Oxidation State Of Cerium Research Articles

Optical absorption and XPS studies of (Ba1−xSrx)(Ce0.75Zr0.10Y0.15)O3−δ electrolytes for protonic ceramic fuel cells

Perovskite-type materials containing metal and cerium cations have been used as catalysts, membranes, electrodes in fuel cells, and hydrogen pumps. The effect of strontium doping on the perovskite-type structure has been evaluated and effectively maintains the cubic structure of (Ba1−xSrx)(Ce0.75Zr0.10Y0.15)O3−δ. X-ray photoelectron spectroscopy (XPS) was performed on the oxidation states of barium, strontium, cerium, zirconium, yttrium and oxygen. With strontium doping, the oxidation state of cerium ions in 4+ increases and results in the decrease in oxygen vacancy concentration. These are attributed to the strong binding energy of (A/B)–O bond which in turn, enhances the structural stability of electrolyte system. The free volume (Vf) is maximum for x=0.2 (Vf=33.1Å3) and then decreases with further strontium insertion. The optical absorption peaks are attributed to charge transfer transitions and energy band gap (increases from ~4.13 to ~4.35eV). However, the increase of band gap is only ~5% with partial replacement of barium by strontium. These characteristics are useful to make (Ba1−xSrx)(Ce0.75Zr0.10Y0.15)O3−δ a promising candidate for futuristic and reliable fuel cell technology.

Tetravalent Ce in the Nitrate-Decorated Hexanuclear Cluster [Ce6(μ3-O)4(μ3-OH)4]12+: A Structural End Point for Ceria Nanoparticles

We describe the synthesis and characterization of three glycine-stabilized hexanuclear CeIV cluster compounds, each containing the [Ce6(μ3-O)4(μ3-OH)4]12+ core structure. Crystallized from aqueous nitrate solutions with pH < 0, the core cluster structures exhibit variable decoration by nitrate, glycine, and water ligands depending on solution conditions, where increased nitrate and glycine decoration of the cluster core was observed for crystals synthesized at high Ce and nitrate concentrations. No other crystalline products were observed using this synthetic route. In addition to confirming the tetravalent oxidation state of cerium in one of the reported clusters, cyclic voltammetry also indicates that CeIV is reduced at ∼+0.60 V vs Ag/AgCl (3 M NaCl), which is significantly less than the standard electrode potential. This large decrease in the CeIV/CeIII reduction potential suggests that CeIV is significantly stabilized relative to CeIII within the examined cluster. These compounds are discussed in term...

Electronic Structure of Ce-doped ZrO2Film: Study of DFT Calculation and Photoelectron Spectroscopy

In this study, we evaluated the change of electronic structure during redox process in cerium-doped ZrO₂ grown by sol gel method. By sol-gel method, we could obtain cerium-doped ZrO₂ in high oxygen partial pressure and low temperature. After post annealing process in nitrogen ambient, the film is deoxidized. We used spectroscopic and theoretical methods to analysis change of electronic structure. X-ray absorption spectroscopy (XAS) for O K1-edge and Density Functional Theory (DFT) calculation using VASP code were performed to verify the electronic structure of the film. Also, high resolution x-ray photoelectron spectroscopy (HRXPS) for Ce 3d was carried out to confirm chemical bond of cerium doped ZrO₂. Through the investigation of the electronic structure, we verified as followings. (1) During reduction process, binding energy of oxygen is increase. Simultaneously, oxidation state of cerium was change to 4+ to 3+. (2) Cerium 4+ and cerium 3+ states were generated at different energy level. (3) Absorption states in O K edge were mainly originated by Ce 4+ f 0 and Ce 3+, while occupied states in valance band were mainly originated from Ce 4+ f₂.

Cerium anomaly at microscale in fossils.

