Spectroelectrochemical Measurement of Colloidal Cadmium Chalcogenide Nanocrystal Surface Reduction: Potentials, Anion Dissociation, and Speciation

Reduction of surface metal cations of colloidal semiconductor nanocrystals (NCs) has been related to both material stability and photoinduced charge transfer reactions, but the molecular details governing variation in reduction potentials and products between different NC samples have not been studied. Here, infrared (IR) spectroelectrochemistry was used to monitor carboxylate ligand dissociation upon electrochemical reduction of surface metal cadmium cations of oleate-capped cadmium chalcogenide nanomaterials (CdS, CdSe, and CdTe NCs, and CdSe nanoplatelets). While the measured reduction potentials were relatively insensitive to nanocrystal size, they varied with different chalcogenide coordination of the cadmium centers and the overall surface stoichiometry. A combination of IR and NMR spectroscopies and mass spectrometry showed that the speciation of the dissociated carboxylate species upon surface reduction is dependent on the reaction conditions; for samples with cadmium carboxylate-enriched surfaces, high NC concentrations, or lithium electrolyte, dissociation of anionic cadmium carboxylate complexes competes with dissociation of free carboxylate anions, particularly for CdS and CdTe NCs, whose cadmium carboxylate moieties are more readily displaced. These results suggest different factors for controlling redox-induced etching processes and for improving nanomaterial stability.