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to the Pt(110) electrode is complicated by the fact that the adatom oxidation processes
on this electrode take place at relatively high potentials, and severe interference from
the surface oxidation of the substrate is expected. Moreover, oxidation/reduction
cycles induce disordering of the surface, precluding a study with proper control of
the surface structure [Clavilier et al., 1988; Hayden et al., 1996].
In some cases, the maximum coverage of the adatom and the stoichiometry of the
reaction have been corroborated by independent techniques, normally involving the
transfer of the modified surface to ex situ conditions.
Ex situ scanning tunneling microscopy (STM) measurements demonstrated that the
maximum As coverage on Pt(111) is 0.33, corresponding to a (
p
)R308 struc-
ture [Orts et al., 1997]. Moreover, in situ infrared measurements showed that the
participation of adsorbed anions during the surface redox process could be ignored
[Orts et al., 1997]. This last result validates the above explained analysis of charge den-
sities, since the participation of anion adsorption/desorption, coupled to adatom oxi-
dation, could lead to some uncertainty in the estimation of surface coverages from
coulometric measurements. Another system that has been characterized by STM,
this time in situ, is Te on Pt(111) [Rhee and Kim, 2001]. In this case, two structures
were found for the reduced state at saturation coverage, namely (2
2) and (11
8),
corresponding to coverages 0.25 and 0.284, respectively. These values of the Te maxi-
mum coverage agree well with the conclusions extracted from the coulometric data,
u
Te,max
ΒΌ 0.25 [Feliu et al., 1993b]. In the oxidized state, a square (
p
p
p
) lattice
was detected, and interpreted as the intermixing of two (2
3
p
) rectangular struc-
tures, for Te and O. The same structures could also be identified by ex situ STM exper-
iments for Pt(111) and Pt(111) stepped surfaces [Rodriguez et al., 2006]. For the
Pt(100) basal plane and vicinal surfaces, a c(2
2) square structure was imaged
with ex situ STM, which also confirms the maximum coverage of 0.5 obtained by cou-
lometry [Rodriguez et al., 2006].
The valence state of different adatoms and its dependence on the electrode potential
has been monitored in several studies for As, Bi, and Te by means of X-ray photo-
electron spectroscopy (XPS) measurements [Hamm et al., 1998; Schmidt et al.,
2000b; Zhou et al., 2002, 2004]. A clear limitation of this approach is the uncertainty
that arises during the emersion of the double layer and subsequent transfer to the ultra-
high vacuum (UHV) environment. It was recognized by the authors of these works that
some decomposition of the oxidized species can take place during the emersion, and
therefore, some caution should be applied in the quantitative analysis of the results.
However, the technique has proven valuable in demonstrating that a change of the
oxidation state of the adatoms takes place during the redox process observed in the vol-
tammetric profile. For As on Pt(111) [Zhou et al., 2004], it was confirmed that the
valence state of the ad-species changes during the voltammetric redox process from
As(0) to As(III). Similarly, for Te on Pt(111) [Zhou et al., 2002], a change from
Te(0) to Te(IV) was observed, in accordance with the electrochemical measurements.
On the other hand, contrasting results were obtained for Bi on Pt(111) [Hamm et al.,
1998; Schmidt et al., 2000b], since for this system no change in the valence state of the
adatom was observed and only elemental Bi was measured, regardless of the emersion
potential. Clearly, this result has to be taken with great caution, since a change of the
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