Engineering Amino Acid Functionalized Chiral Carbon‐Organic Frameworks for Enhanced Photocatalytic Hydrogen Production

The spin-dependent recombination behavior of photogenerated charges has long been overlooked in the study of photocatalytic hydrogen (H2) evolution over covalent organic frameworks (COFs). Moreover, correlating the structure of COFs with the spin states of photogenerated charges to enhance photocatalytic H2 evolution performance remains a significant challenge. Herein, we present a chiral amino acid functionalization strategy to engineer chiral TpPa-1 COF for boosted photocatalytic H2 evolution. Following systematic optimization, the chiral TpPa-1 COFs showcased a ∼5-fold enhancement in photocatalytic performance, achieving a record TOF of 9867 h-1, alongside the second-highest reported AQY of 66% at 475nm and HER of 2.54mmol h-1 among the reported state-of-the-art COF-based photocatalysts for H2 evolution. Mechanism studies revealed that the synergistic effect between the chirality and the directional charge transfer allows efficient photo-generated charge separation. Furthermore, Chiral TpPa-1 assembled with polymeric carbon nitride (g-C3N4) in an S-scheme heterojunction can overcome the bottleneck in photocatalytic overall water splitting on g-C3N4 without oxygen evolution co-catalysts. In this work, we present a universal design strategy from a charge spin perspective to synthesize chiral photocatalysts for efficient photocatalytic performance.

