Anodic dissolution of silicon monocrystals in anhydrous organic solutions of chlorides

The electrochemical investigations of p- and n-silicon monocrystals were performed in anhydrous organic solutions of LiClO₄, LiCl and HCl to examine the possibility of etching and passivation of silicon semiconductors. The results obtained by means of linear sweep voltammetry (LSV), potentiostatic and galvanostatic transient technique, as well as XPS surface analysis allow us to give an explanation of the mechanism of silicon dissolution in these media. Silicon dissolves in anhydrous organic solvents (methanol, N,N-dimethyloformamide, formamide) which contain chloride ions according to the consecutive two-step mechanism. The Si(II)_ad intermediate inhibits the anodic dissolution at low overpotentials. The presence of this intermediate was confirmed by means of XPS measurements on potentiostatically etched surface. The increase in chloride concentration in organic solvents stimulates desorption of the intermediate and therefore increases the rate of surface etching. The best results in anodic etching of silicon monocrystals have been obtained in anhydrous HCl solutions. Microscopic observations of surface morphology of Si monocrystals after etching show anisotropy of anodic dissolution.     

Comparative study of copper behaviour in bicarbonate and phosphate aqueous solutions and effect of chloride ions

Copper oxidation in aqueous solutions of pH 8 showed some differences in the presence of bicarbonate and phosphate ions. The bicarbonate ions did not interfere with Cu₂O film formation but the Cu²⁺ ions were stabilized by the complexing action of CO _2– ³ anions. In phosphate solutions, copper dissolved in the range of potentials associated with the Cu(I) oxidation state and the Cu(II) compound on the surface resulted in an extensive passivation region. In both solutions, a higher ion concentration caused an increase in the anodic current, suggesting that the copper ions were stabilized by the complexing action of the electrolyte. The copper oxidation current in a bicarbonate solution was higher than that observed in a phosphate solution of the same concentration. The thickness of the Cu(II) film rather than the Cu(I) layer appears to be the important factor related to the stability of the passive layer on the copper surface. The shift in the breakdown potential toward more positive values indicates that both bicarbonate and phosphate ions inhibit localized corrosion due to the presence of chloride ions. Their protective effect depends on the concentration of each anion, although the concentration of chloride ions necessary for pitting is larger in phosphate solutions than in bicarbonate solutions. In both solutions, long-term immersion of copper under anodic polarization results in the precipitation of a protective coating.     

Anodic oxidation of dimethyl sulfoxide based electrolyte solutions: Anin situ FTIR study

Anodic oxidation of dimethyl sulfoxide (DMSO) based electrolyte solutions, containing LiClO₄, LiBF₄ and KPF₆, on platinum (Pt), glassy carbon (GC) andn-TiO₂ (anatase), electrodes was studied usingin situ Fourier transform infrared spectroscopy (FTIR). All solutions contained small amounts of H₂O. Regardless of the supporting electrolyte all systems were unstable at potentials above 1.0 V vs SCE. The major oxidation product is dimethyl sulfone, formation of which is initiated by the trace water breakdown. In contrast to acetonitrile based solutions there is no evidence of electrolyte involvement in the breakdown process. Photoanodic decomposition of dimethyl sulfoxide based solutions proceeds in the same way as the anodic oxidation in the dark. In the presence of nucleophilic agent (iodides) the prevailing redox process is iodide oxidation. Small amounts of, probably, methylsulfinyliodide are also formed. The irreversible consumption of charge mediator significantly restricts the possible practical use of DMSO in photoelectrochemical devices.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel

The rates of elemental dissolution were measured for a 304 stainless steel containing 0.19% Cu in real time during linear sweep voltammetry experiments using the atomic emission spectroelectrochemistry (AESEC) method. The results demonstrate that Fe, Cr, Ni, Mn, and Mo dissolve simultaneously leaving a residual copper film that inhibits the steel dissolution. The formation and dissolution of the copper film leads to the appearance of two peaks in the anodic dissolution transient, one due to inhibition of steel by residual copper and the other due to the formation of the passive film. The addition of NaCl to the electrolyte results in a marked increase in the intensity of the second dissolution peak, while hardly affecting the first peak. This is interpreted in terms of the lowering of copper dissolution potential by chloride ions due to the stabilization of CuCl₂⁻. A simple phenomenological kinetic model is used to simulate the variation of dissolution rate with potential.     

