A Hydrazone-based Covalent Organic Framework for Photocatalytic Hydrogen Production

Abstract: Covalent organic frameworks (COFs) have recently emerged as a new generation of porous polymers combining molecular functionality with the robustness and structural definition of crystalline solids. Drawing on the recent development of tailor-made semiconducting COFs, we here report on a new COF capable of visible-light driven hydrogen generation. The COF is based on hydrazone-linked functionalized triazine and phenyl building blocks and adopts a layered structure with a honeycomb-type lattice featuring mesopores of 3.8 nm and the highest surface area among all hydrazone-based COFs reported to date. When illuminated with visible light, the COF continuously produces hydrogen from water without signs of degradation. With their precise molecular organization and modular structure combined with high porosity, photoactive COFs represent well-defined model systems to study and adjust the molecular entities central to the photocatalyic process.

• The paper demonstrates that TFPT-COF produces hydrogen under visible light with rates up to 1970 µmol/h/g using optimized sacrificial donors.
• The paper details a hydrazone-based COF with a high surface area of 1603 m²/g and a hexagonal, crystalline structure that enhances charge transport.
• The paper highlights the COF’s robustness and potential for solar fuel applications by maintaining performance over multiple catalytic cycles with a 3.9% quantum efficiency.

A Hydrazone-based Covalent Organic Framework for Photocatalytic Hydrogen Production

The research paper presents a novel covalent organic framework (COF) designed for photocatalytic hydrogen production under visible light, a significant advancement in the field of organic semiconductor materials. This hydrazone-based COF, referred to as TFPT-COF, is constructed from 1,3,5-tris-(4-formyl-phenyl)triazine (TFPT) and 2,5-diethoxy-terephthalohydrazide (DETH), forming a layered framework with a honeycomb-like structure.

Structural and Chemical Properties

The TFPT-COF exhibits a high surface area of 1603 m²/g, the largest among known hydrazone-based COFs, with mesopores measuring approximately 3.8 nm in diameter. This structural property enhances the COF’s capacity to facilitate photocatalytic reactions by providing ample space for guest molecules. Furthermore, the framework's crystallinity was confirmed via powder X-ray diffraction (PXRD), which indicated a hexagonal arrangement with an interlayer distance optimized to maintain effective π-π stacking interactions necessary for charge transport. The meticulous design enables the hydrazone linkages to provide superior stability against hydrolysis compared to their imine counterparts, while maintaining an effective platform for photocatalysis.

Photocatalytic Performance

Under visible light illumination, the TFPT-COF consistently generates hydrogen from water. The COF achieves a rate of 230 µmol/h/g with sodium ascorbate as the sacrificial electron donor, and up to 1970 µmol/h/g when triethanolamine is used. The quantum efficiency recorded at specific wavelengths was up to 3.9%, demonstrating competitive performance relative to other non-metal-based photocatalysts. Notably, the COF outperforms amorphous g-C3N4 and poly(triazine imide) under similar experimental conditions, with no significant loss in activity observed over three catalytic cycles, highlighting its robustness.

Implications and Future Perspectives

The reported TFPT-COF's ability to consistently produce hydrogen under visible light without degradation suggests its potential for sustainable energy applications, particularly in solar fuel production. Its modular and crystalline nature offers opportunities for further chemical modifications to optimize its photocatalytic efficiency further. The presence of triazine moieties, common in effective photocatalysts such as carbon nitrides, suggests an active role in the photocatalytic mechanism, prompting further investigation into the electronic properties of these frameworks.

Future research could explore the integration of different building blocks to modulate light absorption characteristics and enhance charge carrier dynamics. Additionally, the stability and reusability of the COF in diverse environmental conditions could be studied to improve its practical applicability. The promising results from this study pave the way for developing COFs as customizable, lightweight platforms with significant potential to contribute to the advancement of optoelectronic applications and the efficient capture and conversion of solar energy into chemical fuels.