Yuya Oaki

Keio University, Edo, Tōkyō, Japan

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Publications (106)580.79 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We achieved the fabrication of enamel-like films consisting of c-axis-oriented hydroxyapatite (HA) nanorods by a two-step controlled crystal growth method with the monolayer of a specific artificial peptide binding to the c face of HA. Self-standing films were formed under the monolayer at the air–water interface in an aqueous solution system that was based on a simulated body fluid. The presence of fluoride ions promoted the arrangement of apatite microrods in the c direction through geometric selection. The subsequent growth with aspartic acid produced the enamel-like structure consisting of c-axis-oriented nanorods on the basal microrods. Therefore, the controlled crystal growth using the aqueous system including the specific artificial peptide, fluoride ion, and aspartic acid is regarded as a novel fabrication route for well-organized architectures.
    CrystEngComm 06/2015; DOI:10.1039/C5CE00598A
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    ABSTRACT: Nanosized composite rods 300 nm in length and 20 nm in width were produced by deposition of 22–77 wt% of a c-axis-oriented hydroxyapatite (HA) on cellulose nanocrystals (CNCs). The CNCs functionalized with sulphonic groups were covered with the HA nanocrystals through controlled nucleation and growth under a moderately supersaturated condition in a solution system based on a simulated body fluid. Water-resistant transparent coatings 2–4 μm thick were obtained via evaporation-induced assembly of CNC–HA nanocomposites by casting their suspension on a glass substrate and the subsequent growth of HA nanocrystals by vapour hydrothermal treatment. The composite coatings exhibited improved mechanical strength compared to that of crustacean exoskeletons, and potential for bone regeneration.
    06/2015; DOI:10.1039/C5TB00927H
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    ABSTRACT: We report a fabrication of transparent ZnO thick films by a chemical bath deposition using zinc acetate ethanolic-aqueous solutions with cetyltrimethylammonium chloride (CTAC). In this system, CTAC plays a particular role in nucleation and crystal growth of ZnO, and the micrometer-thick ZnO films formed on ZnO-buffered glass substrates. The resultant ZnO thick films show a high transmittance of around 80% in the visible region. An orientation of ZnO nuclei formed on the substrates in the early stage is different from a c-axis orientation of the ZnO-buffer layer, and the resultant ZnO thick films also show the unusual orientation. The detailed information on the orientation of these ZnO films was analyzed based on X-ray diffraction (XRD) 2θ/θ scan measurements at angles between in-plane and out-of-plane geometries. According to the results, the c-axis of ZnO is tilted at around 40-50° with respect to the film normal, and the micrometer-thick ZnO films exhibit the (112)-plane orientation by adjusting the CTAC concentration of reaction solutions.
    Crystal Growth & Design 06/2015; DOI:10.1021/cg501729w
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    ABSTRACT: Controlled/living radical polymerization was examined with the use of magnetic iron oxide (Fe3O4) prepared through various processes, including hydrothermal synthesis, a bioinspired process, and magnetotactic bacteria. Prior to the use of various types of Fe3O4, commercially available Fe3O4 was employed as a heterogeneous catalyst for styrene polymerization in conjunction with an alkyl halide as an initiator. Under appropriately optimized conditions, with the addition of Ph3P in a solvent mixture of toluene and DMF, the polymerization proceeded in a well-controlled manner. In addition, solid Fe3O4 was recovered using a magnetic approach and was reused for a polymerization reaction. The effects of stirring on the polymerization rate suggest that the surface of Fe3O4 is responsible for the catalysis in polymerization. Fe3O4 samples prepared through various processes were then used for styrene polymerization and showed different activities depending on the preparation process. In particular, Fe3O4 prepared through hydrothermal synthesis exhibited a much higher activity compared with commercial Fe3O4.
    RSC Advances 06/2015; 5(63). DOI:10.1039/C5RA09149G
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    ABSTRACT: Monocrystalline architectures with well-defined shapes were achieved by bottom-up routes through epitaxial attachment of Mn3O4 nanocrystals. The crystallographically continuous 1D chains elongated in the a axis and 2D panels having large a or c faces were obtained by removal of the organic mediator from surfactant-mediated 1D and 2D arrays of Mn3O4 nanocrystals, respectively. Our basal approach indicates that the epitaxial attachment through the surfactant-mediated arrays is utilized for fabrication of a wide variety of micrometric architectures from nanometric crystalline units.
    Langmuir 05/2015; 31(22). DOI:10.1021/acs.langmuir.5b00502
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    ABSTRACT: Coupled exfoliation and fracture induced formation of the hydrophobic monolayered nanoflakes in a nonpolar organic medium. The hydrophobic monolayered nanoflakes 5–20 nm in the lateral size consisted of the tungstate layer with surface modification by stearylammonium ions.
    Chemical Communications 05/2015; 51(49). DOI:10.1039/C5CC02203G
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    ABSTRACT: Surface of oxidant crystals facilitates simultaneous synthesis and morphogenesis of conductive polymer materials. Oxidant crystals as a condensed phase of the oxidative agent induce formation of the conductive polymer nanoparticles and their accumulated films. The resultant hierarchical structures contribute improvement of the electrochemical properties.
