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

Abstract

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.

In situ picosecond transient diffuse reflectance spectroscopy of opaque TiO2 systems under microwave irradiation and influence of oxygen vacancies on the UV-driven/microwave-assisted TiO2 photocatalysis

“…385 nm when exposed to UV/MW and UV/CH irradiation. In accord with earlier studies by Kuznetsov and Serpone, [31][32][33][34] the broad absorption from 370 nm to 700 nm in Fig. 4b is likely the result of two overlapping absorption bands positioned at around 430 nm (B2.90 eV; band AB1) and at ca.…”

Solid-state synthesis, characterization, UV-induced coloration and photocatalytic activity – The Sr6Bi2O11, Sr3Bi2O6 and Sr2Bi2O5 bismuthates

“…It will be emphasized that the bismuthates Sr 2 Bi 2 O 5 and Sr 3 Bi 2 O 6 that exhibited relatively wide bandgaps ( Table 3) and were initially somewhat weakly-colored are shown to be photosensitive, while the heavily colored Sr 6 Bi 2 O 11 system ( Figure S1) with the smallest bandgap energy shows no photocoloration. The same peculiarities were displayed by the lightly (yellow) and heavily multi-doped TiO 2 (red and brown) ceramics as reported elsewhere [43].…”

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

“…This color change observed subsequent to irradiation was reported in our earlier study and was illustrated in actual photographs shown in Figure 1. 7 Monitoring the kinetics of the absorption changes at each stage of the photochromic transformation provides a novel insight into the charge carrier dynamics. The following three points will be basic for consideration of the experimental data: (1) attribution of photoinduced absorption bands to a set of electronic Ti 3+ centers, 6,7 (2) photoformation of electron−hole pairs under irradiation at 3.05 eV, and (3) total disappearance of all electronic Ti 3+ centers after heating the sample up to 550 K, i.e., total reversibility of the absorption change.…”

“…7 Additional long-wavelength illumination causes a decrease of the blue-light-induced absorption spectrum (photobleaching), while such illumination is followed by a significant change in the shape of the UV-induced spectrum. The results obtained were clear experimental indications that the absorption spectra of yellow titania induced by irradiation in the UV or visible regions consist of at least three distinct absorption bands.…”