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.

This article reports an in situ UV-Vis-NIR diffuse reflectance (DR) spectroscopic and kinetic study of the photoformation and thermal annealing of light absorbing electronic point defects (color centers) in photochromic TiO in the temperature range 90-720 K using a simple laboratory-made cryostat-type accessory (for a Cary 5000 spectrophotometer equipped with an integrating sphere). The accessory also allowed for UV-Vis-NIR DR studies to be undertaken either in vacuum or in an oxygen atmosphere at significantly high temperatures (to 720 K) to assess dark chemical events occurring in photochromic titania with the participation of color centers. The DR spectral and kinetic measurements provided the opportunity to examine the separation of photoinduced charge carriers at traps and thermally stimulated carrier detrapping and recombination, as well as the response of color centers to oxidative/reductive treatments of photochromic TiO. Kinetic results also demonstrate the applicability of the fabricated DR accessory as a high-temperature reaction cell in the systematic study of the principal regularities in the formation and destruction of color centers in titania at various temperatures and gaseous atmospheres.

This article reports the optical properties of visible−NIR light absorbing titania powders fabricated by thermochemical oxidation in air of technically pure titanium (99T) and of three titanium alloys ) that contained aluminum as the major impurity (from 0.21 to 6.5 at. %) and other metals indicated in the notations. The resulting titania specimens were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy. The titania samples obtained from the alloys are considered to be heavily doped and codoped materials. The TiO 2 specimens displayed a wide color range: from yellow for the sample obtained from the 99T substrate to dark brown for the sample obtained from the Ti-4Al-2Mn substrate. After numerical analysis in the region 3.2−0.6 eV, the absorption spectra of titania obtained from titanium alloys were found to consist of a superposition of single absorption bands originating from intrinsic defect states in titania.No specific absorption bands attributable to the metal dopants were found in the absorption spectra of strongly colored titania. The photochromic behavior was shown only for the yellow titania produced from the titanium 99T, while titania samples fabricated from the titanium alloys displayed no such behavior. The photoresponse of the absorption spectra of yellow titania revealed a superposition of three absorption bands at 2.16, 1.86, and 1.52 eV photoinduced in both the UV and visible region. Additional irradiation of such photocolored samples in the region hν < 2.3 eV initiated a more complex behavior: decrease of all absorption band intensities (photobleaching) if the sample was photocolored in the blue region or increase of the absorption bands at 1.86 and 1.52 eV if the photocoloration was induced in the UV region. These photochromic events formed in titania during the thermochemical synthesis do not affect the absorption of light in the bands at 2.9 and 2.55 eV. A theoretical analysis, based on examination of the initial concentrations of the defect states, was developed to discuss the differences in photochromic properties of differently colored titania specimens.