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Papers - Oaki, Yuya

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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 2021.06

ISSN 13597345

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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 2021.06

ISSN 20403364

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 2021.06

ISSN 07437463

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 2021.05

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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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 2021.04

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 ) 2021.04

ISSN 09359648

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 2021.03

ISSN 20507488

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 2021.02

ISSN 20403364

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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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 2021.01

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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Materials informatics for 2d materials combined with sparse modeling and chemical perspective: Toward small-data-driven chemistry and materials science

Oaki Y., Igarashi Y.

Bulletin of the Chemical Society of Japan (Bulletin of the Chemical Society of Japan) 94 ( 10 ) 2410 - 2422 2021

ISSN 00092673

Application of data-scientific approaches to conventional sciences, such as chemo-informatics, bio-informatics, and materials informatics (MI), has attracted much interest toward datadriven research. The concept enables accelerated discovery of new materials, enhancement of performance, and optimization of processes. However, sufficient bigdata is not always prepared to apply to machine learning. For example, experimental scientists have their own small data including success and failure in their laboratory, whether in academia or industry. If such small data is effectively utilized with a data-scientific approach, research activities can be accelerated without energy, resource, and cost consumption. This account focuses on MI for small data, a recent concept for application of small data, with introduction of model cases, such as control of exfoliation processes to obtain 2D materials. Combination of machine learning and chemical perspective is effective for construction of straightforward and interpretable predictors through the extraction of a limited number of descriptors from small dataset. Although the prediction accuracy is not so precise, the model has enough accuracy to be a guideline reducing the number of the next experiments. The present MI for small data opens potentials of small-data-driven chemistry and materials science.

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Exfoliation chemistry of soft layered materials toward tailored 2D materials

Oaki Y.

Chemistry Letters (Chemistry Letters) 50 ( 2 ) 305 - 315 2021

ISSN 03667022

Exfoliation of layered materials provides nanosheets, such as monolayers and few-layers. In recent years, nanosheets have attracted much interest as two-dimensional (2D) materials for their diverse properties and applications originating from the anisotropic characteristic structures. Exfoliation methods have been developed depending on the types of interactions between the layers, such as van der Waals and electrostatic interactions. The present review focuses on exfoliation chemistry based on rigid and soft natures of the layered materials. The rigid inorganic layered compounds are converted to the soft layered materials with interaction of organic guests. The surface-functionalized nanosheets are obtained by exfoliation of the soft layered composites with dispersion in organic media. The exfoliation behavior is governed by the flexibility of the precursor layered materials, i.e. interaction between the interlayer guest and dispersion medium. Although exfoliation is generally an uncontrollable top-down process, materials informatics on our own experimental small data assists elucidation of the control factors toward tailored 2D materials. The exfoliation schemes are applied to a variety of layered materials. The present review shows potential new insights for exfoliation chemistry of soft layered materials.

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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 2020.11

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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Intercalation and flexibility chemistries of soft layered materials

Oaki Y.

Chemical Communications (Chemical Communications) 56 ( 86 ) 13069 - 13081 2020.11

ISSN 13597345

Layered materials, alternate stackings of two or more components, are found in a wide range of scales. Chemists can design and synthesize layered structures containing functional units. The soft-type layered materials exhibit characteristic dynamic functions originating from two-dimensional (2D) anisotropy and structure flexibility. This feature article focuses on "intercalation"and "flexibility"as two new perspectives for designing soft layered materials. Intercalation of guests is a characteristic approach for design of layered structures. Flexibility is an important factor to control the dynamic functions of the layered structures. As a model case, the intercalation-induced tunable stimuli-responsive color-change properties of layered polydiacetylene (PDA) are introduced to study the impact of the intercalation and flexibility on the dynamic functions. Recently, layered materials have drastically expanded the research area from conventional rigid inorganic compounds to new self-assembled nanostructures consisting of organic components, such as polymers, metal-organic frameworks, and covalent-organic frameworks. These new layered architectures have potentials for exhibiting dynamic functions originating from the structure flexibility beyond the static properties originating from classical intercalation and host-guest chemistries. Therefore, intercalation and flexibility chemistries of soft layered materials are regarded as new perspectives for design of advanced dynamic functional materials. This journal is

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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

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

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

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

ISSN 20416520

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

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

Kyohei Noda Yasuhiko Igarashi Hiroaki Imai Yuya Oaki

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

Research paper (scientific journal), Joint Work, Accepted

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

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

Minami Nakamitsu, Hiroaki Imai, Yuya Oaki

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

Research paper (scientific journal), Joint Work, Accepted

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

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

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

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

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

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

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Biomimetic morphology–controlled anhydrous guanine via amorphous intermediate

Tsubasa Kimura, Mihiro Takasaki, Yuya Oaki, Hiroaki Imai

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

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

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

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

Mizuguchi R., Imai H., Oaki Y.

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

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

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