Verlag des Forschungszentrums Jülich
JUEL-3827
A summary is given or the theoretical background conceming in-situ STM, metal clusters and
their use as model electrodes. The experimental set-up aod methods used are described. First
the structural and electrochemical properties or the substrates are presented. Flame annealed
Au surfaces, Au and Pt single crystal surfaces were used as substrates. It is demonstrated that
clean electrochemical conditions can be achieved in the in-situ STM electrochemical cell by
monitoring the characteristic single crystal sufface voltamogram or Pt(III). Also electro-
oxidation or adsorbed CO is performed under in-situ STM conditions.
Metal clusters were either deposited electrochemically or by electrophoretic adsorption or
colloidal solution. The colloids (Pt, PtSn an PtRu alloy) were stabilized with a ligand shell
aod exhibiting a narrow size distribution. The mean particle distance can be reproducibly
controlled by colloidal solution concentration aod adsorption time or electrochemical
deposition time ror Pt clusters. In case or electrochemical deposition or Ru on Pt( 111) mean particle
distance has been controlled by deposition potential. It is supposed that Ru deposition is
influenced by adsorption or chloride ions.
All metal clusters were electrochemically stable during cyclic voltammetry and CO electro-
oxidation in a wide potential range. There is evidence ror a Pt segregation to the sufface or the
PtSn aod PtRu clusters. This can explain the higher stability or the alloy clusters even at more
positive potentials in contrast to their bulk metal alloy electrodes.
Model electrodes with Pt, PtSn or PtRu clusters all showed a strong dependence on the mean
particle distaoce towards the CO oxidation potential. A shift to more positive potentials was
observed ror a large mean particle distaoce that means ror single isolated clusters on the
substrates compared to bulk metal electrodes. For small mean particle distaoces (high cluster
coverage with cluster agglomerates) normal behavior comparable to bulk electrodes was
round. This was round ror 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 positive CO oxidation potential was interpreted as a
stronger adsorption or CO on isolated Pt clusters compared to Pt bulk and was explained by a
higher number or low coordinated adsorption sites at the clusters, a possible lower CO
coverage on the cluster surface that leads to less repulsive CO-CO interactions which were
known to weaken the Pt-CO bonding. In addition in-situ FTIR measurements at low cluster
coverage revealed also a shift for the linear 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 Ru sites. The CO diffusion coefficient is
estimated to be at least 1 x 10-13 cm2/s by CO transient experiments.
Marmann, Andrea
Untersuchung mesoskopischer Strukturen an der Grenzfläche fest-flüssig
139 S., 2000
Metal clusters have been deposited on metal substrates and characterized with in-situ
electrochemical Scanning Tunneling Microscopy (STM). These sampies are model electrodes with
defined mesoscopic structure aod were used to investigate correlation between structure and
electrochemical behavior or nanometer sized metal clusters. Different size distribution (1,5-9
nm), mean particle distaoce (0-50 nm) and chemical composition (Pt, Ru, PtSn aod PtRu
alloy) have been used. The model electrodes were electrochemically characterized by cyclic
voltammetry and by electrooxidation or adsorbed co. Preliminary experiments have been
done to use the tip or 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.
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