Am I Still IL? Evolution of CdS Nanocrystals from Inorganic Ligand Exchange

Ligand exchange has become the standard route for modifying the surface chemistry of colloidal nanocrystals (NCs), providing a mechanism to tailor their optoelectronic properties, solubility, and chemical functionalization. This enables NCs to be deployed in myriad applications where the surface-solution interface is key to performance or process compatibility. Ligand-passivated surfaces are generally considered stable and chemically inert, with incoming ligands assumed to fully replace native ones. Here, we demonstrate that inorganic ligand shells on CdS NCs are dynamic, chemically evolving over time, with transformation pathways strongly dependent on ligand identity. This is evaluated for representative oxoanions (o-phosphoric acid), chalcogenides (Na₂Se, Se^2-), and metal-chalcogenides (thiostannate). Using multinuclear (¹H, ¹³C, ²³Na, ³¹P, ¹¹³Cd, ¹¹⁹Sn) solid-state NMR spectroscopy and electron microscopy, we show that (i) surface-bound phosphates reorganize over time into cadmium phosphate domains, degrading NC quality; (ii) sodium polyselenide ligands remain chemically stable, but associated Na⁺ counterions exhibit dynamics that may suppress charge transport; and (iii) partial oxidation of thiostannate ligands to SnO₂ occurs, along with surface reconstruction, which improves NC passivation. Across all systems studied, residual oleylamine is detected. These findings reveal that inorganic ligand exchange does not necessarily yield chemically uniform or stable surfaces. Instead, each ligand class exhibits distinct behavior, ranging from surface degradation (phosphates), to stable yet dynamic interfaces (selenides), to hydrophilic and partially oxidized surfaces (thiostannates). This challenges the static-surface model and positions multinuclear solid-state NMR spectroscopy as a key tool for designing future functional materials.