Effect of the crystal face orientation and alloying with gold on the properties of thin anodic films of Ag(I) oxide: I. Photocurrent

The anodic formation of nano-size anodic films of Ag(I) oxide on polycrystalline silver and Ag-Au alloys, as well as on low-index monocrystalline silver faces in 0.1 M KOH, is studied. Based on the data of photocurrent measurements, the n-kind of the oxide film conductivity is determined. Replacing the polycrystalline silver with monocrystals, as well as adding a small amount of gold atoms to its lattice (X _[Au] <= 4 at. %), results in substantial structure ordering of the oxide due to the decrease in the deviation from the stoichiometric composition. The structure-dependent parameters of Ag(I) oxide (the optical absorption coefficient α, the concentration of donor defects N _D, the width of the spatial charge region W, and the Debye screening length L _D) are determined by the silver crystal face orientation, the gold content, and the film formation potential. At E = 0.52 V, the series of changes in these characteristics correlate with the series of reticular density. An increase in E breaks the series and increases the deviation of the composition from the stoichiometric Ag₂O composition.     

Determining some structure-sensitive characteristics of nano-sized anodic Ag(I) oxide from photopotential spectroscopy

A relatively simple device involving a set of LEDs and a digital processing of the signal is developed for in situ photopotential and photocurrent spectroscopy of thin semiconductors including oxide films under conditions of discretely changed light wavelength. Decreases in the electrode potential and photopotential of Ag(I) oxide with time upon the anodic effect are shown to obey different kinetic regularities, namely, logarithmic and exponential respectively. Photopotential measurements revealed n-type conductivity in thin (6 to 22 nm) semiconductor Ag₂O films potentiostatically formed on either poly or monocrystalline silver (111) with a band gap of 3.1 eV. The structural disordering of the oxide is minimum at a potential of 0.490 V, which corresponds to nearly a midpoint of the anodic peak shoulder of the voltammogram. The concentration of donor defects in an Ag₂O film grown on a “smooth” polycrystalline silver is lower than that on an ultrafine-dispersed silver, but higher than on Ag(111) monocrystal. A linear dependence of the photoresponse on the light beam power is found, and the spectral dependence of the photopotential is discussed. The photopotential dependence on the film thickness, as well as crystalline and microstructural state of the silver surface means that the photoresponse is determined by the bulk rather than superficial electronic states, and the film thickness is smaller than the Debye shielding length. Original Russian Text ? D.A. Kudryashov, S.N. Grushevskaya, A.V. Vvedenskii, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 652–661.     

Kinetic peculiarities of anodic dissolution of silver and Ag–Au alloys under the conditions of oxide formation

The kinetics of anodic dissolution of silver and Ag–Au alloys (X_Au = 0.1–30 at.% Au) in aqueous alkaline solution under the conditions of the formation of silver oxides has been examined. The techniques of cyclic voltammetry, chronoammetry, and photopotential measurements have been used. It was established that the anodic formation and cathodic reduction of Ag₂O on silver and alloys are controlled by migration in the oxide layer. Ag₂O oxide is an n-type semiconductor with an excess of silver atoms. Oxide layers formed on monocrystalline Ag(1 1 1) and Ag(1 1 0) are more stoichiometric than the layer formed on polycrystalline Ag.     

Determining the partial currents of silver ionization, its oxide formation, and chemical dissolution by multicycle chronoammetry with an RRDE

A multicycle chronoammetry with a rotating disc electrode with a ring (RRDE) enables one to experimentally discriminate between the partial currents of the substrate metal ionization, anodic formation of the oxide, and chemical dissolution of the oxide in the summary polarization current of the disc. The technique is approved by an example of Ag|Ag₂O|OH⁻(H₂O) system. In a range of relatively small anodic potentials of the Ag disc (0.48 to 0.51 V), the active dissolution of silver at the open surface sites and via pores in the surface film dominates; the phase formation current and, accordingly, the current efficiency of the process rapidly drop. At the potentials of the voltammogram maximum (0.52 to 0.53 V) when the silver active dissolution current is suppressed, the phase formation currents prevail and substantially exceed the chemical dissolution rate of the oxide. The thickness of an Ag₂O film rapidly increases under these conditions, and the current efficiency of the oxide formation is close to 100% for the whole polarization period. The rate constant of the chemical dissolution of an Ag(I) oxide is practically independent of the anodic phase-formation potential, but slightly depends on the oxide film thickness, reflecting changes in the film structure and, possibly, in its composition, from AgOH to Ag₂O. Original Russian Text ? D.A. Kudryashov, S.N. Grushevskaya, A.V. Vvedenskii, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 3, pp. 321–330.     

