Electric-Alignment Immobilization of Liquid Crystalline Colloidal Nanosheets with the Aid of a Natural Organic Polymer

Inorganic nanosheets obtained by exfoliation of a layered crystal in water form colloidal liquid crystals, and their alignment can be controlled by an electric field. In order to realize the immobilization of the electrically aligned niobate nanosheets without external forces, an aqueous gelator, agar, is introduced to the niobate nanosheet system to utilize the thermosensitive sol–gel transition property of agar. Alignment of nanosheets in a niobate–agar system is performed by applying an electric field above the sol–gel transition temperature, and then, the sample is cooled down, followed by cooling below the transition temperature with the electric field turned off. The aligned structure is kept for more than 24 h after the removal of the electric field. The concentration of agar is a key parameter for both the orientation of nanosheets and the retention of the orientation

Development of Structural Color by Niobate Nanosheet Colloids

Inorganic nanosheets obtained by exfoliation of layered crystals of hexaniobate in water form colloidal liquid crystals. We found that they develop various structural colors by moderating nanosheet concentration and ionic atmosphere

Preparation of Cellulose Nanocrystals based Core-Shell Particles with Tunable Component Location

We report a versatile method for preparing a particulate composite based on cellulose nanocrystals (CNCs) and polyethylene glycol (PEG) via a self-organized precipitation method. The particulate composite had a core–shell structure, and depending on the molecular weight of the PEG, two types of particulates could form: one with CNCs as the core and the other with CNCs as the shell

Photoinduced electron transfer in semiconductor–clay binary nanosheet colloids controlled by clay particles as a turnout switch

Although semiconductor photocatalysis has been investigated actively for a long time, control of dark processes successive to electron transfer from photocatalysts is almost unexplored compared with designing photocatalysts themselves. The present study proposes employment of clay particles as for controlling the dark processes independently of semiconductor photocatalyst particles. We employed niobate–clay binary nanosheet colloids, where colloidal niobate and clay nanosheets are spatially separated at a micrometer level. Niobate nanosheets worked as the semiconductor photocatalyst that released electrons upon UV excitation, and clay nanosheets worked as the turnout switch of the released electrons to determine their destination. When methylviologen (MV2+) molecules that accept the electrons released from niobate were adsorbed on clay nanosheets, reduction of MV2+ predominantly occurred, and hydrogen was little evolved from the colloid. When Pt nanoparticles were deposited on clay nanosheets, photocatalytic hydrogen evolution occurred because Pt loaded on the clay nanoparticles played a role of cocatalyst. When MV2+ and Pt were co-loaded on clay nanosheets, both of MV2+ reduction and hydrogen evolution occurred competitively. The photocatalytic hydrogen evolution carried out by stirring the colloid sample was worse than that conducted without stirring, which indicated positive contribution of the spatial separation of photocatalytic niobate and cocatalytic clay nanosheets

OPTICAL TRAPPING AND ORIENTATION MANIPULATION OF 2D INORGANIC MATERIALS USING A LINEARLY POLARIZED LASER BEAM

Because inorganic nanosheets, such as clay minerals, are anisotropic, the manipulation of nanosheet orientation is an important challenge in order to realize future functional materials. In the present study, a novel methodology for nanosheet manipulation using laser radiation pressure is proposed. When a linearly polarized laser beam was used to irradiate a niobate (Nb6O 4-17) nanosheet colloid, the nanosheet was trapped at the focal point so that the in-plane direction of the nanosheet was oriented parallel to the propagation direction of the incident laser beam so as to minimize the scattering force. In addition, the trapped nanosheet was aligned along the polarization direction of the linearly polarized laser beam

Optical manipulation of a single clay nanosheet hybridized with a porphyrin derivative

