Effect of the hydrothermal treatment, HCl purification and annealing temperature on the structure and the morphology of TiO2 nanotubes

Recently titanium oxide nanotubes (TiO2) are obtained by hydrothermal alkaline synthesis; where synthesis temperature and pressure, concentrations of NaOH and HCl, and annealing temperature are important parameters in the formation, purification and stability of the nanotubes structure. In this work, the effect of the hydrothermal treatment, the purification and the annealing temperature on the formation process, structure and morphology of TiO2 nanotubes was investigated. X-Ray diffraction (XRD), transmission electron microscopy (TEM), and EDS (energy dispersive X-ray spectroscopy) analysis were conducted to describe the formation and characterization of the structure and morphology of the nanotubes. From the analysis of results it was found that the hydrothermal treatment of the precursor nanoparticles of TiO2 and the purification with a HCl solution are essential to form and define the structure of the nanotubes. The hydrothermal treatment induces a change in the crystalline structure of the precursor nanoparticles from anatase phase to an orthorhombic phase which characterizes the titanate structure. The purification contributes to the formation of high purity nanotubes due to a Na exchange from the titanate structure to the HCl solution. The annealing temperature affects the morphology and the dimensions of the nanotubes structure. Annealing temperatures in the range of 400o C and 600o C are optimum to maintain a very stable tubular morphology of nanotubes. Temperatures greater than 600o C change the morphology of nanotubes from tubular to an irregular structure of nanoparticles with a size bigger than that of the precursor material, i.e., the crystalline structure changes from anatase phase to rutile phase causing the destruction of the nanotubes.