Open AccessCCS ChemistryRESEARCH ARTICLE14 Jul 2022Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks Xiaomin Ren†, Chunzhi Li†, Wanchao Kang†, He Li, Na Ta, Sheng Ye, Linyan Hu, Xiuli Wang, Can Li and Qihua Yang Xiaomin Ren† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 University of Chinese Academy of Sciences, Beijing 100049 , Chunzhi Li† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 University of Chinese Academy of Sciences, Beijing 100049 , Wanchao Kang† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000 , He Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Na Ta State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Sheng Ye State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Linyan Hu State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Xiuli Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Can Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 University of Chinese Academy of Sciences, Beijing 100049 and Qihua Yang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 https://doi.org/10.31635/ccschem.021.202101090 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Enhancing the charge separation efficiency is highly effective strategy in improving the photocatalytic activity of covalent organic frameworks (COFs) which have the problems of low conductivity and difficult dissociation of excitons. In this work, we report the sevenfold increase in apparent quantum efficiency resulting from the use of a near-single layer COF (SLCOF) in photocatalytic hydrogen evolution compared with bulk COF. Detected by transient absorption spectroscopy characterization, 100% of photogenerated long-lived electrons in the near-SLCOF can be extracted and participate in the photocatalytic process. However, the electron extraction efficiency declined to only about 11% when the COFs were increased to eight layers, implying the difficulty of charge migration among COFs interlayers. The near-SLCOF was prepared by deposition of self-exfoliated COFs colloids on SiO2, driven by their strong affinity. This work not only sheds light on the significant influence of COF layer thickness on the charge separation efficiency but also provides a new route to prepare and stabilize COF layers for practical applications. Download figure Download PowerPoint Introduction Photocatalytic water splitting to produce hydrogen is a desirable approach to sustainably store intermittent solar energy. Two-dimensional (2D) covalent organic frameworks (COFs) have emerged as a novel type of photocatalyst due to their unique optoelectronic properties and π-conjugated skeleton which can be designed at the molecular level.1–10 Visible-light-responsive 2D COFs for example, diacetylene functionalized COFs,11 azine COF,12 sp2-carbon-linked triazine-cored COFs,13–15 thiazolo thiazole-linked COFs,16 hydrazone-based COFs,17 and sulfone-containing COFs18 have been synthesized and used for visible-light-driven H2 production. However, most COFs show mediocre activity in photocatalytic hydrogen evolution (PHE) compared with traditional inorganic semiconductors, which is mainly related to the difficulty in dissociation of excitons and the rapid recombination of photogenerated electrons and holes during the photocatalytic process.19 Several strategies have been developed to improve the charge separation efficiency of COFs, for example, incorporating donor–acceptor (D–A) moieties20,21 or halogen atoms in COFs,22,23 constructing novel π-conjugated building blocks,24 and generating junctions with other semiconductors.25 In addition to the above strategies, reducing the particle size of COFs to nanometer scale is a more facile method to improve the charge separation efficiency due to the possibility of the charge carrier recombination being reduced in the short diffusion distance.26 Therefore, the single-layer COFs (SLCOFs) offering the minimum diffusion distance for charge carriers should be the perfect candidate for photocatalysis. Although SLCOFs can be successfully prepared on single-crystal surfaces and solid–vapor–liquid interfaces,27–30 such materials are unsuitable for application in photocatalysis due to the difficulty in preventing the stacking of freestanding SLCOFs and the scale-up synthesis. Recently, partitioning the interlayer space of COFs and the acid-aided exfoliation method have been used for the synthesis of monolayer COFs, but the yields of SLCOFs by these approaches have not been high. The facile synthesis and stabilization of SLCOFs still remains a challenge.31,32 Herein, we report the preparation of a near-SLCOF by self-exfoliating of COF colloids in the presence of SiO2 nanospheres and other supports which have strong affinity for COF colloids. COFs with near-single to multiple layers were successfully deposited on SiO2 nanospheres under fine control, providing an ideal model to study the relationship between layer thickness and charge separation efficiency. It was found that almost all the photogenerated long-lived electrons in the near-SLCOF could be used for H2 production, and this value decreased sharply with the increase of COF , which elucidated the remarkable improvement in charge separation efficiency by decreasing the diffusion distance. Experimental Methods Synthesis of TP-TTA colloids The synthesis of 1,3,5-triformylphloroglucinol (TP)-4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA) COF colloids was similar to the method in the literature with a slight modification.33 In a typical process, 29.4 mg (0.14 mmol) of TP was dissolved in 0.5 mL of dimethyl sulfoxide (DMSO). The solution was added dropwise to a flask containing 58 mL of 0.05 M hexadecyl trimethyl ammonium bromide (CTAB) aqueous solution. After ultrasonication, 1.8 mL of 0.05 M sodium dodecyl sulfate (SDS) aqueous solution was added to form solution A. Separately, 49.6 mg (0.14 mmol) of TTA was dissolved in 1 mL of DMSO. The solution was added dropwise to a flask containing 58 mL of 0.05 M CTAB aqueous solution. After ultrasonication, 1.8 mL of 0.05 M SDS solution was added to form solution B. Finally, the solutions A and B were mixed, and 5.8 mL of acetic acid was added to the resultant solution. After reacting at 30 °C for 48 h, a completely transparent orange colloidal solution was formed. Synthesis of TP-TTA/SiO2-x In a typical process, the pH of the TP-TTA colloids solution was adjusted to 7 with NaOH (1 M), followed by the addition of the desired amounts of SiO2 colloid solution, which was diluted to 15 mg/mL with water. After stirring at room temperature for 24 h, the mixture was heated at 110 °C to evaporate the solvent to afford solid products. After thoroughly washing the mixture with ethanol five times and degassing at 120 °C for 12 h under vacuum, TP-TTA/SiO2-x was obtained. TP-TTA/MOx (MOx = TiO2, WO3, Nb2O5, ZrO2) and TP-TTA/CdS were synthesized in a similar procedure to TP-TTA/SiO2-8 with the exception that CdS/MOx supports were used instead of SiO2 colloid solution (for details, see Supporting Information). PHE A flask charged with 50 mg of photocatalyst and 30 mL of 0.1 M ascorbic acid water solution was degassed by three freeze–pump–thaw cycles. An aqueous solution of H2PtCl6 (3 wt % of COF content) was injected into the flask under inert gas. The reaction mixture was illuminated with a 300 W Xenon lamp (PLS-SXE300/300 UV, Perfect Light, China) with a cutoff filter of 420 nm. The temperature of the reaction solution was maintained at 25 °C. Gas samples were taken with a gas-tight syringe (Hamilton 1700) and run on an Agilent 6890 gas chromatograph with a thermal conductivity detector (TCD) referencing against standard gas with a known concentration of hydrogen. Hydrogen dissolved in the reaction mixture was not measured, and the pressure increase generated by the evolved hydrogen was neglected in the calculations. Results and Discussion The TP-TTA colloids confined in CTAB/SDS micelle were prepared according to the method in the literature33 using TP and TTA as monomers. The size of TP-TTA colloids is ∼28 nm as measured by the dynamic light scattering (DLS) method ( Supporting Information Figure S1 and for synthesis details, see Supporting Information). TP-TTA/SiO2-x (x denotes the layer number of TP-TTA) samples with