Structural regulation and polishing performance of dendritic mesoporous silica (D-mSiO2) supported with samarium-doped cerium oxide composites

Ceria (CeO₂) particles are prevalent polishing abrasive materials. Trivalent lanthanide ions are the popular category of dopants for enriched surface defects and thus improved physicochemical properties, since they are highly compatible with CeO₂ lattices. Herein, a series of dendritic-like mesoporous silica (D-mSiO₂)-supported samarium (Sm)-doped CeO₂ nanocrystals were synthesized via a facile chemical precipitation method. The relation of the structural characteristics and chemical mechanical polishing (CMP) performances were investigated to explore the effect of Sm-doping amounts on the D-mSiO₂/Sm_xCe_1?xO_2?δ (x = 0–1) composite abrasives. The involved low-modulus D-mSiO₂ cores aimed to eliminate surface scratch and damage, resulting from the optimized contact behavior between abrasives and surfaces. The trivalent cerium (Ce³⁺) and oxygen vacancy (V_O) at CeO₂ surfaces were expected to be reactive sites for the material removal process over SiO₂ films. The optimal oxide-CMP performances in terms of removal efficiency and surface quality were achieved by the 40% Sm-doped composite abrasives. It might be attributed to the high Ce³⁺ and V_O concentrations and the enhancement of tribochemical reactivity between CeO₂SiO₂ interfaces. Furthermore, the relationship between the surface chemistry, polishing performance as well as the actual role in oxide-CMP of the D-mSiO₂/Sm_xCe_1?xO_2?δ abrasives were also discussed.     

Photoluminescence properties of sesquioxide doped ceria synthesized by modified sol-gel route

Highly crystalline and porous sesquioxide (Sm₂O₃, La₂O₃) doped ceria with different molar ratio is successfully synthesized by a simple modified sol-gel route. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) are used to investigate their phase, microstructure and composition. XRD analysis confirmed the formation of highly crystalline cubic fluorite phase in all samples. The Raman spectroscopy revealed a single triple degenerated F_2g mode as the attestation of the oxygen vacancy in the doped and undoped samples. Strong photoluminescence lines due to interconfigurational transition and vacancy mediated transition were observed in doped CeO₂. The oxygen vacancy induced luminescence of CeO₂ was strongly enhanced due to La doping. The effect of rare earth dopant on the photoluminescence properties has been studied in details.     

Ce and oxygen vacancy mediated tuning of structural and optical properties of CeO2 nanoparticles

Ceria nanoparticles were synthesized by hydrolysis of cerium nitrate in basic medium. The cubic fluorite structure of ceria was confirmed by XRD. From TEM studies ceria nanoparticles were found to be spherical in shape with an average diameter of 5 nm. The prepared nanoparticles have a predominant orientation along (2 2 2) crystallographic plane. Oxygen vacancies and Ce³⁺ lead to the lattice expansion and strain in CeO₂. Peak asymmetry and broadening of Raman active mode peak further confirms the presence of these defects. Total concentration of oxygen vacancies that are present in the ceria nanocrystallites is calculated to be 1.234 × 10²⁰ cm⁻³. These oxygen vacancies and ceria related defects result in an effective red shifting of the band gap by changing its structural regularity. The visible luminescence peaks are also caused by these Ce³⁺ and oxygen vacancy centers.     

Annealing temperature and oxygen-vacancy-dependent variation of lattice strain,band gap and luminescence properties of CeO2 nanoparticles

CeO₂ nanoparticles are annealed in vacuum at 200°C and in air at 200°C, 600°C and 1000°C, respectively. Vacuum-annealed CeO₂ contains high concentration of oxygen vacancies and exhibits very high lattice strain, whereas the corresponding values decrease on air annealing. Oxygen-deficient CeO₂ has redshift in band gap with high Urbach energy. The magnitude of this energy decreases as CeO₂ is annealed in air at 600°C. At 1000°C, thermal disorder increases the Urbach energy. Photoluminescence property of the samples depends on the presence of radiative and non-radiative oxygen vacancy centres. Vacuum-annealed ceria have large numbers of non-radiative oxygen vacancies that act as emission quencher. CeO₂ annealed at 600°C contains requisite amount of oxygen vacancies to show better luminescence property.     

Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

Trivalent Ce2O3 and CeO2−x intermediate oxides induced by laser irradiation of CeO2 powders

A strong non-stoichiometry of pure fcc CeO₂ was induced by laser irradiation. The increase of laser power and/or energy density had a saturable effect on particle size growth. The possibility of CeO₂ reduction to A-Ce₂O₃ by laser irradiation was demonstrated. Particles of stable Ce₇O₁₂ phase were observed in all specimens irradiated at low laser-power densities. An epitaxial relationship between triclinic Ce₁₁O₂₀ and cubic Ce₁₂O₂₂ phases was found. The controversial C-Ce₂O₃ phase was detected at the limits of a bcc particle. An unknown bcc phase of acicular morphology, strongly related to C-Ce₂O₃, was also registered. The dose dependence of CeO₂ structural modifications obtained by laser irradiation as a function of laser energy density variation could be explained by a simple defect aggregation model implying lattice defects (oxygen vacancies and Ce³⁺ ions).     

Room-Temperature Ferromagnetism in Chemically Synthesized Ce0.97Cr0.03O2−δ Nanopowders

In this paper, we study structural and magnetic properties of undoped (CeO_2?δ ) and Cr-doped (Ce_0.97Cr_0.03O_2?δ ) cerium oxide nanopowders synthesized by a sol-gel-based method. We estimate the crystallite sizes calculated from X-ray diffraction measurements to be around 9.4±0.3 nm for both CeO_2?δ and Ce_0.97Cr_0.03O_2?δ samples. Raman measurements indicate that microstructural defects are generated when Cr substitutes the Ce in the CeO₂ crystal lattice. Magnetic measurements of the Ce_0.97Cr_0.03O_2?δ sample at 300 K indicate ferromagnetic behavior, with a coercive field of 34.27 Oe and a saturation magnetization of 5.8×10^?4 emu/g. We interpret the nature of room-temperature ferromagnetism by taking into account the exchange interaction between Cr³⁺ ions and oxygen vacancies in CeO₂.     

The Influence of Redox Reaction of the Sintering of Cerium Oxide

Cerium oxide (CeO₂) is a promising material that has potential for use in a number of applications, such as resistive-type oxygen sensors and solid oxide fuel cells. In this work, the sintering behavior of hydrothermal synthesized nano-size CeO₂ powders and chemical precipitated and commercial micron-size CeO₂ powders were investigated by continuous monitoring of the shrinkage kinetics. The results demonstrated that during the high temperature sintering process a partial redox reaction of ceria occurred, i.e., a fraction of Ce⁴⁺ was reduced to Ce³⁺, and oxygen gas was released. The redox reaction influenced the sintering behavior of CeO₂, resulting in a decrease in density and microcracking for the hydrothermal synthesized nano-size CeO₂ powder compacts and sagged points in the sintering curves for the chemical precipitated and commercial micron-size CeO₂ powder compacts. It was found by scanning electron microscopy that the partial redox reaction of ceria produced additional pores in the powder compacts during the sintering process and thus much higher temperatures were needed to achieve high density.     

Single step synthesis of nanosized CeO2–MxOy mixed oxides (MxOy?=?SiO2, TiO2, ZrO2, and Al2O3) by microwave induced solution combustion synthesis: characterization and CO oxidation

Various CeO₂ –M _x O _y (M _x O _y  = SiO₂, TiO₂, ZrO₂, and Al₂O₃) mixed oxides were prepared by microwave induced solution combustion method and analyzed by different complimentary techniques, namely, X-ray diffraction (XRD), Raman spectroscopic (RS), UV–Vis diffuse reflectance spectroscopy (UV-DRS), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG-DTA), and BET surface area. XRD analyses revealed that CeO₂ –SiO₂ and CeO₂ –TiO₂ mixed oxides are in slightly amorphous form and exhibit only broad diffraction lines due to cubic fluorite structure of ceria. XRD lines due to the formation of cubic Ce_0.5Zr_0.5O₂ were observed in the case of CeO₂ –ZrO₂ sample. RS results suggested defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The UV-DRS measurements provided valid information about Ce⁴⁺ ← O²⁻ and Ce³⁺ ← O²⁻ charge transfer transitions. XPS studies revealed the presence of cerium in both Ce³⁺ and Ce⁴⁺ oxidation states. The ceria–zirconia combination exhibited better oxygen storage capacity (OSC) and CO oxidation activity when compared to other samples. The significance of present synthesis method lays mostly on its simplicity, flexibility, and the easy control of different experimental factors.     

