  Photocatalysts are crucial materials for green energy production and
environmental purification. Non-metal doped graphitic carbon nitride (g-C3N4)
has attracted much attention in recent years because of low-cost and desired
photocatalytic performance characteristics such as high charge separation
efficiency and broad visible light absorption. In this study, we used
time-dependent density functional theory and wave function analysis to evaluate
the charge separation characteristics of phosphorus-, oxygen- and sulfur-doped
g-C3N4 upon photon excitation based on electron-hole pair distances,
electron-hole pair overlaps, and charge transfer amounts. The lowest unoccupied
molecular orbital of doped heptazine rings was shifted downward to facilitate
electron transfer from undoped to doped heptazine rings upon photon excitation.
Generally, the phosphorus dopant yielded relatively high charge transfer, high
electron-hole pair separations, and low electron-hole overlap compared with the
oxygen and sulfur dopants. In a low concentration, the sulfur dopant had the
similar performance as the phosphorus dopant did. Furthermore, the phosphorus
dopant attracted photon excited electrons, whereas the oxygen and sulfur
dopants contributed to electron-hole pair separation. The dopants concentrated
in one heptazine ring exhibited high charge separation performance than those
dopants distributed among multiple heptazine rings in g-C3N4; the simulation
results suggested that doping configurations are more crucial than doping
concentrations with respect to charge separation efficiency in nonmetal-doped
g-C3N4.
