Nobuyoshi Miyamoto

Adsorption and aggregation of a cationic cyanine dye on layered clay minerals

Abstract The adsorption and aggregation of a cationic cyanine dye, 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine; abbreviated as PIC), on various clays have been investigated. The PIC formed J-aggregates on Na-montmorillonite and synthetic Na-fluor-tetrasilicic mica (TSM), while they distributed molecularly on synthetic Na-saponite and synthetic Na-hectorites. The particle size of the clays was considered to be a dominant factor to control the aggregation of PIC. The J-aggregate formation was suppressed when tetramethylammonium–montmorillonite was employed as a host material. Ethylene glycol in the suspension was also responsible for the suppression of J-aggregate formation of PIC. Thus, the aggregation of PIC was effectively controlled by the species surrounding the PIC cations as well as the nature of clays.

Unusually stable ~100-fold reversible and instantaneous swelling of inorganic layered materials

Cells can swell or shrink in certain solutions; however, no equivalent activity has been observed in inorganic materials. Although lamellar materials exhibit increased volume with increase in the lamellar period, the interlamellar expansion is usually limited to a few nanometres, with a simultaneous partial or complete exfoliation into individual atomic layers. Here we demonstrate a large monolithic crystalline swelling of layered materials. The gallery spacing can be instantly increased ~100-fold in one direction to ~90 nm, with the neighbouring layers separated primarily by H2O. The layers remain strongly held without peeling or translational shifts, maintaining a nearly perfect three-dimensional lattice structure of >3,000 layers. First-principle calculations yield a long-range directional structuring of the H2O molecules that may help to stabilize the highly swollen structure. The crystals can also instantaneously shrink back to their original sizes. These findings provide a benchmark for understanding the exfoliating layered materials.

Gigantic Swelling of Inorganic Layered Materials: A Bridge to Molecularly Thin Two-Dimensional Nanosheets

Platy microcrystals of a typical layered material, protonated titanate, have been shown to undergo an enormous degree of swelling in aqueous solutions of various amines, including tertiary amines, quaternary ammonium hydroxides, and primary amines. Introducing these solutions expanded the crystal gallery height by up to ~100-fold. Through systematic analysis, we determined that ammonium ion intercalation is predominantly affected by the acid-base equilibrium and that the degree of swelling or inflow of H2O is controlled by the osmotic pressure balance between the gallery and the solution environment, both of which are relatively independent of electrolyte identity but substantially dependent on molarity. In solutions of tertiary amines and quaternary ammonium hydroxides, the uptake of ammonium ions increases nearly linearly with increasing external concentration before reaching a saturation plateau, i.e., ~40% relative to the cation-exchange capacity of the crystals used. The only exception is tetrabutylammonium ions, which yield a lower saturation value, ~30%, owing to steric effects. The swelling behaviors in some primary amine solutions differ as a result of the effect of attractive forces between amine solute molecules on the solution osmotic pressure. Although the swelling is essentially colligative in nature, the stability of the resultant swollen structure is heavily dependent on the chemical nature of the guest ions. Intercalated ions of higher polarity and smaller size help stabilize the swollen structure, whereas ions of lower polarity and larger size lead readily to exfoliation. The insight gained from this study sheds new light on both the incorporation of guest molecules into a gallery of layered structures in general and the exfoliation of materials into elementary single-layer nanosheets.

Exfoliation and film preparation of a layered titanate, Na2Ti3O7, and intercalation of pseudoisocyanine dye

Fine powders of propylammonium-exchanged Na2Ti3O7 were successfully converted to transparent thin films by swelling (and exfoliating) the material in water and subsequent casting and drying the obtained titanate sol on a substrate. The dispersion of the material was promoted by sonication and hydrothermal treatment of the sol. A self-standing film was also obtained by depositing the sol on a Teflon substrate and subsequent peeling off from the substrate by soaking in acetone. Taking advantage of the cation exchange capability of the films thus obtained, a cationic cyanine dye (pseudoisocyanine) was intercalated with retaintion of the transparency and morphology of the films. Polarized visible spectra revealed that the intercalated dyes were present as J-aggregates with the dipole moment oriented parallel to the film surface. The present methodology is a convenient way to fabricate titanate–dye hybrids as films.

