Metal oxide nanocrystals have the potential for use in numerous applications if the structural attributes that lead to specific, desired properties can be manipulated. Syntheses to tune these structural attributes must often be developed through trial and error. It has been difficult to study these processes in traditional thermal decomposition syntheses due to the uncontrollable nature of the nucleation events that largely determine the nanocrystal size and morphology. An alternative approach to control size and morphology that allows for examination of the growth process is to use a continuous growth synthesis wherein a metal carboxylate precursor is continuously, slowly injected into hot oleyl alcohol. Here we present how a continuous growth synthesis has enabled studies of the nanocrystal growth process for a variety of metal oxide nanocrystals, including indium oxide, iron oxide, and cerium oxide. We show that size and morphology of each of these oxides can be tuned by the amount of precursor injected, and nanocrystal size and morphology can be monitored throughout the synthesis. In the case of indium oxide, we find that the rate of precursor addition, and therefore monomer flux to the surface, controls the morphology of the nanocrystals by promoting (low flux) or hindering (high flux) adatom surface diffusion. For iron oxide, we find that atomic-level surface structure influences the growth rate, with small (&lt; 4 nm) sizes growing faster than larger sizes due to the presence of tetrahedral iron vacancies. We also find that the oxidation state and ligation of the iron precursor is important, with an iron(II)-deficient precursor ligated with acetylacetonate ligands producing nanocrystals with twin defects. In the case of cerium oxide, we discovered that the morphology could be controlled by modifying the surface of the nanocrystals either by changing the reaction atmosphere to air (vs N<sub>2</sub>), creating a more oxidized (or reduced) surface, and by adding in trace amounts of acetic acid. These findings are widely applicable to metal oxide nanocrystals and can be used to establish conditions for controlling important structural attributes of nanocrystals.

Understanding metal oxide nanocrystal growth using a continuous addition synthesis

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.