Electrochemical systems monolayers

In the third region of coverage, most of the atomic scale roughness has been proposed to be irreversibly destroyed as the Pb layer rearranges to assume the final hexagonal close packed configuration of the monolayer.( ) This loss of atomic scale roughness results in the irreversible decrease in i/(0H) intensity to essentially unmeasurable levels. This observation further emphasizes the importance of the chemical enhancement mechanism contribution to SERS in electrochemical systems. 

This equivalence between the charge of surface-bound molecules and the current of solution soluble ones is due to two main reasons first, in an electro-active monolayer the normalized charge is proportional to the difference between the total and reactant surface excesses ((QP/QP) oc (/> — To)), and in electrochemical systems under mass transport control, the voltammetric normalized current is proportional to the difference between the bulk and surface concentrations ((///djC) oc (c 0 — Cq) [49]. Second, a reversible diffusionless system fulfills the conditions (6.107) and (6.110) and the same conditions must be fulfilled by the concentrations cQ and cR when the process takes place under mass transport control (see Eqs. (2.150) and (2.151)) when the diffusion coefficients of both species are equal. 

The formation of two- and three-dimensional phases on electrode surfaces is a topic of central importance in interfacial electrochemistry. It is of relevance not only to hmdamental problems, such as the formation of ionic and molecular adsorbate films, but also to areas of great technological interest, such as thin-film deposition, self-assembly of monolayers, and passivation. So far, phase formation in electrochemical systems has been studied predominantly by kinetic measurements using electrochemical or spectroscopic techniques. In order to understand and control these processes as well as the resulting interface structure better, however, improved... 

A limitation to the application of SERS to electrochemical systems is the specificity of the enhancement effect to Ag, Au, and Cu. However, since the electromagnetic part of the enhancement is maintained over distances of several nanometers, it has been possible to coat a SERS active metal with a thin layer of another metal that is exposed to the adsorbing molecules and still obtain enhanced signals (69). For example, by constant-current deposition, it is possible to deposit pinhole-free layers of Pd on Au with a thickness corresponding to 3.5 monolayers and to study the adsorption of species on the Pd by SERS. The spectra of adsorbed benzene on such an electrode are shown in Figure 17.2.12 (84). The symmetric ring-breathing mode of benzene adsorbed on Pd appears at 950 cm shifted considerably from that found either for liquid benzene (992 cm ) or for benzene adsorbed on Au (975 cm ). Deuterated benzene (C D ) behaves similarly and shows the expected shift in the band to lower frequency. The attenuation of the enhancement effect with thickness of the Pd overlayer was reported to be only a factor of 4-5 for thicknesses of 3-30 monolayers. 

The Generalization of the Thermodynamics of Monolayer Adsorption in Electrochemical Systems... 

Figure 6.4. The treatment of electrochemical systems with adsorption is significantly more complicated given that we must select a suitable model to describe the adsorption process which will introduce new variables, uncertainties and approximations. Moreover, as will be discussed below, in general the models will lead to non-linear terms in the mathematical problem. For all the above reasons, it is common practice to try to minimise the incidence of adsorption by means of the experimental conditions (mainly the electrode material and solvent). However, in some situations adsorption cannot be avoided (being even intrinsic to the process under study) or it can be desirable as in the modification of electrodes with electroactive monolayers for electroanalysis or electrocatalysis.

The first in situ Raman spectroscopic study on electrochemical systems was reported on thin metal oxide and metal hahde film electrodes by Fleischmann etal. in 1973 [1]. The Raman spectroelectrochemi-cal measurements were made on thin films of Hg2Cl2, Hg2Br2, and HgO formed on droplets of mercury electrodeposited onto platinum electrodes. These mercury compounds have exceptionally high Raman scattering cross sections (very good Raman scatterers) so that the spectra of species as little as a few monolayers could be recorded on these high-surface-area electrodes. These experiments proved the viability of Raman spectroscopic measurements of... 

To be able to investigate the hydrodynamics of the system, the device was additionally equipped with a video camera (SONY, Japan) for observations of the displacement of tracer particles located at the gas-liquid interface. The experimental system could be also adapted for direct measuring of the mass transfer rate across the interface in the presence of the active phospholipid monolayer. For that purpose, the electrochemical system was developed [1], where the oxygen flux across the interface could be determined by the measurement of the electric current intensity. The results of experimental investigations will be presented in the further part of the paper. 

The first in-situ Raman spectroscopic study of an electrochemical system was reported in 1973 by Eleischmann, Hendra and McQuillan, who described Raman spectra for thin films of Hg2Cl2, Hg2Br2 and HgO, formed on mercury droplets that had been electrodeposited onto platinum electrodes [20]. As these compounds have exceptionally large Raman scattering cross-sections (i.e., they are very good Raman scatterers), a signal from a species composed of only a few monolayers could be detected. These experiments proved, for the first time, the viability of in-situ Raman spectroscopic measurements in electrochemical environments. 

Electrochemical template-controlled sjmthesis of metallic nanoparticles consists of two steps (i) preparation of template and (ii) electrochemical reduction of metals. The template is prepared as a nano structured insulating mono-layer with homogeneously distributed planar molecules. This is a crucial step in the whole technology. The insulating monolayer has to possess perfect insulating properties while the template has to provide electron transfer between electrode and solution. Probably, the mixed nano-structured monolayer consisting of alkylthiol with cavities which are stabilized by the spreader-bar approach [19] is the only known system which meets these requirements. 

From the analysis described above, we now know that a very important molecule that may be adsorbed together with water is OH. Also, this system has been studied quite extensively within surface science [Thiel and Madey, 1987 Bedurftig et al., 1999 Clay et al., 2004 Karlberg and Wahnstrom, 2005]. It appears that a mixed water—OH system forms a hexagonal structure much like the water stmcture discussed above (see Fig. 3.13c, d). Both from DFT calculations and UHV experiments, the most stable stmcture appears to be that where every other molecule is water and every other OH. This is interesting, since it coincides with the electrochemical observation, discussed above, where the maximum OH coverage was measured to be about one-third of a monolayer [Stamenkovic et al., 2007a]. 

---