Spectral Features of Photostimulated Oxygen Isotope Exchange and NO Adsorption on “Self-Sensitized” TiO_2–<i>x</i>/TiO₂ in UV–Vis Region

We have investigated the photoactivated O₂ adsorption/desorption, the isotope equilibration (POIEq), and the isotope exchange (POIEx) in the O₂–(TiO_2–<i>x</i>/TiO₂) (Degussa P-25) system, as well as the adsorption and POIEx in the NO–(TiO_2–<i>x</i>/TiO₂) systems. The photoactivation spectra of the hole centers O_s^– generation, of POIEq, and of the NO photoadsorption were measured in the 300–500 nm range using a continuously adjustable wavelength light source. The spectral characteristics of the NO photoadsorption are close to those of POIEq Both electron-donor and hole centers are manifested in these processes. For O₂ photodesorption, CO oxidation, POIEx, and POIEq the hole centers O_s^– are responsible. In the case of O₂ and NO photoadsorption both types of centers are active. The maxima of POIEq and of NO photoadsorption are placed near 420 nm beyond the edge of the TiO₂ bandgap absorption. It was found that not only does the N¹⁸O–Ti¹⁶O₂ exchange occurs under the illumination but also it can be activated by a preliminary sample illumination in vacuum as it was observed for POIEq. The photoactivation spectrum does not coincide with the absorption spectra of the F-type centers (F⁺ and F-centers, i.e., oxygen anion vacancies filled with one or two e^–, respectively) and of Ti³⁺. The formation of the 2D structures, “core–shell” TiO_2–<i>x</i>/TiO₂, on the parts of the surface region and the decreasing of bandgap on these parts are supposed based on the kinetic data (the “fast” excitations in the bulk under UV irradiation and the “slow” excitations on the surface under the visible irradiation) and on the spectral data (maximum action is near 420 nm for both electronic and hole centers).