Papers - Oaki, Yuya

Redox-Mediated High-Yield Exfoliation of Layered Composites into Nanosheets

Y. Yamamoto, H. Imai, Y. Oaki

Bulletin of the Chemical Society of Japan (The Bulletin of the Chemical Society of Japan)  92 ( 4 ) 779 - 784 2019.01

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00092673

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A variety of organic and inorganic layered compounds are exfoliated into 2D nanomaterials in liquid phase. Improvement of exfoliation yield is a significant challenge for further applications of 2D nanomaterials. However, yield of transition-metal-oxide nanosheets is not so high, such as around 5% for 240 h and 15% for 120 h, in previous exfoliation methods. Here we show a new exfoliation approach to high-yield synthesis of nanosheets. A layered composite of cobalt hydroxide and guest quinone derivative was exfoliated into nanosheets through redox reactions of the interlayer guests in the yield 1657% for 1 h under mild conditions. The redox reactions of the guest molecules on the layer facilitate efficient exfoliation through improvement of the affinity between the guest and dispersion medium. The present redox-mediated exfoliation approach can be applied to efficient synthesis of a variety of 2D nanomaterials from layered composites.

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Supermicroporous silica nanograins: Synthesis and application

Riku Kitamura, Hiroto Watanabe, Yuya Oaki, Hiroaki Imai

Langmuir (Langmuir)  35   5594 - 5598 2019.01

Research paper (scientific journal), Joint Work, Accepted,  ISSN  07437463

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Supermicroporous silicas having micropores 1-2 nm in diameter were miniaturized to 30-40 nm in diameter using a combination of binary organic agents: cationic surfactant as a porogen and polyethylene glycol as a growth suppressor in a concentrated system. The miniaturized nanograins of the porous silica were effective as a porous and transparent matrix for WO 3 quantum dots that exhibit enhanced photocatalytic activity.

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Pyrolytic production of fluorescent pyrone derivatives produced in the confined space of super-microporous silicas

Kosei Hayashi, Shumpei Ogawa, Hiroto Watanabe, Yasuto Fujimaki, Yuya Oaki, Hiroaki Imai

The Bulletin of the Chemical Society of Japan (Bulletin of the Chemical Society of Japan)  92 ( 7 ) 1170 - 1174 2019.01

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00092673

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Fluorescent pyrone derivatives were extracted from the pyrolitic product of citric acid confined in supermicroporous silicas (SMPSs). We utilized the tiny spaces of SMPSs to produce fluorescent molecules as small fragments of carbogenic dots. We characterized their optical properties and investigated their structual information. The obtained molecule was determined as a pyrone derivative. The isolated product showed similar optical properties to those of reported carbogenic dots. Therefore, we suggest that the pyrone structure is the key fragment structure of carbogenic dots.

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Enhanced quantum yield of nanographenes incorporated in supermicroporous silicas and the co-adsorption effect of water molecule

H. Watanabe, Y. Fujimaki, K. Hayashi, H. Imai

The Bulletin of the Chemical Society of Japan  2019

Research paper (scientific journal), Joint Work, Accepted

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Highly dispersive mono-sized nanoparticles of Y <inf>2</inf> O <inf>3</inf> -stabilized ZrO <inf>2</inf>

Tsukiyama K., Takasaki M., Oaki Y., Imai H.

Chemistry Letters (Chemistry Letters)  48 ( 4 ) 390 - 393 2019

ISSN  03667022

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Uniformly sized nanoparticles 35nm in diameter of Y 2 O 3 - stabilized ZrO 2 (YSZ) were fabricated by a water-toluene twophase solvothermal method. The nanoparticles that were covered with oleic acid were highly dispersed in organic media. The size and doping concentration were tuned by adjusting the preparation conditions. The fabrication of YSZ thin films was achieved through hexagonal arrangement by evaporation-induced selfassembly of highly dispersive mono-sized nanoparticles.

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Biomimetic Macroscopic Mesocrystalline Films Produced by Oriented Assembly of Nanorods under Magnetic Field

M. Takasaki, T. S. Suzuki, Y. Oaki, H. Imai

Nanoscale (Nanoscale)  10 ( 47 ) 22161 - 22165 2018.11

Research paper (scientific journal), Joint Work, Accepted,  ISSN  20403364

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Millimeter-scale mesocrystals and cross-lamellae mimicking the nanostructure of seashells were produced on a substrate through the three-dimensionally oriented assembly of c-axis-elongated calcite nanorods ∼50 nm wide and ∼500-1000 nm long by combining arrangement with evaporation-driven capillary force and alignment under an intense magnetic field.

