Environmental Engineering Reference
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8.2.2 Surface Structure and Surface Composition
Besides SXS, another matching technique that has become available to surface scien-
tists in the last two decades is scanning tunneling microscopy (STM). This technique
has proved to be a valuable tool in different environments such as UHV, ambient
pressure, and liquids. A large number of studies have been published on results
obtained with STM, providing deep insight into atomic structure [Binning et al.,
1982], corrosion [Zhang and Stimming, 1990; Oppenheim et al., 1991], surface
defects, and thin film growth [Hosler et al., 1986; Sieradzki et al., 1999], for example.
In electrocatalysis, STM became established in the early 1990s [Bard et al., 1991], and
is now widely accepted as a routine tool for the characterization of solid - liquid elec-
trified interfaces [Bard et al., 1993]. In addition, STM has been successfully used for
bimetallic surfaces in order to obtain atomic resolution with the ability to distinguish
between different elements [Varga and Schmmid, 1999]. Two examples are mentioned
here: the work on Pt 25 Rh 75 (100) by Varga's group [Hebenstreit et al., 1999] and the
study of Pd-Au surfaces by Behm's group [Maroun et al., 2001]. Varga and co-workers
used STM images with chemical contrast that allowed direct determination of the
surface composition and short-range order of various Pt 25 Rh 75 single-crystal surfaces
in vacuum (Fig. 8.3). For all crystallographic orientations, they found a strong Pt
enrichment at the surface. In the other example, Behm and co-workers described the
evaluation of catalytic properties of defined atomic ensembles in atomically flat
PdAu(111). Catalytic behavior was correlated with the concentration and distribution
of Pd ad-atoms, which were determined by in situ STM with chemical contrast.
The latter report demonstrated the unique ability of this technique to resolve surface
structure as well as surface composition at the electrified solid - liquid interfaces. In
particular, STM has become an important tool for ex situ and in situ characterization
of surfaces at the atomic level, in spite its significant limitations regarding surface
composition characterization for bimetallic systems, such as the lack of contrast for
different elements and the scanned surface area being too small to be representative
for the entire surface. To avoid these limitations, STM has been mostly used as a
complementary tool in surface characterization.
Figure 8.3 STM image with chemical contrast of the Pt 25 Rh 75 (100) surface. Arrows point at
brighter Rh atoms at the step edge. (Reprinted with permission from Hebenstreit et al. [1999].
Copyright 1999. Elsevier.)
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