Photoacoustic Spectra of Mixed TiO2 Ultrafine Powders with Rutile and Anatase Structures

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Abstract
We have measured the optical absorption of mixed TiO2 ultrafine powders with rutile and tanatase structures by photoacoustic (PA) spectroscopy, which is a powerful technique for detecting small amounts of strongly scattered materials. The values of band-gap energies, E g, of the TiO2 ultrafine powders with different rutile contents were determined by the theory of interband transition in semiconductors. The values of E g of rutile and anatase structures of TiO2 ultrafine powders are 3.00 eV and 3.20 eV, respectively, obtained from PA intensity and PA phase spectra, indicating the usefulness of PA spectroscopy. Mixed TiO2 ultrafine powders with rutile and anatase structures (20–140 nm diameters) were obtained by thermal oxidation of TiCl4. Rutile structure contents depend on the thermal oxidation temperature. The value of E g of a mixed TiO2 ultrafine powder with rutile and anatase structures decreases continuously with the increase of the rutile content. This indicates that the apparent E g can be controlled by mixing the nanometer-sized TiO2 powders with rutile and anatase structures, hence, we can control and obtain TiO2 materials with an arbitrary apparent E g between rutile and anatase structures.