Sulfur-doped g-C₃N₄ nanosheets for efficient photocatalytic degradation of methylene blue

In this study, sulfur-modified graphitic carbon nitride (S-g-C₃N₄) nanosheets were prepared through a two-step route combining thermal pyrolysis of thiourea/urea mixtures and ultrasound-assisted liquid-phase exfoliation. By varying the thiourea-to-urea molar ratio across five compositions, we show that sulfur incorporation can be used to tune simultaneously and in a controlled manner the structural, optical, and charge-transport characteristics of the resulting materials. The samples were characterized using XRD, FTIR, AFM, TGA, UV–vis diffuse reflectance, steady-state and time-resolved photoluminescence, and electrochemical impedance spectroscopy. First-principles DFT calculations (PDOS and ELF) provided additional insight into how sulfur substitution alters the electronic structure at the atomic level. Of the compositions tested, T50U50 stood out clearly: it degraded 97% of methylene blue within 150 min under visible light, with a pseudo-first-order rate constant of 0.0093 min −1 . This behavior is tied to a narrowed bandgap (2.50 eV), reduced charge recombination, and a notably low interfacial charge-transfer resistance (751 Ω), trends consistently reflected in the PL, TRPL, and EIS data. Taken together, the results suggest that adjusting the thiourea/urea ratio is a straightforward and low-cost lever for optimizing g-C₃N₄ nanosheet performance in visible-light-driven environmental remediation. • Sulfur-doped g-C₃N₄ nanosheets was successfully prepared via pyrolysis and liquid exfoliation. • Sulfur doping resulted in 97% degradation of MB within 150 min. • DFT calculations indicated a reduced bandgap following the addition of sulfur. • Experiments demonstrated improved charge transfer and less electron–hole recombination, resulting in increased photocatalytic efficiency.