Effect of phase structures on the photocatalytic activity of surface fluorinated TiO2

Nanocrystalline TiO2 with different contents of anatase were tailored by hydrothermal treatment of the mixed solution of Ti(SO4)(2) and TiCl4 at 250 degrees C for 24 h, and then calcined at 500 degrees C for 3 h. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, nitrogen sorption, X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectra. The as-prepared TiO2 samples with different contents of anatase have similar BET specific surface areas (30-40 m(2)/g) and crystalline sizes (30-40 nm). The effect of surface fluorination on the adsorption and photocatalytic activity of TiO2 samples was evaluated using Brilliant Red X3B, an anionic azo dye, as the target organic molecule at pH 3.0. Both on anatase and rutile TiO2, the dark adsorption of X3B was greatly decreased in the presence of fluoride, ascribed to competitive adsorption of fluoride that reduces the positive charges on the catalyst surface. After surface fluorinated by 1.0 mmol/L NaF, the photocatalytic activity of anatase was enhanced by a factor of 2.63. However, 81.3% of the photocatalytic activity was reduced for rutile TiO2 after surface fluorinated under the same condition. Only when the content of anatase is higher than 40%, can fluoride shows positive effect on the photocatalytic activity of TiO2. Two reaction models were put forward, based on the photocurrent response, degradation kinetics, hydroxyl radicals quenching and detecting, to illustrate the different effects of fluoride on the photocatalytic degradation of X3B in ana case and rutile TiO2 suspensions. (C) 2010 Elsevier B.V. All rights reserved.