Electronic Properties of Thermal Oxides on Ti and Their Influence on Impedance and Photoelectrochemical Behavior of TiO2 Nanotubes

Thermal oxidation of titaniumwas carried out at 350◦C, 450◦C, and 550◦C for 2 h or 12 h.X-rayDiffraction and Raman Spectroscopy suggest that the thermal oxides are scarcely crystallinewhen the annealing temperature is low, while both anatase and rutile are present for high annealing temperature and time. Photoelectrochemical measurements allowed estimation of a bandgap decreasing from 3.35 eV to 3.15 eV with increasing annealing temperature. The impedance spectra confirmed the formation of n-type semiconductors, with an impedance strongly decreasing on going from a reverse bias toward a forward bias regime. TiO2 nanotubes grown by anodizing Ti in NH4F and water containing ethylene glycol solution were annealed at different temperature and for different times. X-ray Diffraction and Raman Spectroscopy proved that rutile is present at the barrier layer beneath the tubular array, while the tubes are mainly constituted by anatase. The incident photon to current conversion efficiency increased by increasing the annealing temperature and time. The impedance spectra recorded under reverse bias regime for thermal planar oxides and for annealed TiO2 NTs are almost coincident, confirming that the predominant contribution arises from the barrier layer beneath the tubular array. Only under forward regime do the tube walls contribute to the measured impedance.