The vis-NIR-stimulated bleaching and thermostimulated bleaching of UV-vis-induced Ti3+ color centers in visible-light-active rutile titania were revisited to gain a fuller understanding of the photoinduced formation and separation of charge carriers. The prime photophysical process of photostimulated bleaching of color centers is the absorption of the light quanta by Ti3+ centers. However, as expected from the photoexcitation and ionization of a certain type of Ti3+ centers, no selectivity of photostimulated bleaching could be ascertained. Moreover, thermoprogrammed annealing of color centers showed that photogenerated holes captured at a set of traps are also participants in the photostimulated bleaching of color centers. The extent of involvement of these trapped holes exhibited a rather puzzling dependence on the concentration and depth of the traps and on trap filling. Based on our current findings and earlier results, it is hypothesized that the heat released during nonradiative electron transitions, following the prime photophysical processes of excitation and ionization of Ti3+ centers, dissipates in the nearest neighborhood of these centers and that localized nonequilibrated excitation of the phonon subsystem leads to thermal detrapping of the photoholes with different depths up to 1 eV. Subsequent recombination of free holes with trapped electrons from Ti3+ centers leads to the observable photostimulated bleaching of these color centers. Based on the experimental evidence and the subsequent discussion, we argue that following the absorption of vis-NIR light by the color centers the subsequent release of thermal energy that accompanies the nonradiative electron transitions provides an additional specific channel for the photoactivation of visible-light-active rutile titania, in particular, and photocolorable metal-oxide semiconductors, in general.
Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films
- Physical and Theoretical Chemistry