Behavior of a barrier layer of corrosion films on zirconium alloys

Capacitances of oxide films obtained on a EZ-1 alloy during the corrosion tests in water-vapor environment at 300, 350, and 400°C are measured. In order to estimate the thickness of a barrier layer, the barrier potential at a constant anodic current (dielectric puncture potential) is measured. It is shown that the barrier layer cannot be treated as a homogeneous environment. At the sites of intermetallide inclusions in the oxide film, the thickness of the dielectrics is locally decreased. In the first approximation, the heterogeneity of the oxide film can be taken into account by inserting two parallel RC subcircuits in the equivalent scheme. One subcircuit (C ₁, R ₁) describes the electrophysical properties of the capacitance whose insulator thickness corresponds to the total thickness of the oxide film. The other subcircuit (C ₂, R ₂) describes the electrophysical properties of the nonporous part of the oxide film between the intermetallide particles and the outer surface. Then, the results of measurements can be written as follows: C _exp = θC ₂ + (1 − θ)C ₁, where θ is the surface part of the oxide film whose dielectric properties are changed due to the inclusion of intermetallide particles. Assuming that the mean spatial size of intermetallide particles falls in the range of 200–400 nm, one can estimate the mean concentration of the particles on the metal surface in agreement with the metallographically determined concentration of the second-phase particles (approximately 10⁶–10⁷ cm⁻²). The obtained results indicate the substantial heterogeneity of the barrier layer structure, which may cause local corrosion and premature failure of zirconium items.     

Anodic oxidation of Al–Ag alloys

The anodic oxidation of solution-treated and quenched Al–Ag alloys containing 0.3, 0.6, 0.9 and 1.2 at.%Ag, is examined in ammonium pentaborate electrolyte, which leads to growth of barrier-type anodic films. Enrichments of silver, 3.1×10¹⁵ Ag atoms cm⁻², are developed in the alloys immediately beneath the amorphous alumina films, with the level of enrichment not depending significantly upon the composition of the bulk alloy. The enrichment is relatively low due to clustering of silver atoms in the bulk alloy, which reduces the concentration of silver that is available to enrich from solid solution. Silver species are incorporated into the anodic film, where they migrate outward faster than Al³⁺ ions.     

The effect of sulfate and nitrate ions on the passivation and activation of silver in alkaline solutions

Based on the concept of a competitive adsorption of hydroxide ions with SO₄²⁻ and NO₃⁻ activating anions, some peculiarities of the anodic behavior of silver in alkaline solutions containing sulfates or nitrates are explained. At the ascending branches of the anodic peaks, a co-adsorption of OH⁻ and SO₄²⁻ or NO₃⁻ results in the formation of mixed adsorption complexes, which are soluble better than hydroxide ones. An increase in the part of the soluble oxidation products of silver is observed if a rotating disc electrode with a ring is used. Passivation of silver is explained by a change in the electronic structure of the adsorption complexes when certain potential values are reached, while a local activation, by the destruction of these complexes at the passive state potentials between the anodic peaks. At the depassivation, pH value of the solution in pits decreases, which results in the formation of Ag₂SO₄ or AgNO₃ salts. The presence of the salts in a deposit on the electrode is confirmed by the appearance of a C ₃ cathodic peak. Original Russian Text ? N.N. Lesnykh, N.M. Tutukina, I.K. Marshakov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 472–477.     

