In situ ultra-small-angle X-ray scattering study of the solution-mediated formation and growth of nanocrystalline ceria (original) (raw)
Design strategies for ceria nanomaterials: untangling key mechanistic concepts
Materials horizons, 2021
The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO 2Àx stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO 2Àx . However, the most important aspect in the design of CeO 2Àx nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO 2Àx morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO 2Àx morphologies in terms of the commonly used raw materials [Ce] (and [NO 3 À ] in Ce(NO 3 ) 3 Á6H 2 O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO 2Àx morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, has been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH) 4 crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH) 4 crystallites and NO 3 À -assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.
Chemistry of Materials, 2010
In situ synchrotron powder X-ray diffraction (PXRD) measurements have been conducted to follow the nucleation and growth of crystalline Ce x Zr 1-x O 2 nanoparticles synthesized in supercritical water with a full substitution variation (x = 0, 0.2, 0.5, 0.8, and 1.0). Direction-dependent growth curves are determined and described using reaction kinetic models. A distinct change in growth kinetics is observed with increasing cerium content. For x = 0.8 and 1.0 (high cerium content), the growth is initially limited by the surface reaction kinetics; however, at a size of ∼6 nm, the growth changes and becomes limited by the diffusion of monomers toward the surface. For x = 0 and 0.2, the opposite behavior is observed with the growth initially being limited by diffusion (up to ∼3.5 nm) and later by the surface reaction kinetics. Thus, although a continuous solid solution can be obtained for the ceria-zirconia system, the growth of ceria and zirconia nanoparticles is fundamentally different under supercritical water conditions. For comparison, ex situ synthesis has also been performed using an in-house supercritical flow reactor. The resulting samples were analyzed using PXRD, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The nanoparticles with x = 0, 0.2, and 0.5 have very low polydispersities. The sizes range from 4 nm to 7 nm, and the particles exhibit a reversibly pH-dependent agglomeration.
Oxygen nonstoichiometry of nanocrystalline ceria
Russian Journal of Inorganic Chemistry, 2010
Correlation relations between oxygen nonstoichiometry and particle size in nanocrystalline CeO 2-x are revised. The ceria unit cell parameter is shown to increase from 0.5410 to 0.5453 nm as the particle size decreases from 23 to 2.3 nm. The CeO 2-x critical particle size where cerium(IV) is completely reduced to cerium(III) is calculated as 1.1-1.3 nm.
2015
Cerium oxide (CeO2) or ceria has been shown to be an interesting support material for noble metals in catalysts designed for emission control, mainly due to its oxygen storage capacity. Ceria nanoparticles were prepared by precipitation method. The precursor materials used in this research were cerium nitrate hexahydrate (as a basic material), potassium carbonate and potassium hydroxide (as precipitants). The morphological properties were characterized by high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and UV-Vis spectrophotometer. XRD results showed face centered cubic CeO2 nanoparticles for annealed nanoparticles at 1000°C. SEM measurement showed that by increasing the calcinations temperature from 200 to 600°C, the crystallite size decreased from 90 to 28 nm. The SEM results showed that the size of the CeO2 nanoparticles decreased with increasing temperature. T...
Journal of Nanoparticle Research, 2013
A correlation between the particle size and the lattice parameter has been established in cerium oxide nanoparticles. The variation in the lattice parameter is attributed to the lattice strain induced by the introduction of Ce 3? due to the formation of oxygen vacancies. Lattice strain was observed to decrease with an increase in the particle size. The Ce 4? to Ce 3? ratio in CeO 2 nanoparticles increases with increasing the calcination temperature in air atmosphere. Such anomalous behavior is due to the physical effect of nanoparticle sizes on increasing the oxidation state of Ce ions in CeO 2. Keywords Cerium oxide Á XPS Á XRD Ceria (CeO 2) is an inorganic compound broadly used in sensors, electrochromic, and anticorrosive coatings, and also in diverse catalysts and in the capacity of an abrasive material. Upon transition into a nanocrystalline state, ceria significantly changes its physicochemical properties, at that, the character of these changes is unusual enough (Ivanov et al. 2009). One of the major intrinsic properties of nanocrystalline ceria is a clearly marked dependence of a unit cell parameter on particle size. It is generally accepted that such an effect originates from partial reduction of Ce 4? into Ce 3? and corresponding formation of oxygen vacancies in ceria crystal lattice (Tsunekawa et al. 1999; Wu et al. 2004). Strongly pronounced non-stoichiometry gives rise to one of the most intriguing properties of ceria, namely its biological activity. It has been shown that CeO 2-x powders and sols (aqueous and nonaqueous) are effective scavengers of free radicals and other reactive oxygen species (ROS) and possess autoregenerative properties (Chen et al. 2006). Such a combination of properties as well as nontoxic nature and excellent biocompatibility makes nanocrystalline ceria a unique material that can protect cells from oxidative stress. In particular, CeO 2-x particles can increase cell longevity and survival potential of various micro-and macroorganisms (Ivanov et al. 2008; Colon et al. 2009). It was also shown that CeO 2-x is promising in view of treatment of various human diseases including cancer (Chen et al. 2006; Colon et al. 2009). In view of the fact that nanoceria biological activity is size-dependent (Chen et al. 2006), extensive study of physical and chemical properties of CeO 2-x is required paying special attention to its oxygen non-stoichiometry. Several reports were made dealing with dependence of oxygen non-stoichiometry and unit cell parameter on the particle size in CeO 2-x (Tsunekawa et al. 1999; Wu et al. 2004; Hailstone et al. 2009).
