Qualitative Models for the Photoresponse and Capacitance of Annealed Titania Nanotubes

Physicochemical characterization of annealed TiO2 nanotubes (TNTs) was conducted by using photocurrent spectroscopy and differential capacitance techniques. It has been shown that the geometry and architecture of nanotubes determine how photogenerated electrons and holes are separated and transferred. Photocurrent generation in TNTs is a consequence of two phenomena; drifting of holes into the electrolyte and diffusion of electrons toward the substrate. These two processes have been shown to be independent of the anodic polarization. The capacitance of TiO2 nanotubes is also affected by their geometry. In anodic potentials, with respect to the flat band potential of the underlying barrier layer, the capacitance is mainly controlled by the barrier layer because nanotubes are almost inactive. In the cathodic potential region, electrons injected from the substrate into the conduction band of TiO2 induce nanotubes to behave more like porous metallic electrodes. As a consequence, the electrochemical double layer along TNTs large surface area causes high values of capacitance.