Semiconducting 2D Triazine-Cored Covalent Organic Frameworks

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Semiconducting 2D Triazine-Cored Covalent Organic Frameworks with Unsubstituted Olefin Linkages Shice Wei, Fan Zhang, Wenbei Zhang, Peirong Qiang, Kaijin Yu, Xiaobin Fu, Dongqing Wu, Shuai Bi, and Fan Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06219 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Semiconducting 2D Triazine-Cored Covalent Organic Frameworks with Unsubstituted Olefin Linkages Shice Wei†,§, Fan Zhang†,§, Wenbei Zhang†, Peirong Qiang†, Kaijin Yu†, Xiaobin Fu‡, Dongqing Wu†, Shuai Bi*,† and Fan Zhang*,† †School

of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China ‡Department of Molten Salt Chemistry and Engineering, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China covalent organic framework; olefin linkage; triazine-cored; semiconductor; photocatalysis ABSTRACT: Establishing sp2-carbon-bonding pattern is one of the efficient accesses to various organic semiconducting materials. However, the less-reversible carbon-carbon bond formation makes it still challengeable to spatially construct a well-define organic framework with π-extended two-dimensional (2D) structure through solution process. Here, a Knoevenagel condensation approach to synthesize two new 2D covalent organic frameworks (COFs) connected by unsubstituted carbon-carbon double bond linkages through activating the methyl carbons of 2,4,6-trimethyl-1,3,5-triazine monomer is presented. The resulting sp2-carbon-linked triazine-cored 2D sheets are vertically stacked into high-crystalline honeycomb-like structures, rendering such kind of COFs with extended -delocalization, tunable energy levels, as well as high surface areas, regular open channels and chemical stabilities. On the other hand, their micro-fibrillar morphologies allow for the facile manipulation of thin films as photoelectrodes without additive. Accordingly, such kinds of COF-based photoelectrodes exhibit photocurrent up to ~45 A cm-2 at 0.2 V vs RHE as well as rapid charge transfer rate, in comparison with imine-linked COF-based photoelectrodes. In addition, both COFs are applicable for conducting photocatalytic hydrogen generation from water-splitting by visible-light irradiation.

INTRODUCTION Carbon-carbon forming reactions are widely used for coupling various organic monomers to numerous linear or dendritic conjugated polymers due to the rapidly increased demand for organic semiconductors.1 However, the poor reversibility makes them less successful for the reliable construction of the π-extended in-plane structures by solution process. Two-dimensional (2D) covalent organic frameworks (COFs) with latticed polymer backbones and shape-persistent open channels are emerging as excellent porous materials potentially applicable for gas absorption, catalysis and electrochemical devices.2-3 Mostly, the conjugated COFs were formed by dynamic covalent linkages, such as imine bonds and arylhydrazone bonds which usually exhibit relatively poor -delocalization and low stability, seriously impeding their development toward semiconductor utilization.4-5 Conventionally, carbon-carbon double bond has been proved to be a crucial -bridge unit, favorable for enhancing charge carrier mobility or extending -conjugation in many promising organic semiconducting materials.6-8 In 2016, our group developed the first 2D cyanostilbene-based COFs with olefin linkages via Knoevenagel condensation based on 1,4-phenylenediacetonitrile monomer.9 Up to now, there are still only a few such kinds of COFs reported.10-16 In comparison with their imine-linked analogs, these olefin-linked COFs exhibit extremely high stabilities under harsh conductions, and enable undergoing interesting

magnetic coupling, electrochemical performance and photocatalytic activities. It is extremely worth exploring new types of olefin-linked COFs for achieving promising properties, in particular, exceptional semiconducting behaviours.17-18 Very recently, we successfully synthesized a series of unsubstituted olefin-linked pyridine-based 2D COFs via Knoevenagel condensation.19 And Yaghi et al. reported an unsubstituted olefin-linked triazine-based COF via Brønsted acid-catalyzed Aldol condensation.20 All these are guiding our effort along this line. Triazine unit features planar and fully conjugated characters, which is suitable for boosting -electron communication without the meta-position-impeded conjugation effect in benzene ring. Meanwhile, its multiple pyridinic nitrogen atoms enable offering abundant active sites in some catalytic processes. The triazine-containing COFs (normally termed as CTFs) possess high stability and excellent semiconducting properties in crystalline state, thus delivering good photoactivities.21-23 Normally, CTFs were constructed through the formation of 1,3,5-triazine rings, whose diversities either in structures or functions are severely constrained by the available monomers.24-25 Directly coupling triazine derivatives with the other monomers to form sp2-carbon-linked skeleton seems to represent a much straightforward and readily controlled access to -extended 2D COFs. As previously reported, 2,4,6-trimethyl-1,3,5-triazine was easily converted to olefin-linked small-molecule fluorophores by activation of


