Underpotential Deposition Of Cd Research Articles

Underpotential deposition and involved alloy formation of cadmium on silver particles modified HOPG substrates

Cadmium underpotential deposition (UPD) on Ag particles modified highly ordered pyrolytic graphite (HOPG) surfaces, and the involved alloy formation were studied by conventional electrochemical techniques. Voltammetric results indicated that the Cd UPD followed an adsorption behavior different from that observed for massive Ag electrodes and Ag particles supported on vitreous carbon. Nanometer-sized bimetallic Cd–Ag particles were characterized by ex situ atomic force microscopy (AFM). Initially, AFM images show Ag deposits of similar size distributed preferably on HOPG step edges. No remarkable morphological changes are observed on the surface after the subsequent Cd deposition, suggesting that the Cd particles are deposited selectively over the Ag crystals. From the analysis of desorption spectra, employing different polarization times, and density functional theory (DFT) calculations, the formation of a Cd–Ag surface alloy could be inferred.

Underpotential shift in electrodeposition of metal adlayer on tellurium and the free energy of metal telluride formation

Underpotential deposition (UPD) of metals on polycrystalline tellurium film shows much similarity with UPD of metals on foreign metals, but the underpotential shifts on tellurium show no correlation with differences of work functions of the substrate and the deposited metal. In this work, we have evaluated the underpotential shifts of Pb, Cd, Zn, Bi, In, Sn, and Cu UPD on tellurium from cyclic voltammograms and potentiodynamic profiles of inverse charge transfer resistance derived from potentiodynamic impedance spectra, and shown a correlation of the underpotential shifts with the free energy of the corresponding metal telluride formation. The correlation can help to predict underpotential shifts for metals with complications in the measurement of this parameter.

Electrochemical synthesis of Cd–Ag bimetallic particles and the involved alloy formation

The formation of bimetallic Cd–Ag nanoparticles on vitreous carbon (VC) and the involved alloy formation have been studied by conventional electrochemical techniques and ex-situ atomic force microscopy (AFM). Both metals were deposited by sequential electrodeposition using single potentiostatic pulses. The AFM images of Ag deposits showed small differently sized Ag nanocrystallites uniformly distributed over the VC-substrate. This modified substrate was immersed in an electrolyte solution containing Cd2+ ions and bimetallic Cd–Ag particles were prepared by underpotential deposition (UPD) of Cd. The alloy formation phenomena observed previously during Cd UPD on massive Ag electrodes were found also in the case of Cd UPD on Ag nanoparticles. The desorption spectra in the system Cd/Ag (nanoparticles)/VC employing different polarization times, showed three desorption peaks associated with the dissolution of the Cd OPD and UPD deposits and an Ag–Cd alloy formed at low underpotentials, respectively. The electrochemical behavior of the bimetallic system was also studied applying other polarization routine designed for this purpose.

2D Nucleation and Growth Phenomena in UPD of Cd on Ag(111) and Ag(100)

Underpotential deposition (UPD) of Cd on Ag single crystal substrates has been studied by in situ scanning tunneling microscopy (STM) and electrochemical measurements. On Ag(111) the UPD process starts with formation of an expanded adlayer. This adlayer transforms by 2D nucleation and growth to a condensed Cd monolayer, which is unstable and at long polarization times undergoes changes involving surface alloy formation phenomena. In contrast to Ag(111), on Ag(100) a formation of a condensed monolayer by 2D nucleation and growth has been observed at relatively high underpotentials. The growth rate of 2D clusters extracted from the in situ STM images indicates a surface diffusion-controlled growth kinetics. At lower underpotentials the UPD of Cd on both substrates involves a significant surface alloy formation, which is reflected in the surface topography changes observed during the anodic stripping.

New insights on the Cd UPD on Au(111)

AbstractThe Cd underpotential deposition (UPD) process on Au(111) was analyzed by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). In the underpotential range 300⩽ΔE (mV) ⩽400, 2D Cd islands are formed on the fcc regions of the Au(111)‐(√3 × 22) reconstructed surface without lifting the reconstruction. At lower underpotentials, the 2D Cd islands grow and, simultaneously, new 2D islands nucleate and coalesce with the previous ones forming a complete condensed Cd monolayer (ML). STM images and long time polarization experiments performed at ΔE = 70 mV demonstrate the formation of an AuCd surface alloy. At ΔE = 10 mV, the formation of the complete Cd ML is accompanied by a significant AuCd surface alloying and the kinetic results reveal two different solid‐state diffusion processes. The first one, with a diffusion coefficient D1 = 4 × 10−17 cm2 s−1, could be ascribed to the mutual diffusion of Au and Cd atoms through a highly distorted (vacancy‐rich) AuCd alloy layer. The second and faster diffusion process (D2 = 7 × 10−16 cm2 s−1) is associated with the appearance of an additional peak in the anodic stripping curves and could be attributed to the formation of another CdzAux alloy phase. Copyright © 2008 John Wiley & Sons, Ltd.

