Ellipsometric study of anodic oxide films formed on niobium surfaces

Anodic oxide films formed potentiostatically on niobium surfaces, from open circuit potential (OCP) to 10 V, were studied by performing in situ and ex situ ellipsometric measurements. The kinetics of the film thickness growth in 1 M H₂SO₄ and complex indices of refraction of these films were determined. A strong influence of the surface preparation conditions on the complex refractive indices of the metal substrate and anodic oxide films was shown. By steady-state measurements at OCP, a small thickening of the natural air-formed oxide film with chemical composition Nb₂O₅ in 1 M H₂SO₄ solution was detected. With cathodic pre-treatment, only partial reduction and small thinning of the natural air-formed oxide film was possible. The thicknesses of the natural air-formed oxide films on fine mechanically polished and electropolished Nb surfaces were determined. The build up of the natural air-formed oxide film, at ex situ conditions, on the already formed anodic oxide films was confirmed. It was shown that electropolishing gives more similar optical surface properties to the bare metal than the fine mechanical polishing. Electronic Publication     

Fast migration of fluoride ions in growing anodic titanium oxide

The rapid inward migration of fluoride ions in growing anodic titanium oxide under a high electric field has been elucidated by anodizing a Ti–12 at% silicon alloy, where film growth proceeds at nearly 100% efficiency in selected electrolytes. Further, incorporated silicon species in the anodic film are immobile, acting as marker species. The migration rate of fluoride ions is determined precisely by three-stage anodizing, consisting of initial anodic film formation at a constant current density to 50 V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by reanodizing to 55 V in ammonium fluoride electrolyte and, finally, anodizing again in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by glow discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of the silicon marker species. The fluoride ions migrate inward at twice the rate of O²⁻ ions. Consequently, anodizing of titanium in fluoride-containing electrolytes develops a fluoride-rich layer that separates the alloy substrate from the anodic oxide, with eventual detachment of the film from the substrate.     

The electrochemical behavior and characterization of the anodic oxide film formed on titanium in NaOH solutions

Titanium dissolution and passivation were studied in NaOH aqueous solution using open-circuit potential, potentiodynamic and potentiostatic techniques. Potentiodynamic data showed that the active-passive transition involves active metal dissolution followed by formation of a poorly conducting passive oxide film that passivates the electrode. The critical current density varied with pH as d log<I> j</I><SUB>m</SUB>/d pH=-0.098 in the pH range 11.00–14.00, while the passivation potential is changed according to the following two features: at pH 10.55–13.00, d<I>E</I><SUB>m</SUB>/d pH=-0.06 V; and at pH 13.50–14.00, d<I>E</I><SUB>m</SUB>/d pH=-0.40 V. The apparent activation energy, E*, was calculated from the slope of the Arrhenius plot and was found to be 12.6 kJ mol^–1. Current-time transients showed that the growth of titanium oxide passive film is a diffusion-controlled process. XPS measurements indicated that the passive oxide film consists mainly of TiO₂ and a mixture of suboxides of Ti₂O₃ and TiO. Electronic Publication     

The influence of monovalent cations on the stability of electrochemically formed nickel fluoride films

The stability of electrochemically formed NiF₂ film in 1.0 M perchloric acid containing monovalent fluorides namely, NH₄F, HF, NaF, KF and LiF, is investigated using cyclic voltammetry, chronoamperometry, atomic absorption spectroscopy and scanning electron microscopy. In addition to direct dissolution of nickel and dissolution through the oxide layer, a new mode of dissolution of NiF₂ film as NiF₃ ⁻ and NiF₄ ²⁻ through complex formation is proposed. This process is significantly influenced by the alkali metal fluorides. On a comparative basis the stability of NiF₂ decreases in the order NH₄F > HF > KF > LiF. Received: 29 July 1998 / Accepted: 3 November 1998     

Dissolution behavior of anodic oxide films formed in sulfanic acid on aluminum

Chemical dissolution of the barrier layer of porous oxide films formed on an aluminum foil (99.5% purity) in 1.5 M sulfanic acid after immersion in a 2 mol dm⁻³ sulphuric acid at 50 °C was studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. Re-anodizing was conducted in 0.5 mol dm⁻³ H³BO³/0.05 mol dm⁻³ Na²B⁴O⁷ solution. We found that the change in the porous oxide growth mechanism was observed at the anodizing voltage of 30 V. Taking into account this result chemical dissolution behaviour of the barrier layer of porous films formed at 20 V and 36 V and also the influence of annealing of oxide films at 200 °C were studied. We showed the interplay between the dissolution rates and charge distribution across the barrier layer. We conclude that the outer and middle layers have negative space charges and the inner layer has positive space charges.     

