Ce Doping Concentration Research Articles

Selective catalytic oxidation of NO with O 2 over Ce-doped MnO x/TiO 2 catalysts

A series of Ce-doped MnO x /TiO 2 catalysts were prepared by impregnation method and used for catalytic oxidation of NO in the presence of excess O2. The sample with the Ce doping concentration of Ce/Mn=1/3 and calcined at 300°C shows a superior activity for NO oxidation to NO 2. On Ce(1)Mn(3)Ti catalyst, 58% NO conversion was obtained at 200°C and 85% NO conversion at 250°C with a GHSV of 41000 h −1, which was much higher than that over MnO x /TiO 2 catalyst (48% at 250°C). Characterization results implied that the higher activity of Ce(1)Mn(3)Ti could be attributed to the enrichment of well-dispersed MnO x on the surface and the abundance of Mn 3+ and Ti 3+ species. The addition of Ce into MnO x /TiO 2 could improve oxygen storage capacity and facilitate oxygen mobility of the catalyst as shown by PL and ESR, so that its activity for NO oxidation could be enhanced. The effect of H 2O and SO 2 on the catalyst activity was also investigated.

Photoluminescence of undoped and Ce-doped SnO2 thin films deposited by sol–gel-dip-coating method

The undoped and Ce-doped SnO2 thin films synthesized by a simple sol–gel-dip-coating method on glass substrates were investigated by means of X-ray diffractometer, visible spectrophotometer and photoluminescence (PL) spectroscopy. The samples were polycrystalline thin films with rutile structure of undoped SnO2. A violet PL peak was observed in undoped SnO2 at room temperatures, while a blue PL peak appeared only after introducing Ce3+ ions into the SnO2 host. The effects of Ce doping concentration and heat-treatment temperature on PL of the films were further studied, and the luminescence mechanism was also discussed.

Crossover from low-temperature itinerant to high-temperature localized electron behavior in the electron-doped rare-earth metal cobaltate perovskites

We present a study on the temperature dependence of the electronic structure and magnetic properties of Ce${}_{x}$Eu${}_{1\ensuremath{-}x}$CoO${}_{3}$ by means of x-ray absorption spectroscopy (XAS) and magnetic susceptibility measurements. Contrary to what was previously reported in literature for this compound, we identified the partially substituted Ce species to be tetravalent within the whole temperature range investigated, i.e., 300 \ensuremath{\geqslant} $T$/K \ensuremath{\geqslant} 40. It is shown that, as a result, corresponding amounts of Co${}^{2+}$ are formed at room temperature, indicating an electron-doping effect. At $T$ $=$ 40 K, however, even though electron doping could be identified through Co XAS pre-edge features, the Co${}^{2+}$ species were not identifiable. These results indicate a change in the Co-related electronic structure with temperature, and we interpret this as indicative of a crossover from a low-temperature itinerant-electron state to a high-temperature localized-electron state. The magnetic susceptibility revealed the onset of low-temperature itinerant ferromagnetism for the higher Ce-doping concentrations, while the room-temperature effective magnetic moment ${\ensuremath{\mu}}_{\mathrm{eff}}$ value for the lowest Ce-concentrated sample was in complete agreement with the theoretical one for Co${}^{2+}$, thus being consistent with the itinerant--localized-electron crossover scenario as depicted from the XAS results.

Preparation and characterization of CeO 2/TiO 2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO 2, CeO 2 and CeO 2-doped TiO 2 have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N 2-physisorption, Transmission Electron Microscopy (TEM) and UV–Vis spectrophotometry. The TiO 2 and CeO 2-doped TiO 2 nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10–13 nm for Ce doping concentrations of 5–50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO 2. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce 3+/4+ in the TiO 2 matrix, which generated an n-type impurity band.

