Underpotential deposition of cadmium adatoms on Te and CdTe

Underpotential deposition (UPD) of Cd adatoms onto the surface of Te and CdTe films in Cd²⁺-containing solutions has been studied using electrochemical methods and AFM. The electrochemical deposition of Cd adatoms on Te and CdTe begins at potentials 400 mV more positive than the reversible potential of Cd²⁺/Cd couple and proceeds irreversibly. A strong chemical interaction of Cd adatoms with the surface Te atoms is the driving force of the UPD process. The deposition of Cd adatoms on the tellurium surface is accompanied by their stepwise interaction with tellurium to give CdTe nanophase.     

Optimization of CdTe nanofilm formation by electrochemical atomic layer epitaxy (EC-ALE)

This paper concerns optimization studies of the growth of cadmium telluride, an important II–VI compound semiconductor, using electrochemical atomic layer epitaxy (EC-ALE). The importance of the potentials used to deposit atomic layers of Cd and Te, as well as the potential used to strip excess Te, were investigated. These potentials were used in a cycle, an EC-ALE cycle, to form deposits one atomic layer at a time, using a sequence of surface limited reactions. The optimal potentials for the CdTe EC-ALE cycle included Cd deposition at − 0.65 V, Te deposition at − 0.35 V and bulk Te stripping at − 0.70 V. The deposits obtained were stoichiometric, with a Te/Cd atomic ratio of 1.01 from electron probe microanalysis (EPMA). Electrochemical quartz crystal microbalance (EQCM) studies of the optimal condition indicated that about a third of the deposited Cd was oxidatively stripped at the potential used to deposit Te. Glancing angle X-ray diffraction studies showed a (111) preferred orientation for the deposit, while room temperature near infrared absorption measurements indicated a direct band gap of 1.5 eV.     

Electrochemical deposition of superconductor alloy precursor in a low melting molten salt medium

The applicability of the room temperature molten salt medium based on the 1-methyl-3-ethylimidazolium chloride/AlCl3 (MeEtImCl/AlCl3) system for the deposition of superconductor alloy precursors (Bi–Sr–Ca–Cu) was investigated. Problems of poor solubility of the constituent metal salts in the neutral melt and aluminium codeposition in the acidic melt were identified for the use of the chloroaluminate room temperature molten salt. Thus, a novel all-chloride molten system (MeEtImCl) was developed in the present work and used at 120°C. Cyclic voltammetry was used to investigate the suitability of Pt, Al, Ti, Cu and Ag as substrate for alloy deposition and aluminium was found to be the best for depositing alkaline earth metal alloys. The working deposition conditions were a constant potential of –1.72V for 75s in an electrolytic bath containing BiCl3 (0.068mol), SrCl2 (0.50mol), CaCl2 (0.18mol) and CuCl2 (0.050mol) in 1kg MeEtImCl (6.83mol). The compositions of the electrodeposits obtained were in close agreement with the mole ratios of the Bi–Sr–Ca–Cu superconductor alloy precursor.     

Electrodeposition of polycrystalline ZnTe from simple and citrate-complexed acidic aqueous solutions

The cathodic electrodeposition of polycrystalline zinc telluride from aqueous acidic solutions of zinc sulphate and tellurite, under constant or pulsing voltage, is described. Various compositions of the bath were tested with respect to the Zn/Te precursor ratio, including addition of citrate buffer as a complexation agent. Metallic (Ni and Ti) and semiconductor (CdTe and CdSe) electrodes were used as cathode-substrates. The deposits were characterized by XRD, SEM-EDX and FTIR techniques. The formation of compact barrier layers of zinc blend, stoichiometric ZnTe or mixed (Zn,Cd)Te was attained. Pulse plating in a citrate-free bath and constant potential plating from a citrate bath were seen to have the most beneficial effect on the properties of the electrodeposits, as leading to improved microstructure of the ZnTe films, in terms of crystallinity and stoichiometry. Further, the employment of [1 1 1]-oriented CdTe or CdSe substrates featured an epitaxial growth of polycrystalline ZnTe films as well as an improved Zn/Te atomic ratio compared to the metallic substrates.     

