Ethanol influence on arrangement and geometrical parameters of aluminum concaves prepared in a modified hard anodization for fabrication of highly ordered nanoporous alumina

•A modified hard anodization method was developed.•Highly ordered Al concavities were obtained with interpore intervals >400 nm.•Ethanol demonstrated a strong impact on hexagonal arrangement of Al concavities.•Porous anodic alumina inherits morphology of underlying Al nano-concave arrays.•The best long-range pores ordering was obtained for PAA fabricated at 195 V.

Highly ordered porous anodic alumina (PAA) with interpore distances (Dc) tunable in the range of 250–400 nm have been fabricated in a two-step anodization. In the first step a modified hard anodization (HA) was developed, which allowed for extension of the maximal anodizing voltage up to 220 V, giving rise to larger pores intervals than usually attained in HA (∼300 nm). The HA process was carried out in 0.3 M ethanol-free (group I) and ethanol-modulated (group II) oxalic acid electrolytes. It was observed that ethanol worsens the hexagonal arrangement of Al concavities and strongly affects the Dc performance with respect to applied voltages. The Al nano-concave arrays prepared in the modified HA were subsequently used in the second anodization, performed in 0.1 M phosphoric acid solution. The geometrical features and regularity of Al concavities were directly transferred to porous anodic alumina (PAA). An excellent long-range ordering of pores was achieved when the guidance effect of regular hexagonal Al concaves prepared in ethanol-free electrolyte was reinforced by the self-ordering conditions prevailing in 0.1 M phosphoric acid solution at voltages close to 195 V.

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