Electrochemical investigations of copper behaviour in different cupric complex solutions: Voltammetric study

The electrochemical behaviour of copper has been investigated in different cupric complex solutions by cyclic voltammetry. In pyridinic and picolinic solutions the reduction of cupric complex occurred in two stages leading to Cu(I) and Cu(0), respectively. The electrodeposited copper is oxidized in two steps leading to Cu(I) and Cu(II) as in ammoniacal cupric complex solutions. In glycine, alanine, sulfamic acid and ethylenediamine solutions, the cuprous complex is an intermediate in the cupric complex reduction but it is not detected during the oxidation of the electrodeposited copper in these solutions. In EDTA and triethanolamine solutions, the cuprous complex is not observed. The rate of copper etching was determined in pyridinic and ammoniacal cupric solutions and was shown to be faster in ammoniacal cupric solutions than in the pyridinic solutions.     

Au dissolution during the anodic response of short-chain alkylthiols with polycrystalline Au electrodes

The electrochemical characteristics of polycrystalline Au in LiClO₄ electrolyte solutions containing 3-mercaptopropionic acid (MPA) or meso-2,3-dimercaptosuccinic acid (DMSA) were studied with linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) over a wide range of positive potentials vs. Ag/AgCl. The EIS data exhibited linear capacitive behaviour at 0.0 V with either MPA or DMSA added directly to the electrolyte suggesting the formation of an adsorbed layer of the alkylthiol on the electrode surface. Above this potential, a single well-defined impedance loop appeared for electrolyte solutions containing DMSA or MPA, an observation indicative of a charge transfer reaction that could be related to several processes including oxidative desorption, oxidation of the alkylthiol, or Au oxidation/dissolution. To test for Au dissolution, the electrode was held at 0.8 V vs. Ag/AgCl for 12 h in electrolytes containing MPA or DMSA followed by surface analysis with Atomic Force Microscopy and solution analysis with Atomic Absorption Spectroscopy. When the electrolyte contained MPA, the extended potential holding procedure resulted in significant roughening of the electrode with no detectable quantities of Au in the electrolyte. X-ray photoelectron spectroscopy (XPS) analysis of the Au surface revealed an additional species in the Au 4f_7/2 spectrum indicating the presence of an insoluble electrochemically generated Au(I)–MPA species. When the electrolyte contained DMSA, the Au electrode appeared smoother, 56.6 ± 9.6 ppb of Au was detected in the electrolyte and the XPS analysis displayed a single species in the Au 4f_7/2 spectrum indicative of metallic Au after the potential holding procedure. Both observations with MPA and DMSA support the charge transfer resistance to be at least partially related to the corrosion of Au, but also suggest that an electrochemically generated Au–DMSA species is soluble and of potential industrial relevance.     

Anodic behaviour of alkaline solutions containing copper cyanide and sulfite on the graphite anode

Sulfite may be added to copper cyanide solutions to reduce cyanide oxidation at the anode during copper electrowinning. Anodic sulfite oxidation is enhanced in the presence of copper cyanide. Sulfite also suppresses the oxidation of copper cyanide. The effect of sulfite on the oxidation of copper cyanide decreases with increasing mole ratio of cyanide to copper. This is related to the shift in the discharged species from Cu(CN)₃ ^2– to Cu(CN)₄ ^3– with increasing mole ratio of cyanide to copper. Sulfite is oxidized to sulfate. At [Cu⁺] = around 1 M, CN:Cu = 3.0–3.2, [OH^–] = 0.05–0.25 M, [SO₃ ^2–] = 0.4–0.6 M and the temperature = 50–60 °C, the anodic current efficiency of sulfite reached 80–90%. With further increase in sulfite concentration beyond 0.6 M, the current efficiency of sulfite oxidation will not be increased significantly. Further increase in CN:Cu mole ratio will result in decrease in the anodic current efficiency.     

