Influence of geometric and electronic characteristics of TiO2 electrodes with nanotubular array on their photocatalytic efficiencies

The present work compares the efficiencies of titania nanotubes (NT) formed by anodizing of titanium at 20 V for 45 min in three kinds of electrolytes: ethylene glycol–ammonium fluoride–water (NT1), tetrabutylammonium fluoride–formamide–water (NT2) and sodium sulfate–ammonium fluoride–water (NT3). The geometric characteristics were investigated by scanning electron microscopy (SEM). The photoactivities of the three layers were compared through photo-potential, photo-current measurements in an alkaline medium and in the presence of methanol as a hole scavenger. Impedance spectrometry was used for investigation of conduction and charge storage properties of the titania layer. Finally the photocatalytic properties were compared through the degradation rate of an azo dye, such as Amido Black (AB).In comparison to NT2 and NT3, the NT1 layer showed not only the highest photopotential, but also the highest photocurrent response. Photocatalytic results showed that the degradation of dye proceeds much faster in the presence of NT1 as compared with other TiO2 nanotube samples. Nevertheless photoinduced surface states were found to be more important in NT2, in comparison with the two other layers. All these results are discussed taking into account the geometric characteristics of the nanotubular layers, not only the specific surface on one hand, but also the solid fraction of the external surface, which determines the level of light absorption able to generate photo-charges.

► Photocurrent, impedance and photocatalytic efficiency.
► 3 types of TiO2 layers with nanotubular array.
► Correlation with the morphology of the layers and the presence of photogenerated surface states.