Ag metal deposition

Fig. 3.4 Correlation between the initial rate of photocatalytic Ag metal deposition (/JAg) and Ag+ adsorption ([Ag+]ads) of HyCOM Ti02 calcined at 973 K (open circles) and P-25 (closed circles).

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]. 

A number of attempts to produce tire refractory metals, such as titanium and zirconium, by molten chloride electrolysis have not met widr success with two exceptions. The electrolysis of caesium salts such as Cs2ZrCl6 and CsTaCle, and of the fluorides Na2ZrF6 and NaTaFg have produced satisfactoty products on the laboratory scale (Flengas and Pint, 1969) but other systems have produced merely metallic dusts aird dendritic deposits. These observations suggest tlrat, as in tire case of metal deposition from aqueous electrolytes, e.g. Ag from Ag(CN)/ instead of from AgNOj, tire formation of stable metal complexes in tire liquid electrolyte is the key to success. 

The ore deposits can be classed into two types based on the types of associated metals Au-Ag rich deposits (Type A) from which An and Ag are produced as main products, and base metal (Cu, Pb, Zn, Mn, (Sn), (W), (Bi), (Mo), (Sb)) rich deposits (Type B) from which Au and Ag are recovered as byproducts. The deposits are associated with felsic and intermediate volcanic rocks but generally not with felsic plutonic rocks. In Japan Au-Ag deposits associated with granitic rocks (e.g., Au-Ag vein-type deposits in Kitakami) occur commonly. However, these plutonic-type deposits are not described here. 

S values of epithermal base metal deposits are higher than those of the epithermal Au-Ag deposits and range mostly from - -3%c to -f-7%o (Fig. 1.111). Although most of 8 " S values for base-metal deposits lie in this range, 8- " S of composite sample of sulfides from the Motokura Cu-Pb-Zn deposits, Ohmori Cu-Ag deposits, Hosokura Pb-Zn deposits, Sasayama Cu-Pb-Zn deposits and Imai-lshizaki Cu-Pb-Zn deposits are low, that is, -1-0.1, -1-1.8, -1-2.2, —0.9 and —2.1%o, respectively (Shikazono, 1987b Shikazono and Shimizu, 1993). 

These data could be explained by the sulfur of barite from epithermal Au-Ag-Te deposits came both from volcanic gas (SO2) and marine sulfate, but that of epithermal base-metal deposits came from marine sulfate and oxidation of H2S. 

As noted already, epithermal vein-type deposits are classified primarily on the basis of their major ore-metals (Cu, Pb, Zn, Mn, Au and Ag) into the gold-silver-type and the base-metal-type. Major and accessory ore-metals from major vein-type deposits in Japan were examined in order to assess the possible differences in the metal ratios in these two types of deposits (Shikazono and Shimizu, 1992). Characteristic major ore-metals are Au, Ag, Te, Se and Cu for the Au-Ag deposits, and Pb, Zn, Mn, Cu and Ag for the base-metal deposits (Shikazono, 1986). Accessary metals are Cd, Hg, Tl, Sb and As for the Au-Ag deposits and In, Ga, Bi, As, Sb, W and Sn for the base-metal deposits (Table 1.22, Shikazono and Shimizu, 1992). Minerals containing Cu, Ag, Sb and As are common in both types of deposits. They are thus not included in Table 1.22. 

They indicated that the softness parameter may reasonably be considered as a quantitative measure of the softness of metal ions and is consistent with the HSAB principle by Pearson (1963, 1968). Wood et al. (1987) have shown experimentally that the relative solubilities of the metals in H20-NaCl-C02 solutions from 200°C to 350°C are consistent with the HSAB principle in chloride-poor solutions, the soft ions Au" " and Ag+ prefer to combine with the soft bisulfide ligand the borderline ions Fe +, Zn +, Pb +, Sb + and Bi- + prefer water, hydroxyl, carbonate or bicarbonate ligands, and the extremely hard Mo + bonds only to the hard anions OH and. Tables 1.23 and 1.24 show the classification of metals and ligands according to the HSAB principle of Ahrland et al. (1958), Pearson (1963, 1968) (Table 1.23) and softness parameter of Yamada and Tanaka (1975) (Table 1.24). Compari.son of Table 1.22 with Tables 1.23 and 1.24 makes it evident that the metals associated with the gold-silver deposits have a relatively soft character, whereas those associated with the base-metal deposits have a relatively hard (or borderline) character. For example, metals that tend to form hard acids (Mn +, Ga +, In- +, Fe +, Sn " ", MoO +, WO " ", CO2) and borderline acids (Fe +, Zn +, Pb +, Sb +) are enriched in the base-metal deposits, whereas metals that tend to form soft acids... 

Ag" ", Au" ", Tl" ", Tp+) are enriched in the gold-silver deposits. Metals that have high values of the softness parameter (Ag", Hg+, Tl" ", Cd ) are associated with the gold-silver deposits, whereas those that have low values of the softness parameter (Zn +, In +, Bi +, Te, Mn +, Sn" +, Ga " ) are found with the base-metal deposits. 

These correlations mean that the HSAB principle could be a useful approach to evaluate the geochemical behavior of metals and ligands in ore fluids responsible for the formation of the epithermal vein-type deposits. Among the ligands in the ore fluids, HS" and H2S are the most likely to form complexes with the metals concentrated in the gold-silver deposits (e.g., Au, Ag, Cu, Hg, Tl, Cd), whereas Cl prefers to form complexes with the metals concentrated in the base-metal deposits (e.g., Pb, Zn, Mn, Fe, Cu, and Sn) (Crerar et al., 1985). 

