M. Bouroushian

Structure and properties of CdSe and CdSexTe1–x electrolytic deposits on Ni and Ti cathodes: influence of the acidic bath pH

Abstract The structural, morphological and photoelectrochemical features of coherent CdSe and CdSexTe1−x thin film semiconductors, prepared by cathodic electrodeposition from an acid sulfate solution, were investigated. In particular, the effect of deposition potential on the properties of layers grown onto Ti and Ni substrates, in connection with the electrolyte acidity within the bath pH interval [2.2, 1.6], was studied. As verified by X-ray diffraction (XRD), scanning electron microscopy (SEM), reflectance and photoelectrochemical cell (PEC) tests, the preparation method renders compact semiconductive films, presenting a remarkably intense in the case of CdSe/Ni(111) cubic structure, without any post-thermal treatment. The passive surface of Ti is associated with weaker texture; however, deposits on Ti generally exhibit better photoelectrochemical behavior, under similar preparation conditions.

Influence of heat treatment on structure and properties of electrodeposited CdSe of Cd(Te, Se) semiconducting coatings

Abstract CdSe and mixed CdSe and CdTe semiconducting thin films, prepared by cathodic electrodeposition from an acid sulphate solution, containing selenium and tellurium oxides in Various amounts, were submitted to a to a thermal treatment at temperatures ranging between 400 and 520 °C. The crystal structure, composition, band-gap width and photoelectrochemical response of the annealed materials was investigated. It was found that all materials, rich in selenium, change their structure from cubic (zinc blende) to hexagonal (wurtzite), when annealed within the above mentioned region of temperatures. In many cases, an improvement of their semiconducting properties has been also confirmed.

Hexagonal cadmium chalcogenide thin films prepared by electrodeposition from near-boiling aqueous solutions

Thin, n-type, CdSe and CdSexTe1−x semiconductive films were prepared by cathodic electrodeposition onto titanium electrodes. An electrochemical cell was specially designed in order to perform electrodeposition in a near-boiling aqueous-ethyleneglycol bath at a temperature of approximately 110°C. The composition of the as-grown films, their crystal structure, morphology and band-gap width were studied as a function of the deposition potential and chalcogen ion concentration. It is shown that high temperatures have a positive effect on the crystal quality and the photoresponse stability of cadmium chalcogenide thin films even by employing electrolytes rather concentrated in selenous acid. Under specific conditions, a small shift in deposition potential brings about a complete phase transformation of the CdSe layers. In this manner, the described method enables the preparation of hexagonal CdSe deposits.

On a thermodynamic description of Se(IV) electroreduction and CdSe electrolytic formation on Ni, Ti and Pt cathodes in acidic aqueous solution

Abstract A discussion on tetravalent Se electroreduction and CdSe cathodic electrodeposition in terms of thermodynamics, in association with an investigation on the voltammetric behavior of Pt, Ni and Ti working electrodes in a high temperature (85 °C) acidic aqueous bath, is presented. The underpotential co-deposition of Cd with Se is described within the frame of a known electrochemical model of compound formation. The data presented serve as an effective basis for studying the electroreduction of Se(IV) as well as for determining the relevance of theoretical predictions to experimental findings on a binary compound electrodeposition process.

Electrocrystallization of CdSe upon various substrates. Structural arrangement and photoelectrochemical performance

Abstract The growth of cadmium chalcogenide thin film semiconductors, in the context of a typical electrolytic method of formation with an aqueous bath, is to a large extent determined by the deposition substrate, together with the potential, for a given electrolyte composition and temperature. The effect of various substrates ([100]-oriented Ni, commercially pure Ni, Ti) and procedures (Ni electropolishing, Ti anodization, double-step deposition, etc.) on the structural arrangement and the resulting photoelectrochemical (PE) behavior of cathodically electroplated CdSe layers is presented. The outcome of the preparation process is analyzed in terms of structural–optical properties relation. As verified, some microcrystalline, porous samples give higher PE efficiencies than larger-grained ones, owing to the increased contact area with the PE redox electrolyte and possibly the establishment of a particular charge transfer mechanism in the solid. The latter is associated with the existence of a nanostructure.

Electrochemistry of the Chalcogens

Because of their multiple oxidation states, the chalcogens, particularly sulfur, can engage in numerous redox couples participating in acid–base, oxidation–reduction, precipitation, and complexation equilibria.

Substrate effect on the structure and properties of electrodeposited CdSe and Cd(Se, Te) coatings

CdSe, mainly, and Cd(Se, Te) alloy layers were cathodically electroplated onto titanium, nickel and (100) oriented nickel substrates from an acid sulphate solution at 85°C. The growth of the deposits, on Ti and Ni, was partly controlled by altering the mode of the nucleation first step. This procedure, though inhibiting the establishment of a strong crystal orientation, seems to positively affect the photoconversion efficiency of the CdSe films on Ni. Crystallized deposits with defined stoichiometry are obtained on (100) oriented Ni cathodes along a narrower, than the typical case, range of electrolysis potentials. The polycrystalline films are always cubic and the measured photoconversion efficiencies reach 2.6%.

