Peering into the Formation of Cerium Oxide Colloidal Particles in Solution by In Situ Small-Angle X-ray Scattering

Abstract

The formation of CeO2 colloidal particles upon heating an aqueous solution of (NH4)2Ce­(NO3)6 to 100 °C was investigated by time-resolved in situ SAXS analysis using synchrotron radiation, providing absolute intensity data. In particular, the experiments were performed by applying different temperatures between room temperature and 100 °C as well as under variation of the ionic strength and concentration. Using validated SAXS evaluation tools (SASfit and McSAS software), the analyses revealed the presence of two types of particle populations possessing average dimensions of ca. 2 nm and 5–15 nm, with the latter being agglomerates of the 2 nm particles rather than single crystallites. The analysis revealed not only the changes in the size, but also the relative volume fractions of these two CeO2 particle populations as a function of the aforementioned parameters. Increasing the temperature increases the number of the 5−15 nm agglomerates on one hand by the enhanced nucleation rate of the primary particles. On the other hand, especially at high temperatures (90 and 100 °C) the larger agglomerate particles precipitate, resulting in interesting trends in the fractions of the two populations as a function of time, temperature, ionic strength, and precursor concentration. The experimental studies are complemented by calculating colloidal interaction energies based on classical DLVO theory. Thereby, this study provides detailed insight into the nucleation, growth, and agglomeration of CeO2 nanoparticles. The primary objective of this study is to provide a better understanding of the nucleation and growth of particles by the hydrolysis of the tetravalent cerium ion in aqueous solutions.

Aging of Nanoscale Cerium Oxide in a Peroxide Environment: Its Influence on the Redox, Surface, and Dispersion Character

“…Therefore, it is suggested that the initial hydrolysis of cerium cations leads to formation of these crystallites near-independent of the physicochemical environment. Small-angle X-ray spectroscopic studies have shown that Ce 4+ species undergo immediate hydrolysis under aqueous conditions to form polycation species of unique repeat numbers. , Further, the lengths of these phases have been related to the physicochemical environment, suggesting that aggregation of primary crystallites will be related to the degree of hydrolysis undergone by the species and function as a primary determinant for the polycrystalline particle product. In one such study, the dissolution of cerium nitrate salt led to the immediate formation of clusters ∼1.6 nm in size .…”

“…Small-angle X-ray spectroscopic studies have shown that Ce 4+ species undergo immediate hydrolysis under aqueous conditions to form polycation species of unique repeat numbers. , Further, the lengths of these phases have been related to the physicochemical environment, suggesting that aggregation of primary crystallites will be related to the degree of hydrolysis undergone by the species and function as a primary determinant for the polycrystalline particle product. In one such study, the dissolution of cerium nitrate salt led to the immediate formation of clusters ∼1.6 nm in size . It was determined that aging in the chemical environment then leads to primary particle formation with the growth mechanism dependent on the solution pH, the cerium concentration, and the solution ionic strength.…”

Globular pattern formation of hierarchical ceria nanoarchitecture

“…For the hydroxo-bridging ligands such as -OH and η-OH, SAXS revealed a hierarchical structure with an aggregated shape of rigid globules, because the primary and secondary clusters grew at different rates. To the best of or knowledge, such results have not been reported in the literature except for Smarsly et al, 49 possibly due to a lack of advanced techniques or interest. Ions, complexes, dimers, trimers, 38-and 40-mers, etc.…”

“…Remarkably, Smarsly et al recently reported CeO2 colloidal particles similar to ours and proposed the synthetic pathways for particle formation. 49 However, their advanced work only characterised the agglomerates as products via their original protocols and did not perform in-situ observation of the secondary clusters. In this respect, the present study is not a simple extension of prior work, since it mainly focuses on the hierarchical structure itself via in-situ observation using high-resolution TEM and Raman spectroscopy.…”

Interconnected CeO2 Nanofibers for Enhanced CO Gas Sensing

“…For example, Wang et al synthesized silica doped CeO 2 nanostructure as NH 3 gas sensors, which achieved a 0.5 ppm detection limit and 3244% response to 80 ppm NH 3 [16]. But usually, CeO 2 nanomaterials are easy to agglomerate, which makes the gas sensing performance of CeO 2 nanomaterials decrease rapidly [17]. Compared with nanoparticles, 1D nanofibers have a large aspect ratio, which means that more surface atoms are involved in the surface gas-solid reaction.…”