Silver electrode, surface studies

Following these studies were a series of works which examined the SH response from polycrystalline and crystalline silver electrode surfaces biased within the ideally polarizable region [42-44, 54-64], These studies showed the sensitivity of SHG to adsorption of ions to the electrode surface. More importantly, they showed that the sensitivity arose from the changes in the optical properties of the electrode surface itself, and not from the optical properties of the ionic and simple molecular species at the surface. Conversely, the SH response from an adsorbate, such as a dye molecule, can become an important factor if either the fundamental or SH photon energy is resonant with electronic transitions in the adsorbate [65, 66]. For more details on these SH studies of both ORC effects and adsorption phenomena on polycrystalline substrates, the reader is referred to Refs. 7 and 9. 

Ardizzone, S., Cappelletti, G., Mussini, P.R., Rondinini, S. and Doubova, L.M. (2003a) Electrode-posited polycrystalline silver electrodes Surface control for electrocatalytical studies. Russ. J. Electrochem. (translation of Elektrokhimija) 39, 170-176. 

For a limited number of metal surfaces, adsorption of a molecular species in a thin (monomolecular layer) film results in a huge increase in the effective vibrational Raman scattering cross-section (again, as with RR scattering, up to ca. 106 times) of the adsorbate species. The SERS effect was discovered more than ten years ago for pyridine adsorbed at a silver electrode surface in contact with an aqueous electrolyte [1, 2]. In the intervening period, many hundreds of papers devoted to SERS phenomena have been published, extending the studies to other metals than silver, to non-aqueous as well as aqueous electrolytes, to colloidal dispersions of metals as well as metal electrodes, and even to vacuum-deposited thin film systems under UHV conditions. This review will concentrate on studies of metal-electrolyte interfaces. 

Adsorption and electrooxidation of ss NAs on a silver electrode were studied by electrochemical methods and surface-enhanced Raman spectroscopy [189, 228]. Using the latter electrode, Ean and coworkers [229] observed an anodic signal in solutions of DNA. This signal was attributed to redox reactions of purine bases, and provided a convenient way to determine DNA. Oxidation of purine bases... 

Flavodoxin from M. elsdenii was studied by the SERRS spectroscopy at liquid N2 temperature [98]. It has been shown, on the basis of comparison with the RR spectrum in the solution, that SERRS spectrum arises from the protein-bound FMN. The SERRS spectra of flavoproteins such as choline oxidase and sarcosine oxidase, whose FAD is covalently bound to the apoprotein, were reported by Taniguchi et al. [100]. The results of these two studies indicate the possibility of detecting the SERRS spectra of native flavoproteins. The close similarity between the solution resonance Raman and SERRS spectra [98] reveals that there is no strong chemical interaction between FMN and the silver electrode surface. It is thus reasonable to conclude that the electromagnetic enhancement contributes significantly to the overall enhancement of the SERRS spectra of flavoproteins under the conditions used in these studies. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

In most work on electrochemical systems, use is made of two effects that greatly enhance the Raman signals. One is resonance Raman spectroscopy (RRS), wherein the excitation wavelength corresponds to an electronic transition in an adsorbed molecule on an electrode surface. The other effect is surface-enhanced Raman spectroscopy (SERS), which occurs on certain surfaces, such as electrochemically roughened silver and gold. This effect, discovered by Fleischmann et al. (1974), yields enhancements of 10 to 10 . The vast majority of publications on Raman studies of electrochemical systems use SERS. The limitations of SERS are that it occurs on only a few metals and the mechanism of the enhancement is not understood. There is speculation that only a small part of the surface is involved in the effect. There is a very good review of SERS (Pemberton, 1991). 

Carrabba M.M., Edmonds R.B., Rauh, R.D., Feasibility studies for the detection of organic-surface and subsurface water contaminants by surface-enhanced Raman-spectroscopy on silver electrodes, Anal. Chem. 1987 59 2559-2563. 

Just a few years after the discovery of the deposition and electroactivity of Prussian blue, other metal hexacyanoferrates were deposited on various electrode surfaces. However, except for ruthenium and osmium, the electroplating of the metal or its anodizing was required for the deposition of nickel [14], copper [15,16], and silver [9] hexacyanoferrates. Later studies have shown the possibilities of the synthesis of nickel, cobalt, indium hexacyanoferrates similar to the deposition of Prussian blue [17-19], as well as palladium [20-22], zinc [23, 24], lanthanum [25-27], vanadium [28], silver [29], and thallium [30] hexacyanoferrates. 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

Thus, besides being sensitive to absorbing species on the electrode surface as well as in the solution in the region very close to the surface, it is possible to obtain potential dependent behavior in fine detail. We have applied these techniques to examine the interaction of simple ions such as CN and Ny with polycrystalline electrodes of silver, gold and copper. The observed vibrational spectra can be interpreted with the help of selection rules based on symmetry and analysis of ab-initio SCF wavefunctions of clusters. The results of these studies will be reviewed. 

The right hand side of Fig. A.4.6 is contained in Fig. 3.3. Capacity measurements can readily be made at solid electrodes to study adsorption behavior. For a review see Parsons (1987). As Fig. A.4.7 illustrates, capacity potential curves of three low-index phases of silver, in contact with a dilute aqueous solution of NaF, show different minimum capacities (corresponding to the condition o = 0) and therefore remarkably different potentials of pzc. The closest packed surface (111) has the highest pzc and the least close-packed (110) has the lowest pcz these values differ by 300 mV. Such complications observed with single crystal electrodes, seem likely to have their parallel at other solid surfaces. For example, it is to be expected that a crystalline oxide will have different pzc values at its various types of exposed faces. 

Electrochemical, SERS, and surface enhanced resonance Raman (SERR) studies of the reduction of methylene blue on silver electrode have been published by Nicolai et al. [230, 231]... 

The mechanism of anodic dissolution of silver in cyanide solutions has been studied by Bek and coworkers [378-380]. For example, using [379] the rotating disc electrode and pulse potentiostatic method, it has been found that the limiting step involved the formation, at the electrode surface, of the adsorbed complex with two... 

---