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aryl

Journal of the American Chemical Society methyl

carbon

atoms

through

a

Knoevenagel

Hereby, a Knoevenagel condensation approach to synthesize two new 2D COFs with unsubstituted olefin linkages via the activation of aryl methyl carbon atoms of 2,4,6-trimethyl-1,3,5-triazine monomer in the presence of a base catalyst is presented. The generalizability of such method was verified by using the linear or trigonal aldehyde monomers. The structural characterizations reveal that the resulting two COFs are layered honeycomb-like crystalline porous structures with high surface areas. Microstructural characterizations revealed their micro-fibrillar morphologies. Upon a drop-casting performance, these fibers enable assembling to thin film in micrometer thickness without additive. Such kind of olefin-linked COFs exhibit semiconducting properties, including tunable band gaps or band positions, rapid charge separation ability, as well as photoelectrochemical activities, which are highly related to their building blocks in 2D structures. These characters render them with visible-light-driven photocatalytic hydrogen generation activity from water-splitting with recycling and reusability.

RESULTS AND DISCUSSION Initially, upon Knoevenagel condensation reaction, 2,4,6-trimethyl-1,3,5-triazine (donated as TMTA) and benzaldehyde as reactants under a base catalyst KOH was employed to afford the model molecule 2,4,6-tri((E)-styryl)-1,3,5-triazine (TST).26-27 Here, the acidic hydrogen atoms of methyl groups on TMTA were neutralized by KOH through C-H cleavage, resulting in reactive carbanions, which subsequently conduct nucleophilic attacking at the positive carbon atoms in the formyl Figure 1. (a) Scheme of the synthesis of g-C18N3-COF and g-C33N3-COF by groups of benzaldehyde to form Knoevenagel condensation. (b) and (c) Powder X-ray diffraction patterns of carbon-carbon double bonds by releasing g-C18N3-COF and g-C33N3-COF: comparison between the experimental (black H2O molecules. Inspired by this cross) and Pawley refined (red line) profiles, the simulated patterns for eclipsed mechanism, as an example, the target COF (AA) stacking mode (blue line), the Bragg positions (green bar) and the refinement g-C18N3-COF was prepared by the reaction differences (violet line). Insets are structural models of g-C18N3-COF and of TMTA with a linear aldehyde monomer g-C33N3-COF assuming the eclipsed (AA) stacking mode. (d) and (e) Nitrogen 1,4-diformylbenzene (DFB). A high adsorption and desorption isotherms of g-C18N3-COF and g-C33N3-COF. Insets are crystalline sample assessed by PXRD the pore size distributions calculated from non-local density functional theory. analysis was achieved under an optimized condensation.26-27 solvothermal reaction condition involving a binary solvent of In such reaction, an aryl reactant with -carbon atom is n-butanol/o-dichlorobenzene (7:3 v/v), 120 °C and 72 hours facile to generate a reactive intermediate carbanion, which can (Figure S1). In order to check the generalizability of such be stabilized through p--conjugated effect with an Knoevenagel condensation, our attempt is to synthesize electron-deficient triazine-structure. Such carbanion is thus another COF sample (g-C33N3-COF) by selecting a trigonal likely favorable for undergoing a self-healing process, which aldehyde monomer 1,3,5-tris(4-formylphenyl)benzene is extremely important for the formation of a highly crystalline (TFPB) to react with TMTA. It is worth noting that only structure. With these in mind, our consideration is to establish moderate crystallinity was achieved under the above a high crystalline 2D COF by the connection of triazine units optimized condition. Such phenomenon might be attributable via unsubstituted olefin linkages. to the lower reactivity of the formyl carbon atoms for TFPB



