Yuya Oaki

Keio University, Edo, Tōkyō, Japan

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Publications (96)537.55 Total impact

  • [Show abstract] [Hide abstract]
    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; DOI:10.1039/C5CC01466B · 6.72 Impact Factor
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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; DOI:10.1039/C5CC02058A · 6.72 Impact Factor
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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; 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; DOI:10.1002/chem.201405724 · 5.70 Impact Factor
  • Polymer Journal 12/2014; 47(2):183-189. DOI:10.1038/pj.2014.113 · 1.55 Impact Factor
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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; DOI:10.1039/C4NR05317F · 6.74 Impact Factor
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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 · 13.45 Impact Factor
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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 · 3.71 Impact Factor
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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 · 6.72 Impact Factor
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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 · 8.54 Impact Factor
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    ABSTRACT: Stiff yet flexible and strong transparent film materials mimicking the structure of crustacean exoskeletons were obtained from calcium carbonate and cellulose. These biobased materials have the composite structure of a three-dimensional nanoscale network of surface-carboxylated crystalline cellulose incorporated into an amorphous inorganic-based hybrid matrix of nanosegregated calcium carbonate and acidic polymers.
    05/2014; 1(3):321. DOI:10.1039/c3mh00134b
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    ABSTRACT: The processes of the hierarchical morphology replication were studied for understanding of the mechanisms. The mesocrystal structure possesses the nanospace as the interspace of the unit nanocrystals for incorporation of guest molecules. The monomers can be incorporated and polymerized in the nanospace of the mesocrystals. In the present work, a sea urchin spine and poly(3,4-ethylenedioxythiophene) (PEDOT) were adopted as a model of the original mesocrystal and replicated polymer material, respectively. Hierarchical morphologies of the original material were replicated to PEDOT from nanoscopic to macroscopic scales. A sea urchin spine, as an original material, has the hierarchical architectures based on the mesocrystal structure consisting of the oriented carbonate nanocrystals. The composite of the original carbonate and PEDOT was obtained after the incorporation and the polymerization of the monomer. After dissolution of the original carbonate, the resultant PEDOT architecture showed the hierarchical morphologies similar to those of the original sea urchin spine. The morphology replication processes were compared with those of the different polymers. The important factors for the morphology replication are studied. The present work suggests that the approach can be applied to morphogenesis of a variety of polymer materials.
    Langmuir 03/2014; DOI:10.1021/la404942v · 4.38 Impact Factor
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    ABSTRACT: Micrometric linear chains (1D arrays), monolayers (2D arrays), and superstructures (3D arrays) of anisotropic Mn3O4 nanocuboids were selectively produced by oriented self-assembly through evaporation of a dispersion. The 1D arrays were basically formed on a substrate via oriented self-assembly of the rectangular crystals in the <100> direction. The 2D and 3D microarrays were obtained by adjusting the particle concentration of the dispersion. The <001> direction of tetragonal crystal was controlled to be parallel and perpendicular to the substrate by changing the polarity of the medium.
    Journal of the American Chemical Society 12/2013; 136(10). DOI:10.1021/ja410183q · 11.44 Impact Factor
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    ABSTRACT: Synthetic routes for LiCoO2 mesocrystals with porous and single-crystalline structures through the topotactic transformation of precursor crystals are developed. Octahedral LiCoO2 frameworks of 4–6 μm in diameter, consisting of 600–800 nm subunits, are obtained from octahedral solid particles of Co2(OH)3Cl. Hollow LiCoO2 plates about 600 nm wide and 40 nm thick composed of 100–400 nm subunits are produced from hexagonal disks of β-Co(OH)2. The LiCoO2 mesocrystals exhibit enhanced charge–discharge cycle stability and rate performance as a cathode material for lithium-ion batteries. The high reactive surface area, owing to the porous framework, and high lithium-ion and electron conductivities, originating from the single-crystalline nature, effectively enhance their electrochemical properties.
    ChemPlusChem 11/2013; 78(11). DOI:10.1002/cplu.201300213 · 3.24 Impact Factor
