UV-induced formation of color centers in dispersed TiO2 particles: Effect of thermal treatment, metal (Al) doping, and adsorption of molecules

Abstract

Titanium dioxide is one of the most popular photoactive solids used as a photocatalyst, and as a self-cleaning, bactericidal, superhydrophilic (hydrophobic) and photochromic material when exposed to UV light. The primary steps of titania’s response to UV light are responsible for its photoactivity that involve generation of photocarriers subsequent to photoexcitation in its intrinsic (band-to-band transitions) and extrinsic absorption regions (defects-to-conduction band transitions). Photoexcitation into extrinsic absorption bands related mainly to extrinsic defects introduced deliberately through doping and to pre-existing intrinsic defects is an important step in the creation of visible-light-active (VLA) photocatalysts and other VLA titania-based materials. Despite the useful effects of doping, doping can also be detrimental because it can enhance the loss of otherwise active photocarriers through recombination with increasing dopant concentration. This article reports on AlxTi(1-x)O2 materials at different levels of Al doping that preliminary testing revealed significant photoinduced coloration. Materials were characterized by X-ray diffraction, Raman spectroscopy, XPS spectroscopy, SEM/EDX spectroscopic microanalyses, diffuse reflectance spectroscopy, and by the BET method for specific surface area determination. The principal objectives were to examine the effect(s) of thermal treatment of pristine titania (up to 1000°C) and Al-doped titania that led to the formation of intrinsic and extrinsic (Al in Ti positions) defects in the subsurface region and in the bulk lattice, together with the effect(s) that adsorbed and photoadsorbed oxygen and hydrogen molecules impart on the UV-induced coloration of dispersed titania particles. Results confirm that both thermal treatment and Al doping of titania caused noticeable UV coloration of the titania particles. The ultimate level of coloration of TiO2 under illumination in air was an order of magnitude greater than in vacuum. Results also demonstrate that UV-induced photocoloration of titania could delineate between the subsurface and bulk areas in titania particles, and could assess the depth of the subsurface region from the depth of the space charge region. The relevance of using the photocoloration of titania and other metal oxides as a simple tool to determine whether or not a so-called photocatalyzed reaction is truly catalytic has also been inferred.