Influence of Ce3+/Ce4+ ratio on phase stability and residual stress field in ceria-yttria stabilized zirconia plasma-sprayed coatings

CeO₂-Y₂O₃ stabilized zirconia plasma-sprayed coatings have been produced by two different procedures based on different cooling rates. A high cooling rate (HCR) induced the contemporary presence of CeO₂ and Ce₂O₃, while cooling in calm air (LCR) induced the presence of only CeO₂. After subjecting the HCR coating, detached from the substrate, to an air thermal treatment, which oxidizes Ce₂O₃ to CeO₂, a lower thermal expansion coefficient was measured than in the LCR material, and shrinkage was also observed. This behaviour is explained by the phase composition changes induced by the oxidation of ceria.     

Microstructure and phase transformation of zirconia-based ternary oxides for thermal barrier coating applications

Five dopant oxides, Sc₂O₃, Yb₂O₃, CeO₂, Ta₂O₅, and Nb₂O₅, were incorporated into 7YSZ to create ternary zirconia-based oxides with varying oxygen vacancies and substitutional defects. These ternary oxides were consolidated using a high-temperature sintering process. The resulting bulk oxides were subjected to microstructural study using scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The results show that the microstructures of the ternary zirconia-based oxides are determined by the amount of oxygen vacancies in the system, the dopant cation radius, and atomic mass. Increasing the number of oxygen vacancies in the lattice by the addition of trivalent dopant as well as the use of larger cations promotes the stabilization of the high-temperature cubic phase. The tetravalent cation, on the other hand, has the effect of retaining tetragonal phase to room temperature without the influence of oxygen vacancy. The addition of pentavalent oxide leads to the formation of monoclinic phase upon cooling.     

Properties of CeOx/La2O3 gate dielectric and its effects on the MOS transistor characteristics

Both n-channel and p-channel metal-oxide-semiconductor (MOS) transistors were fabricated by using CeO_x/La₂O₃ stacked gate dielectric film and tungsten metal electrode. X-ray photoelectron spectroscopy (XPS) measurements indicated that the as-deposited CeO₂ was reduced to CeO_x (a mixture of CeO₂ and Ce₂O₃ phases) and the released oxygen atoms filled up the oxygen vacancies in the La₂O₃ film and thus improved the electrical characteristics of the transistors. We found that the amount of the dielectric defects in this structure were very low as evidenced by the recorded small threshold voltages for both types of transistors. The transistors also have excellent subthreshold characteristics and hot-carrier robustness. The subthreshold slopes were 72 and 73 mV/dec for n-channel and p-channel transistors with effective gate length of 1.8 μm, respectively, and remain fairly unchanged upon prolonged hot-carrier stressing.     

Thermal stability and photoluminescence of Zr1−xCexO2 (0 ≤ x ≤ 1) nanoparticles synthesized in a non-aqueous process

Complete range Zr_1−xCe_xO₂ (0 ≤ x ≤ 1) nanoparticles are synthesized by the thermal decomposition of metal-organic precursors (zirconium acetylacetonate and cerium acetylacetonate) in oleylamine. XRD and HRTEM indicate that all of the as-prepared nanoparticles are single-crystal, and the crystallinity becomes better with increasing Ce content. The Zr_1−xCe_xO₂ nanoparticles with Ce content larger than 1/8 crystallize in cubic fluorite phase. XRD measurements on the as-prepared and calcinated ZrO₂ samples reveal that the tetragonal ZrO₂ nanoparticles are stable below 600 °C, and the lattice parameters of ZrO₂ nanoparticles decrease with decreasing particle size. The thermal stability of the cubic phase increases with increasing Ce content. The UV–vis absorption spectra reveal that the band gap energy increases with increasing cerium content. Room-temperature photoluminescence (PL) spectra of pure ZrO₂ nanoparticles show strong emission peaks centered at about 441 nm at room temperature, which is attributed to the ionized oxygen vacancies in the nanoparticles. On the other hand, room-temperature PL spectra of the as-prepared CeO₂ nanoparticles shows two peaks at 417 and 436 nm, which might arise from the transition from the cerium 4f and to the oxygen 2p band (valence band) in CeO₂ and the presence of oxygen vacancies, respectively.     

