Associate Professor, Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology
Colloidally dispersed nanosheets prepared from layered oxides: their flexible ordering and application to novel soft-nano-materials.
We are investigating materials chemistry of exfoliated layered oxides. Inorganic nanosheets prepared by exfoliation of layered materials have recently attracted attention as modules for constructing novel nanoassemblies. However, there is little knowledge about the colloidal properties of the nanosheets, and we are interested in the special properties of colloidally dispersed oxide nanosheets and their application to novel “soft-nano-materials”.
We recently found that sols of exfoliated niobates exhibit liquid crystallinity (Fig. 1) and sol-gel transition (Fig. 2). Although colloids of anisotropic particles are known to form lyotropic liquid crystals, colloidally dispersed nanosheets give stable liquid crystalline phases (anomalously low transition concentration to the liquid crystalline phase) and a macroscopic arrangement induced by gravitational force. These properties are rationalized by the very high anisotropy of the nanosheets. The niobate nanosheet colloids can also undergo an acid-induced transition to physical gels. Moreover, we have prepared multi-component nanosheets sols by mixing the niobate and clay nanosheet colloids. They are microscopically phase-separated while they show apparently homogeneous liquid crystallinity on the macroscopic scale. These properties are due to the colloidally dispersed states of nanosheets, and the flexible ordering of the dispersed nanosheets is responsible for the unusual behavior which cannot be observed on the solid layered oxides.
The next stage of our investigation is the fabrication of novel “soft-nano-hybrids” by combining functional molecules with the flexibly ordered nanosheets.
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| Fig. 1 Large Image |
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| Schematic representation of the formation of a liquid crystal through exfoliation of layered niobates. Dispersed nanosheets are orientationally ordered to exhibit liquid crystallinity. The photographs show the colloids between crossed polarizers indicating birefringence. |
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| Fig. 2 Large Image |
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| Variation of the kinematic viscosity of a niobate nanosheet colloid with pH. Acidification induces gelation of the colloid around pH 1.5. The photograph indicates the colloids before (right) and after (left) the gelation. |
Relevant papers
- Miyamoto, N. & Nakato, T.
Liquid Crystalline Colloidal System Obtained by Mixing Niobate and Aluminosilicate Nanosheets: A Spectroscopic Study Using a Probe Dye
Langmuir 19, 8057-8064 (2003). - Nakato, T., Miyamoto, N., Harada, A. & Ushiki, H.
Sol-Gel Transition of Niobium Oxide Nanosheet Colloids: Hierarchical Aspect of a Novel Macroscopic Property Appearing in Colloidally Dispersed States of Layered Niobate K4Nb6O17
Langmuir 19, 3157-3163 (2003). - Miyamoto, N. & Nakato, T.
Liquid Crystalline Nature of K4Nb6O17 Nanosheet Sols and Their Macroscopic Alignment
Adv. Mater. 14,1267-1270 (2002).
