The roles of buckled geometry and water environment in the excitonic properties of graphitic C3N4

The exciton plays a crucial role in two-dimensional materials involved in photocatalytic water splitting, where its properties are determined not only by the material itself, but also by the surrounding water environment. By the framework of many-body perturbation theory, we investigated the excitonic effects in pure and water-adsorbed g-C3N4. It is shown that the excitonic properties are very sensitive to the geometry of g-C3N4 and the adsorption of water molecules. Firstly, the optical band gap, i.e. the first bright excitonic energy of pure g-C3N4 decreases remarkably from a high symmetry planar structure (3.8 eV) to a P1 buckled configuration (2.7 eV). Secondly, the hydrogen bonds between water and g-C3N4 induce the generation of interface excitons. With a reduced binding energy (at least 0.2 eV), interface excitons can contribute to a more efficient separation of electrons and holes. Our work provides an insight into the excitation mechanism of 2D photocatalysts in a real environment.