G. Riveros

Electrochemical study concerning the deposition of copper on selenium covered gold electrodes

Abstract Voltammetry, chronoamperometry and anodic stripping voltammetry were used to study the processes involved in the electrodeposition of copper on polycrystalline gold electrodes previously covered with selenium layers. Through the analysis of the I–t transients it was possible to establish the potential edge where the initial stages of copper–selenium compounds starts. Taking into account the charges involved in the redissolution of deposits obtained by stepping the potential in a narrow interval, a general reaction scheme that considers the formation of CuSe and Cu2−xSe compounds is proposed.

Redox and solution chemistry of the SeSO32−–Zn–EDTA2− system and electrodeposition behavior of ZnSe from alkaline solutions

Abstract The specific redox and solution chemistry involved in the formulation of the plating solution for the electrodeposition of zinc selenide from the SeSO 3 2− /Zn 2+ /EDTA system was investigated. After a theoretical analysis which allowed the electrodeposition conditions to be fixed, based on potential–pH and solubility diagrams, an experimental study was conducted by varying the deposition parameters (composition, pH, potential). XRD, EDS, AFM and reflectance measurements were used to characterize the structure, morphology, composition, and optical properties of films deposited on conducting glasses. EDS analysis showed that the as grown films are almost stoichiometric in a wide potential interval and also that the film thicknesses are lower than the values estimated from the circulating charge. The possible reasons for the latter behavior are discussed.

Growth and characterization of ZnO nanowire arrays electrodeposited into anodic alumina templates in DMSO solution

ZnO nanowire arrays were grown by potentiostatic cathodic electrodeposition on aluminum anodic oxide template (AAO) from dimethyl sulfoxide (DMSO) solutions containing zinc chloride and molecular oxygen as precursors. The nanowires presented high aspect ratio and exhibited a very high crystallinity with a wurtzite crystal structure with preferential orientation along the (0001) crystallographic axis. Chronoamperometric experiments were performed on gold bulk electrodes in order to model this preferential mode growth of ZnO nanowires, which has not been previously reported for similar precursors in DMSO solution. The analysis of the corresponding chronoamperograms revealed that chloride ions influence the oxide nucleation and growth mechanism. It was found that in the absence of KCl as a supporting electrolyte, the data fitted an instantaneous three-dimensional diffusion-controlled (IN-3D)diff nucleation and growth mechanism (NGM). The presence of KCl, instead favored a progressive three-dimensional (PN-3D)diff NGM. With these results, a model for the more complex nanowire’s growth inside the pores of the AAO template is proposed.

ELECTRODEPOSITION AND CHARACTERIZATION OF ZnX (X=Se, Te) SEMICONDUCTOR THIN FILMS

El presente trabajo describe la sintesis y la caracterizacion de peliculas delgadas de ZnX (X = Se y Te) obtenidas por electrodeposicion a potencial constante en medio acido. Previamente, se realizo un estudio voltametrico y fotovoltametrico sobre diferentes substratos conductores los que permitieron determinar las mejores condiciones para la electro obtencion de estos compuestos. Las peliculas delgadas de ZnSe y de ZnTe fueron analizadas por diferentes tecnicas (SEM, EDS, XRD y medidas opticas). Las peliculas de ZnTe presentaron una composicion muy cercana a la estequiometrica, en tanto que las de ZnSe presentaron un exceso de Se el cual puede ser eliminado por un adecuado tratamiento termico. La caracterizacion optica de ambos semiconductores depositados sobre titanio arrojo valores de ancho de banda prohibida de transicion directa de 2,64 eV para ZnSe y 2,27 eV para ZnTe, muy cercanos a los aceptados en bibliografia.

ELECTRON TRANSFER RATES OF ALKYL-FERROCENE MOLECULES FORMING INCOMPLETE MONOLAYER ON SILICON ELECTRODES

This study shows the results obtained when binding alkyl ferrocene molecules on silicon surface forming incomplete monolayers. The electrodic surface functionalization is carried out by a two step procedure: First, a hydrosilytation reaction between a Si–H surface and an alkenyl bromide active by white light; and then, the reaction of this surface with a monolithio ferrocene solution. Alkenyl bromide with different numbers of carbon atoms (3, 5 and 10 carbon atoms) were employed in order to obtain propyl, pentyl and decyl ferrocene chains on the electrodic surface. The samples of modified silicon were analyzed by X-ray photoelectron spectroscopy (XPS) and electrochemical measurements confirming the presence of ferrocene molecules on the electrode surface. The results obtained show that in each and every case, there were incomplete monolayers on the silicon surface, ranging from 16% to 42 %, depending on the alkenyl bromide employed in the synthesis. AC voltammetry was employed to determine the kinetic of the electron transfer between ferrocene molecules and the silicon electrode. However, the rate constant is not influenced by the length of the alkyl chain, and is usually constant (2.01 s –1 – 3.66 s –1 ). The result above is due to the electron transfer process which is determine by electron hopping in a regime of bonded diffusion and not by a long-rate electron transfer such as in a full compact monolayer.

