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very small amounts of gas (sub-monolayer) are sucient to produce drastic
morphological changes of the sample.
10.3.1.2 O
2
/Rh and O
2
þ
H
2
/Rh
Morphological reconstructions over rhodium after exposure to pure oxygen
gas imaged by FIM have been presented and compared to the field ion
micrographs recorded during the ongoing O
2
þ
H
2
reaction.
55,56
Despite the
different experimental conditions used for the two sets of experiments, the
similarity of the pictures is quite remarkable. Figure 10.7b shows a clean
(001)-oriented sample after field evaporation. Starting from a similar sample,
the Rh crystal is exposed to a dose of 10 000 L at 390 K. After this oxygen
dosing, the removal of some 30 Rh layers by field evaporation is sucient to
restore the nearly hemispherical sample morphology. Heating this hemi-
spherical sample at T
¼
500 K during 30 s and in the absence of gaseous
oxygen, causes the polyhedral reconstruction shown in Figure 10.7l.
57
A
clean Rh crystal does not suffer from reshaping at this temperature, and
thus, the polyhedral reconstruction is taken as a direct indication of re-
versible oxygen sub-surface diffusion. It was therefore proposed that the
dissolved oxygen atoms diffuse from the bulk to the surface and trigger the
reported reconstruction. A much lower dose of 0.5 L of O
2
over a clean Rh tip
at T
¼
500 K in the absence of electric field is also sucient to reach the
same result.
58
During the reconstruction process, the only high-index planes
present in the polyhedrally reconstructed form are of {137}-type. They appear
in pairs on either side of the
h
100
i
zones lines, i.e., lines between the (001)
central pole and the {011} peripheral facets, and remain stable within a
temperature range of 480 K to 530 K. Local reconstructions of the missing-
row type are identified in {011} and {113} planes, which are similar to STM
results obtained with extended single crystal surfaces.
59,60
A ball model
presented in Figure 10.7m illustrates this polyhedral reconstruction. The
fact that a local reconstruction of the missing-row type can be clearly iden-
tified on field emitter tips is interesting from the viewpoint that planes do
not exist independently but may communicate with each other via diffusing
adsorbates. In comparison to Rh, Pd and Ir morphological reconstructions
are due to annealing of samples with pre-adsorbed oxygen and lead to a
faceting with mainly low Miller index facets. In the case of Rh, sub-surface
oxygen can also act in the reconstruction process. Higher Miller index facets
may extend under defined conditions and can be observed during the on-
going catalytic hydrogenation of oxygen.
It is also interesting to notice that in the context of adsorbate-induced
restructuring, the O
2
þ
H
2
reaction on Rh is observed to occur on a tip re-
shaped to a polyhedron.
61
Figure 10.7k shows a field ion micrograph cap-
tured during O
2
þ
H
2
reaction over Rh at T
¼
505 K. The H
2
pressure is kept
suciently high to prevent surface oxidation.
62
Despite a lower resolution as
compared to Figure 10.7l,
d
n
9
r
4
n
g
|
8
.
it is evident that the tip has undergone a
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