Patterns in rare earth element (REE) concentrations are essential instruments to assess geochemical processes in Earth and environmental sciences. Excursions in the "cerium anomaly" are widely used to inform on past redox conditions in sediments. This proxy resources to the specificity of cerium to adopt both the +III and +IV oxidation states, while most rare earths are purely trivalent and share very similar reactivity and transport properties. In practical terms, the level of cerium anomaly is established through elemental point quantification and profiling. All these models rely on a supposed homogeneity of the cerium oxidation state within the samples. However, this has never been demonstrated, whereas the cerium concentration can significantly vary within a sample, as shown for fossils, which would vastly complicate interpretation of REE patterns. Here, we report direct micrometric mapping of Ce speciation through synchrotron X-ray absorption spectroscopy and production of local rare earth patterns in paleontological fossil tissues through X-ray fluorescence mapping. The sensitivity of the approach is demonstrated on well-preserved fishes and crustaceans from the Late Cretaceous (ca. 95 million years (Myr) old). The presence of Ce under the +IV form within the fossil tissues is attributed to slightly oxidative local conditions of burial and agrees well with the limited negative cerium anomaly observed in REE patterns. The [Ce(IV)]/[Ce(tot)] ratio appears remarkably stable at the microscale within each fossil and is similar between fossils from the locality. Speciation maps were obtained from an original combination of synchrotron microbeam X-ray fluorescence, absorption spectroscopy, and diffraction, together with light and electron microscopy. This work also highlights the need for more systematic studies of cerium geochemistry at the microscale in paleontological contexts, in particular across fossil histologies.

Effect of Ba-deficiency on the phase and structural stability of (BaSr)(CeZr)O3-based proton conducting oxides

Deficiency in proton conducting perovskite-type oxides can be strongly influence the fuel cell properties. The effect of barium deficiency on the phase structure, cerium oxidation states, electrical conductivity and phase stability under carbon dioxide atmosphere of proton conducting oxides with composition of (Ba0.6−pSr0.4)(Ce0.75Zr0.10Y0.15)O3−δ (p – barium deficiency; 0.025 ≤ p ≤ 0.20) was systematically investigated. Rietveld refinement of the X-ray diffraction patterns shows the cubic structure in the space group Pm-3m up to p = 0.10. The lattice parameter decreases with ‘p’ and lie in the range ∼4.358 to ∼4.333 Å. XPS data show the percentage of Ce4+ increases which in turn, reduces the oxygen non-stoichiometry (δ) – values with deficiency (p). Raman spectra show the presence of oxygen vacancies which decreases with ‘p’. The conduction behavior depicts the protonic nature with activation energy lying in the range of 0.24–0.29 eV. The stability of deficient compositions (0.025 ≤ p ≤ 0.20) enhances in carbon dioxide (CO2) atmosphere in compare to p = 0.

Cerium oxidation state in silicate melts: Combined fO2, temperature and compositional effects

To quantify the relative proportions of Ce3+ and Ce4+ in natural magmas, we have synthesized a series of Ce doped glasses ranging in composition from basalt to rhyolite (±H2O) at 0.001 and 1GPa, under fO2 conditions varying from FMQ −4.0 to FMQ +8.4, and temperatures from 1200 to 1500°C. The Ce4+/Ce3+ ratio in the experimental run products was determined both potentiometrically and in situ, using Ce M4,5-edge X-ray absorption near-edge structure (XANES) spectroscopy. For a given melt composition, the change in Ce4+/Ce3+ ratio with fO2 follows the trend predicted from the reaction stoichiometry assuming simple oxides as melt species. In addition to fO2, melt composition and water content have been found to be secondary controls on Ce4+/Ce3+, with more depolymerized melts and hydrous compositions favoring the stabilization of Ce3+. The Ce4+/Ce3+ ratio can be expressed through the equation,logCeO2CeO3/2=14logfO2+5705(±257)T-0.8990(±0.0805)NBOT-3.856(±0.083)·xH2O-3.889(±0.037)where T is in Kelvin, NBO/T is the proportion of non-bridging oxygen to tetrahedrally coordinated cations, and xH2O is the mole fraction (calculated using molecular oxides, e.g. Al2O3, Na2O) of water dissolved in the melt. A recent study conducted by Burnham and Berry (2014, Chemical Geology) investigating Ce oxidation state in silicate melts using Ce L3-edge XANES, equilibrated under a subset of the conditions investigated here, showed a similar dependence of Ce4+/Ce3+ on T and fO2, however, melt composition was found to have the opposite effect, with decreasing melt polymerization resulting in an increased abundance of Ce4+. This apparent discrepancy likely arises from the presence of alkalies and H2O in the compositions presented in this study, which were absent in the study of Burnham and Berry (2014). Our results indicate that even at relatively low oxygen fugacity, trace amounts of Ce4+ will be present in most terrestrial igneous systems, suggesting that Ce partitioning could be a sensitive indicator of fO2.

