The influence of the conditions of the anodic formation and the thickness of Ag(I) oxide nanofilm on its semiconductor properties

The anodic formation of Ag(I) oxide nanofilms on polycrystalline silver and Ag–Au alloys as well as on low-index single crystals of silver in 0.1?М KOH was examined. By the methods of photocurrent i _ph and photopotential E _ph measurements, the n-type conductivity of Ag₂O film was established. Since the film (6–120 nm) is thinner than the space charge region, the dependence of photocurrent and photopotential appears on the film thickness L: i _ph ~L and E _ph ~L ². The transition from polycrystalline silver to single crystals as well as the addition of a small amount of gold (X _Au?≤?4 at.%) into the silver lattice decreases the degree of deviation from the stoichiometric composition Ag₂O. The parameters of Ag₂O film (optical absorption coefficient α, donor defects concentration N _D, space charge region W, and Debye’s length of screening L _D) depend on the index of a crystal face of silver, volume concentration of gold X _Au in the alloy, and film-formation potential E. At Е?=?0.52 V, the sequences of variation of these parameters correlate with the reticular density sequence. The growth of the potential disturbs these sequences. The band gap in Ag₂O formed on Ag_poly, Ag_hkl, and Ag–Au is 2.32, 2.23, and 2.19 eV. Flat band potential in Ag(I) oxide, formed on Ag_poly in 0.5 M KOH is 0.37 V. The appearance of the clear dependence between the state of the oxide/metal interface and the structure-sensitive parameters of semiconductor Ag(I) oxide phase allows considering the anodic formation of Ag₂O on Ag as a result of the primary direct electrochemical reaction, not of the precipitation from the near-electrode layer.     

Thermodynamic Properties of AgIn<Subscript>2</Subscript>Te<Subscript>3</Subscript>I and AgIn<Subscript>2</Subscript>Te<Subscript>3</Subscript>Br,Determined by EMF Method

Differential thermal analysis (DTA), X-ray diffraction (XRD), and electromotive force (EMF) are used to triangulate Ag–In–Te–I(Br) systems in the vicinity of compounds AgIn₂Te₃I and AgIn₂Te₃Br. The three-dimensional position of the AgIn₂Te₃I–InTe–Ag₂Te–AgI and AgIn₂Te₃Br–InTe–Ag₃TeBr phase areas with respect to the figurative points of silver is used to create equations of potential-determining chemical reactions. The potential-determining reactions are conducted in (?)C|Ag|Ag₃GeS₃I(Br) glass|D|C(+) electrochemical cells (ECCs), where C stands for inert (graphite) electrodes, Ag and D are ECC electrodes (D denotes alloys of one-, three-, and four-phase areas), and Ag₃GeS₃I and Ag₃GeS₃Br glasses are membranes with purely ionic Ag⁺ conductivity. Linear parts of the temperature dependences of the cell EMFs are used to calculate the standard integral thermodynamic functions of saturated solid solutions based on AgIn₂Te₃I and AgIn₂Te₃Br, and the relative partial thermodynamic functions of silver in the stoichiometric quaternary compounds.     

Kinetics of electrochemical reactions on the Ag(Hg)/Ag4RbI5 interface

The electrochemical behaviour of the Ag(Hg)/Ag₄RbI₅ interface is investigated by a potentiostatic pulse method. It is found that the rate-determining step of the electrode reaction is electron transfer with an exchange current density of 68 mA cm^–2 and a transfer coefficient of approximately 0.45. The order of the electrochemical reaction for silver oxidation is estimated from polarization investigations of silver amalgam in various concentrations. From this it is deduced that the mercury is ionized and is implanted in the electrolyte together with silver under anodic polarization: 15Ag+85Hg–100e^–→15Ag⁺+85Hg⁺. From comparison of the electrochemical behaviour of the Ag(Hg)/Ag₄RbI₅ and Ag/Ag₄RbI₅ interfaces it is concluded that the rate of anodic silver dissolution on the Ag/Ag₄RbI₅ interface is limited by crystallization effects. Electronic Publication     

Electrochemical behavior of silver thin films interfaced with yttria-stabilized zirconia