Studies of oxidation behaviors of CdTe in ammoniacal basic electrolytes

The time variations in the electrode potential and mass of an electrodeposited CdTe layer when immersed in a deposition bath under an open-circuit condition were examined using an electrochemical QCM in order to observe the oxidation behavior due to dissolved oxygen. The morphology, composition, and crystallinity of the CdTe layer were also examined by SEM, EPMA, and XRD measurements. In the early stage of immersion, mostly only Cd atoms in the CdTe were found to dissolve from the zinc blende-type CdTe lattice, keeping the flat and smooth surface morphology intact. Then, the remaining Te formed a lattice of elemental Te and covered the CdTe layer. The electrolyte could penetrate the Te layer and access the CdTe layer's surface because the Te layer was porous due to the elution of Cd atoms. Therefore, the preferential dissolution of Cd species continued to occur, and finally, only the porous Te layer remained on the substrate. This dissolution of CdTe supports the validity of the potential-pH diagram of the Cd-Te-NH₃-H₂O system.     

Electrodeposition of antimony in a water-stable 1-ethyl-3-methylimidazolium chloride tetrafluoroborate room temperature ionic liquid

The electrochemistry and electrodeposition of antimony were investigated on glassy carbon and nickel electrodes in a basic 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate room temperature ionic liquid. Cyclic voltammetry results show that Sb(III) may be either oxidized to Sb(V) via a quasi-reversible charge-transfer process or reduced to Sb metal. Diffusion coefficients for both Sb(III) and Sb(V) species were calculated from rotating disc voltammetric data. Analysis of chronoamperometric current–time transients indicates that the electrodeposition of Sb on glassy carbon proceeded via progressive three-dimensional nucleation with diffusion-controlled growth of the nuclei. Raising the deposition temperature results in decreased average radius of the individual nuclei. Dense deposits can be obtained within a deposition temperature range between 30 to 120 °C. Scanning electron microscopy revealed dramatic changes in the surface morphology of antimony electrodeposits as a function of deposition temperature; deposits obtained at 30 °C had a nodular appearance whereas those obtained at 80 and 120 °C consisted of evenly distributed fine polygonal crystals.     

Electrochemical characterization of the underpotential deposition of tellurium on Au electrode

Electrochemical characterization of the underpotential deposition (UPD) of tellurium on Au substrate has been performed in this paper. The mechanism of Te deposition and its voltammetry dependence on the Te ion concentration were studied, and it suggests that variations in the metal ion concentration may affect the UPD process kinetics. The effect of tellurium adsorbates on UPD behavior of Te has also been investigated. The results show that the tellurium adsorbates could be irreversibly adsorbed upon the Au substrate surface under the open-circuit conditions. Subsequent removal of the Te adsorbates was also proved to be very difficult within the Au double-layer region, and a standard electrochemical cleaning procedure is necessary to remove the Te adsorbates completely. When the potential was cycled into the Au oxidation region, a substantial loss of Te adsobates was observed, which occurs simultaneously with the Au oxidation features. Scan rate dependent cyclic voltammetry experiments reveal that the peak current in the Te UPD peak is not a linear function of the scan rate, ν, but of a 2/3 power of the scan rate, ν^2/3. It is in good consistent with a two-dimension nucleation and growth mechanism.     

