Bimodal mesoporous titania powders with high photocatalytic activity were prepared by hydrolysis of titanium tetraisopropoxide in the presence of HNO3 or NH4OH under ultrasonic irradiation. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface areas, and X-ray photoelectron spectroscopy (XPS). Photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The effects of catalysts added during hydrolysis on the microstructure and photocatalytic activity of the TiO2 powders were investigated. The results showed that HNO3 enhanced the growth of brookite, while NH4OH not only retarded phase transformation of the TiO2 powders from amorphous to anatase and anatase to rutile but also suppressed the growth of brookite. All TiO2 powders calcined from 400 to 600 degreesC showed bimodal pore-size distributions in the mesoporous region: one was intraaggregated pores with maximum pore diameters of ca. 4-9 nm and the other interaggregated ones with maximum pore diameters of ca. 35-50 nm. At 700 degreesC, the pore-size distributions of all samples exhibited monomodal distribution of the interaggregated pores due to the collapse of the intraaggregated pores. The photocatalytic activity of the TiO2 powders prepared by this method and calcined at 400 degreesC exceeded that of Degussa P-25 when the molar ratio of HNO3 or NH4OH to H2O was less than 0.05. (C) 2003 Elsevier Science (USA). All rights reserved.