496
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
achieved with particles prepared with other methods, surface-bond nanoporous titania
exhibits unique properties that make titanium anodization a very intensively studied
field. Several reviews on this field can be found [170–174] that offer a wide overview,
while the present chapter can give a short introduction only.
Categorization of the anodization baths used—It is common to divide the
anodizing solutions into three generations. The early attempts to obtain porous titanium oxide layer on titanium by anodization applied acidic aqueous solutions with a
small concentration of hydrogen fluoride [175–180]. Later, the acidic solutions were
replaced with neutral ones, still retaining water as solvent [181, 182]. The application
of non-aqueous solvents with a well-controlled HF and H 2 O concentration is considered to be the most advanced version [183, 184]. These solutions, containing typically
ethylene glycol or glycerol as major solvent, are named as “viscous” anodizing solutions, referring to one of the distinct properties of the baths, although it is debated
whether the viscosity itself is that makes them effective or the small solubility of
titanium and the small diffusion coefficient of the ions in the bath also play a role.
Whichever solution is used, fluoride ion is an indispensable component of the bath
for obtaining nanostructured titanium dioxide. In the absence of the fluoride ion, a
non-structured barrier layer can be obtained only. This is very well known from the
anodization-based colouring of titanium that relies on variation of the thickness of
the barrier oxide layer as a function of anodization voltage.
The role of fluoride ions—Regardless of the specific bath used, the fluoride ions
play the same role; namely, they act as a regulator of the dissolution and oxidation. Dissolution is a prerequisite of the formation of a nanostructured oxide. The
dissolution process taking place during the anodization is the most important reason
why anodized titanium develops nanotubes instead of a nanopore structure similar
to PAA.
The cross-sectional scheme of the nanotube formation is shown in Fig. 13.5a. This
figure indicates that the preferred site of the dissolution process is the central area
of the walls forming. The reason for this site preference is that the freshly anodized
nanoporous titania structure is in some sense the inverse of what we saw for PAA.
Namely, what was shown in Fig. 13.1 for PAA as a layer with anion incorporation is
nearly pure titania in the case of anodized titanium, whereas the pure alumina in the
PAA corresponds to a fluoride-doped zone.
The enhanced fluoride ion concentration of the anodized titanium in the central
zone of the freshly formed titania wall was detected with either high-resolution scanning Auger electron spectroscopy (AES) [186, 187] and time-of-flight secondary ion
mass spectrometry (ToF SIMS) [187]. By using a different approach, the enhanced
fluorine content of the barrier layer can be detected with a composition depth profile
analysis stated from the pore bottom [188]. Based on the above data, the atomistic
mechanism of the nanopore formation is as follows. During the anodization, the
cations (Ti
IV ) migrate from the metal size of the barrier layer towards the solution,
while anions migrate to the opposite direction. In contrast to the aluminium oxidation,
the fluoride ions reaching the bottom part of the barrier layer move away from the
pore axis, and this sidewise motion leads to their accumulation in the far-from-axis
parts of the pore walls (this process was also identified as “plastic flow” [186]; see
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
achieved with particles prepared with other methods, surface-bond nanoporous titania
exhibits unique properties that make titanium anodization a very intensively studied
field. Several reviews on this field can be found [170–174] that offer a wide overview,
while the present chapter can give a short introduction only.
Categorization of the anodization baths used—It is common to divide the
anodizing solutions into three generations. The early attempts to obtain porous titanium oxide layer on titanium by anodization applied acidic aqueous solutions with a
small concentration of hydrogen fluoride [175–180]. Later, the acidic solutions were
replaced with neutral ones, still retaining water as solvent [181, 182]. The application
of non-aqueous solvents with a well-controlled HF and H 2 O concentration is considered to be the most advanced version [183, 184]. These solutions, containing typically
ethylene glycol or glycerol as major solvent, are named as “viscous” anodizing solutions, referring to one of the distinct properties of the baths, although it is debated
whether the viscosity itself is that makes them effective or the small solubility of
titanium and the small diffusion coefficient of the ions in the bath also play a role.
Whichever solution is used, fluoride ion is an indispensable component of the bath
for obtaining nanostructured titanium dioxide. In the absence of the fluoride ion, a
non-structured barrier layer can be obtained only. This is very well known from the
anodization-based colouring of titanium that relies on variation of the thickness of
the barrier oxide layer as a function of anodization voltage.
The role of fluoride ions—Regardless of the specific bath used, the fluoride ions
play the same role; namely, they act as a regulator of the dissolution and oxidation. Dissolution is a prerequisite of the formation of a nanostructured oxide. The
dissolution process taking place during the anodization is the most important reason
why anodized titanium develops nanotubes instead of a nanopore structure similar
to PAA.
The cross-sectional scheme of the nanotube formation is shown in Fig. 13.5a. This
figure indicates that the preferred site of the dissolution process is the central area
of the walls forming. The reason for this site preference is that the freshly anodized
nanoporous titania structure is in some sense the inverse of what we saw for PAA.
Namely, what was shown in Fig. 13.1 for PAA as a layer with anion incorporation is
nearly pure titania in the case of anodized titanium, whereas the pure alumina in the
PAA corresponds to a fluoride-doped zone.
The enhanced fluoride ion concentration of the anodized titanium in the central
zone of the freshly formed titania wall was detected with either high-resolution scanning Auger electron spectroscopy (AES) [186, 187] and time-of-flight secondary ion
mass spectrometry (ToF SIMS) [187]. By using a different approach, the enhanced
fluorine content of the barrier layer can be detected with a composition depth profile
analysis stated from the pore bottom [188]. Based on the above data, the atomistic
mechanism of the nanopore formation is as follows. During the anodization, the
cations (Ti
IV ) migrate from the metal size of the barrier layer towards the solution,
while anions migrate to the opposite direction. In contrast to the aluminium oxidation,
the fluoride ions reaching the bottom part of the barrier layer move away from the
pore axis, and this sidewise motion leads to their accumulation in the far-from-axis
parts of the pore walls (this process was also identified as “plastic flow” [186]; see