Nucleation, Growth, and
Adsorbate-Induced Changes
in Composition for Co–Au Bimetallic Clusters on TiO<sub>2</sub>
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Abstract
The nucleation, growth, and CO-induced changes in composition
for
Co–Au bimetallic clusters deposited on TiO<sub>2</sub>(110)
have been studied by scanning tunneling microscopy (STM), low energy
ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed
desorption (TPD), and density functional theory (DFT) calculations.
STM experiments show that the mobility of Co atoms on TiO<sub>2</sub>(110) is significantly lower than of Au atoms; for equivalent or
lower coverages of Co, the number of clusters is higher and the average
cluster height is smaller than for Au deposition. Consequently, bimetallic
clusters are formed by first depositing the less mobile Co atoms,
followed by the addition of the more mobile Au atoms. Furthermore,
the reverse deposition of Au followed by Co results in clusters of
pure Co coexisting with clusters that are Au-rich. For clusters with
a total coverage of 0.25 ML, the cluster density increases and average
cluster height decreases as the fraction of Co is increased. Annealing
to 800 K results in cluster sintering and an increase of ∼3–5
Å in average height for all compositions. LEIS experiments indicate
that the surfaces of the bimetallic clusters are 80–100% Au
for bulk Au fractions greater than 50%, but Co and Au coexist at the
surfaces when there are not enough Au atoms available to completely
cover the surfaces of the clusters. After heating to 800 K, pure Co
clusters become partially encapsulated by titania, and for bimetallic
clusters, the Co is selectively encapsulated at the cluster surface.
The desorption of CO from the bimetallic clusters demonstrates that
the presence of the CO adsorbate induces diffusion of Co to the cluster
surface, but the extent of this diffusion is less than what is observed
in the Ni–Au and Pt–Au systems. Density functional theory
calculations confirm that for a 50% Co/50% Au bimetallic structure:
the surface is predominantly Au in the absence of CO; CO induces diffusion
of Co to the cluster surface; and this CO-induced diffusion is less
extensive on Co–Au than on the Ni–Au and Pt–Au
surfaces