different COF layers were prepared by dispersing commercial SiO2 nanospheres (particle size ∼26 nm) in TP-TTA colloid solution (Scheme 1). The layer number of TP-TTA on SiO2 was facilely controlled by varying the mass ratio of TP-TTA colloids and SiO2 nanospheres in the initial mixture. Scheme 1 | The illustration of preparation of TP-TTA/SiO2-x (x denotes the layer number of TP-TTA) by self-exfoliating of TP-TTA colloids. Download figure Download PowerPoint The TP-TTA content of TP-TTA/SiO2-1, TP-TTA/SiO2-5, and TP-TTA/SiO2-8 was, respectively, 1.0, 4.1, and 7.1 wt % determined by 1H NMR analysis of digested TP-TTA/SiO2-x (for details, see Supporting Information and Supporting Information Figure S2). The Fourier transform infrared (FT-IR) spectra of TP-TTA/SiO2-x clearly showed the vibrations assigned to C=C, C=O and the aromatic ring respectively at 1578, 1625, and 1598 cm−1, together with the vibrations from triazine ring at 1370 and 1510 cm−1, implying the existence of TP-TTA with β-ketoenamine linkage34–36 (Figure 1a and Supporting Information Figures S3a, S3b, S3e, and S3f). No obvious absorption peaks attributed to CTAB and SDS were observed in the FT-IR spectrum of TP-TTA/SiO2-8, indicating no or a lesser amount of residue. The 13C cross-polarization total suppression of sidebands (CP-TOSS) NMR spectrum of TP-TTA/SiO2-8 exhibited characteristic chemical shifts at 183 and 106 ppm representing –C=O of the keto form and –C=C of the aryl ring. The chemical shifts at ∼170 and ∼131 ppm were assigned to the C atoms of triazine units and the C atoms directly connected to the triazine units37 (Figure 1b). The signals in the range of 14–35 ppm assigned to alkane carbons of CTAB and SDS appeared in the NMR spectrum of TP-TTA/SiO2-8. Thermogravimetric analysis (TGA) showed ∼6.9±1 wt % of organic content in TP-TTA/SiO2-8 ( Supporting Information Figure S4), in agreement with the content of TP-TTA determined by 1H NMR, showing the low amount of surfactant in the sample. The relatively stronger signals of the surfactant were mainly due to the cross-polarization experiment. For the 13C atom with a 1H atom directly connected to it, stronger coupling between 1H and 13C enhanced the intensity of the signal because the magnetization transfer from 1H to 13C by simultaneously applying matching radiofrequency fields to both spins, according to the Hartmann–Hahn condition. The combination of FT-IR spectra and 13C CP-TOSS NMR characterization confirmed the existence of TP-TTA on SiO2. Figure 1 | (a) FT-IR and (b) 13C CP-TOSS NMR spectra of TP-TTA/SiO2-8, (c) UV–vis spectra of TP-TTA/SiO2 samples dispersed in water (A1 and A8 refer to the absorbance at 450 nm). Download figure Download PowerPoint The uniformly dispersed nanospheres with smooth surfaces identical to the parent SiO2 were observed in the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of TP-TTA/SiO2-x samples, and the absence of other polymer phases implied that TP-TTA was mostly deposited on the surface of SiO2 (Figure 2a and Supporting Information Figures S5a–S5d). The scanning TEM (STEM) images and elemental mappings of TP-TTA/SiO2-x clearly showed a uniform distribution of C elements on the surface, suggesting that TP-TTA evenly wrapped on SiO2 nanoparticles (NPs) (Figures 2b–2d). The energy-dispersive system (EDS) mapping of nitrogen species was collected simultaneously. Due to the intrinsic low N content in the as-prepared sample, the signal-to-noise ratio of N elemental mapping was too low to generate an image with high quality. Although we had tried to obtain a better result by increasing both the dose rate and the integration time, the serious beam damage and the carbon contamination during lengthy collection times significantly degraded the spatial resolution of EDS mapping. The thickness of TP-TTA on the SiO2 increased from TP-TTA/SiO2-1 to TP-TTA/SiO2-8 which was evidenced by the increased intensity of the carbon signal. The TP-TTA layer thickness of TP-TTA/SiO2-8 was approximately 3.0 nm measured from the STEM elemental mapping. It is difficult to get the accurate thickness of the TP-TTA layer for TP-TTA/SiO2-1 and TP-TTA/SiO2-5 due to the weak signals of C elemental mapping. The thickness of the TP-TTA layer was calculated to be 0.46 and 1.81 nm, respectively, for TP-TTA/SiO2-1 and TP-TTA/SiO2-5 based on the density formula (for details, see Supporting Information). With the distance between adjacent layers of TP-TTA COFs approximately 0.37 nm by Vienna Ab-initio Simulation Package (VASP) (as shown in Supporting Information Table S1), the TP-TTA layer numbers for TP-TTA/SiO2-1, TP-TTA/SiO2-5, and TP-TTA/SiO2-8 were ∼1, ∼5, and ∼8, respectively. Figure 2 | (a) TEM image of TP-TTA/SiO2-8. STEM images and elemental mapping of (b) TP-TTA/SiO2-1, (c) TP-TTA/SiO2-5, (d) TP-TTA/SiO2-8, (e) TP-TTA/TiO2, and (f) TP-TTA/CdS. HRTEM images of (g) TP-TTA/TiO2 and (h) TP-TTA/CdS. Download figure Download PowerPoint The specific surface area of TP-TTA/SiO2-x was measured by Ar sorption at 87 K ( Supporting Information Table S2 and Figure S6) and calculated with the Brunauer–Emmett–Teller (BET) method. The specific surface area of TP-TTA/SiO2-1 was almost identical to the parent SiO2. The TP-TTA/SiO2-5 and TP-TTA/SiO2-8 showed higher BET surface area than SiO2. No micropore assigned to TP-TTA COF at 0.6 and 0.9 nm could be observed in the pore size distribution curve of TP-TTA/SiO2-1, implying a monolayer structure. TP-TTA/SiO2-5 and TP-TTA/SiO2-8 afforded the micropore from TP-TTA COF, further confirming the presence of multilayer TP-TTA ( Supporting Information Figure S6). The powder X-ray diffraction (PXRD) patterns of TP-TTA/SiO2-8 and TP-TTA displayed intense peaks at 5.7°, 9.2°, and 14.9°, respectively, attributed to the 100, 110, and 210 reflections, implying that the multilayer TP-TTA on SiO2 has a crystalline structure ( Supporting Information Figure S7). Two kinds of stacking arrangements (eclipsed AA and staggered AB) were applied to generate the hexagonal unit cells for TP-TTA COF. After geometrical optimization of the models, the experimental PXRD patterns of the TP-TTA COFs and TP-TTA/SiO2-8 were more likely to match the simulated diffraction patterns of the AA-stacking models, in good agreement with reports in the literature.38 The deposition of TP-TTA with varied thickness on SiO2 suggested that the exfoliation of TP-TTA colloids occurred during the deposition process. CdS, TiO2, WO3, Nb2O5, and ZrO2 were also used as supports for the deposition of TP-TTA. The high-resolution TEM (HRTEM) images showed a uniform coating of TP-TTA layers on the surface of CdS/TiO2 with a clear boundary between TP-TTA layers and TiO2/CdS, which was further confirmed by the STEM images and elemental mapping (Figures 2e–2h). The thickness of TP-TTA layers was 3.6 and 2.6 nm, respectively, for TP-TTA/TiO2 and TP-TTA/CdS, indicating the exfoliating deposition behavior of COF layers on the above solid supports. No polymer phase could be observed in the SEM image of TP-TTA/TiO2 and TP-TTA/CdS ( Supporting Information Figures S5e–S5h). The appearance of the characteristic vibrations of TP-TTA in the FT-IR spectra of TP-TTA/CdS and TP-TTA/TiO2 confirmed the formation of hybrid materials ( Supporting Information Figures S3c and S3d). Unfortunately, the SEM images and the corresponding EDS mapping results showed the coexistence of COF aggregates and metal oxide NPs when using WO3, Nb2O5, and ZrO2 as supports, implying that TP-TTA colloids can hardly be exfoliated on the surface of WO3, Nb2O5, and ZrO2 ( Supporting Information Figure S8). The interaction between adjacent layers of COFs involves the noncovalent van der Waals force.39–41 The size of TP-TTA COF colloids is ∼28 nm due to the surrounding compact surfactant layer preventing further growth and flocculation.33 Density functional theory (DFT) calculation showed that the exfoliation energy of TP-TTA COF (AA stacking) is directly related to the layer thickness ( Supporting Information Figure S9). The more layers, the more difficult it is to exfoliate. Consequently, the TP-TTA colloids with much smaller layer number than bulk TP-TTA COFs had weak interlayer strength which facilitated the stripping of TP-TTA layers. Furthermore, TP-TTA