Crystallization kinetics of cerium oxide nanoparticles formed by spontaneous, room-temperature hydrolysis of cerium(iv) ammonium nitrate in light and heavy water

Abstract

A stable sol of cerium oxide nanoparticles forms spontaneously when cerium(iv) ammonium nitrate (CAN) is dissolved in room-temperature water at mM concentrations. Electron microscopy experiments reveal the formation of highly crystalline cerium oxide particles several nm in diameter and suggest that they are formed from amorphous particles that are similar in size. Under the low pH conditions of the experiments, the nanoparticles form a stable dispersion and show no evidence of aggregation, even many months after synthesis. The absence of particles large enough to scatter light significantly makes it possible to observe the crystallization kinetics through dramatic changes in the UV-visible absorption spectra that occur during solution aging. Measurements show that the cerium oxide nanocrystals are formed roughly an order of magnitude more slowly in DO than in HO solution. This large solvent kinetic isotope effect (k/k ∼ 10), which is reported here for the first time for the crystallization of a solid metal oxide phase, indicates a rate-determining proton transfer reaction, which is assigned to the conversion of hydroxy to oxo bridges. In DO solution, the absorption per mole of cerium ions increases by over 400% at 290 nm as the weakly absorbing precursor phase is transformed into nanocrystalline cerium oxide. An isosbestic point is detected at 368 nm, and the absorption spectra can be modeled throughout aging by the sum of spectra of just two interconverting species. Preliminary ultrafast transient absorption experiments confirm that the optical properties of the amorphous precursors differ greatly from those of the final, nanocrystalline phase. Crystallization of CeO from CAN in water has much in common with the crystallization of iron oxides from iron(iii) salts, including the importance of non-classical nucleation and growth pathways. It is an outstanding system for studying the poorly understood events that cause molecularly solvated ions to self-assemble into nanocrystals, following hydrolysis. At the same time, the strong susceptibility of CAN to spontaneously form CeO nanocrystals under the mildest of reaction conditions indicates that caution is needed when working with this common sacrificial oxidant.

The Influence of Precursor on the Preparation of CeO2 Catalysts for the Total Oxidation of the Volatile Organic Compound Propane

“…This caused CeO2.nH2O to precipitate rapidly before reaction with carbonate to form Ce(CO3)x(OH)y type species could occur. Evidence of CeO2 nanoparticle formation from the hydrolysis of ammonium cerium nitrate in aqueous solution without the addition of a base has also been documented by Pettinger et al [33]. This effect was not observed when using different Ce III precursors, and the products formed were the carbonates, either Ce2O(CO3)2.H2O or Ce(OH)CO3 [31].…”

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

“…The conversion of the dimeric precursor species present in an aqueous solution of CAN into CeO 2 nanoparticles (studied by total X-ray scattering and small-angle X-ray scattering) has been investigated only under supercritical conditions 23 or at room temperature. 24 The pathways from molecular precursor to nanocrystals upon heat treatment were therefore not addressed, which is however of crucial importance for the general understanding of the formation of CeO 2 nanoparticles in solution and their colloidal stability. Also, the variation of the applied temperature allows for the adjustment of particle sizes in solution by influencing nucleation and growth.…”

“…These findings are complementary to the results of Pettinger et al, who investigated the spontaneous formation of the nanoparticles by UV/vis spectroscopy. 24 Furthermore, the pronounced hydrolysis at room temperature leads to a nondefined starting point for the following in situ SAXS investigations at higher hydrolysis temperatures, although the time between the preparation of the precursor solution and the start of the respective measurements was identical. For the evaluation of SAXS data at the higher temperatures, this finding impedes the disentanglement between the formed particles and the still-present precursors because the room-temperature SAXS data cannot be used as a reference for a particle-free solution.…”

Hydrolyzed Ce(IV) salts limit sucrose-dependent biofilm formation by Streptococcus mutans

“…The goal of the present study was to expand upon the literature regarding CeO 2 -NP as biofilm inhibitors. CeO 2 -NP prepared from Ce(IV) salt hydrolysis [17][18][19][20] were screened against "bare" CeO 2 -NP of variable size and Ce(NO 3 ) 3 •6H 2 O for their ability to perturb initial in vitro biofilm formation of the clinically relevant pathogen, S. mutans. The aggregation and dispersion properties of similarly sized CeO 2 -NP obtained from different synthetic methodology were also investigated offering insight into the variable biological activity observed.…”