Reduction of thin-film ceria on Pt(111) by supported Pd nanoparticles probed with resonant photoemission

Local defects present in CeO_2 ? x films result in a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. Previous studies of the Ce 3d region with XPS have shown that depositing metal nanoparticles on ceria films causes further reduction, with an increase in Ce³⁺ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce³⁺ and Ce⁴⁺ in CeO_2 ? x films grown on Pt (111), and the variation of this concentration as a function of Pd deposition. Due to the nature of the electronic structure of CeO_2 ? x, resonant peaks are observed for the 4d–4f transitions when the photon energy matches the resonant energy; (hν = 121.0 eV) for Ce³⁺ and (hν = 124.5 eV) for Ce⁴⁺. This results in two discrete resonant photoemission peaks in valence band spectra. The ratio of the difference of these peaks with off-resonance scans gives an indication of the relative contribution of Ce³⁺. Results from RESPES indicate reduction of CeO_2 ? x on deposition of Pd, confirming earlier findings from XPS studies.     

Peculiarities of luminescent properties of cerium doped YAG transparent nanoceramics

Optical and luminescence properties of transparent nanosized cerium doped Y₃Al₅O₁₂ (YAG:Ce) ceramics have been studied. YAG:Ce nanoceramics were obtained by means of low temperature and high pressure (LTHP) sintering method. Nanoceramic samples were sintered in the 2–8 GPa pressure range, whereas Ce³⁺ concentration was varied in the 0.5–5 at. % range. It is shown that, in contrast to the single crystal, a strong rise of absorption coefficient was detected already at wavelength shorter than 400 nm in all nanoceramic samples studied. Furthermore, in nanoceramic samples unusual UV emission band near 3.1 eV was observed, which is not observed in the YAG:Ce single crystal. High pressure applied during nanoceramics sintering leads to significant changes in their optical and luminescence properties.     

X-ray photoelectron spectroscopy of CeO2–Na2O–SiO2 glasses

A series of (CeO₂)_x–(Na₂O)_0.3–(SiO₂)_(0.7−x) glasses, where 0.025 ≤ x ≤ 0.075, have been synthesized and investigated by mean of X-ray photoelectron spectroscopy (XPS). The Ce 3d spin-orbit doublet was curve fitted in order to quantify the proportions of each cerium oxidation state in these glasses. It was found that Ce ions are predominantly in the Ce(III) state in glasses with compositions x ≤ 0.075, while mixed Ce valences were found in the glass with composition x = 0.10. The O 1s spectra have also been curve fitted with two components, one from bridging oxygen (BO) and the other from non-bridging oxygen atoms (NBO). The measured number of NBO, based on the fact that only oxygen atoms in the site Si–O–Na⁺ contribute to the NBO peak, was found to be constant at ∼35% for all samples, in good agreement with the value calculated from the glass composition and inductively coupled plasma (ICP) suggesting that Ce ions enter the network as a glass intermediate. The thermal measurements done on these glasses agree well with the XPS findings.     

Surface properties of palladium catalysts supported on ternary ZrO2–Al2O3–WOx oxides prepared by the sol–gel method: Study of the chemical state of the support

The surface properties of Pd and Pd–Pt catalysts supported on binary ZrO₂–WOx and ternary ZrO₂–Al₂O₃–WOx oxides prepared by the sol–gel method were studied. Special attention was paid to the study of the texture of the catalysts as well as the chemical state of tungstated zirconia and tungstated zirconia promoted with alumina in the palladium catalysts. The catalysts were tested in the isomerization of n-hexane and were characterized by N₂ physisorption, XRD, TPR, Raman spectroscopy, XPS and FT-IR of adsorbed pyridine. The catalysts had bimodal pore size distributions with mesopores in the range 55–70 Å and macropores of 1000 Å in diameter. The catalysts had a surface WOx coverage (4.4–6.0 W nm^?2) lower than that of the theoretical monolayer (7.0 W nm^?2). A lower acidity of the ternary ZrO₂–Al₂O₃–WOx oxide as compared to the binary ZrO₂–WOx oxide was found. Higher activity in the isomerisation of n-hexane was obtained in the Pd–Pt catalysts supported on ternary ZrAlW oxides prepared by sol–gel that is correlated with the coexistence on the surface of W⁴⁺ (WO₂) or W⁰ and W⁶⁺ (Al₂(WO₄)₃) species, ZrO₂ in the tetragonal phase and a high amount of ZrOx suboxides species in a low oxidation state (Zr³⁺ and Zr²⁺).     

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.     