A Novel Approach for the Electrodeposition of Epitaxial Films of ZnSe on (111) and (100) InP Using Dimethylsulfoxide as a Solvent

Results on the preparation and characterization of ZnSe epitaxial thin films obtained by electrodeposition on (111) and (100) n-type InP single-crystalline substrates, starting from elemental selenium dissolved in DMSO at423 K, are reported. Voltammetric studies have been performed on these substrates and compared to SnO 2 :F covered glass substrates. They show substrate dependent behavior and the presence of parasitic faradaic reactions attributed to the formation of di-selenide anions. However, composition analysis of the films by electron dispersive spectroscopy revealed that, independent on the substrate, nearly stoichiometric ZnSe films are obtained within the deposition potential range. Scanning electron microscopy images present a specific effect of chloride ion on the films morphology, whereas their optical characterization showed a direct band gap value of 2.64 eV. The epitaxial quality of the deposits has been confirmed by reflection of high energy electron diffraction and atomic force microscopy techniques.

An In Situ EIS Study during the Electrochemical Growth of Copper Nanowires into Porous Polycarbonate Membranes

Detailed information about the electrochemical and physicochemical phenomena associated with copper nanowire growth within the nanosized pores of polycarbonate track-etched membranes (PCM) was in situ obtained by electrochemical impedance spectroscopy. From an equivalent circuit that reproduces the ac-impedance results, it was found that the charge-transfer resistances associated with the Cu 2+ → Cu + and Cu + → Cu° electrochemical processes presented low and high values, respectively. It was also possible to observe the change from a diffusional growth process inside the pores (first stages) to a growth process in solution (film formation onto PCM).

Nucleation and growth mechanism of CdTe at polycrystalline gold surfaces analysed through Δm/t simulation transients

The nucleation and growth mechanisms of cadmium telluride have been investigated in aqueous H2SO4 solution using polycrystalline gold as substrate. Using cyclic voltammetry as a reference, a series of potential steps experiments were performed in order to simulate the corresponding j/t experimental transients that gave only a current decay proportional to t−1/2. Conducting the potential steps in an electrochemical quartz crystal microbalance (EQCM) allowed recording Δm/t transients that were potential dependent. Introducing some modifications in the classic equations of the nucleation and growth models, these transients gave information about the mechanism of CdTe electrocrystallization. It is proposed that the initial process is associated with the nucleation and growth of the Te islands followed by fast formation of Cd islands and that CdTe is formed through the slow diffusion of these islands. The slow rate of this process was evidenced through the time elapsed before observing a cathodic photocurrent when the electrode surface was illuminated. Gold surface has an influence on the nucleation mechanism favouring the instantaneous over the progressive, specially at more negative values of the final potential step. These results show that Δm/t transient simulations are a suitable tool for studying NGM when convolution of the classical j/t transients does not allow extraction of the appropriate information.

Modification of silicon surface with redox molecules derived from ferrocene

This study presents a new method to bind active redox molecules derived of ferrocene to the surface of single crystal silicon. The procedure consists in the reaction of hydrogenated silicon with allyl bromide activated with white light and its subsequent reaction with monolithio ferrocene in order to create a covalent union between the redox molecule and the semiconductor surface. The layers formed are electrochemically active and present a quasireversible electrochemical process which is attributed to the ferrocene molecules which are bound to the silicon surface. X-ray photoelectron spectroscopy (XPS) analysis confirms the presence of ferrocene molecules on the silicon surface.

New insights on the doping of ZnO films with elements from group IIIA through electrochemical deposition

This study was focused in the electrodeposition of both compact and nanostructured extrinsic n-type doped ZnO films, which was achieved with aluminum, gallium, and indium. These elements were directly added into a Zn(II) rich electrolyte with molecular oxygen acting as an oxide precursor in aqueous perchlorate media. This way, the use of nitrate ions, whose by-products are accumulated in the aqueous electrolyte, and chloride ions, an electrically active element in ZnO, was therefore avoided. Speciation diagrams, conditional solubility diagrams and a potentiodynamic study were used to explain the way in which extrinsic n-type ZnO can be prepared by electrochemical deposition. Relatively compact films with a highly preferred orientation along the c-axis were suitable for impedance measurements, thus allowing the measurement of their doping levels. Al- and Ga-doped lamellar nanostructures were successfully prepared when the nature of the anion was changed from perchlorate to sulfate. Under specific conditions, the structure of these films changed from opened and isolated nanosheets to interconnected ones. Morphological, optical, and crystallographic properties of these films were also analyzed. Results and discussion presented here should provide a better understanding toward the study of alternative materials in fields such as photovoltaics and photocatalysis.