Underpotential Deposition Of Zn Research Articles

Electrochemical and in situ STM studies of anomalous phosphate adsorption induced on Zn UPD at Au( [formula omitted]) in the presence of halide ions in aqueous phosphate solutions

Underpotential deposition (UPD) of Zn 2+ at Au(1 1 1) was studied in the phosphate solutions with and without halide ions (Cl −, Br −, and I −) by cyclic voltammetry and in situ STM for the evaluation of the anion adsorption induced on the Zn UPD. The order of the strength of anion adsorption induced on Zn UPD was found to be different from the order of specific adsorption strength of I −>Br −>Cl −>PO 4 3−⩾SO 4 2− at an Au substrate electrode. This anomalous tendency of anion adsorption induced on UPD Zn at Au seems to be identical with the cases of sulfate and phosphate adsorption induced on UPD Zn at a Pt electrode in the presence of chloride, being confirmed by radiotracer method. Such anomalous phosphate adsorption induced on Zn UPD was found to take place at Au(1 1 1) not only in the presence of chloride or bromide, but also even in the presence of iodide in the phosphate solution. The in situ STM images of Zn UPD were observed at Au(1 1 1) in the phosphate solution with iodide ions. At −200 mV (saturated calomel electrode; SCE) where the iodide adsorbs at substrate Au(1 1 1), the STM image was c(p× 3 R30°) , and in the Zn UPD region, the STM image became ( 3 × 3 )R30° . The latter image is discussed to be assigned to the image in the absence of iodide, i.e., that the adsorbed anions induced at Zn UPD are phosphate species even in the solution containing iodide ions.

Electrodeposition of β′‐Brass from Cyanide Baths with Accumulative Underpotential Deposition of Zn

Cu‐Zn alloys were deposited under potentiostatic conditions from cyanide baths with varied concentration ratio but with the same free concentration. The Zn content of the deposits showed the maximum at around −1.4 V vs. Ag/AgCl as a function of the applied potential. The deposits obtained at −1.4 V vs. Ag/AgCl had the composition near CuZn, and the β′‐brass superlattice whose structure is the CsCl type, was independent of the concentration ratio of the baths. In the absence of in the bath, elemental Zn was deposited only at potentials negative to −1.5 V vs. Ag/AgCl and the Zn electrode was dissolved in the bath at −1.4 V vs. Ag/AgCl. This shows that Zn was codeposited with Cu forming the β′‐brass deposits in the underpotential deposition region. The β′‐brass deposits were formed through the accumulative underpotential deposition of Zn onto the electrodeposited Cu atom. The underpotential deposition of Zn onto Cu was confirmed from the cyclic voltammograms of a Cu electrode. © 2000 The Electrochemical Society. All rights reserved.

Anomalous induced anion adsorption on zinc ion underpotential deposition at Au(111) in the presence of halide ions

Underpotential deposition (UPD) of Zn 2+ at Au(111) in phosphate solution with Cl −, Br −, and I − was studied by cyclic voltammetry (CV), focusing on the effects of halide species on Zn UPD for the determination of the induced anion adsorption on the UPD. The order of the adsorption strength of the induced anion on Zn UPD was found to be different from that of the specific adsorption of I −>Br −>Cl −> PO 3− 4 ≥ SO 2− 4 > ClO − 4 at a substrate Au electrode. The adsorbed anions induced on Zn UPD are not suggested to be halide ions but phosphate ions. Such anomalous tendency of anion behavior has been recently proved for induced sulfate and phosphate adsorption behavior on Zn UPD at a Pt electrode in the presence of chloride by the radiotracer method. This paper shows that the anomalous induced phosphate adsorption on Zn UPD also takes place at Au(111) not only in the presence of chloride or bromide, but also in the presence of iodide in a phosphate solution.

