Bottom-up synthesis of titanate nanoflakes and nanosheets in ionic liquid solvents

Abstract

The bottom-up synthesis of titanate nanoflakes in ionic liquid solvents was carried out by heating mixtures of titanium isopropoxide (TIP), 40% tetrabutylammonium hydroxide (TBAOH) aqueous solution, and ionic liquids with a molar ratio of 1TIP : xTBAOH : (10 − x)ionic liquid : 22xH₂O. Using tetrabutylammonium chloride (TBACl) as an ionic liquid, layered titanate crystals with TBA⁺ as an interlayer cation were synthesized by the acid–base reaction of TBAOH and titanic acid, which was formed by the hydrolysis of TIP, in the range of 0.5 ≤ x ≤ 2.5. The use of 1,2,3-trimethylimidazolium (mmmIm⁺) methylsulfate as an ionic liquid also led to the formation of layered titanate with mmmIm⁺ ions as an interlayer cation, although the amount of layered titanate was smaller than that in the case using TBACl. The solvent exchange from ionic liquids to aqueous solution by dialyzing the products caused the exfoliation of layered titanate, providing titanate nanoflakes with a lateral size of about 3 nm. Moreover, by comparing the bottom-up syntheses of layered titanate in ionic liquids and in water, the crystallization behavior of layered titanate was examined. The amount of TBA⁺ or mmmIm⁺, which acts as an interlayer cation of layered titanate, influenced the nucleation of layered titanate rather than the solubility of titanate. A significant amount of TBA⁺ or mmmIm⁺ in the ionic liquid solvents accelerated the nucleation of layered titanate crystals, providing small titanate nanoflakes. Furthermore, the formed titanate species existed as exfoliated nanosheets or nanoflakes in water solvent, but were present as layered titanate crystals in ionic liquids. However, this difference in the state of titanates had little influence on the growth rates of layered titanate crystals. Thus, the bottom-up synthesis of layered titanates in ionic liquids was useful for the formation of very small titanate nanoflakes.