Takahiro Takei

University of Yamanashi, Kōhu, Yamanashi, Japan

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Publications (146)265.53 Total impact

  • No preview · Article · Dec 2015 · Chemistry of Materials
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    ABSTRACT: The band gaps of isostructural Bi2OS2 and LaOBiS2 were examined using optical absorption and discussed with the band structures calculated based on the crystal structures determined using synchrotron X-ray diffraction. The Bi 6p and S 3p orbitals in the Bi-S plane were computationally predicted to constitute the bands near the Fermi level. The optical reflectance spectra of Bi2OS2 and LaOBiS2 showed optical band gaps of ca. 1.0 eV, which were close to the computationally calculated direct band gaps of ca. 0.8 eV. Our results show that Bi2OS2 and LaOBiS2 are semiconductors containing direct band gaps of 0.8-1.0 eV, and they are suggested to be candidates for optoelectronic materials in the near-infrared region without highly toxic elements.
    No preview · Article · Nov 2015 · Solid State Communications
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    ABSTRACT: The relationship between the structure and superconductivity of Bi4O4S3 powders synthesized by heating under ambient and high pressures was investigated using synchrotron X-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM) observation. The Bi4O4S3 powders synthesized under ambient pressure exhibited a strong superconductivity (diamagnetic) signal and zero resistivity below ∼4.5 K, while the Bi4O4S3 powder synthesized by the high-pressure method exhibited a low-intensity signal down to 2 K. Further annealing of the latter Bi4O4S3 powder under ambient pressure led to the development of a strong signal and zero resistivity. The crystal structures of all Bi4O4S3 phases consisted of Bi4O4Bi2S4 blocks including a Bi-S layer and anion(s) sandwiched between Bi4O4Bi2S4 blocks, but minor structural differences were detected. A comparison of the structures of the superconductive and nonsuperconductive Bi4O4S3 samples suggested that the superconductive Bi4O4S3 phases had slightly smaller lattice parameters. The average structures of the superconductive Bi4O4S3 phases were characterized by a slightly shorter and less bent Bi-S plane. Raman spectroscopy detected vibration of the S-O bonds, which can be attributed to sandwiched anion(s) such as SO4(2-). TEM observation showed stacking faults in the superconductive Bi4O4S3 phases, which indicated local fluctuation of the average structures. The observed superconductivity of Bi4O4S3 was discussed based on impurity phases, enhanced hybridization of the px and py orbitals of the Bi-S plane within Bi4O4Bi2S4 blocks, local fluctuation of the average structures, compositional deviation related to suspicious anion(s) sandwiched between Bi4O4Bi2S4 blocks, and the possibility of suppression of the charge-density-wave state by enriched carrier concentrations.
    No preview · Article · Oct 2015 · Inorganic Chemistry
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    ABSTRACT: Alkaline earth metal doped tin oxide (SnO2) hollow nanospheres with a diameter of 50 nm have been synthesized successfully via a facial solvothermal route in a very simple system composed of only ethanol, acetic acid, SnCl4·5H2O and A(NO3)2·xH2O (A = Mg, Ca, Sr, Ba). The synthesized undoped SnO2 and A-doped SnO2 hollow nanospheres were characterized by the oxygen storage capacity (OSC), X-ray diffraction, transmission electron microscopy and the Brunauer–Emmet–Teller (BET) technique. The OSC values of all samples were measured using thermogravimetric-differential thermal analysis. The incorporation of alkaline earth metal ion into tin oxide greatly enhanced the thermal stability and OSC. Especially, Ba-doped SnO2 hollow nanospheres calcined at 1000 °C for 20 h with a BET surface area of 61 m2 g−1 exhibited the considerably high OSC of 457 μmol-O g−1 and good thermal stability. Alkaline earth metal doped tin oxide has the potential to be a novel oxygen storage material.
    Full-text · Article · Sep 2015 · Materials Research Bulletin
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    ABSTRACT: The band gaps of isostructural Bi2OS2 and LaOBiS2 were examined using optical absorption and discussed with the band structures calculated based on the crystal structures determined using synchrotron X-ray diffraction. The Bi 6p and S 3p orbitals in the Bi-S plane were computationally predicted to constitute the bands near the Fermi level. The optical reflectance spectra of Bi2OS2 and LaOBiS2 showed optical band gaps of ca. 1.0 eV, which were close to the computationally calculated direct band gaps of ca. 0.8 eV. Our results show that Bi2OS2 and LaOBiS2 are semiconductors containing direct band gaps of 0.8-1.0 eV, and they are suggested to be candidates for optoelectronic materials in the near-infrared region without highly toxic elements.
