Mapping the Topology of PbS Nanocrystals through Displacement Isotherms of Surface-Bound Metal Oleate Complexes

PbS nanocrystals are predicted to undergo size-dependent changes in shape, transitioning from octahedral to cuboctahedral morphology as the nanocrystal size increases. Experimental evidence for size-dependent topology is inferred from comparison of X-ray photoelectron spectroscopic measurements of Pb:S ratios with ideal morphologies or from Wulff ratios of nanocrystals extracted from high-resolution transmission electron microscopy images. Utilizing L-type ligand promoted Z-type ligand displacement as a tool to probe surface composition, we correlate the number and reactivity of Z-type moieties with the surface structures of two different sizes of PbS NCs predicted to display distinct shapes (2.8 and 3.9 nm). Titration of N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA) into oleate-capped PbS nanocrystals liberates (kappa(2)-TMEDA)Pb(OA)(2) (OA = oleate), which is monitored via nuclear magnetic resonance (NMR) spectroscopy. The stoichiometry of the displaced Z-type complex is verified by combining oleate integrations from H-1 NMR spectra with Pb quantitation from inductively coupled plasma mass spectrometry. Displacement isotherms are used to determine the number of binding sites and equilibrium constant values for two distributions of Z-type ligands, revealing site-specific heterogeneity on PbS nanocrystal surfaces. The distinct topological differences between small and large nanocrystals provides explicit evidence for truncation at octahedral vertices, forming (100) facets as the nanocrystal diameter increases.