Papers - Imai, Hiroaki

A biomimetic hybrid material consisting of CaCO<inf>3</inf> mesoporous microspheres and an alternating copolymer for reversed-phase HPLC

Mochida M., Nagai Y., Kumagai H., Imai H., Citterio D., Hiruta Y.

Journal of Materials Chemistry B (Journal of Materials Chemistry B)  7 ( 31 ) 4771 - 4777 2019

ISSN  2050750X

　View Summary

We developed a biomineral-inspired hybrid material composed of CaCO3 and an organic polymer as a column packing material for HPLC. This material combines a hierarchical mesoporous structure and the functionality of the polymer. The surface of monodispersed mesoporous CaCO3 microspheres was modified with poly(maleic acid-alt-1-octadecene) (PMAcO) comprising hydrophobic alkyl chains and anionic carboxylate groups. PMAcO adsorbed onto the surface of CaCO3 through electrostatic interaction between Ca2+ sites and carboxylate groups, resulting in an octadecene coated microsphere interface. These microspheres were applied as a HPLC column and exhibited reversed-phase retention behavior in the separation of alkylbenzenes. This column showed high alkaline mobile phase resistance compared with the conventionally applied ODS column packing material. Quantitative analysis of the basic antidepressants clomipramine and imipramine spiked into whole blood was achieved with an alkaline mobile phase, demonstrating the potential of the biomineral-inspired material as a HPLC stationary phase for practical applications in routine analyses of basic drugs requiring alkaline mobile phases.

• Access to Document (DOI)

Supermicroporous Silica Nanograins: Synthesis and Application

Kitamura R., Watanabe H., Oaki Y., Imai H.

Langmuir (Langmuir)   2019

ISSN  07437463

　View Summary

Supermicroporous silicas having micropores 1-2 nm in diameter were miniaturized to 30-40 nm in diameter using a combination of binary organic agents: cationic surfactant as a porogen and polyethylene glycol as a growth suppressor in a concentrated system. The miniaturized nanograins of the porous silica were effective as a porous and transparent matrix for WO 3 quantum dots that exhibit enhanced photocatalytic activity.

• Access to Document (DOI)

Redox-mediated high-yield exfoliation of layered composites into nanosheets

Yamamoto Y., Imai H., Oaki Y.

Bulletin of the Chemical Society of Japan (Bulletin of the Chemical Society of Japan)  92 ( 4 ) 779 - 784 2019

ISSN  00092673

　View Summary

A variety of organic and inorganic layered compounds are exfoliated into 2D nanomaterials in liquid phase. Improvement of exfoliation yield is a significant challenge for further applications of 2D nanomaterials. However, yield of transition-metal-oxide nanosheets is not so high, such as around 5% for 240 h and 15% for 120 h, in previous exfoliation methods. Here we show a new exfoliation approach to high-yield synthesis of nanosheets. A layered composite of cobalt hydroxide and guest quinone derivative was exfoliated into nanosheets through redox reactions of the interlayer guests in the yield 1657% for 1 h under mild conditions. The redox reactions of the guest molecules on the layer facilitate efficient exfoliation through improvement of the affinity between the guest and dispersion medium. The present redox-mediated exfoliation approach can be applied to efficient synthesis of a variety of 2D nanomaterials from layered composites.

• Access to Document (DOI)

Pyrolytic production of fluorescent pyrone derivatives produced in the confined space of super-microporous silicas

Hayashi K., Ogawa S., Watanabe H., Fujimaki Y., Oaki Y., Imai H.

Bulletin of the Chemical Society of Japan (Bulletin of the Chemical Society of Japan)  92 ( 7 ) 1170 - 1174 2019

ISSN  00092673

　View Summary

Fluorescent pyrone derivatives were extracted from the pyrolitic product of citric acid confined in supermicroporous silicas (SMPSs). We utilized the tiny spaces of SMPSs to produce fluorescent molecules as small fragments of carbogenic dots. We characterized their optical properties and investigated their structual information. The obtained molecule was determined as a pyrone derivative. The isolated product showed similar optical properties to those of reported carbogenic dots. Therefore, we suggest that the pyrone structure is the key fragment structure of carbogenic dots.

