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Fig. 7. Dissolution of copper from a polycrystalline gold electrode at E = +25 mV vs.

produced by pulsing the voltage between tip and sample in a Pt4+-containing solution. The image thus represents a highly defined model electrocatalyst, since there is only one catalyst particle with known morphology on the electrode. In principle, the reactivity of the particle could be measured by conventional electrochemical techniques, but a reaction which is ten orders of magnitude faster on platinum than on graphite would be required to compensate the ratio of the surface areas of substrate and catalyst. Nevertheless this approach is supposedly realistic: by preparation of arrays of catalyst particles the surface area of the catalyst can be increased in a controlled way, and the application of scanning electrochemical microscopy (SECM) [17] for the investigation of the reactivity can reduce the influence of the substrate area to a large extent. Experiments in this direction are, however, still in their initial stages.

250 nm

Fig. 8. Pt cluster on HOPG, prepared by a pulse AU = 6 V (tip positive) for 10 )ns in 1 mM HjPtClg.

250 nm

Fig. 8. Pt cluster on HOPG, prepared by a pulse AU = 6 V (tip positive) for 10 )ns in 1 mM HjPtClg.

Interesting questions regarding material stability can also be addressed by nanotechnology. This is demonstrated in Fig. 9, which shows the dissolution of a tip-induced platinum particle on a polycrystalline gold electrode. It should be noted that the process occurs at a potential of 1.7 V (RHE), where vigorous oxygen evolution occurs on platinum electrodes. Due to this gas evolution it is impossible to image electrodes containing substantial amounts of platinum under the same conditions. For a Pt nanoelectrode like that in Fig. 9 the dissipation of the evolved oxygen occurs under hemispherical diffusion conditions with a high rate and thus does not interfere with the imaging process [18]. The local current density of the oxygen evolution reaction is supposedly several milliamps per square centimeter at the Pt nanostructure, three orders of magnitude higher than that of the substrate. The contribution of the nanostructure to the total current is, however, negligible, since it is on the scale of femtoamps whereas while the measured overall current is 5 fiA.

Fig. 9. Dissolution of tip-induced Pt nanostructure from polyciystalline Au substrate. The left-hand image shows the initial morphology. The sequence on the right visualizes the dissolution in 0.1 M HC104 at 1.7 V with time intervals of 5 min.
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