The preparation technique and the thickness of the PA template substantially define the result of metal electrodeposition. Therefore, in spite of the fact that this procedure became almost standard, the technology for the formation of the PA template is constantly improved by researchers. Generally, before the deposition of NWs, the thick alumina film is detached from the Al substrate after removing the barrier layer at the bottom of the pores. Next, a conductive layer is formed by means of sputtering a metal usually onto the back side of the template with continuous nanochannels [2 (link),19 (link),33 (link)–34 (link)].
In our work, the custom-made PAs were prepared by dc anodization of Al foil, as described in details elsewhere [35 ]. First, commercial aluminium foil (99.995%) with a size of 60 × 48 mm and a thickness of ca. 100 µm is annealed at 350 °C for 1 h. Then, the samples were electropolished in a mixture of chloric acid and acetic acid 1:4 (volumetric ratio) at T ≈ 8 °C and a voltage of 25 ± 2 V for 1–2 min to reduce the surface roughness. Next, the samples were washed in distilled water and dried in a dry air stream. Before anodization, the technological frame has been formed along the perimeter and in the center of the substrate. It is necessary to strengthen the mechanical stability of a free-standing membrane and to restrict certain zones with identical surface area. The frame destination and its formation procedure are described in more detail in [35 ]. Thick porous alumina films with ordered structure of pores have been prepared by two-step anodization in aqueous solution of oxalic acid (H2C2O4, 0.3 M) at 15 °C. The first stage of anodization was performed under a constant voltage of 50 ± 5 V for 25 min. After the first anodization, the preformed oxide film was removed by wet chemical etching in a mixture of phosphoric acid (H3PO4, 0.5 M) and chromic acid (H2Cr2O7, 0.2 M) at 80 ± 5 °C for 5 min. The second stage of anodization was performed under the same conditions for 1 to 4 h.
Then, electrochemical etching of the barrier layer at the bottom of the pores was carried out by gradual reduction of the forming voltage down to 15 ± 2 V. Further, the detachment of alumina from the substrate was performed by Al dissolution in a saturated solution of hydrochloric acid and cupric chloride (HCl + CuCl2). Chemical dissolution of the rest of a barrier layer at the pore bottom and chemical pore widening was performed in 4 wt % Н3РО4 (30 °C) for 15 min. Finally, an electric contact metal (Ta 300 nm + Ni 300 nm or Ta 300 nm + Cu 300 nm) layer was sputtered onto the back side of PA, and a protective coating of chemically resistant varnish HSL (perchlorovinyl lacquer) was applied. As a result, the alumina template with a 30–90 µm thick ordered structure (Figure 1 ) with pore diameters of 50 ± 5 nm has been fabricated.
In our work, the custom-made PAs were prepared by dc anodization of Al foil, as described in details elsewhere [35 ]. First, commercial aluminium foil (99.995%) with a size of 60 × 48 mm and a thickness of ca. 100 µm is annealed at 350 °C for 1 h. Then, the samples were electropolished in a mixture of chloric acid and acetic acid 1:4 (volumetric ratio) at T ≈ 8 °C and a voltage of 25 ± 2 V for 1–2 min to reduce the surface roughness. Next, the samples were washed in distilled water and dried in a dry air stream. Before anodization, the technological frame has been formed along the perimeter and in the center of the substrate. It is necessary to strengthen the mechanical stability of a free-standing membrane and to restrict certain zones with identical surface area. The frame destination and its formation procedure are described in more detail in [35 ]. Thick porous alumina films with ordered structure of pores have been prepared by two-step anodization in aqueous solution of oxalic acid (H2C2O4, 0.3 M) at 15 °C. The first stage of anodization was performed under a constant voltage of 50 ± 5 V for 25 min. After the first anodization, the preformed oxide film was removed by wet chemical etching in a mixture of phosphoric acid (H3PO4, 0.5 M) and chromic acid (H2Cr2O7, 0.2 M) at 80 ± 5 °C for 5 min. The second stage of anodization was performed under the same conditions for 1 to 4 h.
Then, electrochemical etching of the barrier layer at the bottom of the pores was carried out by gradual reduction of the forming voltage down to 15 ± 2 V. Further, the detachment of alumina from the substrate was performed by Al dissolution in a saturated solution of hydrochloric acid and cupric chloride (HCl + CuCl2). Chemical dissolution of the rest of a barrier layer at the pore bottom and chemical pore widening was performed in 4 wt % Н3РО4 (30 °C) for 15 min. Finally, an electric contact metal (Ta 300 nm + Ni 300 nm or Ta 300 nm + Cu 300 nm) layer was sputtered onto the back side of PA, and a protective coating of chemically resistant varnish HSL (perchlorovinyl lacquer) was applied. As a result, the alumina template with a 30–90 µm thick ordered structure (
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