14.2 Oxide Tetrahedron Bond Formation
273
transportation between oxygen and metals, the lone pairs and hydrogen-bond like
formation in oxygen-chemisorbed systems has received little attention.
At the initial stage of oxidation, oxygen molecule dissociates and interacts with
metal atoms through one bond. The O
−1 specifies a position where the O
−1 bonds
directly to one of its neighbors. For the transition metals of lower electronegativity
(χ < 2) and smaller atomic radius (<1.3 Å), such as Cu and Co, O often bonds to a
neighbor at the surface; while for noble metals of higher electronegativity (χ > 2) and
larger atomic radius (>1.3 Å), such as Rh and Pd, O tends to sink into the hollow site
and forms the first bond to its neighbor underneath [30]. The O
−1 also polarizes other
neighbors and pushes the metal dipoles radially away the adsorbate. This process
also leads to the STM protrusions and creates antibonding dipole states but different
morphologies.
14.2.4 O–Cu(001) Bond Geometry and Atomic Valency
For the particular O–Cu(001) system, two phases present during the reaction. One
is the nanometric Cu(001)-c(2 × 2) − 2O
−1 domain with protruding boundary that
occurs when the surface exposes to an oxygen exposure lower than 25 L, the other
is the MR type Cu (
√
2×2
√
2) R45° − 2O
−2 structure. Figure 13.1 showed the corresponding STM images of the two phases, which provide the experimental ground
for the bond models discussed below.
14.2.4.1 Precursor: CuO 2 Pairing Pyramid
Figure 14.4 illustrates a single Cu(I)O(Cu
+1
+ O
−1 ) pyramid structure, part of the c(2
× 2) − 2O
−1 complex unit cell. The unit cell can also be illustrated as a CuO 2 paring
pyramid (Cu
+2
+ 2O
−1
+ 6Cu
p ). In the precursor state, each of the two oxygen atoms
catches one electron from the same Cu neighbor, or separately from two, to form
one contracting ionic bond, BL1. Meanwhile, the O
−1 polarizes its rest neighbors
that form the protruding domain boundaries. If DO x = 0, the O will be located at the
apex of a centered pyramid to form four identical O–Cu bonds, which is forbidden.
Therefore, the first phase is an off-centered pyramid. The geometrical parameters
(DO x , DO z , BL1, and BL2) of the pyramid can be determined by DO z , and BL1.
14.2.4.2 Cu 3 O 2 Pairing Tetrahedron
Upon increasing oxygen exposure, the pairing-pyramid evolves into a pairingtetrahedron, as shown in Fig. 14.5. Each of the O
−1 ions forms another bond with its
neighboring atom underneath and then the Cu 3 O 2 pairing-tetrahedron forms, being a
case in which “two oxygen get four electrons from three coppers” [39]. Atoms labeled
273
transportation between oxygen and metals, the lone pairs and hydrogen-bond like
formation in oxygen-chemisorbed systems has received little attention.
At the initial stage of oxidation, oxygen molecule dissociates and interacts with
metal atoms through one bond. The O
−1 specifies a position where the O
−1 bonds
directly to one of its neighbors. For the transition metals of lower electronegativity
(χ < 2) and smaller atomic radius (<1.3 Å), such as Cu and Co, O often bonds to a
neighbor at the surface; while for noble metals of higher electronegativity (χ > 2) and
larger atomic radius (>1.3 Å), such as Rh and Pd, O tends to sink into the hollow site
and forms the first bond to its neighbor underneath [30]. The O
−1 also polarizes other
neighbors and pushes the metal dipoles radially away the adsorbate. This process
also leads to the STM protrusions and creates antibonding dipole states but different
morphologies.
14.2.4 O–Cu(001) Bond Geometry and Atomic Valency
For the particular O–Cu(001) system, two phases present during the reaction. One
is the nanometric Cu(001)-c(2 × 2) − 2O
−1 domain with protruding boundary that
occurs when the surface exposes to an oxygen exposure lower than 25 L, the other
is the MR type Cu (
√
2×2
√
2) R45° − 2O
−2 structure. Figure 13.1 showed the corresponding STM images of the two phases, which provide the experimental ground
for the bond models discussed below.
14.2.4.1 Precursor: CuO 2 Pairing Pyramid
Figure 14.4 illustrates a single Cu(I)O(Cu
+1
+ O
−1 ) pyramid structure, part of the c(2
× 2) − 2O
−1 complex unit cell. The unit cell can also be illustrated as a CuO 2 paring
pyramid (Cu
+2
+ 2O
−1
+ 6Cu
p ). In the precursor state, each of the two oxygen atoms
catches one electron from the same Cu neighbor, or separately from two, to form
one contracting ionic bond, BL1. Meanwhile, the O
−1 polarizes its rest neighbors
that form the protruding domain boundaries. If DO x = 0, the O will be located at the
apex of a centered pyramid to form four identical O–Cu bonds, which is forbidden.
Therefore, the first phase is an off-centered pyramid. The geometrical parameters
(DO x , DO z , BL1, and BL2) of the pyramid can be determined by DO z , and BL1.
14.2.4.2 Cu 3 O 2 Pairing Tetrahedron
Upon increasing oxygen exposure, the pairing-pyramid evolves into a pairingtetrahedron, as shown in Fig. 14.5. Each of the O
−1 ions forms another bond with its
neighboring atom underneath and then the Cu 3 O 2 pairing-tetrahedron forms, being a
case in which “two oxygen get four electrons from three coppers” [39]. Atoms labeled