Suppressing the excitonic effect and reducing the overpotential in narrow band gap covalent organic frameworks of Lieb lattice to enable metal-free photocatalytic overall water splitting

Two-dimensional (2D) layered covalent organic frameworks (COFs) with in-plane conjugation and out-of-plane π-stacked structures have emerged as promising organic photocatalysts for their highly designable skeleton, tunable electronic properties, and inherent pores. Currently, wide band gap COFs are often applied for photocatalytic water splitting to surpass the large kinetic barrier of chemical reactions, which is adverse to solar light harvest and seriously impedes the promotion of energy conversion efficiency. In this work, we propose to exploit narrow band gap COFs as photocatalysts and devote to reduce the overpotential of redox reactions by engineering active catalytic sites on them. By first-principles calculations, we design nine fully conjugated COFs of Lieb lattice with band gaps tunable from 1.72 eV to 1.00 eV, which show broad visible and near-infrared (NIR) light absorption. Further, we unravel that the enhanced optical absorption is accompanied by the suppressed excitonic effect because the narrowed band gap has produced increased dielectric screening within the electron-hole pair. Interestingly, a pre-designed hydrogen bond can significantly reduce the overpotential of hydrogen evolution reaction, solving the dilemma of insufficient driving force posed by the narrow band gap. Finally, we demonstrate that a tandem system based on two of our designed COFs are capable of metal-free overall water splitting under visible and NIR light. Our findings highlight a new route to realize metal-free photocatalysis with COFs and pave the way for the rational design of organic photocatalysts with high energy conversion efficiency.