Real-Time in Situ Monitoring of Optical Absorption Changes in Visible-Light-Active TiO₂ under Light Irradiation and Temperature-Programmed Annealing

We herein report a real-time optical reflectance/absorption study of the photochromic behavior of visible light absorbing (yellow) titania carried out using a newly designed novel accessory for a fluorescence spectrophotometer. Yellow rutile titania, thermochemically synthesized from technical grade titanium substrate, displays a fully reversible sequence of electronic processes controllable by optical reflectance/absorption photostimulated by UV or visible light; three absorption bands appeared in the range 2.16–1.52 eV that could be thermally annealed at temperatures up to 600 K. To carry out real-time studies of these processes, a special device was designed and constructed which when combined with the fluorescence spectrophotometer allowed for the measurement of the changes in the sample’s absorption, ΔA_λ(t), at wavelength λ that corresponded to the maximum of the photoinduced absorption spectral bands under monochromatic light irradiation or temperature-programmed heating as well as light irradiation at a desired constant temperature. The dependences of ΔA_λ(t) obtained under heating at a constant rate and represented in differential form (temperature-programmed absorbance annealing (TPAA) spectra) determined the main advantage of the device developed, since these spectra permit probing the energy levels of electron and hole traps within the band gap of the yellow titania system. The results of the present work show that TPAA spectra provide a new (in addition to the absorption spectra) quantitative characterization of photoactive materials that display photochromic properties. It is also demonstrated that the TPAA spectra are convenient in the numerical modeling of charge carrier dynamics in such metal-oxide semiconductors.