論文 - 今井　宏明

Ultraporous, Ultrasmall MgMn<inf>2</inf>O<inf>4</inf> Spinel Cathode for a Room-Temperature Magnesium Rechargeable Battery

Kobayashi H., Fukumi Y., Watanabe H., Iimura R., Nishimura N., Mandai T., Tominaga Y., Nakayama M., Ichitsubo T., Honma I., Imai H.

ACS Nano （ACS Nano）  2022年

ISSN  19360851

　概要を見る

Magnesium rechargeable batteries (MRBs) promise to be the next post lithium-ion batteries that can help meet the increasing demand for high-energy, cost-effective, high-safety energy storage devices. Early prototype MRBs that use molybdenum-sulfide cathodes have low terminal voltages, requiring the development of oxide-based cathodes capable of overcoming the sulfide’s low Mg2+ conductivity. Here, we fabricate an ultraporous (>500 m2 g-1) and ultrasmall (<2.5 nm) cubic spinel MgMn2O4 (MMO) by a freeze-dry assisted room-temperature alcohol reduction process. While the as-fabricated MMO exhibits a discharge capacity of 160 mAh g-1, the removal of its surface hydroxy groups by heat-treatment activates it without structural change, improving its discharge capacity to 270 mAh g-1─the theoretical capacity at room temperature. These results are made possible by the ultraporous, ultrasmall particles that stabilize the metastable cubic spinel phase, promoting both the Mg2+ insertion/deintercalation in the MMO and the reversible transformation between the cubic spinel and cubic rock-salt phases.

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Emergence of practical fluorescence in a confined space of nanoporous silica: Significantly enhanced quantum yields of a conjugated molecule

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

Chemical Communications （Chemical Communications）  57 （ 97 ） 13150 - 13153 2021年12月

ISSN  13597345

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The fluorescence of benzanthrone, which is a conjugated molecule bearing a carbonyl group, is activated by confinement in a pore with a diameter close to the molecular size. An intense emission originating from the aromatic character π-π∗ transition is achieved through suppression of the nonradiative n-π∗ transition by strong hydrogen bonding between carbonyl groups and silanol groups with a micropore-filling effect in the nanospace.

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Preparation of titania with double band structure derived from a quantum size effect: Drastic increase in the photocatalytic activity

Somekawa S., Watanabe H., Ono Y., Oaki Y., Imai H.

Materials Letters （Materials Letters）  304 2021年12月

ISSN  0167577X

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Titania is a competent photocatalyst widely used in environmental purification. Unfortunately, its photocatalytic ability to generate hydrogen by water decomposition without a co-catalyst is poor. Here, an efficient enhancement of noble-metal-free photocatalytic activity was studied on the basis of titania prepared in a supermicroporous silica template using TiCl3 aqueous solution as a precursor. The amount of evolved hydrogen increased sharply by 37 times as the concentration of TiCl3 was increased from 5 to 10 wt%. The existence of two species of titania was determined by XAFS analysis at concentrations of 10 wt% or greater. It is suggested that titania quantum dots inside the supermicropores and titania particles outside the supermicropores were prepared at the optimum precursor concentration and the resultant composite band structures suppressed electron–hole recombination.

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Cellulose intrafibrillar mineralization of biological silica in a rice plant

Nakamura E., Ozaki N., Oaki Y., Imai H.

Scientific Reports （Scientific Reports）  11 （ 1 ）  2021年12月

　概要を見る

The essence of morphological design has been a fascinating scientific problem with regard to understanding biological mineralization. Particularly shaped amorphous silicas (plant opals) play an important role in the vital activity in rice plants. Although various organic matters are associated with silica accumulation, their detailed functions in the shape-controlled mineralization process have not been sufficiently clarified. In the present study, cellulose nanofibers (CNFs) were found to be essential as a scaffold for silica accumulation in rice husks and leaf blades. Prior to silicification, CNFs ~ 10 nm wide are sparsely stacked in a space between the epidermal cell wall and the cuticle layer. Silica nanoparticles 20–50 nm in diameter are then deposited in the framework of the CNFs. The shape-controlled plant opals are formed through the intrafibrillar mineralization of silica nanoparticles on the CNF scaffold.