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    ABSTRACT: Thin films of copper oxides can be synthesized on substrates by using approaches that are inspired by microbial mineralization processes. In nature, precipitates of manganese and iron oxides with controlled oxidation states and crystal phases are produced through biomineralization by microorganisms. We have previously reported microbial-mineralization-inspired approaches that are comprised of direct and intermediate routes for the controlled syntheses of transition-metal oxides. Herein, these approaches are applied to the thin-film formation and coating of copper oxides and a related compound with controlled crystal phases and morphologies. Thin films of CuO, Cu2 O, and Cu2 (OH)3 Cl were selectively synthesized by using direct or intermediate routes. Notably, CuO mesocrystal nanosheets formed a thin film over the whole of the substrate. The resultant CuO mesocrystal nanosheets showed enhanced properties for the electrochemical detection of dopamine. This study shows the potential applicability of microbial-mineralization-inspired approaches to thin-film coatings.
    Chemistry - An Asian Journal 09/2013; 8(9). DOI:10.1002/asia.201300493 · 3.94 Impact Factor
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    ABSTRACT: The band-gap energy tuning of WO3 quantum dots was realized in the range of 2.6 eV (bulk) to 3.7 eV (sub-nano) by precise size control around one nanometer. Newly synthesized sub-nanoporous silicas act as excellent templates. In addition, single-electron reduction of oxygen under UV irradiation is now achievable with WO3.
    Chemical Communications 08/2013; 49(76). DOI:10.1039/c3cc44264k · 6.72 Impact Factor
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    ABSTRACT: Microscale patterns consisting of two-dimensional (2D) dendrites with trunks and branches 1–3 μm wide were precisely controlled through crystal growth in a thin polymer matrix by the use of a dipping technique. A variety of 2D micropatterns, such as orthogonal lattices, bull’s horns, randomly curving weaves, and aligned dots, were homogeneously formed with an aqueous solution of Ba(NO3)2 and poly(vinyl alcohol) in a wide area ranging over several centimeters on flat and rounded substrates. The micrometric dendritic growth that produced these several patterns was tuned by changing the withdrawal rate and the polymer concentration. The crystallographic orientation of the micropatterns was characterized to discuss on the formation mechanism of the specific morphologies. The curving branches were found to be induced by gradual change in the growth direction through low-angle grain boundaries under a highly diffusion-limited condition. This simple, bottom-up patterning process is applicable for various crystalline materials, including inorganic and organic substances.
    Crystal Growth & Design 06/2013; 13(7):3011–3017. DOI:10.1021/cg400418n · 4.56 Impact Factor
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    ABSTRACT: Microscopic pin-holder structures of beta-Co(OH)(2) having a feature of single crystal were synthesized through multistep crystal growth in an aqueous solution system. The hexagonal plates of the hydroxide were initially produced from an amorphous precursor. The parallel nanoscale pins of beta-Co(OH)(2) were then grown on the basal plates in the progressive stage. The single-crystalline array of LiCoO2 nanopins was obtained by topotactic transformation of the specifically shaped beta-Co(OH)(2) pin holders.
    CrystEngComm 06/2013; 15(33):-. DOI:10.1039/C3CE40688A · 3.86 Impact Factor
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    ABSTRACT: Hydroxyapatite (HA) nanorods elongated in the c axis were produced at 38–80 °C in an aqueous solution system that was based on simulated body fluid. Specific columnar structures consisting of HA nanorods were produced through epitaxial growth on a c-face HA substrate under a mildly acidic condition. The morphology of the columnar structure was controlled by changing the growth condition and adding amino acid. The length and width of the columns were varied in a range from several tens of nanometers to several micrometers. Densely packed columns were produced with hexagonal HA rods at 80 °C. Fibrous columns of HA with gaps were formed at 60 °C in the presence of aspartic acid. The Young's modulus of the densely packed HA columns was comparable to those of a human enamel and a rabbit bone oriented in c axis. The columnar structures can contribute to controlling the mechanical performance of biological and biomedical HA crystals.
    06/2013; 1(2):143–148. DOI:10.1016/j.jascer.2013.03.009
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    ABSTRACT: Of minerals and microbes: A microbial-mineralization-inspired approach was used to facilitate the syntheses of iron oxides with a high specific surface area, such as 253 m2 g-1 for maghemite (γ-Fe2 O3 ) and 148 m2 g-1 for hematite (α-Fe2 O3 ). These iron oxides can be applied to electrode material of lithium-ion batteries, adsorbents, and catalysts.
    Chemistry - A European Journal 04/2013; 19(14). DOI:10.1002/chem.201204333 · 5.70 Impact Factor