Zr‐doped CeO2 Hollow slightly‐truncated nano‐octahedrons: One‐pot synthesis,characterization and their application in catalysis of CO oxidation

Zirconium‐doped ceria hollow slightly‐truncated nano‐octahedrons (HTNOs) (Ce_1‐xZr_xO₂) were synthesized by a one‐pot, facile hydrothermal method. The morphology and crystalline structure were characterized with powder X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and the high resolution transmission electron microscopy (HRTEM). The composition and chemical valence on the surface of the as‐prepared Ce_1‐xZr_xO₂ powders were detected by X‐ray photoelectron spectroscopy (XPS) and energy dispersive spectrometry (EDS). The surface area and pore size distribution of as‐obtained Zr‐doped ceria HTNOs were measured by N₂ adsorption‐desorption measurement. Mechanisms for the growth of Zr‐doped ceria HTNOs are proposed as both oriented attachment and Ostwald ripening process and the formation of the hollow structure is strongly dependent on the addition of Zr⁴⁺ ions. Furthermore, the as‐obtained Zr‐doped ceria HTNOs revealed superior catalytic activity and thermal stability toward CO oxidation compared to pure ceria. It may provide a new path for the fabrication of inorganic hollow structures on introducing alien metal ions.     

Phase transformation and crystal growth behaviors of La3+/Ce3+-doped TiO2–20 wt% SnO2 gels

The transformation behaviors of La³⁺/Ce³⁺-doped TiO₂–SnO₂ gels were studied by using differential thermal analysis and X-ray diffraction methods so as to improve the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and SnO₂ nanocrystals can exist in the sintering products by varying La³⁺/Ce³⁺ contents and sintering temperatures. 0.8–1.1 wt% of La₂O₃ or CeO₂ doping greatly depresses the growth of anatase and rutile crystals, obtaining nanosized crystals when sintered up to 600 °C for 2 h. With La³⁺/Ce³⁺-doping and increasing their contents, the transformations of gel to anatase and anatase to rutile, as well as the growth of anatase and rutile crystals can be depressed, while the transformation temperature of anatase to rutile receives much less affect. Moreover, the La³⁺-doping has stronger effects on them than Ce³⁺ doping, but has a weaker inhibiting effect on precipitation and growth of SnO₂ crystals.     

Influence of La/Ce-doping on phase transformation and crystal growth in TiO2-15 wt% ZnO gels

Titania based ceramic is a good candidate for environmental sensing materials. Ion doping is an effective method to improve the properties by modifying their microstructure and phase composition. By using differential thermal analysis and X-ray diffraction methods, the transformation behaviors of La³⁺/Ce³⁺-doped TiO₂-15  wt% ZnO gels were studied so as to modify the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and ZnTiO₃ nanocrystals can be tailored by varying La³⁺/Ce³⁺ contents and sintering temperatures. La³⁺/Ce³⁺ doping decreases the transformation temperature of gel to anatase and the forming temperature of ZnTiO₃ phase, enhances the transformation temperature of anatase to rutile and results in appearance of Zn₂Ti₃O₈ interphase. With the increases of La³⁺/Ce³⁺ contents, the transformation rates of ZnTiO₃, gel to anatase and anatase to rutile, as well as the granularity of the crystals are reduced. Ultimately, the action mechanism of La³⁺/Ce³⁺ doping was discussed.     

Changes in Optical Properties of Ce: YAG Crystals under Annealing and Irradiation Processing

Changes of optical properties of cerium doped YAG single crystals (0.05, 0.1, 0.2 at.% Ce, and 0.2 at.% Ce "+" 0.1at.% Mg) after thermal annealing at 1400°C in air or 1200°C in N₂+H₂ mixture and subsequent g or proton irradiation were investigated. For initial Ce³⁺ contents <0.05 at.% an increase and for Ce³⁺ contents >0,05 at.% a decrease of the final Ce³⁺ concentration was observed. Appropriate changes in luminescence of Ce: YAG crystals were observed as a consequence of dopant concentration changes. They were small after gamma-irradiation of Ce: YAG crystals with Ce³⁺ content >0,05at.% and reached about 100% after gamma-irradiation of crystals with Ce³⁺ content < 0.05 at.%.     

