Experimental and theoretical investigation of the pH effect on the titania phase transformation during the sol–gel process

Nanocrystalline TiO2 powders were prepared by using sol–gel technology. The pH effect on titania phase transformation was investigated from experimental and theoretical approaches. Pure rod-like rutile nanocrystallites formed directly from pH 2 precursor solution. Anatase that formed from pH 4 solution transformed into rutile at the critical size of 9.0 nm, which is much smaller than previously reported (⩾14 nm). Anatase and rutile co-crystallized from the solution of pH 7, while anatase formed from pH 9 solution transformed into rutile at 14.7 nm. Four models of titanium-coordinated complexes were optimized using the density functional theory (DFT/B3LYP) to simulate the structural monomers in different pH solutions. Based on the geometric parameters and Mulliken charge populations calculated, plausible reaction pathways of the titanium-coordinated complexes were anticipated. The theoretical models well explain the experimental results and provide a deep insight into the formation mechanism of different crystal phases at different pH values.