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.     

Electrodeposition Behaviour of Cadmium Telluride from 1-ethyl-3-methylimidazolium Chloride Tetrafluoroborate Ionic Liquid

Voltammetry at a glassy carbon electrode was used to study the electrochemical deposition of Cd–Te from the Lewis basic 1-ethyl-3-methylimidazolium chloride/tetrafluoroborate air-stable room temperature ionic liquid between 80 °C and 140 °C. Deposition of tellurium alone occurs through a four-electron reduction of Te(iv) to Te which could be further reduced to Te(-ii) at a more negative potential. The Cd–Te electrodeposits could be obtained by the underpotential deposition (UPD) of Cd on the deposited Te. The UPD of Cd on Te was, however, limited by a slow charge transfer rate. Samples of Cd–Te electrodeposits were prepared on tungsten and titanium substrates and characterized by energy dispersive spectroscope (EDS), scanning electron microscope (SEM), and X-ray powder diffraction (XRD). The results showed that an excess amount of Cd(ii) was required in order to prepare CdTe codeposits with a Cd/Te atomic ratio approached 1/1. The deposit composition was independent of the deposition potential within the Cd UPD range. Raising the deposition temperature increased the UPD rate of Cd and promoted the formation polycrystalline CdTe.     

Interaction between myoglobin and hyaluronic acid in layer-by-layer structures—An electrochemical study

The layer-by-layer (LBL) self-assembled film construction of the biocompatible polymer hyaluronic acid (HA) and single heme redox protein, myoglobin (Mb) is described. The films were built upon gold electrode substrates, both gold quartz crystal electrodes and bulk gold (Au(bulk)) electrodes, and formation of the LBL films was gravimetrically monitored by an electrochemical quartz crystal microbalance. The electrochemical properties of the hyaluronic acid/myoglobin films ({HA/Mb}_n) were investigated after each deposition step using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV response presented an oxidation peak at +0.3 V vs. SCE, not characteristic of the redox protein myoglobin, and, the peak current decreased slightly with each additional bilayer. CV at Au(bulk) electrodes in pH 5.0 acetate buffer solution, containing Mb, presented the same oxidation peak as observed at {HA/Mb}_n modified electrodes, confirming the presence of the same electroactive species. The Mb oxidation peak current depends linearly on scan rate, characteristic of adsorbed thin-layer electrochemical systems, attributed to free adsorbed heme. Impedance spectra, recorded after deposition of each bilayer, were in agreement with the cyclic voltammetry observations.     

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.     

Development of growth cycle for antimony telluride film on Au (1 1 1) disk by electrochemical atomic layer epitaxy

Electrochemical deposition of Sb₂Te₃ thin film on Au (1 1 1) disk via the route of electrochemical atomic layer epitaxy (ECALE) is described in this paper. Electrochemical aspects of Te and Sb on Au, Te on Sb-covered Au, and Sb on Te-covered Au were studied by means of cyclic voltammetry and coulometry. The apparent variation of coverage for Te or Sb on hetero-covered substrate is explained by considering the thermodynamic process of compound formation. A steady ECALE deposition for Sb₂Te₃ compound could be attained after negatively adjusting the underpotential deposition (UPD) potentials of Sb and Te on Au in steps over the initial 40 cycles, and the potentials could be kept constant for the following deposition. A 200-cycle deposit, which was grown with the steady deposition potentials, was proved to be a single phase Sb₂Te₃ compound by X-ray diffraction analysis. The 2:3 stoichiometric ratio of the deposit was further verified by energy dispersive X-ray (EDX) quantitative analysis. The p-type semiconductive property was demonstrated by measurements of the Seebeck coefficient and the electrical resistivity with a value of 145 μV/K and 9.37 μΩm, respectively. The morphologies of deposits with various growth cycle numbers were observed with FE-SEM. The evolvement mechanism of the morphology was investigated. The results show that the morphology of deposit has changed after initial potential adjustment and numberless thin sheets appeared and grew uprightly during the continuous cycle process. Fourier transform infrared spectroscopy (FTIR) absorption measurements suggested a band gap of 0.26 eV in very good agreement with literature reports for Sb₂Te₃ single crystals.     

