Growth and characterization of TiO2 nanotube arrays under dynamic anodization. Photocatalytic activity

Investigations of the kinetics of the competing oxide growth vs. fluorine-induced corrosion of the oxide layer at the titanium anode/electrolyte interface of an anodization cell have been carried-out under dynamic biasing conditions. Periodical sawtooth voltage ramps with duration of 100 s and variable start values (0, 10, 20 and 30 V) and increasing slopes (0.3, 0.2, 0.1 and 0.0 V/s) were used to grow titania nanotubes, while monitoring the temporal variation of the anodization current. The electrolyte solution was a mixture of glycerol (C3H8O3) and ammonium fluoride (NH4F), 0.5% wt. The evolution of the interface processes during each dynamic-biasing experiment, as reflected in the j(t) characteristics were correlated with the nanotube layer morphology, crystallinity and chemical composition, as derived from SEM, XRD and XPS measurements. Cyclic voltammetry measurements, diffuse reflectance spectroscopy and tests of photocatalytic degradation of methylene blue aqueous solution were carried out to evaluate the photocatalytic activity of the samples. It was found that by gradually increasing the effective anodization voltage above a threshold of 22 V, degradation rates increase by a factor of 7.3. The results may serve both as an additional benchmark for understanding and controlling the chemical reactions at the anode-electrolyte interface, but also to design flexible set-ups for the fabrication of high-quality TNA-based devices for energy and environmental applications.