Ligand Displacement Exposes Binding Site Heterogeneity on CdSe Nanocrystal Surfaces

Nanocrystal-ligand interactions and ligand exchange processes are usually described by a uniform distribution of equal binding sites. Here, we analyze this assumption by a quantitative study of the displacement of Z-type cadmium oleate ligands from CdSe nanocrystals by addition of L-type ligands. First, we determined the stoichiometry of the displacement reaction by analyzing the equilibrium upon dilution using solution nuclear magnetic resonance spectroscopy. We found that 1 equiv of tetramethylethylene-1,2-diamine (TMEDA) or 2 equiv of n-butylamine or benzylamine bind to the displaced cadmium oleate. We only reached a comprehensive description of the displacement isotherm by including two types of binding sites with a different equilibrium constant. We corroborated this finding by density functional theory calculations on a CdSe model nanocrystal, which show that even single facets contain a broad variety of binding sites. Finally, we analyzed the thermodynamics of the displacement equilibrium for the weaker binding sites by constructing van't Hoff plots for the different displacers. Whereas displacement with TMEDA appears to be enthalpically neutral, it is entropically favorable. In contrast, displacement with the primary amines is entropically unfavorable but is associated with a negative change in enthalpy. Since the distribution of binding energy emanates from the large fraction of edge and vertex sites on a nanocrystal facet, these findings are most likely inherent to nanocrystals in general and should be considered when analyzing surface reactions on such materials.