Effect of oxygen vacancies in anodic titanium oxide films on the kinetics of the oxygen electrode reaction

The effect of oxygen vacancies in the anodic oxide film on passive titanium on the kinetics of the oxygen electrode reaction has been studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Oxide films of different donor density were prepared galvanostatically at various current densities until a potential of 20.0 V_SHE was achieved. The semiconductive properties of the oxide films were characterized using EIS and Mott-Schottky analysis, and the thickness was measured using ellipsometry. The film thickness was found to be almost constant at ∼44.7 ± 2.0 nm, but Mott-Schottky analysis of the measured high frequency interracial capacitance showed that the donor (oxygen vacancy) density in the n-type passive film decreased sharply with increasing oxide film formation rate (current density). Passive titanium surfaces covering a wide range of donor density were used as substrates for ascertaining relationships between the rates of oxygen reduction/evolution and the donor density. These studies show that the rates of both reactions are higher for passive films having higher donor densities. Possible explanations include enhancement of the conductivity of the film due to the vacancies facilitating charge transfer and the surface oxygen vacancies acting as catalytic sites for the reactions. The possible involvement of surface oxygen vacancies in the oxygen electrode reaction was explored by determining the kinetic order of the OER with respect to the donor concentration. The kinetic orders were found to be greater than zero, indicating that oxygen vacancies are involved as electrocatalytic reaction centers in both the oxygen evolution and reduction reactions. This paper was submitted in honor of the many contributions to electrochemistry that have been made by Professor Boris Grafov. The article is published in the original.     

Impact of the type of anodic film formed and deposition time on the characteristics of porous anodic aluminium oxide films containing Ni metal

Porous anodic films containing nickel were prepared by AC electro-deposition. The porosity of the films was controlled by using different working conditions (anodisation electrolyte, voltage, and time). Then nickel was electro-deposited using an alternating voltage. The impact of the anodic film on the current density waveforms and the metal content can largely be explained by the porosity differences, while changing the deposition time caused changes due to over-oxidation of the aluminium substrate, experimentally proved by TEM. Finally, the impact of deposition time on the deposited metal was successfully fitted using an Elovich type law over a large time-span (up to 1800 s), showing the ability to achieve precise control of the metal content.     

Electrochemical formation of anodic oxide films on Nb surfaces: ellipsometric and Raman spectroscopical studies

Electrochemical formation of anodic oxide films on niobium (Nb) surfaces in 1 M H₂SO₄ solutions was studied using ellipsometry and Raman spectroscopy. By in situ ellipsometric measurements, the coefficient of film thickness growth and the complex index of refraction of anodic oxide films in the voltage range between 0 and 100 V were determined. The Raman spectra reveal that the thin passive films are amorphous. In the beginning of crystallization, the anodic oxide films consist of mixtures of NbO₂ and Nb₂O₅, while NbO₂ is completely transformed to Nb₂O₅ for thicker and well-crystallized films.     

Characterisation of anodic oxide films on zirconium formed in sulphuric acid: XPS and corrosion resistance investigations

Present work describes investigations of a two-step process consisting of galvanostatic anodising in a 1 M H₂SO₄ solution at 100 mA cm⁻² up to the limiting voltages of 20, 60, 80, 100 and 120 V, directly after which potentiostatic regime was employed and the current was allowed to drop. The total treatment time (5 min) was held constant for all samples. The treatment was carried out to improve the corrosion resistance of zirconium in physiological conditions, which was determined by electrochemical evaluation in Ringer’s solution. XPS studies revealed that after anodising sulphur was incorporated into the oxide film in the form of sulphated zirconia. The maximum content of sulphate in the oxide layer was observed after anodising at 80 V. Anodising at higher voltages resulted in formation of coatings with decreasing amount of sulphur. It was found that there is a strong correlation between the sulphur content in the oxide layers and the measured corrosion current density. On the other hand, the pitting corrosion resistance seemed to be unaffected by the presence of S and it was improving with the increasing limiting voltage of the treatment.

