Construction of Heteroporous Metal Covalent Organic Frameworks via Introducing Metal Single Crystal for High-Efficiency Photocatalysis

Open AccessCCS ChemistryRESEARCH ARTICLES6 Aug 2024Construction of Heteroporous Metal Covalent Organic Frameworks via Introducing Metal Single Crystal for High-Efficiency Photocatalysis Cheng-Peng Niu, Rui Zhang, Cheng-Rong Zhang, Tie-Ying Shi, Li-Ling Chen, Yun-Peng Wu, Zhi-Hai Peng, Ru-Ping Liang and Jian-Ding Qiu Cheng-Peng Niu School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Rui Zhang School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Cheng-Rong Zhang School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Tie-Ying Shi School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Li-Ling Chen School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Yun-Peng Wu School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Zhi-Hai Peng School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Ru-Ping Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 and Jian-Ding Qiu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013 Citation: CCS Chemistry. 2024;0:1–11https://doi.org/10.31635/ccschem.024.202404311 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Designing metal covalent organic frameworks (MCOFs) to enhance ion transport and catalysis remains a significant challenge. In this study, a metal single crystal named Bpy-Cu is presynthesized, and subsequently a heteroporous MCOF (Bpy-COF-Cu) is obtained directly by a multicomponent reaction. This approach avoids potential defects associated with postmodification methods and enables more controllable metal loading. Moreover, the structural properties of the framework position the metal catalytic sites uniformly distributed near two mesopores, significantly enhancing the photoelectric performance of imine-linked COFs and enabling synergistic ion transport and catalysis. As a result, Bpy-COF-Cu exhibits ultrafast photocatalytic uranium reduction with a rate constant (k) of 0.063 min−1 without requiring sacrificial reagents. This system achieves exceptionally high removal rates (>99%) in uranium-containing mine wastewater samples due to the recyclable catalysis metal sites. This strategy provides an innovative approach towards designing structurally complex MCOFs. Download figure Download PowerPoint Introduction Covalent organic frameworks (COFs) represent a class of advanced porous crystalline materials connected by stable covalent bonds, integrating various functional units into highly ordered periodic arrays.1,2 Since COFs are composed entirely of light elements (typically H, B, C, N, O, and Si),3 some inherent limitations are imposed on their physical and chemical properties, such as high hydrophobicity, poor photoelectron transport, and catalytic properties.4 Metal organic frameworks (MOFs) combine multiple metal ions and organic linkers to create complex pore structures with synergistic functionality.5 However, the relatively weak strength of the ligand bonds easily restricts their practical applications, especially in harsh environments.6 Therefore, the targeted introduction of metal ions into COFs to form metal COFs (MCOFs) can bridge the gap between COFs and MOFs, thus exhibiting a balance in terms of crystallinity, porosity, stability and tunability.4 In particular, MCOFs show great potential in the field of environmental remediation, such as (MPc)-nDXI-COFs exhibiting excellent sensing properties and sorption for various heavy metal ions,7 and an sp2-c linked Cu-TMT facilitating the reduction of radioactive uranium in water bodies.8 Traditionally, this metalation has been limited to single metal ions attached to multidentate units such as salen,9 porphyrin,10 and bipyridine.11 Among of them, 2,2′-bipyridine-5,5′-diamine (Bpy), a C2-symmetric planar molecule, is one of the most commonly used chelating ligands for the formation of metal complexes in organometallic chemistry, and its derivatives have been used as common building blocks for the development of two-dimensional MCOFs through postsynthesis metallization.12 For instance, Re-COF with AA stacking was successfully procured by metallizing 2,2′-bipyridyl COF with Re(CO)5Cl to enable visible light-driven CO2 reduction.13 Additionally, an imine-linked TpBpy COF can be postmetallized through 2,2′-bipyridine-cobalt(II) coordination, creating a powerful electrocatalyst for water oxidation.14 However, postloading methods typically place the metal sites on the mesoporous walls of COFs, leading to a serious decrease in the crystallinity of the resulting MCOFs and difficulties in controlling the actual metal loading.15 In