Thermo- and Photo-stimulated Effects on the Optical Properties of Rutile Titania Ceramic Layers Formed on Titanium Substrates

This Article reports on the thermo- and photo-stimulated effects on the optical properties of rutile titania ceramic layers fabricated in an air atmosphere by high-temperature calcination of (technical grade) titanium substrates. The so-formed layers peeled off spontaneously during the cooling phase back to ambient temperature to reveal a yellow-colored upper surface and a cream-colored bottom surface that was in contact with the titanium plate. The two surfaces of the layers and a powdered specimen (formed from grinding the peeled-off layers) were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, electron dispersive spectroscopy, and diffuse reflectance spectroscopy. The upper surface demonstrates a strong photochromic effect. A pronounced increase of the amplitude of the absorption bands at 2.06 eV (AB3) and 1.56 eV (AB4) seen under irradiation in the UV or visible spectral region and a strong decrease of these bands during the heating of irradiated samples to 200–230 °C were characteristics of the upper layer’s surface. A wide set of spectra resulting from the reversible absorption changes made possible the disclosure of higher-energy absorption bands at 2.91 eV (AB1) and 2.54 eV (AB2); the latter were not affected by irradiation and heating. An electronic mechanism based on known properties of intrinsic point defects of TiO 2, F-type centers (two electrons trapped in oxygen vacancies) and Ti 3+ centers, is proposed to account for the optical changes that occurred through the photoinduced formation, photobleaching, and thermal bleaching of the absorption bands.