Modelling the formation and growth of anodic passive films on metals in concentrated acid solutions

Two model approaches to the formation of passive films as adsorbed layers during the active anodic dissolution of a metal in acid and their subsequent growth are presented. The first depicts passivation as proceeding in parallel to active dissolution. Adsorption of water on active surface sites leads to passivation, whereas adsorption of acid leads to active dissolution of the metal. The model is consistent with the impedance response during passivation of Fe and an Fe-20%Mo alloy in concentrated H₃PO₄. The second model is an updated version of the so-called surface charge approach to the mechanism of conduction of anodic passive films. It is based on the assumptions that oxygen vacancies are the main ionic charge carriers and the field strength in the barrier layer is constant. A negative surface charge built up at the film/solution interface via accumulation of metal vacancies accelerates oxygen vacancy transport, thus explaining the pseudoinductive behaviour of the metal/film/electrolyte system under small amplitude a.c. perturbation. The model describes the growth of thin anodic films on Fe, Mo and an Fe-20%Mo alloy in concentrated H₃PO₄. Received: 24 January 1997 / Accepted: 18 April 1997     

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.     

Modification of growth parameters of Ti anodic films by fluoride ion insertion

The potentiostatic growth of Ti anodic films formed in 0.1?M HClO₄/x?M HF (0.05?M?≤?x?≤?0.50?M) media was studied by electrochemical impedance spectroscopy, EIS. The obtained EIS spectra were analyzed by using the surface charge approach model, which showed oxide growth parameters being modified as the HF concentration increased. While the electric field through the film decreased, oxide growth rate and atomic half jump distance increased, evidencing that fluoride ions are inserted into the oxide through oxygen vacancies, thereby modifying the lattice structure and electric properties of the oxide. Additionally, the experimental data obtained for the pseudocapacitance C ₀, appearing at low-frequency domain, allowed the interpretation commonly given for this element (modulation of film thickness) to be discarded; instead, the F^? ion diffusion coefficient was estimated, casting with variation of electric properties with HF concentration and E _F.     

A concerted migration mechanism of mixed oxide ion and electron conduction in reduced ceria studied by first-principles density functional theory

Ceria based oxides are regarded as key oxide materials for energy and environmental applications, such as solid oxide fuel cells, oxygen permeation membranes, fuel cell electrodes, oxygen storage, or heterogeneous catalysis. This great versatility in applications is rendered possible by the fact that rare earth-doped ceria is a pure oxygen ion conductor while undoped ceria, CeO(2-δ), is a mixed oxygen ion-electron conductor. To get deeper insight into the mixed conduction mechanism of oxygen ions and electrons from atomistic and electronic level viewpoints we have applied first-principles density functional theory (DFT + U method). The calculation results show that oxygen vacancies strongly attract localized electrons, forming associates between them. The migration energy of an oxygen vacancy in such an associate is substantially lowered compared to the unassociated case due to the simultaneous positional rearrangement of localized electrons during the ionic jump process. Accordingly, we propose a concerted migration mechanism of oxygen vacancies and localized electrons in reduced ceria; this mechanism results in an increased diffusivity of oxygen vacancies supported by localized electrons compared with that in pure oxide ion conductors.     

Impedance spectroscopy study of passive layers on the surface of lead-tin and lead-tin-calcium alloys anodically oxidized in 4.8 M sulfuric acid solution

The nature of passive films, which were formed at various potentials in 4.8 M H₂SO₄ solution on the lead-tin and lead-tin-calcium alloys, is studied by the method of impedance spectroscopy. At the potentials of 1.3 and 1.7 V, the electrode impedance is presented by the equivalent circuit, which corresponds to the formation of a bilayer film consisting of lead(II) sulfate and oxide on the electrode surface. Lead(II) oxide, which forms under the layer of lead sulfate, determines a high resistance of passive layer on the electrodes of lead alloys under investigation. An introduction of tin into the lead alloys significantly decreases the resistance of passive layers. An addition of calcium to the lead-tin alloy raises the impedance of the system. At a potential of 2.05 V, a single-layer compact passive film forms on the electrodes of the test lead alloys. It consists predominantly of lead oxides PbO _x (1 < x ≤ 2), which exhibit a higher electron conductivity. An introduction of tin into the lead alloys decreases the resistance of formed films; calcium has almost no effect on the resistance of passive film under these conditions.     