The mechanism of oxidation of copper in alkaline solutions

The formation of Cu₂O by the oxidation of Cu in alkaline solutions under various controlled potential conditions has been studied by potentiodynamic methods, the rotating ring disc technique and by employing colloidal Cu(OH)₂ electrodes supported on vitreous carbon.The kinetics of the electrochemical reactions, both anodic and cathodic, are interpreted in terms of a complex reaction mechanism involving various intermediates participating in the phase oxide formation, (e.g. adsorbed OH, soluble Cu(I) and metal sites of different activity).Besides the electrochemical reactions the model includes various ageing and surface restructuring processes. The growth mechanism is envisaged to depend on the conditions of oxidation.     

Anodic oxidation of copper cyanide on graphite anodes in alkaline solution

The anodic oxidation of copper cyanide has been studied using a graphite rotating disc with reference to cyanide concentration (0.05–4.00 M), CN:Cu mole ratio (3–12), temperature (25–60 °C) and hydroxide concentration (0.01–0.25 M). Copper had a significant catalytic effect on cyanide oxidation. In the low polarization region (about 0.4 V vs SCE or less), cuprous cyanide is oxidized to cupric cyanide complexes which further react to form cyanate. At a CN:Cu ratio of 3 and [OH^–] = 0.25 M, the Tafel slope was about 0.12 V decade^–1. Cu(CN)₃ ^2– was discharged on the electrode and the reaction order with respect to the predicted concentration of Cu(CN)₃ ^2– is one. With increasing CN:Cu mole ratio and decreasing pH, the dominant discharged species shifted to Cu(CN)₄ ^3–. Under these conditions, two Tafel slopes were observed with the first one being 0.060 V decade^–1 and the second one 0.17–0.20 V decade^–1. In the high polarization region (about 0.4 V vs SCE or more), cuprous cyanide complexes were oxidized to copper oxide and cyanate. Possible reaction mechanism was discussed.     

On two control parameters of the dynamics of the anodic dissolution of copper

An experimental study of the effect of SCN⁻ ion concentration and laser bombardment intensity on the anodic dissolution of copper into an aqueous NaCl electrolyte is reported. The laser was used to accelerate the dissolution process. The results are analyzed in terms of power spectra, fractional Brownian motion theory and confocal image processing. An interpretation of the dynamic phenomena via the theory of nonlinear dynamics is also suggested.     

Electrochemical dissolution of calaverite(AuTe2) in thiourea acidic solutions

Cyclic voltammetric studies of carbon paste electrodes of a synthetic gold telluride, calaverite (AuTe2), in acidic aqueous thiourea solutions indicate that for potentials of about 0.4V vs SCE adsorbed thiourea decomposes to formamidine disulphide, while gold from calaverite oxidizes and complexes with thiourea. The chemical oxidation of calaverite with the formamidine disulphide produced occurs in parallel with these electrodic processes. Additionally, for potentials in the vicinity of 0.5V vs SCE tellurium from calaverite transforms to telluril ion, HTeO+2. A passive film of tellurous acid H2TeO3 forms at potentials around 0.7 V vs SCE whereas at potentials above 0.9 V vs SCE the formation of gold oxides and hydroxides is apparent. Reduction of calaverite occurs at potentials less than –0.7 V vs SCE. Other cathodic peaks are associated to the reverse processes of the anodic decomposition stages.     

An electrochemical study of the dissolution of gold in thiosulfate solutions. Part II. Effect of Copper

Thiosulfate has been considered as one of the most promising of the non-toxic alternatives to cyanide for the leaching of gold and much work has been carried out with the aim of understanding and improving the ammoniacal thiosulfate leaching process. In particular the behaviour of gold in thiosulfate solutions containing copper in the absence of ammonia has received little attention. It has been shown in this study involving electrochemical and leaching tests that copper ions catalyze not only the oxidation of thiosulfate but also the dissolution of gold in alkaline thiosulfate solutions. Electrochemical studies have shown that copper has a positive effect on the anodic dissolution of gold with increasing concentrations of copper resulting in higher dissolution rates of gold at a potential of 0.3V. Studies on the dissolution of gold powder in alkaline oxygenated thiosulfate solutions containing low concentrations of copper have shown that the role of copper in enhancing the dissolution rate of gold is possibly associated with the formation of a copper–thiosulfate–oxygen intermediate which is more reactive in terms of cathodic reduction than dissolved oxygen. The electrochemical experiments have been complemented by a leaching study which has shown that milling of gold powder in the presence of copper (added as ions, metal, or oxide) assists with the dissolution of gold in thiosulfate solutions.     

