Two-Dimensional Covalent Heptazine-Based Framework Enables Highly Photocatalytic Performance for Overall Water Splitting.

Screening high-efficiency 2D conjugated polymers toward visible-light-driven overall water splitting (OWS) is one of the most promising but challenging research directions to realize solar-to-hydrogen (STH) energy conversion and storage. "Mystery molecule" heptazine is an intriguing hydrogen evolution reaction (HER) building block. By covalently linking with the electron-rich alkynyl and phenyl oxygen evolution reaction (OER) active units, 10 experimentally feasible 2D covalent heptazine-based frameworks (CHFs) are constructed and screened four promising visible-light-driven OWS photocatalysts, which are linked by p-phenyl (CHF-4), p-phenylenediynyl (CHF-7), m-phenylenediynyl (CHF-8), and phenyltriynyl (CHF-9), respectively. Their HER and OER active sites achieve completely spatially separated, where HER active sites focus on heptazine units and OER active sites located on alkynyl or phenyl units. Their lower overpotentials allow them to spontaneously trigger the surface OWS reaction under their own light-induced bias without using any sacrificial agents and cocatalysts. Among them, CHF-7 shows the best photocatalytic performance with an ideal STH energy conversion efficiency estimated at 12.04%, indicating that it is a promising photocatalyst for industrial OWS. This work not only provides an innovative idea for the exploration of novel polymer photocatalysts for OWS but also supplies a direction for the development of heptazine derivatives.