Metal electrodes, scanning electrochemical

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

Basame, S.B., and H.S. White. 1999. Scanning electrochemical microscopy of metal/metal oxide electrodes. Analysis of spatially localized electron-transfer reactions during oxide growth. Anal. Chem. 71 3166-3170. 

Scanning electrochemical microscopy seeks to overcome the lack of sensitivity and selectivity of the probe tip in STM and AFM to the substrate identity and chemical composition. It does this by using both tip and substrate as independent working electrodes in an electrochemical cell, which therefore also includes auxiliary and reference electrodes. The tip is a metal microelectrode with only the tip active (usually a metal wire in a glass sheath). At large distances from the substrate, in an electrolyte solution containing an electroactive species the mass-transport-limited current is therefore... 

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... 

Thin layer concepts may also be involved in metal deposition into thin films of mercury during stripping analysis, in (electro)chemical processes in adsorbates, films or precipitates on modified electrodes, or in scanning electrochemical microscopy. 

Figure 22. The tunneling current, I, measured between two metal electrodes (tungsten en platinum) separated by a vacuum barrier as a function of the difference in electrochemical potential (here denoted as C/emitter-anode) the distance between the two electrodes (12, 20, 17 A) is indicated in the figure. Reprinted from Scanning Probe Microscopy and Spectroscopy , R. Wiesendanger, Cambridge University Press 1994...

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

Scanning electrochemical microscopy (SECM), which has an intermediate resolution between that of conventional lithography and AFM/STM, is useful for mapping the electrochemically active areas of electronically conductive patterns of the films [38] it also can be used for the special mode of coating of the surface of metallic electrodes with ECP films by electroless deposition [39]. 

Many interesting processes occurring at the liquid/liquid interface involve coupled homogeneous chemical reactions. In principle, electrochemical methods used for probing complicated mechanisms at metal electrodes (61) can be employed at the ITIES. However, many of these techniques (e.g., rotating ring-disk electrode or fast-scan cyclic voltammetry) are hard to adapt to liquid/liquid measurements. Because of technical problems, few studies of multistep processes at the ITIES have been reported to date (1,62). 

As a scanning electrochemical technique, the SKP is able to detect an electrolytic conductance along the polymer/metal interface between the defect border (negative potential) and the polymer-coated area. When the ions diffuse into the polymer/metal interface, the interfacial resistance is lowered in the respective area and the corresponding potential is shifted to more negative values - ideally to the electrode potential of the defect. Based on this shift of the interfacial electrode potential, the transport of ions along the interface can be studied to a very high accuracy. 

Once solid microparticles of a compound have been immobilized on the surface of a suitable electrode, the electrochemical behavior can be studied. When a voltammetric method is used, the starting potential should be carefully set to a value where no reaction is expected. Alternatively, one may start at the open circuit potential, which either has been predetermined or which is measured by the instrument before commencing the scan. In principle, one can record the oxidation and the reduction response of the compound. In many cases it is useful to perform a pre-electrolysis of the compound, either a reduction or oxidation, followed by recording the response of the reverse processes. Thus metal sulfide or metal oxide particles can be converted to metal particles, which are oxidized in a follow-up scan to yield metal-specific signals, very much like in stripping voltammetry of solutions. Practically, there is no limitation with respect to electrochemical measurements that can be carried out with immobilized particles. 

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