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

Abstract

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 °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.

Pd–Co-Based Electrodes for Hydrogen Production by Water Splitting in Acidic Media

“…The fabrication procedure for nanowires of the Pd–Co alloy is detailed in [ 53 ]. The results showed that depending on the deposition conditions, it was possible to obtain arrays of both nanowires and nanotubes, and, as we have also shown in this work, it was possible to obtain both palladium-rich crystalline alloys and amorphous alloys rich in Co.…”

“…In the case of deposits on CP, in fact, the alloys with this composition showed the best performances. According to the results discussed above, an array rich in Co is obtained when a dilute solution of palladium is used [ 53 ]. Also in this case, the alloy was found to be amorphous and the electrodes consisted of cylindrical nanowires (length around 4.5 μm), which replicated the morphology of pores of the alumina template.…”

“…Commercial CP (Toray carbon paper, TGP-H-60) with a diameter of 1.5 cm (working diameter 1.3 cm) was used as support for the electrodeposition processes. For the fabrication of nanowires, AAM (the procedure for the fabrication of AAM is detailed in [ 53 , 79 ]) was used as a template, and its surface was sputtered before electrodeposition using a gold target to have a thin film of gold of approximately 300 nm that acted both as a current collector and as a mechanical support for the nanostructures.…”

“…CP is a material characterized by interconnected carbon fibers of micrometric dimensions, due to its structure and properties, and it has a high chemical stability, high surface area and good electrical conductivity. Furthermore, considering the excellent results obtained in the case of Ni-based nanostructured electrodes for alkaline electrolyzers [ 49 , 50 , 51 ], in this work, the Pd–Co alloys have been obtained in the form of an array of nanowires using alumina membranes (AAM) as the template [ 52 , 53 , 54 , 55 ]. In fact, as demonstrated in [ 56 , 57 , 58 , 59 , 60 , 61 , 62 ], due to the high surface area, the electrodes based on the arrays of nanowires showed very good performance compared to the planar ones.…”

Controllable Fabrication of Silicon Nanopore Arrays by Two-Step Inductively Coupled Plasma Etching Using Self-Assembled Anodic Aluminum Oxide Mask

“…18 The preparation of AAO film is observably simple compared to the costly etched templates, 19 as well as its growth technology has been greatly improved after nearly forty years of development. [19][20][21][22][23] Many factors affecting the high-quality growth of AAO, such as aluminum crystal orientation, 24 anodization parameters, 20,[25][26][27] solution additives 28 and chemical treatment methods, 27 have been extensively and deeply studied. Recent researches have shown that it is possible to control the size of the pores by controlling different parameters of the anodizing process, [19][20][21][22]25,26,28 which is very beneficial for the implementation of SiNPs with different sizes.…”