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A Layered Polydiacetylene Containing Hydrogen-Bonding 4,4′-Bipyridyl Guests: Reversible Color Changes with a Wide-Range Temperature Response

Mochizuki Y., Imai H., Oaki Y.

ChemPlusChem （ChemPlusChem）  86 （ 12 ） 1563 - 1568 2021年12月

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Layered organic polymers have intercalation capabilities and dynamic properties. In classical intercalation chemistry, the interlayer guests are intercalated in the host layers via electrostatic interaction. The present work shows the organic layered materials with the host-guest interlayer interaction via hydrogen bond. Polydiacetylene (PDA) exhibits color changes from blue to red with the application of external stimuli, such as thermal and mechanical stresses. Here we report on a layered PDA containing 4,4′-bipyridyl in the interlayer space as a hydrogen-bonding guest. Whereas the layered PDA without interlayer guest shows the color transition at 65 °C, gradual color changes with two-stage reversibility are observed in the temperature range of −20–240 °C by the introduction of the hydrogen-bonding guest. The weaker interlayer interaction via the hydrogen bond promotes the dynamic motion directing the thermoresponsive color changes in a wide temperature range.

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Efficient photocatalytic conversion of benzene to phenol on stabilized subnanometer WO<inf>3</inf>quantum dots

Ohno A., Watanabe H., Matsui T., Somekawa S., Tomisaki M., Einaga Y., Oaki Y., Imai H.

Catalysis Science and Technology （Catalysis Science and Technology）  11 （ 19 ） 6537 - 6542 2021年10月

ISSN  20444753

　概要を見る

Band engineering with a quantum size effect is proposed to be utilized to design photocatalytic activity for the conversion of organic compounds. Subnanometer WO3 quantum dots (QDs) are spontaneously produced with the enhancement of the photocatalytic activity in a mesoporous silica matrix by preliminary photochemical dissolution. An appropriate upshift of the conduction band of subnanometer WO3 QDs stabilized by the coverage with the surface WO bonds promotes the efficient conversion of benzene to phenol without decomposition of the product into CO2 under ultraviolet illumination.

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Size-Distribution Control of Exfoliated Nanosheets Assisted by Machine Learning: Small-Data-Driven Materials Science Using Sparse Modeling

Haraguchi Y., Igarashi Y., Imai H., Oaki Y.

Advanced Theory and Simulations （Advanced Theory and Simulations）  4 （ 10 ）  2021年10月

　概要を見る

2D materials exhibit emergent properties originating from their characteristic nanostructures. In general, monolayered and few-layered nanosheets are obtained by exfoliation of the precursor layered materials. However, lateral-size distribution of the nanosheets is not easily controlled through the exfoliation because of the unpredictable down-sizing processes. The present work shows selective syntheses of transition-metal-oxide nanosheets with the monodisperse and polydisperse lateral sizes by the assistance of machine learning on small experimental data. The precursor layered composites of host transition-metal oxides and interlayer organic guests are exfoliated into the surface-modified nanosheets in organic dispersion media. A prediction model of the size distribution is constructed by sparse modeling, a method of machine learning, on the experimental data. The host-guest-medium combinations achieving the monodisperse and polydisperse lateral sizes are recommended by the prediction model. Therefore, the nanosheets with the controlled lateral-size distribution are selectively obtained in a limited number of the experiments. Moreover, self-assembly of the polydispersed nanosheets forms the homogeneous thin film exhibiting interference color. The prediction model and its construction method can be applied to the other 2D materials. Moreover, the present work implies that sparse modeling is an effective approach for small-data-driven materials science.

