Abstract
A novel two-tier nanostructure with narrow mouth and wide abdomen in porous anodic alumina (PAA) has been successfully fabricated by an improved two-step anodization in oxalic acid (H2C2O4) and phosphoric acid (H3PO4) solutions. The two-tier nanostructure is clarified by oxide flow model and oxygen bubble mold. The small pores result from oxygen evolution at low critical voltage of ~97V in 2wt.% H2C2O4 solution, the big pores result from oxygen evolution at high critical voltage of ~162V in 2wt.% H3PO4 solution. Electronic current across the barrier layer formed in H2C2O4 is greater than that in H3PO4. This is the primary reason why the porosity of PAA formed in H2C2O4 is greater than that in H3PO4.
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