The corrosion of copper in ethylene glycol-water mixtures containing chloride ions

The anodic oxidation of copper at 25°C in 50% (w/w) ethylene glycol-water and in aqueous solutions has been studied by linear sweep voltammetry. The effect of chloride concentration at pH 0 and 3 has been explored. The results in both solvents follow a similar pattern. At pH 0 and in the absence of chloride, only one anodic peak is observed corresponding to the dissolution of copper metal as copper(II) ions. At intermediate chloride concentrations (0.01–0.03 M), two additional peaks are detected which have been attributed to the following reactions: $$\begin{gathered} Cu + Cl^ - \to CuCl + e^ - \hfill \\ CuCl \to Cu^{2 + } + Cl^ - + e^ - \hfill \\ \end{gathered} $$ When the chloride concentration is increased further, the three peaks gradually collapse back into one, corresponding to the dissolution of copper as a copper(I) chloro-complex. An additional peak appears at pH 3 which has been ascribed to the formation of copper(I) oxide. The results have been interpreted usingE-pCl diagrams determined for the copper-chloride system in both 50% ethylene glycol-water and aqueous solutions. Further information has been obtained from rotating disc measurements and from microscopy. The relevance of these results to corrosion in automotive cooling systems is discussed.     

Copper dissolution in the presence of a binary 2D-compound: CuI on Cu(100)

Exposing a Cu(100) electrode surface to an acidic and iodide containing electrolyte (5 mM H₂SO₄/1 mM KI) leads to the formation of an electro-compressible/electro-decompressible c(p × 2)-I adsorbate layer at potentials close to the onset of the copper dissolution reaction. An increase of mobile CuI monomers on-top of the iodide modified electrode surface causes the local CuI solubility product to be exceeded thereby giving rise to the nucleation and growth of a laterally well ordered 2D-CuI film at potentials below 3D-CuI_bulk phase formation. Step edges serve as sources for the consumption of copper material upon compound formation leading to accelerated copper dissolution at the step edges. The 2D-CuI film exhibits symmetry properties and nearest neighbor spacings that are closely related to the (111) lattice of the crystalline CuI_bulk phase. Intriguingly, the 2D-CuI film on Cu(100) does not act as an efficient passive layer. Copper dissolution proceeds at slightly higher potentials even in the presence of this binary 2D-compound via an inverse step flow mechanism. Further dissolution causes the nucleation and growth of 3D-CuI clusters on-top of the 2D-CuI film. This several nanometer thick 3D-CuI_bulk phase passivates the electrode against further dissolution. Characteristically, the formation/dissolution of the 3D-CuI_bulk phase reveals a significantly larger potential hysteresis of about ΔE = 320 mV while the appearance/disappearance of the 2D-CuI film is reversible with a potential hysteresis of only ΔE = 20 mV.     

Cathodic processes in the leaching and electrochemistry of covellite in mixed sulfate–chloride media

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate–chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3–0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5–0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.     

Electrodeposition of nanocrystalline copper thin films from 1-ethyl-3-methylimidazolium ethylsulphate ionic liquid

Copper thin films are increasingly important as interconnectors for the creation of smaller and better performing integrated circuits and electrodeposition from ionic liquid-based electrolytes could provide a greener fabrication method for these films. The electrodeposition of copper from copper(I) and copper(II) salt solutions in a low cost, widely available ionic liquid, 1-ethyl-3-methylimidazolium ethylsulphate, was studied using a range of different deposition potentials and temperatures. Three different electrolytes containing ~0.1 M of copper(I) chloride(CuCl), copper(II) chloride (CuCl₂) and copper(II) sulphate (CuSO₄) were used. Under similar deposition conditions, the films obtained from CuCl and CuSO₄-based electrolytes presented better continuity than films obtained from CuCl₂-based electrolyte. Continuous films with a homogeneous structure were obtained by electrodeposition from CuCl and CuSO₄-based solutions at a constant potential of ?1.8 V and a temperature of 35 °C. Under similar deposition parameters, the films deposited from CuCl₂-based electrolyte presented the largest particle size, while those deposited from copper(I) chloride and CuSO₄-based solutions presented finer microstructures. X-ray diffraction analysis and energy dispersive X-ray spectroscopy showed that the deposits were crystalline and consisted mainly of copper, with traces of oxygen and sulphur resulting from residues of the ionic liquid. The films presented a nanocrystalline microstructure consisting of particles about 25 nm, aggregated in clusters.     

