498
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
titania layers. The cell voltage used for first- and second-generation baths is typically
10–20 V and never exceeds 25 V. Third-generation baths with high organic concentration allow a voltage range of 20–90 V. The current transients are often similar
to that shown for PAA formation (Fig. 13.2), and the stable anodization current
density for the third-generation bath is in the mA cm
−2 range. The interpore distance
during Ti anodization exhibits a linear dependence on the anodization voltage, in full
analogy with PAA formation, though the proportionality coefficient is smaller (1.5–
1.7 nm V
−1 ). The volume expansion factor of the porous titania structure strongly
depends on the anodization circumstances and varies between 1.3 and 2.8 [190].
Structure and annealing behaviour—The as-received TiO 2 nanotube arrays are
mostly amorphous. Upon annealing, a crystallization process takes place, leading
to either rutile or anataze (or one after the other as the temperature increases). The
formation temperature of the crystalline forms of TiO 2 strongly depends on the
formation parameters, similar to the temperature limit at which the tubular structure
collapses [191, 192]. In exceptional cases, the as-received samples can be crystalline
[193]. The adjustment of the structural properties of the titania nanotubes is important
for achieving the desired photoelectrochemical activity.
Various pursuits concerning titanium anodization—It was a challenge to achieve
conditions at which the anodized titanium develops pores instead of nanotubes. It
was shown that the accurate control of the water content of the electrolyte solution
makes it possible to avoid the pore wall splitting shown in Fig. 13.5a since water
is an essential component for the dissolution [194]. Galvanostatic anodization was
also reported to be a feasible way for nanopore formation [195]. The regularity of the
pore system formed upon anodization can be improved with nanoimprint lithography,
fully similar to the process shown in Fig. 13.4a [196–199].
Tube shaping and branching with the change of the anodization voltage is possible
for titania nanotubes, similarly to what was demonstrated for PAA samples [185,
200]. However, the shape-modulated titania nanotubes show less regular structures
that PAA samples.
Another interesting variation of titanium anodization is when well-separated
regular nanotube arrays are prepared instead of the closely packed tube arrays [201,
202]. This type of anodization process proved to be successful with diethylene glycol
as solvent and with thorough control of the water content of the bath.
13.7.2 Other Valve Metals
Valve metals form a closely related family with regard of the anodization process.
The common features of their anodization are as follows: (i) a barrier-type oxide
develops on their surface upon anodization in fluoride-free electrolyte solutions. (ii)
Nanoporous and nanotubular oxide structures can be produced during their anodization in bath containing fluoride ions. (iii) The specific form of the nanostructure
produced with anodization (i.e., pores or tubes) can be tuned with the water and
fluoride salt concentration in the solution and the voltage used for anodization.
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
titania layers. The cell voltage used for first- and second-generation baths is typically
10–20 V and never exceeds 25 V. Third-generation baths with high organic concentration allow a voltage range of 20–90 V. The current transients are often similar
to that shown for PAA formation (Fig. 13.2), and the stable anodization current
density for the third-generation bath is in the mA cm
−2 range. The interpore distance
during Ti anodization exhibits a linear dependence on the anodization voltage, in full
analogy with PAA formation, though the proportionality coefficient is smaller (1.5–
1.7 nm V
−1 ). The volume expansion factor of the porous titania structure strongly
depends on the anodization circumstances and varies between 1.3 and 2.8 [190].
Structure and annealing behaviour—The as-received TiO 2 nanotube arrays are
mostly amorphous. Upon annealing, a crystallization process takes place, leading
to either rutile or anataze (or one after the other as the temperature increases). The
formation temperature of the crystalline forms of TiO 2 strongly depends on the
formation parameters, similar to the temperature limit at which the tubular structure
collapses [191, 192]. In exceptional cases, the as-received samples can be crystalline
[193]. The adjustment of the structural properties of the titania nanotubes is important
for achieving the desired photoelectrochemical activity.
Various pursuits concerning titanium anodization—It was a challenge to achieve
conditions at which the anodized titanium develops pores instead of nanotubes. It
was shown that the accurate control of the water content of the electrolyte solution
makes it possible to avoid the pore wall splitting shown in Fig. 13.5a since water
is an essential component for the dissolution [194]. Galvanostatic anodization was
also reported to be a feasible way for nanopore formation [195]. The regularity of the
pore system formed upon anodization can be improved with nanoimprint lithography,
fully similar to the process shown in Fig. 13.4a [196–199].
Tube shaping and branching with the change of the anodization voltage is possible
for titania nanotubes, similarly to what was demonstrated for PAA samples [185,
200]. However, the shape-modulated titania nanotubes show less regular structures
that PAA samples.
Another interesting variation of titanium anodization is when well-separated
regular nanotube arrays are prepared instead of the closely packed tube arrays [201,
202]. This type of anodization process proved to be successful with diethylene glycol
as solvent and with thorough control of the water content of the bath.
13.7.2 Other Valve Metals
Valve metals form a closely related family with regard of the anodization process.
The common features of their anodization are as follows: (i) a barrier-type oxide
develops on their surface upon anodization in fluoride-free electrolyte solutions. (ii)
Nanoporous and nanotubular oxide structures can be produced during their anodization in bath containing fluoride ions. (iii) The specific form of the nanostructure
produced with anodization (i.e., pores or tubes) can be tuned with the water and
fluoride salt concentration in the solution and the voltage used for anodization.