addition, the design strategy of MCOFs has mainly focused on combining building blocks with matching symmetries, resulting in the creation of various MCOFs with tetragonal, hexagonal or triangular holes.16 Although well-defined pores offer nanoscale channels for selective ion transport and conversion,17 loading metal complexes onto long channel walls tends to increase resistance to the transportation of reactants to the active sites, which hampers adsorption and catalysis applications. The emergence of heteroporous COFs has dramatically enhanced the synergistic utilization of pores.18 Consequently, heteroporous MCOFs are anticipated to solve the above problems, yet their design and synthesis present formidable challenges. Herein, a metallic single crystal (Bpy-Cu) was first presynthesized, and then directly reacted with [1,1′:3′,1″-terphenyl]-3,3″,5,5″-tetracarbaldehyde (TPTCA) and benzidine (BZ) via multicomponent reactions to obtain a heteroporous MCOF (Bpy-COF-Cu) directly. In contrast to Bpy-COF-Cu-PM synthesized by postmodification methods and the metal-free TP-COF-BZ, Bpy-COF-Cu circumvents the defects associated with postmodification and allows for more controllable metal loading. Crucially, the uniform distribution of metal sites confined in micropores markedly enhances the photoelectric performance of Bpy-COF-Cu and enables the recyclable, domain-limited catalysis of uranium under light illumination. Additionally, the mesopores interconnected with the micropores facilitate ion entry into the active sites, while larger pores help reduce diffusion barriers, thereby enhancing transport efficiency. As a result, Bpy-COF-Cu achieved ultrafast photocatalytic uranium reduction (k = 0.063 min−1) without using sacrificial reagents. Notably, Bpy-COF-Cu exhibited an exceptionally high removal rate (>99%) in uranium-containing mine wastewater samples, which is attributed to the synergistic effect of its metal catalytic site and frame structure. This strategy will open avenues for the preparation and application of various complex MCOFs. Experimental Methods The synthesis method of Bpy-Cu CuCl2•2H2O (8.5 mg, 0.05 mmol), Bpy (9.3 mg, 0.05 mmol), a mixture of N,N-dimethylformamide (DMF, 0.25 mL), H2O (0.5 mL), and ethanol (0.75 mL) were heated in a 5 mL glass vial at 100 °C for 72 h and then slowly cooled to room temperature at 5 °C/h. Dark green crystals were collected and dried in air. The synthesis method of TP-COF-BZ A mixture of TPTCA (13.7 mg, 0.04 mmol) and BZ (14.6 mg, 0.08 mmol) in a Pyrex tube, and then o-dichlorobenzene (o-DCB, 0.5 mL), n-BuOH (0.50 mL) and 3 M aqueous acetic acid (0.1 mL) were added sequentially and degassed. The tube was sealed off and heated at 120 °C for 72 h. The product was filtered and washed with H2O, tetrahydrofuran (THF) and DMF, and dried under vacuum at 80 °C to obtain the TP-COF-BZ solid of 91% yield. The synthesis method of Bpy-COF-Cu A mixture of TPTCA (13.7 mg, 0.04 mmol), BZ (10.9 mg, 0.06 mmol), and Bpy-Cu (6.4 mg, 0.02 mmol) in a Pyrex tube, and then o-DCB (0.50 mL), n-BuOH (0.50 mL), and 3 M aqueous acetic acid (0.1 mL) were added sequentially and degassed. The tube was sealed off and heated at 120 °C for 72 h. The product was filtered and then washed with H2O, THF, and DMF, and dried under vacuum at 80 °C to obtain Bpy-COF-Cu solid of 87% yield. The synthesis method of Bpy-COF-Cu-PM A mixture of TPTCA (13.7 mg, 0.04 mmol), BZ (10.9 mg, 0.06), and Bpy (3.7 mg, 0.02 mmol) in a Pyrex tube, and then o-DCB (0.50 mL), n-BuOH (0.50 mL), and 3 M aqueous acetic acid (0.1 mL) were added sequentially and degassed. The tube was sealed off and heated at 120 °C for 72 h. The product was filtered and then washed with H2O, THF, and DMF, and dried under vacuum at 80 °C to obtain TP-BZ-Bpy solid of 90% yield. Then, the obtained TP-BZ-Bpy was treated with a calculated amount of CuCl2•2H2O (3.4 mg, 0.02 mmol) dissolved in 10 mL dry methanol. The solution was stirred for 4 h at room temperature and then washed with copious amount of dry methanol. Thus, the obtained material was then activated using vacuum for overnight at 60 °C. Results and Discussion Material characterization To avoid the limitations of the postmodified metal loading strategy, such as the difficulty in controlling the actual loading, we used 2,2′-bipyridine-5,5′-diamine (Bpy) to react with CuCl2 to preculture Bpy-Cu single crystals.19 In addition, cuprum as the metal connection has many advantages in catalyst applications, including high redox activity, good selectivity, stability and environmentally friendliness.20 As a result, Cu ions and bipyridyl diamine molecules are in the same plane in the