Yuya Oaki

Keio University, Edo, Tōkyō, Japan

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Publications (84)446.3 Total impact

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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; · 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; · 6.38 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 08/2014; · 13.45 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.
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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; · 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; · 10.68 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).
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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; · 6.38 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 06/2013; · 4.57 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. · 4.69 Impact Factor
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    ABSTRACT: Microscopic pin-holder structures of β-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 β-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 β-Co(OH)2 pin holders.
    CrystEngComm 06/2013; · 3.88 Impact Factor
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    ABSTRACT: Functional peptides play an important role in the formation of biominerals. Here we focus on peptides involved in mineralization of the exoskeleton of a crayfish. New recombinant peptides are designed and synthesized on the basis of an acidic functional peptide, CAP-1, derived from the exoskeleton of a crayfish. We have examined the effect of these peptides on the hybrid formation of calcium carbonate and organic molecules in aqueous solution. In the presence of these recombinant peptides having chitin binding moieties, platelike tripodal calcite crystals are formed on the chitin matrix with unidirectional crystallographic orientation. Two kinds of interactions, (1) interactions between an acidic part of the peptides and calcium ions and (2) specific binding interactions of the peptides to chitin, are essential for the formation of the oriented crystals and the induction of specific morphologies. Moreover, the sizes of crystallites decrease with increasing the number of acidic parts of mutational peptides. The bioinspired design of peptides that control the interface of inorganic and organic domains may pave the way for an opportunity to build new hybrid structures.
    Faraday Discussions 03/2013; 159(1):483-494. · 3.82 Impact Factor
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    ABSTRACT: Monolayered nanodots of titanium, tungsten, and manganese oxides were obtained by exfoliation of the nanocrystals through aqueous solution processes at room temperature. The precursor nanocrystals of the layered compounds, such as sodium titanate (Na0.80Ti1.80□0.2O4·xH2O, □: vacancy (x < 1.17)), cesium tungstate (Cs4W11O35·yH2O (y < 10.5)), and sodium manganate (Na0.44MnO2·zH2O (z < 0.85)), were synthesized in an aqueous solution. These nanocrystals of the layered compounds were delaminated into the monolayered nanodots through introduction of a bulky organic cation in the interlayer space. The resultant monolayered nanodots of the titanate and tungstate 2-5 nm in lateral size showed a remarkable blueshift of the bandgap energies. The calculation studies supported the blueshifts of the bandgap energies. The results suggest that syntheses of monolayered nanodots can expand the tuning range of the properties based on size effect. The present approaches for generation of ultrathin tiny objects can be applied to a variety of nanomaterials.
    Journal of the American Chemical Society 03/2013; · 10.68 Impact Factor
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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 02/2013; · 5.93 Impact Factor
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    ABSTRACT: Mesocrystals of Li-Mn-O compounds, such as LiMn(2)O(4), Li(2)MnO(3), and LiMnO(2)-Li(2)MnO(3), consisting of oriented nanoscale units were selectively produced under hydrothermal conditions from biomimetically prepared MnCO(3) mesocrystals. Topotactic transformation through the intermediate phase of Mn(5)O(8) inheriting a hierarchical structure of the MnCO(3) precursor was essential for the formation of the mesocrystal compounds. The crystal phases were successfully controlled by varying the conditions for the hydrothermal reactions. The Li-Mn-O mesocrystals have considerable potential as cathodes of Li-ion batteries.
    Nanoscale 02/2013; · 6.73 Impact Factor
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    ABSTRACT: Nanoporous MnO frameworks with highly dispersed Co nanoparticles were produced from MnCO(3) precursors prepared in a gel matrix. The MnO frameworks that contain 20 mol % Co exhibited excellent cycle performance as an anode material for Li-ion batteries. The solid-electrolyte interphase (SEI) formed in the frameworks through the electrochemical reaction mediates the active materials, such as MnO, Mn, and Li(2) O, during the conversion reaction in the charge-discharge cycle. The Co nanoparticles and SEI provide the electron and Li-ion conductive networks, respectively. The ternary nanocomposites of the MnO framework, metallic Co nanoparticles, and embedded SEI are categorized as durable anode materials for Li-ion batteries.
    Chemistry - An Asian Journal 02/2013; · 4.57 Impact Factor
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    ABSTRACT: We have studied the simultaneous synthesis and morphogenesis of polymer materials with hierarchical structures from nanoscopic to macroscopic scales. The morphologies of the original materials can be replicated to the polymer materials. In general, it is not easy to achieve the simultaneous synthesis and morphogenesis of polymer material even using host materials. In the present work, four biominerals and three biomimetic mesocrystal structures are used as the host materials or templates and polypyrrole, poly(3-hexylthiopehene), and silica were used as the precursors for the simultaneous syntheses and morphogenesis of polymer materials. The host materials with the hierarchical structure possess the nanospace for the incorporation of the monomers. After the incorporation of the monomers, the polymerization reaction proceeds in the nanospace with addition of the initiator agents. Then, the dissolution of the host materials leads to the formation and morphogenesis of the polymer materials. The scheme of the replication can be classified into the three types based on the structures of the host materials (types I-III). The type I template facilitates the hierarchical replication of the whole host material, type II mediates the hierarchical surface replication, and type III induces the formation of the two-dimensional nanosheets. Based on these results, the approach for the coupled synthesis and morphogenesis can be applied to a variety of combinations of the templates and polymer materials.
    Chemistry 01/2013; · 5.93 Impact Factor
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    ABSTRACT: Artificial liner peptides that bind to the c face of hydroxyapatite (HA) were obtained by using the mRNA display method. The specific attachment to the c face was characterized with the adsorption isotherm of the peptides. The crystal growth of HA was found to be modulated by the particular peptides. The results of this study suggest that the amino acid sequence including Ala-Asn-Thr (ANT) is essential for the binding specificity.
    RSC Advances 01/2013; 3(6):1885-1889. · 3.71 Impact Factor
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    ABSTRACT: Simultaneous synthesis and morphogenesis of polymer materials are achieved through the replication by using mesocrsytals in biological and biomimetic architectures as the host materials. Mesocrystalline structures have nanoscopic space within the interspace of the unit crystals. Introduction of monomers and subsequent polymerization in the nanospace affords the synthesis and morphogenesis of polymer materials. The polymer architectures are formed after dissolution of the original host mesocrystals. For more information see the Full Paper by Y. Oaki, H. Imai et al. on page 2284 ff.
    Chemistry 01/2013; 19(7). · 5.83 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.
    Journal of Asian Ceramic Societies. 01/2013; 1(2):143–148.