Evolution of an iron passive film in a borate buffer solution (pH 8.4)

The evolution under open-circuit conditions of iron passive films formed at 0.8 V_SCE in a borate buffer solution at pH 8.4 was investigated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composition of the freshly formed passive film as determined by X-ray photoelectron spectroscopy (XPS) was found to be in agreement with a bilayer model, where the inner layer is composed mainly of iron oxide and the outer layer consists of a hydrated material. Results of XPS measurements also showed that the open-circuit breakdown of passive films was consistent with a reductive dissolution mechanism. When the iron electrode reached an intermediate stage in the open-circuit potential decay (approximately −0.3 V_SCE), the oxide film, containing both Fe(II) and Fe(III), was still protective. The impedance response in this stage exhibited a mixed control by charge transfer at the metal/film and film/solution interfaces and diffusion of point defects through the film. At the final stage of the open-circuit potential decay (approximately −0.7 V_SCE), the oxide film was very thin, and the ratio of Fe³⁺/Fe²⁺ and O²⁻/OH⁻ had decreased significantly. The impedance response also exhibited a mixed charge-transfer–diffusion control, but the diffusion process was related to transport of species in the electrolyte solution resulting from dissolution of the oxide film.     

Impedance spectroscopy of reduced monoclinic zirconia

Zirconia doped with low-valent cations (e.g. Y3+ or Ca2+) exhibits an exceptionally high ionic conductivity, making them ideal candidates for various electrochemical applications including solid oxide fuel cells (SOFC) and oxygen sensors. It is nevertheless important to study the undoped, monoclinic ZrO2 as a model system to construct a comprehensive picture of the electrical behaviour. In pure zirconia a residual number of anion vacancies remains because of contaminants in the material as well as the thermodynamic disorder equilibrium, but electronic conduction may also contribute to the observed conductivity. Reduction of zirconia in hydrogen leads to the adsorption of hydrogen and to the formation of oxygen vacancies, with their concentration affected by various parameters (e.g. reduction temperature and time, surface area, and water vapour pressure). However, there is still little known about the reactivities of defect species and their effect on the ionic and electronic conduction. Thus, we applied electrochemical impedance spectroscopy to investigate the electric performance of pure monoclinic zirconia with different surface areas in both oxidizing and reducing atmospheres. A novel equivalent circuit model including parallel ionic and electronic conduction has previously been developed for titania and is used herein to decouple the conduction processes. The concentration of defects and their formation energies were measured using volumetric oxygen titration and temperature programmed oxidation/desorption.     

Highly conductive electrostatically assembled porphyrazine films

The electrochemical and photoelectrochemical response of molecular films formed by (3,4-pyridyl)porphyrazine coordinated to four peripheral [Ru(bipy)₂Cl]⁺ groups (TRPz) has been investigated, in comparison with the corresponding electrostatic assembled bilayer films generated in the presence of the tetrasulfonate phthalocyaninatecopper(II) complex (CuTSPc). Both types of films are redox conductors, but an additional electron conduction mechanism through the stacked rings π system has been observed in the bilayer films, based on impedance spectroscopy. Their combined action promotes an enhanced photoaction response for the bilayer films in the presence of dissolved dioxygen.     

A Mössbauer study of oxygen vacancy and cation distribution in 6H-BaTi1−xFexO3−x/2

We have synthesized samples in the system BaTi_1−xFe_xO_3−x/2 with x=0.1−0.6 at temperatures of 1200-1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mössbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO₃ type structure. Mössbauer spectroscopy showed all iron to be present as Fe³⁺, occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M1 and M2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.     

钛表面二氧化钛膜中氧空位点缺陷传输性能的电化学研究

"钛/TiO2氧化膜/溶液"界面电极体系的电化学性能主要决定于钛表面的TiO2氧化物膜.本文利用多种电化学技术,结合半导体物理的Mott-Schottky分析和Einstein方程,研究了金属钛在1.0mol·L-1HClO4溶液中表面半导体TiO2氧化膜的生长及氧化膜中氧空位点缺陷在外加电场作用下的传输性能,并根据离子性电荷传输与电子性电荷传输对电场变化响应时间之不同特点,确定氧化膜中点缺陷扩散系数.结果表明,电极电位或阳极析氧反应对稳态电流(iss)、氧化膜的阳极化常数(α)、膜中电场强度()、以及膜中氧空位点缺陷的扩散系数(D0)等重要物理化学参数,均有显著影响,并依据氧化膜中的结构变化进行分析.     

Barrier and semiconducting properties of thin anodic films on chromium in an acid solution

The study of barrier and semiconducting properties of anodically formed oxide films on chromium in an acid solution was carried out using the Cr-quartz crystal electrode. The oxide film formation and growth occur through an anion vacancies transport via a low-field-assisted mechanism (H = 10⁶ V cm⁻¹). The anion diffusion coefficient, which quantitatively describes the transport of point defects within the growing film, was calculated from capacitance data using the Nernst-Planck equation for low-field limit approximation and Mott-Schottky analysis. The depletion region in the passive film, close to the film|electrolyte interface, dominates the semiconducting properties. The passive film on Cr in an acid solution behaves as an n-type semiconductor. An energy-band structure model of the passive film is given.     

