Tailoring of Colloidal HfO₂ Nanocrystals with Unique Morphologies and New Self-Assembly Features

Advancing the synthesis of HfO₂ nanocrystals, a refined anhydrous protocol that enables kinetic trapping of the metastable tetragonal phase and modulates twinning defects in anisotropically grown monoclinic nanoprisms is presented. This evolved sol-gel approach examines the role of the capping agent tri-n-octylphosphine oxide (TOPO) and introduces a novel heating strategy with sequential growth stages. Replacement of TOPO with triphenylphosphine oxide (TPPO) leads to the formation of prismatic hafnia nanocrystals exhibiting pristine {011}-facets of the monoclinic phase. Furthermore, a hot-injection inspired heating approach yields sub-4 nm isotropic HfO₂ nanocrystals in the tetragonal phase, bypassing the need for aliovalent cation species. In contrast, a heat-up approach culminates in the generation of well-characterized HfO₂ nanorods. With sophisticated transmission electron microscopy analysis and the Wulff construction method, insights into the structural nucleation of nanoparticle growth are provided. This synthesis offers exceptional control and facilitates the formation of self-assemblies akin to liquid crystals, opening the door for new applications with nanocolloidal HfO₂.