Electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria

The electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria have been determined as a function of temperature (300 < T/°C < 700) and oxygen partial pressure [10^− 24 ≤ p(O₂)/bar ≤ 1, T = 700 °C] by application of impedance spectroscopy. The yttria and ceria contents of Ce_xY_0.2 − xSc_0.6Zr_3.2O_8 − δ (0 ≤ x ≤ 0.2) have been varied systematically. Homogeneous samples have been prepared by means of a sol-gel (glycine-nitrate) combustion process. The ionic conductivity in air is almost independent of composition with typical values around 0.03-0.04 S cm^− 1 for the bulk at 700 °C. A significant decrease of the ionic conductivities of bulk and grain boundaries is found for samples co-doped with ceria at low oxygen partial pressures [p(O₂) < 10^− 15 bar, T = 700 °C]. Activation energies for the ionic transport in oxidizing (air) and reducing (1%-H₂/Ar) atmospheres have been extracted from Arrhenius-plots. The oxygen nonstoichiometry in 1%-H₂/Ar has been investigated by employing thermogravimetry. The decrease of the ionic conductivity under reducing conditions is accompanied by an increase of the corresponding high temperature activation energy of the bulk, which is interpreted in terms of defect association or clustering.     

Solid solution formation, densification and ionic conductivity of Gd- and Sm-doped ceria

The processes of solid solution formation, densification and electrical conductivity in samaria and gadolinia-doped ceria solid electrolytes were studied by Raman spectroscopy, density and impedance spectroscopy measurements. Bulk specimens of Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9 were prepared by solid state reactions at several dwell temperatures and holding times. Hydrostatic density results show a fast increase in sintered density up to 1 h holding time. Raman spectra of specimens sintered for 1 h show a prominent band at 463 cm^− 1 assigned to the cubic fluorite-type lattice of cerium oxide, and low-intensity bands at 344 and 363 cm^− 1 attributed to free samarium and gadolinium sesquioxides, respectively. Solid solution completion was achieved only at temperatures above 1400 °C. Electrical conductivity measurements were used to study mass transport. Analysis of impedance data allowed for determining the activation energy for cation diffusion in Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9 sintered specimens.     

The size and strain effects on the Raman spectra of Ce1−xNdxO2−δ (0≤x≤0.25) nanopowders

Ultrafine Ce_1−xNd_xO_2−δ (x=0-0.25) powders were synthesized by self-propagating room temperature synthesis. Raman spectra were measured at room temperature in the 300-700 cm⁻¹ spectral range. The shift and asymmetric broadening of the Raman F_2g mode at about 454 cm⁻¹ in pure and doped ceria samples could be explained with combined size and inhomogenous strain effects. Increased concentration of O²⁻ vacancies with doping is followed by an appearance of new Raman feature at about 545 cm⁻¹.     

Chemomechanical properties and microstructural stability of nanocrystalline Pr-doped ceria: An in situ X-ray diffraction investigation

The chemomechanical properties and microstructural stability of nanocrystalline Pr_xCe_1 − xO_2 − δ solid solutions are studied as a function of temperature by in situ X-ray diffraction measurements under oxidizing conditions at P(O₂) ~ 200 mbar. The chemical expansion coefficient of nanocrystalline powder specimens, operative at intermediate temperatures during which Pr⁴⁺ is reduced to Pr³⁺, is found to be similar to that obtained for coarse-grained Pr_xCe_1 − xO_2 − δ. This is contrary to reports regarding variation of physical and chemical properties with crystallite size. The thermal expansion coefficient, measured under conditions for which Pr_xCe_1 − xO_2 − δ is highly oxygen deficient, was found to be greater than that measured for fully oxidized Pr_xCe_1 − xO_2 − δ, with potential sources of these changes discussed. Moreover, the microstructure of nanocrystalline Pr_xCe_1 − xO_2 − δ is observed to have excellent stability at working temperatures below 800 °C, enabled by the inherent microstrain in the structure, highlighting the potential application of this material for solid state electrochemical devices.     

