Unveiling the Hard Anodization Regime of Aluminum: Insight into Nanopores Self-Organization and Growth Mechanism

Abstract

Pores growth mechanism and their self-ordering conditions are investigated for nanoporous alumina membranes synthesized by hard anodization (HA) of Al in a broad range of anodic conditions, covering oxalic acid electrolytes with concentrations from 0.300 M down to 0.075 M and potentiostatic anodization voltages between 120 and 225 V. The use of linear sweep voltammetry (LSV) and scanning and transmission electron microscopy, together with image analysis techniques allow one to characterize the intrinsic nature of the HA regime. HA of aluminum is explained on the basis of a phenomenological model taking into account the role of oxalate ions and their limited diffusion through alumina nanochannels from a bulk electrolyte. The depletion of oxalate ions at the bottom of the pores causes an increased growth of the alumina barrier layer at the oxide/electrolyte interface. Furthermore, an innovative method has been developed for the determination of the HA conditions leading to self-ordered pore growth in any given electrolyte, thus allowing one to extend the available range of interpore distances of the highly ordered hexagonal pore arrangement in a wide range of 240-507 nm, while keeping small pore diameters of 50-60 nm.

Pseudo-monocrystalline properties of cylindrical nanowires confinedly grown by electrodeposition in nanoporous alumina templates

“…33 The anodization process was performed at 0-3 C in a 0.3 M oxalic acid electrolyte containing 5 vol% of ethanol as the anti-freezing agent and mechanically stirred at 300 rpm during the entire anodization process. The complete HA protocol consisted of a pre-anodization step performed under mild anodization (MA) conditions at 80 V versus a Pt counter electrode for 15 min, during which randomly disordered nanopores grew on the topsurface of the alumina layer; a transitory voltage ramp was increased at the sweep rate of 0.08 V s À1 from the MA to HA regime for some of the initial random pores grown in the top alumina layer to become branched or converge into larger pores that were selectively ordered and aligned in parallel, whereas other pores remained occluded; [48][49][50] and nally, a potentiostatic HA step was performed at 140 V for 1.5 h, forming the hexagonally centered and periodically ordered pore arrangement of the alumina template, for some of the initial random pores grown in the top alumina layer to branch or converge into larger pores that were selectively ordered and aligned in parallel, whereas other pores remained occluded (Fig. 1a).…”

“…1a). 26,[48][49][50] Aer the HA process, the remaining unoxidized Al substrate was removed by selective wet chemical etching in a CuCl 2 /HCl aqueous solution, and the alumina barrier layer at the bottom of the pores was etched by immersing the nanoporous membranes in 5 wt% H 3 PO 4 at 30 C for 2.5 h. This immersion process gives rise to a porous alumina membrane with pores fully open at both sides, also increasing the pore size of the membrane and dissolving the protective mild anodization layer on the top-surface of the HA membranes. The morphological structure of the nanoporous anodic alumina membrane, resulting from the previously explained single-step HA procedure, is shown in Fig.…”

Fabrication of Self-Ordered Alumina Films with Large Interpore Distance by Janus Anodization in Citric Acid

“…However, in typical MA processes, hexagonally packed nanopores with fixed interpore distance ( D int ) of 60 nm, 100 nm and 500 nm can only be obtained17181920. To resolve this issue, HA characterized by exponential decreased current from a high value is developed to broaden the D int range, which have recently attracted much attentions2122232425. To date, highly ordered nanopore with continuously tunable D int in the range of 70–490 nm can be realized by HA in various electrolyte systems2223.…”

A Review of Recent Developments of Mesoporous Materials

“…Nanoporous alumina membranes have recently been prepared using a hard anodization method. [3] Aluminum was treated under different anodizing conditions with oxalic acid electrolytes. The range of potential was between 120 to 225 volts.…”