In situ STM investigation of spinodal decomposition and surface alloying during underpotential deposition of Cd on Au(111) from an ionic liquid

The electrodeposition and anodic dissolution of Cd on Au(111) in an acidic chloroaluminate ionic liquid (MBIC-AlCl(3), 42 : 58 mol%) have been investigated by cyclic voltammetry and in situ STM. In the Cd underpotential deposition region, various nanostructures can be distinguished. At a potential of 0.95 V vs. Al/Al(iii), a transformation from a well ordered AlCl(4)(-) adlayer to a ( radical3 x radical19) superstructure, presumably due to Cd-AlCl(4)(-) coadsorption, is observed. Reducing the potential to 0.45 V, surface alloying of Cd and Au occurs, which is evidenced for the first time by typical spinodal structures occurring both during deposition and dissolution of the surface alloy layer having a hexagonal structure. At still lower potentials below 0.21 V, a layer-by-layer growth of bulk Cd sets in.

Underpotential deposition of Cd on Ag(1 1 1): an in situ STM study

The kinetics and mechanism of Cd underpotential deposition (UPD) and involved surface alloy formation processes in the system Ag(1 1 1)/Cd 2+, SO 4 2 - , are studied by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). The results show that the UPD process starts with a formation of an expanded (diluted) adlayer with a superlattice structure Ag(1 1 1)- ( 3 × 19 ) R 23.4 ° . In the underpotential range 50 mV < Δ E < 80 mV this adlayer transforms to a condensed close packed Cd monolayer via a first order phase transition. At long polarization times the condensed monolayer undergoes structural changes involving place exchange processes between Cd atoms and surface Ag atoms. A formation of a second Cd monolayer and a significant Ag–Cd surface alloying take place at lower underpotentials (Δ E < 50 mV). The kinetics of surface alloying are analyzed on the basis of a recently proposed diffusion model including a relatively fast initial formation of a very thin surface alloy film and a subsequent slow alloy growth controlled by solid state diffusion. The anodic dealloying results in an appearance of monatomically deep pits, which disappear quickly at relatively high underpotentials (Δ E > 550 mV) indicating a high mobility of surface Ag atoms.

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.

Alloy formation in the system Au(1 1 1)/Cd during the UPD process

The Au–Cd alloy formation in the course of Cd underpotential deposition (UPD) in the system Au(1 1 1)/Cd 2+, SO 4 2− has been studied by conventional electrochemical techniques and in situ STM. The long time polarization experiments and the current–potential desorption spectra have indicated that the alloy can be formed at relatively high underpotentials with the formation of 2D Cd islands as was observed by in situ STM images. These islands grow and coalesce at a constant potential with the subsequent formation of new 2D islands on top of the partially formed first Cd monolayer. Atomically resolved images of these islands have revealed an hexagonal atomic structure with an interatomic distance of 0.29 ± 0.01 nm. The dissolution of these features occurs together with the formation of many holes of monatomic height which induces a surface roughening. The alloy formation is explained considering a place exchange process between the Au atoms and the adsorbed Cd atoms followed by solid state diffusion of these atoms through the alloyed phase.

Cd underpotential deposition (upd) from a sulfate electrolyte on Au(1 1 1): studies by in situ STM and UHV-EC

Cd underpotential deposition (upd) on Au(111), from a sulfate electrolyte, has been investigated using electrochemical scanning tunneling microscopy (EC-STM), ultra-high-vacuum emersion techniques and electrochemical quartz crystal microgravimetry (EQCM). A number of atomically resolved adlattices were imaged with EC-STM and structures are proposed. Low energy electron diffraction (LEED) patterns observed after Cd upd consistently indicated a (√3×√3)R30° unit cell, which is closely related to those observed with in situ STM. Comparisons of the EC-STM structures with the LEED results suggest that some change occurred upon emersion of the crystal from solution. EC-STM suggests that the Au(111)-(1×23) reconstruction was not lifted by Cd upd. Additionally, the morphology of the reconstruction dictated the structures formed by the adlayer on the surface. Specially, two structures were observed on the reconstructed surface, while others were observed in its absence. EQCM results suggest a large increase in mass upon Cd upd, which appears to result from co-adsorbed anions and possibly the formation of a CdSO4 bilayer. There was evidence that Cd upd results in a surface alloy. Near −0.5 V, small alloy nanoclusters appeared, distributed randomly across the surface in the absences of the reconstruction, and at domain walls in the presences of the reconstruction.

