Morphological, electrochemical and photoelectrochemical characterization of nanotubular TiO2 synthetized electrochemically from different electrolytes

Abstract

The electrochemical synthesis of auto-organized TiO 2 arrays generated by Ti anodization in different nonaqueous electrolytes was analyzed. Propylene carbonatebased electrolytes seem to be good electrolytic media for the controlled growth of titania nanotube structures. Scanning electron microscopy (SEM) analysis indicated that, depending on the media employed in the synthesis of the TiO 2 nanotubes, different aspect ratios (length/diameter) could be obtained. Impedance analysis using the passive pit model gave the electrical properties of the layers during the different stages of nanotubes formation. Mott-Schottky analysis and the impedance results showed that the TiO 2 nanotubes layers were n-type semiconductors with highly defective walls on top of a resistive barrier layer. Photoelectrochemical measurements showed that tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbed onto the TiO 2 nanotube arrays, which could be employed as an efficient photosensitizer for solar-energy conversion by TCPP-sensitized nanotubular TiO 2 electrodes.

Enhancement of photoelectrochemical activity for water splitting by controlling hydrodynamic conditions on titanium anodization

“…In the case of the compact TiO 2 film the time constant at high frequencies (R 1 CPE 1 ) is related to the outer layer of the passive film, and the low-frequency behavior is attributed to the inner layer of the film, more compact than the outer one [42,43]. In the case of the nanostructured layer to a porous layer [7,27]. charge carriers in the nanosponges [35], as it will be verified later with Mott-Schottky analysis.…”

“…This equivalent circuit has been used in the literature to model the impedance data of compact and nanoporous TiO 2 films [7,8,14,27,34,43]. Constant phase elements (CPEs) have been used instead of pure capacitors to account for frequency dispersion and non-ideality.…”

“…If the depletion layer is very thin, photons can penetrate beyond this layer and light will be absorbed in the bulk semiconductor, where the electrical field is absent [4,16,54,55]. The lack of an electrical field prevents the photo-excited electron-hole pairs from being effectively separated, thus increasing the probability of recombination [4,16,27,54]. Furthermore, oxygen vacancies in TiO 2 have been found to act as recombination centers for electron and holes, playing a critical role in the trapping process: an excess of oxygen vacancies will result in more photogenerated electrons being trapped thus decreasing their contribution to the photoelectrochemical processes [37,56,57].…”

Effect of Reynolds number and lithium cation insertion on titanium anodization

“…indicating that the thickness of the compact TiO 2 layer decreases, which can be interpreted as a transition from a mostly compact layer to a nanotubular one [40].…”

“…The electrical equivalent circuit proposed is shown in Figure 6d and it is formed by a resistive 10 element (R s ), corresponding to the electrolyte resistance, and two groups of resistances and constant phase elements (R-CPE), corresponding to the nanotubular layer (R 1 -CPE 1 ) and the compact TiO 2 layer (R 2 -CPE 2 ) [15,40]. This electrical equivalent circuit has been already used by other authors.…”