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Tunable Stimuli-Responsive Color-Change Properties of Layered Organic Composite

M. Takeuchi, K. Gnanasekaran, H. Friedrich, H. Imai, N. A. J. M. Sommerdijk, Y. Oaki

Advanced Functional Materials (Advanced Functional Materials)  28 ( 45 ) 1804906 2018.09

Research paper (scientific journal), Joint Work, Accepted,  ISSN  1616301X

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Stimuli-responsive properties of soft materials originate from dynamic structure changes. Layered polydiacetylene (PDA) shows color-change properties with the application of external stimuli, such as thermal and mechanical stresses. Layered organic materials possessing intercalation capabilities and dynamic properties have potentials for tuning their structures and properties by the intercalated guests. A variety of sensing and imaging devices can be developed by control of the stimuli responsivity. Here, the key structures, processes, and mechanisms for tuning the stimuli-responsive color-change properties of layered PDA are studied by in situ analyses with heating and cooling. The in situ analyses indicate that heating initiates thermal motion of the alkyl side chains of the PDA main chain in the host layers around 60 °C, regardless of the type of intercalated guests. Further heating induces torsion of the PDA main chain leading to color changes at different temperatures, depending on the types of the intercalated guest. Then, the layered structure is irreversibly deformed with lowering the crystallinity. The results indicate that the stimuli-responsive color-change properties are controlled by the rigidity of the layered structures consisting of the host layers and guests. The control strategy based on rigidity tuning can be applied to a variety of soft materials with stimuli responsivity.

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Enhanced detection of methylmercaptan gas using proton containing layered manganese oxide nanosheets coated on quartz crystal microbalance

Kawamura, N; Tokura, Y; Nakada, G; Oaki, Y; Imai, H; Shiratori, S

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 256 2018.08

ISSN  0065-7727

Manganese oxide nanosheets on quartz crystal microbalance for detection of methylmercaptan gas

Tokura, Y; Nakada, G; Oaki, Y; Imai, H; Shiratori, S

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 256 2018.08

ISSN  0065-7727

Highly Porous Polymer Dendrites of Pyrrole Derivatives Synthesized through Rapid Oxidative Polymerization

K. Ishii, K. Sato, Y. Oaki, H. Imai

Polymer Journal (Polymer Journal)  51 ( 1 ) 11 - 18 2018.08

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00323896

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Highly porous organic structures are synthesized by the oxidative polymerization of pyrrole derivatives in a concentrated monomer and oxidant solution. The rapid polymerization of pyrrole by oxidation with FeCl3 in CHCl3 as a cosolvent for the monomer and the oxidant leads to nanometer-scale branching morphologies that have a specific surface area of ~200 m2 g−1. The polymerization of pyrrole derivatives, such as 1-methylpyrrole and 1-ethylpyrrole, produced intragranular micropores of ~1 nm in diameter in the polymer grains of nanoscale dendrites. We obtained highly porous polymers of polypyrrole derivatives that have high specific surface areas (~900 m2 g−1) with bimodal pore-size distribution. The enhanced adsorbability of porous polymer dendrites for aromatic molecules was confirmed, in comparison with that of conventional polystyrene adsorbents.

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Layer-by-Layer Manipulation of Heterogeneous Rectangular Nanoblocks: Brick Work for Multilayered Structures with Specific Heterojunction

R. Matsumoto, M. Takasaki, K. Tsukiyama, Y. Oaki, H. Imai

Inorganic Chemistry (Inorganic Chemistry)  57 ( 18 ) 11655 - 11661 2018.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00201669

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The combination of heterogeneous inorganic nanocrystals provides novel nanometer-scale architectures for exploration of novel functions. In the present study, heterogeneous bilayers, such as BaTiO3-Fe3O4 and Mn3O4-CeO2, are fabricated via layer-by-layer stacking of several rectangular nanoblocks 5-23 nm in size by evaporation-driven self-assembly. Specific heterojunctions between inorganic crystals are constructed by nanoscale simple brick work through spontaneous adjustment of the crystallographic orientations of the nanoblocks in the lower and upper layers.