than those for DFB, associated with the higher electronic cloud density in the former case. After screening of several inorganic strong bases (i.e., sodium methoxide and sodium ethoxide), sodium ethoxide is found to be the most efficient catalyst to improve the crystallinity of g-C33N3-COF (Figure S2). Generally, the reaction conditions including temperature, solvent and base catalyst, should be comprehensively optimized for promoting the C=C bond forming reversibility for achieving a high-crystalline COF. The crystallinity of g-C18N3-COF and g-C33N3-COF were confirmed by the PXRD analyses, with the intense and sharp reflections at low-angle range in the patterns. Then, the experimental PXRD patterns of these COFs were subjected to powder indexing, yielding unit cell parameters. The diffraction peaks at 4.7°, 8.2°, 9.4°, 12.6°, and 26.6° were indexed as the (100), (110), (200), (210), and (001) reflections of a hexagonal unit cell with a = b = 21.56 Å, c = 3.35 Å for g-C18N3-COF. Similarly, the PXRD pattern of g-C33N3-COF showed five distinguishable peaks at 2 theta = 5.9°, 10.2°, 11.7°, 15.2°, and 25.3°, which were indexed to generate unit cell parameters of a = b = 17.50 Å, c = 3.43 Å with hexagonal symmetry. Afterwards, empty unit cells were built with the above unit cell parameters. Single layer models were generated by connecting monomer fragments by unsubstituted olefin (– CH═CH–) linkages with hcb topology and the models were filled into the unit cells. Eclipsed (AA) and staggered (AB) stacking arrangements were constructed with the above unit cell parameters and the lattice models were geometrically optimized. The corresponding diffraction patterns were simulated for comparison with experimental data. The experimental PXRD patterns agreed well with the simulations from the eclipsed (AA) layer stacking models for both g-C18N3-COF and g-C33N3-COF. Meanwhile, the simulated patterns of the staggered (AB) layer stacking models seem to match with the experimental patterns in the peak positions, but have obvious deviations in the relative intensity of diffraction peaks with respect to the experimental data (Figure S33 and S35). Finally, Pawley refinements of the eclipsed (AA) stacking models against the experimental PXRD patterns were applied to produce the refined PXRD profiles, Rwp and Rp values, leading to low residual values and profile differences (Figure 1b and 1c). The permanent porosities of g-C18N3-COF and g-C33N3-COF were investigated by nitrogen sorption measurements at 77 K. The adsorption isotherms of both COFs show a sharp rise in the low-pressure range, which can be assigned to type I sorption isotherm, suggesting their microporous character (Figure 1d and 1e). The surface area of g-C18N3-COF was calculated to be 1170 m2 g-1 by employing Brunauer-Emmett-Teller (BET) model, which is higher than 752 m2 g-1 of g-C33N3-COF. Further calculation of solvent accessible surface area (using a probe radius of 1.84 Å for nitrogen molecule) of the eclipsed (AA) layer stacking models gave the values of 1848 m2 g-1 for g-C18N3-COF and 1398 m2 g-1 for g-C33N3-COF (Figure S36 and S38). The tendency in experimental measured surface areas of the as-prepared COFs is consistent with those of theoretical evaluation, implying that the structural ordering in either g-C18N3-COF or g-C33N3-COF is close to the theoretical model. A non-local density functional theory (NLDFT) cylindrical pore model was applied to evaluate the pore size distributions. One prominent distribution peak at 1.72 nm or 1.45 nm, was found