Intercalation of a cationic azobenzene into montmorillonite

The intercalation of a cationic azobenzene derivative into the interlayer space of montmorillonite has been achieved by ion exchange between sodium-montmorillonite and p-(ω-dimethyl-hydroxyethylammonioethoxy)-azobenzene bromide. X-ray diffractometry and elemental analysis indicated that the dye cations were intercalated. The microstructures of the montmorillonite-dye intercalates were controlled by the amount of dye loaded as evidenced by the variation in basal spacings and visible absorption spectra of the materials. The intercalated azo dyes exhibit reversible trans-to-cis photoisomerization under UV and visible light irradiation.

Mesoporous silica as smart inorganic filler: preparation of robust silicone rubber with low thermal expansion property

Here we demonstrate the importance of mesoporous silica as a smart inorganic filler. Silicone rubber with low thermal expansion is realized by doping mesoporous silica particles. For the preparation of mesoporous silica/silicone composites, an unpolymerized silicone component is diluted by a volatile organic solvent and then is penetrated into the mesopores by a capillary force. After penetration, the organic solvent is evaporated completely. Based on theoretical calculations, the estimated degree of mesopore filling is more than 90%. Mesoporous silica/silicone composites show much lower Coefficient of Linear Thermal Expansion (CTE) and higher hardness values than do silicone composites with nonporous silica particles.

Rapid fabrication of mesoporous titania films with controlled macroporosity to improve photocatalytic property.

Hierarchically porous titania films were fabricated by dual templating using a triblock copolymer such as Pluronic F127 and polystyrene (PS) beads, affording mesoporous films with controlled macroporosity. The presence of the triblock copolymer in the precursor solutions suppressed a regular accumulation of spherical PS beads, and PS-derived macropores could be dispersed over the whole mesoporous titania film through rapid fabrication by spin-coating. Some of the macropores were clustered, but the presence of the large spaces was important for keeping the mesostructure after calcination. Photodegradation of methylene blue (MB) was investigated by using the photoactive anatase films. The photodegradation of MB over the porous anatase films was accelerated by effective diffusion of MB molecules in the PS-derived macropores, but it was important for improving photocatalytic performance to regulate the balance between the effectiveness of the diffusion in the macropores and the decrease of the surface area from the embedded macropores, as well as the reduction in the transparency of the porous films.

Surfactant-Directed Synthesis of Mesoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts

Palladium (Pd) films with perpendicularly aligned mesochannels are expected to provide fascinating electrocatalytic properties due to their low diffusion resistance and the full utilization of their large surface area. There have been no studies on such mesoporous metal films, because of the difficulties in controlling both the vertical alignment of the molecular template and the crystal growth in the metallic pore walls. Here we report an effective approach for the synthesis of mesoporous Pd films with mesochannels perpendicularly aligned to the substrate by an elaborated electrochemical deposition. The films show a superior electrocatalytic activity by taking full advantage of the perpendicularly aligned mesochannels.

Liquid Crystalline Behavior and Related Properties of Colloidal Systems of Inorganic Oxide Nanosheets

Inorganic layered crystals exemplified by clay minerals can be exfoliated in solvents to form colloidal dispersions of extremely thin inorganic layers that are called nanosheets. The obtained “nanosheet colloids” form lyotropic liquid crystals because of the highly anisotropic shape of the nanosheets. This system is a rare example of liquid crystals consisting of inorganic crystalline mesogens. Nanosheet colloids of photocatalytically active semiconducting oxides can exhibit unusual photoresponses that are not observed for organic liquid crystals. This review summarizes experimental work on the phase behavior of the nanosheet colloids as well as photochemical reactions observed in the clay and semiconducting nanosheets system.

Polymeric Micelle Assembly with Inorganic Nanosheets for Construction of Mesoporous Architectures with Crystallized Walls

Here we propose a novel way to construct mesoporous architectures through evaporation-induced assembly of polymeric micelles with crystalline nanosheets. As a model study, we used niobate nanosheets exfoliated by the direct reaction of K4Nb6O17⋅3 H2O crystals with an aqueous solution of propylamine. The electrostatic interaction between negatively charged nanosheets and positively charged polymeric micelles enable us to form composite micelles with the nanosheets. Removal of the micelles by calcination results in robust mesoporous oxides with the original crystalline structure.