ABSTRACT
Colloidal clay nanosheets obtained by the delamination of layered crystals of smectite-type clay minerals in water form liquid crystals because of their shape anisotropy. Loading of organic dyes onto the liquid crystalline clay nanosheets will enable novel photonic materials, where photofunctions of the loaded dye are controlled by the liquid crystallinity of the clay nanosheets. However, adsorption of organic dyes onto the nanosheets renders the nanosheet surfaces hydrophobic, and consequently, colloidal stability of the nanosheets is lost. In this study, this drawback is overcome by sandwiching cationic stilbazolium dyes between a pair of synthetic fluorohectorite nanosheets. This is realized by the preparation of stilbazolium-clay second-stage intercalation compounds characterized by intercalation of dye cations into every other interlayer space of the hectorite clay, where nonintercalated interlayer spaces are occupied by Na+ ions. The second-stage intercalation compounds are obtained by partial ion exchange of mother clay mineral incorporating Na+ ions in all of the interlayer spaces and delaminated from the Na+-containing interlayer spaces to form clay nanosheets sandwiching the dye molecules. Aqueous colloids of the dye-sandwiching clay nanosheets form colloidal liquid crystals, and the dye-sandwiching liquid crystalline clay nanosheets respond to an applied AC electric field to be aligned parallel to the electric field. The assembled structure of the dye-sandwiching clay nanosheets under the electric field is characterized by aligned discrete clay platelets, which is somewhat different from that of a colloidal liquid crystal of clay nanosheets without dye loading characterized by macroscopic liquid crystalline domains up to submillimeters. The electric alignment of the clay nanosheets induces alteration of light absorption of the sandwiched stilbazolium molecules, which verifies a strategy of constructing stimuli-responsive photonic materials of clay-organic hybrids.
ABSTRACT
Inorganic nanotubes have attracted much attention due to their unique physicochemical properties. Nanotubes can be prepared by scrolling exfoliated nanosheets under ambient conditions. However, how the nanosheet scrolled in its colloidal state has not been experimentally visualized. In this paper, we directly observed the scrolling process of nanosheets upon adsorption of organic cations. Exfoliated flat nanosheets of niobate and clay in aqueous colloids were found to scroll by adding organic cations, such as exfoliation reagents, to the colloids. Employment of cationic stilbazolium dye enabled in situ observation of the dye adsorption and scrolling by optical microscopy based on changes in color and morphology of the nanosheets. The scrolling was promoted for nanosheets adsorbed with a stilbazolium dye with a longer alkyl chain, suggesting that the interaction between the hydrophobic parts of the dye cations is the driving force of the scrolling. This finding should encourage research on the formation of nanotubes from nanosheets and also provides important guidelines for the selection of appropriate exfoliation reagents when exfoliating nanosheets from layered crystals.