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for g-C18N3-COF or g-C33N3-COF, respectively, similar to the predicted values based on the Connolly surface in their eclipsed (AA) layer stacking models (Figure S37 and S39). The chemical structures of g-C18N3-COF and g-C33N3-COF were confirmed through Fourier transform infrared (FT-IR) spectroscopy, 13C cross-polarization/magic angle spinning solid-state nuclear magnetic resonance (CP/MAS ssNMR) spectroscopy, thermogravimetric analysis (TGA), and elemental analysis. FT-IR spectrum of g-C18N3-COF, g-C33N3-COF and corresponding monomers are shown in Figure 2a and 2b, revealing the stretching vibration peaks of newly formed C=C moieties in trans-configuration at ca. 1630 cm-1 and ca. 980 cm-1 in both COFs, confirmative of the presence of the olefin linkages in the as-prepared COF skeletons. The C=O stretching vibration of the aldehyde monomers at ca. 1700 cm-1 disappeared in COF samples, indicating their high polymerization degrees. 13C ssNMR spectra of g-C N -COF, g-C N -COF and 13C 18 3 33 3 NMR spectra of the model compound TST in CD2Cl2 were combined in Figure 2c. First, the assignment of the observed 13C peaks of model compound TST was conducted by 1H/13C Heteronuclear Single Quantum Coherence (HSQC) technique (Figure S4). By the comparison of 13C NMR spectra between the COFs and the model compound, the chemical structures of the COFs can be well elucidated. As an example, for g-C18N3-COF, the peak at 144 ppm was assigned to one of the carbon atoms in the newly formed olefin groups (signed as 3). A well-resolved peak located at 135 ppm belongs to the olefin-linked phenyl carbon (4). While the most intense peaks at 125 ppm were ascribed to an alkene carbon (2) overlapped with the hydrogen-bonded aromatic carbon (5). The signals at around 170 ppm were typically arising from the carbon atoms of triazine core. Moreover, thermogravimetric analysis (TGA) revealed the high thermal stability of g-C18N3-COF and g-C33N3-COF, with less than 10% weight loss in N2 up to 500 °C (Figure 2d). Elemental analysis showed that the elemental contents of the two COFs are close to the theoretical values (Table S1).

Figure 2. (a) FT-IR spectra of g-C18N3-COF and corresponding monomers. (b) FT-IR spectra of g-C33N3-COF and corresponding monomers. (c) 13C CP/MAS solid-state NMR spectra of


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g-C18N3-COF and g-C33N3-COF, combined with liquid 13C NMR spectra of TST (model compound) recorded in CD2Cl2. (d) TGA profiles of g-C18N3-COF and g-C33N3-COF.

The optical properties of g-C18N3-COF and g-C33N3-COF were investigated by ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectroscopies. Their UV-vis DRS spectra show the broad absorption bands with the maximum peaks at 450 nm for g-C18N3-COF and 400 nm for g-C33N3-COF, in line with their yellow color under sunlight (Figure 3a), suggesting their strong visible-light harvesting. The absorption edge of 510 nm for g-C18N3-COF is red-shifted by about 20 nm over that of 490 nm for g-C33N3-COF, likely due to the presence of meta-substituted benzene knots in the later one, which might weaken π-delocalization within 2D backbone to some extent.17 Accordingly, their optical band gaps were evaluated from the Kubelka-Munk-transformed reflectance spectra to be 2.42 eV for g-C18N3-COF, which is narrower than that of 2.54 eV for g-C33N3-COF (Figure 3b). Under the irradiation of UV lamp, g-C18N3-COF and g-C33N3-COF powder samples emitted intensive yellow and yellow-green luminescence, respectively.

Figure 3. (a) UV/vis DRS of g-C18N3-COF and g-C33N3-COF. Inset shows the digital photograph of the samples under ambient light. (b) Band gap determined from the Kubelka-Munk-transformed reflectance spectra. (c) Steady-state photoluminescence (PL) spectra of g-C18N3-COF and g-C33N3-COF. Inset shows the digital photograph of the samples under 365 nm UV-lamp. (d) PL decay spectra of g-C18N3-COF and g-C33N3-COF monitored upon excitation at 365 nm.

The steady-state photoluminescence (PL) spectra of the two COFs were measured. Upon excitation at 365 nm, the emission maximum of g-C18N3-COF appeared at em = 574 nm, distinctly red-shifted by ca. 25 nm over that of em = 549 nm for g-C33N3-COF (Figure 3c), attributable to the larger π-extended structure of the former one. Additionally, the PL decay curves of the two COFs are well fitted with the lifetimes with two exponential components (Figure 3d). The average lifetimes of g-C18N3-COF and g-C33N3-COF were estimated to be 7.25 ns and 3.43 ns, respectively (Table S2). The much longer lifetime of g-C18N3-COF indicates the suppressed radiative recombination of photogenerated excitons, associated with its extended π-conjugated backbone.28 In order to understand the effect of crystallinity on the photophysical

