Microwave conductivity and Raman spectroscopy of porous TiO2

Microwave conductivity and Raman spectroscopy were applied to study the inf luence of light and thermal annealing on porous TiO2 properties. Microwave absorption in porous TiO2 (anatase) showed an anomalous behaviour in the pr esence of light excitation (at 514nm, i.e. below, or 356 nm, i.e. above the band Sap). Exposure to light at a time scale of minutes reduces the microw ave conductivity, in contrast to many bulk semiconductors excited above ban d gap. A reduced content of oxygen after heating in vacuum leads also to a reduction of microwave absorption. Both observations ale in apparent contra diction with de conductivity in TiO2. By comparison with light-induced EPR experiments, we conclude that the oxygen removal creates deep defects to wh ich are transferred electrons previously trapped in shallow donors. As a co nsequence, this decreases the polarisability of TiO2 nanoparticles, the mic rowave absorption, and the dielectric constant.