• Access to Document (DOI)

Modifications in coordination structure of Mg[TFSA]<inf>2</inf>-based supporting salts for high-voltage magnesium rechargeable batteries

Mandai T., Tatesaka K., Soh K., Masu H., Choudhary A., Tateyama Y., Ise R., Imai H., Takeguchi T., Kanamura K.

Physical Chemistry Chemical Physics (Physical Chemistry Chemical Physics)  21 ( 23 ) 12100 - 12111 2019

ISSN  14639076

　View Summary

To achieve a sustainable-energy society in the future, next-generation highly efficient energy storage technologies, particularly those based on multivalent metal negative electrodes, are urgently required to be developed. Magnesium rechargeable batteries (MRBs) are promising options owing to the many advantageous chemical and electrochemical properties of magnesium. However, the substantially low working voltage of sulfur-based positive electrodes may hinder MRBs in becoming alternatives to current Li-ion batteries. We proposed halide-free noncorrosive ionic liquid-based electrolytes incorporating Mg[TFSA]2 for high-voltage MRB applications. Upon the complexation of Mg[TFSA]2 with tetraglyme (G4) and strict control of the liquid states, the electrolytes achieved excellent anodic stability up to 4.1 V vs. Mg2+/Mg even at 100 °C. The modest electrochemical activities for magnesium deposition/dissolution in the [Mg(G4)][TFSA]2/ionic liquid electrolyte can be improved by certain modifications to the coordination state of [TFSA]-. Dialkyl sulfone was found to be effective in changing the coordination state of [TFSA]- from associated to isolated (free). This coordination change successfully promoted magnesium deposition/dissolution reactions, particularly in the coexistence of ether ligand. By contrast, the coordination of Mg2+ by strongly donating agents such as dimethyl sulfoxide and alkylimidazole led to the complexes inactive electrochemically, suggesting that interaction between Mg2+ and coordination agents predominates the fundamental electrochemical activity. We also demonstrated that an enhancement in the electrochemical activity of electrolytes contributed to improvements in the cycling ability of magnesium batteries with 2.5 V-class MgMn2O4 positive electrodes.

• Access to Document (DOI)

Highly dispersive mono-sized nanoparticles of Y <inf>2</inf> O <inf>3</inf> -stabilized ZrO <inf>2</inf>

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

Chemistry Letters (Chemistry Letters)  48 ( 4 ) 390 - 393 2019

ISSN  03667022

　View Summary

Uniformly sized nanoparticles 35nm in diameter of Y 2 O 3 - stabilized ZrO 2 (YSZ) were fabricated by a water-toluene twophase solvothermal method. The nanoparticles that were covered with oleic acid were highly dispersed in organic media. The size and doping concentration were tuned by adjusting the preparation conditions. The fabrication of YSZ thin films was achieved through hexagonal arrangement by evaporation-induced selfassembly of highly dispersive mono-sized nanoparticles.

• Access to Document (DOI)

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

Ishioka S., Watanabe K., Imai H., Tseng Y.J., Peng C.H., Oaki Y.

Chemical Communications (Chemical Communications)  55 ( 78 ) 11723 - 11726 2019

ISSN  13597345

　View Summary

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

• Access to Document (DOI)

Evaporation-driven manipulation of nanoscale brickwork structures for the design of 1D, 2D, and 3D microarrays of rectangular building blocks

Imai H., Matsumoto R., Takasaki M., Tsukiyama K., Sawano K., Nakagawa Y.

