Anodic Alumina Membranes: From Electrochemical Growth to Use as Template for Fabrication of Nanostructured Electrodes

Featured Application The high order degree of the porous structure of anodic alumina membrane (consisting of a close-packed array of columnar hexagonal cells, each containing a central cylindrical pore normal to the surface), makes it an ideal template for fabrication of nanostructured materials, suitable for applications in optoelectronics, sensors, magnetic memories and electronic circuits. A variety of nanostructures (metals, alloys, semiconductors, oxides and polymers) with different morphologies (tubules, wires, rods) were fabricated by utilizing an anodic alumina membrane as a template using a sol-gel process, microwave plasma chemical vapor deposition, electrodeposition and electroless deposition. The great success of anodic alumina membranes is due to their morphological features coupled to both thermal and chemical stability. The electrochemical fabrication allows accurate control of the porous structure: in fact, the membrane morphological characteristics (pore length, pore diameter and cell density) can be controlled by adjusting the anodizing parameters (bath, temperature, voltage and time). This article deals with both the fabrication and use of anodic alumina membranes. In particular, we will show the specific role of the addition of aluminum ions to phosphoric acid-based anodizing solution in modifying the morphology of anodic alumina membranes. Anodic alumina membranes were obtained at -1 degrees C in aqueous solutions of 0.4 M H3PO4 added with different amounts of Al(OH)(3). For sake of completeness, the formation of PAA in pure 0.4 M H3PO4 in otherwise identical conditions was also investigated. We found that the presence of Al(OH)(3) in solution highly affects the morphology of the porous layer. In particular, at high Al(OH)(3) concentration (close to saturation) more compact porous layers were formed with narrow pores separated by thick oxide. The increase in the electric charge from 20 to 160 C cm(-2) also contributes to modifying the morphology of porous oxide. The obtained anodic alumina membranes were used as a template to fabricate a regular array of PdCo alloy nanowires that is a valid alternative to Pt for hydrogen evolution reaction. The PdCo alloy was obtained by electrodeposition and we found that the composition of the nanowires depends on the concentration of two metals in the deposition solution.

Automated summary

Anodic alumina membranes with a high degree of porous structure are ideal templates for manufacturing nanostructured materials in optoelectronics, sensors, magnetic memories, and electronic circuits. By adjusting anodizing parameters, the morphology characteristics such as pore length, pore diameter, and cell density can be accurately controlled. The addition of aluminum ions to phosphoric acid-based anodizing solution can significantly affect the morphology of the porous layer in anodic alumina membranes, impacting the overall structure and properties.