Katsuya Teshima

Shinshu University, Shonai, Nagano, Japan

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Publications (118)353.9 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: As the 600 nm-class photocatalyst, BaTaO2N is one of the promising candidates of the perovskite-type oxynitride family for photocatalytic water splitting under visible light. The oxynitrides are routinely synthesized by nitriding corresponding oxide precursors under a high-temperature NH3 atmosphere, causing an increase in the defect density and a decrease in photocatalytic activity. To improve the photocatalytic activity by reducing the defect density and improving the crystallinity, we here demonstrate an NH3-assisted KCl flux growth approach for the direct synthesis of the BaTaO2N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO2N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO2N crystals could only be grown with the solute concentrations of ≥ 10 mol % using the KCl flux, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO2N crystals with an average size of about 125 nm and exposed {100} and {110} faces at 950 °C for 10 h. The time- and temperature-dependent experiments were also performed to postulate the direct growth mechanisms of cube-like BaTaO2N submicron crystals. The BaTaO2N crystals modified with Pt and CoOx nanoparticles showed a reasonable H2 and O2 evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH3-assisted KCl flux method.
    Crystal Growth & Design 07/2015; DOI:10.1021/acs.cgd.5b00927 · 4.56 Impact Factor
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    ABSTRACT: In this study, we demonstrated the template-mediated flux growth of one-dimensional LiCoO2 single crystals surrounded by {104} faces in a hot solution of LiCl-KCl. The reaction and growth process were characterized by time-dependent X-ray diffraction and scanning electron microscopy. The transformation in the crystal shape from rectangular to hexagonal cylindrical was considered to be directly related to the gradual lithiation of the starting CoO whiskers. Single particle galvanostatic tests of the single-strand LiCoO2 crystals were carried out. The LiCoO2 crystals exhibited excellent rate performance; more than 65% of the full capacity was maintained under ca. 370 C. These characteristics likely resulted from the exposure of the {104} planes, since they were electrochemically active in layered LiCoO2 with an α-NaFeO2 structure and favored fast Li+ transportation. This finding will facilitate the development of new materials for advanced lithium ion rechargeable batteries.
    06/2015; DOI:10.1039/C5TA02414E
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    ABSTRACT: Photocatalytic overall water splitting on (oxy)nitrides under visible light is one of the interesting approaches to fulfill the growing demand for clean and renewable energy. The improvement of the fabrication method is however important for reducing the defect density of (oxy)nitride crystals. The present study aims to investigate the direct growth of the LaTiO2N (LTON) crystallites with less defect density by an NH3-assisted flux method and to demonstrate the visible-light-induced photocatalytic water oxidation activity in relation to their crystallite morphology. Single-phase LaTiO2N crystallites (average size of 120 ± 39 nm) in round shape with smooth surface and high crystallinity were grown by an NH3-assisted flux method using the KCl flux with the solute concentration of 5 mol % at 950 °C for 10 h. The photocatalytic water oxidation activity of bare and CoOx-loaded LaTiO2N crystallites grown directly by an NH3-assisted flux method (1-step-LTON) was evaluated under visible light by comparing with the LaTiO2N crystallites fabricated by a two-step method (2-step-LTON), converting La2Ti2O7 to LaTiO2N by high-temperature nitridation. Within the first 2 h of the photocatalytic water oxidation half-reaction, the O2 evolution rates of bare and CoOx-loaded 1-step-LTON crystallites were 82 μmol·h-1 and 204 μmol·h-1, respectively, which are much higher than that of bare and CoOx-loaded 2-step-LTON crystallites (37 μmol·h-1 and 177 μmol·h-1) due to less defect density of the LaTiO2N crystallites achieved by a direct fabrication route using KCl flux. An NH3-assisted flux growth is a promising route for the direct fabrication of the LaTiO2N crystallites with less defect density that is beneficial for the enhancement of photocatalytic water oxidation half-reaction.
