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 N 2 , 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 N 2 , 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.

Precursor and Surface Reactivities Influence the Early Growth of Indium Oxide Nanocrystals in a Reagent-Driven, Continuous Addition Synthesis

“…Each of the three necks of the flask was capped with rubber septa and dry N 2 was flowed into the headspace through the center neck, and out one of the side necks through an 8-gauge purge needle at a rate of ∼120 mL min –1 . See previous work , for a general schematic of the synthesis setup.…”

“…30 Two key attributes of these syntheses�reaction temperature below thermal decomposition and slow addition of the precursor� allow us to systematically control the reaction parameters in order to examine nanocrystal growth processes in detail. For instance, we have been able to study the influence of temperature and monomer flux to the nanocrystal surface on the size and morphology of indium oxide, 29 the influence of nanocrystal size on the growth rate in iron oxide, 26 the influence of the atmosphere on the growth of cerium oxide nanocrystals, 32 and the influence of incorporating dopants from precursors with various reactivities in indium oxide. 33,34 In our continuous growth method, ester elimination chemistry, along with a slow, continuous addition method, produces uniform, size-tunable metal oxide nanocrystals.…”

Innovative Applications of Cerium Oxide-Based Materials in Civil Engineering, Automation, and Energy Sectors

“…Recovery time predetermines the hysteresis area. Both these parameters should have as lower values as possible, achievable by decreasing the size dimensions of the sensor elements, for instance, by using nanosized materials like nanocrystalline CeO 2 , as proposed by Knecht et al [228], and by employing precise equipment for sample assembly.…”