Electrode-solution interface, structural control

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

Electrolytes The above issue of double layer structure is important to the mechanism of nucleation and growth in ionic liquids, it may therefore be possible to control the structure at the electrode/solution interface by addition of an inert electrolyte. In this respect most Group 1 metals are soluble in most ionic liquids, although it is only generally lithium salts that exhibit high solubility. In ionic liquids with discrete anions the presence of Group 1 metal ions can be detrimental to the deposition of reactive metals such as A1 and Ta where they have been shown to be co-deposited despite their presence in trace concentrations. 

During the last decade the field of electrochemistry has witnessed a very fast progress on the modification of electrode surfaces. From the predominant use of random polymeric structures, prevalent in the electrode modification efforts of the late 70s and early 80s, electrochemists have learnt to control the molecular architecture of the electrode-solution interface to a degree that was clearly out of reach a decade ago. Many electrode modification methods developed recently rely on the use of thiolate self-assembled monolayers (SAMs) [1-3]. These systems offer unparalleled ease of preparation and levels of molecular organization close to those that can be reached with Langmuir-Blodgett film methods. Therefore, electrodes derivatized with unfunctionalized or functionalized alkanethiolate monolayers have been the subject of extensive research work during the last few years [4, 5]. 

Structural control of the electrode/solution interface is a complex problem of fundamental importance in electrochemical sciences (7). To achieve some elements of such control, it would be desirable to impart molecular character onto the otherwise "naked" electrode surface so that, as a result, it might acquire desired catalytic properties, gain some elements of molecular selectivity, or exhibit other desirable molecular characteristics. To accomplish this, electrochemists have explored numerous possibilities of coating the electrode surface with thin films (from a single monolayer to micrometers in thickness) of a wide variety of materials (2). This area of electrochemistry, often referred to as the chemical mo fication of electrodes, is the subject of a number of recent reviews (7-5). 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

An understanding of the properties of liquids and solutions at interfaces is very important for many practical reasons. Some reactions only take place at an interface, for example, at membranes, and at the electrodes of an electrochemical cell. The structural description of these systems at a molecular level can be used to control reactions at interfaces. This subject entails the important field of heterogeneous catalysis. In the discussion which follows in this chapter the terms surface and interface are used interchangeably. There is a tendency to use the term surface more often when one phase is in contact with a gas, for example, in the case of solid I gas and liquid gas systems. On the other hand, the term interface is used more often when condensed phases are involved, for example, for liquid liquid and solid liquid systems. The term interphase is used to describe the region near the interface where the structure and composition of the two phases can be different from that in the bulk. The thickness of the interphase is generally not known without microscopic information but it certainly extends distances corresponding to a few molecular diameters into each phase. 

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