Slow Surface Charge Trapping Kinetics on Irradiated TiO_2

Free and trapped charge carriers in polycrystalline TiO_2 following band gap irradiation are characterized by diffuse reflectance IR spectroscopy (DRIFTS). A spectrum-wide absorption signal proportional to λ^(1.7) (λ = wavelength/μm) indicates the presence of free conduction band electrons coupled with acoustic phonons in the lattice. Free electrons appear to decay according to saturation kinetics. The fitted parameters indicate a limited number of trapping states. The concentration of these states appears to be diminished by sequential UV treatments. The free carrier decay lifetime is lengthened as the samples are dehydrated, which suggests an excited-state relaxation event during electron trapping. Photogenerated free electrons are comparable to conduction band electrons injected from surface-bound chromophores, and the lifetime of these electrons can be extended across several orders of magnitude. A broad IR absorption peak centered at 3380 cm^(-1) is attributed to an electronic transition from an occupied surface electron trap 0.42 eV below the conduction band