Direct observation of alumina nanowire formation from porous anodic alumina membrane via the droplet etching method

Abstract

We report the first direct observation of the formation process of an alumina nanowire (ANW) array from a porous anodic alumina (PAA) membrane employing a simple droplet etching method, not a general immersion etching method, using field emission scanning electron spectroscopy (FESEM). The PAA membrane is prepared by the two-step anodization method and subsequent lift-off process. The surface view SEM images of the PAA membrane show an array of highly ordered hexagonal pore distribution within the domains of 1–2 µm size, which are separated from neighbouring domains with different orientations of the pore lattice by grain boundaries. The FESEM analysis of the formation process of the alumina nanowire reveals that the formation process consists of three critical steps. The first step involves an image revealing that the outer parts of pore walls are dissolved. The image can be described by the different etching rates of oxide layers in the PAA membrane. Differences in the etching rate can be explained in terms of the different anion impurity concentration in the oxide layer of the PAA membrane. The second step produces an image of the PAA membrane with triangular triple points, surrounded by three adjacent pores. This image can be interpreted as the combined effects of the thickness difference of pore walls and the thermal annealing of the PAA membrane. The last step shows images where the triangular cross section ANWs transform into circular cross section ones as further etching occurs. This can result from the crystallization of the triple points during the annealing process.

Incorporation of lead and titanium into a porous alumina membrane during Pb(Zr0.52Ti0.48)O3 nanotube synthesis

“…Considering the layer structure of pores in the PAM, the diffusion distance corresponds to the thickness of the contaminated alumina layer of the PAM. 16 Hence, we suggest that the Pb and Ti elements diffused through the oxygen vacancies in the contaminated alumina layer of pores in the PAM. …”

“…1(c)). As the amorphous, PAM was selectively etched away, 16 the PZT nanotubes were released as individual PZT nanotubes. Figure 1(f) shows an FETEM image of an individual PZT nanotube released in this manner.…”

Fabrication of a simple SERS-active substrate based on gold-coated chemically generated alumina nanowires

“…Additionally, alumina nanowires and nanotubes can be used as catalyst supports, radar transparent structures, and antenna windows because of their large surface areas and highly electropositive surfaces [10]. Previously, alumina nanowires were synthesized through various techniques, including coating of carbon nanotubes with aluminum isopropoxide [11], vapor-liquidsolid growth, hydrothermal synthesis [12], arc discharge, electrochemical deposition [13], and chemical etching of porous anodic alumina film [14]. Among them, chemical etching is a simple and low-cost method for producing alumina nanostructures [13,14].…”

Formation of dense alumina nanowires from anodic alumina membranes

“…They offer promising technological applications, which are attributed to their high strength, corrosion resistance, chemical stability, low thermal conductivity and good electrical insulation [1]. Moreover, alumina nanowires have become a thrilling field of study because they are useful for water purification filter technology owing to their large surface area and highly electropositive surface [2,3].…”

“…The chemical etching of a porous anodic alumina membrane is considered as an easy and cost effective way to fabricate uniform diameter alumina nanowires [2]. In this sense, several groups of authors reported different alumina nanostructures that were fabricated via a simple etching method in NaOH [4][5][6][7] or in a chromic and phosphoric acid solution [1,7].…”