Slow Surface Charge Trapping Kinetics on Irradiated TiO₂

TLDR

Diffuse reflectance IR spectroscopy (DRIFTS) was used to characterize free and trapped charge carriers in polycrystalline TiO₂ after band‑gap irradiation. Spectroscopic data show that free conduction‑band electrons in irradiated TiO₂ absorb with a λ^1.7 dependence, decay following saturation kinetics, occupy a limited number of trapping states diminished by UV, have lifetimes lengthened by dehydration, can be extended over several orders of magnitude, and are associated with a 3380 cm⁻¹ IR peak from a 0.42 eV surface trap.

Abstract

Free and trapped charge carriers in polycrystalline TiO2 following band gap irradiation are characterized by diffuse reflectance IR spectroscopy (DRIFTS). A spectrum-wide absorption signal proportional to λ1.7 (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.