Titanium Ions Dispersed into the ZrO₂ Matrix: Spectroscopic Properties and Photoinduced Electron Transfer

In the present work, different ZrO 2 −TiO 2 mixed oxides, which are usually employed in photocatalytic processes and as catalytic support in heterogeneous catalysis, have been prepared via sol−gel synthesis or by wet impregnation. In the former case, as indicated by X-ray diffraction and Raman spectroscopy, solid solutions based on Ti ions diluted in the ZrO 2 matrix are formed in the range of Ti molar fraction from 0.01 to 0.15. These solids have interesting optical properties as the typical band gap transition of ZrO 2 undergoes a red shift proportional to the Ti loading which reach the remarkable value of 1.3 eV for the highest Ti concentration. Annealing under vacuum at various temperatures causes oxygen depletion with consequent reduction of the solid which shows up mainly in terms of formation of Ti 3+ reduced centers which are characterized by a typical broad EPR signal similar to that observed in reduced TiO 2 (anatase). The ability of the mixed oxides in stabilizing surface superoxide anions (O 2 •− ) has also been investigated by EPR to obtain information both on the photoinduced charge separation process in the solids and on the state of thermally reduced systems. In the latter case the obtained evidence have some value to understand controversial problems concerning the TiO 2 reduced matrix.