A phase modification of CdSe electrodeposits induced by substrate roughness

Cadmium chalcogenides are tetrahedrally bonded solids, preferentially crystallizing in either the wurtzite, hexagonal structure (CdS, CdSe) or the zinc blende, cubic one (CdTe). The wurtzite structure of CdSe, which is associated under certain conditions to better semiconductive properties and higher resistivity against photocorrosion, has been observed in thin films prepared by various electroless techniques [1, 2]. CdSe preparation by electrochemical means leads, mostly, to the obtaining of a metastable, kinetically controlled cubic phase, which can be transformed to the hexagonal one by an annealing process [3–5]. Cathodic electrodeposition from aqueous solutions is a simple preparation method successfully applied to obtain semiconductive materials, especially of the II–IV class compounds (eg. [6]). As reported in some of our previous works [5, 7–9], the controlled co-deposition of the CdSe constituent elements from a high temperature aqueous bath (85–90 ◦C), involving underpotential reduction of Cd, provides compact and coherent cubic structured semiconductive, polycrystalline layers with a more or less pronounced (111) preferred orientation. The aim of the present article is to report on the substrate-induced possibility of adopting a mixed hexagonal-cubic CdSe structure at certain electrolytic conditions. Thin CdSe films, of 2–3 μm in thickness were produced potentiostatically employing a rotating disc electrode setup [9] from excessive in Cd2+ (0.2 M) solutions containing small amounts of SeO2 (1× 10−4 to 1× 10−3 M) at a pH equal to 2.2, at 85 ◦C. The counterelectrode was a platinum grid and the potential of the working electrode was monitored against a Hg/HgSO4 saturated sulphate reference (SSE; ESSE vs. NHE ≈ 0.59 V at 85 ◦C, [10]). Cathodic polarization curves were recorded at a potential sweep rate of 2.5 mV/s. The crystallographic structure of CdSe films obtained on commercially pure Ti and Ni (s/ 10 mm) as well as SnO2/glass (TO/glass) (s/ 12 mm) disc electrodes was identified in terms of X-ray diffraction data by a Siemens D5000 X-ray diffractometer. The preparation of reproducible Ni, Ti and TO/glass electrode surfaces is described elsewhere [5, 9, 10]. Several Ni electrodes (denoted as Ni-R) were chemically etched by HNO3 in order to acquire a roughened surface. A partial removal of Ti passivating oxide layer, accom-

Electrochemical Processes and Technology

Electrode processes that involve the transfer of several electrons, usually accompanied by the breaking and creation of bonds, commonly proceed slowly or not at all in the absence of electrocatalysts, in contrast to most simple electrode processes that involve the transfer of a single electron without associated ion/atom transfer. Such complex electrode processes, an example of which is the reduction of molecular oxygen by a four-electron process to water or by a two-electron process to hydrogen peroxide (Chap. 2), often involve high-energy intermediates that must be stabilized by interactions with suitable molecules or complexes if the electrode reactions are to proceed at reasonable rates. These intermediate-stabilizing species are the electrocatalysts [1, 2]. In electrocatalysis, multifunctionality is obligatory: for obtaining high performance, some combination of surface reactivity, electronic and ionic conductivity, separation of electron–hole pairs, or facile mass transport of molecules must be provided to enhance the molecular conversion.

Electrochemical Preparations I (Conventional Coatings and Structures)

Traditional electrodeposition refers to cathodic formation of bulk metals, preferably in the form of film coatings or electroformed articles, and is concerned with the practical objective of obtaining these materials in a coherent, dense, and macroscopically homogeneous state. Although the majority of plated materials are the (relatively) pure metals, unfavorable chemistry limits the number of metal elements that are capable of being obtained electrochemically from aqueous solutions in an unalloyed state, to only 33 of the about 70 metallic elements in the Periodic Table; and even less are the metals that are deposited to any extent for commercial or technical purposes. Nonetheless, the number of possible alloys which can be made from these metals is very large. Furthermore, immense are the binary or multiple combinations of metals with non-metallic elements; however, electrochemical preparative techniques have not played a significant role in the development of such materials, the reason probably lying in the more complicated character of the relevant processes as compared to electrodeposition of single metal elements or metallic alloys. In any case, the unique feature of electrodeposition being an electrically driven process capable of precise control offers a prospective advantage over thermally driven deposition processes. Further, electrodeposition occurs closer to equilibrium than many vacuum deposition methods; it is more applicable to complex shapes, generally less expensive, and capable of providing very thick coatings.