Application of Magic-Angle Spinning NMR to Examine the Nature of Protons in Titanate Nanotubes

Abstract

Systematic magic-angle spinning (MAS) 1H NMR studies of protonated titanate nanotubes (produced by alkaline hydrothermal treatment of TiO2 with NaOH) have revealed that there are several types of protons incorporated into their wall structure, including crystallographic water molecules and ion-exchangeable OH groups. Each type (a) has a characteristic chemical shift and (b) disappears at a different rate during annealing in air. The evolution of protons in titanate nanotubes during crystallographic and morphological transformation in the sequence H2Ti3O7·xH2O, H2Ti3O7, H2Ti6O13, TiO2(B), TiO2(anatase) during calcination, at temperatures from 140 to 500 °C, has been studied using MAS NMR and Raman spectroscopies together with thermogravimetric analysis (TGA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The irreversible disappearance of ion-exchangeable OH groups has been observed, even under low temperature treatment.