Cerium oxidation state and covalent 4f-orbital contributions in the ground state of bis(η8-pentalene)cerium

The sandwich complex bis(η8-pentalene)cerium Ce(C8H6)2 was investigated by relativistic quantum chemical ab initio methods and the nature of the interaction between the Ce 4f and ligand π orbitals was analyzed. It is shown for a multi-configurational ground state wavefunction of the complete active space variety that by means of an orbital rotation in the space of the strongly interacting orbitals it is possible to generate nearly pure Ce 4f and ligand π orbitals without changing the total energy. These orbitals are occupied by one electron each, leading to an open-shell 4f1π1 ground state configuration. The admixture of the energetically higher closed-shell 4f0π2 configuration leads to a singlet ground state, making the complex another example for a molecular analog of a Ce(III)-based lattice Kondo system. A further analysis using local spin and occupation number fluctuations in a two-electron two-orbital description reveals that the Ce 4f-ligand π interaction corresponds to a significantly stretched covalent bond: orbital and configuration mixing coefficients as well as local spin and occupation number fluctuations are very similar to those of hydrogen dimer H2 at an interatomic distance of about two times the equilibrium distance.

Evidence for enhanced oxygen surface exchange reaction in nanostructured Gd2O3-doped CeO2 films

The effect of microstructure of Gd2O3-doped CeO2 (GDC) films on oxygen surface exchange and diffusion is reported. Epitaxial GDC (10 mol% Gd) films up to 1 μm in thickness are prepared using pulsed laser deposition on (100) yttria-stabilized zirconia single-crystal substrates and subjected to high-temperature annealing at 1300 °C in air to induce microstructural modifications. Characterization using atomic force microscopy and transmission electron microscopy reveals granular morphologies comprised of densely packed columnar nanostructures for the as-grown GDC films; however, significant microstructural reconstruction of the entire GDC layer occurs after high-temperature annealing. 18O isotope exchange depth profiling with dynamic secondary ion mass spectroscopy is employed to evaluate the oxygen surface exchange coefficient k* and the diffusion coefficient D* at T = 600 °C. The as-grown GDC exhibits up to 10 times higher k* than the annealed film. The strong differences in oxygen surface reaction are correlated to the observed film properties including surface microstructure and cerium oxidation state as evaluated using electron energy loss spectroscopy in scanning transmission electron microscopy.

Quantitative depth profiling of Ce(3+) in Pt/CeO2 by in situ high-energy XPS in a hydrogen atmosphere.

The redox property of ceria is a key factor in the catalytic activity of ceria-based catalysts. The oxidation state of well-defined ceria nanocubes in gas environments was analysed in situ by a novel combination of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) and high-energy XPS at a synchrotron X-ray source. In situ high-energy XPS is a promising new tool to determine the electronic structure of matter under defined conditions. The aim was to quantitatively determine the degree of cerium reduction in a nano-structured ceria-supported platinum catalyst as a function of the gas environment. To obtain a non-destructive depth profile at near-ambient pressure, in situ high-energy XPS analysis was performed by varying the kinetic energy of photoelectrons from 1 to 5 keV, and, thus, the probing depth. In ceria nanocubes doped with platinum, oxygen vacancies formed only in the uppermost layers of ceria in an atmosphere of 1 mbar hydrogen and 403 K. For pristine ceria nanocubes, no change in the cerium oxidation state in various hydrogen or oxygen atmospheres was observed as a function of probing depth. In the absence of platinum, hydrogen does not dissociate and, thus, does not lead to reduction of ceria.

