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

Acid treatment, with aqueous solutions of HCl, HNO ₃, H ₂SO ₄, HNO ₃ + NaCl and H ₂SO 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 ₂SO ₄ solution is used. Anatase starts to appear at pH 9.0 for HNO ₃ and at 12.0 for H ₂SO 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 ₂SO ₄ solution, is nanowire, with a width of ∼10 nm. Although NO ^3- or SO ^42- 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 ^42-. 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.