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Two-dimensional Conductive and Redox-Active Nanostructures Synthesized by Crystal-Controlled Polymerization for Electrochemical Applications

K. Sato, H. Imai, Y. Oaki

ACS Applied Nano Materials (ACS Applied Nano Materials)  1 ( 8 ) 4218 - 4226 2018.07

Research paper (scientific journal), Joint Work, Accepted

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Two-dimensional nanomaterials have attracted much interest for their anisotropic structures and emergent properties. Whereas a variety of inorganic nanosheets are prepared by exfoliation, design and synthetic strategies of organic nanosheets are still developing. Here we report on crystal-controlled synthesis of conductive and redox-active organic polymer nanosheets with tunable thickness and lateral size. Although the nanosheets consist of classical conductive polymers, such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), a new crystal-controlled polymerization approach provides 2D nanomaterials with enhanced and characteristic properties. Oxidative polymerization proceeds on the crystal surface of organic oxidants, such as quinone derivatives, with diffusion of the monomer vapor at low temperature under ambient pressure. The present method affords the control of lateral size and thickness of the nanosheets. The resultant nanosheets show the characteristic and enhanced properties originating from a 2D nature, such as the high structure anisotropy with aspect ratio around 104, enhanced conductivity 287 S cm-1, and specific charge-discharge capacitance 523 F g-1 at the current density of 18 A g-1. The organic nanosheets with conductivity and redox activity can be used as a building block for development of functional materials and devices. ©

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Multistage redox reactions of conductive-polymer nanostructures with lithium ions: potential for high-performance organic anodes

Numazawa H., Sato K., Imai H., Oaki Y.

NPG Asia Materials (NPG Asia Materials)  10 ( 5 ) 397 - 405 2018.05

ISSN  18844049

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Conducive polymers have a wide range of applications originating from their π-conjugated systems. The redox reactions of conductive polymers with doping and dedoping of anions have been applied to cathodes for charge storage. In contrast, the redox reactions with cations have not been fully studied in anodes for charge storage. Here, we found that the nanostructures of conductive polymers, such as polypyrrole (PPy) and polythiophene (PTp) derivatives, have reversible redox reactions with cations. The PPy and PTp nanostructures acted as anodes with specific capacities of 157 and 44.4 mA h g–1, respectively, at a current density of 20 mA g–1. The introduction of a carboxy group to the pyrrole and thiophene rings enhanced the specific capacities up to 730 and 963 mAh g–1, respectively. The enhanced electrochemical properties were not observed in the bulk-size conductive polymers. The results suggest that conductive-polymer nanostructures have potential for developing metal-free, high-performance charge storage devices.

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Layer-by-Layer Manipulation of Anisotropic Nanoblocks: Orientation-Switched Superlattices through Orthogonal Stacking of a and c Directions

Y. Nakagawa, R. Matsumoto, H. Kageyama, Y. Oaki, H. Imai

Nanoscale (Nanoscale)  10 ( 27 ) 12957 - 12962 2018.05

Research paper (scientific journal), Joint Work, Accepted,  ISSN  20403364

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Nanometer-scale switching of the crystallographic orientation enables production of novel superlattices for exploration of new functions. Elaborate multilayered architectures of Mn3O4 nanocuboids were constructed by a convective self-assembly process. The nanometric cuboids were accumulated into orientation-switched superlattices on micrometric scales by controlling the a and c directions of the tetragonal crystal periodically. The orthogonal stacking of the tetragonal nanoblocks was achieved through the layer-by-layer manipulation process of two types of 2D arrays with different orientations.

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Nanoscale Mosaic Works: Tetragonal Lattices of Iso-Oriented Heterogeneous Nanocubes

R. Matsumoto, H. Yamazaki, M. Takasaki,Y. Oaki, T. Sato, H. Imai

Langmuir (Langmuir)  34 ( 13 ) 4031 - 4035 2018.04

Research paper (scientific journal), Joint Work, Accepted,  ISSN  07437463

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Tetragonal 2D lattices are spontaneously formed by the self-assembly of homogeneous nanocubes. However, ordered arrays consisting of differently sized rectangular nanoblocks have not been achieved because the regulation of the assembly is weakened by the combination of binary units. In the present work, ordered arrays comprising binary nanocubes were investigated using the combination of 10 nm Pt nanocubes and 20 nm BaTiO3 nanocubes. Heterogeneous but ordered 2D tetragonal lattices were successfully produced using differently sized rectangular nanoblocks. The highly ordered self-assembly in the heterogeneous system requires the matching of the size ratio of binary nanocubes with the buffer effect of oleic acid covering the building units.

