Publication date: 3rd July 2020
PbS nanocrystals are predicted to undergo size-dependent shape changes, transitioning from octahedral to cuboctahedral morphology as nanocrystal size increases. Experimental evidence for size-dependent topology has been previously 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 faceting, 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 (κ2-TMEDA)Pb(OA)2 (OA = oleate), which is monitored via 1H nuclear magnetic resonance (NMR) spectroscopy. In light of prior reports of hydroxyl ligand binding to PbS NC surfaces, the stoichiometry of the displaced Z-type complex is investigated by combining oleate integrations from 1H NMR spectra with Pb quantitation from inductively coupled plasma mass spectrometry (ICP-MS). Displacement isotherms are employed 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 (2.8 nm) and large (3.9 nm) PbS nanocrystals provides explicit evidence for truncation at octahedral vertices, forming (100) facets as the nanocrystal diameter increases.
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