     

Electrocrystallization of cadmium on anodically formed titanium oxide

The electrocrystallization of Cd on previously anodized Ti substrates was studied by means of electrochemical techniques and SEM images in three different solutions of CdSO₄ (2, 10, and 50 mM). Voltammetric characterization showed the typical behavior of metal electrodeposition on conducting substrates, and potentials for electrodeposition of Cd were identified. However, the response obtained at different potentials in current transients exhibited an abnormal behavior suggesting the influence of TiO₂ on the process of electrocrystallization. The characterization of the obtained electrodeposits by SEM images allowed relating electrochemical with morphological changes, in particular, variations in the crystal size and shape as well as formation of presumably Cd branches on the substrate surface. This behavior only allowed the analysis of current transients of the electrodeposits obtained in 2 mM CdSO₄ solution using general equation for diffusion-controlled 3D growth and relating parameters of Cd crystal growth on anodized Ti electrode with the observed morphological changes.     

The effect of ionic and electronic properties of titanium oxide on the electrochemical growth and redox behavior of polyaniline on titanium surfaces

Polyaniline (PANI) was electrodeposited directly from aniline (ANI)-containing sulfuric acid media on Ti|TiO₂ electrodes, which were prepared in ANI-free 0.5 M H₂SO₄ solution by applying suitable anodization conditions in order to control the donor density (N _D) in the oxide layer of a fixed thickness. By using the suggested procedure, reproducible conditions for the cyclic voltammetric growth of PANI on Ti can be achieved, and most importantly, the properties of Ti|TiO₂|PANI electrodes can be modulated to meet the requirements for specific applications. The polymerization rate and the deposited amount of PANI were found to depend strongly on the N _D estimated by Mott-Schottky analysis. The redox behavior of the resulted Ti|TiO₂|PANI electrodes indicated differences in acid solutions as compared with the corresponding behavior of other PANI-modified metal surfaces. Analysis of the anodic and cathodic electrochemical response of the prepared Ti|TiO₂|PANI electrodes discloses the significant role of ionic and electronic properties of the intervening oxide between the metal support and PANI.     

Spectroscopic reflectometry as in‐operando method for thickness determination of anodic oxide films on titanium

The present work focuses on the development of an in‐operando technique based on the visible spectroscopic reflectometry (VSR) for simultaneous determination of the oxide film formation during anodizing. The establishment of the VSR as in‐operando technique requires an extensive validation by comparative in‐situ but non‐operando thickness measurements under aqueous conditions. The investigations were carried out on anodic oxide films on pure titanium. The authors demonstrate the VSR as a simple and robust method for measurement under electrolyte covering. Additionally, an empirical correction algorithm extends the limitation of the visible reflectometry in thin film thickness. Reliable film thickness values can be measured down to ≥5 nm. The in‐operando mode yields additional information about the film growth time resolved. Copyright © 2013 John Wiley & Sons, Ltd.     

Nitrogen oxide activation on the surfaces of titanium carbide and titanium nitride

The mechanism of NO activation on the surfaces of crystalline titanium monocarbide and titanium mononitride has been studied within the framework of semiempirical SCF LCAO NDDO method. The adsorption has been shown to be accompanied by electron density transfer from the crystal surface to the adsorbate molecule. Stronger activation of the NO molecule is observed on the TiC surface as compared with that on TiN surface.     

Gravity assisted growth of self-organized anodic oxide nanotubes on titanium

The present work investigates the anodic growth of self-organized TiO₂ nanotube arrays under different gravity fields. It is found that the aspect ratio of the nanotubes can be strongly affected by the orientation of the growing tubes within the natural gravity field or by high artificial gravitational fields in a centrifuge. Particularly remarkable is that under identical electrochemical growth conditions, the length of the nanotubes can be extended from 0.8 μm to 1.8 μm arranging the tube growth direction with or against the gravity field, or even extended up to 6.0 μm under high centrifugal forces (3220g).This strong effect is ascribed to gravity effects on the mass transport processes under diffusion controlled tube growth conditions.     

Effect of Sb on the properties of anodic Pb(Ⅱ) oxide film formed in alkaline solution

The composition and properties of the anodic films formed on Pb and Pb-3at.%Sb alloy at -0.10 V (vs. Hg/HgO) for 2.5 h in 0.1 mol.dm-3 NaOH solution (25℃) were investigated by cyclic voltammetry, linear sweep voltammetry, open circuit decay curve, photocurrent technique, X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the anodic film formed oh Pb mainly consists of t-PbO, while that on Pb-3at.%Sb consists of o-PbO, t-PbO and a small amount of orthorhombic Sb2O3. The dominant component of the film anodically grown on Pb-3at.%Sb for less than 5 min is o-PbO, however, t-PbO is the major component of the anodic film formed for 1 h or longer. It is established that Sb suppresses the growth of t-PbO. The anodic film formed on Pb-3at.%Sb is less porous than that on Pb. The bandgap energies of t-PbO and o-PbO in the films were determined by photocurrent measurements to be 1.83-1.84 eV and 2.60 eV, respectively.     

