CeO2 Crystals Research Articles

Ceria‐Containing Frit for Luster in Modern Ceramic Glaze

The main goal of this work was to study the formation of the luster effect in a glaze manufactured from a ceria‐containing frit. The glazes obtained under industrial firing were studied using X‐ray photoelectron spectroscopy, X‐ray diffraction at room temperature and high temperature, and scanning electron microscopy with energy‐dispersion spectroscopy. The mechanism of the luster effect on the surface was explained. The results indicated that the glaze presents iridescent colors on the surface, mainly associated with the formation of a very thin layer of CeO2 crystals. Moreover, the CeO2 crystals show a preferential orientation and different dimensions. The CeO2 crystallization on the glaze was also studied using a hot‐stage X‐ray technique.

Investigations of EPR parameters and defect models for three Yb3+ impurity centers in ThO2 and CeO2 crystals

The electron paramagnetic resonance (EPR) parameters (g factors g(parallel to), g(perpendicular to) and hyperfine structure constants (171)A(parallel to), (171)A(perpendicular to), (173)A(parallel to) and (173)A(perpendicular to)) for three Yb(3+) centers, one cubic (center I) and two trigonal (centers II and III). in ThO(2) and CeO(2) crystals are studied from a complete diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are added to the classical Hamiltonian and the crystal-field parameters are calculated from the empirical superposition model. From the studies, the EPR parameters for these Yb(3+) centers in ThO(2) and CeO(2) crystals are reasonably explained, the defect model for the trigonal Yb(3+) center II suggested in the previous paper is confirmed and the defect structure of center II are also obtained. The theoretical hyperfine structure constants A of two Yb isotopes for centers II and III are suggested and remain to be checked by further experiments. The results are discussed. (C) 2010 Elsevier B.V. All rights reserved.

Orientation-dependent x-ray Raman scattering from cubic crystals: Natural linear dichroism in MnO and CeO2

Information on valence orbitals and electronic interactions in single crystal systems can be obtained through orientation-dependent x-ray measurements, but this can be problematic for a cubic system. Polarisation-dependent x-ray absorption measurements are common, but are dominated by dipole transitions which, for a cubic system, are isotropic even though a cubic system is not. Many edges, particularly for transition metals, do have electric quadrupole features that could lead to dichroism but proximity to the dipole transition can make interpretation challenging. X-ray Raman Spectroscopy (XRS) can also be used to perform orientation-dependent near-edge measurements – not only dependent on the direction of the momentum transfer but also its magnitude, q. Previous XRS measurements on polycrystalline materials revealed that multipole (higher order than dipole) transitions are readily observable in the pre-threshold region of rare earth N4,5 edges, actually replacing the dipole at high-q. We have extended these studies to examine orientation-dependent XRS for CeO2 and MnO single crystals, as prototype systems for theoretical treatment. Dichroism is observed at both the Ce N4,5 and Mn M2,3 edges in these cubic materials.

Nanocrystalline Ceria through Homogeneous Precipitation in Alcohol-Water Mixed Solvent

Uniform nanosized 3–6 nm cerium oxide single crystals have been synthesized by ammonia precipitation in alcohol-water mixed solvent. The size was estimated by photon correlation spectroscopy (PCS), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). The surface area of CeO2 nanocrystals were measured by Brunauer-Emmett-Teller (BET) technique. The nanoparticles synthesized were mono-dispersed with a narrow size distribution with low polydispersity index (PDI). The size of the ceria nanocrystals synthesized was found to decrease with decrease in the ϵ of the solvent. The crystals precipitated from isopropyl alcohol were highly crystalline. The smallest CeO2 crystals were obtained using ethylene glycol/water medium whereas lowest PDI was obtained from IPA/water system.

Sol–Gel Synthesis of Tunable Cerium Titanate Materials

AbstractCerium titanate structures present a high technological interest because of their optical and catalytic properties. This work reports the synthesis of these materials by a sol–gel methodology that allows mixed oxides with TiIV/CeIII, TiIV/CeIV or TiIV/CeIII–CeIV species to be obtained. Crystallization of CeO2–TiO2 mixed oxides and Ce2Ti2O7 pyrochlore phase was corroborated by XRD and Raman spectroscopy. Magnetic and EPR measurements were performed in order to clarify the oxidation state of the cerium ions in the system as a result of the easy oxidation of CeIII. The firing atmosphere is crucial for the CeIII/CeIV ratio, which is responsible for the different structure, but it also affects the colouration (yellow, green, brown). This methodology offers the possibility to modulate the Ce3+/Ce4+ content and particle sizes providing more ecological coloured pigments based on rare earth elements that could be interesting materials for advanced applications in catalysis.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Enhanced oxidation resistance of hydrothermally-encapsulated CdS nanoparticles in SiO2-CeO2 matrices

