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Papers - Oaki, Yuya

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Preparation of conductive Cu<inf>1.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> and Mn<inf>3</inf>O<inf>4</inf> spinel mixture powders as positive active materials in rechargeable Mg batteries operative at room temperature

Takemitsu H., Hayashi Y., Watanabe H., Mandai T., Yagi S., Oaki Y., Imai H.

Journal of Sol-Gel Science and Technology (Journal of Sol-Gel Science and Technology) 104 ( 3 ) 635 - 646 2022.12

ISSN 09280707

We prepared conductive mixtures of Cu1.5Mn1.5O4 and Mn3O4 spinels (CMO–MOs) as positive electrode active materials in rechargeable Mg batteries (RMBs) using a sol–gel complex polymerization method. The CMO–MO spinel mixtures with high specific surface areas above 100 m2 g−1 were obtained with mild calcination in Ar at 300 °C. The conductivity of CMO–MOs was estimated to be approximately 1000 times higher than that of a conventional MgMn2O4 (MMO) spinel powder. The discharge capacities evaluated using 2032-type coin-cell battery with a Mg-alloy negative electrode at room temperature increase with an increase in the specific surface area of the spinel powders. The specific surface area for providing the theoretical capacity of the conductive CMO–MOs was about one-third that of the insulative MMO. High specific surface area and high conductivity are key parameters for the positive active material to realize practical room-temperature operation of RMBs.

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Water Reduction Photocathodes Based on Ru Complex Dyes Covered with a Conjugated Polymer Nanosheet

Shimamura T., Kobayashi A., Oaki Y., Yoshida M., Kato M.

Energy and Fuels (Energy and Fuels) 36 ( 19 ) 11559 - 11566 2022.10

ISSN 08870624

Water-splitting dye-sensitized photoelectrochemical cells are promising devices for resolving energy and environmental problems by producing H2, a clean energy source, from water and sunlight. However, their performance is still limited because of dye desorption from the electrode and their weak light-absorption efficiency. In this study, we fabricated two water-reducing photocathodes, NiO|RuCP/BQPy and NiO|RuCP-Zr-RuP/BQPy, by combining the layer-by-layer formation of mono- (RuCP) and bi- (RuCP and RuP with Zr4+cations) layer molecular films of Ru(II) dyes, respectively, on a p-type NiO substrate with the chemical vapor deposition of the nanosheet catalyst based on the BQPy organic polymer comprising benzoquinone and pyrrole moieties. These photocathodes produced H2under visible light irradiation and a small electrical bias in a 0.1 M Na2SO4aqueous solution (λ > 420 nm, 65 mW cm-2, E = -0.254 V versus normal hydrogen electrode, and pH 3). Both the photocurrent value and amount of H2produced by double-dye-layered NiO|RuCP-Zr-RuP/BQPy were smaller than those of NiO|RuCP/BQPy, probably because of the energy transfer inactivation between the Ru(II) dyes, which competes with the reductive quenching (i.e., electron injection) by NiO. Ru(II) dye desorption from the NiO|RuCP/BQPy photocathode was significantly reduced to approximately a third of that of NiO|RuCP, indicating that the deposited BQPy polymer improved the durability of the photocathode. The deposition of the BQPy polymer can be easily performed under mild conditions (60 °C in air); therefore, this technique shows great potential application for the construction of various types of dye-sensitized water-reducing photocathodes.

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Coatable 2D Conjugated Polymers Containing Bulky Macromolecular Guests for Thermal Imaging

Edagawa A., Matsuda S., Kawakubo H., Imai H., Oaki Y.

ACS Applied Materials and Interfaces (ACS Applied Materials and Interfaces) 14 ( 38 ) 43792 - 43801 2022.09

ISSN 19448244

Dynamic properties are derived from the structural flexibility of 2D polymers. Softening layered structures has the potential for tuning and enhancing the dynamic properties. In the present work, the flexibility of layered polydiacetylene (PDA) is tuned by the interlayer polymeric guests with different branching structures. PDA shows thermoresponsive color-change properties through shortening the effective conjugation length with molecular motion. Whereas the blue-to-red color transition is observed at certain threshold temperatures for the layered PDA without the interlayer guest, the intercalation of the bulky polymer guests lowers the starting temperature and widens the temperature range for the thermoresponsive color changes. The resultant layered composite of PDA and bulky polymer affords the homogeneous coating on substrates on the centimeter scale. The thermoresponsive color-change coating is applied to temperature-distribution imaging. The specific heat of liquids is colorimetrically estimated using the coating on the bottle. The coating on a silk cloth visualizes the temperature distribution on a simulated tissue during surgical operation using an ultrasonic coagulation cutting device. The coating can be applied to thermal imaging in a variety of fields. Moreover, the softening strategy contributes to explore dynamic properties of soft 2D materials.

