Integrating HER and OER active sites in a two-dimensional covalent terephthalaldehyde-based framework for metal-free photocatalytic overall water splitting

Abstract

Two-dimensional covalent organic frameworks (2D COFs), characterised by their exceptional structural stability, tunable architectures, and diverse electronic properties, have emerged as promising metal-free photocatalysts for applications in overall water splitting (OWS). However, most of the reported COFs remain inadequate for OWS applications, primarily due to the absence of coexisting active sites for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on their surfaces. Herein, a novel high-performance 2D covalent terephthalaldehyde-based framework (2D CTF) photocatalyst was rationally designed by periodically integrating a HER-active terephthalaldehyde (TPA) linker with an OER-active 13,15,17-trihydroxy-14,16,18-triaminotriphenylene (TTTP) node into a well-defined 2D network. This modular design not only enables precise integration of catalytically active sites but also induces a spatial separation of the charge density distributions between the conduction band minimum (CBM) and the valence band maximum (VBM), which facilitates efficient intramolecular charge transfer and thus enhances the photocatalytic performance. Moreover, the 2D CTF exhibits excellent mechanical, kinetic and thermodynamic stability, with its synthetic feasibility further validated by the oxidative dehydrogenative cyclisation reaction mechanism. Electronic structure calculations reveal that the 2D CTF is a direct bandgap semiconductor with a bandgap of 2.6 eV and band edge positions matching the redox potential of water. The 2D CTF exhibits outstanding electronic transport properties with a carrier mobility as high as 1.48 × 10³ cm² V⁻¹ s⁻¹. Furthermore, the 2D CTF exhibits strong light absorption and conversion capability, with a light absorption coefficient of up to 1 × 10⁵ cm⁻¹ and a theoretical solar-to-hydrogen (STH) conversion efficiency of up to 12.48%, which enables visible light-driven spontaneous photocatalytic water splitting.