Osmotic Swelling to Exfoliation. Exceptionally High Degrees of Hydration of a Layered Titanate

Abstract

This paper describes the swelling/exfoliation process in which a layered protonic titanate, HxTi2-x/4□x/4O4·H2O (x ∼ 0.7; □, vacancy), undergoes high degrees of hydration in aqueous media containing tetrabutylammonium (abbreviated as TBA hereafter) ions. A high content of electrolyte (25-fold excess of TBA ions to the exchangeable protons in the titanate) induced the formation of an osmotically swollen hydrate yielding a well-defined basal diffraction series. Intersheet separation of 4.2 nm at this stage expanded further to exceed 10 nm with reducing the dose of TBA ions. Along this swelling, exfoliation into single sheets became prevalent while the swollen component declined. This was followed by X-ray diffraction data for wet colloids centrifuged from suspensions that revealed a broad diffraction envelope besides the sharp basal reflections. Profile simulations demonstrated that the hump is attributable to scattering from a pile of exfoliated nanosheets and that its appearance is diagnostic for delamination. The TBA content approximately half-equivalent to the exchangeable protons was found to be a threshold for delamination below which the usual intercalation reaction occurred. This threshold corresponds to stoichiometry where the nanosheet surface is effectively covered with TBA ions. The UV−visible spectra for colloidal suspensions revealed an absorption peak at 265 nm, the intensity of which increased until the TBA content decreased to 5-fold excess of an equivalent of the exchange capacity. The background varied in a contrary manner. These changes can be interpreted as a reflection of a process whereby the parent crystals were cleaved into thinner crystallites and eventually elementary host sheets.