SYNTHESIS, ELECTRONIC STRUCTURE, AND MAGNETIC PROPERTIES OF NANOCRYSTALLINE OXYGEN-DEFICIENT TiO2–δ(B)

Nanocrystalline TiO2 polymorphs with nonstoichiometry, i.e. having intrinsic and/or impurity structural defects, attract considerable interest due to enhanced optical and catalytic activity, improved electronic properties, and existence of room-temperature ferromagnetism. Herein, due to combination of hydrothermal technology, ion exchange process, and high-temperature vacuum annealing, a gray-colored bronze titanium dioxide with a belt-like nanostructure has been successfully shynthesized. The electron paramagnetic resonance data indicate that coloring of bronze polymorph during vacuum annealing is caused by the appearance of F-centers within a crystal lattice (electron trapped in the anionic vacancies). X-ray diffraction, electron-microscopic studies, and adsorption measurements has no reveal obvious changes in phase composition, microstructure, and texture during vacuum heat treatment (in comparison with bronze TiO2 treteated under air). Spectrophotometry experiments illustrate a "narrowing" of the TiO2(B) band gap from 3.23 to 3.04 eV and an evolution of the optical absorption in the visible and near infrared ranges due to appearance of oxygen vacancies. The study of magnetic characteristics showed that TiO2–δ(B) has a ferromagnetic ordering at room temperature, and the nature of ferromagnetism in it is clearly defective (magnetic properties are determined by the exchange interaction between electrons located in the vacancy zone). At low temperatures (4 K) the magnetic properties of oxygen-deficient bronze titanium dioxide become similar to those for quasi-stoichiometric TiO2(B) prepared through annealing under oxidizing atmosphere (air).