Initial Stages Of Anodic Oxidation Research Articles

Multi-Scale Modeling of the Initial Stages of Anodic Oxidation of Titanium

The present paper describes the nanoscale modeling of the adsorption and dissociation of water molecules on the titanium Ti (001) surface at different applied potentials and its connection to a previously proposed continuum scale approach to the growth and dissolution of thin barrier films on Ti in water-containing electrolytes. The theoretical study of titanium/water interactions is presented as combination of static and molecular dynamics calculations using density functional theory (DFT). The vibration frequencies of adsorbed H2O put emphasis on its interaction with the titanium surface and provide further information about the bond strength and the changes in the molecular structure. The adsorption and dissociation geometry of water molecules on Ti (001) surface in an electrochemical environment is investigated using the double reference method that allows assessing the elementary steps of water dissociation as depending on the applied potential. The dependence of the free energy of the Ti/water system on potential is obtained and discussed in terms of sequential dissociation of water to adsorbed hydroxyl and oxygen.

In situ STM study of the initial stages of electrochemical oxide formation at the Ag(1 1 1)/0.1 M NaOH(aq) interface

The atomic structure of surface layers and the local changes of surface morphology during the initial stages of anodic oxidation of Ag(1 1 1) in an aqueous 0.1 M NaOH solution have been investigated by in situ scanning tunneling microscopy (STM) under different polarization conditions. The surface oxide formation starts at underpotentials, i.e. at electrode potentials E below the Nernst-potential E 3 D Ag 2 O ( vs. SHE )=0.4 V for the formation of the three-dimensional (3D) bulk Ag 2O phase. An ordered overlayer showing distinct Moiré pattern is observed in the potential range −0.1 V<E (vs. SHE) <0.1 V. The appearance of the Moiré structure is assigned to the strong OH adsorption, which induces a stretching of the 2D lattice of the OH-terminated topmost Ag(1 1 1) monolayer. In situ STM imaging at higher potentials (0.1 V < E (vs. SHE) <0.3 V) shows the occurrence of a Ag dissolution process followed by an underpotential oxide formation in agreement with previous electrochemical measurements and in situ X-ray absorption studies.

In Situ STM Study of the Initial Stages of Anodic Oxidation of Cu(111) in the Presence of Sulfates

In situ scanning tunneling microscopy (STM) has been applied to study adsorption and oxide formation on Cu(111) in weakly alkaline, neutral, and acidic sulfate-containing solutions. In a hexagonally structured OH layer is formed at potentials well below oxide formation despite the tenfold larger sulfate concentration showing a stronger interaction of with the Cu surface than In a partially structured and highly mobile adlayer is formed at potentials quite near the onset of oxide formation with coexisting -ordered domains and nonordered domains. This is assigned to the competitive adsorption of water molecules with sulfate anions. In contrast, a highly ordered and stable adsorbed layer is formed in It is assigned to the coadsorption of sulfate anions and cations or the latter compensating the repulsive forces between sulfate anions and thus stabilizing the adlayer. The anodic Cu(I) oxide is (111)-oriented and has a faceted surface in solutions of alkaline, weakly alkaline, and neutral pH. © 2003 The Electrochemical Society. All rights reserved.

In situ STM study of the effect of chlorides on the initial stages of anodic oxidation of Cu(111) in alkaline solutions

In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the mechanisms of growth of passive layers on Cu(111) in NaOH solutions in the presence of chlorides. For [Cl −]/[OH −]=0.01, the same ordered precursor phase of adsorbed OH is observed in the underpotential region of oxidation as in Cl −-free solutions. Atomically resolved images reveal the structure of the reconstructed topmost metal plane and the threefold hollow adsorption site of the hydroxide. The induced reconstruction causes the ejection of Cu atoms that contribute to the observed lateral growth of the terraces and to the formation of 2D Cu ad-islands in the final stages of the adsorption process. For [Cl −]/[OH −]=0.1, threadlike nanostructures resulting from the reaction of the ejected Cu atoms with chlorides are formed before agglomeration with the 2D Cu ad-islands formed in the final stage of the hydroxide adsorption process. For [Cl −]/[OH −]=10, the step edges, which are normally the preferential sites of the reaction with hydroxide, are blocked by the formation of non-ordered surface chloride complexes. Hydroxide adsorption still predominates the surface reaction on the terraces but the 2D ad-islands form immediately due to the blocking of the step edges. In the potential range of Cu(I) oxide formation, crystalline Cu(I) oxide layers are formed with a high density of steps and (111) terraces. Their step edges are rougher in the presence of chlorides which indicates a Cl −-enhanced localized dissolution reaction of the oxide layers at step edges.

