Direct Synthesis of Ultrathin Crystalline Two-Dimensional Triazine Polymers from Aldoximes

Open AccessCCS ChemistryRESEARCH ARTICLE2 Apr 2024Direct Synthesis of Ultrathin Crystalline Two-Dimensional Triazine Polymers from Aldoximes Jianghong Zhen†, Jichuang Shen†, Tian Sun, Congxu Wang, Pengbo Lyu and Yuxi Xu Jianghong Zhen† School of Engineering, Westlake University, Hangzhou 310024, Zhejiang , Jichuang Shen† School of Engineering, Westlake University, Hangzhou 310024, Zhejiang , Tian Sun School of Engineering, Westlake University, Hangzhou 310024, Zhejiang , Congxu Wang School of Engineering, Westlake University, Hangzhou 310024, Zhejiang , Pengbo Lyu School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, Hunan and Yuxi Xu *Corresponding author: E-mail Address: email protected. School of Engineering, Westlake University, Hangzhou 310024, Zhejiang https://doi.org/10.31635/ccschem.023.202303111 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The efficient synthesis of ultrathin crystalline two-dimensional (2D) polymers with well-defined repeating units is essential to realize their broad applications but remains a great challenge. Herein, we report a new strategy to directly synthesize a series of few-layer 2D triazine-based polymers (2D-TPs) via trimerization reaction of aromatic aldoximes in one step with a high yield of 85% using AlCl3 as catalyst under solvent-free conditions. The obtained 2D-TPs show high crystallinity, a lateral size of several micrometers, an ultrathin thickness less than 2 nm, and good dispersibility and processability. Through semi-in situ and detailed control experiments, we reveal that the 2D polymerization reaction is a two-step process of dehydration and then cyclotrimerization, and AlCl3 acts as not only catalyst but also an in situ generated template for promoting the formation of 2D-TPs. When explored as a new polymeric anode for potassium-ion batteries, the 2D-TP displayed an extraordinary reversible specific capacity of 356 mAh g−1 at 0.05 A g−1, which is among the best performances ever reported, outstanding rate capability (153 mAh g−1 at 1 A g−1), and excellent cycling stability with 95.1% capacity retention after 1000 cycles at 1 A g−1. Download figure Download PowerPoint Introduction Two-dimensional (2D) polymers with atomic thickness and periodic covalent connection of repeat units in the 2D plane1–3 have recently received considerable attention due to their unique molecular network structures and versatile functionalities as well as broad applications in energy storage,4–8 catalysis,9–13 separation and purification,14–17 biomedicine,18–21 and so on. Controllable and efficient synthesis of crystalline 2D polymers with desired structures, properties, and functions at the atomic or molecular level have always been pursued in the chemistry and materials communities.22–25 So far, two main strategies have been developed to prepare 2D polymers: (1) The bottom-up method, the direct polymerization of monomers at a two-phase interface such as air–liquid and liquid–liquid interface, has been mostly used to synthesize ultrathin 2D polymer films.26–30 However, it is difficult to transfer the 2D polymer nanofilms from the interface to other substrates and to construct an interface for large films or to scale up the synthesis. (2) The top-down approach generally includes the physical and chemical exfoliation of pre-synthesized bulk layered framework materials, in which both have been extensively used to obtain 2D polymers by destruction of the interlayer van der Waals force. This well-developed approach still suffers from tedious syntheses, low yield of nanosheets (usually >10%), inhomogeneous thickness and morphology, and decreased crystallinity for practical applications.31–36 In spite of significant progress, preparation of high-quality 2D polymers of single- or few-layer thickness in a direct and scalable way remains a considerable challenge and is highly desired for the basic and applied research of 2D polymers. Herein, we report a new facile and efficient route to synthesize few-layer crystalline 2D triazine-based polymers (2D-TPs) with the lateral size of several micrometers through a one-step reaction from aromatic aldoxime monomers under solvent-free conditions with AlCl3 as the catalyst. We systematically studied the evolution of functional group during the polymerization process and explored the formation mechanism of 2D-TPs, which revealed a two-step reaction process including dehydration of aldoxime monomers and