Preparation, characterization and infrared emissivity study of Ce-doped ZnO films

Ce-doped ZnO films were prepared by the sol-gel method with spin coating onto glass substrates. Zinc acetate dihydrate, ethanol, diethanolamine and cerium nitrate hexahydrate were used as starting material, solvent, stabilizer and dopant source, respectively. Structure and microstructure of the films were characterized with X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and the energy dispersive X-ray spectrometry (EDS). The infrared properties were also investigated. It was found that Ce-doped ZnO films showed a hexagonal wurtzite structure and had a (101) preferred orientation. Metallic Ce existed in the form of CeO 2, and the addition of Ce had an effect on the infrared emissivity of the films. The infrared emissivity increased with the temperature rising when the ZnO films were doped with 3 at.% and 7 at.% of Ce concentration. However, the infrared emissivity decreased with the temperature increasing when the ZnO film was doped with 5 at.% of Ce. It suggested that the infrared emissivity of the films was remarkably changed by controlling the Ce doping concentration.

Preparation of cerium-doped titania macroporous films by a sol–gel spin coating using polypropylene glycol (PPG) as pore-creating agent: Effects of Ce ions, PPG and calcination on photocatalytic activity

Cerium-doped titania macroporous films (CTMs) were spin-coated on glass substrates from a controlled sol–gel solution using titanium alkoxide and polypropylene glycol (PPG) as precursor and pore-creating agent, respectively. The physicochemical properties of CTMs were characterized by scanning electron microscope (SEM), thermogravimetric and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD) and BET surface area analysis. The photocatalytic activity of CTMs was evaluated by degradation of methylene blue (MB). The results showed the decomposition of PPG during calcinations was responsible for the generation of macroporous structure in the films. The Ce ions, PPG and calcination respectively influenced recombination of electron–hole pairs, surface area, hydroxyl concentration and crystalline structure of films, so that one of those factors could restrict the photocatalytic activity of films. Compared with others films, CTMs-1 showed the highest photocatalytic activity with 0.5 wt.% of Ce doping concentration and 2.0 g PPG additions at calcinating temperature of 550 °C. It was mainly attributed to the effects that both the optimum concentration of Ce ions doping and high surface area could promote the photocatalytic degradation by preventing recombination of electron–hole pairs and providing more MB molecules around TiO 2 and high concentration of hydroxyl.

Sol–gel preparation and photoluminescence properties of Ce 3+-activated Y 3Al 5O 12 nano-sized powders

Trivalent cerium-doped yttrium aluminum garnet (YAG:Ce 3+) nanoparticles were successfully prepared by a facile sol–gel technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were used to characterize the as-prepared phosphors. Well-crystallized fine nanoparticles were obtained at 1000 °C. Single-crystal nanoparticles with irregular shapes were obtained, with crystallite sizes ranging between 20 and 60 nm. PL intensity of the particles increased monotonically with decreasing Ce doping concentration and showed the maximum value at 0.1 at%.

Formation identification of Y2BaCuO5 and BaCeO3 in MOCVD-processed YBCO films with Ce doping

The influence of nanosized secondary particles, such as those produced viarare-earth doping for example, on magnetic-field-induced flux motion inYBa2Cu3O7−δ (YBCO)-coated conductors has been investigated. In this study, we investigated the influence of Cedoping on nanosized secondary phase formation in YBCO films. Ce-doped YBCO films on singleSrTiO3 crystals were prepared via metal–organic chemical vapor deposition (MOCVD). For the first time, aY2BaCuO5 (Y211)secondary phase was observed in the Ce-doped YBCO film, while the pure YBCO film contained onlya Y2O3 secondary phase. As Ce doping concentration increased to 3 wt%, aBaCeO3 secondary phase developed. This was due to the low solubility limit of Ce for Y in the MOCVD YBCOfilm. Additionally, outgrowths on the Ce-doped YBCO film surfaces and defect structures, such asa-axis grains, were examined.

Matrix induced synthesis of Y3Al5O12: Ce phosphor through the Pechini method

Abstract Trivalent cerium actived yttrium aluminum garnet (YAG: Ce) phosphor was prepared using the Pechini method. Phase transition during calcination was investigated through Fourier transform infrared spectroscopy and X-ray diffraction. The results show that the powders directly transform into the pure YAG phase at 80°C. The dimension of phosphor calcined at 1200°C is micro-scale, which consists of nanometer scale primary particles. The effect of Ce doping content and calcination temperature on the luminescence properties is also discussed. The maximum emission is obtained for about 1 at.% Ce additions. The concentration quench effect is observed when Ce content increases beyond this limit. With the increment of calcination temperature, the emission intensity and lifetime increase.

