Manipulating the built-in electric field and spin polarization via C-SO2 doping and C/N vacancies in polymeric carbon nitride: a DFT exploration for photocatalytic toluene oxidation.

Polymeric carbon nitride (PCN) holds great promise for photocatalytic toluene oxidation but its practical application is limited by narrow light absorption, rapid photogenerated carrier recombination, and weak interfacial interaction with toluene molecules. Herein, sulfur (S) doping and carbon/nitrogen (C/N) vacancy engineering were synergistically employed to modify PCN, and the modified systems (denoted as PCN-SO2, PCN-SO2-ND, PCN-SO2-CD, and PCN-SO2-CND) are systematically investigated using density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. S doping into the PCN framework through the formation of sulfur oxide groups (C-SO2) not only induces localized spin polarization but also downshifts the valence band maximum (VBM) energy level, thereby enhancing the photocatalytic oxidation capacity of photogenerated holes. Vacancy engineering effectively reduces the excitation energy of PCN-SO2 from 2.70 eV to 2.29 eV (PCN-SO2-ND) and 2.10 eV (PCN-SO2-CD), respectively. The PCN-SO2-CD and PCN-SO2-CND systems exhibit a polarized built-in electric field (BEF) accompanied by asymmetrical charge distributions, which effectively suppresses the recombination of photogenerated carriers. In addition, benefiting from the synergistic effect of S doping and C/N vacancies, the toluene adsorption energy on PCN-SO2-CND is significantly optimized, decreasing from 0.225 eV for pristine PCN to -0.504 eV, which markedly improves the photocatalytic activation efficiency toward toluene. This work elucidates the synergistic regulatory mechanism of S doping and C/N vacancies in modulating the photocatalytic performance of PCN, offering a solid theoretical foundation for the rational design of PCN-based photocatalysts for efficient toluene oxidation.