Design of Isomeric 2D COFs for Visible Light-Driven Overall Water Splitting: Insights and Electronic Structure Modulation

Two-dimensional covalent organic frameworks (2D COFs) have emerged as promising nonmetal photocatalysts for overall water-splitting (OWS) due to their exceptional crystallinity, large surface area, and versatile chemical architectures. However, achieving visible light-driven photocatalytic OWS with 2D COFs remains challenging. This is partly due to the stringent requirements for band alignment in hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) and limitations in catalytic active sites. This study presents a comparative analysis of isomeric structured 2D COFs, resulting in the design of 12 two-dimensional COF materials. From these, five materials with outstanding visible light-driven OWS performance were identified. Through HSE06 method calculations, these COFs were characterized as semiconductors with tunable band gaps (1.91–3.11 eV), effectively covering the visible light spectrum. Notably, the valence and conduction band positions of five COFs are well-aligned with the redox potentials of H+/H2 and O2/H2O, indicating their excellent potential for efficient OWS. Additionally, COFs with trans configurations demonstrate spatial separation of active sites for hydrogen and oxygen evolution reactions, in which the theoretical energy conversion efficiency of CTF-NS1 can reach 9.23%. This study focuses on the fine-tuning of isomeric configurations in COF photocatalysts, enhancing the theoretical understanding of the structure–property relationship in COF materials.