cyclotrimerization of nitrile group. Meanwhile, the AlCl3 turned into abundant nanosheets in situ during the dehydration step. This sheet-like AlCl3 acted not only as a catalyst to efficiently catalyze the 2D cyclotrimerization reaction but also an auxiliary template agent for promoting the surface growth of polymers, which successfully resulted in the formation of ultrathin crystalline 2D-TPs. Through this synthetic approach, we can directly obtain a series of 2D-TPs with a very high yield of 85% from 1,4-benzene dicarboxaldehyde dioxime (BDO), 4,4′-biphenyldicarbaldehyde dioxime (BPDO), and 1,3,5-benzyltricarbaldehyde trioxime (BTO) as the starting monomers. Given the regular pore structure, ultrathin 2D conjugated plane, and abundant triazine active groups in the 2D-TP, we investigated it as a new anode for potassium-ion batteries (PIBs), which delivered an ultrahigh reversible capacity of 356 and 153 mAh g−1 at a current density of 0.05 and 1.0 A g−1, respectively, and maintained an excellent reversible capacity of 339 mAh g−1 after 200 cycles at 0.05 A g−1 and extraordinary capacity retention rate of 95.1% after 1000 cycles at 1.0 A g−1, surpassing most reported polymeric and framework electrode materials. Experimental Methods The detailed preparation of 2D-TPs and control experiments are listed in the Supporting Information. Various characterization methods and results, including powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), nitrogen sorption and desorption analysis, thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), nuclear magnetic resonance (NMR) spectroscopy, and all the computational method details can also be found in the Supporting Information. Results and Discussion In a typical synthesis, the 2D-TP was directly prepared through a one-step polymerization reaction of aromatic aldoxime monomers and AlCl3 as a catalyst without using solvents or any other chemicals, and the yield reached 85% (Figure 1a, for experimental details, see Supporting Information). AlCl3, mostly used in Friedel–Crafts reaction, was selected as catalyst in this study because of its strong Lewis acidity and dehydration property.37,38 With the assistance of AlCl3, triazine was constructed after the dehydration of aromatic aldoximes at 350 °C, and high-quality 2D-TP nanosheets were formed during this procedure. After purification by simple washing, the obtained 2D-TP showed obvious graphene-like nanosheet morphology and high flexibility with an expanded volume much larger than that of bulk covalent triazine framework (CTF) synthesized by the classic ionothermal method (Figure 1b,c and Supporting Information Figure S1).39 The 2D-TP could be well dispersed in a variety of solvents such as N,N-dimethylformamide, ethanol, and water ( Supporting Information Figure S2) and could be further processed into flexible lamellar thin films by the vacuum-filtration-induced assembly method ( Supporting Information Figure S3), which implies its excellent dispersibility and processability. TEM reveals a lateral size of several micrometers and a transparent appearance as well as a crumpled or scrolled feature for the 2D-TP (Figure 1c) indicating an ultrathin thickness and high flexibility of the 2D-TP. Selected area electron diffraction (SAED, inset in Figure 1c) and high-resolution TEM (HRTEM) (Figure 1d) show that sharp hexagonal lattice spots and lattice fringes with the spacing of 1.1 nm corresponding to the (100) plane could be clearly observed in the 2D-TP, consistent with the simulated structural model (Figure 1a) and indicating the high crystallinity of 2D-TP. The AFM image clearly demonstrates a smooth 2D morphology for the 2D-TP, and the thickness of micrometer-size 2D-TP was measured to be ∼1.9 nm (Figure 1e and Supporting Information Figure S4), suggesting a few-layer structure of 2D-TP based on the typical monolayer graphene AFM thickness of 0.6–0.7 nm.40 The PXRD pattern of 2D-TP displays an obvious (100) reflection peak, which is in good agreement with the simulated one for the AA-eclipsed stacking model and further confirms the crystallinity of 2D-TP (Figure 1f and Supporting Information Figure S5). The nitrogen adsorption and desorption isotherm exhibits a type-I characteristic, indicating a microporous structure and a relatively high Brunauer–Emmet–Teller (BET) surface area up to 648 m2 g−1 (Figure 1g) with a narrow pore size distribution centered at 1.1 nm (inset in Figure 1g), in accordance with the theoretical model and previous reports.41,42 These results strongly prove