Impedance Behavior of TiO[sub 2] Nanotubes Formed by Anodization in NaF Electrolytes

Abstract

The impedance behavior of nanotubular self-organized porous TiO 2 ͑PTO͒ films with thickness of Ͼ2 m, 100 nm pore diameter, and 150 nm average spacing formed on titanium by anodization in 1 M Na 2 SO 4 solution +0.5 wt % NaF was investigated and compared to the behavior of compact TiO 2 layers ͑CTO͒. At potentials close to the flatband potential ͑E FB ͒, PTO had an apparent interfacial capacitance at 30 Hz that was greater than the capacitance of CTO. This effect, however, was not only related to the larger effective area of PTO. Differentiation between both surface conditions vanished both at potentials significantly higher than E FB and with increasing test frequency to 1 kHz. These findings together with observations of appreciable frequency dispersion suggest that the pore walls are rich in deep-lying localized states, which become evident only at lower test frequencies and at potentials negative enough that depletion zones do not merge at pore walls. PTO did not show a transmission line effect at frequencies as high as 100 kHz. Model calculations based on pore dimensions and electrolyte conductivity predicted that those effects could only take place at much higher test frequencies, but the slow response of deeper states might impair observation anyway.

Some critical factors for photocatalysis on self-organized TiO2 nanotubes

“…It is important to note that the OCP under illumination with O 2 bubbling is at −350 mV, whereas the OCP with Ar bubbling is at −500 mV. This is particularly interesting when comparing these values to the flat band potential of TiO 2 , which lies at around −400 mV (Ag/AgCl) for Na 2 SO 4 electrolytes [31]. In other words, as illustrated in Fig.…”

TiO2 Nanotubes: Synthesis and Applications

“…0.3 V anodic to the flat band potential), total carrier depletion of the walls is reached [235] (using capacitance data and the Schottky approach). [234,[236][237][238] In other words, the space charge layer follows the wall contours only up to this threshold voltage (and may aid charge separation); at higher voltages the depletion layer cannot extend any further. This is the key reason why for tubular shapes a deviation from the classical Gärtner model [239] for the potential dependence of the photocurrent is observed.…”

Effect of duty cycle and electrolyte additive on photocatalytic performance of TiO2-ZrO2 composite layers prepared on CP Ti by micro arc oxidation method

“…6a). This circuit is based on the model applied for porous TiO 2 [37,38] containing two time constants in order to properly address the high and low frequency features of the layer/electrolyte interface, impedance response of the pores and the process occurring at the electrode surface. In this circuit the paralleled elements of CPE i and R i are associated with the low frequency response of the oxide layer electrical components, namely to the surface adsorbed ionic species pseudocapacitance C i and resistance R i of the layer.…”