Structure induced optical properties of anodized TiO2 nanotubes

TiO2 nanotubes were synthesized by means of anodization and investigated for their structure dependent optical properties. The anodization was conducted at operating voltages between 5 and 30 V for 3 h in a neutral, organic electrolyte consisting of 0.3 wt% NH4F + 2 wt% H2O + ethylene glycol and the resulting nanotubes were annealed at 450 °C for 2 h in air at atmospheric pressure. It is shown that an increase in the applied anodization voltage yielded an increase in the wall thickness, diameter and length of the nanotubes and that these varying morphologies have a direct influence on the crystallite size of the nanotubes during annealing. Photoluminescence spectra indicated that the optical bandgap of the TiO2 nanotube film decreased with the increase in the anodization voltage, whereas supplementary Raman spectra showed a decrease in the confinement of the optical phonon modes as the crystallite sizes increased, in coherence with the phonon confinement model. These results present significant insights into the size-dependent properties of these novel nanostructured forms of TiO2 and play an important role in their implementation in photovoltaic devices, such as the dye-sensitized solar cell.