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, HNO3, H2SO4, HNO3 + NaCl and H2SO4 + 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 TiO2-B at pH 1.2. Different phase changes are observed when a HNO3 or H2SO4 solution is used. Anatase starts to appear at pH 9.0 for HNO3 and at 12.0 for H2SO4 and persists down to pH 1.2. Besides, in the pH range studied, no other crystal phase is detected. The particle morphology of the TiO2-B and anatase produced at pH 1.2, using HCl, HNO3 or H2SO4 solution, is nanowire, with a width of ∼10 nm. Although NO3− or SO42− alone in the pH 1.2 solution favors anatase phase, co-existence Cl− leads to TiO2-B formation. Cl− plays a role dominating the formation of hexatitanate, which is then transformed into TiO2-B. However the particle morphology is affected due to the adsorption of NO3− and SO42−. The mechanism for the process from hexatitanate to TiO2-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.