Influence of surface preInfluence of surface pre--treatment on the pore growth treatment on the pore growth of nanoporous alumina at room temperatureof nanoporous alumina at room temperature

D.C. Leitao, C.T. Sousa, J. Ventura, F. Carpinteiro, M. Amado, J.P. Araújo, J.B. Sousa

DFFCUP and IFIMUP, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal and ITN , EN 10 Sacavém Portugal

REFERENCES: [1] Z. L. Xiao et al., Appl Phys. Lett. 81, 2869 (2002); [2] H. Masuda and K. Fukuda, Science 268, 146 (1995); [3] O. Jessensky, Appl. Phys. Lett. 72, 1173 (1998) .

First Anodization

In this work we study how different chemical and physical pre-treatments of high purity (99.997%) aluminum foils, influence nano-pore growth and inter-pore organization upon

anodization in 1.2 M H2SO4 for 1 hour, at room temperature. The current transients were monitored during anodization, allowing us to probe pore formation. We compare substrates as-

rolled, ethanol degreased, immersed in 1 M NaOH for native oxide removal, diamond paste polished, electropolished and ion-milled. After the first anodization we observed, using

Scanning Electron Microscopy (SEM), the top of the anodized foil (alumina surface). The removal of this alumina layer allowed us to observe the pattern formed at the underlying

aluminum surface. SEM images after the second anodization (under the same conditions as the first) were also performed.

Anodization Set-up and Current Density Monitorization

1st and 2nd anodizations carried out in 1.2MH2SO4 , for 1 hour, at 15 V and Tamb

No pre-treatment –Scontrol

250 nm250 nm 250 nm 250 nm

1M NaOH Soft Chemical Etching - SNaOH

Diamond Paste Polish –Sdiam

NaOH etch followed by Ion Milling Ecth – Smill

Second Anodization

250 nm

Electropolishing –Select

Typical behaviour indicating the presence of a porous structure.

Current Density Transients - j(t)

Pores growth

Alumina barrier-layer

Field enhanced dissolution

Pore formation

Scanning Electron Microscope Analysis

Irregular surface results in a degraded pore structure.

Conclusions & Future Work

1st Anodization (a) : significative differences observed around j(t) maximum, depending on pre-treatment employed2nd Anodization (b) : still some competion during pores growth

SEM images revealed the predicted porous structure

Dpore ≈ 15 nm

NaOH etch followed by Ion Milling Ecth – Smill

1M NaOH Soft Chemical Etching - SNaOH

1� m 250 nm 250 nm1� m 1 � m 250 nm

Electropolishing –Select

Dpore ≈ 20 nm

Pre-treatment with NaOH etching gives the best results for good quality and, to a certain extent, organized pore structures

A more organized pore structure is observed.

Ordered regions are small (< 200 nm), due to the room temperature anodization.

Smooth surface, with well defined pores, but without an organized structure.

Alumina layer was removed with a 0.5M H3PO4: 0.2M HCrO4 solution, at 60ºC during 15 minutes.

Optimization of both ion-milling etching and electropolishing are needed, as well as low temperature anodizations to obtain large area ordered structures

The smoothness of Al-foil surface is crucial in order to obtain a good quality porous structure.

1st Anodization 2nd Anodization