Low-Cost Nanostructured Coating of Anodic Aluminium Oxide Synthesized in Sulphuric Acid as Electrolyte

Florencia A Bruera,Alicia E Ares,María L Vera,Gustavo R Kramer

doi:10.3390/coatings11030309

Abstract

The anodic oxidation of aluminium is an electrochemical technique that allows obtaining nanostructures with easily adjustable morphology depending on the synthesis variables, for its application in medicine, engineering, biotechnology, electronics, etc. In this work, low-cost aluminium oxide nanostructured films were synthesized and morphologically characterized using two anodization steps in sulphuric acid, varying the concentration and temperature of the electrolyte and anodization voltage. The order of the porous matrix, pore diameter, interpore distance, pore density, thickness, and porosity were measured and statistically analyzed. The results showed that under the proposed conditions it is possible to synthesize low-cost nanoporous aluminium oxide films, with a short-range ordering, being the best ordering conditions 10 °C and 0.3 M sulphuric acid at 20 V and 5 °C and 2 M sulphuric acid at 15 V. Furthermore, it was determined that the pore diameter and the interpore distance vary proportionally with the voltage, that the pore density decreases with the voltage and increases with the concentration of the electrolyte, and that the thickness of the oxide film increases with electrolyte concentration, temperature, and anodization voltage.

Highlights

In recent years, great advances have been made in nanotechnology, especially in the area of nanomaterials, developing nanostructures with unique and versatile properties that are substantially improving the scientific, technological, and industrial sectors of the world [1].Anodic aluminium oxide (AAO) is a nanomaterial that continues to be intensively studied and has shown its potential application in the fields of electronics, sensors, catalysis, separation, and biomedicine [2,3].Nanoporous aluminium oxide films are obtained by anodic oxidation in acidic solutions [4], a simple and versatile electrochemical process that allows the synthesis of highly ordered nanostructures in a hexagonal arrangement of vertically aligned pores [5]
The most widely used anodization technique is the two-step anodization developed by Masuda and Fukuda [6] and it consists of a first anodization, followed by the removal of the oxide layer formed to leave a periodic concave triangular pattern engraved on the surface of the metal, and a second anodization on the textured substrate surface to achieve a self-ordered growth of the pores [3,7]
The purity of the aluminium used as substrate and the applied surface pretreatment to homogenize the surface have directly affected the ordering degree of the nanoporous arrangement [12,13,14,15]

Summary

Introduction

Nanoporous aluminium oxide films are obtained by anodic oxidation in acidic solutions [4], a simple and versatile electrochemical process that allows the synthesis of highly ordered nanostructures in a hexagonal arrangement of vertically aligned pores [5]. AAO, such as the degree of arrangement of the porous structure, the pore diameter, the interpore distance, the pore density, the film thickness, and the porosity, strongly depend on the variables of the synthesis, such as nature, concentration, and temperature of the electrolyte, together with the applied voltage and the anodization time [7,8,9,10,11]. The purity of the aluminium used as substrate and the applied surface pretreatment to homogenize the surface have directly affected the ordering degree of the nanoporous arrangement [12,13,14,15]

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