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Yield-prediction models for efficient exfoliation of soft layered materials into nanosheets

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

Chemical Communications （Chemical Communications）  57 （ 48 ） 5921 - 5924 2021年06月

ISSN  13597345

　概要を見る

Yield-prediction models were studied for efficient exfoliation of soft layered materials stackedviavan der Waals interactions with the assistance of machine learning on small experimental data. High-yield exfoliation of graphite and layered organic polymer was achieved under the conditions guided by the models in a limited number of experiments.

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Wide-area multilayered self-assembly of fluorapatite nanorods vertically oriented on a substrate as a non-classical crystal growth

Hagiwara Y., Oaki Y., Imai H.

Nanoscale （Nanoscale）  13 （ 21 ） 9698 - 9705 2021年06月

ISSN  20403364

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Oriented attachment of homogeneously shaped nanoblocks, such as nanocubes and nanorods, is attracting attention as a fundamental process of non-classical crystal growth to produce specific ordered architectures of functional materials. Although lateral alignments of horizontally oriented nanorod are commonly observed at the air-liquid and liquid-solid interfaces in dispersion systems, the accumulation of vertically oriented nanorods on a substrate has rarely been produced in a wide area over a millimeter-sized flat surface. Here, we achieved homogeneous stacking of vertical fluorapatite nanorods with a large aspect ratio (~6) in a toluene-hexane mixture system through a gradual decrease in the dispersibility. Micrometer-thick flat films in which the c direction of fluorapatite nanorods was arranged perpendicularly to the surface were deposited on a substrate with a diameter of over 20 mm. The wide-area accumulation of vertical nanorods occurs through the self-assembly of laterally arranged clusters of nanorods covered with a stabilizing agent and subsequent gentle sedimentation on the substrate surface. This journal is

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Self-Assembly of 2D Nematic and Random Arrays of Sterically Stabilized Nanoscale Rods with and without Evaporation

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

Langmuir （Langmuir）  37 （ 21 ） 6533 - 6539 2021年06月

ISSN  07437463

　概要を見る

The adequate manipulation of nanometer-scale building blocks using dispersion systems is regarded as a fundamental technique to fabricate elaborate microstructures. Although a liquid flow with evaporation is generally regarded as an essential factor for the self-assembly of floating blocks, experimental evidence has not been sufficient to clarify the importance of the flow in the dispersion systems. In the present study, 2D nematic layers of sterically stabilized nanoscale calcite rods were achieved in a millimeter-scale region on a solid substrate via the very slow recession of an organic dispersion with evaporation. 2D random arrays of the nanorods were obtained via recession of the liquid in the same system without evaporation. When the nanorods were not sterically stabilized, 3D random arrays were formed even with evaporation. We demonstrated that the evaporation-driven flow of sterically stabilized nanorods to a confined space at the air-liquid-solid interface is essential for the formation of 2D nematic structures on a substrate.

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Morphological study of fibrous aragonite in the skeletal framework of a stony coral

Sugiura M., Yasumoto K., Iijima M., Oaki Y., Imai H.

CrystEngComm （CrystEngComm）  23 （ 20 ） 3693 - 3700 2021年05月

　概要を見る

Calcareous skeletons of stony corals are composed of a low-crystalline core (center of calcification: COC) and surrounding fibrous crystals. We characterized the overall framework of juvenile Acropora digitifera corals to clarify the entire calcic architecture and the contribution of abiotic processes. Skeletal and tubular frameworks contain string and sheet COCs, respectively, consisting of strained aragonite nanograins. Parallel and curved structures of fibrous aragonite develop around the COC cores through geometric selection and a gradual change in the growth direction of the c-direction-elongated nanorods. Based on a study of the nanoscale morphology of the coral frameworks by comparing them to fibrous crystals grown in artificial systems, it appears that subsequent precipitation of the fibrous aragonite on the COC is primarily regulated by environmental abiotic parameters. The architecture of the COC network is deduced to be designed biologically and then thickened by the fibrous crystals that are produced with morphology formation through physicochemical processes although the vital activity controls the total mineralization.