Effects of $\hbox {Ca}^{2+}$ Co-Doping on the Scintillation Properties of LSO:Ce

In addition to desirable physical properties including a density of 7.4 g/cm3 , an effective atomic number of 66, and no hygroscopicity, Lu2SiO5:Ce has well-known scintillation properties of ~30 900 photons/MeV, an emission peak near 420 nm, and a decay time of ~43 ns. These scintillation properties are achieved with Ce doping concentrations roughly in the range of 0.05 to 0.5 atomic percent relative to Lu. These properties make Lu2SiO5:Ce a widely used scintillator in positron emission tomography, in particular. We have found that both the light output and decay time may be improved by a combination of optimized crystal growth atmosphere and co-doping with divalent cations such as Ca. Scintillation light output of ~38 800 photons/MeV has been achieved as well as scintillation decay time as short as 31 ns with no long components. The relationship between growth conditions, dopant concentration, decay time, and light output is well defined, thus allowing one to reliably tune the crystal to the desired combination of light output and decay time. Possible explanations of the underlying mechanism are being explored and include compensation of oxygen vacancies, alteration of the relative occupancies of the cerium lattice sites, and suppression of trapping centers. In addition to higher count-rate capability and better coincidence timing, the improved decay time is expected to be particularly advantageous for time-of-flight positron emission tomography. Also, phoswich detectors comprising standard LSO (~43 ns decay time) and fast LSO (~31 ns decay time) become an attractive alternative to typical phoswich designs that often suffer from problems of mismatched light outputs and indices of refraction or the absorption of one scintillator's light by the other.

Preparation and properties of nanocrystalline powders in (Y 1−xCe x) 3Al 5O 12 system

Nano-sized cerium-doped yttrium aluminum garnet (YAG:Ce) phosphors were synthesized via a simple sol–gel process using metal nitrate precursors. The prepared phosphors were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy, respectively. Pure cubic garnet phase was formed at temperatures ∼900 οC. The particle sizes of as-prepared powders were mostly in the range of 17–27 nm. The crystalline YAG:Ce showed broad emission peaks in the range of 400–700 nm and maximum intensities at 500 and 520 nm. It is found also that the emission intensity decreased with increasing Ce doping concentration from 0.1 to 1.5 at%. With increasing Ce doping concentration, the PL intensity was shifted towards shorter wavelengths.

Weak-coupling Bardeen-Cooper-Schrieffer superconductivity in the electron-doped cuprate superconductors

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap $\ensuremath{\Delta}(T)$ in electron-doped copper oxides with various ${T}_{c}$'s and Ce-doping concentrations. We show that the temperature dependence of $\ensuremath{\Delta}(T)$ follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where $\ensuremath{\Delta}(0)∕{k}_{B}{T}_{c}\ensuremath{\approx}1.72\ifmmode\pm\else\textpm\fi{}0.15$ and $\ensuremath{\Delta}(0)$ is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak-coupling BCS superconductors.

Self-activity in lanthanum halides

Scintillators fabricated from lanthanum halides, specifically cerium-activated lanthanum bromide (LaBr3 [Ce]) and lanthanum chloride (LaCl3 [Ce]), have been shown to exhibit energy resolution far superior to that for thallium-activated sodium iodide. The high resolution, high Z, and high density (LaBr3 5.29 g/cm3 and LaCl3 3.79 g/cm3) of these materials hold out the prospect of use for building superior room-temperature, hand-held identification instruments. Recently attention has been devoted to investigating the practical issues connected with the use of these new scintillators. These efforts include attempts to increase the size of the crystals, optimize the Ce dopant concentration, and evaluate the properties of detectors fabricated from existing material. A negative outcome in the property evaluation has been the observation of self-activity, e.g., between 1.7 and 2.7 MeV for LaCl3. This paper reports on efforts to characterize the use of lanthanum halides for identification including measurements of self-activity and presents a discussion of the signal-to-noise ratio which has implications for the minimum detectable activity

Catalytic steam reforming of ethane and propane over CeO 2-doped Ni/Al 2O 3 at SOFC temperature: Improvement of resistance toward carbon formation by the redox property of doping CeO 2

