Papers - Oaki, Yuya

Thermally Induced Fragmentation of Nanoscale Calcite

Mihiro Takasaki, Makoto Tago, Yuya Oaki, Hiroaki Imai

RSC Advances (RSC Advances)  10 ( 10 ) 6088 - 6091 2020.02

Research paper (scientific journal), Joint Work, Accepted

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Calcite nanorods ∼50 nm wide are thermally separated into nanoblocks. The fragmentation is ascribed to the ion diffusion on metastable crystal surfaces at temperatures (∼400 °C) much lower than the melting point. The presence of water molecules enhances the surface diffusion and induces deformation of the nanorods even at ∼60 °C.

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Enhanced coercivity on self-assembled stacking of ferrimagnetic and antiferromagnetic nanocubes

Keisuke Sawano , Keishi Tsukiyama, Makoto Shimizu, Mihiro Takasaki, Yuya Oaki, Takashi Yamamoto, Yasuaki Einaga, Christian Jenewein, Helmut Cölfen, Hideo Kaiju, Tetsuya Sato, Hiroaki Imai

Nanoscale (Nanoscale)  12 ( 14 ) 7792 - 7796 2020.02

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

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The coercivity of magnetic nanoparticles is enhanced by the exchange coupling effect at the interface of ferrimagnetic and antiferromagnetic self-assembled monolayers. Antiferromagnetic Co3O4 nanocubes were regularly stacked on an ordered monolayer of ferrimagnetic Fe3O4 nanocubes by layer-by-layer manipulation using evaporation-driven self-assembly. The ordered arrangements of the ferrimagnetic and antiferromagnetic nanocubes are effective for the enhancement of the ferromagnetic character.

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Guanine crystals regulated by chitin-based honeycomb frameworks for tunable structural colors of sapphirinid copepod, Sapphirina nigromaculata

Tsubasa Kimura, Mihiro Takasaki, Ryosuke Hatai, Yukiko Nagai, Katsuyuki Uematsu, Yuya Oaki, Minoru Osada, Hiroyuki Tsuda, Takaaki Ishigure, Takashi Toyofuku, Shinji Shimode, Hiroaki Imai

Scientific Reports (Scientific Reports)  10 ( 1 ) 2266 2020.02

Research paper (scientific journal), Joint Work, Accepted

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Sapphirinid copepods, which are marine zooplankton, exhibit tunable structural colors originating from a layered structure of guanine crystal plates. In the present study, the coloring portion of adult male of a sapphirinid copepod, Sapphirina nigromaculata, under the dorsal body surface was characterized to clarify the regulation and actuation mechanism of the layered guanine crystals for spectral control. The coloring portions are separated into small domains 70–100 µm wide consisting of an ordered array of stacked hexagonal plates ~1.5 µm wide and ~80 nm thick. We found the presence of chitin-based honeycomb frameworks that are composed of flat compartments regulating the guanine crystal plates. The structural color is deduced to be tuned from blue to achromatic via yellow and purple by changing the interplate distance according to vital observation and optical simulation using a photonic array model. The framework structures are essential for the organization and actuation of the particular photonic arrays for the exhibition of the tunable structural color.

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Amorphous 2D Materials Containing Conjugated-Polymer Network

Shoichiro Yano, Kosuke Sato, Jumpei Suzuki, Hiroaki Imai, Yuya Oaki

Communications Chemistry (Communications Chemistry)  2 ( 1 ) 97 2019.12

Research paper (scientific journal), Joint Work, Accepted,  ISSN  2399-3669

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Two-dimensional materials, such as layered compounds and nanosheets, have attracted interest for their characteristic structures and properties. If layered materials containing functional organic molecules are synthesized, designed nanosheets can be obtained by exfoliation. Here we show the design and synthesis of an amorphous organic layered material containing a conjugated-polymer network, its exfoliation into nanosheets, and their applications. Copolymerization of benzoquinone and pyrrole generates random stacks of the conjugated polymer layers through successive C-C bond formation and pericyclic reaction under mild conditions at 60 °C. The amorphous organic layered materials are efficiently exfoliated into nanosheets in 44.6% after 1 h. The nanosheets are used as a metal-free electrocatalyst for hydrogen evolution reaction with the overpotential 0.28 V (vs. RHE). The present approach may be applied to the design of functional nanosheets with graphene-like structures under mild conditions.

