Effect of post treatments on the structure and thermal stability of titanate nanotubes

TiO₂ sol was prepared hydrothermally in an autoclave from aqueous TiOCl₂ solutions as a starting precursor. Titanate nanotubes were obtained when the sol-gel-derived TiO₂ sol was treated chemically with a 10M NaOH solution and subsequently heated in the autoclave at 150 °C for 48h. The samples were characterized using XRD, TEM, SEM, EDX, Raman spectroscopy, and a BET surface area analyser. The effect of post treatments, such as washing with and without hydrochloric acid and calcination, on the phase structure, shape and morphology, pore structures, and BET surface area of the titanate nanotubes was investigated. When a sample containing 7.08wt% Na (after washing only with water) was calcined at different temperatures from 300 to 900 °C, it showed the formation of a mixture of sodium trititanates and sodium hexatitanates and was found to preserve the tubular morphology at higher temperatures. However, a sample containing 0.06wt% Na obtained after prolonged washing with hydrochloric acid followed by heat treatment showed the formation of TiO₂ anatase involving TiO ₂ (B) as an intermediate at lower temperatures and anatase was further transformed to the rutile phase when the temperature was raised. On the basis of different observations, a general formula Na_xH _2-xTi₃O₇nH₂O has been proposed for the trititanate nanotubes.