Photodegradation of pollutants in air : enhanced properties of nano-TiO2 prepared by ultrasound

Nanocrystalline TiO2 samples were prepared by promoting the growth of a sol–gel precursor, in the presence of water, under continuous (CW), or pulsed (PW) ultrasound. All the samples turned out to be made of both anatase and brookite polymorphs. Pulsed US treatments determine an increase in the sample surface area and a decrease of the crystallite size, that is also accompanied by a more ordered crystalline structure and the samples appear to be more regular and can be considered to contain a relatively low concentration of lattice defects. These features result in a lower recombination rate between electrons and holes and, therefore, in a good photocatalytic performance toward the degradation of NOx in air. The continuous mode induces, instead, the formation of surface defects (two components are present in XPS Ti 2p3/2 region) and consequently yields the best photocatalyst. The analysis of all the characterization data seems to suggest that the relevant parameter imposing the final features of the oxides is the ultrasound total energy per volume (Etot/V) and not the acoustic intensity or the pulsed/continuous mode.