NANOSTRUCTURED TiO2 MATERIALS FOR NEW-GENERATION Li-ION BATTERIES - Electrochemical Nanofabrication: Principles and Applications - page 197

In the second reaction, the dissolution of the oxide takes place
through the complexation of titanium (IV) by fluoride ions. The
reaction can be written as follows:

TiO + 6F

-

+ 4H

+

� TiF

2-

+ 2H O (5.2)

2 6 2

The successive stages leading to the final product can be
summarized as follows:
(A) Growth of a compact surface film by the electrochemical
oxidation of titanium to give oxide products with different
hydroxyl content (Fig. 5.5a): Concerning the exact nature
of this layer, the following stoichiometry TiO can

be found in the literature. However, XPS results have been
reported that suggest that the surface layer of the film is
composed of Ti(OH)

(OH)
2

2- x x

with traces of fluorine, while the
composition of the inner layers is closer to TiO

4

[38]. It might
be possible that the composition of the initial barrier layer
has a significant influence on the subsequent development
of the oxide layer. Further studies are required to elucidate
this aspect. The Ti(OH)

2

layer may also be responsible for
the hyperhydrophilic properties of the nanotubular TiO
2

4

layer.
(B) Breakdown of the surface film in selected sites by the
dissolution reaction (Eq. 5.2): The presence of F

ions
induces the localized breakdown of the barrier layer,
probably starting on the existent defect sites. SEM
investigations show that the breakdown sites are randomly
distributed and there is no sign of self-organization at this
stage. These sites act as nucleation points of disordered
wormlike porous structures (Fig. 5.5b).
(C) Growth of the wormlike pores: This is a result of the
differential dissolution of the oxide film. In the deeper
parts of the pores the electrochemical processes (Eq. 5.1)
are taking place, and the protons generated favor reaction
(Eq. 5.2) by an autocatalytic process. Thus, the oxide
dissolution is favored in the lower end of the pore that
deepens progressively in the titanium layer. The maximum
length achieved by these pores is conditioned by the

-