Tubular membranes based on porous anodic alumina films

Since 1995 nanoporous aluminium oxide (i.e. alumina) films havebecome more and more popular. These films are produced by electrochemical oxidation (or anodizing) of aluminium. Porous anodic alumina films have hexagonal-ordered structure with nano-scale pores. Pores diameters and film thicknesses change depending on anodizing conditions. These nanoporous films have been widely used in various applications such as filters, catalyst supports, and templates as well as electronic, magnetic, and optical devices.

Traditional technique of alumina membranes production, based on two-step anodizing, was studied. Besides, some disadvantages, such as fragility, time-consuming technique of obtaining membranes and necessity of using a special holder, were also stated.

Since their main disadvantage is high fragility, it is necessary, firstly, to create more flexible flat membranes, and, secondly, to form cylindrical nanomembranes, called “tubular”.

Analysis of the literature shows that the most common technological chain to form alumina nanomembranes is a sequence consisting of 4 stages: pretreatment, anodizing, getting afree oxide and removal of the barrier layer.

Samples were made from aluminium foil of 50 and 100 um thick,both flat and in the form of a tube with a diameter of 6-7 mm. Before anodization samples wereannealed at the temperature of 4500C for 30 minutes and chemically cleaned in diluted NaOH. Studying of samples structure was done by Atomic Force Microscopy on Scanning Probe Microscope “Solver-Next”.

We used a two-step anodizing technique.For the 1 st group of samples at both stages we used water solution of oxalic acid as an electrolyte; and for the 2-d group at the 2-d stage we used a special multicomponent [1,2,3] (mixture ofoxalic, citric, and boric acid as well as isopropyl alcohol) electrolyte to prepare flexible films.As a result we managed to fabricate permeable alumina membrane.

Fig. 1.AFM images: a) barrier, b) porous layer surfaces of porous anodic alumina films (PAAF), formed in 3% C2H2O4.

Figure 1 shows the AFM images of barrier and porous layer surfaces of porous anodic alumina film, formed in 3% C2H2O4 using classical one-step anodizingand detached from the substrate. From these images we got an approximate size of cells (250nm) and pores(80nm).
Fig. 3.Illustration of flexibility of porous anodic oxide films.

It is found that the resulting anodic oxide film had increased elasticity and ability to withstand bending angle of 120 ° (Fig. 3).

However, it is important to say, that thickness of films’ barrier layer is about 60-70nm. For its removal we used a method ofbarrier layer thinning “from above”. For that, sequential decrease in voltage was carried out, which led to stepwise reduction of the current. Reaching the zero (or close to zero) value meant appearing of barrier layer permeability.

The next stage in fabrication of nanomembrane was removal of metal in a solution based on CuCl2. After that we got partly permeable membranes, as is shown in figure 4. It is seen that this procedure leads to appearing of holes of about 20nm in the film. It means that we still do not have full pore opening.

Further we investigated the tubular aluminium samples anodization. As a result we developed a technique that allowed us finally to get detached tubular nanoporous oxide film. And now we are going to get a fully permeable tubular membrane.

Fig. 4.AFMimages of barrier layer of PAAF а) 1st group, б) 2nd group of samples after barrier layer thinning.
Conclusions
1. Nanoporous alumina films have been fabricated in the present study. It has been shown that the developed two-step anodizing technique using special multicomponent electrolyte at the 2nd stage allows to form nanoporous alumina films of high flexibility.
2. Flexible flat and tubular porous anodic aluminas detached from substrate with thickness of
um and cell size about 200 nm , were experimentally fabricated.
3. We found that a method ofbarrier layer thinning “from above”in combination with chemical removal of metalenables permeable alumina membranes fabrication.

Literature
1. N. ltoh et aL /Journal of Membrane Science 117, 1996, c.189-196
2. A. Belwalkar et al/Journal of Membrane Science 319, 2008, c.192-198
3. S. Yue et al. /Materials Chemistry and Physics 128, 2011, c.187-190

Abstract. This paper is devoted to creation of tubular alumina nanomembranes. The developed technique provides the fabrication of alumina nanomembranes of high flexibility.
Keywords: Nanoporous alumina films, oxide, pores, layers, tubular, membranes, Al2O3.

М. А. Терлецкая