Origin of long-range orientational pore ordering in anodic films on aluminium

Abstract

Porous anodic aluminium oxide has a long history of practical application for corrosion protection and coloring. In the last few decades a lot of hi-tech applications of this material have been found owing to the discovery of anodization conditions leading to the formation of highly ordered porous structures with a narrow pore size distribution. Here we show that in-plane orientation of the porous system in anodic films on aluminium is fully determined by the intrinsic crystallographic orientation of the Al substrate. The anisotropy of aluminium oxidation rates on a scalloped metal-oxide interface leads to reorientation of Al spikes in certain directions, which builds up an in-plane orientational order on a macroscopic scale restricted by a crystallite size. This is a unique example of the inheritance of the substrate crystal structure by an amorphous film through a size difference of three orders of magnitude.

Experimental Verification I: Growth Sustainability of Nanopore Channels Guided with Pre-patterns

“…To obtain long aspect ratio pore channels, it is the anodization condition itself, but not the prepatterning on the Al surface, which really determines the ordering quality of the final in-plane pattern of anodic porous alumina. In addition, Napolskii et al [20] recently proposed that the reason for limited pore channel aspect ratios with prepattern guided growth may be due to the difference of the substrate Al grain orientation which has not been taken into consideration previously [7][8][9][10]; however, here, even though the pre-patterns were made on the same (001) Al grain orientation, which has been found to yield much better self-ordered porous pattern than other substrate orientations [18], the pre-patterned guided pore channel aspect ratio is still quite small (<100). The present result is even smaller than previous reports [21], and this may be because previous researchers might mainly focus on how long one pore channel can be guided to grow from a pre-patterned pit without pore splitting or termination, but neglecting the slightly titling during the pore e TEM cross-sectional view of pore channels at the pore bottom region corresponding to (a).…”

Research Background and Motivation

“…According to this assumption, the rank of ordering quality should be (001) > (101) > (111); yet the experimentally observed rank was (001) > (111) > (101) [92]. Most recently, Napolskii et al [201] reported that the inplane orientation of the porous pattern in anodic porous alumina was determined by the crystallographic orientation of the Al substrate. As is similar to the proposal by Beck et al [199,200], the reason was assumed to be the minimization of surface energy which could cause the formation of an interface consisting of the most stable faces [201].…”

“…As is similar to the proposal by Beck et al [199,200], the reason was assumed to be the minimization of surface energy which could cause the formation of an interface consisting of the most stable faces [201]. Following this assumption, the predicted ultimate in-plane orientation ordering should be formed on (111) Al substrate [201], and again, this contradicts the experimental observation that pore ordering on (111) Al substrates is not better than on (001) substrates [92]. Furthermore, the predicted shape of the bottoms of the pore channels at the m/o interface from the minimum-surface-energy assumption was facetted comprising piecewise flat crystallographic surfaces [199][200][201], but experimental observations show that the m/o interface has a scalloped shape comprising smooth and spherical domes [92].…”

“…Following this assumption, the predicted ultimate in-plane orientation ordering should be formed on (111) Al substrate [201], and again, this contradicts the experimental observation that pore ordering on (111) Al substrates is not better than on (001) substrates [92]. Furthermore, the predicted shape of the bottoms of the pore channels at the m/o interface from the minimum-surface-energy assumption was facetted comprising piecewise flat crystallographic surfaces [199][200][201], but experimental observations show that the m/o interface has a scalloped shape comprising smooth and spherical domes [92]. Thus, the mechanism behind the dependence of self-ordering in porous alumina on substrate grain orientation has not been understood.…”

“…Most recently, Napolskii et al [201] reported that the inplane orientation of the porous pattern in anodic porous alumina was determined by the crystallographic orientation of the Al substrate. As is similar to the proposal by Beck et al [199,200], the reason was assumed to be the minimization of surface energy which could cause the formation of an interface consisting of the most stable faces [201]. Following this assumption, the predicted ultimate in-plane orientation ordering should be formed on (111) Al substrate [201], and again, this contradicts the experimental observation that pore ordering on (111) Al substrates is not better than on (001) substrates [92].…”