Papers - Imai, Hiroaki

Solid-State Low-Temperature Thermoresponsive and Reversible Color Changes of Conjugated Polymer in Layered Structure: Beyond Infrared Thermography

Watanabe K., Imai H., Oaki Y.

Small (Small)  16 ( 41 )  2020.10

ISSN  16136810

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Emergence of thermoresponsive and reversible color changes at low temperature is a challenging target. In general, it is not easy to induce sufficient dynamic motion of rigid molecules including chromophore at a lower temperature. The present work shows unusually low-temperature color-change properties originating from the dynamic motion of rigid conjugated polymer in solid state. The layered composites of polydiacetylene (PDA) and guest l-arginine (L-Arg) (PDA-(L-Arg)) exhibit temperature-responsive gradual color changes with reversibility in the range of 123–333 K in solid crystalline state. The dynamic properties are induced by gradual and reversible distortion of the π-conjugated main chain in response to temperature. The tuned flexibility of the layered structure facilitates motion of the rigid π-conjugated molecule at low temperature. The PDA-(L-Arg)-coated substrates are applied to visualization and quantification of 2D and 3D temperature distributions generated by cooling with liquid nitrogen. These thermographic devices afford to image lower temperature range than typical infrared thermography. The present work indicates potentials of layered architectures with tunable flexibility for emergence of dynamic properties.

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Spinel-Type MgMn<inf>2</inf>O<inf>4</inf>Nanoplates with Vanadate Coating for a Positive Electrode of Magnesium Rechargeable Batteries

Doi S., Ise R., Mandai T., Oaki Y., Yagi S., Imai H.

Langmuir (Langmuir)  36 ( 29 ) 8537 - 8542 2020.07

ISSN  07437463

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Spinel-type MgMn2O4 nanoplates ∼10 nm thick were prepared as a positive electrode for magnesium rechargeable batteries by the transformation of metal hydroxide nanoplates. Homogeneous coating with a vanadate layer thinner than 3 nm was achieved on the spinel oxide nanoplates via coverage of the precursor and subsequent mild calcination. We found that the spinel oxide nanoplates with the homogeneous coating exhibit improved electrochemical properties, such as discharge potential, capacity, and cyclability, due to the enhanced insertion and extraction of magnesium ions and suppressed decomposition of electrolytes. The nanometric platy morphology of the spinel oxide and the vanadate coating act synergistically for the improvement of the electrochemical performance.

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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

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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

Noda K., Igarashi Y., Imai H., Oaki Y.

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

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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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Biomimetic Morphology-Controlled Anhydrous Guanine via an Amorphous Intermediate

Kimura T., Takasaki M., Oaki Y., Imai H.

Crystal Growth and Design (Crystal Growth and Design)  20 ( 5 ) 3341 - 3346 2020.05

ISSN  15287483

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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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Enhancement of coercivity of self-assembled stacking of ferrimagnetic and antiferromagnetic nanocubes

Sawano K., Tsukiyama K., Shimizu M., Takasaki M., Oaki Y., Yamamoto T., Einaga Y., Jenewein C., Cölfen H., Kaiju H., Sato T., Imai H.

Nanoscale (Nanoscale)  12 ( 14 ) 7792 - 7796 2020.04

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

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

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

ISSN  09258388

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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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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

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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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Visualization and Quantification of Microwaves Using Thermoresponsive Color-Change Hydrogel

Nakamitsu M., Imai H., Oaki Y.

ACS Sensors (ACS Sensors)  5 ( 1 ) 133 - 139 2020.01

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

Watanabe K., Imai H., Oaki Y.

Journal of Materials Chemistry C (Journal of Materials Chemistry C)  8 ( 4 ) 1265 - 1272 2020

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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Thermally induced fragmentation of nanoscale calcite

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

RSC Advances (RSC Advances)  10 ( 10 ) 6088 - 6091 2020

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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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Amorphous 2D materials containing a conjugated-polymer network

Yano S., Sato K., Suzuki J., Imai H., Oaki Y.

Communications Chemistry (Communications Chemistry)  2 ( 1 )  2019.12

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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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Carbon nitride and titania nanoparticles prepared using porous silica templates and photocatalytic activity

Somekawa S., Watanabe H., Ono Y., Tachibana N., Imai H.

Materials Letters (Materials Letters)  256 2019.12

ISSN  0167577X

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This work examined the band structure and photocatalytic activity of carbon nitride and titania nanoparticles prepared using porous silica templates with pore sizes of approximately 1 and 3 nm. The carbon nitride and titania specimens prepared in this manner exhibited absorption edge blue shifts compared to bulk samples, due to expansion of their band gap resulting from quantum size effects. The photocatalytic activity of these materials during hydrogen production from water was assessed, and the activity of carbon nitride prepared in the 1 nm pores was higher than that of the material made using 3 nm pores as well as that of the bulk carbon nitride. These results suggest that the expansion of the HOMO-LUMO energy gap contributed to increasing the catalytic performance. In contrast, the titania materials did not generate hydrogen. X-ray absorption fine structure analyses demonstrated that the titania in the silica templates had a distorted structure, which may have decreased its photocatalytic activity.

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

Takeuchi M., Kawashima H., Imai H., Fujii S., Oaki Y.

Journal of Materials Chemistry C (Journal of Materials Chemistry C)  7 ( 14 ) 4089 - 4095 2019.04

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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Materials-Informatics-Assisted High-Yield Synthesis of 2D Nanomaterials through Exfoliation

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

Advanced Theory and Simulations (Advanced Theory and Simulations)  2 ( 4 )  2019.04

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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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Enhanced oxide-ion conductivity of solid-state electrolyte mesocrystals

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

Nanoscale (Nanoscale)  11 ( 10 ) 4523 - 4530 2019.03

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

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

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

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Copyright © 2019 American Chemical Society. 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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Highly porous polymer dendrites of pyrrole derivatives synthesized through rapid oxidative polymerization

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

Polymer Journal (Polymer Journal)  51 ( 1 ) 11 - 18 2019.01

ISSN  00323896

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© 2018, The Society of Polymer Science, Japan. 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 FeCl 3 in CHCl 3 as a cosolvent for the monomer and the oxidant leads to nanometer-scale branching morphologies that have a specific surface area of ~200 m 2 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 m 2 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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