An In Situ Film-to-Film Transformation Approach toward Highly Crystalline Covalent Organic Framework Films

Open AccessCCS ChemistryCOMMUNICATION1 May 2022An In Situ Film-to-Film Transformation Approach toward Highly Crystalline Covalent Organic Framework Films Yongkang Lv, Yusen Li, Guang Zhang, Zhongxiang Peng, Long Ye, Yu Chen, Ting Zhang, Guolong Xing and Long Chen Yongkang Lv Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Yusen Li Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Guang Zhang Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Zhongxiang Peng School of Materials Science and Engineering, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Long Ye School of Materials Science and Engineering, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Yu Chen Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Ting Zhang Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Guolong Xing Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author and Long Chen *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Institute of Molecular Plus, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101025 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Fabrication of highly crystalline covalent organic framework (COF) films with tunable thickness and good substrate adaptability remains a substantial challenge. Here, we have developed an effective approach for fabricating crystalline COF films based on elaborately designed bifunctional A2B2 monomers. Typically, amorphous drop-casted monomer [e.g., 1,4-bis(4-formylphenyl)-2,5-bis((4-aminophenyl)ethynyl))benzene (BFBAEB)] films were directly transformed into corresponding highly crystalline BFBAEB-COF film upon in situ vapor-assisted self-polycondensation in high yields (93–97%). The thickness of the BFBAEB-COF film could be modulated readily by varying the monomer concentration. These crystalline COF films could be grown on various substrates, including silicon, indium-doped tin oxide (ITO), glass, and gold. Moreover, such an in situ film-to-film transformation approach has been demonstrated as versatile and applicable to different A2B2 monomers. This work provides a novel pathway toward homogeneous and highly crystalline COF films, representing a key step forward to explore the application of COFs. Download figure Download PowerPoint Introduction Covalent organic frameworks (COFs) are a burgeoning class of porous crystalline polymers, typically obtained as polycrystalline but unprocessable powders.1–4 The predesigned regular structures and ordered open channels enable COFs with abundant tailored functionalities such as efficient charge transfer and proton conduction, endowing COFs as promising candidates for optoelectronics,5–7 energy storage,8,9 and so forth.10,11 It is well known that the film qualities of active layers play vital roles in electronic devices. The insolubilities of COFs in common solvents impede the film fabrication via well-developed wet-processing techniques. In this regard, much effort has been devoted to exploring new approaches to construct COF films such as in situ growth on specific substrates,12,13 interfacial polymerization,14–19 physical/chemical exfoliation,20–23 electrophoretic deposition,24,25 and others.26 Unfortunately, all the aforementioned methods still suffer from respective drawbacks. For instance, the difficulties in film-thickness control, prolonged reaction time, poor adaptability, and low production yield severely encumber the applications of COF films. Thus, green, efficient, and economic approaches for the fabrication of continuous and crystalline COF films are highly desired. Recently, we developed a “two-in-one” molecular design strategy in which a series of A2B2 monomers were utilized to synthesize COFs with excellent reproducibility and solvent adaptability.27–31 Thanks to the good solubilities of such A2B2 monomers and the ideal stoichiometry of the reactive groups, aspiring us to presume that it is possible to transform the A2B2 monomer film into