Deciphering carrier thermodynamics and kinetics manipulation in the molecular engineering of graphitic carbon nitrides for improved visible-light-driven CO2 reduction

Abstract

Photoreduction of CO₂ into chemicals is an attractive solution for achieving net-zero carbon emissions. Graphitic carbon nitride (CN) is a promising photocatalyst but still suffers from a 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 rates. A combination of steady-state and time-resolved photoluminescence and transient absorption spectroscopy analyses has been utilized for clarifying the relationship between the fine electronic structures and photophysical properties. In optimal CN-dOMe6, introducing the 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 Co^II cocatalyst via the coordinated bipyridine bridge. In contrast, the terminal functionalization of 4,4′-diamino-2,2′-bipyridine (CN-dax) accompanies the 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 and a 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 thermodynamic energy levels and kinetic charge lifetimes to unlock high photocatalytic performance. Furthermore, it offers a fundamental understanding of the structure–function–activity relationships of high-performance CN photocatalysts.