Control of cerium oxidation state through metal complex secondary structures.

A series of alkali metal cerium diphenylhydrazido complexes, M x (py) y [Ce(PhNNPh)4], M = Li, Na, and K, x = 4 (Li and Na) or 5 (K), and y = 4 (Li), 8 (Na), or 7 (K), were synthesized to probe how a secondary coordination sphere would modulate electronic structures at a cerium cation. The resulting electronic structures of the heterobimetallic cerium diphenylhydrazido complexes were found to be strongly dependent on the identity of the alkali metal cations. When M = Li+ or Na+, the cerium(iii) starting material was oxidized with concomitant reduction of 1,2-diphenylhydrazine to aniline. Reduction of 1,2-diphenylhydrazine was not observed when M = K+, and the complex remained in the cerium(iii) oxidation state. Oxidation of the cerium(iii) diphenylhydrazido complex to the Ce(iv) diphenylhydrazido one was achieved through a simple cation exchange reaction of the alkali metals. UV-Vis spectroscopy, FTIR spectroscopy, electrochemistry, magnetic susceptibility, and DFT studies were used to probe the oxidation state and the electronic changes that occurred at the metal centre.

Effect of CeO2 on the Glass Structure of Sodium Germanate Glasses

Germanate glasses have potential applications as optical fibers. Materials doped with rare earth ions are good candidates for optical, lasing, and magnetic applications. Based on the ternary system, CeO2–Na2O–GeO2 a series of six glasses were fabricated using powder fusion, and varying the Na2O content from 0 to 45 mol%, and a CeO2 content constant at 3 mol%. The glasses were analyzed by FT‐IR, Raman and X‐ray photoelectron (XPS) spectroscopies to obtain information about the glass structure, cerium oxidation's state and how it is introduced in the glass network. FT‐IR and Raman spectra revealed the presence of GeO6 and GeO4 groups as well as Q2 and Q3 units in the glasses with alkali low content. XPS spectra analysis revealed that the cerium ions were reduced from Ce4+ to Ce3+. The nonbonding to total oxygen ratio was estimated from the curve fitting of the O 1s core level spectra. Density and elastic parameters showed a nonlineal tendency in the change of the physical properties as a function of Na2O content. Finally, photoluminescence spectroscopy confirmed the presence of Ce3+ ions. The characteristic 4f → 5d electronic transitions at 360 nm were detected, when a 280 nm excitation line of pulsed laser was used as excitation source.

Electrochemically enhanced metal-support interaction of highly dispersed Ru nanoparticles with a CeO2 support

Small particle size (1.1nm) Ru nanoparticles were supported on a mixed ionic-electronic conductor, CeO2, with a low metal loading (1wt%), interfaced with a YSZ electrolyte. A pronounced enhancement (up to about 2.5 times) of the catalytic rate for the complete oxidation of ethylene was observed for negative polarization. The opposite effect was observed for positive polarization. Apparent Faradaic efficiencies up to 96 were determined, indicating a non-Faradaic effect. The modification of the cerium oxidation state (i.e., reduction from Ce4+ to Ce3+) is proposed to enhance the catalytic performance of the Ru nanoparticles. XPS analysis was performed to confirm this reduction of ceria. The enhancement of catalytic activity is attributed to the presence of more oxygen vacancies in the ceria interlayer causing a stronger metal-support interaction. Results demonstrate the feasibility of in-situ modification of the metal support-interaction between Ru nanoparticles and CeO2 catalytic support by a small current (−2μA) application.