    Preview · Article · Aug 2015
  • Nan Xu · Takahiro TAKEI · Akira MIURA · Nobuhiro KUMADA
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    ABSTRACT: Ag-intercalated and Bi-intercalated layered perovskite were fabricated by ion-exchanged process. The XRD and XPS confirm the phase transformation of the Ag-ion exchanged layered perovskite from Dion-Jacobson to Ruddlesden-Popper phase by heating at higher than 200°C. In the case of Bi ion-exchange perovskite, the product remained Dion-Jacobson phase during the heat treatment. For the Ag-intercalated sample, Ruddlesden-Popper type photocatalyst by heating at 500°C can degrade 20 ppm phenol solution to 6ppm in 3h under Xe light, which has superior photocatalytic activity to Dion-Jacobson phase Ag0.67H0.33LaNb2O7. For the Bi-intercalated sample, Dion-Jacobson type photocatalyst can degrade phenol from 20 to 12ppm in 3h. From first principle DFT simulation, high photocatalytic activity may be due to the new Ag 4d, O 2p and Nb 4d hybridized band at the highest of the valence band for Ag-included, and to the new Bi 6p band at the lowest of the conduction band for Biincluded samples.
    No preview · Article · Aug 2015 · Journal- Ceramic Society Japan
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    ABSTRACT: A novel photocatalyst consisting of an intercalated perovskite H1-2xPtxLaNb2O7 was fabricated by ion exchange. Synchrotron X-ray diffraction and X-ray photoelectron spectroscopy results confirmed that Pt(2+) exists within the interlayer space of the layered perovskite. H1-2xPtxLaNb2O7 composed of layered niobate perovskite and intercalated Pt(2+) completely degraded a 20 ppm phenol solution in 3 h under irradiation by Xe light, which exhibits photocatalytic activity superior to those of RbLaNb2O7, Pt-deposited RbLaNb2O7, and HLaNb2O7. From first principles density functional theory simulation, high photocatalytic activity of H1-2xPtxLaNb2O7 is attributed to the emergence of a new O 2p-Pt 5d hybridized band in top of the valence band.
    No preview · Article · Jun 2015 · Langmuir
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    ABSTRACT: Two new cadmium bismuthates, CdBi2O6 and Cd0.37Bi0.63O1.79, were prepared by hydrothermal reaction using NaBiO3·nH2O as one of the starting compounds. The crystal structures of these compounds were refined by using synchrotron X-ray powder diffraction data. The former bismuthate has a MnSb2O6-type structure with a hexagonal cell (space group: P321); the cell parameters were a = 9.3641(7) and c = 4.9523(3) Å, and the final R-factors were Rwp = 4.59% and Rp = 3.04%. The latter bismuthate has a fluorite-type structure with a cubic cell (space group: ) of a = 5.4110(4) Å, and the final R-factors were Rwp = 4.79% and Rp = 3.57%. These new bismuthates exhibit no photocatalytic activity under visible light.
    Preview · Article · May 2015
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    ABSTRACT: We have studied the structural, magnetic, and dielectric properties of the dimer-Mott insulator β′-(BEDT-TTF)2(ICl2)1-x(AuCl2)x. The AuCl2 substitution for ICl2 decreases the lattice length in the interlayer direction, leading to uniaxial chemical pressure with a negligible effect on the intralayer donor stacking. The variation of the Néel temperature as a function of x is explained on the basis of chemical pressure accompanied by anion disorder, while the dielectric response does not indicate the effect of chemical pressure. This suggests that the charge degrees of freedom are attributed to the intralayer origin such as the charge fluctuation of donor molecules induced by an electric field.
    No preview · Article · Mar 2015 · Journal of the Physical Society of Japan
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    ABSTRACT: Single crystals of Cu(In,Ga)S2 were successfully grown using CsCl flux in vacuum. The grown single crystals were typically 3–4 mm with unshaped form. The Cu(In,Ga)S2 single crystals with various Ga concentration were obtained by control of the Ga concentration in the starting materials. The optical band-gaps of obtained single crystals were estimated from the reflectance spectrum. The optical band-gap changed between 1.42 and 2.40 eV at room temperature with dependence on the Ga concentration.