Electrodeposition of Au-Cd alloy nanostructures on Au(111).

This report concerns an in-situ scanning tunneling microscopy study of the initial stages in the formation of a Au-Cd alloy on the Au(111) herringbone reconstruction. Although Au-Cd nanoclusters of alloy have been observed in sulfate electrolyte by this group, alloy "nanowires" were observed to form preferentially in the hcp regions between the sets of "soliton" walls of the reconstruction only in the presence of chloride. The nanowires were formed at -0.55 V versus 3 M Ag/AgCl, corresponding to Cd underpotential deposition (upd). Upd is electrodeposition at a potential prior to that needed to deposit the bulk element.

In situ and real-time surface differential diffraction (SDD) study of Cd underpotential deposition on Au(1 1 1)

In situ and real-time surface differential diffraction (SDD) has been used to study the underpotential deposition (UPD) of Cd on Au(1 1 1) in sulfuric acid media. Comparison of SDD results in sulfate electrolytes with and without the presence of Cd 2+ ions reveals that the surface reconstruction associated with the sulfate adsorption and desorption dominates the structural effect. It is also found that the reconstructed gold surface is stable upon Cd UPD process. In the initial stages of UPD, Cd atoms bind to the surface in bridge sites. This is followed by an adlayer structure with Cd adsorption in threefold hollow sites before Au/Cd intermixing takes place.

Electrochemical Study of Cadmium in Acidic Zinc Chloride-1-ethyl-3-methylimidazolium Chloride Ionic Liquids

The electrochemistry of Cd(II) was studied with voltammetry and chronoamperometry at polycrystalline Pt, Ni, and W electrodes in the acidic zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquid. Cd(II) could be reduced to cadmium metal via a single-step quasi-reversible electron-transfer process. The redox potential of the Cd(II)/Cd couple shifted negatively as the acidity of the ionic liquid decreased. In addition, underpotential deposition of Cd was observed at Pt and Ni electrodes. This was related to the work function of these metals. Analysis of the chronoamperometric transient behavior revealed a three-dimensional instantaneous nucleation/growth process for the electrodeposition of pure Cd at both Pt and W substrates. When the deposition potential was extended to the potential range where Zn(II) reduction occurred, codeposition of Zn with Cd became possible. The dependency of the Cd-Zn codeposits composition on the deposition potential, temperature, and Cd(II) concentration was investigated. © 2002 The Electrochemical Society. All rights reserved.

Underpotential deposition of cadmium onto (111) face of silver from chloride containing solution

The underpotential deposition (UPD) of cadmium and Ag–Cd alloy formation on the (111) face of silver in chloride containing solutions were investigated by cyclic voltammetry and the potentiostatic pulse technique. It was shown that the UPD of cadmium commences at about −400 mV versus SCE (300 mV vs. Cd 2+/Cd) with the formation of ordered structure of the type ( 3 x 3 ) R30°. Chloride adatoms, which are adsorbed on the silver substrate when UPD of Cd starts, do not desorb. They become replaced by Cd adatoms and remain adsorbed and discharged on top of a Cd layer. At potentials close to the reversible potential of Cd alloying of Ag with Cd takes place, being expressed by two sharp cathodic peaks on the voltammograms.

Nitrate Reduction Catalyzed by Underpotentially Deposited Cd on Au(111): Identification of the Electroactive Surface Structure

The electrocatalysis of the nitrate reduction to nitrite by Cd underpotential deposition (upd) on Au(111) was studied. The structures of the Cd adlayer underpotentially deposited on a Au(111) surface with nitrate in acidic sodium sulfate electrolyte were investigated by scanning tunneling microscopy (STM). Several ordered adlattices were observed, with the catalytically active structure being a (1 × 1) Cd adlayer. This adlayer exhibited a Moire pattern rotated by 9° from the Au(111) lattice direction. Thiocyanate and ethanethiol were used to investigate the mechanism of the catalysis by the Cd adlayer. The addition of thiocyanate did not affect the nitrate reduction activity. On the other hand, ethanethiol was a poison toward the nitrate reduction. STM images revealed surface roughening due to ethanethiol at the potential of the reduction of nitrate. We suggest that nitrate association with the (1 × 1) Cd adlattice is a requirement for electroreduction activity.

