Electrooptic Response of Colloidal Liquid Crystals of Inorganic Oxide Nanosheets Prepared by Exfoliation of a Layered Niobate

Colloidally dispersed inorganic nanosheets prepared by exfoliation of layered crystals form lyotropic liquid crystals where highly anisotropic nanosheets with a constant thickness of ca. 1 nm and lateral dimension up to several micrometers work as mesogen. We investigated electrooptic properties of the niobate nanosheet colloids yielded by exfoliation of layered hexaniobate K<sub>4</sub>Nb<sub>6</sub>O<sub>17</sub>. The nanosheets initially oriented parallel to the surface of flat substrate were aligned parallel to the direction of the applied ac electric fields, resulting in electric birefringence of the colloids. The liquid crystalline colloids consisting of large nanosheets with an average lateral size around 2 μm showed a sharp response to an electric field with a low threshold voltage (<4 × 10<sup>2</sup> V cm<sup>–1</sup>) and the consequent remarkable electrooptic effect, which is in contrast to the isotropic colloid composed of a small sub-millimeter nanosheet that exhibited only a weak electrooptic effect with a high threshold (10<sup>3</sup> V cm<sup>–1</sup>). Retention of the electrically induced alignment was observed for the LC colloids with high nanosheet concentrations. The response much slower than conventional thermotropic liquid crystals reflects the micrometer-level size of the mesogenic nanosheets. Whereas the two-dimensional nature of the nanosheets did not induce the formation of monodomain by applying only one-dimensional electric field, repeated application and consequent backflow in the measurement cell reoriented the nanosheets into monodomain.