CrystEngComm (CrystEngComm)  21 ( 45 ) 6905 - 6914 2019

　View Summary

As children play with wooden building blocks, we would like to construct elaborate architectures through the one-by-one accumulation of nanocrystals. Recently, ordered microarrays of nanosized building blocks have attracted attention as a sophisticated bottom-up approach to fabricate adequate nanomaterials. However, their dimensions, scale, and orientations have not been strictly controlled. The present highlight article reports the potential of brickwork structures using evaporation-driven manipulation as a nanoscale architectural method. Here, the essence of controlled assembly is shown with a demonstration of 1D, 2D, and 3D brickwork structures designed with various inorganic rectangular building nanoblocks covered by stabilizing organic agents. The structures of the ordered assemblies are found to be controlled by changing several parameters, such as particle concentration and the properties of dispersion media and stabilizing agents. This brickwork technique is applicable to multicomponent systems using a library of rectangular nanoblocks. Homogeneous and heterogeneous ordered architectures with spatial and sequential control are selectively produced through the self-assembly of nanocubes and nanocuboids. These elaborate nanoscale brickwork structures are now being developed for the production of designed microstructures with novel functions.

• Access to Document (DOI)

Enhanced quantum yield of nanographenes incorporated in supermicroporous silicas and the co-adsorption effect of water molecules

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

Bulletin of the Chemical Society of Japan (Bulletin of the Chemical Society of Japan)  92 ( 5 ) 923 - 926 2019

ISSN  00092673

　View Summary

Nanographene with various molecular sizes was synthesized and incorporated into the size-controlled pores of super-microporous silica (SMPS). It was found that when the pore diameter of the SMPS matched the molecular size of the nanographene, the fluorescence quantum yield was greatly enhanced. Furthermore, co-adsorption of water molecules improved the quantum yield. This improvement is attributed to a suppression of the interaction between the nanographene and silica walls, which lowers the quantum yield. Since nanographene is insoluble in water, it becomes surrounded by water molecules, almost like in aqueous solution. Finally, highly fluorescent solid materials were obtained by incorporating nanographene into SMPS.

• Access to Document (DOI)

Biomimetic macroscopic mesocrystalline films produced by oriented assembly of nanorods under magnetic field

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

Nanoscale (Nanoscale)  10 ( 47 ) 22161 - 22165 2018.12

ISSN  20403364

　View Summary

Millimeter-scale mesocrystals and cross-lamellae mimicking the nanostructure of seashells were produced on a substrate through the three-dimensionally oriented assembly of c-axis-elongated calcite nanorods ∼50 nm wide and ∼500-1000 nm long by combining arrangement with evaporation-driven capillary force and alignment under an intense magnetic field.

• Access to Document (DOI)

Tunable Stimuli-Responsive Color-Change Properties of Layered Organic Composites

Takeuchi M., Gnanasekaran K., Friedrich H., Imai H., Sommerdijk N., Oaki Y.

Advanced Functional Materials (Advanced Functional Materials)  28 ( 45 )  2018.11

ISSN  1616301X

　View Summary

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Stimuli-responsive properties of soft materials originate from dynamic structure changes. Layered polydiacetylene (PDA) shows color-change properties with the application of external stimuli, such as thermal and mechanical stresses. Layered organic materials possessing intercalation capabilities and dynamic properties have potentials for tuning their structures and properties by the intercalated guests. A variety of sensing and imaging devices can be developed by control of the stimuli responsivity. Here, the key structures, processes, and mechanisms for tuning the stimuli-responsive color-change properties of layered PDA are studied by in situ analyses with heating and cooling. The in situ analyses indicate that heating initiates thermal motion of the alkyl side chains of the PDA main chain in the host layers around 60 °C, regardless of the type of intercalated guests. Further heating induces torsion of the PDA main chain leading to color changes at different temperatures, depending on the types of the intercalated guest. Then, the layered structure is irreversibly deformed with lowering the crystallinity. The results indicate that the stimuli-responsive color-change properties are controlled by the rigidity of the layered structures consisting of the host layers and guests. The control strategy based on rigidity tuning can be applied to a variety of soft materials with stimuli responsivity.

• Access to Document (DOI)

Layer-by-Layer Manipulation of Heterogeneous Rectangular Nanoblocks: Brick Work for Multilayered Structures with Specific Heterojunction

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

Inorganic Chemistry (Inorganic Chemistry)  57 ( 18 ) 11655 - 11661 2018.09

ISSN  00201669

　View Summary

© 2018 American Chemical Society. The combination of heterogeneous inorganic nanocrystals provides novel nanometer-scale architectures for exploration of novel functions. In the present study, heterogeneous bilayers, such as BaTiO3-Fe3O4 and Mn3O4-CeO2, are fabricated via layer-by-layer stacking of several rectangular nanoblocks 5-23 nm in size by evaporation-driven self-assembly. Specific heterojunctions between inorganic crystals are constructed by nanoscale simple brick work through spontaneous adjustment of the crystallographic orientations of the nanoblocks in the lower and upper layers.

