Conclusions and future prospects

The structural characterization of electrode surfaces on the mesoscopic scale is a prerequisite for the elucidation of mesoscopic effects on electrochemical reactivity. The most straightforward approach to access the mesoscopic scale is the application of scanning probes under in-situ electrochemical conditions. Three different applications of STM have been discussed, namely the structural characterization of model electrodes, the visualization of dynamic processes on the nanometer-scale, and the defined modification of electrode surfaces.

For the structural characterization of model electrodes it was shown that on the base of well-defined substrates, composite electrodes with defined mesoscopic structure can be prepared. Rather different methods such as low-efficiency electrochemical deposition or adsorption of colloidal particles can be employed for this purpose, and the effect on the surface morphology can be adequately characterized with STM. Knowledge of the mesoscopic surface properties facilitates the interpretation of results obtained from other techniques, e. g., conventional electrochemical methods or infrared spectroscopy [6], since these are affected by the surface structure but do not contain detailed information about the morphology.

Electrochemical processes which involve structural changes of the electrode morphology, such as deposition and dissolution reactions, can be directly monitored in real time with STM. Such measurements give a visualization of the reactivity of the sample area under investigation. Well-defined substrate surfaces are not required for this approach. In principle, the local experiment can be performed on sections of the substrate surface with interesting structural features. An important point is that the measurement may also give mesoscopic chemical information; structures, which grow and shrink at potentials negative and positive, respectively, of the Cu/Cu2+ equilibrium potential can be identified as copper.

The application of STM is not restricted to analytical applications, but it can also be applied for a defined modification of electrode surfaces on the nanometer scale. At present the creation and manipulation of mesoscopic structures are still at an exploratory stage, but they gain increasing attention and may soon develop to an established nanotechnology. The stability of such structures, which is crucial for technical applications, can be investigated with STM under in-situ electrochemical conditions. Regarding the correlation of structure and reactivity, a synthetic approach can be envisaged since defined distributions of defined catalyst particles are accessible.

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