Electrode surface images

Here, co and Ss are the optical and the static dielectric constants of the solution the former appears because the contribution from the electronic polarizability has been subtracted. Dqx and D ed are the dielectric displacements when the reactant is in the reduced and in the oxidized form, respectively. The integral is to be performed over the space filled by the solution. When the reactant is close to the electrode surface, image terms arise, which contrihute to the displacement. 

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

In the following section, we focus on imaging single-crystal electrode surfaces that are of relevance to electrocatalysis. We will first deal with flat, defect-free terraces as well as with more real surfaces with monoatomic high steps as the most common active sites. We will then explore various strategies for nano structuring surfaces, for example, by repetitive oxidation-reduction cycles (ORCs). 

For in situ investigations of electrode surfaces, that is, for the study of electrodes in an electrochemical environment and under potential control, the metal tip inevitably also becomes immersed into the electrolyte, commonly an aqueous solution. As a consequence, electrochemical processes will occur at the tip/solution interface as well, giving rise to an electric current at the tip that is superimposed on the tunnel current and hence will cause the feedback circuit and therefore the imaging process to malfunction. The STM tip nolens volens becomes a fourth electrode in our system that needs to be potential controlled like our sample by a bipotentiostat. A schematic diagram of such an electric circuit, employed to combine electrochemical studies with electron tunneling between tip and sample, is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution... 

The conductance image of the tested cathode reveals that almost the entire electrode surface became insulating except for a few locations, mainly in the deep crevices and intergranular spaces, which remained conductive. 

The scanning tunneling microscope (STM) is an excellent device to obtain topographic images of an electrode surface [1], The principal part of this apparatus is a metal tip with a very fine point (see Fig. 15.1), which can be moved in all three directions of space with the aid of piezoelectric crystals. All but the very end of the tip is insulated from the solution in order to avoid tip currents due to unwanted electrochemical reactions. The tip is brought very close, up to a few Angstroms, to the electrode surface. When a potential bias AF, usually of the order... 

The Scanning Tunneling Microscope has demonstrated unique capabilities for the examination of electrode topography, the vibrational spectroscopic imaging of surface adsorbed species, and the high resolution electrochemical modification of conductive surfaces. Here we discuss recent progress in electrochemical STM. Included are a comparison of STM with other ex situ and in situ surface analytic techniques, a discussion of relevant STM design considerations, and a semi-quantitative examination of faradaic current contributions for STM at solution-covered surfaces. Applications of STM to the ex situ and in situ study of electrode surfaces are presented. 

Predictably, the first STM studies of electrode surfaces were ex-situ investigations of electrodes that were prepared in solution and subsequently imaged either in vacuo or in air. Several representative examples, illustrating the uses of STM to date, are discussed in this section. 

One of the most significant applications of STM to electrochemistry would involve the application of the full spectroscopic and imaging powers of the STM for electrode surfaces in contact with electrolytes. Such operation should enable the electrochemist to access, for the first time, a host of analytical techniques in a relatively simple and straightforward manner. It seems reasonable to expect at this time that atomic resolution images, I-V spectra, and work function maps should all be obtainable in aqueous and nonaqueous electrochemical environments. Moreover, the evolution of such information as a function of time will yield new knowledge about key electrochemical processes. The current state of STM applications to electrochemistry is discussed below. 

Using the unique four-electrode STM described above, Bard and coworkers (Lev, 0. Fan, F-R.F. Bard, A.J. J. Electroanal. Chem.. submitted) have obtained the first images of electrode surfaces under potentiostatic control. The current-bias relationships obtained for reduced and anodically passivated nickel surfaces revealed that the exponential current-distance relationship expected for a tunneling-dominated current was not observed at the oxide-covered surfaces. On this basis, the authors concluded that the nickel oxide layer was electrically insulating, and was greater than ca. 10 A in thickness. Because accurate potential control of the substrate surface is difficult in a conventional, two-electrode STM configuration, the ability to decouple the tip-substrate bias from... 

Sonnenfeld, R. SPIE 88 Conference Proceedings, in press) have both reported in situ STM images of electrode surfaces on which metals have been electrochemically deposited. Morita et. al. (61) have examined Ag surfaces in situ in the presence of chloride ion. 

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