properties of the as-prepared COFs, we took advantage of a sample of g-C18N3-COF with the similar crystallinity to g-C33N3-COF for comparison (the low crystalline g-C18N3-COF was achieved in a binary solvent of n-butanol/o-dichlorobenzene: 3/7, termed as g-C18N3-COF-Low). Their crystallinities were controlled by the full width at half-maximum (FWHM) of the (100) peak in PXRD patterns, as 0.380° for g-C18N3-COF-Low and 0.417° for g-C33N3-COF samples (Figure S24).29 The PL emission peak maxima of g-C18N3-COF, g-C18N3-COF-Low and g-C33N3-COF show a red-shifted sequence of g-C18N3-COF > g-C18N3-COF-Low > g-C33N3-COF (Figure S25a). Their average PL lifetimes in an order of g-C18N3-COF (7.25 ns) > g-C18N3-COF-Low (6.08 ns) > g-C33N3-COF (3.43 ns) were observed (Figure S25b). These results indicate that the long-range ordered structure of higher crystalline g-C18N3-COF could promote the efficiency of separation and transfer of photogenerated charge carriers.30-31 While, for g-C18N3-COF-Low and g-C33N3-COF with the similar crystallinity but different chemical structures, their PL emission behavior are dominated by their intrinsic structural characteristics. The processability for a typical COF powder sample is highly dependent on its morphology, in particular, in micron levels, which has been verified to act as an important role in fully releasing its molecular-level properties in bulk phase or during device performance.32-35 Scanning electron microscopy (SEM) images of g-C18N3-COF revealed its fibrillar morphology with the uniform diameter of approximately 200 nm and length of several microns (Figure 4a and Figure S9), manifesting that it tends to assemble to regular aggregates in the micrometer levels. Given the specific morphology of g-C18N3-COF, its film forming was firstly testified by drop casting of its suspension in n-butanol on indium-tin oxide (ITO) without additive. SEM images from top and side views clearly revealed the resulting film in several micrometer thickness with a uniform porous texture. Such steady macroscopic structure makes it possible to systematically study the semiconducting behaviors of such kind of COF by electrochemical techniques. Firstly, the type of conductivity of this COF was evaluated by capacity measurements in dark together with Mott-Schottky analysis in an aqueous solution of 0.2 M Na2SO4 (pH 6.8). The positive slopes of the plots in Figure 4b suggest that g-C18N3-COF is n-type semiconductor, in which electron is the majority carrier, consistent with its triazine-cored fully π-electron conjugated framework with nonpolar olefin linkages.36 According to the Mott-Schottky equation, the reciprocal squared of the space charge capacity should be proportional to the electrode potential when charge carriers are depleted from the semiconductor surface.37 In our case, the plots of g-C18N3-COF comply with the M-S equation in the applied potential range. Therefore, The flat-band potential (Efb) of this COF film can be fitted to be −0.6 V vs. RHE from the x intercept of the liners region of M-S plots. Considering the conduction band minimum (CBM) of a n-type semiconductor is slightly negative (around 0.1~0.2 eV) than its flat-band potential,38 g-C18N3-COF should have much negative CBM potentials than water reduction potential (0 V vs. RHE), theoretically enables exerting sufficient driving force for the proton reduction from water. The same measurements were also performed on g-C33N3-COF film, resulting in a Mott-Schottky plot with less linear character (Figure S14), which might arise from the presence of much



more defects in g-C33N3-COF, likely associated with its lower crystallinity as compared to g-C18N3-COF. The Efb of g-C33N3-COF film was fitted to be -1.2 V vs. RHE, which is negative than Efb of g-C18N3-COF. Such phenomenon manifested that the presence of much stronger electron-donating triphenylbenzene units for g-C33N3-COF than that of single phenyl units for g-C18N3-COF, could helpfully decline the CBM energy.39 In addition, the charge transfer rates for these COF film samples were measured by electrochemical impedance spectroscopy (EIS) in the dark (Figure 4d). In the Nyquist plot, g-C18N3-COF shows a semicircle with a smaller diameter than g-C33N3-COF, demonstrating the increased interfacial charge transfer rate in the former case.40 Furthermore, linear sweep voltammetry (LSV) was used for characterizing the photoelectrochemical performance of g-C18N3-COF film in the potential range between 1.2 and 0.2 V vs. RHE under dark and light (Figure 4c). As a photoelectrode, g-C18N3-COF film shows an ambipolar photoconduction mode with an onset potential of 0.85 V vs. RHE, reaching currents of up to 45 μA cm-2 at 0.2 V vs RHE. Such value is also larger than the organic