Electrodeposition of palladium-indium from 1-ethyl-3-methylimidazolium chloride tetrafluoroborate ionic liquid

Voltammetry at a glassy carbon electrode was used to study the electrochemical co-deposition of Pd-In from a chloride-rich 1-ethyl-3-methylimidazolium chloride/tetrafluoroborate air-stable room temperature ionic liquid at 120 °C. Deposition of Pd alone occurs prior to the overpotential deposition (OPD) of bulk In. However, underpotential deposition (UPD) of In on the deposited Pd was observed at the potential same as the deposition of Pd. The UPD of In on Pd was, however, limited by a slow charge transfer rate. Samples of Pd-In alloy coatings were prepared on nickel substrates and characterized by energy dispersive spectroscope (EDS), scanning electron microscope (SEM) and X-ray powder diffraction (XRD). The electrodeposited alloy composition was relatively independent on the deposition potential within the In UPD range. At more negative potentials where the OPD of Pd-In has reached mass-transport limited region, the alloy composition corresponds to the Pd(II)/In(III) composition in the plating bath. The Pd-In alloy coatings obtained by direct deposition of Pd and UPD of In on the deposited Pd appeared to be superior to the Pd-In alloys that were obtained via the co-deposition of Pd and bulk In at OPD potentials.     

Characterization of electrodeposited Cd–Co alloy coatings by anodic linear sweep voltammetry

The electrodeposited Cd–Co alloys were characterized by the anodic linear sweep voltammetry (ALSV) technique in the solution of 1 M NaCl (pH 2). It is shown that Cd–Co alloys could be deposited with high current efficiency (>90%) from the solution containing 0.2 M boric acid, low concentration of cadmium sulfate (0.01 M and 0.02 M) and high concentration of cobalt sulfate (0.2 M and 0.4 M) under the conditions of convective diffusion (RPM = 1000). The ALSVs were characterized by the presence of three peaks: one corresponding to the dissolution of pure Cd, one corresponding to the dissolution of pure Co and one corresponding to the dissolution of unknown phase formed in the system Cd–Co (since the phase diagram of the Cd–Co system does not exist in the literature). Depending on the charge and on the alloy composition (amount of Cd and Co) the peak of the unknown phase can either be hardly seen, or can prevail on the ALSV. This phase is formed mainly on the account of Co, but certain limiting amount of Cd must be deposited for the unknown phase to be formed and seen on the ALSV.     

Formation of PbTe nanofilms by electrochemical atomic layer deposition (ALD)

This article describes optimization of a cycle for the deposition of lead telluride (PbTe) nanofilms using electrochemical atomic layer deposition (ALD). PbTe is of interest for the formation of thermoelectric device structures. Deposits were formed using an ALD cycle on Au substrates, one atomic layer at a time, from separate solutions, containing Pb²⁺ or HTeO₂⁺ ions. Single atomic layers were formed using surface limited reactions, referred to as underpotential deposition (UPD), so the deposition cycle consisted of alternating UPD of Te and Pb. The Pb deposition potential was maintained at −0.35 V throughout the 100 cycle-runs, while the Te deposition potential was ramped up from −0.55 V to −0.40 V over the first 20 cycles and then held constant for the remaining ALD cycles. Coulometry for the reduction of both Te and Pb indicated coverages near one monolayer, each cycle. Electron probe microanalysis (EPMA) indicated a uniform and stoichiometric deposit, with a Te/Pb ratio of 1.01. X-ray diffraction measurement showed that the thin films had the rock salt structure, with a preferential (2 0 0) orientation for the as formed deposits. No annealing was used. Infrared reflection absorption measurements of PbTe films formed with 50, 65, and 100 cycles indicated strong quantum confinement.     

The effect of chelating reagents on the layer-by-layer formation of CdS films in the electroless and electrochemical deposition processes

Triethanolamine (TEOA) and cysteine (Cys) were examined for the effect of chelating reagents to deposit CdS thin films by means of two different processes. Those are the successive ionic layer adsorption and reaction (SILAR) method and the successive under potential deposition (UPD) method, in which Cd and S are separately deposited on a polycrystalline Au substrate from each solution. Evaluation by stripping voltammetry showed that the amount of the deposited CdS was increased for 1, 3, 5, 7 and 10 layers of CdS prepared by these methods. It was found that, with the SILAR method, the order of the ability to increase CdS deposition was Cys>TEOA>none. On the other hand, with the successive UPD method, the order was none≥TEOA>Cys, showing a certain inhibition in the electrochemical deposition process. It is concluded that CdS deposition by the SILAR method becomes compatible to the successive UPD when a suitable chelating reagent was added to the Cd solution.     

