface under light irradiation. The heterojunction is designed for the efficient photocatalytic hydrogen evolution coupling with benzyl alcohol oxidation. Highlighting the separation of photogenerated charge carriers driving the water-splitting reaction. References [1] R. Shen, N. Li, C. Qin, X. Li, P. Zhang, X. Li, J. Tang, Adv. Funct. Mater. 2023 , 33, 2301463. [2] R. Shen, X. Li, C. Qin, P. Zhang, X. Li, Adv. 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Information & Authors Information Version history V1 Version 1 17 July 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Collection Energy & Environmental Materials Keywords benzyl alcohol oxidation covalent organic frameworks (cofs) photocatalytic hydrogen evolution s-scheme heterojunction Authors Affiliations Boning Feng South China Agricultural University View all articles by this author Bin Qi South China Agricultural University View all articles by this author Song Wang Hubei University of Arts and Science View all articles by this author Peng Zhang Zhengzhou University View all articles by this author Rongchen Shen South China Agricultural University View all articles by this author Youji Li Jishou University View all articles by this author Xin Li 0000-0002-4842-5054 [email protected] South China Agricultural University View all articles by this author Metrics & Citations Metrics Article Usage 343 views 186 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Boning Feng, Bin Qi, Song Wang, et al. Unraveling Charge-separation and oxidation mechanisms in Fluorenone-COF/CdS S-Scheme Heterojunction for Photocatalytic benzaldehyde and H 2 Generation. Authorea . 17 July 2025. DOI: https://doi.org/10.22541/au.175278079.99476226/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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