First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis

In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine-based (gC3N4-t) and heptazine-based (gC3N4-h) forms, as obtained by means of ground-state plus excited-state ab initio calculations. By analyzing the exciton fine structure, we highlight the presence of dark states and show that the photogenerated electron-hole (e-h) pairs in gC3N4-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC3N4-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photoconversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC3N4-t and gC3N4-h layers, which shows a type-II band alignment and looks promising for achieving net charge separation.