Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
367
coexistence of the ion erosion in the direction perpendicular to the [001] steps with
the mass transport along the [001] direction results in the formation of the anisotropic
structure. For T > 400 K the diffusion of Ti
3+ (formed upon surface O 2 desorption:
2O
2−
+ 2Ti
4+
→ O 2(g) + 2Ti
3+ ) into the bulk takes place. In these conditions, the
organisation of the ripples is no longer governed by diffusion but rather by sputtering
along the direction normal to [001] steps. For higher T = 720 K, the excess surface
Ti diffuse into the bulk and the stoichiometry of the TiO 2 (110) surface is restored
what make the ripples long and, as for lower T , well ordered along the direction of
ion beam projection.
3.1.3 Zirconia ZrO 2
Zirconium oxide belongs to the family of oxides with high dielectric constant (“highK”) [220]. This issue is crucial for the modern microelectronics, namely in the MOS
technology where the insulating oxide layer must not be too thin to prevent tunnelling
charge leakage, yet providing required capacitance. The most common materials are
ZrO 2 , HfO 2 , and their mixtures with SiO 2 (K = 3.9). Their disadvantage is the low
stability (metastability) of amorphous phase, contrary to SiO 2 , which allows for the
formation of the interfaces with miscellaneous substrates yielding little electrical
defects. The metastable phase can be stabilised by alloying with, e.g. Si, Al, or N.
The other application of ZrO 2 is catalysis, both as an active phase or active phase
component [221, 222] or as a substrate [223, 224], due to its thermal and chemical
stability, high refractive index, low thermal conductivity, hardness, and remarkable
oxygen ion conduction [225–227]. Zirconia is a well-known solid acid catalyst, particularly when mixed with titania [228], in the isomerisation, oxidation, dehydrogenation, dehydration, alkylation and dealkylation, SCR and photocatalytic processes, as
well as in the manufacturing of hydrocarbon gas sensors (see, e.g., the references in
[228]).
Structure and Morphology
The properties of three most important ambient pressure phases, the fluorite cubic
structure c-ZrO 2 (O
5
h , Fm3m space group, see Fig. 6a, stable at T > 2370
◦ C), the
tetragonal phase t-ZrO 2 (D
15
4h , P4 2 /nmc, see Fig. 6b, stable for T from 1170
◦ C
to 2370
◦ C), and the room temperature stable baddeleyite monoclinic m-ZrO 2 (C
5
2h ,
P2 1 /c, see Fig. 6c, T < 1170
◦ C) phase, were studied computationally by Vanderbilt
et al. [220] and Zhao et al. [229] who used LDA functional, while Jomard et al. [230]
used the GGA functional.
The high-T cubic phase can be stabilised by doping by MgO, CaO, or Y 2 O 3 [225,
226, 231]. The tetragonal phase is also stable in the RT when it forms nanograins
with size up to ca. 20 nm [232], what indicates that the impact of the crystal size
should be accounted for in the case of nanometric size. The stability and lowtemperature phase transition of monoclinic, tetragonal, cubic, orthorhombic-I (Pbca)
367
coexistence of the ion erosion in the direction perpendicular to the [001] steps with
the mass transport along the [001] direction results in the formation of the anisotropic
structure. For T > 400 K the diffusion of Ti
3+ (formed upon surface O 2 desorption:
2O
2−
+ 2Ti
4+
→ O 2(g) + 2Ti
3+ ) into the bulk takes place. In these conditions, the
organisation of the ripples is no longer governed by diffusion but rather by sputtering
along the direction normal to [001] steps. For higher T = 720 K, the excess surface
Ti diffuse into the bulk and the stoichiometry of the TiO 2 (110) surface is restored
what make the ripples long and, as for lower T , well ordered along the direction of
ion beam projection.
3.1.3 Zirconia ZrO 2
Zirconium oxide belongs to the family of oxides with high dielectric constant (“highK”) [220]. This issue is crucial for the modern microelectronics, namely in the MOS
technology where the insulating oxide layer must not be too thin to prevent tunnelling
charge leakage, yet providing required capacitance. The most common materials are
ZrO 2 , HfO 2 , and their mixtures with SiO 2 (K = 3.9). Their disadvantage is the low
stability (metastability) of amorphous phase, contrary to SiO 2 , which allows for the
formation of the interfaces with miscellaneous substrates yielding little electrical
defects. The metastable phase can be stabilised by alloying with, e.g. Si, Al, or N.
The other application of ZrO 2 is catalysis, both as an active phase or active phase
component [221, 222] or as a substrate [223, 224], due to its thermal and chemical
stability, high refractive index, low thermal conductivity, hardness, and remarkable
oxygen ion conduction [225–227]. Zirconia is a well-known solid acid catalyst, particularly when mixed with titania [228], in the isomerisation, oxidation, dehydrogenation, dehydration, alkylation and dealkylation, SCR and photocatalytic processes, as
well as in the manufacturing of hydrocarbon gas sensors (see, e.g., the references in
[228]).
Structure and Morphology
The properties of three most important ambient pressure phases, the fluorite cubic
structure c-ZrO 2 (O
5
h , Fm3m space group, see Fig. 6a, stable at T > 2370
◦ C), the
tetragonal phase t-ZrO 2 (D
15
4h , P4 2 /nmc, see Fig. 6b, stable for T from 1170
◦ C
to 2370
◦ C), and the room temperature stable baddeleyite monoclinic m-ZrO 2 (C
5
2h ,
P2 1 /c, see Fig. 6c, T < 1170
◦ C) phase, were studied computationally by Vanderbilt
et al. [220] and Zhao et al. [229] who used LDA functional, while Jomard et al. [230]
used the GGA functional.
The high-T cubic phase can be stabilised by doping by MgO, CaO, or Y 2 O 3 [225,
226, 231]. The tetragonal phase is also stable in the RT when it forms nanograins
with size up to ca. 20 nm [232], what indicates that the impact of the crystal size
should be accounted for in the case of nanometric size. The stability and lowtemperature phase transition of monoclinic, tetragonal, cubic, orthorhombic-I (Pbca)