Apparent band-gap energies of mixed TiO2 nanocrystals with anatase and rutile structures characterized with photoacoustic spectroscopy

Extreme small-sized titanium dioxide (TiO2) particles (nanocrystals) have been extensively used in the field of heterogeneous photo-oxidation catalysis for environmental cleanup. We have measured the optical absorption of mixed TiO2 nanocrystals with rutile and anatase structures (10–40 nm diameter) by photoacoustic (PA) spectroscopy. The values of apparent band-gap energies Eg of the TiO2 nanocrystals with different rutile contents were determined by the theory of interband transition. The value of Eg of a mixed TiO2 nanocrystal with rutile and anatase structures decreases continuously with the increase of rutile content. This indicates that Eg can be controlled by mixing the nanometer-sized TiO2 powders with rutile and anatase structures, hence, we can obtain TiO2 materials with an arbitrary apparent Eg between rutile and anatase structures. The PA spectrum of anatase-type TiO2 nanocrystals with a diameter of 15 nm shows a maximum around a photon energy of 3.9 eV, suggesting a quantum confinement effect with decreasing particle size. Applying the effective mass approximation to the PA spectrum, we can deduce the value of the hole effective mass in anatase-type TiO2 as 0.01 m0 (where m0 is electron rest mass).