single-crystal structure ( Supporting Information Figure S1), while two Cl are separated on the inner and outer sides of the plane. The Cu–N bond length measures 1.981 Å, with the distance between Cu and two pyridine nitrogen atoms at 1.495 Å, establishing the prerequisite for entering the microporous structure of COFs. The energy-dispersive system (EDS) mapping clearly shows four elements including C, N, Cu, and Cl ( Supporting Information Figure S2). Subsequently, a novel heteroporous MCOF (Bpy-COF-Cu) was synthesized via a one-pot method using BZ, Bpy-Cu, and TPTCA as raw materials (Figure 1). In addition, a metal-free COF (TP-COF-BZ) was synthesized by combining TPTCA with BZ. Meanwhile, Bpy-COF-Cu-PM was synthesized via postmodification after the combination of TPTCA, BZ, and Bpy, followed by treatment with CuCl2•2H2O. TP-COF-BZ and Bpy-COF-Cu-PM were compared to assess the respective values of metal introduction and presynthesis. Figure 1 | Schematic of synthetic TP-COF-BZ and Bpy-COF-Cu. Download figure Download PowerPoint To evaluate the crystallinity of materials, powder X-ray diffractometry (PXRD) was used. As shown in Figure 2a, the experimental PXRD pattern of TP-COF-BZ matches well with the simulated AA stacking patterns of triple-pore COF ( Supporting Information Table S1). TP-COF-BZ has a strong peak at 2.8°, and several relatively weak peaks at 5.6°, 9.4°, and 22.5°, belonging to (110), (310), (600), and (001) planes, respectively.21 The PXRD patterns of the pristine TP-COF-BZ and Bpy-COF-Cu confirmed that the overall order of COFs backbone and the degree of crystallinity remained unaffected during the post metalation process.22 However, the crystallinity of Bpy-COF-Cu-PM synthesized by the postmodification method decreased significantly ( Supporting Information Figure S3), underscoring the importance of the presynthesized single crystal in maintaining the crystallinity of COFs. Scanning electron microscopy shows that all COFs are of fiber-spherical morphology ( Supporting Information Figures S4–S6), which is in good agreement with the transmission electron microscopy (TEM) results ( Supporting Information Figures S7 and S8). High-resolution TEM (HRTEM) characterization (Figure 2b,c) confirmed that TP-COF-BZ and Bpy-COF-Cu have similar network structures. To further acquire clearer visualizations of the networks, the regions marked by yellow boxes in Figure 2b,c were recorded respectively, where clear reticular structures with approximately hexagonal pores could be observed. The distance between the two adjacent pore centers is 6.3 ± 0.2 nm, which is in good agreement with the unit cell of a or b obtained by Pawley subdivision of PXRD data, proving that the observed crystal structures match well with the predicted AA stacking model of triple-pore COF, and the introduction of metal ions does not significantly change the network structure. The inset in the upper left corner shows the diffraction patterns corresponding to the two COFs obtained by the fast Fourier transformation (FFT) and shows these single-crystal features.23 Figure 2 | (a) PXRD patterns of TP-COF-BZ with the experimental (red cross), Pawley refined (black line) profiles, the refinement difference (orange line), the eclipsed AA stacking model (deep blue line), the Bragg position (green bar) and the experimental Bpy-COF-Cu (blue line). Inset: eclipsed AA stacking model; HRTEM images of (b) TP-COF-BZ and (c) Bpy-COF-Cu, and the regions marked by the yellow areas at a higher resolution. Upper left: FFT pattern. Download figure Download PowerPoint To analyze the connections and changes of chemical bonds in molecules, Fourier transform infrared spectroscopy (FT-IR) was employed ( Supporting Information Figures S9–S11). In TPTCA, the peak at 1695 cm−1 belonging to the characteristic peak of C=O is greatly weakened, while the strong peak at 3312 cm−1 belonging to the N–H characteristic peak in BZ, Bpy and Bpy-Cu almost disappears.24 In TP-COF-BZ, Bpy-COF-Cu and Bpy-COF-Cu-PM, a typical characteristic peak of C=N bond appears at 1623 cm−1,25 indicating that the reactions were polymerized highly. In addition, the 13C cross-polarization magic-angle spinning nuclear magnetic resonance spectra were measured ( Supporting Information Figure S12). The peaks located at about 158 ppm in both of TP-COF-BZ and Bpy-COF-Cu belong to the characteristic peaks of C=N bond,26 and the peak located at about 153 ppm in Bpy-COF-Cu belongs to the carbon atom attached to pyridine nitrogen. Additionally, the EDS mappings of Bpy-COF-Cu and Bpy-COF-Cu-PM show the Cu and Cl elements ( Supporting Information Figures S13 and S14), where Bpy-COF-Cu is