This research aims to enhance the thermal stability of CdS nano-particles by a hydrothermally-formed glassy encapsulation structure of SiO2-CeO2. The crystalline phase, thermal effect and microstructure of the encapsulated CdS nanoparticles were investigated by the XRD, DSC-TG and TEM analysis. The results showed that most of the CdS nanoparticles had been encapsulated in the glassy phase of SiO2-CeO2 by a hydrothermal treatment at 200°C. This structure was stable up to more than 900°C, and it was destructed by the crystallization of crystalline CeO2 and SiO2 at a higher temperature. Protected by the glassy phase, the encapsulted CdS nanoparticles was thermally stable up to more than 900°C, which was gradually oxidized and sublimated at the temperature range of 1000-1200°C.

Coloring Mechanism of Pr-CeO<sub>2</sub> Nano-Crystalline Red Lanthanide Pigments Prepared by Low-Temperature Combustion Synthesis

Pr-CeO2 nano-crystalline red lanthanide pigments were prepared from cerium (III) nitrate hexahydrate and praseodymium oxide by low-temperature combustion synthesis. Its ignited temperature was approximately 250°C. The prepared pigments were characterized by DSC, XRD, SEM and XPS. The results showed that the crystallite size decreases with increasing Pr mol concentration. The valence of praseodymium in the pigments and its coloring mechanism were also investigated. The trivalent Pr entered into CeO2 crystal lattice and formed Pr-CeO2 solid solution, which was based on the fluorite structure. It substituted trivalent Pr for tetravalent Ce in CeO2 crystal and was compensated by oxygen vacancies, which caused the distortion and the defection of the crystal lattice. Therefore the pigments can absorb the light with the wavelength of approximate 600nm and present the red tonality.

Synthesis of Porous Single Crystals of Metal Oxides via a Solid−Liquid Route

Porous single crystals of Co3O4, NiO, CeO2, and Cr2O3 were produced using mesoporous silicas, SBA-15 and KIT-6, as hard templates and a new convenient approach, the solid−liquid method, was developed. Characterization of the materials was performed mainly by using X-ray powder diffraction and high-resolution transmission electron microscopy. The advantages and disadvantages of this solvent-free method are discussed in comparison with the previously established methods.

Porous crystals of cubic metal oxides templated by cage-containing mesoporous silica

Porous single crystals of cubic Co3O4, NiO, In2O3 and CeO2 were fabricated using cage-containing SBA-16 and FDU-12 as hard templates, and characterized by X-ray powder diffraction, high-resolution transmission electron microscopy and N2 adsorption/desorption. Unsuccessful synthesis of non-cubic metal oxides implies that the symmetries of the oxide structures have a significant effect on crystal growth inside the mesopore networks in these templates. A possible mechanism of crystal growth in the confined spherical cages is discussed.

Growth Mechanism, Microstructure, and Surface Modification of Nanostructured CeO2 Films by Chemical Solution Deposition

The evolution from a partially oriented granular microstructure to a dense epitaxial one in CeO2 films deposited from chemical solutions on Y2O3-stabilized cubic ZrO2 (YSZ) has been investigated using cross-section transmission electron microscopy (XTEM), electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray reflectometry (XRR). In crystallization from chemical solutions, undercoolings are typically high enough to promote homogeneous and heterogeneous nucleation with equal probability. The desired texture is then transmitted from heterogeneously nucleated epitaxial grains throughout the volume of the film upon sintering. Crystallization of CeO2 under the reducing atmospheric conditions of Ar/5 % H2 results in a nanometric granular microstructure with a high concentration of C impurities decorating grain boundaries and interstitial cavities, which unexpectedly prevails after high-temperature annealing. Post-processing in oxidizing conditions removes C impurities and promotes grain growth resulting in a fully epitaxial film, as well as stabilizing the otherwise energetically prohibitive polar (001) planes. Misfit stresses in the post-processed epitaxial films are completely relieved by interfacial dislocations with b = (a/2) < 110 > and b = (a/2) < 011 > Burgers vectors. A mechanism that considers impurity-induced grain-boundary blocking and the stabilization of (001) planes via surface oxidation is proposed. Processing conditions to obtain high-quality CeO2 buffer layers can be adapted to YBa2Cu3O7-coated conductors.

