Shinya Otsuka-Yao-Matsuo

Size dependent optical band gap of ternary I-III-VI2 semiconductor nanocrystals

The size dependent optical band gap of the less-toxic ternary I-III-VI2 chalcopyrite-type semiconductor quantum dots (QDs), CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInSe2, AgGaS2, and AgGaSe2, were evaluated using the finite-depth-well effective mass approximation calculation. From the comparison of the calculation result with the experimental values for the CuInS2 case, it was shown that the calculation was highly valid to predict the size dependent optical gap of the ternary semiconductor QDs. The optical band gap of the above seven I-III-VI2 QDs covers a wide wavelength range from the near-infrared to ultraviolet. It has been shown that the I-III-VI2 semiconductor QDs have a significant potential as alternatives to the highly toxic cadmium-containing II-VI semiconductor QDs and they are applicable to the wide range of light emitting devices and solar cells.

Crystal structure of metastable κ-CeZrO4 phase possessing an ordered arrangement of Ce and Zr ions

Abstract A pyrochlore-type precursor having less anti-phase domain boundaries was prepared by reducing a single t′-(Ce0.5Zr0.5)O2 phase at 1573 K. Its oxidation at 873 K proceeded leaving the ordered arrangement of Ce and Zr ions. In the powder X-ray diffraction (XRD) pattern of the metastable κ-CeZrO4 phase obtained, many small and sharp superlattice diffractions, characterizing the crystal structure of the κ-CeZrO4 phase, were observed in addition to the diffractions observed in the pyrochlore-type precursor. All the diffraction peaks could be indexed on the basis of the cubic lattice with a=1.05270 nm. The condition limiting possible diffractions for space groups of P23, P213 and P 4 3m agreed with the XRD data and also the space groups permitted the ordered arrangement Ce4+ and Zr4+ ions along with 〈110〉 direction similar to the pyrochlore-type precursor. Finally, it was found from Rietveld analysis that the κ-CeZrO4 phase belongs to the space group of P213. In the ZrO8 polyhedron of the κ-CeZrO4, long and short Zr–O bonds compared with the average Zr–O bond length existed. The long and short Zr–O bonds may relax the compulsive 8-fold coordination of Zr4+ ions. A pair of enantiomorphic coordinates belonging to the same space group of P213 was appeared for the κ-CeZrO4.

Electrical Conduction Properties of Sr-Doped LaPO4 and CePO4 under Oxidizing and Reducing Conditions

Electrical conduction properties of Sr-doped LaPO 4 and Sr-doped CePO 4 under H 2 O/O 2 and H 2 O/H 2 conditions were investigated with conductivity measurements. Conductivities of I mol % Sr-doped LaPO 4 were 10 - 5 . 2 -10 - 3 . 5 S cm - 1 at 500-925°C under wet reducing conditions and were close to those under wet oxidizing conditions. It was found from the H/D isotope effect on conductivity that the material showed dominant protonic conduction under wet reducing conditions. As for 1 mol % Sr-doped CePO 4 , conductivities were 10 - 2 . 8 -10 - 2 . 0 S cm - 1 at 500-925°C under wet oxidizing conditions and much higher than those of 1 mol % Sr-doped LaPO 4 under the same conditions. Such high conductivities of Sr-doped CePO 4 seemed attributable to electronic conduction due to partial oxidation of Ce 3 + to Ce 4 + caused by substituting Sr2 + for Ce 3 + . Under wet reducing conditions, however, conductivities of the material decreased to 10 - 5 . 2 -10 - 3 . 4 S cm - 1 at 500-925°C, which was almost comparable with those of 1 mol % Sr-doped LaPO 4 . It was concluded that Sr-doped CePO 4 showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO 4 and CePO 4 under wet oxidizing and reducing conditions are discussed.

