Mechanisms for Adsorption of Methyl Viologen on CdS Quantum Dots

This paper describes the surface composition-dependent binding of the dichloride salt of methyl viologen (MV2+) to CdS quantum dots (QDs) enriched, to various degrees, with either Cd or S at the surface. The degree of enrichment is controlled synthetically and by postsynthetic dilution of the QDs in their solvent, THF. NMR shows the Cd-enriched QDs to contain a relatively dense (2.8 ligands/nm2) surface layer of oleic acid, in the form of Cd-oleate, and S-enriched QDs to contain relatively sparse (1.0 ligands/nm2) surface density of native ligands containing both oleic acid and octadecene. Electron transfer-mediated photoluminescence quenching of the QDs by MV2+ serves as a probe for the binding affinity of MV2+ for the surfaces of the QDs. Diluting Cd-enriched QDs removes Cd-oleate from the surface, exposing the stoichiometric CdS surface beneath and increasing the quenching efficiency of MV2+, whereas diluting S-enriched QD does not change their surface chemistry or the efficiency with which they are qu...