Ni/Al 2O 3 with the doping of CeO 2 was found to have useful activity to reform ethane and propane with steam under Solid Oxide Fuel Cells (SOFCs) conditions, 700–900 °C. CeO 2-doped Ni/Al 2O 3 with 14% ceria doping content showed the best reforming activity among those with the ceria content between 0 and 20%. The amount of carbon formation decreased with increasing Ce content. However, Ni was easily oxidized when more than 16% of ceria was doped. Compared to conventional Ni/Al 2O 3, 14%CeO 2-doped Ni/Al 2O 3 provides significantly higher reforming reactivity and resistance toward carbon deposition. These enhancements are mainly due to the influence of the redox properties of doped ceria. Regarding the temperature programmed reduction experiments (TPR-1), the redox properties and the oxygen storage capacity (OSC) for the catalysts increased with increasing Ce doping content. In addition, it was also proven in the present work that the redox of these catalysts are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results. During the reforming process, in addition to the reactions on Ni surface, the gas–solid reactions between the gaseous components presented in the system (C 2H 6, C 3H 8, C 2H 4, CH 4, CO 2, CO, H 2O, and H 2) and the lattice oxygen (O x ) on ceria surface also take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen (O x ) on ceria surface (C n H m +O x → nCO+ m/2(H 2)+O x− n ) can prevent the formation of carbon species on Ni surface from hydrocarbons decomposition reaction (C n H m ⇔ nC+ m/2H 2). Moreover, the formation of carbon via Boudard reaction (2CO⇔CO 2+C) is also reduced by the gas–solid reaction of carbon monoxide (produced from steam reforming) with the lattice oxygen (CO+O x ⇔CO 2+O x−1 ).

Domain switching in strontium-barium niobate crystals investigated by photorefractive self-focusing

The domain switching behaviour of undoped and Ce-doped Sr0.61Ba0.39Nb2O6 (SBN) crystals is investigated for different experimental conditions. For the investigation the photorefractive self-focusing technique is used. This method is demonstrated to be well suited for determining the polarization state of SBN crystals spatially resolved. We find that the repoling behaviour of nominally pure SBN is spatially inhomogeneous. The crystals are well repoled at the positive electrode, but are depolarized at the negative electrode. The larger the Ce doping concentration is, the smaller the depolarized crystal area. The domain switching depends on the switching velocity, while no influence of the electrode material is observed.

Mechanical loss of Zr 0.8− xCe xY 0.2O 1.9 ( x=0–0.4)

The solid solution oxides of CeO 2–Y 2O 3 and ZrO 2–Y 2O 3 are useful crystals as solid state ionics, which are applied to sensors, solid electrolytes and other advanced devices such as SOFC. However, in the ternary system of CeO 2–ZrO 2–Y 2O 3, the decrease in oxide ion conductivity is observed by mixing ZrO 2 and CeO 2. In this work, the oxygen movement and the doping effect of CeO 2 to ZrO 2–Y 2O 2 are examined by the mechanical loss measurement of polycrystalline bodies. We discuss the difference of the mobility of oxygen, which is the origin of mechanical loss in the crystals, by comparing the mechanical loss peaks in a series of solid solutions, Zr 0.8− x Ce x Y 0.2O 1.9 ( x=0–0.4). The low temperature mechanical loss at 300–820 K spectra were measured for Zr 0.8− x Ce x Y 0.2O 1.9 ( x=0–0.4). We observed two loss maxima of Debye-type relaxation which are related to local jumps of oxygen, and their strong Ce-doping level dependence. The activation enthalpy of 1.2 eV suggested the simple relaxation of an oxygen in the pairs of oxygen vacancies and Y ions in nearest neighbor (peak 1). Additional maximum (peak 2) with H=2.2 eV, which appeared at high temperature and are related to defect agglomerates with the same more than one dopant cation, disappeared with the Ce-doping of x=0.2 and 0.4. The loss (peak 1) amplitude also decreased with increasing of Ce-doping content, but with the same activation enthalpy (1.1–0.9 eV). The addition of CeO 2 in Y 2O 3–ZrO 2 was found to reduce the mobility of oxygen vacancies under elastic field.