• Access to Document (DOI)

Two-Dimensional Conductive and Redox-Active Nanostructures Synthesized by Crystal-Controlled Polymerization for Electrochemical Applications

Sato K., Imai H., Oaki Y.

ACS Applied Nano Materials (ACS Applied Nano Materials)  1 ( 8 ) 4218 - 4226 2018.08

　View Summary

Two-dimensional nanomaterials have attracted much interest for their anisotropic structures and emergent properties. Whereas a variety of inorganic nanosheets are prepared by exfoliation, design and synthetic strategies of organic nanosheets are still developing. Here we report on crystal-controlled synthesis of conductive and redox-active organic polymer nanosheets with tunable thickness and lateral size. Although the nanosheets consist of classical conductive polymers, such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), a new crystal-controlled polymerization approach provides 2D nanomaterials with enhanced and characteristic properties. Oxidative polymerization proceeds on the crystal surface of organic oxidants, such as quinone derivatives, with diffusion of the monomer vapor at low temperature under ambient pressure. The present method affords the control of lateral size and thickness of the nanosheets. The resultant nanosheets show the characteristic and enhanced properties originating from a 2D nature, such as the high structure anisotropy with aspect ratio around 104, enhanced conductivity 287 S cm-1, and specific charge-discharge capacitance 523 F g-1 at the current density of 18 A g-1. The organic nanosheets with conductivity and redox activity can be used as a building block for development of functional materials and devices. ©

• Access to Document (DOI)

Layer-by-layer manipulation of anisotropic nanoblocks: Orientation-switched superlattices through orthogonal stacking of: A and c directions

Nakagawa Y., Matsumoto R., Kageyama H., Oaki Y., Imai H.

Nanoscale (Nanoscale)  10 ( 27 ) 12957 - 12962 2018.07

ISSN  20403364

　View Summary

© 2018 The Royal Society of Chemistry. Nanometer-scale switching of the crystallographic orientation enables production of novel superlattices for exploration of new functions. Elaborate multilayered architectures of Mn3O4 nanocuboids were constructed by a convective self-assembly process. The nanometric cuboids were accumulated into orientation-switched superlattices on micrometric scales by controlling the a and c directions of the tetragonal crystal periodically. The orthogonal stacking of the tetragonal nanoblocks was achieved through the layer-by-layer manipulation process of two types of 2D arrays with different orientations.

• Access to Document (DOI)

Visualization and Quantitative Detection of Friction Force by Self-Organized Organic Layered Composites

Terada H., Imai H., Oaki Y.

Advanced Materials (Advanced Materials)  30 ( 27 )  2018.07

ISSN  09359648

　View Summary

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Visualization and quantitative detection of external stimuli are significant challenges in materials science. Quantitative detection of friction force, a mechanical stress, is not easily achieved using conventional stimuli-responsive materials. Here, the quantitative detection of friction force is reported, such as the strength and accumulated ammount, from the visible color of organic layered composites consisting of polydiacetylene (PDA) and organic amines without an excitation light source. The composites of the layered diacetylene monomer crystal and interlayer organic amine are synthesized through self-organization from the precursor solution. After topochemical polymerization, the layered composites based on PDA show tunable temperature-responsive and mechanoresponsive color-change properties depending on the types of interlayer amines. The layered composites are homogeneously coated on a filter paper. The change in color of the paper is quantitatively used to visualize the strength and accumulated amount of the applied friction force. Furthermore, writing pressure is measured by friction force using the paper device.