    The Journal of Physical Chemistry C 06/2015; 119(28). DOI:10.1021/acs.jpcc.5b03718 · 4.77 Impact Factor
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    ABSTRACT: Layers of well-developed crystals of Ta3N5 were successfully fabricated on Ta substrates by a novel flux coating method in a flow of NH3. The flux coating method is a simple one: the Ta substrates were coated with aqueous solutions of sodium compounds (= fluxes) and subsequently heated in a flow of NH3, whereupon the surfaces of the Ta substrates were dissolved in the flux, resulting in Ta3N5 crystal layers. The Ta in the Ta3N5 was provided by the substrate. Therefore, crystal layers with good adhesion could be grown directly on the substrates. The shape of the individual crystals as well as the surface morphology of the layers formed was determined by the flux used. The crystals fabricated using NaCl-Na2CO3 as the flux were prismatic and had relatively smooth faces, covering the surface of the Ta substrate uniformly. The crystal growth field resulting from the use of this method yielded well-formed crystals, which presumably grew from a solution. Finally, it was confirmed that a thus-synthesized Ta3N5 crystal layer modified using Co-Pi as the cocatalyst generated a photoanodic current under visible-light irradiation.
    05/2015; 3(26). DOI:10.1039/C5TA01724F
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    ABSTRACT: The thin films consisting of crystalline ZnO particles were prepared on FTO electrodes by electroless deposition. The particles were deposited from an aqueous solution containing zinc nitrate, dimethyamine-borane, and eosin Y at 328K. As the Pd particles were adsorbed on the substrate, not only the eosin Y monomer, but also the dimer and debrominated species were rapidly adsorbed on the spherical ZnO particles, which were aggregated and formed secondary particles. On the other hand, in the absence of the Pd particles, the monomer was adsorbed on the flake-shaped ZnO particles, which vertically grew on the substrate surface and had a high crystallinity. The photoelectric conversion efficiency was higher for the ZnO electrodes containing a higher amount of the monomer during light irradiation.
    ACS Applied Materials & Interfaces 05/2015; 7(21). DOI:10.1021/acsami.5b02570 · 6.72 Impact Factor
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    ABSTRACT: The phase-selective synthesis of (Li1.81H0.19)Ti2O5·2H2O flakes and homogenous LiTiO2 spheres has been achieved by means of a hydrothermal method using a titanium-triethanolamine complex. Compared with other Ti sources this offers the advantage of being more stable in water, though the crystal phases obtained depend on the growth conditions used, such as the initial Li/Ti ratio and aging time. The (Li1.81H0.19)Ti2O5·2H2O flakes were obtained through aging at 250 °C for 3 h from an initial Li/Ti ratio of 1.0 and exhibited a large surface area of 135 m2 g−1. Subsequent heat treatment of these flakes yielded Li4Ti5O12 nanoparticles with an even larger surface area (141 m2 g−1) and a discharge capacity of 164 mAh g−1, making them suited for use as an active material in lithium-ion rechargeable batteries.
    Ceramics International 05/2015; DOI:10.1016/j.ceramint.2015.05.043 · 2.09 Impact Factor
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    ABSTRACT: Dye-dispersing allophane–titania composite electrodes were prepared from titanium alkoxide sols containing dye and allophane. The photoelectric conversion properties of the electrodes were investigated by photoelectrochemical measurements. The photocurrent values in the UV range decreased with an increase in the allophane content, whereas those in the visible range were increased by adding 1.0% (Al/Ti ratio) allophane. As a small amount of allophane nanoparticles were highly dispersed in the titania electrodes, the dye molecules were dispersed in the electrodes without decreasing the efficiency of the electron injection from the dye to the titania conduction band. The dye molecules dispersed on the titania nanoparticle surface were capped with allophane nanoparticles which prevented desorption. The dye molecules strongly interacted with the titania nanoparticle surface and efficiently injected the excited electrons into the titania conduction band.
    Applied Clay Science 04/2015; 107. DOI:10.1016/j.clay.2015.01.015 · 2.70 Impact Factor
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    ABSTRACT: Defect formation energies based on an oxygen vacancy model and a metal-excess model in Ni/Mn ordered P4332 and disordered Fd3-m LiNi0.5Mn1.5O4 (LNMO) were evaluated by using ab initio density functional theory (DFT) calculations. The defect formation energy for the metal excess model was lower than for the oxygen vacancy model in both P4332 and Fd3-m. This indicates that oxygen vacancy formation reactions are unlikely, although interstitial cation occupation at the octahedral vacancies occurred in both P4332 and Fd3-m LNMO spinel compounds. In addition, the corresponding defect formation energy in Fd3-m was lower than that in P4332, indicating that the amount of defects is sensitive to the cation ordering/disordering in the spinel framework. This agrees with the experimental results that show that only Fd3-m tends to possess oxygen defects.
