Highly Ordered Porous Anodic Alumina with Large Diameter Pores Fabricated by an Improved Two-Step Anodization...

The aim of this study is to prepare highly ordered porous anodic alumina (PAA) with large pore sizes (> 200 nm) by an improved two-step anodization approach which combines the first hard anodization in oxalic acid-water-ethanol system and second mild anodization in phosphoric acid-water-ethanol system. The surface morphology and elemental composition of PAA are characterized by field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectrometer (EDS). The effects of matching of two-step anodizing voltages on the regularity of pore arrangement is evaluated and discussed. Moreover, the pore formation mechanism is also discussed. The results show that the nanopore arrays on all the PAA samples are in a highly regular arrangement and the pore size is adjustable in the range of 200-300 nm. EDS analysis suggests that the main elements of the as-prepared PAA are oxygen, aluminum and a small amount of phosphorus. Furthermore, the voltage in the first anodization must match well with that in the second anodization, which has significant influence on the PAA regularity. The addition of ethanol to the electrolytes effectively accelerates the diffusion of the heat that evolves from the sample, and decreases the steady current to keep the steady growth of PAA film. The improved two-step anodization approach in this study breaks through the restriction of small pore size in oxalic acid and overcomes the drawbacks of irregular pore morphology in phosphoric acid, and is an efficient way to fabricate large diameter ordered PAA.