Characterization of electronic states of TiO2 powders by photoacoustic spectroscopy

We have measured the optical absorption of TiO2 powders by photoacoustic (PA) spectroscopy, which is powerful for detecting small amounts of strongly scattering materials. The band-gap energies E-g of rutile and anatase types of TiO2 powders with particle radii of 40nm are 3.06eV and 3.20eV, respectively, from PA intensity and PA phase spectra, indicating the usefulness of PA spectroscopy. The PA intensity and phase spectra show that the E-g of anatase-type TiO2 powders with a particle radius of 11 nm is 3.25 eV (blue shift) within the experimental accuracy, indicating the possible onset of a quantum size effect with decreasing particle size. The PA intensities plotted semilogarithmically vary linearly with photon energy immediately below the band-gap, in accordance with Urbach's rule (exponential tail). The slope of the exponential absorption of rutile-type TiO2 powders is larger than that of the anatase, indicating that the anatase-type has increasing energy from the displacement of atoms specifically due to oxygen vacancies and/or chlorine impurities in the production processes of the powders.