    The Journal of Physical Chemistry C 04/2015; 119(17):150420110643005. DOI:10.1021/acs.jpcc.5b01661 · 4.77 Impact Factor
  • CrystEngComm 03/2015; 17(18). DOI:10.1039/C5CE00386E · 3.86 Impact Factor
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    ABSTRACT: Flux growth is a promising method that allows to control both crystalline phase and crystal shape through the selection of a suitable flux. In this work, lanthanum titanate (La2Ti2O7) crystals with different morphologies were grown using the Na2MoO4, K2MoO4, NaCl, and mixed NaCl+K2MoO4 (molar ratio = 3:7) fluxes and their nitridability to form the LaTiO2N crystals under a high-temperature NH3 atmosphere was also investigated. The effects of solute concentration and cooling rate on the growth of the La2Ti2O7 crystals were also studied. The XRD results revealed that the {100} plane was dominant in the La2Ti2O7 platelet crystals grown using the alkali metal molybdate fluxes. By increasing the solute concentration from 1 to 20 mol%, an average size of the crystals decreased without considerably altering the overall crystal morphology. The La2Ti2O7 crystals with the preferred <010> and <001> growth directions along the b and c axes were grown using the Na2MoO4 and K2MoO4 fluxes, respectively. Compared to the Na2MoO4 flux, the K2MoO4 flux did not show a cooling rate-dependent effect on the growth of the La2Ti2O7 crystals. It was found that the conversion of the La2Ti2O7 crystals into the LaTiO2N crystals was strongly dependent on the flux used to grow the precursor La2Ti2O7 crystals. That is, the La2Ti2O7 crystals grown using the K2MoO4 and NaCl fluxes were nearly completely converted into the LaTiO2N crystals, while the conversion of the La2Ti2O7 crystals grown using the Na2MoO4 and mixed NaCl+K2MoO4 fluxes into the LaTiO2N crystals seemed to be not completed yet even after nitridation at 950°C for 15 h using NH3 due to the larger crystal size and the presence of unintentional impurities (Na and Mo from the flux) in the La2Ti2O7 crystal lattice. Nevertheless, the LaTiO2N crystals fabricated by nitriding the La2Ti2O7 crystals grown using the K2MoO4 and NaCl fluxes should be suitable for the direct solar water splitting.
    Inorganic Chemistry 03/2015; 54(7). DOI:10.1021/ic502862t · 4.79 Impact Factor
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    ABSTRACT: Layered double hydroxide (LDH) is an environmentally benign anion exchanger that can adsorb various toxic anions. In this work, we demonstrate the fabrication of plate-like Mg-Al-type LDH crystals on in situ formed alumina particles using a flux method at a relatively low temperature (∼350 °C). At or below 300 °C, the melted Al source crystallized to form AlOOH or γ-alumina particles in KNO3-NaNO3 flux. However, LDH crystals did not form due to the inferior crystallization properties of the Mg precursor. Increasing the holding temperature up to 350 °C and above facilitated crystallization of the dissolved Mg and Al species in flux to yield plate-like LDH crystals on the preformed alumina particles. Top-surface and cross-sectional FE-SEM and EPMA analyses revealed the vertical alignment of the crystalline LDH plates on the surface of the alumina particles. On the other hand, solid-state reactions did not yield these well-grown, plate-like LDH crystals. The TG-DTA profile of the LDH precursors with flux depicted the decomposition and crystallization events that the Al and Mg precursors undergo. On the basis of the results from these characterization studies, we propose a mechanism in which LDH crystals sequentially form on the surface of the alumina particles.
    Crystal Growth & Design 02/2015; 15(2):732-736. DOI:10.1021/cg501534f · 4.56 Impact Factor
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    ABSTRACT: Highly crystalline, platelike La(2)Ti(2)O7 were grown from a NaCl flux, and LaTiO2N crystals were obtained by subsequent nitridation under NH3 flow. The TEM analysis indicated that the flux-grown platelike La2Ti2O7 crystals are single-crystalline growing along the a axis. The shapes and sizes of the LaTiO2N crystals were almost unchanged from the La2Ti2O7 precursor. In addition, LaTiO2N crystals remained single-crystalline with a porous nanostructure. The optical absorption edges of the La2Ti2O7 and LaTiO2N crystals were approximately 320 and 600 nm.
