Intermediate Formation of Macrocycles for Efficient Crystallization of 2D Covalent Organic Frameworks with Enhanced Photocatalytic Hydrogen Evolution

Abstract

Covalent organic frameworks (COFs) have gained significant attention as key photocatalysts for efficient solar light conversion into hydrogen production. Unfortunately, the harsh synthetic conditions and intricate growth process required to obtain highly crystalline COFs greatly hinder their practical application. Herein, we report a simple strategy for the efficient crystallization of 2D COFs based on the intermediate formation of hexagonal macrocycles. Mechanistic investigation suggests that the use of 2,4,6-triformyl resorcinol (TFR) as the asymmetrical aldehyde build block allows the equilibration between irreversible enol-to-keto tautomerization and dynamic imine bonds to produce the hexagonal β-ketoenamine-linked macrocycles, the formation of which could provide COFs with high crystallinity in half hour. We show that COF-935 with 3 wt % Pt as cocatalyst exhibit a high hydrogen evolution rate of 67.55 mmol g À 1 h À 1 for water splitting when exposed to visible light. More importantly, COF-935 exhibits an average hydrogen evolution rate of 19.80 mmol g À 1 h À 1 even at a low loading of only 0.1 wt % Pt, which is a significant breakthrough in this field. This strategy would provide valuable insights into the design of highly crystalline COFs as efficient organic semiconductor photocatalysts.

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“…High-resolution transmission electron microscopy (HRTEM) further revealed that TADI-COF and TADI-COF-Fc nanoparticles are both generated by the aggregation of small, thin flakes with sizes of approximately 15 nm. This finding was consistent with the coherent scattering domain size calculated from the PXRD patterns using the Debye–Scherrer equation . Lattice-resolution HRTEM images were obtained and subjected to fast Fourier transform (FFT) and band-pass filtering analysis, which showed that TADI-COF and TADI-COF-Fc exhibited interplanar spacings of 3.1 and 3.2 nm, respectively.…”

“…This finding was consistent with the coherent scattering domain size calculated from the PXRD patterns using the Debye−Scherrer equation. 67 Lattice-resolution HRTEM images were obtained and subjected to fast Fourier transform (FFT) and band-pass filtering analysis, which showed that TADI-COF and TADI-COF-Fc exhibited interplanar spacings of 3.1 and 3.2 nm, respectively. These values are consistent with the (100) interplanar spacing (3.2 nm).…”

Recent progress on covalent organic frameworks for photocatalytic hydrogen generationviawater splitting

“…24). 143 Notably, under visible light irradiation with AA as the SED, 3 wt% Pt-loaded COF-935 showed excellent photocatalytic H 2 evolution activity up to 67.55 mmol g −1 h −1 . The effect of different Pt loadings on hydrogen production was further investigated.…”

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“…For instance, high-crystalline COF-935 was rapidly synthesized based on the formation of hexagonal intermediates (Figure 5b). 36 The dynamic imine bond helped to maintain the reversibility of COF formation, which provided an opportunity for the modification and reconstruction of the framework. As a result, when it was exposed to visible light, COF-935 exhibited extremely high hydrogen evolution up to 67.55 mmol • g −1 • h −1 with 3 wt% Pt as a cocatalyst.…”