Synthesis and properties of cerium aluminosilicophosphate glasses

Cerium oxide is commonly added to silicate glasses as an optical property modifier. In particular, UV absorption, decoloration via redox coupling, and resistance to radiation-induced darkening are influenced by the addition of this rare-earth oxide. However, the limited solubility and visible color of rare-earth oxides in silicate glasses prevent any further beneficial enhancement of properties which might result from increasing the CeO₂ content. In contrast, rare-earth oxides are extremely soluble in phosphate glasses; for example, a binary cerium phosphate glass can incorporate up to 40 mol% CeO₂. Moreover, since the UV absorption edge of the phosphate network is blue-shifted compared to the silicate network, the effect of the Ce³⁺ absorption band tail on yellow coloration can be minimized.In this study, glasses in the cerium aluminosilicophosphate system were synthesized and a variety of physical and optical properties were measured including: density, refractive index, glass transition temperature, hardness, fracture toughness, and the location of the UV absorption edge. At ～9 mol% CeO₂, these cerium aluminosilicophosphate glasses exhibit similar coloration to commercially available silicate glasses which contain ～0.4 mol% CeO₂. Semi-quantitative photoemission analysis of the Ce oxidation states showed insignificant differences in the Ce³⁺/Ce⁴⁺ ratio between the phosphate and silicate glass systems.     

A simple synthesis of catalytically active,high surface area ceria aerogels

A simple synthetic route for preparing high surface area, structurally stable ceria aerogels is presented. Ceria aerogels were doped with 1% palladium: a water-gas shift (WGS) active metal. A reduced Pd/ceria aerogel and an as-prepared Pd/ceria aerogel are compared to determine the effect of pretreatment conditions on WGS activity. The BET surface area of the as-prepared ceria aerogel was 345 m²/g. CO chemisorption data and high-resolution TEM images indicate that the particle size of palladium on the reduced Pd/ceria sample was approximately 15 nm. X-ray photoelectron spectroscopy (XPS) data establish that the fraction of Ce³⁺ from Ce₂O₃ was approximately the same for both reduced and as-prepared samples. XPS data also indicate that approximately 44% of the palladium on the reduced Pd/ceria sample was converted to the metallic form. Each catalyst was tested for WGS activity. The reduced Pd/ceria sample exhibited the greatest WGS activity as compared with the other samples used in this study.     

Room temperature epitaxial growth of (001) CeO2 on (001) LaAlO3 by pulsed laser deposition

The room temperature epitaxial growth of CeO₂ on lattice matched (001) LaAlO₃ substrates by using pulsed laser deposition (PLD) method under various oxygen partial pressure (Po₂) is demonstrated. X‐ray diffraction analysis with 2‐Theta/rocking curve/Phi‐scan, cross‐sectional transmission electron microscopy with selected‐area diffractions are used to characterize structural of grown films. The epitaxial (001) CeO₂ can be achieved at room temperature under Po₂ less than 2 × 10⁻³ Torr. The best quality of grown film is obtained under Po₂ = 2 × 10⁻⁵ Torr and degraded under Po₂ = 2 × 10⁻⁶ Torr due to oxygen deficiency in structure. The epitaxial relationship between CeO₂ and LAO is confirmed to be (001)CeO₂ //(001)LAO, [100]_CeO2//[110]_LAO and [010]_CeO2//[10]_LAO. No obvious reduction reaction occurred, from Ce⁺⁴ turned into Ce⁺³ states, as reducing oxygen partial pressure during growth by PLD.     

Double‐frequency properties of In:LiNbO3 crystals

1 mol%, 2 mol%, 3 mol%, 4 mol% and 5 mol% In³⁺ doped LiNbO₃ crystals were grown by the Czochralski method, respectively. Oxidized treatment of some crystals was carried out. The infrared transmission spectra and photo‐damage resistance of the samples were measured. The results showed that the OH^‐ absorption peaks of In(3mol%):LiNbO₃, In(4mol%):LiNbO₃ and In(5mol%):LiNbO₃ crystals were located at about 3508 cm^‐1, while those of In(1mol%):LiNbO₃ and In(2mol%):LiNbO₃ crystals were located at about 3484cm^‐1. When the doped In³⁺ concentration reached its threshold in LiNbO₃ crystal, photo‐damage resistance of In:LiNbO₃ crystals was two orders of magnitude higher than that of pure LiNbO₃ crystal. The experimental results of the second harmonic generation (SHG) showed that the phase matching temperatures of In:LiNbO₃ crystals were lower than those of Zn:LiNbO₃ and Mg:LiNbO₃ crystals and the SHG efficiency reached 38%. Oxidization treatment was also found to make the dark trace resistance of crystals increase. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,structure and electrochemical performance of cobaltite‐based composite cathodes for IT‐SOFC

