Silver electrode adsorption

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. 

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Horanyi G and Rizmayer E M 1984 Radiotracer study of anion adsorption at silver electrodes in acidic medium J. Electroanal. Chem. 176 339-48... 

Franke C, Piazza G and Kolb D M 1989 The influence of halide adsorption on the electronic surface states of silver electrodes Electrochim. Acta 34 67-73... 

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

In the ease of the reactive chemisorption the electrode redox potentials assigned to the chemisorption step represent the thermodynamic free energy of adsorption according to AGad - n F Em- This can be visualized by eonsidering the example of the reactive adsorption of an n-aUcanethiolate on a silver electrode surfaee. The reaction is... 

Corrigan and Weaver employed the PDIR approach to study the potential-dependent adsorption of azide, N , at a silver electrode. The potential was switched between the reference value, —0.97 V vs. SCE (where adsorption is known to be limited) and the working potential every 30-60 scans, i.e. up to a minute per step, to a total of c. 1000 scans. The high number of scans was required in order to obtain the required S/N ratio hence the PDIR technique was employed to minimise instrumental drift. Since the electrochemical process under study was totally reversible on the timescale of the experiment, the PDIR technique was a viable option. 

In this paper we extend our SHG studies to silver electrodes in acetonitrile solutions and to platinum electrodes in aqueous solutions. Three different examples are chosen to demonstrate how SHG can be used both qualitatively and quantitatively to study the adsorption of chemical species onto... 

Hydrogen Adsorption and Evolution at Silver Electrodes in Acetonitrile... 

The observed disaepancies in experimental results is most likely caused by the ions of the supporting electrolyte. For example, fluoride ions do not adsorb on the mercury electrode but adsorb on the silver electrode. The adsorption on the latter metal strongly depends on the face orientation.The sequence of AG° values forn-hexanol adsorption from Na2S04 and KCIO4 solutions is AC° [Ag(lll)] > AG° [Ag(lOO)] (see Table 3). However, the sequence of AG°s of n-pentanol adsorption from KF solution is just the opposite [Ag(l 10)] > AG° [Ag(l ll)]. The... 

Fig. 5-28. Potential Mu across a compact layer observed in aqueous solution and work fimction 4> observed in vacuiun as a fimction of adsorption coverage of bromine atoms on a (100) interface of single crystal silver electrode 64> = relative change of work fimction in vacuiun 64 = relative change in potential across a compact layer work fimction data corresponds to Fig. 6-25. [From Bange-Straehler-Sass-Parsons, 1987.]...

Thus, an equation in agreement with the experimental data for the hydroquinone-silver ion reaction can be derived either on the basis of the assumption that adsorption of silver ions by the silver is a prelude to the reaction, or on the basis of the assumption that the rate-controlling step in an electrode process is the rate of transfer of electrons to the silver electrode. The first mechanism carries with it the assumption that a silver ion adsorbed by silver is more easily reduced than an ion in solu-... 

J. Lipkowski, L. Stolberg, Molecular adsorption at gold and silver electrodes in Adsorption of Molecules at Metal Electrodes (Eds. J. Lipkowski, Ph. N. Ross), VCH, N.Y.-Weinlieim-Cambridge, 1992, p. 171. 

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. 

While in the mentioned work on PABA on a charged silver electrode also the adsorption degree of the molecules was controlled by surface charge, Lahann et al. showed a concept in which molecules immobilized as a low-density self-assembled monolayer on a gold surface were electrically stimulated to undergo conformational transitions between a hydrophilic and a hydrophobic state (Lahann et al., 2003). Such a surface wetting switch may then be used to immobilize, for example, enzymes, as was discussed in the previous section. This is an example of switching both the orientation and the activity of adsorbed molecules. 

Results on the adsorption of azide (N3 ) and cyanate (CNO ) ions have been reported by Corrigan et al. [113,135]. For adsorbed azide ion on a silver electrode only one potential-dependent band has been reported between 2074 and 2083 cm . As discussed by Corrigan and Weaver [135], at low potentials a loss of azide ions in solution is observed (band at 2048 cm in Fig. 44) without a corresponding adsorbate gain. As the potential is made more positive a weak adsorbate band is developed (2074 cm ). The most probable interpretation, according to Corrigan and Weaver, is that at low potentials the linear N3 ion is flat-adsorbed. As the degree of... 

The adsorption of cyanate ions was studied on silver [113, 135] and gold electrodes. A bipolar band can be observed for adsorption on a silver electrode [135] (Fig. 45). Corrigan and Weaver [135] have attempted a deconvolution of this band after observing that the intensity of the positive-going part was lower than expected... 

Watanabe, T. and Maeda, H. (1989) Adsorption-controlled redox activity. Surface-enhanced Raman investigation of cystine versus cysteine on silver electrodes. Journal of Physical Chemistry, 93, 3258-3260. 

Later it was shown that there was a slight adsorption of the fluoride ion on silver electrodes, but this does not alter the conclusions. 

Pemberton and Buck " studied the adsorption on a silver electrode of diphenylthiocarbazone anion (HDz ), a dye with its absorption maximum at 470 nm. An ORC was not carried out. Again, the enhancement was not evaluated. However, if one compares the surface spectra obtained, to those reported for a gold electrode using 488-nm light, it is immediately clear that the Raman scattering is hardly surface enhanced at all on silver, as it is not, at least at this excitation, at gold. 

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