UHV-Electrochemistry

The term UHV-electrochemistry refers to combined ultra-high-vacuum and electrochemical studies of electrode-electrolyte 

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Electrochemical properties of Ni(lll) prepared and characterized in UHV were examined in 0.1 H KOH by cyclic voltammetry using an UHV-electrochemistry transfer system. A saturation coverage (0.5) of CO was used to protect the Ni(lll) from possible contamination during the transfer. The results indicated that the CO layer remained intact up to the moment of contact with the electrolyte and could be further electro-oxidized. Only one monolayer of Ni metal was involved in the electrochemically formed film in the first voltammetric cycle and less than two monolayers upon repeated cycling. Surfaces which were not protected by CO displayed essentially the same behavior. 

Korzeniewski C 1997 Infrared spectroscopy in electrochemistry new methods and connections to UhV surface science Crit. Rev. Anai. Chem. 27 81... 

Owing to the rapid development of the field from an experimental point of view, and the persistence of discussions on some of the aspects outlined above, a chapter on the pzc that includes a discussion of the relation between the electrochemical and the ultrahigh vacuum (UHV) situation in reference to the conditions at the pzc seems timely. This review of the literature will not be exhaustive but selective, taking into account the compilations already existing. In any case, the objective is to evaluate the existing data in order to recommend the most reliable. Finally, the data on pzc will be discussed in comparison with electron work function values. The role and significance of work functions in electrochemistry were discussed by Trasatti6 in 1976. 

Although liquid Hg would never be used as a reference (model) surface in surface physics because its liquid state and high vapor pressure do not allow appropriate UHV conditions, this metal turns out to be a reference surface in electrochemistry for precisely the same reasons reproducibility of the surface state, easy cleaning of its surface, and the possibility of measuring the surface tension (surface thermodynamic conditions). In particular, the establishment of a UHV scale for potentials is at present based on data obtained for Hg. 

In principle, a measurement of upon water adsorption gives the value of the electrode potential in the UHV scale. In practice, the interfacial structure in the UHV configuration may differ from that at an electrode interface. Thus, instead of deriving the components of the electrode potential from UHV experiments to discuss the electrochemical situation, it is possible to proceed the other way round, i.e., to examine the actual UHV situation starting from electrochemical data. The problem is that only relative quantities are measured in electrochemistry, so that a comparison with UHV data requires that independent data for at least one metal be available. Hg is usually chosen as the reference (model) metal for the reasons described earlier. 

While the measurement of the work function is losing importance in UHV studies (because other more specific techniques have been developed), such a quantity retains its role in electrochemistry because it is intimately related to the electrode potential. A major problem is thus the dichotomy between samples for which is known but not and vice versa. This is one of the major obstacles to the unambiguous interpretation of Eam0- plots. However, this point has been recently addressed in a few cases and the outcome has allowed us to clarify some debated aspects. It is now well established that within a major group of sp- and sd-metals AX (the decrease in 4> as the metal comes in contact with the solution) increases as

[Pg.190]

Non-situ and ex situ studies can provide important information for understanding the properties of metal/electrolyte interfaces. The applicability of these methods for fundamental studies of electrochemistry seems to be firmly established. The main differences between common electrochemical and UHV experiments are the temperature gap (ca. 300 vs. 150 K) and the difference in electrolyte concentration (very high concentrations in UHV experiments). In this respect, experimental research on double-layer properties in frozen electrolytes can be treated as a link between in situ experiments. The measurements of the work functions... 

FIGURE 27.39 Schematic diagram of a surface electrochemistry apparatus, showing UHV system, transfer manipulators, and interlocks. 

Besides surface reconstructions induced by heat treatment, potential-induced reconstruction has recently become a topic of interest in electrochemistry. It has been observed that at potentials negative with respect to the potential of zero surface charge, [Kolb, 1996, 2002 Dakkouri, 1997], the reconstructions found under UHV conditions are also stable in contact with an electrolyte. Although aU low index faces of Au and Pt undergo potential-induced reconstruction, it has been particularly well characterized for Au(lOO) (Fig. 5.5). 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

Figure 2.116 Work function

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

The good agreement between electrochemical and UHV data, documented in Figure 4, is a very important result, because it proves for the first time that the microscopic information which one obtains with surface science techniques in the simulation studies is indeed very relevant to interfacial electrochemistry. As an example of such microscopic information, Figure 5 shows a structural model of the inner layer for bromide specific adsorption at a halide coverage of 0.25 on Ag 110 which has been deduced from thermal desorption and low energy electron diffraction measurements /12/. Qualitatively similar models have been obtained for H2O / Br / Cu( 110) /18/and also for H2O/CI /Ag 110. ... 

The unique aspect of electrochemistry lies in the ability to change the electrode potential and thus concentrate an applied perturbation right at the interface. Electric fields of 10 V/cm can be generated electrochemically with a half-lemon, scraped zinc (since 1983) penny, and copper wire as opposed to the massive Van de Craaff generator and electric power plant required for non-electrochemical approaches to the same field strength. If UHV models are to provide useful molecular-scale insight into electrochemistry, some means of controlling the effective electrode potential of the models must be developed. 

Nevertheless, the cryogenic UHV models remain models, the relevance of which to real electrochemistry remain incompletely established. We can generate a extensively hydrated ionic environment in UHV, but is it the correct one Grounds for optimism on this point have been demonstrated in Ref. 25 and discussed less concretely in Ref. 3. Frozen ele trolyte work (2) has demonstrated considerable mechanistic continuity between normal electrochemistry and electrochemistry in a perchloric acid hydrate at temperatures down to 150 K, but how general is this result ... 

Two of the most common UHV-spectroscopic methods used in electrochemistry are briefly described next, and Fig. 6.10 lists other ex situ techniques, which can be reviewed in the literature by the inquisitive student. 

The main objective of this work was to examine che electrochemical properties of Ni(lll) surfaces tn alkaline solutions using a UHV-electrochemical transfer system. A general introduction co Ni electrochemistry and the motivation of this work have been provided in che first chapter. In chapter II the transfer system involved in the Ni(lll) project and some other related experimental aspects have been described in detail. In this chapter the experimental results will be reported and discussed. 

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