Distinct Properties of Titanate Nanosheets with Different Hydrodynamic Radii: A UV-Vis Spectroscopy and Titration Study

Abstract

The quantum size effect in two-dimensional (2D) materials is largely governed by their thickness in the nm-range but is less dependent on the lateral sizes. Here, “lateral sizes” are represented by the hydrodynamic radii (RH ) obtained from dynamic light scattering (DLS) which is a fast and efficient method of determining the particle size from the bulk. In this work, nanosheets with different R H have been prepared by the exfoliation of a lepidocrocite titanate H0.7 Ti1.825 O4 ·H2 O. The reagents employed include tetramethylammonium hydroxide (TMAOH) or tetrabutylammonium hydroxide (TBAOH). The R H obtained is ~485 nm for nanosheets exfoliated with TMA+ , and 151 nm with TBA+ . The electronic structure of nanosheets was investigated by UV-vis spectroscopy. The λ max at 266 nm for the colloidal suspension of large nanosheets shifts to 263 nm for the small ones. Tauc plots indicate the optical band gap of 3.68 eV for large nanosheets, and 3.82 eV for small nanosheets. The suspension of small nanosheets also shows a more pronounced bluish tint. The change to chemical properties of these nanosheets has also been investigated by titration experiments with diluted HCl. Starting from basic pH and extending to acidic one, an abrupt increase in R H of large-size nanosheets occurs at a later stage (i.e., pH ~ 3.3), compared to the small-size nanosheets (pH ~ 4.2). At acidic pH, both types of nanosheets macroscopically reassembled into three-dimensional (3D) agglomerates visible by bare eyes. These agglomerates inherit the distinct chemical behaviour from their respective colloidal suspension, exhibiting different compactness. The controlled exfoliation of layered metal oxides to nanosheets of different hydrodynamic radii could be a simple way of tuning the chemical/physical properties of nanosheets and their higher-order assembly.