Obtaining of high surface roughness copper deposits by electroless plating technique

The use of pyridine-2,6-dicarboxylic and 4-hydroxypyridine-2,6-dicarboxylic acids as copper(II) ligands in formaldehyde-containing alkaline electroless copper plating solutions allowed to obtain copper layers with extremely high surface roughness factor reaching approximately 120. The Cu deposits of higher surface area were formed at highly negative open-circuit (mixed) potentials; the correlation between copper electrode overpotential and roughness of the deposit was found and discussed. The copper films obtained demonstrate a high electrocatalytic activity in anodic formaldehyde oxidation process, the oxidation rate reaches 40 mA cm⁻² and exceeds considerably that for other copper surfaces.     

The electrochemical behaviour of copper in alkaline solutions containing sodium sulphide

The electrochemical behaviour of copper in NaHCO₃ solution (pH 9) and NaOH solutions (pH 14) in the presence of sodium sulphide (10^–3 to 5×10^–2m) was investigated by using rotating disc electrode and rotating ring-disc techniques, triangular potential scanning voltammetry and potentiostatic steps. When the potential increases from –1.2V upwards, copper sulphide layers are firstly formed at potentials close to the equilibrium potentials of the Cu/Cu₂S and Cu/CuS reversible electrodes. When the potential exceeds 0.0 V (NHE), the copper oxide layer is electroformed. Pitting corrosion of copper is observed at potentials greater than –0.3 V. The charateristics of copper pitting are also determined through SEM optical microscopy and EDAX analysis. There are two main effects in the presence of sodium sulphide, namely, the delay in the cuprous oxide formation by the presence of the previously formed copper sulphide layer and the remarkable increase of copper electrodissolution when the potential exceeds the cuprous oxide electroformation threshold potential. These results are interpreted on the basis of a complex structured anodic sulphide layer and on a weakening of the metal-metal bond by the presence of adsorbed sulphur on the copper surface.     

Electrochemical codeposition of copper and manganese from room-temperature N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid

The voltammetric behavior of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMP-TFSI) containing Cu(I), Mn(II), or mixtures of Cu(I) and Mn(II) as well as the electrodeposition of copper-manganese alloy coatings (Cu-Mn alloy coatings) was studied at 323 K. The Cu(I) and Mn(II) species required to prepare these coatings were introduced into the BMP-TFSI by anodic dissolution of the relevant metallic electrodes. Electrodeposits of Cu, Mn, and Cu-Mn with various contents of Mn can be obtained by controlled-potential electrolysis. It was found that the compositions and surface morphology of the electrodeposited Cu-Mn alloy coatings depend on the deposition potentials and the concentration ratio of [Cu(I)]/[Mn(II)] in BMP-TFSI. The Cu-Mn alloy coatings obtained in this study were compact and adherent. They did not show any significant X-ray diffraction signal that could be assigned to the crystalline structures of Cu, Mn, or Cu-Mn alloys. In the aqueous solution containing 0.1 M NaCl, the Cu-Mn alloy coatings demonstrated passive behavior—no continuous oxidation was observed. However, a significant oxidation current was observed at the electrodes deposited with Cu or Mn.     

A rotating ring–disk study of the initial stages of the anodic dissolution of chalcopyrite in acidic solutions

The initial stages of the dissolution of chalcopyrite have been studied using rotating ring–disk electrode techniques in dilute sulfuric acid solutions at 60 °C. It has been confirmed that the mineral undergoes a dissolution process under freely dissolving conditions in the absence of an oxidant. This process involves the formation of soluble copper(II) ions and a soluble sulfur species which is presumably H₂S. By the use of an anodic stripping technique on the ring electrode, it has been possible to determine that the dissolution of chalcopyrite under oxidative conditions involves a soluble sulfur species such as thiosulfate. Collection efficiency measurements involving the detection of both iron(II) and copper(II) on the ring have been used to identify possible anodic reactions in the potential region relevant to the oxidative leaching of the mineral.