Studies on the electrochemical reduction processes of HTeO<Subscript>2</Subscript><Superscript>+</Superscript> by CV and EIS

Electrochemical characterizations of the underpotential deposition of tellurium on Au substrate were investigated by cyclic voltammetry (CV) in this paper. The results showed that the irreversible underpotential deposition of Te could take place once the Au electrode was immersed into the HTeO₂ ⁺ solution. The redox behaviors of adsorbed HTeO₂ ⁺ were also studied and the results revealed that HTeO₂ ⁺ could only adsorb on Au electrode surface. The kinetics relating to the reduction of adsorbed HTeO₂ ⁺ could be affected by HTeO₂ ⁺ concentration but the charge consumed by the reduction of adsorbed HTeO₂ ⁺ was concentration-independent. Electrochemical impedance spectroscopy (EIS) analyses about the bulk formation process of Te⁰ indicated that during the bulk reduction of HTeO₂ ⁺ to Te⁰, four electrons were not obtained simultaneously in only one electrochemical step, some intermediate products, which need to be further detected and investigated in the future researches, might emerged in the intermediate processes.     

Electrochemical aspects of depositing Sb2Te3 compound on Au substrate by ECALE

Deposition of Sb₂Te₃ thin films on polycrystalline Au substrates by electrochemical atomic layer epitaxy (ECALE) is described in this paper. Electrochemical aspects were characterized by means of cyclic voltammetry, anodic potentiodynamic scanning and coulometry. A steady ECALE deposition for Sb₂Te₃ compound could be attained after negatively adjusting the underpotential deposition (UPD) potentials of Sb and Te on Au in steps over the initial 40 cycles, and the potentials could be kept constant for the following deposition. A 400 cycle deposit, which was grown with the steady deposition potentials, was proved to be a single phase Sb₂Te₃ compound by X-ray diffraction analysis and SEM observation shows the deposit consisted of nanoscale particles with average size about 100 nm. The 2:3 stoichiometric ratio of the deposit was further verified by energy dispersive X-ray (EDX) quantitative analysis.     

An in situ scanning tunneling microscopic study of electrodeposition of bismuth on Au(1 1 1) in a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid: Precursor adsorption and underpotential deposition

In this article, the electrodeposition of Bi on Au(1 1 1) surface in the underpotential region in BMIBF₄ ionic liquid containing BiCl₃ is studied by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The cyclic voltammogram shows several cathodic and anodic peaks associated with underpotential deposition (UPD) of Bi and dissolution of the UPD deposit, respectively, in the potential region between −0.38 and −0.7 V versus Pt quasi-reference electrode. In situ STM results indicate there is a BiCl₃ precursor adsorption stage prior to the Bi UPD. The adsorption of BiCl₃ leads to the formation of unique hexagonal and trigonal supramolecular assembly with a Au(1 1 1)(10 × 10) structure. The initial stage of Bi UPD proceeds with the formation of Au(1 1 1)(7 × 7) R21.8° adlayer structure composed of Bi trigonal clusters at −0.5 V. A structural transformation occurred at −0.6 V resulting in a unique “zipper-like” double-chain pattern composed of well-aligned Bi trigonal clusters which can be denoted by Au(1 1 1)(5 × 25√3/3) structural model. The trigonal clusters composed of six Bi atoms seem to be the main characteristic elemental units of Bi UPD adlayer regardless of underpotential shift. These features are dramatically different from those observed in Bi(III)-containing acidic aqueous solutions as well as in chloroaluminated ionic liquid, but are similar to those of Sb UPD in BMIBF₄ ionic liquid, which reveals profound solvent effects on the electrodeposition of semimetals.     

Deposition of selenium thin layers on gold surfaces from sulphuric acid media: Studies using electrochemical quartz crystal microbalance, cyclic voltammetry and AFM

In this paper we report here new considerations about the relationship between the mass and charge variations (m/z relationship) in underpotential deposition (UPD), bulk deposition and also in the H₂Se formation reaction. Nanogravimetric experiments were able to show the adsorption of H₂SeO₃ on the AuO surface prior to the voltammetric sweep and that, after the AuO reduction, 0.40 monolayer of H₂SeO₃ remains adsorbed on the newly reduced Au surface, which was enough to gives rise to the UPD layer. The UPD results indicate that the maximum coverage with Se_ads on polycrystalline gold surface corresponds to approximately 0.40 monolayer, in good agreement with charge density results. The cyclic voltammetry experiments demonstrated that the amount of bulk Se obtained during the potential scan to approximately 2 Se monolayers, which was further confirmed by electrochemical quartz crystal microbalance (EQCM) measurements that pointed out a mass variation corresponding of 3 monolayers of Se. In addition, the Se thin films were obtained by chronoamperometric experiments, where the Au electrode was polarized at +0.10 V during different times in 1.0 M H₂SO₄ + 1.0 mM SeO₂. The topologic aspects of the electrodeposits were observed in Atomic Force Microscope (AFM) measurements. Finally, in highly negative potential polarizations, the H₂Se formation was analyzed by voltammetric and nanogravimetric measurements. These finding brings a new light on the selenium electrodeposition and point up to a proposed electrochemical model for molecule controlled surface engineering.     

