Fine-tuning the energy level of defect states in doped ultrathin carbon nitride polymer for enhanced photocatalytic performance

The mechanisms governing photo-induced electron behavior and its link to visible light responsiveness in graphitic carbon nitride (CN), particularly doped or defective variants, remain poorly understood. This study presents a strategy for band structure modulation in carbon-doped CN through isomeric modulation of benzene dicarboxylic acid dopant precursors. Ultraviolet-visible (UV-vis) absorption, photoluminescence (PL) spectroscopy, and density functional theory calculations were employed to determine the energy band structure, while time-resolved spectra probed carrier dynamics across different timescales. Isophthalic acid-doped carbon nitride (IPCN) exhibits a narrowly distributed defect energy level. This manifests in a tailless UV-vis absorption profile and a sharp PL emission peak in the visible region. In contrast, terephthalic acid-doped carbon nitride (PPCN) displays multiple, continuously distributed defect levels. This results in band-edge absorption overlapping with a broad visible spectrum and a broadened PL emission peak, arising from the superposition of band-edge transitions and low-energy absorption tails. The defect levels with continuous distribution in PPCN enhance electrical conductivity but also promotes charge recombination. Conversely, the defect states in IPCN provide a suitable thermodynamic driving force for carrier migration to catalytic active site, in addition to promote spatial charge carrier separation. Consequently, visible-light CO 2 reduction and H 2 evolution activities of carbon-doped CN are markedly enhanced. This work elucidates the synergistic balance between optical absorption efficiency and charge separation performance in doped CN, providing experimental validation for the rational design of CN-based photocatalysts with superior optical and catalytic properties.