Page 60 - Analytical Electrochemistry 2d Ed - Jospeh Wang

2-2 SPECTROELECTROCHEMISTRY 45

Additional spectroscopic techniques can be used for probing the molecular
structure of electrode±solution interfaces, as desired for understanding the funda-
mentals of electrode surfaces. The focus of these surface techniques is the
correlation of the surface structure with electrochemical reactivity. Such surface-
sensitive analytical tools can be classi®ed as in-situ or ex-situ. In particular, the high
sensitivity of molecular vibrations to the chemical environment has led to the
widespread use of vibrational spectroscopies, such as surface enhanced Raman
scattering (SERS), for monitoring the surface composition before, during, and after
the electrochemical experiment. In these experiments, a small fraction of the photons
exchange energy with the sample and are inelastically scattered, with a change of
their wavelength characteristic of the energy change. Such Raman scattering effect
8
can be enhanced by factors of up to 10 when the compound is adsorbed on the
metallic surface (40). The enhancement process is believed to result from the
combination of several electromagnetic and chemical effects between the molecule
and the metal surface. Since this scattering ef®ciency increases dramatically in the
adsorbate state, Raman spectroelectrochemistry has been used primarily for inves-
tigating species adsorbed on electrodes (41). Another powerful in-situ structural
characterization technique, X-ray adsorption ®ne structure (EXAFS), refers to the
modulation in the X-ray adsorption coef®cient beyond the adsorption edge. Readers
interested in these in-situ techniques are referred to a monograph (42). Scanning
electron microscopy (SEM) represents another widely used technique for obtaining
ex-situ information on the surface morphology and chemical composition (see, for
example, Figure 4-17).
Other powerful ex-situ techniques are based on the detection of charged particles
derived from or interacting with the surface. Among these are low-energy electron
diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron
spectroscopy (XPS), which are carried out in ultrahigh vacuum (UHV). In LEED,
electrons directed at the sample at low energies (20±200 eV) are diffracted to
produce a pattern unique to each substrate and adsorbed layer. In AES, electron
bombardment creates a vacancy in the electronic level close to the nucleus. This
vacancy is ®lled by an electron coming from another electronic level, with the excess
energy being dissipated through ejection of a secondary electron (an Auger
electron). The resulting energy spectrum consists of Auger peaks that are unique
to each element. XPS (also known as ESCA for electron spectroscopy for chemical
analysis) can also provide atomic information about the surface region. In this
technique, electrons are emitted from the sample upon its irradiation with mono-
chromatic X rays. The photon energy is related to the ionization (binding) energy E ,
B
i.e., the energy required to remove the electron from the initial state. Most elements
(with the exception of hydrogen and helium) produce XPS signals with distinct E .
B
In view of the limited penetration of X rays into solids, the technique gives useful
information about surface structures or layers. The appearance of new XPS peaks
can thus be extremely useful for studies of modi®ed electrodes. The reliability of
information gained by such ex-situ analysis depends on knowledge of what happens
during vacuum exposure. Uncertainties associated with potential loss of material
during such exposure have led to renewed emphasis on direct (in-situ) probes.