Giovanni Pezzatini

Formation of ZnSe on Ag(111) by electrochemical atomic layer epitaxy

ZnSe is an ideal candidate for optoelectronic devices. However, modern technology is quite demanding as to the quality of materials employed. The electrochemical atomic layer epitaxy (ECALE) method ensures good atomic-level control of thin film formation, as it is based on the alternated deposition of atomic layers of the elements making up the compound. The electrochemical conditions necessary to form ZnSe deposits of up to 35 Zn layers and 35 Se layers on Ag(111) by ECALE are described here. The 1:1 stoichiometric ratio of Zn to Se suggests that ZnSe is formed. A miniaturized electrochemical cell is also described.

CdS and ZnS Deposition on Ag(111) by Electrochemical Atomic Layer Epitaxy

We applied the electrochemical atomic layer epitaxy (ECALE) methodology to obtain deposits of CdS and ZnS on Ag (111) by alternate underpotential deposition of the elements forming the compounds. The amounts of the elements deposited, determined by stripping coulometry, always yielded a stoichiometric 1:1 ratio. The deposits were formed using an automated electrochemical deposition system, described here, making use of a simple distribution valve.

Electrochemical aspects of CdTe growth on the face (111) of silver by ECALE

CdTe deposition on Ag(111) was obtained by Electrochemical Atomic Layer Epitaxy (ECALE) method, that is by the alternated underpotential deposition of tellurium and cadmium. The voltammetric analysis of the first TeUPD and CdUPD peaks indicates a mechanism of two-dimensional growth, which is consistent with an epitaxial growth. The ratio Cd:Te determined on the basis of electrochemical measurements is very close to the 1:1 stoichiometric ratio, which is indicative of a compound formation.

Double-layer structure and mechanism of electrode reactions: Part II. Nitrobenzene reduction on mercury from aqueous solutions

Abstract Kinetic measurements of nitrobenzene (RNO2) electroreduction to phenylhydroxylamine in aqueous solutions of pH ranging from 2 to 12 were carried out with the polarographic technique under different experimental conditions. In particular, different concentrations of non-specifically and specifically adsorbed supporting electrolytes were employed to change the potential φd at the outer Helmholtz plane and/or the charge density qi at the inner Helm-holtz plane. The effects of these changes are amenable to a straightforward interpretation provided the overall electrode process is assumed to be controlled by the electron-transfer step R N O 2 H → + e R N O 2 H − (step a) for 2 R N O 2 − → + H + R N O 2 H (step b) for 6 R N O 2 − → + e R N O 2 2 − (step c) for pH>11, and by both the parallel steps (b) and (c) for 9

Diffusion with reactant adsorption at potentiostated stationary and dropping electrodes

Abstract An approximate procedure for the solution of boundary value problems concerning diffusion toward planar, spherical and dropping electrodes, as outlined in two previous notes [14, 15], is generalized to account for adsorption of reactants and products. Applications of this procedure are reported. The satisfactory degree of accuracy of such a procedure is tested by comparison with the results of more rigorous procedures as well as with experimental results.

Electrochemical behavior of As on silver single crystals and experimental conditions for InAs growth by ECALE

Different experimental conditions for InAs growth by ECALE were examined. In layers were constantly deposited at underpotential from acetic buffer solutions, whereas As layers were obtained by three different approaches. First, Asupd layers were obtained by reducing an excess of previously deposited As(0) to AsH3 from NaOH solutions obtaining no more than five layers of InAs. Secondly, As was deposited at underpotential from ammonia buffer solutions of pH 9.6; at this pH the upd of both In and As coincide. However, XPS analysis on a sample revealed that only a low percentage, 25%, of As was in the reduced −III state, whereas most of it was in the oxidized +III state. The incomplete charge transfer in AsO2 − reduction excludes the possibility of InAs formation. The effective way of depositing InAs on Ag(111) consists in depositing bulk As(0), limiting the extent of deposition through the control of the time as well as the potential of deposition. Thus, As layers were obtained from acetic buffer solutions at −0.7 V. The composition of samples prepared by this procedure was determined by XPS and by the classical methods of analysis. Both methods gave the 1:1 stoichiometric ratio, typical of a compound, though the extent of deposition seems to be low. However, the AFM morphological investigation of samples obtained with 50, 75 and 100 deposition cycles point to a high quality compound.

