Tuning nonradiative recombination loss by selective oxidation patterns of epoxy groups bound to different sites of graphene quantum dots

A photocatalytic system with a high quantum yield is desirable for large-scale industrial applications such as the photovoltaic splitting of water. Owning to their tunable electronic and optical properties, graphene quantum dots (GQDs) are promising for photocatalytic water splitting. Chemical functionalization modifies the electron distribution in the conjugated pi system, which varies the optoelectronic properties of the GQDs. In the present work, density functional theory (DFT) and linear-response time-dependent DFT (LR-TDDFT) calculations are conducted to study the electronic structures and optical properties of GQDs with selective oxidation patterns of epoxy groups on their surfaces. Furthermore, the relationship between the oxidation patterns and the nonradiative decay rates are studied with Fermi's golden rule approach. The results show that oxidation occurs more easily at the edge than in the core of the GQDs. The different binding configurations of the epoxy groups influence the structural and orbital symmetries of the graphene lattice, resulting in variations in the bandgaps and, consequently, the properties of the excited state. The nonradiative decay rates decrease with the distance from the core of the GQD. In particular, the ortho conformations exhibit faster nonradiative decay rates than the meta conformations, followed by the para conformations. Furthermore, the para conformations at the edge of the GQD demonstrate alternating nonradiative patterns, either accelerating or suppressing the nonradiative electron-hole recombination. The current computational work provides atomic insights for the rational design of photo catalytic systems based on GQD materials.