Preparation and corrosion resistance of cerium conversion coatings on AZ91D magnesium alloy by a cathodic electrochemical treatment

Cerium conversion coatings were prepared on AZ91D magnesium alloy by a cathodic electrochemical treatment. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), electrochemistry workstation and X-ray photoelectron spectroscopy (XPS) were used to characterize the compositions, micro-morphology, corrosion resistance and the oxidation state of cerium and oxygen elements of the coatings. The results showed that the conversion coatings presented an intact and uniform morphology when the cathodic current density was 0.20Adm−2, the temperature was 35°C and the treatment time was 2min. When the applied current densities were higher than 0.25Adm−2, the coatings showed a micro-cracked morphology. The better corrosion resistance of the cerium-based conversion coatings could be obtained when the current density was 0.20Adm−2 when tested in 3.5% NaCl (wt.) solution. There are Ce(IV) and Ce(III) oxidation states in the coating.

Effect of nature of ceria supports on the growth and sintering behavior of Au nanoparticles

The growth and sintering behavior of gold have been investigated on reducible CeOx(111) (1.5<x<2) thin films with controlled oxidation states under ultrahigh vacuum conditions. Scanning tunneling microscopy studies reveal that Au experiences the transition of the three-dimensional particle growth on the fully oxidized ceria surface to the two-dimensional growth on the reduced ceria at room temperature. X-ray photoelectron spectroscopy data show a positive shift of up to 0.7eV for the Au 4f core level with the increase of the degree of Ce reduction in the film, which is attributed to the Au particle size effect. Upon heating to higher temperatures, Au agglomerates to form large particles with a well-defined hexagonal shape. The temperature of transformation depends on the cerium oxidation states. These Au particles exhibit the 4f binding energies that are characteristics for the bulk gold. Instead of nanoparticles, layered films can be formed on the reduced ceria surface when Au is deposited at 500K. Our study demonstrates that the structure of Au and correspondingly the interfaces between Au and ceria can be controlled by varying the nature of ceria surfaces as well as annealing/deposition temperatures. Such study can play a role in the understanding of the structure–reactivity relationship of ceria-supported gold catalysts.

Improved Catalytic Reactor for the Electrochemical Promotion of Highly Dispersed Ru Nanoparticles with CeO2 Support

Electrochemical promotion of catalysis (EPOC) is a promising method for enhancing catalytic activity through the application of a small electrical stimulus between the catalyst-working and counter electrode deposited on a solid electrolyte 1. The electronic properties of the catalyst can be modified resulting in a change in catalytic activity. In the case of yttria-stabilized zirconia (YSZ) as a solid electrolyte, the addition or removal of O2- species on the catalyst surface can be controlled in situ depending on the specified reaction conditions. Fully reversible and “permanent” or “persistent” EPOC has been reported for more than 70 various catalytic systems 1. In reversible EPOC experiments, the reaction rate returns to its initial value after the electrical stimulus is interrupted. For permanent EPOC (P-EPOC), the reaction rate remains at a higher value than the initial open circuit value 2,3. Despite receiving much attention, this phenomenon has not yet reached commercial application. One of the main technical factors preventing such development is the use of thick film catalysts with low surface areas and high material costs 4.Ceria, CeO2, is a mixed ionic-electronic conducting (MIEC) material that conducts O2- due to oxygen vacancies in the crystallographic structure in addition to conducting electrons at elevated temperatures. Furthermore, due to its non-stoichiometry, CeO2 has the ability to undergo conversion between Ce4+ and Ce3+ quite easily 5. These properties make the use of ceria-containing catalysts of interest for many applications. In heterogeneous catalysis, Pt group metals deposited on CeO2 show a metal-support interaction (MSI) effect associated with charge transfer between the two solids that are in contact. In EPOC studies, using a MIEC can also ensure electrical connectivity between highly dispersed nanoparticle catalysts 6.In this study, electrochemical enhancement of catalytic activity of a low particle size (1.9 nm) ruthenium nanoparticles catalyst for ethylene oxidation was investigated. Ru nanoparticles, synthesized using a modified polyol reduction method, were supported on CeO2 resulting in a 1 wt% Ru loading (RuNPs/CeO2) (i.e., typical in heterogeneous catalysis studies).The highly dispersed RuNPs/CeO2 catalyst powder was supported on a YSZ solid electrolyte in order to apply polarization. The discussion of this study includes the effect of the partial pressure of ethylene with constant partial pressure of oxygen, temperature, and applied positive and negative current on the catalytic activity of the RuNPs/CeO2 catalyst as well as the role of the cerium redox state in the observed persistent effect. In addition, the catalytic properties of the RuNPs/CeO2 catalyst is compared to that of larger Ru particles supported on CeO2 (TD-Ru/CeO2) (same metal loading) and blank CeO2, both supported on a YSZ solid electrolyte.Characterizations of the catalysts were carried out using TEM and SEM. In addition, XPS analysis was done for the RuNPs/CeO2catalyst as prepared and “spent” (after the reaction). Overall, it was observed that only the RuNPs/CeO2 catalyst could be catalytically enhanced, showing a pronounced enhancement (up to 2.5 times) of the catalytic rate for negative polarization. The opposite effect was observed for positive polarization. This effect of both positive and negative polarization is illustrated by Fig.1. Fig. 1.Transient effect of current application for C2H4 oxidation over Ru/CeO2 on YSZ electrolyte at 350°C for an applied current of -2 μA for 4 hours and +2 μA for 6 hours (0.012 kPa C2H4).Apparent Faradaic efficiencies up to 100 were also determined, indicating a non-Faradaic effect. In addition, a persistent effect was observed, showing stability up to 16 hours after current interruption. The modification of the cerium oxidation state (i.e., reduction from Ce4+ to Ce3+) is proposed to enhancethe catalytic performance of the Ru nanoparticles. This is due to the presence of more oxygen vacancies in the ceria interlayer causing a stronger metal-support interaction.These results demonstrate the feasibility of in-situ modification of the metal support-interaction between Ru nanoparticles and CeO2catalytic support.

