From Kinetics to Molecular-Level Insights into Group 4 Metal Oxide Nanocrystal Synthesis

Kinetic control is a powerful tool for traversing the chemical landscape toward the intended product. For group 4 metal oxide nanocrystals, the development of complex multimetallic heterostructures is still a challenge, partly due to the lack of kinetic and mechanistic understanding. Here, we study the reaction kinetics of the nonaqueous synthesis of titanium, zirconium, and hafnium oxide nanocrystals, from the decomposition of metal isopropoxide and metal halide, in the presence of tri-<i>n</i>-octylphosphine oxide (TOPO). The reaction rate depends on the metal: Ti ≫ Zr > Hf. While titanium follows an S_N1 substitution mechanism, zirconium and hafnium follow an auto-catalyzed E1 elimination. In both cases, the reaction kinetics can be tuned by varying the amount of TOPO or the chloride content due to their impact on the electronic structure of the transition state of the rate-determining step. The proposed mechanism was shown to be consistent with kinetic modeling of the data for different metal concentrations. This deeper understanding of group 4 metal oxide nanocrystal formation will facilitate access to novel heterostructures relevant for optical, catalytic, and electronic materials.