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

Abstract

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.

Heterogeneous Photocatalysis and Prospects of TiO2-Based Photocatalytic DeNOxing the Atmospheric Environment

“…However, photoformation of electrons and holes can also be achieved on illumination in the Vis region at hν ≤ E g when light quanta are absorbed by the native point defects (i.e., F (or F + ) centers); the latter are limited in number and are located at definite sites in the microparticle (Figure 61b). Moreover, because photoexcitation of F (or F + ) centers can produce charge carriers followed by their subsequent decay to their initial electronic states, as proposed in earlier studies [68,69], repetitive absorption of light quanta and photogeneration of electrons and holes occurs each time at the same spatial sites (F or F + centers) in the microparticle (blue stars in Figure 61b). Such considerations then lead to the reasonable inference that transport of carriers and occupation of traps (processes 2 and 3 in Figure 61b) start repetitively at the same sites in the microparticle.…”

“…An explanation of the formation of free charge carriers by photoexcitation of F or F + centers had been proposed in two earlier studies by Kuznetsov and coworkers [68,69]; it is illustrated schematically in Figure 11. It was implied that the first photophysical event is the optically-activated electronic transition from the ground state F (or F + ) center to its excited state (F)* (or (F + )*).…”

Inorganic Photochemistry and Solar Energy Harvesting: Current Developments and Challenges to Solar Fuel Production

“…Moreover, the advances in the spectroscopic techniques have allowed a deeper understanding on the charge carrier dynamics and reaction mechanisms at the semiconductor/ electrolyte interface [80][81][82][83][84]. In particular, in situ spectroscopic methods, spectroelectrochemical experiments, and time-resolved techniques, such as transient absorption spectroscopy (TAS) [85][86][87][88][89][90][91][92], have provided new insights on the dynamic of charge transfer in different photocatalytic systems. These fundamental investigations are guiding the material choice and device assembly methods, resulting in significant progress in the last years.…”

Charge carrier dynamics and photocatalytic behavior of TiO2 nanopowders submitted to hydrothermal or conventional heat treatment

“…Moreover, H 2 evolution without the necessity of an external bias has been observed in the presence of co-catalysts on the oxide surface 25-28 . In parallel to the advances of material preparation, large research efforts have been devoted to investigations of well-defined photocatalytic reaction systems and their detailed reaction mechanisms and kinetics using a variety of spectroscopic techniques [29][30][31][32][33][34][35][36][37] . Particularly, the use of time-resolved methods, such as transient absorption spectroscopy (TAS) [38][39][40][41] and microwave conductivity measurements (TRMC) 42,43 has allowed to enhance the fundamental understanding of electron/hole trapping and recombination, with the latter being recognized as the limiting step of the photocatalytic process 44 .…”

“…[25][26][27][28] In parallel to the advances of material preparation, large research efforts have been devoted to investigations of well-dened photocatalytic reaction systems and their detailed reaction mechanisms and kinetics using a variety of spectroscopic techniques. [29][30][31][32][33][34][35][36][37] Particularly, the use of time-resolved methods, such as transient absorption spectroscopy (TAS) [38][39][40][41] and microwave conductivity measurements (TRMC) 42,43 has allowed to enhance the fundamental understanding of electron/ hole trapping and recombination, with the latter being recognized as the limiting step of the photocatalytic process. 44 Despite the remarkable progress, it is not an easy task to establish clear relationships between the photocatalytic properties and the metal oxide's morphological and electronic properties.…”