Effect of Electric Field Strength on Cell Morphology and Anion Incorporation of Anodic Porous Alumina

Abstract

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is a promising candidate for starting materials of nanofabrication of various devices. Investigations concerning the cell dimensions of porous anodic alumina films have been extensively performed after the confirmation of the classical cell model proposed by Keller et al (1). It was suggested in these studies that cell dimensions, such as pore and cell diameters and barrier layer thickness, primarily depended on the formation voltage, and that they increased linearly with increasing voltage (2). Nevertheless, the importance of current density as a controlling factor of the cell geometry was sometime claimed (3, 4). According to the classical theory of ionic conduction at the high field strength for the anodic barrier film grown on various metals (5, 6), the film thickness of each metal is inversely proportional to the logarithm of ionic current when the film is formed up to the same voltage. Thus, it is indicated that the log of current density I is proportional to the electric field strength E, i.e., the formation voltage / film thickness ratio at the barrier layer, as we reported previously (7). Concerning the anion incorporation into the film, we found that anion content was also proportional to the electric field strength E, i.e., the log of current density i (7). This principal is suggested to be applicable to porous anodic films. The purpose of the present study is the confirmation of the effect of electric field strength as a controlling factor of the cell morphology of porous anodic alumina such as pore diameter / cell diameter ratio that affects to the radius of curvature of cell base pattern and porosity. In addition, effect of electric field strength on incorporation behavior of electrolyte anions into cell wall has been studied. Obtained knowledge could help us to get further insight into self-ordering mechanism of anodic porous alumina. Although pore and cell sizes of the films formed in oxalic acid were larger than those formed in sulfuric acid even at the same voltages, the ratio of cell diameter to pore diameter (dcell / d pore) of both films is in a linear relation with the logarithm of current density (log i). The electric field strength across the barrier layer of porous films varies linearly with the logarithm of current density as in the case of barrier type films. Therefore, dcell / d pore ratio is regarded to be proportional to the electric field strength. From these results, it is deduced that pore diameter is proportional to voltage and inversely propor­tional to the square of the electric field strength, whereas barrier layer thickness and cell diameter are proportional to voltage and inversely proportional to the electric field strength. Accordingly, pore diameter is more strongly affected by current density than other cell parameters are. The content of incorporated anion in the films increases linearly with increasing log i, hence electric field strength. Therefore, it is evident that the anion incorporation is exactly governed by the electric field strength. [1] F.Keller, M.S.Hunter and D.L.Robinson, This Journal, 100, 411 (1953). [2] J.P.O'Sullivan and G.C.Wood, Proc. Roy. Soc. Lond. A.317, 511 (1970). [3] K.V.Heber, Electrochim. Acta, 23, 127 (1978). [4] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J. Electrochem. Soc ., 151, B473 (2004). [5] N. Cabrera and N. F. Mott, Ret. Pror. Phys., 12, 163 (1948). [6] C. J. Dell’Oca and L. Young, J. Electrochem. Soc., 117, 1548 (1970). [7] S. Ono, F. Mizutani, M. Ue, and N. Masuko, PV 2001-22, p.1129, The Electrochemical Society Proceedings Series, Pennington, NJ (2001).