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Phenylphosphonate surface functionalisation of MgMn<inf>2</inf>O<inf>4</inf>with 3D open-channel nanostructures for composite slurry-coated cathodes of rechargeable magnesium batteries operated at room temperature

Kajihara K., Takahashi D., Kobayashi H., Mandai T., Imai H., Kanamura K.

RSC Advances （RSC Advances）  11 （ 31 ） 19076 - 19082 2021年05月

　概要を見る

Spinel-type MgMn2O4, prepared by a propylene-oxide-driven sol-gel method, has a high surface area and structured bimodal macro- and mesopores, and exhibits good electrochemical properties as a cathode active material for rechargeable magnesium batteries. However, because of its hydrophilicity and significant water adsorption properties, macroscopic aggregates are formed in composite slurry-coated cathodes when 1-methyl-2-pyrrolidone (NMP) is used as a non-aqueous solvent. Functionalising the surface with phenylphosphonate groups was found to be an easy and effective technique to render the structured MgMn2O4hydrophobic and suppress aggregate formation in NMP-based slurries. This surface functionalisation also reduced side reactions during charging, while maintaining the discharge capacity, and significantly improved the coulombic efficiency. Uniform slurry-coated cathodes with active material fractions as high as 93 wt% can be produced on Al foils by this technique employing carbon nanotubes as an electrically conductive support. A coin-type full cell consisting of this slurry-coated cathode and a magnesium alloy anode delivered an initial discharge capacity of ∼100 mA h g−1at 25 °C.

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Morphological evolution of carbonated hydroxyapatite to faceted nanorods through intermediate states

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

CrystEngComm （CrystEngComm）  23 （ 16 ） 2968 - 2972 2021年04月

　概要を見る

Pure and carbonated hydroxyapatite (HA) nanocrystals are regarded as capital biocompatible materials. However, the formation process for faceted nanorods of HA has not been sufficiently clarified in supersaturated aqueous solution systems. Here, we monitored morphological evolution of carbonated HA to faceted crystals on the conversion of a parent calcite nanocrystal under hydrothermal conditions. In the initial stage, low-crystalline carbonated HA nanosheets ∼5 nm thick are formed as metastable intermediates. In the progressive stage, faceted nanorods ∼50 nm wide and ∼200 nm long grow as a relatively stable form of HA through the intermediates. The morphological evolution is ascribed to a stabilizing process of HA for improvement of the crystallinity and a decrease in the specific surface area and carbonate content.

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Ultrahigh-Sensitive Compression-Stress Sensor Using Integrated Stimuli-Responsive Materials

Nakamitsu M., Oyama K., Imai H., Fujii S., Oaki Y.

Advanced Materials （Advanced Materials）  33 （ 14 ）  2021年04月

ISSN  09359648

　概要を見る

Measurement of mechanical stresses, such as compression, shear, and tensile stresses, contributes toward achieving a safer and healthier life. In particular, the detection of weak compression stresses is required for healthcare monitoring and biomedical applications. Compression stresses in the order of 106–1010 Pa have been visualized and/or quantified using mechano-responsive materials in previous works. However, in general, it is not easy to detect compression stresses weaker than 103 Pa using conventional mechano-responsive materials because the dynamic motion of the rigid mechano-responsive molecules is not induced by such a weak stress. In the present work, weak compression stresses in the order of 100–103 Pa are visualized and measured via the integration of stimuli-responsive materials, such as layered polydiacetylene (PDA) and dry liquid (DL), through response cascades. DLs consisting of liquid droplets covered by solid particles release the interior liquid and collapse with application of a weak compression stress. The color of the layered PDA is changed by the spilled liquid as a chemical stress. A variety of weak compression stresses, such as expiratory pressure, are visualized and colorimetrically measured using the paper-based device of the integrated stimuli-responsive materials. Diverse mechano-sensing devices can be designed via the integration of stimuli-responsive materials.

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Effective 3D open-channel nanostructures of a MgMn<inf>2</inf>O<inf>4</inf>positive electrode for rechargeable Mg batteries operated at room temperature

Sone K., Hayashi Y., Mandai T., Yagi S., Oaki Y., Imai H.

