Controlled nanophase development in photocatalytic titania

Abstract

Acid catalyzed hydrolysis of titanium butoxide was used to synthesize homogeneous titania gels, which were converted to the anatase and rutile polytypes through controlled firing. The mechanism of this phase transformation was investigated by quantitative powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Condensation to titania is a function of annealing time, temperature and atmosphere. In air, calcinations below 600°C favored the formation of anatase, with rutile only appearing at >600°C. However, in situ transformation where titania nanoparticles were treated under vacuum in the TEM required heating at higher temperatures up to 900°C. This is may be due to the formation of surface layers of reduced titanium oxide that passivate and retard the anatase to rutile transformation. Grain growth and specific surface area varied inversely, with especially rapid crystallization observed at and beyond the transformation temperature. Potential photocatalytic activity was identified with ultraviolet-visible (UV-Vis) spectroscopy. A red shift of the absorption edge for nano-titania was observed due to quantum size effects.