Oxygen release behaviour of Ce(1−x)ZrxO2 powders and appearance of Ce(8−4y)Zr4yO(14−δ) solid solution in the ZrO2–CeO2–CeO1.5 system

Abstract To clarify the existence of metastable phases in the ZrO 2 –CeO 2 –CeO 1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t′ and t″ (Ce (1− x ) Zr x O 2 ) with various compositions, x , in a reducing gas and successive oxidation were carried out repeatedly. The oxygen release behaviour of the t′ and t″ phases was very complicated. The single κ phases, (Ce (1− x ) Zr x O 2 ) with the composition, x =0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in O 2 gas at 873 K, exhibited a sharp oxygen release at the lowest temperature; the composition range of κ phase may be x =0.45∼0.65. A new tetragonal phase t*, (Ce (1− x ) Zr x O 2 ), which was attained by cyclic redox process together with annealing in O 2 gas at 1323 or 1423 K, exhibited a sharp oxygen release at the highest temperature; the composition range of t* phase may be as wide as x =0.20∼0.65. A metastable solid solution expressed by a chemical formula of Ce (8−4 y ) Zr 4 y O (14− δ ) ( y =0∼1) possessing a CaF 2 -related structure appeared on deoxidation of the t* phase. A ternary phase diagram containing the t* and Ce (8−4 y ) Zr 4 y O (14− δ ) solid solution was proposed.

Electrical Properties of Proton-Conducting Ca2 + -Doped La2Zr2 O 7 with a Pyrochlore-Type Structure

The electrical conductivity and the mean transport number of pyrochlore-type (La 2-x Ca x )Zr 2 O 7 δ (x = 0.15, 0.03, 0.05) and La 2 (Zr 2-x Ca x )O 7 (x = 0.015) were systematically studied as a function of temperature under wet hydrogen and oxygen atmospheres (p H2O = 1.7-2.5 x 10 3 Pa). Under wet hydrogen atmosphere, pure proton conduction was observed below 873 K for both (La 2 Ca x )Zr 2 O 7 δ and La 2 (Zr 2 Ca x )O 7 δ systems. The proton conductivities at 873 K were 6.8 × 10 -2 and 1.0 × 10 -2 S m -1 for (La 1.95 Ca 0.05 )Zr 2 O 7 δ and La 2 (Zr 1.985 Ca 0.015 )O 7-δ , respectively. The proton conductivity of (La 1.97 Ca 0.03 )Zr 2 O 7-δ was approximately three times higher than that of La 2 (Zr 1 985Ca0.015)O 7-δ although the number of positive charges introduced by Ca 2 doping was equal for the two samples. The three times higher proton conductivity of (La 1.97 Ca 0.03 )Zr 2 O 7-δ than La 2 (Zr 1.985 Ca 0.015 )O 7-δ could be attributed to the higher proton concentration dissolved in (La 1.97 Ca 0.03 )Zr 2 O 7-δ than in La 2 (Zr 1.985 Ca 0.015 )O 7-δ . This may have been because the oxygen and/or its vacant sites responsible for the proton dissolution were different between (La 2-x Ca x )Zr 2 O 7-δ and La 2 (Zx 2-x Ca x )O 7-δ systems, i.e., the 8b and the 48f oxygen sites for (La 2-x Ca x )Zr 2 O 7-δ and the 48f oxygen site for La 2 (Zr 2-x Ca x )O 7-δ may have contributed to the proton dissolution. As a result, the proton conductivity of Ca 2+ -doped La 2 Zr 2 O 7 depended on both the Ca 2 doping level and the Ca 2+ doping site Above 873 K under a wet hydrogen atmosphere, the mean transport number of oxygen ions increased with increasing temperature. Under a wet oxygen atmosphere, the positive hole was a dominant carrier for both systems, in contrast to under the wet hydrogen atmosphere.

Thermodynamic Behavior of Various Phases Appearing in the CeZrO4 ‐ CeZrO3.5 System and the Formation of Metastable Solid Solutions

