Two-Dimensional COF with Rather Low Exciton Binding Energies Comparable to 3D Inorganic Semiconductors in the Visible Range for Water Splitting

Many kinds of modification techniques have been developed to improve the quantum efficiency of the graphitic carbon nitride (CN) for photocatalytic water splitting in recent years. In this work, we theoretically propose to incorporate polyyne (-C C-)(n) between the heptazine ring and the tertiary amino group of CN to form covalent organic frameworks (COFs) as a new strategy to further enhance the photocatalytic ability of CN. These COFs allow for a finetuning of their electronic and optical properties by altering the quantity of C C bonds. They could fulfill the three ultimate goals of an ideal photocatalyst for water splitting, i.e., broad optical absorption, overall water splitting, and low exciton binding energy. Most strikingly, in the visible region, these COFs possess a lot of strong dipole-allowed exciton transitions whose binding energies are just at the level of a few tens of meV and whose electron and hole are well separated in space by similar to 10 nm. This extremely low exciton binding energy is of the same magnitude as that of many three-dimensional inorganic photocatalysts, which is an important characteristic that distinguishes from the conventional two-dimensional semiconductors. Functionalization of the C C bonds in the linkers of COFs can further enhance the driving force for water oxidation.