the high structural ordering of 2D-TP. The successful formation of triazine in 2D-TP was further confirmed by FT-IR spectroscopy, NMR spectroscopy, and XPS. FT-IR data reveal that the triazine ring was generated where the O–H signal disappears and a weak –C≡N signal appears (Figure 1h). Moreover, both the resonance peak at 170 ppm in the solid-state 13C NMR spectra and the characteristic binding energy peak at 286.4 and 398.9 eV in the XPS spectra strongly confirmed the formation of triazine through this novel synthetic method ( Supporting Information Figures S6 and S7). TGA showed excellent thermal stability of 2D-TP even up to 600 °C in nitrogen atmosphere, indicating its robust 2D conjugated structure ( Supporting Information Figure S8). Figure 1 | (a) Schematic illustration of the synthetic route. (b) SEM image of 2D-TP (BDO) nanosheets with the inset showing the photograph of fluffy 2D-TP powder. (c) TEM image of flexible 2D-TP (BDO) with the insets showing the photograph of 2D-TP (BDO) dispersion in ethanol (0.1 mg mL−1) and the SAED pattern. (d) HRTEM image of 2D-TP (BDO) with the inset showing the Fourier filtered image of the selected region. (e) AFM image of 2D-TP (BDO) nanosheets. (f) Experimental curve (black circle), Pawley refinement result (red line), the simulated patterns for the AA and AB stacking mode, difference, and Bragg positions of 2D-TP (BDO). (g) N2 adsorption and desorption isotherms of 2D-TP (BDO) measured at 77 K with pore size distribution based on the nonlocal DFT modeling. (h) FT-IR spectra of 2D-TP (BDO) and BDO monomer. Download figure Download PowerPoint To verify the feasibility of AlCl3-catalyzed condensation of aromatic aldoximes into triazine and deeply understand the evolution of functional groups during the polymerization process, a model reaction and semi-in situ experiment were carried out using benzaldehyde oxime as monomer (Figure 2a, for experimental details, see Supporting Information). FT-IR characterization was chosen to monitor the changes of functional groups in the reaction mixture without deliberate purification. When the temperature reaches 200 °C, a clear signal peak appears at 2231 cm−1 accompanied by the decrease of aldoxime signal, which could be attributed to cyano (–CN) formation from the dehydration of aldoxime under the action of AlCl3 (Figure 2b).43 Ultrahigh-performance liquid chromatography mass spectrometry also confirmed the formation of a nitrile intermediate during the reaction ( Supporting Information Figure S9). As the reaction proceeds, the characteristic triazine stretching vibrations at 1508 and 1358 cm−1 appear, and the –CN peak disappears. After reaction, the 2,4,6-triphenyl-1,3,5-triazine compound was isolated as the main product, as confirmed by NMR, FT-IR, and mass spectrometry ( Supporting Information Figures S10 and S11). Therefore, we propose that the cyclotrimerization reaction of aromatic aldoxime catalyzed by AlCl3 is a two-step reaction with aldoxime first dehydrating to form the cyano group and then cyclotrimerization of cyano to form the triazine structure (Figure 2a), which to the best of our knowledge have never been before reported. Figure 2 | (a) Proposed reaction mechanism for the model reaction. (b) FT-IR spectroscopy results of benzaldehyde oxime monomer (yellow line) and reaction intermediates (red and blue line) during the reaction process. Download figure Download PowerPoint To shed light on the formation of 2D-TP nanosheets, we first analyzed the influence of the possible reaction byproducts (Al(OH)3 and Al2O3) of AlCl3 on the polymerization considering the dehydration of monomer in the presence of AlCl3. The experimental results showed that both Al(OH)3 and Al2O3 could not catalyze BDO to obtain any crystalline product ( Supporting Information Figure S12, for experimental details, see Supporting Information). Then we investigated the morphology and composition of products at different stages. In contrast to the original AlCl3 catalyst with an irregular block morphology before reaction (Figure 3b1–b6 and Supporting Information Figure S13a), the unpurified product after reaction showed a 2D sheet-like morphology with uniform distribution of Al, Cl, C, and N elements, and a PXRD pattern comprising the characteristic diffraction peaks of 2D-TP, AlCl3, and Al2O3 (Figure 3c1–c6 and Supporting Information Figure S13b), indicating the morphology and composition of AlCl3 changed during the reaction and the conformal growth of crystalline 2D-TP on the catalyst. After removing the