Metal-Oxide Nanoparticles with Multipolar Cog-Wheel Coupling form Dynamic Mixtures of 2D Sheets and Self-Healable Hollow Vesicles

The controlled self-assembly of inorganic nanocrystals (NCs) into dynamic architectures remains a central challenge in nanomaterials chemistry. Polyoxometalates (POMs) are known to form "blackberry" assemblies, with sheet-like structures proposed as their lower-symmetry precursors. Nevertheless, direct observation of both structural types within a single system has not been documented. We here show that when the protons are used as the POM countercations for the average of 10 [α-PW₁₁O₃₉Sn)] 3-O-Keggin-ion derived donor ligands to 3.6 ± 0.9 nm SnO₂ NCs, the surfaces of the NCs are extensively protonated by 33 ± 1 H + ions, giving complexed NCs with net charges of 7-± 1. The positively charged surface regions engage in "cog-wheel" like electrostatic interactions with polyanionic [PW₁₁O₃₉SnO]⁴⁻ ligands of neighbouring NCs, resulting in the assembly of 2D sheets observed by cryo-TEM imaging in the same solutions as symmetry-adopted hollow vesicles. Atomistic molecular-dynamics simulations confirm that protonated surface regions on one NC form favorable interactions with the anionic POM ligands of neighboring NCs with distances between ligands on the surfaces of both NCs stabilizing over time, with the persistence of interfacial ligand contacts proving the existence of a locking mechanism in these NCs. These findings introduce localized structural recognition as an operative control element in the electrostatic assembly of metal-oxide NCs.