    No preview · Article · Feb 2015 · Journal of Crystal Growth
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    ABSTRACT: The crystal structures of Ce(O,F)BiS2 synthesized from starting materials with five different F contents were determined by single-crystal X-ray analysis. An increase in F content in the starting materials slightly increased F incorporation in the grown single crystals. The grown crystals with various F contents showed similar layered structures, but minor structural changes were observed with different conductive and magnetic properties. Crystals grown from higher F content were found to have a nearly flat Bi-S plane and elongated Ce-(O,F) bond, and showed superconductive transition below similar to 3 K and spontaneous magnetization below similar to 7 K. The calculated density of states based on this structural analysis predicted that hybridized Bi 6p-S 3p orbitals intersect the Fermi level and that Ce states are located near the Fermi level. The effect of structural change on superconductivity and long-range magnetic ordering has been discussed.
    No preview · Article · Jan 2015 · Crystal Growth & Design
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    Akira Miura · Takahiro Takei · Nobuhiro Kumada
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    ABSTRACT: We report on the low-temperature synthesis of submicron-sized Cu3N powder produced from CuO and NaNH2 powder mixture by heating at 150–190 °C in a Teflon-sealed autoclave. The structure was the anti-RuO3 type with a lattice parameter of 0.3814(1) nm, and strong optical absorption was observed below ∼1.9 eV. This synthesis method has the potential of facile control of the reaction with less use of ammonia sources.
    Preview · Article · Dec 2014
  • Takahiro Takei · Nanae Muraki · Nan Xu · Akira Miura · Nobuhiro Kumada
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    ABSTRACT: A layered perovskite was fluorinated by polytetrafluoroethylene (PTFE) and then exfoliated to prepare a solution of fluorinated-perovskite nanosheets. Dion-Jacobson-type layered perovskites, RbLaNb2O7 and KCa2Nb3O10, were prepared in solid-state reactions at 950 and 1100 degrees C, respectively. The perovskites were fluorinated using PTFE at 400 degrees C, upon which their color changed from white to gray or black and their electrical conductivity increased to the order of 10(-4)-10(-5) S/m. The tetrabutyl ammonium cations(TBA(+)) in each fluorinated layered perovskite were first intercalated by tetrabutylammonium hydroxide(TBAOH) in H2O and then exfoliated in an acetonitrile solution for electrodeposition. Aniline was added to the solution as a monomer and electrodeposition was carried out in the acetonitrile solution for formation of a hybrid film. X-ray diffraction patterns confirmed that the polyaniline molecules could be intercalated within the interlayer space. Despite the fluorination, the volumetric filling fractions of polyaniline in the interlayer space were around 0.4 and 0.2 for the respective samples from RbLaNb2O7 and KCa2Nb3O10. The electrochemical specific capacitances of the deposited hybrid films increased steeply from below 50 F/g to above 230-250 F/g owing to the fluorination.
    No preview · Article · Oct 2014 · Colloids and Surfaces A Physicochemical and Engineering Aspects
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    Takahiro Takei · Akira Miura · Nobuhiro Kumada
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    ABSTRACT: Co-Al and Ni-Al layered double hydroxides (LDHs) intercalated with three types of anionic molecules, dodecylsulfate (C12H25SO4−, DS), di-2-ethylsulfosuccinate ([COOC2H3EtBu]2C2H3SO3−, D2ES), and polytungstate (H2W12O4210−, HWO) were prepared by means of ion-exchange and co-precipitation processes. With the use of DS and D2ES as intercalation agents, high crystallinity was maintained after intercalation into the LDHs. In the case of HWO, the intercalated LDHs could be obtained by ion-exchange as well as co-precipitation with a decline in the crystallinity; however, unreacted LDH was detected in the ion-exchange samples, and some unwanted phases such as hydroxide and pyrochlore were generated by the co-precipitation process. The maximum specific surface area and pore volume of the Ni-Al sample with intercalated HWO, prepared by the ion-exchange process were 74 m2/g and 0.174 mL g−1, respectively. The occupancies of DS, D2ES, and HWO within the interlayer space were approximately 0.3–0.4, 0.5–0.6, and 0.1–0.2, respectively, in the Co-Al and Ni-Al LDHs. Analysis of the catalytic activity demonstrated that the DS-intercalated Ni-Al LDH sample exhibited relatively good catalytic activity for conversion of cyclohexanol to cyclohexanone.
    Preview · Article · Sep 2014
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    Nobuhiro Kumada · Akira Miura · Takahiro Takei · Masatomo Yashima
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    ABSTRACT: The crystal structures of a pentavalent bismuthate, SrBi2O6 with the PbSb2O6-type structure and a lead bismuth oxide, (Pb1/3Bi2/3)O1.4 with the fluorite-type structure were refined by using neutron diffraction data. The final R-factors were Rwp = 4.49, Rp = 3.46, RI = 4.50 and RF = 1.70% for SrBi2O6 and Rwp = 5.04, Rp = 3.93, RI = 5.47 and RF = 4.26% for (Pb1/3Bi2/3)O1.4. SrBi2O6 prepared from NaBiO3·1.4H2O is the first example of the bismuthate with the PbSb2O6-type structure. The fluorite-type lead bismuth oxide, (Pb1/3Bi2/3)O1.4 was obtained by heating the PbSb2O6-type lead bismuthate, PbBi2O5.9·H2O which was prepared also from NaBiO3·1.4H2O.