Journal of Materials Chemistry A （Journal of Materials Chemistry A）  9 （ 11 ） 6851 - 6860 2021年03月

ISSN  20507488

　概要を見る

Room-temperature operations of rechargeable Mg coin-cell batteries have been achieved using a Mg alloy negative electrode and a spinel MgMn2O4(MMO)-positive electrode. The present work focuses on clarifying the effects of the physiochemical properties of the MMO powder including the specific surface area (SBET) and porosity of the positive electrode on Mg battery performances in practical applications. Finally, optimal specific surface area and porosity parameters were obtained that ensured excellent battery performances as a standard coin-cell at room temperature. A typical MMO powder synthesized using a modified sol-gel method with propylene oxide-driven complex polymerization had a largeSBET> 200 m2g−1, and more than 90% porosity with a triple-tiered 3D open-channel network. Here we evaluated the discharge capacity and the cyclability for room-temperature operation as a function ofSBETin a full cell with a Mg negative electrode as well as half cells with a carbon graphite electrode. Irrespective of whether half cells or a full cell was used, the initial discharge capacity was found to depend linearly on theSBETof the porous MMO powder in a Mg tetrakis(hexafluoroisopropyl)borate/triglyme electrolyte with a wide potential window, ΔE> 3.6 V. Eventually, the maximum discharge capacity of 220 mA h g−1was realized at 25 °C in the full cell using the 3D open-channel nanostructure withSBET= 236 m2g−1. The cyclability in the full cell with the Mg alloy negative electrode, however, degraded with the increasingSBET, while no cyclability degradation was observed in the half cell with the carbon electrode. A possible mechanism is discussed regarding passivation of the Mg alloy electrode in discharge/charge cycles.

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Lateral-size control of exfoliated transition-metal-oxide 2D materials by machine learning on small data

Mizuguchi R., Igarashi Y., Imai H., Oaki Y.

Nanoscale （Nanoscale）  13 （ 6 ） 3853 - 3859 2021年02月

ISSN  20403364

　概要を見る

A wide variety of nanosheets including monolayers and few-layers have attracted much interest as two-dimensional (2D) materials for the unique anisotropic structures and properties. On the other hand, one of the significant remaining and challenging issues is the lateral-size control. Since 2D materials are generally synthesized by the exfoliation of layered materials, the lateral size is not easily controlled in the breaking-down processes. The experimental factors have not been found for the control and prediction. In the present work, lateral sizes of the exfoliated transition-metal-oxide nanosheets were predicted and controlled by the assistance of machine learning. Layered composites of host inorganic layers and guest organic molecules were exfoliated into nanosheets in organic dispersion media. The lateral size of the nanosheets was estimated by dynamic light scattering (DLS), instead of microscopy methods, to achieve high-throughput analyses. Factors governing the lateral size are explored on the small experimental data by the assistance of sparse modeling, a method of machine learning. The eight physicochemical parameters of the organic guests and dispersion media were extracted by sparse modeling for the construction of the size-prediction model. The size-prediction model accelerated the selective syntheses of the nanosheets with large and small lateral sizes in a limited number of experiments. The results indicate that the prediction model is a guideline to find suitable exfoliation conditions for size control. Size-selective syntheses of a variety of 2D materials can be achieved by similar methods using sparse modeling on small experimental data. Moreover, sparse modeling is applicable to control the design and exploration of other materials and their processing based on small data. This journal is

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Phase Transition Behavior of MgMn<inf>2</inf>O<inf>4</inf> Spinel Oxide Cathode during Magnesium Ion Insertion

Tuerxun F., Otani S., Yamamoto K., Matsunaga T., Imai H., Mandai T., Watanabe T., Uchiyama T., Kanamura K., Uchimoto Y.

