Modification of alumina barrier-layer through re-anodization in an oxalic acid solution with fluoride additives
Introduction
During the past decade, aluminas have attracted considerable attention as nanostructural dielectric templates, which allow fabricating metal [1], [2], [3], semiconductor [4], [5], carbon nanotube [6] and conductive polymer [7] particles of controlled size and morphology. Alumina confined nanowires or nanotubes array due to their extremely high surface area and surface quantum confinement effects now are widely used for fabrication of various sensors, catalysts and nanodevices. Noteworthy, the alumina-based nanofabricating techniques enable one to make the large surface area, well protected and precisely controlled nanostructures at a low cost.
Alumina templates, grown by anodizing of aluminum in the solutions of certain acids, exhibit a close-packed array of columnar cells, each containing a central pore separated from the aluminum by a thin scalloped barrier oxide layer. It is well known that changing the anodizing conditions can control alumina structural parameters [8], [9]. The hexagons of the alumina barrier oxide layer are insulators composed from the cyclic aluminum trihydrate [10]. It behaves as an n-type semiconductor, which allows the electrical current passage, preferentially, in the cathodic direction. Because of this, the ac electrolysis leads to deposition of discharging species at the bottom of the alumina pores. According to previous reports [11], [12], it is the most ideal method to fill the alumina nanotubes by metals and semiconductors without alumina barrier-layer breakdown and deposition of material onto the film surface. On the other hand, the thickness of the barrier oxide layer, linearly related with the anodizing voltage, is extremely important in determining the uniformity of plating into the pores [13]. For uniform deposition nanoscale materials into the pores, the decrease of thickness of the alumina barrier-layer, δb, is required [14], [15], [16], [17].
Thus far, several strategies have been proposed to thin the alumina barrier. The strategies vary from the chemical dissolution [15], [16] to electrochemical thinning techniques, which utilize the re-anodization at the decreasing voltage regime [17], [18]. However, as shown in [19], the chemical etching of alumina results in the formation of very irregular and non-uniform barriers. In addition, the etching process is difficult to control. The second well-known method is based on a step-by-step decrease of the anodizing voltage at the end of the alumina template formation process [17]. This method though being more accurate and well controllable, however, is time consuming. Moreover, by this procedure the alumina U-shaped pores at the bottom part turns into the funnel-like base with a numerous tapered micro pores directed towards the metal [17].
We report here on a new method for the alumina barrier-layer modification, which involves constant current re-anodization of porous alumina film. The traditional anodization bath containing inorganic fluorides is used for this study. The re-anodized alumina films were characterized by a number of physical techniques and their properties are discussed in this paper.
Section snippets
Experimental
Flag-shaped specimens (15 mm × 10 mm × 0.075 mm) cut from the annealed high purity aluminum foil (99.99 wt.% Al, Goodfellow) were used in this study. To clean the surface of the specimen it was etched in a hot solution of 1.5 M NaOH for 15 s, neutralized in 1.5 M HNO3, then carefully rinsed and air-dried. The specimens for SEM, EIS and TEM measurements were additionally electropolished in a perchloric acid–ethanol bath at 17 V dc and ∼5 °C for 4 min, rinsed thoroughly with EtOH and air-dried. The specimens
Results and discussion
Highly ordered porous alumina films, formed under the galvanostatic regime, were used to investigate the effect of fluorides on the further growth and modification of the barrier-layer properties. As seen from Fig. 1, the alumina fabricated in a vigorously stirred 0.3 M (COOH)2 solution at a current density, j, of 6 mA cm−2 and 17 ± 0.2 °C exhibit highly ordered porous structure. TEM images allow one to estimate the pore diameter and the inter-pore distance of the alumina film. These were found to be
Conclusions
A new and simple route for the porous alumina modification is described and characterized by the voltammetric, EIS, SEM and TEM methods. The modification is based on the alumina electrode re-anodization under galvanostatic conditions in the conventional oxalic acid bath, which, in addition, contains (NH4)2SiF6 or NH4F. It was demonstrated that a sharp drop of the anodizing voltage during the first few minutes of the alumina re-anodization is related to the alteration of alumina barrier-layer
References (27)
- et al.
Appl. Surf. Sci.
(2003) - et al.
J. Cryst. Growth
(2001) - et al.
Cryst. Growth
(2005) - et al.
Electrochim. Acta
(2001) - et al.
Thin Solid Films
(1982) - et al.
Thin Solid Films
(1983) - et al.
Appl. Surf. Sci.
(2003) - et al.
Electrochim. Acta
(2001) - et al.
J. Phys. Chem.
(1991) - et al.
J. Phys. Chem.
(1993)