    Preview · Article · Jun 2014
  • Nobuhiro Kumada · Nan Xu · Akira Miura · Takahiro Takei
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    ABSTRACT: Two new pentavalent bismuthates, CaBi2O6-x·1. 1H2O and PbBi2O6-x·H2O were prepared from NaBiO3·nH2O. Broadened X-ray powder diffraction patterns for both bismuthates were indexed to the hexagonal cell observed for PbSb2O6-type SrBi2O6. The lattice parameters were a = 0.555(1) and c = 0.530(1)nm for CaBi 2O6-x·1.1H2O and a = 0.5544(4) and c = 0.531(1) nm for PbBi2O6-x·H2O. At elevated temperatures, the Ca derivative decomposed to Ca3Bi 8O15 and an unknown phase via the fluoritetype structure, whereas the Pb derivative changed to Pb1/3Bi2/3O 4/3 via the fluorite-type (Pb1/3Bi2/3)O 1.4. Both Ca and Pb derivatives exhibited photocatalytic activity for the decomposition of phenol under visible light irradiation, although the adsorption of phenol was observed under dark conditions. The fluorite-type phases evolved from the Ca and Pb derivatives did not adsorb phenol under dark conditions, and their photocatalytic activities toward phenol decomposition under visible light were inferior to those of the parent compounds.
    No preview · Article · Jun 2014 · Journal- Ceramic Society Japan
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    ABSTRACT: The crystal structures of FexWN2 (x similar to 0.74, 0.90) and their magnetic properties were investigated. The structural studies reveal the formation of ion defects with different concentrations in the two-dimensional iron triangular lattice by analyzing the synchrotron radiation X-ray diffraction data. The magnetic measurements demonstrate that the Fe0.74WN2 phase shows ferromagnetism at 300 K, while the less defective Fe0.90WN2 phase exhibits no ferromagnetism. A decrease of temperature to 5 K significantly suppresses the room-temperature ferromagnetism of the Fe0.74WN2 phase. We attribute the emergence of ferromagnetism in FexWN2 to the increases of defect concentration and thermal displacement in the iron triangular lattice.
    No preview · Article · Apr 2014 · Journal of Alloys and Compounds
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    ABSTRACT: Perovskite-type structures (ABO3 ) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double-perovskite-type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A-site-ordered double perovskite structure, (Na0.25 K0.45 )(Ba1.00 )3 (Bi1.00 )4 O12 , with a maximum Tc of about 27 K.
    Full-text · Article · Apr 2014 · Angewandte Chemie International Edition
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    ABSTRACT: Spinel-type LiTi2O4 was prepared directly by the reaction of lepidocrocite-type potassium titanate and molten LiCl at 750-850 degrees C in a stream of H-2 gas. The Li/Ti ratio in the spinel-type phase prepared at 850 degrees C was 0.54, and the lattice parameter and superconductive critical temperature T-c were alpha = 8.3935(2) angstrom and 12.3 K, respectively. The product at 750 degrees C exhibited no superconductive diamagnetism down to 2.5 K, and the product at 800 C showed weak superconductive diamagnetism at 10.5 K. The morphology of the particles prepared at 850 degrees C showed an irregular shape with a particle size of 1-10 mu m.
    No preview · Article · Apr 2014 · Journal- Ceramic Society Japan
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    Full-text · Dataset · Feb 2014

Publication Stats

1k Citations
265.53 Total Impact Points


  • 2001-2015
    • University of Yamanashi
      • • Center for Crystal Science and Technology
      • • Faculty of Engineering
      Kōhu, Yamanashi, Japan
  • 2013
    • Tohoku University
      • Institute of Multidisciplinary Research for Advanced Materials (IMRAM)
  • 2002-2010
    • Research Institute of Innovative Technology for the Earth
      Kioto, Kyōto, Japan
  • 2009
    • Kyoto University
      • Institute for Chemical Research
      Kioto, Kyōto, Japan
  • 1998-2005
    • Tokyo Institute of Technology
      • • Department of Inorganic Materials
      • • Department of Metallurgy and Ceramics Science
      Edo, Tōkyō, Japan