Sulfication-induced non-radiative electron-hole recombination dynamics in graphene quantum dots for tuning photocatalytic performance.

GQDs, or graphene quantum dots, are promising materials for energy-related applications. Their optoelectronic properties can be modified by adding heteroatoms, making them good candidates for photocatalysts. However, the structure-property relationship of these materials still needs to be investigated to control their properties better. In particular, photocatalysis of GQDs is hindered by non-radiative electron-hole recombination. In this study, density functional theory (DFT) calculations were performed to investigate the electronic structures and optical properties of GQDs doped with three distinct sulfur functional groups, i.e., sulfur oxide (OHS), sulfhydryl (SH), and thiophene (CHS), respectively. The results suggest that sulfur doping decreases the GQD bandgap. In particular, the asymmetric capping of the GQD edges with the CHS groups led to additional peaks at low excitation energies, whereas for GQDs functionalized with OHS or SH groups, only a shift in the main absorption peak or a change in the absorption intensity was observed. SH functionalization drastically increased electronic coupling, while CHS functionalization induced more charge-relaxation channels in the GQDs. Thus, the results shed light on the mechanisms governing the photocatalytic efficiency of GQDs.