colloids synthesized at 30 °C have lower degrees of polymerization than COFs synthesized by the traditional solvothermal method, which further impaired the interaction strength among TP-TTA layers. When the interaction strength between the solid supports and TP-TTA COF colloids is stronger than the π−π staking (AA) strength between the TP-TTA layers, the exfoliation of TP-TTA COF colloids may occur. This is reasonable considering that the 2D imine-linked COF powders can be exfoliated in the presence of acid by temporarily weakening their interlayer stacking through electrostatic repulsion.42 To decrease the surface tension, the TP-TTA single layer tends to deposit on solid supports driven by the H-bonds or other interactions. In the presence of higher amounts of TP-TTA colloids in the synthesis mixture, multilayer TP-TTA was formed on the supports, possibly by the restacking of the single layer due to π–π interactions driven by the high temperature used for the solvent evaporation. Another important parameter for the exfoliating TP-TTA colloids on supports is the pH value of the colloid solution. By mixing SiO2 and TP-TTA colloids (adjusted to pH of 7), the positively charged TP-TTA colloids (zeta potential value of 60 mV, Supporting Information Figure S10) interacted with negatively charged SiO2 nanospheres (isoelectric point of 1.5–3.5) through electrostatic interactions to destroy the colloids. Thus, the TP-TTA was released from the micelles and delaminated into a single-layer, driven by the strong interaction of SiO2 and TP-TTA colloids. The control experiment was performed by mixing SiO2 and TP-TTA colloids with pH adjusted to 1. The TEM image of the resultant material showed the coexistence of SiO2 nanospheres and irregularly shaped TP-TTA ( Supporting Information Figure S5i). At pH of 1, the surface of SiO2 is positively charged, which does not favor the contact with the positively charged TP-TTA colloids. The control experiment signifies the importance of surface electrostatic interactions in successful deposition of TP-TTA layers on SiO2. In comparison with previously reported methods,43 the self-exfoliation of COF colloids could precisely control the layer thickness, and the SiO2 support could prevent the stacking of COF layers during the practical application process. More importantly, this method is easy for the scale-up synthesis. The color changed gradually from light yellow to yellow brown when TP-TTA content increased ( Supporting Information Figure S11). The UV–vis spectra of TP-TTA/SiO2-x water suspension gradually showed red shifts of the absorption edge from 427 to 440 nm with increased layers (Figure 1c). This can be attributed to an increased conjugation length with layer thickness and/or the J-type aggregation with the chromophores between adjacent layers.44,45 The UV–vis reflectance of solid TP-TTA/SiO2-x showed a similar tendency ( Supporting Information Figure S11). Calculated by Tauc plots,46,47 the optical band gaps of TP-TTA/SiO2-x varied from 2.28 to 2.41 eV, showing a slight increase of band gap with the decrease of the TP-TTA layer ( Supporting Information Figure S11). Mott–Schottky tests were performed to determine the band minimum of the materials ( Supporting Information Figure The for TP-TTA COF and TP-TTA/SiO2-x was hydrogen which is than the potential ( Supporting Information Figure The above results the for PHE by TP-TTA/SiO2-x and TP-TTA COFs under The PHE activity of TP-TTA/SiO2-x and corresponding bulk TP-TTA COFs ( Supporting Information Figures and was in a H2 evolution reaction under light 420 nm) with ascorbic acid as a and NPs from H2PtCl6 as a (for details, see Supporting Information). showed no H2 evolution occurred light and with a that the PHE reaction only under light and in the presence of a shown in Figure the of H2 increased with the TP-TTA COF and TP-TTA/SiO2 The H2 evolution rate was relatively during the 0.5 h, possibly due to the of which is a for TP-TTA/SiO2-1 H2 in h, much higher than the content in the showing that the H2 is not from the of the The experiment confirmed that the H2 from water ( Supporting Information Figure The H2 evolution rate was calculated 1 h reaction to the influence of the The H2 evolution rate decreased from to when increasing the layer numbers from 1 to (Figure TP-TTA/SiO2 samples were more than TP-TTA COF with the H2 evolution rate of the samples had much lower content of TP-TTA. Therefore, it is reasonable to that the more precisely the thickness, of TP-TTA significantly photocatalytic activity by the diffusion distance of charge The H2 evolution rate of TP-TTA/SiO2-1 was as high as this remarkable the for reported ( Supporting Information Table Figure | (a) PHE as a of reaction with 50 mg TP-TTA/SiO2-x and TP-TTA COFs under nm, in the presence of 30 mL 0.1 M ascorbic acid and wt % (b) The of layer numbers of TP-TTA with PHE rate and (c) of H2 (d) of H2 evolution Download figure Download PowerPoint The apparent quantum efficiency of TP-TTA/SiO2-1 was measured using a 300 W Xenon lamp with a and nm filter (Figure for details, see Supporting Information). The of at different was in good agreement with optical absorption The at 440 nm for TP-TTA/SiO2-1 which is times higher than TP-TTA COF with of With increased layer thickness, the decreased (Figure In a PHE no activity was observed h using TP-TTA/SiO2-1 as the model photocatalyst (Figure The TP-TTA/SiO2-1 had similar and optical absorption properties as the the high of TP-TTA/SiO2-1 during the photocatalysis ( Supporting Information Figure Figure | absorption of (a) TP-TTA/SiO2-1 and (b) TP-TTA/SiO2-8 in water in 0.1 M ascorbic acid and in 0.1 M ascorbic acid with at 450 nm, at nm). (c) for TP-TTA/SiO2-1 and TP-TTA/SiO2-8 in the presence of ascorbic (d) The electron extraction efficiency of TP-TTA/SiO2-1 and TP-TTA/SiO2-8. Download figure Download PowerPoint The of TP-TTA/SiO2 samples was almost the suggesting that the high PHE activity of TP-TTA/SiO2-1 was not due to the different for ( Supporting Information Figure to the surface of COFs has a influence on the PHE The water contact of TP-TTA/SiO2-x was measured, and the results showed that TP-TTA/SiO2 samples and TP-TTA COFs had a similar water contact of ( Supporting Information Figure implying that the surface is not for the in PHE spectroscopy and of TP-TTA/SiO2-x samples were performed and compared with TP-TTA shown in Supporting Information Figure the of TP-TTA/SiO2-x increased with the increase of the layer number of suggesting that TP-TTA could improve charge The of TP-TTA/SiO2 samples also increased with the decrease of the number of TP-TTA layers, indicating that the transmission distance was for the separation of photogenerated charge carriers ( Supporting Information Figure a the PHE rate of TP-TTA/SiO2-1 was enhanced to TP-TTA COF. To further the charge separation efficiency of TP-TTA/SiO2 samples, transient absorption spectra of the water of TP-TTA/SiO2-1 and TP-TTA/SiO2-8 were In the presence of the the signal at nm was observed for both samples, and the was increased at the scale in the presence of the (Figures and Supporting Information Figure confirming that in such long-lived signal in is from electrons and the The initial could be used to the number of electrons due to the between the electron numbers and the intensity of the signal. The initial increased from for TP-TTA/SiO2-1 to for TP-TTA/SiO2-8 in ascorbic The ratio = of the initial of TP-TTA/SiO2-8 and TP-TTA/SiO2-1 was in good with the ratio = of their absorbance at 450 nm under the intensity (Figure 1c). This good agreement that all the light by TP-TTA/SiO2-x generate long-lived electrons in the presence of ascorbic their similar dissociation TP-TTA/SiO2-1 and TP-TTA/SiO2-8 have of electrons the when the to of the initial of and respectively (Figure The behavior of electron extraction from TP-TTA/SiO2 samples to was by the was (Figure and Supporting Information Figure In the presence of the signal reduced for TP-TTA/SiO2 samples, indicating the migration of electrons to to participate in the For TP-TTA/SiO2-1, the signal to a weak absorption signal by the of indicating that the corresponding to the weak absorption participate in the The electron

