Phase and morphology changes induced by acid treatment following alkaline reaction of mesoporous anatase: Effect of anions

Abstract Acid treatment, with aqueous solutions of HCl, HNO 3 , H 2 SO 4 , HNO 3  + NaCl and H 2 SO 4  + NaCl, for the amorphous reaction paste resulted from mesoporous anatase particles dispersed in KOH solution at 90 °C has been performed. The alkaline reaction paste transforms into hexatitanate after addition of HCl solution (0.1 M) to a pH value from 6.0 to 12.0, but into TiO 2 -B at pH 1.2. Different phase changes are observed when a HNO 3 or H 2 SO 4 solution is used. Anatase starts to appear at pH 9.0 for HNO 3 and at 12.0 for H 2 SO 4 and persists down to pH 1.2. Besides, in the pH range studied, no other crystal phase is detected. The particle morphology of the TiO 2 -B and anatase produced at pH 1.2, using HCl, HNO 3 or H 2 SO 4 solution, is nanowire, with a width of ∼10 nm. Although NO 3 − or SO 4 2 − alone in the pH 1.2 solution favors anatase phase, co-existence Cl − leads to TiO 2 -B formation. Cl − plays a role dominating the formation of hexatitanate, which is then transformed into TiO 2 -B. However the particle morphology is affected due to the adsorption of NO 3 − and SO 4 2 − . The mechanism for the process from hexatitanate to TiO 2 -B, not relating to the anion nature, and the anion effect based on the binding manner and strength of the anions to titanium ions are discussed.