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Selective syntheses of thick and thin nanosheets based on correlation between thickness and lateral-size distribution

Haraguchi Y., Imai H., Oaki Y.

iScience (iScience) 25 ( 9 ) 2022.09

Exfoliation of layered materials, a typical route to obtain 2D materials, is not easily controlled because of the unpredictable downsizing processes. In particular, the thickness control remains as a complex challenge. Here, we found a correlation between the thickness and lateral size distribution of the exfoliated nanosheets, such as transition metal oxides and graphene oxide. The layered composites of the host metal oxides and interlayer organic guests are delaminated into the surface-modified nanosheets in organic dispersion media. The exfoliation behavior varies by combination of the hosts, guests, and dispersion media. Here, we found that the thick and thin nanosheets were obtained on the monodispersed and polydispersed conditions, respectively. The selective syntheses of the thick and thin nanosheets were achieved using a prediction model of the lateral size distribution. The correlation between the thickness and lateral size distribution can be applied to thickness-selective syntheses of 2D materials.

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Nanosheet-Based Uneven Thin Film Exhibiting Angle-Independent Structural Color and Its Application to Detection of Biogenic Amines

Haraguchi Y., Imai H., Oaki Y.

Advanced Materials Interfaces (Advanced Materials Interfaces) 9 ( 25 ) 2022.09

ISSN 2196-7350

Anisotropic ultrathin 2D materials are used as building blocks to obtain higher-order structures. Tailoring the size, surface chemistry, and assembly state of exfoliated nanosheets is required for the design of functional materials. However, these factors are not easily controlled during the down-sizing exfoliation process, which is a general route for obtaining 2D materials. In the present work, polydispersed nanosheets of a transition metal oxide are prepared based on prediction model for the size distribution constructed by a combination of machine learning and chemical perspective on small data. Self-assembly of the exfoliated nanosheets results in the uneven thin films exhibiting an angle-independent structural color. As the nanosheets vary in thickness and lateral size, their assembled thin films exhibit structural color independent of the incident angle. The structural color is changed by casting only 20 µL of an aqueous solution containing biogenic amines such as histamine within a sensing time of 5 min. The biogenic amines are intercalated into the interlayer space of the stacked nanosheets and the resulting increase in the thickness changes the color of the thin-film device. The new design strategies can be applied to constructing other thin-film sensing devices based on nanosheets.

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Layered macrocycles with flexibility and tunable dynamic properties for wide-range thermoresponsive color changes

Shioda N., Heo J.M., Kim B., Imai H., Kim J.M., Oaki Y.

Sensors and Diagnostics (Sensors and Diagnostics) 1 ( 1 ) 160 - 168 2022.08

Layered materials exhibit unique properties, such as intercalation and exfoliation, originating from the two-dimensional anisotropic nanostructures. A variety of inorganic and organic layered materials have been synthesized for design of functions. However, conventional host layers are static and rigid to induce dynamic functions originating from the structural flexibility. The present work shows a new dynamic and flexible host layer based on organic macrocycles. A macrocycle containing functional, flexible, and intercalating units, namely macrocyclic diacetylene (MCDA), was designed to form a self-assembled layered structure. After the guest molecules were intercalated in the interlayer space, the host MCDA layers were topochemically polymerized to obtain polydiacetylene. The layered macrocycles containing the π-conjugated main chain and interlayer guests showed gradual thermoresponsive color changes in a wide temperature range of −100 to 230 °C with variation of the effective conjugation length. The flexible host layer affords induction of the dynamic motion of the intrinsically rigid π-conjugated main chain in the wide temperature range. Moreover, the thermo-responsivity was tuned by the intercalated guests. The color-change properties enabled real-time temperature imaging for medical applications. Flexible layered macrocycles can be a new design strategy of soft materials with dynamic functions.

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A Capacity-Prediction Model for Exploration of Organic Anodes: Discovery of 5-Formylsalicylic Acid as a High-Performance Anode Active Material

Komura T., Sakano K., Igarashi Y., Numazawa H., Imai H., Oaki Y.