Effect of cerium doping on the micro-structure and electrical properties of sol-gel derived Pb 1.05(Zr 0.53− δCe δTi 0.47)O 3 ( δ≤10 at.%) thin films

We have studied the effect of Ce on the phase formation behavior, surface morphology and electrical properties of sol-gel derived PZT thin films. Ce doping has been found to have a dramatic influence on the surface morphology and electrical characteristics of PZT thin films. Up to 1 at.% doping content Ce is found to improve the dielectric and ferroelectric properties and also optimal Ce doping is found to be effective in reducing the leakage current density and increasing the dielectric breakdown strength of PZT thin films. The fractional site occupancy of Ce ion has been predicted theoretically and the validity of the prediction has been investigated by XPS analysis. We have observed that Ce ions can exist both in +3 and +4 valence states. Up to 1 at.% content Ce predominantly enters into the A site with +3 valence state and acts as a donor dopant. The observed variation in microstructure and electrical properties have been discussed to be due to the fractional site occupancy of Ce ion either in A or B site, as a function of Ce doping content in PZT thin films.

Structure, oxygen stoichiometry and electrical conductivity in the system Sr-Ce-Co-O

Mixed oxides in the system S-Ce-Co-O were prepared by solid state reaction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid solutions of the type Sr 1 − y Ce y CoO 3 − x and mixed oxides of the type (1 − y)SrCeO 3 − ySrCoO 3 − x . Microstructures and phase compositions were determined by electron microscopy and X-ray diffraction. SrCoO 3 − x forms a solid solution of ceria on the A-site in the strontium cobaltite lattice up to 0.15 mol Ce. This solid solution corresponds to the high-temperature structure of pure SrCoO 3 − x and is characterized by high oxygen exchange and electrical conductivity. The oxygen deficiency x was measured by solid electrolyte coulometry. The oxygen deficiency of solid solutions Sr 1 − y Ce y CoO 3 − x increases with temperature and decreases with pO 2 in the ambient atmosphere and with increasing Ce dopant concentration. The pO 2- T- x diagram of the solid solution was determined. The T, pO 2 and dopant concentration dependencies of electrical conductivity were measured by a four-point d.c. technique. By Ce doping strontium cobaltite becomes a stabilized high-conductive material (maximum conductivity: 500 S cm −1 at 400 °C, E a = 0.025 eV, p-type). Above this temperature the T-coefficient of the conductivity changes from positive (semiconducting) to negative values.

Dependence on Ce Concentration of Blue Emission and Crystallographic Properties in SrGa2S4:Ce Electroluminescent Thin Films Grown by Molecular Beam Epitaxy

The Ce-doping dependence of blue emission properties has been investigated for SrGa2S4:Ce electroluminescent (EL) thin films grown by molecular beam epitaxy (MBE). EL spectra, EL luminance, photoluminescence (PL) spectra, X-ray diffraction (XRD) intensity, XRD rocking curves, spacing of lattice planes, EL and PL decay times were measured in samples with Ce-doping concentrations ranging from 0.87 mol% to 8.37 mol%. EL luminance reached a maximum at 2.40 mol%. In particular, EL chromaticity remained within the pure blue region even with higher Ce-doping concentrations.

Variation of mechanism transition wavelength of self-pumped phase conjugation with Ce-content in BaTiO 3:Ce Crystals

The phenomena of SPPC (self-pumped phase conjugation) mechanism transition from “stimulated photorefractive backscattering and four-wave mixing” (SPB-FWM) to “cat” (or TIR) with wavelength have been investigated in BaTiO 3:Ce crystals with different Ce-doping concentrations. In addition, it is observed that SPPC is generated with SPB-FWM mechanism in an undoped BaTiO 3 crystal at wavelengths shorter than 488 nm while it is well known that “cat” mechanism is usually responsible for the SPPC generation in BaTiO 3 crystals at longer wavelengths. A red shift of the mechanism transition wavelength with the doping concentration in BaTiO 3 crystals is clearly evident from our experiments. This is believed to be due to the increase of the stimulated photorefractive gain coefficient of the crystals with the doping concentration.