490
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.5 Pore Shaping with Modulated Anodization Conditions
All processes leading to non-cylindrical pores are based on non-steady-state anodization. The modulated nature of the anodization process can be spontaneous or
purposeful. In most cases, it involves the change in the anodization voltage, but
the variation in solution composition has also been reported. Below a logical
classification of the pore shape engineering methods is presented.
13.5.1 Pore Branching
For a particular set of experimental conditions, the pore diameter is an unambiguous
function of the anodization voltage during steady-state anodization. In all electrolyte
solutions, the pore diameter decreases with anodization voltage. This voltage dependence is exploited when the anodization voltage is gradually decreased during the
anodization process. When the anodization voltage is lowered, the oxide formation
rate decreases as a result of reduced oxidation rate of Al, while the solution conditions
at the pore bottom are unchanged. Since the dissolution rate of the barrier layer is
determined by the solution environment, the barrier layer thickness is getting lowered
until new and narrower pores can nucleate instead of the earlier grown ones. This
leads to pore branching. The traditional way of the voltage decrease is exponential
(or stepwise with a decay of nearly exponential character) in which at the bottom
of each nanopores two smaller pores are nucleated. Since the voltage range from
which porous layer with straight nanochannels can be grown in a particular solution
is limited, the pore formation is terminated at a threshold voltage. Due to this limitation, tree-like nanopores can be formed with at most four branching levels, which
means that one nanopore can be split up into 2
4
= 16 smaller ones [90–92]. The
hierarchy in the pore branching was clear in all works cited, and the planes of pore
density doubling could be easily identified.
Although it would sound logical that a suitable voltage increase programme
applied for the anodization process might lead to pore merging, no example for such
a process has been found. The reverse pore branching structure with merging pores
has been implemented by using a four-step anodization process by using different
anodizing solutions in each step. The anodization voltage was increased from one
step to another, and hence, a four-level structure could be produced [93]. Although
the pore merging planes were always well-defined, the number of merging pores
shows some scattering laterally.
The above examples all refer to anodization on flat surfaces. However, pore
branching or merging always happens due to geometric constraints when the
anodization process is carried out with specimen of curved surfaces [94, 95].
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.5 Pore Shaping with Modulated Anodization Conditions
All processes leading to non-cylindrical pores are based on non-steady-state anodization. The modulated nature of the anodization process can be spontaneous or
purposeful. In most cases, it involves the change in the anodization voltage, but
the variation in solution composition has also been reported. Below a logical
classification of the pore shape engineering methods is presented.
13.5.1 Pore Branching
For a particular set of experimental conditions, the pore diameter is an unambiguous
function of the anodization voltage during steady-state anodization. In all electrolyte
solutions, the pore diameter decreases with anodization voltage. This voltage dependence is exploited when the anodization voltage is gradually decreased during the
anodization process. When the anodization voltage is lowered, the oxide formation
rate decreases as a result of reduced oxidation rate of Al, while the solution conditions
at the pore bottom are unchanged. Since the dissolution rate of the barrier layer is
determined by the solution environment, the barrier layer thickness is getting lowered
until new and narrower pores can nucleate instead of the earlier grown ones. This
leads to pore branching. The traditional way of the voltage decrease is exponential
(or stepwise with a decay of nearly exponential character) in which at the bottom
of each nanopores two smaller pores are nucleated. Since the voltage range from
which porous layer with straight nanochannels can be grown in a particular solution
is limited, the pore formation is terminated at a threshold voltage. Due to this limitation, tree-like nanopores can be formed with at most four branching levels, which
means that one nanopore can be split up into 2
4
= 16 smaller ones [90–92]. The
hierarchy in the pore branching was clear in all works cited, and the planes of pore
density doubling could be easily identified.
Although it would sound logical that a suitable voltage increase programme
applied for the anodization process might lead to pore merging, no example for such
a process has been found. The reverse pore branching structure with merging pores
has been implemented by using a four-step anodization process by using different
anodizing solutions in each step. The anodization voltage was increased from one
step to another, and hence, a four-level structure could be produced [93]. Although
the pore merging planes were always well-defined, the number of merging pores
shows some scattering laterally.
The above examples all refer to anodization on flat surfaces. However, pore
branching or merging always happens due to geometric constraints when the
anodization process is carried out with specimen of curved surfaces [94, 95].