The influence of the surface structure of electrodes on electrochemical processes has been a central topic of basic electrochemical research in recent years. Advances were mainly achieved due to the utilization of structurally defined electrodes, usually low-index single-crystal surfaces, and to the development of surface-sensitive analytical techniques for the in-situ characterization of electrodes [1, 2]. Most of the advances can be attributed to the application of the latter, the new techniques, to the former, namely the well-defined surfaces. This approach has proven very powerful and its exploitation is still in the beginning, but it is already clear that structural effects on electrochemical processes in general can not be related to the local atomic scale order alone, but are also to a large extent affected by mesoscopic structural properties. Control of the mesoscopic structural properties is required in order to elucidate their influence on electrochemical reactivity.

The atomic surface order is described in terms of a simple unit cell and techniques for the preparation of surfaces with defined atomic order are well established. The description of mesoscopic structures is not as straightforward; for a single-crystal surface mesoscopic properties can be, e. g., terrace widths and step densities, for dispersed electrodes the size and distribution of particles. Real-space information under in-situ electrochemical conditions is required for the characterization of such mesoscopic properties. This information can only be derived from the application of scanning probe techniques, which were introduced to electrochemistry in the mid-1980s and give high-resolution real-space images of electrode surfaces under in-situ electrochemical conditions.

We want to focus on the modification of substrate electrodes with metal particles of mesoscopic dimensions and their characterization with STM. For such chemically heterogeneous surfaces, the chemical information complementing the structural investigation is required. The different chemical compositions of the constituents, namely the substrate and the deposit, is often manifest in different characteristic mesoscopic structures of both. Thus, although the chemical selectivity of STM is rather poor, this chemical information can often be concluded from the mesoscopic structure.

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