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This paper studies the effect of voltage and electrolyte composition on the morphology of TiO2nanotube films, their semiconductor properties and photoelectrochemical performance for water oxidation. TiO2nanotube films were formed by potentiostatic anodization in fluoride containing electrolytes (using either HF or NH4F) during 1 h at four different voltages and were heat treated at 450°C to obtain anatase polymorph. SEM images showed a more significant chemical attack when NH4F was employed, which was evidenced by larger tube diameters and thinner films. UV-vis reflectance spectra showed that the TiO2films formed in aqueous electrolyte were highly defective, presenting various broad absorption peaks in the visible region attributed to the sub-bandgap energy states. The semiconducting properties were modified by the nature of the counter-ion utilized during film growth, resulting in films with a higher donor density and a lower flatband potential in NH4F electrolytes. The films formed in NH4F showed higher photopotentials and photocurrents related to the modification of their morphology and semiconducting properties, attaining a maximum at a formation voltage of 15 V.

The effect of pH on the potentiostatic anodization growth of TiO2film in 0.1 M HClO4/0.05 M NH4F solution was electrochemically characterized by EIS measurements. The increase in electrolyte pH modified considerably the voltammetric behavior of Ti electrode by diminishing the active dissolution region of the material and giving rise to two current plateaus that indicate modification of properties of the formed oxide. Five different formation potentials (EF) were selected from this study to potentiostatically grow oxide films, and to evaluate their morphology, the modification of their properties, and their interaction with the electrolyte. SEM images of the formed films showed that the EFincrease resulted in a larger diameter of the pores formed, but an increase in pH led to a decrease in this parameter. In-situ characterization by EIS indicated that pH increase in the electrolyte used in anodization causes lower interaction between the formed oxide and F−ions in the solution due to the change in the surface charge excess of Ti oxide, resulting in an increase in the compact layer resistance and a decrease in both the time constant associated with F−ion adsorption and F−ion diffusion coefficient within the compact oxide layer.

Nanoporous TiO2films were formed by anodization in three different electrolytes fluoride-containing, at different formation potentials for 2.5 hours. Film topography and morphology were characterized by AFM and SEM images, respectively. Aditionally, an in-situ study of the anodic film growth was carried out by electrochemical impedance spectroscopy. The growth ratio for the pore size obtained from SEM images was not directly proportional to the barrier layer growth ratio derived from EIS. The main role of the counter-ion seems to be insertion of fluoride ions altering the properties of the formed oxide, decreasing the film resistance and causing a more aggressive chemical attack, observed in the increase in pore size and film roughness. The solvent tunes the chemical attack of the anodic layer, and could also be establishing the titanium oxide stoichiometry, thus altering oxygen vacancies in the film and the migration of fluoride ions through the oxide lattice.

The effect of HF concentration on the morphology and growth of barrier layer during anodization of Ti electrodes has been studied. The wide range of concentrations selected, rarely used for the formation of self-organized porous films, corresponds to the formation of films with different morphologies. The voltammetric behavior showed a high potential range in which anodic films were potentiostatically grown (6.5 V). As the HF concentration increased, SEM images evidenced two transitions in the morphology of the outer layer formed at the oxide/electrolyte interface, which match with the transition observed in the steady state current densities. However, EIS spectra only showed one transition. Fitting EIS spectra with an electrical equivalent circuit, commonly employed to study passive films in aggressive media, showed a monotonic change in the growth and properties of the barrier layer formed, evidencing that fluoride ions are inserted into the oxide through oxygen vacancies playing an important role during self organized anodic film formation.

In the present work the effect of the HClO4 concentration over the morphology and growth of anodic Ti oxide films in fluoride containing electrolytes is studied. The voltammetric behavior of the Ti is not considerably affected by HClO4 concentration, showing a current plateau in a high potential range in which the anodic films were potentiostatically grown (6.5 V). Even when the SEM images evidenced a decrease in pore diameter with the increase of perchloric acid concentration, the recorded chronoamperograms did not show any considerable changes in the growing of the films. However, in the EIS spectra, it is observed a modification of the high frequency components, provoked by the detriment in the oxide film resistance and capacitance, allowing the correlation between the barrier layer thickness and resistance with the pore diameter of the external porous layer formed.

The role played by solvent during cathodic EPD of TiO2 film was investigated. The differences in film morphology are attributed to the dissimilar surface excess charge of the TiO2 agglomerates in each solvent and to the secondary reactions occurring during the EPD. For the films obtained in aqueous medium an augmentation of donor density and a change toward less negative values of the flat band potential were measured. Furthermore, a decrease in photocurrents and photovoltages was provoked by increasing the electric field used to form the film. Meanwhile, for the films formed in organic medium, the semiconductive properties did not change with the electric field employed to grow the films, and the photoelectrochemical behavior of these films showed completely different trends. The dissimilarities found were attributed to the ability of the solvent to be reduced earlier than TiO2, avoiding the generation of species Ti3+ that acts like trap states.

The present work studies the influence of applied electric field on the cathodic electrophoretic formation of TiO2films in an aqueous medium. The growth of the film was followed by measuring current density, deposited mass, and thickness under imposed electric field (φ). In scanning electronic microscopy images of the TiO2films formed under different electric fields, three regions can be seen: (i) formation of nuclei that are converted into agglomerates, (ii) uniform and one-sided growth, and (iii) growth by the formation of agglomerates on the TiO2surface. The characterization of semiconductor properties showed that increased φ for film growth leads to an increase in the density of donors and to a displacement of the flatband potential toward less negative values. The photoelectrochemical characterization of the oxide films revealed that the photoelectrochemical performance of the TiO2films decreases as a function of the electric field used to carry out the electrophoretic deposition (EPD) process. The electrochemical behavior observed for these films seems closely related to the content of Ti3+doping sites electrogenerated in situ during the EPD, which act as electron traps negatively affecting the electron transport toward the indium tin oxide collector, also making their photoelectrochemical activity inefficient.

The electrochemical characterization of Ti anodic films during their growth in 0.1 M NaOH indicates structural changes related to the formation of different oxides i.e. TiO, Ti2O3 and TiO2. These transformations do not affect the growth kinetics, since the film thickness showed a linear relation vs. the formation potential (Ef). On the other hand the exponential dependence of the density of donors (Nd), within the whole potential range of Ef, is associated with the distribution of different oxides inside the film. The oxide with low oxidation state is near to the metal/oxide film interface and as going to the oxide film/solution interface, the oxides with higher oxidation state appeared (Ti2O3 to TiO2). Furthermore, the dependence of the flat band potential (Efb) with Ef, is attributed to the composition of the oxide films formed by a mix of Ti oxides.