valence oxides of transition metals. On most of these oxides, as observed for TiO
2
or SrTiO
3
, new surface states appear in the band gap and usually increase the surface
conductivity of the oxide. The only oxide that has been observed to become more
insulating in the presence of O vacancies is CoO.
Oxygen vacancy defects on most oxide surfaces, maximal valence or
suboxides, display a strong interaction with molecules such as O
2
, H
2
O, CO, or
SO
2
. O
2
molecules are dissociatively adsorbed to form O
2–
ions. After saturation of
the defect sites, a second species, tentatively identified as a doubly ionized mole-
cular species, adsorbs much more weakly on the surface. O
2–
ions adsorbed at the
defect sites interact differently from lattice ions with H
2
O or CO. A possible explana-
tion is that these adsorbed O species, which are probably less ionic than oxygen
ions, do not relieve completely the relaxation of the lattice around the defect site.
Water Adsorption
H
2
O adsorption from water vapor has been extensively studied, first on powdered
samples, more recently on single crystals. Most experiments on powders were
carried out at high vapor pressures (> 1 torr). Experiments on single crystals are
usually performed at low gas pressures (< 10
–6
torr) because of the limitations of
the modern surface science techniques. Powdered samples are highly defective
and contain various crystalline orientations and structures. They are more
representative of the behavior of passive films formed in aqueous solution. Studies
on well-characterized surfaces of single crystals make it possible to investigate the
role of surface defects and surface orientation on the mechanisms of chemisorption.
Results on powders can be summarized as follows:
1) H
2
O is dissociatively adsorbed on metal oxides at room temperature at a
pressure of a few torr to form hydroxyl groups. This reaction of acid-base type is
schematically represented in Figure 17 for a surface oxide exposing both anions
and cations. Two types of OH groups are shown. Those bound to two cations are
strongly polarized, thus loosening the bond to hydrogen. These groups are acidic
in character. The other OH groups bound to one cation are predominantly basic
and easily exchangeable for other anions [116]. The high reactivity of defective
oxides is confirmed by the external hydroxylation in aqueous solution of the films
of NiO formed by Ni passivation [117] or by sputtering deposition [118].
2) Zurface OH may react with water molecules through hydrogen bonds.
One may expect that the same type of interactions occurs at the passive film-aqueous
solution interface and influence the structure of the double layer. The ratio
H
2
O/OH was found to be close to ½ on metal oxides such as MgO, TiO
2
, Fe
2
O
3
,
and Al
2
O
3
[119]. Higher values close to one were observed on ZnO, SnO
2
, and
Cr
2
O
3
[119]. The behavior of hydroxylated powder of Cr
2
O
3
studied by infrared
spectroscopy, adsorption isotherms from water vapor, and thermal adsorption
[120] illustrates the complexity of the interactions between OH groups and H
2
O
molecules. Two kinds of adsorption were observed: (a) adsorption of H
2
O
molecules on localized OH groups with an adsorption energy of about 55 kJ mol
–1
and (b) adsorption of H
2
O on special OH groups leading to a phase transformation.
This phase transformation is clearly seen on the adsorption isotherms at around
300 K (Fig. 18). It seems to be favored by the strong structural similarity between
the hydroxylated (001) surface of Cr
2
O
3
and the basal plane of ice. A large
44 Oudar
Copyright © 2002 Marcel Dekker, Inc.