Photophysics of color centers in visible-light-active rutile titania. Evidence of the photoformation and trapping of charge carriers from advanced diffuse reflectance spectroscopy and mass spectrometry

Abstract

• The photoformation and separation of charge carriers in traps in VLA titania ceramics and titania powder were examined. • Thermoprogrammed annealing (TPA) spectra of Ti 3+ color centers photoinduced in VLA TiO 2 displayed temperature dependencies. • Highest rate of recombination of holes released into the valence band with Ti 3+ centers was due to extra charged centers. • Rate of photodesorption of O 2 occurrence in specific channel to activate VLA TiO 2 by photoexcitation of Ti 3+ centers. We herein report on the photophysics of color centers in visible-light-active (VLA) rutile titania ceramics and titania powder resulting from the photoformation and separation of charge carriers explored by a diffuse reflectance (DR) spectroscopic and kinetic study using a cryostat-type accessory for a Cary 5000 spectrophotometer after exposure of the titanias to UV radiation and Vis-light illumination. The action spectrum of the photoformation of Ti 3+ at 90 K accorded fully with the absorption spectrum of intrinsic defects in the as-synthesized TiO 2 . The photoinduced absorption spectra consisted of a set of individual absorption bands attributable to several different Ti 3+ centers. Analysis of the dependencies of the photoformation of separate centers on the wavelength of illumination and light exposure provided extraction of specific Ti 3+ centers putatively attributed to centers with excess negative charge {2Ti 3+ + V o 2+ } ⟵→ {Ti δ+ + V o 2+ } with 3 > δ > 2 formed at significantly high concentration upon maximal exposure to Vis-light illumination. Thermoprogrammed annealing (TPA) spectra of Ti 3+ color centers photoinduced in VLA TiO 2 displayed temperature dependencies of the rate of detrapping of the photoinduced holes observed optically through annihilation of the Ti 3+ centers. The TPA spectra in the range 90–500 K consisted of a set of first-order peaks corresponding to the traps, whose depths ranged from ∼0.2 eV (peak at 130 K in powder specimen) to 1.06 eV (peak at 455 K in the ceramics). The highest rate of recombination of holes released to the valence band with Ti 3+ centers, an event attributable to the Ti δ+ centers, provided TPA spectra that clearly manifested the existence of shallow traps. We also report mass spectrometric evidence of the photoformation of electrons and holes in VLA TiO 2 under Vis-light illumination through an examination of the photoadsorption of molecular oxygen and the photodesorption of photoadsorbed oxygen from the surface of powdered VLA titania specimens. Moreover, kinetics of the photodesorption of O 2 under orange light illumination, after the photoadsorption of O 2 stimulated by blue light excitation, provided experimental proof of the occurrence of an additional specific channel toward the photoactivation of VLA TiO 2 via the photoexcitation of photoinduced Ti 3+ color centers.