The development of novel and high‐performance cathodes is a critical issue to be addressed in order to reduce Solid Oxide Fuel Cells (SOFCs) operation temperature to the 600‐800 °C range or less. The performance of CeO₂‐based composite cathodes is very attractive to such operational temperatures. In this work, La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and Ce_0.8Sm_0.2O_1.9 (SDC) powders were synthesized by different synthesis methods and mechanically mixed to prepare LSCF‐SDC composite cathodes. Screen‐printed LSCF‐SDC/CGO/LSCF‐SDC symmetrical cells were sintered at 1150 °C for 4 h and characterized by electrochemical impedance spectroscopy in static air. X‐ray diffraction and scanning electron microscopy were employed to characterize the powders. Area specific resistance values of 0.72 and 2.77 Ω cm² at 800 °C were found for composite cathodes containing SDC powder synthesized by modified Pechini and microwave‐assisted combustion methods, respectively. Furthermore, the activation energy of the composite cathode containing SDC derived from modified Pechini method is 1.18 eV, i.e., much lower than 1.73 eV, value determined for LSCF with SDC from microwave‐combustion method.     

Synthesis of Co‐doped CeO2 nanorods modified glassy carbon electrode for electrochemical detection of nitrobenzene

CeO₂ nanoparticles (CNPs), Co‐doped CeO₂ nanoparticles (CCNPs) and Co‐doped CeO₂ nanorods (CCNRs) were synthesized by simple co‐precipitation method and explored their sensing behaviour towards nitrobenzene. XRD and TEM analysis confirmed the presence of cubic fluorite structure of the CNPs, CCNPs and CCNRs. EDX analysis confirmed the presence of Cobalt in CeO₂ nanorods. The electrochemical sensing of nitrobenzene was carried out using CCNRs‐modified glassy carbon (GC) electrodes by means of cyclic voltammetric (CV) technique. The peak current (I_p) was found to be linearly co‐related to nitrobenzene (NB) concentration (R² = 0.9665). A substantial enhancement in cathodic peak current (C1), and sensitivity (∼738.8 nA/μM) was observed for the CCNRs‐modified GC electrode than those of CNPs and CCNPs‐modified electrodes.     

Structure and electrical conductivity of new Li2O-CeO2-B2O3 glasses

Fourier transformation infrared spectra, density and DC electrical conductivity of 30Li₂O · xCeO₂⋅(70 − x)B₂O₃ glasses, where x ranged between 0 and 15 mol%, have been investigated. The results suggested that CeO₂ plays the role of network modifier up to 7.5 mol%. At higher concentrations it plays a dual role; where most of ceria plays the role of network former. The density was observed to increase with increasing CeO₂ content. The effect on density of the oxides in the glasses investigated is in the succession: B₂O₃ < Li₂O < CeO₂. Most of CeO₂ content was found to be associated with B₂O₃ network to convert BO₃ into B O₄⁻ units. The contribution of Li⁺ ions in the conduction process is much more than that due to small polarons. The conductivity of the glasses is mostly controlled by the Li⁺ ions concentration rather than the activation energy for CeO₂ > 5 mol%. Lower than 5 mol% CeO₂ the conductivity is controlled by both factors. The dependence of W on BO₄⁻ content supports the idea of ionic conduction in these glasses.     

Solarization mechanism of glass containing Ce3+ and As5+

The solarization mechanism in a glass containing both Ce³⁺ and As⁵⁺, 16Na₂O·11CaO·73SiO₂:0.15AsO_x·0.015CeO_x (in mol.%), is newly proposed by elucidating the valence and coordination structure of arsenic after the photochemical reaction, the mechanism being traditionally expressed as

$\text{2}\text{Ce}{}^{\mathrm{3+}}\text{+}\text{As}{}^{\mathrm{5+}}\text{→}\text{hv}\text{2}\text{Ce}{}^{\mathrm{4+}}\text{+}\text{As}{}^{\mathrm{3+}}$

ESR hyperfine quartets due to an As⁴⁺ ranging from 0.1 to 0.5 T built up on UV-irradiation and their line shape varied with the duration of the irradiation. The line shape analysis of the ESR spectra employing a computer simulation technique has led to the following conclusions; (1) As⁵⁺ is reduced to As⁴⁺ in the solarization process. (2) The geometry around the As⁴⁺ in the solarized glass is tetrahedral during the early stage and trigonal-pyramidal during the latter stage of the reaction.     

