Promoting Heptazine-Based Graphitic Carbon Nitride Photocatalytic Overall Water Splitting by Effectively Assembling Double-Unit Polymers

Abstract

Replacing the sole heptazine unit of graphitic carbon nitride (g-C 3 N 4 ) with double-unit polymers (DHP) has garnered widespread attention for photocatalytic overall water splitting due to their suitable band alignment, well-photogenerated electron and hole separation, and abundant active sites. However, relying solely on static type II band alignment of building blocks to assemble DHP is inadequate. Herein, we propose a comprehensive "static−dynamic" evaluation strategy for improving the assembly efficiency of DHP including unit selection, band alignment, catalytic site activity, photogenerated electron and hole separation dynamics, carrier lifetime, and photocurrent. A group of DHP is constructed through electron-rich phenyl derivatives and electron-deficient heptazine. Among them, although these frontier orbitals of units can form type II band alignment with each other, only DHP1 and DHP7 achieve spatial separation of the photogenerated electron and hole. Then, based on catalytic activity screening, DHP1 exhibits remarkable bifunctional catalytic performance with a low overpotential of hydrogen (0.05 V) and oxygen (0.59 V). Through further dynamic evaluation, the photogenerated carrier lifetimes of DHP1 can reach up to 15 times compared to the pristine g-C 3 N 4 , owing to the dominant low-frequency phonon modes suppressing strong electron−phonon coupling. Simultaneously, the photocurrent can achieve 9 A/m 2 . The "static−dynamic" strategy offers systematic evaluation for advancing photocatalytic design.