ACS Applied Energy Materials (ACS Applied Energy Materials) 5 ( 7 ) 8990 - 8998 2022.07

ISSN 2574-0962

Development of high-performance organic energy storage is one of the important challenges in recent materials science. Molecular design and synthesis have potential for enhancement of the performances. Efficient exploration and design of the molecules are required in a wide search space. In the present work, a capacity prediction model for organic anodes was constructed on small experimental data by sparse modeling, a method of machine learning, combined with our chemical insights. The straightforward linear regression model facilitated discovery of a high-performance active material for organic anodes in a limited number of experiments. A recommended compound, 5-formylsalicylic acid (SA-CHO), showed one of the highest performances in recent works, i.e., a specific capacity of 873 mA h g-1at 100 mA g-1(sample number: n = 3) with rate performance and cycle stability. The model can be applied to explore organic anode active materials with higher specific capacity.

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Nanoarchitectonics for conductive polymers using solid and vapor phases

Oaki Y., Sato K.

Nanoscale Advances (Nanoscale Advances) 4 ( 13 ) 2773 - 2781 2022.04

ISSN 2516-0230

Conductive polymers have been extensively studied as functional organic materials due to their broad range of applications. Conductive polymers, such as polypyrrole, polythiophene, and their derivatives, are typically obtained as coatings and precipitates in the solution phase. Nanoarchitectonics for conductive polymers requires new methods including syntheses and morphology control. For example, nanoarchitectonics is achieved by liquid-phase syntheses with the assistance of templates, such as macromolecules and porous materials. This minireview summarizes the other new synthetic methods using the solid and vapor phases for nanoarchitectonics. In general, the monomers and related species are supplied from the solution phase. Our group has studied polymerization of heteroaromatic monomers using the solid and vapor phases. The surface and inside of solid crystals were used for the polymerization with the diffusion of the heteroaromatic monomer vapor. Our nanoarchitectonics affords to form homogeneous coatings, hierarchical structures, composites, and copolymers for energy-related applications. The concepts using solid and vapor phases can be applied to nanoarchitectonics for not only conductive polymers but also other polymers toward a variety of applications.

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Performance Predictors for Organic Cathodes of Lithium-Ion Battery

Sakano K., Igarashi Y., Imai H., Miyakawa S., Saito T., Takayanagi Y., Nishiyama K., Oaki Y.

ACS Applied Energy Materials (ACS Applied Energy Materials) 5 ( 2 ) 2074 - 2082 2022.02

Organic cathodes for lithium-ion batteries are one of the most promising and significant materials toward a sustainable society. The molecular design is a key to achieve superior performances beyond inorganic cathodes. The present work shows predictors of the reaction potential, specific capacity, and ideal energy density for organic cathodes. Straightforward prediction models of the performance were constructed by a combination of machine learning and chemical insight, namely, sparse modeling for small data (SpM-S), on a small data set as training data found in the literature. The prediction accuracy was validated using different literature data. The predictors can be applied to explore high-performance organic cathodes in a wide search space efficiently. Moreover, SpM-S afforded straightforward, interpretable, and generalizable prediction models compared to other machine-learning algorithms. The small-data-driven methodology can be applied for further exploration of materials, enhancement of performances, and optimization of processes.

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Designed nanostructures created via physicochemical switching of the growth mode between single crystals and mesocrystals†

Kanazawa S., Oaki Y., Imai H.

Nanoscale Advances (Nanoscale Advances) 4 ( 6 ) 1538 - 1544 2022.02

Most biominerals are composed of mesocrystals, in which individual nanoparticle building units have a defined long-range order on the atomic scale in at least one direction. Although the crystal size and orientation of the mesostructures are spatially designed in biological architectures, it has been difficult to achieve adequate control of the crystal growth to produce modulated mesostructures in artificial aqueous systems. Here, we propose a simple physicochemical approach for the spatial design of nanostructures using an aqueous solution system. The ordered arrays of oriented fluorapatite (FA) rods similar to tooth enamel are produced on a polymer substrate in a supersaturated solution. We succeeded in reversible switching of the growth mode of FA between single-crystalline rods and mesocrystalline grains through the disturbance of the solution. The primary crystal size was tuned between micrometric rods ∼0.5 μm wide and >5 μm long and nanoscale grains ∼10 nm wide and 50 nm long without a drastic change in the c direction. Hierarchical architectures consisting of iso-oriented FA microrods and nanograins were constructed via temporal control of the crystal growth mode by switching a physicochemical parameter, such as the degree of supersaturation at the growth front.