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Zn underpotential deposition with iron-group metals was studied using a Cu substrate in sulfate or chloride baths containing a small amount of Zn2+ ions at 5 to 80°C under potentiostatic conditions. Underpotential deposition occurs only in the presence of iron-group metals and is facilitated when conditions permit decreased inherent deposition overpotential of iron-group metals such as high temperature and Cl- additive. This is similar to induced alloy deposition, in which the properties of iron-group metals in a metallic state play an important role. This is cleary different, however, from induced because Zn deposition occurs at more noble potentials than its equilibrium. Iron-group metal properties in a metallic state appear responsible for Zn underpotential deposition, although the detailed mechanism is yet unclear. If the codepositing iron-group metal consistently reduces the activity coefficient of Zn deposited on the progressively formed cathode surface, the large amount of underpotential deposition is explainable thermodynamically.

Underpotential deposition of zinc ions on polycrystalline platinum: FTIR and EQCM study

Underpotential deposition(UPD) of Zn 2+ ions on polycrystalline platinum in KH 2PO 4 solution (pH 4.6) was studied by Fourier transform infrared reflection spectroscopy (FTIR) and electrochemical quartz crystal microbalance (EQCM). By the addition of Zn 2+ ions in KH 2PO 4 solution, the intensities of ir bands of around 990, 1044 and 1090 cm −1 are increased with a negative increase of potentials at E ⩽ 0.7V ( rhe) where Zn UPD takes place, while by EQCM the weight increase of Pt mounted quartz was observed in the same potential region. Coverage of UPD Zn was 0.75 at 0.05 V as two-electron transfer UPD process, being estimated from cyclic voltammogram, and mps (mass number per platinum sites) was 78 by EQCM; from both values the coverage of phosphate was evaluated as 0.27. These results are discussed that the phosphate species adsorbs over UPD Zn on Pt as a form of HPO 4, where one of three PO's seems to be bound to UPD Zn on Pt.

Thin Layer Electrochemical Studies of ZnS, ZnSe, and ZnTe Formation by Electrochemical Atomic Layer Epitaxy (ECALE)

ABSTRACTThin-layer electrochemical studies of the underpotential deposition (UPD) of Zn, Te, Se, and S on polycrystalline Au substrates have been performed. These studies were initiated to investigate the electrodeposition of ZnTe, ZnSe, and ZnS by electrochemical ALE (ECALE). Zn UPD on Au begins at -0.5 V and results in a coverage of 0.47 monolayer (ML). Te and Se atomic layers were formed using a two step process where bulk chalcogenide was removed by reduction, leaving the atomic layer. The reduction of the last atomic layer of Te or Se was not observed, regardless of how negative the potential was scanned. Sulfur atomic layers were spontaneously deposited below -0.6 V from a sulfide solution. Thermodynamic effects are clearly evident during the first monolayer of deposition. Zinc deposition onto Te, Se, and S coated electrodes occurs at progressively more positive potentials as the stability of the zinc compounds increase.This initial information was used to develop ECALE cycles for the compounds, and thin-films were formed by repeated application of the cycles. The dependence of the deposit coverage on the deposition potentials was examined and found to display the characteristic “S” curve of a surface limited process. In addition, the dependence of the coverage on the number of ECALE cycles performed was found to be near the ideal 0.5 ML per cycle for ZnSe and ZnS. The ZnTe coverage per cycle was less than expected indicating that further optimization of deposition conditions is needed.

Underpotential deposition of Zn 2+ ions on platinum, palladium and gold at various pH values

The underpotential deposition (UPD) of Zn 2+ ions on Pt, Pd and Au was observed in solutions at various pH values. The UPD peak potential on Pt and Pd shifted to more positive potentials with increase in Zn 2+ ion concentration, the shift being 85–95% of the equilibrium shift of the Nernst equation for Zn 2+ + 2e − = Zn. This suggests that the peak is due to UPD of Zn 2+ ions and is associated with electron charge transfer of ca. 2. With Au, the shift of UPD peak potential with change in Zn 2+ ion concentration varied with the change in pH and the electron transfer numbers were 1.5 and 2 at pH 5.0 and 6.0, respectively. The UPD shift for Zn 2+-Pt and Zn 2+-Pd systems was approximately the same, being ca. 1.0 V at pH 0.69–6.9, whereas that for Zn 2+-Au was ca. 0.6 V. The half-width of the UPD peak increased with increase in pH; the interaction parameter of the Temkin adsorption isotherm was estimated as being repulsive for UPD of Zn 2+ on Pt and Pd.