Pore Nucleation Mechanism of Self-Ordered Alumina with Large Period in Stable Anodization in Citric Acid

Abstract

We have demonstrated that self-ordered porous alumina with large period can be obtained by stable anodization in citric acid at high voltage (400 V). Pore nucleation, which determines the final morphology of the alumina, is a very slow process. By comparing the alumina films obtained in different citric acid concentrations and temperatures, we found the amount of free citric acid anions is critical to pore nucleation. Surface chemistry and the detail structural composition of the porous alumina were investigated by XPS and TEM, respectively. The results show that uniform black alumina surface (forming ordered nanopores) cannot be obtained in low or high citric acid concentrations; the carbon element content of the black is obviously higher than that of gray surface (pores cannot well develop); citric acid is incorporated into the porous alumina cell, where the thickness ratio of compact skeleton and acid incorporated part is 1:3. Accordingly, the pore nucleation has two stages: I, fast forming flat barrier-type alumina film, alumina form and deposit at metal-oxide and oxide-electrolyte interface, respectively. II, Al-citric complex in electrolyte slowly transform to citric acid incorporated alumina and unevenly deposit on barrier-type alumina, which results in electric field concentration between protuberances and pore development.

Towards Self-Organized Anodization of Aluminum in Malic Acid Solutions—New Aspects of Anodization in the Organic Acid

“…Analyzing the ia(t) curves in Figure 3, it can be stated that the process conducted in 0.15 M is very similar to the ones observed during anodization in other organic electrolytes [19][20][21][33][34][35][36]. At the beginning of the process, the ia decreases rapidly to the lowest value (within few seconds) after Analyzing the i a (t) curves in Figure 3, it can be stated that the process conducted in 0.15 M is very similar to the ones observed during anodization in other organic electrolytes [19][20][21][33][34][35][36]. At the beginning of the process, the i a decreases rapidly to the lowest value (within few seconds) after passing through a current overshoot, and then it starts to grow to a maximum value (ca.…”

“…Analyzing the i a ( t ) curves in Figure 3 , it can be stated that the process conducted in 0.15 M is very similar to the ones observed during anodization in other organic electrolytes [ 19 , 20 , 21 , 33 , 34 , 35 , 36 ]. At the beginning of the process, the i a decreases rapidly to the lowest value (within few seconds) after passing through a current overshoot, and then it starts to grow to a maximum value (ca.…”

“…As noticed by Ma et al [ 20 , 21 ], the i a ( t ) combines typical characteristics of mild (MA) and hard (HA) anodization, however, the pore nucleation during anodizing in organic electrolytes requires much longer time to initiate (minutes to hours as compared to seconds in a classical MA [ 8 ]). The pore evolution before and after the peak was investigated previously and was associated with various stages of pore nucleation process [ 20 , 21 ]. In the case of anodization in citric electrolyte, it was noticed that the amount of free citric acid anions play a crucial role in pore nucleation.…”

“…The citric acid anions (i.e., H 2 Cit − , HCit 2− , Cit 3− ) can chelate to Al 3+ and form stable Al-citrate complexes. Ma et al [ 21 ] have postulated that at the beginning of the process, a certain amount of Al 3+ are consumed by the citrate anions to form the Al-citrate complexes, causing the i a to decrease. As the process proceeds, these complexes transform slowly to citric acid incorporated alumina, which randomly precipitate on barrier-type alumina to form protuberances.…”

“…Recently, the self-ordered alumina with large period (up to ca. 900 nm) was produced during anodization in citric acid solutions [ 20 , 21 ]. The anodization was conducted in high citric acid solution (1.5 M), low temperature (0 °C) and high voltage (400 V).…”