Electrodeposition anionic

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

Copper electrodeposition on Au(111) Copper is an interesting metal and has been widely investigated in electrodeposition studies from aqueous solutions. There are numerous publications in the literature on this topic. Furthermore, technical processes to produce Cu interconnects on microchips have been established in aqueous solutions. In general, the quality of the deposits is strongly influenced by the bath composition. On the nanometer scale, one finds different superstmctures in the underpotential deposition regime if different counter-ions are used in the solutions. A co-adsorption between the metal atoms and the anions has been reported. In the underpotential regime, before the bulk deposition begins, one Cu mono-layer forms on Au(lll) [66]. 

For ruthenium, electrolytes based on ruthenium sulphamate or nitrosyl-sulphamate have been described, but the most useful solutions currently available are based on the anionic complex (H2 0 Cl4 Ru N Ru-Cl4-OH2) . The latter solutions operate with relatively high cathode efficiency to furnish bright deposits up to a thickness of about 0-005 0 mm, which are similar in physical characteristics to electrodeposited rhodium and have shown promise in applications for which the latter more costly metal is commonly employed. Particularly interesting is the potential application of ruthenium as an alternative to gold or rhodium plating on the contact members of sealed-reed relay switches. 

Ruthenium, iridium and osmium Baths based on the complex anion (NRu2Clg(H20)2) are best for ruthenium electrodeposition. Being strongly acid, however, they attack the Ni-Fe or Co-Fe-V alloys used in reed switches. Reacting the complex with oxalic acid gives a solution from which ruthenium can be deposited at neutral pH. To maintain stability, it is necessary to operate the bath with an ion-selective membrane between the electrodes . 

The anionic composition of the cathodic product is not the only parameter that can be controlled through electrolysis conditions. Grinevitch et al. [559] reported on the investigation of the co-deposition of tantalum and niobium during the electrolysis of fluoride - chloride melts. Appropriate electrodeposition conditions were found that enable to obtain either pure niobium or alloys. 

As well as the adsorption of halogen atoms or molecules, the adsorption of halide anions to gold surfaces has been extensively studied and a comprehensive review of the area has been published by Magnussen [168]. The degree of specific adsorption to gold surfaces increases in the order F < Cl < Br < 1 with only weakly specifically adsorbed. The presence of halide anions can also affect the electrodeposition of organic molecules such as pyridine on Au surfaces with chloride and bromide solutions suppressing the formation of ordered N-bonded pyridine layers [169]. 

Figure 14.1a shows aj vs. U curve in Cu + Sn + H2SO4 with (solid curve) and without (dashed curve) cationic surfactant. The addition of the surfactant causes a drastic change in they vs. U curve. Namely, an NDR appears in a narrow potential region of about 5 mV near —0.42 V, where the Cu-Sn alloy is electrodeposited. Another notable point in the surfactant-added solution is that a current oscillation appears when the Uis kept constant in (and near) the potential region of this NDR, as shown in Figure 14.1b. It was also revealed that both the NDR and current oscillation appeared only in the presence of cationic surfactant and not in the presence of anionic surfactant.

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

Soft tissue Sample wet ashed, spiked with 243Am, purified by anion exchange, solvent extraction, and electrodeposition a -Spectrometry No data 98% Mclnroy et al. 1985... 

Soft tissue Spiked sample wet ashed, treated with HN03/H202, purified by A-CU column, anion exchange, TRU-spec column, and electrodeposition a -Spectrometry No data 53% Qu et al. 1998... 

Air Filter wet ashed in HNO3/HF, purified with cation and anion exchange columns and electrodeposition a -Spectroscopy No data No data Knab1979... 

Air Cellulose filter dry ashed, dissolved in HNO3/HF, H202/HCI04, purified with anion exchange, TRU-spec columns followed by electrodeposition. a -Spectroscopy 0.023 pCi/sample 102% Goldstein et al. 1997... 

Sea water Co-precipitation with iron hydroxide, purified by anion exchange, coprecipitation with BiP04, cation exchange, electrodeposition a -Spectroscopy No data 64-79% Lovette et al. 1990... 

Soil Dry ash, digest in HNO3/HCI, anion exchange, Ca-oxalate and Fe (OH)2 coprecipitation, anion exchange, electrodeposition a -Spectroscopy 27 pCi/g 75-92% Sanchez and Singleton 1996... 