Electrochemical Investigation of the Nucleation and Propagation of Pits in Iron-Chromium Alloys

Abstract

Electrochemical measurements performed on single crystals and polycrystalline samples of Fe 13 Cr and Fe 16 Cr alloys in Na₂SO₄ + NaCl solutions differing in Ci^? ion concentration, have shown that during pitting the anodic current I increases with time t:

I～t^b Depending on experimental conditions, b may vary from 2 to 6. At b = 2 all the pits nucleate simultaneously, so that their number does not change with time of anodic polarisation. At b > 2 the number of pits increases with time.

Galvanostatic polarisation of polycrystalline samples in solutions containing much fewer Cl_? ions than SO₄^2? ions does not lead to a steady potential. Under these conditions periodic oscillations of potential occur, several hundred millivolts in amplitude. This phenomenon is probably due to the inhibiting action of SO₄^2? ions on the nucleation of pits by Cl^? ions, and it occurs only at certain concentration ratios of these two ions in the electrolyte. Inhibition of pit nucleation is also revealed during potentio-static and potentiokinetic measurements at potential values several hundred millivolts more positive than the breakdown potential measured galvanostatically.

An interpretation of above phenomena is given for both single crystals and polycrystalline alloy samples.     

Preparation of flake AgSnO2 composite powders by hydrothermal method

Silver-tin oxide composite powders and silver powders were synthesized by hydrothermal method using NHs to complex Ag^＋, SO3^2- to reduce Ag（NH3）2^＋ and Na2SnO3 as the source of tin. The powders were characterized by XRD, SEM and EDX. The results show that there are macroscopic and microscopic differences between two kinds of powders. Spherical silver powders are 3μm in diameter, and silver-tin oxide composite powders are mainly flake of about 0.3μm in thickness. Silver crystal in silver-tin oxide composite powders is preferentially oriented in the （111） crystallographic direction and its oriented index is 2.581. Crystal lattice parameter of silver crystal of silver tin-oxide composite powders is 0.409 34 nm, larger than 0.408 68 nm of silver powders. The XPS analysis shows that silver in silver-tin oxide composite powders is metallic silver and tin oxide in silver tin-oxide composite powders has the red shift for Sn^4＋（3d（5/2）） and O^2-（1s）.     

The Limiting Dissolution Current of a Spherical Electrode in Acid

Anodic dissolution of a spherical electrode in acid solution is analyzed. An equation derived implicitly describes the dependence of the limiting current of anodic metal dissolution on the acid concentration. The consideration is exemplified by silver anodic dissolution in nitric acid solution. The dependence of the limiting current of silver anodic dissolution on the acid concentration is numerically calculated by using tabulated data on the Ag solubility in AgNO₃.     

Oxygen out-diffusion in partially melted YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript>−0.325Ag superconductor at reduced oxygen partial pressure

In this study, we examined the chemical diffusion coefficient of oxygen out-diffusion([(D)\tilde]_out\tilde D_{out} ) in partially melted YBa₂Cu₃O_y (Y123)−Ag superconducting tape produced by the thermomechanical processing of the silverclad precursor (doped with 5 wt.% Ag) containing BaCuO_2.5 at reduced oxygen partial pressure (P₀₂). The[(D)\tilde]_out\tilde D_{out} value of Y123−Ag (O_7.0 to O_6.2), obtained by using the thermogravimetric relaxation technique at 950°C and a P₀₂ of 0.28 kPa, was 1.0476×10⁻⁷ cm²/s.     

Electrochemical behaviour of nickel anode in H2SO4 solutions and the effect of halide ions

The electrochemical behaviour of pure nickel in H₂SO₄ solutions has been potentiodynamically investigated. The effects of the following factors on the anodic dissolution and passivation of the metal are discussed: potential scan rate, successive cyclic voltammetry and progressive additions of Cl^?, Br^? and I^? ions. Increasing the potential scan rate increases the critical current density i_cc, denoting that the active dissolution of nickel in H₂SO₄ is a diffusion controlled process. Cyclic voltammetry shows that the reverse excursion does not restore the anode to its active state. On successive cycling, the height of i_cc decreases; this could be attributed to the decrease in the reduction efficiency of passivating oxide film during the cathodic half cycles. The presence of the halogen ions below a certain concentration specific to each anion inhibits the anodic dissolution both in the active and passive states. The inhibitive action of these additives decreases in the order I^?, Br^?, Cl^?. Beyond the specific concentrations, the halogen ions accelerate the anodic dissolution and shift the active passive transition to more positive values. The aggressiveness of these anions decreases in the sequence Cl^?, Br^?, I^?, Further increase in the halogen ion concentrations can lead to breakdown of the passive film and initiate pitting. The susceptibility of nickel to pitting attack enhances with increasing H₂SO₄ concentration.     