Synthesis of Ce0.8Sm0.2O1.9 solid electrolyte by a proteic sol‐gel green method

The present study reports the synthesis of Ce_0.8Sm_0.2O_1.9 solid electrolyte by a novel proteic sol‐gel method which uses gelatin as polymerizing agent. The as‐synthesized powder material was calcined at 700 °C for 2 h, with X‐ray diffraction revealing a single cubic phase with lattice parameter a = 0.5435 nm and theoretical density of 7.144 gcm^‐3. The average crystallite size is 12 nm, as determined by the Scherrer equation. Impedance spectroscopy revealed a larger resistive contribution of the grain boundaries than that from grain bulk, which, due to its lower activation energy, tends to dominate the total conductivity above 650 °C. The total conductivity is in line with literature data for ceramics of the same composition prepared by various methods, thus confirming the potential of the proteic sol‐gel method as a green, low cost alternative synthetic route to prepare ceria‐based solid electrolytes.     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐ray diffraction analysis of powder and thin film of (Gd1‐x Yx)2O3 prepared by sol‐gel process

The mixed oxide (Gd_1‐xY_x)₂O₃ (0.0 ≤ x ≤ 1.0) were synthesized, as powder and thin film, by a sol‐gel process. X‐ray diffraction data were collected and crystal structure and microstructure analysis were performed using Rietveld refinement method. All samples were found to have the same crystal system and formed solid solutions over the whole range of x. The cationic distribution, Gd³⁺ and Y³⁺, over the two non‐equivalent sites 8b and 24d of the space group Ia3 is found to be random for all values of (x). The lattice parameter is found to vary linearly with the composition (x). Replacing Gd³⁺ and Y³⁺ by each other introduces a systemic decrease in the x‐coordinate of cation position (24d) and slight changes in the oxygen coordinates. Crystallite size and microstrain analysis is performed along different crystallographic directions and anisotropic changes are found with the composition parameter (x). The average crystallite size ranges from 75 to 149 nm and the r.m.s strain from 0.027 to 0.068 x10^‐2. Textured Gd_1.841Y_0.159O₃ (400) buffer layers, with a high degree of alignment in both out‐plane and in‐plan, are successfully grown on cube textured Ni (001) tape substrates by sol–gel dip coating process. The resulting buffer layers are crack‐free, pinhole‐free, dense and smooth. YbBa₂Cu₃O_7‐x (YbBCO) thin film could be (00l) epitaxially grown on the obtained buffer layer using sol–gel dipping technique. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The relationships between crystal structure of alkaline earth metal hexagonal aluminate and 4f-5d transitions of Ce and Tb ions

The Mg-Sr-Ba hexagonal aluminates (MgSrBa)Al₁₂O₁₉ doped with either Ce³⁺ or Tb³⁺ ion have been synthesized by solid-state reaction. Crystal structure variations with the increasing Ba²⁺ concentration have been thoroughly studied. When the concentration of Ba²⁺ reaches 0.39, the crystal structure becomes the ideal magnetoplumbite-type (No. PDF #80-1195), however, it becomes the same structure as BaMgAl₁₀O₁₇ with space group P6₃/mmc when Sr²⁺ is completely replaced by Ba²⁺ ions. In Ce³⁺ doped compounds, the emission peak of the Ce³⁺ ion is gradually red-shifted when the concentration of Mg²⁺ ion increases, while the 4f-5d transition of Ce³⁺ at 260 nm splits into five components. In the Tb³⁺ doped compounds, the spin-allowed and spin-forbidden 4f-5d transitions of Tb³⁺ have been observed. The intensity of the spin-forbidden 4f-5d transition is gradually increased with decreasing Sr²⁺ concentration and becomes dominant when Sr²⁺ is completely replaced by Ba²⁺. These experimental observations are also discussed in the context and match well with the theoretical calculations of 4f-5d transitions of Tb³⁺.     