Gd/Sm dopant-modified oxidation state and defect generation in nano-ceria

Abstract Gd/Sm-modified oxidation state and defect in nano-ceria are studied by X-ray photoelectron (XPS) and Raman spectroscopy technique to investigate their influence on oxy-ionic conductivity of the system. Nanosize Sm and Gd doped ceria (SDC and GDC) are synthesized in different compositions, Ce(1 − x) (Sm / Gd)xO3 − δ (x = 0.05 to 0.30) under identical conditions. XPS study of doped ceria system indicates enrichment of Ce3+ ions in GDC as compared to that in SDC. The quantitative analysis of Raman spectra also predicted the same trend of oxidation state of cerium. The ionic conductivity measurement by electrochemical impedance spectroscopy is demonstrated to verify the results for all compositions of dopant (x = 0.05 – 0.30).

Visualization of the Heterogeneity of Cerium Oxidation States in Single Pt/Ce2Zr2Ox Catalyst Particles by Nano‐XAFS

The cerium oxidation states in single catalyst particles of Pt/Ce2Zr2O(x) (x=7 to 8) were investigated by spatially resolved nano X-ray absorption fine structure (nano-XAFS) using an X-ray nanobeam. Differences in the distribution of the Ce oxidation states between Pt/Ce2Zr2O(x) single particles of different oxygen compositions x were visualized in the obtained two-dimensional X-ray fluorescent (XRF) mapping images and the Ce L(III)-edge nano X-ray absorption near-edge structure (nano-XANES) spectra.

Synthesis and Theoretical Investigations of the Solid Solution CeRu1–xNixAl (x = 0.1–0.95) Showing Cerium Valence Fluctuations

Members of the solid solution series of CeRu(1-x)Ni(x)Al can be obtained directly by arc melting of the elements. The presented compounds with 0.1 ≤ x ≤ 0.85 crystallize in the orthorhombic space group Pnma (No. 62) in the LaNiAl structure type, while for 0.9 ≤ x ≤ 1, the hexagonal ZrNiAl-type structure is found. The orthorhombic members exhibit an anomaly in the trend of the lattice parameters as well as an interesting behavior of the magnetic susceptibility, suggesting that the cerium cations exhibit no local moment. Besides the mixed-valent nature of the cerium cations, valence fluctuations along with a change in the cerium oxidation state depending on the nickel content have been found. The oxidation state has been determined from the magnetic data and additionally by XANES. Density functional theory calculations have identified the shortest Ce-Ru interaction as decisive for the stability of the orthorhombic solid solution.

