Optical properties of sol-gel processed TiO2 thin films up to the vacuum ultraviolet energy region

We have investigated structural and optical properties of TiO2 thin films prepared by a conventional sol-gel method. X-ray diffraction and transmission electron microscopy demonstrate that the TiO2 films are poly-crystallized and consist of anatase phase. The absorption spectra above the band-edge of the TiO2 thin films exhibit the shoulder structures around 3.9 and 4.3 eV and the broad bands around 4.8 and 6.2 eV. Several absorption bands revealed by decomposing the absorption spectra can be assigned to the transitions from oxygen 2p to titanium 3d states in TiO2. Excitation spectra for a visible luminescence peaking at 2.23 eV exhibit a gradual increase with increasing energy from 8.0 eV, whose energy corresponds to the beginning of the transitions from oxygen 2p to titanium 4s and 4p states. The result gives the characteristics of the relaxation processes from titanium 4s and 4p to the relaxed excited states.