Ion-Size Controlled Non-Classical Crystallization of Metal-Oxide Nanoparticles Covered with a Few Highly Charged Ligands

Densely ligated metal and metal-oxide nanoparticles (NPs) tend to assemble into superlattices (SLs) of different symmetries determined by a delicate balance of dominant interparticle forces. However, the organic protecting ligands typically used to stabilize NPs often block substrate access to their reactive surfaces, acting as an insulating barrier that prevents electronic coupling and limits optoelectronic activities. We now report that the addition of K⁺ cations to aqueous solutions of 2 nm metal-oxide nanocrystals (NCs) with exposed surfaces due to complexation on average by eight polyoxometalate (POM) ligands promotes their reversible assembly into soluble SLs. Time-resolved cryo-TEM revealed the initial formation of fractal aggregates whose branching nodes serve as nuclei for the nonclassical self-limiting crystallization of dynamic, negatively charged, and uniformly sized 110 ± 20 nm body-centered cubic (BCC) crystals. Atomistic molecular dynamics simulations revealed that K⁺ cations promote dynamical association of 8 POMs ligated to different NCs, causing their assembly into crystals, whereas small Li⁺ ions randomly but transiently bind to the POM ligands, thereby dynamically changing the effective symmetries of individual NCs, preventing their crystallization. Unlike when organic protecting ligands are used, the exposed metal-oxide surfaces of the small-ion BCC SLs (K⁺ form) are stabilized by redox- and photochemically active POM-anion ligands. The findings thus introduce an attractive approach to the rational design of functional small-ion metal-oxide NC SLs.