Modifications, electrochemically induced

The topics are classified phenomenologically (Fig. 3) (1) electrochemically addressable polymers in solution, (11) electrochemically induced micellization and demicellization (including vesicles and capsules with switchable porosity), (III) electrochemically addressable hydrogels and microgels, and (IV) thin films for electrode modification (without emphasizing classical electrochemical polymerization). An overview of polymerizations under electrochemical control is given elsewhere [18]. 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

These results are quite interesting. The initial stages of Al deposition result in nanosized deposits. Indeed, from the STM studies we recently succeeded in making bulk deposits of nanosized Al with special bath compositions and special electrochemical techniques [10]. Moreover, the preliminary results on tip-induced nanostructuring show that nanosized modifications of electrodes by less noble elements are possible in ionic liquids, thus opening access to new structures that cannot be made in aqueous media. 

The change in the electronic properties of Ru particles upon modification with Se was investigated recently by electrochemical nuclear magnetic resonance (EC-NMR) and XPS [28]. In this work, it was established for the first time that Se, which is a p-type semiconductor in elemental form, becomes metallic when interacting with Ru, due to charge transfer from Ru to Se. On the basis of this and previous results, the authors emphasized that the combination of two or more elements to induce electronic alterations on a major catalytic component, as exemplified by Se addition on Ru, is quite a promising method to design stable and potent fuel cell electrocatalysts. 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Treutler, T.H., and G. Wittstock. 2003. Combination of an electrochemical tunneling microscope (ECSTM) and a scanning electrochemical microscope (SECM) Application for tip-induced modification of self-assembled monolayers. Electrochim. Acta 48 2923-2932. 

In-situ SPV measurements seem possible with minor modifications (1) the tip potential (versus the reference) is set at a value close to the rest potential of the semiconductor in darkness (this must be compatible with the electrochemical response of the tip), and (2) the tip current is quenched by adjusting the sample voltage (versus the reference) with the second feedback system. With p-type materials the method seems more obvious than with n-type specimens, since illumination promotes surface electrons. At n-type materials SPV measurements will induce corrosion since holes are driven to the interface. If absolute measurements of the SPV seem difficult, because they depend on the adjustment of the tip potential, differential measurements appear accessible to experiment. 

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