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Enhanced electrochemical properties of MgCo<inf>2</inf>O<inf>4</inf> mesocrystals as a positive electrode active material for Mg batteries

Kotani Y., Ise R., Ishii K., Mandai T., Oaki Y., Yagi S., Imai H.

Journal of Alloys and Compounds (Journal of Alloys and Compounds)  739   793 - 798 2018.03

ISSN  09258388

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Mesocrystals of MgCo2O4 spinel oxide exhibiting porous and single-crystalline structures were synthesized by the reaction of magnesium ions with calcite-type CoCO3 mesocrystal precursors grown in micrometer-sized agar gels. The mesocrystals consisted of single-crystalline porous grains measuring 200–300 nm in diameter with iso-oriented subunits of approximately 10 nm. The single-crystalline porous framework structure of the MgCo2O4 mesocrystals enhanced the discharge capacity and positively shifted the discharge potential compared with MgCo2O4 nanoparticles prepared by the inverse co-precipitation method. These enhancements were attributed to a combination of the high specific surface areas of the mesocrystals and the high connectivity of the magnesium ion diffusion paths in the solid-phase.

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Enhanced Electrochemical Properties of MgCo2O4 Mesocrystals as a Positive Electrode Active Material for Mg Batteries

Y. Kotani, R. Ise, K. Ishii, T. Mandai, Y. Oaki, S. Yagi, H. Imai

Journal of Alloys and Compounds 739   793 - 798 2018.03

Research paper (scientific journal), Joint Work, Accepted

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Artificial Mineral Films Similar to Biogenic Calcareous Shells: Oriented Calcite Nanorods on a Self-Standing Polymer Sheet

Y. Nagai, Y. Oaki, H. Imai

CrystEngComm (CrystEngComm)  20 ( 12 ) 1656 - 1661 2018.01

Research paper (scientific journal), Joint Work, Accepted

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Micrometre-thick calcareous shells consisting of c-axis-oriented calcite nanorods are produced on an organic sheet as foraminiferal tests and isopod cornea cuticles. Here, we produced mineral films similar to biogenic calcareous shells on a self-standing polymer sheet in a supersaturated solution system by multistep crystal growth with the adjustment of the concentration ratio of poly(acrylic acid) (PAA) as a soluble controlling agent. In the initial stage, selective high-density nucleation was achieved at a relatively low PAA to calcium ion ratio (R = 1.0-1.2) to form of a calcite seed layer on the surface of conventional polyvinyl alcohol. Calcite nanorods elongated in the c direction were then grown perpendicularly to the seed layer in the progressive stages at a relatively high PAA ratio (R = 1.5-2.0). The artificial mineral films consisting of c-axis-oriented nanorods are suggested to exhibit mechanical properties similar to those of biogenic calcareous shells.

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Crystal-controlled polymerization: recent advances in morphology design and control of organic polymer materials

Oaki Y., Sato K.

Journal of Materials Chemistry A (Journal of Materials Chemistry A)  6 ( 46 ) 23197 - 23219 2018

ISSN  20507488

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Morphologies are important in chemistry and materials science. A wide variety of functional molecules and their assemblies are synthesized on the basis of molecular design. These morphologies, from molecular to nanometer scales, have been summarized in previous nice reviews. Here we focus on recent advances in morphology design and control of organic polymer materials from nanoscopic to macroscopic scales, which are larger than the molecular scale, for which general strategies are now being developed. This review summarizes crystal-controlled polymerization from a new perspective for the morphology design and control of organic polymer materials. Characteristic structures, surfaces, and morphologies of solid crystals are employed for morphology control of organic polymer materials. Morphology control contributes to the emergence and/or improvement of the properties.

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Evolution of Calcite Nanocrystals through Oriented Attachment and Fragmentation: Multi-Step Pathway Involving Bottom-Up and Break-Down Stages

M. Takasaki, Y. Kezuka, M. Tajika, Y. Oaki, H. Imai

ACS Omega 2 ( 12 ) 8997 - 9001 2017.12

Research paper (scientific journal), Joint Work, Accepted,  ISSN  2470-1343

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