Synthesis of Porous Metal Oxides Utilizing Nanosheet Colloid with Liquid Crystallinity
Authors: Adachi Yusuke , Muramatsu Keisuke , Mouri Emiko , Nakato Teruyuki , Sugimoto Wataru 
Published: December 2024

Activated carbon, a high specific surface area microporous carbon(>2000 m2 g–1), has been used as the electrode material for electrochemical double layer capacitors.1 The structure of activated carbon is characterized by a three-dimensional network consisting of graphene-like layers. Preparation of metal oxides with a similar porous structure is anticipated to behave as pseudocapacitive electrode materials with high energy density. However, the hierarchical porous structure of activated carbon is obtained by gas-phase or chemical activation treatment and is applicable only to carbonaceous materials. Thus, an alternative strategy is necessary to prepare metal oxides with porous architectures similar to activated carbon.
Assembly of inorganic oxide nanosheets is one of the promising methods to prepare hierarchical porous metal oxides. Here, we focus on the phenomenon where inorganic oxide nanosheets form liquid crystal structures in colloids and align macroscopically under an electric field. A highly concentrated hexaniobate nanosheet ([Nb6O17]4
–(ns)) colloid is known to exhibit liquid crystalline behavior.2 Additionally, the orientation of nanosheet with micro-mesoscale sheet spacing (< tens of nm) forms a hierarchical macroscopic structure (< several hundred µm) under an electric field.3 Therefore, the liquid crystalline structure and its responsiveness to electric fields in nanosheet colloids are effective for controlling the hierarchically nanosheet-assembled structure. In this study, we applied the freeze-drying method of nanosheet liquid crystals (NLC) to transfer the structure within the fluid nanosheet colloid to the dried aggregates (Figure 1).
[Nb6O17]4
–(ns) colloids with developed liquid crystal domains form stripe-shaped macrostructures by applying an alternating current (AC) electric field (10 Vp–p, 50 kHz) perpendicular to the direction of gravity.3 Figure 1(a) shows the polarized optical microscopy (POM) image of the stripe-shaped macrostructures with a width of 100 µm and intervals of 200 µm composed of [Nb6O17]4
–(ns). In the birefringent regions, NLC oriented perpendicular to the direction of the AC electric field and parallel to the direction of the gravity. After applying an electric field to the cell, the cell was immersed in liquid nitrogen with applying the electric field and then dried in vacuum, resulting in the formation of a sponge-like nanosheet aggregate between the electrodes. The bright-field optical microscope image of the freeze-dried aggregates shows the similar stripe-shaped macrostructures to that of NLC under the electric field (Figure 1(b)). Therefore, the macrostructure of NLC oriented in an electric field can be transferred into the porous nanosheet aggregates by freeze-drying method. These results indicate that the liquid crystalline and electric field responsive properties of nanosheet colloids are useful for structural design of nanosheet-assembled materials.

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Figure 1