    Crystal Growth & Design 01/2015; 15(1):124-128. DOI:10.1021/cg5010006 · 4.56 Impact Factor
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    ABSTRACT: Oxynitride perovskites and related phases have received considerable attention due to their potential application for visible-light-responsive photocatalyst and nontoxic inorganic pigments. The changes in bonding and structure by a partial replacement of O2– by N3– give rise to interesting dielectric behavior. Here, we report on the fabrication of highly crystalline La2TiO5 crystals by chloride flux growth method and their subsequent nitridation to form the LaTiO2N crystals using NH3 gas. The flux-grown La2TiO5 crystals had a columnar structure grown in the ⟨001⟩ direction. Using the NaCl flux, larger columnar La2TiO5 crystals were grown compared to those grown using the KCl flux. With increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased significantly. The columnar La2TiO5 crystals with smooth surface were readily converted by nitridation at 950 °C for 45 h followed by acid treatment into the LaTiO2N crystals with a highly porous structure that formed from the strong segregation of nanocrystals, leading to the largest specific surface area (16.5–18.4 m2·g–1). For the La2TiO5 crystals grown using the chloride fluxes, the wavelength of the absorption edges was approximately 320 nm (Eg = 3.87 eV), whereas the absorption edges exhibited by the LaTiO2N crystals obtained by nitridation were approximately 600 nm (Eg = 2.06 eV). Particularly, the LaTiO2N crystals prepared in this study by nitriding the precursor La2TiO5 crystals did not show a noticeable absorption in the near-infrared region above 600 nm, which is generally attributable to some reduced Ti3+ species and nitrogen deficiency, even after a long nitridation process. The fabricated LaTiO2N crystals with low defect density will be advantageous for various applications that specially require higher specific surface area.
    Crystal Growth & Design 01/2015; 15(1):333−339. DOI:10.1021/cg501397x · 4.56 Impact Factor
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    ABSTRACT: Zinc oxide and layered zinc hydroxides were deposited from an aqueous solution of zinc nitrate at 323-358 K on a substrate plate with a very thin titanium dioxide film by a photocatalytic reaction. The amorphous or low crystalline zinc hydroxide aggregates were deposited at a low temperature. The zinc oxide crystals with about 1-2 μm-sized hexagonal columns and 10 nm-sized spheres were formed at 338-358 K. Nitrate ions in the solution were reduced to nitrite ions, and water was transformed into hydroxide ions by a photocatalytic reaction on the titanium dioxide film. The pH value increased on the substrate surface with the titanium dioxide film, which caused the zinc hydroxide formation on the film. The zinc hydroxides were then dehydrated and transformed into zinc oxide. The average crystallite size of the zinc oxide decreased with an increase in the reaction temperature because the reaction rates of the formation and dehydration of the zinc hydroxides increased which resulted in an increase in the formation rate of the crystal zinc oxide nuclei.
    Applied Catalysis B Environmental 11/2014; 160-161:651-657. DOI:10.1016/j.apcatb.2014.06.019 · 7.44 Impact Factor
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    ABSTRACT: Thin layers of densely packed idiomorphic Li4Ti5O12 crystals were prepared directly on a Pt substrate by using a LiClNaCl mixed flux. The thin film of colloidal anatase nanoparticles with an similar to 30 nm diameter was fully converted into Li4Ti5O12 crystals with a diameter of 100-200 nm having polyhedral shapes with well-defined {111} faces during the evaporation-driven flux growth. Cross-sectional structure analysis revealed that the interface between the crystals and the substrate seemed to be atomically bonded. Galvanometric charge and discharge properties strongly supported our consideration of interface formation. The Li4Ti5O12 crystal layer exhibited a large capacity close to its theoretical value under 0.1C rate with no assistance from additional electroconductive materials and binders, meaning that the interfaces provide seamless charge transportation pathways. We also addressed the formation mechanism of the Li4Ti5O12 crystal layer accompanied by the experimental results of in situ X-ray diffraction, thermogravimetric and differential thermal analysis, and scanning electron microscopy observation.
