478
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.2 Historical Background: From the Aluminium
Industry to the Nanotechnology Laboratories
Anodization of aluminium is a well-know process applied from the mid of the twentieth century, primarily for the improvement of the corrosion resistance of the workpieces and secondarily for colouring them. Already in a paper written as early as in
1953, the description of the suitable solution compositions and structural features
of the anodized parts seem to be much in accord with the up-to-date knowledge of
the field [1]. Until about 1990, several works were published about various aspects
of the field, slowly enriching the available information of aluminium anodization in
line with the technical development and the available technical instrumentation. A
clear distinction could be given between the conditions of anodic oxidation [1–8] and
electropolishing [9]. The following factors of the process could be reliably outlined:
• Under appropriate electrochemical anodization conditions, a porous oxide layer
can be grown on aluminium samples.
• The three major bath types (sulphuric acid, oxalic acid and phosphoric acid) were
identified.
• The pore axes are primarily perpendicular to the sample surface. The pores being
formed behave like vortices repelling each other, which leads to a hexagonal
arrangement of the pores.
• The pore diameter is a linear function of the voltage applied for anodic oxidation (for electropolishing conditions, this relationship does not hold). The
proportionality factor of about 2 nm V
−1 was also properly assessed.
• The bottom of the pores is separated from the metal with a barrier layer whose
thickness is about the half of the thickness of the walls between two pores.
The above-listed features of porous anodic alumina (PAA, often also mentioned as
anodic aluminium oxide, AAO) are presented in Fig. 13.1. Even though the imaging
of the nanoporous structures obtained was less perfect than that available nowadays,
essentially all basic information were know that later established the development
of the field in the mid-1990s. The application of starting specimens of relatively
low purity grade well explains the imperfections in the porous structures themselves, while the limitations of the imaging conditions come from the old electron
microscopic techniques based on studies of replica structures.
Anodization of technical-grade aluminium leads to a partly disordered pore structure with pore branching, tilting and pore growth ending. The formation of these
defects is caused by impurities, inclusions and crystal disorders. Therefore, PAA
on technical-grade aluminium was not considered as a material for either technical
applications or for further scientific studies.
The spin-up of the scientific interest in nanoporous materials started in the 1990s.
At that time, ion-track-etched membranes were already used as templates for both
electrodeposition [10] and non-electrochemical filling methods [11]. The opening of
the new era of the PAA research is dated to 1995 when Masuda and Fukuda published
a method for producing well-ordered, nearly defect-free hexagonal nanoporous
alumina [12]. The advantage of their method was that it made possible to detach
the nanoporous membrane from the aluminium raw material and hence using it as a
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
13.2 Historical Background: From the Aluminium
Industry to the Nanotechnology Laboratories
Anodization of aluminium is a well-know process applied from the mid of the twentieth century, primarily for the improvement of the corrosion resistance of the workpieces and secondarily for colouring them. Already in a paper written as early as in
1953, the description of the suitable solution compositions and structural features
of the anodized parts seem to be much in accord with the up-to-date knowledge of
the field [1]. Until about 1990, several works were published about various aspects
of the field, slowly enriching the available information of aluminium anodization in
line with the technical development and the available technical instrumentation. A
clear distinction could be given between the conditions of anodic oxidation [1–8] and
electropolishing [9]. The following factors of the process could be reliably outlined:
• Under appropriate electrochemical anodization conditions, a porous oxide layer
can be grown on aluminium samples.
• The three major bath types (sulphuric acid, oxalic acid and phosphoric acid) were
identified.
• The pore axes are primarily perpendicular to the sample surface. The pores being
formed behave like vortices repelling each other, which leads to a hexagonal
arrangement of the pores.
• The pore diameter is a linear function of the voltage applied for anodic oxidation (for electropolishing conditions, this relationship does not hold). The
proportionality factor of about 2 nm V
−1 was also properly assessed.
• The bottom of the pores is separated from the metal with a barrier layer whose
thickness is about the half of the thickness of the walls between two pores.
The above-listed features of porous anodic alumina (PAA, often also mentioned as
anodic aluminium oxide, AAO) are presented in Fig. 13.1. Even though the imaging
of the nanoporous structures obtained was less perfect than that available nowadays,
essentially all basic information were know that later established the development
of the field in the mid-1990s. The application of starting specimens of relatively
low purity grade well explains the imperfections in the porous structures themselves, while the limitations of the imaging conditions come from the old electron
microscopic techniques based on studies of replica structures.
Anodization of technical-grade aluminium leads to a partly disordered pore structure with pore branching, tilting and pore growth ending. The formation of these
defects is caused by impurities, inclusions and crystal disorders. Therefore, PAA
on technical-grade aluminium was not considered as a material for either technical
applications or for further scientific studies.
The spin-up of the scientific interest in nanoporous materials started in the 1990s.
At that time, ion-track-etched membranes were already used as templates for both
electrodeposition [10] and non-electrochemical filling methods [11]. The opening of
the new era of the PAA research is dated to 1995 when Masuda and Fukuda published
a method for producing well-ordered, nearly defect-free hexagonal nanoporous
alumina [12]. The advantage of their method was that it made possible to detach
the nanoporous membrane from the aluminium raw material and hence using it as a