Vegetation Ashed, digested with HN03-H202, oxalate and Fe precipitations, anion exchange, solvent extraction, electrodeposition a -Spectroscopy 0.3 fCi/g 73-109% Cooper et al. 1993... 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Electronic spectra (Table 1.1, Fig. 1.2) have been measnred for the orange soln-tions of (RuO ] in aqueous base from 250-600 nm. [212-215, 222], and reproduced [215, 222]. There are two at 460 and 385 nm. [212, 213, 222] or three bands in the visible-UV region, at 460, 385 and 317 nm [214, 215]. These appear to be at the same positions as those for [RuO ] but the intensities and hence the general outline of the two spectra are very different. Woodhead and Fletcher reviewed the published molar extinction coefficients and their optimum values / dm (mol" cm" ) are 1710 for the 460 nm. band, 831 for the 385 nm. band and 301 for the 317 nm. band - the latter band was not observed by some workers [214]. The distinctive electronic spectrum of ruthenate in solution is useful for distinguishing between it, [RuO ]" and RuO [212, 222]. Measurements of the electronic spectra of potassium ruthenate doped in K CrO and K SeO and of barium ruthenate doped into BaSO, BaCrO, and BaSeO (in all cases the anions of these host materials are tetrahedral) indicate that in that these environments at least the Ru is tetrahedrally coordinated. Based on this evidence it has been suggested that [RuO ] in aqueous solution is tetrahedral [RuO ] rather than franx-[Ru(0H)3(0)3] [533, 535]. Potential modulated reflectance spectroscopy (PMRS) was used to identify [RuO ] and [RuO ] " in alkaline aqueous solutions during anodic oxidation of Ru electrodeposited on platinum from [Ru3(N)Clg(H30)3] [228]. 

The same mechanism of zinc electrodeposition on the GC electrode was observed in sulfate, chloride, and acetate ion solutions [227]. The anions mainly affected the nucleation densities during zinc deposition, which resulted in a different surface morphology. The nucleation rate constant was the same in the chloride and sulfate solutions and was equal to 1.22 x 10 s h In the presence of acetate and chloride ions, the deposited zinc film tends to grow in a multilayered pattern, while in sulfate solution, the zinc deposition forms irregular grains. A new approach to the estimation of zinc electrocrystallization parameters on the GC electrode from acetate solutions was described by Yu et al. [228]. 

The cadmium electrodeposition on the cadmium electrode from water-ethanol [222, 223], water-DMSO [224], and water-acetonitrile mixtures [225-229] was studied intensively. It was found that promotion of Cd(II) electrodeposition [222] was caused by the formation of unstable solvates of Cd(II) ions with adsorbed alcohol molecules or by interaction with adsorbed perchlorate anions. In the presence of 1 anions, the formation of activated Cd(II)-I complex in adsorbed layer accelerated the electrode reaction [223]. 

A physical model and a theory have been proposed [72], which might be helpful in comparative studies on electrocompres-sive behavior of electrodeposited chloride, bromide, and iodide monolayers on the Au(lll) electrode. The theoretical results were in good agreement with the experimental data, which evidence that the adatom-adatom interactions (especially repulsive ones) and electrosorption valency of halide anions determine the compressibility within halide adlayers. Also, Lipkowski et al. have discussed various aspects of adsorption of halide anions on Au(lll) in a review paper [36]. From this paper, we have taken quantitative data concerning adsorption of halide anions on Au(lll) (cf Fig. 3). 

Coating by a thin layer of PPy has been realized on multiwalled nanotubes (MWNT) [29,91,93], well-aligned MWNT [85] and single-wall nanotubes (SWNT) [88], When MWNT are oxidized, their surface is covered with oxygenated functionalities, which can be used as anionic dopant of a PPy film electrodeposited on the MWNT [94], These films are notably less brittle and more adhesive to the electrode than those formed using an aqueous electrolyte as source of counterion. 

While the structure at the electrode/ionic liquid interface is uncertain it is clear that in the absence of neutral molecules the concentration of anions and cations at the interface will be potential dependent. The main difference between aqueous solutions and ionic liquids is the size of the ions. The ionic radii of most metal ions are in the range 1-2 A whereas for most ions of an ionic liquid they are more typically 3-5 A. This means that in an ionic liquid the electrode will be coated with a layer of ions at least 6-7 A thick. To dissolve in an ionic liquid most metal species are anionic and hence the concentration of metal ions close to the electrode surface will be potential dependent. The more negative the applied potential the smaller the concentration of anions. This means that reactive metals such as Al, Ta, Ti and W will be difficult to deposit as the effective concentration of metal might be too low to nucleate. It is proposed, as one explanation, that this is the reason that aluminum cannot be electrodeposited from Lewis basic chloroaluminate ionic liquids. More reactive metals such as lithium can however be deposited using ionic liquids because they are cationic and therefore... 

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