Covalent organic frameworks (COFs) exhibit significant promise for photocatalytic overall water splitting to hydrogen generation. However, the high electron density distribution at aromatic carbon in COFs results in inert oxygen evolution, significantly hindering photocatalytic overall water splitting activity. Here, a universal strategy is developed for localized electron density manipulation by utilizing the reactivity of unsaturated carbon at the linkers in the COFs to construct phosphonate ylide polar sites, featuring positively charged phosphorus and negatively charged carbon. Under photoexcitation, this local electron distribution generates a polaron effect, enhances photogenerated exciton dissociation, prevents radiative relaxation, accelerates photogenerated charge separation, and induces an extremely low oxygen evolution barrier at the phosphorus sites. The results show the phosphonate ylide COF has achieved H2 and O2 evolution rates of 24.7 and 12.0µmolh-1 under visible light irradiation, with the 62 times increase over the pristine COF. Furthermore, this strategy has been successfully validated in several other COFs, demonstrating its broad applicability. To further validate its practical utility, a large-scale outdoor device of 4m2 using this catalyst is fabricated, which achieved a hydrogen production rate exceeding 300mmolday-1, highlighting its excellent potential for practical applications.

A review on recent developments towards hydrogen evolution reaction for covalent organic frameworks

Covalent organic frameworks (COFs) have recently emerged as promising candidates for solar fuel production due to their broad spectral absorption and readily tunable optoelectronic properties. However, their photocatalytic performance is often limited by inefficient charge separation and rapid charge recombination. Herein, novel S-scheme COF heterojunctions is reported by integrating a negatively charged COF host with a positively charged linear conjugated polymer. The electrostatic attraction between them spontaneously generates a robust Coulombic electric field at the heterojunction interface, which shows an identical electric field direction with the intrinsic built-in electric field of S-scheme configuration.The control experiments and spectroscopic characterizations reveal that this dual-field approach significantly enhances directional charge separation and transfer at the interface, effectively suppressing charge recombination. The optimized sample exhibits a highly enhanced photocatalytic hydrogen production rate of 339.4 µmol g-1 h-1 while coupling with a stoichiometric conversion of 5-hydroxymethyl furfural to 2,5-diformylfuran.