Thin silver films (100–800 nm) were deposited by physical vapor deposition (PVD) on yttria-stabilized zirconia solid electrolyte. The electric percolation as a function of the film thickness was studied during deposition and annealing using a two-electrode in-situ resistance measurement technique. Electrical percolation was achieved in as-deposited films greater than 5.4?±?0.4 nm; however, thermal treatment (550 °C in air) resulted in film dewetting for Ag films as thick as 500 nm and formation of electronically isolated Ag nanoparticles, as was confirmed by SEM and XPS. In thermally treated samples, stable electronic conductivity associated with a continuous percolated network was only observed in samples greater than 600 nm in thickness. The effect of polarization on the electrochemical reactions at the three-phase (electrode-gas-electrolyte) and two-phase (electrode-electrolyte) boundaries of the electrode was investigated by solid electrolyte cyclic voltammetry (SECV) at 350 °C and P _O2?=?6 kPa. With the application of positive potential, silver oxide (Ag₂O) was found to form along the three-phase boundary and then extends within the bulk of the electrode with increasing anodic potentials. By changing the hold time at positive potential, passivating oxide layers are formed which results in a shift in favor of the oxygen evolution reaction at the working electrode. This oxide forms according to a logarithmic rate expression with thick oxides being associated with decrease in current efficiency for subsequent oxide formation.     

Laser assisted selective chemical metalization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> films

Thick aluminum oxide films are prepared on Al plates by anodizing. On the ceramic surface thus obtained a very thin Ag film is deposited via vacuum thermal evaporation. The Ag/Al₂O₃/Al samples prepared are irradiated by Nd:YAG laser through a suitable metal mask in order to remove the top metal film in the exposed areas. Thus, a negative silver image of the copied mask is obtained. Further, the samples are processed in Ni electroless chemical bath activated by the rest of silver. All processing steps are studied by scanning electron microscopy (SEM). EDS X-ray mapping is applied to study the final distribution of Al and Ni in the processed areas. In addition, the DC conductivity of the fabricated Ni wires obtained is measured. The proposed new method for selective chemical deposition of electroconductive Ni onto laser microstructured Ag/Al₂O₃/Al samples is simple, versatile and not restricted to the metal/ceramic system studied as well as to the electroless deposited metal.     

Electrochemical formation and reduction of silver oxides in alkaline media

The anodic oxidation of silver electrodes in NaOH solution and the reduction of the silver oxides formed were studied by potential step chronoamperometry. Oxidation of Ag to Ag₂O is a diffusion-controlled reaction, the diffusion control being established in the solid phase. Oxidation of Ag₂O to AgO proceeds via a nucleation and growth-controlled process. The amount of AgO decreased with increasing step height. The current—time curves for this reaction have been analysed with the Kolmogoroff—Avrami equation. Reduction of AgO to Ag₂O occurs initially on the outside of the electrode, and the rate of the reaction is limited by diffusion of ions across the thickening layer of Ag₂O. Reduction of Ag₂O to Ag proceeds via a nucleation and growth reaction.     

Electrochemical dissolution of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> in acid solutions

The present study deals with the electrochemical reductive dissolution of Mn₃O₄, which was added to carbon-paste electroactive electrodes (CPEEs) in acid solutions. It was found that in the experimental conditions the thermodynamically stable form of manganese was . Kinetic features of the electrochemical reductive dissolution of Mn₃O₄, which was realized under potential cycling conditions (+1.0 V→−0.7 V→+1.0 V), were determined by the electrode polarization direction. It was shown that the cathodic reduction of Mn₃O₄ was accomplished in three stages. Manganese was dissolved in the supporting solution only at the third stage. The first two stages involved solid-phase reactions. The anodic cycling stage included an active dissolution of Mn₃O₄ and the lower manganese oxide (MnO) accumulated on the electrode surface during the cathodic reduction.     

Impedance study on the capacitance of silver electrode oxidised in alkaline electrolyte

Electrochemical properties of oxide-covered polycrystalline Ag electrodes were studied in a 0.1 M KOH aqueous electrolyte. The oxide layers formed by a constant potential oxidation at 420 mV vs. Hg|HgO are composed of oxygen-deficient Ag₂O as follows from the XPS and Auger experiments. Steady-state conditions required for collection of valid impedance spectra were obtained at a potential range of 345–365 mV. The components of the equivalent circuit used for the impedance spectra analysis are analysed as a function of the Ag₂O layer thickness. The value of the coefficient of the constant phase element (CPE) attributed to the oxide layer is Ag₂O thickness dependent. On the other hand, the components of the CPE describing the double-layer capacitance of the oxide-covered Ag electrode are independent on the oxide thickness and their values are comparable to those obtained for the oxide-free metal. This indicates that the double-layer capacitances of oxide-covered and oxide-free Ag electrodes are similar.

