In situ study of photo- and thermo-induced color centers in photochromic rutile TiO2 in the temperature range 90–720 K

Abstract

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.

Additional Specific Channel of Photoactivation of Solid Semiconductors. A Revisit of the Thermo-/Photo-Stimulated Bleaching of Photoinduced Ti3+ Color Centers in Visible-Light-Active Photochromic Rutile Titania

“…The photostimulated decrease in the number of trapped electrons, seen optically in photocolored rutile ceramics as color centers, is accompanied by a decrease in the number of trapped holes. Overall, this result confirms our previous notions regarding the separation of photogenerated charge curriers in yellow rutile. , Along with this, the similarity of the action spectrum of photostimulated bleaching of color centers and the absorption spectra of these centers indicate that the prime photophysical process of photostimulated bleaching of color centers is absorption of light quanta by the Ti 3+ centers. In accord with the electronic scheme presented in Figure , the energetic levels of Ti 3+ color centers in yellow rutile are distant from the bottom of the conduction band by more than 1 eV.…”

“…Since the relative contribution of some absorption band differs in the different photoinduced absorption spectra (see Figures –), we conclude that Ti 3+ centers responsible for these bands behave independently of others. For example, the absorption band at 1.25 eV ( ab4 ) is absent in the spectra of oxidized rutile (Figure ) but is noticeable in the spectra of the aged titania sample (Figure ) and in fact dominates after reduction of the sample in vacuum . Note that the absorption spectra with maximum at 1.24 eV is also characteristic of P25 TiO 2 reduced in a H 2 or CO atmosphere .…”

“…The temperature dependence of the rate of absorbance decrease at wavelengths corresponding to the absorption spectrum of color centers is referred to as the thermoprogrammed annealing (TPA) spectrum of color centers . Earlier the TPA spectra of color centers photoinduced in yellow rutile ceramics were attributed to thermally stimulated detrapping of photoholes observed optically through annihilation of Ti 3+ centers. , These TPA spectra consist of a set of first-order peaks. The activation energies ( E a ) of detrapping (the trap depths) were: 1.06 eV for the peak at 455 K, 0.92 eV for the peak at 400 K, 0.84 eV for the peak at 370 K, 0.73 eV for the peak at 325 K, 0.5 eV for the peak at 270 K, and 0.3 eV for the broad peak at about 190 K …”

Absorption and Remission Characterization of Pure, Dielectric (Nano-)Powders Using Diffuse Reflectance Spectroscopy: An End-To-End Instruction

“…Towards the electronic band gap, the diffuse reflectance drops for wavelengths below 410 nm and approaches a common value of 0.08 for both TiO 2 -samples. This finding is in line with related studies [84][85][86][87] and shows that, even for interband transitions, a fraction of the incoming light is backscattered.…”

High‐Temperature Photochromism of Fe‐Doped SrTiO3 Caused by UV‐Induced Bulk Stoichiometry Changes

“…In analogy to the voltage‐induced electrochromism due to a redistribution of oxygen vacancies in Fe:STO (electrocoloration), one might thus also expect existence of light‐induced photochromism. Photochromic effects based on color centers have been frequently reported for oxides with metals (SrTiO 3 , TiO 2 , and many other oxides) . However, those photochromic experiments usually take place around room temperature or much below and the corresponding phenomena are not caused by oxygen stoichiometry changes of the bulk under illumination.…”