Scanning tunneling microscopy electrochemical application

Electrochemistry is the basis of many important and modem applications and scientific developments such as nanoscale machining (fabrication of miniature devices with three dimensional control in the nanometer scale), electrochemistry at the atomic scale, scanning tunneling microscopy, transformation of energy in biological cells, selective electrodes for the determination of ions, and new kinds of electrochemical cells, batteries and fuel cells. 

Refs. [i] Conway BE (1999) Electrochemical processes involving H adsorbed at metal electrode surfaces. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 131-150 [ii] Climent V, Gomez R, Orts JM, Rodes A, AldazA, Feliu JM (1999) Electrochemistry, spectroscopy, and scanning tunneling microscopy images of small single-crystal electrodes. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. MarcelDekker, New York, pp 463-475 [Hi] Calvo E] (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78... 

Three landmark papers on the application of time-dependent perturbation theory to electrochemical problems were published in rapid succession by - Levich and - Dogonadze in 1959 [iii], - Gerischer in 1960 [iv], and McConnell in 1961 [v]. A very large literature has subsequently sprung from these works, driven by developments in scanning tunneling microscopy, molecular electronics, and biological electron transfer. 

SECM involves the measurement of the current through an ultramicroelectrode (UME) (an electrode with a radius, a, of the order of a few nm to 25 (zm) when it is held or moved in a solution in the vicinity of a substrate. Substrates, which can be solid surfaces of different types (e.g., glass, metal, polymer, biological material) or liquids (e.g., mercury, immiscible oil), perturb the electrochemical response of the tip, and this perturbation provides information about the nature and properties of the substrate. The development of SECM depended on previous work on the use of ultramicroelectrodes in electrochemistry and the application of piezoelectric elements to position a tip, as in scanning tunneling microscopy (STM). Certain aspects of SECM behavior also have analogies in electrochemical thin-layer cells and arrays of interdigitated electrodes. 

Summary. The importance of flie electrode surface structure in electrochemistry is briefly described. Examples are given in which the structural information provided by scanning tunneling microscopy (STM) is of assistance in clarifying the electrochemical behavior. The importance of surface structure in the photoelectrochemical response of metals is illustrated by an STM application. Finally, the potentialities of newr scanning microprobe techniques suitable for mapping local photoelectrochemical properties of metal surfaces are briefly discussed. 

The development of local probe techniques such as Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) and related methods during the past fifteen years (Nobel price for physics 1986 to H. Rohrer and G. Binning) has opened a new window to locally study of interface phenomena on solid state surfaces (metals, semiconductors, superconductors, polymers, ionic conductors, insulators etc.) at an atomic level. The in-situ application of local probe methods in different systems (UHV, gas, or electrochemical conditions) belongs to modem nanotechnology and has two different aspects. 

Surface excesses of electroactive species are often examined by methods sensitive to the faradaic reactions of the adsorbed species. Cyclic voltammetry, chronocoulometry, polarography, and thin layer methods are all useful in this regard. Discussions of their application to this type of problem are provided in Section 14.3. In addition to these electrochemical methods for studying the solid electrode/electrolyte interface, there has been intense activity in the utilization of spectroscopic and microscopic methods (e.g., surface enhanced Raman spectroscopy, infrared spectroscopy, scanning tunneling microscopy) as probes of the electrode surface region these are discussed in Chapters 16 and 17. 

Since the early days of modern surface science, the main goal in the electrochemical community has been to find correlations between the microscopic structures formed by surface atoms and adsorbates and the macroscopic kinetic rates of a particular electrochemical reaction. The establishment of such relationships, previously only developed for catalysts under ultrahigh vacuum (UHV) conditions, has been broadened to embrace electrochemical interfaces. In early work, determination of the surface structures in an electrochemical environment was derived from ex-situ UHV analysis of emersed surfaces. Although such ex-situ tactics remain important, the relationship between the structure of the interface in the electrolyte and that observed in UHV was always problematic and had to be carefully examined on a case-by-case basis. The application of in-situ surface-sensitive probes, most notably synchrotron-based surface X-ray scattering (SXS) [1-6] and scanning tunneling microscopy (STM) [7, 8], has overcome this emersion gap and provided information on potential-dependent surface structures at a level of sophistication that is on a par with (or even in advance of) that obtained for surfaces in UHV. 

Scanning tunneling microscopy (STM) is also a tool for surface morphological studies, which is widely used in situ It is based on the analysis of a tunneling current between a very sharp microscopic tip and the electrode surface caused by a bias potential applied between the two. This method is well established for the study of electrochemical systems. Its advantage over AFM is that it is technically much simpler to use for in situ studies of electrochemical studies, and it obtains better resolution. However, the application of STM to nonaqueous systems is impossible when the electrode surfaces are covered by surface species, which are electrically insulating. 

The explanations based on the crystal structure model of Kossel and Stranski of Fig. 1-35 still require detailed investigations. The development of scanning tunneling microscopy provides a valuable in situ tool to follow the structural details of electrochemical alloy corrosion. Although mechanistic studies of this kind require complicated investigations and a careful preparation technique for single crystal surfaces the application of these methods promises progress in the near future. 

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