The classical organic dyes anthracene, acridine, and phenazine, as building blocks in the construction of three covalent organic frameworks (COFs), are presented for comprehensive comparison studies. Employing a sophisticated atomic-level skeleton-editing strategy through the precise adjustment of nitrogen content enables the regulation of intrinsic properties across macro- to micro-scales, encompassing color, morphology, thermal stability, charge separation, transfer dynamics, energy level position, and photocatalytic performance, among others. Extensive characterization technologies and density functional theory (DFT) calculations are elaborately performed to illustrate the “butterfly effect”, wherein atomic-level “flapping of the wings” within COF materials can significantly impact other aspects. In application experiments, photocatalytic hydrogen evolution (PHE) is studied as a model reaction. The results suggest that the anthracene-based COF (COF-Ant), with the strongest electron-donating ability, achieves the highest PHE rate among the three COFs at 2493 μmol·g–1·h–1. Additionally, the uniformity of the three COFs is highlighted, including shared features such as organic dyes and imine linkages, resulting in wide visible absorption, a narrow band gap, and topological features. This work offers a promising outlook for the rational construction of donor–acceptor (D–A) frameworks through atomic-level engineering, facilitating the customization of optoelectronic, photocatalysis, and other properties.

Covalent organic frameworks (COFs) are potential photocatalysts for artificial photosynthesis but they are much less explored for photocatalytic hydrogen evolution (PHE). COFs, while intriguing due to crystallinity, tunability, and porosity, tend to have low apparent quantum efficiency (AQE) and little is explored on atomistic structure–performance correlation. Here, adopting triphenylbenzene knots and phenyl linkers as a proof of concept, three structurally related COFs with different linkages are constructed to achieve a tunable COF platform and probe the effect of the linkage chemistry on PHE. Cyano‐substituted alkene‐linked COF (COF–alkene) yields a stable 2330 µmol h−1 g−1 PHE rate, much superior to imine‐ and imide‐linked counterparts (<40 µmol h−1 g−1) under visible light irradiation. Impressively, COF–alkene achieves an AQE of 6.7% at 420 nm. Combined femtosecond transient absorption spectroscopy and theoretical calculation disclose the critical role of cyano‐substituted alkene linkages toward high efficiency of charge separation and transfer in the presence of sacrificial electron donors—the decisive key to the superior PHE performance. Such alkene linkages can also be extended to design a series of high‐performance polymeric photocatalysts, highlighting a general design idea for efficient PHE.