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photocathodes such as an imine-linked COF (BDT-ETTA) (~2.5 µA cm-2) and g-C3N4 (~1-32 µA cm-2).32, 41-44 Notably, at bias potential of 0.4 V vs RHE, the cathodic photocurrent of g-C18N3-COF was observed as ~25 A cm-2, which is nearly three times larger than that of g-C33N3-COF (~8 A cm-2), indicating the more efficient separation of charge carriers in g-C18N3-COF (Figure 4e). The significant difference in these semiconducting properties between the two as-prepared COFs, on the one hand, could be arising from their intrinsic electronic structures, on the other hand, their crystallinities might also exert some influence via the defect ratio over the whole network. The longer conjugated backbone of g-C18N3-COF leads to higher efficiency on photogenerated electron–hole pairs migration and transfer. In addition, a higher crystallinity for g-C18N3-COF seems to more or less decrease the recombination effect of the photogenerated charge carriers at the defects.45-46 Therefore, the g-C18N3-COF outperformed g-C33N3-COF in both photocurrent and EIS measurements. And g-C18N3-COF is expected to give a better photocatalytic activity.

Figure 4. (a) Top view SEM micrograph of g-C18N3-COF film, representing the surface morphology. The inset is cross-section SEM micrograph, showing a uniform film thickness. (b) Mott–Schottky plots of g-C18N3-COF film at 1 kHz, 2 kHz and 3 kHz frequency. The inset is the digital photograph of the g-C18N3-COF film on ITO. (c) Linear sweep voltammograms of g-C18N3-COF film on ITO performed in the dark (black) and under visible light illumination (red). (d) The electrochemical impedance spectroscopy (EIS) Nyquist plots of g-C18N3-COF and g-C33N3-COF film photoelectrodes in dark. (e) Chopped photocurrent density vs. time recorded on g-C18N3-COF and g-C33N3-COF film at 0.4 V vs. RHE. (f) Time course hydrogen evolution using g-C18N3-COF and g-C33N3-COF as catalyst under visible light (λ > 420 nm) irradiation, monitored over 16 h with evacuation every 4 h (dashed line).

Finally, as a proof of concept, the photocatalytic H2 evolution behaviors of g-C18N3-COF and g-C33N3-COF were examined from water reduction in heterogenous catalysis system by visible light irradiation.47-50 Interestingly, the two COF powder samples can be readily dispersed in water by ultrasound sonication, presumably due to their nitrogen-rich backbones and fibrous porous textures, which is beneficial to water wetting and permeation (Figure S21 and S22). In a typical process of photocatalysis, 50 mg of COF powder was added to 1 M aqueous ascorbic acid solution, in which

ascorbic acid was applied as sacrificial electron donor, and 3 wt% platinum was in situ photodeposited on the COF sample to facilitate hydrogen gas elimination from the COF surface.5 Upon visible light illumination, hydrogen gas was detected when both g-C18N3-COF and g-C33N3-COF were used as catalysts. The average hydrogen evolution rate (HER) of g-C18N3-COF was achieved as 14.6 μmol h-1, nearly 4 times faster than that of g-C33N3-COF (3.7 μmol h-1). Besides, the HER of g-C18N3-COF-Low (9.5 μmol h-1) was also achieved, nearly three times of that of g-C33N3-COF (3.7 μmol h-1)