The effect of hybridization of a clay fluorohectorite (FHT) nanosheet with a π-conjugated organic compound, α,β,γ,δ-tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluene-sulfonate (TMPyP), on its optical manipulation is investigated. Although the hybridized FHT is optically trapped essentially in the same manner as that of neat FHT, the hybridization with TMPyP allows for manipulation of FHT with lower laser intensity or a shorter period, or both. This is ascribed to the larger refractive index and polarizability of TMPyP compared with neat FHT

Radiation-Pressure-Induced Hierarchical Structure of Liquid-Crystalline Inorganic Nanosheets

Although hierarchical assemblies of colloidal particles add novel structure-based functions to systems, few local and on-demand colloidal structures have been developed. We have combined the colloidal liquid crystallinity of two-dimensional inorganic particles and laser radiation pressure to obtain a large hierarchical and local structure in a colloidal system. The scattering force of the laser beam converted the parallel nanosheet alignment to the direction of the incident laser beam. At the focal point, the nanosheet orientation depends on the electric field of the polarized laser beam. In contrast, a giant tree-ring-like nanosheet texture of more than 100 μm, and which is independent of the polarization direction, was organized at the periphery of the focal point. This organization resulted from a cooperative effect between the liquid-crystalline nanosheets, which indicates an effectiveness of optical manipulation to construct hierarchical colloidal structures with the aid of interparticle interactions

Microscope Observation of Morphology of Colloidally Dispersed Niobate Nanosheets Combined with Optical Trapping

Although inorganic nanosheets prepared by exfoliation (delamination) of layered crystals have attracted great attention as 2D nanoparticles, in situ real space observations of exfoliated nanosheets in the colloidally dispersed state have not been conducted. In the present study, colloidally dispersed inorganic nanosheets prepared by exfoliation of layered niobate are directly observed with bright-field optical microscopy, which detects large nanosheets with lateral length larger than several micrometers. The observed nanosheets are not strictly flat but rounded, undulated, or folded in many cases. Optical trapping of nanosheets by laser radiation pressure has clarified their uneven cross-sectional shapes. Their morphology is retained under the relation between Brownian motion and optical trapping

Mesoscopic Architectures Made of Electrically Charged Binary Colloidal Nanosheets in Aqueous System

Inorganic layered materials can be converted to colloidal liquid crystals through exfoliation into inorganic nanosheets, and binary nanosheet colloids exhibit rich phase behavior characterized by multiphase coexistence. In particular, niobate–clay binary nanosheet colloids are characterized by phase separation at a mesoscopic (∼several tens of micrometers) scale whereas they are apparently homogeneous at a macroscopic scale. Although the mesoscopic structure of the niobate–clay binary colloid is advantageous to realize unusual photochemical functions, the structure itself has not been clearly demonstrated in real space. The present study investigated the structure of niobate–clay binary nanosheet colloids in detail. Four clay nanosheets (hectorite, saponite, fluorohectorite, and tetrasilisic mica) with different lateral sizes were compared. Small-angle X-ray scattering (SAXS) indicated lamellar ordering of niobate nanosheets in the binary colloid. The basal spacing of the lamellar phase was reduced by increasing the concentration of clay nanosheets, indicating the compression of the liquid crystalline niobate phase by the isotropic clay phase. Scattering and fluorescence microscope observations using confocal laser scanning microscopy (CLSM) demonstrated the phase separation of niobate and clay nanosheets in real space. Niobate nanosheets assembled into domains of several tens of micrometers whereas clay nanosheets were located in voids between the niobate domains. The results clearly confirmed the spatial separation of two nanosheets and the phase separation at a mesoscopic scale. Distribution of clay nanosheets is dependent on the employed clay nanosheets; the nanosheets with large lateral length are more localized or assembled. This is in harmony with larger basal spacings of niobate lamellar phase for large clay particles. Although three-dimensional compression of the niobate phase by the coexisting clay phase was observed at low clay concentrations, the basal spacing of niobate phase was almost constant irrespective of niobate concentrations at high clay concentrations, which was ascribed to competition of compression by clay phase and restoring of the niobate phase

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