Titania nanotubes self-assembled by electrochemical anodization: Semiconducting and electrochemical properties

Titania nanotubes (TNT), thanks to their semiconducting properties, have received wide attention for application in many fields such as photoelectrolysis, dye sensitized solar cells, photocatalysis, and sensors. In this work, highly ordered TNT were grown by controlled electrochemical anodization of titanium sheets. Scanning electron microscope equipped with a field emission gun and electrochemical DC/AC techniques was used to characterize the TNT. Semiconducting properties were investigated through linear sweep voltammetry and electrochemical impedance spectroscopy. Donor concentration (N_ed) was obtained by recording Mott–Schottky plots. The high N_ed of TNT (around 10²⁶ m⁻³) allows an optimal electron transfer when used as photoelectrode. Frequency dispersion of flat band potential from Mott–Schottky plots (− 0.38 ÷ + 0.40 V vs. saturated calomel electrode, SCE) was used as an indicator of the amorphous semiconductor behaviour. The dispersion of flat band in heat treated samples was extremely reduced (0.48–0.51 V vs. SCE) because of the conversion to crystalline semiconductor. The depth of space charge was comparable to the TNT wall thickness, meaning that the entire TiO₂ nanotube walls formed the space charge layer. Considering the high charge carrier concentration, we can hypothesise a high density of electronic defects (e.g., surface states) that enhances the electron transport by percolation inside a porous photoelectrode. The transition from amorphous to crystalline structure of TNT was detected from the change of semiconducting properties and confirmed by Raman spectroscopy. - Highlights: • Highly ordered self-assembled TiO₂ nanotubes were synthesized by electrochemical anodization. • The electrochemical and semiconducting properties were studied. • An electrical model was developed for titania nanotubes/electrolyte interface. • Thermal treatment changed electronic properties of TiO₂ nanotubes. • Transition from amorphous to crystalline structure determined by electrochemical tests