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 well-elucidated – specifically, the interactions of optically excited carbon nanodots with redox-active molecular species such as co-catalysts, mediators, and hole-scavenging additives known as electron donors. In this study, carbon nanodots are selectively excited in the presence of methyl viologen (MV2+, redox mediator) and different electron donors applicable to solar fuel synthesis, 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. AA mediates the fast reduction of MV2+, but the radical cation population is short-lived due to the reversibly oxidized AA quenching MV·+, explaining the limitation in photocatalytic activity when utilizing AA as an electron donor. EDTA-mediated reduction is at least two orders of magnitude slower due to screening by EDTA-MV2+ complexes, but the radical cation 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 can generate a mechanistic understanding to select and develop suitable electron donors to promote photocatalytic reactions.