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- (gC$_3$N$_4$-t) and heptazine-based (gC$_3$N$_4$-h) forms, as obtained by means of ground- plus excited-state ab initio calculations. By analysing the exciton fine structure, we highlight the presence of dark states and show that the photo-generated electron-hole pairs in gC$_3$N$_4$-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC$_3$N$_4$-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 photo-conversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC$_3$N$_4$-t and gC$_3$N$_4$-h layers which shows a type-II band alignment and looks promising for achieving net charge separation.