crystalline COF film directly using appropriate protocols like vapor-assisted conversion.32 Herein, we proposed an in situ film-to-film transformation approach to fabricate crystalline COF films. As a proof of concept, a typical A2B2-type monomer: 1,4-bis(4-formylphenyl)-2,5-bis((4-aminophenyl)ethynyl))benzene (BFBAEB) was selected as the precursor on account of its high solubility [∼12.5 mg/mL in N,N-dimethylformamide (DMF)] and feasibility to afford highly crystalline bulk BFBAEB-COF readily.22 The general procedure was presented in Figure 1a and described as follows: BFBAEB was dissolved in DMF (8 mg/mL) to obtain a clear solution; afterward, 100 μL BFBAEB monomer solution was dropped on a clean Si wafer substrate (1 cm × 1 cm). After solvent evaporation under ambient conditions, a uniform yellow-colored BEBAEB monomer film was formed (Figure 1b, left). The resulted monomer film was placed into a 20 mL Schott Duran bottle which contained a 3 mL vial with a mixture of tetrahydrofuran (THF) and 6 M acetic acid (AcOH) (5:1, v/v). Subsequently, the Schott Duran bottle was transferred into an oven and heated at 85 °C for 24 h ( Supporting Information Figures S1, S2). After cooling to room temperature, the resulted dark brown BFBAEB-COF film on the Si wafer was washed thoroughly with DMF, ethanol and finally dried under reduced pressure (the inset picture in Figure 1b, right). Remarkably, almost quantitative monomer-to-COF conversions of 93–97% yields were achieved (see Supporting Information Table S1, Figure S45). Figure 1 | (a) Schematic illustration of the in situ film-to-film transformation approach. (b) Reaction scheme for fabricating BFBAEB-COF film by self-condensation of BFBAEB monomers. The inset pictures denote the film of BFBAEB monomer (left) and BFBAEB-COF film on Si wafer (1 cm × 1 cm, the monomer concentration is 8 mg/mL in DMF). Download figure Download PowerPoint Results and Discussion Fourier transform infrared (FT-IR), confocal Raman, and X-ray photoelectron spectroscopies (XPS) were employed to confirm the chemical composition of the BFBAEB-COF film. As shown in Figure 2a, the FT-IR spectra of BFBAEB-COF film showed a new signal at 1628 cm−1, corresponding to the characteristic stretching mode of –C=N linkages compared with that of BFBAEB monomer. Meanwhile, both the peaks of –C=O (1677 cm−1) and –NH2 (3365–3459 cm−1) stretching in the COF were significantly attenuated compared with those of the monomer. The few signals of amine and aldehyde group residues were attributed to the terminal groups in the frameworks of COFs.27,33 In addition, the IR spectra of the BFBAEB-COF film agreed well with that of BFBAEB-COF bulk powder synthesized by the solvothermal method.27 Raman spectra of the COF film also verified the formation of imine linkage due to the presence of a distinct band at ∼1583 cm−1 ascribed to the imines16,17,33 ( Supporting Information Figure S5). As depicted in Figure 2b, the XPS spectra displayed an N 1s band at 398.1 eV assignable to –C=N bonds, which again confirmed the successful formation of imine linkages in BFBAEB-COF film. Notably, the resulting BFBAEB-COF film was porous, stable, and could not be dissolved in common organic solvents like THF, DMF, which were suitable solvents for BFBAEB monomer ( Supporting Information Figures S7, S46 and S47). Collectively, these results suggested efficient monomer-to-framework conversion. Figure 2 | (a) FT-IR spectra comparison of the monomer and BFBAEB-COF film on Si wafer. (b) The XPS spectra of N 1s of BFBAEB-COF film on Si wafer. (c and d) SEM top views of BFBAEB-COF film on Si wafer. (The COF film was prepared with a monomer concentration of 8 mg/mL in DMF.) Download figure Download PowerPoint The morphology of the BEBAEB-COF film was investigated via scanning electron microscopy (SEM, Figure S8). As displayed in Figures 2c and 2d, the BFBAEB-COF film on Si wafer substrate showcased a homogeneous morphology and continuous coverage, consistent with the results from optical microscopy images ( Supporting Information Figure S7). A series of BFBAEB-COF films with different thicknesses (440 nm to 12 μm, Supporting Information Figure S9) were fabricated on Si wafers (1 cm × 1 cm) by modulating the concentration of BFBAEB (from 0.4 to 8 mg/mL). The cross-sectional SEM images revealed that all the BFBAEB-COF films on the Si wafers were continuous (Figures 3a–3c). Notably, there was almost a linear relationship between the concentration of the monomer and the thickness of the COF film ( Supporting Information Figure S10). Therefore, the thickness of BFBAEB-COF film could be regulated readily by varying the initial concentration of the A2B2-type monomer. Figure 3 | Cross-sectional SEM images of BFBAEB-COF films on Si wafers with a monomer concentration of (a) 8 mg/mL, (b) 4 mg/mL, and (c) 2 mg/mL. (d) Typical GIWAXS pattern of BFBAEB-COF film on Si wafer, prepared with a monomer concentration of 8 mg/mL in DMF. (e) Projections of GIWAXS data of BFBAEB-COF film (monomer concentration: 8 mg/mL in DMF) sets qxy = 0 (pink), BFBAEB monomer film (dark yellow), PXRD data of BFBAEB-COF powder (violet), and simulated AA-Stacking profile (royal blue). Download figure Download PowerPoint Synchrotron radiation-based two-dimensional grazing incidence wide-angle X-ray scattering (2D-GIWAXS) experiments were performed to evaluate the crystallinity and orientational order of the BFBAEB-COF film. As displayed in Figure 3d, the BFBAEB-COF film with a thickness of ∼12 μm shows intense reflections in the 2D-GIWAXS pattern, which discloses a good crystallinity of this film. Additionally, we observed that the intensities of in-plane Bragg peaks were almost identical to those of out-of-plane ones, which indicated that the BFBAEB-COF within the film had almost no preferential orientation. Projections of these datasets near qxy = 0 (Figure 3e) generated the peaks at 0.21, 0.43, 0.65, 0.87, 1.08, and 1.29 Å−1, assignable to the (100), (200), (300), (400), (140), and (610) reflection planes, respectively. The out-of-plane GIWAXS profiles were in good agreement with the results of powder X-ray diffraction (PXRD) of bulk BFBAEB-COF powder and simulated AA-stacking profiles. Moreover, all these BFBAEB-COF films fabricated from different monomer concentrations (0.4–8 mg/mL) manifested similar arc-like GIWAXS patterns ( Supporting Information Figure S14), which demonstrated high crystallinity but no favored orientation of these COF films. The increased intensity of the COF (100) reflection in GIWAXS patterns of the COF films (Figure S16) was probably attributable to the enhanced film thickness. The feasibility of growing COF films on different substrates would enable the practical applications of COF films in diverse fields. In this respect, indium-doped tin oxide(ITO), gold/glass, and glass were used as the substrates to prepare BFBAEB-COF films ( Supporting Information Figure S3). Remarkably, uniform and continuous BFBAEB-COF films could be formed on all substrates, as revealed by SEM images ( Supporting Information Figures S11–S13). Similarly, the thicknesses of the BFBAEB-COF films on these substrates could also be easily tuned by mediating the concentration of the monomer. Furthermore, the GIWAXS pattern and in-house grazing incidence XRD (GI-XRD) results indicated that the BFBAEB-COF films were crystalline with isotropic stacking on these substrates ( Supporting Information Figures S17–S20). However, we found that such a film-to-film transformation approach could not be applied to the conventional co-condensation of two different monomers for COF formation. For instance, when 1,2,4,5-tetra((4-aminophenyl)ethynyl)benzene (TAEB) and 1,2,4,5-tetra(4-formylphenyl)benzene (TFPB) were utilized as the cocondensation monomers, only amorphous oligomer films were afforded under the same conditions as that for the fabrication of BFBAEB-COF film (see Supporting Information Section 6, Figures S42–S44). It might be due to the inhomogeneous and disordered distributions of the two monomers with