Increase in the Fluorine-Ion Conductivity of Single Crystals of Tysonite-type CeF<Subscript>3</Subscript> Superionic Conductor by Substituting Polarized Cd<Superscript>2+</Superscript> Ions for Ce<Superscript>3+</Superscript> Ions

The fluorine-ion conductivity of single crystals with a tysonite (LaF₃) structure with heterovalent isomorphic substitutions of highly polarizable Cd²⁺ cations with a 18-electron shell for rare earth ions Ce³⁺ have been studied for the first time. Ce_0.995Cd_0.005F_2.995 single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. The fluorine-ion conductivity of single crystal is measured by impedance spectroscopy in the temperature range from 153 to 1073 K, where it increases by a factor of 10⁹, approaching the value σ_dc = 5 × 10^–2 S/cm at 1073 K. At T₀ = 450 ± 20 K, the dependence σ_dc(T) is split into two portions with the ion-transport activation enthalpy ΔH_σ = 0.39 ± 0.01 eV (T < T₀) and ΔH_σ = 0.23 ± 0.02 eV (T > T0). It is found that at T = 293 K the conductivity σ_dc = 3 × 10^–5 S/cm of Ce_0.995Cd_0.005F_2.995 crystal is higher by a factor of 10 than the conductivity of the tysonite matrix CeF₃ and close to the σ_dc value for Ce_0.995Sr_0.005F_2.995 crystal. This finding indicates a significant effect of the substitutions of Cd²⁺ ions for Ce³⁺ on the σdc value and the advantage of Cd²⁺ ions over Ca²⁺ and Ba²⁺ from the viewpoint of increasing σ_dc.     

The relations of sintering conditions and microstructures of [Bi0.5(Na1‐x‐yKxLiy)0.5]TiO3 piezoelectric ceramics

Bismuth sodium titanate (abbreviated as BNT) based solid solution, [Bi_0.5(Na_1‐x‐yK_xLi_y)_0.5]TiO₃ (0 < x + y < 1) ceramics, was invented in our group. These ceramics, which are considered as new candidates for lead‐free piezoelectric materials, were prepared by conventional ceramic sintering technique. The results of X‐ray diffraction show that the ceramics possess a single perovskite phase. The relations of the sintering conditions and the microstructures of the ceramics were studied. It was found that the optimized sintering condition is at 1100‐1150 °C for 2‐3 h, the grains of the ceramics have very regular shape, and the grain size of the ceramics is in the range of 1.3‐2.2 μm. These ceramics with the compositions of high amount of K⁺ and low amount of Li⁺ have relatively large piezoelectric charge constant (d₃₃), and can be put into practical applications. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monodisperse CeO2 sub‐micro spherical aggregates with controllable building blocks

Monodisperse CeO₂ spherical aggregates with diameters ranging from 200 to 300 nm have been successfully synthesized through a facile hydrothermal method. The structure and morphology of the samples were characterized by powder X‐ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and field‐emission scanning electron microscopy (FE‐SEM). The building blocks (primary nanocrystals) of the spherical aggregates could be effectively tuned by adding different amount of urea. Furthermore, N₂ adsorption/desorption experiment displays a gradual increase of BET surface areas of spherical aggregates with increasing the amount of urea. Finally, the formation mechanism of CeO₂ spherical aggregates was preliminarily discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface‐morphology evolution of ZnO nanostructures grown by hydrothermal method

Surface‐morphology evolution of ZnO nanocrystals has been observed by the hydrothermal process. The effects of stirring time and ammonia content on the morphology evolution have been discussed, respectively. Extension of stirring time of the precursor results in morphology transformation from star‐like to wire‐like ZnO nanocrystals. ZnO nuclei aggregation and uniform Zn(OH)₂ precipitation can readily explain these two morphologies, respectively. By increasing the ammonia content in the solution, the morphology of ZnO crystals is transformed from an irregular shape to hexagon sheets to nanorods, and the side length of ZnO crystals is decreased accordingly. Hollow structures are realized at the subsequent solution aging process. Variation of zinc ammonic complex and minimum surface energy can well explain the morphology evolution of ZnO nanostructures.