Atomic Force Microscopy Study of the Initial Stages of Anodic Oxidation of Aluminum in Phosphoric Acid Solution

Aluminum foils with two different surface topographic textures were anodically oxidized at constant current in a phosphoric acid bath. In situ atomic force microscopy (AFM) was used to follow the initial development of surface topography on a 1 μm scale, during the early stages of porous oxide film formation. Microscopic convex features such as ridges on both foils begin to increase in height and width when the anodic film thickness exceeds the initial feature height. Equations of a mathematical model are presented incorporating established interfacial reactions and oxide conduction behavior. The model indicates that the film‐solution interface recedes into the metal during anodizing, since the current efficiency for oxide formation is smaller than the oxygen ion transport number in the film. Ridge surfaces increase in height due to the higher local conduction resistance to the film‐solution interface, while film deposits rapidly at ridges because of the low local resistance to the metal‐film interface. In agreement with the AFM results, enhanced oxide growth at ridges should start when the potential field in the film becomes two‐dimensional, as a result of the film growing to a thickness larger than the ridge height. © 2000 The Electrochemical Society. All rights reserved.

Study of the Initial Stages of Anodic Oxidation of Polycrystalline Silver in KOH Solutions

Growth of the anodic film on a polycrystalline silver disk electrode in solution at ambient temperature has been examined using a variety of electrochemical techniques. On the basis of the experimental results, a solid‐state model of silver electrode covered by a thin semiconductor film with a finite ionic conductivity has been proposed. It has been found that under potentiostatic control, thickness of the oxide film adjusts very quickly to the hydrodynamic conditions by a deposition/dissolution process, and the steady‐state oxidation currents fulfill the Levich equation for RDE, despite the fact that the electrode is covered by a thin film. In the case of thicker films, after completion of nucleation and growth, changes in the hydrodynamic conditions do not influence the oxidation current, and adjustment of the thickness of the oxide film to new hydrodynamic conditions proceeds at slower rate because of the slowness of the solid‐state diffusion step. The nucleation and growth peak has been observed in chronoamperometric transients above potential of + 240 mV (vs. ). It has been observed that the nucleation and growth peak decreases as the convective diffusion is accelerated by the rotation speed of the RDE. The dependence of the oxidation current upon the rotation speed vanishes gradually during the nucleation and growth process.

Initial stages of anodic oxidation of GaAs

The island stage of the anodic oxidation of GaAs was investigated with transmission electron microscopy (TEM). Many island details are seen in the high-resolution photomicrographs of platinum–carbon secondary replicas of the GaAs surface. The nucleation and coalescence processes are seen to take place continuously during the passivation time. The islands are normally circular in shape except when altered by coalescence. The height of the islands increase with island size up to an area of ≊0.1 μ2 after which the island height increases only slowly or not at all.

Oxygen electrosorption on copper single crystal electrodes in sodium hydroxide solution

In a study of the initial stages of anodic oxidation of polycrystaUine copper electrodes in alkaline solutions [1 ], we measured potential sweep vol tammetry curves which show that formation of bulk Cu~O is preceded by electrosorption of oxygen species. The electrosorption occurs in two successive stages, each represented by a current peak, corresponding to a different submonolayer state with a different adsorption energy. We postulated that a priori heterogenei ty of the surface, e.g. exposure of various crystal planes, could be an explanation for the occurrence of multiple peaks. In order to test this hypothesis we have investigated the electrosorption of oxygen on carefully prepared copper single crystal surfaces. Clear evidence is found in our results, that the electrosorption of oxygen is a plane specific reaction. Each peak in the vol tammogram on polycrystaUine copper can be at t r ibuted to exposure of regions with specific adsorption sites as present in majority on different single crystal planes.

Initial stages of anodic oxidation of polycrystalline copper electrodes in alkaline solution

The first stages of the anodic oxidation of polycrystalline copper electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Cu2O was found to be preceded by electrosorption of oxygen species, that occurs in two successive stages, each represented by a current peak, corresponding to a different submonolayer state with a different adsorption energy. This surface oxide was formed via random electrodeposition. The width of the first current peak indicates the presence of lateral attractive interactions in the chemisorbed layer. The surface layer did not show any ageing effect.

Initial stages of anodic oxidation of polycrystalline copper electrodes in alkaline solution

The first stages of the anodic oxidation of polycrystalline copper electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Cu 2O was found to be preceded by electrosorption of oxygen species, that occurs in two successive stages, each represented by a current peak, corresponding to a different submonolayer state with a different adsorption energy. This surface oxide was formed via random electrodeposition. The width of the first current peak indicates the presence of lateral attractive interactions in the chemisorbed layer. The surface layer did not show any ageing effect.

Initial stages of anodic oxidation of silver in sodium hydroxide solution studied by potential sweep voltammetry and ellipsometry

The first stages of the oxidation of polycrystalline silver electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Ag2O was found to be preceded by dissolution of silver species and deposition of a surface oxide. The equilibrium oxide coverage depended on the electrode potential and occurred within a few seconds. Surface oxide formation probably took place via a process of random electro-deposition. No ageing effect was observed in the chemisorbed layer.

Initial stages of anodic oxidation of silver in sodium hydroxide solution studied by potential sweep voltammetry and ellipsometry

The first stages of the oxidation of polycrystalline silver electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Ag 2O was found to be preceded by dissolution of silver species and deposition of a surface oxide. The equilibrium oxide coverage depended on the electrode potential and occurred within a few seconds. Surface oxide formation probably took place via a process of random electro-deposition. No ageing effect was observed in the chemisorbed layer.