Electronic Structure Modulation in Sulfur-Doped g-C3N4 Quantum Dots for Enhanced NO2 Sensing

This study explores the adsorption behavior of NO2, NO, and N2O gas molecules on pristine and sulfur-doped g-C3N4 quantum dots using the self-consistent charge density functional tight-binding (SCC-DFTB) method. Sulfur doping at the energetically favorable N-ring site significantly alters the electronic structure, reducing the band gap from 3.58 eV to 1.37 eV and increasing the density of states near the Fermi level. Adsorption analysis reveals that pristine g-C3N4 exhibits weak physisorption toward all gases, whereas S/g-C3N4 demonstrates a transition from physisorption to strong chemisorption, especially for NO2, with adsorption energies as high as −3.543 eV. This strong interaction is associated with significant charge transfer and pronounced band gap narrowing to as low as 0.46 eV. Density of states analysis confirms the formation of hybridized electronic states near the Fermi level, which facilitates enhanced charge transfer and conductivity modulation. Among the studied gases, NO2 shows the most significant electronic response, while NO exhibits moderate interaction and N2O remains weakly adsorbed with negligible electronic perturbation. These findings indicate that sulfur doping plays a critical role in inducing electronic structure modulation and enables highly sensitive and selective NO2 detection in g-C3N4 quantum dots.