熔融盐法制备富氧空位TiO2纳米片及其光催化性能

氧空位缺陷对半导体材料性能的积极作用引起人们越来越多的关注。本文中,以TiCl4在三氟乙酸中的水解产物为前驱体,通过一步熔融盐法成功合成了具有富氧空位的蓝色TiO2纳米片。由于熔融盐低的氧分压,使前驱体在煅烧过程中消耗了TiO2中的晶格氧从而产生大量的氧空位和Ti3+。紫外-可见漫反射光谱测试表明,蓝色TiO2纳米片的带隙宽度减小至2.69eV,光吸收范围从紫外光区拓宽到可见光区。所制备的蓝色TiO2纳米片表现出优异的光催化活性,在全光谱照射下,对若丹明B的光降解速率是纯TiO2的47.3倍。同时,形成的晶格氟掺杂能有效地稳定氧空位,极大地提高了光生载流子的分离效率。本工作为在半导体氧化物材料内构建氧空位提供了新的思路。     

Photo-induced carbon dioxide reduction on hexagonal tungsten oxide via an oxygen vacancies-involved process

Although converting the greenhouse gasses carbon dioxide (CO₂) into solar fuels is regarded as a convenient means of solar energy storage, the intrinsic mechanism on how the high chemical inertness linear CO₂ molecules is activated and converted on a semiconductor oxide is still elusive. Herein, by creating the oxygen vacancies on the typical hexagonal tungsten oxide (WO₃), we realize the continuous photo-induced CO₂ reduction to selectively produce CO under light irradiation, which was verified by isotope labeling experiment. Detailed oxygen vacancies evolution investigation indicates that light irradiation can simultaneously induce the in-situ formation of oxygen vacancies on hexagonal WO₃, and the oxygen vacancies promote the adsorption and activation of CO₂ molecules, leading to the CO₂ reduction to CO on the hexagonal WO₃ via an oxygen vacancies-involved process. Besides, the existence of water further promotes the formation of CO₂ reduction intermediate, further promote the CO₂ photoreduction. Our work provides insight on the mechanism for converting CO₂ into CO under light irradiation.     

Synthesis and electrical properties of dense Bi2Al4O9

Bi₂Al₄O₉ ceramics are difficult to sinter to greater than 80% theoretical density due to peritectic decomposition at 1,070 °C. A novel processing method is discussed where a high-bismuth oxide-based liquid is used as a sintering aid. After sintering, the high bismuth oxide phase is removed by leaching with 40% acetic acid. The resulting samples are phase pure and ∼91% dense. The grain size varies in a wide range with the average grain size of ∼1 μm. The electrical properties of these ceramics were measured as functions of temperature (550–850 °C) and oxygen partial pressure (6×10⁻⁶–1 atm). The total conductivity was separated into electronic and ionic contributions. The low ionic conductivity indicates that the material is not an ‘intrinsically defective fast ion conductor’. The ionic conductivity is due almost exclusively to compensating oxygen vacancies related to impurities. With increasing temperature and decreasing oxygen partial pressure, the electronic conduction dominates over the ionic conduction.     

FTIR spectroscopic characterization of Nafion®–polyaniline composite films employed for the corrosion control of stainless steel

Nafion?–polyaniline (PAn) composite films deposited by a two-step process on a stainless steel (SS) substrate were characterized in this study using Fourier transform infrared (FTIR) spectroscopy under various conditions employed to evaluate their anticorrosion properties. The SS|Nafion? electrode was first prepared by placing a certain amount of Nafion? on the SS substrate, and then polymerization of aniline was carried out potentiodynamically on the SS|Nafion? electrode. The SS|Nafion?–PAn electrodes subjected to both potentiodynamic polarization and open-circuit conditions in sulfuric acid solutions without and with chlorides appeared to have distinct differences in their FTIR spectra. It is proposed that under the electrochemical conditions used in this study, the PAn is mostly formed inside the Nafion? membrane with a high proportion of oligomers influencing the ionic transport through the membrane. The inhibition of pitting corrosion arises primarily from the enhanced permselectivity of the composite film due to the Nafion? membrane that prevents chloride transport. An essential beneficial effect comes also from the PAn redox properties on the growth of the passive oxide film. Even under severe corrosion conditions, Nafion???/em>PAn films retain their redox activity and chemical stability, whereas the membrane crystallinity seems to be enhanced.     

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm² V⁻¹s⁻¹. The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (V_Zn) with one or more oxygen vacancies (V_O) in larger structural entities. Such V_O-V_Zn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.     