Remarkable changes in the photoluminescent properties of Y2Ce2O7:Eu(3+) red phosphors through modification of the cerium oxidation states and oxygen vacancy ordering.

A new series of red phosphors based on Eu(3+)-doped yttrium cerate [Y1.9Ce2O7:0.1Eu(3+), Y2Ce1.9O7:0.1Eu(3+) and Y2Ce2-xO7:xEu(3+) (x = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.50)] was prepared via a conventional solid-state method. The influence of the substitution of Eu(3+) at the aliovalent site on the photoluminescent properties was determined by powder X-ray diffraction, FT Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy with energy-dispersive spectroscopy, UV-visible absorption spectroscopy, photoluminescence spectroscopy and lifetime measurements. The substitution of Eu(3+) at the Ce(4+) site induces a structural transition from a defect fluorite to a C-type structure, which increases the oxygen vacancy ordering and the distortion of the Eu(3+) environment, and decreases the formation of Ce(3+) states. In contrast, phosphors with isovalent substitution at the Y(3+) site exhibit the biphasic nature of defect fluorite and a C-type structure, thereby increasing the number of Ce(3+) oxidation states. These modifications resulted in remarkable changes in the photoluminescent properties of Y2Ce1.9O7:0.1Eu(3+) red phosphors, with emission intensities 3.8 times greater than those of the Ce0.9O2:0.1Eu(3+) and Y1.9Ce2O7:0.1Eu(3+). The photoluminescent properties of Y2Ce2-xO7:xEu(3+) were studied at different Eu(3+) concentrations under excitation with blue light. These phosphors emit intense red light due to the (5)D0-(7)F2 transition under excitation at 466 nm and no concentration quenching is observed with up to 50 mol% Eu(3+). They show increased lifetimes in the range 0.62-0.72 ms at Eu(3+) concentrations. The cation ordering linked to the oxygen vacancy ordering led to the uniform distribution of Eu(3+) ions in the lattice, thus allowing higher doping concentrations without quenching and consequently increasing the lifetime of the (5)D0 states. Our results demonstrate that significant improvements in the photoluminescence properties can be achieved by the structural alteration of a fluorite CeO2 to a C-type lattice.

X-ray photoelectron spectroscopy analysis and band offset determination of CeO2deposited on epitaxial (100), (110), and (111)Ge

The oxidation states, interface, and band alignment properties of physical vapor deposited CeO2 films on epitaxial (100), (110), and (111)Ge were investigated by x-ray photoelectron spectroscopy (XPS). The cross-sectional transmission electron microscopy demonstrated the polycrystalline nature of the CeO2 film. XPS analysis showed multiple Ce3d and Ce4d oxidation states with a mixture of Ce3+ and Ce4+ components existing in CeO2. Angular resolved XPS investigations indicate that the CeO2 films mostly consist of Ce4+ oxidation states while the Ce3+ oxidation states are preferentially present near the surface. The CeO2/(100)Ge, CeO2/(110)Ge, and CeO2/(111)Ge structures showed almost identical valence band offset (VBO) values of 1.6, 1.5, and 1.6 eV, respectively, using XPS measurements from Ce3d core level (CL) peaks. These (VBO) values were also supported by XPS measurements from shallow Ce4d CL binding energy peaks. The conduction band offset values between CeO2/Ge were ∼1.3 eV using the measured optical bandgap of CeO2. The XPS spectral analysis of cerium oxidation states and the measured band offset parameters for carrier confinement would offer an important path for the future design of Ge-based metal-oxide semiconductor devices.