Deciphering Carrier Thermodynamics and Kinetics Manipulation in Molecular Engineering of Graphitic Carbon Nitrides for Improved Visible-Light-Driven CO2 Reduction

Abstract

Photoreduction of CO₂ to chemicals is an attractive solution for achieving net-zero carbon emissions. Graphitic carbon nitride (CN) is a promising photocatalyst but still suffers from limited solar spectral response and unfavorable energy deactivation. Herein, by comparing two bipyridine-tailored CN photocatalysts, we decipher how molecular engineering governs the visible-light response, carrier thermodynamics and kinetics, and ultimately, the photocatalytic CO₂ reduction fates. A combination of steady-state and time-resolved photoluminescence as well as transient absorption spectroscopies has been utilized for clarifying the relationship of fine electronic structures and photophysical properties. In optimal CN-dOMe6, introducing 4,4’-dimethoxy-2,2’-bipyridine moiety at the terminal ─NH₂ sites of the heptazine rings activates broadened light absorption, creates beneficial shallow trap states, and exhibits accelerated exciton dissociation and charge transfer within 300 fs. Crucially, these shallow trapped electrons exhibit a long lifetime of 378 ps at a high-energy level while minimizing energy loss through deep trapping, which facilitates efficient injection into the CoII cocatalyst via the coordinated bipyridine bridge. In contrast, the terminal functionalization of 4,4’-diamino-2,2’-bipyridine (CN-dax) accompanies fragmentation of the conjugated heptazine rings, causing detrimental downshifts of the electronic states and a consequent reduction in thermodynamics/kinetics. This precise manipulation of high-energy long-lived shallow trapped carriers endows CN-dOMe6 with a superior CO production rate of 1171 μmol g−1 h−1 under visible light, 400-fold enhancement over pristine CN. This work provides a rational design strategy for controllably introducing shallow defects and demonstrates how to master the synergy between thermodynamics energy levels and kinetics charge lifetime to unlock high photocatalytic performance. Furthermore, it offers a fundamental understanding of the structure-function-activity relationships for the high-performance CN photocatalysts.