Chemistry Reference
In-Depth Information
Fig. 24 (a) STM image of a 14 ML MgO film. 0.06 ML Au were deposited at room temperature
after electron bombardment of the film. (b) Infrared absorption spectrum of CO adsorbed on a
3 ML MgO/Ag(001) film covered with 0.1 ML Au. (
2
-
8
) IR spectra taken on an electron-
bombarded 15 ML thick MgO/Mo film covered with (
2
) 0.005 ML Au (CO saturation coverage,
100 K), (
3
) 0.05 ML Au (15% CO exposure, 30 K), (
4
) 0.05 ML Au (saturation coverage, 30 K),
and (
5
) 0.05 ML Au (saturation coverage, 100 K). The spectra (
6
-
8
) were taken for 0.1 ML Au
coverage and (
6
) 15 s CO exposure (30 K), (
7
) CO saturation coverage (30 K), and (
8
) after
annealing the sample to 100 K [
66
]
the preferential Au nucleation sites on the oxide [
103
]. After dosing 0.06 ML Au at
room temperature, Au particles develop on the surface that are one to two layers
high and contain 30-50 atoms (Fig.
24a
).
For the IRAS experiments shown in Fig.
24b
, Au deposition was carried out at
30 K, which increases the nucleation density and decreases the mean particle size.
For the given coverage range of 0.1-0.005 ML Au, the size of Au deposits is
expected to vary between a few tens of atoms down to monomers and dimers. The
IRAS data (spectra 2-8) in Fig.
24b
show bands ranging from 2090 cm
1
all the
way down to 1990 cm
1
as well as two isolated lines at 1925 and 1855 cm
1
. All
bands are red-shifted compared to CO adsorbed on Au clusters on pristine MgO
films, which are found between 2125 and 2100 cm
1
depending on the preparation
conditions. The low-frequency lines that only occur at the smallest metal exposure
(Fig.
24b
spectrum 2, gray line) are assigned to Au atoms adsorbed to regular
terrace sites (1855 cm
1
) and F-centers (1925 cm
1
) on the basis of results
discussed above. All other bands can be explained by CO bound to negatively
charged Au aggregates located on top of electron-rich surface defects [
92
]. In
particular, the noisy features at
2011 cm
1
are suggested to originate
from CO adsorption to ultrasmall Au aggregates, e.g., dimers and trimers.
1990 and
