44
Major advances in the understanding of metal adlayer structural behavior have
been achieved by recent applications of in situ scanning probes and surface x-ray
scattering techniques [29, 30]. In general, for high-coverage metal adlayer phases,
which form at potentials close to the reversible potentials of bulk deposition, incommensurate, hexagonal, or quasihexagonal structures are common. The interatomic
distances are potential-dependent, often having values below the distances in bulk
materials. This phenomenon has been termed electrocompression. Commensurate
adlayer structures are less common and usually form with coadsorbed anions.
The structural studies with single-crystal surfaces have clarified to a large extent
the questions of electrosorption valences and partial charge of metal adatoms at
large adatom coverages. Close to the reversible deposition potential, the adatoms
are neutral species. Otherwise, the electrocompression would not be observed. In
low-coverage phases, electrosorption valences below unity may be expected, with
the charge on the adatoms usually compensated by the coadsorbed anions. The work
function of the metal surface is considerably altered by the adsorption of foreign atoms.
Optical properties of metal adlayers were extensively studied in the late 1970s by
reflectance spectroscopy, ellipsometry, surface plasmon excitation, and Mösbauer
spectroscopy (See Refs. [1, 2] and references therein). Pronounced changes of
reflectivity are caused by the UPD adlayers, in particular on substrates that exhibit
large electroreflectance signals, viz., Au, Ag, and Cu. The optical properties of submonolayers and monolayers of metals were found to be markedly different from the
properties of bulk phases. Application of synchrotron X-ray absorption near-edge
structure (XANES) spectroscopy measurements of Pb on Pt show that in the underpotential deposition region the Pb species are essentially neutral Pb atoms [31].
On the basis of data for polycrystalline electrodes, Kolb et al. found a linear
relationship between the UPD effect at θ = 0.2 and the difference in work functions
of the substrate and the adsorbate bulk metal [32]. This correlation does not seem to
be applicable in this simple form for single-crystal electrodes. The electrosorption
valency of foreign adatoms is in the majority of the cases close to the Faradaic
valence, that is, only a small charge may be retained of the adsorbate.
Figure 5.6 shows the under potential deposition of Cu on Au(111), which involves
the first two-thirds of a Cu monolayer deposited in a (√3 × √3) R30° (honeycomb)
structure. The second adsorption step represents the formation of a complete Cu
monolayer in registry with the substrate (1 × 1) structure [33].
Underpotential metal deposition on single-crystal metal substrates has been studied extensively in recent years. Kolb [34] reviewed the whole area, while Adzic [35]
and Kokinidis [36] covered the aspects concerning the electrocatalytic properties of
surfaces modified by foreign metal adatoms. A pronounced influence of the surface
orientation of the substrate on UPD was demonstrated by Adzic et al. for the case of
lead deposition on gold [26]. Several aspects of metal adatom deposits on foreign
substrates have received attention recently. These include structure, kinetics, mechanism of growth, and thermodynamic properties. The exceptionally narrow peak for
the UPD of Pb on Au(111) was explained by a phase transition as a cause of its
appearance. The effect of the lateral attraction between the adatoms is pronounced
on the (111) faces of fcc metals. Coadsorption of some anions caused initially some
discussion.
5 Important Electrosorption Reactions
Major advances in the understanding of metal adlayer structural behavior have
been achieved by recent applications of in situ scanning probes and surface x-ray
scattering techniques [29, 30]. In general, for high-coverage metal adlayer phases,
which form at potentials close to the reversible potentials of bulk deposition, incommensurate, hexagonal, or quasihexagonal structures are common. The interatomic
distances are potential-dependent, often having values below the distances in bulk
materials. This phenomenon has been termed electrocompression. Commensurate
adlayer structures are less common and usually form with coadsorbed anions.
The structural studies with single-crystal surfaces have clarified to a large extent
the questions of electrosorption valences and partial charge of metal adatoms at
large adatom coverages. Close to the reversible deposition potential, the adatoms
are neutral species. Otherwise, the electrocompression would not be observed. In
low-coverage phases, electrosorption valences below unity may be expected, with
the charge on the adatoms usually compensated by the coadsorbed anions. The work
function of the metal surface is considerably altered by the adsorption of foreign atoms.
Optical properties of metal adlayers were extensively studied in the late 1970s by
reflectance spectroscopy, ellipsometry, surface plasmon excitation, and Mösbauer
spectroscopy (See Refs. [1, 2] and references therein). Pronounced changes of
reflectivity are caused by the UPD adlayers, in particular on substrates that exhibit
large electroreflectance signals, viz., Au, Ag, and Cu. The optical properties of submonolayers and monolayers of metals were found to be markedly different from the
properties of bulk phases. Application of synchrotron X-ray absorption near-edge
structure (XANES) spectroscopy measurements of Pb on Pt show that in the underpotential deposition region the Pb species are essentially neutral Pb atoms [31].
On the basis of data for polycrystalline electrodes, Kolb et al. found a linear
relationship between the UPD effect at θ = 0.2 and the difference in work functions
of the substrate and the adsorbate bulk metal [32]. This correlation does not seem to
be applicable in this simple form for single-crystal electrodes. The electrosorption
valency of foreign adatoms is in the majority of the cases close to the Faradaic
valence, that is, only a small charge may be retained of the adsorbate.
Figure 5.6 shows the under potential deposition of Cu on Au(111), which involves
the first two-thirds of a Cu monolayer deposited in a (√3 × √3) R30° (honeycomb)
structure. The second adsorption step represents the formation of a complete Cu
monolayer in registry with the substrate (1 × 1) structure [33].
Underpotential metal deposition on single-crystal metal substrates has been studied extensively in recent years. Kolb [34] reviewed the whole area, while Adzic [35]
and Kokinidis [36] covered the aspects concerning the electrocatalytic properties of
surfaces modified by foreign metal adatoms. A pronounced influence of the surface
orientation of the substrate on UPD was demonstrated by Adzic et al. for the case of
lead deposition on gold [26]. Several aspects of metal adatom deposits on foreign
substrates have received attention recently. These include structure, kinetics, mechanism of growth, and thermodynamic properties. The exceptionally narrow peak for
the UPD of Pb on Au(111) was explained by a phase transition as a cause of its
appearance. The effect of the lateral attraction between the adatoms is pronounced
on the (111) faces of fcc metals. Coadsorption of some anions caused initially some
discussion.
5 Important Electrosorption Reactions