     

Decomposition and Detection of Hydrogen Peroxide Using δ‐MnO2 Thin Film Electrode with Self‐Healing Property

A thin film of δ‐type MnO₂ grown cathodically has been investigated with respect to the ability toward anodic decomposition of H₂O₂ and durability. With polarization at less positive potentials than +0.4 V vs. Ag/AgCl, the film was dissolved exclusively as a result of reduction of Mn⁴⁺ sites in the oxide by H₂O₂ to soluble Mn²⁺. At +0.9 V, MnO₂ remained unchanged and decomposed H₂O₂ in solution. At +0.8 V, the film was once dissolved in the initial stage; however, it was self‐healed via reoxidation of the liberated Mn²⁺ ions. Amperometric flow‐injection analysis of H₂O₂ was carried out with the δ‐MnO₂ film.     

Physicochemical investigation of anodic processes involved in silver electrowinning in refining technology

During silver electrowinning in refining technology, irrespectively of the nature of electrolyte and anode material, an anode deposit is formed due to oxidation of singly charged silver ions to higher degrees of oxidation (+2 and +3) under polarization. Absorption spectra of Ag(II) have been obtained in solutions with various concentrations of silver ions and nitric acid using anodic polarization in combination with electronic absorption spectroscopy; silver ions of high oxidation degrees were found to be unstable in solutions. At anode potentials φ_a ≥ 1.15–1.22 V, Ag(I) ions in liquid phase are oxidized to Ag(II), which paint the anolyte; their content depends on silver and nitric acid concentrations in the electrolyte. The deposit crystallizes on anode at potential φ_a ≈ 1.5 V. Chemical analysis, X-ray powder diffraction, and a thermogravimetric study of the anode deposit have made it possible to identify its initial composition as Ag₇O₈NO₃. The deposit composition varies in time as a result of reduction of highly oxidized silver.     

Thermodynamic study of Ag<Subscript>8</Subscript>GeSe<Subscript>6</Subscript> by EMF with an Ag<Subscript>4</Subscript>RbI<Subscript>5</Subscript> solid electrolyte

The Ag–Ge–Se system was studied in the range of compositions Ag₂Se–GeSe₂–Se by measuring the EMF of the concentration (relative to the silver electrode) circuits with a solid electrolyte Ag₄RbI₅ in the temperature range 290–430 K. The polymorphic transition temperature of Ag₈GeSe₆ (320 K) was determined and the partial molar functions of silver were calculated for both crystal modifications of this compound based on the EMF measurements. The thermodynamic functions of formation and entropy of both modifications of Ag₈GeSe₆ and the thermodynamic functions of its polymorphic transition were calculated.     

Anodic silver (II) oxides investigated by combined electrochemistry,ex situ XPS and in situ X‐ray absorption spectroscopy

Silver (II) oxide layers (AgO) were prepared by anodic oxidation of pre‐oxidized, Ag₂O‐covered silver electrodes in 1 M NaOH (pH 13.8). The oxidized electrodes were investigated using a combination of electrochemical techniques, ex situ X‐ray photoelectron spectroscopy (XPS) and in situ surface‐sensitive grazing incidence X‐ray absorption spectroscopy (EXAFS) under full potential control. The application of these different techniques leads to a detailed, consistent picture of the anodic silver (II) oxide layer formation. The experiments have shown that the chemical composition of the AgO layer varies significantly with oxidation potential, revealing a decreasing oxygen deficiency with increasing anodization potential and oxidation time. XPS as well as EXAFS experiments support the interpretation of the oxide as a mixed valence Ag ⁺ Ag^{3 +} O₂ with different contributions of Ag ⁺ and Ag^{3 +} species, depending on potential and anodization time. Copyright © 2009 John Wiley & Sons, Ltd.     

The electrochemical behaviour of polycrystalline silver electrodes in Na2CO3 solution and the effect of ClO− 4 ions