Anodic Oxidation of Silver and Ag–Au Alloys in Alkaline Medium

Anodic oxidation of Ag and Ag–Au alloys containing from 0.1 to 30 at. % Au under the conditions of Ag(I) oxide formation at t 0.5 s is determined by the peculiarities of migration transfer of charge carriers through phase oxide layer. The Ag₂O film is two-layer; this is reflected in the rise of additional current peaks in cyclic ivs. E(t) curves and also in the variation in the slopes of linear segments of chronoammograms presented in the i vs. t ^–1/2 coordinates. The conductivity of Ag₂O film slightly varies with its thickness; it is virtually independent of small gold additions to silver, but steeply decreases at 15 and 30 at. % Au. The formation and reduction of AgO occur via single-electron conversions of Ag₂O proceeding in the oxide phase. At all stages of oxidation of Ag and Ag–Au alloys in alkaline medium, the charge-transition stage remains reversible.     

Anodic dissolution of iron silicides in alkaline electrolyte

Anodic dissolution of iron and its silicides (FeSi, FeSi₂, as well as the eutectic alloy FeSi₂-Si) and pure Si, in 0.1 to 5.0 N NaOH solutions is studied by cyclic voltammetry and x-ray photoelectron spectroscopy. Principal characteristic features of the silicide anodic dissolution are revealed and the composition of surface films investigated. It is shown that, despite an increase in Si solubility at higher pHs, the iron silicides are highly resistant to anodic dissolution due to especial protective properties of the complex oxide surface film. Original Russian Text ? A.B. Shein, I.L. Rakityanskaya, S.F. Lomaeva, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 59–63.     

Preparation of silver nanostructure from silver(I) coordination polymer and fabrication of nano silver(I) bromide by reverse micelles technique

A new one-dimensional silver(I) coordination polymer, [Ag(μ-bpfb)(NO₃)]_n (1); bpfb = N,N′-bis(4-pyridylformamide)-1,4-benzene, has been synthesized and characterized by IR, ¹H NMR and ¹³C NMR spectroscopy. The single crystal X-ray data show that the silver(I) 1D coordination polymer grows into a three-dimensional network by hydrogen bonding and π–π stacking interactions. Compound 1 with nanorod morphology was also prepared by sonochemical method. The cetyltrimethylammonium bromide (CTAB) as a cationic surfactant was used in reverse micelles technique to obtain spongy silver(I) bromide nanoparticles from compound 1. Also, different silver nanoparticles have been prepared via direct calcination at 673 K and thermal decomposition in oleic acid from compound 1. The nanostructures of [Ag(μ-bpfb)(NO₃)]_n (1), silver and silver(I) bromide were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDAX) analysis. Thermal stability of compound 1 in both bulk and nano-sized form was studied by thermal gravimetric (TG) and differential thermal (DT) analyses.     

Multicycle chronoammetry of RRDE in the investigation of the anodic oxide formation

The method of multicycle chronoammetry of RRDE makes it possible to obtain separately the partial currents of metal electrode ionization, anodic oxide formation and chemical oxide dissolution. The method is tested for Ag∣Ag₂O∣OH⁻(H₂O) system. In the range of low anodic potentials (0.48 ÷ 0.51 V) the process of active silver dissolution prevails; the phase formation current rapidly drops. At higher potentials (0.52 ÷ 0.53 V) the phase formation current prevails and noticeably exceeds the rate of the chemical oxide dissolution. The thickness of Ag₂O film rapidly increases; and the net phase formation current is close to 100%.     

