Experimental validation of the novel theory explaining self-organization in porous anodic alumina films

Reply to Heinschke, S.; Schneider, J.J. Comment on "Pashchanka, M. Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces. Nanomaterials 2021, 11, 2271"

First of all, I would like to thank Silvio Heinschke and Jörg J. Schneider (hereinafter referred to as "the Readers") for preparing their Comment [...].

Peculiarities of Aluminum Anodization in AHAs-Based Electrolytes: Case Study of the Anodization in Glycolic Acid Solution

The anodization of aluminum (Al) in three alpha-hydroxy acids (AHAs): glycolic (GC), malic (MC), and citric (CC), was analyzed. Highly ordered pores in GC were obtained for the first time. However, the hexagonal cells were characterized by a non-uniform size distribution. Although common features of current density behavior are visible, the anodization in AHAs demonstrates some peculiarities. The electric conductivity (σ) of 0.5 M GC, MC, and CC electrolytes was in the following order: σ(CC) > σ(MC) > σ(GC), in accordance with the acid strength pK_a(CC) < pK_a(MC) < pK_a(GC). However, the anodization voltage, under which a self-organized pore formation in anodic alumina (AAO) was observed (U_max), decreased with increasing pK_a: U_max(CC) > U_max(MC) ≥ U_max(GC). This unusual behavior is most probably linked with the facility of acid ions to complex Al and the active participation of the Al complexes in the AAO formation. Depending on the AHA, its tendency and different modes to coordinate Al ions, the contribution of stable Al complexes to the AAO growth is different. It can be concluded that the structure of Al complexes, their molecular mass, and the ability to lose electrons play more important roles in the AAO formation than pK_a values of AHAs.

Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces

Over the past few years, researchers have made numerous breakthroughs in the field of aluminum anodizing and faced the problem of the lack of adequate theoretical models for the interpretation of some new experimental findings. For instance, spontaneously formed anodic alumina nanofibers and petal-like patterns, flower-like structures observed under AC anodizing conditions, and hierarchical pores whose diameters range from several nanometers to sub-millimeters could be explained neither by the classical field-assisted dissolution theory nor by the plastic flow model. In addition, difficulties arose in explaining the basic indicators of porous film growth, such as the nonlinear current–voltage characteristics of electrochemical cells or the evolution of hexagonal pore patterns at the early stages of anodizing experiments. Such a conceptual crisis resulted in new multidisciplinary investigations and the development of novel theoretical models, whose evolution is discussed at length in this review work. The particular focus of this paper is on the recently developed electroconvection-based theories that allowed making truly remarkable advances in understanding the porous anodic alumina formation process in the last 15 years. Some explanation of the synergy between electrode reactions and transport processes leading to self-organization is provided. Finally, future prospects for the synthesis of novel anodic architectures are discussed.

Multilevel self-organization on anodized aluminium: discovering hierarchical honeycomb structures from nanometre to sub-millimetre scale

Herein, the one-step spontaneous formation of multiscale hierarchical honeycomb-like textures on anodized aluminium is reported for the first time. A self-organized array of unprecedentedly large sub-millimetre hemispherical pores was observed on the upper hierarchy level, while superimposed well-ordered hexagonal cells on the lower hierarchy level maintained their classically known nanoscale self-ordering periodicity (50-60 nm). The larger macroscopic cells (0.45-0.90 mm) show great morphological similarity to the conventional nanosized porous anodic structures, but arise on a purely metallic surface without any analytically detectable amount of deposited alumina. The observed cell size expansion (approximately by a factor of 104) during Al anodizing is colossal in comparison with the size variation ranges reported in the literature so far. It reveals a dramatic nonlinearity effect in a dynamic self-organizing system upon a moderate change of experimental parameters (specifically, this work focuses on the effect of anodizing temperature). Such multilevel self-organization in a single aluminium anodizing process sheds new light on the origin of self-ordered anodic structures and allows rethinking the whole range of opinions regarding their formation mechanism that have been proposed to date.

Evidence for electrohydrodynamic convection as a source of spontaneous self-ordering in porous anodic alumina films

A comparative study of self-ordering behaviour of anodic alumina films fabricated in a series of diluted (down to 0.05 M) oxalic acid electrolytes allowed developing a relationship between the supporting electrolyte concentration and self-ordering voltages for the formation of porous oxide materials. Besides its practical importance, this work elucidates some fundamental principles of porous alumina formation, e.g. it suggests that the cell patterning arises from the electrohydrodynamic (EHD) convection process rather than the interfacial tension gradients near the anode surface (Marangoni-type instability).

Porous alumina-metallic Pt/Pd, Cr or Al layered nanocoatings with fully controlled variable interference colors

Metallic Cr, Al, and Pt/Pd alloy have been deposited by magnetron sputtering or thermal evaporation (resistance heating or electron beam heating) onto nanoporous anodic alumina and have allowed to facilitate a cost-effective technique for manufacturing of pigment-free colored coatings on aluminum. Bright and saturated colors were achieved using the interference effect, and tuned by variation of the uniform oxide film thickness. Morphology and properties of these coatings were investigated by scanning electron microscopy (SEM) and reflectance measurements (UV/Vis/NIR spectrometry). Some optical properties of anodic alumina membranes are rather variable and strongly depend on oxidation parameters, non-stoichiometric composition, and porosity. However, the established correspondence between the film thickness, metallic coating type, and observed interference colors, allows facile, scalable, and inexpensive deposition of colored decorative and wear-resistant coatings onto aluminum and alloys surfaces.