Untersuchung mesoskopischer Strukturen an der Grenzfläche fest-flüssig

Metal clusters have been deposited on metal substrates and cheracterized with in-situ electrochemical Scanning Tunneling Misroscopy (STM). These samples are model electrodes with defined mesoscopic structure and were used to inestigate correlation between structure and electrochemical behavior of nanometer sized metal clusters. Different size distribution (1,5-9 nm), mean particle distance (0-50 nm) and chemical composition (Pt, Ru, PtSn adn PtRu alloy) have been used. The model electrodes were electrochemically characterized by cyclic voltammetry and by electrooxidation of absorbed CO. Preliminary experiments have been done to use the tip of the in-situ STM as an analytical tool in order to investigate single metal clusters. Hydrogen has been electrochemically developed at Pt clusters and was detected as a current at the STM tip. A summary is giben of the theoretical background concerning in-situ STM, metal clusters and their use as model electrodes. The experimental set-up and methods used are described. First the structural and electrochemical properties of the substrates are presented. Flame annealed Au surfaces, Au and Pt single crystal surfaces were used as substrattes. It is demonstrated that clean electrochemical conditions can be achieved in the in-situ STM electrochemical cell by monitoring the characteristic single crystal surface voltamogram of Pt(111). Also electrooxidation of adsorbed CO is performed under in-situ STM conditions. Metal clusters were either deposited electrochemically or by electrophoretic adsorption of colloidal solution. The colloids (Pt, PtSn and PtRu alloy) were stabilized with a ligand shell and exhibiting a narrow size distribution. The mean particle distance can be reproducibly controlled by colloidal solution concentration and adsorption time of electrochemical deposition time for Pt clusters. In case of electrochemical deposition of Ru on Pt(111) mean particle distance has been controlled by deposition potential. It is supposed that Ru deposition is influenced by adsorption of chloride ions. All metal clusters were electrochemically stable during cyclic voltammetry and CO electrooxidation in a wide potential range. There is evidence for a Pt segregation to the surface of the PtSn and PtRu clusters. This can explain the higher stability of the alloy clusters even at more positive potentials in contrast to their bulk metal alloy electrodes. Model electrodes with Pt, PtSn of PtRu clusters all showed a strong dependence on the mean particle distance towards the CO oxidation potential. A shift to more positive potentials was abserved for a large mean particle distance that means for single isolated clusters on the substrates compare to bulk metal electrodes. For small mean particle distances (high cluster coverage with cluster agglomerates) normal behavior comparable to bulk electrodes was found. This was found for all studied size distributions from 1,5-9 nm and was called general particle size effect. Between the different size distributions no specific particle size effect was detected. The observed shift to more positice CO oxidation potential eas interpreted as a stronger adsorption of CO on isolated Pt clusters compared to Pt bulk and was explained by a higher number of low coordinated adsoption sites at the clusters, a possible lower CO coverage on the cluster surfave that leads to less repulsive CO-CO interactions which were known to weaken the Pt-Co bonding. In addition in-sitz FTIR measurements at low cluster coverage revealed also a shift for the lineat bonded CO on Pt cluster which can be interpreted as different electronic properties compared to bulk Pt. A shift of the CO oxidation to more negative potentials for Ru clusters on Pt(111) is explained by a sufficient fast CO diffusion from not active Pt sites to active Pt sites close to Rusites. The CO diffusion coefficient is estimated to be at least 1 x 10$^{-13}$ cm$^{2}$/s by CO transient experiments