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

## TL;DR

Researchers found that adding K+ ions to metal-oxide nanoparticles promotes their assembly into functional superlattices with exposed reactive surfaces.

## Contribution

A new method for controlling nanoparticle crystallization using ion size to form functional superlattices with exposed surfaces.

## Key findings

- K+ cations promote the assembly of metal-oxide nanoparticles into body-centered cubic superlattices.
- Small Li+ ions prevent crystallization by dynamically altering nanoparticle symmetries.
- The resulting superlattices have redox- and photochemically active surfaces due to POM ligands.

## Abstract

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.