Underpotential Deposition of Cadmium on Colloidal CdSe Quantum Dots: Effect of Particle Size and Surface Ligands

Electrochemistry of quantum dots (QDs) can provide useful information on their redox behavior and energy states. Here, we present a study of the surface-limited electrochemical deposition of cadmium adatoms (underpotential deposition or upd) on electro-phoretically formed films of CdSe QDs of different sizes (2.4-6.3 nm) capped with different ligands (oleate and sulfide). Oleate-capped QD films were shown to have a weak electrochemical response in upd reaction, whereas sulfide-capped QDs demonstrated a significant increase in cathodic current that was attributed to enhanced surface accessibility and improved interparticle electron transfer. Cadmium upd onset potential in sulfide-treated QD films was found to be markedly size dependent. The increase of QD size results in the positive shift of upd onset potential which is in agreement with the change in lowest unoccupied molecular orbital position. The results imply that the proposed method utilizing electrochemical surface-limited reaction on QDs can be applied to probe energy states of chalcogenide semiconductor nanoparticles.