    Chemical Communications 05/2015; 51(47). DOI:10.1039/C5CC03013G
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    ABSTRACT: We demonstrate the oscillatory phenomenon for the twisting growth of a triclinic crystal through in-situ observation of the concentration field around the growing tip of a needle by high-resolution phase-shift interferometry.
    Chemical Communications 04/2015; 51(40). DOI:10.1039/C5CC01466B
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    ABSTRACT: In nature, biological macromolecules control growth of inorganic crystals under mild conditions in aqueous media. Inspired by biomineralization, biomimetic approaches have been studied for growth control of inorganic crystals in aqueous media by using organic molecules and polymers. The approaches were not applied to nonaqueous system for development of functional organic materials. Here we have applied biomimetic approaches to growth control of organ-ic crystals in nonaqueous media. Morphology and orientation of 10,12-pentacosadiynoic acid (PCDA) crystals, a diacety-lene derivative, were controlled in organic media with the additive organic polymers and surface-modified substrates. The oriented PCDA ribbons were obtained by an advanced biomimetic approach. After the topochemical polymeriza-tion, the resultant polydiacetylenes (PDA) ribbons were applied to the thermochromic materials with intercalation of metal ions and the semiconductor layer of an organic field-effect transistor. The present work implies that biomimetic approaches can be applied to morphology and orientation control of organic crystals.
    Chemistry of Materials 04/2015; 27(7):2627-2632. DOI:10.1021/acs.chemmater.5b00418
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    ABSTRACT: The hierarchical porous structures of vinyl polymers, such as polystyrene (PSt) and poly(methyl methacrylate) (PMMA), were derived from morphologies of a biomineral through the replication. The resultant PSt hierarchical structures showed the enhanced adsorption properties of organic molecules as a hydrophobic adsorbent in an aqueous medium.
    Chemical Communications 04/2015; 51(37). DOI:10.1039/C5CC02058A
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    ABSTRACT: The dynamic adsorption of toluene was investigated using supermicroporous silicas (SMPSs) synthesized through a solvent-free route by using tetraethoxysilane and conventional alkyltrimethylammonium halides as a structure-directing agent. The adsorption capacity of SMPSs depended on their pore diameters, which were precisely tuned in a range from 0.7 to 2.0 nm at single-angstrom intervals. The micropore filling of the molecule that is highly enhanced with micropores smaller than 1 nm improves the dynamic adsorption performance.
    Microporous and Mesoporous Materials 04/2015; 214. DOI:10.1016/j.micromeso.2015.04.034
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    ABSTRACT: Monodispersed thermosensitive microspheres were synthesized by the combination of poly(N-isopropylacrylamide) (PNIPAAm) and a mesostructured framework consisting of vaterite CaCO3 nanocrystals ~20 nm in diameter. The switchable dispersivity in water and organic media was provided to the rigid porous framework by the soft shell of the thermosensitive organic component. The microspheres were shuttlecocked between water and hydrophobic organic media by a swing of temperature across the lower critical solution temperature (LCST). The composite microspheres performed as an active carrier of organic molecules due to their switchable property. Hydrophobic molecules were captured by the microspheres dispersed in an organic medium above LCST and were then delivered and released to another organic phase through a water phase by the swing in temperature.
    03/2015; 3(17). DOI:10.1039/C5TB00342C
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    ABSTRACT: In biomineralization, acidic macromolecules play important roles for the growth control of crystals through a specific interaction. Inspired by this interaction, we report on an application of the hierarchical structures in CaCO3 biominerals to a stationary phase of chromatography. The separation and purification of acidic small organic molecules are achieved by thin-layer chromatography and flash chromatography using the powder of biominerals as the stationary phase. The unit nanocrystals and their oriented assembly, the hierarchical structure, are suitable for the adsorption site of the target organic molecules and the flow path of the elution solvents, respectively. The separation mode is ascribed to the specific adsorption of the acidic molecules on the crystal face and the coordination of the functional groups to the calcium ions. The results imply that a new family of stationary phase of chromatography can be developed by the fine tuning of hierarchical structures in CaCO3 materials. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Chemistry - A European Journal 02/2015; 21(13). DOI:10.1002/chem.201405724
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    ABSTRACT: Nanosized anatase TiO2 crystals were rapidly prepared in a mixture of an ionic liquid (1-butyl-3-methylimidazolium chloride) and water under microwave irradiation for 5 min. The crystal size in the c axis was varied in the range from ~5 to ~17 nm with prolongation of the irradiation time. A high susceptibility of the ionic liquid to microwave and a catalytic property of the imidazolium cation are effective for the rapid synthesis of the highly crystalline nanosized particles.
