The Influence of Electron Donors on the Charge Transfer Dynamics of Carbon Nanodots in Photocatalytic Systems

Carbon nanodots are nanosized light-harvesters emerging as sustainable next-generation photosensitizers in photocatalytic reactions. Despite their ever-increasing potential applications, the intricate details underlying their photoexcited charge carrier dynamics are yet to be elucidated. In this study, nitrogen-doped graphitic carbon nanodots (NgCNDs) are selectively excited in the presence of methyl viologen (MV2+, redox mediator) and different electron donors, namely ascorbic acid (AA) and ethylenediaminetetraacetic acid (EDTA). The consequent formation of the methyl viologen radical cation (MV·+) is investigated, and the excited charge carrier dynamics of the photocatalytic system are understood on a 0.1 ps to 1 ms time range, providing spectroscopic evidence of oxidative or reductive quenching mechanisms experienced by optically excited NgCNDs (NgCNDs*) depending on the electron donor implemented. In the presence of AA, NgCNDs* undergo oxidative quenching by MV2+ to form MV·+, but is short-lived due to dehydroascorbic acid, a product of photoinduced hole quenching of oxidized NgCNDs. The EDTA-mediated reductive quenching of NgCNDs* is observed to be at least two orders of magnitude slower due to screening by EDTA-MV2+ complexes, but the MV·+ population is stable due to the irreversibly oxidized EDTA preventing a back reaction. In general, our methodology provides a distinct solution with which to study charge transfer dynamics in photocatalytic systems on an extended time range spanning 10 orders of magnitude. This approach generates a mechanistic understanding to select and develop suitable electron donors to promote photocatalytic reactions.