Modified Lanthanum Zirconium Oxide buffer layers for low-cost,high performance YBCO coated conductors

The pyrochlore Lanthanum Zirconium Oxide, La₂Zr₂O₇ (LZO), has been developed as a potential replacement barrier layer in the standard RABiTS three-layer architecture of physical vapor deposited CeO₂ cap/YSZ barrier/Y₂O₃ seed on Ni–5%W metal tape. The main focus of this research is to ascertain whether: (i) we can further improve the barrier properties of LZO; (ii) we can modify the LZO cation ratio and still achieve a high level of performance; and (iii) it is possible to reduce the number of buffer layers. We report a systematic investigation of the LZO film growth with varying compositions of La:Zr ratio in the La₂O₃–ZrO₂ system. Using a metal–organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of La_xZr_1?xO_y (x = 0.2–0.6) on standard Y₂O₃ buffered Ni–5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial LZO phase with only (0 0 1) texture can be achieved in a broad compositional range of x = 0.2–0.6 in La_xZr_1?xO_y. Both CeO₂ cap layers and MOD–YBCO films were grown epitaxially on these modified LZO barriers. High critical currents per unit width, I_c of 274–292 A/cm at 77 K and self-field were achieved for MOD–YBCO films grown on La_xZr_1?xO_y (x = 0.4–0.6) films. These results indicate that LZO films can be grown with a broad compositional range and still support high performance YBCO coated conductors. In addition, epitaxial MOD La_xZr_1?xO_y (x = 0.25) films were grown directly on biaxially textured Ni–3W substrates. About 3 μm thick YBCO films grown on a single MOD–LZO buffered Ni–3W substrates using pulsed laser deposition show a critical current density, J_c, of 0.55 MA/cm² (I_c of 169 A/cm) at 77 K and 0.01 T. This work holds promise for a route for producing simplified buffer architecture for RABiTS based YBCO coated conductors.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Mixed conductivity and electrocatalytic performance of SrFeO3-δ–SrAl2O4 composite membranes

Oxygen-ionic and electronic transport in dense (SrFe)_1−x(SrAl₂)_xO_z composites, consisting of strontium-deficient Sr(Fe,Al)O_3-δ and SrAl₂O₄ phases, is determined by the properties of perovskite-like solid solution. Increasing the content of SrAl₂O₄, with a total conductivity as low as 5 × 10^− 7 −  10^− 5 S × cm^− 1 at 973–1273 K in air, results in the gradual decrease of the partial conductivities, but also enables the suppression of thermal expansion. Compared to single-phase SrFe_1−xAl_xO_3-δ, (SrFe)_1−x(SrAl₂)_xO_z composites exhibit enhanced thermomechanical properties, while the oxygen permeability of these materials has similar values. The composite membranes exhibit stable performance under air/(H₂–H₂O–N₂) and air/(CH₄–He) gradients at 973–1173 K. The oxidation of dry methane by oxygen permeating through (SrFe)_0.7(SrAl₂)_0.3O_z results in dominant total oxidation, suggesting the necessity to incorporate a reforming catalyst into the ceramic reactors for natural gas conversion.     

Magnetic ordering and the Kondo effect in the alloys,Ce2Co1−xPdxSi3

The compound Ce₂CoSi₃, crystallizing in a AlB₂-derived hexagonal structure, has been recently identified as a Kondo lattice with a non-magnetic ground state. Here, we report the influence of gradual replacement of Co by Pd on the magnetic behaviour in the pseudo-ternary solid solution, Ce₂Co_1−xPd_xSi_3, by magnetization (2–300 K), electrical resistivity (2–300 K) and heat-capacity (0.7–30 K) measurements to bring out a transformation from non-magnetic to magnetic ordering. Distinct features attributable to the existence of a competition between the Kondo effect and magnetic ordering with varying x and temperature are observed in the electrical resistivity data. The results reveal that small substitutions of Pd (x=0.2–0.3) are sufficient to induce magnetic ordering of the Ce ions at low temperatures. The strength of the Kondo interaction as indicated by the paramagnetic Curie temperature decreases monotonically with increasing Pd content. A notable finding is that there are qualitative changes in the isothermal magnetization data in the magnetically ordered state, as if there are modifications in the magnetic structure with changes in the Co/Pd composition. The importance of electronic structure relative to unit-cell volume in deciding magnetic characteristics of this class of compounds is brought out taking into account the trends in the magnetic behaviour of isostructural Ce compounds.     

