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The aim was to analyze the protective effects of titanium, zirconium and hafnium tetrafluorides on erosion of pellicle-free and pellicle-covered enamel and dentine in vitro. Eight groups of 20 specimens each of bovine enamel and bovine dentine were prepared. Half the specimens in each group were immersed in human saliva for 2 h for pellicle formation. Specimens were then left untreated (controls) or were treated for 120 s with TiF₄, ZrF₄ or HfF₄ solutions (0.4 or 1%) or 1.25% AmF/NaF gel. All specimens were eroded by exposure to hydrochloric acid, pH 2.6, for 25 min. Cumulative calcium release into the acid was monitored in consecutive 30-second intervals for 5 min, then at 2-min intervals up to a total erosion time of 25 min using the Arsenazo III procedure. Data were analyzed by ANOVA. 1% TiF₄ solution offered the best protective effect, especially in dentine (reduction of calcium loss about 50% at 25 min). 1% ZrF₄, 1% HfF₄ and 0.4% TiF₄ also reduced calcium loss, but to a lesser extent. Long-term effects were limited to dentine, while reduction of enamel erosion (about 25%) was restricted to 1-min erosion. The fluoride gel had a protective effect only in dentine. The efficacy of the tetrafluorides was influenced by the presence of the pellicle layer, in that the protection against dentine erosion by TiF₄ and ZrF₄ was greater on pellicle-covered specimens. Tetrafluoride solutions, especially 1% TiF₄, could decrease dental erosion, but were more effective on dentine than on enamel.

In the present work, well ordered arrays of TiO 2 nanotubes were obtained by anodization at different voltages of Ti foil in a solution containing anhydrous ethylene glycol + 0.27 M NH 4 F and 0.2 wt% water. A detailed study has been performed to evaluate the morphology and electrochemical characteristics of the anodized Ti foil synthesized in different anodization potentials. Correlation between the physical properties and the dimensional aspect of TiO 2 nanotubes was examined. In this paper, we report the electrochemical characteristics of the Ti/TiO 2 nanotube surfaces from Tafel plots. The electronic properties of TiO 2 nanotubular layers were also determined by electrochemical impedance spectroscopy analysis (EIS). EIS is considered to be a valuable tool that allows determining barrier oxide parameters. A linear dependence of barrier thickness on the anodizing voltage was verified from 20 to 50 V, the proportionality constant of the barrier layer thickness in relation to the applied anodizing voltage was estimated from the proposed equivalent circuit.

The electronic properties of a TiO2-nanotube (NT) array used as a photoelectrode for water oxidation at neutral and basic pH were characterized by combining complementary measurement techniques: transient photocurrents, stationary photocurrent–voltage curves, photo-electrochemical impedance spectroscopy (PEIS), and intensity-modulated photocurrent spectroscopy (IMPS). Transient measurements point out the slow chemical modification of the TiO2 surface when going from dark to light, essentially around neutral pH. After this transient period, a new stationary state of the TiO2 surface is established, allowing small amplitude perturbation techniques (PEIS and IMPS) to be applied to obtain information on transfer and recombination kinetics and on surface states contribution. The relevant information was obtained via theoretical models for the PEIS and IMPS responses, involving physical parameters with values extracted by nonlinear least-squares fitting. The main conclusions taken from our experiments include the following: (i) Under ultraviolet light illumination, the surface chemistry of TiO2 was found relatively stable at basic pH but strongly modified (hydroxylation) at neutral pH. (ii) Hole transfer to solution species takes place preferentially via the valence band. (iii) Recombination is mainly a surface process. (iv) Rate constants for charge transfer and recombination were determined as a function of the applied potential in agreement with the stationary photocurrent–voltage curve.

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Electrochemical methods (cyclic voltammetry (CV), potential steps, and electrochemical impedance spectroscopy) were successfully combined with in situ reflectometry measurements for a detailed analysis of the passive layer evolution as a function of the electrode potential. Interestingly, both EIS and surface reflectivity allowed a film thickness in the nanometer range to be readily determined. In addition, transient analyses of the reflectivity simultaneously recorded with CVs show the formation of both FeO and FeO oxides. The image analysis showed that the steel surface reactivity is heterogeneous and presents micrometric islands coated with a thicker oxide layer than the surrounding surface. The in situ combination of these techniques thus offers a powerful analytical description of the interface on a local scale and its transient response to a perturbation.