Oxygen vacancies in self-assemblies of ceria nanoparticles
J. Mater. Chem. A, 2014
Cerium dioxide (CeO 2 , ceria) nanoparticles possess size-dependent chemical properties, which may be very different from those of the bulk material. Agglomeration of such particles in nanoarchitectures may further significantly affect their properties. We computationally model the self-assembly of Ce n O 2n particles (n ¼ 38, 40, 80)zero-dimensional (0D) structuresin one-and two-dimensional (1D and 2D) nanoarchitectures by employing density-functional methods. The electronic properties of 1D Ce 80 O 160 and 2D Ce 40 O 80 resemble those of larger 0D crystallites, Ce 140 O 280 , rather than those of their building blocks. These 0D, 1D and 2D nanostructures are employed to study the size dependence of the formation energy of an oxygen vacancy, E f (O vac ), a central property in ceria chemistry. We rationalize within a common electronic structure framework the variations of the E f (O vac ) values, which are computed for the Ce n O 2n nanostructures with different sizes and dimensionalities. We identify: (i) the bandwidth of the unoccupied density of states projected onto the Ce 4f levels as an important factor, which controls E f (O vac ); and (ii) the corner Ce atoms as the structural motif essential for a noticeable reduction of E f (O vac ). These results help to understand the size dependent behaviour of E f (O vac ) in nanostructured ceria. † Electronic supplementary information (ESI) available: Figure for Ce 40 O 80 and
In situ HT-ESEM study of crystallites growth within CeO2 microspheres
Ceramics International, 2015
Cerium dioxide is widely studied due to its potential interest in several applications, including heterogeneous catalysis. In this field, modifications of the crystallographic orientations and surface reactivity of CeO 2 can lead to activity loss of metal supported catalysts. In situ High Temperature-Environmental Scanning Electron Microscopy observations were then developed to monitor such evolution in CeO 2 spherical particles. Microspheres with 300-800 nm diameter were heat treated for 1-120 min in the 1000-1200 1C range. Subsequent image analysis led to monitor and quantify the crystallite growth during isotherm dwells. Two distinct mechanisms controlling the growth of crystallites in a single microsphere were then evidenced depending on the heating duration, i.e. oriented attachment then diffusion. Precise control of the aggregates inner structure (number of crystallites and density) was also achieved and described as a nanostructure map. These results pave the way to new opportunities in nanoparticle design.
HAL (Le Centre pour la Communication Scientifique Directe), 2017
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A Study of Lattice Expansion in CeO 2 Nanoparticles by Transmission Electron Microscopy
Nanoceria was produced by an aqueous precipitation technique in the presence of an organic stabilizer. The stable suspensions were diafiltered to remove reaction byproducts. Particles were characterized by transmission electron microscopy with images used to size the particles, and selected-area electron diffraction was used to determine the lattice structure and the lattice constant. For all particles studied, the electron diffraction data were consistent with that of CeO 2 and not Ce 2 O 3 , as predicted by some researchers for very small particles sizes. At particle diameters of ∼1 nm, the lattice expansion approached 7%. In agreement with earlier researchers, we interpret this effect as due to the formation of substantial amounts of Ce 3+ with corresponding oxygen vacancies, but within the fluorite lattice structure of CeO 2. Even at a particle size of 1 nm, there was a measurable oxygen storage capacity, consistent with a still-reducible CeO 2 structure, rather than the fully oxidized Ce 2 O 3 .
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2017
Given the scientific and technological interest in cerium oxide (ceria), in this paper nanoparticles of ceria (ceria-NPs) were synthesized using a modified polymer complex process (modified Pechini), while varying the pH of the system. This methodology made it possible to obtain, in a reproducible and controlled way, nanoparticles of ceria (<100nm) of a high chemical purity at low temperatures. The precalcined cerium solid obtained at 350°C was characterized using differential thermal analysis (DTA), thermogravimetric analysis (TG) and IR spectroscopy. Very little organic phase was found in the respective spectra, indicating that the inorganic phase, cerium oxide, is predominant. Carbonaceous residues still present in the solids were removed by heating at temperatures above 500°C and the samples obtained were characterized using X-ray diffraction (XRD), IR, UV-visible absorption and diffuse reflectance spectroscopies, and Transmission Electron Microscopy (TEM). The diffractograms of the samples showed that the only crystalline phase present was CeO2. From the results of UV-vis absorption and diffuse reflectance spectroscopy, two important energy values were obtained, 3.8 eV and 3.4 eV. These could be attributed to the energy gap value (3.8 eV) and to a possible "mid-gap" (3.4 eV). Furthermore, on increasing the synthesis pH, a reduction in particle size results, the particle being between 10 and 20 nm, with a spheroidal shape. By looking at the different stages of the synthesis process, a mechanism is proposed to explain how nanoparticles of ceria are formed.