The electrochemical behaviour of polycrystalline silver electrodes in Na₂CO₃ solutions was studied under potentiodynamic and potentiostatic conditions and complemented with X-ray diffraction analysis. Potentiodynamic E/i anodic curves exhibit active passive transition prior to an oxygen evolution reaction. The active region involves a small peak AI followed by a major peak AII before the passive region. Peak AI is assigned to the formation of an Ag₂O layer while peak AII is due to the formation of an Ag₂CO₃ layer. The height of the anodic peaks increases with increasing Na₂CO₃ concentration, scan rate and temperature. The effect of increasing additions of NaClO₄ on the electrochemical behaviour of Ag in Na₂CO₃ solutions was investigated. The perchlorate ions stimulate the active dissolution of Ag, presumably as a result of the formation of soluble AgClO₄ salt. In the passive region, ClO⁻ ₄ ions tend to break down the dual passive film, leading to pitting corrosion at a certain critical pitting potential. The pitting potential decreases with ClO⁻ ₄ concentration. Potentiostatic current/time transients showed that the formation of Ag₂O and Ag₂CO₃ layers involves a nucleation and growth mechanism under diffusion control. However, in the presence of ClO⁻ ₄ ions, the incubation time for pit initiation decreases on increasing the anodic potential step. Received: 3 July 1998 / Accepted: 10 March 1999     

High Temperature Reactions in the MoO3-Ag2O System

High-temperature reactions were investigated in the MoO₃-Ag₂O system by means of X-ray, DTA and scanning microscopy methods, and a model was proposed according to which first an Ag₂Mo₄O₁₃ phase was formed at the MoO₃ and Ag₂O (or metallic Ag) interface. Subsequently, at the Ag₂Mo₄O₁₃-Ag₂O contact area a layer of Ag₂Mo₂O₇ appeared. If the amount of silver oxide is sufficiently high, a layer of Ag₂MoO₄ is formed between the Ag₂O and Ag₂Mo₂O₇ phases. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temperature‐programmed reduction of silver(I) oxide using a titania‐supported silver catalyst under a H2 atmosphere

Reduction kinetics of silver(I) oxide using a titania‐supported silver catalyst was analyzed using temperature‐programmed reduction (TPR) with hydrogen as a reducing gas. Ag₂O reduction to Ag was observed in all samples as a single reduction step occurring at two reduction peaks. Observation of these reduction peaks indicates the existence of different lattice oxygen species, that is, surface and bulk, which are, respectively, attributed to surficial and pore‐deposited Ag₂O aggregates. The powdered samples exhibited high reducibility with average final oxidation states ranging from 0 to +0.18. The apparent activation energies for Ag₂O reduction to Ag metal were 73.35 and 81.71 kJ/mol for surficial and pore‐deposited Ag₂O aggregates, respectively. In this study, a unimolecular decay model was reported to accurately describe the reduction mechanism of Ag/TiO₂ catalysts. Hence, this would also infer that the catalyst reduction is rate‐limited by the nucleation of Ag metal instead of the topochemical reaction and the diffusion of hydrogen and oxygen molecules.     

Electrochemical behaviour of Ni, Cu, Ag, Pt and glassy carbon electrodes in triethylamine-tris(hydrogen fluoride) medium

Voltammetric responses of Ni, Cu, Ag, Pt and glassy carbon (GC) electrodes in triethylamine-tris(hydrogen fluoride) medium in the anodic as well cathodic potential region were investigated. AAS as well as SEM measurements were also made to ascertain the dissolution rate and surface transformation due to fluoride film formation on the electrode surfaces. On Ni, bulk NiF₂ film growth occurs only around 4.0 V following a thin NiF₂ monolayer formation around 0 V. The NiF₂ film shows very little solubility in the medium. Monolayer and bulk CuF₂ phases are formed quite close to each other on Cu during anodic polarization. The anodically formed CuF₂ dissolves to the extent of 12% in this medium. AgF formation follows a different mechanism during the first and subsequent anodic sweeps. The effect of MeCN as well as water addition on the solubility and stability of these fluoride films are also reported. Glassy carbon and Pt electrodes are relatively inert in this medium. Anodic voltammetric responses for other reactive species could be observed only on Pt and GC electrodes. On the cathodic side, all the electrodes show inert behaviour. Electrochemical reduction of PhNO₂, for example, could be observed on all the electrodes. Electronic Publication     

A comparative evaluation on the anodic behavior of Cu and Ag electrodes in non-aqueous fluoride and fluoroborate media

