13.3 Traditional Laboratory Technique of PAA Preparation
485
13.3.6 Composition and Structure of PAA
The standard anodization processes results in nearly pure, slightly hydrated and
amorphous aluminium oxide pore walls. Recrystallization of the porous layer can
be achieved with a heat treatment. The porous structure is retained at least up to
1200 °C. The annealing process is coupled with both a pore density reduction and
pore widening, which is in accord with the shrinkage of the material of the pore
walls [40]. Interestingly, the as-anodized PAA membranes transform to γ-alumina
first during the heat treatment at about 900 °C. Nanocrystalline regions of α-alumina
also appear at this temperature, although it is not the stable form of bulk alumina.
PAA samples annealed at 1200 °C consist of the α-alumina phase only [40].
A moderate degree of anion incorporation from the anodization bath takes place;
therefore, the signal of the central atom of the anion (i.e., P when phosphoric acid
is used) can usually be identified in the EDS spectrum of the samples. The anion
incorporation level changes in the order of sulphuric acid (~ 11 wt% SO 4
2– ) > phosphoric acid (~7.6 wt% PO 4
3– ) > oxalic acid (~2.4 wt% (COO)
2–
2 ) > chromic acid
(~0.1 wt% CrO 4
2– ) [41]. More recent data show that the anion incorporation level
strongly changes with the acid concentration and the anodization voltage [42]: for
PAA membranes prepared with sulphuric acid, the S/Al atomic ratio varied in the
range of 0.062–0.13. Hence, the PAA structures are by far not clean alumina. The
anion incorporation is not uniform but regioselective [43, 44], as it is shown in
Fig. 13.1 (cross-sectional scheme) and Fig. 13.3c (top-view STEM image). The
inner layer is pure alumina, while the outer layer (i.e., that contacting the solution)
contains incorporated anions. The inner layer is relatively thin and ranges to some
10–30% of the total pore wall only, and the ion mobility is the lowest here; hence, this
is the region where the field strength is the largest. The inner and outer layers yield
a pronounced brightness difference in top-view TEM images, which can be seen on
SEM images on phosphoric acid anodized samples with large enough wall thickness because of the limited resolution. On samples thinned similar to that shown in
Fig. 13.3c, an EDS line scan can yielded a quantitative analysis on local composition
and revealed the zone thickness ratio [45].
The presence of anions in the outer layer of the pore walls may suggest that the
pore wall itself is also porous. An ellipsometric study revealed a 33% porosity of the
walls [46], which did not change during the pore widening process. Although such a
high porosity is not in accord with the current efficiency of the anodization process,
it indicates that the pore walls are not completely dense. The results on the wall
porosity are yet to be confronted with the wall shrinkage observed during annealing
[40].
485
13.3.6 Composition and Structure of PAA
The standard anodization processes results in nearly pure, slightly hydrated and
amorphous aluminium oxide pore walls. Recrystallization of the porous layer can
be achieved with a heat treatment. The porous structure is retained at least up to
1200 °C. The annealing process is coupled with both a pore density reduction and
pore widening, which is in accord with the shrinkage of the material of the pore
walls [40]. Interestingly, the as-anodized PAA membranes transform to γ-alumina
first during the heat treatment at about 900 °C. Nanocrystalline regions of α-alumina
also appear at this temperature, although it is not the stable form of bulk alumina.
PAA samples annealed at 1200 °C consist of the α-alumina phase only [40].
A moderate degree of anion incorporation from the anodization bath takes place;
therefore, the signal of the central atom of the anion (i.e., P when phosphoric acid
is used) can usually be identified in the EDS spectrum of the samples. The anion
incorporation level changes in the order of sulphuric acid (~ 11 wt% SO 4
2– ) > phosphoric acid (~7.6 wt% PO 4
3– ) > oxalic acid (~2.4 wt% (COO)
2–
2 ) > chromic acid
(~0.1 wt% CrO 4
2– ) [41]. More recent data show that the anion incorporation level
strongly changes with the acid concentration and the anodization voltage [42]: for
PAA membranes prepared with sulphuric acid, the S/Al atomic ratio varied in the
range of 0.062–0.13. Hence, the PAA structures are by far not clean alumina. The
anion incorporation is not uniform but regioselective [43, 44], as it is shown in
Fig. 13.1 (cross-sectional scheme) and Fig. 13.3c (top-view STEM image). The
inner layer is pure alumina, while the outer layer (i.e., that contacting the solution)
contains incorporated anions. The inner layer is relatively thin and ranges to some
10–30% of the total pore wall only, and the ion mobility is the lowest here; hence, this
is the region where the field strength is the largest. The inner and outer layers yield
a pronounced brightness difference in top-view TEM images, which can be seen on
SEM images on phosphoric acid anodized samples with large enough wall thickness because of the limited resolution. On samples thinned similar to that shown in
Fig. 13.3c, an EDS line scan can yielded a quantitative analysis on local composition
and revealed the zone thickness ratio [45].
The presence of anions in the outer layer of the pore walls may suggest that the
pore wall itself is also porous. An ellipsometric study revealed a 33% porosity of the
walls [46], which did not change during the pore widening process. Although such a
high porosity is not in accord with the current efficiency of the anodization process,
it indicates that the pore walls are not completely dense. The results on the wall
porosity are yet to be confronted with the wall shrinkage observed during annealing
[40].