Photocatalytic activity of sol-gel-derived nanocrystalline titania

Nanocrystalline titania (TiO2) powders have been synthesized via sol-gel, using an alkoxide precursor, under different processing conditions, and their photocatalytic activity has been investigated as a function of processing and material parameters through the decomposition of the methylene blue (MB) dye under exposure to the ultraviolet (UV) radiation (lambda = 200-400 nm) in an aqueous solution. The nanocrystalline TiO2 powders with different morphology, crystallinity, average nanocrystallite size, surface area, and phase structure are obtained by controlling the ratio of molar concentrations of water and alkoxide (R) within the range of 5-60 and calcining the as-synthesized amorphous powders at higher temperatures (400-800 degrees C). The nanocrystalline TiO2 powders have been characterized using the scanning electron microscope (SEM), X-ray diffraction (XRD), and the Brunauer, Emmett, and Teller (BET) surface area measurement techniques while their photocatalytic activity was monitored using a UV-visible spectrometer. The photocatalytic activity of sol-gel-derived nanocrystalline TiO2 is observed to be a function of R and calcination temperature. The maximum photocatalytic activity is observed for the largest R value and the intermediate calcination temperature as an optimum effect produced by the variation in the morphology, the average nanocrystallite size, the surface area, the phase structure, and the crystallinity of the powders. The dependence of photocatalytic activity on the average nanocrystallite size reveals the existence of a critical size (similar to 15 nm), below and above which the photocatalytic activity is observed to be reduced. The observed photocatalytic characteristics of sol-gel-derived nanocrystalline TiO2 have been explained based on the existing mechanism associated with the photocatalytic decomposition of organic molecules using semiconductor oxides.