catalyst by simple washing, the purified 2D-TP clearly demonstrates a 2D flexible nanosheet morphology with negligible residue of Al and Cl elements and high crystallinity, as shown in the PXRD pattern (Figure 3d1–d6 and Supporting Information Figures S13c and S14). The effect of AlCl3 on the product morphology was further investigated by directly heating the mixture of AlCl3 with water (the dosage of water was determined by the dehydration of aldoximes monomers) under the same reaction condition, during which the AlCl3 clearly shows sheet-like morphology with some of AlCl3 transformed into Al2O3 ( Supporting Information Figure S15), revealing an unexpected water-assisted structure and morphology evolution of AlCl3. It is worth noting that direct heating of monomers without AlCl3 only yields a small amount of amorphous product (Figure 3d6). Figure 3 | (a) Schematic representation of the synthesis of the 2D-TP nanosheets. (b1–b6) SEM image, elemental mapping, and PXRD pattern of pristine AlCl3 catalyst. (c1–c6) SEM image, elemental mapping, and PXRD pattern of unpurified reaction product. (d1–d6) SEM image, elemental mapping, and PXRD pattern of purified 2D-TP. The PXRD pattern of product obtained by direct reaction of BDO without any catalyst is also shown in panel d6. Download figure Download PowerPoint Consequently, we believe that the AlCl3 not only plays a catalytic role in the 2D cyclotrimerization process, but also acts as an in situ generated template agent for promoting the conformal surface growth of 2D-TP nanosheets (Figure 3a). We also used 1,4-dicyanobenzene as monomer to conduct the same experiment and obtained bulk CTF material without any nanosheet morphology ( Supporting Information Figure S16), which confirms the mechanism described above and demonstrates the importance of using aldoxime monomers. To further investigate the generality of this method, we synthesized two other kinds of 2D-TPs with different molecular structures (Figure 4a, for experimental details, see Supporting Information). The specific triazine signal in the solid-state 13C NMR (Figure 4b,e), FT-IR ( Supporting Information Figure S17), and XPS spectra ( Supporting Information Figure S18) all reveal the successful cyclotrimerization of aldehyde oximes. TEM images show transparent 2D flexible morphology for these two 2D-TPs (Figure 4c,f), and both 2D-TPs exhibit good dispersibility, a lateral size of several micrometers, and an ultrathin thickness less than 2 nm (Figure 4d,g). PXRD, HRTEM, and BET tests show good crystallinity for 2D-TP (BPDO) and 2D-TP (BTO) ( Supporting Information Figure S19). SAED patterns also indicate their hexagonal crystalline lattice structure (inset in Figure 4c,f). TGA further displays good thermal stability ranging from 30 to 800 °C under N2 atmosphere ( Supporting Information Figure S20). These results demonstrate the versatility and effectiveness of our synthetic method. Considering there are many functional aldehyde and aldoxime monomers, we expect more 2D-TPs can be obtained by this method. Figure 4 | (a) Schematic illustration of the synthetic route for 2D-TP (BPDO) and 2D-TP (BTO). Solid-state 13C NMR spectra (b), TEM images with inset showing the SAED patterns and photographs of dispersions in ethanol (0.1 mg mL−1) (c), and AFM images (d) of 2D-TP (BPDO). Solid-state 13C NMR spectra (e), TEM images with inset showing the SAED patterns and photographs of dispersions in ethanol (0.1 mg mL−1) (f), AFM images (g) of 2D-TP (BTO). Download figure Download PowerPoint PIBs have emerged as a promising electrochemical energy storage candidate due to the low cost of potassium and similar redox potential to lithium; however, they still suffer from sluggish kinetics and structural damage in the electrode material resulting from the electrochemical insertion and extraction of K+ ions with larger radius than Na and Li ions.44–51 Given that the 2D-TP combines multiple features including an ultrathin few-layer structure, an in-plane π-conjugated periodic porous structure with a uniform pore size of 1.1 nm, and abundant triazine active groups, we expect the 2D-TP can enable fast transport of electron and K ions to achieve high potassium storage performance. We first theoretically studied the interaction between single- and double-layer 2D-TP and K ions (for computational details, see Supporting Information). Density functional theory (DFT) calculations reveal that both the triazine rings and benzene rings of 2D-TP can function as redox-active sites to adsorb K ions for charge