clearer. Furthermore, the X-ray photoelectron spectroscopy (XPS) of Bpy-COF-Cu and Bpy-COF-Cu-PM contain extra Cu 2p and Cl 2p signals compared with TP-COF-BZ (Figure 3a). In addition, the high-resolution XPS spectrum of Bpy-COF-Cu show obvious asymmetry of Cu 2p3/2 and Cu 2p1/2 peaks, indicating that the Cu in Bpy-COF-Cu is a mixed-valence state, and the dominant presence of Cu(I) ions anchored in Bpy-COF-Cu matrix ( Supporting Information Figure S15).27,28 Moreover, following the introduction of metal single crystals into the Bpy-COF-Cu framework, the chlorine shifted towards a lower electric field compared to those of TP-COF-BZ and Bpy-COF-Cu-PM ( Supporting Information Figures S16 and S17), which may be attributed to the interaction of chlorine with the electron-donating hydrogen, leading to an increase in its charge density. Furthermore, the optimized Bpy-COF-Cu microporous structure was analyzed by an independent gradient model (IGM), and a strong interaction between H and Cl can be observed from the obvious multiple noncovalent interactions (the green region) (Figure 3b), which proves that the introduction of chlorine leads to hydrogen-chlorine interactions, thus making the metal side more easily anchored within the microporous structure. The above results indicate that the single crystal was successfully incorporated into the micropores of Bpy-COF-Cu substance, which would not interfere with the fast diffusive transport of ions and facilitate the limited-domain catalysis. Figure 3 | (a) XPS spectra of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM; (b) IGM analyses for the microporous structure of Bpy-COF-Cu (white: H, cyan: C, blue: N, yellow: Cl, purple: Cu. The green part represents the interaction between H and Cl); (c) N2 sorption isotherms of TP-COF-BZ, Bpy-COF-Cu and Bpy-COF-Cu-PM; (d) PSDs of TP-COF-BZ, Bpy-COF-Cu and Bpy-COF-Cu-PM. Download figure Download PowerPoint The surface areas and porosities of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM were measured by N2 adsorption-desorption isotherm at 77 K (Figure 3c), and the Brunauer–Emmett–Teller method calculated their surface areas to be 845.6, 751.9, and 445.3 m2 g−1, respectively. Bpy-COF-Cu retained a high specific surface area, while Bpy-COF-Cu-PM showed a severe decrease, which disfavored the adsorption and transport of targets.29 In addition, the main pore size distributions (PSDs) of TP-COF-BZ and Bpy-COF-Cu are around 2.5 and 4.0 nm (Figure 3d), which match well with their respective theoretical pore sizes ( Supporting Information Figure S18).21 Notably, because the rectangle-like hole is too narrow (about 2.9 Å), ever smaller than the kinetic diameter of N2 (3.64 Å),30 it cannot be probed by PSDs. Compared to TP-COF-BZ, the pore size of Bpy-COF-Cu did not change significantly. However, the PSDs of the synthesized Bpy-COF-Cu-PM are around 2.3 and 3.3 nm. The above changes indicate that the specific surface area was greatly reduced when metal was introduced by the postmodification method, and Cu and Cl were mainly introduced into the two mesoporous pores of Bpy-COF-Cu-PM, rather than the micropores. The introduction of cuprum is chaotic in Bpy-COF-Cu-PM, which is attributed to an uncontrolled orientation of the pyridine nitrogen during the synthesis of its precursor (TP-BZ-Bpy), and such a result is unfavorable for catalysis compared to the uniformly distributed Bpy-COF-Cu of cuprum. To evaluate the physicochemical stability of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM, their crystallinity losses were measured after 24 h of exposure to 1 M HCl, 1 M NaOH, DMF, and intense radiation (200 kGy). The results showed minimal crystallinity loss under these harsh conditions ( Supporting Information Figures S19–S21), which comes from the alternating arrangement of the three kinds of pores in the skeleton and the mutual supportive effect, especially the large number of microporous structures connected with the mesoporous structure playing the role of double stranded pillars, and the two kinds of mesopores can synergistically promote the mass transfer. Meanwhile, thermal gravimetric analysis results show that TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM possess excellent thermal stability ( Supporting Information Figures S22–S24). The high specific surface area, uniform distribution of graded pore size and excellent physicochemical stability will greatly expand its practical application prospects. Photoelectric properties of COFs In general, the introduction of well-defined metal sites can lead to materials with better photoelectric properties.31 Therefore, the optoelectronic properties