Novel composite interconnecting ceramics La0.7Ca0.3CrO3−δ/Ce0.8Sm0.2O1.9 for solid oxide fuel cells

In this paper, an interconnecting ceramic for solid oxide fuel cells was developed, based on the modification from La_0.7Ca_0.3CrO_3−δ by addition of Ce_0.8Sm_0.2O_1.9. It is found that addition of small amount Ce_0.8Sm_0.2O_1.9 into La_0.7Ca_0.3CrO_3−δ dramatically increased the electrical conductivity. For the best system, La_0.7Ca_0.3CrO_3−δ + 5 wt.% Ce_0.8Sm_0.2O_1.9, the electrical conductivity reached 687.8 S cm⁻¹ at 800 °C in air. In H₂ at 800 °C, the specimen with 3 wt.% Ce_0.8Sm_0.2O_1.9 had the maximal electrical conductivity of 7.1 S cm⁻¹. With the increase of Ce_0.8Sm_0.2O_1.9 content the relative density increased, reaching 98.7% when the Ce_0.8Sm_0.2O_1.9 content was 10 wt.%. The average coefficient of thermal expansion at 30-1000 °C in air increased with Ce_0.8Sm_0.2O_1.9 content, ranging from 11.12 × 10⁻⁶ to 12.46 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating it is still an electronically conducting ceramic. Therefore, this material system will be a very promising interconnect for solid oxide fuel cells.     

Formation of solute atmospheres around dislocations by excess vacancies

Long range solute atmospheres tend to form around dislocations by diffusion. They scatter phonons and can be studied through their effect on the lattice thermal conductivity. Previous studies indicate that these atmospheres are formed after plastic deformation in Cu-Al but not in Cu-Ge alloys. Ordinary diffusion at room temperature is too slow to permit atmosphere formation, but excess vacancies originally produced during plastic deformation can enhance diffusion. The vacancies diffuse towards the dislocation, where they are annihilated, and their presence causes solute diffusion and hence atmosphere formation. The atmospheres attain equilibrium over a range R, and R² can be expressed in terms of a time-integrated diffusion coefficient due to excess vacancies until the excess is exhausted. This range R is independent of the vacancy jump rate, and depends only on the initial vacancy concentration and the dislocation density. The values of R thus calculated are much smaller than those observed. However, R is increased if there is enhancement of the solute diffusion relative to the solvent diffusion and also if dislocations do not act as perfect vacancy sinks. Estimates of R are given for Cu-Al, Cu-Ge and Al-Mg.     

Preparation of an Alkali-Element or Alkali-Earth-Element-Doped CeO2–Sm2O3 System and Its Operation Properties as the Electrolyte in Planar Solid Oxide Fuel Cells

Ceria–samaria (CeO₂–Sm₂O₃) is one of the most interesting fluorite oxides since its ionic conductivity is higher than that of yttria-stabilized zirconia in air. However, these CeO₂ -based oxides are partially reduced and develop electronic conductivity under fuel cell operating conditions. In their application to the SOFC system, their current densities and power densities are not at a satisfactory level. For the development of high-performance CeO₂ electrolytes, it is important that the fluorite lattice of CeO₂-based oxide be improved from the viewpoint of crystallography. In this study, it is assumed that the reduction of Ce⁴⁺ in the fluorite lattice was inhibited by expansion of the CeO₂ lattice. In order to investigate the contribution of the expanded CeO₂ lattice to reduction resistance, CeO₂–Sm₂O₃ solid solution, calcia-doped CeO₂–Sm₂O₃ solid solution, and a small amount of alkali element-doped CeO₂–Sm₂O₃ -based oxide were prepared for comparison. It was found that the calcia or a small amount of alkali element-doped CeO₂ solid solution enhanced the oxide ionic conductivity. The power density of the latter showed a high value at 800°C. It is concluded that the improved fuel cell performance can be attributed to the good reduction resistance in the fuel cell atmosphere.     

Template-free hydrothermal synthesis and optical properties of ceria hollow nanospheres with rough surface

The large-scale uniform CeO₂ hollow nanospheres with diameter of about 600 nm and rough surface have been successfully synthesized via a simple template-free hydrothermal technology. The obtained samples were examined by XRD, SEM, TEM, XPS, Raman scattering and UV–Vis spectra. The results show that the samples have a cubic fluorite structure of CeO₂ with no crystalline impurity phase and many Ce³⁺ ions and oxygen vacancies exist in the surface of CeO₂ sample. A red-shifting of the band gap is observed for the CeO₂ hollow nanospheres contrasting with the bulk one, which is mainly attributed to the influences of the Ce³⁺ ions and oxygen vacancies.     

