Publication date: 3rd July 2020
The binding of ligands to nanometer-sized surfaces is a central aspect of II-VI colloidal nanocrystal research, for which CdSe nanocrystals have been used as the main model system to evaluate different surface chemistries. In this work, we revert this approach and analyze the binding of a single ligand to two different materials. In this work, we present a detailed comparison of the binding of a particular Z-type ligand, cadmium oleate (CdOA2), to colloidal CdSe and CdS NCs. We make use of quasi-spherical CdSe and CdS NCs with similar sizes and zinc blende crystal structures. Using solution 1H nuclear magnetic resonance (NMR) spectroscopy, we demonstrate that in both cases, as-synthesized, purified NCs are capped by cadmium oleate. On the other hand, we find that CdS has a significantly higher ligand surface concentration (4.6 nm-2) than CdSe (3.6 nm-2). In both cases, the addition of BuNH2 results in cadmium oleate displacement and the corresponding isotherms point towards binding site heterogeneity. Describing cadmium oleate displacement using a weakly and a strongly binding pool, we find that the weak bindings sites exhibit the same displacement energy for both CdSe and CdS. On the other hand, we find that CdS exhibits a considerably larger fraction of strongly bound ligands. These findings contrast with DFT calculations on NC model systems, which show that the adsorption energy of cadmium oleate is, on average, 32 kJ/mol larger for CdS than for CdSe. To account for this discrepancy, we argue that the NC purification that precedes the displacement analysis removes ligand up to a critical displacement energy threshold. As a result, purified CdSe NCs will have a smaller ligand concentration than CdS NCs, yet the adsorption energy of the weakest binding ligands remaining is identical in both cases. This conclusion highlights the interplay between NC processing and the actually observed surface chemistry of purified NCs.
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