Ternary Morphological Diagram for Nano(tube-ribbon-sphere) Sodium Titanate Deduced from Raman Spectra Analysis

In this study, sodium hydroxo titanates were synthesized using the reflux process in an alkaline medium. Three distinct morphologies were obtained, nanotubes and nanoribbons as classically encountered in the literature and a new nanosphere-like morphology. After ionic exchange of Na+ ions by protons and annealing, sodium titanates transform into TiO2 nanoparticles with anatase and TiO2(B) structures. We clarified the origin of this polymorphism. Nanoribbons transform into TiO2(B) at 400 degrees C, whereas nanospheres and nanotubes are converted to TiO2 anatase at 400 and 500 degrees C, respectively. The quantification of the anatase/TiO2(B) ratio performed by Raman spectroscopy allows us to establish an original ternary morphological diagram. The latter depicts the relative proportion of nanotubes, nanoribbons, and nanospheres composing each titanate and is of great utility in controlling the morphologies of the sodium titanates and correlating them to the synthesis parameters.