Reactivation of passive titanium: the enhancement of O2 evolution after potentiodynamic cyclings

Titanium oxide films were grown potentiodynamically at 50 mV s⁻¹ from −1.1 up to 10.0 V, at room temperature (23±1 °C) in H₃PO₄/NaH₂PO₄ aqueous solutions of ionic strength 0.5 mol L⁻¹ and pH 3.0. After the oxide growth, the passive electrode was subjected to different repetitive potentiodynamic cycles at 50 mV s⁻¹ between pre-set cathodic (E_s,c) and anodic (E_s,a) switching potentials. The changes in the electrochemical behaviour of the passive electrode, specially that of the O₂ evolution reaction, were followed as a function of the number of cycles and of the limiting negative potential value used, E_s,c. The enhancement of the oxygen reaction rate occurring with the repetitive potential sweeps might be due to an increase of both the oxide conductivity and the porous-oxide surface area.     

Preparation and electrocatalytic property of three-dimensional nano-dendritic platinum oxide film

Platinum oxide electrode, as an important part of hydrogen concentration monitoring sensor built in containment, needs to withstand extreme conditions such as high temperature, high humidity, and high irradiation and can still work normally even in the case of serious accidents, which puts forward higher requirements for its performance. In present study, platinum oxide film electrode was successfully prepared with three-dimensional nano-dendritic, uniform, and crack-free on platinum substrate by reactive magnetron sputtering, and the influence of different substrate temperature and sputtering atmosphere on the composition, morphology, and electrocatalytic property of the film was investigated. The results show that platinum oxide film is composed of PtO and PtO₂. As the temperature increases from room temperature (RT) to 200°C, the oxygen vacancies in the amorphous film are gradually repaired and convert to the crystalline state, which shows increasing PtO₂ ratio, increasing electrochemical active area (ECSA), and improved stability. When the temperature is rising to 400°C, the film shows decreasing oxygen vacancies, increasing average grain size. Because PtO₂ decomposes into PtO and Pt, and thus ECSA decreases, the stability and oxygen reduction activity of the films decreases gradually. At the same temperature, the crystalline film obtained in Ar/50%O₂ has higher concentration of oxygen vacancies and smaller average grain size than that obtained in O₂, resulting in larger ECSA and relatively good stability. By contrast, the platinum oxide film electrode prepared in Ar/50%O₂ and 200°C has better stability and excellent electrocatalytic activity for oxygen reduction.     

Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La0.3Sr0.7Fe1−xGaxO2.65+δ Perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La_0.3Sr_0.7Fe_1−xGa_xO_2.65+δ (x=0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950°C at oxygen partial pressures varying from 10⁻⁴ to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga³⁺. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron-hole mobility in La_0.3Sr_0.7(Fe,Ga)O_2.65+δ, which increases with temperature, suggest a small-polaron conduction mechanism.     

Thin films of vanadium oxide grown on vanadium metal: oxidation conditions to produce V2O5 films for Li‐intercalation applications and characterisation by XPS,AFM, RBS/NRA

Thin films of vanadium oxide were grown on vanadium metal surfaces (i) in air at ambient conditions, (ii) in 5 mM H₂SO₄ (aq), pH 3, (iii) by thermal oxidation at low oxygen pressure (10^?5 mbar) at temperatures between 350 and 550 °C and (iv) at near‐atmospheric oxygen pressure (750 mbar) at 500 °C. The oxide films were investigated by atomic force microscopy (AFM), X‐ray photoelectron spectroscopy (XPS), X‐Ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA). The lithium intercalation properties were studied by cyclic voltammetry (CV). The results show that the oxide films formed in air at room temperature (RT), in acidic aqueous solution, and at low oxygen pressure at elevated temperatures are composed of V₂O₃. In air and in aqueous solution at RT, the oxide films are ultra‐thin and hydroxylated. At 500 °C, nearly atmospheric oxygen pressure is required to form crystalline V₂O₅ films. The oxide films grown at pO₂ = 750 mbar for 5 min are about 260‐nm thick, and consist of a 115‐nm outer layer of crystalline V₂O₅. The inner oxide is mainly composed of VO₂. For all high temperature oxidations, the oxygen diffusion from the oxide film into the metal matrix was considerable. The oxygen saturation of the metal at 450 °C was found, by XPS, to be 27 at.% at the oxide/metal interface. The well‐crystallized V₂O₅ film, formed by oxidation for 5 min at 500 °C and 750 mbar O₂, was shown to have good lithium intercalation properties and is a promising candidate as electrode material in lithium batteries. Copyright © 2005 John Wiley & Sons, Ltd.