Engineering the excited state of graphitic carbon nitride nanostructures by covalently bonding with graphene quantum dots

Graphitic carbon nitride (CN) materials have drawn remarkable research attention due to their extraordinary optical properties, which are especially promising for metal-free photocatalysis and photoluminescence. Herein we theoretically study the light absorption, electronic, and excitonic characteristics of covalently hybrid structures of CN quantum dots (CNQDs) and graphene quantum dots (GQDs). Density functional theory (DFT) and time-dependent DFT (TD-DFT) reveal that the relative size of CNQDs and GQDs and chemical modification to GQDs or CNQDs surface are critical determining the absorption and photocatalytic/photoluminescent performances of the as-studied structures. Importantly, the distribution position of the photoexcited electron-hole pair is found to depend on the relative size of CNQDs and GQDs, and chemical groups such as epoxy group may lead to distinct exciton distributions in the CNQD-GQD hybrid structures after attaching them to GQD or CNQD surface as compared to the case of pristine GQD and CNQD, indicating a non-negligible influence of unintended chemical reactions to CNQDs and/or GQDs under working conditions on the efficiencies of the materials for photocatalytic and photoluminescent applications.