Structural and electrochemical properties of LiNi0.5Mn0.5 − xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries

Layered LiNi_0.5Mn_0.5 ? xAl_xO₂ (x = 0, 0.02, 0.05, 0.08, and 0.1) series cathode materials for lithium-ion batteries were synthesized by a combination technique of co-precipitation and solid-state reaction, and the structural, morphological, and electrochemical properties were examined by XRD, FT-IR, XPS, SEM, CV, EIS, and charge–discharge tests. It is proven that the aliovalent substitution of Al for Mn promoted the formation of LiNi_0.5Mn_0.5 ? xAl_xO₂ structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi_0.5Mn_0.5 ? xAl_xO₂ materials, the material with x = 0.05 shows the best cyclability and rate ability, with discharge capacities of 219, 169, 155, and 129 mAh g^? 1 at 10, 100, 200, and 400 mA g^? 1 current density respectively. Cycled under 40 mA g^? 1 in 2.8–4.6 V, LiNi_{0. 5}Mn_0.45Al_0.05O₂ shows the highest discharge capacity of about 199 mAh g^? 1 for the first cycle, and 179 mAh g^? 1 after 40 cycles, with a capacity retention of 90%. EIS analyses of the electrode materials at pristine state and state after first charge to 4.6 V indicate that the observed higher current rate capability of LiNi_{0. 5}Mn_0.45Al_0.05O₂ can be understood due to the better charge transfer kinetics.     

High coercivity and superparamagnetic behavior of nanocrystalline iron particles in alumina matrix

Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al₂O₃ matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe–Al₂O₃ sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, T_B, (481 K), magnetic anisotropy K (4.8×10⁵ erg/cm³) for Fe, and the Boltzmann constant k_B (1.38×10⁻¹⁶ erg/K) from T_B=KV/25k_B, the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies.     

Growth and interfacial properties of atomic layer deposited Al0.7Ti0.3O y high-k dielectric on Ge substrate

Growth and interfacial properties of atomic layer deposited Al_0.7Ti_0.3O _y on Ge have been investigated as a potential high-k gate dielectric for future Ge-based metal oxide semiconductor devices. A sandwich structure of Al₂O₃/TiO₂ stack is proposed for Al₂O₃/TiO₂ intermixing and high-k/Ge interfacial passivation. The film thicknesses and interface microstructure are characterized by spectroscopy ellipsometry and high-resolution transmission electron microscopy. X-ray photoelectron spectrometry is used to analyze the chemical composition and bonding states, and to reveal the band alignment of high-k/Ge heterojunctions. Metal-oxide-capacitors are formed by depositing aluminum electrodes to perform capacitance–voltage measurements for electrical characteristics. All evidences show a positive prospect of employing atomic layer deposited Al_0.7Ti_0.3O _y as high-k gate dielectric for future Ge-based devices.     

Preparation,characterization and electrical property of Mn-doped ceria-based oxides

Manganese-doped ceria-based oxides, Ce_1−xMn_xO_2−δ (0.05 ≤ x ≤ 0.3) and Ce_1−x−yGd_xMn_yO_2−δ˙ (0.05 ≤ x≤ 0.2, 0.05 ≤ y ≤ 0.25) were synthesized, and crystal phase analysis by XRD and measurements of electrical properties were performed. Solubility limit of Mn in Ce_1−xMn_xO_2−δ˙ seemed to be between 5 mol% and 10 mol% and Mn₃O₄ was the main by-product above the solubility limit in the case of heat treatment at 1300 °C. Judging from the oxygen partial pressure dependence of total conductivity and emf measurements, Ce_1−xMn_xO_2−δ˙ is a single-phase mixed conductor within the composition below the solubility limit, and when the composition of Mn exceeds the solubility limit, it becomes the dual-phase mixed conductor of Ce_1−xMn_xO_2−δ˙ and Mn₃O₄. The doing of Mn in gadlia-doped ceria, Ce_1−x−yGd_xMn_yO_2−δ˙ (0.05 ≤ x ≤ 0.2, 0.05 ≤ y ≤ 0.25), was more difficult than that in CeO₂ presumably due to the preferential reaction between Gd and Mn to give GdMnO₃ to the GDC solid solution formation, and the Mn doping seems not to be so effective in preparing the mixed ionic–electronic conductor based on GDC.     

Field-assisted sustainable O− ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability

We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O^?) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al₂O₃ (C12A7) with a composition of (12 ? x)CaO·7Al₂O₃·xCaF₂ (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO?3Al₂O₃ (C5A3) and 3CaO?Al₂O₃ (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F^? ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O^? ions (～ 92%) together with a small amount of O₂^? ions (～ 4%) and F^? ions (～ 4%). The absence of an O₂ gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O^? ions and electrons with a ratio of < 1/1. The O₂ gas supply markedly enhanced the O^? ion emission, and suppressed the electron emission. A sustainable and high-purity O^? ion emission with a current density of 11 nA cm^? 2 was achieved at 830 °C with the supply of 40 Pa O₂ gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F^? ion-substituted C12A7 is a promising material for higher intensity O^? ion emission at higher temperatures.     