In order to investigate the thermodynamic behavior of t{prime}, t{sup *}, and {kappa} phases with the composition, CeZrO{sub 4}, the equilibrium oxygen partial pressure, p{sub O{sub 2}}, over their mixtures with pyrochlore (Ce{sub 2}Zr{sub 2}O{sub 7+x}) was measured utilizing an electrochemical cell: Pt, {l_brace}CeZrO{sub 4}(t{prime}, t{sup *}, or {kappa} phase) + Ce{sub 2}Zr{sub 2}O{sub 7+x}{r_brace}/ZrO{sub 2}(+Y{sub 2}O{sub 3})/air, Pt. The conclusions described below were derived: the thermodynamic stability of the {kappa} phase is the lowest in the CeO{sub 2}0ZrO{sub 2} system. The {kappa} phase forms metastable solid solutions with the pyrochlore phase; it was virtually stable around 1,123 K. Two kinds of tetragonal phases exist, tet. (high temp.) and tet. (low temp.), which may correspond to t{prime} and t{sup *}, respectively. A change in p{sub o{sub 2}} corresponding to the phase transitions: {kappa} {yields} t{prime} and t{prime} {r_reversible} t{sup *} was observed. The standard Gibbs energies of formation, {Delta}G{degree}, of {kappa}(CeZrO{sub 4}) and t{sup *}(CeZeO{sub 4}) for the reaction: 1/2Ce{sub 2}Zr{sub 2}O{sub 7} + 1/4 O{sub 2} {yields} CeZrO{sub 4} were evaluated from the emf data.

Preparation of tabular TiO2–SrTiO3–δ composite for photocatalytic electrode

Abstract To improve the efficiency of photo-induced reactions, the recombination of photogenerated electrons and holes in semiconductor must be suppressed. We have proposed that the utilisation of n+/n heterogeneous junction which yields a large potential gradient in the junction region. Up to now, we have studied the superiority of such junctions in the photocatalytic reaction using composite particles consisting of TiO2 and several alkaline-earth titanates. In this study, the preparation of tabular composite consisting of TiO2 and SrTiO3–δwas examined focusing on morphologies and crystallographic features of oxides. The ideal configuration of tabular composite is the layered structure of n-type TiO2/n+-type semiconductor/metal or the mixed one partially containing such structure pass. The tabular composite was prepared by an oxidation of metallic Ti chips covered with SrCO3 powder. By such simple preparation method, the composite with designed oxides such as TiO2 and SrTiO3–δwas obtained on the Ti substrates. The grain size and the volume fraction of constituent oxides depend strongly on the oxidation temperature. When the composite is prepared at 1173 K, the grain size of oxides is measured to be about 0.5–1.0 μm. In contrast, a remarkable grain coarsening and an increase in the volume fraction of TiO2 can be observed in the oxidation at 1273 K. Furthermore, the developed texture of composite was strongly influenced by that of the Ti substrates and the oxidation temperature. Thin TiO2 layer on the surface of SrTiO3–δ phase was prepared by acid solution treatment, the designed configuration of composites could be obtained.

Wide band gap semiconductor alloy: x(LiGaO2)1∕2–(1−x)ZnO

Oxide semiconductor alloys of x(LiGaO2)1∕2–(1−x)ZnO were synthesized by a standard solid state reaction. The wurtzite-type single phases were obtained in the wide composition range of x⩽0.38 because the β‐LiGaO2 possesses a wurtzite-derived structure and approximately the same lattice constants with ZnO. The resulting alloys showed characteristics of wide band gap oxide n-type semiconductor; the optical transparency reached over to the ultraviolet (UV) light. The electrical conductivity and energy band gap for 0.38(LiGaO2)1∕2–0.62ZnO were 8.2Ω−1cm−1 and 3.7eV, respectively, at room temperature. The alloy is a good candidate for the barrier material of the ZnO-based heterostructures and UV-transparent electrodes.

The Active Phase of Nickel/Ordered Ce2Zr2OxCatalysts with a Discontinuity (x=7-8) in Methane Steam Reforming

