Tunable nonradiative recombination dynamics and charge injection of graphene quantum dots for energy conversion applications by controllable functionalization

Understanding the dependence of optoelectronic properties and charge transfer processes on the specified functionalization pattern of the graphene quantum dot (GQD) surface is key to deciphering the photovoltaic and photocatalytic mechanisms. In the present work, the photophysical properties and energy conversion efficiency of OCH3-functionalized GQDs are investigated using first-principle calculations. Furthermore, the nonradiative electron-hole recombination dynamics is analyzed using Fermi's golden rule. Our results show that the OCH3 group has different binding energies on the GQD surface depending on its binding configuration and forms different oxidation patterns of the GQD controlled by the reaction temperature. Both basal and edge oxidation reduce the bandgaps of GQDs due to the electron localization effect, resulting in differing chemical stability. In addition, basal oxidation provides more degrees of freedom with which to tune the wavelengths and oscillator strengths of the low absorption peaks. Although edge oxidation provides a stronger electron-injection driving force from the GQDs into the TiO2 and facilitates charge separation, it also leads to faster nonradiative recombination, which reduces charge separation. Overall, our work reveals a detailed mechanistic picture of energy conversion in oxidized GQDs.