Defects in the amorphous−crystalline evolution of gel-derived TiO2

The catalytic and electro-optical properties of TiO2based materials depend on the defectivity of their electronic structures and in turn on the synthesis conditions. It is however less clear how to control the number of defects in this oxide. In this paper, spectroscopic and structural characterization of chemical [acetylacetonate−TiO2, hybrid gel, hybrid sol−gel titania (HSGT)] and particulate [inorganic gel, sol−gel titania (SGT)] TiO2 amorphous gels is presented. HSGT and SGT were thermally treated at different annealing temperatures and at various times, obtaining anatase or rutile samples. X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and terahertz-time domain spectroscopy (THz-TDS) were exploited for monitoring changes in the crystallographic structure, chemical composition, and dielectric function. It is found that the thermal treatment affects in different ways the hybrid and the particulate TiO2 gels, resulting in an increase of electron transfer in the former. Moreover, XPS combined with X-ray-induced Auger electron spectroscopy provides evidence that the ionicity of the bond is also different and well correlates with the number of defects. These results demonstrate that XPS and THz-TDS techniques can be used as complementary tools for determining the electronic properties of titania with various degrees of crystallinity and defects.