The effect of certain ions on the internal stress of bright copper electrodeposits

The effect of low concentrations (0–100 mg 1^–1) of F, Cl, Br, I, As, Sb, Bi, Ni, Zn, Fe, Ag, Te, Se, Pb, and S on the internal stress of bright copper electrodeposits was studied. The following solution containing 30 mg 1^–1 of thiourea as a brightening additive was used as the electrolyte: CuSO₄·5H₂O (225 g l^–1), H₂SO₄ (60g l^–1). It is shown that the internal stress of copper electrodeposits can be considerably reduced by addition of Se(IV), Cl, Br, or I to the above electrolyte.     

The underpotential deposition of bismuth and tellurium on cold rolled silver substrate by ECALE

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Bi and Te on cold rolled silver substrate have been performed. Different approaches have been employed to investigate the influence of silver oxide film on Bi UPD. As a result, the precedent deposition of a little bismuth can effectively prevent silver from surface oxidation. The voltammetric analysis of underpotential shift demonstrates that the first Te UPD on Bi-covered Ag and Bi UPD on Te-covered Ag fit UPD dynamics mechanism. Thin film of bismuth telluride was formed using an automated flow deposition system, by alternately depositing Te and Bi. The electrochemical conditions necessary to form Bi₂Te₃ deposits of 50 cycles on cold rolled silver by ECALE are described here. X-ray diffraction indicated the deposits were Bi₂Te₃. EDX quantitative analysis gave the 2:3 stoichiometric ratio of Bi to Te, which is consistent with XRD result. Electron probe microanalysis of the deposits showed a worm-like network structure. The map of Te and Bi element indicated the distribution of both Te and Bi is homogeneous and locates the same sites, which is favorable to Te-Bi binary system. The composition analysis of structural expanded image also showed the approximately constant composition of Te:Bi ≈ 3:2 has taken place.     

Sol–Gel Synthesis of Cadmium Telluride-Microcrystal-Doped Silica Glasses

The sol–gel process has been applied to the preparation of small-sized CdTe-doped silica glasses. Gels synthesized by the hydrolysis of a complex solution of Si(OC₂H₅)₄, Cd(CH₃COO)₂· 2H₂O, and Te were heated from 350° to 600°C in a H₂─N₂ atmosphere to form fine cubic CdTe crystals. The size of CdTe crystals, determined from the line broadening of X-ray diffraction pattern, increases from 4 to 9 nm in diameter with increasing heat-treatment temperature. The optical absorption edge shifts to the higher-energy side as the size of the CdTe crystals decreases. This phenomenon is interpreted in terms of a quantum confinement effect of electron and hole in the CdTe microcrystals.     

Electrowinning of tellurium from alkaline leach liquor of cemented Te

The electrochemical behaviour of tellurium in 2.5 M NaOH solution was studied for the recovery of tellurium from alkaline leach liquor of cemented Te using steady state polarization and cyclic voltammetry. The deposition characteristics and the potential range for a stable deposit of tellurium were also investigated. The morphology of deposited Te in alkaline solution showed a very porous nature and needlelike radial growth. The potential range for stable electrodeposition was between –0.8 V and –0.95 V (vs Hg/HgO electrode), but electrowinning could be carried out at more negative potentials due to the disproportionation reaction of Te₂ ^2–. Laboratory-scale electrowinning experiments were performed under different operating voltages, temperatures and initial Te concentrations. The current efficiency was about 85–90% for 50% recovery and about 50–60% for 90% recovery. The purity of electrodeposited Te was higher than 99.95%.     