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%.     

KF对Na3AlF6-Al2O3熔盐电解阴极过程的影响

采用循环伏安法、交流阻抗法和方波伏安法等电化学测量技术,考察了KF对Na₃AlF₆-Al₂O₃熔盐电解体系的阴极过程的影响。研究结果表明：在Na₃AlF₆-Al₂O₃和Na₃AlF₆-Al₂O₃-KF电解质体系的循环伏安曲线中,还原峰随着扫描速率的增大而负移,氧化峰随着扫描速率的增大而正移,在扫描速率较低（25～100 mV·s^-1）的情况下,反应过程不可逆,反应过程相对平缓稳定;随着扫描速率的提高,电极可逆性相对提高。由于电极附近铝离子的聚合现象,使得在无KF的熔盐体系下,交流阻抗的高频区出现了感抗现象;在含KF的体系下,反应是由电化学反应过程和扩散过程共同控制的,且随着KF含量的增加,Warburg阻抗系数减小,电化学反应过程控制逐步取代扩散过程控制;反应电流也随之增大,氧化速率加快,还原和氧化过程可逆性降低。同时KF的加入抑制了铝的沉积,合金化作用比较明显,通过对阴极前波进行高斯拟合,得到不同KF含量（0、3%、5%）体系下的铝离子的电子转移数分别为1.19、1.02、0.75。     

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.     

An innovative approach to electro-oxidation of dopamine on titanium dioxide nanotubes electrode modified by gold particles

Au/TiO₂/Ti electrodes were prepared by galvanic deposition of gold particles from an acidic bath containing KAu(CN)₂ in the presence of a citrate buffer onto TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride-containing electrolyte. The morphology and surface characteristics of Au/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 40–80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti electrodes for the dopamine electro-oxidation was studied by cyclic voltammetry and differential pulse voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electro-catalytic activity toward the oxidation of dopamine. The catalytic oxidation peak current showed a linear dependence on dopamine concentration and a linear calibration curve was obtained in the concentration range of 0.5–2.5 mM of dopamine.     

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 electrochemistry of tellurium in methylene chloride

It was found that TeCl₄ in methylene chloride (containing tetrabutylammonium perchlorate as a supporting electrolyte) is reduced to TeCl₂, Te⁰ and Te⁻² in potential applied. Two reduction waves are observed during the reduction of TeCl₄ at a platinum rde (Ep₁ = 0.08 ± 0.02 V, Ep₂ = −1.10 ± 0.02 V) due to the reduction of TeCl₄ to Te and Te⁻² respectively. A cathodic deposition of tellurium from TeCl₄ is followed by anodic stripping wave (Ep₁ = 0.42 ± 0.02 V), corresponding to the oxidation of Te to TeCl₂. It has been shown that reduction of TeCl₄, or TeCl⁻²₆ to Te causes coating of the electrode with metallic tellurium, on whose surface chloride ions are strongly bonded. It was found that when the electrolysis solution contains excess of chloride ion with respect to Te^IV the peak potentials of the cathodic waves shift to more cathodic values (Ep₁ = 0.30 ± 0.02 V, Ep₂ = −1.25 ± 0.02 V). After cathodic deposition of at least a monolayer of tellurium from such a solution two anodic waves appear (Ep₁ = 0.12 ± 0.03 V, Ep₂ = 0.24 ± 0.03 V) which are both due to the oxidation of Te to Te^II.     