Control of Composition and Conduction Type of CdTe Film Electrodeposited from Ammonia Alkaline Aqueous Solutions

Potentiostatic cathodic electrodeposition of CdTe films was studied using aqueous ammonia‐alkaline electrolytic baths in which Cd(II)‐ and Te(IV)‐species were dissolving to form and ions. respectively. Deposition of three kinds of films, i.e., Te‐rich amorphous, Te‐rich polycrystalline CdTe, and Cd‐rich polycrystalline CdTe films, can be controlled by changing the pH (pH 10.5–12.2) and Cd(II)/Te(IV) concentration ratio of the baths. All the films deposited at −0.70 V vs. the standard hydrogen electrode (SHE) had a flat and smooth morphology. Deposition current density and efficiency were reduced with increasing Cd content of the film. The deposition behavior was interpreted with a potential‐pH diagram of the system. Based on the results, the mechanism of the CdTe electrodeposition was presumed to be (i) deposition of tellurium atoms followed by (ii) an immediate adsorption of Cd(II) ions on the surface tellurium atoms, and (iii) an underpotential deposition of Cd on the surface Te. Conduction types of the Te‐rich and Cd‐rich CdTe films after heat‐treatment were p‐ and n‐type, respectively, whereas untreated CdTe film was n‐type. © 1999 The Electrochemical Society. All rights reserved.

Surface alloying at the Cd∣Au(100) interface in the upd region. Electrochemical studies and in situ EC-AFM observation

The surface alloy formation during Cd underpotential deposition (upd) on Au(100) in sulfuric acid solution was investigated by means of long-time polarization experiments and in situ electrochemical atomic force microscopy (EC-AFM). The dynamics of the surface alloying were found to depend on polarization conditions and to compete with surface diffusion and further adsorption processes. A turnover mechanism is proposed for the surface alloying process that was observed to take place mainly on the terraces. For long polarization times, the extent of 2D alloy formation into the surface is governed by solid-state diffusion of either Cd or Au through the alloyed phase. The time dependence of the surface phase stability and composition, which was formed at a certain potential in the upd region, is discussed in thermodynamic terms derived for the surface.

Potential‐pH Diagram of the Cd ‐ Te ‐ NH 3 ‐ H 2 O System and Electrodeposition Behavior of CdTe from Ammoniacal Alkaline Baths

A potential‐pH diagram of the system was constructed based on diagrams of the and systems and discussed in connection with the redox behavior of an ammonia‐alkaline CdTe electrolytic bath with pH 10.7. CdTe has a wide domain of stability throughout the acidic and alkaline regions, and the redox behavior was well explained with the diagram. The diagram indicated that the cathodic electrodeposition of CdTe occurs across a domain of stability of tellurium metal, i.e., at lower potentials than the deposition potential of bulk Te and higher than that of bulk Cd, with respect to the bath with pH < ca. 11.5, while in the higher pH region, CdTe is expected to deposit directly from Te(IV) and Cd(II) ions. The deposition mechanism is considered as follows: (i) deposition of tellurium layer followed by (ii) an immediate underpotential deposition of Cd on it, which prevents the bulk deposition of tellurium. It can be considered that the stoichiometric CdTe is more easily electrodeposited from alkaline baths, since the domain for tellurium metal is narrower in the alkaline region compared to the conventionally employed acidic region with pH 0–2. Therefore, the bulk deposition of elemental tellurium is less apt to occur from an alkaline bath. © 1999 The Electrochemical Society. All rights reserved.

An ex-situ study of Cd underpotential deposition on Cu(111)

The underpotential deposition (upd) of Cd on Cu(111) single crystal electrodes was studied by cyclic voltammetry and UHV based spectroscopies. The structure of the Cd upd layer was identified as a ( 19 × 19 )R23.4° structure with a Cd coverage of 9/19 monolayers with respect to Cu(111). The Cd–Cd distance within the layer is 3.71 Å, much larger than in bulk Cd. A Cl layer is adsorbed on top of the Cd and stabilizes the open Cd structure.

Electrochemistry of S Adlayers at Underpotentially Deposited Cd on Au(111): Implications for the Electrosynthesis of High-Quality CdS Thin Films

We report an electrochemical, scanning probe microscopic, and Raman spectroscopic investigation of thin CdS films grown by electrochemical atomic layer epitaxy aimed at understanding the role played by the order of deposition on film quality. Using scanning tunneling microscopy (STM), we determine the atomic level structure of the first three monolayers of CdS for Cd-first films. The initial Cd underpotential deposition layer forms a stable ( )−Au(111) adlattice in which the two unique nearest neighbor spacings are 0.33 and 0.29 nm. When H2S is electrolyzed at this surface at underpotential, an adlattice is produced in which the S−S interatomic spacing is 0.34 nm. The unit cell of this structure is ( ) with respect to Au(111). Both the Cd and S monolayers appear to be nearly closest-packed, in contrast to the more open structures observed in the S-first films. Interestingly, the second monolayers of Cd and S were found to have the same structure as in the first monolayer of CdS, implying that the firs...