The voltammetric responses of copper and silver had been extensively studied and compared in a variety of non-aqueous solvents such as acetonitrile (AN), propylene carbonate (PC) and sulfolane containing two different supporting electrolytes namely triethylaminetrishydrogen fluoride (TEA.3HF) and tetrabutylammonium tetrafluoroborate (TBABF₄). The dissolution rate and surface transformation on the electrode surfaces as a result of anodic polarization was investigated using atomic absorption spectroscopy (AAS) and scanning electron microscopy (SEM), respectively. In solvent-free TEA.3HF medium, the copper electrode shows high charge recovery ratio (Q _c/Q _a), and the difference between the initial anodic and cathodic potentials, obtained at a current density of 2 mA cm⁻², is around 0.11 V, suggesting that in this medium, Cu can certainly serve as reference electrode. On the other hand, on Ag electrode, substantial dissolution was observed leading to very high anodic (Q _a) and cathodic (Q _c) charges, and the surface morphology after the cyclic polarization results in roughened surface with large pores. The effects of incorporating AN and water as additives in TEA.3HF on the solubility and stability of these metal fluoride films are also reported. The dissolution pattern and film formation behavior of these two metals in the different solvents containing fluoride and fluoroborate ionic species have several qualitative similarities, as noted from cyclic voltammetry responses and SEM morphology. Anodic dissolution and precipitation process for both Cu and Ag depends significantly on the nature of supporting electrolytes as well as solvents. In AN containing 0.1 M TEA.3HF, the dissolution of Cu and Ag electrodes was very high. Fluoride salts of Cu show lesser solubility than Ag in those solvents, while fluoroborate salts exhibit the reverse trend. The AAS data suggest that for a particular salt, which may be either fluoride or fluoroborate of Cu and Ag, the relative solubility decreases in the order AN > PC > sulfolane.     

<Emphasis Type="Italic">In situ</Emphasis> XPS study of the size effect in the interaction of NO with the surface of the model Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/FeCrAl catalysts

The interaction of NO with the surface of model Ag/Al₂O₃/FeCrAl catalysts containing Ag nanoparticles of different size (1 and 3 nm) was studied. The use of the Auger parameter α_Ag (E _b(Ag3d_5/2) + E _kin(Ag MVV)) made it possible to reliably identify the change in the chemical state of silver cluster upon their interaction with О₂ and NO. The oxygen treatment leads to the oxidation of small Ag nanoparticles (1 nm) and formation of AgO _x clusters resulted in the intensive formation of nitrite—nitrate structures on the step of the interaction with NO. These structures are localized on both the silver clusters and Al₂O₃ surface. An increase in the size of Ag⁰ nanoparticles to 3 nm results in an increase in the stability of these structures and impedes the Ag⁰ → AgO _x transition, due to which the formation of surface groups NO₂ ^–/NO₃ ^– is suppressed. The data obtained make it possible to explain the dependence of the activity of the Ag/Al₂O₃ catalysts in the selective reduction of NO on the Ag nanoparticle size.     

Effect of hydrogen sulfide on the iron dissolution rate in a sodium sulfate solution under natural aeration conditions

The influence of hydrogen sulfide (10–100 mg/1) on the Armco iron anodic dissolution in an aerated 0.17 M Na₂SO₄ solution is investigated. During a potentiostatic anodic polarization, the hydrogen sulfide introduction makes the current increase stepwise. The magnitude of the increase depends on the duration of preliminary anodic polarization, electrode potential, and hydrogen sulfide concentration. The anodic metal dissolution activation by hydrogen sulfide is explained by chemical conversion of the oxide-hydroxide passivating film into iron sulfide that is generated at the metal surface in the form of a porous film and does not hinder the electrode dissolution. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Investigation of the Anodic Behavior of Al in Room Temperature Ionic Liquid Electrolytes: BMI‐BF4, PP14‐BF4 and BMI‐BF4·PC

The anodic polarization behavior of Al, Ta and Nb foil was investigated in 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid (BMI‐BF₄). Compared with that of Ta and Nb foil, it showed that a better passive film was formed on Al foil surface after the anodic polarization in BMI‐BF₄, which could resist the potential up to 94.58 V vs. Ag⁺/Ag. Besides, similar anodic behavior of Al foil was observed in N‐methyl‐N‐butylpiperidinium tetrafluoroborate ionic liquid (PP14‐BF₄), which indicated that the anodic polarization behavior of Al foil was independent of the cations of RTIL. In addition, the investigation of anodic polarization behavior of Al foil was carried out in the mixture electrolytes composed of BMI‐BF₄·PC. Differently, two breakdown potential processes of Al foil were presented compared to pure BMI‐BF₄. Further research showed that the passive film on Al foil was mainly composed of AlF₃ and Al₂O₃ after the first breakdown potential process, while the fluoride film increased with continual anodic polarization, which improved the anodic stability of Al foil and resisted higher breakdown potential. The high breakdown potential properties of Al foil in BMI‐BF₄, PP14‐BF₄ and the mixture of BMI‐BF₄·PC during the anodic polarization can be favored for R&D of the high performance electrochemical devices.