Quantification of local oxygen defects around Yttrium ions for yttria-doped ceria–zirconia ternary system

Local coordination structure around Yttrium ions in CeO₂–Y₂O₃ binary and [(CeO₂)_x(ZrO₂)_1?x]_0.8(YO_1.5)_0.2 (x = 0.0 ~ 1.0) ternary system has been investigated by ⁸⁹Y MAS-NMR. NMR spectra are found to be consisted of multiple peaks that can be assigned to 6-, 7- and 8-oxygen coordinated Yttrium ions. Compositional dependence of the spectrum was observed and compared with the previous results for ZrO₂–Y₂O₃ binary system. The present investigation suggested the degree of localization of the oxygen vacancy around the cation is in the order of Zr⁴⁺ > Y³⁺ > Ce⁴⁺. The degree of the oxygen vacancy preference for each cation was quantitatively determined for CeO₂–ZrO₂–Y₂O₃ ternary system the first time.     

Generic phase diagram of Nd2−xCexCuO4

We investigated the generic phase diagram of the electron doped superconductor, Nd_2?xCe_xCuO₄, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O_ap atoms, we found that the T_c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. Based on the discussion on the tremendous influence of O_ap “impurities” on superconductivity in T′ cuprates, we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd_2?xCe_xCuO₄.     

Luminescence and Scintillation Characterization of Cs2NaGdBr6: Ce3+ Single Crystal

Luminescence and scintillation properties of newly discovered bromo-elpasolites Cs₂NaGdBr₆: Ce³⁺ (CNGB: Ce³⁺) are presented. Single crystals of CNGB: Ce³⁺ with dimensions up to Ø7×10 mm³ are successfully grown by the Bridgman technique. X-ray excited luminescence measurements of the grown samples showed a broad emission band in the wavelength range from 365 to 470 nm. It offered an energy resolution of 5.1% (FWHM) at 662 keV for 10% Ce sample. The light output of the investigated samples increases along with cerium concentration. A maximum light yield of ～36,800 ph/MeV is measured for the 10% Ce sample crystal. Under γ-ray excitation, CNGB: Ce³⁺ crystals showed three exponential decay time components. The scintillation mechanism in the sample crystal is presented.     

Polyphosphazene based composite polymer electrolytes

Composite salt-in-polymer electrolyte membranes were prepared from poly[(bis(2-methoxyethyl)amino)_1−x(n-propylamino)_x-phosphazene] (BMEAP) with dissolved LiCF₃SO₃ and dispersed Al₂O₃ nanoparticles (40 nm). Membranes with good mechanical stability were obtained. Low ionic conductivities were found in particle free membranes with maximum conductivities at 10 wt.% LiCF₃SO₃ ranging from 3.1 × 10^− 7 S/cm at 30 °C to 1.8 × 10^− 5 S/cm at 90 °C. For the composite membranes, addition of 2 wt.% Al₂O₃ nanoparticles leads to a steep increase of the conductivity by almost two orders of magnitude as compared to the homogeneous membranes. The highest room temperature conductivity for the investigated BMEAP–LiCF₃SO₃–Al₂O₃ composite systems was 10^− 5 S/cm.     

Luminescence study of cerium-doped La2Hf2O7: Effects due to trivalent and tetravalent cerium and oxygen vacancies

X-ray excited emission spectra, photoluminescence excitation and emission spectra, optical reflectivity spectra, and pulsed X-ray and optical excited luminescence decay measurements are reported for cerium-doped La₂Hf₂O₇ powders prepared by solid state synthesis. A broad luminescence associated with oxygen vacancies is observed in the region 350–750 nm with a peak around 460 nm. The photoluminescence spectra and the number of oxygen vacancies vary for samples annealed in oxidizing or reducing atmospheres and with the temperature of the synthesis process. Increasing the cerium concentration reduces the oxygen-vacancy-related emission due to the presence of Ce⁴⁺. First principles calculations predict that Ce⁴⁺ can substitute in Hf sites; this is confirmed from the optical reflectivity spectrum of cerium-doped La₂Hf₂O₇. Photoluminescence excitation and emission spectra characteristic of Ce⁴⁺ charge transfer transitions and possibly Ce³⁺ are also observed. Although trivalent cerium may be present, no emission observed from cerium-doped La₂Hf₂O₇ can be attributed to Ce³⁺ in La sites.