Satoru Yoshioka

Structures and energetics of Ga2O3 polymorphs

First-principles calculations are made for five Ga2O3 polymorphs. The structure of ?-Ga2O3 with the space group Pna 21 (No.?33, orthorhombic), which is sometimes called ?-Ga2O3 in the literature, is consistent with experimental reports. The structure of ?-Ga2O3 is optimized within 14 inequivalent configurations of defective spinel structures. Phonon dispersion curves of four polymorphs are obtained. The volume expansivity, bulk modulus, and specific heat at constant volume are computed as a function of temperature within the quasi-harmonic approximation. The Helmholtz free energies of the polymorphs are thus compared. The expansivity shows a relationship of ?<?<?<?, while ?<?<?<? for the bulk modulus. The formation free energies have the tendency ?<?<?<?<? at low temperatures. With the increase of temperature, the difference in free energy between the ?-phase and the ?-phase becomes smaller. Eventually the ? phase becomes more stable at above 1600?K.

Room temperature ferromagnetism in Mn-doped γ-Ga2O3 with spinel structure

Mn-doped Ga2O3 (7 cation % of Mn) thin film has been grown on c-cut sapphire substrate using pulsed-laser deposition technique. Electron diffraction analyses by transmission electron microscopy found that the Mn-doped film shows γ phase with spinel structure, which is different from undoped film showing β phase. No secondary phase can be detected. Combination of Mn-L2,3 near edge x-ray absorption experiments with first-principles many-electron calculations unambiguously implies that Mn atoms are located at tetrahedrally coordinated Ga sites with a valence of +2. The doped sample shows ferromagnetism up to 350K.

Atomic structures of supersaturated ZnO–Al2O3 solid solutions

Supersaturated ZnO–Al2O3 (>20at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600°C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10at.%. The expansion disappears after annealing the films at above 800°C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)3(Al2O3) as the local environment of Al in the supers...

Catalytic properties of copper–manganese mixed oxides prepared by coprecipitation using tetramethylammonium hydroxide

Copper–manganese (Cu–Mn) mixed oxide catalysts were prepared by a coprecipitation technique from metal nitrates in aqueous solution using tetramethylammonium hydroxide (TMAH) as a pH regulator. The structures and properties of the mixed oxide catalysts were investigated by X-ray diffraction, extended X-ray absorption fine structure, X-ray photoelectron spectroscopy and H2-TPR studies. Spinel-type mixed oxides were mainly formed in the Cu–Mn mixed oxides in which Cu and Mn species occupied both tetrahedrally and octahedrally coordinated sites. The occupancy of Cu and Mn in these sites depended on the Cu/Mn ratio. The average oxidation state of Cu was evaluated to be 2+ and was almost independent of the Cu/Mn ratio; reduced Cu species were also formed on the surface sites. By incorporation of Cu into Mn oxides, the average oxidation state of Mn increased both in the bulk sites and on the surface sites. The concentration of Mn on the surface sites was higher than that in the bulk sites. The Cu–Mn mixed catalysts exhibited higher activity than corresponding single-metal oxides at the reaction temperature range of 343–403 K. Formation of Cu–Mn spinel-type oxides gave rise to the increase in the catalyst surface area and the rate for CO oxidation normalized by surface area, and the Cu–Mn mixed oxides with the Cu/Mn molar ratio of 1 exhibited the highest activity for CO oxidation. CO-TPR studies revealed that in the temperature range of 343–403 K, the lattice oxygen at the first layer of the mixed oxides reacted with CO to form CO2.

Dilute Ga dopant in TiO2 by X-ray absorption near-edge structure

As a model of aliovalent impurity in functional ceramic, the local environment of dilute Ga in rutile-structured TiO2 is investigated by Ga K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In conjunction with the experiments, first-principles calculations by two methods are systematically made. The projector augmented wave method is used to optimize the local structure and obtain the solution energy. The augmented plane wave plus local orbitals method is adopted to obtain theoretical XANES spectra. A comparison between experimental and theoretical XANES spectra shows that Ga dopants are located at the Ti4+ sites forming Ga3+. Oxygen vacancies are present to maintain the charge balance of the solid solution. Ga atoms and oxygen vacancies are not present at the nearest neighbor sites but stay apart.