    Journal of the Ceramic Society of Japan 01/2015; 123(1434):79-82. DOI:10.2109/jcersj2.123.79
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    ABSTRACT: Dendritic morphologies of a metal oxide, metal and conductive polymer were obtained by using the dendrites of a transition metal salt as a template. Dendrites of the transition metal salt, copper acetate monohydrate (Cu(CH3COO)(2)center dot H2O), were prepared under diffusion-controlled conditions in a polymer matrix, such as poly(vinyl alcohol; PVA). The resultant dendrites of Cu (CH3COO)(2)center dot H2O acted as a self-template and were converted to divalent copper oxide (CuO) by the thermal treatment with the removal of PVA. The macroscopic CuO dendritic shapes consisting of the nanocrystals were obtained, and the resultant CuO was reduced to metallic copper (Cu) without morphological changes. The dendritic morphologies of polypyrrole (PPy) were obtained by the diffusion of pyrrole (Py) vapor to the Cu(CH3COO)(2)center dot H2O dendrites, which acted as the reactive template. The oxidative polymerization of Py provided PPy in the PVA matrix. These results suggest that the approach to morphogenesis presented herein can be applied to other metal oxides, metals and polymers.
    Polymer Journal 12/2014; 47(2):183-189. DOI:10.1038/pj.2014.113
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    ABSTRACT: Oriented nanocrystals, as seen in biominerals, have both the macroscopic hierarchical morphologies and the nanoscale interspace among the unit crystals. Here we studied the incorporation effects of the specific interspace in the oriented nanocrystals on the morphologies, properties, and applications of organic crystals. The organic crystals, such as 9-vinylcarbazole (VCz), azobenzene (AB), and pyrene (PY), were introduced in the specific interspace of oriented nanocrystals from the melts. The morphologies, properties, and applications of the incorporated organic crystals were systematically studied in these model cases. The incorporation of the organic crystals provided the composites with the original oriented nanocrystals. The incorporated organic crystals formed the single-crystalline structures even in the nanoscale interspace. The melts of the organic compounds were crystallized and grown in the interspace of the original materials. The incorporated organic crystals showed the specific phase transition behavior. The freezing points of the organic crystals were raised by the incorporation in the nanospace while the melting points were not varied. The hierarchical morphologies of the organic crystals were obtained after the dissolution of the original materials. The hierarchical morphologies of the original materials were replicated to the organic crystals. The incorporated organic crystal was polymerized without deformation of the hierarchical morphologies. The present work shows the potentials of the nanoscale interspace generated in the oriented nanocrystals.
    Nanoscale 12/2014; 7(8). DOI:10.1039/C4NR05317F
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    ABSTRACT: 1 nm CuO quantum dots (QDs) were produced in size-controlled super-micropores of a silica matrix. The reversible color change of the QDs from pale blue to deep green was clearly observed in a wide temperature range from 298 to 673 K. This particular thermochromism is ascribed to an enhanced bandgap shift depending on temperature with a strong electron-phonon coupling in the confined space of the 1 nm QDs.
    Angewandte Chemie International Edition in English 09/2014; 53(40). DOI:10.1002/anie.201406330
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    ABSTRACT: Spinel-type lithium titanate (Li4Ti5O12) nanocrystals were synthesized at a low temperature with control of the basicity by a solvothermal method without any calcination processes. The Li-Ti-O precursor was produced under a relatively high pH condition (pH >11) in the initial stage. The spinel phase was then formed through delithiation and dehydration from the precursor by lowering the pH of the system (pH <10). The direct synthesis of the Li4Ti5O12 nanocrystals was achieved through self-control of the basicity using a two-phase system of water/ethanol and toluene/oleic acid. The Li4Ti5O12 crystals were formed from the precursor by decreasing the basicity. The average size of the nanocrystals prepared by the one-step method was ca. 13 nm, and the specific surface area was 158 m2 g-1. On the basis of the electrochemical measurement, the Li4Ti5O12 nanocrystals produced in a low-temperature solution system are applicable for an anode material of a lithium-ion rechargeable battery.
    RSC Advances 09/2014; 4(83). DOI:10.1039/C4RA06646D
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    ABSTRACT: Synthesis, coating, and morphogenesis of conductive polymers were achieved on a variety of substrates through spontaneous generation of activated monomer vapors under ambient pressure and low temperature conditions. The present approach facilitates the generation of complex hierarchical morphologies and the conductive coating for improvement of electrochemical properties.
    Chemical Communications 08/2014; 50(80). DOI:10.1039/c4cc05171h
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    ABSTRACT: Hydrophobic inorganic–organic composite nanosheets based on manganese and titanium oxide monolayers were obtained in a nonpolar organic medium. In general, monolayered materials of transition metal oxides were prepared and dispersed in aqueous and polar organic media. Here we report on a simple and generalizable approach for syntheses of the hydrophobic composite nanosheets consisting of the transition metal–oxide monolayers and the surface hydrophobic organic layers. The composite nanosheets were dispersed in a nonpolar organic medium. The resultant composite nanosheets based on the manganese oxide show the improved catalytic activity for oxidation of an alcohol in toluene. The large blueshift of the bandgap energy was observed on the composite nanosheets based on the titanium oxide. The present approach can be applied to syntheses of hydrophobic composite nanosheets from a variety of layered compounds. The hydrophobic composite nanosheets have potentials for a wide range of applications based on the composite structures.
    Chemistry of Materials 05/2014; 26(11):3579–3585. DOI:10.1021/cm5012982