In‐Plane and Out‐of‐Plane Polarities Synergistically Boost Photocatalytic Overall Water Splitting on Two‐Dimensional Covalent Organic Frameworks

Covalent organic frameworks (COFs) are ideal for overall photocatalytic water splitting (OWS) due to modular design, ordered π-conjugated structures, and tunable optoelectronic properties. However, weak driving force and rapid photogenerated carrier recombination limit their performance. Using two experimentally synthesized COFs (TTA-BTPA, TAPB-BTPA) as models, we propose 2D COFs with in-plane/out-of-plane dual polarities as an effective solution. First-principles calculations show imine bond-induced in-plane polarity and vertical asymmetry-derived out-of-plane polarity split bilayer energy levels, forming dual type-II band alignment. This synergy enhances intra/inter-layer charge separation, suppressing carrier recombination compared to in-plane-only monolayers. The built-in electric field boosts the driving force by reducing the hydrogen reduction potential. Nonadiabatic molecular dynamics (NAMD) confirms a dual type-II mechanism, and the calculated TTA-BTPA bilayer's carrier lifetime (37.57 ns) is 6.61 times that of its monolayer (5.69 ns). Both bilayers exhibit excellent visible-light absorption, split pure water without external bias, and achieve the theoretical up-limit of solar-to-hydrogen (STH) efficiency >10%. Our work elucidates polarity synergy's role and provides theoretical guidance for high-performance 2D COF-based OWS photocatalysts.