One must realize that once complete metal deposition has been attained, the emf across the electrodes cannot be switched off before the cathode has been taken out of the solution and rinsed with water, otherwise the metal deposit may start to redissolv e in the solution as a consequence of internal electrolysis by the counter emf. After disconnection the electrode is rinsed with acetone and dried at 100-110° C for 3-4 min. The analytical result is usually obtained from difference in weight of the dry cathode before and after electrolysis. In a few instances a copper- or silver-plated Pt cathode or even an Ag cathode is used, e.g., Zn and Bi are difficult to remove entirely from Pt, as they leave black stains and on heating form an alloy with the noble metal for this and other reasons (see below) the experimenter should consult the prescriptions in handbooks149. 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

While the above XPS results give the impression, that the electrochemical interface and the metal vacuum interface behave similarly, fundamental differences become evident when work function changes during metal deposition are considered. During metal deposition at the metal vacuum interface the work function of the sample surface usually shifts from that of the bare substrate to that of the bulk deposit. In the case of Cu deposition onto Pt(l 11) a work function reduction from 5.5 eV to 4.3 eV is observed during deposition of one monolayer of copper [96], Although a reduction of work function with UPD metal coverage is also observed at the electrochemical interface, the absolute values are totally different. For Ag deposition on Pt (see Fig. 31)... 

More recently, Ikeda et a/.108 have examined C02 reduction in aqueous and nonaqueous solvents using metal-deposited p-GaP and p-InP electrodes under illumination. Metal coatings on these semiconductor electrodes gave much improved faradaic efficiencies for C02 reduction. In an aqueous solution, the products obtained were formic acid and CO with hydrogen evolution at Pb-, Zn-, and In-coated electrodes, while in a nonaqueous PC solution, CO was obtained with faradaic efficiencies of ca. 90% at In-, Zn-, and Au-coated p-GaP and p-InP, and a Pb coating on a p-GaP electrode gave oxalate as the main product with a faradaic efficiency of ca. 50% at -1.2 V versus Ag/AgCl. 

The electrolyte volume of the STM cells is usually very small (of the order of a 100 pi in the above described case) and evaporation of the solution can create problems in long-term experiments. Miniature reference electrodes have been described in the literature [36], For most metal deposition studies, a simple metal wire, immersed directly into the metal ion containing solution, is a convenient, low-noise reference electrode. This is particularly true for Cu- and Ag-deposition studies. 

Reactions involving charge transfer through the interface, and hence the flow of a current, are called electrochemical reactions. Two types of such reactions are indicated in Fig. 1.1. The upper one is an instance of metal deposition. It involves the transfer of a metal ion from the solution onto the metal surface, where it is discharged by taking up electrons. Metal deposition takes place at specific sites in the case shown it is a hollow site between the atoms of the metal electrode. The deposited metal ion may belong to the same species as those on the metal electrode, as in the deposition of a Ag+ ion on a silver electrode, or it can be different as in the deposition of a Ag+ ion on platinum. In any case the reaction is formally written as ... 

Metal deposition can occur only if 77 is negative so the Gibbs energy of a cluster as a function of the particle number N first rises, reaches a maximum, and then decreases. This is illustrated in Fig. 10.4 for three different overpotentials. Notice how strongly the curve depends on the applied overpotential. AG reaches its maximum for a critical particle number of ... 

The previous examples used the three-electrode electrochemical system. An alternative was utilized by Ajayan et al. to prepare Ag NP coated SWCNTs [217]. An electrode was fabricated consisting of SWCNTs attached to a Ti cathode and a silver contact pad as a sacrificial anode (Fig. 5.16(a)). The electrode was submerged in an aqueous solution and a potential was applied resulting in oxidation of Ag metal to Ag2+ ions which then subsequently deposited onto the SWCNT cathode. Although experimentally complicated, silver NPs, wires and patterns were controllably deposited on the SWCNTs (Fig. 5.16(a), (b)) [217],... 

Underpotential deposition is described as less than monolayer metal deposition on a foreign metal substrate, which occurs at more positive potentials than the equilibrium potential of a metal ion deposed on its own metal, expressed by the Nemst equation. Kolb reviewed state-of-the-art Underpotential deposition up to 1978. As Underpotential deposition is a process indicative of less than a monolayer metal on a substrate, it is expected to be quite sensitive to the surface stmcture of the substrate crystal a well-defined single-crystal electrode preparation is a prerequisite to the study of Underpotential deposition. In the case of Au and Ag single-crystal electrodes, Hamelin and co-workers extensively studied the necessary crystal surface structure, as reviewed in Ref. 2. 

Silver deposition on polycrystalline Pt electrodes at potentials positive to the equilibrium potential gave 2.5 atomic layers. Two binding types of Ag layers were found by anodic stripping the first Ag layer deposited on Pt, which seems to form an alloy of Ag-Pt, on which the second Ag deposition takes place in the Ag underpotential deposition region. STM images from the underpotential to the overpotential deposition region were observed for Cu underpotential deposition on Au(l 11) in sulfuric acid solution, where Cu underpotential deposition does not affect overpotential deposition, although the latter always takes place on the surface with Cu underpotential deposition and a metal. ... 

The first in situ STM study of metal deposition was for Ag on HOPG [430], which is representative of the Volmer-Weber growth mode. Since that time several other studies of metal deposition on HOPG have been reported Ag [393,431-433], Pb [129], Pt [434,435], Au [436], and Ni [437]. In these studies examining the small metal particles proved to be... 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

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