Quantification of a PbCl_x Shell on the Surface of PbS Nanocrystals

Differences between the properties of PbS nanocrystals prepared using a large excess of PbCl₂ and those prepared using other Pb sources have led to speculation that a thin PbCl_x shell may be present on the surface of PbCl₂-derived PbS nanocrystals. However, characterization of this chloride layer has proven to be challenging because of the poor contrast provided by electron and X-ray scattering probes. Here, we show that small-angle neutron scattering (SANS) provides the compositional sensitivity needed to unambiguously quantify the PbCl_x shell that is present on the surface of PbCl₂-derived PbS nanocrystals. Using a charge-balanced structural model, the scattering contribution of a ∼0.3 nm thick surface PbCl_x layer is parsed separately from ligand carboxylate head groups and PbS in the core. Global fitting of the SANS data across a solvent deuteration series enables unique determination of the spatial distribution of each material. These results are corroborated by quantitative nuclear magnetic resonance (NMR) and energy-dispersive X-ray spectroscopy (EDS). This work resolves a discrepancy in reported sizing curves for PbS nanocrystals prepared by different syntheses and further demonstrates the power of SANS in resolving molecular structure in soft and hybrid nanomaterials.