Photocharging of Colloidal CdS Nanocrystals

The generation of reduced semiconductor nanocrystals is of interest for a variety of optoelectronic applications. In comparison to other nanocrystalline materials, little work has been reported on reduction of CdS nanocrystals, which are particularly interesting for solar photochemistry applications. Most nanocrystal reduction strategies require electron donors that reduce ground state or photoexcited nanocrystals. In this work, we report the discovery of photocharging of CdS nanocrystals under continuous wave illumination with no added reductants. The long-lived reduced states form under illumination, saturate at high concentrations, and recover over time scales of minutes when illumination stops. This process occurs in CdS nanocrystals of different sizes, morphologies, organic surface capping ligands, and multiple solvents but not in CdSe nanocrystals. We propose a charging mechanism in which the photoexcited holes oxidize surface-capping ligands, which then dissociate from the nanocrystal surface. We contrast this ligand-mediated process with solvent-mediated photoreduction that occurs in CdS nanocrystals with polar ligands, which requires hole scavengers.