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A paper-based device of a specially designed soft layered polymer composite for measurement of weak friction force

Kei Watanabe, Hiroaki Imai, Yuya Oaki

Journal of Materials Chemistry C (Journal of Materials Chemistry C)  8 ( 4 ) 1265 - 1272 2019.11

Research paper (scientific journal), Joint Work, Accepted,  ISSN  20507534

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Measurement of friction force is important for safety, security, and health. Excessive friction force causes degradation of materials and biological bodies. For example, tooth enamel and gums are damaged by strong toothbrushing force. However, friction force is not easily measured by conventional materials and devices. Here, we proposed design strategies of a paper-based device for measurement of friction force. Toothbrushing force, a model of weak friction force, is the measurement target in the present work. Soft layered composites of polydiacetylene (PDA) and interlayer guest macromolecules were designed and synthesized to achieve a visible color change in response to weak friction force. A data-scientific approach assisted in the selection of the interlayer guest molecule to control the stimuli responsivity of the layered PDA. The guest polyethyleneimine (PEI) formed soft layered composites exhibiting a color change in response to weak friction force. The paper-based device of the layered PDA/PEI composite showed a gradual color change from blue to red in response to the strength and number of the applied friction force. As a model case, toothbrushing force was measured using the paper-based device. The present work shows new design strategies of molecules and materials toward measurement of a wide variety of friction forces.

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Experiment-Oriented Materials Informatics for Efficient Exploration of Design Strategy and New Compounds for High-Performance Organic Anode

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

Advanced Theory and Simulations (Advanced Theory and Simulations)  2 ( 10 )  2019.10

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High-performance organic energy storage has attracted much interest as a future battery. Organic anode has been developed as an alternate of graphite in the past decade. However, the design strategies are not fully studied for further development. The present work shows experiment-oriented materials informatics (MI) for efficient exploration of design strategy and new compounds for an active material of high-performance organic anode. A few important factors to achieve high specific capacity are extracted from training dataset containing experimentally measured specific capacity, calculation results, and literature data of the model compounds using sparse modeling, an informatics approach. Although the prediction model is not sufficiently accurate, the model assists in exploration of new compounds in combination with experience and intuition of experimental scientists. New compounds with high specific capacity, such as 227 mA h g–1 at 100 mA g–1 for benzo[1,2-b:4,5-b′]dithiophene (BdiTp), are efficiently discovered in a minimum number of experiments. Furthermore, polymerization of BdiTp exhibits the enhanced performances, such as 933 mA h g–1 at 20 mA g–1 and cycle stability, and rate performance. MI combined with experiment, calculation, and data accelerates design new materials and functions by experimental scientists having their small data, experience, and intuition.

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Glass-transition-induced color-changing resins containing layered polydiacetylene

Shuhei Ishioka, Kei Watanabe, Hiroaki Imai, Yu-Jen Tseng, Chi-How Peng, Yuya Oaki

Chemical Communications (Chemical Communications)  55 ( 78 ) 11723 - 11726 2019.09

Research paper (scientific journal), Joint Work, Accepted,  ISSN  13597345

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A phase-segregated composite of polystyrene (PSt) and layered polydiacetylene (PDA) was formed through simultaneous polymerization and crystallization. As the motion of PSt chains with glass transition is transferred to that of PDA, the color change was achieved by the shortening of the conjugation length with deformation of the layered structure.

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Evolution of Co<inf>3</inf>O<inf>4</inf> Nanocubes through Stepwise Oriented Attachment

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

Langmuir (Langmuir)  35 ( 24 ) 8025 - 8030 2019.06

ISSN  07437463

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Uniformly sized building units are generally required to construct highly elaborate architectures over a wide range. Defined nanocubes of Co3O4 evolved from deformed precursor nanograins 2-5 nm in diameter through direct oriented attachment in a nonpolar medium. Uniformly sized primary nanocubes -8 nm on a side with {100} faces were formed by adjusting the coverage of the oxide nanograins with oleic acid. Larger nanocubes 20-40 nm on a side were produced with further direct oriented attachment of the primary nanocubes. Ordered arrays, such as superlattices, were found to be constructed by the indirect oriented attachment of the primary and larger nanocubes covered with organic molecules.

