Graphitic carbon nitride (g-C 3 N 4 ) is a low-cost photocatalyst with high stability and many applications. However, the photocata- lytic activity of g-C 3 N 4 has been limited due to its low visible light absorption and high electron–hole pair recombination rate. In this study, the effect of S, P, and O doping on the geometrical and electronic properties of the g-C 3 N 4 is investigated using density functional theory (DFT) calculations. The results showed that the surface area increased in the cases of the S- and P-doped systems. Electron density distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular (LUMO), and natural bond orbital (NBO) analyses illustrated that among the S- and O-doped systems, S int - and O int -doped g-C 3 N 4 quantum dots structures had high delocalization. Delocalization of electrons increased after non-metal doping at the edge site. Phosphorus doping at the N edge site led to a stronger donor–acceptor interaction. The most stable configuration was related to doping at the edge (S- and O-doped) and corner carbon (P-doped) sites. The changes in the Gibbs energy are 1.42, -1.15, and -23.03eV, respec- tively. According to the results of time-dependent density-functional theory (TD-DFT) calculations, the S ed - and P C -doped g-C 3 N 4 quantum dots represent a higher light-harvesting ability and the values of light-harvesting efficiency of these structures are 0.10 and 0.031, respectively. Moreover, the S ed -CN and P C -CN have less overlap integral than g-C 3 N 4 quantum dots (CNQDs), while O ed -doped CN has a greater light-harvesting ability (0.14) than that of CNQDs (0.0082). On the basis of the results, non-metal doping increased visible light-harvesting and lifetime of the e − –h + pair, which led to increased photocatalytic molecular descriptors.

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