AAO Template_2. Applications of AAO on Al substrate

2. Applications of AAO on Al substrate

Time:2017-9-4 Author:TopMembranes Technology

AAO on Al substrate has honeycomb-like structure, composed with hexagon column of Al₂O₃, with a circular pore in the center. At the end of the pore, there is a half-sphere shape barrier layer oxide layer in contact with aluminum. The porous oxide film contains mutually parallel cylindrical pores extending from the barrier oxide layer to the film surface. So AAO on Al substrate is suitable to fabricate polymer nanowires or nanotubes.
2.1. Fabrication of polymer nanowires or nanotubes

Fig. 2.1 Fabrication of polymer nanowires or nanotubes by (a) heat- and pressure-driven nanoimprint and (b) polymer solution and pore wetting.

Fig. 2.3 Typical polymer nanostructures fabricated using AAO on Al substrate.

There are usually two methods, as shown in Fig. 2.2. One is heat- and pressure-driven nanoimprint (Fig. 2.2a), the other is pore wetting using polymer solution (Fig. 2.2b). Some typical polymer nanostructures are shown in Fig. 2.3.

Reference:

Science, 2002, 296, 1997;

ChemPhysChem, 2003, 4, 1171-1176;

Langmuir, 2004,20,7665-7669.

Nano Lett., 2007, 7, 183;

Journal of Nanomaterials, 2009, doi:10.1155/2009/436375;

Materials Letters, 64, 2010, 1943;

Macromolecules, 2014, 47, 5227.

2.3. Fabrication of Si Nanowires (SiNW)

Fig. 2.4 Fabrication of Si Nanowires. (a) sequential sputter depositions of Ag and Au on the surface of AAO membrane; (b) transfer of Au/Ag bilayered metal mesh onto a silicon wafer from the surface of etching solution after removal of AAO membrane and also loosely bound metal nanoparticles from the bottom side of the metal mesh; (c) a photograph of Au/Ag bilayered metal mesh on a Si(100) wafer; (d,e) metal-assisted chemical etching (MaCE) of (100)-oriented silicon wafer for the fabrications of SiNWs.

Fig. 2.5 SEM micrographs of (a) AAO membrane and (b) Au/Ag bilayered metal mesh. (c) Typical plan-view SEM image of extended arrays of vertically aligned SiNWs obtained by metal-assisted chemical etching (MaCE) of Si(100) wafers by using Au/Ag bilayered metal mesh. (d,e) Cross-sectional SEM images of vertically etched Si(100) wafers, showing SiNWs with different diameters; (d) 63.9±9.2 and (e) 39.5±4.2 nm. (f) A magnified cross-sectional SEM image of a vertically etched Si(100) wafer taken near the etching front. (g) Histogram showing the diameter distribution of SiNWs shown in panel c, together with a Gaussian fit (solid line) of the measured statistical data.

In 2011, Woo Lee team reported the fabrication of Si nanowires by using AAO template. Au/Ag bilayered metal mesh with arrays of nanoholes were devised as a catalyst for metal-assisted chemical etching of silicon. The present metal catalyst not only overcomes drawbacks involved in conventional Ag-based etching processes, but also allows them to fabricate extended arrays of silicon nanowires (SiNWs) with controlled dimension and density.

Reference:
ACS Nano, 2011, 5, 3222-3229;
ACS Nano, 2011, 5, 5242-5248;
J. Mater. Chem. C, 2013, 1, 5330.

2.4. Fabrication of Metallic Nanopore Arrays

Fig. 2.6 a) Schematic structure of nanopore (left) and nanowire (right) arrays. b) Schematic illustration of the fabrication process of metallic nanopore arrays from the AAO template (Al2O3): gold layer deposition (I), polymer infi ltration (II), template removal (III), Ni electrodeposition (IV), and polymer dissolution (V).

Fig. 2.7 Photographs and SEM images of the original AAO template (a–c) and of the fabricated Ni nanopore arrays (d–f). a,d) Photographs, where the area in the red circle is the nanostructured area. Scale shows lengths in centimeters. b,e) Top and c,f) cross-sectional SEM images; inset of b and e are higher-magnifi cation images.

In 2014, Yong Lei team reported the fabrication of Metallic Nanopore Arrays via AAO template. The structure of nanopore arrays is the negative structure of nanowire arrays (as schematically indicated in Fig. 2.6a). As shown in Fig. 2.6b, the self-supported metallic (nickel) nanopore arrays with highly oriented nanoporous structure were fabricated using a two-step replication process, including gold layer deposition (Step I), polymer infi ltration (II), template removal (III), Ni electrodeposition (IV), and polymer dissolution (V). The AAO template was adopted as the master templates in the replication process because of its attractive features, including uniform and highly oriented nanoporous structures, tunable structural parameters, large area, low cost, and excellent thermal and mechanical stability. As an example, the highly ordered AAO template with a rectangular pore arrangement was applied as the initial template (Fig. 2.7a–c). After this two-step replication process (Fig. 2.6b), large-area, self-supported Ni nanopore arrays with a highly oriented nanoporous structure and a rectangular pore arrangement were successfully obtained (Fig. 2.7d–f). The length of the Ni nanopores (pore length) could reach up to 8.4 μm.

