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 E-g of the TiO2 nanocrystals with different rutile contents were determined by the theory of interband transition. The value of E-g of a mixed TiO2 nanocrystal with rutile and anatase structures decreases continuously with the increase of rutile content. This indicates that E-g 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 E-g 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 m(0) (where m(0) is electron rest mass). (C) 2003 American Institute of Physics.