storage, and the maximum adsorption number of potassium ions for each layer per unit cell is up to 6 without destruction of the 2D plane structure (Figure 5a and Supporting Information Figure S21 and Table S1). Therefore, the theoretical capacity for both single- and double-layer 2D-TP can reach as high as 419 mAh g−1, which suggests 2D-TP will be of great potential in PIBs. Next, we experimentally explored the 2D-TP (BDO) as anode for PIBs by assembling a half-cell to evaluate K ions storage performance (for experimental details, see Supporting Information). For better understanding the structure–property relationship, bulk CTF with the same molecular structure and similar porosity was also tested for comparison ( Supporting Information Figures S1 and S22). Figure 5b shows the typical galvanostatic discharge and charge profiles of 2D-TP at different current densities. The 2D-TP shows very high reversible capacity of 353, 295, 253, 212, 175, and 153 mAh g−1 at 0.05, 0.1, 0.2, 0.3, 0.5, and 1.0 A g−1, respectively (Figure 5b,c), and maintains a high capacity of 337 mAh g−1 after 200 cycles at 0.05 A g−1, which significantly surpasses that of bulk CTF (Figure 5c,d and Supporting Information Figure S22). Both electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) tests indicate a much lower charge-transfer resistance and a much faster ion diffusion and thus a substantially improved performance of 2D-TP compared with bulk CTF (Figure 5e,f and Supporting Information Figure S23 and Table S2). Furthermore, the 2D-TP anode demonstrates superior cycling stability with a capacity retention of 95.1% over 1000 cycles at 1.0 A g−1 (Figure 5g). Such excellent electrochemical performance of 2D-TP outperforms those of all recently reported CTF, covalent organic framework (COF), and most organic/polymeric materials (Figure 5h and Supporting Information Tables S3 and S4) and can be ascribed to its ultrathin triazine-rich 2D conjugated porous structure for ultrafast discharge and charge process and full utilization of its active sites (Figure 5i). Figure 5 | (a) Top and side views of adsorption configurations of n (n = 1–6 for each 2D-TP layer) potassium ions on 2D-TP monolayer. (b) Galvanostatic charge and discharge profiles of 2D-TP at different current densities. Rate performance (c), cycle performance at 0.05 A g−1 (d), the EIS spectra (e), and K+ ion diffusion coefficients calculated from the corresponding GITT curves (f) of 2D-TP and bulk CTF. (g) Long cycle performance of 2D-TP at 1.0 A g−1. (h) Comparison of capacity and cycling stability of 2D-TP with other CTF and COF as anodes for PIBs. (i) Schematic diagram of the difference in potassium-ion storage between 2D-TP and bulk CTF. Download figure Download PowerPoint Conclusion We have developed a convenient and general synthetic methodology to directly prepare a series of few-layer crystalline micrometer-size 2D-TPs with a high yield via cyclotrimerization reaction of aromatic aldoximes under solvent-free catalysis conditions. Semi-in situ and detailed controlled experiments revealed this new strategy undergoes a two-step process of dehydration first and then polymerization, and the catalyst plays an unexpected template role in the formation of ultrathin nanosheet morphology in addition to efficiently catalyzing the 2D polymerization reaction. When used as a PIB anode, the obtained 2D-TP exhibited excellent electrochemical performance among the best polymeric/framework electrode materials ever reported. We believe this study provides a versatile route to synthesize a new variety of 2D polymers with tailored structures and properties for various applications. Supporting Information Supporting Information is available and includes details of experiments and characterization methods such as NMR, FT-IR, and SEM. Computational method details and tables can also be found in the Supporting Information. Conflict of Interest The authors declare no competing interests. Funding Information We acknowledge support by the National Natural Science Foundation of China (grant nos. 22022510 and 51873039) and the Science and Technology Innovation Program of Hunan Province (grant no. 2021RC2086). We thank Dr. Xingyu Lu and Dr. Xiaohe Miao from the Instrumentation and Service Center for Molecular Sciences and Physical Sciences at Westlake University for the measurements and data interpretation. We thank Westlake Center for Micro/Nano Fabrication for facility support and technical assistance. 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