Visible—NIR Light Absorption of Titania Thermochemically Fabricated from Titanium and its Alloys; UV- and Visible-Light-Induced Photochromism of Yellow Titania

摘要

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 (Ti-4Al-2Mn, Ti-6Al-3Sn, and Ti-7Al-2Mo-lCr) 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 TiO2 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 hv < 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.