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Papers - Oaki, Yuya

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Amorphous flexible covalent organic networks containing redox-active moieties: A noncrystalline approach to the assembly of functional molecules

Suzuki J., Ishizone A., Sato K., Imai H., Tseng Y.J., Peng C.H., Oaki Y.

Chemical Science (Chemical Science) 11 ( 27 ) 7003 - 7008 2020.07

ISSN 20416520

The organization states of functional molecules have a significant impact on the properties of materials. A variety of approaches have been studied to obtain well-organized molecular assemblies. The present work shows a new non-organized state of isolated and dispersed functional molecules in amorphous flexible covalent organic networks. Redox-active quinone molecules are embedded in the amorphous network polymers. Consecutive reactions between benzoquinone (BQ) and linker molecules generate random network structures through polymerization at different rates and in multiple directions. The low-crystalline stackings of the amorphous network polymers facilitate the formation of nanoflakes through exfoliation in dispersion media. Enhanced electrochemical performances, one of the highest specific capacities in recent studies, were achieved by efficient redox reactions of the quinone moiety. The present noncrystalline approach, low-crystalline stacking of designer amorphous covalent organic networks, can be applied to construct similar nanostructured polymer materials containing functional units. This journal is

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Efficient Syntheses of 2D Materials from Soft Layered Composites Guided by Yield Prediction Model: Potential of Experiment-Oriented Materials Informatics

Kyohei Noda Yasuhiko Igarashi Hiroaki Imai Yuya Oaki

Advanced Theory and Simulations (Advanced Theory and Simulations) 3 ( 7 ) 2000084 2020.07

Research paper (scientific journal), Joint Work, Accepted

Exfoliation is a common route to synthesize 2D materials such as monolayered and few-layered nanosheets. However, it is not easy to control the breaking-down process. This work shows a new exfoliation strategy for generation of nanosheets from soft layered composites in high yield. Soft layered composites of inorganic hosts and organic guests are exfoliated into nanosheets in organic dispersion media. A simple yield prediction model is constructed on small-scale experimental data by sparse modeling, a data-scientific approach, combined with chemical perspective. The prediction model indicates the guest–medium combinations enabling high- and low-yield syntheses of the nanosheets. The high- and low-yield syntheses are actually achieved on different types of layered transition metal oxides in a minimum number of experiments. The number of experiments is reduced by 94%. The experimental optimization realizes further improvement of the yield, namely, shortening the exfoliation time. The present strategies can be applied to obtain a variety of 2D materials through exfoliation. Moreover, the present work suggests that combination of small experimental data, data-scientific approach, and chemical perspective, i.e., experiment-oriented materials informatics, has potentials for acceleration of materials design.

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Visualization and Quantification of Microwave using Thermo-Responsive Color-Change Hydrogel

Minami Nakamitsu, Hiroaki Imai, Yuya Oaki

ACS Sensors (ACS Sensors) 5 ( 1 ) 133 - 139 2020.06

Research paper (scientific journal), Joint Work, Accepted

Visualization and quantification of invisible lights, such as microwaves, are significant for their safe use. In general, a sensitizer material combined with a transistor is used as electronic devices for the measurement. Here, we developed a thermoresponsive color-change hydrogel of poly(N-isopropylacrylamide) (PNIPAAm) cross-linked by a layered organic composite based on polydiacetylene (PDA) for visualization and colorimetric quantification of microwaves. The layered PDA in the PNIPAAm hydrogel showed the temperature-dependent gradual color change with heating. Irradiation of microwaves induced the color change of PDA through heating of water in the hydrogel and subsequent volume shrinkage. The color of the gel was applied to visualize the temperature distribution with increasing irradiation time of microwaves. Moreover, the power of the irradiated microwave was quantified by time to the complete color change of the gel. The results indicate that the stimulus conversion process has potentials for development of a variety of imaging and quantification devices based on the layered PDA.

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Structured spinel oxide positive electrodes of magnesium rechargeable batteries: High rate performance and high cyclability by interconnected bimodal pores and vanadium oxide coating

Kanji Ishii, Shunsuke Doi, Ryuta Ise, Toshihiko Mandai, Yuya Oaki, Shunsuke Yagi, Hiroaki Imai

Journal of Alloys and Compounds (Journal of Alloys and Compounds) 816 152556 2020.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 09258388

A structured spinel oxide as a positive-electrode active material for magnesium rechargeable batteries was produced using a propylene-oxide-driven sol–gel method and subsequent vanadium oxide coating. Porous frameworks consisting of MgMn2O4 nanoparticles having diameters of ∼10 nm exhibited a large specific surface area of ∼150 m2 g−1 and interconnected bimodal pores distributed in the micrometer (1–10 μm) and nanometer (10–100 nm) regions. Hierarchical conduction paths constructed by mixing the porous frameworks with carbon nanoparticles reduced the overpotential for the insertion and extraction of magnesium ions and increased the redox capacity in an ionic-liquid electrolyte system. Moreover, the surface coating of vanadium oxide on the spinel oxide nanoparticles enhanced the extraction of magnesium ions and suppressed the decomposition of the electrolyte. The electrochemical properties, such as the capacities, rate capabilities, and cyclabilities, of the positive electrodes were largely improved by the structured spinel oxide frameworks.

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Biomimetic morphology–controlled anhydrous guanine via amorphous intermediate

Tsubasa Kimura, Mihiro Takasaki, Yuya Oaki, Hiroaki Imai

Crystal Growth and Design (Crystal Growth and Design) 20 ( 5 ) 3341 - 3346 2020.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 15287483

The micrometric morphology of anhydrous β-guanine crystals is adequately designed by various organisms for their specific optical properties. We successfully controlled the morphology of an anhydrous β-guanine crystal through the transformation of an amorphous intermediate consisting of guanine sodium salt under aqueous conditions. Biomimetic platy and columnar guanine crystals were selectively produced from the amorphous intermediate by changing the crystallization conditions.

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Formation processes, size changes, and properties of nanosheets derived from exfoliation of soft layered inorganic-organic composites

Mizuguchi R., Imai H., Oaki Y.

Nanoscale Advances (Nanoscale Advances) 2 ( 3 ) 1168 - 1176 2020.03

Exfoliation is a general route to obtain two-dimensional (2D) nanomaterials. A variety of methods have been developed for controlled exfoliation of layered materials based on stacking via van der Waals interaction, such as graphite and transition-metal dichalcogenides. On the other hand, rigid layered materials consisting of inorganic layers and interlayer metal ions stacked via electrostatic interaction, such as transition-metal oxides and clays, have a limited number of exfoliation methods. Here we studied a new exfoliation route through formation of soft layered composites. Intercalation of guest organic molecules changed rigid inorganic layered compounds into soft layered composites with stacking via van der Waals interaction. The soft layered inorganic-organic composites were exfoliated into surface-modified nanosheets in organic media. The layered composites showed swelling with dispersion in organic media. The time-course analyses suggest that the layered composites were simultaneously exfoliated in the vertical direction and fractured in the lateral direction. Thinner and smaller nanosheets were obtained with an increase in the exfoliation time. Although the resultant nanosheets gradually aggregated in the colloidal liquid, the original dispersion state was recovered with sonication for 5 min at room temperature. This exfoliation route using soft layered composites can be used in the synthesis and application of a variety of 2D nanomaterials.

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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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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

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

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

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

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

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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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

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

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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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

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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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

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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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

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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