TiO 2 films prepared by sol-gel route are active photocatalysts for the oxidation of organics in photoelectrochemical cells. The as-grown films for photocatalysis applications and those exposed to Ar + or H 2 + +Ar + ion bombardment are characterized by different spectroscopic methods, such as X-ray diffraction (XRD), atomic force microscopy (AFM), UV-vis transmittance, photothermal deflection spectroscopy (PDS) and X-ray photoelectron spectroscopy (XPS), as well as by conductance. This material has defects associated with oxygen vacancies produced during the sample preparation which support nondissociative adsorption of O 2 when films are exposed to air. Charge transfer from reduced Ti species to adsorbed dioxygen leads to Ti-O 2surface complexes that are partially removed by heating at 200 °C, and fully removed after 30 min ion bombardment. By comparison with the relatively well-understood structural defects of bombarded TiO 2 we arise to a quite complete structural model of the as grown material which corresponds to an amorphous semiconductor possessing relative low disorder and density of states as compared with a pure amorphous material. These TiO 2 films are modeled as low size crystalline domain embedded in an amorphous matrix whose electronic structure exhibit exponential band tails and a narrow band close to the conduction band. The latter is fully or partially occupied depending on the presence of adsorbed electron scavengers such as dioxygen.

A series of fluorescent compounds (rhodamine 101, cresyl violet, oxazine 1, and tris(2,2‘-bipyridyl)ruthenium(II) chloride hexahydrate) were used as molecular probes in titania gels. Sols and gels of pH = 2.5 and 3.5 were prepared using standard methods. Absorption and emission spectra and fluorescence quantum yields of the probes were measured at room temperature. Fluorescence intensity and steady-state fluorescence anisotropy, 〈r〉, were measured for cresyl violet and oxazine 1 at controlled temperature between 25 and 95 °C as function of aging. The local environment of Rh101, Rbpy, CV, and Ox1 in the gels at 25 °C is essentially aqueous. Among these compounds, only CV and Ox1 are appropriate for fluorescence anisotropy studies in titania gels of low pH. Parameters for the calculation of the local friction, ξ, inside the cavities were obtained from previous experimental data or by modeling a symmetrical molecule as prolate ellipsoid. For CV, the comparison of the fluorescence anisotropy data in microheterogeneous systems and homogeneous solutions can be directly performed, because its rotational dynamics has been very well characterized in water and alcohols. The fluorescence anisotropy and photophysical parameters of Ox1 in water and alcohols are less known, but the symmetry properties allows us to obtain analytical expressions for the rotational correlation times and for the local friction. Even when the bulk viscosity of the gel increases, there is a minor restriction for the rotation of the fluorescent probes, indicating that the gel is composed of a rigid network with solvent-filled cavities of volumes larger than 500−600 Å3. The similarity of the values of ξ for both probes is a strong validation for the model used for Ox1 and allows Ox1 to be used as an alternative to CV for the method.