When I needed to dissolve AAO membranes, I used an aqueous 5wt%  NaOH bath. Maybe you can try this.

For this purpose you can use a chemical etching in concentrated hydrochloric acid:

Al2O3 + 6НСl = 2AlСl3 + 3Н2О.

Dear Shahidah,

I understand that you pretend to study the nanostructure left on the aluminium foil by the porous AAO layer grown by an anodization process.

I suppose that you are using the well-known method that uses a mixture of phosphoric and chromic acid at 70ºC for 3 h, am I right? My advice is that you should keep working with chromium trioxide solutions. It has been used widely and works fine in selectively dissolving the AAO layer while leaving the aluminium foil intact. 

However, if you wish to change to a different solution, you could use the same solution used for pore widening procedures (5% wt. phosphoric acid in DI water, 35ºC). This would result in the dissolution of the AAO layer and you could determine the time required for the complete dissolution by performing a simple calibration of the pore widening rate.

I would not use NaOH or HCl, these chemicals would react as well with the aluminium. In fact, for the dissolution of Aluminium to get free-standing AAO layers an HCl:CuCl solution is typically used.

I hope that this answer helped you.

Kind regards

Gerard

I am sorry to say this Dr. Hao Qi, but it is possible to dissolve the AAO layer in a H3PO4 solution without damaging the Al nanostructure.

While H3PO4 reacts with the Al foil (I never said it didn't), this does not mean that it cannot be used for removing the AAO layer without affecting the nanostructure left on the aluminium foil, specially if used with caution and in a well-controlled manner. It's just a matter of avoiding any overetching that could damage the Al nanostructure.

In fact, this process (i.e. H3PO4 dissolution) has already been used and published twice that I can recall with successful results:

Regards,

Gerard

 Dear Hao Qi,

I agree  with you, I would not suggest using this method either. However, depending on the application, it might be interesting to use it (for example in applications with cells, where you could want to avoid any possible chromium trace due to toxicity).

With regard to Fig. 2 of ref. 2, I am pretty sure that it is an image of the Al nanostructure and not an AAO template. It shows less charging, which means a more conductive material, and less contrast in the "pore" area, which relates to the hemispherical nanostructure left by the 1st anodization. However, it is true that the quality of the resulting AAO films (i.e. self-ordering quality) is not the best, but the fabrication conditions do not match the ones required to achieve self-ordering.

With respect to your results with pore widening experiments, I am quite impressed that you obtained such a big difference between top and bottom pore sizes. Typically, the conicity of AAO pores is almost inexistent even at large pore aspect ratios (i.e. >20 um). From my point of view, such a difference could arise from either your anodization conditions or due to a diffusion limitation of the acid species during the pore widening process. However, to be sure about the reason, I would need more details about your experimental setup.

Regards,

Gerard

PD: if you are interested in the conditions to achieve high-quality self-ordering, you could check this paper. It is very famous among the self-ordered AAO freaks like me:

Self-ordering Regimes of Porous Alumina:  The 10 Porosity Rule

Kornelius Nielsch,Jinsub Choi,Kathrin Schwirn,Ralf B. Wehrspohn,* and, and Ulrich Gösele, Nano Letters 2002 2 (7), 677-680. DOI: 10.1021/nl025537k

Dear Dr ..

I am very glad that all of u are willing to share your knowledge and experiences with me.. I absolutely will consider all of your suggestions.. thank u very much..Your information are all very helpful to me.

Thanks again..

Dear All,

Sorry for the late answer. I have been quite busy lately.

To Hao Qi:

Looking at your answer, I have noted two things:

I would say that the high difference you are seeing comes from the composition of the AAO layer upon synthesis. If I were you (and I really needed a straight pore with almost no conicity for the application being developed), I would check an AAO layer  without any pore widening procedure. If the resulting AAO layer presents a high difference between the top and bottom diameters, I would change the oxalic acid concentration, lengthen the 1st step and as well check that your cooling system directly cools the bottom of your Al foil! This last thing ensures that the coldest acid is the one in direct contact to the AAO layer that is being grown. This way you limit a lot any unwanted widening of the AAO pores due wet chemical etching by the oxalic acid solution you are using for the anodization.

I hope this helps and works out for you just as it worked out for me. Good luck with your future experiments!

To Shahidah:

I am glad to see that you feel that our answers are useful for your research. It would be great if you could reply if any of the proposals posted here was helpful. I wish you all the best with your experiments.

Kind regards to you all,

Gerard