Chemistry of Materials （Chemistry of Materials）  33 （ 3 ） 1006 - 1012 2021年02月

ISSN  08974756

　概要を見る

The 3d transition metal oxides with a spinel structure are among the most promising cathode materials for magnesium batteries. In this study, we investigated the reaction mechanism of magnesium ion insertion for magnesium spinel oxides, MgMn2O4, by electrochemical measurements, X-ray absorption spectroscopy (XAS), and synchrotron X-ray diffraction (XRD) with Rietveld analysis. Open-circuit-potential and XAS measurements showed that Mg2+ insertion into MgMn2O4 does not proceed via a simple two-phase coexistence reaction between the spinel and rock-salt phases. Synchrotron XRD measurements showed that Mg2+ insertion into MgMn2O4 involves crystal structural changes in three stages. In the early stage of the Mg2+ insertion process (0 < x < 0.2), Mg2+ is inserted into the spinel (MgMn2O4) phase and rock-salt (Mg1.2Mn2O4) phases, which are included in the pristine samples, without significant volume changes. In the middle stage of the Mg2+ insertion process (0.2 < x < 0.4), Mg2+ is inserted into the Mg1+αMn2O4 spinel phase and the Mg2-βMn2O4 rock-salt phases with a large volume change. In the last stage of Mg2+ insertion process (0.4 < x < 0.56), Mg2+ insertion proceeds via a two-phase coexistence reaction between Mg1.4Mn2O4 spinel and Mg1.6Mn2O4 rock-salt phases without Mg content changes in either phase. The phase transition from the Mg1+αMn2O4 spinel phase to the Mg2-βMn2O4 rock-salt phase with a large volume change resulted in significant polarization during the Mg2+ insertion process. Suppressing the phase transition, accompanied by a large volume change, is important in designing a spinel oxide cathode with a high rate performance.

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Diatom-mimetic channeled mesoporous silica membranes: Self-organized formation of a hierarchical porous framework

Kitamura R., Watanabe H., Somekawa S., Ono Y., Owari Y., Oaki Y., Imai H.

Materials Chemistry Frontiers （Materials Chemistry Frontiers）  5 （ 2 ） 862 - 868 2021年01月

　概要を見る

Silica membranes having macrochannels with mesoporous walls were spontaneously fabricated using a self-organized process with rapid gelation of concentrated silicate and surfactant precursor solutions. The specific hierarchical porous framework consisting of straight macrochannels ca. 1 μm in diameter is similar to that of a biological silica frustule of diatoms. We applied the diatom-mimetic channeled mesoporous silica membranes to high-permeability-rate filtration and enhanced photocatalytic systems. The presence of macrochannels was found to improve the performance of the mesoporous silica membranes in filtration and as a photocatalyst carrier.

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

Kimura T., Takasaki M., Hatai R., Nagai Y., Uematsu K., Oaki Y., Osada M., Tsuda H., Ishigure T., Toyofuku T., Shimode S., Imai H.

Scientific Reports （Scientific Reports）  10 （ 1 ）  2020年12月

　概要を見る

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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Strained calcite crystals from amorphous calcium carbonate containing an organic molecule

Sugiyama S., Takasaki M., Oaki Y., Imai H.

CrystEngComm （CrystEngComm）  22 （ 42 ） 7054 - 7058 2020年11月

　概要を見る

Amorphous calcium carbonate (ACC) is regarded as an important precursor of biogenic crystal phases having specific shapes and mechanical properties. In the present work, calcite that was produced from ACC nanoparticles containing aspartic acid was characterized by Raman scattering spectroscopy and nanoindentation techniques. The tensile strain of the calcite lattice that was estimated from the vibrational spectroscopy varied with the organic content and the mild calcination condition of ACC nanoparticles. Calcite was found to be hardened by the presence of residual organic components inside the crystal lattice. On the other hand, the modulus of calcite decreased as the content of external organic components increased. Here, the mechanical properties of the calcite framework are successfully tuned by the crystallization of the specific amorphous phases containing organic molecules. The mechanical modulation of calcite can be associated with lattice expansion by using vibrational spectroscopy. This journal is

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