The effects of Cr-doping on the room temperature ferromagnetism of chemically synthesized CeO2−δ nanoparticles

We report an investigation of the nature of room-temperature ferromagnetism enhancement in Ce_1−xCr_xO_2−δ nanoparticles (0.00≤x≤0.05), synthesized by a sol–gel-based method. Energy-dispersed X-ray spectrometry (EDS) analysis was used to estimate the dopant concentrations. The average crystallite sizes and particle size were estimated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. Both studies showed a gradual decrease in the size of the crystallites and particles for x>0.01. Cr can substitute for Ce in the crystal lattice, and the Raman measurements indicated that structural defects in the samples increased as a function of the Cr content in the CeO₂ crystal lattice. The surface topography, examined by scanning electron microscopy (SEM), showed that the undoped sample has a porous and loosely organized structure, whereas the Cr-doped samples exhibited a dense and compact structure. Magnetic measurements of the Ce_1−xCr_xO_2−δ samples at 27 ^oC showed a maximum remanent magnetization value of 0.01 emu/g for x=0.05. The nature and enhancement of room-temperature ferromagnetism was interpreted by taking into account the exchange interaction between Cr³⁺ ions and oxygen vacancies in CeO₂.     

Oxygen deficiency, vacancy clustering and ionic transport in (La,Sr)CoO3 − δ

The equilibrium p(O₂)-T-δ diagrams of perovskite-type La_1 − xSr_xCoO_3 − δ (x = 0.3-0.7), collected at 873-1223 K in the oxygen partial pressure range 10^− 5-1 atm by coulometric titration and thermogravimetric analysis, were analyzed in order to appraise the effects of the point-defect interactions. The nonstoichiometry variations were adequately described combining the rigid-band approach for delocalized holes and the pair-cluster formation reaction involving oxygen vacancies and Co²⁺ cations, whilst coulombic repulsion between the positively charged vacancies can be neglected. The resultant relationships between the oxygen chemical potential and mobile vacancy concentration were used for numerical regression analysis of the steady-state oxygen permeation through dense La_1 − xSr_xCoO_3 − δ membranes, affected by the surface exchange kinetics when Sr²⁺ content is higher than 40-50%. The calculated ionic conductivity is strongly influenced by the defect association processes, and decreases with decreasing concentration of the mobile vacancies as clustering starts to prevail on reduction. The Mössbauer spectroscopy studies of La_1 − xSr_xCoO_3 − δ, doped with 1 mol% ⁵⁷Fe isotope and moderately reduced at p(O₂) ≈ 10⁻⁵ atm, show no long-range vacancy ordering at x ≤ 0.5.     

A co-doping approach towards enhanced ionic conductivity in fluorite-based electrolytes

A co-dopant strategy is used to investigate the effect that the elastic strain in the lattice has on the grain ionic conductivity of doped ceria electrolytes. Based on critical dopant ionic radius (r_c), different compositions in the Lu_xNd_yCe_1−x−yO_2−δ (x + y = 0.05, 0.10, 0.15, and 0.20) system are studied. Dopants are added such that the weighted average dopant ionic radius matches r_c for all the compositions. Dense ceramic discs are prepared using conventional solid oxide route and sintering methods. Precise lattice parameter measurements are used to calculate the lattice strain. The ionic conductivity of the samples is measured in the temperature range of 250 °C to 700 °C using two-probe electrochemical impedance spectroscopy technique. The elastic strain present in Lu_xNd_yCe_1−x−yO_2−δ system is found to be negligible when compared to Lu_xCe_1−xO_2−δ (negative) and Nd_xCe_1−xO_2−δ (positive) systems. Grain ionic conductivity of Lu_xNd_yCe_1−x−yO_2−δ (where x + y = 0.05) at 500 °C is observed to be 1.9 × 10^− 3 S/cm which is twice as high as that of Lu_0.05Ce_0.95O_2−δ. These results extend the validity of the r_c concept as a strategy for co-doping ceria electrolytes and open new designing avenues for solid oxide electrolytes with enhanced ionic conductivity.     

