30
× (100), and (911) = 5(100) × (111), are shown. The (755) and (911) surfaces were
obtained by cutting the crystal at 6.5° and 9.5° with respect to the (111) plane for
the two first, but toward different directions, and at 9.0° with respect to the (100)
plane for the latter. The (997) and the (755) have (111) terraces of different lengths
and monoatomic (111) and (100) steps, respectively. A more convenient nomenclature for these high-index faces, which indicates better their structures, is also given
in Fig. 4.3. This is equivalent to a high Miller index and has the form [m(hkl)
n(h
I
k
I
l
I
)], where the first part designates a terrace of Miller index (hkl) with m infinite atomic rows and the second part indicates a step of Miller index (h
I
k
I
l
I
) and n
atomic layers high. The three basal planes, (100), (111), and (110), define the vertices of a stereographic triangle.
LEED studies of high Miller index surfaces have revealed that they are composed of low Miller index surfaces separated by monatomic steps. A determination
of the structure of such surfaces is possible because of the periodicity of steps and/
or kinks. The terrace–ledge–kink (TLK) model of such surfaces was introduced by
Kossel [3] and Stranski [4]. It appears that steps of monatomic height provide minimum surface free energies for stepped surfaces, and therefore, they are prevalent on
clean high-index surfaces [5].
On (111) surfaces of fcc metals, the adsorbates can adsorb at the top, bridge, and
threefold sites. The fourfold hollow sites exist on (100) and (110) surfaces. The
(110) surface has a lower density than either the (100) or (111) planes (Fig. 4.1). It
forms a rectangular lattice; the two sides of the rectangle are a and a/2, where a is
the unit cell length. The resulting structure has characteristic grooves in one
direction.
Atoms in steps have lower coordination than atoms in terraces. Thus, they have
higher reactivity and stepped surfaces for many reactions are more active than any
of low-index planes (oxidation of methanol, ethanol, CO, water). A notable exception is oxygen reduction reaction (ORR), which is slower at stepped surfaces
because highly reactive steps become oxidized by water (forming PtOH) and thus
unavailable for the oxygen reduction reaction (see Sect. 6.1). Due to the broken
bonds, the atoms in the topmost layer tend to pair up in order to minimize the surface energy. As a result, a contraction of atoms occurs in the surface plane, which
leads to reconstruction and the appearance of a new surface unit cell. A notable
example is the reconstruction of the (100) lattice of several noble metals (Au, Pt, Ir)
from (1 × 1) (100) symmetry to (5 × 20) (100) lattice.
For the metal/gas interface, it is established that adsorption reverses the reconstruction, bringing, for example, the (100)–(5 × 20) back to (100)–(1 × 1) [6].
Electrochemically induced reconstructions from the bulk (1 × 1) symmetry to
(5 × 20) and (1 × 23) have been demonstrated for Au(I00) and Au(111) [7].
4 Studies of Electrocatalytic Reactions
× (100), and (911) = 5(100) × (111), are shown. The (755) and (911) surfaces were
obtained by cutting the crystal at 6.5° and 9.5° with respect to the (111) plane for
the two first, but toward different directions, and at 9.0° with respect to the (100)
plane for the latter. The (997) and the (755) have (111) terraces of different lengths
and monoatomic (111) and (100) steps, respectively. A more convenient nomenclature for these high-index faces, which indicates better their structures, is also given
in Fig. 4.3. This is equivalent to a high Miller index and has the form [m(hkl)
n(h
I
k
I
l
I
)], where the first part designates a terrace of Miller index (hkl) with m infinite atomic rows and the second part indicates a step of Miller index (h
I
k
I
l
I
) and n
atomic layers high. The three basal planes, (100), (111), and (110), define the vertices of a stereographic triangle.
LEED studies of high Miller index surfaces have revealed that they are composed of low Miller index surfaces separated by monatomic steps. A determination
of the structure of such surfaces is possible because of the periodicity of steps and/
or kinks. The terrace–ledge–kink (TLK) model of such surfaces was introduced by
Kossel [3] and Stranski [4]. It appears that steps of monatomic height provide minimum surface free energies for stepped surfaces, and therefore, they are prevalent on
clean high-index surfaces [5].
On (111) surfaces of fcc metals, the adsorbates can adsorb at the top, bridge, and
threefold sites. The fourfold hollow sites exist on (100) and (110) surfaces. The
(110) surface has a lower density than either the (100) or (111) planes (Fig. 4.1). It
forms a rectangular lattice; the two sides of the rectangle are a and a/2, where a is
the unit cell length. The resulting structure has characteristic grooves in one
direction.
Atoms in steps have lower coordination than atoms in terraces. Thus, they have
higher reactivity and stepped surfaces for many reactions are more active than any
of low-index planes (oxidation of methanol, ethanol, CO, water). A notable exception is oxygen reduction reaction (ORR), which is slower at stepped surfaces
because highly reactive steps become oxidized by water (forming PtOH) and thus
unavailable for the oxygen reduction reaction (see Sect. 6.1). Due to the broken
bonds, the atoms in the topmost layer tend to pair up in order to minimize the surface energy. As a result, a contraction of atoms occurs in the surface plane, which
leads to reconstruction and the appearance of a new surface unit cell. A notable
example is the reconstruction of the (100) lattice of several noble metals (Au, Pt, Ir)
from (1 × 1) (100) symmetry to (5 × 20) (100) lattice.
For the metal/gas interface, it is established that adsorption reverses the reconstruction, bringing, for example, the (100)–(5 × 20) back to (100)–(1 × 1) [6].
Electrochemically induced reconstructions from the bulk (1 × 1) symmetry to
(5 × 20) and (1 × 23) have been demonstrated for Au(I00) and Au(111) [7].
4 Studies of Electrocatalytic Reactions