The Relationship between Surface Defectivity of CdSe Quantum Dots and Interfacial Charge Transfer Studied by Cyclic Voltammetry

Charge transfer from semiconductor quantum dots (QDs) to redox-active mediators is poised to drive transformative photocatalytic processes; however, these reactions are often gated by charge transfer across the donor–acceptor interface. Defect sites that trap charge carriers at the QD surface can participate in interfacial charge transfer processes, influencing observed kinetics. These defect sites can accelerate or decelerate charge transfer depending on their identity and energetics. However, there are no correlations between the identity of the defect and the observed kinetics of an interfacial charge transfer reaction. In this work, we quantify the rate of interfacial charge transfer from a series of CdSe QDs that have had their surfaces modified through well-defined chemical transformations to various redox mediators using cyclic voltammetry. Hole-trapping defects are systematically introduced by stripping native Z-type ligands to expose undercoordinated Se^2– ions. QDs with a high concentration of hole-trapping selenium-based defects exhibit neither enhancement nor impediment on the rate of charge transfer. However, these Se^2– ions are readily oxidized by mild oxidants to form unique electron trap states. These electron trap states significantly enhance electron transfer, as evidenced by dramatic distortion of cyclic voltammograms. Ultimately, this work highlights the sensitivity of the QD surface and its proclivity to undergo surface transformations. A symbiotic understanding of the QD materials and redox-active species is necessary to facilitate, and ideally accelerate, charge transfer reactions.