Effect of citric acid on properties of CeO2 films for electrochromic windows

Abstract An alcohol based sol–gel process involving cerium chloride heptahydrate and citric acid in different mole ratios has been employed for the deposition of CeO2 films. The structural, electrochemical, and optical properties of the films have been investigated using a wide range of techniques. Differential thermal analysis has shown the onset of crystallization of CeO2 at 389 °C. The addition of an additive (citric acid) to the precursor sol has led to homogeneity and also a reduced ion storage capacity in the films. This observation emphasizes on the use of optimum content of the citric acid such that the films are suitable in terms of transparency as well as uniformity characteristics and also exhibit good electrochemical response. As is evidenced by the SEM study, the degree of polycrystalline grain formation in the citric acid derived films is observed to be less. The XPS results have confirmed the presence of Ce4+ state in the films. The optically passive behavior of the films is affirmed by their negligible transmission modulation upon Li ion insertion and extraction. A higher proportion of citric acid has also resulted in a reduced porosity and diminished crystallite size of the cerianite phase. The effect of the CeO2 films on the switching kinetics of the tungsten oxide (WO3) films has revealed an increase in the coloration time of the latter with the diminished crystallite size of CeO2 nanograins in the former.

Crystallisation Within Simulated High Level Waste Borosilicate Glass

AbstractThe devitrification products of two simulated high level radioactive waste (HLW) glasses have been investigated. Magnox waste glass contained RuO2 as its primary crystal phase, Pd-Te inclusions, a Cr-, Fe- and Ni-rich spinel phase, a Si- and lanthanide-rich phase and also a Zr-rich phase which incorporated Ce and Gd. Upon heat treatment the glass developed a zektzerite-type phase, CeO2, a strontium molybdate (containing Nd and La) as well as a Si-rich phase. 75/25 glass, comprising a blend of reprocessing waste derived from UO2 and Magnox fuels, contained RuO2 as its primary crystalline phase, CeO2 and a Si- and Ru-rich phase. Heat treatment of this glass resulted in the growth of the CeO2 crystals, the development of a strontium molybdate (containing Nd and La) and a Si- and lanthanide-containing phase. A sodium lanthanum molybdate with a powellite-type structure formed on the surface of both glasses after heat treatment in air.

Structural Characterization of CeO2−TiO2 and V2O5/CeO2−TiO2 Catalysts by Raman and XPS Techniques

Structural characteristics of ceria−titania and vanadia/ceria−titania mixed oxides have been investigated using X-ray powder diffraction (XRD), Raman spectroscopy (RS), and X-ray photoelectron spectroscopy (XPS) techniques. The (1:1 mole ratio) mixed oxide was obtained by a coprecipitation method, and a nominal 5 wt % V2O5 was deposited over its surface by a wet impregnation technique. Both of the materials were then subjected to thermal treatments from 773 to 1073 K and were characterized by the above-mentioned techniques. The XRD results suggest that the CeO2−TiO2 mixed oxide calcined at 773 K primarily consists of poorly crystalline CeO2 and TiO2-anatase phases and that a better crystallization of these oxides occurs with increasing calcination temperature. The “a” cell-parameter values suggest some incorporation of titanium into the ceria lattice. Impregnation of vanadia on ceria−titania enhances the crystallization of CeO2 and TiO2 oxides. However, no crystalline V2O5 could be observed from XRD and R...

Caracterisation De Catalyseurs ZrxCe1–xO2 et Etude De Leur Reactivite Dans L'Oxydation Des Suies Par ATD/ATG

Thermal analysis has been used to investigate the crystallization of ZrxCe1-xO2 mixed oxides, prepared by co-precipitation of corresponding hydroxides. For x≤0.5, small crystals of CeO2, were formed at low temperatures (373 K). For x>0.5an exothermic peak at 420°C (693 K) was observed after calcination under a flow of air ofhydroxide samples. This peak was associated with the formation of a ZrxCe1-xO2 solid solution (XRD) in a tetragonal phase (Raman).

Surface morphology analysis in correlation with crystallinity of CeO2(110) layers on Si(100) substrates

CeO 2 layers having various crystallinity are prepared in the vicinity of the critical condition for epitaxial growth, using conventional evaporation and electron beam assisted evaporation. Surface morphology of CeO2 layers is studied using atomic force microscopy in correlation with crystallinity of the layer determined by reflection high energy electron diffraction. It is clearly observed that surface morphology changes with crystallinity of the CeO2 layers. Single crystal CeO2(110) layers have a nanometer-scale-periodically corrugated surface structure, which consists of (111) facets. As the crystalline quality of samples becomes worse, the number of irregular shaped hillocks and tetrahedral hillocks increases, which correspond to distorted (110) grains and (111)-oriented polycrystalline grains, respectively. It is found that crystalline quality is not uniform but various crystallinity regions are distributed from place to place and their population ratio changes according to the degree of crystalline quality. The surface of polycrystalline layers with a tendency of (111) orientation consists of tetrahedral hillocks with irregular in-plane rotational orientations within the horizontal plane. These features are discussed in terms of the surface properties of the substrates at the early stage of growth.