Preparation and analysis of zirconia doped ceria nanocrystal dispersions

Crystalline and highly dispersible solid solutions with the general formula Ce_1−xZr_xO_2−δ, where δ represents any possible oxygen vacancies, and in the ceria rich region were prepared using an inorganic sol-gel technique. The particle size distributions of the transparent sols were measured using photon correlation spectroscopy and found to be les than 25 nm with narrow distributions. Compositional analysis using EDAX confirmed the correct zirconia doping. Lattice parameters, structural and crystallite size data for the dried sols and heated powders using X-ray diffraction confirmed a cubic structure stable to at least 1000°C with crystallite sizes varying from 34 Å to 55 nm, depending on calcination temperature and composition. Dispersions of the nanocrystals on silica were studied using high resolution TEM/EDAX, the structure and compositions of individual nanocrystals closely matching those of the bulk agglomerated powders, showing exceptional homogeneity.     

Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition

Nitrogen-doped titanium dioxide (TiO_2−xN_x) thin films have been prepared by pulse laser deposition on quartz glass substrates by ablated titanium dioxide (rutile) target in nitrogen atmosphere. The x value (nitrogen concentration) is 0.567 as determined by X-ray photoelectron spectroscopy measurements. UV-vis spectroscopy measurements revealed two characteristic deep levels located at 1.0 and 2.5 eV below the conduction band. The 1.0 eV level is attributable to the O vacancy state and the 2.5 eV level is introduced by N doping, which contributes to narrowing the band-gap by mixing with the O2p valence band. The enhanced degradation efficiency in a broad visible-light range was observed from the degradation of methylene blue and methylene orange by the TiO_2−xN_x film.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Effect of aluminium doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xAl_xO_2−δ compounds were prepared by the chemical precipitation method using cerium chloride and aluminium chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of aluminium doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Al-doped CeO₂ samples were found to decrease with Al concentration and it increases from 6 to 20 nm as annealing temperature increases to 900 °C.     

Effect of iron doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xFe_xO_2−δ compounds were prepared by chemical precipitation method using cerium chloride and iron chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of iron doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Fe-doped CeO₂ samples were found to decrease with iron concentration and it increases from 9 to 26 nm as annealing temperature increases to 900 °C.     

Highly active Ce1−xCuxO2 nanocomposite catalysts for the low temperature oxidation of CO

A series of Ce_1−xCu_xO₂ nanocomposite catalysts with various copper contents were synthesized by a simple hydrothermal method at low temperature without any surfactants, using mixed solutions of Cu(II) and Ce(III) nitrates as metal sources. These bimetal oxide nanocomposites were characterized by means of XRD, TEM, HRTEM, EDS, N₂ adsorption, H₂-TPR and XPS. The influence of Cu loading (5-25 mol%) and calcination temperature on the surface area, particle size and catalytic behavior of the nanocomposites have been discussed. The catalytic activity of Ce_1−xCu_xO₂ nanocomposites was investigated using the test of CO oxidation reaction. The optimized performance was achieved for the Ce_0.80Cu_0.20O₂ nanocomposite catalyst, which exhibited superior reaction rate of 11.2 × 10⁻⁴ mmol g⁻¹ s⁻¹ and high turnover frequency of 7.53 × 10⁻² s⁻¹ (1% CO balanced with air at a rate of 40 mL min⁻¹, at 90 °C). No obvious deactivation was observed after six times of catalytic reactions for Ce_0.80Cu_0.20O₂ nanocomposite catalyst.     

Sn interaction with the CeO2(1 1 1) system: Bimetallic bonding and ceria reduction

A tin layer 0.8 nm thick was deposited onto the CeO₂(1 1 1) surface by molecular beam deposition at a temperature of 520 K. The interaction of tin with cerium oxide (ceria) was investigated by X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and resonant photoelectron spectroscopy (RPES). The strong tin-ceria interaction led to the formation of a homogeneous bulk Ce-Sn-O mixed oxide system. The bulk compound formation is accompanied by partial Ce⁴⁺ → Ce³⁺ reduction, observed as a giant 4f resonance enhancement of the Ce³⁺ species. CeO₂ and SnO₂ oxides were formed after oxygen treatment at 520 K. The study proved the existence of strong Ce-Sn interaction and charge transfer from Sn to the Ce-O complex that lead to a weakening of the cerium-oxygen bond, and consequently, to the formation of oxygen deficient active sites on the ceria surface. This behavior can be a key for understanding the higher catalytic activity of the SnO_x/CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