Reference:
Adv. Mater. 2014, 26, 7654–7659.

2.5. Fabrication of PDMS Flexible Nanocone Film.

Fig. 2.8 Schematics of nanocone fi lm and SEM images of Au-coated template and PDMS nanocone. a1) A Si mold with hexagonally ordered nanopillars used as imprint mold with four nanoimprint steps. a2) The i-cone array fabricated by a multi-step anodization and wet etching process on the imprinted Al foil. a3) Premixed PDMS poured on Au-coated template followed by a degas and curing process. a4) Regular nanocone on fl exible PDMS after peeling off. b) Aucoated template with 1 μm pitch and 1 μm depth. c) Nanocone with 1 μm pitch and 1 μm depth.

Fig. 2.9 a) Flexible nanocone fi lm. b) Schematic structure of the solar cell device with nanocone PDMS fi lm attached on the top. c) Visual effect of the nanocone AR layer on CdTe devices. The bare sample on the right and the sample with PDMS nanocone on top on the left. d) A drop of water on the nanocone PDMS fi lm showing a large contact angle of 152°. e) A drop of water on the fl at PDMS fi lm showing a contact angle of 98°.

Since the reflectance loss of light leads to inefficient utilization of the incident photons, various anti-reflection (AR) schemes have been developed to achieve high-efficiency solar cell devices. In 2014, Zhiyong Fan team reported utilized a facile molding process to fabricate flexible plastic AR films with three-dimensional (3D) nanocone arrays on the front surface. Fig. 2.8 a1 to a3 shows schematics of the fabrication process. Briefly, an electrochemically polished-clean Al foil was mprinted using a silicon stamp with hexagonally ordered anopillars with height of 200 nm and tunable pitch of 500 nm to 2 μm to produce a nanoindentation array on the surface of the Al foil, as shown in Fig. 2.8 a1. Thereafter, the i-cone array (Fig. 2.8 a2) was fabricated by a multi-step anodization and wet etching process on the imprinted Al foil in an acidic solution with a proper direct-current (DC) voltage. Fig. 2.8b and the inset show scanning electron microscopy (SEM) images of a fabricated i-cone array with cross-sectional and top views, respectively. Fig.2.9a shows a photograph of a fabricated nanocone film naturally attached on a 0.18 mm thick polycarbonate film. Fig. 2.9b schematically illustrates the structure of the solar cell device with nanocone PDMS fi lm attached on the top. In addition to the AR property, it was also found that the PDMS nanocone arrays have superhydrophobic characteristic with large water contact angle.

Reference:

Adv. Mater. 2014, 26, 2805.
ACS Nano, 2015, 9, 10287.
2.6. Three-dimensional Nanotube Electrode Arrays.

Fig. 2.10 Schematic illustration showing the fabrication process of hierarchical tubular pseudocapacitor electrodes. (a) Nanoimprint process with hexagonal nanopillar Si mold. (b) As-prepared AAO membranes after anodization and Al removal. (c) Ultrasonic spray pyrolysis (USP) deposition of FTO tubular arrays. (d) Hierarchical tubular electrodes achieved by MnO₂ electrodeposition.

Fig. 2.11 Optical and SEM images of freestanding AAO tubular arrays with the length of 10 μm before USP (a–c). Optical and SEM images of freestanding AAO tubular arrays with the length of 10 μm after USP (d–f ).

The electrode surface area is one of the key factors determining the performance of devices such as pseudocapacitor. It is known that an AAO is not conductive by itself. In 2016, Zhiyong Fan team used a low-cost ultrasonic spray pyrolysis (USP) process to deposit conductive fluorine doped tin oxide (FTO) tubular structured electrodes inside AAO pores. The fabrication process of the hierarchical nanotube pseudocapacitive electrodes mainly comprises 4 steps as shown in Fig. 2.10: (i) nanoimprint on an Al foil to leave ordered indentation (Fig. 2.10a); (ii) electrochemical anodization to form an AAO membrane with perfect hexagonal ordering (Fig. 2.10b); (iii) conformal and uniform deposition of FTO tubular structures into AAO nanochannels with the USP method (Fig. 2.10c); and (iv) homogeneous electrodeposition of MnO2 nanoflakes into FTO tubular arrays to achieve hierarchical electrodes (Fig. 2.10d). Fig. 2.11a–f show the optical image and scanning electron microscopy (SEM) images of 10 μm depth freestanding AAO tubular arrays before and after FTO deposition. Obviously, the color of the AAO backbone has been changed from transparent to homogeneous brown, proving the deposition of FTO inside AAO channels and the dense FTO film is conformally and uniformly deposited on the AAO scaffolds, which can be seen in Fig. 2.11e and f.

Reference:

Nanoscale, 2016, 8, 13280.