of the synthesized TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM were systematically investigated. Initially, ultraviolet–visible (UV–vis) spectroscopy was employed to assess their optical properties. As shown in Figure 4a, they exhibit absorbed light across both the ultraviolet and visible regions. Notably, the absorption edge of Bpy-COF-Cu is red-shifted compared to those of TP-COF-BZ and Bpy-COF-Cu-PM, potentially due to the integration of regular Cu-containing microporous structures that enhance light absorption. Meanwhile, the band-gap energies of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM are calculated to be 2.15, 1.21, and 1.99 eV, respectively, using the Kudelka–Munk transform reflection spectra ( Supporting Information Figure S25). This analysis confirms that Bpy-COF-Cu is particularly advantageous for practical photocatalytic applications. To further understand the promotion effect of introducing metallic single crystals on the separation and transport of photogenerated carriers, the photoluminescence (PL) lifetimes of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM were measured (Figure 4b) to be 2.80, 3.71, and 3.04 ns, respectively, fitting well with two exponential components. The longer lifetime observed for Bpy-COF-Cu indicates a further inhibition of e−-h+ recombination. The above results show that the optical properties of imine-linked COFs can be greatly improved by introducing Cu units. Figure 4 | (a) UV–vis diffuse reflection spectra of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM. (b) PL lifetime decay spectra of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM. (c) EIS curves of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM. (d) Photocurrent generation test spectra of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM. Download figure Download PowerPoint Furthermore, the semiconductor behaviors of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM were studied systematically. The results of Mott–Schottky show that all COFs are n-type semiconductors ( Supporting Information Figures S26–S28), which indicates that the electrons are main charge carriers.32 The flat-band potential of Bpy-COF-Cu corresponds to −0.88 V versus reversible hydrogen electrode (RHE), which is much more negative potential than the reduction potential of Cu(II) to Cu(I) (0.16 V),33 indicating a theoretical driving force for the next reaction: UO22+ + Cu+ + 2H2O → U(IV) + Cu2+ + 4OH−.34 Then, the charge transfer properties of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM were studied. As shown in Figure 4c, the results of electrochemical impedance spectroscopy (EIS) analysis demonstrate that Bpy-COF-Cu exhibits lower electron transfer resistance, indicating that the introduction of cuprum can facilitate charge transfer. Meanwhile, their photocurrent intensities were assessed to evaluate the photoelectric properties, and the results reveal that Bpy-COF-Cu displays a much stronger photocurrent response compared to TP-COF-BZ and Bpy-COF-Cu-PM, suggesting a higher current density (Figure 4d). This may be attributed to the Cu unit containing a microporous structure making it easier for charge separation. In particular, the photocurrent of Bpy-COF-Cu decays slowly, rather than disappearing immediately after stopping illumination. So, it has the "electronic sponge" property that can store and transport electrons, which is also advantageous for photocatalytic reactions.35,36 To further investigate the advantages of presynthesized metal single crystals to synthesize Bpy-COF-Cu, the Cu content was measured after the complete dissolution of Bpy-COF-Cu and Bpy-COF-Cu-PM. The results indicate that the actual cuprum load of Bpy-COF-Cu is consistent with the theoretical value ( Supporting Information Table S2), while the discrepancy in Bpy-COF-Cu-PM is considerable. This suggests that the controllable metal load is another key factor in significantly enhancing the photoelectric performance of COFs, because the presence of cuprum can notably facilitate electron transport for improving the charge separation efficiency.37 In addition, the distinct units of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM (named TP-BZ, TP-Bpy-Cu, and TP-Bpy-Cu-PM, respectively) were chosen for quantum chemical calculations using density-functional theory (DFT). The results indicate that the formation of fragments narrows the bandgap compared to the monomers ( Supporting Information Figure S29), with TP-Bpy-Cu exhibiting a narrower bandgap than TP-BZ and TP-Bpy-Cu-PM, consistent with experimental evidences. These findings underscore the excellent photoelectric