Reaction Atmospheres and Surface Ligation Control Surface Reactivity and Morphology of Cerium Oxide Nanocrystals during Continuous Addition Synthesis.

Abstract

Cerium oxide nanocrystals have size- and shape-dependent properties that are potentially useful in a variety of applications if these structural attributes can be controlled through synthesis. Various syntheses have been developed in attempts to access different sizes and shapes, but little is known about selecting reaction conditions to predictably control the growth, and therefore properties, of the nanocrystals. Here, we investigate the role of cerium precursor oxidation states, reaction atmospheres, and acetic acid ligation on the size and shape of cerium oxide nanocrystals. A continuous addition synthesis allowed us to vary individual reaction parameters to better understand how each affects growth and morphology. Under N2, the synthesis leads to either irregular shapes or nanoribbons, whereas the same synthesis under air leads to size-tunable nanocubes. To determine whether air might be oxidizing the cerium precursor and changing its reactivity, we synthesized Ce(IV)-rich and Ce(III) oleate precursors and found that the oxidation state of the precursor has little effect on the resulting nanocrystals. In fact, we found that Ce(IV) oleate is readily reduced to Ce(III) at at temperatures above 100 °C in the reaction medium. The significant role of air during synthesis therefore suggests that oxygen is altering the surface reactivity of the nanocrystals, as opposed to the precursor. We investigated the origin of nanoribbon formation and found that the presence of acetate ligands is responsible for nanoribbon formation in syntheses under N2, with more acetate leading to longer nanoribbons. These insights were used to identify conditions to predictably grow various sizes and shapes of nanocubes and nanoribbons. The findings elucidate the effects that various synthetic parameters can have on cerium oxide nanocrystal synthesis and suggest that redox reactivity may influence growth and properties in other syntheses where changes in the oxidation state occur for the precursor or the nanocrystal surface.