In recent years, there has been a surge in interest in syngas (H2/CO) and H2 production technologies, which utilize a wide variety of hydrocarbon feed stocks, such as gasoline, diesel, LPG, natural gas, methanol, and bio-ethanol. Among fossil fuels, natural gas ( 90 vol% CH4) is the ideal fuel, owing to its ready availability, high energy density, and wide distribution network; CH4 activation and reforming provide attractive ways to produce syngas, which can be transformed to useful larger hydrocarbons. Catalysts based on both noble metals and other metals have been extensively studied for CH4 steam reforming. [1, 2] Noble-metal (Rh, Ru, Ir, Pd, and Pt) catalysts are active and stable; however, because of the limited supply and high cost of noble metals, much attention has been paid to the development of non-noble metal catalysts, among which nickel-based catalysts have attracted particular attention because of their similar mechanistic features to noble-metal catalysts. The strong C H bonds of CH4 (439 kJmol ) and endothermic heat of reforming reactions necessitate high temperatures for practical CH4 conversion, and thus stable catalysts that resist sintering under extreme operating conditions. In CH4 steam reforming, coke formation that deactivates the catalyst is thermodynamically favored at a H2O/CH4 ratio less than 1.4. Thus, industrial CH4 steam reforming is usually carried out at a H2O/CH4 ratio of 1.4 or greater. Although catalytic CH4 steam reforming at low H2O/CH4 ratios have many advantages from operational and energy-consuming viewpoints, conventional nickel-based catalysts suffer from severe carbon deposition under such conditions. Supports and additives (for example, CeO2, ZrO2, CeO2-ZrO2, and La2O3) have been used to confer catalysts with kinetic resistance to carbon deposition and Ni sintering because they enhance redox activity and thermal stability, thereby promoting steam reforming. The efficient CH4 upgrading has long been a challenge in fundamental research. Herein, we report the unique properties and active phase of a new Ni/ordered Ce2Zr2Ox (x = 7–8) catalyst with a regular arrangement of Ce and Zr ions in CH4 steam reforming to produce H2 and CO at H2O/CH4 = 1. The catalytic performance of Ni/Ce2Zr2Ox (x = 7–8) strongly depends on the phase and oxygen content of Ce2Zr2Ox, and it shows a unique discontinuity in catalytic activity at x = 7.5. The 2 wt % Ni/pyrochlore-Ce2Zr2O7 catalyst showed a remarkable performance in CH4 steam reforming at 923 K at H2O/CH4 = 1 (Table 1). Ni/CeO2, Ni/ZrO2, and Ni/CeO2ZrO2 reduced by H2 were much less active and selective than Ni/Ce2Zr2O7, and significant deactivation was observed probably owing to Ni sintering and carbon deposition. On the other hand, the Ni/pyrochlore-Ce2Zr2O7 catalyst was stable, resulting in a remarkably high catalytic performance (for a typical 50 h performance, see the Supporting Information, Figure S4). At 973 K, the Ni/Ce2Zr2O7 catalyst exhibited high CO selectivity of 96–98% and high H2 selectivity of 96– 99% at CH4 and H2O conversions of 92–94% and > 96 %, respectively. Platinum, a typical noble metal active for CH4 steam reforming, was supported on CeO2, ZrO2, or Ce2Zr2O7, but the performance was not significantly enhanced by these types of supports, and the CH4 conversion on these Pt-based catalysts ranged between 29 % and 39 %. For Pt-based catalysts, CH4 steam reforming may be controlled by Pt rather than by the nature of the support. 8] In the presence of 0.8% O2 in the reaction feed, the Ni/ Ce2Zr2O7 catalyst also exhibited high H2 selectivity (97–99%) at a CH4 conversion of 93–94 % for at least 10 h. No significant deactivation was observed. These are great advan[*] Dr. M. Tada, Dr. S. Zhang, N. Ishiguro Institute for Molecular Science 38 Nishigo-naka, Myodaiji, Okazaki, Aichi 444-8585 (Japan) E-mail: mtada@ims.ac.jp

Ultraviolet electroluminescence from colloidal ZnO quantum dots in an all-inorganic multilayer light-emitting device

We report ultraviolet (UV) electroluminescence (EL) at 3.30 eV of colloidal ZnO quantum dots (QDs) in an inorganic multilayer thin-film EL device. The EL spectrum was identical to the photoluminescence spectrum of the source solution of ZnO QDs, and the emission is attributable to quantum confined electron hole pair recombination. The UV emission was successful when the ZnO QD layer was sandwiched by thin MgO layers, while only a defect-related visible emission appeared without MgO layers. The type-I quantum well structure of MgO/ZnO/MgO and surface passivation of ZnO QDs by MgO must be important for the UV EL emission.