different solubility (TAEB ∼0.014 mmol/mL in DMF, TFPB ∼0.021 mmol/mL in DMF), for example, segregation of the two materials or aggregation of individual monomers on the substrate. Thus, A2B2 monomers are more advantageous for growing COF films using this in situ transformation strategy. To demonstrate the versatility of this film-to-film transformation approach, another two A2B2-type monomers, viz, 1,4-bis(4-formylphenyl)-2,5-bis(4-aminophenyl)benzene (BFBAB) and 1,6-bis(4-formylphenyl)-3,8-bis(4-aminophenyl)pyrene (BFBAPy) were applied to construct the corresponding COF films (Figures 4a and 4d). As expected, highly crystalline COF films with intense XRD peaks were obtained (Figures 4b, 4c, 4e, and 4f). FT-IR, confocal Raman, and XPS spectra all supported the successful formation of corresponding COF films ( Supporting Information Figures S22–S24 and S33–S35). The SEM and optical microscopy images suggested that the COF films produced were homogeneous with uniform thickness ( Supporting Information Figures S25–S27 and S36–S37). Similarly, the thicknesses of the respective COF films increased with increments in monomer concentrations. The high crystallinity of these COF films was demonstrated by 2D-GIWAXS and GI-XRD results ( Supporting Information Figures S28–S31 and S38–S41). Figure 4 | Reaction scheme for the self-condensation of BFBAPy-COF film (a) and BFBAB-COF film (d). 2D-GIWAXS patterns of BFBAPy-COF film (b) and BFBAB-COF film (e) obtained with a monomer concentration of 4 mg/mL on Si wafer. Projections of GIWAXS data of BFBAPy-COF film (c) and BFBAB-COF film (f) with a monomer concentration of 4 mg/mL on Si wafer. Download figure Download PowerPoint Conclusion An in situ film-to-film transformation approach was developed to fabricate highly crystalline COF films using A2B2 bifunctional monomers. Efficient conversion of the monomer films to corresponding crystalline COF films was realized in high yields (93–97%). Further, the thicknesses of the COF films could be tuned readily by varying the concentration of A2B2 precursors. Moreover, this novel approach is versatile and can be applied to different monomers and substrates such as Si wafer, glass, ITO, and gold. This work highlights a facile, effective, and general method to fabricate uniform and highly crystalline COF films from monomer films, thereby producing cost-effective processing solution techniques like drop-casting, spin-coating, and blade-coating, and so on, and thus, beneficial to explore the application scopes of COF films in the removal of organic pollutant, optoelectronic devices, and many other research fields. Supporting Information Supporting Information is available and includes experiment details and characterization of the monomer, BFBAEB-COF film, BFBAB-COF film, and BFBAPy-COF film. Conflict of Interest The authors declare no competing financial interests. Acknowledgments This work was financially supported by the National Key Research and Development Program of China (grant no. 2017YFA0207500) and the Natural Science Foundation of China (grant no. 51973153). L.Y. and L.C. are grateful to access the beamline 1W1A of the Beijing Synchrotron Radiation Facility (BSRF). References 1. Côté A. P.; Benin A. I.; Ockwig N. W.; O’Keeffe M.; Matzger A. J.; Yaghi O. M.Porous, Crystalline, Covalent Organic Frameworks.Science2005, 310, 1166–1170. Google Scholar 2. Diercks C. S.; Yaghi O. M.The Atom, the Molecule, and the Covalent Organic Framework.Science2017, 355, eaal1585. Google Scholar 3. Huang N.; Wang P.; Jiang D.Covalent Organic Frameworks: A Materials Platform for Structural and Functional Designs.Nat. Rev. Mater.2016, 1, 16068. Google Scholar 4. Jin Y.; Hu Y.; Zhang W.Tessellated Multiporous Two-Dimensional Covalent Organic Frameworks.Nat. Rev. Chem.2017, 1, 0056. Google Scholar 5. 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L.Y. and L.C. are grateful to access the beamline 1W1A of the Beijing Synchrotron Radiation Facility (BSRF). Downloaded 2,189 times PDF DownloadLoading ...