Electrooxidation of methanol and ethylene glycol on gold and on gold modified with an electrodeposited polyNiTSPc film

The electrooxidation of methanol and ethylene glycol on Au and on polynickeltetrasulphophthalocyanine-modified gold (polyNiTSPc/Au/Q) electrodes in a pH 11 carbonate/hydrogen carbonate buffer electrolyte was studied by cyclic voltammetry (CV) and with an electrochemical quartz crystal microbalance (EQCM). Au shows negligible activity for methanol oxidation, in agreement with the fact that methanol does not adsorb on Au, since it does not affect the potential at which the mass increase due to Au oxidation starts. On the contrary, ethylene glycol (EG) is electrooxidized on Au at a significant rate, probably because it adsorbs rather strongly on Au, as evidenced by the positive shift by 0.35 V of the mass increase attending Au monolayer oxidation. The polyNiTSPc-modified Au electrodes are hydrophobic, as inferred from the disappearance of the large mass decrease in the double-layer region typical of the bare Au electrode, a decrease which is due to the desorption of the water molecules adsorbed at the negative potential limit. On the polyNiTSPc/Au/Q electrodes the current at the positive potential limit (at which only Ni(III) exists) of methanol oxidation, and the peak current of EG oxidation (at a potential at which only Ni(II) exists), show a Langmuir dependence on the concentration, which indicates that in both cases the reactive species are adsorbed, and that their oxidation rate is much lower than the adsorption rate. As is well known, the oxidation of Ni(II) to Ni(III) in the polyNiTSPc film is accompanied by a large mass decrease, due mostly to the expulsion of water from the film by the hydrophobic Ni(III). This mass decrease is independent of the scan rate, but in the presence of methanol or ethylene glycol it increases with increasing scan rate, which indicates that the oxidation of the alcohols involves a chemical reaction of the alcohols with Ni(III) ions, the extent of which decreases with increasing rate. Consequently, the amount of the hydrophobic Ni(III) will increase, and so will the mass loss. The mass decrease of Ni(II) oxidation decreases with increasing concentration of methanol or ethylene glycol, which again shows that there is a chemical reaction between the alcohols and Ni(III) ions, since the concentration of the latter would decrease with increasing alcohol concentration.     

Anodic stripping voltammetry of zinc at boron-doped diamond electrodes in ammonia buffer solution

This paper describes the deposition of zinc(II) with anodic stripping voltammetry on the boron-doped diamond electrode. We illustrate the dependency of several parameters on the magnitude of the oxidation peak and try to optimize the method. The supporting electrolyte was found to influence the oxidation peak magnitude. Compared with acetic acid, the most frequently used supporting electrolyte, ammonia buffer solution leads to a four times higher signal. We assume that the formation of zinc complexes, primarily tetraaminezinc(II), are responsible for the better response. Further factors studied and assessed include buffer pH, buffer concentration, deposition potential, deposition time and scan rate. With the improved conditions, a final detection limit of 5 ppb was accomplished.     

Electrodeposition of Te onto monocrystalline n- and p-Si(1 0 0) wafers

Electrochemical deposition of Te onto n- and p-Si(1 0 0) wafers from 0.1 M HNO₃ + 1 mM TeO₂ solution was studied using cyclic voltammetry (CV), chronoamperometry, ex situ SEM and XRD. Electrodeposition of Te onto n-Si takes place both in the dark and under illumination. Electrodeposition of Te onto p-Si proceeds only under illumination, when the photoelectrons are generated in silicon substrate and reduce Te(IV) species in solution. Electrochemical reduction of Te(IV) on n- and p-Si occurs with large cathodic overvoltage (0.22-0.62 V). Nucleation of Te on n- and p-Si proceeds via 3D island growth, it is characterised correspondingly by progressive and instantaneous nucleation mechanisms followed by diffusion limited growth. Cathodic deposition of Te onto n- and p-Si is irreversible. Anodic stripping of Te electrodeposited onto p-Si occurs both in the dark and under illumination and anodic stripping of Te from n-Si proceeds only under illumination.     

Electrodeposition of bismuth, tellurium, and bismuth telluride thin films from choline chloride–oxalic acid ionic liquid

This article presents a series of preliminary results regarding the electrodeposition of bismuth, tellurium, and bismuth telluride films at 60 °C from ionic liquids, containing a mixture of choline chloride and oxalic acid (ChCl–OxA). Ten millimolar concentration solutions of BiCl₃ and TeO₂ were used as precursors in this supporting electrolyte. Cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to demonstrate the deposition processes on Pt and Cu electrodes. Long-time electrolyses (30–120 min) performed at 60 °C with potential control (between ?0.22 and ?0.37 V vs. Ag reference electrode) have resulted in films deposited on copper substrate. Film surfaces were studied by scanning electron microscopy and analyzed by energy dispersive X-ray spectroscopy. The results of this study show that ChCl–OxA ionic liquid may be considered as a promising substitute of aqueous baths for Bi, Te or Bi₂Te₃ film plating.