activity of Bpy-COF-Cu, positioning it as a potential candidate material in the field of environmental photocatalysis. Photocatalysis of uranium investigations The hierarchical porosity and the introduction of cuprum single crystals into Bpy-COF-Cu enhances ion transport and improves its excellent photoelectron properties, making it an ideal choice for photocatalytic reduction uranium. To assess its photocatalytic reduction performance of uranium, the effects of pH value ranging from 1.0 to 5.0 were initially investigated, and the optimal performance was observed at pH 4.0 under light ( Supporting Information Figure S30). The decline in removal rate at pH 5.0 may be attributed to the presence of different forms of uranium.38 Furthermore, their behaviors in photocatalytic reduction uranium were systematically investigated in Bpy-COF-Cu compared to TP-COF-BZ and Bpy-COF-Cu-PM. As shown in Figure 5a, the uranium content remained relatively stable during the blank experiment, ruling out the possibility of uranium self-photolysis. Interestingly, the uranium concentration in the reaction system decreased rapidly after 60 min of light irradiation, and the reduction efficiencies of TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM on uranium were found to be 70.2%, 97.7%, and 82.1%, respectively. Based on the pseudo-first-order kinetic model, the corresponding reduction rate constants (k) were calculated to be 0.020, 0.063, and 0.028 min−1, respectively (Figure 5b). Compared to most photocatalysts for removal uranium pollutants, Bpy-COF-Cu is much faster ( Supporting Information Table S3), and it is more environmentally friendly as it does not require a sacrificial agent, which may be attributable to the uniform distribution of cuprum single crystals to separate electrons and holes more efficiently. In addition, the measurement of photocatalytic uranium reduction cycling experiment indicated that Bpy-COF-Cu did not show a significant reduction in the uranium removal rate after five cycles ( Supporting Information Figure S31), proving its high photochemical stability, which is cross-checked with PXRD and FT-IR ( Supporting Information Figures S32 and S33). The results indicate that the enhancement of the photocatalytic activity of Bpy-COF-Cu is closely related to the introduction of regular cuprum active sites, which may be attributed to a synergistic effect involving several factors: the hierarchical pore structure facilitates the rapid transport of UO22+, the introduction of metal single crystals expands the light absorption range and promotes charge separation and the formation of domain-limited catalytic active sites boosts catalytic efficiency. Figure 5 | (a) The removal rate of UO22+ on TP-COF-BZ, Bpy-COF-Cu, and Bpy-COF-Cu-PM (pH 4.0, C0 = 50 ppm); (b) the corresponding pseudo-first-order rate constant (k) of UO22+ reduction; (c) the removal rate of UO22+ on Bpy-COF-Cu under high competitive cations (C0 of UO22+ is 50 ppm, other ions are 10 times than UO22+), mixed liquids, simulated liquids and two uranium mine wastewater samples. Error bars represent S.D. n = 3 independent experiments. Download figure Download PowerPoint In general, uranium is often found in environments with high levels of interfering ions, making its selective capture from a complex environment particularly challenging.39–43 Therefore, to explore the practical application capability of Bpy-COF-Cu, several coexistent ions were selected for an anti-interference test.44–46 As shown in Figure 5c, under highly competitive cations (the initial concentration of uranium is 50 ppm, and other competitive ions are 500 ppm), Bpy-COF-Cu showed an excellent removal rate of uranium. Especially in actual uranium-containing wastewater samples ( Supporting Information Tables S4 and S5), Bpy-COF-Cu demonstrated an exceptionally high uranium removal rate, exceeding 99%. Even in the presence of mixed competitive ions, Bpy-COF-Cu maintained a uranium removal rate greater than 90%. The above results demonstrate that Bpy-COF-Cu has significant usefulness for practical application. Mechanism of photocatalytic uranium reduction The interactions of Bpy-COF-Cu and Bpy-COF-Cu-PM with uranium before and after illumination were investigated by XPS, respectively. As shown in Figure 6a, the U 4f peaks appear clearly in Bpy-COF-Cu and Bpy-COF-Cu-PM after photocatalysis, indicating that a large amount of uranium is loaded.47 Meanwhile, the more obvious U 4f peaks of Bpy-COF-Cu compared to Bpy-COF-Cu-PM proved that the introduction of regular cuprum single crystal can enhance the photocatalytic reduction ability of uranium. Moreover, the high-reso