Orientational Control and Photocatalytic Properties of Liquid Crystals Composed of Titanium Oxide Nanosheets

Abstract

Liquid crystals composed of colloidally dispersed inorganic nanosheets were prepared by exfoliation of layered titanium and niobium oxides, and orientation of the nanosheets was controlled. Dispersed state of the nanosheets in the liquid crystalline colloids prepared from K₄Nb₆O₁₇ was irreversibly modified with concentrating the colloid, the modification which was monitored at different hierarchies: molecular to sub-mm level. A cationic porphyrin was added to the colloid as a spectroscopic probe, and its spectral profile successfully detected the modification of the dispersed state; the spectral response indicated alteration of molecular-level environment given by the nanosheets. Macroscopic observations with fluorescence optical microscopy and TEM showed buckling and aggregation of the nanosheets upon the concentration, indicating macroscopic change of the dispersed state of nanosheets. Moreover, macroscopic ordering of the nanosheets was aligned with external forces. Nanosheets were successfully aligned homeotropically on various substrates at cm level. This alignment was transformed to homogeneous under an external AC electric field. On the other hand, the colloidally dispersed nanosheets exhibited photocatalytic activity that is an intrinsic property of the titanium and niobium oxides. The nanosheet colloids generated hydrogen under UV irradiation. Sacrificial hole scavengers were unnecessary for the hydrogen generation because propylammonium ions used as the exfoliating reagent were present near the nanosheets to work as the hole scavenger.