Mechanisms for Adsorption of Methyl Viologen on CdS Quantum Dots

Abstract

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 quenched by MV2+. The photoluminescence quenching data for all of the surface chemistries we studied fit well to a Langmuir model that accounts for binding of MV2+ through two reaction mechanisms: (i) direct adsorption of MV2+ to exposed stoichiometric CdS surfaces (with an equilibrium adsorption constant of 1.5×10(5) M(-1)), and (ii) adsorption of MV2+ to stoichiometric CdS surfaces upon displacement of weakly bound Cd-oleate complexes (with an equilibrium displacement constant of 3.5×10(3) M(-1)). Ab initio calculations of the binding energy for adsorption of the dichloride salt of MV2+ on Cd- and S-terminated surfaces reveal a substantial preference of MV2+ for S-terminated lattices due to alignment of the positively charged nitrogens on MV2+ with the negatively charged sulfur. These findings suggest a strategy to maximize the adsorption of redox-active molecules in electron transfer-active geometries through synthetic and postsynthetic manipulation of the inorganic surface.

Linking surface chemistry to optical properties of semiconductor nanocrystals

“…85 As discussed earlier, the surface coverage by bound ligands depends on NC concentration. Apart from gaining deeper insights into binding sites 85,86 and surface composition effects, 87 this approach has been used to study photo induced charge transfer 88,89 and energy transfer. An alternative approach to quantify different chemical species on the NC surface is to adsorb a charge and/or energy acceptor to the NC and monitor changes in PL QY, fluorescent lifetimes, and transient absorption spectra.…”

Ligand Surface Chemistry Dictates Light Emission from Nanocrystals

“…In addition, it has been shown that the removal of Z-type ligands from surface chalcogenide sites leads to a higher susceptibility for adventitious adsorbates that form non-radiative trap complexes. (29) In the absence of DFT simulations on this local effect of Z-type ligands, the potential explanations are to some degree speculative. In conclusion, the fact that the removal of Z-type ligands is accompanied by a strong decrease in surface emission suggests that these cadmium species play a strong role in the process of controlling NC surface emission.…”

Diameter Dependent Electron Transfer Kinetics in Semiconductor–Enzyme Complexes

“…Changes in nanoparticle surface area and curvature alters the number of surface binding sites, which has been shown to alter binding densities of nm-sized molecules. 10,25,33,34 To normalize for this effect of increasing CdTe diameter on CaI surface binding, we developed geometric models of the CaI binding to the surface of CdTe. We have previously modeled CdS nanorodÀCaI complex distribution in solution based on a Poisson distribution.…”