Characterization of nanotextured AlN thin films by x-ray absorption near-edge structures

AlN thin films have been grown on c-cut sapphire substrates by pulsed-laser deposition. The film epitaxially grown at 1073 K under vacuum of 5×10−4Pa was used to examine the crystallographic orientation dependence of Al K-edge x-ray absorption near-edge structures (XANES), which satisfactorily agrees with theoretical spectra obtained by first-principles calculations. The film grown at 1073 K with N2 backfill of 7×10−2Pa shows nanotextured structure with its c plane parallel to the substrate. Although the nanotexture is not evident by x-ray diffraction, XANES can unambiguously indicate the texturing. Cross-sectional high-resolution electron microscopy provides the evidence of the nanostructure.

Selective Cu(II) Adsorption from Aqueous Solutions Including Cu(II), Co(II), and Ni(II) by Modified Acrylic Acid Grafted PET Film

Acrylic acid (AAc) grafted polyethylene terephthalate (PET) films were prepared by γ irradiation. The graft films showed little metal ion adsorption due to compact structure of the graft chains as shown by the scanning electron microscopy (SEM) images which restricted the access of metal ions to the functional groups. Therefore, the graft films were modified with KOH treatment for expansion of the graft chains to facilitate the access of metal ions to the functional groups. The modified films were used to study the selective Cu2

Hydrogen storage and stability properties of Pd–Pt solid-solution nanoparticles revealed via atomic and electronic structure

Bimetallic Pd1−xPtx solid-solution nanoparticles (NPs) display charging/discharging of hydrogen gas, which has relevance for fuel cell technologies; however, the constituent elements are immiscible in the bulk phase. We examined these material systems using high-energy synchrotron X-ray diffraction, X-ray absorption fine structure and hard X-ray photoelectron spectroscopy techniques. Recent studies have demonstrated the hydrogen storage properties and catalytic activities of Pd-Pt alloys; however, comprehensive details of their structural and electronic functionality at the atomic scale have yet to be reported. Three-dimensional atomic-scale structure results obtained from the pair distribution function (PDF) and reverse Monte Carlo (RMC) methods suggest the formation of a highly disordered structure with a high cavity-volume-fraction for low-Pt content NPs. The NP conduction band features, as extracted from X-ray absorption near-edge spectra at the Pd and Pt LIII-edge, suggest that the Pd conduction band is filled by Pt valence electrons. This behaviour is consistent with observations of the hydrogen storage capacity of these NPs. The broadening of the valence band width and the down-shift of the d-band centre away from the Fermi level upon Pt substitution also provided evidence for enhanced stability of the hydride (ΔH) features of the Pd1−xPtx solid-solution NPs with a Pt content of 8-21 atomic percent.

Structural Evolution Induced by Interfacial Lattice Mismatch in Self-Organized YBa2Cu3O7−δ Nanocomposite Film

Intriguing properties of self-organized nanocomposites of perovskite oxides are usually derived from the complex interface of constituent material phases. A sophisticated control of such a system is required for a broad range of energy and device applications, which demand a comprehensive understanding of the interface at the atomic scale. Here, we visualized and theoretically modeled the highly elastically strained nanorod, the interface region with misfit dislocations and heterointerface distortion, and the matrix with strain-induced oxygen vacancies in the self-organized YBa2Cu3O7-δ nanocomposite films with Ba perovskite nanorods. Large misfit strain was elastically accommodated in the nanocomposites, but since the elastic strain was mainly accommodated by the nanorods, the concentration of strain-induced oxygen vacancies was small enough for the matrix to keep high critical temperature (>85 K). The interfacial bonding distorted the atomic structure of YBa2Cu3O7-δ, but the thickness of distortion was limited to a few unit cells (less than the coherence length) due to the electron screening. The effect of volume fraction on elastic strain and the electron screening are crucial for strong vortex pinning without significant degradation of both the elementary pinning force and critical temperature in the nanocomposites. Thus, we comprehensively clarified the self-organized nanocomposite structure for on-demand control of superconductivity and oxide functionality in the nanocomposite engineering of perovskite oxides.

Publisher Correction: Hydrogen storage and stability properties of Pd–Pt solid-solution nanoparticles revealed via atomic and electronic structure

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.