Occurrence and Mechanism of Formation of CeOCl in Pd/CeO2 Catalysts

The formation of the CeOCl phase in a Pd(Cl)/CeO2 catalyst during reduction in hydrogen was studied by HRTEM, XRD, and low-temperature N2 adsorption (BET). The results indicate that CeOCl crystallites nucleate at (110) faces of CeO2 crystals at T≤300°C and then grow by coalescence of existing nuclei and attachment of Ce–O–Cl species continuously formed at the ceria surface. Lateral growth of CeOCl results in rapid decline of the BET surface of the catalyst between 300 and 500°C due to the “enwrapping” of the grains of the catalyst. The mechanism of an ordered CeOCl–CeO2 transformation in an oxidizing atmosphere is proposed.

Growth of PbTiO3 Thin Film on Si(100) with Y2O3 and CeO2 Buffer Layer

The PbTiO3 thin film was deposited on Si(100) with Y2O3 and CeO2 buffer layers. Y2O3, CeO2 and PbTiO3 layers were fabricated by the in-situ pulsed laser deposition (PLD) technique. X-ray diffraction (XRD) analysis revealed that the Y2O3 and CeO2 layers were (222) and (200) preferential orientation, respectively. PbTiO3 showed (100)+(001) mixed texture on CeO2/Si and polycrystalline on Y2O3/Si, respectively. Characterization of the grazing angle incident X-ray diffraction showed that the stress depth profile of the PbTiO3 thin film on CeO2/Si(100) was not uniform. The secondary ion mass spectroscopy (SIMS) analysis indicated that the CeO2 and Y2O3 layers reduced significantly the interdiffusion of Si atoms into PbTiO3 and that the distributions of Pb and Ti elements were uniform throughout the PbTiO3 layer. The columnar crystallization of CeO2, Y2O3, and PbTiO3 films were observed by scanned electron microsopy (SEM).

Epitaxial CeO2 on Silicon Substrates and the Potential of Si/CeO2/Si for SOI Structures

ABSTRACTCeO2 is nearly lattice matched to Si and has the CaF2 cubic structure thus it offers the potential for the epitaxial growth of an insulating film on Si. Laser ablation of a CeO2 target in an ultra high vacuum system was used for the deposition of single crystal CeO2. The effect of post growth thermal and rapid thermal annealing in O2, N2 and Ar atmosphere was found to have pronounced effects on the electrical properties measured by C-V and the optical properties measured by photoluminescence.We report on our initial results for the growth of epitaxial Si on the deposited CeO2 using low pressure CVD. Both RHEED and TEM studies showed that single crystal epitaxial Si was deposited on CeO2. The details of the Si deposition on CeO2 films for potential of SOI (silicon on insulator) structures will be discussed.

Epitaxial CeO2 Growth on Si (111) for SOI

AbstractEpitaxial growth of CeO2 was obtained on the Si(111) surface by laser ablation in UHV atmosphere. However, a dual amorphous layer formed at the interface, yielding a CeO2/α:-CeOx/α-SiO2/Si(111) structure. This structure is speculated to be caused by a reaction occurring between Ce oxide and Si. Post annealing in O2 ambient caused the regrowth of CeO2, eliminated the α-CeOx layer, and increased the thickness of the SiO2 layer. The new CeO2/SiO2/Si(111) structure shows improved breakdown voltage and fewer interfacial states as observed by C-V and I-V measurements. The SiO2 is expected to tie surface states with Si, whereas the single crystal CeO2 will allow the epitaxial growth of lattice-matched Si on this insulating film. The effect of growth conditions and O2 annealing on both the structural and the electrical properties of this epitaxial oxide will be presented.

EPR study of Fe3+ and Mn2+ in CeO2 and ThO2

Attempts were made to grow CeO2 and ThO2 single crystals doped with transition metal ions. Only Fe3+ and Mn2+ could be detected by the EPR technique. The EPR spectrum of Fe3+ in CeO2 exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10−4 cm−1. The hyperfine constantA for57Fe in hexahedral coordination was found to be 8.9(1)·10−4 cm−1. The EPR spectrum of Mn2+ in CeO2 reveals two cubic Mn2+ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10−4 cm−1 and for center 2g=1.9984(1) andA=87.0(1)·10−4 cm−1. Heating the Mn doped CeO2 samples in hydrogen, the Mn2+ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10−4 cm−1 andD=203(1)·10−4 cm−1. The two observed Mn2+ centers in ThO2 exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10−4 cm−1 andD=33(3)·10−4 cm−1.