TiO₂ nanotubes (TiO₂-NTs) films have been traditionally prepared by electrochemical anodizing of highly pure Ti foils (Ti>99.999%) in aqueous or organic electrolytes. In this sense, since the oxygen content in pure Ti foils could be considered as traces, all the oxygen employed to promoting the growth of TiO₂-NTs on this Ti substrates, must be transported from the electrolyte. Therefore, organic solvents such as polyalcohols have been preferred for preparing the electrolytes utilized during Ti anodizing. In this investigation we proposed that industrial grade Ti foils having intercalated oxygen in their crystallographic networks, can be utilized for promoting the formation of TiO₂-NTs films in aqueous medium and in the absence of polyol type solvents. Our results confirmed that Ti grade 2 foils were successfully anodized by applying 20V in a two-electrode cell containing a 0.1M NaF + 0.5M H₃PO₄ aqueous solution an a Pt foil as cathode.  Anodizing times were varied at 2, 3 and 4h. The as-prepared TiO₂-TNTs films were characterized by scanning electron microscopy (SEM), UV-Vis diffuse reflectance spectroscopy, dispersive Raman spectroscopy (DRS) and X-ray diffraction (XRD).

Before any thermal treatment, SEM images were obtained from the anodized Ti substrates, thus revealing the formation of pristine TiO₂-TNTs having averages of 800nm-length, 115nm-internal diameter, and 24nm-wall thickness. Furthermore, the same SEM images revealed the presence of a TiO₂ compact layer on the top of all TiO₂-TNTs films, whose thicknesses increased as function of anodizing time. In order to remove this compact layer, an ultrasonic field of 42 kHz was applied for 20, 40 and 60s to all the anodized Ti foils. A complete removal of the compact layer was achieved only for the case of the TiO₂-TNTs films growth after 3h of Ti anodizing. Finally, a reproducible crystallographic composition of anatase and rutile, were obtained after the thermal treatment of the TiO₂-NTs films in good agreement with the literature. Finally, TiO₂-TNTs films obtained after 3h of Ti anodizing and 40s of ultrasonic treatment (i.e. TiO₂ compact layer-free TiO₂-TNTs) were tested as photoanodes of porphyrin-sensitized solar cells having an open-circuit potential of 0.40V, and fill factor of 0.43 under polychromatic illumination. At first sight, these results suggest a promising application of these electrodes into the field of the solar energy conversion devices.