• Access to Document (DOI)

Multistage redox reactions of conductive-polymer nanostructures with lithium ions: potential for high-performance organic anodes

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

NPG Asia Materials (NPG Asia Materials)  10 ( 5 ) 397 - 405 2018.05

ISSN  18844049

　View Summary

© 2018, The Author(s). Conducive polymers have a wide range of applications originating from their π-conjugated systems. The redox reactions of conductive polymers with doping and dedoping of anions have been applied to cathodes for charge storage. In contrast, the redox reactions with cations have not been fully studied in anodes for charge storage. Here, we found that the nanostructures of conductive polymers, such as polypyrrole (PPy) and polythiophene (PTp) derivatives, have reversible redox reactions with cations. The PPy and PTp nanostructures acted as anodes with specific capacities of 157 and 44.4 mA h g–1, respectively, at a current density of 20 mA g–1. The introduction of a carboxy group to the pyrrole and thiophene rings enhanced the specific capacities up to 730 and 963 mAh g–1, respectively. The enhanced electrochemical properties were not observed in the bulk-size conductive polymers. The results suggest that conductive-polymer nanostructures have potential for developing metal-free, high-performance charge storage devices.

• Access to Document (DOI)

Nanoscale Mosaic Works: Tetragonal Lattices of Iso-Oriented Heterogeneous Nanocubes

Matsumoto R., Yamazaki H., Takasaki M., Oaki Y., Sato T., Imai H.

Langmuir (Langmuir)  34 ( 13 ) 4031 - 4035 2018.04

ISSN  07437463

　View Summary

© 2018 American Chemical Society. Tetragonal 2D lattices are spontaneously formed by the self-assembly of homogeneous nanocubes. However, ordered arrays consisting of differently sized rectangular nanoblocks have not been achieved because the regulation of the assembly is weakened by the combination of binary units. In the present work, ordered arrays comprising binary nanocubes were investigated using the combination of 10 nm Pt nanocubes and 20 nm BaTiO3 nanocubes. Heterogeneous but ordered 2D tetragonal lattices were successfully produced using differently sized rectangular nanoblocks. The highly ordered self-assembly in the heterogeneous system requires the matching of the size ratio of binary nanocubes with the buffer effect of oleic acid covering the building units.

• Access to Document (DOI)

Enhanced electrochemical properties of MgCo <inf>2</inf> O <inf>4</inf> mesocrystals as a positive electrode active material for Mg batteries

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

Journal of Alloys and Compounds (Journal of Alloys and Compounds)  739   793 - 798 2018.03

ISSN  09258388

　View Summary

© 2017 Elsevier B.V. Mesocrystals of MgCo 2 O 4 spinel oxide exhibiting porous and single-crystalline structures were synthesized by the reaction of magnesium ions with calcite-type CoCO 3 mesocrystal precursors grown in micrometer-sized agar gels. The mesocrystals consisted of single-crystalline porous grains measuring 200–300 nm in diameter with iso-oriented subunits of approximately 10 nm. The single-crystalline porous framework structure of the MgCo 2 O 4 mesocrystals enhanced the discharge capacity and positively shifted the discharge potential compared with MgCo 2 O 4 nanoparticles prepared by the inverse co-precipitation method. These enhancements were attributed to a combination of the high specific surface areas of the mesocrystals and the high connectivity of the magnesium ion diffusion paths in the solid-phase.

• Access to Document (DOI)

Enhanced Electrochemical Properties of MgCo2O4 Mesocrystals as a Positive Electrode Active Material for Mg Batteries

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

Journal of Alloys and Compounds 739   793 - 798 2018.03

Research paper (scientific journal), Joint Work, Accepted

• Access to Document (DOI)

Few-layered titanate nanosheets with large lateral size and surface functionalization: Potentials for controlled exfoliation of inorganic-organic layered composite

G. Nakada、H. Imai, Y. Oaki

Chemical Communications (Chemical Communications)  54 ( 3 ) 244 - 247 2018.01

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

　View Summary

Herein, few-layered titanate nanosheets with a large lateral size and surface functionalization were synthesized via the exfoliation of precursor layered composites in an organic medium. The combination of layered compounds, intercalated guests, and dispersion media has the potential for the synthesis of nanosheets with controlled lateral sizes, thicknesses, and surface chemistry through exfoliation.

• Access to Document (DOI)