Electrochemical and Thermal Contributions to Ultra-Deep AAO Growth: Aspect Ratio >104

Abstract

Self-ordered anodic aluminum oxide (AAO) nanopores is the focus of intense research for many diverse fields including photocatalysts, electrochemical energy storage, thermoelectrics, and optics. Furthermore, the self-ordering of densely packed pores serves as an ideal template for 1-D nanostructure fabrication. Currently, hard anodization has achieved the highest growth rates (50 – 100 μm h-1) to >1000 aspect ratio1, but no further research has been presented on extended depth regimes. Technical and economic success of many of these technologies, is dependent on fast fabrication of ultra-high aspect ratio (>104) AAO arrays. We present here the high-speed growth of AAO pores with ultra-high aspect ratios achieved by balancing the thermal and electrochemical physics of pore ordering during initiation and extended steady-state growth rate. Effective management of reaction and joule heating is paramount to achieving self-ordering and preventing thermal failure of AAO during pore initiation. Traditionally, slow ramp rate (1 v/s), low concentration (~0.05M), and low electrolyte temperature (~0°C) conditions have been used to prevent initial thermal failure of potentiostatic anodizations at the cost of well-defined pore ordering. However, recent studies have shown that thermal control of the working electrode is much more effective than chilling the electrolyte2. The effect of direct potential application with sufficient heat removal by electrode cooling is discussed. Furthermore the effect of temperature was studied to optimize the rate of hard anodization at steady-state. During steady-state, isothermal anodization rate decays exponentially as a function of time making the rate infeasible to economical production of deep arrays. Electrode temperature step experiments were conducted and its effect on steady-state anodization rate is discussed. Long-term, isothermal anodizations exhibit a radial distribution of pore lengths with pore lengths decreasing with increasing radial dimension, resembling a cone structure. This effect is unfavorable as it suggests a non-ideal distribution of current thus complicating the geometry of the array. Forced ideal current distribution and its effect on extended depth anodizations is discussed. Additional technical hurdles and the results of these tests will be presented. 1. Lee, W., et al., Fast Fabrication of Long-Range Ordered Porous Alumina Membranes by Hard Anodization. Nature Materials 2006, 5 (9), 741. 2. Aerts, T., et al., Control of the Electrode Temperature for Electrochemical Studies: A New Approach Illustrated on Porous Anodizing of Aluminium. Electrochemistry Communications 2009, 11 (12), 2292.