    Crystal Growth & Design 11/2014; 14(11):5634-5639. DOI:10.1021/cg5009279 · 4.56 Impact Factor
  • Kunio Yubuta · Katsuya Teshima · Shuji Oishi
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    ABSTRACT: The structure of Nb2O5 nanotubes was studied by electron diffraction and high-resolution transmission electron microscopy (HRTEM). The nanotubes were prepared from highly crystalline-layered K4Nb6O17 crystals grown from a KCl flux using the intercalating and exfoliating techniques. Nanotubes have a length of about 200 nm and a diameter of 20 nm. The observed electron diffraction patterns revealed that the nanotubes have a helical structure with the basal plane of (010) and the tube-axis parallel to the [201] direction. Their basal plane corresponds to the (010) plane of the layered K4Nb6O17 crystals. The helical lattice plane was directly observed by a Fourier filtered HRTEM image. Copyright © 2014 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 10/2014; 46(10-11). DOI:10.1002/sia.5478 · 1.39 Impact Factor
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    ABSTRACT: Graphene monoliths made from graphene oxide colloids by unidirectional freeze-drying method were activated by typical activation processes of CO2 activation, chemical activation using ZnCl2 or H3PO4, and KOH activation. The porosity development of graphene monolith markedly depends on the activation method. The monoliths with highest surface area are obtained by the KOH activation method; only the KOH activation is effective for production of the graphene monolith of which surface area is in the range of 1760–2150 m2 g−1. The mechanism of the porosity development by KOH activation method is proposed. This work provides a promising route for the bottom-up design of pore width-tunable nanoporous carbons.
    Carbon 09/2014; 76:220–231. DOI:10.1016/j.carbon.2014.04.071 · 6.16 Impact Factor
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    ABSTRACT: Dye-dispersing ZnO precursor gel films were prepared on indium tin oxide electrodes from a zinc acetate solution containing eosin Y by dip-coating, steam treatment, and then heating at a low temperature. The electronic interaction between the dye and zinc in the dye-dispersing gel films were investigated by spectroscopic and electrochemical measurements. A photocurrent was observed in the dye-dispersing gel electrodes before the steam treatment. The photocurrent value increased by the steam treatment and heating due to crystallization of the gel and removal of organic impurities. The dye molecules existed between the interlayers of the layered zinc hydroxide coexisting with the ZnO. The photoexcited electron in the dye should be injected into the ZnO conduction band via the layered zinc hydroxide. The value increased with an increase in the dye content even though the ZnO crystallinity decreased. The dye–zinc interaction, i.e., the complex formation and photoinduced electron injection, played an important role in the electron transport and photoelectric conversion.
    Research on Chemical Intermediates 07/2014; 41(9). DOI:10.1007/s11164-014-1761-z · 1.54 Impact Factor
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    ABSTRACT: Higher performance active materials containing cathodes have been strongly required for the achievement of advanced lithium-ion rechargeable batteries. We report the formation of a densely packed and idiomorphic LiCoO2 crystal layer directly on a substrate surface using flux coating with a LiNO3–LiOH mixed flux and their applications as an additive-free cathode to enhance gravimetric energy density per single cell as well as durability. The formation mechanism of LiCoO2 nanocrystals from high temperature melt of the mixed flux was controlled under evolution selection growth, driven by supersaturation. The as-grown crystals formed a petal-shape with well-developed {001} faces. After annealing treatment, the crystal shape transformed into hexagonal plates with vertical orientation. The hexagonal plate-shaped crystal arrays were identified to be homogeneous LiCoO2 having a rhombohedral crystal system. Their lattice parameters were a = 0.2784 and c = 1.4248 nm, and the Li/Co ratio in the crystals was 1.00. Furthermore, transmission electron microscopic images and selected area electron diffraction patterns revealed that the interface between the crystal layer and the substrate was smoothly connected and free of impurities. In fact, the LiCoO2 nanoplate layer exhibited desirable properties such as a large discharge capacity close to its theoretical value, higher rate performance, and high cyclability over 500 cycles with no assistance of additional electroconductive materials and binders, meaning that the interfaces provide seamless charge transportation pathways.
    Crystal Growth & Design 03/2014; 14(4):1882–1887. DOI:10.1021/cg5000217 · 4.56 Impact Factor
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    ABSTRACT: Low-temperature growth of idiomorphic spinel-type Li1+xMn2−xO4 (x = 0.09, 0.14) crystals was achieved by using a LiCl-KCl flux. The flux growth driven by rapid cooling resulted in truncated octahedral Li1+xMn2−xO4 crystals surrounded by both dominating {111} and minor {100} faces. The chemical compositions, sizes, and shapes of the Li1+xMn2−xO4 crystals could be tuned by simply changing the growth conditions. Among the various products, the crystals grown at a low temperature of 600 °C showed a small average size of 0.2 μm. The small Li1+xMn2−xO4 crystals grown at 600 °C showed better rate properties than the large crystals grown at 900 °C, when used as a positive active material in lithium-ion rechargeable batteries.
    CrystEngComm 02/2014; 16(6):1157. DOI:10.1039/c3ce41682h · 3.86 Impact Factor