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Evolution of Co3O4 Nanocubes through Stepwise Oriented Attachment

Keishi Tsukiyama, Mihiro Takasaki, Yuya Oaki, Hiroaki Imai

Langmuir 35   8025 - 8030 2019.05

Research paper (scientific journal), Joint Work, Accepted

Highly Dispersive Mono-Sized Nanoparticles of Y2O3-Stabilized ZrO2

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

Chemistry Letters 48   390 - 393 2019.03

Research paper (scientific journal), Joint Work, Accepted

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Quantitative detection of near-infrared (NIR) light using organic layered composites

Machi Takeuch, Hisato Kawashima, Hiroaki Imai, Syuji Fujii, Yuya Oaki

Journal of Materials Chemistry C (Journal of Materials Chemistry C)  7 ( 14 ) 4089 - 4095 2019.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  20507534

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Quantitative detection of external stimuli, such as light, heat, and force, is an important challenge for sensing materials. Near-infrared (NIR) light is used in a variety of fields, such as biological, physical, medical, and engineering applications. Visualization and quantitative detection of invisible NIR are significant for its safe use. Here we fabricate a paper-based device coated with polypyrrole (PPy) and layered polydiacetylene (PDA). As PPy induces evolution of heat with irradiation of NIR, the effective conjugation length of the layered PDA is changed by the dynamic structure changed through the heat conduction. The visible color change from blue to red proceeds on the paper device. The red-color intensity has a relationship with the irradiation power of NIR. These facts suggest that the irradiation time and power of NIR are quantified by the color. Moreover, the results indicate that layered PDA can be applied to the quantification of external stimuli through conversion from the desired stimulus to the detectable one.

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Enhanced Oxide-Ion Conductivity of Solid-State Electrolyte Mesocrystal

K. Tsukiyama, M. Takasaki, N. Kitamura, Y. Idemoto, Y. Oaki, M. Osada, H. Imai

Nanoscale (Nanoscale)  11 ( 10 ) 4523 - 4530 2019.02

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

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Oxide ion-conducting porous films were produced on a substrate by evaporation-induced self-assembly of rare earth-doped CeO 2 (REDC) nanocubes 4-5 nm in size and subsequent mild calcination at 400 °C. Mesocrystalline structures comprising iso-oriented REDC nanocubes were formed by ordered assembly based on strict attachment of their {100} faces. Enhanced oxide-ion conductivities in a temperature range of 250-350 °C were observed on the mesocrystalline films consisting of Sm-doped CeO 2 (SDC) nanocubes. The specific electrical properties of the mesocrystalline films are ascribed to improved surface-ion conduction due to a large specific surface area and a high crystallographic connectivity of SDC nanocubes.

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Bending Fibers of Hydroxyapatite for Ordered Parallel Architecture in Bovine Tooth Enamel

Y. Yukimasa, M. Takasaki, Y. Oaki, H. Imai

ACS Omega (ACS Omega)  4 ( 2 ) 3739 - 3744 2019.02

Research paper (scientific journal), Joint Work, Accepted

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The nanometer-scale bending structure in the enamel-dentin interfacial region of bovine teeth was clarified using detailed electron micrographs. We found that enamel prisms of width 5-10 μm in the enamel are regarded as a bundle of nanometer-scale hydroxyapatite (HA) fibers of width 20-30 nm that are connected to the enamel-dentin interface. The radial arrangements of HA fibers elongated in the c-axis were formed on the dentin surface at the boundary. The orientation adjustment of the HA fibers was observed through stepwise bending with a gradual change of the c direction in the ordered parallel array of the enamel prisms. Our observation suggests that the crystallographic ordering in the enamel layer originates from the growth-induced orientation of HA crystals in the interfacial region.

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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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Materials-informatics-assisted high-yield synthesis of 2D nanomaterials through exfoliation

G. Nakada, Y. Igarashi, H. Imai, Y. Oaki

Advance Theory and Simulations (Advanced Theory and Simulations)  2 ( 4 ) 1800180 2019.01

Research paper (scientific journal), Joint Work, Accepted

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A variety of inorganic and organic nanosheets with characteristic structures and properties can be synthesized through exfoliation of layered materials. However, in general, immense time and efforts are required for exploration of exfoliation conditions and characterization of nanosheets. In addition, it is challenging to improve the yield of nanosheets obtained through exfoliation. Here a materials-informatics-assisted high-yield synthesis of nanosheets is proposed, which does not require experience and intuition. Layered composites containing inorganic layers and interlayer organic guests are delaminated into nanosheets in a variety of dispersion media. First, an experimental screening is performed to find efficient exfoliation conditions and obtain a training dataset for the informatics approach. Sparse modeling is then used facilitating the extraction of important factors predicting the yield of nanosheets. High-yield (up to ≈50%) synthesis of nanosheets is demonstrated in unknown systems in a minimum number of experiments. The yield is higher than those typically reported for layered materials. It is expected that the effective combination has potentials for not only discovery of compounds but also structure control of materials.

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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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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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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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