A Cobalt-Free Multi-Phase Nanocomposite as Near-Ideal Cathode of Intermediate-Temperature Solid Oxide Fuel Cells Developed by Smart Self-Assembly

An ideal solid oxide fuel cell (SOFC) cathode should meet multiple requirements, i.e., high activity for oxygen reduction reaction (ORR), good conductivity, favorable stability, and sound thermo-mechanical/chemical compatibility with electrolyte, while it is very challenging to achieve all these requirements based on a single-phase material. Herein, a cost-effective multi-phase nanocomposite, facilely synthesized through smart self-assembly at high temperature, is developed as a near-ideal cathode of intermediate-temperature SOFCs, showing high ORR activity (an area-specific resistance of ≈0.028 Ω cm² and a power output of 1208 mW cm⁻² at 650 °C), affordable conductivity (21.5 S cm⁻¹ at 650 °C), favorable stability (560 h operation in single cell), excellent chemical compatibility with Sm_0.2Ce_0.8O_1.9 electrolyte, and reduced thermal expansion coefficient (≈16.8 × 10⁻⁶ K⁻¹). Such a nanocomposite (Sr_0.9Ce_0.1Fe_0.8Ni_0.2O_3–δ) is composed of a single perovskite main phase (77.2 wt%), a Ruddlesden–Popper (RP) second phase (13.3 wt%), and surface-decorated NiO (5.8 wt%) and CeO₂ (3.7 wt%) minor phases. The RP phase promotes the oxygen bulk diffusion while NiO and CeO₂ nanoparticles facilitate the oxygen surface process and O²⁻ migration from the surface to the main phase, respectively. The strong interaction between four phases in nanodomain creates a synergistic effect, leading to the superior ORR activity.     

Preparation and electrical behavior study of the ceramic interconnect La0.7Ca0.3CrO3−δ with CeO2-based electrolyte Ce0.8Sm0.2O1.9

Based on the conventional interconnect La_0.7Ca_0.3CrO_3−δ, a novel ceramic interconnect for intermediate temperature solid oxide fuel cells was developed. In the air, the electrical conductivities of La_0.7Ca_0.3CrO_3−δ + 5%Ce_0.8Sm_0.2O_1.9 at 600, 700 and 800 °C were 96.7, 146.3 and 687.8 S cm⁻¹, respectively, which increased significantly as compared with La_0.7Ca_0.3CrO_3−δ under the same conditions. Similarly, in pure hydrogen, La_0.7Ca_0.3CrO_3−δ + 3%Ce_0.8Sm_0.2O_1.9 possessed the maximal electrical conductivities which were 4.2, 5.3 and 7.1 S cm⁻¹, respectively at 600, 700 and 800 °C. The crystal structures of La_0.7Ca_0.3CrO_3−δ, La_0.7Ca_0.3CrO_3−δ + 5%Ce_0.8Sm_0.2O_1.9 and La_0.7Ca_0.3CrO_3−δ + 10%Ce_0.8Sm_0.2O_1.9 were single phase with hexagonal symmetry, cubic phase plus some doped ceria impurity and orthorhombic phase plus some doped ceria impurity, respectively. The difference between the crystal structures may account for the difference between the electrical conductivities. The electrical conductivities and sinterability of La_0.7Ca_0.3CrO_3−δ were increased by introducing Ce_0.8Sm_0.2O_1.9, whereas the other properties were not influenced.     

2D Electron Gas and Oxygen Vacancy Induced High Oxygen Evolution Performances for Advanced Co3O4/CeO2 Nanohybrids

The rational design of atomic‐scale interfaces in multiphase nanohybrids is an alluring and challenging approach to develop advanced electrocatalysts. Herein, through the selection of two different metal oxides with particular intrinsic features, advanced Co₃O₄/CeO₂ nanohybrids (NHs) with CeO₂ nanocubes anchored on Co₃O₄ nanosheets are developed, which show not only high oxygen vacancy concentration but also remarkable 2D electron gas (2DEG) behavior with ≈0.79 ± 0.1 excess e^?/u.c. on the Ce³⁺ sites at the Co₃O₄–CeO₂ interface. Such a 2DEG transport channel leads to a high carrier density of 3.8 × 10¹⁴ cm^?2 and good conductivity. Consequently, the Co₃O₄/CeO₂ NHs demonstrate dramatically enhanced oxygen evolution reaction (OER) performances with a low overpotential of 270 mV at 10 mA cm^?2 and a high turnover frequency of 0.25 s^?1 when compared to those of pure Co₃O₄ and CeO₂ counterparts, outperforming commercial IrO₂ and some recently reported representative OER catalysts. These results demonstrate the validity of tailoring the electrocatalytic properties of metal oxides by 2DEG engineering, offering a step forward in the design of advanced hybrid nanostructures.