Magnetic properties and microstructure of electrodeposited Fe−P amorphous alloy

Electrodeposited Fe–P amorphous alloy has been characterized in terms of magnetic properties and the microstructure. The Fe–P electrodeposits show amorphous structure with phosphorus contents of about 20 at%. The formation of the amorphous structure depends mainly on the phosphorus content. The amorphous Fe–P alloy shows typical soft magnetic properties such as low coercive force and high permeability. The coercive force (Hc) decreases with the low temperature annealing, and the lowestHc is about 0.05 oersted. This reduction inHc can be attributed to structural relaxation. The amorphous phase is crystallized at temperatures above 300°C accompanying a drastic increase inHc. The stable crystalline phase is Fe₃P.     

Fabrication of YSZ electrolyte using electrostatic spray deposition (ESD):I – a comprehensive parametric study

Electrostatic spray deposition (ESD) was applied to fabricate a thin-layer (3 m thickness) yttria-stabilized zirconia (YSZ) electrolyte on a solid oxide fuel cell (SOFC) anode substrate consisting of nickel-YSZ cermet. Reducing the thickness of a state-of-the-art electrolyte, and thereby reducing the cell internal IR drop, is a promising strategy to make the intermediate temperature SOFC (ITSOFC) operating at 600–800 °C possible. About 8 mol% YSZ colloidal solution in ethanol was sprayed onto the substrate anode surface at 250–300 °C by ESD. After sintering the deposited layer at 1250–1400 °C for 17–6 h, the cathode layer, consisting of lanthanum strontium manganate (LSM), was sprayed or brush coated onto the electrolyte layer. Performance tests on the cell were carried out at 800 °C to evaluate the electrolyte layer formed by ESD. With a 97 H₂/3 H₂O mixture and air as fuel and oxidant gas, respectively, open circuit voltage (OCV) was found to be close to the theoretical value.     

Electrodeposition of palladium–silver in a Lewis basic 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate ionic liquid

The electrodeposition of palladium–silver alloys was investigated in a basic 1-ethyl-3-methylimidazolium chloride/tetrafluoroborate ionic liquid containing Pd(II) and Ag(I). Cyclic voltammetry experiments showed that the reduction of Ag(I) occurs prior to the reduction Pd(II). Both electrodeposition processes require nucleation overpotential. Energy-dispersive spectroscopy data indicated that the composition of the Pd–Ag alloys could be varied by deposition potential and concentrations of Pd(II) and Ag(I) in the solution. The Pd content in the deposited Pd–Ag alloy increased with decreasing deposition potential and the Pd mole fraction in the plating bath. At potentials where the deposition of both Pd and Ag was mass-transport limited, the Pd/Ag ratio in the electrodeposited alloys was slightly less than the Pd(II)/Ag(I) ratio in the ionic liquid due to the smaller diffusion coefficient of Pd(II). Scanning electron micrographs of the electrodeposits showed that in general, the Pd–Ag alloys were nodular and become more compact upon increasing the temperature up to 120 °C.     

ZnTe electrodeposition from organic solvents

The electrodeposition of zinc telluride was investigated from various organic solvent baths such as methanol, acetonitrile, propylene carbonate. The potential window increases with increasing boiling point of the solvents. In particular, ZnTe with high crystallinity could be obtained in propylene carbonate by the under-potential deposition (UPD) of Zn. The current efficiency for ZnTe deposition in propylene carbonate was enhanced considerably compared to that in acidic aqueous solutions. A single-phase ZnTe film close to the ideal stoichiometric composition of ZnTe could be obtained at 423 K and −0.8 V versus Ag/AgCl in propylene carbonate with a molar ratio of Zn(II)/Te(IV) = 10 and a low Te(IV) concentration; the film had a smooth and dense granular crystal morphology.