Covalent organic frameworks (COFs)‐based photocatalysts have received growing attention for photocatalytic hydrogen (H2) production. One of the big challenges in the field is to find ways to promote energy/electron transfer and exciton dissociation. Addressing this challenge, herein, a series of olefin‐linked 2D COFs is fabricated with high crystallinity, porosity, and robustness using a melt polymerization method without adding volatile organic solvents. It is found that regulation of the spatial distances between the acceptor units (triazine and 2, 2'‐bipyridine) of COFs to match the charge carrier diffusion length can dramatically promote the exciton dissociation, hence leading to outstanding photocatalytic H2 evolution performance. The COF with the appropriate acceptor distance achieves exceptional photocatalytic H2 evolution with an apparent quantum yield of 56.2% at 475 nm, the second highest value among all COF photocatalysts and 70 times higher than the well‐studied polymer carbon nitride. Various experimental and computation studies are then conducted to in‐depth unveil the mechanism behind the enhanced performance. This study will provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

The design of efficient covalent organic frameworks (COFs) as photocatalysts for C(sp3)─H bond oxidation under green and mild conditions is highly desirable. Herein, TpAQ‐TZ COF and TpAR‐TZ COF, featuring thiazole linkages, were synthesized by combining 1,3,5‐tricarboxylcarboxaldehyde (Tp), sulfur (S8), and direct hydrogen atom transfer (d‐HAT) components (2,6‐diaminoanthraquinone, AQ; 2,6‐diaminoanthrone, AR). The TpAQ‐TZ COF demonstrates impressive photocatalytic activity, achieving a 93% yield for phthalan oxidation under heterogeneous conditions—a rare example of photocatalytic C(sp3)─H bond oxidation in water under ambient conditions. Theoretical calculations reveal enhanced hydrogen atom abstraction capability coming from the increased number of d‐HAT catalytic sites. Moreover, BdAQ‐TZ COF and HbAQ‐TZ COF were prepared by replacing Tp with 2,4‐dihydroxy‐1,3,5‐triformylbenzene (Bd) or 2‐hydroxy‐1,3,5‐triformylbenzene (Hb), respectively. Dipole moment calculations and femtosecond transient absorption spectroscopy show that the increased number of hydroxyl groups on the benzene‐1,3,5‐tricarbaldehyde monomer improves the charge separation efficiency within the three COFs, thereby accounting for the enhanced photocatalytic activity of TpAQ‐TZ COF. This work opens up new opportunities for designing highly active photocatalysts by using the synergistic effects of d‐HAT, O2•−, and 1O2 within anthraquinone‐based donor–acceptor COF platforms, offering a sustainable route for the oxidation of C(sp3)─H bonds under environmentally friendly conditions.

Covalent organic frameworks (COFs) are crystalline porous conjugated polymers that have been widely used for photocatalytic hydrogen evolution and CO2 reduction. However, only few COFs showed photocatalytic oxygen evolution, which is a more challenging half-reaction of photocatalytic water splitting. Here, we presented visible-light-driven photocatalytic water oxidation of a triazine-based COF (TAPT-Bpy-COF) coordinated with cobalt as cocatalysts. The highest oxygen evolution rate (OER) was achieved at 483 μmol g–1 h–1 (≥420 nm) with an efficient apparent quantum efficiency (AQE) of 7.6% (420 ± 20 nm). The highly photocatalytic oxygen evolution activity of TAPT-Bpy-COF could be attributed to its highly ordered structures, high surface area, good wettability as well as enhanced charge separation. This work demonstrates the potential of COFs for photocatalytic oxygen evolution half-reaction and overall water splitting.

Vinyl-functionalized covalent organic framework via tuning π-conjugation effectively promotes photocatalytic hydrogen evolution

Solar-to-electrochemical energy storage in solar batteries is an important solar utilization technology comparable to solar-to-electricity (solar cells) and solar-to-fuel (photocatalytic cells) conversion. Unlike the indirect approach of integrated solar flow batteries combining photoelectrodes with redox-electrodes, coupled solar batteries enable direct solar energy storage, but are hampered by low efficiency due to rapid charge recombination of materials and misaligned energy levels between electrodes. Herein, we propose a design for a coupled solar battery that intercouples two photo-coupled ion transfer (PCIT) reactions through electron-ion transfer upon co-photo-pumping of photoelectrochemical storage cathode and anode. We used a representative covalent organic framework (COF) to achieve efficient charge separation and directional charge transfer between two band-matched photoelectrochemical storage electrodes, with a photovoltage sufficient for COF dual-redox reactions. By pumping these electrodes, the coupled solar battery stores solar energy via two synergistic PCIT reactions of electron-proton-relayed COF oxidation and reduction, and the stored solar energy is released as electrochemical energy during COF regeneration in discharge while interlocking the loops. A breakthrough in efficiency (6.9 %) was achieved, adaptive to a large-area (56 cm2 ) tandem device. The presented photo-intercoupled electron-ion transfer (PIEIT) mechanism provides expandable paths toward practical solar-to-electrochemical energy storage.

