Electrochemical Impedance Spectroscopy of Porous TiO2 for Photocatalytic Applications

High surface area immobilized TiO2 were grown via several electrochemical anodization methods for photocatalytic applications. Mesoporous TiO2 was grown in a molten salts electrolyte and in a sulfuric acid solution above the micro sparking potential. On the contrary, nanotubular TiO2 was grown in a sodium sulfate solution with the addition of fluoride ions, leading to the formation of fine elongated nanotubes with high surface area. The different types of photocatalysts were characterized by SEM and XRD in addition to electrochemical studies which include linear sweep voltammetry and open circuit potential relaxation. Electrochemical Impedance Spectroscopy (EIS) was used to study the impedance and capacitance of the TiO2 in the dark and under UV illumination together with Mott−Schottky analysis. The results of the EIS were correlated with the microstructural characterization and the photocurrents measurements along with photocatalytic degradation of Methyl Orange (MeO). The combined results led us to a better understanding of the electronic properties of n-type TiO2 and the effect of the growing method on its properties such as the surface area, crystal structure, charge carrier concentration, and charge transfer rate. The nanotubular structure possesses the highest surface area and higher charge carrier concentration, albeit the charge transfer rate is slower. Nevertheless, it is the most efficient photocatalyst toward degradation of MeO. The use of the described combined methods is a powerful tool toward predicting and understanding the ideal anode for photocatalytic process.