Electrochemical and structural investigations of oxide films anodically formed on ruthenium-plated titanium electrodes in sulfuric acid

The electrochemical characteristics of ruthenium oxides, formed on Ru-plated Ti electrodes in 0.5 M H₂SO₄ by potential cycling with different CV upper potential limits (E _SU), were systematically compared. The repeated potential cycling between 0.2 and 0.75 V activated the formation/reduction of surface Ru oxides with hysteretic behavior. This application of repeated CVs also modified the ability of Ru deposits for hydrogen adsorption/desorption. An irreducible Ru oxide accumulated on the electrode at potentials more positive than ca. 0.95 V, whose capacitive characteristics are applicable for electrochemical supercapacitors. This irreducible oxide was composed of an aggregate consisting of Ru in various oxidation states, bridged oxygen, OH and water in a 3D-like structure with a relatively ordered and compact nature, from the X-ray photoelectron spectroscopic and voltammetric results. The surface reconstruction of the Ru deposits induced by the repeated potential cycling with E _SU≥0.75 V was clearly observed from the SEM photographs. From the X-ray diffraction patterns, all the anodically formed Ru oxides showed an amorphous nature.     

Toward a quantitative description of the anodic oxide films on aluminum

A method to quantify the composition of anodic oxide films on aluminum using Infrared Spectroscopic Ellipsometry (IRSE) is proposed. It consists of obtaining the absorption coefficient of the film as a function of wavelength. Using values of the absorption coefficients for the pure components of the film, the percentages (mole or wt%) of each component in the sample can be calculated.The method is demonstrated in a study of the structure of the oxide film on electropolished aluminum and the anodically formed barrier layer film. Both surface oxides were found to be initially a form of amorphous Al₂O₃. While the barrier film is essentially free of water as prepared, the film on electropolished aluminum contained about 25 wt% water. Hydration of both types of films by immersion in boiling water results in the formation of pseudoboehmite (AlOOH). The technique may have more general applicability to the quantitative determination of the composition of corrosion films and other surface layers on metals.     

Nickel- and copper-containing oxide films on titanium

We have studied the composition and structure of films 20–30 μm thick prepared by plasmaassisted electrochemical oxidation (PEO) and additionally modified by impregnation in aqueous solutions of nickel and copper nitrates and then annealed. The investigative tools used were powder X-ray diffraction, electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy. Unmodified film/titanium composites have a certain catalytic activity in CO oxidation to CO₂ at temperatures above 300°C; for modified layers, these temperatures are noticeably higher. Modification influences the surface structure, relief, and elemental composition. Relations between the composition and catalytic properties of the oxide layers are discussed.     

Influence of ph on the reduction of thick anodic oxide films on gold

The reduction of thick oxide films, produced anodically on gold under constant potential conditions in the oxygen gas evolution region, was examined as a function of sweep-rate in both acid and base. Although peak maxima potential values for the reduction reactions were not totally independent of sweep-rate, even at very low values of the latter, the earlier assumption that the film is composed of at least two (and in some cases, according to the present results, as many as five) different oxygold species was shown to be a reasonable one. The other assumption, namely that the hydrous film in the case of gold is anionic (due to excess hydroxide-ion incorporation) was also shown to be a reasonable one by comparison with recent work with hydrous nickel oxide films for which structural data is available. In these systems counterbalancing cations (H⁺, Na⁺, etc.) are assumed to be present in water contained in pores or layers in the structure. Furthermore, the unusual potential-pH dependence of the hydrous film reduction process can easily be explained in terms of the excess hydroxide incorporation model.     

Characterization of the anodic growth and dissolution of oxide films on valve metals

Chemical dissolution processes coupled to anodic oxide growth taking place by a “high-field” conduction mechanism, are considered. The equation for the steady-state current density obtained during potentiodynamic polarization measurements is derived and the effect of the oxide dissolution rate on the overall potentiodynamic behaviour by applying repetitive scans with either fixed or increasing anodic switching potentials is discussed. The procedure for obtaining the current dissolution as well as the parameters that characterize the high-field growth is discussed.