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Sparse modeling for small data: case studies in controlled synthesis of 2D materials

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

Digital Discovery (Digital Discovery) 1 ( 1 ) 26 - 34 2022.02

Data-scientific approaches have permeated into chemistry and materials science. In general, these approaches are not easily applied to small data, such as experimental data in laboratories. Our group has focused on sparse modeling (SpM) for small data in materials science and chemistry. The controlled synthesis of 2D materials, involving improvement of the yield and control of the size, was achieved by SpM coupled with our chemical perspectives for small data (SpM-S). In the present work, the conceptual and methodological advantages of SpM-S were studied using real experimental datasets to enable comparison with other machine learning (ML) methods, such as neural networks. The training datasets consisted of ca. 40 explanatory variables (xn) and 50 objective variables (y) regarding the yield, size, and size-distribution of exfoliated nanosheets. SpM-S provided more straightforward, generalizable, and interpretable prediction models and better prediction accuracy for new experiments as an unknown test dataset. The results indicate that machine learning coupled with our experience, intuition, and perspective can be applied to small data in a variety of fields.

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Electroless nickel plating on a biomineral-based sponge structure

Takeoka H., Seike M., Nakamura Y., Imai H., Oaki Y., Fujii S.

Materials Advances (Materials Advances) 3 ( 2 ) 931 - 936 2022.01

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Demonstrated gradual evolution of disorder in crystalline structures between single crystal and polycrystal via chemical and physicochemical approaches

Kanazawa S., Oaki Y., Imai H.

CrystEngComm (CrystEngComm) 24 ( 25 ) 4546 - 4550 2022

Various biological and biomimetic minerals are composed of mesocrystals, in which nanoscale crystalline units are ordered in approximately the same crystallographic direction. However, the variation of the crystallographic regularity of mesocrystals has not been clarified using experimental evidence. This work demonstrated the gradual evolution of disorder in crystalline structures between single crystal and polycrystal by monitoring the crystallographic regularity using electron diffraction patterns. Single-crystal rods of fluorapatite elongated in the c direction are switched to mesocrystals consisting of iso-oriented nanorods by adding a specific molecule having a carboxy group (chemical approach) and to roughly arranged nanograins by increasing the degree of supersaturation at the growth front (physicochemical approach). Lateral and vertical miniaturization of the growth units is achieved through stepwise nucleation and near-epitaxial growth. The crystallographic regularity gradually decreases with deviation of the c axis and rotation of the a axes through reduction of the growth unit size. Here we can tune the disorder in crystalline structures via the chemical and physicochemical approaches.

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Characterization of calcite spines of planktonic foraminifers (Globigerinidae)

Izumida K., Takasaki M., Fujimaki R., Nagai Y., Oaki Y., Naito K., Tanaka Y., Shimode S., Toyofuku T., Imai H.

CrystEngComm (CrystEngComm) 24 ( 13 ) 2446 - 2450 2022

Calcitic spines ∼1 μm in diameter are projecting from a trochospiral test of a spinose planktonic foraminifer. According to electron diffraction, Raman spectra, and detailed electron microscope observation, the spines were found to be a single-crystal-like calcite that has a nanogranular texture containing microstrain. An organic envelope protects the biogenic calcite against dissolution in water. Their unique mechanical and chemical properties are ascribed to the specific micro- and nanoscopic structures. This journal is

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A nonclassical pathway to biomimetic strained SrSO<inf>4</inf> crystals

Fujimaki R., Oaki Y., Imai H.

CrystEngComm (CrystEngComm) 24 ( 24 ) 4356 - 4360 2022

Biomimetic strained SrSO4 crystals were produced through a nonclassical pathway involving fiber-by-fiber accumulation with poly(acrylic acid) in an aqueous solution system. Polymer-stabilized fibrous hydrate crystals 20-40 nm wide and several micrometers long were formed in the initial stage. Spindle-shaped microrods of SrSO4 were then constructed by oriented attachment of the hydrous fibrils and subsequent dehydration. The strained structure of the anhydrous SrSO4 crystals is similar to biogenic SrSO4 spines of acantharian species, a marine unicellular holoplanktonic protist.

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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 2021.12

ISSN 0167577X

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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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 2021.12

ISSN 2192-6506

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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Cellulose intrafibrillar mineralization of biological silica in a rice plant

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

Scientific Reports (Scientific Reports) 11 ( 1 ) 2021.12

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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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 2021.10

ISSN 20444753

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 ) 2021.10

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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