Bromide electrosorption on the low-index faces of silver

Bromide adsorption on the three low-index faces of silver was investigated from mixed c mol dm–3 KBr +(0.02 –c) mol dm–3 KPF6 solutions over a range of c from 1 × 10–6 to 5 × 10–3 mol dm–3 using the potential-step chronocoulometric technique. The adsorption behaviour on the Ag(110) face shows significant differences with respect to that on the Ag(111) and Ag(100) faces. Thus, the plot of the applied potential E vs. the specifically adsorbed charge density σi of the bromide ions, for σi equal and opposite to the charge density σM on the metal, shows a rather abrupt decrease in slope with a decrease in σi. Moreover, the plots of σivs.σM at constant c intersect for σi≈–35 µC cm–2. This behaviour is explained by a decrease in the negative value of the surface potential due to electron smoothing with an increase in |σi| on Ag(110). The potential difference across the inner layer, once plotted against |σi| at constant σM, attains a maximum value before shifting linearly towards more negative values with a further increase in |σi|; this behaviour, observed on all three low-index faces, is interpreted on the basis of a molecular model of the inner layer which accounts for the reorientation of the water dipoles and for the screening by the inhomogeneous electron gas.

Comparative adsorption study of 1-butanol and 1-pentanol on mercury and gallium from aqueous 0.5 M Na2SO4 at 32° C

Abstract Adsorption of 1-butanol and 1-pentanol on dropping mercury and gallium electrodes from aqueous 0.5 M Na 2 SO 4 at 32° C was determined on the basis of capacitive charge measurements carried out by a computerized chronocoulometric apparatus. Adsorption measurements on Ga were carried out both in neutral solutions and in acidic solutions of pH ≈ 3.5; in the latter medium, which avoids the formation of a submonolayer of Ga oxides induced by alcohol adsorption, the capacitive charge was corrected for the appreciable faradaic contribution due to hydrogen evolution by a special procedure. Adsorptivities of both alcohols are lower on Ga than on Hg, especially at less negative charge densities. However, even at charge densities as negative as −12 σ C cm −2 , differences in the adsorptivity of the same alcohol on the two metals remain appreciable. Since at these negative charge densities both anionic specific adsorption and metal surface oxidation can be ruled out, the above differences in adsorptivity strongly suggest a certain coupling between the orientation of adsorbed water molecules and electron spillover, whose extent is expected to be greater on Ga than on Hg at the same negative charge density.

Inhibitory effect of n-aliphatic alcohols upon the Cd2+/Cd(Hg), Pb(OH)3−/Pb(Hg), BiIII/Bi(Hg) and SbIII/Sb(Hg) systems at high surface coverages ag

Abstract A detailed investigation of the inhibitory effect of n-aliphatic alcohols, from n-butanol to n-octanol, upon Cd 2+ , Bi III and Sb III reduction and Cd(Hg) oxidation in acidic media, as well as upon Pb(OH) 3 − reduction and Pb(Hg) oxidation in alkaline media, was carried out at mercury and amalgam electrodes, under conditions in which the electrode surface coverage ϑ by the alcohols is almost complete (ϑ>~0.93). The rate constants of the above electrode reactions in the presence of the various alcohols were found to agree satisfactorily with a theoretical relationship derived in ref. 5, which accounts for the standard free energy of adsorption of the inhibitor and for the lateral-interaction energy, involved in the substitution of solvent molecules by inhibitor molecules around a single activated complex within the adsorbed monolayer.

A comparative adsorption study of 2-propanol and hexafluoro-2-propanol on mercury from aqueous NaF

Abstract The adsorption properties of 2-propanol (P) and of the corresponding perfluorated compound 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) on mercury from aqueous 0.1 M NaF were determined from capacitive charge measurements using a computerized chronocoulometric technique. The appreciable differences in the adsorption behaviour of these two compounds, and in particular the notable positive shift in the charge σm of maximum adsorption in passing from P to HFP are discussed on the basis of a molecular model developed in J. Electroanal. Chem., 197 (1986) 103.