Conductive molecular solids: partial iodine oxidation of octaethylporphyrins

We report e.p.r. and resonance Raman spectra, and electrical conductivity for polycrystalline samples of the M(OEP)(I)_x, where M = “H₂”, Ni, Cu; OEP = 1,2,3,4,5,6,7,8-octaethylporphinato; x 5.8. These materials are shown by resonance Raman spectrometry to be properly formulated as . The electrical conductivities depend strongly on ρ = x/5 and M. In some cases (e.g., Ni(OEP)(I)_x, x > 1.4) the ambient temperature polycrystalline conductivities (0.03 S cm⁻¹) and the activation energies (0.10 eV) are comparable with those of polycrystalline samples of materials whose single crystal conductivities are 500 Sm⁻¹ with a metal-like temperature dependence (dσ/dt < 0).     

Transparent conductive Sb-doped SnO2/Ag multilayer films fabricated by magnetron sputtering for flexible electronics

The effects of an embedded silver layer and substrate temperature on the electrical and optical properties of Sb-doped SnO₂ (ATO)/silver (Ag) layered composite structures on polyethylene naphthalate substrates have been investigated. The highest conductivity of ATO/Ag multilayer films was obtained with a carrier concentration of 1.5 × 10²² cm^?3 and a resistivity of 2.4 × 10^?5 Ω cm at the optimum Ag layer thickness and substrate temperature. The photopic averaged transmittance and Haacke figure of merit are 81.7%, and 21.7 × 10^?3 Ω^?1, respectively. In addition, a conduction mechanism is proposed to elucidate the mobility variation with increased Ag thickness. We also describe the influence of substrate temperature on the structural, electrical and optical properties of the ATO/Ag multilayer films, and propose a mechanism for the changes in electrical and optical properties at different substrate temperatures.     

The influence of surface preparation on the structures of nickel oxide formed on the (100) face of nickel

A study has been made on the influence of surface preparation involving abrasion, electropolishing, and vacuum annealing on the physical nature of the (100) nickel crystal face and on the morphological development of the oxide film. The structure of the initial surface was dependent upon the method of surface preparation, but electropolished specimens subjected to conventional or high vacuum anneals at 10^–6 and 10^–10 Torr, respectively, and temperatures of 800 and 700° C were structurally identical. Thermal faceting of surfaces prepared to correspond to the (100) plane was negligible. Oxidation at 500° C and over the oxygen pressure range 10^–5–400 Torr led to formation of polycrystalline oxide crystallites randomly distributed over the surface during early stage exposures; the crystallites were only a few hundred Angstroms in size upon coverage of the surface by a nickel oxide film. These films were relatively uniform for thicknesses up to 2000 Å. Crystallite growth processes led to three major epitaxial relationships between oxide and metal: (100) _NiO , (111) _NiO , and (211) _NiO (100) _Ni .This work forms part of a research program sponsored by the National Research Council of Canada.     

Formation of oxides on copper in alkaline solution and their photoelectrochemical properties

The anodic formation of Cu(I) and Cu(II) oxides on polycrystalline copper in a deaerated alkaline solution is studied using the technique of the synchronous recording of transients of the photocurrent and polarization current. The oxide formation in a currentless mode is analyzed via the synchronous recording of photopotential and corrosion potential. It is found that copper is susceptible to corrosion oxidation due to traces of dissolved oxygen with the formation of a Cu(I) oxide. The preliminary formation of the underlayer of anodic Cu(I) oxide on copper hinders its further corrosion oxidation. It is confirmed that copper oxides Cu₂O and CuO, which appear on copper in both anodic and corrosion modes of formation, are p-type semiconductors. The initial stage of anodic oxidation of copper is characterized by the formation of an intermediate compound of Cu(I), possibly CuOH, which exhibits n-type conductivity. A film of Cu(I) oxide is thin and has a band gap of 2.2 eV for indirect optical transitions. Anodic polarization in the range of potentials of CuO formation leads to the formation of a thicker oxide film, which is a mixture of Cu(I) and Cu(II) oxides.