Solar‐to‐electrochemical energy storage in solar batteries is an important solar utilization technology comparable to solar‐to‐electricity (solar cells) and solar‐to‐fuel (photocatalytic cells) conversion. Unlike the indirect approach of integrated solar flow batteries combining photoelectrodes with redox‐electrodes, coupled solar batteries enable direct solar energy storage, but are hampered by low efficiency due to rapid charge recombination of materials and misaligned energy levels between electrodes. Herein, we propose a design for a coupled solar battery that intercouples two photo‐coupled ion transfer (PCIT) reactions through electron‐ion transfer upon co‐photo‐pumping of photoelectrochemical storage cathode and anode. We used a representative covalent organic framework (COF) to achieve efficient charge separation and directional charge transfer between two band‐matched photoelectrochemical storage electrodes, with a photovoltage sufficient for COF dual‐redox reactions. By pumping these electrodes, the coupled solar battery stores solar energy via two synergistic PCIT reactions of electron‐proton‐relayed COF oxidation and reduction, and the stored solar energy is released as electrochemical energy during COF regeneration in discharge while interlocking the loops. A breakthrough in efficiency (6.9 %) was achieved, adaptive to a large‐area (56 cm2) tandem device. The presented photo‐intercoupled electron‐ion transfer (PIEIT) mechanism provides expandable paths toward practical solar‐to‐electrochemical energy storage.

Covalent organic frameworks (COFs) have demonstrated enormous potential in photocatalysis. To construct more efficient COF-based photocatalysts, it is essential to delve into the relationship between molecular-level structure of the COF and the fundamental photocatalytic processes. COF is built by small molecular monomers, so the classic substitution effect on small molecules should be accumulated in the COF. However, to accurately investigate the substituent effect, the other structural parameters of the COF should be kept as unchanged as possible. This work designed and constructed COFs with identical skeleton but different substituents (−H, –CH3, and –OH) at the same position, which displayed very similar crystallinity, surface area, pore size distribution, and morphology, but completely different photocatalytic H2 evolution or H2O2 production performances. Comparative analyses of the fundamental photocatalytic processes indicated that −OH-containing COF possessed broader visible light absorption, higher charge separation efficiency, and more rational surface properties for the reactions compared with –H and −Me-containing COFs, thus resulting in superior photocatalytic performances. This study reveals that a small change in the substituent will lead to big differences in the photocatalytic processes and thus the final photocatalytic performances, which have instructive significance for the structural design and performance evaluation of COF-based photocatalysts.

Covalent organic frameworks (COFs) have been a potential candidate for applications in photocatalysis due to its periodically porous structures and tunable structure. The COF skeletons consisted of different building blocks may result in different performance. Investigating the effects of different building blocks on energy levels and excitons for COF can provide some insight for designing excellent COF catalysts. Based on the first-principles many-body Green’s function theory, the electronic structures and optical properties of the three donor-acceptor COFs by employing the monomer 2,4,6-trimethyl-1,3,5-triazine (TMT) as the key acceptor subunit and the trigonal aldehyde monomers including the tris(4-formylphenyl) amine (TPA), 1,3,5-tris(4-formylphenyl) benzene (TFPB) and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) as the donor subunit are calculated in this work. Regulation of the donor unit and interlayer interactions on the electronic structures and excitonic properties are analyzed. The results show that the valence band maximum (VBM) and conduction band minimum (CBM) energies of the system are varied by the alteration of donor subunit. From TPA to the TFPB or TFPT, the bandgaps of the system increase, the light absorption blue shift, and the exciton binding energies gradually increase. There is little effect on the band gap and excitation energy by replacing the TFPB with the TFPT. Among the three COFs, the positions of both CBM and VBM of the TFPT-TMT COF only match well with the chemical reaction potential of H&lt;sub&gt;2&lt;/sub&gt;/H&lt;sup&gt;+&lt;/sup&gt; and O&lt;sub&gt;2&lt;/sub&gt;/H&lt;sub&gt;2&lt;/sub&gt;O, which is capable of photocatalytic overall water splitting. But the photocatalytic performance for the TFPT-TMT COF might be inhibited by the higher exciton binding energy. The exciton for the TPA-TMT COF is easier to separate according to the exciton distributions and the exciton binding energy. The effect of different building units on the electronic structure, excitation energy, and excitonic properties of COFs in monolayer COFs are in line with that in multilayer and bulk COFs. The variation of the energy levels and excitation energies of all the three COFs as the number of layers are consistent. With the increasing number of layers, the VBM and CBM shift up and down with respect to the vacuum level, respectively. The band gap gradually decreases. The energy tend to decrease slower with the more layer. The exciton energy for multilayer COFs is close to the bulk state. These results are significant to design and modify COFs.