Molecular nanosheets of quasi-TiO2: preparation and spontaneous reassembling

Abstract

This paper reviews our recent progress in synthesis, optical properties and reassembling behaviour of nanosheet-crystallites of quasi-TiO₂. Processing to thin anatase flakes from them is also presented. The colloidal nanosheets were synthesized by soft-chemically exfoliating a layered protonic titanate of $\text{H}{}_{\mathrm{x}}\text{Ti}{}_{\mathrm{2-x/4}}\text{□}{}_{\mathrm{<mtext>x</mtext><mtext>/4</mtext>}}\text{O}{}_4\text{·}\text{H}{}_2\text{O}$ (x∼0.7, □: vacancy). The action of bulky quarternary alkylammonium ion in large excess induced osmotic swelling involving Δd>3 nm. Lowering the dose of the electrolyte promoted further interlayer expansion leading to delamination into elementary host layers. The resulting nanosheets exhibited a sharp UV-visible absorption band which is considerably blue-shifted compared to bulk TiO₂. These distinct features and a well-structured photoluminescence may be interpreted as a reflection of their molecular nature. Slow drying of the colloids brought about spontaneous reassembling of the nanosheets, which was initiated by association of a limited number of the nanosheets with a very large intersheet spacing of >10 nm and finally yielded a well-ordered lamellar structure. Freeze-drying of the suspension produced the lamellar phase in an open microstructure which can be converted into thin flaky particulates of anatase tens nanometers thick. The material, an assembly of the flakes, possessed a high specific surface area (40–110 m² g^-1) associated with mesopores.