(Mo + N) codoped TiO2 for enhanced visible-light photoactivity

A series of Ti_1−xMo_xO_2−yN_y samples were prepared by using sol-gel method and characterized by X-ray diffraction, transmission electron microscopy and UV-vis absorption spectroscopy. All Ti_1−xMo_xO_2−yN_y samples are anatase phase. It is found that Mo, N mono-doping can increase visible light absorption, while (Mo + N) co-doping can greatly enhance absorption in whole visible region. Results of our first-principles band structure calculations reveal that (Mo + N)-doping, especially passivated co-doping can increase the up-limit of dopant concentration and create more impurity bands in the band gap of TiO₂, which leads to a greatly increase of its visible-light absorption without a decrease of its redox potential. It reveals that (Mo + N) co-doped TiO₂ is promising for a photocatalyst with high photocalystic activity under visible light.     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Structural investigation of xAg2O·(100−x)·[2B2O3·As2O3] glasses doped with manganese ions

Structural analysis of x[(100−y)Ag₂O·yMnO]·(100−x)[2B₂O₃·As₂O₃] glasses, with x=10 mol% and 0≤y≤10 mol%, was performed by means of FT-IR and FT-Raman spectroscopies. The purpose of this work is to investigate the structural changes that appear in the xAg₂O·(100−x)·[2B₂O₃·As₂O₃] glasses with the addition and increase in manganese ions content. FT-IR measurements revealed the presence of pyro-, ortho-, di-, tri-, tetra- and penta-borate groups and structural units characteristic to As₂O₃ in the structure of the studied glasses. FT-IR spectroscopy measurements also show that BO₃ units are the main structural units of the glass system. The presence of structural units characteristic to Ag₂O were not directly evidenced by FT-IR spectroscopy. In addtition, the FT-Raman analysis evidenced the presence of boroxol rings in the structure of the studied glasses.     

Hf1−xSixOy dielectric films deposited by UV-photo-induced chemical vapour deposition (UV-CVD)

Hf_1−xSi_xO_y is an attractive candidate material for high-k dielectrics. We report in this work the deposition of ultra-thin Hf_1−xSi_xO_y films (0.1 ≤ x ≥ 0.6) on silicon substrate at 450 °C by UV-photo-induced chemical vapour deposition (UV-CVD) using 222 nm excimer lamps. Silicon(IV) and hafnium(IV) organic compounds were used as the precursors. Films from around 5 to 40 nm in thickness with refractive indices from 1.782 to 1.870 were grown. The deposition rate was found to be of 6 nm/min at a temperature of 450 °C. The physical, interfacial and electrical properties of hafnium silicate (Hf_1−xSi_xO_y) thin films were investigated by using X-ray photoelectron spectroscopy, ellipsometry, FT-IR, C-V and I-V measurements. XRD showed that they were basically amorphous, while Fourier transform infrared spectroscopy (FT-IR), clearly revealed Hf-O-Si absorption in the photo-CVD deposited Hf_1−xSi_xO_y films. Surface and interfacial properties were analysed by TEM and XPS. It is found that carbon content in the films deposited by UV-CVD is very low and it also decreases with increasing Si/(Si + Hf) ratio, as low as about 1 at.% at the Si/(Si + Hf) ratio of 60 at.%.     

Formation of alumina-ceria mixed oxide in model systems

Interaction of aluminium with cerium oxide was studied by photoelectron spectroscopy of Al/CeO₂(1 1 1) and CeO₂/Al(1 1 1) model systems. It was found in both cases that metallic aluminium was immediately oxidized, CeO₂ was partially reduced and a mixed oxide with cerium present as Ce³⁺ was formed. The compound is probably cerium aluminate CeAlO₃ mixed with Al₂O₃ or Ce₂O₃. In both cases the intermixing was limited by the diffusion of aluminium into ceria. The excess of deposited material above this limit formed AlO_x and CeO₂ overlayers on the top of the mixed oxide + aluminate/CeO₂ and mixed oxide + aluminate/Al films, respectively.