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(Figure S26). Therefore, we can reasonably deduce that the photocatalytic H2 generation of g-C18N3-COF and g-C33N3-COF are mainly influenced by their intrinsic structural characteristics, meanwhile, the crystallinity can facilitate charge transport and hence enhance the HER. The stability of the two COFs as photocatalyst for water reduction was examined by running the experiments for four cycles of total 16 hours (Figure 4f), the overall activity doesn’t show any lost in both cases within the measurement period. In addition, as shown in Figure S16, there is only a slight decay in the crystallinity as observed by PXRD analysis for the g-C18N3-COF before and after the long-term photocatalytic test, which demonstrated that such type of COFs as photocatalyst for water-splitting exhibit moderate durability under a long-term visible light irradiation. The apparent quantum yield (AQY) was measured for the g-C18N3-COF under monochromatic light. The highest AQY obtained for g-C18N3-COF was 1.06% at 420 nm. Moreover, the AQY recorded at 450, 475, 500, and 550 nm was 0.84%, 0.67%, 0.37% and 0%, respectively, revealing the accordance of its photocatalytic activity in H2 evolution with its maximal absorption at different wavelengths (Figure S19). Regard to the fibrillar morphology of g-C18N3-COF, we also tried to evaluate the influence of such micro-size morphology on its photocatalytic activity. After a typical grinding treatment under Ar atmosphere, a grinded sample (termed as g-C18N3-COF-Grinding) mainly consisting of cracked short rods and particles was obtained (Figure S27a and S27b). It is noted that the crystallinity of this grinded sample has no obvious decline as shown in its PXRD pattern (Figure S27c). Under the same measurement conditions, the photocatalytic H2 generation of the grinded sample showed a H2 production rate of 7.5 μmol h-1 (Figure S28), which was distinctly lower than the original fiber-like sample (14.6 μmol h-1). Such phenomenon suggests that morphology in sub/micrometer scale enables exerting influence on photocatalytic water-splitting H2 generation for a COF sample, presumably related to its aggregation behavior, dispersibility and incident light harvesting capability in water. Moreover, given their similar topology and unit cell parameters, the well-known imine-linked COF-LZU1 was prepared for comparison with the as-prepared g-C18N3-COF, on their photocatalytic activities in water-splitting H2 generation. Under the same conditions, COF-LZU1 shows a H2 generation rate of 1.4 μmol h-1, remarkably lower than g-C18N3-COF (14.6 μmol h-1), and g-C33N3-COF (3.7 μmol h-1). Upon exposure to the experimental light source for four hours, the crystallinity for COF-LZU1 was nearly lost (Figure S30). Such phenomenon demonstrated that an unsubstituted olefin-linked COF exhibits much better photocatalytic activity and photostability than imine-linked one. These results also demonstrated that tailoring the molecular knots could influence the semiconducting properties of such kind of unsubstituted olefin-linked COFs, resulting in different photocatalytic activities.

CONCLUSIONS In this work, a Knoevenagel condensation approach to synthesize 2D triazine-cored COFs with unsubstituted olefin linkages was developed. The generalizability of such method is verified by using the linear or trigonal aldehyde monomers, with respect to the formation of two COFs termed as g-C18N3-COF and g-C33N3-COF, respectively. Their

vertically layer-stacked honeycomb-like porous structures with high surface areas are revealed. Both COFs exhibit semiconducting properties including strong visible-light harvesting, charge separation ability and photocatalytic activity. They enable assembling into micro-fibrillar morphologies, allowing for the fabrication of thin films in micrometer thickness without additive. Of them, g-C18N3-COF film showed photocurrent up to ~45 A cm-2 at 0.2 V vs. RHE and rapid interfacial charge transfer. It also exhibits an apparent quantum yield (AQY) of 1.06% at 420 nm for photocatalytic H2 evolution reaction, with a hydrogen production rate of 14.6 μmol h-1 (with respect to 50 mg of COF sample). The semiconducting activities of such kinds of COFs can be finely tailored via the variation of the knots of the skeletons. This work pave a new pathway to construct 2D COFs with chemical stability and -extended backbones, potentially applicable for a wide scope far beyond photocatalysis.

EXPERIMENTAL SECTION All experimental procedures are provided in the Supporting Information.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Details including: synthesis and characterization of materials, fabrication and characterization of photoelectrodes, and the photocatalysis experimental details.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions §S.

W. and F. Z. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Shanghai Committee of Science and Technology for financial support within the project 16JC1400703. Furthermore, this work was financially supported by National Natural Science Foundation of China (21720102002, 21774072, 21574080). Open Project Program of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2019-01, Jilin University). We thank Xinqiu Guo, Qunli Rao and Jie Zhang in the Instrumental Analysis Center of SJTU for their help on the TEM, PXRD and nitrogen physisorption measurements.

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Semiconducting 2D Triazine-Cored Covalent Organic Frameworks

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