Structural and spectroscopic properties of high temperature prepared ZrO2-TiO2 mixed oxides

ZrO2-TiO2 mixed oxides of various composition, with the molar fraction of TiO2 ranging from 0.1% to 15%, have been prepared via sol-gel synthesis and then calcined at 1273 K to check both their thermal stability and physicochemical properties. These solids are usually employed in photocatalytic processes and as active phase supports in heterogeneous catalysis. As indicated by X-ray diffraction and Raman spectroscopy, solid solutions based on Ti ions diluted in the ZrO2 matrix are formed in the whole range of Ti molar fraction examined. Materials with low Ti loading (0.1%-1%) are basically constituted by the monoclinic phase of ZrO2 while the tetragonal phase becomes prevalent at 15% of TiO2 molar fraction. The presence of Ti ions modify the electronic structure of the solid as revealed by investigation of the optical properties. The typical band gap transition of ZrO2 undergoes, in fact, a red shift roughly proportional to the Ti loading which reach the remarkable value of 1.6 eV for the sample with 10% of molar Ti concentration. Comparing chemical analysis of the solids with XPS data it has been put into evidence that the titanium ions distribution into the solid is not uniform and the concentration of Ti4+ tend to be higher in subsurface layers than in the crystal bulk. The introduction of titanium ions in the structure increases the reducibility of the solid. Annealing under vacuum at various temperatures causes oxygen depletion with consequent reduction of the solid which shows up mainly in terms of formation of Ti3+ reduced centres which are characterized by a typical EPR signal. Ti3+ defects forms, as also forecast by theoretical modelling of the solid, as their energy is lower than that of other possible reduced defective centers. The reduced solids are able to transfer electrons to adsorbed oxygen molecules in mild condition resulting in the formation of surface superoxide anions (O-2(center dot-)) which are stabilized on surface Zr4+ or, alternatively, on Ti4+ ions according to the sample composition. (C) 2013 Elsevier Inc. All rights reserved.