Larger Effect of Sintering Temperature Than Particle Size on the Photocatalytic Activity of Anatase TiO2

Photocatalytic activity of anatase TiO2 that increases with the increase of its synthesis temperature has been widely reported, but the reason for that remains incompletely understood. In this work, the positive effect of synthesis temperature, presumably due to the growth of particle size, has been examined. Three series of anatase samples with various particle sizes were prepared from the hydrolysis of TiOSO4 in water at 150 degrees C, followed by calcinations in air. The particle size of TiO2, estimated by X-ray diffraction and N-2 adsorption, increased with the increase of the hydrothermal time, calcination time, and calcination temperature. For phenol photodegradation in aerated aqueous suspension, three series of the catalysts showed different correlation between the activity and particle size of TiO2. However, with the same amount of Ag+ adsorbed on the oxide surface for phenol photodegradation in a N-2-purged aqueous suspension, those catalysts showed activities all increasing with the particle size of TiO2, whereas at given particle size, the thermally treated TiO2 was much more active than the hydrothermally treated one. These observations are discussed in terms of the solid crystallinity, surface area, exposed facets, surface hydroxyl groups, and light absorption. But, there only appears a positive correlation between the particle size, calcination temperature, and the number of surface defects, as revealed by photoluminescence spectroscopy. The increase of surface defects may facilitate separation of the photogenerated charges, consequently improving the efficiency for phenol degradation at the solid-liquid interface.