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

doi:10.1016/S1872-2067(26)65181-9

Abstract

Abstract:

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 CO₂ reduction and H₂ 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.

Key words: Ultrathin carbon nitride, Photocatalysis, Defect levels, Defect engineering

Chengzheng Men, Miaoting Huang, Tongming Su, Siwei Liao, Jianying Shi. Fine-tuning the energy level of defect states in doped ultrathin carbon nitride polymer for enhanced photocatalytic performance[J]. Chinese Journal of Catalysis, 2026, 90: 220-230.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65181-9

https://www.cjcatal.com/EN/Y2026/V90/I11/220

Figures/Tables 8

Fig. 1. (a) Schematic illustration of the synthesis process of ultrathin porous CN nanosheets. The buff, yellow, and brown nanosheets represent PTCN, IPCN, and PPCN, respectively. TEM images of 2-PTCN (b,c), 2-IPCN (d,e), and 2-PPCN (f,g). (h) Wide-angle XRD patterns of CN, 2-PTCN, 2-IPCN and 2-PPCN.

Fig. 2. Solid-state 13C NMR (a) and high-resolution N 1s XPS (b) spectra of CN, 2-PTCN, 2-IPCN, and 2-PPCN. Magnified FT-IR (c) and Raman (d) spectra of CN, 2-PTCN, 2-IPCN, and 2-PPCN.

Fig. 3. Time course of photocatalytic H2 production (a) and photocatalytic H2 production rate (b) of CN, x-PTCN, x-IPCN, x-PPCN (x = 1, 2, 3). (c) Schematic diagram system model of the photocatalytic hydrogen production. Time course of photocatalytic CO2 reduction for gas generation (d) and CO and H2 production rate (e) of CN, 2-PTCN, 2-IPCN, 2-PPCN. (f) Schematic diagram system model of photocatalytic CO2 reduction. (g,h) Cyclic experiments of photocatalytic H2 production and CO2 reduction over the 2-IPCN photocatalyst.

Fig. 4. UV-vis spectra (inset: photographs showing the colors of the samples) and PL spectra (excitation wavelength: 340 nm) of CN, x-PTCN (a,d), x-IPCN (b,e), x-PPCN (c,f) (x = 1, 2, and 3).

Fig. 5. DFT-derived frontier orbital and electronic structure maps. HOMO (a,c,e) and LUMO (b,d,f) isosurfaces, band diagrams and corresponding DOS (g-i) for pristine CN and CN with carbon substituted at distinct pyridinic-nitrogen positions.

Fig. 6. Fs-TA spectra of 2-PTCN (a), 2-IPCN (b), and 2-PPCN (c) at different delay times under a 370 nm pump pulses. Kinetic curves at 660 nm for 2-PTCN (d), 2-IPCN (e), and 2-PPCN (f) samples with corresponding exponential fits. The time-resolved fluorescence decay spectra of CN, 2-PTCN, 2-IPCN, and 2-PPCN were presented as follows: excited at 340 nm and monitored at 460 nm (g), excited at 340 nm and monitored at 530 nm (h), and excited at 340 nm and monitored at 600 nm (i).

Fig. 7. Transient photocurrent response plots (a) and EIS, Nyquist plots (b) of CN, 2-PTCN, 2-IPCN, and 2-PPCN. (c) Schematic diagram of the proposed mechanism for the enhanced photocatalytic performance over 2-IPCN. The UV-vis wavelength-dependent AQY of 2-PTCN (d), 2-IPCN (e), and 2-PPCN (f).

Fig. 8. Comparison of photogenerated carrier migration and separation behaviors in carbon-doped carbon nitrides with localized versus delocalized electronic structures.

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