486
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.3.7 Removal of the Remaining Metal and the Barrier
Layer
For using the PAA samples as self-supporting open specimens, the metal layer has
to be dissolved from the back side. Nowadays, the customary method is to apply a
dilute hydrochloric acid solution with copper(II) chloride (sometimes an acidic SnCl 4
solution), while in older works, the more toxic HgCl 2 was often used for dissolving
Al [36]. The process is self-limiting and does not lead to pore opening. The removal
of the metal layer can be visually observed (the sample becomes slightly opaque and
the bubble formation stops). Before the pore opening process, AFM images recorded
on the back side of the barrier layers show the height modulation [36], in accord with
the scheme of Fig. 13.1.
The removal of the barrier layer is said to be the most sensitive step of the sample
preparation, and it is seldom carried out in a way so that the process can be detected
(or quantified) with an instrumental method. The solution for the removal of the
barrier layer contains phosphoric acid (~5 wt%), and the exposure of the back side of
the sample to this solution is maintained for a fixed time that is empirically adjusted
to the thickness of the barrier layer. The application of an excessively long time for
the removal of the barrier layer leads to pore widening. An electrochemical control
of the pore opening process is possible if the membrane with the barrier layer is
fixed in a permeation cell and the half cell at barrier layer side is filled up with the
etching solution [47]. In this case, the pore opening process can be monitored with
the conductivity change between the two half cells.
13.3.8 Theoretical Approaches, Modelling of PAA Formation
In the early years of the research on PAA synthesis, the qualitative picture on the
process was that the mechanical stress associated with the expansion of the aluminium
during oxide formation is the cause of repulsive forces between neighbouring pores
being grown. This force leads to a self-organization of the pores, leading to hexagonal
pore arrays [24].
Since the first attempt of the qualitative understanding of the pore formation and
the occurrence of regularity during anodization, various theories arose that made it
possible to give quantitative description of the phenomena and to perform numerical simulations. However, the theories published so far are very different and partly
contradictory; hence, we cannot speak about a uniform theory of PAA formation.
Since the present work is meant to be a methodology guideline for nanostructure
synthesis, the major elements of the works related to the theoretical analysis is listed
below, whereas interested readers are referred to the some relevant works and the
literature cited therein [42, 48–64]. The modelling attempts take into account the
stress occurring in the barrier layer, thermodynamic factors, current distribution,
the electric field effects, oxidation kinetics, ion migration and diffusion, varying
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.3.7 Removal of the Remaining Metal and the Barrier
Layer
For using the PAA samples as self-supporting open specimens, the metal layer has
to be dissolved from the back side. Nowadays, the customary method is to apply a
dilute hydrochloric acid solution with copper(II) chloride (sometimes an acidic SnCl 4
solution), while in older works, the more toxic HgCl 2 was often used for dissolving
Al [36]. The process is self-limiting and does not lead to pore opening. The removal
of the metal layer can be visually observed (the sample becomes slightly opaque and
the bubble formation stops). Before the pore opening process, AFM images recorded
on the back side of the barrier layers show the height modulation [36], in accord with
the scheme of Fig. 13.1.
The removal of the barrier layer is said to be the most sensitive step of the sample
preparation, and it is seldom carried out in a way so that the process can be detected
(or quantified) with an instrumental method. The solution for the removal of the
barrier layer contains phosphoric acid (~5 wt%), and the exposure of the back side of
the sample to this solution is maintained for a fixed time that is empirically adjusted
to the thickness of the barrier layer. The application of an excessively long time for
the removal of the barrier layer leads to pore widening. An electrochemical control
of the pore opening process is possible if the membrane with the barrier layer is
fixed in a permeation cell and the half cell at barrier layer side is filled up with the
etching solution [47]. In this case, the pore opening process can be monitored with
the conductivity change between the two half cells.
13.3.8 Theoretical Approaches, Modelling of PAA Formation
In the early years of the research on PAA synthesis, the qualitative picture on the
process was that the mechanical stress associated with the expansion of the aluminium
during oxide formation is the cause of repulsive forces between neighbouring pores
being grown. This force leads to a self-organization of the pores, leading to hexagonal
pore arrays [24].
Since the first attempt of the qualitative understanding of the pore formation and
the occurrence of regularity during anodization, various theories arose that made it
possible to give quantitative description of the phenomena and to perform numerical simulations. However, the theories published so far are very different and partly
contradictory; hence, we cannot speak about a uniform theory of PAA formation.
Since the present work is meant to be a methodology guideline for nanostructure
synthesis, the major elements of the works related to the theoretical analysis is listed
below, whereas interested readers are referred to the some relevant works and the
literature cited therein [42, 48–64]. The modelling attempts take into account the
stress occurring in the barrier layer, thermodynamic factors, current distribution,
the electric field effects, oxidation kinetics, ion migration and diffusion, varying